enhanced metal recovery from a modified caron leach of mixed nickel-cobalt hydroxide. ·...
TRANSCRIPT
ENHANCED METAL RECOVERY FROM A
MODIFIED CARON LEACH OF MIXED
NICKEL-COBALT HYDROXIDE.
Andrew Jones
B.Sc. (Applied Chemistry), Hons (Mineral Science)
This thesis is presented for the degree of Doctor of Philosophy of Murdoch
University
2013
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I declare that this thesis is my own account of my research and contains as
its main content work which has not previously been submitted for a degree
at any tertiary educational institution.
………………………
Andrew Jones
December 2013
iii
ABSTRACT
In the last 20 years nickel laterites have become a popular resource due to
the economic expansions of China and India, an improvement in processing
technologies and the large unexploited orebodies around the world. The
development of a split process, producing a metal hydroxide intermediate, is
becoming popular as it lowers technical risk and capital costs. Following on
from Cawse, BHP Billiton have been instrumental in developing this process,
and produced a mixed hydroxide precipitate for approximately a year (2008)
at Ravensthorpe in Western Australia, which was processed in an ammonia
solution at the existing Yabulu refinery in Townsville Queensland. This PhD
project focused on the ageing of the precipitate which would occur during
transportation, and the subsequent leaching in an ammonia-ammonium
carbonate solution with a sulphide (CoNiS) reductant.
Metal ion hydroxides were discovered to precipitate within the pores of
magnesium hydroxide (precipitant). This meant that the precipitate particle
size was relatively large, oxidation of cobalt and manganese occurred
throughout the particles and the dissolution rate followed a shrinking core
model. Although cobalt and manganese oxidation was envisaged to be a
problem, only ~8% of cobalt and ~52% of manganese oxidised in a
Ravensthorpe sample after 12 weeks and was leached in 45 minutes in the
presence of a reductant. All oxidation occurred during precipitation, filtration
and preparation of the precipitate.
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The formation of stable slow leaching nickel-magnesium hydroxide and
hydrotalcite-like structures did affect nickel and cobalt recoveries. Reducing
the incorporation of magnesium, increasing the manganese concentration
and drying the precipitate all reduced the effect of the nickel-magnesium
hydroxide. Drying the precipitate could result in a saving in transportation
costs, while increasing the manganese concentration would lower reagent
costs and energy consumption. Aluminium, chromium(III) and sulphate
concentrations needed to be minimised to reduce the effect of hydrotalcite-
like structures. Sulphate may need to be precipitated from solution prior to
metal hydroxide precipitation.
The reaction mechanism of the reduction of high valent metal ions by mixed
cobalt nickel sulphide reductant (CoNiS) produced on-site at Yabulu was
investigated. The extent of reduction was directly related to the Co:S ratio,
however the presence of NiS was crucial as it had a faster rate of dissolution
and introduced sulphur species into solution. The ideal ratio of cobalt to
nickel was between 2:1 and 3:1. The site survey of Yabulu revealed the
potential of the leach liquors needed to be monitored to ensure cobalt existed
in the trivalent state, which is more soluble. HPLC (High Performance Liquid
Chromatography) results showed that numerous cobalt ammine species
were present in solution. As unwanted cobalt precipitation is a major cause
of lower metal recoveries and the final product is influenced by solution
chemistry, the results will help improve cobalt recovery and product grade.
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ACKNOWLEDGEMENTS
This project was originally proposed and constructed by Dr. Nicholas
Welham (Murdoch University) and Peter Anderson (BHP Billiton Yabulu
Refinery). It was Nick who suggested I commence the PhD, and through his
supervision over the years is the main reason it has actually come together.
His trust and his relaxed, honest style of supervision made him a delight to
work with. Associate Professor Gamini Senanayake was adopted as the
primary supervisor in 2008 when Nick moved to Ballarat University. Although
extremely busy, he always had time for me and was incredibly patient. His
vast experience with PhD students meant his advice through the writing
process was very valuable.
John Fittock at the Yabulu Refinery was the industry supervisor. John has
been very helpful over the years, dedicating a large amount of time to spend
with me, to answer questions and organise site visits. A genuine person and
with over 25 years of experience at the Refinery he was very knowledgeable.
Kirsten Smith, Joy Morgan, Leslie Chegwidden, Chris Nethercott and Sandra
Bessel have all helped in some way on visits to the refinery, particularly
Kirsten and Joy who spent an awful amount of time with me.
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The network of the Mineral Science department, friends and family are
another major reason this project has come to completion. Advice from staff
and fellow students, and enthralling conversations over a cup of coffee or
lunch made it a joy to be at the university. My network of friends, always
interested in doing something, has made life outside of uni very enjoyable.
Finally, Greg, Kerry, Sarah and Amy your love and support is felt, and greatly
appreciated.
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RESEARCH PUBLICATIONS
Jones, A.N. and Welham, N.J.
Properties of aged mixed nickel-cobalt hydroxide intermediates produced
from acid leach solutions and subsequent metal recovery.
Hydrometallurgy 103(2010): 173-179.
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GLOSSARY Term Definition λmax wavelength of maximum absorption Area 340 Ore leaching and washing Area 345 MHP leaching and CoNiS precipitation Area 352 Stripping stills and gas recovery ASX ammoniacal solvent extraction
CCD Counter Current Decantation; a process for separating pregnant leach liquor from tailings in a series of thickeners
CoNiS cobalt-nickel sulphide ECoR Enhanced Cobalt Recovery EN European Nickel FLL fresh leach liquor
Free NH3 free NH3 = titrated NH3 – 6 x 17 / 58.7 x [Ni + Co] – 2 x 17 / 44 x [CO2]
Hexammine (hexa) hexamminecobalt(III), [CoIII(NH3)6]3+
HPLC high performance liquid chromatography Hydrotalcite Mg6Al2(OH)16CO3.4H2O ICP inductively coupled plasma Leached pulp mix of leached ore and leachate (leach discharge solution)
MES Report Online lab results submitted by the Quantitative Analysis Laboratory
Metsim Program used to model concentrations and flow rates for the refinery
MHP mixed hydroxide precipitate nm nanometer ORP oxidation-reduction potential
Oxidise to increase the oxidation state of an element or compound, remove electrons
Pentammine (penta) pentammine(carbonato)cobalt(III), [CoIII(NH3)5CO3]+
PL product liquor ppm parts per million
Preboil process step in which Product Liquor is steam stripped to lower the ammonia content from ~90 g/L to ~40 g/L
Preboil solids precipitate formed in the preboil process, comprising manganese, iron, nickel, cobalt and magnesium hydroxides and carbonates
Reduce to decrease the oxidation state of an element or compound, add electrons
RNO Ravensthorpe Nickel Operations RCPT reductive complexing predictor leach test RPT reductive predictor leach test RSPT reductive soak predictor leach test SAC synthetic ammonium carbonate SPL special product liquor
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SPT standard predictor leach test SSPT standard soak predictor leach test Sulfato pentammine(sulphato)cobalt(III), [CoIII(NH3)5SO4]+ Sulfito pentammine(sulphito)cobalt(III), [CoIII(NH3)5SO3]+ Tailings final residue from the leaching process Tetrammine (tetra) tetrammine(carbonato)cobalt(III), [CoIII(NH3)4CO3]+
Thiosulphato pentammine(thiosulphato)cobalt(III), [CoIII(NH3)5S2O3]+
Titratable NH3 ammonia content of a solution determined by direct acid titration
Total NH3 NH3 content of a solution determined by Kjeldahl analysis, includes NH4
+ XRD X-ray diffraction YEP Yabulu Expansion Project
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TABLE OF CONTENTS 1 INTRODUCTION ................................................................................. 1-1
1.1 Nickel Ores .................................................................................. 1-1 1.2 Processing Laterite Ores ........................................................... 1-2
1.2.1 Major Routes ......................................................................... 1-2 1.2.2 Pressure Acid Leaching (PAL) Process ................................. 1-5 1.2.3 Ammoniacal Carbonate Leaching (Caron) Process .............. 1-8
1.3 Commercial PAL Processes .................................................... 1-10 1.3.1 Proposed and Piloted Processes ........................................ 1-11 1.3.2 Murrin Murrin, Bulong and Cawse ....................................... 1-12 1.3.3 Ravensthorpe Project and Yabulu Extension ...................... 1-13 1.3.4 Current/Future Projects ....................................................... 1-18
1.4 Project Aim................................................................................ 1-19
2 LITERATURE REVIEW ....................................................................... 2-1
2.1 Laboratory Synthesis of Metal Hydroxides .............................. 2-1 2.1.1 Nickel Hydroxide ................................................................... 2-1 2.1.2 Cobalt Hydroxide ................................................................... 2-8 2.1.3 Manganese Hydroxide ........................................................... 2-9 2.1.4 Magnesium Hydroxide ......................................................... 2-11 2.1.5 Mixed Metal Hydroxides ...................................................... 2-12 2.1.6 Comparison of Precipitating Agents .................................... 2-16
2.2 Commercial Production of Mixed Nickel-Cobalt Hydroxide . 2-16 2.2.1 Cawse – Original Flowsheet ................................................ 2-16 2.2.2 Ravensthorpe Process ........................................................ 2-17 2.2.3 Ramu Process ..................................................................... 2-19 2.2.4 European Nickel Process .................................................... 2-20 2.2.5 Niquel do Vermelho Process ............................................... 2-20 2.2.6 Comparison of Flowsheets .................................................. 2-21
2.3 ‘Ageing’ of MHP ........................................................................ 2-22 2.3.1 Formation of High-Valent oxides ......................................... 2-23 2.3.2 Formation of Insoluble or Slow-Leaching Compounds ........ 2-27 2.3.3 Other Possible Ageing Processes/Influences ...................... 2-30
2.4 Drying MHP ............................................................................... 2-31 2.5 Chemistry of Leaching of MHP in SAC Solutions .................. 2-32
2.5.1 Three Stage Leaching Process ........................................... 2-32 2.5.2 Metal Ammine Complexes ................................................... 2-33 2.5.3 Measured Metal Ion Solubility ............................................. 2-39 2.5.4 Leach Kinetics ..................................................................... 2-42 2.5.5 Impurities in MHP ................................................................ 2-43 2.5.6 Reductive Leaching of MHP ................................................ 2-45 2.5.7 Effect of Soaking ................................................................. 2-47
2.6 Metal Sulfides as Reducing Agents ........................................ 2-48 2.6.1 Precipitation Process ........................................................... 2-48 2.6.2 Precipitation Kinetics ........................................................... 2-50 2.6.3 Practical Difficulties ............................................................. 2-51 2.6.4 Reducing Properties ............................................................ 2-52
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3 MATERIALS AND METHODS .......................................................... 3-1 3.1 Reagents and Industry Samples ............................................... 3-1 3.2 Synthesis of Mn3O4 ..................................................................... 3-1 3.3 Synthesis of MnOOH .................................................................. 3-4 3.4 Precipitation of MHP .................................................................. 3-5
3.4.1 Precipitates for the Effect of Composition ............................. 3-5 3.4.2 Precipitates for the Effect of Drying ....................................... 3-8 3.4.3 Simple Metal Hydroxides ....................................................... 3-8 3.4.4 Nickel-Magnesium Hydroxide for Solubility Testing ............... 3-9 3.4.5 Transformation of MgO to Mg(OH)2 ...................................... 3-9 3.4.6 Influence of Magnesium Content ........................................... 3-9 3.4.7 Influence of Ageing of Mixed Nickel-Magnesium Hydroxide .. 3-9 3.4.8 Influence of Co, Mn, Al and Cr ............................................ 3-10 3.4.9 Influence of Cobalt(II) and Cobalt(III) Valency ..................... 3-11 3.4.10 Influence of Crystallinity ....................................................... 3-12 3.4.11 Precipitates for Oven Ageing ............................................... 3-12 3.4.12 Elevated Temperature Precipitation .................................... 3-14 3.4.13 Precipitation Mechanism ..................................................... 3-15
3.5 CoNiS Preparation .................................................................... 3-16 3.6 Leach Tests ............................................................................... 3-19
3.6.1 Synthetic Ammonium Carbonate (SAC) Leach Solution ...... 3-19 3.6.2 Predictor Leach Tests ......................................................... 3-20 3.6.3 Modified Predictor Leach Tests ........................................... 3-22 3.6.4 Reductive Leaching of Oxidised Mn and Co Hydroxides ..... 3-24 3.6.5 Batch Leach Tests ............................................................... 3-24 3.6.6 Kinetic Leach Tests ............................................................. 3-25 3.6.7 Effect of Anions on Ni(II) Solubility ...................................... 3-26
3.7 Analysis ..................................................................................... 3-27 3.7.1 Moisture Content ................................................................. 3-28 3.7.2 Determination of Extent of Oxidation ................................... 3-29 3.7.3 Atomic Absorption Spectrometry ......................................... 3-30 3.7.4 Inductively Coupled Plasma Mass Spectrometry ................ 3-30 3.7.5 X-Ray Diffraction ................................................................. 3-30 3.7.6 Neutron Diffraction .............................................................. 3-31 3.7.7 Scanning Electron Microscopy ............................................ 3-32 3.7.8 Optical Microscopy .............................................................. 3-33 3.7.9 Thermogravimetric Analysis ................................................ 3-33 3.7.10 Laser Size Analysis ............................................................. 3-33 3.7.11 BET Surface Area Tests ...................................................... 3-34 3.7.12 Infrared and Raman Spectroscopy ...................................... 3-34 3.7.13 High Performance Liquid Chromatography ......................... 3-35 3.7.14 X-Ray Photoelectron Spectroscopy ..................................... 3-35
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4 SYNTHESIS, CHARACTERISATION AND REDUCTIVE LEACHING OF OXIDISED MANGANESE AND COBALT HYDROXIDES ........... 4-1
4.1 Introduction and Experimental .................................................. 4-1 4.2 PrecipitationCharacterisation of a Single Phase MnOOH ....... 4-3 4.3 Reductive Leaching of MnOOH and Mn3O4 with NH2OH and
Co(II). ........................................................................................... 4-6 4.4 Reductive Leaching of Mn3O4 with Sulfite and Co(II). ........... 4-11 4.5 Reductive Leaching of Mixed Oxidised Mn-Co Hydroxide
with Sulphite and Co(II). .......................................................... 4-14 4.6 Summary ................................................................................... 4-18
5 CHARACTERISICS AND PROPERTIES OF MgO AND SYNTHETIC MIXED HYDROXIDE PRECIPITATES ............................................... 5-1
5.1 Introduction and Experimental .................................................. 5-1 5.2 Composition and Properties of MgO ........................................ 5-3
5.2.1 Chemical Analysis and Size Distribution ............................... 5-3 5.2.2 Dissolution of MgO and Reprecipitation Mg(OH)2 ................. 5-4 5.2.3 Rate of Hydration of MgO ...................................................... 5-7
5.3 Synthetic MHP ............................................................................ 5-8 5.3.1 Mechanism of Precipitation ................................................... 5-8
5.4 Effect of pH and Initial Metal Solution Concentration on MHP Composition .................................................................... 5-14
5.4.1 Precipitation Diagrams ........................................................ 5-14 5.4.2 Effect of Initial Metal Ion Concentration ............................... 5-17 5.4.3 Effect of Cobalt and Manganese ......................................... 5-20 5.4.4 Discussion of Assay Results ............................................... 5-22 5.4.5 Effect of Cation Softness ..................................................... 5-26 5.4.6 Variation of Ni/Mg and Co/Mn Molar Ratio .......................... 5-29
5.5 Size Distribution of MHP .......................................................... 5-32 5.6 Moisture Content ...................................................................... 5-37 5.7 Extent of Oxidation During Ageing ......................................... 5-40 5.8 X-Ray Diffraction Patterns ....................................................... 5-47
5.8.1 Effect of Ageing of MHP ...................................................... 5-47 5.8.2 Effect of Ageing on Crystalline Ni-Mg Hydroxide ................. 5-58 5.8.3 Effect of Anions on Oven Ageing of Mixed Hydroxides ....... 5-60 5.8.4 Effect of Precipitation at Elevated Temperatures ................ 5-64
5.9 Scanning Electron Microscopy ............................................... 5-71 5.9.1 Synthetic MHP ..................................................................... 5-71
5.10 Summary ................................................................................... 5-75
6 LEACHING OF SYNTHETIC HYDROXIDE PRECIPITATES………6-1 6.1 Introduction and Experimental .................................................. 6-1 6.2 Effect of Ageing on Leaching .................................................... 6-3 6.3 Effect of metal Ion Composition on Leaching .......................... 6-5
6.3.1 General Comparison ............................................................. 6-5 6.3.2 Effect of Magnesium, Cobalt and Manganese on Leaching .. 6-8 6.3.3 Nickel-Cobalt Correlation ..................................................... 6-16 6.3.4 Effect of Al, Fe, Cr(VI), Zn, Cu & Si in the Absence of Mn.. 6-21
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6.4 X-Ray Diffraction of Leach Residues ...................................... 6-25 6.5 Effect of Drying, Ageing and Heating ..................................... 6-31
6.5.1 Effect of Moisture Content ................................................... 6-31 6.5.2 Effect of Ageing Dried Precipitates ...................................... 6-39 6.5.3 Effect of Heating Precipitates .............................................. 6-41
6.6 Leaching Kinetics of Synthetic MHP ...................................... 6-45 6.6.1 Mathematical Expressions for Kinetic Analysis ................... 6-45 6.6.2 Porosity of Starting Material ................................................ 6-47 6.6.3 Effect of Crystallinity ............................................................ 6-49 6.6.4 Effect of Particle Size .......................................................... 6-53 6.6.5 Effect of Magnesium Content .............................................. 6-54 6.6.6 Effect of Oxidation of Co(II) ................................................. 6-58 6.6.7 Effect of Other Metal Ions and Crystallinty .......................... 6-61
6.7 Summary and Conclusions ..................................................... 6-70
7 CHARACTERISATION AND LEACHING OF COMMERCIAL MIXED HYDROXIDE PRECIPITATES ........................................................... 7-1
7.1 Introduction and Experimental .................................................. 7-1 7.2 Composition and Characterisation ........................................... 7-3
7.2.1 Chemical Analysis ................................................................. 7-3 7.2.2 Collection and Size Analysis of RNO MHP ............................ 7-5 7.2.3 X-Ray and Neutron Diffraction Analysis of RNO MHP .......... 7-7 7.2.4 SEM and EDS of RNO MHP ............................................... 7-11
7.3 Oxidation States of Mn and Co in RNO MHP.......................... 7-16 7.4 Ageing and Drying of RNO-MHP ............................................. 7-20 7.5 Leaching Kinetics of RNO MHP ............................................... 7-22 7.6 Predictor Leach Test Results .................................................. 7-31
7.6.1 General Comparison of Different Commercial Precipitates . 7-31 7.6.2 Effect of composition on Standard Predictor Test Results .. 7-35 7.6.3 Predictor Leach Test Results - Preboil Solids Sample ........ 7-38 7.6.4 Predictor Leach Test Results – RNO Pilot Plant MHP ........ 7-39 7.6.5 Predictor Leach Test Results - Cawse MHP ....................... 7-42 7.6.6 Predictor Leach Test Results - European Nickel MHP ........ 7-44 7.6.7 Predictor Leach Test Results of RNO-June Sample ........... 7-48
7.7 Summary and Conclusions ..................................................... 7-52
8 REDUCTIVE LEACHING OF MIXED HYDROXIDE PRECIPITATE WITH COBALT-NICKEL-SULFIDES (CoNiS) ................................... 8-1
8.1 Introduction ................................................................................. 8-1 8.2 Precipitation and Characterisation of CoNiS .......................... 8-2
8.2.1 Precipitation Diagrams .......................................................... 8-2 8.2.2 Precipitation and Analysis ..................................................... 8-3 8.2.3 XRD and SEM of CoNiS ........................................................ 8-8
8.3 Redox Behaviour of Sulfides in SAC solutions ..................... 8-10 8.3.1 Redox Behaviour of Elemental Sulphur and Sulphide ions . 8-11 8.3.2 Redox/dissolution Behaviour of NiS and CoS ..................... 8-15 8.3.3 Redox/dissolution Behaviour of CoNiS ................................ 8-21 8.3.4 Relative Dissolution of Ni(II) and Co(II) from CoNiS ............ 8-24 8.3.5 ORP of NiS, CoS and CoNiS in SAC Solutions ................... 8-25
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8.4 Reductive Leaching of MnOOH by CoNiS in SAC Solution .. 8-27 8.5 Summary and Conclusions ..................................................... 8-34
9 YABULU REFINERY PLANT SURVEY .............................................. 9-1 9.1 Introduction and Experimental .................................................. 9-1 9.2 Yabulu Flowsheets ..................................................................... 9-3 9.3 Oxidation Reduction Potentials (ORP) ..................................... 9-7 9.4 XRD of Plant Solids .................................................................. 9-11 9.5 Cobalt Speciation in Plant Liquors ......................................... 9-18 9.6 Cobalt Speciation in Batch Leach Tests of MHP ................... 9-24 9.7 Secondary Leaching of MHP with CoNiS ............................... 9-33 9.8 Summary and Conclusions ..................................................... 9-35
10 SUMMARY, CONCLUSIONS AND FUTURE WORK ....................... 10-1 10.1 Precipitation Mechanism ......................................................... 10-1 10.2 Composition of Precipitates .................................................... 10-2 10.3 Oxidation During Precipitation ................................................ 10-2 10.4 Slow Leaching Compounds in MHP ....................................... 10-3 10.5 Remedies to Improve MHP Leaching ...................................... 10-4 10.6 Leaching Kinetics of MHP........................................................ 10-6 10.7 Precipitation of CoNiS .............................................................. 10-7 10.8 Yabulu Plant Survey ................................................................. 10-7 10.9 Future Work .............................................................................. 10-8
REFERENCES APPENDIX
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LIST OF FIGURES Figure 1.1. Simplified flowsheet of Caron process. .................................... 1.9 Figure 1.2. Ravensthorpe flowsheet. ....................................................... 1.15 Figure 1.3. Flowsheet of the Yabulu refinery with MHP processing circuit .................................................................................................................. 1.16 Figure 2.1. A solubility diagram of metal hydroxides at 25°C .................... 2-12 Figure 2.2. Brucite and hydrotalcite structure. .......................................... 2-24 Figure 2.3. Eh-pH diagram of Co-H2O and Mn-H2O systems for 0.01 M Mn(II) and 0.1 M Co(II). ....................................................................................... 2-25 Figure 2.4. Hydrotalcite structure. ............................................................. 2-29 Figure 2.5. Potential-pH diagrams for Ni-NH3-H2O system at 25°C and 1 atm. 1. Ni(NH3)2+; 2. Ni(NH3)2
2+; 3. Ni(NH3)32+; 4. Ni(NH3)4
2+; 5. Ni(NH3)52+;
6. Ni(NH3)62+. a) activity of ionic species is unity, b) activity of ionic species is
10-2, c) activity of ionic species is 10-4. ...................................................... 2-36 Figure 2.6. Potential-pH diagrams for Co-NH3-H2O system at 25°C and 1 atm. 1. Co(NH3)2+; 2. Co(NH3)2
2+; 3. Co(NH3)32+; 4. Co(NH3)4
2+; 5. Co(NH3)5
2+; 6. Co(NH3)62+. a) activity of ionic species is unity, b) activity of
ionic species is 10-2, c) activity of ionic species is 10-4. ............................. 2-37 Figure 2.7. Eh-pH diagram of Ni-ammonia-carbonate system at 30°C. .... 2-38 Figure 2.8. Eh-pH diagram of Co-ammonia-carbonate system at 30°C .... 2-38 Figure 2.9. Eh-pH diagram of Mn-ammonia-carbonate system at 30°C ... 2-39 Figure 2.10. Nickel(II) carbonate solubility at 45°C depending on ammonia concentration and NH3:CO2 ratio .............................................................. 2-40 Figure 2.11. Cobalt(II) hydroxide solubility at 45°C depending on ammonia concentration and NH3:CO2 ratio .............................................................. 2-40 Figure 2.12. Manganese(II) chloride solubility at 45°C depending on ammonia concentration and NH3:CO2 ratio............................................... 2-41 Figure 2.13. Iron(II) chloride solubility at 45°C depending on ammonia concentration and NH3:CO2 ratio .............................................................. 2-41 Figure 2.14. Sulphide solubility diagram at 25°C ...................................... 2-49 Figure 3.1. Dropwise addition of NaOH to manganese solution. ................ 3-3
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Figure 3.2. Manganese hydroxide precipitate and solution after overnight air sparging ...................................................................................................... 3-3 Figure 3.3. Refluxing to produce MnOOH. .................................................. 3-4 Figure 3.4. Precipitation of mixed hydroxides. ............................................ 3-6 Figure 3.5. Precipitates stored in sample jars. ............................................ 3-7 Figure 3.6. Bottles used for oven ageing. ................................................. 3-13 Figure 3.7. Picture of elevated temperature precipitation.......................... 3-15 Figure 3.8. Effect of H2S:Co stoichiometry in thickener-2 overflow on Yabulu-CoNiS composition .................................................................................. 3-18 Figure 3.9. Reactors used for leach tests. ................................................ 3-21 Figure 3.10. Mill drive used for modified predictor tests. ........................... 3-23 Figure 3.11. Clips on mill drive holding centrifuge tubes. .......................... 3-24 Figure 3.12. Vessel in oven used for drying. ............................................. 3-28 Figure 3.13. Stubs prepared for SEM ....................................................... 3-32 Figure 3.14. Precipitates embedded in resin blocks for SEM and EDS analysis ..................................................................................................... 3-33 Figure 4.1. XRD scans of various products formed during manganite precipitation ................................................................................................ 4-5 Figure 4.2. XRD scans of Mn3O4, MnOOH and a mixture. .......................... 4-6 Figure 4.3. Extent of reduction of Mn3O4, a mixture and MnOOH in SAC solution, under reducing conditions using either cobalt(II) or hydroxylamine sulphate. ..................................................................................................... 4-7 Figure 4.4. Eh-pH diagram for Mn-Co-NH3-H2O system. (a) 10-6 Mn and 1 M NH3 at 250C (b) 10-6 Co and 1 M NH3 at 250C ............................................ 4-9 Figure 4.5. XRD scans of MnOOH/Mn3O4 mixed phase and leach residues using Co(II) and hydroxylamine sulphate as reductants ........................... 4-10 Figure 4.6. XRD scans of MnOOH and leach residues using Co(II) and hydroxylamine sulphate as reductants ...................................................... 4-11 Figure 4.7. XRD scan of original sample, and leach residues after reduction of Mn3O4 in SAC with sulphite, cobalt(II) or hydroxylamine sulphate ........ 4-12
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Figure 4.8. Extent of reduction of Mn3O4 using SO32- or Co2+ as reducing
agents in a SAC (carbonate) or sulphate solution ..................................... 4-12 Figure 4.9. Eh-pH Diagram of Mn-Co-O2-H2O system under standard conditions at 25oC ..................................................................................... 4-15 Figure 4.10. XRD scans of the mixed Mn, Co oxidised hydroxide before and after leaching. ........................................................................................... 4-16 Figure 4.11. Extent of reduction of a mixed Mn, Co oxidised hydroxide using SO3
2- or Co(II) in a SAC solution. .............................................................. 4-16 Figure 4.12. XRD scans of a mixed Mn3O4 and CoOOH precipitate before and after leaching. .................................................................................... 4-18 Figure 5.1. Size analysis of MgO. ............................................................... 5-4 Figure 5.2. SEM Image of MgO. ................................................................. 5-4 Figure 5.3. MgO dissolution at 25°C in SAC solution and water. ................ 5-6 Figure 5.4. XRD scans of 60% MgO/water mixture after 1, 2, 3 & 4 days .. 5-7 Figure 5.5. Change in size distribution of precipitates at 25°C over 240 minutes. ...................................................................................................... 5-9 Figure 5.6. SEM and EDS images of precipitate at 25°C after 5 minutes. 5-11 Figure 5.7. SEM and EDS images of precipitate at 25°C after 30 minutes. ..... .................................................................................................................. 5-11 Figure 5.8. SEM and EDS images of precipitate at 25°C after 240 minutes. ... .................................................................................................................. 5-12 Figure 5.9. HRTEM image of MgO-1-520N ............................................... 5-13 Figure 5.10. Cross section SEM image of MgO after 30 minutes in water at 25°C. ......................................................................................................... 5-13 Figure 5.11. Precipitation of metals with rising pH at 25°C. ...................... 5-15 Figure 5.12. A solubility diagram of metal hydroxides based on KSP at 25°C .................................................................................................................. 5-16 Figure 5.13. Ni/Mg or Co/Mn molar ratios in precipitates .......................... 5-21 Figure 5.14. Effect of Mn(II) in solution on Mn in synthetic MHP .............. 5-25 Figure 5.15. Effect of Mn(II) in solution on ratio of nickel and cobalt incorporation in synthetic MHP. ................................................................ 5-25
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Figure 5.16. Effect of covalent radii on cation softness ............................. 5-28 Figure 5.17. Effect of cation softness on pKSP of hydroxides of M(II) and M(III) ......................................................................................................... 5-28 Figure 5.18. Effect of cation softness on metal assays of dry precipitates of groups 1, 2 and 4 ...................................................................................... 5-29 Figure 5.19. Effect of different metal ion compositions on Ni/Mg molar ratio in dry precipitates in Groups 1, 2 and 4 ........................................................ 5-30 Figure 5.20. Effect of initial Mn(II) concentration on Ni/Mg molar ratio in dry precipitates in Group 3 .............................................................................. 5-31 Figure 5.21. Effect of initial Mn(II) concentration on Ni/Mg molar ratio in dry precipitates in Group 5 .............................................................................. 5-32 Figure 5.22. Size distribution of MgO. ....................................................... 5-33 Figure 5.23. Size distribution of MHP’s, A-H – 6 weeks – cumulative percent passing. .................................................................................................... 5-33 Figure 5.24. Size distribution of MHP’s, A, B, I-N – 6 weeks – cumulative percent passing. ....................................................................................... 5-33 Figure 5.25. Size distribution of MHP’s, A-H – 6 weeks – percent passing. ..... .................................................................................................................. 5-34 Figure 5.26. Size distribution of MHP’s, A, B, I-N – 6 weeks – percent passing. .................................................................................................... 5-34 Figure 5.27. Size distribution of precipitates O – AA over time. ................ 5-35 Figure 5.28. Photo of Ni, Co, Mn precipitate after 2 days (left) and a year (right), precipitate was in a sealed plastic jar. ........................................... 5-36 Figure 5.29. Percent passing, precipitates O - AA – week 1. .................... 5-37 Figure 5.30. Percent passing over time – precipitate O. ........................... 5-37 Figure 5.31. Percent solids of precipitates A – N over time. ..................... 5-39 Figure 5.32. Percent solids of precipitates O – AE over time. ................... 5-39 Figure 5.33. Extent of oxidation titration results (EO%) over time, precipitates A - N.......................................................................................................... 5-42 Figure 5.34. Unoxidised % of Co(II) over time in precipitates A - N. ......... 5-44
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Figure 5.35. Effect of sulphate ion concentration in initial solution on Unoxidised % of Co(II) over time in precipitates A - N. ............................. 5-44 Figure 5.36. Eh-pH Diagram of Co-Si-O2-H2O system under standard conditions at 25oC. .................................................................................... 5-45 Figure 5.37. Ni/Mg and Co/Mn molar ratio in dry precipitate. .................... 5-46 Figure 5.38. XRD scans of precipitate A (Ni, Co, Mg) over 9 weeks. ........ 5-48 Figure 5.39. XRD scans of precipitate B (Ni, Co, Mg, Mn) over 9 weeks. . 5-48 Figure 5.40. XRD scans of precipitate C (Ni, Co, Mg, Mn, Al) over 9 weeks. .................................................................................................................. 5-49 Figure 5.41. Percentage of MgO in precipitates (rough calculation: height of MgO peak at 43° divided by total height of MgO and metal hydroxide peaks at 43° and 38°, respectively). .................................................................... 5-50 Figure 5.42. Hydrotalcite structures (a) general formula and structure, (b) Mg6Al2(CO3)(OH)16.4H2O, and (c) other structures with trivalent cations similar to hydrotalcite. ............................................................................... 5-52 Figure 5.43. Ni/Mg hydroxide peaks at 38° of precipitate A at times 16, 25, 36, 63 and 84 days. .................................................................................. 5-54 Figure 5.44. Ni/Mg and Co/Mn molar ratio in dry precipitates-S and AB-AE .... .................................................................................................................. 5-55 Figure 5.45. Percentage of MgO in precipitates O – S (rough calculation based on peak heights). ............................................................................ 5-57 Figure 5.46. Percentage of MgO in precipitates AB – AE (rough calculation based on peak heights). ............................................................................ 5-57 Figure 5.47. XRD scans of a mixed Ni-Mg(OH)2 precipitate immediately after precipitation and after ageing for approximately a year. ........................... 5-59 Figure 5.48. XRD scans of a mixture of Ni(OH)2 and Mg(OH)2 after precipitation and after ageing for approximately a year. ........................... 5-59 Figure 5.49. XRD scans after oven ageing - batch 1, 12 weeks………….5-61 Figure 5.50. XRD scans after oven ageing, batch 2, introduction of anions (SO4
2-, CO32-, Cl-), 12 weeks ageing. ........................................................ 5-62
Figure 5.51. XRD scans after oven ageing, batch 3, 12 weeks ageing with 2 g/L SO4
2- (from metal sulphate) ............................................................. 5-62
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Figure 5.52. XRD scans after oven ageing, batch 3, 12 weeks of ageing with 5 g/L CaCO3. ............................................................................................ 5-63 Figure 5.53. XRD scans after oven ageing, batch 3, 12 weeks of ageing with 15 g/L NaCl. .............................................................................................. 5-63 Figure 5.54. XRD scans of 12 precipitates, batch 4. ................................. 5-65 Figure 5.55. XRD scan of Ni precipitate. ................................................... 5-66 Figure 5.56. XRD scans of Ni/Co and Ni/Mn precipitates. ........................ 5-67 Figure 5.57. XRD scans of Ni/Fe and Ni/Al precipitates. ........................... 5-68 Figure 5.58. XRD scans of Ni/Ca, Ni/Cr, Ni/Si and Ni/Zn precipitates....... 5-69 Figure 5.59. XRD scans of Ni/Cu precipitate. ........................................... 5-69 Figure 5.60. XRD scan of a Ni/Mn precipitate. .......................................... 5-71 Figure 5.61. Back scattered electron image of precipitate P – week 1. .... 5-73 Figure 5.62. Back scattered electron image of precipitate P – week 3. .... 5-74 Figure 5.63. Back scattered electron image of precipitate P – week 12. .. 5-74 Figure 5.64. Elemental mapping of precipitate P – week 1. ...................... 5-75 Figure 6.1. Nickel leaching results in Modified Standard Predictor Test in SAC over 12 weeks– precipitates A – D. .................................................... 6-3 Figure 6.2. Nickel leaching results in Modified Reductive Predictor Test in SAC with hydroxylamine sulphate over 12 weeks– precipitates A – D. ...... 6-4 Figure 6.3. Effect of Mg% on Ni/Mg molar ratio and Ni% in precipitate (data from Table 6.4) ........................................................................................... 6-9 Figure 6.4. Effect of Mg, Co and Mn content in precipitate on Ni leaching in SPT, RPT and RSPT ................................................................................ 6-10 Figure 6.5. Comparison of Ni and Co leach results (% and molar ratio) in SPT and RPT. ........................................................................................... 6-18 Figure 6.6. Ni-Co Correlations based on leaching results of precipitates A-AE in STT, RPT and RST ............................................................................... 6-20 Figure 6.7. Effect of metal ions in SPT of Group 4 precipitates on (a) Ni/Mg molar ratio, (b) nickel leaching, and (c) cobalt leaching. ........................... 6-22
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Figure 6.8. Effect of metal ions in RPT of Group 4 precipitates on (a) nickel leaching, and (b) cobalt leaching. ............................................................. 6-23 Figure 6.9. Effect of metal ions in RSPT of Group 4 precipitates on (a) nickel leaching, and (b) cobalt leaching. ............................................................. 6-24 Figure 6.10. XRD scans of standard predictor test residues A – D after 6 and 12 weeks (37 and 85 days) ageing. .......................................................... 6-27 Figure 6.11. XRD scans of reductive predictor test residues A - D after 6 and 12 weeks ageing. ...................................................................................... 6-28 Figure 6.12. XRD scans of reductive predictor leach residues – 12 weeks – T, U, V, W. ................................................................................................ 6-28 Figure 6.13. XRD scans of reductive soak predictor test residues after 12 weeks ageing. ........................................................................................... 6-29 Figure 6.14. XRD scans of precipitates MHP1-MHP7............................... 6-29 Figure 6.15. XRD scans of standard predictor leach test residues of MHP1-MHP7. ....................................................................................................... 6-30 Figure 6.16. Standard predictor leach test results - effect of drying for 5 and 20 hours at 50°C, and % solids on nickel recovery. .................................. 6-32 Figure 6.17. Microscope picture of Ni/Mg precipitate. ............................... 6-33 Figure 6.18. XRD scans of Ni/Mg precipitate of 56% solids and 68% and 95% solids obtained after drying for 5 and 20 hours at 50°C. ................... 6-34 Figure 6.19. Analysis of XRD peak at 38° of Ni/Mg precipitate of different % solids. ........................................................................................................ 6-35 Figure 6.20. XRD scans of Ni/Co/Mg/Al precipitate of 45% solids 73% and 81% solids obtained after drying for 5 and 20 hours at 50°C. ................... 6-36 Figure 6.21. XRD scans of Ni/Co/Mg/Al leach residues. ........................... 6-36 Figure 6.22. XRD scans of Ni/Co/Mg/Fe precipitate of 42% solids and 52% and 97% solids obtained after drying for 5 and 20 hours at 50°C. ............ 6-38 Figure 6.23. XRD scans of Ni/Co/Mg/Fe leach residues ........................... 6-38 Figure 6.24. Standard predictor leach test results showing the effect of drying on nickel recovery from aged precipitates. ................................................ 6-39 Figure 6.25. Reductive predictor leach test results showing the effect of drying on nickel recovery from aged precipitates. ..................................... 6-41
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Figure 6.26. TGA plots for Ni/Mg, Ni/Mg/Co, Ni/Mg/Al and Ni/Mg/Fe precipitates after 6 weeks ageing. ............................................................ 6-42 Figure 6.27. XRD scans of Ni, Mg precipitate after heating at 200, 450 and 1000°C. ..................................................................................................... 6-42 Figure 6.28. Slope (Wt %/°C) of TGA plot for Ni/Mg, Ni/Mg/Co, Ni/Mg/Al and Ni/Mg/Fe precipitates. ............................................................................... 6-43 Figure 6.29. Nickel recovery from precipitates after drying at 200°C. ....... 6-44 Figure 6.30. XRD scans of precipitates dried at 200°C. ............................ 6-45 Figure 6.31. Ratio of BET surface area:laser sizer surface area vs. time of leaching. ................................................................................................... 6-49 Figure 6.32. XRD scans of nickel magnesium precipitates of varying crystallinity. ............................................................................................... 6-50 Figure 6.33. Nickel recovery from precipitates over a 20 minute period. .. 6-51 Figure 6.34. Leaching of Ni/Mg precipitates NiMg1-NiMg5: (a) effect of initial Ni(II) concentration on initial rates, (b) testing of a shrinking sphere model, (c) testing of a shrinking core model ......................................................... 6-52 Figure 6.35. Nickel leaching from precipitate – influence of particle size at 20 g/L solid/liquid ratio. ............................................................................. 6-53 Figure 6.36. Applicability of a shrinking core model for Ni leaching from Ni-Mg hydroxide precipitates of different particle sizes: (a) 25-38 μm, (b) 38-53 μm, (c) 53-75 μm, (d) plot of apparent rate constant as a function of 1/r2. ........................................................................................................... 6-54 Figure 6.37. Effect of Ni/Mg ratio on nickel recovery over time. ................ 6-55 Figure 6.38. Effect of Ni/Mg ratio in Ni-Mg-hydroxide precipitate on Ni leaching kinetics: (a) Log-Log plot of initial rates as a function of Ni/Mg ratio; (b) Shrinking sphere model; (c) Shrinking core kinetic model ................... 6-57 Figure 6.39. XRD scans of cobalt precipitates .......................................... 6-59 Figure 6.40. Cobalt leaching from unoxidised and oxidised cobalt hydroxide precipitates in a SAC solution. .................................................................. 6-60 Figure 6.41. Cobalt leaching from unoxidised and oxidised cobalt hydroxide precipitates in a SAC solution. .................................................................. 6-60 Figure 6.42. Applicability of shrinking core kinetic model for Co(II) and Ni(II) leaching in SAC solutions: (a) from precipitate of low Co(III), (b) from NiMg5. .................................................................................................................. 6-61
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Figure 6.43. Nickel leach results for elevated temperature precipitates at 20 g/L in a SAC solution. .......................................................................... 6-63 Figure 6.44. Effect of metal ions on initial rates and final Ni leaching after 60 minutes ..................................................................................................... 6-64 Figure 6.45. Testing the applicability of shrinking sphere or core kinetic models for nickel hydroxide precipitates containing Si or Cr. .................... 6-67 Figure 6.46. Testing the applicability of a shrinking core kinetic model for nickel hydroxide precipitates containing other metal ions. ........................ 6-69 Figure 6.47. Effect of metal ions on the apparent rate constants and nickel leaching in SAC solutions ......................................................................... 6-69 Figure 7.1. Size distribution of RNO MHP samples. ................................... 7-6 Figure 7.2. Size distribution of RNO MHP collected June 2008. ................. 7-6 Figure 7.3. XRD scans of MHP Samples .................................................... 7-8 Figure 7.4. XRD scan of RNO MHP collected June 2008 – 1 week. ........... 7-8 Figure 7.5. Neutron Diffraction pattern of RNO MHP – 1 week. ................ 7-10 Figure 7.6. XRD pattern of Preboil Solids ............................................... 7-11 Figure 7.7. Back scatter electron SEM image of precipitate 1A (MgO addition point). ........................................................................................................ 7-12 Figure 7.8. SEM and EDS images of precipitate 1A (MgO addition point) ....... ................................................................................................................. .7-13 Figure 7.9. SEM and EDS images of precipitate 2A (outside 1st Tank). .... 7-14 Figure 7.10. SEM and EDS images of precipitate 3A (2nd tank). ............... 7-14 Figure 7.11. SEM and EDS images of precipitate 4A (3rd tank). ............... 7-15 Figure 7.12. XPS scan of RNO MHP June 2008, Mg Kα1 source, Mn 2p doublet. ..................................................................................................... 7-16 Figure 7.13. XPS scan of RNO MHP June 2008, Mg Kα1 source, Co 2p doublet. ..................................................................................................... 7-17 Figure 7.14. Laser size analysis of Ni/Co/Mn/Mg precipitate. ................... 7-18 Figure 7.15. XPS scans of Ni/Co/Mn/Mg precipitate, Al Kα1 source, Co 2p doublet. ..................................................................................................... 7-19
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Figure 7.16. XPS scans of Ni/Co/Mn/Mg precipitate, Al Kα1 source, Mn 2p doublet. ..................................................................................................... 7-19 Figure 7.17. XRD scans of RNO MHP over time – 57, 81 & 100% solids. 7-21 Figure 7.18. SEM and EDS images of RNO MHP after 4 days – particles embedded in resin. ................................................................................... 7-22 Figure 7.19. Effect of S/L ratio on nickel leaching from RNO-MHP in SAC solutions. ................................................................................................... 7-25 Figure 7.20. Effect of temperature on nickel leaching from RNO- MHP in SAC solutions ........................................................................................... 7-25 Figure 7.21. Effect of agitation on nickel leaching from RNO-MHP in SAC solutions .................................................................................................... 7-26 Figure 7.22. Effect of particle size on nickel leaching from RNO MHP, 10 g/L. .................................................................................................................. 7-26 Figure 7.23. XRD scans of RNO-MHP of different size fractions. ............. 7-27 Figure 7.24. Arrhenius plot for Ni(II) dissolution from RNO-MHP in SAC solution (500 rpm, 38-53 μm, 10 or 20 g/L solids) ..................................... 7-29 Figure 7.25. Effect of particle size on initial rates of Ni(II) dissolution from RNO-MHP and Ni,Mg(OH)2. ..................................................................... 7-30 Figure 7.26. Comparison of kinetic models for Ni(II) dissolution from (a) Ni,Mg(OH)2 , and (b) MHP-RNO in SAC solution at 25oC, 500 rpm, 20 g/L solids and particle size range of 38-53 μm. .............................................. 7-30 Figure 7.27. Comparison of metal leaching from different commercial MHP’s under different leach conditions ................................................................ 7-33 Figure 7.28. Effect of metal ion composition in Cawse, PS-44 and S-22 samples on Ni leaching in SAC solution under standard conditions. ........ 7-35 Figure 7.29. Effect of metal ion composition in Cawse, PS-44 and S-22 samples on Co leaching in SAC solution under standard conditions. ....... 7-36 Figure 7.30. Effect of metal composition in Yabulu-Preboil, RNO-Pilot and RNO-June samples on Ni leaching in SAC solution under standard conditions. ................................................................................................. 7-37 Figure 7.31. Effect of metal composition in Yabulu-Preboil, RNO-Pilot and RNO-June samples on Co leaching in SAC solution under standard conditions. ................................................................................................. 7-38 Figure 7.32. XRD scans of Preboil Solids and leach residues .................. 7-39
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Figure 7.33. XRD scans of RNO Pilot MHP and leach residue. ................ 7-41 Figure 7.34. XRD scans of Cawse MHP and leach residue ...................... 7-44 Figure 7.35. XRD scans of PS-44 leach residues ..................................... 7-47 Figure 7.36. XRD scans of SS-22 leach residues ..................................... 7-48 Figure 7.37. XRD scans of standard and reductive predictor leach test residues after 12 weeks (85 days) ageing. ............................................... 7-52 Figure 8.1. Sulfide solubility diagram at 25°C ............................................. 8-2 Figure 8.2. Sulfide solubility diagram at 45°C. ............................................ 8-3 Figure 8.3. XRD scans of CoNiS samples – effect of cobalt oxidation state at 25°C. ........................................................................................................... 8-9 Figure 8.4. SEM image of unseeded CoNiS produced at 25°C with a divalent oxidation state and sulphidation ratio of 2.2:1. .......................................... 8-10 Figure 8.5. ORP (vs. Ag/AgCl) of sodium sulphide with 1 L/min oxygen in 1 L of SAC at 25°C. ........................................................................................ 8-12 Figure 8.6. Potential-pH diagrams of Ni-NH3-S-H2O system..................... 8-14 Figure 8.7. CoS and NiS dissolution in SAC at 25°C with 1 L/min N2. ...... 8-16 Figure 8.8. Effect of pH on the speciation of (a) CO2 and SO2, (b) NH3 and S2-. ............................................................................................................ 8-16 Figure 8.9. Effect of anions on nickel(II) dissolution from Ni,Mg(OH)2 in 90 g/L ammonia with 1.47 mol/L of the anion solutions at 25°C. .............. 8-18 Figure 8.10. NiS dissolution in SAC at 25°C with 1 L/min N2 or air. .......... 8-20 Figure 8.11. CoS dissolution in SAC at 25°C with 1 L/min N2 or air, and sulphite with N2. ........................................................................................ 8-20 Figure 8.12. Ni and Co dissolution from 50-50 CoNiS in SAC solutions at 25°C with 1 L/Min N2. ................................................................................ 8-22 Figure 8.13. Fraction of Ni and Co dissolution in SAC at 25°C from different sulphides with 1 L/Min N2 and air .............................................................. 8-23 Figure 8.14. Cobalt dissolution from RNO-MHP in SAC solution in an open vessel, or with 500 mL/min N2 or air. ........................................................ 8-23
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Figure 8.15. (a) Co/Ni ratio in solution, and (b) Co/Ni ratio of fraction dissolved from CoNiS precipitates produced at 25°C with varying sulphiding ratios in SAC solution under N2, in the absence of MnOOH. .................... 8-25 Figure 8.16. Reductive leaching of MnOOH: effect of temperature, cobalt oxidation state and sulphidation ratio. ....................................................... 8-29 Figure 8.17. Reductive leaching of MnOOH: effect of Co/S ratio. ............. 8-30 Figure 8.18. Eh-pH diagrams for Ni(II) and Co(II)/(IIII) in ammonia solutions at 25oC and 6 M NH3, 0.1 M Ni(II) and 0.01 M Co(II)/(IIII) ......................... 8-33 Figure 8.19. Reductive leaching of MnOOH: effect of drying. ................... 8-34 Figure 9.1. Yabulu refinery YEP flowsheet ................................................. 9-4 Figure 9.2. MHP reslurry ............................................................................. 9-5 Figure 9.3. MHP primary leach ................................................................... 9-5 Figure 9.4. CoNiS precipitation and thickening ........................................... 9-6 Figure 9.5. MHP secondary leach and leach residue.................................. 9-6 Figure 9.6. Plant Survey – ORP. Conducted over 3 weeks, blue: week 1, red: week 2 and green: week 3. ......................................................................... 9-7 Figure 9.7. Eh-pH diagram for Co-ammonia-carbonate system at similar solution concentrations to YEP at 30°C. ..................................................... 9-8 Figure 9.8. XRD scans of MHP’s and preboil solids.................................. 9-12 Figure 9.9. Comparison of XRD scans of preboil solids collected in May 08 and June 06 .............................................................................................. 9-14 Figure 9.10. XRD scans of plant solid samples ........................................ 9-15 Figure 9.11. XRD scans of secondary leach slurries ................................ 9-16 Figure 9.12. XRD scan of CoNiS .............................................................. 9-17 Figure 9.13. XRD scans of thickener residues .......................................... 9-17 Figure 9.14. HPLC – secondary leach tank 3345-1913, sampled 19/5 ..... 9-19 Figure 9.15. HPLC – cobalt ammine species concentrations .................... 9-20 Figure 9.16. Cobalt(III) concentration determined by solvent extraction and ICP. Error bars represent difference in concentration determined by laboratory method and HPLC. .................................................................. 9-22
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Figure 9.17. Total cobalt(III) concentration determined by HPLC in batch test and SPT liquors ........................................................................................ 9-28 Figure 9.18. Cobalt(II)/cobalt(III) concentration ratio determined by SX and ICP of batch leach test liquors .................................................................. 9-28 Figure 9.19. Distribution of cobalt(III) speciation in batch leach liquors (after 1, 2,3 or 4 h) based on HPLC analysis ..................................................... 9-31 Figure 9.20. Distribution of cobalt(III) speciation in standard predictor leach test of MHP1-4 with batch leach liquors (after 0.75 h). ............................. 9-31
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LIST OF TABLES Table 2.1. Hydrotalcite-like structures ...................................................... 2-30 Table 2.2. Stability constants (Kn) of metal ammine complexes. ............. 2.34 Table 3.1. List of reagents. ......................................................................... 3-2 Table 3.2. List of industry samples ............................................................. 3-3 Table 3.3. Solution compositions prior to precipitation of MHP, g/L. .......... 3-7 Table 3.4. Solution composition for precipitation of samples similar to RNO-MHP, g/L. .................................................................................................... 3-8 Table 3.5. Solution compositions for precipitation of samples for drying, g/L. . .................................................................................................................... 3-8 Table 3.6. Solution compositions for varying cobalt content, g/L. ............ 3-10 Table 3.7. Solution composition for varying Co, Mn, Al and Cr contents, g/L. .................................................................................................................. 3-11 Table 3.8. Solution volume and nickel composition for varying crystallinity of Ni,Mg(OH)2 ............................................................................................... 3-12 Table 3.9. Solution compositions for precipitates produced for oven ageing tests in batch 1-2, g/L. .............................................................................. 3-13 Table 3.10. Solution compositions for precipitates for oven ageing tests in batch 2, g/L. .............................................................................................. 3-14 Table 3.11. Solution compositions for precipitation at elevated temperature (80°C), g/L. ............................................................................................... 3-15 Table 3.12. CoNiS precipitation conditions. ............................................. 3-17 Table 3.13. RNO kinetic leach test conditions. ........................................ 3-26 Table 3.14. AAS conditions for analysis. .................................................. 3-30 Table 4.1. Equilibrium constants for the reactions of manganese oxides...4-8 Table 5.1. Assay of Queensland Magnesia’s MgO (Emag 45). .................. 5-3 Table 5.2. Particle size (P80 ) of precipitates at 25 and 40°C over 4 hours . 5-9 Table 5.3. Composition of precipitates of Groups 1-5 (dry basis) ............. 5-19 Table 5.4. Composition of precipitates of Group 6 (dry basis). ................. 5-20
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Table 5.5. Assay results of cobalt and manganese rich precipitates. ....... 5-21 Table 5.6. Ratio of % metal in MHP over % metal in solution. .................. 5-24 Table 5.7. Atomic radii of selected metals, pm ......................................... 5-26 Table 5.8. Effect of Eh on Mn and Co species .......................................... 5-40 Table 5.9. Percentage of possible oxidised metals. .................................. 5-43 Table 5.10. Extent of Oxidation ................................................................. 5-47 Table 5.11. Assay results for precipitates O–R and AB-AE, %. ................ 5-55 Table 6.1. Summary of predictor leach test results – standard, reductive, soak ............................................................................................................ 6-6 Table 6.2. Confidence intervals of leach results. ........................................ 6-7 Table 6.3. Soak test – leach residue analysis. ............................................ 6-8 Table 6.4. Effect of manganese and cobalt on leach results from SPT, RPT and RST ...................................................................................................... 6-9 Table 6.5. Effect of Co in the absence or presence of Mn on Ni and Co leaching in MSPT and MRPT .................................................................... 6-11 Table 6.6. Modified standard and reductive predictor leach test results - 95 % confidence interval, %. .............................................................................. 6-12 Table 6.7. Effect of increasing Co, Mn, Al and Cr on composition of precipitates. .............................................................................................. 6-14 Table 6.8. Effect of increasing Co, Mn, Al and Cr on leaching of metals .. 6-14 Table 6.9. Mathematical expressions for heterogeneous kinetic models .. 6-46 Table 6.10. Effect of Ni/Mg ratio on initial rates of leaching. ..................... 6-50 Table 6.11. Chemical analysis of precipitates formed at elevated temperature, %. ........................................................................................ 6-62 Table 7.1. Age of Precipitate Samples. ....................................................... 7-2 Table 7.2. BHP Billiton Chemical Analysis of Aged MHP Samples ............. 7-4 Table 7.3. Assay of RNO MHP Collected in June 2008. ............................. 7-5 Table 7.4. P80 of Ravensthorpe MHP’s. ...................................................... 7-6
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Table 7.5. Effect of leach conditions on the initial leach rates of June 2008 RNO-MHP ................................................................................................. 7-23 Table 7.6. Assay results for size fractions of RNO-MHP, mass %. ........... 7-26 Table 7.7. Predictor leach test results of commercial precipitates. ........... 7-32 Table 7.8. Comparison of assays of different types of RNO samples and Yabulu Preboil sample .............................................................................. 7-37 Table 7.9. Predictor leach test results from Preboil Solids. ....................... 7-39 Table 7.10. Predictor leach test results from RNO-MHP over time. .......... 7-41 Table 7.11. Predictor leach test results from Cawse MHP over time. ...... 7-43 Table 7.12. Predictor leach test results for EN Pilot Plant MHP. ............... 7-46 Table 7.13. Standard predictor leach test results of RNO-MHP June 2008 - % leached and 95 % confidence interval. ................................................. 7-49 Table 7.14. Reductive predictor test results of RNO-MHP June 2008 – % leached and 95 % confidence interval. ................................................. 7-49 Table 7.15. Reductive soak predictor test results of RNO-MHP June 2008 – % leached and 95 % confidence interval. ................................................. 7-49 Table 7.16. Modified standard predictor test results of RNO-MHP June 2008 – % leached and 95 % confidence interval. .............................................. 7-50 Table 7.17. Modified reductive predictor test results of RNO-MHP June 2008 – % leached and 95 % confidence interval. .............................................. 7-51 Table 8.1. Ksp values at 45°C…………………………………………………..8-3 Table 8.2. Preparation conditions and composition of CoNiS ..................... 8-4 Table 8.3. Molar ratios and formula of CoNiS ............................................. 8-4 Table 8.4. Precipitation Reactions for Co-Ni-S ........................................... 8-5 Table 8.5. Possible reactions of sulphides with Mn(III) and Co(III) oxides 8-11 Table 8.6. Non-Oxidative or Oxidative dissolution of NiS and CoS. .......... 8-17 Table 8.7. Metal ion concentrations in SAC during leaching of sulphides . 8-22 Table 8.8. Oxidation half cell reactions of nickel sulphides ....................... 8-27
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Table 8.9. ORP and extent of leaching of MnOOH with CoNiS ................ 8-29 Table 8.10. Reduction reactions of Mn(III) and Co(III) oxides ................... 8-32 Table 9.1. Thiosulphate concentrations in plant liquors. ............................. 9-9 Table 9.2. Nickel and cobalt concentrations in plant liquors. .................... 9-11 Table 9.3. Ammonia and carbonate concentrations in plant liquors. ......... 9-11 Table 9.4. Composition of Preboil Solids from June 06 and May 08 ......... 9-13 Table 9.5. HPLC peaks in plant liquors. .................................................... 9-23 Table 9.6. Composition of MHP used in batch tests ................................. 9-25 Table 9.7. Percentage composition of Co(II) in batch leach liquors based on solvent extraction ...................................................................................... 9-25 Table 9.8. Composition of Co(III) in batch test leach Liquors based on HPLC .................................................................................................................. 9-27 Table 9.9. Speciation of Co(III) in Standard Predictor Leach Tests based on HPLC. ....................................................................................................... 9-27 Table 9.10. Peaks present in HPLC plots of plant liquors. ........................ 9-30 Table 9.11. Peaks present in HPLC plots of secondary leach liquors of MHP. .................................................................................................................. 9-35
1-1
1 INTRODUCTION
Nickel is used for the manufacture of stainless steel (60% total
production 2008), alloys, and for electroplating. Therefore the demand, price
and production of nickel are influenced by industrial activity, which, until
recently, had been growing at 5-6% pa for the last 20 years. World
consumption of stainless steel is being driven in largely by continued rapid
economic expansion in China and India (ABARE, 2008).
Australia is the third largest nickel producer behind Russia and Canada
with the majority of production occurring in WA from around ten operations.
Currently, most of the nickel is produced from sulphide ores. Nickel laterite
ores represent over 70% of onshore nickel resources, yet currently account
for less than 30% of global production (Brand et al., 1998). Thus, there is an
increasing trend towards the processing of nickel laterites.
1.1 Nickel Ores
Nickel sulphide deposits are typically found at depths of hundreds of
meters in a hard rock geological environment. The principal minerals present
in the ore are: pyrrhotite (Fe1-xS), pentlandite ((Fe,Ni)9S8), chalcopyrite
(CuFeS2), and magnetite (Fe3O4). In addition they often contain pyrite (FeS2),
cubanite (CuFe2S3), polydymite (NiNi2S4), niccolite (NiAs), millerite (NiS),
violarite (FeNi2S4) and minerals of the platinum group metals. Sulphide
bearing nickel ore is currently being mined economically with grades of 1.5%
to 4% in Russia, Canada and Australia.
1-2
Nickel laterites are formed from the weathering of nickel-bearing mafic
and ultramafic rocks that leads to the concentration of the nickel in a band
close to the original water table. The overall depth of deposits generally
ranges from 20 to 150 m. A laterite orebody typically consists of two different
ore types described below:
• Limonite ore (upper horizon) containing low silica, low magnesia,
typically 1.4% Ni, 0.15% Co and >40% Fe.
• Saprolite ore (lower horizon) containing high silica, high magnesia,
typically 2.4% Ni, 0.15% Co and <15% Fe.
Moving down the profile the nickel, silica and magnesia levels increase and
the cobalt and iron levels decrease (Reid, 1996). Unfortunately the relative
ease and low cost of open pit mining laterites is offset by the difficulties of
processing. Nickel is typically dispersed throughout the orebody preventing
further concentration, and high levels of impurities (Fe, Mg, Mn, Cr, Al) cause
problems when leaching (Reid, 1996).
1.2 Processing Laterite Ores
1.2.1 Major Routes
There are four major commercial routes available for processing nickel
laterites, these are: ferronickel smelting, matte smelting (sulphur is added),
the Caron process (reduction roast/ammoniacal leaching) and pressure acid
leaching (PAL). However, over the last decade considerable effort has been
spent on the development of atmospheric and low pressure leach processes.
1-3
Ultimately, the process selection would depend highly on ore mineralogy and
the level of adventurousness of the potential producer.
Smelting requires some magnesia to create slag, and lower levels of
iron (saprolite), whilst acid leaching requires low levels of magnesia
(limonite) to avoid excessive acid consumption. The Caron process achieves
a relatively high nickel recovery when processing the mid-zone transition ore
that is unacceptable as either acid leach or smelter feed. Limonite is an
acceptable feed and saprolite has been processed resulting in lower
recoveries.
One of the reasons for the resurgence of pressure acid leaching has
been the realisation that whilst many of the smelting grade saprolite
orebodies around the world are being exploited, there are still a large number
of untapped limonite resources. The main reasons for preferring the PAL
process are:
• Lower energy requirement.
• Higher nickel and cobalt recoveries.
• Lower capital cost.
Although the high metal recovery and low energy requirement of PAL are
attractive features, the ammoniacal leach is more common. Since Moa Bay
(first PAL process) in the late 1950’s five greenfield plant project groups have
selected the ammonia leach route. Perhaps the known advantages were
1-4
insufficient at the time to convince the project owners to proceed with PAL.
The main reason would appear to be the relatively high magnesia content in
the limonite ore (3%) compared to the Moa project (<1%). Because acid is
the highest cost component, a significant increase in consumption due to
higher Mg content could have dire consequences. Another reason has been
the problem of effluent disposal with the acid solutions containing high levels
of impurities in comparison. Finally, the ammonia leach option produces a
nickel oxide or metal product which is directly saleable to stainless steel
mills, whereas the mixed nickel cobalt sulphide precipitate produced by PAL
is an intermediate, which requires relatively expensive further processing to
produce an end use nickel product (Reid, 1996).
More recently there have been significant technological developments in
the design and operation of high-pressure autoclaves and high rate
thickeners, as well as solvent extraction and electrowinning technology. This
has helped reduce capital costs and technical risks, and has allowed the
development of alternative flowsheets to the earlier laterite operations (Kyle,
1996).
Willis (2012) presented a comprehensive paper on the developments
and trends in hydrometallurgical processing of nickel laterites. The paper
examines trends and new developments, and discusses the relative merits of
each option. Atmospheric and low pressure leaching processes, along with a
wide array of downstream processing options (Mixed Hydroxide
1-5
Precipitation, Mixed Sulphide Precipitation, Direct Solvent Extraction), seems
to be the direction most potential producers are heading in.
1.2.2 Pressure Acid Leaching (PAL) Process
(a) Leaching
Laterite ores do not require grinding for metal liberation as the
leaching process is fairly insensitive to particle size (<250 μm) due to high
porosity. The grinding circuit is therefore used to slurry the ore. The slurried
ore is thickened to about 35 to 40% solids prior to feeding to the autoclaves
for pressure acid leaching. Nickel and cobalt are readily solubilised in
sulphuric acid solutions at around 250°C, with pressures of around 4500
kPa. Unfortunately, other metals present in the ore are also solubilised.
These include iron, magnesium, manganese, chromium, and aluminium, with
minor elements silica, copper and zinc. However, aluminium and iron is
precipitated as gibbsite (Al(OH)3), haematite, basic iron sulphate and jarosite.
Acid consumption is related to the magnesium and aluminium content of the
ore (Kyle, 1996; Mayze, 1999; Whittington & Muir, 2000). The dissolution of
nickel, cobalt and other divalent metals can be described by the following
general equation (Krause et al., 1998). General reactions for the dissolution
and precipitation of iron and aluminium are also shown.
OHHSOMeSOHMeO 242
42 22 ++→+ −+
OHSOFeSOHFeOOH 224
342 43232 ++→+ −+
++ +→+ HsOFeOHFe 6)(32 3223
1-6
OHAlOOHOHAl 23)( +→
OHSOAlSOHAlOOH 224
342 43232 ++→+ −+
+−+ +→++ HOHSOOAlHOHSOAl 5)()(723 62433224
3
426243422342 6)()(212)(3 SOHOHSONaFeSONaOHSOFe +→++
)()()(236 62434242 overallOHSONaFeSONaSOHFeOOH →++
Laterites can also be leached at atmospheric pressure; however, to
achieve high recoveries the nickel/cobalt bearing iron minerals need to be
dissolved, which generally requires a longer leaching time and results in a
higher acid consumption. The major concern is the downstream problems
associated with high ferric ion concentrations in solution. The Ravensthorpe
process incorporated an atmospheric leach on the saprolite, which would
contain much lower concentrations of iron (Kyle, 1996; Mayze, 1999;
Whittington & Muir, 2000).
(b) Separation
A counter-current decantation (CCD) wash process is used to
separate nickel and cobalt from the leach residue (flocculants generally
aren’t required). The free acid in the pregnant liquor is neutralised with
limestone, calcrete or magnesia depending on cost and availability. This
stage may be done prior to CCD, so that precipitated solids are rejected with
the tailings stream, although care must be taken so that nickel and cobalt are
not co-precipitated with the iron. Air is also added to oxidise iron(II) to iron(III)
(Mayze, 1999).
1-7
The leaching stage is followed by a precipitation / re-leach / solvent
extraction process or direct solvent extraction. The precipitation step is used
to separate the nickel and cobalt from many of the impurities present in
solution. The precipitate can either be a sulphide or hydroxide. Although this
process is more complicated it separates the flowsheet resulting in a more
robust process (Mayze, 1999).
The mixed hydroxide precipitate is usually referred to as MHP and
typically contains Ni and Co with a concoction of other metals including: Mg,
Mn, Al, Fe, Zn, Cr, Cu, Ca and Si. The advantage of the hydroxide
precipitate is the much simpler dissolution process using ammonia at low
temperature and atmospheric pressure. Although the precipitation process is
non-selective, the ammonia re-leach is highly selective, with excellent
rejection of iron, manganese and magnesium.
The major advantage of sulphide precipitation is its selectivity with
excellent rejection of all major impurities. However, the re-dissolution proves
more difficult requiring oxidative pressure leaching at 165°C and 1100 kPa
(Kyle, 1996). After precipitation / re-leach / solvent extraction or direct solvent
extraction, the final products are obtained by electrowinning, sulphide
precipitation or hydrogen reduction.
1-8
1.2.3 Ammoniacal Carbonate Leaching (Caron) Process
The Caron process was developed in Holland in the 1920’s and is a
combined pyrometallurgical/hydrometallurgical process. The nickel, cobalt
and iron are reduced to their metallic forms by heating the ore above 700°C
in a reducing atmosphere using fuel oil or a gaseous reductant. Simplified
chemical equations are listed below:
OHNiHNiO 22 +→+
OHOFeHOFe 243232 23 +→+
OHFeOHOFe 2243 3 +→+
OHFeHFeO 22 +→+
2CONiCONiO +→+
24332 23 COOFeCOOFe +→+
243 3 COFeOCOOFe +→+
2COFeCOFeO +→+
The cooled calcine is treated with an ammonia-ammonium carbonate
solution to selectively leach nickel and cobalt. These two metal ions form
complexes with ammonia whilst iron is oxidised to the ferric state and
precipitated as a hydroxide:
−++ ++→++++ 234
2632232 22])([(28
21 CONHNHNiOHCONHONi
−++ ++→++++ 234
2632232 22])([(28
21 CONHNHCoOHCONHOCo
1-9
OHNHNHCoNHONHCo 233
63422
63 21])([
41])([ ++→++ +++
−++ ++→++++ 234
2432232 22])([26
21 CONHNHFeOHCONHOFe
−+++ +++→++++ 234
243
2632232 32])([(])([(312 CONHNHFeNHNiOHCONHOFeNi
−++ ++→+ OHNHOHFeOHNHFe 416)(416)(4 432243
The pregnant leach liquor is separated from the barren solids by a
series of CCD washing stages. Boiling removes the ammonia and
precipitates basic nickel carbonate, which upon calcination at 1200°C
produces nickel oxide. The cobalt is recovered by sulphide precipitation
(Whittington & Muir, 2000). Figure 1.1 shows the generalised flowsheet for
the roasting, leaching and CCD stages of the Caron process.
Figure 1.1. Simplified flowsheet of Caron process (Nikoloski, 2002).
The Caron process is able to treat a mixture of limonite and saprolite
ores at a relatively low reagent cost. Standard construction materials can be
1-10
used as corrosion problems are minimal in alkaline medium. As mentioned
previously, the technology is well proven and the nickel and cobalt products
are ideal. However, the drying and reduction roasting is energy intensive and
the metal recovery is relatively low with a significant loss of cobalt by
co-precipitation with ferric hydroxide. The overall process is sensitive to the
mineral composition requiring careful mining and blending (Whittington &
Muir, 2000; Reid, 1996).
Although developed in the 1920’s the first commercial plant was not
built until 1943. Four more ammonia leach plants have been built since the
nickel boom of the late 1960’s, but all performed badly and were unprofitable
for many years. Their operating performance and profitability record has
tarnished the image of the process, particularly the Cuban plants with poor
dust containment practices. The exception is the Yabulu refinery in
Queensland, Australia, which is regarded as demonstrating the real potential
of the ammonia leach process due to being modified in a number of ways by
incorporating technological advances (Reid, 1996).
1.3 Commercial PAL Processes
Up until 1997 there were no other commercial PAL processes since the
construction of the Moa Bay plant in 1957. Thanks to ongoing research and
development, including the piloting of the AMAX and SURAL processes, and
the significant rise in price and demand of nickel in the mid 1990’s new
producers, particularly in Australia and the South Pacific region were
1-11
attracted to the PAL processes. In 1997, three PAL plants; Bulong, Cawse
and Murrin Murrin were constructed in Western Australia (Flett, 2002).
1.3.1 Proposed and Piloted Processes
(a) AMAX Process
The PAL process applied at Moa Bay is only suitable for low
magnesia ore. Higher Mg concentration results in higher acid consumption.
In the 1970’s the AMAX process was developed to be suitable for the entire
deposit taking the high magnesia content of saprolite into account. The feed
ore is split into a fine low magnesia fraction and a coarse high magnesia
fraction by screening. The fine fraction is leached at 270°C while the coarser
ore is first calcined and then leached with pregnant leach liquor at
atmospheric pressure. This magnesia rich portion neutralises the free acid in
the liquor removing the need for and cost of lime as in the traditional process.
The solid residue, which contains depleted magnesium and remaining nickel
is fed into the autoclave. After the leach liquor is separated by counter
current decantation (CCD) the nickel and cobalt are recovered as a mixed
sulphide. The process reduced energy consumption, autoclave scaling and
improved sulphide precipitation conditions (Monhemius, 1987).
(b) SURAL Process
The SURAL (Sulzer Regenerative Acid Leach) process also
addresses the inclusion of saprolite ores, energy reduction and scale
minimisation. The process proposed the use of batch autoclaves and the
1-12
production of a mixed hydroxide product using magnesia as a precipitant.
The product can simply be dissolved in an ammonia solution and solvent
extraction and electrowinning can be applied for the recovery of the metals
(Monhemius, 1987).
1.3.2 Murrin Murrin, Bulong and Cawse
The Murrin Murrin, Bulong and Cawse projects in Western Australia
have/had a similar leaching step to that employed at Moa Bay but differ in
the subsequent metal recovery circuits. Unfortunately none of the production
ramp-ups went smoothly, with a variety of problems arising at all three
plants. These appear to have been the least serious at Cawse and Murrin
Murrin as the plants are still operating although owned by different
companies since start-up (Norilsk and Glencore-Xstrata). After CCD, Murrin
Murrin produces a mixed sulphide by neutralising the liquor with calcrete and
precipitating under pressure with H2S. This precipitate is then pressure
leached with sulphuric acid and air. The nickel and cobalt are separated by
solvent extraction and then recovered as powders by hydrogen reduction
from ammoniacal solutions (Whittington & Muir, 2000).
The Cawse operation was particularly lucky being able to upgrade the
ore by 30-40% by an initial screening process prior to leaching. The initial
leaching stage was much the same as other PAL plants however, after the
CCD’s the solution was neutralised with limestone and any residual iron
oxidised to Fe(III) by air. The precipitate was recycled to recover any nickel
1-13
and cobalt co-precipitated with iron during this stage. A mixed nickel/cobalt
hydroxide was then precipitated with MgO and redissolved in an ammonia-
ammonium carbonate solution. The separation of nickel from cobalt was
achieved by solvent extraction and metals produced by electrowinning.
Residual cobalt in solution was treated with ammonium sulphide to produce
a cobalt sulphide product (Whittington & Muir, 2000). The Cawse flowsheet
has changed significantly since start-up. Since the take-over of Centaur
Mining in 2002 by OM Group Inc. and subsequent takeover by Norilsk in
2006, the solvent extraction and electrowinning plants have been sold. Until
recently, the plant produced a nickel-cobalt carbonate for shipment to
Finland for smelting and refining.
Bulong’s major downfall was the use of a direct solvent extraction
process rather than the production of an intermediate. Front-end plant
availability was affected by a variety of autoclave equipment failures. While
downstream, silica-crud and gypsum precipitation caused major problems in
the Cyanex 272 cobalt extraction circuit and the versatic acid solvent
extraction for nickel (Flett, 2002).
