Selective Thorium Separation from Rare Earth Leach Solutions: Hydrometallurgical Strategies, Process Limitations, and Future Perspectives

Thorium is a critical radioactive impurity in rare earth element (REE) processing and poses a major environmental, regulatory, and technological challenge to expanding sustainable REE supply chains. Selective removal of thorium from REE leach solutions is particularly difficult because efficient separation must be achieved while minimizing losses of valuable REE and maintaining process stability. This critical review analyzes recent advances (2020–2026) in thorium removal strategies for REE hydrometallurgical systems, including precipitation, solvent extraction, ion exchange, adsorption, membrane separation, and hybrid integrated processes. The fundamental chemical differences between Th(IV) and REE(III) species, including hydrolysis, complexation, and coordination behavior, are discussed to elucidate the mechanisms of separation selectivity. The review critically compares removal efficiency, REE losses, operational conditions, scalability, environmental impacts, and industrial feasibility across technologies. Particular attention is given to the limitations of laboratory-scale studies, the lack of pilot-scale validation, and challenges in radioactive residue management. Emerging trends involving functional materials, selective extractants, advanced membranes, and integrated flowsheets are also evaluated. The analysis demonstrates that future progress in REE processing depends on developing selective, scalable, and environmentally responsible strategies for thorium management compatible with industrial operations.