Potentiometric pH Measurements in the Pressure Acid Leaching of Nickel Laterites

229 Pages·2010·1.3 MB·English

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POTENTIOMETRIC pH MEASUREMENTS IN THE PRESSURE
ACID LEACHING OF NICKEL LATERITES

Zoran Jankovic
Doctor of Philosophy 2010
Graduate Department of Chemical Engineering and Applied Chemistry
University of Toronto

ABSTRACT
An electrochemical cell consisting of a flow-through yttria-stabilized zirconia (YSZ)
sensor and a flow-through Ag/AgCl reference electrode has been employed to measure
pH of high-temperature acidic sulphate solutions relevant to the pressure acid leaching
(PAL) of nickel laterites. In a previous study, this cell was used to measure pH of H SO ,
2 4
Al (SO ) -H SO and MgSO -Al (SO ) -H SO solutions at 250oC. In this work, the
2 4 3 2 4 4 2 4 3 2 4
solutions range in complexity from the binary MgSO -H SO , NiSO -H SO , and
4 2 4 4 2 4
Al (SO ) -H SO , through the ternary MgSO -Al (SO ) -H SO and NiSO -Al (SO ) -
2 4 3 2 4 4 2 4 3 2 4 4 2 4 3
H SO , to the PAL process solutions, whereas the temperature ranges from 200oC to
2 4
250oC. The measured and theoretical pH values typically agree within less than 0.1 pH
unit and 0.2 pH units in synthetic solutions and PAL solutions, respectively. This is an
improvement over the results of the previous study in synthetic solutions, which show
differences between theory and experiment as high as 0.4 pH units. The conversion of
measured potentials into pH values is based on the new mixed-solvent electrolyte (MSE)
ii
speciation model of the OLI Systems software calibrated independently based on
solubility measurements. Both Henderson’s equation and the exact definition of the
diffusion potential were employed in treating the obtained experimental data.
Experimental pH values calculated using the diffusion potentials evaluated by either
approach are essentially the same. This finding suggests that Henderson’s equation,
which is based on readily available limiting ionic mobilities, can be effectively used.
Lithium chloride is found to be a suitable alternative to sodium chloride as the reference
electrode solution for the measurement of pH of aluminium-containing solutions, because
it did not induce precipitation of aluminium as an alunite-type compound. The
experimental results indicate that the high-temperature behaviour of Ni, Co and Mn
sulphates can be satisfactorily approximated with that of MgSO . The experimental
4
findings also support the postulation that acid should be added to a PAL process so that
the solution pH is around 1 at the leach temperature, regardless of the feed composition.
The cell can be used for hydrometallurgical process research and development on a
laboratory scale with very satisfactory performance, provided that a well-behaved YSZ
sensor is available.

iii
ACKNOWLEDGEMENTS

This thesis was conducted under the supervision of Professor V.G. Papangelakis. I
would like to thank him for his guidance, advice and support during the course of this
thesis.
I would like to express my thanks to Professor S.N. Lvov for helpful discussions
and advice on pH measurement.
I would like to thank my reading committee, Professors D.W. Kirk and R.C.
Newman, for their advice, suggestions and comments. I would also like to thank
Professor S.J. Thorpe for constructive criticism at my Departmental Defence. I would like
to especially thank Professor A. Alfantazi for reviewing my thesis and providing valuable
feedback.
I would like to thank Dr. M.J. Collins for useful discussions on Pressure Acid
Leach solutions.
Thanks are also due to the Aqueous Process Engineering and Chemistry Group. I
would like to thank Dr. D. Seneviratne, Dr. J. Adams, Mr. G. Singh, Mr. J. Brown, Mr.
M. Reid, Dr. I. Bylina, Mr. S. Peters, Mr. R. Saini and Mr. I. Perederiy for their help in
the lab. I would also like to thank Dr. H. Liu, Dr. M. Huang, Dr. G. Azimi and Mr. S.
Roshdi for their support on OLI.
I would finally like to thank my wife and my family for their support and
patience.

