Xiao YANG, Hiroyuki MATSUURA and Fumitaka TSUKIHASHI

Transcription

1 , pp Reaction Behavior of P 2 O 5 at the Interface between Solid 2CaO SiO 2 and Liquid CaO SiO 2 FeO x P 2 O 5 Slags Saturated with Solid 5CaO SiO 2 P 2 O 5 at K Xiao YANG, Hiroyuki MATSUURA and Fumitaka TSUKIHASHI Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo, Kashiwanoha, Kashiwa, Chiba Japan. (Received on December 21, 2009; accepted on February 12, 2010) It is important to understand the role of solid phases such as solid CaO or 2CaO SiO 2 in hot metal dephosphorization. In the present study, the reaction behavior of P 2 O 5 at the interface between solid 2CaO SiO 2 and liquid CaO SiO 2 FeO x P 2 O 5 slags saturated with solid 5CaO SiO 2 P 2 O 5 at K was investigated. The result shows that the solid 2CaO SiO 2 reacts with the liquid phase in the two phase mixture to form the P 2 O 5 condensed phases. Compared with the results by using the homogeneous slag as reported in the previous papers, the formation of P 2 O 5 condensed phases is more evident and the back dissolution of P 2 O 5 condensed phases into the surrounding slag is restrained in this study. These results indicate that the dissolution of P 2 O 5 condensed phases can be considerably restrained in the case that the bulk slag is saturated with solid 5CaO SiO 2 P 2 O 5 before reaction. KEY WORDS: multi phase flux; dephosphorization; dicalcium silicate; P 2 O 5 condensed phases; dissolution. 1. Introduction In the hot metal dephosphorization, the generated CaObased slag contains both liquid phase and solid CaO and 2CaO SiO ) The existence of solid phases causes problems such as increase of slag volume and difficulty of slag recycling. Fluorspar (CaF 2 ) was ever commonly used as an additive to promote the melting of CaO-based flux and improve its fluidity. However owing to the release of hazardous fluoride ion from the disposed slag, the use of fluorspar is strictly limited based on the concept of ecofriendly steel production. The reduction of CaO consumption and slag emission by an approach without adverse technical and environmental effects is the existing problem that the steel makers confront. Since many studies 1,4 9) had confirmed that phosphorus in the liquid phase can be fixed into the 2CaO SiO 2 3CaO P 2 O 5 solid solution or compounds by CaO contained in the steelmaking slag, the idea is proposed to innovate the present refining process by promoting the transfer of phosphorus from liquid phase to solid phase for better dephosphorization. Reduction of the lime consumption and slag emission can be achieved by improving the utilization efficiency of solid CaO or 2CaO SiO 2 phase in the steelmaking slag. In order to develop the new technology, it is firstly indispensible to understand the microscopic reaction mechanism involving the solid phase in the multi phase flux. Hamano et al. 10) clarified the reaction mechanism between solid CaO and CaO SiO 2 FeO x P 2 O 5 slag by dipping solid CaO into the slag for different time at K and analyzing the interface after reaction. It was observed that P 2 O 5 in the liquid slag is condensed into the firstly precipitated 2CaO SiO 2 phase to form the 2CaO SiO 2 3CaO P 2 O 5 solid solution or compound. Since many researchers had confirmed the enrichment of P 2 O 5 in 2CaO SiO 2 phase in steelmaking slag, it is considered that the precipitation of 2CaO SiO 2 phase in the CaO based flux is necessary for the formation of P 2 O 5 rich phase. However, an integrated explanation of the formation mechanism of the phosphate compound in 2CaO SiO 2 phase in steelmaking slag is still unavailable. Upon the unsolved problem, the reaction between solid 2CaO SiO 2 and CaO SiO 2 FeO x P 2 O 5 slag is investigated by the present authors. In the previous study, 11,12) the reaction between solid 2CaO SiO 2 and homogeneous CaO SiO 2 FeO x P 2 O 5 slag was studied by dipping solid 2CaO SiO 2 disc into the liquid slag for different time at or K. It is understood that the P 2 O 5 condensed phases (2CaO SiO 2 3CaO P 2 O 5 solid solution or compounds) are formed at the 2CaO SiO 2 /slag interface in less than 1 s. The occurrence of back dissolution of the solid P 2 O 5 rich phase soon after its formation was observed, indicating that the transfer of phosphorus in flux from liquid phase to solid phase is reversible. In the practical production, this property can be utilized to increase the concentration of phosphorus in the solid phase in hot metal dephos- 702

