2004 Volume 44 Issue 12 Pages 1999-2007
Reactions in carbonaceous material composite iron ore pellets of 18 mm heated at elevated temperatures were studied. The overall reduction rate is controlled by a complex combination of direct reactions and indirect reactions and never by gasification reaction. The final reduction of iron oxide is controlled by carburized carbon. The carburization of the reduced iron is mainly progressed by the direct carburization reaction, C(s)=[C]. As the carbon content approaches to the solidus line of the phase diagram of Fe-C system, the reduced iron particles are gradually forced to coalesce to grow in much larger particles. The coalescing rate is increased with temperature. Its driving force is the large surface energy of reduced iron due to its large specific surface area. Due to the coalescence, the particles of carbon are apt to lose the contact state with the reduced iron particles and to lose the ability to carburize the reduced iron. Therefore, the carburization exceeding the solidus line can be severely depressed. Consequently, the carburization up to the solidus line can be achieved at relatively lower temperatures, i.e. 1200-1250°C. As a result, the carbon content of reduced iron decreases with increasing temperature. Since the coal char has a very poor ability to directly carburize reduced iron, the carbon contents of reduced iron were very low and increased with increasing temperature. Graphite and coke possess the strongest and the intermediate carburizing abilities, respectively. And the carbon contents of the shell part and core part of reduced iron decreases with increasing temperature.