Effect of Sinter Reducibility on Softening-Melting-Dripping Behaviors of Burden: Sinter and Coke Layered Charging

Abstract

Since the 20th century, the pressure on carbon emission reduction has increased gradually in blast furnace iron-making. Using the high reducibility sinter is considered as the way to further reduce carbon emissions. In this study, the effect of sinter reducibility on the softening-melting-dripping behavior is investigated based on sinter-coke-sinter-coke-sinter layered charging. It provides theoretical guidance for optimizing the burden structure. The results show that the softening-melting behavior is determined by the upper layer sinter under experimental conditions. With increasing sinter reducibility, softening start temperature drops from 1150°C to 1046°C, softening end temperature drops from 1270°C to 1185°C, melting start temperature decreases from 1270°C to 1197°C, dripping temperature increases from 1477°C to 1516°C. The cohesive zone of high reducibility sinter widens. Gas permeability index increases from 1042.20 Kpa·°C to 1199.56 Kpa·°C. Under 1150°C, gas utilization ratio increases with increasing sinter reducibility, and then the gas utilization ratio decreases as the melting begins. Therefore, the packing structure has a significant impact on the use of high reducibility sinter. High reducibility sinter can only be used to reduce emissions under a reasonable burden structure.

1. Introduction

Since the 20th century, the pressure on carbon emission reduction has increased gradually in the steel industry, low-carbon iron-making technology has developed rapidly. However, blast furnace (BF) will still be the most major iron-making process worldwide. In 2021, China’s pig iron output is 868 million tons. And the iron-bearing burden put into furnace is 1.42 billion tons, including about 70% sinter, 15% acid pellet and 15% imported rich lump ore. New low-carbon iron-making technologies, such as carbon-iron composite burden, injection hydrogen-rich medium, top gas circulation and metallized burden, provide new possibilities for carbon emission reduction of BF.^{1,2,3,4,5,6,7)} High basicity sinter is the main component of iron-bearing burden put into furnace. According to the current China National Standard YB/T 421-2014, the reducibility index (RI) of high-quality sinter is ≥ 70.0%, and w(FeO) is ≤ 9.0%. As the main component of iron-bearing burden put into furnace, the properties of sinter have an important impact on the production and energy consumption of BF. The burden structure of high reactivity coke and high reducibility sinter are considered as the way to further save energy and reduce emissions.^8,9,10)

The gas flow distribution is the comprehensive reflection of various operation indexes of BF. And reasonable gas flow distribution is the premise to ensure the long-term stable operation. The gas flow distribution and burden permeability are determined by the cohesive zone. The research shows that the cohesive zone is the area with the largest pressure loss in the BF,^11,12) and more than 60% of the total pressure loss comes from cohesive zone. Therefore, the shape and position of the cohesive zone are critical factors to ensure the reasonable gas flow distribution, improve the permeability and achieve low-carbon operation.^13,14) Sinter, as the main component of iron-bearing burden put into furnace, is widely used due to the excellent metallurgical properties. The softening-melting and dropping characteristics of sinter determine the position and thickness of the cohesive zone as to affect the gas flow distribution, indirect reduction, and fuel consumption.^15,16)

The effects of sinter with different properties on the softening and melting behaviors of the burden have been studied extensively. Tang et al.¹⁷⁾ studied the effect of MgO content in sinter on the smelting mechanism of chromium-bearing vanadium-titanium magnetite. The results show that the softening and melting region decrease with increasing MgO content as to make the cohesive zone moves down slightly and tends to narrow. Liu et al.¹⁸⁾ studied the softening and melting behaviors of comprehensive burden with different mixing degrees of sinter and pellet. They believe that adjusting the mixing degree of sinter and pellet can refrain the interaction between the burdens as to improve the permeability of the cohesive zone. Lan et al.¹⁹⁾ studied the effect of CI on the softening-melting-dropping behaviors of iron-bearing burden by increasing the CI content in sinter. With increasing CI content in sinter, the softening start and softening end temperature increase, the softening range decrease, the melting start temperature increase, the dropping temperature decrease, the cohesive zone tends to narrow as to improve the permeability of the burden. Matsumura et al.²⁰⁾ improved the reducibility and softening property of sinter by controlling the chemical compositions. They believe that sinter with low SiO₂, low CaO, high MgO and high FeO content has high reducibility and good softening property, indicating that sinter is beneficial to the smelting process of BF. In addition, the effect of mixed lump coke charging in the ore layer on gas permeability has been studied extensively.^21,22,23) These are mainly being studied using laboratory experiments²²⁾ or DEM-CFD (Discrete Element Method and Computational Fluid Dynamics) models.^21,23) Ueda et al.²⁴⁾ studied the softening and melting behaviors of the CaO–FeO–SiO₂–Al₂O₃–MgO system for improving gas permeability in a blast furnace. The temperature of the oxide is increased to 1450°C or 1500°C in a CO/CO₂ mixture. They found that the reduction of the sample progresses in an atmosphere with a high CO/CO₂ ratio, and thus, the softening temperature increases. Ohno et al.²⁵⁾ investigated the effect of pre-reduction degree on softening behavior of simulant sinter iron ore. They found that peripheral structure has a significant effect on deformation resistance. The existence of molten slag phase could make it easy to deform the sample particle shape, and metallic solid Fe phase helped to strengthen the particle’s deformation resistance. Existing studies have mainly investigated the effects of composition, burden structure and burden mixing degree on the softening-melting-dropping behaviors of iron-bearing burden. The results of these studies are important for understanding the cohesive zone and rational BF operation, both theoretically and practically. However, there are few studies on softening-melting-dropping behaviors and gas utilization ratio of sinter with different reducibility based on ore-coke layered charging.