1.3.3 Ravensthorpe Project and Yabulu Extension
One of the more recent developments in the processing of laterites
was the Ravensthorpe project. Production commenced early in 2008 and
ceased a year later. The laterite ore in the south of Western Australia was
leached with sulphuric acid at atmospheric and pressurised conditions and a
1-14
nickel-cobalt hydroxide was produced. The Ravensthorpe flowsheet is shown
in Figure 1.2. The mixed hydroxide precipitate (MHP) was shipped to the
existing Yabulu refinery in Townsville (QLD) for further processing.
First Quantum Minerals acquired and adapted the processing plant,
and are now producing a mixed hydroxide precipitate for sale.
1-15
Figure 1.2. Ravensthorpe flowsheet (BHP Billiton, 2004).
1-16
BNC
air
PreboilSolids to
MHP Leach
NH 4 HS
(Cu,Ni)Sto disposal
PreboilStills
MagmaStills
steam
QN Nickel HiGrade & QN Nickel Compact
Products (76,000 t/yr Ni)
syngas syngas
SinterFurnaces
ReductionFurnaces
Coal seammethane
gascleaning
LaroxFilter
steam
steam
air
CompactorTables
Filter
Filter
CombinedProductLiquor
(22 g/L Ni)air
Leaching
residue toleaching
O 2
NH4HS
Filter
QN ChemGradeCobalt Product(3,500 t/yr Co)
steam
Larox Filter
air
Drying
Flash
Co LSLPreboil Still
steam
Ion ExchangeCa & Mg Removal
Solvent ExtractionZn & Fe Removal
Solvent ExtractionTransfer to Ammine
Solvent ExtractionNi & Cu Removal
Oxidation
Flash
Calcination& Reduction
Kilns
Thickener
Precipitation
Thickener Cobalt
SulphideStills
Thickener
to NH 3 Recovery
to NH 3 Recovery
to G.C.C.’s
Thickener
Thickener
water
to A.S.X. Plant
water
H 2 SO 4 Zn,Fe to disposal
H2O2
Oxidation
sawdust
Belt Vacuum Filter
Belt Vacuum Filter
powercoal Power Plant
steamwater
E1E2E3
S1S2
S3S4
E1E2
S1S2
S3
ScalpA.S.X.
Absorbers Gas CoolerCondensers
Thickener
to generalprocess water
usesto generalprocess
water uses
FollowA.S.X.
Filter
PrimaryLeach
RNO MHP0.19 M wt/yr
(44,000 t/yr Ni& 1,500 t/yr Co)
SecondaryLeach
air
Thickener
Thickener
SyngasPlant
Coal seam methane
air
H2SPlantsulphur
CO2syngas (3H 2/N2)NH3
Converter
NH3NH4HS
1 2 3
6
Tailings Stills steam
air
air fuel oil
fuel oil gas cleaning
Ball Mills
DustBypass
gas cleaning
Dryers air coal
Imported Ore3.5 M wt/yr
(32,000 t/yr Ni& 2,000 t/yr Co)
4 5
7 8
Product Liquor
10 g/L Ni
Leaching
Ore Reduction Furnaces (12)
Solar Drying
Tailings Ponds Brine Pond
Reverse Osmosis
Plant clear
effluent
Process water
to NH 3 Recovery
Coolers
Refrigeration Plant
Clarifier
NH 4 HS
FLL
FLL
CoNiS
air
air
Figure 1.3. Flowsheet of the Yabulu refinery with MHP processing circuit (Fittock, 2004).
1-17
At the Yabulu refinery the MHP from Ravensthorpe was leached in an
ammonia-ammonium carbonate solution and refined (Figure 1.3). Essentially
it was a very similar process to Cawse, with the exception of laterite
atmospheric leaching and the transportation of the MHP. The most
significant problem of this process was the ‘ageing’ of the mixed nickel-cobalt
hydroxide precipitate (MHP), which occurred whilst shipping. Although
Cawse produced a similar intermediate, leaching generally occurred soon
after production. The precipitate from the Ravensthorpe plant was predicted
to be a complex material that could undergo various solid-state
transformations and oxidation reactions in the presence of Mg, Mn, Al, Fe,
Cr, Cu, Si, Zn and other impurities. This had the potential to cause significant
problems and metal losses in downstream processing (Muir, 2003).
In 2007 the Yabulu Expansion Project (YEP) was completed in order
to accept a mixed nickel cobalt hydroxide from BHP’s Ravensthorpe project.
To overcome the oxidation of metals that occurred during transportation
Yabulu incorporated a reductive leach, using the mixed cobalt-nickel
sulphide (CoNiS) produced on site, into its expanded process. Many parts of
the plant were expanded and a separate leaching circuit, consisting of a
primary and secondary leach, was installed which flowed into the existing
CCD circuit. The primary leach was open to the atmosphere while the
secondary leach consisted of a reductive stage followed by aeration. To
overcome the oxidation of metals that occurred during transportation a cobalt
nickel sulphide (CoNiS), produced on site, was added as a reductant to
reduce any oxidised metals, releasing associated nickel and cobalt. As well
1-18
as benefitting MHP leaching, the precipitation of CoNiS also improved cobalt
recoveries in the roast-leach section of the Yabulu refinery. However, CoNiS
is also a complex material as the precipitation conditions and composition
have a significant effect on its reducing ability.
1.3.4 Current/Future Projects
The Goro development by Vale-Inco in New Caledonia is a fully
integrated flowsheet using solvent extraction on the product liquor, with the
option of producing a mixed hydroxide intermediate. Production commenced
in 2011; initial annual production target is 22,000 tonne of Ni (Mining News).
The Chinese built Ramu operation in Papua New Guinea is currently
in the implementation phase with commissioning expected in 2013. After
high-pressure acid leaching, a hydroxide intermediate will be produced for
sale (Mining News; O’Shea, 2003).
Sherritt, Sumitomo and Korea Resources are developing the
$5.5 billion Ambatovy nickel project in Madagascar, with a forecast start-up
in 2013. The process includes the precipitation of a mixed sulphide as an
intermediate. ERAMET’s Weda Bay project in Indonesia is currently in the
financial feasibility stage. The ore will be leached under atmospheric
conditions, and nickel and cobalt recovered by direct solvent extraction.
Metallica is currently in the piloting stage; recovery of scandium will make the
process more viable (Mining News).
1-19
The Caldag Nickel Heap Leach Project went into care and maintenance
in December 2010 due to delays in being granted a forestry permit. Vale-
Inco’s Nickel De Vermelho suffered a similar fate in 2008 (Mining News).
Ferronickel is currently being produced in New Caledonia, Greece,
Kosovo and Macedonia. Two new projects (Barro Alto and Onca Puma) were
commissioned in March 2012.
Although there are other proposed nickel laterite projects, construction
and production is many years away and information on proposed flowsheets
isn’t available.
1.4 Project Aim
The production and transportation of a mixed nickel-cobalt hydroxide to
an existing refinery is a novel process designed to simplify processing and
reduce capital costs. BHP Billiton was the first to implement this technology.
Unfortunately, ‘ageing’ of the mixed metal hydroxide is a significant,
complicated problem as the MHP contained at least 10 metals which were
known to oxidise and/or restructure.
This thesis aims to investigate the oxidation and ageing which could
occur during the transportation of MHP, and subsequent leaching in an
ammonia-ammonium carbonate solution at conditions similar to those used
in the Yabulu Refinery. A cobalt nickel sulphide reductant made on-site at the
1-20
Yabulu refinery was also investigated. Although BHP has conducted
comprehensive reviews and testwork (Nikoloski et al., 2005 and Muir, 2003
to name a few), no precipitates have been produced in the laboratory to be
studied while ageing. This will be a significant study for the thesis.
The study of mixed metal hydroxide can be divided into several
sections and will be discussed accordingly: precipitation; oxidation, ageing
and influence of impurities; and leaching in an ammonia solution.
Precipitation conditions (i.e. temperature, solution composition and the
neutralising agent) will be investigated for single and multiple metal
hydroxides in the laboratory and industrial scale. The oxidation of metals,
re-structuring of metal hydroxide structures and influence of impurities (Mg,
Mn, Al, Fe, Zn, Cr, Cu, Ca and Si) will also be discussed. Finally the solution
chemistry, leach conditions, metal ammine complexes, metal impurities and
properties of reducing agents will be investigated in relation to nickel and
cobalt leach kinetics and recoveries in ammonia-ammonium carbonate
solutions. A plant survey on the MHP leaching circuit of the Yabulu refinery
will also be conducted.
Characterisation of the ageing process, investigation of sulphide
intermediates and a better understanding of the ammonia-ammonium
carbonate leach will help improve nickel and cobalt recoveries in current and
future lateritic nickel projects. With various nickel refineries around the world
willing to accept a mixed hydroxide product, many companies/projects have
1-21
the opportunity to produce the intermediate for sale. Further opportunities
exist due to the continuing developments of laterite heap leaching.
2-1
2 LITERATURE REVIEW
2.1 Laboratory Synthesis of Metal Hydroxides
Although industrial processes already exist and are being developed,
collated information of single and multiple metal hydroxides from laboratory
synthesis and pilot plant trials will still be particularly useful. Reports by
Nikoloski et al. (2005) and Muir (2003) were very informative.
2.1.1 Nickel Hydroxide
Nickel hydroxide has been used in rechargeable nickel-cadmium
batteries for over 10 years and has been actively studied by a large number
of solid state chemists and electrochemists. Despite the simple formula, its
reactions are quite complex due to the various phases and crystallographic
forms. These reactions become more complex when impurities such as Co,
Mn, Fe, Mg and Al are present. Furthermore the nature of the precipitate is
influenced by the conditions of precipitation and various anions present in
solution.
(a) Effect of reagents and conditions
In a thermodynamic study, the precipitate was produced by dropwise
addition of 0.8 M sodium hydroxide to 0.4 M nickel nitrate in a CO2-free
atmosphere over 1 hour, to produce ~10 g of nickel hydroxide (Chickerur et
al., 1980). The precipitate was left overnight in contact with the mother liquor
to improve crystallinity. Once dried, the nickel content of the sample was
2-2
determined by a complexometric titration with Na2EDTA using murexide as
the indicator.
Ramesh and Vishnu Kamath (2005) explored a large matrix of
precipitation conditions to generate a wide range of nickel hydroxide samples
for analysis by X-ray diffraction and infrared spectroscopy. Nickel hydroxide
was precipitated by addition of NaOH and ammonia under a variety of
conditions. While most precipitation reactions resulted in the formation of
β-Ni(OH)2, the samples differed from one another in the degree of structural
disorder. Bonding in nickel hydroxide is anisotropic. While intra-layer bonding
is strongly iono-covalent in nature, bonding between layers is of the weak
van der Waal’s type. The orientation of layers is affected by the precipitation
conditions and hence it is important to explore different regimes of pH,
concentration and temperature, as well as the conditions of digestion.
(b) Effect of pH and temperature
Interstratification of α-motifs in the matrix of β-Ni(OH)2 was noted
when precipitating with NaOH from solutions at pH > 9. Precipitation at a
constant high pH (>13) resulted in the formation of a structure known as βbc
(bc: badly crystalline)-nickel hydroxide. This sample was replete with
different types of structural disorder which could not be eliminated by ageing
at different temperatures. Precipitates formed in solutions of lower pH values
were deficient in hydroxyl ions requiring the inclusion of nitrates for charge
neutrality, whilst the precipitates formed above pH 13 contained intercalated
water (Ramesh & Kamath, 2005).
2-3
The crystallinty of the product is adversely affected using a different
nickel source. The XRD trace of precipitates formed from nickel sulphate and
sulphamate exhibited broad peaks, while the nickel ammonium sulphate
(NAS) product was X-ray amorphous. Infrared spectroscopy revealed the
precipitate to exhibit all the features of α-Ni(OH)2. Interstratified nickel
hydroxide was obtained using NAS at pH 13 (Ramesh & Kamath, 2005).
Ammonia-induced precipitation at a low temperature (4°C) yielded a
poorly ordered α-Ni(OH)2, while at high temperature (25-65°C), β-Ni(OH)2
was obtained with a surprisingly high degree of crystallinity. When the
α-Ni(OH)2 was aged in concentrated ammonia at ambient temperature it
transformed into β-Ni(OH)2. While ageing in concentrated alkali (6 M KOH) it
transformed into βbc-Ni(OH)2. Clearly the pH during ageing has a profound
affect on structural disorder (Ramesh & Kamath, 2005).
During precipitation of nickel hydroxide the solid formed immediately
on precipitation is the α-amorphous phase. This phase is metastable and
transforms into other phases of progressively greater order and
thermodynamic stability. Ramesh and Kamath (2005) discovered the
transformation in the order:
α-Ni(OH)2 (amorphous) → βbc → β-Ni(OH)2.
In strong alkali, once the first step is completed, further transformation
becomes very slow and can only be accelerated at high alkali concentrations
and/or high temperature. The conditions required suggest dissolution-
2-4
reprecipitation as the reaction mechanism. In ammonia the solubility can be
enhanced, therefore the transition takes place under milder conditions.
Delahaye-Vidal et al. (1990) produced the α-Ni(OH)2 chemically by
mixing a nickel nitrate solution with an ammonia solution. The resulting
precipitate was disordered and had a much larger interlamellar distance than
the β equivalent due to incorporation of water, nitrates and carbonates. The
β-Ni(OH)2 precipitate with brucite structure was prepared by adding a nickel
sulphate solution to a sodium hydroxide solution. Alternatively, it was also
produced by ageing an α-Ni(OH)2 in an aqueous medium or a KOH solution,
as this phase is known to be unstable under certain conditions.
Sist & Demopoulos (2003) precipitated nickel hydroxide from a sulphate
solution using either NaOH or MgO. The analysis showed that the recovery
of nickel was greater with the use of MgO. It was postulated that the slow
release of hydroxyl ion by MgO, relative to NaOH, contributes to a lower
supersaturation environment which in turn favours particle growth and
crystallinity, resulting in a lower final nickel concentration. The pH control
with NaOH was found to be far more erratic resulting in a pH overshoot and
a higher final nickel concentration.
(c) Effect cations and anions
The effect of aluminium ions on nickel hydroxide precipitation was
examined by Hengbin et al. (2002). It was found that stable aluminium-
substituted α-Ni(OH)2 was formed when soaked in strong alkali for 6 months.
2-5
This stability could be related to the many anions and hydrogen bonds
between the layers. The substitution of Al3+ for Ni2+ resulted in anions such
as CO32- entering the space between layers for charge neutralisation. Bing et
al. (1999) conducted similar research.
The substitution of aluminium for nickel in the lattice of nickel
hydroxide, prepared by co-precipitation, leads to a hydrotalcite-like
([Ni6Al2(OH)16].[CO3.4H2O]) compound of α-Ni(OH)2. This compound has
been used as the electrochemical active material in the positive electrodes of
rechargeable alkaline batteries as it proves to display better stability and
reversibility of the redox couple.
The physicochemical and electrochemical characteristics of nickel
hydroxides are greatly influenced by the nature of precipitating agent used.
The precipitate obtained using urea was poorly crystalline in nature and was
of α-motif. It contained low nickel content, high degree of hydration and a
significant amount of intercalated anions. The nickel hydroxide samples
prepared using sodium hydroxide and ammonia showed the formation of
β-Ni(OH)2 phase. These samples contained comparatively high nickel
content, low degree of hydration and much less intercalated anions. The
samples prepared with ammonia were more crystalline (Acharya et al.,
2003).
2-6
In a joint venture between Outokumpu Research and a Finnish
university (Abo Akademi), the nickel hydroxide sulphate precipitate obtained
during hydrogen reduction of nickel hydroxide slurries was studied. The
precipitate was discovered to be present and insoluble in sulphuric acid
medium at pH 2.0 when a slurry, obtained by neutralisation with sodium
hydroxide, was being reduced by hydrogen gas at an elevated temperature
(160°C) and pressure (21 bar). The XRD investigations showed that the
phase composition of the solid changed drastically through hydrogen
reduction. Chemical analysis revealed that the total sulphur content of the
precipitate varied from 1.5% prior to reduction to about 5% during the
reduction. As no sulphur was present, the precipitate was presumed to be a
basic sulphate. No attempt was made to determine the exact phases or
crystal structures present in the solids. The precipitate disappears by the
completion of reduction or when the temperature exceeds 70°C. The
formation of this insoluble precipitate probably explains why the reduction
time depends on neutralisation (Saarinen et al., 1996).
The infrared study of magnesium-nickel hydroxide solids solutions by
De Oliveira and Hase (2003) is particularly relevant for this review. Pure and
mixed precipitates were obtained by dropwise addition of 1 M ammonium
hydroxide to a 0.15 M nickel nitrate/magnesium perchlorate solution at 40-
50°C over 5 hours. Like previous investigations the precipitate was left in
contact with the mother liquor for 3 days. The IR spectra resembled those of
Mg(OH)2 and β-Ni(OH)2, while certain differences were noted when
compared to mechanically mixed samples of the ‘same’ compounds. Such
2-7
behaviour may imply formations of monophase solid solutions which have a
brucite-like (Mg(OH)2) crystal structure. This tendency is discussed in terms
of polarisation of the O-H bond and partial covalency of the M-O bonds.
Partial substitution of magnesium by nickel atoms increases the covalency of
the M-O interactions which decreases the mean bond distance. This
changes the hybridisation of the oxygen atoms which polarises the O-H bond
slightly more. The polarisation in the O-H bonds results in a decrease of the
bond strength.
The article by McEwens (1971) offers a review on nickel hydroxide
structures for the nickel-cadmium battery, as well as a study on a variety of
phases and incorporation of water. Nickel hydroxide is capable of
incorporating water molecules between its layers of nickel-oxygen polyhedra,
which then binds the crystallites tightly together at a fixed distance. Usually,
only a fraction of the total number of layers separate to admit water.
(d) Ageing
Ageing of nickel hydroxides precipitated from sulphate and chloride
parent solutions by the slow addition of NaOH was investigated by Suoninen
et al. (1973) using XRD. NaOH was added at stoichiometric amounts, R, of
0.8, 1 and 1.2 to solutions consisting of (NiSO4)n + (NiCl2)1-n with n values of
0, 0.1, 0.9 and 1. The precipitates were aged in the parent solution over 90
days. The growth of primary Ni(OH)2 particles was found to be strongly
dependant on the value of R. From sulphate solutions a smaller size and a
relatively long incubation time for the growth of fresh nuclei was observed
2-8
when R < 1. For all solutions when R ≥ 1, the crystal growth was prevented
because of the free OH- ions in solution.
Suoninen et al. (1973) explained these observations by the behaviour
of the double layer formed at the [001] surfaces of the primary particles. The
[001] faces of the crystals are first covered with Ni2+ ions, which attract
anions from solution forming an electric double layer on the surface of the
crystals. The ageing of the precipitates is caused by the desorption of the
anions, followed either by their substitution with hydroxide ions or by
subsequent desorption of the Ni2+ as well. The results of the investigation
indicate the adsorption of the sulphate ions to the double layer is stronger
than that of the chloride ions. A likely bonding mechanism is the formation of
a complex involving the top layer atoms of the hydroxide precipitate. Finally,
the growth behaviour of the precipitates corresponding to R ≥ 1 suggests a
virtually permanent double layer which inhibits the growth due to the excess
of the OH- ion Suoninen et al. (1973).
2.1.2 Cobalt Hydroxide
Like nickel, cobalt also forms α and β-hydroxides. The α-Ni(OH)2
phase has a higher electrochemical activity due to its larger interlayer
spacing and the inclusion of anions (NO3-, Cl-, AcO- (acetate), SO4
2- and
CO32-) into its amorphous structure. The distinguishing features of the
α-Ni(OH)2 has caused considerable interest in synthesising a Co(II)
precipitate similar in structure and composition. Rajamathi et al. (2000)
2-9
produced a cobalt hydroxide phase that is structurally and compositionally
similar; however, the materials were poorly ordered. Various cobalt salts
(NO3-, Cl-, AcO-, SO4
2-) were added instantaneously to a 0.5 M ammonia
solution and characterised by powder X-ray diffraction, infrared spectroscopy
and thermogravimetry. The absence of Co3+ was confirmed by dissolving a
known amount of the hydroxide in excess of a standard solution of ferrous
ammonium sulphate and backtitrating the excess Fe2+ with a standard
solution of K2Cr2O7 potentiometrically. Upon analysis, the hydroxyl content
was found to be less than expected indicating the possible presence of other
anions to restore charge balance. The XRD and IR analysis confirmed that
poorly ordered α-Co(OH)2 was produced.
As mentioned in the previous investigation and as found for other
cations (Ni2+, Al3+, Fe3+), the reaction between a cobalt sulphate solution and
soda first produces a blue α-Co(OH)2 precipitate, which spontaneously
transforms into a pink β-Co(OH)2 precipitate of higher stability and
crystallinity. Gaunanad and Lim (2002) discovered that a weak
supersaturation leads to a crystalline β precipitate in the form of sub-micron
hexagonal platelets which become larger for a higher sulphate concentration.
2.1.3 Manganese Hydroxide
To produce manganese hydroxide a potassium hydroxide solution
was mixed with a manganese chloride solution in the absence of oxygen
(Moore et al., 1950). The manganese content was determined by reaction
2-10
with standard ferrous sulphate after all the manganese had been oxidised to
permanganate. The sample was treated with dilute sulphuric acid containing
ferrous sulphate, while the oxidation of the manganese from a valency of two
to seven was accomplished by the use of sodium bismuthate in nitric acid.
“Active” oxygen was determined by making use of the quantitative reaction of
manganese in valence states greater than two with ferrous ions. A known
excess of standard ferrous ammonium sulphate was used for the dissolution
of the sample and the excess was then titrated with standard permanganate.
The “active” oxygen was calculated by multiplying the number of equivalents
of ferrous ion oxidised by manganese by the factor 8.
Zhang and Cheng (2007) provide a useful review of manganese
metallurgy. The report by Nikoloski et al. (2005) focussed on the reduction
and recovery of metals which would oxidise during preparation and
transportation of Ravensthorpe’s mixed hydroxide precipitate. To study
possible reductants, a high-valent manganese oxy/hydroxide was
precipitated from a Mn(II) solution using an aerated sodium hydroxide
solution. This high-valent manganese oxy/hydroxide was assumed to have
the structure of manganite (MnOOH). The precipitate proved to be difficult to
reduce as a result of a manganese carbonate passivating layer formed upon
reduction (Nikoloski et al., 2005).
2-11
2.1.4 Magnesium Hydroxide
The production of magnesium compounds from seawater is a well
known industrial process in which calcium hydroxide in the form of slaked
lime or dolomite is used to precipitate magnesium hydroxide. Magnesia is
used in the refractory, pharmaceutical, pulp and paper, and waste water
treatment industries. The use of dolomite as a precipitant instead of lime
increases product yield due to the presence of both calcium and magnesium
carbonate in the uncalcined ore (Carson and Simandil, 1994).
The chemical precipitation of magnesium from sulphate solution,
resulting from heap leaching of nickeliferous laterites with sulphuric acid was
studied by Karidakis et al. (2005). Magnesium was removed using Ca(OH)2,
which produced a precipitate consisting of magnesium hydroxide and
gypsum (CaSO4.2H2O). Magnesium removal and the specific surface area
(m2/g) of the precipitate was measured varying the temperature and the
stoichiometric quantity of Ca(OH)2. The use of the precipitate as a filler
material was also examined. The results obtained showed that the chemical
precipitation using Ca(OH)2 was a very quick process resulting in 90-99%
magnesium removal over 30-270 minutes depending on precipitation
conditions. Temperature only had a significant effect when Ca(OH)2 was
used in stoichiometric quantity. Therefore, it is possible to achieve optimum
conditions of magnesium removal at temperatures as low as 20°C with the
use of at least 1.1 times the stoichiometric quantity of calcium hydroxide.
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2.1.5 Mixed Metal Hydroxides
Monhemius (1977) published the precipitation diagrams for a variety
of metal compounds including hydroxides. The plot in Figure 2.1 illustrates
the pH at which metal hydroxides precipitate depending on activity.
Figure 2.1. A solubility diagram of metal hydroxides at 25°C (Monhemius, 1977).
The substitution of metal ions in manganites was investigated by
Sinha et al. (1957). Although the precipitates were not hydroxides the paper
is relevant to the present study. The divalent cations Cd2+, Mg2+, Co2+, Fe2+,
Cu2+ and Ni2+ were all substituted into the spinel structure of Mn3O4. All metal
ions except nickel, iron and cobalt had a strong tendency to form sp3 bonds
in the tetrahedral sites. Nickel tended to occupy octahedral sites, while cobalt
and iron occupied both. If the Ni2+ ions also formed dsp2 bonds, all the
octahedral sites tended to distort the lattice, and the observed cubic
symmetry becomes anomalous. The strange occupancy of iron was
explained by a redox reaction between Fe2+ and Mn3+. Although this
mechanism also seems likely for cobalt, it was not mentioned in the article.
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Hydroxide precipitation of heavy, complexed metals was investigated
by Tünay and Kabdaşli (1994). Since precipitation was performed at alkaline
pH well above 7 from solutions containing ethylenediaminetetraacetic acid
(EDTA), nitrilotriacetic acid (NTA) and succinic acid, most of the investigation
is irrelevant to this review. However, NaOH was utilised as the neutralising
agent and the degree of removal of metal ions from solution was checked by
chloride determination. This procedure could be useful when performing
precipitation in the laboratory.
The removal of iron, aluminium, manganese and trace metals from
acid mine drainage using NaOH was reported by Giehyeon et al. (2002). It
was discovered that Fe, Al and Mn ions form precipitates over different pH
ranges. Iron hydroxides were formed at pH ~4, while Al and Mn compounds
were formed at pH ~5 and ~8, respectively, as expected from the solubility
diagram in Figure 2.1. Consequently, the relative abundance of Fe, Al and
Mn ions in solution determines the pH range for the removal of trace metals
from natural waters by sorption. It generally coincides with the precipitation of
the most abundant element (ion); therefore, the pH dependence is caused
not only by changes in the sorption coefficients but also by the fact that the
formation and composition of the sorbent is controlled by the pH and
chemistry of water. In general the sorption coefficient describes: (i) the
‘Henry sorption coefficient’ in case of linear equilibrium sorption is applied to
a model, or (ii) the first parameter of the non-linear Freundlich or Langmuir
sorption isotherms.
2-14
Zhu et al. (2010) investigated the precipitation of impurities from
synthetic laterite leach solutions, using MgO and NaOH, within the pH range
of 4-7. Below a pH of 4.5 the precipitation order was
Cr(III)>Al(III)>Cu(II)>Fe(III), above pH 4.5 the order changed to
Cr(III)~Fe(III)>Al(III)>Cu(II). Nickel and cobalt were discovered in precipitates
as low as pH 4 assumed to be due to hydroxide co-precipitation and as
sulphate precipitates resulting from chemical adsorption. Manganese,
magnesium and calcium were not precipitated within the pH range.
Precipitation parameters of mixed hydroxide precipitation were
evaluated by Harvey et al. (2011) using a fine magnesia powder.
Thermodynamic analysis and experimental results showed that some of the
manganese reports to the solids by an oxidative precipitation reaction. Also,
contacting the MHP with feed solution improved nickel and cobalt
concentrations significantly, whilst decreasing the manganese and
magnesium content.
Oustadakis et al. (2007) studied the precipitation of nickel and cobalt
ions from sulphate leach liquors by adding CaO slurry. A mixed Ni,Co(OH)2
precipitate was produced from a solution of typical composition produced by
a heap leach operation of a low grade nickel laterite ore with sulphuric acid.
After the removal of iron, chromium and aluminium by chemical precipitation,
the authors were able to precipitate over 99.8% of nickel and cobalt at 40°C
at a pH of 8.7 using a 10% CaO pulp. At the specified conditions manganese
2-15
precipitation reached 68%, while only 9% of magnesium was incorporated.
The precipitate was easily filterable as it had a P80 of 10 μm.
Likewise, Packter and Upplaladinni (1984) investigated the
precipitation of mixed Ni-Mg(OH)2. They monitored the precipitation process
by potentiometric titration and characterised the precipitate by chemical
analysis, infrared spectrophotometry and thermal analysis. The precipitates
were prepared by the addition of excess sodium hydroxide to the mixed
metal nitrate solutions (passed the equivalent volume) at varying rates.
The addition of sodium hydroxide to a solution of mixed metal nitrate
produces a complex precipitate. As various metal ions begin precipitation at
different pH values as shown in Figure 2.1, it is possible that the metal
hydroxides may form layers rather than mixed phases. In research
conducted by Comet Resources Ltd. in 2001, it was discovered that cobalt(II)
was precipitated at a lower pH than nickel(II) followed by manganese(II).
When magnesia was used as the precipitating agent the unreacted MgO
particles were coated with metal ion hydroxides. The SEM analysis of the
mixed hydroxide precipitate from the pilot plant runs 2 and 3 of the
Ravensthorpe Operations confirmed that some MgO particles were coated
with Ni(OH)2 (Muir, 2003). Elemental mapping also confirmed that Ni, Mg, Mn
and Co were intimately mixed in solid solution. Muir (2003) stated: “It is
anticipated that SEM analysis of freshly precipitated MHP would show more
coatings and rims of different metal hydroxides that slowly transform and
rearrange to solid solutions over time”.
2-16
2.1.6 Comparison of Precipitating Agents
The slow dissolving action of magnesia as a neutralising agent is an
appealing feature. This action produces a precipitate of larger particle size
which is easier to wash and filter. Unfortunately, the mechanism of magnesia
dissolution and its application to remove hydroxide precipitates from solution
is quite complex. The high pH outer layer of magnesia encourages co-
precipitation of several metal hydroxides, which can form impermeable
surface layers and inhibit dissolution of underlying magneisa. Moreover, the
reactivity varies according to its origin, impurities present, calcination
temperature, particle size and storage conditions. Due to its high porosity
water and CO2 are readily absorbed from the atmosphere lowering its
reactivity. Furthermore, the calcination temperature affects the surface area
and porosity of the particle. (Frost et al., 1990). The influence of calcination
temperature and starting material on crystallinity and porosity is discussed
further by Guan et al. (2006), Ardizzone et al. (1997), Hartman et al. (1993)
and TecEco.
2.2 Commercial Production of Mixed Nickel-Cobalt Hydroxide
2.2.1 Cawse – Original Flowsheet
The original Cawse flowsheet included the precipitation of a mixed
nickel-cobalt hydroxide. The leach slurry was neutralised at pH 3.5 then 6
with limestone to oxidise and remove iron from solution. After recycling the
precipitate to recover any co-precipitated nickel and cobalt, the hydroxide
intermediate was produced by addition of MgO (Whittington & Muir, 2000).
2-17
The re-dissolution of the precipitate using ammonia at atmospheric
pressure was much simpler. Although the precipitation process was non-
selective, the ammonia re-leach process was highly selective for nickel and
cobalt, with excellent rejection of iron, manganese and magnesium.
However, copper(II) and zinc(II) precipitates were also dissolved in ammonia
solution. Although the caustic calcined magnesia (MgO) was effective for
precipitation, care needed to be taken upon addition to avoid magnesium
build up in the process water (Whittington & Muir, 2000).
2.2.2 Ravensthorpe Process
(a) Leaching of Limonitic and Saprolitic Ore
The Ravensthorpe process involved both pressure acid leaching
(PAL) and atmospheric leaching running in parallel (flowsheet – Figure 1.2).
The PAL trains treated the limonite ore using similar conditions to other
plants, while the saprolite was leached under atmospheric conditions. The
saprolite was pre-leached, which entailed a high intensity stage where the
ore was blended with concentrated sulphuric acid for four hours. This stage
breaks down some of the more refractory ore structures, eliminates
carbonate from the ore and heats the slurry to a temperature greater than
95°C.
The slurry from the pre-leach stage contained a significant residual of
acid, iron and other impurities in solution. Output streams from the PAL and
pre-leach stages were combined and allowed to react at temperatures
2-18
around 95 – 100°C. As acid levels decreased iron started to precipitate as
sodium or potassium jarosite (Na/KFe3(SO4)2(OH)6) which simultaneously
regenerated acid maintaining a driving force for continued nickel and cobalt
leaching:
OHSOFeSOHFeOOH 234242 4)(32 +→+
426243422342 6)()(212)(3 SOHOHSONaFeSONaOHSOFe +→++
(b) Precipitation of MHP
After leaching, limestone slurry was added to increase the pH from 2
to 2.5 (primary neutralisation), which precipitated the bulk of the remaining
ferric ion, along with portions of the aluminium, chromium and other
impurities. Limestone was further added to the countercurrent decantation
(CCD) washing stage, raising the pH from 4 to 5 (secondary neutralisation),
to remove the remnant iron and aluminium before proceeding to the mixed
hydroxide precipitation (MHP) stage. This was achieved by oxidation and
precipitation of the ferrous ion as ferric hydroxide and by hydrolysis of the
aluminium ion.
The MHP stage recovered the bulk of the nickel and cobalt from
solution as a mixed hydroxide using MgO powder as the precipitant.
Magnesia was added to a seeded tank at a nominal ratio of 0.82 kg MgO
per kg of Ni+Co with a residence time of 3 hours. The seeding improved solid
settling characteristics and reduced the amount of unreacted MgO in the
2-19
precipitate. The solids were collected in a thickener, washed and filtered and
dispatched to further refining.
The thickener overflow was treated by slaked lime addition as it still
contained approximately 5% of the incoming nickel and cobalt. After
complete nickel/cobalt precipitation the clarifier solution was treated by
further addition of slaked lime and mild aeration to precipitate the bulk of the
remaining manganese so the solution could be recycled.
2.2.3 Ramu Process
China Metallurgical Construction are developing a PAL nickel laterite
project in Papua New Guinea; commissioning expected in 2013. After a
substantial piloting program a flow sheet has been proposed which entails a
pressure acid leach followed by metal hydroxide precipitation.
The hydroxide precipitation occurs in 2 stages. The first stage
precipitates the bulk of the nickel and cobalt while minimising co-precipitation
of manganese. Piloting with a tight pH control established that over 90%
nickel and 85% cobalt with no more than 25% of manganese can be
precipitated. In the second stage, again with tight pH control, over 99% of the
nickel and cobalt were precipitated with about 40% of the manganese. The
remaining manganese was precipitated down to less than 50 mg/L by
increasing the pH further with aeration to increase the oxidation state of the
metal. Lime was used for pH adjustment. Although magnesia may be the
2-20
preferred precipitant, the local availability of good quality limestone results in
a strong economic preference (Mason & Hawker, 1998; Mining News).
2.2.4 European Nickel Process
The Caldag nickel operation proposed by European Nickel in Western
Turkey went into care and maintenance in 2010 due to the lack of a forestry
permit. The proposed process involved heap leaching then precipitation of a
nickel hydroxide precipitate for shipping. The process claimed to leach over
75% of nickel and cobalt in a recirculating leach using dilute sulphuric acid
over a period of around 15 months. The recirculation would neutralise some
of the acid and maximise the metal content, before being pumped to the
precipitation plant where the iron is precipitated by raising the pH. The liquor
is further treated by raising the pH with soda ash to produce a mixed nickel-
cobalt hydroxide with a nickel content of above 30% (Purkiss, 2006;
Proactive Investors, 2009, Mining News).
2.2.5 Niquel do Vermelho Process
A comprehensive pre-feasibility study was conducted by CVRD for the
Niquel do Vermelho nickel laterite project in 2004. Five flowsheet options
were evaluated via batch and pilot campaigns at Lakefield Oretest. The
viability of the beneficiation of several ore types and each flowsheet/stage
was demonstrated successfully (Adams et al., 2004). The processing stages
involved the integrated pressure acid leach/mixed hydroxide precipitation
(PAL/MHP) and mixed sulphide precipitation (PAL/MSP) circuits, and the
2-21
treatment of barren liquors. A hydroxide intermediate will be produced and
re-leached in ammonia, purified by solvent extraction then electrowon on
site. Little information is available about the project to date, however nothing
seems to have been reported since the drop in nickel price in 2008 (Mining
News).
2.2.6 Comparison of Flowsheets
Hydrometallurgy Research Laboratories conducted a series of pilot
plant trials for a hydroxide precipitation/ammoniacal releach circuit
(Steemson, 1999). Some Ramu piloting results were also included for
comparison and evaluation of lime and magnesia as precipitating agents.
The main findings are listed below:
• The precipitation behaviour of nickel and manganese using lime and
magnesia were similar in a single stage precipitation (over 90% nickel,
~30% manganese).
• Magnesia may be slightly superior for cobalt precipitation with
recoveries generally over 90% using magnesia compared to 80-90%
using lime.
• In a two stage precipitation circuit, over 99.5% nickel and 99% cobalt
can be recovered using either neutralising agent.
• The nickel-cobalt content of a magnesia based filter cake is
significantly higher compared to that of a lime based filter cake. This is
a result of the presence of gypsum in a lime based filter cake.
2-22
Although magneisa based precipitates proved to be of better quality,
lime was eventually chosen due to cost considerations. Further work has
since been conducted utilising magnesia in a more efficient manner.
Australian patent 701829 (Cawse) describes the precise addition (based on
stoichiometry) of MgO to an acid sulphate liquor to minimise the manganese
content of the mixed hydroxide precipitate. The recovery of nickel and cobalt
suffered as a result. Consequently, the process required further
neutralisation using Ca(OH)2. This addition had the potential to cause
gypsum scale. Moreover, the ‘ageing’ of the mixed Mn, Ni and Co hydroxides
precipitate inhibited the redissolution in ammoniacal liquors.
The BHP Billiton European patent WO0248042 involves the treatment
of acidic sulphate feed liquor with MgO to produce a mixed hydroxide. This
hydroxide is contacted with further acidic feed liquor to re-dissolve unreacted
MgO in the precipitate, and to precipitate additional nickel and cobalt.
2.3 ‘Ageing’ of MHP
A general observation is that the efficiency of nickel and cobalt
dissolution from a stored sample of a mixed hydroxide precipitate is lower
than that from a freshly prepared sample. This behaviour has been
historically termed as ageing. Although ‘ageing’ was not an issue with the
Cawse project, as the precipitate was processed soon after production, it
becomes a significant issue for developing projects where the refinery is
located off-site.
2-23
The reactions of Ni(OH)2 are complex due to its various phases and
crystallographic forms. This is further complicated by the existence of
impurities such as Co, Mn, Mg, Fe, Cr and Al which are found in most mixed
hydroxide precipitates. Anions such as sulphate, carbonate and chloride also
influence the nature of Ni(OH)2 (Muir, 2003). Thus, a mixed hydroxide
precipitate is thought to age in various ways described below.
2.3.1 Formation of High-Valent Oxides
Nickel hydroxide forms as either an α or β phase and may oxidise to
γ-NiOOH. The α and β phases precipitate at temperatures below 60°C or
around 90°C, respectively. They exhibit a layered brucite-like (Mg(OH)2)
structure as shown in Figure 2.2. The α-Ni(OH)2 contains significant
quantities of intercalated water containing up to 20% mono or divalent anions
(e.g. SO42-, CO3
2- and Cl-) in place of OH-. This phase is often amorphous
and unstable in water as it slowly transforms into β-Ni(OH)2 with a significant
increase in particle size and the appearance of XRD peaks and IR spectra
due to its well defined structure. Intercalated anions are desorbed in the
process (Muir, 2003). Cobalt(II) and manganese(II) hydroxides readily
oxidise in air to the high valent oxyhydroxide M(III)OOH. These oxides
induce transformation to a hydrotalcite-like structure
((MII1-xMIII
x)8(OH)16(An-)8x/n.4H2O). An example of the two structures is shown
in Figure 2.2.
2-24
Figure 2.2. Brucite and hydrotalcite structure (Alcaraz et al., 1998).
The species involved in the oxidation of Mn(OH)2 and Co(OH)2 under
neutral or alkaline pH conditions is illustrated by the Eh-pH diagram in Figure
2.3. In fact, cobalt is used as an additive for battery grade Ni(OH)2 to
promote nickel oxidation and improve charge capacity. Although one metal is
predicted to oxidise before another according to Eh values, simultaneous
oxidation is likely with solid solutions (Muir, 2003; Zhang et al., 2002).
2-25
Figure 2.3. Eh-pH diagram of Co-H2O and Mn-H2O systems for 0.01 M Mn(II) and 0.1 M Co(II) (Zhang et al., 2002).
The oxidation of the co-precipitated Co and Mn induces slow lattice
re-arrangement to the hydrotalcite like structure where nickel may be present
as Ni(III) or even Ni(IV). Existence of trivalent cations including Al, Fe and Cr
(all present in mixed hydroxide precipitates) also encourages further
re-arrangement into hydrotalcite like phases. In this form, both Co and Ni are
likely to be insoluble in an ammoniacal leach (Muir, 2003).
Unfortunately, the oxidation of cobalt and manganese hydroxide is not
as simple as mentioned above. The pathways followed during the oxidation
of Mn(II) are complicated as noted by previous researchers (Murray et al.,
1985; Burns & Burns, 1977,1979; Giovanoli, 1976, 1980). The initial products
can be Mn3O4, β-MnOOH, γ-MnOOH or a Na-Mn-oxide-hydrate depending
on the conditions (Feitknecht et al., 1962; Oswald et al., 1964; Bricker, 1965;
2-26
Hem & Lind, 1983). In the investigation by Murray et al. (1985) the initial solid
formed at atmospheric conditions in slurry was Mn3O4. This was converted
completely to γ-MnOOH after eight months, with β-MnOOH appearing to be
an intermediate in the transformation.
The aeration of 0.01 M solutions of MnCl2, Mn(NO3)2, MnSO4, or
Mn(ClO4)2 at pH in the range 8.5-9.5 (or higher) at 25°C produced Mn3O4 as
the predominant oxide. At temperatures near 0°C the product was
β-MnOOH. However, when the initial solution was MnSO4 the product was a
mixture of γ-MnOOH and α-MnOOH. All of these metastable oxides were
altered to highly oxidised species by irreversible processes during ageing in
aerated solution. Relatively unstable β-MnOOH was most readily converted
to MnO2. Some preparations of β-MnOOH aged in solution at 5°C attained a
manganese oxidation state of +3.3 or more after 7 months. The ageing of
Mn3O4 at 25°C produced γ-MnOOH. The latter was more stable than α or
β-MnOOH, and manganese oxidation states above 3.0 were not reached in
Mn3O4 precipitates during 4 months of ageing (Hem & Lind, 1983).
In terms of oxidation from 2+ to 3+, cobalt seems to behave in a
similar manner to manganese. Hem et al. (1985) suggest that the cobalt
hydroxide oxidises to cobalt oxide (like manganese) which reacts with
protons:
++ +↔+ 243 22 CoCoOOHHOCo
This could be accompanied by reoxidation of the released Co2+:
2-27
++ +→++ HOCoOHOCo 24331
22612
This intermediate phase is a spinel with a tetragonal structure and formula of
CoO.Co2O3 (Ardizzone et al., 1998).
If the manganese and cobalt are precipitated separately, oxidation
would probably occur as discussed above. However, in a complex solid state
system where the metals ions are likely to co-precipitate, and the precipitate
is amorphous, it would be hard to determine the phases formed upon
oxidation. It is reasonable to assume that the various oxide phases of cobalt
or manganese formed during aeration would behave differently during the
leaching of MHP. Nevertheless, despite the wealth of information on the
nature of the oxidation products of manganese and cobalt hydroxides
described above, the presence or absence of such oxides in MHP has not
been established. Moreover, the lack of information on the nature, stability
and the ammoniacal leaching behaviour of various oxide phases present in a
mixed hydroxide precipitate highlight the importance of the present study.
2.3.2 Formation of Insoluble or Slow-Leaching Compounds
It is anticipated that α-Ni(OH)2 will be the predominant nickel phase
upon precipitation. Due to the high levels of sulphate, it can be assumed that
significant concentrations of nickel ions will be incorporated in the
intercalated water layers. Therefore this α-phase will slowly transform into β-
Ni(OH)2 after several days. The sulphate ions will be rejected while the
crystallite size improves dramatically and peaks will become visible in XRD
2-28
and IR spectra. It has been discovered that some nickel(II) was in solid
solution with Mg(OH)2, when precipitated with MgO. Packter and Uppaladinni
(1984) state that the solid solution would react slowly in ammonium
carbonate via a thickening layer of MgCO3 or (NH4)2CO3.MgCO3 double salt.
A review of MHP characteristics by BHP Billiton in 2003, revealed that
the phases responsible for slow leaching after ageing are crystalline
Ni,Mg(OH)2 and NiMnO3. However, these phases were leached when either
soaked for 72 hours in an ammonia/ammonium carbonate solution or
reductively leached using hydroxylamine sulphate with a complexing agent
(EDTA). It was also stated that a proportion of the cobalt may be
incorporated into a crystalline phase that is more resistant to leaching (Muir,
2003).
In 2005-6 BHP Billiton tested MHP’s from Polymet and European
Nickel as potential suppliers for the Yabulu Refinery from 2013. The Polymet
sample was produced from a sulphide deposit while the European Nickel
samples were produced by neutralisation with sodium carbonate. Overall, the
results were poor mainly due to the fact that the precipitates contained high
levels of impurities. On a ‘fresh’ Polymet sample, 5% of Ni and Co remained
after leaching for 45 minutes at 30°C in an ammonia/ammonium carbonate
solution under reducing conditions using hydroxylamine sulphate as the
reductant. The ‘poor’ results were due to the presence of hydrotalcite-like
compounds containing Fe3+, Al3+ and Cr3+ ions. Hydrotalcite-like compounds
((MII1-xMIII
x)8(OH)16(An-)8x/n.4H2O) are refractory to standard leach conditions.
2-29
The structure is shown in Figure 2.4. The slow leaching mixed hydroxide
(Ni,Mg)(OH)2 was also present in the leach residue (Bessel, 2006b). Similar
results were obtained when leaching ‘aged’ European Nickel samples. Again,
the loss of nickel and cobalt was associated with the presence of
hydrotalcite-like compounds. The formation of MnCO3 was also suggested as
a possible inhibitant (Bessel, 2006a).
Figure 2.4. Hydrotalcite structure (Forano et al., 2006).
The ‘preboil solids’ produced at the Yabulu refinery also contain
significant compositions of the slow leaching hydrotalcite-like compounds.
These solids are produced when product liquor is prepared for solvent
extraction by reducing the ammonia content in the liquor via steam stripping
(flowsheet – Figure 1.1). This step is essential to the process as it is the only
means available to control the manganese content of the liquor. The solids
are separated from the liquor and recycled to the ore stockpile. However, the
slow leaching characteristics of the solids are responsible for nickel and
cobalt losses at the refinery (Bolden, 1997).
2-30
Although hydrotalcite-like structures involving Fe, Al and possibly Cr
have been observed in various intermediates, Mn and Co in the trivalent
state are also known to form similar structures. Table 2.1 lists some of the
known structures. As manganese and cobalt oxidise fairly readily, the
transformation from the brucite-like structure to a hydrotalcite-like compound
could be occurring in the first few weeks after production. This is complete
speculation as the precipitates are generally amorphous in the first few
weeks; hence these phases have never been observed. Moreover, Mn and
Co hydrotalcite structures have not been observed in leach residues,
suggesting that they are leachable in an ammonia solution. This poses the
question: why are some structures more stable, crystalline and resistant to
leaching than others?
Table 2.1 Hydrotalcite-like structures (Forano et al., 2006). Hydrotalcite Mg6Al2(OH)16CO3.4H2O Pyroaurite Mg6Fe2(OH)16CO3.4H2O
Desautelsite Mg6Mn2(OH)16CO3.4H2O Woodallite Mg6Cr2(OH)16Cl2.4H2O Stichtite Mg6Cr2(OH)16CO3.4H2O
Reevesite Ni6Fe2(OH)16CO3.4H2O Honessite Ni6Fe2(OH)16SO4.4H2O Takovite Ni6Al2(OH)16CO3.4H2O Comblainite Ni6Co2(OH)16CO3.4H2O
2.3.3 Other Possible Ageing Processes/Influences
The absorption of CO2 from air during precipitation should also be
considered as a possible influence on ageing. The transformation of
α-Ni(OH)2 to β-Ni(OH)2 is inhibited by the carbonate ion. Unfortunately it is
2-31
not known how much carbonate is required to affect this transformation. It is
also unknown whether carbonate affects transformation into hydrotalcite like
structures reported in Figure 2.2 (Muir, 2003).
2.4 Drying MHP
Jones (2000a) found drying of MHP for 4 hours at 75°C had little or no
effect on leaching efficiencies. However, drying at 95°C for the same period
of time decreased the leaching efficiencies by over 8% Ni and 10% Co. No
explanation was given for the difference in reactivity of MHP dried at different
temperatures. However, it was recommended to repeat the tests at 75°C and
use a MHP sample which would be more representative of solids from a full
scale operation.
Experiments conducted at Lakefield Oretest showed that Ni and Co
dissolutions were not significantly affected by drying at 70 or 85°C (Furfaro et
al., 2000). Metal ion dissolution efficiencies with the standard
ammonia/ammonium carbonate (SAC) solution were 98-99% for Ni and
86-88% for Co. Nickel and cobalt dissolution in the product liquor (PL) of the
Yabulu refinery (Figure 1.1) were slightly lower. The ageing of a dried (90%
solids) precipitate for 25 days had no significant effect on metal dissolution.
Jones (2001a) found that a sample of dried MHP, from the Lakefield pilot
plant, had lower metal recoveries (88% Ni, 92% Co) compared to fresh,
undried material (95% for Ni and Co). Unfortunately, there were no
comments about the drying temperature, percentage solids or age of sample.
2-32
Researchers at the joint venture between SNC-Lavalin Australia and
Worley Limited (SLW, 2001) also examined the effect of drying. They found
that the nickel dissolution decreased with decreasing moisture level in the
dried product from 25% to 10% for MHP dried at 70 and 85°C. These
contradictions are a little unsettling. More testwork at a wider range of
temperatures is required to examine the effect of drying and the moisture
content on metal ion leaching efficiency. It is also important to consider the
chemistry of leaching of metal ions in ammonia/ammonium carbonate
solutions in order to rationalise the effect of drying and ageing.
2.5 Chemistry of Leaching of MHP in SAC Solutions
2.5.1 Three Stage Leaching Process
The ammonia/ammonium carbonate leaching of the Ravensthorpe
MHP at the Yabulu refinery was conducted in a three stage process
(Figure 1.3). Firstly, the MHP was repulped in Product Liquor (PL, typically
10 to 12 g/L Ni, 0.6 g/L Co, 95 g/L NH3, 60 g/L CO2 and pH 10.5). Then the
slurry was combined with the remaining PL in four aerated agitated tanks.
Overflow from the secondary MHP leach thickener was also added to the
first reactor so that the configuration was counter-current. The combined
overflow from the primary leach was typically 23 g/L Ni and 1.1 g/L Co.
Fresh Leach Liquor (FLL, typically >120 g/L NH3 and >60 g/L CO2) and
a cobalt-nickel sulphide reductant (CoNiS, typically Co:Ni >2) were used in
the secondary leach stage to enhance the dissolution of nickel and cobalt.
2-33
The CoNiS reductant served to undo any MHP ageing effects. Of the five
reactors, the last three were aerated in order to solubilise the unreacted
CoNiS (Fittock, 2004). The ammonia/ammonium carbonate leaching of MHP
can be rationalised on the basis of the published information on metal
ammine complexes, Eh-pH diagrams and salt solubilities described below.
2.5.2 Metal Ammine Complexes
Nickel and cobalt hydroxide (M(OH)2) dissolves in ammoniacal
solutions by forming ammine complexes (M(NH3)n2+), exchanging its hydroxyl
group for ammonia molecules. The overall general reaction is as expressed
below:
OHNHMNHnNHOHM n 22
3342 2)()2(2)( +→−++ ++
where n is an integer having a value from 1 to 6 depending on the
concentration of ammonia present and on the pH of solution. Stability
constants of the ammine complexes of various transition metal ions of
interest in this thesis listed in Table 2.2 show that Ni(II), Co(II) and Co(III) are
most stable as either the pentammine or hexammine complexes. Stability
constants of Mn(II) and Co(II) ammines were also determined in two
separate Russian papers (Isaev et al., 1990a & 1990b) while Isaev et al.
(1990c) also investigated the influence of ammonia on the hydrolysis of
cobalt(III) hexammine.
2-34
Table 2.2 Stability constants (Kn) of metal ammine complexes
at 298 K for an ionic strength of 0.5 (Smith & Martell, 1989).
Ammonium carbonate is preferred over ammonium sulphate since
both ammonia and carbon dioxide can be regenerated and recycled (Figure
1.1). Nevertheless, the chemistry of the leaching is complex due to the fact
that sulphate, carbonate, sulphite, thiosulphate and other anions in the
process liquor will also form substituted ammine complexes depending on
leaching conditions and concentrations of anions. The existence and
formation of these complexes is basically undocumented as they are
extremely hard to synthesise for analysis by High Performance Liquid
Chromatography (HPLC). Moreover, various other metal ions which exist in
MHP may act differently in an ammonia solution and some metals will oxidise
upon leaching.
Osseo-Asare and Asihene (1979) discuss the equilibria of metal ions
(Ni, Co, Fe, Mn and Mg) in ammonium carbonate solutions. In terms of
cobalt, the pentammine complex is found to be the major ammine species
even though thermodynamics predict a greater stability for hexamine
complex. According to the authors, hexammine is more likely to occur in the
presence of a catalyst like carbon. Hang and Meng (1993) thoroughly review
2-35
the thermodynamic and kinetic aspects of nickel and cobalt leaching. The
behaviour of cobalt in ammonia solutions is also discussed in detail by
Osseo-Asare (1980). More recently, Asselin (2008) published the
thermodynamics for the Caron Process in relation to Fe-Ni-Co alloy
passivation. A brief review of thermodynamic data was conducted and
Pourbaix diagrams were constructed for cobalt(II), cobalt(III), nickel(II) and
iron(II).
The chemistry of nickel and cobalt during the Caron leaching process
is portrayed by Eh-pH diagrams, as shown in Figures 2.5 and 2.6 (Han and
Meng,1993). The Eh-pH diagrams from Nikoloski et al. (2005) shown in
Figures 2.7-2.9 consider the hydroxides and carbonates in solid state, but do
not include all the metal ammine and metal oxide species. However, they
provide a good indication of phases present at typical leaching conditions
using an ammonia-ammonium carbonate solution. Although the diagram for
cobalt in Figure 2.8 shows a possible transition between Co(III) hydroxide
precipitate and the ammine complex, the dissolution of cobalt(III) hydroxides
requires prior reduction. Conversely, the transition between the cobalt(II)
ammine and cobalt(III) ammine complexes occurs readily in an oxidising
environment (e.g. dissolved oxygen).
2-36
(a)
(b)
(c)
Figure 2.5. Potential-pH diagrams for Ni-NH3-H2O system at 25°C and 1 atm. 1. Ni(NH3)2+; 2. Ni(NH3)2
2+; 3. Ni(NH3)32+; 4. Ni(NH3)4
2+; 5. Ni(NH3)52+; 6.
Ni(NH3)62+. a) Activity of ionic species is unity, b) activity of ionic species is
10-2, c) activity of ionic species is 10-4 (Han & Meng, 1993).
2-37
(a)
(b)
(c)
Figure 2.6. Potential-pH diagrams for Co-NH3-H2O system at 25°C and 1 atm. 1. Co(NH3)2+; 2. Co(NH3)2
2+; 3. Co(NH3)32+; 4. Co(NH3)4
2+; 5. Co(NH3)5
2+; 6. Co(NH3)62+. a) Activity of ionic species is unity, b) activity of
ionic species is 10-2, c) activity of ionic species is 10-4 (Han & Meng., 1993).
The manganese diagram in Figure 2.9 illustrates that MnCO3 is the
most stable of the manganese species at low Eh. Any un-oxidised Mn(OH)2
present would be expected to dissolve to form an ammine complex
(Mn(NH3)42+) prior to precipitation as carbonate. Like cobalt, the
manganese(III) in the form of MnOOH needs to be reduced before
dissolution and precipitation will occur. The dissolution of
2-38
precipitated/oxidised manganese in MHP is essential for the dissolution of
nickel and cobalt. The reductive dissolution of MnOOH and reprecipitation of
MnCO3 may be necessary in the overall MHP leach process. This will be
discussed in a later section.
Figure 2.7. Eh-pH diagram of Ni-ammonia-carbonate system at 30°C (Nikoloski et al., 2005).
Figure 2.8. Eh-pH diagram of Co-ammonia-carbonate system at 30°C (Nikoloski et al., 2005).
2-39
Figure 2.9. Eh-pH diagram of Mn-ammonia-carbonate system at 30°C (Nikoloski et al., 2005).
2.5.3 Measured Metal Ion Solubility
Nickel(II) carbonate, cobalt(II) hydroxide, iron(II) chloride and
manganese(II) chloride solubilities were determined in the laboratory as
functions of NH3 concentration and the NH3:CO2 ratio relevant for the Yabulu
refinery (Benjamin, 2003). Tests were conducted at 45°C with three different
ammonia concentrations. Results summarised in Figures 2.10-2.13 are
valuable for the refinery and this investigation as they highlight the
importance of maintaining the ammonia:carbonate ratio.
As shown by Figures 2.10-2.13, the NH3:CO2 ratio has a significant
influence on metal ion solubility. Nickel(II) and cobalt(II) have the highest
solubilities at NH3:CO2 ratio of approximately 0.8 and 1, respectively
(Figures 2.10-2.11). While iron(II) and manganese(II) solubilities increase as
the NH3:CO2 ratio increases, thus lower ratios would be ideal. The Yabulu
2-40
refinery operates at a ratio between 1.6 and 1.8, so the lower ratio could only
be achieved by the construction of an additional carbon dioxide plant. It is
still not known how the carbonate improves nickel and cobalt solubility:
whether the carbonate affects the pH and/or complexes with the metal ions.
Further testwork is required.
0
20
40
60
80
100
120
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0NH3:CO2 (wt/wt)
Ni (
g/L)
120g/L NH3
80g/L NH3
35g/L NH3
Figure 2.10. Nickel(II) carbonate solubility at 45°C depending on ammonia concentration and NH3:CO2 ratio (Benjamin, 2003).
0
5
10
15
20
25
30
35
40
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5NH3:CO2 (wt/wt)
Co2+
(g/L
)
120 g/L NH3
80g/L NH3
40g/L NH3
Figure 2.11. Cobalt(II) hydroxide solubility at 45°C depending on ammonia concentration and NH3:CO2 ratio (Benjamin, 2003).
2-41
Figure 2.12. Manganese(II) chloride solubility at 45°C depending on ammonia concentration and NH3:CO2 ratio (Benjamin, 2003).
Figure 2.13. Iron(II) chloride solubility at 45°C depending on ammonia concentration and NH3:CO2 ratio (Benjamin, 2003).
2-42
2.5.4 Leach Kinetics
Leach kinetics are reviewed in detail in the comprehensive article by
Meng and Han (1995). Kinetic studies of ammonia pressure leaching of
metallic nickel powders with oxygen revealed the diffusion of oxygen through
the leach solution was the rate determining step. In one of the studies the
dissolution was described in terms of a shrinking core model with a first order
reaction at the solid/liquid interface. The initial rate of dissolution was a
function of the oxygen concentration, mass transfer coefficient, density and
particle size. In another investigation rate equations were derived using a
rotating disk electrode. Dissolution of nickel exhibited a linear relationship,
with the rate being increased with the increase in disc rotation speed. At low
temperatures the reaction seems to be limited by a surface chemical
reaction, while at temperatures 35-65°C the reaction was limited by mass
transfer control.
Meng and Han (1995) have also discussed cobalt dissolution. At
ambient conditions and high ammonia concentrations (0.8 M) the dissolution
exhibited a linear relationship, whilst at lower ammonia concentrations the
dissolution seemed to be limited by the equilibrium concentrations of cobalt
and ammonia in the bulk solution. At pH 11.5, passivation is thought to occur
due to the presence of some form of cobaltic oxide. Dissolution behaviour
was very similar to that of nickel whereby the reaction is diffusion limited at
higher temperatures (>25°C) and chemical reaction controlled at lower
temperatures.
2-43
Nickel(II) and nickel(III) hydroxides were leached in ammoniacal
solutions by Bhuntumkomol et al. (1982). The article is more comprehensive
in relation to nickel oxide leaching behaviour. Unfortunately, little is
discussed, apart from the fast dissolution rates. In fact complete dissolution
had occurred within 60 minutes. This was achieved with 1 M ammonia
solution and a slurry density of ~0.2 g/L. Nickel(III) hydroxide was seen to
reduce upon dissolution, producing gaseous nitrogen and a nitrite ion
(oxidation products of ammonia).
Senaputra et al. (2008) discovered thiosulphate, Cu(II), excess
oxygen and agitation had a beneficial effect on nickel dissolution. Copper(II)-
thiosulphate improved nickel leaching via redox mediation by Cu(II) and the
involvement of thiosulphate ions in the surface reaction. Other anions also
assisted nickel dissolution, influencing in the following order S2O32- > HS- >
HCO3- > SO4
2- > SO32-.
2.5.5 Impurities in MHP
(a) Detrimental effects of impurities
According to Jones (2000b) high concentrations of Fe, Al and Mn in
‘fresh’ MHP resulted in lower releach recoveries of nickel and cobalt.
However, in comparison to typical leach liquor, the concentrations of Cr, Al
and Fe in the primary releach liquor remained unchanged. In contrast, the
concentrations of Zn, Cu, Mg and S were elevated. The elevated levels of
readily leached Zn(II) in the ammonia liquor remain in solution during aerobic
2-44
and anaerobic leaching. Zinc and copper would have a significant effect on
the cobalt plant due to high loading capacity in the solvent extraction stage.
The elevated levels of Mg may affect the scale formation, depending on the
sensitivity of the Mg solubility to the expected temperature changes in the
flowsheet.
As previously mentioned, high-valent manganese oxides in fresh MHP
or oxidation of manganese(II) in the primary stage can become an issue.
The high-valent manganese oxides produce MnCO3 in the reductive
secondary leach as revealed by the Eh-pH diagram in Figure 2.9, which can
coat and inhibit the leaching of other particles. Sulphur is also a significant
problem as it is leached readily from MHP, resulting in high concentrations of
(NH4)2SO4, causing reduced ammonia recovery at the residue steam strip
step (Moroney, 2002).
(b) Removal of impurities
The manganese remaining in solution after the secondary stage is
removed at the preboil step. Manganese oxide is anaerobically leached from
MHP and precipitated from the liquor as either anaerobic or aerobic leaching
progresses. Previous research by BHP Billiton (Moroney, 2002) showed that
there is a maximum solubility limit for manganese in process liquors. Within
several minutes of leaching, iron is precipitated as ferric hydroxide from initial
solutions of low concentrations.
2-45
2.5.6 Reductive Leaching of MHP
The Kennecott Cuprion Process, which involves the reduction of
manganese ocean nodules, is relevant to the reductive leaching of aged
MHP. As discussed by Agarwal et al. (1979a & b), it was possible to reduce
MnO2 to MnCO3 through the oxidation of copper(I) in the presence of
ammonia. Copper(II) is a by-product from the process, and is regenerated for
further oxidation by sparging carbon monoxide through the leach solution.
Likewise, the reductive leaching of high-valent oxides of manganese
and cobalt in MHP releases cobalt(II) and some manganese(II) to solution
hence improving overall cobalt and nickel recovery. The patented invention
WO2004090176 for BHP Billiton involves contacting a nickel, cobalt or a
mixed nickel/cobalt hydroxide, carbonate, basic carbonate or basic sulphate
material with an ammoniacal ammonium carbonate solution and a reductant
at atmospheric pressure between 30 and 90°C. The reductant is preferably
selected from hydroxylamine, a mixed cobalt/nickel sulphide (CoNiS), or
cobalt sulphide. The metal sulphides can be obtained by treating process
liquor with ammonium hydrosulphide or sodium hydrosulphide.
In 2005 Queensland Nickel investigated the following reagents as
possible reductants for the patented process using aged Cawse MHP: Co(II)
ions, Fe(II) ions, spent hydroxylamine liquor, sulphite, spent acid liquor
streams containing Co(II) and Fe(II) ions, CoNiS slurry, WMC Resources
CoNiS and sodium dithionite (Na2S2O4). The project proved that all of these
2-46
reagents significantly increased the extractions of nickel and cobalt. The
reductants based on CoNiS were superior. However, dithionite,
hydroxylamine and Co(II) ions may also have potential for industrial
application (McGregor, 2005). Nikoloski et al. (2005) also investigated the
effectiveness of Co(II), sulphide, sulphite and thiosulphate ions for reducing
the oxidised manganese species present in the primary leach underflow (PU)
from the Yabulu refinery. In the same investigation, Co(II) ions, sulphide ion,
thiosulphate, sulphite, elemental sulphur and hydrazine were tested on a
synthetic oxidised Mn-hydroxide, but to no avail. The unexpected results
suggest that the formation of a MnCO3 layer on the surface of the hydroxide
particles may be inhibiting the reduction.
The patented process involves an initial anaerobic period of at least
10 minutes, and then air or oxygen is injected into the mixture to ensure
complete oxidative dissolution of the excess metal sulphide used for
reduction. The weaknesses of the invention are:
(i) Unknown extent of oxidation in the primary leach and how
much CoNiS to be added.
(ii) Unknown duration of leaching under oxidising conditions to
ensure complete extraction. Extensive oxidation can lead to re-
oxidation of precipitated MnCO3 or remaining Mn(OH)2 and lock
up of cobalt (Muir, 2003).
Mohanty et al. (1996) discovered that the adsorption of cobalt can be
minimised by raising the temperature to about 55°C and minimising free
ammonia in solution. Although high free ammonia resulted in less
2-47
manganese in solution more cobalt was adsorbed on the manganese(III)/(IV)
oxides. Nevertheless, the reductive leaching is particularly advantageous to
the Yabulu refinery and future projects due to several reasons:
(i) It eliminates impurities such as manganese, magnesium and
iron that may be present in the intermediate by eliminating
them from the enriched product liquor solution.
(ii) It improves the settling and filtration characteristics of the leach
residue.
(iii) It improves the leaching of cobalt in the ore leach stage by
depleting the cobalt content of the process stream.
2.5.7 Effect of Soaking
In the proposed Yabulu MHP leaching process (flowsheet in
Figure 1.3), the leach residue is sent to the counter current decantation
section (CCD) where it will exist in contact with the solution for a further ~72
hours. This section should leach the residual nickel and cobalt retained in the
form of slow leaching phases such as hydrotalcite-like compounds and
crystalline Ni,Mg(OH)2. A soak predictor leach test was developed by BHP
Billiton, whereby the residue from a 45 minute reductive leach was left in
‘fresh’ leach liquor at 50°C for a further 72 hours to determine the effect of
‘soaking’ on the overall recovery. The beneficial effect of soaking observed
by previous researchers is listed overleaf.
2-48
(i) Nickel and cobalt extractions from a ‘fresh’ Ravensthorpe pilot
plant MHP sample were increased from 87.6% and 98.7%, at
the end of the reductive leach, to >99% and >99.5%,
respectively after soaking (Hultgren, 2003b).
(ii) Likewise, the nickel extraction from an aged (~28 weeks)
Ravensthorpe pilot plant MHP sample was greatly improved
from ~92% to >99% upon soaking (Hultgren, 2003b).
(iii) Similar results were also reported when conducting the soak
test on a different pilot plant sample. On aged samples, nickel
recovery improved by between 5-12% depending on age
(Anderson, 2003a).
(iv) Alternatively, nickel and cobalt recovery from Polymet MHP
samples improved by over 6-10% upon soaking the reductive
leach residue (Bessel, 2006b).
(v) The results from alternate investigation (Bessel, 2006a) were
even more significant. Nickel and cobalt recovery from aged
(~6 months) European Nickel MHP samples, after soaking,
improved by between 6-14% and 15-25%, respectively.
2.6 Metal Sulphides as Reducing Agents
2.6.1 Precipitation Process
Monhemius (1977) computed metal concentrations in solution as a
function of pH in the presence of sulphide ions using the solubility product of
various metal sulphides. Figure 2.14 shows how the low solubility of CoS and
2-49
NiS allows the selective precipitation of these two sulphides from solutions
containing iron(II) and manganese(II).
Figure 2.14. Sulphide solubility diagram at 25°C (Monhemius, 1977).
The Yabulu Refinery currently precipitates a mixed cobalt-nickel
sulphide from a process stream using ammonium hydrosulphide (flowsheet
in Figure 1.3). The precipitation has two benefits to the process. Lowering
the cobalt concentration in solution improves recovery by minimising cobalt
co-precipitation during the removal of iron and an improvement in solvent
extraction efficiency. Secondly the metal sulphide could be used as a
reductant to improve metal recoveries with MHP leaching. Sherritt Gordon
patented this method of improving cobalt recovery in 1969. The process was
conducted at the Punta Gorda, Nicaro and Marinduque nickel refineries, and
trialled during piloting of the Yabulu Refinery in 1971 (Chappell, 2003).
Understanding the precipitation of metal sulphides and the reduction
mechanism of the particular sulphide is a crucial part of this thesis.
2-50
Sulphide precipitation has been commercially practiced in Murrin
Murrin and Moa Bay nickel laterite processes (Motteram et al., 1996), and
has been planned to be conducted at Goro and Ambatovy (Mining News,
2009). Jandova et al. (2005) studied a controlled sulphide precipitation for
recovery of copper and nickel-cobalt concentrates from leach solutions
produced by reducing manganese nodules with acidic ferrous sulphate.
2.6.2 Precipitation Kinetics
Kinetic testing of sulphide precipitation by Bryson and Bijsterveld
(1991) proved that manganese precipitation followed first order kinetics, and
existed as either MnS.H2O or MnOH.SH. The precipitation of cobalt sulphide
was a little more complex. Three kinetic regions were discovered; an
induction period, followed by rapid precipitation and then a slow approach to
equilibrium. Seeding eliminated the induction period. Analysis proved difficult
due to the extremely small particle size, its tendency to form agglomerates
and the amorphous nature of the precipitates. Similar results were achieved
in an investigation of the kinetics of zinc and cobalt sulphide precipitation
using sodium sulphide by Mishra and Das (1992). Zinc exhibited first order
precipitation kinetics whilst cobalt showed three kinetic regions. More
recently, Huang et al. (2007) investigated the precipitation of metal sulphides
using Na2S.
2-51
2.6.3 Practical Difficulties
Like all processes the sulphide precipitation route has various
difficulties including: formation of fine particles, and excess consumption of
the sulphide reagent due to polysulphide complexes. Lewis and Hille (2006)
discussed and provided solutions to these problems. High and low
supersaturations resulted in the formation of a significant quantity of fines.
This could be altered by process control or the use of a gaseous sulphide as
mass transfer would be limited.
Poor mixing and thus localised areas of high sulphide concentrations
was the probable cause for the formation of polysulphides. Karbanee et al.
(2008) investigated the controlled precipitation of NiS using gaseous
hydrogen sulphide. Results suggested that the hydrosulphide ion (HS-) was
responsible for NiS precipitation. The precipitation was also limited by the
availability of NaOH. Aqueous sulphide, which was attributed to the formation
of polysulphide complexes, accumulated in the system when NaOH addition
was limited.
Olivas et al. (1999) managed to produce crystalline NiS1.03 and NiS
(millerite) by following a particular method set out by a previous author
(Candia et al., 1981). It was discovered that a longer homogenization time
(36 h) caused a change from millerite to NiS1.03, whilst a temperature
increase lead to a sintering of sulphides and therefore a lower surface area.
2-52
Senaputra et al. (2008) discovered that the dissolution of nickel
sulphide in acid was hindered by thiosulphate ions whilst aided by sulphite,
suggesting NiS2 could be forming a passivating layer. In a sulphuric acid
solution Danielson and Baer (1989) discovered a layer of sulphur in reduced
form on the surface of NiS increased the rate of nickel dissolution.
2.6.4 Reducing Properties
A multitude of work on the production of a sulphide reductant for
cobalt(III) was conducted at the Yabulu Refinery during 2001-2005. A
scoping study by BHP Billiton and Hatch achieved positive results (Chappell,
2001). The most significant process variables were the cobalt oxidation state
and temperature. At 20°C the optimum NH4HS to Co(III) ratio was 2:1 to 3:1.
Seeding improved cobalt selectivity. Moroney (2003) conducted a thorough
investigation for the design conditions for producing mixed CoNiS. Each
mole of cobalt was treated with 2.2 moles of ammonium hydrosulphide.
Seeding was discovered to be beneficial, while oxygen ingress with
laboratory testwork was a problem.
Pre-treatment tests revealed the sulphur species associated with the
Ni component in the reductant were predominantly responsible for cobalt
reduction in the process stream (thickener 2 overflow). Consequently, nickel
was dissolved from CoNiS seed hence increasing Co/Ni ratio. Further work
(McGregor, 2004) found over-sulphiding produced a lower Co/Ni ratio, while
under sulphiding the opposite. Seeding reduced the quantity of ammonium
2-53
hydrosulphide required and improved kinetics of sulphiding, probably due to
the reduction of cobalt by the seed, which is considered the rate determining
step.
Anderson (2003b) lead a project which investigated a number of
possible reductants produced from various plant streams using ammonium
hydrosulphide. Nickel rich CoNiS was found ineffective whilst both CoS and
CoNiS achieved satisfactory nickel and cobalt extraction from Cawse MHP.
