Mechanisms and rates of sulphide oxidation in relation to the problems of acid rock (mine) drainage

The aqueous oxidation of metal sulphide minerals in natural rocks, minewastes or mineworkings generates acidic waters, often containing elevated concentrations of toxic metals, and known as acid mine drainage (AMD) or, more generally, as acid rock drainage (ARD). Understanding the mechanisms and rates of oxidation of key sulphide minerals is the essential first stage in understanding the processes giving rise to ARD. In this chapter, our knowledge of the aqueous oxidation of the most important sulphide minerals (pyrite, pyrrhotite, galena, chalcopyrite, sphalerite, marcasite and arsenopyrite) is considered in the context of problems associated with ARD. In certain cases, qualitative or semi-quantitative data concerning oxidation rates are available (for example, in tailings impoundments the sequence from most to least reactive is generally pyrrhotite > galena - sphalerite > pyrite - arsenopyrite > chalcopyrite) and a substantial body of data (some conflicting) exists concerning the products of oxidation. It is acknowledged that surface reaction control is the key to oxidation reaction mechanism. However, as reviewed here, the data and models currently available to describe the oxidation of particular sulphides do not, as yet, yield a consistent picture. Fundamental understanding of oxidation mechanisms remains sketchy, therefore, but the tools are now available to make progress in this field through in situ studies of oxidation processes at atomic resolution.