Behavior of Phosphate in Uranium-Bearing Organophosphorus Solvents in the Pyrolysis and Combustion Reactors

This study investigated the behavior of phosphate in uranium-bearing organo phosphorus solvents under pyrolysis and combustion conditions. The volatilization characteristics of phosphate and the reaction between uranium oxide and phosphate oxide were established based on the results of a thermogravimetric analysis of tributyl phosphate (TBP) and uranium-bearing TBP-dodecane solutions. Uranium dioxide, UO2, functioned as a sorbent for the capture of vaporized phosphate oxide, P4O10(g). A significant fraction of phosphate remained in the form of uranium pyrophosphate (UP2O7) and uranium metaphosphate (U(PO3)4). The P4O10(g) capturing capacity of UO2 was 0.96-0.84 g/g for pyrolysis and 0.60 g/g for combustion, respectively. The relatively low phosphate-capturing capacity of combustion was due to the dissociation of P4O10(g) from (U(PO3)4) at flame temperatures higher than 1300K. Pyrolysis appears to be a better method than combustion for a thermal destruction of uranium-bearing TBP solutions while affording minimized problems associated with the condensation of corrosive phosphoric acid.