Electrochemical Amalgamation Optimization as Part of the Development of the OREATE Process

Abstract

Oxide reduction by electrochemical amalgamation and thermal extraction (OREATE) is an electrochemical process in which metal oxides are converted to highly pure metals. This process that is currently being developed at LANL involves the dissolution and chlorination of the metal oxide into an acidic medium. This metal cation is then electrochemically amalgamated via reduction using a mercury pool cathode. Finally, mercury's high vapor pressure allows an easy thermal extraction process to obtain the metal. In addition to its ability to electrochemically form amalgams, mercury is used because it has a high hydrogen evolution overpotential, allowing for a higher current efficiency for reduction of the metals in acidic solution. As a critical step of the OREATE process, the electrochemical amalgamation reaction must be optimized in order to advance the overall performance.In this work, we studied the electrochemical amalgamation of cerium through anodic stripping voltammetry (ASV) experiments using a silver-based mercury film electrode (SBMFE). The SBMFE allows for the electrochemical amalgamation to be optimized at a smaller scale than the mercury pool, which not only minimizes the amount of mercury needed, but also drastically reduces the experiment time. This study included several different experimental parameters such as the pH of the buffered solution, concentration of cerium, reduction potential, and temperature. For each parameter, the reaction rate and current efficiency were evaluated to determine the optimal conditions for the electrochemical amalgamation reaction.The results showed that the pH of the buffered solution must be 3.5 or lower to avoid the formation of cerium hydroxides, which can passivate the surface of the mercury electrode. However, an increasingly acidic solution promotes the hydrogen evolution reaction, which directly competes with the electrochemical reduction of cerium, decreasing the overall current efficiency. Likewise, it has been demonstrated that an increase of the cerium concentration in solution drops the current efficiency of the electrochemical amalgamation reaction. This effect can be explained by the fact that cerium is a small cation with a +3 charge that leads to a high degree of solvation. Therefore, as the cerium concentration in solution increases, more water molecules are brought to the electrode surface during electrochemical amalgamation. This favors the water reduction reaction that increases hydrogen evolution and lowers current efficiency. However, increasing the concentration of cerium enhances the reaction rate of the electrochemical amalgamation, resulting in more cerium entering the mercury film. Furthermore, as the reduction potential is moved towards more negative potentials, the reaction rate increases. However, a more negative reduction potential promotes a faster hydrogen evolution reaction, which diminishes the current efficiency. Finally, it has been demonstrated that temperature increases the reaction rate, though it does not have a significant effect on the current efficiency.In conclusion, a variety of system parameters were explored for their effect on current efficiency and reaction rate of the electrochemical amalgamation reaction. This allowed for the determination of the optimal parameters to be used during the electrochemical amalgamation in the large-scale OREATE process.