Effect of CO Gas Concentration on Reduction Rate of Major Mineral Phase SINTER

7/28/2019 Effect of CO Gas Concentration on Reduction Rate of Major Mineral Phase SINTER

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2013 ISIJ 570

ISIJ International, Vol. 53 (2013), No. 4, pp. 570575

Effect of CO Gas Concentration on Reduction Rate of Major

Mineral Phase in Sintered Iron Ore

Daisuke NOGUCHI,1)* Ko-ichiro OHNO,2) Takayuki MAEDA,2) Kouki NISHIOKA3) and Masakata SHIMIZU2)

1) Graduate Student, Kyushu University, Motooka, Nishi-ku, Fukuoka, 819-0395 Japan.

2) Department of Materials Science & Engineering, Kyushu University, Motooka, Nishi-ku, Fukuoka, 819-0395 Japan.

3) Formerly Department of Materials Science & Engineering, Kyushu University. Now at Nippon Steel & Sumitomo Metal

Corporation, 16-1, Sunayama, Kamisu, Ibaraki, 314-0255 Japan.

(Received on October 31, 2012; accepted on January 4, 2013; originally published in Tetsu-to-Hagan,

Vol. 97, 2011, No. 11, pp. 548553)

As a fundamental study for clarifying the reduction phenomena of iron ore sinter in blast furnace, iron

oxide (H) and quaternary calcium ferrite (Cf) were prepared and these kinetic behaviors at the final stage

of reduction with COCO2 gas mixture were studied.Reduction rate increased with increasing reduction temperature. Moreover, it increased with increasing

partial pressure of CO gas. Difference of reduction rate caused by gas composition is much larger than

reduction temperature. From comparisons of weight loss curves, reduction rate of H samples was faster

than that of Cf samples under the same or similar conditions.

Reduction reaction of H and Cf samples proceeded topochemically at higher temperature (1100C),

and didnt proceed topochemically at lower temperature (1000C). Besides, the reduction reaction of

samples with CO rich gas proceeded more topochemically. Structure of iron layer in H samples was

affected by temperature and gas composition. On the other hand, structure of iron layer in Cf samples

was almost the same in all experimental conditions.

Reduction data were analyzed based on one interface unreacted core model, and chemical reaction rate

content kc and effective diffusion coefficient in product layer De were determined. The values of kc show

Arrhenius-type temperature dependency, and were approximately same tendency except for Cf samples

with near equilibriums gas compositions. The values of De of H samples show the temperature and gas

composition dependencies, and that of Cf samples were approximately constant in all experimental con-

ditions.

KEY WORDS: iron oxide; quaternary calcium ferrite; sinter; reduction rate; kinetic analysis; unreacted core

model.

1. Introduction

It is necessary to know reduction behavior of self-fluxing

iron ore sinter that is main burden of iron, in order to clarify

the reaction behavior in a blast furnace. Many experimentsfor iron ore sinter that produced in sinter plant or pod were

carried out to clarify the reduction behavior. However, iron

ore sinter has various mineral phases which consist of iron

oxide, calcium ferrite, slag and so on. Therefore structure of

iron ore sinter is very complex. Analysis of simulated iron

ore sinter that has simplified structure is required for the

quantitative analysis of reduction rate of iron ore sinter. In

particular, clarifying the reducibility of iron oxide and cal-

cium ferrite is most important for understanding the reduc-

ibility of iron ore sinter. Many reduction experiments were

carried out with pure CO or H2 gas. But actual atmosphere

in blast furnace is COCO2 gas mixture, therefore experi-ments with COCO2 gas mixture are required. Rate analysis

of reduction with COCO2 gas mixture near the FeOFe

equilibrium gas composition is important especially.

Calcium ferrite in iron ore sinter is multicomponent

calcium ferrite including SiO2, Al2O3 and so on, and has

different reduction mechanism with Fe2O3CaO binary cal-cium ferrite. One of the authors13) synthesized CaOFe2O3

SiO2Al2O3 quaternary calcium ferrite, then studied its

reduction sequence and equilibrium constants in COCO2gas mixture. In consequence, quaternary calcium ferrite is

reduced to iron by way of magnetite and wustite containing

CaO, SiO2 and Al2O3 without producing intermediate prod-

ucts CaOFeOFe2O3, CaO3FeOFe2O3 and 2CaOFe2O3,

which are produced in the reduction of binary calcium fer-

rite. The equilibrium CO concentrations for the reduction of

quaternary calcium ferrite were higher than those for the

reduction of pure iron oxide.

