Effect of Chemical Species of Silicon Oxide on Carburizing and Melting Behaviors of Carbon-Iron Oxide Composite

Abstract

In order to achieve a low-carbon operation of the blast furnace, it is important to promote not only the reduction of iron oxides but also the carburizing and melting of reduced iron. In this study, the use of highly reactive carbon-iron oxide composite was focused on. The effect of chemical species of silicon oxides which are main components of ash in coal and gangue minerals in iron ores on the carburizing and melting phenomena of iron was examined in detail. The silicon transfer to metallic iron was also thermodynamically investigated under the condition with low carbon activity in iron.

The composites prepared by hematite reagent, carbonaceous materials and silicon oxides such as SiO₂, CaSiO₃ and Ca₂SiO₄ having different SiO₂ activity were heated up to 1300°C under inert gas flow. The reduction, carburization and melting behaviors were examined.

The silicon concentration of the reduced iron in the composite with high SiO₂ activity was high, while the carbon concentration was low. It was thermodynamically confirmed that silicon transfer precedes carburization of iron in the reduced carbon-iron oxide composite under high SiO₂ activity condition.

1. Introduction

The reduction of carbon dioxide emissions of the iron and steelmaking industry has been strongly required. Low-carbon operation of blast furnace (BF) with low coke ratio (CR) is an effective method to achieve it. However, low CR operation of BF tends to lead low permeability of BF¹⁾ and degrade its operation stability, since coke is the gas flow spacer as well as the reducing agent. To improve the permeability, thinning of the cohesive zone by means of narrowing the range of the softening and melt-down temperatures is required. In order to increase the softening temperature, decrease in FeO content is effective, since it lowers the melting point of slag.²⁾ It could be achieved by improving the reducibility of iron ores. In order to decrease the melt-down temperature, on the other hand, promoting the carburization of reduced iron is important. In this study, the use of carbon–iron oxide composite which shows high reactivity due to close distance among carbonaceous material and ore particles³⁾ was focused on.

A number of studies on the carburizing and melting behaviors of iron have been reported. It is known that the solid carbon contacts with the metallic iron in the molten slag and proceeds the carburization.⁴⁾ Such a direct contact carburization proceeds more rapidly than that by CO gas.⁵⁾ However, the ash enrichment at the interface of carbonaceous material prevents the carburization because it obstructs the contact between carbon and metallic iron.^6,7) It was also reported that rapid carburizing and melting after the reduction of carbon–iron oxide composite can be achieved at low temperature by dividing the role of carbonaceous materials into reduction and carburization.⁸⁾ In addition, the reduction and carburization behaviors of the coal-iron ore composite under similar conditions of the BF was investigated.⁹⁾ It has pointed out the suitable carbon to oxygen ratio of the composite for the BF operation was 0.4. It was also reported that the carbon concentration in reduced iron increased with an increase in the amount of gangue in carbon-iron ore composite.¹⁰⁾ However, the effect of chemical species of gangue minerals on the carburization behavior of carbon–iron oxide composite has not been reported.

One of major components of the gangue in iron ores is silicon oxide which exists as silica or aluminum silicate. Furthermore, a certain amount of CaO is contained in the cold-bonded carbon-iron ore composite as a binder.^11,12,13) Silica is reduced by carbon to SiO gas in BF as shown in Eq. (1).   

$${\text{SiO}}_2+\text{C}=\text{SiO}+\text{CO}$$(1)

Then, SiO adsorbs on Fe–C interface and is reduced by dissolved carbon. The reduced silicon dissolves into iron. These reactions are expressed by Eq. (2)^14,15,16)   

$$\text{SiO}+\underset{\_}{\text{C}}=\underset{\_}{\text{Si}}+\text{CO}$$(2)

Our group conducted carburizing experiments using Fe–Si alloys in contact with small pieces of graphite and reported that the melting temperature of the alloys increased with increasing the silicon concentration in the alloys.¹⁷⁾ This implies that melting of iron is suppressed when silicon transfer reaction precedes carburization. While, their detailed phenomenon and mechanisms have not been investigated. Therefore, the objective of this study is to examine the effects of chemical species of silicon oxide and basicity (mass ratio of CaO/SiO₂) of oxides on the reduction and carburization of carbon–iron oxide composite, especially on the silicon transfer reaction.

