Recent advances in metal extraction improvement: Mixture systems consisting of ionic liquid and molecular extractant

Ionic liquids (ILs), also termed “green” and “designer” solvents, have been proposed as promising solvents for diverse applications. This review focused on the improvements brought about by mixing ILs with molecular extractants (MEs) in solvent extraction processes for metal extraction and separation. Mixture systems consisting of IL with ME were collected and discussed on the basis of the functional cations of ILs, such as ammonium-type, phosphonium-type and imidazolium-type ILs. IL was found to play a role as an active component by generating a new acid/base coupling bi-functional ionic liquid (ABC-BIL) or as a diluent when combined with acidic or neutral ME in IL-ME mixture systems. The merits and drawbacks of IL-ME systems were reviewed and compared with conventional ME systems alone in terms of the preparation route, separation behavior and extraction mechanism. Further approaches to the improvement of IL-ME systems, such as extraction enhancement, stripping and regeneration, were discussed. The summaries and comparisons made in the present review may indicate future directions for the development and application of ILs in metal solvent extraction technology.