An Investigation of the Role of Sodium Carbonate and Silica in the Neutral/Alkaline Pressure ...

121 Pages·2010·18.8 MB·English

by  Sammy Peters

An Investigation of the Role of Sodium Carbonate and Silica in
the Neutral/Alkaline Pressure Oxidation of Pyrite
by
Samuel Peters
A thesis submitted in conformity with the requirements
for the degree of Master of Applied Science
Chemical Engineering and Applied Chemistry
University of Toronto
© Copyright by Samuel Peters 2010
An Investigation of the Role of Sodium Carbonate and Silica
in the Neutral/Alkaline Pressure Oxidation of Pyrite
Samuel Peters
Master of Applied Science
Department of Chemical Engineering and Applied Chemistry
University of Toronto
2010
Abstract
Pressure oxidation of refractory gold ores containing carbonate minerals is conducted
under neutral/alkaline conditions in order to promote fast kinetics, reduced reagent consumption
and suppressing the formation of elemental sulphur and CO (which reduces the effectiveness of
2
the process). In this work, both the addition of sodium carbonate and the presence of silica were
investigated during the pressure oxidation of pyrite in the presence of calcium carbonate. It was
found that the shift to an alkaline leaching environment favours the formation of soluble sulphate
products over anhydrite (an industrial scale), but that the increase in kinetics is likely due to an
increase in pH and carbonate/bicarbonate concentrations. The presence of silica in the autoclave
induces the formation of an in situ iron oxyhydroxide silicate coating and a significant reduction
in pyrite oxidation, which was minimized by addition of sodium carbonate.
ii
Acknowledgments

I would like to thank my supervisor, Professor Vladimiros Papangelakis, for entrusting
me this project. More importantly, he provided me with the tools and guidance to personally
develop and refine my thought process in a more logical and communicable manner. His
reminders that research does not follow a straight and smooth course but results always tell a
story, expected or not, in addition to his encouragement and belief in me provided a constant
source of motivation to tell the story taking place in front of me. For this and more I am sincerely
thankful.
I would like to thank the members of the APEC group, namely, Georgiana Moldoveanu,
Ghazal Azimi, Ilya Perederiy and Ramanpal Saini for their continued support and assistance. The
time spent in the lab would not have been as valuable and enjoyable without you.
I would like to thank Dr. John Graydon for his time spent reviewing my thesis.
To all the students, faculty and staff, I want to thank you for creating such a great
environment to come to everyday. The relationships, knowledge and memories you have all
endowed me with is a treasure very dear to me.
I thank Barrick Gold Corporation and the University of Toronto for funding this research.
And finally, I want to acknowledge my family, for their love, support and encouragement
in all my endeavours.
iii
Table of Contents
Acknowledgements iii
Table of Contents iv
List of Tables v
List of Figures vi
List of Appendices vii
Definitions viii
Chapter 1 1
1 Introduction 1
Chapter 2 6
2 Experimental 6
2.1 Materials, Preparation and Characterization of Pyrite Samples 6
2.2 Apparatus 7
2.3 Experimental Conditions 7
2.4 Experimental Procedure 8
2.5 Analysis 9
2.6 Thermodynamic Modeling 10
Chapter 3 11
3 Results and Discussion 11
3.1 Initial Test Work 11
3.2 Dissolution Tests in the Absence of Silica 15
3.2.1 Calcium Carbonate 16
3.2.2 Sodium Carbonate 18
3.2.3 Sodium Carbonate Additions to Calcium Carbonate 19
3.3 Dissolution Tests in the Presence of Silica 26
Chapter 4 34
4 Conclusion 34
Chapter 5 36
5 References 36

iv
List of Tables

Table 1. Summary of pyrite oxidation control mechanism 2
Table 2. Simplified composition of an ore body 7
Table 3. Summary of simulations using OLI Stream Analyzer 10
Table 4. Matrix of initial slurry compositions 14

v
List of Figures

Figure 1. Pyrite oxidation from preliminary test work 12
Figure 2. SEM-BSE image of a pyrite particle coated in a hematite shell containing silicon 13
Figure 3. Pyrite oxidation and soluble sulphur in absence of silica 15
Figure 4. SEM-BSE image of a cross section of a pyrite particle surrounded by a ferric oxyhydroxide 17
coating in calcium carbonate medium after 1 h at 200°C
Figure 5. SEM-BSE image of a feed pyrite particle with several cracks 17
Figure 6. SEM-BSE image of reaction products of pyrite in sodium carbonate solutions after 1 hour 18
Figure 7. Soluble sulphate vs stoichiometric addition of Na CO at 60°C and 200°C modeled by OLI 20
2 3
and experimental values
Figure 8. Sulphate concentration (molal) versus extent of pyrite oxidation at 200°C modeled by OLI 21
Figure 9. Initial pH as a function of stoichiometric addition of Na CO to CaCO and the substitution 22
2 3 3
of Na CO for CaCO at 200°C modeled by OLI
2 3 3
Figure 10. pH (at 200°C) versus extent of pyrite oxidation as modeled by OLI 22
Figure 11. Bicarbonate concentration (molal) versus extent of pyrite oxidation at 200°C modeled by 23
OLI
Figure 12. Carbonate concentration (molal) versus extent of pyrite oxidation at 200°C modeled by 24
OLI
Figure 13. Dissolved CO concentration (molal) versus extent pyrite oxidation at 200°C modeled by 25
2
OLI
Figure 14. O partial pressure (kPa) versus extent pyrite oxidation at 200°C modeled by OLI 26
2
Figure 15. Pyrite oxidation and soluble sulphur in the presence of silica 27
Figure 16. Silicon concentration (molal) versus extent of pyrite oxidation at 200°C modeled by OLI 28
Figure 17. pH (at 200°C) versus extent of pyrite oxidation in the presence of silica modeled by OLI 29
Figure 18. Pyrite particle after 1 hour with CaCO and silica 29
3
Figure 19. Pyrite particle after 1 hour with Na CO and silica 30
2 3
Figure 20. Proposed dissolution mechanism of silica in water (Iler, 1979) 31
Figure 21. Precipitation of the surface of a metal hydroxide (Iler, 1979) 32
Figure 22. Silicon concentration versus temperature for various pH levels 33
vi
List of Appendices

