Method for producing reduced iron and reduction furnace

Abstract

Provided is a method for producing reduced iron, said method making it possible to accurately control the final carbon content of reduced iron. The composition of a carburizing gas is changed in accordance with the composition of a reducing gas.

Description

Method for producing reduced iron and reduction furnace

[0001] The present invention relates to a method for producing reduced iron and a reduction furnace.

[0002] In the steelmaking process, for example, a method is used in which iron oxide is reduced in a vertical reduction furnace (shaft furnace) to produce reduced iron. In this method, agglomerated iron ore such as sintered ore or pellets (hereinafter simply referred to as iron oxide) is charged into the reduction furnace through an iron oxide charging port at the top of the reduction furnace as the iron oxide raw material. Then, CO and H ２ Reduced iron is produced by injecting a reducing gas containing the above-mentioned element into the furnace to reduce iron oxide. In this method, natural gas or the like is used as the raw material gas for the reducing gas. This raw material gas is heated and reformed together with the furnace top gas in a reformer. This generates the reducing gas. The furnace top gas is the gas remaining after the reducing gas has been used to reduce iron oxide in the reduction furnace, and is generally discharged from the furnace top. The generated reducing gas is injected into the reduction furnace and reacts with iron oxide supplied from above the reduction furnace. The iron oxide is then reduced to form reduced iron. The reduced iron is then discharged from a reduced iron outlet at the bottom of the reduction furnace.

[0003] As a technology related to the production of such reduced iron, for example, Patent Document 1 discloses the following: "A method for producing reduced iron by reducing iron oxide, comprising: a reduced iron production step of bringing the iron oxide into contact with a reducing gas while causing it to fall from a top of a reduction furnace in order to reduce the iron oxide to produce reduced iron, and discharging the reduced iron from the bottom of the reduction furnace; a reformed gas production step of extracting furnace top gas from the reduction furnace, adjusting the moisture content thereof, and performing dust removal treatment thereon to produce a process gas, and supplying at least the process gas into a reformer to produce a reformed gas containing carbon monoxide and hydrogen in the reformer; a reducing gas supply step of supplying the produced reformed gas to the reduction furnace as the reducing gas; a cooling step of introducing a cooling gas into a cooling region set in a lower part of the reduction furnace to cool the cooling region; and a reformed gas introduction step of extracting a portion of the reformed gas and introducing it into the cooling region to increase the carbon content of the reduced iron passing through the cooling region."

[0004] JP 2017-88912 A

[0005] In the production of reduced iron, as in Patent Document 1, there are cases where the reduced iron is carburized in a region below the reducing gas injection position of the reducing furnace up to the reduced iron discharge port (hereinafter also referred to as the lower part of the reducing furnace; the region above the reducing gas injection position of the reducing furnace up to the iron oxide charging port is also referred to as the upper part of the reducing furnace) before the reduced iron is discharged from the reducing furnace. Here, carburization is a process of increasing the carbon content of the reduced iron by using carburizing gas. Carburization can be carried out, for example, by injecting CH ４ The reduction is carried out by blowing in a gas containing CO as a main component and bringing the gas into contact with the reduced iron.

[0006] When the carbon content of reduced iron increases, carbon serves as an auxiliary heat source when melting the reduced iron. Furthermore, the melting point of the reduced iron decreases. As a result, the energy required to melt the reduced iron can be reduced. On the other hand, when transporting reduced iron by sea, for example, the reduced iron must be compression-molded into hot briquette iron (HBI) to prevent spontaneous combustion. Here, if the carbon content of the reduced iron increases excessively, the formability of the reduced iron to HBI decreases. Therefore, in carburization in the lower part of the reducing furnace, it is necessary to control the carbon content of the reduced iron discharged from the reducing furnace (hereinafter also referred to as the final carbon content of the reduced iron) so that it falls within a target range.

[0007] However, with the technique of Patent Document 1, it is difficult to accurately control the final carbon content of reduced iron, and improvements in this respect are currently required.

[0008] The present invention has been developed in view of the above-described current situation, and aims to provide a method for producing reduced iron that enables accurate control of the final carbon content of reduced iron. Another aim of the present invention is to provide a reducing furnace that can be suitably used in the above-described method for producing reduced iron. In this disclosure, any numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits, respectively.

[0009] The inventors have conducted extensive research to solve the above-mentioned problems. As a result, the inventors have found that the composition of the reducing gas, in particular the ratio of H to CO concentration in the reducing gas, ２ The ratio of concentrations is H ２ / CO (hereinafter simply referred to as reducing gas H ２ It has been found that the above problems can be solved by changing the composition of the carburizing gas depending on the amount of carbon dioxide (CO).

