Solubility limits of dibutyl phosphoric acid in uranium-nitric acid solutions

The Savannah River Site has enriched uranium (EU) solution that has been st ored since being purified in its solvent extraction processes. The concentr ations in solution are approximately 25 mM U and 0.1 M nitric acid. Residua l tributylphosphate in solution has slowly hydrolyzed to form dibutyl phosp horic acid (HDBP) at concentrations averaging 0.14-0.2450 mM. Dibutyl phosp horic acid, in turn, is in equilibrium with (HDBP)(2) and DBP-. Uranium can form compounds with the dibutylphosphate ion (DBP-) which have limited sol ubility, thereby creating a nuclear criticality safety issue. Literature reports and earlier SRTC tests have shown that it is feasible to precipitate U-DBP solid during the storage and processing of EU solutions. As a result, a series of solubility experiments were run at nitric acid co ncentrations from 0-4.0 M HNO3, uranium at 0-378 mM, and temperatures from 0-30 degreesC. The data show temperature and HNO3 concentration dependence consistent with what would be expected. With respect to uranium concentrati on, U-DBP solubility passes through a minimum between 25 and 50 mM U at the HNO3 concentrations and temperatures studied. However, the minimum shows a slight shift toward lower uranium concentrations at lower HNO3 concentrati ons. The shifts in solubility are strongly dependent upon the overall ionic strength of the solution. These data also reveal a shift to higher DBP solubility above 0.5 M HNO3 fo r both 25 mM and 50 mM uranium solutions. Analysis of U-DBP solids from the tests identified distinct differences between precipitates from <0.5 M sol utions and those from >4 M acid. Analyses identified UO2(DBP)(2) as the dom inant compound present at low HNO3 concentrations in accordance with litera ture reports. As the acid concentration increases, the crystalline UO2(DBP) (2) shows molecular substitutions and an increase in amorphous content.