Production method of molten iron

Abstract

To provide a production method of molten iron by which the generation amount of CO2 is suppressed, and the reduction and melting of reduced iron can be further promoted.SOLUTION: In a production method of molten iron by melting reduced iron by arc heating, plant-derived biomass coal is used as a carbon source for melting the reduced iron, and preferably, the biomass coal is charged into a position within a radius of 1.0 m around a point where the reduced iron is supplied to the molten iron. The ratio of carbon derived from the fixed carbon of the biomass coal to the carbon content in the additive to be added as the carbon source is 30 mass% or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing molten iron using an electric furnace. [Background technology]

[0002] Conventionally, in order to reduce the amount of power consumed by arc generation in an electric furnace, a carbonaceous material is charged as a heat source to increase the carbon concentration in the molten metal and lower the melting point. Patent Document 1 discloses a technology in which a carbonaceous material is injected using an immersion lance when a cold iron source such as direct reduced iron is melted by arc heating. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-145393 Summary of the Invention [Problem to be solved by the invention]

[0004] When melting reduced iron such as DRI (Direct Reduced Iron) or HBI (Hot Briquetted Iron) in an electric furnace, carbonaceous materials are added to promote the reduction and melting of iron oxide. In recent years, there has been an increase in reduced iron with low or no carbon content, such as that produced by hydrogen reduction, and more carbonaceous materials are required to melt this reduced iron. On the other hand, reducing CO2 emissions is also important from an environmental perspective.

[0005] In view of the above-mentioned problems, an object of the present invention is to provide a method for producing molten iron that can suppress the amount of CO2 generated and further promote the reduction and dissolution of reduced iron. [Means for solving the problem]

[0006] The present invention is as follows. [1] 1. A method for producing molten iron by melting reduced iron by arc heating, comprising: using plant-derived biomass charcoal as a carbon source for melting the reduced iron; and introducing the biomass charcoal into a position within a radius of 1.0 m from a point where the reduced iron is supplied to the molten iron. [2] The method for producing molten iron according to the above [1], characterized in that the proportion of carbon derived from fixed carbon in the biomass charcoal among the carbon content in the additive added as the carbon source is 30 mass% or more. [3] The method for producing molten iron according to the above [1] or [2], characterized in that the proportion of carbon derived from the mold pig iron in the carbon content of the additive added as the carbon source is 50 mass% or less. [4] The method for producing molten iron according to any one of the above [1] to [3], wherein the reduced iron is solid reduced iron having a carbon content of 1 mass % or less. [5] The method for producing molten iron according to [4] above, wherein the reduced iron is solid reduced iron having a carbon content of 0.05 mass % or less and a metallization rate of 90% or more. [Effects of the Invention]

[0007] According to the present invention, it is possible to suppress the amount of CO2 generated and further promote the reduction and dissolution of reduced iron. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 2 is a diagram for explaining an operation method of an electric furnace according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Recently, reduced iron with a low carbon content has been widely produced in order to reduce CO2 emissions. Such reduced iron with a low carbon content, particularly reduced iron produced by hydrogen reduction, contains substantially no carbon, and carburization is therefore important to promote reduction and dissolution. Furthermore, when performing carburization, the use of plant-derived biomass charcoal as a carbon source, rather than carbonaceous materials such as general coal or coke, can further reduce the amount of CO2 emissions.

[0010] Because biomass charcoal is derived from plants, even if it emits CO2 when burned, the CO2 is absorbed by the plants through photosynthesis during their growth process, so the amount of CO2 emitted is considered to be essentially zero.In contrast, general coal and coke are fossil fuels, so the amount of CO2 emitted is not considered to be zero.

[0011] Like conventional coal, biomass charcoal derived from plants contains volatile matter, so when it is added to the furnace, it rapidly generates gas, which contributes to the stirring of the slag and molten iron. Furthermore, the gas generation breaks down the biomass charcoal, increasing the reaction interface area and promoting carburization of the molten iron. Furthermore, adding biomass charcoal near the point where reduced iron (DRI or HBI) is added can locally increase the carbon concentration of the molten iron, promoting the melting of the reduced iron. This is because the reduced iron is carburized by the carbon in the molten iron, lowering its melting point. Furthermore, CO gas and methane gas are generated, so gas stirring also contributes to promoting the melting. This is because the boundary layer between the reduced iron and the molten iron becomes thinner, facilitating mass transfer.

