organic degradation in uranium and cobalt solvent extraction: the...

ORGANIC DEGRADATION IN URANIUM AND COBALT SOLVENT EXTRACTION 295

Introduction

In 2002, Rössing Uranium mine in Namibia reported that organic phase breakdown productsmay be the cause of problems in the strip section of the solvent extraction plant, whichseemed to relate to the presence of nitrosamines3. Extensive crud formation, poor strippingefficiency and excessive organic entrainment were noted. ChemQuest was approached as thesupplier of the isodecanol in use as a phase modifier, and as link to the supplier of thediluents, which at that time was Shell Chemicals.

At the same time, a small cobalt extraction facility in South Africa reported very poorperformance on the nickel/cobalt separation stage of their process4, which also appeared to belinked to breakdown products of the organic extractant and diluents.

VAN RENSBURG, D.M. MUNYUNGANO, B., HAIG, P., LOUIS, P., and STOLTZ, J. Organic degradation inuranium and cobalt solvent extraction: the case for aliphatic diluents and antioxidants. HydrometallurgyConference 2009, The Southern African Institute of Mining and Metallurgy, 2009.

Organic degradation in uranium and cobaltsolvent extraction: the case for aliphatic

diluents and antioxidants

D.M. VAN RENSBURG*, B. MUNYUNGANO†

*ChemQuest (Pty) Ltd, South Africa†Rössing Uranium Limited, South Africa

In the solvent extraction of cobalt, nickel, uranium and zinc, variousparties1,2 have reported over the years that a certain amount ofdegradation of the organic phase has taken place, affecting kinetics,loading and phase separation times.

When the degradation products of the organic phase were detectedat a uranium operation in Namibia3, a cobalt refinery in South Africa,and a cobalt pilot plant in the DRC4, a number of steps were taken tocombat the problem, including the use of an aliphatic diluent and theaddition of butyl hydroxy toluene as an antioxidant.

Through a combination of laboratory testing and plant observation,it was concluded that plant management to identify, isolate andcontrol the source of the oxidant material was sufficient to combat theproblem, and that neither the diluent change nor the use of theantioxidant was proven to have made any significant difference to thedegree of organic degradation

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HYDROMETALLURGY CONFERENCE 2009296

As ChemQuest was extensively involved in the supply of organic reagents, and a number ofprojects in the DRC were beginning to develop, we decided to invest some time inresearching the subject of organic degradation in certain solvent extraction operations, and themeasures taken to combat the problem.

Organic degradation in uranium solvent extraction

The organic phase at Rössing is made up as follows: • Alamine 336, a tertiary amine: 7% by volume • Isodecanol phase modifier: 3% by volume • Diluent, initially Shellsol 2325: 90% by volume

The tertiary amine extractant R3N functions as follows:

First it protonates in sulphuric acid 2R3N + H2SO4 ⇔ (R3NH)2SO4

Then it exchanges/extracts uranyl sulphate: (3(R3NH)2SO4 + UO2 (SO4)22- ⇔ (R3NH)6UO2(SO4) 4 +SO42-

This is a process of ion association and differs from copper extraction (chelation, orcomplex formation) and systems using acidic extractants such as D2EHPA and Cyanex 272(solvation). It is neither highly selective nor as pH dependent as the chelation and solvationprocesses, and in practice ion exchange with many anionic species takes place, including co-extraction of nitrates to form nitrosamines:

The presence of the nitrosamines and other organic degradation products, detected by gaschromatographic analysis, was correlated directly with the upset conditions on the SX plant.In turn, the formation of the nitrosamines was initially linked to two factors: the ingress ofnitrates with process water originating from the explosives in the open pit and high redoxpotentials from the leach circuit. The presence of Mo as a catalyst was also identified as acontributor.

Nitrates and nitrosamine formation

Laboratory tests indicated that the presence of nitrous acid at a concentration of 1 g/l and at aredox potential of 550 mV led to the formation of nitrosamines.

Water is a scarce resource in the Namib Desert and the recycling of water is unavoidable—there simply is not enough water to ‘bleed and top-up’ to keep species like nitrates undercontrol cost-effectively. The use of ‘pit’ water in the elution of the ion exchange resin(upstream from the SX, to produce the pregnant leach feed solution to the SX) is inevitableand thus also the ingress and build-up of nitrates. However, the reduction plant managers havefound that controlling the nitrates in the PLS to SX at 0.5–0.7 g/l and loadings on the organicto below 1 g/l controls the nitrosamine formation to acceptable levels.

It was found that the nitrosamines were scrubbed from the organic phase during the solventregeneration step using sodium carbonate.

