Ironmaking
Comprehensive Research about Critical Interaction Region Named Cohesive Zone in Series of Dissected Blast Furnaces
2021 Volume 61 Issue 6 Pages 1758-1767
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2021 Volume 61 Issue 6 Pages 1758-1767
Ironmaking industry occupies significant responsibility of resource-saving and environment-protecting in the national economy. As the largest monomer smelting unit in ironmaking process, the efficiency of blast furnace can directly determine the consumption of energy. Researchers have conducted vast investigations about cohesive zone due to its energy redistribution and interaction-concentrated region role in blast furnace. Amidst this backdrop, the behavior of cohesive zone obtained through the most direct mothed, the dissection investigation, is introduced in this article, including blast furnaces from Japan, European countries and China. Moreover, a brief introduction about the way to cool down working blast furnace is also covered. The relationship between operation and macro-profile of cohesive zone, the behavior of various burden in cohesive zone and the effect of harmful elements on the softening & melting properties were mostly conducted in Japanese dissected blast furnace. The charge properties in cohesive zone were investigated through German Mannesmann No. 5 dissected blast furnace and Swedish 8.2 m3 dissected experimental blast furnace. China had conducted three dissection research including 23 m3 Shougang blast furnace, 0.8 m3 Pangang blast furnace and 125 m3 Laigang blast furnace, while a damage investigation in 1050 m3 blast furnace was also selected. The existing studies witness the recognition process of cohesive zone in several European countries, as well as Japan and China, reflecting the development of ironmaking technology.
As the backbone of the national economy, the iron & steel industry shoulders the important task of saving resources and reducing energy consumption.1) Among which, the energy consumption of ironmaking system occupies a large proportion.2) Hence the efficiency and longevity of blast furnace has a significant impact on the overall economic benefits.3) Maintaining the stability of the blast furnace status is the primary target as well as the fundament of high-efficiency and low-consumption operation. The distribution of gas flow is the comprehensive reflection of various operational indexes in blast furnace. It proved that the cohesive zone is the concentrated area of pressure loss inside the blast furnace, which can occupy more than 60% of the total pressure loss.4)
Since the first cohesive zone detected in the dissection of No. 5 blast furnace in Tobata, Japan and No. 1 blast furnace in the former Soviet Union, researchers have continuously deepened their understandings of the cohesive zone by means of sampling, testing, experiments and simulations. Scholars obtained cohesive zone samples through blast furnace dissection to clarify the basic characteristics,5) and then carried out experiments to simulate the working conditions of blast furnace.6,7,8) Numerical mathematical model gradually become another tool to investigate cohesive zone with the development of technology.9,10)
The formation of cohesive zone is governed by the temperature, pressure, softening & melting properties, et al. Liu11) had summarized current experimental methods used to evaluate the softening & melting behavior of burden, including experiment equipment and scheme. Plenty of mathematical models have been developed to describe the behavior of fluid flow heat and mass transfer in cohesive zone.12,13,14,15,16,17,18) However, among the variable research methods, the blast furnace dissection is the most straightforward way to study cohesive zone. Through sampling from the dissection blast furnace, the actual information about cohesive zone can be clarified, which then would provide reference for the experimental and mathematical simulations.
The first detection of cohesive zone improved that some reduced charge consolidated together to formulate an interval of softening and melting. Japan took advantage of the renewing and technology innovation of ironmaking industry to carry out dissection research on several blast furnaces during the late 1960s to the 1980s.5,19,20,21,22,23,24,25,26,27) The dissection results showed that the main shape of cohesive zone was “V-type”, “W-type” and “inverted V-type”. Then Germany and Sweden had conducted blast furnace dissection in 1981 and 2001, respectively.28,29) The first dissection of Chinese blast furnace was a 23 m3 experimental blast furnace in Shougang Group.30,31,32) Then a 0.8 m3 experimental blast furnace for vanadium-titanium magnetite smelting was dissected at Panzhihua Iron and Steel Co., LTD., which filled the blank of the internal smelting status of vanadium-titanium magnetite smelting furnace in the world at that time.33) The latest dissection happened on a 125 m3 blast furnace of Laigang Group.34) Besides the aforementioned blast furnace dissection surveys, the damage investigation about hearth has gradually become a new way to study the internal status of blast furnace.35,36) However, it is difficult to obtain ideal information about the cohesive zone during damage survey as a result of the measures were taken to lower burden surface before shutting down the furnace.