Thus, as a fundamental study for clarifying the reductionphenomena of iron ore sinter in blast furnace, experiments

on iron oxide and quaternary calcium ferrite were carried

out with COCO2 gas mixture, and relationship between the

reduction rate at the final stage of reduction of the samples* Corresponding author: E-mail: [email protected]

DOI: http://dx.doi.org/10.2355/isijinternational.53.570


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and the compositions of COCO2 gas mixture was studied.

We investigated at the final stage of reduction of the sam-

ples because about 70% of total reduced oxygen in iron

oxide and quaternary calcium ferrite are removed in this

stage and this reaction is main reduction at chemical reserve

zone or more under.

2. Experiments

2.1. Samples

Two kinds of samples that were made from iron oxide (H

samples) and calcium ferrite (Cf samples) were used in the

experiments. Table 1 shows chemical compositions of each

samples.

H samples were prepared from a reagent grate powder

(45m) of Fe2O3. About 3.0 g of the powder was weighed

out and it was made spherical shape about 1 cm by hand

roll method. Then the sample was heated up to 1200C at

the rate of 0.33 K/s (20C/min). After being kept for 1 h at

1200C, the samples were cooled in furnace. This sphericalpellet was used as H samples to the reduction experiments.

The porosity of H samples were 2732%.

Quaternary calcium ferrite was prepared from reagent

grade powders of Fe2O3, CaCO3, SiO2 and Al2O3. The pow-

ders were mixed to be the composition as shown in Table 1.

The mixed powder was fired at 1 000C for 1 h in the air

and was followed by crushing and mixing. The operation of

firing, crushing and mixing was repeated three times. The

powder was put into a magnesia crucible (3 cm 10 cm)

and it was then heated up to 1300C at the rate of 0.17 K/s

(10C/min) using a silicon carbide resistance furnace. After

being kept for 0.5 h at 1 300C, the synthesized sample was

cooled down to 1 100C at the rate of 0.33 K/s (20C/min)

and was finally quenched in water. Synthesized sample was

crushed into powder of 4575 m in diameter. About 2.3 g

of the powder was weighed out and pressed into a briquette

of about 1 cm 1 cm. The briquette was used for the

reduction experiment without sintering. The porosity of Cf

samples were 4449%.

Porosity of H and Cf samples were calculated from the

apparent and true density respectively.

2.2. Experimental Procedure

Reduction experiments carried out at 900, 1 000 and

1100C using a thermal balance and it was heated up toeach experiment temperature in N2 gas stream. Then, a sam-

ple was hung on the thermal balance under N2 atmosphere.

At first, the sample was reduced to wustite with 50%CO

50%CO2 gas mixture. Next, the sample was reduced to iron

with prescribed COCO2 gas mixture. All gas flow rates

were 3.33 105 Nm3/s (2 NL/min).

Experimental gas composition determined as follows.

Equations (1) and (2) show the reaction at final stage reduc-

tion of iron oxide and its equilibrium constant KH.4)

FeO (s) + CO (g) = Fe (s) + CO2 (g)............ (1)

KH = exp (2.706 + 2289 / T) ................. (2)

Where T is absolute temperature. Equilibrium gas composi-

tion derived from Eq. (2) because total gas pressure was 1 atm.

Likewise, Eqs. (3) and (4) show the reaction at final stagereduction of the quaternary calcium ferrite and its equilibri-

um constant KCf.1) Therefore, equilibrium gas composition

derived from Eq. (4).

FeO (s) + CO (g) = Fe (s) + CO2 (g)........... (3)

KCf = exp (2.785 + 2 042 / T)................. (4)

Figure 1 shows equilibrium gas compositions as close and

open squares. Experimental gas compositions were equilib-

rium gas composition + 2% CO gas (as reverse triangles),

100% CO gas (as circles) and intermediate composition of

the two (as regular triangles). Table 2 shows equilibriumand experimental gas compositions of H and Cf sample

respectively.

3. Results

3.1. Reduction Curves

Figures 24 show fractional reduction curves of H and Cf

samples with 100% CO, intermediate CO% and equilibrium

+2% CO respectively. Reduction rates of both samples

increased with increasing reduction temperature with same

Table 1. Chemical composition of iron oxide (H) and quaternary

calcium ferrite (Cf) samples (mass%).