[0010] The present invention has been completed based on the above findings and further investigations. That is, the gist and configuration of the present invention are as follows.

[0011] 1. A method for producing reduced iron, comprising: a reduction step in which iron oxide is reduced to form reduced iron by using a reducing gas blown into a reduction furnace; a carburization step in which the carbon content of the reduced iron is increased by using a carburizing gas blown into the reduction furnace; and a control step in which the composition of the carburizing gas is changed in accordance with the composition of the reducing gas.

[0012] 2. In the control step, H of the reducing gas ２ H concentration and CO concentration ２ The ratio of concentrations is H ２ / (H ２ the composition of the carburizing gas is changed so that the amount of carburization of the reduced iron in the carburizing step increases as the deviation of the H content of the reducing gas (H + CO) from 0.5 increases, ２ / (H ２ 2. The method for producing reduced iron according to item 1, wherein the composition of the carburizing gas is changed so that the amount of carburization of the reduced iron in the carburizing step decreases as the deviation of the ratio (Cu + CO) from 0.5 decreases.

[0013] 3. In the control step, the H of the reducing gas ２ H concentration and CO concentration ２ The ratio of concentrations is H ２ / (H ２ +CO), the CH of the carburizing gas ４ 3. The method for producing reduced iron according to 1 or 2 above, wherein the concentration is changed.

[0014] 4. The method for producing reduced iron according to any one of 1 to 3 above, wherein the control step satisfies the following relationship: X ０ +95×|R-0.5|-3≦X≦X ０ +115×|R−0.5|+3 Where, X: CH of carburizing gas ４ Concentration (volume%), X ０ : Standard CH for carburizing gas ４ concentration (volume %) and R: H of reducing gas ２ / (H ２ +CO).

[0015] 5. A distribution step in which the furnace top gas discharged from the reduction furnace is distributed into a first furnace top gas and a second furnace top gas, and the furnace top gas is a gas obtained after the reducing gas has been used for reducing the iron oxide in the reduction step; and ４ a reforming step of generating the reducing gas from a containing gas.

[0016] 6. A reducing furnace comprising: a reducing section that reduces iron oxide with a reducing gas to produce reduced iron; a carburizing section that increases the amount of carbon contained in the reduced iron with a carburizing gas; and a control section that changes the composition of the carburizing gas in accordance with the composition of the reducing gas.

[0017] 7. The control unit adjusts the H of the reducing gas. ２ H concentration and CO concentration ２ The ratio of concentrations is H ２ / (H ２ the composition of the carburizing gas is changed so that the amount of carburization of the reduced iron in the carburized portion increases as the amount of deviation of the H content of the reducing gas (H + CO) from 0.5 increases, ２ / (H ２ 7. The reducing furnace according to claim 6, wherein the composition of the carburizing gas is changed so that the amount of carburization of the reduced iron in the carburized portion decreases as the deviation of the ratio (R / R) of the carbon dioxide gas to the carbon dioxide gas (CO) from 0.5 decreases.

[0018] 8. The control unit controls the H ２ H concentration and CO concentration ２ The ratio of concentrations is H ２ / (H ２+CO), the CH of the carburizing gas ４ 8. The reduction furnace according to 6 or 7, wherein the concentration is changed.

[0019] 9. The reduction furnace according to any one of 6 to 8, wherein the following relationship is satisfied by the control of the control unit. X ０ +95×|R-0.5|-3≦X≦X ０ +115×|R−0.5|+3 Where, X: CH of carburizing gas ４ Concentration (volume%), X ０ : Standard CH for carburizing gas ４ concentration (volume %) and R: H of reducing gas ２ / (H ２ +CO).

[0020] The method for producing reduced iron of the present invention allows for accurate control of the final carbon content of reduced iron, which is extremely advantageous industrially. Furthermore, the method for producing reduced iron of the present invention does not require large-scale expansion of facilities, which is extremely advantageous in terms of cost.

[0021] FIG. 1 is a schematic diagram showing an example of the schematic configuration of a reduction furnace and its auxiliary equipment. ２ / (H ２ 1 is a diagram showing an example of the relationship between the amount of carbon monoxide (CO + CO) and the amount of carburization in the reduction step. FIG. 2 is a schematic diagram showing an example of the schematic configuration of a reduction furnace having a control unit and its auxiliary equipment. FIG. 3 is a schematic diagram showing an example of the functional blocks of the control unit.