2 phorization slag. Therefore, it is required to understand the reaction behavior of phosphorus in the heterogeneous flux in which the liquid phase is saturated with the solid P 2 O 5 condensed phase. Little information on this topic can be found in the literature. In the present study, the authors try to clarify the reaction mechanism between solid 2CaO SiO 2 and the slag saturated with solid 5CaO SiO 2 P 2 O 5, and also to verify the assumption that the dissolution of P 2 O 5 condensed phases can be considerably restrained when the bulk slag is saturated with solid 5CaO SiO 2 P 2 O 5 before reaction. However, since the phase diagram for the CaO SiO 2 FeO x P 2 O 5 quaternary system is unavailable, it is impossible to make sure the accurate composition of the slag saturated with solid 5CaO SiO 2 P 2 O 5 in the experiment. Therefore, the heterogeneous CaO SiO 2 FeO x P 2 O 5 slags containing solid 5CaO SiO 2 P 2 O 5 were used. The solid 2CaO SiO 2 disc was reacted with the slags at K. The formation mechanism of P 2 O 5 condensed phases at the interface between solid 2CaO SiO 2 and the heterogeneous slag was discussed. The results were also compared with the findings in the previous study. 11,12) 2. Experimental The CaO SiO 2 FeO x P 2 O 5 slag was prepared by mixing synthesized CaO, FeO x, reagent grade SiO 2 and 3CaO P 2 O 5. Powder of CaO was obtained by the calcination of reagent grade CaCO 3 for 24 h at K in air. FeO x was synthesized by sintering an equimolar mixture of reagent grade Fe 3 O 4 and electrolytic Fe powders in iron crucible at K with CO CO 2 atmosphere (CO/CO 2 1) for 24 h. The initial composition of slags are listed in Table 1. The solid 2CaO SiO 2 disc was prepared by the following method. Firstly, synthesized CaO and reagent grade SiO 2 were mixed on molar ratio of 2 : 1 and sintered in a platinum crucible at K for 24 h in air. About 1.0 mass% 3CaO P 2 O 5 was added into the mixture to prevent the phase transformation from b-2cao SiO 2 to g-2cao SiO 2 with volume expansion during the sintering, followed by pressing the powder at 50 MPa into a die of diameter 10 mm, thickness 1 mm. Prepared tablet was sintered once more in the platinum crucible at K for 200 h in air to obtain the solid b-2cao SiO 2 sample. The last step was the confirmation of the formation of b-2cao SiO 2 by X-ray diffraction analysis. The reaction between solid 2CaO SiO 2 and the heterogeneous CaO SiO 2 FeO x P 2 O 5 slag was conducted in the center of a vertical mullite reaction tube (I.D.: 52 mm, O.D.: 60 mm, Length: mm) in an electric furnace. Ten grams of slag was charged in an alumina crucible (I.D.: 34 mm, O.D.: 38 mm, Height: 45 mm) with the coexistence of electrolytic iron (about 3 g) to maintain the Fe 3 /Fe 2 ratio in the slag constant. The crucible was set in the hot zone of the reaction tube at K. In the previous study on reaction between CaO and slag, 8) both Al 2 O 3 crucible and Fe crucible were used. The results proved that the dissolved Al 2 O 3 did not affect the reaction behavior of P 2 O 5 significantly. Accordingly, it is supposed that the influence of Al 2 O 3 impurity on the reaction behavior is also negligible in the present study. High purity Ar with a flow rate Table 1. Initial compositions of the heterogeneous slags. 