Abstract:The electrochemical separation of Mn(II) impurity from molten NaCl-KCl-MgCl₂ was systematically investigated to facilitate the electrolytic production of high-purity magnesium. The reduction of Mn(II) to Mn metal on tungsten electrode was a quasi-reversible process controlled by diffusion. The apparent standard potential and exchange current density of Mn(II)/Mn(0) electrode reaction were determined at temperatures ranging from 973 to 1048 K. Solid Mn metal generated during electrolysis aggregated into irregular clumps and adsorbed some needle-like MgO, imposing a detrimental effect on both the aggregation and the purity of magnesium metal. After electrolysis at -1.5 V in molten NaCl-KCl-MgCl₂-0.62wt.%MnCl₂ for 8 h, the concentration of MnCl₂ impurity decreased to 0.037 wt.%, achieving a removal efficiency of 94.14%. When direct electrolysis was performed in molten NaCl-KCl-MgCl₂-0.62wt.%MnCl₂, the obtained magnesium metal was small blocks with a caviar-like appearance, and the purity was just 98.59%. In contrast, a large globule of magnesium metal was obtained when electrolysis was performed in the purified electrolyte, and its purity was improved to 99.94%. The controlled-potential electrolysis proposed in this work has been verified to be a green and practically effective method to separate the metal ion impurities from molten electrolyte for high purity magnesium extraction.

Key words: electrochemical separation; Mn(II) impurity; high purity magnesium; removal efficiency; molten salt electrolyte