[0012] 1 is a diagram illustrating a method for operating an electric furnace 1 according to this embodiment. The method for producing molten iron in the electric furnace 1 will be described in detail below. In the present invention, the electric furnace may be of either a DC or AC type, and the method of electrification is not particularly limited.

[0013] First, the furnace lid is rotated with the seed molten metal 2 remaining, and pig iron 6 is charged from the bucket 5 as a carbon source. Note that charging of pig iron may be omitted depending on the type of reduced iron and carbonaceous material to be charged subsequently. Next, the mouth of the bucket 5 is closed, the furnace lid is closed, an electrode (not shown) is lowered to the vicinity of the molten iron surface, an arc is generated from the bottom end of the electrode, and charging of reduced iron begins. At this time, biomass charcoal is also charged as a recarburizer. As a result, iron oxide (FeO) in the reduced iron is reduced by the carbon in the reduced iron and pig iron, or by the carbon in the charged recarburizer.

[0014] The metallic iron in the reduced iron then melts to form molten iron, and the gangue components (SiO2 and Al2O3) in the reduced iron also melt and become part of the slag. Furthermore, the reduction and dissolution of the reduced iron generates CO gas and volatile gases from the recarburizer, causing the slag to foam. These CO gas and methane gas are discharged through an exhaust section (not shown), and some of the carbon dissolves in the molten iron. As the reduction and dissolution of the reduced iron progresses, the amounts of molten iron and slag increase. When the slag surface reaches a certain height, the slag discharge port 4 is opened and the slag is discharged. When the amount of molten iron reaches a predetermined level, the tapping port 3 is opened and the molten iron, excluding the seed molten metal, is discharged.

[0015] The reduced iron used in this embodiment may be DRI or HBI, and the production method is not particularly limited, but when reducing reduced iron produced using natural gas or the like, it is preferable to use reduced iron with a carbon content of 1 mass % or less in order to reduce the amount of CO2 generated. Furthermore, reduced iron produced by hydrogen reduction contains substantially no carbon, and is therefore more preferable when the aim is to reduce the amount of CO2 generated.

[0016] Currently, reduced iron is produced in a shaft furnace by reducing pelletized iron ore with natural gas. Because the main component of natural gas is methane (CH4), the iron ore is reduced by the carbon and hydrogen in the methane, and some of the carbon carburizes into the reduced iron. Therefore, the carbon content in the reduced iron is determined by operating conditions such as temperature and pressure, as well as the metallization rate. In other words, when producing reduced iron using natural gas, if the carbon content in the reduced iron is kept below 1% by mass, the metallization rate is likely to be low.

[0017] On the other hand, in the case of reduced iron produced by hydrogen reduction, only hydrogen is used as the reducing agent, and the reducing agent does not contain carbon, so the reduced iron is substantially free of carbon and can have a high metallization rate. Therefore, in this embodiment, it is more preferable to use reduced iron produced by hydrogen reduction. The reduced iron produced by hydrogen reduction has, for example, a carbon content of 0.05 mass% or less and a metallization rate of 90% or more.

[0018] As described above, when reducing and melting reduced iron with a low carbon content, carburization is important because the carbon source is likely to be insufficient. On the other hand, if a large amount of steam coal is added as a recarburizer, a large amount of CO2 is generated due to the carbon in the recarburizer, and CO2 cannot be reduced even when reduced iron with a low carbon content is used. Therefore, in this embodiment, an example will be described in which pig iron and a recarburizer (biomass charcoal) are used as additives added as carbon sources. Note that pig iron is produced by solidifying molten iron produced using coke in a blast furnace, so the CO2 generated from the carbon in the pig iron cannot be considered to be zero.

[0019] Biomass charcoal is charged from a hopper or the like (not shown), and the charging position of the biomass charcoal is within a radius of 1.0 m from the position where the reduced iron is charged. If the charging position of the biomass charcoal is farther away from the position where the reduced iron is charged than a radius of 1.0 m, the carburization effect and the stirring effect due to the decomposition of volatile matter become smaller, and the effect of promoting the dissolution of the reduced iron decreases.

