1. Introduction

Since steel come in use under various conditions itsphosphorus content must be as low as possible in order toensure that it is of high quality. Because of that, the demandfor the production of low-phosphorus steel has been in-creasing, however production of low-phosphorous steelcause production of large amount of slag in the steel ren-ing process. Recently, because of the increasing concern forenvironmental conservation, much attention has been de-voted to the hot metal dephosphorization process as ameans to decrease the amount of slag generated, reduce theproduction cost, and increase the productivity during theproduction of high-purity steel.

Normally, hot metal dephosphorization slag consists ofthe CaOSiO2FeOP2O5 system, and the industrial opera-tion is mainly carried out in the dicalcium silicate (C2S) sat-urated region. It is well known that C2S forms a solid solu-tion with the main product of dephosphorizationtrical-cium phosphate (C3P)at the treatment temperature over awide composition range.13) Measurements of the distribu-tion ratio of P2O5 between the solid and liquid phases insemisolid slag (coexistence of the solid solution phase in

the liquid slag phase) indicated that P2O5 is concentrated inthe solid solution with a high distribution ratio.4,5) Kitamuraet al.6,7) pointed out the importance of a solid solution inachieving efcient dephosphorization. Based on their re-sults, it may be considered that dicalcium silicate and itssolid solution with tricalcium phosphate can act as a sinkfor phosphorus and lower the phosphorus content of the liq-uid phase, which make it more capable for future dephos-phorization.

Some researches, consider steelmaking slag as a sourceof phosphorus.2) The C2SC3P solid solution in the slag,which has high phosphorus content, can be used as an alter-native source of phosphorus. Therefore, if steelmaking slagis to be used as an alternative source of the phosphate ore, itis important to increase the distribution ratio of phospho-rous between the liquid slag and the solid solution.

Recently, Kitamura et al.6,7) studied the effect of P2O5content of the CaOSiO2Fe2O3P2O5 slag system on thedistribution ratio of phosphorus and also claried the inu-ence of oxides such as MgO and MnO on the distributionratio. Shimauchi et al.7) found that if the P2O5 content issufciently high and other conditions such as T Fe are con-trolled in order to obtained a high distribution ratio, the

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ISIJ International, Vol. 50 (2010), No. 6, pp. 822829

Distribution of P2O5 between Solid Solution of2CaOSiO23CaOP2O5 and Liquid Phase

Farshid PAHLEVANI, Shin-ya KITAMURA, Hiroyuki SHIBATA and Nobuhiro MARUOKA

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577Japan. E-mail: farshid@tagen.tohoku.ac.jp

(Received on February 5, 2010; accepted on April 6, 2010 )

It is desirable to decrease the phosphorous content of steel and the amount of slag in steel rening. Forsatisfying this requirement recently, the importance of multiphase slag in steel rening has been given con-siderable attention. Normally, hot metal dephosphorization slag consists of the CaOSiO2FeOP2O5 sys-tem, and the industrial operation is mainly carried out in the dicalcium silicate (C2S) saturated region. It iswell known that C2S forms a solid solution with the main product of dephosphorizationtricalcium phos-phate (C3P)at the treatment temperature over a wide composition range, and a high distribution ratio ofP2O5 between the solid solution and the liquid phase has been reported. In order to determine the rulingfactors on the distribution ratio, the inuences of MgO, MnO, and Al2O3 were investigated for various slagcompositions in the case of FeO or Fe2O3 as the iron oxide. First, a mixture of a standard regent was heatedto the melting temperature in order to produce homogenous liquid slag. Next, it was cooled down to a semi-solid state: during cooling, the solid solution of C3PC2S was precipitated under the equilibrium condition.For clarifying the inuence of slag composition on the distribution ratio, the ruling factors on the activity co-efcient of P2O5 in the solid solution and liquid slag phase were evaluated. It is the contention of this re-search that the activity coefcient of P2O5 in the solid solution was largely inuenced by P2O5 and the totalsolved oxide content of the solid solution. (log gP2O5SS)cal was obtained empirically in order to represent the ac-tivity coefcient in the solid solution. On the other hand, the activity coefcient of P2O5 in the liquid phasewas strongly inuenced by the CaO content of the liquid phase. On the basic of these correlations, it wasshown that the CaO content of the liquid phase and (log gP2O5SS)cal are the ruling factors on distribution ratio.

