Ferric Sulphate Leaching of Pyrrhotite Tailings

FERRIC SULPHATE LEACHING OF PYRRHOTITE TAILINGS

Nazanin Samadifard

A thesis submitted in conformity with the requirements for the degree of Master of Applied Science Graduate Department of Chemical Engineering and Applied Chemistry University of Toronto

Ferric Sulfate Leaching of Pyrrhotite Tailings

Nazanin Samadifard

Master of Applied Science

Department of Chemical Engineering and Applied Chemistry University of Toronto

2015

Abstract

The present study investigates the potential to recover nickel from pyrrhotite tailings produced at the smelting operations of Vale in Sudbury, Ontario. Leaching tests were performed in acidic ferric sulphate media with 0.14 wt.% solids. The temperature was varied between 30 °C and

55 °C, and the ferric concentration was in a range 0.02- 0.3 M. The shrinking core model (SCM) was applied on the nickel extraction data. The dominant reaction mechanism was found to be diffusion control. The Arrhenius plot yielded an activation energy of Ea=62.12 kJ/mol based on apparent reaction rates obtained by the SCM. The reaction order with respect to ferric ion was found to be 1.0 at the high concentration range. SEM images of partially leached tailings confirmed the presence of elemental sulphur around the pyrrhotite particles, which was responsible for the observed non-linear leaching kinetics (diffusion control).

Acknowledgments

I would like to express my sincere appreciation to my supervisor Dr. Vladimiros Papangelakis for his constant support, guidance and patience throughout the course of my study.

Financial support of Vale Canada, Glencore, the Ontario Centers of Excellence (OCE), the Centre for Excellence in Mining Innovation (CEMI), and the Natural Sciences and Engineering Research Council of Canada (NSERC) are gratefully acknowledged.

I would extend my grateful thanks to Dr. Cheryl Washer for being a great consultant in the later stages of my project, and other members of APEC laboratory for their friendship, guidance and generosity.

The most sincere gratitude goes to my parents and my lovely family for their support and encouragement in all aspects of my life.

Finally, I dedicate this thesis to my dear husband Mohammadreza Dadkhah for his unconditional love and support during the completion of my master’s degree.

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Table of Contents

Acknowledgments ...... iii

List of Tables ...... vii

List of Figures ...... viii

Abbreviations ...... x

Chapter 1: Introduction ...... 1

1.1 Overview ...... 1

1.2 Research scope ...... 2

1.3 Objectives ...... 2

1.4 Thesis organization ...... 2

Chapter 2: Literature review ...... 4

2.1 Nickeliferous pyrrhotite tailings ...... 4

2.2 Pyrrhotite ...... 6

2.3 Dissolution pathways of pyrrhotite ...... 6

2.3.1 Oxidative dissolution ...... 6

2.3.2 Non-oxidative dissolution ...... 7

2.4 Factors affecting pyrrhotite oxidation ...... 8

2.4.1 Crystal structure ...... 8

2.4.2 Oxygen ...... 9

2.4.3 Ferric ion concentration ...... 9

2.4.4 Temperature ...... 9

2.5 Oxidation-reduction potential ...... 10

2.6 Shrinking core model ...... 11

2.7 Passivation ...... 13

Chapter 3: Materials and experimental procedures ...... 15

3.1 Materials ...... 15

3.2 Characterization of pyrrhotite tailings ...... 15

3.2.1 Particle size distribution ...... 15

3.2.2 Chemical characterization ...... 16

3.2.3 Mineralogical characterization ...... 17

3.3 Leaching tests ...... 18

3.4 Analytical techniques ...... 19

3.4.1 ICP analysis ...... 19

3.4.2 Potassium dichromate titration ...... 20

3.4.3 HPLC analysis ...... 20

3.4.4 SEM analysis ...... 21

3.5 Elemental sulfur determination ...... 21

Chapter 4: Results and discussions ...... 23

4.1 Effect of temperature on nickel extraction ...... 23

4.2 Effect of ferric sulphate concentration on nickel extraction ...... 27 v

4.3 Elemental sulphur generation during leaching...... 30

4.4 Morphology of elemental sulphur ...... 36

Chapter 5: Conclusions ...... 39

References...... 41

Appendices ...... 45

Appendix A. Nickel extraction kinetics at different temperatures...... 45

Appendix B. Ferric sulphate concentration effect on nickel extraction ...... 53

Appendix C. Elemental sulphur determination ...... 63

Appendix D. SEM images and EDX Spectra ...... 72

List of Tables

Table 2-1. Activation energies obtained for oxidation of pyrrhotite and pentlandite...... 10

Table 2-2. ORP correction factors for different reference electrodes...... 11

Table 2-3. Shrinking core model mechanisms...... 13

Table 3-1. Elemental composition of pyrrhotite tailings...... 17

Table 3-2. Mineralogical composition of pyrrhotite tailings...... 17

Table 4-1. Diffusion-control apparent rate constant values at various temperatures...... 26

Table 4-2. Chemical reaction-control apparent rate constant values at various temperatures. . 26

Table 4-3. Diffusion-control apparent rate constants at various ferric sulphate concentrations.

...... 29

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List of Figures

Figure 2-1. Schematic diagram of the SCM...... 12

Figure 3-1. Particle size distribution of pyrrhotite tailings...... 16

Figure 3-2 Experimental setup for the leaching experiments...... 19

Figure 3-3. Leached residues for elemental sulphur analysis...... 22

Figure 4-1. Effect of temperature on nickel extraction kinetics. Conditions: 0.14 wt.% solids,

0.12 M Fe2(SO4)3, and 0.2 M H2SO4. The error bars represent standard deviation from triplicate tests...... 23

Figure 4-2. Plot of the diffusion-control SCM vs. time at different temperatures...... 25

Figure 4-3. Plot of the chemical reaction-control SCM vs. time at different temperatures...... 24

Figure 4-4. Arrhenius plot for the diffusion-control process...... 26

Figure 4-5. Effect of Fe2(SO4)3 concentration on nickel extraction kinetics. Conditions: 0.14 wt.% solids, 0.2 M H2SO4, and 55 °C. The error bars represent standard deviation from triplicate tests...... 29

Figure 4-6. Determination of reaction order with respect to ferric ion...... 30

Figure 4-7. Extraction of Ni and S0 formation from leaching of pyrrhotite tailings. Conditions:

55 °C, 0.14 wt.% solids, 0.2 M Fe2(SO4)3, and 0.2 M H2SO4...... 31

Figure 4-8. Molar ratios of Fe2+/S0 and Ni2+/S0 generated during leaching of pyrrhotite tailings.

Conditions: 55 °C, 0.14 wt.% solids, 0.2 M Fe2(SO4)3 , 0.2 M H2SO4. The lines are least square linear fits to the data excluding the first time point (1 h)...... 32

viii

Figure 4-9. Extraction of Ni and S0 formation from leaching of pyrrhotite tailings. Conditions:

40 °C, 0.14 wt.% solids, 0.2 M Fe2(SO4)3, and 0.2 M H2SO4 ...... 34

Figure 4-10. Molar ratios of Fe2+/S0 and Ni2+/S0 generated during leaching of pyrrhotite tailings.

Conditions: 40 °C, 0.14 wt.% solids, 0.2 M Fe2(SO4)3 , 0.2 M H2SO4. The lines are least square linear fits to the data...... 34

Figure 4-11. ORP and pH values of the leach solution measured at different times for 55 °C and

40 °C. Conditions: 0.14 wt.% solids, 0.2 M Fe2(SO4)3, and 0.2 M H2SO4 ...... 36

Figure 4-12. SEM-BSE images of partially leached pyrrhotite particles after (a) time 0 (0%), (b)

19h (50%), (c) 72 h (80%) and (d) 144 h (90%) of leaching. Conditions: 55 °C, 0.56 wt.% solids,

0.2 M Fe2(SO4)3, and 0.2 M H2SO4...... 37

Figure 4-13. Elemental maps of a pyrrhotite particle after 90% nickel extraction...... 38

Abbreviations

AES Auger Electron Spectroscopy

AMD Acid Mine Drainage

CV Coefficient of Variation

HPLC High Pressure Liquid Chromatography

ICP-OES Inductively Coupled Plasma- Optical Emission Spectrometry

PCE Perchloroethylene

PSD Particle Size Distribution

QEMSCAN Quantitative Evaluation of Minerals by Scanning Electron Microscopy SCM Shrinking Core Model

SEM Scanning Electron Microscopy

SHE Standard Hydrogen Electrode

SIMS Secondary Ion Mass Spectrometry

UV Ultraviolet

XPS X-ray Photoelectron Spectroscopy

XRD X-ray Diffraction

Chapter 1: Introduction

1.1 Overview

Due to continuous exploitation, high-grade sulphide ores are gradually being exhausted. Low- grade sulphide minerals, including mining wastes are being considered as alternative metal resources for the future. Over the past 50 years, the Sudbury region in Ontario, Canada, has accumulated 50-100 million tonnes of pyrrhotite tailings as a consequence of the local smelting operations [1]. These tailings would be a significant low-grade nickel resource if a low-cost process for nickel recovery could be developed. In addition, environmental hazards like acid mine drainage (AMD) could be reduced by treating the pyrrhotite tailings. Previously, several pyrometallurgical processes were investigated for the treatment of the pyrrhotite tailings [1, 2,

3, 4]; however, most were abandoned due to the production of SO2 gas during the roasting process and the introduction of additional Fe in the slag. On the other hand, limited laboratory studies have been performed using hydrometallurgical techniques such as acid leaching by hydrochloric and nitric acid, or by high-pressure oxidation [5, 6, 7].

Although hydrometallurgical methods offer the advantage of avoiding SO2 emissions, none of these lab-scale processes has ever been commercialized. Recently, a bioleaching process was investigated as an alternative hydrometallurgical process in our lab [8]. In this process concept, pyrrhotite and pentlandite, the two principal nickel-bearing minerals in the tailings, are leached by ferric sulphate which is regenerated by microbial assistance.

Ferric ion is a strong oxidizing agent that plays an important role in oxidation of sulphide ores.

Only few studies have been conducted on the leaching of pyrrhotite in ferric sulphate solution

[9, 10, 11], whereas the direct effect of ferric ion on the oxidation of the pyrrhotite tailings has not been systematically studied. Earlier studies have shown that elemental sulphur is the dominant product of the ferric sulphate leaching of pyrrhotite [12, 13]. This is advantageous over the production of sulphuric acid as a by-product that would require recovery, storage, or neutralization, and impoundment of vast amounts of gypsum, which would make the process uneconomical.

1.2 Research scope

The aim of this work is to study the leaching kinetics of the nickeliferous pyrrhotite tailings in concentrated ferric sulphate media under abiotic conditions and develop a kinetics model that can be used for process design. The results of this study would be very useful in any follow-up bioleaching study, as it would set a baseline for the chemical performance of the system.

1.3 Objectives

The main objectives of this study are the following:

1. Characterize the mineralogy and chemistry of pyrrhotite tailings

2. Evaluate and explain the effects of ferric ion concentration and temperature on the

kinetics of nickel extraction

3. Determine the dissolution mechanism of pyrrhotite tailings

4. Investigate the formation and morphology of elemental sulphur produced during ferric

sulphate leaching of pyrrhotite tailings

1.4 Thesis organization

Chapter 2 provides a literature review on nickeliferous pyrrhotite tailings and pyrrhotite 2

oxidation. Chapter 3 introduces the experimental procedures and analytical techniques. Chapter

4 discusses the experimental results, and Chapter 5 summarizes the key findings of this study.

