A comparative study of the dissolution of hexagonal and monoclinic pyrrhotites in cyanide solution

Abstract

The extent and rate of dissolution of two natural pyrrhotite samples, one monoclinic (Fe 0.88S) and the other hexagonal (Fe 0.90S), were studied in alkaline cyanide solutions (pH 10). The effects of dissolved oxygen, particle size, and temperature were investigated. The extent of dissolution was followed by monitoring the dissolved sulfur levels in solution. The initial rates of dissolution were determined by fitting the conversion data to a second-order polynomial regression equation. The rate of dissolution of the monoclinic pyrrhotite under the variety of conditions was found to be greater than that of the hexagonal pyrrhotite. The activation energies for the oxidative dissolution of monoclinic and hexagonal pyrrhotites were found to be 65.3 and 71.9 kJ/mol, respectively, which indicated that the initial dissolution of pyrrhotite is under chemical control. The reduced reactivity of the hexagonal form was attributed to its greater iron content, which, during oxidation, was proposed to form a more compact iron-oxyhydroxide coating, which passivated the sulfide and made it less susceptible to cyanide ion attack. No passivation effect was evident in the case of the monoclinic pyrrhotite.