The galvanic effect of pyrite enhanced (bio)leaching of enargite (Cu3AsS4)

Abstract

Enargite is a representative arsenic-bearing copper resource, and pyrite is often found co-existed with it. Understanding the effect of pyrite on enargite (bio)leaching is of vital importance for the recovery of copper from enargite. The galvanic interaction between enargite and pyrite in bioleaching and electrochemical systems was investigated. Bioleaching experiment showed that the addition of pyrite significantly promoted the enargite leaching rate due to the galvanic effect. However, the pyrite promotion effect on the enargite leaching rate decreased due to the oxidation of pyrite and the surface area of pyrite reduced in the bioleaching process. The dissolution rate of enargite without pyrite was controlled by diffusion through a product film, suggesting a passivation layer formed around the enargite surface. The presence of pyrite changed the rate-limiting factor of enargite dissolution to the surface chemical control, indicating that the addition of pyrite eliminated the passive effect. Electrochemical studies showed the mixed potentials (EM) and dissolution current densities (Idiss) of enargite electrodes increased when the enargite was in contact with a pyrite electrode, and the Emix and Idis further increased with increasing the surface area of the pyrite electrode. The effect of the pyrite electrode surface area on the dissolution of enargite electrodes was quantified by the kinetics of the half-cell reactions occurring in the pyrite-enargite galvanic couple under mixed potentials with oxygen and ferric ions as oxidants. Electrochemical impedance spectroscopy studies demonstrated a significant decrease in the charge transfer resistance and passivation film resistance of the enargite electrode in contact with pyrite, indicating that the cathodic reduction controlled the dissolution of enargite electrode and galvanically coupled pyrite continued to act as a cathode to facilitate the oxidation of enargite by oxygen and ferric ions even after enargite was passivated.