[0022] [1] Manufacturing method of reduced iron Hereinafter, a manufacturing method of reduced iron according to one embodiment of the present invention will be described with reference to Fig. 1. Fig. 1 is a schematic diagram showing an example of the general configuration of a reducing furnace and its auxiliary equipment used in a manufacturing method of reduced iron according to one embodiment of the present invention. In the figure, reference numeral 1 denotes a reducing furnace, 1a denotes iron oxide, 1b denotes reduced iron, 3 denotes a dust removal device, 4 denotes a dehydration device, 5 denotes a natural gas supply section, 6 denotes an air supply section, 7 denotes a reforming device, 8 denotes a reducing gas injection device, 9 denotes a carburizing gas injection device, 10 denotes a reduction section, and 11 denotes a carburizing section.

[0023] In a method for producing reduced iron according to one embodiment of the present invention, iron oxide is charged into a reducing furnace through an iron oxide charging port and gradually lowered. Reducing gas is then blown into the reducing furnace through a reducing gas inlet, and the iron oxide is reduced to produce reduced iron in the reduction zone by the reducing gas. Carburizing gas is then blown into the reducing furnace through a carburizing gas inlet at the bottom of the reducing furnace, and the carbon content of the reduced iron is increased by the carburizing gas in the carburizing zone. The reduced iron is then discharged from the reduced iron outlet of the reducing furnace. At this time, mainly CO and CO are discharged from the top of the reducing furnace. ２ , H ２ and H ２ The furnace top gas containing O is discharged. This furnace top gas is subjected to dust removal in a dust removal device and moisture adjustment in a dehydration device. Then, a part of the furnace top gas is sent to a reformer as a first furnace top gas that becomes a raw material gas. The reformer is supplied with CH 3 O 4 together with the first furnace top gas. ４ A gas containing, for example, natural gas is supplied from a natural gas supply. Then, in the reformer, the supplied gas is heated. Then, a reforming reaction occurs, and mainly CO and H are produced. ２ A high-temperature reducing gas containing CO is generated. This reducing gas is then blown into the reducing furnace. The remaining part of the furnace top gas is used as a second furnace top gas, for example, as a heating fuel in the combustion chamber of the reformer. The second furnace top gas after being combusted as a heating fuel usually contains CO ２ The carburizing gas after being used for carburizing (hereinafter also referred to as "reacted carburizing gas") is discharged to the outside of the reducing furnace from a carburizing gas outlet located at a position lower than the reducing gas inlet. Next, the reacted carburizing gas is, for example, dusted by a dust removal device, and then appropriately mixed with CH4 gas such as natural gas supplied from a natural gas supply unit. ４ After adding the containing gas, the gas is blown into the reduction furnace again as carburizing gas.

[0024] Hereinafter, each step of the method for producing reduced iron according to one embodiment of the present invention will be described in more detail.

[0025] (Reduction step) In the reduction step, a reducing gas is blown into a reduction furnace, and the iron oxide charged into the reduction furnace is reduced by the reducing gas blown into the reduction furnace to produce reduced iron. The conditions for the reduction step are not particularly limited, and may be those of a conventional method.

[0026] For example, the gas composition of the reducing gas is CO: 1 to 70% by volume, H ２ The balance is preferably 0 to 10% by volume. ２ The reducing gas blowing temperature is 800 to 1100°C, and the blowing amount of reducing gas is 1400 to 3000 Nm ３ The amount of iron oxide charged is preferably 1,300 to 1,500 kg / t.

[0027] (Carburizing Process) In the carburizing process, carburizing gas is injected into the reduction furnace, and the carbon content of the reduced iron is increased by the carburizing gas injected into the reduction furnace. The amount of carburization in the carburizing process is the amount of carbon increase (mass%) in the reduced iron in the carburizing process, and can be calculated, for example, by the following formula: [Amount of carburization in the carburizing process (mass%)] = [Final carbon content of reduced iron (mass%)] - [Carbon content of reduced iron at the end of the reduction process (mass%)]

[0028] The composition of the carburizing gas may be determined by the control step described later. ４ The balance may be determined in the range of 0 to 70% by volume. ２ The amount of carburizing gas blown may be constant or may be determined by control in the control step described below. ３ / t or more 500Nm ３ / t. Other carburizing step conditions are not particularly limited. For example, the carburizing gas blowing temperature is preferably 0°C or higher and 100°C or lower.

[0029] (Control Step) In the control step, the composition of the reducing gas, particularly the H ２ / (H ２ It is important to change the composition of the carburizing gas depending on the amount of carbon dioxide (CO).

