Utilizing rare-earth elements (REEs), such as for permanent magnets in renewable energy applications, requires costly chemical separations that generate large volumes of waste. A process featuring new diglycolamide (DGA)-based ligands and liquid-liquid extraction exhibits more effective separation of individual REEs, and will allow lower-cost processing of REE-containing ores.
The process was developed by Oak Ridge National Laboratory (ORNL; Oak Ridge, Tenn.; www.ornl.gov) and Idaho National Laboratory (Idaho Falls; www.inl.gov) through the U.S. Department of Energy’s Critical Materials Institute (CMI), an Energy Innovation Hub. The technology was recently licensed by Marshallton Research Laboratories Inc. (Tobaccoville, N.C.; www.marshalltonlabs.com).
Traditional REE separation uses liquid-liquid extraction, in which a non-polar solvent containing specific ligand molecules is mixed with an aqueous REE solution. Ligand-REE complexes are extracted from the aqueous phase to the solvent. Purified REE ions are then recovered from the solvent, which is reused.
“REEs can be separated according to their ionic radius, but since the selectivity obtained is not very high with existing ligands, often based on organophosphorus extractants, a large number of extraction stages are required to obtain desirable purities of REEs, like neodymium and praseodymium, that are used in permanent magnets,” explains Bruce Moyer, an ORNL corporate fellow and member of the CMI. “The new DGA-based ligands we have developed have alkyl groups that can be tailored to create different steric and electronic environments around the complexed ions.”
Moyer points out that the new ligands have the ability to Increase the REE selectivity compared to traditional ligands, so separation can occur with far fewer extraction stages. This reduces both capital and operating costs.
Key to the improved selectivity of the DGA ligands for different REEs is their ability to form more rigid structures. “The rigidity of the DGA ligand structures makes it more difficult for the molecules to twist and adjust to accommodate elements with varying ionic radii as the ligands form coordination complexes,” Moyer says. “Because of this, the ligand-ion binding strength has more variability across the series of REEs, making possible much greater selectivity among ions.”
Marshallton and CMI are looking to scale up the process and optimize the extraction.