2023 Volume 63 Issue 5 Pages 828-834
The state of deadman in hearth is important for the long campaign life of blast furnace (BF). In order to clarify the influence of deadman on sidewall erosion, the corresponding relationship between the shape of deadman and sidewall erosion was studied, the fundamental reason for the difference of sidewall erosion was analyzed, the influence mechanism of slag-coke interface and iron-coke interface on the coke in deadman was explored based on the phase distribution in deadman. The results show that: The shape of circumferential bulge at the root of deadman is a common feature of BFs, this feature is the main reason for the serious erosion of sidewall, and the serious erosion area of sidewall corresponds to the root position of deadman. The deadman shows a dynamic evolution law of sinking and floating under the action of force in the smelting process of BF, the depth of slag and iron level in hearth has the greatest influence on the deadman. The difference of central voidage of deadman is the fundamental reason for the difference of sidewall erosion for BFs in which the deadman is in the same floating state. Slag-coke interface and iron-coke interface commonly exist in hearth. The dissolution reaction at iron-coke interface makes carbon in coke continuously migrate into molten iron, which makes the graphitization degree of coke increase, the coke is easy to pulverize, this is the main mechanism of coke deterioration and renewal in deadman. The enrichment of harmful element K at slag-coke interface makes the coke expand and crack, the coke matrix is easier to peel off, which accelerates the deterioration process of coke.
In 2021, China’s pig iron production was 869 million tons, most of which was produced by blast furnace (BF).1) BF ironmaking is facing the challenge of safety and low-carbon smelting under the background of carbon peaking and carbon neutralization.2,3) Hundreds of BFs in China need safety maintenance every year, which affects the utilization coefficient of BF, and increases the fuel consumption and protection cost. Prolonging the service life of BF is one of the fundamental measures to save energy and reduce carbon emission in ironmaking.4,5)
Deadman is an important part in BF.6) In the smelting process of BF, the deadman is immersed in liquid slag and iron, its state directly affects the working state of hearth. The state of deadman is related to the smooth discharge of slag and iron, the hearth activity, and even plays a key role in the stable running of BF.7,8,9) The slag and iron will react with the coke in deadman, which will affect the slag composition and molten iron composition.10) In addition, the state of deadman affects the flow of molten iron and has a significant impact on the sidewall erosion in hearth.11,12) In recent years, many scholars and operators have paid attention to the state of deadman, a lot of studies have been carried out on the deadman through thermocouple temperature monitoring and numerical simulation.13,14,15,16) The influence of the state of deadman on the circulation of molten iron has been analyzed, the depth of salamander has been appropriately increased in the design of new BFs. However, the sidewall erosion of BFs is obviously different. The previous studies have not thoroughly analyzed the reasons for the differences, the understanding of the phase distribution characteristics in deadman is not enough, and the current situation of long campaign life of BF in China has not been fundamentally improved.
On the basis of previous studies, this paper intends to clarify the corresponding relationship between the shape of deadman and sidewall erosion, analyze the dynamic evolution law of deadman sinking and floating, and explore the fundamental reason for the difference of hearth sidewall erosion, combined with the dissection investigation of three different BFs.
The dissection investigation of three BFs was carried out. The volumes of the BFs are 2200 m3, 3200 m3 and 4350 m3, respectively. The basic information of BFs is shown in Table 1. The hearth diameter and the salamander depth of three dissected BFs are (10.7 m 2.0 m), (13.3 m 3.0 m) and (14.2 m 3.0 m), respectively. The three BFs were shut down without releasing residual molten iron, and cooling with N2 to maintain the state of deadman during production to the greatest extent. Among them, the 2200 m3 BF was shut down without lowering charge level, and the materials in BF were well preserved. After BFs were shut down, the method of core-boring was used to study the deadman. The core-boring was carried out at different heights and angles of serious erosion area in hearth, and the length of samples could reach the center of hearth. Moreover, the modular investigation method of rope saw cutting in hearth was adopted by 3200 m3 BF and 4350 m3 BF, which was conducive to the measurement and image processing of the interface, a large amount of information about deadman could be obtained. After further cutting, sample preparation and grinding, the samples of core-boring were analyzed through XRD and SEM-EDS.