[0030] That is, the inventors have made the following findings as a result of extensive research: (a) Reducing gas usually contains CO. Therefore, the carbon content of reduced iron increases even in the reduction process. In other words, the final carbon content of reduced iron depends not only on the carburizing conditions but also on the composition of the reducing gas, particularly the H content of the reducing gas.２ / (H ２ (b) As shown in Figure 2, the reduction gas H ２ / (H ２ As CO + H approaches 0.5, the carbon increase (mass %) in the reduced iron during the reduction process (hereinafter also referred to as the carburization amount during the reduction process), that is, the carbon amount (mass %) of the reduced iron at the end of the reduction process, increases. That is, the carburization amount during the reduction process increases according to the following formula (1): CO + H ２ →C+H ２ O ... (1) Here, if the CO concentration of the reducing gas is H ２ It becomes higher with increasing concentration, ２ / (H ２ When the ratio of H + CO becomes smaller than 0.5, black carbon powder is discharged together with the reduced iron from the outlet of the reduction furnace. As a result, the amount of carburization in the reduction process is controlled by the H ２ / (H ２ The amount of carburization in the reduction process is smaller than that in the case where the CO concentration in the reducing gas is 0.5. The inventors believe that the reason for this is that although the amount of carbon due to the reaction of the following formula (2) increases with an increase in the CO concentration in the reducing gas, most of the carbon derived from the reaction of the following formula (2) does not adhere to the reduced iron, that is, most of the carbon derived from the reaction of the following formula (2) does not carburize the reduced iron but precipitates on the surface of the reduced iron. 2CO → C + CO ２ ... (2) Also, H ２ / (H ２ +CO) is greater than 0.5, the amount of CO serving as a carbon source decreases, resulting in a decrease in the amount of carburization in the reduction step.

[0031] From the above, in the control step, the composition of the reducing gas, particularly the H ２ / (H ２ In one preferred embodiment, the composition of the carburizing gas is changed depending on the amount of H in the reducing gas. ２ / (H ２ The composition of the carburizing gas is changed so that the greater the deviation of the H+CO ratio from 0.5, the greater the amount of carburization of the reduced iron in the carburizing process (in other words, the amount of carburization of the reduced iron in the carburizing section of the reducing furnace). ２ / (H ２The composition of the carburizing gas is changed so that the amount of carburization in the carburizing process decreases as the deviation of the carbon dioxide (CO) from 0.5 decreases. ４ CH concentration during the carburizing process ４ It can be controlled by changing the supply amount. ４ has the effect of promoting carburization of reduced iron through the decomposition reaction of the following formula (3). ４ Increasing the concentration increases the amount of carburization during the carburizing process. ４ Decreasing the concentration reduces the amount of carburization during the carburization process. ４ →C+2H ２ ...(3)

[0032] In addition, the CH ４ Among the gas species other than H ２ and H ２ O, CO ２ , CO, and hydrocarbons with two or more carbon atoms affect the amount of carburization in the carburization process. Therefore, the amount of carburization in the carburization process can also be controlled by changing the concentration of these gas species.

[0033] For example, H ２ affects the reaction equilibrium of the above formula (3), and as a result, affects the amount of carburization in the carburizing process. ２ It is advantageous to reduce the concentration.

[0034] Also, H ２ O and CO ２ When the atmosphere is particularly hot, CH ４ reacts with H ２ and CO. In this case, the increase in the amount of carburization in the carburizing process is due to the increase in H ２ O concentration and CO ２ It is advantageous to reduce the concentration. ４ +CO ２ → 2CO + 2H ２ ... (4) CH ４ +H ２ O → CO + 3H２ ...(5)

[0035] In addition, CO and hydrocarbons with two or more carbon atoms generate C according to the following formulas (1), (2), and (6), which increases the amount of carburization in the carburization process: CO + H ２ →C+H ２ O...(1) 2CO→C+CO ２ ... (2) C ｎ H ｍ →nC+m / 2H ２ (6) Here, n in the above formula (6) is an integer of 2 or more, and m is an integer corresponding to the number n (the number of H atoms bonded to n C atoms).

[0036] It is also preferable that the relationship of the following formula (7) is satisfied. In particular, the H of the reducing gas is preferably used so as to satisfy the relationship of the following formula (7). ２ Depending on the / CO, the CH of the carburizing gas ４ It is more preferable to change the concentration. ０ +95×|R-0.5|-3≦X≦X ０ +115×|R−0.5|+3 (7) Wherein, X: CH of carburizing gas ４ Concentration (volume%), X ０ : Standard CH for carburizing gas ４ concentration (volume %) and R: H of reducing gas ２ / (H ２ +CO).

[0037] X is more preferably X ０ +100×|R−0.5|−3 or more. X is more preferably X ０ +110×|R−0.5|+3 or less. Note that the right-hand side value of the above formula (7) is X １ If the value exceeds the upper limit, the right-hand side of the formula (7) is X １ In addition, the left side value of the above formula (7) is X １ If the value exceeds X, the left-hand side value (lower limit value) and the right-hand side value (upper limit value) of the above formula (7) are both X １ In other words, the range of the above formula (7) is X １ However, the inclusion of unavoidable impurities of less than 1% by volume is acceptable. １ is the upper limit of carburizing gas CH４ concentration (vol %), and the CH ４ For example, if there is a sufficient amount of CH ４ If you can supply X １ becomes 100. In this case, it is preferable to satisfy the following equation (8) in addition to the above equation (7).

