WO2025211000A1 - Method for producing reduced iron - Google Patents

Abstract

This method for producing reduced iron comprises: a raw material charging step for charging a raw material into a shaft furnace; a reduction step for causing the raw material to react with a reducing gas in the shaft furnace to obtain reduced iron and exhaust gas after reduction; and an exhaust gas circulation step for circulating the exhaust gas after reduction so as to be used as part of the reducing gas. The raw material charging step includes, in order, a depressurization step, a first purge step, a raw material charging step, a second purge step, a pressure equalization step, and a raw material discharge step. The exhaust gas circulation step includes a dehydration step for removing water from the exhaust gas after reduction. The gas for purging is an inert gas. A pressure equalization gas is obtained by removing water from the exhaust gas in the dehydration step.

Description

Reduced iron manufacturing method

The present invention relates to a method for producing reduced iron.
This application claims priority based on Japanese Patent Application No. 2024-059600, filed on April 2, 2024, the contents of which are incorporated herein by reference.

Currently, reduced iron is produced using shaft furnace operation with natural gas (NG) as the reducing gas, as in the HyL and Midrex processes.

In the production of reduced iron, the raw material, iron oxide, is charged into the shaft furnace from the top through several hoppers. When the raw material is loaded into the charging hopper, the pressure inside the hopper must be atmospheric pressure, but when the raw material is fed from the hopper to the shaft furnace, the pressure inside the hopper must be the pressure of the shaft furnace. Therefore, before the raw material is transferred from the hopper to the shaft furnace, the pressure must be equalized to ensure that the pressure is the same.

The gas used for pressure equalization is an inert gas (usually nitrogen gas) for the purging operation to prevent oxidation reactions with the reducing gas remaining in the pressure equalization hopper after the raw materials are added, and for the pressurization operation to equalize the pressure inside the hopper with that inside the furnace before the raw materials are charged.

This inert gas enters the furnace along with the raw materials and becomes mixed with the reducing gas. It then accumulates in the gas as it circulates, reducing the reducing ability of the gas. For this reason, it was previously necessary to appropriately vent the circulating gas to prevent this concentration.

In contrast, the method for manufacturing carburized sea-surface iron disclosed in Patent Document 1 uses hydrogen gas as the raw material gas and carbon dioxide as the pressure-equalizing gas.

International Publication No. 2022/115024

However, the method used in Patent Document 1 uses carbon dioxide gas as the equalizing gas, which means there is a risk that more carbon dioxide than is consumed in carburizing will enter the shaft furnace. This creates the problem that as the carbon dioxide concentration increases inside the shaft, the reducing power decreases.

The present invention was developed in consideration of the above circumstances, and aims to provide a method for producing reduced iron that can suppress the concentration of unnecessary components such as nitrogen and carbon dioxide and maintain reduction performance. Here, unnecessary components refer to components that are not used in the reduction reaction, such as nitrogen, carbon dioxide, and water.

In order to solve the above problems, the present invention proposes the following means.
(1) A method for producing reduced iron using a shaft furnace according to aspect 1 of the present invention includes a raw material charging step of charging raw materials from a charging hopper that stores raw materials into a shaft furnace that operates at a pressure higher than atmospheric pressure via a pressure equalizing hopper that adjusts the pressure;
a reduction step of reacting the raw materials with a reducing gas in the shaft furnace after the raw material charging step to obtain reduced iron and a post-reduction exhaust gas;
an exhaust gas circulation step in which, after the reduction step, the exhaust gas after the reduction step is circulated and used as part of the reducing gas;
Including,
The raw material charging step
a depressurization step of reducing the pressure in the pressure equalizing hopper from the pressure in the shaft furnace to atmospheric pressure;
a first purging step of replacing the gas in the pressure equalizing hopper with a purging gas after the depressurizing step;
a raw material charging step of transferring the raw material from the charging hopper to the pressure equalizing hopper whose air has been replaced with the purge gas after the first purging step;
a second purging step of replacing the air introduced into the pressure equalizing hopper in the raw material charging step with a purging gas after the raw material charging step;
a pressure equalization step of increasing the pressure in the pressure equalization hopper from atmospheric pressure to the pressure in the shaft furnace with an equalization gas after the second purge step;
a raw material discharge step of transferring the raw material from the pressurized pressure equalizing hopper into the shaft furnace after the pressure equalization step;
Including,
The exhaust gas circulation step
a dehydration step for removing water from the exhaust gas after reduction;
the purge gas is an inert gas,
The equalizing gas is the gas obtained by removing water from the exhaust gas in the dehydration step.
(2) Aspect 2 of the present invention is the method for producing reduced iron according to Aspect 1, further comprising:
the exhaust gas circulation step further includes a carbon dioxide removal step of removing carbon dioxide from the exhaust gas,
The equalizing gas is the gas obtained by removing water and carbon dioxide from the exhaust gas in the dehydration step and carbon dioxide removal step.

The above aspects of the present invention provide a method for producing reduced iron that can suppress the concentration of unnecessary components such as nitrogen and carbon dioxide and maintain reduction performance.

FIG. 1 is a flow diagram showing an example of a direct reduction device according to a first embodiment of the present invention. FIG. 1 is a flow diagram of a method for producing reduced iron according to a first embodiment of the present invention. FIG. 6 is a flow diagram showing an example of a direct reduction device according to a second embodiment of the present invention. FIG. 5 is a flow diagram of a method for producing reduced iron according to a second embodiment of the present invention.