The complete set of results obtained from the experiments is given in the appendices.

2 Chapter 2: Literature review

2.1 Nickeliferous pyrrhotite tailings

The Sudbury basin in Ontario contains ore reserves from which, currently, both nickel and copper concentrates are produced. Both Vale and Glencore have been mining this ore type for more than 100 years. The ore contains the sulphide minerals pentlandite, chalcopyrite, pyrite, and large amounts of nickeliferous pyrrhotite. After grinding the ore, the minerals are concentrated into various fractions using flotation processes. Chalcopyrite is recovered as a copper concentrate for smelting, while pentlandite is recovered as a nickel concentrate. Since the nickel content of nickeliferous pyrrhotite is low (~0.8%) compare to pentlandite (~32%), it is removed from the nickel concentrate using pyrrhotite rejection circuits at the mills and disposed of in tailing ponds. The tailings are predominantly pyrrhotite with trace amounts of other sulphide minerals, as well as some gangue minerals (silicates). The majority of nickel is found within the pyrrhotite matrix, and the remainder is present as an interlocked fine-grained pentlandite fraction [1].

As mentioned earlier, pyrometallurgical processes for the treatment of the pyrrhotite tailings were abandoned due to the production of SO2 gas. In 1947-1954 Inco (Vale) developed two processes based on the dead roasting of the pyrrhotite tailings to produce SO2 gas and iron- nickel oxide. The SO2 gas was processed in an acid plant to produce saleable high-grade sulphuric acid. The iron-nickel oxide was selectively reduced to an iron-nickel alloy (FeNi) in a rotary kiln and then leached in ammonia-ammonium carbonate solution. The nickel was recovered as nickel oxide (NiO) by precipitation, then the iron oxide was indurated in a sintering machine. From this process the overall nickel recovery and the sulphur content of the iron ore

were about 85% and 0.25%, respectively. Thomson et al. has provided some details about the

Inco roasting process [2].

In 1960, Falconbridge (Glencore) began the operation of a pilot plant using a selective sulphation roast. The nickel from leaching was precipitated with fresh pyrrhotite, iron, and sulphur to form a matte that was treated in a smelter for nickel recovery. The iron ore and nickel produced from this process represented 97.2% and 87.5% recovery, respectively. Despite its low sulphur content (0.47%), the iron ore did not meet the quality standards as a salable product.

The sulphation conditions in the process produced high concentrations of SO3 and caused extensive aerial pollution [1, 3, 4].

Furthermore, limited lab-scale studies have been performed using hydrometallurgical techniques. Van Weert et al. [5] conducted an investigation on leaching of nickeliferous pyrrhotite in 6-8 N hydrochloric acid at 60-90 °C. Relatively fast leaching rates were obtained.

The main drawback of this method was that pentlandite, the other nickel-bearing mineral, could not be leached under these conditions; also part of the solubilized nickel reprecipitated with iron as nickeliferous marcasite (FeS2). In general, acid leaching is the least known practiced application since it involves the formation and handling of hydrogen sulphide (H2S).

Falconbridge performed a preliminary study on oxidation of pyrrhotite tailings using an autoclave at 100 psi air pressure and 110 °C. Although, there were no mineralogical restrictions in this route, the process never progressed beyond the experimental stage [7].

In order to design and optimize a hydrometallurgical treatment flowsheet for nickeliferous pyrrhotite tailings, a good understanding of chemistry and dissolution pathways of pyrrhotite is necessary.

2.2 Pyrrhotite

Pyrrhotite is one of the most abundant sulphide minerals in mining environments. It has the generic formula Fe1-xS, where x varies between 0 and 0.125 based on the vacancy of iron atoms in the crystal structure. Pyrrhotite with the stoichiometric formula FeS is called troilite, and it has a hexagonal structure. The monoclinic and more common structure of pyrrhotite (Fe7S8) is magnetic due to the deficiency of iron atoms in the structure.

2.3 Dissolution pathways of pyrrhotite

For most sulfide minerals, two main pathways of dissolution were suggested, namely oxidative and non-oxidative dissolution [10, 12, 14].

2.3.1 Oxidative dissolution

Both oxygen and ferric ion are important oxidants of sulphide minerals. When oxygen is the primary oxidant of pyrrhotite, the oxidation reaction can proceed by reaction 2-1, where x varies between 0 and 0.125.

2+ 2− + Fe1−xS + (2 − 0.5x)O2 + xH2O → (1 − x)Fe + SO4 + 2xH 2-1

2+ + 3+ 2Fe + 0.5O2 + 2H → 2Fe + H2O 2-2

3+ + Fe + 3H2O → Fe(OH)3 + 3H 2-3

3+ 2+ 2− + Fe1−xS + (8 − 2x)Fe + 4H2O → (9 − 3x)Fe + SO4 + 8H 2-4

In reaction 2-2, ferrous ion (Fe2+) is oxidized to ferric ion (Fe3+) by oxygen. At pH > 3, ferric ion precipitates as ferric hydroxide (Fe(OH)3), shown in reaction 2-3, but if the reaction occurs

under acidic conditions, a significant quantity of ferric ion will remain in the solution and can act as the oxidant of pyrrhotite (reaction 2-4). However, the oxidation reactions may not proceed to completion, and partial oxidation may occur instead [10, 13, 15]. In this case, a small proportion of sulphide in the mineral is transformed to sulphate ions, and the remainder is accumulated as elemental sulfur, shown in reactions 2-5 and 2-7. Reactions 2-6 and 2-8 show that elemental sulphur from the partial oxidation of pyrrhotite can further oxidize to sulphate ions; however, this can occur only at temperatures above the melting point of sulphur (~120 °C)

[16, 17].

+ 2+ 0 Fe1−xS + (0.5 − 0.5x)O2 + (2 − 2x)H → (1 − x)Fe + S + (1 − x)H2O 2-5

0 2− + S + 1.5O2 + H2O → SO4 + 2H 2-6

3+ 2+ 0 Fe1−xS + (2 − 2x)Fe → (3 − 3x)Fe + S 2-7

0 3+ 2− + 2+ S + 6 Fe + 4H2O → SO4 + 8H + 6Fe 2-8

2.3.2 Non-oxidative dissolution

Non-oxidative dissolution of sulphide minerals is another pathway of dissolution, which is an acid consuming reaction and produces a significant amount of hydrogen sulphide (H2S) according to reaction 2-9.

+ 2+ Fe1−xS + 2H → H2S + (1 − x)Fe 2-9

Tewari and Campbell [18] studied the dissolution of commercial troilite in dilute sulfuric acid and reported that the dissolution rate constant of troilite in pH values between 2 and 5.3 at 25

°C is 2.7 ± 0.2 ×10 -6 m/s. Van Weert et al. [5] investigated the kinetics of pyrrhotite dissolution under various conditions in hydrochloric acid, and they found that relatively rapid dissolution is 7

preceded by an induction period, during which no hydrogen sulphide is evolved. Thomas et al.

[19] studied the oxidation of synthesized pyrrhotite by 1 M perchloric acid and observed an induction period followed by hydrogen sulphide production at different temperature ranges.

Temperature, acid strength, and the amount of surface oxidation products were found to be the controlling factors of the length of the induction period.

2.4 Factors affecting pyrrhotite oxidation

Several variables have been identified as the main controlling factors of the rate of pyrrhotite oxidation.

2.4.1 Crystal structure

There is a lack of consensus in the literature about whether monoclinic or hexagonal pyrrhotite is more reactive towards oxidation. Conventionally, it is thought that the monoclinic structure is more reactive phase of pyrrhotite; this can be attributed to the vacancy of iron atoms in the crystal structure, which promotes electron transfer between the oxidant and the crystal lattice.

Lehmann et al. [20] showed that in a cyanide solution, monoclinic pyrrhotite has a higher dissolution rate than the hexagonal structure under various experimental conditions. Orlova et al. [21] reported the opposite result, finding that hexagonal pyrrhotite is more reactive than monoclinic pyrrhotite. Janzen et al. [10] studied the oxidation behaviour of twelve pyrrhotite samples, ranging from pure hexagonal to pure monoclinic. They measured the oxidation rates in terms of sulphate production and iron release and found no consistent correlation between the oxidation rate of pyrrhotite and the pyrrhotite structure.

2.4.2 Oxygen

Oxygen is the primary oxidant of pyrrhotite in basic environments. Knipe et al. [22] investigated the interaction of water and the surface of pyrrhotite in the absence of oxygen, concluding that no oxidation is observed upon exposure of pyrrhotite to deoxygenated water. There are relatively few studies that relate the oxidation rate of pyrrhotite to the amount of dissolved oxygen. Many studies have related the oxidation rates of sulphide ores to the partial pressure of oxygen [23,

10, 24]. Filippou et al. [24] studied the oxidative pressure leaching of monoclinic pyrrhotite at two different temperature regimes: below and above the melting point of sulphur (119 °C). They found that for temperatures above 119 °C, the rate of pyrrhotite oxidation shows first order dependency on oxygen partial pressure, whereas in the low temperature regime, the reaction rate is proportional to the square root of oxygen partial pressure.

2.4.3 Ferric ion concentration

At low pH values (pH <3) and atmospheric pressure, sulfides are mainly oxidized by ferric ion, and the kinetics of pyrrhotite oxidation by oxygen are relatively slow [10]. Accordingly, oxygen is considered to be the oxidizing agent for oxidation of ferrous ion to ferric ion as shown in reaction 2-2.

2.4.4 Temperature

The oxidation kinetics of pyrrhotite increases with increasing temperature. Generally, chemical leaching follows the Arrhenius law that describes the reaction rate constant (k) as a function of temperature (T) and activation energy (Ea) according to Equation 2-10. Several studies have reported activation energies for oxidation of pyrrhotite and pentlandite under different leaching conditions. Some of these values are listed in Table 2-1. 9

E − a 2-10 k = A e RT

Table 2-1. Activation energies obtained for oxidation of pyrrhotite and pentlandite.

Activation energy Sample Condition Oxidant Reference (kJ/mol) Nickeliferous 4-37 °C Bacteria 40 [15] Pyrrhotite pH=2.5

12 Museum-grade 25-45 °C Oxygen 47.7-62.5 a [10] pyrrhotite samples pH=2.5 79.1-106 b

Museum-grade 7.5-35 °C Ferric 22.8-63 [11] pyrrhotite samples pH=2.5

Museum-grade 10-33 °C Oxygen 58.1 [11] pyrrhotite samples pH=2 52.4 pH=4 100.4 Pyrrhotite ore 30pH=6-45 ° C Ferric 99 [9] from Bolivia pH=2

Nickeliferous 80-180 °C Oxygen 68.5 [24] pyrrhotite from 0.5 M H2SO4 Sudbury Pentlandite 30, 80 °C Ferric 61 [25] PO2 1,100%

a. Based on iron release b. Based on sulphate release

2.5 Oxidation-reduction potential

Ferric ion is a strong oxidizing agent and the relative abundance of ferric and ferrous ions in solution is the principal factor that determines oxidation-reduction potential (ORP) of a leach solution (also known as redox potential). Equations 2-11 and 2-12 are used to determine the

ORP at 25 °C, where the activities of ferrous and ferric irons are replaced by their concentrations in dilute solutions.

2-11 Fe3+ + 푒̅ → Fe2+ 휀0 = 0.771

[Fe3+] 2-12 E = 0.771 + 0.059 log h [Fe2+]

ORP is defined by Equation 2-12 when the standard hydrogen electrode (SHE) is the reference electrode. Correction factors should be used to convert the ORP measurements with different reference electrodes to Eh, shown in Table 2-2.

