4th International Conference on Molten Slags and. Fluxes, 1992, Sendai, ISIJ

EQUILIBRIUM BETWEEN CaO-SATURATED FetO-CaO-P205-CaF2 SLAG AND LIQUID IRON

Shigeta Hara*, Tetsuji Tokonami** and Kazumi Ogino***

*Department of Materials Science and Processing, Osaka University,Suita,Osaka **Graduate School of Osaka University, now Kakogawa Works, Kobe Steel Ltd.Kakogawa ***Department of Materials Science and Processing, Osaka University, now Kagawa polytechic

Collage,Marugame, Japan.

Synopsis: Equilibrium between molten FetO-CaO-P205-CaF2 slags and liquid iron were studied under a Cao-saturated condition at 1873K to clarify a role of CaFz in the molten slags. An

increase of CaF2 in the slags containing lOmass%P205, increases the activity coefficient of

iron oxide, YFetO and also descreases the YP205 as follows, log Y P205=2.52NFeto-1.32NcaFZ-19.4

Under a Cao-saturated condition, CaF2 is effective to dephosphorization of liquid iron and most suitable slags for the dephosphorization of iron are those containing 40-50mass% CaFz.

Key words: Cao-saturated slag, dephosphorization, effect of CaFz, YP205.YFeto. solubility

of Cao, oxygen distribution.

1. Introduction Calcium fluoride(CaF2) has been used as an additive to promote slagging of fluxes in

steelmaking processes. However, a role of CaF2 in fluxes is not entirely clear yet. Some reports showed that the addition of CaF2 promoted dephosphorization of liquid iron, but others didn't so ,in other words, the CaF2 helps only to decrease slag viscosity and hence its addition results in increase of the dephosphorization rate of steel.

In the present work, we measured equilibria of oxygen and phosphorus between FetO-CaO­

P205-CaF2 slags and iron under a Cao-saturated condition at 1873K to clarify a role of CaF2 in the molten slags.

Temperature(°C)

-o.5.-__ 1_1,_.0_0 __ 1....,s_50 __ 1_6,_.0_0 __ 1~55_o ___ _ I I I I

-o- Present work

o Sanbong,watanabe[l]

~._ • Fischer. Ende[2]

. : ::·~1: -~I ~ -1.0t-"' ...::. t>O 0 - 0 3

-1.5~~1 ____ ~1 ___ ____,l'-----....L.'---' 5.0 5.2 5.4 5.6

Fig. 1 Change of oxygen content in liquid iron equilibrated with CaO-satrated iron oxide melt with temperature.

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Cofi

o Present work

20

Cao (mass•/.)

Fig. 2 Solubility line of Cao in Fe,O-CaO-CaF 2

system equilibrated with liquid iron at 1873K.

cao

4th International Conference on Molten Slags and Fluxes, 1992, Sendai, ISIJ

2. Experimental procedures About 20 to 25 g of iron sample and lOg of slag sample were melted in a sintered Cao

crucible under Argon. A molybdenum crucible was used a susceptor for a induction heating furnace, that provided for an uniform temperature in the Cao crucible. Metal samples are electrolytic iron for phosphorization run and an iron-0.03mass%P alloy for dephosphorization run. Slag samples were prepared by Ar-arc-melting of mixtures of chemical reagents. Usually, more than 30min. was required to established the slag-metal equilibria.

3. Oxygen content in liquid iron equilibrated with Cao-saturated FetO-CaO slag. Figure 1 shows oxygen content in liquid iron equilibrated with Cao-saturated iron oxide

melt . The results are as follows, log[%Q)=-5240/T+l.743 ( 1848<T/K<1933 ) (1)

They were slightly higher than those reported by Sanbongi and Watanabe[!), but are in good agreement with those by Fischer and Ende[2).

4. Solubility of Cao in FetO-Ca0-CaF2 melt at 1873K. Solubility line of Cao in the system, FetO-CaO-CaF2 is shown in Fig. 2. Our result is

fairly far from that reported by Wrampelmeyer et al. (3), especially in a higher CaFz region, but it is similar to that equilibrated with solid iron at 1723K reported by Hawkins and Oavies[4]. Phase diagrams of the systems,CaO-FetO and CaO-CaFz [5,6)indicate that by means of chemical analysis, it is very difficult to separate a suspended lime in the slag melts, namely exfoliated lime, from that crystallizing out from melts during the solidification. Althogh a very dense lime crucible we used, EDX-analysis of solidified slags showed that Ca in samples obtained near the crucible wall, was higher than that far from the wall . This suggests existence of the suspended Cao near the crucible wall, and hence slag com­positions were determined for samples obtained from the central part of the solidified slag by chemical analysis, even if the slags were apparently seemed to be uniform.

