Mineralogical Characterisation

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Introduction

Pyrrhotite Fe(1-x)S is one of the most commonly occurring metal sulfide minerals and is recognised in a variety of ore deposits including nickel-copper, lead-zinc, and platinum group element (PGE). Since the principal nickel ore mineral, pentlandite, almost ubiquitously occurs coexisting with pyrrhotite, the understanding of the behaviour of pyrrhotite during flotation is of fundamental interest. For many nickel processing operations, pyrrhotite is rejected to the tailings in order to control circuit throughput and concentrate grade and thereby reduce excess sulfur dioxide smelter emissions (e.g. Sudbury; Wells et al., 1997).
However, for platinum group element processing operations, pyrrhotite recovery is targeted due to its association with the platinum group elements and minerals (e.g. Merensky Reef; Penberthy and Merkle, 1999; Ballhaus and Sylvester, 2000). Therefore, the ability to be able to manipulate pyrrhotite performance in flotation is of great importance. It can be best achieved if the mineralogical characteristics of the pyrrhotite being processed can be measured and the relationship between mineralogy and flotation performance is understood. The pyrrhotite mineral group is non-stoichiometric and has the generic formula of Fe(1-x)S where 0 ≤ x < 0.125. Pyrrhotite is based on the nickeline (NiAs) structure and is comprised of several superstructures owing to the presence and ordering of vacancies within its structure.
Numerous pyrrhotite superstructures have been recognised in the literature, but only three of them are naturally occurring at ambient conditions (Posfai et al., 2000; Fleet, 2006). This includes the stoichiometric FeS known as troilite which is generally found in extraterrestrial localities, but on occasion, has also been recognised in some nickel deposits (e.g. Voisey’s Bay, Bushveld Igneous Complex; Liebenberg, 1970; Naldrett et al., 2000). The commonly occurring magnetic pyrrhotite is correctly known as 4C pyrrhotite, has an ideal composition Fe7S8 and monoclinic crystallography (Powell et al., 2004). Non-magnetic pyrrhotite is formally described as NC pyrrhotite where N is an integer between 5 and 11 (Morimoto et al., 1970).
Non-magnetic NC pyrrhotite has a range of ideal compositions varying from Fe9S10, Fe10S11 to the least iron deficient composition of Fe11S12. Although NC pyrrhotite is generally known as “hexagonal pyrrhotite”, it has been argued to be pseudohexagonal and may actually be monoclinic or orthorhombic (Carpenter and Desborough, 1964; Morimoto et al., 1970; Koto et al., 1975; Posfai et al., 2000). Pyrrhotite is known to be a reactive mineral which is highly prone to oxidation (Rand, 1977; Belzile et al., 2004). The downstream consequences of this during minerals beneficiation may be quite severe, an example of which is the Nickel Rim site in Sudbury described by Johnson et al. (2000). The Nickel Rim site is actively suffering from the release of low pH waters, high concentrations of iron, and dissolved metals such as nickel and aluminium and will likely suffer from this continual discharge for at least another half century.
Pyrrhotite from the Sudbury ore deposit was acknowledged as the source of the acid mine drainage (AMD). However, in terms of flotation, the reactivity of pyrrhotite towards excessive oxidation may have a detrimental effect on its flotation performance due to the formation of hydrophilic iron hydroxides that render surfaces of pyrrhotite particles less floatable. Therefore, the influence of pyrrhotite mineralogy upon oxidation and oxidation rates is also of interest. Accounts in the literature are somewhat contradictory, but more frequently attribute monoclinic pyrrhotite to be the more reactive phase (e.g. Vanyukov and Razumovskaya, 1979; Yakhontova et al., 1983; both in Russian and quoted by Janzen, 1996).
More recently, using ToF-SIMS, Gerson and Jasieniak (2008) similarly showed that the oxidation rate of monoclinic pyrrhotite was greater than “hexagonal” pyrrhotite. Orlova et al. (1988) however (in Russian, quoted by Janzen, 1996), suggested that “hexagonal” pyrrhotite was more reactive. Janzen (1996) showed no correlation between pyrrhotite crystal structure and oxidation rate. Kwong (1993) suggested that the oxidation rate of pyrrhotite enriched in trace metals was slower than those with low nickel and cobalt contents. In the leaching study of Lehmann et al. (2000) it was shown that “hexagonal” pyrrhotite from two Australian locations was less susceptible to cyanide leaching than monoclinic pyrrhotite. Accounts in the literature with respect to the flotation behaviour of magnetic and nonmagnetic pyrrhotite are similarly quite varied. According to Iwasaki (1988), it was noted by Harada (1967; In Japanese) that samples of freshly ground monoclinic pyrrhotite were more floatable than “hexagonal” pyrrhotite although the reverse occurred on more oxidised .

