Reduction and Carburization Behaviors of Iron Oxide Composite with Iron Carbide and Free Carbon

Abstract

The blast furnace ironmaking process relies heavily on fossil fuels, posing challenges to achieving carbon neutrality. New methods, like hydrogen reduction ironmaking, face limitations such as the need for high-grade iron ore and issues with DRI melting. To address these, biomass char has been explored as a carbon-neutral carburizing agent, yet practical application is difficult due to biomass supply limitations in East Asia. Thus, Carbon Capture and Utilization, CCU becomes essential. Examples include iACRES and carbon recycling blast furnaces, which recycle CO₂ for reducing agents but have limited scope.

Direct carburization with solid carbon is significantly faster than gas carburization, suggesting solid carbon recovery from CO₂ is preferable. However, issues for fine carbon powder handling necessitate a new process: Carbon Recycling Ironmaking Process using Deposited Carbon-Iron Composite (CRIP-D). This process reforms exhaust gases into CO-rich gas, depositing carbon into porous iron whiskers, which can produce hot metal carbon-neutrally.

The composites using hematite and deposited carbon reduced at lower temperatures and exhibited more extensive melting and coalescence at 1300°C compared to those with less the carbon. The presence of cementite was crucial for promoting carburization and melting.

CRIP-D aims to integrate carbon recycling into steel production, utilizing high-porosity iron whiskers and recovered carbon to achieve carbon-neutral steelmaking. The findings show the importance of co-existing of free carbon and iron carbides in enhancing the efficiency of the reduction and melting of carbon-iron oxide composite.

1. Introduction

Blast Furnace ironmaking process has been adopted as the main process at integrated steel mills because of its high efficiency. However, it needs a large amount of fossil fuel derived carbon for heat source, reducing agent of iron ores and carburizing agent of reduced iron. Thus, it is essential to develop a new process to achieve carbon-neutral steelmaking. Currently, research and development for the commercialization of hydrogen reduction ironmaking with shaft furnace have been conducted around the world1.^1,2,3) However, ironmaking processes based on DRI production such as MIDREX process, are largely limited by the grade of the iron ore.⁴⁾ This is because the gangue in the DRI increases the amount of molten slag in the melting process of DRI with EAF.⁵⁾ To improve this problem, it is effective to produce hot metal separated from molten slag before the refining process. Carbon, however, must be added to reduced iron for the production of the hot metal. Therefore, the attempts using biomass char as a carburizing agent have been made to produce hot metal carbon-neutrally.^6,7,8) Practical application, however, is difficult in countries with large crude steel production such as China and Japan. This is because of difficulty to supply the required amount of carbon with biomass. As long as the amount of available biomass char is limited, the application of CCU is inevitable to achieve carbon neutrality in ironmaking process.

Examples of CCU application in steel mills include iACRES⁹⁾ and carbon recycling blast furnace.¹⁰⁾ iACRES is designed to recycle CO reformed by high-temperature electrolysis of CO₂ (2CO₂ → 2CO + O₂) and the reverse water gas shift reaction (CO₂ + H₂ → CO + H₂O). CH₄ produced by the methanation reaction (CO₂ + 4H₂ → CH₄ + 2H₂O), on the other hand, is reused in the carbon recycling blast furnace. In both cases, the carbon cycle is achieved by reforming and reusing the CO₂ recovered from the exhaust gas, but its scope is limited to the reduction process. To extend the scope of the carbon cycle to oxidation refining, carburizing the reduced iron is inevitable.

There are mainly two routes for carburization of iron. One is direct carburization by solid carbon and another is indirect one by CO gas. The reaction equations are described as Eq. (1) and Eqs. (2), (3), (4), respectively.

