Method for dissolving reduced iron

Abstract

This invention provides a method for suppressing the occurrence of icebergs in electric furnace operations using reduced iron as a raw material, even when the input rate Qs of reduced iron is increased. [Solution] A method for dissolving reduced iron using an electric furnace equipped with at least one raw material input pipe provided on the side or top surface of the furnace body, wherein reduced iron is introduced into molten metal through the raw material input pipe, and the method for dissolving reduced iron is such that, for the input pipe into which reduced iron is introduced at an input rate Qs of 33 [kg / min / MW] or more, an upward flow of molten iron is formed at the point where the introduced reduced iron is deposited. The upward flow of gas flotation by a gas blowing device is such that an upward flow of molten iron is formed at the point where the reduced iron is deposited.

Description

[Background technology]

[0001] This invention relates to a method for dissolving reduced iron using an electric furnace. [Technical Field]

[0002] In recent years, the technology of molten steel production using electric furnaces has attracted attention. While waste scrap and reduced iron can be used as raw materials for electric furnaces, using scrap presents challenges such as the formation of trump elements, making it necessary to use reduced iron as a raw material for the production of high-grade steel.

[0003] Reduced iron generally has a lower apparent density than molten steel, so it tends to float at the molten steel / slag interface and does not easily sink into the molten steel. It is known that when a large amount of such reduced iron is introduced into an electric furnace, a huge mass of suspended matter called an iceberg is formed. Icebergs are formed when a large amount of reduced iron condenses, and since their dissolution requires a lot of time and energy, it is necessary to avoid their occurrence from an operational standpoint. Empirically, it is known that the formation of icebergs cannot be avoided unless the rate of reduced iron input is limited to 27-33 [kg / min / MW] or less per MW of power input (e.g., Non-Patent Documents 1 and 2).

[0004] On the other hand, when reducing iron with a specific heat of 0.700 [kJ / kg / K] and latent heat of 268 [kJ / kg] is heated from room temperature of 25°C to its melting point of 1500°C and melted, sensible heat of 1050 [kJ / kg] and latent heat of 268 [kJ / kg] are required, and the theoretical maximum input rate is 43 [kg / MW / min] per MW of input power. In contrast, in conventional electric furnace operation, as mentioned above, it is necessary to limit the input rate of reduced iron to 27-33 [kg / min / MW] or less per MW of input power, meaning that productivity is sacrificed in order to suppress iceberg formation, and iceberg formation has been a challenge when aiming for high-productivity operation.

[0005] Patent Document 1 discloses a method for dissolving reduced iron introduced into an arc-type electric furnace by utilizing the stirring of molten steel by bottom-blowing gas. This method involves having an electric furnace body including a circular portion, and arranging three or more bottom-blowing nozzles at the vertices of a regular polygon with the center of gravity of the circular portion at the bottom of the furnace body, with a raw material input means for introducing reduced iron into a predetermined area of ​​the polygon. However, this method does not suppress the formation of icebergs immediately after the introduction of reduced iron. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2024-115816 [Non-patent literature]

[0007] [Non-Patent Document 1] Kaneko Dentaro et al.: Iron and Steel, 73 (1987), 2116. [Non-Patent Document 2] A. Chatterjee: Beyond the Blast Furnace, CRC Press, Boca Raton, (2017), 213. [Non-Patent Document 3] K. Kihara and N. Okada: Chem. Eng. Sci., 270 (2023), 118507. [Non-Patent Document 4] J. Ni and C. Beckermann: Metall. Mater. Trans. B, 22 (1991), 349. [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] Reduced iron, introduced through the input pipe, falls through the furnace and reaches the molten metal surface. This process of reduced iron reaching the molten metal surface is called "soaking." The location where the reduced iron soaks is called the soaking location. Icebergs occur when the molten iron surrounding the soaked reduced iron solidifies, and the solidified material becomes a binding phase, causing adjacent reduced iron particles to bond together. The higher the rate at which reduced iron is introduced, the more likely the molten iron temperature at the soaking location is to decrease, making it easier for the molten iron surrounding the reduced iron to solidify, and also making it easier for the reduced iron particles to cluster together, thus increasing the likelihood of iceberg formation. Therefore, to suppress the formation of icebergs, it is necessary to mitigate the solidification of the molten iron surrounding the reduced iron at the soaking location or the clustering of reduced iron particles.