2. Experimental

2.1. Sample Preparation

α–Fe₂O₃ reagent (purity 99.9%, average particle size 1 μm) was mixed with carbonaceous materials and gangue components. Non-coking coal and graphite were used as the reducing agent and the carburizing agent, respectively.⁸⁾ In order to remove the ash in coal, non-coking coal was ground and sieved to be less than 53 μm in particle size. Then, de-ashed coal was obtained by immersing the coal powder in mixed acid (HCl:HF=1:1).¹⁸⁾ The particle size of graphite was 150–250 μm.⁸⁾ The results of proximate analysis of carbonaceous materials are shown in Table 1.

Table 1. Results of proximate analysis of carbonaceous materials used (mass%).

	Fixed Carbon	Volatile Matter	Ash
De-ashed coal	63.69	35.71	0.60
Graphite	99.99	–	–

The additives as gangue components were prepared using SiO₂ reagent (purity 99.9%, average particle size 4 μm) and CaO obtained by calcinating CaCO₃ reagent (purity 99.5%) at 1200°C for 7.2 ks. Ca₂SiO₄ (C2S) and CaSiO₃ (CS) were synthesized by heating the mixture of SiO₂ reagent and CaO at 1300°C for 259.2 ks in air. Chemical species and basicity of oxides used for the samples are shown in Table 2. Total Fe of each sample is 48.8 mass%. The effect of silicon activity on the carburization was examined using the samples of group A (C2S, CS+CaO and SiO₂+2CaO). In the samples of group B (B0, B1, B2 and B3), the effect of basicity of oxides was investigated. The ratio of Ca and Si oxides was constant at 15 mass% of the Fe amount in Fe₂O₃ reagent. The powder mixture was thoroughly mixed with Fe₂O₃ reagent using alumina mortar. The mixing ratio, C/O, which was defined as the molar ratio of fixed carbon in carbonaceous material to oxygen in iron oxide, was set as 1.0. C/O for de-ashed coal and graphite were 0.8 and 0.2, respectively.⁸⁾ These carbonaceous materials were well mixed without decrease in the particle size. The mixed powder was press-shaped under a pressure of 98 MPa, and a composite sample with 10 mm in diameter and 10 ± 0.5 mm in height was obtained.

Table 2. Chemical species and basicity of oxides used for the samples.

	Sample	Ca2SiO4	CaSiO3	SiO2	CaO	Basicity
		(mass%)
Group A	C2S	7.32	–	–	–	1.87
	CS+CaO	–	4.94	–	2.38	1.87
	SiO2+2CaO	–	–	2.55	4.77	1.87
Group B	B0	–	–	7.32	–	0.00
	B1	–	–	3.66	3.66	1.00
	B2	–	–	2.44	4.88	2.00
	B3	–	–	1.83	5.49	3.00

2.2. Reduction and Carburization Experiment

The sample was set in the experimental apparatus¹⁹⁾ as shown in Fig. 1. After evacuating air in the chamber, Ar-5%N₂ gas was introduced at the rate of 8.33×10⁻⁶ Nm³/s under atmospheric pressure. Then, the sample was heated up to 1200°C or 1300°C at a heating rate of 0.33°C/s using an infrared image furnace, and cooled down by turning off the power. The temperature at 1 mm upper the surface of the sample was measured using an R-type thermocouple. The concentrations of CO, CO₂, H₂O and N₂ of the outlet gas were measured during the experiment at 90 s intervals by a gas chromatography. N₂ gas was used as a tracer to estimate the amount of gas generated from the sample. Reduction degree (R.D.) of the sample was calculated by Eq. (3) using the amount of generated gas.   

