Effects of Temperature and Gas Composition on Reduction and Swelling of Magnetite Concentrates
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NTRODUCTION
HIGH-QUALITY hematite ores have iron content 56 to 64 pct. However, steel industry is facing the depletion of high-grade ores and easily accessible hematite ore deposits, which stimulates a more broad utilization of magnetite ores. In general, magnetite ores have relatively low iron content (25 to 40 pct), and must be concentrated before they can be used in ironmaking. Despite the additional cost involved in the ore beneficiation, magnetite ore will continue to emerge as an important source of iron. Concentration of iron in magnetite concentrates is comparable with that of high-grade hematite ores; magnetite concentrates also contain impurities of SiO2, Al2O3, S, and others.[1] It is known that these impurities affect not only reduction kinetics but also swelling and degradation of magnetite pellets. A strong effect of Al2O3 on the reduction behavior of chemical grade magnetite was demonstrated in previous studies.[2–4] Paananen et al.[2] found that the addition of Al2O3 caused swelling and degradation of magnetite briquettes during the reduction at 1223 K (950 C), while it was
YURY KAPELYUSHIN, Ph.D. Student, YASUSHI SASAKI, Visiting Professor, JIANQIANG ZHANG, Associate Professor, and OLEG OSTROVSKI, Emeritus Professor, are with the School of Materials Science and Engineering, UNSW Australia, Sydney, NSW 2052, Australia. Contact e-mail: [email protected] SUNKWANG JEONG, Principal Researcher, is with Technical Research Laboratories, POSCO, Pohang, South Korea. Manuscript submitted March 14, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
not observed at 1023 K to 1123 K (750 C to 850 C).[3,4] Obviously, the effects of impurities depend on the chemical and physical properties of magnetite samples and reduction conditions. The swelling mechanism and effects of swelling on the reduction kinetics have not been well understood. Very often swelling of pellets is attributed to the whiskers formation. However, they are not always observed. Other mechanisms have been proposed, such as the breakdown or bursting of a dense iron layer formed in the process of the ore reduction. Wieberg[5,6] observed the bursting of the iron layer in the reduction of magnetite by CO-H2 gas mixture in 1940. Edstro¨m[7] came to the conclusion that cyclic reactions of dissolved carbon with CO2 and the reduction of wu¨stite by CO increase pressure in the voids between wu¨stite and iron which caused the breakout of the iron layer. Rieke and Bohnenkamp[8] calculated that the total pressure in the voids at the iron/wu¨stite interface at 1163 K (890 C) is 4 to 5 atm, which is in excess of the critical pressure at which the iron layer bursts. St. John[9] also considered the bursting of the dense iron layer using the Edstro¨m model. Nasar et al.[10] showed that the onset of swelling corresponded to the metallic iron formation. The aim of this paper is to study the effects of temperature and gas composition on the reduction of magnetite concentrates and swelling observed in the reduction process.
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EXPERIMENTAL
In this paper, two m
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