In present work, four kinds of sinters with different reducibility are used to investigate the effects of sinter reducibility on softening-melting-dripping behaviors, permeability index and gas utilization ratio based on sinter-coke-sinter-coke-sinter layered charging. The apparatus of softening-melting-dripping experiment is used to simulate the temperature, atmosphere and load at different positions of BF. Characteristic curves can be recorded. The CO and CO₂ content of tail gas can be analyzed by using a gas analyzer. It provides theoretical guidance for optimizing the burden structure.

2. Experiment

2.1. Raw Materials

Four sinters with different reducibility and one coke are used in this study. Sinters are obtained by using the sintering pot. Use the various ore powders (OP) from steel enterprises to sintering allocating-ore. Under the same process system, the reducibility of the finished sinter is controlled by changing the different dosage of coke powder in the raw material. That will lead to differences in FeO content of the finished sinter. Under the other composition of sinter remains unchanged, the reducibility of sinter is also different. Table 1 presents the chemical composition of the various ore powders. Table 2 shows the industrial analysis and ash composition of coke powder. Table 3 shows the sintering allocating-ore structure. The carbon allocations for the four experiments are 8%, 7%, 5% and 3% respectively. Figure 1 presents the schematic view of the apparatus of sintering pot experiment process. The inner diameter of the sintering pot is 200 mm. The total mass of raw materials is 45 kg for each group experiment. Firstly, use the small mixing granulator to mix and make balls. During the mixing process, the amount of water added is controlled to 7% to achieve granulation. The mixing granulation time is 7 minutes. Then, screen the mixture to ensure that the particle with 3–5 mm reaches 70–80%. The bottom of the sinter pot is paved with 1.5 kg sinter with a particle size of 10–20 mm as the bedding material to improve air permeability. The height of the material layer is 750 mm. 60 g coke powder lay on the upper part of the material layer for combustion support. Upper ignition, lower suction. The sintering process parameters are shown in Table 4.

Table 1. Main chemical composition of the various ore powders (wt%).

OP	TFe	CaO	MgO	SiO2	Al2O3	P	S
1#	61.47	0.65	9.56	3.45	2.25	0.05	0.11
2#	56.00	0.74	0.39	1.93	0.53	0.05	0.02
3#	56.50	0.19	0.10	3.78	2.20	0.10	0.10
4#	65.30	0.01	0.10	6.60	3.10	0.10	0.10
5#	66.50	0.04	0.10	6.50	6.00	0.01	0.10
6#	70.00	0.20	0.60	7.30	1.00	0.005	0.17
7#	43.00	0.20	0.60	2.00	1.00	0.005	0.47
8#	55.20	3.00	40.00	2.50	0.56	0.01	0.85

Table 2. Industrial analysis and ash composition of coke powder (wt%).

Component	FCad	Aad	Vad	Mad	Ash composition
					TFe	SiO2	CaO	Al2O3	MgO
coke powder	79.17	12.81	4.08	0.50	15.10	36.58	6.68	24.61	1.20

Note: FC_ad-fixed carbon content; V_ad -volatile matter content on dry ash free basis; A_ad-ash content on air dry basis; M_ad-moisture on air dry basis.

Table 3. Sintering allocating-ore structure (wt%).

OP	1#	2#	3#	4#	5#	6#	7#	8#
Content	17.81	10.96	9.59	13.70	36.99	2.05	3.42	5.48

Fig. 1. Schematic view of the apparatus of sintering pot experiment process. (Online version in color.)

Table 4. Sintering process parameters.

