Cuprous hydrometallurgy Part IV. Rates and equilibria in the reaction of copper sulphides with copper(II) sulphate in aqueous acetonitrile

Abstract

Copper(II) sulphate in solutions of aqueous acetonitrile leaches copper from copper sulphides to form stable copper(I) sulphate solutions. Covellite and chalcopyrite are oxidised and leached more rapidly in the early stages of leaching with acidic CuSO 4/CH 3CN/H 2O than with acidic iron(III) sulphate in water. A redox equilibrium between copper(I) sulphate, copper(II) sulphate and partially leached solid copper sulphide, Cu x S, is established. The equilibrium concentration of Cu 2SO 4 and the value of x in Cu x S, in solutions saturated with CuSO 4, are interdependent if the acetonitrile concentration is constant. This behaviour is considered in terms of the structural and electrochemical changes which occur, in the solids Cu 2S and CuS, as leaching proceeds. According to the activities of acetonitrile, of copper(I) sulphate and of copper(II) sulphate, i.e. according to the redox potential of the solution, CuS either can be oxidised by copper(II) sulphate to a less copper-rich copper sulphide and even to sulphur, or reduced by copper(I) sulphate to a more copper-rich sulphide, up to Cu 2S, in acidic solutions containing CuSO 4, Cu 2SO 4, CH 3CN and water. This observation leads to an easy method of generating Cu 2S from CuS or from sulphur.