Method for operating an electric furnace and method for manufacturing reduced iron briquettes for producing molten steel

Abstract

The reduced iron briquette has a nitrogen content of less than 50 ppm.

Description

[Technical Field]

[0001] The present invention provides Method for operating an electric furnace and method for manufacturing reduced iron briquettes for producing molten steel This application claims priority from Japanese Patent Application No. 2024-024032, filed on February 20, 2024, the contents of which are incorporated herein by reference. [Background technology]

[0002] For example, techniques for producing sponge-like reduced iron (porous reduced iron), such as a reduced iron production method using a shaft furnace, are known (Patent Documents 1 and 2). In these techniques, instead of producing liquid molten pig iron (pig iron) as in the blast furnace method, iron ore or its agglomerates are reduced in solid form to produce porous, highly porosity sponge-like reduced iron (hereinafter also referred to as sponge iron). In these techniques, some or all of the produced sponge-like reduced iron is briquetted to produce reduced iron briquettes. Reduced iron briquettes are agglomerates produced by compressing multiple pieces of sponge-like reduced iron together. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japan Utility Model Publication No. 63-167162 [Patent Document 2] Japanese Patent Publication No. 2000-204419 Summary of the Invention [Problem to be solved by the invention]

[0004] Incidentally, steel materials used in the production of high-grade steel are required to have an extremely low nitrogen content of less than 50 ppm by mass, and steel materials used in the production of deep-drawn sheets, in particular, are required to have an extremely low nitrogen content of less than 20 ppm. Note that, hereinafter, ppm by mass will be simply referred to as ppm.

[0005] In the blast furnace / converter process, the following process is known to produce steel with such low nitrogen content. Specifically, a carbon source such as a carbonaceous material is added to the molten pig iron in the converter, and then an oxygen-containing gas is blown into the converter to promote the decarburization reaction, causing N to adsorb onto the surface of the CO bubbles. This process aims to denitrify the molten pig iron. The nitrogen content of the molten pig iron tapped from the converter is approximately 30 ppm. This process is possible because the molten pig iron contains a high carbon content. In other words, coke is used to produce molten pig iron in a blast furnace, which increases the carbon content of the molten pig iron. Furthermore, when this molten steel is treated in an RH degasser, the molten steel is denitrified by reducing the pressure, further reducing the nitrogen content of the molten steel. As a result, the nitrogen content of the molten steel is reduced to less than 20 ppm.

[0006] On the other hand, the nitrogen content of sponge-like reduced iron produced in a shaft furnace is 20 ppm or higher. Furthermore, the nitrogen content of reduced iron briquettes produced by briquetting sponge-like reduced iron is 50 ppm or higher. The reduced iron briquettes are transported to an electric furnace to be turned into molten steel, where impurities are removed, but nitrogen is not sufficiently removed. Even if a carbon source is added to the molten steel in the electric furnace, as in the blast furnace / converter process, the denitrification reaction does not proceed sufficiently. Because shaft furnaces do not use coke, the carbon content of the reduced iron briquettes transported to the electric furnace is low, and the CO bubbles necessary for denitrification are not sufficiently generated. Furthermore, because electric furnaces are less airtight than converters, atmospheric nitrogen may be mixed into the electric furnace and ultimately into the molten steel. Therefore, it has been difficult to produce steel with low nitrogen content using shaft furnaces and electric furnaces. For example, the nitrogen content of molten steel tapped from an electric furnace is approximately 50–100 ppm.

[0007] Therefore, the present invention has been made in view of the above problems, and an object of the present invention is to provide a reduced iron briquette having a nitrogen content of less than 50 ppm and a method for producing the reduced iron briquette. [Means for solving the problem]