Table 2-2. ORP correction factors for different reference electrodes.

Reference electrode Electrode potential with respect to SHE (mV) Standard hydrogen electrode (SHE) 0

Saturated calomel electrode (SCE) +241

Ag/AgCl , 1 M KCl +192

Ag/AgCl , 4 M KCl + 228

Ag/AgCl , sat. KCl +236

2.6 Shrinking core model

Shrinking core model (SCM) is the most relevant kinetic model applied to the leaching kinetics of low-grade ores. The model is based on the reaction zone on a particle, which shrinks inwards as the reaction proceeds and leaves a solid product layer behind. The SCM assumes that the

reaction is first order with respect to the reactant, and particles are of uniform size [26].

Considering this model, five steps contribute to the overall rate of reaction as shown in Figure

2-1.

1) Diffusion of the reactant through the liquid film to the surface of the particle.

2) Diffusion of the reactant through the solid product layer to the surface of the unreacted

core.

3) Reaction of the reactant with the surface of the unreacted core.

4) Diffusion of the liquid products through the layer to the exterior surface of the particle.

5) Diffusion of the liquid products through the liquid film back into the main fluid.

Liquid Film

Surface of the particle R Solid Product rc Unreacted Core

Figure 2-1. Schematic diagram of the SCM.

In cases where the reaction is irreversible and no liquid product is formed, steps 4 and 5 do not contribute to the overall reaction rate, therefore the reaction would be controlled by one or a combination of the following mechanisms:

1) Liquid film diffusion 12

2) Chemical reaction

3) Diffusion through the product layer

Table 2-3 shows the mathematical description of the shrinking core kinetic models, where

XB is the fraction of ore that is converted.

Table 2-3. Shrinking core model mechanisms.

Rate controlling Reaction equation The time of Parameters mechanism complete reaction (τ)

Film diffusion t 푅휌 kg: mass transfer coefficient = X 휏 = 퐵 τ B between fluid and particle 3푏푘푔퐶퐴 b: stoichiometric reaction coefficient τ: the time of complete reaction 1 k : first order rate constant for Chemical reaction t 푅휌퐵 s = 1 − 3(1 − X )3 휏 = the surface reaction τ B 푏푘 퐶 푠 퐴 De: effective diffusion coefficient of A in the product layer CA: concentration of A in the Diffusion through t 2 푅2휌 fluid = 1 − 3(1 − X )3 + 2(1 − X ) 퐵 ρ molar density of B in the τ B B 휏 = B: 6푏퐷푒퐶퐴 solid the product layer R: initial particle radius

2.7 Passivation

Passivation is one of the most common problems of leaching of sulphide minerals. It describes the situation in which the mineral surface is covered with a layer of by-product like an oxide or other insoluble species. The product layer hinders the diffusion of reactants and products in/out of the sulphide particles and slows down the leaching kinetics. The existence of a passivation layer may not be proven based on only the chemical and thermodynamic properties of the leach solution, and it would only be directly detectable by the sensitive surface analysis methods such 13

as X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES or SAM) or secondary ion mass spectrometry (SIMS).

The formation of a product layer on pyrrhotite and pentlandite has been investigated by Legrand et al. [27]. They examined the oxidation of pyrrhotite and pentlandite in an aqueous solution with pH=9.3 and showed that a thin layer of Fe (III)-hydroxide (probably FeOOH) is formed on the surface of both minerals. Lu et al. [28] studied the kinetics and mechanism of pentlandite leaching in aerated acidic chloride/sulphate solutions and observed that pentlandite experiences passivation by elemental sulphur at low potentials. Ahonen and Tuovinen [15] performed a number of column bioleaching experiments with a complex sulphide ore and observed a compact ferric oxide/oxy-hydroxide, elemental sulphur, and covelite layer on pyrrhotite surfaces.

3 Chapter 3: Materials and experimental procedures

This chapter describes material characterization, experimental procedure, and analytical techniques used in this work.

3.1 Materials

Upgraded pyrrhotite tailings were obtained from Vale’s nickel smelting operations in Sudbury,

Ontario. The slurry was delivered in a large bucket, containing deoxygenated water due to the reducing and reactive nature of pyrrhotite. A core sample was taken and homogenized using a heavy-duty impeller. It was subsequently vacuum-filtered through Whatman quantitative filter paper and collected as several small cakes. The cakes were kept in double-sealed plastic bags in a freezer at -20 °C to minimize surface oxidation by oxygen in air. One day prior to any experiment, samples were thawed and their moisture content was determined. All chemicals used for preparing the solutions were reagent grade, and MilliQ deionized water was used.

3.2 Characterization of pyrrhotite tailings

3.2.1 Particle size distribution

The average particle size distribution of the pyrrhotite taillings were determined using a light scattering technique with Malvern Mastersizer 2000. Figure 3-1 shows the particle size distribution (PSD) of the pyrrhotite tailings. The distribution shows a tail on the left side indicating the presence of fine particles at about 20 Vol% below 10 μm. The average particle size is about 32 μm with a d50 of 28 μm and a coefficient of variation (CV) of 0.82.

Figure 3-1. Particle size distribution of pyrrhotite tailings.

3.2.2 Chemical characterization

Elemental composition of the pyrrhotite tailings was determined by digestion in aqua regia (1:3

HNO3: HCl) at high temperature. Approximately 0.4 g of the tailings was dissolved in a 20 mL aqua regia solution, transferred to a sealed vessel, and heated to 200 °C in a microwave digester

(Ethos EZ Microwave Digestion System). The mixture was diluted with 5% trace metal nitric acid in a volumetric flask. Multiple sets of dilutions were made based on the expected concentrations of base metals in the solution and were analyzed using Inductively Coupled

Plasma-Optical Emission Spectrometry (ICP-OES), Agilent Technologies 700. Although the exact composition of the pyrrhotite tailings produced at the mill varies with variations inside the ore body, the general composition is shown in Table 3-1. Fe (52.8%), S (32.0%), and Ni (1.0%), were found to be the most abundant elements in the pyrrhotite tailings.

Table 3-1. Elemental composition of pyrrhotite tailings.

Element Concentration (wt.%) Fe 52.81 S 32.00 Ni 1.00 Al 0.38 Cu 0.28 Ca 0.28 Mg 0.18 Co 0.02 Mn 0.02 Cr 0.01

3.2.3 Mineralogical characterization

Mineralogical characterization of the tailings was determined by Quantitative Evaluation of

Materials by Scanning Electron Microscopy (QEMSCAN) at SGS Canada Inc., Lakefield, ON, and summarized in Table 3-2. Monoclinic Pyrrhotite (Fe7S8) is dominant at 86.2 wt.%, whereas pentlandite accounts for only 1.2 wt.% of the tailings although it holds 40% of the total nickel.

The balance of 60% is associated with pyrrhotite.

Table 3-2. Mineralogical composition of pyrrhotite tailings.

Mineral Chemical Formula wt. %

Pyrrhotite Fe7S8 86.2 Pentlandite (Fe,Ni)9S8 1.2 Chalcopyrite CuFeS2 0.7 Pyrite FeS2 0.4 Silicates Mg2SiO4 3.8 Fe-(Ti) Oxides Fe3O4, FeTiO3 4.6 Quartz SiO2 3.1

3.3 Leaching tests

Leaching experiments were conducted in baffle-based Erlenmeyer flasks. Each flask was filled with 500 mL solution containing 0.2 M sulphuric acid and different ferric sulphate concentrations. The ferric sulphate concentrations tested were 0.02, 0.1, 0.2, and 0.3 M, corresponding to 1.3, 6.4, 12.7, and 19.1 excess stoichiometric levels with respect to monoclinic pyrrhotite based on the following overall reaction.

3+ 2+ 0 Fe7S8 + 14Fe → 21Fe + 8S 3-1

Before adding the pyrrhotite sample, the solution was heated to the desired temperature (30, 40, or 55 °C) on an orbital shaker set at 250 rpm. 250 rpm was enough to ensure solid suspension.

The reaction was initiated by adding about 0.7 g of the pyrrhotite tailings to the solution, unless otherwise stated. The small solids content ensured that leaching was performed under constant ferric concentration, which is a prerequisite to test the SCM on the extraction values. The flasks were then capped with aluminum foil to prevent cross-contamination. At regular time-intervals, the shaker was stopped, and the flasks were weighed to determine the mass of water that had evaporated. The evaporated water loss was made up by adding an equal volume of deionized water. The flasks were mixed thoroughly by shaking to keep the pulp density uniform, and 5 mL of slurry samples were taken for analysis. The samples were then immediately filtered using

0.45 μm syringe filters and analyzed for ORP, pH, nickel and ferrous ion concentration. Figure

3-2 shows the experimental setup for the leaching experiments.

Figure 3-2 Experimental setup for the leaching experiments.

3.4 Analytical techniques

Liquid samples taken during the leaching experiments were analyzed for the dissolved ferrous ion (Fe2+), dissolved nickel (Ni2+) and other trace metal elements. Selected solid samples after filtration were used for acid digestion or elemental sulphur analysis.

3.4.1 ICP analysis

The concentrations of trace metals in the liquid samples were analyzed via ICP-OES.

Wavelength calibration and torch alignment were routinely applied to ensure the normal operation of the ICP system. To prepare the calibration standards, a multi-element stock solution containing 250 mg/L Fe, Ni, S, Mg, and 100 mg/L Al, Cu, Co, Mn, Zn, and Cr was diluted with

5% trace metal nitric acid. The dilution factors were selected according to the expected concentrations of the elements of interest and the detection limit of the ICP.

3.4.2 Potassium dichromate titration

Ferrous ion concentration in the liquid samples was determined potentiometrically with 0.01 M potassium dichromate solution (K2Cr2O7). In the redox potential between potassium dichromate

2- 3+ and ferrous ion, dichromate ion (Cr2O7 ) gets reduced to two chromium III (Cr ) ions in an acidic environment with enough hydrogen ions (H+), shown in reactions 3-2 and 3-3.

2− + 3+ Cr2O7 + 14H + 6푒̅ → 2Cr + 7H2O 3-2

Fe3+ + 푒̅ → Fe2+ 3-3

2− 2+ + 3+ 3+ Cr2O7 + 6Fe + 14H → 6Fe + 2Cr + 7H2O 3-4

2- 2+ Reaction 3-4 represents the net ionic reaction. The 1:6 mole ratio of Cr2O7 : Fe provides the stoichiometric basis for all of the titration calculations.

3.4.3 HPLC analysis

All HPLC analyses were performed by High Performance Liquid Chromatography (HPLC) analysis with a DIONEX 3000 HPLC system having a 20 μL injection volume and a DIONEX

UV detector operating at 254 nm with a 5 nm bandwidth. A Dionex Acclaim 120 C18 reverse- phase column (4.6 x 250 mm; 5 μm) was used with a mobile phase composed of 95% methanol

(HPLC Grade, min. purity 99.9%) and 5% water (0.2 μm filtered) at a flow rate of 1 mL/min.

Because the presence of air bubbles in the mobile phase accounts for a variety of interferences, it was degassed using helium sparging for about 30 min before passing it through the column.

Each measurement lasted for 25 min, and the peak corresponding to the elemental sulphur appeared somewhere between 15 and 20 min, due to small changes in the eluent composition.

