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

By Terence Edwin Warner

Abstract

Pentlandite (Fe4.5Ni4.5S8) and violarite (FeNi2S4) were\ud synthesized by dry in vacuo techniques. The products were\ud analysed by reflected light microscopy, powder X-ray\ud diffraction and electron microprobe analysis. The synthetic\ud pentlandite was found to have an average stoichiometry of\ud Fe4.35Ni4.65S8. A partial phase segregation of pentlandite\ud into heazlewoodite and pyrrhotite was observed. The\ud synthetic violarite grains showed a zonal separation into a\ud Fe1.2Ni1.8S4 core, and a Fe0.5Ni2.5S4 rim. Trace amounts of\ud pyrite and millerite were also detected.\ud \ud From a critical review of the thermodynamic data in the\ud literature, several Eh-pH diagrams were constructed for the\ud Fe-Ni-S aqueous system. These were compared with\ud mineralogical evidence obtained from naturally occuring\ud mineral assemblages.\ud \ud A study of the oxidative dissolution of pentlandite by\ud electrochemical techniques was made to clarify the mechanism\ud by which pentlandite is leached in acid FeCl3 solution. The\ud techniques used included: potentiometry, linear sweep cyclic\ud voltammetry, intermittent galvanostatic polarization,\ud chronopotentiometry and chronoamperometry. The products were\ud analysed using scanning electron microscopy, powder X-ray\ud diffraction, electron microprobe analysis, atomic absorption\ud spectroscopy and gravimetric analysis. The fitting of\ud experimental results to a simple electron transfer model via\ud the Sand equation was tested and found to be inappropriate.\ud \ud A mechanism for the oxidative dissolution of pentlandite\ud is postulated. In acid solution, pentlandite decomposes\ud spontaneously, liberating aqueous metal ions and H2S. Under\ud potentiostatic conditions akin to FeC13 leaching,\ud pentlandite is oxidized directly to elemental sulphur,\ud without the formation of any intermediate phases. The lack\ud of formation of violarite indicates that the system is\ud substantially perturbed from equilibrium due to slow solid\ud state diffusion of metal atoms within the sulphur sublattice. The formation of metastable amorphous sulphur as\ud the alternative product is further evidence of this\ud perturbation. The physical properties of the sulphur product\ud layer cause an impediment to mass transport between the bulk\ud aqueous solution and the mineral surface. However, the\ud oxidation involves an intrinsically slow 'electron transfer\ud for the So, Fe2+, Ni2+ / Fe4.5Ni4.5S8 couple which, within the potential range relevant to FeCl3 leaching, is rate\ud determining for an appreciable part of the reaction. The\ud implication for extractive hydrometallurgy is discussed.\ud \ud The use of a convolution transform of voltammetric\ud currents with a (πt)-1/2 function as applied to simple\ud electron transfer is described. In addition, the derivation\ud of a functional form for the treatment of chronoamperometric\ud data is given. These models were applied in the\ud determination of the heterogeneous electrochemical\ud parameters and diffusion coefficients for the FeC13/FeC12\ud couple in 1M HC1 solution on platinum at 293K, using\ud computer controlled chronoampermetric techniques. The\ud results show quasi-reversible behaviour (at 293K), which\ud implies that electron transfer for this couple would not be\ud rate determining in the leaching of pentlandite

Publisher: School of Chemical and Process Engineering (Leeds)
Year: 1988
OAI identifier: oai:etheses.whiterose.ac.uk:1075

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