2025 Volume 65 Issue 6 Pages 825-832
Blast furnace blowing break and re-blowing is a regular operation in the smelting process, However, some blast furnace conditions fluctuate for a long time due to improper operation of blast furnace blowing break and re-blowing, and preventing rapid attainment of production capacity. This paper first analyzes the influence of hydrogen-rich on the cohesive zone. Subsequently, it simulates the conditions of ferrous burden during partial and complete tuyere blowing break under hydrogen-rich conditions, followed by re-blowing. The study explores the influence of these operational changes on the softening and melting behaviors of the ferrous burden. The results indicate that with a 10% hydrogen enrichment, the melting range of ferrous burden narrows and shifts to higher temperatures, improving the permeability of the burden. During partial tuyere blowing break, this promotes the reduction of the ferrous burden and the carburization of metallic iron, increasing the melting start temperature and decreasing the dropping temperature by 29°C, thereby narrowing the cohesive zone. Both maximum pressure difference (ΔPmax) and permeability index (S) values decrease. In contrast, with a complete tuyere blowing break, the dropping temperature of the ferrous burden gradually increases from 1459°C to 1478°C as the isothermal duration extends, widening the melting interval and leading to an increase in both ΔPmax and S values.
The iron and steel industry is an important basic industrial sector and the material basis for the development of the national economy and national defense.1,2) At present, the iron and steel industry is still dominated by blast furnace production, and its stable smooth operation is very important to production.3,4,5) Blast furnace smelting is a continuous and uninterrupted physical and chemical reaction process. The more stable the smelting conditions, the more conducive to the stable and smooth operation of the blast furnace.6,7) During the process of a blast furnace from production to blowing break, and subsequently from blowing break to re-blowing, there are significant changes and redistributions in the gas flow and temperature fields within the furnace. If the blowing break is improper operation, it can easily lead to abnormal furnace conditions, hindering the re-blowing process.8) To avoid repeated issues in furnace condition recovery due to improper operations during the re-blowing process, it is necessary to analyze the impact of partial and complete tuyere blowing break on the softening and melting properties of ferrous burden. This facilitates the implementation of appropriate measures to ensure the rapid restoration of normal furnace conditions, thereby mitigating the adverse effects of blowing break on furnace performance. Such measures are beneficial for achieving energy savings and emissions reduction, and for promoting the stable and smooth operation of the blast furnace.9)
Peng et al.10) analyzed the blowing break and re-blowing process of the No. 6 blast furnace at Chang gang, concluding that as the blowing break duration extends, the internal heat continuously dissipates, causing the temperature in the block and cohesive zones to decrease, and the dripping zone to gradually disappear. The cohesive zone becomes progressively thicker, significantly reducing the overall permeability of the burden column, necessitating the reestablishment of gas channels. Liu et al.6) conducted a comprehensive analysis of the re-blowing operating practices of several domestic blast furnaces with favorable outcomes. The results indicate that the primary challenges in the blowing break and subsequent re-blowing processes lie in controlling the heat within the hearth after blowing break, managing the pressure differential across the entire furnace upon re-blowing, and monitoring the progress of furnace condition recovery during the re-blowing process. Bi et al.11) analyzed the multiple blowing break and re-blowing in different blast furnaces of Cheng Steel, and concluded that the determination of the number of blowing break burden, their location and distribution, and correction are the key technologies for blowing break and re-blowing. Han et al.,12) using numerical simulations, calculated the temperature distribution and heat loss within the No. 5 blast furnace at Meigang during blowing break periods of 1, 4, and 7 days, and optimized the burden addition strategy for the blowing break period.
