Establishment of product morphology during the initial stages of wustite reduction
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I.
INTRODUCTION
THE microstructural
changes which occur during the reduction of pure dense wustite (Fe~_,.O) in H2/H20 gas mixtures have recently been reported. ~'~ Three types of product morphology were identified: type A (porous iron), type B (porous wustite covered with dense iron), and type C (dense wustite covered with dense iron). The study of the reduction of wustite has now been extended to cover reactions in CO/CO2 gas mixtures. The results of the present investigation and those obtained during reduction in H2/H20 gas mixtures are discussed, and transformation mechanisms which account for the observed microstructures are proposed.
II. RESULTS OF THE REDUCTION OF WUSTITE IN CO/CO2 GAS MIXTURES Using the same experimental techniques as were used in the previous investigation,~'2 samples of pure, dense wustite having initial iron ion vacancy concentrations y = 0.05 and y = 0.10 were reduced in CO/CO2 gas mixtures (Pco + Pco2 --- 1 atm) at temperatures between 1073 and 1373 K. Controlled reduction below 1073 K was found to be impossible because of the deposition of carbon on the samples. The reduced samples were examined under the scanning electron microscopy (SEM) and mounted for metallographic examination of their cross-sections. Examples of the partially reduced wustite structures that were formed in various gas compositions at 1273 K are shown in Figures 1 and 2. It is apparent that the product microstructures are markedly dependent upon the reducing gas composition; however, no significant differences in structure are observed between samples having initial composition Fe0950 and Fe0.900 for any given reducing gas composition. A similar range of structures to those obtained at 1273 K was observed at the other reduction temperatures investigated. Since the final
microstructures strongly resemble those obtained in H2/H20 reductions,~2 the same classification into types A, B, and C is used in the present paper. A summary of the microstructures observed on reduction of wustite in CO/CO2 gas mixtures between 1073 and 1373 K is given in Figure 3. Several features of the product morphologies are worth noting. Firstly, type A morphologies (porous iron) are formed in gas mixtures containing up to 7.5 pct CO2. The transition from type A to type B morphologies appears to be almost independent of reaction temperature. This can be contrasted with the morphologies of samples reduced in H2/H20 gas mixtures, ~2 where, below 1173 K, only gas compositions approaching pure H2 produced type A structures, while above 1173 K the transition from type A to type B occurred at progressively higher H20 contents as the temperature was increased. Secondly, all of the type B morphologies formed in CO/CO2 gas mixtures were similar to those obtained in Fe0.950 samples reduced in Hz/H20 mixtures. Thirdly, metallographic examination of sample crosssections indicates that whether the final product morphology is type A, B, or C, a dense iron layer covers the surface of all samples (Figures 1 and 2). At high CO2 contents the surface loo
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