Fe2O3 CaO SiO2 Al2O3

H 100 0 0 0

Cf 65 23.3 7.8 3.9

Fig. 1. Equilibrium gas composition-temperature diagram for

reduction of quaternary calcium ferrite with COCO2 gas

mixture.

Table 2. Experimental gas composition (vol%).

H (Eq.)

900C 68.0 70.0 81.4 100

1 000C 71.3 73.3 82.9 100

1100C 73.9 75.9 84.0 100

Cf (Eq.)

900C 74.0 76.0 88.0 100

1 000C 76.5 78.5 89.3 100

1100C 78.5 80.5 90.3 100


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gas composition.

Figures 57 show fractional reduction curves of H and Cf

samples at 9001100C respectively. Reduction rates of

both samples decreased with decreasing partial pressure of

CO at same temperature, and they were especially small

near by equilibrium CO%. Furthermore, influence of gas

composition on the reduction rate was larger than that of

temperature as shown Figs. 24.

Reduction rates of H and Cf samples cannot be comparedbased on the fractional reduction curves, because H and Cf

samples were different from initial weight and total reducible

oxygen. For this reason, weight loss curves were prepared

and reduction rates of H and Cf samples were compared

based on these curves. Figure 8 shows the weight loss

curves at 1100C. Reduction rates of H and Cf samples

were almost the same near by equilibrium CO%. On the oth-

er hand, reduction rate of H samples was faster than that of

Cf samples in higher CO%. Same tendency was also

observed below 1 000C. The reason for the difference of

the reduction rate between H and Cf samples was consid-

ered to be not only reactivity of raw materials but also gasdiffusivity depend on the sample structure, driving force of

reaction depend on gas composition and so on.

Fig. 2. Reduction curves of FeO to Fe with 100% CO gas.

Fig. 3. Reduction curves of FeO to Fe with COCO2 gas mixture

of intermediate CO%.

Fig. 4. Reduction curves of FeO to Fe with COCO2 gas mixture

of equilibrium +2%CO.

Fig. 5. Reduction curves of FeO to Fe with COCO2 gas mixture at

1100C.


1000C.


900C.


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3.2. Macro and Microscopic Observations of Partially

Reduced Samples

Macroscopic observations were carried out on the partial-ly reduced samples which were prepared by interrupting the

reduction at about 70% reduction. Figures 9 and 10 show

the cross sections of H and Cf samples. Percentages that

shown in these figures represent the fractional reduction of

each samples. Cf samples that reduced with pure CO are not

shown here because one of authors5) reported that Cf reduc-

tions proceeded topochemically with pure CO at 900C or

higher.

These figures show that the reduction of H and Cf sam-

ples proceeded topochemically at higher temperature and

CO concentration. By contrast, reduction reaction of H and

Cf samples at lower temperature and CO concentration pro-ceeded not topochemically.

Figures 1114 show the microstructure of partially

reduced H and Cf samples at 900, 1 000 and 1100C with

prescribed COCO2 gas mixture respectively. In these fig-

Fig. 8. Weight loss curves of FeO to Fe with COCO2 gas mixture

at 1100C.

Fig. 9. Cross-sectional view of iron oxide samples partially reduced

with COCO2 gas mixture.

Fig. 10. Cross-sectional view of calcium ferrite samples partially

reduced with COCO2 gas mixture.

Fig. 11. Microstructure of iron oxide samples partially reduced

with 100% CO gas.

Fig. 12. Microstructure of iron oxide samples partially reduced

with COCO2 gas mixture of equilibrium +2%CO.


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ures, an, bn and cn show the microstructure at near the sur-

face, reaction interface and center of samples respectively.In the case of H samples, sintering of reduced iron was

observed in all samples. And the morphology of reduced

iron and wustite were different. Sintering of reduced iron

more proceeded at higher temperature, and grain shapes and

porosity appreciably changed especially at 1 100C. On the

other hand, when CO gas concentration was low, the sinter-

ing of produced iron was more proceeded because the reduc-

tion time became longer. Moreover, when the reduction was

carried out with CO rich gas and at 1000C and below, wus-

tite grains surrounded by reduced iron were observed.

In the case of Cf samples, sintering of reduced iron was

observed in all samples as well as H samples. The morphol-ogy of grain and porosity of iron and wustite layer were

almost the same. Beside, wustite grain surrounded by

reduced iron that observed in H samples were not observed

in all Cf samples.