(First embodiment)
Hereinafter, a direct reduced iron manufacturing apparatus 100 for carrying out a reduced iron manufacturing method according to a first embodiment will be described with reference to the drawings. Fig. 1 is a flow diagram showing an example of a direct reduction apparatus according to the first embodiment of the present invention. The direct reduced iron manufacturing apparatus 100 includes a charging hopper 12 for storing iron oxide as a raw material, a pressure equalizing hopper 14 for adjusting the pressure between the atmosphere and the pressure inside the shaft furnace, a shaft furnace 20 for producing direct reduced iron using hydrogen as a raw material gas, a dehydration device 30 for dehydrating exhaust gas from the shaft furnace 20 to obtain a circulating gas, a pressure booster 40 for boosting the pressure of the circulating gas, and a heating device 50 for heating the dehydrated circulating gas together with the raw material gas to produce a reducing gas.

(Charging hopper 12)
The charging hopper 12 stores iron oxide. The charging hopper 12 transfers the raw material iron oxide to the pressure equalizing hopper 14, which is at atmospheric pressure, via a raw material charging port 14a.

(Pressure equalizing hopper 14)
After transferring the raw materials into the pressure equalizing hopper 14, the pressure therein is adjusted. After adjusting the pressure, the pressure equalizing hopper 14 sends the raw materials to the raw material charging section 24 of the shaft furnace 20. The pressure equalizing hopper 14 is equipped with a raw material charging inlet 14a through which the raw materials are transferred from the charging hopper 12, a raw material discharge outlet 14b through which the raw materials are sent to the raw material charging section 24 of the shaft furnace 20 after pressure equalization, a purge gas inlet 14c through which purge gas is introduced, a gas outlet 14d through which the purge gas and the like are discharged, and a pressure equalizing gas inlet 14e through which the pressure equalizing gas is introduced into the pressure equalizing hopper 14. The raw material charging inlet 14a is connected to the charging hopper 12, and the raw material discharge outlet 14b is connected to the raw material charging section 24 of the shaft furnace 20.

The operation of the pressure equalizing hopper 14 will be described below. The pressure equalizing hopper 14 closes the raw material charging inlet 14a, raw material discharge outlet 14b, purge gas inlet 14c, and pressure equalizing gas inlet 14e, and opens the gas outlet 14d to reduce the pressure inside the pressure equalizing hopper 14 to atmospheric pressure. Next, the purge gas inlet 14c is opened, and an inert gas serving as a purge gas is introduced into the pressure equalizing hopper 14. This allows the furnace gas remaining in the pressure equalizing hopper 14 to be replaced with the inert gas. Next, the pressure equalizing hopper 14 closes the purge gas inlet 14c and opens the raw material charging inlet 14a, transferring the raw materials from the charging hopper 12 to the pressure equalizing hopper 14, which is filled with inert gas. After the raw materials are transferred, the purge gas inlet 14c is opened, and the inert gas is introduced into the pressure equalizing hopper 14. This allows the air introduced into the pressure equalizing hopper 14 along with the raw materials to be replaced with the inert gas. Examples of the inert gas include nitrogen gas, rare gas, CO ₂ , H ₂ O gas, and mixtures thereof. Nitrogen gas is generally used as the inert gas. The inert gas used as the purge gas is supplied from outside the system.

Here, purge gas refers to an inert gas introduced into the pressure equalizing hopper 14 to prevent contact between the flammable reducing gas and the air. The pressure of the purge gas is, for example, 0.2 to 1.0 MPa, and the amount of purge gas injected is 1V to 3V relative to the volume V of the pressure equalizing hopper 14. The pressure equalizing gas is introduced into the pressure equalizing hopper to reduce the difference between the pressure inside the shaft furnace 20 and the pressure inside the pressure equalizing hopper 14, and refers to gas from which the moisture content has been reduced to approximately 2 vol% during the dehydration process. The pressure of the pressure equalizing gas is, for example, 0.2 to 0.8 MPa, and the amount of pressure equalizing gas injected is 1V to 8V relative to the volume V of the pressure equalizing hopper 14. Here, purging refers to sending gas A (here, an inert gas) into a certain space (here, the pressure equalizing hopper 14) and replacing gas B (here, a reducing gas) that has been accumulating in that space with gas A, thereby removing gas B. Pressure equalization means making the pressure uniform.

After replacing the air with inert gas, the pressure equalizing hopper 14 closes the raw material charging inlet 14a, purge gas inlet 14c, and gas outlet 14d, opens the pressure equalizing gas inlet 14e, and introduces pressure equalizing gas into the pressure equalizing hopper 14 to increase the pressure. This pressure equalizing gas is pressurized to the pressure inside the shaft furnace 20 by the pressure increase device 40, which will be described later. It is preferable to leave the gas outlet 14d open for a certain period of time to discharge any inert gas remaining inside at the beginning of the pressure increase. For example, it is possible to monitor whether the inert gas has been discharged using gas chromatography or the like, and close the gas outlet 14d when the amount of inert gas discharged from the gas outlet 14d no longer fluctuates.

