Stability of Uranium Complexes in Supercritical Fluids and their Effective Recovery(The joint research report about precedence basic engineering research)

Tomiyasu, Hiroshi*; Nomura, Mitsuo; Yamazaki, Hitoshi; Hayashibara, Kenichi

This study has been performed to establish a method to recover uranium from uranium containing NaF and CaF2 wastes without forming any secondary wastes. For this purpose the use of supercritical carbon dioxide was primarily considered. Although TBP has been generally used with supercritical carbon dioxide to extract uranium, the use of TBP was ruled out in the present study because of the formation of secondary wastes due to phosphorous oxides. Among a variety of ligands, we have choused acetylacetone as a ligand for the extraction of uranyl ion. Acetylacetone is a strong chelate ligand, which dose not gives any solid residual after vaporization. As a matter of fact, our experimental result indicates that acetylacetone is a stronger ligand for uranyl ion compared with TBP., because uranyl TBP complex is replaced by acetylacetonate resulting in the formation of acetylaetonate complex. Experiments were carried out to recover uranium from the NaF waste by use of supercritical carbon dioxide containing acetylacetone, and it was found that uranium was only partly extracted to carbon dioxide phase. This might be attributed to the strong uranium fluoride complexes and also to the distribution of uranium to the inside of NaF pellets. In order to recover uranium from the NaF waste, another method was also carried out as follows: dissolve NaF waste by hot water, then add NaOH to the solution as to form the precipitation of uranium hydrolyzed species, where uranium can be recovered at high yield using centrifugation, and finally the addition of ethanol yields the NaF powder. In conclusion, the recovery of uranium from fluoride wastes seems to be difficult by the method using supercritical carbon dioxide; however, we have established a new method to recover both uranium and NaF in high yield.