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TABLE OF CONTENTS :

  • SYNOPSIS
  • DECLARATION
  • STATEMENT OF ORIGINALITY
  • LIST OF PUBLICATIONS AND PRESENTATIONS
  • ACKNOWLEDGEMENTS
  • TABLE OF CONTENTS
  • LIST OF FIGURES
  • LIST OF TABLES
  • LIST OF ELECTRONIC APPENDICES
  • LIST OF ABBREVIATIONS
  • GLOSSARY
  • Chapter 1: INTRODUCTION
    • 1.1 Introduction
    • 1.2 Aim of this Study
    • 1.3 Key Questions
    • 1.4 Scope of Research
    • 1.5 Organisation of the Thesis
  • Chapter 2: LITERATURE REVIEW
    • 2.1 Overview of Pyrrhotite Ore Deposits
    • 2.1.1 Bushveld Igneous Complex, South Africa
    • 2.1.2 Uitkomst Complex, South Africa
    • 2.1.3 Phoenix Deposit, Botswana
    • 2.1.4 Sudbury Igneous Complex, Canada
    • 2.2 Pyrrhotite Mineralogy and Crystallography
    • 2.2.1 Building blocks of pyrrhotite structures
    • 2.2.2 Metastable 1C pyrrhotite
    • 2.2.3 2C Troilite
    • 2.2.4 Metastable NA and MC pyrrhotites
    • 2.2.5 Magnetic 4C pyrrhotite
    • 2.2.6 Non-magnetic NC pyrrhotite
    • 2.2.7 Relationship between pyrrhotite, pentlandite and the platinum group elements
    • 2.2.8 Analytical methods for discrimination between pyrrhotite types
    • 2.3 Electrochemical Properties of Pyrrhotite
    • 2.3.1 Pyrrhotite Oxidation Reactions
    • 2.3.2 Mechanism of Pyrrhotite Oxidation
    • 2.3.3 Factors affecting Pyrrhotite Oxidation
    • 2.3.3 Electrochemical measurements of pyrrhotite
    • 2.4 Pyrrhotite Flotation
    • 2.4.1 Principles of Flotation
    • 2.4.2 Collectorless flotation of Pyrrhotite
    • 2.4.3 Flotation with Xanthate Collectors
    • 2.4.4 Activation of Pyrrhotite
    • 2.4.5 Pyrrhotite Rejection
    • 2.4.6 Comparison of plant operating strategies for pyrrhotite flotation and rejection
    • 2.5 Process Mineralogy
    • 2.6 Critical Review of the Literature
    • 2.6.1 Pyrrhotite Mineralogy
    • 2.6.2 Pyrrhotite Reactivity
    • 2.6.3 Pyrrhotite Flotation
    • 2.6.4 Approach of this Thesis
  • Chapter 3: SAMPLING AND ANALYTICAL METHODS
    • 3.1 Pyrrhotite sampling
    • 3.1.1 Merensky Reef
    • 3.1.2 Nkomati
    • 3.1.3 Phoenix
    • 3.1.4 Sudbury Copper Cliff North
    • 3.1.5 Sudbury Gertrude and Gertrude West
    • 3.2 Mineralogical Characterisation
    • 3.2.1 Optical Microscopy
    • 3.2.2 Powder X-ray Diffraction
    • 3.2.3 Single Crystal X-ray Diffraction
    • 3.2.4 Electron Microprobe Analysis
    • 3.2.5 Automated SEM
    • 3.3 Development of methodology for discrimination of pyrrhotite types
    • 3.3.1 Analysis of pyrrhotite types using QXRD
    • 3.3.2 Analysis of pyrrhotite types using QEMSCAN
