2025 Volume 65 Issue 6 Pages 739-748
It has been reported that an increasing H2 gas ratio of the reducing gas in the blast furnace promotes the low temperature reduction disintegration of the iron ore pellet. Reduction of the pellet sample proceeds uniformly under higher H2 condition at 500°C. Microcracks with the size of several micrometer form at the primary particles of iron oxide and it promote fine particles formation after the drum test. Macrocracks with the size of several millimeter form inside of the pellet after reduction and it promotes the volumetric destruction. In this study, the temperature dependence of reduction disintegration of self-fluxing pellet under higher H2 condition was examined.
Pellet sample was reduced under CO and CO–H2 gas conditions at 600°C and 700°C. After reduction, the disintegration test was conducted using the drum. Under higher H2 condition at 600°C, the density of microcracks decreases with increasing reduction degree and it leads to lower of the RDI value than that at 500°C. The reason is that the volumetric expansion by the reduction from hematite to magnetite at 600°C is not significant compared with that at 500°C. The difference of reduction degree at the center and near the surface of the pellet increases with increasing the reduction temperature. Reduction reaction proceeds more topochemically by increasing reduction temperature and the addition of hydrogen. This change leads the macrocrack formation, which is same mechanism under CO gas condition at 500°C. At 700°C, on the other hand, this microcrack was not observed because the volumetric expansion by the reduction is lower than that at lower temperature. Therefore, the effect of hydrogen addition on disintegration is not significant at 700°C.
To take urgent action to combat climate change and its impact is one of SDGs, and reduction in CO2 emission is its global mission. In Japan, the iron and steelmaking industry accounts for approximately 14% of total CO2 emissions.1) In particular, CO2 emission from the blast furnace ironmaking process accounts for more than half in the steel industry.2) Therefore, its reduction from blast furnace is urgent issue. Currently, Technological development to reduce 30% of domestic CO2 emissions through the development of an innovative ironmaking process (COURSE50), which focuses on H2 reduction and CO2 capture technology3) was carried out in Japan. In this project, 10% reduction of CO2 emissions from blast furnaces by replacing direct reduction with H2 reduction. Therefore, it will be more important to study the hydrogen reduction.
According to the estimation of temperature distribution in blast furnace by Nogami et al.4) using mathematical model, it is reported that the low temperature region from 400°C to 600°C in blast furnace expands with increasing hydrogen concentration. The disintegration of iron ore pellet accelerated under hydrogen enriched blast furnace condition at 500°C is reported.5,6,7) The gas permeability in the shaft region of the blast furnace deteriorates with an increase in the amount of particle size of 5 mm or less. This phenomenon results in the abnormality of the burden descent.8) Therefore, the permeability in blast furnace is an important factor for stable operation.9,10,11,12,13) Furthermore, reduction disintegration can be a major issue for hydrogen enriched blast furnace operation.14)
In the previous report,5) the disintegration mechanism of self-fluxed pellet at 500°C was examined by observing the cross section and CT images of the sample before and after reduction by CO and CO–H2 gases. It is clarified that the formation of fine cracks called microcracks. Microcrack was defined as the crack width is smaller than 5 μm. The formation of microcracks leads to an increase in the formation amount of fine powder with the size of smaller than 0.25 mm. The cracks with the length and width were longer than 0.5 mm and 10 μm, respectively, which is called macrocracks, form by not only the stress difference between near the surface and the center but also combining formed microcracks under CO–H2 condition. It indicates that macrocracks lead to the structural destruction of the pellet. On the other hand, it is important to clarify the effect of the reduction temperature on the reduction disintegration mechanism of the pellet in the hydrogen enriched blast furnace. In this study, therefore, the temperature dependence of reduction disintegration mechanism of the pellet is examined.
