Mechanisms of porous iron growth on wustite and magnetite during gaseous reduction
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INTRODUCTION
W H I L E the formation of porous or sponge iron during the gaseous reduction of wustite and magnetite has long been recognized, rl-6] the mechanisms of formation and the morphologies of the products have yet to be fully characterized. Previous microstructural studies have been carried out using optical, tHu scanning electron, t12-141 and transmission electron microscopy, l~Sl The micrographs presented in these publications indicate significant disparities between the microstructures of the polished sections and those prepared for electron microscopy; the pore sizes and apparent porosities, in particular, are very different. Indirect measurements of the pore size variation with gas composition and reduction temperature have been reported, [16A71 but these experiments were carried out on large magnetite and hematite pellets. The rate-limiting effects of gas film mass transfer and porous diffusion cast some doubt as to the actual gas compositions at the reaction interface during reduction of these large samples. In addition, little attention has been paid to the possibility of sintering of the pore structure during treatment. The aim of the present investigation, therefore, is to provide detailed information on the structure of the iron/ iron oxide interface during reduction under a range of controlled gas conditions and to deduce the growth mechanisms which are operative. Detailed descriptions of the apparatus and experimental procedure used in the present investigation are given in a previous publication. Im These techniques were specifically designed to overcome the limitations imposed by previous procedures.
S.P. MATTHEW, formerly Graduate Student, Department of Mining and Metallurgical Engineering, University of Queensland, is Research Metallurgist, Process Development Group, Mount Isa Mines, Mt. Isa, Queensland 4825, Australia. T.R. CHO, formerly Graduate Student, Department of Mining and Metallurgical Engineering, University of Queensland, is Lecturer in Metallurgy, Choong-nam National University, Tae Jeon, Republic of Korea. P.C. HAYES, Associate Professor in Extractive Metallurgy, is with the Department of Mining and Metallurgical Engineering, University of Queensland, St. Lucia, Brisbane, Queensland 4067, Australia. Manuscript submitted June 19, 1989. METALLURGICAL TRANSACTIONS B
Dense 1-mm 3 oxide samples were prepared and reduced in a furnace designed to give high local gas velocities over the sample surface. The high gas velocities overcame gas film mass transfer limitations, and the small sample sizes reduce the risk of variations in gas composition from sample surface to reaction interface. The samples are quenched into liquid nitrogen to preserve the interface structure for analysis by scanning electron microscopy. In H2/H20 and CO/COz mixtures, experiments have been carried out at a number of reduction potentials, marked "A," "B," "C," "D," and "E," in Figures 3, 5, and 9. Each of these reduction potentials represents conditions of constant chemical driving force with respect to t
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