Routine monitoring of nitrates in the concentrated eluate (effectively the SX PLS), theammonium sulphate, the strip solvent and at other points has been implemented (see Figures 1and 2). Control of the pH in the strip and scrub sections was integral to the removal of thenitrosamines, and better management measures were also introduced.

Obviously, the reduction of the amount of nitrates in the incoming water sources, althoughnearly impossibly expensive, is an ongoing exercise.

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Redox potential

The link between high redox potential and the degradation of the organic phase wasinvestigated, both on the plant and by a survey of similar circumstances reported in theliterature.

The acid leaching of the Rössing uranium-bearing ore requires the oxidation of tetravalenturanium oxide to the acid soluble hexavalent form. This is achieved by indirect oxidation viaferric ions. The ferric is reduced to ferrous, then re-oxidized to ferric by the addition ofpyrolusite, manganese dioxide. The process is controlled by the maintenance of the ferric toferrous ratio. The carry-over of high redox potentials from the leach to the IX and in turn tothe SX is controlled by the addition of iron metal (wire) to the pregnant solution.

Figure 2. Rössing plant monitoring

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Figure 1. Rössing plant monitoring

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Despite the link found in research and in the laboratory between high redox potentials andorganic degradation/formation of nitrosamines, no significant correlation was found betweenthe redox potential in the leach and the redox potential in the SX circuit. The poor milling ofpyrolusite and associated carry-over of MnO2 to the IX did not influence the redox potentialin the SX. Varying redox potentials in the leach and in the concentrated eluate from IX did notreflect in the measured redox potential anywhere in the SX circuit, nor did it correlate withnitrosamine formation or SX performance upsets.

Other organic degradation factors/products

The literature survey and the experience of some applications (see below) led to Rössinginvestigating the possibility of the degradation of the other organic constituents5,6, namely theisodecanol and the paraffinic diluent. Dr. Gordon Ritcey7 noted from personal experience asfollows:

‘Isodecanol (CH3(CH2)9OH) was oxidized in the presence of air and high redox potential toaldehyde, carboxcyclic acid and eventually primary alcohol which will result in poorcoalescence.

Iso-tridecanol (CH3(CH2)12OH) which is inherently more stable and has lower solubility inboth the aqueous phases and in many organics, had been used in applications wheredegradation was noted and possible.’

Isodecanol is an essential phase modifier in uranium solvent extraction systems as itimproves the solubility of the tertiary amine in the diluent5. Rössing have a standard methodfor testing the kinetics, loading and phase separation characteristics of their fresh organicconstituents, and tested a sample of isotridecanol. They found it offered no significantdifference to isodecanol, and as it was considerably more expensive, they did not pursue itspossible use. Furthermore, in the examinations of the both the fresh organic and degradedorganic chromatographic scans, no difference in the peaks of the isodecanol could be detected.

Without further evidence, it was decided that no isodecanol-linked degradation productscould be detected, and that no change would be made to the isodecanol use or regime.

The diluent presented another scenario entirely. Following the reported experience of cobalt-catalysed degradation of diluents (discussed below), and laboratory testing under acceleratedoxidative conditions, it was decided that the aromatic diluent in use, Shellsol 2325 containing16–23% aromatics, would be replaced by Sasol SSX 210, with < 0.1% aromatic fraction.

Initial testing was performed by ChemQuest in the laboratory, using a jacketed glassreaction vessel, similar to the Acorga apparatus used for testing copper solvent extractionreagents, together with a variable speed stirrer. A number of extraction and strip tests werecarried out using a synthetic solution containing uranyl sulphate, and in the case of thedegradation tests, some permanganate ions as the oxidizing agent (5 mg/l as KMnO4) andbutyl hydroxy toluene (BHT) as an antioxidant (0.2% m/v). The starting pH was maintainedexactly by adjustment with NH4OH.

The purpose of the test was to determine, inter alia, the effect of organic oxidation onextraction and phase separation times with the type of diluent and the presence or absence ofBHT as the variables.

The accelerated oxidation was done at an elevated temperature (45°C) using the samesynthetic solution used by Rössing to perform their own kinetic tests, but with the addition ofthe permanganate ions. Organic to aqueous ratios were kept at exactly 1:1 and mixing timesand stirrer speed kept constant (180 minutes, 220 rpm). Air was bubbled through uniformlyduring the oxidation phase using a constant pressure air pump.



The organic phase was removed after the oxidation and tested for extraction and phasedisengagement using the same synthetic PLS solution but without the permanganate ion, atambient temperature, and using the same apparatus. (See Figure 3.)

The complete test methods, solution concentrations, test parameters and all results areavailable from the author in a separate document8 which includes work on copper, zinc andcobalt/nickel solvent extraction systems.

The key results related to uranium were as follows: (Table I)The most significant indication was that, with or without BHT anti-oxidant, the aliphatic

diluent showed far fewer deleterious effects after oxidation than was seen by the diluentcontaining a portion of aromatics.

