Solvent extraction of rare earth elements using phosphonic/phosphinic acid mixtures

Abstract

This work examines the extraction of rare earths (Ce, Pr, Nd, Sm, Tb, Dy, Ho, Er, Yb, Lu and Y) by 2-ethylhexyl phosphonic acid 2-ethylhexyl mono-ester (EHEHPA), Cyanex 272, Cyanex 572 and mixtures of EHEHPA and Cyanex 272, to determine whether the mixed extractants could be beneficial to industrial rare earth separations. Analysis of the effect of pH and extractant concentration on distribution ratios indicated that addition of the phosphinic acid to EHEHPA resulted in an antagonistic effect. The antagonistic effect was confirmed using the method of continuous variation, and is thought to be due to an association between the phosphinic and phosphonic acids which reduces the free dimer concentrations of each component. Examination of 31P{1H} NMR spectra showed that for the mixed extractant the extracted yttrium complex was predominantly composed of EHEHPA. However, some Cyanex 272 was also found to be associated with yttrium, which suggests the formation of a mixed yttrium–EHEHPA–Cyanex 272 complex. Separation factors for the mixed extractants were similar to those of EHEHPA, while the maximum loading for equimolar extractants was approximately halved. The results suggest potential savings resulting from a reduction in the stripping acid required for the mixed extractant relative to equimolar EHEHPA; however equipment size would likely increase due to lower overall metal loading.