Theory Study of AlCl Disproportionation Reaction Mechanism on Al (110) Surface
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INTRODUCTION
DURING the past 50 years, numerous attempts have been made to produce aluminum using carbothermic reduction to take the place of the traditional Hall cell. The major driving force to develop carbothermic reduction is a decrease in energy consumption. The Pechiney and Reynolds processes reached a significant level of development, although Aloca technologies should be noted in particular.[1] Recently, Kunming University of Science and Technology with support from Joint Funds of the National Natural Science Foundation of China has been researching and developing the technology to produce aluminum by carbothermic reduction as well as by chlorination of alumina under vacuum condition, which consists of two stages. In stage 1, AlCl gas is formed by carbon reduction and AlCl3 chlorination of alumina at about 1110 C and 60 Pa. In stage 2, the Al metal is formed by a disproportionation reaction of AlCl gas at lower temperatures. It can be shown by the following reactions[2,3]: AlCl3 ðgÞ þ 3CðsÞ þ Al2 O3 ðsÞ ! 3COðgÞ þ 3AlClðgÞ ½1 3AlClðgÞ ! 2AlðsÞ þ AlCl3 ðgÞ
½2
This method is different from direct carbothermic reduction of alumina to produce aluminum. In any direct carbothermic reduction process for aluminum, species like Al4C3, Al4O4C, Al2O3, and carbon—which usually cannot react completely—are associated with XIU-MIN CHEN, Associate Professor, and BIN YANG and YONG-NIAN DAI, Professors, are with the National Engineering Laboratory of Vacuum Metallurgy, Kunming University of Science and Technology, Kunming 650093, P.R. China. Contact e-mail: [email protected] DONG-PING TAO, Professor, is with the Faculty of Materials and Metallurgy Engineering, Kunming University of Science and Technology, Kunming 650093, P.R. China. Manuscript submitted July 30, 2009. Article published online November 24, 2009. METALLURGICAL AND MATERIALS TRANSACTIONS B
aluminum, and aluminum cannot be separated from the mixture. So, using direct carbothermic reduction of alumina to produce aluminum is too difficult to realize under regular conditions. Carbothermic reduction and chlorination of alumina have solved the problem of separating aluminum from the mixture by chlorinating aluminum into AlCl gas, which is easy to separate from the mixture in a vacuum.[4] Using AlCl disproportionation to produce Al at a lower temperature is a key step to producing aluminum through carbothermic reduction and chlorination of alumina under a vacuum condition, so it is important to study the reaction mechanism of AlCl disproportionation to control the disproportionation reaction process effectively. A large amount of experimental and theory studies exist for low-valent aluminum compounds. The enthalpies of formation[5] and the electric quadrupole moment[6] for AlCl and AlF were calculated with quantum chemistry methods. The results of high-level quantum chemical calculations on the [AlHF]+ (protonation of gas-phase compounds AlF yields a complex of Al+ with a hydrogenated ligand [AlHF]+) potential energy surface indicate that aluminum, not the hal
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