Distribution of impurities in magnesium production via silicothermic reduction

The desire for light weight materials, particularly in the automotive field, is fuelling greater magnesium production. The Pidgeon process is currently the most widely used process for the production of magnesium. This batch process involves reduction of dolomite by ferro-silicon, carried out at temperature between 1100-1200oC under vacuum in a retort, producing magnesium vapour which is then cooled and collected as a condensate. The major attractions of the process are its simplicity and low capital cost; however, the process is also labour and energy intensive. The Magnetherm process is also a vacuum batch process based on similar chemistry but operates at higher temperatures with magnesia dissolved in a slag. Various operational issues relating to the batch and vacuum nature of the process have limited its development. Higher temperatures will improve the kinetics of the silicothermic reduction but will also promote greater impurities into the vapour. Thermodynamic analysis of the distribution of impurities during condensation of magnesium vapour has been carried using FactSage thermodynamic package. The results from the multi-stage equilibrium model show that at equilibrium it should be possible to condense solid impurities before the majority of the magnesium vapour is condensed below 750oC, depending on the operating pressure of the system and the particular chemistry of the process. The model predicts that purities above 99.8wt% can be achieved for the Pidgeon process and 99.4wt% for the Mintek process. Without multi-stage condensation, the impurities in the metal are predicted to be significantly higher for the Mintek process (2.14wt% compared to 0.6 wt%). These results are broadly consistent with existing industrial data.