Activity Measurement of CaO SiO 2 AlO 1.5 MgO Slags Equilibrated with Molten Silicon Alloys

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1 ISIJ International, Vol. 40 (000), No. 6, pp Activity Measurement of CaO SiO AlO 1.5 MgO Slags Equilibrated with Molten Silicon Alloys Kousuke KUME, Kazuki MORITA 1), Takahiro MIKI ) and Nobuo SANO 3) Formerly Graduate Student, Department of Metallurgy, The University of Tokyo. Now at Oita Works, Nippon Steel Corporation, Nishinosu, Oita-shi, Oita, Japan. 1) Department of Metallurgy, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo, Japan. ) Formerly Graduate Student, Department of Metallurgy, The University of Tokyo. Now at Department of Metallurgy, Tohoku University, Aramaki, Aoba-ku, Sendai-shi, Japan. 3) Emeritus Professor, Department of Metallurgy, The University of Tokyo. Now Executive Advisor, Nippon Steel Corporation, Shintomi, Futtsu-shi, Chiba, Japan. (Received on November 9, 1999; accepted in final form on December, 1999 ) The activities of components in the CaO SiO AlO 1.5 MgO slags at 1 83 and K were directly measured by equilibrating the slags with Si based alloys. For the CaO SiO, CaO SiO AlO 1.5 and CaO SiO MgO systems, thermodynamic properties of Si based alloys and the activity data of SiO obtained from the distribution of each element between slags and the alloys in the previous paper were used to derive the activities of AlO 1.5, CaO and MgO. For the CaO SiO AlO 1.5 MgO quaternary system, the activities of components other than SiO on the 10 mass% MgO plane were determined at K by employing the SiO activities available in the literature. Accordingly, it was confirmed that the activities of components other than SiO in multi-component silicate slag systems can be determined by this technique as long as the activity datum of at least one component in the systems is available. KEY WORDS: activity measurement; CaO SiO AlO 1.5 MgO slags; Si based alloy; ironmaking; steelmaking. 1. Introduction The CaO SiO AlO 1.5 MgO slags, which are the typical by-products of ironmaking processes, are also commonly used in the treatment of high quality steels such as desulfurization and deoxidation processes. Thermodynamic analyses for the refining reactions in such processes need reliable data for the activity of each component in the slag system over a wide range of liquid composition. For instance, the erosion of a refractory is considered to be significantly affected by such activities. They are also very important in controlling the composition of inclusions in steels. Recently, the formation of spinel inclusion, MgAl O 4, has been focused and several researches 1,) have been performed in order to clarify the critical condition for their formation, which is strongly influenced by the activities of respective components in the CaO SiO AlO 1.5 MgO system. As described in the previous work 3), several researchers 4 6) determined the activity of SiO in the CaO SiO AlO 1.5 MgO system by chemical equilibration techniques, and subsequently calculated the activities of other components for the ternary systems by Schuhmann s tangent intercept procedure. However, this method essentially involves the errors caused by the accuracy of tangent slopes of the artificially drawn iso-activity lines, and also requires the activity data of one component over a wide range of liquid composition together with the activity values of the other components as integration limits. For quaternary or more complicated systems, it is difficult to apply this method. Ohta and Suito 7,8) determined the activity of SiO and AlO 1.5 on the liquidus lines at the 0, 10, 0, 30, 40 mass% AlO 1.5 planes and the 0, 10, 0 mass% MgO planes of CaO SiO AlO 1.5 MgO slags at K by equilibrating the slag and Fe alloy in an AlO 1.5 or CaO crucible. However, the reliable data are restricted to the liquidus composition and the activities in the liquid region had to be estimated by approximation. On the other hand, by applying a slag metal equilibration technique using a Si based alloy, the activities of components other than SiO can be obtained directly at each experimental point when SiO activity of the slag and thermodynamic properties of the Si based alloy are available. Accordingly, in the present work the activities of CaO, AlO 1.5 and MgO in the CaO SiO, CaO SiO AlO 1.5, CaO SiO MgO and CaO SiO AlO 1.5 MgO (on 10 mass% MgO plane) systems over a wide range of liquid composition were experimentally determined at 1 83 and K by equilibrating Si based alloys as the reference metals with the slags ISIJ

