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Production practice of increasing tundish life at high casting speed
Correctly view the influence mechanism of raw fuel quality on the smelting process of blast furnace, rationally analyze the operation trend of furnace condition, strengthen the screening management under the trough, do a good job in the tracking management of raw fuel quality, formulate a reasonable "attack, defense and retreat" operation policy according to the quality of raw fuel, eliminate the adverse factors affecting the stable and smooth operation of furnace condition in the bud, and escort the stable and smooth operation of blast furnace.
In the past, the ironmaking industry often said that the operation of "seven parts of raw materials and three parts of raw materials" means that the importance of raw materials accounts for 70% of the smooth running of the blast furnace. Now, with the large-scale blast furnace, ironmaking pays more attention to raw materials, and equipment and quality inspection play a more and more important role in production. Therefore, it is now three parts plus quality inspection, that is, the accuracy of operation, equipment, raw materials and quality inspection data is equally important. With the development of China's metallurgical industry in recent decades, the large-scale blast furnace and the evolution of equipment, ironmaking has formed a very standardized production process, and the impact of labor is basically small, unless there is a corresponding problem with the equipment. The quality of raw materials has a great impact on the quality of products. Therefore, by controlling the particle size, grade, metallurgical properties and physical properties of raw fuels, the probability of better product quality and stable and smooth blast furnace conditions can be obtained. In fact, this sentence emphasizes the importance of raw materials (including sinter, pellet and lump ore) and fuel quality (including coke and pulverized coal injection) to the technical and economic indicators of blast furnace in blast furnace ironmaking. In order to let more colleagues and newly graduated students of metallurgy department systematically understand the impact mechanism of raw material and fuel quality on blast furnace smelting, the author shares the summary of the impact of raw material and fuel quality changes on blast furnace smelting for more than ten years to many readers. This article will start with raw material screening and introduce them one by one as follows.
1. Importance of raw material screening under tank
Screening of raw materials: many articles and enterprises often mention to strengthen the management of raw materials and fuels, strengthen the screening management under the trough, and do a good job in the tracking management of raw materials and fuels quality in the process of strengthening smelting and treating furnace conditions. It can be seen that the quality of raw materials and fuels is of great importance to blast furnace smelting. I believe that friends from the same industry often hear about strengthening the quality management of raw fuels and supervising and following up the screening management under the trough in many conference venues. When the furnace condition fluctuates, the first sentence asked by the leaders is often what is the situation of raw fuels. Have you read the materials? However, few people can say clearly what is the purpose of strengthening the screening of raw materials and fuels under the tank and what is the purpose of strengthening the screening under the tank.
The author believes that the purpose and intention of screening under the trough should not only be clear to the blast furnace operator, but also to the operator responsible for raw material screening. Only by letting him understand the purpose and significance of raw material screening and improving his initiative and enthusiasm can he ensure the quality of raw fuel screening. It is generally understood that the purpose of raw fuel screening is to screen out the small particle size powder and reduce the powder content of the material into the furnace, which will help to improve the permeability and daily output of the material column, and reduce the probability of difficult operation of the material column pressure difference and furnace conditions. There is no mistake in this understanding, but it is one-sided and many things cannot be explained.
In this regard, the author's understanding is that to screen out small particle size materials, first, to reduce the powder content of the materials in the furnace and prevent the small particle size powder from blocking the gap of large particle materials. Once the powder forms a blocking body, it will prevent the flow of high heat reduction gas, making it difficult to heat and restore some materials in the upper block zone. When these materials reach the soft melting zone and begin to melt, the remaining oxides need to be removed by direct reduction, This will lead to an increase in the ratio of iron to fuel per ton or cooler furnace conditions.
Second, operators often see "unmelted" materials similar to slag skin from the tuyere. Many inexperienced operators often think that the slag skin falls off. The slag skin fall off can be verified from the temperature curve changes throughout the furnace body. If the temperature curve around the furnace body is stable, the "unmelted" materials falling from the tuyere can basically be determined as the materials that fail to melt at the upper part, that is, the "falling" of the tuyere. When the "production drop" accounts for a large proportion, it is inevitable that the ratio of iron to fuel per ton will rise and the furnace temperature will fall.
