Formation Mechanisms of the 2CaO·SiO2-3CaO·P2O5 Solid Solutionat Hot Metal Dephosphorization Process
Fumitaka TSUKIHASHI
Professor
Department of Advanced Materials Science,
Graduate School of Frontier Sciences,
The University of Tokyo
1. Introduction
Decrease of phosphorus content in the melt at steelmaking processes is one of the important issues
and thus development of hot metal dephosphorization process has been extensively conducted last
three decades to meet the demand of high quality grade steels. Since dephosphorization reaction
proceeds with high basic slag, at oxidizing condition, and at lower temperature, CaO based fluxes havebeen used due to the basic property of CaO and abundance as domestic resource with low cost.
Oxygen gas or iron oxide is used as oxidizer and thus the dephosphorization flux is the FeO-CaOSiO2-P2O5 system. However, the practical flux contains large amount of solid CaO dispersed in liquid
flux, because of relatively low temperature around 1573 K during hot metal pretreatment resulting low
solubility of CaO into the flux. Currently large amount of CaO have been added as a flux to enhancethe dissolution of solid CaO. Therefore, utilization efficiency of CaO is low and unreacted CaO
remains in the slag, which increases the slag volume and prevents smooth slag recycling. Although
CaF2 was previously used to dramatically increase the solubility of CaO, the addition of CaF2 to theflux is strictly regulated from the environmental view point. Reduction in the volume ofdephosphorization slag without using CaF2 is strongly required.
Many researches regarding the dissolution of CaO in the liquid slag have been reported. Schlitt etal.1) found the dissolution rate of CaO increased significantly with increasing FeO content. Dissolution
rate is also affected by the additives2) such as CaF2, CaCl2, Al2O3, B2O3, or by CaO particle size.3) It is
well known P2O5 in steelmaking slag after solidification is condensed to 2CaO·SiO2 phase.4,5) Ito etal.6) and Hirosawa et al.7) measured the partition of phosphorus between liquid slag and solid
2CaO·SiO2 at hot metal pretreatment temperature. Inoue et al.5) reported that the formation of
CaO-SiO2-P2O5 occurs within 5 s. The 2CaO·SiO2-3CaO·P2O5 pseudo binary system has very widesolid

solution

range

and

also

Silicocarnotite(5CaO·SiO2·P2O5)

andNagelschmidtite

8)(7CaO·2SiO2·P2O5) are formed at hot metal pretreatment temperature . Therefore, efficient hot metal
dephosphorization process will be achieved with multi phase fluxes containing solid CaO if P2O5 inliquid slag could be condensed to 2CaO·SiO2 phase.
The objective of this research is to elucidate the formation mechanism of the 2CaO·SiO23CaO·P2O5 solid solution in the dephosphorization process and the following two topics are focusedon, namely (1) reaction behavior between solid CaO or 2CaO·SiO2 and FeO-CaO-SiO2-P2O5 liquid,and (2) phase equilibria between solid 2CaO·SiO2-3CaO·P2O5 and liquid phase for the FeO-CaO-SiO2－ 43 －

