高温顕微鏡を利用したその場の観察による CaO 滓化メカニズムの解明
Clarification of the Mechanism of CaO-containing Slag Formation in situ
Observed by High Temperature Optical Microscope
研究代表者 名古屋大学大学院工学研究科 助教授 楊 健
共同研究者 名古屋大学大学院工学研究科 教授

桑原 守

1. Introduction
The reaction between lime-containing slag and molten metal plays an important role in desulfurization anddephosphorization of molten iron. It is highly desirable that lime could rapidly melt with other flux components topromote the reaction between slag and metal.
A great number of researches have contributed to clarify the mechanisms involved in dissolution of lime intoslag and develop the new methods to promote dissolution of lime into slag. Schlitt et al.1) conducted a kinetic
study on lime dissolution into CaO-FeO-SiO2 slag by lowering specimens of CaO into the molten slag for aprescribed time, and then withdrawing them to the upper section of tube to cool. They found that the dissolution
rate of lime markedly increased with increasing the FeO content in slag. The mechanism of CaO dissolution intoslag was also investigated by observing the lime-containing flux heated on Pt-filament directly through a
microscope.2) It is indicated that the layer of 2CaO・SiO2 is formed on the surface of CaO particles, anddissolution of CaO proceeded by migration of the molten slag through cracks on the layer of 2CaO・SiO2. In thecase of higher Fe2O3 content, the layers of 2CaO・SiO2 and 3CaO・SiO2 compounds formed around the CaO
crystal were not continuous but looked more like particles, which promoted dissolution of CaO.3) The rate ofdissolution of solid lime into liquid slag was determined from the decrease in diameter of a lime cylinder rotatedin CaO-SiO2-Al2O3 or FeO-CaO-SiO2 slag.4) The diffusion of calcium through a slag boundary phase wasrevealed to be the rate-determining step. By use of a rotating-disc apparatus, influence of lime properties on rateof dissolution in CaO-SiO2-FeO slag was investigated.5) The more porous lime was found to show the higher
reactivity in the experiments. In addition, influence of various fluxing agents, such as BOF slag and dust, BF slag,
on lime dissolution was also studied.6)
In recent years, there is an increasing concern about environment. Fluorspar(CaF2) is a commonly used additive
to promote melting of lime-containing flux. Owing to fluorine containing species released into the atmosphere anddissolved into the soil from the disposed slag, utilization of fluorspar is increasingly limited. On the other hand, inorder to reduce the amount of steelmaking slag, higher reactivity of lime is also strongly demanded. Therefore, thealternative additives and methods are required for facilitating the melting of lime-containing flux.
Lee et al.7) developed a new flux of burnt lime(CaO) coated with di-calcium ferrite(2CaO・FeO). It was claimed
that this new kind of flux had a high resistance to hydration and a capacity for fast and complete dissolution into
the steelmaking slag of BOF and EAF. Hamano et al studied dissolution rate8) of solid lime into the molten slag
used for hot-metal dephosphorization, and the reaction mechanism9) between solid lime and the slag. They
concluded that the effect of the additives to FetO-CaO-SiO2 melt on increasing the dissolution rate of CaO was
CaF2>CaCl2>B2O3>Al2O3. Amini et al10) indicated that addition of CaF2, MnOx, FeOx, and TiO2 into
－ 11 －

Al2O3-CaO-SiO2 slag increased the dissolution rate of lime into slag, while SiO2 had an opposite effect.
In contrast to most of the previous studies on dissolution rate of block lime into the molten slag, the present
work concentrates on promotion of the melting of CaO-containing flux particles. By use of high temperature
optical microscope(HTOM), the melting processes of flux particles are in situ observed. The flux melting is also
confirmed from observation using a scanning electron microscope (SEM) and analysis with an energy dispersive
spectrometer(EDS). The flux melting temperature is determined by a differential scanning calorimeter (DSC).
Furthermore, a novel method is proposed to promote the melting of CaO-containing flux, by taking advantage ofheat generated by thermit reaction between Fe2O3 and Al.
2.

