The influence of specific impurities on the nucleation and growth of magnetite during reduction of artificially prepared
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INTRODUCTION
H E M A T I T E (a-Fe203) is reduced in a blast furnace through two intermediate oxide stages, magnetite (Fe304) and wustite (Fel.xO), into pig iron. The last reduction stage, from wustite to iron, is the main step for the whole reduction reaction. [q During this step the major part of the bound oxygen is removed. This is one of the reasons why this step has attracted so many investigators. On the other hand, one cannot neglect the importance of the two earlier stages of reduction because of their influence on the progress of the overall reaction. During the first reduction step, in which hematite is converted into magnetite by carbon monoxide, several changes of varying importance take place, which are typical for this transition. First of all, the main crystallographic transformation occurs during this first reduction step; the hexagonal hematite is converted into cubic magnetite, which has an inverse spinel structure. Second, depending on both reduction temperature and gas composition, there is a difference in morphology of the magnetite formed, i.e., magnetite can occur with a porous morphology, or as dense plates (lamellae), and these morphologies can transform into each other. Following discrepancies among reports of various authors tz-sl regarding the exact conditions for the formation of porous magnetite or dense magnetite lamellae, Et-Tabirou et al.[61 conclusively established that lamellar magnetite is favored at high temperature and low CO content of the gas. At low temperature and higher CO content, the porous form of magnetite is formed. The pores in this structure are very small and vary from several nanometers up to 0 . 1 0 0 / z m . A third morphology, resembling the porous one, is the formation of a porous magnetite exhibiting severe cracking, tl,61 It is favored at low temperatures and low CO content of the reducing gas. Finally, a volume increase of the formed magnetite in comparison with the volume of the original hematite can be calculated and has been observed by several authors I3,4'71 during reduction experiments. Calculated valR. CHAIGNEAU, Research Assistant, and R.H. HEEREMA, Professor, are with the Department of Raw Materials Technology, Delft University of Technology, Delft, The Netherlands. Manuscript submitted November 14, 1990. METALLURGICAL TRANSACTIONS B
ues indicate a range in volume increase for single crystals between 10 and 12.5 pct v / v , depending on the data used and method of calculation. Measured values on single crystals are somewhat lower, ranging between 8 and 10 pct v / v . An explanation for this small discrepancy in the volume increase may be related to the sintering of the pores during the reduction process. The potential breakdown of iron-bearing raw materials as a result of such expansion is tested for, in practice, with ISO standard test #4698 for low temperature disintegration. Aside from the above three changes, two additional phenomena occurring during this transition, based on observation, are reported. The first phenomenon is the existenc
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