Apparatus for producing molten iron and method for producing molten iron

Abstract

The present invention provides an apparatus for producing molten iron and a method for producing molten iron that make it possible to efficiently utilize energy to preheat a cold iron source and produce molten iron.　An apparatus 1 for producing molten iron comprises a preheating chamber 7 into which a cold iron source is charged, a gate 11 which is provided to the preheating chamber 7, and an arc type electric furnace 8 into which a cold iron source 2 in the preheating chamber 7 is charged by opening the gate 11 and which melts the charged cold iron source 2 with arc heat to produce molten iron. Said apparatus comprises a coil 9 which is wound on an outer peripheral surface of the preheating chamber 7 and a power source 10 which supplies electric current at a frequency that has been set in advance for the coil 9.

Description

Molten iron manufacturing apparatus and method for manufacturing molten iron

[0001] The present invention relates to an apparatus for producing molten iron and a method for producing molten iron.

[0002] The global drive for carbon neutrality is driving a shift in the steel industry from the blast furnace method to the electric furnace method for producing molten iron. The blast furnace method primarily uses lump ore, sintered ore, and coke as raw materials. Coke serves as fuel for heating lump iron ore (hereinafter referred to as lump ore) and sintered ore, while also reducing the iron oxide contained in the lump ore and sintered ore to produce low-melting-point molten iron, i.e., molten iron with a carbon concentration of approximately 4% to 5% by mass. Therefore, large amounts of coke are charged into blast furnaces. The carbon in the molten iron produced in the blast furnace is removed in the converter process and ultimately converted into carbon dioxide. As a result, crude steel production using the blast furnace and converter methods generates approximately 2 tons of carbon dioxide per ton of iron. Hereinafter, "mass %" will be simply referred to as "%."

[0003] In contrast, the electric furnace process primarily uses scrap as a raw material, which is heated and melted using electrical energy to produce crude steel. Since the electric furnace process uses already reduced iron as a raw material, no reduction energy is required, and the energy required to heat the raw material is provided by electricity. Therefore, the amount of carbon dioxide generated is approximately 0.5 tonnes per tonne of iron, which is less than that generated by the blast furnace and converter processes.

[0004] Meanwhile, the amount of scrap shipped domestically is approximately 30 million tons per year. Of that, approximately 23 million tons is used domestically, and the remaining approximately 7 million tons is exported overseas. Even if the approximately 7 million tons of exported scrap were to be distributed domestically, the amount of high-quality scrap suitable for high-grade steel production would be limited, and there is a possibility of competition for scrap between the company and other companies, including electric furnace manufacturers. Therefore, even if a company were to simply switch from the blast furnace method to the electric furnace method for the purpose of promoting carbon neutrality, it may be difficult to secure a large amount of new, stable, high-quality scrap for use in the electric furnace method domestically.

[0005] Therefore, it is conceivable to import reduced iron produced overseas and use it together with scrap as a raw material in the electric furnace process. Reduced iron is metallic iron obtained by reducing iron ore. Regarding the method of producing reduced iron, the MIDREX process uses natural gas as the raw material and a gas containing a large amount of hydrogen gas as the reducing agent. Therefore, the amount of carbon dioxide generated can be reduced by the amount of reduction by hydrogen gas. In addition, a method of producing reduced iron that directly uses hydrogen gas as the reducing agent is currently under development, and if realized, it will enable completely zero-carbon steelmaking.

[0006] However, when reduced iron is melted using an electric arc furnace, which is an example of an electric furnace process, the proportion of reduced iron in the raw materials increases, resulting in a decrease in the power consumption and productivity compared to when all the raw materials are scrap. This is due to the additional energy required to dissolve gangue from the iron ore contained in the reduced iron and to dissolve lime added to neutralize the gangue. Gangue refers to impurities other than iron contained in the reduced iron, and is present in an amount of 2% to 10%. Furthermore, reduced iron is more difficult to melt than scrap. Therefore, when the reduced iron is heated in an electric arc furnace, the reduced iron aggregates and forms large clumps. This further reduces the melting of the reduced iron in the electric arc furnace, resulting in an extended steelmaking time.

[0007] Various methods and devices for preheating scrap and reduced iron have been developed to reduce the power consumption rate and improve productivity in arc-type electric furnaces. For example, a conventionally known scrap preheating method is the SPH (Scrap Preheating) method. Furthermore, CONSTEEL has been developed as a horizontal preheating furnace, and ECOARC (registered trademark), Quantum, SHARC, and other vertical preheating furnaces have been developed to reduce the power consumption rate and improve productivity. The power consumption rate refers to the amount of power used to produce 1 ton of iron.

[0008] An example of the above-mentioned preheating device is disclosed in Patent Document 1. The device includes a conveying device that continuously transports metal material to an arc electric furnace, and an induction heating unit that induction heats the metal material being transported by the conveying device. The conveying device includes a cylindrical housing and a belt conveyor or plate conveyor disposed inside the housing. Multiple induction heating units are provided outside the housing. The device disclosed in Patent Document 1 is configured to induction heat the metal material using the induction heating unit while continuously transporting the metal material inside the housing toward the arc electric furnace.

[0009] Patent No. 6316972

[0010] In recent years, in the operation of electric arc furnaces, there has been a strong demand for improved energy utilization efficiency and power saving in view of global environmental issues. In response to such demands, by using the above-mentioned conventionally known preheating method or the device disclosed in Patent Document 1 in the operation of an electric arc furnace, it is possible to obtain a certain degree of effect in terms of energy utilization efficiency.

