An X-ray photoelectron spectroscopy study of the mechanism of oxidative dissolution of chalcopyrite

Abstract

An alternative mechanism based on the application of simple chemical principles involving hydrated (hence realistic) solution species is proposed for the oxidative acid leaching of copper and iron from chalcopyrite. The consistent surface speciation for abiotic and microbial, aerobic and anaerobic conditions fits with the need for a common mechanism. X-ray photoelectron spectroscopy detected the expected sulphur and sulphate surface species, plus a disulphide phase. The sulphate species was found to be a basic ferric sulphate akin to jarosite. A key conclusion is that chalcopyrite dissolves by oxidation of the disulphide phase that forms rapidly on freshly fractured chalcopyrite surfaces and also persists on leached chalcopyrite surfaces. The oxidation of the disulphide phase is thought to occur by electron transfer to ferric ions, via surface protonation and hydration, with the likely consequent production of thiosulphate. The thiosulphate being further oxidized to sulphate producing a basic ferric sulphate phase which acts as a nucleation template for jarosite formation that ultimately leads to chalcopyrite “passivation”. The copper ions thus take no part in the oxidation mechanism and are simply solubilised as the chalcopyrite surface is destroyed. Neither thiosulphate nor polysulphide species were directly detectable as surface intermediates. Synthesis and XPS examination of model polysulphides indicates that they do not play any role in inhibiting chalcopyrite dissolution. For the microbes examined they appeared to play no active role in the oxidation process at the solid solution interface. The same surface speciation for abiotic conditions suggests the microbial role is one of a solution oxidant of the ferrous ions produced. An examination of silver catalysis indicates an entirely different mechanistic pathway.