Reactivity was found to vary with precipitation temperature and Co(II)/Co(III)
ratio in the original liquor. McGregor (2005) tested Yabulu CoNiS, KNR
(Kalgoorlie Nickel Refinery) CoNiS, cobalt(II) ions, iron(II) ions, spent
hydroxylamine solution and spent acid solution, sulphite and dithionite. All
reagents proved to be effective. However Yabulu-CoNiS and KNR-CoNiS
were superior. Dithionite, hydroxylamine and cobalt(II) ions may also have
potential for use in an industrial application.
Alternative processes (Nicaro and Marinduque) used H2S:Co ratios
between 3:1 and 4.5:1. The Co:Ni ratio in the solids was between 0.34:1 and
0.53:1. Both refineries precipitate sulphide at higher temperatures (43-57°C)
than at the Yabulu refinery (28°C) (Chappell, 2003).
Nikoloski et al. (2005) produced MnOOH by bubbling air through a
Mn(OH)2 precipitate slurry in order to have a simple standard precipitate to
test a multitude of reductants. On this precipitate, cobalt(II) was effective, but
sulphide, sulphite and thiosulphate were all ineffective. The difference was
2-54
attributed to the formation of a passivating MnCO3 layer. In a similar test,
using the same precipitate, none of the ions used by McGregor (2004) were
effective, nor were elemental sulphur and hydrazine. Most of the reductants
were successful with the primary leach underflow from the Yabulu Refinery.
In subsequent tests, with the primary leach underflow, cobalt sulphides gave
a higher metal extraction than nickel sulphides. The reactivity seemed to be
related to the concentration of cobalt. Assuming that S is the product of
oxidation of CoNiS and the oxidised Mn solid can be represented as Mn(III)
the overall reaction was summarized as follows:
SIINiIICoIIICoCoNiS ++→+ )()(5)(4
)()()()( IIMnIIICoIICoIIIMn +→+
According to the reactions, the oxidation of CoNiS reduces cobalt which in
turn reduces manganese. The oxidation-reduction potential (ORP) seemed
to be controlled by the Co(III)/Co(II) couple, which would depend on the
relative rates of the above reactions. Although elemental sulphur is shown as
a product, it is probably oxidised to a soluble state. Further work is essential
for a better understanding of the reductive leaching process by sulphides.
3-1
3 MATERIALS AND METHODS
3.1 Reagents and Industry Samples
Table 3.1 lists the reagents used in all experimental work in their
as-received form. All solutions were prepared with analytical or lab grade
reagents and deionised water. Samples of MHP, preboil solids and CoNiS
obtained from various processing plants for testing are described in
Table 3.2. Each MHP sample was different in age and composition. The
Cawse sample was 5 years old, European Nickel samples were 11 months
old and the Ravensthorpe samples were either fresh or 4 years old. The
composition of the precipitates will be discussed in Chapter 7. Preboil solids
and CoNiS samples were collected from BHP Billiton’s Yabulu Refinery.
3.2 Synthesis of Mn3O4
The solutions required for the synthesis of Mn3O4 was prepared by
dissolving approximately 100 g of NaOH in 1 L, and 250 g of MnCl2.4H2O in
250 mL of DI water (5 M). The NaOH solution was added dropwise from a
separating funnel (Figure 3.1) over approximately 2 hours to the stirred
manganese solution in order to allow time for crystal growth. A milky
coloured precipitate was produced with each drop of NaOH, which oxidised
almost immediately to a light brown colour. Once addition was complete, air
was bubbled through the solution overnight to ensure complete oxidation.
The brown precipitate (Figure 3.2) was filtered and dried at 45°C in an inert
atmosphere. The procedure was based upon a method used by Nikoloski et
al. (2005).
3-2
Table 3.1. List of reagents. Reagents Formula Purity SupplierAcetylene C2H2 Industrial grade BOC
Air - Industrial grade BOCAluminium AAS standard - 1000 mg/L MERCK
Aluminum sulphate Al2(SO4)3.18H2O AR AJAXAmmonia NH3 28% Chem-Supply
Ammonium carbonate (NH4)2CO3+NH3CO2 AR AJAXAmmonium chloride NH4Cl AR AJAX
Ammonium hydrogen sulphide NH4HS Industry sample BHP Billiton Yabulu RefineryAmmonium nitrate NH4NO3 AR AJAX
Ammonium sulphide (NH4)2S 20% MERCKAmmonium sulphate (NH4)2SO4 AR AJAX
Argon Ar High purity BOCBarium chloride BaCl2.6H2O AR Biolab
Calcium AAS standard - 1000 mg/L ScharlauCalcium sulphate CaSO4.2H2O LR Chem-Supply
Chromium AAS standard - 1000 mg/L MERCKChromium (III) sulphate Cr4(SO4)5(OH)2 TG Chem-SupplyCobalt AAS standard - 1000 mg/L MERCK
Cobalt sulphate CoSO4.5H2O AR AJAXCoNiS - Industry sample BHP Billiton Yabulu Refinery
Copper AAS standard - 1000 mg/L MERCKCopper sulphate CuSO4.5H2O AR AJAX
Ethanol CH3CH2OH 95% BiolabFerric sulphate Fe2(SO4)3.xH2O LR Chem-Supply
Hydrochloric acid HCl 35% MERCKHydrogen peroxide H2O2 30% Biolob
Hydroxylamine sulphate H6N2O2.H2SO4 Industry sample BHP Billiton Yabulu RefineryIron AAS standard - 1000 mg/L MERCK
Kerosene - - DiggersLIX 84 - 50% BHP Billiton Yabulu Refinery
Magnafloc 351 - Industry sample CibaMagnesium oxide MgO Industry sample Qmag
Magnesium AAS standard - 1000 mg/L MERCKMagnesium sulphate MgSO4.7H2O LR AJAX
Manganese AAS standard - 1000 mg/L MERCKManganese sulphate MnSO4.H2O AR AJAX
MHP - Industry sample Ravensthorpe, Cawse, European NickelNickel AAS standard - 1000 mg/L MERCK
Nitric acid HNO3 70% LabscanNitrogen N2 Industrial grade BOC
Nitrous oxide N2O Industrial grade BOCNickel chloride NiCl2.6H2O AR Chem-Supply
Nickel carbonate NiCO3.2Ni(OH)2.4H2O AR Chem-SupplyNickel sulphate NiSO4.6H2O AR Chem-Supply
Potassium permanganate KMnO4 LR AJAXSilicon AAS standard - 1000 mg/L Australian Chemical Reagents
Preboil solids - Industry sample BHP Billiton Yabulu RefinerySodium silicate solution 2SiO2:Na2O to 3.2SiO2:Na2O TG Chem-Supply
Sodium hydroxide NaOH AR AJAXSodium sulphide Na2S.9H2O AR AJAXSodium sulphite Na2SO3 AR Sigma ChemicalsSodium oxalate (COONa)2 AR AJAXSulphuric acid H2SO4 98% MERCK
Vaseline - - VaselineZinc AAS standard - 1000 mg/L MERCK
Zinc sulphate ZnSO4.7H2O AR Hayashi
3-3
Table 3.2. List of industry samples Sample Source Precipitant No. Samples Age
MHP Cawse MgO 1 5 yearsEuropean Nickel Na2CO3 2 11 months
Ravensthorpe MgO 2 0 and 4 yearsPreboil solids BHPB Yabulu Refinery Boiled 2 -
CoNiS BHPB Yabulu Refinery Ammonium hydrogen sulphide 5 -
Figure 3.1. Dropwise addition of NaOH to manganese solution.
Figure 3.2. Manganese hydroxide precipitate and solution after overnight air sparging.
3-4
3.3 Synthesis of MnOOH
A sample of MnOOH was synthesised by mixing 900 mL of 0.2 M
NH3 with 3 L of 0.06 M MnSO4 hydrate and approximately 60 mL of 30%
hydrogen peroxide. It was important to mix the manganese solution with the
peroxide before adding the ammonia, as ammonia and hydrogen peroxide
react vigorously. The mixture was refluxed (90-100°C) for over 6 hours,
filtered, washed and dried at 100°C overnight (Figure 3.3) (Ardizzone et al.,
1998; Wang & Stone, 2006). Precipitates of cobalt hydroxide and 1:1 mixed
cobalt-manganese hydroxide were also synthesised using the same method
with the equivalent molar ratios of metal ions to NaOH and a cobalt chloride
salt.
Figure 3.3. Refluxing to produce MnOOH.
3-5
3.4 Precipitation of MHP
3.4.1 Precipitates for the Effect of Composition
Metal hydroxides were precipitated by adding a stoichiometric
quantity (2 mole MgO for every 1 mole Ni+Co) of ‘fresh’ MgO to 6 L of a
metal ion sulphate solution (Figure 3.4). Fresh MgO (EMag-45) was supplied
by Queensland Magnesia Pty Ltd. In addition to Ni(II) and Co(II) the initial
solutions contained Mn(II), Fe(III), Al(III), Zn(II), Cr(III), Cr(VI), Cu(II), Ca(II)
and Si(IV), all added as a sulphate salt with no pH adjustment. Experimental
conditions (MgO stoichiometry and solution metal concentrations) were
based on two reports prepared by SGS Lakefield Oretest (Jayasekera,
2003a & b). The nature of the metal ions salts and concentrations are listed
in Tables 3.1 and Table 3.3, respectively. To ensure complete dissolution of
MgO the solution was left stirring for 4 hours at ambient conditions before
being filtered to produce a cake of approximately 50% solids. Each filter
cake was divided into fractions, which were stored separately in plastic
sample jars ready for analysis at various stages over 12 weeks (Figure 3.5).
After the initial precipitation and analysis it was discovered that the
pH from MgO addition did not rise above 8. According to Miller (2005) a pH
of 8.8 is required to remove 100% manganese. Therefore, some precipitate
samples may not contain ideal manganese levels for investigation. Another 4
precipitates containing manganese(II) were produced using similar solutions
but raising the pH from 8 to 8.3 with lime (Table 3.3, precipitates AB - AE).
3-6
In order to establish a pattern of metal ion precipitation in the
Ravensthorpe process, the metal ion concentrations in solutions were
monitored over a pH range of 2.5 to 9 at two temperatures. The solution was
prepared to be of similar composition to the Ravensthorpe process liquor
(Table 3.4). The addition of magnesia followed by lime was used to raise the
pH.
The dissolution of magnesia was also monitored by adding 6 g of
magnesia to 1 L of water at 25, 45 and 80°C, and at 25°C with 150%
salinity. Slurry samples were taken after 5, 30, 60, 120 and 240 seconds and
filtered.
Figure 3.4. Precipitation of mixed hydroxides.
3-7
Figure 3.5. Precipitates stored in sample jars.
Table 3.3. Solution compositions prior to precipitation of MHP, g/L. Precipitate Ni2+ Co2+ Mn2+ Al3+ Fe3+ Cr3+ Cu2+ Zn2+ Si4+
A 4 0.4B 4 0.4 0.4C 4 0.4 0.4 0.1D 4 0.4 0.4 0.1 0.1E 4 0.4 0.4 0.1 0.1 0.1F 4 0.4 0.4 0.1 0.1 0.1 0.1G 4 0.4 0.4 0.1 0.1 0.1 0.1 0.1H 4 0.4 0.4 0.1 0.1 0.1 0.1 0.1 0.1I 4 0.4 0.1J 4 0.4 0.1K 4 0.4 0.1L 4 0.4 0.1M 4 0.4 0.1N 4 0.4 0.1O 4 0.4 0.15P 4 0.4 0.66Q 4 0.4 1R 4 0.4 2.5S 2 0.4 4T 4U 4 1.25V 4 0.4 0.8W 4 0.4 0.5X 4 0.4 0.6Y 4 0.4 0.8Z 4 0.4 0.8
AA 4 0.4 0.33AB 4 0.4 0.15AC 4 0.4 0.66AD 4 0.4 1AE 4 0.4 2.5
3-8
Table 3.4. Solution composition for precipitation of samples similar to RNO-MHP, g/L.
Ni2+ Mg2+ Co2+ Mn2+ Al3+ Fe3+ Cr3+ Cu2+ Zn2+ Si4+
4.60 24.0 0.45 1.00 0.45 0.34 0.25 0.45 0.44 0.02
3.4.2 Precipitates for the Effect of Drying
Four precipitates of Ni+Mg in the absence or presence of Co, Al and
Fe were precipitated with MgO using the method described in section 3.4.1
and filtered to 50% solids. The initial solution compositions are listed in Table
3.5. Each precipitate was split into three sections, whereby two were dried
further in the oven at 50°C in inert conditions either overnight or for
approximately five hours. Drying resulted in precipitates containing
approximately 20% and <5 % moisture.
Table 3.5. Solution compositions for precipitation of samples for drying, g/L. Precipitate Ni2+ Co2+ Al3+ Fe3+
Ni, Mg 4.0Ni, Mg, Co 4.0 0.4
Ni, Mg, Co, Al 4.0 0.4 0.8Ni, Mg, Co, Fe 4.0 0.4 0.5
3.4.3 Simple Metal Hydroxides
Simple metal hydroxides of Ni(II), Co(II), Mn(II), Mg(II), Fe(III), Al(III),
Ca(II), Cu(II), Cr(III) and Zn(II) were precipitated separately from 6 L of a
4 g/L metal sulphate solution (Table 3.1) at ambient conditions using a 2:1
mole ratio of NaOH to metal. Solutions were stirred for four hours before
being filtered and dried. XRD analysis and kinetic leach tests were performed
on the precipitates to distinguish rates of metal dissolution.
3-9
3.4.4 Nickel-Magnesium Hydroxide for Solubility Testing
The mixed Ni,Mg(OH)2 was precipitated from 20 L of a 2.75 g/L
nickel sulphate solution at 80°C using a 1:1 mole ratio of MgO:Ni(II). After
four hours stirring, the solution was filtered and dried for solubility testing.
3.4.5 Transformation of MgO to Mg(OH)2
Six mixtures of MgO and water (10, 20, 40, 50, 60 and 90% solids)
were prepared and analysed by XRD over a week. The purpose was to
examine the effect of moisture content on the rate of transformation of MgO
to Mg(OH)2.
3.4.6 Influence of Magnesium Content
The influence of magnesium content on the kinetics of dissolution of
Ni(II) from Ni,Mg(OH)2 was investigated using four precipitates. Nickel(II)
concentration in the initial 3 L solution was maintained constant at 4 g/L while
magnesia was added in four differing quantities: 40, 20, 15 and 10 g to vary
the mole ratio of MgO:Ni to 5:1, 2.5:1, 1.8:1 and 1.25:1, respectively.
Solutions were stirred for four hours then filtered. The solids were dried and
subjected to leach tests described in section 3.6.3.
3.4.7 Influence of Ageing of Mixed Nickel-Magnesium Hydroxide
Nickel hydroxide, magnesium hydroxide and a mixed nickel
magnesium hydroxide were precipitated from 6 L of 4 g/L Ni(II) and/or Mg(II)
solution. Sodium hydroxide was added at a NaOH:M(II) molar ratio of 2:1 for
3-10
the precipitation of Ni(OH)2, Mg(OH)2 and Ni,Mg(OH)2. The solutions were
stirred for four hours and filtered to approximately 50% solids. A fourth
sample was prepared by mixing the Ni(OH)2 and Mg(OH)2 precipitates at a
mole ratio of 1:1.
The leachability of Ni(II) from all precipitates was monitored by
predictor leach tests and XRD analysis of solids over approximately a year.
The purpose of this experiment was to determine if nickel and magnesium
form a stable slow leaching compound during precipitation or afterwards
during ageing.
3.4.8 Influence of Cobalt, Manganese, Aluminium and Chromium
Eight precipitates were produced with varying levels of cobalt with
and without manganese. Table 3.6 lists the initial composition of the eight
solutions used in the experiment using the procedure described previously in
section 3.4.1. Slurries were filtered to ~50% moisture and the precipitates
were aged for 6 weeks and subjected to predictor leach tests.
Table 3.6. Solution compositions for varying cobalt content, g/L. Precipitate Ni2+ Mn2+ Co2+
Ni, 1% Co 4.0 0.19Ni, 2% Co 4.0 0.38Ni, 5% Co 4.0 0.77
Ni, 10% Co 4.0 1.75Ni, Mn, 1% Co 4.0 2.7 0.19Ni, Mn, 2% Co 4.0 2.7 0.38Ni, Mn, 5% Co 4.0 2.7 0.77
Ni, Mn, 10% Co 4.0 2.7 1.75 Column 1 lists the targeted metal incorporation.
3-11
Seven precipitates were produced to examine the effect of cobalt,
and compare the effect of manganese, aluminium and chromium. The initial
concentrations of metal ions are listed in Table 3.7 and the precipitates were
produced from 6 L solution at ambient conditions using a MgO:Ni mole ratio
of 2:1. The precipitates were aged for six weeks for analysis by HPLC at the
Yabulu Refinery. Predictor leach tests were also performed on these
precipitates.
Table 3.7. Solution composition for varying Co, Mn, Al and Cr contents, g/L.
Precipitate Ni2+ Co2+ Mn2+ Al3+ Cr3+
Ni, 1% Co 4.0 0.2Ni, 2% Co 4.0 0.4Ni, 5% Co 4.0 1.0
Ni, 10% Co 4.0 1.8Ni, Mn 4.0 2.7Ni Al 4.0 0.8Ni Cr 4.0 1.7 Column 1 lists the targeted metal incorporation.
3.4.9 Influence of Cobalt(II) and Cobalt(III) Valency
Two precipitates were produced from 6 L of 4 g/L cobalt(II) solution
at ambient conditions using a 2:1 mole ratio of NaOH to cobalt. Hydrogen
peroxide was added to one solution at a 1:1 mole ratio to cobalt(II) prior to
precipitation in order to oxdise Co(II) to Co(III). After precipitation, solutions
were stirred for four hours then filtered and dried. The precipitates were
subjected to XRD and leach tests to compare the kinetics. The extent of
oxidation of Co(II) to Co(III) was also tested using the titration method
described in section 3.7.2. The purpose of these experiments was to
determine the effect of cobalt oxidation on the rate of leaching and recovery.
3-12
3.4.10 Influence of Crystallinity
Five precipitates were produced at 80°C using solutions of varying
nickel(II) concentrations and solution volumes listed in Table 3.8. In all cases
the precipitation was conducted using a MgO:Ni(II) mole ratio of 1:1. Leach
tests were conducted on the dried precipitates (50°C) to examine the effect
of crystallinity of Ni,Mg(OH)2 on leaching kinetics. A second batch of
precipitate four (in Table 3.8) was also made in bulk for nickel solubility
studies.
Table 3.8. Solution volume and nickel composition for varying crystallinity of Ni,Mg(OH)2
Precipitate Volume, L Ni2+, g/L1 20.0 0.252 20.0 0.703 20.0 1.404 15.0 2.755 5.00 5.50
3.4.11 Precipitates for Oven Ageing
In order to improve crystallinity and increase the speed of structural
reordering, precipitates were produced and placed in sealed bottles in
solution at 50°C for a period of months (Figure 3.6). The influence of metal
ions and anions listed in Tables 3.9 and 3.10 (Ni(II), Co(II), Mg(II), Mn(II),
Fe(III), Al(III), Zn(II), Cr(III), Cu(II), Ca(II) and Si(IV), Cl-, SO42- and CO3
2-)
were investigated.
3-13
Figure 3.6. Bottles used for oven ageing.
The first batch of precipitates were produced from 6 L of solution
using the same procedure described in section 3.4.1 and the solution
compositions listed in Table 3.9. After filtration the solids were repulped in
500 mL of DI water and placed in the oven. The metal ion concentrations
(using metal sulphates) were the same for the second batch, which were
precipitated from 1 L solution containing 5 g of CaCO3 and 15 g NaCl
(~150% salinity). After precipitation ~750 mL of solution was decanted and
the slurry was placed in the oven in sealed bottles. The third batch of
precipitates were precipitated in 250 mL of DI using a 2:1 mole ratio of MgO
to Ni and left in solution for the oven ageing (Table 3.10). The influence of
sulphate, chloride and carbonate was investigated by adding the appropriate
nickel salt to achieve 4 g/L.
Table 3.9. Solution compositions for precipitates produced for oven ageing tests in batch 1-2, g/L.
Precipitate Ni2+ Mn2+ Co2+
Mn, Co 1.0 1.0Co 2.0
Mn, Co 1.0 1.0Ni, Co 1.0 1.0Ni, Mn 1.0 1.0
3-14
Table 3.10. Solution compositions for precipitates for oven ageing tests in batch 2, g/L.
Precipitate Ni Salt Mn2+ Co2+ Fe3+ Al3+ Cr3+
1 SO42- 0.8
2 SO42- 0.8
3 SO42- 0.8
4 SO42- 0.8
5 SO42- 0.8
6 CO32- 0.8
7 CO32- 0.8
8 CO32- 0.8
9 CO32- 0.8
10 CO32- 0.8
11 Cl- 0.812 Cl- 0.813 Cl- 0.814 Cl- 0.815 Cl- 0.8
3.4.12 Elevated Temperature Precipitation
To examine the effect of improved crystallinity precipitates were
produced at 80°C over 4 hours from 20 L of solution with a relatively low
nickel concentration (0.15 g/L) (Figure 3.7). The precipitates were produced
to study the formation and crystallinity of various metal hydroxides, and the
subsequent influence on leach kinetics. The compositions of various metal
ions are listed in Table 3.11. Magnesia was added at a 2:1 mole ratio. Four
batches were produced before the desired results were achieved. In these
attempts the metal ion concentration in solution was lowered after each test,
while the volume and temperature was increased. Seeding, length of stirring
and precipitation with NaOH at a pH 8.3 was also investigated.
3-15
Figure 3.7. Picture of elevated temperature precipitation.
Table 3.11. Solution compositions for precipitation at elevated temperature (80°C), g/L.
Precipitate Ni2+ Mn2+ Co2+ Fe3+ Al3+ Cr3+ Ca2+ Si4+ Cu2+ Zn2+
1 0.152 0.15 0.0253 0.15 0.0254 0.15 0.0255 0.15 0.0256 0.15 0.0257 0.15 0.0258 0.15 0.0259 0.15 0.025
10 0.15 0.02511 0.15 0.025 0.012512 0.15 0.0125 0.0125
3.4.13 Precipitation Mechanism
In total 18 samples were analysed: eight from Ravensthorpe and ten
synthetic precipitates. Samples from the Ravensthorpe plant (flowsheet in
Figure 1.2 – only 1 tank shown for MHP precipitation) were collected near
the precipitation point (1A), from the outside of the tank (2A), and from the
following two tanks (3A & 4A). The samples were collected in a sponge and
3-16
were placed in the oven almost immediately. The second batch was taken in
the same manner but washed prior to drying (1-4B).
Two precipitates were produced from 1 L solutions containing 4 g/L
Ni(II), 0.4 g/L Co(II) and 1.25 g/L Mn(II) at 25 and 40°C using a 2:1 MgO:Ni
mole ratio. After the addition of MgO, 50 mL of slurry was removed, filtered
and dried after 5, 30, 60, 120 and 240 minutes of stirring. The conditions of
testing were based on the Ravensthorpe process, where MgO is added to a
solution of similar composition at 45°C with approximately 4 hours of reaction
time.
The MgO precipitate was also analysed in the same manner at 25
and 40°C, by adding MgO without any metals in solution.
All solids were assayed, sized and analysed by XRD and SEM.
3.5 CoNiS Preparation
The mixed cobalt-nickel sulphide was precipitated from 1 L of
solution containing 1 g/L Co and 10 g/L Ni using ammonium sulphide with a
N2 blanket (reactors: Figure 3.9). The solids were dried at 50°C overnight in
air after being settled from solution using Magnafloc at 1 mg/L. Flocculant
was required due to the poor settling and filtering characteristics of the
precipitates. In total 9 precipitates were produced by varying production
temperatures, the oxidation state of cobalt and the sulphidation ratio (mole
3-17
ratio H2S:Co) (Table 3.12). The ratio (2.2:1) was based upon the precipitation
conditions used in the Yabulu refinery.
Table 3.12. CoNiS precipitation conditions. CoNiS Temperature, oC Sulfidation Ratio Attempted Percent of Co3+
1 25 2.2:1 02 25 2.2:1 503 25 2.2:1 1004 40 2.2:1 05 40 2.2:1 506 40 2.2:1 1007 25 1:1 08 25 1.5:1 09 25 3:1 0
The oxidation state of cobalt was determined by extracting Co(II)
from 10 mL of solution with 10 mL of 20% LIX84 (80% kerosene). Analysis of
the initial and final solutions by atomic absorption spectroscopy allows for the
determination of the fraction of cobalt ions in oxidation state II or III. In order
to prepare a solution of approximately 50% Co(III), the oxidation state of
cobalt was monitored over a 30 hour period. Air was sparged (1 L/min)
through solutions at 25 and 40°C containing 1 g/L cobalt and 10 g/L Ni, the
oxidation state of cobalt was determined after 2, 5, 7, 12, 25 and 30 hours.
The Yabulu-CoNiS precipitate was produced using 1 L of thickener 2
overflow solution (flowsheet in Figure 1.3), ammonium hydrogen sulphide
produced on Yabulu site and Magnafloc 351. Based on the previous test
work by BHP Billiton the quantity of ammonium hydrogen sulphide added
was determined by the desired nickel, cobalt and sulphide concentrations
(Figure 3.8). After the initial precipitation, Magnafloc 351 (1 mg/L) was
3-18
added. After the solids had settled the solution was decanted, fresh solution
added, and precipitation repeated. Precipitation was repeated numerous
times in order to produce enough CoNiS for testing. Six precipitates were
produced in this manner.
y = -1.177x + 5.238R2 = 0.731
0
1
2
3
4
5
6
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
H2S:Co in T2 o/f (mol/mol)
Co:N
i in
CoN
iS (m
ol/m
ol)
Co/NiLinear (Co/Ni)
Figure 3.8. Effect of H2S:Co stoichiometry in thickener-2 overflow on Yabulu-CoNiS composition (McGregor 2004).
Samples of CoS, NiS and CoNiS were also precipitated from a 1 g/L
metal ion solution with a 1:1 mole ratio of ammonium hydrogen sulphide to
metal ion. During the precipitation nitrogen gas was sparged at a rate of 1
L/min. The concentrations ratios Co(II) and Ni(II) in solutions were 3:1, 1:1
and 1:3.
After precipitation, the solids were settled using 1 mg/L Magnafloc,
decanted and leached in 1 L of SAC solution (see section 3.6.1) with 1 L/min
oxygen or nitrogen and 10 g of sodium sulphite. The oxidation reduction
3-19
potential (ORP) was measured, and solution samples (5 mL) were taken
after 2, 5, 10 and 20 minutes for kinetic analysis. This procedure was
developed to ensure there was no oxygen ingress and therefore no loss of
the reducing ability of the reductant.
3.6 Leach Tests
3.6.1 Synthetic Ammonium Carbonate (SAC) Leach Solution
The typical lixiviant in the Caron process (SAC solution) contains
93 g/L ammonia and 65 g/L CO2. A SAC solution of 1 L was prepared by
mixing 220 g (or 245 mL, assuming a density of 0.9 g/L) of 25% ammonia
solution, 115 g ammonium carbonate ((NH4)2CO3 + NH4HCO3) and adjusting
the total volume to 1 L with deionised water at room temperature. The
concentrations were confirmed by titration, whereby a 5 mL sample of the
solution was added to 50 mL of deionised water and titrated with
standardised (0.05 M or 0.1 N) sulphuric acid solution. The volumes at pH
8.1 (T1) and 4.2 (T2) were used to calculate the concentration of NH3 and
CO2:
)(17
)/( 4223 mLvolumealiquot
NTLgNH SOH ××
=
)(44)(
)/( 42122 mLvolumealiquot
NTTLgCO SOH ××−
=
The relevant reactions with sulphuric acid are:
2NH3 + H2SO4 = (NH4)2SO4 (1)
3-20
(NH4)2CO3 + 0.5H2SO4 = NH4HCO3 + 0.5(NH4)2SO4 (2)
Reactions 1 and 2 were assumed to be complete when pH reaches 8.1,
which could be determined by inflection in the titration curve of pH vs.
volume.
NH4HCO3 + 0.5H2SO4 = 0.5(NH4)2SO4 + CO2 + H2O (3)
Combining reactions 2 and 3 gives:
(NH4)2CO3 + H2SO4 = (NH4)2SO4 + CO2 + H2O (4)
Reaction 3 was taken to be complete at pH 4.2. In reactions 1 and 4, at the
end point, 1 mole of H+ equates to 1 mole of nitrogen. The concentration of
CO2 was determined using reaction 3, which occurs between pH 4.2 and 8.1.
3.6.2 Predictor Leach Tests
(a) Standard Predictor Test (SPT)
Predictor leach tests were developed by Hultgren at the Yabulu
Refinery in 2003(a) for two purposes: (i) to represent achievable recoveries
for the refinery, and (ii) to have a standard test to compare various
precipitates. All tests were conducted in triplicate in 1 L leach vessels
(reactors: Figure 3.9). Synthetic material or commercial MHP samples were
passed through a 2 mm screen before leaching to ensure no large lumps
existed in the precipitate. The standard predictor test (SPT) entailed a 45
minute leach of 4 g (Ni + Co) dry basis in 500 mL of SAC at 30°C. The test
was designed to replicate recoveries from the first (oxidative) stage of the
Yabulu process.
3-21
Figure 3.9. Reactors used for leach tests.
(b) Reductive Predictor Test (RPT)
The reductive predictor test (RPT) was the same except nitrogen
was sparged into the leach vessel and a calculated quantity of hydroxylamine
sulphate was added. The mass of hydroxylamine sulphate required was
calculated as follows:
57,)()(. gMHPdryinMnComassMHPdryinMnComolesNo +
=+
5.1164)(., ××+= MnComolesNogrequiredmass
This test was designed to replicate results from the second, reductive stage
of leaching at the Yabulu Refinery (flowsheet in Figure 1.1). The difference
between the standard and reductive predictor tests gives a good indication of
the quantity of oxidised cobalt and manganese.
3-22
(c) Reductive Complexing Predictor Test (RCPT)
The reductive complexing predictor test (RCPT) procedure was the
same as previous, but 50 g of the sodium salt of ethylenediaminetetraacetic
acid (Na2EDTA) was also added. This complexing agent was added to
ensure that there were no solubility issues. The leaching recoveries from this
test represent total, achievable recoveries.
(d) Reductive Soak Predictor Test (RSPT)
Reductive soak and standard soak tests were developed
subsequently. These followed the procedures in standard and reductive
predictor tests. However, after the 45 minute leaching period and filtration,
the leach residue was transferred to a plastic sample jar with 250 mL of SAC
solution and retained at 50°C for 72 hours. The jar was shaken initially and
after 24 and 48 hours to break up compacted solids. These tests were aimed
to represent the ~72 hour Counter Current Decantation (CCD) circuit at the
refinery with and without the second stage of reductive leaching (flowsheet in
Figures 1.1 & 1.3).
3.6.3 Modified Predictor Leach Tests
The modified predictor leach test used was a scaled down version of
the tests developed by BHP Billiton researchers for the Yabulu Refinery. The
tests were modified due to there being a limited quantity of sample. A 0.2 g
(Ni+Co) sample of dry MHP was leached in triplicate at ambient conditions
with 25 mL of SAC liquor (93 g/L NH3, 65 g/L CO2) for 45 minutes in 30 mL
3-23
centrifuge tubes. The tubes were placed in a mill drive which rotates the
tubes end over end at 100 rpm (Figures 3.10 and 3.11).
Figure 3.10. Mill drive used for modified predictor tests.
Figure 3.11. Clips on mill drive holding centrifuge tubes.
The reductive predictor test used 0.2 g of hydroxylamine sulphate.
This was in gross excess of the required amount according to BHP Billiton
procedure. However, the tests were conducted in 30 mL vessels, leaving
5 mL of air. The excess ensured the oxygen will be consumed, leaving
sufficient hydroxylamine sulphate for reduction of cobalt and manganese.
3-24
3.6.4 Reductive Leaching of Oxidised Mn and Co Hydroxides
Samples of 0.5 g of the synthesised manganese, cobalt and mixed
oxidised hydroxides were leached in 500 mL of either ammonia-ammonium
carbonate (93 g/L NH3 and 65 g/L CO2) or sulphate (93 g/L NH3 and 104 g/L
SO4, equivalent moles of carbonate to sulphate) solutions at 55°C and
500 rpm for 2 hours with a steady flow of nitrogen using various reductants
(reactors: Figure 3.9). This was thought to best represent the secondary
leach conditions of the proposed Yabulu Extension Project (YEP). Sodium
sulphite, cobalt(II), hydroxylamine sulphate and Yabulu-CoNiS (refinery plant
sample) were tested as reductants. They were added to the leach in excess
at twice the calculated mass. The mass of Yabulu-CoNiS added ensured the
sulphur in CoNiS to metal in MHP ratio was 2.2:1 molar unless stated
otherwise. Samples during the tests were introduced to vials that had been
sparged with nitrogen.
3.6.5 Batch Leach Tests
The batch leach tests were designed to simulate the primary and
secondary leaching of MHP at the Yabulu refinery. A sample of 4 g of Ni+Co
was leached in 500 mL of product liquor for two hours at 45°C open to the
atmosphere. Product liquor is the overflow from the first thickener. According
to plant simulation data it should contain 10.5 g/L Ni, 0.6 g/L Co, 88 g/L NH3
and 57 g/L CO2 (Yabulu Refinery Metsim data 2008).
3-25
After two hours, the solution was filtered and the solids returned to
the leach vessel with 500 mL of fresh leach liquor and a calculated quantity
of CoNiS at 55°C for three hours. The fresh leach liquor is produced in the
stills after solvent extraction (flowsheet in Figure 1.3). The liquor contains
minimal nickel and cobalt (<0.02 g/L), and 132 g/L NH3 and 90 g/L CO2.
Nitrogen was sparged at a rate of 0.5 L/min for the first hour, and then
replaced with air for the final two. The quantity of CoNiS was calculated to
ensure a 2:1 mole ratio of sulphur in CoNiS to Co and Mn in the MHP.
3.6.6 Kinetic Leach Tests
Kinetic leach tests were performed on the RNO-MHP collected in
June 2008 (flowsheet in Figure 1.2) for 1 hour using the reactors described in
Figure 3.9 and 250 mL SAC solution. The effect of pulp density, temperature,
rotation speed and size fractions (listed in Table 3.13) were investigated.
Slurry samples (5 mL) were removed after 2, 5, 10, 20 and 60 minutes,
filtered and diluted immediately, then analysed by Atomic Absorption
Spectroscopy (AAS).
3-26
Table 3.13. RNO kinetic leach test conditions.
Leach Tests 2 5 10 20 25 40 60 500 600 750 25-38 38-53 53-75123456789
10111213141516
Pulp Density, g/L Temperature, oC Rpm Size Fraction, µm
Kinetic leach tests were also performed on the Ni,Mg(OH)2,
Co,Mg(OH)2 and CoOOH precipitates, as well as all of the precipitates
produced at elevated temperature. Due to limited quantity, small samples of
0.5 g were leached in 25 mL (20 g/L solids, w/v) of SAC in 30 mL centrifuge
tubes for 2, 5, 10, 20 and 60 minutes (separate samples) on the mill drive at
100 rpm (Figures 3.10 and 3.11). After the designated leaching time 5 mL of
slurry was filtered for analysis by AAS, while the solids were washed and
centrifuged for analysis by X-Ray Diffraction (XRD) and Scanning Electron
Microscopy (SEM). Unfortunately, the influence of temperature and rotation
speed could not be investigated using this method. However, the leaching of
samples of three size fractions: 25-38, 38-53 and 53-75 μm was examined.
3.6.7 Effect of Anions on Ni(II) Solubility
A sample of 20 g of Ni,Mg(OH)2 was added to 250 mL of leach liquor
in a 250 mL Schott bottle and agitated on an orbital shaker for six hours.
3-27
Testing was conducted in triplicate. After 6 hours, a solution sample was
taken immediately and diluted appropriately for analysis by AAS. The leach
liquors tested contained 90 g/L of ammonia and either carbonate (60 g/L),
sulphate (142 g/L), chloride (52 g/L) or nitrate (91 g/L). The concentration of
the anion in each leach liquor was calculated to ensure that the number of
moles was consistent (1.47), so the influence of complexing could be
determined. The solution pH’s ranged from 10.50 to 10.65.
3.7 Analysis
The following analysis was conducted as part of this thesis:
• Moisture content by gravimetry after drying.
• Extent of oxidation by titration.
• Solution composition by Atomic Absorption Spectrometry (AAS),
Inductively Coupled Plasma Optical Emission Spectrometry and Mass
Spectrometry (ICP-OES and ICP-MS), and High Performance Liquid
Chromatography (HPLC).
Characterisation of the precipitates utilised the following techniques:
• X-ray diffraction (XRD) and neutron diffraction.
• Infrared (IR) and Micro-Raman spectroscopy.
• Scanning Electron Microscopy (SEM) and optical microscopy.
• Thermogravimetric Analysis (TGA).
• Laser size analysis and BET surface area testing.
• X-Ray Photoelectron Spectroscopy (XPS).
3-28
The X-ray Absorption Near Edge Structure Spectroscopy (XANES),
Secondary Ion Mass Spectroscopy (SIMS), Electron energy-loss
spectroscopy (EELS), Synchrotron X-Ray Diffraction and Polarisation tests
were considered but not conducted for a variety of reasons (discussed in
results).
3.7.1 Moisture Content
Samples were dried at 50°C in an inert atmosphere (0.5 L/min N2)
(Figure 3.12). Moisture content determinations were conducted in triplicate,
whereby a known mass of sample was dried overnight, placed in a
desiccator to cool, and weighed.
Figure 3.12. Vessel in oven used for drying.
3-29
3.7.2 Determination of Extent of Oxidation
The quantity of oxidised manganese and cobalt in the mixed
hydroxide precipitate (MHP) was determined using the method developed by
Hultgren (2003a) described in details by Nikoloski et al. (2005). After drying
the sample under a N2 blanket at 45°C for 1-2 hours, 0.1 to 0.3 g of sample
was dissolved in 50 mL of 0.05 M oxalate solution and 50 mL of 1 M
sulphuric acid at 50°C. The solution was titrated with a 0.02 M potassium
permanganate solution. The titration was conducted in triplicate along with
blank titrations (without MHP). The reaction equations were:
222
423 222 COMOCM +→+ +−+ (digestion)
222
424 22 COMOCM +→+ +−+ (digestion)
where M = Mn or Co.
OHCOMnHOCHMnO 222
4224 8102652 ++→++ ++− (titration)
The percentage of oxidised Mn, Co and Fe in the sample was
calculated by determining the quantity of unreacted reducing agent (oxalic
acid). It was assumed that cobalt and manganese did not exist in the 4+
state. These titrations were performed on a number of metal hydroxide
precipitates. However, its best use was on MnOOH before and after
reduction with CoNiS or an alternative reagent.
3-30
3.7.3 Atomic Absorption Spectrometry
Metal ion concentrations were determined using a GBC Avanta AAS
Model 933AA. Standard solutions were prepared from commercially available
1000 mg/L solutions. Conditions for analysis are listed in Table 3.14.
Table 3.14. AAS conditions for analysis. Metal Flame Wavelength, nm Working Range, mg/L Sensitivity, mg/L
Aluminium Nitrous oxide-acetylene 396.2 25 - 110 0.55Calcium Nitrous oxide-acetylene 422.7 1 - 4 0.02
Chromium Air-acetylene 357.9 2 - 15 0.05Cobalt Air-acetylene 240.7 2.5 - 9 0.05Copper Air-acetylene 327.4 2.5 - 10 0.05
Iron Air-acetylene 248.3 2 - 9 0.05Magnesium Air-acetylene 202.6 5 - 20 0.1Manganese Air-acetylene 279.5 1 - 3.6 0.02
Nickel Air-acetylene 232.0 1.8 - 8 0.04Silicon Nitrous oxide-acetylene 251.6 68 - 275 1.5Zinc Air-acetylene 213.9 0.4 - 1.5 0.008
3.7.4 Inductively Coupled Plasma Mass Spectrometry
This technique was used for the analysis of complex solutions and
for sulphur in solution. The instrument used was the Varian ICP Model
Liberty 200 through the Marine and Freshwater Research Laboratory
(MAFRL) of Murdoch University.
3.7.5 X-Ray Diffraction
Over the course of the project XRD was conducted on three different
machines. Initially, at Murdoch University, a Phillips Model 1050 theta-theta
diffractometer with a Co Kα1 source was used. Sample was ground in a
mortar and pestle (<5 μm) and smeared onto a glass slide using ethanol and
placed into the machine. Results were relatively poor as the signal to noise
3-31
ratio was small, an amorphous lump occurred around 15° due to the glass
slide, and background noise increased in intensity as the angle increased.
The Siemens D500 Bragg Brentano Diffractometer with a Cu Kα1
source at Curtin University provided better results. Samples were ground in a
mortar and pestle (<5 μm) and predominantly prepared in packed sample
holders. Low background slides were used when sample size was limited:
sample was sprinkled and pressed onto the centre of a Vaseline smeared
silicon slide. The silicon wafer was cut at an angle so no crystal faces were
aligned with the surface. Typical step sizes were 0.04 or 0.08° with a 1
second count time. Smaller step sizes and longer count times were used
when appropriate.
Finally the GBC Enhanced Mini-Materials Analyser (EMMA) theta-
theta diffractometer was used with a Cu Kα1 source (Murdoch University).
Preparation of samples and equipment settings were the same as in the
case of the D500 Bragg Brentano Diffractometer.
3.7.6 Neutron Diffraction
Neutron Diffraction was performed, on one sample (RNO-MHP,
collected in June 2008), at the Australian Nuclear Science and Technology
Organisation (ANSTO). The source was 1.54 Angstroms which was
equivalent to Cu Kα. This technique was thought to enable the identification
of new phases for a number of reasons: (i) provide a better signal to noise
3-32
ratio, (ii) higher intensity source than traditional diffraction, and (iii) interaction
with the nucleus of the atom rather than the electron cloud.
3.7.7 Scanning Electron Microscopy
Scanning Electron Microscopy (SEM) and Energy Dispersive
Spectroscopy (EDS) was conducted on a Phillips XL30 instrument. Samples
were prepared on stubs (Figure 3.13) and in resin blocks (Figure 3.14), and
coated with carbon prior to analysis.
Cross sections of MHP were prepared by drying the precipitate at
50°C under nitrogen, then embedding the particles in a resin block. The resin
block was ground and polished to reveal cross sections of particles
(Figure 3.14). The EDS analysis used an Everhart-Thornley detector (back
scattering electron detector) with optimum conditions at 20 keV, a spot size
of 5 nm and a working distance of 8.5 mm.
Figure 3.13. Stubs prepared for SEM
3-33
Figure 3.14. Precipitates embedded in resin blocks for SEM and EDS analysis.
3.7.8 Optical Microscopy
A Nikon EPIPHOT 200 optical microscope was used to examine
precipitates, reagents and SEM samples. A camera was attached to the lens
to capture the image viewed.
3.7.9 Thermogravimetric Analysis
Thermogravimetric Analysis (TGA) was conducted using a TA
Instruments SDT 2960. The temperature was raised to 1000°C at 10°/min
with either argon or air injection at 100 mL/min.
3.7.10 Laser Size Analysis
A Microtrac SRA150 laser sizer was used for size analysis. Samples
were suspended in deionised water in an ultrasonic bath prior to analysis.
Experiments were conducted in triplicate.
3-34
3.7.11 BET Surface Area Tests
Brunauer-Emmett-Teller (BET) surface area tests were conducted by
Nancy Hanna at the Particle Analysis Service of CSIRO Waterford.
3.7.12 Infrared and Raman Spectroscopy
Infrared (IR) spectroscopy was performed using a Nicolet Magna-IR
System 850 spectrometer. Samples were dried under nitrogen and mixed in
a KBr disk prior to analysis.
Raman spectroscopy was the preferred technique as sample could
be analysed wet. However, sample heating could alter the structure of the
precipitate as nickel is known to absorb around 500 nm. A rotating test tube
mount for Raman on the same machine was manufactured; however heating
was still a problem. To avoid sample heating, a Dilor Labram 1B Micro-
Raman spectrometer was used which operates at higher wavelengths.
Sample was smeared onto a glass slide for analysis.
The analysis by IR and Raman spectroscopy was performed weekly
for 12 weeks on the first batch of 12 precipitates. Results did not exhibit any
trends and were not reproducible probably due to the small sample area
(approximately 10 x 10 x 1 μm).
3-35
3.7.13 High Performance Liquid Chromatography
High Performance Liquid Chromatography (HPLC) was conducted at
the Yabulu Refinery using a Hamilton PRP-X 200 cation exchange column
with a Waters 2996 Photodiode Array (PDA) detector. Separation was
achieved using a gradient of eluents with increasing ionic strength at
1.00 mL/minute: 4 g/L (NH4)2CO3 for 1.4 minutes, 3.2 g/L (NH4)2CO3 /
13.2 g/L (NH4)2SO4 for 1.1 minutes, 66 g/L (NH4)2SO4 for 3.5 minutes and
4 g/L (NH4)2CO3 for 4 minutes.
Solution for testing was filtered through a 0.45 μm millipore filter and
diluted with 4 g/L (NH4)2CO3 to ensure that the cobalt concentration was less
than 1 g/L. A sample of 20 μL was injected into the column with a 50 μL
syringe. Analysis of results was conducted using Waters Empower software.
Method validation and quantification of three complexes was conducted by
Smith (2007) as part of a BHP Billiton funded honours project through James
Cook University.
3.7.14 X-Ray Photoelectron Spectroscopy
This technique was initially conducted on the RNO-MHP collected in
June 2008 by Craig Klauber at CSIRO Waterford. Two monochromatic
sources Mg Kα1 (1253.6 eV) and Al Kα1 (1486.6 eV) were tested with 0.1 eV
increments and a 0.1 s dwell time.
3-36
XPS analysis was also conducted at Murdoch University on two
samples using a Kratos Ultra Axis Spectrometer with a monochromatic Al
Kα1 (1486.6 eV) source. Data for the manganese and cobalt 2p doublet
peaks was collected using 0.2 eV increments and a 0.2 s dwell time five
times and averaged. The sample tested was a simple precipitate containing
~18% Ni and ~3% cobalt and manganese. The second run was conducted
on the same sample after it was ground in a mortar and pestle for a minute in
an inert atmosphere (5 L/min N2 into a sealed vessel). Exposure to oxygen
occurred for less than 30 seconds during sample preparation.
4-1
4 SYNTHESIS, CHARACTERISATION AND REDUCTIVE
LEACHING OF OXIDISED MANGANESE AND COBALT
HYDROXIDES
4.1 Introduction and Experimental
The oxidation of manganese(II) and cobalt(II) during transportation
and ageing was thought to be the most significant problem of MHP leaching
at the Yabulu refinery (Muir, 2003; Nikoloski et al., 2005). This lead to the
development of reducing agents for the reductive leaching of commercial
MHP’s in ammoniacal ammonium carbonate solutions in the Yabulu refinery.
The reactive MnOOH would be ideal as a standard material to test the
reductants, as it is important to understand the effect of formation of the
oxidised products on leaching. Thermodynamic calculations based on
HSC 6.1 database (Roine, 2001) in the present study also confirms the
possibility of the oxidation of M(OH)2 to MOOH or M(OH)3 and M3O4 by
dissolved oxygen for both Mn(II) and Co(II), as revealed by the large
equilibrium constants at 25oC:
)(2)()(2)(2 24)(4 lsaqs OHMOOHOOHM +→+ (log KMn = 51.8)
)(2)(43)(2)(2 62)(6 lsaqs OHOMOOHM +→+ (log KMn = 54.4 , log KCo = 49.2)
)(32)(2)(2 )(42)(4 saqs OHCoOHOOHCo →++ (log K = 16.7)
The production of a stable, reproducible oxidised hydroxide of
manganese or cobalt could be used as a standard to investigate the
reductants developed by BHP Billiton for the Ravensthorpe MHP and to
4-2
possibly find alternative reducing agents for MHP’s produced in other
processes.
Quantifying the oxidation states of Co, Mn and Fe would provide
useful information to understand the leaching behaviour and the formation of
slow leaching compounds. Unfortunately, this has proven to be difficult.
The titration method for the determination of the extent of oxidation of
M(II) (M = Co, Mn, Fe), developed by BHP Billiton, was useful for simple
metal precipitates; i.e. only one reducible metal (Hultgren, 2003a). When Co,
Mn and Fe existed together in a precipitate, it was impossible to distinguish
between the extent of oxidation of each metal ion. Moreover, the titrations of
solutions containing cobalt were difficult due to the interference of the pink
colour of the cobalt solution with the colour change at the end point. Also, the
experimental error was relatively large and the overall theory and method
were questionable. Firstly, the calculation assumes no metal exists in a 4+
oxidation state. Secondly, at 50°C upon dissolution in solution containing
oxygen and metals in the divalent state, further oxidation/reduction may be
occurring in solution prior to analysis.
In order to understand the oxidation of these two metals, Co(II) and
Mn(II) hydroxides were precipitated, oxidised, characterised, analysed and
leached according to the procedure described in Chapter 3
(sections 3.3.1-3.3.2). The precipitate was leached in both ammonia-
ammonium carbonate and sulphate solutions using sulphite, Co(II) or
4-3
hydroxylamine sulphate (NH2OH.H2SO4) as the reducing agents. This
chapter describes the results of synthesis, characterisation and reductive
leaching of manganese and cobalt oxides/hydroxides.
4.2 Precipitation and Characterisation of a Single Phase MnOOH
The synthesis of Mn3O4 using the procedure described in section 3.2
was successful. However, the six attempts to produce a single phase of
MnOOH (manganite) using a procedure adapted from Ardizzone et al. (1998)
and Wang and Stone (2006), described in Chapter 3, were all unsuccessful.
The sample ‘MnOOH October’ was produced by mixing 900 mL of
0.2 M NH3 with 3 L of 0.06 M MnSO4 hydrate and approximately 60 mL of
30% hydrogen peroxide at ~60°C. It was important to mix the manganese
solution with the peroxide before adding the ammonia, as ammonia and
hydrogen peroxide react vigorously. The large equilibrium constants based
on HSC 6.1 (Roine, 2001) calculated in the present study show the oxidising
ability of H2O2:
)(2)()(22)(2 22)(2 lsaqs OHMnOOHOHOHMn +→+ (log K = 37.7 at 60oC)
)(2)(43)(22)(2 4)(3 lsaqs OHOMnOHOHMn +→+ (log K = 39.3 at 60oC)
)(2)(2)(22)(3 632 lgaqaq OHNOHNH +→+ (log K = 151 at 60oC)
4-4
The mixture was refluxed (90-100°C) for over 6 hours, filtered,
washed and dried at 100°C overnight. Solution volumes and concentrations
were the same as in previous reports by Ardizzone et al. (1998) and Wang
and Stone (2006). However, argon sparging, addition of reagents at 60°C
and 95°C (both reagents were added at ~60°C), leaving the solids to cool in
solution overnight, and washing the filtrate 10 times according to Wang and
Stone (2006) was not conducted. Refluxing was performed for between 6
and 24 hours (Ardizzone et al., 1998 state 24 hours, Wang and Stone, 2006
state 6 hours).
MnOOH can exist as three phases: feitknechtite (MnOOH,
hexagonel), groutite (MnOOH, orthorhombic) and manganite (MnOOH,
monoclinic). The XRD traces of different samples shown in Figure 4.1 exhibit
a mixture of all 3 phases in varying concentrations depending on the
conditions used in synthesis. The different preparation conditions produced
mixtures of differing compositions. Of the six attempts two syntheses were
performed over 12 hours instead of 6 (B and C in Figure 4.1), the addition of
reagents was changed (D: 30 mL H2O2 instead of 60 mL; E: 30 mL H2O2
combined with ammonia solution and added together), and one was left in
the vessel for a further day (F). All precipitates were washed thoroughly prior
to analysis. The peak seen around 33° in some of the precipitates (E,
MnOOH-Oct, MnOOH-Jan) is most likely Mn3O4. The transformation is
incomplete either due to lack of time refluxing or evaporation of ammonia.
4-5
As a final attempt, the method described by Wang and Stone (2006)
was followed precisely to produce the sample MnOOH-Jan. This attempt was
also unsuccessful as the XRD trace of MnOOH-Jan exhibited peaks
belonging to all three MnOOH phases (Figure 4.1).
10 20 30 40 50 60 70 802 Theta
MnOOH oct B C DE F MnOOH Jan Feitknechtite, MnOOHGroutite, MnOOH Manganite, MnOOH
Figure 4.1. XRD scans of various products formed during manganite precipitation (B, C, D, E, F are products of different attempts, see text).
The first attempt at production, following the procedure in Wang and
Stone (2006), was mildly successful, while the second (B) worked well. The
difference in the two methods was only the chemical used for neutralisation:
(A) sodium hydroxide and (B) ammonium hydroxide. Examination of the
appropriate Eh-pH diagram (Nikoloski et al., 2005) revealed that the
manganese ions form complexes with ammonia in its divalent state. Thus,
the presence of ammonia was discovered to be essential.
4-6
4.3 Reductive Leaching of MnOOH and Mn3O4 with NH2OH and Co(II).
Two of the precipitates produced (Mn3O4 and MnOOH (B)) were
leached with various reductants and compared. The XRD trace (Figure 4.2)
shows the three precipitates which were leached: Mn3O4, MnOOH or a
mixture of the two (labelled ‘MnOOH mixed phase’). The trace was of better
quality due to the use of a different machine (Siemens D500), purchased by
Murdoch University. The various phases of MnOOH were labelled separately
as the change in precipitation conditions produced different structures. The
precipitate ‘MnOOH’ consisted of all three structures with a manganite
dominance, while ‘MnOOH mixed phase’ consisted of a mixture of MnOOH
structures and Mn3O4.
10 20 30 40 50 60 70 80
2 Theta
Mn3O4 MnOOH mixed phase MnOOH Mn3O4Feitknechtite, MnOOH Groutite, MnOOH Manganite, MnOOH
Figure 4.2. XRD scans of Mn3O4, MnOOH and a mixture.
4-7
Figure 4.3. Extent of reduction of Mn3O4, a mixture and MnOOH in SAC solution, under reducing conditions using either cobalt(II) or hydroxylamine
sulphate.
As shown in Figure 4.3, hydroxylamine sulphate was effective as a
reductant on all three samples with almost 100% of Mn reduced. The
relevant half cell reaction and standard reduction potential is shown below:
3N2(g) + 2H2O +4H+ + 2e- = 2NH3OH+ (Eo = -1.87 V)
The Eh-pH diagrams for Mn and Co systems in ammoniacal solutions
based on the HSC 6.1 database in Figures 4.4a and 4.4b show that the
reduction potentials for Mn3O4/Mn(NH3)42+ and MnOOH/Mn(NH3)4
2+ are
much higher than that for N2/NH3OH+ couple noted above. This explains the
very effective reduction of MnOOH, Mn3O4 and mixed MnOOH/Mn3O4 by
hydroxylamine sulphate shown in Figure 4.3. The overall reaction with
MnOOH is: 2NH2OH2+ + 2MnOOH + 5H+ = 2Mn2+ + N2 + 6H2O.
4-8
Cobalt(II) was not as effective as a reductant for any of the samples,
as shown in Figure 4.3. However, it gave a better indication of the stability of
precipitates as it would have a similar reduction potential to CoNiS
(described in Chapter 8). Figure 4.4a shows the predominant ammonia
complexes of manganese while Table 4.1 lists the equilibrium constants for
the reduction of MnOOH and Mn3O4 by Co(NH3)62+. The reduction of
MnOOH or Mn3O4 by cobalt(II) ions is thermodynamically feasible as Eh for
MnOOH/Mn(NH3)42+ is higher than that for Co(NH3)6
3+/Co(NH3)62+. This
explains the partial reduction of MnOOH by Co(II) in Figure 4.3. However,
the equilibrium constants listed in Table 4.1 show that some of the reactions
for the conversion on Mn3O4 to MnOOH and the reduction of MnOOH is
possible as a result of the involvement of carbonate ions and the
precipitation of MnCO3 or CoCO3. A proper understanding of the cobalt and
manganese speciation in solution and solid phases is essential in order to
rationalise the leaching results in Figure 4.3.
Table 4.1. Equilibrium constants for the reactions of manganese oxides
No. Reaction Log K 1 Mn3O4 + SO3
2- + 3NH4+ + 3HCO3
- = 3MnCO3 + SO42- + 3NH3 + 3H2O 35.5
2 Mn3O4 + SO32- + 6HCO3
- = 3MnCO3 + SO42- + 3CO3
2- + 3H2O 31.3 3 Mn3O4 + SO3
2- + 6NH4+ + 6NH3 = 3Mn(NH3)4
2+ + SO42- + 3H2O 13.3
4 Mn3O4 + HCO3- + NH4
+ = MnCO3 + 2MnOOH + NH3 3.77 5 MnOOH + HCO3
- + Co(NH3)62+ + 2NH4
+ = Co(NH3)63+ + MnCO3 + 2H2O + 2NH3 1.49
6 MnOOH + 2HCO3- + 3Co(NH3)6
2+ + NH4+ = Co(NH3)6
3+ + Mn(NH3)42+ + 2CoCO3 +
2H2O + 9NH3 1.07
7 Mn3O4 + 7HCO3- + 9Co(NH3)6
2+ + NH4+ = 2Co(NH3)6
3+ + 3Mn(NH3)42+ + 7CoCO3 +
4H2O + 30NH3 8.81
8 4MnOOH + O2 = 4MnO2 + 2H2O 15.9 9 Mn3O4 + O2 = 3MnO2 23.6 10 4Mn3O4 + O2 + 6H2O = 12MnOOH 46.7 11 Co(NH3)6
2+ + MnO = CoO + Mn(NH3)42++ 2NH3 0.55
12 Co3O4 + 2Mn(NH3)42+ + 2NH4
+ = 3Co(NH3)62+ + 2MnOOH + 2NH3 5.30
NH3 and O2 represents NH3(aq) and O2(aq); Log K based on HSC6.1 database.
4-9
14121086420
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
Mn - N - H2O - System at 25.00 C
C:\HSC6\EpH\MnN25.iep pH
Eh (Volts)
H2O LimitsMn
MnO2
Mn3O4
Mn(OH)2
MnO*OH
Mn(NH3)3(+2a)
MnO2(-2a)
MnO4(-a)
MnO4(-2a)
Mn(+2a)
ELEMENTS Molality PressureMn 1.000E-06 1.000E+00N 1.000E+00 1.000E+00
14121086420
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
Co - N - H2O - System at 25.00 C
C:\HSC6\EpH\CoN25.iep pH
Eh (Volts)
H2O Limits
Co
Co3O4
Co3O4
Co(OH)2
Co(OH)3Co(OH)3
Co(NH3)6(+2a)
Co(NH3)6(+3a)Co(+3a)
Co(+2a)
ELEMENTS Molality PressureCo 1.000E-06 1.000E+00N 1.000E+00 1.000E+00
(a)
(b)
Figure 4.4. Eh-pH diagram for Mn-Co-NH3-H2O system. (a) 10-6 Mn and 1 M NH3 at 250C (b) 10-6 Co and 1 M NH3 at 250C.
4-10
The XRD traces of the MnOOH/Mn3O4 and MnOOH leach residues
are shown in Figures 4.5 and 4.6, respectively. Manganese carbonate was
present in significant concentrations in all the residue traces. The absence of
any characteristics of the original phases in the traces of leach residues
produced in the presence of hydroxylamine sulphate in both figures proves
successful reduction. Interestingly a small peak shift of manganese
carbonate was observed in Figures 4.5 and 4.6 when cobalt was present in
the system as it substituted for manganese. The substitution of Co(II) to
manganese precipitates is also supported by the residue analysis described
later.
10 20 30 40 50 60 70 802 theta
Original Co Hydroxylamine sulfate Mn3O4 MnOOH MnCO3
Figure 4.5. XRD scans of MnOOH/Mn3O4 mixed phase and leach residues using Co(II) and hydroxylamine sulphate as reductants.
4-11
10 20 30 40 50 60 70 802 Theta
MnOOH Co Hydroxylamine sulphate MnOOH MnCO3 CoCO3
Figure 4.6. XRD scans of MnOOH and leach residues using Co(II) and hydroxylamine sulphate as reductants.
4.4 Reductive Leaching of Mn3O4 with Sulphite and Co(II).
The leach results of Mn3O4 with different reducing agents, and the
XRD on leach residues are summarised in Figures 4.7-4.8. The precipitation
of manganese hydroxide and subsequent oxidation was originally assumed
to produce MnOOH (Nikoloski, et al., 2005). However, XRD analysis showed
the precipitate from this study to consist of mainly Mn3O4 (Figure 4.7), which
is a spinel with a tetragonal structure and a formula of MnO.Mn2O3
(Ardizzone et al., 1998).
4-12
10 20 30 40 50 60 70 802 theta
Original SO3 Co hydroxylamine Mn(OH)2 Mn3O4 MnOOH MnCO3
Figure 4.7. XRD scan of original sample, and leach residues after reduction of Mn3O4 in SAC with sulphite, cobalt(II) or hydroxylamine sulphate.
-40
-30
-20
-10
0
10
Exte
nt o
f Red
uctio
n, %
SAC, Co2+ SAC, SO32- Sulphate, Co2+ Sulphate, SO3
2-
Figure 4.8. Extent of reduction of Mn3O4 using SO32- or Co2+ as reducing
agents in a SAC (carbonate) or sulphate solution.
4-13
As the extent of reduction should be a positive value, the negative
results in Figure 4.8 indicates that the oxidation has occurred during leaching
in the presence of any of the reducing agents: Co(II) or SO32-. Nikoloski et al.
(2005) observed a similar phenomenon, and suggested that the passivation
of the surface by manganese carbonate was inhibiting the reduction. The
comparison between carbonate (SAC) and sulphate (SAS) solutions in this
study proves this was not the case, as the presence of carbonate has no
effect on the XRD pattern or reduction (Figures 4.7-4.8). Clearly there seems
to be an oxidant present in the system, or there was an ingress of oxygen
into the vessel, or during sampling. Reactions 9 and 10 in Table 4.1 show
that oxidation of Mn3O4 to MnOOH or MnO2 is thermodynamically feasible.
Regardless of this, the Mn3O4 appears to be stable and difficult to reduce.
According to XRD patterns, the original sample consists
predominantly of Mn3O4 with perhaps a trace amount of MnOOH, existing in
the same structure as the mineral feitknechtite (MnOOH, hexagonal)
(Figure 4.7). This mineral has a tetragonal structure, and according to PDF
XRD card 180804, forms along with hausmannite (Mn3O4), when Mn(OH)2 is
oxidised. Figure 4.8 also shows little oxidation has occurred after leaching in
the presence of sulphite and cobalt(II). There was no significant reduction in
peak size and no new phases were formed. Hydroxylamine sulphate proved
to be much more effective as a reductant with 90% of the oxidised
manganese reduced over the 2 hours of leaching. The hydroxylamine trace
shows a significant reduction (disappearance) in the Mn3O4 peaks while new
4-14
crystalline peaks of manganese carbonate have become evident
(Figure 4.7).
4.5 Reductive Leaching of Mixed Oxidised Mn-Co Hydroxide with
Sulphite and Co(II).
As described in Chapter 3 (Section 3.3.1), a hydroxide was
precipitated from a cobalt-manganese solution containing equal
concentrations of metals, and oxidised overnight by bubbling air through the
solution. The oxidation of Mn(II) has a lower potential than Co(II) as shown
by the Eh-pH diagram in Figure 4.9 constructed in this study based on the
thermodynamic data from the HSC 6.1 database (Roine, 2002).
Thus, the solids were expected to contain some remaining Co(II) that
would aid the reduction of the manganese upon leaching in SAC solutions.
The mixed precipitate was leached in a SAC solution with and without
reductants. The XRD traces before and after leaching and results on the
extent of reductive leaching are shown in Figures 4.10 and 4.11. The XRD
analysis (Figure 4.10) shows that the original sample contained
predominantly CoMn2O4 (Co substituted Mn3O4) along with some MnOOH,
CoOOH and Co(OH)2. The phase of MnOOH has the same structure as the
mineral feitknechtite which has a tetragonal crystal system. The CoOOH
present in the sample appears to consist of a mixture of hexagonal and
rhombohedral crystal structures for heterogenite-3R or heterogenite-2H
shown in Fig. 4.10.
4-15
Figure 4.9. Eh-pH Diagram of Mn-Co-O2-H2O system under standard conditions at 25oC.
The extent of reduction of mixed oxidised hydroxides (Figure 4.11)
was significantly better, compared to manganese oxide alone described in
Figure 4.8. Cobalt(II) in the precipitate has aided the reaction, with between
25-30% reduction of manganese achieved without an added reductant. The
added cobalt(II) in solution was a more effective reductant, achieving almost
80 % reduction, compared to <40% in the presence of SO32- (Figure 4.11).
4-16
10 20 30 40 50 60 70 802 theta
Original No Reductant SO3Co CoMn2O4 MnOOHCo(OH)2 MnCO3 CoCO3Heterogenite-3R syn, CoOOH Heterogenite-2H, CoOOH
Figure 4.10. XRD scans of the mixed Mn, Co oxidised hydroxide before and after leaching.
0
10
20
30
40
50
60
70
80
90
100
Exte
nt o
f Red
uctio
n, %
No Reductant Co2+ SO32-
Figure 4.11. Extent of reduction of a mixed Mn, Co oxidised hydroxide using SO3
2- or Co(II) in a SAC solution.
4-17
The XRD traces of leach residue (Figure 4.10) show a cobalt
substituted manganese carbonate has precipitated. While the Co(OH)2 peaks
have disappeared, and the MnOOH and CoOOH peaks have reduced in
size, CoMn2O4 seemed to be present in all residues. These observations
indicate that MnOOH and CoOOH have been reduced while the CoMn2O4
remained unreacted. It can be concluded that MnOOH was formed in more
significant concentrations in the presence of cobalt and was either less
stable or more susceptible to reduction in an ammonia solution than Mn3O4.
To investigate the possible formation of MnOOH and the effect of
Co(II) in solid form on reduction, the original Mn3O4 precipitate was mixed
with a CoOOH precipitate (for synthesis refer to section 3.3) and leached as
previously. The XRD patterns of the mixed Mn3O4+CoOOH precipitate before
and after leaching are shown in Figure 4.12 which shows that (i) the Co(OH)2
peaks have disappeared, (ii) the CoOOH peaks reduced in size, and (iii) the
Mn3O4 peaks remained unchanged. Although cobalt(II) was present in the
solids, and would expect to act as a reductant, less reduction has occurred in
the present study. This suggests that Mn3O4 remains stable and unreactive
when attempting to reduce with sulphite or cobalt(II), while MnOOH, which is
formed at atmospheric conditions in the presence of CoOOH, is susceptible
to reduction. These results are consistent with the log K values listed in
Table 4.1.
4-18
10 20 30 40 50 60 70 802 theta
Original No Reductant Co2+ Mn3O4 Co(OH)2 CoOOH MnCO3
Figure 4.12. XRD scans of a mixed Mn3O4 and CoOOH precipitate before and after leaching.
4.6 Summary
• Production and oxidation, by bubbling air through the solution overnight,
of simple precipitates provided some useful information. Bubbling air
through solutions containing manganese and cobalt overnight resulted in
the oxidation of 100% manganese and up to 60% of cobalt. Based on this
observation, the occurrence of complete oxidation of cobalt during the
production of MHP at the Ravensthorpe plant is unlikely.
• Manganese(II) hydroxide precipitate can be oxidised to MnOOH, Mn3O4
or a mixed MnOOH+Mn3O4 precipitate by selecting the oxidation
procedure. Manganese as a simple hydroxide precipitate (Mn(OH)2)
oxidises to predominantly Mn3O4 (or MnO.Mn2O3). However, in the
presence of cobalt, using the same procedure, the predominant product
4-19
is Feiknechtite (MnOOH). Manganite and Groutite (other MnOOH mineral
structures) are also more predominant than Mn3O4.
• Mn3O4 proved difficult to leach in ammonia in the presence of mild
reductants (Co(II) and sulphite), while the MnOOH structures leached
readily. The MnO.Mn2O3 spinel tetragonal structure is reported to be slow
leaching (Ardizzone et al., 1998). This structure was also observed in the
present study by XRD in a cobalt substituted form (CoMn2O4), which also
did not leach. If this compound is formed in MHP it will prove difficult to
leach at the Yabulu refinery.
• If only ~60 % of cobalt oxidised under the extreme conditions used in the
present study, it is unlikely that all of the cobalt(II) in the MHP would
oxidise during precipitation and transportation. Divalent cobalt in the
precipitate facilitates the dissolution of oxidised manganese as it would
act as a reductant when in solution and destroy the crystal lattice upon
leaching.
• Some of the reduction products appear to be MnCO3 and CoCO3,
supported by the large equilibrium constant predicted for their formation
and XRD analysis of leach residues.
5-1
5 CHARACTERISICS AND PROPERTIES OF MgO AND
SYNTHETIC MIXED HYDROXIDE PRECIPITATES
5.1 Introduction and Experimental
Pilot plant studies by BHP Billiton revealed that the mixed hydroxide
precipitate, produced from pressure acid leach liquors of Ravensthorpe
laterite in Western Australia, contained at least 11 metals of typical assays:
40.0% Ni, 1.38% Co, 2.75% Mn, 1.75% Mg, 0.2% Ca, 0.05% Al, 0.15% Fe,
0.01% Cr, 0.015% Cu, 0.23% Zn, 0.5% Si. During the operation under BHP
Billiton, the MHP of the Ravensthorpe plant was transported to the Yabulu
refinery in Townsville, Queensland for further processing using ammoniacal
ammonium carbonate leaching.
Reducing the moisture content of MHP from 40% to less than 1% could
result in transport savings up to $5.6 million per year (Fittock, 2008).
However, oxidation of Co, Mn and Fe, and formation of stable slow leaching
phases was thought to inhibit nickel and cobalt recoveries in the Yabulu plant
(Muir, 2003). Thus, wet MHP (60% solids) was transported to the Yabulu
plant in order to minimise the ‘ageing’. The decision was based upon
significant testwork conducted by multiple companies; however, none of this
work utilised a reductive leach.
Precipitates produced with MgO were different from those produced
with alternative neutralising agents. It was unknown whether metal ions
precipitated together or separately, and if it was by nucleation or precipitation
5-2
on the MgO particle. Examining the changes in crystallinity during
precipitation and over time during ageing/drying, in the presence of various
metal ions relevant to the Ravensthorpe MHP, would help to explain the
behaviour of ‘ageing’ precipitates and formulate remedies.
In an effort to understand the influence of metal ions on the leaching of
nickel and cobalt from transported (aged) MHP, multiple precipitates were
produced from a variety of solutions. Metal hydroxides were produced,
introducing one metal ion at a time in one group, and various combinations of
metal ions in several other groups to simulate the Ravensthorpe MHP.
Precipitates were also produced and aged at 50°C (oven ageing), and were
also produced at elevated temperatures (80°C) from solutions of low metal
concentration to improve crystallinity. As part of another batch of work, the
influence of drying the precipitates was investigated.
A heavy focus was placed on the major metal ions: Ni, Mg, Co and Mn,
and their influence on crystallinity. In total, 29 precipitates were synthesized
in 5 groups and analysed over a period of 12 weeks to examine the effect of
ageing. Not all precipitate analyses are included as some provided little
information. Whilst these precipitates were ‘ageing’ over 12 weeks they were
examined in a multitude of ways and leached under oxidising and reducing
conditions. Initially, IR and Micro-Raman spectroscopy, and testing including:
moisture tests, and extent of oxidation titrations were performed weekly,
while XRD was performed daily for the first week. Analysis of results showed
this was unnecessary, as little occurred in the first few days. The
5-3
characterisation of precipitates is described in this chapter, followed by the
leaching results in Chapter 6.
5.2 Composition and Properties of MgO
5.2.1 Chemical Analysis and Size Distribution
To investigate the precipitation mechanism of MHP, the logical place
to begin was an analysis of the composition, properties and solubility of the
precipitating agent MgO in water and SAC solution. Chemical analysis of
MgO used in the precipitation process (QMag) is listed in Table 5.1.
Unfortunately, the minor quantities of SiO2, Fe2O3, Al2O3 and Mn3O4 can
contaminate the synthetic precipitates as described later. Size analysis of
MgO (Figure 5.1) revealed that it has an 80% passing size (P80) of
approximately 18 μm. A surface area of 1.0 m2 cm-3 (~0.28 m2 g-1) was
calculated by the laser sizer, assuming all particles were spherical. Scanning
Electron Microscopy shows the material to have a jagged fluffy appearance
(Figure 5.2). The BET surface area (determined by QMag) was 35 m2 g-1,
proving that the material is extremely porous.
Table 5.1. Assay of Queensland Magnesia’s MgO (Emag 45).
5-4
0
20
40
60
80
100
1 10 100
Size, um
Cum
% P
assi
ng
0
2
4
6
8
10
% C
hanc
e
Cumulative % Passing % Chance
Figure 5.1. Size analysis of MgO.
Figure 5.2. SEM Image of MgO.
5.2.2 Dissolution of MgO and Reprecipitation Mg(OH)2
During the precipitation process MgO transforms to Mg(OH)2,
probably by two mechanisms: (i) hydrolysis/dissolution, and (ii) dissolution-
hydrolysis.
