The kinetics of the copper-catalysed oxidation of iron(II) in chloride solutions

An alternative approach to the heap leaching of copper sulfide minerals involves the use of chloride ions as the lixiviant. The facile oxidation of copper(I) ions in a chloride system by atmospheric oxygen offers an alternative to bio-oxidation of iron(II) in the sulfate system. In the chloride system, it is well-known that both copper(II) and iron(III) ions can act as the oxidants for these minerals. However, regeneration of these species by oxidation with dissolved oxygen is required and this is possible in a sulfate system by the use of bacterially catalysed oxidation of iron(II). However, in the chloride system, re-oxidation of copper(I) and iron(II) ions can only economically be achieved by chemical reaction with dissolved oxygen. The kinetics of the reduction of dissolved oxygen by iron(II) and copper(I) in acidic chloride solutions have been previously studied with the former being considerably slower than the latter. This paper will focus on the kinetics of the copper-catalysed reaction of iron(II) with dissolved oxygen. It will be shown that this reaction can be described in terms of the rapid equilibrium Fe(II) + Cu(II) ⇆ Fe(III) + Cu(I) followed by relatively rapid auto-oxidation of copper(I). The inhibiting effect of iron(III) ions in terms of the above equilibrium is demonstrated and the effects of acidity and chloride ion concentration on the rate described. An overall kinetic model has been developed which is consistent with the published data on the individual reactions involved in this mechanism. The application of this model to the prediction of maximum possible leach rates for copper sulfide minerals in aerated systems under ambient conditions is discussed.