Separation of rare earth elements using chelation-assisted electrodialysis

Abstract

Rare earth elements (REE) find crucial applications in electronics, clean energy, aerospace, automotive, and defense sectors. However, the conventional solvent extraction (SX) technique currently dominates the process of isolating individual REE into pure elements, presenting challenges due to its extensive stage requirements and environmental footprint. To address these limitations, this study explores chelation-assisted electrodialysis (ED) as a promising alternative to SX for efficiently separating light and medium REE from binary and tertiary solutions. By incorporating chelating agents into the ED process, we aim to enhance separation efficiency while mitigating the drawbacks associated with SX. A laboratory-scale ED system with a microflow cell consisting of five pairs of Neosepta® cation and anion exchange membranes in an alternating configuration was utilized. Aminopolycarboxylic acids such as Ethylenediaminetetraacetic acid (EDTA), diaminocyclohexanetetraacetic acid (DCTA), N-(2-hydroxyethyl) ethylenediaminetriacetic acid (HEDTA) and diethylenetriamine-pentaacetic acid (DTPA) were employed as chelating agents to increase the selectivity of REE separation. The effects of operating voltage, operation mode, pH, type of electrolytes, and membrane configuration on the separation process were investigated. ED was conducted at varying concentrations of REE and chelating agents, and the system was optimized for the highest separation factors. It was observed that the separation factors increased by increasing the voltage due to the higher ionic mobility of non-chelated REE at higher voltages. Precipitation of REE in ED compartments was one of the main challenges in the tests. To prevent the precipitation, a modified membrane configuration and an optimized pH window of 3.0–4.5 were applied. At optimized operating conditions, the REE separation factors of up to 42 were achieved, which were up to 20 times greater than those reported for the conventional SX process. The final REE concentrations in the concentrate compartment were comparable with those calculated from the dissociation equilibrium equations and in most cases, less than 10 % error was observed between the experimental results and the calculations.