P2O5 system.
2. Experimental
2.1. Reaction between Solid CaO/2CaO·SiO2 and FeO-CaO-SiO2-P2O5 Slag
A SiC resistance furnace with a mullite reaction tube (O.D. 70 mm, I.D. 63 mm, L 1000 mm) was
employed for the experiment. The slag sample was produced by mixing synthesized wüstite, calcined
CaO, and reagent grade SiO2 and 3CaO·P2O5. A CaO piece of 3 g was cut from a chunk of CaO(purity: 99.9 %, density: 3.3×103 kg/m3) and used as solid CaO specimen. A 2CaO·SiO2 piece wasproduced by pressing and heating a mixture of calcined CaO and reagent grade SiO2 on molar ratio of
2:1, together with 1 mass% 3CaO·P2O5 to prevent the dusting of 2CaO·SiO2 during cooling. All solid
specimens were polished to obtain the plain reaction surface.
Ten grams of mixed slag sample was charged in an alumina crucible (O.D. 38 mm, I.D. 34 mm, H
45 mm), and the crucible was put inside a reaction tube heated at experimental temperature. The slag
was held for one hour to ensure the equilibrium at CO-CO2 atmosphere (CO/CO2=100/1), or Ar
atmosphere with a solid electrolytic iron piece submerged in the slag. The solid CaO or 2CaO·SiO2
piece attached to the tip of the ceramic tube was firstly inserted in the reaction tube and held near theslag to preheat the piece for 120 s, and then dipped into the liquid slag to react. The count of reaction
time started when the solid piece was dipped, and after the prescribed reaction time the solid sample
was quickly taken out from the furnace and quenched in flushing Ar gas or by immersing into liquid
nitrogen. Quenched sample was embedded in the polyester resin and the cross section of the interface
between solid piece and slag was exposed by polishing the embedded sample and coating with 13 nm
thickness Au layer. The interface was observed and the chemical composition was analyzed by
SEM(JEOL JSM-6060LV) with EDS(JEOL EX-54175JMU). For the calculation of slag compositions
from element compositions determined by EMS, FeO, CaO, SiO2, P2O5, and Al2O3 oxides wereassumed. The spot size of electron beam was about 2 μm.
2.2. Equilibria between Solid 2CaO·SiO2-3CaO·P2O5 and Liquid Phase for the FeO-CaO-SiO2-P2O5
SystemA MoSi2 resistance furnace with an alumina reaction tube (O.D. 60 mm, I.D. 50 mm, L 1000 mm)
was employed for the experiment. A sample was put in a Pt crucible (O.D. 5.1 mm, I.D. 4.9 mm, H 5.0mm). After increasing the hot zone temperature to 1923 K and changing the inside of the tube to Ar
gas, a Pt crucible containing 0.1 g oxide specimen prepared by mixing FeO, CaO, SiO2 and
3CaO·P2O5 was put in the hot zone by suspending with Pt wire (φ0.5 mm). After 1 h for pre-melting,the furnace temperature decreased to 1673 K with 6 K/min and then CO-CO2 gas (CO/CO2=5/1) was
introduced with 200 cm3/min. The sample was held for 5 h, where the oxygen partial pressure wascontrolled to be 9.1×10−11 atm by Eq. (1). After the sample was equilibrated, the crucible was quickly
pulled up to the top of the tube and quenched by flushing Ar gas inside the tube. The quenched sample
was embedded in the polyester resin and the slag was exposed by polishing with SiC papers anddiamond suspensions up to 1 μm. After coating with carbon, the sample was analyzed by SEM with－ 44 －EDS as previously mentioned. Since solidification structure was observed in several samples, the area
composition analysis of 300 s was applied for such samples. The EDS analyses were conducted for 5
to 30 positions for each phase to calculate the average composition.
CO (g) + 1/2 O2 (g) = CO2 (g)J/mol9)ΔG ° = −281000 + 85.23T(1)

3. Results and Discussion
3.1. Reaction between Solid CaO/2CaO·SiO2 and Molten FeO-CaO-SiO2-P2O5 Slag(a) Reaction between CaO and slag10)
Figures 1(a) to (d) show the SEM images around the interface between solid CaO and 25
mass%FeO-31%CaO-33%SiO2-11%P2O5 slag at 1573 K reacted for 2 to 30 s. The numbered positions
in figures were analyzed by EDS to obtain the chemistry. The CaO-FeO phase was observed adjacent
to solid CaO, and the CaO-SiO2 or CaO-SiO2-P2O5 phase surrounded by liquid FeO-CaO-SiO2 withhigh FeO content was formed next to the CaO-FeO layer. SiO2 content in the CaO-FeO phase was lessthan 5 mass% and the ratio of CaO/FeO was approximately unity. Solid CaO-SiO2 surrounded byliquid slag was identified as 2CaO·SiO2. CaO-SiO2-P2O5 particles coexisting with FeO-CaO-SiO2 slag
contained from 1 to 10 mass% of P2O5, and CaO-SiO2-P2O5 composition was a binary mixture of
2CaO·SiO2 and 3CaO·P2O5. 2CaO·SiO2 and 3CaO·P2O5 make wide solid solution region.8) Therefore,
it is considered that P2O5 was taken into 2CaO·SiO2 as 3CaO·P2O5.

