Solubility of uranous sulfate in aqueous sulfuric acid solution
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I.
INTRODUCTION
U R A N O U S sulfate is an important intermediate in the process of uranium fluoride production by the hydrometallurgical method. At the Ningyo-toge plant of the Power Reactor and Nuclear Fuel Development Corporation (PNC), Tomata-gun, Japan, uranous sulfate is currently produced by an electrolytic reduction method, as reported previously, t1'21 Uranous sulfate often precipitates during the treatment of acidic uranous sulfate solution. Once the precipitation occurs, it plugs the connecting pipes or causes sedimentation on the diaphragms installed in the bipolar electrolytic cells. Therefore, it is very important to know the conditions under which the precipitation occurs. However, no available data exist on the solubility of uranous sulfate as a function of free sulfuric acid concentration and temperature. The solubilities of uranous sulfate were determined as a function of free sulfuric acid concentration, as well as temperature, to provide important fundamental data for use in uranium hydrometallurgy.
II.
EXPERIMENTAL PROCEDURES
A. Chemicals To prepare the stock uranous sulfate solution, uranium dioxide with a purity of 99.9 + pct was dissolved in an
SHIGERU SUZUKI, Deputy General Manager, West Isolation and Ore Processing Division, is with the Ningyo-toge Works, Power Reactor and Nuclear Fuel Development Corporation, Tomata-gun, Okayama Prefecture 708-06, Japan. SHUICHIRO HIRONO, formerly Director, Mining and Ore Processing Division, Ningyo-toge Works, Power Reactor and Nuclear Fuel Development Corporation, is Director, Ningyo-toge Nuclear Service Company, Tomata-gun, Okayama Prefecture 708-06, Japan: YASUHIRO AWAKURA, Associate Professor, and HIROSHI MAJIMA, Professor, are with the Department of Metallurgy, Kyoto University, Sakyo-ku, Kyoto 606, Japan. Manuscript submitted January 16, 1990. METALLURGICALTRANSACTIONS B
aqueous H 2 S O 4 solution to approximately 100 g d m -3 of uranium [U]T. This total concentration of uranium is currently employed in the process of electrolytic reduction of U(VI) at the Ningyo-toge plant of PNC. The acidic uranyl sulfate solution was then electrolytically reduced to the uranous state at a sufficiently low temperature. At this stage, it is important to completely reduce U(VI) to U(IV), as the presence of even a small amount of U(VI) will cause a pseudo-stable behavior of uranous sulfate in the precipitation experiments. The uranous sulfate solution, thus prepared, was used as the stock solution. All chemicals other than uranium were commercially supplied reagent grade products. Deionized water with a specific resistivity of 5 • 106 ohm cm was used to prepare all aqueous solutions.
B. Experimental Procedures Two sets of experimental work were performed to determine the solubility of uranous sulfate in aqueous sulfuric acid solutions. One set is the precipitation experiments in which concentrated uranous sulfate solution was mixed with sulfuric acid of suitable concentration to a total solution volume of 100 c m 3. Each 100 c m 3 solution was stoppered in
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