organic addittives in zinc electrowinning and ... · organic additives in zinc electrowinning and...
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Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as
Cathodes for Chlorate Production
Mémoire
NABIL SOROUR
Maîtrise en génie des matériaux et de la métallurgie Maître ès sciences (M.Sc.)
Québec, Canada
© Nabil Sorour, 2016
Organic Additives in Zinc Electrowinning and Electrodeposition of Fe-Mo-P Alloys as
Cathodes for Chlorate Production
Mémoire
NABIL SOROUR
Sous la direction de :
Edward Ghali, directeur de recherche
iii
Résumé
Ce projet de travail est divisé en deux études principales: (a) l'influence des certains additifs
organiques sur la consommation d'énergie et la pureté du métal de zinc déposé dans le
processus d'extraction électrolytique, et (b) l’électrodéposition des alliages binaires et
ternaires de Fe-Mo et Fe-Mo-P sur des substrats d’acier doux afin d’agir comme cathodes
pour la production de chlorate.
(a) Parmi les sept différents additifs organiques examinés, les sels des liquides ioniques
ont réussi à augmenter le rendement du courant jusqu'à 95,1% comparé à 88,7% qui a
obtenu à partir de l'électrolyte standard en présence des ions de Sb3+. La réduction
maximale de la consommation d'énergie de ~173 kWh tonne-1 a été obtenue en ajoutant de
3 mg dm-3 du chlorure de 1-butyl-3-méthylimidazolium dans le même électrolyte. La teneur
en plomb dans le dépôt de zinc est réduite de 26,5 ppm à 5,1-5,6 ppm en utilisant les sels
des liquides ioniques.
(b) Des différents binaires Fe-Mo et ternaires Fe-Mo-P alliages ont été électrodéposés
sur des substrats d’acier doux. Les alliages préparés ont une tenure en Mo entre 21-47 at.%
et une tenure en P de 0 à 16 at.%. L'activité électrocatalytique de ces alliages vers la
réaction de dégagement d'hydrogène (RDH) a été étudiée dans des solutions de chlorure de
sodium. La réduction maximale de la surtension de RDH de ~313 mV a été obtenue par
l’alliage ternaire préparé Fe54Mo30P16 par rapport à celle obtenue pour l'acier doux. La
rugosité de surface et l'activité intrinsèque des revêtements de Fe-Mo-P peuvent être
l'origine du comportement prometteur de ces électrocatalyseurs vers la RDH.
iv
Abstract
This work project is divided into two main studies: (a) the influence of certain organic
additives on the power consumption and the purity of deposited zinc during electrowinning
process, and (b) the electrodeposition of binary and ternary alloys of Fe-Mo and Fe-Mo-P
on mild steel substrates to act as cathodes for chlorate production.
(a) Among seven different examined organic additives, the ionic liquid salts succeeded
to increase the current efficiency up to 95.1% compared to 88.7% obtained from standard
electrolyte in presence of Sb3+ ions. Maximum reduction of power consumption of ~173
kWh ton-1 was observed by addition of 3 mg dm-3 of 1-butyl-3-methylimidazolium chloride
to the same electrolyte. Lead content in the zinc deposit is reduced from 26.5 ppm to 5.1-
5.6 ppm by using the ionic liquid salts.
(b) Different binary Fe-Mo and ternary Fe-Mo-P alloys have been electrodeposited on
mild steel substrates. The prepared alloys have Mo content between 21-47 at.% and P
content from 0 to 16 at.%. The electrocatalytic activity of these alloys towards the hydrogen
evolution reaction (HER) was investigated in sodium chloride solutions. The maximum
reduction of HER overpotential of ~313 mV was achieved from the prepared ternary alloy
Fe54Mo30P16 compared to that obtained from mild steel. The surface roughness and intrinsic
activity of Fe-Mo-P coatings could be the origin of the promising behavior of these
electrocatalysts towards the HER.
v
Table of Content
Résumé ............................................................................................................................................... iii
Abstract .............................................................................................................................................. iv
Table of Content .................................................................................................................................. v
List of Tables ...................................................................................................................................... ix
List of Figures .................................................................................................................................... xi
Acknowledgments ............................................................................................................................. xv
Forward ............................................................................................................................................ xvi
CHAPTER 1 ........................................................................................................................................ 1
INTRODUCTION ............................................................................................................................... 1
1.1. Background .............................................................................................................................. 2
1.2. Zinc Electrowinning ................................................................................................................. 2
1.2.1. Zinc Metal ......................................................................................................................... 2
1.2.2. Methods of Extraction of Zinc Metal ................................................................................ 3
1.2.3. Uses of Zinc ...................................................................................................................... 4
1.3. Electrodeposition of Alloys as Cathodes in Chlorate Production ............................................ 5
1.3.1. Chlorate Production ........................................................................................................... 5
1.3.2. Cathodes in Chlorate Production ....................................................................................... 6
1.4. Objectives and Detailed Approaches ....................................................................................... 7
1.4.1. Effect of Certain Organic Additives on Zinc Electrowinning Process .............................. 7
1.4.2. Performing the Electrodeposition of Fe-Mo & Fe-Mo-P Alloys as Cathodes .................. 8
CHAPTER 2 ........................................................................................................................................ 9
LITERATURE REVIEW .................................................................................................................... 9
2.1. Zinc Electrowinning Process .................................................................................................. 10
2.1.1. Lead-Based Anodes ......................................................................................................... 11
2.1.2. Corrosion of Lead-Based Anodes ................................................................................... 12
2.1.3. Oxygen Overpotential of Lead-Based Anodes ................................................................ 13
2.1.4. Role of Manganese Ions in the Electrolyte ..................................................................... 14
2.1.5. Surface Structure and Crystallographic Orientation........................................................ 16
2.1.6. Metallic Impurities in Zinc Electrowinning .................................................................... 17
2.1.6.1. Effect of Lead Impurity on Zinc Deposition ............................................................ 18
2.1.6.2. Effect of Antimony Impurity on Zinc Deposition .................................................... 19
vi
2.1.6.3. Effect of Copper, Nickel and Cobalt Impurities on Zinc Deposition ....................... 21
2.1.7. Additives in Zinc Electrowinning ................................................................................... 23
2.1.7.1. Effect of Glue ........................................................................................................... 23
2.1.7.2. Effect of Natural Products and Surfactants .............................................................. 25
2.1.7.3. Effect of Synthetic Polymers .................................................................................... 26
2.1.7.4. Effect of Quaternary Ammonium Salts .................................................................... 27
2.1.7.5. Effect of Ionic Liquid Salts ...................................................................................... 28
2.2. Electrodeposition of Alloys as Cathodes for Chlorate Production ......................................... 31
2.2.1. Chlorate Production ......................................................................................................... 31
2.2.2. Mild Steel Cathodes ........................................................................................................ 33
2.2.3. Fe-Based Alloys Cathodes .............................................................................................. 34
2.2.4. Ni-Based Alloys Cathodes .............................................................................................. 35
2.2.5. Molybdenum Co-deposition ............................................................................................ 37
2.2.6. Phosphorous Co-deposition ............................................................................................. 38
2.3. Electrochemical Test Methods (Approach and Evaluation) ................................................... 40
2.3.1. Galvanostatic Polarization Technique ............................................................................. 41
2.3.2. Potentiodynamic Polarization Technique ........................................................................ 41
2.3.3. Cyclic Voltammetry Technique ...................................................................................... 43
2.3.4. Electrochemical Impedance Spectroscopy Technique .................................................... 44
2.4. Summary ................................................................................................................................ 46
CHAPTER 3 ...................................................................................................................................... 48
EXPERIMENTAL ............................................................................................................................ 48
3.1. Electrolyte and Set-up ............................................................................................................ 49
3.1.1. Zinc Electrolyte and Materials Preparation ..................................................................... 49
3.1.2. Fe-Mo & Fe-Mo-P Electrolytes and Materials Preparation ............................................ 50
3.1.3. Set-up .............................................................................................................................. 51
3.2. Electrochemical Techniques and Measurements ................................................................... 52
3.2.1. Galvanostatic Polarization ............................................................................................... 52
3.2.2. Current Efficiency Calculations ...................................................................................... 52
3.2.3. Power Consumption Calculations ................................................................................... 52
3.2.4. Potentiodynamic Polarization ......................................................................................... 53
3.2.5. Cyclic voltammetry ......................................................................................................... 54
3.2.6. Electrochemical Impedance Spectroscopy ...................................................................... 55
vii
3.3. Deposit Examination Techniques ........................................................................................... 55
3.3.1. Scanning Electron Microscopy (SEM) ............................................................................ 55
3.3.2. Energy Dispersive Spectroscopy (EDS) .......................................................................... 55
3.3.3. X-ray Diffraction (XRD) ................................................................................................. 56
3.3.4. Inductively Coupled Plasma (ICP) .................................................................................. 56
CHAPTER 4 ...................................................................................................................................... 57
INFLUENCE OF DIFFERENT ORGANIC ADDITIVES IN ZINC ELECTROWINNING FROM ACIDIC SULPHATE ELECTROLYTE .......................................................................................... 57
Résumé .............................................................................................................................................. 58
Abstract ............................................................................................................................................. 59
4.1. Introduction ............................................................................................................................ 60
4.2. Experimental .......................................................................................................................... 61
4.2.1. Electrolyte and Experimental Setup ................................................................................ 61
4.2.2. Deposit Examination ....................................................................................................... 62
4.2.3. Potentiodynamic Polarization and Cyclic Voltammetry ................................................. 62
4.3. Results and Discussion ........................................................................................................... 63
4.3.1. Power Consumption and Current Efficiency ................................................................... 63
4.3.2. Characterization of Deposits ........................................................................................... 67
4.3.3. Potentiodynamic Polarization ......................................................................................... 70
4.3.4. Cyclic Voltammetry Measurements ................................................................................ 73
4.4. Conclusions ............................................................................................................................ 77
CHAPTER 5 ...................................................................................................................................... 79
ELECTROCHEMICAL STUDIES OF IONIC LIQUID ADDITIVES DURING THE ZINC ELECTROWINNING PROCESS ..................................................................................................... 79
Résumé .............................................................................................................................................. 80
Abstract ............................................................................................................................................. 81
5.1. Introduction ............................................................................................................................ 82
5.2. Experimental .......................................................................................................................... 84
5.2.1. Electrolysis ...................................................................................................................... 84
5.2.2. Deposit Examination ....................................................................................................... 85
5.2.3. Electrochemical Measurements ....................................................................................... 85
5.3. Results and Discussion ........................................................................................................... 86
5.3.1. Cell Voltage and Power Consumption ............................................................................ 86
5.3.2. Current Efficiency ........................................................................................................... 88
viii
5.3.3. Deposit Examination ....................................................................................................... 89
5.3.4. Polarization Studies ......................................................................................................... 92
5.4. Conclusions ............................................................................................................................ 98
CHAPTER 6 ...................................................................................................................................... 99
ELECTRODEPOSITION AND STUDY OF THE ELECTROCATALYTIC ACTIVITY OF Fe-Mo-P ALLOYS FOR HYDROGEN EVOLUTION DURING CHLORATE PRODUCTION ....... 99
Résumé ............................................................................................................................................ 100
Abstract ........................................................................................................................................... 101
6.1. Introduction .......................................................................................................................... 102
6.2. Experimental ........................................................................................................................ 104
6.3. Results and Discussion ......................................................................................................... 105
6.3.1. Deposit Characterization ............................................................................................... 105
6.3.2. Steady-State Polarization Curves .................................................................................. 108
6.3.3. Electrochemical Impedance Spectroscopy .................................................................... 112
6.4. Conclusions .......................................................................................................................... 115
CHAPTER 7 .................................................................................................................................... 117
CONCLUSIONS AND OUTLOOK ............................................................................................... 117
7.1. Conclusions .......................................................................................................................... 118
7.2. Outlook ................................................................................................................................. 121
Bibliography .................................................................................................................................... 122
ix
List of Tables Table 2.1. Electrode potential (V/SCE) vs. current density (A m-2) of anodes from lead and its
alloys in 1.8 M H2SO4 at 30oC [30] ................................................................................ 14
Table 2.2. Variation of current efficiency and preferred crystalline orientation of zinc deposit at
different concentrations of lead at 400 A m-2 and 35oC for zinc electrolyte of 55 g dm-3
Zn2+ + 150 g dm-3 H2SO4 [37]......................................................................................... 18
Table 2.3. Variation of current efficiency and preferred crystalline orientation of zinc deposit at
different concentrations of antimony at 400 A m-2 and 35oC for zinc electrolyte of 55 g
dm-3 Zn2+ + 150 g dm-3 H2SO4 [37] ................................................................................ 20
Table 2.4. Effect of copper on current efficiency and crystal orientation of zinc deposit at different
concentrations and different current densities for electrolysis in 55 g dm-3 Zn2+, 150 g
dm-3 H2SO4 at 35oC [43] ................................................................................................. 21
Table 2.5. Variation of current efficiency and preferred crystalline orientation of zinc deposit at
different concentrations of nickel at 400 A m-2 and 35oC for zinc electrolyte of 55 g dm-
3 Zn2+ + 150 g dm-3 H2SO4 [37] ...................................................................................... 22
Table 2.6. Effect of [BMIM]HSO4 and Gelatin on current efficiency and power consumption during
zinc electrodepsotion [70] ............................................................................................... 29
Table 2.7. Effect of Sb3+ on current efficiency in absence and in presence of [BMIM]HSO4 during
zinc electrowinning [72] ................................................................................................. 31
Table 2.8. Kinetic parameters for the HER on the Ni-Cu-Fe electrode [88] .................................... 37
Table 3.1. Fe-Mo and Fe-Mo-P electrolytes compositions .............................................................. 50
Table 4.1. Effect of PAM, [BMIM]Cl, TBABr, BKCl and Chitin on cell voltage, CE and PC in
absence and in presence of Sb3+ during zinc electrodeposition for 2 h at 50 mA cm-2 and
38оC ................................................................................................................................ 65
Table 4.2. Effects of PAM, [BMIM]Cl, TBABr, BKCl and Chitin on surface morphology, crystal
orientation and lead contamination in absence and in presence of Sb3+ during zinc
electrodeposition for 2h at 50 mA cm-2 .......................................................................... 67
Table 4.3. Effects of additives on Tafel slopes, cathodic overpotential at 50 mA cm-2 obtained from
potentiodynamic polarization versus Ag,AgCl/KCl(sat) and NOP obtained from cyclic
voltammetry .................................................................................................................... 76
Table 5.1. Effect of gelatin, [EMIM]MSO3 and [BMIM]Br on CE and PC in absence and in
presence of Sb(III) during zinc electrodeposition for 2h at 50 mA cm-2 ....................... 87
x
Table 5.2. Crystallographic orientations and lead concentration of zinc deposits obtained by adding
3mg of gelatin, [EMIM]MSO3 and [BMIM]Br in absence and in presence of Sb(III)
during zinc electrodeposition for 2h at 50 mA cm-2 ....................................................... 92
Table 5.3. Effect of [EMIM]MSO3, [BMIM]Br and gelatin on Tafel slopes, cathodic overpotential
at 50 mA cm-2, exchange current density and NOP ........................................................ 95
Table 6.1. The compositions of the coatings from four different electrolytes after 6 hours of
electrodeposition at 20 mA cm-2 and 30oC ................................................................... 106
Table 6.2. The measured kinetic parameters of HER for MS, Fe-Mo and Fe-Mo-P electrodes in
sodium chloride solution at 80oC and pH 6.4 ............................................................... 110
Table 6.3. The electrochemical data obtained by the Nyquist plots of MS, Fe-Mo and different Fe-
Mo-P alloys ................................................................................................................... 114
xi
List of Figures
Figure 1.1. Typical roast-leach-electrowinning processes for zinc [7] .............................................. 3
Figure 1.2. The major uses of zinc [13] ............................................................................................. 5
Figure 2.1. (a) Simple electrolysis cell for zinc (b) Aluminum cathodes deposited by zinc ......... 11
Figure 2.2. Potential-pH diagram obtained according to the ionic activities in an actual anodic film
for the Pb-H2O-H2SO4 system at 25oC (potential vs. SHE) [27] ................................... 12
Figure 2.3. Lead-based anode; (a) Before electrolysis and (b) After 5 hours of electrolysis ........... 16
Figure 2.4. Zinc deposit shows HCP Lattice among the three most important lattices .................... 17
Figure 2.5. SEM photomicrographs (X 385) showing the effect of current density on the
morphology of zinc deposits from addition-free electrolyte using unconditioned Pb-Ag
anodes. (a) 215 A m-2, 60 min, 0.125% Pb; (b) 323 A m-2. 60 min, 0.076% Pb; (c) 430
A m-2, 60 min, 0.04% Pb; (d) 538 A m-2, 60 min, 0.021% Pb; (e) 1076 A m-2, 30 min,
0.019% Pb; (f) 2152 A m-2, 15 min, 0.011% Pb [40] .................................................... 19
Figure 2.6. SE micrographs showing the morphology of 6h zinc deposits electrowon at 500 A m-2
and 38oC from electrolytes containing; (a) and (b) 0.02, (c) and (d) 0.04 mg dm-3 Sb
[42] ................................................................................................................................. 20
Figure 2.7. Quaternary ammonium salts; (a) Non-aromatic, (b) Aromatic ...................................... 27
Figure 2.8. Examples of ionic liquids salts; (a) Cationic, and (b) Anionic ...................................... 29
Figure 2.9. Schematic process of chlorate production (Chemetics Inc. B.C., Canada) .................... 32
Figure 2.10. Scanning micrographs of developed cathodes [86] ..................................................... 36
Figure 2.11. Potentiodynamic polarization plot [110] ...................................................................... 43
Figure 2.12. Theoretical cyclic voltammogram [102] ...................................................................... 44
Figure 2.13. (a) Simple electrified electrode/electrolyte interface, (b) Electronic components for the
same interface [112] ...................................................................................................... 45
Figure 2.14. Nyquist impedance plot for the showed circuit for Rs, Rct and Cdl [114] ..................... 46
Figure 3.1. Schematic experimental set-up for three-electrode cell [116] ....................................... 51
Figure 3.2. Electrolysis cell set-up ................................................................................................... 51
Figure 3.3. Polarization curve potential vs current density (log i) [119] .......................................... 54
Figure 4.1. Effects of PAM, [BMIM]Cl, TBABr, BKCl and Chitin on current efficiency: (a) in
absence of Sb3+ and (b) in presence of 0.0055 mg dm-3 of Sb3+ during zinc
electrodeposition for 2h at 50 mA cm-2 and 38оC .......................................................... 66
Figure 4.2. Scanning electron micrographs (x1000) of zinc deposits in absence of Sb3+; (a) SE, (b)
PAM, (c) [BMIM]Cl 3mg dm-3, (d) TBABr 3mg dm-3, (e) BKCl 3mg dm-3 and (f)
Chitin 3mg dm-3 ............................................................................................................. 68
xii
Figure 4.3. Scanning electron micrographs (x1000) of zinc deposits in presence of 0.0055 mg of
Sb3+; (a) SE, (b) PAM 3mg dm-3, (c) [BMIM]Cl 3mg dm-3, (d) TBABr 3mg dm-3, (e)
BKCl 3mg dm-3 and (f) Chitin 3mg dm-3 ....................................................................... 69
Figure 4.4. Effects of the additives on the cathodic polarization during zinc electrodeposition with
different concentrations in absence and in presnce of antimony; (a) PAM, (b)
[BMIM]Cl, (c) TBABr, (d) BKCl and (e) Chitin .......................................................... 73
Figure 4.5. Cyclic voltammograms during zinc electrowinning using aluminum cathode with
different concentrations of 0,1,5 and 40 mg dm-3 of: (a) PAM, (b) [BMIM]Cl, (c)
TBABr, (d) BKCl and (e) Chitin ................................................................................... 75
Figure 5.1. Effect of gelatin, [EMIM]MSO3 and [BMIM]Br on CE: (a) in absence of Sb(III) and (b)
in presence of 0.0055 mg of Sb(III) during zinc electrodeposition for 2h at 50 mA cm-2
....................................................................................................................................... 88
Figure 5.2. Scanning electron microscopy photomicrographs (x1000) of zinc deposit in absence of
Sb(III); (a) blank, (b) 3mg gelatin, (c) 3mg [EMIM]MSO3 and (d) 3mg [BMIM]Br ... 90
Figure 5.3. Scanning electron microscopy photomicrographs (x1000) of zinc deposit in presence of
0.0055mg of Sb(III); (a) blank, (b) 3mg gelatin, (c) 3mg [EMIM]MSO3 and (d) 3mg
[BMIM]Br ..................................................................................................................... 91
Figure 5.4. XRD patterns of zinc deposit in absence of Sb(III); (a) 3mg [EMIM]MSO3, (b) 3mg
[BMIM]Br ..................................................................................................................... 91
Figure 5.5. Effect of [EMIM]MSO3 on the cathodic polarization during zinc electrodeposition
using aluminum cathode with different concentrations; (a) in absence of Sb(III), (b) in
presence of Sb(III) ......................................................................................................... 94
Figure 5.6. Effect of [BMIM]Br on the cathodic polarization during zinc electrodeposition using
aluminum cathode with different concentrations; (a) in absence of Sb(III), (b) in
presence of Sb ................................................................................................................ 94
Figure 5.7. Cyclic voltammograms of [EMIM]MSO3 during zinc electrodeposition using aluminum
cathode with different concentrations; (a) in absence of Sb(III), (b) in presence of
Sb(III) ............................................................................................................................ 96
Figure 5.8. Cyclic voltammograms of [BMIM]Br during zinc electrodeposition using aluminum
cathode with different concentrations; (a) in absence of Sb(III), (b) in presence of
Sb(III) ............................................................................................................................ 97
Figure 6.1. Scanning electron micrographs (X500) of deposits; (a) Fe53Mo47, (b) Fe70Mo21P9, (c)
Fe61Mo26P13, (d) Fe54Mo30P16 after electrodeposition during 6 hours at 20 mA cm-2 at
30oC ............................................................................................................................. 107
xiii
Figure 6.2. XRD spectra of Fe-Mo and Fe-Mo-P deposits obtained from electrodeposition of 6
hours at 20 mA cm-2 and 30oC ..................................................................................... 108
Figure 6.3. Polarization curves of Fe-Mo and three Fe-Mo-P deposited electrodes compared to MS
in chlorate solution at 80oC and pH 6.4 ....................................................................... 109
Figure 6.4. The electrical equivalent circuit used for simulation of the impedance spectra for the
HER [171] .................................................................................................................... 112
Figure 6.5. The Nyquist plots for the HER process on a) MS, b) Fe53Mo47, c) Fe70Mo21P9, d)
Fe61Mo26P13 and e) Fe54Mo30P16 ................................................................................... 113
xiv
“To my kind mother and the memory of my great father.
To my adorable sister and beloved brothers who are my support in life.’’
xv
Acknowledgments
I would like to express my deepest appreciation to my supervisor Prof. Edward Ghali for
giving me the opportunity to work on such interesting subject also for his valuable advices
and guidance. His dedication and diligent work ethics are always my inspiration during this
study.
I wish to acknowledge Dr. Georges Gabra for his advices and participation during choosing
the working materials and their testing. Sharing his experience with me was really valuable.
Dr. Fariba Safizadeh and Dr. Wei Zhang are gratefully acknowledged for their support,
assistance, and contribution during this work. I’m really thankful to Mr. Georges Houlachi
for his insightful comments and valuable contribution.
Zinc Électrolytique du Canada (CEZinc) Limitée, Hydro-Québec, and Natural Sciences and
Engineering Research Council of Canada (NSERC) are gratefully acknowledged for their
financial support. I would like also to thank Mrs. Vicky Dodier, Mr. André Ferland, Mr.
Jean Frenette, and Mr. Alain Brousseau for their professional technical participation. Kind
help, friendships, and ideas of Deniz Bas, Chaoran Su, and Ramzi Ishak, are really
appreciated.
xvi
Forward
This thesis is composed of seven chapters and presented as articles insertion form. The first
chapter provides a brief introduction of two main electrometallurgical processes; the
electrowinning of zinc, and the electrodeposition of cathodes in chlorate production. In the
second chapter, a targeted literature review covers the previous studies concerning the
problems in zinc electrowinning process such as the metallic impurities also the beneficial
effect of organic additives on this process. Chapter two discusses as well the utilization of
different electrodeposited cathodes in the chlorate industry and their effect on hydrogen
evolution reaction overpotential. The experimental steps and conditions are explained in
chapter three. Chapters four, five, and six present the results of this work in the form of
three scientific papers as the following:
Chapter four
Influence of Different Organic Additives on Zinc Electrowinning from Acidic Sulphate Electrolyte
N. Sorour1,*, W. Zhang1, G. Gabra1, E. Ghali1, and G. Houlachi2
1Department of Mining, Metallurgical and Materials Engineering, Laval University, Québec, Canada, G1V 0A6.
2Hydro-Québec research centre (LTE), Shawinigan, QC, Canada, G9N 7N5.
This paper was presented in the 54th annual Conference of Metallurgists - COM, Toronto,
Canada (Aug. 23-26, 2015) and published in the proceeding by Canadian Institute of
Mining, Metallurgy and Petroleum. CIM-COM, paper #8986 pp 1-13, ISBN: 978-1-
926872-32-2.
In this work, different five additives from different organic groups have been studied in the
zinc electrowinning process. The experimental measurements and analysis along with paper
writing and presentation were performed by the first author. The scientific revision was
done by Dr. W. Zhang, Dr. G. Gabra, and Prof. E. Ghali. The project was supervised by
Prof. E. Ghali and Mr. G. Houlachi.
xvii
Chapter five
Electrochemical Studies of Ionic Liquid Additives during the Zinc Electrowinning Process
N. Sorour1,*, W. Zhang1, G. Gabra1, E. Ghali1, and G. Houlachi2
1Department of Mining, Metallurgical and Materials Engineering, Laval University, Québec, Canada, G1V 0A6.
2Hydro-Québec research centre (LTE), Shawinigan, QC, Canada, G9N 7N5.
This paper is published in the journal of Hydrometallurgy, Vol. 157, 2015, pp 261-269.
In this work, the effect and importance of ionic liquids on zinc deposits and lead
contamination have been highlighted by using certain electrochemical techniques. The
experimental measurements and analysis along with paper writing were performed by the
first author. The scientific revision was done by Dr. W. Zhang, Dr. G. Gabra, and Prof. E.
Ghali. The project was supervised by Prof. E. Ghali and Mr. G. Houlachi.
Chapter six
Electrodeposition and Study of the Electrocatalytic Activity of Fe-Mo-P Alloys for Hydrogen Evolution during Chlorate Production
F. Safizadeh1,*, N. Sorour1, G. Houlachi2, and E. Ghali1
1Department of Mining, Metallurgical and Materials Engineering, Laval University, Québec, Canada, G1V 0A6.
2Hydro-Québec research centre (LTE), Shawinigan, QC, Canada, G9N 7N5.
This paper is submitted to the International Journal of Hydrogen Energy, February, 2016.
Different alloys of Fe-Mo and Fe-Mo-P have been prepared to study their effects as
cathodes on hydrogen evolution reaction in similar conditions to chlorate production. The
experimental measurements were carried out by the second author, analysis was done by
the first and second authors while, paper was written by the first author. The scientific
revision and project supervision were done by Prof. E. Ghali and Mr. G. Houlachi.
Finally, chapter seven provides complete conclusions for this thesis as well as few
recommendations for future work plan.
1
CHAPTER 1
INTRODUCTION
2
1.1. Background
Electrolysis is a process by which an electric current is moved through a substance
to do a chemical change. This chemical change is occurred when the substance gains or
losses electrons (reduction or oxidation) and this is always preformed in an electrolytic cell.
