Confirmation and elimination of cyclic electrolysis of uranium ions in molten salts

Abstract

The cathodic collection efficiency (CCE) for the exhaustive electrolysis of uranium is very low; however, determination of the mechanism responsible for this phenomenon has been hindered by the lack of information about the uranium species present in molten salts. Herein, we used in situ X-ray absorption spectroscopy and quasi in situ electronic absorption spectroscopy to study the chemical speciation of uranium during the electrolysis process. For the first time, uranium was revealed to be continuously converted between [UCl6]2− and [UCl6]3−, leading to problematic cyclic electrolysis. This cyclic electrolysis process was the main cause of the low CCE during exhaustive electrolysis, and also hindered the full recovery of uranium. To eliminate this cyclic electrolysis process, we adopted a sacrificial active lanthanum metal anode. The lanthanum metal dissolved at the anode to form La3+ ions, which hindered the conversion between [UCl6]2− and [UCl6]3−. Thus, the uranium cycle between the cathode and the anode was eliminated, resulting in a significant improvement in the CCE for exhaustive electrolysis.