  

$$\text{Fe(s)}+\text{C(s)}=\text{Fe(s)}+\underset{\_}{\mathrm{C}}$$(1)

  

$$\text{CO(g)}={\text{C}}^{*}+{\text{O}}^{*}$$(2)

  

$$\text{CO(g)}+{\text{O}}^{*}={\text{CO}}_2(\text{g})$$(3)

  

$${\text{C}}^{*}=\underset{\_}{\mathrm{C}}$$(4)

Here, (*) and (_) mean the atom absorbing on and dissolving in Fe, respectively. Murakami et al.¹¹⁾ compared the melting rate of metallic iron plate by direct carburization with a graphite piece and gas carburization with CO at 1250°C and reported that the melting proceeded more than 1000 times faster with direct contact carburization than with gas carburization. This is because the mass transfer of carbon to molten iron at the boundary between the carburizing agent and solid iron is the rate-limiting factor for iron melting. Indirect carburization, which requires the desorption process of gas molecules, supplies carbon to molten iron slower than direct one. Therefore, considering hot metal production in carbon recycling ironmaking, it is desirable to reform the CO₂ in the exhaust gas to solid carbon. However, the solid carbon is recovered in a fine powder state. This leads handling problems since the fine powder is easily scattered during transportation. It is preferable to load the fine powder onto support carrier. Therefore, our group propose the following new ironmaking process named Carbon Recycling Ironmaking Process using Deposited Carbon-Iron ore Composite (CRIP-D), which extends the scope of carbon recycling to steel production without fossil fuel derived carbon.¹²⁾ The conceptual diagram of CRIP-D is shown in Fig. 1. CRIP-D comprises four processes; (1) gas reforming, (2) porous iron production, (3) carbon deposition and (4) reduction and melting of carbon-iron ore composite. In (1) the gas reforming process, hydrogen and energy are added to the gas generated from iron ore reduction process and hot metal refining process. The gas is reformed to CO-rich gas through reverse water gas shift reaction catalyzed by porous iron. Regarding (2) the porous iron production, it has been reported that metallic iron whisker was produced by reducing carbon-iron ore composite prepared using charcoal.¹³⁾ Due to intertwining the fibrous structure, the porous iron showed about 95% of porosity. This porous iron is also used as a substrate for carbon deposition reaction using reformed CO-rich gas in (3) the carbon deposition process. The carbon in the gas, therefore, can be recovered as not only free carbon but also iron carbide. Higashi et al.¹⁴⁾ reported that porous iron whisker is carbonized by CO gas approximately three times faster compared to the direct reduced iron pellet. Furthermore, the reaction rate can be improved by increasing the catalytically active area, and the handling of carbon powder can be more easily by incorporating the deposited carbon into the pores. Nishihiro et al.¹⁵⁾ conducted carbon deposition reaction catalyzed by metallic iron particles. They reported that a larger amount of carbon was deposited using CO–H₂ mixed gas than pure CO gas. Sato et al.¹⁶⁾ conducted melting experiment under elevated temperature using the tablets made of metallic iron powder with cementite or graphite powder. The melting rate of the tablet with cementite was higher than that with graphite. This indicates iron carbides are suitable for a carburizing agent. Murakami et al.¹⁷⁾ also conducted elevated temperature test on the carbon-iron ore composite using coal and graphite. The reduction and carburization reactions were reported to be accelerated by dividing the function of carbonaceous materials as a reducing agent and a carburizing agent. Thus, Deposited Carbon-Iron ore Composite (DCIC) with high reactivity may be prepared by using the free carbon and iron carbides recovered in the carbon deposition process as reducing and carburizing agents, respectively. Hot metal can be produced by (4) reduction and melting of carbon-iron ore composite in rotary hearth furnaces and shaft furnaces.^18,19) By replacing some of the raw materials in the blast furnace with DCIC, it is expected to reduce CO₂ emissions while using the facilities in the proven integrated steelmaking technology. The gas after the carbon deposition reaction is introduced in to hot metal production process as a reducing agent and heat source since the gas contains certain amount of CO and H₂. Carbon recycling is achieved by reforming the gases emitted in the reducing, hot metal production and refining process. Our group has revealed that DCIC exhibits more rapid reduction and significant melting behaviors compared to the composite using fossil fuel-derived carbons.¹²⁾ However, the mechanism has not been explained. Clarifying the effects of iron carbide and free carbon on the reduction and carburization behaviors of the composite facilitates the development of effective principles for their ingredient design.

Fig. 1. Conceptual diagram of Carbon Recycling Ironmaking Process using Deposited Carbon-Iron ore Composite (CRIP-D).¹¹⁾ (Online version in color.)

