Method for manufacturing reduced pellets

Abstract

To provide a blast furnace reduced pellet production method capable of efficiently producing blast furnace reduced pellets by a hydrogen reduction shaft furnace using self-fluxing pellets.SOLUTION: A reduced pellet production method has a process in which self-fluxing pellets are reduced with a reducing gas essentially consisting of hydrogen by a shaft furnace, and has: a first step in which samples of the self-fluxing pellets are subjected to a reduction test with the reducing gas at plural reduction temperatures; a second step in which, based on the test result of the reduction test, a relation between arrival times from a state where the self-fluxing pellets are non-reduced or a state where they are pre-reduced to wustites to a state where they reach a prescribed reduction rate and the reduction temperatures is obtained; and a third step in which, based on the relation, a reduction temperature in which the arrival time is shortest and reduction efficiency is highest in a range of 1,000°C or less is determined, and based on the reduction temperature, the blowing temperature of the reducing gas upon producing reduced pellets in the shaft furnace is set.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to a method for producing reduced pellets for blast furnaces. [Background technology]

[0002] In recent years, CO2 reduction has been demanded from the perspective of preventing global warming, and the required reduction amount is steadily increasing. In the steel industry, a large portion of CO2 emissions come from the ironmaking process, and in particular, there is a demand for reduction in CO2 emissions from blast furnaces. Reducing CO2 in blast furnaces is possible by reducing the amount of reducing agents (coke, pulverized coal, etc.) used in the blast furnace, and one method for doing so is charging with reduced iron. By charging with iron ore raw materials that have been reduced to iron (reduced iron), the reaction FeO + C → Fe + CO (direct reduction), which is a major endothermic reaction in the blast furnace, can be reduced. This reduces the amount of heat required to produce 1 ton of molten iron, making it possible to reduce the amount of reducing agent.

[0003] On the other hand, as the quality of iron ore resources deteriorates, the proportion of fine powder raw materials is increasing, and there is a need to increase the amount of pellets used that can effectively utilize fine powder raw materials. Under these circumstances, in order to achieve a reduction in CO2 emissions in the ironmaking process, it is necessary to use pellets that have been reduced to iron (hereinafter also referred to as "reduced pellets") as reduced iron for charging into the blast furnace, and to have a method for efficiently producing these reduced pellets. There are several methods for producing reduced iron and processes for utilizing them, but for example, as described in Patent Document 1, there is a method for producing reduced pellets using a shaft furnace. In Patent Document 1, coal gasification gas or the like is blown in from the bottom of the furnace as a reducing gas, and reduced iron oxide in pellet form is produced.

[0004] Furthermore, a process is envisioned in which hydrogen reduction is performed as a reduction treatment in the shaft furnace to produce reduced pellets, and these reduced pellets are then used in the blast furnace. In this case, since the hydrogen reduction shaft furnace is primarily hydrogen reduction, CO2 emissions can be minimized. And by using the reduced pellets in the blast furnace, CO2 emissions from the blast furnace can also be significantly reduced.

[0005] Traditionally, in the production of reduced pellets in shaft furnaces, acidic pellets with a basicity of less than 0.5 have been used. Here, basicity refers to the CaO / SiO2 ratio, which is the mass ratio of CaO to SiO2 in the pellet's composition. When large quantities of these acidic pellets (including reduced acidic pellets) are charged into a blast furnace, it becomes necessary to charge carbonates, mainly limestone (CaCO3) or dolomite (CaCO3·MgCO3), as a CaO source in order to adjust the basicity of the blast furnace slag. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Patent No. 5880790 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, these carbonates undergo an endothermic decomposition reaction in the blast furnace, represented by the reaction equation CaCO3 → CaO + CO2, which causes a delay in the heating of the bulk zone in the blast furnace. Generally, the reduction rate of iron oxide increases in proportion to the temperature, so this delay in heating reduces the gaseous reduction (indirect reduction) rate of iron oxide. When the indirect reduction rate decreases, the proportion of direct reduction increases in the lower part of the blast furnace, which in turn increases the amount of reducing agent used in the blast furnace.