[0008] The gist of the present invention is as follows. (1) One aspect of the present invention Electric furnace operation method has a nitrogen content of less than 50 ppm Reduced iron briquettes are used. . (2) The above (1) Electric furnace operation method has a nitrogen content of less than 20 ppm Alternatively, reduced iron briquettes of . (3) A method for producing reduced iron briquettes for producing molten steel according to another aspect of the present invention includes the steps of: determining the relationship between the nitrogen content of sponge-like reduced iron and the denitrification temperature and holding time; determining the denitrification temperature and holding time of the sponge-like reduced iron from the relationship; denitrification treatment step of holding the sponge-like reduced iron at the denitrification temperature and for the holding time to denitrify the sponge-like reduced iron; and briquetting step of briquetting the denitrified sponge-like reduced iron. (4) In the method for producing reduced iron briquettes for producing molten steel described in (3) above, the nitrogen content of the sponge-like reduced iron after the denitrification treatment step may be less than 20 ppm. (5) In the method for producing reduced iron briquettes for producing molten steel according to (3) or (4), the sponge reduced iron may be briquetted at 650° C. or higher in a nitrogen-free atmosphere in the briquetting step. (6) In the method for producing reduced iron briquettes for producing molten steel according to (3) or (4), the sponge reduced iron at a temperature of less than 650° C. may be briquetted in the briquetting step. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide reduced iron briquettes having a nitrogen content of less than 50 ppm and a method for producing reduced iron briquettes. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic diagram showing an apparatus for producing sponge reduced iron. [Figure 2]10 is a graph showing simulation results of the retention time and nitrogen content of sponge-like reduced iron for each initial nitrogen content when the denitrification atmosphere was H2 100% by volume, 1.013×10 Pa, and 700° C. [Figure 3] 10 is a graph showing simulation results of the retention time and nitrogen content of sponge-like reduced iron for each initial nitrogen content when the denitrification atmosphere was H2 90% by volume, N2 10% by volume, 1.013 × 105 Pa, and 700°C. [Figure 4] 10 is a graph showing simulation results of the retention time and nitrogen content of sponge-like reduced iron for each initial nitrogen content when the denitrification atmosphere was H2 80 vol %, N2 20 vol %, 1.013 × 105 Pa, and 700°C. [Figure 5] 10 is a graph showing simulation results of the retention time and nitrogen content of sponge-like reduced iron for each initial nitrogen content when the denitrification atmosphere was H2 70 vol %, N2 30 vol %, 1.013 × 105 Pa, and 700°C. [Figure 6] 10 is a graph showing simulation results of the retention time and nitrogen content of sponge-like reduced iron for each initial nitrogen content when the denitrification atmosphere was H2 50 vol %, N2 50 vol %, 1.013 × 105 Pa, and 700°C. [Figure 7] 10 is a graph showing simulation results of the retention time and nitrogen content of sponge-like reduced iron for each initial nitrogen content when the denitrification atmosphere was set to 36% by volume of H2 and 64% by volume of N2, 1.013×105 Pa, and 700°C. [Figure 8] FIG. 1 is a schematic diagram showing an apparatus for producing molten steel. DETAILED DESCRIPTION OF THE INVENTION

[0011] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0012] <1. Reduced iron briquette manufacturing equipment> 1 is a schematic diagram showing an apparatus for manufacturing reduced iron briquettes 1. The apparatus for manufacturing reduced iron briquettes 1 according to this embodiment includes a shaft furnace 1a, a reducing gas heating device 2, a dust removing device 3, a dehydration device 4, a denitrification device 5, and a briquette molding machine 6.

[0013] The shaft furnace 1a is an apparatus for producing sponge-like reduced iron by reducing iron oxide raw materials. The shaft furnace 1a, for example, operates as follows: First, iron oxide raw materials (e.g., iron oxide pellets) are charged into the shaft furnace 1a from above, and reducing gas is blown into the shaft furnace 1a from below. The reducing gas is heated to a predetermined temperature (e.g., about 900 to 950°C) by a reducing gas heater 2 and then blown into the shaft furnace 1a. The reducing gas blown into the shaft furnace 1a reduces the iron oxide raw materials in the shaft furnace 1a. This direct reduction process produces sponge-like reduced iron (DRI). The sponge-like reduced iron is discharged from the bottom of the shaft furnace 1a.