[0011] However, the inventors' investigations revealed that the energy utilization efficiency is insufficient, i.e., there is waste, and a new preheating method is needed. Specifically, with the SPH and CONSTEEL preheating furnaces, the preheating temperature is approximately 200°C or higher and 300°C or lower, leaving room for improvement in terms of reducing the power consumption and improving productivity. Furthermore, vertical preheating furnaces such as the ECOARC (registered trademark) cannot hold the reduced iron within the preheating furnace. Therefore, the reduced iron cannot be sufficiently preheated within the preheating furnace, and there is a possibility that the reduced iron may be charged into the electric arc furnace without being sufficiently preheated. As a result, the power consumption cannot be sufficiently reduced, and there is a possibility that the productivity of molten iron may deteriorate or that electrode breakage may occur.

[0012] Furthermore, it was found that the equipment disclosed in Patent Document 1, in which a preheating device alone is not sufficient to achieve high energy utilization efficiency, is insufficient. That is, the reduced iron filling ratio in the preheating device, preheating temperature, frequency, reduced iron charging rate, amount of molten iron in the furnace, and temperature of molten iron in the furnace are also important factors for achieving high energy utilization efficiency. It was found that optimization of operating conditions, including these factors, is necessary. Specifically, in the equipment disclosed in Patent Document 1, space is inevitably required between the conveyor and the metal material to move them relative to the housing. This space may prevent a high metal material filling ratio relative to the housing's internal volume. In other words, the metal material filling the housing is induction heated while the filling ratio is low and there is ample space. Therefore, there is still room for improvement in terms of energy utilization efficiency. The same problem exists even if the conveying device disclosed in Patent Document 1 uses a horizontal conveyor or a selectively rotatable drum. The charging rate of reduced iron is calculated by dividing the charging amount t of reduced iron by the charging interval of reduced iron.

[0013] Furthermore, in the device disclosed in Patent Document 1, the inside of the housing of the conveying device cannot be sealed, so the metal material comes into contact with air during transportation, which can cause the metal material to oxidize or cool. Furthermore, the conveying device is induction heated together with the metal material, causing a temperature rise, which increases the risk of problems occurring due to the temperature rise and can also increase the frequency of maintenance.

[0014] Furthermore, since reduced iron is less easily heated by induction heating than conventional cold iron sources such as scrap, the frequency design of the alternating current supplied to the induction heating unit is important. Therefore, the device disclosed in Patent Document 1 may not be able to sufficiently preheat the reduced iron.

[0015] The present invention has been made to solve the above-mentioned problems, and has an object to provide an apparatus for producing molten iron and a method for producing molten iron that can preheat a cold iron source using energy efficiently to produce molten iron.

[0016] Means for solving the above problems are as follows: [1] An apparatus for manufacturing molten iron, comprising: a preheating chamber into which a cold iron source is charged, a gate provided in the preheating chamber, and an electric arc furnace that charges the cold iron source in the preheating chamber by opening the gate and melts the charged cold iron source with arc heat to produce molten iron, the apparatus having a coil wound around an outer periphery of the preheating chamber and a power source that supplies a current of a predetermined frequency to the coil. [2] The apparatus for manufacturing molten iron according to [1], further comprising an actuator that opens and closes the gate, and a control device that controls the actuator to control the opening degree of the gate so that the charging rate of the cold iron source preheated in the preheating chamber satisfies the following equations (1) to (3): v Ｉ : Charging speed of cold iron source (t / min) P: Amount of electric power input to arc type electric furnace (MW) T ｐｈ : Target preheat temperature of cold iron source (℃) T ｍ W: Temperature of molten iron in the melting furnace of an electric arc furnace (℃) Ｉ W: Amount of cold iron source charged from the preheating chamber to the melting furnace of the electric arc furnace per time (t) ｍ : amount of molten iron in the melting furnace (t) C: charging interval of cold iron source (min) [3] The apparatus for producing molten iron according to [2], wherein the control device calculates a charging rate of the cold iron source from the preheating chamber to the electric arc furnace based on either the temperature of the cold iron source in the preheating chamber or the temperature of the molten iron in the electric arc furnace. [4] The apparatus for producing molten iron according to [2] or [3], wherein the cold iron source is scrap and reduced iron, and the control device controls the amount of power input from the power source to the electric arc furnace based on a mixing ratio of the scrap and the reduced iron. [5] The apparatus for producing molten iron according to any of [2] to [4], wherein the control device controls the power source so that the frequency of the current input to the coil satisfies the following equations (4) and (5): f: frequency (Hz) σ: electrical conductivity of the cold iron source at the target preheating temperature (1 / Ωm) μ ｒ: Relative permeability μ of reduced iron at the target preheating temperature ０ : Magnetic permeability of vacuum (4π×10 －７ ) t: thickness of the cold iron source (m) w: width of the cold iron source (m) l: length of the cold iron source (m) [6] The apparatus for producing molten iron according to any one of [1] to [5], wherein the winding height and winding diameter of the coil wound around the outer peripheral surface of the preheating chamber satisfy the following formula (6): 0.5≦H_Coil / D_Coil≦2.5 ... (6) The winding height H_Coil is the distance between the lowest coil in the height direction of the apparatus for producing molten iron and the highest coil in the height direction. The winding diameter D_Coil is the diameter of the coil wound around the outer peripheral surface of the preheating chamber 7. [7] A method for producing molten iron using an apparatus for producing molten iron, the apparatus including a preheating chamber into which a cold iron source is charged, a gate provided in the preheating chamber, and an electric arc furnace that charges the cold iron source into the preheating chamber by opening the gate and melts the charged cold iron source by arc heat to produce molten iron, wherein the cold iron source filled in the preheating chamber is induction heated to a target preheat temperature by supplying a current of a predetermined frequency from a power source to a coil wound around the outer periphery of the preheating chamber. [8] A method for producing molten iron according to [7], wherein the cold iron source includes scrap and reduced iron. [9] A method for producing molten iron, the method including: ３ 3.5t / m or more ３

