New perspective for indium recovery from e-waste by electrodeposition in room temperature ionic liquids

Faced with explosive growth in demand for indium and, faced with challenges to both socio-economic and political potential it represents, the recycling of indium content in the equipments of end of life remains the only alternative to address risk of shortages. Beyond the economic and strategic aspects, recycling of indium can help preserving the environment by preventing large-scale exploitation of mineral ores containing indium. In addition, the toxicity of indium alone justifies the development of methods for treating waste containing indium. However, the recovery of indium from electronic waste is currently fairly limited, except in Japan where there are several processes at an industrial scale. However, these processes are not environmentally friendly and are energy-intensive In this PhD work, recycling of indium by liquid / liquid extraction in an ionic liquid followed by its electrodeposition in situ appeared to us as a promising process, to overcome the often difficult step of de-extraction of the metal cation. Among the ionic liquids that we have synthesized and characterized in terms of structure and of physicochemical properties, 1-butyl-1-ethyl-piperidinium bis(trifluoro-methylsulfonyl) imide (BEPipNTf₂) has been shown most suitable because of its cathodic stability, its low viscosity, its hydrophobic and weakly hygroscopic character. We have shown that in synergy with the trioctylphosphine oxide (TOPO) as extractant, it is possible to extract more than 90% of the indium contained in a 10^-2 M HCl aqueous phase. The electrochemical system In(III)/In(0) in the BEPipNTf₂ was then studied in the presence of chlorides, of water, of oxygen and of TOPO, which are the species present after the liquid/liquid extraction step of the In(IIII). A detailed study of the influence of chloride ions has been particularly carried out, highlighting the formation of chloro-complexes of indium when applying a cathodic potential of reduction of In(III), which significantly changes the electrochemical characteristics of the system In(III)/In(0), The results obtained show that indium (III) can be electrodeposited in its metallic form in the range of electrochemical stability of the ionic liquid and that this is non-reversible in presence of TOPO, whose electroactivity in the corresponding potential range suggests that the reduction could result in the presence of organic impurities in the deposition of indium and could limit the amount of metal deposited. (author)