Fundamentals of pentlandite mineralogy and its effect on its electrochemical behaviour

Pentlandite [(Fe,Ni)9S8] is the primary source of nickel and an important carrier of palladium and rhodium. Previous mineralogical investigations indicated compositional variations of natural pentlandite. However, the effect these mineralogical effects may have on the electrochemical behaviour of pentlandite is not clear. In this study, compositional variations of natural single pentlandite particles (∼100 μm in size) sourced from the Bushveld Igneous Complex were shown. Experimental difficulties when measuring pentlandite microelectrodes are discussed. As a result only massive pentlandite samples were considered. The electrochemical investigations of the massive electrodes were carried out in an oxygen-free and air saturated 0.05 M borate solution using polarisation resistance, mixed potential, linear anodic voltammetry, electrochemical impedance spectroscopy and capacitance measurements. compositional variations of the massive pentlandite electrodes influenced the electrochemical behaviour. Iron-enriched pentlandites were less reactive after progressive oxidation. Electrochemical investigations established that as pentlandite oxidises, the oxides develop and grow at the surface decreasing the reactivity of the mineral. The decrease in reactivity of the electrodes was related to the mechanism of the reduction of oxygen at oxidised passive electrode surfaces. A variation in the electronic properties of the formed oxide film was observed. Effects of pentlandite Fe/Ni ratio and cobalt were evident.