Third, due to its flow characteristics, the powder will also deposit in a certain area, resulting in distribution segregation. When the raw materials are loaded into the charging tank on the furnace top, the powder will stay at the collision point of the loaded materials and rise with the material level, while the coarse-grained materials will quickly roll down under the influence of the rest angle. When the material tank is unloaded, the material will be fed from the materials in the center, which account for a large proportion of small particle materials, and then the large particle materials left from the edge of the material tank, so that the chute also produces segregation of coarse and fine materials. The small particle material is inside the falling curve, and the large particle material rapidly rolls down to the lower part outside the curve, resulting in that the materials at the throat collision point are basically small particle materials, and the edge air flow will also change under the influence of small particle materials.
2. Influence mechanism of raw material quality on blast furnace smelting
Sinter quality: for the evaluation of sinter quality, its physical properties and metallurgical properties are often compared in the industry. The physical properties of sinter include drum index, anti-wear index and screening index, that is, the sintering drum test under cold strength conducted daily by the sintering plant and quality inspection room. Metallurgical properties mainly include reducibility, low-temperature reducibility, pulverization, soft melting, etc.
1. The most direct impact of the physical properties of sinter on blast furnace smelting is in the upper block zone, which is roughly reflected in the following aspects:
(1) After the collision and falling of multiple material transfer stations, the particle size grade has changed and the proportion of medium and small particle size will increase. This change will have a certain impact on the gas flow distribution in the block zone;
(2) After the small particle size ore is fed into the furnace, especially in the block zone, the small particle size sinter is unevenly filled in the gap, which blocks the normal rising channel of the gas flow, and it is very easy to generate local gas flow in the blast furnace. If it is not controlled, it is likely to cause pipe travel;
(3) The small-size sinter will fill the gap of large-size material when it is put into the furnace, resulting in the gap of furnace material becoming smaller, and the iron containing raw materials can not fully contact with the gas, reducing the gas reduction amount of the sinter in the upper block zone. Due to the reduction of gas reduction amount, the utilization rate of gas decreases, which directly leads to the increase of the lower direct reduction amount, the increase of the consumption of tons of iron fuel, and the reduction of the blast furnace output;
(4) The normal preheating of the block zone charge is completely realized by the heat transfer and heat exchange between the falling charge and the rising high-temperature gas in the reverse movement. Due to the uneven filling of the small particle size ore in the gap after entering the furnace, the normal rising channel of the gas flow is blocked, and the gas flow distribution is uneven, so that part of the charge cannot be fully preheated, and this part of the charge cannot be melted normally when it reaches the soft melting zone due to insufficient temperature, It can only be directly reduced at the lower part. With the increase of direct reduction, the proportion of coke consumed will also increase. The proportion of coke gasified and burned at the tuyere will inevitably decrease, resulting in the reduction of high-temperature gas volume and the decline of comprehensive load.
2. The influence of the metallurgical properties of sinter on the smelting operation of blast furnace starts from below the isotherm at the beginning of the reduction reaction. At this time, the surface water in the charge has been evaporated. With the contact between the charge and CO in the rising gas, Fe2O3 (hematite) in the charge begins the oxygen removal reaction first. This reaction process is an exothermic process, followed by Fe3O4 (magnetite). This process is an endothermic process, Then there are FeO (faustite) and feo1/2. The reaction process is exothermic. In actual production, FeO can only be directly reduced in the lower high temperature zone, and the FeO content also means the difficulty of the upper sinter reduction. The reducibility also means the working load of the hearth and the fuel consumption. Low temperature reduction pulverization (RDI). After Fe2O3 (hematite) in the charge contacts and reacts with CO in the rising gas, the reaction will generate internal stress in the crystal structure of the charge, and may also cause the crystal structure to crack, making the charge become smaller particles. This characteristic of the charge is called low temperature reduction pulverization. If the low temperature reduction pulverization rate is high, the porosity of the charge will be reduced, and the air permeability of the upper charge column of the furnace body will be deteriorated first, The risk of furnace body nodulation will also increase, the normal distribution of gas flow will also be damaged, and the utilization of gas will become worse. In the upper part, due to the uneven filling of small particle size ore in the gap, the distribution of gas flow is uneven, resulting in that some furnace materials cannot be fully preheated and deoxidized. This part of furnace materials can not be gradually melted or even partially melted until they reach the soft melting zone. Finally, the tuyere will drop, the ratio of iron to fuel per ton will increase, and the output will decrease. Through continuous analysis and Research on sinter composition, permeability of iron making blast furnace, gas utilization and other parameters, it is finally determined that the influence factors of Al2O3, R2, FeO and TiO2 on sinter rdi+3.15 are roughly as follows:
