Ionic Liquids and Deep‑Eutectic Solvents in Extractive Metallurgy: Mismatch Between Academic Research and Industrial Applicability

The past 10–20 years have seen numerous academic papers describing the benefits of ionic liquids (ILs) and deep-eutectic solvents (DESs) for leaching, solvent extraction and electrowinning. The scientific community—including the authors of this opinion article—have frequently proclaimed these neoteric solvents as game-changers in extractive metallurgy. Despite this, there have been no commercial breakthroughs. In this paper we reflect on the reasons why ILs and DESs seem to have failed to impact on the metallurgical industry. These include: (1) issues with high viscosity; (2) limited chemical stability under the conditions of metallurgical processes; (3) difficulties with recycling and reuse; (4) a lack of demonstrated unit processes and flowsheets on the pilot scale; (5) insufficient material-property data available for engineering purposes; (6) the administrative burden of obtaining licenses and safety permits; (7) very high costs for large-scale operations; and (8) minimal added value compared to state-of-the-art hydrometallurgical processes. Our belief is that innovations in hydrometallurgy based on ILs or DESs are unlikely. Instead, we should be aiming for a deeper understanding of hydrometallurgical processes at the molecular level. This is because advances are more likely to derive from the refocused efforts of experienced IL/DES researchers investigating the speciation and chemical thermodynamics of hydrometallurgical solutions, which will then hasten the transition from linear to low-energy-input, circular hydrometallurgy.