Studies on the kinetics of carbon tetrachloride chlorination of tantalum pentoxide
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e k2 is the rate constant and c is another constant. However, in the latter periods of the chlorination, the activation energy in the lower-temperature range was calculated to be 31 kJ/mole, while that in the higher-temperature range was found to be 142 kJ/mole. Further, in the lower-temperature range, k2 was found to be nearly independent of PCCl4, while in the higher-temperature range, it was found to be proportional to P0.9CCl4, as observed in the initial periods. Based on these kinetics results, two different mechanisms for the chlorination of Ta2O5 by CCl4 vapor have been suggested for the two temperature ranges studied. It was possible to chlorinate ⬎92 pct of Ta2O5 (⫺105 ⫹ 74 m) at 833 K in 210 minutes, by using CCl4 at a partial pressure of 0.6 atm.
I. INTRODUCTION CHLORINATION, in the case of most of the refractory metal oxides, is one of the important intermediate steps to produce the respective metals. Chlorination of these metal oxides is generally carried out by gaseous chlorine at a high temperature, in the presence of carbon or carbon monoxide gas. However, thermodynamically as well as kinetically, chlorination of some refractory metal oxides by carbon tetrachloride gas, even at lower temperatures and partial pressures, has been found to be highly promising. Jena and coworkers have carried out extensive studies[1–4] on the kinetics of the chlorination of anatase, zirconium dioxide, and niobium pentoxide by carbon tetrachloride gas in dilution with nitrogen. The kinetics of chlorination of briquetted anatase by carbon tetrachloride,[1] in the temperature range from 523 to 673 K, have been found to follow the topochemical reaction model P.K. JENA, Senior Visiting Professor, is with the Department of Materials Science and Metallurgy, Rio de Janeiro Catholic University, Gavea, CEP 22453-900, Rio de Janeiro, Brazil. E.A. BROCCHI, Associate Professor and Head of Department, and M.P.A.C. LIMA, Postgraduate Student, are with the Department of Materials Science and Metallurgy, Rio de Janeiro Catholic University. Manuscript submitted February 15, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS B
1 ⫺ (1 ⫺ ␣)1/3 ⫽ kt
[1]
where ␣ is the fraction of the briquetted anatase reacted in time t (in minutes), and k is the rate constant at a particular temperature and partial pressure of carbon tetrachloride (PCCl4). At 673 K, nearly 90 pct of the anatase has been found to be chlorinated by using 0.6 atm of PCCl4 in 60 minutes. Subsequently, similar studies have been reported on CCl4 chlorination of ilmenite, which propose the aforementioned type of topochemical reaction.[5] Studies of the kinetics of chlorination of ZrO2 by gaseous CCl4 have been carried out[2] in the temperature range from 650 to 825 K. In the temperature range from 700 to 750 K, the kinetics results have been found to fit the topochemical reaction model, as given in Eq. [1] for anatase. However, the chlorination results at lower temperatures (650 and 675 K) seem to follow a diffusion-controlled reaction model of Jander’s type: (1 ⫺ (1 ⫺ ␣)1/3)2 ⫽ kt
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