(Shaft furnace 20)
The shaft furnace 20 includes a raw material charging section 24 for charging the raw material iron oxide, a reduced iron discharge section 25 for discharging reduced iron, a reducing gas inlet 28 disposed at the bottom of the shaft furnace 20 for blowing in reducing gas, and an exhaust gas outlet 29 disposed at the top of the shaft furnace 20 for discharging exhaust gas. The raw material iron oxide is charged from the pressure equalizing hopper 14 through the raw material charging section 24 at the top of the shaft furnace 20. The furnace pressure in the shaft furnace 20 is higher than atmospheric pressure. For example, the pressure near the reducing gas inlet 28 is 0.1 to 1 MPa, and the iron oxide is discharged from the exhaust gas outlet 29 at a pressure reduced by the pressure loss within the furnace. The iron oxide is reduced by the reducing gas as it descends within the shaft furnace 20, becoming reduced iron. The reduced reduced iron is discharged from the reduced iron discharge section 25. The reducing gas is heated to 900°C or higher by a heating device 50 and blown into the shaft furnace 20 through the reducing gas inlet 28. The iron oxide is reduced by the blown reducing gas. When the reducing gas becomes hydrogen gas, the specific reaction between the iron oxide and hydrogen gas is as shown in the following formula (1A). As shown in the following formula (1A), the hydrogen gas reacts with the iron oxide to become water (water vapor). The unreacted hydrogen gas (H ₂ ) and water vapor (H ₂ O) are sent to the dehydration device 30 via the exhaust gas outlet 29.
Fe ₂ O ₃ +3H ₂ →2Fe+3H ₂ O (1A)

(Dehydration device 30)
The dehydrator 30 dehydrates the exhaust gas discharged from the exhaust gas outlet 29. The exhaust gas discharged from the exhaust gas outlet 29 contains unreacted hydrogen gas and water vapor generated by the reduction reaction of iron oxide, etc. The dehydrator 30 dehydrates the exhaust gas, for example, by cooling the exhaust gas after dust removal. Because water inhibits the reduction reaction, it is preferable that the moisture concentration in the exhaust gas be as low as possible. For example, if the moisture concentration in the exhaust gas is 25 vol%, it is preferable to remove moisture to a moisture concentration of 2 vol% or less by dehydration. The exhaust gas (circulation gas) after dehydration is sent to the pressure booster 40. The exhaust gas may be dust-removed in a dust removal device (not shown) before being dehydrated by the dehydrator 30. The dust removal method is not particularly limited, and examples include a cyclone and a scrubber.

(Boost device 40)
The pressure booster 40 boosts the pressure of the circulating gas that has been dehydrated in the dehydration device 30 to atmospheric pressure or higher and sends it to the heating device 50. A portion of the pressurized circulating gas is introduced as a pressure equalizing gas into the pressure equalizing hopper 14 via the pressure equalizing gas inlet 14e. The pressure booster 40 is, for example, a compressor.

(Heating device 50)
The heating device 50 heats the circulating gas and raw material gas pressurized by the pressure booster 40, and injects them into the reducing gas inlets 28. The reducing gas is a gas that reduces the raw material iron oxide, and in this embodiment, serves as the circulating gas and raw material gas. The temperature of the injected reducing gas is approximately 700 to 1000°C. The injection rate of the reducing gas is approximately 1000 to 2000 Nm ³ /t-DRI. The reducing gas is injected through the reducing gas inlets 28, which are evenly arranged around the circumference of the shaft furnace 20. It is preferable to use hydrogen gas obtained by electrolysis of water or the like as the raw material gas.

(Method for producing reduced iron)
Next, a method for producing reduced iron according to the first embodiment will be described. Fig. 2 is a flowchart of the method for producing reduced iron according to the first embodiment. The method for producing reduced iron according to the first embodiment includes a raw material charging step S10 in which raw materials are charged from a charging hopper 12 that stores raw materials via a pressure equalizing hopper 14 that adjusts the pressure into a shaft furnace 20 that operates at a pressure higher than atmospheric pressure; a reduction step S20 in which the raw materials are reacted with a reducing gas in the shaft furnace 20 after the raw material charging step S10 to obtain reduced iron and a post-reduced exhaust gas; an exhaust gas circulation step S30 in which the post-reduced exhaust gas is circulated and used as part of the reducing gas after the reduction step S20; and a raw material gas supply step S40. Each step will be described below.

(Raw material charging process S10)
In the raw material charging step S10, raw materials are charged from a charging hopper 12 that stores raw materials through a pressure equalizing hopper 14 that adjusts the pressure into a shaft furnace 20 that operates at a pressure higher than atmospheric pressure. The raw material charging process S10 includes: a depressurization process S1 in which the pressure in the pressure equalizing hopper 14 is reduced from the pressure inside the shaft furnace 20 to atmospheric pressure; a first purging process S2 in which the furnace gas in the pressure equalizing hopper is replaced with a purge gas after the depressurization process S1; a raw material introduction process S3 in which the raw materials are transferred from the charging hopper 12 to the pressure equalizing hopper 14 whose air has been replaced with the purge gas after the first purging process S2; a second purging process S4 in which the air introduced into the pressure equalizing hopper 14 in the raw material introduction process S3 is replaced with a purge gas after the second purging process S4; a pressure equalizing process S5 in which the pressure in the pressure equalizing hopper 14 is increased from atmospheric pressure to furnace pressure with an equalizing gas after the second purging process S4; and a raw material discharge process S6 in which the raw materials are transferred from the pressurized equalizing hopper 14 into the shaft furnace 20 after the pressure equalizing process S5.

