Nonisothermal gravimetric investigation on kinetics of reduction of magnesia by aluminum
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I. INTRODUCTION
MAGNESIUM is the eighth-most abundant element. Magnesium and its alloys are known for their excellent machinability and good hot formability, which make them widely used. Aluminum alloying is the largest application for magnesium, accounting for over half of its total consumption, and about 10 pct of the total consumption of magnesium is used in metallurgy as an agent for desulfurization, deoxidation, and other processes, because of its high reactivity. However, its production is energy intensive. Magnesium is produced by two principal processes: electrolysis of molten magnesium chloride and thermal reduction of magnesium oxide by a reductant. Electrolysis is the predominant route, accounting for some 77 pct of total production. In electrolysis, the manufacture of magnesium chloride from seawater or salt brines is characterized by high energy requirements. In pyrometallurgical routes, the chemical stability of magnesia requires that the reduction be performed at high temperature, or low pressure, or a combination of both.[1,2,3] In both the electrolysis and pyrometallurgical routes, the reactivity of magnesium can lower the yields further due to the reaction with chlorine or oxygen. Aluminum has a low melting point (933 K), high boiling point (2723 K), and high affinity with oxygen. Aluminum has been used as the reductant for many oxides, which otherwise are reduced by carbon with difficulty.[4,5,6] If aluminum is chosen as the reductant for magnesia, it can be used at above the boiling point of magnesium (1393 K), and the magnesium produced can be removed from the reaction site as vapor. Thermogravimetry has long been accepted as a useful technique for studying the kinetics of a solid-state reaction. The kinetic study is normally conducted at constant temperature to obtain the reaction-rate constant, and the activation LAN HONG, formerly Graduate Student, Department of Materials Process Engineering, Graduate School of Engineering, is Postdoctoral Fellow, Research Center for Advanced Waste and Emission Management, Nagoya University. KEIJI OKUMURA, Assistant Professor, and MASAMICHI SANO, Professor, are with the Department of Materials Process Engineering, Graduate School of Engineering, Nagoya University, Aichiken, 464-8603 Japan. Manuscript submitted October 21, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS B
energy is evaluated by an Arrhenius plot. This is called the isothermal technique. But for a reaction sensitive to temperature, the reaction may proceed before the sample is heated to the predetermined temperature. The reaction interface area also changes during the reaction. In this case, one has to investigate the kinetics under nonisothermal conditions using samples with a changing reaction interface area. While theoretical analyses of isothermal kinetic studies are well developed, few attempts have been made to describe nonisothermal experiments in a systematic way. In the present work, a nonisothermal technique is applied to investigate the reaction kinetics, because the reduction of m
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