Studies on the kinetics of carbon tetrachloride chlorination of tantalum pentoxide

Studies on the kinetics of chlorination of tantalum pentoxide powder by car bon tetrachloride vapor in dilution with nitrogen have been carried out by weight-loss measurements in two temperature ranges, i.e., 698 to 773 K and 793 to 853 K. The effect of time, temperature, and partial pressure of CCl4 (P-CCl4) on the kinetics of chlorination of the powder Ta2O5, (-105 + 74 m um) has been investigated. In both the temperature ranges, the chlorination results have been found to fit the following relationship during the "init ial periods:" -ln (1 - alpha) = k(1)t where alpha is the fraction of the oxide reacted in time t and k(1) is the rate constant. In the low- and high-temperature ranges, the activation-ener gy (E) values have been calculated and were found to be 145 and 71 kJ/mole, respectively. The k(1) value was found to be proportional to p(CCl4)(1.5) in the lower-temperature range, while in the higher-temperature range, it w as found to be proportional to P-CCl4(0.9). n the "latter periods" of the chlorination of Ta2O5, the following relation ship was found to be applicable in both the temperature ranges: ln alpha /1-alpha = k(2) t + c where k(2) is the rate constant and c is another constant. However, in the latter periods of the chlorination, the activation energy in the lower-temp erature range was calculated to be 31 kJ/mole, while that in the higher-tem perature range was found to be 142 kJ/mole. Further, in the lower-temperatu re range, k(2) was found to be nearly independent of P-CCl4, while in the h igher-temperature range, it was found to be proportional to P-CCl4(0.9), as observed in the initial periods. Based on these kinetics results, two different mechanisms for the chlorinat ion of Ta2O5 by CCl4 vapor have been suggested for the two temperature rang es studied. It was possible to chlorinate >92 pct of Ta2O5 (-105 + 74,um) at 833 K in 2 10 minutes, by using CCl4 at a partial pressure of 0.6 atm.