[0014] The sponge-like reduced iron immediately after being discharged from the shaft furnace 1a has a nitrogen content of 20 ppm or more. Furthermore, if the sponge-like reduced iron is briquetted without denitrification, the nitrogen content of the reduced iron briquettes will be 50 ppm or more. The nitrogen content of the sponge-like reduced iron and the reduced iron briquettes is measured in accordance with JIS G1228:1997, an inert gas dissolution-thermal conductivity method (ISO / DIS 10720).

[0015] The sponge-like reduced iron may be discharged in a heated state or in a cooled state. The sponge-like reduced iron discharged in a heated state is also called HDRI (Hot Direct Reduced Iron), and the sponge-like reduced iron discharged in a cooled state is also called CDRI (Cold Direct Reduced Iron). The temperature of HDRI is, for example, 650°C or higher, and the temperature of CDRI is, for example, lower than 650°C. The temperatures of HDRI and CDRI are measured, for example, by a radiation thermometer or a thermocouple (the thermocouple is inserted into the DRI).

[0016] The sponge-like reduced iron is porous reduced iron, and preferably has a porosity of 40% or more. The porosity is measured by the method specified in JIS M8716:1990.

[0017] Furnace top gas (exhaust gas) is discharged from the top of the shaft furnace 1a. The furnace top gas is, for example, hydrogen gas and steam gas, or carbon monoxide gas and carbon dioxide gas, or a mixture of these. The furnace top gas may be discharged to the outside of the system, or may be circulated as in this embodiment.

[0018] The furnace for producing sponge reduced iron is not limited to a shaft furnace, and may be, for example, a kiln furnace.

[0019] The dust remover 3 removes dust from the furnace gas using a dust filter, etc. Thereafter, a part of the furnace gas is used as fuel gas for the reducing gas heating device 2, and the remainder is introduced into the dehydration device 4 as circulation gas.

[0020] The dehydration device 4 dehydrates the circulating gas. Thereafter, the circulating gas is mixed with a reducing gas introduced from the outside and introduced into the reducing gas heating device 2. Although not shown, carbon dioxide gas may be removed from the circulating gas.

[0021] The reducing gas heating device 2 heats the reducing gas with heat generated by burning the fuel gas. The heated reducing gas is blown into the shaft furnace 1a. The combusted fuel gas is discharged outside the system as exhaust gas.

[0022] The sponge-like reduced iron discharged from the shaft furnace 1a is introduced into the denitrification unit 5. The denitrification unit 5 denitrifies the sponge-like reduced iron using hydrogen gas (denitrification treatment process). Denitrification proceeds according to the following reaction formula: The gas used for denitrification may be mixed with other types of gases as long as they do not interfere with denitrification. Examples of such gases include argon gas, carbon monoxide gas, and carbon dioxide gas. N (in sponge-like reduced iron) + (3 / 2)H2 → NH3 The conditions required for denitrification (e.g., the temperature of the hydrogen gas) may be determined, for example, by following the research of Mori et al. (Iron and Steel, 58 (1972), 1264). The denitrification unit 5 performs denitrification so that the nitrogen content of the reduced iron briquette becomes less than 50 ppm, preferably less than 20 ppm. For example, the denitrification unit 5 performs denitrification until the nitrogen content of the sponge-like reduced iron becomes less than 20 ppm. The nitrogen content of the sponge-like reduced iron can be adjusted to any value by adjusting the operating conditions of the denitrification unit 5. The denitrification treatment is performed, for example, at 600°C or higher. The gas generated from the denitrification unit 5, i.e., the denitrification treatment exhaust gas, contains hydrogen gas and ammonia gas, and may be mixed with a reducing gas introduced from an external source. The installation of the denitrification unit 5 enables denitrification of any form of sponge-like reduced iron.

[0023] The denitrification conditions for sponge-like reduced iron will now be described in detail. The inventors performed a simulation calculation of the denitrification behavior of sponge-like reduced iron using hydrogen. The denitrification reaction shown in the above chemical reaction formula was assumed to be surface-limited by the sponge-like reduced iron, and the reaction rate was determined by the value presented by Grabke (Ber. Bunsenges. Physik. Chem., (1968), 533, 541).