[0008] Therefore, it is preferable that the reducing pellets used in blast furnace operation be self-fluxing pellets with a basicity of 0.5 or higher.

[0009] Therefore, the present invention aims to provide a method for producing reduced pellets for blast furnaces using a hydrogen reduction shaft furnace with self-fluxing pellets. [Means for solving the problem]

[0010] This invention was made to solve the above-mentioned problems, and the gist of the invention is as follows.

[0011] (1) A method for manufacturing reduced pellets, comprising reducing self-fluxing pellets with a reducing gas mainly composed of hydrogen in a shaft furnace, the method comprising: a first step of performing a reduction test with a reducing gas on a sample of the self-fluxing pellets at multiple reduction temperatures; a second step of determining the relationship between the time it takes for the self-fluxing pellets to reach a predetermined reduction rate from an unreduced state or a state pre-reduced to wustite, and the reduction temperature, based on the test results of the reduction test; and a third step of determining the reduction temperature that has the shortest time to reach the target and the highest reduction efficiency in the range of 1000°C or less, based on the relationship, and setting the injection temperature of the reducing gas when manufacturing the reduced pellets in the shaft furnace based on the determined reduction temperature.

[0012] (2) The method for manufacturing reduced pellets according to (1) above, characterized in that, in the third step, calculations are performed using a shaft furnace model to which the relationship obtained in the second step is applied, and the injection temperature of the reducing gas corresponding to the reduction temperature with the shortest arrival time is calculated and set as the injection temperature of the reducing gas when manufacturing reduced pellets.

[0013] (3) The method for manufacturing reduced pellets according to (1) above, characterized in that, in the third step, the reduction temperature with the shortest time to reach is applied as the pellet condition in the shaft furnace model, and operational calculations are performed for multiple reducing gas injection temperatures, and the reducing gas injection temperature that has the highest reduction efficiency in the calculated operation is set as the reducing gas injection temperature when manufacturing reduced pellets.

[0014] (4) For the test results for each of the plurality of reduction temperatures, perform reaction analysis using the unreacted core model to obtain a relational expression between the reduction temperature and the chemical reaction rate coefficient, and apply the relational expression as the conditions of the pellets in the shaft furnace model for calculation. The method for manufacturing a reduced pellet according to (2) or (3) above, characterized in that calculation is performed.

[0015] (5) The method for manufacturing a reduced pellet according to any one of (1) to (4) above, characterized in that the predetermined reduction rate is 90%.

[0016] (6) The method for manufacturing a reduced pellet according to any one of (1) to (5) above, characterized in that the basicity of the self - soluble pellet is 1.2 or less.

Advantages of the Invention

[0017] According to the present invention, it is possible to provide a method for manufacturing a reduced pellet that efficiently manufactures reduced pellets for a blast furnace using a hydrogen reduction shaft furnace with self - soluble pellets.

Brief Description of the Drawings

[0018] [Figure 1] It is a flowchart showing the method for manufacturing a reduced pellet of the present embodiment. [Figure 2] It is a graph showing the change over time of the reduction rate of each pellet at a reduction temperature of 800°C. [Figure 3] It is a graph showing the change over time of the reduction rate of each pellet at a reduction temperature of 900°C. [Figure 4] It is a graph showing the change over time of the reduction rate of each pellet at a reduction temperature of 1000°C.

Embodiments for Carrying Out the Invention

[0019] The hydrogen reduction shaft furnace according to this embodiment (hereinafter also simply referred to as "shaft furnace") produces reduced pellets by blowing hydrogen gas into the furnace from the bottom of the furnace. The hydrogen gas supplied in this embodiment is not limited to pure hydrogen gas composed solely of hydrogen gas, but also includes hydrogen-based gases in which hydrogen is the main component. For example, it may be a mixed gas with carbon monoxide, etc., containing 70 mol% or more of hydrogen gas, produced by reforming natural gas. Even with such a hydrogen-based gas, the pellets of this embodiment are reduced in the same way as with pure hydrogen gas, so the method for producing reduced pellets of this embodiment can be applied.

