An Electrochemical Study of the Oxidative Dissolution of Synthetic Nickel-Iron Sulphide Minerals in Aqueous Media

Abstract

Pentlandite (Fe4.5Ni4.5S8) and violarite (FeNi2S4) were synthesized 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 Fe4.35Ni4.65S8. A partial phase segregation of pentlandite into heazlewoodite and pyrrhotite was observed. The synthetic violarite grains showed a zonal separation into a Fe1.2Ni1.8S4 core, and a Fe0.5Ni2.5S4 rim. Trace amounts of pyrite and millerite were also detected. From a critical review of the thermodynamic data in the literature, several Eh-pH diagrams were constructed for the Fe-Ni-S aqueous system. These were compared with mineralogical evidence obtained from naturally occuring mineral assemblages. A study of the oxidative dissolution of pentlandite by electrochemical techniques was made to clarify the mechanism by which pentlandite is leached in acid FeCl3 solution. The techniques used included: potentiometry, linear sweep cyclic voltammetry, intermittent galvanostatic polarization, chronopotentiometry and chronoamperometry. The products were analysed using scanning electron microscopy, powder X-ray diffraction, electron microprobe analysis, atomic absorption spectroscopy and gravimetric analysis. The fitting of experimental results to a simple electron transfer model via the Sand equation was tested and found to be inappropriate. A mechanism for the oxidative dissolution of pentlandite is postulated. In acid solution, pentlandite decomposes spontaneously, liberating aqueous metal ions and H2S. Under potentiostatic conditions akin to FeC13 leaching, pentlandite is oxidized 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 sublattice. The formation of metastable amorphous sulphur as the alternative product is further evidence of 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 So, Fe2+, Ni2+ / Fe4.5Ni4.5S8 couple which, within the potential range relevant to FeCl3 leaching, is rate determining for an appreciable part of the reaction. The implication for extractive hydrometallurgy is discussed. The use of a convolution transform of voltammetric currents with a (πt)-1/2 function as applied to simple electron transfer is described. In addition, the derivation of a functional form for the treatment of chronoamperometric data is given. These models were applied in the determination of the heterogeneous electrochemical parameters and diffusion coefficients for the FeC13/FeC12 couple in 1M HC1 solution on platinum at 293K, using computer controlled chronoampermetric techniques. The results show quasi-reversible behaviour (at 293K), which implies that electron transfer for this couple would not be rate determining in the leaching of pentlandite