700 cm 3 /min was introduced from the bottom of the reaction tube. After the slag had been maintained for s at experimental temperature, the solid 2CaO SiO 2 disc attached to the tip of a ceramic tube by the platinum wire was inserted in the reaction tube, suspended slightly above the liquid slag for 120 s to ensure thermal equilibrium, and then dipped into the slag to react. The reaction time was defined as the duration that the solid sample was kept in the slag. After reaction for 1 to 600 s, the solid 2CaO SiO 2 sample with adhered slag was rapidly withdrawn from the reaction tube and quenched by immersing in liquid nitrogen, followed by embedding in the polyester resin. The cross-section was polished and the interface between 2CaO SiO 2 and slag was observed and analyzed by SEM/EDS. 3. Results 3.1. Mineralogy of the Heterogeneous Slag before the Reaction with 2CaO SiO 2 In the experiment at K, the heterogeneous slag before the reaction with 2CaO SiO 2 was sampled by the mullite tube to study the mineralogy of the slag. The solid phases can be easily identified by microscope due to their large size, as shown in Fig. 1. The numbers shown in the figures correspond to the positions analyzed by EDS as listed in Table 2, where the iron oxide is calculated as FeO. The dark grey particles were proved to be with the composition of 5CaO SiO 2 P 2 O 5 from EDS analysis at positions 1, 4, 6, 7, 9, 12 and 17 in the SEM images as shown by the open circles. It is considered that the coexisting liquid phase such as position numbers 2, 3, 5, 8, 10, 13, 14 and 16 is saturated with the solid 5CaO SiO 2 P 2 O 5. The compositions of the liquid phase in the heterogeneous slag were calculated by averaging the EDS results as shown in Table Interface between 2CaO SiO 2 and Slag A (FeO x 20 mass%, CaO/SiO 2 1.3) The SEM images of the interfaces between solid 2CaO SiO 2 and the heterogeneous slag A after reaction for 1, 10, 60 and 180 s at K are shown in Fig. 2 where the left side is the original solid 2CaO SiO 2, while the bulk slag is on the right. The boundary can be clearly identified. The composition analysis by EDS at different positions across the interface was conducted. During EDS analysis, the originally precipitated 5CaO SiO 2 P 2 O 5 were avoided to detect and analyze only newly precipitated P 2 O 5 condensed phase. The existence of high P 2 O 5 content positions was revealed through EDS analysis. As mentioned in previous studies, 11,12) the precipitated P 2 O 5 condensed phase was smaller than the electron beam size and thus the obtained compositions by EDS were the average composition between P 2 O 5 condensed phase and surrounding liquid phase. Therefore, in the present study the positions where the FeO content is less than 16.0 mass%, CaO/SiO 2 molar ratio is larger than 703

3 ISIJ International, Vol. 50 (2010), No. 5 Fig. 1. SEM images of the heterogeneous slags. Table 2. Results of EDS analysis for Fig. 1. while plotting the concentration profiles. Similar in four cases, CaO content is decreasing from 2CaO SiO2 to slag and FeO content shows the incremental tendency while SiO2 content almost keeps constant. The positions with the formation of P2O5 condensed phases are labeled by open symbol, while other positions are labeled with solid symbols. The solid line is drawn from the tendency of FeO content. In order to describe the location of the P2O5 condensed phases, the area near the interface can be divided into three regions based on the variation of FeO: solid 2CaO SiO2 area (constant low FeO content), multi phase area with coexistence of solid 2CaO SiO2, 2CaO SiO2 3CaO P2O5 solid solution/compounds and liquid slag (increasing FeO content), and bulk slag area (constant high FeO content), as illustrated in Fig. 4.12) As mentioned in the previous study,12) the multi phase area where FeO content increases from 2CaO SiO2 side toward bulk slag side is formed by the penetration of bulk slag inside 2CaO SiO2 piece, and solid 2CaO SiO2 continuously dissolves in the bulk slag Table 3. Compositions of the liquid phases in the heterogeneous slags. 