A critical review of the surface chemistry of acidic ferric sulphate dissolution of chalcopyrite with regards to hindered dissolution

The present understanding of the surface chemistry of acidic ferric sulphate dissolution of chalcopyrite is critically reviewed with regard to hindered dissolution and how the hydrometallurgical limitations, especially for microbial heap leaching operations, might be overcome. In particular the surface science investigations of what surface phases might be responsible for hindered dissolution are reviewed. Some other mechanistic issues are also considered which require further investigation. The possible phase candidates for hindered dissolution are examined, with most discussion focussed on elemental sulphur and jarosites. Phases such as polysulphides are rejected as candidates. The physical reality of metal-deficient sulphides is also questioned.A conceptual 4-stage model is proposed which explains all the general dissolution behaviour that is widely observed, i.e. of an induction period and a parabolic rate curve that may or may not be followed by linear rate behaviour. The general conclusion is that thick over-layers of sulphur cause the initial parabolic behaviour, and a thin systemic sulphur layer is responsible for the rate-limiting step, even in the linear region. Depending upon solution conditions, either unhindered near linear dissolution may occur, or jarosite precipitation that will cause a second parabolic region. Sulphur formation remains a systemic phase in the context of heap bioleaching but is not a problem of any consequence for mixed culture systems unlike jarosites. Suggestions are made as to a low-cost jarosite precipitation pond for iron removal and advantages that could result.