TTT Diagram of 10 pct CaO-90 pct Fe 2 O 3 in Low-Temperature Region Below 1499 K
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TRODUCTION
THE role of the sintering process of iron ores has increased in recent times due to the yearly decrease in their quality. The increase in the proportion of fine ores necessitates the need for a crashing process to remove the gangue components. An important property of a sinter is its ability to withstand the load in a blast furnace (BF). Hence, the main reaction of a sintering process occurs between CaO and Fe2O3, where the combustion of coke yields a high temperature and a liquid phase of the CaO-FeO-Fe2O3 system. During cooling, compounds among CaO, FeO, and Fe2O3 precipitate, thereby affecting the quality of the sinter. Actually, gangue components such as SiO2, Al2O3, and MgO has an important role in determining the strength and reducibility, in which the microstructure consists of a CaO-Fe2O3-SiO2-Al2O3 system referred to as ‘‘SFCA’’ (silico-ferrite of calcium and aluminum). Although the formation reaction of SFCA
YOSHIAKI KASHIWAYA is with the Department of Energy Science and Technology, Graduate School of Energy Science, Kyoto University, Yoshida Honmachi, Sakyoku, Kyoto, Kyoto Fu 606-8501 Japan. Contact e-mail: [email protected] SOTA YANAI is with the Faculty of Engineering, Kyoto University. Manuscript submitted March 11, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
is complicated, even a reaction between CaO and Fe2O3 is also complicated. Because the CaO-Fe2O3 binary system changes to the CaO-FeO-Fe2O3 ternary system during melting under air atmosphere (Eq. [1]), as shown by Kashiwaya.[1] Furthermore, the FeO compound in the melt is changed based on the distance from the coke combustion front due to a lower oxygen potential produced by the combustion in the sinter bed. In addition, the formed FeO is re-oxidized to Fe2O3 during cooling, such that Eq. [1] proceeds to the left-hand side. However, since the diffusion of oxygen controls the reaction, the system cannot attain its equilibrium state of the CaO-Fe2O3 binary system, and many compounds of the CaO-FeO-Fe2O3 system are produced during cooling.[2] Hence, the phenomena should be considered using time-temperature-transformation (TTT) and/or continuous cooling transformation (CCT) diagrams since the reaction system is time-dependent. 1 Fe2 O3 $ 2FeO þ O2 2
½1
In this study, a 10 pct CaO-90 pct Fe2O3 sample was melted completely using a hot thermocouple (HTC)[1,3,4] and cooled to a desired temperature and held for a given time. After cooling to the ambient temperature, the precipitated phases in the sample was examined by Xray diffraction (XRD). From the results of XRD with experimental conditions, the TTT diagram was developed for the 10 pct CaO-90 pct Fe2O3 sample.
Theoretically, if there is enough time for oxygen diffusion and reaction of compound formation in the sample, the formed compounds are in agreement with the known binary phase diagram of CaO-Fe2O3.
II.
MATERIALS AND METHOD
A. Sample Composition A 10 pct CaO-90 pct Fe2O3 sample was used in this study, which is shown by a dashed line in Figure 1,[1,2] where F
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