Kinetic factors for oxidative and non-oxidative dissolution of iron sulfides

The dissolution behaviour of iron sulfides is significant in the minerals industry for both the recovery of more precious metals and the treatment of waste materials (pyrite being the major contributor to acid rock drainage). The dissolution behaviour of pyrrhotite, Fe_(1−x)S, and troilite, FeS, in deoxygenated acid was studied using approaches established for the study of binary metal oxides in acid conditions. A feature of pyrrhotite (Fe_(1−x)S) is that a relatively slow dissolution rate (10⁻⁸ mol m⁻² s⁻¹) can increase suddenly by three orders of magnitude (to 10⁻⁵ mol m⁻² s⁻¹) in the same solution. The sudden increases have been shown to be due to the onset of non-oxidative dissolution.Correlation was found between the non-oxidative dissolution of pyrrhotite and that of ionic and semi-conducting binary metal oxides dissolving in acid with reaction controlled kinetics. In both situations -log (Rate) was proportional to pH, implying that the rate-determining step is ionic transfer across the surface - electrolyte interface.The strongly ionic nature of the S²⁻ bonding within semi-conducting troilite is made evident by dissolution rates of the order of 10⁻⁴–10⁻³ mol m⁻²s⁻¹. As with ionic oxides with dissolution rates of this magnitude, the dissolution kinetics of troilite are no longer reaction-controlled, but are controlled by bulk solution diffusion factors.