Effect of silica on reduction of calcium ferrite with CO-N2 gas mixtures

Abstract

Silica is a significant chemical component in the formation of SiO2-Fe2O3-CaO-Al2O3 from CaO·Fe2O3 (CF). The influence of silica on the reducibility of the CaO-Fe2O3-SiO2 system was fully examined in this study. The isothermal reduction kinetics of CF, CF2S, CF4S, and CF8S were investigated through thermogravimetric analysis at 1 123, 1 173, and 1 223 K with 30% CO and 70% N2 gas mixtures. CF2S and CF4S presented a slight degrader in reduction degree but activated the reaction faster than CF reduction. The reduction of the samples with 8% silica was not only highly accelerated but also proceeded easily. Rate analysis revealed that CF and CF8S reduction occurs in two stages, whereas CF2S and CF4S reduction occurs in three stages, the Fe3O4-to-FeO stage overlaps with the previous Fe2O3-to-Fe3O4 stage and tends to approach the following FeO-to-Fe stage with an increase in silica content. The apparent activation energy values of CF, CF2S, CF4S and CF8S reduction are 46.89, 24.48, 44.84, and 8.71 kJ·mol-1. CO diffusion is the rate-controlling step of the entire process of CF8S reduction, whereas the rate-controlling step of CF, CF2S and CF4S reduction are performed as inner gas diffusion, inner gas diffusion and interface chemical reaction mixed control then interface chemical reaction in turns with reduction degree increasing. Sharp analysis indicated that CF and CF8S reduction was expressed by the Avrami-Erofeev equation presenting a 2D shrinking layer reaction, whereas CF2S and CF4S reduction was expressed initially by a 2D shrinking layer reaction and then by a 2D phase boundary-controlled reaction.