Photolytic effects in alumina chlorination
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I.
INTRODUCTION
IN the
course of a recent study ~ of the chlorination of a-alumina with CO/CI 2 gas mixtures, an anomaly was observed in the Arrhenius plot of the reaction rate at temperatures below 835 ~ a decrease in the temperature from 950 ~ to 835 ~ resulted in a gradual decline in the rate of the chlorination reaction, as predicted by the Arrhenius equation; however, as the temperature was lowered further, the reaction rate began to increase attaining a local maximum at around 670 ~ and below this temperature the rate began to decrease again. Figure 1 shows the general shape of the Arrhenius plot, constructed with the rate data reported in the literature. Landsberg 2 reported an anomaly in the chlorination rate of gamma-alumina, with a local maximum in the rate constant occurring between 450 and 500 ~ The effect was less noticeable in the case of alpha-alumina where a small maximum occurred at about 700 ~ Alder 3 found a similar anomaly in the chlorination of gamma-alumina particles, with the maximum lying between 600 ~ and 700 ~ However, no explanation was offered in either case as to the cause of such anomalies. Landsberg et al. 4 observed a marked rate inversion with rising temperature in the chlorination of ZrO2 with CO/C12 mixtures. In the transition region, the maximum rate occurred between 600 ~ and 700 ~ they 4 attributed the effect to the transition in the CO-CO2 equilibrium. Milne 5 ascribed a similar anomaly he observed in the chlorination of gamma-alumina with CO/C12 gas mixtures to mass-transfer effects. The plot in Figure 1 is analogous to the plots obtained by Bertoti et al. 6 for the chlorination of gamma-alumina and by Landsberg 2 for the chlorination of clay, with phosgene (COC12). Dunn 7 reported a very similar Arrhenius plot for the chlorination of ilmenite (TiOz 9 FeO) with phosgene. The fact that A1203 (a and y), ZrO2, and TiO2 exhibit similar anomalous effects in chlorination reactions with CO/C12 mixtures is a strong indication that the observed anomaly is independent of the solid oxide reactant. The similarities in the Arrhenius plots for oxide chlorination with CO/Clz mixtures to those with phosgene point to the presM.K. SOLEIMAN, Postdoctoral Research Associate, and Y. K. RAO, Professor, are with the Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195. Manuscript submitted October 7, 1986. METALLURGICAL TRANSACTIONS B
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Fig. l - - A schematic Arrhenius plot for the chlorination rate (r) of alumina with CO/CI2 mixtures.
ence of phosgene in the CO/C12 gas mixtures; the phosgene is formed by the electromagnetic irradiation of the CO/C12 mixture. The purpose of the present paper is to provide experimental evidence for such an effect. No phosgene was intentionally introduced into the initial mixture which was composed of CO, He, and Clz gases. II.
EXPERIMENTAL
The TGA apparatus (Figure 2) is basically the same as that used in the recent chlorination st
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