[0020] Next, the raw material pellets that are charged into the shaft furnace in this embodiment will be described. The raw material pellets are manufactured by mixing iron ore and auxiliary materials, granulating them, and then firing them using pellet manufacturing equipment. The auxiliary materials include limestone and binders.

[0021] The pellets used as raw materials in this embodiment are self-solubilating pellets. Furthermore, it is preferable that the self-solubilating pellets in this embodiment have a basicity of 0.75 or higher. As mentioned above, basicity is the mass ratio (CaO / SiO2) of CaO to SiO2 in the pellet's composition. Generally, pellets with a basicity of less than 0.5 are acidic pellets, while pellets with a basicity of 0.5 or higher are self-solubilating pellets.

[0022] When using large quantities of pellets with a basicity of less than 0.5, it is necessary to charge carbonates such as limestone (CaCO3) or dolomite (CaCO3·MgCO3) into the blast furnace to adjust the basicity. As mentioned above, the added CaCO3 causes a delay in the blast furnace's heating due to the endothermic reaction CaCO3 → CaO + CO2. This reduces the indirect reduction rate of iron oxide gas and increases the direct reduction rate, resulting in an increase in the amount of reducing agent, which is undesirable.

[0023] In contrast, in the case of the self-fluxing pellets of this embodiment, even if a large amount of the reduced pellets is used as raw material for the blast furnace, the amount of carbonate charged can be suppressed, thereby reducing the increase in the reducing agent ratio.

[0024] Furthermore, in the case of self-fluxing pellets with a basicity of 0.75 or higher, increasing the reduction temperature in the shaft furnace does not increase the reduction rate; on the contrary, the reduction efficiency may decrease. As described later, there is an optimal reduction temperature at which the reduction efficiency is maximized. Therefore, when using pellets with a basicity of 0.75 or higher, the pellets can be efficiently reduced by setting and controlling the reduction temperature in the shaft furnace using the method of this embodiment.

[0025] While there is no particular upper limit to the basicity of the pellets to be reduced, at least at a basicity of 1.2 or less, the reduction efficiency changes depending on the reduction temperature, and as described later, there is a reduction temperature at which the reduction efficiency is maximized. Therefore, by setting the reduction temperature to the one that maximizes the reduction efficiency using the method of this embodiment and manufacturing the reduced pellets, the reduced pellets can be manufactured efficiently.

[0026] Next, the method for manufacturing reduced pellets according to this embodiment will be explained based on Figure 1. Figure 1 is a flowchart of the method for manufacturing reduced pellets. First, as the first step, hydrogen reduction tests are performed on iron oxide pellets to be hydrogen-reduced at multiple reduction temperatures (S101). Next, as the second step, the relationship between each reduction temperature and the time it takes for the reduction rate to reach 90% is determined from the test results in S101 (S102). Next, as the third step, based on the relationship determined in S102, the operating conditions (hydrogen gas injection temperature) are determined such that the reduction temperature in the shaft furnace is 1000°C or lower and the temperature at which the reduction efficiency is highest is determined (S103). The above is an overview of the method for manufacturing reduced pellets according to this embodiment.

[0027] The details of each process will be explained. In the first process, S101, hydrogen reduction tests are performed on the raw material pellets at various reduction temperatures. These hydrogen reduction tests are conducted on samples extracted from the actual raw material pellets to estimate the appropriate reduction temperature for the entire pellet. Specifically, a small sample (a few pellets) is prepared from the pellets to be used as raw material, and heated in the reaction tube of the reduction test apparatus under a pure hydrogen gas atmosphere to reduce iron oxide to metallic iron. The weight change at this time is measured, and the change in the reduction rate over time is determined based on the weight change. Reduction tests are performed on the same type of pellet at multiple reduction temperatures (temperatures inside the reaction tube), and the change in the reduction rate over time is determined for each reduction temperature. The pellets used for the reduction tests may be used in an unreduced state or in a state that has been pre-reduced to wustite, but the reduction tests are performed on pellets in the same reduced state at multiple reduction temperatures. As described later, when applying the reaction analysis results using an unreacted nucleus model to the reduction test results from wustite to metallic iron to the shaft furnace model to determine operating conditions, the test in S101 should also be conducted using pellets that have been pre-reduced to wustite.