The melting behavior of ferrous burden dictates the internal morphology of the cohesive zone in the blast furnace, thereby influencing the furnace’s permeability and gas distribution. Consequently, researchers have investigated the softening and melting behaviors of blast furnace burden.13) Nogueira et al.14,15) examined the softening and melting behaviors of ferrous burden at different stages. Pan et al.16) investigated the impact of varying reduction degrees on the cohesive zone and permeability of mixed burden. The results indicated that as the degree of reduction increases, the cohesive zone becomes thinner and shifts downward, leading to improved permeability. Sunahara et al.17) analyzed the influence of coke reactivity on the softening and melting performance of sintered ore. An18) and Zhang19) studied the softening and melting behaviors of composite burden in oxygen blast furnaces. Yang et al.20) employed computational fluid dynamics (CFD) and discrete element method (DEM) simulations to model the softening and melting processes of multilayered burden. Numerous studies have demonstrated that under hydrogen-rich conditions, the initial melting temperature and dropping temperature of the burden increase, the melting zone narrows and shifts to higher temperatures, enhancing the permeability of the burden. The optimal hydrogen enrichment level is found to be 10%–15%.21,22,23,24,25,26)
In summary, there has been extensive research on the operational protocols and the distribution of thermal fields and heat losses within the blast furnace during blowing break and re-blowing processes, as well as on the softening and melting behavior of ferrous burden under steady-state furnace conditions. However, further in-depth studies are needed on the softening and melting properties of ferrous burden during the blowing break and re-blowing processes in blast furnaces. This paper first analyzes the softening and melting properties of ferrous burden under hydrogen-rich conditions in a blast furnace. Based on hydrogen-rich conditions, the softening-melting characteristics of the ferrous burden were studied under both partial and complete tuyere blowing break scenarios. The study examined the influence of different isothermal times on the softening-melting characteristic parameters, permeability, and dropping residues of the ferrous burden, providing a theoretical basis for blast furnace production.
The raw materials of this experiment were from an iron and steel plant. The raw materials were broken into 10–12.5 mm and dried at 120°C for 2 h. The chemical compositions of sinter, pellet, and lump ore are shown in Table 1. The industrial analysis, reactivity and post-reaction strength of the coke are shown in Table 2.
| Chemical compositions | TFe | FeO | SiO2 | Al2O3 | CaO | MgO |
|---|---|---|---|---|---|---|
| Pellet | 62.19 | 0.772 | 6.25 | 1.06 | 0.45 | 1.4 |
| Sinter | 55.44 | 7.69 | 4.51 | 2.27 | 7.91 | 1.01 |
| Lump ore | 63.99 | 29.21 | 3.14 | 2.83 | 1.85 | 0.28 |
| Name | Industrial analysis/% | CRI/% | CSR/% | ||
|---|---|---|---|---|---|
| C | Volatile | Ash | |||
| Coke | 80.5 | 7.85 | 14.73 | 40.3 | 45.2 |
Figure 1 shows a schematic diagram of the high-temperature furnace used in the experiment and the heating program. The gases used in the experiment had a purity of 99.99% and were mixed in the gas mixing cabinet in specified proportions before being introduced into the high-temperature furnace. Throughout the experiment, the temperature of the burden, the shrinkage rate, and the pressure differential were recorded using a computer. To simulate the burden conditions of a blast furnace, 500 g ferrous burden was randomly mixed (380 g of sinter, 60 g of pellets, and 60 g of lump ore) into a graphite crucible of 94×210 mm, and the burden layer thickness was measured and recorded. Additionally, 80 g of coke was placed beneath the burden and 40 g of coke was placed above it. The burden load during the experiment was set at 1 kg/cm2.
This study established six experimental schemes based on partial and complete tuyere blowing break scenarios in a blast furnace. Case 1 involved the standard softening and melting performance test of the ferrous burden, while Case 2 tested the softening and melting performance with a 10% hydrogen-rich. Cases 3 and 4 involved partial tuyere blowing break, while Cases 5 and 6 involved complete tuyere blowing break. The temperatures during the blowing break periods in Cases 3 to 6 were determined based on Case 2, ensuring that the ferrous burden remained in the softening-melting transition phase. The atmospheres for Case 3 and Case 4 were N2-4.8 L/min, CO-2.4 L/min, H2-0.8 L/min, and the atmospheres for Case 5 and Case 6 were N2-5.0 L/min. The experimental scheme is shown in Table 3. The experiment began by heating under an N2 (5 L/min) to 900°C, after which the gas was switched to a reducing gas (12 L/min) and the heating continued. The atmosphere was adjusted according to the experimental scheme after reaching the target isothermal temperature. After the isothermal phase ended, the gas was switched back to the reducing gas (12 L/min) and heating continued. Once the mass of the dripping material reached 20 g, heating was stopped, and the furnace was cooled to room temperature under an N2 (2.0 L/min). Finally, the slag in the dripping material was subjected to phase analysis using X-ray diffraction (XRD), and the carbon content in the metallic iron was measured using a carbon-sulfur analyzer.
