Semi-continuous vacuum distillation for high-purity magnesium: Thermodynamic assessment, impurity separation behavior, and direct ingot formation

The demand for magnesium with purity exceeding 99.99% has increased owing to the increasing use of magnesium in advanced applications. Commercial crude magnesium produced via electrolytic and thermal reduction contains metallic impurities that reduce the corrosion resistance through micro-galvanic coupling. Conventional vacuum distillation offers a promising refinement pathway; however, it exhibits limited scalability and promotes nonuniform condensation and the formation of oxidatively unstable fine crystals or crown-shaped deposits. In this study, a laboratory-scale semi-continuous vacuum distillation system was designed to enable liquid-phase condensation and direct magnesium ingot formation. Thermodynamic analyses, including saturated vapor pressures, separation coefficients, activity-based multicomponent equilibrium partial pressures, and vapor–liquid equilibrium behavior, were conducted to investigate impurity separation tendencies. The calculations indicated that most impurities except zinc possess significantly lower vapor pressures than magnesium, enabling effective thermodynamic separation. Distillation experiments were conducted at evaporation temperatures of 973–1123 K, condensation temperatures of 873–1073 K, and holding times of 30–360 min. Under optimized conditions (evaporation zone temperature of 1023 K, condensation zone temperature of 923 K, and holding time of 300 min), magnesium vapor formed a dense cylindrical ingot-like structure through localized liquid-phase condensation and accumulation. A Mg recovery ratio of 84.7% was achieved. Inductively coupled plasma optical emission spectroscopy revealed that the magnesium purity increased from 99.96% in the Crude-Mg to above 99.99%. Zn removal was limited because of its high intrinsic volatility and relative enrichment in the vapor phase, consistent with the thermodynamic predictions. Furthermore, the detectable Si in the recovered ingot despite its low equilibrium partial pressure suggested a transport behavior that could not be fully explained by the equilibrium thermodynamic analysis. The laboratory-scale semi-continuous system demonstrates stable magnesium condensation, high recovery, and effective impurity removal, providing a basis for future scale-up of high-purity magnesium ingot production.