Electrolysis is used enormously in many metallurgical processes, such as: extraction
(Electrowinning), deposition of metals or alloys (Electrodeposition), metal purification
(Electrorefining), and substrates plating (Electroplating) [1].
Electrowinning or Electrodeposition is an electrochemical process by which an adherent
film of desired metal or alloy can be deposited onto an electrode by electrolysis of a
solution containing the desired metals ions or their complexes [2].
→
Accordingly, the Electrowinning of zinc and electrodeposition of different metals or alloys
as cathodes in chlorate production are two major electrometallurgical processes which have
been always the concern of many studies and researches.
1.2. Zinc Electrowinning
1.2.1. Zinc Metal
Zinc is considered as the fourth most widely used metal following iron, aluminum
and copper [3]. Zinc is the 24th most abundant element in the earth’s crust, with an average
concentration of 65 g ton-1 (0.0065%) [4]. The discovery of pure metallic zinc was done by
the German chemist, Andreas Marggraf in 1764 [5]. The metal has silvery blue-gray color
with relatively low melting point of 419oC and boiling point of 907oC with density of 7.14
g cm-3 at 20oC. Zinc has medium strength and hardness properties which are greater than
those of tin and lead but less than those of aluminum and copper [6]. Global zinc
consumption grew from around five million tonnes per year in 1970 to over eight million
tonnes per year by the end of the 20th century and to 9.7 million tonnes per year in 2003 [7].
Canada is number seven globally in producing zinc with total production of 550,000 tons in
2013 according to The US Geological Survey.
3
1.2.2. Methods of Extraction of Zinc Metal
The world primary zinc industry employs five different processes: Retorting
processes (I) Electrothermic, (II) Vertical, and (III) Horizontal which were the main
methods at beginning of last century but had been gradually declined by 1950. (IV)
Imperial Smelting ISF (blast furnace) which represents ≈15% of zinc metal production.
However, currently ≈85% of production of zinc metal is produced by (V) Electrolytic
process (Roast - Leach - Electrowinning) [7].
The production of zinc from sulphides is predominantly conducted through roast-leach-
electrowinning process which was used firstly in 1916 by Cominco - BC, Canada and
Zinifex - Hobart, Australia.
Figure 1.1. Typical roast-leach-electrowinning processes for zinc [7]
The preparation of the purified zinc solution for electrowinning plant is schematically
presented (Figure 1.1) and starts with the concentrate of the sulphide ore, in which
sphalerite (ZnS) is the predominant component, is roasted in a fluidized bed furnace
forming zinc oxide (ZnO) at 900-1000oC. The zinc oxide is then fed into the leaching tanks
together with sulphuric acid solution in the spent electrolyte from electrowinning. During
Zinc Sulphide Concentrate
Roasting
Leaching /Iron Purification
Purification
Electrowinning
Cathode Zinc
Acid Plant
Spent Electrolyte
SO2
4
leaching, temperature and pH should be carefully adjusted and manipulated to encourage
the precipitation of ferric hydroxide which acts as metal ion collector and remove others
impurities such as; arsenic, antimony and germanium [7]. Further purification step is
carried out by zinc dust precipitation in 2-3 stages, in which the leached solution is mixed
with a fine dust of zinc which caused the reductive precipitation of the metal ions
electropositive to zinc, while the zinc metal is oxidized. Then, the purified zinc solution is
circulated through electrowinning plant.
The zinc metal is deposited on the cathode in solid form, while the anodic reaction is
the oxygen evolution. The metal deposition rate is always related to the available surface
area, maintaining properly working cathodes is important. Two cathode types exist, flat-
plate and reticulated cathodes, each with its own advantages. Flat-plate cathodes can be
cleaned and reused, and deposited metal is recovered. Reticulated cathodes have a much
higher deposition rate compared to flat-plate cathodes. These cathodes are not the best
choice as they are not reusable and must be recycled [8]. For zinc electrodeposition,
aluminum cathodes are usually used as they proved their high performance. During the
electrolysis the deposit is adherent to the aluminum cathodes while, it is separated easily by
mechanical methods after the electrodeposition [9].
However, actually one of the main challenges faced by the zinc electrowinning industry is
the presence of the metallic impurities in the electrolyte even after several purification
steps. These metallic impurities with very small concentrations of ppm or even ppb affect
negatively the current efficiency, power consumption, and the purity of the deposited zinc
metal [10].
1.2.3. Uses of Zinc
The electrochemical properties of zinc are very important in production as well as
applications of zinc. Electrowinning process in zinc refining, electroplating, zinc batteries
and zinc coating for corrosion protection of steel are all based on the electrochemical
properties [11].
The uses of zinc can be divided into six categories: (a) galvanic coatings for steel, (b) die
casting, (c) alloys, (d) zinc chemicals, (e) rolled zinc, and (f) miscellaneous including zinc
5
oxides and others. As shown in Figure 1.2 the most important application of zinc is its
action as protective coatings against corrosion for steel structures due to its relative
corrosion resistance in atmospheric and other environments. Approximately, one-half of
zinc production is used for this purpose [12-13].
Figure 1.2. The major uses of zinc [13]
Galvanic protection is also called ‘’sacrificial protection’’ as the metal used is sacrificing
itself to protect the structure. This type of protection utilizes a galvanic cell consisting of an
anode made from more active metal than the structure [12]. So, zinc is the most common
metal used to protect steel due to its position in the electromotive series, [Zn/Zn2+:
-0.76V/SHE] VS [Fe/Fe2+: -0.44V/SHE] as zinc is more active than iron, accordingly it
starts sacrificing itself first before iron.
1.3. Electrodeposition of Alloys as Cathodes in Chlorate Production
1.3.1. Chlorate Production
Chlorate, chlor-alkali, and water electrolysis operations are among the largest
consumers of electricity in electrolytic industries. Sodium chlorate (NaClO3) is produced
industrially by an electrochemical process, where chloride ions (Cl-) are oxidized to
chlorine (Cl2) on the anode then dissolved in water forming chlorate ions and hydrogen gas
is evolved on the cathode. The selectivity of main reactions, as well as the energy required
by the chlorate process, depends on the electrode materials and surface state also on the
electrolyte composition [14].
Galvanizing - 58%
Die Casting - 14%
Brass / Bronze - 10%
Compounds - 9%
Rolled Zinc - 6%
Miscellaneous - 3%
6
The electrolytic hydrogen is quite pure and is acceptable for various applications.
Normally, hydrogen gas recovery process involves many steps: (a) cooling, (b) boosting,
(c) compression, and (d) purification. The obtained hydrogen is used as fuel or as raw
material for the production of HCl [15]. The hydrogen evolution reaction (HER) on
different metal cathodes in acidic or alkaline media is one of the most investigated reactions
in the electrochemistry field. The HER was always place of interest due to: (i) hydrogen is
an interesting candidate, energy carrier, for fuel cells applications, (ii) it is one of the main
reaction products during chlorine production and, (iii) the HER provides the high pure
hydrogen gas. This reaction is the main reaction produced in alkaline water electrolysis,
hydrogen-based fuel cells, and during some industrial practices such as chlorate cells. Due
to the high consumption of energy; reducing the hydrogen evolution overpotential is always
one of the challenges and purpose of many studies [15-16].
1.3.2. Cathodes in Chlorate Production
Cathode materials in the first years of chlorate manufacture were copper, nickel and
even platinum. Recently, mainly mild steel and in some plants titanium or a Ti-0.2% Pd
alloys are used. Chlorate electrolyte containing the oxidizing agents hypochlorite and
chlorate is extremely corrosive and oxidises most of those metals when they are not under
cathodic protection [17]. Mild steel is one of the most popular used cathodes in chlorate
production due to its low cost. However, these cathodes are not the best choice due to the
high overpotential values of HER reaching 850-950 mV, depending on the surface
roughness also due to low corrosion resistance in the aggressive chlorate electrolyte [18].
Therefore, efforts and attempts are made by many researchers in order to obtain binary and
ternary mild steel coated alloys with electrocatalysts exhibiting low hydrogen evolution
overpotential as well as improving the corrosion resistance and mechanical properties.
Molybdenum and phosphorus are among the different elements that employed to fabricate
new cathodes having a positive electrocatalytic behavior towards HER and corrosion
resistance. These two elements cannot be electrodeposited directly from the aqueous
solutions; therefore, they require another metal to stimulate its co-deposition [19].
7
1.4. Objectives and Detailed Approaches
The aim of this work is divided into two main objectives:
1.4.1. Effect of Certain Organic Additives on Zinc Electrowinning Process
‐ The main challenge faced by zinc electrowinning industry is the presence of
metallic impurities; lead is one of the major impurities as far as lead-based anodes
are still used in this process. Pb2+ ions are usually reduced and co-deposited as
elemental lead with zinc metal on the cathode which reduces the purity of the
obtained zinc metal. Organic additives proved their good performance in improving
this process by reducing the detrimental effect on power consumption, current
efficiency and the purity of deposited zinc as well as modifying the surface
morphology.
‐ In this study, certain organic additives are chosen from different organic groups:
(1) Polyacrylamide [PAM] is one of the well known organic polymers used in
industry and has been tested previously as additive in copper electrowinning,
showing a good effect in improving morphology of the surface. (2) Tetra-
butylammonium bromide [TBABr] is one of the quaternary ammonium salts group
which have been examined also as additives. (3) Benzalkonium chloride [BKCl] is
a cationic surface-acting agent belonging to the quaternary ammonium salts with
aromatic group. (4) Chitin is also selected as it is one of the natural polymer
compounds which can be found in crabs, lobsters and shrimps.
‐ Ionic liquid salts are currently used in many chemical and hydrometallurgical
applications due to their chemical and physical properties. Ionic liquids are widely
used in liquid–liquid extraction and electrodeposition of some metals. Also, they are
considered as a medium in the electrodeposition of aluminum on a stainless steel
cathode. In this work also, different ionic liquid salts are selected in order to
examine their effects on zinc electrowinning process. (5) 1-butyl-3-
methylimidazolium chloride [BMIM]Cl, (6) 1-butyl-3-methylimidazolium bromide
[BMIM]Br, and (7) 1-Ethyl-3-methylimidazolium methanesulfonate [EMIM]MSO3,
are chosen to be examined as additives in zinc sulphate electrolyte with different
concentrations.
8
‐ The combination between the selected organic additives and antimony as metallic
impurities is also considered in this work.
‐ Various electrochemical techniques and other techniques are employed to evaluate
the efficiency of these additives in zinc electrodeposition. Galvanostatic
polarization, potentiodynamic, cyclic voltammetry are conducted in order to
examine the electrochemical activity and the cathodic behavior. Scanning electron
microscopy (SEM), X-ray diffraction (XRD), and inductively coupled plasma
spectroscopy (ICP) are used as well to determine the surface morphology,
crystallographic orientation and lead content in zinc deposit, respectively.
1.4.2. Performing the Electrodeposition of Fe-Mo & Fe-Mo-P Alloys as Cathodes
‐ Chlorate production process is one of the largest consumers of energy in electrolytic
industries. Due to this high consumption of energy; reducing the hydrogen evolution
overpotential is always one of the challenges and purposes of many studies.
Therefore, improving the cathodic materials exhibiting lower hydrogen evolution
overpotential is one of the targets of many studies in order to reduce the power
consumption.
‐ In this work, the electrodeposition of different Fe-Mo and Fe-Mo-P coatings on
mild steel substrates is carried out in order to study the effect of different
phosphorous and molybdenum contents on the HER. The electrocatalytic activities
of these cathodes are assessed in simulated conditions of chlorate industry.
‐ Potentiodynamic polarization and electrochemical impedance (EIS) techniques are
employed in this work to examine the electrocatalytic activity of the
electrodeposited coatings in alkaline solution. Also, (SEM) and (XRD) are used to
determine the surface morphology and state, respectively.
9
CHAPTER 2
LITERATURE REVIEW
10
Based on the previous reviews and research studies, this chapter is divided into
three main parts concerning: (a) additives and impurities in zinc electrowinning, (b) alloyed
metals and elements as cathodes used for chlorate production, and (c) recommended
electrochemical techniques for evaluation of additives and cathodes during electrolysis.
Also, at the end a short summary is given based on the reviewed literature and the
objectives of this project.
2.1. Zinc Electrowinning Process
Zinc ores are roasted, dissolved in sulphuric acid and highly purified by zinc dust as
explained in chapter 1. Then, metallic zinc is won from the purified zinc sulphate solution
by electrolysis using aluminum cathodes and lead-based anodes. Normally, many zinc
electrowinning plants operate with current densities of 400-500 A m-2 at temperature of
38±2oC [20].
It is important to understand and determine the electrochemical reactions occur in this
electrolysis process in order to measure the potentials and power consumed for
electrowinning of zinc. Figure 2.1a shows a simple cell of electrolysis by using aluminum
cathode and lead-silver anode in acidic zinc sulphate solution.
The cathodic reactions with standard potentials are:
2 → Eo = -0.763V (2.1)
2 2 → Eo = 0.00V (2.2)
The anodic reactions with standard potentials are:
→ 2 2 Eo = -1.229V (2.3)
The overall reaction is:
→ 2 Eo= -1.992V (2.4)
11
As many plants add manganese ions to the sulphate solution due to its remarkable effect in
forming compact layers of MnO2 on the anode to reduce the lead contamination, so,
following reaction cannot be neglected.
2 → 4 2 Eo= -1.208V (2.5)
Approximately, 90% of the cathodic current is consumed to produce zinc metal as reaction
(2.1), while, 99% of the anodic current is consumed to produce oxygen gas as reaction
(2.3). There are several factors that affect these reactions and their potentials such as: Zn2+
concentration, pH, current density, and temperature [20]. Therefore, those variables must be
considered in the electrowinning process in any plant.
Figure 2.1. (a) Simple electrolysis cell for zinc (b) Aluminum cathodes deposited by zinc
2.1.1. Lead-Based Anodes
As far as lead anodes are used so, many problems occur due to the weakness and
ductility which cause buckling and sagging of the anodes resulting in current distortion in
the elctrowinning cell. Also, the corrosion of lead based anodes causes lead contamination
of the zinc deposit. When this contamination exceeds the normal level fixed by the plant,
maintenance or replacement of the anodes are required [21-22].
Instead, lead alloys containing 0.37 to 1% silver have been used as anodes in zinc
electrowinning industry since 1909 [23]. Silver is alloyed with lead anode to reduce the rate
of corrosion and improve conductivity of the anode. The addition of silver also reduces the
oxygen evolution overpotential by approximately 120 mV compared to pure lead [24]. A
12
small amount of silver oxide maybe formed on the surface of the anode along with lead
oxides. The poor mechanical property is one of the disadvantages of Pb-Ag anodes
accordingly, they are relatively weak and bended quite easily when struck by aluminum
cathode sheets when they are removed or inserted to the cell. Therefore, calcium is
sometimes added to the alloy by percentage of 0.05 to 0.08% in order to improve the
mechanical properties [25].
2.1.2. Corrosion of Lead-Based Anodes
Lead metal can be dissolved by oxidizing acidic solutions with the formation of
divalent plumbous ions Pb2+. Further oxidation can result in conversion of divalent
plumbous ions into brown quadrivalent lead dioxide PbO2. In the absence of passivating
substances such as carbonates, and oxidizing action can cause lead to corrode, except at
high electrode potentials where PbO2 is stable [26].
Figure 2.2. Potential-pH diagram obtained according to the ionic activities in an actual anodic film for the Pb-H2O-H2SO4 system at 25oC (potential vs. SHE) [27]
13
Guo Y. [27] determined the potential-pH relation of lead in sulphuric acid as Figure 2.2.
This diagram includes the basic lead sulphates PbO.PbSO4, 3PbO.PbSO4.H2O and the
tetragonal oxide, PbO (PbOt). When a lead electrode is immersed in sulphuric acid solution
and polarized anodically to potentials in the area of stability of PbO2.
It has also been observed by X-ray diffraction (XRD) analysis that two forms of PbO2 are
found with rhombic (α-form) being stable at lower potentials than the tetragonal (β-form).
α-PbO2 they found that they show more dense deposits, composed of large and closely
packed crystals. On the other hand, β-PbO2 deposits are less compact being composed of
poorly bonded, fine, needle shaped crystals [28]. However, the problem of lead
contamination of zinc deposit is still the solution by lead ions is the most critical problem in
zinc electrowinning.
2.1.3. Oxygen Overpotential of Lead-Based Anodes
Two main reactions occur on the anode which are; the evolution of oxygen gas O2
and the oxidation of PbSO4 to PbO2 [27]. The oxidation of water to oxygen is theoretically
possible at 1.23 V according to reaction (2.3), but production of oxygen is only observed at
potentials more positive than the equilibrium potential for the PbO2/PbSO4. Therefore, the
oxidation of lead sulphate to lead dioxide and the evolution of oxygen gas require
overpotentials.
During the electrowinning, lead alloys are immersed in the zinc electrolyte, the reaction
(2.6) takes place on the fresh anode surface at first;
→ 2 E= -0.356 V/SHE (2.6)
The anode surface is covered with time by non-conducting layer of PbSO4. The anodic
current density and the potential on that part of the anode surface of non-covering with
PbSO4 increase. The following reaction is expected on the Pb surface at atmospheric
temperature:
2 → 2 2 E= 1.685 V/SHE (2.7)
14
This reaction takes place instead of reaction (2.6), thus, a well conducting PbO2 occurs
instead of PbSO4, so the current density and the anodic potential decrease. The reaction of
the oxygen evolution starts on the layer of PbO2 and sulphuric acid in the renewed
electrolyte. After the anodic film formation, ≈ 99.20% of the electricity goes to the oxygen
evolution, ≈ 0.67% for formation of PbO2 and ≈ 0.13% for the other reactions [29].
The overpotential of oxygen can be reduced if a good lead anode is used, or alloyed with
another metal. As discussed previously that silver is the major metal could be alloyed with
lead at certain limit due to its high cost to reduce oxygen overpotential. It has shown also
that anodic potential of Pb and Pb-Ag anodes becomes more positive with the increase of
electrolyte acidity. The overpotential of Pb-Ag (1% Ag) anodes is 80-120 mV less than that
of pure Pb [30]. Table 2.1 illustrates the relation of electrode potential by changing
different alloys of anode at different current densities.
Table 2.1. Electrode potential (V/SCE) vs. current density (A m-2) of anodes from lead and its alloys in 1.8 M H2SO4 at 30oC [30]
Current density (A m-2)
Electrode potential vs. V/SCE
Pb Pb-Sn 1%- Ca 0.07%
Pb-Ag 0.37% Ca 0.12% - Ti 0.99%
Pb-Ag 0.97% Sn 0.63%
Pb-Ag 0.76%
Pb-Ag 0.9% Ca 0.04%
250 1.915 1.925 1.880 1.850 1.835 1.810
500 1.940 1.955 1.915 1.885 1.870 1.850
1000 1.975 1.980 1.945 1.925 1.915 1.885
2.1.4. Role of Manganese Ions in the Electrolyte
Usually, manganese ions are added to electrolyte to reduce lead contamination as
mentioned previously. Mn2+ ions are active electrochemically at lead or lead alloyed anodes
and the manganese oxide may be formed after the formation of PbO2 on the anode before
extensive evolution of oxygen occurs. It has been shown that the electrochemical
deposition of manganese on the anode may act favorably to minimize disintegration of the
anode scale by decreasing the amount of lead dioxide PbO2 formed on the anode [31].
Usually, manganese ions are added continuously to the industrial electrolyte in the form of
MnSO4, and accordingly the followings reactions are expected on the anode [30]:
15
2 → (2.8)
4 → 6 5 (2.9)
2 → 3 (2.10)
3 2 2 → 5 3 (2.11)
The lead anode is protected from corrosion by MnO2 and PbO2 layers, since the well
adherent oxide film of MnO2 increases the thickness and oxide layer PbO2-MnO2 which
acts as barrier on the anode surface. The presence of Mn2+ ions affects slightly the anodic
potential since the potential decreases as a result of depolarization effect during the
oxidation of Mn2+ on the anode. The potential decreases also as a result of the formation of
a protective layer of manganese oxides. Moreover, the presence of Cl- ions in the
electrolyte leads to a considerable decrease of the anodic potential for lead-based anodes.
Also, the increase of temperature results in a remarkable decrease of the anodic potential in
the presence of Cl- ions [30,32]. Figure 2.3 shows the lead-based anode before and after 5
hours of electrolysis as the corrosion of the anode is very remarkable with formed MnO2
layers.
However, studies proved that Mn2+ ions have also an effect on the cathodic reactions at
high concentrations. Zhang and Hua [33] revealed that adding Mn2+ ions in the
concentration range of 1-10 g dm-3 has no significant effect on the current efficiency (CE),
while a decrease in CE of more than 35% was happened at addition of high concentration
of 50 g dm-3. This decrease in CE was due to the strong depolarizing effect of MnO4- ions
and other oxidized products of manganese on hydrogen evolution reaction. The addition of
Mn2+ ions was also observed to change the surface morphology and deposit quality of the
electrodeposited zinc, affecting the crystallographic orientation.
16
Figure 2.3. Lead-based anode; (a) Before electrolysis and (b) After 5 hours of electrolysis
2.1.5. Surface Structure and Crystallographic Orientation
Almost all electroplated or electrodeposited metals are crystalline, which means that
the atoms are arranged in a regular three dimensional pattern called ‘’Lattice’’. The most
known lattices are: (i) face centered cubic (FCC), (ii) body centered cubic (BCC), and (iii)
hexagonal close packed (HCP) (Figure 2.4). Normally, Zn atoms are arranged in lattice
type hexagonal close packed (HCP) [34]. The crystal structure resulting from an
electrodeposition process is strongly dependent on the relative rate of formation of crystal
nuclei and growth of existing crystals. Finer-grained deposits are the result of conditions
that favourite crystal nuclei formation, while larger crystals are obtained in those cases that
favourite growth of existing crystals. Generally, a decreasing crystal size is the result of
factors which increase the cathodic polarization such as: increasing current density,
different electrolytes, and addition of colloids or additives [35].
Texture, which is preferred distribution of grains (individual crystallites) having a particular
crystallographic orientation with respect to a fixed reference frame, is an important
structural parameters for bulk materials and coatings. It is important to be able to specify
certain planes in crystal lattices. Miller indices signify a single plane or set of parallel
planes which are always presented in parentheses such as (100).
17
Figure 2.4. Zinc deposit shows HCP Lattice among the three most important lattices
Electrochemical parameters appear to be the only controlling factor. For example, texture
mainly depends on the cathodic potential and pH of the solution for a given electrolyte
composition, this also applies to current density if temperature is constant. It has been
revealed that electrodeposits have the (111) direction normal to the surface for BCC crystal
structures and the (110) direction for FCC substrates, independent of substrate orientation.
With hexagonal closed packed HCP metals such as zinc, the (101) direction is predominant
[36].
2.1.6. Metallic Impurities in Zinc Electrowinning
The presence of metallic impurities in zinc sulphate electrolyte is a critical problem
for zinc electrowinning industry. Low concentrations of metallic impurities influence
negatively the zinc deposition on the cathode; leading to a decrease in current efficiency, a
change in deposit morphology as well as an increase in cell voltage [37-38]. Actually, the
reduction of hydrogen ions in solution is affected in the presence of the impurities. Certain
impurities, e.g. Ge and Sb are hybrid formers may facilitate the hydrogen evolution reaction
HER, other impurities such as Ni and Co, more noble than zinc, cause re-dissolution of the
zinc deposit (low current efficiency) [39].
There have been many studies over the past decades dealing with the harmful effect of
impurities in zinc electrowinning. While, most of electrolytic zinc plants follow the same
general procedures in order to keep the optimal operating conditions which have usually
been arrived by experience and depend on the type of zinc ore treated and its impurity
content [37].
18
2.1.6.1. Effect of Lead Impurity on Zinc Deposition
As mentioned previously that lead impurity is one of the challenges in zinc
electrowinning industry so, some studies were done to investigate the effect of lead ions in
the solution. Mackinnon et al. [40] conducted several investigations of the influence of lead
in an industrial zinc solution (55 g dm-3 Zn2+ + 150 g dm-3 H2SO4). They found that the
effect of lead (6 mg dm-3 at 35oC) on current efficiency was current density dependent,
producing an increase of ~0.7% at 400 A m-2 and a reduction of ~1.5% at 800 A m-2.
Increasing amounts of lead in the zinc deposits progressively changed the preferred
crystalline orientation from (112) to (101) to (100) to finally a poorly (002) crystalline
structure. The same results trend was also proved by Ault and Frazer [37]. The lead content
of the zinc deposits was dependent on the solution concentration of lead, the form in which
lead was added, the current density as well as presence of Sb and glue. Table 2.2 shows the
variation of current efficiency and preferred orientation of deposits in different
concentrations of lead. Also, Figure 2.5 shows the deposit morphology with different
additions of lead [37,40].
Table 2.2. Variation of current efficiency and preferred crystalline orientation of zinc deposit at different concentrations of lead at 400 A m-2 and 35oC for zinc electrolyte of 55 g dm-3 Zn2+ + 150 g dm-3 H2SO4 [37]
Initial Pb (mg dm-3)
Final Pb (mg dm-3)
Pb removed (%)
Change in CE (%)
Preferred orientation
0 0.01 - - Random
1.0 0.25 75 0.3 (102) (103) (104)
2.0 0.6 70 0.8 (004) (002)
3.0 0.9 70 0.9 (004) (002)
4.0 1.4 65 1.0 (004) (002)
5.0 1.7 66 1.1 (004) (002)
19
Figure 2.5. SEM photomicrographs (X 385) showing the effect of current density on the morphology of zinc deposits from addition-free electrolyte using unconditioned Pb-Ag anodes. (a) 215 A m-2, 60 min, 0.125% Pb; (b) 323 A m-2. 60 min, 0.076% Pb; (c) 430 A m-
2, 60 min, 0.04% Pb; (d) 538 A m-2, 60 min, 0.021% Pb; (e) 1076 A m-2, 30 min, 0.019% Pb; (f) 2152 A m-2, 15 min, 0.011% Pb [40]
2.1.6.2. Effect of Antimony Impurity on Zinc Deposition
Antimony has been known as one of the most toxic solution impurities with respect
to current efficiency (CE). Ault and Frazer [37] and Lafront et al. [41] studied the effect of
different concentrations of Sb3+ of 0.0055-19 mg dm-3 in high purity-solutions on CE and
morphology; they found that it has a dramatic effect on CE with decrease of ~5.2 to 62.3%.
With such small concentrations of antimony present in the solution, this decrease can be
assumed due to the catalytic production of hydrogen which inhibits the reduction of Zn2+.
The variation of current efficiencies and preferred orientation of deposit with addition of
antimony are shown in Table 2.3. Also, antimony had a dramatic grain-refining effect on
zinc deposit, reducing platelet size even at low concentrations of 0.02 - 0.04 mg dm-3
(Figure 2.6).
20
Table 2.3. Variation of current efficiency and preferred crystalline orientation of zinc deposit at different concentrations of antimony at 400 A m-2 and 35oC for zinc electrolyte of 55 g dm-3 Zn2+ + 150 g dm-3 H2SO4 [37]
Initial Sb (mg dm-3)
Final Sb (mg dm-3)
Sb removed (%)
Change in CE (%)
Crystal orientation
0 4.0 - - Random
4.0 4.0 0 -5.2 (112) (212)
7.0 5.0 29 -10.0 (112) (211)
10.0 9.0 10 -23.6 (112) (101)
14.0 10.0 29 -52.4 (104) (101)
19.0 13.0 32 -62.3 (004) (103)
Figure 2.6. SE micrographs showing the morphology of 6h zinc deposits electrowon at 500 A m-2 and 38oC from electrolytes containing; (a) and (b) 0.02, (c) and (d) 0.04 mg dm-3 Sb [42]
Although antimony has negative effect on zinc deposit, it showed very good effect when
combined with some organic additives such as glue and gelatin.