Proper control of cohesive zone is the key to determining the sustainable operation and low-carbon smelting of blast furnace, and it is of great significance for energy saving and emission reduction of ironmaking industry. In recent years, driven by the goal of reducing carbon emissions in iron industry, the raw material structure and smelting characteristics of blast furnace have undergone major changes, such as the application of high-proportion pellets, pure oxygen, and low-grade ore. Hence mastering the optimal control of the cohesive zone under different operation conditions is an important subject facing the current ironmaking industry.
The purpose of this paper is to provide a current review of cohesive zone in dissection blast furnace. As the one of the necessary means to deeply understanding the cohesive zone, Japan, German, Sweden and China had conducted several typical blast furnace dissection researches. The way to shut down blast furnace before dissection is comprehensively described in section 2. As the country with the most dissected blast furnace in the world, section 3 has summarized the relationship between operation and the type of cohesive zone, the behavior of feeding and the effect of harmful element in the cohesive zone. Section 4 describes two dissected blast furnaces in Europe and section 5 depicted characteristics of cohesive zone in several blast furnaces in detail, including a 23 m3 blast furnace, an experimental blast furnace for smelting vanadium-titanium magnetite ores with the inner volume of 0.8 m3, a 125 m3 blast furnace and a 1050 m3 blast furnace.
For sampling from the blast furnace safely, cooling down the furnace is the primary task. Water-cooling and inert gas-cooling are usually used in dissection. Water-quenched method can lower the furnace temperature within a short time, and also save the dissection costs. Higashida No. 5 blast furnace and Atsumi No. 1 blast furnace injected cooling water from the top of the furnace after releasing gas. The experimental blast furnace of Shougang Group was also shut down through 102 t cooling water within 21 h.32) Another 125 m3 blast furnace in Laigang Group was cooled down through 602 t cooling water in 144 h.
A 8.2 m3 experimental blast furnace (EBF) in Lulea was cooled through nitrogen gas from the top of furnace. The charge in EBF had been cooled for more than 10 days, while the chemical reaction in EBF stopped within one minute after the nitrogen injection.29) Besides this one, the Bureau’s experimental blast furnace in Pittsburgh was also cooled down by replacing the blast air with nitrogen.37) The nitrogen was blasted into furnace to replace same amount of cold-blast air. About 7.6 × 104 m3 nitrogen was consumed.38) The Mannesmann No. 5 blast furnace was cooled down through about 2.0 × 106 m3 N2.39) The 0.8 m3 experimental blast furnace of Pangang Group was shut down through N2–Ar gas-cooling method.40) Cooling through inert gas could avoid re-oxidation of burden during dissection, so that the original morphology can be retained as much as possible. However, gas-cooling method takes more time and costs than water-cooling, hence inert gas-cooling is more suitable for smaller blast furnace. The basic situation of the following series of dissected blast furnaces is listed in Table 1.
| Blast furnace | Volume, m3 | Diameter of hearth, m | Average utilization factor, t/m3·d | Burden, % | Ore-coke ratio | Dissection time | Cooling method | Shape of CZ | ||
|---|---|---|---|---|---|---|---|---|---|---|
| Sinter | Pellet | Lump ore | ||||||||
| Atsumi No. 1, Japan | 1407 | 8.5 | 1.85 | 50.4 | 15.2 | 34.4 | 3.0 | 1970 | Water | Inverse V |
| Kukioka No. 4, Japan | 1279 | 7.9 | 1.65 | 68.7 | 11.6 | 19.7 | 3.9 | 1971 | Water | W |
| Kawasaki No. 4, Japan | 922 | 7.1 | 1.77 | 79.0 | 2.0 | 19.0 | 3.5 | 1974 | Water | Inverse V |
| Kawasaki No. 2, Japan | 1148 | 7.4 | 1.63 | 61.0 | 10.0 | 29.0 | 3.1 | – | Water | Inverse V |
| Kawasaki No. 3, Japan | 936 | 7.1 | 1.53 | 58.0 | 13.0 | 29.0 | 3.0 | – | Water | Inverse V |
| Tsurumi No. 1, Japan | 1150 | 7.6 | 1.46 | – | 70.0 | 30.0 | 3.1 | – | Water | Inverse V |
| Kokura No. 2, Japan | 1350 | 8.4 | 1.87 | 60.2 | – | 39.8 | 3.3 | 1974 | Water | Inverse V |
| Amagasaki No. 1, Japan | 721 | 6.7 | 1.79 | 39.9 | 39.9 | 20.2 | 3.6 | 1976 | Water | Inverse V |
