A critical review of the passivation and semiconductor mechanisms of chalcopyrite leaching

The idea that there is a consensus in the field of hydrometallurgy that chalcopyrite leaching is hindered by a passivation layer is challenged in this review. Not all studies of chalcopyrite leaching propose that surface layers are passivating. Some claim that there are passivating and non-passivating surface species, but with directly opposing opinions as to which plays the passivating role. The more common passivation proposals are variants of a metal-deficient sulfide or a metal-deficient polysulfide. Sometimes it is claimed these are distinctly different species with different passivating capabilities, at other times they are simply different names for the same thing. These are not well defined and are usually given a general name such as Cu1-xFe1-yS2 or CuFe1-xS2, which is often inferred from electrochemistry and dissolution stoichiometry. In contrast to leaching in acidic solutions, it has been shown that in alkaline solutions with a complexing agent there is no passivating effect. This is despite the same supposedly passivating species being present on the surface as is found in acid solutions. An alternative explanation to the various passivation proposals is that the electronic structure of chalcopyrite dictates the effects observed in electrochemistry and leaching. Natural chalcopyrite has the electronic structure of an n-type semiconductor. As such it is not expected to conduct significant current at modest anodic potentials until a 'breakdown' potential is reached. This semiconducting property is often observed in electrochemistry where very low currents are observed under an applied potential up to about 1.0 V (vs SHE), where a rapid increase in current is observed. Other observations such as photocurrents, trends in capacitance and limiting currents have also been attributed to the semiconducting nature of chalcopyrite. This approach has however attracted extremely strong criticism. These critics instead propose that chalcopyrite should be regarded as "pseudo-metallic" rather than semiconducting due to expected metal impurities in a natural sample. Any semiconducting effects observed are said to be due to improper operation of laboratory equipment or are experimental artefacts. However, a careful analysis of the claims of these critics shows that they are mostly speculative - indeed, several are based on incomplete data and misinterpretations of other published work. More objective experimental studies are needed both to verify these criticisms and to prove that surface layers are indeed passivating.