Thermodynamic estimation on the reduction behavior of iron-chromium ore with carbon
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INTRODUCTION
Recently, a number of efforts have been made to produce a crude stainless steel melt by direct smelting of ironchromium ore in a basic oxygen furnace (BOF) without use of ferrochromium alloys, in order to save electric energy and production costs.[1–4] It is necessary to have information on the thermodynamic properties of chromium ore for a better understanding of this new process, especially the prereduction of ore in the process. It is known that the ironchromium ore generally consists of Fe3O4, FeO z Cr2O3, MgO z Cr2O3, MgO z Al2O3, and (Cr,Al)2O3, and these components form a complex spinel-structure solid solution. Since Fe3O4 in the ore would be easily reduced by the solid carbon or reducing gas, iron-chromium ore can be considered as a FeO z Cr2O3-MgO z Cr2O3-MgO z Al2O3 pseudoternary spinel-structure solid solution coexisting with (Cr,Al)2O3 solid solution in the prereduction process. The raw average compositions of iron-chromium ore mined in the world are actually on the plane of the FeO z Cr2O3-MgO z Cr2O3-MgO z Al2O3 pseudoternary in the compositional tetrahedron of the FeO-MgO-Cr2O3-Al2O3 system, as shown in Figure 1. This indicates that the iron-chromium ore in the prereduction process can be reasonably approximated as the (Fe,Mg)O z (Cr,Al)2O3 pseudoternary iron-chromite spinel-structure solid solution, and the thermodynamics of this iron-chromite solid solution is important for the reduction of iron-chromium ore. However, there is little activity data on the iron-chromium ore itself available in the literature.[5,6,7] MITSUTAKA HINO, Professor, and TETSUYA NAGASAKA, Associate Professor, are with the Department of Metallurgy, Graduate School of Engineering, Tohoku University, Sendai 980-77, Japan. KENICHI HIGUCHI, formerly Graduate Student, Department of Metallurgy, Graduate School of Engineering, Tohoku University, is Researcher with the Ironmaking Laboratory, R&D Bureau, Nippon Steel Corporation, Futtsu 299-12, Japan. SHIRO BAN-YA, Professor Emeritus, Department of Metallurgy, Graduate School of Engineering, Tohoku University, is President, Akita National College of Technology, Akita 011, Japan. Manuscript submitted March 7, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS B
The activities of constituents in (Fe,Mg)O z (Cr,Al)2O3 iron-chromite spinel-structure solid solution had been previously investigated by the authors.[8,9] In the first step of the previous work,[8] the authors had determined the free energy of formation of FeO z Cr2O3 from solid iron, O2 gas, and solid Cr2O3 in the temperature range of 1423 to 1723 K. The phase equilibria and the activity of FeO z Cr2O3 in FeO z Cr2O3-MgO Cr2O3 binary solid solution saturated with Cr2O3 had also been measured at 1573 K. In the second step, the activity of FeO z Cr2O3 in FeO z Cr2O3-MgO z Cr2O3-MgO z Al2O3 pseudoternary iron-chromite spinelstructure solid solution saturated with (Cr,Al)2O3 solid solution was measured and the mutual solid solubilities between the two phases were determined at 1573 K. It has been shown from the res
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