[10] The method for producing molten iron according to any one of [7] to [9], wherein the cold iron source is charged from the preheating chamber to the electric arc furnace so that a charging rate of the cold iron source from the preheating chamber to the electric arc furnace satisfies the following formulas (1) to (3): v Ｉ : Charging speed of cold iron source (t / min) P: Amount of electric power input to arc type electric furnace (MW) T ｐｈ : Target preheat temperature of cold iron source (℃) T ｍ W: Temperature of molten iron in the melting furnace of an electric arc furnace (℃) ＩW: Amount of cold iron source charged from the preheating chamber to the melting furnace of the electric arc furnace per time (t) ｍ : Amount of molten iron in the melting furnace (t) C: Charging interval of cold iron source (min)

[11] The method for producing molten iron according to any one of [7] to

[10] , wherein the frequency satisfies the following formulas (4) and (5): f: frequency (Hz) σ: electrical conductivity of the cold iron source at the target preheating temperature (1 / Ωm) μ ｒ : Relative permeability μ of reduced iron at the target preheating temperature ０ : Magnetic permeability of vacuum (4π×10 －７ ) t: Thickness of the cold iron source (m) w: Width of the cold iron source (m) l: Length of the cold iron source (m)

[0017] According to the present invention, it is possible to produce molten iron by preheating a cold iron source using energy efficiently, thereby reducing the cost of producing molten iron.

[0018] It is a diagram showing an example of an apparatus for manufacturing molten iron according to an embodiment of the present invention. It is a diagram for explaining the configuration of a control device. It is a diagram for explaining the operation of the apparatus for manufacturing molten iron shown in FIG.

[0019] Hereinafter, an embodiment of the present invention will be specifically described. The embodiment described below shows a preferred example of the present invention, and the present invention is not limited to this embodiment.

[0020] FIG. 1 is a diagram showing an example of an apparatus for producing molten iron according to an embodiment of the present invention. The apparatus for producing molten iron 1 shown in FIG. 1 is an apparatus for producing molten iron using a cold iron source 2 as a raw material. The cold iron source 2 can be, for example, scrap 3 generated in the city or reduced iron 4. The scrap 3 can be recycled materials such as construction steel and cans. The construction steel can be H-beams and automotive steel. It is preferable that the scrap 3 be scrap specified in the "Unified Standard for Iron Scrap Inspection" of the Japan Iron and Steel Association.

[0021] The reduced iron 4 refers to metallic iron obtained by reducing iron ore. The reduced iron 4 may be either domestically produced or imported. Examples of the reduced iron 4 include hot briquette iron (HBI). The reduced iron 4 comes in various shapes and sizes, such as chunks, pellets, and briquettes. Reduced iron 4 in any one of these shapes may be charged into the preheating chamber 7 (described later), or reduced iron 4 in multiple shapes may be mixed in a predetermined ratio and charged into the preheating chamber 7. When charging the reduced iron 4 into the preheating chamber 7, the scrap 3 described above may be mixed in a predetermined ratio.

[0022] The cold iron source 2 may include in-house scrap generated at a steelworks, crop generated during the rolling of steel materials such as steel strips, and pig iron solidified from molten pig iron. In-house scrap refers to the unsteady portion of a slab cast by continuous casting or ingot casting. Examples of in-house scrap include slabs that are generated at the start or end of casting and do not become products. Pig iron solidified from molten pig iron refers to solidified molten pig iron obtained from raw materials such as iron ore and coke in a blast furnace or other blast furnace. The cold iron source 2 is stored in a scrap yard (not shown) by the above-mentioned type or quality of the cold iron source 2.

[0023] The cold iron source 2 is transported from a scrap yard to a bottom-opening bucket 5 shown in Figure 1 by a conveyor (not shown) and charged into the bottom-opening bucket 5. A gate configured to open downward in the height direction of the molten iron manufacturing apparatus 1 (the up-and-down direction in Figure 1) is provided at the bottom of the bottom-opening bucket 5. The bottom-opening bucket 5, filled with the cold iron source 2, is transported by a traveling cart 6 from the scrap yard to a predetermined position above the preheating chamber 7. When the traveling cart 6 stops at the predetermined position, the gate of the bottom-opening bucket 5 is opened and the cold iron source 2 inside drops down and is charged into the preheating chamber 7.

[0024] The preheating chamber 7 is a crucible-shaped container configured to preheat the cold iron source 2 by induction heating and charge it into the electric arc furnace 8. The inside sides and bottom of the preheating chamber 7 are covered with a refractory material. The refractory material may be a conventionally known material such as firebrick. To improve the efficiency of preheating the cold iron source 2 in the preheating chamber 7, it is preferable to provide a heat insulating material with a lower thermal conductivity than the refractory material inside the furnace wall of the preheating chamber 7. In the example shown in FIG. 1 , the preheating chamber 7 is located above the electric arc furnace 8 in the height direction of the molten iron manufacturing apparatus 1 and at a position off-center from the center of the electric arc furnace 8.

