Carbide Capacity of CaO SiO 2 CaF 2 ( Na 2 O) Slags at K

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1 ISIJ International, Vol. 44 (004), No., pp. 3 8 Carbide Capacity of CaO SiO CaF ( Na O) Slags at K Joo Hyun PARK and Dong Joon MIN 1) Stainless Steel Research Group, Technical Research Laboratory, POSCO, Pohang , Korea. 1) Department of Metallurgical System Engineering, Yonsei University, Seoul , Koreea. basicity@posco.co.kr (Received on March 4, 003; accepted in final form on September 15, 003 ) The influence of CaF, Na O, and basicity on the solubility of carbon in the CaO SiO CaF Na O slag at K was investigated on the basis of carbide capacity concept. The carbide capacity of the CaO satd SiO CaF system increases with increasing CaF content up to about 40 mass%, followed by nearly constant value. The capacity of the slags saturated with Ca SiO 4 also increases with increasing the content of CaF and this trend can be explained from the depolymerization reaction of silicate network by F ions. The carbide capacity of the slags increases by increasing the ratio of (%CaF )/(%SiO ) up to about 0.8, followed by slight increase or nearly constant value at a fixed CaO content. The capacity also increases by increasing the ratio of (%CaO)/(%SiO ) at a fixed CaF content. The carbide capacity of the Na O-containing slags linearly increases with increasing the basicity, B ( {(%CaO) (%Na O)}/(%SiO )) and the dependence of the capacity on the basicity decreases by increasing the CaF content. On the other hand, the capacity decreases with increasing CaF content at basicity, B of the slags greater than about.0 and vice versa in the composition of B.0. The capacity of the slags containing CaF and Na O could quantitatively be described by the activity of CaO rather than optical basicity. KEY WORDS: calcium fluoride; sodium oxide; basicity; carbon; carbide capacity; depolymerization; activity; optical basicity. 1. Introduction The content of carbon in stainless steel should severely be controlled because some physical properties and formability of products become deteriorative as carbon content increases. However, the content of carbon in molten steel would generally increase in the ladle treatment process after oxygen blowing in AOD (Argon Oxygen Decarburization) converter, probably resulting from the low quality of burnt lime or fluorspar added. 1) In this background, the appropriate slag systems with high solubility of carbon would be designed, which could minimize the transfer of carbon from slag to metal phase. The solubility of carbon in the basic slags has been investigated by employing the carbide capacity (Eq. ()) defined from the dissolution reaction (Eq. (1)). C (s) O (slag) C (slag) O (g)...(1) C C K () 1 1 a f C O (mass%c a C ) p 1 / O...() where K (1), a i, f C, and p O are, respectively, the equilibrium constant of Eq. (1), the activity of i, the activity coefficient of C ions, and the oxygen partial pressure. Hence, the carbide capacity is a function of basicity and the stability of carbide ions in molten slags at a fixed temperature. The solubility of carbon and the carbide capacity of the slags containing CaF has recently been reported by research groups including one of the authors. 7) Park et al. 5,6) reported that the carbide capacity of the CaO Al O 3 CaF slags was strongly affected by CaO/Al O 3 ratio at a fixed CaF content. Song et al. 1,7) reported that the carbide capacity of the calcium silicate melts could be increased by addition of about 0 mass% CaF. However, because the composition of slags investigated in these studies was limited to the unsaturated liquid phase, the carbide capacity of silicate melts saturated with solid phase such as CaO or Ca SiO 4 has not been understood yet. Therefore, in the present study, the carbide capacity of the CaO SiO CaF slags through the wide composition region has been measured at K to understand the effect of slag composition on the solubility of carbon in the slags. Furthermore, the effect of small addition of Na O on the carbide capacity has been investigated, because the Na O is known to increase the basicity of molten slags.. Experimental A super-kanthal electric furnace was used for an equilibration of the CaO SiO CaF ( Na O) slag and gas phases. The temperature was controlled within K using an R-type (Pt 13%Rh/Pt) thermocouple and a proportional integral differential controller. The slag samples were prepared using reagent-grade SiO, CaF, Na O, and CaO calcined from CaCO 3. A schematic diagram of the experimental apparatus is available in previous article. 8) ISIJ