21
2.1.6.3. Effect of Copper, Nickel and Cobalt Impurities on Zinc Deposition
Although copper readily removed from zinc electrolyte by zinc dust cementation
purification, it can re-enter the electrolyte via corrosion of the bus bars. It is known also
that copper co-deposit with zinc leading to a reduction of metal quality at certain
concentrations. Ault and Frazer [37] also Mackinnon [43] studied the effect of Cu on
current efficiency and crystal orientation at different concentrations and different current
densities (Table 2.4).
It is shown that content of electrodeposited zinc increased with increasing copper
concentration in the electrolyte and with decreasing current density. Although copper co-
deposited with zinc, it did not result in a dramatic decrease in the current efficiency but co-
deposited copper reduced the grain size of the zinc deposits [43].
Table 2.4. Effect of copper on current efficiency and crystal orientation of zinc deposit at different concentrations and different current densities for electrolysis in 55 g dm-3 Zn2+, 150 g dm-3 H2SO4 at 35oC [43]
Current density (A m-2)
Copper (mg dm-3)
CE (%)
Cathode Copper (%)
Crystal Orientation
430 0 93.6 - (112)(103)(102)
5 93.0 0.025 (112)(110)
10 95.3 0.060 (112)(110)
20 95.1 0.095 (112)
30 92.3 0.157 (114)(112)
50 92.5 0.254 (002)(101)
323 0 96.0 - (112)
10 94.0 0.041 (112)
20 92.4 0.129 (101)(103)
30 94.8 0.240 (002)(101)
50 93.0 0.399 (002)
215 0 94.3 - (112)
10 92.1 0.065 (101)(102)(103)
20 93.9 0.161 (101)(002)(103)
30 90.7 0.232 (101)(002)
50 88.5 0.393 (101)
22
Nickel is one of the most injurious impurities in the electrolytes. During the electrowinning
of zinc from sulphate electrolytes in the presence of nickel, a re-dissolution process of the
deposited zinc takes place. Nickel co-deposits with zinc and forms numerous galvanic
micro-batteries. Hydrogen evolves on the nickel zone and surrounding zinc re-dissolves,
causing spongy and dark deposits [44]. After many formation and dissolution cycles of the
zinc deposits, the frequency of the cycle increases, since the cathode is polluted gradually
by nickel until zinc deposition can no longer occur. Ault and Frazer [37], Stefanov and
Ivanov [45], and Morrison et al. [46] studied the effect of Ni impurity on current efficiency
and deposit structure during zinc electrowinning (Table 2.5). Briefly, the more noble co-
deposited metals with Zn enhance hydrogen evolution in acidic medium and their effect is
expected to be in the following order: Ni, Co, and then Cu.
Table 2.5. Variation of current efficiency and preferred crystalline orientation of zinc deposit at different concentrations of nickel at 400 A m-2 and 35oC for zinc electrolyte of 55 g dm-3 Zn2+ + 150 g dm-3 H2SO4 [37]
Initial Ni (mg dm-3)
Change in CE (%)
Crystal Orientation
0 - Random
0.25 -0.1 (114) (102)
0.5 -0.1 (114) (102)
1.0 -0.2 (114) (102)
1.5 -0.1 (211) (105)
2.0 -0.2 (114) (102)
5.0 -0.3 (204) (102)
The CE declined very slowly, with increasing nickel concentration, with most of the
decrease occurring in the 0-1 mg dm-3 range. The crystal orientation changed from a
relatively random pattern with (102), (104), (114), (204) as major plans, to an orientation
dominated by the (114), (102), (204), (203) plans, when the nickel concentration was in the
range 0.25-2 mg dm-3. At 5 mg dm-3 Ni the (114) plane was replaced by the (204) plane.
Cobalt combined with nickel is difficult to be removed from the electrolyte, can have
disastrous effect on zinc electrowinning under certain conditions [47]. Maja and Spineli
[48], and Maja et al. [49] have studied the effect of both impurities on induction time in
23
zinc electrowinning. The term “induction time or period” is used in zinc electrowinning.
During this induction time, which coincides with the beginning of zinc electrodeposition,
the zinc deposits are uniform and adhere firmly to the cathode. The typical current
efficiency is 93‐95%. Following this induction time, zinc re‐dissolution occurs with
hydrogen evolution. After the zinc is completely dissolved, deposition restarts. The
induction time depends on several factors such as temperature, cathodic current density,
and the concentrations of sulphuric acid, zinc and impurities [50-51]. It has shown that an
induction period more than one hour exists before cobalt and nickel begin to have an effect
on CE and zinc deposit. After the induction period CE decreases rapidly with time. The
length of induction period decreases with increasing temperature, increasing acid
concentration and with decreasing current density [49].
2.1.7. Additives in Zinc Electrowinning
The presence of high concentrations of impurities in the industrial electrolytes
decrease the induction period associated with zinc electrowinning process resulting in
deterioration of zinc deposit quality and in decrease the current efficiency [51]. High
quality and high current efficiency of zinc deposit are always obtained from pure
electrolytes. However, various electrolyte purification steps are nonviable economically.
Accordingly, an alternative method to reduce the detrimental effect of metallic impurities is
to use suitable organic additives [52]. These additives may be classified into non-ionic,
anionic and cationic types. Most of used additives are organic materials with high
molecular weight which could be adsorbed on the cathode and act as a diaphragm
(hydrogen inhibitor and crystal growth modifier). In industrial zinc deposition the most
commonly used additives are the naturally occurring, gums, gelatins or glues which in
acidic solutions are cationic [53].
2.1.7.1. Effect of Glue
Animal glues are most known additives in the zinc electrowinning industry. Such
additives are often added to the electrolyte at low concentrations in order to have smooth
24
and compact zinc deposit. Although glues have beneficial leveling effect on zinc deposit
but when increasing the addition alone to the purified electrolyte usually result in a
decrease in the current efficiency of zinc deposit. Also, addition of glues alone to the pure
electrolyte leads to cathodic overpotential this is due to the re-arrangement of
crystallographic orientations which consumes over voltage [39]. Moreover, glues also
interact in a beneficial way with certain impurities in the electrolyte; one of those famous
impurities is antimony. Addition of glue to an electrolyte contains low concentrations of Sb
(≤ 0.02 mg dm-3) optimizes zinc deposition CE and modify the zinc deposit morphology
and preferred crystallographic orientation. In spite of the detrimental effect of Sb on CE, a
small concentration of antimony is usually added to the electrolyte to reduce zinc deposit
adherence to the aluminum cathode and because its beneficial interaction with glue [37].
Glue in presence of antimony has also a significant effect on the zinc deposition
overpotential [54]. The addition of glue alone increases the overpotential; that is polarizes
zinc deposition while increasing the concentrations of antimony decrease the overpotential
due to the high hydrogen evolution. Accordingly, balanced additions of glue and antimony
produce zinc deposit potential or (nucleation overpotential) that results in an optimum
values for the CE with uniform deposit [22,55]. The term ‘’Nucleation Potential’’
corresponds to the commencement of the reduction of Zn2+ at the cathode. This potential
can be easily determined by cyclic voltammetry technique. While, the potential difference
between the crossover point and the point where the Zn2+ ions are started to be reduced on
the cathode is known as nucleation overpotential (NOP). NOP is used to elucidate the
extent of polarization of a cathode, and high NOP values indicate strong polarization. It is a
convenient parameter to show the effects of various additives on zinc electrowinning. The
number of nuclei can be calculated by the following equation:
. ɳ
Where; α and b are constants, ɳ is the nucleation overpotential. It indicates that the higher
nucleation overpotential, the much more fine-grained zinc deposits can be obtained with
good crystallographic orientation [56].
25
It was found that, adding 0.02 mg dm-3 of Sb3+ ions to standard zinc electrolyte reduced CE
from 91% to 86.7% while adding 5 mg dm-3 of glue to this electrolyte in presence of same
quantity of antimony succeeded to increase CE from 86.7% to 92.4%. Presence of
antimony reduced NOP by 75 mV which indicates non-smooth and very small grain size as
well as distortion in crystallographic orientation while addition of 5 mg dm-3 of glue
restored the normal values of NOP and formed medium grain sizes [42].
2.1.7.2. Effect of Natural Products and Surfactants
Natural products and surfactants were always the concern of many studies as
additives in zinc electrowinning. Saponins, Licorice, Tennafroth 250, and Dowfroth 250
were studied as additives by different concentrations (0, 5, 10, and 15 ppm) also their
effects on acid mist suppression have been reported [57].
‐ Saponins: are found in various plants, they are amphipathic glycosides grouped
phenomenologically by the soap-like foaming.
‐ Licorice: is the root of Glycyrrhiza glabra from which a sweet flavour can be
extracted. The scent of licorice root comes from a complex and variable
combination of compounds, of which anethole is up to 3% of total volatiles. Much
of the sweetness in liquorice comes from glycyrrhizin, which has a sweet taste, 30–
50 times the sweetness of sugar.
‐ Tennafroth 250 and Dowfroth 250: are products of Dow Company which used as
foam sealants.
Studies reported that, none of the additives succeeded to increase the CE%. While,
Tennafroth 250 and Dowfroth 250 appeared to achieve high acid mist suppression
efficiency (66% and 62% respectively). The high suppression efficiency with low power
consumption for both additives was supported by surface tension results (67.6 and 67.9
mN/m respectively) and polarization behavior obtained by cyclic voltammetry (NOP at 66
and 48 mV [57].
Sodium lignin sulphonate had been studied by Alfantazi and Dreisinger [58]. The addition
of sodium lignin sulphonate up to 10 ppm had no negative effect on zinc electrowinning
26
process, nor on the quality of the zinc deposits. CE% maintained constant with addition of
this surfactant.
Also, the extract of horse-chestnut tree (HCE) was tested as an additive in zinc
electrowinning. The additives increased the cathodic polarization and promoted leveling.
HCE had a beneficial influence on the zinc deposit quality, being a good leveling agent by
increasing the nucleation overpotential NOP and the deposition rate of zinc on the cathode
[59].
However, studies proved that gelatin acts more or less as glue in presence of small traces of
antimony in the zinc electrolyte. Small concentrations of antinomy (0.0055 mg dm-3)
reduced the CE by ≈7%, with depolarization by ≈40 mV, while addition 1 mg dm-3 of
gelatin to this electrolyte restored back the normal values of current efficiencies [41].
2.1.7.3. Effect of Synthetic Polymers
The behavior of zinc electrodeposition and Zn deposit morphology were studied in
electrolytes containing polymer additives such as polyethylene glycol (PEG) [60]. PEG
with molecular weight of 1.54x103 was added to the electrolyte with different
concentrations (0, 0.001, 0.01, 0.1, and 1 mg dm-3). Results showed that, PEG acts as a
polarizer to shift the deposition potential of zinc in a less noble direction. It was found that,
increasing the concentration of additive shifted the cathodic potential to more negative
values starting from ~-0.87 V at zero addition to ~-0.93 V at 1 mg of PEG. Another series
of experiment have been done in order to observe the effect of molecular weight of polymer
on the cathodic potential and polarization resistance. As the molecular weight of PEG
increases, the overpotential and the polarization resistance for Zn deposition first increased,
but then decreased when the molecular weight exceeded 1x104.
When the molecular weight of the polymer is less than 1x104, almost all the oxygen
radicals are utilized for adsorption to inhibit zinc deposition effectively. So, the electrolytic
solution contains a smaller number of longer chains as the molecular weight increases. As a
result, the Zn deposition potential is shifted in a less noble direction and the polarization
27
resistance for Zn deposition increases. It can be concluded that, the degree of polarization
depends on the molecular weight of the additive.
The electrodeposits obtained from additive-free solution are composed of hexagonal
platelets with medium grain size, while the deposits obtained from solution containing PEG
are found to have grain size smaller than that obtained from free-addition electrolyte [60].
2.1.7.4. Effect of Quaternary Ammonium Salts
Quaternary ammonium salts are characterized by having positively charged nitrogen
(cation) covalently bonded to four alkyl group substituents (non-aromatic) (Fig. 2.7a)
and/or benzyl substituents (aromatic) (Fig. 2.7b). R = CnH2n+1, where n=8 to 18, with
mixture of carbon chain lengths, predominantly 12, 14 or 16. Quaternary ammonium salts
are known with their stability under neutral or acidic conditions up to 150oC, but
decomposition can occur with the quaternary ion acting as an alkylating agent in its
reaction with anion [61].
Figure 2.7. Quaternary ammonium salts; (a) Non-aromatic, (b) Aromatic
The effect of some quaternary ammonium salts represented in cetyltrimethyl ammonium
bromide (CTABr) and tetrabutyl ammonium bromide (TBABr) on zinc electrowinning
have been investigated [62]. Results indicated that CTABr has approximately similar
properties to glue the commonly used additive in industry. CTABr has been found to have
the same polarization behavior, crystallographic orientation and surface morphology like
glue while TBABr has less useful properties. This could be explained due to the higher
molecular weight. Addition of small concentrations of CTABr (1 mg dm-3) to the standard
electrolyte increased CE from 89.3% to 93% while adding same quantity of TBABr has no
28
effect on CE in absence of antimony. In presence of antimony both additives succeeded to
increase CE up to 94.2%. Crystallographic orientation has not been changed for both
additives as the main orientation was (101) [62].
Another quaternary ammonium salt triethybenzylammonium chloride (TEBACl) has been
examined also in zinc electrowinning [52,63]. TEBACl was found to decrease the screening
effect of hydrogen bubbles responsible for the formation of local galvanic cells in presence
of some metallic impurities such as Ni2+ ions. Addition of small concentration of 0.2 mg
dm-3 alone to the standard electrolyte increased CE from 89.3% to 92.8% with total
reduction in power consumption of ≈178 kWh ton-1, while adding the same quantity in
electrolyte containing 0.01-0.02 mg dm-3 of Sb3+ increased the CE up to 95.6% with total
reduction in power consumption of ≈317 kWh ton-1.
2.1.7.5. Effect of Ionic Liquid Salts
Recently, ionic liquids have been used in many chemical and hydrometallurgical
applications due to their chemical and physical properties, as they are salts where the ions
are poorly coordinated, leading to being liquids below the boiling point and even at room
temperature [64]. Ionic liquids consist of an organic cation and inorganic or organic anion
(Figure 2.8); they have a wide range of solubility and miscibility. For example, some of
them are hydrophobic while others are hydrophilic; most of them are non-inflammable and
non-toxic [65-66]. Ionic liquids are widely used in liquid-liquid extraction and
electrodeposition of some metals due to their low melting point and the thermal degradation
properties which are important in the electrochemical media [67]. Also, as a medium in the
electrodeposition of aluminum on a stainless steel cathode [68]. They are used as organic
solvents in electroplating of a range of metals impossible to deposit in water due to
hydrolysis e.g. Al, Ti, Ta, Nb, Mo, and W [69].
29
(a) Cationic ionic liquids
(a) Anionic ionic liquids
Figure 2.8. Examples of ionic liquids salts; (a) Cationic, and (b) Anionic
The ionic liquids salts in the form of 1-butyl-3-methylimidazolium hydrogen sulphate
[BMIM]HSO4 had been studied as an additive in zinc electrowinning from acidic sulphate
electrolyte [70]. Also, the effects of temperature and current density on zinc
electrodeposition in presence of [BMIM]HSO4 had been reported [71].
Table 2.6. Effect of [BMIM]HSO4 and Gelatin on current efficiency and power consumption during zinc electrodepsotion [70]
Additives mg dm-3
Current Efficiency Cell Voltage Power Consumption kWh ton-1 % V
Blank 89.3 2.89 2655 [BMIM]HSO4
1 90.5 2.78 2520 2 91.6 2.84 2543 5 92.7 2.84 2513 10 91.8 2.87 2564 50 87.8 2.90 2709
Gelatin 1 2
89.5 88.8
2.84 2.85
2603 2633
5 87.4 2.87 2694 10 86.1 2.92 2782 50 81.6 2.92 2935
Combined* 91.8 2.85 2547
* Combined addition of 5 mg dm-3 [BMIM]HSO4 and 1 mg dm-3 gelatin
30
Studies showed that, addition of [BMIM]HSO4 to the electrolyte by concentrations of 0,1,2
and 5 mg dm-3 increased the CE from 89.3% to 92.7%, while, increasing the concentration
up to 50 mg dm-3 reduced the CE to 87.8% (Table 2.6) due to the excessive adsorption of
additive on the cathode surface which block the active sites.
It was found also, there is sharp decrease in cell voltage by addition low concentration of
additive (1 mg dm-3) from 2.89 V to 2.78 which affect positively the power consumption
from 2655 kWh ton-1 to 2520 kWh ton-1, while cell voltage is increased slightly by
increasing the concentration (50 mg dm-3) to reach 2.90 V which affect negatively the
power consumption.
It was reported that, ionic liquid [BMIM]HSO4 maintained the medium grain size of the
obtained zinc deposit as the standard electrolyte. It is increased the nucleation overpotential
(NOP) gradually by increasing the concentrations of additive. By addition of 0,1,2,5 and 10
gm dm-3 increased the NOP by 112, 115, 123,131 and 160 mV, respectively which
indicates smooth deposit could be obtained at high concentrations of additive [70].
The effect of antimony (III) on zinc electrodeposition in presence of [BMIM]HSO4 has
been examined in acidic sulphate electrolyte [72]. The presence of Sb3+ decreased the
current efficiency and decreased also the cell voltage due the hydrogen evolution reaction.
As shown in Table 2.7, current efficiency decreased from 90.8% to 68.2% by addition Sb3+
up to 0.08 mg dm-3. Also cell voltage decreased from 2.89 V to 2.86 V. The addition of
additive in the electrolyte containing antimony is found to inhibit the hydrogen evolution
reaction which favourites the zinc electrodeposition. Accordingly, CE values were
increased with increasing of the cathodic potentials. The presence of Sb3+ ions in the
electrolyte decreased the NOP values to 60 mV which is an indication of small grain size of
zinc deposit is obtained from, while the addition of additive restored back the normal
values of NOP [72].
31
Table 2.7. Effect of Sb3+ on current efficiency in absence and in presence of [BMIM]HSO4 during zinc electrowinning [72]
Sb(III) mg dm-3
[BMIM]HSO4 mg dm-3
CE %
Cell Voltage V
PC kWh ton-1
0 0 90.8 2.89 2611 0.01 0 91.3 2.88 2588 0.02 0 90.5 2.87 2601 0.04 0 85.8 2.87 2744 0.08 0 68.2 2.86 3440 0.01 5 93.4 2.85 2503 0.02 5 94.7 2.84 2460 0.04 5 91.5 2.84 2546 0.01 10 92.6 2.86 2534 0.02 10 93.5 2.86 2509 0.04 10 90.4 2.86 2586
2.2. Electrodeposition of Alloys as Cathodes for Chlorate Production
2.2.1. Chlorate Production
Sodium chlorate is produced industrially by an electrochemical process in a typical
chlorate electrolyte consisting of 100-120 g dm-3 NaCl, 1-4 g dm-3 NaClO, Cr(VI)
corresponding to 1-6 g dm-3 Na2Cr2O7, at a bulk pH of 6.0-6.5 and a temperature of 70-
85oC. The electrolyte can also contain NaClO4 at concentrations that should not exceed 100
g dm-3. A high chlorate concentration (500-650 g dm-3 NaClO3) is essential for the
separation of NaClO3(s) by crystallization, and a high chloride concentration is important for
the anode operation [14,73]. The chloride ions are oxidized to chlorine on the anodes and
hydrogen gas is evolved on the cathodes (Figure 2.9). In this process, sodium chloride is
oxidized to sodium chlorate as global reaction explained in Eq. (2.12); at the cathode water
is reduced to hydrogen gas as explained in Eq. (2.13). While, at the anode chlorine is
formed and dissolved as Equations (2.14 & 2.15). Chlorate is formed by disproportionation
reaction (Eq. 2.17) [14].
3 → (Global Reaction) (2.12)
2 2 → 2 (Cathodic Reaction) (2.13)
2 → 2 (Anodic Reaction) (2.14)
32
→ (2.15)
↔ (2.16)
2 → 2 2 (2.17)
In practice, the cell voltage of chlorate electrolysis is in the range of 2.75-3.5 V at
approximately 95% of average current efficiency (CE). Accordingly, for chlorate cell
operating at 3.3 V with CE of 95%, the power consumption will be equivalent to 5245 kWh
ton-1 according to following equation [14,16,73,74].
. /
Where, PC is power consumption (kWh ton-1), E is the cell voltage (V), and CE is the
current efficiency (%).The efficiency of the main reactions as well as the energy required
by the chlorate process depend on the electrodes materials and on the electrolyte
compositions [14]. Figure 2.9 illustrates the schematic chlorate production process.
However, this process consumes large amounts of energy due to the hydrogen evolution
reaction (HER) overpotential. Hence, the reduction of cathodic overpotential depends
mainly on the cathodic materials. Accordingly, various materials were studied to develop
durable cathodic electrocatalysts to reduce the overpotential of HER [16].
Figure 2.9. Schematic process of chlorate production (Chemetics Inc. B.C., Canada)
33
2.2.2. Mild Steel Cathodes
One of the most used cathodes in chlorate production is mild steel. However, these
cathodes are not the preferable choice due to the following inconvenients: (i) when the
surface of mild steel is fresh, the overvoltage values of hydrogen evolution reaction at 250
mA cm-2 (η250) are between 850 to 950 mV depending on surface roughness [18]. During
electrolysis, the cathode surface is gradually covered by chromium oxide added in the
electrolyte in order to increase the current efficiency. This can lead to Ca and Mg
containing precipitation giving an increase of 1100 mV in overpotential, (ii) due to the
thermodynamic instability of iron, the steel cathodes are significantly corroded with time,
(iii) corrosion products create many problems during operation and the cathode life is
considerably shortened [16,18].
It has been found that a corroded steel surface requires higher chromate concentrations (>3
g dm-3 Na2Cr2O7) for a high current efficiency for hydrogen evolution. Then, chromate is
added to the chlorate electrolyte, mainly to hinder the side reactions of reduction of
hypochlorite and chlorate on the cathode as explained in Equations (2.18 & 2.19). During
electrolysis Cr(VI) is reduced and forms a thin film, less than 10 nm thick, of Cr(OH)3.H2O
on the cathode. This film hinders also some other cathodic reactions as oxygen reduction,
whereas hydrogen evolution can take place though with changed kinetics compared to that
on a bare electrode surface [15]. As Cr (VI) is harmful and not environmentally
recommended, a replacement of chromate addition is required. Therefore, efforts and
attempts are made by many researchers in order to have binary and ternary mild steel
coated alloys with electrocatalysts exhibiting low hydrogen evolution overpotential as well
as improving the corrosion resistance [75-76].
2 → 2 (2.18)
3 6 → 6 (2.19)
An ideal cathode as an electrocatalyst may include several properties as the following: (i)
low hydrogen overvoltage at industrial current density, (ii) no potential drift with time, (iii)
good chemical and electrochemical stability: long lifetime and no release of deleterious
products during electrolysis, (iv) low sensitivity towards presented impurities in the
34
electrolyte, (v) low sensitivity to current shut down (short-circuit) or modulation, (vi) no
safety or environmental problems in the manufacturing process, and (vii) easy to prepare at
a low cost in comparison with its life time [16,77]. However, many attempts were
conducted on fabrication of the new cathodes for chlorate production based on only one
metal or alloys of several elements such as Ni, Cu, Co, Mo, Pd, Rh, Ti-Ru-O, Ni-Mo-P and
Ni-P in order to reduce the hydrogen reaction overpotential [78-79].
2.2.3. Fe-Based Alloys Cathodes
Fe metal alone or alloyed with other elements are always considered in certain
fields of research as enhanced catalysts. Mo is one of the most remarkable elements that is
alloyed with Fe for its high rate of hydrogen evolution due to the considerable real surface
area. Elezovic et al. [75] succeeded to deposit electrochemically Fe-Mo alloys which
showed a reduction in HER overpotential by 0.15-0.30 V as compared to mild steel in
chlorate electrolyte. They reported that with increasing the current density during the
electrodeposition, the Mo content is increased, while Fe content is reduced in the prepared
alloys. It was found that lowest HER overpotential was obtained for electrode containing
40.7 at.% and 59.3 at.% of Fe and Mo, respectively [16,75].
The electrocatalytic properties of some Fe–R (R = rare earth elements) crystalline alloys,
Fe90Ce10, Fe90Sm10, Fe90Y10, and Fe90MM10 (MM = mischmetal), have been studied for the
hydrogen evolution reaction (HER) in 1 M NaOH solution at 25oC. Those alloys were
compared with the G14 (Fe60Co20B10Si10) amorphous alloy, which is a good electrocatalyst
material for the HER. High catalytic efficiencies for the HER were achieved on the
Fe90Ce10 and Fe90MM10 electrodes, the latter being a better catalyst as compared with the
G14 alloy. The improvement of the electrocatalytic performances of the Fe90Ce10 and
Fe90MM10 electrodes as compared with the Fe90Y10 and Fe90Sm10 ones was attributed to
synergetic composition effects of these alloys [80].
The research team in Germany, Müller et al. [81] prepared several amorphous melt-spun
Fe-alloys such as Fe82B18, Fe80Si10B10, and Fe60Co20Si10B10. The electrolcatlystic activities
of the obtained amorphous structure of alloys were compared to the crystalline Ni and Fe in
35
1M KOH solution at 25oC. The obtained overpotential values at 300 mA cm-2 (ɳ300) were
430, 430, and 360 mV for alloys Fe82B18, Fe80Si10B10, and Fe60Co20Si10B10, respectively,
compared to 480 mV for Fe or Ni. Maximum reduction of overpotential of 120 mV was
obtained for the alloy Fe60Co20Si10B10.
2.2.4. Ni-Based Alloys Cathodes
Ni and Ni-based alloys are the most interesting materials as electrodes for hydrogen
evolution reaction applications. In spite of the performance of Ni as a catalyst is not
remarkable as steel towards the electrocatalytic activity, but it has an excellent resistance to
corrosion in highly concentrated alkaline solutions and at elevated temperatures [15-16].
This is the reason why there are many studies to use nickel or its alloys as cathode for HER.
Ni is more stable than the other transition metals such as Fe or Co in alkaline media. Thus,
different binary and ternary Ni-metal electrodes with different configurations including
nanopowders, spinel or perovskite structures, and different preparation methods such as
electrodeposition or electroless plating have been reported in order to increase the surface
area of the electrodes and to improve electrocatalytic performance. Among these, Ni-Ti,
Ni-Zn, Ni-Co, Ni-W, Ni-P, Ni-Mo, Ni-Cu, Ni-Al, Ni-Fe, Ni-Mo-P, Ni-Mo-Cu, Ni-Mo-Zn
and Ni-Mo-Cr have been extensively considered [82-85].
Different binary Ni-based alloys such as: Ni-Co and Ni-W have been electrodeposited on
stainless steel substrates in order to investigate their activities towards HER [86].
Electrodeposition at very high current densities provided macro-porous materials due to the
fact that the metallic deposition takes place simultaneously during the gas bubbling at high
current densities (1000 A cm-2) (Figure 2.10).
