Solvent extraction systems for selective isolation of light rare earth elements with high selectivity for Sm and La

Abstract

This study details a comprehensive approach focusing on the effective separation of light rare earth elements (REEs) via solvent extraction technique. A stock solution containing lanthanum, cerium, neodymium, praseodymium, and samarium was prepared by dissolving their pure mixed oxide (reclaimed from spent Ni-MH batteries) in a diluted HCl solution. Key extractants, including bis (2,4,4-trimethylpentyl) phosphinic acid (Cyanex 272), Cyanex 572, trialkylphosphine oxide (Cyanex 923), and 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (PC 88A), along with tributyl phosphate (TBP) as a phase modifier, were utilized to form organic systems. The extraction behavior and separability of these systems at various pH levels as well as their extraction mechanisms were investigated. The results demonstrated a direct relationship between the extraction trend and the experimental pH value, with enhanced selectivity when TBP was added. Notably, Nd and Pr exhibited similar extraction behaviors, with minor deviations from Ce, making their separation difficult to achieve. Sm extraction followed a distinct trend, allowing for its separation from other elements at pH ≤ 2. In contrast, La exhibited a low affinity for coordination with extractants when pH was ≤ 3.5, facilitating the separation of other elements from La, which could then be isolated in the raffinate. Among the studied organic systems, combinations of Cyanex 572 and PC 88A with TBP demonstrated superior performance in element separation. Optimum separation factors were calculated with β_Ce/La = 12, β_Nd/La = 87, β_Pr/La = 127, and β_Sm/La = 3191 for the former, and β_Sm/Ce = 54, β_Sm/Nd = 20, and β_Sm/Pr = 14 for the latter. These findings provide valuable insights for selecting extraction systems and designing experiments for the effective solvent extraction separation of light REEs from their mixture.

Graphic Abstract