Laboratory reduction rate and current efficiency studies of batch type electrolytic reduction of U(VI) in a sulfate system

The electrolytic reduction of U(VI) was investigated to improve the batch type electrolytic reduction of uranyl sulfate. For this purpose, theoretical considerations were made on the time variation of reduction rate and of current efficiency. A monitoring device consisting of two titanium electrodes and one platinum electrode was developed to determine the reduction ratio and to detect the end point of the reduction of U(VI). The monitoring device worked well for these purposes. The reduction rate in a batch type electrolytic reduction of U(VI) at constant current was larger than theoretically expected. This phenomenon was attributed to the increase in the mass transfer rate of U(VI) species toward the cathode due to the disturbance of the concentration boundary layer by hydrogen gas evolved from the cathode surface. The deterioration of titanium cathode experienced in an operating plant was investigated, and was found to be caused by platinum plating onto the titanium cathode surface during the operation. The deteriorated cathode could be restored by immersing it in a hot aqua regia or a mixture of hydrofluoric acid and phosphoric acid for a few minutes.