| Chiba No. 1, Japan | 966 | 7.6 | 1.29 | 89.0 | 16.0 | 5.0 | 2.8 | 1977 | Water | Inverse V |
| Mannesmann No. 5, German | 821 | 7.0 | – | 73.0 | 13.0 | 12.0 | 3.6 | 1981 | N2 | Inverse V |
| LKAB experimental blast furnace, Sweden | 8.2 | 1.2 | – | – | – | 1998–2008 | N2 | – | ||
| Shougang 23 m3, China | 23 | 1.8 | 1.93 | 100 | – | – | 2.6 | 1979 | Water | V |
| Panggang 0.8 m3, China | 0.8 | 0.7 | – | 100 | – | – | 1.4 | 1982 | N2–Ar | Inverse V |
| Laigang 125 m3, China | 125 | 3.2 | 3.5 | 65.0 | 35.0 | – | 1.7 | 2007 | Water | Inverse V |
| Fangda No. 1 China | 1050 | 8.4 | 3.1 | 77.8 | 22.2 | – | 3.5 | 2017 | Water | Inverse V |
| Blast furnace | Gas flow distribution | New characteristic |
|---|---|---|
| Atsumi No. 1, Japan | Central gas flow development | •Plotting temperature curve and establishing thermal transfer model of cohesive zone |
| •Reduction rate: 50%–70% | ||
| •Clarifying the distribution and behavior of cyclic elements and coke | ||
| Kukioka No. 4, Japan | Marginal gas flow development | •The softening and semi-melting of burden leads to the formation of soft layer |
| •The wear of brick may lead to the appearance of mixed burden layer | ||
| •Reduction rate: 40%–75% | ||
| Kawasaki No. 4, Japan | Flat gas flow distribution | |
| Kawasaki No. 2, Japan | Central gas flow development | •Clarifying the reason why the strengthen of coke decent sharply in lower part of stack |
| Kawasaki No. 3, Japan | Flat gas flow distribution | •The behavior of raceway is related to the consumption of coke |
| Tsurumi No. 1, Japan | – | •Testing the accuracy of charge motion model |
| •The movement of coke in hearth is due to the tapping and the change of force | ||
| Kokura No. 2, Japan | Central gas flow development | •The charge melts and falls faster in the direction of the larger tuyere |
| •The gasification catalyzed by alkali metals of coke leads to the decrease of strengthen | ||
| Amagasaki No. 1, Japan | Marginal gas flow development | •The blast volume is one of significant factors to affect the shape of charge level |
| Chiba No. 1, Japan | Central and marginal gas flow development | •The erosion of brick is related to the position of taphole |
| •Clarifying the relationship between porosity and permeability resistance | ||
| Mannesmann No. 5, German | – | •Deepening the understanding of charge behavior in furnace |
| •Metal accounts for 20% of the furnace volume | ||
| LKAB EBF, Sweden | – | •Testing different process concepts with a flexible design |
| Shougang 23 m3, China | Central gas flow development | •Injecting magnesia powder to preserve tuyere raceway during dissection |
| •Reduction rate: ~90% | ||
| Panggang 0.8 m3, China | – | •Clarifying the reduction mechanism of vanadium titanium sinter |
| Laigang 125 m3, China | Central gas flow development | •Illustrating burden behavior, slag and iron formation process, gas distribution in detail |
| •Reduction rate: 40%–90% | ||
| Fangda No. 1 China | Central gas flow development | •Obtaining relatively complete cohesive zone samples in recently years |
| •Investigating the evolution mechanism of phase in cohesive zone via microstructure | ||
| •Reduction rate: 40%–80% |
The dissection investigation showed that different kinds of charge could be distinguished clearly in softening part. Pellet in cohesive zone adhered to each other via contact surface in the form of molten slag and iron, while sinter only adhered through contact angle with molten slag. The whole burden contracted in the function of charge pressure in semi-melting part of cohesive zone. The content of iron increased obviously, while the gangue in burden had not participate in the formation of slag.
The relationship between operation and the shape of cohesive zone shown in Fig. 1 has been summarized through a series of blast furnace dissection in Japan.41) The formation of inverse V-type cohesive zone is controlled through the distribution of gas flow in the central region. The higher the melting intensity, the higher location of the inverse V-type cohesive zone. When the ratio of burden/coke in the central region increasing, the height of cohesive zone will decrease, which is contributed to the formation of stable V-type cohesive zone. The mixed layer of coke and burden can improve the permeability of the edge, and then the W-type cohesive zone formed.