2 ISIJ International, Vol. 40 (000), No. 6. Experimental.1. Experimental Procedure Experiments were carried out as described in the previous paper. 3) For the CaO SiO, CaO SiO AlO 1.5 and CaO SiO MgO systems, using the thermodynamic properties of Si based alloys and SiO activities derived in the previous study, the activities of CaO, AlO 1.5 and MgO in the slags at 1 83 and K were calculated according to the principle shown below. In the case of the CaO SiO AlO 1.5 MgO quaternary system, experiments were carried out at K in the same manner as those for binary and ternary systems, and the activity data of SiO reported by Rein and Chipman were employed since it was difficult to obtain such data by the method previously reported due to inaccuracy of the integration... The Principle of the Activity Determination At equilibrium, the exchange reactions with respect to each component between a molten Si alloy and a slag may be written as follows: Table 1. Slag and metal compositions after equilibrium and the activities of components in the CaO SiO system. 1 Si(l) CaO(s) 1 SiO() s Ca(l)...(1) 1 a X DG RT ln SiO Ca in Si Ca 1/ a a / γ Si CaO 156, T (J/mol) 9 11)...() 1 1 Si(l) MgO(s) SiO s) Mg(l)...(3) ( 1 / a X DG RT ln γ SiO Mg in Si Mg 1/ asi amgo 134, T (J/mol) 9,10)...(4) 3 3 Si(l) AlO (s) SiO s) Al(l)...(5) 1.5 ( 4 4 3/ 4 a X DG RT ln γ SiO Al in Si Al 3/4 a a 13, T (J/mol) 9,10)...(6) where g i is the activity coefficient of i in a molten Si based alloy relative to pure liquid i. The activity coefficient of Ca, Al and Mg in molten Si based alloys have been reported by Miki et al., 1,13) hence the activities of all the elements in the alloy may be obtained as previously reported. 3) Accordingly, the activities of CaO, AlO 1.5 and MgO in the slags can be calculated from the SiO activity of the slag and the thermodynamic properties of the alloy. Here, the same data for the standard Gibbs energy change of respective reactions as were used in the derivation of the activity coefficients of Ca, Al and Mg in Si based alloys were selected in order to obtain more accurate values by canceling the intrinsic errors. Si AlO 1.5 Fig. 1. Relationship between slag composition and the activities of CaO and SiO for the CaO SiO system at 1 83 and K. 3. Results and Discussion 3.1. The CaO SiO System The activities of SiO were derived from the experimental results as shown in the previous paper. 3) Using the thermodynamic properties of Si Ca alloys and SiO activities of the CaO SiO slags, the activities of CaO at 1 83 and K were derived as shown in Table 1. The value reported by Miki et al. 1) for the SiO saturated composition at 1 83 K was also employed. Relationship between slag composition and CaO activity is shown in Fig. 1. Similarly to SiO activities, CaO activities show little temperature dependence in the present study. When they are compared with those reported by other researchers, 4,14 18) the present results are in reasonable agreement with those by Sawamura 14) at K and Sharma and Richardson 15) at K, showing slightly lower values than the rest of other results (Fig. ). 3.. The CaO SiO AlO 1.5 System The activities of SiO, which were also derived in the previous paper, and the experimental results on equilibrating molten Si Ca Al alloys with the CaO SiO AlO 1.5 slags were used to obtain the activities of AlO 1.5 and CaO at 1 83 K. The results are listed in Table. Iso-activity lines for CaO are drawn in Fig. 3. Although the curvatures of iso-activity contours agree well with those 000 ISIJ 56