Experimental Apparatus and Procedures
The materials used for the experiment were calcium oxide powders (10 µm in average diameter and purity of

99.9%), aluminium oxide powders (50 µm in average diameter and purity of 99.9%), silicon oxide powder(0.8 µm
in average diameter and purity of 99.9%), α-iron oxide powders (15 µm in average diameter and purity of 99.5%),
and aluminum powder (75－150µm in diameter and purity higher than 99.5%). After mixing, pellets were formedby use of a cold isostatic press(CIP) under a pressure of 150 MPa for one hour, having the diameter of 3 mm andthe length of 2 mm.
The components of pellets are shown in Table 1. Run B had the common constitution of slag employed in ironand steel making process, which had a relatively low melting point of about 1573 K from the CaO-Al2O3-SiO2
ternary phase diagram.11) In Run A, Fe2O3 and Al powders were added at the stoichiometric mole ratio of 1:2
according to thermit reaction between Fe2O3 and Al. After the thermit reaction, the components of Run A would
be almost the same as those in Run B except for iron produced. On the other hand, Run D had the components thatare possible to be used as a dephosphorization flux in pretreatment of hot metal and had a low melting point of
about 1473 K.12) In Run C, Al powders were added instead of Al2O3 powders in Run D. After thermit reaction
between Fe2O3 and Al, the constitution of Run C would be almost the same as that in Run D. In Run E, aminimum amount of Al powders was added for compensating the heat absorbed during the flux melting and its
temperature rising to the melting point. The calculation method of the addition amount of Al powders will be
described in the later part of this paper.
Figure 1 is a schematic diagram of high temperature optical microscope. It has components of a hot stage andan optical microscope equipped with a digital video camera, monitor and computer. The in-situ observation of theflux melting process was performed by placing an examining pellet in a platinum crucible of 6.0 mm i.d. and 2.0
mm in height. Then the crucible was set in a sapphire sample holder equipped with B-type thermocouple, and thewhole assembly was located in the hot zone of the platinum-image furnace.
Top view of hot stage
4

6

3

11

Table 1. Pellet components in the experiments.

7
5

Run A
Run B
Run C
Run D
Run E

CaO
(%)
29.6
34.0
18.1
20.2
19.5

Al2O3
(%)
12.1
27.7
0
8.6
6.9

SiO2
(%)
32.9
38.3
0
0
0

Fe2O3
(%)
18.8
0
77.2
71.2
72.6

Al
(%)
6.6
0
4.7
0
1.0

8
10
1. Hot stage
2. Microscope
3. Video Camera
4. Video
5. Monitor
6. Heating unit

92
1

7. Thermocouple
8. Cooling water
9. Crucible(C)
10. Gas inlet
11. Gas outlet

PC

Fig. 1. Schematic diagram of high temperature
optical microscope.

－ 12 －

Before heating, the chamber was evacuated with a diffusion pump and backfilled with ultrapure Ar gas, inwhich oxygen and water vapor were minimized by passing through heated Mg chips and a gas-cleaning system.
This purified Ar gas at a flow rate of 3.3×10-6 m3/s was also used as the protective gas during the experiment. The
sample was observed from top surface through a top quartz window. The temperature on the sample surface wascalibrated by use of an optical pyrometer. In Runs A and B, pellet masses were 0.0190 g. In Runs C, D and E,
pellets of 0.020 g were used.
After observation with HTOM, the cooled pellet was observed using SEM. The microscopic compositions at
various positions on the pellet surface were determined by EDS. The melting temperatures of different kinds offlux were further determined by use of differential scanning calorimeter (DSC).

3.

Experimental Results and Discussion

3.1. Melting of CaO-Al2O3-SiO2 Flux Assisted by Thermit Reaction
By use of HTOM, the change in temperature with time is shown in Fig. 2. The specimen was heated to 1073 K
at the rate of 50 K/min, and then to 1623 K at the rate of 10 K/min. During the heating, it was held for 10 min at
the temperatures of 1273, 1373, 1473 and 1573 K.
Figure 3 presents the morphologies of all kinds of materials before formation of pellets. Calcium oxide
powders were relatively uniform in diameter. The diameters of aluminium oxide powders were varied, ranging
from 20 µm to 80 µm. An aluminum oxide particle was usually in a cluster formed by many fine aluminum oxide
particles. Silicon oxide powders were very fine in size with the average diameter of only 0.8 µm. α-iron oxide
powders had greatly varied diameters and irregular shapes. Aluminum powders had dense surface with thediameters ranging from 75 µm to 150µm.
20001273 K

1500

1373 K

1473 K

1573 KT (K)1073 K

1000
10 K/min
50 K/min

500

0

0

20

40

60
Time (min)

80

100

120

Fig. 2. Change in experimental temperature withtime by use of high temperature optical
microscope.

