Kinetics and mechanism of carbothermic reduction of magnesia
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11/7/03
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Kinetics and Mechanism of Carbothermic Reduction of Magnesia LI RONGTI, PAN WEI, and MASAMICHI SANO The reaction between MgO and graphite powders under flowing argon atmosphere was studied using a dynamic thermogravimetric method. In the temperature range 293 to 1973 K, the effects of compacting pressure, magnesia/carbon ratio, heating rate, Ar carrier-gas flow rate, and CO-partial pressure were investigated. An experimentally determined reaction mechanism was proposed and discussed. The reduction process could be divided into two stages. The first stage includes the direct reaction between MgO and graphite particles and partial gas-solid reaction at relatively low temperature (below 1750 K). The overall reaction rate depends on the solid phase-boundary reaction between magnesia and carbon particles. The second stage is the gas-solid reaction between CO and MgO, which determines the overall reaction rate. The apparent activation energies of the two stages were estimated to be 208.29 and 374.13 kJ/mol, respectively.
I. INTRODUCTION
MAGNESIUM is the lightest of all the commonly used structured metals. This property entices automobile manufacturers to replace denser materials, not only steels, cast irons, and copper-based alloys, but even aluminum alloys with magnesium-based alloys. The requirement to reduce the weight of car components and further introduction of legislations to limit emissions have triggered renewed interest in magnesium.[1] Magnesium is produced by two principal processes: electrolysis of molten magnesium chloride and thermal reduction of magnesia. Electrolysis is the predominant route, accounting for about 77 pct of total production, but this process is characterized by high-energy requirements.[2] The reaction between MgO and C may provide a new route toward the production of magnesium. Furthermore, MgO/C refractory bricks are widely used in the steelmaking industry as basic oxygen furnace (BOF) linings because of their good resistance both to slag corrosion and to thermal stresses.[3] However, a serious problem in using BOF bricks is that the reaction between magnesia and carbon takes place when they are used under the steelmaking condition at about 1873 K for a long time, which leads to a decrease of their strength and lifetime. Pickering found that weight and strength losses were observed experimentally in the 1773 to 1873 K temperature range, and the reduction of magnesia by carbon produced extensive microstructural changes.[4] Despite the importance of the performance of the BOF magnesia bricks in the steelmaking industry, a systematic study dealing with reduction kinetics of magnesia by carbon has not been reported. To obtain the high-quality steels and so-called “clean steel,” their content of oxygen, sulfur, phosphorus, and nitrogen must be decreased to ultralow concentration. Alkaline LI RONGTI, Graduate Student, and PAN WEI, Professor, are with the Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of Chi
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