Hydrofluorination plant

Uranium(IV) oxide is the starting material for uranium(lV) fluoride production in which uranium(lV) oxide is generally reacted with anhydrous hydrogen fluoride. This difficult to carry out exothermic reaction proceeds either in a fluidized bed, in moving bed reactors, or in screw-reactors. To achieve as complete as possible reaction in fluidized bed reactors, two fluidized bed reactors are connected in series. Screw-reaetors are also preferably connected in series. In moving bed reactors the reduction zone and the hydrofluorination are arranged above one another in a plant. The uranium(IV) oxide produced by the reduction of uranium(VI) oxide with hydrogen is very reactive and is eompletely reaeted with HF at temperatures between 500 and 650°C to uranium(lV) fluoride. 

In U.S. plants hydrofluorination is carried out in two stirred fluidized-bed reactors in series, with counterflow of solids and gases. The bed to which UO2 is fed and from which exhaust gases are discharged runs at 300°C, partially converts UO2 to UF4, and reduces the HF content of the effluent gases to around 15 percent. The bed to which anhydrous HF and the partially converted UO2 are fed runs at 500°C and converts more than 95 percent of the UO2 to UF4. To prevent caking of the fluidized beds, it has been found necessary to provide each reactor with a vertical-shaft, slow-speed stirrer to scrape the reactor walls. Production rates around 700 to 900 kg/h are obtained in 0.75-m-diameter reactors. Effluent gases are filtered to remove entrained solids, cooled to condense aqueous HF, and scrubbed to remove the last traces of HF. 

In the Comurhex plant at Malvesi [B5], reduction of UO3 and conversion of UO2 to UF4 are carried out in a single L-shaped, moving-bed reactor. Reduction takes place in a vertical section and hydrofluorination in a horizontal section. Practically complete utilization of HF is obtained. 

In the U.S. Department of Energy (DOE) plant at Paducah and the Comurhex plant at Pierrelatte [B5], UF4 is converted to UF by reaction with fluorine in a tower reactor. Solid UF4 and a slight excess of fluorine gas are fed at the top of a monel tower with walls cooled to around 500°C. Most of the UF4 reacts almost instantaneously with a flame temperature of around 1600°C. Small amounts of unreacted UF4 and uranium oxides are removed from the bottom of the tower and recycled to the hydrofluorination step. 

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

A larger pilot plant has also operated at the Argonne National Laboratory based upon reduction in a four-stage, 5 in. diameter, stainless-steel reactor, followed by hydrofluorination in a five-stage, 6 in. diameter, monel reactor. This is shown in Fig. 5.13 and is based upon principles similar to those employed in the smaller plant. 

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