5. Application of Ionic Liquid Extractants on Rare Earth Green Separation

A green solvent extraction process for the separation of rare earth elements (REEs) in different media (such as hydrochloric acid media, nitric acid media, and sulfuric media) was developed using bifunctional ionic liquid extractants (Bif-ILEs) especially containing quaternary ammonium and phosphonium-based ILs. The characteristics of the Bif-ILEs are simple synthesis, high extraction ability and selectivity, low extraction acid and base consumption, and easy stripping. Bif-ILEs could be synthesized by acid/base neutralization, and this reaction process was under mild condition with easier purification. The extraction thermodynamics showed that the separation factor (β) values for REEs were changed with the different Bif-ILEs or reaction media. The inner synergistic effect between cations and anions in Bif-ILEs has greatly facilitated the β values of REEs and also avoided the production of ammonia nitrogen caused by the acidity extractant saponification. The kinetic experimental data on the separation of REEs by Bif-ILEs, with a constant interfacial area cell with laminar flow, showed that there exist real possibilities for increasing the efficiency of the separation by the use of kinetic factors. The kinetics model could be deduced from the rate-controlling step. Serving as extractants, additives, and templating materials, ILs were used in the preparation of separation materials, including silica-supported ionic liquids (S-SILs), membrane-supported ionic liquids (M-SILs), and polymer-supported ionic liquids (P-SILs). ILs were usually attached or doped into the solid support and offered high extraction efficiency and excellent stability for REEs separation. Using such separation materials may indeed enable us to substantially reduce the steps for the separation of REEs and thus decrease both the separation time and waste production.