Uranium dioxide reducing conditions

In TBP extraction, the yeUowcake is dissolved ia nitric acid and extracted with tributyl phosphate ia a kerosene or hexane diluent. The uranyl ion forms the mixed complex U02(N02)2(TBP)2 which is extracted iato the diluent. The purified uranium is then back-extracted iato nitric acid or water, and concentrated. The uranyl nitrate solution is evaporated to uranyl nitrate hexahydrate [13520-83-7], U02(N02)2 6H20. The uranyl nitrate hexahydrate is dehydrated and denitrated duting a pyrolysis step to form uranium trioxide [1344-58-7], UO, as shown ia equation 10. The pyrolysis is most often carried out ia either a batch reactor (Fig. 2) or a fluidized-bed denitrator (Fig. 3). The UO is reduced with hydrogen to uranium dioxide [1344-57-6], UO2 (eq. 11), and converted to uranium tetrafluoride [10049-14-6], UF, with HF at elevated temperatures (eq. 12). The UF can be either reduced to uranium metal or fluotinated to uranium hexafluoride [7783-81-5], UF, for isotope enrichment. The chemistry and operating conditions of the TBP refining process, and conversion to UO, UO2, and ultimately UF have been discussed ia detail (40). 

Fig. 10. Uranium dioxide solubility data obtained under nominally reducing conditions, corresponding to values extracted from the literature as given in the figure. Thermodynamic database for uranium from Grenthe et al. (1992) and Bruno Puigdomenech (1989).

A second test was conducted under the above conditions. Reaction of the uranium dioxide was assumed to be complete when gas evolution ceased. At that point, the temperature of the melt was reduced to 200°C, and nitric acid vapor was added to the melt. The nitric acid vapor was carried from a heated vessel containing 100% nitric acid with the inert gas sparge. Transfer lines were heated to minimize condensation. The quantity and transfer rate of the nitric acid were not determined. Addition of the nitric acid produced a reaction with the solids present, shown by gas evolution from the solid s surface, which yielded a soluble uranium species in the nitrate melt. The total solids were dissolved, which produced a characteristic uranyl color in the melt. After complete dissolution of the uranium species, the nitric acid sparge was removed, and the melt was open to the atmosphere. 

Derivation Finely ground ore is leached under oxidizing conditions to give uranyl nitrate solution. The uranyl nitrate, purified by solvent extraction (ether, alkyl phosphate esters), is then reduced with hydrogen to uranium dioxide. This is treated with hydrogen fluoride to obtain uranium tetrafluoride, followed by either electrolysis in fused salts or by reduction with calcium or magnesium. Uranium can also be recovered from phosphate sand. 

The uranium anion present in carbonate solutions is [U02(C03)8] and this is associated with few other impurity anions. Absorption capacities as high as 100 to 200 mg/g of dried resin have been obtained on Amberlite IRA-400 2 and Dowex I , under conditions where competing anionic impurities such as phosphate and aluminate ions have only absorbed to an insignificant extent. The resin capacity, in both cases, is greatest at low sodium carbonate concentrations. Vanadate ion absorption can take place to an appreciable extent when vanadium is present in the carbonate leach liquor from the ore. It is, however, readily separated from the uranium, e.g. by a preliminary elution with a saturated solution of sulphur dioxide. This removes the vanadium from the resin by reducing it to a lower valency state. 

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