ISIJ International, Vol. 49 (2009), No. 4, pp. 505511

1. Introduction

The hot metal dephosphorization process was first in-vented in Japan. Further development of this process is stillunderway, since it can be used to decrease slag generationand production costs and increase productivity during theproduction of high-purity steel. Initially, a highly basic slagwas formed by the injection of CaOCaF2 flux into the de-siliconized hot metal. However, recently, treatment withless basic slag containing a high amount of T Fe has be-come more common, as the use of fluorspar has been elimi-nated because of concerns for the environment. Normally,the hot metal dephosphorization slag consists of CaOSiO2FeOP2O5 system, and the industrial operation ismainly carried out in the dicalcium silicate (C2S) saturatedcomposition range. It is well known that C2S forms a solidsolution with tricalcium phosphate (C3P) at the treatmenttemperature over a wide composition range.1) This impliesthat the product of the dephosphorization reaction can bedissolved into the solid phase in slag. Measurements of theequilibrium distribution ratio of P2O5 between the solid so-lution and the liquid phase revealed that P2O5 is concen-trated in the solid solution with a high distribution ratio.2,3)

On the basis of these results the solid solution plays an im-portant role in efficient dephosphorizationby acting as asink for phosphorus and lowering the phosphorus content in

the liquid phaseas pointed out by Kitamura et al.4)

In addition, research to utilize the slag as a resource ofphosphate has been reported.5) Although phosphorus is oneof the indispensable elements for the growth of crops, theobtainable natural resource of high grade phosphate orescarce in Japan. The C2SC3P solid solution in the slag,which has a high phosphorous content, can be looked uponas an alternative source of phosphate. The use of magneticforces to separate the solid solution from the bulk phase forthe slag which was powdered after the solidification hasbeen investigated.5)

In order to increase the efficiency of dephosphorizationand to use the slag as an alternative source of phosphateores, it is important to increase the distribution ratio ofphosphorous between the liquid slag and the solid solution.Ito et al. measured this distribution ratio for CaOSiO2P2O5FeO and CaOSiO2P2O5Fe2O3 systems, which con-tained 3% and 8% of P2O5.

2) They clarified that the distri-bution ratio expressed in Eq. (1) is proportional to the T Fecontent in the liquid phase and that this relation remainedunchanged, irrespective of the type of iron oxide, i.e., FeOor Fe2O3.

............................(1)LP2 5 SS

2 5 L

(%P O

(%P O

)

)

Distribution of P2O5 between Solid Dicalcium Silicate and LiquidPhases in CaOSiO2Fe2O3 System

Ken-ichi SHIMAUCHI,1) Shin-ya KITAMURA2) and Hiroyuki SHIBATA2)

1) Formerly Student, Graduate School of Engineering, Tohoku University. Now, Sumitomo Steel Wire Corp.,1-1-1 Koya-kita,Itami, Hyogo 6640016 Japan. 2) Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 9808577 Japan.

(Received on November 12, 2008; accepted on January 14, 2009; originally published in Tetsu-to-Hagan, Vol. 95, 2009, No. 3, pp. 229235)

In most cases, the slag used in hot metal dephosphorization is saturated with dicalcium silicate (C2S) andcontains two phasessolid C2S and liquid. It is known that C2S and tricalcium phosphate (C3P) form a solidsolution over a wide range of composition. The distribution ratio of P2O5 between the solid solution and theliquid slag phase has been reported to be very high. In order to determine the maximum possible concentra-tion of P2O5 in the solid solution, the distribution ratio of P2O5 in slag containing a high concentration of P2O5was measured in this study, and the influence of MgO and MnO on the distribution ratio was also investi-gated. CaOSiO2Fe2O3 slag containing up to 18% P2O5 was melted at 1 873 K and then cooled to 1 673 Kat a rate of 10 K/min. During cooling, the solid solution of C2S and C3P precipitated from the liquid slag. A lin-ear relationship, which was independent of the lime/silica ratio and P2O5 content, was found to exist be-tween the distribution ratio of P2O5 and the T Fe content. On the contrary, the concentration of P2O5 in thesolid solution was strongly influenced by the lime/silica ratio and P2O5 content. If the P2O5 content was highenough and the T Fe content was controlled to show the high distribution ratio, the concentration of C3P inthe solid solution can be increased to 100%. No significant change was observed in the distribution ratioupon the addition of MgO and MnO.

