A model for silver ion catalysis of chalcopyrite (CuFeS2) dissolution

Atmospheric leaching of chalcopyrite in sulfuric acid solutions is a slow process due to its passivation. Silver ions have been shown to catalyze this process and increase the oxidation rate. In this study the interaction between silver ions and chalcopyrite has been investigated in silver-containing sulfuric acid solutions. The silver concentration of the test solutions was between 0 and 10 ppm. In situ Mott-Schottky analysis and X-ray photoelectron spectroscopy (XPS) were respectively applied to study chalcopyrite's semiconductor properties and silver diffusion into the chalcopyrite passive film. Results of the Mott-Schottky experiments showed that the addition of silver to the solution increases the apparent capacitance of the chalcopyrite passive film owing to changes in the semiconductor properties of the film. The measured capacitance was shown to be time dependent for the first 15 min of the experiment (i.e. sample immersion in solution) and after this elapsed time the capacitance stayed constant. XPS results showed that silver atoms diffuse about 18 nm below the chalcopyrite surface. A model is proposed for silver ion catalyzed chalcopyrite electro-dissolution. In this model it is assumed that silver cations adsorb to the electrode surface and form Ag2S. The formation of each Ag2S molecule on the surface requires the creation of a sulfur vacancy and a pair of holes in the passive film. The effect of both the sulfur vacancy and the pair of holes is an increase in the dissolution kinetics of chalcopyrite. (C) 2015 Elsevier B.V. All rights reserved.