3 ISIJ International, Vol. 40 (000), No. 6 Fig.. Comparison of the CaO activities for the CaO SiO system at 1 83 and K reported by several researchers. Fig. 3. Iso-activity contours for CaO in the CaO SiO AlO 1.5 system at 1 83 K. Table. Slag and metal compositions after equilibrium and the activities of components in the CaO SiO AlO 1.5 system at 1 83 K. Fig. 4. Iso-activity contours for AlO 1.5 in the CaO-SiO-AlO 1.5 system at 1 83 K. by Rein and Chipman, 4) the numerical values of the present results are about two thirds of their values. Iso-activity lines for AlO 1.5 are drawn in Fig. 4. The curvatures of the lines also agree well with those by Rein and Chipman. 4) However, activity values are smaller than their results when AlO 1.5 content is higher than 30 mol%. The discrepancies in both activities are considered to be due to the errors in their graphical treatment for the Gibbs Duhem integration and those of the activity data at the integration limits. The activities of components in the CaO AlO 1.5 system can not be measured by the present method. Hence, the activities were estimated by extrapolating the iso-activity lines for CaO and AlO 1.5 in the CaO SiO AlO 1.5 system. The activities of CaO and AlO 1.5 in the CaO AlO 1.5 system at 1 83 K are shown in Figs. 5 and 6 together with those by other researchers, 4,19 ) respectively. The present results for the CaO activities are in good agreement with those by Cameron et al. 19) and Fincham and Richardson, 1) but showing lower values than those by the others. In the case of AlO 1.5 activities, the present results at 1 83 K were among the other data and show good agreement with those by Rein and Chipman 4) at 1 83 K, Carter and Macfarlane 16) at K and Fujisawa et al. 0) at K The CaO SiO MgO System The activities of CaO and MgO for the CaO SiO MgO slags at K were similarly derived and listed in Table ISIJ

4 ISIJ International, Vol. 40 (000), No. 6 Table 3. Slag and metal compositions after equilibrium and the activities of components in the CaO SiO MgO system at K. Fig. 5. Comparison of the CaO activities for the CaO AlO 1.5 system at 1 773, 1 83 and K reported by several researchers. Fig. 6. Comparison of the AlO 1.5 activities for the CaO AlO 1.5 system at 1 773, 1 83 and K reported by several researchers. Iso-activity lines for CaO and MgO obtained in the present study are shown in Fig. 7(a), compared with those by Rein and Chipman. 4) (Fig. 7(b)) Although the curvatures of constant contours for CaO and MgO activities agree well with those by Rein et al., the CaO activities of the present work are about a half of their values, while the MgO activities are twice as large as theirs. This may be due to the inaccuracy in their graphical treatment of the Gibbs Duhem integration with SiO activities. These discrepancies would be large enough to cause considerable difference in estimating the slag basicity or the errosion of refractories in steelmaking processes The CaO SiO AlO 1.5 MgO System Considering the availability of SiO activity data, the activity of each component was determined in the CaO SiO AlO 1.5 MgO system on 10 mass% MgO plane whose phase relations had been reported by Prince. 3) The experimental results for the CaO SiO AlO 1.5 MgO slag compositions, the Si Ca Al Mg alloy compositions and Fig. 7. Iso-activity contours for CaO and MgO in the CaO SiO MgO system at K. (a) Present work. (b) After Rein and Chipman. 4) the activities of respective components at K are summarized in Table 4. For the present system, the activity data of SiO reported by Rein et al. 4) (Fig. 8) were used because it was difficult to employ the same method as ternary systems considering the accuracy of calculation. Iso-activity lines for CaO, AlO 1.5 and MgO are drawn in the CaO SiO AlO 1.5 pseudo-ternary triangle as shown in Figs. 9, 10 and 11, respectively. For the activities of CaO 000 ISIJ 564

5 ISIJ International, Vol. 40 (000), No. 6 Table 4. Slag and metal compositions after equilibrium and the activities of components in the CaO SiO AlO 1.5 MgO system at K. Fig. 8. The SiO activity data 4) employed in the present study for the CaO SiO AlO mass%mgo system at K. Fig. 10. Iso-activity contours for AlO 1.5 in the CaO SiO AlO 1.5 pseudo-ternary system on the plane of 10 mass% MgO at K. Fig. 9. Iso-activity contours for CaO in the CaO SiO AlO 1.5 pseudo-ternary system on the plane of 10 mass%mgo at K. Fig. 11. Iso-activity contours for MgO in the CaO SiO AlO 1.5 pseudo-ternary system on the plane of 10 mass% MgO at K ISIJ

