Separation of Rare Earths from the Transition Metals Using a Novel Ionic-Liquid-Based Aqueous Two-Phase System: Toward Green and Efficient Recycling of Rare Earths from the NdFeB Magnets

An ionic-liquid-based aqueous two-phase system (iL-ATPS) consisting of an ionic liquid, tributylmethylammonium nitrate (N4441NO3), and NaNO3 aqueous solutions was developed for the extraction and separation of rare-earth ions, for example Nd­(III), from transition-metal ions. The excellent selectivity of a green nonfluorinated ionic liquid and the low-viscosity feature of an aqueous two-phase system were combined in our suggested iL-ATPS. The results indicated that efficient separation of Nd­(III) from transition-metal ions, Fe­(III), Ni­(II), and Co­(II), could be obtained in our suggested iL-ATPS. Furthermore, different parameters of the separation process, including the concentrations of HNO3 and NaNO3 and the added amount of N4441NO3, were optimized. Notably, the extraction kinetics of Nd­(III) could be improved significantly in the present iL-ATPS compared with the conventional organic-solvent-based ionic-liquid extraction systems. The similar environment across the liquid–liquid interface was revealed by the ultralow viscosity and interfacial tension of the ionic-liquid-rich phase in the iL-ATPS. This feature was responsible for the enhanced extraction kinetics of Nd­(III). In addition, the structures of the extraction complexes were elucidated by using NMR spectroscopy and molecular simulations. It was demonstrated that NO3– ions were in the inner coordination shell of Nd­(III) ions to interact with Nd­(III) ions, while N4441+ surrounded Nd­(III) ions at the outer coordination shell of Nd­(III) ions. Moreover, 2 mol/L HNO3 aqueous solutions could be used to effectively strip Nd­(III) from the loaded ionic-liquid-rich phase. After 5 cycles, the extraction performance of the ionic liquid remained nearly unchanged. The present work highlights an environmental method to selectively separate rare-earth ions from the complicated solutions coexisting with other transition-metal impurity ions. It provides a basis for the future application in recycling of rare-earth elements from the spent NdFeB magnets