[0024] Figure 2 shows the denitrification atmosphere with 100% H2 by volume and 1.013 x 10 5 10 is a graph showing simulation results of the holding time and nitrogen content of sponge-like reduced iron for each initial nitrogen content at 100 Pa and 700° C. The denitrification atmosphere refers to the atmosphere in which the sponge-like reduced iron is denitrified. H2100 vol%, 1.013 x 10 5 In a denitrification atmosphere at 700°C and 200 Pa, it was found that the nitrogen content of sponge-type reduced iron with an initial nitrogen content of 30 ppm was reduced to 20 ppm in 3.6 minutes. The nitrogen content of sponge-type reduced iron with an initial nitrogen content of 50 ppm was reduced to 20 ppm in 8 minutes, and the nitrogen content of sponge-type reduced iron with an initial nitrogen content of 100 ppm was reduced to 20 ppm in 13.8 minutes.

[0025] Figure 3 shows the denitrification atmosphere: H2 90% by volume, N2 10% by volume, 1.013 × 10 5 10 is a graph showing simulation calculation results of the retention time and nitrogen content for each initial nitrogen content of sponge reduced iron at Pa and 700° C. Figure 4 shows the denitrification atmosphere with 80% H2 by volume, 20% N2 by volume, and 1.013 × 10 5 10 is a graph showing simulation calculation results of the retention time and nitrogen content for each initial nitrogen content of sponge reduced iron at Pa and 700° C. Figure 5 shows the denitrification atmosphere with 70% H2 by volume, 30% N2 by volume, and 1.013 × 10 5 10 is a graph showing simulation calculation results of the retention time and nitrogen content for each initial nitrogen content of sponge reduced iron at Pa and 700° C. Figure 6 shows the denitrification atmosphere with 50% H2 by volume, 50% N2 by volume, and 1.013 × 10 5 10 is a graph showing simulation calculation results of the retention time and nitrogen content for each initial nitrogen content of sponge reduced iron at Pa and 700° C. Figure 7 shows the denitrification atmosphere with 36% H2 by volume, 64% N2 by volume, and 1.013 × 10 5 10 is a graph showing simulation calculation results of the retention time and nitrogen content for each initial nitrogen content of sponge reduced iron at Pa and 700° C.

[0026] As the N2 concentration in the denitrification atmosphere increases, the time required to reduce the nitrogen content of the sponge-like reduced iron to 20 ppm or less increases. The time required to reduce the nitrogen content of the sponge-like reduced iron from 30 ppm to 20 ppm is 6.1 minutes when the N2 concentration in the denitrification atmosphere is 10% by volume, as shown in Figure 3; 9.1 minutes when the N2 concentration is 20% by volume, as shown in Figure 4; 13.8 minutes when the N2 concentration is 30% by volume, as shown in Figure 5; and 38.6 minutes when the N2 concentration is 50% by volume, as shown in Figure 6. When the N2 concentration is 64% by volume, the N2 concentration (equilibrium concentration) at which the above reaction equation reaches equilibrium is 20 ppm, as shown in Figure 7, so it is not possible to reduce the nitrogen content below 20 ppm.

[0027] The treatment time is longer as the H2 concentration is lower. For example, the time required to reduce the nitrogen content of the sponge-like reduced iron from 30 ppm to 20 ppm is 9.8 minutes for 50 vol% H2 and 50 vol% Ar, and 71 minutes for 10 vol% H2 and 90 vol% Ar.

[0028] As described above, the retention time for the denitrification treatment can be calculated by simulation. Therefore, the retention time for holding the sponge reduced iron at the denitrification temperature can be determined based on the relationship between the nitrogen content and the retention time for each condition, such as the denitrification temperature and the denitrification atmosphere.

[0029] The nitrogen content of the sponge-like reduced iron to be denitrified is 20 ppm or more. The nitrogen content of the sponge-like reduced iron to be denitrified may be, for example, 50 ppm or more, or 100 ppm or more. The nitrogen content of the sponge-like reduced iron to be denitrified may be, for example, 60,000 ppm (6 mass%) or less. In consideration of productivity, the nitrogen content of the sponge-like reduced iron to be denitrified is preferably 2,000 ppm or less. The nitrogen content of the sponge reduced iron after the denitrification treatment step is, for example, less than 20 ppm.