[0025] A coil 9 is wound around the outer periphery of the preheat chamber 7. A power supply unit 10 is connected to the coil 9. The power supply unit 10 has an AC power supply 10A and a frequency converter 10B. The power supply unit 10 converts the AC current supplied from the AC power supply 10A to a predetermined frequency using the frequency converter 10B, and supplies the AC current converted to the predetermined frequency to the coil 9. The frequency of the AC current supplied from the power supply unit 10 to the coil 9 will be described later. The power supply unit 10 of this embodiment is an example of a power supply. The AC power supply 10A and the frequency converter 10B of the power supply unit 10 are controlled by a control device, which will be described later.

[0026] (Coil) The ratio of the winding height H_Coil of the coil 9 wound around the outer periphery of the preheating chamber 7 to the winding diameter D_Coil satisfies the following formula (6). In the following description, this ratio will be referred to as the coil ratio: 0.5≦H_Coil / D_Coil≦2.5 (6) The winding height H_Coil is the distance (m) between the lowest coil 9 in the height direction of the molten iron manufacturing apparatus 1 and the highest coil 9 in the height direction. The winding diameter D_Coil is the diameter (m) of the coil 9 wound around the outer periphery of the preheating chamber 7 and is equal to the diameter of a circle inscribed in the coil 9 wound around the outer periphery of the preheating chamber 7. In other words, the winding diameter D_Coil of the coil 9 is approximately the same as the outer diameter of the preheating chamber 7. The coil ratio is an index for determining the height region in the preheating chamber 7 where the cold iron source 2 can be efficiently preheated, and the coil length. If the coil ratio is less than 0.5, the area in the height direction described above will be excessively narrow, and it may not be possible to uniformly and efficiently preheat the cold iron source 2 in the preheating chamber 7. On the other hand, if the coil ratio exceeds 2.5, the coil length will be excessively long, increasing the power loss caused by energizing the coil, and there is a possibility that the power consumption rate of the preheating device will deteriorate.

[0027] The charging port formed at the top of the preheating chamber 7 is opened and closed by a lid (not shown). The lid is opened to open the charging port, and the cold iron source 2 is charged into the preheating chamber 7 from the open-bottom bucket 5. When the cold iron source 2 is preheated by the preheating chamber 7, the charging port is closed by the lid. This is to avoid the possibility that if the charging port is left open while the cold iron source 2 is being preheated, the cold iron source 2 will be exposed to the outside air and cooled, making it difficult for the temperature of the cold iron source 2 to rise.

[0028] A gate 11 configured to open downward in the height direction of the molten iron manufacturing apparatus 1 is provided at the bottom of the preheating chamber 7. The gate 11 is configured to close when the cold iron source 2 is charged into the preheating chamber 7 and when the cold iron source 2 is preheated, and to open when the cold iron source 2 that has been preheated inside the preheating chamber 7 is charged into the electric arc furnace 8. The supply of AC current to the coil 9 of the preheating chamber 7 and the opening and closing of the gate 11 may be performed by a control device or by an operator.

[0029] An actuator 30 is connected to the gate 11 so as to be capable of transmitting power. The actuator 30 is a device that transmits power to the gate 11 to open and close the gate 11, and also adjusts the opening degree of the gate 11. Examples of the actuator 30 include a hydraulic cylinder and a motor. A control device 40 is connected to the actuator 30. The control device 40 is also connected to the AC power supply 10A and frequency converter 10B of the power supply unit 10 described above. The control device 40 controls the operations of the actuator 30, the AC power supply 10A, and the frequency converter 10B. For example, the control device 40 may be configured as a general-purpose computer such as a workstation or a personal computer.

[0030] 2 is a diagram illustrating the configuration of the control device 40. As shown in FIG. 2, the control device 40 includes an acquisition unit 41, a calculation unit 42, a storage unit 43, and an output unit 44.

[0031] (Acquisition Unit) The acquisition unit 41 acquires various data measured by various sensors (not shown) and data stored in the memory unit 43, and outputs the acquired data to the calculation unit 42 and the memory unit 43. Examples of such data include the charging speed of the cold iron source 2 preheated in the preheating chamber 7 when it is charged into the arc electric furnace 8, and data for calculating the appropriate frequency of the AC current supplied from the power supply unit 10 to the coil of the preheating chamber 7. Other examples of the data include the electrical conductivity σ of the cold iron source 2, the mixing ratio, the thickness t, width w, and length l of the cold iron source 2, the target preheating temperature of the cold iron source 2, the amount of the cold iron source 2 charged from the preheating chamber 7 to the arc electric furnace 8, the charging interval, and the amount of power input to the arc electric furnace 8. The acquisition unit 41 then outputs the acquired data to the calculation unit 42 and the memory unit 43. The acquisition unit 41 may be connected to a data input device (not shown) that is operated by an operator, so that various data can be input from the data input device.

[0032] The storage unit 43 may be, for example, an updatable flash memory, a built-in hard disk or a hard disk connected via a data communication terminal, an information recording medium such as a memory card, or a read / write device thereof. The storage unit 43 stores programs for the calculation unit 42 to execute each function, data used by the programs, and the various data described above.

[0033] (Calculation Unit 42) The calculation unit 42 is, for example, a CPU or the like, and executes various programs stored in the memory unit 43, causing the calculation unit 42 to function as a calculation unit 45. The calculation unit 45 calculates an appropriate frequency of AC current that satisfies Equations (4) and (5), described below, based on data stored in the memory unit 43 acquired via the acquisition unit 41. The calculation unit 45 also calculates a charging speed of the cold iron sources 2 from the preheating chamber 7 to the arc electric furnace 8 based on the target preheating temperature of the cold iron sources 2, the amount of cold iron sources 2 charged from the preheating chamber 7 to the arc electric furnace 8, the charging interval, and the amount of power input to the arc electric furnace 8. The calculation unit 45 then calculates a control command signal corresponding to the charging speed and outputs the control command signal to the output unit 44. Examples of the control command signal include a signal for operating the AC power source 10A, i.e., a signal for turning the AC power source 10A ON / OFF, a target frequency (i.e., appropriate frequency) of the AC current output from the frequency converter 10B, and an operating amount of the actuator 30.

