Catalytic and galvanic effects of pyrite on ferric leaching of sphalerite

Abstract

The catalytic and galvanic effects of pyrite on the dissolution kinetics of a zinc sulfide concentrate in an acidified ferric sulfate medium were assessed. The effects of particle size, oxidant concentration (ferric ion), and amount of pyrite added with respect to the original amount of zinc concentrate was explored, while temperature (70°C), hydrogen potential (pH=1.0) and agitation speed (600rpm) of the leach solution were held constant. A process based on the galvanic coupling between pyrite and sphalerite at solution potentials above 500mV to ensure rapid and complete sphalerite dissolution in a ferric sulfate medium is proposed. As a result, the highest zinc extraction in the shortest possible time was obtained through minimizing the formation of a passivating film, which inhibits further reaction, and by a synergetic effect between pyrite and sphalerite. The rate of zinc leaching was considerably improved by doping with pyrite in the presence of Fe3+. The results have demonstrated that it was possible to achieve 98.9% zinc extraction with doping pyrite in 6h and 85% zinc extraction without pyrite in 7h, both results being for a fine grain size. Ferric leaching reached 75% Zn extraction with doped pyrite and 70% without it, both in 7h using a coarse grain size. Scanning electron microscopy (SEM) images show a passivating layer on the sphalerite surface after ferric leaching without pyrite, while in presence of pyrite this layer was not observed and the pyrite surface was not attacked. The galvanic interaction between pyrite and sphalerite particles can be explained by the difference between their Fermi energies and the electrochemical potential of the solution. During this process pyrite remains mostly unleached, acting as a redox catalyst (electron-accepting) for sphalerite (electron-donating) dissolution. This diminishes the typical passive behavior of sphalerite in a ferric solution enhancing considerably the kinetics of sphalerite. This confirms that the difference between the rest potentials of pyrite and sphalerite is the driving force that improves zinc dissolution. For an industrial hydrometallurgical operation, using an appropriate sphalerite-pyrite concentrate does not necessarily represent a significant cost because the ore contains both sulfides.