(Decompression step S1)
In the depressurization step S1, the pressure in the pressure equalizing hopper 14 is reduced from the pressure inside the shaft furnace 20 to atmospheric pressure. Specifically, the raw material discharge port 14b is closed and the gas outlet port 14d is opened, thereby reducing the pressure in the pressure equalizing hopper 14 from the pressure inside the shaft furnace 20 to atmospheric pressure.

(First purge step S2)
In the first purge step S2, after the depressurization step S1, the gas in the pressure equalizing hopper 14 is replaced with a purge gas. Specifically, the purge gas inlet 14c is opened to introduce an inert gas, which is a purge gas, into the pressure equalizing hopper 14. This allows the furnace gas in the pressure equalizing hopper 14, which was mixed in when the raw materials were charged from the pressure equalizing hopper 14 into the shaft furnace 20, to be replaced with the inert gas. It is preferable to replace the gas with an inert gas whose volume is at least three times the volume of the pressure equalizing hopper 14. The end of the first purge step S2 may be determined by measuring the gas components and confirming that the concentration of the gas to be replaced (reducing gas) has fallen below a specified value.

(Raw material input process S3)
In the raw material charging step S3, after the first purging step S2, the raw material iron oxide is transferred from the charging hopper 12 to the pressure-equalizing hopper 14, which has been purged with a purge gas. Specifically, the purge gas inlet 14c is closed, the raw material charging port 14a is opened, and the raw material is transferred from the charging hopper 12 to the pressure-equalizing hopper 14, which has been depressurized and purged.

(Second purge step S4)
In the second purge step S4, after the raw material charging step S3, the air introduced into the pressure equalizing hopper 14 in the raw material charging step S3 is replaced with a purge gas. The purge gas is an inert gas. Specifically, the purge gas inlet 14c is opened to introduce the inert gas as the purge gas into the pressure equalizing hopper 14. This allows the air introduced into the pressure equalizing hopper 14 together with the raw materials in the raw material charging step S3 to be replaced with the inert gas.

(Pressure equalization step S5)
In the pressure equalization step S5, after the second purge step S4, the pressure in the pressure equalization hopper 14 is increased from atmospheric pressure to the pressure inside the shaft furnace 20 using an equalization gas. The pressure inside the shaft furnace 20 is usually controlled. The pressure of the equalization gas may be increased with the control value of the pressure in the shaft furnace 20 as the target.
Specifically, the raw material charging port 14a and the gas outlet 14d are closed, the equalizing gas inlet 14e is opened, and equalizing gas is introduced into the equalizing hopper 14 to increase the pressure. The equalizing gas is pressurized to the pressure inside the shaft furnace (furnace pressure) by the pressure booster 40. The equalizing gas is part of the circulating gas dehydrated by the dehydration device 30. It is preferable to use the equalizing gas to push out the inert gas (nitrogen gas) introduced in the second purge step S4 at the beginning of the equalizing step S5, as this prevents the inert gas (nitrogen gas) from accumulating inside the shaft furnace 20. When using equalizing gas to push out the nitrogen gas, the gas outlet 14d is left open for a while to exhaust the inert gas (nitrogen gas). The time for which the gas outlet 14d is open can be determined by checking the fluctuations in the inert gas using gas chromatography or the like.

(Raw material discharge process S6)
In the raw material discharge step S6, after the pressure equalization step S5, the raw materials are transferred from the pressurized equalizing hopper 14 into the shaft furnace 20. Specifically, the pressure equalizing gas inlet 14e is closed and the raw material discharge port 14b is opened, thereby transferring the raw materials in the pressurized equalizing hopper into the shaft furnace. After the raw material discharge step S6 is completed, the process returns to the depressurization step S1, allowing the raw materials to continue to be fed into the shaft furnace 20.

(Reduction step S20)
In the reduction step S20, after the raw material charging step S10, the raw material is reacted with a reducing gas in the shaft furnace 20 to produce reduced iron and post-reduced exhaust gas. Specifically, in the shaft furnace 20, the raw material is reduced by the reducing gas blown in through the reducing gas blowing port 28 to produce reduced iron (direct reduced iron), which is then discharged from the reduced iron discharge section 25. When the raw material gas is hydrogen gas, the iron oxide reacts with the hydrogen gas as shown in formula (1A) above to produce reduced iron and water. The unreacted hydrogen gas and water vapor are discharged as exhaust gas (post-reduced exhaust gas) from the exhaust gas discharge port 29.

(Exhaust gas circulation step S30)
In the exhaust gas circulation step S30, after the reduction step S20, the exhaust gas discharged in the reduction step S20 (exhaust gas after reduction) is circulated and used as part of the reducing gas. When hydrogen gas is used as the raw material gas, the exhaust gas becomes hydrogen gas and water. The exhaust gas circulation step S30 includes a dehydration step in which water is removed from the exhaust gas after reduction. The exhaust gas discharged from the exhaust gas outlet 29 is dehydrated using a dehydration device 30. The dehydrated exhaust gas (circulation gas) is pressurized by a pressure booster 40, and a portion of the circulation gas is used as an equalization gas in the equalization step S5. In other words, the equalization gas is gas obtained by removing water from the exhaust gas in the dehydration step. The remaining circulation gas not used in the equalization step S5 is heated by a heating device 50 and recycled as a reducing gas. In other words, the remaining circulation gas not used in the equalization step S5, which is heated by the heating device 50 and introduced into the shaft furnace 20, is used as part of the reducing gas.

