Diffusion and reaction mechanism of limestone and quartz in fluxed iron ore pellet roasting process

The increase to the proportion of fluxed pellets in the blast furnace burden is a useful way to reduce the carbon emissions in the ironmaking process. In this study, the interaction between calcium carbonate and iron ore powder and the mineralization mechanism of fluxed iron ore pellet in the roasting process were investigated through diffusion couple experiments. Scanning electron microscopy with energy dispersive spectroscopy was used to study the elements’ diffusion and phase transformation during the roasting process. The results indicated that limestone decomposed into calcium oxide, and magnetite was oxidized to hematite at the early stage of preheating. With the increase in roasting temperature, the diffusion rate of Fe and Ca was obviously accelerated, while the diffusion rate of Si was relatively slow. The order of magnitude of interdiffusion coefficient of Fe₂O₃–CaO diffusion couple was 10⁻¹⁰ m²·s⁻¹ at a roasting temperature of 1200°C for 9 h. Ca₂Fe₂O₅ was the initial product in the Fe₂O₃–CaO–SiO₂ diffusion interface, and then Ca₂Fe₂O₅ continued to react with Fe₂O₃ to form CaFe₂O₄. With the expansion of the diffusion region, the sillico-ferrite of calcium liquid phase was produced due to the melting of SiO₂ into CaFe₂O₄, which can strengthen the consolidation of fluxed pellets. Furthermore, andradite would be formed around a small part of quartz particles, which is also conducive to the consolidation of fluxed pellets. In addition, the principle diagram of limestone and quartz diffusion reaction in the process of fluxed pellet roasting was discussed.