KEY WORDS: distribution ratio; activity coefcient of P2O5; added oxide to slag; multi phase slag; steel re-ning.

concentration of C3P in the solid solution can be increasedto 100%. They also indicated that there is no signicant ef-fect of the addition of MgO and MnO on the distributionratio of P2O5.

In this research effect of different oxides in the slag sys-tem on the distribution ratio of P2O5 was studied. Becausemost industrial slags contain Al2O3, in this research rst,effect of different Al2O3 contents in the CaOSiO2Fe2O3P2O5 slag system was studied. Then based on the perviousresults of the authors effect of changing the iron oxide inthe slag system from Fe2O3 to FeO was studied. Effect ofdifferent P2O5 contents in the CaOSiO2FeOP2O5 slagsystem on the distribution ratio of phosphate and also theinuence of oxides such as MgO, MnO, and Al2O3 on thedistribution ratio was investigated. On the basic of thesedata and their analysis by a regression method, an equationis proposed to evaluate the activity coefcient of P2O5 in asolid solution. Finally, the ruling factors on the distributionratio were discussed.

2. Experimental Procedure

Regent-grade SiO2, Fe2O3, Fe, CaCO3 and [3CaO P2O5] Ca(OH)2 were used to produce slag in this experiment.First, CaCO3 was heated to 1 273 K for 60 min under an airatmosphere in an Al2O3 crucible so as to produce CaO.Next, for the conditions that require FeO, Fe was mixedwith Fe2O3 and heated to 1 673 K for 60 min under an Ar at-mosphere in an iron crucible so as to produce FeO. Then,the reagents were mixed in various ratios in order to pro-duce the CaOSiO2FeOP2O5 or CaOSiO2Fe2O3P2O5slag system. For investigating the effect of other elementson the distribution ratio of phosphorous, Al2O3, MgO andMnO were added in some cases. Table 1 shows the differ-ent mixing conditions which was used in this research. Fig-ure 1 also shows the composition range of the used slag onthe CaOSiO2Fe2O3 phase diagram for samples A, B, and

C and on the CaOSiO2FeO phase diagram for samples D,E, F and G. The desired criterion for the used slag systemswas that the slag should be in a homogenous liquid state at1 673 K in the case of FeO and 1 873 K in the case of Fe2O3and it should be in a semisolid state at 1 573 K in the caseof FeO and 1 673 K in the case of Fe2O3.

The mixed regents containing FeO were placed in a steelcrucible with dimensions of F 10 mm20 mm and thosecontaining Fe2O3 were placed in a Pt crucible with dimen-sions of F 5 mm5 mm. The steel crucible was heated inan electric resistance furnace up to 1 673 K for producingFeO-containing slag and the Pt crucible was heated up to1 873 K in an electric resistance furnace for producingFe2O3-containing slag. Each crucible was maintained at itsabove mentioned temperature for 60 min in order to pro-duce a homogenous slag in liquid phase. Then, FeO-con-tained slag was cooled to 1 573 K at a cooling rate of3 K/min and Fe2O3-contained slag was cooled to 1 673 K ata cooling rate of 10 K/min. Each crucible was maintained atits particular temperature for 60 min so as to produce semi-solid slag (solid solution of C2SC3P in equilibrium withliquid slag). Finally, the slag sample thus produced wasquenched in oil. The heat treatment pattern of samples isshown in Fig. 2. It should be mentioned that when FeO wasused as iron oxide, the heat treatment was performed in anAr atmosphere, and in the case of Fe2O3, it was performedin an air atmosphere.

The quenched sample was mounted and analyzed by fol-lowing a standard metallographic procedure. The composi-tion of each phase was measured by using an EPMA (elec-tron probe micro analyzer). Because the average particle di-ameter of the solid solution phase exceeded 10 mm and theliquid phase became glassy after quenching their composi-tion could be directly analyzed by EPMA. Then, the distri-bution ratio of P2O5 (LP) was calculated by Eq. (1).

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Table 1. Mixing conditions of slag (mass%).

