Effective acid recovery from rare earth stripping solution by diffusion dialysis

The consumption of substantial alkali and emission of high-salinity wastewater during the precipitation step of stripping solutions in rare earth (RE) element extraction processes remain critical challenges. This study proposes an energy-efficient and environmentally benign diffusion dialysis (DD) process for separating H⁺ and rare earth ions (RE³⁺) in simulated rare earth stripping solutions. Six commercial anion exchange membranes (AEMs) were first evaluated for separating RECl₃/HCl mixtures. The dense AEM with single polyethylene terephthalate string structure (M6) demonstrated the optimal DD performance for H⁺ and RE³⁺ separation. Among three typical RE³⁺ ions with different radii (i.e., Y³⁺, La³⁺, and Yb³⁺), the M6 AEM exhibited the lowest selectivity between H⁺ and La³⁺, which is correlated with the higher tendency of La³⁺ hydrated ions to dehydrate during transmembrane transfer. Under optimized operation conditions (feed solution: 0.2 mol L⁻¹ HCl and 0.25 mol L⁻¹ La³⁺), the DD process using the M6 AEM achieved an H⁺ recovery rate of 69.77 % and a La³⁺ leakage rate of 0.05769 % with a H⁺/La³⁺ selectivity of 5519.2. Importantly, the M6 AEM maintained consistently stable DD performance for separation of H⁺ and RE³⁺ ions over a 30-cycle consecutive operation in the stimulated rare earth stripping solution containing typical organic pollutants (i.e., P507 and sulfonated kerosene), showing an impressive antifouling property. This study provides an insightful guideline for practical acid recovery from the rare earth stripping solutions.