Parameters	Values	Parameters	Values
Ignition temperature/°C	1100	Ignition timing/s	90
Ignition suction/kPa	8.0	Negative pressure/kPa	15
Layer thickness/mm	750	Return fines ratio/%	20

Sinters are labeled sinter A, B, C, and D, respectively. The main chemical composition and properties of sinters are listed in Table 5. The FeO content of sinter A, B, C and D are 15.97%, 10.32%, 8.98% and 5.26%, respectively. The reducibility of sinter is determined by Eq. (1) according to current China National Standard of the GB/T 13241-2017. The reducing atmosphere is 4.5 L/min CO+10.5 L/min N₂. The reducing temperature is 900°C. The reducing time is 3 h. The reducibility is 80%, 82%, 85% and 90%, respectively. Coke comes from China’s iron and steel company. Proximate analyses and properties of coke are listed in Table 6. The coke reactivity index (CRI) and post reaction strength (CSR) of coke are determined by Eqs. (2) and (3) according to current China National Standard of the GB/T 4000-2017. The reaction atmosphere is 5 L/min CO₂. The reaction temperature is 1100°C. The reaction time is 2 h. After the reaction, all the coke is loaded into the I-type drum and rotated at a speed of 20 r/min for 30 min. The CRI, CSR and fixed carbon content are 24.75%, 70.08% and 79.66%, respectively. The sizes of the sinter and coke are both between 10–12.5 mm. Samples are dried at 105°C in a dry furnace. Based on on-site production conditions, the ore-to-coke ratio of the burden is 3.5:1. Each group test is conducted with an equal amount of sinter (450 g) and coke (128 g).

  

$$R_3(\%)=\left(\frac{0.111W_1}{0.430W_2}+\frac{m_1-m_3}{m_0\times 0.430W_2}\times 100\right)\times 100$$(1)

where m₀ is the mass of the sinter, g; m₁ is the mass of the sinter before the start of reduction, g; m₃ is the mass of the sinter after 3 h of reduction, g; W₁ is the content of FeO in the sinter before the test, %; W₂ is the total iron content of the sinter before the test, %.

  

$$CRI(\%)=\frac{m-m_2}{m}\times 100$$(2)

  

$$CSR(\%)=\frac{m_4}{m_2}\times 100$$(3)

where m is the mass of the coke, g; m₂ is the mass of the coke after the reaction, g; m₄ is after drum the mass of coke with a particle size greater than 10 mm.

Table 5. Main chemical composition and property of sinter with different reducibility (wt%).

Sinter	TFe	FeO	CaO	MgO	SiO2	Al2O3	P	S	Reducibility
A	56.81	15.97	10.35	1.60	4.41	1.55	0.057	0.180	80
B	56.78	10.32	10.30	1.60	4.52	1.54	0.051	0.175	82
C	56.75	8.98	10.25	1.59	4.35	1.52	0.042	0.162	85
D	56.23	5.26	9.98	1.68	4.67	1.54	0.058	0.173	90

Table 6. Proximate analyses and properties of coke (wt%).

Mad	Vad	FCad	Aad	S	CRI	CSR
<0.01	0.82	88.21	10.97	0.5750	24.75	70.08

2.2. Experiment Methods

The equipment used to measure the softening-melting and dripping behavior of sinter is shown in Fig. 2(a). The equipment consists of a furnace body, thermocouples, air-operated pressure device, position detection device, gas distribution system, pressure detection device and weight detection device. 450 g sinter and 128 g coke are layered into a graphite crucible. The packing structure is 150 g sinter + 64 g coke + 150 g sinter + 64 g coke + 150 g sinter. The inner diameter of graphite crucible is 75 mm. Dropping holes with inner diameter of 8 mm are set on the bottom of graphite crucible to ensure molten slag-iron and gas passing through packed bed. The softening-melting-dripping experiment conditions all simulate the real BF. The experimental conditions of softening-melting-dripping experiment are shown in Table 7. Differential pressure pass though the packed bed, shrinkage of the sinter, weight of dripping slag-iron and temperature are continuously recorded, respectively.

Fig. 2. Schematic view of the apparatus of softening-melting-dripping experiment and gas analyzer, (a) softening-melting-dripping apparatus, (b) gas analyzer. (Online version in color.)

Table 7. Experimental conditions of softening-melting-dripping experiment.

temperature range, °C	heating rate, °C/min	furnace top load, MPa	Gas composition, L/min
			CO	CO2	N2
25–400	10	0.18	0.0	0.0	5.0
400–900	10	0.18	3.9	2.1	9.0
900–1020	3	0.35	4.5	0.0	10.5
1020–1550	5	0.35	4.5	0.0	10.5

The effect of different reducibility sinter on the tail gas composition is investigated using Gasboard-3100 gas analyzer (Hubei Cubic-Ruiyi Instrument Co., Ltd., Hubei) under the same experimental conditions. Schematic diagram of gas analyzer is shown in Fig. 2(b). The analysis system consists of sampling transmission unit, preprocessing unit, analysis and data output unit. The tail gas enters from the sampling transmission unit. The pretreatment unit is used to remove dust, tar and moisture from the tail gas so that the tail gas meets the use requirements of the analysis unit. The analysis and data output unit consists of an on-line infrared gas analyzer. The analyzer is equipped with 4–20 mA DC output interface and RS232 serial interface. Gas composition analysis data can be transmitted to the user’s central control unit through shielded cable.

