Recovery of Metals using Deep Eutectic Solvents

The mineral processing industry produces vast amounts of metallurgical waste streams which are increasing every year. However, this waste still contains a variety of economically important metals and could constitute a valuable secondary source for their production. Recovery of metals from aqueous solutions can be achieved by electrodeposition, cementation, precipitation and ion exchange. Due to the high operating costs and the associated environmental hazards, many of these processes have been characterized as inefficient. This study aims to introduce the DESs as an alternative medium for the recovery of metals from metallurgical wastes.

The ionometallurgical recovery of Ag, Fe, Ni, Co, Sn, Cu, Zn and Pb is studied by electrowinning and cementation from low concentration solutions. It is shown that different metals exhibit different deposition rates and this can be linked to their electrochemical behaviour. Chemically reversible metals such as Ag, Sn, Cu and Pb exhibit relatively high deposition efficiencies with low impurity deposits in contrast with Fe, Ni, Co and Zn which show a non-reversible behaviour. Side reactions such as the degradation of the DES and the reduction of the water contained in it can cause issues during the deposition processes and lead to passivation.

Fe, Zn and Pb are primarily found in most of the metallurgical wastes and their separation was further studied by solvent extraction and adsorption. During the former, the isolation of Fe and Zn from a mixture of the three metals is achieved by commercial extractants providing high selectivity. Waste materials from the egg industry showed adsorption capabilities towards the three metals with chemical reactions between the metal and the adsorbate to control the process. Selectivity of Fe over the others is also demonstrated as it has low market value and can negatively impact the deposition efficiencies of the other elements. A previously developed process for the recovery of metals from industrial jarosite waste is modelled using the HSC Chemistry® software to provide insight for the electrodeposition and cementation of metals. It was found that the recovery rates of Fe, Pb and Zn were low and that the process cannot be cost efficient under the current conditions. Overall recovery of metals using DESs was found to be potentially economically viable for high value elements from primary and secondary materials.