CaO-FeO x
CaO-FeO

CaO-FeO x
CaO-FeO

CaO-SiO2(-P2O5)

CaO

CaO-SiO2-P2O5

CaO

14
23 4 5 6 7

1

16
9

8

10

11 12 13
1 3 4 5 6 9

FeO-CaO-SiO
FeO
x -CaO-SiO
2 2

7 10

FeO-CaO-SiO
FeO
22x -CaO-SiO

15

2

8

11 12 13 14

15

17

CaO-SiO2
5 μm

5 μm

(a) CS-2 (reaction(a) CS-2time 2 s)

1

3
2

4

CaO-FeO x
CaO-FeO

CaO

CaO-FeOx
CaO-FeO
CaO

CS-10(b) (b)CS-10(10 s)

8

5 6

7

CaO-SiO2

FeO-CaO-SiO
FeO
22x -CaO-SiO
9

1

2 3 4

5 6

FeO-CaO-SiO
FeO
x -CaO-SiO
22
789

10 12

13

CaO-SiO2
11

5 μm

5 μm

(d) CS-30(d) CS-30(30 s)

CS-20(20 s)(c)(c)
CS-20

Figure 1

SEM images at the interface between solid CaO and molten slag at 1573 K.

－ 45 －

Figure 2

Projection of the chemical compositions analyzed by EDS for the FeO-CaO-(SiO2+P2O5)
system at 1573 K.

The compositions obtained by EDS analysis are plotted on the FeO-CaO-(SiO2+P2O5) pseudo
ternary diagram as shown in Fig. 2. The solid curves represent the liquidus for the FeO-CaO- SiO2
system equilibrated with iron at 1573 K.11) The phases were classified into solid CaO, CaO-FeO,
2CaO·SiO2, FeO-CaO-SiO2 liquid with high FeO, and 2CaO·SiO2 saturated phases. The compositionsof FeO-CaO-SiO2 liquid phase with high FeO content, coexisting with 2CaO·SiO2, were close to theliquidus composition of 2CaO·SiO2 saturation. Therefore, it is considered CaO content in liquid slagaround solid CaO increased by CaO dissolution and local slag composition reached to the liquidus of
2CaO·SiO2 saturation. Dissolved CaO and SiO2 in the liquid were consumed to precipitate 2CaO·SiO2
and the liquid phase composition changed along the liquidus resulting increase of FeO content.
The activities of FeO and CaO in the CaO-FeO phase and those in the liquid phase saturated by
2CaO·SiO2 were estimated by using the data after Takeda and Yazawa,12) and by the regular solution
model,13) respectively. The activity of FeO in the 2CaO·SiO2 saturated liquid phase is higher than thatin CaO-FeO phase and bulk slag. Therefore, FeO diffuses from 2CaO·SiO2 saturated liquid phase toboth the CaO-FeO phase and bulk slag. On the other hand, the activity of CaO in the CaO-FeO phaseis much higher than that in other phases. Therefore, CaO diffuses from solid CaO toward bulk slag
through the CaO-FeO and 2CaO·SiO2 saturated liquid phases.

－ 46 －

Figure 3

Schematics of reaction mechanisms.