3. Results and Discussion

3.1. Experiment Result Characterization

The readout process for the characteristic temperature is shown in Fig. 3. The softening-melting characteristic temperatures mainly incorporates softening start temperature T₄, softening end temperature T₄₀, melting start temperature T_S and dripping temperature T_D. T₄ is the temperature at which the burden shrinks by 4%. T₄₀ is the temperature at which the burden shrinks by 40%. T₄₀-T₄ is the softening temperature range. T_S is the temperature at which the sudden rise of burden differential pressure. T_D is the start dropping temperature of slag and iron from the graphite crucible. T_D-T_S is the melting temperature range (cohesive zone). The permeability index S is used to describe the permeability of the burden. The S value can be calculated by integrating the differential pressure curve from T_S to T_D, as shown in Eq. (4).

  

$$S={\int }_{T_S}^{T_D}(\mathrm{\Delta }{P}_T-\mathrm{\Delta }{P}_S)\cdot dT$$(4)

Fig. 3. Readout process for the characteristic temperature. (Online version in color.)

Where T_S and T_D are the softening start temperature and dripping temperature, respectively. ΔP_T and ΔP_S are the pressure drop at certain temperature and the pressure drop at T_S, respectively. The small S value represents the better the gas permeability of the burden.

3.2. Effect of Sinter Reducibility on Softening-melting Behaviors

The characteristic curves of the softening-melting and dripping process are shown in Fig. 4. According to the shrinkage ratio curve, shrinkage behavior can be divided into three stages. In stage I, the shrinkage rate of the burden is relatively fast. The stage II is a slow shrinkage process. The stage III is a fast shrinkage process. The three stages correspond to the gas-solid reduction, the softening-melting, and the slag and iron dripping processes of the burden, respectively. According to the differential pressure curve, the platform section of the differential pressure curve corresponds to the stage II of the shrinkage behaviors. The platform range and the full width at half maximum (FWHM) of the differential pressure curve increase with the increase of the sinter reducibility.

Fig. 4. Characteristic curves of softening-melting and dripping process of sinter, (a) sinter reducibility of 80%, (b) sinter reducibility of 82%, (c) sinter reducibility of 85%, (d) sinter reducibility of 90%. (Online version in color.)

The effect of sinter different reducibility on the softening-melting behaviors are shown in Fig. 5. With increasing reducibility from 80% to 90%, softening start temperature T₄ drops from 1150°C to 1046°C, softening end temperature T₄₀ drops from 1270°C to 1185°C, softening temperature range (T₄₀-T₄) is shifted towards lower temperatures. Above test data and regularity show that the gas-solid reaction rate accelerates at a relatively low temperature with increasing sinter reducibility, shrinkage of burden speed up, temperatures corresponding to shrinkage ratios of 4% and 40% decrease. With increasing reducibility, melting start temperature T_S decreases from 1270°C to 1197°C, dripping temperature T_D increases from 1477°C to 1516°C and melting region (T_D-T_S) becomes wider. In this study, the melting region is regarded as cohesive zone. Based on the above analysis, the cohesive zone corresponding to high reducibility sinter widens from both ends of the temperature range. The cohesive zone of the burden has a great influence on the permeability and the distribution of the gas flow. Thicker cohesive zone will lead to a poor permeability and uneven distribution of the gas flow.

Fig. 5. Effect of sinter different reducibility on the softening-melting behaviors. (Online version in color.)

3.3. Effect of Sinter Reducibility on Burden Permeability

Figure 6 shows that the effect of sinter different reducibility on the highest differential pressure (ΔPmax) and S value. As the sinter different reducibility is varied from 80% to 90%, ΔPmax are 7.15 KPa, 9.24 KPa, 6.85 KPa and 5.54 KPa, respectively. There is no significant tendency for ΔPmax. Since the ΔPmax is an instantaneous value, it is unreasonable to judge the permeability of the burden according to the instantaneous characteristic value. S value increases from 1042.20 KPa·°C to 1199.56 KPa·°C and the FWHM of the differential pressure curve increases with increasing sinter reducibility. It shows that the burden corresponding to a high reducibility sinter has a poor permeability. Under the sinter-coke-sinter-coke-sinter packing structure, the cohesive zones that are not favorable for smelting will form and the gas permeability of the burden will become poor with increasing sinter reducibility.

Fig. 6. Effect of sinter reducibility on S-value of burden, (a) sinter reducibility of 80%, (b) sinter reducibility of 82%, (c) sinter reducibility of 85%, (d) sinter reducibility of 90%. (Online version in color.)

