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

The dissolution behaviour of iron sulfides is significant in the minerals i ndustry 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 d eoxygenated acid was studied using approaches established for the study of binary metal oxides in acid conditions. A feature of pyrrhotite (Fe1-xS) is that a relatively slow dissolution rate (10(-8) mol m(-2) s(-1)) can incre ase suddenly by three orders of magnitude (to 10(-5) mol m(-2) s(-1)) in th e same solution. The sudden increases have been shown to be due to the onse t of non-oxidative dissolution. Correlation was found between the non-oxidative dissolution of pyrrhotite a nd that of ionic and semi-conducting binary metal oxides dissolving in acid with reaction controlled kinetics. In both situations -log (Rate) was prop ortional to pH, implying that the rate-determining step is ionic transfer a cross the surface - electrolyte interface. The strongly ionic nature of the S2- bonding within semi-conducting troilit e is made evident by dissolution rates of the order of 10(-4)-10(-3) mol m( -2) s(-1). As with ionic oxides with dissolution rates of this magnitude, t he dissolution kinetics of troilite are no longer reaction-controlled but a re controlled by bulk solution diffusion factors. (C) 2000 Published by Els evier Science Ltd. All rights reserved.