............................(1)

3. Results

From microstructural study of samples, it is clear that inthe case of FeO contained slag at 1 673 K and in the case ofFe2O3 contained slag at 1 873 K the samples was in uniformliquid phase which cause uniform glassy structure in thequenched samples. Also at 1 573 K for FeO contained slagand 1 673 K for Fe2O3 contained slag solid solution phasewas precipitated under all experimental conditions. In somecases, because of high FeO content, pure Fe phase was alsoprecipitated in addition to the solid solution phase. Thecomposition and area fraction of the precipitated phase andmatrix are summarized in Table 2. This table also presentsthe distribution ratio of P2O5 (LP) and the average composi-tion of samples after precipitation of Fe and solid solution.For calculating the average composition, rst, the area frac-tion of each phase was calculated by image analyzing themicrostructure of samples. Then, based on the compositionof each phase, which was obtained from EPMA analysis theaverage composition under each mixing condition was cal-culated. It should be mentioned that, for some cases inTable 2 the sum of area fraction for matrix and precipitatedsolid solution is not equal to one. In these cases this differ-ence refers to area fraction of precipitated Fe phase.

3.1. Effect of Al2O3 on the CaOSiO2P2O5Fe2O3 Sys-tem

Figure 3 shows the effect of adding different oxides onthe distribution ratio of P2O5 for different T Fe in the liquidphase. The results of adding MgO and MnO to the slag sys-tem were taken from pervious paper by Shimauchi et al.7)

As compared with the results of adding MgO and MnO tothe CaOSiO2P2O5Fe2O3 slag system, the addition ofAl2O3, caused LP to become large in the low T Fe condi-tion.

If the activity of P2O5 in a solid solution does not changewidely, LP depends on the basicity of the liquid phase. InFig. 4, the CaO content of the liquid slag was selected torepresent the index of slag basicity. LP decreased on the ad-dition of MgO and MnO, but it did not change on the addi-tion of Al2O3.

3.2. Comparison between CaOSiO2P2O5Fe2O3 andCaOSiO2P2O5FeO Systems

Figure 5 shows the distribution ratios at different T Fecontents by changing the iron oxide in the slag system. Asshown for high T Fe content, LP of the CaOSiO2P2O5FeO system is lower than that of the CaOSiO2P2O5Fe2O3 system.

Figure 6 shows the inuence of the CaO content of theliquid phase on the distribution ratio. The difference be-tween two systems becomes more clear. Inoue5) has showed

L SS

LP

2 5

2 5

(%P O

(%P O

)

)

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Fig. 1. Mixing conditions of slag are shown on the ternary (a) CaOSiO2Fe2O3 phase diagram for the case in whichFe2O3 was used and (b) CaOSiO2FeO phase diagram for the case in which of FeO was used.

Fig. 2. Experimental conditions for precipitation of C2SC3P solid solution, with (a) Fe2O3 used as the iron oxide, and (b)FeO was used as the iron oxide.

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Table 2. Composition of liquid and solid phase in slag (mass%).

Fig. 3. Inuence of adding Al2O3 on the P2O5 distributionratio between the liquid and solid solution phases atdifferent T Fe contents in liquid phase.

Fig. 4. Changes produced in the P2O5 distribution ratio between the liquid andsolid solution phases by adding MgO, MnO and Al2O3 at different CaOcontents in liquid phase in the case of Fe2O3 as iron oxide.

that the temperature dependence of LP is very small, acord-ingly it is expected that a change in LP will be caused by thechange in the slag system (Fe2O3 or FeO).

3.3. Effect of Added Oxides in the CaOSiO2P2O5FeO System

Figure 7 shows the effect of adding different oxides

when FeO is used as iron oxide. By comparing this gurewith Fig. 4, it is found that the effect of adding MgO, MnOor Al2O3 on LP is smaller in the case of CaOSiO2P2O5FeO system than that in the case of CaOSiO2P2O5Fe2O3system.

4. Discussion

In order to achieve better understanding of the phenome-non of distribution of P2O5 between the solid solution andthe liquid slag phase and to nd out the effect of differentoxides, the results of the present research and the previousresearch by Shimauchi et al.7) was used for this discussion.

Because the liquid slag phase and solid solution are inequilibrium the activities of P2O5 in the solid solution andliquid phase are same. Then, the observed distribution ratiois proportional to the activity coefcient of P2O5 in eachphase, as shown in Eq. (2). Where a is the activity; g , theactivity coefcient; and k, the coefcient for the conversionof the mass percentage to the mol fraction. SubscriptsP2O5(Liq) and P2O5(SS) denote the activities and activity coef-cients of P2O5 in the liquid phase and the solid solution,respectively.

