Mechanism of Abnormal Swelling in the Process of Prereduced Pellet Firing

　An investigation has been made on the growth mechanism of metallic filaments formed at the reduction stage of FeO to Fe, in the process of prereduced pellet firing. Six kinds of iron ore were tested in the respective reducing atmosphere of CO and H₂. Using a scanning electron microscope, an observation was made on the phase of ore during reduction. In H₂ atmosphere, no abnormal swelling occurred. But, in CO atmosphere, abnormal swelling occurred in pellets due to the formation of metallic filaments.
　There is a big difference in microstructure between ore having swelling properties and ore having no such properties. The former usually shows a fine and orderly phase such as intergrowth, and the latter is usually of irregular mosaic structure. Prior to swelling, fine steps parallelled to (111) of FeO appear during the period of reduction to Fe. At an early stage of reduction of FeO to Fe in CO atmosphere, cracking takes place, and then ore particles break down. After further reduction, neeale-like FeO phase seems to change into fibrous iron. The microscopic observation revealed that there were considerabele destorted and branched filaments but little straight whisker-like iron crystals. An electron diffraction pattern of filament shows the (111) plane of α-Fe, which coincides with the plane of steps displayed on FeO. The carbon combined with metallic iron is contained in much higher percent in the metallic filament than in granular iron produced by topochemical reduction of FeO.