Synthesis Of Uranium Fluorides From Uranium Dioxide With Ammonium Bifluoride And Ammonolysis Of Uranium Fluorides To Uranium Nitrides

An experimental study was conducted to determine the practicality of various unit operations for fluorination of uranium dioxide. The objective was to prepare ammonium uranium fluoride double salts from uranium dioxide and ammonium bifluoride, then decompose these salts to uranium tetrafluoride through heating to temperatures near 425 °C in either a fluidized bed or a stirred bed. Fluorination in the stirred bed was attempted without pretreatment of the reagents. For the fluidized bed experiments, reagents were ball-milled prior to being heated in the bed. Experiments were conducted in either argon or 4% hydrogen in argon. The ball mill appeared to be an effective technique for fluorinating uranium dioxide with ammonium bifluoride. Samples changed color from brown to bright green, and no oxides could be detected in the x-ray diffraction pattern of the product. It was found that stainless steel is a suitable material of construction for reaction vessels, whereas mild steel parts corroded quickly. Only a small degree of fluidization provided adequate mixing in fluidized beds, but a paddle mixing the stirred beds left an unmixed region around the bed perimeter. Results from the stirred beds showed the initial fluorination reaction completed only when the reagents were heated to 110 °C for at least three hours under argon. Decomposition took place under argon with a temperature ramp up to 425 °C. The product UF4 contained less than 1% oxide as an impurity, and the decomposition appeared to be complete. Fluidized beds were run with both argon and 4% hydrogen in argon as carrier gases. Experiments with 4% hydrogen in argon produced uranium tetrafluoride, with ammonium uranium pentafluoride and uranium dioxide as impurties. Experiments in argon produced uranium tetrafluoride, with uranyl difluoride, ammonium diuranyl pentafluoride and triuranium octoxide as impurities. Minimum temperatures and times needed to decompose the double salt in the fluidized beds were 200 minutes at 115 °C, a 500-minute ramp to 425 °C, and 200 minutes at 425 °C. The intermediate double salt produced at 110 °C appeared to be triammonium uranium septafluoride.

Chemical and infra-red spectrophotometric methods were used to analyse the off-gas from the reaction in which sulphur dioxide is bubbled into a heated solution containing ammonium fluoride and formic acid with or without the presence of uranyl ions. It was found that carbon dioxide is evolved for a relatively short period at a constant rate during the lengthy reaction between sulphur dioxide and formic acid. It is believed that the accompanying constant production of a reducing agent that is capable of reducing uranyl ions is responsible for the zero-order characteristics that have been found when precipitating ammonium uranous fluoride from such a system. From the qualitative observations made when studying off-gas evolution by infra-red spectrophotometric methods, several suggestions have been made that might make the process more attractive economically.