Kinetics of chlorination of tantalum pentoxide in mixture with sucrose carbon by chlorine gas
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4/27/04
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Kinetics of Chlorination of Tantalum Pentoxide in Mixture with Sucrose Carbon by Chlorine Gas J.A. GONZÁLEZ, J.B. RIVAROLA, and M. DEL C. RUIZ The mechanism and kinetics of -Ta2O5 chlorination, mixed with sucrose carbon, have been studied by a thermogravimetric technique. The investigated temperature range was 500 °C to 850 °C. The reactants and reaction residues were analyzed by scanning electronic microscopy (SEM), X-ray diffraction (XRD), and Brunauer-Emmett-Teller method for surface area (BET). The effect of various experimental parameters was studied, such as carbon percentage, temperature, chlorine partial pressure, and flow, use of the multiple sample method, and carbon previous oxidation. The carbon percentage and previous treatment have an effect on the system reactivity. The temperature has a marked effect on the reaction rate. In the 500 °C to 600 °C temperature interval, the apparent activation energy is 144 kJ/mol of oxide, while at higher temperatures, the activation energy decreases. With high chorine partial pressures, the order of reaction is near zero. The kinetic contractile plate model, X ⫽ kt, considering carbon oxidation as the controlling stage, is the one with the best fit to the experimental data. A probable mechanism for the carbochlorination of -Ta2O5 is proposed: (1) activation of chlorine on the carbon surface, (2) chlorination of Ta2O5, (3) oxidation of carbon, and (4) recrystallization of -Ta2O5.
I.
INTRODUCTION
THE chlorination of Nb- and Ta-containing materials in the presence of carbon, i.e., carbochlorination, yields the respective chlorides, from which the metals or the pure oxides can be produced commercially. Carbochlorination of a metal oxide using carbon as the reducing element and Cl2 as the chlorinating element is a complex process that involves two solids and Cl2 gas. The role of carbon in this type of reaction was originally considered to be that of a reducing element, which, combined with O2, led to the formation of CO or CO2 shifting the reaction to the formation of the respective metal chlorides. It has now been demonstrated that carbon has a double effect on this type of reaction; on the one hand, it allows the formation of active species that act as chlorinating agents, and on the other, it acts as a reducing element that combines with O2 to form CO/CO2 or other types of compounds, such as phosgene.[1–8] Few studies have dealt with the study of the mechanism and kinetics of Ta2O5 carbochlorination in the presence of carbon and using Cl2 as chlorinating agent. Most of the research has focused on the relative effectiveness of chlorination using different experimental methods.[9,10] Nb2O5 and Ta2O5 chlorination with Cl2 in the presence of carbon in the 320 °C to 1000 °C temperature range has been investigated by Fairbrother et al.[11] and Lind and Ingles.[12] These authors identified the obtained compounds as a mixture of tantalum and niobium chlorides and oxichlorides, and observed that NbOCl3 is more stable than TaOCl3. Mehra et al.[13,
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