1.5 and P2O5 content is more than 1.0 mass% are defined as P2O5 condensed positions and labeled by open circles in the figure. It is regarded that P2O5 condensed phases are formed at these positions. The positions without formation of P2O5 condensed phases are labeled by solid circles. The concentration profiles of the components in the system are plotted in Fig. 3. An arbitrary reference location was chosen inside the 2CaO SiO2 phase. The composition at each analyzed position was recorded as the function of its distance from the reference location as shown in the figure. As mentioned above, the initially existing solid 5CaO SiO2 P2O5 before the reaction were not considered 704

4 ISIJ International, Vol. 50 (2010), No. 5 Fig. 2. SEM images of the interface between 2CaO SiO2 and slag A at K. Fig. 3. Concentration profiles of components against the position at the interface between 2CaO SiO2 and slag A at K. during the penetration. Therefore, it is considered that the reaction interface between solid 2CaO SiO2 and the slag moves inside the solid as reaction time increases. The location of the region with P2O5 condensed phases is obtained with the concentration profiles. The P2O5 condensed phases are only located in the multi phase area after reaction for 1 s. After reaction for longer time, the P2O5 condensed phases can be observed in both the multi phase area and the bulk slag area, and the region with P2O5 condensed phase is expanding. The width of the region in each case is marked in the figure. Moreover, it can be noticed that the P2O5 content at the positions with the formation of 705

5 P 2 O 5 condensed phases shows an increasing tendency from solid 2CaO SiO 2 to the bulk slag area in each case. Fig. 4. Partition of the area near the interface. 12) 3.3. Interface between 2CaO SiO 2 and Slag B (FeO x 30 mass%, CaO/SiO 2 1.3) Slag B contains more FeO x than slag A, while the molar ratio of CaO/SiO 2 is the same. The concentration profiles of the components across the interfaces between 2CaO SiO 2 and slag B after reaction for 1, 10, 60, 180 and 600 s at K are shown in Fig. 5. The P 2 O 5 condensed phases were observed in both the multi phase area and the bulk slag area in all these five cases. After reaction for longer time, P 2 O 5 condensed phases were formed in wider region. The P 2 O 5 content at the positions with the formation of P 2 O 5 condensed phases is not simply increasing from 2CaO SiO 2 to the bulk slag Fig. 5. Concentration profiles of components against the position at the interface between 2CaO SiO 2 and slag B at K. 706

6 Fig. 6. Concentration profiles of components against the position at the interface between 2CaO SiO 2 and slag C at K. area in some cases such as reaction for 10, 60 and 180 s Interface between 2CaO SiO 2 and Slag C (FeO x 30 mass%, CaO/SiO 2 1.5) The molar ratio of CaO/SiO 2 in slag C is larger than that of the above two slags. The concentration profiles for the components across the interfaces between 2CaO SiO 2 and slag C after reaction for 1, 10, 60, and 600 s at K are shown in Fig. 6. The P 2 O 5 condensed phases are observed in both the multi phase area and the bulk slag area in all these four cases, and the region with P 2 O 5 condensed phases is expanding. The P 2 O 5 content at the positions with the formation of P 2 O 5 condensed phases is very clearly increasing from 2CaO SiO 2 to the bulk slag area in each case. 4. Discussion 4.1. The P 2 O 5 Condensed Phases at the Interface Since the initial slag contains both solid phase (5CaO SiO 2 P 2 O 5 ) and liquid phase before the reaction, it is considered that the P 2 O 5 condensed phases were newly formed by two mechanisms after the dipping of solid 2CaO SiO 2. On one hand, according to the phase