Phase Equilibrium for the CaO SiO 2 FeO 5mass%P 2 O 5 5mass%Al 2 O 3 System for Dephosphorization of Hot Metal Pretreatment

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1 , pp Phase Equilibrium for the CaO SiO 2 FeO 5mass%P 2 O 5 5mass%Al 2 O 3 System for Dephosphorization of Hot Metal Pretreatment Xu GAO, 1) Hiroyuki MATSUURA, 1) * Masaki MIYATA 2) and Fumitaka TSUKIHASHI 1) 1) Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo, Kashiwanoha, Kashiwa, Chiba, Japan. 2) Nippon Steel & Sumitomo Metal Corporation, 16-1 Sunayama, Kamisu, Ibaraki, Japan. (Received on February 1, 2013; accepted on April 18, 2013) Recently, after the restriction of the use of CaF 2, dephosphorization process often generates large amount of slag, due to the neglect of refining functions of solid phases. Consequently, this brings environmental issues and influences refining. In order to improve the utilization efficiency of solid CaO and its compounds in the dephosphorization slag, multiphase flux refining has been proposed by considering the enrichment of phosphorus within the solid phases. As to provide theoretical fundamentals for both understanding on the reaction mechanism of phosphorus and practical slag control, phase relationship for the CaO SiO 2 FeO 5mass%P 2O 5 5mass%Al 2O 3 system has been studied based on chemical equilibration technique with oxygen partial pressure of atm at K. In current work, the liquidus saturated with P 2O 5-rich solid solution has been firstly deduced on the CaO SiO 2 FeO ternary system, and the discussions on the relationship between solid solution and liquid phase has been proceeded. It has been found that the existence of Al 2O 3 enlarges the liquid phase area, but does not affect the composition of solid solution. On the other hand, the equilibrium solid phase has been confirmed as 2CaO SiO 2 3CaO P 2O 5 solid solution, while the ratio between both varies along with liquidus. As expected, the large equilibrium partition ratio of phosphorus between solid solution and liquid slag has also been found and discussed. KEY WORDS: dephosphorization; multiphase flux; phase relationship; thermodynamics; steelmaking. 1. Introduction In steelmaking process, dephosphorization often requires lime addition to achieve high slag basicity. As to accelerate the lime dissolution, fluorite has been used as a flux agent. But due to serious environmental issues, the application of it has been restricted. Consequently, much more lime is required to maintain high dephosphorization ability of molten slag. As a result, the slag amount increases and much solid phases remain after hot metal pretreatment. In order to promote the utilization efficiency of solid phases, the refining function of solid phases such as remained CaO and condensed 2CaO SiO 2 solid solution has been reconsidered. Based on this perspective, it is expected that the lime consumption could be reduced. Afterwards, an innovative refining process for dephosphorization by using multiphase flux has been proposed. Lots of efforts have been made to understand the multiphase flux system. 1 19) However, some thermodynamic studies, especially for the equilibrium phase relationship between solid phases and molten slag, yet remain insufficient. * Corresponding author: matsuura@k.u-tokyo.ac.jp DOI: In this study, the phase relationship for the CaO SiO 2 FeO 5mass%P 2O 5 5mass%Al 2O 3 slag system has been studied experimentally with constant low oxygen partial pressure at hot metal pretreatment temperature. Components of CaO, SiO 2, FeO and P 2O 5 in the slag system stand for the basic dephosphorization slag, while Al 2O 3 which commonly exists in refining slag systems has been added due to the consideration of promoting the melting condition. Based on current experimental results and discussions, liquidus at high CaO/SiO 2 ratio region has been deduced, and also the solid phases equilibrated with it have been studied. 