| Effective inner volume | 2200 m3 | 3200 m3 | 4350 m3 |
|---|---|---|---|
| Started | 2001.05.19 | 2010.03.16 | 2006.10.13 |
| Shut down | 2019.10.13 | 2022.01.02 | 2020.06.19 |
| Number of tuyeres | 26 | 36 | 38 |
| Number of taphole | 2 | 4 | 4 |
| Hearth height | 4.5 m | 5.0 m | 5.4 m |
| Hearth diameter | 10.7 m | 13.3 m | 14.2 m |
| The depth of salamander | 2.0 m | 3.0 m | 3.0 m |
| Hearth structure | Large carbon brick + high alumina brick | Small carbon brick + ceramic cup | Small carbon brick + ceramic cup |
The serious erosion area of carbon brick in hearth has always attracted more attention. The sidewall of hearth has become a restricted area of long campaign life of BF, which has been widely recognized.17,18) However, there are different opinions on the height of serious erosion area. Some believe that the serious erosion area is located at the sidewall of the junction of hearth and bottom. While, some think that it is fixed at the sidewall 1.5 m below the center line of taphole. Others believe that small BFs are at the junction of hearth and bottom, and large BFs are at the sidewall 1.5 m below the center line of taphole.19,20,21,22) Actually, the height of serious erosion area is closely related to the shape of deadman. The shape of deadman in BF is a circumferential bulge at the root, the root is not the bottom of deadman, but the place with the most circumferential bulges. The results of dissected BFs in China are shown in Fig. 1, the deadman is also a circumferential bulge shape at the root, which indicates that the circumferential bulge shape of root is the common feature of deadman at home and abroad.
The structure of the commercial BF hearth. (Online version in color.)
As shown in Fig. 1, the serious erosion area of sidewall is slightly below the root of deadman, and the erosion curve is similar to the root shape of deadman. After BF is shut down, the charge level drops and the gravity of the whole charge column decreases, resulting in a small floating of deadman. So, the serious erosion area is located at the corresponding position of the root of deadman, it is not at the junction of hearth and bottom, nor is it fixed 1.5 m below the center line of taphole. Therefore, the position of serious erosion area depends on the position of deadman root. In the smelting process of BF, molten iron is easy to flow through the coke free area in hearth. The distance between the root and the sidewall is the smallest due to the root of deadman is bulge (as presented in Fig. 2). The convection heat transfer between the sidewall of hearth and molten iron can be expressed by Nusselt number:23,24)
| (1) |
| (2) |
The corresponding relationship between the shape of deadman and sidewall erosion. (Online version in color.)
The distance between the lower part of the deadman and the sidewall can be expressed as:
| (3) |
Equations (4) and (5) can be obtained according to the above equations:
| (4) |
| (5) |
The convective heat transfer coefficient between molten iron and sidewall increases with the decrease of the distance between deadman and sidewall. The convection heat transfer coefficient is the highest at the root of deadman due to the distance between the root of deadman and the sidewall is the smallest. Moreover, the convection heat transfer coefficient increases with the increase of BF smelting intensity (the degree of smelting process intensification, which refers to the coke consumption per cubic meter of BF volume every day), the quantitative relationship is shown in Table 2. The temperature of iron-carbon interface increases with the increase of convective heat transfer coefficient, which accelerates the corrosion process of carbon brick, so the erosion is the most serious here. Therefore, the bulge shape at the root of deadman is the main reason for serious erosion of sidewall, the height of the serious erosion area corresponds to the position of deadman root. The reason why it is believed that the serious erosion area is located at the junction of hearth and bottom is that the salamander depth of early BFs (small BFs) is designed to be shallow, the deadman sits on the bottom for a long time, and the root position is just at the junction of hearth and bottom. Modern BFs (large BFs) have consciously deepened the salamander depth, the root position of deadman moves up, so there is a view that it is fixed at the sidewall 1.5 m below the center line of taphole.