The relationship between operation and the shape of cohesive zone. (Online version in color.)
Besides the research about the macroscopic morphology of cohesive zone, Fig. 2 has summarized the behavior of different burden in cohesive zone. The reducibility of sinter could increase sharply to 65% in cohesive zone, while that of pellet could be 80%. The study on the behavior of acid pellet in furnace showed that some low melting phases like K2O–FeO–SiO2 was formed at the edge of pellet in the early softening & melting stage. When the temperature was higher than 1000°C, the slag at the edge part disappeared and the pores on the surface of pellet were blocked. At about 1150°C, plenty iron oozed out and the shrinkage rate of burden reached more than 50% at 1350°C. The slag and iron were separated completely in acid pellet. The mixture of 2CaO·Al2O3·SiO2 and iron was the dominant phase in sinter of the early softening & melting stage. When the shrinkage rate of burden reached about 30%, the edge of sinter was slag with the composition close to 2CaO·SiO2 and low amount of Al2O3. And the content of Ca and Mg were higher at the slag-iron interface. The shrinkage of self-fluxing pellet and lump ore were calculated on the basis of experimental results. Compared with acid pellet and sinter, the shrinkage of lump ore was lower, and some (Fe, Mg) O was found in self-fluxing pellet.44,45) Researchers also have investigated the softening and melting behavior of high titanium sinter (HTS),46) and the results showed that the shrinkage rate of HTS was apparently lower than that of other kinds of burden and the phase basically unchanged during the process of softening and melting.
3.3. The Effect of Harmful Element on the Softening & Melting Properties of BurdenSulfur in cohesive zone was mainly existed in the form of FeS, which had little impact on the softening and melting properties on burden as the dissection investigation showed.47) The alkali metal was mainly concentrated in the lower part of cohesive zone and the shape of cohesive zone can be directly related to the distribution of alkali metal as the dissection results showed.38) The softening & melting experiment showed that at the beginning of softening, the shrinkage of burden was much more obvious in samples containing alkali metal compared with normal samples. Then the shrinkage of acid pellet decreased within higher temperature. The higher alkali metal absorption ability of basic sinter made its softening & melting property much more easily affected than that of acid pellet.48) The effect of K2O on the acid pellet has been presented in Fig. 3. The circulated region of zinc phase spanned from the lumpy zone to the cohesive zone with the temperature interval of 900–1250°C.
The effect of alkali metal on the acid pellet. (Online version in color.)
The burden structure of Mannesmann No. 5 blast furnace was 73% sinter, 13% pellet and 12% lump ore.28) The sinter in furnace was found to be interwoven with each other and mingled with fibrous metal iron through the dissection in 1981. The type of its cohesive zone was a mixture of “V-type” and “inverse V-type”. The reduction rate of burden in cohesive zone was above 80% and the permeability was poor, with little pathway for gas flow. Some mixture layers composed of charge and coke were also found in the cohesive zone. The main composition of spongy iron was small amount of [C] and [Si], while that of slag was related to the primary gangue.49)
Un-melted pellet in cohesive zone was always surrounded by the rim of iron as shown in Fig. 4, while sinter was wrapped around the the gangue and the iron transferred to the inside. Magnesite phase was usually found in the center of pellet. The bonding between sinter and pellet occurred at the FeO enriched surface. On the edge of cohesive zone in Mannesmann No. 5 blast furnace, the content of [C] in molten iron increased, indicating the separation between slag and iron. The mentioned region was also the enrichment area of alkali metal, while the ascending powders always blocked the pores of coke layers, leading to the poor permeability of the cohesive zone.
The information about cohesive zone in the No. 5 blast furnace. (Online version in color.)