$$\text{R}.\text{D}.=\frac{{\text{M}}_{\text{CO}}+2{\text{M}}_{{\text{CO}}_2}+{\text{M}}_{{\text{H}}_2\text{O}}-{\text{M}}_{\text{vol}}}{{\text{M}}_{\text{total}\mathrm{\hspace{0.17em} } \text{O}}}$$(3)

M_CO, M_CO2, and M_H2O are the molar amounts of CO, CO₂, and H₂O gases detected by the gas chromatography, respectively. M_vol is the molar amount of oxygen originated from volatile matter in de-ashed coal. M_{total O} is the molar amount of oxygen in the Fe₂O₃ reagent.

Fig. 1.

Schematic diagram of an experimental apparatus for reduction of the sample. (Online version in color.)

2.3. Sample Observation and Analysis

Cross-sectional observation and analysis of carbon concentration of the metallic iron were carried out for the reduced samples. In the cross-sectional observation, the sample embedded in resin was cut and polished using 1 μm of diamond paste. Then, the microstructure was observed using an optical microscope and SEM. Chemical composition of the composite sample was measured by EDX. Silicon concentration of the reduced iron was also measured by EPMA. In the analysis of carbon concentration,⁸⁾ the sample was cooled in liquid nitrogen before ground using alumina mortar. The ground sample was put into distilled water to remove remaining carbon. After drying, the powder was ultrasonically cleaned in ethanol, and then the black turbid ethanol was excluded. This operation was repeated several times to obtain the metallic iron powder. Then, the carbon concentration of the metallic iron was measured by the infrared absorption method after combustion.

3. Results and Discussion

3.1. Reduction Behavior

Figure 2 shows changes in the reduction degrees with temperature obtained for the group A samples. Reduction degrees of all the samples start to increase at approximately 500°C. The reduction degrees of the sample C2S are stagnated at 10% around 700°C. This indicates magnetite was not reduced to wustite in the sample C2S around 700°C. The reduction degrees at 1200°C of the samples “CS+CaO” and “SiO₂+2CaO” are slightly higher than that of C2S. This is because adding CaO accelerates the surface chemical reaction of FeO.^20,21)

Fig. 2.

Changes in the reduction degrees with temperature obtained for the group A samples. (Online version in color.)

Figure 3 shows changes in the reduction degrees with temperature obtained for the group B samples. Reduction starts at approximately 500°C same as group A. And above 700°C, the reduction degrees of the samples tend to increase with an increase in basicity because the amount of CaO is increased. The reduction degrees for all the samples at 1200°C reached over 90%. It indicates that almost all iron oxide has been reduced to metallic iron.

Fig. 3.

Changes in the reduction degrees with temperature obtained for the group B samples. (Online version in color.)

3.2. Carburization Behavior

In order to examine the carburizing and melting behaviors of the samples, the effect of carburization by CO gas was first discussed. Figures 4 and 5 show the phase diagram for the Fe–C–O system with the changes in equilibrium oxygen partial pressures obtained for the samples of the groups A and B, respectively. The oxygen partial pressures of C2S and B0 around 700°C are below the equilibrium line of Fe₃O₄–FeO. This indicates the partial pressure of CO₂ was higher than those of the other samples because of the reduction by CO was stagnated.⁸⁾ Dashed lines correspond to iso-activity curves of carbon in iron, a_c=0.05, 0.10, 0.30 and 0.50, in the graphite standard. The equilibrium oxygen partial pressure was calculated assuming that Eq. (4) had reached equilibrium.   

$$2{\text{CO}}_2=2\text{CO}+{\text{O}}_2$$(4)

Fig. 4.

Changes in equilibrium oxygen partial pressures obtained for the group A samples on the Fe–C–O phase diagram. (Online version in color.)

Fig. 5.

Changes in equilibrium oxygen partial pressures observed for the group B samples on the Fe–C–O phase diagram. (Online version in color.)

