An electrochemical study of the oxidative dissolution of synthetic pentlandite in aqueous media

Abstract

Pentlandite (Fe 4.5Ni 4.5S 8) and violarite (FeNi 2S 4) were synthesised from their elements by dry in vacuo techniques. The products were analysed by reflected light microscopy, powder X-ray diffraction and electron microprobe analysis. The synthetic pentlandite was found to have an average stoichiometry of Fe 4.35Ni 4.65S 8. A partial phase segregation of pentlandite into heazlewoodite (Ni 2.7Fe 0.2S 2) and monosulphide solid-solution (Fe 0.74Ni 0.21S) was observed. The synthetic violarite grains showed a zonal separation into a Fe 1.2Ni 1.8S 4 core, and a Fe 0.5Ni 2.5S 4 rim. Trace amounts of pyrite (FeS 2) and millerite (NiS) were also detected. A study of the oxidative dissolution of pentlandite by electrochemical techniques including potentiometry, linear sweep cyclic voltammetry, intermittent galvanostatic polarisation, chronopotentiometry and chronoamperometry was made to clarify the mechanism by which pentlandite is leached in acidic iron(III) chloride solution. The products were analysed using scanning electron microscopy, powder X-ray diffraction, electron microprobe analysis, atomic absorption spectroscopy and gravimetric analysis. A critical comparison of similar studies by other workers on metal sulphide minerals is included. The fit of experimental results to a simple electron transfer model via the Sand equation was tested and found to follow non-Arrhenius behaviour. A mechanism for the oxidative dissolution of pentlandite is proposed. In acid solution, pentlandite decomposes spontaneously, liberating aqueous metal ions and hydrogen sulphide. Under potentiostatic conditions akin to iron(III) chloride leaching, pentlandite is oxidised directly to elemental sulphur, without the formation of any intermediate phases. The lack of formation of violarite indicates that the system is substantially perturbed from equilibrium due to slow solid-state diffusion of metal atoms within the sulphur sub-lattice. The formation of metastable amorphous sulphur as the alternative product is further evidence for this perturbation. The physical properties of the sulphur product layer cause an impediment to mass transport between the bulk aqueous solution and the mineral surface. However, the oxidation involves an intrinsically slow electron transfer for the S,Fe 2+, Ni 2+ Fe 4.5 Ni 4.5 S 8 couple which, within the potential range relevant to iron(III) chloride leaching, is rate-determining for an appreciable part of the reaction. The implications for extractive hydrometallurgy are discussed.