The Swedish experimental blast furnace was built for testing different process concepts and various types of ferrous burden.50,51,52) The first campaign was started in 1997 for the pellet development purpose.53) The cohesive zone started from the 20th layer (near the tuyere) in which most reduced pellets melt together and individual pellets can non longer be distinguished.29) The cohesive zone usually occupied one fourth of the furnace working height. The experimental blast furnace had experienced nine campaigns and at the end of seven of them were conducted dissection, while the burden structure was summarized in Table 2.
| Servie life | Burden structure | Composition (%) | |||
|---|---|---|---|---|---|
| Fe | SiO2 | CaO | MgO | ||
| No. 1 | 100% olivine pellets MPBO | – | |||
| No. 2 | 100% olivine pellets KPBO | 66.8 | 2.20 | 0.39 | 1.60 |
| No. 3 | 100% lime/olivine fluxed pellet LOFP | 66.9 | 1.25 | 1.25 | 0.85 |
| No. 4 | 100% lime fluxed pellet LFBP | 67.2 | 1.50 | 1.50 | 0.30 |
| No. 5 | 100% quartzite/lime pellet KPBA | 67.0 | 2.50 | 0.60 | 0.52 |
| No. 6 | 70% sinter; | 60.5 | 4.33 | 7.16 | 1.01 |
| 30% KPBA | 67.0 | 2.50 | 0.60 | 0.50 | |
Five of the campaigns were made with 100% pellets, while the sixth was a mixture of sinter and pellet. Researchers also had conducted LKAB melt-down test, and the results showed that the cohesive zone was highest in the furnace with 100% KPBA and 100% LFBF, while lowest for 100% KPBO. The thickness of cohesive zone in blast furnace with mixed burden and KPBO was largest, and least for LFBP.
According to the several dissection research, a list of key parameters named “dS-T”, “dT-Re”, “dT-Rs” and “% Rh” were summarized to characterize the cohesive zone as presented in Fig. 5. The lowest cohesive zone was found in the furnace with KPBO as fed burden, while the lowest root position of cohesive zone was detected in the No. 6 campaign of experimental furnace. The KPBO made the shortest cohesive zone, while the root part occupied one third of the whole cohesive zone height. The height of cohesive zone in No. 2 campaign blast furnace was about three fifths of that in No. 4 campaign.
The characteristics of experimental blast furnace. (Online version in color.)
A large cavity in height of 1000 mm was found below the top of cohesive zone in No. 3 campaign blast furnace, which taken up 20% working volume of the furnace. Most of the pellets with little deformation welded together to form a grid. However, there were still some voids served as gas channels in the gird, which made the gird was fused layer more than cohesive layer. The burden in grid had higher degree of metallization than that of the rest of the burden.
The main burden in Shougang blast furnace was sinter before dissection, and there were 10 cohesive layers in total from the middle part of the belly to the lower part of bosh.30) The reduction degree of sinter in lumpy zone ranged from 10% to 35%, while that in cohesive zone increased sharply, especially in layer 4. The sinter could hardly be distinguished in cohesive zone, and the top of cohesive zone was a whole block with a diameter of 300 mm. layer 2 to 6 were ring-shaped and layer 7 to 10 were irregular shape as listed in Fig. 6. The root of cohesive zone was detached from the furnace wall because that the wall was filled with coke. The middle part of cohesive zone was near to the tuyere. The outside of cohesive zone was thicker and upward, while the inside was thinner. The whole cohesive zone was inverse V-type.
The characteristics of cohesive zone in Shougang 23 m3 blast furnace. (Online version in color.)
The amount of iron oxide decreased with the increase of temperature. According to the experimental results, the content of iron oxide in the outer side of cohesive zone was higher than that in the inner side of cohesive zone, while the change of reduction degree was opposite.31) Due to the low gas permeability of ore layer in cohesive zone, the gas flow was regarded to flow along the coke bed. Hence the temperature of inside was higher than that of outside in the same layer of cohesive zone, and the temperature gradually increased from the outside to the inside in the radial direction of cohesive zone. The content of iron oxide in outside of upper cohesive zone was about 44% and the reduction degree was about 50%, while that in the middle part of cohesive zone was about 80% and 90%.30)
The separation between slag and iron mainly happened in cohesive zone, and part of slag phase contained wustite. The structure of molten iron changed from vermicular shape to dense block, and the content of carbon content was low. According to the comprehensive analysis results of temperature measuring pieces and graphitization degree of coke, the softening temperature, melting temperature and dropping temperature of burden were about 1100°C, 1200–1250°C and 1450°C, respectively. Phases with low melting point in higher part of cohesive zone would softening and melting ahead of molten iron. With the reducing of wustite, the melting point of slag phases would increase, then the slag phase would melt with iron and drop near the front of ore layer.31,54) The dissection results also showed that the content of alkali metal in cohesive zone was several times of that in feeding, and that in coke was more than ten times, indicating that alkali metal was obviously enriched in cohesive zone, and the enrichment area corresponded to the position of cohesive zone.