36
Figure 2.10. Scanning micrographs of developed cathodes [86] Hydrogen evolution on these electrodes was evaluated in 30 wt.% KOH solution by means
of steady-state polarization curves and electrochemical impedance spectroscopy (EIS) at
different temperatures 30, 50, and 80oC. At 80oC, the measured hydrogen evolution
overpotential values at 100 mA cm-2 of Ni-Co and Ni-W were compared to Ni catalyst, the
overpotential values were reduced by 52 and 33 mV, respectively. Values of exchange
current densities for these electrodes were in the range of 103 orders of magnitude higher
than those of commercial Ni electrode. This significant catalytic performance can be
attributed to the increase of real the surface area [86-87].
Giz et al. [88] carried out the electrodeposition of Ni-Cu-Fe film on mild steel substrates
from an acetate bath at current density of 25 mA cm-2, temperature of 45oC and at low pH
of 3.2. They succeeded to obtain crystalline surface composition of Ni = 49, Cu = 43 and
Fe = 8 at.%. They examined the catalytic activities toward HER in brine solution (160 g
dm-3 NaCl + 150 g dm-3 NaOH) at different temperatures through steady-state polarization
curves.
Analyzing the values of the overpotential measured at a current density of 210 mA cm-2 in
Table 2.8. An enhanced electrocatalytic activity to the HER is observed for the Ni-Cu-Fe
co-deposit. At 80oC (the operational temperature used in industrial electrolysers) the
overpotential for the Ni-Cu-Fe electrode is 249 mV lower than that of mild steel (404 mV),
and 19 mV lower than that of the previously obtained Ni-Fe material by Carvalho et al.
[89]. The values of the exchange current density (i0) are higher for Ni-Cu-Fe compared to
Ni-Fe and this indicates that the poor intrinsic catalytic activity of this material is
37
compensated by a larger improvement due to its larger surface area. It is also found that the
larger active surface area is the main factor responsible for the enhanced activity of this
material.
Table 2.8. Kinetic parameters for the HER on the Ni-Cu-Fe electrode [88]
Temperature (oC)
Kinetic Parameters
(-bc) mV dec-1
(io) A cm-2
(ɳ(210)) mV
25 66 6.65 x 10-5 231
40 70 2.08 x 10-4 210
60 73 8.03 x 10-4 179
80 71 1.52 x 10-3 155
Also, it has been found that alloying Co with Ni has a significant effect on the oxygen
evolution reaction (OER) and that the actual effect depends on its content, with an optimum
value in the Ni-Co based materials could be easily prepared by means of electroless-plating
deposition leading to very active surfaces on any type of support, either conducting or non-
conducting. In addition, this synthetic approach has the advantage of covering substrates
with complex surface morphologies, resulting in strongly adherent metal deposits [90].
2.2.5. Molybdenum Co-deposition
Due to the superior properties of Mo towards HER, various investigations and
studies have been conducted during the past three decades about Mo containing alloys. Mo
containing alloys showed a high resistance to corrosion, and low hydrogen overpotential
[16,91]. The electrodeposition of molybdenum in the pure state from aqueous solutions has
not been performed yet, as Mo cannot be reduced alone in aqueous solutions. Accordingly,
Mo can be easily co-deposited with other elements such as: Fe, Ni and Co. Most of studies
suggested multi-steps reduction of Mo species for induced co-deposition. For example,
during the co-deposition with nickel, the first slow step molybdenum oxide (MoO2) reduces
into a mixed oxide with nickel via electrochemical reduction (Eq. 2.20).
4 8 ↔ (2.20)
38
During this slow reaction, a surface compound that inhibits the hydrogen evolution is
produced by mixed oxides. This slow reaction could be coupled by fast global reaction
producing deposited alloy of Ni3Mo (Eq. 2.21) [92].
3 8 4 → 4 (2.21)
Podlaha and Landolt [93] showed that the deposition rate of Mo might be limited by mass
transport either Mo or Ni species depending on the relative concentrations. They observed
that in citrate electrolyte in the presence of low concentration of Mo ions and excess of Ni
ions, Mo co-deposition followed-up a mass control mechanism. In this way, Mo content
increases in the alloy with the increase of rotation rate of a disc electrode while it reduces
with the increase of current density [93].
Electrodeposition of Ni-Mo alloy coatings has been developed and their characterization as
cathode for HER has been studied as well [94]. K2P2O7 was used as complexing agent as it
was found that higher percentage of Mo could be co-deposited with Ni from such type of
electrolyte in comparison with the citrate-based electrolyte. It was confirmed that, the
percentage of Mo in the deposit increases with increasing deposition current density, from
about 28 at.% at 20 mA cm-2 to about 41 at.% at 100 mA cm-2. The electrodeposited
coatings of Ni-Mo exhibit porous surface morphology and much better activity toward the
HER than pure Ni electrode. The main contribution toward the apparent acitivity is a
consequence of the increase of the real surface area [94].
2.2.6. Phosphorous Co-deposition
The incorporation of phosphorous can give amorphous effect to alloys. Amorphous
alloys possess good mechanic and magnetic properties and high corrosion resistance
because of their special structure [95-96]. The sodium hypophosphite supplies the H2PO2-
anions that serve as the source of phosphorus during coating (Eq. 2.22) [97].
3 3 → 3 (2.22)
The amorphous characteristics can be observed for alloys with P content over than 8-10%
considering to different contributions [98]. The incorporation of very low contents of
39
phosphorous (0.0003 to 0.5% by weight) showed also an improvement in corrosion
resistance [99]. Ordine et al. [100] reported also the presence of P in Zn-Ni or Zn-Fe alloy
deposits, even at very low contents would improve effectively the anti-corrosive properties
of deposits. They deduced that this phenomenon is due to amorphous characteristics of
these alloys. They observed fastest corrosion rate for crystalline structure of Zn-Fe-P alloy.
Other research group, Shibli and Dilimon [79] confirmed that the presence of phosphorus
enhances the corrosion resistance, hardness and wears resistance of Ni-based plates. They
studied the role of phosphorous content on physicochemical and electrocatalytic
characteristics of electroless Ni-P plates. They obtained amorphous structure in the
presence of phosphorous content (more than 13%) and crystalline structure in lower
amounts (less than 7%). They found optimum P content of 10% in the plates yielding high
electrocatalytic activity during HER. They also deduced that the plates containing higher
surface roughness produces large number of electrochemical active sites. The presence of
active sites facilitates maximum extent of hydrogen adsorption on the surface [79].
Shervedani and Lasia [101] electrodeposited adherent metallic films of Ni-Mo-P from
citrate-based electrolyte with pH 9 and temperature of 30oC at current density of 200 mA
cm-2. The obtained alloys were with phosphorous percentage between 2 to 10%. The
electrocatalytic activities of Ni-Mo-P alloy toward hydrogen evolution have been studied in
alkaline solution of 1M of NaOH by steady-state polarization. At 70oC, highest
overpotential value of 591 mV at 250 mA was obtained from pure Ni electrode. While, a
reduction of 252 mV was achieved from the electrode Ni74Mo16P10 which contains the
highest percentage of phosphorous. However, the highest reduction of overpotential of 436
mV was obtained from electrode Ni50Mo45P5 which means that HER depends on combined
percentage of Mo and P.
Different coatings of Co-Mo-P were electroplated from citrate baths containing ammonia
and hydrazine in order to study the relation between the composition, morphology and
corrosion resistance [76]. The Mo content in the alloy hardly changes with an increase in
the current density, while the P increases with increasing current density. The corrosion
resistance of Co-Mo-P coatings is found to be increased with increasing the molybdenum
content and decreased with increasing the phosphorous content in the alloy which means
40
the phosphorous content does not produce a positive effect on the corrosion resistance of
Co-Mo-P alloy [76].
2.3. Electrochemical Test Methods (Approach and Evaluation)
Electrochemistry concerns the study of the chemical response of a system to an
electrical stimulation. Electrochemistry studies the loss of electrons (oxidation) or gain of
electrons (reduction) that a material undergoes during the electrical stimulation. These
reduction and oxidation reactions are commonly known as redox reactions and can provide
information about the concentration, kinetics, reaction mechanisms, chemical status and
other behaviors of species in the electrolyte. Similar information can be obtained
concerning the electrode surface or the electrode/electrolyte interface. In an electrochemical
experiment, many parameters such as: potential (E), current (i), charge (Q), resistances (Ω)
and time (t) can be measured. The response of a system depends on which parameter is
used as the excitation signal. Useful information can be obtained by plotting different
parameters in different ways [102].
In most electrochemical techniques, there are three electrodes: the working electrode, the
reference electrode and the counter (or auxiliary) electrode. The three electrodes are
connected to a potentiostat, an instrument which controls the potential of the working
electrode and measures the resulting current. In one typical electrochemical experiment, a
potential is applied to the working electrode and the resulting current measured, then
plotted versus time. In another, the potential is varied and the resulting current is plotted
versus the applied potential [103].
During electrolysis, there are certain useful electrochemical techniques such as: (1)
Galvanostatic Polarization, (2) Potentiodynamic Polarization, (3) Cyclic Voltammetry, and
(4) Electrochemical Impedance Spectroscopy (EIS) to study the kinetics in this process.
Kinetics is the study of the rate at which reactions occur. The oxidation and reduction
reactions on a metal each at a potential polarized from its equilibrium value. A general
definition of polarization is ‘’the deviation in potential of an electrode as a result of the
41
passage of current’’ the amount of polarization is refereed to overpotential and generally is
assigned by the symbol (ɳ) [104].
2.3.1. Galvanostatic Polarization Technique
The galvanostatic polarization technique measures the polarization behavior of an
electrode by applying a constant current or controlling a constant current scan rate while
monitoring the potential response to the current. Galvanostatic polarization is used also to
carry out the electrodeposition of metals at constant current through their dissolved ions in
an electrolyte. The potential of a metal in an aqueous solution is a function of the inherent
reactivity of the metal and the oxidizing/reducing power of the solution. The goal of metal
potential measurements is to measure the potential without affection, in any way,
electrochemical reactions on the metal surface. Accordingly, the potential measurements
are necessary to be made with respect to a stable reference electrode so that any changes in
the measured potential can be attributed to changes at the metal/solution interface
[103,105].
2.3.2. Potentiodynamic Polarization Technique
Potentiodynamic polarization technique permits the measurement of polarization
behavior by continuously scanning the potential while monitoring the current response.
This experimental method permits the easy automation of curves and real time plots of the
experimental data [103,106]. Measurements of current-potential relations under carefully
controlled conditions can yield information on corrosion rates, coatings and films,
passivity, pitting tendencies as well as kinetics studies. When a metal specimen is
immersed in a solution, both reduction and oxidation processes occur on its surface.
Typically, the specimen oxidizes (corrodes) and the medium (solvent) is reduced. In acidic
media, hydrogen ions are reduced and hydrogen gas is evolved. The specimen must
function as both anode and cathode and both anodic and cathodic currents occur on the
specimen surface [106].
42
Any corrosion processes that occur are usually a result of anodic currents. When a
specimen is in contact with a corrosive liquid and the specimen is not connected to any
instrumentation – as it would be “in service” – the specimen assumes a potential (relative to
a reference electrode) termed the corrosion potential, Ecorr. A specimen at Ecorr has both
anodic and cathodic currents present on its surface. However, these currents are exactly
equal in magnitude so there is no net current to be measured. The specimen is at
equilibrium with the environment even though it may be visibly corroding. Ecorr can be
defined as the potential at which the rate of oxidation is exactly equal to the rate of
reduction. If the specimen is polarized slightly more positive than Ecorr, then the anodic
current predominates at the expense of the cathodic current. As the specimen potential is
driven further positive, the cathodic current component becomes negligible with respect to
the anodic component. A mathematical relationship exists which relates anodic and
cathodic currents to the magnitude of the polarization. Obviously, if the specimen is
polarized in the negative direction, the cathodic current predominates and the anodic
component becomes negligible. Experimentally, one measures polarization characteristics
by plotting the current response as a function of the applied potential. Since the measured
current can vary over several orders of magnitude, usually the log current function is
plotted vs. potential on a semi-log chart. This plot is termed a potentiodynamic polarization
plot or curve (Figure 2.11) [107-110]. This plot can provide many kinetics parameters such
as overpotential (ɳ), Tafel Slope (b), and exchange current density (I0 or J0) which are
important parameters to understand the electrochemical reaction and electrode behavior in
known medium.
43
Figure 2.11. Potentiodynamic polarization plot [110]
2.3.3. Cyclic Voltammetry Technique
Cyclic voltammetry (CV) technique is normally used to study qualitative
information about electrochemical processes at stationary non-agitated interface under
various conditions, such as the presence of intermediates in oxidation-reduction reactions,
the reversibility of a reaction through the determined peaks in the obtained E-I curve during
the polarization of the electrode [111]. A single CV experiment only hints at the events that
constitute the electrochemical reaction at the electrode. However, multiple CV experiments
can be used for a variety of applications, including:
The determination of reversible or irreversible behavior of a redox reaction.
The number of electrons transferred in an oxidation or reduction.
Nucleation overpotential potential for reduction reaction.
Formal potentials.
Rate constants.
Formation constants.
Reaction mechanism.
In a CV experiment, the potentiostat applies a potential ramp to the working electrode to
gradually change potential and then reverses the scan, returning to the initial potential
(Figure 2.12) [102].
44
Figure 2.12. Theoretical cyclic voltammogram [102]
Single CV experiment could then reflect the influence of an additive or change in the
electrolyte composition on the electrochemical properties of the interface. This is
considered mainly in zinc electrowinning process to determine the formal reduction and
nucleation overpotential (NOP) [42].
2.3.4. Electrochemical Impedance Spectroscopy Technique
An electrochemical reaction at the electrode/electrolyte interface cannot be fully
understood by using traditional electrochemical measurements. A complete description
requires impedance measurements made over a broad frequency range at various potentials
and determination of all the electrical characteristics on the interface which can be thought
of as a thin capacitor that forms between the charged electrode and the counter ions lined
up parallel to it [112]. Electrochemical Impedance Spectroscopy (EIS) or ac impedance
methods have seen tremendous increase in popularity in recent years. Initially applied to the
determination of the double-layer capacitance and in ac polarography, they are now applied
to the characterization of electrode processes and complex interfaces. EIS studies the
system response to the application of a periodic small amplitude ac signal. These
measurements are carried out at different ac frequencies and, thus, the name impedance
spectroscopy was later adopted. Analysis of the system response contains information about
the interface, its structure and reactions taking place there. EIS is now described in the
general books on electrochemistry, and there are also numerous articles and reviews [113].
45
EIS is an electrochemical method in which an ac signal is used. This signal is applied to an
electrode, and the response is measured. Usually a small voltage signal is applied and the
resulting current is measured. The measuring equipment processes the current-time and the
voltage-time measurements to provide the impedance at different frequencies, the
impedance spectrum [114].
Representations of the electrified interface have gradually evolved from repeated
modifications of the model first proposed by Helmholtz (Figure 2.13a) [115]. In a simple
case, the interface can be modeled by an equivalent circuit (Figure 2.13b). In Figure 2.13a,
the oxidants (red) with a positive charge diffuse toward the negatively charged electrode
(cathode) at the interface, the oxidants is also a counterion to the electrode. IHP and OHP
are the inner and outer Helmholtz planes, respectively. In Figure 2.13b, an equivalent
circuit representing each component at the interface and in the solution during an
electrochemical reaction is shown for comparison with the physical components; double
layer capacitor (Cd or Cdl), polarization resistor (Rp), Warburg resistor (W), and solution
resistor (Rs) [112].
Figure 2.13. (a) Simple electrified electrode/electrolyte interface, (b) Electronic components for the same interface [112]
Nyquist plot, sometimes known as a complex plane plot, this is a plot of imaginary part of
the impedance Z'' against the real part Z'. Since the majority of the responses of corroding
metals have negative Z'', it is conventional, for corrosion studies, the plot –Z'' against Z'.
46
Consider the response of the resistor-capacitor circuit in the top part of Figure 2.14. The
expressions for the real and the imaginary parts of the impedance are given in equations
(2.23 & 2.24) and, when plotted, give Figure 2.14. Each point of the plots corresponds to
the impedance (or admittance) at one frequency [114]. The points trace out of a semicircle,
with center at Z' = Rs + Rp/2, Z''=0, and diameter Rp. As apparent from equations 2.23 &
2.24 the solution resistance can be obtained from the high frequency intercept with the real
axis, and the total resistance, Rs+Rp, from the low frequency intercept. Where, Rct,
represents the charge transfer resistance.
Z = R/1+ω2C2R2 – jωCR2 /1+ω2C2R2 = Z' + jZ'' (2.23)
Z' = Rs + R/1+ω2C2R2 – jωCR2 /1+ω2C2R2 (2.24)
This curve is easy to be obtained by potentiostat in order to determine all components of the circuit.
Figure 2.14. Nyquist impedance plot for the showed circuit for Rs, Rct and Cdl [114]
2.4. Summary
Many studies and directed researches projects have been done on the effect of
additives during zinc electrowinning. Since the effect of Mn2+ has been reported on the
cathode behavior, and barely of that on the cathode [30,31,33]. So, it is interesting then to
examine the effect of organic additives in presence of manganese ion on the cathode
behavior. A common average concentration of 8 g dm-3 of Mn2+ is chosen as currently
considered by the Canadian industrial electrolytes.
47
Considering Sb3+ alone, it has a very harmful effect on current efficiency and morphology
of zinc deposit, however it shows a good effect once it is combined at low concentrations
with some additives such as gelatin and glue [37,41]. So, it is interesting also to study in
detail the combined effect of Sb3+ and Mn2+ in presence of the selected organic additives on
zinc electrodeposition process. Lead contamination of the zinc deposit should be considered
due to its importance on the zinc deposit quality.
Recently, ionic liquids have been used in many chemical and hydrometallurgical
applications due to their chemical and physical properties [64]. As reported here, only one
team studied the effect of ionic liquid on zinc electrodeposition in standard basic
electrolyte. So, the goal is to examine the effect of different cationic and anionic groups of
ionic liquids in presence of Pb2+, Sb3+, and Mn2+ during electrowinning and zinc deposit
contamination by lead.
Considering the HER during the sodium chlorate production by using Fe-Mo coated
steel cathode, a positive effect of phosphorous addition to the sodium chloride brine
electrolyte on HER was reported [75]. Also, Fe, Mo, and P were vastly doped with different
elements in the form of binary and ternary alloys as cathodes for chlorate and chlor-alkali
productions [97,101]. However, to our knowledge, the electrodeposition and
electrocatalytic activity of Fe-Mo-P alloys towards HER have not been reported yet.
During electrolysis, the electrochemical techniques such as: (1) galvanostatic
polarization, (2) potentiodynamic polarization, (3) cyclic voltammetry, and (4)
electrochemical impedance spectroscopy are found to be appropriate in studying the
kinetics in this process. Also other physical techniques such as SEM, XRD, and ICP are
found to be supporting techniques for evaluation and better understanding of the
electrometallurgical process.
48
CHAPTER 3
EXPERIMENTAL
49
This chapter discusses the assigned experimental protocol including all
experimental steps and conditions such as: electrolyte preparation, electrodes fabrication,
cell set-up as well as electrochemical and physical techniques.
3.1. Electrolyte and Set-up
3.1.1. Zinc Electrolyte and Materials Preparation
(a) Standard Electrolyte Preparation:
A standard acidic zinc sulphate electrolyte (SE) was prepared from the following content:
60 g dm-3 of Zn2+ (ZnSO4.7H2O), 180 g dm-3 of H2SO4 (Conc. 98%) and 8 g dm-3 of Mn2+
(MnSO4.H2O). All chemicals were dissolved in distilled water with continuous stirring at
room temperature till the solution became totally homogenous. The determined pH of
solution was very acidic (≤0.5) (H). The prepared electrolyte SE contained the following
impurities: Pb (0.003% = 30 ppm), Fe (0.001% = 10 ppm), and Na (0.05% = 500 ppm).
Reagents were supplied from Lab mat and VWR Canada.
(b) Introduced Additives to the Electrolyte:
The examined additives were dissolved separately in distilled water by using resonator in
order to have homogenous solutions. A quantity of 100 mg of each additive was dissolved
individually in 100 ml of distilled water giving then 1 mg dm-3 for each 1 ml. Additives
were added to the SE with concentrations (1,3,5,10 and 40 mg dm-3) individually. Also, the
effect of 1 and 3 mg of each additive was studied in combination with 0.0055 mg dm-3 of
Sb3+ (KSbC4H4O7.5H2O) as another impurity. The selected additives were supplied from
Sigma-Aldrich USA.
(c) Electrolysis Conditions and Electrodes Fabrication:
The electrolysis was performed in 1000 cm3 solution in double-glazed beaker. The solution
was heated by the flow of thermostated water in the double-glazed wall to reach the
working temperature 38 ± 1oC and solution was agitated by using magnetic agitator at 60
rpm. All used electrodes of pure Aluminum (>99.95%), Pb-Ag (Ag, 0.7%) and Platinum
50
were connected to electric wires using a conductive two components silver epoxy adhesive
(MG Chemicals No. 8331) and were casted in polyester resin (SamplKwick 20-3566
Buehler) in order to have only the required exposed surface. The metallic electrodes were
manually polished by several grits of SiC papers (80, 320, 600 and 1200) to give uniform
shiny surface. Then, they were washed by distilled water, ethanol and dried few seconds
before immersion in the solution. Ag, AgCl/KCl(sat) (0.199 V/SHE) was used as reference
electrode.
3.1.2. Fe-Mo & Fe-Mo-P Electrolytes and Materials Preparation
FeSO4.7H2O, Na2MoO4.2H2O and NaH2PO2.H2O were employed as the sources of
Fe, Mo and P, respectively. Along with trisodium citrate dihydrate (Na3C6H5O7.2H2O) used
as a complexing agent and controller of the reduction rate. Four different electrolytes were
prepared according to concentrations in Table 3.1. All reagents were dissolved in double
distilled water with continuous magnetic stirring at room temperature. pH 6 was adjusted
by using citric acid. During the electrolysis, the electrolyte was heated at 30oC by the flow
of thermostated water in the double-glazed wall and magnetically agitated at 300 rpm.
Table 3.1. Fe-Mo and Fe-Mo-P electrolytes compositions
Electrolyte Electrolytes compositions (g dm-3)
Na3C6H5O. 2H2O
FeSO4. 7H2O
Na2MoO4. 2H2O
NaH2PO2.H2O
I 120 10 50 0
II 120 10 50 10
III 120 10 70 10
IV 120 10 70 30
Electrodes were prepared by the same method described in the previous section. Mild steel
and platinum foil with the surface of 1 cm2 were used as cathode and anode electrodes,
respectively. Both anode and cathode electrodes were mounted in epoxy resin and
assembled in a three-electrode cell. The reference electrode was a silver chloride electrode
with a saturated KCl double junction Ag, AgCl/KClsat (0.199 V vs. standard hydrogen
electrode (SHE).
51
3.1.3. Set-up
The electrodes were mounted in Three-electrode cell as the schematic Figure 3.1. The
cathode and anode were adjusted with inter-electrode distance of 2 cm. The reference
electrode Ag, AgCl/KCl(sat) was adjusted in the cell to be far from the working electrode
few millimetres. The three-electrode cell was connected to a potentiostat, Gamry Reference
3000 – Gamry USA to carry out the electrodeposition and the other electrochemical tests.
The potentiostat was connected to Dell PC for data output (Figures 3.2).
Figure 3.1. Schematic experimental set-up for three-electrode cell [116]
Figure 3.2. Electrolysis cell set-up
52
3.2. Electrochemical Techniques and Measurements
3.2.1. Galvanostatic Polarization
The galvanostatic polarization technique is typically used for special applications
such as measuring the potential of an electrode during the electrodepoistion process using
constant current and time [103]. During the current study of zinc electrowinning process,
galvanostatic polarization has been employed to carry out the zinc electrodeposition on Al
cathode at 50 mA cm-2 for 2 hours also to measure the corresponding cathodic and anodic
potentials during electrodeposition. For Fe-Mo and Fe-Mo-P alloys electrodeposition was
performed on mild steel (MS) substrates at 20 mA cm-2 for 6 hours. Each experiment is
done in duplicates and average was taken.
3.2.2. Current Efficiency Calculations
After electrolysis the cathode was dried and current efficiency was calculated by
weight using Faraday’s law:
% . .. .
%
Where W is the weight of deposit (g), F is Faraday’s constant (96500 C mol-1), n is the
number of electrons (2 electrons for Zn), I is the total cell current (A), t is the time of
electrodeposition in seconds and M is the atomic weight of metal [117].
3.2.3. Power Consumption Calculations
From the combination of Joule’s law and Faraday’s law the power consumption is
calculated by following relation:
/
Where, PC is power consumption (kWh ton-1), Vcell is total cell voltage (volt) obtained from
galvanostatic polarization, Ic the applied current (A), t is the deposition time (seconds) and
W is the weight of deposited metal (g) [117].
53
3.2.4. Potentiodynamic Polarization
The potentiodynamic polarization for the deposit has been carried out by using 1
cm2 of deposited cathode as working electrode and 1 cm2 of platinum as auxiliary electrode,
also, Ag, AgCl/KCl(sat) was used as reference electrode. The three-cell electrode cell was
connected to the potentiostat Gamry Reference 3000 – Gamry USA. Based on the
experimental approach and previous studies, polarization for zinc deposit was carried out
from -1.05 to -1.25 V with scan rate of 5 mV s-1, while for MS, Fe-Mo and Fe-Mo-P was
carried out from -1.2 to 0.5 V with a scan rate of 1 mV s-1 in a solution containing 300 g
dm-3 of NaCl and 4 g dm-3 of K2Cr2O7 at 80oC and magnetic agitation of 80 rpm [75]. The
pH 6.4 was always adjusted using NaOH. The parameters measured from this technique
are:
i. Cathodic Tafel slope (bc): This can be obtained by selecting two points on the
cathodic curve, first point is far by ~20-40 mV from the corrosion potential and the
other point is far by one decade of current density [118]. Also it is an important
parameter to measure the I0 (exchange current density) (Figure 3.3).
ii. Exchange current density (I0 or J0): is defined as the current flowing in both
directions per unit area when an electrode reaction is at equilibrium (and, hence, at
its equilibrium potential). If I0 is small, then little current flows and the reactions at
dynamic equilibrium are generally slow. Likewise, a high I0 gives a fast reaction.
The metal itself affects the value of I0, even if the reaction does not involve the
metal directly. I0 can be estimated by extrapolating the Tafel slopes to the
corresponding zero current-potentials (Figure 3.3).
iii. Cathodic overpotential (ɳc): overpotential is the potential difference between a half-
reaction's thermodynamically determined reduction potential and the potential at
which the redox event is experimentally observed in the same conditions of
electrolytes. It can be determined from following equations:
, 0.763 2⁄ / (3.1)
, 0.0 ⁄ / . (3.2)
ɳ , , (3.3)
ɳ , (3.4)
54
Where; Ee is the equilibrium potential, R is the gas constant (equal to 8.314 mol-1 K-1), F is
the Faraday’s constant (equal to 96 500 C mol-1) and Em is the measured potential at 50 mA
in case of zinc electrodeposits or 250 mA in case of Fe-Mo and Fe-Mo-P deposits
[106,117].
Figure 3.3. Polarization curve potential vs current density (log i) [119]
3.2.5. Cyclic voltammetry
Cyclic voltammetry (CV) technique is normally used to study qualitative
information about electrochemical processes at stationary non-agitated interface under
various conditions, such as the presence of intermediates in oxidation-reduction reactions
[111]. CV could then reflect the influence of an additive or change in the electrolyte
composition on the electrochemical properties of the interface. This is considered mainly in
this work to determine the formal reduction and nucleation overpotential (NOP). The effect
of each additive alone or combined with antimony on the reduction of zinc ions on
aluminum cathode was studied by using cyclic voltammetry polarization [42].
