Fundamental Study of Size Degradation Mechanism of Agglomerates during Reduction

Abstract

The mechanism of degradation of agglomerates during reduction is studied with pellets made from powdery ingredients of chemical reagent grade, and the changes in mineral phases due to certain oxides leading to degradation, are examined quantitatively.The results obtained are summarized as follows:(1) Microscopic observations suggest that cracking occurs in the skeletal hematite and the calcium ferrite phases. Analysis by line-broadening measurements of X-ray diffraction reveals no strain both in the bulky and in the skeletal hematites regardless of the amount of alumina held in solid solution. Micropores are generated in magnetite as it is reduced from hematite to give rise to a stress when reduced at a low temperature, and stress concentration occurs around inclusions of small radius of curvature to cause cracking of skeletal hematite grains.(2) The quaternary calcium ferrite is expected to be reduced to metallic iron in the blast furnace through dicalcium ferrite or wustite. Nonferrous oxides form gehlenite and β-dicalcium silicate. Calcium ferrite is not only fragile but its amount to increase with the basicity under existence of Al2O3.(3) The amount of skeletal hematite depends mainly on the degree of supersaturation of iron ions in slag and on the dissociation temperature of hematite. The addition of TiO2 in sinters enhanced greatly the crystal growth of skeletal hematite in the case of induration at above THM, whereas bulky hematite at below THM.