[0038] Here, X ０ is the reducing gas H ２ / (H ２ +CO) is 0.5, especially H in the reducing gas ２ The concentration of CO is equal to the concentration of CO, and the balance (balance concentration: 10% by volume or less, preferably 5% by volume or less, more preferably 0% by volume) is N ２ Under the conditions of inert gases such as those mentioned above, the CH of the carburizing gas when the final carbon content of reduced iron targeted in actual operation (hereinafter also referred to as the target final carbon content of reduced iron) is obtained is ４ Concentration (volume %).

[0039] In addition, X ０ In deriving (2), the conditions other than the composition of the reducing gas may be set based on the base operating conditions. The base operating conditions are the amount of iron oxide charged, the composition of the reducing gas, the reducing gas injection temperature, and the reducing gas injection amount specified or recommended for the reducing furnace to be used in actual operation (hereinafter also referred to as the reducing furnace to be used). The base operating conditions can be confirmed, for example, from the specifications (operating manual) of the reducing furnace to be used or from past operating records. Note that conditions that cannot be confirmed from specifications, such as the composition of the iron oxide to be used and the carburizing gas injection temperature and injection amount, may be set according to the conditions planned for actual operation.

[0040] For example, X ０ can be obtained by conducting a preliminary operation test of the reducing furnace to be used. ２ : 50 vol% and CO: 50 vol%. Other conditions depend on the base operating conditions of the reduction furnace to be used. ４Preliminary operation tests are conducted on the reduction furnace to be used, with the concentration being varied in various ways. Conditions that cannot be confirmed in specifications, such as the composition of the iron oxide to be used and the injection temperature and injection amount of the carburizing gas, are set according to the conditions planned for actual operation. Then, from the results of the preliminary operation tests, the CH of the carburizing gas when the target final carbon content of the reduced iron is obtained is calculated. ４ Calculate the concentration, X ０ The error of each value is allowable within the range of ±5%, preferably ±3%. ０ can take various values ​​depending on the type (structure, size, etc.) of the reduction furnace to be used. ４ The remainder other than ２ Inert gas such as

[0041] The target final carbon content of the reduced iron is set, for example, in the range of 0.50 to 7.00 mass %.

[0042] In addition, the left side value of the above formula (7) is X １ If it exceeds this, the CH ４ In addition to the concentration, it is preferable to change (increase) the injection amount of the carburizing gas. ４ Concentration X １ While maintaining the above, it is preferable to change (increase) the injection amount of the carburizing gas so as to satisfy the following formula (8): ０ × (X ０ +95×|R-0.5|-3)÷X １ ≦V≦V ０ × (X ０ +115×|R-0.5|+3)÷X １ ... (8) In the formula, V: amount of carburizing gas blown in (Nm ３ / t), V ０ : Standard injection amount of carburizing gas (Nm ３ / t), X: CH of carburizing gas ４ Concentration (volume%), X ０ : Standard CH for carburizing gas ４ Concentration (volume%), X １ : Upper limit of carburizing gas CH ４ concentration (volume %) and R: H of reducing gas ２ / (H ２ +CO).

[0043] In addition, V ０ is, for example, the amount of carburizing gas blown in the above-mentioned preliminary operation test (standard CH of carburizing gas). ４ The amount of carburizing gas to be blown when the concentration is obtained may be determined based on the amount of carburizing gas to be blown under the operating conditions planned for actual operation. １ is the CH ４ For example, if there is a sufficient amount of CH ４ If you can supply X １ becomes 100.

[0044] (Distribution Step) In the distribution step, as described above, it is preferable to distribute the furnace gas into a first furnace gas and a second furnace gas. The furnace gas is the gas obtained after the reducing gas has been used to reduce iron oxide in the reduction step. In one example, the first furnace gas is supplied to a reformer and used as a raw material gas for the reducing gas in the reforming step. The second furnace gas is used as heating fuel in the combustion chamber of the reformer. Furthermore, a portion of the furnace gas may be distributed to a location other than the first furnace gas and the second furnace gas, for example, to be supplied to another device or stored. The distribution amount may be determined appropriately depending on the operating conditions.

[0045] The means for distributing and controlling the flow rate of the furnace gas is not particularly limited, and may be any conventional method, such as a mass flow controller.

