BOF Slag

Name: BOF Slag
Diagram No.: 758
Type of diagram: CCT
Chemical composition in weight %: Original slag: 44.5% CaO, 20.6% Fe (total iron in the oxides), 10.2% Fe2+, 10.4% Fe3+, 10.14% SiO2, 4.78% MnO, 2.18% MgO, 2.05% Al2O3, 2.43% P2O5, 0.89% TiO2, 0.37% V2O5
Group: Slags
Note: (a) CCT diagram of the original and the modified BOF slags. The solid and open symbols represent the starting and ending crystallization temperatures, respectively. T1, T2, T3 and T4 indicate the calculated liquidus temperature of original, 5 , 10 , and 15 wt% Al2O3 modified BOF slags. The line is drawn as a guide for reading. (b) Undercooling of the original and modified BOF slags under various cooling rates. 5 A, 10 A, and 15 A indicate the 5, 10, and 15 wt% Al2O3 modified BOF slags, respectively.

To modify the chemistry, 5, 10 and 15 wt% Al2O3 (Sasol, Germany, 25 m) were respectively added to the master slag in ethanol and each of the mixtures was homogenized in a multidirectional mixer (Turbula type) for 24 h, followed by drying in a rotating evaporator at 60 C and further dried in a muffle furnace at 80 C for 24 h.

Using the images captured from the CSLM crystallization experiments, the CCT diagrams of the different slags (i.e. onset crystallization temperature as a function of time under continuous cooling conditions) are constructed and given in Figure (a). 5 A, 10 A, and 15 A refer to the 5, 10, and 15 wt% Al2O3 modified slags, respectively. In the CCT diagram, the solid symbols and corresponding open symbols respectively represent the onset and ending temperature of crystallization under a certain cooling rate. The narrow temperature range between the onset and ending temperatures for each slag implies rapid crystallization of the slags. The fast crystallization of BOF slag is not significantly changed by adding Al2O3. For a given slag, an increase in the cooling rate results in a decrease of the onset crystallization temperature as indicated by the solid lines in Figure 4.8 (a). The theoretical crystallization temperatures of slags with different Al2O3 addition were calculated using FactSage, and the results are given by the horizontal dashed lines in Figure (a). The liquidus temperature of the original and the Al2O3 modified slags are, respectively 1527 C(T1, original slag), 1450 C(T2, 5 A), 1400 C(T3, 10 A), 1365 C(T4, 15 A). Based on these calculated liquidus temperature data, the undercooling (temperature difference between the liquidus temperature and actual solidification temperature) of each slag at various cooling rates can be estimated as shown in Figure (b). The undercooling is increased with increasing cooling rate and Al2O3 addition. For the original slags, by increasing the cooling rate from 29.4 to 1260 Cmin-1, the undercooling increased from 80 to 247 C. While for the 15 wt% Al2O3 modified slag, the undercooling increased from 128 to 277 C by increasing the cooling rate from 29.4 to 1260 Cmin-1. According to the classical nucleation theory, the undercooling implies the resistance of a liquid for solid precipitate nucleation, which is much larger than the resistance of crystal growth. It has been reported that Al2O3 can increase the viscosity of the BOF slag at high temperature, where the slag is mostly liquid.
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