2 ISIJ International, Vol. 44 (004), No. Table 1. Dependence of carbide capacity of the CaO SiO CaF ( Na O) system on slag composition at K. Fig. 1. Carbide capacity of the CaO satd SiO CaF system as a function of CaF content at K. The slag sample of 10 g was maintained in a graphite crucible under a CO atmosphere to control the oxygen partial pressure by C/CO equilibrium reaction, as given in Eq. (3). 9) 1 C (s) O (g) CO (g) DG T (J/mol)...(3) The impurities in the CO gas were removed by passing through CaSO 4, soda lime, and silica gel. The equilibration time was predetermined to be 1 to 18 h according to slag compositions. After equilibrating, the sample was quenched by Ar gas blowing and crushed for chemical analysis. The contents of carbon and each component in the slags were determined by a LECO (CS-300) analyzer and conventional titration methods, respectively. 3. Results and Discussion 3.1. Effect of CaF Addition at CaO Saturation Condition The carbide capacity of the CaO satd SiO CaF system at K is shown as a function of CaF content in Fig. 1 along with the result in previous work. 6) The capacity slightly increases with increasing CaF content up to about 40 mass%, followed by nearly constant value of about C C ; the similar tendency was observed in the CaO satd Al O 3 CaF system. 6) These trends could be understood based on the depolymerization reaction of silicate network by F ions (e.g. Eq. (4)), resulting in an increase in the basicity of molten slags. 10) Si O 6 7 F [SiO 3 F] 3 O...(4) It is of interest in Fig. 1 that the values of carbide capacity in both slag systems are close to each other, indicating that the ratio a O /f C in Eq. () would be identical in these CaO-saturated systems. In previous work, 11) it was also observed that the carbide capacity of the CaO satd SiO system was similar to that of the CaO satd Al O 3 system at 1 83 K. Thus, it is suggested that the acidic oxides in the slags would not significantly affect the slag capacity when the activity of CaO (a CaO ) is unity. This can be explained based on the simple thermodynamics as follows: If it is assumed that the a O in the CaO satd Al O 3 ( CaF ) system is greater than that in the CaO satd SiO ( CaF ) system, the activity of Ca ions (a Ca ) in the former should be lower than that in the latter because of a CaO 1 from Eq. (6). CaO Ca O...(5) a a K( 5) Ca O...(6) a where K (5) and a i are, respectively, the equilibrium constant of Eq. (5) and the activity of i. Thus, the value of f C in the CaO satd SiO ( CaF ) system would be less than that in the CaO satd Al O 3 ( CaF ) melts, because the stability of the C anions would mainly be affected by the Ca cations. Therefore, it is suggested that the greater the a O in the slag system A than that in B, the greater the f C in the system A than that in B at a fixed a CaO in a similar extent, based on the present experimental results shown in Fig. 1, although the absolute values of them are not known within thermodynamics. 3.. Effect of CaF Addition at Ca SiO 4 Saturation Condition The carbide capacity of the CaO SiO CaF slag saturated with Ca SiO 4 at K is shown as a function of CaF CaO 004 ISIJ 4