[0034] (Output Unit) The output unit 44 outputs the control command signal calculated by the calculation unit 45 to the AC power supply 10A, the frequency converter 10B, and the actuator 30. Therefore, the output unit 44 is communicatively connected to these devices 10A, 10B, and 30. The output unit 44 may also have a monitor (not shown). The monitor may display the target frequency of the AC current supplied to the coil 9 of the preheat chamber 7 and the actual measured value of the AC current frequency to notify the operator.

[0035] As described above, the charging speed of the cold iron sources 2 in the preheating chamber 7 into the electric arc furnace 8 can be controlled by controlling the opening and closing of the gate 11. Controlling the opening and closing of the gate 11 means adjusting the opening degree of the gate 11, adjusting the time for which the gate 11 is open, and adjusting the time interval until the gate 11, which was closed at a previous time, is opened at the current time. This time interval can be referred to as the time interval for charging the cold iron sources 2 from the preheating chamber 7 into the electric arc furnace 8. The charging speed of the cold iron sources 2 is preferably a charging speed that can prevent the cold iron sources 2 charged into the electric arc furnace 8 from becoming large chunks. The charging speed of the cold iron sources 2 will be described later. In the following explanation, the time interval for charging the cold iron sources 2 from the preheating chamber 7 into the electric arc furnace 8 will be referred to as the charging interval C.

[0036] A chute 12 is provided below the preheating chamber 7 in the height direction of the molten iron manufacturing apparatus 1. The chute 12 is configured so that the cold iron source 2 can be charged into the melting furnace 14 of the arc type electric furnace 8 even when the lid 13 of the arc type electric furnace 8 is closed. In other words, a through hole 15 that penetrates the lid 13 in the thickness direction of the lid 13 is formed in the lid 13 of the arc type electric furnace 8, and the chute 12 is connected to the through hole 15. Therefore, when the gate 11 of the preheating chamber 7 is opened, the preheated cold iron source 2 can be directly charged into the melting furnace 14 of the arc type electric furnace 8 via the chute 12 without having to retract the lid 13 to open the opening of the melting furnace 14.

[0037] The electric arc furnace 8 shown in FIG. 1 is a DC electric arc furnace and includes a melting furnace 14 for melting the charged cold iron source 2 and a furnace lid 13 for opening and closing the opening of the melting furnace 14. In the example shown in FIG. 1, an upper electrode 16 is provided at approximately the center of the furnace lid 13, penetrating the furnace lid 13. The upper electrode 16 is made, for example, mainly of graphite and extends in the height direction of the molten iron manufacturing apparatus 1. An oxygen blowing lance 17 is provided at a position off the center of the furnace lid 13. Oxygen is blown into the melting furnace 14 from the oxygen blowing lance 17. The oxygen may be pure oxygen or a mixed gas containing a predetermined ratio of oxygen, for example, a mixed gas of pure oxygen and air.

[0038] A carbonaceous material injection lance 18 and a supporting burner 19 are provided penetrating the furnace wall of the melting furnace 14. The carbonaceous material injection lance 18 uses air, nitrogen, or the like as a carrier gas and injects carbonaceous material such as coke, char, coal, charcoal, and graphite into the melting furnace 14. The supporting burner 19 charges fossil fuels such as heavy oil, kerosene, pulverized coal, propane gas, and natural gas into the melting furnace 14 together with oxygen, air, or oxygen-enriched air, and burns them within the melting furnace 14. For example, if unmelted cold iron source 2 remains when molten iron is tapped, the unmelted cold iron source 2 is melted by the supporting burner 19.

[0039] A bottom electrode 20 is provided at approximately the center of the bottom of the melting furnace 14, penetrating the bottom. The bottom electrode 20 is made mainly of graphite, for example, and extends in the height direction of the molten iron manufacturing apparatus 1. A predetermined gap is set between the upper electrode 16 and the bottom electrode 20 in the height direction. This is to generate an arc between the upper electrode 16 and the bottom electrode 20.

[0040] A tapping port 21 is formed through the bottom of the furnace at a position off-center. A tapping door 22 for opening and closing the tapping port 21 is provided on the outside of the bottom of the melting furnace 14. The tapping door 22 is configured to close the tapping port 21 when molten iron is being produced. The tapping door 22 is configured to open the tapping port 21 when the amount of molten iron in the melting furnace 14 reaches a predetermined amount, and the molten iron is tapped from the tapping port 21 into a ladle (not shown). It is preferable to sample the molten iron when it is being tapped and analyze its components.