| Item | The quantitative relationship | |||
|---|---|---|---|---|
| BF smelting intensity | 0.69 | 0.79 | 0.86 | 0.93 |
| The convection heat transfer coefficient, W/(m2·K) | 68.12 | 73.12 | 76.21 | 79.30 |
The deadman will not remain stationary in hearth, but will sink and float and change dynamically with the smelting process. The floating height of deadman can be calculated:25)
| (6) |
With the progress of tapping, the slag and iron level in hearth will drop, and the buoyancy to the deadman will decrease. The taphole is in the pressure relief state, the deadman will sink. While, after the taphole is blocked, the slag and iron continues to generate and drip, and the slag and iron level in hearth rises, making the deadman float. In addition, the sinking and floating of deadman is related to raw and fuel parameters such as ore density and coke density, and operation parameters such as coke load, blast pressure and blast momentum. Among them, the slag and iron in hearth have the greatest impact on the sinking and floating of deadman. The quantitative relationship between floating height of deadman and slag iron liquid level is shown in Fig. 3. As shown in the figure, the floating height of deadman increases with the liquid level depth of slag iron, and the liquid level depth of slag iron is closely related to the design of salamander depth. For large BFs with deep salamander depth, the position of the deadman root will also sink and float, which will reduce the erosion of molten iron on the same position. For small BFs with a shallow salamander depth, even if the slag and iron level rises, the buoyancy is not enough to make the deadman float. The deadman sits at the bottom for a long time without dynamic changes of sinking and floating, which will make the deadman root always correspond to the same position of sidewall, resulting in serious erosion at the same position. This is the essential reason why it is generally believed that deepening the depth of salamander can prolong the service life of hearth.
The quantitative relationship between floating height of deadman and slag iron liquid level in 2200 m3 BF.
The dynamic evolution law of deadman has a great influence on the sidewall erosion. A large number of numerical simulation studies also show that when the deadman floats, molten iron is easy to flow through the coke free area at the lower part of deadman, so as to reduce the effect of molten iron circulation and the erosion of sidewall by molten iron circulation.26,27) However, many BFs have deep salamander depth and the deadman can float, but the difference of sidewall erosion is obvious. Some BFs sidewall erosion is light, while some sidewall erosion is still serious.
The coke in deadman in the actual hearth is not all densely stacked, and the coke compactness in different areas is also different. Figure 4 shows the morphology of the lower part of the deadman in 3200 m3 BF. The coke free area at the lower part of deadman has a small space. Loose coke is accumulated above the coke free area, and dense coke is accumulated at the upper part of deadman. In this case, the amount of molten iron flowing through the coke free area is limited, it must pass through the lower part of deadman with coke. However, the coke accumulation degree at the lower part of deadman in different BFs may be different, which leads to different molten iron flow and different degree of molten iron circulation. Therefore, although the deadman in different BFs is floating, there are differences in sidewall erosion.
Morphology of the lower part of the deadman in 3200 m3 BF. (Online version in color.)
The interface of residual iron cut in the investigation was pickled to make the interface more clear, the interface after pickling was recorded by camera. The obtained images were binarized and calculated using Photoshop software and Image Pro software to obtain the particle size of coke and voidage of deadman. The results show that the particle size of coke decreases from top to bottom along the height of BF, and gradually decreases from the center to the edge in hearth. The voidage of deadman shows the same law, and the particle size of coke plays a key role in the voidage.
The distribution diagram of slag, iron and coke in deadman obtained from hearth dissection is presented in Fig. 5. As shown in Fig. 5, there is a certain depth of coke free area at the lower part of deadman, which is the feature of deadman floating. It is speculated that this is caused by the circulation of molten iron during tapping, and the coke free area is also found at the same position of the other two BFs. As shown in Fig. 1, a lot of broken coke and even coke powder are found in the root area of deadman, which proves that the coke at this position has a high degree of deterioration. These crushed coke and coke powder are easily swept away by molten iron due to the circulation. This makes the coke continuously degraded and renewed, forming a coke free area between deadman root and sidewall.