3.4. Effect of Sinter Reducibility on Gas Utilization Ratio

Gasboard-3100 gas analyzer is utilized to analyze the content of the CO and CO₂ during the softening-melting and dropping processes. Analysis of CO and CO₂ contents in the tail gas are shown in Fig. 7. The tail gas change process is divided into three main stages. In stage I, the volume fraction of reaction atmosphere is 26% CO, 14% CO₂ and 60% N₂. With increasing temperature, the CO content decreases and CO₂ content increases below 850°C. It is caused by the gas-solid reduction reaction. From 850°C to 900°C, CO content slightly decreases and CO₂ content slightly increases due to the reduction products formation. In stage II, cut off CO₂ and increase CO volume fraction. And the volume fraction of reaction atmosphere becomes 30% CO and 70% N₂. The CO content increases and the CO₂ content decreases from 900°C. At the same time, all of the CO₂ in the tail gas is produced by iron oxide reduction. In stage III, the CO content increases rapidly from approximately 1300°C. Two reasons can explain this phenomenon. First, as the degree of reduction increases, the amount of product gradually increases and the amount of unreacted burden decreases. This suppresses reduction reactions as to reduce the CO consumption. Second, the burden had reached the temperature of coke gasification reaction, and the oxidizing agent CO₂ comes from reduction reaction of the iron oxide. Coke gasification reaction produced CO.

Fig. 7. Analysis of CO and CO₂ contents in the tail gas, (a) sinter reducibility of 80%, (b) sinter reducibility of 82%, (c) sinter reducibility of 85%, (d) sinter reducibility of 90%. (Online version in color.)

Gas utilization ratio (η_CO) can be calculated by Eq. (5).

  

$${\eta }_{CO}=\frac{\phi ({\text{CO}}_{\text{2}})\%}{\phi (\text{CO})\%+\phi ({\text{CO}}_{\text{2}})\%}\times 100\%$$(5)

Here, η_CO is the gas utilization ratio, %, φ(CO) is CO volume fraction in tail gas, φ(CO₂) is CO₂ volume fraction in tail gas, %.

Effect of sinter reducibility on gas utilization ratio in softening-melting and dripping process are shown in Fig. 8. Since the fixed burden in the experiment is charged before the experiment, the product gradually increases and the unreacted sinter gradually decreases with the reaction process. Therefore, the gas utilization ratio decreases with increasing temperature. The relation between gas utilization ratio and sinter reducibility is not monotonically increasing during softening-melting and dropping process. From 900°C to 1150°C, gas utilization ratio increases with sinter reducibility. At about 1200°C, the gas utilization curve corresponding to high reducibility sinter is no longer at the highest position. When the softening-melting of the burden begins, the gas utilization ratio decreases with increasing sinter reducibility. Based on the above analysis, before the softening-melting of the burden, the use of high reducibility sinter can improve the gas utilization ratio. However, the high reducibility sinter can deteriorate the properties of the cohesive zone under experimental conditions and packing structure. This can reduce gas utilization ratio. Therefore, the charging method has a significant impact on the use of high reducibility sinter. High reducibility sinter can only be used to reduce carbon emission under a reasonable burden structure.

Fig. 8. Effect of sinter reducibility on gas utilization ratio, (a) sinter reducibility of 80%, (b) sinter reducibility of 82%, (c) sinter reducibility of 85%, (d) sinter reducibility of 90%. (Online version in color.)

3.5. Mechanism Analysis and Discussion of Softening-melting Behaviors

The different reducibility of sinter in this study is achieved by the carbon content in the sintering process. Sinter with different FeO content can be produced by using different carbon content in the sintering process. The reducibility of the sinter decreases with increasing FeO content. The reasons are as follows. FeO in sinter is converted from Fe²⁺. Fe²⁺ exists in the phase that is not easy to be reduced, such as magnetite (Fe₃O₄), olivine (2FeO·SiO₂), calcium-iron olivine (CaO·FeO·SiO₂), etc. As the FeO content increases, the fraction of the phase that is not easily reduced increases. The amount of easily reducible phases such as hematite and calcium ferrite decreases. During the sintering process, the high FeO content can form a liquid phase with high fluidity and low melting point, which makes the structure of the sinter compact. Consequently, sinters with high FeO content have low reducibility. Conversely, high reducibility sinter has more hematite and calcium ferrite phases, and the reduction reaction is easier to carry out. Under the action of the load, the shrinkage rate of the burden becomes faster. Temperatures corresponding to shrinkage ratios of 4% and 40% decrease. Therefore, the softening start temperature T₄ and softening end temperature T₄₀ decrease with increasing sinter reducibility. The effect mechanisms schematic diagrams of sinter reducibility on the T₄ and T₄₀ is shown in Fig. 9.

Fig. 9. Effect mechanism schematic diagrams of sinter reducibility on the T₄ and T₄₀. (Online version in color.)

