Effect of Ore Type and Gangue Content on Carburization and Melting Behavior of Carbon-Iron Ore Composite

Abstract

In order to reduce energy consumption in the blast furnace ironmaking process, it is important to promote not only rapid reduction of ore but also carburization and melting of metallic iron. In this study, the use of carbon-iron ore composite which is an agglomerate of fine iron ore and carbonaceous material was focused on, and the effects of ore type and slag content on carburization and melting of metallic iron were examined.

The reduction, carburization, and melting behavior of the composite samples prepared using carbonaceous materials and iron oxide such as various fine ores, goethite and hematite reagents were evaluated under heating condition up to 1573 K. The composite of hematite reagent with the additions of SiO₂ and Al₂O₃ reagents was also used to control the gangue composition.

Carburization and melting phenomena of metallic iron due to direct contact with graphite were observed in the composite samples made of pisolite ore and hematite reagent at the same slag composition. Carburization phenomenon was accelerated with increase in slag content.

1. Introduction

Reduction of energy consumption in the steel industry has been required against a background, e.g., increasing in energy cost and the demand to reduce carbon dioxide emissions. Especially, reduction in the ironmaking process is essential because of huge amount of energy consumption. Lowering the thermal reserve zone temperature of blast furnace is one of the effective methods to achieve it.^1,2) The use of carbon-iron ore composite has been attracted attention, since they show high reactivity due to short distance among carbonaceous material and ore particles.^3,4,5,6,7)

One of the important factors for stable operation of blast furnace is the gas permeability. Improvement of the permeability results in the decrease of reducing agent ratio (RAR) and the increase in production ratio of hot metal.⁸⁾ Acceleration of carburization and melting of iron at the cohesive zone is focused on to achieve it.

Various studies on the melting behaviors of iron by carburization have been reported.^9,10,11,12) It was found that carburization through direct contact with solid carbon is more preferential than that with CO gas.¹³⁾ Carburization between metallic iron and carbonaceous materials proceeds through molten slag including FeO,¹⁴⁾ and the carburization is prevented due to the enrichment of ash on the surface of coke by gasification reaction.^15,16) Carburization proceeds through high FeO slag even when metallic iron and carbonaceous material are not in contact directly, but carbon concentration in metallic iron is not enough to be melted at low temperature such as 1573 K.¹⁷⁾ Murakami et al. also reported that acceleration of carburization and melting after reduction of carbon-iron ore composite can be achieved using two types of carbonaceous material as reducing and carburizing agents.¹⁸⁾ Thus, property control of carburizing agent is very important. Characteristics of iron ore are also important because it affects reduction and slag formation behaviors.^19,20) In other words, carburizing and melting behavior seem to be affected by ore type and gangue composition.

It is reported that ores with high combined water content enhanced low temperature reduction and the reduction temperature decreases 150 K compared to that using hematite ore.²¹⁾ Maeda et al. studied the effect of SiO₂ and Al₂O₃ on the high temperature reduction of iron ore. The high temperature reduction was suppressed when FeO is surrounded by 2FeO–SiO₂ slag having lower reducibility.²²⁾ However, the effect of ore type and the gangue minerals contained in the ores on the carburization and melting behaviors of reduced iron in carbon-iron ore composite has not been reported. In this study, the effects of ore types and gangue composition on carburization and melting behaviors was examined using iron ores with different characteristics and reagents to control the gangue compositions.

2. Experimental

In this study, pisolite (PO) and hematite (HO) ores and goethite (FeOOH, purity 99%) and hematite (α–Fe₂O₃, purity 99.9%) reagents were used. The chemical composition of the ores is listed in Table 1. LOI is loss on ignition, which corresponds to the amount of combined water. The content of combined water of PO is 10.1%. It means iron oxide in this ore is FeOOH.

Table 1. Chemical composition and particle size of iron ores used (mass%).