Polarization for zinc deposit was carried from initial potential of -1.30 V to areversible
potential of -0.60 V at 38oC in presence of atmospheric air without agitation. NOP is the
55
difference between the crossover potential and the start of the dissolution and the point at
which the Zn begins to deposit. This could be a useful parameter to identify the best
additive concentration ratio with antimony ions.
3.2.6. Electrochemical Impedance Spectroscopy
EIS measurements for MS, Fe-Mo and Fe-Mo-P deposits were preformed in a
solution containing 300 g dm-3 of NaCl and 4 g dm-3 of K2Cr2O7 at 80oC and magnetic
agitation at 80 rpm. It was scanned over the frequency range from 100 kHz to 0.01 Hz, an
ac signal of 50 mA for galvanostatic mode at 250 mA cm-2.
3.3. Deposit Examination Techniques
3.3.1. Scanning Electron Microscopy (SEM)
A scanning electron microscopy (SEM) is a type of electron microscope that
produces images of a sample by scanning it with a focused beam of electrons. The electrons
interact with atoms in the sample, producing various signals that can be detected and that
contain information about the sample's surface morphology and composition. The electron
beam is generally scanned in a raster scan pattern, and the beam's position is combined with
the detected signal to produce an image. SEM can achieve resolution better than 1
nanometer. Specimens can be observed in high vacuum, in low vacuum, in wet conditions
(in environmental SEM), and at a wide range of cryogenic or elevated temperatures [120].
In this study the deposits were washed by distilled water then dried after the
electrodeposition process, X500 and X1000 images have been taken by using JEOL JSM-
840a in order to examine the morphology of deposited metals.
3.3.2. Energy Dispersive Spectroscopy (EDS)
An energy-dispersive (EDS) detector is used to separate the characteristic x-rays of
different elements into an energy spectrum, and EDS system software is used to analyze the
energy spectrum in order to determine the abundance of specific elements. EDS can be
56
used to find the chemical composition of materials down to a spot size of a few microns,
and to create element composition maps over a much broader raster area. Together, these
capabilities provide fundamental compositional information for a wide variety of materials
[121].
The Fe-Mo and Fe-Mo-P deposits were analyzed by using EDS detectors models JEOL
JSM-840a and FEI Quanta FEG 250 in order to determine the atomic composition
percentage of each element.
3.3.3. X-ray Diffraction (XRD)
X-ray diffraction (XRD) relies on the dual wave/particle nature of X-rays to obtain
information about the structure of crystalline materials. A primary use of the technique is
the identification and characterization of compounds based on their diffraction pattern
[122]. All deposited were analyzed using X-ray diffractor model Siemens - D5000 to
determine the crystallographic orientation and crystal/amorphous state.
3.3.4. Inductively Coupled Plasma (ICP)
Inductively coupled plasma (ICP) techniques can be very powerful tools for
detecting and analyzing trace and ultra-trace elements. Over the past years, ICP has become
the technique of choice in many analytical laboratories for providing the accurate and
precise measurements needed for today’s demanding applications and for providing
required lower limits of detection [123].
The zinc deposits were analyzed by using ICP model Optima 4300 Perkin-Elmer in order to
determine the lead concentrations in zinc deposits.
57
CHAPTER 4
INFLUENCE OF DIFFERENT ORGANIC ADDITIVES IN ZINC ELECTROWINNING
FROM ACIDIC SULPHATE ELECTROLYTE
58
Influence of Different Organic Additives in Zinc Electrowinning from Acidic Sulphate Electrolyte
N. Sorour1,*, W. Zhang1, G. Gabra1, E. Ghali1, and G. Houlachi2
1Department of Mining, Metallurgical and Materials Engineering, Laval University, Québec, Canada, G1V 0A6.
2Hydro-Québec research centre (LTE), Shawinigan, QC, Canada, G9N 7N5.
*Corresponding author: Tel: 418 656-2131 - Fax: 418 656-5343 ([email protected])
Published by Canadian Institute of Mining, Metallurgy and Petroleum. CIM-COM, paper
no. 8986, pp 1-13, ISBN: 978-1-926872-32-2.
Résumé
Les additifs polyacrylamide, chlorure de 1-butyl-3-méthylimidazolium, bromure de tétra-butylammonium, chlorure de benzalkonium et chitine ont été évalués durant l'électrolyse du zinc à partir de l’électrolyte synthétique acide de sulfate contenant des ions de Mn2+, en absence et en présence de 0,0055 mg dm-3 des ions de Sb3+. Des expériences de polarisation galvanostatique pendant 2 heures à 50 mA cm-2 et 38°C ont été effectuées afin de déterminer les potentiels cathodique et anodique, et le rendement du courant de zinc déposé. Les expériences de polarisation potentiodynamique et voltamétrie cyclique ont également été utilisées pour étudier le comportement électrochimique de chaque additif sur la déposition de zinc sur l'électrode d'aluminum. Les résultats montrent qu’en présence de Sb3+, la tension de la cellule augmente d’environ 0 à 7 mV en ajoutant de 1 à 3 mg de chlorure de 1-butyl-3-méthylimidazolium. Le rendement du courant a été augmenté d’environ 4,9-6,4%; aussi la consommation d'énergie a été réduite de ≈147-173 kWh tonne-
1 en ajoutant de 1 à 3 mg. Cependant, en absence de Sb3+, une diminution de la tension de la cellule d’environ 7-28 mV, une augmentation du rendement du courant d’environ 0,70-1,50%; une diminution de la consommation d'énergie de ≈41-47 kWh tonne-1 ont été réalisées en ajoutant de 1 à 3 mg de chlorure de 1-butyl-3-méthylimidazolium. L'effet des autres additifs sur la tension de la cellule, le rendement du courant, la teneur en plomb et la morphologie du dépôt de zinc a également été examiné. La morphologie de surface et l'orientation cristallographique du dépôt ont été étudiées en utilisant le microscope électronique à balayage (MEB) et la diffraction des rayons-X (DRX). La teneur en plomb dans le dépôt a également été mesurée en utilisant la spectroscopie de plasma à couplage inductif (PCI).
59
Abstract
The additives Polyacrylamide, 1-Butyl-3-methylimidazolium chloride, Tetra-butylammonium bromide, Benzalkonium chloride and Chitin are evaluated during zinc electrolysis from synthetic acidic sulphate electrolyte containing Mn2+ ions, in absence and in presence of 0.0055 mg dm-3 of Sb3+ ions. Galvanostatic polarization tests for 2 hours at 50 mA cm-2 and 38oC were carried out to determine the cathodic and anodic potentials, and current efficiency of the deposited zinc. Potentiodynamic polarization and cyclic voltammetry tests have also been employed to study the electrochemical behavior of each additive on the zinc deposit on aluminum electrode. Results showed that, in presence of Sb3+, adding 1 to 3 mg of 1-butyl-3-methylimidazolium chloride increases the cell voltage by ≈0-7 mV. Current efficiency is increased by ≈4.9-6.4%; power consumption is reduced by ≈147-173 kWh ton-1 by adding 1-3 mg, respectively. However, in absence of Sb3+, cell voltage is decreased by ≈7-28 mV, current efficiency is increased by ≈0.70-1.50%; power consumption is reduced by ≈41-47 kWh ton-1 by adding 1-3 mg. The effect of the other additives on cell voltage, current efficiency, lead content and morphology of zinc deposit has also been examined. Surface morphology and crystallographic orientation of the deposit was studied using scanning electron microscopic (SEM) and X-ray diffraction (XRD), respectively. The content of lead in the deposit has also been measured using inductively coupled plasma spectroscopy (ICP).
60
4.1. Introduction
Zinc is a common base metal with wide uses; it is used for fabrication of metal
components in the form of diecasting alloys and brasses. The major use of zinc which
cannot be neglected is the corrosion protection or galvanising of steel, this protection is
achieved by forming surface barrier as well as by corroding preferentially to the coated
steel [7,12]. Most of global pure zinc metal is produced via electrowinning process from
acidic sulphate electrolyte [124]. This process is very sensitive to the harmful effect of Pb
impurity coming from the used lead-based anodes and to the other presented metallic
impurities in the electrolyte such as: Sb, Fe, Cu, Co, Ni ...etc. [59]. Most of these metallic
impurities can reduce the zinc current efficiency, change in deposit’s morphology, and
change the cathodic and anodic polarizations. They can also assist the evolution of H2 gas
when sufficient amounts are presented in the electrolyte.
One of the considerable goals in zinc electrowinning is minimizing the power consumption
(PC). The two important factors which can determine the energy requirements are the
current efficiency (CE) and cell voltage which are affected negatively by the presence of
impurities [125]. Depending on the electrolysis conditions, one or several organic additives
may be added to the electrolyte in order to counteract the detrimental effects caused by
impurities [10]. The effect of organic additives in the electrolyte on the nature of the
crystallization presents one of the important aspects. Additives could be adsorbed
preferentially on the cathode to completely alter the growth of the deposit. Additives also
reduce the grain size by creation of more nucleation sites during the electrodeposition
[126]. They are also susceptible to decomposition by the presence of a large amount of
Mn2+ which is gradually oxidized to MnO4- or MnO2; this also can cause further alterations
in the electro-crystallization.
The most commonly used additives in industry are glues and gelatin which prompt the
deposit growth and minimize the negative effect of metallic impurities [54]. Gelatin
showed very good results in increasing current efficiency, reducing overpotentials,
producing better smooth and compact deposits in presence of traces of antimony [41].
Sodium lauryl sulphate [SLS] with low concentrations in presence of Sb (III) had been
examined as additive by Tripathy et al. [53]. It showed good results in increasing current
61
efficiency, reducing power consumption and improving surface morphology. In addition,
Triethyl benzylammonium [TEBA] [127-128], 1-butyl-3-methylimidazolium hydrogen
sulfate [BMIM]HSO4 [70], and Perfluorinates surfactant [129] have been considered as
additives to investigate their effects on the electrodeposition characteristics of zinc from
acidic sulphate electrolytes.
In the present work, five additives have been chosen from different organic groups, in order
to examine their effects individually on zinc electrowinning process in absence and in
presence of antimony ions. (1) Polyacrylamide [PAM] is one of the well known organic
polymers used in industry and has been tested as additive in copper electrowinning,
showing a good effect in improving morphology of the surface [130]. (2) 1-Butyl-3-
methylimidazolium chloride [BMIM]Cl represents the ionic liquids group. Ionic liquids are
widely used in liquid-liquid extraction and electrodeposition of some metals due to their
low melting point and the thermal degradation properties which are important in the
electrochemical media [67]. (3) Tetra-butylammonium bromide [TBABr] is one of the
quaternary ammonium salts group which have been examined also as additives. (4)
Benzalkonium chloride [BKCl] is a cationic surface-acting agent belonging to
the quaternary ammonium salts with aromatic ring. Finally, (5) Chitin has been studied as it
is one of the natural polymer compounds which can be found in
crabs, lobsters and shrimps.
4.2. Experimental
4.2.1. Electrolyte and Experimental Setup
A standard electrolyte (SE) was prepared from the following content: 60 g dm-3 of
Zn2+ (ZnSO4.7H2O), 180 g dm-3 of H2SO4 (Conc. 98%) and 8 g dm-3 of Mn2+ (MnSO4.H2O)
dissolved in double distilled water. The effect of different concentrations of 0,1,3,5,10 and
40 mg dm-3 of each additive added to the SE was studied individually. Also, the effect of 1
and 3 mg of each additive was studied in combination with 0.0055 mg dm-3 of Sb3+
(KSbC4H4O7.5H2O). The following impurities were determined in the SE: Pb (0.003% = 30
ppm), Na (0.05% = 500 ppm), Mg (0.005% = 50 ppm) and Fe (0.002% = 20 ppm).
Reagents were supplied from Lab mat and VWR Canada, while, the selected additives were
62
supplied from Sigma-Aldrich USA. An electrolysis cell was performed in 1000 ml solution
in double-glazed beaker. The solution was heated by the flow of thermostated water in the
double-glazed wall. The three-electrode cell used consisted of two plates of pure Al
(>99.95%) and one plate of Pb-Ag (Ag, 0.7%) as cathode and anode, respectively. The two
plates were cast in polyester resin with total exposed surface area of 1 cm2, they were
assembled in a Teflon holder. The inter-electrode distance was 2 cm. Ag, AgCl/KCl(sat) was
used as reference electrode. Both electrodes were manually polished by several grits of SiC
papers (80, 320, 600 and 1200) to give uniform shiny surface. Then, they were washed by
distilled water, ethanol and dried few seconds before immersion in solution.
The three-electrode cell was connected to a potentiostat, Gamry Reference 3000 – Gamry
USA. All elecrodeposition experiments were carried out for 2 hours at 50 mA cm-2 and 38
± 1oC with magnetic agitation of solution of 60 rpm. After electrolysis the cathode was
dried and current efficiency was calculated by weight using Farady’s law: CE% =
(W.F.n/I.t.M) x 100%; where W is the weight of deposit (g), F is Faraday’s constant, n is
the number of electrons, I is the total cell current (A), t is the time of electrodeposition and
M is the atomic weight of zinc.
4.2.2. Deposit Examination
Morphology of the surfaces of deposits was examined by scanning electron
microscopy (SEM) using JEOL JSM-840a. The crystal orientation of the zinc deposit was
determined using X-ray diffractometer model Siemens - D5000. Lead concentration in the
deposit has been measured using inductively coupled plasma (ICP) model Optima 4300
Perkin-Elmer.
4.2.3. Potentiodynamic Polarization and Cyclic Voltammetry
Electrochemical studies such as potentiodynamic polarization and cyclic
voltammetery measurements were preformed. The three-electrode cell consisted of 0.30
cm2 of Al (>99.95%) as working electrode and 1 cm2 of Pt as auxiliary electrode. Also a
Ag, AgCl/KCl(sat) was used as reference electrode. The cell was connected to the
63
potentiostat Gamry Reference 3000 – Gamry USA. The cathodic potentiodaynamic
polarization was carried out from -1.00 to -1.25 V with a scan rate of 5 mV s-1 at agitation
of 60 rpm. Cyclic voltammetric scanning was conducted from an initial potential of -1.30 to
the final potential of -0.60 V at a constant scanning rate of 10 mV s-1 without agitation.
Both tests were done at 38oC under atmospheric conditions. Working electrodes were
manually polished with SiC abrasive paper (Leco Corporation) down to 1200 grit before
each experiment and washed by distilled water, ethanol and dried few seconds prior to the
experiment.
4.3. Results and Discussion
4.3.1. Power Consumption and Current Efficiency
The effects of addition of PAM, [BMIM]Cl, TBABr, BKCl and Chitin on current
efficiency, cell voltage and accordingly on power consumption have been studied in the
range of 0-40 mg dm-3. Table 4.1 shows the variance by changing the quantity of each
additive to the SE as it varies from one additive to another. Results showed that cell voltage
was increased gradually by low concentrations of 1-5 mg dm-3 of additives in absence of
antimony then started to be increased significantly by the quantity of additives from 10 to
40 mg dm-3. Addition of [BMIM]Cl showed the best results in reducing cell voltage by ≈28
and 7 mV by adding 1 and 3 mg dm-3, respectively, as compared to that obtained from the
standard electrolyte, SE. However, the other additives had negative effect on reducing cell
voltage, leading to an increase of overpotential from ≈2 to 127 mV. The Highest
overpotential was caused by adding 40 mg dm-3 of BKCl. In presence of antimony,
additives showed also an increase in cell voltage from range of ≈0-47 mV.
Effects of additives on power consumption (PC) were also investigated in absence and in
presence of antimony (Table 4.1). In absence of antimony, the maximum reduction of PC
was obtained from [BMIM]Cl as it was reduced by ≈41-47 kWh ton-1 due to the addition of
1-3 mg dm-3, respectively. On the other hand, the highest value of PC of 3303 kWh ton-1
was observed by adding 40 mg dm-3 of PAM, followed by 2952 kWh ton-1 with 40 mg dm-3
of BKCl as compared to that of SE (2560 kWh ton-1). Most of the additives showed good
results in decreasing PC in presence of 0.0055 mg dm-3 of antinomy. Results showed that
64
the maximum reduction of ≈147-173 kWh ton-1 was obtained from adding 1-3 mg dm-3 of
[BMIM]Cl, respectively, followed by a reduction of ≈111-139 kWh ton-1 obtained by
adding 1-3 mg dm-3 of PAM. Although addition of PAM showed negative results in
reducing PC in absence of Sb3+, it showed good results in presence of Sb3+, acting more or
less as gelatin in removing the harmful effect of antimony on hydrogen evolution reaction
(HER). Antimony addition to the electrolyte decrease the HER overpotential as it catalyzes
this reaction leading to a decrease in the current efficiency and then a porous deposit is
obtained. Addition of the additive inhibits the HER and favorites the reduction of zinc ions
due to their adsorption on the cathode and smooth deposit can be obtained accordingly.
Addition of BKCl showed a minimum reduction of PC of ≈33-49 kWh ton-1, while addition
of TBABr and chitin has given a medium reduction values. Accordingly, the power
consumption values in SE with additives in presence of Sb3+ decreased in order of:
[BMIM]Cl > PAM > TBABr > Chitin > BKCl.
The values of current efficiencies are also plotted in Figure 4.1. Results showed that,
current efficiency (CE) values obtained from SE in absence and in presence of Sb3+ were
92.8% and 88.7%, respectively. Figure 4.1a shows that maximum values of current
efficiencies obtained in the absence of antimony were 94.3% and 94.0% by adding 3 and 5
mg dm-3 of [BMIM]Cl to SE solution, respectively. Addition of 10 mg dm-3 of TBABr
showed also an increase of 1.2% more than that obtained from SE. Addition of 1 mg dm-3
of [BMIM]Cl and 5 mg dm-3 of TBABr showed the same value of 93.5%. Generally,
increasing the concentrations of additives ≥10 mg dm-3 in the electrolyte leads to a decrease
in current efficiency values which could be explained by high adsorption of additives on the
cathode surface which could block the active sites and prevent further nucleation, such that
deposits start to be dissolved again in the acidic electrolyte.
Figure 4.1b shows the values of current efficiencies obtained by combination of 1 and 3 mg
dm-3 of each additive with 0.0055 mg of Sb3+. Results showed that the maximum CE
obtained was 95.1% from addition of 3 mg dm-3 of [BMIM]Cl to SE + Sb3+ as compared to
88.7% obtained from SE + Sb3+. This is followed by 94.8% from addition of 3 mg dm-3 of
PAM. This could explain since PAM has a very good effect on CE and accordingly on PC
in presence of antimony.
65
Table 4.1. Effect of PAM, [BMIM]Cl, TBABr, BKCl and Chitin on cell voltage, CE and PC in absence and in presence of Sb3+ during zinc electrodeposition for 2 h at 50 mA cm-2 and 38оC
Additive /mg dm-3
Sb (III) /mg dm-3
Cell voltage /V
CE /%
PC /kWh ton-1
SE 0 0 2.898 92.8 2560 0 0.0055 2.873 88.7 2655
PAM 1 0 2.900 92.0 2584 3 0 2.905 90.2 2640 5 0 2.920 87.8 2726 10 0 2.930 82.3 2918 40 0 2.970 73.7 3303 1 0.0055 2.892 93.2 2544 3 0.0055 2.910 94.8 2516
[BMIM]Cl 1 0 2.870 93.5 2516 3 0 2.891 94.3 2513 5 0 2.898 94.0 2527 10 0 2.908 93.4 2552 40 0 2.935 91.3 2635 1 0.0055 2.870 93.8 2508 3 0.0055 2.880 95.1 2482
TBABr 1 0 2.902 92.4 2575 3 0 2.912 93.0 2567 5 0 2.917 93.5 2557 10 0 2.965 94.0 2586 40 0 2.972 88.4 2756 1 0.0055 2.890 93.2 2542 3 0.0055 2.920 94.4 2536
BKCl 1 0 2.943 92.2 2617 3 0 2.982 91.0 2686 5 0 2.995 90.1 2725 10 0 3.010 89.3 2763 40 0 3.025 84.0 2952 1 0.0055 2.887 90.2 2606 3 0.0055 2.920 91.1 2622
Chitin 1 0 2.911 93.1 2563 3 0 2.933 93.9 2560 5 0 2.937 93.6 2572 10 0 2.944 92.9 2598 40 0 2.968 89.0 2734 1 0.0055 2.872 92.8 2537 3 0.0055 2.890 93.1 2545
66
Addition of TBABr followed by chitin also showed an increase in CE in presence of
antimony while, addition of BKCl had a slight effect on the CE as compared to the other
additives. Comparing the results of Figures 4.1a and 4.1b, it could be deduced that
additives in presence of antimony can increase the current efficiencies, while their addition
to SE frequently decrease the current efficiencies. This indicates that these additives
counteract the harmful effect of Sb and have synergetic effects on current efficiencies.
Figure 4.1. Effects of PAM, [BMIM]Cl, TBABr, BKCl and Chitin on current efficiency: (a) in absence of Sb3+ and (b) in presence of 0.0055 mg dm-3 of Sb3+ during zinc electrodeposition for 2h at 50 mA cm-2 and 38оC
70
74
78
82
86
90
94
98
0 5 10 15 20 25 30 35 40 45
CE
%
Qty. of additives in (mg)
SE
PAM
[BMIM]Cl
TBABr
BKCl
Chitin
(a)
88
90
92
94
96
98
0 1 2 3 4
CE
%
Qty. of additives in (mg)
SE + Sb
PAM
[BMIM]Cl
TBABr
BKCl
Chitin
(b)
67
4.3.2. Characterization of Deposits
The effect of 3 mg dm-3 of each additive in absence and in presence of Sb3+ on
deposit’s morphology and crystal orientation during 2 hours of zinc electrowinning was
examined using SEM and XRD, respectively. Results are listed in Table 4.2, and the
scanning electron micrographs are shown in Figures 4.2 and 4.3.
Results revealed that crystallographic orientation obtained from SE is (101) (102) (103)
(002) which changed to (101) (112) (102) (103) by adding antimony showing a decrease in
platelet size (Figures 4.2a and 4.3a). Addition of BKCl also has the same effect as antimony
alone as it showed a porous and small grain size. This could explain the decrease of current
efficiency by increasing the concentration of BKCl in the electrolyte, since hydrogen
evolution on cathode caused the porosity in the zinc deposit.
Table 4.2. Effects of PAM, [BMIM]Cl, TBABr, BKCl and Chitin on surface morphology, crystal orientation and lead contamination in absence and in presence of Sb3+ during zinc electrodeposition for 2h at 50 mA cm-2
Additive /mg dm-3
Sb (III) /mg dm-3
Crystal orientation /hkl
SEM (Figure)
Pb Conc. /ppm
SE 0 0 (101) (102) (103) (002) 4.2a 26.49 0 0.0055 (101) (112) (102) (103) 4.3a 03.64 PAM 3 0 (100) (101) (110) (201) 4.2b 23.80 3 0.0055 (101) (110) (112) (100) 4.3b 11.60 [BMIM]Cl 3 0 (101) (102) (103) (002) 4.2c 10.40 3 0.0055 (101) (102) (110) (112) 4.3c 10.90 TBABr 3 0 (101) (102) (100) (201) 4.2d 23.00 3 0.0055 (101) (100) (102) (201) 4.3d 12.40 BKCl 3 0 (101) (112) (102) (103) 4.2e 11.80 3 0.0055 (101) (002) (102) (103) 4.3e 11.10 Chitin 3 0 (101) (102) (103) (112) 4.2f 11.40 3 0.0055 (101) (102) (103) (112) 4.3f 11.30
68
Figure 4.2. Scanning electron micrographs (x1000) of zinc deposits in absence of Sb3+; (a) SE, (b) PAM, (c) [BMIM]Cl 3mg dm-3, (d) TBABr 3mg dm-3, (e) BKCl 3mg dm-3 and (f) Chitin 3mg dm-3
69
Figure 4.3. Scanning electron micrographs (x1000) of zinc deposits in presence of 0.0055 mg of Sb3+; (a) SE, (b) PAM 3mg dm-3, (c) [BMIM]Cl 3mg dm-3, (d) TBABr 3mg dm-3, (e) BKCl 3mg dm-3 and (f) Chitin 3mg dm-3
The addition of [BMIM]Cl to the SE had no effect in changing the preferred crystal
orientation in absence of Sb3+, while it was changed to (101) (102) (110) (112) in presence
of Sb3+. The given deposit had a moderate platelet size with compact and smooth surface
(Figure 4.3c). Addition of TBABr in absence of antimony had very high peak intensity at
2θ = 43.247 with crystallographic orientation (101) (102) (100) (201) (Figure 4.2d) which
slightly modified in presence of antimony. Addition of PAM to the SE changed the most
70
preferred orientation from (101) to (100) showing the highest peak intensity at 2θ = 39.013,
resulting in a needled deposit with low current efficiency (Figure 4.2b). Addition of PAM
in presence of Sb3+ restored the most preferred orientation to (101) giving smooth and
compact deposit which could explain the obtained high current efficiency of zinc in
presence of PAM combined with Sb3+ (Figure 4.3b). Addition of chitin showed a good
surface morphology in absence and in presence of Sb3+ with crystallographic orientation of
(101) (102) (103) (112) (Figures 4.2f and 4.3f).
The zinc deposits obtained were analyzed using inductively coupled plasma spectroscopy
(ICP) to determine the lead concentration, in order to examine the effect of each additive on
counteracting the lead contamination caused by the anode. Results in Table 4.2 show that
lead concentration found in zinc deposit obtained from SE was 26.49 ppm. Additions of
PAM and TBABr have approximately no effect in reducing this value, while other additives
succeeded in reducing it to the range of 10.40-11.80 ppm. The greatest reduction was in
presence of 3 mg of [BMIM]Cl. The lead concentration found in zinc deposit obtained from
SE with Sb was 3.64 ppm. This decrease in lead contamination in presence of Sb3+ could be
explained due to the effect of antimony in reducing the HER overpotential which cannot
reach to the required potential for the co-deposition of lead with zinc. Also, the effect of
high evolution of hydrogen bubbles could limit the access of reducible ions to the interface.
None of the additives succeeded in reducing this concentration in presence of antimony, but
rather they increased it in the range of 10.90-12.40 ppm.
4.3.3. Potentiodynamic Polarization
The effect of additives in the zinc electrolyte in absence and presence of antimony
ions on the cathodic polarization were examined by potentiodyamic polarization and cyclic
voltammetry scanning. Results of potentiodnamic polarization are plotted in Figure 4.4.
Addition of 1 mg dm-3 of [BMIM]Cl and chitin shifted slightly the polarization curves to
less negative potentials. The behavior of addition of 1 mg dm-3 of PAM and 3 mg dm-3 of
[BMIM]Cl gave more or less the same values as that of the standard electrolyte. With the
addition of TBABr and BKCl, even at low concentration of 1 mg dm-3, overpotentials were
increased during polarization. At high concentrations of the five additives, a remarkable
71
polarization occurred and the polarization curves are shifted to more negative potentials,
which increased the overpotentials. However in presence of Sb3+, addition of all additives
except for 1 mg dm-3 of [BMIM]Cl into SE increased the overpotentials on zinc deposit
during cathodic polarization.