3.4.4 SEM analysis

The presence of elemental sulphur on the reacted pyrrhotite tailings was visualized on polished epoxy mounts using JEOL JSM-840 Scanning Electron Microscope (SEM) utilizing back- scattered electrons (BSE) at 15 keV accelerating potential difference. Energy-dispersive X-ray

(EDX) spectra were obtained with the same instrument. About 1 g of the solid sample was gently dispersed in epoxy and adequate time was allowed to ensure that the material is fully cured

(about 24 hours). The sample mount was gently ground and polished with a 1200 grit silicon carbide paper under running water and then subjected to SEM analysis.

3.5 Elemental sulphur determination

To extract elemental sulphur from the leach slurry, a combination of the techniques described by Dutrizac, and McGuire and Hamers [29, 30] was used. At the end of the leaching test the contents of the flask were vacuum filtered with a 10 cm Buchner filter using 15 cm Whatman filter paper. To remove any sulphur from the walls of the flask, it was rinsed several times with water and re-filtered. The residue was then washed twice with a weak sulphuric acid solution

(pH=3) to remove metal ions adsorbed by the residue. The filtrates were analyzed for nickel content using ICP. The filter paper with the leach residue was dried in a vacuum oven at 40 °C

(overnight) as shown in Figure 3-3. It was then folded several times and placed in a beaker containing 50 mL of PCE for 24 h. After sulphur extraction, the PCE solution was filtered using a 0.2 µm nylon syringe filter, and, if necessary, sequential dilutions were performed with PCE to bring the concentration of the sample to within the linear range of the standard solutions (6 -

240 mg/L of sulphur). The samples and the standards were then analyzed for elemental sulphur using HPLC. To measure any elemental sulfur that might exist on the pyrrhotite surface prior to

leaching, a known amount of sample (~0.25 g) was dissolved in 25 mL PCE and subjected to the same HPLC analysis. Standard solutions for elemental sulphur analysis were prepared within the range of 6-240 mg/L by a procedure similar to that employed by McGuire and Hamers [30].

Figure 3-3. Leached residues for elemental sulphur analysis.

4 Chapter 4: Results and discussions

4.1 Effect of temperature on nickel extraction

Figure 4-1 shows the effect of temperature on the kinetics of nickel extraction from the pyrrhotite tailings. These results were obtained by leaching about 0.14 wt.% solids in acidic ferric sulphate solution at 30, 40, and 55 °C, 0.12 M Fe2(SO4)3, and 0.2 M H2SO4. At 55 °C the nickel extraction is about 90% within 50 h, whereas at 30 and 40 °C the nickel extraction rate is slower, indicating a possible leveling off before complete dissolution. The curve fitting in Figure

4-1 was made with the diffusion-control SCM which is discussed below. The raw extraction data is reported in Appendix A, Tables A1.2, A2.2, and A3.2.

Figure 4-1. Effect of temperature on nickel extraction kinetics. Conditions: 0.14 wt.% solids, 0.12 M Fe2(SO4)3 and 0.2 M H2SO4. The error bars represent standard deviation from triplicate tests.

The extraction of nickel from the pyrrhotite tailings involves a reaction between Fe2(SO4)3 and

the nickel-containing sulfide minerals of pyrrhotite and pentlandite, which exist as a mixture.

Although tailings are non-uniform particulate material in terms of composition and size, the

SCM was tested as a semi-empirical kinetics model for the overall nickel dissolution kinetics

[26]. The nickel extraction vs. time data at different temperatures were plotted for three rate-

controlling forms of the SCM: liquid film diffusion (Equation 4-1); diffusion through the

product layer (Equation 4-2), and chemical reaction (Equation 4-3); where km, kd and kr are the

apparent reaction rate constants, and x is the fraction of the solid converted. The data is reported

in Appendix A, Tables A1.4, A2.4, and A3.4.

x = kmt 4-1

2⁄ 4-2 1 − 3(1 − x) 3 + 2(1 – x) = kdt

1⁄ 4-3 1 − (1 − x) 3 = krt 4-3

Figure 4-2. Plot of the diffusion-control SCM vs. time at different temperatures.

Figure 4-3. Plot of the chemical reaction-control SCM vs. time at different temperatures.

Equation 4-1 was ruled out because the curves in Figure 4-1 are not linear. The straight-line plots in Figure 4-2 produce higher correlation coefficients for all the temperatures in comparison to the chemical reaction-control SCM (Figure 4-3). Moreover, in Figure 4-2, the fitted lines at different temperatures extrapolate through the origin or are very close to the origin, whereas in

Figure 4-3 they do not. Apparent diffusion-control and chemical reaction-control rate constants

(kd and kr) at various temperatures were obtained from the slopes of the linear fits in Figures 4-

2 and 4-3 as shown in Tables 4-1 and 4-2, respectively. These values were used in an Arrhenius plot shown in Figure 4-4. The linear fit to the data in Figure 4-4 gave an activation energy value

2 of Ea=62.12 kJ/mol with a correlation coefficient close to unity (R =0.99). This is a high activation energy value commonly attributed to chemical reaction-control processes rather than diffusion-control processes since the former are more sensitive to temperature than the latter

[26].

Table 4-1. Diffusion-control apparent rate constant values at various temperatures.

-1 2 Temperature (°C) kd (h ) R 55 1.05×10-2 0.99 40 0.35×10-2 0.99 30 0.16×10-2 0.94

Table 4-2. Chemical reaction-control apparent rate constant values at various temperatures.

-1 2 Temperature (°C) kr (h ) R 55 0.96×10-2 0.96 40 0.45×10-2 0.92 30 0.25×10-2 0.83

Figure 4-4. Arrhenius plot for the diffusion-control process.

However, a number of kinetic studies on different systems have reported high values of activation energy for diffusion-control dissolution of sulphide minerals [25, 31]. Gbor and Jia 26

[32] proposed that this behaviour is due to the wide particle size distribution (PSD) of the particles. They showed mathematically that when the coefficient of variation (CV) of the PSD is large (0.7

The CV of the PSD of the pyrrhotite tailings was found to be 0.82, which is within the range indicated above.

The fact that the tailings are composed of two individual nickel-bearing minerals, each containing almost half of the total nickel, may complicate things. The present study was set to develop an overall model accounting for the overall kinetics behaviour of nickel in both pyrrhotite and pentlandite in the tailings. To obtain an accurate kinetic model for nickel extraction, the oxidation kinetics of pyrrhotite and pentlandite need to be known separately. This information needs to become available by tracking the conversion of each individual mineral in the mixture. A method proposed by Ingraham et al. [33] based on the idea that pyrrhotite is an acid soluble mineral, while pentlandite is essentially not acid soluble could be applied to the pyrrhotite tailings. Given the very small amount of pentlandite (1.2 wt.%) in the tailings and the experimentation under very low percent solids (~1.4 wt.%), it was not possible to achieve this in the present study, and therefore it was decided to treat the tailings as a uniform mixture until we develop a mineral separation (physical or chemical) protocol.

4.2 Effect of ferric sulphate concentration on nickel extraction

Figure 4-5 shows the effect of ferric sulphate concentration on the percent extraction of nickel at 55 °C. The results show that the rate of nickel extraction increases with increasing ferric sulphate concentration from 0.02 to 0.3 M. At 0.3 M ferric sulphate about 30 h of retention time is needed to achieve 96% extraction. At lower concentrations of ferric sulphate, the kinetics are 27

slower and the extraction curves tend to reach a plateau before complete dissolution of the tailings was achieved. The raw extraction data is reported in Appendix B, Tables B1.2, B2.2,

B3.2, and B4.2. The curve fitting in Figure 4-5 was made with the diffusion-control SCM. The plot of the diffusion-control SCM vs. time produced linear fits with higher correlation coefficients for all the ferric sulphate concentrations as to the chemical reaction-control SCM.

Apparent diffusion-control rate constants (kd) were obtained from the slopes of the linear fits and are shown in Table 4-3. (More details of the SCM data and the corresponding plots are provided in Appendix B, Tables B1.4, B2.4, B3.4, and B4.4 and Figures B-2 and B-3).

Using the mixed solvent-electrolyte (MSE) chemical model of the OLI software (OLI Analyzer

Studio 3.2), the concentrations of different iron species were calculated at 55 °C, 0.2 M H2SO4, and different ferric sulphate concentrations (0.02-0.3 M). It can be seen in Table 4-3 that ferric ion was found to be the dominant species, while Fe(OH)2+ concentration was low and did not vary significantly over the range of ferric sulphate concentrations tested. Therefore, the order of reaction was determined with respect to ferric ion concentration.

Figure 4-5. Effect of Fe2(SO4)3 concentration on nickel extraction kinetics. Conditions: 55 °C, 0.14 wt.% solids, and 0.2 M H2SO4, and. The error bars represent standard deviation from triplicate tests.

Table 4-3. Diffusion-control apparent rate constants at various concentrations of ferric sulphate and other iron species.

3+ 2+ -1 2 Fe2(SO4)3 Fe Fe(OH) kd (h ) R (M)0.3 0.502 0.0021 2.50×10-2 0.99 0.2 0.311 0.0025 1.00×10-2 0.97 0.1 0.153 0.0024 0.71×10-2 0.98 0.02 0.039 0.0011 0.39×10-2 0.99

Figure 4-6 7 shows a plot of the log10 of kd against the log10 of ferric concentration, and the linear fit to the data indicates that the order of reaction with respect to ferric ion is about 0.6.

However, the order of reaction with respect to ferric ion within the range of 0.1 to 0.3 M ferric sulphate gives a value of 1.0 which is consistent with the expected value for a diffusion- control process, since Fick’s law is first order with respect to diffusing species. This discrepancy is due to the fact that at 0.02 M ferric sulphate the stoichiometric ratio of ferric

sulphate: pyrrhotite is 1.2:1 and does not meet the requirements for the use of the SCM, where the concentration of the reactant is required to remain constant during the reaction. Therefore, the lowest ferric sulphate concentration (0.02 M) was ruled out for determination of the reaction order with respect to ferric ion.

Figure 4-6. Determination of reaction order with respect to ferric ion.

4.3 Elemental sulphur generation during leaching

Figure 4-7 shows the extraction of nickel and elemental sulphur formation at 55 °C, 0.2 M

Fe2(SO4)3 and 0.2 M H2SO4. Each pair of points represents a separate experiment since the low percent solids employed did not permit sufficient slurry sampling for sulphur analysis. It can be seen in Figure 4-7 that the pyrrhotite tailings dissolve relatively fast, and after about 50 h 85% extraction of nickel is achieved, and about 75% of the sulphide in the pyrrhotite tailings has been converted into elemental sulphur. Although the extraction curves follow a similar trend, the elemental sulphur extraction is consistently lower. The difference should be the amount of

sulphide oxidized to sulphate ions, although the amount of sulphate could not be determined due to the high sulphate background in the ferric sulphate solution. The data of nickel extraction and elemental sulphur conversion is reported in Appendix C, Tables C1.2, C1.3, and C1.4.

Figure 4-7. Extraction of Ni and S0 formation from leaching of pyrrhotite tailings. Conditions: 55 °C, 0.14 wt.% solids, 0.2 M Fe2(SO4)3, and 0.2 M H2SO4.

Figure 4-8 shows the molar ratios of Fe2+/S0 and Ni2+/S0 with time from data in Figure 4-7.