		T–Fe	FeO	CaO	SiO2	Al2O3	LOI	Particle size
①	PO	53.2	–	0.01	5.51	2.54	10.1	<45 μm
②	HO	68.0	0.21	0.01	1.32	0.74	0.52	<45 μm

Ore was pulverized in a mortar and adjusted to −45 μm using the sieve. The average particle size of the reagent is 1 μm. Samples G and H were prepared to add SiO₂ (99.9% purity, particle size 0.8 μm) and Al₂O₃ (99.9% purity, particle size 1 μm) reagents to goethite and hematite reagents to control in the same gangue component as PO. Samples H0, H5, H10, and H15 were also prepared with the different amount of SiO₂ and Al₂O₃ reagents while the ratio of these components was constant. The chemical compositions of these samples are listed in Table 2.

Table 2. Chemical composition of sample mixtures (mass%).

		T–Fe	SiO2	Al2O3
③	G	57.2	5.51	2.54
④	H	63.6	6.13	2.82
⑤	H0	69.9	–	–
⑥	H5	67.5	2.43	0.95
⑦	H10	65.3	4.70	1.83
⑧	H15	63.2	6.83	2.66

Non-coking coal as the reducing agent and graphite as the carburizing agent were used similarly to the previous report.¹⁸⁾ The results of industrial analysis of carbonaceous materials together with the particle size which was adjusted by the sieve and chemical composition of ash in coal are shown in Tables 3 and 4, respectively. The particle size of graphite selected in the present study as the best results for carburization in the previous report.¹⁸⁾ Samples were well mixed without decrease in the particle size. The mixing ratio, C/O, which was defined as the molar ratio of fixed carbon in carbonaceous material to oxygen in iron oxide, was 1.0. C/O for reducing and carburizing agents were 0.8 and 0.2, respectively. The mixed powder was press-shaped under a pressure of 90 MPa, and a composite sample with a 10 mm in diameter and 10 ± 0.5 mm in height was obtained

Table 3. Composition and particle size of coal and graphite used.

	Fixed carbon (mass%)	Volatile matter (mass%)	Ash (mass%)	Particle size (μm)
Coal	55.5	36.1	8.4	45–105
Graphite	99.9	–	–	150–250

Table 4. Chemical composition of ash in coal (mass%).

	T–Fe	CaO	SiO2	Al2O3	MgO	P	S
Ash (coal)	3.6	3.3	35.63	23.43	0.55	0.49	0.31

The composite was set in the experimental apparatus²¹⁾ as shown Fig. 1. After evacuating air from the chamber, Ar-5%N₂ gas was introduced at the rate of 8.33 × 10⁻⁶ Nm³/s under atmospheric pressure. The sample was heated up to 1573 K at a heating rate of 0.33 K/s using an infrared image furnace, and then it was cooled down by turning off the power. The temperature at 1 mm upper the surface of the composite sample was measured using an R-type thermocouple. The concentrations of CO, CO₂, H₂, H₂O, CH₄, C₂H₄, C₂H₆, and N₂ gases emitted from the sample during the experiment were measured at 90 s intervals by gas chromatography. The amount of generated gas was calculated from N₂ balance. Reduction degree (R.D.) of the composite sample was calculated by the Eq. (1) using the amount of generated gas   

$$\text{R}.\text{D}.=\frac{{\text{M}}_{\text{CO}}+2{\text{M}}_{{\text{CO}}_2}+{\text{M}}_{{\text{H}}_2\text{O}}-{\text{M}}_{\text{LOI}}-{\text{M}}_{\text{vol}}}{{\text{M}}_{\text{total}\mathrm{\hspace{0.17em} }\text{O}}}$$(1)

M_CO, M_CO2, and M_H2O are the molar amounts of oxygen in the CO, CO₂, and H₂O gases emitted from the composite, respectively. M_LOI and M_vol are the molar amount of oxygen originated from combined water in ore and reagent and that generated from volatile matter in coal, respectively. These values were obtained to analyze generated gas from ore (reagent)-alumina composite and coal-alumina composite, respectively. M_{total O} is the molar amount of oxygen in the iron oxide sample. Microstructure observation was conducted for the reduced composite sample cross-sections by using optical microscope. The composite sample was ground using alumina mortar after cooling in liquid nitrogen. The ground powder was put into distilled water to remove the residual 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. XRD profile of the obtained sample showed that it consisted of single phase of metallic iron. Then, the carbon concentration in the metallic iron was measured by the infrared absorption method.