From the above considerations, the reaction mechanisms between solid CaO and molten
FeO-CaO-SiO2-P2O5 slag are considered to be as follows; (1) Dissolution of CaO into the slag
resulting the increase of CaO content in the melt (Fig. 3(a)), (2) Formation of 2CaO·SiO2 from liquidslag, decrease of CaO and SiO2 contents in the liquid, and relatively increase of FeO content (Fig.
3(b)), (3) Diffusion of FeO from FeO rich phase to both solid CaO and bulk slag (Fig. 3(c)), (4)Formation of CaO-FeO phase adjacent to solid CaO (Fig. 3(d)), and (5) Diffusion of CaO to bulk slag
through formed CaO-FeO layer (Fig. 3(e)).
Figure 4 shows the relationship between P2O5 content in the 2CaO·SiO2-3CaO·P2O5 phase and the
distance from CaO-slag interface to the 2CaO·SiO2-3CaO·P2O5 phase after dipping CaO in 25
mass%FeO-36%CaO-29%SiO2-10%P2O5 slag at 1673 K for 2 to 10 s. P2O5 content increased from the
CaO-slag boundary toward the bulk slag. After longer reaction time, P2O5 content in the 2CaO·SiO23CaO·P2O5 phase became larger. From this result, the formation of the 2CaO·SiO2-3CaO·P2O5 phase isconsidered to proceed through two steps; firstly, P2O5 in the slag is condensed as 2CaO·SiO23CaO·P2O5 solid solution precipitated from the molten slag. Afterward, P2O5 is absorbed as
3CaO·P2O5 in the existing 2CaO·SiO2-3CaO·P2O5 phase.

－ 47 －

Figure 4

Relationship between P2O5 content in 2CaO·SiO2-3CaO·P2O5 phase and the distance from
CaO-slag interface to 2CaO·SiO2-3CaO·P2O5 phase at 1673 K.

(b) Reaction between 2CaO·SiO2 and slag14-17)
Figure 5 shows the SEM image around the interface between 2CaO·SiO2 and 20 mass%FeO38%CaO-32%SiO2-10%P2O5 slag after the reaction for 60 s at 1673 K. The left side is the original
solid 2CaO·SiO2 and the right side is the bulk slag, though the interface is not seen clearly because thefabricated 2CaO·SiO2 piece was porous and the slag penetrated easily inside the 2CaO·SiO2 piece.
Chemical compositions at positions represented in Fig. 5 were analyzed by EDS and plotted on thepseudo ternary diagram for the FeO-CaO-(SiO2+P2O5) as shown in Fig. 6 to examine the phase
distribution and relationship at the interface. In this figure, solid lines indicate the liquidus for the
FeO-CaO-SiO2 system equilibrated with solid iron at 1673 K.11) All phases appeared along the jointline between 2CaO·SiO2 and the original bulk slag shown as dashed line in Fig. 6. Observed phases
are categorized to solid 2CaO·SiO2, 2CaO·SiO2 saturated liquid slag, and the solid-liquid coexisting
phase.

Figure 5

SEM image around the interface between solid 2CaO·SiO2 and FeO-CaO-SiO2-P2O5 slag
at 1673 K.

－ 48 －

Figure 6

Phase distribution at the interface between solid 2CaO·SiO2 and the bulk slag at 1673 K.

Figure 7

Concentration of oxides around the interface as a function of position after 60 s reaction at1673 K.

The composition of each oxide was shown in Fig. 7 as a function of position. The left end of the
SEM image in Fig. 5 corresponds to position 0 in Fig. 7. CaO content decreased from 2CaO·SiO2 (left)
toward bulk slag (right), while FeO content increased. SiO2 content was almost constant. Positions insolid-liquid coexisting region in Fig. 6 with high P2O5 content were represented as open symbols in
Fig. 7. P2O5 condensed phase was observed at the region where a gradient for CaO and FeO contents
was seen.
The profile of FeO content showed a clear trend, and thus the region in Fig. 7 was separated bydashed line; low FeO content (left), FeO increasing region (center) and high constant FeO content(right), and these three regions correspond to solid 2CaO·SiO2, liquid slag penetration region into
2CaO·SiO2, and liquid slag region, respectively. Therefore, it is considered there was an initialinterface between solid 2CaO·SiO2 and liquid slag around the right side of the slag penetration layer.
The P2O5 condensed phase, namely 2CaO·SiO2-3CaO·P2O5 phase, was observed around this－ 49 －

