Amides as Phase Modifiers for N,N′-Tetraalkylmalonamide Extraction of Actinides and Lanthanides from Nitric Acid Solutions

Abstract

The N,N′-tetraalkylmalonamides are a class of compounds under development for transuranic (TRU) separations under high nitric acid conditions. There are several issues that challenge the further development of these ligands. One is the development of improved synthetic procedures that lend themselves to commercial scale-up. Another major issue is the third-phase formation that occurs when the N,N′-tetraalkylmalonamides are contacted with medium-to-high nitric acid concentrations in hydrocarbon solvents. To address the synthesis issue we have developed a new synthetic approach for preparing these materials. Third-phase formation can be eliminated by addition of diluent modifiers such as tributylphosphate (TBP). TBP is inappropriate if a nonphosphate-containing process stream is required. Amides have been proposed as alternatives for TBP in a variety of applications because of their ease of synthesis and the variety of substituents that can be generated. We have been able to develop an amide phase-modified system that extends the working process range of alkylmalonamides (0.5 M) in dodecane (unbranched hydrocarbon) from 3.5 M to 7.5 M nitric acid and in Isopar H (branched hydrocarbon) from 4.0 M to 10.0 M nitric acid using 1.0 M di-2-ethylhexylacetamide/0.5 M alkylmalonamide. The K_d values were comparable to extraction with alkylmalonamide in Isopar H or hydrogenated tetrapropylene (TPH) solvents. The overall extraction system was more robust than the phase-unmodified system allowing for greater temperature and acid concentration fluctuations without third-phase formation.