Mechanism of pyrite oxidation in copper(II)-ethylenediamine-thiosulphate gold leaching system

Abstract

Although the Cu2+–ethylenediamine–S2O32− system is a promising alternative to cyanide-based gold leaching systems because of its low cost, nontoxicity, and environmental friendliness, its performance is strongly affected by the presence of pyrite. Therefore, a deep understanding of the mechanism of pyrite oxidation in this system is of high practical importance. Herein, this mechanism was probed to demonstrate the occurrence of rapid reactions on the pyrite surface at high S2O32− concentrations and reveal that the dissolution of bulk pyrite was most favoured at a certain optimal S2O32− concentration, i.e., S2O32− ions strongly affected the surface properties of pyrite. Anodic processes occurring in active, passive, and transpassive dissolution regions were studied by linear sweep voltammetry, with additional insights into the properties of pyrite electrodes and the related electrochemical reactions obtained by electrochemical impedance spectroscopy and equivalent circuit fitting. With increasing potential at range of 0.20 to 0.80 V, the reactions on the pyrite surface accelerated, and its roughness significantly increased. The products of pyrite surface oxidation were identified by Raman spectroscopy and X-ray photoelectron spectroscopy, and a corresponding mechanism was proposed. Thus, this work provides a theoretical basis for the regulation and control of gold leaching reagents in the Cu2+–ethylenediamine–S2O32− system.