[0009] The present invention aims to solve the above problems and provides a method for suppressing the occurrence of icebergs even when the rate of inputting reduced iron is increased in electric furnace operation using reduced iron as a raw material. [Means for solving the problem]

[0010] In other words, the gist of this invention is as follows: [1] A method for dissolving reduced iron using an electric furnace equipped with at least one raw material input pipe provided on the side or top surface of the furnace body, wherein reduced iron is introduced into molten metal through the raw material input pipe, and the method for dissolving reduced iron is such that, with respect to the input pipe into which reduced iron is introduced at an input rate Qs of 33 [kg / min / MW] or more, an upward flow of molten iron is formed at the point where the introduced reduced iron is introduced. [2] The method for dissolving reduced iron according to [1], wherein an electric furnace having at least one gas injection device provided on the underside of the furnace bottom is used, and the flow of gas flotated by the gas injection device causes an upward flow of molten iron to form at the location where the reduced iron is deposited. [3] The method for dissolving reduced iron according to [2], wherein the gas blowing device is operated such that the distance d [mm] between a vertical line drawn from at least one nozzle of the gas blowing device and the center of the reduced iron hot water placement position satisfies the following relationship with the amount of gas blown from the nozzle Qg [NL / min] and the rate at which reduced iron is introduced into the reduced iron hot water placement position Qs [kg / min / MW]. Qs ≤ 33 + A×d + B×Qg C Here, A = -0.0063, B = 0.33, and C = 0.44.

Advantages of the Invention

[0011] According to the present invention, since the reduced iron charged into the electric furnace can be immediately dispersed after being splashed with water, it is difficult for an iceberg formed by the aggregation and condensation of the reduced iron after being splashed with water to occur, and the charging rate Qs of the reduced iron can be increased compared with the prior art. The limit value Qs,limit of the charging rate is obtained from the following formula based on the amount of blown gas Qg and the offset distance d of the water splash position. Qs,limit = 33 + A×d + B×Qg C Here, A = -0.0063, B = 0.33, and C = 0.44.

Brief Description of the Drawings

[0012] [Figure 1-1] It is a diagram showing the distribution of the space filling rate fs of the solid phase and the distribution of the solidification rate Rv at the molten iron interface, where (A-1)(B-1) represent the space filling rate fs and (A-2)(B-2) represent the solidification rate Rv. [Figure 1-2] It is a diagram showing the distribution of the space filling rate fs of the solid phase and the distribution of the solidification rate Rv at the molten iron interface, where (C-1)(D-1) represent the space filling rate fs and (C-2)(D-2) represent the solidification rate Rv. [Figure 2] It is a diagram showing the distribution of the condensation index at the molten iron interface when charging reduced iron into the electric furnace, showing the cases where the charging rate is the rate described in each of the diagrams (A)(B)(C)(D). [Figure 3] It is a diagram showing the relationship between the charging rate of reduced iron and the in-furnace integral value of the condensation index. [Figure 4] It is a diagram showing the relationship between the charging rate of reduced iron and the in-furnace integral value of the condensation index. [Figure 5] It is a diagram showing the relationship between the amount of blown gas and the limit charging rate. [Figure 6] It is a diagram showing the relationship between the charging rate of reduced iron and the in-furnace integral value of the condensation index. [Figure 7] It is a diagram showing the relationship between the input rate of reduced iron and the in-furnace integrated value of the condensation index. [Figure 8] It is a diagram showing the relationship between the input rate of reduced iron and the in-furnace integrated value of the condensation index. [Figure 9] It is a diagram showing the relationship between the offset distance and the critical input rate.

Embodiments for Carrying out the Invention

[0013] The inventor carried out a numerical simulation capable of reproducing the behavior of injecting and melting a large number of solid particles from above the molten iron pool surface in an electric furnace vessel. The numerical simulation was carried out by combining a solid-gas-liquid three-phase flow numerical analysis model (Non-Patent Document 3) capable of simulating the behavior of a large number of solid particles developed by the inventor and a phase change model (Non-Patent Document 4) generally applied to a multi-fluid model.

[0014] In addition, the simulation model used in this numerical simulation can also simulate the behavior of floating on the molten iron / slag interface (hereinafter also simply referred to as the "molten iron interface") as described above, and can predict how the group of solid particles that have fallen into the molten iron float and move on the molten iron interface, and where the solid particles and the surrounding molten iron solidify and dissolve.

