Understanding of the Reduction Behavior in Direct Reduction of High-Phosphorus Oolitic Hematite Using a High Volatile Bituminous Coal

Abstract

The utilization of direct reduction roasting and magnetic separation is an efficient method to treat the high-phosphorus oolitic hematite (HPOH). The reduction behavior using a low-rank bituminous coal was compared with a coke. The related mechanisms were investigated using thermogravimetric–differential scanning calorimetry analysis (TG-DSC), X-ray diffraction (XRD), and scanning electron microscope–energy dispersive spectroscopy (SEM-EDS). The findings revealed that the total iron (TFe) grade and TFe recovery rate of direct reduced iron (DRI) produced using low-rank bituminous coal were slightly lower than those produced using coke. The utilization of 25% low-rank bituminous coal combined with 30% limestone resulted in high-quality DRI, with TFe grade of 93.44 wt.%, TFe recovery rate of 93.31%, and phosphorus (P) content of 0.18 wt.%. The low-rank bituminous coal with a high H element content of 5.76 wt.% and volatile content of 36.81 wt.% released a lot of volatiles, predominantly comprising H₂, gaseous hydrocarbons, and CO, which were employed for the reduction of HPOH. The mixture of low-rank bituminous coal and HPOH sample showed higher weightlessness rate below about 800°C, indicating a greater reactivity. Above 800°C, coke with higher fixed carbon (97.93 wt.%) displayed enhanced reactivity, primarily through C and CO to reduce HPOH. The P content of DRI produced using low-rank bituminous coal was lower than that using coke. High quartz content in the inorganic component of coke would promote the reduction of phosphorite and entered metal iron to form Fe-P alloy. The calcite presented in low-rank bituminous coal decomposed CaO, which acted as the dephosphorization agent to inhibit the reduction of phosphorite. This study established a research foundation for the direct reduction of HPOH utilizing low-rank bituminous coal as reductant.