[0028] The reduction test apparatus can consist of an electric furnace, a gas supply unit that supplies hydrogen gas into the reaction tube of the electric furnace, and a weight change measuring unit that measures the weight of the sample; for example, a thermobalance may be used. The reduction rate is determined from the oxygen balance contained in the pellet (the mass of oxygen contained in the iron oxide before reduction and the mass of oxygen removed by reduction) based on the weight change of the sample before and after the start of reduction.

[0029] Furthermore, when using a hydrogen-based gas containing components other than hydrogen to manufacture reduced pellets in a shaft furnace, it is preferable to use a gas with the same or similar components as the gas used in manufacturing during the reduction test. Although testing and evaluation are possible using pure hydrogen gas, conducting the hydrogen reduction test with a gas with similar components to the reducing gas used in the shaft furnace allows for a more accurate evaluation of the reduction temperature at which reduction efficiency is highest.

[0030] Next, in S102, the reduction efficiency for each reduction temperature is evaluated based on the change in the reduction rate obtained. The evaluation of reduction efficiency is based on the time required to reach a predetermined reduction rate. When reduction tests are performed on the same type of pellet at different reduction temperatures, a shorter time to reach a predetermined reduction rate indicates a reduction temperature with higher reduction efficiency up to that rate. In this embodiment, the relationship between each reduction temperature and the time it takes to reach a 90% reduction rate is determined from the test results in S101. Specifically, the time it takes to reach a 90% reduction rate is determined for each reduction temperature from the data on the change in the reduction rate over time for each reduction temperature obtained in the test in S101. A shorter time to reach a 90% reduction rate indicates higher reduction efficiency.

[0031] As a result of diligent research by the inventors, it was found that in reduction using hydrogen gas, when the reduction rate is less than 90%, the reduction efficiency increases with increasing reduction temperature. However, when the reduction rate is 90% or higher, the time required to reach the target reduction rate (a predetermined reduction rate) decreases up to a certain temperature in the range of 1000°C or below, but beyond that temperature, the required time may increase. In other words, it was found that when the reduction rate is 90% or higher, there is a reduction temperature at which the reduction efficiency is maximized (the above required time is minimized). In the higher temperature range above 1000°C, the reduction efficiency improves again, but the energy consumption for raising the hydrogen gas temperature increases, reducing energy efficiency, which is undesirable. Therefore, by performing the reduction at the reduction temperature at which the above reduction efficiency is maximized, the highest reduction efficiency is achieved in the range of 1000°C or below. In this embodiment, unless otherwise specified, "highest reduction efficiency" means the highest reduction efficiency in the range of 1000°C or below.

[0032] Based on this finding, the method of this embodiment makes it possible to efficiently produce pellets with a reduction rate of 90% or higher at the reduction temperature with the highest reduction efficiency. Therefore, the predetermined reduction rate only needs to be 90% or higher, but in this embodiment, the predetermined reduction rate will be described as 90%. When producing pellets with a reduction rate higher than 90% in a shaft furnace, the predetermined reduction rate should be set according to that reduction rate.

[0033] Here, as an example of the S101 and S102 processes described above, we will explain the results of hydrogen reduction tests conducted on multiple types of pellets, A to E. In the tests, three pellets of each type A to E, having the chemical compositions shown in Table 1, were used as samples in a reduction test apparatus and reduced from an unreduced state to metallic iron under a pure hydrogen gas atmosphere. A thermobalance was used as the reduction test apparatus. Hydrogen gas was supplied at 5 NL / min. Reduction tests were conducted at reaction tube temperatures of 800°C, 900°C, and 1000°C, which are the reduction temperatures. The reduction rate was determined from the weight change of the pellets as described above. The reduction rate was calculated with the case where all the iron oxide in the pellet was hematite being considered as 0%.

