Characterization of surface layers formed during pyrite oxidation

Abstract

Surface electrochemical reactions of pyrite have been studied using cyclic voltammetry, in situ laser Raman spectroscopy and potentiostatic measurements. A surface sulfur layer was identified on pyrite surfaces during transpassive oxidation. Sulfur layer formation at high anodic potentials was confirmed by subsequent surface reactions during the cathodic and return anodic cycle in cyclic voltammetry and by in situ Raman spectroscopy. Initially, polysulfides (FeS x of variable x) form as intermediates during anodic oxidation in both acidic and basic solutions. The nature of these intermediates depends upon pH and the rate of surface film growth. The end product of the film growth is an active form of sulfur, S 0 act, which is readily reducible. Raman spectra obtained during an extended period of time indicated the film was mainly S 0 act. The conditions such as applied potentials and pH that lead to the build-up of the surface oxidation layer have been determined. Film growth kinetics obey the paralinear rate law, indicating uniform film growth of the sulfur-rich layer to a steady state thickness. The thickness is controlled by the simultaneous rate of diffusion of cations through the polysulfide layer, with parabolic rate of constant k p, and the rate oxidation of the active-sulfur outer layer, with the linear rate constant k 1, to soluble sulfur species in solution. The effects of temperature, applied potential and pH on k p and k 1 were determined using the paralinear rate law.