Metal oxides processing using deep eutectic solvents

Metal oxides are the form from which most metals are extracted. They are found in natural ores and many industrial residues and end-of-life products, which makes their efficient processing an important topic. The state-of-the art processes for the extraction of metal oxides include either the pyrometallurgy or hydrometallurgy, which both have a significant environmental footprint. The investigation of more efficient alternatives for their extraction is crucial, in order to develop sustainable flowsheets for recycling materials like cathodes from lithium ion batteries.

The understanding of the dissolution mechanism of selected metal oxides was attempted using deep eutectic solvents. In general, Pourbaix diagrams have shown that metal oxides can be digested either through protonation, complexation or by redox processes. The two former methods were investigated to determine their effect on solubility, and it was shown that the surface complexation had a greater impact on their solubility compared to the proton activity of the solvent. Speciation is known to be the key to designing selective processes, so the ability to tune the deep eutectic solvents to selectively dissolve some metals over others is a great asset. The selective extraction of Co and Mn over Ni from cathode materials of lithium ion batteries and Y and Eu from spent fluorescent lamp phosphors was demonstrated.

Apart from the chemical dissolution, the electrochemical oxidation of metal oxides was also investigated as this was previously shown to be efficient for the dissolution of metal sulfides, tellurides and selenides. It was found that a significant enhancement of the metal oxide dissolution rate could be obtained in solvents that are neither acidic nor consist of complexing agents. Indeed, the rate of dissolution, which was dependent upon the band gap of the metal oxides, was strongly enhanced, sometimes even more than 10000 times.