The self-fluxing pellet produced in the commercial plant was used in this study. Its chemical composition is listed in Table 1. The pellet particles were sieved to the grain size of 9.5–11.1 mm. The primary particle size of the pellet is less than several hundred micrometers based on the images of microstructure of the pellet. The value of Reduction Disintegration Index (JIS-RDI) that the pellet was reduced at 550°C for 1.8 ks under N2-30%CO gas was 2.6%.
| T.Fe | FeO | CaO | SiO2 | Al2O3 | MgO |
|---|---|---|---|---|---|
| 66.90 | 0.27 | 1.64 | 1.71 | 0.34 | 0.50 |
Figure 1 shows the schematic diagram of the reduction experimental apparatus used in this study.15) The same reduction furnace was used as in the previous report.5)
Alumina balls of 100 ± 1.0 g with the particle size of 10 mm was inserted in the sample holder. Subsequently, the pellet sample with the total weight of 100 ± 1.0 g were randomly picked up and inserted in the sample holder. The sample was heated up to 600°C and 700°C in a N2 gas flow with 13 NL/min and kept for 20 min. Subsequently, the gas was changed to a reducing gas with the same flow rate and the reduction experiment was carried out for selected time.
Table 2 shows the reducing gas composition. Two types of the reducing gas composition were used: N2-20%CO-20%CO2 (0%H2), and N2-12%CO-8%H2-x%CO2-y%H2O (8%H2). The x and y values were determined by the equilibrium conditions of the water–gas–shift reaction expressed in Eq. (1) at reduction temperature. After the reduction experiment, the reducing gas was changed to N2 again, and then the reduced sample was cooled down below 200°C.
| (1) |
| N2 | H2 | CO | H2O | CO2 | |
|---|---|---|---|---|---|
| 0%H2 | 60 | 0 | 20 | 0 | 20 |
| 8%H2-600°C | 60 | 8.0 | 12 | 4.0 | 16 |
| 8%H2-700°C | 60 | 8.0 | 12 | 5.8 | 14.2 |
The reduction degree was calculated from the weight change of the sample before and after reduction using Eq. (2)
| (2) |
where W0 and W1 are the weights of the sample before and after reduction, respectively. WS is the weight of oxygen coursed by iron oxide in sample.
The disintegration test was conducted based on ISO 4696-2. Reduced sample was charged into a tumbling drum with an inner diameter of 130 mm and rotated for 1.8 ks with 30 rpm. After that, the sample was sieved with 2.8 mm and determined the weight of the sample. The RDI value was calculated using the following Eq. (3)
| (3) |
where W2 and W3 are the weights of the sample after the reduction experiment and that of particles smaller than 2.8 mm respectively. After the drum test, the samples were sieved to the particle size of −0.25 mm, 0.25–2.8 mm, 2.8–8.0 mm, and +8.0 mm and the particle size distribution was obtained. The particles which sizes of −0.25 mm and 2.8–8.0 mm were defined as fine powder and intermediate grain, respectively. Furthermore, the drum test of the sample reduced at 600°C was carried out for not only 1.8 ks but also 0.6 ks and 1.2 ks.
X-Ray Diffraction (XRD) was carried out to identify the mineral composition of samples before and after reduction, using the Fe anode target with the wavelength of 0.194 nm (scan step: 0.01 deg, diffraction speed: 3.0 deg/s).
Brightness reduction degree (BRD) and calculated according to Eq. (4) in same method in previous report.5) It is confirmed that pellet reduced at 600°C is also consists of mainly hematite and magnetite.
| (4) |
The density of macrocracks and microcracks were calculated from the same method in previous report.5) Microcrack was observed at the area within 0.5 mm from the sample surface by SEM. Macrocrack was determined by inner structure observation using CT images which was obtained using the BL28B2 beamline at Spring-8 in Japan. Comparing CT images of same sample before and after reduction, the propagated cracks during the reduction were observed. The macrocrack existed before reduction is defined as “initial macrocrack”.