[0034] [Table 1]

[0035] Based on the above test results, Figures 2 to 4 show graphs illustrating the change in reduction rate over time at each reduction temperature for each sample. Figure 2 shows the graph for reduction temperatures of 800°C, Figure 3 for 900°C, and Figure 4 for 1000°C, corresponding to the reduction test results in the first step. Table 2 shows the time required to reach 90% reduction in the reduction rate changes shown in Figures 2 to 4. Table 2 corresponds to the relationship between each reduction temperature derived in the second step and the time required to reach a reduction rate of 90%.

[0036] [Table 2]

[0037] Comparing the case of a reduction temperature of 800°C shown in Figure 2 with the case of a reduction temperature of 900°C shown in Figure 3, both pellets A and B (acidic pellets) with a basicity of less than 0.5, and pellets C, D, and E with a basicity of 0.75 or higher according to this embodiment, take less time to reach a reduction rate of 90% at the higher reduction temperature of 900°C. Furthermore, at a reduction temperature of 1000°C shown in Figure 4, pellets A and B with a basicity of less than 0.5 took even less time to reach a reduction rate of 90% than at 900°C. On the other hand, for pellets C to E with a basicity of 0.75 or higher according to this embodiment, the time to reach a reduction rate of 90% at 1000°C was longer than at 900°C, and for pellets C and D, it was longer than at 800°C. In addition, it was confirmed that for pellets C to E according to this embodiment, if the reduction rate is 90% or higher, the time to reach that reduction rate is faster at 900°C than at 1000°C.

[0038] From the above test examples, it can be seen that, for self-solubilating pellets with a basicity of 0.75 or higher according to this embodiment, when the reduction rate is 90% or higher, the reduction efficiency does not improve by increasing the reduction temperature as with acidic pellets, but rather there is a specific reduction temperature at which the reduction efficiency is improved and maximized. Furthermore, the above tests confirmed that the reduction temperature at which the reduction efficiency is highest below 1000°C is around 900°C.

[0039] Furthermore, it can be seen that the reduction efficiency drops significantly when the reduction temperature is set higher than around 900°C, to 1000°C. Increasing the reduction temperature requires a lot of energy to raise the hydrogen gas injection temperature. Therefore, even if reduction is performed at a reduction temperature higher than the temperature around 900°C where the reduction efficiency is highest, the reduction efficiency decreases despite consuming more energy.

[0040] The above test examples evaluated three reduction temperatures, but by conducting tests at more reduction temperatures, the relationship between reduction temperature and the time to reach 90% can be determined more accurately, and the reduction efficiency can be evaluated more precisely. As described later, when calculating the hydrogen gas injection temperature using model calculations, more accurate calculations can be achieved by conducting reduction tests at intervals of approximately 10°C and using the test results for model calculations.

[0041] Next, in S103, based on the relationship obtained in S102, the operating conditions are determined such that the reduction temperature in the shaft furnace is the temperature at which the reduction efficiency is highest. Specifically, the reduction temperature T with the highest reduction efficiency in the range of 1000°C or less is determined. R Determine the reduction temperature T R The injection temperature of the reducing gas is determined based on the following. In the above test example, the reduction time is shortest when the reduction temperature is 900°C within the range of 1000°C or less, and therefore the temperature T is the temperature at which the reduction efficiency is highest. R Set the temperature to 900°C to determine the injection temperature of the reducing gas.

[0042] Specifically, the hydrogen gas injection temperature can be determined by model calculations of the shaft reactor model. The process of determining the hydrogen gas injection temperature using the model involves (1) obtaining the rate equation for the pellet reduction reaction (relationship between reduction temperature and chemical reaction rate coefficient) using the unreacted nucleus model, and determining the reduction temperature T that yields the highest reduction efficiency. R The process of determining the relationship between (2)(1) and the reduction temperature T that yields the highest reduction efficiency. R This is done by performing calculations (simulations) using a shaft furnace model that applies the values ​​of the pellets to be charged, and by determining an appropriate hydrogen gas injection temperature based on the calculation results of (3)(2).