5.2. A 0.8 m3 Blast Furnace in Pangang GroupThe small height of Pangang 0.8 m3 blast furnace (only 2.1 m from the tuyere to the throat) led to the irregular distribution of cohesive zone: only 4 softening & melting layers were formed between the lower part of the stack to the middle part of the bosh, with an inverted V-shaped distribution.55) The slag and molten iron could hardly be detected in cohesive zone and the separation of slag and iron mainly happened in a short range from the lower part of bosh to the tuyere. Small amount of Ti(C, N) was found in higher part of cohesive zone, in which the change of FeO can be neglected, suggesting that the formation of Ti(C, N) can be inhibited even at high temperature as long as keeping the certain oxygen potential.
The microstructure analysis found that the existence of molten slag and iron containing Ti(V) in coke made the porosity of coke gradually decreased with height decreasing, which was different from the phenomenon in Shougang dissected blast furnace. The enrichment of alkali metal in cohesive zone was also proved through this dissection.56) The content of alkali metal was 7.05 times in upper part of cohesive zone of that in feeding and gradually became the second most primary component in coke after SiO2.
The temperature range of cohesive zone in 0.8 m3 blast furnace was 1250–1300°C, the main phase was molten iron, perovskite, ilmenite and brookite solid solution.33) The following reaction may occur in cohesive zone.
The iron oxide was reduced in cohesive zone and some low melting point phases were formed. Figure 7 showed that TiO2 reacted with coke to form Ti(C, N) and some vanadium was also reduced to form V(N, C) in cohesive zone.
The main reaction occurred in Pangang dissected blast furnace. (Online version in color.)
The whole height of cohesive zone formed in Laiga ng dissected blast furnace was 5000 mm with 10 layers and inverse V-type distribution.42) The diameter of lowest part was 3600 mm and the height-diameter ratio was 1.38. The cross section of cohesive zone increased with the descending of height and the center of it leant toward tuyere due to the strengthening of gas flow as shown in Fig. 8. The top two layers of cohesive zone were solid spherical shape and the others were ring shape. The distance between the root of cohesive zone and the top of tuyere raceway was 500 mm. An unbonded layer with the thickness of 200–300 mm appeared between the root of cohesive zone and furnace wall.
The overlooking projection of cohesive zone in Laigang blast furnace.59) (Online version in color.)
Figure 9 presented the evolution of burden in cohesive zone of Laigang dissected blast furnace. The first layer of cohesive zone was mainly composed of pellet, in which the main composition was FeO and slag. The shape of wustite phase was irregular, which was surrounded by slag. The main composition of slag phase in pellet were O, Fe and Si. Tiny metal iron was reduced in slag of layer 1. The reduced iron increased apparently in layer 3 and layer 7, and the content of FeO decreased. Iron in pellet of cohesive zone mainly existed in the forms of FeO and Fe. The centre of pellet was loosened FeO, while the margin was dense metal iron during the reduction. The difference of pellet microstructure was tiny, but the pellet in the same layer showed apparent discrepancy. The main composition of outside pellet was FeO and only small amount of metal iron was reduced. The centre of inside pellet was FeO and slag, with a layer of metal iron wrapping.
The change of pellet and sinter in cohesive zone of Laigang dissected blast furnace. (Online version in color.)
The main composition of sinter in layer 1 were FeO and slag phase, while the content of iron increased obviously in layer 3. Slag phase was composed of O, Si, Ca and Fe. Slag and iron came to separate in layer 8 and only small amount of wustite had not been reduced. Lath-shaped 2CaO·SiO2 phase could be detected in slag, which indicating the inside temperature of cohesive zone was near 1500°C. Altogether, the iron in pellet and sinter was already reduced to FeO on the upper part of cohesive zone, and only small amount of metal iron existed in layer 1. The change of microstructure of burden in cohesive zone was non-obvious in the height direction, which was mainly reflected in the content of metal iron. The content of Si was higher than that of Ca in cohesive zone, indicating that the primary molten slag formed in cohesive zone was acid slag.
The reduction degree of sinter in outside of cohesive zone was about 40%–70%, while that of pellet was 40%–50%. The reduction degree of sinter in inside part of cohesive zone was above 80%. The reduction degree of sinter from outside to inside can increase 20%–45% in diameter direction, while that of pellet was about 30%–40%, which can be directly related to the microstructure change of burden in cohesive zone.57) The metallization ratio of sinter in outside of cohesive zone was about 20%–45%, while that of pellet was only half of sinter. The metallization ratio of sinter inside of cohesive zone was about 70%–95%, indicating the end of reduction. The lower metallization ratio of pellet manifested the lower reducibility, while main iron oxide in cohesive zone was FeO.