KEY WORDS: hot metal pretreatment; slag; dephosphorization; solid solution; precipitation.

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where the subscripts SS and L denote the solid solution andliquid phase, respectively.

Inoue et al. also measured the distribution ratio of P2O5in slag prepared by various procedures.3) Their reportedvalues were very similar to those reported by Ito et al., andthey have shown that the distribution ratio reached the equi-librium value in less than 10 s, irrespective of the experi-mental procedures.

In these experiments the maximum P2O5 content in thesolid solution of C2SC3P was 20%, so that the upper limitof P2O5 content in the solid solution has not yet been deter-mined. For this purpose, it is necessary to carry out distri-bution ratio measurements using slag containing a veryhigh P2O5 content. Industrial slag also contains other oxidessuch as MgO and MnO; therefore, the influence of theseoxides on the distribution ratio must also be clarified. Inthis research, the distribution ratios for CaOSiO2P2O5Fe2O3 systems containing a maximum of 18 mass% P2O5were measured to determine the maximum P2O5 content inthe solid solution. The influence of oxides such as MgOand MnO on the distribution ratio was also investigated.

2. Experimental Methods

Reagent-grade SiO2, Fe2O3, CaCO3, and 3[3CaOP2O5] Ca(OH)2 were used to produce slag. CaCO3 was first heatedat 1 273 K for 60 min or more under air atmosphere in anAl2O3 or MgO crucible to produce CaO. Then, the reagentswere mixed in various ratios. The slag consisted of CaOSiO2Fe2O3 system and 6, 12 and 18 mass% of P2O5 wasadded to it. To study the influence of MgO and MnO on thedistribution ratio,10 mass% of MgO or MnO was added tothe slag. The mixing conditions are summarized in Table 1.Figure 1 shows the composition ranges of the slag on theCaOSiO2Fe2O3 phase diagram, and named A, B and Crespectively. The initial composition of CaO, SiO2, andFe2O3 was decided after preliminary experiments so thathomogeneous liquid was in fact formed at 1 873 K and sothat a C2S saturated liquid and solid were formed during thecooling to 1 673 K. The composition of the slag quenchedfrom 1 873 K analyzed by EPMA (electron prove micro an-alyzer) were almost the same as the initial composition.Therefore, in this study, the initial slag composition wasused.

The mixed reagents were put in a Pt crucible whose di-mensions were F5 mm5 mm. The Pt crucible was in-serted into an Al2O3 crucible and heated in an electric re-sistance furnace. First, the sample was heated up to 1 873 Kat a rate of 10 K/min and kept for 60 to 120 min. Then, itwas cooled to 1 673 K at a rate of 10 K/min and kept for 0to 60 min. Finally, the sample was quenched into the oilbath. The heat treatment pattern is shown in Fig. 2.

According to the following reasons, Fe2O3 was used asiron oxide in this experiment and the heating was carriedout under the air atmosphere, although in steelmaking con-dition, most of the iron oxide would be in the form of FeO.1) Ito et al.2) has shown that the distribution behavior of

P2O5 between solid solution and liquid phases was notdifferent, when the iron oxide was changed from FeOto Fe2O3.

2) By the preliminary experiments, it was found that FeO

was solved into the solid solution but the content ofFe2O3 in solid solution was negligible small.

3) When FeO was used as iron oxide, Pt crucible can notbe used as Fe was precipitated in the slag. If MgO cru-cible was used, MgO content in the slag would becomevery high and the experiments in CaOSiO2P2O5FeOx system were not be able to carried out.

The quenched sample was mounted and polished. The

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Table 1. Slag composition (mass%).

Fig. 1. Observed slag composition in the ternary phase diagramof the CaOSiO2Fe2O3 system.

Fig. 2. Experimental conditions for precipitation of C2SC3Psolid solution.

composition of each phase was measured by EPMA. Thedistribution ratio of P2O5 (LP) was calculated from theEPMA analysis using the following methods.