In this study, carbon-iron oxide composite prepared using deposited carbon is investigated to elucidate the mechanism of the reduction and melting by carburization of DCIC.

2. Experimental

2.1. Deposited Carbon Preparation

Iron carbide and free carbon were recovered by proceeding carbon deposition reaction (2CO → C + CO₂) on porous iron whisker.¹⁴⁾ The porous iron whisker was prepared by carbothermic reduction of hematite-biomass char mixture.¹²⁾ The schematic diagram of experimental apparatus for carbon deposition is shown in Fig. 2. N₂, H₂ and CO were supplied from the bottom of the reaction tube made of Inconel 601. A balance was installed at the bottom of the apparatus. Fused silica tubes used for sample positioning is placed on the balance. The sample holder (O.D.: 63 mm, I.D.: 53 mm, height: 100 mm) was set on the silica tube. The bottom of the sample holder has 34 evenly spaced vent holes with 4 mm in diameter for the gas flow. The prepared porous iron was cut out into cylindrical blocks with 18 mm in diameter and 10 mm in height.

Fig. 2. Schematic diagram of an experimental apparatus for carbon deposition. (Online version in color.)

The iron whisker blocks were set on the alumina ball layer in the holder. The sample was heated up to 600°C at 10°C/min while under N₂ gas flow of 2.5 × 10⁻⁴ Nm³/s. Then, the supplied gas was switched to N₂-10%H₂ gas to remove the remained oxide layer of the sample by holding the temperature for 1.8 ks. Carbon deposition reaction was proceeded after purging N₂ gas for 0.6 ks by switching the supplied gas to CO at 6.7 × 10⁻⁵ Nm³/s. The samples with carbon deposition reaction times of 6.0 ks, 2.4 ks and 0.6 ks were defined as samples A, B and C, respectively. The composition of Fe compounds and C in each sample, determined by Rietveld analysis, are shown in Table 1. The sample D is iron whisker before proceeding carbon deposition reaction. Figure 3 shows SEM images of the samples A–D. Each sample shows whisker structure with approximately 2 μm in diameter. Free carbon particles are observed in the sample A and B.

Table 1. Composition of the samples (mass%).

Sample	Fe3C	Fe	FeO	Free C
A	83.4	2.1	0.0	14.5
B	88.4	3.6	0.0	8.0
C	64.8	34.7	0.0	0.5
D	0.0	99.0	1.0	0.0

Fig. 3. SEM images of the samples.

2.2. Reduction Experiment

The samples A–D were ground and sieved as to the particle size under 106 μm. The carbon-iron oxide composite sample were prepared under four conditions. Table 2 shows the molar ratios of Fe, O and C for each composite samples. The molar ratio of C is shown divided into that in iron carbide, free carbon and carbon black. Fe₂O₃-A was prepared by press-shaping the mixture of hematite reagent (1 μm size, purity 99.9%) and sample A. Sample B, C and D were mixed with hematite reagent to same molar ratio of C to Fe with Fe₂O₃-A, respectively. In addition, carbon black (−106 μm, F. C=99.6%) was added to adjust the C/O ratio to 1.0. The composite sample Fe₂O₃-B, Fe₂O₃-C and Fe₂O₃-D were prepared by press-shaping those mixtures, respectively.

Table 2. Molar ratio of the composites.

	Fe	O	C in Fe3C	Free C	C in C.B.
Fe2O3-A	0.44	0.28	0.08	0.20	0.00
Fe2O3-B	0.44	0.28	0.07	0.11	0.10
Fe2O3-C	0.44	0.28	0.05	0.01	0.22
Fe2O3-D	0.44	0.28	0.00	0.00	0.28

The composite was set in the experimental apparatus as shown in Fig. 4. After evacuating air in the chamber, Ar-5%N₂ gas was introduced at the rate of 8.3 × 10⁻⁶ Nm³/s under atmospheric pressure. Then, the composite was heated up to 1300°C at a heating rate of 0.17°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 composite was measured using an R-type thermocouple. The concentrations of CO, CO₂, 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 composite. Reduction degree (R.D.) of the composite was calculated by Eq. (5) using the amount of generated gas.