Distribution characteristics of slag, iron and coke in the deadman of 2200 m3 BF. (Online version in color.)
The lower part of deadman is mostly the coexistence of molten iron and coke, while the upper part of deadman is the coexistence of slag and coke, which is related to the natural stratification of the liquid level caused by the different slag and iron density. After the analysis of coke particle size, the coke in deadman can be divided into large particle size coke (30–40 mm), medium particle size coke (20–30 mm) and small particle size coke (10–20 mm). The distribution of coke particle size in deadman is shown in Fig. 5. The hearth center is distributed from top to bottom as slag+large coke, molten iron+medium coke. The hearth edge is distributed from top to bottom as slag+medium coke, molten iron+small coke. The particle size of coke gradually decreases from top to bottom, and gradually increases from edge to center, which is related to the deterioration of coke in BF.
The voidage is an important parameter to reflect the liquid permeability of deadman.28) The voidage distribution of the deadman can be calculated through image processing of the samples. As presented in Table 3, the voidage of 2200 m3 BF is significantly higher than that of other BFs, and the voidage decreases from the center to the edge in hearth, which is different from other BFs. Compared with Fig. 5, the particle size of coke plays a key role. The central coke charging technology is adopted during the smelting process of this BF, which will ensure the particle size of coke in the center of hearth. Even if the coke deteriorates in BF, the coke particle size in the center is significantly larger than that at the edge. It can provide a wider channel for the flow of slag and iron between the coke with large particles, which will increase the voidage, and enhance the liquid permeability of deadman. However, several other BFs, especially small BFs, development of marginal gas flow is adopted for the smooth movement, which makes the voidage in the center of deadman insufficient. When the voidage in the center of deadman is high, the flow of molten iron from the lower part of deadman is more, which will greatly reduce the circulation of molten iron and the erosion of sidewall. While, when the voidage in the center of deadman is low, the situation is completely opposite, and the erosion of sidewall is will be serious. Therefore, the difference of the central voidage of deadman is the fundamental reason for the difference of sidewall erosion in which deadman is in the same floating state.
| Distance from hearth center, m | Voidage of deadman in hearth | ||
|---|---|---|---|
| 2200 m3 BF | 3200 3 BF | 4350 m3 BF | |
| 0.0 | 53.01 | 24.32 | 25.31 |
| 1.0 | 51.36 | 24.98 | 29.03 |
| 2.0 | 46.89 | 29.86 | 38.95 |
| 3.0 | 43.01 | 33.08 | 37.88 |
The slag-coke interface is common in the samples at the upper part of deadman, but it is also found in the samples below the center line of taphole. There are two ways to generate slag-coke interface here. Firstly, the slag-coke interface is formed when coke and slag come into contact for the first time, and then it drops to the lower part of hearth. Some slag-coke interfaces are preserved after contacting with molten iron. Secondly, the dissolution reaction between molten iron and coke causes the ash in coke to be released. CaO, SiO2 and other oxides similar to the slag exist in the coke ash, resulting in the slag-coke interface. The hearth dissection proves the existence of both ways.