[0043] First, in step (1), a reaction analysis using an unreacted nucleus model is performed on the results of hydrogen reduction tests at multiple reduction temperatures conducted in S101. This allows for the calculation of a relationship between the reduction temperature of the pellet and the chemical reaction rate coefficient, which is used as the pellet condition for application to the shaft reactor model.

[0044] Also, from the test results, the reduction temperature T at which the pellets applied to the shaft furnace model have the highest reduction efficiency is determined as the condition for the pellets. R In the above test example, the reduction temperature T at which the reduction efficiency is the highest R is 900°C. When determining the reduction temperature T at which the reduction efficiency is the highest R it is preferable to conduct the reduction test by changing the reduction temperature as finely as possible, and more accurately determine the reduction temperature at which the time to reach a reduction rate of 90% is the shortest. For example, conduct the reduction test by changing the reduction temperature in 10°C increments, and determine the reduction temperature T at which the reduction efficiency is the highest R That's all. Also, when determining the reduction temperature T R it may be determined from among the multiple reduction temperatures at which the actual tests were conducted, or the reduction temperature at which the reduction efficiency is the highest may be obtained from an approximate curve showing the relationship between the reduction temperature and the reduction efficiency of the test results.

[0045] Next, in the step (2), as the conditions for the charged pellets, the operation of the shaft furnace is calculated using the relational expression obtained in (1) and the shaft furnace model using the reduction temperature T at which the reduction efficiency is the highest. The calculation by the model is performed under multiple hydrogen gas injection temperature conditions. The multiple hydrogen gas injection temperatures are set based on the hydrogen reduction test results obtained in S101 and S102. For example, when the reduction temperature T at which the reduction efficiency is the highest R is 900°C, the multiple hydrogen gas injection temperatures include at least the hydrogen gas injection temperature such that the reduction temperature in the shaft furnace is around 900°C. Since the hydrogen gas is deprived of sensible heat by the pellets in the furnace, the reduction temperature in the shaft furnace is lower than the gas injection temperature. The temperature difference should be confirmed in advance by model calculation or the like, and the hydrogen gas injection temperature corresponding to the reduction temperature may be set according to the temperature difference. R That's all.

[0046] In calculating the shaft furnace model, the calculation is performed by setting equipment conditions such as the dimensions of the shaft furnace, pellet conditions such as particle size and charging temperature, other conditions such as the composition and airflow rate of the hydrogen gas used for injection, and the production volume of reduced pellets. Furthermore, a known model can be used as the shaft furnace model, and in the method for manufacturing reduced pellets in this embodiment, a one-dimensional shaft furnace model can be used.

[0047] Next, in step (3), the gas injection temperature that results in the operation with the highest reduction efficiency is determined from the model calculation results for multiple hydrogen gas injection temperatures performed in step (2). The method for evaluating the reduction efficiency is not particularly limited as long as it can be confirmed that the operation has a high reduction efficiency, but for example, the average reduction rate of the produced reduced pellets can be calculated and evaluated using model calculations. The hydrogen gas injection temperature that yielded the highest average reduction rate can be said to be the injection temperature with the highest reduction efficiency under those operating conditions. When producing reduced pellets using the pellets evaluated in an actual shaft furnace, if hydrogen gas is injected at the determined hydrogen gas injection temperature, reduced pellets can be produced efficiently.

[0048] In step (3) of S103 described above, the gas injection temperature was determined from the average reduction rate obtained by performing calculations at multiple reduction temperatures. However, the reduction temperature in the shaft furnace is the reduction temperature T which has the highest reduction efficiency. R Alternatively, the gas injection temperature that results in the desired outcome can be directly calculated using model calculations, and that temperature can be determined as the actual gas injection temperature.