  

$$\text{R}.\text{D}.=\frac{{\text{M}}_{\text{CO}}+2{\text{M}}_{{\text{CO}}_2}}{{\text{M}}_{\text{total}\mathrm{\hspace{0.17em} }\text{O}}}$$(5)

Fig. 4. Schematic diagram of an experimental apparatus for reduction experiment. (Online version in color.)

M_CO, M_CO2, and M_H2O are the molar amounts of CO, CO₂, and H₂O gases detected by the gas chromatography, respectively. M_{total O} is the molar amount of oxygen in the Fe₂O₃ reagent. The reduced composite was cross-sectioned and polished using a 1 μm diamond paste. Selected samples underwent etching for iron carbide using sodium picrate.^20,21) For the carbon concentration analysis,²²⁾ the reduced composite was cooled in liquid nitrogen and then ground using alumina mortar. The ground particles were immersed in distilled water to remove remaining carbon. After drying, the particles were ultrasonically cleaned in ethanol, and then the black turbid ethanol was discarded. The compounds present in the obtained particles were identified by X-ray diffraction (XRD) method with Fe anode (tube voltage: 40 kV, tube current: 30 mA, wave length: 0.193597 nm) as the X-ray source. Subsequently, the carbon concentration in the particles was measured by the infrared absorption method after combustion.

3. Results and Discussion

3.1. Reduction Behavior

Figure 5 shows the changes in the generation rate of CO and CO₂ gases from each composite with temperature. CO gas is generated above 700°C in Fe₂O₃-A, Fe₂O₃-B and Fe₂O₃-C. In contrast, Fe₂O₃-D begins to generate CO gas at approximately 800°C. It has a possibility that CO can be generated by the decomposition of Fe₃C,²³⁾ as described in Eq. (6), at a lower temperature.

  

$${\text{Fe}}_3\text{C}+{\text{CO}}_2=3\text{Fe}+\text{CO}$$(6)

Fig. 5. Changes in the generation rate of CO and CO₂ gases from each composite. (Online version in color.)

As the temperature rises further, the gases begin to be generated abruptly at approximately 830°C, especially in Fe₂O₃-A and Fe₂O₃-B. The details of this gasification behavior are discussed below.

Figure 6 shows the changes in reduction degrees for each composite with temperature. The composites prepared using deposited carbon starts to be reduced at lower temperature than Fe₂O₃-D. This indicates CO generated by Fe₃C decomposition attributed the reduction of iron oxides. The reduction curves of Fe₂O₃-A, Fe₂O₃-B and Fe₂O₃-C nearly overlap. However, the reduction degree of Fe₂O₃-C is lower than those two samples above approximately 870°C. The reduction degrees at 1100°C are 97.9%, 96.3%, 93.7% and 95.3% for Fe₂O₃-A, Fe₂O₃-B, Fe₂O₃-C and Fe₂O₃-D, respectively. Figure 7 shows the changes in the partial pressure ratio of CO to CO+CO₂ gas (CO gas ratio) generated from the sample with temperature drawn on the Fe–C–O phase diagram. The CO gas ratio starts to shift closer to the equilibrium of FeO reduction, as described in Eq. (7), at around 850°C for Fe₂O₃-A, Fe₂O₃-B and Fe₂O₃-D.

  

$$\text{FeO}+\text{CO}=\text{Fe}+{\text{CO}}_2$$(7)

Fig. 6. Changes in reduction degrees for the composites with temperature. (Online version in color.)

Fig. 7. Changes in the partial pressure ratio of CO to CO+CO₂ gas (CO gas ratio) generated from the sample with temperature drawn on the Fe–C–O phase diagram. (Online version in color.)