[0015] The inventors conducted a numerical simulation in which reduced iron was introduced into an electric furnace at a constant rate while applying arc heat to the electrodes, with a certain amount of residual molten iron stored in the furnace as the initial condition. The electric furnace used in the numerical simulation had a diameter of approximately 7 [m], the arc was a three-phase alternating current, and the three electrodes were positioned at a radius of approximately 1 [m] from the center of the furnace at 120° intervals. The reduced iron was HBI (hot briquette iron), which has an apparent density lower than that of molten iron and higher than that of slag. The initial amount of molten iron was 187 [tons], the slag thickness was 500 [mm], and the arc heat input was 200 [MW]. The initial temperatures of the molten iron and slag were 1570°C, the temperature of the reduced iron at the time of introduction was 30°C, the carbon concentration of the initial molten iron was 0.5 mass%, and the carbon concentration of the reduced iron at the time of introduction was 1.5 mass%. The apparent density of the reduced iron was 5.5 [kg / m³]. 3 ], bulk density is 2.65 [kg / m³ 3 The spherical equivalent diameter was set to 68 mm.

[0016] To evaluate the solidification of reduced iron, the inventors focused on the solid phase space packing ratio fs[-], which is the total volume of unmelted reduced iron and solidified molten iron per unit volume, as a quantity representing the degree of solid phase density in the region containing reduced iron. They also focused on the solidification rate Rv[1 / s] of the molten iron around the reduced iron as a quantity representing the degree of solidification of the molten iron around the reduced iron. The space packing ratio fs and the solidification rate Rv are obtained within the above simulation model. From the above definition, in the region where Rv has a positive value when the solid phase is completely stationary and the reduced iron has not dissolved, fs increases. However, this is not the case in the region where the solid phase is moving; that is, there may be regions where fs does not appear to change even if Rv has a positive value. The behavior of the solid phase space packing ratio fs and the solidification rate Rv was then investigated in detail.

[0017] In Figures 1-1 and 1-2 (A) to (D), the input rate Qs is the rate described in Figures (A), (B), (C), and (D). Figures (A-1) to (D-1) show the distribution of the solid phase space filling ratio fs[-] at the molten iron interface, and Figures (A-2) to (D-2) show the distribution of the solidification rate Rv[1 / s]. The outline of each figure indicates the location of the molten iron interface in the electric furnace, and the "×" position labeled "Isolation Position" in (A-2) indicates the center of the position where reduced iron is molten. In all cases of Figures 1-1 and 1-2 (A) to (D), reduced iron is molten around the "×" position shown in Figure 1-1 (A-2).

[0018] Since reduced iron floats at the molten iron interface until it completely dissolves after being added to the molten metal, a certain amount of reduced iron is concentrated near the point of addition. At the point of addition of reduced iron, the surrounding molten iron solidifies immediately after the addition of reduced iron, but then quickly begins to melt in its vicinity. However, as shown in (D-1) and (D-2), when the input rate Qs increases to 36 [kg / min / MW], as shown in (D-2), the area of ​​the region where the solidification rate Rv has a positive value increases, indicating that solidification is occurring in a wide area other than the point of addition of reduced iron. As a result, the solid phase space packing ratio fs is also high over a wide area, as shown in (D-1).

[0019] The product of the solid phase space packing ratio fs[-] and the solidification rate Rv[1 / s] at the molten iron interface will be referred to as the "coagulation index." Figure 2 shows the distribution of the coagulation index. In Figures 2(A) to (C), the input rate Qs is 26 to 33 [kg / min / MW], and the region where the coagulation index is positive is small. In contrast, in Figure 2(D), the input rate Qs is 36 [kg / min / MW], and the region where the coagulation index is positive is widely distributed, meaning that reduced iron is densely concentrated and the molten iron is solidifying over a wide area. When this state occurs, many of the densely concentrated reduced iron particles are linked together by the solidification of the molten iron, forming an iceberg. According to the results of this numerical simulation, this state transitions between input rates Qs[kg / min / MW] of 33 to 36 [kg / min / MW], supporting the empirically known input rate limit of 27 to 33 [kg / min / MW].