[0049] According to the method of this embodiment described above, when producing reduced pellets by hydrogen reduction in a shaft furnace using self-fluxing pellets with a basicity of 0.75 or higher, where there is a reduction temperature at which the reduction efficiency is maximized, it is possible to determine a gas injection temperature that is highly efficient in reducing the reduction, and thus efficiently produce reduced pellets. Furthermore, according to this embodiment, since the hydrogen gas is injected at a temperature that is highly efficient in reducing the reduction without raising the temperature of the hydrogen gas to a high temperature that would reduce production efficiency, the energy required to heat the hydrogen gas can also be reduced.

[0050] In other words, according to this embodiment, by setting the operating temperature of the shaft furnace within an appropriate range, it is possible to achieve high production volume while reducing energy consumption, and to reduce CO2 emissions throughout the entire process. Furthermore, by producing reduced pellets from the self-fluxing pellets with a basicity of 0.75 in this embodiment and using them as blast furnace raw materials, the amount of carbonate used in the blast furnace can be reduced compared to when acidic pellets are used. This also makes it possible to reduce the blast furnace reducing agent ratio.

[0051] In this embodiment, the method for determining the operating conditions in S103 is shown as determining the hydrogen gas injection temperature by calculation using a shaft furnace model, but the embodiment is not limited to this.

[0052] For example, based on the relationship obtained in S102, the operating conditions such as the hydrogen gas injection temperature may be determined so that the temperature distribution in the vertical direction within the shaft furnace becomes such that the region of temperatures where the reduction efficiency decreases is small and the region of temperatures where the reduction efficiency is high is large. The temperature distribution can be confirmed, for example, by measuring the temperature at multiple positions in the vertical direction within the shaft furnace.

[0053] Alternatively, based on the relationship obtained in S102, a heat flow ratio that results in a temperature distribution with high reduction efficiency may be determined, and this heat flow ratio may be determined as the operating condition for the shaft furnace. [Examples]

[0054] The present invention will be described in more detail below with reference to examples. In the examples, the hydrogen gas injection temperature was determined by performing calculations (simulations) using a shaft furnace model.

[0055] First, using pellet D as shown in the example of the embodiment, hydrogen reduction tests from wustite to metallic iron were conducted at multiple reduction temperatures, and the relationship between the reduction temperature and the time to reach a reduction rate of 90% was determined. From this relationship, the reduction temperature T that yielded the highest reduction efficiency was determined. R It was set to 920℃.

[0056] Next, a reaction analysis was performed on the test results using a single-interface unreacted nucleus model. Through this reaction analysis, the relationship between the reduction temperature and the chemical reaction rate coefficient in the reduction reaction from iron oxide to reduced iron was calculated.

[0057] Then, the calculated relationship and the reduction temperature T that yields the highest reduction efficiency were determined. R The operation of hydrogen reduction of pellets was calculated using a shaft furnace model to which the following was applied. The model calculations were performed by changing the hydrogen gas injection temperature in 10°C increments within the range of 900°C to 1030°C. In the model calculations of this embodiment, a shaft furnace with an inner diameter of 0.1m and a height of 4m was used.

[0058] As the shaft furnace model, we used a one-dimensional shaft furnace model (Reference: Hara, et al., "Mathematical Model of a Shaft Furnace for Iron Ore Reduction," Iron and Steel, The Iron and Steel Institute of Japan, March 1976, Vol. 62, No. 3, pp. 315-323).

[0059] In the model calculations, the reduction temperature T that yields the highest reduction efficiency, as described above, is the one that yields the highest reduction efficiency. R The temperature was set to 920°C. The pellet particle size was 12.5 mm, and the pellet charging temperature was 25°C. The reducing gas was pure hydrogen gas, and the airflow rate was 356 NL / min. The production rate of reduced pellets was 12.56 kg / h.