The composition of the solid solution could be directlyanalyzed by EPMA, as the average diameter of the solid so-lution precipitate was more than 10 mm. When the matrixphase became glassy, liquid phase composition was ana-lyzed by EPMA, also. In this case, LP could be directly cal-culated from the results obtained from the analysis. Thismethod was termed Method a.

In some cases, some crystallized phases were observedin the matrix phase although the slag was quenched. Whenthe particles in each phase in the matrix were large enoughto be analyzed by EPMA, the liquid phase composition attemperature was calculated from Eq. (2), using the compo-sition and area fraction of each phase.

................(2)

where Ai and (%P2O5)i are the area fraction and P2O5 con-centration of phase i in the matrix, respectively.

In this case, LP was calculated using the liquid phase

composition from Eq. (2)this method was termedMethod b.

When the particles in each phase in the matrix were toosmall to be analyzed by EPMA, the matrix phase composi-tion was calculated by Eq. (4) using the mass balance ofP2O5 shown in Eq. (3). Again, LP was calculated under theassumption that the matrix corresponds to the liquid phaseat 1 673 K; this method was called Method c.

(%P2O5)T(%P2O5)LAM(%P2O5)SSASS........(3)

........(4)

where (%P2O5)T and (%P2O5)SS represent the P2O5 concen-tration in the total slag and the solid solution, respectively.AM and ASS represent the area fraction of the matrix and thesolid solution, respectively.

3. Experimental Results

When the slag was quenched from 1 873 K, a uniformglassy structure was observed, but when it was quenchedfrom 1 673 K, the solid phases had precipitated under all the

(% )(% ) (% )

P OP O P O

2 5 L2 5 T 2 5 SS SS

M

A

A

(% ) [(% ) ]P O P O2 5 L 2 5 i i

i

A

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

experimental conditions. The compositions of the precipi-tated solid phases and matrix liquid phases at temperatureare summarized in Table 2. In this table, the distributionratio of P2O5 and the method of calculating it are alsoshown. It can be seen that most of the solid phases are solidsolution of C2SC3P.

Figure 3 shows the influence of holding time on the P2O5content in the solid and liquid phases in sample A-1 (6%P2O5). Since the composition of each phase did not show anotable change with the holding time, it was presumed thatequilibrium was attained between the liquid phase and theprecipitated solid solution during the cooling from 1 873 to1 673 K. The rate of P2O5 distribution between the solid so-lution and liquid phase by the precipitation can be consid-ered to be quite high. Also, it can be said that the precipita-tion as the solid solution of C2SC3P is more stable than theprecipitation as C2S without C3P.

The results obtained for the CaOSiO2Fe2O3 system forvarious P2O5 content are summarized in Figs. 4 and 5. InFig. 4, the composition of the liquid phase is plotted in theCaOSiO2Fe2O3 phase diagram. The observed liquid com-position was close to the liquidus at 1 673 K even for theP2O5-rich slag. This implies that the addition of P2O5 doesnot cause any notable change in the composition of the liq-uidus. In Fig. 5, the composition of the solid solution isplotted in the CaOSiO2P2O5 phase diagram. The compo-

sition of the solid solution was close to the pseudo-binaryrelation of C2S and C3P. This result can be considered to bereasonable because the Fe2O3 content in the solid solution isless than 2%, as shown in Table 2.

As shown in Fig. 6, the P2O5 concentration in the solidsolution increased with the P2O5 content and also when theslag composition changed from C to B to A. In the case ofslag A with 18% P2O5, the precipitated solid was found benearly pure C3P, whose composition is shown by the dottedline in Fig. 6. This implies that the concentration of C3P inthe solid solution can be increased to 100% by controllingthe slag composition. A linear relationship was found to

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Fig. 4. Composition of liquid phase for each slag with variousP2O5 content in CaOSiO2Fe2O3 ternary phase diagram.

Fig. 3. Change in the concentration of P2O5 in the solid solutionand liquid phases with the holding time.

Fig. 6. Influence of slag composition on P2O5 content in solidsolution.

Fig. 7. Relationship of P2O5 distribution ratio between the solidsolution and the liquid phases with T Fe content.

