The electrochemical behaviour of enargite in ammoniacal solutions. I. Anodic reactions

Abstract

The leaching of enargite in acidic solutions is known to be particularly slow and, as in the case of chalcopyrite, the use of alternative lixiviants has been extensively studied. In particular, the use of ammoniacal solutions has been a focus due to the faster kinetics in such media. However, there have not been any fundamental studies of the electrochemistry of enargite in such solutions. A fundamental study has therefore been made of the electrochemical behaviour of enargite in both ammonium sulfate and ammonium chloride solutions at pH values from 8 to 10 and comparisons have been drawn with the behaviour of chalcopyrite under similar conditions. Mixed potential measurements have shown that copper(II) is an effective oxidant for enargite in ammoniacal solutions and the potential region in which enargite is oxidised have been defined. Voltammetry under various conditions has shown that partial passivation also occurs in such solutions and that the anodic behaviour in chloride solutions is similar to that in sulfate. Potentiostatic measurements at potentials in the region of the mixed potentials in copper(II) solutions have been used to derive pseudo-steady-state curves that reveal Tafel-like behaviour with slopes of approximately 120 mV/decade suggesting a one-electron rate determining step. A first-order dependence of the rate of anodic dissolution on the ammonia concentration has been derived from this data. Coulometric measurements suggest that thiosulfate is the main product of anodic oxidation under these conditions. The relative rates of dissolution of chalcopyrite and enargite in ammoniacal solutions have been estimated using mixed potential theory that show that enargite dissolves more rapidly than chalcopyrite in such systems.