solid-liquid interface. It is considered the formation reaction of 2CaO·SiO2-3CaO·P2O5 phase is fast,
because the phase was observed after the reaction for 1 s. The amount of P2O5 condensed phase was
not large compared to that in the case of CaO dipping experiments, neither was P2O5 content in thephase.
The reaction mechanisms between solid 2CaO·SiO2 and liquid FeO-CaO-SiO2-P2O5 slag areconcluded from the present results as illustrated in Fig. 8; (1)The solid 2CaO·SiO2 dissolves into theliquid slag and also the liquid slag penetrates into the solid sample (Fig. 8(a)), (2)The rim of the solid
2CaO·SiO2 changes into multi phase area where solid and liquid phases are coexisting, while the liquidslag turns to be saturated with 2CaO·SiO2 (Fig. 8(b)), (3)P2O5 reacts with solid 2CaO·SiO2 to form theP2O5 condensed phase in the multi phase region (Fig. 8(c)), (4)The multi phase region shifts towards
the side of 2CaO·SiO2 and new P2O5 condensed phases are formed because of continuous 2CaO·SiO2
dissolution as well as penetration of slag into solid 2CaO·SiO2 (Fig. 8(d)), (5)The previously formed
P2O5 condensed phase would remain (Fig. 8(e1)), fully dissolve (Fig. 8(e2)), or partly dissolve into theslag (Fig. 8(e3)), depending on the temperature and slag composition.

Figure 8

Reaction behavior of phosphorous at the interface between 2CaO·SiO2 and FeO-CaOSiO2-P2O5 slag. C2S is short for 2CaO·SiO2.
－ 50 －

3.2. Phase Equilibria between Solid 2CaO·SiO2-3CaO·P2O5 and Liquid Slag for the FeO-CaO-SiO2P2O5 System18,19)
Figure 9 shows the measured liquidus at 1673 K, with the liquidus for the FeO-CaO-SiO2 system
equilibrated with solid iron.11) P2O5 was considerably concentrated in the solid phase compared to theliquid, and the large phosphorus partition ratio between solid and liquid phases was observed. The
compositions of solid phases were found to be the solid solution of 2CaO·SiO2-3CaO·P2O5.
Composition of solid phase was shifted to CaO corner from 2CaO·SiO2 on the FeO-CaO-SiO2 ternary
phase diagram, because the increase of P2O5 content in the 2CaO·SiO2-3CaO·P2O5 phase results in the
CaO/SiO2 ratio larger than 2. Since small solubility of FeO in the solid phase was observed, it seems
that small amount of CaO is substituted by FeO. P2O5 content in liquid phase was small. Therefore, thedifference between measured and reported liquidus is due to the difference in oxygen partial pressure.
In the figure, the liquidus for the FeO-CaO-SiO2 system with PO2 = 1.8×10−8 atm at 1573 K20) is also
shown. Similar liquidus were observed at the FeO content between 20 and 60 mass% regardlesstemperature or oxygen partial pressure differences. On the other hand, considerable differences wererecognized at high FeO content range. The author previously clarified that the decrease in the oxygen
partial pressure makes the liquid area at high FeO content region wider.20) Therefore, the further
expansion of the liquid area was expected in the present study due to the decrease of (%Fe3+)/(%Fe2+)
ratio. However, the shrinkage of the liquid area at high FeO content region was observed oppositely.
Some FeO might be reduced to form metallic Fe because the oxygen partial pressure in the present
study is quite close to that of Fe-FeO equilibrium. If the reaction takes place, the formed Fe causes theoverestimation of Fe content in the slag which makes the liquid area narrower than the actual.