In all samples, the partial pressure tends to increase with increasing temperature, and the carbon activity decreases. In the cases of the sample “SiO₂+2CaO”, B2 and B3, the oxygen partial pressure decreases rapidly at 1000°C. This is because of the rapid increase in CO partial pressure caused by the gasification promoted by CaO.^20,21) The maximum carbon activity above the eutectic temperature of the Fe–C system is about 0.10. The equilibrium carbon concentration in iron at 1300°C with a_c=0.10 is about 0.5 mass%. Since the solidus concentration of the Fe–C system at 1300°C is about 1.3 mass%, it can be concluded that the melting of iron by CO gas carburization cannot occur.

Figure 6 shows appearances of the group A samples after the reduction experiment heating up to 1200°C and 1300°C, respectively. In all samples, iron nuggets are considered to be formed mainly between 1200°C and 1300°C. Their number and size vary depending on the chemical species of silicon oxide. In the case of the sample “CS+CaO”, several iron nuggets seeped out from the surface of the sample. However, only one larger nugget is observed for the sample C2S. Although a large iron nugget is also observed for the sample “SiO₂+2CaO”, the iron nugget was formed inside the sample and the cylindrical sample was broken. Figure 7 shows the appearance of the group B samples after the reduction experiment heating up to 1200°C and 1300°C, respectively. The samples B0 and B1 do not show any significant change in shape, but only some shrinkages are observed. The exposed iron nuggets, on the other hand, are observed for the samples with higher basicity, i.e., B2 and B3 heated up to 1300°C. Furthermore, a large iron nugget is considered to have already seeped out below 1200°C in the sample B3.

Fig. 6.

Appearances of the group A samples after the reduction experiment (1200°C and 1300°C). (Online version in color.)

Fig. 7.

Appearances of the group B samples after the reduction experiment (1200°C and 1300°C). (Online version in color.)

Figure 8 shows the cross-sectional microstructures of group A samples after heated up to 1300°C. In all the samples, contact points between graphite and coalesced metallic iron are observed as shown in the circled area of Fig. 8. It means that the reduced iron in the sample was melted by direct contact carburization. However, the sample “SiO₂+2CaO” has less coalesced iron particles than the other samples. It suggests that chemical species of silicon oxide affects the carburizing and melting behaviors of iron. Figure 9 shows cross-sectional microstructures of the group B samples after heated up to 1300°C. The contacts between metallic iron and graphite are observed in the samples when basicity of the sample above 1.0 while the sponge shape iron is observed for the sample B0. This is because the melting point of the slag is lowered by adding CaO.²²⁾

Fig. 8.

Cross-sectional microstructures of the group A samples after heating up to 1300°C. (Online version in color.)

Fig. 9.

Cross-sectional microstructures of the group B samples after heating up to 1300°C. (Online version in color.)

Considering that the carburization of iron proceeds in the molten slag as explained by Kim et al.,⁴⁾ it is necessary to examine the slag composition in the initial melting stage of iron. Figures 10 and 11 show the SEM images of the cross-section of the samples of the groups A and B after heated up to 1200°C, respectively. The metallic iron contacts with slag in all the samples. Also, some of iron grains were coalescing. It suggests that the iron started to melt.

Fig. 10.

SEM images of the cross-section of the group A samples heating up to 1200°C.

Fig. 11.

SEM images of the cross-section of the group B samples heating up to 1200°C.

The composition of each slag was determined by a point analysis using EDX. Figure 12 shows the composition of the slag at 1200°C for the group A samples plotted on the state diagram of the CaO–SiO₂–FeO system calculated by the thermodynamic calculation software FactSage. In the cases of the samples C2S and “CS+CaO”, most of the plots are in the three phases coexistence region of FeO, Ca₂SiO₄ and Ca₃Si₂O₅ and liquid, Ca₃Si₂O₅ and CaSiO₃, respectively. On the other hand, there are oxides having wide composition range in the sample “SiO₂+2CaO”. It indicates that even though the average composition of the sample is same, composition of locally formed slags is different. Oxides with high SiO₂ activity seemed to be coexisted with the reduced iron in the sample “SiO₂+2CaO” heated up to 1200°C. The same plots for the group B samples are shown in Fig. 13. The slag compositions of the samples B0 at 1200°C are concentrated in the area near SiO₂. The ratio of CaSiO₃ and Ca₂SiO₄ increases in the slag compositions of the samples added CaO. These calcium silicates were produced by the reaction between SiO₂ and CaO. Especially, there seems to be no SiO₂ as a single substance in the sample B3.

