Formation of a Network Structure in the Gaseous Reduction of Magnetite Doped with Alumina
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A study of the reduction of magnetite doped with alumina by CO–CO2 gas mixture[1,2] has demonstrated that alumina has a strong effect on the magnetite reduction. Magnetite and alumina form a magnetite–hercynite solid solution Fe3O4–FeAl2O4, which has a miscibility gap with a critical temperature of 1133 K (860 C). The reduction of magnetite from this solution to wu¨stite occurred along certain lattice planes with the formation of a network-like structure. With the progress of the magnetite reduction, the Al2O3 content in the solid solution gradually increased and finally reached to the miscibility gap (the solubility limit). Then the magnetite reduction stopped. After that, the reduction of the formed wu¨stite in the plates of the network structure started and was converted to metallic Fe. The formation of lamellae or network-like structure was also observed by Paananen et al.[3] during the reduction of magnetite doped with 3.78 mass pct of Al2O3 at 1223 K (950 C) by CO–CO2 gas mixture (90 vol pct CO). The network structure is observed in ironsand (titania-ferrous ore) and other minerals.[4–9] The formation of the network structure in minerals is generally explained by the exsolution mechanism, e.g., exsolution of titano-hematite solution formed by the weathering of YURY KAPELYUSHIN, YASUSHI SASAKI, JIANQIANG ZHANG, and OLEG OSTROVSKI are with the School of Materials Science and Engineering, UNSW Australia, Sydney, NSW 2052, Australia. Contact e-mail: [email protected] SUNKWANG JEONG is with Technical Research Laboratories, POSCO, South Korea. Manuscript submitted September 5, 2016. Article published online January 3, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS B
ironsand from the titano-magnetite solution.[4] Exsolution lamellae are often planar and their orientation is crystallographically controlled. Mechanism of formation of the network-like structure in the reduction of magnetite doped with alumina has not been established. The reduction of un-doped magnetite proceeds topochemically with the formation of a dense iron layer [with CO–CO2 gas mixture (80 vol pct CO) at 1023 K and 1123 K (750 C and 850 C)],[1,2] which hinders further reduction. However, the formation of a network-like structure in the reduction of magnetite doped with alumina to wu¨stite was found to enhance the reduction process.[1] Therefore, establishment of the network formation mechanism is important for further understanding of reduction reactions and industrial applications. The aim of this study is to investigate the mechanism of formation of the network structure in the reduction of magnetite doped with alumina by CO–CO2 gas mixture.
II. EXPERIMENTAL STUDY OF REDUCTION OF MAGNETITE DOPED WITH ALUMINA USING CO–CO2 GAS MIXTURE The reduction of un-doped magnetite and magnetite doped with alumina was studied in the fixed bed reactor[1] and using in situ high-temperature XRD (HT-XRD) analysis.[2] The experimental set-ups and procedures were presented elsewhere.[1,2] High-purity magnetite (97.7 mass pct) and alumina powders were mixed (
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