New synergistic solvent mixture of DNPPA and bidentate octyl (phenyl) CMPO for enhanced extraction of uranium (VI) from phosphoric acid medium

The synergistic extraction of uranium from phosphoric acid solutions with di-nonyl phenyl phosphoric acid (DNPPA) and bidentate n-octyl (phenyl)-N,N-di-isobutyl carbamoyl methyl phosphine oxide (CMPO) in n-dodecane has been investigated and effect of varying concentrations of phosphoric acid, phosphate ions, DNPPA and CMPO on the extraction of uranium were determined. The results demonstrated that synergistic combination of DNPPA with bidentate CMPO significantly enhances the extraction efficiency when compared to monodentate extractant TOPO (tri-n-octyl phosphine oxide). Stoichiometry of complex obtained by slope analysis method indicated the formation of neutral species of the type [UO2(HA2)2 · CMPO], where (HA)2 is the dimer form of DNPPA. Extraction reaction was found to be exothermic with enthalpy change (ΔH) of 43.3 kJ/mol. A mole ratio of DNPPA/CMPO = 2 was found to be suitable for effective extraction of uranium. Under comparable condition, the extraction of uranium from phosphoric acid with various synergistic reagents followed the order: DNPPA/CMPO > DNPPA/TOPO > DIDPA (di-isodecyl phosphoric acid)/CMPO > DIDPA/TOPO > D2EHPA (di-2-ethyl hexyl phosphoric acid)/CMPO > D2EHPA/TOPO. The loading capacity of DNPPA/CMPO was found to be greater than DNPPA/TOPO for both WPPA (wet process phosphoric acid) and MGA (merchant grade phosphoric acid). Stripping of uranium with various reagents was in the order: Na2CO3 > H3PO4 + Fe > H3PO3 > HCOOH. DNPPA and CMPO solvent mixture was found to be stable even after 20 cycles of repeated extraction and stripping operations.