Fig. 12.

Compositions of the slags in the group A samples at 1200°C plotted on the state diagram of the CaO–SiO₂–FeO system. (Online version in color.)

Fig. 13.

Compositions of the slags in the group B samples at 1200°C plotted on the state diagram of the CaO–SiO₂–FeO system. (Online version in color.)

Figure 14 shows the silicon and carbon concentrations of the metallic iron recovered from the group A samples heated up to 1300°C. The dashed lines correspond to the solidus and liquidus lines of the binary phase diagram of Fe–C system at 1300°C. These concentrations were measured separately for the iron in the reduced composite and the exposed iron nuggets, respectively. The carbon concentrations of metallic iron exceed the liquidus line for the samples C2S and “CS+CaO”, although iron in the composite was not completely melted as shown Fig. 10. This may be because the graphite in contact with the iron could not be completely removed. While the carbon concentrations of the samples C2S and “CS+CaO” tend to be higher than that of the sample “SiO₂+2CaO”, the silicon concentration of iron in the sample “SiO₂+2CaO” is higher than that of the others. Also, there is no significant difference between the silicon concentrations of iron remained in the composite and iron nuggets for all the samples. This tendency was also observed in the case of sample B3 heated up to 1200°C. Therefore, silicon is considered to be transferred to iron before the iron is melted. These suggest that silicon transfer to metallic iron progresses and carburizing and melting seem to be inhibited when SiO₂ exists as a high active state. The silicon and carbon concentrations in metallic iron measured for the group B samples are shown in Fig. 15. Since iron nugget formation was not observed in the samples B0 and B1, only the analytical results of iron remained in the sample are shown. The carbon concentration in the iron nugget exceeds the liquid phase concentration. The carbon amount in the iron nuggets of the sample B3 is considered to be larger than that of the sample B2 although the carbon concentration is almost same. This is because the amount of iron nuggets of the sample B3 is larger than that of the sample B2. The silicon concentration of the iron in the sample decreases with increasing basicity, while the carbon amount in the iron tends to increase.

Fig. 14.

Comparisons of carbon and silicon concentrations of metallic iron in the group A samples heated up to 1300°C. (Online version in color.)

Fig. 15.

Comparisons of carbon and silicon concentrations of reduced iron in the samples of group B heated up to 1300°C. (Online version in color.)

3.3. Thermodynamics of Si Transfer Reaction

Thermodynamic calculations were carried out to examine the Si transfer to the iron in the reduced carbon-iron oxide composite. P_SiO is calculated by Eq. (5).   

$$P_{\text{SiO}}=\frac{a_{\text{C}}\cdot a_{{\text{SiO}}_2}}{P_{\text{CO}}}exp\left(-\frac{\mathrm{\Delta }{G}_1^{°}}{RT}\right)$$(5)

SiO₂ activity of slag in the sample, $a_{{\text{SiO}}_2}$ calculated by FactSage. ΔG_i represents standard Gibbs energies of the chemical reaction of Eq. (i). a_C is the carbon activity of graphite. P_CO is the partial pressure of CO measured by a gas chromatography. Figure 16 shows changes in the partial pressure of SiO at 1300°C with SiO₂ activity of the slag. SiO partial pressure is increased as the SiO₂ activity is increasing. It showed about 10⁻⁵ atm of SiO in the case of the highest SiO₂ activity in this study. This indicates SiO₂ could be partially reduced to SiO gas when the SiO₂ activity is high even around 1300°C.

Fig. 16.