Fig. 3. Morphology and size of each kind of powders
to be mixed to form pellets.

Fig. 4. Optical micrographs of pellet for Run A
and B before experiment.
Fig. 5. Change in pellet morphology with temperature(Run A).
－ 13 －

The optical microscope images of the whole pellet before the experiment are presented in Fig. 4. The pellet hada diameter of 3.5 mm and a height of 1.0 mm. The pellet of Run A had a red color of Fe2O3 powders dotted withwhite color of the other added particles. The pellet of Run B had a gray white color. Both pellets of Runs A and B
had a mass of 0.0190 g.
The results of in situ observation of melting process of lime-containing flux assisted by thermit reaction for Run
A are shown in Fig . 5. The frame size is 0.73 mm by 0.97 mm. The morphological change of the pellet was notdetected before 1373 K. After rising to 1473 K, the white parts on the pellet surface decreased in their areas. And
it was observed that the surface of pellet had begun to move. After rising to 1573 K, the flow on the pellet surface
was clearly observed, and the specimen was fully melting.
The thermit reaction between Fe2O3 and Al usually involves a highly exothermic reaction along with vigorous
combustion.13-15) However, in the present experiment, the ignition and combustion phenomenon was not observed.
This is because the components of CaO, Al2O3 and SiO2 acted as the inactive materials, the reaction heat was
deprived and the ignition and combustion were obstructed.
After in situ observation, the cooled specimen was further observed with SEM and analyzed with EDS. Figure
6 shows the results. Before experiment as shown in diagram(a), the individual particle made a distinct appearance.
After 10 min holding at 1373 K, a few of the particles were partly melting as shown in diagram(b). The EDS
analysis results show that at points of A, B and C, the particles almost remained their original compositions. After
10 min holding at 1473 K, diagram(c) clearly shows that the pellet was fully melting and the individual particle
disappeared. The EDS results also show that at points of D and E, all kinds of compositions in the pellet were
obviously detected. The melting of pellet was confirmed. These results are in good agreement with those in situ
observed by use of HTOM, indicating the lime-containing flux was fully melting at the temperature of 1473 K
with assistant from thermit reaction.
For comparison, experiment of Run B was conducted. Run B had nearly the same components as those of Run
A after thermit reaction. The SEM images and EDS analysis results after experiment are given in Figure 7. Before
experiment, the particles appeared individually. After 10 min holding at 1473, there was no change in the particle
morphology and no sign of melting. After 10 min holding at 1573 K, a few of particles seemed to be partly
melting. EDS results shows most of the particles remained their original compositions on the pellet surface. At
point C, CaO combined with Al2O3. In comparison to Run A, it is clear that the melting of lime-containing flux isgreatly promoted with assistant from thermit reaction.

Fig. 6. SEM image and EDS analysis of pellet
after experiment(Run A).
－ 14 －

Fig. 7. SEM image and EDS analysis of pellet
after experiment(Run B).

