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I. INTRODUCTION AND PREVIOUS INVESTIGATIONS

THEreduction of iron oxides by hydrogen or carbon monoxide (or their mixtures) is one of the most frequently studied topics in extractive metallurgy. The reader is referred to reviews 1-4 for a detailed account of research carried out on a macroscopic scale; that is, on pellets, sinter, or lump ore such as are fed to the iron blast furnace or a direct reduction unit. The present investigation is concerned with microstructural aspects of iron oxide reduction, in particular the nucleation, growth, and morphology of second solid phases on a scale of micrometers. Macroscopic studies of iron oxide reduction at temperatures encountered in industrial processes have frequently led to the conclusion that pore diffusion of gases is rate controlling, except for small particles ( e . g . , References 9, 10). The pore diffusion of gases referred to here is the diffusion of gaseous reactants and products through porous reactant or product. The pore structure (porosity, pore size distribution) is therefore of significance in determining the reaction rate under conditions of pore diffusion or mixed controlled. Turkdogan e t al. 11 have shown that the structure of the porous iron formed on reduction is very dependent on reduction temperature, being coarser at higher reduction temperature. Koo and Evans ~2 showed that the structure of porous iron ores can be markedly altered by holding at temperature prior to reduction. An important microstructural aspect of iron oxide reduction is therefore the development of porosity in solid reaction products. Edstrom ~-~and Edstrom and Bitsianes ~4 used optical microscopy to study the relation between microstructure and reduction rate. They observed that the reduction of hematite occurred more rapidly when there was early and extensive formation of pores in the solid reaction products. BrillMANN-FU RAU is Materials Engineer, Morgan Semiconductor, 2623 National Circle, Garland, TX 75041. DAVID RIECK is with the Department of Animal Science, South Dakota State University, Brookings, SD 57006. JAMES W. EVANS is Professor and Chairman, Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720. Manuscript submitted November 21, 1985. METALLURGICALTRANSACTIONS B

Edwards e t al. 15 studied the reduction of polycrystalline hematite and observed randomly distributed spherical pores for reduction temperatures between 400 ~ and 700 ~ with elongated pores between 700 ~ and 900 ~ A second microstructural aspect of iron oxide reduction is the nucleation and growth of the solid reaction products. A recent paper on the reduction of wustite is that of E1-Rahaiby and Rat. 5 These investigators used polycrystalline specimens of wustite about 50/xm in thickness and carried out hydrogen reduction at 238 to 417 ~ They observed a sigmoidal relationship between extent of reaction and time. That is, on contacting the oxide with the gas there was an initial "incubation" period where little reaction took place, followed by an interme