Recycling of Magnesium Alloy Employing Refining and Solid Oxide Membrane (SOM) Electrolysis
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GNESIUM is the least dense engineering metal, with an excellent stiffness-to-weight ratio. For this reason, U.S. auto makers would like to replace 283.5 kg of steel and aluminum parts per vehicle with 153 kg of magnesium alloy by the year 2020. This would reduce vehicle weight by 130.5 kg. It is estimated that 22.5 kg of mass reduction would improve fuel efficiency by around 1 pct.[1–3] Additionally, the fact that magnesium works well as an electromagnetic shield makes it very attractive to the audio and electronics industry.[4,5] As magnesium consumption increases in automotive, aerospace, and electronics industries, magnesium recycling will become ever more important for both economic and environmental reasons.[6–9] Several magnesium recycling processes are currently in use. Some are based on the use of refining fluxes and others are flux-free methods. Hydro magnesium[10,11] has a system for continuous melting in flux. Magnesium alloy is melted in one end of a large multi-chamber furnace and pumped out or cast from the other end. This process uses sufficient flux to absorb all the oxides XIAOFEI GUAN, Graduate Student, is with the Division of Materials Science and Engineering, Boston University, Boston, MA 02215. PETER A. ZINK, Research Assistant Professor and Lecturer, is with the Department of Mechanical Engineering, Boston University. UDAY B. PAL, Professor, is with the Division of Materials Science and Engineering, Boston University and also with the Department of Mechanical Engineering, Boston University. Contact e-mail: upal@ bu.edu ADAM C. POWELL, Chief Technology Officer, is with the Metal Oxygen Separation Technologies, Inc., Natick, MA 01760. Manuscript submitted May 18, 2012. Article published online January 30, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS B
present in the molten metal, but it cannot separate pure magnesium from aluminum and trace impurities such as iron and nickel. These impurities decrease the corrosion resistance of magnesium.[12] Zhu et al.[13] demonstrated vacuum distillation for magnesium recycling. Nevertheless, 20 to 25 pct of the original scrap remained as residue with a magnesium mass content as high as 60 pct; magnesium distillation was hindered by magnesium oxide (MgO) and aluminum oxide (Al2O3) scale on the surface of the residue. If there is relatively little magnesium in an Al-Mg scrap stream, as is the case for beverage can stock or today’s automobiles, then chlorine bubbling through the liquid metal can remove it by forming MgCl2 and impurity chlorides. This process leaves purer and highervalue aluminum.[14–16] However, as the magnesium content of vehicles increases, this will require large amounts of chlorine, energy, and new markets that can consume the MgCl2. This method also wastes the energy content of the magnesium metal: Reducing MgCl2 back to magnesium would require additional energy, and the low purity of the chloride might make even that impractical. In this paper, an innovative process for recycling pure magnesium metal from impure magnesium scrap alloy at high efficienc
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