Influence of Slag Composition on Slag Iron Interfacial Tension

Transcription

1 , pp Influence of Slag Composition on Slag Iron Interfacial Tension Haiping SUN, 1) Kunihiko NAKASHIMA 2) and Katsumi MORI 2) 1) CRC for Coal in Sustainable Development, School of Materials Science and Engineering, University of New South Wales, Sydney NSW 2052 Australia. 2) Department of Materials Process Engineering, Kyushu University, Fukuoka Japan. (Received on June 1, 2005; accepted on December 8, 2005) Interfacial tensions between liquid iron and slag were examined for CaO SiO 2 binary and CaO SiO 2 Al 2 O 3 ternary slag systems. The influences of FeO, MgO, B 2 O 3, TiO 2, BaO and ZrO 2 in slag on the interfacial tension were also investigated by varying these oxides from 1 30 wt% in the CaO SiO 2 binary or CaO SiO 2 Al 2 O 3 ternary slag systems. The interfacial tensions were determined by the sessile drop technique under argon atmosphere at K. An X-Ray radiographic technique was employed to image the profile of molten iron drop in a slag bath and the interfacial tensions between slag and iron were obtained from the drop shape analysis. In examining the effect of slag composition on the interfacial tension, the oxygen in liquid iron and its impact on the interfacial tension was taken into consideration. The interfacial tension was found slightly increased with increasing CaO/SiO 2 ratio in CaO SiO 2 slag, and with increasing MgO content in the slag. A decrease of interfacial tension was found with increasing FeO in slag. Only minor changes in the interfacial tension were observed with varying the slag composition in the CaO SiO 2 Al 2 O 3 ternary slag system and the influences of B 2 O 3, TiO 2, BaO, and ZrO 2 in slag on the interfacial tension were found insignificant. KEY WORDS: interfacial tension; continuous casting; flux; inclusions; slag, iron. 1. Introduction Interfacial phenomena play important roles in materials design since many materials contain more than two phases. Interfacial properties provide information for understanding of the interfacial nanostructure or internal nanostructure of materials at high temperatures. In iron and steel making processes where molten iron and slag are coexisting, interfacial properties affect the interaction between iron and slag. For example, the processes of the formation of metal slag emulsion in steel making, the generation of oxide or sulfide inclusions and their separation from molten steel are related to the interfacial properties between the two phases. In continuous casting, marangoni convection due to the gradient of the interfacial tension between flux and steel enhances heat transfer at meniscus level. Since the measurement of the interfacial tension between slag and metal is very difficult, there are considerable disagreements on the interfacial tension among different investigations in the past. One of the reasons account for these disagreements were the difficulties in controlling metal composition, i.e., the degree of purity of liquid metal samples, particularly the oxygen content in the metal samples. It is well known that, oxygen is a surface or interfacial active element in the melt, this means that the oxygen, even a small amount present in liquid iron, will has a great impact on the surface or interfacial properties. In this work, the measurement system was defined by the composition of final slag and metal oxygen. Slag systems investigated in the present work were CaO SiO 2, CaO SiO 2 Al 2 O 3 and CaO SiO 2 Al 2 O 3 M x O y, where M x O y was FeO, MgO, B 2 O 3, TiO 2, BaO or ZrO 2. The oxygen in liquid iron and its impact on the interfacial tension was taken into consideration when examining the effect of slag composition. 