(i) Hydrolysis/dissolution
MgO + H2O = Mg(OH)2 (log K = 4.76)
Mg(OH)2 + 2NH4+ = Mg2+ + 2NH3 + H2O (log K = -1.65)
5-5
(ii) Dissolution/hydrolysis
MgO + 2NH4+ = Mg2+ + 2NH3 + H2O (log K = 3.11)
Mg2+ + 2H2O + 2NH3 = Mg(OH)2 + 2NH4+ (log K = 1.65)
The calculated concentration of Mg2+ in equilibrium with different solids
at 25oC, based on the equilibrium constants from HSC 6.1 database are
listed below:
MgO + H2O = Mg2+ + 2OH-, log K = -6.396, [Mg2+] = 112 mg/L.
Mg(OH)2 = Mg2+ + 2OH-, log K = -11.15, [Mg2+] = 2.90 mg/L.
MgCO3 = Mg2+ + CO32-, logK = -5.07, [Mg2+] = 0.204 mg/L.
MgCO3 + H2O = Mg2+ + HCO3- + OH-, log K = -8.73, [Mg2+] = 0.297 mg/L
The calculation was made on the basis of stoichiometry of the
dissolution of solids at saturation and the resultant pH assuming unit activity
coefficients which allows the use of concentrations (mol L-1) in dilute
solutions.
The solubility of the MgO sample measured in the present study is low
(<10 mg/L) in both water and SAC solution as shown in Figure 5.3. The initial
increase in magnesium concentration in SAC solution in Figure 5.3 suggests
that the dissolution/hydrolysis mechanism is more likely in an ammoniacal
solution. The decrease in magnesium concentration in SAC solution after 30
seconds indicates the precipitation due to hydrolysis.
5-6
Dissolution-precipitation of magnesium would also result in a higher
incorporation of magnesium in the synthetic metal hydroxide precipitate.
Therefore, the solubility of MgO would have a significant influence on
magnesium content in the final product. Temperature, and to a smaller
degree, ionic strength would also influence the equilibrium constants and
hence the magnesium incorporation.
Figure 5.3. MgO dissolution at 25°C in SAC solution and water.
Silicon was present as SiO2 (Table 5.1) so was unlikely to dissolve
and precipitate, while calcium was present as CaO which would probably
convert to Ca(OH)2. These predictions are supported by the low equilibrium
constant at 25oC for the conversion of SiO2 to Si(OH)4(aq) compared to the
high equilibrium constant for the conversion of CaO to Ca(OH)2, based on
the HSC 6.1 database:
SiO2 + 2H2O = Si(OH)4(a) log K = -4.03
CaO + H2O = Ca(OH)2 log K = 10.1
5-7
5.2.3 Rate of Hydration of MgO
In order to examine the effect of moisture on the rate of MgO
hydration, six mixtures of MgO and water (10, 20, 40, 50, 60 and 90% solids)
were prepared and analysed by XRD over a week. In synthetic MHP’s
(Section 5.4.2) MgO was present in the precipitate for up to 16 days. The
90% (solids) sample was not analysed as the small amount of water did not
create a homogenous sample. Examining the XRD trace in Figure 5.4, it is
evident that most of the hydration has occurred in the first three days. A
similar rate was observed with the other samples, so the XRD traces were
not included.
10 15 20 25 30 35 40 45 50
2 Theta
1 2 3 4 MgO Mg(OH)2 CaO
Figure 5.4. XRD scans of 60% MgO/water mixture after 1, 2, 3 and 4 days.
5-8
5.3 Synthetic MHP
5.3.1 Mechanism of Precipitation
Magnesia is the superior precipitant for metal hydroxide precipitation
as it forms a product with a larger particle size and improved crystallinity due
to the slow release of the hydroxyl ion (Schiller & Khalafalla, 1984; Frost et
al., 1990; Sist & Demopoulos, 2003). Due to its slow dissolution and
reprecipitation in SAC solution (Figure 5.3) it was unknown whether complete
dissolution would occur or if metal hydroxides would coat unreacted MgO
particles during precipitation. Also, as various metal ions begin precipitation
at different pH values, it is possible that the metal hydroxides may form
layers rather than mixed phases. In research conducted by Comet
Resources Ltd. Muir (2003) stated: “It is anticipated that SEM analysis of
freshly precipitated MHP would show more coatings and rims of different
metal hydroxides that slowly transform and rearrange to solid solutions over
time”.
The metal hydroxides were produced at 25 and 45°C from solutions
containing 4 g/L Ni(II), 0.4 g/L Co(II) and 1.25 g/L Mn(II) by adding a 2:1
mole ratio of MgO to Ni(II) (Chapter 3). The conditions of testing were based
on the Ravensthorpe process, where MgO is added to a solution of similar
composition at 45°C with approximately 4 hours of reaction time. In order to
determine the precipitation mechanism, samples were collected over the 4
hour precipitation period of stirring (5, 30, 60, 120 and 240 minutes) and
analysed with a laser sizer and by SEM.
5-9
Results from size analysis of the precipitate particles over the initial 4
hour period were surprising. According to the P80’s displayed in Table 5.2 all
the crystal growth had occurred in the first 30 minutes. Also, there didn’t
seem to be a significant difference between size of precipitates formed at the
two temperatures 25oC and 45oC.
Table 5.2. Particle size (P80 ) of precipitates at 25 and 40°C over 4 hours
0 5 30 60 120 24025°C 18 29 43 44 44 4745°C 18 30 45 45 47 52
Time, minutes
0
1
2
3
4
5
6
7
8
9
10
1 10 100
% P
assi
ng
Size, µm
0 5 30 60 120 240
Figure 5.5. Change in size distribution of precipitates at 25°C over 240 minutes.
A comparison of the size distribution of precipitates produced after
different time intervals is shown in Figure 5.5. Although the P80 of the
material did not increase significantly after 30 minutes, there seems to be a
small shift to the right over the first hour, shown in Figure 5.5. This was
5-10
probably due to the agglomeration of smaller particles or the dissolution of
unreacted MgO. The change between bimodal and unimodal distribution over
time (after 30 minutes) would also be due to the dissolution of the smaller
MgO particles and agglomeration.
The SEM and EDS analysis of precipitates after 5, 30 or 240 minutes
compared in Figures 5.6-5.8 revealed that a different precipitation
mechanism is occurring over the 4 hour reaction time. Although SEM was
conducted on all samples, there did not seem to be an obvious difference
between samples at 25 and 45°C. The three images displayed in
Figures 5.6, 5.7 and 5.8 are representative of all the images taken. Over the
4 hour period, there was a general increase in nickel, and decrease in
magnesium concentrations with an even distribution of the two metal ions.
Also, in all samples, there were a number of particles with a bright ring on the
edge of the particle. As the images were taken using a back scatter electron
detector, the bright ring would consist of predominantly nickel as it has a
higher atomic number than magnesium. The particles become more jagged
and agglomeration occurs over time (Figure 5.8). These particles are
probably cemented together by fresh precipitate. Mapping was attempted on
the binding precipitate; however the resolution required could not be
achieved. Three different precipitation mechanisms are possible on the basis
of the SEM images and EDS results: (i) precipitation in pores, (ii) dissolution-
nucleation, (iii) crystal growth.
5-11
Figure 5.6. SEM and EDS images of precipitate at 25°C after 5 minutes.
Figure 5.7. SEM and EDS images of precipitate at 25°C after 30 minutes.
5-12
Figure 5.8. SEM and EDS images of precipitate at 25°C after 240 minutes.
According to a BET test conducted by QMag (supplier of MgO), the
magnesia has a surface area of 35 m2 g-1 compared to ~0.28 m2 g-1
determined in this study using a laser sizer. Although the calculation for the
laser sizer assumes spherical particles, the large difference between values
is due to high porosity. Previous researchers (Ardizzone et al., 1997;
Hartman et al., 1993; Guan et al., 2006; Tececo) discuss the porous nature
of this material and the effect of feed and calcination temperature on the
surface area. It was thought that the expulsion of carbon dioxide during
calcination was the cause for porosity. Guan et al. (2006) published a High
Resolution Transmission Electron Microscopy (HRTEM) image (Figure 5.9)
which illustrates the porosity of the material. An image of QMag MgO
(Figure 5.10) also shows the porous nature of the material.
5-13
Figure 5.9. HRTEM image of MgO-1-520N (Guan et al., 2006).
Figure 5.10. Cross section SEM image of MgO after 30 minutes in water at 25°C.
Due to this feature, metal hydroxides are probably precipitating within
the pores of the particles. This would explain high P80 value of the 5 minute
sample and the even metal distributions of the large rounded particles
(Figure 5.6). The remaining MgO in these particles would either hydrolyse or
dissolve. The hydration of MgO usually takes between 3 days and a number
of weeks. This material, with nickel substitution seemed to form a stable,
crystalline, slow leaching material. This slow reacting MgO would probably
exist towards the core of the larger particles where it is inaccessible by
5-14
solution. If this is the case, no amount of washing would eliminate the
problem.
Dissolution-nucleation was also occurring. Small particles with a high
nickel concentration were observed in all SEM samples (Figures 5.6 – 5.8),
while magnesium rich particles were more predominant in the early stages of
precipitation (Figure 5.6). Nucleation would probably occur in the first few
minutes when metal concentrations are high and there is a large driving force
for precipitation.
Finally, crystal growth onto existing particles, known as Ostwald
ripening (Ratke & Voorhees, 2002), was also observed by SEM. The brighter
ring seen around a significant number of the particles is a material of higher
atomic mass (high electron density). These nickel rich rings seem to become
more predominant over the 4 hours.
5.4 Effect of pH and Initial Metal Ion Concentration on MHP
Composition
5.4.1 Precipitation Diagrams
In order to examine the procedure to produce mixed hydroxide
precipitates with the desired composition of metals, a plot of precipitation %
of metal ions as a function of pH was developed for a multi-metal ion solution
similar to Ravensthorpe’s plant liquor. Solution samples were taken at pH
intervals to determine the percentage of metal precipitated at each pH. The
5-15
experimental data points were joined together to allow for easier viewing of
the precipitation behaviour of each metal ion in Figure 5.11. It should be
noted that with this test less than 0.1 g of MgO raised the pH from 3.8 to 6.6
which gave some of the metals a linear precipitation-pH relationship rather
than the expected curve similar to the curves for nickel and cobalt.
Experimental work revealed that MgO raised the pH to around 8,
depending on metal ions in solution. At this pH, around 90% of nickel and
cobalt and only about 10% manganese precipitated. These estimates would
depend on the initial concentration of metal ions in solution. In addition to the
Ksp values, the interaction with the sulphate anion would influence the
solubility. The curve for silicon has an unusual shape, compared to other
metal ions in Figure 5.11, as it is difficult to analyse for Si using AAS.
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
2 3 4 5 6 7 8 9 10
Frac
tion
Prec
ipita
ted
pH
Al Co Cr Cu Fe Mn Ni Si * Zn
Figure 5.11. Precipitation of metals with rising pH at 25°C.
5-16
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0 2 4 6 8 10 12 14 16
pH
Frac
tion
Prec
ipita
ted
Fe(III) Al(III) Pb Cu Zn Fe(II) Cr(II) Ni Co Mn Mg Ca
Figure 5.12. A solubility diagram of metal hydroxides based on KSP at 25°C (Monhemius, 1977). (Metals plotted from left to right, Pb and Cu overlap,
thermodynamic predictions are based on unit activity coefficients of species involved).
Monhemius (1977) published a solubility diagram of metal hydroxides
in which the logarithm of metal ion concentration was plotted as a function of
pH using literature data on solubility products (KSP). This was reviewed in
Chapter 2 (Figure 2.1). Figure 5.12 shows a modified version to show the
extent of precipitation as a function of pH. The plot in Figure 5.12 is
significantly different to the precipitation plot produced experimentally shown
in Figure 5.11. The pH for complete precipitation from the multi-metal ion
solution seemed to be different in the two figures for a given metal ion. For
example, nickel(II) required a pH of approximately 8 in Figure 5.11,
compared to 6.8 in Figure 5.12. Likewise aluminium(III) required pH~6.5 in
Figure 5.11 compared to 3.5 in Figure 5.12. Moreover, some metal ions were
observed to precipitate together in Figure 5.11 (iron(III) and aluminium(III),
5-17
and nickel(II) and cobalt(II)). Clearly, the precipitant and metal ions present in
solution have an effect on the pH for complete precipitation.
This can be related to the association of metal ions with sulphate
anions producing ion-pairs such as NiSO40, CoSO4
0 and FeSO4+ with logK
values, based on the HSC 6.1 database, in the range 1 - 2.5 as shown
below. The difference in equilibrium constants for the precipitation reactions
of Ni2+(aq) or NiSO40(aq) to produce Ni(OH)2(s) is also noticeable.
Ni2+ + SO4
2- = NiSO40 (log K = 2.29 )
Co2+ + SO42- = CoSO4
0 (log K = 2.42)
Fe3+ + SO42- = FeSO4
+ (log K =1.94 )
Ni2+ + H2O = Ni(OH)2 + 2H+ (log K = -12.8 )
NiSO40 + H2O = Ni(OH)2 + 2H+ + SO4
2- (log K = -15.1)
The lower equilibrium constant for the precipitation of NiSO40 as
Ni(OH)2 (log K = -15.1 ), compared to the precipitation of Ni2+ as Ni(OH)2
(log K = -12.8), can enhance the pH of precipitation in the presence of
sulphate ions.
5.4.2 Effect of Initial Metal Ion Concentration
The initial metal ion concentrations required for the precipitation of
mixed hydroxides were selected on the basis of the % precipitation vs. pH
information in Figure 5.11 and the cobalt and manganese precipitation work
conducted earlier, described in Chapter 4. Precipitates were produced and
5-18
aged for 6 weeks, before selected predictor leach tests described in
Chapter 6 were performed on the samples. The initial solution compositions
for various metal ion mixtures described in Table 3.3 are summarised below:
Group 1 (A-H) Major components : 4 g/L Ni(II), 0.4 g/L Co(II) with 0 or 0.4 g/L Mn(II) Minor components : 0.1 g/L Al(III), Fe(III), Cr(III), Cu(II), Zn(II) and Si(IV). pH adjusted by adding MgO Group 2 (I-N) Major components : 4 g/L Ni(II), 0.4 g/L Co(II) Minor components : 0.1 g/L Al(III), Fe(III), Cr(III), Cu(II), Zn(II) or Si(IV). pH adjusted by adding MgO
Group 3 (O-S) Major components: 4 or 2 g/L Ni(II), 0.4 g/L Co(II) Minor components: 0.1 g/L Al(III), Fe(III), Cr(III), Cu(II), Zn(II) or Si(IV). Mn(II) : 0.15-4 g/L pH adjusted by adding MgO
Group 4 (T-Z, AA) Major components : 4 g/L Ni(II), 0.4, 1.25 or 0 g/L Co(II) Minor components : 0.8-0.33 g/L Al(III), Fe(III), Cr(III), Cu(II), Zn(II) or Si(IV). pH adjusted by adding MgO
Group 5 (AB-AE) (Repeat of O-R with CaO added to raise pH to 8.3) Major components: 4 g/L Ni(II), 0.4, 1.25 or 0 g/L Co(II) Minor components: 0.8-0.33 g/L Al(III), Fe(III), Cr(III), Cu(II), Zn(II) or Si(IV). pH adjusted by adding MgO and then CaO
Group 6 (MHP1-MHP7) Major component: 4 g/L Ni(II), Minor components: 0-1.8 g/L Co(II); 20 or 2.7 g/L Mn(II), 0 or 1.7 g/l Cr(VI)
The metal ion composition of the precipitates (groups 1-6) is shown in
Tables 5.3 and 5.4. The precipitates A-N (Table 5.3) were the first
precipitates produced, when metal incorporation was unknown. Solution
compositions for precipitates O-AA were adjusted based on previous results,
5-19
and to raise the metal content in the precipitate to around 5%. Precipitates
AB-AE were repeats of O-R except the pH was raised further to 8.3 using
lime to raise manganese content in the precipitate. All precipitates contained
minor quantities of Al, Si and Fe, although these ions were absent in the
initial liquors. This is a result of the contamination from MgO (added dry)
which contained 0.1% Al2O3, 1% SiO2 and 0.1% Fe2O3 as noted in Table 5.1.
The precipitates MHP1-MHP7 were synthesised to examine the effect of
increasing cobalt content and the introduction of high levels of Mn, Al or Cr.
Table 5.3. Composition of precipitates of Groups 1-5 (dry basis) Sample Ni Co Mg Mn Al Fe Cr Cu Zn Si Ca S C
A 33.21 3.03 14.95 0.16 0.08 0.10 <0.01 0.001 0.02 0.52 0.21 3.27 0.291B 30.97 2.89 13.48 2.43 0.08 0.09 <0.01 0.001 0.02 0.49 0.23 3.73 0.169C 27.52 2.68 12.43 2.95 0.67 0.10 <0.01 0.001 0.02 0.48 0.24 3.79 0.185D 31.36 2.84 11.65 2.56 0.68 0.53 <0.01 0.001 0.02 0.50 0.26 3.82 0.153E 27.15 2.70 10.81 2.70 0.71 0.61 0.34 0.007 0.02 0.43 0.26 3.92 0.249F 27.14 2.57 10.38 2.55 0.69 0.57 0.32 0.604 0.02 0.52 0.26 3.88 0.233G 25.99 2.59 10.23 2.53 0.67 0.60 0.32 0.590 0.58 0.39 0.27 3.88 0.192H 26.37 2.58 10.62 2.29 0.66 0.55 0.31 0.576 0.57 0.58 0.24 3.73 0.226I 27.39 2.61 11.94 0.07 0.67 0.09 <0.01 <0.001 0.02 0.54 0.27 3.24 0.522J 27.73 2.70 12.15 0.07 0.08 0.58 <0.01 <0.001 0.02 0.41 0.22 3.07 0.450K 27.71 2.60 12.88 0.07 0.08 0.09 0.31 <0.001 0.02 0.44 0.23 3.08 0.473L 28.56 2.63 13.38 0.07 0.07 0.08 <0.01 0.624 0.02 0.43 0.22 3.13 0.453M 27.38 2.79 13.24 0.07 0.08 0.09 <0.01 <0.001 0.58 0.46 0.25 3.37 0.220N 27.37 2.75 13.07 0.07 0.08 0.09 <0.01 <0.001 0.03 0.67 0.23 3.06 0.247O 22.27 2.00 10.80 0.74 0.17 0.06 <0.01 0.001 <0.01 0.29 0.17 3.05 0.476P 20.61 1.93 10.78 1.31 0.12 0.05 <0.01 <0.001 <0.01 0.23 0.15 2.99 0.449Q 20.90 1.88 10.38 1.76 0.07 0.05 <0.01 <0.001 <0.01 0.21 0.14 3.13 0.452R 20.58 1.95 10.55 2.05 0.15 0.07 <0.01 <0.001 <0.01 0.25 0.13 3.13 0.365S 12.43 2.30 15.60 3.94 0.12 0.06 <0.01 <0.001 <0.01 0.26 0.16 2.58 0.375T 23.22 <0.01 12.68 0.02 0.12 0.06 <0.01 0.001 <0.01 0.26 0.18 2.77 0.485U 20.23 6.38 11.68 0.01 0.08 0.05 <0.01 <0.001 <0.01 0.23 0.17 3.33 0.324V 14.36 1.73 10.25 0.02 5.32 0.07 <0.01 <0.001 <0.01 0.29 0.20 4.34 0.288W 21.78 2.02 8.12 0.03 0.15 4.23 <0.01 <0.001 <0.01 0.26 0.19 3.06 0.509X 20.20 1.94 7.66 <0.01 0.12 0.06 4.21 <0.001 <0.01 0.21 0.26 4.24 0.461Y 16.32 1.65 16.83 0.02 0.06 0.07 <0.01 7.612 <0.01 0.49 0.20 3.08 0.431Z 18.97 1.86 9.69 0.01 0.14 0.06 <0.01 0.002 4.28 0.22 0.17 3.52 0.524
AA 18.62 1.73 10.94 0.01 0.15 0.07 <0.01 <0.001 <0.01 2.55 0.18 2.18 0.077AB 15.40 1.46 20.34 2.66 0.05 0.09 <0.01 0.001 <0.01 0.30 0.31 2.88 0.61AC 18.97 1.82 14.54 4.39 0.05 0.10 <0.01 0.002 <0.01 0.26 0.34 4.44 0.35AD 18.68 1.81 12.34 6.43 0.04 0.07 <0.01 <0.001 <0.01 0.23 0.25 4.19 0.28AE 17.38 1.66 8.19 12.21 0.04 0.06 <0.01 <0.001 <0.01 0.20 0.24 4.87 0.27
5-20
Table 5.4. Composition of precipitates of Group 6 (dry basis).
The desired concentration of nickel and cobalt, to replicate
Ravensthorpe MHP, should have been 40% and 4%, respectively. The Ni
(25-33%) and Co (2.6-3.0%) compositions of the precipitates, particularly
with the first batch of tests (A – N), was lower than the expected values
(Table 5.3). The levels of magnesium (>10%) were extremely high; ideally
the precipitates should have contained less than 3% Mg. The percentage of
minor metals in precipitates A-N (first batch) should have been close to 1%.
Precipitates O to S were meant to have an increased concentration of
manganese in order to quantify its effect. Table 5.4 shows that precipitates
MHP1 to MHP4 contain higher compositions of cobalt. An inverse
relationship exists between cobalt and magnesium incorporation in MHP1 to
MHP4. The precipitates MHP5 to MHP7 contain nickel and magnesium along
with manganese, aluminium or chromium(III).
5.4.3 Effect of Cobalt and Manganese
Precipitates were produced with increasing levels of cobalt, with and
without manganese, to examine the effect of cobalt oxidation on nickel
dissolution. Manganese was precipitated with four of the precipitates as the
metal has been observed to interact with cobalt by altering the structure upon
5-21
precipitation. According to assay results (Table 5.5) each metal was
competing for precipitation. When increasing levels of cobalt existed in the
starting solution, less nickel, manganese and magnesium precipitated.
However, as shown in Figure 5.13 the Ni/Mg molar ratios in different
precipitates remain close to unity indicating precipitation of mixed Ni(II)-
Mg(II)-hydroxide in most cases.
Table 5.5. Assay results of cobalt and manganese rich precipitates. Precipitate Ni Mn Co MgNi, 1% Co 30.73 1.29 15.64Ni, 2% Co 29.17 2.54 14.92Ni, 5% Co 28.02 6.16 12.33
Ni, 10% Co 25.24 10.62 9.98Ni, Mn, 1% Co 25.9 5.43 1.17 11.72Ni, Mn, 2% Co 26.93 4.94 2.71 10.73Ni, Mn, 5% Co 26.37 3.67 5.25 9.97
Ni, Mn, 10% Co 23.99 2.31 11.29 8.67
0
1
2
3
4
5
Ni,C
o-1
Ni,C
o-2
Ni,C
o-5
Ni,C
o-10
Ni,M
n,C
o-1
Ni,M
n,C
o-2
Ni,M
n,C
o-5
Ni,M
n,C
o-10
Initial Co composition
Ni/M
g or
Co/
Mn
mol
ar ra
tio
Ni/Mg molar ratioCo/Mn molar ratio
Figure 5.13. Ni/Mg or Co/Mn molar ratios in precipitates
5-22
5.4.4 Discussion of Assay Results
The precipitation and particularly incorporation of metals in a mixed
metal hydroxide is an extremely complicated process, therefore, almost
impossible to predict. While each metal hydroxide has a unique value of KSP,
metal ions also interact with sulphate anions and each other and precipitate
together in some cases (co-precipitation). High levels of magnesium in the
sample were unavoidable when precipitating on a small scale at ambient
conditions. In a study conducted by SGS Lakefield Oretest Pty Ltd. in 2003
for Ravensthorpe Nickel Operations (Jayasekera, 2003a and 2003b), nickel
and cobalt were precipitated out of similar solutions using various magnesia
samples. The magnesium incorporation varied from 2.6% to 12.4% after
precipitating at 50°C and 100% stoichiometry for 4 hours. The lower
magnesium content of 2.6% was a result of using Emag 45, which was the
magnesia with a composition described in Table 5.1, used in this
investigation.
The temperature has a significant effect on magnesia dissolution and
the kinetics of precipitation. Unfortunately, heating 6 L vessels up to 50°C at
Murdoch University laboratory facilities was unfeasible at the time. The
magnesium incorporation could possibly have been reduced by lowering the
stoichiometric quantity of magnesia added. However, competition for
incorporation would be increased, thus, resulting in lower minor metal
compositions and a more complicated system to achieve desired levels. For
the purpose of this investigation the high levels of magnesium were deemed
5-23
insignificant. In terms of monitoring crystalline (Ni,Mg)(OH)2 of a molar ratio
of Ni/Mg = 1, as in Figure 5.13, the higher levels are favourable.
The lower nickel, cobalt and minor metal concentrations were
probably also a result of slower precipitation associated with ambient
conditions. Also, as higher concentrations of minor metals existed in solution,
less nickel and cobalt precipitate. Minor metals which precipitate at lower pH
values seem to have preference over the desired major metals (Ni, Co).
The concentration of manganese in precipitate A (0.16%) is an
anomaly, as the magnesia sample contained Mn (Table 5.1). Sulfur would
probably be present as sulphate, as the metal ions added to solution were in
the form of metal sulphates. Concentration of sulphate in precipitate would
depend on the effectiveness of washing after filtration. The relatively high
silicon (0.20-0.54%) and calcium (0.13-0.34%) levels were also due to these
metal ions being present in the magnesia sample (Table 5.1). As there was
little difference between base levels of silicon in precipitate A and when the
metal was incorporated in precipitates H and N, they should not be
compared. Precipitate AA has a much higher composition of Si (2.55 %).
In precipitates AB-AE the nickel concentrations were relatively
constant, while the concentrations of manganese increased and that of
magnesium decreased. Although the Ravensthorpe process was designed to
produce a precipitate with manganese below 3%, higher levels were thought
to be of interest, based on experimental results to be discussed later.
5-24
Precipitates U-AA showed accepted levels of the desired metals. By
raising the concentrations of Co, Al, Fe, Cr, Cu, Zn and Si in the precipitate
to around 5% their effect is expected to be more observable. Table 5.6
shows the metal incorporation ratio (metal in precipitate / metal in solution)
for the precipitates O-S, T-Z and AA. Some metals may be incorporated into
the precipitate more readily than others, hence competing with the desired
metals, nickel and cobalt. It was discovered experimentally, when
precipitating at pH 8, the manganese incorporation was limited. This can be
observed in Table 5.6 which lists the ratio of % metal in MHP over % metal in
solution, compared to total metal. Figure 5.14 shows that the incorporation of
manganese into the precipitate reached a limit of about 5%. The limiting of
metal precipitation means that the inclusion ratio of nickel and cobalt actually
increases with increasing manganese in solution. This is shown in both Table
5.6 and Figure 5.15.
Table 5.6. Ratio of % metal in MHP over % metal in solution. Precipitate Ni Co Mg Mn Al Fe Cr Cu Zn Si
O 0.63 0.57 0.56P 0.68 0.63 0.26Q 0.72 0.65 0.24R 0.91 0.86 0.14S 1.05 0.97 0.17T 0.58U 0.62 0.63V 0.51 0.61 0.94W 0.73 0.68 1.03X 0.78 0.75 0.89Y 0.45 0.46 1.06Z 0.63 0.61 0.71
AA 0.57 0.56 1
5-25
Figure 5.14. Effect of Mn(II) in solution on Mn in synthetic MHP.
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 10 20 30 40ratio
of m
etal
inco
rpor
atio
n
% Mn of total metals in solution
Ni Co
Figure 5.15. Effect of Mn(II) in solution on ratio of nickel and cobalt incorporation in synthetic MHP.
Aluminium, copper and silicon had an adverse effect, where each
metal seemed to compete with nickel and cobalt for precipitation. Aluminium
and copper were particularly detrimental as shown by the lowest Ni
compositions of 0.51% or 0.45% in Table 5.6. Zinc did not seem to affect
5-26
nickel and cobalt precipitation, while iron and chromium actually improved
incorporation (Table 5.6).
The ionic properties such as radius, charge density and degree of
hydration can affect the softness of ions which in turn can affect the solubility
products as described in the next section. However; in these results it seems
as though metal incorporation is mostly related to the atomic radius
(Table 5.7). Aluminium, copper and silicon generally have smaller radii than
nickel and cobalt, while iron and chromium are larger. The smaller atoms
would probably be more desirable for precipitation with MgO. In Table 5.7 the
empirical and calculated radii agree reasonably well. Empirical numbers vary
by ±5 pm, and the dash represents unavailable data.
Table 5.7. Atomic radii of selected metals, pm (WebElements, 2009) Empirical Calculated Van der Waals Covalent Metallic Radii
Ni 135 149 163 121 124Mg 150 145 173 130 160Mn 140 161 - 139 127Co 135 152 - 126 125Fe 140 156 - 125 126Al 125 118 - 118 143Cr 140 166 - 127 128Cu 135 145 140 138 128Zn 135 142 139 131 134Ca 180 194 - 174 197Si 110 111 210 111 -
5.4.5 Effect of Cation Softness
The complex formation behaviour of metal ions with different ligands
and precipitation behaviour of metal ions in the form of different salts can be
5-27
related to the variation of cation or anion softness (Senanayake, 2011).
Softness of an ion is related to the polarizability. The values of softness of
ions of interest in this thesis, in the increasing softness order, are shown
below (Marcus, 1997).
Ca2+(-0.66), Mg2+ (-0.41),
Al3+ (-0.31), Cr3+ (-0.10), Fe3+ (0.33), Mn3+ (0.33), Co3+ (0.50),
Fe2+ (-0.16), Mn2+ (-0.15), Ni2+ (-0.11), Co2+ (-0.11), Zn2+ (0.35), Cu2+ (0.38),
Hard cations soft cations
CO32- (-0.50), SO4
2- (-0.38), Cl- (-0.09), OH- (0.0), SO32- (0.66), S2- (1.09),
Hard anions soft anions
For example, Figure 5.16 shows that the softness of cations generally
increases with the increase in covalent radius. Figure 5.17 shows a linear
relationship of a plot pKSP (= -log KSP) of metal hydroxides as a function of
softness of cations. Figure 5.18 plots the % metal in dry residues in
precipitates of groups 1, 2 and 4 as a function of softness to show that the
incorporation of minor metal ions in Table 4.2 is generally affected by the
pKSP of hydroxides, which in turn is governed by the softness of cations.
5-28
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
100 120 140 160 180 200
Covalent radii (pm)
Softn
ess
Figure 5.16. Effect of covalent radii on cation softness (Marcus, 1997)
y = 12.07x + 15.42R2 = 0.82
0
10
20
30
40
-1 -0.5 0 0.5Softness of Cations M(II) or M(III)
pKsp
of M
(OH
) 2 or
M(O
H) 3
pKsp M(II) pKsp M(III)
Figure 5.17. Effect of cation softness on pKSP of hydroxides of M(II) and M(III) (pKSP from HSC 6.1 database (Roine, 2001); cation softness from
Marcus, 1997)
5-29
0.1
1
10
-1 -0.5 0 0.5Softness of cations
% Metal (Group 1)% Metal (Group 2)% Metal (Group 4)
% M
etal
in d
ry p
reci
pita
tes
Figure 5.18. Effect of cation softness on metal assays of dry precipitates of groups 1, 2 and 4 (data from Table 5.3 and Figure 5.16).
5.4.6 Variation of Ni/Mg and Co/Mn Molar Ratio
Although Groups 1, 2 and 4 contained a variety of cations in different
concentrations, the Ni/Mg molar ratio was close to 1±0.1 in most cases
(Figure 5.19). This indicates that a mixed Ni(II)-Mg(II) hydroxide is
precipitating, which will be discussed in more detail later. The Lowest values
of Ni/Mg molar ratios of 0.6 and 0.4 in Group 4 are shown by tests V and Y in
Figure 5.19 corresponding to the presence of Al3+ and Cu2+, respectively.
This may be related to the very high values of pKSP of these hydroxides: 30.7
and 21.6 for Al(OH)3 and Cu(OH)2, respectively (HSC 6.1 data base).
Therefore, it is likely that the precipitation of these hydroxides on or with
MgO (or Mg(OH)2) particles is likely to decrease the extent of precipitation of
Ni(OH)2 due to surface blockage.
5-30
0.1
1
10
100
1000
A B C D E F G H I J K L M N T U V W X Y Z
AA
Test
Ni/Mg (Groups 1,2,4)Co/Mn (Groups 1,2,4)
Ni/M
g or
Co/
Mn
mol
ar ra
tio
in d
ry p
reci
pita
te
Figure 5.19. Effect of different metal ion compositions on Ni/Mg molar ratio in dry precipitates in Groups 1, 2 and 4 (data from Table 5.3)
.
The Co/Mn molar ratio of tests A to H (all with Mn(II)) in Figure 5.19 is
also close to 1, indicating co-precipitation. However, tests T and X (without
added Mn(II)) also showed a Co/Mn molar ratio of 1, indicating contamination
from MgO or other sources as described previously.
In the case of precipitates of group 3 (O-S) and group 5 (AB-AE) the
% Mn in the dry precipitates increased (Table 5.3) with the increase of Mn(II)
in each solution. Fig 5.20 shows the effect of initial Mn(II) concentration in
Group 3 (O-S) on % Mn and the molar ratios of Ni/Mg and Co/Mn in the dry
precipitates. Likewise, Figure 5.21 shows the same variables in Group 5 (AB-
AE) where the pH was increased by adding lime to enhance the precipitation
of Mn(OH)2.
5-31
The pKSP values of the four hydroxides considered in
Figures 5.20-5.21 decrease in the order: Co(OH)2 (15.6) > Ni(OH)2 (15.2) >>
Mn(OH)2 (12.8) > Mg(OH)2 (11.2) > Ca(OH)2 (5.41). Despite these
differences the measured Ni/Mg and Co/Mn molar ratios of the precipitates
follow the same trend in Figure 5.20 at higher concentrations of Mn(II) in
solution (Group 3). As noted previously, the addition of lime enhanced the %
Mn in dry precipitate; this can be observed by the difference between the
precipitates in Figures 5.20 (without lime) and 5.21 (with lime). Moreover,
unlike in Figure 5.20, the Ni/Mg molar ratio increases while Co/Mn molar
ratio decreases in Figure 5.21. Further work is essential to rationalise these
trends.
0
1
2
3
0 1 2 3 4
[Mn(II)] in initial solution (g/L)
0
0.02
0.04
0.06
0.08Ni/Mg (molar ratio)Co/Mn (molar ratio)
Mn%
Ni/M
g or
Co/
Mn
mol
ar ra
tio
in d
ry p
reci
pita
te
% M
n in
dry
pre
cipi
tate
Figure 5.20. Effect of initial Mn(II) concentration on Ni/Mg molar ratio in dry precipitates in Group 3
5-32
0.0
0.2
0.4
0.6
0.8
1.0
0 1 2 3 4
[Mn(II)] in initial solution (g/L)
0.00
0.05
0.10
0.15
0.20
0.25
Ni/Mg (molar ratio)Co/Mn (molar ratio)
Mn%
Ni/M
g or
Co/
Mn
mol
ar ra
tio
in d
ry p
reci
pita
te
% M
n in
dry
pre
cipi
tate
Figure 5.21. Effect of initial Mn(II) concentration on Ni/Mg molar ratio in dry precipitates in Group 5 (data from Tables 3.3 and 5.3)
5.5 Size Distribution of MHP
Laser size analysis was performed on MgO (Figure 5.22) and
numerous precipitates over the 12 week period (Figures 5.23-5.26). The P80
of the MHP’s ranged between 23 and 30 μm while the P80 of the magnesia
was 18 μm. There were no observable trends between particle size, metal
concentrations and percent solids. Both distributions were bimodal and
exhibit a similar shape suggesting that the metal hydroxides coated the MgO
particles upon neutralisation. The Ravensthorpe pilot plant runs conducted in
2002/2003 produced MHP’s with a mean particle size of ~40 μm and a P80
around 50 μm. The larger particle size could be attributed to seeding and a
continuous precipitation process (Shrestha et al., 2003). Further work,
where the particle size was monitored over time and Scanning Electron
Microscopy (SEM) was performed, could confirm this theory. These aspects
are discussed later.
5-33
0
2
4
6
8
10
0
20
40
60
80
100
1 10 100
% C
hanc
e
Cum
% P
assi
ng
Size, µm
Cumulative % Passing % Chance
Figure 5.22. Size distribution of MgO.
0
10
20
30
40
50
60
70
80
90
100
1 10 100
Cum
% P
assi
ng
Size, µm A B C D E F G H
Figure 5.23. Size distribution of MHP’s, A-H – 6 weeks – cumulative percent passing.
0
10
20
30
40
50
60
70
80
90
100
1 10 100
Cum
% P
assi
ng
Size, µm A B I J K L M N
Figure 5.24. Size distribution of MHP’s, A, B, I-N – 6 weeks – cumulative percent passing.
5-34
0
1
2
3
4
5
6
1 10 100
% P
assi
ng
Size, µm
A B C D E F G H
Figure 5.25. Size distribution of MHP’s, A-H – 6 weeks – percent passing.
0
1
2
3
4
5
6
1 10 100
% P
assi
ng
Size, µm A B I J K L M N
Figure 5.26. Size distribution of MHP’s, A, B, I-N – 6 weeks – percent passing.
5-35
Figure 5.27. Size distribution of precipitates O – AA over time.
Size analysis on precipitates O-AA was conducted over the 12 week
period in order to monitor crystal growth. Results are summarised in
Figure 5.27. The overall particle size of the precipitate increased after
production. This was related to the presence and hydration of MgO and
probably coagulation of smaller metal hydroxide particles. As the variability of
the P80’s was not related to atomic radius, it must be due to differences in
conditions during precipitation, filtration and storage. The decrease in particle
size observed with seven of the precipitates in Figure 5.27 was unusual.
Precipitates were observed to become visibly drier as water was
incorporated into the crystal lattice over time. Drying the precipitates for size
analysis could result in cracking and splitting of particles and hence a lower
P80. This effect was visible with SEM and photography (discussed later). The
sample represented in Figure 5.28 has become dryer and has changed
colour, due to oxidation of cobalt and manganese. The oxidation reactions
5-36
have large equilibrium constants (log KM) based on the HSC 6.1 database for
M = Mn and Co (Roine, 2001):
)(2)()(2)(2 24)(4 lsaqs OHMOOHOOHM +=+ (log KMn = 51.8)
)(2)(43)(2)(2 62)(6 lsaqs OHOMOOHM +=+ (log KMn = 54.4, log KCo = 49.2)
4Co(OH)2(s) + O2(aq) +2H2O(l) = 4Co(OH)3(s) (log K = 16.7)
Figure 5.28. Photo of Ni, Co, Mn precipitate after 2 days (left) and a year (right), precipitate was in a sealed plastic jar.
Figure 5.29 shows the percent passing of all precipitates in week 1.
Figure 5.30 shows the percent passing of precipitate O over the 12 weeks of
ageing. Like the first batch of precipitates (A – N, Figures 5.25 & 5.26) some
of the samples exhibited a bimodal distribution similar to MgO, suggesting
the metal hydroxide precipitate is coating the MgO particles (Figure 5.22).
Figure 5.30 demonstrates the size distribution over time. With all precipitates,
there was a general increase in size, and a change of the shape of
distribution. However, no patterns were observed, so all other plots were
omitted from the thesis.
5-37
Figure 5.29. Percent passing, precipitates O - AA – week 1.
Figure 5.30. Percent passing over time – precipitate O.
5.6 Moisture Content
The moisture content was measured over the full period to ensure that
the loss of moisture by evaporation was not occurring and therefore not
affecting the crystal structure of the precipitates. Moisture content,
5-38
summarised in Figures 5.31 and 5.32, was measured to constant mass in
triplicate by weighing ~2 g of sample before and after drying overnight at
50°C.
Although the values fluctuate in Figures 5.31 and 5.32, most of the
precipitates were between 40% and 60% solids. As each test was performed
on a different sample, the fluctuations were expected; the line was drawn
between points to allow for easier viewing of singular precipitates.
Experimentally, producing a precipitate of 50% solids proved to be difficult
using a laboratory Buchner filter. For the purpose of this investigation a
variation of ±10% absolute was acceptable. Although no evaporation
occurred, the precipitates became observably drier over the 12 week period.
This was probably due to the incorporation of water into the crystal lattice.
The water content in precipitates AA – AE, produced from solutions
with higher concentrations of Mn, was significantly different. Although the
same procedure was used, the precipitates contained considerably more
water. Due to the high water content the precipitates had a more gelatinous
appearance. The precipitates AC, AD and AE were each approximately 20%
solids while AA and AB were around 35% solids. The difference in moisture
content between the samples may cause structural changes and effect
subsequent metal recovery.
5-39
Figure 5.31. Percent solids of precipitates A – N over time.
Figure 5.32. Percent solids of precipitates O – AE over time.
5-40
5.7 Extent of Oxidation During Ageing
In a typical batch of MHP the ageing process is thought to be
associated with the oxidation occurring over a period of weeks. It is believed,
that the manganese(II) and cobalt(II) on the outer of the hydroxide sample
oxidise almost immediately while the metal ions towards the centre remain
unreacted or oxidise slowly (Fittock, 2007). The Eh-pH diagram for Co-Mn-
hydroxide-oxide system in Figure 4.9 show the possibility of co-existence of
the hydroxides/oxides listed in Table 5.8.
Table 5.8. Effect of Eh on Mn and Co species _______________________________________________________
Mn Species Co Species _______________________________________________________
(vi) MnO4
- Co(OH)3
(v) MnO4- Co3O4
(iv) MnO2 Co3O4
(iii) MnOOH Co3O4
(ii) Mn3O4 Co(OH)2
(i) Mn(OH)2 Co(OH)2
______________________________________________________ (based on Figure 4.9) Some important points to note are: (i) the co-existence of Co(II) and
Mn(II) hydroxides is a possibility at low potentials, (ii) the co-existence of
Co(OH)2 and Mn3O4 indicates the preferential oxidation of Mn(OH)2 to
Mn3O4, (iii) the co-existence of MnOOH and Co3O4 indicates that it is
reasonable to assume Mn(II) is oxidised to only Mn(III) state.
High Eh
Low Eh
5-41
Using the extent of oxidation titration, the oxidation state of cobalt and
manganese was monitored over 85 days (12 weeks). The titration was an
oxalate-permanganate reaction, after the oxalate had reduced trivalent
metals existing in solution. Dissolution of precipitate occurred in an acidic
solution at 50°C (Nikoloski et al., 2005).
)()(2)(2
)(3
)(422 2222 aqaqaqaqaq HCOMMOCH +++ ++=++
(M can be any reducible trivalent metal)
)(2)(2)(2
)(3)(4)(422 14102625 laqaqaqaqaq OHCOMnOHMnOOCH ++=++ ++−
The extent of oxidation (EO) expressed as a percentage (EO%) with
time was calculated using the method described in Section 3.7.2 and the
results are plotted in Figure 5.33. The EO% ranged between 0.5% and 6%
depending on the concentration of oxidised metals in Tests A-N, assuming
Mn(IV) doesn’t occur. Although chemically possible (at high Eh, Table 5.8),
manganese(IV) was not observed in any of the XRD traces reported in
previous investigations which studied the oxidation of Mn(OH)2 (Murray et al.,
1985; Burns & Burns, 1977, 1979; Giovanoli, 1976, 1980; Feitknecht et al.,
1962; Oswald et al., 1964; Bricker, 1965; Hem & Lind, 1983).
The two precipitates M and N had the lowest EO% after 1 day. In all
other precipitates, oxidation has occurred in the first day to differing extents
(Figure 5.33). This was not considered unusual as the small quantity of
solids (~100 g wet) allowed the intrusion of air to the whole sample. The
5-42
oxidation probably occurred during filtration as a large volume of air would
have passed through and around the precipitate. The fluctuation in results
would be due to experimental error, as each test only used ~0.1 g of
precipitate from a different container at each age. The 95% confidence
intervals ranged up to ±0.5 % of the calculated values.
0
1
2
3
4
5
6
7
0 10 20 30 40 50 60 70 80 90
Days
Perc
ent O
xidi
sed
A B C D E F G H I J K L M N
Figure 5.33. Extent of oxidation titration results (EO%) over time, precipitates A - N.
Using the percentage of possible oxidised metals (Co(III), Mn(III),
Fe(III), Cr(VI)), the quantity of unoxidised Co(II) could be determined
assuming manganese only oxidises to Mn(III) using the equations described
in Section 3.7.2. The results are listed in Table 5.9.
5-43
Table 5.9. Percentage of possible oxidised metals. Possible % Oxidised Approx % Oxidised Approx % Co(II) % of Co in 2+ State
A 3.0 1.5 1.5 51B 5.3 3.5 1.8 63C 6.3 4.5 1.8 67D 6.6 5.0 1.6 57E 6.0 4.5 1.5 56F 6.3 4.0 2.3 89G 6.3 4.0 2.3 89H 6.0 3.0 3.0 116I 2.6 1.0 1.6 62J 3.3 2.0 1.3 47K 2.6 1.5 1.1 42L 3.3 1.5 1.8 67M 2.8 1.0 1.8 64N 2.8 1.0 1.8 64
The % of Co(II) which remains unoxidised in Tests A-N from Table 5.9
is plotted in Figure 5.34. The unoxidised % of Co in the precipitate is 50% in
Test A (without additives), and varies in other tests depending on the
additives. Thus, Figure 5.35 plots the unoxidsed % of Co as a function of
total SO42- ion concentration in the initial solution based on the added salt
concentrations. The increase in total SO42- concentration enhances the
unoxidised % Co(II) in the precipitate in Tests A-N in Figure 5.35. This may
be related to stabilisation of Co(II) as CoSO40 (ion-pair), or lower dissolved
oxygen concentration in concentrated solutions and warrants further studies.
5-44
0
20
40
60
80
100
A B C D E F G H I J K L M N
Test
Uno
xidi
sed
% C
o(II)
Figure 5.34. Unoxidised % of Co(II) over time in precipitates A - N.
40
50
60
70
80
90
100
0.07 0.08 0.09 0.10[SO4
2-]total (mol/L)
Uno
xidi
sed
% C
o(II
) in
pre
cipi
tate
Tests A-HTests I-N
A
BC
D E
F G
H
N
L
M
KJ
I
Figure 5.35. Effect of sulphate ion concentration in initial solution on Unoxidised % of Co(II) over time in precipitates A - N.
The presence of Cu(II) and Si(IV) also seems to enhance the
unoxidised %Co(II) in the precipitate (Tests L, F, G and H in Figure 5.35).
The Eh-pH diagram in Figure 5.36 shows that in the presence of Si(IV),
Co(II) can precipitate as 2CoO.SiO2 which may resist oxidation to Co(III)
status. In the case of Tests I to N (Group 2) in Figure 5.35, the presence of
Fe(III) and Cr(VI) in Tests J and K leads to lowest % of unoxidised Co(II)
indicating that these cations facilitate the oxidation of Co(II) to Co(III). Large
5-45
equilibrium constants predicted for some of the redox reactions which involve
Cu(II), Fe(III) and Cr(VI), listed below, warrant further studies.
2Co(OH)2 + 2Cu(OH)2 = 2Co(OH)3 + Cu2O + H2O (log K = -14.7)
3Co(OH)2 + 2Cu(OH)2 = Co3O4 + Cu2O + 5H2O (log K = 1.55)
3Co2+ + 2Cu2+ + MgO = Co3O4 + Cu2O + 5Mg2+ (log K = 59.5)
3Co2+ + 2Fe3+ + 6MgO + 2H2O = Co3O4 + 2Fe(OH)2 + 6Mg2+ (log K = 74.1)
4.5Co2+ + CrO42- + 3.5MgO + 1.5H2O
= 1.5Co3O4 + Cr(OH)3 + 3.5Mg2+ (log K = 54.6)
Figure 5.36. Eh-pH Diagram of Co-Si-O2-H2O system under standard conditions at 25oC.
Figure 5.37 shows the change in Ni/Mg and Co/Mn molar ratios in dry
precipitates in Tests A-N in Groups 1 and 2, based on the assay results of
the precipitates tested (A – N) listed in Table 5.3. The Ni/Mg and Co/Mn
5-46
molar ratios remain close to unity in Tests A-H (Group 1) in the precipitates
produced in the presence of added Mn(II). However, in the absence of added
Mn(II) the Co/Mn molar ratio is higher due to very low content of Mn in the
precipitate (contaminated from MgO, see Table 5.1).
0.1
1
10
100A B C D E F G H I J K L M N
Test
Ni/Mg Co/Mn
Ni/M
g or
Co/
Mn
mol
ar ra
tio
in d
ry p
reci
pita
te
Figure 5.37. Ni/Mg and Co/Mn molar ratio in dry precipitate.
Between 40% and 100% of cobalt remained in its divalent state
throughout the 12 weeks of ageing (Table 5.9). If up to 60% of cobalt existed
in its trivalent state, the oxidative leach tests should yield poor results
assuming Co(III) doesn’t leach in ammonia. This will be discussed later in
Chapter 6.
Extent of oxidation titrations were also performed on precipitates
produced with MgO containing Mn, Co and Fe (Table 5.10). All precipitates
tested were oxidised. The extent of oxidation of cobalt seems to be less than
that of manganese and iron. These trivalent metals would either form
separate metal oxides/hydroxides or hydrotalcite-type structures. In fact XRD
5-47
on the precipitates showed hydrotalcite-type structures were present
(Section 5.8.1).
Table 5.10. Extent of Oxidation Ni Co Ni Mn Ni Fe Ni Co Mn Ni Co Fe
Percent Oxidised 67 89 88 65 6095 % Confidence Interval 5.8 2.5 2.6 2.7 4.6
5.8 X-Ray Diffraction Patterns
5.8.1 Effect of Ageing of MHP
The XRD analysis was performed on all precipitates over 9 weeks.
Only those showing significant trends were displayed and used for
comparison. For example, Figures 5.38-5.40 show a comparison of the effect
of ageing of the three precipitates in Tests A, B and C. The precipitates
consisted of Ni(OH)2, Mg(OH)2 and MgO. Although MgO was present in the
precipitate in the first few days it was transformed to Mg(OH)2. The mixed
nickel-magnesium hydroxides exhibited sharper peaks with age as the
precipitates became more crystalline.
In some precipitates the transformation of MgO to Mg(OH)2 seemed to
take longer. Based on XRD peak heights, the quantity of MgO in the
precipitate was plotted with time (Figure 5.41).
5-48
10 20 30 40 50 60 70 80
2 Theta
1 day 2 days 3 days 4 days 8 days 16 days25 days 36 days 63 days Ni,Mg(OH)2 MgO
Figure 5.38. XRD scans of precipitate A (Ni, Co, Mg) over 9 weeks.
10 20 30 40 50 60 70 80
2 Theta
1 day 2 days 3 days 4 days 8 days 16 days25 days 36 days 63 days Ni,Mg(OH)2 MgO
Figure 5.39. XRD scans of precipitate B (Ni, Co, Mg, Mn) over 9 weeks.
5-49
10 20 30 40 50 60 70 80
2 Theta
1 day 2 days 3 days 4 days 8 days 16 days25 days 36 days 63 days Ni,Mg(OH)2 MgO
Figure 5.40. XRD scans of precipitate C (Ni, Co, Mg, Mn, Al) over 9 weeks.
Manganese was found to have an effect on the time taken for the
conversion of MgO to Mg(OH)2. In the presence of manganese (samples
B - H) conversion occurred over at least 16 days, while alternatively the
transformation only took 4 days in the absence of manganese (Figure 5.38
vs 5.39). Figure 5.41 shows that there is a significant difference between
samples with and without manganese. Comparison of peak heights was
acceptable as peak width did not change noticeably. These calculations were
based on the height of MgO peak at 43o divided by total height of MgO and
metal hydroxide peaks at 43o and 38o, respectively.
5-50
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40
Time (Days)
Perc
ent o
f MgO
in P
reci
pita
te
A (Ni, Co , Mg)
B (Ni, Co, Mg, Mn)
C (Ni, Co, Mg, Mn, Al)
D (Ni, Co, Mg, Mn, Al, Fe)
E (Ni, Co, Mg, Mn, Al, Fe, Cr)
F (Ni, Co, Mg, Mn, Al, Fe, Cr, Cu)
G (Ni, Co, Mg, Mn, Al, Fe, Cr, Cu, Zn)
H (Ni, Co, Mg, Mn, Al, Fe, Cr, Cu, Zn, Si)
I (Ni, Co, Mg, Al)
J (Ni, Co, Mg, Fe)
K (Ni, Co, Mg, Cr)
L (Ni, Co, Mg, Cu)
M (Ni, Co, Mg, Zn)
N (Ni, Co, Mg, Si)
Figure 5.41. Percentage of MgO in precipitates (rough calculation: height of MgO peak at 43° divided by total height of MgO and metal hydroxide peaks
at 43° and 38°, respectively).
Despite the equimolar ratios of Ni/Mg and Co/Mn of precipitates B-H
in Figure 5.37), the slow conversion of MgO to Mg(OH)2 in the presence of
manganese may be related to the reactions listed below:
4M(OH)2 + O2(aq) = 4MOOH + 2H2O (M= Mn, during precipitation)
6M(OH)2 + O2(aq) = 2M3O4 + 6H2O (M = Mn, during precipitation)
MgO + H2O = Mg(OH)2 (during ageing)
The formation of a hydrotalcite-type structure upon oxidation of
manganese can also be considered. Hydrotalcite has the generalised
formula of (MII1-xMIII
x)8(OH)16(An-)8x/n.4H2O (Forano et al., 2006). This
structure (Figure 5.42) is a result of incorporation of trivalent metals into the
brucite (Mg(OH)2) structure, and the subsequent splitting of layers when
anions (i.e. CO32-, SO4
2-, Cl-) and water are incorporated to balance the
5-51
charge. This type of structure is probably more stable than brucite which
would inhibit the incorporation of Mg into the existing metal hydroxide crystal
lattice. Hydrotalcite is not visible in XRD scans from this study, suggesting it
is either not present or it is X-ray amorphous. The latter is more likely, as the
precipitate is relatively fresh and the formation of a hydrotalcite-like structure
would absorb water and anions into the crystal lattice.
5-52
(a)
(b)
(c)
Desautelsite Mg 6
Mn3+2
(CO 3
)(OH) 16 · 4H
2 O
Hydrowoodwardite(Cu,Al) 9(SO
4 ) 2
(OH) 18 · nH
2 O
Iowaite Mg
6 Fe3+
2 (OH) 16
Cl 2 · 4H
2 O
Pyroaurite Mg
6 Fe3+
2 (CO
3 )(OH) 16
· 4H 2
O
Stichtite Mg 6
Cr 2 (CO
3 )(OH) 16
· 4H 2
O
Takovite Ni 6 Al 2(CO
3 )(OH) 16
· 4H 2O
Figure 5.42. Hydrotalcite structures (a) general formula and structure from Forona et al. (2006), (b) Mg6Al2(CO3)(OH)16.4H2O from
http://en.wikipedia.org/wiki and http://www.kyowa-chem.co.jp/products/page901_e.html, and (c) other structures with trivalent
cations similar to hydrotalcite.
5-53
The shared nickel-magnesium hydroxide XRD peaks at around 19 and
38° have an asymmetric shape. The peaks seem to become more
asymmetric with time suggesting that the mixed nickel-magnesium hydroxide
was separating, or the separate nickel hydroxide phase present in the
precipitate was becoming more crystalline over time. This can be observed in
Figure 5.43.
The transformations observed in Figure 5.43 occurred with all the
precipitates. The values sitting on either side of the peaks are the peak
positions of the metal hydroxides. According to the Joint Committee on
Powder Diffraction Standards (JCPDF) Mg(OH)2 has a peak at 38.02° and
Ni(OH)2 at 38.54°. The peaks seem to be separating over time until 25 days,
after which they seem to merge back together. Perhaps the peaks separate
as the nickel hydroxide becomes more crystalline NiO, whilst the nickel and
magnesium form a mixed hydroxide in the later stages.
5-54
8 Days
35 37 39 41
38.21 38.34
16 Days
35 37 39 41
38.23 38.36
25 Days
35 37 39 41
38.16 38.28
36 Days
35 37 39 41
38.14 38.27
63 Days
35 37 39 41
38.2538.10
84 Days
35 37 39 41
38.20 38.26
Figure 5.43. Ni/Mg hydroxide peaks at 38° of precipitate A at times 16, 25, 36, 63 and 84 days. (Mg(OH)2 – green, Ni(OH)2 – blue).
5-55
Precipitates O – R and AB – AE were produced to quantify the effect
of manganese on the transition over 84 days (Table 5.11). As shown in
Figure 5.44 the Co/Mn molar ratio of precipitates O-R and AB-AE decrease
due to the enhanced %Mn in the precipitates noted in Table 5.11. In the case
of precipitates O, P, Q and R formed by adding MgO (to adjust pH to 8) the
Ni/Mg molar ratio remains close to unity (Figure 5.44). However, when CaO
was added to increase pH to 8.3, in order to enhance the precipitation of
manganese, the Ni/Mg molar ratio increases from low values to high values
for AB, AC, AD and AE.
Table 5.11. Assay results for precipitates O–R and AB-AE, %. Sample Ni Co Mg Mn
O 22.27 2.00 10.80 0.74P 20.61 1.93 10.78 1.31Q 20.90 1.88 10.38 1.76R 20.58 1.95 10.55 2.05
AB 15.40 1.46 20.34 2.66AC 18.97 1.82 14.54 4.39AD 18.68 1.81 12.34 6.43AE 17.38 1.66 8.19 12.21
0.1
1
10
O P Q R S
AB
AC
AD
AE
Test
Ni/Mg Co/Mn
Ni/M
g or
Co/
Mn
mol
ar ra
tio
in d
ry p
reci
pita
te
Figure 5.44. Ni/Mg and Co/Mn molar ratio in dry precipitates-S and AB-AE
5-56
These trends suggest a relationship between initial manganese
concentration used in tests O-R and AB-AE and time of transformation
plotted in Figures 5.45 & 5.46, respectively, for group 2 (O-R) and group 4
(AB-AE). Again, the XRD peak heights were used to calculate the change in
%MgO as a function of time in both figures, representing the rate of
transformation of MgO to Mg(OH)2.
There was an increase in transformation time for precipitates O, P, Q
and R as expected, due to the increase in initial manganese concentration
which lead to increasing %Mn of the precipitate in the same order:
O<P<Q<R (Table 5.11). For precipitates AB, AC, AD and AE, the Ni/Mg
molar ratio increases from 0.2 to ~1.0 but the Co/Mn molar ratio decreases
from 0.5 to ~0.1 (Figure 5.44). The precipitate AE has the slowest
transformation after 1 day, indicated by the highest MgO/Mg(OH)2 ratio. This
may be related to the high Mn content (Table 5.11). However, after 4 days
the precipitate AC has the slowest transformation, while the transformation is
complete in all cases after 8 days.
In general, the transformation of MgO to Mg(OH)2 is much faster in
the precipitates represented in Figure 5.46 (≤ 8 days), compared to those in
Figure 5.45 (>25 days). The precipitates AD and AE somehow seemed to
take less time, which may be a result of the increase in Ni/Mg molar ratio as
well as the high Mn content (Figure 5.44). These trends also suggest that
there was an ‘ideal’ quantity of manganese to slow down the transfer time. In
this case it was around 4.4% when there was about 19% Ni and 1.8% Co
5-57
with ~20% moisture. This percentage would probably vary depending on
metal concentrations and moisture content.
0.00.10.20.30.40.50.60.70.80.91.0
0 20 40 60 80 100
MgO
/Mg(
OH
) 2 R
atio
Time (Days)
O (Ni, Co, Mg, Mn) P (Ni, Co, Mg, Mn)
Q (Ni, Co, Mg, Mn) R (Ni, Co, Mg, Mn)
Figure 5.45. Percentage of MgO in precipitates O – S (rough calculation based on peak heights).
0.00.10.20.30.40.50.60.70.80.91.0
0 5 10 15 20
MgO
/Mg(
OH
) 2 R
atio
Time (Days)
AB (Ni, Co, Mg, Mn) AC (Ni, Co, Mg, Mn)
AD (Ni, Co, Mg, Mn) AE (Ni, Co, Mg, Mn)
Figure 5.46. Percentage of MgO in precipitates AB – AE (rough calculation based on peak heights).
5-58
5.8.2 Effect of Ageing on Crystalline Nickel Magnesium Hydroxide
As discussed in previous sections, the crystalline nickel magnesium
hydroxide would form upon precipitation within the pores of MgO, according
to the overall equations shown below:
MgO(s) + NiSO4(a) = NiO(s) + MgSO4(a) (log K = 9.00)
Mg(OH)2(s) + NiSO4(a) = Ni(OH)2(s) + MgSO4(a) (log K = 3.92)
Ni(OH)2 + MgO = Mg(OH)2 + NiO (log K = 5.07)
However, as this structure has been observed to appear and/or
improve in crystallinity over time it is not known if this structure could form
after precipitation. In order to test the difference, a mixed nickel-magnesium
hydroxide precipitate and a mixture of nickel hydroxide and magnesium
hydroxide were analysed by XRD after approximately a year
(Figures 5.47 & 5.48). The plots were surprisingly similar. Peaks have
increased in intensity as the metal hydroxides have improved in crystal order.
However, no peak shifting or change in peak intensity ratios has occurred.
The lack of change in peak patterns proves that the nickel-magnesium
hydroxide is formed during initial precipitation, rather than after ageing, but
the crystallinity improves during ageing.
5-59
10 20 30 40 50 60 70
2 Theta
PPT 1 year Ni(OH)2 Mg(OH)2 Ni,Mg(OH)2
Figure 5.47. XRD scans of a mixed Ni-Mg(OH)2 precipitate immediately after precipitation and after ageing for approximately a year.
10 20 30 40 50 60 70
2 Theta
Phys Mix 1 year Ni(OH)2 Mg(OH)2 Ni,Mg(OH)2
Figure 5.48. XRD scans of a mixture of Ni(OH)2 and Mg(OH)2 after precipitation and after ageing for approximately a year.
5-60
As hydrotalcite is a similar structure, the formation of this type of
compound is also unlikely after precipitation. This structure would form upon
precipitation or upon oxidation of one of the metals in the brucite-like
structure. Like the Ni-Mg hydroxide, the structural reordering over time would
result in improved crystallinity and observation by XRD.
5.8.3 Effect of Anions on Oven Ageing of Mixed Binary Hydroxides
Oven ageing of single and binary hydroxides of Ni(II), Co(II) and Mn(II)
was conducted in the absence or presence of sulphate, carbonate or
chloride, as it was an easy method to determine when hydrotalcite-like
structures exist or appear in a precipitate. Precipitates were left in solution in
sealed bottles in the oven at 50°C. This caused a dramatic improvement in
crystallinity as shown by the XRD analysis in Figure 5.49. Anions (SO42-,
CO32-, Cl-) were also introduced separately to determine the individual
influence (Figures 5.50, 5.51 & 5.52).
After 12 weeks of ageing there was a significant improvement in
crystallinity (Figure 5.49 vs. XRD traces in figures in section 5.8.1), and there
seemed to be hydrotalcite present with the Mn/Co and the Ni/Mn precipitates
(Figure 5.49). A manganese oxyhydroxide was observed in the Mn
precipitate. The introduction of sulphate ions (from metal sulphates, 2 g/L),
calcium carbonate (5 g/L) and sodium chloride (15 g/L) to the ageing solution
improved hydrotalcite formation (Figure 5.50).
5-61
The increase in hydrotalcite-type structure formation was due to the
introduction of anions which would balance the charge of the trivalent metal
involved in the structure (see Figure 5.42). Hydrotalcite-like structures were
observed in all plots. The manganese structures are more crystalline,
indicating high oxidation. It can be concluded that anions other than
hydroxide are required for the formation of hydrotalcite structures.
10 20 30 40 50 60 70 80
2 Theta
1. Mn/Co 2. Co 3. Mn 4. Ni/Co5. Ni/Mn Ni(OH)2 Mg(OH)2 beta-Co(OH)2MnOO.15H2O Hydrotalcite (Mg,Al) Desautelsite (Mg,Mn) Comblainite (Ni,Co)
Figure 5.49. XRD scans after oven ageing - batch 1, 12 weeks of ageing.
5-62
10 20 30 40 50 60 70 80
2 Theta
1. Mn/Co 2. Co 3. Mn 4. Ni/Co5. Ni/Mn Ni(OH)2 Mg(OH)2 beta-Co(OH)2MnOO.15H2O Hydrotalcite (Mg,Al) Desautelsite (Mg,Mn) Comblainite (Ni,Co)
Figure 5.50. XRD scans after oven ageing, batch 2, introduction of anions (SO4
2-, CO32-, Cl-), 12 weeks ageing.
10 20 30 40 50 60 70
2 Theta
Ni Mn Ni Co Ni Fe Ni AlNi Cr Ni(OH)2 Mg(OH)2 beta-Co(OH)2MnOO.15H2O Hydrotalcite (Mg,Al) Desautelsite (Mg,Mn) Comblainite (Ni,Co)
Figure 5.51. XRD scans after oven ageing, batch 3, 12 weeks ageing with 2 g/L SO4
2- (from metal sulphate)
5-63
10 20 30 40 50 60 70
2 Theta
Ni Mn Ni Co Ni Fe Ni AlNi Cr Ni(OH)2 Mg(OH)2 beta-Co(OH)2MnOO.15H2O Hydrotalcite (Mg,Al) Desautelsite (Mg,Mn) Comblainite (Ni,Co)Magnesium Chlorate
Figure 5.52. XRD scans after oven ageing, batch 3, 12 weeks of ageing with 5 g/L CaCO3.
10 20 30 40 50 60 70
2 Theta
Ni Mn Ni Co Ni Fe Ni AlNi Cr Ni(OH)2 Mg(OH)2 beta-Co(OH)2MnOO.15H2O Hydrotalcite (Mg,Al) Desautelsite (Mg,Mn) Comblainite (Ni,Co)Magnesium Chlorate Al(OH)3
Figure 5.53. XRD scans after oven ageing, batch 3, 12 weeks of ageing with 15 g/L NaCl.
5-64
There was little difference between the XRD plots for sulphate,
carbonate and chloride precipitates (Figures 5.51, 5.52 & 5.53). Hydrotalcite
structures were formed in the presence of Mn, Co, Fe and Al. The cobalt
hydrotalcite is always less crystalline and of lower intensity, as not all of the
metal would oxidise. From these tests it is clear that hydrotalcite-like
structures could be forming in the Ravensthorpe MHP with nickel or
magnesium, and manganese, cobalt, iron or aluminium with sulphate,
carbonate or chloride. Approximate concentrations of sulphate, calcium
carbonate and sodium chloride in solution prior to precipitation at the
Ravensthorpe plant are 180 g/L, 3 g/L and 33 g/L, respectively. Washing the
precipitate thoroughly with desalinated, demineralised water to remove these
anions would be crucial to avoid the formation of these stable, slow leaching
hydrotalcite compounds.
5.8.4 Effect of Precipitation at Elevated Temperatures
Precipitates were produced at 80°C from 20 L of solution with low
metal concentrations (0.15 g/L Ni) in an effort to improve the crystallinity, and
to produce enough sample for kinetic leach tests. The improvement in
crystallinity would also make phases more visible by XRD.
Comparison between the XRD patterns of the precipitates
(Figure 5.54) indicated that the metal incorporation influences crystallinity to
various degrees depending on the metal introduced. Manganese, aluminium
and iron seem to have a significant effect, while cobalt and zinc a lesser
5-65
effect. Calcium, silicon, chromium and copper seem to have no influence.
The metal substitution for nickel is likely to alter the size of the crystal unit
cell. The diffraction patterns of the precipitate formed in the presence of Mn,
Al, Fe and Cu contain extra peaks which do not match the sharp metal
hydroxide peaks. Either the metal ions are forming the separate structures
when the brucite-like unit cell is filled, or they have a preference to form due
to stability or atomic radius (Table 5.7). The XRD plots from Figure 5.54 were
separated into groups for further discussion (Figures 5.55 - 5.58).
10 20 30 40 50 60 70
2 Theta
Ni Ni Mn Ni Co Ni Fe Ni Al Ni Cr Ni CaNi Si Ni Cu Ni Zn Ni Co Mn Ni Co Fe
Figure 5.54. XRD scans of 12 precipitates, batch 4.
5-66
10 20 30 40 50 60 70
2 Theta
Ni Ni,Mg(OH)2 Ni(OH)2 Mg(OH)2 Nickel hydroxide hydrate
Figure 5.55. XRD scan of Ni precipitate.
In all plots, due to similar peak positions, it was difficult to distinguish
between nickel and magnesium hydroxides. In the nickel rich precipitate
(Figure 5.55), the peak at about 33° can only be due to nickel hydroxide or a
mixed nickel-magnesium hydroxide. It can therefore be safely stated that no
pure magnesium hydroxide was present in significant concentrations in the
precipitate (i.e. <3 %). Nickel hydroxide and nickel hydroxide hydrate also
seemed to be present.
5-67
10 20 30 40 50 60 70
2 Theta
Ni Mn Ni Co Ni,Mg(OH)2 Mn(OH)2 Hydrotalcite Co(OH)2
Figure 5.56. XRD scans of Ni/Co and Ni/Mn precipitates.
In Figure 5.56 manganese and cobalt appear to be forming a mixed
metal hydroxide with the nickel and a hydrotalcite-type structure. The
presence of a hydrotalcite-type structure indicates a portion of the metal has
oxidised. With both precipitates, the Ni-Mg(OH)2 peak at 33° was lower in
height compared to the peak at 38° and broader than the simple nickel
hydroxide precipitate (Figure 5.55). This indicates that the presence of cobalt
and manganese is likely to inhibit the formation of Ni-Mg(OH)2.
5-68
10 20 30 40 50 60 70
2 Theta
Ni Fe Ni Al Ni,Mg(OH)2 Hydrotalcite
Figure 5.57. XRD scans of Ni/Fe and Ni/Al precipitates.
A hydrotalcite-type structure and nickel hydroxide seem to be the only
species present in the diffraction pattern for the iron and aluminium rich
precipitates (Figure 5.57). All of the iron or aluminium and a significant
portion of nickel and magnesium would be bonded together in the
hydrotalcite-like structure, while the remaining metal would form metal
hydroxides. These forms of hydrotalcite appear to have a higher degree of
crystallinity than other precipitates, and are known to cause significant
problems upon leaching. As aluminium(III) is virtually insoluble in the SAC
solution and cannot be reduced like cobalt(III) and manganese(III), the nickel
is probably locked in the structure. Iron(III) can be reduced, which would
release any bonded cobalt or nickel. Clearly, calcium, chromium, silicon and
zinc form a crystalline metal hydroxide with nickel (Figure 5.58). Crystalline
5-69
Ni,Mg hydroxide was also present in the copper rich precipitate along with
hydrated copper sulphate (Figure 5.59).
10 20 30 40 50 60 70
2 Theta
Ni Ca Ni Cr Ni Si Ni Zn Ni,Mg(OH)2
Figure 5.58. XRD scans of Ni/Ca, Ni/Cr, Ni/Si and Ni/Zn precipitates.
10 20 30 40 50 60 70
2 Theta
Ni Cu Ni,Mg(OH)2 Cu4SO4(OH)6.H2O
Figure 5.59. XRD scans of Ni/Cu precipitate.
5-70
An attempt was made in this study to produce hydrotalcite-type
structures with nickel and another metal, without magnesium, at conditions
similar to the Ravensthorpe precipitation process. Solutions of compositions
similar to previous tests were prepared and hydroxides were precipitated by
raising the pH to 8.3 with NaOH. The binary precipitates Ni/Mn, Ni/Co, Ni/Al,
Ni/Fe and Ni/Cr were produced.
As all the XRD patterns were similar in appearance, only the Ni/Mn
precipitate XRD is shown in Figure 5.60. Significant hydrotalcite peaks were
observed when manganese and aluminium were present, while broad peaks
were observed in all other diffraction patterns. Although the procedure was
similar, the precipitates produced using NaOH were less crystalline
compared to those produced with MgO. It is clear that the precipitation
mechanism is different. Precipitation occurs within the MgO particles due to
their porous nature. This type of precipitation would result in larger crystals
and sharper XRD peaks.
5-71
10 20 30 40 50 60 70 80
2 Theta
Ni Mn Ni,Mg(OH)2 Mn(OH)2 Hydrotalcite Hausmannite, Mn3O4 manganese oxide hydrate
Figure 5.60. XRD scan of a Ni/Mn precipitate.
5.9 Scanning Electron Microscopy
5.9.1 Synthetic MHP
Scanning Electron Microscopy on cross sections of the precipitates
was performed over the 12 weeks of ageing to monitor the metal composition
throughout the precipitate. Two different mechanisms were proposed to
occur upon precipitation of metal hydroxides with MgO (Fittock, 2007), as
described previously. The first was the complete dissolution of MgO and
subsequent nucleation of metal hydroxide, the second was the precipitation
of the metal hydroxides on the MgO particles. These two mechanisms may
be represented by the equations:
5-72
Mechanism 1 (metal evenly dispersed)
MgO + H2O = Mg2+ + 2OH- (complete dissolution)
Mn+ + nOH- = M(OH)n (subsequent nucleation)
Mechanism 2 (Mg rich core)
MgO + H2O = Mg(OH)2 (hydration of MgO)
Mn+ + n/2Mg(OH)2 = M(OH)n + n/2Mg2+ (hydroxide of Mn+ precipitation on
magnesia surface)
It was also not known whether metals precipitated separately, forming
small single metal hydroxide layers, or if mixed hydroxides were present from
the beginning. If complete dissolution and subsequent nucleation occurred,
elemental mapping with SEM would show metals evenly dispersed
throughout the precipitate (mechanism 1). Alternatively, elemental mapping
would reveal a magnesium rich core if the hydroxides were precipitating upon
the magnesia (mechanism 2).