The effects of additives on Tafel slopes and cathodic overpotential have been also
investigated. Results are given in Table 4.3. Results revealed that by increasing the
concentrations of additives the Tafel slope values gradually increased, and the values of
cathodic overpotential at 50 mA cm-2 were varied depending on the concentration of the
additive used. It could be stated that the addition of 1 or 3 mg dm-3 of [BMIM]Cl
corresponds to the best concentration in the zinc electrolyte as cathodic overpotential is
decreased to 3-14 mV. However, the other additives showed negative effect in reducing the
cathodic overpotential. The higest overpotential value of 401 mV was obtained in presence
of 40 mg dm-3 of BKCl in absence of antimony ions compared to 360 mV that obtained
from SE.
72
0
20
40
60
80
100
-1.25 -1.20 -1.15 -1.10 -1.05 -1.00
Cu
rren
t (m
A c
m-2
)
Potential vs Ag, AgCl/KCl (V)
SEPAM 1mgPAM 5mgPAM 40mgSE + SbPAM 1mg + SbPAM 3mg + Sb
(a)
0
20
40
60
80
100
-1.25 -1.20 -1.15 -1.10 -1.05 -1.00
Cu
rren
t (m
A c
m-2
)
Potential vs Ag, AgCl/KCl (V)
SE
[BMIM]Cl 1mg
[BMIM]Cl 5mg
[BMIM]Cl 40mg
SE + Sb
[BMIM]Cl 1mg + Sb
[BMIM]Cl 3mg + Sb
(b)
0
20
40
60
80
100
-1.25 -1.20 -1.15 -1.10 -1.05 -1.00
Cu
rren
t (m
A c
m-2
)
Potential vs Ag, AgCl/KCl (V)
SE
TBABr 1mg
TBABr 5mg
TBABr 40mg
SE + Sb
TBABr 1mg + Sb
TBABr 3mg + Sb
(c)
73
Figure 4.4. Effects of the additives on the cathodic polarization during zinc electrodeposition with different concentrations in absence and in presnce of antimony; (a) PAM, (b) [BMIM]Cl, (c) TBABr, (d) BKCl and (e) Chitin
4.3.4. Cyclic Voltammetry Measurements
The effect of each additive alone or combined with antimony on the reduction of
zinc ions on aluminum cathode was studied by using cyclic voltammetry polarization.
Polarizations were carried out from initial potential of -1.30 V to final potential of -0.60 V
without agitation at 38oC in presence of an electrolyte saturated with atmospheric air.
Figure 4.5 shows these results; also nucleation overpotential (NOP) values are listed in
Table 4.3.
0
20
40
60
80
100
-1.25 -1.20 -1.15 -1.10 -1.05 -1.00
Cu
rren
t (m
A c
m-2
)
Potential vs Ag, AgCl/KCl (V)
SE
BKCl 1mg
BKCl 5mg
BKCl 40mg
SE + Sb
BKCl 1mg + Sb
BKCl 3mg + Sb
(d)
0
20
40
60
80
100
-1.25 -1.20 -1.15 -1.10 -1.05 -1.00
Cu
rren
t (m
A c
m-2
)
Potential vs Ag, AgCl/KCl (V)
SEChitin 1mgChitin 5mgChitin 40mgSE + Sbchitin 1mg + Sbchitin 3mg + Sb
(e)
74
-400
-300
-200
-100
0
100
200
300
400
-1.30 -1.20 -1.10 -1.00 -0.90 -0.80 -0.70 -0.60
Cu
rren
t (m
A c
m-2
)
Potential vs Ag, AgCl/KCl (V)
SEPAM 1mgPAM 5mgPAM 40mg
A
BD C
(a)
-400
-300
-200
-100
0
100
200
300
400
-1.30 -1.20 -1.10 -1.00 -0.90 -0.80 -0.70 -0.60
Cu
rren
t (m
A c
m-2
)
Potential vs Ag, AgCl/KCl (V)
SE[BMIM]Cl 1mg[BMIM]Cl 5mg[BMIM]Cl 40mg
A
B CD
(b)
-400
-300
-200
-100
0
100
200
300
400
-1.30 -1.20 -1.10 -1.00 -0.90 -0.80 -0.70 -0.60
Cu
rren
t (m
A c
m-2
)
Potential vs Ag, AgCl/KCl (V)
SETBABr 1mgTBABr 5mTBABr 40mg
(c)
A
B CD
75
Figure 4.5. Cyclic voltammograms during zinc electrowinning using aluminum cathode with different concentrations of 0,1,5 and 40 mg dm-3 of: (a) PAM, (b) [BMIM]Cl, (c) TBABr, (d) BKCl and (e) Chitin
-400
-300
-200
-100
0
100
200
300
400
-1.30 -1.20 -1.10 -1.00 -0.90 -0.80 -0.70 -0.60
Cu
rren
t (m
A c
m-2
)
Potential vs Ag, AgCl/KCl (V)
SEBKCl 1mgBKCl 5mBKCl 40mg
(d)
A
B CD
-400
-300
-200
-100
0
100
200
300
400
-1.30 -1.20 -1.10 -1.00 -0.90 -0.80 -0.70 -0.60
Cu
rren
t (m
A c
m-2
)
Potential vs Ag, AgCl/KCl (V)
SEChitin 1mgChitin 5mgChitin 40mg
(e)
A
BD C
76
Table 4.3. Effects of additives on Tafel slopes, cathodic overpotential at 50 mA cm-2 obtained from potentiodynamic polarization versus Ag,AgCl/KCl(sat) and NOP obtained from cyclic voltammetry
Additive mg dm-3
Sb (III) mg dm-3
Tafel slope (bc) mV/decade
Cathodic Overpotential -ɳ(50) / (mV/Ref)
NOP mV
PAM
0 0 -123 360 62 1 0 -120 360 100 5 0 -126 364 108
40 0 -132 377 110 0 0.0055 -101 330 45 1 0.0055 -113 342 68 3 0.0055 -116 348 104
[BMIM]Cl 0 0 -123 360 62 1 0 -117 346 72 5 0 -121 357 70
40 0 -131 373 98 0 0.0055 -101 330 45 1 0.0055 -102 321 48 3 0.0055 -97 332 50
TBABr 0 1
0 0
-123 -125
360 362
62 76
5 0 -129 369 72 40 0 -141 390 79 0 0.0055 -101 330 45 1 0.0055 -120 369 60 3 0.0055 -124 374 68
BKCl 0 0 -123 360 62 1 0 -126 362 68 5 0 -131 377 70
40 0 -144 401 132 0 0.0055 -101 330 45 1 0.0055 -106 335 58 3 0.0055 -128 366 94
Chitin 0 0 -123 360 62 1 0 -119 352 60 5 0 -126 366 69
40 0 -138 380 98 0 0.0055 -101 330 45 1 0.0055 -106 337 56 3 0.0055 -118 350 82
77
The voltammograms were initiated at point (A) at potential of -1.30 V vs Ag, AgCl/KCl(sat),
scanned in the positive direction, and then reversed at -0.60 V in the negative direction,
crossed-over at point (B). No significant current was observed until the potential reached
the point (B), corresponding to the reduction of Zn2+ ions. NOP is the difference between
the crossover potential (B), the start of the dissolution and the point at which Zn begins to
deposit (D). This could be a useful parameter to identify the best additive concentration or
ratio with antimony ions [42]. Results revealed that measured NOP in SE was 62 mV, this
value was found to be increased gradually by increasing the concentration of additive in the
electrolyte. The value range of 60 mV was observed at 1 mg of chitin to 110 mV at 40 mg
dm-3 of PAM. The best combinations with antimony ions are found at 1 and 3 mg of
[BMIM]Cl corresponding to NOP range of 48-50 mV, respectively.
4.4. Conclusions
Five different additives from different organic groups have been examined
individually during zinc electrowinning process in order to study their effects on power
consumption (PC), current efficiency (CE), surface morphology, lead impurity and
electrochemical behavior. The results are as follows:
- The presence of additives in the standard electrolyte containing antimony could
increase the current efficiencies and counteract the harmful effect of Sb, while additives
decreased the current efficiencies in most of the cases in absence of antimony. In
presence of 0.0055 mg of antimony ions, maximum reductions of PC of ≈147-173 kWh
ton-1 were obtained from adding 1-3 mg dm-3 of [BMIM]Cl, respectively, followed by a
reduction of ≈111-139 kWh ton-1 by adding 1-3 mg dm-3 of PAM.
- The addition of [BMIM]Cl to the SE did not change the preferred crystal orientation in
absence of Sb3+, but it changed the orientation to (101) (102) (110) (112) in presence of
antimony, giving compact and smooth deposit with moderate platelet size. Addition of
PAM to the SE changed the most preferred crystal orientation from (101) to (100)
direction, showing a needled deposit with weak current efficiency, while in presence of
antimony, it restored the (101) giving deposit with very small grain size.
78
- The highest reduction of lead concentration in the deposit was obtained from 3 mg dm-3
of [BMIM]Cl in absence of antimony. Lead concentration found in zinc deposit
obtained from SE with Sb was 3.64 ppm. None of the additives succeed in reducing this
concentration in presence of antimony, but rather they increased it in the range of
10.90-12.40 ppm.
- Potentiodynamic technique showed that addition of 1 or 3 mg dm-3 of [BMIM]Cl
corresponds to an appropriate concentration in the zinc electrolyte as cathodic
overpotential at 50 mA cm-2 in presence of antimony was decreased by 3-14 mV,
respectively.
- Cyclic voltammetry technique revealed that the best combinations of additives with
antimony ions are found at 1 and 3 mg dm-3 of [BMIM]Cl, corresponding to nucleation
overpotential “NOP” range of 48-50 mV, respectively.
Acknowledgements
Zinc Électrolytique du Canada (CEZinc) and Natural Sciences and Engineering Research
Council of Canada (NSERC) are gratefully acknowledged for their financial support. The
authors would like to express their sincere thanks and appreciation to Mr. Gary Monteith
from CEZinc for his interest and fruitful discussions. Also, thanks to Mr. André Ferland for
SEM analysis, Mr. Jean Frenette for XRD analysis and Mr. Alain Brousseau for ICP
analysis.
79
CHAPTER 5
ELECTROCHEMICAL STUDIES OF IONIC LIQUID ADDITIVES DURING THE ZINC
ELECTROWINNING PROCESS
80
Electrochemical Studies of Ionic Liquid Additives during the Zing Electrowinning Process
N. Sorour1,*, W. Zhang1, G. Gabra1 and E. Ghali1, G. Houlachi2
1Department of Mining, Metallurgical and Materials Engineering, Laval University, Québec, Canada, G1V 0A6.
2Hydro-Québec research centre (LTE), Shawinigan, QC, Canada, G9N 7N5.
*Corresponding author: Tel: 418 656-2131 - Fax: 418 656-5343 ([email protected])
Published in journal of Hydrometallurgy, Vol. 157, 2015, pp 261-269.
Résumé
1-éthyl-3-méthylimidazolium méthanesulfonate [EMIM]MSO3 et bromure de 1-butyl-3 imidazolium [BMIM] Br ont été évalués individuellement comme des additifs par rapport de la gélatine et de l'additif précédemment étudié [BMIM]Cl dans l’électrolyse du zinc à partir de l’électrolyte acide de sulfate contenant 8 g dm-3 des ions de Mn2+. Une impureté métallique de 0,0055 mg dm-3 des ions de Sb3+ a été examinée en combinaison avec de 1 à 3 mg de chaque additif. Des mesures galvanostatiques ont été utilisées dans un électrolyte acide de sulfate pour étudier les potentiels cathodique et anodique individuellement, aussi que le rendement du courant de métal de zinc déposé dans l'électrolyte sulfate acide pendant 2 heures à 50 mA cm-2 et 38°C. L’effet de chaque additif sur la morphologie de surface et l’orientation cristallographique a été étudié par la microscopie électronique à balayage (MEB) et la diffraction des rayons-X (DRX). Les impuretés de plomb dans le dépôt ont été analysées en utilisant le plasma à couplage inductif (ICP). Parmi les cinq différentes concentrations examinées de chaque additif (1,3,5,10 et 40 mg dm-3), les résultats ont révélé que l'addition de 1 et 3 mg dm-3 de [EMIM]MSO3 réduit la tension de la cellule d’environ 10-15 mV, respectivement, tandis que [BMIM]Br réduit la tension de la cellule par 5-10 mV en ajoutant de 1 et 3 mg dm-3, respectivement. Les efficacités de courant de 93,6-94,4% ont été obtenus en ajoutant 1-3 mg dm-3 de [EMIM]MSO3 ou 1-3 mg dm-3 de [BMIM]Br par rapport à 92,8% qui a obtenue à partir de l'électrolyte standard. La réduction maximale de la consommation d'énergie de ≈165 kWh tonne-1 a été obtenue en ajoutant de 3 mg dm-3 de [EMIM] MSO3 en présence des ions de Sb3+, suivie par une réduction de ≈154 kWh tonne-1 en ajoutant de 3 mg dm-3 de [BMIM] Br. La polarisation potentiodynamique et les études voltamétriques montrent que la polarization de l’électrodéposition de zinc a été diminuée en présence d'antimoine. Apparemment, les deux additifs ont un comportement similaire de la polarisation sur l'électrode d'aluminum dans l'électrolyte acide de sulfate.
81
Abstract
1-ethyl-3-methylimidazolium methanesulfonate [EMIM]MSO3 and 1-butyl-3 methylimidazolium bromide [BMIM]Br were evaluated individually as additives compared to gelatin and previously studied additive [BMIM]Cl in zinc electrowinning from synthetic acidic sulphate electrolyte containing 8 g dm-3 of Mn2+ ions. A metallic impurity of 0.0055 mg dm-3 of Sb3+ ions was examined in combination with 1 and 3 mg of each additive. Galvanostatic measurements have been employed to investigate the cathodic and anodic potentials individually also current efficiency of the deposited zinc metal in acidic sulphate electrolyte for 2 hours at 50 mA cm-2 and 38oC. Effect of each additive on surface morphology and crystallographic orientation was studied using scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. Lead impurities in the deposit have been measured by using inductively coupled plasma (ICP). Among five different concentrations tested of each additive (1,3,5,10 and 40 mg dm-3), results revealed that addition of 1 and 3 mg dm-3 of [EMIM]MSO3 reduced the cell voltage by ≈15 and 10 mV, respectively; while [BMIM]Br reduced the cell voltage by ≈10 and 5 mV by adding 1 and 3 mg dm-3, respectively. Current efficiencies of 93.6% - 94.4% have been obtained by adding 1-3 mg dm-3 of [EMIM]MSO3 or 1-3 mg dm-3 of [BMIM]Br as compared to 92.8% obtained from the standard electrolyte. Maximum reduction of power consumption of ≈165 kWh ton-1 was obtained from adding 3 mg dm-3 of [EMIM]MSO3 in presence of Sb3+ ions followed by a reduction of ≈154 kWh ton-1 by adding 3 mg dm-3 of [BMIM]Br. Potentiodynamic polarization and voltammetric studies indicate that polarization for zinc electrodeposition decreased in presence of antimony. Apparently, the two additives have approximately similar polarization behavior on the aluminum electrode in the acidic sulphate electrolyte.
82
5.1. Introduction
Zinc is considered as the fourth most widely used metal, following iron, aluminum
and copper [3]. Zinc is processed by many methods in order to obtain the metal in high pure
state, among these methods, electrowinning process which is the most commonly used
[131]. Electrowinning uses an electrolytic cell to reduce the zinc on an aluminum cathode
and electric current is run through a lead anode. During the electrolysis of zinc sulphate
electrolyte, two main reactions are competing on the cathode, one is zinc reduction, and the
other is hydrogen evolution reaction (HER) [132]. On the anode, oxygen gas is produced
through the overall electrochemical reaction; H2O → 2H+ + 2e- + ½O2(g) Eo= 1.229V.
Approximately, 99% of the anodic current is used for oxygen evolution reaction (OER),
consuming ≈40% of total cell voltage [20].
As far as lead anodes are used in electrowinning so this process is very sensitive to the
detrimental effect of Pb impurities and to the presented metallic impurities in the electrolyte
such as: Sb, Fe, Cu, Co, Ni ...etc. Low concentrations of these impurities substantially
affect negatively the zinc deposition process. This leads to a decrease of zinc current
efficiency (CE), change in deposit morphology, cathodic polarization and even anodic
polarization [59]. Although the costly steps used for purification, the zinc electrolyte is
usually contaminated with many metal ion impurities [134]. Zinc deposits contaminated
with lead were found to have characteristic morphologies and orientations as well as
negative effect on current efficiency. The overpotential which depends on the amount of
lead present in the zinc deposits and to the presence of other impurities such as antimony
and nickel in the deposits cannot be neglected [40]. One of the considerable goals in zinc
electrowinning is minimizing the power consumption. The two important factors which can
determine the energy requirements are the current efficiency and cell voltage which are
affected negatively by the presence of impurities [125]. Accordingly, additives are used to
reduce the negative effect on current efficiency, cell voltage and deposit morphology
through their adsorption on the surface of electrode [134].
Natural products and surfactants have always been the focus of attention as additives in
zinc electrowinning process. Among these additives; animal glues and Arabic gums which
showed a positive influence on the CE and deposit orientation in the presence of traces of
83
Sb3+ ions in the industrial electrolyte [42,135]. The effects of saponin alone and in
combination with antimony and glue have been investigated; saponin alone decreased the
CE and was weakly polarized but in combination with glue + antimony at low
concentrations resulted in increase in CE [135]. Addition of sodium lignin sulphonate
alone into the industrial electrolyte at a range of 3-10 ppm had no negative impact on CE,
nor on the zinc electrowinning process [58]. Also many organic additives have been deeply
studied in zinc electrowinning process; Zhang et al. [63] have reported the beneficial effect
of triethyl benzyl ammonium chloride (TEBACl) and malonic acid on CE and cell voltage
in presence of Ni as impurity. Quaternary ammonium bromides in forms of
cetyltrimethylammonium bromide (CTABr) and tetrabutyl ammonium bromide
(TBABr) were studied by Tripathey et al. [62], as they increased the CE and reduced the
power consumption in presence of antimony. Mathieu et al. [136] have investigated the
effect of 2-butyene-1,4-diol and reported its significant effect on improving current
efficiency. Al2(SO4)3 and the horse-chestnut tree extract (HCE) showed their beneficial
effects on the deposit quality, being good levelling agents [64].
Recently, ionic liquids have been used in many chemical and hydrometallurgical
applications due to their chemical and physical properties, as they are salts where the ions
are poorly coordinated, leading to being liquids below boiling point and even at room
temperature [65]. Ionic liquids consist of an organic cation and inorganic or organic anion;
they have a wide range of solubility and miscibility. For example, some of them are
hydrophobic while others are hydrophilic; most of them are non-flammable and non-toxic
[65-66]. Ionic liquids are widely used in liquid-liquid extraction and electrodeposition of
some metals due to their low melting point and the thermal degradation properties which
are important in the electrochemical media [67]. Also, as a medium in the electrodeposition
of aluminum on stainless steel cathode [68]. They are used as an organic solvent in
electroplating of a range of metals impossible to deposit in water due to hydrolysis e.g. Al,
Ti, Ta, Nb, Mo, W [137]. Ionic liquids in the form of 1-butyl-3-methylimidazolium
hydrogen sulphate [BMIM]HSO4 showed their effects on the kinetics of oxygen evolution
as additive during zinc electrowinning process [138]. [BMIM]HSO4 is found to have good
influence in increasing current efficiency, reducing power consumption and producing
smooth and compact zinc deposits similar to that obtained from gelatin [70]. In other
84
previous studies [139-140], the influence of the ionic liquid salt 1-butyl-3-
methylimidazolium chloride [BMIM]Cl as additive and its electrochemical activity has
been studied and showed its good ionic conductivity and its effect in reducing power
consumption.
This study investigates the effect of two different ionic liquid salts, 1-ethyl-3-
methylimidazolium methanesulfonate [EMIM]MSO3 and 1-butyl-3-methylimidazolium
bromide [BMIM]Br as additives compared to gelatin and [BMIM]Cl on CE, cell voltage,
morphology and electrochemical activity during electrowinning process. The current ionic
liquids additives are chosen in this research paper in order to study the effect of different
anion (Br-) compared to the previously studied one (Cl-) presented in [BMIM]Cl [139], and
also to examine the effect of different ionic liquid [EMIM]MSO3 which could show a good
performance among the examined additives.
5.2. Experimental
5.2.1. Electrolysis
A synthetic standard electrolyte (SE) similar to that used in the Canadian zinc
electrowinning industry. was prepared by dissolving 60 g dm-3 of Zn2+ (ZnSO4.7H2O), 180
g dm-3 H2SO4 and 8 g dm-3 of Mn2+ (MnSO4.H2O) in distilled water. Manganese ions are
added to the solution due to their remarkable effect in reducing the anodic potential and
forming compact layers of MnO2 on the anode. The supplied zinc sulphate containing the
following impurities: Pb (0.003%), Na (0.05%) and Fe (0.001%). The effect of additives
was studied individually with different concentrations of 0,1,3,5,10 and 40 mg dm-3 added
to the standard electrolyte. The influence of 1 mg and 3 mg of the selected additives on CE,
cell voltage and morphology was studied in presence of 0.0055 mg dm-3 of Sb3+ as
impurity. Reagents are supplied from Laboratoire MAT and VWR Canada while additives
are supplied from Sigma-Aldrich USA.
Small-scale galvanostatic electrolysis experiment was performed in an 800 cm3 solution in
double-glazed beaker heated by a flow of thermostated water in the double wall in order to
maintain the working temperature constant. Two plates of aluminum and Pb-Ag (Ag, 0.7%)
were used as cathode and anode, respectively. They were casted in polyester resin with total
85
exposed surface area of 1 cm2 mounting in Teflon cell with an inner distance of 2 cm
between the two electrodes. Ag, AgCl/KCl(sat) (0.199 V/SHE) was used as reference
electrode. Electrodes were manually polished in several steps by several grits SiC papers
(80, 320, 800, 1000 and 3000) to give uniform surface. Then, they were washed by distilled
water, ethanol and dried few seconds before the experiment.
All electrowinning experiments were carried out for 2 hours at 50 mA cm-2 and 38 ± 1oC
with magnetic agitation at 60 rpm using magnetic bar (L=38 mm & D=10 mm). After
electrolysis the cathode was dried and current efficiency was calculated by weight using
Faraday’s law: CE% = (W.F.n/I.t.M) x 100%; where W is the weight of deposit (g), F is
Faraday’s constant, n is the number of electrons, I is the total cell current (A), t is the time
of electrodeposition and M is the atomic weight of zinc.
5.2.2. Deposit Examination
The surface morphology of the deposits was examined by scanning electron
microscopy (SEM) using JEOL JSM-840a. The crystal orientations of the zinc deposits
were determined using X-ay diffractometer model Siemens - D5000. Lead concentrations
in the deposits have been measured using inductively coupled plasma (ICP) model Optima
4300 Perkin-Elmer.
5.2.3. Electrochemical Measurements
Electrochemical studies were done on the base of potentiodynamic polarization and
cyclic voltammetery measurements. 1 cm2 of Aluminum as working electrode and 1 cm2 of
platinum as auxiliary electrode, Also, Ag, AgCl/KCl(sat) was used as reference electrode.
The three-electrode cell was connected to the potentiostat Gamry Reference 3000 – Gamry
USA. Potentiodaynamic polarization was carried out from -1.05 to -1.25 V with a scan rate
of 5 mV s-1. Cyclic voltammetric scanning was scanned from initial potential of -1.30 to a
reversible potential of -0.60 V at a constant scanning rate of 10 mV s-1. Both tests were
done at 38oC under atmospheric conditions. Working electrodes were manually polished
86
before each experiment and washed by distilled water, ethanol and dried few seconds prior
the experiment.
5.3. Results and Discussion
5.3.1. Cell Voltage and Power Consumption
The effect of different concentrations of [EMIM]MSO3, [BMIM]Br and gelatin on
cell voltage and power consumption (PC) during zinc electrowinning from acidic zinc
sulphate solution were studied. Results are listed in Table 5.1. Results showed that in
absence of Sb3+ ions, a reduction of cell voltage of about 15 and 10 mV was obtained by
adding 1 and 3 mg of [EMIM]MSO3 to the standard solution, respectively, while the cell
voltage was reduced only by 10 and 5 mV by adding 1 and 3mg of [BMIM]Br,
respectively. Results of cell voltage are approximately compatible to that obtained from
adding [BMIM]Cl [139], as it reduced cell voltage by 7 mV for 3 mg dm-3 addition. Total
cell voltage was found to be increased gradually by increasing the concentrations of
additives reaching to overpotential of 32-40 mV at 40 mg dm-3. This could be explained
due to the strong adsorption of additives on the surface of electrode at high concentrations
which increases the potential of zinc reduction and decreases strongly the hydrogen
evolution reaction (HER). In presence of Sb3+ ions, the addition of [EMIM]MSO3 or
[BMIM]Br has no observed remarkable decrease in cell voltage compared to that obtained
from SE + Sb3+. However, according to the previous study [139], the maximum reduction
of 28 mV in cell voltage was reported by adding 1 mg dm-3 of [BMIM]Cl, this could be
explained due to its higher ionic conductivity and electrochemical stability [139-140].
Power consumption calculations revealed that the maximum reduction of PC of ≈52 kWh
ton-1 was obtained in zinc electrolyte without Sb(III) by addition of 3 mg of [EMIM]MSO3,
and reduction of ≈45 kWh ton-1 by addition of 3 mg of [BMIM]Br. The PC of zinc
electrolysis from zinc electrolyte in the presence of Sb(III) alone and combined with
gelatin, [EMIM]MSO3 and [BMIM]Br in the electrolytes during the electrodeposition of
zinc is also shown in Table 1. By the addition of very low concentration of Sb(III), the PC
is extremely increased due to the sharp decrease in current efficiency of obtained deposit,
while, the PC is found to decrease rapidly in the addition of tested two additives. Addition
of 1 mg and 3 mg of [EMIM]MSO3 to the zinc electrolyte in presence of antimony
87
decreased the PC by ≈150 and 165 kWh ton-1, respectively, while, addition of 1 mg and 3
mg of [BMIM]Br decreased the PC by ≈141 and 154 kWh ton-1, respectively. However,
the maximum reduction of PC was obtained by addition of 3 mg of [EMIM]MSO3, as
compared to that obtained by addition of standard additive. Adding 1 mg and 3 mg of
gelatin to the zinc electrolyte in presence of Sb(III), the PC is reduced by ≈142 and only
122 kWh ton-1, respectively.
Table 5.1. Effect of gelatin, [EMIM]MSO3 and [BMIM]Br on CE and PC in absence and in presence of Sb(III) during zinc electrodeposition for 2h at 50 mA cm-2
Additive/ mg dm-3
Sb(III)/ mg dm-3
Cell voltage/ V
CE/ %
PC/ kWh ton-1
SE 0 0 2.898 92.8 2560 0 0.0055 2.873 88.7 2655
Gelatin 1 0 2.904 91.1 2613 3 0 2.909 90.6 2632 5 0 2.922 89.1 2688 10 0 2.928 85.3 2814 40 0 2.937 77.5 3107 1 0.0055 2.885 94.1 2513 3 0.0055 2.895 93.7 2533
[EMIM]MSO3 1 0 2.883 93.9 2518 3 0 2.888 94.4 2508 5 0 2.900 93.8 2534 10 0 2.904 93.1 2557 40 0 2.930 91.6 2622 1 0.0055 2.870 93.9 2505 3 0.0055 2.873 94.6 2490
[BMIM]Br 1 0 2.888 93.6 2529 3 0 2.893 94.3 2515 5 0 2.915 92.8 2575 10 0 2.928 92.4 2598 40 0 2.938 90.7 2655 1 0.0055 2.877 93.8 2514 3 0.0055 2.880 94.4 2501
88
5.3.2. Current Efficiency
The effect of the two additives [EMIM]MSO3 and [BMIM]Br on current efficiency
compared to gelatin in absence and in presence of antimony are plotted in Figure 5.1. CE
was studied over range of concentrations of 0-40 mg dm-3 in absence of Sb(III) (Figure
5.1a). Results revealed that, current efficiency obtained from the standard solution is
92.8%. CE was increased gradually from 93.9% to 94.4% by adding 1 and 3 mg dm-3 of
[EMIM]MSO3, to standard zinc electrolyte respectively, while decreased consequently to
91.6% by increasing the concentration to 40 mg dm-3. Also, CE was increased from 93.6%
to 94.3% by adding 1 and 3 mg of [BMIM]Br to the standard zinc electrolyte, respectively.