diagram for the 2CaO SiO 2 3CaO P 2 O 5 binary system, 9) solid 2CaO SiO 2 and solid 5CaO SiO 2 P 2 O 5 can form stable solid solution or another phosphate compound 7CaO 2SiO 2 P 2 O 5 in wide composition range at K. Therefore, it is considered that one of the mechanisms is the reaction between solid 2CaO SiO 2 and the initially existing solid 5CaO SiO 2 P 2 O 5 in the slag. On the other hand, solid 2CaO SiO 2 also reacts with the liquid phase in the slag to form the P 2 O 5 condensed phases. Both solid solid and solid liquid reactions are the mechanisms to form the P 2 O 5 condensed phases at the interface between solid 2CaO SiO 2 and the heterogeneous slag. However, since the solid 5CaO SiO 2 P 2 O 5 is dispersed in the molten slag, the extent of solid solid reaction is not as intense as that of the solid liquid reaction. Therefore, it is regarded that the reaction between solid 2CaO SiO 2 and liquid phase in the slag is the dominant mechanism to form the P 2 O 5 condensed phases in the present study. The composition obtained by EDS analysis at the positions with the formation of P 2 O 5 condensed phases and liquid phase in the bulk slag are normalized to the CaO SiO 2 P 2 O 5 system and plotted in the ternary composition triangle as shown in Fig. 7. The composition points are mostly off the tie line between 2CaO SiO 2 and 3CaO 707

7 ISIJ International, Vol. 50 (2010), No. 5 Fig. 7. Composition of the positions with P2O5 condensed phases at the interface between 2CaO SiO2 and heterogeneous slags at K. Fig. 8. Interfaces between solid 2CaO SiO2 and the heterogeneous slag at K. P2O5, which is the same as reported in the previous study.11,12) Such result indicates that the composition obtained by EDS analysis at the positions with the formation of P2O5 condensed phases is the average composition of 2CaO SiO2 3CaO P2O5 solid solution or compound and the surrounding liquid phase because the size of the formed solid P2O5 rich particle is not large enough to be detected separately by the present analysis method. The ratio of solid P2O5 condensed phases in certain area for slag C is larger than that for the other two cases on account of the fact that comparatively the composition points are closer to the tie line while far away from the liquid phase in the case of slag C. Thus it can be said that the formation of P2O5 condensed phases is the most intense when using slag C. solid P2O5 condensed phases which was confirmed by the EDS analysis from the liquid phase in the slag is clearly observed in the region next to the rim of solid 2CaO SiO2. The P2O5 condensed phases were not formed as a uniform layer, but scattered unequally in a certain region next to the rim of solid 2CaO SiO2. According to the concentration profiles across the interface, the characteristics of this region including the width and location for each slag after reaction for various time are summarized in Table 4. For slag A at K, the P2O5 condensed phases are only located in multi phase area after reaction for 1 s while the condensed phases turn to be located in both the multi phase area and the slag area after reaction for longer time. For slags B and C at K, the condensed phases can be observed in both multi phase area and bulk slag area after various reaction time. This tendency indicates that the formation of P2O5 rich phase with slag A at K is not as fast as other two slags. In all the three cases, the region with P2O5 condensed phases is expanding with the increase of reaction time. It seems that 4.2. Characteristics of the Region with P2O5 Condensed Phases Formation of P2O5 condensed phases at the interface between solid 2CaO SiO2 and the heterogeneous slag can be explained more easily with Fig. 8. The precipitation of the 708

