Kinetics of chlorination of zirconia in mixture with petroleum coke by chlorine gas
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k ? K ? pCl2 1 1 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.
I. INTRODUCTION
ZIRCONIUM metal has certain unique properties such as compatibility with uranium fuel in nuclear reactor, resistance to corrosion in boiling water and liquid metal coolants, resistance to radiation damage, very low absorption cross section for thermal neutrons, retention of strength at moderately high temperature, and favorable response to alloying modifications. Because of these properties, zirconium and its alloys are being used increasingly in nuclear and other metallurgical industries. Zirconium sponge is mostly produced from its tetrachloride through magnesium reduction or fused salt electrolysis in presence of alkali chlorides. Zirconium tetrachloride is produced by chlorination of pure zirconium oxide by chlorine in the presence of carbon. In static bed chlorination, briquettes of the mixture of ZrO2 and carbon are used, or in fluidized bed chlorination, their powder mixture is used.[1,2] The kinetics of chlorination of a number of refractory [5] [6] [7] metal oxides such as Nb2O[3,4] 5 , Ta2O5 , TiO2 , MoO3 , [9,10,11] V2O[8] , and ZrO have been studied in order to establish 5 2 the mechanisms of the processes involved. In case of ZrO2 chlorination, a number of diverse results have been reported by different workers on the kinetics of the processes studied. O’Reilly et al.[9] in their studies on the chlorination of ZrO2 in a static bed with carbon and chlorine found that the P.K. JENA, Senior Visiting Professor, E.A. BROCCHI, Associate Professor and Head of the Department, and M.L. DOS REIS, Consultant, are with the Department of Materials Science and Metallurgy, Rio de Janeiro Catholic University, Ga´vea CEP 22453-900, Rio de Janeiro, Brazil. Manuscript submitted February 26, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
rate of reaction followed the topochemical model with the reaction interface advancing linearly with time. The rate depended on the partial pressure of chlorine to a power of 0.64. They used the mixture of ZrO2 and a considerable excess of petroleum coke powder in the weight ratio of 1:4 (20 pct ZrO2 and 80 pct coke, by wt.). Landsberg et al.[10] studied the kinetics of chlorination of ZrO2 in the presence of carbon and carbon monoxide separately. In the case of carbothermic chlorination, they found that the rate of reaction was proportional to the partial pressure of Cl2; for the rate of chlorination of ZrO2 in CO and Cl2, i
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