4. Kinetic Analysis

Reduction data were analyzed by one interface unreacted

core model because reduction reactions proceeded

topochemically at 1100C. Applying unreacted core model

were not suitable because reactions did not proceed

topochemically at 1000C and below. In this study, however,

reduction data at all temperature were analyzed by unreact-

ed core model for comparison. Cf sample was analyzed as

a spherical approximation based on volume.

Chemical reaction rate constants kc and effective diffusiv-

ities in product layers De were obtained by mixed-control

plot.6)Figures 15 and 16 show the temperature dependencyof kc and De. The values of kc of H samples with each CO

concentrations show the Arrhenius-type temperature depen-

dency. The values of kc of Cf samples with each CO con-

centrations also show the Arrhenius-type temperature

dependency, but that with near the equilibrium gas compo-

sition shows the quite different trend from the others. It is

considered that the reduction of Cf sample with the near

equilibrium gas composition was not proceeded topochem-

ically compared to H sample. On the other hand, the values

of De of H samples show the Arrhenius-type temperature

dependency and the values of De of Cf samples did not show

the temperature dependency.Figures 17 and 18 show the gas composition dependency

of kc and De. The values of kc did not show the gas compo-

sition dependency except for the Cf sample reduced with the

near equilibrium gas composition, and these values were

Fig. 13. Microstructure of calcium ferrite samples partially

reduced with COCO2 gas mixture of intermediate CO%.

Fig. 14. Microstructure of calcium ferrite samples partially

reduced with COCO2 gas mixture of equilibrium

+2%CO.

Fig. 15. Temperature dependency of chemical reaction rate con-

stants kc.

Fig. 16. Temperature dependency of effective diffusivities De.


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almost constant at same temperature. On the other hand, thevalues of De of H samples show the gas composition depen-

dency, but the values of De of Cf samples did not show the

gas composition dependency.

According to the microscopic observations, the morphol-

ogy of product layers in H samples changed by temperature

and CO gas concentration, in contrast, it in Cf samples did

not change. They suggests that the values of De of H sam-

ples have the temperature and gas composition dependen-

cies, and the values of De of Cf samples dont have those.

According to the magnitude correlation of kc and De of H

and Cf samples, the difference of reduction rate between H

and Cf samples as shown in Fig. 8 was influenced by diffu-

sivity rather than reactivity of samples.

5. Conclusions

Iron oxide (H) and quaternary calcium ferrite (Cf) sam-

ples were prepared and these kinetic behaviors at the finalstage of reduction with COCO2 gas mixture were studied.

Obtained results are summarized as follows.

Reduction rate increased with increasing reduction tem-

perature. Moreover, it increased with increasing partial pres-

sure of CO gas. Difference of reduction rate caused by gas

composition is much larger than reduction temperature.

From the comparisons of weight loss curves, reduction rate

of H samples was faster than that of Cf samples in same

conditions.

Reduction reaction of H and Cf samples proceeded

topochemically at higher temperature (1100C), and didnt

proceed topochemically at lower temperature (1000C).Besides, the reduction reaction of samples used CO rich gas

proceeded more topochemically. Structure of iron layer in H

samples was changed by both temperature and gas compo-

sition. On the other hand, structure of iron layer in Cf sam-

ples was almost the same in all experimental conditions.

Reduction data were analyzed by one interface unreacted

core model, and chemical reaction rate content kc and effective

diffusion coefficient in product layer De were determined.

The values of kc show the Arrhenius-type temperature

dependency, and were approximately same tendency except

for Cf samples in near equilibriums gas compositions. The

values of De of H samples show the temperature and gas

composition dependencies, and that of Cf samples were

approximately constant in all experimental conditions.

REFERENCES

1) T. Maeda and Y. Ono: Tetsu-to-Hagan, 75 (1989), 416.2) T. Maeda, S. Masumoto and Y. Ono: Technol. Rep.Kyushu Univ., 62

(1989), 697.3) T. Maeda and Y. Ono: Tetsu-to-Hagan, 80 (1994), 451.4) W. S. Chung, T. Murayama and Y. Ono:J. Jpn. Inst. Met., 51 (1987),

659.5) T. Maeda and Y. Ono: Tetsu-to-Hagan, 77 (1991), 1569.6) J. Yagi and Y. Ono: Trans. Iron Steel Inst. Jpn., 8 (1968), 377.

Fig. 17. Gas composition dependency of chemical reaction rate

constants kc.

Fig. 18. Gas composition dependency of effective diffusivities De.

Effect of CO Gas Concentration on Reduction Rate of Major Mineral Phase SINTER

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