[0030] The sponge-like reduced iron discharged from the denitrification device 5 is transported by transport means 7 to a storage hopper (not shown) of the briquette molding machine 6 in a nitrogen-free atmosphere. The transport means 7 may be a transport pipe connecting the denitrification device 5 and the storage hopper of the briquette molding machine 6, or may be a sealed container. Here, the nitrogen-free atmosphere includes not only a nitrogen-free atmosphere of 0% but also a nitrogen-free atmosphere containing a small amount of nitrogen. A small amount of nitrogen means a nitrogen-free atmosphere containing nitrogen as an impurity, and specifically, a nitrogen-free atmosphere containing 10% or less of nitrogen. -3This means that the nitrogen-free atmosphere contains approximately 100% nitrogen. A nitrogen-free atmosphere is, for example, an atmosphere that does not contain nitrogen, does not react with the sponge-like reduced iron, or hardly induces any harmful reactions to the sponge-like reduced iron, such as reoxidation. Examples of nitrogen-free atmospheres include hydrogen gas, carbon dioxide gas, carbon monoxide gas, argon, and mixtures thereof. Strictly speaking, argon does not react with the sponge-like reduced iron. While hydrogen gas may reduce the sponge-like reduced iron, this reaction is not harmful to the sponge-like reduced iron. Carbon monoxide gas may reduce and carburize the sponge-like reduced iron, but these reactions are not harmful to the sponge-like reduced iron. Carbon dioxide gas may reoxidize the sponge-like reduced iron, but the rate of reoxidation is sufficiently low. If the interior of the transportation means 7 contains nitrogen, the nitrogen-containing atmosphere is replaced with the nitrogen-free atmosphere described above.

[0031] Here, when the sponge reduced iron becomes HDRI, the sponge reduced iron may be cooled while being transported to the briquetting machine 6. For example, the sponge reduced iron may be cooled until the temperature of the sponge reduced iron becomes less than 650°C (i.e., until the sponge reduced iron becomes CDRI).

[0032] The briquetting machine 6 briquettes the denitrified sponge-like reduced iron (briquetting process). That is, the briquetting machine 6 compresses the sponge-like reduced iron to produce reduced iron briquettes. The structure of the briquetting machine 6 is not particularly limited, and any known briquetting machine can be used. Here, HDRI or CDRI is transported to the briquetting machine 6 as the sponge-like reduced iron. Therefore, the briquetting machine 6 briquettes the sponge-like reduced iron at 650°C or higher in a nitrogen-free atmosphere, or briquettes the sponge-like reduced iron at a temperature below 650°C. This allows hot briquetted iron (HBI) or cold briquetted iron (CBI) with the desired nitrogen content to be produced.

[0033] Reduced iron briquettes produced by briquetting HDRI are also called HBI, and reduced iron briquettes produced by briquetting CDRI are also called CBI. HBI and CBI may have a structure conforming to JIS M8700, for example. That is, HBI is a briquette made at a temperature of 650°C or higher, and has a density of 5 g / cm. 3 The CBI may be reduced iron having an apparent density of 5 g / cm or more, briquetted at a temperature of less than 650°C. 3 The reduced iron may have an apparent density of less than 10 ...

[0034] It is preferable to denitrify the sponge-like reduced iron before briquetting it, as in the present embodiment. If briquetting is performed before denitrification, the reduced iron briquettes will have a dense structure and may not be sufficiently denitrified.

[0035] FIG. 8 is a schematic diagram showing an apparatus 10 for producing molten steel. As shown in FIG. 8, reduced iron briquettes discharged from a briquetting machine 6 are transported to an electric furnace 9 by, for example, transport means 8. The transport means 8 may be a transport pipe connecting the briquetting machine 6 and the electric furnace 9, or may be a sealed container. The reduced iron briquettes are then melted in the electric furnace 9 to produce molten steel. This allows the production of steel materials used in the production of high-grade steel and, further, steel materials used in the production of thin plates for deep drawing.