[0020] Furthermore, the VM (volatile matter) content of the biomass coal added as a recarburizer is preferably 20% by mass or more and 35% by mass or less. This is because if the VM content is less than 20% by mass, the amount of gas generated is small, the recarburizer is difficult to break down into small pieces, and the stirring effect is reduced. On the other hand, if the VM content exceeds 35% by mass, the amount of gas generated becomes too large, increasing the loss of the recarburizer due to scattering outside the furnace and resulting in a large yield loss. Furthermore, biomass coal and steam coal may be used in combination as a recarburizer. In this case, since the VM content of steam coal is relatively high, it is preferable to use biomass coal with a relatively low VM content and set the weighted average VM content of the biomass coal and steam coal to 35% by mass or less.

[0021] Next, the proportion of carbon in the entire additive will be described. The proportion of carbon derived from fixed carbon in biomass charcoal in the carbon content of the additive, which is a carbon source, is preferably 30% by mass or more and 80% by mass or less. If the proportion of carbon derived from fixed carbon in biomass charcoal in the carbon content of the additive is less than 30% by mass, the proportion of pig iron and the like increases, thereby reducing the effect of reducing the amount of CO2 generated. Furthermore, if the proportion of carbon derived from fixed carbon in biomass charcoal in the carbon content of the additive exceeds 80% by mass, the proportion of volatile matter decreases accordingly, resulting in a smaller amount of gas generated, which may reduce the stirring effect and make it difficult for the reduced iron to dissolve. When biomass charcoal and steam coal are used in combination, the decomposition reaction of volatile matter also occurs in the steam coal, so the total proportion of carbon derived from fixed carbon in biomass charcoal and carbon derived from fixed carbon in steam coal is preferably 80% by mass or less.

[0022] Furthermore, it is preferable that the proportion of carbon derived from the pig iron in the carbon content of the additive, which is a carbon source, be 50 mass% or less. As mentioned above, the carbon in the pig iron is not derived from plants, and this increases the amount of CO2 generated.

[0023] As described above, in this embodiment, biomass charcoal is used as a recarburizer when reducing and melting reduced iron in an electric furnace. This allows the reduction and melting of reduced iron to be promoted while substantially suppressing the amount of CO2 generated. In particular, even when reducing and melting reduced iron with a low carbon content, the amount of CO2 generated can be further suppressed and the reduction and melting of reduced iron can be promoted. [Example]

[0024] Next, an example of the present invention will be described, but the conditions are merely examples for confirming the feasibility and effects of the present invention, and the present invention is not limited to the description of this example. The present invention can be implemented in various ways to achieve the object of the present invention without departing from the gist of the present invention.

[0025] Experiments were conducted in a 100-ton electric furnace, as shown in Figure 1. First, the furnace lid was rotated with 10 t of molten iron remaining as a seed molten metal, and pig iron was charged from the bucket. The lid was then closed, the electrode was lowered, and an arc was generated from the bottom of the electrode to initiate the charging of reduced iron and recarburizer. Three types of biomass charcoal or steam coal with the compositions shown in Table 1 were used as recarburizers. All charged pig iron had the composition shown in Table 2. The charged reduced iron was DRI with a metallization rate of 90%, and three types of reduced iron with different carbon contents were charged. The reduced iron was charged for one hour while adjusting the feed rate to prevent the reduced iron from sticking and becoming layered. The recarburizer and pig iron were added so that the carbon concentration of the resulting molten iron was 0.3% by mass. The amount of biomass charcoal or steam coal added was varied depending on the proportion of carbon derived from the pig iron in the carbon source. In Example 9, biomass coal B and general coal were used in combination as the carbon additive, and the proportion of carbon derived from the fixed carbon of biomass coal B and the proportion of carbon derived from the fixed carbon of general coal in the additive were 30 mass% and 20 mass%, respectively.

[0026] In the examples, the melting characteristics were evaluated as an index of the reduction and melting of reduced iron, and the amount of CO2 generation was also evaluated. Furthermore, the amount of carbon emission was also evaluated as an index of the yield of the recarburizer. For the melting characteristics, the reduced iron was charged for one hour, and the difference in the amount of reduced iron charged at that time was evaluated, and the relative amount of charge was evaluated with Comparative Example 1 set to 1. For the amount of CO2 generation, the amount of carbon required to produce 1 ton of molten iron with a specified C concentration was calculated using the following formula (1), and the relative amount of CO2 generation was evaluated with Comparative Example 1 set to 1. Note that the C concentration in the recarburizer is the sum of fixed carbon (FC) and volatile matter (VM). When biomass coal was used as the recarburizer, the amount of recarburizer charged was set to 0 in the calculation. Carbon content (t) = mold pig input amount (t) × C concentration in mold pig iron (mass %) + reduced iron input amount (t) × C concentration in reduced iron (mass %) + recarburizer input amount (t) × C concentration in recarburizer (mass %)... (1)