iv
TABLE OF CONTENTS

ABSTRACT ii
ACKNOWLEDGEMENTS iv
TABLE OF CONTENTS v
LIST OF TABLES ix
LIST OF FIGURES xii
LIST OF APPENDICES xvii
1 INTRODUCTION 1
1.1 Background 1
1.2 Objectives 4
2 THEORETICAL SECTION 6
2.1 Pressure acid leaching of nickel laterites 6
2.2 Low-temperature pH measurement 9
2.2.1 Definition of pH 9
2.2.2 Primary method of measurement and primary pH standards 9
2.2.3 Secondary methods of measurement and secondary pH standards 11
2.2.4 Glass electrode cells and their calibration 11
2.3 High-temperature potentiometric pH measurement techniques 12
2.3.1 Reference electrodes for high-temperature measurements 12
2.3.2 High-temperature indicator electrodes 14
2.3.2.1 Hydrogen electrode 14
2.3.2.2 Yttria-stabilized zirconia pH sensor 20
v
2.4 Principle of operation of the yttria-stabilized zirconia pH sensor 32
2.4.1 Structural properties of yttria-stabilized zirconia 32
2.4.2 Electrical properties of yttria-stabilized zirconia 32
2.4.3 Mechanism of pH response 35
2.4.4 Effects of chemical purity 37
2.4.5 Resistance to chemical attack 38
2.5 Thermodynamics of the high-temperature electrochemical cell 40
2.5.1 Thermoelectric potential 42
2.5.2 Diffusion potential 42
2.5.3 Thermal diffusion potential 45
2.5.4 Streaming potential 45
2.6 Calculation of pH 46
2.7 Speciation models 49
3 EXPERIMENTAL SECTION 57
3.1 Measurement of pH 57
3.1.1 Apparatus 57
3.1.1.1 Flow-through yttria-stabilized zirconia pH electrode 59
3.1.1.2 Flow-through Ag/AgCl reference electrode 61
3.1.2 Measurement procedure 61
3.2 Pressure acid leaching 63
4 RESULTS AND DISCUSSION 64
4.1 Effect of nickel sulphate and magnesium sulphate on pH of sulphuric acid
solutions at elevated temperatures 66
vi
4.1.1 Effect of MgSO on the pH of H SO solutions at 250oC with
4 2 4(aq)
NaCl as the reference electrode solution 66
(aq)
4.1.2 Effect of NiSO on the pH of H SO solutions at 250oC with
4 2 4(aq)
NaCl as the reference electrode solution 75
(aq)
4.1.3 Effect of MgSO on the pH of H SO solutions at 200oC with
4 2 4(aq)
NaCl as the reference electrode solution 77
(aq)
4.1.4. Effect of MgSO on the pH of H SO solutions at 250oC and
4 2 4(aq)
200oC with LiCl as the reference electrode solution 80
(aq)
4.1.5 Effect of NiSO on the pH of H SO solutions at 250oC with
4 2 4(aq)
LiCl as the reference electrode solution 82
(aq)
4.2 Measurement of pH in aluminium-containing acidic sulphate solutions 88
4.2.1 Measurements at 250oC 88
4.2.1.1 Al (SO ) -H SO solutions at 250oC 88
2 4 3 2 4
4.2.1.2 MgSO -Al (SO ) -H SO solutions at 250oC 93
4 2 4 3 2 4
4.2.1.3 NiSO -Al (SO ) -H SO solutions at 250oC 97
4 2 4 3 2 4
4.2.2 Measurements at 200oC 99
4.2.2.1 Al (SO ) -H SO solutions at 200oC 99
2 4 3 2 4
4.2.2.2 MgSO -Al (SO ) -H SO solutions at 200oC 102
4 2 4 3 2 4
4.3 Measurement of pH in high-temperature nickel laterite pressure acid leach
process solutions 104
4.3.1 Pressure acid leach solutions 110
4.3.2 Diluted, acid-adjusted pressure acid leach solutions 111
4.3.3 Synthetic NiSO -H SO solutions 116
4 2 4
vii
4.3.4 Process implications 119
4.4 Sensitivity analysis 124
4.5 Potential variability 139
4.6 Comparison of calculations using different speciation models 145
4.7 Feasibility of using an internal reference electrode 155
5 CONCLUSIONS 160
6 RECOMMENDATIONS 165
7 NOMENCLATURE 167
8 REFERENCES 169
APPENDICES 178
Appendix A: Dielectric constants of bisulphate ion and water at temperatures
to 250oC 178
Appendix B: Data for pH measurements with yttria-stabilized zirconia electrodes
made from etched tubes 180
Appendix C: Data for pH measurements with yttria-stabilized zirconia electrodes
made from as received tubes 187
Appendix D: Resistivities of yttria-stabilized zirconia tubes 209
Appendix E: Data for experiments with an internal reference electrode 211

viii
LIST OF TABLES

Table No. Title Page

4.1 Calibration data for the MgSO -H SO -H O and NiSO -H SO -
4 2 4 2 4 2 4
H O systems at 250oC with 0.1 mol kg-1 NaCl as the reference
2
electrode solution. 67

