The anodic behaviour of chalcopyrite in chloride solutions: Potentiostatic measurements

This paper summarises the results of the first part of an electrochemical study of the anodic characteristics of chalcopyrite in the potential region relevant to ambient temperature heap leaching in chloride solutions during which both iron(II) and copper(II) act as oxidants for the mineral.Mixed potential (Em) measurements in concentrated chloride solutions in the presence of iron(II) or copper(II) have enabled the potential region of 0.65 V to 0.8 V to be defined as important in the leaching process. The systematic variations in Em with pH and oxidant concentrations suggest that increases in both should result in increased rates of dissolution. This conclusion is also supported by simultaneous decreasing solution potentials due to reduction of the oxidants on the dissolving mineral surface. Slower rates of dissolution can be expected for iron(II) than copper(II) as the oxidant.Potentiostatic measurements at various potentials in the above range at different pH values have confirmed that chalcopyrite undergoes a slow “passivation” that is not complete even after 24 h. The current densities after prolonged oxidation vary exponentially with potential and increase with increasing pH but are not affected by the concentration of chloride in the range 1-5 M. Measurements with chalcopyrite samples from 3 different locations showed only minor variations in anodic reactivity.Analysis of both the current-time transients and the re-activation of the mineral surface under zero current conditions after potentiostatic oxidation have been interpreted in terms of a growing copper-rich sulphide layer under anodic polarization. Solid-state diffusion through this layer is suggested as being responsible for the “passivation” process by analogy with the well known de-alloying and back-alloying processes in some binary alloys.