Study On Preparation Of High Purity Magnesium By Vacuum Distillation

Magnesium is the lightest material of structural metals, China’s rich magnesium resources ranking first in the world. Magnesium was produced for low energy consumption and low price, which is a new generation of green materials. However, the problem that poor corrosion resistance of magnesium largely limited the application of magnesium products, because the purity of magnesium was one of the most important factors affecting the performance of magnesium metal, in particular the harmful impurities content of magnesium. Thus high purity purification of magnesium had became one of the core issues for magnesium research, and was one of the key to expand the scope of application for magnesium and magnesium alloy, also this was a very difficult task.In this paper, the magnesium(≥96.54%) from factory using Pidgeon process, which was a raw material to prepare high-purity magnesium metal by vacuum distillation method. On the basis of theoretical analysis of vacuum distillation, the advantage of vacuum distillation was used to remove the impurity elements from magnesium, and crude magnesium was purified. Finally the preparation process of high-purity magnesium was achieved by vacuum distillation method.Fisrst, from the point of view of thermodynamic, it analysed the degree of vacuum distillation separation between magnesium and other impurities. For the boiling point and the saturated vapor pressure of pure substances, the boiling point of impurity element zinc below the boiling point of magnesium volatilized out in the distillation process, into the gas phase; but boiling points of silicon, nickel, iron, tin, cooper, aluminum, manganese, lead and calcium higher than the boiling point of magnesium was not substantially volatile during the distillation, enriched in the liquid phase. The magnesium could be theoretically separated from other impurities. In addition it analyzed gas-liquid equilibrium phase diagram or phase diagram between magnesium and other impurities for magnesium, where activity coefficients in wireless dilute solution were introduced. Activity coefficients in dilute solution of impurity elements at different temperatures could be calculated to establish a gas-liquid equilibrium diagram. It derived that the degree of separation among calcium, lead, aluminum, cooper, tin, nickle and magnesium increased, the majority of these types of impurity elements left behind, magnesium was more volatile than the impurity elements into the vapor phase and enriched in the condensate, so as to achieve the separation between above impurity elements and magnesium; at different temperatures, zinc concentration was0.0035%in the liquid phase and3.13×10-5%in the gas phase only two orders of magnitude, indicating that zinc and magnesium were not easily separated; the lack of parameters in magnesium-based binary system, through phase diagram of magnesium and impurities for the analysis and prediction:the same temperature, the solubility of iron, silicon, manganese increased in magnesium; increased by increasing temperature, so873～1023K for vacuum distillation of crude magnesium was more suitable.Secondly, from the point of view of kinetics, it analyzed the rate of evaporation and condensation for component in the melt. The evaporation rate of magnesium increased with increasing temperature; at the temperature of873K, the condensation rate of magnesium was3.717g·cm-2·h-1, which was basically the same as the actual evaporation rate of magnesium3.346g·cm-2·h-1. According to the actual conditions, the testing process using water to condense, condensation temperature below the melting point of magnesium, and larger condenser (condensing area is4～5times than volatile area), therefore condensation rate did not affect the entire distillation process; the degree of vacuum was far below the saturation vapor pressure of magnesium, the distillation process and the gas phase mass transfer process did not matter much. So the whole process was affected by the evaporation process and the liquid phase mass.The experimental study of vacuum distillation for removing impurities was carried out, which investigated that the influences of distillation temperature and holding time on evaporation rate and removing impurities effects of materials. It got that the temperature was873K, vacuum degree was8-15Pa and insulation time was30-45min. Under this condition, the calcium content in the crude magnesium variationed from0.56%to0.0016%, cooper from0.00064%to0.00018%, zinc from0.0035%to0.0033%, manganese dropped from0.0097%to0.00099%, lead from0.0011%to0.00083%, iron down to0.0068percent from the0.066percent. By this condition, we could get high-purity magnesium for3N8.