Revaluating solvent extraction equilibria of trivalent rare earths with EHEHPA (PC-88A) using Alstad equation

Abstract

The equilibria associated with the solvent extraction of trivalent rare earth elements (REEs) with 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (EHEHPA) have been previously studied by many researchers because of the industrial importance of REEs. However, quantitative analysis has been limited to relatively low concentrations of EHEHPA because of the nonideal characteristics of the organic phase. In the present paper, we used an empirical equation to correct the nonideality of the organic phase and calculated extraction equilibrium constants for some trivalent REEs (La, Ce, Pr, Nd, Sm, Eu, Tb, Dy, and Y) that are particularly important in RE recycling from end-of-life products and in-plant scraps. Calculations based on use of EHEHPA in a nonpolar diluent and a nitrate medium were used to systematically investigate equilibria at a constant ionic strength of 1 mol/L, temperature of 298 K, EHEHPA dimeric concentrations that ranged from 0.04 to 0.33 mol/L, and a range of pHs. Apparent extraction equilibrium constants that reproduced the distribution ratios of the REEs with good accuracy were obtained. Further experiments at higher EHEHPA concentrations revealed that the empirical equation was effective up to an EHEHPA dimer concentration of 0.5 mol/L, which is significantly higher than the upper bound for the conventional dimer model. Separation factors defined as the ratios of the distribution ratios of two REEs were calculated from the extraction equilibrium constants for the possible pairs of REEs and compared critically with literature values. Differences in the separation factors depended on the measurement conditions and were most pronounced when the difference in atomic numbers of the REEs was large. The results are expected to facilitate designing processes for the separation and purification of REEs.