Effect of iron ore type on the reduction behavior at multi-stage reduction by H2

The fluidised bed reduction process is noted as one of the representative clean ironmaking process to replace the blast furnace. Since the multi-stage fluidise bed reduction process operate without sintering, it is necessary to clarify the effect of iron ore properties on the reduction behaviour. The current study employed the thermogravimetric analyses to characterise the four-stage reduction of four kinds of iron ores which are classified by phase of iron oxide and gangue amount. At the first reduction stage where iron ores are reduced to magnetite, iron ore of larger surface area showed higher reduction rate, which is in good agreement with the kinetic analysis results. The diffusion was identified to the rate-controlling step where the pores in the ores acted as channels for hydrogen diffusion into the internal core of the ores. However, the final reduction degrees (RDs) evaluated after four-stage reduction were not correlated with the surface area. The kinetic analyses identified the reduction in wustite to metallic iron to be controlled by chemical reaction. As the amount of SiO2 in the ores increased, the final RDs were relatively smaller. According to X-ray diffraction, it was observed that SiO2 reacted with wustite during reduction to metallic iron to cause the decrease in crystallite size and distortion in the lattice structure of wustite. This was supported by the elemental distribution measured employing electron probe micro-analysis, that is, the area containing Si and Fe in the reduced iron ore expanded as SiO2 amount increased. SiO2 reacted with wustite to form fayalite of low reducibility compared with wustite, which resulted in decreased degree of final reduction. The current study measured that the gangue within the iron ore played a significant role in the reduction of wustite. In addition, different rate-determining steps were identified for each reduction stage, which might be utilised for determining suitable reduction conditions for each stage.