Oreate - Oxide Reduction by Electrochemical Amalgamation and Thermal Extraction

Abstract

Establishing a capability for generating consistent, small-scale, and high-purity metals (i.e., transition metals, lanthanides and actinides) would have significant impact in many levels from fundamental science to national security related applications. Among the many procedures to purify transition and f-block metals (direct oxide reduction, electrolysis in molten salts, etc), there does not exist a U.S. process that can respond to the diverse needs of small- and large-scale science campaigns. Hence, there is widespread need to develop a capability that can supply a broad range of customers with “craft” metals of interest. The main goal is to consistently convert metal oxides to chemically pure metals in near quantitative yield.Traditional separation methods of actinides and lanthanides from aqueous media has been based on structure and bonding differences between these metals with various ligands. Although liquid-liquid extractions have proven to be quite efficient, they are expensive and require complex engineering. On the other hand, separation by pyro-chemistry based on the difference of their reductive extraction to metallic phase provides a good separation performance but requires the use of high temperatures, which adds technological difficulties to the process. An alternative method that has grown interest in the last few years is the utilization of the difference in the reduction potentials from ionic state to metallic state. However, this electrochemical approach has been believed difficult because of the highly negative reduction potentials of these metals (more negative than hydrogen evolution) and the challenging control of their electrolytic deposition onto solid cathodes.In this work we present a process comprising the oxide reduction by electrochemical amalgamation and thermal extraction (OREATE) that intends to be an optimized version of the traditional electrochemical amalgamation process in aqueous medium for trivalent f-elements. A mercury pool is used as cathode material to overcome most of the difficulties related to the use of solid cathodes: (i) Once reduced to the metallic state, the metal quickly dissolves into the liquid mercury to form amalgam, which ensures more stable surface condition of the cathode. (ii) Higher hydrogen over-voltage on mercury enables more effective reduction of the metals in a slightly acidic solution. (iii) Mercury's high vapor pressure allows an easy thermal extraction process to obtain a highly pure metal.As part of our study, we have maximized the yield of the Ce electrochemical amalgamation (yield > 99%) by monitoring dissolved Ce3+ concentration in solution by UV-vis, optimized the mercury recycling process during the thermal extraction (Hg recycling > 99%), as well as improved the purity of the Ce content in the product (Ce purity > 99%). The proposed OREATE method appears easier and simpler than traditional separation and purification methods and is suitable for laboratory scale preparation of high purity metals.