Effect of Alumina on the Gaseous Reduction of Magnetite in CO/CO₂ Gas Mixtures

Abstract

Reduction of magnetite doped with alumina (3, 6 and 12 mass pct Al₂O₃) was studied using CO/CO₂ gas mixture (80 vol pct CO) at 1023 K and 1123 K (750 °C and 850 °C). The reduction rate and degree of reduction were evaluated from the weight loss of a sample with time. The reduction behavior was analyzed using the results of XRD and SEM–EDS measurements and thermodynamic analysis. Effect of alumina on the magnetite reduction depended on the alumina content and temperature. Magnetite reduction at 1023 K (750 °C) was accelerated by the addition of 3 mass pct Al₂O₃, however, the rate of reduction significantly decreased with the further increase in the alumina content to 6 and 12 mass pct. Different effect of alumina was observed in reduction at 1123 K (850 °C); the rate of reduction of the Fe₃O₄-Al₂O₃ mixture with 6 mass pct Al₂O₃ was the fastest. Reduction of un-doped magnetite was developed topochemically with the formation of a dense iron shell. However, reduction of alumina-doped magnetite to wüstite started along certain lattice planes with the formation of network-like structure. In the course of reduction, Al³⁺ ions diffused from wüstite to the Fe₃O₄-FeAl₂O₄ solution enriching hercynite content in the solution at the reaction interface. Further reduction of alumina-rich Fe₃O₄-FeAl₂O₄ solution resulted in the formation of micro-cracks which enhanced the rate of the reduction process.