The gaseous reduction of solid calciowustites in Co/Co 2 and H 2 /H 2 O gas mixtures
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INTRODUCTION
IN previous investigations I'2 of the reduction of pure wustite (FeO) it was found that the product microstructures were dependent on the reducing gas composition, the initial oxide stoichiometry, and the reaction temperature. Three types of product microstructure were identified, porous iron (Type A), porous wustite covered with dense iron (Type B), and dense wustite covered with dense iron (Type C). The reduction characteristics of each of these microstructural types are very different. With a porous iron product (Figure l(a)) fast reduction is observed whereas the dense iron morphologies (Figure l(b)) will always result in extremely slow reduction rates. 3 Thus with only minor changes in gas composition, e.g., I to 2 pet CO2 in CO, the time to complete reduction can change by an order of magnitude or more. The actual reduction time will be dependent on the initial size of the oxide particles. These studies clearly indicate the need for a more detailed investigation of the critical conditions for the formation of each of the microstructural types. Many iron ore deposits consist of high purity iron oxides, oxides which contain very low levels of impurity actually dissolved in the oxide lattice. However, during preparation of the raw materials for ironmaking, in sintering or pelletizing processes, the oxides are heated to high temperature. At temperature, gangue minerals and other second phase additions may dissolve in the iron oxides forming dilute solid solutions. The presence of these impurities in the oxide may have an effect on the chemical reaction rates between the reduction gases and the oxide and, by altering the defect structures of the solids, the mass transport processes within the oxides. Since the mechanisms of formation of the various product microstructures are also affected by the relative rates of the chemical reactions and mass transport processes in the solid, 2'4 it can be argued that the presence of impurity elements will alter the conditions for the formation of the various microstructural types. The present E NAKIBOGLU, formerly Graduate Student, Department of Mining and Metallurgical Engineering, University of Queensland, Brisbane, Australia, is Postdoctoral Fellow, Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, AB, Canada. D.H. St. JOHN, formerly Postdoctoral Fellow, University of Queensland, is now Lecturer, Department of Metallurgy and Mining, RMIT, Melbourne, Australia. P.C. HAYES is Senior Lecturer in Metallurgy, University of Queensland, Brisbane, Australia. Manuscript submitted November 13, 1984. METALLURGICALTRANSACTIONSB
investigation is aimed at determining the effects of CaO on the microstructures formed during the reduction of wustite.
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EXPERIMENTAL TECHNIQUES
Calciowustites, (Fe, Ca)l-yO, were prepared from high purity iron powder (Johnson Matthey spectrographically pure iron sponge, Mn 3 ppm, Na 1 ppm, Cu 1 ppm, Mg 1 pprn, Ag I ppm) and CaO powder (BDH A.R. grade). About 1.5 g of iron powder were mixed with appropriat
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