The relationship between the RDI value of the pellet reduced at 500°C5) and 600°C and reduction degree is shown in Fig. 3. The solid and dotted lines indicate the RDI values at 600°C and 500°C, respectively. “Unreduced” data means the value of the pellet only for the drum test without heating and reduction. RDI shows the constant value as 4% until reduction degree reaches to about 4% and 2% under 0%H2 and 8%H2 conditions, respectively. After further reduction, the RDI value increases parabolically under both gas conditions. However, their behaviors are different at 500°C and 600°C because the distribution of reduction degree inside of the pellet and that of the generated stress are different. RDI value increases drastically again when reduction degree reaches to about 6% under 8%H2 condition whose reduction time is 600 s. It is caused by the formation of intermediate grains described after. Although RDI value of 8%H2 at 500°C continues to increase until reduction degree reaches about 11%, where the reduction from hematite to magnetite almost finishes, the value at 600°C is saturated to 23% after reduction degree reaches 6.5%. Maximum RDI value decreases as reduction temperature increases. Focused on reduction degree of 11%, the RDI values are 28% and 23% at 500°C and 600°C. The latter value is about 5% lower than the former on. This result corresponds to the previous study that the temperature dependence can be recognized on RDI value under hydrogen reduction condition.6)
The change in the particle size distribution of the pellet reduced by 0%H2 and 8%H2 after the drum test for 1.8 ks is shown in Fig. 4. Average reduction degree and its dispersion of several samples are indicated in the parentheses. The weight ratio of fine powder tends to increase as reduction time increases. The particles with the size of smaller than 2.8 mm, which affects to the RDI value, is accounted by fine powder with less than 0.25 mm. The formation of fine powder accelerates under 8%H2 condition, compared with that under 0%H2 condition. On the other hand, the weight ratio of intermediate grains at 600°C is higher than that at 500°C.5) Compared at the reduction time in 11% of reduction degree under 8%H2 condition, the weight ratio of intermediate grains at 600°C in the reduction time of 900 s is approximately 7%, which is about twice higher than that at 500°C, which is approximately 4% at the reduction time of 3.6 ks. The weight ratio of intermediate grains under 0%H2 condition at 600°C in the reduction time of 900 s (reduction degree is 8%) is 8.5%. It is estimated that the volumetric destruction of the pellet proceeds regardless of hydrogen addition. This is a different tendency with reduction at 500°C.
In Fig. 3, some samples show the different RDI value, especially under 0% condition. Sample A and B have low and high RDI values, respectively in Fig. 3. All pellets used disintegration test of point A were scanned by X-ray CT. The number of pellets with the initial macrocrack was 10 in 29, which was about 34%. On the other hand, the average ratio of this pellet is approximately 43% in this study. It is estimated that the sample A had smaller number of such pellet with initial macrocrack. The weight ratio of intermediate grains of sample A was 1.9%. It is smaller than 6.2%, which is the average value of the pellet with similar reduction degree. It means that the formation of intermediate grains was suppressed because the number of the pellet with initial macrocrack in sample A is smaller. Furthermore, it results in the suppression of fine powder formation. On the other hand, the weight ratio of intermediate grains of sample B was not so different with the average ratio of almost same reduction degree. The weight ratio of the particle size of 0.25–2.8 mm after drum test was about twice as high as the average ratio. It indicates that the number of the pellet with initial macrocrack in sample B is larger than that of the average although there is no CT image of sample B before reduction. Therefore, it is estimated that the reason of the large difference of the RDI value is caused by the number of the pellet with the initial macrocrack. The dispersion of RDI value under 8%H2 condition is also significant at the reduction degree range of 4–5%, where fine powder formation accelerates by intermediate grains formation. It also may be due to the initial macrocrack. Therefore, the initial condition of the pellet should be considered when the RDI and reduction behavior of the pellet is discussed.
Figure 5 shows the change in reduction degree of the pellet reduced at 600°C under 0%H2 and 8%H2 conditions with time. The RDI values of both samples are not significantly different with other samples with same reduction conditions. Reduction degree was calculated based on the weight change measured using a balance installed in the furnace as shown in Fig. 1. “0 s” means the time when the gas was changed to reducing one. At early stage, reduction degree increases faster under 0%H2 condition than 8%H2 condition. This is the same trend with that at 500°C. Under both conditions, reduction degree reached approximately 11% after reduction for 1.8 ks. It implies that reduction from hematite to magnetite is completed. On the other hand, although the reducing gas has a potential to reduce to wustite, reduction degree did not increase over 11%. Furthermore, XRD profiles of the pellet before and after reduction at 600°C for 3.6 ks under 8%H2 gas condition (R.D.: 10.7%) is shown in Fig. 6. Major compound in the pellet before reduction is only hematite. After reduction, all hematite reduced to magnetite and no wustite phase is detected. This also corresponds that reduction degree of sample reduced for 3.6 ks at 600°C reaches 11%. Based on the above results, it can be concluded that the main phases in the pellet reduced at 600°C were hematite and magnetite.
