Reduction kinetics of MgO-doped calcium ferrites under CO–N2 atmosphere

Abstract

The effect of magnesia on calcium ferrite (CaO·Fe2O3) reduction by CO was examined by isothermal thermogravimetry. Samples of calcium ferrite added with 0, 2, 4, and 8 wt.% magnesia (abbreviated as CF, CF2M, CF4M, and CF8M) were prepared. Phase composition was analyzed by X-ray diffraction, and the results indicated that CF2M and CF4M are reduced to lower reduction degree and with lower apparent activation energy than CF; and CF8M with more MgO·Fe2O3 is reduced to a lower degree and with more difficulty compared with CF. Reduction rate analysis revealed that CF, CF2M, CF4M, and CF8M reductions are all typical two-step reactions with the order of CF → CWF (CaO·FeO·Fe2O3) → Fe. The apparent reduction activation energies of CF, CF2M, CF4M, and CF8M are 46.89, 37.30, 17.30, and 29.20 kJ/mol, respectively. Sharp analysis depicted that CF2M, CF4M, and CF8M reductions are all described by 2D Avrami–Erofeev (A–E) equation (A2) in the whole process, while CF reduction is first expressed by A2 and then by 3D A–E equation (A3). Different from shrinking core model, a new kinetic model for powdery samples reduction was proposed to illustrate the relationship among reduction rates, reduction routes, and model functions.