Fig. 5. Composition of solid solution for each slag with variousP2O5 content in CaOSiO2P2O5 ternary phase diagram.

exist between the distribution ratio and the T Fe content inthe liquid phase, which was independent of the lime/silicaratio and P2O5 content, as shown in Fig. 7. This result is al-most the same as that obtained by Ito et al.2) The change inthe distribution ratio with the slag composition, which isshown in Fig. 6, can be explained by the change in the T Fecontent in each slag.

The results obtained by the addition of 10 mass% ofMgO or MnO to the abovementioned slag systems are dis-cussed in the following paragraphs.

The compositions of the liquid phase and solid solutionsare summarized in Fig. 8 for the MgO-containing slag, andin Fig. 9 for the MnO-containing slag. The composition ofthe liquid phase was almost the same as the liquidus line ofthe CaSSiO2Fe2O3 system, and no notable effect of theMgO or MnO addition was seen. On the other hand, thecomposition of the solid solution did not lie on theC2SC3P pseudo-binary. As the MgO and MnO can be dis-solved in the solid solution up to 6 mass%, shown in Table2, the composition of the solid solution became differentfrom that of the pure solid solution of C2S and C3P. The ad-

dition of MgO or MnO brought about a slight decrease inthe P2O5 content in the solid solution, as shown in Fig. 10.No clear influence of these additions was observed on thedistribution ratio of P2O5, as can be seen in Fig. 11.

4. Discussion

As the liquid phase and precipitated solid phase are inequilibrium, the activities of P2O5 in both phases are thesame. Then, the observed distribution ratio is proportionalto the activity coefficient of P2O5 in each phase, as shown inEq. (5), where a is the activity; g , the activity coefficient;and k, the coefficient for the conversion of mass percentageto mol fraction. Subscripts P2O5(Liq) and P2O5(SS) denotethe activity and activity coefficient of P2O5 in the liquidphase and the solid solution, respectively.

....(5)

The activity of P2O5 in the liquid phase is calculated

L ka

akP

2 5 SS

2 5 L

P O (SS) P O (L)

P O (L) P O (SS)

P O (L)

P O (SS)

P O

P O2 5 2 5

2 5 2 5

2 5

2 5

(% )

(% )

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Fig. 8. Composition of liquid phase in the ternary phase diagram of the CaOSiO2Fe2O3 system (a) and solid solutionin the ternary phase diagram of the CaOSiO2P2O5 system (b) of the MgO-containing slag.

Fig. 9. Composition of liquid phase in the ternary phase diagram of the CaOSiO2Fe2O3 system (a) and solid solutionin the ternary phase diagram of the CaOSiO2P2O5 system (b) of the MnO-containing slag.

using a regular solution model6) under the assumption thatall iron oxides are in the form of Fe2O3. Equations (6) and(7) are used for the calculation of PO2.5 for the CaOSiO2Fe2O3P2O5MgO and CaOSiO2Fe2O3P2O5MnO sys-tems, respectively, where X is the mol fraction. The conver-sion from PO2.5 to P2O5 is shown in Eq. (8), where the sub-script indicates the reference state of the activity, and (R.S.)indicates the pure PO2.5 liquid under the assumption thatthe regular solution and (L) represent the pure P2O5 liquid.

...............(6)

..................(7)

..........(8)

Figure 12 shows the relationship between the activity co-efficients of P2O5 in the liquid phase obtained using the reg-ular solution model and T Fe. The activity coefficient in-creases with the T Fe content when P2O5 content is thesame and with P2O5 content when the T Fe is in the similarconcentration range. By the addition of MgO and MnO, theactivity coefficient increased, although it was not so clear inthe case of MnO addition.

An increase in the T Fe content was expected to causean increase in the activity coefficient of P2O5 because of adecrease in the CaO concentration and the basicity of theliquid slag. Ito et al.2) considered that the decrease in theCaO concentration caused by an increase in the T Fe con-tent is the probable reason for the linear relationship be-tween the T Fe content and the distribution ratio, which isshown in Figs. 7 and 11. From the result shown in Fig. 12,it could be stated that the activity coefficient of P2O5 in theliquid phase was affected not only by the T Fe content butalso by the addition of P2O5, MgO, and MnO.