(1) Effect of mgo/al2o3 on rdi+3.15 during the cooling process of sinter, magnetite around the pores is oxidized to form skeletal hematite; This skeletal hematite is very easy to be reduced in the process of low temperature reduction of sinter and produces large structural stress, which makes the sinter powdered. According to the mineral phase analysis, the volume content of skeletal hematite in sinter under different mgo/al2o3 conditions was determined; According to the relationship between the volume content of skeletal hematite and the low-temperature reduction pulverization performance of sinter, when mgo/al2o3 is 1.22, the content of skeletal hematite is low, and the sinter has good low-temperature reduction pulverization performance. The reason is that when the mgo/al2o3 of sinter is appropriate, its mineral composition is more reasonable, which greatly inhibits the formation of skeletal hematite and improves the low-temperature reduction pulverization performance. When mgo/al2o3 is too high, on the one hand, it will promote the development of magnetite in sinter; On the other hand, the addition of more dolomite also makes the large pores of sinter increase, which provides conditions for the formation of skeletal hematite, resulting in the deterioration of low-temperature reduction and pulverization performance of sinter. When the solid solution of Al2O3 in hematite increases and the sintering temperature is high, part of fe3+ in the secondary Hematite crystal lattice in the melt is replaced by al3+, which promotes the recrystallization and continuous crystallization of Fe2O3 from granular to flaky. Several single particles are combined into flake crystalline state, which makes the expansion stress generated during Fe2O3 reduction change from relatively dispersed to relatively concentrated, causing crystal plane contraction, generating internal stress and reducing the strength of sinter. New cracks are easy to occur during reduction, and the cracks are easy to expand, which promotes the intensification of expansion. Considering the improvement of low-temperature pulverization index of sinter, mgo/al2o3 of sinter should be controlled in the range of 1.06-1.22 as far as possible to ensure that rdi+3.15 index can meet the needs of blast furnace production.
(2) The effect of FeO on rdi+3.15 with the increase of ferrous iron, the low-temperature reduction and pulverization performance of sinter is effectively improved. Due to the effect of FeO on rdi+3.15, the low-temperature reduction and pulverization performance of sinter is effectively improved with the increase of ferrous iron from 8.3% to 9.8%. Due to the increase of fuel during the sintering process, the sintering temperature is increased, the dissolution of hematite is increased, and more liquid phases are generated, so that the structure of sinter becomes compact, The low-temperature reduction pulverization index of sinter has been improved. However, with the gradual increase of ferrous iron, the low-temperature reduction pulverization index of sinter has not increased much, but the reduction index of sinter has begun to decline. Therefore, ferrous iron is properly controlled within 10% on the premise that the low-temperature reduction pulverization index of sinter can be met.
(3) Effect of alkalinity r on rdi+3.15 by increasing the amount of quicklime, the alkalinity of sinter is increased from 1.83% to 1.96%, the low-temperature reduction pulverization performance of sinter is effectively improved, and the permeability of sinter layer is improved. When the R2 of sinter is lower than 1.85, every 0.1 decrease in alkalinity will affect the fuel ratio and output by 3.0%~3.5%. It is understood that in practice, the impact of reducing alkalinity on the fuel ratio of blast furnace is much higher than the ratio of 3.5%. The reason is that after the basicity r of the sinter decreases, the binder minerals change, which are mainly composed of calcium iron olivine and a small amount of monocalcium silicate, dicalcium silicate, calcium ferrite and glass, and the metallurgical properties of the sinter are poor. However, with the increase of alkalinity, dicalcium silicate and calcium ferrite increase significantly, while calcium iron olivine and glass gradually decrease, thus improving the rdi+3.15 index of sinter. Therefore, it is suggested that the alkalinity of sinter should be properly increased while considering the balance of sintering output, so as to improve the low-temperature reduction and pulverization performance of sinter.
