EFFECT OF VOLATILE MATTER OF COAL ON VOLUME CHANGE AND REDUCTION OF IRON ORE/COAL COMPOSITE PELLETS

摘要

In the ironmaking processes via the carbothermic reduction with the tall-bed reactor, the composite pellets can be rapidly reduced to highly metallized direct reduced iron (DRI) with high productivity under high temperature. However, the volume change of the pellets during the reduction will affect the efficiency of heat transfer from the freeboard to the tall-bed pellets inside the reactor. In this study, coals with different volatile matter (VM) content but with fixed C/O =0.95 were used to prepare the composite pellets for the reduction experiments. The in-situ digital camera was used to record the morphological changes and volume change of pellets throughout the reduction experiments. The microstructure and phase composition of DRI specimens were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The metallization degree of DRI specimens was also analyzed. Differential scanning calorimetry (DSC) was applied to investigate the characteristics of VM pyrolysis in coal. In this study, it was shown that the swelling of pellets could be resulted from the quick and large VM released from the coal pyrolysis due to the rapid rise of internal gas pressure of pellets. It was also found that the shrinkage degree of pellets increased as the VM content of the coal increase. And, more metallic iron grains can be found in the microstructure of the DRI specimens reduced from the composite pellets with high VM coal.