Behavior Analysis of CaF 2 in Magnesia Carbothermic Reduction Process in Vacuum
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INTRODUCTION
MAGNESIUM is the lightest structural metal in common use.[1] The properties such as high specific strength and low density[2] entice automobile manufacturers to replace denser materials, not only steels, cast irons, and copper-based alloys, but even aluminum alloys, with magnesium-based alloys.[3] Magnesium alloys have attracted an increasing interest for mass reduction in aerospace, automobile industry, and transportation.[4] This requirement triggered a renewed interest in magnesium.[5] Magnesium is produced essentially either by molten salt electrolysis or by silicothermic reduction of oxides. Electrolysis is the predominant route, accounting for approximately 77 pct of total production, but this process is characterized by high-energy requirements.[6] In recent years, production of magnesium in China by the Pidgeon process has dominated world production. In this process, ferrosilicon reduces magnesia from calcined dolomite under vacuum. The process suffers from high energy usage and low productivity.[7] However, C can also be used as a reductant for magnesia; the reaction between MgO and C may provide a new route toward the production of magnesium. The carbothermic reduction of calcined dolomite is an alternative to both silicothermic and electrolytic YANG TIAN, PhD Graduate, TAO QU, Lecturer, BIN YANG, Professor, YONG-NIAN DAI, Academician, BAO-QIANG XU, Lecturer, and SEN GENG, Undergraduate, are with the National Engineering Laboratory of Vacuum Metallurgy, Key Laboratory of Nonferrous Metals Vacuum Metallurgy of Yunnan Province, and with the Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, P.R. China. Contact e-mail: [email protected] Manuscript submitted September 21, 2011. Article published online February 15, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS B
processes for the production of magnesium. Numerous studies have been conducted on the mechanism of carbothermic reduction.[8–11] The reduction temperature is more than 2000 K (1727 C) at atmospheric pressure, however, the reaction can proceed at lower temperatures by operating under reduced pressure.[12] Corresponding to the parameters of thermodynamic analysis, metal magnesium can be obtained under the condition of the reduction temperature of 1553 K and the pressure between 30 and 100 Pa, which is approximately 450 K lower than normal pressure.[13,14] Recently, our research conducted a thermodynamic analysis of the magnesia carbothermic reduction with CaF2. It was found that the rate of reduction was significantly low compared with the experimental data when no flux was used. The reduction ratio increased along with increasing CaF2. CaF2 played a catalytic role during the carbothermic reduction magnesia. The reaction kinetics catalytic mechanism of CaF2 and a comprehensive experiment of calcined dolomite will be carried out in future research. The aim of the current work was to investigate the behavior of CaF2 in the magnesia carbothermic reduction process. The behaviors of CaF
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