Initially (at ~ 1 h), the molar ratios of Fe2+/S0 and Ni2+/S0 are high suggesting that the dissolution of the pyrrhotite tailings at the beginning of leaching does not produce as much elemental sulphur as at later stages of reaction. This is likely due to the direct acid attack of pyrrhotite with the formation of dissolved hydrogen sulphide, shown in reaction 4-4 (two electrons are added to account for the non-stoichiometric nature of pyrrhotite). However, no noticeable gaseous hydrogen sulphide was detected, it is possible that subsequent oxidation of hydrogen sulphide by ferric ions occurs at a slower rate according to reaction 4-5. In any case, initial surface

oxidation of sulphides in the tailings could not account for the initial high ratios. These values were determined to be about 2% of total nickel in the tailings samples. The lines in Figure 4-8 are least-square linear fits to the data points (excluding 1 h), where the average Fe2+/S0 ratio is

2.26 and the Ni2+/S0 ratio is 0.022. The data is provided in Appendix C, Table C1-6.

Figure 4-8. Molar ratios of Fe2+/S0 and Ni2+/S0 generated during leaching of pyrrhotite tailings. Conditions: 55 °C, 0.14 wt.% solids, 0.2 M Fe2(SO4)3, and 0.2 M H2SO4. The lines are least square linear fits to the data excluding the first time point (1 h).

+ 2+ 0 Fe7S8 + 14H → 7H2S + 7Fe + S 4-4

3+ 2+ + 0 H2S + 2Fe → 2Fe + 2H + S 4-5

3+ 2+ 0 Fe7S8 + 14Fe → 21Fe + 8S 4-6

It can be seen in reaction 4-6 that if monoclinic pyrrhotite were completely dissolved, the molar ratio of Fe2+/S0 would be 2.6. The fact that the pyrrhotite tailings are not pure and contain small quantities of other sulphide minerals (pentlandite, pyrite and chalcopyrite) may explain the difference between the expected and observed values.

Pyrrhotite is the dominant mineral of the tailings (86.2 wt.%), and therefore most of the elemental sulphur produced is the by-product of pyrrhotite dissolution. However, nickel in the tailings is attributed to both pentlandite and pyrrhotite, accounting for 40 and 60% of the total nickel, respectively. Accordingly, if pyrrhotite and pentlandite react at different rates, the

Ni2+/S0 ratio is expected to change during the reaction, but it can be seen in Figure 4-8 that the

Ni2+/S0 ratio remains relatively constant over time. This shows that there is a similarity between the dissolution rates of both pyrrhotite and pentlandite, indicating that no galvanic interaction has occurred between the two minerals. This supports the use of the SCM for the analysis of the dissolution kinetics. The theoretical value of Ni2+/S0 obtained based on the assumption that pyrrhotite and pentlandite react at a similar rate is 0.02 which is comparable with the molar ratios obtained for both 55 and 40 °C (Ni2+/S0 = 0.022).

To examine whether ferrous ion is re-oxidized to ferric ion due to oxygen entrainment, a control experiment was conducted in which 0.02 M ferrous ion was added to 0.2 M ferric sulphate solution at 55 °C and 250 rpm. After one week, the concentration of ferrous ion remained reasonably constant over time (1% overall reduction). This suggests that there was no oxygen entrainment in these tests, and all oxidative reactions are due to ferric attack.

Figure 4-9 shows the extraction of nickel and elemental sulphur formation from the leaching of the pyrrhotite tailings at a lower temperature of 40 °C, 0.2 M Fe2(SO4)3, and 0.2 M H2SO4, where each pair of points represents a separate experiment. It can be seen in Figure 4-9 that the pyrrhotite tailings dissolve relatively fast, and after about 100 h 80% of nickel is extracted, while only about 70% of sulphide is extracted as elemental sulphur. Although the extraction curve for elemental sulphur follows a similar trend to that of nickel, the elemental sulphur extraction is

consistently lower due to reasons explained above. The data of nickel extraction and elemental sulphur conversion is reported in Appendix C, Tables C2.2, C2.3, and C2.4.

Figure 4-9. Extraction of Ni and S0 formation from leaching of pyrrhotite tailings. Conditions: 40 °C, 0.14 wt.% solids, 0.2 M Fe2(SO4)3, and 0.2 M H2SO4.

Figure 4-10. Molar ratios of Fe2+/S0 and Ni2+/S0 generated during leaching of pyrrhotite Tailings. Conditions: 40 °C, 0.14 wt.% solids, 0.2 M Fe2(SO4)3, and 0.2 M H2SO4 . The lines are least- square linear fits to the data.

Figure 4-10 shows the molar ratios of Fe2+/S0 and Ni2+/S0 with time from data in Figure 4-9.

The lines are least-square linear fits to the data points, where the average ratios of Ni2+/S0 and

Fe2+/S0 are 2.62 and 0.022, respectively. Here, the relatively constant Ni2+/S0 ratio shows that the rates of nickel extraction from both pyrrhotite (60% of total Ni) and pentlandite (40% of total Ni) must be very similar. The constant molar ratios of Ni2+/S0 and Fe2+/S0 are similar at both temperatures of 40 and 55 °C indicating that the measured elemental sulphur yield was essentially independent of the temperature of the leach solution. Figure 4-11 shows that the ORP and pH values of the leach solution remain relatively constant at both temperatures. The data is provided in Appendix C, Tables C2-5 and C2-6.

Figure 4-11. ORP and pH values of the leach solution measured at different times for 55 °C and 40 °C. Conditions: 0.14 wt.% solids, 0.2 M Fe2(SO4)3, and 0.2 M H2SO4.

4.4 Morphology of elemental sulphur

To observe the morphology of elemental sulphur on partially leached pyrrhotite particles at different percent extractions of nickel, three additional experiments were performed by adding about 2.8 g of the pyrrhotite tailings into 500 mL leach solution containing 0.2 M ferric sulphate and 0.2 M sulphuric acid at 55 °C. After 19, 72, and 144 h of leaching, the contents of the flasks were vacuum filtered, and the residues were dried, epoxy mounted, polished, and subjected to

SEM analysis. The ICP analysis of the leach solutions showed that the 19, 72, and 144 h of leaching correspond to 50, 80 and 90% nickel extractions, respectively. These values are lower than those obtained with 1.4 g/L of initial solids loading due to different stoichiometric excess of ferric.

Figure 4-12 shows the SEM-BSE images of the leached pyrrhotite particles at 0% (a), 50% (b),

80% (c), and 90% (d) nickel extractions. Clearly, a distinct and relatively porous sulphur layer forms on the pyrrhotite particles that allows the diffusion of the reactants to the surface of the 36

particles. This is consistent with the postulated SCM for diffusion through the product layer- control processes. It seems that as the layer grows, it reaches a point that halts the reaction. This phenomenon is less dominant as the temperature increases and accelerates the rate of allowing sulphur to remain amorphous and porous for a longer time. Elemental mapping of a pyrrhotite particle after 90% nickel extraction was obtained with the help of EDX and are shown in Figure

4-13.

Figure 4-12. SEM-BSE images of partially leached pyrrhotite particles after (a) time 0 (0%), (b) 19h (50%), (c) 72 h (80%) and (d) 144 h (90%) of leaching. Conditions: 55 °C, 0.56 wt.% solids, 0.2 M Fe2(SO4)3, and 0.2 M H2SO4.

20 20 µm

Figure 4-13. Elemental maps of a pyrrhotite particle after 90% nickel extraction.

5 Chapter 5: Conclusions

The present work investigated the dissolution kinetics of pyrrhotite tailings from Sudbury,

Ontario, Canada, in concentrated ferric sulphate media. The effect of temperature on the nickel extraction kinetics was investigated, and the dissolution mechanism of the tailings was determined using the SCM. The diffusion-control kinetic model best fit the nickel extraction data within the temperature range studied (30- 55 °C) with an activation energy value of 62.12 kJ/mol. The high activation energy was attributed to the wide PSD and the non-homogeneity of the tailings (two nickel-bearing minerals). The reaction order with respect to ferric ion was found to be 0.65 for the whole range of concentrations studied. However, the order of reaction with respect to ferric ion was found to be 1.0 for ferric sulphate concentrations ranging from 0.1 to

0.3 M.

The results of this study suggested that the overall dissolution process of the pyrrhotite tailings was governed by diffusion through the sulphur product layer. The molar ratios of Fe2+/S0 and

Ni2+/S0 at 40 °C were about 2.62 and 0.022, respectively, regardless of time. Similar values of the molar ratios were obtained at 55 °C, where Fe2+/S0 ratio was 2.26 and Ni2+/S0 ratio was

0.022. The Fe2+/S0 ratios compare well with the theoretical value of 2.6 based on reaction stoichiometry, whereas the constant Ni2+/S0 ratio is indicative of a similarity between the dissolution rates of both pyrrhotite and pentlandite.

SEM-BSE images showed a distinct and relatively porous layer of elemental sulphur around the leached pyrrhotite particles. Continuation of the leaching process resulted in a progressive thickening of the sulphur layer to the point of reaction stoppage at certain extraction levels

depending on temperature. This is consistent with the observed non-linear leaching kinetics

(diffusion control).

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Appendices Appendix A. Nickel extraction kinetics at different temperatures

Figure 4-1 in chapter 4 shows the effect of temperature on the kinetics of nickel extraction from the pyrrhotite tailings. The experimental conditions performed at 55, 40, and 30 °C are shown in Tables A1.1, A2.1, and A3.1, respectively. The raw data corresponding to the extraction curves in Figure 4.1 are shown in tables A1.2, A2.2, and A3.2.

Table A1.1 Experimental conditions for nickel extraction at 55 °C.

Temperature 55 °C

Fe2(SO4)3.5H2O (g) 28.7 Fe2(SO4)3.5H2O (M) 0.12 Shaking speed (rpm) 250 H2SO4 (M) 0.2 Reaction volume (mL) 500 Sample IDs Exp-1, Exp-2, Exp-3 Wet weight (g) Exp-1. 0.72 Exp-2. 0.72 Exp-3. 0.72 Sample moisture content (wt%) 8.26 Dry weight (g) Exp-1. 0.67 Exp-2. 0.67 Exp-3. 0.67 Ni weight in the dry sample (mg) Exp-1. 6.7 (1 wt.%) Exp-2. 6.7 Exp-3. 6.7

Table A1.2 Nickel extraction data at 55 °C, 0.2 M H2SO4, 0.12 M ferric sulphate. Time (h) Ni (mg) Ni extraction (%) 0 Exp-1 Exp-2 Exp-3 Exp-1 Exp-2 Exp-3 Average 1 1.37 1.37 1.36 20.44 20.58 20.38 20.45±0.10 3 2.01 2.01 2.14 29.94 30.12 31.98 30.68±1.13 6 2.79 2.68 2.72 41.59 40.14 40.70 40.81±0.73 9 3.08 3.41 2.99 46.00 51.07 44.66 47.24±3.38 24 4.69 5.06 4.49 69.95 75.72 67.11 70.92±4.39 31 5.10 5.01 4.83 76.30 75.00 72.16 74.39±2.06 49 6.44 - 5.50 95.96 - 82.14 89.05±9.77 59 5.88 5.99 - 87.66 89.69 - 88.67±1.43 76 - 6.07 6.02 - 91.30 90.28 90.79±0.72 97 5.99 6.05 - 88.66 90.77 - 89.71±1.49

Table A2.1 Experimental conditions for nickel extraction at 40 °C. Temperature 40 ° C

Fe2(SO4)3.5H2O (g) 28.7 Fe2(SO4)3.5H2O (M) 0.12 Shaking speed (rpm) 250 H2SO4 (M) 0.2 Reaction volume (mL) 500 Sample IDs Exp-1, Exp-2 Wet weight (g) Exp-1. 0.72 Exp-2. 0.73 Sample moisture content (wt%) 8.26 Dry weight (g) Exp-1. 0.67 Exp-2. 0.68

