Blast Furnace Slag

Name: Blast Furnace Slag
Diagram No.: 1134
Type of diagram: CCT, TTT
Chemical composition in weight %: 42.99% CaO, 33.52% SiO2, 13.77% Al2O3, 6.28% MgO, <0.37% Fe2O3, 0.27% MnO, C/S=1.28
Group: Slags
Note: In the present study, using SHTT (Single Hot Thermocouple Technique), TTT and CCT diagrams of BF slags were measured. Crystallization behavior in the TTT and CCT diagrams of BF slag were clarified by XRD analysis, SEM observation and EDS analysis together with the in situ observation. Crystal phases in the TTT diagram for BF slag used were Gehlenite (2CaO Al2O3 SiO2) and Merwinite (3CaO MgO 2SiO2). The Merwinite precipitated faster than the Gehlenite. The nose position of Merwinite was 4 s at 1090C and the nose of Gehlenite was 8 s at 1230C. CCT diagram had wider glass region than TTT diagram and the temperature of crystal region decreased to 1340C at 1000 s and 1160C at 14 s.

Figure 1 shows the tentative TTT diagram of the BF slag obtained. The horizontal axis is the time in a natural logarithm and the vertical one is the temperature that a crystal precipitated.
The nose position of BF slag was about 9 s at 1250C and was 4 s at 1050C, which meant the double nose TTT diagram. In addition, the melting point of BF slag was about 1400C. Generally, the double nose TTT diagram will have more than two kinds of crystals. In the case of BF slag, there is no information about the kind of crystal phase, only TTT diagram having a close chemical composition of BF slag was published by P. Rocabois et al.) in which several crystal phases were reported such as pseudowollastonite (CS: CaO SiO2), anorthite (CAS2: CaO Al2O3 2SiO2), Gehlenite (C2AS: 2CaO Al2O3 SiO2) and Akermanite (C2MS2: 2CaO MgO2SiO2), but there is a little information about optical characteristics during precipitation, which is related to the result of in situ observation. As shown in Fig. 1, there are relatively large scattering of the data. The causes on the results will come from the followings factor; (1) Many impurities in the BF slag will affect the behavior of the crystallization. Especially, the beginning of crystal precipitation will be changed by the impurities such as Fe2O3, FeO, MnO, P2O5, S, Na2O, K2O, and so on. (2) The content of H2O will enhance the crystallization behavior.) (3) Tthe optical property will be important factor for the visible crystal. (4) The size of crystal is also important, and the small size at the beginning of crystallization can not see, depending on the resolution of the optical system. The distinct definition of the beginning of the crystallization will be necessary to deduce with a theoretical procedure.)

Finally, the TTT diagram of BF slag was determined as shown in Fig. 2. The upper nose in the tentative TTT diagram obtained by the in situ observation would be the nose of Gehlenite which located inside the TTT diagram of Merwinite. The crystal of Merwinite precipitated in high temperature was a transparent or a translucent which was difficult to see in high temperature. The nose position of Merwinite located around 4 s at 1090C. The reason why the Merwinite precipitates faster than the Gehlenite will be resulted from the lower free energy, however, the behavior of crystallization until a stable state (100% of crystallization degree) will be affected by the difference of the diffusibility of cations that construct the Merwinite and the Gehlenite. Especially, as Ca and Si are common elements in the both compounds, the difference of the diffusibility of Mg and Al will be important to clarify the crystallization behavior around 1300C.

CCT diagram of BF slag was measured by various cooling rate from 70C/s to 0.3C/s (typical cooling curves of 35C/s, 3.5C/s and 0.35C/s were shown by the broken lines in Fig. 3). The temperatures of the beginning of crystallization on the cooling curves were plotted on the TTT diagram obtained. In the CCT diagram, glass region became large and the minimum temperature of crystallization decreased to 1160C at 14 s and the maximum one was 1340C at 1000 s, which were general tendency of CCT v.s. TTT.

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