Separation of heavy (dysprosium) and light (praseodymium, neodymium) rare earth elements using electrodialysis

This work reports electrodialysis studied to separate dysprosium (Dy, a heavy rare earth element (REE)) from praseodymium and neodymium (Pr, Nd, a mixture of light REEs). A unique electrodialysis setup comprising a cation exchange membrane, a PC-400D membrane (anion exchange membrane that transfers large organic anions), and an anion exchange membrane is used. To achieve the separation of these REEs, ethylenediaminetetraacetic acid disodium salt dihydrate is used as the chelating agent. Because of the higher stability constant of EDTA with Dy(III) compared with Pr(III) and Nd(III), most of EDTA form [Dy-EDTA]− chelate which upon applying electric potential move from the feed to the concentrate compartment through the PC-400D membrane, leaving behind free Pr and Nd cations in the feed compartment. The effect of rinse solution concentration (sodium sulfate, 0.05–0.07 mol/L), applied electric potential (10–14 V), feed pH (2–5), and EDTA concentration (EDTA/Dy molar ratio of 0.9–1.2) on key performance indicators of electrodialysis, i.e., purity, yield, separation factor, and Dy removal efficiency is investigated. Rinse solution concentration has a negligible effect on the separation performance in the studied range. Applied potential shows a positive effect on the Dy removal efficiency, but separation factor decreases with increasing potential above 12 V. The suitable pH range for maximum separation performance is found to be above 3 and below 5. Increasing EDTA/Dy molar ratio increases the amount of Pr and Nd chelates with EDTA, which also move to the concentrate compartment and decrease the product purity. The ratio 1:1 is found to result in the highest Dy purity and separation factor. The rinse solution concentration of 0.05 mol/L with 12 V applied potential, pH 4, and EDTA/Dy molar ratio of 1.0 result in a separation factor of 125 which is promising. Under this set of conditions, a Dy purity of 93% and Dy yield of 77% is achieved.