Ni weight in the dry sample (mg) Exp-1. 6.7 (1 wt.%) Exp-2. 6.8

Table A2.2 Nickel extraction data at 40 °C, 0.2 M H2SO4, 0.12 M ferric sulphate. Time (h) Ni (mg) Ni extraction (%) 0 Exp-1 Exp-2 Exp-1 Exp-2 Average 1 1.36 1.32 20.30 19.75 20.02±0.39 3 1.59 1.78 23.76 26.70 25.23±2.08 7 1.95 - 29.05 - 29.05 24 3.27 3.01 48.85 45.15 47.00±2.61 31 3.35 3.20 50.07 48.01 49.04±1.46 48 3.96 3.92 59.17 58.82 58.99±0.25 61 4.40 - 65.62 - 65.62 73 4.46 4.50 66.57 67.45 67.01±0.62 96 5.01 4.68 74.79 72.10 72.44±3.32 120 5.44 5.39 81.13 80.75 80.94±0.27

Table A3.1 Experimental conditions for nickel extraction at 30 °C. Temperature 30 °C

Fe (SO ) .5H O (g) 28.7 2 4 3 2 Fe2(SO4)3.5H2O (M) 0.12 Shaking speed (rpm) 250

H2SO4 (M) 0.2 Reaction volume (mL) 500 Sample IDs Exp-1, Exp-2, Exp-3 Wet weight (g) Exp-1. 0.72 Exp-2. 0.72 Exp-3. 0.72 Moisture content (wt%) 8.26 Dry weight (g) Exp-1. 0.67 Exp-2. 0.67 Exp-3. 0.67 Ni weight in the dry sample (mg) Exp-1. 6.7 (1 wt.%) Exp-2. 6.7 Exp-3. 6.7

Table A3.2 Nickel extraction data at 30 °C, 0.2 M H2SO4, 0.12 M ferric sulphate. Time(h) Ni (mg) Ni extraction (%) 0 Exp-1 Exp-2 Exp-3 Exp-1 Exp-2 Exp-3 Average 3 1.37 1.32 1.37 20.44 19.74 20.33 20.17±0.37 7 1.55 - - 23.12 - - 23.12 10 1.63 1.66 1.70 24.30 24.86 25.23 24.80±0.47 24 2.15 2.02 2.15 31.96 30.32 31.91 31.40±0.93 31 2.69 2.24 2.89 39.98 33.55 42.94 38.83±4.80 49 2.75 2.53 3.31 40.95 37.92 49.05 42.64±5.75 59 3.03 3.10 - 45.08 46.47 - 45.78±0.98 76 3.01 2.72 2.74 44.78 40.68 40.67 42.04±2.37 97 3.21 3.10 3.13 47.72 46.44 46.49 46.88±0.72 120 3.50 3.29 3.41 52.08 49.28 50.64 50.67±1.40

Ferrous ion concentration and pH values of the leach solutions, corresponding to Figure 4-1 in chapter 4, were measured during the ferric leaching of the pyrrhotite tailings. Figure A-1 shows the ferrous ion concentration curves at 55, 40, and 30 °C, and the raw data are shown in tables

A1.3, A2.3, and A3.3, respectively.

Figure A-1. Ferrous ion concentration at different temperatures and 0.12 M ferric sulphate-0.2 M H2SO4 media. 48

Table A1.3 Ferrous ion concentrations and pH values at 55 °C, 0.2 M H2SO4, 0.12M ferric sulphate.

Samples Time (h) Fe2+ (M) pH Average Fe2+ Average pH Exp.1 1 0.0014 0.87 0.0015±0.0001 0.84±0.030 Exp.2 0.0017 0.81 Exp.3 0.0014 0.84 Exp.1 3 0.0057 0.79 Exp.2 0.006 0.83 0.0057±0.0002 0.82±0.028 Exp.3 0.0055 0.83 Exp.1 6 0.0091 0.78 Exp.2 0.0096 0.80 0.0093±0.0003 0.79±0.013 Exp.3 0.0091 0.80 Exp.1 9 0.0110 0.76 Exp.2 0.0115 0.79 0.0114±0.0003 0.79±0.028 Exp.3 0.0115 0.82 Exp.1 24 0.0168 0.81 Exp.2 0.0173 0.87 0.0171±0.0003 0.82±0.045 Exp.3 0.0173 0.79 Exp.1 31 0.018 0.78 Exp.2 0.0195 0.83 0.0185±0.0008 0.82±0.045 Exp.3 0.018 0.87 Exp.1 49 0.021 0.84 Exp.2 0.021 0.85 0.021 0.84±0.002 Exp.3 0.021 0.84 Exp.1 59 0.021 0.79 Exp.2 0.021 0.79 0.021 0.80±0.022 Exp.3 0.021 0.83 Exp.1 76 0.021 0.80 Exp.2 0.021 0.85 0.021 0.83±0.022 Exp.3 0.021 0.83

Table A2.3 Ferrous ion concentrations and pH values at 40 °C, 0.2 M H2SO4, 0.12 M ferric sulphate.

Samples Time (h) Fe2+ (M) pH Average Fe2+ Average pH Exp.1 3 0.0031 0.88 Exp.2 0.0035 0.86 0.0033±0.0003 0.87±0.009 Exp.1 7 0.0058 0.891 Exp.2 - - 0.0058 0.89 Exp.1 24 0.0108 0.81 Exp.2 0.0117 0.82 0.0112±0.0006 0.81±0.009 Exp.1 31 0.012 0.79 Exp.2 - - 0.012 0.79 Exp.1 48 0.015 0.85 49

Exp.2 0.0154 0.79 0.0152±0.0003 0.82±0.042 Exp.1 61 0.0171 0.81 Exp.2 - - 0.0171 0.81 Exp.1 73 0.0182 0.78 Exp.2 0.0187 0.74 0.0185±0.0004 0.76±0.029 Exp.1 96 0.02 0.82 Exp.2 0.0199 0.80 0.0199±0.0001 0.81±0.018

Table A3.3 Ferrous ion concentrations and pH values at 30 °C, 0.2 M H2SO4, 0.12 M ferric sulphate.

Samples Time (h) Fe2+ (M) pH Average Fe2+ Average pH Exp.1 3 0.0019 0.90 Exp.2 0.0019 0.87 0.00192 0.89±0.004 Exp.3 0.0019 0.90 Exp.1 7 0.0041 0.92 Exp.2 0.0034 0.82 0.0038±0.000 0.86±0.011 Exp.3 0.0041 0.83 42 Exp.1 10 0.0055 0.77 Exp.2 0.0048 0.78 0.0054±0.000 0.78±0.047 Exp.3 0.0058 0.76 5 Exp.1 24 0.0096 0.86 Exp.2 0.0084 0.87 0.009±0.0006 0.86±0.015 Exp.3 0.0091 0.84 Exp.1 31 0.0096 0.89 Exp.2 0.009 0.86 0.0094±0.000 0.84±0.069 Exp.3 0.0096 0.76 35 Exp.1 49 0.012 0.80 Exp.2 0.0114 0.73 0.012±0.0006 0.74±0.054 Exp.3 0.0126 0.70 Exp.1 59 0.0132 0.80 Exp.2 0.0126 0.73 0.0128±0.000 0.79±0.536 Exp.3 0.0126 0.84 35 Exp.1 76 0.0144 0.81 Exp.2 0.0132 0.79 0.014±0.0007 0.83±0.051 Exp.3 0.0144 0.89 Exp.1 97 0.0156 0.85 Exp.2 0.0144 0.79 0.0152±0.000 0.81±0.037 Exp.3 0.0156 0.78 7

The shrinking core model (SCM) was applied to the nickel extraction data at different temperatures (30, 40, and 55 °C), and the results were plotted in Figures 4-2 and 4-3 in Chapter

4 for diffusion-control and chemical reaction-control kinetic models. The raw data of the figures is summarized in Tables A1-4, A2-4, and A3-4.

Table A1.4 SCM fitted to nickel extraction data at 55 °C, 0.2 M H2SO4, 0.12 M ferric sulphate.

Time (h) Conversion (x) 1-(1-x)1/3 1-3(1-x)2/3+2(1-x) 0 0 0 0 1 0.205 0.073 0.015 3 0.307 0.115 0.037 6 0.408 0.160 0.069 9 0.453 0.182 0.087 24 0.685 0.320 0.241 31 0.744 0.364 0.302 49 0.890 0.521 0.532

Table A2.4 SCM fitted to nickel extraction data at 40 °C, 0.2 M H2SO4, 0.12 M ferric sulphate.

Time (h) Conversion (x) Chemical-control Diffusion-control model model 1-(1-x)1/3 1-3(1-x)2/3+2(1-x) 0 0 0 0 1 0.200 0.072 0.015 3 0.252 0.092 0.024 7 0.290 0.108 0.032 24 0.470 0.191 0.095 31 0.490 0.201 0.105 48 0.590 0.257 0.164 61 0.656 0.299 0.215

Table A3.4 SCM fitted to nickel extraction data at 30 °C, 0.2 M H2SO4, 0.12 M ferric sulphate. Time (h) Conversion (x) Chemical-control Diffusion-control model model 1-(1-x)1/3 1-3(1-x)2/3+2(1-x) 0 0 0 0 3 0.201 0.072 0.015 7 0.231 0.088 0.022 10 0.247 0.091 0.023 24 0.314 0.118 0.038 31 0.389 0.151 0.061 49 0.426 0.169 0.076 59 0.458 0.185 0.090

Appendix B. Ferric sulphate concentration effect on nickel extraction

Figure 4-1 in chapter 4 shows the effect of ferric sulphate concentration on the kinetics of nickel extraction from the pyrrhotite tailings. The experimental conditions performed at 0.3, 0.2, 0.1, and 0.02 M ferric sulphate are shown in Tables B1.1, B2.1, B3.1, and B4.1, respectively. The raw data corresponding to the extraction curves in Figure 4.5 is shown in tables B1.2, B2.2,

B3.2, and B4.2.

Table B1.1 Experimental conditions.

Temperature 55 °C Fe2(SO4)3.5H2O (g) 72.8 Fe2(SO4)3.5H2O (M) 0.3 Shaking speed (rpm) 250 H2SO4 (M) 0.2 Reaction volume (mL) 500 Sample IDs Exp-1, Exp-2, Exp-3 Wet weight (g) Exp-1. 0.681 Exp-2. 0.680 Exp-3. 0.682 Sample moisture content (wt.%) 8.26 Dry weight (g) Exp-1. 0.625 Exp-2. 0.621 Exp-3. 0.626 Nickel content in the dry samples Exp-1. 6.25 (mg) (1 wt.%) Exp-2. 6.21 Exp-3. 6.26

Table B1.2 Nickel extraction data at 55 °C, 0.2 M H2SO4, 0.3 M ferric sulphate Time (h) Ni (mg) Ni Extraction (%) 0 Exp-1 Exp-2 Exp-3 Exp-1 Exp-2 Exp-3 Average 1 2.56 2.61 2.63 40.90 42.09 42.08 41.69±0.69 3 3.13 3.29 3.32 50.06 52.97 53.09 52.04±1.72 6 3.66 3.61 3.67 58.50 58.06 58.73 58.43±0.34 9 4.09 4.05 3.95 65.36 65.17 63.11 64.54±1.25 24 5.57 5.76 5.82 89.12 92.69 93.14 91.65±2.20 30 5.91 6.08 6.06 94.62 97.88 97.05 96.52±1.69

Table B2.1 Experimental conditions.

