(389d) Measurement of Electrochemical Transport Properties of Manganese Ions in Eutectic LiCl-KCl Using High Temperature Voltammetry
Measurement of Electrochemical Transport Properties of Manganese Ions in Eutectic LiCl-KCl using High Temperature Voltammetry
D. Horvathand M.F. Simpson
University of Utah, Department of Metallurgical Engineering
A study of electrochemical deposition of manganese from molten salt mixtures of LiCl-KCl-MnCl2 was carried out using a variety of voltammetry methods in a high temperature electrochemical test system. Deposition was studied on both tungsten and molybdenum working electrodes in electrodes in eutectic LiCl-KCl melts at 773 K. This study was motivated by a need to minimize transition metal impurity concentrations in electrorefined uranium metal. Though manganese is electrochemically nobler then uranium, its standard reduction potential is relatively close to that of U. And it is present in the stainless steel hardware that is immersed in the molten salt in an electrorefiner along with spent fuel or other sources of impure uranium. Study of manganese is expected to provide insight to the behavior of a number of different transition metals that may contaminate the uranium, including zirconium, chromium, and iron. For this study, transient electrochemical techniques such as cyclic voltammetry, chronopotentiometry, and chronoamperometry were used to investigate the deposition behavior of Mn2+ ions from LiCl-KCl with varying concentrations of MnCl2. Molybdenum was found to be the preferred working electrode, as double anodic peaks were measured for cyclic voltammetry using tungsten electrodes. This has been attributed to possible formation of an intermetallic W-Mn compound. Diffusion coefficient, exchange current density, and other properties have been evaluated at MnCl2 concentrations ranging from 0.05 to 0.75wt%. Even over this relatively short range, concentration was found to affect these properties. Diffusion coefficient calculations also varied depending on which voltammetry method was used for the measurement. Cyclic voltammetry tests yielded D values from 6.8x10-6 to 1.8x10-5 cm2/sec. Standard exchange current density was likewise determined using CV based on the slope of the reducing peak near the apparent reduction potential--ranging from 0.18 to 0.58 A/cm2. These properties can ultimately be used to model the electrorefining process, estimate the extent of transition metal contamination of uranium, and provide insight in how to minimize this contamination.