The hydrofluorination reaction to convert U02 to UF4,

U02 + 4HF ^ UF4 + 2H20

is exothermic. It proceeds rapidly at 500°C, but the equilibrium mixture of H20 and HF contains around 35 percent HF. At 300°C, nearly complete utilization of HF can be obtained, but the reaction rate is slow. Problem 5.3 illustrates calculation of the HF content of the equilibrium mixture of HF and H20 at these two temperatures from free-energy data.

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 U02 is fed and from which exhaust gases are discharged runs at 300°C, partially converts U02 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 U02 are fed runs at 500°C and converts more than 95 percent of the U02 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 U03 and conversion of U02 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.