A model for the role of carbon on carbochlorination of TiO 2
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I. INTRODUCTION
THE chlorination of metallic oxides in the presence of carbon (carbochlorination) is an example of gas-solid reactions of industrial importance. The mechanism is not known even though carbochlorination has been applied since 1940 as a step in the industrial extraction of Zr and Ti.[1] The carbochlorination stoichiometry at high temperatures (800 8C to 1000 8C) is well represented by the following equation:[2–6] MeO2(s) 1 2Cl2(g) 1 2C(s) 5 MeCl4(g) 1 2CO(g) [1] where Me is Zr or Ti. The chlorination of ZrO2 and TiO2 performed in the presence of carbon makes the formation of the respective chlorides thermodynamically favorable at all temperatures since direct chlorination is not feasible. From this point of view, carbon has a thermodynamic effect on the chlorination of oxides. In addition, an accelerating effect of carbon on reaction has been reported (kinetic effect).[2,5,7–10] Although the carbochlorination mechanisms are incompletely understood, most reports[2,6,8,11] have agreed that physical contact between carbon and oxide particles is not necessary. Hence, it has been proposed that carbochlorination occurs by means of gaseous intermediates formed during the reaction. Among these intermediates, phosgene (COCl2),[3] chlorine atoms (Cl),[2,4,6,8,12] and oxychlorides[7] have been the principal species proposed, even though they have not been detected in the bulk gas phase. If reaction intermediates were formed after interaction of Cl2 with carbon (as in the case of COCl2 or Cl formation), the overall reaction rate should be proportional to carbon content in the sample due a higher carbon surface area, while at the same time, the intermediate concentration in the gaseous phase would also be enhanced. Hence, the observed kinetic effect JULIO ANDRADE-GAMBOA, Associate Professor, National University of Comahue, Argentina, is also Researcher, Chemical Kinetics Division, National Commission of Atomic Energy. DANIEL M. PASQUEVICH, Senior Researcher and Head of the Chemical Kinetics Division, is with National Commission of Atomic Energy, San Carlos de Bariloche (8400). Argentina. Manuscript submitted October 13, 1999.
METALLURGICAL AND MATERIALS TRANSACTIONS B
of carbon on carbochlorination suggests its participation in the formation of active species for chlorination.[2] There are few studies about the effect of carbon content on carbochlorination rate. Dunn[7] found that on varying carbon contents in TiO2-C mixtures, the TiO2 chlorination rate increased linearly up to about 25 wt pct of carbon. Biceroglu and Gauvin[5] examined the chlorination of ZrO2-C pellet and proposed a model where the reaction rate is proportional to the initial carbon concentration. Recently, in a kinetic study on TiO2 chlorination, using alternatively carbon and CO as reductants,[13] the effect of carbon content on reaction rate was included as a factor in a pseudo rate constant, but the analysis was only qualitative. However, no general reaction models have been applied to carbochlorination in order to account quantitatively
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