6 ISIJ International, Vol. 40 (000), No. 6 and AlO 1.5, the results of the present slag showed little difference compared with those of the CaO SiO AlO 1.5 slag system which corresponds to the CaO SiO AlO 1.5 MgO slag with 0 mass% of MgO. Since there is some scatterings in MgO content after the experiments, the activity data plotted in Figure 11 were adjusted to those of 10.0 mass% MgO with making the activity coefficients of MgO constant. The activity of MgO increases with increasing slag basicity, and its value varies from 0.03 to 1 and the curvatures of iso-activity lines for MgO are similar to those for CaO at a constant MgO content of 10 mass%. Therefore, only 10 mass% of MgO addition to the CaO SiO AlO 1.5 slag may increase the activity of MgO to a large extent, depending upon the matrix composition without changing the activity coefficients of CaO and AlO 1.5. Since the activities other than SiO, such as CaO, AlO 1.5 and MgO, could hardly be determined due to the difficulty in the thermodynamic derivation from those of SiO, no other reliable data are available for such complicated slag systems that are of practical importance in iron- and steelmaking processes. Accordingly, it has been confirmed that any activities in multi-component silicate slag systems can be determined by the technique developed in the present study as long as the activity datum of one component at a desired composition in the systems is available. 4. Conclusions By a slag metal equilibration technique using Si based alloys as reference metals, the activities of CaO, AlO 1.5 and MgO in the CaO SiO, CaO SiO AlO 1.5, CaO SiO MgO, CaO SiO AlO 1.5 MgO systems were directly measured at 1 83 and K. The activities of SiO, which were preliminary obtained by integrating equilibrium distribution data between Si based alloys and slags, were used in the derivation of other component activities for the binary and ternary systems, while available data of SiO activities were employed for the CaO SiO AlO 1.5 MgO quaternary system. It is considered to be the largest advantage of the present procedure that any activities in multi-component silicate slag systems can be determined as long as the activity datum of at least one component at the desired composition is available. Acknowledgement This research was conducted with the financial support of the research program, Progressive and Active Commitments to Universities by 5 Companies. REFERENCES 1) H. Ohta and H. Suito: ISIJ Int., 36 (1996), 983. ) H. Itoh, M. Hino and S. Ban-ya: Metall. Mater. Trans. B, 8B (1997), ) K. Morita, K. Kume and N. Sano: ISIJ Int., 40 (000). 4) R. H. Rein and J. Chipman: Trans. AIME, 33 (1965), ) D. A. R. Kay and J.Taylor: Trans. Faraday Soc., 56 (1960), ) D. A. R. Kay and J. Taylor: J. Iron Steel Inst., 01 (1963), 67. 7) H. Ohta and H. Suito: Metall. Mater. Trans. B, 7B (1996), ) H. Ohta and H. Suito: Metall. Mater. Trans. B, 9B (1998), ) R. G. Berman, T. H. Brown and H. J. Greenwood: Report No.TR- 377, Atomic Energy of Canada Limited, Mississauga, Ontario, (1985). 10) JANAF Thermochemical Tables, 3rd ed.: J. Phys. Ref. Data, 14 (1985), suppl ) T. Wakasugi and N. Sano: Metall. Trans. B, 0B (1989), ) T. Miki, K. Morita and N. Sano: Metall. Mater. Trans. B, 9B (1998), ) T. Miki, K. Morita and N. Sano: Mater. Trans., JIM, 40 (1999), ) K. Sawamura: Tetsu-to-Hagané Overseas, (196), ) R. A. Sharma and F. D. Richardson: J. Iron and Steel Inst., 00 (196), ) P. T. Carter and T. G. Macfarlane: J. Iron Steel Inst., 185 (1957), ) J. D. Baird and J. Taylor: Trans. Faraday Soc., 54 (1958), ) G. Eriksson, P. Wu, M. Blander and A. D. Pelton: Can. Metall. Q., 33 (1994), ) J. Cameron, T. B. Gibbons and J. Taylor: J. Iron Steel Inst., 06 (1968), 13. 0) T. Fujisawa, M. Yamauchi and H. Sakao: CAMP-ISIJ, 1 (1988), ) C. J. B. Fincham and F. D. Richardson: Proc. Roy. Soc., A33 (1954), 40. ) S. Ueda, K. Morita and N. Sano: ISIJ Int., 38 (1998), 19. 3) T. Prince: J. Am. Ceram. Soc., 37 (1954), ISIJ 566