Fig. 1.

Schematic diagram of experimental apparatus for reduction of composite.

3. Results and Discussion

3.1. Effect of Ore Type on Reduction Behavior

Figure 2 shows the changes in the generation rates of gases the composite ① using PO with temperature. The rate of H₂O gas shows the peak value at 600 K, which is attributed to the decomposition of combined water of the ore.²¹⁾ CH₄ gas originated from volatile matter in coal starts to generate at 700 K. Its rate peaked out at about 800 K and decreases with increasing temperature and it was not detected above 1100 K. The rate of H₂ gas shows two peaks at about 700 and 1000 K, and the latter is broad and large. The rate of CO₂ gas also shows two peaks at about 1100 and 1300 K, while many peaks of CO gas generation rate are observed. These behaviors are consistent with those found by the previous study.¹⁸⁾

Fig. 2.

Changes in the generation rates of gases obtained for ①PO. (Online version in color.)

Figure 3 shows the change in the gas generation rate of composite ② using HO with temperature. Since the composite ② contains little amount of combined water, the peak of H₂O gas at 600 K is not observed. The generation behavior of H₂ gas is similar to that of composite ①, although it shows a steep peak. The rate of CO gas shows one peak at 1300 K, while the peaks of CO₂ gas appear at 870, 1200, and 1300 K. The generation behavior of hydrocarbon gases is similar to that of composite ①, expect for the larger generation of CH₄ gas.

Fig. 3.

Changes in the generation rates of gases obtained for ②HO. (Online version in color.)

The curves of reduction degrees and rates for each composite are shown in Figs. 4 and 5, respectively. Reduction degree of the composite ③ and ④ with reagents shows higher value than those with the ores. This is because decrease in the particle size leads to increasing reactivity and the size of reagent is smaller than that of the ores. Comparing composites ① and ②, the reduction of composite ① proceeds at 80 K lower temperature than composite ② below 1000 K while reduction behavior is similar. Reduction rate of composite ① increases at 1000 K and decreases at 1350 K. That of composite ②, on the other hand, increases at 1200 K and reduction degree reaches 92% at 1450 K. This indicates that the composite ① has higher reactivity than composite ② at lower temperature. It was reported that the dehydration reaction of pisolite ore starts at approximately 573 K and become porous.²³⁾ Thus, composite ① has larger specific surface area than composite ②, and The reduction reaction of composite ① proceeds at lower temperature. Reduction degrees of both composite ① and ② at 1473 K are 92%. The reason that reduction reaction of composite ③ proceeds at lower temperature than that of composite ④ can be similarly explained. The values of reduction degrees of composites ③ and ④ at 1473 K are 99% and 100%, respectively. These results suggest that the reduction reaction completes at 1473 K, and an enough amount of metallic iron forms to proceed the carburization and melting reactions.

Fig. 4.

Changes in reduction degrees of the composites with temperature. (Online version in color.)

Fig. 5.

Changes in reduction rates of the composites with temperature. (Online version in color.)

Figure 6 shows the change in oxygen partial pressure calculated using CO and CO₂ gas concentrations generated from the composites with temperature. The oxygen partial pressure of all composites at 1250 K is about 10⁻¹¹ Pa. The pressures show the different behavior above 1250 K. However, the carbon activities in iron are below 0.3. This implies that iron melting did not proceed by CO gas carburization. In other words, the melting of iron is promoted by the direct carburization with solid carbon.

Fig. 6.

Changes in oxygen partial pressure calculated considering CO and CO₂ gas concentrations. (Online version in color.)

3.2. Effect of Ore Type on Carburization and Melting Behavior

Figure 7 shows the appearances of composite ①–④ heated up to 1573 K. Composite ① maintains the shape of cylindrical after experiment, but its surface darkened and several small spherical metallic iron particles are observed on the composite. On the other hand, there is no significant change in the shape of the composite ②. The white balls on the right side of the figure are alumina balls which were used in the sample holder. Composite ③ and ④, which have the same chemical composition have spherical metallic iron particle as well as composite ①. The number of the particles in composite ③ is larger than that in composite ④.

Fig. 7.

Appearances of the composite ①PO, ②HO, ③G and ④H heated up to 1573 K. (Online version in color.)

