An electrochemical study of the dissolution of chalcopyrite in ammonia–ammonium sulphate solutions

Ammoniacal solutions are an effective lixiviant for the oxidative dissolution of some mineral sulphides. A study of the anodic dissolution of chalcopyrite in ammonium sulphate-ammonium hydroxide solutions has been carried out using cyclic voltammetry, chrono-amperometry and rest-potential measurements. The role of the copper(II)/copper(I) redox couple in the oxidation process has been evaluated. Rest potentials have been found not to be affected by oxygen but to increase with an increase in initial copper(II) concentration. This trend remains more or less unchanged with increasing total ammonia (NH3+NH4+) concentration. Cyclic voltammetry analysis shows an oxidation peak/shoulder when anodically polarising the chalcopyrite after attaining rest potential in the presence of copper(II) ions. All reverse sweep curves are more or less identical, but the Tafel slope decreases from around 150 mV/decade at 1 M total ammonia concentration to around 120 mV/decade at higher ammonia concentrations. Pseudo-steady-state anodic current densities measured in the absence of copper(II) ions at the rest potentials obtained in the presence of copper(II) ions increase with increasing concentration of copper(II) ions, confirming the positive effect of copper(II) ions on the rate of dissolution of the mineral. Cyclic voltametric and chrono-amperometric data show that in the absence of initial copper(II) ions, the rate of dissolution in the presence of dissolved oxygen is significantly lower. These results confirm that the effective oxidant for the mineral under the conditions of the study is copper(II) and not dissolved oxygen. Coulometric measurements have been used to establish the stoichiometry of the anodic reaction at different potentials as involving approximately seven electrons per mole of chalcopyrite, suggesting the formation of thiosulphate, although thiosulphate ions have not been tested for or identified in solution. SEM and energy dispersive X-ray (EDX) analysis of the mineral surface left to equilibrate over time shows a copper-depleted surface, rich in iron but relatively low in sulphur. The iron in the surface layer can easily be removed by short contact with concentrated acid, which is consistent with the formation of a secondary iron-based precipitation layer rather than an altered mineral phase.