The SEM pictures of the particles shown in Figures 5.61, 5.62 & 5.63
were representative of the whole sample. Elemental mapping shown in
Figure 5.64 revealed that the particles consist of a mixed metal hydroxide
core with an outer nickel and cobalt hydroxide layer. This was shown by the
brighter ring around the particle as nickel and cobalt are of higher atomic
number (electron density) than magnesium.
5-73
These scans demonstrated that the metals were present as mixed
hydroxides rather than in separate phases, as there was no ‘banding’
observed. It can be seen from Figures 5.62 & 5.63 that a decrease in the
size of the outer layer has occurred by week 3 and week 12. This would
occur as the nickel and cobalt move in solution to form more stable phases.
By week 12 (Figure 5.63) the particles had grown in size and cracking had
occurred upon drying. This was attributable to the adsorption of water into
the metal hydroxide structure over time. Assuming that the precipitate
particles only consist of MgO and Mg(OH)2, the calculation based on density
and molar mass predicts the particle to grow in diameter by approximately a
third.
Figure 5.61. Back scattered electron image of precipitate P – week 1.
5-74
Figure 5.62. Back scattered electron image of precipitate P – week 3.
Figure 5.63. Back scattered electron image of precipitate P – week 12.
5-75
Figure 5.64. Elemental mapping of precipitate P – week 1.
5.10 Summary
• In search for an acid neutralising agent for the precipitation of metal
hydroxides, surface area would be the most important quality. The
feed and calcination temperature of MgCO3 for the production of MgO
have a significant effect on the surface area and porosity of MgO,
which is thought to be due to the expulsion of carbon dioxide during
calcination (Ardizzone et al., 1997; Hartman et al., 1993; Guan et al.,
2006; Tececo, 2009).
• BHP Billiton selected QMag’s Emag 45 as its neutralising agent at the
Ravensthorpe Plant based on significant precipitation testing. It was
used in the preparation of synthetic MHP’s of different composition in
the present study. Emag 45 typically contains 95% MgO, 3% CaO and
5-76
1% SiO2, and minor quantities of Fe2O3 and Mn3O4. Calcium would
contribute towards neutralisation and would hydrolyse, while silicon
would remain as insoluble SiO2. Thus, calcium and silicon were
incorporated into the MHP along with other minor elements from
QMag’s Emag 45. The effect of calcium and silicon incorporation in
minor quantities on nickel and cobalt recoveries upon dissolution in
ammonia will be discussed in Chapter 6.
• The high concentration of magnesium in the synthetic precipitate
(between 2.6 and 12.4%), using magnesia as the precipitant, was
unavoidable on a small scale laboratory tests (6 L) at ambient
conditions. Precipitation of metal hydroxides within the pores of the
magnesia particles resulted in the formation of stable-slow leaching
(Ni,Mg)(OH)2.
• SEM images on synthetic precipitates over the four hour precipitation
period revealed two mechanisms were occurring; dissolution-
nucleation-agglomeration and precipitation within the pores of MgO.
Metals were distributed evenly throughout the particles. Due to these
mechanisms the size distribution was relatively large and did not
change significantly over the period. Towards the end of the
precipitation period, probably when the pores were filled, metals
precipitated on the outside of the Mg rich particles giving a higher
overall nickel and cobalt content. Synthetic MHP’s grew in size over
time (12 weeks) as MgO hydrated and smaller particles agglomerated.
5-77
• Interaction between metal ions influenced the precipitation/pH
relationship based on KSP. Thus, precipitation using MgO raised the
pH to between 8.0 and 8.3 depending on the solution composition and
temperature, where over 90% of nickel and cobalt precipitated with
less than 40% of the manganese.
• Using a solution of composition similar to that of the Ravensthorpe
process at 50°C, 20-40% of manganese was precipitated between a
pH of 8.0 and 8.3. Due to this effect, manganese incorporation into the
precipitate seemed to reach a maximum; at 25°C it was around 5.5%.
• Inclusion of nickel and cobalt into the precipitate was improved when
manganese(II), iron(III) or chromium(VI) were present in solution.
Manganese probably formed a hydrotalcite-type structure with nickel
and magnesium as it would oxidise readily to Mn(III). Precipitates also
changed colour from green to brown (oxidation of manganese and
cobalt) and became visibly drier without losing moisture (intercalation
of water and hydration of MgO).
• Aluminium(III), silicon(IV) and copper(II) had an adverse effect on the
inclusion of nickel and cobalt into the precipitate . This may be due to
large pKSP causing the surface precipitation of these hydroxide and
surface blockage. Atomic radius correlated with these findings.
• All oxidation of Mn(II) and Co(II) in the precipitate seems to have
occurred in the first day, and according to the extent of oxidation
results (assuming complete oxidation of manganese) less than 60% of
cobalt oxidised over the 12 week period.
5-78
• Hydrotalcite has the brucite structure (Mg(OH)2) with the incorporation
of trivalent metals. The formation of a hydrotalcite-type structure upon
the oxidation of Mn(II) to Mn(III) is likely. This results in the splitting of
layers when anions (i.e. CO32-, SO4
2-, Cl-) and water are incorporated
into the structure to balance the charge. This type of structure is
probably more stable than brucite which would inhibit the incorporation
of Mg into the existing metal hydroxide crystal lattice.
• Both drying the precipitate and the incorporation of manganese
minimised the formation of the nickel-magnesium hydroxide. The
transformation of MgO to Mg(OH)2 and consequently Ni(OH)2 to NiO
according to the reaction MgO + Ni(OH)2 = Mg(OH)2 + NiO (log K =
5.07) was also significantly slower when the precipitate contained
manganese.
• Hydrotalcite-like structures were usually not present in XRD traces of
synthetic precipitates, suggesting they either did not exist in large
quantities or were X-ray amorphous. As the precipitates were
relatively fresh and the formation of a hydrotalcite-like structure would
absorb water and anions into the crystal lattice, the latter was very
possible.
• Hydrotalcite-type structures were observed by XRD only after weeks
of ageing or when the precipitate was produced at elevated
temperatures (80°C) from solutions with low metal concentrations
(0.15 g/L Ni) to improve the crystallinity.
5-79
• Manganese slowed the hydration of magnesia, and hence limited the
formation of nickel-magnesium hydroxide. This produced higher nickel
and cobalt recoveries upon leaching (as described in Chapter 6).
• Drying the sample also slowed the hydration of magnesia hence
inhibited nickel-magnesium hydroxide formation, and slowed or
inhibited the formation of hydrotalcite-type structures.
6-1
6 LEACHING OF SYNTHETIC MIXED HYDROXIDE
PRECIPITATES
6.1 Introduction and Experimental
In prior investigations, only the standard predictor leach tests (SPT)
developed at the Yabulu refinery (Hultgren, 2003(a)), were used in order to
predict the leaching of metals from the Ravensthorpe MHP. However, the
leaching behaviour of the mixed nickel-magnesium hydroxide and various
hydrotalcite structures which are present in the mixed hydroxide precipitate
remains unknown. It is important to understand the effect of such
compounds and the role of other metal ions in the mixed hydroxide
precipitate on nickel and cobalt leaching in ammoniacal ammonium
carbonate (SAC) solutions used in the Yabulu refinery.
The oxidised hydroxide of manganese or cobalt have been used as a
standard to investigate the success of reducing agents in Chapter 4. As
noted in chapter 3 the predictor leach tests were modified to suit the small
sample size of the synthetic precipitates of different metal ion composition
obtained in this study. The modified predictor leach tests were conducted
numerous times on the synthetic precipitates over the 12 weeks of ageing.
The modified standard (MSPT) and reductive (MRPT) predictor tests were
conducted in triplicate whereby samples of 0.2 g (Ni + Co) were leached at
ambient conditions in 25 mL of SAC solution in centrifuge tubes on a mill
drive rotating at 100 rpm for 45 minutes. The modified reductive predictor
test also included 0.2 g of hydroxylamine sulphate as the reductant.
6-2
Difference in leaching recoveries between the two tests (MSPT & MRPT)
would be due to the oxidation of cobalt, manganese and iron that would
occur during precipitate ageing, as noted in Chapter 4.
Modified reductive predictor leach tests (MRPT) were only conducted
on synthetic precipitates after 6, 9 and 12 weeks of ageing, as results before
6 weeks would not provide any further information. After 12 weeks of ageing,
the Yabulu standard (SPT), reductive predictor (RPT) and reductive soak
predictor (RSPT) leach tests were conducted in triplicate. The standard
predictor test entailed a 45 minute leach of 4 g of Ni+Co (dry basis) in
500 mL of SAC at 30°C. The reductive predictor test was the same except
nitrogen was sparged into the leach vessel and a calculated quantity of
hydroxylamine sulphate was added. The reductive soak predictor test
involved combining the residue from the reductive predictor test with 250 mL
of synthetic ammonia solution (SAC) at 50°C for 72 hours.
Kinetic studies conducted on Ni(II)/Mg(II) hydroxides and Co(II)/(III)-
hydroxides are also described in this chapter. Effect of different variables
such as solid/liquid ratio, temperature, particle size, agitation speed was
examined and the applicability of the shrinking sphere/core kinetic models
was tested. After a significant testing program on laboratory based
precipitates described in this chapter, pilot plant samples and commercial
precipitates were aged, analysed and leached, as described in Chapter 7.
6-3
6.2 Effect of Ageing on Leaching
Typical leaching results from modified standard predictor leach tests for
precipitates A-D aged over 12 weeks are shown in Figure 6.1. Similar figures
were produced for all precipitates with leach tests for both nickel and cobalt.
The general trend in Figure 6.1 is that the nickel extraction decreases with
ageing, most likely due to the change in structure and/or crystallinity of the
precipitates. However, the presence of manganese appears to have a
beneficial effect on nickel leaching, as the nickel extraction from the
precipitates B, C and D was higher (Figure 6.1).
Figure 6.1. Nickel leaching results in Modified Standard Predictor Test in SAC over 12 weeks– precipitates A – D.
6-4
Figure 6.2. Nickel leaching results in Modified Reductive Predictor Test in SAC with hydroxylamine sulphate over 12 weeks– precipitates A – D.
Results from the modified reductive predictor leach tests are shown in
Figure 6.2. The presence of NH2OH.H2SO4/N2 in the lixiviant also has a
beneficial effect, especially in the presence of manganese(II) added during
precipitation. This is evident from the higher extraction of over 90% Ni from
precipitates B, C and D in Figure 6.2. The leaching recoveries of cobalt and
manganese also diminished over time with the modified reductive predictor
tests, especially in the presence of iron. This indicates that the improvement
in crystallinty due to restructuring over time has a detrimental effect on
leaching. These observations highlight the need to compare and contrast the
leach results in the three types of tests SPT, RPT and RSPT in order to
rationalise the role of the presence of manganese and other metal ions on
the leaching of nickel and cobalt. Thus, after 12 weeks of ageing, the Yabulu
standard, reductive and soak predictor leach tests were conducted in
triplicate. As the information in Figures 6.1 and 6.2 can be conveyed in
6-5
tabular form the figures were omitted and the tabulated results are discussed
in the next section.
6.3 Effect of Metal Ion Composition on Leaching
6.3.1 General Comparison
The results from different tests A-H (Group 1), I-N (Group 2), O-S
(Group 3), T-AA (Group 4) and AA-AE (Group 5) are listed in Tables 6.1. The
confidence levels of the leach results and the metal ion assays of the
residues produced in soak tests are listed in Tables 6.2 and 6.3, respectively.
In most cases the standard and reductive predictor tests exhibited poor
recoveries, while results with the soak tests were above 94% Ni and 84%
Co, excluding sample V (Tables 6.1, 6.3). Although the oxidation of cobalt
and manganese could be a problem, it was overcome by the reductive leach.
The stable slow leaching phases seemed to be far more detrimental to nickel
recovery. Crystalline nickel/magnesium hydroxide detected in XRD traces
described previously was a likely cause. Some important points to note are
described under different headings.
6-6
Table 6.1. Summary of predictor leach test results – standard, reductive, soak
Precipitate Metals % Solids Ni Co Ni Co Ni CoA Ni,Co,Mg 52 63 47 60 68 99 99B Ni,Co,Mg,Mn 53 85 82 90 89 100 99C Ni,Co,Mg,Mn,Al 52 79 75 95 95 99 98D Ni,Co,Mg,Mn,Al,Fe 52 80 77 93 93 99 97E Ni,Co,Mg,Mn,Al,Fe,Cr 48 76 72 91 89 97 96F Ni,Co,Mg,Mn,Al,Fe,Cr,Cu 50 74 66 91 87 97 96G Ni,Co,Mg,Mn,Al,Fe,Cr,Cu,Zn 52 72 65 89 86 97 96H Ni,Co,Mg,Mn,Al,Fe,Cr,Cu,Zn,Si 47 71 64 84 79 97 96I Ni,Co,Mg,Al 46 43 31 58 67 98 97J Ni,Co,Mg,Fe 49 39 28 59 72 99 99K Ni,Co,Mg,Cr 50 47 35 54 62 98 98L Ni,Co,Mg,Cu 51 54 37 59 67 100 100M Ni,Co,Mg,Zn 50 60 51 61 74 100 100N Ni,Co,Mg,Si 45 54 45 52 64 100 100O Ni, Co, Mg, Mn 54 75 68 80 78 99 99P Ni, Co, Mg, Mn 56 82 75 86 81 99 99Q Ni, Co, Mg, Mn 53 74 64 79 70 99 99R Ni, Co, Mg, Mn 53 81 63 86 75 100 98S Ni, Co, Mg, Mn 51 81 60 91 78 99 97T Ni, Mg 57 67 - 53 - 98 -U Ni, Co, Mg 57 80 74 69 61 99 99V Ni, Co, Mg, Al 47 40 18 42 28 87 61W Ni, Co, Mg, Fe 45 39 33 55 59 98 84X Ni, Co, Mg, Cr 43 58 39 49 36 96 88Y Ni, Co, Mg, Cu 62 88 59 94 90 98 98Z Ni, Co, Mg, Zn 41 80 70 76 73 100 100
AA Ni, Co, Mg, Si 31 46 21 40 34 94 89AB Ni, Co, Mg, Mn 34 70 60 86 81 99 95AC Ni, Co, Mg, Mn 18 89 79 99 96 100 97AD Ni, Co, Mg, Mn 20 78 69 96 94 100 96AE Ni, Co, Mg, Mn 19 90 82 99 97 100 91
Metal Recovery After 12 Weeks, %Standard Reductive Soak
6-7
Table 6.2. Confidence intervals of leach results.
Precipitate Metals Ni Co Ni CoA Ni,Co,Mg 2.308 2.321 0.004 0.034B Ni,Co,Mg,Mn 0.885 1.413 0.038 0.046C Ni,Co,Mg,Mn,Al 3.589 5.229 0.027 0.035D Ni,Co,Mg,Mn,Al,Fe 0.598 0.673 0.021 0.037E Ni,Co,Mg,Mn,Al,Fe,Cr 1.162 1.449 1.104 1.544F Ni,Co,Mg,Mn,Al,Fe,Cr,Cu 1.258 1.413 0.39 0.5G Ni,Co,Mg,Mn,Al,Fe,Cr,Cu,Zn 0.412 0.875 0.236 0.501H Ni,Co,Mg,Mn,Al,Fe,Cr,Cu,Zn,Si 1.316 2.364 1.95 2.243I Ni,Co,Mg,Al 3.11 2.294 0.378 0.804J Ni,Co,Mg,Fe 2.724 1.185 1.567 1.278K Ni,Co,Mg,Cr 1.276 0.106 0.969 1.177L Ni,Co,Mg,Cu 1.238 0.761 3.542 3.955M Ni,Co,Mg,Zn 0.675 3.301 0.721 1.163N Ni,Co,Mg,Si 1.029 3.709 1.344 1.371O Ni, Co, Mg, Mn 1.006 0.399 4.449 5.306P Ni, Co, Mg, Mn 2.129 2.352 1.598 2.506Q Ni, Co, Mg, Mn 1.028 0.959 1.525 2.134R Ni, Co, Mg, Mn 2.007 2.770 1.641 2.797S Ni, Co, Mg, Mn 1.052 2.474 1.857 3.165T Ni, Mg 5.862 - 1.923 -U Ni, Co, Mg 1.632 1.284 0.445 0.516V Ni, Co, Mg, Al 1.420 0.934 2.710 1.892W Ni, Co, Mg, Fe 1.304 1.638 1.963 2.082X Ni, Co, Mg, Cr 1.256 2.010 8.780 8.502Y Ni, Co, Mg, Cu 1.514 2.412 1.386 2.428Z Ni, Co, Mg, Zn 0.639 0.938 0.662 0.745
AA Ni, Co, Mg, Si 9.161 12.679 2.536 3.513AB Ni, Co, Mg, Mn 3.553 5.010 5.370 6.955AC Ni, Co, Mg, Mn 2.624 2.373 0.161 0.351AD Ni, Co, Mg, Mn 8.960 13.001 0.964 1.313AE Ni, Co, Mg, Mn 1.876 3.841 0.488 1.353
95 % Confidence IntervalStandard Reductive
6-8
Table 6.3. Soak test – leach residue analysis.
Precipitate Metals Ni Co Mn Mg FeA Ni,Co,Mg 0.50 0.04 0.06 22.1 0.12B Ni,Co,Mg,Mn 0.29 0.06 2.46 21.7 0.13C Ni,Co,Mg,Mn,Al 0.86 0.18 3.40 19.0 0.15D Ni,Co,Mg,Mn,Al,Fe 1.24 0.22 2.81 19.0 0.98E Ni,Co,Mg,Mn,Al,Fe,Cr 1.78 0.24 2.42 20.0 0.62F Ni,Co,Mg,Mn,Al,Fe,Cr,Cu 1.87 0.27 2.78 18.8 0.75G Ni,Co,Mg,Mn,Al,Fe,Cr,Cu,Zn 2.03 0.30 2.93 19.4 0.86H Ni,Co,Mg,Mn,Al,Fe,Cr,Cu,Zn,Si 2.04 0.31 2.93 18.8 0.85I Ni,Co,Mg,Al 0.98 0.18 0.07 16.9 0.13J Ni,Co,Mg,Fe 0.52 0.05 0.06 14.2 0.60K Ni,Co,Mg,Cr 0.85 0.08 0.03 13.6 0.07L Ni,Co,Mg,Cu 0.14 0.01 0.04 15.7 0.13M Ni,Co,Mg,Zn 0.11 0.01 0.04 21.8 0.18N Ni,Co,Mg,Si 0.35 0.04 0.05 23.1 0.16O Ni, Co, Mg, Mn 0.44 0.06 0.81 21.7 0.15P Ni, Co, Mg, Mn 0.15 0.04 1.36 20.7 0.12Q Ni, Co, Mg, Mn 0.19 0.08 2.43 21.1 0.13R Ni, Co, Mg, Mn 0.11 0.10 3.79 20.3 0.13S Ni, Co, Mg, Mn 0.12 0.12 3.48 20.2 0.09T Ni, Mg 1.14 <0.01 0.11 22.2 0.18U Ni, Co, Mg 0.96 0.40 0.05 22.7 0.16V Ni, Co, Mg, Al 3.66 1.33 0.07 16.7 0.14W Ni, Co, Mg, Fe 1.31 0.84 0.09 17.6 11.41X Ni, Co, Mg, Cr 1.67 0.48 0.04 17.2 0.13Y Ni, Co, Mg, Cu 0.42 0.07 0.05 23.2 0.17Z Ni, Co, Mg, Zn 0.11 0.01 0.05 22.5 0.13
AA Ni, Co, Mg, Si 2.32 0.37 0.06 19.5 0.10AB Ni, Co, Mg, Mn 0.07 0.05 1.39 11.6 0.02AC Ni, Co, Mg, Mn 0.04 0.09 6.45 14.9 0.03AD Ni, Co, Mg, Mn 0.13 0.14 7.33 16.7 0.04AE Ni, Co, Mg, Mn 0.02 0.35 18.87 11.6 0.06
Metal Assay, %
6.3.2 Effect of Magnesium, Cobalt and Manganese on Leaching
Table 6.4 summarises the results from 4 selected Tests (T, A, U and
B) in order to compare the composition of the precipitates and leaching
results under different conditions (SPT, RPT and RSPT) in the absence or
presence of cobalt and/or manganese. These results plotted in Figure 6.3
show that the nickel content in the precipitate increases with the increasing
magnesium content. Moreover, the Ni/Mg molar ratio reaches a value close
to unity at higher magnesium contents indicating the existence of mixed
Ni/Mg-hydroxide at higher magnesium contents. As a result the nickel
6-9
leaching in SPT decreases with increasing magnesium content of the
precipitate (Figure 6.4a). The presence of the reducing agent in RPT causes
a further decrease in nickel leaching. However, higher contents of cobalt in
the precipitate do not cause a detrimental effect as shown in Figure 6.4b.
Table 6.4. Effect of manganese and cobalt on leach results from SPT, RPT and RST
Test
Initial composition ( g/L)
Assay of precipitate (%)
Ni (%) Leached under different conditions
Co (%) Leached under different conditions
Ni Co Mn Ni Co Mn Mg SPT RPT RSPT SPT RPT RSPT T 4 23.2 <0.01 0.02 12.7 67 53 98 A 4 0.4 33.2 3.03 0.16 14.9 63 60 99 47 68 99 U 4 1.3 20.2 6.38 0.01 11.7 80 69 99 74 61 99 B 4 0.4 0.4 30.9 2.89 2.43 13.5 85 90 100 82 89 99
20
30
40
11 13 15Mg content (%) in precipitate
Ni c
onte
nt (%
) in
prec
ipita
te
0.7
0.8
0.9
1.0Ni %Ni/Mg molar ratio
Ni/M
g m
olar
ratio
Figure 6.3. Effect of Mg% on Ni/Mg molar ratio and Ni% in precipitate (data from Table 6.4)
6-10
(a)
40
60
80
100
11 13 15Mg content (%) in precipitate
Ni L
each
ed (%
) SPTRPTRSPT
(b)
40
60
80
100
0.01 0.1 1 10Co content (%) in precipitate
Ni L
each
ed (%
) SPTRPTRSPT
(c)
40
60
80
100
0.01 0.1 1 10Mn content (%) in precipitate
Ni L
each
ed (%
)
SPTRPTRSPT
Figure 6.4. Effect of Mg, Co and Mn content in precipitate on Ni leaching in SPT, RPT and RSPT (data from Table 6.4)
6-11
It is of interest to note that the increase in manganese content in the
precipitate at low levels is detrimental for nickel leaching, as shown by
Figure 6.4(c). However, higher contents of manganese in the precipitate
improve the nickel dissolution, even in the absence of a reducing agent. This
is a result of the changes in the structure of the precipitate discussed in
Chapter 5. The soak leach tests give nearly 100% leaching of nickel,
irrespective of the composition of Mg or Mn in the precipitate (RSPT)
(Figures 6.4a-c). The higher nickel leaching in RSPT compared to low values
in RPT suggests the soaking is effective on the mixed Ni-Mg-hydroxide
precipitate.
Table 6.5 considers the effect of increased quantities of Co in the
initial solution and hence in the precipitate, in the absence or presence of
Mn, on Ni and Co leaching in SPT and RPT. The beneficial role of Mn is
further exemplified from these results where Ni and Co leaching in
precipitates AK-AN is much higher compared to those in precipitates AF-AJ.
Table 6.5. Effect of Co in the absence or presence of Mn on Ni and Co leaching in MSPT and MRPT
Test
Initial composition
( g/L)
Assay of precipitate (%)
Ni (%) Leached from precipitate under different
conditions
Co (%) Leached from precipitate under different
conditions Ni Co Mn Ni Co Mn Mg MSPT MRPT MSPT MRPT
AF 4 0.19 30.7 1.29 15.6 51.4 48.0 16.3 72.1 AG 4 0.38 29.2 2.54 14.9 44.5 52.7 21.0 59.1 AH 4 0.77 28.0 6.16 12.3 27.7 53.3 1.90 44.7 AJ 4 1.75 25.2 10.6 9.98 4.10 42.8 -17.8 27.1 AK 4 0.19 2.7 25.9 1.17 5.43 11.7 95.1 98.5 77.3 96.9 AL 4 0.38 2.7 26.9 2.71 4.94 10.7 92.4 97.7 83.1 95.2 AM 4 0.77 2.7 26.4 5.25 3.67 9.97 94.4 96.7 89.8 93.9 AN 4 1.75 2.7 24.0 11.3 2.31 8.67 98.0 97.4 95.4 95.2
6-12
Table 6.6. Modified standard and reductive predictor leach test results - 95 % confidence interval, %.
Precipitate Ni Co Ni CoNi, 1% Co 3.0 2.6 0.7 1.9Ni, 2% Co 2.6 4.2 2.9 3.3Ni, 5% Co - - - -
Ni, 10% Co 2.1 3.1 1.6 2.1Ni, Mn, 1% Co 0.9 1.3 0.1 0.2Ni, Mn, 2% Co 0.5 0.9 0.4 0.3Ni, Mn, 5% Co 2.1 5.4 1.0 2.3
Ni, Mn, 10% Co 0.3 0.3 1.5 1.7
SPT RPT
Important points to note in Table 6.5 are listed below:
(i) The negative value for Co% in the precipitate AJ in Table 6.5 indicates
that the solubility limit for cobalt has been reached and precipitation
has occurred. A 95% confidence interval of 3.1 (Table 6.6) confirms
the number is true.
(ii) In the absence of manganese, the increase in cobalt concentration in
the initial liquor increases the cobalt content of the precipitate but
decreases the magnesium and nickel content. This also lowers the
nickel leaching in MSPT (Table 6.5) indicating the effect of oxidation of
cobalt. The nickel dissolution from precipitates AF-AJ in MRPT is
higher than that from MSPT due to reductive leaching.
(iii) Due to the presence of manganese, the magnesium content in
precipitates AK-AN is lower, compared to that in AF-AJ. In previous
tests, manganese has been proven to limit the formation of the slow
leaching nickel-magnesium hydroxide. Thus, nickel and cobalt
dissolution is higher in MSPT of precipitates AK-AN, even in the
absence of reducing agents. A further increase in cobalt extraction
6-13
occurred in RPT due to reductive leaching of cobalt. Clearly cobalt
appears to be associated with the manganese structure causing low
leaching in the absence of reducing agents.
(iv) Based on differences between the standard (SPT) and reductive
(RPT) predictor test results in Table 6.5, up to 55% of cobalt could
have oxidised in the absence of manganese and only up to 20% in the
presence of manganese, during MHP precipitation. When the two
metals (Mn & Co) were present, manganese oxidised preferentially
due to its lower reduction potential as described in the Eh-pH
diagrams in previous chapters (1.5 vs. 1.92 V in solution or -0.25 vs.
0.42 V as hydroxides). In fact, oxidation of manganese could reduce
cobalt i.e. Mn(II) acts as a reductant for Co(III).
Tables 6.7 (assays) and 6.8 (leach results) show data obtained in
another set of experiments conducted to test the effect of increasing Co in
the absence of Mn (AP-AS) and the effect of Mn, Al or Cr in the absence of
Co (AT-AV). As the number of leach tests was limited by the quantity of
precipitate, only two tests (SPT and RSPT) could be run singularly on each
sample.
6-14
Table 6.7. Effect of increasing Co, Mn, Al and Cr on composition of precipitates.
Test PPT
Initial composition
( g/L)
Assay of precipitate
(%)
Ni Co Mn Al Cr Ni Co Mn Al Cr Mg Ni/Mg
AP MHP1 4 0.2 30.7 1.29 15.6 0.81
AQ MHP2 4 0.4 29.2 2.54 14.9 0.81
AR MHP3 4 1 28.0 6.16 12.3 0.94
AS MHP4 4 1.8 20.2 10.6 9.98 0.84
AT MHP5 4 2.7 25.2 5.19 12.8 0.81
AU MHP6 4 0.8 25.9 8.58 8.84 1.21
AV MHP7 4 1.7 26.9 6.36 12.5 0.89
Table 6.8. Effect of increasing Co, Mn, Al and Cr on leaching of metals
Test PPT
Metal Leached from precipitate under different conditions
Ni% Co% Mn% Al% Cr% Mg%
SPT RSPT SPT RSPT SPT RSPT SPT RSPT SPT RSPT SPT RSPT
AP MHP1 94.8 99.7 94.7 100 5.0 8.2
AQ MHP2 95.1 99.9 94.4 99.8 14.0 19.8
AR MHP3 93.4 99.9 91.1 99.8 15.9 44.0
AS MHP4 88.9 99.9 88.0 99.7 15.4 47.3
AT MHP5 80.7 99.8 -18.1 26.6 -7.6 8.6
AU MHP6 57.1 98.7 29.6 86.7 30.0 58.9
AV MHP7 99.8 100 99.9 99.9 24.7 36.7
The main points of interest in Table 6.8 are listed below:
(i) The reductive soak tests (RSPT) were very effective as revealed by
very high percentages of nickel and cobalt leached (99.7%) from all
precipitates except the aluminium rich sample (precipitate AU). Poor
nickel leaching (57%) was also apparent with the same precipitate
with the standard predictor leach test. Aluminium could be forming a
strong stable phase with nickel. This was most likely a hydrotalcite-
6-15
type structure, as evident from XRD analysis described in Chapter 5
and later.
(ii) If manganese oxidises and forms a similar structure, it does not have
an effect on the recovery upon soaking. However, in the standard
predictor test nickel recovery is around 15% lower (for Ni/Mn
precipitate AT) than the equivalent Ni/Co precipitate (for precipitate
AP). Cleary, manganese oxidises, but does not influence recovery
when reduced. If a hydrotalcite-like structure were forming with
manganese it would be destroyed upon reduction. Trivalent cations
that cannot be reduced would therefore be far more detrimental to
nickel and cobalt recovery than manganese and other equivalents.
(iii) Standard predictor test results for precipitates AP-AS show that the
cobalt content in precipitate (1.3-10.6%) does not have a significant
influence on nickel leaching. There appears to be a decline in
recovery as the metal to solution ratio (g/L) was increased. Also, high
cobalt recoveries (over 89%) proved that if cobalt did oxidise over
time, less than 11% of the metal had oxidised in 6 weeks. This was
assuming the accepted view that cobalt(III) oxide would not leach, as
noted in Chapter 4.
(iv) The best metal recoveries were observed with the chromium(VI) rich
precipitate AV. In this precipitate it was assumed that chromium
remained in its 6+ oxidation state as a metal would need to oxidise in
order to reduce chromium, but only nickel and magnesium were
present. However, in Ravensthorpe MHP this would occur with
6-16
divalent manganese, iron and cobalt, represented by M2+ in the
following equation:
)()(3
)(3
)(2)(2
)(2
4 8343 aqaqaqlaqaq OHMCrOHMCrO −+++− ++→++
Previous results and discussion has shown (see Figures 6.3 and 6.4a)
that nickel and magnesium form a slow leaching mixed hydroxide
(Ni,Mg(OH)2), which inevitably decreases nickel dissolution, especially with
short term leaching tests. Chromium(VI) has somehow lowered this effect by
interacting with the metal ions. An alternative structure like nickel chromate
(NiCrO4) was a possibility, however there was no sign of it in the XRD traces
as described later (Figure 6.14, precipitate 7).
6.3.3 Nickel-Cobalt Correlation
The relative effects of different metal ions in various groups can be
examined by comparing the leaching efficiency (%) of Ni and Co and the
molar ratios of Ni/Co leached as shown in Figures 6.5a-c. The molar ratio of
Ni/Co leached under standard and reductive conditions is close to unity in
many precipitates, as shown in Figure 6.5c, indicating the coexistence of
these metal ions in the solid phase. Precipitates in Group 1 (B-H), Group 3
(O-S) and Group 5 (AB-AE) show higher leaching of Ni (Figure 6.5a) and Co
(Figure 6.3b) due to the presence of Mn in the precipitate. The X-Ray
Diffraction studies showed that manganese inhibits formation or
crystallisation of the mixed Ni-Mg-hydroxide (Chapter 5). This inhibition could
be linked to the higher metal dissolution. The most likely explanation for this
6-17
inhibition is the oxidation of manganese causing the brucite to transform to a
hydrotalcite structure. If this were to occur, the structure must be X-ray
amorphous and also more leachable under reducing conditions than the
nickel bearing brucite.
6-18
(a)
0
10
20
30
40
50
60
70
80
90
100
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
AA AB AC AD AE
Test
% N
i lea
ched
Ni(SPT)Ni(RPT)
(b)
0102030405060708090
100
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
AA
AB
AC
AD AE
Test
% C
o le
ache
d
Co(SPT)
Co(RPT)
(c)
0.0
0.5
1.0
1.5
2.0
2.5
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
AA AB AC AD AE
Test
Lea
ched
mol
ar ra
tio
Ni/Co ratio (SPT)Ni/Co ratio (RPT)
Figure 6.5. Comparison of Ni and Co leach results (% and molar ratio) in SPT and RPT.
6-19
The difference in Ni/Co leaching ratios in Figure 6.5c and the effect of
reduction and soaking is further examined in Figures 6.6a, b and c which
summarise the leach results as plots of cobalt extraction (%) as a function of
nickel extraction (%) for the leaching under different conditions (SPT, RPT
and RST). If both nickel and cobalt leaches in the same manner the data
points should fit into a line of slope ~1. In Figure 6.5a, corresponding to
standard predictor tests in the absence of reductant, the cobalt extraction is
much lower than that expected from the line of slope ~1 indicating the
existence of Co(III) in the precipitate as a result of the oxidation of cobalt(II)
during MHP precipitation.
In Figure 6.6b corresponding to RPT the beneficial effect of reductive
leaching is evident in Groups 1, 3 and 5. This is a result of the presence of
Mn in the precipitate. This shows that the presence of a reducing agent
facilitates the Ni-Co leaching by reductive leaching of Mn in the precipitate.
Thus, Co-Ni correlation in Figure 6.6b is much more close to the line of unit
slope, but generally Co extraction is lower than Ni.
The leaching is further improved to close to 100% by soaking in RST
as evident from Figure 6.6c, except in the case of precipitates in Group 4.
The outliers of Group 4 in Figure 6.6c, corresponding to lower Ni-Co
leaching, are those precipitates which contained Al, Fe, Cr and Si (see
Table 6.1). A further discussion of the effect of these metal ions is presented
in the next section.
6-20
(a) SPT
0
20
40
60
80
100
0 20 40 60 80 100Ni Leached ( %)
Co
Leac
hed
(%)
Group 1Group 2Group 3Group 4Group 5
(b) RPT
0
20
40
60
80
100
0 20 40 60 80 100Ni Leached ( %)
Co
Leac
hed
(%)
Group 1Group 2Group 3Group 4Group 5
(c) RST
0
20
40
60
80
100
0 20 40 60 80 100Ni Leached ( %)
Co
Leac
hed
(%)
Group 1Group 2Group 3Group 4Group 5
Figure 6.6. Ni-Co Correlations based on leaching results of precipitates A-AE
in STT, RPT and RST
6-21
6.3.4 Effect of Al, Fe, Cr(VI), Zn, Cu and Si in the Absence of Mn.
As noted previously, reductive leaching was effective when
manganese was present in the precipitate. Figure 6.7 compares the effect of
different metal ions (Group 4), in the absence of manganese, on nickel
leaching. The molar ratio of Ni/Mg in the precipitate in Figure 6.7a changes in
the order Cu(II) < Zn(II) < no metal ion < Cr(VI) ~ Fe(III) > Si(IV) >Al(III). The
decreasing order of nickel extraction in Figure 6.7b follows the opposite
order: Cu(II) > Zn(II) > no metal ion > Cr(VI) > Fe(III) ~ Si(IV) ~ Al(III). This
indicates two effects: (i) the beneficial effect of Cu(II) and Zn(II), and (ii) the
detrimental effect of other ions. This is consistent with Figure 6.3 and 6.4a
which showed that the nickel extraction decreases when the Ni/Mg ratio in
the precipitate increases. Thus, the higher extraction of nickel in the
presence of Cu(II) and Zn(II), in the order: Cu(II) > Zn(II) > no metal ion
(Figure 6.7b) can be related to the lower Ni/Mg molar ratio in the precipitates
containing Cu(II) and Zn(II) (Figure 6.7a). The low extraction of nickel in the
presence of Si(IV) may also be due to the formation of a nickel(II)-hydroxy
silicate. The cobalt extraction in the presence of metal ions also decreases in
the order: No metal ion >Al > Si > Fe > Cr (Figure 6.7c). While these metal
ions cause a decrease in the Ni/Mg molar ratio in the same order
(Figure 6.7a), Al, Fe and Cr are known to form slow leaching hydrotalcite
type structures ((MII1-xMIII
x)8(OH)16(An-)8x/n.4H2O), described previously
(Forano et al., 2006), which are stable and slow leaching.
6-22
(a)
(b)
0
20
40
60
80
100
Cu Zn None Cr Fe Si Al
Metal Ion
Nic
kel l
each
ed (%
)
(c)
0
20
40
60
80
100
Cu Zn None Cr Fe Si AlMetal Ion
Cob
alt
leac
hed
(%)
Figure 6.7. Effect of metal ions in SPT of Group 4 precipitates on (a) Ni/Mg molar ratio, (b) nickel leaching, and (c) cobalt leaching.
6-23
As shown in Figure 6.8a and b, the reductive predictor tests with the
Fe rich precipitate (W) exhibited slightly better nickel and cobalt leaching
than in the case of Al and Cr rich precipitates (V and X). This behaviour is
similar to the nickel and cobalt leaching from laterite ores where the
reductive leaching of iron enhances the leaching of both nickel and cobalt
(Senanayake et al., 2011). Similar results obtained from synthetic
precipitates in this study indicate the co-precipitation of nickel and oxidised
cobalt with iron(III) and the beneficial role of iron(II) produced in RPT.
(a)
0
20
40
60
80
100
Cu Zn None Cr Fe Si AlMetal Ion
Nic
kel l
each
ed (%
)
(b)
0
20
40
60
80
100
Cu Zn None Cr Fe Si AlMetal Ion
Cob
alt l
each
ed (%
)
Figure 6.8. Effect of metal ions in RPT of Group 4 precipitates on (a) nickel leaching, and (b) cobalt leaching.
6-24
Figure 6.9 compares the reductive soak leach test results on nickel
and cobalt leaching. Despite the significant increase in the leaching of both
metals (~100%), the presence of Cr, Fe, Al and Si has an unfavourable effect
on cobalt leaching (Figure 6.9b), in the absence of manganese. Although Al,
Fe and Cr are known to form slow leaching hydrotalcite type structures
(Forano et al., 2006), the conditions of the leach should not affect recovery
as Cr, and Al cannot be reduced. Thus, results in Figure 6.9b suggest that
these structures must also contain a reducible metal (Mn, Co or Fe) which
would break the crystal structure upon reduction.
(a)
0
20
40
60
80
100
Cu Zn None Cr Fe Si AlMetal Ion
Nic
kel l
each
ed (%
)
(b)
0
20
40
60
80
100
Cu Zn None Cr Fe Si AlMetal Ion
Cob
alt l
each
ed (%
)
Figure 6.9. Effect of metal ions in RSPT of Group 4 precipitates on (a) nickel leaching, and (b) cobalt leaching.
6-25
Comparison of predictor leach results between precipitates T and U
after 12 weeks of ageing revealed that the presence of cobalt actually
improved nickel recovery by 13% and 15% for the standard and reductive
tests respectively (Table 6.1). Thus, the presence of cobalt, like manganese,
might somehow limit the formation of the slow leaching Ni-Mg hydroxide. It
was also observed with precipitates T and U that the nickel recovery was
higher with the standard test than with the reductive test. Precipitates X, Z
and AA also had a similar trend, while T, U and X had the same trend for
cobalt recoveries. The oxidation of metals (in a simple system) was not an
issue, and in some cases where metals could not be reduced (T, U, X, Z and
AA), the oxidative process was more ideal.
It should be noted that the precipitates produced and tested in the
present study are aged longer and contain higher levels of metals than the
precipitate produced at the Ravensthorpe and other plants, as discussed in
Chapter 7. Moreover, the BHP Billiton Yabulu Refinery flowsheet was
considered to be more robust than laboratory leach tests. Therefore,
aluminium in high levels seems to be the only metal that would cause
significant problems at Yabulu.
6.4 X-Ray Diffraction of Leach Residues
XRD analysis was conducted on all leach test residues in order to
distinguish which structures exist in the precipitate after leaching. There was
no value including all of the XRD traces as they all appeared essentially
6-26
identical. Selected traces are shown in Figures 6.10-6.15. A crystalline mixed
nickel/magnesium hydroxide was present in all residues, and therefore can
be deemed to be the slow leaching component of these precipitates. There
were a few small unidentified peaks in some of the residues which could
possibly be hydrotalcite type compounds. However, the phase was
indistinguishable as they were lone peaks. The amorphous nature of the
compound and the low metal impurity concentration did not help the matter.
The traces of V, W (Figure 6.12) were of interest as they contained higher
concentrations of Al and Fe, and for that reason hydrotalcite-like structures
were observed in the XRD plots. These structures could have existed from
the production of precipitate and have slowly become more crystalline over
time. If this was the case, various undetectable phases could be present in
all hydroxide precipitates which would have a significant effect on ‘ageing’
and metal recoveries. Broad peaks in the 11-13° range were visible in most
traces suggesting a poorly ordered hydrotalcite-like structure was present.
However, there were no secondary peaks for confirmation.
As over 96% of nickel and cobalt were leached by the reductive soak
predictor test on precipitates A – N (Table 6.1), it was not surprising to see a
little trace of phases containing these metals in the XRD plots. The plots
consist of forms of magnesium carbonate, as magnesium is known to have a
low solubility in ammonia. According to Fittock (2006) the solubility of
magnesium is below 100 mg/L in the ammonia ammonium carbonate
solutions in Yabulu refinery. It is also stated that magnesium precipitates in
different forms such as MgCO3, (NH4)2CO3.MgCO3.4H2O or
6-27
4MgCO3.Mg(OH)2.4H2O depending on temperature and solution
composition.
10 20 30 40 50 60 70 80
2 theta
37a 37b 37c 37d 85a 85b 85c85d Ni(OH)2 Mg(OH)2 Hydrotalcite MgCO3 Ni,MgCO3
Figure 6.10. XRD scans of standard predictor test residues A – D after 6 and 12 weeks (37 and 85 days) ageing.
6-28
10 20 30 40 50 60 70 80
2 theta
Red37a Red37b Red37c Red37d Red85a Red85bRed85c Red85d Ni(OH)2 Mg(OH)2 Hydrotalcite MgCO3
Figure 6.11. XRD scans of reductive predictor test residues A - D after 6 and 12 weeks ageing.
10 20 30 40 50 60 70 80
2 theta
T UV WNi,Mg(OH)2 Mg Fe Hydrotalcite structure (sjogrenite)Mg Al Hydrotalcite structure (hydrotalcite)
Figure 6.12. XRD scans of reductive predictor leach residues – 12 weeks – T, U, V, W.
6-29
10 20 30 40 50 60 70 80
2 Theta
A B C DE F G HI J K LM N MgO MgCO3Mg(OH)2 MgCO3.5H2O Hydrotalcite (NH4)2Mg(CO3)2.4H2O
Figure 6.13. XRD scans of reductive soak predictor test residues after 12 weeks ageing.
10 20 30 40 50 60 70 80
2 Theta
1 2 3 4 5 6 7 Ni(OH)2 Mg(OH)2 Ni,Mg(OH)2 MgO
Figure 6.14. XRD scans of precipitates MHP1-MHP7.
6-30
10 20 30 40 50 60 70 80
2 Theta
1 2 3 4 5 6 7 Ni,Mg(OH)2 MgO Hydrotalcite
Figure 6.15. XRD scans of standard predictor leach test residues of MHP1-MHP7.
The XRD trace of the seven precipitates MHP1-7 are shown in
Figure 6.14 and compared with the XRD traces of leach residues in
Figure 6.15. Mixed nickel magnesium hydroxide was present in all
precipitates (MHP1-MHP7) (Figure 6.14), and all residues (Figure 6.15) other
than the residue of precipitate MHP7. This suggests the chromium
incorporation in the precipitate may have influenced the stability of the nickel
magnesium hydroxide. The mixed nickel-magnesium hydroxide is the slow
leaching phase associated with these precipitates. The three large peaks
present in residue from precipitate MHP7 in Figure 6.15 could not be
identified as no structures in the database seemed to correspond. As
magnesium was the only metal ion remaining, the peaks could be associated
with this metal ion, some sort of metal oxide, hydroxide or carbonate were
the most likely possibilities.
6-31
The slow leaching hydrotalcite-like structures were undoubtedly present
in the soak leach residue of the aluminium rich (MHP6) precipitate
(Figure 6.15). The broad nature of the peaks, suggests the structure is poorly
ordered. The structure may have been improving in order over the six-week
period, as the precipitate was stored as a wet filter cake. This type of
structure may also be present in the leach residue of precipitate 4 (MHP4) as
the XRD trace shows a slightly larger ‘lump’ around 11 degrees. If this was
the case, cobalt has oxidised as it is the only metal present in the precipitate
to constitute the trivalent metal required for the hydrotalcite compound.
6.5 Effect of Drying, Ageing and Heating
6.5.1 Effect of Moisture Content
The effect of drying MHP on nickel leaching was investigated by using
the four simplified precipitates prepared in the laboratory which contained
approximately 30% nickel and 10% magnesium (Ni/Mg) with 5% cobalt
(Ni/Co/Mg), 5% aluminium (Ni/Al/Mg) or 5% iron (Ni/Fe/Al). As noted
previously, after filtering, the moist precipitate (~50 %) was dried at 50°C for
5 and 20 hours, to make 3 precipitate samples of varying moisture content.
Standard and reductive predictor tests were performed on the 3 samples of
each precipitate.
Figure 6.16 is a summary of the results on nickel leaching from these
four precipitates initially, and after drying for 5 and 20 hours (overnight) at
50°C with a N2 blanket. The recoveries of the various precipitates cannot be
6-32
compared as they contain different compositions of nickel. For the simple
Ni/Mg and Ni/Co/Mg precipitates, it can be seen that the nickel leaching
actually improved as the precipitate became drier. This seemed unusual for
the precipitate containing cobalt, as drying was thought to encourage the
oxidation of the metal and entrain nickel. If cobalt was present in its trivalent
state the lack of oxidation upon drying suggests all the oxidation has
occurred during filtration.
Figure 6.16. Standard predictor leach test results - effect of drying for 5 and 20 hours at 50°C, and % solids on nickel recovery.
The mixed nickel/magnesium hydroxide has been discovered to be
the predominant slow leaching component in the precipitate. It was thought
that drying the precipitate would improve its crystallinity and stability. This
was not the case. Observing the nature of the precipitates, ‘clumping’ was
0
10
20
30
40
50
60
70
80
90
100
%N
i Lea
ched
56 % 68 % 95 % 68 % 82 % 98 % Ni/Mg Ni/Co/Mg
97 % 89 % 44 %
Ni/Co/Al/Mg Ni/Co/Fe/Mg
97 % 88 % 42 %
6-33
thought to be the cause of the conflicting results. However, further leach
tests in an ultrasonic bath produced similar results, and a microscope picture
(Figure 6.17) showed small round accessible particles.
Figure 6.17. Microscope picture of Ni/Mg precipitate.
The XRD scans of precipitates of differing moisture contents in
Figure 6.18 revealed that the increase in Ni leaching is due to a slower
transformation MgO to Mg(OH)2 due to drying. This is expected to lower the
quantity of Ni/Mg hydroxide in the dried product and lower the stability and
enhance the ability to leach Ni(II). This is evident from the presence of MgO
in the driest sample at 2θ ≈ 43o; there may be a tiny peak of MgO in the
sample containing 68% solids (Figure 6.18). Also, the Ni/Mg hydroxide peak
at ~38° became lower in intensity as the precipitate was dried longer. These
results indicate that drying retards the following transformation: Ni(OH)2 +
MgO = NiO + Mg(OH)2 (log K = 5.07)
This effect was also observed with the precipitate containing
Ni/Co/Mg, and would probably correspond to the lack of MgO hydration
6-34
associated with drying. The peak at 38° for both precipitates was slightly
asymmetric, suggesting some of the nickel and magnesium were present in
separate phases when the precipitate was dried. A low moisture content
would inhibit or slow down the process of transformations and re-structuring.
The separate phases can be observed in Figure 6.19 indicating separation of
phases during the drying of the precipitate.
10 20 30 40 50 60 70 80
2 Theta
56% 68% 95% Ni,Mg(OH)2 MgO
Figure 6.18. XRD scans of Ni/Mg precipitate of 56% solids and 68% and 95% solids obtained after drying for 5 and 20 hours at 50°C.
6-35
56 % Solids
35 36 37 38 39 40 41 42
38.39 38.40
68 % Solids
35 36 37 38 39 40 41 42
38.31 38.34
95 % Solids
35 36 37 38 39 40 41 42
38.18 38.23
Figure 6.19. Analysis of XRD peak at 38° of Ni/Mg precipitate of different % solids.
There were no observable trends with the precipitates containing
aluminium and iron. These precipitates were studied as both metals exist in a
trivalent state and are known to form stable hydrotalcite-like structures
(Forano, 2006). If these stable phases were present upon precipitation,
drying should improve crystal order and stability. Figures 6.20 and 6.22 show
the XRD traces of initial solids and the solids dried for 5 or 20 hours at 50oC.
The XRD traces of the leach residues are shown in Figures 6.21 and 6.23.
Improvement of crystallinity doesn’t seem to occur since drying did not inhibit
nickel leaching (Figure 6.16) and there was little difference between the XRD
plots of the precipitate before and after drying.
6-36
10 20 30 40 50 60 70 80
2 Theta
45% 73% 81% Ni(OH)2 Mg(OH)2 MgO Mg Al Hydrotalcite structure (hydrotalcite)
Figure 6.20. XRD scans of Ni/Co/Mg/Al precipitate of 45% solids 73% and 81% solids obtained after drying for 5 and 20 hours at 50°C.
10 20 30 40 50 60 70 80
2 Theta
45% 73% 81% Ni(OH)2 Mg(OH)2 MgO Mg Al Hydrotalcite structure (hydrotalcite)
Figure 6.21. XRD scans of Ni/Co/Mg/Al leach residues.
6-37
Figures 6.20 and 6.21 show the XRD traces of the precipitates and
the leach residues for the samples containing aluminium. There doesn’t
seem to be any significant differences between the plots before and after
drying. The precipitate in Figure 6.20 is poorly ordered, while in Figure 6.21
the predominant phase was a hydrotalcite-like structure. This confirmed the
stable crystalline phase was the cause for poor leach results.
The XRD traces of the iron rich precipitates (Figure 6.22) were more
ordered than the aluminium equivalents (Figure 6.20). Like the Ni/Mg and
Ni/Mg/Co precipitates, MgO is present in the driest sample. The same
phenomenon was probably occurring, whereby drying slows the
transformation to a hydroxide. In the leach residue traces (Figure 6.23) the
mixed metal hydroxide was still present along with a hydrotalcite-like
structure. The figures confirm that both structures are stable and responsible
for slow leaching.
6-38
10 20 30 40 50 60 70 80
2 Theta
42% 52% 97% Ni,Mg(OH)2 MgO Mg Al Hydrotalcite structure (hydrotalcite)
Figure 6.22. XRD scans of Ni/Co/Mg/Fe precipitate of 42% solids and 52% and 97% solids obtained after drying for 5 and 20 hours at 50°C.
10 20 30 40 50 60 70 80
2 Theta
42% 52% 97% Ni(OH)2 Mg(OH)2 MgO Mg Al Hydrotalcite structure (hydrotalcite)
Figure 6.23. XRD scans of Ni/Co/Mg/Fe leach residues
6-39
6.5.2 Effect of Ageing Dried Precipitates
After an ageing period of 4 to 12 weeks the precipitates tested in the
previous section were leached again to determine if moisture content had an
influence on the ageing and subsequent nickel recovery (Figure 6.24 & 6.25).
The error bars in Figure 6.24 show the initial recoveries (before ageing
shown previously in Figure 6.16) to indicate the effect of ‘ageing’. After over
9 weeks of ‘ageing’, the nickel recovery only decreased by a few percent for
the Ni/Mg and Ni/Co/Mg precipitates. The slight decrease can probably be
related to an improvement in crystal structure and stability. No further
oxidation has occurred with cobalt, so the metal ion was either stable in its
divalent state, or complete oxidation occurred during filtration. Based on
ageing results, the first option is far more likely.
Figure 6.24. Standard predictor leach test results showing the effect of drying on nickel leaching from aged precipitates (error bars show recoveries before
ageing).
0
10 20 30 40 50 60 70 80 90
100
% N
i Lea
ched
58 % 68 % 99 % 69 % 83 % 98 % Ni/Mg 12 weeks
Ni/Co/Mg 9 weeks
95 % 87 % 42 % Ni/Co/Al/Mg
4 weeks Ni/Co/Fe/Mg
4 weeks
96 % 87 % 41 %
6-40
After 4 weeks of ‘ageing’, the % nickel leaching from the aluminium
rich precipitates and iron rich precipitates decreased by up to 15% and 30%,
respectively. As noted previously, the improvement of crystal structure that
occurs with hydrotalcite-like phases over time is detrimental to nickel
leaching. This improvement in crystal structure seems to be more significant
for the iron rich precipitate, suggesting the aluminium hydrotalcite structure is
slower to form or less stable. With both precipitates, the largest decrease in
Ni leaching occurred with the wetter samples. Higher moisture content must
allow for quicker or more effective crystal ordering. Given the large influence
of the moisture content, a dissolution-nucleation precipitation mechanism is
likely.
As shown in Figure 6.25, the reductive leaching of the aged
precipitates was ineffective for all but the iron rich precipitate. Therefore,
cobalt was probably in its divalent state and the aluminium hydrotalcite-like
structure was extremely stable and unaffected by the reductive conditions.
Nickel recovery improved by up to 16% with the iron rich precipitate.
Evidently, the ferric ion is reducible, which would destroy the hydrotalcite-like
structure and consequently improve nickel recovery. It should also be noted,
in Figures 6.24 and 6.25, nickel recovery was the highest with the iron and
aluminium rich precipitates when dried for 5 hours. This seems unusual and
is unexplainable at this stage.
6-41
Figure 6.25. Reductive predictor leach test results showing the effect of drying on nickel recovery from aged precipitates.
6.5.3 Effect of Heating Precipitates
Thermal gravimetric analysis (TGA), conducted on some of the
precipitates, showed that they had three stages of weight loss (Figure 6.26).
The XRD scan in Figure 6.27 shows that the precipitate consists of a metal
hydroxide at 200°C, and a metal oxide at 450oC and 1000°C. Evidently, the
first stage was the loss of moisture and dehydration, while the second stage
was the conversion to a metal oxide and the third was loss of some of the
oxide to improve the crystallinity.
)(2)(2)(22 )(,.)(, lsheat
s OxHOHMgNiOxHOHMgNi +⎯⎯→⎯
)(2)()(2 ,)(, gsheat
s OHMgONiOHMgNi +⎯⎯→⎯
0
10 20
30 40 50 60 70 80 90
100
%
Ni
58 % 68 % 99 % 69 % 83 % 98 %
Ni/Mg 12 weeks
Ni/Co/Mg9 weeks
95 % 87 % 42 %
Ni/Co/Al/Mg4 weeks
Ni/Co/Fe/Mg 4 weeks
96 % 87 % 41 %
6-42
0102030405060708090
100
0 200 400 600 800 1000
Wei
ght %
Temp, C Ni/Mg Ni/Mg/Co Ni/Mg/Co/Al Ni/Mg/Co/Fe
Figure 6.26. TGA plots for Ni/Mg, Ni/Mg/Co, Ni/Mg/Al and Ni/Mg/Fe precipitates after 6 weeks ageing.
10 20 30 40 50 60 70
2 Theta
Ni, Mg 200 deg Ni, Mg 450 deg Ni, Mg 1000 deg Ni(OH)2 Mg(OH)2 MgO NiO
Figure 6.27. XRD scans of Ni, Mg precipitate after heating at 200, 450 and 1000°C.
6-43
XRD was performed on the other precipitates. However, traces were
very similar. The second mass loss was significantly different between
precipitates as observed in Figure 6.28. The decomposition temperature for
Ni/Mg and Ni/Mg/Co precipitates start around 300°C, while decomposition
temperatures for iron and aluminium rich precipitates were around 250 and
200°C. This could be due to a difference in strength of metal to hydroxide
bonds. Also, the aluminium rich precipitate had two mass losses between
200 and 500°C.
-0.06
-0.05
-0.04
-0.03
-0.02
-0.01
0
200 300 400 500 600 700 800 900 1000Temperature, oC
Slop
e -
Ni/Mg Ni/Mg/Co Ni/Mg/Al Ni/Mg/Fe
Figure 6.28. Slope (Wt %/°C) of TGA plot for Ni/Mg, Ni/Mg/Co, Ni/Mg/Al and Ni/Mg/Fe precipitates.
The mass loss with the iron and aluminium rich precipitates is poorly
defined compared to Ni/Mg and Ni/Mg/Co. This is probably related to water
occupying multiple sites, making it more difficult to remove. Hydrotalcite
((MII1-xMIII
x)8(OH)16(An-)8x/n.4H2O) is likely to be present, which would lose its
6-44
intercalated water. This would probably result in a splitting of the compound
as the water is required for stability upon formation.
In order to determine the effect of dehydration on nickel leaching, the
precipitates were dried at 200°C before standard predictor leach tests were
performed. Error bars are included in Figure 6.29 to indicate the recovery by
the same test on the original precipitates. In 3 out of 4 precipitates the nickel
dissolution was decreased due to drying. The XRD scans of the precipitates
prior to leaching (Figure 6.30) showed that the material consisted of mixed
metal hydroxides (Ni,Mg(OH)2 and Ni,Mg,Co(OH)2). With all precipitates the
peaks became more intense and narrower, indicating an improvement in
crystal order (Figure 6.30 vs. 6.18, 6.20 and 6.22).
Figure 6.29. Nickel recovery from precipitates after drying at 200°C.
0102030405060708090
% N
i Lea
ched
Ni/Mg Ni/Co/Mg Ni/Co/Al/Mg Ni/Co/Fe/Mg
6-45
10 20 30 40 50 60 70 80
2 Theta
Ni, Mg Ni, Mg, Co Ni, Mg, Al Ni, Mg, Fe Ni(OH)2 Mg(OH)2 MgO
Figure 6.30. XRD scans of precipitates dried at 200°C.
6.6 Leaching Kinetics of Synthetic MHP
6.6.1 Mathematical Expressions for Kinetic Models
Kinetic studies based on the dissolution of metals from a particle B in
batch reactors according to the reaction A(aq) + bB(s) → products, where A
is the active reagent of the lixiviant, can be interpreted by using the
established mathematical expressions listed in Table 6.9. Reaction rate for
the dissolution of metal M (RM) and order (n) with respect to the
concentration of different reagents (Y) are given by Eqs. 1-2 in Table 6.9,
where [M] and XM are the concentration or fraction of dissolved metal after
time t and kap is the apparent rate constant.
6-46
The determination of activation energy (Ea), based on the effect of
temperature on rates or rate constants in conjunction with the Arrhenius
equation (Equation 3), reveals the chemical controlled (high Ea) or diffusion
controlled (low Ea) nature of the leaching reaction (Levenspiel, 1972). A
chemically controlled surface reaction of suspended particles is independent
of the agitation speed ω representing rotation speed of the impellor (s-1).
Table 6.9. Mathematical expressions for heterogeneous kinetic models
No. Equation
(1) napM Yk
dtdXor
dtMdR ][][
==
(2) apM kYnR += ]log[log
(3) RTEaAek /−= (Arrhenius Equation)
(4) tkt
rcbkX ap
i =⎟⎟⎠
⎞⎜⎜⎝
⎛=−−
ρ3/1)1(1 (Shrinking Sphere Model)
(5) tkt
rbDcXX ap=⎟⎟
⎠
⎞⎜⎜⎝
⎛−
=−+−− 23/2
)1(6)1(2)1(31
ρε(Shrinking Core Model)
(6) ( ) ( ))1(2)1(316
)1(13
3/22
3/1 XXbcDrX
cbkrX
ckrt
il
−+−−⎟⎟⎠
⎞⎜⎜⎝
⎛+−−⎟⎟
⎠
⎞⎜⎜⎝
⎛+⎟⎟
⎠
⎞⎜⎜⎝
⎛=
ρρρ
The heterogeneous kinetic models for the interpretation of the fraction
of metal leached from suspended particles after time t are given by Eqs. 4-6,
where ki = intrinsic rate constant of the surface reaction (cm s-1), b =
stoichiometric factor, c = concentration of Y (mol cm-3), ρ = molar density
6-47
(mol cm-3) of the dissolving metal in the particle, r (cm) = particle radius, D =
diffusivity (cm2 s-1) of the species through a product layer and ε = particle
porosity (Levenspiel, 1972; Gergeou and Papangelakis, 1998).
A shrinking sphere model assumes that a surface reaction is the rate
controlling step while a shrinking core model assumes that the diffusion of a
reactive species or product through a porous solid layer of increasing
thickness is the rate controlling step. However, if the reacting species is
present in more than one chemical form, this may lead to more than one rate
controlling step depending on the particle size. For example, the rate of
nickel leaching from NiO (fast, Ea = 57 kJ mol-1) in fine particles of NiO.Al2O3
catalyst is controlled by the slow diffusion of species through the fluid film. In
contrast, the rate of nickel leaching from NiAl2O4 (slow, Ea = 65 kJ mol-1)
from the porous structure of the more coarse particles of the same catalyst is
controlled by a surface chemical reaction. These findings are based on the
analysis of rate data according to the combined rate expression in Eq. 6
which considers three rate controlling steps (Nazemi et al., 2011), instead of
Eq. 4 (Abdel-Aal and Rashad, 2004) or Eq. 5 (Mulak et al., 2005).
6.6.2 Porosity of Starting Material
The precipitated metal hydroxides were thought to leach at different
rates in ammonia, leaving pores in the MHP particles. Unfortunately this was
difficult to justify as some metal ions have a low solubility in the ammoniacal
lixiviant used in the Yabulu refinery. Single metal hydroxides were
6-48
precipitated with sodium hydroxide and leached in ammonia. The residues
were weighed and analysed by XRD. The complete dissolution of single
hydroxide precipitates occurred in the first 2 minutes. This behaviour is
different to the dissolution behaviour of mixed hydroxides precipitated by
adding MgO. It was proven with the precipitation studies in Chapter 5 that
metal ions precipitate within the pores of MgO. The initial dissolution of MgO
in SAC solutions and the reprecipitation of Mg(OH)2 or MgCO3, as described
in Chapter 5, can also affect the dissolution of other metal ions in the MHP
matrix if such precipitates coat the dissolving MHP particles.
In order to determine the changes in porosity, surface area tests were
conducted on a precipitate after 2, 5, 10, 20 and 60 minutes of leaching in
SAC solution. The surface area of the precipitate of particle size ranging
38-53 μm, measured by the laser sizer (assuming round particles) was
0.143 m2/g while the BET surface area was 8.3 m2/g. This indicates that the
material was porous prior to leaching. The ratio of BET surface area to laser
sizer surface area was plotted over the 60 minutes of leaching (Figure 6.31).
Although the particles were getting smaller during the test, the BET surface
area increased substantially. The precipitate has become more porous over
time, indicating the leaching was occurring within the pores of the precipitate.
This leaching mechanism therefore appears to fit a shrinking core kinetic
model, as described later.
6-49
0
200
400
600
800
1000
1200
1400
0 20 40 60 80
Surf
ace
Area
Rat
io:
BET
/Las
er S
izer
Time, s
Figure 6.31. Ratio of BET surface area:laser sizer surface area vs. time of leaching.
6.6.3 Effect of Crystallinity
After numerous attempts, five simple precipitates containing nickel
and magnesium (Table 3.8) were produced with varying Ni/Mg ratios
(Table 6.10) and varying crystallinity. Nickel and magnesium composition
ranged between 52-60% and 2.3-3.7%, respectively. The XRD scan in
Figure 6.32 showed peak height to be changing while peak width almost
remaining constant indicating the higher cystallinity of the precipitate in
NiMg1 compared to that of NiMg5.
The nickel leaching curves of these precipitates were significantly
different from each other (Figure 6.33), proving that the crystallinity has a
large influence on rate. This conclusion can be applied to any brucite
structure in MHP and probably hydrotalcite-like structures. Table 6.10
summarises the initial rates of dissolution expressed as RNi (g L-1 s-1) based
6-50
on solution analysis or dXNi/dt (s-1) based on the leach curves in Figure 6.34
where X = fraction of Ni leached = % Ni leached /100.
Table 6.10. Effect of Ni/Mg ratio on initial rates of leaching. Initial Composition used for
precipitation
Ni/Mg ratio in
precipitate
Initial rates of Ni(II)
leaching
PPT Volume
(L)
Ni(II)
(g L-1)
Ni(II):Mg
mole ratio
Mass
Ratio
Mole
Ratio
Ri
(mg L-1 s-1)
104*dXNi/dt
(s-1)
NiMg1 20 0.25 1:1 16.8:1 7.0:1 7.40 5.35
NiMg2 20 0.70 1:1 15.6:1 6.4:1 17.5 13.0
NiMg3 20 1.45 1:1 19.5:1 8.1:1 33.3 24.5
NiMg4 15 2.75 1:1 19.5:1 8.1:1 41.7 31.4
NiMg5 5 5.5 1:1 23.2:1 9.6:1 41.7 34.4
Leach conditions: Lixiviant SAC, T = 25oC, size as produced, S/L = 10 g/L, rpm 500 (Leach curves in Figure 6.33)
10 15 20 25 30 35 40 45 50 55 60
Ni Mg 1 Ni Mg 2 Ni Mg 3 Ni Mg 4 Ni Mg 5
Figure 6.32. XRD scans of nickel magnesium precipitates of varying crystallinity.
6-51
0102030405060708090
100
0 5 10 15 20
% N
i Lea
ched
Time, mins
Ni Mg 1 Ni Mg 2 Ni Mg 3 Ni Mg 4 Ni Mg 5
Figure 6.33. Nickel recovery from precipitates over a 20 minute period.
Figure 6.34a shows a log-log plot of initial rate of nickel dissolution
from the precipitate (dXNi/dt) as a function of the initial Ni(II) concentration
used for the precipitation of NiMg1-NiMg5. The first order dependence at low
nickel(II) concentrations indicates the relationship between the incorporation
of Ni(II) from solution into the mixed Ni/Mg precipitate during precipitation
and it’s dissolution back into the SAC solution during nickel leaching. This
can be further examined using the shrinking sphere and core models based
on the mathematical expressions listed in Table 6.9.
The applicability of a shrinking sphere model for the precipitate NiMg1
with R2 = 0.96 in Figure 6.34b indicates that the surface reaction between the
lixiviant and Ni(II) in the precipitate is rate controlling. However, a shrinking
core model shows a better fit for the two precipitates NiMg1 and NiMg2 with
slightly higher values of R2 = 0.97 or 0.98 (Figure 6.34c). Nevertheless, these
6-52
models can be applied only for a group of particles of narrow size range (r in
Eqs. 5-6).
(a)
-3.5
-3.0
-2.5
-2.0
-1.0 -0.5 0.0 0.5 1.0
Log {Initial Ni(II) concentration g L-1}
log{
(dX N
i(II)/d
t) / s
-1}
Slope ~1
(b)
y = 0.01xR2 = 0.96
0
0.1
0.2
0.3
0.4
0 5 10 15
Time / minutes
1-(1
-X)1/
3
NiMg1NiMg2NiMg3NiMg4NiMg5
(c)
y = 0.002xR2 = 0.973
y = 0.003xR2 = 0.981
0
0.1
0.2
0.3
0.4
0 5 10 15Time / minutes
1-3(
1-X)
2/3 +
2(1-
X)
NiMg1NiMg2NiMg3NiMg4NiMg5
Figure 6.34. Leaching of Ni/Mg precipitates NiMg1-NiMg5: (a) effect of initial Ni(II) concentration on initial rates, (b) testing of a shrinking sphere model,
(c) testing of a shrinking core model
6-53
6.6.4 Effect of Particle Size
Three size fractions of the Ni/Mg Ni/Mn/Mg, Ni/Al/Mg, and Ni/Fe/Mg
precipitates were leached to ensure that the particle size had an influence on
leach kinetics. Only the nickel magnesium precipitate was displayed as they
all exhibited similar trends (Figure 6.35).
Figure 6.35. Nickel leaching from precipitate – influence of particle size at 20 g/L solid/liquid ratio.
Figures 6.36a-c show the applicability of a shrinking core kinetic
model for nickel leaching from these precipitates of different particle sizes.
The apparent rate constants determined from the slopes of the linear
relationships are plotted as a function of 1/r2 in Figure 6.36d. The linear
relationship at lower values of 1/r2 (i.e. higher particle sizes) confirms the
validity of a shrinking core kinetic model expressed by Eq. 5 in Table 6.9.
6-54
(a) (b)
y = 0.0028xR2 = 0.975
0
0.1
0.2
0.3
0 5 10 15
Time / minutes
X, 1
-(1-
X)1/
3 ,
or 1
-3(1
-X)2/
3 +2(1
-X) X
Sphere
Core
y = 0.0023xR2 = 0.9905
0
0.1
0.2
0.3
0 5 10 15
Time / minutes
X, 1
-(1-X
)1/3 ,
or 1
-3(1
-X)2/
3 +2(1
-X) X
Sphere
Core
(c) (d)
y = 0.0007xR2 = 0.87150
0.1
0.2
0 5 10 15
Time / minutes
X, 1
-(1-X
)1/3 ,
or 1
-3(1
-X)2/
3 +2(1
-X)
X
Sphere
Core0
0.001
0.002
0.003
0 0.002 0.004 0.006
1/r2 (μm-2)
kapp
aren
t / m
in-1
Figure 6.36. Applicability of a shrinking core model for Ni leaching from Ni-Mg hydroxide precipitates of different particle sizes: (a) 25-38 μm, (b)
38-53 μm, (c) 53-75 μm (data from Figure 6.35); (d) plot of apparent rate constant as a function of 1/r2.
6.6.5 Effect of Magnesium Content
When nickel dissolves from the mixed Ni,Mg-hydroxide the build up of
the residual Mg(OH)2 on the dissolving particle would affect the leaching rate
of Ni(II). In order to determine the influence of magnesium and nickel
contents on nickel dissolution, four precipitates with increasing
concentrations of magnesium were produced, sized, aged for 4 weeks, and
6-55
leached in SAC solutions at ambient conditions. Results shown in
Figure 6.37 demonstrate that the nickel leaching is retarded with the increase
in magnesium content in the precipitate. The extent of Ni leaching is low at a
low Ni/Mg ratio of 0.63 and reaches a plateau after 20 minutes. However, at
a higher content of Ni, despite the retardation after about 10 minutes, Ni
leaching continues to higher values close to 100%.
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40 50 60 70
% N
i Lea
ched
Time, mins
0.63 1.2 1.51 1.59
Figure 6.37. Effect of Ni/Mg ratio on nickel recovery over time.
These results also support the view that magnesium forms a stable
slow leaching mixed hydroxide with nickel during precipitation. The
retardation of kinetics could be due to the surface blockage by Mg(OH)2 or a
product layer of MgCO3. However, no MgCO3 peaks were observed in XRD
scans. As the shape of Figure 6.37 is similar to previous kinetic plots, and
magnesium is known to have a much lower solubility than nickel, it can be
concluded that the retardation of nickel dissolution was a result of the
6-56
leaching behaviour of a shrinking core. This is further tested and confirmed in
Figures 6.38a-c.
A logarithmic plot of the initial rate of Ni dissolution dXNi/dt as a
function of Ni/Mg ratio of the precipitate in Figure 6.38a shows a linear
relationship of slope close to 1. This indicates that the initial rate of the
surface reaction is governed by the Ni content of the precipitate.
Figure 6.38b shows that the Ni leaching follows a shrinking sphere model at
low nickel contents, again supporting the view that the rate is controlled by a
surface reaction. However, as the reaction proceeds the build up Mg(OH)2
blocks the surface and retards the reaction. Figure 6.38c shows that Ni
leaching from precipitates of higher Ni contents obeys a shrinking core model
indicating considerable initial surface blockage. However, high porosity (at
higher Ni content) facilitates the reaction to continue and leach nickel, as
shown in Figure 6.37.
6-57
(a)
-1.6
-1.4
-1.2
-1.0
-0.8
-0.4 -0.2 0.0 0.2 0.4Log{Ni/Mg ratio}
Log{
(dX N
i/dt)/
min-1
}
Slope ~1
(b)
y = 0.011xR2 = 0.9911
0.0
0.1
0.2
0.3
0 5 10 15Time / minutes
1-(1
-X)1/
3
Ni/Mg = 0.63/1Ni/Mg = 1.20/1Ni/Mg = 1.51/1Ni/Mg = 1.59/1
(c)
y = 0.0113xR2 = 0.989
0.0
0.1
0.2
0 5 10 15Time / minutes
1-3(
1-X
)2/3 +2
(1-X
)
Ni/Mg = 0.63/1
Ni/Mg = 1.20/1
Ni/Mg = 1.51/1
Ni/Mg = 1.59/1
Figure 6.38. Effect of Ni/Mg ratio in Ni-Mg-hydroxide precipitate on Ni leaching kinetics: (a) Log-Log plot of initial rates as a function of Ni/Mg ratio; (b) Shrinking sphere model; (c) Shrinking core kinetic model (data and other
conditions from Figure 6.37)
6-58
6.6.6 Effect of Oxidation of Co(II)
Two samples of cobalt hydroxides were precipitated from 6 L of a
4 g/L cobalt(II) solution using sodium hydroxide. Hydrogen peroxide was
added to one of the solutions to produce an oxidised cobalt hydroxide. Based
on the titrations for the determination of the extent of oxidation described in
Chapter 3, the two precipitates contained a 7% (low) and 60% (high)
compositions of cobalt(III). According to the XRD-traces shown in Figure
6.39 the two precipitates were poorly ordered. Cobalt dihydroxide was
present in the first precipitate, while the three lumps in the scans of the
oxidised precipitate were due to either an oxidised cobalt hydroxide or a
hydrotalcite-type structure. The first precipitate also had lumps in XRD scans
corresponding to a hydrotalcite-type structure, which according to the extent
of oxidation titrations makes up 7% of cobalt. As cobalt oxidation was not
possible prior to precipitation, this structure has formed afterwards.
