Structural Effect of Phosphoric Esters Having Bulky Substituents on the Extraction of Rare Earth Elements

Article abstract of DOI:10.1246/bcsj.69.589

Phosphoric esters having bis(4-ethylcyclohexyl) (D4ECHPA) and bis(4-cyclohexylcyclohexyl) (D4DCHPA) groups as the alkyl substituents were synthesized in order to investigate their selectivity in the extraction of rare earth elements from acidic nitrate media, together with the apparent molecular weights of the extradants in toluene, the acid dissociation constants and the aqueous distribution coefficients of the extractants. These extractants were found to exist as dimeric species in toluene. The acid dissociation constants of the extractants are nearly the same as that of bis(2-ethylhexyl) hydrogenphosphate (D2EHPA), which is a typical commercial acidic phosphorus extradant. The aqueous distribution coefficients of the extractants become greater in the order D4ECHPA>D2EHPA>D4DCHPA. From the concentration dependencies of the hydrogen ion and extradant on the distribution ratio of rare earth elements, it was concluded that the rare earth elements (M³⁺) were extracted with D4ECHPA and D4DCHPA (HR) as M(HR₂)₃ in the organic phase; also, the extraction equilibrium constants (K_ex) were evaluated. The extraction equilibrium constants of rare earth elements become greater in the order D4DCHPA>D4ECHPA>D2EHPA. It was very noticeable that the separation factor between Ho^III and Gd^III, β(Ho/Gd), and β(Yb/Y) with D4ECHPA, and β(Ce/La), β(Pr/La), and β(Ho/Gd) with D4DCHPA are significantly greater than those with D2EHPA. The structural effects of the extradant on the selectivity of rare earth elements are discussed in terms of the molecular modelling. Furthermore, we found that the loading capacity of Nd^III with these extractants is excellent compared with that of D2EHPA, due to preventing gelation of the organic phase.