Temperature 55 °C

Fe2(SO4)3.5H2O (g) 49 Fe2(SO4)3.5H2O (M) 0.2 Shaking speed (rpm) 250

H2SO4 (M) 0.2 Reaction volume (mL) 500 Sample IDs Exp-1, Exp-2 Wet weight (g) Exp-1. 0.6821 Exp-2. 0.6823 Sample moisture content (wt.%) 7.00

Dry weight (g) Exp-1. 0.634 Exp-2. 0.634

Nickel content in the dry samples Exp-1. 6.34 (mg) (1 wt.%) Exp-2. 6.34

Table B2.2 Nickel extraction data at 55 °C, 0.2 M H2SO4, 0.3 M ferric sulphate. Time (h) Ni (mg) Ni extraction (%) 0 Exp-1 Exp-2 Exp-1 Exp-2 Average 1 1.88 1.87 29.63 29.47 29.55±0.11 3 2.39 2.40 37.61 37.78 37.69±0.12 6 2.90 3.00 45.74 47.33 46.54±1.12 8.5 3.14 3.25 49.53 51.20 50.37±1.18 23 4.49 4.53 70.70 71.37 71.04±0.48 30 4.83 4.87 76.13 76.72 76.43±0.42 48 5.30 5.40 83.55 85.18 84.36±1.15 72 5.69 5.73 87.82 88.43 88.13±0.43 96 5.70 5.66 89.87 89.23 89.55±0.45

Table B3.1 Experimental conditions.

Temperature 55 °C Fe2(SO4)3.5H2O (g) 24.5 Fe2(SO4)3.5H2O (M) 0.1 Shaking speed (rpm) 250

H2SO4 (M) 0.2 Reaction volume (mL) 500 Sample IDs Exp-1, Exp-2 Wet weight (g) Exp-1. 0.681

Exp-2. 0.682 Sample moisture content (wt.%) 7.00 Dry weight (g) Exp-1. 0.634 Exp-2. 0.634 Nickel content in the dry samples Exp-1. 6.34 (mg) (1 wt.%) Exp-2. 6.34

Table B3.2 Nickel extraction data at 55 °C, 0.2 M H2SO4, 0.1 M ferric sulphate.

Time (h) Ni (mg) Ni extraction (%) Exp-1 Exp-2 Exp-1 Exp-2 Average 1 1.18 1.18 18.55 18.55 18.55±0.0016 3 1.69 1.70 26.63 26.83 26.73±0.13 6 2.27 2.26 35.86 35.60 35.73±0.18 8.5 2.57 2.68 40.56 42.19 41.38±1.15 23 3.83 3.89 60.51 61.29 60.90±0.55 30 4.41 4.22 69.63 66.54 68.09±2.19 48 4.78 4.77 75.45 75.22 75.34±0.17 56.5 4.79 4.86 75.65 76.66 76.15±0.72 72 4.93 5.04 77.81 79.51 78.66±1.21 96 5.70 5.16 80.40 81.31 80.85±0.64

Table B4.1 Experimental conditions.

Temperature 55 °C Fe2(SO4)3.5H2O (g) 4.9

Fe2(SO4)3.5H2O (M) 0.02 Shaking speed (rpm) 250

H2SO4 (M) 0.2 Reaction volume (mL) 500

Sample IDs Exp-1, Exp-2 Wet weight (g) Exp-1. 0.6826 Exp-2. 0.6825

Sample moisture content (wt%) 7 Dry weight (g) Exp-1. 0.6348 Exp-2. 0.6347 Ni weight in the dry sample (mg) (1 Exp-1. 6.3482 wt.%) Exp-2. 6.3472

Table B4.2 Nickel extraction data at 55 °C, 0.2 M H2SO4, 0.02 M ferric sulphate. Time (h) Ni (mg) Ni Extraction (%) 0 Exp-1 Exp-2 Exp-1 Exp-2 Average 1 0.69 0.63 10.90 9.93 10.42±0.68 3 1.13 1.07 17.82 16.92 17.36±0.64 6 1.66 1.57 26.25 24.74 25.49±1.07 8.5 1.93 1.81 30.33 28.59 29.47±1.23 23 3.12 3.04 49.10 47.89 48.49±0.86 30 3.44 3.36 54.21 52.96 53.59±0.89 48 4.10 4.06 64.65 64.00 64.32±0.46 72 4.46 4.42 70.27 69.61 69.94±0.47 96 4.63 4.54 72.98 71.50 72.24±1.05

Ferrous ion concentration and pH values of the leach solutions, corresponding to Figure 4-5 in chapter 4, were measured during the ferric leaching of the pyrrhotite tailings. Figure B-1 shows the ferrous ion concentration curves at 0.3, 0.2, 0.1, and 0.02 M and the raw data are shown in tables B1.3, B2.3, B3.3, and B4.3, respectively.

Figure B-1. Ferrous ion concentration at different ferric sulphate concentrations, 55 °C and 0.2 M H2SO4.

Table B1.3 Ferrous ion concentrations and pH values at 55 °C, 0.2 M H2SO4, 0.3 M ferric sulphate.

Samples Time (h) Fe2+ (M) pH Average Fe2+ Average pH Exp.1 1 0.0018 0.649 Exp.2 0.0018 0.739 0.0018 0.7037±0.048 Exp.3 0.0018 0.723 Exp.1 3 0.0054 0.625 Exp.2 0.0054 0.706 0.0056±0.000 0.6743±0.043 Exp.3 0.006 0.692 3 2 Exp.1 6 0.0072 0.769 Exp.2 0.009 0.697 0.0084±0.001 0.7503±0.046 Exp.3 0.009 0.785 0 9 Exp.1 9 0.0096 0.583 Exp.2 0.0102 0.667 0.0102±0.000 0.638±0.0476 Exp.3 0.0108 0.664 6 Exp.1 24 0.015 0.611 Exp.2 0.0156 0.706 0.0153±0.000 0.6386±0.058 Exp.3 - 0.599 4 6 Exp.1 30 0.0162 0.689 Exp.2 0.0168 0.716 0.0166±0.000 0.7013±0.013 Exp.3 0.0168 0.699 5 6

Table B2.3 Ferrous ion concentrations at 55 °C, 0.2 M H2SO4, 0.2 M ferric sulphate.

Samples Time (h) Fe2+ (M) Average Fe2+ (M) Exp.1 1 0.0012 Exp.2 0.0012 0.0012 Exp.1 3 0.0039 Exp.2 0.0042 0.00405±0.0002 Exp.1 8.5 0.0084 Exp.2 0.009 0.0087±0.0004 Exp.1 23 0.0132 Exp.2 0.0138 0.0135±0.0004 Exp.1 30 0.0144 Exp.2 0.0144 0.0144 Exp.1 48 0.0156 Exp.2 0.0162 0.0159±0.0004 Exp.1 56.5 0.0162 Exp.2 0.0162 0.0162 Exp.1 72 0.0168 Exp.2 0.0168 0.0168 Exp.1 96 0.0174 Exp.2 0.0174 0.0174

Table B3.3 Ferrous ion concentrations at 55 °C, 0.2 M H2SO4, 0.1 M ferric sulphate.

Samples Time (h) Fe2+ (M) Average Fe2+ (M) Exp.1 1 0.0009 Exp.2 0.0012 0.00105±0.0002 Exp.1 3 0.0036 Exp.2 0.0036 0.0036 Exp.1 8.5 0.0078 Exp.2 0.0078 0.0078 Exp.1 23 0.0132 Exp.2 0.0126 0.0129±0.0004 Exp.1 30 0.0132 Exp.2 0.0138 0.0135±0.0004 Exp.1 48 0.0156 Exp.2 0.0150 0.0153±0.0004 Exp.1 56.5 0.0156 Exp.2 0.0156 0.0156 Exp.1 76 0.0162 Exp.2 0.0162 0.0162 Exp.1 96 0.0168 Exp.2 0.0168 0.0168

Table B4.3 Ferrous ion concentrations at 55 °C, 0.2 M H2SO4, 0.02 M ferric sulphate.

Samples Time (h) Fe 2+ (M) Average Fe2+ Exp.1 3 0.0027 Exp.2 0.0024 0.00255±0.0002 Exp.1 8.5 0.0054 Exp.2 0.0054 0.0054±0 Exp.1 23 0.009 Exp.2 0.0096 0.0093± Exp.1 30 0.0102 Exp.2 0.0102 0.0102±0 Exp.1 48 0.012 Exp.2 0.012 0.012±0 Exp.1 56.5 0.0123 Exp.2 0.012 0.01215±0.0002 Exp.1 79 0.0132 Exp.2 0.0138 0.0135±0.0004 Exp.1 96 0.0138 Exp.2 0.0138 0.0138±0

The shrinking core model (SCM) was applied to the nickel extraction data at different ferric sulphate concentrations (0.3, 0.2, 0.1, and 0.02 M), and the results are plotted in Figures B-2 and B-3 for diffusion-control and chemical reaction-control kinetic models. The raw data of these figures is summarized in Tables B1-4, B2-4, B3-4, and B4-4. The straight-line plots in

Figure 4-2 produce higher correlation coefficients for all the temperatures in comparison to the chemical reaction-control SCM (Figure 4-3). Moreover, in Figure 4-2, the fitted lines at different temperatures extrapolate through the origin or are very close to the origin, whereas in

Figure 4-3 they do not. Therefore, the process is diffusion control.

Table B1.4 SCM fitted to nickel extraction data at 55 °C, 0.2 M H2SO4, 0.3 M ferric sulphate.

Time (h) 0.3M Conversion (x) 1-(1-x)1/3 1-3(1-x)2/3+2(1-x) 0 0 0 0 1 0.42 0.07 0.16 3 0.52 0.12 0.22 6 0.58 0.16 0.25 9 0.65 0.21 0.29 24 0.92 0.59 0.56 30 0.97 0.75 0.67

Table B2.4 SCM fitted to nickel extraction data at 55 °C, 0.2 M H2SO4, 0.2 M ferric sulphate.

Time (h) 0.2M Conversion (x) 1-(1-x)1/3 1-3(1-x)2/3+2(1-x) 0 0 0 0 1 0.30 0.03 0.11 3 0.38 0.06 0.15 6 0.47 0.09 0.19 8.5 0.50 0.11 0.21 24 0.71 0.27 0.34 30 0.76 0.33 0.38 48 0.84 0.44 0.46

Table B3.4 SCM fitted to nickel extraction data at 55 °C, 0.2 M H2SO4, 0.1 M ferric sulphate.

Time (h) Conversion (x) 1-(1-x)1/3 1-3(1-x)2/3+2(1-x) 0 0 0 0 1 0.19 0.01 0.07 3 0.27 0.03 0.10 6 0.36 0.05 0.14 8.5 0.41 0.07 0.16 23 0.61 0.18 0.27 30 0.68 0.24 0.32 48 0.75 0.31 0.37

Table B4.4 SCM data fitted to nickel extraction data at 55 °C, 0.2 M H2SO4, 0.02 M ferric sulphate.

Time (h) Conversion (x) 1-(1-x)1/3 1-3(1-x)2/3+2(1-x) 0 0 0 0 1 0.1 0.003 0.04 3 0.17 0.01 0.06 6 0.25 0.02 0.09 8.5 0.29 0.03 0.11 23 0.48 0.10 0.20 30 0.53 0.13 0.23 48 0.64 0.20 0.29 72 0.69 0.25 0.33

Figure B-2. Plot of chemical reaction-control SCM at various ferric sulphate concentrations.

