Spectroelectrochemical investigation of chalcopyrite leaching

Abstract

This thesis describes an experimental investigation, utilising primarily spectroelectrochemical techniques, into the mechanism of chalcopyrite leaching and the properties of the metal-deficient product layer. A systematic comparison of the leaching behaviour and products in chloride and sulfate lixiviants was undertaken, and variation of leach conditions was also considered. The product layer distribution, structure and morphology was examined, and various model systems and compounds investigated to aid in identifying the product layer composition and properties. A range of chalcopyrites, and other pertinent sulfide-minerals, were investigated. The behaviour of chalcopyrite in both chloride and sulfate electrolytes was investigated in situ and ex situ. Electrochemical experiments demonstrated similar initial behaviour in both chloride and sulfate systems. Potentiostatic techniques combined with normal Raman interrogation of the mineral surface showed much thicker product layers formed on the chalcopyrite leached in chloride electrolytes, over the entire surface. The product layer formed on chalcopyrite leached in chloride solutions consisted of octasulfur and/or a Raman-inactive product phase that could be activated under 442 nm laser irradiation to form polymeric sulfur. The parent phase could not be positively identified but is probably an amorphous metal-deficient remnant lattice on the oxidised mineral surface, which can be restructured under specific conditions to polymeric sulfur. Model compound investigations demonstrated that the induced phase converted to elemental sulfur at ~70°C. Polymeric sulfur was present on leached surfaces even in the absence of laser-inducement but was generally overwhelmed by the octasulfur signal (but identified on samples kept under ultrahigh vacuum conditions). Similar product was rarely observed over the sulfate-leached chalcopyrite surface, as the product layer was too thin to be detectable. However, at 'active' sites (cracks, fissures and grain boundaries) product with a lower ?(SS) Raman shift was encountered, indicating longer sulfur bonds and probably less metal-deficiency. Polysulfides, polysulfanes, jarosite and sulfoxy anions were not detected on acid-leached samples. Model sulfide compounds, and proposed intermediates in chalcopyrite oxidation, were investigated using spectroelectrochemical and neutron reflectometry techniques. The same laser-induced polymeric sulfur phase was identified on high-Fe sphalerite and pyrite surfaces during acid-chloride leaching, though not on covellite surfaces. Sulfoxy anion intermediates were observed on pyrite oxidation in acid solution, confirming a different mechanism to that observed for chalcopyrite oxidation. CuS showed distinctly different spectroelectrochemical behaviour to chalcopyrite and thus is not an intermediate in chalcopyrite oxidation. More aggressive leaching of chalcopyrite was investigated at circumneutral pH. Raman, Environmental Scanning Electron Microscopy (ESEM) and X-ray Photoelectron Spectroscopy (XPS) indicated the presence of highly soluble sulfate salts on the sample surface, though the majority of the product consisted of ferric oxyhydroxides and elemental sulfur. Optical and electron microscopy revealed that the product layer thickness and properties varied as a function of grain orientation. Dynamic Secondary Ion - Mass Spectroscopy (SIMS) and XPS were used to yield elemental composition and valence-state information of oxidation products, and how these varied with depth. Oxidation appeared to be incongruent in the early stages of oxidation, with iron more deficient on the surface than copper. The metal-deficiency extended to some depth (up to hundreds of nanometres) on both chloride and sulfate leached samples. Copper maintained a formal univalent oxidation state on the corroding chalcopyrite surface, while iron was present as ferric ion and sulfur present in oxidation states intermediate between sulfide and elemental sulfur. Submonolayer in situ investigations of chalcopyrite oxidation in acid chloride and sulfate solutions were undertaken via development of a technique to facilitate surface-enhanced Raman scattering (SERS) from the chalcopyrite surface. Both ex situ and in situ investigations were undertaken and showed an amorphous product containing sulfur bonds but distinct from polysulfides, polythionates or elemental sulfur allotropes. The product spectrum was similar in sulfate and chloride solutions, in good agreement with electrochemical observations. Normal Raman and SERS investigations indicate that chalcopyrite oxidation proceeds via a mechanism of cation removal and concomitant oxidation of the remnant sulfide lattice. Sulfur bonding does not appear to commence immediately, and when it does occur, an amorphous structure is formed. This amorphous leached layer is stable for relatively long periods of time at room temperature and extends to some depth. In the case of the chloride, the leached layer ages to sulfur, though of a somewhat amorphous nature that does not volatilise extensively under vacuum. The greater aging of the leached layer on chloride-leached samples is probably facilitated by the ability of copper to leave the lattice as a cuprous complex.