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Experimental Procedure FUNDAMENTAL ELECTROCHEMICAL BEHAVIOUR OF PENTLANDITE by Getrude Marape Submitted in partial fulfillment of the requirements for the degree Masters in Engineering (Metallurgical Engineering) in the Faculty of Engineering, Built Environment and Information Technology Supervisor: Dr M.K.G. Vermaak August 2010 © University of Pretoria Acknowledgements I would like to thank God for His support and the strength He has given me throughout this research project. I would like to express my sincere gratitude to the following people: My supervisor, Dr Thys Vermaak for his continuous support, guidance and patience Prof Chris Pistorius, for his support and the use of his samples My siblings, Joshua, Jacoline and Aubrey, for their continuous support and patience Sarah Havenga and the Department of Materials Science and Metallurgical Engineering, for their support to complete this research All my friends, for their support, advice and friendship Maggy Loubser and Sabine Verryn, for the XRD and XRF analyses Prof R.W. Merkle, for his sample and support Peter Graser, for the microprobe analyses Luukkanen Saaija of Geological Survey of Finland (GTK) for pentlandite samples Mintek, for the financial assistance that allowed me to conduct the research i FUNDAMENTAL ELECTROCHEMICAL BEHAVIOUR OF PENTLANDITE by Getrude Marape Supervisor: Dr M.K.G. Vermaak Department of Materials Science and Metallurgical Engineering Masters Degree in Engineering ABSTRACT Previous research indicates compositional variation of pentlandite [(Fe,Ni)9S8] and the effect this variation may have on the electrochemical behaviour of pentlandite is poorly understood. Pentlandite is the primary source of nickel and an important base metal sulfide (BMS) in the platinum industry. It hosts significant amounts of PGEs especially palladium and rhodium when compared to chalcopyrite and pyrrhotite. The aim of the project was to investigate the possible compositional variations of natural pentlandite and the effect of these variations on the electrochemical behavior thereof. To study possible compositional variations, single pentlandite particles - in the order of 100m in size from flotation concentrates (PGM deposits) and massive samples (massive ore bodies) - from various sources were employed. Electron microprobe analysis indicated a compositional variation of the pentlandite particles hand-picked from the flotation concentrate samples. Variation was observed in the cobalt, iron and nickel content and this was independent of the deposit. A slight compositional variation was observed from the massive pentlandite samples. The effect the compositional variation may have on the electrochemical reactivity of pentlandite was investigated using electrochemical techniques, i.e. measurement of the polarisation resistance and mixed potential as well as performing linear anodic voltammetry, current density–transients and electrochemical impedance spectroscopy (i.e. capacitance). ii Poor electrochemical response of the pentlandite microelectrodes was observed. Pre– existing pores, deep pores, cracks and the brittle nature of pentlandite microelectrodes may have contributed to the poor electrochemical response of natural pentlandite particles hand-picked from the flotation concentrate. Slight compositional variations of the massive pentlandite sample influenced the electrochemical behaviour. In aerated solutions, iron enriched pentlandites were less reactive after progressive oxidation. The lower reactivity of the electrodes was a result of thick oxide films formed. This was illustrated by polarisation resistance and capacitance measurements. The lower reactivity of the electrodes was also related to the mechanism of the reduction of oxygen at oxidised passive electrode surfaces. It is however difficult to distinguish if the differences in the reactivity was a result of the Fe/Ni ratio or the influence of cobalt. Current density transients confirmed that the reactivity of a pentlandite electrode to be time dependent. The reactivity of the electrode decreased during oxidation. A variation in the electronic properties of the formed oxide film was observed. Slight compositional variation of pentlandite did not have a significant effect on the rest potential values as do changes in the type of sulfides (e.g. pyrite vs. pentlandite). This was confirmed by similar rest potential values of various pentlandite electrodes. The oxidation of synthetic pentlandite may be influenced by the chemical composition. In de-aerated solutions, anodic oxidation (as indicated by the linear anodic voltammogram) of synthetic pentlandite started at a potential lower than of the natural electrodes. In aerated solutions, the synthetic pentlandite was less reactive and formed thicker oxide films. Keywords: Pentlandite, pyrrhotite, Cu–Ni sulfide ores, oxidation, flotation, electron microprobe, electrochemical impedance spectroscopy, electrochemistry iii TABLE OF CONTENTS ABSTRACT ....................................................................................................................... ii TABLE OF CONTENTS ................................................................................................ iv 1. INTRODUCTION......................................................................................................... 1 2. LITERATURE REVIEW ............................................................................................ 3 2.1 Platinum - group element (PGE) Mineralogy ........................................................... 3 2.1.1 PGE dominant deposits ...................................................................................... 3 2.1.2 Chromite deposits with PGEs ............................................................................ 5 2.1.3 Nickel–copper dominant deposits ...................................................................... 6 2.2 Pentlandite ................................................................................................................. 7 2.2.1 Chemical composition of pentlandite ................................................................ 7 2.2.2 Oxidation of pentlandite .................................................................................. 13 2.2.3 Oxidation of a synthetic pentlandite ................................................................ 15 2.2.4 Flotation of pentlandite .................................................................................... 16 2.3 Electrochemistry ..................................................................................................... 23 2.3.1 Cyclic voltammetry .......................................................................................... 24 2.3.2 Rest potential ................................................................................................... 27 2.3.3 Electrochemical Impedance Spectroscopy....................................................... 29 3. CONCLUSIONS ......................................................................................................... 35 4. RESEARCH OBJECTIVE ........................................................................................ 36 iv 5. EXPERIMENTAL PROCEDURE................................................................ ............ 37 5.1 Samples employed .................................................................................................. 37 5.2 Analysis of the flotation concentrate samples ........................................................ 39 5.2.1 X-ray diffraction (XRD) analysis, X-ray fluorescence spectroscopy (XRF) and scanning electron microscopy (SEM) ....................................................................... 39 5.3 Identification and separation of pentlandite particles from the flotation concentrate ....................................................................................................................................... 40 5.3.1 Pre–concentration of pentlandite particles ....................................................... 40 5.3.2 Hand-picking of pentlandite particles .............................................................. 41 5.4 Electrode preparation .............................................................................................. 43 5.4.1 Massive electrode preparation ......................................................................... 43 5.4.2 Microelectrode preparation .............................................................................. 44 5.5 Electron microprobe analysis .................................................................................. 46 5.6 Electrochemical measurements ............................................................................... 47 5.7. TOF-SIMS ............................................................................................................. 51 6. RESULTS AND DISCUSSION ................................................................................. 52 6.1. Composition of pentlandite particles from flotation concentrate .......................... 52 6.1.1 Composition of pentlandite particles from Platreef ......................................... 52 6.1.2 Composition of pentlandite particles from Lebowa Merensky flotation concentrate ................................................................................................................ 54 6.1.3 Composition of pentlandite particles from the Lebowa UG-2 flotation concentrate ...............................................................................................................
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