2. Experimental Procedures The interfacial tension was measured using the X-ray sessile drop technique at K. An electrical resistance furnace was used for heating and melting iron and slag samples. The vertical alumina tube was purged with a purified argon gas during the experiments. A crucible located in the hot zone of the reaction tube was observed by an X-ray device through a window opened in the midway of the furnace. About 20 g of pure iron and 25 g of the slag were melted in an alumina or zirconia crucible. Alumina crucible was used for the slags containing alumina, and zirconia crucible was used for the slags without alumina. Once metal and slag melted, the iron drop was formed in the slag phase. Twenty minutes was allowed for slag/metal system to reach equilibrium. The iron drop in slag was photographed by the X-ray device to provide the drop-image for obtaining the interfacial tension. After photographing, the crucible was swiftly withdrawn from the furnace and quenched in an ice water. The slag and metal samples collected from the quenched crucible were used for the determination of the composition of final slag and oxygen content in the metal samples. The interfacial tensions were obtained from the numerical analysis of the profile of molten iron drop in the slag ISIJ

2 obtained from the X-ray photography. The experimental apparatus and the procedure to determine the interfacial tension from a X-ray image of a droplet using Laplace s equation were detailed in the previous report. 1) The density of the pure iron used in these calculations was chosen as 6.94 g cm 3. The density values for the molten slag were estimated from final slag composition by the regular solution approximation model 2) ; it was assumed that the interaction effects of B 2 O 3, TiO 2, BaO and ZrO 2 with other components in the slag were negligible. 3. Results and Discussions In the present study, the interfacial tension between slag and liquid iron was investigated using slag systems as presented in Table 1. The oxygen contents were determined for the liquid iron, and were taken into account in assessing the interfacial tensions for these slags. The slag compositions, oxygen contents in the metal obtained by the chemical analysis of the quenched samples, and the interfacial tensions obtained from the analysis of metal drop profiles are presented in Tables 2 and 3. ZrO 2 was found in the final slag where zirconia crucible was used, indicating the extent of the dissolution of crucible materials in the slag. The densities of the molten slag used in the calculation of the interfacial tension were also given in these tables Influence of Oxygen in Metal Oxygen is well known as a surface/interfacial active element, the interfacial tension will be markedly reduced by the oxygen present in the molten iron. To remove oxygen in iron samples, the metals were prepared by remelting electrolyte iron with small amount of carbon in an induction furnace under vacuum condition. Most of oxygen and carbon in the iron was removed as CO gas during melting and casting under induction current stirring and/or vacuum conditions. Great attention was also paid to prevent the oxygen pickup of the iron during high temperature experiments. However, as seen in the Tables 2 and 3, oxygen was found in the final metal samples, even for those runs where slag only contained less reducible components. The oxygen sources are considered to be the oxygen generated from the decomposition of slag component (particularly for slag C), and air contamination during slag addition. The interfacial tensions obtained from all slag systems are plotted against oxygen content in the metal in Fig. 1. Regardless of the dissimilar slag systems, the interfacial tension was found decreases with increasing oxygen content in the iron. This suggests that, slag composition affects the slag metal interfacial tension through two ways: one is the direct influence of slag composition on the interfacial tension, and the other is the influence of slag composition on the oxygen content in the metal which reduces the interfacial tension. Before investigating the influence of slag composition on the interfacial tension, the influence of oxygen in metal has to be quantified. Assuming only one solute, oxygen, is present in the metal, the interfacial adsorption in mol/m 2, G, can be obtained from Gibbs adsorption isotherm by, Γ 1 dσ...(1) RT d ln a O where oxygen activity, a O, is referred to the infinite dilution for oxygen in metal based on the Henrian law. Langmuir isothermal equilibrium gives K 0 OaO Γ Γ...(2) 1 KOaO where G 0 is the surface or interfacial excess quantity at full Table 1. Slag systems. Table 2. Interfacial tension between iron and slag at K ISIJ 408