(Source gas supply step S40)
In the raw material gas supply step S40, the amount of hydrogen gas consumed in the reduction of the raw material (iron oxide) in the reduction step S20 is introduced as raw material gas into the shaft furnace 20 via the heating device 50. The amount of hydrogen gas introduced is determined by analyzing the hydrogen concentration in the exhaust gas, and if the hydrogen concentration exceeds the upper limit of a specified range, the amount of hydrogen gas introduced is reduced, and if the hydrogen concentration is below the specified range, the amount of hydrogen gas introduced is increased.

The above describes the method for producing reduced iron according to the first embodiment and the direct reduced iron production apparatus 100 used in said method. According to the method for producing reduced iron according to the first embodiment, purging flammable gases with an inert gas (nitrogen gas) allows safe operation even when using a circulating gas containing hydrogen as the equalizing gas. By using the exhaust gas after reduction as the equalizing gas, it is possible to prevent the concentration of nitrogen, carbon dioxide, and other substances that accompanies the circulation of the exhaust gas, and therefore the reducing power of the reducing gas is not reduced. Furthermore, since there is no concentration, there is no need to periodically release the exhaust gas.

Second Embodiment
Next, a direct reduced iron manufacturing apparatus 100A for carrying out the reduced iron manufacturing method according to the second embodiment will be described. Fig. 3 is a flow diagram showing an example of a direct reduction apparatus according to the second embodiment of the present invention. The direct reduced iron manufacturing apparatus 100A includes a charging hopper 12 for storing iron oxide as a raw material, a pressure equalizing hopper 14 for adjusting the pressure between the atmosphere and the shaft furnace internal pressure, a shaft furnace 20 for producing direct reduced iron using methane gas as a raw material gas, a dehydration device 30 for dehydrating the exhaust gas from the shaft furnace 20 to obtain a circulating gas, a carbon dioxide removal device 60 for removing carbon dioxide from the circulating gas, a pressure booster 40 for pressurizing the circulating gas after removing water and carbon dioxide, and a heating device 50 for heating the pressurized circulating gas together with the raw material gas to produce a reducing gas.

(Charging hopper 12)
The charging hopper 12 stores iron oxide, which is a raw material. The charging hopper 12 transfers the stored iron oxide to the pressure equalizing hopper 14, which is at atmospheric pressure, through a raw material charging port 14a.

(Pressure equalizing hopper 14)
The pressure equalizing hopper 14 adjusts the pressure of the raw materials after transferring them, and then sends the pressure-adjusted raw materials to the raw material charging section 24 of the shaft furnace 20. The pressure equalizing hopper 14 includes a hopper body, a raw material charging inlet 14a through which the raw materials are charged from the charging hopper 12, a raw material discharge outlet 14b through which the raw materials are equalized and then sent to the raw material charging section 24 of the shaft furnace 20, a purge gas inlet 14c through which purge gas is charged, a gas outlet 14d through which the purge gas and other gases are discharged, and a pressure equalizing gas inlet 14e through which the pressure-equalizing gas is introduced into the pressure equalizing hopper 14. The raw material charging inlet 14a is connected to the charging hopper 12, and the raw material discharge outlet 14b is connected to the raw material charging section 24 of the shaft furnace 20.

The operation of the pressure equalizing hopper 14 will be described below. The pressure equalizing hopper 14 closes the raw material charging inlet 14a, raw material discharge outlet 14b, purge gas inlet 14c, and pressure equalizing gas inlet 14e, and opens the gas outlet 14d to reduce the pressure inside the pressure equalizing hopper 14 to atmospheric pressure. Next, the purge gas inlet 14c is opened, and an inert gas serving as a purge gas is introduced into the pressure equalizing hopper 14. This allows the furnace gas remaining in the pressure equalizing hopper 14 to be replaced with the inert gas. Next, the pressure equalizing hopper 14 closes the purge gas inlet 14c, opens the raw material charging inlet 14a, and transfers the raw materials from the charging hopper 12 to the pressure equalizing hopper 14, which is filled with inert gas. After the raw materials are transferred, the purge gas inlet 14c is opened, and the inert gas is introduced into the pressure equalizing hopper 14. This allows the air introduced into the pressure equalizing hopper 14 along with the raw materials to be replaced with the inert gas. Examples of the inert gas include nitrogen gas, rare gas, CO ₂ gas, H ₂ O gas, and mixtures thereof. Nitrogen gas is preferred as the inert gas.

After replacement with inert gas, the pressure-equalizing hopper 14 closes the raw material inlet 14a and gas outlet 14d, opens the pressure-equalizing gas inlet 14e, and introduces the equalizing gas into the pressure-equalizing hopper 14 to increase the pressure. Completion of replacement can be determined when more than three times the volume of inert gas has been introduced into the pressure-equalizing hopper 14, or by measuring the gas components and confirming that the concentration of the gas to be replaced (reducing gas) has fallen below a specified value. This equalizing gas is pressurized to the pressure inside the shaft furnace 20 by the pressurization device 40, described below. The gas outlet 14d may be left open for a certain period of time to discharge any inert gas remaining inside during the initial period of pressurization. For example, gas chromatography or other means may be used to monitor whether the inert gas has been discharged, and the gas outlet 14d may be closed when the amount of inert gas discharged from the gas outlet 14d no longer fluctuates.

