Effect of Hydrogen Concentration in Reducing Gas on the Changes in Mineral Phases during Reduction of Iron Ore Sinter

Abstract

In order to decrease CO₂ emission from the ironmaking process, an increasing use of hydrogen in blast furnace (BF) ironmaking is a promising way. In this case, the properties of iron ore sinter such as reducibility and strength need to be optimized because hydrogen reduction of iron oxide is an endothermic reaction and temperature distribution in BF drastically changes. In this study, the effect of hydrogen concentration in the reducing gas on the changes in mineral phases during reduction of iron ore sinter is evaluated. Mineral composition of the ten types of sinter samples was analyzed by XRD and image analysis. Sinter sample was reduced under the simulated conditions such as Low-H₂ (N₂ − 48%(CO + CO₂) − 5.8%(H₂ + H₂O)) and High-H₂ (N₂ − 48%(CO + CO₂) − 13%(H₂ + H₂O)). After reduction, microstructure of the sample was observed. Iron ore sinters usually consist of mineral phases such as hematite, magnetite, calcium-ferrites and slag. Furthermore, calcium-ferrite phases are roughly divided into four types: 1) acicular texture coexisted with primary hematite (1H-ACF), 2) columnar texture coexisted with secondary hematite (2H-CF), 3) small and 4) large columnar textures coexisted with magnetite (M-FCF and M-CCF). An increase in hydrogen concentration of reducing gas accelerates the reduction of hematite, 1H-ACF, and 2H-CF in all sinter samples, while it does not affect the reduction of magnetite, and calcium-ferrite coexisted with magnetite.

1. Introduction

Significant reduction of carbon dioxide emissions is required to the iron and steel industry to mitigate the global warming, since a large amount of coal and coke is used in the ironmaking process of blast furnace (BF). In this process, such carbonaceous materials are used as a reducing and carburization agents as well as heat source. Therefore, it is not very easy to substantially replace them with other substances.

Lowering the thermal reserve zone temperature in the blast furnace by decreasing the distance between the iron oxide and carbonaceous particles may be one of the effective ways to achieve this. Mixed charging of iron ore with coke in the blast furnace,¹⁾ the utilization of highly reactive coke such as iron coke,²⁾ and the utilization of iron ore–carbon composites³⁾ have attracted significant attention. The researches on the properties of the composites are reported by many research groups.^4,5,6) However, the decrease amount of carbon dioxide emission is less than 10%.

Use of hydrogen as reducing gas is other promising ways to decrease the amount of carbon dioxide emissions, since hydrogen reduction produces not carbon dioxide but water. When hydrogen concentration in the reducing gas of BF increases, not only the reduction behavior of iron burdens but also the temperature distribution drastically changes because the reduction by hydrogen is endothermic reaction.⁷⁾ Increase in the hydrogen concentration leads to a decrease in the shaft region temperature of BF. Therefore, higher reducibility of iron ore burden at lower temperature is required under higher hydrogen condition.

Iron ore sinter, pellet, and lump ore are typical iron burdens of BF, and especially sinter is major one in East Asian countries. It is well known that main mineral phases of sinter are hematite, magnetite, calcium ferrite, and slag. The reducibility of hematite and magnetite was reported by many researchers.^8,9,10) Hida et al. reported that calcium-ferrite which is an artificial mineral can be sorted into two main types by shape and chemical compositions.¹¹⁾ One is acicular calcium-ferrite which coexists with primary hematite and pores. The other is columnar one with skeletal secondary hematite and slag. However, they did not refer calcium-ferrite coexisted with magnetite in detail. Iron ore properties have drastically changed compared to those before 1990’s. It will affect the mineral phases formed in their sinter. In this study, mineral phase composition of practical sinters produced by a Japanese steel mill was examined.

