Kinetics of chlorination and carbochlorination of pure tantalum and niobium pentoxides

Abstract

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.