(Shaft furnace 20)
The shaft furnace 20 includes a raw material charging section 24 for charging the raw material iron oxide, a reduced iron discharge section 25 for discharging the reduced iron, a reducing gas inlet 28 disposed at the bottom of the shaft furnace 20 for injecting reducing gas, and an exhaust gas outlet 29 disposed at the top of the shaft furnace 20 for discharging exhaust gas. The raw material iron oxide is charged from the pressure equalizing hopper 14 through the raw material charging section 24 at the top of the shaft furnace 20. The internal furnace pressure in the shaft furnace 20 is higher than atmospheric pressure. For example, the internal furnace pressure is 0.1 to 1 MPa. As the iron oxide descends within the shaft furnace 20, it is reduced to reduced iron by carbon monoxide (CO) and hydrogen gas (reducing gas) generated from methane gas. The reduced reduced iron is discharged from the reduced iron discharge section 25. The reducing gas is heated by a heating device 50 and injected into the shaft furnace 20 through the reducing gas inlet 28. The injected reducing gas reduces the iron oxide. Next, the production of reduced iron based on methane gas in the shaft furnace 20 will be described.

Methane gas introduced as raw material gas reacts with water in the shaft furnace 20 to produce carbon monoxide and hydrogen as shown in formula (2A). Similarly, methane gas introduced as raw material gas reacts with carbon dioxide in the shaft furnace 20 to produce carbon monoxide and hydrogen as shown in formula (2B). The carbon monoxide produced in the above reaction reacts with iron oxide as shown in formula (2C) to produce reduced iron and carbon dioxide. As shown in formula (2D), the reduced iron further reacts with methane gas, contributing to an increase in the carbon concentration in the reduced iron. Increasing the carbon concentration in the reduced iron lowers the melting point of the reduced iron, making it easier to use in electric furnaces, etc.

The hydrogen gas produced in reaction (2D) is used to reduce iron oxide, producing reduced iron and water. Specifically, the reaction between iron oxide and hydrogen gas is as shown in formula (1A) above. Unreacted methane gas ( _CH4 ), hydrogen gas ( _H2 ), carbon monoxide, carbon dioxide, and water vapor ( _H2O ) produced by the reaction are sent to the dehydration device 30 via the exhaust gas outlet 29. _H2O + _CH4 → CO + _3H2 (2A)
_CO2 + _CH4 →2CO+ _2H2 (2B)
_Fe2O3 + _3CO →2Fe+ _3CO2 (2C)
3Fe+CH ₄ →Fe ₃ C+2H ₂ (2D)

(Dehydration device 30)
The dehydrator 30 dehydrates the flue gas discharged from the flue gas outlet 29. The flue gas discharged from the flue gas outlet 29 contains unreacted methane gas and water vapor, hydrogen gas, carbon monoxide, and carbon dioxide generated by the reduction reaction of iron oxide, etc. The dehydrator 30 dehydrates the flue gas, for example, by cooling the flue gas after dust removal. Because water inhibits the reduction reaction, it is preferable that the moisture concentration in the flue gas be as low as possible. For example, if the moisture concentration in the flue gas is 25 vol%, it is preferable to remove moisture to a moisture concentration of 2 vol% or less by dehydration. The dehydrated flue gas (circulation gas) is sent to the carbon dioxide removal device 60. The flue gas may be dedusted in a dust removal device (not shown) before being dehydrated by the dehydrator 30. The dust removal method is not particularly limited, and examples include a cyclone and a scrubber.

The carbon dioxide removal device 60 removes carbon dioxide from the circulating gas after dehydration. Because carbon dioxide inhibits the reduction reaction, it is preferable to remove as much carbon dioxide as possible. The carbon dioxide removal device 60 preferably removes 90 vol% or more of carbon dioxide from the circulating gas. This prevents the carbon dioxide from concentrating in the circulating gas. Carbon dioxide can be separated from the circulating gas using, for example, chemical adsorption. The circulating gas after carbon dioxide separation is sent to the pressure booster 40.

(Boost device 40)
The pressure booster 40 boosts the circulating gas after carbon dioxide removal to atmospheric pressure or higher and sends it to the heating device 50. A portion of the pressurized circulating gas is introduced as a pressure equalizing gas into the pressure equalizing hopper 14 via the pressure equalizing gas inlet 14e. The pressure booster 40 is, for example, a compressor.

(Heating device 50)
The heating device 50 heats the circulating gas and raw material gas pressurized by the pressure booster 40, and blows them into the reducing gas blowing port 28. The temperature of the blown reducing gas is approximately 700 to 1000°C. The blowing amount of the reducing gas is approximately 1000 to 2000 Nm ³ /t-DRI. It is preferable to use methane gas as the raw material gas.

