T.G. Levitskaia, D.H. Altavilla, N.P. Bessen, S. Jansone-Popova, B. O’Kelley, P. Keller
Pacific Northwest National Laboratory,
United States
Keywords: rare earth, DGA, organophosphorus acids
Summary:
Separations of the rare earth element (REE) remain a grand challenge, accounting for the major cost in producing purified individual REEs from ores. The current technology for industrial REE separations utilizes solvent extraction with phosphorus-based acidic extractants, which are highly effective in partitioning of the bulk REE into organic aliphatic diluent but lack selectivity to discriminate individual REE. This results in a complex multistage process notorious for its excessive chemical consumption, wastewater effluents, and hundreds of processing steps required to produce individual purified REEs. We investigate a novel approach combining the phosphorus acidic and neutral diglycolamide (DGA) extractants. Previous research has demonstrated that (N’- bis(3,6-dimethyloctyl)-N’,N-bis(n-hexyl) DGA (DGA-6) extractant offers improved selectivity for separation of the adjacent light REE. In combination with the weakly acidic extractant, this selectivity can be amplified during the stripping process by manipulating acidity of the aqueous phase. We demonstrated that the organic solvent comprised of (2-ethylhexyl)phosphonic acid mono-2-ethylhexyl ester (HEH(EHP) and DGA-6 in the aliphatic diluent tolerates high concentration of REEs. High loading of this extraction phase has been achieved when using the feed material derived from the monazite ore. The aqueous buffer was used to mediate the ion exchange of HEH(EHP), allowing complete extraction of REEs from aqueous to organic. Our current research is focused of the development of the sequential stripping of the light REE accomplished by increasing the acidity of the aqueous solutions.