Kinetics of chlorination of niobium pentoxide by carbon tetrachloride
- PDF / 300,157 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 92 Downloads / 212 Views
I.
INTRODUCTION
REFRACTORY metals like titanium, zirconium, hafnium, niobium, and tantalum are generally produced by metallothermic or hydrogen reduction of their chlorides. Most of their oxides can be chlorinated by Cl2 in the presence of a reducing agent like carbon or carbon monoxide. A number of studies have been carried out to optimize the chlorination parameters and also to establish the mechanism of the reactions involved.[1–5] Jena and co-workers[6,7] have studied the kinetics of the chlorination of pure pentoxides of niobium and tantalum by chlorine in the presence of an excess of graphite powder. Based on these results, by applying absolute reaction rate theory, mobile adsorption of chlorine was suggested to be the rate-controlling step. Subsequently, Srinivasan and Jena[8] studied the kinetics of the chlorination of the pentoxides of niobium and tantalum by chlorine and carbon monoxide. On the basis of the absolute reaction rate theory, immobile adsorption or a first-order chemical reaction of COCl2 molecules with the surface sites was suggested to be the rate determining step for both the oxides. Gaballah et al.[9] studied the kinetics of chlorination of some refractory metal oxides, including Nb2O5 by Cl2 in mixture with N2. In the temperature range of 973 to 1123 K, the activation energy E for Nb2O5 was reported to be 208 kJ/mole and in the temperature range of 1123 to 1273 K it was 86 kJ/mole. The rate of chlorination was found to be proportional to p0.82 Cl2 . They suggested that formation of NbO2Cl before the production of NbOCl3 might be an intermediate step and that the chlorination by monoatomic chlorine was more favorable than the chlorine molecule. In another investigation, Allain et al.[10] made detailed studies P.K. JENA, Senior Visiting Professor, E.A. BROCCHI, Associate Professor and Head, and R.I. GARCIA, Doctorate Student, are with the Department of Materials Science and Metallurgy, Catholic University of Rio de Janeiro, Ga´vea, CEP 22453-900, Rio de Janeiro, RJ, Brazil. Manuscript submitted June 16, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS B
of the chlorination of some refractory metal oxides, including Nb2O5 by Cl2 in the presence of CO and N2. As suggested by Srinivasan and Jena,[8] the authors also found that the rate of chlorination was proportional to individual partial pressures of Cl2 and CO equal to one. Their results fitted to a topochemical reaction model equation 1 2 (1 2 x)1/3 5 kt
[1]
where x is the fraction of the oxide chlorinated in time t and k is the rate constant. Two activation energy values, 74.4 kJ/mole (in the range of 658 to 873 K) and 78 kJ/mole (in the range of 1073 to 1273 K), were found, with an anomaly between 873 and 1073 K. Besides C and Cl2 or CO and Cl2, thermodynamically carbon tetrachloride is considered to be another efficient chlorinating agent. Jena et al.[11,12] have studied the kinetics of the chlorination of TiO2 in the temperature range of 523 to 673 K and ZrO2 in the temperature range of 650 to 825 K by carbon tetrachloride vapor. In
Data Loading...
