Effects of CaO/CaCO 3 on the Carbothermic Reduction of Titanomagnetite Ores
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DUE to the depletion of high grade hematite ores, it has been undertaken to utilize magnetite ores in the conventional blast furnace ironmaking or direct reduced iron (DRI) production.[1] The proportion of DRI usage in electric arc furnace (EAF) operation has been increasing because high quality and low-residual scrap, which had traditionally been used in EAFs to produce high quality steel, is in shortage.[2] The gas-based DR processes such as MIDREX or HYL III are becoming more competitive due to the supply of shale gas at lower price. Nevertheless, coal-based DRI processes such as rotary kiln or rotary hearth furnace are still receiving great attention for producing flat product of steel using virgin iron.[1] However, extensive understanding of DRI production technology using magnetite ores is limited in alternative ironmaking processes from a viewpoint of catalytic additives. The fundamental investigations on the kinetics and mechanism of carbothermic reduction of iron oxide were extensively carried out for the carbon composite iron oxide agglomerates. Rao[3] found that the availability of CO governs the reduction process in hematite and carbon powder mixtures and that the carbon gasification reaction constitutes the rate-limiting step for the overall process. Furthermore, he investigated the effect of addition of promotive (Li2O) or inhibitive (FeS) SUNG-MO JUNG, Associate Professor, is with the Graduate Institute of Ferrous Technology, POSTECH, Pohang 790-784, Korea. Contact e-mail: [email protected] Manuscript submitted February 3, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
reagent on reduction kinetics. Fruehan[4] confirmed that the overall rate of carbothermic reduction of Fe2O3 is controlled by the carbon gasification. He also reported that the iron formed by the reduction does not significantly catalyze the carbon gasification. Szendrei and Berge[5] studied the reduction of hematite and graphite composite using thermogravimetric analysis (TGA) equipped with differential thermal analysis, simultaneously conducting direct in situ analysis of reaction product gases during reduction. They concluded that the catalytic effect of Na on solid direct reduction derives primarily from its acceleration of the rate of carbon gasification. Deventer and Visser[6] showed that the activation energies of both gasification and reduction were decreased by the addition of either Na2CO3 or K2CO3 and that K2CO3 is more effective catalyst than Na2CO3. And they reported that the apparent activation energy of reduction is on the average 29 pct less than that for the corresponding gasification reaction. Recently, Chen et al.[7] showed that the metallization degree of titanomagnetite (TTM) concentrates are significantly improved by the pre-oxidation and by adding additives such CaF2, Na2CO3, and Borax in the solid state reduction of TTM ores with pulverized coal. In most of the previous researches, continuous change of fractional reduction of raw iron ores and char composite was not clearly evaluated in terms of the weight change
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