Carbochlorination kinetics of tantalum and niobium pentoxides

The carbochlorination kinetics of pure Nb2O5 and Ta2O5 by gas mixture (Cl-2 + CO + N-2) between 380 and 1,000 degrees C is studied. A calculation of the standard free energy of the carbochlorination reactions is made. A diagram of the phases stability is drawn. The influence of the gas flow, temperature and the partial pressure of Cl-2 and CO at temperatures below 650 degrees C on the reaction rate is studied. The apparent activation energy is approximately 75 and 110 kJ/mol for Nb2O5 and Ta2O5, respectively. At temperatures above 650 degrees C the Arrhenius diagram presents an anomaly which may be attributed to the decomposition of the COCl2 formed in situ. The apparent reaction order of the carbochlorination of these oxides against Cl-2+CO is approximately 2. The carbochlorination rates of these oxides are much greater than those of chlorination by Cl-2 + N-2. The carbochlorination kinetics of tin furnace slag leaching concentrates containing tantalum and niobium compounds are also studied and compared with the carbochlorination kinetics of the pure oxides.