－ 51 －

SiO2 (%)
0

100

20

40

CaO•SiO2

80

Liquidus of FeO-CaO-SiO2
coexisting with solid Fe11)60
Liquidus after Kimura et al.20)

3CaO•2SiO2
60

40

2CaO•SiO2
3CaO•SiO2
80

20

Present study100

CaO (%) 0

Figure 9

0
20

40

60

80

100

FeO (%)

Measured liquidus for the FeO-CaO-SiO2-P2O5 system saturated with solid 2CaO·SiO23CaO·P2O5 at 1673 K with PO2 = 9.1×10−11 atm, together with the liquidus for the
FeO-CaO-SiO2 system equilibrated with solid iron at 1673 K,11) and that with PO2 =1.8×10−8 atm at 1573 K.20)

4. Conclusions
The present study focuses on the formation mechanisms of the 2CaO·SiO2-3CaO·P2O5 solid
solution in the FeO-CaO-SiO2-P2O5 system. Firstly the reaction behavior between solid
CaO/2CaO·SiO2 and liquid FeO-CaO-SiO2-P2O5 slag has been observed. Secondly, the phaseequilibria between solid 2CaO·SiO2-3CaO·P2O5 and liquid phase for the FeO-CaO-SiO2-P2O5 systemhave been measured at 1673 K with PO2 = 9.1×10−11 atm.
By immersing solid CaO or 2CaO·SiO2 in the liquid slag containing P2O5, 2CaO·SiO2- 3CaO·P2O5
solid solution was formed at the interface between solid and liquid and the fast formation rate of solid
solution was expected. Efficient and continuous dephosphorization reaction is possible by enhancing
the condensation of P2O5 in solid phase and maintaining P2O5 content in liquid phase low. Reaction
mechanisms between solid CaO/2CaO·SiO2 and liquid slag were clarified.
Measurement of the phase equilibria proved the large phosphorus partition ratio between solid andliquid phases. The partition ratio became larger with larger FeO content.
Acknowledgment
This research was supported by JFE 21st Century Foundation for FY 2009. The author expresses hisgreat gratitude to their support.
－ 52 －

References
1) W. J. Schlitt and G. W. Healy: Am. Ceram. Soc. Bull., 50 (1971), 954.
2) T. Hamano, M. Horibe and K. Ito: ISIJ Int., 44 (2004), 263.
3) J. Yang, M. Kuwabara, T. Asano, A. Chuma and J. Du: ISIJ Int., 47 (2007), 1401.
4) H. Suito, Y. Hayashida and Y. Takahashi: Tetsu-to-Hagané, 63 (1977), 1252.
5) R. Inoue and H. Suito: ISIJ Int., 46 (2006), 174.
6) K. Ito, M. Yanagisawa and N. Sano: Tetsu-to-Hagané, 68 (1982), 342.
7) S. Hirosawa, K. Mori and T. Nagasaka: CAMP-ISIJ, 17 (2004), 868.
8) W. Fix, H. Heymann and R. Heinke: J. Am. Ceram. Soc., 52 (1969), 346.
9) E. T. Turkdogan: Physical Chemistry of High Temperature Technology, Academic Press, New York,(1980), 7.
10) R. Saito, H. Matsuura, K. Nakase, X. Yang and F. Tsukihashi: Tetsu-to-Hagané, 95 (2009), 258.
11) E. F. Osborn and A. Muan: Phase Equilibrium Diagrams of Oxide Systems, the American CeramicSociety and the Edward Orton Jr., Columbus, Ohio, (1960), Plate 7.
12) Y. Takeda and A. Yazawa: J. Min. Metall. Inst. Jpn., 96 (1980), 901.
13) S. Ban-ya: ISIJ Int., 33 (1993), 2.
14) X. Yang, H. Matsuura and F. Tsukihashi: Tetsu-to-Hagané, 95 (2009), 268.
15) X. Yang, H. Matsuura and F. Tsukihashi: ISIJ Int., 49 (2009), 1298.
16) X. Yang, H. Matsuura and F. Tsukihashi: ISIJ Int., 50 (2010), 702.
17) X. Yang, H. Matsuura and F. Tsukihashi: Mater. Trans., 51 (2010), 1094.
18) X. GaO, H. Matsuura and F. Tsukihashi: CAMP-ISIJ, 23 (2010), 951.
19) X. GaO, H. Matsuura and F. Tsukihashi: Proc. Fourth Baosteel Biennial Academic Conference,(2010), B12.
20) H. Kimura, S. Endo, K. Yajima and F. Tsukihashi: ISIJ Int., 44 (2004), 2040.

－ 53 －