Kinetics of oxychlorination of magnesium oxide
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I. INTRODUCTION
MAGNESIUM is a major element of the chromite ores. It is present in the spinel structure of chromite, which has the general formula (Fe21, Mg)(Cr, Al, Fe31)2O4, and also in the gangue of chromite concentrates, which are essentially composed of magnesium silicates such as olivine and serpentine.[1] Recently, chlorination was studied as a potential method to improve the quality of poor-chromite concentrates and for the recovery of specific elements (Cr, Fe, etc.) from chromite concentrates.[1,2,3] The oxychlorination of chromite concentrates by Cl2 1 O2 at about 900 8C allowed almostfull extraction of iron and partial recovery of chromium as chromium oxychloride (CrO2Cl2). It was found[4] that the kinetics parameters of the chromite oxychlorination, such as activation energies and reaction orders, changed with the reaction extents. Magnesium metal is produced mainly either by the electrolysis of purified MgCl2 from natural brine or by silicothermy. The use of lightweight alloys of magnesium by the transport industry allows the production of lighter-weight vehicles and, consequently, reduces the fuel consumption and CO2 emissions.[5] Moreover, magnesium oxide is employed in some applications, such as rubber processing, pharmaceuticals, and electrical insulator uses, which require a high-purity MgO.[6] The common impurities contained in MgO are oxides such as Fe, Si, and Ca. For these reasons, kinetic studies of the reactions of MgO, Cr2O3, and Fe2O3 with Cl2 1 O2 were undertaken. This article focuses on the oxychlorination kinetics of magnesium oxide in isothermal conditions using thermogravimetric analysis (TGA). No studies concerning the oxychlorination of magnesium oxide were found in the literature. Most of the investigations were devoted to the thermodynamical and kinetics aspects of the reactions of MgO with Cl2, Cl2 1 C, HCl, and HCl 1 C, and they have been summarized in Reference 7. N. KANARI, Researcher, and I. GABALLAH, Senior Researcher, are ยด with the Laboratoire Environnement et Mineralurgie, Mineral Processing and Environmental Engineering Team, INPL-ENSG, LEM, 54501 Vandoeuvre Cedex, France. E. ALLAIN, Scientist, is with the Center of Pyrometallurgy, Department of Metallurgical Engineering, University of Missouri Rolla, Rolla, MO 65409-1460. Manuscript submitted November 19, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS B
II. MATERIALS AND EXPERIMENTAL PROCEDURE The sample used in this study was MgO powder with a purity of 98 pct, which was supplied by PROLABO (Paris). Results of scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses of the magnesium oxide sample are shown in Figure 1. The main identified phase was MgO. The TGA tests for the oxychlorination of MgO were performed using 40 mg of sample in a vertical experimental set-up schematized previously.[7] Its main unit was a CAHN 1000 microbalance with a sensitivity of 10 mg. The oxychlorination gas mixture, composed of Cl2 1 O2, was purified through H2SO4 and CaCl2 columns before its introduction in the reactor.
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