5. Oxygen content in liquid iron equilibrated with molten FetO-CaOsat.-CaF2 slag at 1873K. Figure 3 shows oxygen content in liquid iron equilibrated with FetO-CaOsat.-CaF2 slags

at 1873K as a function of F/(Fe+F) mass ratio, because solubility line of the CaD by \Vram­pelmeyer et al. [3] is differ from ours, as stated above. Activity of FetO. aFetD in the slags (refered to iron saturated pure iron oxide) are calculated from the oxygen content in

E a. a.

_g 600 ClJ E c ·- 400 c ClJ O' >.

200 x 0

00

o Present work

£>- Wrompelmeyer et al.(1550°C)3>

... Wrompelmeyer etal. (1600°C )3>

0.2 0.4 0.6 o.s F

(moss ratio) F •Fe

...

Fig. 3 Oxygen content in iron equilibrated with Fe,0-CaO •• t. -

CaF, slag at 1873K as a function of the mass ratio, F/(Fe+F).

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6 6

1.0

4th International Conference on Molten Slags and Fluxes, 1992, Sendai, ISIJ

combination with thermo-chemical data[7,8,9] and is shown in Fig . 4. The Wrarnpelrneyer's

results are in fairly good agreement with ours . The figure also shows those reported by Iwase et al. [10] and Kay et al[ll]. using different method at lower temperature, which are extrapolated to 1873K by the Masson's method[12]. The activity coefficient of FetO, y FetO in the Cao-saturated melts is shown in Fig. 5. It is concluded that CaF2 in the melts con­tributes to increase the Y FetO .

'?. .. "-

0

0.5

0.4

0.3

0.2

0.1

0 0

0 .. ,,. 0

o Present work "'Wrampelmeyer et al.(1550'Cl

.. Wrampelmeyer et al.(1600'C)

o Iwase et al. o Kay t al.

0.2 0.4

F F• Fe

0.6

(mass ratio)

..

0.8

Fig.4 Activity of iron oxide, are to in Fe,o-cao •• ,. -CaF2 slag saturated with iron at 1873K.

1.0

3.o~--~------,-------,-------,-----,

2.0 o Present work • Kay et al. o lwase et al.

0

D D

/

/ ,./ /

/

/ /

/

OOL-----'0.-2---~0.-4---0~.6-::----~0~.8,------;-'1.0

F F. Fe

(mass ratio)

Fig. 5 Activity Coefficient of iron oxide, r •eto in Fe,O-CaO,,, , -CaF2 slag saturated with iron at 1873K.

6. Equilibrium phosphorus and oxygen content between liquid iron and FetO-CaOsat.-P205-CaF2 slags at 1873K. Figures 6 and 7 shows ~and t contents in irons equilibrated with CaF2-free FetO­

CaOsat.-P205 slags at 1873K. Solid and open circles in the figure are from dephosphorization and phosphorization runs, respectively. Below lOmass%P205, oxygen in the metals decreased and phosphorus in it increased with increasing the P205 content. However, above 10mass%P205 ,effect of P205 in the slags on the oxygen and phosphorus became small.

Effects of CaF2 in slags containing lOmass%P205 on equilibrium ~ and £ in metals are shown in Fig.8 and Fig.9. Under Cao saturation, increase of CaF2 means decrease of FetO in slags. These figures suggest that increase of CaF2 contribute to decrease phosphorous in metals, instead of decrease of oxidation power of the slags.

E a. 2-

:g .. E c: c: .. O> ,.. 6

1000

• •

"'"' ·~ "'CS, C/'i5.--,.-t:.;.-

"' l!.l!. 500

t:. Na go bayashi et al.

• Present wor1<(dephosphorization)

o Present work(phosphorization)

0 0 10 20 30

(P20s) (mass'/,)

Fig. 6 Oxygen content in liquid iron equilibrated with Fe,O-CaO .. ' · -P20o slag at 1873K.

0.03

"' ;.: Ill "' Ill "' Nagabayashi etal. "' 0

"' t;l!. ..s • Present work(dephosphorization) "' o Present work(phosphorization) "' c 0.02

"'Lb o; "'"' "' "' E "' l!. "' "' !:A

.!: "' "' !S fl. Ill

"'"' 4. (l~.9 ~ "' 2 "' "'"'a cf>t:. 0 0.01 "' .I: Ot;~ o0 "' a. I a o ·

l!. "' 0 .s:: a. "'"' •••

•o"' "'- • • l!.

"' • 0

0 10 20 30

(P2 05) (mass'/,)

Fig. 7 Phosphorus content in liquid iron equilibrated with Fe,O-Cao •••. -P20o slag at 1873K.

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4th International Conference on Molten Slags and fluxes, 1992, Sendai, ISIJ

o! 0.02 E l/\ a. l/\

CaO saturation,(P20.sl:10mass'/, a. I Ci 0 0 Dephosphorizalion

a; a; • Phosphoriza t ion

E O Dephosphorization E

c 400 c 0.01 -c • Posphorizalion l/\ <IJ :J en 5 • >. ( P 2 Os ) = 10 mass'/, .r. 6 200 a.

