Selective recovery of rare earth elements from e-waste via ionic liquid extraction: A review

Increasing generation of electronic waste (e-waste) is a major concern for emerging rare earth elements (REEs) pollution if not appropriately disposed of. Nevertheless, e-waste could be considered as a secondary source for critical REEs mining. Under the context of circular economy, this review focuses on selective recovery of REEs from e-waste using ionic liquid (IL) extraction. ILs demonstrate a sustainable alternative to organic volatiles used in traditional solvent extraction process. Despite insufficient literature on applications of ILs for REEs extraction from limited categories of real e-waste, the review comprehensively summarizes the latest studies on recovery of REEs from e-waste (mostly uncontaminated) using IL extraction and the existing data indicate encouraging outcomes in terms of extractability and selectivity. Roles of ILs in REEs recovery have been critically analyzed. The review has systematically illustrated the fundamental design of ILs for selective extraction of REEs, including bifunctional and task-specific ILs, incorporation of dilutant/extractant, non-fluorinated ILs, etc. This fundamental knowledge could greatly facilitate cost-effective and selective extraction of REEs from e-waste. ILs must be completely assessed in terms of extractability, selectivity, and reusability. Synergistic extraction based on quaternary phosphonium salts are discussed to realize excellent extraction of REEs into IL phase leaving non-REEs in the aqueous phase for easier separation. Overall, regeneration of spent ILs and subsequent separation of extracted REEs from ILs are also crucial to selective recovery of REEs from e-waste using novel ILs. Nevertheless, industrialization of this technology would require more fundamental data on extraction efficiency and recovery rate of REEs from real e-waste.