...(2)

The activity of P2O5 in the liquid phase was calculated byusing a regular solution model,8) and from that the activitycoefcient of P2O5 in solid solution was calculated.

4.1. Activity Coefcient of P2O5 in Solid Solution

Figure 8 shows the relation between the activity coef-cient of P2O5 and the P2O5 content of the solid solutionwhen Fe2O3 or FeO is used as the iron oxide. From these re-sults, it can be said that the activity coefcient increaseswith increasing the P2O5 content. In addition, it is clearfrom the gure that the activity coefcient is larger in thecase of FeO than that in the case of Fe2O3.

Figure 9 shows the effect of the added oxide on the ac-tivity coefcient of P2O5 in solid solution in the case ofFe2O3 iron oxide. Figure 10 also shows this effect in thecase of FeO as iron oxide. The addition of MgO and MnOcauses an increase in the activity coefcient of P2O5, while

L ka

akSS

L

SS L

L SS

L

SS

P2 5

2 5

P O P O

P O P O

P O

P O

(%P O

(%P O2 5 2 5

2 5 2 5

2 5

2 5

)

)( ) ( )

( ) ( )

( )

( )

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Fig. 5. Inuence of different iron oxides on the P2O5 distributionratio between the liquid and solid solution phases at dif-ferent T Fe contents in liquid phase in the case of Fe2O3as iron oxide.

Fig. 6. Change produced in the P2O5 distribution ratio betweenthe liquid and solid solution phases by changing the usediron oxide at different CaO contents in liquid phase.

Fig. 7. Changing on the P2O5 distribution ratio between the liq-uid and solid solution phase by adding MgO, MnO andAl2O3 at different CaO content in liquid phase in the caseof FeO as iron oxide.

Fig. 8. Activity coefcient of P2O5 vs. P2O5 content of the solidsolution for two types of iron oxides (Fe2O3 and FeO).

that of Al2O3 leads to a decrease in the activity coefcient.Table 3 summarizes the composition of the solid solution

and the activity coefcient of P2O5 in the solid solution forall the samples. The results for the CaOSiO2P2O5Fe2O3slag system are taken from the previous research by Shi-mauchi et al.7) From Table 3, it is clear that in theCaOSiO2P2O5Fe2O3 slag system, the solid solution ispure 2CaOSiO23CaOP2O5 when the Fe2O3 content is less

than 2%. However, in the CaOSiO2P2O5FeO slag sys-tem, the FeO content of the solid solution is 38%. Further,in the case of Al2O3 addition, the Al2O3 content of the solidsolution is less than 1%, but in the case of MgO and MnOaddition, the MgO and MnO content is 17%. Therefore,the presence of an oxide other than CaO, SiO2, and P2O5 ina solid solution (solved oxide) will inuence the activitycoefcient of P2O5.

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Fig. 10. Effect of different added oxides on the relation betweenthe activity coefcient of P2O5 and the P2O5 content ofsolid solution in CaOSiO2P2O5FeO system.

Table 3. Composition of the solid solution phase (mass%) and the activity coefcient of P2O5.

Fig. 9. Effect of different added oxides on the relation betweenthe activity coefcient of P2O5 and the P2O5 content ofsolid solution in CaOSiO2P2O5Fe2O3 system.

For more precise understanding of the effect of a solvedoxide on the activity coefcient of P2O5 its relation with thetotal solved oxide content in the range of 2733% of theP2O5 content was extracted. This relation is shown in Fig.11. After analyzing these data by a regression method thecorrelation between the oxide content and the activity coef-cient of P2O5 was evaluated and is presented in Eq. (3).

..............(3)

where W is the total solved oxide content (in mass%) in thesolid solution.

4.2. Partition of P2O5 between Liquid Slag and SolidSolution

The distribution ratio of P2O5 is proportional to the ratioof the activity coefcients of P2O5 in the two phases. Basedon the above discussion, because the activity coefcient ofP2O5 in the solid solution changes with the P2O5 contentand the solved oxide, nding a simple relation between thedistribution ratio and T Fe would be limited.