As there was no funding or incentive available for ChemQuest to continue these tests, theywere limited to single results and the single set of oxidizing conditions. Rössing, however,accepted the trends indicated and decided to continue with a plant change-over to the aliphaticdiluent and to test the use of the BHT.

Extraction of uranium, % Phase disengagement time, secs

Before oxidation After oxidation Before oxidation After oxidation

Shellsol 2325 73.1 33.8 35 95

Sasol SSX 210 72.9 54.9 38 78

Shellsol 2325 with 0.2% BHT 72.2 63.8 35 51

Sasol SSX 210 with 0.2% BHT 72.9 71.8 36 40

Table I

Results related to uranium

Figure 3. Apparatus used in accelerated degradation tests phase disengagement and kinetic apparatus

B 24 Sockets

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To date, the aliphatic diluent has not been shown to provide any significant changes in theoperation of the solvent extraction facility. A number of factors, such as strip efficiency, crudformation at low temperature, and bacterial growth may or may not be linked to the diluentchange, especially as other influencing parameters have also changed.

The BHT has not yet been tested on the plant.As Rossing have simultaneously instituted corrective measures and tests to control the

ingress of nitrates, and therefore the formation of nitrosamines, the influence of the diluentchange cannot be measured or quantified, and no conclusion drawn.

What is clear from this exercise, however, is that it remains better to tackle the source of theproblem, rather than treat the symptoms and effects—i.e. controlling the formation ofnitrosamines is probably far more beneficial and effective than trying to adapt the organicreagents to handle the oxidative conditions.

Organic degradation in cobalt/nickel separation solvent extraction

ChemQuest was alerted to a problem encountered in a very small solvent extraction facility inSouth Africa. Though not much more than a pilot plant, the facility was part of a plant used toproduce a pure cobalt carbonate product, which in turn was calcined to cobalt oxide for sale inEurope.

The solvent extraction part of the facility used di-ethyl hexyl phosphoric acid (BayerD2EHPA) to extract zinc and manganese from cobalt-rich leachate. A cobalt nickel separationwas then performed using bis (2,4,4-trimethylpentyl) phosphinic acid (Cyanex 272).

The D2EHPA circuit showed increased viscosity, poor phase separation, and an increase inorganic entrainment. Although the zinc and manganese extraction was not significantlydifferent, it was seen that co-extraction of nickel and increased extraction of cobalt was takingplace, both of which are not expected from the extraction curves for D2EHPA at the pH levelsused in the plant. The Cyanex 272 circuit was performing within normal parameters initially,but started demonstrating poorer extraction kinetics.

Even though the above plant was shut down by Umicore before resolution of the problem,samples were obtained for laboratory testwork.

At the same time, Kasese Cobalt in Uganda was reporting crud runs that occurred withoutobvious reason in both their D2EHPA and Cyanex 272 (later replaced by Ionquest 290)circuits9, Chambishi Metals in Zambia reported crud problems in their D2EHPA circuit stripsection10 and Namzinc Skorpion in Namibia was experiencing silicaceous gels in the ZincexD2EHPA SX and downstream coalescers and filters linked to high acidity in the leach plant11.

As ChemQuest was in the process of introducing the product Chemorex D2EHPA to themarket, and was assisting on a pilot-plant testing Ionquest 290, it was decided to do a study onorganic degradation in these SX circuits as part of the technical service.

Figures 4 and 5 give extraction curves generated with actual leached ore samples from theCongo Cobalt Corporation (now Boss Mining) Kakanda Cobalt Facility, which produces aheterogenite based cobalt concentrate in an oxide flotation circuit.

The literature1,2, as well as a study done by Barnard12 indicated that problems could begenerated by aromatic portions of the diluent breaking down. This could not be the case ateither the Umicore circuit or at Kasese Cobalt, as they were both using aliphatic diluents(Shellsol D70 and Sasol SSX 210 at Umicore, Escaid 110 at Kasese). However, a commonfactor observed was the presence of manganese in high oxidation states at both plants. AtKasese, the manganese was oxidized in the electrowinning plant and returned with spent



electrolyte as the strip solution in the cobalt/nickel SX. At Umicore, the leach section usedstrong oxidants, including peroxide, which were probably carried into the PLS entering theD2EHPA circuit.

At Kasese, however, parallel testing and plant observations linked the crud formation tobiological growth and high levels of acid-soluble silica. No evidence of permanganate-induced degradation was proven.

