Comprehensive appraisal and application of novel extraction system for heavy rare earth separation on the basis of coordination equilibrium effect

Several novel extraction systems were reported since 2-ethylhexyl phosphonic acid mono-(2-ethylhexyl) ester (P507) had been confronted with low selectivity and incomplete stripping for heavy rare earths (HREs). This paper makes a comprehensive appraisal of P507, bis (2,4,4-trimethylpentyl) phosphinic acid (Cyanex272), mixture of P507 and Cyanex272, P507 adding isooctanol and bifunctional ionic liquid extractant (Bif-ILE) [trialkylmethyl-ammonium][di-(2-ethylhexyl)orthophosphinate] ([A336][P507]). Thermodynamics study, apparent kinetics study and theoretical calculation for the cascade extraction are all included. We put forward coordination equilibrium effect for the first time to illustrate the inner and outer environment during the coordination. It could be useful to explain the extraction ability and selectivity of different extractants. All these would provide separation of HREs with more feasible and informative understanding and guidance.