Chalcopyrite (Bio)leaching in Sulphate Solutions An Investigation into Hindered Dissolution with a Focus on Solution Redox Potential

Abstract: Chalcopyrite (CuFeS2) is the most abundant and the most economically important copper mineral. Increasing worldwide demand for copper accompanied by exhaustion of copper resources necessitate the development of new processes for treating lower-grade copper ores. Heap (bio)leaching of copper oxides and secondary sulphides (covellite (CuS) and chalcocite (Cu2S)) is a proven technology and a convenient process nowadays. However, chalcopyrite is recalcitrant to leaching and bioleaching in conventional leaching systems in sulphate media. Slow dissolution of chalcopyrite is attributed to the formation of compounds on the surface of the mineral during its dissolution and is often termed “passivation” or “hindered dissolution”. There is still no consensus about the nature of the passivation layer. There are, however, four proposed candidates suggested in the literature: metal deficient sulphides, polysulphides, jarosite and elemental sulphur. This project was aimed to further investigate the chalcopyrite dissolution and its passivation under strictly controlled redox potential conditions. The leaching experiments of the aged and fresh chalcopyrite concentrate under identical conditions showed that copper dissolution was significantly lower from the aged concentrate. The common understanding of reductive leaching mechanism (i.e. higher recoveries at lower redox potentials) was not valid for aged concentrates. Aged concentrates gave steadily increasing recoveries with increased redox potential. The hindering effect exerted from the atmospheric oxidation products on the surface of the aged concentrates was found to be responsible for this behaviour. It was also shown that the reductive leaching mechanism would be beneficial in the presence of an active galvanic interaction. Experiments using a pyritic concentrate resulted in higher recoveries at low redox potential while the dissolution rates were similar at low and high redox potentials using a relatively pure concentrate. In addition, the effect of initial copper concentration had no influential effect on the leaching rates for possible industrial processes. Redox potential development during moderately thermophilic bioleaching experiments of a pyritic chalcopyrite concentrate and a relatively pure chalcopyrite concentrate were chemically/electrochemically mimicked in the absence of microorganisms. The copper recoveries in absence and presence of microorganisms were similar. In some of the abiotic experiments, jarosite precipitated due to a loss of control of the redox potential. However, presence of bulk jarosite did not hamper the copper recovery compared to the bioleaching experiments where there was no bulk jarosite formation. Bio-oxidation of elemental sulphur did not have a positive effect on the leaching behaviour compared to the abiotic experiments where bulk elemental sulphur accumulated. Isotopic fractionations of copper and iron during the bioleaching and abiotic experiments showed that regardless of presence or absence of microorganisms the copper and iron isotopes fractionation followed a similar trend and that such analyses could be used in natural systems as an indicator of the oxidation extent. Surface analyses using X-ray photoelectron spectroscopy (XPS) measurements revealed that common phases on the surface of the samples leached for different durations were iron-oxyhydroxides and elemental sulphur. The elemental sulphur on the surface of the samples was bound to the surface rigidly in a way that it did not sublimate in the ultra-high vacuum environment of the XPS spectrometer at room temperature measurements. Surface jarosite was observed in only one sample but no correlation between its presence and the hindered leaching could be made. It is proposed that iron-oxyhydroxides are the main precursor of chalcopyrite hindered dissolution in sulphate media where their inevitable formation entraps surface elemental sulphur resulting in a consolidated phase on the surface. It was shown that when suitable conditions are met, high copper recoveries can be obtained before the surface is finally hindered.