2. Experimental Chemical equilibration technique 20 25) has been adopted to investigate the phase relationship, and the detail experimental procedures have been described in our previous work. 26) Reagent grade of SiO 2, 3CaO P 2O 5 and Al 2O 3, and synthesized CaO and FeO were used for preparing the initial oxide mixtures. The concentrations of CaO, SiO 2 and FeO were determined by using the phase diagram of the CaO SiO 2 FeO x ternary system, and the concentrations of P 2O 5 and Al 2O 3 were both set to be 5 mass% constantly according to practical refining condition. The compositions of initial oxide mixtures are shown in Table ISIJ

2 Table 1. Compositions of initial oxides mixture. Table 2. Compositions of observed phases. No. FeO CaO SiO 2 For each experiment, about 0.1 g of prepared oxide mixture were firstly loaded in a platinum crucible (height: 5 mm; diameter: 5 mm), and then it was heated to K for premelting with an atmosphere of argon gas. After 1 to 3 hours preservation at K, temperature decreased to K by 10 K/min, and during this period the CO CO 2 gas mixture was introduced. The volume ratio between CO and CO 2 gases was maintained to be approximately 5:1 according to Eq. (1) to control the oxygen partial pressure of atm which could reflect the practical thermodynamic condition within the multiphase flux system during dephosphorization. Therefore, the precise oxygen partial pressure for experiments was atm. J/mol 27)... (1) Time to achieve chemical equilibration was 3 to 24 hours according to preliminary experiments. After equilibration, the crucible containing multiphase flux was quickly taken out and quenched by liquid nitrogen. Finally, Scanning Electron Microscope (SEM) with Energy Dispersive Spectroscopy (EDS) was used to analyze the phase compositions. In order to diminish the uncertainty of EDS analysis, quantity repetitions of analysis for the same phase and also area analysis were employed. 3. Results and Discussion P 2O 5 Al 2O 3 CaO/SiO 2 mole ratio CO(g) + 1/2O 2(g) = CO 2(g) ΔG = T No. Phase Compositions Al 2O 3 SiO 2 P 2O 5 CaO FeO Liquid CaO SiO 2 3CaO P 2O 5 solid solution Liquid CaO SiO 2 3CaO P 2O 5 solid solution Liquid CaO SiO 2 3CaO P 2O 5 solid solution Liquid CaO SiO 2 3CaO P 2O 5 solid solution Liquid CaO SiO 2 3CaO P 2O 5 solid solution Liquid CaO SiO 2 3CaO P 2O 5 solid solution Liquid Liquid CaO SiO 2 3CaO P 2O 5 solid solution Liquid CaO SiO 2 3CaO P 2O 5 solid solution Liquid CaO SiO 2 3CaO P 2O 5 solid solution Liquid CaO SiO 2 3CaO P 2O 5 solid solution Liquid CaO SiO 2 3CaO P 2O 5 solid solution Liquid CaO SiO 2 3CaO P 2O 5 solid solution Liquid Solid CaO Liquid CaO SiO 2 3CaO P 2O 5 solid solution Solid CaO The compositions of the observed phases are listed in Table 2. By using the observed data of the elements, the corresponding oxide compositions were calculated. Since the valence of iron ions cannot be detected by EDS, and also due to the low Fe 3+ /Fe 2+ ratio under current experimental conditions, 20) only Fe 2+ has been considered for presentation purpose. The equilibrium status shown in Table 2 has been determined by comparing current phase compositions to the wellstudied ternary systems such as the CaO SiO 2 