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

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 UO₂F_3-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.