Molybdenum Electrorefining in Ionic Liquids

Molybdenum is a valuable refractory metal used to harden and protect metal alloys. Additionally, certain nuclear reactors can be used to produce Mo 99 , a valuable radioactive isotope with important medical applications. To qualify for medical use, the isotope must be isolated in high purity molybdenum. One method of metal purification is electrorefining. Electrorefining consists of anodically dissolving a target metal of inadequate purity, and then depositing it onto a substrate at a higher purity. Conventional aqueous-based electrolytes are not suitable for Mo electrorefining owing to their instability in the potential range needed for Mo deposition and the tendency of molybdenum to form insoluble oxides. Ionic liquids offer the wider electrochemical window needed to process Mo. Such liquids are often referred to as ‘designer solvents’ due to the plethora of possible constituents that can render different properties. The purpose of this study was to explore Mo electrorefining in ionic liquids, and to identify solvents with properties suitable for this type of molybdenum purification. In this study, the effectiveness of molybdenum electrorefining was examined in a few different ionic systems, including choline-chloride-based deep eutectic systems. Cyclic voltammetry was utilized to determine oxidation and reduction potentials given against an Ag/Ag+ reference electrode. Deposits were characterized with EDX and morphologies examined with SEM. Furthermore, faradaic efficiencies and methods for optimizing deposition were determined. Our findings show that the effectiveness of a solvent for Mo electrorefining is determined by the coordinating nature of the solvent. Weakly coordinating ligands, such as BF - 4 , are not ideal for electrorefining because Mo ion replenishment via anodic dissolution is inhibited by low solubility. Ethylene glycol-choline chloride, which is more strongly coordinating, has shown effective dissolution at room temperature. In addition, deposition has been observed in this DES at temperatures above 80° C with aggressive stirring, indicating that it is transport controlled. Despite low faradaic efficiencies for deposition, deposit morphology and adherence can be improved through pulse deposition. This is believed to be the first report of molybdenum deposition or electrorefining in a deep eutectic solvent.