Synergistic effects of Ionquest 801 and Cyanex 572 on the solvent extraction of rare earth elements (Pr, Nd, Sm, Eu, Tb, and Er) from a chloride medium

Abstract

A binary mixture of Ionquest 801 and Cyanex 572 was employed to study their synergistic effect on the solvent extraction and separation of rare earth elements (praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), terbium (Tb) and erbium (Er)) dissolved in a chloride solution. The results demonstrated a significant synergistic effect of 50% vol. Ionquest 801 and 50% vol. Cyanex 572 at pH of 1.5 in the extraction of heavy rare earth elements (HREEs). The maximum percentage extraction of the HREEs, Er3+ and Tb3+, under the optimized condition, were 95.7% and 71.7%, respectively; also, their distribution coefficient was improved, DEr (22.61) and DTb (2.54). Under that optimum condition, the extraction efficiencies of Nd (light rare earth element, LREE), Pr (LREE), Eu (middle rare earth element, MREE), and Sm (MREE) were 28%, 27%, 40%, and 38%, respectively. Additionally, the distribution coefficients of the HREEs to those of LREEs and MREEs were improved. For example, the measured separation factor for some of the couples were: Er/Pr: 61.31, Er/Nd: 57.77, Er/Sm: 36.44, Er/Eu: 33.19, Tb/Pr: 6.88, Tb/Nd: 6.48, Tb/Sm: 4.09 and Tb/Eu: 3.72. Both extractants individually showed similar performance in the extraction of HREEs. However, the highest efficiencies of the extraction of LREEs and MREEs were obtained solely with Cyanex 572. In contrast, Ionquest 801 showed little tendency towards extracting LREEs and MREEs, especially towards LREEs. Overall, the addition of Ionquest 801 to Cyanex 572 resulted in increased extraction of HREEs compared to the same in the sole Cyanex 572 and Ionquest 801. Additionally, the selectivity of the separation of HREEs from LREEs and MREEs was improved. The synergistic effect was concluded to result from changes in the phosphinic and phosphonic acids ratio in the extractant. By plotting McCabe-Thiele diagrams, it was confirmed that the complete extraction of Er3+ and Tb3+ occurred at the 3rd and 6th extraction stages, respectively. Stripping REE3+ from the loaded organic solution, using HCl with different concentrations, was investigated and the maximum stripping percentages reached were 92.2% for Er3+ and 55.8% for Tb3+ using a 4 M HCl solution.