The ratio of these phases was calculated from the distribution of BRD. Figure 7 shows the distribution of BRD of the pellet reduced for 0.6 ks. The value of reduction degree by BRD does not correspond to the total one calculated by the weight change of sample of about 100 g because BRD is calculated by observing cross section of one pellet particle. Under both gas conditions, BRD is higher near the surface than that at the center of the pellet. It is implied that reduction proceeds topochemically at 600°C although BRD showed the constant value in the sample reduced under 8%H2 condition at 500°C.5) It is estimated that the diffusion is one of the rates limiting step at 600°C. The topochemical reaction means that there is stress difference between the surface and the center of pellet5) under both gas condition at 600°C.
The reason why the RDI value of 8%H2 reduction at 600°C is lower than that at 500°C in high reduction degree is as follows. At 600°C, reduction proceeds topochemically in the pellet, and reduction of hematite primary particles near the surface is completed faster. At 500°C, on the other hand, reduction of hematite particles continues until higher reduction degree. This may affect the disintegration of the pellet.
SEM image of near the surface of pellet reduced under 0%H2 condition and 8%H2 condition for 3.6 ks at 500°C5) and 600°C is shown in Fig. 8. Microcracks which can be recognized in the matrix are indicated by white arrows in the figure. At both temperatures, more microcracks form in the pellet reduced under 8%H2 condition than 0%H2 condition.
Figure 9 shows the change in micro cracks density (Pmicro) with reduction time at 600°C. Microcrack does not form in eary stage of reduction. It is considered that a certain stress is accumulated after further reduction proceeds, then microcracks form.5) The approximation curve of Pmicro was extrapolated considering the timing that the RDI value starts to increase and the tendency of RDI increasing. It is estimated that Pmicro reaches maximum value after reduced for 1.8 ks because the reduction from hematite to magnetite is completed. Compared with the results at 500°C, the value of Pmicro is 1/6 and 1/4 under 8%H2 and 0%H2 conditions, respectively. It is known that the volumetric expansion due to reduction from hematite to magnetite is most significant at 525°C.10) It is considered that a decrease in stress generation by increasing reduction temperature attributes to decreasing in Pmicro. Figure 10 shows the effect of microcracks density on the weight ratio of fine powder at 500°C5) and 600°C. For comparison, the results at 500°C is also shown. As same as at 500°C, the weight ratio of fine powder increases as microcrack density increases under both reducing gas condition. The slope of this relation is almost same at same reduction temperature under both gas condition. The increase in surface area resulting from the formation of intermediate grains contributes to fine powder generation. Therefore, the ratio of fine powder varies even when the microcrack density remains the same. The ratio of fine powder increases at lower microcracks density at 600°C because the region near the surface is reduced more quickly and the area with higher microcracks density is localized near the surface. CT images of the same pellets before and after reduction under 0%H2 and 8%H2 for 0.3 ks at 600°C are shown in Fig. 11. Any macrocracks cannot be recognized in the pellet before reduction. On the other hand, several macrocracks can be recognized in the sample after reduction under both gas conditions. However, there is a few macrocracks with thick and long in the sample reduced under 8%H2. Figure 12 shows the change in macrocrack density with reduction degree at 600°C. Macrocrack density of the pellet reduced by 8%H2 is higher than that by 0%H2. It is different from the results at 500°C. Macrocrack density increases as reduction proceeds under 8%H2 condition. It indicates that the formation of intermediate grains is accelerated as shown in Fig. 4.
Figure 13 shows the change in the RDI value of the pellet with reduction degree at 700°C. The approximative curve does not include the origin because reduction proceeds faster at this temperature, and reduction degree of the pellet is considered to be over 5% when the pellet reaches to the region at 700°C in BF. The significant difference cannot be recognized between the results of 0%H2 and 8%H2. The RDI value increases as reduction degree increases, then the increase in the RDI value is retarded at 20% after reduction degree reaches over 10%.
Figure 14 shows the particle size distribution of the pellet reduced for a certain time periods at 700°C. Average reduction degree and its dispersion of several samples used for drum test is shown in the parentheses. For 0.3 ks reduction, there is the difference of the weight ratio of fine powder between two gas conditions. It can be explained by the difference of the RDI value although reduction degree is similar. The weight ratio of intermediate grains is also different. On the other hand, the particle size distribution of the sample reduced for 0.6 ks is almost same under both gas condition. The weight ratio of intermediate grains in the reduction degree of about 8% at 700°C is higher than those at 600°C as shown in Fig. 4. It implies that more volume destruction occurs at 700°C.