8 Table 4. Characteristics of the region with P 2 O 5 condensed phase at the interface between 2CaO SiO 2 and the heterogeneous slags at K. with slag C, the region with P 2 O 5 condensed phases is the widest among these three cases. Therefore, it can also be said that the formation of P 2 O 5 condensed phase is the most intense when using slag C. As explained in the previous section, the P 2 O 5 condensed phases are mostly formed by the reaction between solid 2CaO SiO 2 and the liquid phase in the heterogeneous slag. The compositions of the liquid phase in three slags are shown in Table 2. The liquid phase in slag C with the smallest CaO/SiO 2 ratio contains more FeO while much less P 2 O 5 than the other two cases. However, the condensation of P 2 O 5 by using slag C is the most intense and the P 2 O 5 content in the condensed phase is as large as in other cases. Such tendency is in agreement with the findings by Ito et al. 4) who measured the equilibrium partition ratio (L P ) of P 2 O 5 between solid 2CaO SiO 2 and molten CaO SiO 2 FeO x P 2 O 5 slags at the temperature ranging from to K. It was found that the increase of total iron content in the slag increases L P by making the activity coefficient of 3CaO P 2 O 5 in the slag larger. Similarly, it is considered that the formation of P 2 O 5 condensed phases in the present study also has close relationship with the activity coefficient of 3CaO P 2 O 5 in the slag which varies with the liquid slag composition. Further study should be conducted to explain this relationship in detail. Comparing the present results with the previous ones, 11,12) the region with P 2 O 5 condensed phases at the interface between 2CaO SiO 2 and the heterogeneous slag is much wider than that at the interface between 2CaO SiO 2 and the homogeneous slag. Such difference indicates that the formation of P 2 O 5 condensed phases is enhanced and the back dissolution of P 2 O 5 condensed phases into the surrounding slag is restrained by adjusting the liquid slag to be saturated with 5CaO SiO 2 P 2 O Dissolution of P 2 O 5 Condensed Phases In the present study, solid 2CaO SiO 2 reacts with the liquid phase in the heterogeneous slag to form the P 2 O 5 condensed phases. Even though the liquid phases in the heterogeneous slags were saturated with solid 5CaO SiO 2 P 2 O 5 before the reaction, the back dissolution of the newly formed P 2 O 5 condensed phases into the surrounding slag can still be noticed by observing the profile of P 2 O 5 content at the positions with the formation of P 2 O 5 condensed phases. The profile of P 2 O 5 content at the positions with the formation of P 2 O 5 condensed phases is the indication of the dissolution of P 2 O 5 condensed phases into the slag. The increasing tendency for P 2 O 5 content at the positions with the formation of P 2 O 5 condensed phases from 2CaO SiO 2 to the bulk slag should be observed in the case of formation of P 2 O 5 condensed phase, while dissolution brings decreasing tendency. The concentration profiles of P 2 O 5 across the interfaces with different slags are compared as shown in Fig. 9. As can be seen, only for slag B after reaction for 10, 60 and 180 s, P 2 O 5 content at the positions with the formation of P 2 O 5 condensed phases exhibits a distinct tendency which is not simply increasing. Such result indicates the occurrence of dissolution when using slag B. However, the influence of dissolution for heterogeneous slag B at K is not as remarkable as for the homogeneous slags introduced in the previous study. 