Separation of adjacent rare earth elements enhanced by “external push-pull” extraction system: An example for the separation of Pr and Nd

Separation of adjacent rare-earth elements, such as Pr and Nd, is extremely difficult due to the similarity in their physicochemical properties . Enhanced separation of adjacent rare-earth elements based on the “push-pull effect” by addition of water-soluble complexing agents was an effective strategy in traditional organic-aqueous two-phase extraction. However, environmental pollution is serious due to the residual of complexing agent in the extraction raffinates. In present work, a novel “external push-pull” extraction system, composed of three coexisting liquid phases, P507 organic phase, ionic liquid (tributyl-methyl ammonium nitrate , N 4441 NO 3 )-rich phase and NaNO 3 aqueous solutions, is suggested to improve the separation between Pr and Nd. It is revealed that Nd and Pr can be enriched respectively into the P507 organic top phase and N 4441 NO 3 -rich ionic liquid middle phase in the external push-pull extraction system, owing to a so-called “external push-pull effect” from a reversed extraction selectivity of P507 organic phase and N 4441 NO 3 ionic liquid-rich phase towards Nd and Pr, respectively. Compared to traditional organic-water two-phase systems, the separation factor of Pr to Nd in the suggested external push-pull extraction system could increase obviously to 3.5 or even more. Various effects from the aqueous pH, NaNO 3 concentration, the initial concentration ratios of Pr to Nd in feed aqueous solutions, and the addition amount of N 4441 NO 3 , the volume of P507 organic phase on the separation between Pr and Nd are discussed. In addition, reversal extraction of rare earth ions in the ionic liquid-rich phase is found to be closely related to the complexation behavior of rare earth ions with NO 3 − ions by using molecular dynamic simulations. Based on this work, a possibility for extraction and separation of 14 rare-earth element ions from their coexisting aqueous solutions using the suggested external push-pull extraction system is also explored. The present work highlights a novel green strategy for improving the separation of adjacent rare-earth elements.