(4) Effect of TiO2 on rdi+3.15 TiO2 in sinter decreased from 0.28% to 0.23%, and rdi+3.15 in sinter increased. At the same time, the titanium content in molten iron of blast furnace was reduced, and the fluidity of molten iron became better. Relevant studies show that TiO2 in sinter mainly exists in the glass phase. TiO2 reduces the fracture toughness of the glass phase, reduces the stress ability of the glass phase to resist the reduction and pulverization, and causes more cracks in the sinter and intensifies the pulverization. Rdi+3.15 in the sinter decreases with the increase of TiO2 content in the sinter, the cracks in the sinter increase during reduction, and the perovskite and pores in the sinter also increase, which not only reduces the strength of the sinter, but also improves the dynamic conditions of reduction and worsens the reduction and pulverization performance of the medium titanium type sinter. Therefore, reducing the titanium content of the charging material is conducive to the improvement of the metallurgical performance of the sinter.
3. Soft melt refers to the softening starting temperature, softening range and permeability of the soft melt zone of the sinter. The softening start temperature and softening end temperature determine the softening range. For the high-temperature performance of sinter, the higher the softening start temperature of sinter is, the better. Once the furnace burden begins to soften and melt, the gas permeability will be greatly reduced. Therefore, the smaller the softening temperature range of furnace burden is, the better. Firstly, the height of the soft melting zone of the blast furnace will be reduced; secondly, when the furnace burden is heated in the furnace, the gas flow channel will not be blocked, It is beneficial to improve the permeability of material column. The softening temperature of materials is generally required to be ≥ 1100 ℃, the softening end temperature is 1300-1350 ℃, the softening temperature range is ≤ 150 ° C, and 100-120 ° C is more appropriate.
The initial softening temperature of sinter depends on its mineral composition and pore structure strength, and the change of initial softening temperature is often the result of the dominant role of pore structure strength. That is to say, the final softening temperature is often dominated by mineral composition. On the contrary, if the softening starting temperature is low, firstly, the softening temperature range naturally becomes larger, and the thickness of the soft melt zone increases. Secondly, it is not conducive to the improvement of the permeability of the soft melt zone, and the stability of the furnace condition decreases with the increase of the column pressure difference.
According to relevant tests, the resistance loss of softening zone accounts for about 25%, which is also an important index to reflect the forward running status of furnace charge at the lower part of furnace stack and waist. For the droplet performance of sinter, the melting start temperature is required to be ≥ 1400 ℃. The higher the temperature, the better. The higher the melting temperature means that the lower the melting range, the smaller the melting layer thickness, the lower the maximum differential pressure, and the smaller the resistance of the soft melting zone to the blast furnace gas flow.
Droplet performance is the most important performance in the metallurgical properties of sinter, because the resistance loss of droplet zone accounts for about 60% of the total resistance loss of blast furnace, and it is the limiting link in the downstream of blast furnace. This is also the reason why the new blast furnace operation concept, which mainly focuses on the upper part of the blast furnace in the past, has been changed to the lower part of the blast furnace.
Pellet quality: the evaluation of sinter quality is usually based on the comparison of chemical composition, cold strength, pellet size, expansion characteristics, softening and melting. In the production process, the most concerned about the pellet properties are particle size, cold compressive strength, expansibility, softening and melting temperature. The particle size fluctuation range of pellets is the smallest block material. Large particle size pellets are more favorable to improve the permeability of the material layer, while small particle size pellets are easier to be reduced. However, when the particle size range is large, it will have a negative impact on the permeability of the material layer.
As for the compressive strength of pellets, there are some big differences. The pellets with good compressive strength can reach 2800N, and the ones with poor compressive strength will be lower than 2000N. As for acid pellets, their metallurgical properties are general, but their strength is good, their reduction index is slightly poor, and their softening and melting temperatures are lower than those of solvent pellets and magnesium based pellets. In terms of expansion, acid pellets are sensitive to Cao. When R2 is greater than 0.25, the expansion trend is very strong, especially when the alkali metal content is in the furnace, the pellet will be over expanded, and after expansion, it will inevitably produce powder, which will have an adverse impact on the permeability and smoothness of the column.