Then, it was found that CE was decreased to 90.7% by increasing the concentration to 40
mg dm-3 of [EMIM]MSO3. This could be explained due to the excessive adsorption of
additive on the cathode surface which could block the active sites and forbids further
reduction of zinc ions.
Figure 5.1. Effect of gelatin, [EMIM]MSO3 and [BMIM]Br on CE: (a) in absence of Sb(III) and (b) in presence of 0.0055 mg of Sb(III) during zinc electrodeposition for 2h at 50 mA cm-2
The effect of Sb(III) on CE had been investigated for concentrations of 1 and 3 mg dm-3 of
additives as shown in Figure 5.1b. The presence of small quantity of antimony showed a
harmful effect on the quantity of obtained deposit from the standard electrolyte, which
could indicate that small traces of Sb3+ facilitate the hydrogen evolution reaction (HER) on
the cathode leading to produce weak and porous deposit with low current efficiency.
76
78
80
82
84
86
88
90
92
94
96
0 5 10 15 20 25 30 35 40 45
Cu
rren
t Eff
icie
ncy
(%)
Concentrations of additives (mg dm-3)
SE
Gelatin
[EMIM]MSO3
[BMIM]Br
(a)
88
89
90
91
92
93
94
95
96
0 1 2 3 4
Cu
rren
t Eff
icie
ncy
(%)
Concentrations of additives (mg dm-3)
SE
Gelatin
[EMIM]MSO3
[BMIM]Br
(b)(
89
Positive effect on the current efficiency was obtained from the two examined additives. For
[EMIM]MSO3, adding 1 mg to standard zinc electrolyte increased the current efficiency to
93.9% while, addition of 3 mg to the zinc electrolyte showed the highest increase of 94.6%
compared to that obtained from the standard electrolyte (88.7%). Also, adding 1 and 3 mg
dm-3 of [BMIM]Br increased CE to 93.8% and 94.4%, respectively. The two studied
additives succeeded to counteract the harmful effect of antimony by their adsorption on the
cathode. The current and previous studied different salts of ionic liquids [EMIM]MSO3,
[BMIM]Br and [BMIM]Cl showed a good synergetic effect in presence of antimony in
reducing PC which complies to that reported by Zhang and Hua [70], from using
[BMIM]HSO4. The obtained reduction in cell voltage and increase of CE by using
[BMIM]HSO4 were higher than that obtained in this study, this could be explained due to
the different working conditions and parameters such as Mn2+ ions addition, current
density, agitation, temperature, lead-silver anode composition as well as different additive
function groups. However, gelatin is still one of the remarkable additives used to increase
the CE in presence of antimony.
5.3.3. Deposit Examination
The zinc deposits obtained were examined by using SEM and X-ray diffraction to
determine surface morphology and crystallographic orientations, respectively. SEM
photomicrographs are shown in Figures 5.2 and 5.3, also the crystallographic orientations
of zinc deposits from zinc electrolyte containing additives in presence and in absence of
Sb3+ are given in Table 5.2. Results revealed that crystallographic orientation obtained from
addition-free electrolyte is (101) (102) (103) (002) which changed to (101) (112) (102)
(103) by adding antimony or gelatin showing a decrease in platelet size (Figures 5.2b and
5.3a). The obtained deposit from SE + Sb3+ showed small platelet size and slightly porous
deposit, this could be explained due to the increase in hydrogen evolution leading to an
increase in agitation on the electrode’s surface. Adding [EMIM]MSO3 and [BMIM]Br to
the SE changed the crystal orientation to (101) (102) (110) (112) and (002) (101) (004)
(103), respectively, leading to an increase in the platelet size. [BMIM]Br changed the most
preferred orientation from (101) to (002) showing the highest peak intensity at 2θ = 36.30
90
(Figure 5.4b). The obtained deposit from adding [BMIM]Br is found more smooth and
compact than that obtained from [EMIM]MSO3, this could be explained that the higher
molecular weight of [BMIM]+ cation helps the additive to be adsorbed and present more on
the cathodic sites. However, addition of 3 mg of [EMIM]MSO3 and [BMIM]Br to the SE in
presence of Sb3+ changed the preferred crystal orientation to (101) (112) (102) (103) and
(101) (102) (112) (103), respectively. The same effect approximately was given to have a
moderate platelet size producing a compact and smooth deposit by 3 mg of both additives
(Figures 5.3c and 5.3d).
Figure 5.2. Scanning electron microscopy photomicrographs (x1000) of zinc deposit in absence of Sb(III); (a) blank, (b) 3mg gelatin, (c) 3mg [EMIM]MSO3 and (d) 3mg [BMIM]Br
91
Figure 5.3. Scanning electron microscopy photomicrographs (x1000) of zinc deposit in presence of 0.0055mg of Sb(III); (a) blank, (b) 3mg gelatin, (c) 3mg [EMIM]MSO3 and (d) 3mg [BMIM]Br
Figure 5.4. XRD patterns of zinc deposit in absence of Sb(III); (a) 3mg [EMIM]MSO3, (b) 3mg [BMIM]Br
The zinc deposits were analyzed by using inductively coupled plasma spectroscopy (ICP)
to determine the lead concentration, in order to study the effect of each additive on
counteracting the lead contamination. Analysis was done in duplicate and average was
calculated. Results in Table 5.2 shows that both additives of [EMIM]MSO3 and [BMIM]Br
have the same effect on reducing lead concentration, as adding 3 mg to the zinc electrolyte
reduced lead concentration from 26.5 ppm to 5.1-5.6 ppm in absence of antimony and to
4.0-5.1 ppm in presence of antimony. The reported concentrations of lead in zinc deposits
by addition of [EMIM]MSO3 and [BMIM]Br were less than that has been achieved by
adding [BMIM]Cl (10.4-10.9 ppm) [139]. This could be explained due to the presence of
Cl- ions which facilitates the dissolution rate of the lead-based anode. On the other hand,
adding 3 mg of gelatin alone to the standard solution reduced lead concentration to 17.4
ppm, showing very good effect on countering lead concentration to 4.1 ppm in presence of
antimony.
0
500
1000
1500
2000
2500
20 30 40 50 60 70 80 90
Inte
nsi
ty
2-Theta
(a)
(101)
(102)
(110)
(112)
0
500
1000
1500
2000
2500
3000
3500
4000
20 30 40 50 60 70 80 90
Inte
nsi
ty
2-Theta
(b)(002)
(101)
(004)(103)
92
Table 5.2. Crystallographic orientations and lead concentration of zinc deposits obtained by adding 3mg of gelatin, [EMIM]MSO3 and [BMIM]Br in absence and in presence of Sb(III) during zinc electrodeposition for 2h at 50 mA cm-2
Additive/ Sb (III)/ Crystal orientation/ SEM/ Lead conc./ mg dm-3 mg dm-3 hkl Figure ppm
SE 0 0 (101) (102) (103) (002) 5.2a 26.5 0 0.0055 (101) (112) (102) (103) 5.3a 3.6
Gelatin 3 0 (101) (112) (102) (103) 5.2b 17.4 3 0.0055 (101) (102) (103) (112) 5.3b 4.1
[EMIM]MSO3 3 0 (101) (102) (110) (112) 5.2c 5.1 3 0.0055 (101) (112) (102) (103) 5.3c 5.1
[BMIM]Br 3 0 (002) (101) (004) (103) 5.2d 5.6 3 0.0055 (101) (102) (112) (103) 5.3d 4.0
5.3.4. Polarization Studies
The electrochemical behavior of [EMIM]MSO3, [BMIM]Br and gelatin in absence
and in presence of Sb(III) on the cathode have been investigated by potentiodynamic
polarization. This technique is very useful to understand the kinetics and electrochemical
behavior of the electrode during the electrowinning process. The cathodic potential –
current curves have been obtained by polarization of the electrode from -1.05 to -1.25 V
with a scan rate of 5 mV s-1 using different concentrations of additives. The kinetic
parameters represented in cathodic Tafel slopes (bc), overpotentials at 50 mA (ɳ(50)) and
exchange current densities (I0) were determined and listed in Table 5.3. The overpotentials
values were calculated from following equations:
, 0.763 2⁄ / (5.1)
, 0.0 ⁄ / . (5.2)
ɳ , , (5.3)
Where; Ee is the equilibrium potential, R is the gas constant (equal to 8.314 mol-1 K-1), F is
the Faraday constant (equal to 96 500 C mol-1) and Em is the measured potential at 50 mA
[117]. The apparent exchange current densities (I0) were estimated by extrapolating the
Tafel lines to the corresponding zero current potentials.
Results revealed that, in absence of Sb3+ Tafel slopes are varying from 117 to 132 mV/
decade by addition of [EMIM]MSO3, this small variance in Tafel slopes indicates that the
charge transfer reaction is not controlled by increasing the concentration of additive up to
93
40 mg dm-3. While, Tafel slopes are increased from 118 to 139 mV/decade by increasing
the concentration of [BMIM]Br up to 40 mg dm-3. At small concentrations of additives the
polarization curves were slightly shifted to less negative potentials, the behavior of addition
of 3 mg of both additives acted more or less as that of the standard electrolyte (Figures 5.5a
& 5.6a). Approximately, the same trend had been obtained from the addition of [BMIM]Cl
to the standard solution [139]. In presence of Sb3+, the addition of [BMIM]Br restored the
regular values of Tafel slopes from 98 to 114 mV per decade compared to [EMIM]MSO3
which could be explained that the higher molecular weight of [BMIM]+ cation has been
adsorbed preferentially on the surface of cathode. This also could be confirmed by the
obtained values of overpotentials as it is increased from 347 to 366 mV and 347 to 372 mV
by adding [EMIM]MSO3 and [BMIM]Br, respectively. 3 mg of [BMIM]Br increased the
overpotential value from 347 to 356 mV which could confirm the resulting fine grain size
obtained [141]. In presence of antimony, 1-3 mg dm-3 of [EMIM]MSO3 increased the
overpotentials from 324 to 334 mV also same quantities of [BMIM]Br increased it up to
337 mV and shifted the polarization curves to more cathodic potentials (Figure 5.6a). This
indicates that the adsorption of low quantities of both additives on the cathode has the
ability to reduce the harmful effect of Sb3+ ions in reducing the hydrogen reaction
overpotential (HER), which leads also to higher current efficiencies of Zn deposition.
The determined exchange current densities (I0) for addition of additives in absence of Sb3+
indicate that at low concentrations of both additives (1-3 mg dm-3) values are close to what
determined for SE (0.079 mA cm-2). This could indicate that at low adsorbed quantities of
additives the active sites are still free and prompt further reduction of Zn2+ ions, while at
high concentrations I0 values are decreased to 0.050-0.060 mA cm-2 at 40 mg dm-3 that
could be related to the strong adsorption of additives on the electrode’s surface, blocking
certain active nucleation sites. In presence of antimony, I0 values are increased up to 0.104
mA cm-2 due to the high evolution of hydrogen. Addition of additives to the electrolyte (3
mg dm-3) decreased this value to 0.083-0.091 mA cm-2.
94
Figure 5.5. Effect of [EMIM]MSO3 on the cathodic polarization during zinc electrodeposition using aluminum cathode with different concentrations; (a) in absence of Sb(III), (b) in presence of Sb(III)
Figure 5.6. Effect of [BMIM]Br on the cathodic polarization during zinc electrodeposition using aluminum cathode with different concentrations; (a) in absence of Sb(III), (b) in presence of Sb Cyclic voltammetry technique is normally used to study qualitative information about
electrochemical processes at stationary non-agitated interface under various conditions,
such as the presence of intermediates in oxidation-reduction reactions, the reversibility of a
reaction through the determined peaks in the obtained E-I curve during the polarization of
the electrode [111]. CV could then reflect the influence of an additive or change in the
electrolyte composition on the electrochemical properties of the interface. This is
considered mainly in this work to determine the formal reduction and nucleation
overpotential (NOP). The effect of each additive alone or combined with antimony on the
0
20
40
60
80
100
-1.25 -1.20 -1.15 -1.10 -1.05
Cu
rren
t (m
A c
m-2
)
Potential vs Ag, AgCl/KCl (V)
0mg1mg3mg5mg10mg40mg
(a)
0
20
40
60
80
100
-1.25 -1.20 -1.15 -1.10 -1.05
Cu
rren
t (m
A c
m-2
)
Potential vs Ag, AgCl/KCl (V)
0mg
1mg
3mg
(b)
0
20
40
60
80
100
-1.25 -1.20 -1.15 -1.10 -1.05
Cu
rren
t (m
A c
m-2
)
Potential vs Ag, AgCl/KCl (V)
0mg1mg3mg5mg10mg40mg
(a)
0
20
40
60
80
100
-1.25 -1.20 -1.15 -1.10 -1.05
Cu
rren
t (m
A c
m-2
)
Potential vs Ag, AgCl/KCl (V)
0mg
1mg
3mg
(b)
95
reduction of zinc ions on aluminum cathode was studied by using cyclic voltammetry
polarization.
Table 5.3. Effect of [EMIM]MSO3, [BMIM]Br and gelatin on Tafel slopes, cathodic overpotential at 50 mA cm-2, exchange current density and NOP
Additive/ mg dm-3
Sb (III)/ mg dm-3
Tafel slope (-bc)/ Overpotential/ I0/ NOP/ mV/decade -ɳ(50) / mV vs Ref (mA cm-2) mV
[EMIM]MSO3
0 0 118 347 0.079 75 1 0 117
123 350 0.081 73
3 0 347 0.074 72 5 0 124 351 0.071 78
10 0 128 354 0.065 80 40 0 132 366 0.060 87 0 0.0055 98 324 0.104 62 1 0.0055 102 329 0.097 60 3 0.0055 104 330 0.091 62
[BMIM]Br 0 0 118 347 0.079 75 1 0 120 349 0.076 78 3 0 127 356 0.065 80 5 0 130 358 0.061 82
10 0 133 364 0.059 84 40 0 139 372 0.050 88 0 1
0.0055 0.0055
98 112
324 337
0.104 0.085
62 63
3 Gelatin
0.0055 114 334 0.083 65
0 0.0055 98 324 0.104 62 1 0.0055 118 344 0.079 66 3 0.0055 121 342 0.076 73
96
Figure 5.7. Cyclic voltammograms of [EMIM]MSO3 during zinc electrodeposition using aluminum cathode with different concentrations; (a) in absence of Sb(III), (b) in presence of Sb(III)
Polarization was carried from initial potential of -1.30 V to a reversible potential of -0.60 V
at 38oC in presence of atmospheric air without agitation. Results are shown in Figures 5.7
and 5.8, and nucleation overpotential (NOP) values are listed in Table 5.3. The
voltammograms were initiated at point (A) at potential of -1.30 V, scanned in the positive
direction, and then reversed at -0.60 V in the negative direction, crossed-over at point (B).
No significant current was observed until the potential reached the point (B), corresponding
to the reduction of Zn2+ ions. NOP is the difference between the crossover potential (B), the
start of the dissolution and the point at which the Zn begins to deposit (D). This could be a
useful parameter to identify the best additive concentration ratio with antimony ions [42]. It
-400
-300
-200
-100
0
100
200
300
400
500
600
-1.30 -1.20 -1.10 -1.00 -0.90 -0.80 -0.70 -0.60
Cu
rren
t (m
A c
m-2
)
Potential Vs Ag, AgCl/KCl (V)
0mg
1mg
3mg
5mg
10mg
40mg
(a)Anodic
CathodicA
BD C
-400
-300
-200
-100
0
100
200
300
400
500
600
-1.30 -1.20 -1.10 -1.00 -0.90 -0.80 -0.70 -0.60
Cu
rren
t (m
A c
m-2
)
Potential Vs Ag, AgCl/KCl (V)
0mg
1mg
3mg
(b)Anodic
CathodicA
BD C
97
was found that at small concentrations of additives, no changes happened on the cathodic
curves which were confirmed by potentiodynamic polarization, while increasing quantity of
additives shifted the cathodic curve to more negatives values leading to an increase in NOP
values. NOP obtained from standard electrolyte was 75 mV which gradually increased to
87-88 mV by adding 40 mg of [EMIM]MSO3 and [BMIM]Br. High NOP values could
indicate strong polarization and fine-grained zinc deposit can be obtained. However, at low
concentrations of additives the observed deposit has medium platelet size unlike the gelatin
which in presence of Sb3+ increased the NOP from 62 to 73 mV (3 mg dm-3) leading to
grain size reduction of the deposit.
Figure 5.8. Cyclic voltammograms of [BMIM]Br during zinc electrodeposition using aluminum cathode with different concentrations; (a) in absence of Sb(III), (b) in presence of Sb(III)
-400
-300
-200
-100
0
100
200
300
400
500
600
-1.30 -1.20 -1.10 -1.00 -0.90 -0.80 -0.70 -0.60
Cu
rren
t (m
A c
m-2
)
Potential Vs Ag, AgCl/KCl (V)
0mg
1mg
3mg
5mg
10mg
40mg
(a)Anodic
CathodicA
BD C
-400
-300
-200
-100
0
100
200
300
400
500
600
-1.30 -1.20 -1.10 -1.00 -0.90 -0.80 -0.70 -0.60
Cu
rren
t (m
A c
m-2
)
Potential Vs Ag, AgCl/KCl (V)
0mg
1mg
3mg
(b)Anodic
CathodicA
BD C
98
5.4. Conclusions
The influence of the ionic liquids additives [EMIM]MSO3, [BMIM]Br as compared
to gelatin and previously studied additive [BMIM]Cl has been examined:
- Maximum power consumption reduction of ≈165 kWh ton-1 is obtained by adding 3 mg
dm-3 of [EMIM]MSO3 to the standard electrolyte containing 0.0055 mg of Sb followed
by a reduction of ≈154 kWh ton-1 from addition of 3 mg dm-3 of [BMIM]Br. However,
gelatin is still one of the best additives in reducing PC in presence of Sb3+ ions, showing
a reduction of ≈142 and 122 kWh ton-1 by adding 1 and 3 mg dm-3, respectively.
- 3 mg of [EMIM]MSO3 and [BMIM]Br added to the standard electrolyte in presence of
Sb3+ changed the preferred crystal orientation giving a moderate platelet size producing
a compact and smooth deposit.
- Both additives of [EMIM]MSO3 and [BMIM]Br showed better effect than [BMIM]Cl
in reducing lead contamination from 26.5 ppm to 5.1-5.6 ppm in the zinc deposits
while, they have almost no bad effect in presence of antimony.
- [EMIM]MSO3 and [BMIM]Br have approximately similar polarization behaviors,
slightly better than gelatin. Increasing concentration of additives shifted the polarization
curves to more negative values leading to an increase in Tafel slope values from -118
mV/decade to -132 & -139 mV/decade at 40 mg dm-3 and NOP from 75 mV to 87 & 88
mV at 40 mg dm-3 for both additives.
Acknowledgements
Zinc Électrolytique du Canada (CEZinc) Limitée and Natural Sciences & Engineering
Research Council of Canada (NSERC) are gratefully acknowledged for their financial
support. The authors would like to express their sincere thanks and appreciation to Mr.
André Ferland, Mr. Jean Frenette and Mr. Alain Brousseau for their professional technical
participations.
99
CHAPTER 6
ELECTRODEPOSITION AND STUDY OF THE ELECTROCATALYTIC ACTIVITY OF Fe-Mo-P ALLOYS FOR HYDROGEN
EVOLUTION DURING CHLORATE PRODUCTION
100
Electrodeposition and Study of the Electrocatalytic Activity of Fe-Mo-P Alloys for Hydrogen Evolution during Chlorate Production
F. Safizadeh1,*, N. Sorour1, G. Houlachi2, E. Ghali1
1Department of Mining, Metallurgical and Materials Engineering, Laval University, Québec, Canada, G1V 0A6.
2Hydro-Québec research centre (LTE), Shawinigan, QC, Canada, G9N 7N5.
*Corresponding author: Tel: 418 6562131-Fax:418 6565343 ([email protected])
This paper is submitted to the International Journal of Hydrogen Energy.
Résumé
Des binaires Fe-Mo et ternaires Fe-Mo-P revêtements différents on été déposés par une méthode électrochimique à partir d’un électrolyte a base de citrate sur des substrats d’acier doux (MS). Des électrodépositions galvanostatiques ont été menées pendant 6 heures à 20 mA cm-2 et 30oC. L’activité électro-catalytique de ces alliages vers la réaction de dégagement d`hydrogène (RDH) a été étudiée en utilisant les techniques de polarisation à l’état-stable et la spectroscopie d'impédance électrochimique (SIE). Les expériences électrochimiques ont été réalisées dans des solutions de chlorure de sodium. Tous les alliages électrodéposés ont produit la structure amorphe, révélée par des diagrammes de diffraction de rayon-X. L’alliage ternaire préparé Fe54Mo30P16 a diminué la surtension de RDH par 30% par rapport à MS à la densité de courant de 250 mA cm-2. Cet électrocatalyseur a porté une amélioration de 16.5% à la surtension de la RDH en comparaison avec l’alliage binaire de Fe-Mo. Les résultats de la polarisation à l’état-stable et de SIE ont révélé que la rugosité de surface et l’activité intrinsèque des alliages Fe-Mo-P pourraient être l’origine du comportement prometteur de cet électrocatalyseur vers la RDH. l'alliage ternaire de Fe-Mo-P pourrait être un candidat considérable pour l'amélioration de la RDH.
101
Abstract
Binary Fe-Mo and ternary Fe-Mo-P coatings have been electrochemically deposited from citrate-based electrolyte on mild steel (MS) substrate. Galvanostatic electrodepositions have been conducted for 6 hours at 20 mA cm-2 and 30oC. The electrocatalytic activity of these alloys towards hydrogen evolution reaction (HER) was assessed using steady-state polarization and electrochemical impedance spectroscopy (EIS) techniques. Electrochemical tests were carried out in sodium chloride solutions. All electrodeposited alloys yielded the amorphous structure, revealed by X-ray diffraction patterns. At a current density of 250 mA cm-2, the Fe54Mo30P16 electrode reduced the HER overpotential by 30% in comparison with mild steel. This electrocatalyst also showed an enhancement of 16.5% for the HER overpotential as compared to the binary alloy of Fe53Mo47. The steady-state polarization and EIS results revealed that both the surface roughness and intrinsic activity could be the origin of the promising behavior of Fe-Mo-P electrocatalyst towards HER. The ternary alloy of Fe-Mo-P could be a considerable candidate in enhancing the HER.
102
6.1. Introduction
Chlorine and sodium chlorate are between the most important chemical products,
being extensively applied in pulp and paper industry, water treatment, agriculture defoliant,
herbicide as well as fabrication of different polymers [14-16,142]. Chlorine production is
performed using chlor-alkali electrolysis. During this process, sodium chloride solution is
electrolysed to form chlorine at the anode and sodium hydroxide and hydrogen at the
cathode (Eq. 6.1).
2 2 → 2 (6.1)
During the electrochemical process of chlorate production, sodium chloride is oxidised to
sodium chlorate while water is reduced to hydrogen gas evolved at the cathode according to
the following reaction (Eq. 6.2) [14].
3 → 3 (6.2)
Mild steel (MS) is a popular cathode used for chlorate production and diaphragm chlorine
cells, owing to its low hydrogen overvoltage and high durability in sodium hydroxide, low
cost and capability of being shaped into different forms [18]. However, MS is not the best
choice as cathode due to some existing drawbacks [16,18]: (i) when the surface of mild
steel is fresh, the overvoltage values of HER are between 850 and 950 mV at 250 mA cm-2
(η250), depending on surface roughness [18]. Since Cr is present during electrolysis as well
as Ca and Mg as impurities in the electrolyte, their precipitations gradually cover the
cathode surface resulting in an increase of ~1100 mV as overpotential. In fact, the Cr(VI) is
reduced during cathodic polarization and formed a thin film of Cr(OH)3.xH2O on the
cathode so-called chromium diaphragm that contained a thickness less than 10 nm. This
film hinders also some other cathodic reactions such as oxygen reduction, whereas
hydrogen evolution can still take place on the surface though with changed kinetics
compared to that occurring on a bare electrode surface [15,18]. Thus, the presence of
chromium oxide is essential for enhancement of the overall cell efficiency. Moreover, the
presence of chromate inhibits the reduction of hypochlorite and chlorate ions on the
cathode (parasitic reactions), reducing the corrosion rate of the steel cathodes and finally
acting as a buffer in the pH range of 5-7 [14-15]. (ii) Due to the thermodynamic instability
103
of iron, the steel cathodes are significantly corroded in hot concentrated caustic solution
with time especially during power shutoff (open circuit potential). (iii) The corrosion
products cause the shortening life of the cathodes. Therefore, replacement of steel cathodes
with new materials has a great economical interest for the industry.
The HER was extensively investigated on several elements such as Ni, Fe, Mo, Cu, P, Ti,
Pd, Mn, Ru, Co, W, Cr and graphite as well as on rare-earth elements for different
applications such as water electrolysis and hydrogen-based fuel cells [16,18,143].
Molybdenum and phosphorous are among new cathode materials exhibiting improved
properties for the HER. These two metals were doped with other elements to enhance the
surface roughness of the cathode or the intrinsic catalytic activity towards the HER [101,
142,144-151]. Molybdenum and phosphorous may yield an amorphous structure, creating
some modifications in the electronic structure and surface properties [145,152]. The
amorphous coating is usually achieved by co-deposition of an element from iron group (Fe,
Ni or Co) with a metal (Mo and W) or a metalloid such as P, B, Ge or Si that initiates the
defects in crystal lattice, leading to suppress the crystallization of the deposit [153-154]. In
general, transition metal phosphides offer interesting features such as enhanced electronic
conductivity and good stability in acidic and basic media, as compared to pure metals
[155].
Electrodeposition is a low cost technique for preparation of thin metal films. However,
preparation of alloys by electrodeposition is not always straightforward since different
experimental parameters such as current density, temperature and pH may influence the
chemical composition of the alloys. Furthermore, when two or more metals are
electrodeposited simultaneously, the elemental composition of the obtained coating does
not necessarily reflect the composition of the starting electrolyte [156]. Thus, the
experimental parameters and chemical composition of the electrolyte should be well
optimized in order to obtain a desired coating.
Krstajic et al. [157] reported the positive effect of phosphorous addition inside the sodium
chloride brine where Fe-Mo coated alloy on steel was used as cathodes during sodium
chlorate production. Although, Fe, Mo and P were vastly doped with different elements in
the form of binary and ternary alloys [95,97,101,154], to our knowledge, the
104
electrodeposition and the electrocatalytic activity of Fe-Mo-P alloy towards HER has not
been reported yet. In this work, three Fe-Mo-P coatings on mild steel, comprising different
phosphorous contents, were prepared by electrodeposition in the presence of citrate ions.
Thereafter, the electrocatalytic activities of the coatings were assessed in the simulated
conditions of chlorate production. The results were compared to mild steel and Fe-Mo alloy
as references.
