Anodic oxidation of pyrrhotite in simulated CIP liquors

Natural pyrrhotite (Fe7S8) can be oxidized in alkali (pH 10) at 25°C at potentials above −0.2 V (SCE). The voltammetry of a ground pyrrhotite disc shows current peaks consistent with the formation of a ferric surface phase such as Fe(OH)3 by air oxidation or by potentiostatic oxidation. In unbuffered solution, proton production can be demonstrated implicating the formation of sulfate, rather than sulfur as another product of reaction. In unbuffered or buffered solution, electrode rotation increases the initial oxidation rate due to the limiting diffusion of base in the hydrodynamic boundary layer between the bulk solution and the mineral surface. At longer reaction times parabolic kinetics apply suggestive of a limiting transport of base through the pores of the thickening product layer on the mineral surface. This layer persists in cyanide solution for conditions in which dissolved iron cyanide species are thermodynamically stable. Dissolved lead (1.5–3 ppm) decreases the oxidation rate probably due to lead sulphate precipitation within the porous layer. The oxidation rate of pyrrhotite is much lower than the cyanidation rate of gold for similar conditions. The most likely mechanism of pyrrhotite interference in carbon-in-pulp (CIP) gold processing plants involves the precipitation of gold on pyrrhotite driven by the oxidation of surface ferrous hydroxide to ferric hydroxide; and this mechanism is discussed using the data of Koch et al.