Block quality: for the influence of block ore, the focus is on the reduction and pulverization performance, melting temperature and physical properties of block ore. As lump ore belongs to green ore, its metallurgical performance is worse than that of sinter and pellet, and generally has the following characteristics:
(1) The softening temperature of lump ore is low, the soft melting zone is wide, the soft melting performance is poor, and the droplet range is large. These properties lead to the poor permeability of the soft melting zone of blast furnace and the increase of the lower differential pressure;
(2) The thermal burst performance of lump ore is poor. After the burst in the medium temperature zone, a large amount of powder is produced by pulverization, which has a serious impact on the permeability and the distribution of gas flow;
(3) The sio2/al2o3 of lump ore is low, which has a great impact on blast furnace slagging. Especially, the primary slag affects the change of air flow in the furnace, resulting in the gradual increase of edge air flow. At the same time, the Al2O3 in the slag increases, which reduces the fluidity of slag, reduces the desulfurization capacity and affects the quality of pig iron;
(4) The reduction of some lump ores is not good, resulting in the increase of blast furnace fuel ratio. Therefore, in consideration of production cost, when it is necessary to greatly increase the use proportion of lump ore, attention should be paid to the metallurgical performance of lump ore to prevent the loss of metallurgical performance from damaging the smooth operation of blast furnace and deteriorating smelting indicators. Influence of alkali metals: Generally speaking, alkali metals refer to potassium, sodium and zinc. The alkali metals in blast furnace have multiple effects on the smelting operation of blast furnace. However, the blast furnace dominated by the central gas flow is generally less affected by alkali metals, because after the temperature of the central gas flow rises to a certain level, some alkali metals and all zinc will be discharged from the blast furnace with the top gas in the form of steam.
This phenomenon used to occur at 1080m ³ And 1280M ³ The same material is used for blast furnace due to 1280M ³ The central air flow of blast furnace is relatively vigorous, and the composition of dust from clean gas and 1080m ³ The blast furnace with strong central air flow is less affected by alkali metals. If the temperature of the top gas is low, alkali metals and zinc will accumulate in the furnace, resulting in the decrease of the forward running degree of the blast furnace.
The effects of alkali metals on blast furnace smelting are as follows:
(1) From the point of view of chemical reaction, alkali metal acts as a catalyst for coke dissolution loss reaction (c+co2 → 2CO), which also means that with the increase of alkali metal content, the ratio of iron to fuel per ton will also increase;
(2) The strength of coke and charge is reduced, the pulverization of coke and charge is intensified, the permeability of charge column is deteriorated, and the smooth running degree and production index of blast furnace are reduced;
(3) Alkali metals and zinc adhere to the surface of solid particles and are bonded to the furnace wall of the blast furnace shaft. The effect of furnace lump formation is that the burden drops and worsens, resulting in suspension and sliding in extreme cases;
(4) Alkali metals will erode refractories, especially carbon based refractories, resulting in "brittleness" of refractories or cracks of varying degrees, which will reduce the service life of furnace lining.
In view of the adverse effect of alkali metal on blast furnace smelting, in order to ensure the stable and smooth operation of blast furnace, gb-50427-2008 code for design of blast furnace ironmaking process requires that the total content of alkali metal in raw fuel should be ≯ 3kg/t. In some regions, some companies are restricted by conditions, and the use of local iron ore and alkali metals may reach a higher level.
3. Influence mechanism of fuel quality on blast furnace smelting
1. Coke quality: in addition to conventional fixed carbon, ash, volatile matter, sulfur and moisture, the commonly used coke quality indicators for blast furnace smelting also include industrial analysis indicators, such as strength index M40, wear resistance index M10, reactivity index CRI and post reaction strength CSR. M40 and M10 are important indexes reflecting the cold state performance of coke. Higher M40 and lower M10 are conducive to improving the permeability of the block zone in the furnace and improving the smoothness of the furnace condition.