6.2. Experimental
Mild steel and platinum foil electrodes with a surface of 1 cm2 were used as cathode
and anode, respectively. Both electrodes were mounted in epoxy resin and assembled in a
three-electrode cell. The reference electrode was a silver chloride with a saturated KCl
double junction of Ag, AgCl/KClsat (0.202 V vs. standard hydrogen electrode (SHE)).
FeSO4.7H2O, Na2MoO4.2H2O and NaH2PO2.H2O were employed as the sources of Fe, Mo
and P, respectively. Trisodium citrate dehydrate (Na3C6H5O7.2H2O) was used as a
complexing agent and the reduction rate controller during deposition. All chemicals were
supplied from Lab Mat–Canada and Sigma-Aldrich – USA. The electrolytes were heated
by the flow of water in a double-glazed wall in order to maintain working temperature at
30oC during electrodeposition. The electrolyte was agitated magnetically at 300 rpm and
pH 6 was adjusted using citric acid. Galvanostatic tests were carried out for 6 hours of at
current density of 20 mA cm-2.
Electrochemical measurements such as potentiodynamic polarization and electrochemical
impedance spectroscopy (EIS) were carried out using potentiostat Gamry Reference 3000.
The potentiodaynamic polarizations were carried out from -1.2 to 0.5 V with a scan rate of
1 mV s-1. The triplicate tests were conducted for each experimental condition in this work.
Close duplicates are considered, however triplicate confirmed the same tendency. All tests
were performed in solution containing 300 g dm-3 of NaCl and 4 g dm-3 of K2Cr2O7 at 80oC
alongside magnetic agitation of 80 rpm. The pH 6.4 was always adjusted using NaOH. EIS
measurements were preformed over the frequency range of 0.01 Hz to 100 kHz. High
amplitude sinusoidal signals were used to overcome the interference of the hydrogen
105
bubbles on the cathode surface. An AC signal of 50 mA was employed for galvanostatic
mode at 250 mA cm-2. Before the test, the electrode was held under cathodic current
density of 20 mA cm-2 for 10 minutes in order to remove the oxide layer on the surface. All
tests were performed under atmospheric conditions.
Surface morphology and elemental composition of the alloys was investigated by scanning
electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) (JEOL JSM-840a
and FEI Quanta FEG 250). X-ray diffraction patterns of the deposits were determined using
an X-ray diffractometer Siemens - D500.
6.3. Results and Discussion
6.3.1. Deposit Characterization
Four different binary and ternary coatings of Fe-Mo and Fe-Mo-P have been
electrochemically deposited using different electrolytes. All deposited films exhibited a
good adherence on the MS substrate and it was not possible to separate them mechanically.
The composition of each electrolyte and that of the produced coatings are listed in Table
6.1.
The electrodeposition of the Fe-Mo binary system is the reflect of “induced co-deposition
mechanism” which is referred to the condition of a metal that can not be deposited alone (in
this case Mo); instead, it will be co-deposited in the presence of another metal; called
inductor metal (in this case Fe) [158]. The chemical composition of the coating obtained by
the electrolyte I, shows that despite the higher concentration of Mo, as compared to Fe (5
times) in the solution, the final content of iron and molybdenum in the coating are close
together. This confirmed that in the competition between Fe and Mo for deposition, iron as
a less noble metal is preferentially adsorbed on the electrode surface, as reported by
Sanches et al. [159]. The comparison of the electrolytes I and II shows that the addition of
phosphorous in the bath resulted in an increase of the phosphorous content in the deposit.
These results are in accordance with the findings in the case of the ternary system of Co-
Mo-P alloy, showing that the increase of hypophosphite anions (H2PO2-) in the electrolyte
increases the phosphorous content in the resulting coating [97]. However, the increase of
106
sodium molybdate concentration from 50 to 70 g dm-3 (electrolytes II and III) not only
reduced the molybdenum and phosphorous content in the deposit but also led to a
significant increase of Fe content in the cathode. This phenomenon could be explained by
the enhancement of the hydrogen evolution reaction after addition of molybdenum to iron
[160].
Table 6.1. The compositions of the coatings from four different electrolytes after 6 hours of electrodeposition at 20 mA cm-2 and 30oC
Electrolyte
Electrolytes compositions (g dm-3)
Deposits compositions (at.%)
Na3C6H5O. 2H2O
FeSO4. 7H2O
Na2MoO4. 2H2O
NaH2PO2. H2O
Fe Mo P
I 120 10 50 0 53 47 0
II 120 10 50 10 54 30 16
III 120 10 70 10 70 21 9
IV 120 10 70 30 61 26 13
Apparently, the HER becomes major reaction in the presence of high concentration of Mo.
High hydrogen evolution inhibits the deposition of ions close to the electrode surface
however, since iron is deposited easier than Mo and probably P, the obtained coating
contained higher amount of Fe than Mo. The hydrogen evolution could also be the reason
for reduction of phosphorous content. It was already indicated that the higher hydrogen
evolution, during the electrolysis process of Ni-P alloy is an undesirable reaction
prohibiting the co-deposition of phosphorous [16]. This phenomenon yields a reduction in
the current efficiency and correspondingly in the deposition rate [160]. This could explain
the slight increase of phosphorous content (from 9 to 13 at.%) in the cathode (electrolytes
III and IV). In fact, despite an important increase of P concentration in the bath (three
times), phosphorous content was increased by only 4 at.%.
Scanning electron microscopy images showed the presence of nodules on all deposits. For
the Fe53Mo47 deposit (Figure 6.1a), the agglomeration of nodules could be clearly observed
in some points while the rest of deposit surface stays nodule-free, having fine cracks. The
presence of the cracks is ascribed to the relaxation of internal tensile stress in coating while
the existence of the cauliflower-shape grains on the surface could be explained by the
107
growth of the secondary nuclei on top of the first layer that was already formed on the
substrate. Cracked morphology for some binary coatings such as Ni-Mo containing more
than 17 at.% Mo was already observed by several research groups [161-163]. The cracks
decrease the hardness of the coating and weaken its resistance to corrosion. The deposits
containing phosphorus, as illustrated in Figures 6.1b, c and d, present almost similar
morphologies. The most important difference between SEM images of the binary Fe-Mo
and ternary Fe-Mo-P electrodes is that the nodules are more homogeneously dispersed on
the electrode surface of the ternary alloys.
Figure 6.1. Scanning electron micrographs (X500) of deposits; (a) Fe53Mo47, (b) Fe70Mo21P9, (c) Fe61Mo26P13, (d) Fe54Mo30P16 after electrodeposition during 6 hours at 20 mA cm-2 at 30oC
The X-ray diffraction patterns of four prepared deposits show a broad peak around 2θ of
43°, indicating the amorphous structure of these alloys (Figure 6.2). As Stepanova et al.
[164-165] reported for the electrodeposition of Mo alloys with iron-group metals in citrate-
(b)
(c)
108
ammonium baths, when the amount of Mo in the deposit exceeds 22-25 at.%, the alloy
contains the amorphous/nanostructured phases. Considering that all prepared Fe-Mo and
Fe-Mo-P coatings contained more than 21 at.% Mo, the observation of an amorphous
structure for all samples confirmed what have been reported previously. Furthermore, it
should be mentioned that the presence of more than 9 at.% phosphorous in ternary alloys
promotes formation of amorphous matrix.
Figure 6.2. XRD spectra of Fe-Mo and Fe-Mo-P deposits obtained from electrodeposition of 6 hours at 20 mA cm-2 and 30oC
6.3.2. Steady-State Polarization Curves
The activity of different coatings towards HER was studied using steady-state
polarization Tafel curves. The potential-current curves are presented in Figure 6.3 where
the current density is plotted against overpotential. As it was shown in Figure 6.3, the
curves follow a typical Tafel behavior. The kinetic parameters derived from the linear part
of the Tafel plots were calculated according to following equation (Eq. 6.3):
.log
.log log| | (6.3)
Inte
nsi
ty /
a.u
Fe54Mo30P16
Fe61Mo26P13
Fe70Mo21P9
Fe53Mo47
20 30 40 50 60 70 80
2θ /degree
MS
109
Where, ɳ is the overpotential and j the current-density for equilibrium at ɳ=0. Using
equation 6.3, the parameters concerning the cathodic Tafel slopes (bc), hydrogen evolution
ovrepotentials at 250 mA cm-2 (ɳ250), charge-transfer coefficients (), exchange current
densities (j0) and the current densities at 200 and 300 mV were determined and listed in
Table 6.2. The apparent current density (j0) values were estimated by extrapolation of Tafel
plots to zero current potentials. The overpotential (ɳ) was corrected considering the ohmic
drop (iRs) where Rs (solution resistance) was obtained employing EIS method.
Figure 6.3. Polarization curves of Fe-Mo and three Fe-Mo-P deposited electrodes compared to MS in chlorate solution at 80oC and pH 6.4
It is well established that the HER in alkaline solutions proceeds via three following steps
[166]:
↔ Volmer step (6.4)
↔ Heyrovsky step (6.5)
2 ↔ 2 Tafel Step (6.6)
Where, M represents electrode material and MH the adsorbed hydrogen on the electrode surface.
-8
-7
-6
-5
-4
-3
-2
-1
0
0 200 400 600 800
Log
(j/
A c
m-2)
-ɳ /mVSHE
MS
Fe53Mo47
Fe70Mo21P9
Fe61Mo26P13
Fe54Mo30P16
110
The first step (Eq. 6.4) is the electro-reduction of water with desorption of hydrogen. This
step is followed by either the electrochemical desorption of hydrogen (Heyrovsky step) or
chemical desorption (Tafel step). It has been stated that the rate-determining step (rds) is
the Volmer or the Volmer coupled with Herovsky or Tafel step if the charge transfer
coefficient is equal to 0.5 [85,167]. Since the charge-transfer coefficient of MS is close to
0.5, it can be concluded that the rds of HER on mild steel may be Volmer or Volmer in
conjunction with other two reactions of Heyrovsky or Tafel. Moreover, the Tafel plots for
all binary and ternary cathodes are linear at negative potentials. This is indicating that the
predominant mechanism of HER on these electrodes appears to be discharge of water
(Volmer reaction) followed by electrochemical desorption step (Heyrovsky)
[85,145,166,168].
The kinetic parameters in Table 6.2 revealed that the Tafel slope decreased significantly
from MS to the binary and ternary catalysts, indicating higher performance of these alloys
for the discharge of the hydrogen on the cathodic surface. Apparently, alloying binary Fe-
Mo catalyst with phosphorous influenced the hydrogen overpotential. The decrease of
overpotential could be observed with the increase of the phosphorous content in the alloys.
The lowest overpotential was obtained for the electrode containing the highest phosphorous
content (Fe54Mo30P16). This electrode showed a decrease of 313 mV in overpotential, an
improvement of 30% as compared to MS. The comparison between Fe53Mo47 and
Fe54Mo30P16 alloys revealed a decrease of 141 mV in overpotential (16.5%) caused by
phosphorous addition.
Table 6.2. The measured kinetic parameters of HER for MS, Fe-Mo and Fe-Mo-P electrodes in sodium chloride solution at 80oC and pH 6.4
Electrode -bc
(mV/dec) -ɳ(250)
(mV vs. SHE) J0
(mA cm-2) J200
(mA cm-2) J300
(mA cm-2)
MS 153 1029 0.46 13×10-3 0.16 0.72
Fe53Mo47 90 857 0.78 538×10-3 43 106
Fe70Mo21P9 79 776 0.88 237×10-3 33 134
Fe61Mo26P13 80 753 0.88 330×10-3 50 186
Fe54Mo30P16 80 716 0.88 429×10-3 84 583
111
The charge-transfer coefficients, , were calculated using equation: = 2.303RT/bF; where
R is the ideal gas constant, T the temperature, bc the cathodic Tafel slope and F the Faraday
constant. It could be observed that the values were increased from 0.46 for MS to 0.78 for
binary Fe-Mo alloy and 0.88 for ternary Fe-Mo-P coatings. The increase of values is an
indication of an improvement of the charge transfer kinetics and a better electrocatalytic
activity of the HER. This parameter is used often as a comparative parameter instead of j0
[80,169].
The exchange current density measurements at the equilibrium potential (zero
overpotential) were also reported in Table 6.2. The apparent exchange current density
provides also the information about the catalytic activity of coatings. It is already known
that the hydrogen evolution reaction could not be occurred at open circuit potential without
certain overpotential [169-170]. Thus, the overpotential at a given current density is more
practical parameter to compare the activity of different electrodes and the current density
value at the equilibrium could not be considered as the solely criteria for evaluation of the
catalytic activity. Regarding the results presented in Table 6.2, an important improvement
in j0 and j could be seen for binary and ternary catalysts as compared to MS. The
comparison of j at two overpotentials, e.g. 200 and 300 mV clearly showed an increase in
current density with increase of the induced phosphorous content in the ternary system. A
direct comparison of two alloys, e.g. Fe53Mo47 and Fe54Mo30P16 containing similar iron
content shows that the substitution of Mo by P could be promising for decreasing the HER
overpotential. Although, the Mo is known for its promising catalytic properties, this
comparison induced that the replacement of the Mo by phosphorous was promising for the
catalytic behavior of the cathode. However, these results are not in accordance with the
results published on Ni-Mo-P ternary alloy. Regarding that both Ni and Fe are from iron
group elements, we may expect the similar behavior for two systems of Ni-Mo-P and Fe-
Mo-P. Nonetheless, it was shown by Shervedani and Lasia [101] that comparing two
electrodes of Ni74Mo16P10 and Ni71Mo27P2 tested in 1M NaOH at 70oC, the electrode
having more molybdenum and less phosphorous content presents better catalytic activity.
They also deduced that any treatment leading to remove of Mo deactivates the Ni-Mo-P
electrode.
112
6.3.3. Electrochemical Impedance Spectroscopy
The electrochemical impedance data were modeled using modified Armstrong
equivalent-circuit along with a constant phase element (CPE) presented in Figure 6.4. This
electrical equivalent circuit diagram was used to model the solid/liquid interfaces and
thereafter the EIS experimental data were fitted using a CNLS program to this model. The
1-CPE (Constant Phase Element) model presented in Figure 6.4 predicts the appearance of
two depressed capacitive as well as two overlapped semicircle-shapes on Nyquist plots
(Figure 6.5).
Figure 6.4. The electrical equivalent circuit used for simulation of the impedance spectra for the HER [171]
0.00
0.04
0.08
0.12
0.16
0.20
0.5 0.6 0.7 0.8 0.9 1.0
Z''
/ Ω
cm
2
Z' / Ω cm2
(a)
0.00
0.04
0.08
0.12
0.16
0.5 0.6 0.7 0.8 0.9 1.0
Z'' /
Ω cm
2
Z' / Ω cm2
(b)
113
Figure 6.5. The Nyquist plots for the HER process on a) MS, b) Fe53Mo47, c) Fe70Mo21P9, d) Fe61Mo26P13 and e) Fe54Mo30P16
This model was already used by several authors to study the behavior of Raney-nickel
composite coated electrodes [166,171-173]. Generally, the CPE is attributed to the
roughness or porosity of the real surface of solid electrodes, causing the depression of the
0.00
0.04
0.08
0.12
0.16
0.5 0.6 0.7 0.8 0.9 1.0
Z''
/ Ω
cm
2
Z' / Ω cm2
(c)
0.00
0.04
0.08
0.12
0.16
0.5 0.6 0.7 0.8 0.9
Z''
/ Ω
cm
2
Z' / Ω cm2
(d)
0.00
0.04
0.08
0.12
0.16
0.6 0.7 0.8 0.9 1.0
Z''
/ Ω c
m2
Z' / Ω cm2
(e)
114
semicircles as a result of inhomogenieties present at a micro or nano (atomic/molecular)
scale [113,174-175]. In the case of MS, two complete semicircles, yet slightly depressed,
appear clearly while for the other electrodes, the overlapped flattened semicircles could be
observed on the complex plane plots. The scattering of the data, especially at low frequency
was observed that could be basically due to vigorous hydrogen evolution on the rough
electrode surface. The fitting parameters were listed in Table 6.3.
The Rct corresponding to the Faradic resistance for electrosorption reaction (charge transfer
process) of the catalyst was reduced from binary to the ternary coating. The lowest Rct was
obtained for Fe54Mo30P16 electrode, indicating the highest electrocatalytical activity for this
alloy. However, it should be noted that the values of the charge transfer resistance were
very close for all ternary catalysts. The highest CPE value was also determined for
Fe54Mo30P16 cathode, comprising lower overpotential according to the steady-state
polarization tests. Since the CPE could be related to the Faradic reaction of the HER, the
increase of CPE values could thus be indicating the increase of the electrocatalytic activity.
The lowest CPE was obtained for MS, presenting maximum overpotential to HER. The
overpotential is indicating the increase of hydrogen adsorption rate on the electrode surface.
In fact, the production of gas bubbles due to excessive hydrogen evolution could result in
blocking of the electrode surface, leading to the CPE reduction [169,176,177]. Adsorption
of the hydrogen gas bubbles on the surface could also reduce the effective surface area.
Table 6.3. The electrochemical data obtained by the Nyquist plots of MS, Fe-Mo and different Fe-Mo-P alloys
Electrode Rct
(Ω cm2) CPE (mF cm-2)
n Rf
MS 0.235 0.457 0.931 11
Fe53Mo47 0.180 261 0.652 2196
Fe70Mo21P9 0.172 271 0.709 3457
Fe61Mo26P13 0.174 361 0.708 5164
Fe54Mo30P16 0.167 437 0.590 3010
The parameter n is generally accepted to be a measure of surface inhomogeneity and
irregularities of the solid surface. The n values lower than 1 for all catalysts suggested a
115
porous structure for coatings. The effective surface area, meaning the electrochemically
accessible surface area on which hydrogen is adsorbed, could be estimated from the double
layer capacitance (Cdl). The Cdl is calculated by following equation (Eq. 6.7) according to
EIS results [170,178].
(6.7)
Where, Rs is the solution resistance (Ω cm2). The roughness factor, that characterizes the
real-to-geometrical surface area, was calculated based on Rf = Cdl/20 µF cm-2 (20 µF cm-2 is
the value considered for the double layer capacitance of a smooth electrode) [179]. Table
6.3 shows that, All ternary catalysts showed higher roughness factor Rf (3010-5164) as
compared to binary catalyst (2196) and MS. Electrochemical impedance studies revealed
that the enhanced behavior of ternary alloys especially the Fe54Mo30P16 coating could be
attributed to both the higher surface roughness and the better intrinsic activity of the alloy
due to the synergetic effect of phosphorous with iron and molybdenum.
6.4. Conclusions
The binary Fe-Mo and ternary Fe-Mo-P electrodes were successfully
electrodeposited and their activities towards the hydrogen evolution reaction (HER) were
evaluated in simulated conditions of chlorate industry. The obtained results in agitated
solution containing 300 g dm-3 of NaCl and 4 g dm-3 of K2Cr2O7 at 80oC and pH 6.4 can be
summarized as follows:
1. The overpotential of the HER determined by steady-state polarization for the
prepared coating Fe54Mo30P16 was decreased by 313 mV compared to mild steel,
and by 141 mV compared to Fe53Mo47 coating. The XRD analysis of the coating
confirmed the presence of amorphous structure for all electrodes.
2. The other kinetics parameters such as charge-transfer coefficient (), and cathodic
Tafel slope (bc) show an improvement in the catalytic activities of the ternary
coatings compared to that of the binary coating. is increased from 0.78 to 0.88
116
while bc is decreased from 90 to 80 mV per decade for Fe-Mo and Fe-Mo-P
coatings, respectively.
3. Electrochemical impedance results revealed that, ternary alloys of Fe-Mo-P
exhibited better catalytic activity as compared to binary alloy of Fe-Mo. Moreover,
the phosphorous-containing electrodes exhibited higher roughness Rf (3010-5164)
as compared to the Fe-Mo (2196) and MS electrodes.
4. The beneficial effect of phosphorus addition in Fe-Mo alloy could be ascribed to the
increase of the effective surface area as well as the intrinsic activity of coatings.
These results revealed that the Fe-Mo-P coatings may be considered as a promising
cathode towards the HER. However, the corrosion performance of this alloy should
be considered in the future.
Acknowledgements
The Hydro-Québec Research Institute and the Natural Sciences and Engineering Research Council of Canada (NSERC) are gratefully acknowledged for their financial support.
117
CHAPTER 7
CONCLUSIONS AND OUTLOOK
118
7.1. Conclusions
Based on the obtained results in this research study, the following points can be
concluded:
(A) The effect of certain organic additives on the zinc electrowinning process;
Seven different organic additives from different groups were chosen to be examined during
the zinc electrodeposition. (1) Polyacrylamide [PAM], is one of the remarkable synthetic
polymers, (2) Tetra-butylammonium bromide [TBABr], is a quaternary ammonium salt, (3)
Benzalkonium chloride [BKCl], is a cationic surface-acting agent belonging to
the quaternary ammonium salts with aromatic group, (4) Chitin, is a natural compound.
Ionic liquid salts were well in this study due to their promising behavior in the
electrochemical media. Among these salts, (5) 1-butyl-3-methylimidazolium chloride
[BMIM]Cl, (6) 1-butyl-3-methylimidazolium bromide [BMIM]Br, and (7) 1-ethyl-3-
methylimidazolium methanosulfonate [EMIM]MSO3 were also chosen as additives
compared to gelatin. The effect of different concentrations of 1,3,5,10 and 40 mg dm-3 of
each additive have been examined in standard electrolyte, also 1 and 3 mg dm-3 have been
examined in electrolyte containing Sb3+ ions as a metallic impurity. The effect of each
additive on zinc electrowinning parameters such as: power consumption (PC), current
efficiency (CE), cell voltage (CV), lead contamination in the deposit, surface morphology,
crystallographic orientation, and polarization behavior has been studied. It was found that:
I. The obtained current efficiency and power consumption from the standard electrolyte
(free-addition) were 92.8% and 2560 kWh ton-1, respectively. The addition of additives
such as [PAM], [TBABr], and [BKCl] to the standard electrolyte (SE) decreased the CE
and increased the PC in most of the cases. The ionic liquids salts succeeded to increase the
CE by ~0.7-1.6% and decrease the PC by ~31-52 kWh ton-1 at low concentrations of 1-3
mg dm-3. Increasing the concentration of additives increased the overpotential and
decreased the CE due to the excessive adsorption which blocks the active sites on the
cathode.
II. The presence of small traces of Sb3+ reduced significantly the CE to 88.7%, also reduced
the cell voltage by 25 mV. This could be attributed to the acceleration of hydrogen
119
evolution reaction (HER) which is catalyzed by the presence of Sb3+ ions and hinder the
Zn2+ ions reduction. All additives added to the SE + Sb3+ increased the CE and
counteracted the harmful effect of Sb3+ on the zinc reduction through their adsorption on
the cathode surface. The ionic liquid salts succeeded to increase the CE up to 95.1% from
standard electrolyte in presence of Sb3+ ions. Maximum reduction of PC of ~173 kWh ton-1
was observed by addition of 3 mg dm-3 of [BMIM]Cl to the same electrolyte. The PC
values in SE with additives in presence of Sb3+ was decreased in the order of: Ionic liquid
salts > PAM > TBABr > Chitin > BKCl.
III. Maximum reduction of lead contamination in zinc deposit from 26.5 ppm to 5.1-5.6
ppm was obtained from adding 3 mg dm-3 of [EMIM]MSO3 and [BMIM]Br individually in
absence of antimony. Both additives showed better effect than [BMIM]Cl in reducing lead
contamination (~10.7 ppm), this could be explained due to the presence of Cl- ions which
facilitate the dissolution of lead-based anode. All examined additives did not succeed in
reducing the Pb concentration in presence of antimony, since lead contamination is already
well reduced.
IV. X-ray diffraction and scanning electron microscope results revealed that, the
crystallographic orientation for zinc deposit obtained from SE was (101) (102) (103) (002)
showing moderate grain size. The addition of antimony showed a decrease in the grain size
with change in the morphology and crystal orientation. Most of additives restored the
crystallographic orientation of standard deposit and gave medium grain size with smooth
compact deposit which could explain the high current efficiency obtained in presence of
antimony.
V. The polarization studies showed that, the cathodic overpotential at 50 mA cm-2 increased
with increasing the additive concentration, this is explained due to the high adsorption of
additive on the cathode surface which blocks the active sites and lead to higher potential for
Zn2+ reduction. Addition of antimony to the electrolyte decreased the overpotential by ~23-
30 mV due to the hydrogen evolution reaction. Nucleation overpotential (NOP) values
obtained by cyclic voltammetry technique were found to be increased with increasing the
120
additive concentration. For example, in SE containing Sb3+, [PAM] increased the NOP
from 68 mV to 104 mV by increasing the concentration from 1 to 3 mg dm-3, respectively.
High NOP values indicate that more fine-grained deposits can be obtained with good
crystallographic orientation.
(B) Fe-Mo and Fe-Mo-P coatings as cathode for chlorate production;
Different coatings of Fe-Mo and Fe-Mo-P were successfully electrodeposited from citrate-
based electrolyte at 20 mA cm-2 for 6 hours. The prepared alloys have atomic percentage of
Fe53Mo47, Fe70Mo21P9, Fe61Mo26P13, and Fe54Mo30P16 which analyzed by energy dispersive
spectroscopy.
I. All obtained deposits are found to have amorphous structure due to the presence of
phosphorous and high content of molybdenum. The hydrogen evolution reaction (HER)
overpotential for these cathodes was examined by potentiodynamic polarization in
simulated conditions as chlorate production electrolyte. The overpotential of HER for
Fe54Mo30P16 was decreased by 30%, compared to mild steel, and by 16.5%, compared to
Fe53Mo47.
II. The electocatalytic activities were also studied by electrochemical impedance
spectroscopy in the same electrolyte. It was found that, charge transfer resistance (Rct)
decreased gradually by increasing the P content in the alloy. The lowest value of Rct of
0.167 Ω cm2 was obtained by Fe54Mo30P16, compared to 0.235 Ω cm2 which obtained by
mild steel cathode. Also the roughness factor and the intrinsic electrolcatalytic activities
were found to be increased with increasing the P and Mo contents in the alloys and this
could explain the origin of this promising behavior of the prepared cathodes toward the
HER.
121
7.2. Outlook
Based on the obtained results in this research study, the following perspectives and
future work could be considered:
Concerning the additives in zinc electrowinning;
- Different salts of ionic liquids could be considered with different anions (I- and F-) and
cations parts (C5H11+, C6H13
+, C7H15+, C8H17
+) in order to examine the effect of each
part individually on zinc electrodeposition.
- Different concentrations of Sb3+ ions (0.01, 0.015, 0.02 mg) could be used to study the
best combinations between the additives and antimony.
- Certain concentrations of Pb2+ ions (0.05, 0.1, 0.15, 0.2 mg) could be added to the
electrolyte to examine the difference between the contamination caused by the lead-
based anode and the soluble lead ions in the electrolyte.
- Ionic exchange membrane could be used to separate the cathodic and anodic
compartments to provide better understanding of the anodic and cathodic reactions also
ions movements towards the electrodes.
- Certain parameters such as current density, temperature, agitation, Mn2+ ion
concentration, Zn2+ ion concentrations and pH could be changed to study the effect of
each parameter individually on zinc electrowinning process.
- Electrochemical impedance spectroscopy (EIS) and electrochemical noise
measurements (ENM) could be conducted in the future work to give better
understanding of the electrochemical activities in presence of additives.
Concerning the deposited cathodes for chlorate industry;
- The corrosion resistance measurements for the prepared alloys could be examined in the
future in the same conditions by using electrochemical noise measurements (ENM) and
scanning reference electrode technique (SRET).
122
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