CaO saturation l/\ 0 0 0 0-.r.

Cl..

0 0 0 10 20 30 40 50 0 10 20 30 40 50

(CaF2) (mass°lo) (CaF2) (mass'!.)

Fig. 8 Effect of CaF, content in Fe,0-Cao .... - Fig. 9 Effect of CaF, content in Fe,O-Cao,, .. -10mass%P,O.-CaF, slags on oxygen in iron. lOmass%P 20,-CaF 2 slags on phosphorus in iron.

From these results, aFetO in the slags was calculated and the results are shown in Fig.10. The Y FetO increases with increasing of CaFz content in the slags containing lOmass%Pz05 as well as in those of Pz05-free FetO-CaO(sat.)-CaFz system.

1.5

1.0 0

0.5

10 20

o Dephosphorizotion • Phosphorization

( P20 5) = 10moss•t.

CaO saturation

30 40 50

CaF2) (mass°lo)

Fig. 10 Activity Coefficient of iron ox,de, r ,, to in Fe,0-CaO., t. -10mass%P,O,-CaF2 slags saturated with iron at l873K.

Combined our results with equilibrium constant of the reaction, 5~+2t=(Pz05) by Turkdogan[ll], the YP205 in the slags was claculated. Fig.11 shows that the higher Pz05 in slags is( that means decrease of FetO content in slags), lower the YP205 in the slags is. Fig.12 shows that at a constant Pz05 content of 10mass%, an addition of CaF2 in slags con­tributes to decrease the Y P205 , which are as follows,

log Yp205=2.52NFeto-1.32NcaFz-19.4 (2)

-16

-17

((Pz05)=lOmass%, at Cao saturation at 1873K)

0 0

9: J18 Q._ ;>-Cl1 ~

-19 -----0

-20L-~-'-~-'-~~-'--~-'-~--"~~-'--~-' 0 0.1 0.2 0.3. 0.4 0.5 0.6 0.7

NFe10

Fig.11 Relation between N,.,o and logr r20• in Fe,0-Cao .... -CaF2 slags at 1873K.

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I I ' I

0.1 0.2 0.3 0.4

NcaF2

Fig. 12 Relation between N,,'° and logr r20• in Fe,O-cao .... -10mass%P,O,-CaF, slags at 1873K.

-

0.5

4th International Conference on Molten Slags and Fluxes, 1992, Sendai, ISIJ

CaF2

60

0.0047 10mass'!. P2o5

80 0.0050 0.0061

Fe10 20 40 60 80 Cao

CaO(mass'lo)

Fig. 13 Calculated phosphorus content in liquid iron equilibrated with Fe,O-CaO.,,. -lOmass%P 20,-CaF 2 slags at 1873K.

Combined this equation with YFetO• shown in Fig.10, we calculate phosphors content in met­als equilibrated with slags containing lOmass%P205 along the line of lime saturation, that is shown in Fig.13. From the figure, we can choose the most suitable composition of FetO­CaOsat.-lOmass%P205-CaF2 slags for the dephosphorization, that contains 40-50mass%CaF2.

Reference [1] K.Sanbongi and S.Watanabe:Tetsu-to-Hagane,42(1956),861 [2] W.A.Fisher and H.Vom Ende: Arch.Eisenhuttenwes.,23(1952),22 [3] J-C.Wrampelmeyer,S.Dimitrov and D.Janke:Steel Research,61(1990),l [4] R.J.Hawkins and M.W.Davies:J.Iron Steel Inst.,209(1971),226 [5] K.C.Mills: NPL report Chem.100,(1979),p.19[teddington,Middlesex,UK] [6] S.Hara,T.Araki and K.Ogino:Proceed.2nd Intern.Symp.Met.Slags and Fluxes(l984),441 [7] Recomended Equilibrium Values on Steelmaking Reactions by the l9th Committee of the

Japan Soc. for Prom.otion of Science, (1984) [8] C.R.Taylor and J.F.Chipman:Trans.Amer.Inst.Min.Met.Eng.,154(1943),228 [9] H.Sakao and K.Sano:J.Japan Inst.Metals,23(1959),667 [10] M.Iwase,E.Ichise and N.Yamada:ISS.Trans.,4(1984),47 [11] D.A.Kay,A.Mitchell and M.Ram:J.Iron Steel Inst., 2(1970),141 [12] C.R.Masson:Proc.Royal Soc.,287A(1965),201 [13]R.Nagabayashi,M.Hino and S.Banya:Tetsu-to-Hagane,74(1988),1577 [14]R.Nagabayashi,M.Hino and S.Banya:Tetsu-to-Hagane,74(1988),1585 [15]E.T.Turkdogan and J.Peason:J.Iron Steel Inst.,176(1954),59.

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