The T_S is related to the slag melting point. The primary slag composition determines the melting behavior of the burden. The initial melting temperature is related to the FeO content of primary slag. Figure 10 shows the phase diagram of FeO-SiO₂-CaO-1.55%Al₂O₃-1.65%MgO drawn by using the phase diagram module in Factsage7.2 software. That can be used to describe the effect of FeO content on the slag formation behavior under the same alkalinity conditions. From Fig. 10, the liquidus temperature of primary slag decreases with increasing FeO content. Therefore, the FeO content before melting can affect the T_S of burden. Figure 11 shows the liquid slag formation in the atmosphere of 100% N₂ and 30% CO+70% N₂ calculated by using the Equilib module in the Factsage7.2 software. The original chemical composition of different reducibility sinters are used as input. As can be seen in Fig. 11(a), the initial formation temperature of the liquid slag increases with increasing sinter reducibility in the 100% N₂ atmosphere. When using the Equilib module to calculate liquid slag formation, the FeO content will still conform to the high reducibility sinter has a low FeO content, while the low reducibility sinter has a high FeO content. Because iron oxides do not undergo a reduction reaction in the atmosphere of 100% N₂. Therefore, the initial formation temperature of the liquid slag increases with increasing sinter reducibility in the 100% N₂ atmosphere.

Fig. 10. Phase diagram of FeO-SiO₂-CaO-1.55%Al₂O₃-1.65%MgO. (Online version in color.)

Fig. 11. Liquid slag formation based on the chemical equilibrium calculation, (a) In 100% N₂ atmosphere, (b) In 30% CO and 70% N₂ atmosphere; (1#) sinter reducibility of 80%, (2#) sinter reducibility of 82%, (3#) sinter reducibility of 85%, (4#) sinter reducibility of 90%. (Online version in color.)

Conversely, the initial formation temperature of the liquid slag decreases with increasing sinter reducibility in the 30% CO+70% N₂ atmosphere. The reason for this phenomenon using thermodynamic calculations may be as follows: When using the Equilib module to calculate liquid slag formation, the original chemical composition of different reducibility sinters are used as input. The high reducibility sinter has a low FeO content and a high Fe₂O₃, while the low reducibility sinter has a high FeO content and a low Fe₂O₃ in the original chemical composition. In the 30% CO+70% N₂ atmosphere, the Equilib module will perform a reduction reaction calculation for all iron oxides at any temperature. For example, the reduction reaction processes from Fe₂O₃ to Fe₃O₄, Fe₃O₄ to FeO, and FeO to Fe may be calculated simultaneously at the same temperature. A low reducibility sinter with high FeO content may be calculated to generate more Fe, and a high reducibility sinter with low FeO content may be calculated to generate less Fe at a certain temperature. The calculation process may manifest as high reducibility sinter having a higher FeO content at a certain temperature, while low reducibility sinter having a lower FeO content. Therefore, the thermodynamic calculation results in the 30% CO+70% N₂ atmosphere are as follows: the initial formation temperature of the liquid slag of high reducibility sinter is relatively low, while the initial formation temperature of the liquid slag of low reducibility sinter is relatively high. This may be inconsistent with the fact that the reducibility of sinter is higher as the FeO content is lower. However, it indicates that the reaction conditions have a significant impact on the chemical composition of the sinter before melting temperature.

Based on the above analysis, it can be concluded that the softening-melting behavior of the sinter cannot be judged according to FeO content in the original composition of sinter. FeO content in the original composition can affect the reducibility of sinter. Under the different conditions, the reduction degree of different reducibility sinter is different, which changes the composition content and subsequently affects the softening-melting behavior of the burden.

To investigate the reduction degree of FeO before the melting of sinters with different reducibility, we conduct an interruption temperature and atmosphere experiment at 1200°C. After rapid cooling in an N₂ atmosphere, we analyzed the macrostructure of the burden and performed an X-ray diffraction (XRD) (MPDDY2094, PANalytical B.V., Almelo, Netherlands) analysis of the primary slag. Copper ka radiation (40 kV, 40 mA, wavelength 0.154 nm) is used as the X-ray source, the scanned angular range varied from 10° to 90° with a scanning speed of 0.2°/s. Utilizing search match software (X’Pert HighScore Plus 3.0, PANalytical B.V., Almelo, Netherlands) analyze the XRD pattern. Figure 12 presents the characteristic analysis of sinter D at 1200°C. From the figure, it shows that the melting degrees of the three layers of the sinter are not the same. The upper layer sinter has a dense structure, while the lower layer sinter still maintains its original shape. This indicates that the melting degree of the upper layer sinter is significantly higher than that of the lower layer sinter. XRD analysis of iron oxides in different layers of samples shows that the reduction degree of FeO in the lower layer sinter is significantly higher than that in the upper layer sinter. This is because the reducing atmosphere of the upper layer sinter weakens with the reduction of the lower layer sinter. At lower temperatures, more FeO is reduced to Fe in the lower layer sinter, and the resulting Fe increases the deformation resistance of the lower layer sinter. Therefore, under this packing structure, the reduction degree of FeO in the upper layer sinter determines the differential pressure of burden. T_S is determined by the FeO content in the upper layer sinter before melting temperature.

Fig. 12. Characteristic analysis of sinter D at 1200°C. (Online version in color.)