The typical microstructures of the composites are shown in Fig. 8. The white and dark grey particles are metallic iron and graphite, respectively. The light grey and black parts are coal and resin, respectively. It is possible to distinguish graphite from coal to observe the shape and color by optical microscopy. Graphite and gasified coal used in this study are dark grey phase with spongiform structure and light grey phase with dense one.¹⁸⁾ In the case of composite ①, the contact between metallic iron and graphite particles is extensively observed, and agglomerated iron particles are observed. This corresponds to the previous report.¹⁸⁾ In the case of composite ②, on the other hand, little contact between these particles is observed. In the cases of composites ③ and ④, such contact are also observed as composite ①, but the aggregation of metallic iron does not proceed.

Fig. 8.

Microstructures of the composite ①PO, ②HO, ③G and ④H heated up to 1573 K. (Online version in color.)

Figure 9 shows the carbon concentrations in metallic iron of each composite heated up to 1573 K. The dashed lines show the liquidus and solidus lines of the Fe–C phase diagram at 1573 K. Composite ① has the highest carbon concentration and is almost on the liquidus line. The carbon concentrations of composites ③ and ④ which have the same slag composition are almost similar and in the solid-liquid coexistence region. This suggests that the type of iron oxide has less effect on the carburizing and melting behaviors.

Fig. 9.

Comparison of carbon concentration of reduced iron in the composites ①PO, ②HO, ③G and ④H heated up to 1573 K.

3.3. Effect of Gangue Content on Carburization and Melting Behavior

Figure 10 shows the appearances of composites ⑤–⑧ heated up to 1573 K. All of them kept their columnar shape and have a darkened surface. Only composites ⑦ and ⑧ show metallic iron grains on the surface of the composite samples. The typical microstructures of composites ⑤ and ⑥ were similar to composite ②. Those of composites ⑦ and ⑧ were also similar to composite ④.

Fig. 10.

Appearances of the composite ⑤H0, ⑥H5, ⑦H10 and ⑧H15 heated up to 1573 K.

Figure 11 shows the effect of the gangue content on the concentration of carbon in metallic iron. The carbon concentration increases with an increase in the gangue content. This is because that an increase in the amount of liquid slag which mediates the contact between metallic iron and carbonaceous material¹⁴⁾ leads to increasing the possibility of this contact. When the gangue content is 0.15, the carbon concentration reaches the liquidus line. This suggests that the presence of a certain amount of slag is necessary to accelerate the carburization to metallic iron.

Fig. 11.

Relation between carbon concentration in the iron of the composites heated up to 1573 K and the amounts of additives with gangue. (Online version in color.)

The chemical compositions of slag phases in the composite ①–④ heated up to 1573 K, which were analyzed by EDX in several spots, are plotted on the FeO–Al₂O₃–SiO₂ ternary diagram as shown in Fig. 12. In all the samples, most of the compositions are in the primary phase region of mullite, and no obvious difference is observed among the samples. However, the carbon concentration is different as shown in Fig. 10. It means that the effect of gangue content on the carburization to metallic iron is greater than that of its composition.

Fig. 12.

Composition of the slag in the composite ①PO, ②HO, ③G and ④H heated up to 1573 K. (Online version in color.)

4. Conclusion

Effects of ore type and gangue content on the melting behavior by carburization after heating up to 1573 K were investigated using the carbon – iron ore composites of the powders of iron ore and the carbonaceous materials such as coal as the reducing agent and graphite as carburizing agent. The following results were obtained:

(1) Direct contact between reduced metallic iron and graphite in the composite using pisolite ore whose main compound is goethite heated up to 1573 K is observed. Carbon concentration of metallic iron reaches the solid-liquid coexistence region in the Fe–C phase diagram at this temperature.

(2) When hematite ore is used, carburization through direct contact among metallic iron and graphite isn’t recognized. Therefore, the carbon concentration in the metallic iron is low and below the solidus line.

(3) Carbon concentration of metallic iron in the composites adding oxide reagents to hematite reagent more higher than that using hematite ore. This suggests that the slag in the composite affect to the carburization behavior, and the effect of the gangue content is more significant than its composition.

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