Kinetics and mechanism of the bacterial and ferric sulphate oxidation of galena

The bacterial oxidation of galena (PbS) has been studied, largely due to its relevance to the bioleaching of sphalerite (ZnS) and other sulphide minerals. Ferric sulphate oxidation experiments were first conducted as an analog to the indirect bio-oxidation mechanism, and the process was found to be diffusion-controlled due to the presence of a product layer. The bacterial oxidation of galena by a mixed culture of iron- and sulphur-oxidising bacterium was found to proceed according to a diffusion-controlled indirect mechanism, in which the primary role of the bacteria is the re-generation of ferric iron. Using quantitative XRD analysis, it was demonstrated that the oxidation product layer consisted solely of lead sulphate and elemental sulphur, in a 1:1 ratio. It was proposed that elemental sulphur and lead ions were formed in a ferric oxidation reaction, while lead sulphate subsequently formed via precipitation of the lead ions, consuming sulphate ions from solution. The galena bio-oxidation experiments demonstrated considerable acid consumption, because of occlusion of the produced elemental sulphur by precipitated lead sulphate. This reveals a new mechanism through which galena oxidation may hinder the bioleaching of other sulphide minerals.