The Umicore D2EHPA (18% by volume in Shellsol D70/Sasol SSX 210 blend) was sent forGCMS scans—both fresh solutions and the sample of degraded organic. Initial observationindicated peaks similar to those found on samples from a Versatic 10 (neo-decanoic acid)nickel pilot circuit at Tati Nickel (now Norilsk) in Botswana. In the degraded plant organic,

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Figure 5. Cyanex 272 extraction curves



carboxylic acid type peaks were detected at a similar wavelength and retention time asdetected by Barnard. These were not present in the freshly prepared organic. We drew aninterim conclusion that the organic breakdown products contained carboxylic acids, which ledto the co-extraction of nickel that had been observed. No accidental cross-contamination ofthe D2EHPA by Cyanex 272 was noted.

Unfortunately, the Umicore plant was shut down before the work could be completed, andas there was no particular financial advantage for ChemQuest to continue the research, thework was not completed.

However, during the course of the study, a more detailed study was done on the effect ofBHT as an antioxidant, as mentioned above and in reference8, but with a D2EHPA systeminstead of the amine, and using only the aliphatic diluents. Using the same methods andapparatus described above, and similar artificial oxidation conditions to degrade the organic,on samples without BHT, the major results of the laboratory study were as follows: (Table II)

The above exercise was aimed at proving firstly that there was no significant differencebetween the two diluents, and secondly that there was a measurable influence on extent ofpossible degradation by using BHT antioxidant. These objectives were partly achieved, butthe project was not completed for the reasons indicated above.

If organic degradation is found or suspected in SX circuits using solvation-type extractantssuch as Cyanex 272, Ionquest 290, D2EHPA or Versatic 10, then the use of an antioxidant isprobably indicated. There is some contradictory evidence on whether aromatic diluents shouldor should not be used, but this becomes largely irrelevant when the antioxidant is used.

Acknowledgement

We would like to thank P. Haig, P. Louis and J. Stoltz for their contribution to this paper.

References

1. FLETT, D.S. and WEST, D.W. The Cobalt Catalysed Oxidation of Solvent ExtractionDiluents Proceedings ISEC ’86, Munich, 1986, vol. II. pp. 3–10.

2. RICKLETON, W.A., ROBERTSON, A.J., and HILLHOUSE, J.H. The Significance ofDiluent Oxidation in Cobalt-Nickel Separation, Solvent Extraction and Ion Exchange,vol. 9, no. 1, 1991. pp. 73–84.

3. Personal correspondence between the author and Brodrick Munyangano and others,Rössing Uranium, 2002–2008.

Extraction of zinc, % Phase disengagement time, secs

Before oxidation After oxidation Before oxidation After oxidation

Shellsol D70 88.2 81.3 27 45

Sasol SSX 210 87.0 80.1 29 49

Shellsol D70 with 0.2% BHT 88.1 88.2 28 33

Sasol SSX 210 with 0.2% BHT 87.4 87.9 28 34

Table II

The effect of BHT as an antioxidant



4. Personal correspondence between the author and various parties at Umicore, WasteProducts Utilisation, Chambishi Mines, SGS Lakefield and Mintek. All written.

5. MACKENZIE, J.M.W. Uranium Solvent Extraction using Tertiary Amines, PresentationUranium Ore Yellowcake Seminar February 1997, Melbourne, Australia

6. MAXWELL, B., RASDELL, S., and CARLIN, P. Oxidative Stability of Diluents inCo/Ni solvent Extraction Presentation ALTA 1999.

7. Personal correspondence between the author, Peter Haig of Shell Chemicals and Dr.Gordon Ritcey. All written correspondence on record.

8. VAN RENSBURG, D.M. Accelerated Degradation Tests on Uranium, Copper, Zinc andCobalt/Nickel Solvent Extraction Solutions Currently under preparation and refereeingfor presentation.

9. Personal correspondence between the author and Moses Mugabe, Amos Silungwe andStanford Saungweme of Kasese Cobalt.

10. Personal correspondence between the author and Hira Singh, Robert Minango andKennedy Mwanza of Chambishi Metals.

11. Personal correspondence between the author and Johan van Rooyen (now with TWM),Herman Fuls and Jurgen Gnoinski of Namzinc and Dr. Kathy Sole of Anglo Research.

12. BARNARD, K.R. (AJ Parker Co-operative Research Centre) Tools for Diagnosis of Crudand Organic Degradation Problems in SX Circuits ALTA 2001.

Deon van RensburgProduct Manager, ChemQuest (Pty) Ltd, South Africa

• 1995 to date: Manager of Solvent Extraction, Ion Exchange andAdsorbents Division at ChemQuest (Pty) Ltd;

– Responsible for the development of Chemorex range ofextractants;

– Solvent Extraction plant technical service and design.• 1988–1995: NCP ACIX Division, manager activated carbon division

• 1983–1988: Allied Colloids, technical representative• 1980–1982: Student• 1975–1979: Anglo American Research, learner official/student


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