It is known that the reducibility of acicular calcium ferrite by CO gas has higher than that of columnar one.¹²⁾ However, the reducibility of calcium-ferrite for CO–H₂ gas mixture, e.g., shaft gas composition of BF, is not clear. Furthermore, there is few data about the reducibility of sinter under actual BF conditions with the gas containing water vapor. Therefore, in this study, the effects of increasing hydrogen concentration in the reducing gas mixture of N₂–CO–CO₂–H₂–H₂O on the reduction behaviors of the sinter and individual mineral phases were evaluated.

2. Experimental

Ten types of practical sinters produced by Japanese steel mills were used in this study. Chemical composition of sinter samples was listed in Table 1. Mineral phase ratio of hematite and magnetite in the sinter was determined by the internal standard method of XRD using NaF regent as a standard material. The ratio of calcium ferrite was made by an image analysis using more than twenty microstructure images of a few sinter particles in each sample. Chemical composition of the calcium ferrite was measured by EDX.

Table 1. Chemical composition and basicity of sinter samples used in this study.

Sinter	T–Fe	FeO	CaO	SiO2	Al2O3	MgO	Basicity
1	57.19	5.29	9.47	6.13	1.71	0.98	1.54
2	56.54	5.89	10.01	5.46	1.83	1.15	1.83
3	57.40	6.36	10.18	5.23	1.60	1.23	1.95
4	57.85	6.37	9.12	5.70	1.57	0.93	1.60
5	56.96	6.91	10.10	5.03	1.92	1.48	2.01
6	58.16	6.93	9.28	5.27	1.79	0.91	1.76
7	57.63	7.30	10.84	4.52	1.55	1.03	2.40
8	56.70	8.00	11.23	5.31	1.86	–	2.11
9	56.88	8.28	9.62	5.28	1.87	1.13	1.82
10	57.16	10.61	9.32	5.73	1.96	1.57	1.63

Sinter sample of approximately 100 g with the particle size of 6.7–9.5 mm was charged in MgO crucible with an inner diameter of 53 mm. This crucible has several pores at the bottom to purge reducing gas, and alumina balls with the diameter of 10 mm was packed on its bottom. Then, it was set in the furnace as shown in Fig. 1 and pre-heated up to 200°C under nitrogen gas flow. Then, the gas was changed to the reducing one, ant the sample was heated up to 700°C with the rate of 10°C/min. After that, it was heated up to 1000°C with the rate of 4°C/min. Two types of the reducing gas composition were applied; Low-H₂ (N₂ − 48%(CO + CO₂) − 5.8%(H₂ + H₂O)) and High-H₂ (N₂ − 48%(CO + CO₂) − 13%(H₂ + H₂O)) conditions. The gas flow rate was set at 13.06 NL/min, which was high enough value that the reduction rate was not affected by flow rate. The ratio of CO/CO₂ and H₂/H₂O was changed with increasing temperature as shown in Fig. 2,¹³⁾ simulating the BF reducing gas composition. In these gas conditions, there is little difference of oxygen partial pressure. For example, oxygen partial pressure at 700°C of High-H₂ and Low-H₂ conditions are 2.8 × 10⁻¹⁷ and 2.9 × 10⁻¹⁷ Pa, respectively. The weight change of sample was measured during sintering to calculate reduction degree.

Fig. 1.

Schematic diagram of experimental apparatus for reduction of sinter. (Online version in color.)

Fig. 2.

Partial pressure of reduction gas on the phase diagram of Fe–C–O system and Fe–H–O system. (Online version in color.)

Reduction degree was calculated from net weight change of sinter sample during heating as shown in the equation.   

$$\text{Reduction}\mathrm{\hspace{0.17em} }\text{degree}=\frac{\mathrm{\Delta }{\text{W}}_1+\mathrm{\Delta }{\text{W}}_{2+}}{{\text{W}}_{3+}}\times 100$$(1)

Here, ΔW₁ is net weight change, ΔW₂₊ is the weight change assuming that Fe²⁺ in original sinter is oxidized to Fe³⁺, and W₃₊ is oxygen weight in sinter caused by iron oxide when all iron oxide is Fe³⁺. Net weight change was calculated from the difference between that of sample and blank test. Blank test was carried out using MgO crucible with only alumina balls using same gas and heating conditions. After reduction up to the target temperature, microstructure of sample was observed, and image analysis was carried out to determine metallization degree.

