Studies on kinetics of low-temperature chlorination of ZrO2 by gaseous carbon tetrachloride

Studies on the kinetics of chlorination of ZrO₂. powder by carbon tetrachloride vapor in mixture with nitrogen in a low-temperature range of 650 to 825 K at different periods and partial pressures of carbon tetrachloride were carried out. The chlorination results at 650 and 675 K seem to follow a diffusion-controlled reaction model of Jander’s type: 1−(1−R)^1/3]² =k ₁t whereR is the fraction of ZrO₂ chlorinated in timet andk ₁ is the rate constant. The approximate activation energy of the process was calculated fromk ₁ values at the previously mentioned two temperatures and found to be 278 kJ/mole. For the chlorination in the temperature range of 700 to 750 K, the topochemical reaction model 1−(1−R) ^1/3 =k ₂t was followed. The rate constant,k ₂, was found to be proportional to the partial pressure of carbon tetrachloride. The activation energy of this reaction was calculated to be 154 kJ/mole. In the temperature range of 775 to 825 K, the rate of chlorination was found to be directly proportional to the time of chlorination following Langmuir’s Adsorption Isotherm. Because of the very high rate of chlorination and thermodynamic possibility of decomposition of CCl₄ above 773 K, the rate-controlling step has been suggested to be the decomposition of the adsorbed complex formed by ZrO₂ with carbon and chlorine atoms, obtained from the decomposition of CCl₄ vapor. The activation energy of the process was 54 kJ/mole. In view of nearly complete chlorination of ZrO₂ by CCl₄ in a very short period of about 15 minutes, at a temperature around 800 K and lesser possibility of formation of toxic product gases, the process is recommended for commercial application.