Current research has focused on the sustainable and efficient recovery of critical/technological metals from secondary raw materials by integrating the use of non-aqueous solvents, such as deep eutectic solvents (DESs) and unconventional brines, with process intensification methodologies such as ultrasound and electrocatalysis. Spent electronic components (SEC), such as printed circuit boards, solar photovoltaic (PV) cells, magnets and thermoelectrical generators, were considered as they are an attractive source for critical/technological metals. Until now, the dissolution and subsequent recovery of metals from SEC considers either pyro- and/or hydrometallurgy. Such processing routes are typically very energy intensive, and are not exempted of producing a vast amount of solid and liquid residues. Consequently, there is a need to develop more energy-efficient and environmentally-compatible processes. Deep eutectic solvents (DESs) have several advantages over traditional solvents, such as low cost and ease of preparation from readily available components. Thus, metals can be directly recovered from complex solid materials by following a similar principle to the traditional chemical processes, but with higher selectivity and lower toxicity.
The utilisation of brines, on the other hand, demonstrated promising results for leaching metals as a replacement for traditional mineral acids and/or highly viscous DESs. Due to their high water content, these liquids have a lower viscosity in comparison to ionic fluids. Hence, they represent a new medium with interesting properties for the dissolution and recovery of metals, which deserve further attention.
Within the ELECTROION project, it was pioneered the catalytic dissolution of metals from meteorite proxies of metal-rich asteroids using a DES. Three types of meteorites were investigated: 2 chondrites and one iron meteorite. Chondrites samples were composed of silicates (olivine, pyroxene, amphibole) with metallic phases occurring as native metal alloys and sulphides. Metallic Fe-Ni and troilite (FeS) were the most abundant metal-bearing phases in all three samples, particularly in the iron-rich meteorite. The samples were subjected to chemical micro-etching experiments with iodine and iron(III) chloride as oxidising agent in a DES formed from the mixture of choline chloride (ChCl) and ethylene glycol (EG). Micro-etching experiments demonstrated that iron-nickel phases were effectively leached out in this solvent, while other mineral phases remained unreactive. Thus, chemical oxidation using DESs has a great potential for metal recovery and separation from NEAs. DESs such as the eutectic system ChCl: 2EG meets some of the criteria for its potential use in space, being an anhydrous solvent and having low vapour pressure. In addition, due to the relatively high chloride concentration (ca. 5 mol dm ̶ 3) and the absence of oxygen gas in space, the formation of less reactive oxide/hydroxide is circumvented. Furthermore, the technology hereby proposed, though in a nascent stage, is very promising with DESs possessing many favourable physical and chemical characteristics. Moving forward, this will require an integrated understanding of how asteroids have been formed, what minerals host potentially useful metals, and what electrochemical methodologies can be implemented for metal recovery.