3. Results and Discussion

3.1. Microstructure and Mineral Phases in Sinter

Calcium-ferrite phases were divided into the following four types considering their morphologies and coexisting minerals as shown in Fig. 3. 1H-ACF is acicular-calcium ferrite with the size of less than 10 μm coexisted with primary hematite. 2H-CF is columnar calcium-ferrite with skeletal hematite and eutectic structure of fine calcium ferrite and silicate phases. In addition, there are fine and course calcium-ferrite phases coexisted with magnetite, namely M-FCF and M-CCF, respectively.

Fig. 3.

Microstructure of a) 1H-ACF and b) 2H-CF phases with hematite and c) M-ACF and d) M-CCF phase with magnetite in sinter. (Online version in color.)

Figure 4 shows phase ratios of these four types calcium-ferrites, hematite and magnetite in sinter. In this study, slag phase was not considered because iron oxide in the slag does not reduce below 1000°C. The trend that the amount of magnetite phase increases with increasing FeO content is roughly obtained. The amount of hematite in Sinter 2 and 10 is largest and lowest, respectively. This is corresponding to the trend of FeO compositions in sinter samples.¹⁴⁾ However, this trend has large error because these sinters were produced by various granulation methods such as simple, separate and exterior coating granulations using various kinds of raw materials.

Fig. 4.

Comparison of mineral composition of sinter samples: hematite, magnetite, and calcium ferrite (1H-ACF, 2H-CF, M-ACF, and M-CCF). (Online version in color.)

Clear trend is not seen between the ratio of calcium-ferrite and FeO compositions. Calcium-ferrite ratios of Sinter 1, 4, 5, 7, and 8 are higher than 40%. It means that higher ratio of calcium-ferrite can be obtained even if basicity of sinter is lower. It indicates that the formation amount of calcium-ferrite is affected by other parameters such as granulation method, temperature profile of the sinter bed, local gas atmosphere. The ratios of 1H-ACF in Sinter 1, 4, and 5 is large. The ratios of M-FCF and M-CCF in sinter 1, 3, 7, 8, and 9 are high. However, there is not clear relation between the ratios of magnetite and these phases.

Figure 5 shows the chemical composition of the four types of calcium-ferrite phases in several sinter samples on the phase diagram of (CaO + MgO)-SiO₂-(Al₂O₃ + Fe₂O₃) pseudo-ternary system. Wang et al.¹⁵⁾ reported that the chemical composition of calcium-ferrite exists in the region of C₄S₃–CA₃–CF₃(C = CaO, S = SiO₂, A = Al₂O₃, F = Fe₂O₃), CS–CA₆–CF₆, S–CA₃–CF₃ faces. Hamilton et al.¹⁶⁾ reported that it exists on CS–CA₃–CF₃ face. Furthermore, Patrick et al.¹⁷⁾ and Murao et al.¹⁸⁾ also reported the phase diagram aiming at SFCA phase. In this study, the chemical composition agrees with the data by Wang et al., while some data is below the line of C₄S₃–CA₃–CF₃.¹⁵⁾ On the other hand, it is assumed that all iron oxide has triad ion when the composition is plotted on this figure. However, calcium ferrite in sinter contains a certain amount of bivalent iron ion.¹⁹⁾ Especially, M-FCF and M-CCF may contain higher amount of Fe²⁺ than 2H-CF because these textures may form under lower oxygen partial pressure. These should be considered although it is difficult to measure the ratio of Fe²⁺/Fe³⁺ in calcium ferrite. In this phase diagram, Fe²⁺ should be treated as CaO + MgO side because FeO is basic oxide. It means that the plots of M-FCF and M-CCF will shift to CaO + MgO side and become closer to the line of C₄S₃–CA₃–CF₃. Calcium-ferrite can be classified as SFCA and SFCA-I based on the crystal structure.²⁰⁾ It is reported that SFCA has higher SiO₂ content than 6 mol%²¹⁾ and its shape is columnar. In this study, 2H-CF, M-FCF and M-CCF may consisted of SFCA because these show columnar shape. On the other hand, acicular calcium-ferrite with lower SiO₂ content than SFCA was reported as SFCA-I.²²⁾ In this study, acicular calcium-ferrite show higher SiO₂ content than the previous report by Sugiyama et al.¹⁹⁾ This difference should be discussed, but it is future work. Figure 6 shows relationship between CaO and MgO contents of calcium-ferrite phases. MgO contents in 1H-ACH, M-FCF, and M-CCF are almost below 5 mol%. MgO content of 2H-CF shows wide range between 5 and 15 mol%, which is higher than other calcium-ferrites. Wang et al. reported the calcium-ferrite with low SiO₂ and High MgO contents.⁶⁾ It has a possibility that MgO in SFCA has different function from CaO.