[0036] As described above, according to this embodiment, it is possible to produce reduced iron briquettes having a nitrogen content of less than 50 ppm, preferably less than 20 ppm. Furthermore, such reduced iron briquettes can be transported to an electric furnace to produce molten steel while ensuring that the nitrogen content does not exceed 50 ppm. This facilitates the production of high-grade steel. Furthermore, it is possible to improve the quality of high-grade steel.

[0037] Even if denitrification of the sponge-like reduced iron progresses in the shaft furnace 1a and the nitrogen content becomes less than 20 ppm, the sponge-like reduced iron will be exposed to a nitrogen-containing atmosphere during cooling and discharge in the shaft furnace 1a, and nitridation will progress, resulting in discharged sponge-like reduced iron with a nitrogen content of 20 ppm or more. Therefore, the nitrogen content of the reduced iron briquettes will be 50 ppm or more.

[0038] According to this embodiment, after denitrification by the denitrification device 5, briquetting is performed in a nitrogen-free atmosphere, making it possible to produce reduced iron briquettes with low nitrogen (less than 50 ppm). Furthermore, since the reduced iron briquettes are transported to the electric furnace 9 with low nitrogen content, it is possible to produce low-nitrogen steel products.

[0039] <2. Manufacturing method of reduced iron briquettes> Next, a method for producing reduced iron briquettes will be described.

[0040] First, sponge reduced iron is produced using a shaft furnace 1a. That is, first, an iron oxide raw material (e.g., iron oxide pellets) is charged into the shaft furnace 1a from above, and a reducing gas is blown into the shaft furnace 1a from below. The reducing gas is heated to a predetermined temperature (e.g., about 900 to 950°C) by a reducing gas heater 2 and then blown into the shaft furnace 1a. The reducing gas blown into the shaft furnace 1a reduces the iron oxide raw material in the shaft furnace 1a. This direct reduction process produces sponge reduced iron (DRI). The sponge reduced iron is discharged from the bottom of the shaft furnace 1a.

[0041] The sponge-like reduced iron immediately after being discharged from the shaft furnace 1a has a nitrogen content of 20 ppm or more. The sponge-like reduced iron may be discharged in a heated state (HDRI) or in a cooled state (CDRI).

[0042] On the other hand, furnace top gas (exhaust gas) containing hydrogen gas, carbon monoxide gas, water vapor, and carbon dioxide gas is discharged from the furnace top of the shaft furnace 1a. Dust is removed from the furnace top gas by a dust remover 3. Thereafter, part of the furnace top gas is used as fuel gas for the reducing gas heating device 2, and the remainder is introduced into a dehydration device 4 as a circulating gas.

[0043] Next, the dehydration device 4 dehydrates the circulating gas. After that, the circulating gas is mixed with reducing gas introduced from the outside and introduced into the reducing gas heating device 2. Although not shown, carbon dioxide gas may be removed from the circulating gas. The reducing gas heating device 2 heats the reducing gas with heat generated by burning fuel gas. The heated reducing gas is blown into the shaft furnace 1a.

[0044] The sponge-like reduced iron discharged from the shaft furnace 1a is introduced into the denitrification unit 5. The denitrification unit 5 denitrifies the sponge-like reduced iron using hydrogen gas (denitrification treatment step). The denitrification unit 5 performs denitrification so that the nitrogen content of the reduced iron briquette becomes less than 50 ppm, preferably less than 20 ppm. For example, the denitrification unit 5 performs denitrification until the nitrogen content of the sponge-like reduced iron becomes less than 20 ppm. As mentioned above, the retention time for holding the sponge-like reduced iron at the denitrification temperature can be determined based on the relationship between the nitrogen content of the sponge-like reduced iron and the retention time. For example, referring to FIG. 2, when H2 100% by volume, 1.013×10 5 In order to denitrify sponge-like reduced iron having a nitrogen content of 50 ppm in an atmosphere of Pa to reduce the nitrogen content of the sponge-like reduced iron to 20 ppm or less, the sponge-like reduced iron should be held at the denitrification temperature of 700°C for 8 minutes or more.