5.3.3. The Coke and Circulating Elements in Cohesive ZoneThe coke in cohesive zone usually had smooth surface and adhered to the burden. Coke layer in cohesive zone provided channel for gas flow and molten melt. The porosity of coke usually decreased 10%–25% in cohesive zone due to the reaction between coke and molten melt and the filling of un-reacted powder. The content of alkali metal and sulfur were higher in cohesive zone compared with that in lumpy zone while the content of zinc showed opposite tendency.34)
5.4. The Damage Investigation in a 1050 m3 Blast Furnace 5.4.1. The Macroscopic Morphology of Cohesive ZoneA 1050 m3 blast furnace was shut down with water-quenched method for damage investigation, and the whole cohesive zone was thus preserved. The shape of this cohesive zone was inverse-V type and the average distance between coke layer and softening & melting layer was 100 mm.58) The mean size of coke in cohesive zone was about 25–40 mm, while thickness of coke layer and and softening & melting layer were 150 mm and 100 mm, respectively. There were 21 layers in cohesive zone from the lower part of stack to the upper part of tuyere as shown in Fig. 10, and 7 typical samples were got from it. The pellet and sinter could be distinguished until sample 4.
The macroscopic morphology of cohesive zone in 1050 m3 blast furnace. (Online version in color.)
The investigation results showed that FeO was the main iron oxide in cohesive zone, and the amount of Fe increased obviously in the height direction. The average value of carbon in iron was about 2.0%, and the slag and iron were basically separated in sample 6. The change of microstructure was shown as Fig. 11. The basicity of slag phase maintains about 1.8 in cohesive zone, and a layer of titanium was observed around iron in the lower part of cohesive zone due to the development of direct reduction.58) The reduction degree of the higher part of cohesive zone was about 50%, while that of the root of cohesive zone was above 85%. The metallization ratio increased about 50% in cohesive zone, which can further prove that the main reduction in cohesive zone was FeO → Fe.
The microstructure of cohesive zone. (Online version in color.)
According to the content of carbon in iron and the thermodynamic condition in cohesive zone, it can be verified that the carburization of coke is inhibited, and the main function of coke was reductant and burden support in cohesive zone. The graphitization degree of coke was different between the surface and the interior of coke, both were higher than that of feeding ones.59) Furthermore, the order of the carbon structure on the coke surface was higher than that inside the coke, which may lead to the formation of fines. The size of coke pores gradually decreased with the position nearer the root of cohesive zone due to the severe gasification reaction. The alkali metal vapors can penetrate into the matrix of coke via pores, hence some aluminosilicates containing alkali metal can be detected near the cracks and pores.60) The adsorption of K was more obvious than that of Na in cohesive zone, and the content of K decreased in the height direction, while that of Na was opposite.
Based on the plenty dissection researches about blast furnace, scholars had already mastered the characteristics and properties of cohesive zone. From the initial understanding of the importance of cohesive zone, the iron-makers can now take some measures to better control it to ensure the smooth operation condition of blast furnace. This paper provides a summary of cognitive process of cohesive zone in several European countries as well as Japan and China. On the basis of this review, the development of ironmaking technology in different countries can be reflected. The properties of cohesive zone from the macro to the micro are summarized systematically. The behavior of different burden, the effect of circulating elements, the phase evolution process is also involved.
However, the investigations about the cohesive zone in dissected blast furnace conducted before were mainly concentrated on the macro-change of charge materials, while the comprehensive study about the actual composition of cohesive zone is still lacking. The interactions between various phases in cohesive zone is still unclear, which brings a gap in the selection of charge composition. As an important heat storage area in the blast furnace, the research on the evolution of heat transfer performance of charge at different locations after softening and reduction need be further investigated.