12) For slags A and C, the increasing tendency is clearly observed after different reaction time. In addition, the gradient of P 2 O 5 content at the positions with the formation of P 2 O 5 condensed phases is diminished after longer time reaction. Such tendency suggests that during the dissolution of solid 2CaO SiO 2 into liquid phase in the heterogeneous slag, P 2 O 5 condensed phases are formed continuously and the back dissolution of P 2 O 5 condensed phases into slag is negligible. The mechanism of the dissolution is attributed to the composition change of the liquid slag surrounding the P 2 O 5 condensed phases. With the increase of reaction time, the firstly formed P 2 O 5 condensed phases stay still with little variation whereas the surrounding slag is changing continuously both by dissolution of 2CaO SiO 2 and by diffusion of components from bulk slag. The back dissolution of the firstly formed P 2 O 5 condensed phases occurs at the moment when the equilibrium between the P 2 O 5 condensed phases and the surrounding slag is lost Reaction Behavior of P 2 O 5 at the Interface between Solid 2CaO SiO 2 and the Heterogeneous Slag The mechanism of reaction between solid 2CaO SiO 2 and the multi phase CaO SiO 2 FeO x P 2 O 5 slag could be concluded from the present results as illustrated in Fig. 10. (1) Since the initial slag is saturated by 5CaO SiO 2 P 2 O 5, activity of 2CaO SiO 2 is lower than that in liquid slag saturated with 2CaO SiO 2. Therefore, the solid 2CaO SiO 2 dissolves into the slag and also the slag penetrates into the solid 2CaO SiO 2. However, the region with gradient of 2CaO SiO 2 content will be formed with very tiny thickness, or never formed, dependent on the composition of the liquid phase. Since the liquid phase in the slag is already saturated with solid 5CaO SiO 2 P 2 O 5, a little increase of 2CaO SiO 2 content in the liquid phase will result in the precipitation of solid P 2 O 5 condensed phases (Fig. 10(a)). (2) The rim of the solid 2CaO SiO 2 changes into multi phase area where solid and liquid phases are coexisting. The P 2 O 5 condensed phases are formed in this area by the increase in 2CaO SiO 2 activity (Fig. 10(b)). (3) The multi phase area shifts towards the side of 2CaO SiO 2 and new P 2 O 5 condensed phases are formed (Fig. 10(c)). 709

9 Fig. 9. Concentration profiles of P 2 O 5 across the interfaces. Fig. 10. Reaction behavior of phosphorus at the interface between 2CaO SiO 2 and the heterogeneous CaO SiO 2 FeO x P 2 O 5 slag. C 2 S and 5CSP are short for 2CaO SiO 2 and 5CaO SiO 2 P 2 O 5, respectively. 710

10 (4) Since the liquid phase in the slag is already saturated with solid 5CaO SiO 2 P 2 O 5, the P 2 O 5 condensed phases formed by dissolution of solid 2CaO SiO 2 is not easy to re-dissolve. Therefore, the region with P 2 O 5 condensed phases expands with reaction time (Fig. 10(d)). 5. Conclusions The reaction behavior of phosphorus at the interface between solid 2CaO SiO 2 and the heterogeneous CaO SiO 2 FeO x P 2 O 5 slags at K was studied. The solid 2CaO SiO 2 reacted with the liquid phase in the two phase slag to form the P 2 O 5 condensed phases. The reaction behavior of P 2 O 5 at the interface was clarified. Comparing with the results in the previous study, 11,12) the formation of P 2 O 5 condensed phases is enhanced and the back dissolution of P 2 O 5 condensed phases into the surrounding slag is restrained. The enrichment of phosphorus in solid phase in the multi phase flux is considerably improved when the bulk slag is saturated with solid 5CaO SiO 2 P 2 O 5 before reaction. REFERENCES 1) H. Suito, Y. Hayashida and Y. Takahashi: Tetsu-to-Hagané, 63 (1977), ) M. Matsushima, S. Yadoomaru, K. Mori and Y. Kawai: Tetsu-to- Hagané, 62 (1976), ) F. Noguchi, Y. Ueda and T. Yanagase: J. Jpn. Inst. Met., 41 (1977), ) K. Ito, M. Yanagisawa and N. Sano: Tetsu-to-Hagané, 68 (1982), ) S. Kitamura, H. Shibata and S. Saito: CAMP-ISIJ, 20 (2007), ) S. Fukagai, T. Hamano and F. Tsukihashi: ISIJ Int., 47 (2007), ) H. Suito and R. Inoue: ISIJ Int., 46 (2006), ) R. Saito, H. Matsuura, K. Nakase, X. Yang and F. Tsukihashi: Tetsuto-Hagané, 95 (2009), ) W. Fix, H. Heyman and R. Heinke: J. Am. Ceram. Soc., 52 (1969), ) T. Hamano, S. Fukagai and F. Tsukihashi: ISIJ Int., 46 (2006), ) X. Yang, H. Matsuura and F. Tsukihashi: Tetsu-to-Hagané, 95 (2009), ) X. Yang, H. Matsuura and F. Tsukihashi: ISIJ Int., 49 (2009),