Kinetics of low-temperature chlorination of vanadium pentoxide by carbon tetrachloride vapor
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I. INTRODUCTION
CHLORIDE metallurgy enjoys a prominent place in the extractive metallurgy of almost all the refractory metals. The chlorides of the metals obtained from their respective oxides are the principal compound for the preparation of the refractory metals through reduction by another suitable metal such as magnesium or a nonmetal such as hydrogen. The chlorides of refractory metals are generally produced through chlorination of their oxides with Cl2, in the presence of a reducing agent such as carbon or carbon monoxide at quite a high temperature around 1200 K. Alternatively, thermodynamically, and also kinetically, it is possible to chlorinate these oxides by carbon tetrachloride vapor at a much lower temperature around 700 K and partial pressure of about 0.25 atm. Jena et al.[1–6] have carried out extensive studies on the kinetics of chlorination of TiO2, ZrO2, Nb2O5, and Ta2O5, as well as the tin slag containing these oxides, by carbon tetrachloride at a much lower temperature and partial pressure of CCl4 in nitrogen. Besides establishing the mechanism of the chlorination process, these studies have made it possible to arrive at optimum experimental conditions to produce the metal chlorides. In all cases studied, the kinetics of chlorination of the refractory metal oxides with CCl4 vapor in dilution with nitrogen have been found to be much faster compared to those using Cl2 gas in the presence of a reducing agent such as carbon or carbon monoxide. Secondly, it is observed that the chlorination of the refractory metal oxides by CCl4 vapor around and above 773 K is much faster than that below this temperature. This is very likely due to the decomposition of CCl4 at these temperatures to elemental carbon and chlorine, which are very reactive in nature. P.K. JENA, Professor and Chairman, is with the Institute of Advance Technology & Environmental Studies, Bhubaneswar 751002, India. Contact e-mail: [email protected] E.A. BROCCHI, Associate Professor, and J. GONZÁLEZ, Postdoctorate Fellow, are with the Department of Materials Science and Metallurgy, Rio de Janeiro Catholic University, Rio de Janeiro, 22453-900, Brazil. Manuscript submitted October 22, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS B
In the present investigation, studies on the kinetics of low temperature (553 to 788 K) chlorination of vanadium pentoxide powder by carbon tetrachloride vapor (0.1 to 0.6 atm) have been undertaken. The effect of time, particle size of V2O5, temperature, and partial pressure of CCl4 on the extent of chlorination of V2O5 has been studied. Based on the kinetics results, the mechanisms of the chlorination reactions, have been suggested in two temperature ranges studied.
II. EXPERIMENTAL Chemically pure vanadium pentoxide powder and analar grade liquid carbon tetrachloride were used in the kinetic studies. The CCl4 vapor is diluted in desired proportion with nitrogen, obtained from the cylinder (nitrogen of purity about 99.6 pct). The liquid CCl4 is converted into vapor by heating it in a water bath at the desire
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