FeO, CaO SiO 2 Al 2O 3 and CaO SiO 2 P 2O 5 systems Liquidus for the CaO SiO 2 FeO 5mass%P 2O 5 5mass%Al 2O 3 System with P O2 of atm at 1673 K Since the liquid phase mainly contains CaO, SiO 2, FeO and nearly constant Al 2O 3, according to Table 2, it can be discussed by projecting on the CaO SiO 2 FeO ternary section as shown in Fig. 1. Ignoring the variation in Al 2O 3 concentration, the solid line shown in Fig. 1 represents the liquidus for the CaO SiO 2 FeO P 2O 5 10mass%Al 2O 3 system with P O2 of atm at K. This 10 mass% Al 2O 3 in liquidus is judged according to Fig. 2, which implies that mostly Al 2O 3 is enriched in liquid phase and its concentration fluctuates around 10 mass%. Fraction of solid phase to total slag in weight which was calculated by the mass balance of Al 2O 3 was in the range between 25 and 2013 ISIJ 1382

3 Fig. 1. Projections of equilibrium phases compositions on the CaO SiO 2 FeO ternary section. Fig. 3. Phase sections for the CaO SiO 2 FeO 5mass%P 2O 5 5mass%Al 2O 3 system with oxygen partial pressure of atm at K on the CaO SiO 2 FeO ternary section. Fig. 2. Distribution of Al 2O 3 between liquid slag and solid solution. 68%, mainly around 50%. The area fraction of solid phase to total slag calculated from SEM images was also around 50%. Comparing with the liquidus for the CaO SiO 2 FeO x system equilibrated with metallic iron, 28) current measured liquid phase area enlarges a little. Although the oxygen partial pressure in current work is higher 3) and the condensed solid solution is different, both of which should lead to the shrinkage of liquid phase area and thus the slight enlargement occurs by the contribution of the existence of Al 2O 3 in liquid phase. Comparing with the CaO SiO 2 FeO Fe 2O 3 5mass%Al 2O 3 system with P O2 of 10 8 atm at K, 29) the liquid phase area also enlarges, especially in the region where SiO 2 content is larger than 20 mass%. However, opposite trend appears near the inflection point of liquidus. The liquidus shifts towards the FeO apex a little, as shown as the solid line locating on the right side of the dash line at the region of FeO content between 60 and 80 mass% in Fig. 1, though current liquid phase contains larger Al 2O 3 content. This difference is due to the condensation of 2CaO SiO 2 3CaO P 2O 5, which consumes more CaO than 2CaO SiO 2. On the other hand, this shrinkage of liquid phase area becomes noticeable only near the inflection point of liquidus where the FeO content reaches the largest value. This implies a stronger promotion of the condensation of P 2O 5- containing solid solution with larger FeO content in liquid phase. The CaO primary region shrinks a lot comparing to the CaO SiO 2 FeO x system equilibrated with metallic iron, and the following two reasons can be considered. One is the enlargement of liquid phase area by the addition of Al 2O 3 as already mentioned. Another one could be that the 3CaO SiO 2 solid solution has not been observed in current work as shown in Table 2, which means that the 3CaO SiO 2 primary region no longer exists. Therefore, the confinement to the liquidus vanishes and the liquid phase could extend to a larger CaO content region. Based on above discussion, the phase sections have been deduced as shown in Fig. 3. Because the projection of 2CaO SiO 2 3CaO P 2O 5 solid solution represents a composition range rather than a definite point, the composition of solid solution is expressed as the dash-dotted-dotted line with two arrows at both ends. On the other hand, due to the low solubility of FeO in solid solution, space has been kept between the boundaries of phase section 1 and the CaO SiO 2 tie line Effect of Al 2O 3 Addition on Liquidus The effect of Al 2O 3 addition on the liquidus is shown in Fig. 4 by comparing with the CaO SiO 2 FeO 5mass%P 2O 5 system equilibrated at the same experimental conditions. 