Rare earth elements (REEs)—encompassing the lanthanides, scandium, and yttrium—are vital for advanced technologies and everyday consumer products, from smartphones to electric vehicles. Yet their separation remains notoriously challenging, largely because these elements all exist in a 3+ oxidation state and differ in ionic radii by only about 0.01 Å per unit increase in atomic number.1 As a result, conventional solvent extraction processes require hundreds of stages to achieve high-purity separation.
The Critical Materials Innovation Hub (CMI), an Energy Innovation Hub of the U.S. Department of Energy, is spearheading efforts to develop more efficient and economically viable REE separation technologies. This presentation will provide an overview of CMI’s initiatives, including bio-based separations,2 neutral-ligand-based solvent extraction of REEs from secondary sources, new methods for concentrating REEs from dilute streams,3 and a two-ligand-based solvent extraction process4. I will delve into the two-ligand approach, reviewing the evolution of the current state-of-the-art acidic extractant, 2-ethylhexylphosphonic acid mono-(2-ethylhexyl) ester, and highlighting how neutral extractants (e.g., diglycolamides5-6) have emerged as promising replacements7. I will also discuss why combining two extractants—one lipophilic and one hydrophilic—can yield more efficient separations than single-extractant processes. By leveraging their reverse selectivity toward REEs, this innovative two-ligand approach enables more effective separations in fewer steps. Moreover, the system is designed to facilitate complete recycling of process chemicals, including the water-soluble ligand, contributing to improved sustainability and reduced environmental impact.
1. Sholl, D. S.; Lively, R. P. Seven chemical separations to change the world. Nature 2016, 533, 316.
2. Dong, Z.; Mattocks, J. A.; Deblonde, G. J.-P.; Hu, D.; Jiao, Y.; Cotruvo Jr., J. A.; Park, D. M. Bridging Hydrometallurgy and Biochemistry: A Protein-Based Process for Recovery and Separation of Rare Earth Elements. ACS Central Science 2021, 7, 1798.
3. Stetson, C., Prodius, D., Lee, H.; Orme, C.; White, B.; Rollins, H.; Ginosar, D.; Nlebedim, I. C.; Wilson, A. D. Solvent-driven fractional crystallization for atom-efficient separation of metal salts from permanent magnet leachates. Commun. 2022, 13, 3789.
4. Johnson, K. R.; Driscoll, D. M.; Damron, J. T.; Ivanov, A. S.; Jansone-Popova, S. Size Selective Ligand Tug of War Strategy to Separate Rare Earth Elements. JACS Au 2023, 3, 584.
5. Stamberga, D.; Healy, M. R.; Bryantsev, V. S.; Albisser, C.; Karslyan, Y.; Reinhart, B.; Paulenova, A.; Foster, M.; Lyon, K.; Moyer, B. A.; Popovs, I.; Jansone-Popova, S. Peripheral Substitution in Diglycolamides Controls Intralanthanide Selectivity. Chem. 2020, 59, 17620.
6. Jansone-Popova, S.; Popovs, I.; Lyon, K. L.; Moyer, B. A. Methods for separation and recovery of rare earth elements from aqueous solutions using diglycolamide derivatives. United States Provisional Application No. 63/048,237, 2020.
7. Pramanik, S. Kaur, I. Popovs, A. S. Ivanov, S. Jansone-Popova, Emerging Rare Earth Element Separation Technologies, EurJIC 2024, e202400064.