3 ISIJ International, Vol. 44 (004), No. Fig.. Dependence of carbide capacity of the CaO SiO CaF slag saturated with Ca SiO 4 on the content of CaF at K. Fig. 3. Relationship between log C C and (%CaF )/(%SiO ) ratio at a fixed CaO contents in the slags at K. content in Fig.. The capacity slightly increases with increasing CaF content up to about 0 mass%, followed by significant increase. The increase in the carbide capacity in this system can also be explained from the depolymerization reaction of silicate network by F ions as explained in Sec In our previous work, 10) the NBO/Si ( Non- Bridging Oxygen per Silicon) in the Ca SiO 4 saturated system was evaluated from an infrared spectra analysis as shown in Fig.. It is of interest that the tendency of the carbide capacity is in good agreement with that of the NBO/Si of the depolymerized structural units, indicating that the above discussion is acceptable Effect of Slag Composition on Carbide Capacity The carbide capacities in the CaO SiO CaF (33.7 and 49. mass% CaO) systems at K are shown in Fig. 3 as a function of the (mass%caf )/(mass%sio ) ratio. The capacity of the system containing 49. mass% CaO is greater than that of the system with 33.7 mass% CaO at a fixed (%CaF )/(%SiO ) ratio, which could be expected. In both of the slags, the carbide capacity increases by increasing the ratio (%CaF )/(%SiO ) up to about 0.8, followed by slight increase or nearly constant value. This trend means that the effect of F ions on an increase in slag basicity due to the depolymerization of silicate network would be restricted within the critical content. In these systems, the activities of CaO vary with slag composition, although the contents of CaO are fixed. Thus, the effect of a CaO on the carbide capacity in both slags is discussed as follows. The dependence of carbide capacity on the activity of CaO at K is shown in Fig. 4. The activity of CaO at K was estimated from the thermodynamic data at K reported by Zaitsev et al., 1) assuming the regular solution behavior of these slag systems. The capacity, log C C of both systems linearly increases with increasing log a CaO with the slope of about 0.4, which is less than the expected value of unity (Eq. (7)) when it is assumed that the a CaO would be in proportion to the a O, and that the f C would not seriously be affected by slag composition. Fig. 4. Relationship between the carbide capacity of the slags containing 49. and 33.7 % CaO and the activity of CaO at K. log C C log a O log f C log K (1)...(7) The Eq. (8) can be deduced from Eq. (7) by using Eq. (6). log C C log a CaO log a Ca log f C Constant...(8) Therefore, it is probably suggested from the experimental results that the slope of not unity but 0.4 in Fig. 4 may be casued by the difference between the a Ca -increasing rate and the f C -decreasing rate in an increase of the activity of CaO. The carbide capacity of the CaO SiO 30.5mass%CaF system at K is shown in Fig. 5 as a function of the (mass%cao)/(mass%sio ) ratio and is compared to that of the CaO Al O 3 7.5mass%CaF system, which is available in the literature. 6) The carbide capacity of the calcium fluorosilicate system is lower than that of the aluminate melt ISIJ

4 ISIJ International, Vol. 44 (004), No. Fig. 5. Effect of (%CaO)/(%SiO ) ratio on the carbide capacity of the slags at a fixed CaF content at K. Fig. 6. Relationship between the carbide capacity of the slags containing 30.5 % CaF and the activity of CaO at K. under the conditions of the fixed (%CaO)/(%SiO ) or (%CaO)/(%Al O 3 ) ratio and the similar CaF contents. This means that the amount of lime added to increase carbide capacity of the silicate slags is much greater than that of the aluminate slags, if other variables are fixed. The capacity of the silicate and aluminate melts can quantitatively be discussed by employing the activity of CaO as an indirect basicity index. Figure 6 exhibits the relationship between the carbide capacity and the activity of CaO in both slag systems at K. 1,13) The capacity, log C C of both slags linearly increases by increasing the log a CaO with the slope of about 0.4. The difference from the expected slope of unity might be explained by the similar reason as discussed above (Fig. 4 and Eq. (8)). In addition, it can be proposed that the activity of CaO could be applied as an indirect basicity index to quantify the carbide capacity of the CaO SiO ( Al O 3 ) CaF slags from the fact that the carbide capacities of both slags are located on a straight line. The relationship between the capacity and the basicity index will be discussed later in detail Effect of Na O and CaF on Carbide Capacity It has generally been known that the refining ability of the CaO SiO CaF slags could significantly be increased by addition of small amount of Na O. 14,15) On the other hand, Niekerk and Dippenaar 15) reported that the CaF (10 35 mass%) decreased the sulfide capacity of the CaO SiO CaF slags containing 5 15 mass% Na O due to its dilution effect. Thus, in the present study, the effects of ( mass%) Na O and CaF on the carbide capacities of the CaO SiO CaF Na O slags saturated with CaO or Ca SiO 4 has been investigated. In order to clarify the effects of CaF and Na O on the carbide capacity, an iso-capacity line for the CaO- and Ca SiO 4 -saturated slag systems could respectively be constructed as shown in Fig. 7. In the slags saturated with CaO, an increase in the CaF content necessitates a simultaneous Fig. 7. Iso-carbide capacity line of the CaO- and Ca SiO 4 -saturated systems at K with basicity B and CaF content as dependent parameters. increase in basicity B (Eq. (9)) to maintain a constant carbide capacity and vice versa in the Ca SiO 4 -saturated slags, indicating the contrary effect of CaF and basicity B on the carbide capacity. (%CaO)(%NaO) B (%SiO...(9) The effect of basicity B and CaF content on the carbide capacity can be separated even better by constructing Fig. 8 from Fig. 7. The carbide capacity linearly increases with increasing basicity B of the slags through the entire composition, and the dependence of capacity on the basicity B decreases by increasing the CaF content. On the other hand, the carbide capacity decreases with increasing CaF content ) 004 ISIJ 6

5 ISIJ International, Vol. 44 (004), No. Fig. 8. Carbide capacity of the CaO SiO CaF Na O slags as a function of basicity B at K. Fig. 9. Carbide capacity of the CaO SiO CaF Na O slags as a function of optical basicity at K (L CaO 1.0, L SiO 0.48, L CaF 0.67, L Na O 1.15, L Al O ). at basicity B of the slags greater than about.0 and vice versa in the composition of B.0. Therefore, it can be proposed that the role of F ions in the depolymerization of silicate network (Eq. (4)) would be negligible in the highly basic slags containing Na O and that the F ions would increase the activity coefficient of the anions, resulting in a decrease in carbide capacity. 5) 3.5. Indirect Basicity Index for Carbide Capacity The discussion in previous section indicates that an indirect basicity index is needed to quantitatively estimate the carbide capacity of the slags by compensating the different effects of each slag component. Hence, in this section, the optical basicity and the activity of CaO will be taken into account as an indirect basicity index for the carbide capacity of the CaO SiO CaF Na O slags. Figure 9 exhibits the relationship between the carbide capacity and the optical basicity of the slags at K. Here, the optical basicity of molten slags (L) can be calculated from Eq. (10). 16,17) L Â Â xn i ili xn...(10) where x i, n i, and L i are, respectively, the mole fraction, the number of oxygen or fluorine in the oxide or fluoride, and the theoretical optical basicity of component i. The theoretical optical basicity of CaF was taken to be about 0.67 from an average electron density. 18) The carbide capacity increases with increasing optical basicity of the slags with some scatters, which are significant as the basicity increases. In Fig. 9, the carbide capacity of the CaO Al O 3 CaF slags, which is available in the literature, 6) is also shown as a function of optical basicity. From the relatively large scatters of experimental data between silicate and aluminate melts, the application of an optical basicity as an indirect basicity index of the aluminosilicate melts containing CaF and Na O would be restricted. Figure 10 exhibits the relationship between the carbide i i Fig. 10. Carbide capacity of the CaO SiO CaF Na O slags as a function of CaO activity at K. capacity and the activity of CaO in the CaO SiO CaF Na O at K. The relationship between log C C and log a CaO in the CaO Al O 3 CaF slags at K are also plotted in Fig ) Those CaO activities were estimated from the reported data in the literature. 