The ratio for Fe₂O₃-C, on the other hand, reaches this equilibrium line at around 750°C, although the reduction degree shows only 5%. This is because metallic iron contained initially in the composite is oxidized by CO₂ generated from the reduction of hematite and magnetite by CO gas generated from Fe₃C decomposition and CO gas generates. This reducing potential can reduce hematite and magnetite. Hence, Fe₂O₃-C shows same reduction curves as Fe₂O₃-A and Fe₂O₃-B up to 870°C at which the reduction reaction to wustite completes. Since the reduction potential of the CO produced by oxidization of metallic iron is not sufficient for the reduction to metallic iron, the reduction degree of Fe₂O₃-C is considered to be lower than Fe₂O₃-A and Fe₂O₃-B above 870°C. When the CO gas ratio is near the equilibrium, metallic iron exhibits a catalytic effect on the carbon gasification.²⁴⁾ Moreover, the iron formed by the decomposition of Fe₃C exhibits nano-sized particles.²⁵⁾ Thus, the catalytic effect is considered to be stronger in Fe₂O₃-A and Fe₂O₃-B compared to the other composites which contain metallic iron whisker particles. This explains why the CO and CO₂ gases were generated drastically around 900°C, especially in Fe₂O₃-A and Fe₂O₃-B. When the CO gas ratio is higher than the equilibrium, the nano-sized iron particles are easily oxidized at around 700°C according to the reaction, as described in Eq. (8).

  

$$3\text{Fe}+4{\text{CO}}_2={\text{Fe}}_3{\text{O}}_4+4\text{CO}$$(8)

The nano-sized iron oxide particles may have catalytic effect on the gasification.²⁶⁾ Hence, the CO gas ratio of Fe₂O₃-A and Fe₂O₃-B also shift near the equilibrium of Fe₃O₄ reduction, as described in Eq. (9).

  

$${\text{Fe}}_3{\text{O}}_4+\text{CO}=3\text{FeO}+{\text{CO}}_2$$(9)

Figure 8 shows the appearances of each composite after reduction experiment. Fe₂O₃-C and Fe₂O₃-D show gray cylindrical shapes after reduction, while iron nuggets are observed on the black surface of Fe₂O₃-A and Fe₂O₃-B. Figure 9 shows the XRD profiles obtained for each composite. Almost all of the iron oxides were reduced to Fe, consistent with the tendency of reduction degrees shown in Fig. 6. The peaks of Fe₃C and Fe₄C are also observed in each sample. However, the intensity of Fe₃C peaks is higher in Fe₂O₃-A and Fe₂O₃-B compared to Fe₂O₃-C and Fe₂O₃-D. Fe₄C was considered to be formed by rearrangement of the carbon dissolved in iron during cooling.²⁷⁾

Fig. 8. Appearances of each composite after heating to 1300°C. (Online version in color.)

Fig. 9. XRD profiles obtained for each composite after heating to 1300°C. (a) Full intensity (b) Lower intensity section. (Online version in color.)

3.2. Carburization and Melting Behavior

Figure 10 shows the relation between the ratio of initial carbon content in the composite by type of carbon and carbon concentration in the iron. The dashed line shows the solidus line of Fe–C binary phase diagram at 1300°C. The carbon concentration exceeds the solidus line and increases with the free carbon ratio. On the other hand, the carbon concentration in the iron does not increase until the cementite content exceeds a certain level. This suggests that the presence of a certain amount or more of cementite is important to promote carburization.

Fig. 10. Relation between the ratio of initial carbon content in the composite by type of carbon (a) Free carbon (b) C in Fe₃C and carbon concentration in the iron. (Online version in color.)

Figure 11 shows the cross-sectional macrostructures of Fe₂O₃-A and Fe₂O₃-B after the reduction experiment. Molten iron structures are observed in both samples. However, more molten iron forms and is coalesced in Fe₂O₃-A compared to Fe₂O₃-B. Magnified images of the indicated sections in Fig. 11 are shown in Fig. 12. Crystallized graphite structures are observed exclusively in the fused iron of Fe₂O₃-A, as depicted in Fig. 11(a). This iron is inferred to possess a higher carbon content and to have been melted at elevated temperatures. Conversely, iron structures without crystallized graphite are also observed in both Fe₂O₃-A and Fe₂O₃-B, suggesting a lower carbon content in these regions at high temperature. The contact between iron with and without crystallized graphite structures is evident in Fe₂O₃-A, as shown in Fig. 11(b). This observation indicates the possibility of carbon diffusion from high-carbon content molten iron to low-carbon content iron through the contact points at elevated temperatures.