Fig. 5.

Chemical composition of each calcium ferrite on phase diagram of (CaO + MgO)-SiO₂-(Al₂O₃ + Fe₂O₃) pseudo-ternary system. C:CaO, S:SiO₂, A:Al₂O₃, F:Fe₂O₃. (Online version in color.)

Fig. 6.

Relationship between CaO and MgO contents in calcium ferrite phase in sinter samples. (Online version in color.)

3.2. Reduction Behavior of Sinter

Changes in reduction degree of the sinter samples under Low-H₂ condition with temperature are shown in Fig. 7. Table 2 shows the value of reduction degree at 1000°C. Reduction degree of all samples below 500°C is not 0 because of its definition described at section 2. Reduction degree starts to increase at approximately 650°C, and it becomes 40–55% when the temperature reaches to 1000°C. Reduction degrees of Sinter 3 and 10 are lower and that of Sinter 1, 2, and 4 are higher. There is no relation between the reducibility and FeO content in sinter. Figure 8 shows changes in reduction rate of sinter samples under Low-H₂ condition with temperature. From 700°C to 800°C, reduction rate increases with increasing temperature, and there is large difference of reduction rate among sinter samples. At this temperature range, the reduction from hematite to magnetite mainly proceeds. However, it decreases with increasing temperature up to 900°C. Effect of sinter type on the reduction rate is not so large. It seems that many types of reduction reaction proceed. Above 900°C, it increases again. At this temperature range, reduction from wustite to metallic iron proceeds.

Fig. 7.

Change in reduction degree of sinter samples with temperature under Low-H₂ conditions.

Table 2. Reduction degree of sinter samples heated up to 1000°C under Low-H₂ and High-H₂ conditions.

Sinter	Reduction degree at 1000°C
	Low-H2 (%)	High-H2 (%)
1	49.4	57.6
2	52.0	61.1
3	43.6	51.2
4	53.4	63.1
5	50.7	60.5
6	51.0	60.3
7	46.8	55.5
8	47.5	55.0
9	52.0	59.1
10	42.5	50.0

Fig. 8.

Change in reduction rate of sinter samples with temperature under Low-H₂ conditions.

Figure 9 shows changes in reduction degree of sinter samples under High-H₂ condition. Reduction behavior under High-H₂ condition is similar to that under Low-H₂. However, Reduction under the High-H₂ condition was faster than that of the Low-H₂ condition, and this trend was seen for all the sinter samples. From only these results of reduction degree, however, it is very difficult to discuss in detail because sinter has very complicated microstructure composed of hematite, magnetite, and 4 types of calcium ferrite and reduction reaction proceeds step by step. At next section, therefore, microstructure of reduced sinter samples was observed.

Fig. 9.

Change in reduction degree of sinter samples with temperature under High-H₂ conditions.