[0046] (Reforming step) In the reforming step, the first furnace gas and CH ４ For example, the reformer may be configured to generate a reducing gas from the first furnace top gas and CH ４ The natural gas containing gas is supplied to the reformer. The supplied gas is then heated in the reformer. The reforming reactions of the following formulas (4)' and (5)' occur, producing mainly CO and H. ２ A hot reducing gas containing CH ４ +CO ２ → 2CO + 2H ２ ΔH=247kJ / mol...(4)' CH４ +H ２ O → CO + 3H ２ ΔH=206kJ / mol...(5)'

[0047] The conditions other than those mentioned above are not particularly limited, and may be those according to conventional methods.

[0048] [2] Reduction Furnace Next, a reduction furnace according to an embodiment of the present invention will be described. The reduction furnace according to an embodiment of the present invention can be suitably used in the above-described method for producing reduced iron according to an embodiment of the present invention.

[0049] A reducing furnace according to one embodiment of the present invention has, for example, a reducing section that reduces iron oxide with a reducing gas to produce reduced iron, a carburizing section that increases the amount of carbon contained in the reduced iron with a carburizing gas, and a control section that changes the composition of the carburizing gas in accordance with the composition of the reducing gas. Figure 3 is a schematic diagram showing an example of the overall configuration of a reducing furnace having a control section and its associated equipment. In Figure 3, reference numeral 12 denotes the control section.

[0050] The reducing section is not particularly limited in its configuration, and a general configuration can be used. For example, the reducing section is a region through which the reducing gas blown in through the reducing gas blowing port flows and which serves as a charging section (a descending path) for the iron oxide charged through the iron oxide charging port at the top of the reducing furnace. The reducing section is connected to the reducing gas blowing port and the top gas outlet, respectively, and forms a path through which the reducing gas (top gas) flows. The reducing section is also connected to the iron oxide charging port at the top and the carburizing section at the bottom, respectively, and forms a descending path for the iron oxide (reduced iron).

[0051] The carburizing section is not particularly limited in its configuration, and a general configuration can be used. For example, the carburizing section is a region through which carburizing gas injected from a carburizing gas inlet flows and which serves as a path for the reduced iron obtained from the iron oxide in the reducing section to fall. The carburizing section is connected to the carburizing gas inlet and the carburizing gas outlet, respectively, to form a path for the carburizing gas to flow. The carburizing section is also connected to the reducing section above and the reduced iron outlet below, respectively, to form a path for the reduced iron to fall. That is, in one example, the iron oxide charging inlet, reducing section, carburizing section, and reduced iron outlet are arranged in this order from top to bottom.

[0052] The control section determines the composition of the reducing gas, particularly the H ２ The composition of the carburizing gas is changed according to the amount of CO. The preferred control mode in the control unit is as described in [1] above.

[0053] As an example, the control unit may have an input unit for inputting various set values ​​and measurement data such as the composition of the reducing gas, a calculation unit for processing the input set values ​​and measurement data, a memory unit for storing the set values ​​and measurement data, and an output unit for outputting an operation signal for changing the composition of the carburizing gas based on the processing results of the calculation unit.

[0054] In this case, specifically, the control unit is an information processing device. Fig. 4 shows an example of functional blocks of the control unit. The reducing furnace may have a control unit as shown in Fig. 4. As shown in Fig. 4, the control unit includes an input unit and an output unit connected to external devices so as to be able to communicate data with each other, a storage unit that stores various data, and a calculation unit, all of which are connected to each other so as to be able to communicate data with each other.

[0055] The input unit and the output unit are, for example, interfaces that are provided to enable data communication with external devices.

[0056] The calculation unit is, for example, a CPU. The calculation unit controls the operation of the entire control unit. The calculation unit calculates how to change the composition of the carburizing gas based on various set values ​​and measurement data input from the outside to the input unit or stored in the memory unit, and generates an operation signal to change the composition of the carburizing gas. The output unit outputs the operation signal. The calculation unit realizes the above-mentioned functions by, for example, executing a program stored in the memory unit.

[0057] The storage unit is, for example, a writable nonvolatile memory such as an EPROM, etc. The storage unit is not particularly limited, but may be, for example, an HDD, an SSD, etc.

[0058] The carburizing gas injection device receives the operation signal output from the output section, controls the composition of the carburizing gas as described in [1] above, and then injects the carburizing gas into the reducing furnace.

[0059] The configuration other than that described above is not particularly limited, and may be the same as that of a conventionally known reduction furnace.