Compared with the change of coke M40, M10 has a greater impact on the blast furnace smelting process, and the impact of M10 on the output is 3.6 times that of M40. Therefore, while improving M40, M10 must be effectively controlled. So far, the most commonly used method for coke reactivity index CRI and post reaction strength CSR is to adopt the "chemical reactivity index test" of Nippon Steel. Through the percentage of coke after 120 minutes of gasification reaction in 100%co2 and 1100 ℃ environment, namely CRI, the greater the reactivity of coke, the greater the weight loss. After the drum test of the coke after the reaction, the percentage of particles greater than 10mm, i.e. CSR, CRI and CSR are inversely proportional. The influence of coke on smelting mechanism most operators generally stay in the understanding of material column framework, heating agent, reducing agent and carburizing agent, which is also one-sided to a certain extent.
The author's understanding is:
(1) Realize the gas rising and distributing through the burden, support the weight of all materials above the soft melting zone of the blast furnace, and ensure the smooth downward flow of slag and iron to the iron mouth;
(2) Generate heat to melt the charge;
(3) Producing reducing gas to remove the oxygen combined with iron in the iron containing furnace charge;
(4) Provide carbon to remove the residual oxygen not removed in the block zone in the iron bearing furnace charge and meet the needs of hot metal carburization.
Based on the above contents and the realistic conditions of increasing coal ratio after the large-scale blast furnace, in order to ensure the stable and smooth operation of blast furnace, the requirements for coke quality are more stringent.
First of all, the coke is in the state of extrusion, mechanical wear and (chemical) erosion in the process of descending in the furnace. The quality of coke M40 index determines the state of coke in the charging zone and the block zone after extrusion. A better M40 can reduce the crushing rate of coke in the charging zone and maintain a better particle size at the lower part of the block zone, which is very important to improve the permeability of the block zone and reduce the pressure difference of furnace charge.
Secondly, from 900 ℃, coke starts from CO2 reaction and continues to 1000 ℃. The deterioration of coke in this area is caused by mechanical wear and mild gasification reaction. The wear condition in this area is determined by the quality of M10 index. A lower M10 indicates that the wear resistance of coke is good, and vice versa.
CRI (coke reactivity index): massive iron ore materials begin to soften and deform in the soft melt zone, forming large agglomerate particle binder. The ore bed materials in this area basically have no air permeability, and the rising gas can only pass through the reserved coke layer (coke window layer). At a higher temperature level, the temperature in this area is in the range of 1000~1300 ℃, the coke reaction rate increases, and the coke reacts with CO2 gas fiercely. The environment and thermal reaction test process are basically similar. The coke blocks in this area are in closer contact with softened or melted materials, which not only causes the weight loss and porous change of coke after reaction, but also reduces the wear resistance of coke particles. The higher the CRI index, the more intense the reaction in the high temperature zone, the greater the weight loss, and the greater the reduction of wear resistance, and vice versa.
With the increase of coke CRI index, the thickness of the coke window in the soft melt zone will be smaller after extrusion, and the permeability of the charge column will also decrease. The pressure difference of the charge column is bound to rise, and the stable and smooth operation of the blast furnace condition will also change significantly, and vice versa.
CSR (coke strength after reaction): the strength after reaction is inversely proportional to the reactivity index. With the increase of CRI, CSR gradually decreases. The influence mechanism on blast furnace is as follows:
(1) It causes the phenomenon that the coke window of the soft melt zone becomes thinner, the permeability of the material column becomes worse, and the pressure difference increases;
(2) As a result, the quantity of coke burning at tuyere decreases, and the heat generated by combustion decreases, aggravating the cooling trend of furnace temperature;
(3) After the decrease of CSR, the ability of blast furnace to accept high coal injection ratio decreases, which leads to the decrease of oxygen enrichment rate and output and the increase of cost per ton of iron;
(4) With the decrease of M40, the coke particle size entering the hearth will also decrease (because the coke entering the center of the hearth is basically not or very limited to be eroded by CO2, and its CSR performance has basically not decreased. The coke keeps the particle size when it is loaded into the furnace more or less, and is only affected by slight wear during the lowering process in the furnace), resulting in the deterioration of the porosity of the hearth dead coke stack, The liquid permeability of the hearth dead coke pile and the activity of the hearth decrease, the circulation erosion of molten iron along the circumference of the hearth increases, and the elephant foot erosion of the hearth carbon brick tends to increase;
(5) With the decrease of M40, the particle size of coke entering the hearth will also decrease, resulting in poor liquid permeability of the hearth coke stack. Under such conditions, the high silicon content of hot metal will lead to high viscosity of hot metal, low mgo/al2o3 in slag, and decreased slag fluidity, which will easily lead to the decline of the liquid drainage capacity of the hearth, resulting in the burning loss of the lower part of the tuyere sleeve and the front end. According to the experience of Jiuquan Iron and Steel Technology Department, the order of influence degree of coke quality on blast furnace smelting process is M10, M40, CRI and CSR.