Figure 15 shows the change in reduction degree of the pellet with time at 700°C under both gas conditions. 0 s is the time when the gas was changed to the reducing one. As same as the results at other temperatures, reduction degree of 0%H2 starts to increase faster than that of 8%H2. That of 8%H2 becomes larger than that of 0%H2 after reduction for 300 s. This is the reason of the difference of the weight ratio of intermediate grains which can be recognized in Fig. 14. Reduction degree continues to increase after reduction degree reaches 11%. It implies the formation of other phases. Then, XRD profiles of the pellet before and after reduction at 700°C for 3.6 ks under 8%H2 gas condition is shown in Fig. 16. Not only magnetite but also wusitite is detected in the pellet after reduction.
SEM image near the surface of the pellet reduced under both gas conditions for 0.6 ks at 700°C is shown in Fig. 17. Few microcrack can be recognized unlike the sample reduced at 500°C and 600°C. The CT images before and after reduction for 0.6 ks at 700°C is shown in Fig. 18. Initial macrocracks which indicate by white arrows are observed in both pellets before reduction. However, not only the growth of initial macrocracks but also new macrocrack formation by reduction, which are indicated in CT images by white arrows are observed. Many initial macrocrack grows. Therefore, initial condition of the pellet is also important for reduction at 700°C. These initial macrocracks lead dispersion of RDI because the intermediate grains formation can accelerate fine powder formation.5) Furthermore, new macrocrack formation also leads to the intermediate grain formation. For 0.6 ks reduction at 700°C, there is no wustite formation. Therefore, it is considered that these macrocracks form by the stress difference between the surface and the center of the pellet. This macrocrack formation may lead to the increase in intermediate grains formation.
Figure 19 shows conceptual images of the effect of reduction temperature on macrocrack formation mechanism. As shown in the results of BRD, reduction of the pellet proceeds more topochemically as reduction temperature increases. Therefore, the stress difference inside the pellet increases as reduction temperature increases, then macrocracks form frequently. It is considered that more macrocracks form as reduction temperature increases, and the volumetric destruction accelerates under 8%H2 condition. It results in increasing intermediate grains formation.
Figure 20 shows conceptual images of the effect of reduction temperature on microcrack formation mechanism. The volumetric expansion due to reduction from hematite to magnetite is most significant at 525°C,16) and the stress generated by reduction is considered as reduction temperature increases. Therefore, it is considered that the generated stress in the primary particle decreases as reduction temperature increases, and microcrack formation is suppressed.
The formations of macrocracks and microcracks lead to the formation of intermediate grains and fine powder, respectively. Therefore, it is considered that the disintegration of the pellet can be clarified by identifying the factor of these crack formations.
In this study, reduction and drum tests were conducted, and the cross section of the pellet sample was observed by CT scanning, SEM, and optical microscopy. The effect of reduction temperature on reduction disintegration mechanism of the basic pellet reduced by the gas with and without hydrogen was examined. The following results were obtained.
(1) Although more microcracks generate by H2 addition, microcracks generation decrease as reduction temperature increase. The stress by the volume expansion decreases as reduction temperature increases. It leads to decreasing the formation number of microcracks. Then, the weight ratio of fine powder and the RDI value decrease as reduction temperature increases.
(2) Reduction proceeds more topochemically with increasing reduction temperature regardless H2 addition. It leads to increasing the difference of reduction degree between the surface and the center of the pellet. Therefore, the number of macrocrack formation tends to increase as reduction temperature increases to 600°C.
(3) The average ratio of this basic pellet with the initial macrocracks is approximately 40%. The formation of intermediate grains and fine powder is suppressed when the ratio of the pellet with initial macrocracks is lower than average. Therefore, the initial condition of the pellet is important for reduction disintegration.
(4) At 700°C, change in the RDI value by both 0%H2 and 8%H2 reduction has same trend. The RDI value shows the constant as approximately 20% at the higher reduction degree. It is almost same value as the pellet reduced under the gas with hydrogen for long time at lower temperature. The effect of H2 addition is not significant at 700°C.
The authors declare no conflicts of interest associated with this manuscript.
CT imaging was conducted at the BL28B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2022B1121).
This work is supported by ISIJ research Group “Lumpy zone control for next generation hydrogen enriched blast furnace”.
