Electrolytic reduction of U(VI) to U(IV) in acidic chloride and acidic sulfate solutions
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I.
INTRODUCTION
THE electrolytic
reduction process of U(VI) to U(IV), originally developed by the Power Reactor and Nuclear Fuel Development Corporation of Japan (PNC) for a chloride solution system, is of interest in the hydrometallurgy of uranium ores. 1 This process uses an electrolytic cell having 60 unit ceils in series; each consists of a titanium cathode, a platinum anode, and a cation exchange membrane. The titanium plate with platinum electrolytically plated on one side is used as both cathode and anode in bipolar form. This process now operates in batch style at the Ningyo-toge plant of PNC. Although the usefulness of this method for the uranyl chloride solution has been demonstrated, the application of this process to uranyl sulfate solutions or for use in continuous operation is desired. We thought that a fundamental study of electrolytic reduction of U(VI) would be the fastest way to approach these purposes. As the first step of this investigation, we calculated the composition of the catholytes at various percentages of reduction and studied the polarization characteristics of electrolytic reduction of U(VI) to U(IV). In conjunction with this, the electrical conductivity of the catholyte, the electrical resistivity of the cation exchange membrane, and the diffusion coefficient of uranyl sulfate were also determined. This paper details the results obtained.
II.
EXPERIMENTAL PROCEDURES
A. Measurement of the Electrical Resistivity of the Cation Exchange Membrane Figure 1 schematically illustrates an experimental apparatus used for measuring the electrical resistivity of the cation exchange membrane employed in this study. A titanium plate and a platinum plated titanium plate of 5.0 cm • 10.0 cm in size were used as cathode and anode, respectively. A cation exchange membrane, type K-401 made HIROSHI MAJIMA, Professor, and YASUHIRO AWAKURA, Lecturer, are with the Department of Metallurgy, Kyoto University, Kyoto, Japan 606. SHUICHIRO HIRONO is Manager, Mining and Ore Processing Division, Ningyo-toge Works, Power Reactor and Nuclear Fuel Development Corporation, 1550 Kamisaibara, Tomata-gun, Okayama prefecture, Japan 708-06. Manuscript submitted May 2, 1985. METALLURGICALTRANSACTIONSB
by the Asahi Kasei Co. Ltd, was placed between the cathode and anode compartments, both made of acryl resin. Two agar salt bridges filled with saturated potassium chloride solution were used for the determination of the potential difference across the membrane. One end of each agar salt bridge was located at a position adjacent to the center of the cation exchange membrane surface, with one bridge in the cathode compartment and one in the anode compartment of the cell. The other ends of the salt bridges were immersed in a saturated potassium chloride solution, in which an AgC1-Ag electrode with 3.3 tool 9 -3 KC1 solution was immersed as a reference electrode as shown in Figure 1. By measuring the potential difference of these two reference electrodes, the potential difference across the cation exchange membrane during the
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