Fig. 11. Cross-sectional macrostructures of Fe₂O₃-A and Fe₂O₃-B after heating to 1300°C. (Online version in color.)

Fig. 12. Magnified images of the indicated sections in Fig. 10. (Online version in color.)

To further investigate the melting behavior, the reduction experiment was conducted on Fe₂O₃-A, heating it up to 1200°C. Figure 13 shows the cross-sectional macrostructures of Fe₂O₃-A heating to 1200°C. The molten iron structure is hardly seen. The iron is considered to be molten and coalesced significantly between 1200°C and 1300°C. Figure 14 shows the cross-sectional microstructures of Fe₂O₃-A heating to 1200°C and 1300°C after etching by sodium picrate. The stained areas represent the Fe₃C phase.^20,21) Pro-eutectoid cementite and pearlite structures can be observed in Fe₂O₃-A after heating to 1300°C. In contrast, in the sample after heating to 1200°C, relatively large cementite structures are observed. This suggests that some amount of cementite remains even after heating Fe₂O₃-A to 1200°C. The remained cementite can contribute to the formation of carbon-saturated molten iron above its melting point of 1252°C.²⁸⁾

Fig. 13. Cross-sectional macrostructures of Fe₂O₃-A after heating to 1200°C. (Online version in color.)

Fig. 14. Cross-sectional microstructures of Fe₂O₃-A heating to 1200°C and 1300°C after etching by sodium picrate. (Online version in color.)

Figure 15 shows the schematic diagram of the carburization and melting mechanism of reduced iron oxide composited containing iron carbide and free carbon. The cementite that remains inside the composite after the reduction reaction is considered to have free carbon attached to its surface. The reduced metallic iron particles exist around it. Above the melting point of cementite, carbon-saturated molten iron comes into contact with reduced iron and donates carbon. If the amount of free carbon is low, its carbon concentration decreases as the molten iron enlarges. Therefore, the gradient of carbon concentration between molten iron and reduced iron becomes small and enlargement stops as shown in Fig. 15(a). Conversely, if there is sufficient amount of free carbon, molten iron originated from cementite is donated carbon from the free carbon. Thus, its high carbon concentration can be maintained even as the molten iron enlarges by the coalescence with solid iron, and carburization and melting of reduced by above mechanism are expected to proceeds further as shown in Fig. 15(b).

Fig. 15. Schematic diagram of the carburization and melting mechanism of deposited carbon – iron oxide composite (a) without and (b) with free carbon. (Online version in color.)

4. Conclusions

This study aimed to evaluate the mechanism of the reduction and melting of DCIC based on a new iron making process, Carbon Recycling Ironmaking Process using DCIC (CRIP-D), to achieve carbon-neutral steelmaking. The following results were obtained.

• The reduction experiments demonstrated that the composite utilized deposited carbon began to be reduced at lower temperature compared to that without deposited carbon. This may be attributed to CO generated by cementite decomposition.

• The carburization and melting behaviors are significantly influenced by the type of deposited carbon in the composites. The composite contained more deposited carbon heated to 1300°C exhibits more extensive melting and coalescence, which correlates with higher intensity of cementite peaks in the XRD profiles. This suggests that the presence of cementite plays a crucial role in promoting carburization and melting.

• The presence of crystallized graphite structures in the composite heated to 1300°C indicates more carbon dissolution in the molten iron. However, molten iron was not observed by heating to 1200°C. This suggests that the iron fused and coalesced significantly between 1200°C and 1300°C.

• The proposed mechanism for carburization and melting involves the transfer of carbon from cementite and free carbon to the reduced iron. To facilitated continued carburization and melting, a sufficient amount of free carbon to ensure a high carbon concentration in the molten iron is important.

These findings provide a fundamental understanding of the reduction, carburization and melting behaviors in this innovative ironmaking process.

Declaration of Competing Interest

The authors declare that they have no known competing financial interest or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This work was supported by JSPS Grant-in-Aid for JSPS Fellows Grant Number 22KJ0282 and Steel Foundation for Environmental Protection Technology.

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