Mechanism of Calcium Fluoride Acceleration for Vacuum Carbothermic Reduction of Magnesia
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LCIUM fluoride has been widely used as a reaction accelerator in ferroalloy production and ferrous metallurgy, including magnesium metallurgy.[1–8] During the industrial production of magnesium using the Pidgeon process in China, calcium fluoride may be added at a nominal rate of 0.2 kg per 1 kg of pure magnesium.[9] Calcium fluoride also obviously enhances the reaction rate of magnesia in the preparation of magnesium using vacuum carbothermic reduction method. Based on the previous thermodynamic analysis, the carbothermic reduction reaction is MgOðsÞ þ CðsÞ ¼ MgðgÞ þ COðgÞ :
½1
The reaction between magnesia and carbon produces magnesium vapor and carbon monoxide. The gaseous mixture is cooled down when it is pulled out from the heating system. The magnesium vapor condenses on the surface of the condenser, and carbon monoxide is expelled from the equipment. The system pressure affects the reaction because solid materials transform
into gaseous products during the reaction. The decrease of the system pressure contributes to the forward reaction. The thermodynamic calculation indicates that the beginning reaction temperature dips to 1387 K (1114 C) when the system pressure in the vacuum oven is 20 Pa.[10] Some experiments on the vacuum carbothermic reduction of magnesia with various amounts of calcium fluoride have been performed. Li[11] noted that the rapid reaction temperature without CaF2 is approximately 1750 K (1477 C) at 150 Pa. Tian[12] confirmed that the reaction with 5 pct CaF2 at 30 to 100 Pa is sharply accelerated only at 1573 K (1300 C). Gao[13] observed that the magnesia reaction rate with 5 pct CaF2 at 1673 K (1400 C) for an hour increased from 31.02 to 91.72 pct. However, the mechanism of calcium fluoride in the experiment is not fully clear and remains worthy of investigation. Therefore, experiments and analyses have been conducted to investigate the effect and mechanism of calcium fluoride on the reaction.
II. YUN JIANG, Lecturer, is with the School of Science, China University of Geosciences, 29 Xueyuan Road, Haidian District, Beijing 100083, China. YU-QIN LIU, Associate Professor, and HONG-WEN MA, Professor, are with the School of Materials Science and Technology, China University of Geosciences, 29 Xueyuan Road, Haidian District, Beijing 100083, China. Contact e-mail: [email protected] WEI-GONG ZHOU, Associate Professor, is with the Great Wall College, China University of Geosciences, 1698 South Second Ring Road, Baoding 071000, China. Manuscript submitted July 21, 2015. Article published online December 14, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B
EXPERIMENTS
A series of experiments were performed. The raw materials were magnesia, coke, and calcium fluoride. Light magnesium oxide and calcium fluoride, produced by the Sinopharm Chemical Reagent Co., Ltd., were both analytical reagent with nominal purities of 98.0 and 98.5 pct, respectively. Senior grade metallurgical coke with some complex components produced in Shanxi, China, was used as the reductant. The chemical compositions of magnesia,
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