2. Quality of bituminous coal and anthracite: in addition to carbon content and hydrogen content, coal quality indicators also include ash, volatile matter, hardness (grindability index), moisture, alkali metal and calorific value. For blast furnace smelting, in addition to hardness (grindability index), the above components are the uniformity of tuyere coal injection and full tuyere coal injection. After all, the coal failure of individual tuyeres does too much harm to the soft melt zone of blast furnace. Coal is special for blast furnace injection. The most valuable heat comes from the "incomplete combustion" of coal into CO and h. CO and H are used to reduce oxygen in iron ore, so they are also called gas "reductants".
It is well known that high-temperature heat is required in the lower part of the blast furnace (enthalpy, the definition of enthalpy was explained in detail in the article "the impact of MgO on iron making production" published by China iron making network on December 16, 2021). The most suitable coal is the coal with low ash content and low oxygen content in the structure. Because there are less carbon oxygen bonds (covalent bonds formed between carbon atoms and oxygen atoms, which is one of the most common chemical bonds in organic chemistry and Biochemistry) in coal, the more heat is generated when CO is formed. From the perspective of heat generation, the lower the content of ash and volatile matter, the better the quality of pulverized coal. In addition to the ash, volatile, fixed carbon and moisture of pulverized coal, the calorific value of pulverized coal should also be paid attention to during operation. When other variables of blast furnace are relatively stable, the change of calorific value of pulverized coal will also cause the fluctuation of furnace temperature, air volume and feed rate.
Moreover, the local coal breaking at the tuyere has a great impact on the soft melt zone. The author's understanding of it is as follows:
(1) In the smelting process, each tuyere receives the same heat per unit time. Once the coal is cut off at some tuyeres, the hot air volume of the tuyere is still the same as that of the adjacent tuyeres. However, this tuyere is used to burn coke, which leads to the increase of coke combustion rate (productivity) and the significant increase of flame combustion temperature;
(2) As the coal of this tuyere is distributed to other tuyeres, the coke combustion rate (productivity) and flame temperature of other tuyeres are reduced, resulting in the asymmetry of the soft melt zone of the blast furnace. According to relevant calculations, the thickness of the local upward movement of the corresponding soft melt zone caused by the coal breaking of individual tuyeres can be twice that of the corresponding soft melt zone of the normal tuyere;
(3) The higher the coal ratio of the blast furnace, the more serious the problem of the asymmetry of the soft melt zone caused by the coal breaking tuyere. If two adjacent tuyeres break coal continuously, the harm to the soft melt zone will be more serious, because in the smelting process of the blast furnace with high coal ratio, the coal quantity of the two tuyeres will average to the other tuyeres, the flame temperature and production rate of the other tuyeres will be lower, and the asymmetry of the soft melt zone will also be greater. In order to ensure the uniformity of the soft melt zone, it is necessary to pay attention to the uniform blowing at the tuyere during the production process. Once the coal is cut off at some tuyeres, measures should be taken quickly to avoid the asymmetry of the soft melt zone, so as to maintain the uniform distribution of the air flow.
4、Summary
1. According to the impact of raw fuel quality on blast furnace smelting, when low-quality furnace charge has to be used to balance production cost or uncontrollable factors in the daily production organization process, the production system can take temporary countermeasures according to the bad characteristics of furnace charge to avoid furnace condition fluctuation;
2. In order to ensure stable and smooth production, control the most important process variables, and reduce the damage to the smooth operation of furnace due to the fluctuation of raw fuel quality, the production system can gradually digest the low-quality furnace burden in an organized and planned way;
3. If the enterprise has to use low-quality raw fuels for a long time due to external factors, the production system shall formulate targeted specific operation policies according to the principles of "ten stabilities in sintering and ironmaking production" and "attack, defend and retreat" in production operation, so as to transition the production of low-quality raw fuels and achieve the purpose of stable and smooth production.
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