As shown in Figs. 4, 6, and 7, there is a good relation be-tween the distribution ratio and the CaO content of the liq-uid phase. Figure 12 shows the relation between the activ-ity coefcient of P2O5 and CaO content of the liquid phase;the activity coefcient was calculated by the regular solu-tion model. Figure 13 shows the effect of different added

oxides on this relation; a good relation is observed, exceptin the case of Al2O3 addition to the slag. Although Al2O3is known as a neutral oxide, in the regular solution model,8) the interaction parameter between P5 and Al3

(261 500 J) has almost the same value as that between P5

and Ca2 (251 040 J). Therefore, in the present calcula-tion, the addition of Al2O3 lowers the activity of P2O5 in theliquid slag. In order to conrm the validity of this conjec-ture, the interaction parameter of Al3 must be evaluated inthe future.

As discussed before, the P2O5 content and the solvedoxide content of a solid solution have large inuence on theactivity coefcient of P2O5 in the solid solution. From theconditions under which the total solved oxide content waslower than 1.5%, an equation was formulated for calcu-lating the activity coefcient of P2O5 in the pure2CaOSiO23CaOP2O5 system. By combining this equa-tion with Eq. (3) the relation between and the oxide contentwas obtained (Eq. (4)).

...(4)

where (log gP2O5,SS)cal indicates the logarithm of the activitycoefcient of P2O5 in a solid solution. Figure 14 summa-rizes the relation between (log gP2O5,SS)cal which was calcu-

(log ) . )

( . . ) .

,

/

P O cal 2 52 5 (%P OSS SSW

0 0409

1 4305 17 567 1 2271 2

log . .,

/ P O2 5 SS W 1 4305 17 5671 2

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Fig. 11. Effect of total oxide content except CaO, SiO2, and P2O5on the activity coefcient of P2O5 in the solid solution.

Fig. 12. Relation between the activity coefcient of P2O5 and theCaO content of the liquid phase for two types of ironoxides (Fe2O3 and FeO).

Fig. 13. Effect of different added oxides on the relation betweenthe activity coefcient of P2O5 and the CaO content ofthe liquid phase.

Fig. 14. Relation between (log gP2O5,SS)cal and logarithm of activ-ity coefcient of P2O5 in solid solution.

lated from Eq. (4) and the logarithm of the activity coef-cient of P2O5 in the solid solution for all data presented inTable 3.

From the above discussion, it is seen that the activity co-efcient of P2O5 in liquid slag is governed by the CaO con-tent of the liquid phase and the activity coefcient of P2O5in solid solution is inuenced by (log gP2O5,SS)cal. Therefore,the distribution ratio can be controlled by these factors.Figure 15 shows the relation between the distribution ra-tion of P2O5 and the CaO content of the liquid phase, forsolid solutions with different value of (log gP2O5,SS)cal. Afairly good relation is obtained in this case.

5. Conclusion

By changing the iron oxide from Fe2O3 to FeO and byadding, different oxides such as Al2O3, MnO, and MgO theeffect of oxides on the distribution ratio of P2O5 between

the solid solution and the liquid slag phases was measuredfor various slag compositions. In order to clarify the inu-ence of slag composition on the distribution ratio, the rul-ing factors on the activity coefcients of P2O5 in the solidsolution and liquid phase were evaluated. By this research,it was claried that the activity coefcient of P2O5 in a solidsolution was largely inuenced by the P2O5 content and thesolved oxide content of the solid solution. (log gP2O5,SS)calwas obtained empirically in order to represent the activitycoefcient of P2O5 in a solid solution. On the other hand,the activity coefcient of P2O5 in a liquid phase was inu-enced by the CaO content of the liquid phase. On the basisof these correlations, it was shown that the CaO content-ment of a liquid phase and (log gP2O5,SS)cal are the ruling fac-tors on distribution ratio.

Acknowledgment

We would like to gratefully acknowledge the ISIJ andSteel Industry foundation for the advancement of environ-mental protection technology which provide the nancialsupport of this research.

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Jeffes: Ironmaking Steelmaking, 11 (1984), 202.4) K. Ito, M. Yanagisawa and N. Sano: Tetsu-to-Hagan, 68 (1982),

342.5) R. Inoue and H. Suito: ISIJ Int., 46 (2006), 174.6) S. Kitamura, H. Shibata and N. Maruoka: Steel Res., 79 (2008), 586.7) K. Shimauchi, S. Kitamura and H. Shibata: Steel Res., 49 (2009),

505.8) S. Ban-ya: ISIJ Int., 33 (1993), 2.

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Fig. 15. Relation between the distribution ratio of P2O5 and theCaO content of the liquid at different (log gP2O5,SS)cal.