Kinetics of chlorination and carbochlorination of pure tantalum and niobium pentoxides

Kinetics of chlorination and carbochlorination of pure Nb₂O₅ and Ta₂O₅ were studied by thermogravimetric analysis between 385 °C and 1000 °C using Cl₂-N₂ and Cl₂-CO-N₂ gas mixtures. Standard free energy changes of the reactions and phase stability diagrams of Nb-O-Cl and Ta-O-Cl systems were calculated. The chlorination reaction order, for both oxides, with respect to Cl₂ in the Cl₂-N₂ gas mixture was 0.82. The apparent activation energies (E _a ) for Nb₂O₅ chlorination were 208 and 86 kJ/mole for temperatures lower and higher than 850 °C, respectively. The experimental data could be described by a shrinking sphere model between 700 °C and 1000 °C. The chlorination mechanism, between 700 °C and 850 °C, was likely controlled by the chemical reaction. For T > 850 °C, the overall Nb₂O₅ chlorination rate was affected by the allotropic transformation of the Nb₂O₅ T form to M form. Between 925 °C and 1000 °C, E _a for Ta₂O₅ chlorination was 246 kJ/mole. In this case, the most appropriate model was also that of shrinking sphere suggesting that the chlorination of Ta₂O₅ was controlled by the chemical reaction. For both oxides, the carbochlorination reaction order with respect to Cl₂+CO partial pressure, in the gas mixture, was about 2. The mathematical analysis of carbochlorination data indicates that Nb₂O₅ and Ta₂O₅ reactions could be described by shrinking sphere or cylinder, respectively. Below 600 °C, the E _a values of Nb₂O₅ and Ta₂O₅ carbochlorination were 74 and 110 kJ/mole, respectively. Chemical reaction was probably the rate controlling step in both cases. An anomaly characterized by a decrease of the reaction rates occurs in the Arrhenius plots between 600 °C and 800 °C. This anomaly could be attributed to the thermal decomposition of COCl₂ formed in situ during the carbochlorination.