The dissolution of chalcopyrite in chloride solutions: Part 3. Mechanisms

In Parts 1 and 2 of this series, which describe the results of a study of the dissolution of chalcopyrite under conditions that could be expected in a heap leaching process for primary copper minerals, it was shown that enhanced leaching of chalcopyrite from several copper concentrates in dilute acidic chloride solutions can be achieved by controlling the potential in a "window" of 560-600 mV (SHE) in the presence of dissolved oxygen. It was also found that the rate is linear and essentially independent of the initial concentration of chloride and cupric ions under these conditions. Furthermore, the rate appears to be largely independent of the source of the mineral and is strongly dependent on the temperature (activation energy = 72 kJ mol^{- 1}). In this part, additional kinetic data on the effects of fine pyrite on the rate complemented by detailed mineralogical analysis of the residues will be used to demonstrate that sulfur forms a soluble intermediate such as H₂S in the dissolution reaction. A summary of the results of a detailed study of the kinetics of the copper ion catalysed oxidation of H ₂S by dissolved oxygen is presented which provides further support for a mechanism for the dissolution of chalcopyrite under heap leach conditions which involves an initial step involving non-oxidative dissolution to form H₂S and either cupric ions or a covellite-like surface as the initial products.