26) It has been found that the enlargement of liquid phase area becomes more distinct at larger FeO content region by the addition of Al 2O 3. Noting that the CaO FeO and high T.Fe phases mentioned in the previous work 26) has been assumed as those formed during quenching Condensed 2CaO SiO 2 3CaO P 2O 5 Solid Solution In Fig. 5, the compositions of solid solutions have been projected on the CaO SiO 2 P 2O 5 ternary section. It is observed that the compositions of condensed solid solutions locate within the 2CaO SiO 2 3CaO P 2O 5 stable region, 28) rather than 3CaO SiO 2 or 4CaO P 2O 5 stable regions. Composition of solid solutions is close to the tie line between 2CaO SiO 2 and 3CaO P 2O 5 as shown in Fig. 5. Therefore, the solid solutions can be discussed by using the ISIJ

4 Fig. 6. Projections of compositions of condensed solid solutions on the 2CaO SiO 2 3CaO P 2O 5 pseudo binary system. Fig. 4. Effect of Al 2O 3 addition on the liquidus with oxygen partial pressure of atm at K. Fig. 7. Relationship between phosphorus partition ratio and T.Fe content in liquid slag. Fig. 5. Projections of compositions of condensed solid solutions on the CaO SiO 2 P 2O 5 ternary section. 2CaO SiO 2 3CaO P 2O 5 pseudo binary system 30) as shown in Fig. 6. At K, only R type solid solution [(α- 2CaO SiO 2 α-3cao P 2O 5) S.S.] equilibrates with liquid phase, and the effect of Al 2O 3 on the composition of solid solution has not been discovered Phosphorus Partition between Solid Solution and Liquid Slag The phosphorus partition between solid solution and liquid slag within multiphase flux system has been studied by many researchers. 7,31 33) In this study, the phosphorus partition has been considered with the equilibrium phase relationship. The phosphorus partition ratio between 2CaO SiO 2 3CaO P 2O 5 and liquid slag (see Sections 1 and 2 in Fig. 3) has been concerned. When liquid slag coexists with solid CaO, 2CaO SiO 2 3CaO P 2O 5 solid solution has not been observed. Therefore, the phosphorus partition ratio has not been calculated. As shown in Fig. 7, current results agree well with the relationship observed by Ito et al. 31) Also, the dashed curve with arrows represents the composition change of liquid slag along the liquidus with decreasing SiO 2 concentration. Fig. 8. Relationship between phosphorus partition ratio and CaO content in liquid slag. As indicated by these arrows, the phosphorus partition ratio firstly increases and then decreases. The smallest value was observed when solid CaO, 2CaO SiO 2 3CaO P 2 O 5 and liquid slag coexist. Comparing with the CaO SiO 2 FeO 5mass%P 2 O 5 system with the same experimental conditions, 26) current results shift to the left side in Fig. 7, which reveals the movement of liquidus against the FeO apex after Al 2 O 3 addition. Similarly, the relationship between phosphorus partition ratio and CaO content in liquid phase is demonstrated in Fig. 8. Opposite to the effect of T.Fe content in liquid slag, the phosphorus partition ratio between solid solution and 2013 ISIJ 1384