Appendix A. SEM and EDS Analysis 41
Appendix B. Values of Tables 103
Appendix C. OLI Vapour Pressure Explanation 109

vii
Definitions
Acid mine drainage (AMD) – refers to the outflow of acidic water from (usually abandoned)
metal mines or coal mines. (Wikipedia)
Cyanidation – a method of leaching solid gold by forming a soluble aurocyanide complex.
Gangue – is the commercially worthless material that surrounds, or is closely mixed with, a
wanted ore. (Wikipedia)
Ore – a metal-bearing mineral or rock, or a native metal, that can be mined at a profit.
(Dictionary.com)
Pressure oxidation (POX) – A process by which high pressures and temperatures are used to
leach metals, dissolve oxides and oxidize sulphides in ore. Carried out in autoclaves, high
pressure reaction vessels.
Refractory gold ore – gold ore bodies from which the gold cannot be recovered by direct
cyanidation and therefore require pretreatment prior to cyanidation.

viii
1
Chapter 1
Introduction
1 Introduction

For high carbonate refractory gold ores alkaline POX is preferable to acid POX because it
does not consume acid and does not generate CO while maintaining fast kinetics under mild
2
conditions that allow the use of more affordable autoclave materials. Alkaline POX also
suppresses elemental sulphur formation (which is responsible for agglomeration and gold losses).
While oxidizing the gold encapsulating sulphide (mainly FeS and FeAsS) alkaline POX
2
improves the oxidative power of oxygen in the autoclave by reducing CO production.
2
A considerable amount of effort has been expended by various investigators over the last
40 years in order to understand the kinetics and mechanism of pyrite oxidation as it relates to
POX, acid mine drainage (AMD) and even coal desulphurization processes. Reactions (1) and
(2) describe the stoichiometry of the overall pyrite, FeS , oxidation in neutral or alkaline
2
conditions (Burkin and Edwards, 1963; Wheelock, 1981; Goldhaber, 1983; Berezowsky and
Weir; 1984; Guilinger et al., 1987; Nicholson et al., 1990; Koslides and Ciminelli, 1994;
Ciminelli and Osseo-Asare, 1995a,b; Descostes et al., 2002; Caldeira et al., 2003). Because of
high pH the iron products precipitate both in the bulk solution and as an iron oxide/hydroxide
coating on pyrite surface.
2 FeS + 7.5 O + 4 H O  Fe O + 4 SO 2- + 8 H+ (1)
2 2 2 2 3 4
FeS + 3.75 O + 2.5 H O  FeOOH + 2 SO 2- + 4 H+ (2)
2 2 2 4
2
Although pyrite oxidation kinetics has been found to conform to the shrinking core model
(SCM) a variety of rate control mechanisms has been identified in the literature and is presented
in Table 1. An analysis of Table 1 reveals that under acidic and low temperature alkaline
conditions, pyrite oxidation is controlled by chemical reaction, whereas under high temperature
and alkaline conditions the oxidation of FeS is under mixed and diffusion through a solid
2
product layer control. It is logical to conclude that the iron oxide coatings produced on the
surface are responsible for this behaviour.
Control
Regime Reference Temperature Solution
Mechanism
Surface Chemical
Acid Warren, 1956 130-210°C Water
Reaction
Papangelakis and Demopoulos,
Acid 140-180°C 0.5 M H SO Chemical Reaction
1991 2 4
1 g/L pyrite -
mixed,
Acid Long and Dixon, 2004 170-230°C 0.5 M H SO
2 4
20 g/L pyrite -
chemical reaction
Alkaline Stenhouse and Armstrong, 1952 93-170°C 1-6 M NaOH Diffusion
Alkaline Wheelock, 1981 100-200°C 0-1.6 M Na CO Diffusion
2 3
Alkaline Goldhaber, 1983 30°C 0.1 M KCl Chemical
60-150°C 0.375 M NaOH Mixed
Alkaline Nicholson et al., 1987 25°C 0.0005 M NaHCO Chemical Reaction
3
NACl, NaClO , NaHCO , Surface Chemical
Alkaline Brown and Jurinak, 1989 25°C 4 3
Na SO , K SO , CaCO Reaction
2 4 2 4 3
Alkaline Nicholson et al., 1990 25°C 0.0005 M NaHCO Mixed
3
Alkaline Koslides and Ciminelli, 1992 100-160°C NaOH Mixed
Ciminelli and Osseo-Asare,
Alkaline 50-85°C NaOH, Na CO Chemical
1995a,b 2 3
Table 1. Summary of pyrite oxidation control mechanism.