Metallic Iron Formation during Iron Ore Reduction

　Here we refer to the difference between normal swelling and abnormal swelling. The normal swelling is a phase transformation from hematite of hexagonal structure to magnetite of cubic structure, and the experiments by Edström, using hematite-single crystal, reported that the coefficient of volume expansion of hematite in the process of reduction from Fe₂0₃ to Fe₃0₄ was 25%. And when the reduction process further proceeds from FeO(in the following FeO indicates Wustite Fe_1-yO, where Y hold concentration) to metallic iron(hereinafter,abbreviated to M. Fe), the volume of hematite single crystal shrunk reversely. The abnormal swelling is a phenomenon where the sample of iron ore swells greatly like cotton in the process of reduction from FeO to M. Fe, and coefficient of volume expansion reaches over 400% under some conditions. It is obvious that this phenomenon has an ill effect on the blast furnace operation. It is considered to be said that abnormal swelling is briefly called “Swelling”,and the expansion whose coefficient of volume expansion in the reduction process is more than 20%, is called abnormal(abnormal or catastrophic) swelling. In short,the expansion in the reduction process to Fe₃0₄ and FeO is not called abnormal swelling. Also, abnormal swelling is obviously observed when iron ore is reduced by CO, while it is generally not observed when the ore reduced by H₂.