The important information get from blast furnace dissection is usually used to guide the experiment and numerical modeling simulation. Researchers have conducted plenty of studies about softening and melting behavior. In recent years, DEM-CFD model and softening & melting under load are widely used to investigate the burden behavior in cohesive zone. Among which, the motion of solid phase is calculated via DEM, and the motion of gas phase is calculated through CFD,17) such as the effect of coke slit,61) the behavior of acid pellets and sinter,62) mixed coke charging in the ore layer.63) Experimental condition can be controlled to simulate the performance of cohesive zone under different smelting, which can provide sufficient research space of researchers from all over the world. Hence a series of studies about the behavior of burden in cohesive zone have been reporting in the past two years, such as the comparison between sinter, olivine pellet and acid pellet,64,65,66) the high temperature interaction between different charge,67,68) the effect of certain content (C2S69)) or certain condition (pre-reduction degree,70) atmosphere and basicity71)) on the phase formation, the properties of some special feed materials like high Al2O3 content feed charge,72) high titanium sinter,46) and titanomagnetite ironsand coal composite hot briquette.73) The cognition process of cohesive zone is a process of mutual support and cooperation of various methods. Among them, due to the intuitiveness and authenticity of dissection investigation itself, it still occupies a significant guiding position. However, the limitations of previous dissection method leading to the fact that the current research rarely involves the nature of the softening & melting behavior of burden, and it is difficult to effectively control the shape and position of the cohesive zone under different operation conditions. With the continuous development of equipment level and cognitive system, more fundamental researches could be carried out to form the overall control technology of blast furnace.
The softening and melting of raw materials leading to the formation of cohesive zone, reflecting the distribution of temperature inside blast furnace. Direct reduction is strengthened as melting molten blocking voids of burden in this region, hence the control of cohesive zone including its location, height, type et al is the critical part of reducing energy consumption. The actual situation of cohesive zone is gradually revealed with the development of blast furnace dissection.
(1) Japan is the country with the largest number of blast furnace dissection. The relationship between operation and the shape of cohesive zone was clarified through series of dissection work. The reduction rate of sinter was found to be 65%, while that of pellet was 80%. Low melting point phases and the mixture of Fe and 2CaO·Al2O3·SiO2 respectively formed in pellet and sinter at the initial stage of softening & melting. Slag and iron then separated completely in acid pellet. Sulfur mainly existed as FeS in cohesive zone with little impact, and alkali metal primarily concentrated at lower part directly related to the shape of cohesive zone.
(2) The main composition of spongy iron in cohesive zone of German Mannesmann No. 5 blast furnace was [C] and [Si]. Dense metal iron shell was found encircle the pellet in cohesive zone, while the interaction between pellet and sinter primarily happened on the interface with higher content of wustite. The effect of different kinds of burden on the softening & melting behavior of cohesive zone was investigated via Swedish 8.2 m3 experimental blast furnace dissection, a series of parameters named “dS-T”, “dT-Re”, “dT-Rs” and “% Rh” were also selected to characterize cohesive zone. The results showed that 100% olivine charge led to the lowest and shortest cohesive zone.
(3) China conducted her first dissection study in 1979 on Shougang 23 m3 blast furnace. The amount of iron oxide was found decreased with increasing temperature in cohesive zone. Slag phases containing higher content of FeO may soft and melt antecedent to Fe at higher cohesive zone, and then slag melted with iron due to the reduction of wustite. Coke layers were regarded to provide channel for ascending gas flow due to the low permeability of ore layer. No obvious cohesive zone was detected in 0.8 m3 vanadium-titanium magnetite smelting blast furnace dissection on account of its low height. The porosity of coke bed decreased with height increasing with the fulfilment of iron containing [Ti/V] in voids. The content of alkali metal could be 7 times at higher cohesive zone of that in feeding.
(4) 10-layers cohesive zone was formed in Laigang 125 m3 blast furnace. The dissection investigation summarized the behaviour of sinter and pellet in different layers. The differences of microstructure of charge was not obvious in the height direction, while that in the same layer showed apparent discrepancy. The reducibility of sinter was higher than that of pellet. The porosity of coke bed decreased 10%–25% in cohesive zone. The content of zinc was lower in cohesive zone than that in lumpy zone. The damage investigation of 1050 m3 blast furnace in China showed that the mean size of coke in cohesive zone was 25–40 mm. FeO was the primary iron oxide in cohesive zone and the content of [C] was 2%. Titanium-ring was founded surrounding reduced iron suggesting the dominated position of direct reduction. The graphitization degree of coke was higher than that of feeding ones, while the adsorption of K was more apparent than that of Na in cohesive zone.
This work was financially supported by the National Science Foundation for Young Scientists of China (51604178, 51704019). Xiaoyue Fan gratefully acknowledges financial support from China Scholarship Council for one-year study at the RWTH Aachen University, Germany.