Representative samples of 0.5 g of each precipitate were leached in
25 mL of SAC with samples taken periodically over 20 minutes. Rates of
metal dissolution represented by the slopes of the curves in Figure 6.40 from
the two precipitates are significantly different. Given that cobalt(III) does not
leach in the SAC solution in the absence of a reducing agent, Figure 6.41
shows the percentage of cobalt(II) leached over time from the two
precipitates for comparison. Although the results are based on % of total
cobalt dissolution, the plots are still different. The slow leaching of Co(II) from
the second precipitate shown in Figure 6.41 is largely due to the low
6-59
composition of Co(II) in this precipitate. Moreover, as discussed in the
previous chapters regarding the precipitation of cobalt and manganese,
cobalt forms the spinel structure Co3O4 which contains both divalent and
trivalent cobalt. Metal ions associated with this structure would leach at a
slower rate than in a simple brucite structure (Co(OH)2).
10 15 20 25 30 35 40 45 50 55 60
2 Theta
Co(OH)2 CoOOH Co(OH)2 CoOOH, Heterogenite-\IT2H\RG Hydrotalcite
Figure 6.39. XRD scans of cobalt precipitates
It is of interest to note that the leaching curve of Co(II) from the first
precipitate (of low 7% Co(III)) in Figure 6.41 is similar to that of Ni(II) leaching
curve of NiMg5 precipitate in Figure 6.37. Therefore, Figure 6.42 examines
the applicability of shrinking sphere or core kinetic models to these two
precipitates. The dissolution of Co(II) from the first precipitate and Ni(II) from
NiMg5 obeys a shrinking core kinetic model with comparable apparent
6-60
constant of 0.04 min-1, based on the slopes of the linear relationships in
Figures 6.42a and b, respectively
.
0102030405060708090
100
0 5 10 15 20
% C
o Le
ache
d
Time, mins
Co(OH)2 CoOOH
Figure 6.40. Cobalt leaching from unoxidised and oxidised cobalt hydroxide precipitates in a SAC solution.
0102030405060708090
100
0 5 10 15 20
% C
o Le
ache
d
Time, mins
Co(OH)2 Co2O3
Figure 6.41. Cobalt leaching from unoxidised and oxidised cobalt hydroxide precipitates in a SAC solution.
6-61
Although the initial rates (dXM/dt) represents the leaching of metal ions
from the surface, prolonged leaching is governed by the diffusion of
reactants (NH4+/NH3) or leach products (Ni(II) or Co(II)) through the
thickening porous layer of Mg(OH)2 or other insoluble components/products
in the MHP matrix. This was further examined by considering the leaching of
Ni(II) from precipitates produced at an elevated temperature of 80oC in the
presence of other metal ions representing the MHP of Ravensthorpe
Operation. Results are described in the next section.
(a) (b)
y = 0.0422xR2 = 0.9998
0
0.2
0.4
0.6
0.8
0 5 10 15
Time / minutes
X, 1
-(1-X
)1/3 ,
or 1
-3(1
-X)2/
3 +2(1
-X)
X
Sphere
Corey = 0.0444xR2 = 0.9841
0
0.2
0.4
0.6
0.8
0 5 10 15
Time / minutes
X, 1
-(1-X
)1/3 ,
or 1
-3(1
-X)2/
3 +2(1
-X)
X
Sphere
Core
Figure 6.42. Applicability of shrinking core kinetic model for Co(II) and Ni(II) leaching in SAC solutions: (a) from precipitate of low Co(III) (data from
Figure 6.41); (b) from NiMg5 (data from Figure 6.37).
6.6.7 Effect of Other Metal Ions and Crystallinty
Twelve precipitates were produced from 20 L solutions with low metal
ion concentrations (<0.15 g/L) at 80°C (using the initial compositions of
solutions described in Table 3.11) in order to improve the crystallinity. Metal
concentrations in the precipitate were at reasonable concentrations for
testing (Table 6.11). Nickel and magnesium compositions ranged from 15.3
6-62
to 20.4% and 18.6 to 23.0%, respectively. The curves for leaching nickel
from synthetic precipitates were similar to the curve for Ravensthorpe MHP
(denoted by RNO in Figure 6.43), but the recoveries from synthetic
precipitates were lower in most cases.
In order to rationalise the effect of metal ions on nickel leaching from
synthetic MHP, the Ni leaching curves in Figure 6.43 can be analysed in a
number of different ways.
Table 6.11. Chemical analysis of precipitates formed at elevated temperature, %.
Sample Ni Mg Co Mn Fe Al Cr Ca Si Cu ZnNi 17.8 23.0
Ni Mn 16.1 21.1 3.1Ni Co 16.4 22.1 2.1Ni Fe 15.3 19.8 3.0Ni Al 16.4 20.1 3.4Ni Cr 16.8 19.6 2.8Ni Ca 17.6 22.0 3.5Ni Si 15.8 21.9 2.3Ni Cu 20.4 18.9 3.3Ni Zn 20.0 20.3 3.2
Ni Co Mn 16.5 18.6 2.9 6.7Ni Co Fe 17.9 18.9 3.1 2.7
6-63
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40 50 60
% N
i Lea
ched
Time, mins
Ni Ni Co Ni Mn Ni Fe Ni AlNi Cr Ni Ca Ni Si Ni Cu Ni ZnNi Co Mn Ni Co Fe RNO
Figure 6.43. Nickel leach results for elevated temperature precipitates at 20 g/L in a SAC solution.
Figure 6.44 shows a plot of %Ni leached after 60 minutes as a
function of initial rate dXNi/dt. The linear relationship indicates that higher
initial rates, based on the Ni leaching in the first 2 minutes, correspond to
higher Ni leaching after 60 minutes. For example, the presence of Fe has a
negative effect on initial leaching rate and thus lowers the final leaching of Ni.
In contrast, the presence of Si and Cr has a beneficial effect on initial
leaching rate and enhances the final leaching of Ni.
6-64
y = 384.25xR2 = 0.99
0
20
40
60
80
100
0.00 0.05 0.10 0.15 0.20 0.25
Initial Rate {(dXNi/dt) / min-1}
Ni l
each
ed a
fter 6
0 m
inut
es (%
)NiCoMn
NiCoFeNi
NiMn
NiCo
NiFeNiZn
NiAl
NiCr
NiSiRNO
Figure 6.44. Effect of metal ions on initial rates and final Ni leaching after 60 minutes (data from Figure 6.43).
The five outliers in Figure 6.44 are the precipitates containing Ni,
NiMn, NiCo, NiCoMn and NiCoFe. A common feature of these outliers is that
the final Ni leaching is higher than the expected value based on the initial
rates and the linear relationship. The presence of Co and Mn in the
precipitate, compared to Fe, is beneficial on initial rates and final Ni leaching.
Some other important points to note are listed below:
(i) The crystalline nickel-magnesium hydroxide denoted by Ni in
Figure 6.44 (in the absence of other metal ions) leaches slowly
and poorly with only 50% Ni leached after 60 minutes.
(ii) Addition of Co, Mn, Al, Zn, Si and Cr improved the initial rate, but
Fe retarded the initial rate.
6-65
(iii) The incorporation of the additional metal would either lower the
crystallinity of the nickel-magnesium hydroxide or form an
alternative structure with nickel which is less stable.
(iv) According to the XRD traces described previously, the three metal
ions cobalt, manganese and aluminium form hydrotalcite-type
structures; copper forms a copper sulphate; and the others must
be incorporated into the brucite structure to lower the structural
ordering (Figures 6.10-6.15).
(v) As well as lowering the crystallinity of the brucite structure, the
leaching of calcium, zinc, silicon and chromium would destroy the
crystal lattice, releasing the nickel.
(vi) Based on initial rates and % leached after 20 minutes the iron
hydrotalcite structure was the most stable and slowest to leach,
followed by the structures nickel-magnesium hydroxide, aluminium
hydrotalcite and manganese hydrotalcite.
(vii) The rate of leaching of hydrotalcite structures can be related to the
crystallinity demonstrated in Figures 5.56 and 5.57, where the
peak height intensities at 11.3° follow the order: 360 for Mn
hydrotalcite <760 for Al hydrotalcite < 990 for Fe hydrotalcite. The
Mn hydrotalcite was the lowest as not all of the manganese would
oxidise. The difference between the effect of iron and aluminium
can be related to the atomic radius as iron is closer in size to nickel
and magnesium (Table 5.8).
6-66
Further analysis of the results in Figure 6.43 can be carried out on the
basis of kinetic models. For example, the fraction of Ni leached (X) from NiSi
and NiCr precipitates follow similar trends but do not obey a shrinking sphere
or kinetic model, as shown by the non-linear relationships in Figure 6.45. In
both cases the initial reaction is faster, leading to about 45% of Ni dissolution
in 2 minutes. However, the dissolution of the next 45% of Ni takes about 60
minutes. Thus, the fast leaching fraction of Ni(II) can be considered to be
associated with finer particles and/or at the surface of the large porous
particles. This implies that the reaction rate is controlled by the surface
reaction or the mass transfer of the lixiviant to the surface. In contrast, the
slow leaching fraction of Ni(II) is associated within the pore structure and the
rate is controlled by the surface reaction as well as the diffusion of reactants
and products through the pores, corresponding to mixed-kinetics expressed
by the mixed-kinetic model in Equation 6 in Table 6.11.
6-67
0.0
0.2
0.4
0.6
0.8
1.0
0 10 20 30 40 50 60 70
Time / minutes
X, 1
-(1-
X)1/
3 ,
or 1
-3(1
-X)2/
3 +2(1
-X)
X
Sphere
Core
NiSi : solid linesNiCr : dashed lines
Figure 6.45. Testing the applicability of shrinking sphere or core kinetic models for nickel hydroxide precipitates containing Si or Cr.
The dissolution of Ni from most of the other precipitates have slower
initial rates and generally follow a shrinking core kinetic model as shown in
Figure 6.46. The apparent rate constants (min-1) based on the slopes of the
linear relationships follow the descending order: NiMn (0.0068) > NiCoMn
(0.0057) > NiAl (0.0039) > Ni (0.0031) > NiCo (0.0013) > NiCoFe (0.0012) >
NiFe (0.0002). From Figures 6.43 and 6.44 and the relative orders of
apparent rate constants in Figure 6.46 it is clear that Si, Cr, Mn and Al have
beneficial effect while Fe has a negative effect on the rate of dissolution.
Figure 6.47 examines the relationship between the apparent rate constant
and the nickel leaching after 60 minutes. The presence of iron gives lowest
rate constant, indicating low porosity, and thus low nickel leaching with NiFe
precipitate. The enhanced rate constant in precipitates Ni, NiCo and NiAl
show improved porosity, but the nickel leaching is about 50%. The low
leaching in all these cases can be related to the presence of Ni,Mg(OH)2.
6-68
However, the presence of Mn causes a larger increase in apparent rate
constant. This confirms the oxidation of manganese, lowering the formation
of Ni,Mg(OH)2 causing a larger porosity in the precipitate NiMn. The deep
sea manganese nodules containing Mn, Fe, Co, Ni oxides also have a large
porosity due to the presence of high-valent manganese oxides (Senanayake,
2011). Relatively higher leaching of Ni from the precipitate NiCoMn,
compared NiCoFe in Figure 6.47 indicates the high porosity and/or the
reductive role of Co(II).
Further analysis of these relative effects based on the particle radius
(r) and porosity (ε) in Equation 6 (Table 6.11) is beyond the scope of this
investigation. However, the similarity of Ni leaching from synthetic
precipitates and the Ravensthorpe precipitate (MHP-RNO) in Figure 6.43
and the higher Ni leaching from MHP-RNO due to higher initial rate in Figure
6.44 can be related to the metal ion composition of commercial precipitates,
as described in the next chapter.
6-69
y = 0.0057x
y = 0.0068x
y = 0.0012xy = 0.0002x
y = 0.0013xy = 0.0031x
y = 0.0039x
0.0
0.2
0.3
0.5
0 20 40 60 80
Time / Minutes
1-3(
1-X
)2/3 +2
(1-X
)
NiCoMnNiMnNiFeNiCoFeNiCoNiNiAl
Figure 6.46. Testing the applicability of a shrinking core kinetic model for nickel hydroxide precipitates containing other metal ions.
0
20
40
60
80
100
0 0.002 0.004 0.006 0.008
Apparent rate constant (min-1)
Ni (
%) l
each
ed a
fter 6
0 m
in
NiFe
NiCo
NiCoFe
Ni NiAlNiMn
NiCoMn
Figure 6.47. Effect of metal ions on the apparent rate constants and nickel leaching in SAC solutions
6-70
6.7 Summary and Conclusions
• Five simple precipitates containing Ni,Mg(OH)2 were produced with
varying crystallinity. The XRD peak height increased with the
crystallinity of precipitates while peak width remained almost constant.
Rate of nickel dissolution was significantly different between
precipitates, proving crystallinity had a large influence on rate.
• Four NiMg(OH)2 precipitates of different magnesium concentrations
were produced, sized, aged for 4 weeks, and leached in an
ammonium carbonate solution at ambient conditions. As the
magnesium concentration of the precipitate increased, rate and extent
of nickel leaching decreased.
• Precipitation using MgO raised the pH to 8.0-8.3 depending on the
solution composition and temperature. Using a solution of similar
composition to Ravensthorpe liquor at 50°C, 20-40 % of manganese
was precipitated between a pH range of 8.0-8.3. Due to this effect,
manganese incorporation into the precipitate seemed to reach a
maximum; at 25°C it was around 5.5%.
• In the absence of manganese, the precipitation of Ni,Mg(OH)2 of high
Ni/Mg molar ratio is detrimental for nickel leaching. However, both
drying and the incorporation of manganese in the precipitate
minimised the formation or influence of the Ni,Mg(OH)2. As
manganese precipitates at a higher pH than all the other metals, it is
likely to precipitate last or at a slower rate, therefore ending up with
the larger, slower growing particles.
6-71
• Manganese formed a hydrotalcite-type structure with nickel and
magnesium as it would oxidise readily to Mn(III). The leaching tests
with simple Ni, Co, Mn, Mg precipitates proved that the incorporation
of manganese lowered the quantity of oxidised cobalt. Due to lower
Eh, manganese is likely to oxidise before cobalt. The ratio of
%Ni/%Co leached in standard predictor tests is close to unity. This
ratio is less than unity in reductive predictor leach tests due to the
higher leaching of cobalt by reductive leaching. This also shows that a
fraction of Co(II) is oxidised during precipitation.
• Although the oxidation of manganese and cobalt was thought to be a
problem during the treatment of Ravenshorpe precipitate in the
Yabulu refinery (Muir, 2003), the oxidised precipitate leached rapidly
under reducing conditions, while the nickel-magnesium hydroxide was
much slower to leach depending upon the magnesium content.
• The extent of nickel leaching from synthetic precipitates depends on
the metal ion composition and ageing. In general, ageing decreases
the nickel leaching efficiency in standard predictor leach test, but the
detrimental effect of ageing is diminished by the presence of
manganese. One of the roles of manganese is to limit the formation of
slow leaching Ni,Mg(OH)2. Thus, the nickel leaching in reductive
predictor leach tests is higher especially from the precipitates
containing manganese. This shows that the oxidation of Mn(II) to
Mn(III) during precipitation affect the structure/porosity of the
precipitate. The destruction of the structure caused by the reduction of
6-72
Mn(III) to Mn(II) is beneficial for nickel leaching. Reductive soak leach
tests are effective even in the absence of manganese leading to high
nickel leaching close to 100%.
• The XRD traces show that all precipitates contained crystalline mixed
Ni,Mg(OH)2. The presence of mixed Ni,Mg(OH)2 in the XRD traces of
leach residues of standard predictor leach tests shows that
Ni,Mg(OH)2 is the slow leaching phase. In contrast, the reductive soak
tests leached ~100% nickel and the leach residues contained
predominantly MgCO3 due to the low solubility of this compound in the
SAC solution. However, the soak leach test residues from precipitates
containing aluminium and cobalt contained hydrotalcite structures
indicating the oxidation of Co(II) to Co(III).
• As confirmed by the XRD traces, drying of the precipitates increase
nickel leaching due to the slow transformation of MgO to Mg(OH)2
caused by drying, i.e. the dried precipitates contain MgO and a lower
quantity of slow leaching Ni,Mg(OH)2.
• The XRD traces of leach residues produced from precipitates with or
without drying showed little difference and confirmed that both
Ni,Mg(OH)2 and hydrotalcite structures are responsible for slow
leaching. Thus, the improvement of the crystal structure that occurs
with hydrotalcite-like phases over time has a detrimental effect on
nickel leaching.
6-73
• The kinetic leaching pattern of synthetic precipitates was very similar
to the Ravensthorpe MHP. In the case of synthetic precipitates the
addition of cobalt, copper, calcium, manganese, aluminium, zinc,
silicon and chromium actually improved the initial rate. The
incorporation of the additional metal would either lower the crystallinity
of the nickel magnesium hydroxide or form an alternative structure
with the nickel which was less stable.
• The reductive leaching of the aged precipitates was inefficient except
in the case of iron rich precipitates. This indicates the existence of
Co(II) and Al(III) in a hydrotalcite structure, which is unaffected by
reductive leaching. However, in iron rich precipitates the reduction of
Fe(III) to Fe(II) destroys the hydrotalcite structure and improve nickel
leaching.
• The analysis of nickel leaching results from synthetic precipitates
show that the nickel leaching kinetics from synthetic precipitates obey
a shrinking core kinetic model due to much lower solubility of
magnesium than nickel. This is further supported by the fact that the
apparent rate constant from precipitates of different particle size (r) is
inversely proportional to r2. According to the mathematical expression
for a shrinking core kinetic model the porosity, affected by the
cryastallinity, composition and structure of the precipitates play
important roles during leaching. Thus, the slow leaching of nickel
associated with Ni,Mg(OH)2 and hydrotalcite structures can be a result
of the low porosity of such material.
6-74
• The leaching of nickel from mixed hydroxide precipitates produced at
80oC in the presence of other metal ions also obey a shrinking core
kinetic model. The leaching of nickel from Ni,Si and Ni,Cr precipitates
did not obey a shrinking core kinetic model, but gave highest leaching
of nickel after 60 minutes. This indicates the very high porosity of
these precipitates.
• Based on initial rates and recovery after 20 minutes of leaching the
iron hydrotalcite structure was the most stable, followed by the nickel
magnesium hydroxide, the aluminium hydrotalcite structure and the
manganese hydrotalcite. The rate of leaching of hydrotalcite
structures was probably related to the crystallinity, as peak size
correlated to leaching rate.
7-1
7 CHARACTERISATION AND LEACHING OF
COMMERCIAL MIXED HYDROXIDE PRECIPITATES
7.1 Introduction and Experimental
BHP Billiton’s concept was to be able to process precipitates from
various sources at the Yabulu refinery. For example, the MHP from
Ravensthorpe plant (RNO-MHP) was transported wet (60% solids) to
Townsville in shipping containers for leaching in the Yabulu refinery. As
noted in chapter 5 the idea of keeping the precipitate moist was to minimise
the technical risk and lower the capital and operating costs associated with
drying. The decision was based on the results of studies conducted by BHP
Billiton (Jones, 2000a & 2001a), SNC-Lavalin and Worley (2001), and
Lakefield Oretest (2000).
Preboil solids have also been a continuing problem in the Yabulu circuit
since the commencement of solvent extraction in 1989. Although essentially
produced to control the Mn and Fe contents in the product liquor, the preboil
solids contain significant quantities of Ni, Co and Mg. Preboil solids are also
known to consist mainly of hydrotalcite-type compounds. The poor leaching
characteristics of these solids in ammonia ammonium carbonate liquor
described in previous chapters preclude recycling preboil solids to the
reduced ore leaching and washing circuit, as that would result in major Ni
and Co losses. Therefore, preboil solids are currently added to the ore
stockpile and reprocessed through the ore roasters.
7-2
After a significant testing program on laboratory based precipitates
described in previous chapters, this chapter considers a systematic study of
characterisation and the leaching behaviour of pilot plant samples and
commercial precipitates. This includes pilot plant samples from
Ravensthorpe (RNO), European Nickel (EN), and MHP samples from Cawse
and eventually Ravensthorpe when production commenced in 2008. The
MHP’s were collected, aged, analysed and leached in order to compare and
contrast the behaviour of different samples. The MHP samples ranged in age
from 11 months to 5 years (Table 7.1). It was thought that the significant
ageing that had occurred would emphasise the problems associated with
leaching MHP. As noted previously, the standard predictor leach tests
(Bolden, 1997) have been designed to examine and improve the leaching
process in the Yabulu refinery. Predictor leach tests were performed to
compare and contrast the leaching behaviour of different samples. It was
found that the neutralising agent used for the precipitation and the
composition of the precipitate influenced the nickel recovery significantly.
Table 7.1. Age of Precipitate Samples. Sample Age
Ravensthorpe Pilot Plant (RNO) ~4 years Cawse ~5 years European Nickel (EN) PS-44 11 months European Nickel (EN) SS-22 11 months Yabulu Preboil Solids 3 months
7-3
The soak test was also conducted in order to determine the total
achievable nickel recovery, as refineries would usually incorporate a
thickening or counter-current decantation (CCD) stage after leaching. For
example, the Yabulu refinery has a 72 hour CCD circuit that operates at
50°C (Figure 1.1). The stable slow leaching compounds in the unsoaked
material will begin to leach in this stage. Therefore, after performing a
reductive predictor test, the residue was stored with 250 mL of SAC for 72
hours at 50°C to simulate and examine the effect of soaking in the CCD
circuit.
7.2 Composition and Characterisation
7.2.1 Chemical Analysis
The chemical analysis of various commercial products from RNO, EN,
Cawse and Yabulu plants are listed in Table 7.2. The RNO typical
specification noted in Table 7.2 was the proposed and achievable MHP
composition, based on previous pilot runs for the production of MHP. The
chosen RNO pilot plant sample had metal concentrations similar to the
typical specification with the exception of Mn and Mg, both being a little
higher.
The RNO-MHP sample collected in June 2008 had the composition
listed in Table 7.3. The June MHP has higher nickel and silicon, and a lower
magnesium concentration than typical MHP (Table 7.3). The lower
magnesium concentration was a significant improvement. Clearly, the stages
7-4
of neutralisation to remove magnesium, and washing of the MHP were more
effective on a commercial scale than at pilot level. The Cawse sample had
significantly higher Co and Mn concentrations. The EN sample PS-44 had
higher concentrations of Al and Fe, while SS-22 had higher Mn and Mg
levels. The Yabulu preboil solids had a much lower composition of Ni, very
high Co, Fe, Mn and Mg, and relatively high concentrations of Al and Cr.
Table 7.2. BHP Billiton Chemical Analysis of Aged MHP Samples
Element Units
RNO
Typical
Specification
RNO
Pilot Cawse
EN
PS-44
EN
SS-22
Yabulu
Preboil
Ni % 40.0 44.33 39.5 30.7 28.6 15.0
Co % 1.38 1.45 3.57 1.06 0.083 3.24
Fe % 0.15 0.08 0.04 1.15 0.39 9.23
Mn % 2.75 3.41 6.79 3.04 3.85 9.66
Mg % 1.75 2.28 0.86 4.07 4.89 5.29
Ca % 0.2 0.15 0.12 1.02 1.82 0.44
Cu % 0.015 0.06 0.038 0.002 <0.001 0.347
Zn % 0.23 0.33 0.19 0.2 0.04 0.07
Al % 0.05 0.05 0.08 0.56 0.34 0.43
Cr % 0.01 <0.01 <0.01 0.01 <0.01 0.16
Si % 0.5 0.48 0.54 0.26 0.26 4.03
S % NA 3.98 4.83 3.95 4.85 0.74
C % NA 0.17 0.12 3.38 1.6 4.7
H2O % NA 53.29 61.58 57.22 71.46 NA
7-5
Table 7.3. Assay of RNO MHP Collected in June 2008.
% Ni Co Fe Mn Mg Ca Cu Zn Al Cr Si S C H2O
Typical 40.0 1.38 0.15 2.75 1.75 0.20 0.015 0.23 0.05 0.01 0.50 N/A N/A N/A
June Sample 42.6 1.32 0.14 2.58 0.94 0.16 0.034 0.27 0.07 <0.01 0.95 3.00 0.39 57.4
7.2.2 Collection and Size Analysis of RNO MHP
Samples of RNO MHP were collected at the addition point of MgO,
and from tanks 1, 2 and 3 (flowsheet in Figure 1.2 – only 1 tank shown for
MHP precipitation). Residence time was approximately 4 hours. The first
batch of samples (A) were collected in a sponge and dried immediately at
50°C in air, while the second batch of samples (B) were collected in the
same manner but were washed with water prior to drying. The measured
size distribution of all RNO precipitates are shown in Figures 7.1 & 7.2. Due
to lack of sample, the measurements on sample 3B (washed) were
unsuccessful. Size distribution did not seem to differ drastically between
samples (Figure 7.1). The P80’s of the precipitates also showed no trends
with values ranging between 73 and 83 μm (Table 7.4). Particle size was
significantly larger than that of the synthetic precipitates described in Chapter
5 (20 μm). All size distributions exhibited a bimodal shape which didn’t
change significantly between samples (Figure 7.1). Clearly, most of the
precipitation, dissolution and agglomeration had occurred before the first
sample. Also, in this case, washing had no influence on particle size
distribution of the precipitate (B).
7-6
0
2
4
6
8
10
12
14
16
10 100
% P
assi
ng
Size, µm 1A 2A 3A 4A 1B 2B 4B
Figure 7.1. Size distribution of RNO MHP samples.
Table 7.4. P80 of Ravensthorpe MHP’s. 1 2 3 4
A 73.1 81.9 71.9 82.7B 79.4 73.3 - 80.8
P80 in µm
0
2
4
6
8
10
12
0102030405060708090
100
1 10 100
% C
hanc
e
Cum
ulat
ive
% P
assi
ng
Size, µm
Cumulative % Passing % Chance
Figure 7.2. Size distribution of RNO MHP collected June 2008.
7-7
7.2.3 X-Ray and Neutron Diffraction Analysis of RNO MHP
As expected the older MHP samples were more crystalline whilst the
two EN pilot plant samples were amorphous (Table 7.1 and Figure 7.3). The
Cawse and RNO Pilot MHP samples seemed to be a mixture of nickel and
cobalt hydroxides and possibly hydrotalcite. The small peak around 37° in
the two EN traces was probably a manganese hydroxide given the high
levels of Mn in the samples.
It should be noted that although theophrastite (Ni(OH)2) was used as a
label for the XRD traces, it is very unlikely to be present in its pure form.
Likewise, Mg(OH)2, Co(OH)2 and Mn(OH)2 also precipitate in the same
brucite structure and will form solid solutions with Ni(OH)2. It is also likely that
the metal hydroxide structures would be hydrated. The Cawse and RNO Pilot
MHP samples were produced by neutralisation with MgO so were expected
to contain higher levels of magnesium, particularly mixed with the nickel and
cobalt hydroxides.
The XRD analysis of RNO June 2008 sample (Figure 7.4) revealed
little information. The broad nature of the peaks suggests the material was
poorly crystalline. This was probably due to the substitution of a variety of
metal ions, and the hydration of the subsequent hydroxides.
7-8
10 20 30 40 50 60 70 802 theta
PS-44 SS-22 CAWSERNO Theophrastite, Ni(OH)2 Nickel Hydroxide HydrateCobalt hydroxide Co(OH)2 Mn(OH)2 Heterogenite, CoOOH
Figure 7.3. XRD scans of MHP Samples
10 20 30 40 50 60 70 80
2 Theta
RNO MHP nickel hydroxide hydrate comblainite
Figure 7.4. XRD scan of RNO MHP collected June 2008 – 1 week.
7-9
The two compounds (nickel hydroxide hydrate and comblainite)
labelled in the scan shown in Figure 7.4 are both likely and are the most
predominant. Peak shifting due to metal ion substitution was expected,
especially with Co, Mn, Mg and Si due to their ‘higher’ composition. Although
comblainite (Ni6Co2(OH)16CO3.4H2O) was listed, the metal composition was
unknown. There are a variety of comblainite-type structures with similar
diffraction peaks, which are commonly called hydrotalcite-like compounds.
The Ni2+ and Co3+ ions in this structure can be substituted by other divalent
ions (Mg2+, Ca2+, Cu2+, Zn2+, Co2+) and trivalent ions (Mn3+, Fe3+, Cr3+ and
Al3+) (Forano et al., 2006). Using the assay data in Table 7.3 and assuming
that cobalt(II) is oxidised, it was possible to calculate that up to 30% of nickel
in RNO MHP was involved in this type of structure. Without cobalt, up to 21%
of nickel could be involved in a hydrotalcite-type compound.
Neutron Diffraction (Figure 7.5) at ANSTO (Australian Nuclear
Science and Technology Organisation) exhibited a similar pattern to XRD.
The radiation had a similar wavelength to the Cu Kα1 sources used for XRD
(1.54 & 1.5406 A°, respectively), so the peaks would appear in similar
positions, however they would exhibit different intensities to traditional
diffraction. No difference was observable using a higher intensity machine,
with better resolution and a different interaction with the atoms. Thus, it was
concluded that there are no phases of lower concentrations which are
undetected and the material is simply ‘poorly crystalline’.
7-10
10 30 50 70 90 110 130 150
2 Theta
RNO MHP Ni(OH)2 Ni,Mg(OH)2Comblainite (Ni,Co) Nickel hydroxide hydrate
Figure 7.5. Neutron Diffraction pattern of RNO MHP – 1 week.
The XRD trace of the preboil solids (Figure 7.6) shows that the
sample consists of crystalline phases of a hydrotalcite-like compound and
manganese carbonate; and a small peak that was possibly magnetite
(Fe3O4). These solids were renowned for being difficult and slow to leach,
and were responsible for some of the losses of nickel and cobalt in the
Yabulu refinery (Bolden, 1997).
7-11
10 20 30 40 50 60 70 802 theta
Preboil Solids Hydrotalcite Rhodochrosite, MnCO3 Magnetite, Fe3O4
Figure 7.6. XRD pattern of Preboil Solids
7.2.4 SEM and EDS of RNO MHP
The Scanning Electron Microscopy (SEM) images of all samples were
similar. There were two types of particles observed: (i) black large rounded
particles, and (ii) smaller jagged, agglomerated particles (Figure 7.7). Using
a back scatter electron detector, the smaller particles appeared brighter
indicating that they contain elements of a higher atomic number. Some of the
rounded particles have a bright ring around the edge. An approximate grain
count, based on size of particles, on Figure 7.7 gives a ratio of 1:8 of dark
round particles to bright agglomerated particles.
7-12
Figure 7.7. Back scatter electron SEM image of precipitate 1A (MgO addition point).
The Energy Dispersive Spectroscopy (EDS) on specific particles
revealed the black round particles are magnesium rich, while the brighter ring
around the edge is nickel rich (Figures 7.8 - 7.11). Clearly, these are
unreacted MgO particles. The brighter, jagged, agglomerated particles are
rich in nickel and cobalt. Images on washed precipitates (B) were not
included as they did not differ from the unwashed equivalents (A).
The SEM images on synthetic material (described previously in
Chapter 4) and Ravensthorpe MHP (described in Figures 7.8 - 7.11) over the
four hour precipitation period revealed that the precipitation was occurring
according to two mechanisms: (i) dissolution-nucleation-agglomeration, and
7-13
(ii) precipitation within the pores of MgO. Metals were distributed evenly
throughout the particles. Due to these mechanisms the size distribution was
relatively large and did not change significantly over the period. With the
Ravensthorpe MHP the P80’s ranged between 71 and 83 μm. The larger
particle size is a desirable quality as it would settle and filter well. Towards
the end of the precipitation period (tank 3), probably when the pores were
filled, metals precipitated on the outside of the Mg rich particles is
responsible for a higher overall nickel and cobalt content (Figure 7.12).
Figure 7.8. SEM and EDS images of precipitate 1A (MgO addition point).
7-14
Figure 7.9. SEM and EDS images of precipitate 2A (Outside 1st Tank).
Figure 7.10. SEM and EDS images of precipitate 3A (2nd tank).
7-15
Figure 7.11. SEM and EDS images of precipitate 4A (3rd tank).
The EDS of particles in the samples progressing through the tanks,
showed the magnesium rich particles were incorporating more nickel. This
was also seen with the synthetic precipitates, described in Chapter 5, where
the porous nature of the MgO allowed metals to precipitate within the core.
Moreover, like the synthetic precipitates, Ostwald ripening and dissolution-
nucleation-agglomeration was occurring. The dissolution-nucleation was the
predominant (80-90%) mechanism with the industrial process, compared to
only about 50% with synthetic precipitates. This would be due to precipitation
in multiple tanks by a continuous process. Continuous precipitation in
multiple tanks also allows greater control and improves metal recovery. In
the first tank, metal concentrations were high causing nucleation to be the
predominant mechanism, while in the subsequent tanks, when
7-16
concentrations are lower, crystal growth was more likely to occur. This also
explains why the Ravensthorpe precipitates had a larger particle size.
7.3 Oxidation States of Mn and Co in RNO MHP
Oxidation states of Mn and Co using X-Ray Photoelectron
Spectroscopy (XPS) are usually determined by the 2p doublet (Figures 7.12
& 7.13). This was relatively easy to see for Mn with MgKα radiation, though
the signal was weak as the sample contained less than 3% of the metal. The
peak position suggests that manganese exists predominantly in the 3+ and
4+ oxidation states.
630635640645650655660665670
Binding Energy, eV
Mn4+ ← Mn2+
Figure 7.12. XPS scan of RNO MHP June 2008, Mg Kα1 source, Mn 2p doublet.
7-17
770775780785790795800805810815820
Binding Energy, eV
Co2+ → Co3+
Figure 7.13. XPS scan of RNO MHP June 2008, Mg Kα1 source, Co 2p doublet.
For Co, the 2p doublet was strongly interfered with by the O KLL
Auger series using Mg Kα radiation and the Ni LMM Auger series using Al Kα
radiation. As MHP consists of predominantly Ni(OH)2 both the O and Ni
Auger features are very intense. That said, cobalt appears to exist primarily
in its divalent state. The poor signal due to low concentrations (<5%) and the
interference by the oxygen and nickel Auger series prevents quantification of
oxidation states.
Previous reports indicated the general view that cobalt and
manganese would only oxidise on the outside of the particle (Fittock, 2008).
In light of this, a precipitate containing ~3% cobalt and manganese with 19%
nickel was analysed by XPS at Murdoch University. A second analysis on the
7-18
same precipitate was conducted after it was ground in a mortar and pestle
and transferred in an inert atmosphere. Size analysis was performed on both
precipitates to ensure there was a significant difference in size between
samples, as shown in Figure 7.14. The P80 for the outer and the core
samples was 59 and 49 μm, respectively.
In Figures 7.15 and 7.16 the signal improved with the core sample due
to an increase in surface area, however the peaks were in the same
positions. Therefore, it can be concluded that cobalt and manganese are
most likely in the same oxidation state throughout the particles.
Figure 7.14. Laser size analysis of Ni/Co/Mn/Mg precipitate.
7-19
760770780790800810
outer core
Co2+ → Co3+
Figure 7.15. XPS scans of Ni/Co/Mn/Mg precipitate, Al Kα1 source, Co 2p doublet.
620625630635640645650655660
outer core
Mn4+ ← Mn2+
Figure 7.16. XPS scans of Ni/Co/Mn/Mg precipitate, Al Kα1 source, Mn 2p doublet.
7-20
7.4 Ageing and Drying of RNO-MHP
The XRD scanning was conducted on the original sample and
samples dried for 5 and 20 hours at 50°C over a 12 week period to see how
the precipitate aged. Diffraction patterns, as shown in Figure 7.17, recorded
over time and between samples were essentially the same. Drying the
precipitate and leaving it over 12 weeks had no effect on crystal structure.
The only difference between XRD traces was the existence of a peak around
63° on the 4 day sample, which was probably MgO remaining from
precipitation.
The SEM and EDS analysis conducted on the precipitate after 4, 11
and 25 days did not reveal any further information or exhibit any trends.
Images from the 4 day sample are displayed in Figure 7.18. Nickel,
magnesium, cobalt, manganese, sulphur and oxygen concentrations were
plotted, where brightness corresponds to high concentration.
Cracking occurred with all samples, where water would have
evaporated upon drying. In Figure 7.18 Ni, Mg, Co, Mn, S and O were all
observed by EDS and in most cases seemed to be distributed evenly
through the particles. The scattered distribution of oxygen was probably
related to an uneven carbon coating, while the bright spot in the magnesium
image was most likely unreacted MgO which was also present in the XRD
pattern (Figure 7.17). Clearly, nickel and oxygen were the most predominant.
7-21
10 20 30 40 50 60 70 80
2 Theta
4 days 100 % - 4 days 11 days 81% - 11 days 100% - 11 days40 days 81% - 40 days 100% - 40 days 85 days 81% - 85 days100% - 85 days Ni(OH)2 Mg(OH)2 MgO Hydrotalcite
Figure 7.17. XRD scans of RNO MHP over time – 57, 81 and 100 % solids.
7-22
Figure 7.18. SEM and EDS images of RNO MHP after 4 days – particles embedded in resin.
7.5 Leaching Kinetics of RNO MHP
Higher initial rates of leaching of nickel from synthetic MHP lead to
higher final leaching efficiencies (%) as noted in Chapter 6. Therefore, the
samples of RNO-MHP collected in June 2008 were leached in SAC solution
under different conditions in order to determine the influence of various
7-23
parameters on nickel dissolution over a period of 20 minutes. The variables
studied and the initial rates per unit mass of precipitate (s-1) under different
conditions are listed in Table 7.5 for general comparison. The initial rates per
unit mass of precipitate (Ri, s-1) were calculated from the ratio of initial
dissolution rate of Ni(II) (g L-1 s-1) by the concentration of solids (g L-1).
Smaller particle size, higher agitation and higher temperatures cause higher
initial rates (Table 7.5). Figures 7.20-7.23 show the effect of sold/liquid ratio
(g L-1), temperature, rotation speed (agitation) and particle size on %Ni
leached over 20 minutes.
Table 7.5. Effect of leach conditions on the initial leach rates of June 2008 RNO-MHP
Solid/Liquid (g/L) Rpm
size (μm)
Temp (oC)
Initial Rate Ri (s-1)
2 500 38-53 25 0.0017 5 0.0015 10 0.0015 20 0.0013 10 500 38-53 25 0.0015
40 0.0024 60 0.0025
20 500 38-53 25 0.0013 40 0.0020 60 0.0021
10 500 38-53 25 0.0015 600 0.0016 750 0.0018
20 500 38-53 25 0.0013 600 0.0018 750 0.0022
10 500 25-38 25 0.0020 38-53 0.0015 53-75 0.0018
20 500 25-38 25 0.0018 38-53 0.0013 53-75 0.0015
Ni/Mg ratio of solid = 40/1.75; Lixiviant = SAC solution
7-24
At each solid/liquid ratio, over 70% of dissolution occurred in the first 5
minutes (Figure 7.19). After this point dissolution slowed considerably, and in
some cases looks to have stopped altogether. The overall recovery and the
point at which dissolution starts to retard (around 5-10 minutes) decreased,
as the mass of precipitate (S/L ratio) increased. The increase in temperature
from 25 to 40°C resulted in an increase in the initial rate of dissolution, while
there was little difference between 40 and 60°C (Figure 7.20). According to
pKa (-log Ka) values for the two reactions: NH4+ = H+ + NH3 and HCO3
- = H+
+ CO32- reported by Olofsson (1975) and Millero (1995), the concentrations
of NH3 and CO32- increase with increasing temperature. Nevertheless,
previous studies conducted at the Yabulu Refinery have related the little
difference in results between 40 and 60°C to the evaporation of ammonia
(Jones, 2000b & 2001b; Nikoloski et al., 2005). This would lower the
solubility limit, and the driving force for nickel dissolution. Clearly there is an
‘ideal’ temperature. At the Yabulu refinery 1st stage of MHP leaching is
operated at about 45°C.
The rotation speed showed expected results (Figure 7.21) while the
effect of size fraction on rates was unusual. The best rate was observed with
the smallest size fraction (25-38 μm), followed by the largest (53-75 μm) then
the middle (38-53 μm) (Figure 7.22). Chemical analysis and XRD scans were
conducted on samples of different size fractions to examine if there was a
difference in composition. The results are summarised in Table 7.6 and
Figure 7.23, respectively.
7-25
0
20
40
60
80
100
0 5 10 15 20
% N
i Lea
ched
Time, mins
2 g/L 5 g/L 10 g/L 20 g/L
Figure 7.19. Effect of S/L ratio on nickel leaching from RNO-MHP in SAC solutions.
0
20
40
60
80
100
0 5 10 15 20
% N
i Lea
ched
Time, mins
25 deg C 40 deg C 60 deg C
Figure 7.20. Effect of temperature on nickel leaching from RNO- MHP in SAC solutions
7-26
0
20
40
60
80
100
0 5 10 15 20
% N
i Lea
ched
Time, mins
500 rpm 600 rpm 750 rpm
Figure 7.21. Effect of agitation on nickel leaching from RNO-MHP in SAC solutions
0
20
40
60
80
100
0 5 10 15 20
% N
i Lea
ched
Time, mins
25-38 µm 38-53 um 53-75 µm
Figure 7.22. Effect of particle size on nickel leaching from RNO MHP, 10 g/L.
Table 7.6. Assay results for size fractions of RNO-MHP, mass %. Sample Al Ca Co Cu Fe Mg Mn Ni S Si Zn
25-38 µm 0.07 2.44 2.12 0.35 0.33 1.44 4.02 51.9 2.96 26.6 0.7738-53 µm 0.09 3.23 2.04 0.31 0.09 1.53 4.12 48.2 2.93 27.0 0.8253-75 µm 0.09 2.43 2.11 0.28 0.14 1.36 5.90 49.6 2.78 27.3 0.76
7-27
10 15 20 25 30 35 40 45 50 55 602 Theta
25-38 µm 53-38 µm 53-75 µm
Figure 7.23. XRD scans of RNO-MHP of different size fractions.
There were no significant differences between the XRD scans of
different size fractions (Figure 7.23) to explain the unusual leach results in
Figure 7.22. The only significant differences in assay results were the higher
Ca concentration for the 38-50 μm sample and the higher Mn concentration
for the 53-75 μm sample (Table 7.6). As calcium has been proven to have no
effect on nickel recovery in this investigation (Chapter 6), the higher
manganese must be the cause of the beneficial effect. Manganese has been
proven to slow or inhibit the formation of Ni,Mg(OH)2. A lower concentration,
or a less crystalline version, of this compound would result in improved
kinetics. As noted previously, manganese precipitates at a higher pH than all
the other metals. Thus, it is likely to precipitate last or at the slowest rate,
therefore ending up with the larger, slower growing particles.
7-28
In general the effect of temperature, stirring and particle size provides
valuable information on the rate controlling step of the leaching process. For
example, chemically controlled reactions are accelerated by an increase in
temperature, but less affected by the stirring speed. In contrast, diffusion
controlled reactions are more affected by the stirring speed, but less affected
by an increase in temperature. Figure 7.24 examines the effect of
temperature on leaching nickel from the RNO-MHP at two pulp densities 10
and 20 g/L, particle size 38-53 μm and 500 rpm. The figure shows the
Arrhenius plot based on the mathematical expression described by Equation
3 of Table 6.9: Ln(Rate) = Ln A – Ea(1000/RT). The values of the activation
energy (Ea) based on the slopes of different sections of the curves are also
shown in Figure 7.24. The low values of Ea = 2.6-2.9 kJ/mol at low
temperatures supports a diffusion controlled reaction. The change in Ea to
higher values of 24-25 kJ/mol supports a mixed chemical-diffusion controlled
reaction at higher temperatures.
Figure 7.25 plots the effect of particle size (geometric mean) on the
initial rates for the dissolution of nickel from RNO-MHP and compares with
the results for Ni,Mg(OH)2 previously considered in Chapter 6 (Figure 6.35).
At a given particle size the initial rate of dissolution of nickel from RNO-MHP
is higher than that of Ni,Mg(OH)2. The reasons for this behaviour can be
examined by considering the heterogeneous kinetic models as shown in
Figure 7.26.
7-29
-4.5
-4.0
-3.5
-3.0
-0.42 -0.40 -0.38 -0.36 -0.34
{-(1000/RT) / (kJ/mol)-1}
Ln {I
nitia
l Rat
e R i
/ (s
-1)}
Ea=25 kJ/mol
Ea=2.6 kJ/mol
Ea=2.9 kJ/mol
Ea=24 kJ/mol
10 g/L
20 g/L
Figure 7.24. Arrhenius plot for Ni(II) dissolution from RNO-MHP in SAC solution (500 rpm, 38-53 μm, 10 or 20 g/L solids)
The nickel dissolution from both precipitates of the same particle size
range 38-53 μm obey a shrinking core model (Figure 7.26) and the apparent
rate constant for MHP-RNO (0.0034 minute-1) is larger than that for
Ni,Mg(OH)2 (0.0028 minute-1). According to Equation 5 in Table 6.9, the
apparent rate constant k = 6bDc/(1-ε)ρr2 depends on the porosity (ε) and
molar density of nickel(II) in the solid (ρ mol cm-3). Thus, a higher value of
kapparent corresponds to a higher porosity of MHP-RNO, compared to a lower
porosity of Ni,Mg(OH)2. This is also supported by the lower dependence of
initial rates of nickel dissolution on the particle size of MHP-RNO noted in
Figure 7.22.
7-30
0.0001
0.0010
0.0100
10 100Mean Particle Radius (μm)
Initi
al R
ate
{ Ri /
s-1}
MHP-RNO, 10 g/L
MHP-RNO, 20 g/L
Ni-Mg-Hydroxide, 20 g/L
Figure 7.25. Effect of particle size on initial rates of Ni(II) dissolution from RNO-MHP and Ni,Mg(OH)2.
(a) (b)
y = 0.0028xR2 = 0.975
0
0.1
0.2
0.3
0 5 10 15
Time / minutes
X, 1
-(1-X
)1/3 ,
or 1
-3(1
-X)2/
3 +2(1
-X) X
Sphere
Core
y = 0.0034xR2 = 0.985
0
0.1
0.2
0.3
0 5 10 15
Time / minutes
X, 1
-(1-X
)1/3 ,
or 1
-3(1
-X)2/
3 +2(1
-X) X
Sphere
Core
Figure 7.26. Comparison of kinetic models for Ni(II) dissolution from (a) Ni,Mg(OH)2 , and (b) MHP-RNO in SAC solution at 25oC, 500 rpm, 20 g/L
solids and particle size range of 38-53 μm.
7-31
7.6 Predictor Leach Test Results
7.6.1 General Comparison of Different Commercial Precipitates
Five types of predictor leach tests were conducted on the precipitate
samples in triplicate and the results are summarised in Table 7.7 and
Figure 7.27. The difference between the standard and reductive predictor
tests gave a good indication of the fraction of oxidised cobalt and
manganese. The addition of Na2EDTA ensured that there were no solubility
problems. Thus, the leaching results from reductive/complexing predictor
leach in the presence of Na2EDTA represent total, achievable recoveries.
The oldest MHP sample, Cawse (~5 years), exhibited the highest
dissolution of Ni and Co in the standard predictor leach test, followed by the
two EN pilot plant samples PS-44 and SS-22 (11 months), the RNO pilot
plant sample (~4 years) and then the Yabulu preboil solids (3 months). This
trend was probably related to the composition of Mg and trivalent metal ion
impurities (Fe, Al, and Mn). They were responsible for the formation of slow
leaching compounds, especially hydrotalcite-like structures, as described in
Chapter 6. It is likely that Ni and Co would be present in these structures.
Therefore, minimising the trivalent metal cations in MHP will reduce the
levels of these compounds and improve leach results. The considerable
influence of these structures on leaching results proves that age becomes
irrelevant after a few months.
7-32
Table 7.7. Predictor leach test results of commercial precipitates. Metal RNO Pilot Cawse PS-44 SS-22 Preboil
% (1) Standard Predictor Leach Test Ni 60.5 88.2 75.5 79.9 6.1 Co 64.5 79.7 62.6 63.2 5.1 Mn 17.2 19.0 16.1 5.9 2.9 Mg 51.3 75.6 39.2 36.7 3.6
(2) Reductive Predictor Leach Test Ni 64.4 98.4 89.8 87.4 23.1 Co 75.7 98.6 79.8 76 16.1 Mn 73.4 59.9 21.7 14.1 8.1 Mg 62.3 84.1 47.1 23.5 10.6
(3) Reductive/Complexing Predictor Leach Test Ni 60.9 99.6 95.4 99.2 49.4 Co 74.1 99.9 96.4 99.2 38.7 Mn 91.4 99.9 96.7 99.0 19.8 Mg 68.1 95.5 98.6 99.4 22.0
(4) Reductive Soak Predictor Leach Test Ni 99.6 99.6 95.7 96.7 34.7 Co 97.6 99.6 90.9 89 23.9 Mn 80.8 72.6 34.2 22.5 8.1 Mg 87.2 86 55.9 24.8 11.3
(5) Standard Soak Predictor Leach Test Ni 99.2 97.9 91.5 92.2 20.4 Co 88.7 88.5 83.6 80.2 15.1 Mn 23.3 28.3 24.8 17.3 4.0 Mg 78.3 74.9 51.3 48.2 6.4
(1) Standard predictor test entailed a 45 minute leach of 4 g (Ni + Co) dry basis in 500 mL of SAC at 30°C.
(2) Reductive predictor test was the same as (a) except nitrogen was sparged into the leach vessel and a calculated quantity of hydroxylamine sulphate was added.
(3) Reductive complexing predictor test was the same as previous plus 50 g of Na2EDTA (sodium ethylenediaminetetraacetic acid).
(4) Reductive soak tests followed the same procedure as (c); however after the 45 minute leach and filter, the leach residue was transferred to a plastic sample jar with 250 mL of SAC and retained at 50°C for 72 hours.
(5) Standard soak tests followed the same procedure as (a); however after the 45 minute leach and filter, the leach residue was transferred to a plastic sample jar with 250 mL of SAC and retained at 50°C for 72 hours.
7-33
020406080
100
1 2 3 4 5
Met
al io
ns le
ache
d (%
)
NiCoMnMg
Sta
ndar
d
Red
uctiv
e
Red
uctiv
e /
Com
plex
ing
Red
uctiv
e /
Soak
Sta
ndar
d /
Soak
(a) RNO
0
20
40
60
80
100
1 2 3 4 5
Met
al io
ns le
ache
d (%
)
NiCoMnMg
Sta
ndar
d
Red
uctiv
e
Red
uctiv
e /
Com
plex
ing
Red
uctiv
e /
Soa
k
Sta
ndar
d /
Soa
k
(c) PS-44
0
20
40
60
80
100
1 2 3 4 5
Met
al io
ns le
ache
d (%
)
NiCoMnMg
Stan
dard
Red
uctiv
e
Red
uctiv
e /
Com
plex
ing
Red
uctiv
e /
Soa
k
Stan
dard
/ S
oak
(d) SS-22
0
20
40
60
80
100
1 2 3 4 5
Met
al io
ns le
ache
d (%
)
NiCoMnMg
Sta
ndar
d
Red
uctiv
e
Red
uctiv
e /
Com
plex
ing
Red
uctiv
e /
Soa
k
Sta
ndar
d /
Soa
k
(e) Yabulu Preboil
Figure 7.27. Comparison of metal leaching from different commercial MHP’s under different leach conditions (data from Table 7.7).
7-34
The Ravensthorpe pilot plant MHP must contain significant quantities
of slow leaching compounds as an extra 35% Ni was leached by the soak
leach test (Figure 7.27a). The effect and presence of slow leaching
compounds was demonstrated by the comparison between the reductive and
reductive soak predictor test results. Although the difference was small for
the Cawse MHP sample (Figure 7.27b), nickel and cobalt recoveries were
significantly different with the other precipitates. The leach results for preboil
solid were very poor with only 35% Ni recovered even after the reductive
soak (Figure 7.27e). The slow leaching compound associated with these
solids is obviously very stable and hinders metal dissolution considerably.
The results also show the effect of oxidation of Mn and Co on the
leaching to be potentially significant. Given the age of the MHP samples it
can be assumed that the bulk of Mn and Co would be in their trivalent state.
This oxidation was overcome by reduction using hydroxylamine sulphate.
The reductive soak predictor test was a good representation of the proposed
Yabulu Extension Project (YEP) flowsheet which included a reductive
leaching step. The results from reductive soak predictor test in Table 7.7
were promising with over 95% Ni recovered from all MHP samples, and over
99.5% Ni recovered from the Cawse and RNO samples.
7-35
7.6.2 Effect of composition on Standard Predictor Test Results
Further analysis of the standard predictor test results for the leaching
of Ni and Co from the Cawse, PS-44 and S-22 precipitate samples is
considered in Figures 7.28 (Ni%) and 7.29 (Co%). Here, the composition of
various metal ions plotted on the y-axis is correlated to the Ni% or Co%
leached, plotted on the x-axis. In Figure 7.28a-b the composition Mn, Fe, Al,
Si, Co, Ni, Mg, S and C in the initial precipitates is plotted as a function of
%Ni leached in the standard predictor test. Higher compositions of Mn, Co
and Si are beneficial for Ni leaching (Figure 7.28a); lower compositions of
Mg, C, Fe and Al are also beneficial for Ni leaching (Figure 7.28b). These
findings are consistent with the findings in Chapter 6, based on the effect of
individual elements in synthetic precipitates on initial leaching rates and Ni
leaching after 1 hour (Figure 6.44).
0
2
4
6
8
70 75 80 85 90%Ni Leached
Com
posi
tion
(%) o
f S
i, M
n, S
or C
o in
MH
P
Mn
S
Co
Si
(a) Mn, Co, Si, S
0
2
4
6
70 75 80 85 90
%Ni Leached
Com
posi
tion
(%) o
f S
i, M
n, S
or C
o in
MH
P
Mg
Fe
C
Al
(b) Mg, Fe, Al, Ca, Zn, C
Ca
Zn
Figure 7.28. Effect of metal ion composition in Cawse, PS-44 and S-22 samples on Ni leaching in SAC solution under standard conditions.
Likewise, higher compositions of Mn, Ni and Si and lower
compositions of Mg, C, Fe and Al are beneficial for Co leaching (Figures 7.29
a-b). The detrimental effect of higher carbon content, as well as Fe and Al
7-36
suggest the presence of hydrotalcite and/or insoluble carbonates in the MHP.
For example, the highest leaching of Mn and Mg from all MHP’s considered
in Figure 7.27 was found in reductive/complexing leaching in the presence of
Na2EDTA which form complexes with both Mn(II) and Mg(II).
0
2
4
6
8
60 70 80
%Co Leached
Com
posi
tion
(%) o
f S
i, M
n, S
or N
i in
MH
P Mn
SNi
Si
(a) Mn, Ni, Si, S
0
2
4
6
60 70 80
%Co LeachedC
ompo
sitio
n (%
) of
Mg,
Fe,
Al o
r C in
MH
P
Mg
Fe CAl
(b) Mg, Fe, Al, C
Figure 7.29. Effect of metal ion composition in Cawse, PS-44 and S-22 samples on Co leaching in SAC solution under standard conditions.
The elemental composition of MHP samples from RNO-Pilot, Yabulu-
Preboil and RNO-June are summarised and compared in Table 7.8. The
leaching of Ni and Co in SAC solutions under standard conditions show
different behaviour with respect to the elemental composition, as shown in
Figures 7.30-7.31. Higher compositions of Mn, Si, Co, Fe, Mg and C appears
to be the reason for low Ni and Co leaching from the Yabulu Preboil sample
in SAC solution under standard conditions (5-6%) in Figures 7.30a and
7.31a. In contrast, the lower composition of these components in the RNO-
June precipitate seem to be beneficial leading to higher Ni and Co leaching
in SAC solutions (> 90%) even without reducing agents or soaking as shown
in Figures 7.30b and 7.31b. Further comparison under different leach
7-37
conditions in the next section would shed more light on the reasons for
different behaviour.
Table 7.8. Comparison of assays of different types of RNO samples and Yabulu Preboil sample
Composition (%)
Element
RNO RNO RNO Yabulu Typ.
Spec. Pilot June sample Preboil Ni 40 44.33 42.58 15 Co 1.38 1.45 1.32 3.24 Fe 0.15 0.08 0.14 9.23 Mn 2.75 3.41 2.58 9.66 Mg 1.75 2.28 0.94 5.29 Ca 0.20 0.15 0.16 0.44 Cu 0.015 0.06 0.034 0.347 Zn 0.23 0.33 0.27 0.07 Al 0.05 0.05 0.07 0.43 Cr 0.01 <0.01 <0.01 0.16 Si 0.5 0.48 0.95 4.03 S NA 3.98 3.00 0.74 C NA 0.17 0.39 4.7
H2O NA 53.29 57.4 NA
0
2
4
6
8
10
0 20 40 60 80 100
% Ni Leached
Com
posi
tion
(%) o
f M
n, C
o, S
i or S
in M
HP Mn
SCo
Si
(a) Mn, Si, Co, S
0
2
4
6
8
10
0 20 40 60 80 100% Ni Leached
Com
posi
tion
(%) o
f Fe
, Al,
C, C
a, M
g, o
r Zn
in M
HP
(b) Fe, Al, C, Ca, Mg, ZnFe
Mg
C
Ca, Al, Zn
Figure 7.30. Effect of metal composition in Yabulu-Preboil, RNO-Pilot and RNO-June samples on Ni leaching in SAC solution under standard
conditions.
7-38
0
2
4
6
8
10
0 20 40 60 80 100% Co Leached
Com
posi
tion
(%) o
f Fe
, Al,
C, C
a, M
g, o
r Zn
in M
HP
(b) Fe, Al, C, Ca, Mg, ZnFe
Mg
C
Ca, Al, Zn
Figure 7.31. Effect of metal composition in Yabulu-Preboil, RNO-Pilot and RNO-June samples on Co leaching in SAC solution under standard
conditions.
7.6.3 Predictor Leach Test Results - Preboil Solids Sample
The predictor leach test results for the Yabulu Preboil solids sample
under different conditions were also very poor, as shown in Table 7.9. Only
35% Ni was leached in the RSPT, which best represents the proposed YEP
process. Even the RCPT results were comparatively low. The hydrotalcite-
like compounds present in the preboil solids are obviously very stable and
slow leaching under all leaching conditions. Thus, the XRD traces of the
leach residues (Figure 7.32) show highly crystalline peaks of hydrotalcite,
MnCO3 and Fe3O4. The smaller peaks could be associated with CoOOH and
MnOOH. No new peaks have formed and no significant reduction in peak
size has occurred. The striking presence of these compounds remaining in
the leach residues emphasises their stability and reluctance to be leached in
an ammonia solution. Hydrotalcite, known to incorporate significant quantities
of Co and Ni, can lower the leach recovery dramatically when present in
MHP’s.
0
2
4
6
8
10
0 20 40 60 80 100
% Co Leached
Com
posi
tion
(%) o
f M
n, C
o, S
i or S
in M
HP Mn
SCo
Si
(a) Mn, Si, Co, S
7-39
Table 7.9. Predictor leach test results from Preboil Solids. Predictor Leach Test Ni% Co% Mn% Mg%
Standard 6.1 5.1 2.9 3.6 Reductive 23.1 16.1 8.1 10.6
Reductive/Complexing (RCPT) 49.4 38.7 19.8 22 Reductive Soak (RSPT) 34.7 23.9 8.1 11.3
Standard Soak 20.4 15.1 4 6.4
10 20 30 40 50 60 70 802 theta
Preboil SPT RPTRCPT Red Soak Std SoakRhodochrosite, MnCO3 Hydrotalcite Magnetite, Fe3O4
Figure 7.32. XRD scans of Preboil Solids and leach residues
7.6.4 Predictor Leach Test Results – RNO Pilot Plant MHP
The RNO pilot plant MHP samples were produced in mid 2002,
utilising MgO as the neutralising agent. Predictor leach tests were performed
on fresh MHP and after ageing for either 2-3 or 7 months (Hultgren, 2003a).
In this project the predictor leach tests were also performed after 4 or 5 years
of ageing. The test results for the particular sample over time are shown in
Table 7.10. The soak component of the predictor leach tests was developed
in 2003 (Hultgren, 2003a). Therefore, the Reductive Soak Predictor Test
7-40
(RSPT) and Standard Soak Predictor Test (SSPT) were not conducted on
the Cawse and RNO pilot plant MHP samples at the time that they were
produced in 2001 and 2002, respectively.
The standard predictor leach test results revealed that considerable
ageing has occurred in the first 7 months as shown by the decrease in Ni-Co
leaching from over 99% (fresh) to <90% (7 months). The ageing of MHP has
continued significantly over time causing a decrease in Ni-Co leaching to
60-65% (~4 years). This ‘poor’ leaching can be related to the presence of Mn
and Mg in higher concentration. This continued ageing of the RNO pilot plant
MHP suggests that the formation of the mixed Mg,Ni(OH)2 phase is
continuing with time.
XRD analysis was conducted on all leach residues and compared with
the original samples in Figure 7.33. A poorly crystalline hydrotalcite-like
phase was present in the RNO Pilot plant MHP while there was no sign of it
in the leach residue traces (Figures 7.33). Clearly this hydrotalcite-like phase
has been leached. Although the crystalline peaks in the RNO Pilot MHP trace
were labelled as Ni(OH)2 (theophrastite), Mg, Mn and Co would be
incorporated into the brucite structure, particularly Mg.
7-41
Table 7.10. Predictor leach test results from RNO-MHP over time. Ageing
Predictor Leach Test
Ni%
Co%
Mn%
Mg%
Fresh Standard 98.9 96.5 34 52.5 Reductive 99.9 99.5 59.6 41.6
7 months
Standard 87 89.1 9.9 39 Reductive 92.5 99.8 81.5 47.1
Reductive/Complexing 92.9 99.9 99.7 83.3 Reductive Soak 99.4 99.6 NA NA
~4 years
Standard 60.5 64.5 17.2 51.3 Reductive 64.4 75.7 73.4 62.3
Reductive/Complexing 60.9 74.1 91.4 68.1 Reductive Soak 99.6 97.6 80.8 87.2
Standard Soak 99.2 88.7 23.3 78.3 (Hultgren, 2003a)
10 20 30 40 50 60 70 802 theta
RNO SPT RPTRCPT Red Soak Std SoakTheophrastite, Ni(OH)2 Cobalt hydroxide Co(OH)2 Mn(OH)2Rhodochrosite, MnCO3 Hydrotalcite
Figure 7.33. XRD scans of RNO Pilot MHP and leach residue.
7-42
The Standard Predictor Test (SPT), Reductive Predictor Test (RPT)
and Reductive Complexing Predictor Test (RCPT) residues also contain this
structure (Figure 7.33), which was assumed to be a mixed Mg,Ni(OH)2. This
mixed Mg,Ni(OH)2 was leached almost completely in the 72-hour RSPT and
leached completely in the SSPT, indicating the crystalline hydroxide phase
was responsible for slow leaching. There was crystalline MnCO3 present in
the RSPT residue, but it was less evident in the SSPT residue. This was
consistent with ~80% of Mn hydroxide being oxidised in the RNO-MHP
sample, thus MnCO3 was formed upon reduction of the oxidised Mn
hydroxide.
7.6.5 Predictor Leach Test Results - Cawse MHP
The Cawse commercial plant MHP sample was produced in mid 2001,
utilising MgO as the neutralising agent. Predictor leach tests were performed
on fresh MHP and after ageing for either 2-3 months (Hultgren, 2003a).
Predictor leach tests were also performed after 4 or 5 years of ageing as part
of this project. The test results for the particular sample over time are shown
in Table 7.11.
The Cawse plant sample has aged relatively well i.e. without
significant oxidation of Mn and Co. Thus, even after 5 years, 99.6% Ni and
Co were leached by the RSPT. Nevertheless, the SPT results show that the
Ni and Co recovery decreased by 8% and 15% respectively. The difference
between the two metal recoveries proves some cobalt has oxidised over the
7-43
5 year period. The positive results were probably due to the low levels of
irreducible trivalent cations present in the sample.
Table 7.11. Predictor leach test results from Cawse MHP over time. Ageing
Predictor Leach Test
Ni%
Co%
Mn%
Mg%
16 days Standard 94.8 95 32 NA Reductive NA NA NA NA
2-3 months
Standard 95.6 89.2 23 NA Reductive 99.1 98.2 45 NA
Reductive/Complexing >99.9 >99.9 100 NA Reductive Soak NA NA NA NA
~5 years
Standard 88.2 79.7 19 75.6 Reductive 98.4 98.6 59.9 84.1
Reductive/Complexing 99.6 99.9 99.9 95.5 Reductive Soak 99.6 99.6 72.6 86
Standard Soak 97.9 88.5 28.3 74.9 (Hultgren, 2003a)
The leaching of cobalt with the standard predictor tests prove, even
after 4-5 years, only 35.5% and 20.3% of the metal has oxidised in the
Cawse precipitates. Over 80% of manganese exists in the trivalent state after
this significant ageing.
XRD analysis was conducted on all leach residues and compared with
the original samples in Figure 7.34. As in the case of the RNO sample
discussed in the previous section (Figure 7.33), a poorly crystalline
hydrotalcite-like phase was present in the Cawse MHP samples, while there
was no sign of it in the leach residue (Figure 7.34). Clearly this hydrotalcite-
like phase has been leached.
7-44
The predictor tests were very effective for the Cawse MHP sample,
especially the RCPT, RSPT and SSPT. No residue was available from the
RSPT, while the XRD traces of the SSPT and RCPT residues in Figure 7.34
indicate that there were no identifiable phases remaining. The peak at
around 14° in the SSPT trace could not be identified as more peaks were
required for matching. However, as the residue consisted of approximately
40% manganese, some sort of hydrated manganese hydroxide was likely.
10 20 30 40 50 60 70 802 theta
Cawse SPT RPTRCPT Std Soak Theophrastite, Ni(OH)2Cobalt hydroxide Co(OH)2 Mn(OH)2 Rhodochrosite, MnCO3Hydrotalcite
Figure 7.34. XRD scans of Cawse MHP and leach residue
7.6.6 Predictor Leach Test Results - European Nickel Pilot Plant MHP
Predictor leach tests were performed by Hultgren (2003a) at the
Yabulu Refinery upon receiving the precipitate samples. Eleven months later,
the same tests were performed as part of this project at Murdoch University
7-45
(Table 7.12). This particular batch of samples was known for their poor
leaching characteristics due to high levels of trivalent cations, as noted in
section 7.2. The two samples PS-44 and SS-22 were selected as they have
lower concentrations of impurities (Table 7.2) and were significantly different
from each other. Both MHP’s were precipitated using sodium carbonate as
the neutralising agent.
The SPT results only decreased by a few percent, whilst the RPT
results actually improved over time. This could be attributed to a higher NH3
concentration in the leach solution (98 g/L, compared to 93 g/L NH3) and/or
experimental error. Little or no ageing has occurred over 6-7 months,
suggesting that most of the ageing with these MHP’s occurred in the first few
months.
The generally higher nickel leaching of SS-22 compared to PS-44
suggests that it has aged better. This was unusual as SS-22 has higher
levels of Mn and Mg (Table 7.2) so more hydrotalcite-type compounds would
be expected to exist. As the quantity of hydrotalcite-type compounds can’t be
responsible, it must be due to stability. This improved stability is probably
associated with the higher Fe and Al concentrations. Moreover, the
aluminium hydrotalcite structures cannot be reduced.
7-46
Table 7.12. Predictor leach test results for EN Pilot Plant MHP. PS-44 SS-22
Metal 4 months 11 months 5 months 11 months Standard Predictor Leach Test
Ni 79.3 75.5 86.1 79.9 Co 71.7 62.6 76 63.2 Mn 15.9 16.1 17.3 5.90 Mg 52.9 39.2 48.5 36.7
Reductive Predictor Leach Test Ni 80.3 89.8 86.7 87.4 Co 67 79.8 74.5 76.0 Mn 16.8 21.7 13.3 14.1 Mg 55.3 47.1 51.7 23.5
Reductive/Complexing Predictor Leach Test Ni 95.4 99.2 Co 96.4 99.2 Mn 96.7 99.0 Mg 98.6 99.4
Reductive Soak Predictor Leach Test Ni 95.7 96.7 Co 90.9 89.0 Mn 34.2 22.5 Mg 55.9 24.8
Standard Soak Predictor Leach Test Ni 91.5 92.2 Co 83.6 80.2 Mn 24.8 17.3 Mg 51.3 48.2
(Hultgren, 2003a)
The XRD scans were conducted on all leach residues (Figures 7.35 &
7.36). The amorphous nature of all the residues made it difficult to distinguish
between particular compounds. Manganese carbonate was present in all the
traces due to sodium carbonate being used as the neutralising agent, while
calcite (CaCO3) was observed in the reductive soak and complexing tests
(Figures 7.35 & 7.36). Although Ni and Co hydroxides were present in the
MHP samples, they were not observed in the XRD traces. A hydrotalcite-like
compound and nickel hydroxide hydrate may be present, but cannot be
7-47
confirmed due to the amorphous nature of the precipitate. The unidentified
peak around 31° in the SS-22 trace was probably manganite (MnOOH).
10 20 30 40 50 60 70 802 theta
PS-44 SPT RPTRCPT Red Soak Std SoakTheophrastite, Ni(OH)2 Nickel Hydroxide Hydrate HydrotalciteRhodochrosite, MnCO3 Calcite
Figure 7.35. XRD scans of PS-44 leach residues
7-48
10 20 30 40 50 60 70 802 theta
SS-22 SPT RPTRCPT Red Soak Std SoakTheophrastite, Ni(OH)2 Nickel Hydroxide Hydrate HydrotalciteRhodochrosite, MnCO3 Calcite
Figure 7.36. XRD scans of SS-22 leach residues
7.6.7 Predictor Leach Test Results of RNO-June Sample
The comparison of Ni leach results from the two samples RNO-Pilot
and RNO-June are shown in Tables 7.12 and 7.13, respectively. Results
indicate that higher Mn and Mg compositions (Table 7.8) are largely
responsible for the low leaching of Ni and Co (60-65%) from RNO-Pilot
sample, compared to higher leaching of Ni-Co (93-95%) from RNO-June
sample. Thus, a range of predictor tests were conducted on the sample over
the 12 week period, whilst XRD was conducted on the leach residues. In
both the standard and reductive predictor tests (Tables 7.13 & 7.14) the
nickel leaching was seen to decrease slightly, while cobalt leaching actually
improved over the 12 week period.
7-49
Table 7.13. Standard predictor leach test results of RNO-MHP June 2008 - % leached and 95 % confidence interval.
Sample Weeks Ni, % Co, % Mn, % Mg, %RNO 4 94.8 (±0.20) 92.5 (±0.26) 34.7 (±1.44) 71.1 (±0.32)
6 95.1 (±0.21) 93.4 (±0.42) 43.9 (±1.32) 72.7 (±6.97)12 93.5 (±0.22) 93.2 (±0.37) 40.4 (±1.45) 67.7 (±8.04)
Table 7.14. Reductive predictor test results of RNO-MHP June 2008 – % leached and 95 % confidence interval.
Sample Weeks Ni, % Co, % Mn, % Mg, %RNO 4 98.5 (±0.02) 98.9 (±0.29) 93.4 (±0.73) 75.9 (±1.02)
6 98.8 (±0.11) 99.5 (±0.13) 93.5 (±1.12) 83.2 (±4.38)12 97.7 (±0.29) 99.4 (±0.25) 92.2 (±0.18) 82.4 (±2.60)
Table 7.15. Reductive soak predictor test results of RNO-MHP June 2008 – % leached and 95 % confidence interval.
Sample Weeks Ni, % Co, % Mn, % Mg, %RNO 6 99.9 (±0.01) 99.9 (±0.01) 98.3 (±0.16) 92.2 (±0.05)
12 99.9 (±0.11) 99.9 (±0.17) 96.3 (±4.79) 89.7 (±7.74)81% Solid 6 99.9 (±0.03) 99.9 (±0.01) 97.5 (±0.55) 89.4 (±3.40)
12 99.8 (±0.02) 99.7 (±0.13) 94.3 (±0.72) 82.0 (±5.33)Dry Solid 6 99.9 (±0.01) 99.9 (±0.01) 99.2 (±0.08) 94.5 (±4.42)
12 99.8 (±0.01) 99.9 (±0.04) 96.1 (±0.36) 84.4 (±2.02)
Cobalt dissolution of over 92% by the standard predictor test in
Table 7.13 proved that less than 8% of the metal had oxidised. This 8%
oxidation occurred in the first 4 weeks. Although metal dissolution improved
in the presence of a reducing agent (Table 7.14), it may be due to the
enhanced reductive dissolution of manganese (from 34-43% to 92-94%)
rather than reduction of cobalt. The same effect was observed with nickel,
which is known to exist in its divalent state under these conditions. Clearly,
manganese is coexisting with nickel and cobalt, as in manganese nodules,
where the reductive acid leaching with SO2 improves the leaching of Mn, Ni
and Co (Senanayake, 2011).