[0060] Table 3 shows the average reduction rates of the produced reduced pellets at different hydrogen gas injection temperatures, as a result of model calculations performed under the above conditions. The average reduction rate here refers to the product reduction rate calculated by the model calculations.

[0061] [Table 3]

[0062] In the 920-940°C range, reduced pellets with an average reduction rate exceeding 90% were produced. However, in the higher gas injection temperature range of 950°C and above, the average reduction rate temporarily decreased and fell below 90%. It is understood that in order to obtain pellets with an average reduction rate of 90% or more in the range above 950°C, the gas injection temperature must be higher than 1030°C. Therefore, the optimal gas injection temperature for operating the shaft furnace under the above conditions was determined to be 920-940°C. By operating the shaft furnace at this hydrogen gas injection temperature in actual operation, reduced pellets can be produced more efficiently while suppressing energy consumption.

[0063] On the other hand, if the gas injection temperature is set higher than 1030°C, even if a reduction rate of 90% or more is achieved, a lot of energy will be consumed to raise the temperature of the hydrogen gas, resulting in poor energy efficiency. Therefore, it has been confirmed that when reducing self-fluxing pellets with a basicity of 0.75 or higher in a hydrogen reduction shaft furnace, reducing pellets can be efficiently produced by setting the gas injection temperature according to the method of the present invention.

Claims

1. A method for producing reduced pellets, comprising reducing self-fluxing pellets having a basicity of 0.75 or more and 1.2 or less with a reducing gas containing 70 mol% or more of hydrogen in a shaft furnace, A first step involves performing a reduction test with a reducing gas on a sample of the self-fluxing pellet at multiple reduction temperatures, A second step involves determining the relationship between the time it takes for the self-solubilating pellet to reach a predetermined reduction rate of 90% or more from an unreduced state or a state where it has been pre-reduced to wustite, and the reduction temperature, based on the test results of the reduction test. A third step is to determine the reduction temperature that has the shortest arrival time and highest reduction efficiency in the range of 1000°C or less based on the aforementioned relationship, and to set the injection temperature of the reducing gas when manufacturing the reduced pellets in the shaft furnace based on the determined reduction temperature. A method for manufacturing reduced pellets, characterized in that, in the third step, calculations are performed using a shaft furnace model to which the relationship obtained in the second step is applied, and the injection temperature of the reducing gas corresponding to the reduction temperature with the shortest arrival time is calculated and set as the injection temperature of the reducing gas when manufacturing reduced pellets.

2. A method for producing reduced pellets, comprising reducing self-fluxing pellets having a basicity of 0.75 or more and 1.2 or less with a reducing gas containing 70 mol% or more of hydrogen in a shaft furnace, A first step involves performing a reduction test with a reducing gas on a sample of the self-fluxing pellet at multiple reduction temperatures, A second step involves determining the relationship between the time it takes for the self-solubilating pellet to reach a predetermined reduction rate of 90% or more from an unreduced state or a state where it has been pre-reduced to wustite, and the reduction temperature, based on the test results of the reduction test. A third step is to determine the reduction temperature that has the shortest arrival time and highest reduction efficiency in the range of 1000°C or less based on the aforementioned relationship, and to set the injection temperature of the reducing gas when manufacturing the reduced pellets in the shaft furnace based on the determined reduction temperature. A method for manufacturing reduced pellets, characterized in that, in the third step, the reduction temperature with the shortest time to reach is applied as the pellet condition in the shaft furnace model, operational calculations are performed for a plurality of reducing gas injection temperatures, including the reducing gas injection temperature corresponding to the reduction temperature with the shortest time to reach, and the reducing gas injection temperature with the highest reduction efficiency in the calculated operation is set as the reducing gas injection temperature when manufacturing reduced pellets.

3. A method for producing reduced pellets according to claim 1 or 2, characterized in that a reaction analysis using an unreacted nucleus model is performed on the test results for each of several reduction temperatures to obtain a relationship between the reduction temperature and the chemical reaction rate coefficient, and the relationship is applied as the pellet condition in the shaft furnace model to perform calculations.

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