Figure 13 presents the XRD analysis (a) and FeO content (b) of upper layer sinter at 1200°C. From the figure, under experimental conditions, the FeO content in the upper layer sinter increases with increasing sinter reducibility at 1200°C. This is because the reducing atmosphere of the upper layer sinter gradually weakens with the reduction of the lower layer sinter. In other words, the reducing atmosphere of the upper layer sinter gradually weakens with increasing sinter reducibility. Therefore, the reduction degree of FeO in the upper layer sinter decreases with increasing sinter reducibility.

Fig. 13. XRD analysis (a) and FeO content (b) of upper layer sinter at 1200°C. (Online version in color.)

Based on the above analysis, the upper layer sinter with high reducibility has a high FeO content before melting temperature under experimental conditions and packing structure. On the contrary, the upper layer sinter with low reducibility has a lower FeO content before melting temperature. We use the polarizing microscope to analyze the mineral phase structure of different samples before and after reduction. Table 8 presents the analysis of the mineral phase structure of the sinter before and after the reduction. For well-known reasons, the exact number of phase content analysis have a small deviation from the true value, but the overall trend analysis is no problem. The intermediate product FeO and unreacted phase can form a mixture with lower melting point. That can reduce the melting start temperature T_S. Thus, under experimental conditions and packing structure, high reducibility sinter has relatively low T₄₀ and T_S, and low reducibility sinter has relatively high T₄₀ and T_S.

Table 8. Analysis of the mineral phase structure of the sinter before and after the reduction (%).

Group		Magnetite	Hematite	Calcium ferrite	Dicalcium silicate	Glassiness	Other
Before reaction	A	35	19	30	3	9	4
	B	35	20	32	3	8	2
	C	31	22	35	3	8	1
	D	28	24	37	3	7	1
Upper layer sinter after reaction at 1200°C	A	16	6	10	2	11	55
	B	13	4	10	2	10	61
	C	10	2	8	2	10	68
	D	7	1	4	2	8	78

4. Conclusions

In this study, the sinters with different reducibility are used to investigate the effects of reducibility on softening-melting-dripping behaviors, permeability index and gas utilization ratio based on sinter-coke-sinter-coke-sinter layered charging. The apparatus of softening-melting-dripping experiment is used to simulate the temperature, atmosphere and load at different positions of BF. Gasboard-3100 gas analyzer is used to analyze the composition of tail gas in the process of softening-melting and dripping. The following conclusions are drawn:

(1) Based on sinter-coke-sinter-coke-sinter layered charging, the softening-melting behavior is determined by the upper layer sinter under experimental conditions. The reducing atmosphere of the upper layer sinter gradually weakens with increasing sinter reducibility due to the reduction of lower layer sinter. Therefore, the reduction degree of FeO in the upper layer sinter decreases with increasing sinter reducibility before initial melting temperature.

(2) With increasing sinter reducibility, softening start temperature T₄ drops from 1150°C to 1046°C, softening end temperature T₄₀ drops from 1270°C to 1185°C, softening temperature range (T₄₀-T₄) is shifted towards lower temperatures, melting start temperature T_S decreases from 1270°C to 1197°C, dripping temperature T_D increases from 1477°C to 1516°C, melting region (T_D-T_S) becomes wider, and gas permeability index S value increases from 1042.20 KPa·°C to 1199.56 KPa·°C. The cohesive zone corresponding to high reducibility sinter widens under the sinter-coke-sinter-coke-sinter packing structure.

(3) The relation between gas utilization ratio and sinter reducibility is not monotonically changing during softening-melting and dropping process. From 900°C to 1150°C, gas utilization ratio increases with increasing sinter reducibility. When the softening-melting of the burden begins, the gas utilization ratio decreases with increasing sinter reducibility.

(4) The charging method has a significant impact on the use of high reducibility sinter. High reducibility sinter can only be used to reduce emissions under a reasonable burden structure.

Acknowledgements

This work was supported by national natural science foundation of china-liaoning Joint Funds: [Grant Number U1808212]; National Natural Science Foundation of China: [Grant Number 52074080]; Major Special Project of Shanxi Province: [Grant Number 20181102020].

