Chlorine fluxing for removal of magnesium from molten aluminum: Part I. Laboratory-scale measurements of reaction rates
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I.
INTRODUCTION AND PREVIOUS INVESTIGATIONS
SIGNIFICANT quantities of aluminum are recycled each year in the United States and in other countries. For example, the Aluminum Association reports that the United States recycled 62.2 pct of its total of 100.7 billion aluminum cans produced in 1995.[1] Beverage cans are made from alloys containing ;1 pct Mg in the body and ;4 pct Mg in the end. Consequently, recycled beverage cans, if simply melted, would yield aluminum containing between 1 and 4 pct of magnesium. Such levels of magnesium are too high for the uses to which recycled cans are frequently put, such as, for example, the production of aluminum alloys for casting, where magnesium specifications are on the order of 0.1 pct. Consequently, various techniques have been developed for magnesium removal (‘‘demagging’’). Nowadays, the most frequently employed technique is chlorine fluxing. At its least sophisticated, this technique involves simply passing chlorine or a chlorine-nitrogen (or argon) mixture down a tube (called the ‘‘lance’’ or ‘‘wand’’) into the molten alloy. In more sophisticated technologies, such as the ALCOA 503 process, rotating impellers disperse chlorine-argon bubbles into the melt. The reactions involved in chlorine fluxing for magnesium removal are Mg 1 Cl2 ⇒ MgCl2, DG 5 2481.7 kJ/mole Cl2
[1]
2 2 Al 1 Cl2 ⇒ AlCl3, DG 5 2354.5 kJ/mole Cl2 3 3
[2]
2 2 AlCl3 1 Mg ⇒ MgCl2 1 Al, 3 3 DG 5 2127.2 kJ/mole Mg
[3]
QIAN FU, Graduate Student, DONG XU, Visiting Scientist, and JAMES W. EVANS, Chancellors Professor, are with the Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720. Manuscript submitted July 23, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS B
where the standard Gibbs energies of reaction (kJ/mol) at 1033 K are shown to the right of each equation.[2] Clearly, all these reactions are favorable and it should be expected that formation of aluminum chloride would compete with formation of magnesium chloride when chlorine is bubbled into an aluminum-magnesium melt. Any aluminum chloride formed would be a gas and, given sufficient contact time, should react with magnesium in the melt according to Eq. [3]. To be specific, for a 0.1 pct magnesium content at 1033 K, when equilibrium is reached at a total pressure of 1 atm, the partial pressure of aluminum chloride is only 8.8 3 1025 atm and the pressure of chlorine is several orders of magnitude less. Nevertheless, emissions of chlorine and chlorides accompany most chlorine fluxing[4] and the kinetics of the reactions are worthy of study, with the objective of minimizing these emissions. Attempts to trap the emissions at the fluxing unit by means of a bell, as in the INTEC bell process,[3] or by a deep salt flux layer, as in the Derham process,[5] have not met with commercial success. Beyond the use of mere lances (henceforth called ‘‘static bubbling’’) or pellets, the accepted technology appears to be the use of rotating impellers and the in-line injection of chlorine at a pump.[6,7] Stationar
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