(Method for producing reduced iron)
Next, a method for producing reduced iron according to a second embodiment will be described. Fig. 4 is a flowchart of the method for producing reduced iron according to the second embodiment. The method for producing reduced iron according to the second embodiment includes a raw material charging step S10A in which raw materials are charged from a charging hopper 12 that stores raw materials via a pressure equalizing hopper 14 that adjusts the pressure into a shaft furnace 20 that operates at a pressure higher than atmospheric pressure, a reduction step S20A in which the raw materials are reacted with a reducing gas in the shaft furnace 20 after the raw material charging step S10A to obtain reduced iron and a post-reduced exhaust gas, an exhaust gas circulation step S30A in which the post-reduced exhaust gas is circulated and used as part of the reducing gas, and a raw material gas supply step S40A. Each step will be described below.

(Raw material charging process S10A)
In the raw material charging step S10A, raw materials are charged from a charging hopper 12 that stores raw materials through a pressure equalizing hopper 14 that adjusts the pressure into a shaft furnace 20 that operates at a pressure higher than atmospheric pressure. The raw material charging process S10A includes: a depressurization process in which the pressure in the pressure equalizing hopper 14 is reduced from the pressure inside the shaft furnace 20 to atmospheric pressure; a first purging process S2 in which the furnace gas in the pressure equalizing hopper is replaced with a purge gas after the depressurization process S1; a raw material introduction process S3 in which raw materials are transferred from the charging hopper 12 to the pressure equalizing hopper 14 whose air has been replaced with the purge gas after the first purging process S2; a second purging process S4 in which the air introduced into the pressure equalizing hopper 14 in the raw material introduction process S3 is replaced with a purge gas after the second purging process S4; a pressure equalizing process S5A in which the pressure in the pressure equalizing hopper 14 is increased from atmospheric pressure to furnace pressure with an equalizing gas after the second purging process S4; and a raw material discharge process S6 in which the raw materials are transferred from the pressurized equalizing hopper 14 into the shaft furnace 20 after the pressure equalizing process S5A.

(Decompression step S1)
In the depressurization step S1, the pressure in the pressure equalizing hopper 14 is reduced from the pressure inside the shaft furnace 20 to atmospheric pressure. Specifically, the raw material discharge port 14b is closed and the gas outlet port 14d is opened, thereby reducing the pressure in the pressure equalizing hopper 14 from the pressure inside the shaft furnace 20 to atmospheric pressure.

(First purge step S2)
In the first purge step S2, after the depressurization step S1, the gas in the pressure equalizing hopper 14 is replaced with a purge gas. Specifically, the purge gas inlet 14c is opened to introduce an inert gas, which is a purge gas, into the pressure equalizing hopper 14. This allows the inert gas to replace the furnace gas in the pressure equalizing hopper 14 that was mixed in when the raw materials were charged from the pressure equalizing hopper 14 into the shaft furnace 20.

(Raw material input process S3)
In the raw material charging step S3, after the first purging step S2, the raw material iron oxide is transferred from the charging hopper 12 to the pressure-equalizing hopper 14, which has been purged with a purge gas. Specifically, the purge gas inlet 14c is closed, the raw material charging port 14a is opened, and the raw material is transferred from the charging hopper 12 to the pressure-equalizing hopper 14, which has been depressurized and purged.

(Second purge step S4)
In the second purge step S4, after the raw material charging step S3, the air introduced into the pressure equalizing hopper 14 in the raw material charging step S3 is replaced with a purge gas. Specifically, the purge gas inlet 14c is opened to introduce an inert gas as a purge gas into the pressure equalizing hopper 14. This allows the air introduced into the pressure equalizing hopper 14 together with the raw materials in the raw material charging step S3 to be replaced with the inert gas.

(Pressure equalization step S5A)
In the pressure equalization step S5A, after the second purge step S4, the pressure in the pressure equalization hopper 14 is increased from atmospheric pressure to the pressure inside the shaft furnace 20 using equalizing gas. Specifically, the raw material charging port 14a and the gas outlet 14d are closed, and the equalizing gas inlet 14e is opened to introduce equalizing gas into the pressure equalization hopper 14 to increase the pressure. The equalizing gas is pressurized to the pressure inside the shaft furnace (furnace pressure) by the pressure increase device 40. The equalizing gas is part of the circulating gas dehydrated by the dehydration device 30. It is preferable to use the equalizing gas to push out the inert gas (nitrogen gas) introduced in the second purge step S4 at the beginning of the pressure equalization step S5A, since this prevents the inert gas (nitrogen gas) from accumulating inside the shaft furnace 20. When pushing out the nitrogen gas using equalizing gas, the gas outlet 14d is left open for a while to exhaust the inert gas (nitrogen gas). The time for which the gas outlet 14d is open can be determined by checking the fluctuation of the inert gas (for example, nitrogen gas) using gas chromatography or the like.

(Raw material discharge process S6)
In the raw material discharge step S6, after the pressure equalization step S5A, the raw materials are transferred from the pressurized equalizing hopper 14 into the shaft furnace 20. Specifically, the pressure equalizing gas inlet 14e is closed and the raw material discharge port 14b is opened, thereby transferring the raw materials in the pressure equalizing hopper from the pressurized equalizing hopper into the shaft furnace. After the raw material discharge step S6 is completed, the process returns to the depressurization step S1, allowing the raw materials to continue to be fed into the shaft furnace 20.