| Case | 900°C heating atmosphere (L/min) | isothermal temperature (°C) | isothermal atmosphere (L/min) | isothermal time/min | ||||
|---|---|---|---|---|---|---|---|---|
| N2 | CO | H2 | N2 | CO | H2 | |||
| 1 | 8.4 | 3.6 | 0 | – | – | – | – | – |
| 2 | 7.2 | 3.6 | 1.2 | – | – | – | – | – |
| 3 | 7.2 | 3.6 | 1.2 | 1280 | 4.8 | 2.4 | 0.8 | 10 |
| 4 | 7.2 | 3.6 | 1.2 | 1280 | 4.8 | 2.4 | 0.8 | 30 |
| 5 | 7.2 | 3.6 | 1.2 | 1280 | 5.0 | 0 | 0 | 10 |
| 6 | 7.2 | 3.6 | 1.2 | 1280 | 5.0 | 0 | 0 | 30 |
To evaluate the softening and melting characteristics of the ferrous burden, this study defines several characteristic temperatures. T10 denotes the initial softening temperature at which the burden shrinks by 10%, and T40 denotes the final softening temperature at 40% shrinkage. Ts indicates the initial melting temperature at which the pressure differential across the burden layer begins to increase sharply. Td signifies the final melting temperature when iron begins to drip. ΔTS and ΔTM are defined as the softening range (T40-T10) and melting range (Td-Ts), respectively. ΔPmax refers to the maximum pressure drop. The comprehensive permeability of the burden layer is expressed using the permeability index (S), which can be calculated using Eq. (1).
| (1) |
where S is the permeability index (kPa·°C) and p is the pressure difference of furnace charge.
The softening behavior of the ferrous burden under hydrogen-rich conditions is shown in Fig. 2(a). As the H2 content increases from 0% to 10%, T10 rises from 1110°C to 1120°C, T40 increases from 1192°C to 1212°C, and ΔTS expands from 82°C to 92°C. It can be seen that hydrogen significantly influences the softening range of the burden. This is because H2 molecules are smaller and have a higher diffusion coefficient, which enhances the gas reduction capacity, thereby accelerating the reduction rate of the burden and raising the softening temperature. According to the FeO–SiO2–CaO ternary phase diagram (Fig. 3), the content FeO of influences the formation of primary slag. Under hydrogen-rich conditions, the mass of iron reduced from the initial FeO in the burden increases, leading to a decrease in the FeO content. This results in a gradual reduction of low-melting-point substances formed in the primary slag reaction,27,28) thereby promoting a gradual increase in T10 and T40. The melting behavior of the ferrous burden under hydrogen-rich conditions is shown in Fig. 2(b). As the H2 content increases from 0% to 10%, Ts rises from 1270°C to 1350°C, Td increases from 1415°C to 1459°C, and ΔTM decreased by 36°C. As the temperature of the ferrous burden increases and the degree of reduction intensifies, the FeO content further decreases, resulting in an increase in the Ts of the burden. However, this also leads to an increase in the melting point of the slag and a deterioration in its fluidity, hindering the dripping of molten iron and thereby raising the dropping temperature of the ferrous burden.25,29) The results indicate that under hydrogen-rich conditions, the onset temperature of melting and the dropping temperature of the burden increase, causing the melting range to narrow and shift towards higher temperatures. This shift improves the permeability of the burden and enhances smelting efficiency. However, if the slag’s melting point becomes too high, it can inhibit slag dripping, leading to an elevated melting point of the burden. Therefore, the hydrogen enrichment level should not be excessively high.30,31)
The impact of partial tuyere blowing break on the softening and melting characteristics of ferrous burden is shown in Fig. 4. It can be observed that the isothermal temperature lies within the softening-melting transition region, and the reduction conditions do not change before the temperature is 1280°C. Therefore, the T10 and T40 temperatures of the ferrous burden remain essentially unchanged compared to the control group, Case 2. During partial tuyere blowing break, Ts gradually increases with extended isothermal holding time, rising from 1350°C to 1371°C, while Td gradually decreases from 1459°C to 1430°C, and ΔTM decreases from 109°C to 50°C. Compared to the control group (Case 2), the reduction degree of ferrous burden during the isothermal phase was significantly higher, leading to a further decrease in FeO content and the formation of a substantial amount of metallic Fe, thereby causing the increase in Ts.
