Intra-Lanthanide Separation Processes Using Neutral Diglycolamide Extractants

Separation of individual rare earth elements (REE) is often regarded as the most difficult processing step in the production of high-purity rare earth oxides for end-use technology applications due to their inherent chemical similarities. Current industrial REE separation practices utilize solvent extraction with organophosphorus extractants, a complex process plagued by poor adjacent-lanthanide selectivity, excessive chemical reagent consumption, and adverse environmental impacts. Consequently, research efforts within the Critical Materials Institute (CMI) are aimed at the development of alternative REE separation technologies that improve economic viability and environmental sustainability to enable domestic supply diversification. Recent efforts have focused on electroneutral solvating diglycolamide (DGA) extractants as an alternative method for the separation and purification of critical rare earth elements. DGAs offer distinct advantages over traditional phosphonic acid extractants used in separations including elimination of saponification to achieve high recovery in a solvent extraction cascade and improved adjacent-lanthanide separation factors, ultimately requiring fewer solvent extraction stages to facilitate the required separations. Novel DGA extractants have been synthesized and tested to maintain high intralanthanide selectivity, high organic-phase loading capacity, and proper phase dispersion behavior for high throughput separations. In this paper, solvent extraction cascade design principles have been tested using the well-known DGA N,N,N’,N’-tetraoctyldiglycolamide as the first test case to validate separation performance in counter-current solvent extraction equipment to obtain high degrees of REE recovery and purity. Finally, challenges and ongoing research associated with this family of neutral extractants are evaluated within the context of domestic rare earth oxide production from bastnäsite ore.