Molten Salt Electrolysis of Magnesium Oxide Using a Liquid-Metal Cathode for the Production of Magnesium Metal

A novel electrolytic process that uses a liquid-metal cathode was investigated to produce high-purity magnesium (Mg) metal using magnesium oxide (MgO). Electrolysis of MgO in a magnesium fluoride (MgF2)-lithium fluoride (LiF) molten salt was carried out with applied voltages of 2.5 V to 3.0 V at 1053 K to 1083 K using tin (Sn), silver (Ag), or copper (Cu) as the cathode and graphite or platinum (Pt) as the anode. After electrolysis, Mg alloys with Mg2Sn, AgMg, and Cu2Mg phases were produced with current efficiencies of 77.2 to 83.8 pct when the concentration of Mg in the Mg alloys was 11.9 to 12.9 mass pct. For the production of high-purity Mg metal directly from the Mg alloys, vacuum distillation was performed. When vacuum distillation was conducted at 1200 K to 1300 K, the concentration of Mg in the Mg alloys feed decreased from 30.2 to 34.1 mass pct to 0.32 to 1.75 mass pct, and Mg metal with a purity of 99.975 to 99.999 pct was obtained. Therefore, this study demonstrates that the production of high-purity Mg metal through an efficient and environmentally sound method using the electrolysis of MgO is feasible.