[0041] Here, the blending ratio of the cold iron source 2, i.e., scrap 3, and reduced iron 4 preheated in the preheating chamber 7 when molten iron is produced using the molten iron production apparatus 1 shown in FIG. 1 will be described. In the following description, the blending ratio of the scrap 3 and reduced iron 4 will be referred to as the "mixing ratio." In this embodiment, only reduced iron or a mixture of the scrap 3 and reduced iron 4 is charged and preheated in the preheating chamber 7, and then the mixture is charged into the arc electric furnace 8. First, the magnetic properties and electrical properties of the reduced iron 4 will be described. Compared to a typical cold iron source 2 such as scrap 3, the reduced iron 4 contains gangue. Therefore, the reduced iron 4 has properties that are unfavorable for induction heating in the preheating chamber 7. For example, the relative permeability μr, which indicates the ease with which magnetic flux passes, is approximately 1500 to 5000 for typical scrap 3 at room temperature, whereas it is approximately 50 to 100 for reduced iron 4. In terms of temperature characteristics, the specific heat of the reduced iron 4 is higher than that of the scrap 3, and the thermal conductivity of the reduced iron 4 is lower than that of the scrap 3. In other words, the reduced iron 4 is more difficult to heat up than the scrap 3. Therefore, when the reduced iron 4 and the scrap 3 are induction heated under the same conditions in the preheating chamber 7, the relationship between the temperature TSc of the scrap 3 and the temperature TDRI of the reduced iron 4 is expressed by the following formula: TSc>TDRI

[0042] Therefore, when a mixture of reduced iron 4 and scrap 3 is charged into the preheating chamber 7 and induction-heated, the scrap 3 heats more quickly than the reduced iron 4, resulting in a temperature increase. Heat from the scrap 3 is transferred to the reduced iron 4 in contact with the scrap 3. Therefore, when the reduced iron 4 and the scrap 3 are mixed and induction-heated, the reduced iron 4 can be heated more efficiently than when the reduced iron 4 alone is induction-heated. On the other hand, the scrap 3 may contain a greater variety of components than the reduced iron 4. When the reduced iron 4 and the scrap 3 are mixed and melted, the reduced iron 4 functions as a clean iron source that dilutes these components. Therefore, the proportion of the scrap 3 in the reduced iron 4, i.e., the mixing ratio of the scrap 3 to the reduced iron 4, is preferably, for example, 50% (upper limit) or less. The upper limit of the mixing ratio is preferably determined depending on the type of scrap 3, and can be determined in advance, for example, through experiments. The mixing ratio is the ratio between the mass of the scrap 3 and the mass of the reduced iron 4 , and is calculated by dividing the mass of the scrap 3 by the mass of the reduced iron 4 .

[0043] The bulk density B of the cold iron source 2 charged into the preheating chamber 7 will now be described. In this embodiment, the bulk density B of the cold iron source 2 is 1.0 to 3.5 t / m ３ First, it is preferable that the bulk density B is 1.0 t / m ３ The case where the bulk density B is less than 1.0 t / m ３ If the bulk density B is less than 3.5 t / m, the proportion of the cold iron source 2 in the internal volume of the furnace, i.e., the preheating chamber 7, will be low, and the proportion of the air region will be high, which may result in a decrease in the heating efficiency of the cold iron source 2 due to heat dissipation to the air region. The amount of cold iron source 2 charged and heated in the furnace will decrease, resulting in a decrease in the preheating processing capacity. ３ The case where the bulk density B exceeds 3.5 t / m will be described. ３If the bulk density B exceeds 3.5 t / m, the cold iron sources 2 will be densely packed in the preheating chamber 7. Furthermore, the size of each cold iron source 2 is very small, and the appropriate frequency of the AC current when inductively heating such a small cold iron source 2 (hereinafter referred to as the "appropriate frequency") will be high. Due to the cold iron sources 2 being densely packed in the preheating chamber 7, the high appropriate frequency, and the skin effect during induction heating, the bulk density B will be 3.5 t / m ３ If the bulk density B of the cold iron sources 2 exceeds 1.0 to 3.5 t / m, the risk of the cold iron sources 2 welding to each other may increase. Also, when the cold iron sources 2 are charged into the preheating chamber 7, the risk of damaging the hearth of the preheating chamber 7 may increase. Furthermore, when the cold iron sources 2 are charged from the preheating chamber 7 into the melting furnace 14 of the electric arc furnace 8, the risk of damaging the hearth of the melting furnace 14 may increase. Therefore, the bulk density B of the cold iron sources 2 is set to 1.0 to 3.5 t / m ３ It is preferable that the ratio is 2.5 to 3.5 t / m ３ It is more preferable that:

[0044] Bulk density B is the mass of the cold iron source 2 placed in the preheating chamber 7 divided by the internal volume of the preheating chamber, and can be expressed by the following formula: B = Σ (mass of cold iron source) / Σ (internal volume of the preheating chamber). Note that bulk density B can also be said to be the packing density of the cold iron source 2 in the preheating chamber 7.

[0045] When scrap 3 and reduced iron 4 are mixed and charged into the preheating chamber 7 as the cold iron source 2, the bulk density of the scrap 3 and the bulk density of the reduced iron 4 are each determined in advance. Then, the bulk density B of the cold iron source 2 to be charged into the preheating chamber 7 is determined based on each bulk density and the mixing ratio of the scrap 3 and reduced iron 4 charged into the preheating chamber 7. Alternatively, the mass and charging height of the reduced iron 4 or the mixture of the reduced iron 4 and scrap 3 charged into the preheating chamber 7 may be measured to determine the actual bulk densities of the scrap 3 and reduced iron 4.

[0046] The bulk density of the reduced iron 4 is higher than that of the scrap 3. Therefore, when the scrap 3 and the reduced iron 4 are mixed and charged into the preheating chamber 7, if the blending ratio of the reduced iron 4 is higher than that of the scrap 3, the number of times the cold iron source 2 is charged per charge into the arc-type electric furnace 8 can be reduced. Furthermore, as will be described later, energy can be efficiently used for melting, thereby reducing the cost of producing molten iron. Therefore, it is preferable to mix the scrap 3 and the reduced iron 4 so that the bulk density and blending ratio of the reduced iron 4 are higher than those of the scrap 3.

