Electrochemical study of oxidative dissolution of synthetic violarite in aqueous media

Abstract

A study of the oxidative dissolution of the nickel-iron sulphide mineral violarite (FeNi₂S₄) by electrochemical techniques including potentiometry, linear sweep cyclic voltammetry, intermittent galvanostatic polarisation and chronoamperometry was made to clarify the mechanism by which violarite is leached in acidic iron (III) chloride solution. This forms a complementary study to that recently reported on the related mineral pentlandite (Fe_4.5 Ni_4.5 S₈). A mechanism for the oxidative dissolution of violarite is proposed. In acid solution, under potentiostatic conditions akin to iron (III) chloride leaching, violarite is oxidized to elemental sulphur. The formation of metastable amorphous sulphur as opposed to ortho-rhombic sulphur indicates that the system is substantially perturbed from equilibrium. 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, Fe²⁺, Ni²⁺ / FeNi₂S₄ couple with heterogeneous rate constant, k_o = 10^-9 m s^-1. Within the potential range relevant to iron(III) chloride leaching, this electron transfer is rate-determining for an appreciable part of the reaction. A comparison with similar studies on violarite is made.