Abstract
The unique physicochemical properties of rare earth elements (REEs) make them imperative for the production and improvement of advanced technology, renewable energy, and national defense applications. As these fields rapidly develop, there has been an increase in the demand for REEs. Due to the similar ionic charge and radii of adjacent REEs it is a challenge to selectively recover individual elements. Currently, the most common method for industrial REE separation is multistep traditional liquid-liquid extraction (T-LLE) systems. However, these systems generate large quantities of hazardous waste including flammable solvents, contaminated wastewater, and materials that cannot be recycled. There has been a recent increase in modifications to T-LLE systems, which has shown promising results in improving REE refinement and decreasing waste. This Review summarizes recent advances in solvent extraction systems other than T-LLE including aqueous two-phase systems (ATPS), nonaqueous systems (NAS) and systems evaluating the synergistic effects of multiple extractants or solvents in each phase. We also discuss developments in magnetophoretic separation methods that could be useful in solvent extraction in the future. Based on the findings of this Review, estimated Technology Readiness Levels (TRLs) for each of these systems are provided. These new systems are currently at TRLs of ≤5. Improving their TRLs and examining their industrial potential could lead to a reduction in operation cost and environmental impacts while improving separation efficiency and overall REE refinement, if practical in industrial settings.