Dioxide hydrofluorination

Fluorination and Reduction. Until recently, plutonium dioxide from the calcination of plutonium peroxide was contacted with HF in a rotary hydrofluorinator. The reaction converted Pu02 to PuFt, according to the following equation ... 

Gas-solid heterogeneous reactions may be noncatalytic. An example is the hydrofluorination of uranium dioxide pellets referred to in Sec. 7-1. Since one reactant is in the solid phase and is consumed, the rate of reaction varies with time. Hence such processes are basically transient, in comparison with the steady-state operation of gas-solid catalytic reactors. The process for smelting ores such as zinc sulfide,... 

Fig. 14-1 Stirred-tank single-pellet reactor for hydrofluorination of uranium dioxide...

The Pu(ni) oxalate is calcined to form the dioxide or hydrofluorinated with HF and oxygen to form Pup4, as has been described above for the tetravalent oxalate. 

Direct calcination of Pu(N03)4 involves no chemical separations that could remove impurities, so a highly pure plutonium nitrate feed solution is required. The plutonium dioxide product can be hydrofluorinated to PUF4, or it can be used as a feed for the formation of PUCI3. Direct calcination has received less industrial-scale application than the precipitation processes described above [C2]. 

The lower fluorides of the actinide elements are prepared by reaction with hydrogen fluoride, either in aqueous solution or at elevated temperatures in the dry way. The latter is the more usual procedure. The methods used for the preparation of UF4 are typical of those used for the preparation of the lower fluorides of any of the actinide elements. The usual starting material for the preparation of uranium tetrafluoride is the trioxide, U03, obtained by thermal decomposition of uranyl nitrate hexahydrate, U02(N03)2 6H20. The trioxide is reduced by hydrogen to uranium dioxide, which is in turn hydrofluorinated to UF4 ... 

For the other actinide elements the usual starting material will be the dioxide. Although these reactions appear to be simple, they are in fact quite complex. Uranium trioxide as prepared by thermal decomposition of uranyl nitrate hexahydrate, uranium peroxide, or ammonium diuranate may exist in any one of at least four crystal modifications and in an important amorphous form. The reduction of U03 appears to be related both to the crystal form and to the surface area, and depending on these and perhaps other still unspecified variables, the hydrofluorination reaction on the... 

Wang and Wen [17] used this model to simulate the burning of coke from fire clay particles of 1.2 cm radius with up to 41 percent by weight of carbon. Figures 4.6-1 and 4.6-2 show the agreement between calculated and experimental conversions and temperatures. Similar balances were used by Costa and Smith [18] to analyze experimental results concerning hydrofluorination cS uranium dioxide. 

The process of calcination of oxides, hydrogen reduction of oxides to lower oxides, hydrofluorination and hydrochlorination of oxides, etc., can often be carried out by conventional chemical engineering methods. However, the high standards of purity usually required for rare metal extraction at least necessitate novel materials of construction. In addition, a considerable development and pioneering effort has been devoted to the improvement of these techniques, particularly applied to the intermediates in the production of uranium metal, uranium trioxide, dioxide and tetrafluoride. It is possible, therefore, that the resulting processes can be more widely employed in the rare metal extraction field in the future. 

Fluidized bed processes have been devised which convert uranyl nitrate solution to uranium trioxide, by thermal denitration, reduce to the dioxide with hydrogen and finally hydrofluorinate to uranium tetrafluoride. Three separate fluidization systems are needed, with solids transfer throughout. In addition, nitric acid can be recovered from the denitration process and the excess of hydrogen fluoride can be recovered from the hydrofluorina-tion process. 

Several U.S. pilot plants have been described for the hydrofluorination of uranium dioxide to uranium tetrafluoride.i >... 

The temperature is controlled between about 450°C and 650°C, the lower part of the range being most suitable for the formation of a highly reactive dioxide which hydrofluorinates rapidly. 

Hydrofluorination of UO to UF. The hydrofluorination reaction is conducted in a series of stirred fluidized-bed reactors with counterflow of solid and gases. During the exothermic reaction the conversion of the uranium dioxide into uranium tetrafluoride occurs with complete consumption of hydrogen fluoride gas according to the following scheme ... 

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