Ammonium Polyuranate Precipitation from Fluoride Solutions: An Experimental Study of the UO3-HF-NH3-H2O System as Applied in UF6 Processing

Abstract

In the ammonium diuranate (ADU) process, UF6 is reacted with water, and the acidic solution of uranyl fluoride is treated with aqueous ammonia to precipitate ammonium polyuranate for subsequent reduction to UO2 and production of fuel pellets for commercial nuclear reactors. Our experiments simulated adding aqueous ammonia to the reaction products of UF6 and water in typical ADU processes. Chemical and X-ray diffraction analysis of products from the experiments are consistent with postulated chemical equilibria in which solids with structures close to that of ammonium polyuranate are formed from co-precipitation of the NH4+(aq) cation with (previously unreported) anions of the form UO2F3-x(OH)x-(aq). More efficient separations of solid products were obtained at NH4OH:UF6 ratios of 19 or greater, with x closer to the value of 3 for the hypothetical formation of pure ammonium polyuranate. Supplementary experiments in the current study and a previous study in our laboratory indicated that nominal uranium concentrations of 90 mg/l in the filtrate resulting from such separations could be reduced to microgram per liter levels by batch mixing a 1-to-2.5 aqueous diluate of the filtrate with the Diphonix® ion exchange resin. Our study further demonstrated that reaction of the purified NH4OH-NH4F diluate with aqueous Ca(OH)2 at 80 to 90°C could produce essentially uranium-free CaF2 and an ammonia distillate, as useful waste-conversion end products from a modified ADU process.