Intrinsic kinetics of the chlorination of hematite powder between 898 and 1023 K
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INTRODUCTION
THE kinetics of the chlorination of hematite has not been studied extensively.[1–5] Bertoti et al.[1] proposed that the chlorination rate of Fe2O3 might be influenced by mass transport in the 873 to 1253 K temperature range, even though it occurs with a high apparent activation energy of 188 kJ/mol. Fruehan and Martonik[2] claimed that the reaction rate was controlled by pore diffusion between 1073 and 1273 K and by chlorine diffusion through the gas film boundary layer around the pellet from 1273 to 1473 K. Szekely et al.[3] also reported that the chlorination of Fe2O3 was controlled by mass transfer at high temperatures and moderate pellet sizes. Gennari and Pasquevich[5] investigated the chlorination of hematite powder by using thermogravimetric techniques. They studied the effect of both chlorine transfer and crucible geometry on the reaction rate between 873 and 1223 K. The reaction occurred under two kinetic regimes, depending on the temperature. From 873 to 1023 K the kinetic regime was characterized by a high activation energy, approximately 200 kJ/mol, under a constant chlorine partial pressure of 35 kPa and with a chlorine flow greater than 4.5 L/h, which was high enough to minimize gaseous-phase diffusion effects into the boundary layer. In contrast, at higher temperatures (between 1023 and 1223 K), the reaction was significantly affected by convective mass transfer of chlorine into the boundary layer.[5] The purpose of this work is to extend the study of the intrinsic kinetics of the chlorination of hematite between 898 and 1023 K in order to propose a reaction model. This work follows the paper published by Gennari and Pasquevich.[5] Therefore, to ensure that the kinetic regime and the reactivity of the Fe2O3 used in this study are the same as those reported,[5] hematite samples were withdrawn from the same batch used by those authors.
DANIEL M. PASQUEVICH, JUAN P. GAVIRI´A, MARCELO ESQUIVEL, and ANA E. BOHE´, Researchers, are with the Comisio´n Nacional de Energı´a Ato´mica, San Carlos de Bariloche (8400), Rı´o Negro, Argentina. Contact e-mail: [email protected] ANA E. BOHE´ is also with the Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas, Universidad Nacional del Comahue. Manuscript submitted November 13, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS B
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EXPERIMENTAL PROCESS
A. Experimental Procedure The material used was a hematite powder (Spex Industries, Inc., Edison, NJ, U.S.A.). Particles have been well characterized by scanning and transmission electron microscopy, as reported elsewhere.[5,6,7] The powder has a BET area of 3.3 m2/g and is formed by nonporous spherical crystalline grains of approximately 0.2 mm in size, which in turn form agglomerates from various microns to 100 microns in size. The chlorination of Fe2O3 progressed with a continuous mass loss due to the volatilization of volatile iron chlorides. The following chemical equations represent the hematite chlorination:[1–5] Fe2 O3 ðsÞ 1 3 Cl2 ðgÞ ¼¼¼¼¼ 2FeCl3 ðgÞ 1 3=2 O2 ðgÞ [I] 2FeCl3 ðgÞ ¼¼¼¼¼ F
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