The typical slag-coke interface which was taken from the upper part of hearth is shown in Fig. 6. Coke matrix is found at the lower right of the sample, and slag is observed at the lower left and upper right of the sample. The obvious slag-coke interface exists in most areas, but the interface in the middle part is not very clear, which is the mixed state of slag and coke. This indicates that the liquid slag may penetrate into the coke matrix at the slag-coke interface in hearth. As shown in Fig. 6(d), the enrichment of harmful element K is found at the slag-coke interface in the upper right corner. The coke matrix is porous and loose. After K enriched slag enters the coke through the pores, it is easy to degrade the coke. The mechanism can be explained from two aspects.29,30) On one hand, the atomic radius of K atom is large. After K enters the coke, the original interlayer structure of carbon will be destroyed and the interlayer distance of carbon will increase, which makes the original ordered structure of carbon easier to be disassembled, making carbon easier to separate from the coke matrix. On the other hand, the oxide of K will react with the minerals in the coke matrix, new minerals such as potassium nepheline or potassium aluminosilicate (KAlSiO4) are generated, resulting in mineral volume expansion and loose coke structure. Under the action of the two mechanisms, K causes cracks in the coke matrix, which makes the coke matrix easier to peel off, and accelerates the deterioration process of coke.
Slag-coke interface of deadman observed by SEM and EDS. (Online version in color.)
A large number of iron-coke interfaces are found in deadman below the center line of taphole. The typical iron-coke interface which was taken from the lower part of hearth is shown in Fig. 7. As presented in Fig. 7, the coke is surrounded by molten iron at the iron-coke interface. The left side of coke has been surrounded by molten iron, and there is a small amount of slag on the right side. There is no obvious boundary between molten iron and coke, they are interwoven at the interface. The interfacial dissolution reaction will inevitably occur due to the carbon content of molten iron is undersaturated, so that the carbon in the coke matrix will continuously migrate to molten iron.31) The density of molten iron is much higher than that of coke, it will permeate along the pores of coke under the static pressure, and dissolve while penetrating, the carbon around the coke matrix gradually enters the molten iron. Therefore, the coke at the interface presents a dendritic distribution, as shown in Fig. 7(a). The molten iron further reacts with the remaining coke to reduce the particle size of coke, the coke is completely dissolved by molten iron or taken away with the circulation of molten iron. This is the main mechanism of coke renewal in deadman.
Iron-coke interface of deadman observed by SEM and EDS. (Online version in color.)
Under the action of dissolution, molten iron dissolves the disordered carbon in coke, and then part of the carbon is precipitated in an orderly structure at the interface through catalytic graphitization. The carbonization bond after dissolution reaction changes from SP3 to SP2, which makes the graphitization degree of coke in deadman increase continuously.32,33) The drum strength of coke decreases and the pulverization rate of coke increases duet to the high graphitization degree. In the deadman of hearth, the coke is densely accumulated, the coke strength decreases after the graphitization degree increases, which makes the coke more easily broken into small pieces under the pressure of material column, so as to create a new iron-coke interface. The new iron-coke interface will accelerate the dissolution of coke in deadman.
(1) The most serious erosion area of BF is located on the sidewall, sidewall has become a restricted area for the long campaign life of BF. The shape of deadman is circumferential bulge at the root, which is a common feature of deadman in hearth. The bulge shape of deadman root is the main reason for the serious erosion of sidewall, the height of serious erosion area corresponds to the root position of deadman during production.
(2) The deadman is acted by buoyancy, gravity and friction in BF. With the change of BF smelting process, the deadman shows a dynamic evolution law of sinking and floating under the action of various forces. The depth of slag and iron level in hearth has the greatest influence on the sinking and floating of deadman.
(3) The voidage is an important parameter to reflect the liquid permeability of deadman, the coke particle size plays a key role. When the voidage in the center of deadman is high, the flow of molten iron from the lower part of deadman is more, which will greatly reduce the circulation of molten iron and the erosion of sidewall. The difference of central voidage of deadman is the fundamental reason for the difference of sidewall erosion.
(4) Slag-coke interface and iron-coke interface commonly exist in hearth. The dissolution reaction at iron-coke interface makes carbon in coke continuously migrate into molten iron, this is the main mechanism of coke deterioration and renewal in deadman. The enrichment of harmful element K at slag-coke interface makes the coke expand and crack, which accelerates the deterioration process of coke.
This work was financially supported by Tangshan Science and Technology Research and Development Plan (22130203H) and Open Fund of The State Key Laboratory of Refractories and Metallurgy (G202005).