1,13) The value of log C C increases with increasing log a CaO. It is of interest that the experimental data of the silicate and aluminate melts are on the same straight line within some scatters, which are much less than those observed in Fig 9. The relationship between the carbide capacity and the activity of CaO in both slags at K can be expressed as follows from the linear regression of all the experimental data: log C C log a CaO (r 0.8)...(11) The relatively large scattering of the data for the systems saturated with CaO in Fig. 10 is originated from the effect ISIJ

6 ISIJ International, Vol. 44 (004), No. of CaF on the carbide capacity, which was shown in Fig. 1. Consequently, the carbide capacity of the aluminosilicate melts containing CaF and Na O could be expressed by using the activity of CaO as an indirect basicity index, although the slope of the line is less than unity due to the effect of slag composition on the a O and f C terms as mentioned in Sec Conclusions The influence of CaF, Na O, and basicity on the solubility of carbon in the CaO SiO CaF Na O slag at K was investigated on the basis of carbide capacity concept. The following conclusions could be obtained: (1) The carbide capacity increases with increasing CaF content up to about 40 mass%, followed by nearly constant value of about log C C 8. in the CaO satd SiO CaF system. () The carbide capacity of the slags saturated with Ca SiO 4 increases by increasing the content of CaF and this can be explained from the depolymerization reaction of silicate network by F ion. (3) The carbide capacity of the slags increases with increasing ratio of (%CaF )/(%SiO ) up to about 0.8, followed by slight increase or nearly constant value at a fixed CaO content. (4) The carbide capacity of the slags increases by increasing the ratio of (%CaO)/(%SiO ) at a fixed CaF content. (5) The carbide capacity of the Na O-containing slags linearly increases with increasing basicity B ( {(%CaO) (%Na O)}/(%SiO )) and the dependence of capacity on the basicity B decreases by increasing the CaF content. On the other hand, the capacity decreases with increasing CaF content at basicity B of the slags greater than about.0 and vice versa in the composition of B.0. (6) The carbide capacity of the slags containing CaF and Na O could quantitatively be expressed by the activity of CaO rather than optical basicity. REFERENCES 1) H. S. Song, D. S. Kim and C. H. Rhee: 85th Steelmaking Conf. Proc., ISS-AIME, Warrendale, PA, (00), 301. ) W. Oelsen, H. Keller, H. G. Schubert and H. Brockmann: Arch. Eisenhüttenwes., 40 (1969), ) W. Oelsen, K. H. Sauer, H. G. Schubert and R. Winzen: Arch. Eisenhüttenwes., 41 (1970), 89. 4) K. Schwerdtfeger and H. G. Schubert: Metall. Trans. B, 8B (1977), ) I. Sohn, D. J. Min and J. H. Park: Steel Res., 70 (1999), 15. 6) J. H. Park, D. J. Min and H. S. Song: ISIJ Int., 4 (00), 17. 7) H. S. Song, C. H. Rhee and D. J. Min: Steel Res., 70 (1999), ) J. H. Park and D. J. Min: Metall. Mater. Trans. B, 30B (1999), ) E. T. Turkdogan: Physical Chemistry of High Temperature Technology, Academic Press, New York, NY, (1980), 7. 10) J. H. Park, D. J. Min and H. S. Song: ISIJ Int., 4 (00), ) J. H. Park and D. J. Min: ISIJ Int., 40 (000), S96. 1) A. I. Zaitsev, A. D. Litvina and B. M. Mogutnov: J. Chem. Thermodyn., 4 (199), ) M. Hino, S. Kinoshita, Y. Ehara, H. Itoh and S. Ban-ya: Proc. 5th Int. Conf. Molten Slags, Fluxes, and Salts, ISS-AIME, Warrendale, PA, (1997), ) H. Kimura, F. Tsukihashi and N. Sano: Metall. Mater. Trans. B, 6B (1995), ) W. H. Van Niekerk and R. J. Dippenaar: ISIJ Int., 33 (1993), ) J. A. Duffy: Ironmaking Steelmaking, 17 (1990), ) K. C. Mills: ISIJ Int., 33 (1993), ) T. Nakamura, T. Yokoyama and J. M. Toguri: ISIJ Int., 33 (1993), ISIJ 8