[0020] Figure 3 shows the in-furnace integrated value of the condensation index on the vertical axis and the input rate Qs on the horizontal axis. The condensation index increases sharply from an input rate Qs of approximately 33 [kg / min / MW], indicating a transition to a state where aggregation and solidification of reduced iron are likely to occur.

[0021] Next, the inventors used numerical simulations to investigate how the transition to an iceberg-forming state changes depending on the operating conditions.

[0022] Then, we conceived the idea that an effective method would be to inject an inert gas near the bottom of the furnace directly below the point where the injected reduced iron ignites the slag or molten iron. By causing the injected gas to rise, a strong flow of molten iron and slag is generated near the ignition point, thereby immediately dispersing the reduced iron after it ignites. It is presumed that if the reduced iron is immediately dispersed after ignition, it will be possible to prevent it from clumping together and solidifying as the molten iron solidifies.

[0023] First, the inventors considered an operation in which inert gas is injected from several locations at the bottom of the furnace, similar to a general electric furnace, and investigated a method for depositing reduced iron at a position approximately directly above one of the gas injection ports (offset by 100 mm from directly above) (offset distance d = 100 mm, as described later). The cross-sectional area of ​​the reduced iron injection pipe is approximately 0.167 m². 2 (The diameter in circular terms is approximately 460 mm), and the area where the reduced iron introduced from the inlet pipe condenses is almost the same as the cross-sectional area of ​​the inlet pipe. In numerical simulations, the amount of gas injected from each gas inlet Qg [NL / min] was varied from 100 to 900 [NL / min], and the rate of reduced iron injection Qs [kg / min / MW] was varied from 23 to 43 [kg / min / MW], and the condensation index was investigated to see how this changed.

[0024] Figure 4 shows the relationship between the in-furnace integral of the condensation index and the injection rate Qs for each of five different injection gas rates, with the vertical axis representing the in-furnace integral of the condensation index and the horizontal axis representing the injection rate Qs. When compared at the same injection rate Qs, the condensation index decreases as the injection gas rate Qg increases. Furthermore, the injection rate Qs at which the condensation index begins to increase sharply also increases as the injection gas rate Qg increases. In other words, by injecting a constant amount of gas from directly below the reduced iron molten metal contact point, it is possible to achieve an operating condition in which reduced iron condensation and iceberg formation are less likely to occur, even at a high injection rate Qs. Figure 4 also shows the calculation results for the case without gas injection. In the case without gas injection, the condensation index is approximately 6.15 × 10⁻¹⁵ at a critical injection rate Qs of 33 [kg / min / MW]. -4 Although it can increase to this extent, if gas is blown in, the condensation index can be kept at a similar level even if the input rate Qs is increased to 33 [kg / min / MW] or even 35-40 [kg / min / MW]. For each amount of blown gas Qg, the condensation index is 6.15 × 10⁻⁶. -4 The injection speed Qs at which this is reached was calculated as the limit injection speed Qs,limit. Figure 5 shows the relationship between the limit injection speed Qs,limit and the amount of injected gas Qg (with an offset distance d = 100 mm, as described later). The larger the amount of injected gas Qg, the larger the limit injection speed Qs,limit.

[0025] As a result, we discovered the following method as an operational method that is less likely to cause icebergs.

[0026] Specifically, this is a method for dissolving reduced iron in an electric furnace equipped with at least one reduced iron input pipe provided on the side or top surface of the furnace body, wherein, for an input pipe among the raw material input pipes that is fed at an input rate Qs of 33 [kg / min / MW] or more, an upward flow of molten iron is formed at the point where the reduced iron is deposited. The point where the reduced iron is deposited can be recognized by observing the liquid level in the electric furnace while the reduced iron is being fed, and identifying the region where the added reduced iron reaches the liquid level.

[0027] Here, as mentioned above, it is preferable to use the floating gas flow from a gas injection device to form an upward flow of molten iron at the location where the reduced iron is deposited.

[0028] Furthermore, the means for forming an upward flow of molten iron at the point where the reduced iron is introduced does not necessarily have to be by gas blowing near the point of entry. For example, an upward flow with a flow velocity of 0.40 to 0.55 [m / s] or higher may be formed by applying an electromagnetic force to the molten iron non-contactually using an electromagnetic force control device. In this case, the presence or absence of an upward flow directly below the point of entry and the flow velocity can be predicted using, for example, a general thermal fluid simulation.