7-50
From the difference between the standard and reductive predictor
leach test results it can be concluded that at least 52% of manganese was
oxidised after 12 weeks of ageing (Tables 7.13-7.14). Like cobalt, it looks as
though all of the oxidation occurred in the first 4 weeks. Over 99.8% Ni and
99.7% Co was leached by the reductive soak predictor test after 12 weeks of
ageing (Table 7.15). This was an improvement of approximately 2.2% Ni and
0.5% Co from the reductive predictor test.
The data in Table 7.15 proves that drying the precipitate had no effect
on recovery. Modified predictor leach test results in Tables 7.16 & 7.17 also
showed that drying had no effect on metal dissolution. In addition there
seems to be little difference between leach results over the 12 week period.
All leach results were lower than those from other commercial products
discussed in previous sections. However, the results exhibited similar trends.
Table 7.16. Modified standard predictor test results of RNO-MHP June 2008 – % leached and 95 % confidence interval.
Sample Weeks Ni, % Co, % Mn, % Mg, %RNO 4 90.1 (±1.26) 88.2 (±1.18) 25.9 (±2.97) 63.4 (±1.29)
6 91.2 (±0.17) 88.2 (±0.38) 31.0 (±0.52) 63.6 (±2.33)12 87.9 (±0.32) 87.7 (±0.46) 19.1 (±1.44) 60.1 (±0.70)
81% Solid 4 88.6 (±0.39) 86.0 (±0.55 17.7 (±0.58) 55.6 (±1.43)6 89.2 (±0.87) 85.7 (±1.01) 22.0 (±1.77) 54.4 (±4.15)
12 89.4 (±0.45) 86.6 (±0.77) 12.0 (±4.76) 50.5 (±4.84)Dry Solid 4 89.7 (±0.69) 88.1 (±0.30) 19.3 (±0.30) 58.0 (±0.48)
6 90.5 (±0.55) 89.1 (±0.40) 27.3 (±0.90) 59.2 (±2.98)12 89.9 (±0.41) 89.3 (±0.30) 18.4 (±1.76) 57.1 (±2.79)
7-51
Table 7.17. Modified reductive predictor test results of RNO-MHP June 2008 – % leached and 95 % confidence interval.
Sample Weeks Ni, % Co, % Mn, % Mg, %RNO 4 97.9 (±0.29) 98.5 (±0.20) 92.5 (±0.84) 79.3 (±2.83)
6 97.4 (±0.06) 98.8 (±0.01) 94.7 (±0.23) 82.5 (±1.99)12 95.2 (±0.28) 98.5 (±0.07) 92.0 (±0.38) 78.0 (±1.85)
81% Solid 4 96.2 (±0.39) 97.6 (±0.20) 89.4 (±0.93) 74.2 (±0.69)6 96.5 (±0.13) 98.2 (±0.05) 93.5 (±0.16) 75.7 (±2.22)
12 95.0 (±1.81) 97.3 (±1.09) 88.0 (±4.85) 62.6 (±10.6)Dry Solid 4 96.8 (±0.19) 97.9 (±0.05) 90.0 (±0.62) 75.7 (±1.06)
6 96.2 (±1.34) 98.0 (±0.71) 91.3 (±3.68) 69.8 (±14.8)12 97.0 (±1.52) 98.5 (±0.76) 93.6 (±3.16) 77.8 (±11.3)
The comparison of XRD scans on leach residues from standard and
reductive predictor tests (Figure 7.37) shows that a crystalline nickel and/or
magnesium hydroxide, and a comblainite-type structure (hydrotalcite)
remaining after leaching. These structures are probably slow leaching.
Although, nickel hydroxide hydrate was also possible, comblainite
(Ni6Co2(CO3)(OH)16•4(H2O)) seemed more likely. The similarity between
diffraction patterns suggests the two structures are not affected by reduction.
7-52
10 20 30 40 50 60 70 80
2 Theta
SPT 85 RPT 85 Ni(OH)2 Ni,Mg(OH)2 Comblainite
Figure 7.37. XRD scans of standard and reductive predictor leach test residues after 12 weeks (85 days) ageing.
7.7 Summary and Conclusions
Precipitation
• The SEM images of RNO-MHP over the four hour period revealed two
mechanisms of precipitation: (i) dissolution-nucleation-agglomeration,
and (ii) precipitation within the pores of MgO. Metal ions were
distributed evenly throughout the particles, as revealed by the SEM
images. Due to these mechanisms the size distribution of the RNO-
MHP was relatively large and did not change significantly over the
period (P80 = 71-83 μm). The larger particle size is a desirable quality
as it would settle and filter well. Towards the end of the precipitation
period, probably when the pores were filled, metals precipitated
on the outside of the Mg rich particles giving a higher overall nickel
and cobalt content.
7-53
• Both drying the precipitate and the incorporation of manganese
minimised the formation or influence of the Ni,Mg(OH)2. Manganese
probably formed a hydrotalcite-type structure with the nickel and
magnesium as it would oxidise readily to Mn(III). Although the
oxidation of manganese and cobalt was thought to be a problem by
BHP Billiton (Muir, 2003), this structure leached rapidly under reducing
conditions.
Characterisation
• The XRD analysis of RNO-MHP collected in June 2008 showed that
the precipitate was poorly crystalline and consisted of predominantly
hydrated nickel hydroxide and comblainite (hydrotalcite-like structure).
Neutron Diffraction had a similar pattern, confirming the poor
crystallinity of the precipitates.
• XPS analysis was conducted on a 12 week old sample containing
~3% cobalt, ~3% manganese and 19% nickel. After the original
analysis, the sample was ground in a mortar and pestle in an inert
atmosphere and analysed again. The lack of difference between the
2p doublet peaks with both analyses for cobalt and manganese
proved the extent of oxidation was the same throughout the particles.
This would be due to the porous nature of the precipitate.
• Surface area tests were conducted on the RNO-MHP (June 2008)
precipitate after 2, 5, 10, 20 and 60 minutes of leaching. The surface
area of the precipitate (38-53 μm) measured by the laser sizer
(assumes spherical particles) was 0.14 m2/g while the BET surface
7-54
area was 8.3 m2/g. This showed the high porosity of the RNO-MHP,
even before leaching. Moreover, the porosity increased over time,
indicating the leaching was occurring preferentially within the pores of
the precipitate (shrinking core kinetic model).
Leaching
• The precipitation of nickel within the MgO pores resulted in the
formation of stable-slow leaching Ni,Mg(OH)2. This structure was one
of the main causes of lower nickel and cobalt leaching from the
Ravensthorpe (as well as synthetic) mixed hydroxide precipitates.
However, the apparent rate constant for the shrinking core model is
larger for the Ravensthorpe MHP, compared to that of the synthetic
Ni,Mg(OH)2. This can be related to the high porosity of the
Ravensthorpe MHP. Moreover, the formation of Ni,Mg(OH)2 could be
minimised by lengthy residence times in the precipitation tanks and an
effective filtration/washing technique.
• Predictor leach tests on extensively aged (11 months – 5 years) pilot
plant and commercial MHP samples (Cawse, RNO Pilot and
European Nickel) revealed composition to be significantly more
influential than age. There was an indirect relationship between the
extent of nickel leaching and trivalent metal (Fe, Al, Cr and Mn)
composition in the MHP. Hydrotalcite-type structures proved to be
difficult to leach in most precipitates. Thus, in the aged MHP samples
from pilot plant and other commercial MHP samples the oxidation of
cobalt and manganese reduced the nickel leaching significantly.
However, this was overcome by the use of a reductant.
7-55
• Drying was also beneficial with precipitates containing cobalt,
aluminium and iron, and did not influence leaching recoveries with the
RNO-MHP collected in June 2008. Less moisture resulted in slower
transformation to stable-slow leaching compounds.
• Drying of RNO-MHP prior to transportation seems to have no
influence on nickel and cobalt leaching if a reductive leach is
conducted. This would result in lower transportation costs, so should
therefore be studied on a pilot or commercial scale prior to
incorporation into a flow-sheet.
• The extent of cobalt leaching was over 92% by the standard predictor
test on RNO-MHP collected in June 2008. This proved less than 8%
of the metal had oxidised over 12 weeks. From the standard and
reductive predictor leach results it was concluded at least 52% of
manganese had oxidised after 12 weeks. For both metals, it looks as
though all of the oxidation has occurred within the first 4 weeks. As
noted in Chapter 5, the titration tests to determine the extent of
oxidation on the synthetic precipitates over 12 weeks did not change
significantly, because the oxidation of cobalt and manganese had
occurred during precipitation and filtration.
• The extent of oxidation and the influence of oxidised manganese and
cobalt on nickel leaching was much less than predicted in the previous
research reports (Muir, 2003). The introduction of the reductive leach
using hydroxyalamine sulphate (or CoNiS described in Chapter 8) at
the Yabulu Refinery, improved the leaching results and proved that
7-56
the detrimental effect of oxidation is removed. Thus, the formation of
Ni,Mg(OH)2 and irreducible hydrotalcite-like structures were the main
causes of lower nickel and cobalt leaching from the RNO-MHP. This
was confirmed by the fact that these types of structures were present
in RNO-MHP as well as the leach residues from the standard and
reductive predictor tests.
• Nickel and cobalt leaching of the RNO-MHP after 12 weeks of ageing
were 97.8% and 99.4%, respectively. This suggests over 2% of nickel
is involved in a hydrotalcite-like structure or a nickel-magnesium
hydroxide. Moreover, 99.9% of both nickel and cobalt were leached by
the soak predictor test. This test replicated the residence time
associated with the CCD’s at the Yabulu refinery which entailed a 72
hour leach. These species are clearly stable and slow leaching. If
more stable structures or a larger quantity formed, or the CCD circuit
ran for less than 72 hours, nickel and cobalt recovery could be
lowered significantly.
• Through experimental work with synthetic precipitates described in
Chapter 5 it was discovered hydrotalcite-like structures require a
divalent and a trivalent metal, and an anion other than hydroxide. In
Ravensthorpe MHP these structures would form with divalent cations
(Ni2+, Co2+, Mg2+, Mn2+, Cu2+, Fe2+, Ca2+ or Zn2+), trivalent cations
(Co3+, Mn3+, Fe3+, Al3+ or Cr3+) and anions such as sulphate,
carbonate or chloride. Hydrotacite-like compounds with trivalent
cobalt, manganese and iron did not influence metal recoveries as the
trivalent metal was reduced during reductive leaching, releasing nickel
7-57
and cobalt for dissolution. However, the Aluminium(III) and
chromium(III) hydrotalcites composed with non-reduceable trivalent
cations gave lower metal recoveries.
• To minimise the formation of harmful hydrotalcite-like compounds,
aluminium(III), chromium(III), sulphate, carbonate and chloride
concentrations need to be minimised. This can be achieved by a
thorough precipitate washing technique, and oxidation of Cr(III) to
Cr(VI).
Proposed Changes for Ravensthorpe Plant
• BHP Billiton selected QMag’s Emag 45 as its neutralising agent at the
Ravensthorpe Plant. The solids remained in the slurry for 3 to 4 hours
after precipitation. A Larox pressure filtration system was used to
separate the solids.
• The RNO-MHP collected in June 2008 contained 2.58% manganese,
while typical MHP (based on pilot plant runs) contained 2.75%. At
Ravensthorpe, manganese was precipitated out of solution to control
its incorporation into MHP. This stage involved aeration and addition
of lime to raise the pH to 8.5 (Figure 1.2). As manganese
incorporation in the precipitate was beneficial, this stage could be
removed or less lime added to lower the pH of precipitation. Without
this stage, manganese incorporation would probably remain lower
than 5% as the pH was only raised to 7.2. Enough manganese may
be removed by MHP precipitation and scavenger precipitation to
7-58
prevent scaling. Removal of this stage or operating at a lower pH
would lower reagent costs and energy consumption.
• At the Ravensthorpe plant, sulphate, chloride and carbonate were in
significant concentrations in solution so were likely to precipitate with
the metal hydroxides. To minimise the formation of hydrotalcite-like
compounds, the concentrations of sulphate, carbonate and chloride in
the precipitate need to be minimised by a thorough washing technique
with deionised water. The incorporation of magnesium,
which forms the stable slow-leaching Ni,Mg(OH)2, was also minimised
by washing.
• The washing of the RNO-MHP during the Larox filtration process was
conducted with desalinated and demineralised water. Chloride and
carbonate concentrations in the precipitate were negligible. However,
sulphate concentrations could be up to 12% (Muir, 2003), whereas the
RNO-MHP collected in June 2008 contained 9.9% sulphate. More
washing could possibly lower the sulphate concentration.
8-1
8 REDUCTIVE LEACHING OF MIXED HYDROXIDE
PRECIPITATE WITH COBALT-NICKEL-SULFIDES
(CoNiS)
8.1 Introduction
Researchers have already developed a mixed Co-Ni sulphide (CoNiS)
reductant in order to leach the oxidised metals in the MHP. Work by BHP
Billiton (Price, 1979; Moroney, 2002; Miller, 1970; McGregor 2003a & 2003b;
Chappell, 2003) had determined that CoNiS was an effective reductant at
certain compositions. Test work by BHP Billiton discovered the ideal
sulphidation ratio of Metal to sulphide was 2.2:1 to remove the majority of
cobalt from the liquor during the precipitation of CoNiS (Price, 1979;
Moroney, 2002; Miller, 1970; McGregor 2003a & 2003b; Chappell, 2003).
The CoNiS precipitated from a mixed solution of Co(II) and Ni(II) in the
presence of CoNiS seed produced a precipitate with a higher cobalt
concentration. Bryson and Bijsterveld (1991) discovered the precipitation of
cobalt had three kinetic regions; an induction period, followed by rapid
precipitation and then a slow approach to equilibrium. Reports based on BHP
Billiton test work (Moroney, 2003) stated that the higher levels of cobalt were
attributed to the reduction of Co(III) by the CoNiS seed. However, little was
known about the reason for the reductive role, reaction mechanism and the
effect of composition and preparation conditions on reactivity of CoNiS.
Thus, CoNiS was investigated as part of this project as it was an integral part
of the MHP leaching process at the Yabulu refinery. The synthesis,
8-2
composition and reactivity based on precipitation conditions and the reaction
mechanism were investigated.
8.2 Precipitation and Characterisation of CoNiS
8.2.1 Precipitation Diagrams
The sulphide solubility diagram published by Monhemius at 25oC
(1977), and the diagram constructed for this study relevant to 45oC,
(Figures 8.1 & 8.2) summarise important aspects relevant to sulphide
precipitation. Cobalt(II) has a slightly lower solubility than nickel(II), and
increasing the temperature from 25oC and 45oC shifts the solubility lines to
higher pH by ~1.5 units on the logarithmic scale. According to Figures 8.1
and 8.2, cobalt(II) sulphide precipitation is preferential at lower temperatures.
Figure 8.1. Sulfide solubility diagram at 25°C (Monhemius, 1977).
8-3
Figure 8.2. Sulfide solubility diagram at 45°C (constructed from data in Monhemius 1977 and Ksp values at 45°C obtained from the HSC 6.1
database: Table 8.1).
Table 8.1. Ksp values at 45°C (obtained from HSC 6.1 database: Roine, 2001)
Ag2S 7.53 x 10-47 Cu2S 5.53 x 10-46 CuS 5.83 x 10-35 PbS 2.18 x 10-27 ZnS 6.95 x 10-24 CoS 3.25 x 10-22 NiS 5.16 x 10-23 FeS 6.70 x 10-17 MnS 1.23 x 10-13
8.2.2 Precipitation and Analysis
The mixed cobalt-nickel-sulphides (CoNiS) were precipitated from a
solution of Ni(II)+Co(II) using ammonium sulphide. A range of precipitates
were produced by varying the (i) temperature, (ii) oxidation state of cobalt (II
& III), and (iii) molar ratio of ammonium sulphide to cobalt (sulphiding ratios).
The first six precipitates were produced at 25 or 40°C with three differing
cobalt(III) concentrations (%) in the range 0%, 53-56% and 100% and a 2.2:1
8-4
sulphiding ratio. The final four precipitates were produced at 25°C with 0%
cobalt(III) at different sulphiding ratios in the range 1:1 to 3:1. Tables 8.2 and
8.3 show the concentration (mM) of cobalt, nickel and sulphide in the initial
solution used for the precipitation as well as the elemental composition
(mass%) of the precipitates, molar ratios and a nominal chemical formula
based on the molar ratios.
Table 8.2. Preparation conditions and composition of CoNiS
Sample Ni(II) mM
Co(II) mM Co(III)% (NH4)2S mM ToC
Co %
Ni %
S %
CoNiS-1 170 17 0 37.4 25 13.1 9.20 32.5 CoNiS-2 170 17 50 37.4 25 11.4 13.4 45.4 CoNiS-3 170 17 100 37.4 25 9.5 14.1 49.0 CoNiS-4 170 17 0 37.4 40 13.1 12.7 45.5 CoNiS-5 170 17 50 37.4 40 9.70 14.2 42.9 CoNiS-6 170 17 100 37.4 40 6.80 14.7 44.4 CoNiS-7 170 17 0 17.0 (1:1) 25 14.8 11.4 39.0 CoNiS-8 170 17 0 25.5 (1.5:1) 25 15.9 9.80 39.9 CoNiS-9 170 17 0 37.4 (2.2:1) 25 13.1 9.20 32.5
CoNiS-10 170 17 0 51.0 (3:1) 25 12.6 12.9 40.2 CoNiS-Yabulu - - - - - 26.2 9.30 32.2
1 g/L Co(II orIII) and 10 g/L Ni(II) as sulphate, in 1 L solutions under a N2 blanket with different ratios of Co to (NH4)2S; Values in parentheses represent the ratio of (NH)2S : Co.
Table 8.3. Molar ratios and formula of CoNiS.
Sample Temp. °C Co/S Ni/S Co/Ni(Co+Ni)/
S Formula CoNiS-1 25 0.22 0.15 1.44 0.37 CoNi0.7S4.6 CoNiS-2 25 0.19 0.16 1.20 0.30 CoNi1.2S7.3 CoNiS-3 25 0.16 0.16 1.03 0.26 CoNi1.5S9.5 CoNiS-4 40 0.22 0.15 1.46 0.31 CoNi1.0S6.4 CoNiS-5 40 0.16 0.18 0.91 0.30 CoNi1.5S8.1 CoNiS-6 40 0.12 0.18 0.64 0.26 CoNi2.2S12 CoNiS-7 25 0.25 0.16 1.58 0.37 CoNi0.8S4.9 CoNiS-8 25 0.27 0.13 2.01 0.35 CoNi0.6S4.6 CoNiS-9 25 0.22 0.15 1.44 0.37 CoNi0.7S4.6
CoNiS-10 25 0.21 0.17 1.22 0.35 CoNi1.0S5.9 CoNiS-Yabulu - 0.44 0.16 2.82 0.60 CoNi0.4S2.3
Based on results reported in Table 8.2
8-5
Table 8.4. Precipitation Reactions for Co-Ni-S No
Reaction Log K at 25oC
1 Ni(NH3)62+ + HS- = NiS + NH4
+ + 5NH3 18.3 2 Ni(NH3)6
2+ + HS- = NiS + NH4+ + 5NH3 (45oC) 17.6
3 Co(NH3)62+ + HS- = CoS + NH4
+ + 5NH3 18.8 4 Co(NH3)6
2+ + HS- = CoS + NH4+ + 5NH3 (45oC) 18.2
5 2Co(NH3)63+ + 3HS- = 2CoS + S + 3NH4
+ + 9NH3 115 6 2Co(NH3)6
3+ + 3HS- = CoS + CoS2 + 3NH4+ + 9NH3 123
7 8Co(NH3)63+ + 10HS- + 3H2O = 8CoS + S2O3
2- + 16NH4+ + 32NH3 308
8 CoS + S2O32- = CoS2 + SO3
2- 2.94 9 NiS + S2O3
2- = NiS2 + SO32- 1.32
10 CoS2 + Ni(NH3)62+ = Co(NH3)6
2+ + NiS2 -2.10 Based on HSC 6.1 database
Equilibrium constants for the precipitation reactions based on the
HSC 6.1 database (Roine, 2001) are listed in Table 8.4. Large equilibrium
constants indicate the feasibility of the precipitation of NiS, CoS and CoS2 as
well as elemental sulphur in some cases. The formation of CoS2 can take
place via direct reactions or via S2O32-/SO3
2- ions. However, the formation of
NiS2 according to reaction 10 is thermodynamically not feasible as revealed
by the very low equilibrium constant. Other important points on Tables
8.2-8.4 are listed below:
(i) The metal and sulphur percentages in Table 8.2 do not add to 100%
due to absorbed water content. As more cobalt was present in its
trivalent state, less cobalt and more nickel were incorporated in the
precipitate.
(ii) The composition of sulphur in the samples exhibited no trends, except
in the first three (at 25oC) where the sulphur content increased with
the increase in Co(III) content in the initial mixture. Trivalent cobalt
needs to be reduced to its divalent state before precipitation as CoS,
as shown by the reactions in Table 8.4, facilitating the formation of S,
8-6
CoS2 or S2O32- by the redox reaction. This also lowers the cobalt
content and enhances the nickel content in the precipitate at each
temperature, as shown in Table 8.2.
(iii) The precipitates produced at a lower temperature contain more cobalt,
due to lower solubility as predicted from the solubility diagrams in
Figures 8.1 and 8.2 and therefore have a higher Co:S and Co:Ni
ratios.
(iv) Adjusting the quantity of ammonium sulphide added also had little
effect on cobalt and nickel incorporation. When sulphide was in
excess, the precipitation of nickel and cobalt seemed to be
independent of the initial sulphide concentration, as shown by the
relatively unaffected ratio of (Co+Ni)/S in Table 8.3 for CoNiS 7 to
CoNiS 10.
In summary, the cobalt precipitation was more favourable at lower
temperatures (25°C) and when cobalt was in its divalent state, as evident
from the assay results (Tables 8.2-8.3), and as noted by the precipitation
diagrams (Figures 8.1-8.2) and equilibrium constants (Table 8.4). The test
work by BHP Billiton revealed that the ideal sulphiding ratio to remove the
majority of cobalt from Ravensthorpe MHP was 2.2:1 (Price, 1979; Moroney,
2002; Miller, 1970; McGregor 2003a & 2003b; Chappell, 2003). Since the
concentration of the sulphide reductant (for Co(III)) seemed to remain in
excess, it would be pointless to use any more than is required to remove the
desired quantity of cobalt.
8-7
Tables 8.2-8.3 also compare the metal composition of CoNiS
produced at Murdoch laboratories (CoNiS-1 to CoNiS-10) and in the Yabulu
processing plant research laboratories (CoNiS-Yabulu). The CoNiS-Yabulu is
produced with seed, resulting in a product with a higher cobalt concentration.
If cobalt composition in CoNiS is the measure of quality, the CoNiS-Yabulu
product is superior. The product liquor at the Yabulu refinery (approximately
0.5 g/L Co and 5 g/L Ni) is similar in ratio to the synthetic liquor (1 g/L Co and
10 g/L Ni, described in Chapter 3) used for the CoNiS precipitation in the
laboratory. Therefore, the difference in metal composition must be due to
experimental technique and reagents used. For example, the sulphiding
reagent used at the Yabulu refinery is H2S, compared to (NH4)2S used in the
laboratory preparation.
Bryson and Bijsterveld (1991) discovered that the seeding eliminated
the induction period. Moroney (2003) stated that the higher levels of cobalt in
the CoNiS precipitate can be attributed to the reduction of Co(III) by the
CoNiS seed which enhance the Co content in the final precipitate. Thus, a
simple test was conducted whereby nickel(II) and cobalt(II) were precipitated
with and without seed. The ratio ([before]/[after]) of concentration was 6.3 for
nickel and 7.7 for cobalt, without seed. These ratios were 6.4 and 7.8,
respectively, with seed. The largest 95% confidence interval was ±0.68. With
this system, it is clear that the seeding had little influence on cobalt
precipitation. Therefore, the differences in cobalt incorporation would be due
to different oxidation states Co(II) and Co(III) in the initial liquor and the
differences in porosity.
8-8
8.2.3 X-Ray Diffraction and Scanning Electron Microscopy of CoNiS
XRD was conducted on the CoNiS samples produced at 25°C with
varying initial cobalt oxidation states. It was not only difficult to determine the
composition of CoNiS but also to distinguish between the three XRD traces
(Figure 8.3). Numerous nickel and cobalt sulphides were present, which
would have varying degrees of substitution, causing a shift in peak positions.
Elemental sulphur seems to be present as a result of the addition of excess
ammonium hydrogen sulphide and reactions described in Table 8.4, which
also describe the possibility of formation of CoS+CoS2. The most intense
peak around 22° for sulphur is missing. The sulphides of different
compositions/formulae in Table 8.3 can also be treated as mixtures of
NiS+CoS+CoS2 of different ratios of Co:Ni:S, noting that the formation of
NiS2 is not feasible as indicated by very low values of log K for reaction 10 in
Table 8.4. Thus, the only distinguishable nickel peak seemed to be that of
NiS (millerite) at around 17 degrees. The two most prominent peaks between
20 and 25 degrees could not be identified. As mentioned in the previous
section, the composition of nickel in the precipitate increases with the
quantity of trivalent cobalt in solution. Consequently more millerite existed in
the top two traces in Figure 8.3. Due to the lack of similarity between traces,
other XRD results were omitted and no further XRD was performed.
8-9
10 20 30 40 50 60 70 80
2 Theta
25 deg C, 0 % 25 deg C, 53 % 25 deg C, 100 % CoS2Co3S4 Co4S3 (cubic) Co4S3 (hexagonal) CoNi2S4NiS Millerite, NiS Ni3S4 Ni3S2 (rhombohedral)NiS2 Ni3S2 (cubic) S
Figure 8.3. XRD scans of CoNiS samples – effect of cobalt oxidation state at 25°C.
SEM was conducted on the CoNiS samples in an effort to determine
physical differences between the precipitates. Figure 8.4 shows that the
precipitate consists of round agglomerated particles, so no preferential
orientation would have occurred. All precipitates looked very similar; they
were ‘fluffy’ in appearance and seemed to consist of small particles
agglomerated together into particles over 20 μm in size.
8-10
Figure 8.4. SEM image of unseeded CoNiS produced at 25°C with a divalent oxidation state and sulphidation ratio of 2.2:1.
8.3 Redox Behaviour of Sulfides in SAC solutions
As noted in the previous section the mixed Co-Ni-sulphide (CoNiS)
precipitate is a complex mixture of NiS, CoS, CoS2 etc. The redox reactions
between MnOOH and CoNiS can take place via the formation of S, S2O32- or
SO42-. Thermodynamic calculations based on the literature data show the
possibility of reduction of both MnOOH and Co(OH)3 by NiS and CoS, largely
due to the stability of elemental sulphur and sulphate anion (Table 8.5).
Although CoNiS acts as a reductant during MHP leaching it is most likely that
the dissolved species from CoNiS in SAC solution are responsible for the
reducing ability of CoNiS, rather than a reaction between CoNiS and MHP in
solid state. Therefore, the examination of the dissolution/redox behaviour of
individual components would provide useful information on the complex
behaviour of CoNiS. This was achieved by testing the dissolution/redox
8-11
behaviour of S, Na2S, NiS, CoS and NiS+CoS in SAC solutions under N2, air
and in the absence or presence of sufite ions.
Table 8.5. Possible reactions of sulphides with Mn(III) and Co(III) oxides Reactions Log K
1 2MnOOH + 6NH4+ + 38NH3 + NiS = 2Mn(NH3)4
2+ + Ni(NH3)62+ + S + 4H2O -3.66
2 8MnOOH + 18NH4++ 26NH3 + 2NiS = 8Mn(NH3)4
2+ + 2Ni(NH3)62+ + S2O3
2- + 13H2O 3.38
3 8MnOOH + 16NH4++ 22NH3 + NiS = 8Mn(NH3)4
2+ + Ni(NH3)62+ + SO4
2- + 12H2O 20.2
4 2MnOOH + 6 NH4++ 8NH3 + CoS = 2Mn(NH3)4
2+ + Co(NH3)62+ + S + 4H2O -7.79
5 8MnOOH + 18 NH4++ 26NH3 + 2CoS = 8Mn(NH3)4
2+ + Co(NH3)62+ + S2O3
2- + 13H2O -4.78
6 8MnOOH + 16NH4++ 22NH3 + CoS = 8Mn(NH3)4
2+ + Co(NH3)62+ + SO4
2- + 12H2O 17.1
7 2Co(OH)3 + 6NH4++ 12NH3 + NiS = 2Co(NH3)6
2+ + Ni(NH3)62+ + S + 6H2O 16.1
8 8Co(OH)3 + 18NH4++ 42NH3 + 2NiS = 8Co(NH3)6
2+ + 2Ni(NH3)62+ + S2O3
2-+ 21H2O 82.2
9 8Co(OH)3 + 16NH4++ 38NH3 + NiS = 8Co(NH3)6
2+ + Ni(NH3)62+ + SO4
2- + 20H2O 96.8
10 2Co(OH)3 + 6NH4++ 12NH3 + CoS = 3Co(NH3)6
2+ + S + 6H2O 11.9
11 8Co(OH)3 + 18NH4++ 42NH3 + 2CoS = 10Co(NH3)6
2+ + S2O32- + 21H2O 73.9
12 8Co(OH)3 + 16NH4++ 38NH3 + CoS = 9Co(NH3)6
2+ + SO42- + 20H2O 92.7
Based on HSC 6.1 database
8.3.1 Redox Behaviour of Elemental Sulphur and Sulphide ions
Although the formation of elemental sulphur is a possibility in the case
of the reduction of MnOOH by CoS (Table 8.5), the oxidation of elemental
sulphur formed in this manner to various sulphur-oxygen species is unlikely
as sulphur is hydrophobic and very difficult to oxidise at ambient conditions
(Habashi & Bauer, 1966; Feng & Van Deventer, 2002). To confirm this,
elemental sulphur was placed in a SAC solution with oxygen sparging or
ferric sulphate with nitrogen sparging. The measured oxidation-reduction
potential (ORP) of a platinum electrode with respect to a silver/silver chloride
8-12
electrode did not change after the addition of elemental sulphur indicating
that there was no redox reaction.
Although the oxidation of metal sulphides can produce sulphate as the
most stable anion, indicated by large equilibrium constants in Table 8.5, the
reaction takes place via a number of sulphur-oxygen species including HS-,
S2O32- and SO3
2- ions. In order to test this, sodium sulphide was added to a
stirred SAC solution with an oxygen flow of 1 L/min for over an hour. As
shown in Figure 8.5, the measured ORP decreased to -450 mV (vs. Ag/AgCl
reference electrode) upon addition of sodium sulphide. The ORP continued
to decrease to -490 mV vs. Ag/AgCl (-280 mV vs. SHE) where it remained
unchanged for the hour. The potential of -280 mV vs. SHE is consistent with
the predicted and measured redox couple HS-/S2O32- in the published Eh-pH
diagram by Aylmore and Muir (2001) and Senaputra et al. (2008) shown in
Figures 8.6a-c.
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0 500 1000 1500 2000 2500 3000 3500 4000
Time, s
Pot
entia
l, V
Figure 8.5. ORP (vs. Ag/AgCl) of sodium sulphide with 1 L/min oxygen in 1 L of SAC at 25°C.
8-13
After one hour of leaching, barium chloride was added to the solution
in excess to precipitate barium sulphate. According to the mass of BaSO4
precipitate, approximately 80% of sulphide had oxidised to sulphate. After
filtration, hydrogen peroxide was added to oxidise sulphite and thiosulphate
with more barium chloride to precipitate sulphate. A further 10% of sulphate
was precipitated. No elemental sulphur was present at the end of the
reaction. The lower potential and the absence of sulphur prove an alternative
reaction is occurring. Sulfide must be directly oxidised to thiosulphate then to
sulphite and sulphate, without forming elemental sulphur as an intermediate
product predicted in Figures 8.6(b) and 8.6(c).
8-14
(c)
Figure 8.6. Potential-pH diagrams of Ni-NH3-S-H2O system (from Senaputra et al. (2008), (a) and (b) at 45oC; Aylmore and Muir (2001), (c) at
25oC). Symbols show measured values of a nickel electrode.
8-15
8.3.2 Redox/dissolution Behaviour of NiS and CoS
The non-oxidative dissolution behaviour of nickel and cobalt from
various sulphides in SAC solutions under N2 was monitored and the results
are summarised in Figure 8.7. Nickel dissolution from NiS was superior to
cobalt dissolution from CoS in the presence of N2 (Figure 8.7). According to
Figure 8.8, which shows the change in anionic speciation with pH, it is
reasonable to assume the presence of NH3, HCO3-, HS- and SO3
2- anions at
pH 10-10.5 (as shown in some of the equations listed in Table 8.6). Thus, the
dissolution of NiS and CoS in ammoniacal solutions can be expressed by
reactions 1 and 2 in Table 8.6.
The calculated saturated solubility M(II) from NiS and CoS in a
solution of 1 M NH4+ and 5 M NH3 according to reactions 1 and 2 are close to
52 mg/L Ni(II) and 0.5 mg/L Co(II), respectively. These values were
calculated using the equilibrium constants, assuming unit activity coefficients,
and ignoring the ion-association between M(NH3)62+ and HS-, HCO3
- or CO32-
anions. The higher concentration of Ni(II) than Co(II) in Figure 8.7 may be
related to the higher equilibrium constant of Eq. 1 compared to that of Eq.2.
The higher measured values in Figure 8.7 indicates the interaction between
the anions (ion-association) and the dissolved cations in actual solutions, as
well as other reactions are responsible for the improved solubility. The ion-
association was investigated in this study by measuring the solubility of
nickel(II) in ammonia solutions with carbonate, sulphate, chloride and nitrate
ligands for comparison.
8-16
Figure 8.7. CoS and NiS dissolution in SAC at 25°C with 1 L/min N2.
Figure 8.8. Effect of pH on the speciation of (a) CO2 and SO2, (b) NH3 and S2- (at 45oC, Senaputra et al., 2008).
8-17
Table 8.6. Non-Oxidative or Oxidative dissolution of NiS and CoS. No. Reaction Log K
1 NiS + NH4+ + 5NH3 = Ni(NH3)6
2+ + HS- -9.6
2 CoS + NH4+ + 5NH3 = Co(NH3)6
2+ + HS- -13.7
3 NiS + Co(NH3)62+ = Ni(NH3)6
2+ + CoS 4.11
4 2NiS + 2O2 + 2NH4+ + 10NH3 = 2Ni(NH3)6
2+ + S2O32- + H2O 122
5 2CoS + 2O2 + 2NH4++ 10NH3 = 2 Co(NH3)6
2+ + S2O32- + H2O 115
6 2CoS + 2.5O2 + 4NH4+ + 8NH3 = 2 Co(NH3)6
3+ + S2O32- + 2H2O 134
7 NiS2 + SO32- = NiS + S2O3
2- -1.31
8 NiS2 + HSO3- + NH3 = NiS + S2O3
2- + NH4+ 0.73
9 NiS2 + HSO3- + NH3 + NiS + NH4S2O3
- 1.66
10 CoS2 + SO32- = CoS + S2O3
2- -2.93
11 CoS2 + HSO3- + NH3 = CoS + S2O3
2- + NH4+ -0.89
12 CoS2 + HSO3- + NH3 = CoS + NH4S2O3
- 0.04
13 2Co(OH)3 + 2S2O32- + 6NH4
+ + 6NH3 = 2Co(NH3)62+ + S4O6
2- + 6H2O 14.7
14 8Co(NH3)63+ + 2HS- + 8NH3 + 3H2O = 8Co(NH3)6
2+ + NH4S2O3- + 7NH4
+ 68.2
Based on HSC 6.1 database
A simple Ni,Mg(OH)2 precipitate was synthesised for testing with
solutions containing 90 g/L ammonia with 1.47 mol/L of the selected anion
using an ammonium salt. This equates to 60 g/L CO2, 142 g/L SO42-, 52 g/L
Cl- and 91 g/L NO32-. The effect of pH, which changed in the range 10.50 and
10.65 in different anion systems, is expected to be negligible. The measured
Ni(II) concentrations after 6 hours of leaching Ni,Mg(OH)2 are shown in
Figure 8.9.
8-18
CO32-
SO42-
Cl- NO32-
0
5
10
15
20
25
30
Nic
kel C
once
ntra
tion,
g/L
Figure 8.9. Effect of anions on nickel(II) dissolution from Ni,Mg(OH)2 in 90 g/L ammonia with 1.47 mol/L of the anion solutions at 25°C.
Molar ratios of NH3/Ni were 17.8 for carbonate, 14.5 for sulphate and
23 for chloride and nitrate. Results prove that the complexation (or ion-
association) with the buffer anions affect Ni(II) solubility (Figure 8.9). Chloride
and nitrate are known to have little complexing ability so the improvement
with carbonate and sulphate would probably be due to the formation of
complex species containing the anion. Sulfate must therefore form a stronger
complex ion than carbonate. There is a 23% improvement in solubility
between sulphate and carbonate, a 58% improvement between sulphate and
chloride and a 29% improvement between carbonate and chloride. Figure
8.8a shows that HSO3- and CO3
2- ions are predominant in solutions of pH 10-
10.5. Therefore, the concentration of carbonate, sulphate and possibly the
other ions such as sulphite and thiosulphate in the Yabulu (Caron) process
liquors will have an influence on metal ion solubility.
8-19
Cobalt(II) ions would probably behave in a similar manner to nickel
shown in Figure 8.9 whereby solubility would depend on the complexing
ability of ligands and anions in solution. However, similar tests conducted
with a Co,Mg(OH)2 precipitate gave results which were not reproducible.
Solubility would probably depend on the dissolved oxygen concentration and
the oxidation state of the starting material. Thus, the concentration of nickel
and cobalt dissolved from NiS and CoS was measured under the flow of air
(instead of N2) and the results are compared in Figures 8.10 and 8.11.
The dissolution of Ni and Co improved significantly in the presence of
air as an oxidant. Nickel dissolution improved by close to 200% (Figure 8.10)
while cobalt dissolution improved by 800%, after 20 minutes (Figure 8.11).
As nickel is unlikely to oxidise, the increase in nickel dissolution would be
related to the oxidation of sulphide by dissolved oxygen from air. In the case
of cobalt sulphide, the dissolved oxygen from air would oxidise both the
sulphide and cobalt causing higher dissolution, as expected from the larger
equilibrium constants for Equations 4-6 in Table 8.6 representing a higher
driving force.
8-20
050
100150200250300350400450500
0 5 10 15 20 25
Ni(I
I) C
once
ntra
tion,
mg/
L
Time, min
N2 air
Figure 8.10. NiS dissolution in SAC at 25°C with 1 L/min N2 or air.
0
50
100
150
200
250
300
350
400
450
0 5 10 15 20 25
Co
Con
cent
ratio
n, m
g/L
Time, min
N2 air SO3
Figure 8.11. CoS dissolution in SAC at 25°C with 1 L/min N2 or air, and sulphite with N2.
Recent studies on the leaching of NiS in ammoniacal ammonium
carbonate solutions have shown that Ni leaching was enhanced by SO32- and
hindered by S2O32- as evident from the descending reactivity of NiS caused
by different anions: SO32- > SO4
2- > HCO3- > HS- > S2O3
2- (Senaputra et al.,
2008). This was related to the passivation by NiS2, where the presence of
8-21
SO32- prevented the passivation by NiS2 due to the reaction with HSO3
- to
produce NH4S2O3- ion (Equations 8 and 9 in Table 8.6). The reason for the
beneficial effect of SO32-
and N2 on CoS dissolution in Figure 8.11 is not
clear, but indicates reductive reaction and/or ion association.
8.3.3 Redox/dissolution Behaviour of CoNiS
The dissolution of CoNiS (with 50:50 NiS and CoS) shows the
influence of sulphur species on cobalt dissolution in Figure 8.12 and
Table 8.7. The concentration of Ni(II) and Co(II or III) after 2 and 20 minutes
are also listed in Table 8.7 for comparison. Even though half the cobalt was
present in the starting material, dissolution of Co from CoNiS after 20
minutes under N2 (50 g/L) in Figure 8.12 was better than that from CoS alone
(34 mg/L) in Figure 8.11. However, the dissolution of Ni from CoNiS under N2
(157 mg/L) was lower than that from NiS (258 mg/L). In the case of NiS and
CoS, air facilitates the dissolution of both Ni and Co after 2 and 20 minutes.
In the case of CoNiS, air facilitates the dissolution of Ni initially (2 min) but
retards later (20 min). The dissolution of Co from CoNiS is retarded by air
throughout the test period of over 2-20 min (Figure 8.12).
8-22
0
20
40
60
80
100
120
140
160
180
0 5 10 15 20 25
Ni(I
I) or
Co
Con
cent
ratio
n, m
g/L
Time, min
Co, air Ni, air Co, N2 Ni, N2
Figure 8.12. Ni and Co dissolution from 50-50 CoNiS in SAC solutions at 25°C with 1 L/Min N2.
Table 8.7. Metal ion concentrations in SAC during leaching of sulphides
Sulfide
Gas Ni(II) mg/L Co(II or III) mg/L
2 min 20 min 2 min 20 min NiS N2 22.1 258 - - NiS Air 41.6 461 - - CoS N2 - - 13.8 33.5 CoS Air - - 18.9 267
CoNiS (50:50) N2 13.6 157 5.85 50.3 CoNiS (50:50) Air 68.3 110 3.13 5.85
Results from Figures 8.11-8.12
A further comparison between CoS, NiS and CoNiS is shown in Figure
8.13a-b on the basis of the fraction of Ni or Co dissolution in each case.
Figure 8.13a shows that the presence of air facilitates the dissolution of Ni
from NiS and Co from CoS. In contrast, Figure 8.13b shows that the
presence of air is detrimental for Co dissolution from CoNiS, but Ni
dissolution from CoNiS is facilitated by air in the initial 10 minutes.
8-23
0.0
0.2
0.4
0.6
0.8
1.0
0 5 10 15 20Time / minutes
Frac
tion
of M
etal
Lea
ched
Ni (NiS)-air
Ni (NiS)-N2
Co (CoS)-air
Co (CoS)-N2
(a) from NiS or CoS
0.0
0.2
0.4
0.6
0.8
1.0
0 5 10 15 20Time / minutes
Frac
tion
of M
etal
Lea
ched
Ni (CoNiS)-air
Co (CoNiS)-N2
Co (CoNiS)-air
Ni (CoNiS)-N2
(b) from CoNiS
Figure 8.13. Fraction of Ni and Co dissolution in SAC at 25°C from different sulphides with 1 L/Min N2 and air
Similar tests were also conducted with RNO-MHP which was leached
in SAC in an open vessel, or with nitrogen or air sparging to examine the
effect of air in the absence of sulphide (Figure 8.14). Dissolution was
improved in the presence of oxygen indicating that the oxidation of Co(II) to
Co(III) is causing the enhanced dissolution.
020406080
100120140160180200
0 5 10 15 20
Co
Con
cent
ratio
n, m
g/L
Time, mins
N2 Open Air
Figure 8.14. Cobalt dissolution from RNO-MHP in SAC solution in an open vessel, or with 500 mL/min N2 or air.
8-24
8.3.4 Relative Dissolution of Ni(II) and Co(II) from CoNiS
The concentrations of dissolved nickel and cobalt were measured,
whilst dissolving the different CoNiS samples in SAC under a N2 blanket, to
see if a relationship existed between metal dissolution and the composition of
CoNiS synthesised using different sulphide/cobalt ratios (Figure 8.15a). Not
all the results were included as there seemed to be too much fluctuation to
make any conclusions based on rates. Due to the faster dissolution of Ni(II)
compared to Co(II) the concentration ratio of Co/Ni in solution was lower than
1 in most cases. However, the Co/Ni ratio increases with time as more cobalt
enters the solutions. It is more realistic to consider the ratio of Co/Ni based
on fraction dissolved due to different Co and Ni compositions of the 4
samples of CoNiS reported in Figure 8.15a. Thus, Figure 8.15b shows the
ratio of the fraction of Co/Ni dissolved as a function of time. Also, the higher
Co/Ni ratio in solution is expected to improve the effectiveness of CoNiS as a
reductant as shown later.
8-25
(a)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0 10 20 30 40 50 60 70
Time, min
Co/
Ni C
once
ntra
tion
Rat
io in
So
lutio
n
1:1 1.5:1 2.2:1 3:1
(b)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 10 20 30 40 50 60 70
Time, min
Co/
Ni R
atio
of F
ract
ion
Dis
solv
ed
1:1 1.5:1 2.2:1 3:1
Figure 8.15. (a) Co/Ni ratio in solution, and (b) Co/Ni ratio of fraction dissolved from CoNiS precipitates produced at 25°C with varying sulphiding
ratios in SAC solution under N2, in the absence of MnOOH.
8.3.5 ORP of NiS, CoS and CoNiS in Contact with SAC Solutions
The ORP during the dissolution of NiS, CoS and CoNiS was
measured, but was variable and not reproducible. Potentials ranged between
-200 and -350 mV vs. SHE. These values are close to some of the predicted
8-26
potentials, indicating that the dissolution takes place via S2O32- ions in the
anodic reactions listed in Table 8.8, leading to the overall reactions listed in
Table 8.6. The potentials of various redox couples in Table 8.8 are lower
than the Eh of MnOOH/Mn(NH3)42+ and Co(NH3)6
3+/Co(NH3)62+ couples in
the range 100-200 mV, based on the Eh-pH diagrams reported in Chapter 4
(Figure 4.4). Therefore, both sulphides are effective in the reduction of
MnOOH. Moreover, the nickel sulphides have a much lower reduction
potential. In both cases, the potentials are more negative at a higher
temperature of 60°C (Table 8.8). Thus, there is no relationship between
reduction potential and metal to sulphide ratio.
The variation of measured potentials in the range -200 to -350 mV (vs.
SHE) was probably due to ammonium hydrogen sulphide remaining in
solution as the precipitates were decanted prior to leaching to avoid ingress
of oxygen and a lowering of sulphide reactivity. The exposure to air during
preparation and storage of CoNiS would lower the reactivity due to the
oxidation of cobalt and sulphide. From the results it is clear that the presence
of nickel sulphide is vital, as it initialises reduction via HS- due to its faster
dissolution to Ni(NH3)62+ and HS-. Cobalt dissolution will then improve when
sulphur species are in solution as any trivalent cobalt would need to be
reduced for dissolution (e.g. reaction 13 in Table 8.6). This would allow for
further reduction of Mn-oxides as described later.
8-27
Table 8.8. Oxidation half cell reactions of nickel sulphides (HSC). No. Reaction Eo(25oC) Eo(60oC)
1 −+−+ +++→++ eHOSNHNiOHNHNiS 63)(36 232
263232 -280 -300
2 −+−+ +++→++ eHOSNHNiOHNHSNi 10122)(3618 232
2632343 -250 -300
3 −+−+ +++→++ eHOSNHNiOHNHNiS 66)(2362 232
26323 -260 -290
4 −+−+ +++→++ eHOSNHNiOHNHNiS 6305)(12157212 232
2632384.0 -260 -290
5 −+−+ +++→++ eHOSNHNiOHNHSNi 106)(3318 232
2632323 -280 -300
6 −+−+ +++→++ eHOSNHCoOHNHCoS 66)(3 232
23232 -150 -220
7 −+−+ +++→++ eHOSNHCoOHNHSCo 10122)(363 232
232343 -140 -210
8 −+−+ +++→++ eHOSNHCoOHNHCoS 6122)(3633 232
232333.1 -40 -100
9 −+−+ +++→++ eHOSNHCoOHNHCoS 66)(2322 232
2323 -180 -240
10 −+−+ +++→++ eHOSNHCoOHNHCoS 6244)(91299 232
232389.0 -140 -200
11 −+−+ +++→++ eHOSNHCoOHNHSCo 22244)(9129 232
232389 -150 -210
8.4 Reductive Leaching of MnOOH by CoNiS in SAC Solution
The MnOOH October precipitate (discussed in Chapter 4, section 4.2)
was leached with each CoNiS sample, with a Mn:CoNiS molar ratio of 2.2:1
unless stated otherwise. This was thought to best represent the proposed
Yabulu Expansion Project (YEP) secondary leach conditions. Size analysis
was performed on the CoNiS precipitates to ensure that surface area would
not influence dissolution rate. The size (P80) ranged between 130-160 μm
with no observable trends between samples. For the purpose of this
investigation it was assumed that the difference in size would not influence
the reaction rate significantly.
8-28
The % reduction of MnOOH by the ten CoNiS samples prepared in
the laboratory (1-10) and the sample from the Yabulu refinery (CoNiS-
Yabulu) is compared in Table 8.9 and Figure 8.16. As the Ni:S ratio of the
ten laboratory sulphides is similar (in the range 0.13-0.17), it was clear that
the reduction depends almost directly on the Co:S ratio which changed in the
range 0.12-0.27 (Table 8.9), depending on the starting oxidation state and
temperature during the precipitation of CoNiS. The most effective sulphide
was CoNiS-1 which resulted in 35% of the Mn leaching (Figure 8.16). This
sulphide was produced at 25°C when the cobalt was in its divalent state in
the initial solution used for precipitation.
The CoNiS sample from the Yabulu Refinery was almost twice as
reactive as the synthetic material and reduced 70% of the Mn (Table 8.9).
This is further highlighted in Figure 8.17 which plots the ORP and %Mn
leached as a function of Co/S ratio in CoNiS. This confirms that the cobalt
content in CoNiS was crucial. Also, the hydrogen sulphide used at the
Yabulu plant was more effective than (NH4)2S used in the laboratory
preparation.
8-29
Table 8.9. ORP and extent of leaching of MnOOH with CoNiS
Sample
Ni/S ratio
Co/S ratio
ORP (mV)
Reduction of MnOOH (%)
Ag/AgCl SHE Wet Dry CoNiS-1 0.15 0.22 -111 99 35 CoNiS-2 0.16 0.19 -100 110 15 CoNiS-3 0.16 0.16 -104 106 10 CoNiS-4 0.15 0.22 -109 101 18 CoNiS-5 0.18 0.16 -96 114 10 CoNiS-6 0.18 0.12 -95 115 6 CoNiS-7 0.16 0.25 -121 89 24 CoNiS-8 0.13 0.27 -115 95 29 CoNiS-9 0.15 0.22 -111 99 35
CoNiS-10 0.17 0.21 -113 97 27 CoNiS-11 41 39
CoNiS-Yabulu 0.16 0.44 -117 93 70 71
Figure 8.16. Reductive leaching of MnOOH: effect of temperature, cobalt oxidation state and sulphidation ratio.
0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
50%
Reduction
25 deg, 0%
25 deg, 53%
25 deg,100%
40 deg, 0%
40 deg, 56%
40 deg, 100%
1:1 1.5:1 3:12.2:1 0
5
10
15
20
25
30
35
40
45
50
Mn
% L
each
ed
25 deg, 0%
25 deg, 53%
25 deg,100%
40 deg, 0%
40 deg, 56%
40 deg, 100%
1:1 1.5:1 3:12.2:1
8-30
y = 182.89x - 9.15R2 = 0.96
85
95
105
115
0 0.2 0.4 0.6Co/S Molar ratio in CoNiS
MnO
OH
Red
uctio
n (%
)by
CoN
iS
0
20
40
60
80
OR
P o
f CoN
iS/S
AC
(mV
, SH
E)
Figure 8.17. Reductive leaching of MnOOH: effect of Co/S ratio in CoNiS.
Anderson (2003) stated that the improved leach results by CoNiS
produced at lower temperatures could be due to more cobalt existing in its
divalent state prior to precipitation. Although changing the sulphidation ratio
does not appear to affect metal composition in the precipitate, there seems
to be a trend in reducing ability. The CoNiS produced using a sulphidation
ratio of 2.2:1 was the most effective. Adding excess sulphide should be
avoided as it could possibly precipitate out of solution as elemental sulphur to
form a passivating layer, and would also add to reagent costs.
The oxidation reduction potential (ORP, using a Pt vs. Ag/AgCl / 3 M
KCl electrode) of the precipitates in ammonia at 25°C under reducing
conditions was also measured to determine the effect of ORP (Table 8.9).
The general trend of the results plotted in Figure 8.17, including the potential
of CoNiS produced at the Yabulu plant on 11/10/06 (-117 mV vs. Ag/AgCl) is
that the % reduction of MnOOH increases with the decrease in ORP. The
8-31
similarity between all the results shows the ORP is controlled by the same
redox couple.
The Eh-pH diagrams of the Ni(II)/Co(II)/(III)-NH3-H2O system
published by Asselin (2008, 2011) are shown in Figure 8.18. The figure
shows that the Co(NH3)63+/Co(NH3)6
2+ couple has a potential of 200 mV at a
molar ratio of [Co(III)]/[Co(II)] = 1 and pH 10. Thus, the ORP values lower
than this value represent solutions of [Co(II)]/[Co(III)] molar ratios greater
than unity. The reductive precipitation of MnOOH as MnCO3 by Co(NH3)62+
according to the reaction 3 in Table 8.10 has a high equilibrium constant of
log K = 1.5 based on the standard free energy data in the HSC 6.1 data
base, assuming that there is no ion-association between cobalt ammine
complexes and anions such as S2O32-, CO3
2- or SO42- which exist in solution.
As the reducing ability of CoNiS is usually tested in a slurry, it is a
possibility that some of its reducing ability is due to species present in
solution. To remove this doubt, similar tests were conducted with dry CoNiS
samples and the results are shown in Figure 8.19. Results summarised in
Table 8.9 and Figure 8.19 prove that drying has no effect on the reductants
reducing ability, and there were no reducing species present in the slurry.
Thus, reducing agents appear to originate from the slow leaching of CoNiS
according to the reactions described previously. For example, both
Co(NH3)62+ (reaction 3) and HS- can reduce MnOOH, while HS- can reduce
Mn3O4 , Co(OH)3 and Co3O4 as shown by some of the other reactions in
Table 8.10. Thus, nickel sulphide is important due to its faster dissolution
8-32
rate producing HS-, while the initial nickel dissolution from CoNiS is faster
than that in the case of NiS alone, as shown in Figure 8.13b. Discussions
with John Fittock (technical services, Yabulu Refinery) revealed that there
was a general perception of the reducing mechanism via the sulphur
intermediates, although further work is necessary to determine the reaction
mechanism.
Table 8.10. Reduction reactions of Mn(III) and Co(III) oxides No. Reactions with aqueous species Log K
1 MnOOH + Co(NH3)62+ + H2O = Mn(NH3)4
2+ + Co(OH)3 + 2NH3 -9.84
2 MnOOH + Co(NH3)62+ + 3NH4
+ + NH3 = Mn(NH3)42+ + Co(NH3)6
3+ + 2H2O -5.51
3 MnOOH + Co(NH3)62+ + 2NH4
+ + HCO3- = Co(NH3)6
3+ + MnCO3 + 2NH3+ 2H2O 1.49
4 2MnOOH + 5NH4+ + 3NH3 + HS- = 2Mn(NH3)4
2+ + S + 4H2O 5.95
5 8MnOOH + 16NH4+ + 2HS- + 16NH3 = 8Mn(NH3)4
2+ + S2O32- + 13H2O 22.6
6 8MnOOH + 15NH4+ + HS- + 17NH3 = 8Mn(NH3)4
2+ + SO42- + 12H2O 27.7
7 2MnOOH + 2S2O32- + 6NH4
+ + 2NH3 = 2Mn(NH3)42+ + S4O6
2- + 4H2O -4.95
8 4Mn3O4 + 2 HS- + 24NH4+ + 48NH3 = 12Mn(NH3)4
2+ + S2O32- + 13H2O 9.43
9 Mn3O4 + 2S2O32- + 8NH4
+ + 4NH3 = 3Mn(NH3)42+ + S4O6
2- + 4H2O -8.20
10 Mn3O4 + 2Co(NH3)62+ + 8NH4
+ + 4NH3 = 3Mn(NH3)42+ + 2Co(NH3)6
3+ + 4H2O -11.5
11 2Co(OH)3 + 5NH4+ + HS- + 7NH3 = 2Co(NH3)6
2+ + S + 6H2O 25.6
12 8Co(OH)3 + 16NH4+ + 2HS- + 32NH3 = 8Co(NH3)6
2+ + S2O32- + 21H2O 101
13 8Co(OH)3 + 15NH4+ + HS- + 33NH3 = 8Co(NH3)6
2+ + SO42- + 20H2O 106
14 2Co(OH)3 + 2S2O32- + 6NH4
+ + 6NH3 = 2Co(NH3)62+ + S4O6
2- + 6H2O 14.7
15 4Co3O4 + 2 HS- + 24 NH4+ + 48NH3 = 12Co(NH3)6
2+ + S2O32- + 13H2O 32.5
16 Co3O4 + 2S2O32- + 8NH4
+ + 4NH3 = 3Co(NH3)62+ + S4O6
2- + 4H2O -2.47
8-33
Figure 8.18. Eh-pH diagrams for Ni(II) and Co(II)/(IIII) in ammonia solutions at 25oC and 6 M NH3, 0.1 M Ni(II) and 0.01 M Co(II)/(IIII) (Asselin,
2008,2011)
8-34
Figure 8.19. Reductive leaching of MnOOH: effect of drying.
8.5 Summary and Conclusions
Precipitation and Characterisation
• Cobalt nickel sulphides (CoNiS) samples were precipitated from 1 L
solutions containing 1 g/L cobalt and 10 g/L nickel at varying
temperatures, cobalt oxidation states (II & III) and sulphiding ratios
using ammonium sulphide.
• The composition (%) of S in CoNiS precipitated under different
conditions exhibited no trends. However, Co/Ni ratio in CoNiS
decreased with increasing temperature. The sulphide solubility
diagrams predicted from thermodynamic data at 25oC and 45oC show
that whilst CoS is less soluble than NiS, the difference is larger at
lower temperatures. Thus, the cobalt precipitation was preferential at
0
10
20
30
40
50
60
70
80
90
100
Reduction
QN CoNiSWet
CoNiSDry
CoNiSWet
QNI CoNiSDry
10
20
30
40
50
60
70
80
90
100
Mn
% L
each
ed
QN CoNiSWet
CoNiSDry
CoNiSWet
QNI CoNiSDry
8-35
lower temperature giving rise to a higher Co/S ratio of 0.22 in CoNiS
at 25oC compared to 0.12 at 45oC. Moreover, the precipitates
produced at a lower temperature contained more cobalt and therefore
had a higher Co:S ratio.
• The Co/Ni ratio in CoNiS was also decreased with the increase in
Co(III)/Co(II) ratio in the starting solution. As more cobalt was present
in its trivalent state before precipitation, less cobalt and more nickel
was incorporated in the CoNiS precipitate. This effect is probably due
to the reduction of the trivalent cobalt to its divalent state prior to
precipitation.
• The ideal molar ratio of (NH4)2S to Co(II) (sulphiding ratio) was found
to be 2.2:1 which gave the highest ratio of (Co+Ni)/S = 0.38 in the
CoNiS precipitate. When sulphide was in excess, the precipitation of
nickel and cobalt seemed to be independent of sulphide
concentration.
• The size (P80) of CoNiS ranged between 130-160 μm with no
observable trends between different samples. For the purposes of this
investigation it was assumed that the difference in size would not
influence the reaction rate significantly.
• The characterisation of CoNiS by XRD was unsatisfactory, but SEM
indicated that CoNiS consists of small particles aggregated into larger
particles (> 20 μm) of high porosity.
8-36
Oxidation and Leaching
• The oxidation of Na2S and S by dissolved oxygen or Fe(III) and the
dissolution of nickel and cobalt from various sulphide (NiS, CoS and
CoNiS) samples in SAC solutions was monitored with nitrogen or air
sparging, in order to determine the leaching behaviour of the metals in
CoNiS.
• The measured ORP, after the oxidation of HS- ions formed by the
reaction: Na2S+H2O = NaOH+NaHS with dissolved oxygen in a SAC
solution was -280 mV. This value is close to the Eh of the S2O32-/HS-
redox couple in the published Eh-pH diagrams of the S-H2O system at
ambient temperatures. The gravimetric analysis by precipitating
BaSO4 after 1 hour of oxidation of Na2S showed that 80% of the
sulphide has oxidised to sulphate. The treatment of filtered liquor with
hydrogen peroxide and subsequent precipitation of BaSO4 indicated a
further conversion of 10%. In contrast, there was no sign of oxidation
of elemental sulphur by dissolved oxygen or Fe(III). This indicates that
the oxidation of HS- to SO42- takes place via intermediate sulphur
species such as S2O32-, SO3
2- as predicted by the published Eh-pH
diagrams.
• The dissolution of NiS in a SAC solution under nitrogen was faster
than that of CoS. The dissolved concentrations of Ni(II) is higher than
that of Co(II) and both are in reasonable agreement with the predicted
solubilities based on the dissolution reaction: MS + NH4+ + 5NH3 =
M(NH3)62+ + HS-. In both cases metal dissolution enhanced in the
8-37
presence of air due to the oxidation of HS- and Co(II) by the dissolved
oxygen.
• It was clear that nickel sulphide was vital, as it initialised reduction of
Mn-oxides due to its faster dissolution to Ni(NH3)62+ and HS-. The
lower potentials of NiS oxidation to S2O32- are also beneficial in the
reduction process. Cobalt dissolution improved when oxidisable
sulphur species were in solution. The regeneration of sulphur species
from NiS would allow further reduction.
• Due to the porous nature of the material, the dissolution of CoNiS in a
SAC solution occurred at a relatively constant rate, especially after the
primary leach.
• The rate of nickel and cobalt dissolution from MHP would be
dependent on the rate of reduction of cobalt and manganese.
• Leach tests of synthetic manganese oxyhydroxide (MnOOH-Oct) with
CoNiS samples synthesised in the laboratory and Yabulu refinery
revealed that the reduction depended directly on the Co/S ratio. A
higher ratio of Co/S in CoNiS had a higher reducing ability towards
MnOOH.
9-1
9 YABULU REFINERY PLANT SURVEY
9.1 Introduction and Experimental
The ammonia/ammonium buffer solution is widely recognised for the
dissolution of nickel, cobalt and a range of other metal ions with the rejection
of magnesium, manganese and iron as oxides/hydroxides at high pH. The
Eh-pH diagrams for nickel and cobalt in ammoniacal solutions discussed in
previous chapters show the nickel and cobalt ions to have the highest
stability around pH 10. The pH is also known to have a significant influence
on the stability of various metal ammine complexes which in turn affect the
solubility of metal ions. In buffer solutions of a given pH, various anions can
also compete with ammonia for the coordination sites of nickel and cobalt
ions, and affect the solubility as described in Chapter 8. The lack of reliable
information on the stability constants restricts the inclusion of such
complexes in the Eh-pH diagrams. Also, as kinetic data is limited, this
chapter will focus on the stability of complexes rather than the speed of
transformation; further studies could help improve the leaching efficiency of
this system.
Carbonate and sulphate are the two most common ammonia buffer
solutions. The direct comparisons of the influence of these two anions on
solution and precipitation chemistry of nickel and cobalt is lacking in the
literature. Carbonate was chosen for the Yabulu process, as it can be
regenerated as CO2 and recycled. In addition to carbonate ions, the leach
liquor in the Yabulu refinery contains various sulphur species originating from
9-2
the sulphur in fuel oil used for the roasting of the laterite ore. For example,
depending on ore characteristics, typical Caron lixiviant at 45oC contains
90 g/L NH3, 62 g/L HCO3-, 0.9 g/L Ni(II), 0.7 g/L Co(III), 2.5 g/L S2O3
2- and
0.05-0.4 g/L Cu, as well as SO42- and other metal ions such as Fe(II) and
Mn(II) (Nikoloski, 2002; Nicol et al., 2004). The HPLC results show evidence
for the formation of a variety of ammine and anion (CO32-, SO4
2-, S2O32-,
SO32- etc.) complexes of cobalt(III) in solution (Hultgren, 2004; Smith, 2007).
At high concentrations of cobalt the pentaammine carbonate was the
predominant structure. If a pentaammine complex is more stable than the
others this would have significant influence on metal ion solubility. It is
reasonable to assume that nickel would form similar complexes.
This chapter describes the effect of anions in the plant liquors and a
site survey of the Yabulu Extension Project (YEP) conducted as part of this
project when MHP was introduced to the Yabulu refinery. The idea of the
survey was to generate a better understanding of the industrial process in
order to improve the plant performance.
The leaching of MHP with ammonia-ammonium carbonate solution at
the Yabulu Refinery was conducted in two stages over approximately 5 hours
followed by a 72 hour counter current decantation (CCD). After the MHP was
reslurried in product liquor (typically 10 g/L Ni, 95 g/L NH3 and 60 g/L CO2), it
was leached for 2 hours at 45°C in more product liquor. The CoNiS
precipitate was then added, at a 2:1 ratio of ‘Co+Mn’ in MHP to S in CoNiS,
9-3
and leached for a further hour in fresh leach liquor (typically >120 g/L NH3
and >60 g/L CO2), before air was sparged into the tank for 2 hours.
As part of this site survey batch Leach tests were conducted on four
synthetic Ni,Co,Mg(OH)2 precipitates with increasing cobalt composition for
comparison with plant data. In the case of batch leach tests, the first 2 hours
of leaching with the product liquor was replicating the Yabulu refinery primary
leach conditions at 45oC, open to the atmosphere. The product liquor used
for leaching was the overflow from the first thickener which contained
10.5 g/L Ni, 0.6 g/L Co, 88 g/L NH3 and 57 g/L CO2 (Chapter 3). The liquor
was analysed using High Performance Liquid Chromatography (HPLC) to
determine the changes in Co(III) speciation with respect to the coordination
numbers 4, 5 and 6 during leaching. After 2 hours the solids were filtered and
leached with CoNiS under reducing conditions for an hour, then oxidised for
another hour until leached. Results are compared and contrasted with the
plant performances.
9.2 Yabulu Flowsheets
A site survey was conducted when MHP was introduced to the refinery.
The survey was carried out in order to investigate the chemistry of the leach
on a continuous, industrial scale. Samples were taken weekly over a 3 week
period. An overview of the YEP flowsheet including MHP reslurry, MHP
Primary Leach, CoNiS precipitation and thickening, and MHP Secondary
9-4
Leach and leach residue treatment are described in Figures 9.1-9.5 (Fittock,
2004). The flowsheets were obtained from the refineries operating software.
BNC
air
PreboilSolids to
MHP Leach
NH 4 HS
(Cu,Ni)Sto disposal
PreboilStills
MagmaStills
steam
QN Nickel HiGrade & QN Nickel Compact
Products (76,000 t/yr Ni)
syngas syngas
SinterFurnaces
ReductionFurnaces
Coal seammethane
gascleaning
LaroxFilter
steam
steam
air
CompactorTables
Filter
Filter
CombinedProductLiquor
(22 g/L Ni)air
Leaching
residue toleaching
O 2
NH4HS
Filter
QN ChemGradeCobalt Product(3,500 t/yr Co)
steam
Larox Filter
air
Drying
Flash
Co LSLPreboil Still
steam
Ion ExchangeCa & Mg Removal
Solvent ExtractionZn & Fe Removal
Solvent ExtractionTransfer to Ammine
Solvent ExtractionNi & Cu Removal
Oxidation
Flash
Calcination& Reduction
Kilns
Thickener
Precipitation
Thickener Cobalt
SulphideStills
Thickener
to NH 3 Recovery
to NH 3 Recovery
to G.C.C.’s
Thickener
Thickener
water
to A.S.X. Plant
water
H 2 SO 4 Zn,Fe to disposal
H2O2
Oxidation
sawdust
Belt Vacuum Filter
Belt Vacuum Filter
powercoal Power Plant
steamwater
E1E2E3
S1S2
S3S4
E1E2
S1S2
S3
ScalpA.S.X.
Absorbers Gas CoolerCondensers
Thickener
to generalprocess water
usesto generalprocess
water uses
FollowA.S.X.
Filter
PrimaryLeach
RNO MHP0.19 M wt/yr
(44,000 t/yr Ni& 1,500 t/yr Co)
SecondaryLeach
air
Thickener
Thickener
SyngasPlant
Coal seam methane
air
H2SPlantsulphur
CO2syngas (3H 2/N2)NH3
Converter
NH3NH4HS
1 2 3
6
Tailings Stills steam
air
air fuel oil
fuel oil gas cleaning
Ball Mills
DustBypass
gas cleaning
Dryers air coal
Imported Ore3.5 M wt/yr
(32,000 t/yr Ni& 2,000 t/yr Co)
4 5
7 8
Product Liquor
10 g/L Ni
Leaching
Ore Reduction Furnaces (12)
Solar Drying
Tailings Ponds Brine Pond
Reverse Osmosis
Plant clear
effluent
Process water
to NH 3 Recovery
Coolers
Refrigeration Plant
Clarifier
NH 4 HS
FLL
FLL
CoNiS
air
air
Figure 9.1. Yabulu Refinery YEP flowsheet (Fittock, 2004).
9-5
Figure 9.2. MHP reslurry
Figure 9.3. MHP primary leach
9-6
Figure 9.4. CoNiS precipitation and thickening
Figure 9.5. MHP secondary leach and leach residue
9-7
9.3 Oxidation Reduction Potentials (ORP)
Although the measured ORP values in the Yabulu refinery fluctuate
across the survey (Figure 9.6), a general pattern was observable. The
primary leach was intended to be oxidative. According to Metsim, 4.5 t/h of
air was added to the process (Figure 9.3). According to the Eh-pH diagram in
Figure 9.7, constructed using the HSC 6.1 software package, equimolar
(0.05 M) cobalt(II) and cobalt(III) exist in solution when the potential is
200 mV vs. SHE or -10 mV vs. Ag/AgCl (3 M KCl). A similar diagram in
Figure 8.18, published by Asselin (2008, 2011), predicts an Eh of 250 mV for
an equimolar (0.01 M) solution of Co(II) and Co(III), corresponding to ORP =
40 mV (vs. Ag/AgCl (3 M KCl).
Thic
kene
r 2 O
/F
SP
L
Thic
kene
r 5 O
/F
MH
P L
each
Slu
rry(3
45-2
4201
)
Prim
ary
Leac
h O
/F(3
45-3
3201
)
CoN
iS U
/F(3
45-3
303)
345-
1910
345-
1912
345-
1913
Sec
onda
ry L
each
O/F
(345
-330
2)
Thic
kene
r 1 O
/F
Thic
kene
r 3 O
/F
Thic
kene
r 8 O
/F
-500
-400
-300
-200
-100
0
100
OR
P, m
V
Figure 9.6. Plant Survey – ORP. Conducted over 3 weeks, blue: week 1, red: week 2 and green: week 3.
9-8
Figure 9.7. Eh-pH diagram for Co-ammonia-carbonate system at similar solution concentrations to YEP at 30°C.
Thus, the negative values (< -10 mV) of ORP for the primary leach in
Figure 9.6, which correspond to the stability region of Co(NH3)5-62+ in
Figure 9.7, were unexpected. The measured ORP indicates that significant
quantities of divalent cobalt exist in solution after the primary and secondary
leaches. This should be minimised by raising the potential. According to the
Nernst equation the potential should be increased to around 53 mV vs.
Ag/AgCl which would ensure a 10:1 ratio of cobalt(III) to cobalt(II) at 45°C.
As the potential was below 53 mV vs. Ag/AgCl in the primary leach, all of the
oxygen has been consumed and some of the cobalt may have been
reduced. The oxidation of thiosulphate and sulphide would result in a
reduction of Co(III) to Co(II). Concentrations of thiosulphate in solution
ranged between 4.4 – 4.7 g/L (Table 9.1), whilst sulphide in the solids had
compositions between 0.31 and 0.43 %.
9-9
Table 9.1. Thiosulphate concentrations in plant liquors. Thiosulfate Concentration, g/L 14-May 18-May 26-MayMHP Leach Slurry (345-24201) 2.0 0.4 1.7Primary Leach O/F (345-33201) 4.4 4.6 4.7CoNiS U/F (345-3303) 5.7 4.8345-1910 12.4 9.0345-1912 10.5 8.3345-1913 9.2 8.1Secondary Leach O/F (345-3302) 10.3 8.5 6.8Thickener 1 O/F 4.5 3.0 4.9Thickener 2 O/F 3.9Thickener 3 O/F 5.3 2.6Thickener 5 O/F 2.4Thickener 8 O/F 2.4 2.2
Using Metsim (plant simulation) data and site survey results, there
was between 6,300 and 8,700 moles of cobalt per hour in the primary leach
overflow compared to only 3,000 moles of oxygen. It is clear that more
oxygen was required in the system, especially when manganese and various
sulphur species would also consume oxygen. Moreover, the calculated
values of equilibrium constants for the dissolution of Co(OH)2, CoCO3 and
Co(OH)3 show that the solubility of Co(III) species would be higher (than
Co(II) species) in the Yabulu process liquor, due to the larger value of log K
at 30oC:
CoCO3 + NH4+ + 5NH3 = Co(NH3)6
2+ + HCO3- (Log K = -3.28)
Co(OH)2 + 2NH4+ + 4NH3 = Co(NH3)6
2+ + 2H2O ( Log K = -0.93)
Co(OH)3 + 3NH4+ = Co(NH3)6
3+ + 3NH3 + 3H2O (log K = 4.60)
Therefore, a reducing environment in the primary leach could result in
a lower leach efficiency. As cobalt(II) is known to be less stable in ammonia
solutions than cobalt(III) exemplified by the low values of log K, the driving
force for dissolution would be lower resulting in slower leaching kinetics
9-10
(Smith & Martell, 1989). A lower recovery in the primary leach puts
unnecessary strain on the secondary leach. Perhaps the input of air should
be linked to an online potential probe, whereby sufficient air is added to
ensure a potential of around 53 mV on slurry leaving the primary leach. The
concentrations of nickel, cobalt, ammonia and carbonate are listed in
Tables 9.2 & 9.3.
