Kinetics of chlorination of zirconia in mixture with petroleum coke by chlorine gas

Abstract

Studies on the kinetics of carbothermic chlorination of zirconium dioxide in gaseous chlorine were carried out with petroleum coke fines in powder form. The amounts of ZrO₂ chlorinated were found to be directly proportional to the time of chlorination in the temperature range studied (973 to 1273 K). The activation energy values for chlorination of ZrO₂, in mixture with petroleum coke, was found to be 18.3 kJ/mole. The influence of particle size of petroleum coke on the chlorination of ZrO₂ (-38 + 25 µm) was studied, and it was found that the rate of chlorination increased up to the size range of -75 to +53 µ m, and the size finer than this produced negligible increase. The amount of petroleum coke in the mixture above 17.41 pct in excess of the stoichiometry resulted in very little increase in the rate. The effect of the partial pressure of chlorine (pCl₂) on the rate of chlorination was found to obey the following relationship, derived from the Langmuir adsorption isotherm: v = k.K.pCl₂ / l+K.pCl₂ where v is the amount of ZrO₂ (g/min) reacted, k is the rate constant, and K is the equilibrium constant for adsorption of two chlorine atoms (obtained from the dissociation of a molecule of Cl₂ at the carbon surface) on the surface of ZrO₂. By plotting 1/v vs 1/pCl₂, a straight-line relationship with an intercept in x-axis is obtained, further supporting the preceding equation. The dissociation of the adsorbed complex, Cl … ZrO₂ … Cl to ZrCl₂ and O₂ is suggested to be the rate-controlling step. Subsequently, the less stable ZrCl₂ combines with Cl₂ to form ZrCl₄ and the oxygen combines with C to form CO and CO₂.