Electrochemical separation of Fe(III) impurity from molten MgCl2-NaCl-KCl for magnesium electrolytic production

Abstract

To effectively separate Fe(III) impurity and purify the electrolyte for magnesium electrolytic production, the electrochemical behavior of Fe(III) on tungsten electrode was systematically examined in molten MgCl2-NaCl-KCl. Fe(III) ions can stably exist in the melt at 700 °C. Fe(III) undergoes a one-step reduction to iron metal on tungsten electrode exchanging three electrons, and the reaction process is reversible and controlled by diffusion. The deposition of iron on tungsten electrode belongs to the progressive nucleation mode. The co-reduction of Fe(III) and Al(III) makes the reduction of Al(III) at a more positive potential via formation of Al-Fe alloy. By electrolysis at −1.5 V in molten MgCl2-NaCl-KCl-0.94 wt%FeCl3-1.04 wt%AlCl3 for 8 h, Fe(III) and Al(III) impurities can be efficiently separated from the melt as AlFe3 and Fe, and the contents of FeCl3 and AlCl3 in the electrolyte can be reduced to a fairly low level of 0.022 wt% and 0.037 wt%, respectively. It is confirmed that the Fe(III) impurity in the electrolyte has a detrimental effect on the aggregation properties and purity of the magnesium metal generated during electrolysis. When electrolysis in the electrolyte purified by pre-electrolysis, the generated liquid magnesium metal can converge together and its purity can improved from 99.48% to 99.88% compared with direct electrolysis in the electrolyte with higher concentration of Fe(III) and Al(III) impurities. Molten salt pre-electrolysis is proven to be a practically effective method to purify the MgCl2-NaCl-KCl electrolyte for magnesium electrolytic production.