The effect complete tuyere blowing break on the softening and melting characteristics of the ferrous burden is shown in Fig. 5. The figure indicates that the isothermal temperature is within the softening-melting transition region, and the reduction conditions unchanged before the temperature is 1280°C, resulting in essentially unchanged T10 and T40 values for the ferrous burden. During complete tuyere blowing break, Ts decreases slightly compared to the control group, Case 2, dropping from 1350°C to 1345°C. The reduction of the ferrous burden during the isothermal stage relies solely on coke, leading to a reduced reduction rate and an increased shrinkage rate of the burden, which in turn causes Ts to decrease. The Td increases with longer isothermal holding time, rising from 1459°C to 1478°C, while ΔTM increases from 109°C to 133°C. Research indicates that the dropping temperature of the ferrous burden is influenced by the carburization of metallic iron and the concentration of molten iron phases. The carburization process of metallic iron primarily involves CO carburization in the granular zone and coke carburization in the melting zone.26,32) During the isothermal stage, the carburization of metallic iron relies solely on coke, resulting in a lower carbon content in the metallic iron compared to Cases 3 and 4. This leads to an increase in the dropping temperature, a broader melting range, and a shift towards higher temperatures, consistent with blast furnace production results following blowing break and re-blowing processes.10) Consequently, during blast furnace operations, complete tuyere blowing break followed by re-blowing can easily cause burden collapse or slipping, with longer blowing break durations increasing the likelihood of such occurrences.
When the partial tuyere blowing break, the ΔPmax of the ferrous burden layer at different isothermal times is shown in Fig. 6(a). As illustrated, ΔPmax initially decreases from 10.3 kPa to 5.1 kPa and then increases to 9.6 kPa as the isothermal time extends.
According to Eq. (1), S is determined by the melting temperature, dropping temperature, and pressure differential. A larger S value indicates poorer permeability of the burden layer, while a smaller S value indicates better permeability. During partial tuyere blowing break, the S values at different isothermal times are shown in Fig. 6(b). The figure demonstrates that the S value initially decreases from 1016.5 kPa·°C to 526.6 kPa·°C and then increases to 727.5 kPa·°C, mirroring the trend observed for ΔPmax.
Compared to Case 2, Case 3 shows that at lower temperatures, iron oxides are already reduced to metallic Fe, preventing the premature formation of low-melting-point materials. This effectively improves the permeability of the blast furnace cohesive zone, resulting in reduced ΔPmax and S values. In Case 4, as compared to Case 3, the prolonged isothermal time increases the shrinkage rate of the iron-bearing burden, causing the pores within the burden to become clogged, thereby reducing permeability. This leads to an increase in ΔPmax and S values, though they still remain lower than those in Case 2. Therefore, when partial tuyere of the blast furnace is blowing break, the state of the burden does not affect the smooth operation of the blast furnace.