In order to obtain the melting temperature more exactly for Runs A and B, a differential scanningcalorimeter(DSC) was used to measure the start melting temperature. The pellet mass was 0.0190 g, which was
the same as that in observation with HTOM. A platinum crucible of 6.0 mm i.d. and 2.0 mm in height was used.
An inert atmosphere was maintained by use of Ar gas at the flow rate of 8.3×10-7 Nm3/s. The heating rate was
0.167 K/s.
Figure 8 presents the DSC results for Runs A and B. For Run B, initiating from 1587 K, a great endothermic
peak appeared. After that, no endothermic peak was detected and the specimen was fully meting after DSC
measurement. Therefore, it is clear that the endothermic peak was caused by the flux melting and its starttemperature was 1587 K, which was close to the melt point(1573 K) of the ternary CaO-Al2O3-SiO2 phase of thepresent components.
For Run A, at 933 K, there was an endothermic peak, which corresponded to melting endotherm of Al.
Initiating from 1413 K, a big exothermic peak appeared, which matched the great heat generation of thermit
reaction between Fe2O3 and Al. After DSC measurement, the specimen was fully melting, but no endothermic
peak was further detected. Therefore, it is reasonable to deduce that the flux melting proceeded simultaneouslywith thermit reaction. The great heat generation of thermit reaction had assisted the melting of lime-containing
flux. Since the heat generated during thermit reaction was much greater than that absorbed during flux melting,the exothermic peak was due to the surplus heat of thermit reaction after compensating the heat absorption of flux
melting. The start melting temperature in Run A was decreased to 1413 K, which was much lower than that in Run
B. Again, it is confirmed that the melting of lime-containing flux was greatly promoted with assistance fromthermit reaction.
3.2. Melting of CaO-Al2O3-Fe2O3 Flux Assisted by Thermit Reaction
The melting of dephosphorization flux assisted by thermit reaction was further studied. The components ofpellets are shown in Table 1. Run D had the constitution of common dephosphorizaiton flux. In Run C, Al
powders were added instead of Al2O3 powders. The optical microscope images of the whole pellet before theexperiment are presented in Fig. 9. The pellet had a diameter of 3.5 mm, a height of 1.0 mm and a mass of 0.020 g.
Both pellets had red color of the Fe2O3 powders dotted with white color of the other added particles.
For Run C, change in pellet morphology with temperature in situ observed by HTOM is given in Fig. 10. After
873 K, the color of the pellet surface changed from red to grey. This is maybe due to the partial decomposition of
Fe2O3. Before 1443 K, no obvious change in morphology was detected. When the temperature was raised to 1523
K, the flow caused by the melting of pellet was observed on its surface. The pellet was fully melting.
DSC(μw)

4.0×104

Run A

2.0×104

Ⅱ0-2.0×104-4.0×104
600
873
0

Ⅰ
800
1073

1000
1273
T(K)

1200
1473

Run B

DSC(μw)-1.0×104-2.0×104-3.0×104

Ⅲ-4.0×104
950
1223

Fig. 8.

1400
1673

1050
1323

1150
1423

T(K)

1250
1523

Fig. 9. Optical micrographs of pellet for Run C
and D before experiment.

1350
1623

DSC results of Run A and B.
－ 15 －

Fig. 10. Change in pellet morphology withtemperature(Run C).

Fig. 11. SEM image and EDS analysis of pellet
after experiment(Run C).

Figure 11 shows the SEM images and EDS analysis results for cooled pellets of Run C after they were heated
to different temperatures. After heated to 1273 K, the pellet was not melting, which could be justified from theappearance of pellet. From EDS analysis results, just a slight combination between CaO and Fe2O3 could berecognized. After heated to 1373 K, the pellet was partly melting, which could be identified from the gaps formedby melting contraction. EDS results also show that CaO and Fe2O3 were well amalgamated. After heated to 1523
K, the pellet was fully melting. CaO and Fe2O3, as well as Al2O3 were completely amalgamated.
For comparison, pellets of Run D, in which Al powders were not added, were also heated for SEM observationand EDS analysis. The results are shown in Fig. 12. After heated to 1273 K, the morphology of pellet did notshow it melting. EDS analysis indicates that a slight amalgamation between CaO and Fe2O3 took place. After
heated to 1373 K, the pellet did not look like melting. But CaO and Fe2O3 were somewhat amalgamated from the
EDS results. After heated to 1523 K, the pellet showed partly melting. But some particles appeared individually
without melting, which could be justified from the appearance of pellet. The EDS results indicate that at someparts on the pellet surface, CaO, Fe2O3 and Al2O3 are amalgamated.

Fig. 12. SEM image and EDS analysis of pellet after
experiment(Run D).

DSC( W)

DSC( W)

5.0×105
Run C
4.0×105
3.0×105
μ 2.0×105
1.0×105
0.0-1.0×1051273
2.0×104
0.0

1673

Run D

4

μ -2.0×10-4.0×104-6.0×104-8.0×104

Fig. 13.

－ 16 －

1473
T (K)

Ⅰ
1273

Ⅱ

1473
1673
T(K)
DSC results of Run C and D.