The CoNiS collected after precipitation had a potential around
-400 mV. In the reductive tank (345-1910), where the CoNiS was added, the
ORP ranged between -160 and -270 mV. In the subsequent tanks (345-1912
and 345-1913), air was added in order to oxidise any unreacted reductant. Of
the three samples taken during the site survey, two of these still had a
negative potential, indicating some Co(II) existed in solution. The remaining
cobalt(II) in solids put unnecessary strain on CCD cobalt recovery. In this
situation, either too much CoNiS was being added or not enough oxidation
was occurring due to lack of leaching time or oxygen availability. Similar to
the primary leach, monitoring the output potential of tank 345-1913 is an
easy way to ensure all cobalt is oxidised.
The ORP’s for the secondary leach overflow and all the thickener
overflows were also negative. These values are most likely related to the
existence of reducing species present from the roasting and leaching of
lateritic ore and oxygen deficiency in solution. The exception to this
relationship is Thickener 8, where the potential has risen compared to
Thickener 5 (Figure 9.6). The reason for the rise is unknown.
9-11
Table 9.2. Nickel and cobalt concentrations in plant liquors. Concentration, g/L
Ni Co Ni Co Ni CoMHP Leach Slurry (345-24201) 49.7 0.718 36.9 0.310 29.4 0.180Primary Leach O/F (345-33201) 21.6 1.390 21.8 1.000 29.2 1.140CoNiS U/F (345-3303) 13.7 0.234 12.9 0.620 12.6 0.210345-1910 28.0 3.110 30.5 2.710 29.6 2.420345-1912 20.6 4.720 27.9 7.000 28.8 4.670345-1913 23.1 5.920 27.4 9.200 26.2 4.590Secondary Leach O/F (345-3302) 26.6 4.960 38.3 5.460 29.2 4.100Thickener 1 O/F 15.3 1.052 19.3 0.962 17.8 0.607Thickener 2 O/F 10.0 0.965Thickener 3 O/F 4.8 0.805 6.3 0.601Thickener 5 O/F 2.9 0.428Thickener 8 O/F 0.6 0.100 0.6 0.087SPL 19.3 1.228
14-May 18-May 26-May
Table 9.3. Ammonia and carbonate concentrations in plant liquors. Concentration, g/L
NH3 CO2 NH3 CO2 NH3 CO2MHP Leach Slurry (345-24201) 56 46 57 45 55 44Primary Leach O/F (345-33201) 103 48 109 48 100 41CoNiS U/F (345-3303) 87 39 103 47 99 40345-1910 98 45 96 44 98 46345-1912 83 43 85 41 88 42345-1913 84 43 91 42 85 39Secondary Leach O/F (345-3302) 100 46 85 39 86 42Thickener 1 O/F 109 49Thickener 2 O/F 118 52Thickener 3 O/FThickener 5 O/F 118 49Thickener 8 O/FSPL 61 33
14-May 18-May 26-May
9.4 XRD of Plant Solids
X-Ray Diffraction was conducted on three MHP samples collected on
the 8th of January and the 15th and 19th of May 2008, and labelled as
MHP 0108, 15-5 and 19-5, respectively in Figure 9.8. The preboil solids
collected during this period were also analysed using XRD, as they have
similar properties to MHP. Most scans contained the characteristic lump of a
hydrotalcite-like structure around 11°, however the broad nature of the peak
and the lack of distinguishable secondary peaks made it impossible to
9-12
determine exactly what phases existed. The peaks for hydrotalcite
(Mg6Al2(OH)16CO3.4H2O) were included for indication.
The XRD traces of different solid samples were similar as shown in
Figure 9.8. However, the peaks around 19° and 38° were missing in the older
sample (MHP 0108). It was not known if these peaks existed initially; if they
did, the metal hydroxide has transformed to a hydrotalcite-like structure. All
three traces exhibit the hydrotalcite characteristic ‘lump’ around 11°, which
was larger for the older precipitate. The Preboil solids also contained this
type of structure along with a metal hydroxide.
10 20 30 40 50 60 70 80
2 Theta
MHP 15-5 MHP 19-5 MHP 0108 Preboil - wet Ni(OH)2Mg(OH)2 MgO Ni,Mg(OH)2 Hydrotalcite
Figure 9.8. XRD scans of MHP’s and preboil solids
9-13
In Figure 9.9, two preboil solid samples collected from two separate
site visits are compared to see if the Yabulu Expansion Project (YEP) had
influenced the solid structure. The sample taken in June 2006 has narrower
more intense peaks indicating the hydrotalcite-like structure is more
crystalline, while the sample collected 2 years later has a lower crystal order
and a metal hydroxide was present. Assay results of the two samples listed
in Table 9.4 show that the nickel and cobalt concentrations have increased
from June-06 to May-08, while iron and manganese have decreased since
the introduction of YEP. The formation of hydrotalcite was probably limited by
the quantity of trivalent metals. The higher concentrations of nickel and
cobalt in the precipitate are probably attributed to higher solution
concentrations due to the introduction of YEP (approximately 28 vs. 22 g/L).
Table 9.4. Composition of preboil solids collected in June 06 and May 08.
Solid Date Ni Co Mg Mn Fe Cu Zn Cr Si Al Ca S CPreboil Solids Jun-06 15.0 3.24 5.29 9.66 9.23 0.35 0.07 0.16 4.03 0.43 0.44 0.74 4.70Preboil Solids 19-May-08 26.1 4.09 4.45 6.06 1.72 0.01 0.25 0.04 1.77 0.17 0.34 0.28 2.45
% Metal
9-14
10 15 20 25 30 35 40 45 50 55 60
2 Theta
Preboil - May 08 Preboil - June 06 Ni,Mg(OH)2 Hydrotalcite MnCO3
Figure 9.9. Comparison of XRD scans of preboil solids collected in May 08 and June 06
Hydrotalcite-like structures became more predominant in the leach
residues as shown in Figure 9.10. Metal hydroxides were also present and
were more ordered. This suggests that hydrotalcite and the more crystalline
metal hydroxides are leaching at a slower rate than the poorly ordered metal
hydroxides. This is consistent with the relationship between crystallinity and
leaching kinetics described in Chapters 6 and 7 where high porosity and low
crystallinity is beneficial for leaching.
The XRD analysis on solids of the secondary leach process is shown
in Figure 9.11 where more peaks are identified. According to Figure 9.11 the
mixed Ni,Mg(OH)2 appears to be present in all solids, along with manganese
carbonate. Thus, Ni,Mg(OH)2 is slow leaching as described in previous
chapters. Moreover, the presence of manganese carbonate proves that
9-15
leaching and reductive precipitation of manganese has occurred. The small
peak around 33 degrees suggests that NiS is present in the secondary leach
underflow. This could be a result of: (i) incomplete leaching of excess CoNiS
added, (ii) insufficient leach time, or (iii) oxygen deficiency. The latter was
also highlighted by the negative ORP discussed earlier.
10 20 30 40 50 60 70 80
2 Theta
MHP Reslurry (345-24201) Primary Leach U/F (345-33201) Secondary Leach U/F (345-3302)Ni(OH)2 Mg(OH)2 Ni,Mg(OH)2Hydrotalcite
Figure 9.10. XRD scans of plant solid samples
9-16
10 20 30 40 50 60 70 80
2 Theta
345-1910 345-1912 345-1913Secondary Leach U/F (345-3302) Ni,Mg(OH)2 NiS2NiCo2S4 Ni3S2 (rhombohedral) HydrotalciteNiS MnCO3 MgCO3
Figure 9.11. XRD scans of secondary leach slurries
XRD analysis was also conducted on plant CoNiS and the thickener
residues; results are shown in Figures 9.12 and 9.13, respectively. Plant
CoNiS was poorly crystalline (Figure 9.12), making it hard to distinguish the
structures present in the sample. The predominant components appear to be
CoS2, NiS and NiCo2S4. It was difficult to determine what structure causes
the ‘lump’ around 12°. Hydrated phases exhibit peaks around this area.
Otherwise, iron sulphate is a possibility as CoNiS has an iron concentration
of around 5%. A significant quantity of non-crystalline material with broad
peaks around 30 and 55 degrees are also present.
9-17
10 20 30 40 50 60 70 80
2 Theta
CoNiS CoS2 NiS NiCo2S4
Figure 9.12. XRD scan of CoNiS
10 20 30 40 50 60 70 80
2 Theta
Thickener 1 Thickener 3 Thickener 8 Ni,Mg(OH)2 Hydrotalcite MgFeAlO4
Figure 9.13. XRD scans of thickener residues
9-18
The predominant structure present in the thickener solids
(Figure 9.13) is a mixed magnesium, iron, and aluminium oxide probably
formed during the reductive roasting of laterite. These metal ions are
considered to have a low solubility in ammonia solutions. The lump around
11 degrees could be due to a hydrotalcite-like compound. As nickel and
cobalt compositions in the solids are below 0.4%, their involvement in any of
the structures present would be minimal.
9.5 Cobalt Speciation in Plant Liquors
High Performance Liquid Chromatography (HPLC) was developed at
the Yabulu Refinery to determine the nature of the cobalt ammine complexes
which exist in the plant liquors. A better understanding of cobalt speciation
would help to improve recoveries and the quality of the final product.
Figure 9.14 is an example of the information obtained from the HPLC
instrument at 490 nm. The instrument scans from 200 to 700 nm. However,
cobalt species tend to absorb between 450 and 600 nm, and nickel above
600 nm. Only the cobalt(III) complexes can be studied, as cobalt(II) tends to
be unstable, and nickel complexes absorb with long peaks across the time
axis. There are many nickel species and they interconvert very quickly.
Studying at 490 nm allows the identification and quantification of cobalt(III)
species without interaction from nickel.
9-19
Figure 9.14. HPLC – Secondary Leach Tank 3345-1913, Sampled 19/5/08
Liquor from Tank 345-1913, from the secondary leach, was used as
an example as it contains 7 of the 9 peaks found when analysing all of the
plant samples (Figure 9.14). The first peaks between 1 and 2 minutes was
the material not retained by the column. The second peak (around 3
minutes) was a tetraammine complex (tetra, [Co(NH3)4CO3]+). The peak
around 7 minutes was the pentaammine complex (penta, [Co(NH3)5CO3]+)
and the peak at about 8 minutes was the hexaammine complex (hexa,
[Co(NH3)6]3+). The broad peak around 4.5 minutes was a
pentaamminesulphito complex (sulphito, [Co(NH3)5SO3]+), the following peak
at 5.6 minutes was a pentaamminethiosulphato complex (thiosulphato,
[Co(NH3)5S2O3]+). The two peaks between the pentaammine and
hexaammine are unknown peaks called 1 and 2 (from left). Complexes were
distinguished by their absorption maxima, which was characteristic for each
9-20
species. Unknown species 1 and 2 absorb at approximately 520 and 500 nm,
respectively.
In a few plant liquor samples a pentaamminesulphato complex
(sulphato, [Co(NH3)5SO4]+) was detected at around 6 minutes. Another
unknown species was detected with a retention time around 6 minutes with
absorption maxima of 450 nm. The pentaamminesulphato complex has a
very distinguishable absorption spectrum where peaks exist at 350 and 515
nm (Yoshikawa et al., 1981; Smith 2007). At the time of the survey it was
possible to quantify the cobalt tetraammine, pentaammine and hexaammine
complexes. These were quantified through the YEP plant over the 3 week
survey period (Figure 9.15).
Figure 9.15. HPLC – Cobalt ammine species concentrations
SP
L
Thic
kene
r 2 O
/F
MH
P L
each
Slu
rry
(345
-242
01)
Prim
ary
Leac
h O
/F(3
45-3
3201
)
CoN
iS U
/F(3
45-3
303)
345-
1910
345-
1912
345-
1913
Sec
onda
ry L
each
O/F
(345
-330
2)
Thic
kene
r 1 O
/F
Thic
kene
r 5 O
/F
0
500
1000
1500
2000
2500
Co(
III) C
once
ntra
tion,
mg/
L
TetraPentaHexa
9-21
The average of three plant surveys (14th, 18th and 26th May) with 95%
confidence intervals are plotted in Figure 9.15. The samples without error
bars were only analysed on May 14. Clearly the total Co(III) concentration
increases with a progression through the MHP leaching circuit and from
Thickener 5 to Thickener 1. The large increase in cobalt concentration from
345-1910 to 345-1912 and 345-1913 can be related to the oxidation of
CoNiS, as described in Chapter 8. The pentaammine and hexaammine
species were always more dominant than the tetraammine, and in most
cases the hexaammine species had a higher concentration than that of the
pentaammine species. This was a good indication that there is a large
availability of ammonia to complex with cobalt(III) ions. In relation to other
samples, the concentration of tetraammine species was significantly higher
in 345-1912, 345-1913 and the secondary leach overflow when cobalt
concentrations were much higher. The increase in tetraammine species
suggests less ammonia is available for complexation. As discussed
previously, it appears that excess CoNiS was being added to the secondary
leach.
Figure 9.16 shows the concentration of Co(III) determined by ICP after
solvent extraction (Chapter 3). As HPLC analysis shown in Figure 9.15 only
quantifies the tetra, hexa and pentaammine species, the difference between
the HPLC values in Figure 9.15 and experimentally determined values in
Figure 9.16 can be related to the species in solution not quantifiable by
HPLC. The negative values for the difference, which should not occur, are
probably the result of oxidation or reduction occurring between sampling time
9-22
and analysis. Time between sampling and analysis could be up to 2 hours,
as samples could take around 30 minutes to collect and HPLC requires
approximately 20 minutes for each sample. Although there seems to be a
significant error associated with both techniques (at least 320 mg/L), the
difference is positive in approximately 70% of the solutions. This suggests
that there are other cobalt complex species in significant concentrations not
quantifiable by HPLC.
Figure 9.16. Cobalt(III) concentration determined by solvent extraction and ICP. Error bars represent difference in concentration determined by
laboratory method and HPLC.
The complex species can be identified by their characteristic
absorption maxima, if previous literature for the complex species exists
(Hurst, 1974; Yoshikawa et al., 1981). Work conducted by the researchers at
the Yabulu Refinery has enabled a further three (sulphito, thiosulphato and
sulphato) species to be distinguished (Hultgren, 2004; Smith, 2007). In order
Thic
kene
r 8 O
/F
Thic
kene
r 3 O
/F
MH
P L
each
Slu
rry
(345
-242
01)
Prim
ary
Leac
h O
/F(3
45-3
3201
)
345-
1910
345-
1912
345-
1913
Sec
onda
ry L
each
O/F
(3
45-3
302)
0
1000
2000
3000
4000
5000
6000
7000
Co(
III) C
once
ntra
tion,
mg/
L
14th May18th May26th May
Thic
kene
r 1 O
/F
Thic
kene
r 2 O
/F
Thic
kene
r 5 O
/F
SP
L
9-23
to quantify these species the structure needs to be produced synthetically to
be used as a standard. Unfortunately, this work has not been successfully
completed to date. Peaks identified through the survey are shown in
Table 9.5.
Before conclusions can be made, it should be mentioned that possible
sulphato peaks have been labelled as thiosulphato. It is difficult to distinguish
between the two as both species have a retention time around 6 minutes and
absorb at approximately 515 nm. The sulphato complex species has an extra
peak at 350 nm; however, so does nickel(II) species. Also, peaks 1 and 2
have varying retention times between 7 and 8 minutes. The absorption peaks
are very broad, making it difficult to determine the absorption maxima.
Table 9.5. HPLC peaks in plant liquors. HPLC - Unknown Peaks
sulfito thiosulfato 1 2 sulfito thiosulfato 1 2 sulfito thiosulfato 1 2MHP Leach SlurryPrimary Leach O/F191019121913Secondary Leach O/FCoNiSThickener 1 O/FThickener 2 O/FThickener 5 O/FSPL
26-May14-May 18-May
There was no consistency of peaks through the site survey
(Table 9.5). Sulfito and thiosulphate complexes seem to be predominantly
involved with the reductive leach where CoNiS would be present. However,
thiosulphato and sulphito complexes were also detected in the MHP reslurry
and primary leach occasionally. As collected MHP contains around 3%
sulphur this was no surprise. The published Eh-pH diagrams (Figure 8.6)
also show the coexistence of S2O32- and SO3
2- in alkaline solutions. Large
9-24
equilibrium constants for the conversion of sulphur species based on the
HSC 6.1 database listed below show the possible conversion:
4S4O62- + 6OH- = 5S2O3
2- + 2S3O62- + 3H2O (log K = 10)
2S3O62- + 6OH- = S2O3
2- + 4SO32- + 3H2O (log K = 56)
S3O62- + 2OH- = S2O3
2- + SO42- + H2O (log K =40)
S4O62- + 3OH- = 1.5S2O3
2- + SO32- + 1.5H2O (log K =19)
Peaks 1 and 2 seem to appear sporadically in all samples. The other
peak which occurs around 6 minutes (mentioned earlier, not in Table 9.5)
with absorption maxima around 450 nm was present in the primary leach
overflow sample taken on May 26 and a CoNiS leach test that will be
discussed in a later section.
According to Yoshikawa et al. (1981), the charge of the eluted species
increases while the symmetry decreases with retention time. It can therefore
be concluded that unknowns 1 and 2 have a higher charge or are less
symmetrical than pentaammine complex. If the species are stable in solution
for a few hours, the structure could be determined by using HPLC combined
with mass spectrometry (LC-MS), nuclear magnetic resonance (NMR) and
infrared spectroscopy (IR).
9.6 Cobalt Speciation in Batch Leach Tests of MHP in Plant Liquors
Batch Leach tests were conducted on four synthetic precipitates
(MHP1-4) with different compositions of cobalt listed in Table 9.6. The MHP
samples were leached with the product liquor, which is the overflow from the
first thickener. This liquor contained 10.5 g/L Ni, 0.6 g/L Co, 88 g/L NH3 and
9-25
57 g/L CO2. The first 2 hours were replicating the Yabulu refinery primary
leach conditions (i.e. oxidative leach under air). The residue was filtered and
leached with fresh leach liquor (227 g/L NH3, 97 g/L CO2) and CoNiS was
added for the leaching under reducing conditions for an hour, then oxidised
until leached. The liquors at different stages were analysed for cobalt
speciation using HPLC. The Co(II) % in leach liquors after 1 hour and 3
hours was analysed using solvent extraction described in Chapter 3 (section
3.5). The composition of initial MHP samples of different cobalt content
(1.3-10.6%) and the Co(II) % in leach liquors are listed in Tables 9.6, and
9.7, respectively.
Table 9.6. Composition of MHP used in batch tests Solid Solids (%) Ni (%) Co (%) Mg (%)MHP 1 45.3 30.7 1.3 15.64MHP 2 48.2 29.2 2.5 14.92MHP 3 48.9 28.0 6.2 12.33MHP 4 46.4 25.2 10.6 9.98
Table 9.7. Percentage composition of Co(II) in batch leach liquors based on solvent extraction
Co2+, % 1 hr 3 hrMHP 1 93 86MHP 2 90 96MHP 3 89 95MHP 4 87 96
Determined by solvent extraction and ICP (procedure: Section 3.5)
The concentrations of Co(III) species Co(NH3)n3+ (n = 4, 5, 6) in leach
liquors after 1,2,3 and 4 hours based on HPLC studies and the % of each
species are summarised in Table 9.8. Standard predictor leach tests were
also conducted on the 4 precipitates in a SAC solution (93 g/L NH3, 65 g/L
CO2) at 30oC for 45 minutes. As noted in Chapter 3, the standard predictor
9-26
test was designed to replicate leach recoveries from the first (oxidative)
stage leaching at the Yabulu Refinery. Results from the standard predictor
tests are summarised in Table 9.9, along with the Co(III) speciation based on
HPLC.
The results in Tables 9.8 (batch tests) and Table 9.9 (SPT) are also
plotted in Figure 9.17 for comparison. As shown in Table 9.9, the total
concentration of Co(III) based on HPLC studies increases with the increase
in Co% of MHP. The low concentrations of Co(III) after the first hour
(Figure 9.17), despite the fact that the leach vessel was open to the
atmosphere, indicates the effect of the presence of reducing species such as
thiosulphate and sulphite in the product liquor of the Yabulu refinery.
Moreover, the oxidising conditions which exist in the first 2 hours and the last
hour causes an increase in Co(III) concentration as revealed by the higher
concentrations after 2 hours and 4 hours in Figure 9.17.
9-27
Table 9.8. Composition of Co(III) in batch test leach Liquors based on HPLC
Precipitate
Initial Co % (mass) in
precipitate
Leach Time / hr
Co(III) Concentration mg/L
Co(III) Concentration %
n=4 n=5 n=6 Total n=4 n=5 n=6 MHP 1 1.3 1 83 368 562 1013 8 36 55
2 120 442 654 1216 10 36 54 3 37 175 157 368 10 48 43 4 166 286 258 709 23 40 36
MHP 2 2.5 1 115 499 595 1208 10 41 49 2 182 566 662 1410 13 40 47 3 29 182 10 221 13 83 4 4 355 489 489 1333 27 37 37
MHP 3 6.2 1 214 838 592 1644 13 51 36 2 312 1068 641 2022 15 53 32 3 82 362 115 559 15 65 21 4 444 773 789 2005 22 39 39
MHP 4 10.6 1 405 1676 649 2730 15 61 24 2 486 1811 649 2946 17 61 22 3 81 351 27 459 18 76 6 4 595 838 1216 2649 22 32 46
Table 9.9. Speciation of Co(III) in standard predictor leach tests based on HPLC.
Precipitate
Initial Co % (mass) in
precipitate Leach Time / hr
Co(III) Concentration mg/L
Co(III) Concentration %
n=4 n=5 n=6 Total n=4 n=5 n=6 MHP 1 1.3 0.75 23 61 1 85 27 72 1 MHP 2 2.5 0.75 52 161 0 213 24 76 0 MHP 3 6.2 0.75 93 295 50 438 21 67 11 MHP 4 10.6 0.75 66 388 0 454 15 85 0
9-28
0
1000
2000
3000
0 4 8 12Co% (mass) in MHP
Tota
l Co(
III) L
each
ed (
mg/
L) Batch Test (1 h)
Batch Test (2 h)
Batch Test (3 h)
Batch Test (4 h)
SPT (0.75 h)
Figure 9.17. Total cobalt(III) concentration determined by HPLC in batch test and SPT liquors (data from Table 9.8 and 9.10).
0
5
10
15
20
25
30
0 2 4 6 8 10 12Co% (mass) in MHP
Rat
io o
f {[C
o(II)
]/[C
o(III
)]}
Co(II)/Co(III) (1 hour)
Co(II)/Co(III) (3 hours)
Figure 9.18. Cobalt(II)/cobalt(III) concentration ratio determined by SX and ICP of batch leach test liquors (data from Table 9.7)
However, the total concentration of Co(III) remains low after 3 hours,
due to the reducing conditions imposed by CoNiS. Figure 9.17 also shows
that the SPT results after 0.75 hours are comparable with the total Co(III)
concentration in the batch leach tests after 3 hours (after adding the CoNiS).
This is further exemplified in Figure 9.18 which shows the ratio of
%Co(II)/%Co(III) based on the liquor analysis using solvent extraction and
ICP (Table 9.7). Higher ratio of Co(II)/Co(III) in solution after 3 hours is
9-29
caused by the reduction of MHP by CoNiS. It was shown that the solubility of
Ni(II) from Ni,Mg(OH)2 precipitate increased in the order Cl- ~ NO3- < CO3
2- <
SO42- due to the coordination of divalent anions with Ni(II) (Chapter 8).This
highlights the importance of considering the distribution of Co(III) species
(n=4,5,6) in all cases for a better understanding of the dissolution behaviour
of MHP in various plant liquors.
In general the coordination of an anion (Y2-) with Co(III) can be
represented by the reaction: Co(NH3)63+ + Y2- = Co(NH3)5Y+ + NH3, where Y
= SO42-, CO3
2-, S2O32-, SO3
2-. A higher concentration ratio of [Y2-]/[NH3] and
higher equilibrium constant for the forward reaction depending upon the
anion Y2- are expected to favour the formation of the pentaammine complex
Co(NH3)5Y+. Whilst S2O32- and SO3
2- anions would exist under reducing
conditions (in the absence of air), they are converted to SO42- under oxidising
conditions. Table 9.10 lists the anionic species identified by HPLC at various
stages. Sulfito, thiosulphato, sulphato complexes and unknowns 1 and 2
were present in solutions. The thiosulphato complex appears to be the most
stable sulphur complex which exists in most solutions, while the others only
appear occasionally.
9-30
Table 9.10. Peaks present in HPLC plots of plant liquors. sulfito thiosulfato sulfato 1 2
MHP 1 1 hour2 hour3 hour4 hour
MHP 2 1 hour2 hour3 hour4 hour
MHP 3 1 hour2 hour3 hour4 hour
MHP 4 1 hour2 hour3 hour4 hour
HPLC - Unknown Peaks
Figure 9.19 plots the distribution of Co(III)-NH3 complex species in
batch leach liquors after 1, 2, 3 and 4 hours based on HPLC analysis from
Table 9.9. The figure shows that pentaammine and hexaammine complexes
are predominant as the concentration of tetraammine complex is less than
25% of the total in all cases. The hexaammine complex is predominant in
batch leach liquor after 1 hour, at low total Co(III) concentration. However,
the pentaammine complex becomes the predominant species at higher total
Co(III) concentrations (Figure 9.19a). At a low concentration of total Co(III)
after 3 hours, due to the reductive conditions imposed by CoNiS, the
pentaammine complex appears to be the predominant species
(Figure 9.19c). Thus, the pentaammine complex is preferred in the presence
of reducing anions and low Co(III) concentrations as shown in Figures 9.19c
and Figure 9.20.
9-31
0
20
40
60
80
100
0 4 8 12
Co% (mass) in precipitate
Co(
III) s
peci
es in
liqu
or (%
) n=4n=5n=6
(a) 1 hour
0
20
40
60
80
100
0 4 8 12
Co% (mass) in precipitate
Co(
III) s
peci
es in
liqu
or (%
) n=4n=5n=6
(b) 2 hours
0
20
40
60
80
100
0 4 8 12
Co% (mass) in precipitate
Co(
III) s
peci
es in
liqu
or (%
) n=4n=5n=6
(c) 3 hours
0
20
40
60
80
100
0 4 8 12
Co% (mass) in precipitate
Co(
III) s
peci
es in
liqu
or (%
) n=4n=5n=6
(d) 4 hours
Figure 9.19. Distribution of cobalt(III) speciation in batch leach liquors (after 1, 2,3 or 4 h) based on HPLC analysis (data from Table 9.8)
0
20
40
60
80
100
0 4 8 12
Co% (mass) in precipitate
Aqu
eous
Co(
III) s
peci
es (%
)
n=4(%)n=5(%)n=6(%)
Figure 9.20. Distribution of cobalt(III) speciation in standard predictor leach test of MHP1-4 with batch leach liquors (after 0.75 h). (data from Table 9.9)
9-32
The hexaammine complex was barely present in the standard leach
tests, despite the absence of reducing anions (Figure 9.20). This can be
explained by the lower ammonia/carbonate mole ratio, as the predictor tests
were using a fresh ammonium carbonate solution (93 g/L NH3, 65 g/L
CO2: NH3/CO2 mole ratio = 5.0) compared to product liquor (109 g/L NH3,
49 g/L CO2: NH3/CO2 mole ratio = 7.8) and fresh leach liquor (227 g/L NH3,
97 g/L CO2: NH3/CO2 mole ratio = 8.2) of higher ammonia content used in
batch leach tests. A higher carbonate/ammonia ratio would favour the
pentaammine complex coordinated with carbonate. Likewise, higher
concentrations of reducing anions would also favour the pentaammine
complex coordinated with these anions. Other fluctuations in Figures 9.19a,
b and d can be related to the relative changes of concentrations of Co(III)
and anions. For example, the concentration of reducing anions would be
lower in oxidising media (after 2 hours and 4 hours) which enhance the
concentration of Co(III) and favour the hexamine complex as shown in
Figures 9.19 b and d. At low concentrations of Co(III) (after 3 h)
pentaammine complex is predominant than the hexamine complex
(Figure 9.19c) . At high total concentrations of Co(III) (results after 1, 2 and 4
hours in Figure 9.19) the hexaammine complex is predominant than the
pentaammine complex (Figures 9.19a, b, d).
9-33
9.7 Secondary Leaching of MHP with CoNiS
In the secondary leach at the Yabulu refinery (Figure 9.1), CoNiS is
added to the primary leach underflow with fresh leach liquor (227 g/L NH3
and 97 g/L CO2) in a vessel at 55°C, closed to the atmosphere. The quantity
of CoNiS added to the secondary leach was tested in the laboratory by
adjusting the ratio of sulphur in CoNiS to ‘Co+Mn’ in the primary underflow.
Ratios 1:1, 2:1, 3:1 and 4:1 were tested in duplicate. After 1 hour of reductive
leaching, oxygen was added to the system in order to oxidise and leach all
the remaining CoNiS. This way the effectiveness of the reductant in the initial
hour can be determined. After the first hour, the time taken to leach the
remaining CoNiS, ranged from 30 minutes to 3 hours. This was determined
by measurement of the ORP.
Unfortunately, the 95 % confidence interval was above 20% for 3 sets
of metal recovery data, so the results were omitted. During experimentation,
taking representative samples of both the CoNiS and primary underflow was
challenging. This would have contributed to the fluctuations in results. The
CoNiS was taken with a syringe from a shaken plastic bottle, while the
primary underflow was originally collected from a 5 L vessel then a 20 L
stirred bucket with a small beaker. These techniques were thought to be
satisfactory at the time but results prove otherwise. As CoNiS reactivity
diminishes with oxygen ingress, the precipitate needs to remain in solution in
an air tight container. There is therefore no alternative sampling technique for
CoNiS.
9-34
Excluding the omitted numbers, the results seemed constant but poor,
with nickel and cobalt recoveries around 60 and 40%. The poor recoveries
are most likely related to the CoNiS being around 2 weeks old and the high
solids content of the primary underflow, giving a lower solid/undeflow ratio
than intended. Exposure to oxygen over time results oxidation of the
reductant and therefore becoming less reactive. With the solids content, the
work instruction was designed to represent the secondary leach, where the
ratio of primary underflow solids to CoNiS and fresh leach liquor was
designed to be similar. Somehow, even though the online plant data reports
were reading ~10% solids, the collected sample was around 17%. The work
instructions were followed, resulting in a higher metal/solution ratio and
hence the lower recoveries. If more time was available during the site visit,
the test could have been revised and corrected given the higher percent
solids.
It seems as though leaching rate was unaffected by the reductant
ratio. It is likely that the dissolution rate of CoNiS or the dissolution/reduction
of the manganese and cobalt in the primary leach would affect the leaching
rate. In these tests there seems to be a gross excess of CoNiS after the first
hour of leaching. The 30 minutes of oxidation required for the 1:1 ratio
proves that some CoNiS remains unreacted. This was also the case with the
site survey.
9-35
Table 9.11. Peaks present in HPLC plots of secondary leach liquors of MHP.
HPLC - Unknown Peaks sulfito thiosulfato sulfato 1 2CoNiS 1:1 1 hour1 hour 30 minsCoNiS 2:1 1 hour1 hour 45 minsCoNiS 3:1 1 hour2 hoursCoNiS 4:1 1 hour4 hours
HPLC analysis on the leach liquors (Table 9.11) showed sulphito and
thiosulphato complexes to be the most common or stable species in solution.
These probably originated from the sulphur in the CoNiS and are related with
the reduction of cobalt and manganese due to the change in oxidation state
of the sulphide to thiosulphate or sulphite, according to the reaction
described previously. The absence of unknown peaks 1 and 2 compared with
other solutions could indicate that they probably do not contain a sulphur
ligand. At the end of leaching of the 3:1 ratio of CoNiS, there was also the
unknown peak at 6 minutes that absorbs at 450 nm. This complex species
was also present in the primary leach overflow. Further systematic studies
are essential to shed light on the secondary leaching.
9.8 Summary and Conclusions
Solution Potential (ORP)
Measurement of the ORP during the site survey revealed a significant
problem. At the end of the primary and secondary leaches the potentials with
respect to an Ag/AgCl probe were negative. According to the relevant Eh-pH
diagram, divalent and trivalent cobalt were in equal concentrations when the
9-36
potential was -10 mV vs Ag/AgCl. Clearly, significant concentrations of
divalent cobalt existed in solution, causing the lower ORP. This should be
avoided as cobalt(II) is known to have a lower solubility in ammonia solutions
than cobalt(III) (Smith & Martell, 1989). It was recommended that the
potential of these leach stages should be monitored and controlled by
addition of air or adjustment of feed rates. The potential needed to be raised
to around 53 mV vs. Ag/AgCl, in order to maintain a 10:1 ratio of cobalt(III) to
cobalt(II) at 45 °C. It is also advisable to experimentally correlate the solution
potential to the cobalt speciation as other species in the liquor could
influence the ORP value.
In the primary leach the negative potential seemed to be due to
oxygen deficiency. In the secondary leach it was obvious from the ORP and
XRD on residues that excess CoNiS was being added to the system. This
was not being recovered during the secondary leach and could be putting
undue strain on the subsequent leaching stage. The negative ORP values
continued into the thickeners, where the roasted ore, which was fed into
Thickener 2, probably controlled the solution potential.
Cobalt Speciation
The HPLC analysis showed cobalt(III) hexaammine to be in higher
concentrations than the pentaammine and tetraammine complexes in most
of the plant liquors. This was due to the fact that ammonia was in
significantly higher concentrations relative to the desired metal ions. The
main exception to this observation was the secondary leach overflow, where
9-37
tetraammine was in significant concentrations due to the excess CoNiS
added to the secondary leach. The lower stability of the tetraammino
complex results in the cobalt being more susceptible to precipitation. The co-
precipitation of cobalt hydroxide during iron removal is a significant cause of
lowered cobalt recoveries at the refinery.
The HPLC analysis indicated the presence of thiosulphato, sulphito
and possibly sulphato complexes and three unknown complexes in plant
liquors. The two unknowns occurred between the pentaammine and
hexaammine peaks in the chromatogram, after approximately 7 minutes.
They were labelled as unknown 1 and 2, which absorb at approximately 520
and 500 nm, respectively. The third unknown complex eluted around 6
minutes and absorbed at 450 nm. Unknowns 1 and 2 either have a larger
charge or are less symmetrical than pentaammine (Smith, 2007; Yoshikawa
et al., 1981). Moreover, as these species were less common in the CoNiS
leach tests, they are probably not sulphur related. If the complexes are
stable, the structure could be determined using HPLC combined with mass
spectrometry (MS), nuclear magnetic resonance (NMR) and infrared
spectroscopy (IR). Of the complexes not quantifiable due to the unavailability
of standards, sulphito and thiosulphato complexes were the most common in
the plant liquors. These two were predominant in the secondary leach,
suggesting they may be involved in the reduction of manganese and cobalt.
10-1
10 SUMMARY, CONCLUSIONS AND FUTURE WORK
10.1 Precipitation Mechanism
The laboratory precipitation of synthetic metal hydroxides seems to
occur evenly within the pores of magnesia due to the dissolution-nucleation
mechanism. Precipitation also occurs on the particle surface due to the
Ostwald ripening, as a result of the high pH of the outer layer of magnesia
particles. This could form impermeable surface layers and inhibit dissolution
of underlying magnesia and cause a high magnesium composition (%) in
synthetic MHP. In contrast, continuous precipitation in multiple tanks allows
for greater control and improved metal ion precipitation in the Ravensthorpe
process. For example, a grain count of SEM images estimated 80–90% of
particles were formed by dissolution-nucleation in the Ravensthorpe
precipitates, compared to only ~50% in the synthetic precipitates. This
explains the larger particle size of the Ravensthorpe material, which is
another desirable quality of precipitates formed by magnesia addition.
Moreover, with synthetic precipitates the unreacted MgO took up to 21
days to convert to Mg(OH)2, causing significant ‘ageing’ problems. This is
due to a relatively high magnesium concentration (~10%) and a simple
washing technique used in the laboratory. This did not occur with
Ravensthorpe MHP as the Larox filtration and washing process was
thorough, and magnesium concentrations were below 1%.
10-2
10.2 Composition of Precipitates
Metal ions tend to precipitate at different pH values depending upon
the solubility products (KSP) of hydroxides. A plot of metal ion concentration
in solution as a function of pH was developed for most metal ions relevant to
the Ravensthorpe process at 25 and 45°C. The precipitation of manganese
was incomplete as MgO only raised the pH to approximately 8. As more
manganese(II) ions were present in solution, more nickel(II) and cobalt(II)
ions precipitated on a relative scale. Aluminium(III), copper(II) and silicon(IV)
ions had an adverse effect. Zinc(II) did not seem to affect nickel(II) and
cobalt(II) precipitation, while iron(III) and chromium(VI) actually improved it.
The competition of various metal ions was related to atomic radius and
softness where the incorporation of smaller ions in the precipitate was more
desirable. Metal ion incorporation also influenced the crystallinity as revealed
by the XRD scans of MHP. Precipitation of cobalt and manganese
oxyhydroxides also had an influence on the structures and composition of
MHP.
10.3 Oxidation During Precipitation
Manganese(II) hydroxide was oxidised to predominantly Mn3O4;
however, in the presence of cobalt(II), CoMn2O4, MnOOH and CoOOH
structures were observed to form. The spinel structures (Mn3O4 and
CoMn2O4) did not leach in ammonia/ammonium carbonate solutions in the
presence of mild reductants (Co(II) and sulphite), but the oxyhydroxide
structures (MnOOH and CoOOH) did leach. During the production of these
10-3
precipitates, bubbling air through the solution overnight oxidised 100% of
manganese(II) and only ~60% of cobalt(II). When leaching the precipitates
under reducing conditions the divalent cobalt aided the reduction of
manganese and trivalent cobalt. Cobalt(II) in the solid state was more
effective than that in solution as it would destroy the crystal lattice upon
leaching.
Titrations for the extent of oxidation and leaching results proved that
all the oxidation of cobalt(II) and manganese(II) occurred during precipitation,
filtration and sample preparation of MHP in the laboratory. XPS results
proved that the oxidation state of the two metals was homogenous
throughout the particles. The porous nature of the precipitate (8.3 m2/g for
RNO-MHP of 38-53 μm) would allow for oxygen ingress. Leach tests on
simple precipitates proved that the incorporation of manganese lowered the
quantity of oxidised cobalt, as manganese would oxidise preferentially due to
its lower oxidation potential (1.5 vs. 1.92 V). Less than 8% of cobalt and at
least 52% of manganese had oxidised in the 12 week old RNO-MHP sample
produced in June 2008.
10.4 Slow Leaching Compounds in MHP
The mixed Ni,Mg(OH)2 formed by the precipitation of nickel within the
MgO pores was observed in all MHP samples produced in the laboratory and
during pilot plant trials. The formation of Ni,Mg(OH)2 and hydrotalcite-like
structures were the causes of lowered nickel and cobalt leaching in
10-4
ammonia/ammonium carbonate solutions. Manganese caused a retardation
of the transition of MgO to Mg(OH)2 by the formation of a hydrotalcite-type
structure with the magnesium. Increasing levels of magnesium and
improvement in crystallinity both reduced nickel and cobalt leaching
significantly.
Stable, slow-leaching hydrotalcite-like structures were observed by
XRD in many of the synthetic precipitates and the RNO-MHP collected in
June 2008. These structures were formed when divalent and trivalent metal
hydroxides were present with an anion. The structure was poorly ordered
and recrystallised at a very slow rate, so was difficult to distinguish by XRD.
10.5 Remedies to Improve MHP Leaching
The formation Ni,Mg(OH)2 could be minimised by ensuring complete
dissolution of MgO. The incorporation of manganese also minimised the
formation of Ni,Mg(OH)2. Thus, all precipitates containing manganese
showed better cobalt and nickel leaching recoveries. This was achieved at
Ravensthorpe by (i) seeding, (ii) extending the tank residence times to 4
hours, and (iii) washing by the Larox filtration process. Thus, the RNO-MHP
sample collected in June 2008 only contained 0.94% Mg.
At Ravensthorpe the incorporation of manganese into MHP was
controlled by precipitating the metal ions out of solution with aeration and the
addition of lime to raise the pH to 8.5. As manganese incorporation was
10-5
beneficial, this stage could be removed or less lime added to run at a lower
pH, resulting in lower reagent costs and energy consumption.
The effect of oxidised cobalt and manganese on nickel leaching was
minimised or removed by the reductive leach. The hydrotalcite structure
containing oxidised metal ions was reductively leached in 45 minutes. As
hydrotalcite-type compounds in synthetic precipitates containing cobalt(III),
manganese(III) and iron(III) were reducible, they did not affect nickel
recovery during reductive leaching with a reducing agent in an ammonia
solution. However, structures containing aluminium(III) and chromium(III) did
not leach. Less than 85% of the nickel and cobalt was extracted from the
synthetic precipitates in a reductive soak predictor test when aluminium(III)
or chromium(III) were present in low concentrations (<10%).
Sulfate, chloride and carbonate were the most likely anions in the
Ravensthorpe process. A thorough washing technique (Larox filtration) was
found to remove all chloride and carbonate. However, sulphate was still high
and contributed to around 10%. To minimise the formation of these
compounds aluminium(III), chromium(III) and sulphate concentrations
needed to be minimised.
Drying the precipitate also minimised the formation of Ni,Mg(OH)2 by
retarding the transition between MgO and Mg(OH)2. With the Ni/Mg and
Ni/Mg/Co precipitates, nickel and cobalt leach recoveries improved with
drying at 50°C. However, nickel and cobalt recoveries were not influenced by
10-6
drying of the Ni/Mg/Co/Fe, Ni/Mg/Co/Al and RNO-MHP produced in June
2008. As transportation costs from Ravensthorpe to Townsville were
approximately $14 million per year, drying MHP to <1 % moisture could save
up to $5.6 million per year, less the capital and operating costs of drying and
dust management.
10.6 Leaching Kinetics of MHP
The rate of RNO-MHP dissolution was affected by the pulp density,
temperature, agitation and particle size. As a result of the evaporation losses
of ammonia at higher temperatures, the dissolution rates were different at 40
and 60°C. There were two leaching mechanisms for MHP. The initial rate
was controlled by a surface chemical reaction, followed by the shrinking core
kinetics due to the porosity of the precipitates.
Similar tests on synthetic precipitates revealed addition of cobalt(II),
copper(II), calcium(II), manganese(II), aluminium(III), zinc(II), silicon(IV) and
chromium(VI) ions actually improved the initial rate. This was a result of
either lowering the crystallinity of Ni,Mg(OH)2 or formation of an alternative
structure which was less stable. In relation to the rate of leaching, the iron(III)
hydrotalcite was the most stable and slowest leaching, followed by the nickel
magnesium hydroxide, aluminium hydrotalcite and manganese hydrotalcite.
10-7
10.7 Precipitation and Reduction Role of CoNiS
The precipitation of CoNiS was investigated by varying the
temperature, cobalt oxidation state and molar ratio of S:Co. Precipitates
produced at lower temperatures, with seed and when cobalt was in its
divalent state had higher cobalt compositions in CoNiS. However, the
addition of excess ammonium sulphide at a S:Co mole ratio greater than
2.2:1 did not affect metal compositions. Leach tests of MHP in the presence
of CoNiS as a reductant proved that the reduction depended directly on the
Co:S ratio. However, nickel sulphide was vital as it initialised reduction with
its faster dissolution and the release of sulphur species which encouraged
cobalt dissolution. The regeneration of sulphur species allowed for further
reduction. It was concluded that the cobalt to nickel ratio should be between
2:1 and 3:1 to achieve maximum reactivity of CoNiS.
10.8 Yabulu Plant Survey for ORP and Speciation
Measurement of the ORP using a Pt-Ag/AgCl probe during the Yabulu
site survey revealed a significant problem. At the end of the primary and
secondary leaches the ORP values with respect to the Ag/AgCl electrode
were negative. Divalent and trivalent cobalt were in equal concentrations
when the potential was -10 mV vs. Ag/AgCl. Significant concentrations of
divalent cobalt existed in solution, as evident from low ORP. This should be
avoided as cobalt(II) is known to have a lower solubility in ammonia. To
achieve a 10:1 ratio of cobalt(III) to cobalt(II) at 45 °C the potential needed to
be raised to 53 mV. Therefore, it was recommended that the potential of
10-8
these leaches should be monitored and controlled by the addition of air or
adjustment of feed rates.
According to the HPLC analysis of the Yabulu plant liquors in the Caron
leach circuit, cobalt(III) hexaammine is of higher concentrations than the
pentammine and tetraammine complexes. In contrast, the secondary leach
overflow contained significant concentrations of tetraammine complex due to
the presence of CoNiS in excessive quantities. As the tetraammine complex
is known to have a lower stability, cobalt co-precipitation with iron would be
more likely during the iron removal process resulting in lower overall cobalt
recoveries. Thiosulphato, sulphito and possibly sulphato and three unknown
complexes were also observed in the plant liquors. The effect of these
anions on Ni(II) solubility was confirmed by the varying solubility of
Ni,Mg(OH)2 in SAC solutions containing different buffer anions. These
findings warrant further studies on the influence of actual nickel and cobalt
speciation on the leach performance of MHP in the Yabulu circuit.
10.9 Future Work
Quantification of oxidation states and crystal phases in precipitates
proved to be extremely difficult due to the amorphous nature of MHP and the
sensitivity of cobalt and manganese towards oxygen. Due to the low
concentrations of cobalt and manganese in the MHP and the interference
from the O KLL and Ni LMM Auger series, quantification by XPS could take
months. The synchrotron surface technique of XANES may be quicker due to
10-9
an improved sensitivity and an alternative X-Ray source. Synchrotron
diffraction may also reveal more information due to an increased flux.
Original prediction according to the work completed by BHP Billiton in
the design of Yabulu extension project for the treatment of Ravensthorpe
MHP was that the ageing of MHP was responsible for low nickel and cobalt
leach recoveries. However, the findings from the ageing of synthetic MHP
and the influence of a variety of metal ions on the extent of ageing and
subsequent nickel and cobalt leach recoveries from this study lead to
different conclusions described in previous sections. A pilot plant study
producing, ageing and leaching MHP would be of value to justify the findings
from this project and to adjust the Yabulu extension circuit accordingly.
The Yabulu refinery plant survey revealed that the oxidation potential in
many of the tanks was lower than the expected value. A plant trial measuring
and controlling the ORP would also be useful as it may be used to improve
cobalt recoveries or at the very least make the process more robust.
Three unknown cobalt ammonia complexes were discovered by HPLC
in Yabulu plant liquors. As unwanted cobalt precipitation is the major cause
for lower cobalt recoveries and the final cobalt product is influenced by
solution chemistry, a better understanding of metal ammine complexes
would assist to improve recovery and product quality. If the species are
stable in solution for a few hours, the structures could be determined by
10-10
using HPLC combined with mass spectrometry (LC-MS), nuclear magnetic
resonance (NMR) and infrared spectroscopy (IR).
As drying did not influence metal recoveries and could save up to $5.6
million per year on transportation costs, it should be investigated further. The
MHP’s with a wide range of metal ion compositions should be dried in
commercial driers and leached using a method similar to that used at the
Yabulu refinery. The capital, running and dust management costs should
also be taken into consideration.
1
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Hydrometallurgy 103 (2010) 173–179
Contents lists available at ScienceDirect
Hydrometallurgy
j ourna l homepage: www.e lsev ie r.com/ locate /hydromet
Properties of aged mixed nickel–cobalt hydroxide intermediates produced from acidleach solutions and subsequent metal recovery
Andrew N. Jones a,⁎, Nicholas J. Welham b
a Murdoch University, South Street, Murdoch, WA 6155, Australiab Ballarat University, University Drive, Mount Helen, Ballarat, Victoria 3353, Australia
⁎ Corresponding author.E-mail address: [email protected] (
0304-386X/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.hydromet.2010.03.017
a b s t r a c t
a r t i c l e i n f oArticle history:Received 7 January 2010Received in revised form 20 March 2010Accepted 21 March 2010Available online 28 March 2010
Keywords:Nickel–cobalt hydroxideMHPAgeingNickel–magnesium hydroxideHydrotalciteNickel leaching
Synthetic mixed nickel–cobalt hydroxide precipitates (MHP) were produced containing varying levels of Ni,Co, Mn, Mg, Al, Fe, Cr, Cu, Zn and Si to understand and characterize the ageing processes and subsequentnickel and cobalt leach recovery. Precipitates were monitored over a 12 week period using X-ray diffraction(XRD), scanning electron microscopy (SEM) and leach tests in ammonia–ammonium carbonate solution.Manganese and cobalt incorporation into MHP was beneficial for subsequent nickel recovery; most likelypreventing or slowing nickel–magnesium hydroxide formation. High magnesium MHP generally loweredsubsequent nickel recovery due to its stability and slow leaching kinetics.Between 94–100% Ni and 84–100% Co were leached from nearly all high magnesium MHP precipitates aftersoaking in the leach solution for 72 h, except for the precipitate containing about 5% Al which only recovered87% Ni and 61% Co. An XRD of this precipitate showed that it was much more amorphous than any otherMHP whilst the XRD of the leach residues revealed a magnesium–aluminium hydrotalcite structure. Bothnickel–magnesium hydroxide and hydrotalcite-like structures appear to inhibit nickel and cobalt recoverybecause of their slow leaching characteristics.
A.N. Jones). Fig. 1. Effect of a
ll rights reserved.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
The BHP Billiton Ravensthorpe operation in Western Australia wasthe first of the third-generation nickel laterite acid leach plants whichcommenced production in late 2007. The operation utilized pressureand atmospheric acid leaching to recover metals into the solution,followed by the production and shipping of an intermediate mixednickel–cobalt hydroxide precipitate (MHP) to BHP Billiton's YabuluRefinery for further processing. The MHP separates the nickel andcobalt from many of the other impurities present in the solution,resulting in a more robust process (Flett, 2002; Mayze, 1999; Willis,2007). The advantage of themixed hydroxide precipitate is the simpledissolution process using ammoniacal ammonium carbonate solutionat atmospheric conditions with excellent rejection of iron, manganeseand magnesium. Unfortunately, the ageing of the intermediate overthe 6 week transportation period introduces significant technical riskdue to ageing and oxidation processes, which are deleterious to thesubsequent dissolution in ammonia–ammonium carbonate liquors(Muir, 2003).
The MHP produced is a complex material that undergoes varioussolid state transformations and oxidation reactions (Muir, 2003). Cobaltandmanganese oxidize readily, whilst nickel has been observed to form
a stable slow leaching phase with magnesium and trivalent metalimpurities which induce re-structuring of nickel hydroxide to formstable crystalline compounds. The effect of ‘ageing’ of MHP producedfrom a Ravensthorpe pilot plant is clearly shown in Fig. 1, whereammoniacal leaching of nickel over a period of time resulted in asignificant decrease in recovery.
The intermediate was precipitated at 50 °C and was transportedwet as a filter cake of approximately 60% solids. Through in-houseinvestigations it was discovered that drying the precipitate was not
geing on the ammonia leaching of MHP (White et al., 2006).
Table 1Approximate concentration of metals in solution for precipitation.
Solutionno.
Metal concentration [g/L]
Ni Co Mn Al Fe Cr Cu Zn Si
1 4 0.4 0.152 4 0.4 0.663 4 0.4 14 4 0.4 2.55 46 4 1.257 4 0.4 0.88 4 0.4 19 4 0.4 1.6610 4 0.4 0.811 4 0.4 0.812 4 0.4 0.33
Table 2Mixed hydroxide precipitation discharge compositions (White et al., 2006).
Stream Analysis
Ni Co Fe Mg Al Mn Ca Si SO4 pH
Liquor (mg/L) 260 1.3 0.8 42,900 2 800 430 2 180 7.4Solids (wt.%) 40.4 1.7 0.1 2.5 0 3 0.7 0.4 17 –
174 A.N. Jones, N.J. Welham / Hydrometallurgy 103 (2010) 173–179
only costly but also detrimental to metal recovery. Upon arrival atYabulu, the precipitate went through successive oxidative andreductive leach stages in agitated tank reactors containing ammo-nia–ammonium carbonate solutions. The reductive leach used acobalt nickel sulfide (CoNiS) produced on site to reduce the oxidizedcobalt and manganese species. Final MHP leaching was achieved bywashing the MHP leach residue solids for approximately 3 days in aneight stage thickener CCD circuit where metal concentrations werecomparatively low and the ammonia concentrations were compara-tively high. Impurities were removed from the leach solution byprecipitation and solvent extraction before various cobalt and nickelprecipitates were produced for sale (Fittock, 2004).
Although significant test work was conducted by BHP Billiton, thecauses of the ageing processes and the effects of various impurities stillrequired further investigation. In this study, mixed hydroxide pre-cipitates were synthesized with varying levels of Ni, Co, Mn, Mg, Al, Fe,Cr, Cu, Zn, Si, C, S and SO4, and monitored over 12 weeks. During thisperiod, X-ray diffraction (XRD), scanning electron microscopy (SEM),leach tests, moisture content and size analysis were performed on thesamples at periodic intervals.
Table 3Assay on precipitates.
2. Experimental
2.1. Production of synthetic metal hydroxides
The metal hydroxides were produced by adding a stoichio-metric quantity of ‘fresh’ magnesia (MgO) (Emag 45—QueenslandMagnesia Pty Ltd.) to 6 L of a metal sulfate solution. To ensurereaction of MgO, the solution was stirred for 4 h under ambientconditions before filtering and washing 3 times with 500 mL ofdeionised water to produce a filter cake containing ∼50% solids(Jayasekera, 2003; White et al., 2006). Each filter cake was dividedinto 7 fractions, which were stored separately in sealed plasticsample jars ready for analysis.
The concentrations of the metals in the solution before precipita-tion are shown in Table 1. Nickel and cobalt concentrations are similarto the Ravensthorpe process, but manganese was added so that theprecipitate would contain between 2 and 15% Mn. Other metalconcentrations were chosen to aim at 5% incorporation into MHPbased on previous experimental work.
After the initial precipitation and analysis, it was discoveredthat the pH did not rise above 8 using MgO due to the presence ofsulfate in the solution from the metal sulfates. According to Miller(2005), a pH of 8.8 was required to remove 100% Mn. Therefore, forsamples 1 to 4 the pH was raised from 8 to 8.3 with lime. Althoughthis introduces calcium into the precipitate, this was consideredinsignificant since the Ravensthorpe MHP already contained 0.7%Ca (Table 2).
2.2. Analysis of precipitates
XRD (Siemens D500), moisture determination and leach testswere performed on the precipitates in weeks 1, 2, 3, 4, 6 and 12. XRDwas also performed on selected leach residues, and a reductive soaktest was conducted in week 12 using the Yabulu refinery procedure.SEM (Phillips XL30) was conducted on a few of the precipitates inweeks 1, 3 and 12; whilst the size distributionwasmonitored over the12 week period using a laser sizer.
Cross sections of the MHP were prepared for SEM by firstly dryingthe precipitate at 50 °C under nitrogen, then embedding the particlesin a resin block. The resin was ground down to reveal cross sections ofparticles and coated with carbon for analysis.
The leach tests were conducted in triplicate and were scaleddown versions of the Yabulu Refinery predictor leach test. In themodified standard leach test, 0.2 g (Ni+Co) dry basis was agitatedfor 45 min at ambient temperatures with 25 mL of syntheticammonia–ammonium carbonate liquor (SAC) containing 93 g/LNH3 and 65 g/L CO2. The reductive leach test was similar to the
Table 4Assay of Queensland Magnesia's MgO (Emag 45).
Chemical analysis [wt.%]
Specification Typical
MgO 92.0 max 95CaO 5.0 max 3SiO2 2.0 max 1Fe2O3 – 0.1Al2O3 – 0.1Mn3O4 – 0.1As – b0.5 ppmPb – b0.2 ppmHg – b0.2 ppmCd – b0.1 ppm
Table 5Metal incorporation into MHP.
Precipitateno.
Ratio [% metal in MHP/% metal in feed solution]
Ni Co Mn Al Fe Cr Cu Zn Si
1 0.40 0.38 1.832 0.53 0.51 0.743 0.57 0.55 0.784 0.66 0.63 0.755 0.586 0.62 0.637 0.51 0.61 0.948 0.73 0.68 0.579 0.78 0.75 0.3910 0.45 0.46 1.0611 0.63 0.61 0.7112 0.57 0.56 0.08
175A.N. Jones, N.J. Welham / Hydrometallurgy 103 (2010) 173–179
standard leach but contained 0.2 g of hydroxylamine sulfate ((NH2-
OH)2H2SO4) as an additional reductant. This was in gross excess ofthe required amount, which ensures that all the oxygen will beconsumed, leaving ample hydroxylamine sulfate for the reduction ofcobalt and manganese.
The reductive soak predictor leach test entailed a 45 minuteagitated leach of 4 g (Ni+Co) dry basis in 500 mL of SAC with acalculated quantity of hydroxylamine sulfate according to Eqs. (1) and(2). The leach residue was transferred to a plastic sample jar with
Fig. 2. Size distribution of pre
250 mL of SAC and retained at 50 °C for 72 h. The jar was shakeninitially and after 24 and 48 h to break up compacted solids.
No:molesðCo + MnÞ in dry MHP =mass Co indry MHP; g
58:9
+massMnindryMHP; g
54:9
ð1Þ
mass required; g = No:molesðCo + MnÞ × 164 × 1:5 ð2Þ
3. Results and discussion
3.1. Assay results
Table 2 demonstrates the ‘typical’ discharge compositions for theRavensthorpe process. In Table 3 the highlighted boxes indicate whichmetals were added to the solution. However, the assay results wereunexpected and significantly different to plant MHP particularly withregard to the nickel and magnesium content. The obtained nickelcontent was about 20%, rather than 40% in Ravensthorpe MHP, whilstthe 10–20% magnesium content was high relative to the 3% in plantMHP. Nevertheless, test work was continued to determine theinfluence of metal incorporation on ageing and leach recovery.
The precipitation and incorporation of metals in a mixed metalhydroxide is an extremely complicated process (Muir, 2003). PreviousBHP Billiton investigations have found that high levels of magnesiumin the sample are unavoidable when precipitating on a small scale atambient conditions. In a study conducted by SGS Lakefield Oretest PtyLtd in 2003 for Ravensthorpe Nickel Operations (Jayasekera, 2003) themagnesium incorporation varied from 2.6 to 12.4% after precipitatingnickel and cobalt at 50 °C at 100% stoichiometry for 4 h.
The temperature seems to have a significant effect on magnesiadissolution and on the kinetics of precipitation. As the precipitates tookabout 8 h to prepare and were produced in 3 batches of 4, the heatingthe 6 L of each solution to 50 °C was considered excessive for thislaboratory investigation. Themagnesiumincorporation could have beenreduced by raising the temperature or by lowering the stoichiometricquantity of magnesia added. However this would result in lower minormetal concentrations and a more complicated system. For the purposeof this investigation, the higher level of magnesium was deemedacceptable to monitor the effect of crystalline nickel–magnesiumhydroxide (Ni,Mg(OH)2). The relatively high levels of silica (0.39–0.54%Si) and calcium(0.21–0.27%Ca) in theprecipitates are due to theirpresence in the magnesia sample (Table 4), but the variability of the
cipitates over 12 weeks.
Fig. 3. XRD pattern of precipitate after 1, 4, 8, 21, 35, 58 and 84 days ageing: (a) precipitate 5 (Ni, Mg), (b) precipitate 6 (Ni, Co, Mg), (c) precipitate 2 (Ni, Co, Mn, Mg) and(d) precipitate 7 (Ni, Co, Mg, Al).
176 A.N. Jones, N.J. Welham / Hydrometallurgy 103 (2010) 173–179
carbon (0.08–0.61%), present as carbonate, is likely due to uptake of CO2
from the air.In precipitates 1–4 the nickel concentrations are reasonably
constant, whilst the manganese increases and magnesium decreases.Although Ravensthorpe MHP typically contains 3% Mn (Table 2),higher levels were used to ensure the effect of manganese isobservable. Precipitates 5–12 show accepted levels of the desiredmetals. By raising the concentrations of Co, Al, Fe, Cr, Cu, Zn and Si,their effect should be easily observable.
Table 5 shows the metal incorporation ratio (% metal inprecipitate/% metal in feed solution) since some metals such as Al
Fig. 4. Percentage of MgO in precipitates 1–4 (a
and Cu are incorporated into the precipitate more readily than others,hence competing with nickel and cobalt.
When more manganese was present in the solution, more nickeland cobalt were incorporated into the precipitate. There is obviouslysome interaction between the metals causing a co-precipitation of thenickel and cobalt. However, aluminium and copper have an adverseeffect and compete with nickel and cobalt for precipitation. Bycomparison, only 8% Si, 39% Cr, 57% Fe and 71% Zn were precipitatedwhich did not significantly affect Ni and Co precipitation.
In terms of magnesium content, precipitates containing copperhave a higher Mg, whilst high iron and chromium precipitates have a
pprox. calculation based on peak heights).
Fig. 5. Back scattered electron image of precipitate 2 after: (a) 1 week, (b) 3 weeks and(c) 12 weeks of ageing.
177A.N. Jones, N.J. Welham / Hydrometallurgy 103 (2010) 173–179
lower Mg contents. The reasons for the effects of these metals onnickel, cobalt and magnesium are unknown. However, the size of thehydrated ions and their hydrolysis pH will likely have an effect ontheir level of inclusion.
3.2. Size distribution
As expected, the overall particle size of the precipitate increasedafter their initial production (Fig. 2). This was related to the presenceand hydration of MgO and probable coagulation of smaller metalhydroxide particles. The decrease in particle size observed with sevenof the precipitates was unusual and inexplicable.
3.3. X-ray diffraction
XRD (SiemensD500) on the filter cakes showed that the precipitatesconsisted of predominantly Ni(OH)2, Mg(OH)2 andMgO. In all patternstherewere no peaks to suggest that a hydrotalcite-like structure existedin which trivalent metal impurities induce re-structuring of Ni(OH)2;(generalised formula: [MII
1− xMIIIx(OH)2]2+with interlayers containing
anions and water molecules). The peak positions of the 3 phases areincluded in Fig. 3. In the plots, MgOwas present in the precipitate in thefirst few days until it transformed to Mg(OH)2, whilst nickel andmagnesium hydroxides became more crystalline over time.
Therewereno conclusive signsof oxidizedmanganese species in anyof theprecipitates.Manganese is known tooxidize readily, and indeed inmany cases the precipitate was observed to become brown over time asthe metal oxidized. Obviously no crystalline oxidized manganesehydroxides exist in the precipitate, so the composition cannot bedetermined.
The XRD pattern of precipitate 7 containing aluminium issignificantly different, as shown in Fig. 6. Although the filter cake isaround 40% solids, like most other precipitates, the pattern shows amaterial that is significantly more amorphous than the others. Thiscould be due to the formation of a hydrotalcite-type structure becauseof the presence of trivalent aluminium.
In the XRD patterns of precipitates 5, 6 and 2 (Fig. 3a, b and crespectively), the transformation of MgO to Mg(OH)2 was slowerwhen manganese and cobalt were present. However the mostsignificant effect was observed for precipitate 2, where MgO wasstill present in the pattern after 84 days of ageing.
Fig. 4 highlights the difference between samples with and withoutmanganese. For precipitates 5 to 12, the MgO was completelyhydrated within 21 days of ageing, whilst the precipitates 6 and 2took over 30 days. There seems to be an ‘ideal’ quantity of about 4.4%Mn to slow the transition which in this case was in precipitate 2.
3.4. Scanning electron microscopy
SEM images on cross sections of the precipitates were performedover the 12 weeks of ageing to monitor the metal compositionthroughout the precipitate. Two mechanisms have been proposed tooccur upon precipitation of metal hydroxides with MgO. The first isthe complete dissolution ofMgO and subsequent nucleation of ametalhydroxide; the second is the precipitation of the metal hydroxides onthe MgO particles. It is also not known whether metal hydroxidesprecipitate separately, forming small metal hydroxide layers, or ifmixed hydroxides are present from the beginning. If completedissolution and subsequent nucleation occurred, elemental mappingwith SEM would show metals evenly dispersed throughout theprecipitate. Alternatively, if the hydroxides are precipitating upon themagnesia, as indicated by Muir (2003) and by Shrestha et al. (2003),elemental mapping would reveal a magnesium rich core.
The particles chosen in Fig. 5a, b and c were representative of thewhole sample. Elemental mapping (not shown) revealed that theparticles consist of a mixed metal hydroxide core containing nickel,
cobalt, manganese and magnesium, with an outer nickel and cobalthydroxide layer. This is shown as a brighter ring around the particle asnickel and cobalt have a higher atomic number thanmagnesium. Thesescans demonstrate that the particles are formed by the partial
Fig. 6. XRD of reductive leach residues 5–12.
178 A.N. Jones, N.J. Welham / Hydrometallurgy 103 (2010) 173–179
dissolution of magnesia and subsequent nucleation and that the metalsare present as a mixed hydroxide rather than in separate layers.
Fig. 5b and c indicates that by week 3 and week 12, the outer layerappears to decrease in size. This occurs when the nickel and cobaltform more stable phases, like a mixed nickel–magnesium hydroxideor a hydrotalcite-like structure. By week 12 (Fig. 5c) the particles havegrown in size and cracking has occurred upon drying. This isattributable to the adsorption of water into the metal hydroxidelayered structure over time. Assuming the precipitate only consists ofMgO and Mg(OH)2, the particle was calculated to grow in size byapproximately a third using density and molar mass.
3.5. Leach tests
Modified standard and reductive leach tests were performed atperiodic intervals over the 12 weeks. In all cases, the nickel and cobaltrecovery was seen to decrease with time. Table 6 presents therecoveries of nickel and cobalt for the standard, reductive and the soaktests performed at the end of the ageing period.
A comparison of leach recoveries between precipitates 5 and 6revealed that thepresenceof cobalt actually improvednickel recoveryby13 and 15% for the standard and reductive tests respectively.Manganesealso improved nickel recovery by between 3 and 23% (precipitates 1–4vs. 5). The presence of manganese and/or cobalt appears to limit theformation of the slow leaching nickel–magnesium hydroxide.
Unusually, the recovery of nickel was greater with the standardleach compared to the reductive leach for precipitates 5, 6, 9, 11, and12. Similarly, for the recovery of cobalt with precipitates 6 and 9.Therefore, the oxidation of cobalt is not an issue and in some caseswhere metals cannot be reduced (precipitates 5, 6, 9, 11 and 12) theoxidative process was better. However, the reductive leach wasbeneficial when the precipitates contained manganese.
These conclusions suggest that the manganese and cobalt could beinteracting with the nickel in different ways. It is suggested that thecobalt forms a mixed hydroxide with the nickel, thus limiting theformation of Ni,Mg(OH)2 whilst the oxidation of manganese induces are-structuring of the Ni(OH)2 to form a hydrotalcite-like structure.
Overall, precipitate 7 containing Ni, Co, Mg and Al exhibited theworst metal recoveries followed by precipitates containing Si, Cr andFe. The trivalent Al, Cr and Fe ions are known to induce transformationto hydrotalcite-type structures, which are stable and slow leaching.However, in the reductive leach test, divalent iron-rich precipitateexhibited better nickel and cobalt recoveries than the aluminium andchromium rich precipitates.
Inmost cases the standard and reductive leach tests exhibited poorrecoveries, whilst the reductive soak test leached N94% Ni and N84%Co, except for precipitate 7. Slow leaching phases, which were leachedupon soaking, were clearly present in all the precipitates. To observewhich phases remained after the various leach tests, an XRD was
Table 6Percent metal recovery from leach tests after 12 weeks of ageing.
Precipitateno.
Metals Standard Reductive Soak
Ni Co Ni Co Ni Co
1 Ni, Co, Mg, Mn 70 60 86 81 99 952 Ni, Co, Mg, Mn 89 79 99 96 100 973 Ni, Co, Mg, Mn 78 69 96 94 100 964 Ni, Co, Mg, Mn 90 82 99 97 100 915 Ni, Mg 67 – 53 – 98 –
6 Ni, Co, Mg 80 74 69 61 99 997 Ni, Co, Mg, Al 40 18 42 28 87 618 Ni, Co, Mg, Fe 39 33 55 59 98 849 Ni, Co, Mg, Cr 58 39 49 36 96 8810 Ni, Co, Mg, Cu 88 59 94 90 98 9811 Ni, Co, Mg, Zn 80 70 76 73 100 10012 Ni, Co, Mg, Si 46 21 40 34 94 89
performed on most leach residues. Apart from precipitates 7, 8 and 10(discussed below), Ni,Mg(OH)2 was the only phase remaining.
The XRD of the reductive leach residue of precipitate 7 containingNi, Co, Mg, Al and precipitate 8 containing Ni, Co, Mg, Fe indicates thepresence of magnesium–aluminium and magnesium–iron hydrotal-cite structures (Fig. 6). The peaks were broad indicating that thestructures were poorly crystalline. Nevertheless, this was a significantfinding in terms of hydrotalcite-like structures, but various amor-phous phases could be present which may have a significant effect on‘ageing’ and metal recoveries.
The leach residue of precipitate 10, which contained copper, hasmany unidentified XRD peaks (Fig. 6). There are many possibilities, sono definite conclusions were made. There are clearly multiple phasesremaining which are all stable and slow leaching.
It should be noted that the synthetic precipitates produced andtested were aged longer and contained higher levels of metalimpurities than the Ravensthorpe MHP, and the laboratory leachtests were not as robust as the Yabulu Refinery MHP leach process.Nevertheless, it is apparent that aluminium at high levels would causea significant loss of nickel recovery in leaching.
3.6. Nickel–magnesium hydroxide leaching kinetics
To observe the kinetics of nickel leaching and to determine theeffect of magnesium on the rate of nickel dissolution, four precipitateswith increasing concentrations of magnesium (17 to 30%) wereproduced, aged for 3 weeks, and then leached in an ammonia–ammonium carbonate solution at ambient conditions.
Fig. 7 demonstrates that as the magnesium concentration increasedthe kinetics slowed and the nickel recovery dropped. This is logical dueto the magnesium forming a stable mixed hydroxide with the nickel.
Fig. 7. Rate of nickel leaching — Ni/Mg ratio.
179A.N. Jones, N.J. Welham / Hydrometallurgy 103 (2010) 173–179
The slowerkineticsmaybedue to aMgCO3product layer formingor dueto shrinking core model kinetics. However, no magnesium carbonatepeaks were observed by XRD.
4. Conclusions
The presence of manganese and cobalt in nickel hydroxide hasbeen found to be beneficial to subsequent recovery of nickel byleaching. The beneficial level of manganese in this study was about4.4%. Manganese is observed to oxidize to an amorphous phase and noX-ray crystalline phases exist in the precipitates. The oxidation stateof cobalt in MHP could not be determined but likely to be Co(III). Over99% Ni and 91% Co were leached from all precipitates containing over4.4% Mn. The formation of a mixed manganese or cobalt hydroxidewith nickel is believed to prevent or slow Ni,Mg(OH)2 formationwhich appears to be the perpetrator for lower nickel recovery due toits stability and slow leaching kinetics. This crystalline phase waspresent in all leach residues.
Fortunately, with all precipitates but one, reductive soaking for72 h was able to leach over 94% Ni and 84% Co. These results weresurprising since the precipitates were aged for 12 weeks andcontained over 10% Mg and 5% other metal impurities which aregross exaggerations of impurities in the MHP produced fromRavensthorpe. The exception was the precipitate containing about5% Al when only 87% Ni and 61% Co were recovered with the soakleach test. An XRD on the precipitate containing aluminium showedthat it was more amorphous than any of the other precipitates and anXRD on the leach residue revealed that it contained a magnesium–
aluminium phase with a hydrotalcite structure. This could be thereason for its poor leaching. The iron-rich leach residue was the onlymaterial where this type of structure was present, but severalunidentified phases were present in the copper-rich leach residue.The hydrotalcite-like XRD peaks observed were broad, indicating lowcrystal order even after 12 weeks of ageing. No doubt these structuresform upon precipitation and slowly become more ordered withageing. In which case, this type of structure could be present in mostmixed metal hydroxide precipitates but not detectable by XRD.
It is concluded that it is important to incorporate the 72 hour CCDcircuit at Yabulu to allow complete nickel dissolution to take place if‘poor’ quality (highMg or Al) precipitates were processed through thecircuit. Magnesium slows the rate of reaction by forming a stablesurface hydroxide as the nickel hydroxide is leached and the slowingof the kinetics is believed to follow the shrinking core model.
Acknowledgements
The work was conducted as part of an Australian Research Council(ARC) linkage project involving BHP Billiton Yabulu Refinery andMurdoch University. Additional funding was provided by theMineralsand Energy Research Institute of Western Australia (MERIWA).Magnesia was supplied by QMag.
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