Sulphur speciation of leached chalcopyrite surfaces as determined by X-ray photoelectron spectroscopy

Abstract

A key factor in improving the bioleaching route for chalcopyrite processing is a better understanding of the surface speciation that exists under chemical leaching conditions that mimic the acid bioleach. The surface sulphur speciation of chalcopyrite under such conditions has been revisited using X-ray photoelectron spectroscopy (XPS). Objectives of the study were to resolve the issue of possible passivation candidates and to understand the relative roles of ferric and ferrous ions in the oxidative leaching process. Neither severely metal-deficient sulphides nor polysulphides were found to be major surface layer components during initial leaching. The primary surface species produced with an acid ferric leach is elemental sulphur. This is largely volatilisable and coats the underlying unleached sulphide mineral but not adjacent minor sulphate domains. It is important to distinguish between the loss of multilayer elemental sulphur and the loss of monolayer or submonolayer quantities of the same species, especially as it impacts on polysulphide identification. The second major leach product on the chalcopyrite surface is disulphide S 2 2−. Although the cation association of the S 2 2− is not known, evidence from the Cu 2p spectra discounts the formation of any CuS 2 type species. Both acidic ferric and ferrous leaches produced the elemental sulphur and disulphide surface, though the more aggressive ferric produced a greater quantity of elemental sulphur. Evidence for polysulphides with a chain length greater than 2 remains an open question. There is some evidence that such polysulphides might form with acidic ferrous leaching, but the prime candidate for any initial leaching inhibition (prior to jarosite formation) is elemental sulphur.