3.3.2. Maximum Pressure Difference and Permeability during Complete Tuyere Blowing BreakWhen the complete tuyere blowing break, the ΔPmax of the ferrous burden at different isothermal holding times is shown in Fig. 7(a). The figure indicates that ΔPmax increases with longer isothermal times, rising from 10.3 kPa to 18.9 kPa. Similarly, the S values as shown in Fig. 7(b), also increase with isothermal time, from 1016.5 kPa·°C to 2292.9 kPa·°C.
Fan et al.33) demonstrated that above 1200°C, liquid phases begin to form within the slag phase. However, the viscosity of the slag remains relatively high, with increased viscosity correlating with decreased FeO content. The influence of FeO on viscosity is particularly pronounced when its content changes within a low range. In Cases 5 and 6, compared to Case 2, the reduction of ferrous burden during the isothermal phase relies solely on coke, leading to a significantly slower reduction rate. Nonetheless, a portion of iron oxides is still reduced to metallic iron at lower temperatures. As the isothermal time increases, the FeO content gradually decreases, resulting in an increasing amount of liquid phase and a corresponding rise in viscosity. This increase in viscosity leads to higher ΔPmax and S values, indicating a deterioration in the permeability of the burden layer. Consequently, fluctuations in furnace conditions are more likely during the re-blowing process in practical operations.
3.4. Effect of Tuyere Blowing Break on Dropping Residues 3.4.1. The Dropping Residues during Partial Tuyere Blowing BreakThe dropping residues of the ferrous burden under different isothermal times are shown Fig. 8. The portions with a metallic luster represent metallic iron, while the slag phase, encircled with a red dashed line, exhibits a rock-like or glassy appearance. The figure indicates that, when partial tuyere blowing break, the slag phase on the surface of the dripping materials in Cases 3 and 4 continuously increases and is significantly larger in area compared to Case 2. When the isothermal time reaches 30 minutes, the dripping slag appears in sheet-like forms. The results suggest that with longer isothermal times, the slag becomes more prone to dripping. This phenomenon occurs because the melting temperature and dropping temperature of the ferrous burden depend on the melting points of the slag and iron, respectively. With the increase of isothermal time, the carbon content of metal iron gradually increases, resulting in metal iron taking the lead in dropping, while the decrease of FeO content results in the melting point of the slag increasing, and the difference between the temperature of dropping and that of the metal iron becomes larger. When a large amount of metallic iron drops, the space within the charge is released in large quantities, and the slag is more likely to gather, forming a situation in which the drops of the slag phase appear as flakes.
The slag from different isothermal times was separated from the droppings and broken into less than 74 μm for XRD analysis. The results are shown in Fig. 9. It can be observed that the slags in Case 2, Case 3, and Case 4 primarily consist of merwinite (3CaO·MgO·2SiO2, melting point of 1570°C), dicalcium silicate (β-2CaO·SiO2, melting point of 1425°C), and gehlenite (Ca2Al2SiO2, melting point of 1593°C). The content of these three phases gradually increases with longer isothermal times.
Table 4 presents the content of carbon in the iron of the dripping materials for different cases. As shown in the table, the carbon contents for Case 2, Case 3, and Case 4 are 1.91%, 2.56%, and 2.97%, respectively. The carbon content in the iron increases with the extension of the isothermal time. The ferrous burden remains lumpy at isothermal temperature, only deformation is produced and it does not melt. At this time, the carburizing method is mainly CO carburizing (Eq. (2)). The longer the isothermal time, the greater the amount of carburization. Additionally, at lower temperatures, the iron oxides in the iron-bearing burden are reduced to metallic iron, leading to a decrease in the FeO content in the slag. This reduction also reduces the amount of carbon consumed in the decarburization reactions at the slag-iron interface (Eq. (3)).34) As a result, the carbon content in the molten iron increases, lowering the dropping temperature.
| (2) |
| Case | 2 | 3 | 4 | 5 | 6 |
|---|---|---|---|---|---|
| C | 1.91 | 2.56 | 2.97 | 2.16 | 2.01 |
Where CO is in the reducing gas and [C] is in the molten iron.
| (3) |
Where (FeO) is in the liquid slag and [C] is in the molten iron.
