Preparation of Anhydrous Magnesium Chloride from Magnesium Chloride Hexahydrate
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INTRODUCTION
THERE are two major industrial methods for magnesium production: the thermal reduction method and the electrochemical method.[1–3] The thermal reduction method has an intensive energy usage and generates a large amount of greenhouse gas (GHG) emissions. The GHG emissions are 37 to 47 kg CO2 eq/kg Mg ingot and the energy consumed by the process is 284 MJ/kg Mg ingot, which are five times higher than steel production.[4–6] On the contrary, the electrochemical method is environment friendly and energy intensive.[1,7] Currently, however, more than 70 pct of global magnesium production is produced by the thermal reduction method because of its relatively lower capital price.[4] The preparation of electrolytic-grade anhydrous magnesium chloride takes up 50 pct of the overall cost, which has limited the development of electrolytic magnesium production.[8,9] Salt lakes in the world have adequate reserve of magnesium chloride hexahydrate (MgCl2Æ6H2O) resources (22 billion tons in the Dead Sea, 3.16 billion tons in Chaerhan Salt Lake in China, 1.2 billon tons in the Kara-Bogaz-Gol Bay in Former Soviet, and 0.6 billion tons in the Great Salt Lake in Utah).[10,11] Preparation of anhydrous magnesium chloride from magnesium chloride hexahydrate is the most ZHIMIN ZHANG and FENG PAN, Ph.D. Students, and YUN WANG and SUPING YANG, Masters, are with the State Key Laboratory of Multi-Phase Complex Systems, Institution of Process Engineering, Chinese Academy of Science, Haidian District, Beijing 100190, P.R. China, and with the Graduate University of Chinese Academy of Science, Beijing 100049, P.R. China. XUCHEN LU, Professor, is with State Key Laboratory of Multi-Phase Complex Systems, Institution of Process Engineering, Chinese Academy of Science, and with the United Research Center for Resource and Materials, Wuhai 016000, P.R. China. Contact e-mail: [email protected]. Manuscript submitted April 23, 2012. Article published online December 1, 2012. 354—VOLUME 44B, APRIL 2013
economical way[12,13]; however, magnesium chloride hexahydrate inevitably hydrolyzes in the dehydration process with harmful oxide-containing compounds (MgO and MgOHCl) as products. In recent years, various methods have been proposed in the literature related to the preparation of anhydrous magnesium chloride from magnesium chloride hexahydrate. These methods can be classified as organic solvent distillation,[14,15] gas protection heating,[16–18] and decomposing the MgCl2-NH3 complex compound[19–26]; however, each of these methods has certain disadvantages. For organic solvent distillation method, the consumption of a considerable quantity of organic solvent increases the capital price. For the gas protection heating method, the purification and transportation of a great deal of corrosive hydrogen chloride are obvious disadvantages. For decomposing the MgCl2-NH3 complex compound method, the complexity of the process and the excessive of a big deal of organic solvent and ammonium chloride have limited its development. In this work, high-purity anhyd
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