[0047] The filling ratio H of the cold iron source 2 to the preheating chamber 7 ＤＲＩ / H ＩＨ In this embodiment, the filling ratio H of the cold iron source 2 in the preheating chamber 7 is ＤＲＩ / H ＩＨ The cold iron source 2 is filled into the preheating chamber 7 so that the value of H is 0.8 or more and 1.0 or less. ＤＲＩ " is the filling height of the cold iron source 2 placed in the preheating chamber 7. ＩＨ " is the height from the bottom of the preheating chamber 7 to the top end of the coil 9. In other words, the cold iron source 2 is filled in the preheating chamber 7 with the top end of the coil 9 as the height limit. ＤＲＩ can be measured by, for example, a laser distance meter (not shown). ＤＲＩ may be calculated based on the internal volume of the preheating chamber 7, the amount of the cold iron source 2 charged into the preheating chamber 7, and the bulk density B.

[0048] Filling ratio H ＤＲＩ / H ＩＨ The reason why the filling ratio H is set to 0.8 or more and 1.0 or less will be explained. In the preheating chamber 7, a magnetic field is generated in the preheating chamber 7 by energizing the coil 9. This generates an eddy current in the cold iron source 2 in the preheating chamber 7, and the cold iron source 2 is heated by the current loss when the eddy current flows through the cold iron source 2. If the preheating chamber 7 is not sufficiently filled with the cold iron source 2, that is, if the filling ratio H is set to 0.8 or more and 1.0 or less, the cold iron source 2 is heated by the current loss when the eddy current flows through the cold iron source 2. ＤＲＩ / H ＩＨIf the filling ratio H in the preheating chamber 7 is less than 0.8, the empty space in the preheating chamber 7 will directly lead to energy loss and a decrease in energy efficiency. ＤＲＩ / H ＩＨ From the viewpoint of suppressing energy loss and improving energy efficiency, the filling ratio H ＤＲＩ / H ＩＨ The filling ratio H is preferably 0.8 or more. The preheating chamber 7 is opened and closed by a lid (not shown). ＤＲＩ / H ＩＨ In order to increase the filling ratio H ＤＲＩ / H ＩＨ If the filling ratio H exceeds 1.0, there is a possibility that the lid will not close. In addition, the cold iron source 2, which is located higher than the upper end of the coil 9, is less exposed to the magnetic field and is therefore less likely to be induction heated. ＤＲＩ / H ＩＨ If the filling ratio H exceeds 1.0, it may result in an extension of the preheating time. Therefore, in order to preheat the cold iron source 2 with energy efficiency and to prevent trouble, it is necessary to set the filling ratio H ＤＲＩ / H ＩＨ It is preferable to load the cold iron source 2 into the preheating chamber 7 so that the value of the cold iron content is 0.8 or more and 1.0 or less.

[0049] Furthermore, simply preheating the cold iron source 2, including the reduced iron 4 and scrap 3, in the preheating chamber 7 may not be enough to efficiently melt the cold iron source 2 in the electric arc furnace 8. The charging rate of the cold iron source 2 into the electric arc furnace 8, the amount of molten iron in the melting furnace 14, and the temperature of the molten iron in the melting furnace 14 are also important factors for efficiently melting the cold iron source 2. For example, even if the cold iron source 2 is sufficiently preheated, if the charging rate of the cold iron source 2 is excessively higher than the melting rate of the cold iron source 2 in the melting furnace 14, the reduced iron 4 and scrap 3 may form large lumps in the melting furnace 14, which may result in a longer steelmaking time or operational problems. Therefore, it is preferable that the charging rate of the cold iron source 2 preheated in the preheating chamber 7 satisfy the following formulas (1) to (3). Satisfying the following formulas (1) to (3) enables efficient operation. The charging rate of the cold iron source 2 is calculated by dividing the charging amount t of the cold iron source 2 per charge by the charging interval of the cold iron source 2 from the preheating chamber 7 to the electric arc furnace 8, and means the charging amount of the cold iron source 2 per unit time. The charging interval is the time between the current point in time at which the cold iron source 2 is charged from the preheating chamber 7 to the electric arc furnace 8 and the point in time at which the cold iron source 2 is charged from the preheating chamber 7 to the electric arc furnace 8 one charge before or one charge after the current point in time.

[0050] "v Ｉ " is the charging speed t / min of the cold iron source. "P" is the amount of electric power MW input to the electric arc furnace 8, and can be determined in advance based on the type and mixture ratio of the reduced iron 4 and scrap 3 charged into the electric arc furnace 8 as the cold iron source 2. ｐｈ " is the target preheating temperature of the cold iron source in °C, and is preferably equal to or lower than 800 °C, which is the Curie point temperature. ｍ " is the temperature of molten iron in the melting furnace 14 of the electric arc furnace 8 in °C, which is the temperature of the cold iron source 2 charged into the melting furnace 14, the amount of charge W Ｉ , and the input power amount P. Ｉ " is the amount t of cold iron source 2 charged from preheating chamber 7 to melting furnace 14 per one time. ｍ " is the amount of molten iron in the melting furnace 14 t, and "C" is the charging interval (min) of the cold iron source 2 .