3 Table 3. Interfacial tension between iron and slag at K. Fig. 1. Variation of iron slag interfacial tension with oxygen in metal at K. coverage in mol/m 2, and K O is equilibrium constant of oxygen adsorption. Combines Eq. (1) with Eq. (2) and integrate, interfacial tension as a function of oxygen activity, a O, is given as, s 0 s RTG 0 ln(1 K O a O )...(3) Assuming that the surface excess quantity at full coverage and equilibrium constant of oxygen adsorption are independent of the slag composition, hypothetical interfacial tensions where oxygen activity in the molten iron is zero, s 0, Fig. 2. Variation of iron slag interfacial tension with oxygen in metal at K. were calculated for the slags from Eq. (3). In this calculation, max adsorption, RTG mn/m 2, and adsorption constant, K O 116, reported for iron contacting CaO SiO 2 Al 2 O 3 slag at K 3) and the interaction parameter, e O O 1 750/T ) were used. To examine the direct influence of slag composition on the interfacial tension, the reduction of interfacial tension by the oxygen in the metal has to be removed. Figure ISIJ

4 Fig. 3. Variation of iron slag interfacial tension with basicity of CaO SiO 2 slag at K. Fig. 4. Variation of iron slag interfacial tension with basicity of CaO Al 2 O 3 SiO 2 slag at K. shows hypothetical interfacial tensions obtained from measured s and oxygen content in the metal from (3) for all slag systems in this study. The difference between s 0 and s, Ds, is the reduction of interfacial tension by oxygen adsorption at the interface. The value of s 0, with which the influence of oxygen is eliminated, is used to assess the direct effect of slag compositions on the interfacial tension between the slag and the liquid iron. s 0 obtained for each slag system was also presented in Tables 2 and Interfacial Tension between Iron and CaO SiO 2 Slags The results of the influence of CaO/SiO 2 ratio of slag A on the interfacial tension between iron and slag are shown in Fig. 3. The solid circles show the measured values of the interfacial tension (s) and the open circles (s 0 ) are the hypothetic values with respect to the oxygen content in the iron based on Eq. (3). For this slag, both interfacial tensions (s and s 0 ) were not varied with CaO/SiO 2 when CaO/ SiO 2 1 and was slightly increased with CaO/SiO 2 when CaO/SiO 2 1. The increasing tendency is agreed with the results reported by Adachi et al. 5) using CaO SiO 2 /Fe 4.3C system at K, El Gammal and Stracke 6) using CaO SiO 2 /CK45 steel (0.038 mass% S) system at K, Mukai et al. 7) using CaO SiO 2 /Fe system at K and Bretonnet et al. 8) using CaO SiO 2 /Fe Si O system at K. The lower literature values than those found in this work are probably because higher oxygen and/or sulfur presented in their metal samples Interfacial Tension between Iron and CaO SiO 2 Al 2 O 3 Slag The influence of CaO/SiO 2 ratio of Slag B on the interfacial tension between iron and slag is shown in Fig. 4. Al 2 O 3 in these slags was approximately 30 wt%. The observed interfacial tension (s) increased with CaO/SiO 2, which is well agreed with those measured by El Gammal and Stracke 6) using CaO 15Al 2 O 3 SiO 2 /CK45 steel (0.038 mass% S) system at K, Popel 9) using CaO 32Al 2 O 3 SiO 2 /Fe 0.6C system at K and Bretonnet et al. 8) using CaO 28 32Al 2 O 3 SiO 2 /Fe Si O system at K. At the experimental temperature, SiO 