The DSC measurement was also conducted for Runs C and D as shown in Fig. 13. The measurement conditions
were the same as those for Runs A and B except for heating rate of 0.50 K/s. For Run D, the first endothermic
peak began at the temperature of 1470 K. From the binary CaO-Fe2O3 phase diagram,12) since the eutectic
temperature is about 1473 K, the first peak should correspond to the amalgamation between CaO and Fe2O3. The
second endothermic peak initiated from 1553, which should match the amalgamation of all the three components
in the pellets. The pointⅠof 1538 K is the temperature just after the first peak and before the second peak, while
pointⅡ of 1663 K is the temperature after the second peak. After heated to temperatures ofⅠandⅡ, the cooled
samples were further observed with SEM and analyzed with EDS. The results are given in Fig. 14. After heated totemperatureⅠ, the SEM image clearly shows some of white particles did not melt, and their compositions weremainly Al2O3. It can be seen that at this temperature, CaO and Fe2O3 were well amalgamated. After heated totempertureⅡ, at every point on the pellet surface, CaO and Fe2O3, as well as Al2O3 were fully melting. Therefore,
it is confirmed that the first peak in Fig. 13(RunD) was caused by the amalgamation between CaO and Fe2O3,
while the second peak corresponded to the entire melting of pellet. The fully melting temperature was obtained to
be 1663 K
The DSC analysis result for Run C is also shown in Fig. 13. Since there was only an exothermic heat peak andthe specimen was fully melting after the measurement, it is clear that flux melting proceeded simultaneously withthermit reaction. The exothermic heat peak was attributed to the surplus heat of thermit reaction after
compensating the endothermic heat of flux melting. The start temperature of the exothermic reaction was 1478 K,
which was lower than the initiating temperature of 1553 K for the entire melting in Run D. The fully melting
temperature of 1503 K is much lower than that of 1663 K without assistance from thermit reaction. This meansthat the melting of dephosphorization flux is also promoted with assistance from thermit reaction.

3.3. Estimation of Heat Generation by Thermit Reaction
In order to obtain the minimum addition amount of Al powders for melting the flux in Run C, heat generated bythermit reaction is estimated. It is assumed that the heat generated by thermit reaction does not escape from thereaction system, being used only for flux melting and its rising in temperature, and the products do notdisintegrate.
The thermit reaction between Fe2O3 and Al can be described as

DSC(

W)

5.0×105 Run E
4.0×105
3.0×105
μ 2.0×105
1.0×105
0.0-1.0×105
1273

1473
T (K)

1673

Fig. 15. DSC results of Run E.
Fig. 14. SEM image and EDS analysis of pellet after
DSC measurement(Run D).
－ 17 －

2Al + Fe2O3 → 2Fe + Al2O3 ………………………………………...………….………………………..(1)If x mol Al powders are added, one can obtainx
x
Fe2O3 → x Fe + Al2O3 ………………………..…………….…………..………………….(2)22x Al +

From the DSC analysis, it is known that the peak temperature for thermit reaction is 1493 K for Run C and themelting point of Al is 933 K,16) the heat generated by thermit reaction can be estimated aso∆H (2)
=

933
1493
x
o
× ∆H 298(2)+ ∫ ∆Cp(2) dT − x × ∆H m(Al) + ∫ ∆Cp(2)' dT
298
933
2

………………………………………….(3)

where
o
o
o
∆H 298(2)= ∆H 298,
Al O − ∆H 298, Fe O
2

32…………………………………………………...……………………(4)3xx∆Cp(2) = x ⋅ Cp, Fe + Cp, Al O − x ⋅ Cp, Al(s) − ⋅ Cp, Fe O
2
2
2

3

2

3

x
x
∆Cp(2)' = x ⋅ Cp, Fe + Cp, Al O − x ⋅ Cp, Al(l) − ⋅ Cp, Fe O
2
2
2

3

2

3

...…………………………………………….………(5)………………………………..……..…………..…(6)oo∆H m(Al) = 8.4(kJ) ,16) ∆H 298,
Al O ＝-1677.4(kJ/mol), ∆H 298, Fe O =-821.3(kJ/mol), Cp, Fe(s) , Cp, Al O (s) , Cp, Fe O (s) and
2

3

2

2

3

3

2

3

o
Cp, Al(l) are the molar specific heat of each component.17) As a result, ∆H (2)＝-451.815x kJ/mol.