(Reduction step S20A)
In the reduction step S20A, after the raw material charging step S10A, the raw material is reacted with a reducing gas in the shaft furnace 20 to produce reduced iron and a post-reduction exhaust gas. Specifically, in the shaft furnace 20, the raw material is reduced by the reducing gas blown in through the reducing gas blowing port 28 to produce reduced iron (direct reduced iron), which is then discharged from the reduced iron discharge section 25. When the raw material gas is methane gas, iron oxide reacts with carbon monoxide produced from the methane gas as shown in the above formulas (2A) to (2D), producing reduced iron and carbon dioxide. Unreacted methane gas and the hydrogen gas, carbon monoxide, carbon dioxide, and water vapor produced by the above reaction are discharged as exhaust gas from the exhaust gas discharge port 29.

(Exhaust gas circulation process S30A)
In the exhaust gas circulation step S30A, the exhaust gas (exhaust gas after the reduction reaction) discharged in the reduction step S20 is circulated and used as part of the reducing gas. When methane gas is used as the raw material gas, the exhaust gas consists of unreacted methane gas and hydrogen gas, carbon monoxide, carbon dioxide, and water vapor produced by the above-mentioned reaction. The exhaust gas discharged from the exhaust gas outlet 29 is dehydrated in the dehydration device 30, and then carbon dioxide is removed in the carbon dioxide removal device 60. That is, the exhaust gas circulation step S30A includes a dehydration step for dehydrating the exhaust gas and a decarbonation step for removing carbon dioxide from the exhaust gas. The circulating gas from which carbon dioxide has been removed is pressurized in the pressure booster 40, and a portion of the circulating gas is used as an equalizing gas in the equalizing step S5A. The remaining circulating gas not used in the equalizing step S5A is heated in the heating device 50 as a reducing gas and circulated and reused. That is, the equalizing gas is the gas obtained by removing water and carbon dioxide from the exhaust gas in the dehydration step and the carbon dioxide removal step. The equalizing gas may be a combustion exhaust gas obtained by burning the gas in a heating furnace or the like outside the direct reduced iron manufacturing apparatus 100 and removing carbon dioxide from the combustion exhaust gas.

(Source gas supply step S40A)
In the raw material gas supply step S40A, the methane gas consumed in the reduction of the raw material (iron oxide) in the reduction step S20 is introduced as raw material gas into the shaft furnace 20 via the heating device 50. The amount of methane gas introduced is determined by calculating the required amount of methane gas to be injected and increasing or decreasing the amount of methane to be injected.

The above describes the reduced iron manufacturing method according to the second embodiment and the direct reduced iron manufacturing apparatus 100A used in said manufacturing method. According to the reduced iron manufacturing method according to the second embodiment, purging flammable gases with an inert gas (nitrogen gas) enables safe operation even when using a circulating gas containing hydrogen as the equalizing gas. By using the exhaust gas after reduction as the equalizing gas, it is possible to prevent the concentration of nitrogen that accompanies the circulation of the exhaust gas, and therefore the reducing power of the reducing gas is not reduced. Furthermore, since there is no concentration, there is no need to periodically release the exhaust gas.

The technical scope of the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. Furthermore, the components in the above-described embodiments can be replaced with well-known components as appropriate, and the above-described modifications can be combined as appropriate, without departing from the spirit of the present invention.

The method for producing reduced iron disclosed herein can suppress the concentration of unwanted components such as nitrogen and carbon dioxide while maintaining reduction performance, making it highly industrially applicable.

12. Charging hopper, 14. Equalizing hopper, 20. Shaft furnace, 24. Raw material charging section, 25. Reduced iron discharge section, 28. Reduction gas inlet, 29. Exhaust gas outlet, 30. Dehydration device, 40. Pressure booster, 60. Carbon dioxide removal device

Claims (2)

a raw material charging step of charging the raw materials from a charging hopper that stores the raw materials through a pressure equalizing hopper that adjusts the pressure into a shaft furnace that operates at a pressure higher than atmospheric pressure;
a reduction step of reacting the raw materials with a reducing gas in the shaft furnace after the raw material charging step to obtain reduced iron and a post-reduction exhaust gas;
an exhaust gas circulation step in which, after the reduction step, the exhaust gas after the reduction step is circulated and used as part of the reducing gas;
Including,
The raw material charging step
a depressurization step of reducing the pressure in the pressure equalizing hopper from the pressure in the shaft furnace to atmospheric pressure;
a first purging step of replacing the gas in the pressure equalizing hopper with a purging gas after the depressurizing step;
a raw material charging step of transferring the raw material from the charging hopper to the pressure equalizing hopper whose air has been replaced with the purge gas after the first purging step;
a second purging step of replacing the air introduced into the pressure equalizing hopper in the raw material charging step with a purging gas after the raw material charging step;
a pressure equalization step of increasing the pressure in the pressure equalization hopper from atmospheric pressure to the pressure in the shaft furnace with an equalization gas after the second purge step;
a raw material discharge step of transferring the raw material from the pressurized pressure equalizing hopper into the shaft furnace after the pressure equalization step;
Including,
The exhaust gas circulation step
a dehydration step for removing water from the exhaust gas after reduction;
the purge gas is an inert gas,
The method for producing reduced iron, wherein the equalizing gas is a gas obtained by removing water from the exhaust gas in the dehydration step. the exhaust gas circulation step further includes a carbon dioxide removal step of removing carbon dioxide from the exhaust gas,
2. The method for producing reduced iron according to claim 1, wherein the equalizing gas is a gas obtained by removing water and carbon dioxide from the exhaust gas in the dehydration step and the carbon dioxide removal step.