2 is most unstable component in Slag B than the other two, CaO and Al 2 O 3 ; the decrease of SiO 2 with increasing CaO/SiO 2 ratio Fig. 5. Interfacial tension (mn/m) for CaO Al 2 O 3 SiO 2 slag with liquid iron at K. The numbers in parentheses refer to s 0. Lines are the results by Bretonnet et al. at K. in slag will decrease oxygen in the liquid. As can been seen in Table 3, the decrease oxygen with increasing CaO/SiO 2 ratio in slag is responsible for the increase in the interfacial tension. When the effect of oxygen in the liquid was taken into the account, the interfacial tension s 0, as presented by the open circle, was essentially not varied with the CaO/SiO 2 ratio. Figure 5 shows the interfacial tension map of CaO SiO 2 Al 2 O 3 slag at K. The measured values (s) are in general agreement with those reported by Bretonnet et al. 8) at K. However, compared to s, s 0 is nearly independent of slag composition except those on SiO 2 CaO line, where alumina was absent. In the other words, the interfacial tension for CaO SiO 2 Al 2 O 3 slag is primarily dominated by the oxygen in the liquid iron Effect of FeO, B 2 O 3, TiO 2, MgO, BaO and ZrO 2 Figure 6 shows the influence of FeO in Slag C on the interfacial tension. The interfacial tension suggested by Kawai et al. 10) using CaO SiO 2 Fe t O MnO MgO slags of various composition at K are lower than those found in present work. The high oxygen in the metal by MnO re ISIJ 410

5 Fig. 6. Variation of iron slag interfacial tension with FeO content in CaO SiO 2 Al 2 O 3 slag at K. Fig. 8. Variation of iron slag interfacial tension with TiO 2 content in CaO SiO 2 slag at K. Fig. 7. Variation of iron slag interfacial tension with B 2 O 3 content in CaO SiO 2 slag at K. duction is considered responsible for the lower interfacial tension for their slag systems. Popel 11) studied the effect of Fe t O ( FeO Fe 2 O 3 ) in oxide slags of CaO SiO 2 Al 2 O 3 MgO at K, their results show a similar tendency but generally lower interfacial tensions. Adachi et al. 12) measured the interfacial tensions for CaO Al 2 O 3 MgO slags at K which is well agreed with present study. FeO is an unstable oxide at high temperatures, FeO will undergo the reaction FeO (in slag) Fe [O] to provide oxygen to the metal phase. An increase in FeO will increase oxygen in liquid iron as can be confirmed in Table 3. The interfacial tension will be reduced by increasing oxygen in iron indirectly. Nevertheless, s 0 also decreased with increasing FeO in the slag as seen in Fig. 6. The variation of the interfacial tension with B 2 O 3 in Slag D is shown in Fig. 7. No significant change in the interfacial tension was found with varying B 2 O 3 content from 0% to 5% in the slag. As seen in Table 3, ZrO 2 varied from 7 16% in slag D due to the dissolution of the crucible material. Further investigation is considered necessary because ZrO 2 could also affect the interfacial tension. Figure 8 shows the influence of TiO 2 in Slag E on the interfacial tension. The measured interfacial tension remained unchanged when TiO 2 was added up to 25% in the slag. The values measured by Sharan and Cramb 13) using CaO Al 2 O 3 /Fe 20Ni system at K, Evseev and Filippov 14) using CaO Al 2 O 3 /steel system, Mao et al. 15) using CaO SiO 2 14Al 2 O 3 5MgO (CaO/SiO 2 1)/Fe 0.3Si 0.14Ti Fig. 9. Variation of iron slag interfacial tension with MgO content in CaO SiO 2 slag at K. 0.07Si system at K and Pham et al. 16) using 50MnO Al 2 O 3 TiO 2 /mild steel system at K are also presented in the figure. The influence of TiO 2 in slag was found insignificant in all studies except that observed by Sharan and Cramb. 