    • 3.4 Pyrrhotite Reactivity
    • 3.4.1 Electrode Preparation
    • 3.4.2 Open Circuit Potential
    • 3.4.3. Cyclic Voltammetry
    • 3.4.4 Oxygen Uptake
    • 3.5 Pyrrhotite Microflotation
    • 3.5.1 Microflotation tests
    • 3.5.2 Analysis of Flotation Performance
  • Chapter 4: PYRRHOTITE MINERALOGY
    • 4.1 Introduction
    • 4.2 Petrography
    • 4.2.1 Merensky Reef Pyrrhotite
    • 4.2.2 Nkomati Pyrrhotite
    • 4.2.3 Phoenix Pyrrhotite
    • 4.2.4 Sudbury Pyrrhotite
    • 4.3 Crystallography
    • 4.3.1 Merensky Reef Pyrrhotite
    • 4.3.2 Phoenix Pyrrhotite
    • 4.3.3 Sudbury Pyrrhotite
    • 4.4 Mineral Chemistry
    • 4.4.1 Merensky Reef Pyrrhotite
    • 4.4.2 Nkomati Pyrrhotite
    • 4.4.3 Phoenix Pyrrhotite
    • 4.4.4 Sudbury Pyrrhotite
    • 4.5 Comparison of the Mineral Chemistry of Pyrrhotite Samples
    • 4.7 Key Findings
  • Chapter 5: PYRRHOTITE REACTIVITY
    • 5.1 Introduction
    • 5.2 Open Circuit Potential
    • 5.2.1 Comparison of the Open Circuit Potentials of Pyrrhotite Samples
    • 5.3 Cyclic Voltammetry
    • 5.3.1 Nkomati MSB Pyrrhotite
    • 5.3.2 Phoenix Pyrrhotite
    • 5.3.3 Sudbury CCN Pyrrhotite
    • 5.3.4 Sudbury Gertrude West Pyrrhotite
    • 5.3.5 Comparison of the Cyclic Voltammetry of Pyrrhotite Samples
    • 5.4 Oxygen Uptake
    • 5.4.1 Nkomati MSB Pyrrhotite
    • 5.4.2 Phoenix Pyrrhotite
    • 5.4.3 Sudbury Copper Cliff North Pyrrhotite
    • 5.4.4 Sudbury Gertrude West Pyrrhotite
    • 5.4.5 Comparison of the Oxygen Uptake of Pyrrhotite Samples
    • 5.5 Key findings
  • Chapter 6: PYRRHOTITE MICROFLOTATION
    • 6.1 Introduction
    • 6.2 Mineralogy of Flotation Feed Samples
    • 6.3 Nkomati MSB Pyrrhotite
    • 6.4 Phoenix Pyrrhotite
    • 6.5 Sudbury Copper Cliff North Pyrrhotite
    • 6.6 Sudbury Gertrude and Gertrude West Pyrrhotite
    • 6.7 Comparison of the Floatability of Pyrrhotite Samples
    • 6.8 Key findings
  • Chapter 7: DISCUSSION
    • 7.1 Introduction
    • 7.2 Variation in Pyrrhotite Mineralogy
    • 7.3 Effect of Ore Deposit Formation on Pyrrhotite Mineralogy
    • 7.4 Effect of Mineralogy on Pyrrhotite Reactivity
    • 7.5 Effect of Mineralogy on Pyrrhotite Flotation Performance
    • 7.6 Implications of this Study
  • Chapter 8: CONCLUSIONS AND RECOMMENDATIONS
    • 8.1 Conclusions
    • 8.2 Recommendations
  • Chapter 9: REFERENCES

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THE MINERALOGY AND CRYSTALLOGRAPHY OF PYRRHOTITE FROM SELECTED NICKEL AND PGE ORE DEPOSITS AND ITS EFFECT ON FLOTATION PERFORMANCE

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