Declaration of Competing Interest

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

References

• 1)   P.  Wang,  J. X.  Li,  L. Y.  Zhou and  H. M.  Long: Ironmak. Steelmak., 40 (2013), 312. https://doi.org/10.1179/1743281212Y.0000000056
• 2)   M. S.  Chu and  J. I.  Yagi: Steel Res. Int., 81 (2010), 1043. https://doi.org/10.1002/srin.201000103
• 3)   H. M.  Ahmed,  N.  Viswanathan and  B.  Bjorkman: Steel Res. Int., 85 (2014), 293. https://doi.org/10.1002/srin.201300072
• 4)   H.  Wang,  M. S.  Chu,  W.  Zhao,  R.  Wang,  Z. G.  Liu and  J.  Tang: Ironmaking Steelmaking, 43 (2016), 571. https://doi.org/10.1080/03019233.2016.1152344
• 5)   W.  Zhao,  M. S.  Chu,  H. T.  Wang,  Z. G.  Liu and  Y. T.  Tang: Int. J. Miner. Metall. Mater., 23 (2016), 501. https://doi.org/10.1007/s12613-016-1261-9
• 6)   R. J.  Yan,  Z. G.  Liu,  M. S.  Chu and  P. J.  Liu: Metall. Res. Technol., 119 (2022), 607. https://doi.org/10.1051/metal/2022089
• 7)   R. J.  Yan,  Z. G.  Liu,  M. S.  Chu and  P. J.  Liu: Can. Metall. Q., 62 (2023), 134. https://doi.org/10.1080/00084433.2022.2049989
• 8)   M. S.  Chu,  X. Z.  Guo,  F. M.  Shen,  J. I.  Yagi and  H.  Nogami: J. Iron Steel Res. Int., 14 (2007), 13. https://doi.org/10.1016/S1006-706X(07)60020-X
• 9)   F. C.  Meng,  L.  Shao and  Z. S.  Zou: Energies, 13 (2020), 5062. https://doi.org/10.3390/en13195062
• 10)   T.  Natsui,  K.  Nakano,  Y.  Matsukura,  K.  Sunahara,  Y.  Ujisawa and  T.  Inada: Tetsu-to-Hagané, 99 (2013), 267 (in Japanese). https://doi.org/10.2355/tetsutohagane.99.267
• 11)   X. Y.  Fan,  K. X.  Jiao and  J. L.  Zhang: ISIJ Int., 61 (2021), 1758. https://doi.org/10.2355/isijinternational.ISIJINT-2020-693
• 12)   S.  Ueda,  T.  Kon,  T.  Miki,  S. J.  Kim and  H.  Nogami: ISIJ Int., 55 (2015), 2098. https://doi.org/10.2355/isijinternational.ISIJINT-2015-269
• 13)   Y. A.  Lu,  S. L.  Wu,  B. B.  Du and  H.  Zhou: ISIJ Int., 60 (2020), 1461. https://doi.org/10.2355/isijinternational.ISIJINT-2019-713
• 14)   P. F.  Nogueira and  R. J.  Fruehan: Metall. Mater. Trans. B, 35 (2004), 829. https://doi.org/10.1007/s11663-004-0077-6
• 15)   S. L.  Wu,  B. Y.  Tuo,  L. H.  Zhang,  K. P.  Du and  Y.  Sun: Steel Res. Int., 85 (2014), 233. https://doi.org/10.1002/srin.201300061
• 16)   S. L.  Wu,  Y. A.  Lu,  Z. B.  Hong and  H.  Zhou: ISIJ Int., 60 (2020), 1504. https://doi.org/10.2355/isijinternational.ISIJINT-2019-833
• 17)   W. D.  Tang,  S. T.  Yang,  G. J.  Cheng,  Z. X.  Gao and  X. X.  Xue: Steel Res. Int., 89 (2018), 1800226. https://doi.org/10.1002/srin.201800226
• 18)   Z. W.  Liu,  B.  Chu and  Y. Z.  Pan: Ironmak. Steelmak., 50 (2023), 226. https://doi.org/10.1080/03019233.2022.2099699
• 19)   C. C.  Lan,  Q. L.  Yu,  X. J.  Liu,  Y. N.  Qie and  S. H.  Zhang: High Temp. Mater. Process., 38 (2019), 273. https://doi.org/10.1515/htmp-2017-0186
• 20)   M.  Matsumura,  M.  Hoshi and  T.  Kawaguchi: ISIJ Int., 45 (2005), 594. https://doi.org/10.2355/isijinternational.45.594
• 21)   M.  Yakeya,  A.  Kasai,  R.  Tadai and  K.  Nozawa: ISIJ Int., 60 (2020), 1438. https://doi.org/10.2355/isijinternational.ISIJINT-2019-699
• 22)   S.  Watakabe,  K.  Takeda,  H.  Nishimura,  S.  Goto,  N.  Nishimura,  T.  Uchida and  M.  Kiguchi: ISIJ Int., 46 (2006), 513. https://doi.org/10.2355/isijinternational.46.513
• 23)   S.  Matsuhashi,  H.  Kurosawa,  S.  Natsui,  T.  Kon,  S.  Ueda,  R.  Inoue and  T.  Ariyama: ISIJ Int., 52 (2012), 1990. https://doi.org/10.2355/isijinternational.52.1990
• 24)   S.  Ueda,  T.  Miki,  S. J.  Kim,  X.  Gao,  H.  Nogami and  S.  Kitamura: ISIJ Int., 60 (2020), 1380. https://doi.org/10.2355/isijinternational.ISIJINT-2019-136
• 25)   K.  Ohno,  H.  Konishi,  T.  Watanabe,  S.  Ishihara,  S.  Natsui,  T.  Maeda and  K.  Kunitomo: ISIJ Int., 60 (2020), 1520. https://doi.org/10.2355/isijinternational.ISIJINT-2020-040