Solid oxide membrane process for magnesium production directly from magnesium oxide
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I. INTRODUCTION
MAGNESIUM is the third most abundant metal in the earth’s crust after aluminum and iron and is widely distributed in almost all parts of the world as oxides, carbonates, sulfates, chlorides, and their combinations. It constitutes 2 wt pct of the earth’s crust and is the third most plentiful element dissolved in seawater with a concentration averaging 0.13 wt pct.[1] Magnesium has been primarily used as an addition in aluminum alloys, for desulfurization of steel and production of ductile cast iron. Recently, magnesium has been gaining attention as a material for use in next generation vehicles with an emphasis on weight reduction.[2] The price of magnesium dictated by its existing primary production processes has been a major barrier to its more extensive use in the automobile industry. For the future hydrogen economy, magnesium hydride slurry is being investigated as a potential medium for hydrogen storage in the form of a pumpable slurry.[3] Hydrogen is generated when needed by hydrolysis of magnesium hydride. The by-product of the hydrolysis reaction is stable magnesium hydroxide. The magnesium hydride slurry technology lends itself well for automotive applications in conjunction with fuel cells. For such high volume applications, the success and long-term economic viability depends on an efficient and cost-effective recycling scheme for converting magnesium hydroxide into magnesium. Two major production routes currently exist for primary magnesium: metallothermic reduction and the direct electrolysis of magnesium chloride from a chloride bath at 675 °C to 800 °C.[4] The metallothermic process for producing magnesium is based on the thermal reduction of calcined dolomite or magnesite at high temperature (1200 °C to 1400 °C) and reduced pressure with ferrosilicon as the reductant. The reduced magnesium vapor is collected in an attached condenser. The A. KRISHNAN, Graduate Student, and U.B. PAL, Professor, are with the Manufacturing Engineering Department, Boston University, Boston, MA 02446. Contact e-mail: [email protected] X.G. LU, Visiting Professor, Manufacturing Engineering Department, Boston University, is on leave from Department of Materials and Engineering, Shanghai University, Shanghai, P.R. China 200072. Manuscript submitted December 10, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
ferrosilicon reductant represents 12 KWh/kg or roughly 40 pct of the total energy required to produce magnesium and roughly a third of its total production cost. Furthermore, this batch process generates about 4 to 5 tons of slag per ton of magnesium, which must be properly disposed. The conventional electrolytic magnesium process is based on an anhydrous chloride feed material that is recovered from brines via an elaborate and expensive front-end dehydration process. The feed preparation process can represent 80 pct of the plant footprint and 30 pct of its capital cost.[4] Anhydrous magnesium chloride for electrolysis can also be produced via high-temperature carbochlorination of oxide magnesium ores,[5] but t
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