[0060] Examples will be described below. The examples of the present invention are based on a numerical analysis model that can simulate heat transfer and reactions in a reduction furnace. The numerical analysis model and analysis conditions used are as follows: (Numerical analysis model and analysis conditions) - Numerical analysis model: A one-dimensional model in which the region inside the reduction furnace is divided into 40 regions at equal intervals in the vertical direction. - Starting from the top of the reduction furnace (the upper side of the reduction furnace in the vertical direction), the first to 20th regions were set as the reduction region, and the 26th to 40th regions were set as the carburization region. (The 21st to 25th regions were set as the transition zone, where only the reduced iron descends (reduction and carburization do not occur).) - Using the above numerical analysis model, the material balance and heat balance of the solids and gases in each region inside the reduction furnace, as well as the solid-gas heat transfer, were calculated sequentially at short time intervals (1 second). - Using the above calculations, the changes in the temperature and composition of the solids and gases in each region inside the reduction furnace were analyzed, and the final carbon content of the reduced iron was calculated. - Source of values ​​used in calculations of reaction rates, etc.: Bechara et al.: Materials 2018, 11(7), 1094 (https: / / doi.org / 10.3390 / ma11071094) Expected operating conditions: As described in Examples 1 and 2 below

[0061] Example 1 Furnace volume as shown in Figure 1: 80 m ３ In the reducing furnace and its auxiliary equipment, reduced iron is produced under the conditions shown in Table 1. That is, in Example 1, the composition of the carburizing gas is changed depending on the composition of the reducing gas under each condition, and reduced iron is produced. In particular, the H ２ / (H ２ +CO), the CH of the carburizing gas is adjusted to satisfy the relationship of the above formula (7). ４ In Example 1, under Condition 5, the amount of carburizing gas injected was also changed so as to satisfy the relationship of Equation (8). In Comparative Example 1, on the other hand, reduced iron was produced under all conditions with the carburizing gas composition and injection amount remaining almost unchanged.

[0062] Under all conditions, the operation period was 7 days, the injection temperature of the reducing gas was 980°C, the target final carbon content of the reduced iron was 2.00 mass%, and X ０ is 61.7% by volume, X １ is 100% by volume, V ０ is 350Nm ３ / t.

[0063] In Table 1, the operational specifications are listed in terms of the basic unit per ton of reduced iron produced. For example, if 1,400 kg of iron oxide pellets are used to produce 1 ton of reduced iron, the amount of iron oxide pellets used is expressed as 1,400 kg / t. If 3,000 t / day of reduced iron is produced, this amount is multiplied by 3,000 to obtain the specifications per day. ０ and V ０ are obtained by a preliminary operation test according to the above-mentioned procedure. The amount of iron oxide charged, the reducing gas injection temperature, the reducing gas injection rate, and the reducing gas composition under the base operating conditions are based on the specifications of the reducing furnace used. The same applies to Example 2 described below.

[0064] Conditions other than those described above and in Table 1 were the same as those used in the conventional methods. The remaining parts of the reducing gas and carburizing gas were N ２ The temperature of the carburizing gas blown in was room temperature (25°C) in all cases. The same applies to Example 2 described later.

[0065] Next, the accuracy of controlling the final carbon content of reduced iron was evaluated using the final carbon content of reduced iron calculated for each condition according to the following criteria. The evaluation results are also shown in Table 1. Pass (excellent): The final carbon content of reduced iron was within the range of 1.90 to 2.10 mass% (target final carbon content of reduced iron ±0.10 mass%) under all conditions. Fail (poor): The final carbon content of reduced iron was outside the range of 1.90 to 2.10 mass% under at least one condition.

[0066]

[0067] As shown in Table 1, Example 1 exhibits excellent accuracy in controlling the final carbon content of reduced iron, while Comparative Example 1 exhibits insufficient accuracy in controlling the final carbon content of reduced iron.

[0068] Example 2 Furnace volume as shown in Figure 1: 100 m ３ In the reducing furnace (having a different furnace volume from that of Example 1) and its auxiliary equipment, reduced iron is produced under the conditions shown in Table 2. That is, in Example 2, the composition of the carburizing gas is changed depending on the composition of the reducing gas under each condition, and reduced iron is produced. In particular, the H ２ / (H ２ +CO), the CH of the carburizing gas is adjusted to satisfy the relationship of the above formula (7). ４ In Example 2, under Condition 7, the amount of carburizing gas injected is also changed so as to satisfy the relationship of Equation (8). In Comparative Example 2, reduced iron is produced under all conditions with the carburizing gas composition and injection amount remaining almost unchanged.

[0069] Under all conditions, the operation period was 7 days, the injection temperature of the reducing gas was 1025°C, the target final carbon content of the reduced iron was 4.70 mass%, and X ０ is 75.1% by volume, X １ is 100% by volume, V ０ is 424 Nm ３ / t.