3.3. Effect of Hydrogen on the Reduction of Mineral Phase

Figure 10 shows the ratio of hematite phase before and after reduction at 700°C under the Low-H₂ and High-H₂ conditions. Reduction of hematite to magnetite proceeded during heating up to 700°C, but it did not finish completely. The ratio of hematite phase after reduction has no relation with FeO content in original sinter sample while it is generally known that sinter with high FeO content has low reducibility.²³⁾ Increasing hydrogen concentration leads to increasing reduction of hematite. Figure 11 shows microstructure of reduced Sinter 5 up to 700°C under both gas conditions. In this figure, [Hem.] means that the phase originated from hematite and it was reduced to other phases, for example magnetite. White lines are cracks. Many cracks are observed in sinter. It is caused by the volume expansion occurs by the reduction from hematite to magnetite at lower temperature such as 500–600°C.²⁴⁾ In the case of the High-H₂ condition, number of formed cracks is larger than that under the Low-H₂ condition. These cracks lead to promotion the reduction reaction of hematite to magnetite because reduction of skeletal hematite accelerates by increasing hydrogen concentration.²⁵⁾ This crack formation may affect the difference of reduction rate at the temperature from 700°C to 800°C as shown in Fig. 8.

Fig. 10.

Ratio of hematite phase in sinter samples before reduction and reduced by each gas condition up to 700°C. (Online version in color.)

Fig. 11.

Microstructures of surface area of Sinter 5 reduced under a) Low-H₂ and b) High-H₂ conditions up to 700°C. [Hem.]:Originally hematite before reduction. White lines are cracks. (Online version in color.)

Figure 12 shows microstructures of the reduced phase originated from skeletal hematite in the center of sinter particle of Sinter 2 after heating up to 1000°C.²⁶⁾ In this figure, [2H-CF] and [Hem.] mean the phases (wustite) originated from 2H-CF and hematite, respectively, and M-Fe is metallic iron. In the case of the Low-H₂ condition, small particles of metallic iron with the size of a few micrometers are observed in the phase originated from skeletal hematite in the center of the figure. In the case of High-H₂ condition, on the other hand, many metallic iron phases are observed. Reduction of the phase originated from 2H-CF under the High-H₂ condition is also faster than that under the Low-H₂ condition. The reason is that the crack forms around skeletal hematite by the reduction from hematite to magnetite under lower temperature described above,²⁵⁾ and it enables the penetration of reducing gas easily into sinter particle. In case of 1H-ACF, fine particles of metallic iron formed by reduction up to 1000°C.¹¹⁾ The amount of formed metallic iron originated from 1H-ACF increased with increasing hydrogen concentration in the reducing gas. It seems that the diffusion of reducing gas, especially hydrogen, passing through the micro pores of 1H-ACF is easy.

Fig. 12.

Microstructure of the reduced phase originated from skeletal hematite in the center of sinter particle of Sinter 2 after heating up to 1000°C.²⁶⁾ [2H-CF]:Reduced 2H-CF and [Hem.]: Reduced hematite. (Online version in color.)

On the other hand, reduction acceleration was not observed for M-FCF and M-CCF. Figure 13 shows microstructure of the reduced phase originated from M-FCF and M-CCF near the surface of sinter particles of Sample 9 and 10, respectively. Figures 13(a) and 13(c) are is Low-H₂, b) and d) are High-H₂. In this figure, [Mag.], [M-FCF], and [M-CCF] mean the phases (wustite) originated from magnetite, M-FCF, and M-CCF, respectively. Metallic iron forms in the reduced sinter. From wustite originated magnetite phase, metallic iron forms on the surface of iron oxide particles, and it covers oxide particles. However, metallic iron formed from M-FCF shows different structure. It may grow from metallic iron originated from wustite as shown in the white arrow in the figure. The amount of formed metallic iron from those original phases in the sinter reduced under Low-H₂ condition is not so different from that under High-H₂ condition. It means that increasing H₂ concentration does not leads to reduction acceleration of magnetite and M-FCF.

