The application of deep eutectic solvent ionic liquids for environmentally-friendly dissolution and recovery of precious metals

The processing of ore by hydrometallurgy or pyrometallurgy typically has a high energy demand, and associated release of carbon dioxide. Thus there is a need to develop more energy-efficient and environmentally-compatible processes. This article demonstrates that deep eutectic solvent (DES) ionic liquids provide one such method since they can be used to selectively dissolve and recover native gold and tellurium, sulphides and tellurides. Ionic liquids are anhydrous salts that are liquid at low temperature. They are powerful solvents and electrolytes with potential for high selectivity in both dissolution and recovery. Deep eutectic solvents are a form of ionic liquid that are mixtures of salts such as choline chloride with hydrogen-bond donors such as urea. DESs are environmentally benign, yet chemically stable and, furthermore, the components are already produced in large quantities at comparable costs to conventional reagents. Electrum, galena and chalcopyrite, as well as tellurobismuthite (Bi2Te3), were soluble in DES through an oxidative leach at 45-50°C. Leaching rates determined by a novel technique employing an optical profiler were very favourable in comparison to the current industrial process of cyanidation. Pyrite was notably insoluble by an oxidative leach. However, pyrite, and indeed any other sulphide, could be selectively dissolved by electrolysis in a DES, thus suggesting a protocol whereby target inclusions could be liberated by electrolysis and then dissolved by subsequent oxidation. Ionometallurgy could thus offer a new set of environmentally-benign process for metallurgy.