Influence of urea concentration in nitric acid solutions of uranium on precipitation of ammonium polyuranates with ammonia

Abstract

A factorial study is made of precipitation of ammonium polyuranates from nitric acid solutions of uranium in the presence of urea, including measuring the effects of the temperature and urea and uranium concentrations in the initial solution on the sedimentation and filtration characteristics of the precipitates. The conditions of the process are optimized to obtain crystalline, readily filterable precipitates.Precipitation of ammonium polyuranates with ammonia from concentrated uranium backwash solutions finds wide use in the commonly accepted extraction3 precipitation reprocessing of spent nuclear fuel [1,2]. That is why intensive studies have been made of the mechanism of precipitation of ammonium polyuranates and of the properties of the resulting precipitates as influenced by the precipitation conditions [3,4].In the extraction3precipitation reprocessing of highly enriched (weapon-grade) uranium at the Siberian Chemical Combine, ammonium polyuranates are precipitated from solutions containing considerable concentrations of urea (urea is used in the backwashing stage, to obtain uranium concentrates) [5,6].There are only limited data in the literature on precipitation of ammonium polyuranates in the presence of urea [7]. Therefore, in this work we thoroughly studied the effect of the urea concentration in nitric acid solutions of uranium on the precipitation of ammonium polyuranates with ammonia. EXPERIMENTAL Stock uranium solutions were prepared by dissolving uranyl nitrate hexahydrate in 0.2 M HNO 3 [ultrapure grade, OST (Branch Standard) K-03-265376]. Then, a fixed amount of urea [GOST (State Standard) 2081392, grade A] was dissolved in the uranium solution.Precipitation of ammonium polyuranates from urea-containing uranyl nitrate solutions was carried out in a continuous precipitator with automatic control of the temperature and pH. Ammonia (25 wt % solution, GOST 6221-82E) and the stock uranium solution were fed simultaneously to the precipitator (slurry residence time in the reactor 1 h). The flow rate of the initial solution was controlled using a batcher, and that of the ammonia solution was automatically controlled by pH. Fixed pH was maintained with a BAT-15 automatic titration unit connected to a pH-121 pH meter (ESL 63-07 pH-metric glass electrode; EVL-1MB Ag/AgCl reference electrode). Stirring was carried out with a propeller mixer. The temperature control was realized using a contact thermometer (GOST 9871361) connected to a temperature controller. Slurry was transferred through an upper discharge to a settling tank. After 3 h of continuous operation of the system, samples of the slurry were taken, and the aqueous phase composition and physicochemical characteristics of the precipitates were analyzed.The completion of precipitation was characterized by the uranium concentration in the mother liquors, and properties of the precipitate, by its moisture content and permeability coefficient. The relative volume of the precipitate was estimated as the ratio of the precipitate volume after settling to ...