Fig. 13.

Microstructure of the reduced phase originated from a) and b) M-ACF in the particle of Sinter 9, and c) and d) M-CCF in the particle of Sinter 10 after heating up to 1000°C. a) and c) are Low-H₂, and b) and d) are High-H₂. [Mag.]: Reduced magnetite, [M-ACF]: Reduced M-ACF, and [M-CCF]: Reduced M-CCF. (Online version in color.)

As for M-CCF, metallic iron forms on the surface of wustite originated from magnetite same as M-FCF. Metallic iron also forms form M-CCF as shown in white circle in the figure. The ratio of metallic iron was very small. The difference of the ratios between two conditions is also small.

Figure 14 shows metallization degree of originally calcium-ferrite phases in Sinter 5 and 10 reduced up to 1000°C. Metallization degree was calculated using the image analysis data of microscopic images of the samples. Increasing hydrogen concentration leads to increasing metallization degree of 1H-ACF and 2H-CF, while M-FCF and M-CCF show few increases. The results show that the reduction of hematite, 1H-ACF, and 2H-CF are accelerated with increasing in hydrogen concentration. Figure 15 shows the effect of the ratio of such mineral phases on the difference of reduction degree of sinters at 1000°C under the High-H₂ and Low-H₂ conditions, whose values are obtained from Figs. 7 and 9. The ratio of this mineral phase is calculated from the summation of the phase ratio of hematite, 1H-ACF, and 2H-CF in sinter as shown in Fig. 4.

Fig. 14.

Metallization degree of calcium ferrite phases in a) Sinter 5 and b) Sinter 10 reduced up to 1000°C. (Online version in color.)

Fig. 15.

Relationship between the difference of reduction degree of sinter sample reduced up to 1000°C under Low-H₂ and High-H₂ conditions, and ratio of the phases of which reduction are accelerated by an increase in the partial pressure of H₂. (Online version in color.)

This difference increases with increasing this ratio in a linear manner. It indicates that the increase in the ratios of such phases leads to acceleration of reduction in BF. However, reduction of skeletal hematite leads to the problem of low-temperature reduction-disintegration of sinter.²⁷⁾ Therefore, 1H-ACF and primary hematite are desirable phases of iron ore sinter and the idea to produce the sinter with higher amount of 1H-ACF and primary hematite is required.

4. Conclusions

Mineral composition analysis of ten sinter samples, and their reduction experiments under two different gas conditions, i.e, High-H₂ and Low-H₂ compositions, were carried out. The results are summarized as follow:

(1) Calcium-ferrite phases are sorted into typical four types. 1H-ACF: acicular calcium-ferrite less than 10 μm in size coexisted with primary hematite. 2H-CF: columnar calcium-ferrite coexisted with skeletal hematite. M-FCF: columnar calcium-ferrite less than 20 μm in size coexisted with magnetite. M-CCF: and columnar calcium-ferrite more than 20 μm in size coexisted with magnetite.

(2) Increasing hydrogen concentration of the reducing gas leads to promoting the reduction of sinter. Increasing hydrogen concentration results in the reduction of hematite, 1H-ACH, and 2H-CF. On the other hand, there is less effect of hydrogen increase on the reduction of magnetite, M-FCF, and M-CCF.

(3) Reduction degree of sinter increases with increase in the ratios of hematite, 1H-ACF, and 2H-CF in sinter. However, the increase in the amount of skeletal hematite brings about low-temperature reduction-disintegration of sinter. Therefore, the increases in the ratios of primary hematite and 1H-ACF are suitable when higher hydrogen concentration is applied for the blast furnace.

Acknowledgment

The present work was carried out as a part of COURSE50 project. Financial support by the New Energy and Industrial Technology Development Organization (NEDO) is gratefully acknowledged. The authors greatly appreciate the experimental work of Mr. H. Hoshino, and Mr. S. Yamazaki who are past members of our laboratory in Tohoku University.

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