Fundamental electrochemical behaviour of pentlandite

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 100μm 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). 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. Copyright