[0045] The gas generated from the denitrification device 5, i.e., the denitrification treated exhaust gas, contains hydrogen gas and ammonia gas, and therefore may be used as a heat source and fuel for the reducing gas heating device 2, or may be mixed with the reducing gas introduced from outside while controlling the nitrogen concentration.

[0046] The sponge-like reduced iron discharged from the denitrification device 5 is transported by transport means 7 to a storage hopper of the briquette molding machine 6 in a nitrogen-free atmosphere.

[0047] Here, when the sponge reduced iron becomes HDRI, the sponge reduced iron may be cooled while being transported to the briquetting machine 6. For example, the sponge reduced iron may be cooled until the temperature of the sponge reduced iron becomes less than 650°C (i.e., until the sponge reduced iron becomes CDRI).

[0048] The briquetting machine 6 briquettes the denitrified sponge-like reduced iron in a nitrogen-free atmosphere (briquetting process). That is, the briquetting machine 6 produces reduced iron briquettes by compressing the sponge-like reduced iron. Here, HDRI or CDRI is transported to the briquetting machine 6 as the sponge-like reduced iron. Therefore, the briquetting machine 6 briquettes the sponge-like reduced iron at 650°C or higher in a nitrogen-free atmosphere, or briquettes the sponge-like reduced iron at a temperature lower than 650°C. The produced reduced iron briquettes are transported to an electric furnace 9 in a nitrogen-free atmosphere by a transport means 8. The reduced iron briquettes are then melted in the electric furnace 9 to produce molten steel. [Example]

[0049] DRI with a nitrogen content of 60 ppm was held at 700°C in a hydrogen gas atmosphere for 10 minutes. The nitrogen content was measured using the method described above. The DRI was then briquettered to produce HBI. The nitrogen content of the HBI was measured using the method described above and found to be 19 ppm. Therefore, it was revealed that the above-described briquetting apparatus and briquetting method can produce reduced iron briquettes with a nitrogen content of less than 50 ppm, preferably less than 20 ppm.

[0050] Although the preferred embodiments of the present invention have been described in detail above, the present invention is not limited to these examples. It is clear that a person skilled in the art to which the present invention pertains can conceive of various modifications and alterations within the scope of the technical ideas set forth in the claims, and it is understood that these also naturally fall within the technical scope of the present invention. [Explanation of symbols]

[0051] 1. Sponge-like reduced iron manufacturing equipment 1a Shaft furnace 2. Reducing gas heating device 3 Dust removal equipment 4 Dehydration equipment 5 Denitrification equipment 6 Briquette molding machine 7 Means of transportation 8 Means of transportation 9 Electric Furnace 10 Molten steel manufacturing equipment

Claims

1. A method for operating an electric furnace using reduced iron briquettes having a nitrogen content of less than 50 ppm.

2. 2. The method for operating an electric furnace according to claim 1, wherein reduced iron briquettes having a nitrogen content of less than 20 ppm are used.

3. determining the relationship between the nitrogen content of the sponge reduced iron and the denitrification temperature and retention time; determining a denitrification temperature and a retention time of the sponge reduced iron from the relationship; a denitrification treatment step of denitrifying the sponge-like reduced iron by holding the sponge-like reduced iron at the denitrification temperature for the holding time; and a briquetting step of briquetting the denitrified sponge-like reduced iron into briquettes.

4. 4. The method for producing reduced iron briquettes for producing molten steel according to claim 3, wherein the sponge reduced iron after the denitrification treatment has a nitrogen content of less than 20 ppm.

5. 5. The method for producing reduced iron briquettes for producing molten steel according to claim 3, wherein in the briquetting step, the sponge-like reduced iron is briquetted at 650°C or higher in a nitrogen-free atmosphere.

6. 5. The method for producing reduced iron briquettes for producing molten steel according to claim 3, wherein the sponge-like reduced iron at a temperature of less than 650°C is briquetted in the briquetting step.

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