From the activity of P2O5 in the liquid phase obtained bythe regular solution model, the activity coefficient of P2O5in the solid solution, where pure liquid P2O5 is in the stan-dard state, can be calculated using Eq. (9).

............................(9)

Figure 13 shows that the activity coefficient of P2O5 inthe solid solution increases with P2O5 content. Further, theactivity coefficient in the solid solution increases with theaddition of MgO and MnO. The increase in the activity co-

a aP O (SS) P O (Liq)2 5 2 5

RT a RT a Tln ln .P O (L) PO (R.S.)2 5 2.5(J / mol)

2 52 720 230 706

RT

X X X

X X X X X

X X X X

X X X X

ln

( )

PO (R.S.)

FeO2

CaO2

MnO2

SiO2

FeO CaO FeO MnO

FeO SiO CaO MnO

CaO SiO MnO SiO

2.5

1.5

2 1.5 1.5

1.5 2

2 2J

14 640 251040 84 940

83 680 140 590 13 820

65 680 243 930

33 470 74 050

RT

X X X

X X X X X

X X X X

X X X X

ln PO (R.S.)

FeO2

CaO2

MgO2

SiO2

FeO CaO FeO MgO

FeO SiO CaO MgO

CaO SiO MgO SiO

2.5

1.5

2 1.5 1.5

1.5 2

2 2(J)

14 640 251040 37 660

83 680 140 590 20 090

65 680 188 280

33 470 112 960

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Fig. 10. Influence of MgO and MnO addition on the P2O5 con-tent in solid solution.

Fig. 11. Influence of MgO and MnO addition on the P2O5 distri-bution ratio between the liquid and the solid solutionphases.

Fig. 12. Relationship between T Fe content and the activity co-efficient of P2O5 in liquid phase calculated by using theregular solution model.

Fig. 13. Influence of P2O5 content on the activity coefficient ofP2O5 in the solid solution.

efficient caused by the addition of MgO is greater than thatcaused by the addition of MnO.

As shown in Fig.12, the activity coefficients of P2O5 inthe liquid phase and the solid solution increase with theP2O5 content and with the addition of MgO and MnO. As aresult, the relationship between the distribution ratio andT Fe is not influenced by the addition of P2O5, MgO, andMnO, as shown in Figs. 7 and 11.

5. Conclusions

In order to determine the maximum concentration ofP2O5 in the solid solution phase, the distribution ratio ofP2O5 in P2O5-rich slag was measured, and the influence ofMgO and MnO on the distribution ratio was also investi-gated. CaOSiO2Fe2O3 slag containing a maximum of18% P2O5 was melted at 1 873 K and cooled to 1 673 K at acooling rate of 10 K/min. During cooling, the solid solutionof C2S and C3P precipitated from the liquid slag. TheEPMA results of the compositions of the precipitated solidphase and the matrix liquid phase are as follows:

(1) The composition of the liquid phase and solid solu-tion hardly changed with the holding time at 1 673 K. Equi-librium was probably established during cooling of the slagfrom 1 873 to 1 673 K. It can be considered that the rate ofP2O5 distribution between the solid solution and liquidphase by the precipitation is quite high.

(2) A linear relationship, which was independent of thelime/silica ratio and P2O5 content, was found to exist be-

tween the distribution ratio of P2O5 and the T Fe content.On the contrary, the concentration of P2O5 in the solid solu-tion was strongly influenced by the lime/silica ratio andP2O5 content. If the P2O5 content was high enough and theT Fe content was controlled to show the high distributionratio, the concentration of C3P in the solid solution can beincreased to 100%.

(3) No significant change was observed in the distribu-tion ratio upon the addition of MgO and MnO.

Acknowledgement

This study was supported by the Iron and Steel Instituteof Japan and Steel Industry Foundation for the Advance-ment of Environmental Protection Technology, Japan. Theauthors would like to thank Professor D. G. C. Robertson,Missouri University of Science and Technology, USA, forthe useful advice during his stay in Japan as a Visiting Pro-fessor of Tohoku University.

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