Non-isothermal reduction kinetics of calcium ferrite and hematite

The reduction behaviors of sinter and lump ores, which account for 90% of the raw materials charged into blast furnace process, are important to coke reduction in the ironmaking industry. The isothermal reduction behaviors of sinter and lump ores have been extensively studied; however, non-isothermal conditions, which are more consistent with the temperature change characteristics in a blast furnace, have been rarely investigated in terms of their reduction processes. Studies on the reduction behaviors of calcium ferrite and hematite, which are individually contained in sinter and lump ores, can provide more significant guidance to the practical operation. Comparisons of the reduction behaviors of calcium ferrite and hematite involves thermodynamic parameters such as the starting reduction temperature and kinetic parameters such as activation energy and model function. Considering reduction processes, hematite has been clearly explored by numerous works to an extent far more than calcium ferrite. In addition, in studying the reduction behaviors of calcium ferrite and hematite, pellet samples of 1-100 mm were the focus in the past and rarely micron-sized powder samples (1-100 μm). However, nowadays, micron-sized particles are extensively applied on iron ores reduction in fluidized bed process and non-blast furnace process, such as FINEX^® method; therefore, in this study, calcium ferrite and hematite were compared, considering their reduction routes and reduction kinetics. Non-isothermal reduction experiments of powdery calcium ferrite and hematite heated up to 1123 K with a rate of 10 K·min^-1 in a continuous stream of 30% CO and 70% N₂ were conducted through thermo-gravimetric analysis. The results show that the reduction processes of calcium ferrite and hematite begin at 873 K and 623 K, respectively. Reduction rate analysis and subsequent X-Ray diffraction measurements at various stages reveal that the reduction of calcium ferrite can be divided into two steps:CaO·Fe₂O₃ → 2CaO·Fe₂O₃ → Fe, whereas that of hematite mainly comprises three steps:Fe₂O₃ → Fe₃O₄ →FeO → Fe. The activation energy was calculated by Freeman-Carroll method, and the average values of calcium ferrite and hematite reduction are 49.88 and 43.74 kJ·mol^-1, respectively. The reduction of calcium ferrite can be described by random instant nucleation and two-dimensional growth of nuclei model; its corresponding model function is Avrami-Erofeev equation with an integral form of-ln(1-α)ⁿ, whereas the reduction of hematite was initially expressed by a tertiary chemical reaction model (reduction degree α=0.1~0.5) with an integral form of 1-(1-α)³, and subsequently by a two-dimensional cylindrical diffusion model (α=0.5~0.9) with an integral form of α+(1-α) ln(1-α).