4.2 Compositions of test solutions and corresponding diffusion
potentials for the MgSO -H SO -H O system at 250oC with 0.1
4 2 4 2
mol kg-1 NaCl as the reference electrode solution. 67

4.3 Comparison between experimental pH values (calculated based on
diffusion potentials evaluated using Henderson’s equation) and
theoretical pH values (calculated with OLI) for the MgSO -H SO -
4 2 4
H O system at 250oC with 0.1 mol kg-1 NaCl as the reference
2
electrode solution. 68

4.4 Comparison between experimental pH values (calculated based on
diffusion potentials evaluated using Harper’s equation) and
theoretical pH values (calculated with OLI) for the MgSO -H SO -
4 2 4
H O system at 250oC with 0.1 mol kg-1 NaCl as the reference
2
electrode solution. 68

4.5 Compositions of test solutions and corresponding diffusion
potentials for the NiSO -H SO -H O system at 250oC with 0.1 mol
4 2 4 2
kg-1 NaCl as the reference electrode solution. 75

4.6 Calibration data for the MgSO -H SO -H O system at 200oC with
4 2 4 2
0.1 mol kg-1 NaCl as the reference electrode solution. 78

4.7 Compositions of test solutions and corresponding diffusion
potentials for the MgSO -H SO -H O system at 200oC with 0.1
4 2 4 2
mol kg-1 NaCl as the reference electrode solution. 78

4.8 Calibration data for the MgSO -H SO -H O and NiSO -H SO -
4 2 4 2 4 2 4
H O systems at 250oC with 0.1 mol kg-1 LiCl as the reference
2
electrode solution. 81

4.9 Calibration data for the MgSO -H SO -H O system at 200oC with
4 2 4 2
0.1 mol kg-1 LiCl as the reference electrode solution. 81

4.10 Compositions of test solutions and corresponding diffusion
potentials for the MgSO -H SO -H O system at 250oC with 0.1
4 2 4 2
mol kg-1 LiCl as the reference electrode solution. 83
ix
4.11 Compositions of test solutions and corresponding diffusion
potentials for the MgSO -H SO -H O system at 200oC with 0.1
4 2 4 2
mol kg-1 NaCl as the reference electrode solution. 83

4.12 Compositions of test solutions and corresponding diffusion
potentials for the NiSO -H SO -H O system at 250oC with 0.1 mol
4 2 4 2
kg-1 LiCl as the reference electrode solution. 87

4.13 Compositions of test solutions and corresponding diffusion
potentials for the Al (SO ) -H SO , MgSO -Al (SO ) -H SO and
2 4 3 2 4 4 2 4 3 2 4
NiSO -Al (SO ) -H SO systems at 250oC. 89
4 2 4 3 2 4

4.14 Compositions of test solutions and corresponding diffusion
potentials for the Al (SO ) -H SO and MgSO -Al (SO ) -H SO
2 4 3 2 4 4 2 4 3 2 4
systems at 200oC. 100

4.15 Equilibrium constants for ion association of divalent metal
sulphates at 25oC. 104

4.16 Ionic mobilities at infinite dilution and 25oC. 104

4.17 Elemental composition (wt%) of laterite feeds. 105

4.18 Pressure acid leaching conditions under which solutions P#1, P#2
and P#3 were obtained. 106

4.19 Compositions of leach solutions P#1, P#2 and P#3 in g L-1 107

4.20 Compositions of leach solutions P#1, P#2 and P#3 in mol kg-1 107

4.21 Compositions of synthetic solutions SP#1 and SP#2. 108

4.22 Compositions of diluted, acid-adjusted leach solutions D#1 to D#5. 109

4.23 Compositions of binary NiSO -H SO synthetic solutions Ni#1 to
4 2 4
Ni#6. 109

4.24 Values for constants A, B and C in Equation (4.3) depending on
the type of equation. 120

4.25 Calibration data for the MgSO -H SO -H O system at 250oC. 147
4 2 4 2

4.26 Calculated values of the calibration coefficient. 147

4.27 Composition of test solutions and corresponding diffusion
potentials for the MgSO -H SO -H O system at 250oC. 151
4 2 4 2
x