[0029] Next, when using the upward floating flow of gas by a gas blowing device as a means to form an upward flow of molten iron, the inventors examined the case where the center of the splashing position of the reduced iron was offset from directly above the gas blowing port. The offset distance d was changed in the range of d = 200 to 1000 [mm], and the resulting change in the condensation index was investigated. Figures 6, 7, and 8 all show the results when the horizontal axis represents the input rate Qs and the vertical axis represents the in-furnace integrated value of the condensation index, for the cases of blown gas amounts Qg = 300, 600, and 900 [NL / min], respectively. In each of Figures 6, 7, and 8, the offset distance d takes on 3 to 5 values. Here, the center of the splashing position of the reduced iron can be recognized as the center position of the region where the input reduced iron reaches the liquid surface in the electric furnace by observing the liquid surface in the electric furnace during the input of the reduced iron. Also, for example, the center of the splashing position of the reduced iron may be predicted using simulations such as those that approximate a group of solid particles as a fluid or simulations that individually track a group of solid particles, like the present simulation.

[0030] Furthermore, in the same manner as described above, when the in-furnace integrated value of the condensation index reaches 6.15×10 -4 the input rate Qs at that time was defined as the limiting input rate Qs,limit, and the limiting input rate Qs,limit was calculated. Figure 9 shows the relationship between the limiting input rate Qs,limit and the offset distance d. The smaller the offset distance d, the larger the limiting input rate Qs,limit.

[0031] Based on the data in Figures 5 and 9, the regression equation for the limiting input rate Qs,limit with respect to the blown gas amount Qg and the offset distance d was obtained as follows. Qs,limit = 33 + A×d + B×Qg C

[0032] Using the least squares method, the coefficients A, B, and C were found to be A = -0.0063, B = 0.33, and C = 0.44. The dashed line in Figure 5 (offset distance d = 100 mm) shows the results of this regression equation. The three dashed lines in Figure 9 show the results of the regression equation for Qg = 300, 600, and 900, respectively, from bottom to top. Therefore, by operating with the input speed Qs, injected gas amount Qg, and offset distance d satisfying the following equation, it is possible to suppress the occurrence of icebergs even when the input speed Qs is increased compared to conventional methods. Qs ≤ 33 + A × d + B × Qg C Here, A = -0.0063, B = 0.33, and C = 0.44.

[0033] Furthermore, as the amount of injected gas Qg increases, the gas injected from the bottom of the furnace is more likely to escape into the atmosphere, so forced convection due to the injected gas cannot be increased indefinitely. According to the present invention, within the range of an injected gas amount of 900 [NL / min] or less, an increase in the amount of injected gas Qg contributes to the enhancement of convection in molten iron.

[0034] Furthermore, more preferably, in order to increase the injection speed Qs to 1.05 times or more than the conventional rate, it is best to set the injected gas volume Qg to approximately 40 to 110 NL / min or more when the offset distance d = 0 to 150 mm, and numerical simulation results showed that the upward flow velocity under these conditions is approximately 0.40 to 0.55 [m / s].

Claims

1. A method for dissolving reduced iron using an electric furnace equipped with at least one raw material input pipe provided on the side or top surface of the furnace body, wherein reduced iron is introduced into molten metal through the raw material input pipe, the method for dissolving reduced iron wherein, with respect to the input pipe into which reduced iron is introduced at an input rate Qs of 33 [kg / min / MW] or more, an upward flow of molten iron is formed at the point where the introduced reduced iron is introduced.

2. The method for dissolving reduced iron according to claim 1, wherein an electric furnace having at least one gas injection device provided on the underside of the furnace bottom is used, and the flow of gas flotated by the gas injection device causes an upward flow of molten iron to form at the location where the reduced iron is deposited.

3. The method for dissolving reduced iron according to claim 2, wherein the gas blowing device is operated such that the distance d [mm] between a vertical line drawn from at least one blowing port and the center of the reduced iron hot water placement position satisfies the following relationship with the amount of gas blown from the blowing port Qg [NL / min] and the rate at which reduced iron is introduced to the reduced iron hot water placement position Qs [kg / min / MW]. Qs≦33+A×d+B×Qg Ｃ Here, A = -0.0063, B = 0.33, and C = 0.44.

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