On the other hand, the components in Run D have a molar raito of CaO:Fe2O3:Al2O3=4:5:1. If x mole of Al
powders is added into pellets, and the products have the same component as that in Run D, the molar ratio of
CaO:Fe2O3:Al2O3:Al in the pellets should be 4:(5+x/2):(1-x/2):x. After thermit reaction, the molar ratio of
CaO:Fe2O3:Al2O3:Fe in the product becomes 4:5:1:x.
From the DSC results, Run D has a melting point of about 1623 K, which is taken as the second peaktemperature in Fig. 13. After thermit reaction, as for the product having a molar ratio of CaO:Fe2O3:Al2O3:Fe =4:5:1:x, the necessary heat for pellet melting and its temperature rising from 1493 K to 1623 K is1623o
∆H (P)=∫

1493( 4 × Cp, CaO + 5 × Cp, Fe O + Cp, Al O + x ⋅ Cp, Fe )dT + ∆H m( CaO − Fe O − Al O )
2

3

2

3

2 3

2 3

………………….…..……(7)Where ∆H m(CaO - Fe O - Al O ) =61.294 kJ, which is obtained by DSC analysis for the present specimen, Cp,CaO , C p ,Fe O ,
2

3

2

3

2 3

o
C p , Al O and Cp,Fe are the molar specific heat of each component. As a result, ∆H (p)= 201.562 + 6.074x (kJ).
2 3o) is equal to the necessary heat for pellet melting and its
If the heat generated by thermit reaction( ∆H (2)temperature rising from 1493 K to 1623 K, the minimum amount of Al powders is x=0.451 (mol). And the masspercent ratio at this component is CaO:Fe2O3:Al2O3:Al=19.5:72.6:6.9:1.0. This means that only if 1.0% of Al
powder is added, the flux will be fully melting after thermit reaction.
The components of Run E have a mass percent ratio of CaO:Fe2O3:Al2O3:Al=19.5:72.6:6.9:1.0. The DSC
measurement for Run E was conducted as shown in Fig. 15. After the measurement, the specimen was fully
melting. Moreover, both of the endothermic and exothermic peaks were very small, indicating that the heatgenerated by thermit reaction just compensated the heat absorbed by the flux melting and its temperature rising.
This is in good agreement with the predication made above.

4. Conclusions
A new method for promoting the melting of lime-containing flux is proposed with assistance from thermit－ 18 －

reaction between Fe2O3 and Al. Two kinds of fluxes are used. One has the component of CaO-Al2O3-SiO2, having
a common constitution of slag employed in iron and steel making process. The other has the component of
CaO-Al2O3-Fe2O3, which is possibly used for dephosphorization in the pretreatment of hot metal. The flux
melting process is in situ observed by use of High Temperature Optical Microscope(HTOM). The cooled samples
are further observed by SEM, and analyzed by EDS. Furthermore, the heat change in the process is determined bydifferential scanning calorimeter (DSC). Following conclusions can be drawn from the present studies:
1) For the first kind of flux of the component of CaO-Al2O3-SiO2, its melting initiates from the temperature of
1587 K. With assistance from thermit reaction between Fe2O3 and Al, the start melting temperature isdecreased to 1413 K. The results of the flux melting in situ observed by use of HTOM are in good agreementwith those measured with DSC. The SEM-EDS results indicate that after holding 10 min at 1473 K, the flux isfully melting and all of the components in the flux are amalgamated assisted by thermit reaction.
2) For the second kind of flux of the component of CaO-Al2O3-Fe2O3, it is revealed that the melting process takeplace in two stages. At the first stage, CaO is combined with Fe2O3; at the second stages, they are further
amalgamated with Al2O3. The fully melting temperature is decreased from 1663 K to 1503 K with assistancefrom thermit reaction between Fe2O3 and Al.
3) An estimation of heat generation during thermit reaction is carried out. The minimum amount of Al powders
necessary for assistance of the flux melting is obtained. The result of testifying experiment is in a good
agreement with the prediction result.

Acknowledgement
This study was supported in part by JFE 21st Century Foundation, Grants-in-Aid for Scientific Research in2006-2007.

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－ 20 －