13) Further investigation is considered necessary because a fairly large diversity was found between different investigations. As seen in Fig. 9, a slight increase in the interfacial tension was found with increasing MgO in the slag (Slag F) up to 20%. This tendency is agreed with the results (s 0 ) obtained by Sun et al. 3) using same slag and metal system. The sharp drop of the interfacial tension with further addition of MgO is considered because the precipitation of solid CaMgSiO 4 at the interface. The lower interfacial tension obtained by El Gammal and Stracke 6) using CaO SiO 2 (CaO/SiO 2 0.8)/CK45 steel (0.038 mass% S) at K and by Deev and Patskevich 17) using 40MnO 60SiO 2 /SV-08 steel at K may be due to the higher sulfur or oxygen contents in their metals. The influence of BaO in Slag G on the interfacial tension is presented in Fig. 10. The complicated behavior of the measured interfacial tensions (s) with varying BaO content in slag is considered because the variation of the oxygen content in the melt. As shown by open circles in the figure, s 0 was nearly independent of BaO content in the slag. The values measured by Adachi et al. 12) for CaO MgO SiO 2 slag at K agreed well with this work, while those by ISIJ

6 Fig. 10. Variation of iron slag interfacial tension with BaO content in CaO Al 2 O 3 SiO 2 slag at K. 4. Conclusions The effect of slag composition on the interfacial tension between slag and liquid iron was examined at K. The oxygen content in the liquid iron was measured and its effect on the interfacial tension was taken into account in the examination of the interfacial tensions. The interfacial tension was slightly increased with CaO/ SiO 2 ratio for CaO SiO 2 slag. For CaO SiO 2 Al 2 O 3 slag, the interfacial tension was found primarily dominated by the oxygen content in the liquid iron. When the additional oxide is present in CaO SiO 2 Al 2 O 3 slag, the interfacial tension was found to increase with increasing MgO content but decrease with increasing FeO content in the slag, while B 2 O 3, TiO 2, BaO and ZrO 2 in the slag were found to have an insignificant effect on the interfacial tension. REFERENCES Fig. 11. Variation of iron slag interfacial tension with ZrO 2 content in CaO Al 2 O 3 SiO 2 slag at K. Liu et al. 18) for CaO Al 2 O 3 SiO 2 (CaO/SiO )/Fe system at and K show slightly lower interfacial tension. The results with varying ZrO 2 in slag are presented in Fig. 11. Changes of both s and s 0 were insignificant and agreed well with those reported in the previous work 3) using similar slag and metal system, and with the results by Linzer and Myslivec 19) from MnO SiO 2 ZrO 2 /Fe 14.4Cr system at K. Considering the relatively small effect of ZrO 2 on the interfacial tension, ZrO 2 dissolution from crucible is considered to have a negligible effect on the results obtained in this study. 1) H. Sun, R. Ito, K. Nakashima and K. Mori: Tetsu-to-hagané, 81 (1995), ) K. Nakajima: Tetsu-to-Hagané, 80 (1994), ) H. Sun, K. Nakashima and K.Mori: ISIJ Int., 37 (1997), ) H. Sakao and K. Sano: Trans. Jpn. Inst. Met., 1 (1960), 38. 5) A. Adachi, K. Ogino and T. Suetaki: Tetsu-to-hagané, 50 (1964), ) T. EI Gammal and P. Stracke: Proc. 3rd Int. Conf. on Molten Slags and Fluxes, Glasgow, The Institute of Metals, London, (1989), ) K. Mukai, T. Kato and H. Sakao: Tetsu-to-Hagané, 59 (1973), 55. 8) J. L. Bretonnet, L. D. Lucas and M. Ollette: Comptes Rendus des Seances de l Academie des Sciences, Serie C: Sciences Chimiques, 280 (1975), 1169 and 285 (1977), 45. 9) S. I. Popel: Izv. V.U.Z. Chernaya. Metall., 8 (1962), 9. 10) Y. Kawai, N. Shinozaki and K. Mori: Can. Metall. Q., 21 (1982), ) S. I. Popel: Sb. Nauch. Tr. Uralskii Politehn. Inst., 126 (1963), 5 12) A. Adachi, K. Ogino, T. Suetaki and T. Saito: Tetsu-to-Hagané, 51 (1965), ) A. Sharan and A. V. Cramb: Metall. Mater. Trans., 26B (1995), ) P. P. Evseev and A. F. Filippov: Izv. V.U.Z. ov, 6 (1965), ) Y. Mao, W. Luo, F. Lu and Y. Zhu: Gangtie, 22 (1987), ) C. K. Pham, J. Krusina and T. Myslivec: Kovové Mater., 16 (1978), ) G. F. Deev and I. R. Patskevich: Avtam. Svarka, 2 (1971), 5. 18) W. Liu, N. J. Themelis, T. EI Gammal and F. L. Zhao: Trans. ISS, (1992), June, ) E. Linzer and T. Myslivec: Kovové Mater., 16 (1978), ISIJ 412