Figure B-3. Plot of diffusion-control SCM at various ferric sulphate concentrations.

Table B5.1 Surface reaction-control apparent rate constant values at various ferric sulphate concentrations obtained from the slopes of linear fits in Figure B-2.

-1 2 Fe2(SO4)3 (M) kr (h ) R 0.3 2.0×10-2 0.90 0.2 0.84×10-2 0.90 0.1 0.72×10-2 0.91 0.02 0.45×10-2 0.91

Table B5.2 Diffusion-control apparent rate constants at various ferric sulphate concentrations obtained from the slopes of linear fits in Figure B-3.

-1 2 Fe2(SO4)3 (M) kd (h ) R 0.3 2.50×10-2 0.99 0.2 1.00×10-2 0.97 0.1 0.71×10-2 0.98 0.02 0.39×10-2 0.98

Appendix C. Elemental sulphur determination

Figure 4-7 in Chapter 4 shows the extraction of nickel and elemental sulphur formation at 55

°C, 0.2 M Fe2(SO4)3 and 0.2 M H2SO4. Table C1.1 provides the experimental conditions of the tests performed. The raw data corresponding to the nickel extraction curve in Figure 4.7 is provided in Table C1.2.

Table C1.1 Experimental conditions Temperature 55 °C

Fe2(SO4)3.5H2O (g) 49 Fe2(SO4)3.5H2O (M) 0.2 Shaking speed (rpm) 250 H2SO4 (M) 0.2 Reaction volume (mL) 500

Sample IDs Exp-1- Exp.5 Moisture content (wt.%) 9 Ni (wt.%) 1.0 Sulphur (wt.%) 32.1 Dry weight 6.72 6.75 6.72 6.67 6.72

Table C1.2 Nickel extraction data at 55 °C, 0.2 M ferric sulphate, and 0.2M H2SO4.

Time (h) Measured Ni (mg) Ni Extraction (%) Ni2+ (M) MW:58.7

1 1.97 29.26 0.07×10-3 4 2.92 43.26 0.1×10-3 9 3.76 55.89 0.13×10-3 24 4.95 74.21 0.17×10-3 49 5.78 86.10 0.20×10-3

As explained in section 3.5 of chapter 3, the concentration of elemental sulphur present in the leached residue was measured using HPLC. Standard solutions for elemental sulphur analysis were prepared within the range of 6-240 mg/L by a procedure similar to that employed by

McGuire and Hamers [30]. The standard HPLC calibration curve is shown in Figure C-1, where the equation of the linear fit is y= 2.35 x – 1.36. In order to calculate the sulphur concentration of the leached sample, the peak area (from HPLC) is inserted in the linear equation of the following calibration curve.

Figure C-1. Standard HPLC calibration curve for sulphur analysis at 55 °C.

Each measurement lasted for 25 min, and the peak corresponding to the elemental sulphur appeared somewhere between 15 and 20 min, due to small changes in the eluent composition.

Figure C-2 shows the HPLC chromatographs of elemental sulphur having a retention time of

15.5 min. The data obtained from the HPLC analysis of the samples leached at 55 °C, 0.2 M ferric sulphate, and 0.2M H2SO is provided in Table C1.3.

Figure C-2. HPLC Chromatograms of elemental sulphur. The elemental sulphur peak appears at 15.5 min.

Table C1.3 HPLC data for the analysis of elemental sulphur kinetics at 55 °C, 0.2 M ferric sulphate, and 0.2M H2SO4.

Peak area Dilution Peak Area Peak area Peak are Sulphur Volume Sulphur HPLC factor x dilution with filter (mg/L) (mL) (mg) factor paper subtracted Filter paper 1 5.54 5.54 0 0 25 0 1 h 50 3.77 188.56 183.02 427.85 25 10.70 4 h 50 22.47 1123.34 1117.80 2620.11 25 65.50 9 h 50 29.00 1449.97 1444.43 3386.11 25 84.65 24 h 50 45.00 2250.12 2244.58 5262.62 25 131.57 49 h 50 52.48 2624.01 2618.47 6139.47 25 153.49

Table C1.4 Elemental sulphur conversion data at 55 °C, 0.2 M ferric sulphate, and 0.2M H2SO4.

Time (h) Measured S0 (mg) S conversion (%) S0 (M) MW:32

1 0 0 0.66×10-3 4 10.70 5.13474 4.1×10-3 9 65.50 31.2611 5.3×10-3 24 84.65 40.61268 8.22×10-3 49 131.57 63.5763 9.6×10-3

Ferrous ion concentration, pH, and ORP values of the leach solutions, corresponding to Figure

4-7 in chapter 4, were measured during the ferric leaching of the pyrrhotite tailings. Figure C-3 shows the ferrous ion concentration curve and the raw data is shown in Table C1.5.

Figure C-3. Ferrous ion concentration at 55 °C, 0.2 M ferric sulphate and 0.2 M H2SO4 media.

Table C1.5 Ferrous ion concentrations, pH and ORP values at 55 °C, 0.2 M ferric sulphate, and 0.2M H2SO4.

2+ Sample Time (h) Fe (M) Eh pH ORP SCE (mV) SHE (V) Exp-1 1 0.0024 550 0.64 791 Exp-2 4 0.0096 523 0.67 764 Exp-3 9 0.0141 510 0.67 751 Exp-4 24 0.0165 505 0.67 746 Exp-5 49 0.0195 501 0.67 742

Figure 4-8 in chapter 4 shows the molar ratios of Fe2+/S0 and Ni2+/S0 with time from data in

Figure 4-7. The calculation of these molar ratios is provided in Table C1.6, where the molarities of, Ni2+, S0, and Fe2+ are from Tables C1.2, C1.4, and C1.5, respectively.

Table C1.6 Analysis of molar ratios of Fe2+/S0 and Ni2+/S0 at 55 °C.

Time (h) Ni2+/S0 Fe2+/S0 1 0.100 3.59 4 0.024 2.34 9 0.024 2.84 24 0.021 2.19 49 0.021 2.03

Figure 4-9 in Chapter 4 shows the extraction of nickel and elemental sulphur formation at 40

°C, 0.2 M Fe2(SO4)3 and 0.2 M H2SO4. Table C2.1 provides the experimental conditions of the tests performed. The raw data corresponding to the nickel extraction curve in Figure 4.9 is provided in Table C2.2.

Table C2.1 Experimental conditions. Temperature 40 °C

Fe2(SO4)3.5H2O (g) 49 Fe2(SO4)3.5H2O (M) 0.2 Shaking speed (rpm) 250 H2SO4 (M) 0.2 Reaction volume (mL) 500

Sample IDs Exp-1- Exp.5 Moisture content (wt.%) 9 Ni (wt.%) 1.0 Sulphur (wt.%) 32.1

Dry weight (g) Exp-1 0.46 Exp-2 0.65 Exp-3 0.65 Exp-4 0.65 Exp-5 0.65

Table C2.2 Nickel extraction data at 40 °C, 0.2 M ferric sulphate, and 0.2M H2SO4.

Time (h) Measured Ni (mg) Ni Extraction (%) Ni2+ (M)

4 1.967179 29.3 0.06×10-3 24 2.924076 43.3 0.13×10-3 48 3.758084 55.9 0.16×10-3 72 4.954033 74.2 0.17×10-3 97 5.778863 86.0 0.19×10-3

McGuire and Hamers [30]. The standard HPLC calibration curve for elemental sulphur analysis at 40 °C is shown in Figure C-4, where the equation of the linear fit is y= 1.84 x – 6.002. In order to calculate the sulphur concentration of the leached sample, the peak area (from HPLC analysis) is inserted in the linear equation of the following calibration curve. The data obtained from the HPLC analysis of the samples leached at 40 °C, 0.2 M ferric sulphate, and 0.2M H2SO4 is provided in Table C2.3.

Figure C-4. Standard HPLC Calibration curve for sulphur analysis at 40 °C.

Table C2.3 HPLC data for the analysis of elemental sulphur kinetics at 40 °C, 0.2 M ferric sulphate, and 0.2M H2SO4.

Peak area Dilution Peak Area Peak area Peak are Sulphur Volume Elemental HPLC factor x dilution with filter (mg/L) (mL) sulphur factor paper (mg) subtracted Filter 4.54 4.54 0 0 0 paper 1 20 4 h 50 16.42 821.22 816.68 1501.17 20 30.02 24 h 50 46.24 2311.93 2307.39 4252.28 25 106.31 48 h 50 25.73 1286.58 1282.02 2360.01 50 118.01 72 h 50 28.60 1430.07 1425.53 2624.80 50 131.24 97 h 50 33.83 1691.43 1686.89 3107.14 50 155.35

Table C2.4 Elemental sulphur conversion data at 40°C, 0.2 M ferric sulphate, and 0.2M H2SO4.

Time (h) Measured S0 (mg) S conversion (%) S0 (M) MW:32

4 30.02 20.16 1.87×10-3 24 106.31 50.85 6.64×10-3 48 118.01 57.73 7.38×10-3 72 131.24 62.95 5.20×10-3

97 155.35 74.33 9.71×10-3

Ferrous ion concentration, pH, and ORP values of the leach solutions, corresponding to Figure

4-9 in chapter 4, were measured during the ferric leaching of the pyrrhotite tailings. Figure C-5 shows the ferrous ion concentration curve and the raw data is shown in Table C2.5

Table C2.5 Ferrous ion concentrations and pH values at 40 °C, 0.2 M ferric sulphate, and 0.2M H2SO4

.Time (h) Fe2+ (M) ORP pH ORP SCE (mV) SHE (V) 4 0.0054 527 0.72 768 24 0.0156 498 0.72 739 48 0.0204 492 0.76 733 72 0.0216 488 0.73 729 97 0.024 480 0.73 721

Figure C-5. Ferrous ion concentration at 40 °C, 0.2 M ferric sulphate and 0.2 M H2SO4 media.

Figure 4-8 in chapter 4 shows the molar ratios of Fe2+/S0 and Ni2+/S0 with time from data in

Figure 4-7. The calculation of these molar ratios is provided in Table C2.6, where the molarities of, Ni2+, S0, and Fe2+ are from Tables C2.2, C2.4, and C2.5, respectively.

Table C2.6 Analysis of molar ratios of Fe2+/S0 and Ni2+/S0 at 40 °C.

Time (h) Ni2+/S0 Fe2+/S0 4 0.032 2.88 24 0.019 2.35 48 0.021 2.77 72 0.021 2.63 97 0.019 2.47

Appendix D. SEM images and EDX Spectra

D.1 Additional SEM image, elemental maps, and EDX spectra of the leached pyrrhotite particle after 80% nickel extraction.

SEM image Fe

2 1 + +

S Ni

1 2

. .

Element Weight % Element Weight %

S 27.20 S 29.15 Fe 46.75 Fe 2.16 Ni 0.7 Ni 0.09 C and O 25.35 C and O 68.60

Total 100 Total 100

D.2 Additional SEM image, elemental maps, and EDX spectra of the leached pyrrhotite particle after 50% nickel extraction

SEM image S

1 +

+ 2

Fe Ni

2 1 .

Element Weight % Element Weight % S 23.67 S 49.15 Fe 38.54 Fe 6.39

Ni 0.9 Ni 0.22 C and O 36.89 C and O 44.24 Total 100 Total 100

D3. Additional SEM-BSE images

80% reacted 80% reacted

90% reacted 50% reacted