3.4.2. The Dropping Residues during Complete Tuyere Blowing BreakWhen the complete tuyere blowing break, the appearance of the dripping residues from the ferrous burden at different isothermal times, as shown in Fig. 10, includes regions with a metallic luster indicative of metallic iron, while the slag phases exhibit a rock-like or glassy texture, delineated with red dashed lines. The figure indicates that the slag phase on the surface of the dripping materials in Cases 5 and 6 continuously decreases and is significantly smaller in area compared to Case 2. At an isothermal time of 30 minutes, only a minimal amount of slag phase is observed on the surface of the dripping materials. The results suggest that as the isothermal time increases, the difficulty of slag dripping also increases, which is unfavorable for blast furnace smelting.
The slag from different isothermal times was separated from the droppings and broken into powder less than 74 μm for XRD analysis. The results are shown in Fig. 11. It is evident that, compared to Case 2, the slag in Case 5 and Case 6 also contains magnesium silicate (MgSiO3, melting point 1890°C).35) The analytical results indicate that, with increasing isothermal time, high-melting-point oxides gradually formed in the slag, leading to an increased slag melting point and a higher iron dropping temperature.
As shown in Table 4, the carbon contents in Cases 2, 5, and 6 are 1.91%, 2.16%, and 2.01%, respectively. The carbon content gradually increases with longer isothermal times. Compared to Case 2, the carburization reactions at the coke interface with slag and iron during the isothermal stage (Eq. (4)) result in higher carbon contents in Cases 5 and 6 than in Case 2. However, the carbon contents in Cases 5 and 6 are lower than those in Cases 3 and 4. The dropping temperature of the ferrous burden continuously increases, indicating that under an N2 atmosphere, the increase in high-melting-point substances in the slag due to prolonged isothermal times has a greater impact on the dropping temperature of the iron-bearing burden than the effect of increased carbon content in iron, which would lower the melting temperature.
| (4) |
Where C is in the coke and [C] is in the iron carbon.
This study analyzed the impact of hydrogen-rich on the cohesive zone in the blast furnace. Subsequently, simulations of the ferrous burden under conditions of partial and complete tuyere blowing break in a hydrogen-rich blast furnace were conducted. The conclusions drawn are as follows:
(1) When the hydrogen content increases from 0% to 10%, the amount of metallic iron reduced from the ferrous burden at lower temperatures increases. This results in a higher softening start temperature and a wider softening range for the burden. The reduction in FeO content leads to an increase in the slag melting point, raising both the melting start temperature and the dropping temperature. Consequently, the melting range narrows and shifts to a higher temperature region, significantly improving the permeability of the blast furnace.
(2) During partial tuyere blowing break, the melting start temperature of ferrous burden increases with extended isothermal time, rising from 1350°C to 1371°C. Conversely, the dropping temperature decreases progressively from 1459°C to 1430°C, resulting in a narrower melting range. Additionally, at lower temperatures, iron oxides were reduced to metallic iron, which facilitated carburization. This led to a reduction in the dropping temperature, maximum pressure differential, and droplet characteristic index of the burden. Consequently, partial tuyere blowing break has a minimal impact on blast furnace operations.
(3) During complete blowing break, the dropping temperature of the ferrous burden increased progressively with the duration of isothermal holding, rising from 1459°C to 1478°C, and the melting interval widened. During the isothermal stage, carburization of the metallic iron relied solely on coke, resulting in a lower carbon content in the metallic iron compared to partial tuyere blowing break. This led to an increase in dropping temperature, maximum pressure differential, and the permeability index. Therefore, in the actual production process, the furnace condition is easy to fluctuate during the re-blowing.
This work was supported by National Natural Science Foundation of China (U1960205), China Baowu Low Carbon Metallurgical Innovation Foundation (BWLCF202101), China Minmetals Science and Technology Special Plan Foundation (2020ZXA01) and Overseas Expertise Collaboration Base for Green and Intelligent Metallurgy (B21004).
On behalf of all authors, the corresponding author states that there is no conflict of interest.