[0070] Next, the accuracy of controlling the final carbon content of reduced iron was evaluated using the final carbon content of reduced iron calculated for each condition according to the following criteria. The evaluation results are also shown in Table 2. Pass (excellent): The final carbon content of reduced iron was within the range of 4.60 to 4.80 mass% (target final carbon content of reduced iron ±0.10 mass%) under all conditions. Fail (poor): The final carbon content of reduced iron was outside the range of 4.60 to 4.80 mass% under at least one condition.

[0071]

[0072] As shown in Table 2, Example 2 is excellent in the accuracy of controlling the final carbon content of reduced iron, while Comparative Example 2 is insufficient in the accuracy of controlling the final carbon content of reduced iron.

[0073] In addition, when various reducing furnaces are used to produce reduced iron under various operating conditions and with various target final carbon contents, the composition of the carburizing gas is changed depending on the composition of the reducing gas. In particular, the H ２ / (H ２+CO), the CH of the carburizing gas is adjusted to satisfy the above formula (7), or the above formula (8) in addition to the above formula (7). ４ By varying the concentration and flow rate, similar results can be obtained.

[0074] REFERENCE SIGNS LIST 1 reduction furnace 1a iron oxide 1b reduced iron 3 dust removal device 4 dehydration device 5 natural gas supply section 6 air supply section 7 reforming device 8 reducing gas injection device 9 carburizing gas injection device 10 reduction section 11 carburizing section 12 control section

Claims

1. A method for producing reduced iron, comprising: a reduction step in which iron oxide is reduced to form reduced iron by using a reducing gas blown into a reduction furnace; a carburization step in which the carbon content of the reduced iron is increased by using a carburizing gas blown into the reduction furnace; and a control step in which the composition of the carburizing gas is changed in accordance with the composition of the reducing gas.

2. In the control step, H of the reducing gas ２ H concentration and CO concentration ２ The ratio of concentrations is H ２ / (H ２ the composition of the carburizing gas is changed so that the amount of carburization of the reduced iron in the carburizing step increases as the deviation of the H content of the reducing gas (H + CO) from 0.5 increases, ２ / (H ２ 2. The method for producing reduced iron according to claim 1, wherein the composition of the carburizing gas is changed so that the amount of carburization of the reduced iron in the carburizing step decreases as the deviation of the ratio (N / (CO) + CO) from 0.5 decreases.

3. In the control step, the H of the reducing gas ２ H concentration and CO concentration ２ The ratio of concentrations is H ２ / (H ２ +CO), the CH of the carburizing gas ４ The method for producing reduced iron according to claim 1 or 2, wherein the concentration is changed.

4. The method for producing reduced iron according to any one of claims 1 to 3, wherein the following relationship is satisfied in the control step: X ０ +95×|R-0.5|-3≦X≦X ０ +115×|R−0.5|+3 Where, X: CH of carburizing gas ４ Concentration (volume%), X ０ : Standard CH for carburizing gas ４ concentration (volume %) and R: H of reducing gas ２ / (H ２ +CO).

5. A distribution step in which the furnace top gas discharged from the reduction furnace is distributed into a first furnace top gas and a second furnace top gas, and the furnace top gas is a gas obtained after the reducing gas has been used to reduce the iron oxide in the reduction step; and ４ The method for producing reduced iron according to any one of claims 1 to 4, further comprising: a reforming step of generating the reducing gas from a containing gas.

6. A reducing furnace comprising: a reducing section that reduces iron oxide with a reducing gas to produce reduced iron; a carburizing section that increases the amount of carbon contained in the reduced iron with a carburizing gas; and a control section that changes the composition of the carburizing gas in accordance with the composition of the reducing gas.

7. The control unit adjusts the H of the reducing gas. ２ H concentration and CO concentration ２ The ratio of concentrations is H ２ / (H ２ the composition of the carburizing gas is changed so that the amount of carburization of the reduced iron in the carburized portion increases as the amount of deviation of the H content of the reducing gas (H + CO) from 0.5 increases, ２ / (H ２ 7. The reducing furnace according to claim 6, wherein the composition of the carburizing gas is changed so that the amount of carburization of the reduced iron in the carburized portion decreases as the deviation of the ratio (R) of the carbon monoxide (CO) from 0.5 decreases.

8. The control unit controls the H ２ H concentration and CO concentration ２ The ratio of concentrations is H ２ / (H ２ +CO), the CH of the carburizing gas ４ The reduction furnace according to claim 6 or 7, wherein the concentration is changed.

9. The reducing furnace according to any one of claims 6 to 8, wherein the following relationship is satisfied by the control of the control unit: X ０ +95×|R-0.5|-3≦X≦X ０ +115×|R−0.5|+3 Where, X: CH of carburizing gas ４ Concentration (volume%), X ０ : Standard CH for carburizing gas ４ concentration (volume %) and R: H of reducing gas ２ / (H ２ +CO).

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