Arsenic sulphides such as enargite and tennantite can represent significant penalty elements in base metals production. There are significant economic advantages to achieving a separation of arsenic bearing minerals at an early stage in processing, but to date no feasible widely applicable commercial method of flotation separation has been developed. A review of the existing literature on the selective flotation of enargite considered its surface properties and floatability. Special consideration was given to the various approaches in the study of enargite surface chemistry, and the implications for the flotation of enargite. Developments in these approaches were critically reviewed and discussed.
The surface oxidation and hydrophobicity of natural enargite (Cu3AsS4) and the formation of oxidation species at the mineral surface were examined by a novel experimental approach that combines electrochemical techniques and atomic force microscopy (AFM). Combined with ex situ cryo X-ray photoelectron spectroscopy (XPS), the surface speciation of oxidised enargite was obtained, and compared with the newly fractured natural enargite surface.
At pH 4, surface layer formations consisting of metal-deficient sulphide and elemental sulphur were identified, associated with a limited increase in root-mean-square (rms) roughness (1.228 to 3.143 nm) and an apparent heterogeneous distribution of surface products as demonstrated by AFM imaging. A mechanism of initial rapid dissolution of Cu followed by diffusion-limited surface layer deposition was identified.
At pH 10, a similar mechanism was identified although the differences between the initial and diffusion-limited phases were less definitive. Surface species were identified as copper sulphate and hydroxide. A significant increase in surface rms roughness was found (0.795 to 9.723 nm). Dynamic (receding) contact angle measurements obtained by a droplet evaporation method found no significant difference between the oxidised surface and a freshly polished surface. A significant difference was found between the polished surface and that oxidized at pH 4, with an increase in contact angle of about 13° (46° to 59°) after oxidation. Competing effects of hydrophilic and hydrophobic species on the mineral surface under oxidizing conditions at pH 4 and the change in surface roughness at pH 10 may contribute to the observed effects of electrochemically controlled oxidation on enargite hydrophobicity.
The effects of X-ray radiation on the surface after electrochemical oxidation were investigated using XPS. Surface species present on unoxidized enargite were compared with those present after oxidation at pH 10, and the effects of X-ray irradiation time as a function of temperature were studied. XPS spectra characteristic of a copper (II) hydroxide surface layer reduced in intensity with increasing X-ray exposure time. Associated changes in the relative concentrations of surface oxygen species were also observed. Temperature was shown to significantly influence the rate of change. A two-stage mechanism involving the dehydration of Cu(OH)2 to CuO, followed by photoreduction of CuO to Cu2O was proposed, and sample cooling was found to reduce these effects.
Oxidation of natural enargite under potentiostatic control and the formation of oxidation species at the surface were investigated at potentials where oxidation was known to occur (+347, +516, +705, +869 and +1100 mV SHE, or Standard Hydrogen Electrode). XPS analysis found a progressively increasing level of oxidation as applied potential increased. Surface layer deposition was linked to potential, with limited evidence of oxidation after treatment at +347 and +516 mV, where no evidence of Cu(II) compounds was found, while a decrease in copper at the surface suggested dissolution as the primary reaction mechanism. At +705 mV, Cu(II) species identified as CuSO4 and Cu(OH)2 were found, although arsenic oxides or sulfides were not found.
After treatment at +869 and +1100 mV significant evidence of oxidation was found, with CuSO4 and Cu(OH)2 identified and additional sulphur and arsenic species (CuS and As2O3) identified. Comparison of these results with published reaction mechanisms for similar treatments showed that they did not account for all species identified in the XPS data. Analysis of buffer solutions post-treatment by ICP (Inductively Coupled Plasma spectroscopy) showed a pattern of change in concentrations of Cu, As and S characterized by a step-change increase in dissolution between the +516 and +705 mV treatment conditions, which correlates with the formation of Cu(II) on the mineral surface.
Investigation of the effects of applied potential on froth flotation collector adsorption was completed to better understand the potential for selective recovery of enargite. Electrochemical techniques and XPS were used to study the adsorption of sodium ethyl xanthate and dialkyl dithiophosphinate at pH 10. Cyclic voltammetry was used to investigate the effect of applied potential on enargite in sodium ethyl xanthate (SEX) and dialkyl dithiophosphinate (3418A) solutions of different concentrations at pH 10 while surface speciation due to anodic and cathodic applied potentials was investigated by XPS. The flotation response of enargite at these potentials was determined using a micro-flotation cell after conditioning in the presence of xanthate for 5 minutes.