5 liquid slag decreases with increasing CaO content in liquid phase, which is consistent with the work of Pahlevani et al. 33) The variation of liquid phase compositions with decreasing SiO 2 concentration along with liquidus is expressed as the dashed curve with arrows, which shows the similar trend shown in Fig Conclusions By using chemical equilibration technique, the phase relationship for the CaO SiO 2 FeO 5mass%P 2O 5 5mass%Al 2O 3 with oxygen partial pressure of atm at K has been investigated. The conclusions are summarized as follows: the liquidus saturated with 2CaO SiO 2 3CaO P 2O 5 solid solution for the above slag system has been deduced. Solid solution almost consists of 2CaO SiO 2 and 3CaO P 2O 5, and the ratio between both varies with the liquid phase composition. Addition of Al 2O 3 enlarges the liquid phase area, while it does not affect the composition of solid solution since Al 2O 3 is mainly distributed to the liquid phase. At equilibrium, the large partition ratio of phosphorus between solid solution and liquid slag is found. However, this partition ratio does not reach larger value compared to that for the CaO FeO SiO 2 system with the concentration of Al 2O 3 in the liquid slag, because FeO content on the liquidus becomes smaller than that for the system without Al 2O 3. REFERENCES 1) F. Tsukihashi: Tetsu-to-Hagané, 95 (2009), ) N. Ishiwata and H. Ito: Tetsu-to-Hagané, 95 (2009), ) K. Miyamoto, K. Naito, I. Kitagawa and M. Matsuo: Tetsu-to- Hagané, 95 (2009), ) A. Matsui, S. Nabeshima, H. Matsuno, N. Kikuchi and Y. Kishimoto: Tetsu-to-Hagané, 95 (2009), ) K. Watanabe, T. Miki, Y. Sasaki and M. Hino: Tetsu-to-Hagané, 95 (2009), ) M. Hasegawa and M. Iwase: Tetsu-to-Hagané, 95 (2009), ) K. Shimauchi, S. Kitamura and H. Shibata: Tetsu-to-Hagané, 95 (2009), ) M. Kami, M. Terasawa, A. Matsumoto and K. Ito: Tetsu-to-Hagané, 95 (2009), ) K. S. Pham and Y. Kashiwaya: Tetsu-to-Hagané, 95 (2009), ) Y. Kashiwaya and K. S. Pham: Tetsu-to-Hagané, 95 (2009), ) R. Saito, H. Matsuura, K. Nakase, X. Yang and F. Tsukihashi: Tetsuto-Hagané, 95 (2009), ) X. Yang, H. Matsuura and F. Tsukihashi: Tetsu-to-Hagané, 95 (2009), ) T. Tanaka, Y. Ogiso, M. Ueda and J. Lee: Tetsu-to-Hagané, 95 (2009), ) N. Saito, S. Yoshimura, S. Haruki, Y. Yamaoka, S. Sukenaga and K. Nakashima: Tetsu-to-Hagané, 95 (2009), ) M. Susa, N. Tsuchida, R. Endo and Y. Kobayashi: Tetsu-to-Hagané, 95 (2009), ) K. Asakura and K. Ito: Tetsu-to-Hagané, 95 (2009), ) H. Kubo, K. Matsubae-Yokoyama and T. Nagasaka: Tetsu-to- Hagané, 95 (2009), ) K. Matsubae-Yokoyama, H. Kubo and T. Nagasaka: Tetsu-to- Hagané, 95 (2009), ) S. Kitamura, K. Miyamoto, H. Shibata, N. Maruoka and M. Matsuo: Tetsu-to-Hagané, 95 (2009), ) H. Kimura, S. Endo, K. Yajima and F. Tsukihashi: ISIJ Int., 44 (2004), ) H. Matsuura, M. Kurashige, M. Naka and F. Tsukihashi: ISIJ Int., 49 (2009), ) H. M. Henao, F. Kongoli and K. Itagaki: Mater. Trans., 46 (2005), ) S. Nikolic, P. C. Hayes and E. Jak: Metall. Mater. Trans. B, 39B (2008), ) T. Hidayat, P. Hayes and E. Jak: Metall. Mater. Trans. B, 43B (2012), ) Y. Kang and H. Lee: ISIJ Int., 45 (2005), ) X. Gao, H. Matsuura, I. Sohn, W. Wang, D. J. Min and F. Tsukihashi: Metall. Mater. Trans. B, 43B (2012), ) E. T. Turkdogan: Physical Chemistry of High Temperature Technology, Academic Press, New York, (1980), 7, ) Verein Deutscher Eisenhüttenleute ed.: SLAG ATLAS, 2nd ed., Verlag Stahleisen GmbH, D-Düsseldorf, (1995), 126, ) H. Kimura, T. Ogawa, M. Kakiki, A. Matsumoto and F. Tsukihashi: ISIJ Int., 45 (2005), ) W. Fix, H. Heyman and R. Heinke: J. Am. Ceram. Soc., 52 (1969), ) K. Ito, M. Yanagisawa and N. Sano: Tetsu-to-Hagané, 68 (1982), ) R. Inoue and H. Suito: ISIJ Int., 46 (2006), ) F. Pahlevani, S. Kitamura, H. Shibata and N. Maruoka: ISIJ Int., 50 (2010), ISIJ