The breakdown of dense iron layers on wustite in CO/CO 2 and H 2 /H 2 O systems
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I.
INTRODUCTION
PREVIOUS investigations ~,2.3 indicated that three distinct final product morphologies can be obtained on the reduction of wustite, (Fe~_~O), in CO/CO> and in H 2 / H 2 0 gas mixtures, namely, type A (porous iron), type B (porous wustite covered with dense iron), and type C (dense wustite covered with dense iron). The experimental evidence presented in a previous paper 3 suggested, however, that irrespective of the final product morphology the initial iron structure is always that of a dense layer. This dense iron morphology prevents direct contact between the reducing gas and the oxide, and the removal of oxygen from the sample can take place only by the solid state diffusion of oxygen through the iron layer. 4 This mechanism results in low overall reduction rates since the oxygen flux continually decreases as the product layer thickness increases. In gas mixtures of high CO2 or U 2 0 contents, for example, at 1273 K in gas mixtures containing more than 7.5 pct CO2 or 25 pct U 2 0 , this dense iron product is maintained throughout the reduction. At lower CO2 or H20 contents, however, despite the initial formation of a dense iron layer, the final structure consists of a porous iron morphology (type A). Observations by previous workers s,6-7have shown that the breakdown of dense iron structures can occur in CO/CO2 gas mixtures. It was postulated 6 that the mechanism for this breakdown involved the carburization of the iron layer by the reducing gas mixture and the subsequent reaction of the carbon in the iron with the oxygen in the wustite. This led to the formation of gas bubbles which, as they filled and expanded, resulted in the separation or bursting of the dense iron layer from the underlying oxide. This reaction exposes the wustite surface to direct contact with the reducing gas mixture once more. The observations of these workers 567 indicated that the breakdown of dense iron layers does not occur in H2/H20 gas mixtures. The aim of the present investigation is to demonstrate that the breakdown of dense iron layers can occur under certain reaction conditions both in CO/CO2 and in Hz/H20 gas mixtures and to determine the mechanisms of these phenom-
D.H. St. JOHN, formerly Postdoctoral Fellow at the Department of Mining and Metallurgical Engineering, University of Queensland, St. Lucia, Brisbane, Australia, is Lecturer in the Department of Metallurgy and Mining, Royal Melbourne Institute of Technology, Melbourne, Australia. S. R MATTHEW, Graduate Student, and P.C. HAYES, Senior Lecturer, are with the Department of Mining and Metallurgical Engineering, University of Queensland, St. Lucia, Brisbane, Australia. Manuscript submitted November 7, 1983. METALLURGICALTRANSACTIONSB
ena. Experiments have been carried out over a wide range of temperatures and gas conditions in both CO/CO2 and H2/H~O gas mixtures using the experimental procedure adopted in previous investigations. 1,2,3
II. THE BREAKDOWN OF DENSE IRON LAYERS Initial experiments were carried out to test whether or not dense iron layers o
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