Isothermal Reduction Kinetics of Calcium Ferrite Containing FeCr₂O₄ from Chromite Ore Processing Residues Under CO–N₂ Atmosphere

The treatment of chromite ore processing residues (COPR), primarily composed of FeCr₂O₄, by the sintering process significantly influences the metallurgical properties of the resulting sinter, such as reduction index. In this study, thermogravimetric analysis (TGA) combined with first-principles calculations was employed to investigate the isothermal reduction kinetics of calcium ferrite (CaFe₂O₄, CF) in the presence of FeCr₂O₄ at 1073 K, 1173 K, and 1273 K, as well as the effect of FeCr₂O₄ addition in the CF on the reduction behavior. The results indicate that the incorporation of FeCr₂O₄ into calcium ferrite decrease the reduction rate and extent of reduction. When the FeCr₂O₄ addition increases from 0 to 10 wt pct, the reduction of calcium ferrite exhibits two distinct kinetic stages. Model-fitting method analysis reveals that the reduction process is initially controlled by interfacial chemical reactions and subsequently by diffusion for CF with less than 7 wt pct FeCr₂O₄ content. In contrast, at FeCr₂O₄ contents of 7 wt pct or higher, the reduction is governed by nucleation and growth mechanisms in the primary stage, followed by solid state diffusion control. Both the model-fitting method and model-free method indicate that the apparent activation energy of calcium ferrite reduction exhibits an overall increasing trend with increasing FeCr₂O₄ content. After Cr was incorporated into the CaFe₂O₄ crystal structure, the CO adsorption energy increased from −1.65 to −0.21 eV, indicating that CO adsorption on Cr-containing calcium ferrite became less favorable.