Studies on the kinetics of carbon tetrachloride chlorination of tantalum pentoxide

Abstract

Studies on the kinetics of chlorination of tantalum pentoxide powder by carbon 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 (PCCl₄) on the kinetics of chlorination of the powder Ta₂O₅, (-105+74 µm) has been investigated. In both the temperature ranges, the chlorination results have been found to fit the following relationship during the "initial periods": - ln (1 - α ) = k₁ t where α 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-energy (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 was found to be proportional to P _CCl4 ^0.9. In the “latter periods” of the chlorination of Ta₂O₅, the following relationship was found to be applicable in both the temperature ranges: ln α /1-α = k₂ 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-temperature range was calculated to be 31 kJ/mole, while that in the higher-temperature range was found to be 142 kJ/mole. Further, in the lower-temperature range, k₂ was found to be nearly independent of P_CCl4, while in the higher-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 chlorination of Ta₂O₅ by CCl4 vapor have been suggested for the two temperature ranges studied. It was possible to chlorinate >92 pct of Ta₂O₅ (-105+74 µm) at 833 K in 210 minutes, by using CCl₄ at a partial pressure of 0.6 atm.