[0027] The amount of carbon released was evaluated based on how much of the fixed carbon (FC) in the recarburizer added over a given time period was transferred to dust. Specifically, since all of the carbon in the dust was caused by the recarburizer scattering, the total amount of dust generated was measured, and the ratio of the fixed carbon released in the recarburizer was calculated using the following formula (2). The relative ratio of the fixed carbon released was evaluated, assuming that Comparative Example 1 was 1. Fixed carbon dispersion rate (mass%) = (total amount of dust generated (t) × C concentration in dust (mass%)) / (amount of recarburizer input (t) × FC concentration (mass%) × 100 (2)

[0028] [Table 1]

[0029] [Table 2]

[0030] [Table 3]

[0031] In Table 3, the position of the recarburizer introduction indicates the distance (m) from the position where the reduced iron was introduced. All of Examples 1 to 9 showed better dissolution characteristics and CO2 generation rates than Comparative Examples 1 and 2. Furthermore, all of the examples, except for Example 4, showed better carbon emissions than Comparative Examples 1 and 2. Note that Comparative Example 2 is an example in which the distance from the position where the reduced iron was introduced to the position where the recarburizer was introduced was outside the range of the present invention. Although the CO2 generation rate was reduced, the distance from the reduced iron introduction position to the position where the recarburizer was introduced reduced the amount of fixed carbon available for carburization, resulting in a large amount of evaporation loss. Furthermore, Comparative Example 2 used biomass coal A, which has a lower ash content than conventional coal, thereby reducing the amount of slag compared to conventional coal and improving the stirring effect. However, the reduction and dissolution of the reduced iron did not proceed sufficiently due to the reduction in the amount of fixed carbon available for carburization due to evaporation, and the dissolution characteristics were also poor.

[0032] The present invention includes the following methods. [Method 1] 1. A method for producing molten iron by melting reduced iron by arc heating, comprising: using plant-derived biomass charcoal as a carbon source for melting the reduced iron; and introducing the biomass charcoal into a position within a radius of 1.0 m from a point where the reduced iron is supplied to the molten iron. [Method 2] The method for producing molten iron according to Method 1, characterized in that the proportion of carbon derived from fixed carbon in the biomass charcoal among the carbon content in the additive added as the carbon source is 30 mass% or more. [Method 3] 3. The method for producing molten iron according to method 1 or 2, characterized in that the proportion of carbon derived from the pig iron in the carbon content of the additive added as the carbon source is 50 mass% or less. [Method 4] 4. The method for producing molten iron according to any one of Methods 1 to 3, wherein the reduced iron is solid reduced iron having a carbon content of 1 mass % or less. [Method 5] 5. The method for producing molten iron according to method 4, wherein the reduced iron is solid reduced iron having a carbon content of 0.05 mass % or less and a metallization rate of 90% or more. [Explanation of symbols]

[0033] 1 electric furnace 2 types of hot spring 3. Tap 4. Slag outlet 5 buckets

Claims

1. 1. A method for producing molten iron by melting reduced iron by arc heating, comprising: using plant-derived biomass charcoal as a carbon source for melting the reduced iron; and introducing the biomass charcoal into a position within a radius of 1.0 m from a point where the reduced iron is supplied to the molten iron.

2. 2. The method for producing molten iron according to claim 1, wherein the proportion of carbon derived from fixed carbon of the biomass charcoal in the carbon content of the additive added as the carbon source is 30 mass% or more.

3. 3. The method for producing molten iron according to claim 1, wherein the proportion of carbon derived from the pig iron in the carbon content of the additive added as the carbon source is 50 mass% or less.

4. 3. The method for producing molten iron according to claim 1, wherein the reduced iron is solid reduced iron having a carbon content of 1 mass % or less.

5. 5. The method for producing molten iron according to claim 4, wherein the reduced iron is solid reduced iron having a carbon content of 0.05 mass % or less and a metallization rate of 90% or more.

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