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UCTION

THE reduction behavior of the ferrous burden in a blast furnace greatly influences both its fuel consumption and productivity. For these reasons, extensive research has been conducted on the reduction processes involved.[1–4] Due to the variation in iron ore composition, including mineral content, the study of the reduction phenomena has previously been focused mainly on the effect of various impurities on the rate of reduction. Typical impurities include CaO, SiO2, MgO, Al2O3, and Na2O, among others. During the sintering process, these impurities may react with, or be dissolved in, the iron oxides, forming complex combinations of crystalline and glassy phases, which can affect the reduction behavior. El-Geassy et al.[5–9] showed that CaO accelerated the reduction of Fe2O3, reaching a maximum and then subsequently decreasing with further addition of CaO. In addition, the reduction of Fe2O3 in the presence of MgO was found to be impeded at the final stage due to the formation of the thermodynamically stable magnesiowu¨stite. Shigematsu and Iwai[10] found that the reduction of dense wu¨stite in H2 below 1373 K (1100 C) was accelerated by small concentrations of SiO2 and/or CaO, but this effect was negligible above 1473 K (1200 C). The influence of the addition of Al2O3 on iron oxide reduction has also been studied.[11–13] The reducibility and the efficiency of the reaction were observed to increase with Al2O3 addition. Thus, it is clear that much work has been carried out on the effects of the major impurities such as WEN PAN, ZE JUN MA, and ZHI XING ZHAO, Researchers, are with the Shougang Research Institute of Technology, Beijing 100041, P.R. China. WAN HO KIM, Graduate Student, and DONG JOON MIN, Professor, are with the Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Korea. Contact e-mail: [email protected] Manuscript submitted March 18, 2012. Article published online October 9, 2012. 1326—VOLUME 43B, DECEMBER 2012

CaO, SiO2, MgO, and Al2O3. However, the effect of alkali oxides, such as Na2O, on the reduction process has yet to be fully understood, even though Na2O content can significantly increase as the quality of iron ore decreases. Barnes and Pickles[14] found that the addition of Li2CO3, Na2CO3, and K2CO3 to ilmenite briquets at 1073 K to 1223 K (800 C to 950 C), using CO gas to increase the reduction rate, was more pronounced with increasing atomic radius of the alkali cation. Sayama et al.[15] found that Na2O and K2O promoted abnormal swelling during the reduction of single-crystal Fe2O3, which could increase the pore gas diffusion and kinetics of reduction. However, the direct cause of the alkali effect on the reduction remains to be fully understood. In the current study, the influence of Na2O on the reduction behavior of Fe2O3 compacts was examined from 1073 K to 1273 K (800 C to 950 C) under a CO/ CO2 gas mixture using thermogravimetric analysis. In addition, morphological observations of quenched, reduced samples were carried out using scanning electron microscopy