Conditions for the formation of a nonequilibrium nonstoichiometric layer on pyrrhotite in acid solutions

Conditions for the formation of a nonequilibrium nonstoichiometric metal-deficient layer (NL) on natural pyrrhotites are studied, together with the NL electroreduction and the role NL plays in a nonoxidizing dissolution of pyrrhotite in solutions of sulfuric and hydrochloric acids. To estimate the NL weight, the charge connected with a cathodic peak at about -0.2 V (Ag/AgCl) is used. The peak reflects the irreversible reduction of NL with the formation of hydrogen sulfide, which is confirmed by SEM and XES data. Bulky NL forms in 0.5 M H2SO4 at -0.1 to 0.0 V, where the nonoxidizing dissolution rate sharply alters, and at 0.5-1.1 V, where the pyrrhotite oxidation rate is high. The NL growth is controlled by solid-state diffusion, whereas nonoxidizing dissolution of iron is limited by diffusion at 20-30degreesC and a kinetic stage of dissolution of sulfur at higher temperatures.