Changes in the partial pressures of SiO at 1300°C with SiO₂ activity of slag.

In the reduced carbon-iron oxide composite, the distance between the reduced iron and the solid carbon is very short. Therefore, silicon may also transfer to reduced iron in the composite as following process shown in Fig. 17. (i) SiO absorbs on reduced iron. (ii) O atom adsorbed at a locally high carbon activity area reacts with CO and desorbs as CO₂. (SiO+CO=Si+CO₂) (iii) Si atom dissolves into iron. (Si=Si) (iv) The carbonaceous material in close proximity to the iron is gasified by the desorbed CO₂. (C+CO₂=2CO) The total reaction equation is expressed as in Eq. (6).   

$$\text{SiO}+\text{C}=\underset{\_}{\text{Si}}+\text{CO}$$(6)

Fig. 17.

Schematic diagram of silicon transfer reaction to metallic iron in the reduced carbon-iron oxide composite.

The equilibrium Si concentration of iron due to the Si transfer reactions expressed in Eqs. (2) and (6) are expressed as Eqs. (7) and (8), respectively.   

$$\left[\%\text{Si}\right]=\frac{a_{\left[\text{C}\right]}\cdot P_{\text{SiO}}}{P_{\text{CO}}\cdot f_{\text{Si}}}exp\left(-\frac{\mathrm{\Delta }{G}_2^{°}}{RT}\right)$$(7)

  

$$\left[\%\text{Si}\right]=\frac{a_{\text{C}}\cdot P_{\text{SiO}}}{P_{\text{CO}}^{\prime }\cdot f_{\text{Si}}}exp\left(-\frac{\mathrm{\Delta }{G}_6^{°}}{RT}\right)$$(8)

Here, a_[C] is the carbon activity in iron based on the mass% Henry’s law. f_Si is the silicon activity coefficient in iron. They are calculated using the model of Ahn et al.²³⁾ $P_{\text{CO}}^{\prime }$ is the partial pressure assumed as that carbon activity is 1. Figure 18 shows changes in carbon and silicon concentration of metallic iron measured and calculated by Eqs. (7) and (8) with SiO₂ activity of slag, respectively. The measured silicon concentration increases with increasing in the SiO₂ activity, while that of carbon decreases. The calculated values by Eq. (7), however, are much lower than that of measured with the high SiO₂ activity condition. This is because the reaction represented by Eq. (2) assumes carbon dissolved in iron reduces SiO. The reaction represented in Fig. 17, in contrast, CO is the reductant of SiO. Therefore, the calculated values by Eq. (8) show a similar tendency to the measured values even with the condition which carbon is hardly dissolved into the iron. This indicates that Si transfer precedes carburization by the reaction represented by Eq. (6) in the reduced carbon-iron oxide composite.

Fig. 18.

Changes in carbon and silicon concentrations of iron measured and calculated by Eqs. (7) and (8) with SiO₂ activity of slag, respectively. (Online version in color.)

4. Conclusion

Reduction and carburization experiments of carbon-iron oxide composite containing different chemical species of silicon oxide and basicity were carried out. Furthermore, silicon transfer reaction was thermodynamically discussed to clarify its effect on the carburizing and melting behavior of the composites. The following results were obtained.

• When silicon and calcium oxides are added as calcium silicates such as Ca₂SiO₄ and CaSiO₃, there was no single substance of SiO₂ remained in the composite after heating up to 1200°C. On the other hand, when both of SiO₂ and CaO are added as single substances at the same basicity, 1.87, SiO₂ phase remains after heating. Therefore, SiO₂ activity of the composite is higher in the latter case.

• When the basicity is 2.0 or higher, SiO₂ activity of the composite is relatively low at 1200°C because SiO₂ reacts with CaO to produce calcium silicates.

• The carbon concentration of the reduced iron tends to be low in the composite with high SiO₂ activity. This is because silicon has transferred into the iron around 1300°C even when the carbon activity in the iron is low. As a result, the carburization to reduced iron is inhibited.

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