[0051] In addition, the scrap 3 and reduced iron 4 are ferromagnetic at room temperature, but as the temperature rises, their relative permeability μ ｒ For example, at the Curie point of the reduced iron 4 (700° C. or higher and 800° C. or lower), the relative permeability μ ｒ becomes almost "1", and the reduced iron 4 becomes a paramagnetic material. As the temperature of the reduced iron 4 increases, it becomes gradually more difficult to inductively heat the reduced iron 4. Therefore, the target preheat temperature T ｐｈ When the temperature of the cold iron source 2 reaches the target preheating temperature T ｐｈ It is necessary to design the frequency of the AC current supplied to the coil 9 in accordance with the magnetization characteristics of the cold iron source 2 when the temperature reaches the Curie point. For example, for a specific reduced iron, the appropriate frequency below the Curie point is approximately 10 to 3,300 Hz, and the appropriate frequency at a preheating temperature above the Curie point is approximately 2,000 to 94,000 Hz. Furthermore, since the frequency of the AC current supplied to the coil 9 is restricted by equipment, the energy utilization efficiency varies depending on the target temperature range and the frequency design of the equipment. In other words, energy is wasted. The magnetization characteristics are defined as the relative permeability μ ｒ and electrical conductivity.

[0052] For this reason, in this embodiment, the reduced iron 4 is induction heated until it nearly reaches its Curie point temperature, that is, at a temperature below the Curie point temperature. The appropriate frequency in the preheating chamber 7 at this time can be expressed by the following formula (4), and the frequency of the AC current supplied to the coil 9 of the preheating chamber 7 is set so as to satisfy the following formulas (4) and (5). The frequency that satisfies the following formula (4) means a frequency that can energy-efficiently induction heat the reduced iron 4 at the Curie point of the reduced iron 4 and in a temperature range below the Curie point.

[0053] "f" is the appropriate frequency Hz of the AC current supplied to the coil 9 of the preheating chamber 7, and "σ" is the target preheating temperature T of the cold iron source 2. ｐｈ The electrical conductivity of the cold iron source 2 at this temperature is 1 / Ωm. ｒ " is the relative permeability of reduced iron 4, "μ ０ " is the magnetic permeability of a vacuum, 4π x 10 －７, "t" is the thickness m of the cold iron source 2, "w" is the width m of the cold iron source 2, and "l" is the length m of the cold iron source 2. The electrical conductivity σ can be calculated based on the electrical conductivity of the scrap 3, the electrical conductivity of the reduced iron 4, and their mixing ratio. The electrical conductivity of the scrap 3 and the electrical conductivity of the reduced iron 4 can be determined experimentally. When only reduced iron 4 is used as the cold iron source 2, the thickness t, width w, and length l of the cold iron source 2 may be the thickness t, width w, and length l of the reduced iron 4. When a mixture of scrap 3 and reduced iron 4 is used as the cold iron source 2, the thickness t, width w, and length l of the cold iron source 2 may be the average value of the thickness t, width w, and length l of the scrap 3 and reduced iron 4. In this embodiment, the "thickness t" refers to the shortest of the three sides (in the three axial directions) of the scrap 3 or reduced iron 4. The "length l" refers to the longest of the three sides (in the three axial directions) of the scrap 3 or reduced iron 4. The "width w" refers to the length of the portion of the three sides (three axial directions) of the scrap 3 or reduced iron 4 between the above-mentioned "thickness t" and "length l."

[0054] The skin effect when induction heating is performed will now be described. The skin effect refers to the property that the current density increases closer to the surface of the material to be heated and then rapidly decreases toward the interior of the material. The thickness of the portion of the material where the current density is high on the surface is called the skin depth. The skin depth is defined as the distance over which the current density at the outermost surface of the material to be heated is 1 and attenuates to 36.7%. In this embodiment, the material to be heated is the cold iron source 2.

[0055] If the frequency of the AC current supplied to the coil 9 of the preheating chamber 7 is higher than the above-mentioned appropriate frequency, the skin depth becomes very thin, and the surface of the cold iron source 2 is heated in a concentrated manner. In other words, the cold iron source 2 cannot be heated uniformly, and the heating efficiency of the reduced iron 4 in particular decreases. On the other hand, if the frequency of the AC current supplied to the coil 9 of the preheating chamber 7 is lower than the appropriate frequency, the skin depth becomes thick. For example, if the skin depth becomes half or more of the thickness of the reduced iron 4, the eddy currents induced inside the reduced iron 4 are canceled out, and the heating efficiency of the reduced iron 4 decreases. From the viewpoint of heating efficiency as well, it is preferable that the frequency of the AC current supplied to the coil 9 of the preheating chamber 7 be the above-mentioned appropriate frequency.

[0056] (Operation) The operation of the apparatus 1 for producing molten iron according to this embodiment will be described. Fig. 3 is a diagram for explaining the operation of the apparatus 1 for producing molten iron shown in Fig. 1. As shown in Fig. 3, only reduced iron 4 stored in a scrap yard or a mixture of scrap 3 and reduced iron 4 is charged into the preheating chamber 7 via a conveyor or a bottom-opening bucket. In addition, the filling ratio H ＤＲＩ / H ＩＨ is set to 0.8 or more and 1.0 or less.

[0057] When molten iron is produced by the molten iron production apparatus 1 shown in Fig. 3, the control device 40 reads out various data from the storage unit 43, such as the electrical conductivity σ of the cold iron source 2 to be charged into the preheating chamber 7, and the thickness t, width w, and length l of the cold iron source 2. Based on this data, the calculation unit 45 of the control device 40 calculates the appropriate frequency of AC current that satisfies the above-mentioned equations (4) and (5). Then, a control command signal corresponding to the appropriate frequency is output from the output unit 44 to the AC power source 10A and the frequency converter 10B. In this way, AC current of the appropriate frequency that satisfies the equations (4) and (5) is supplied to the coil 9 of the preheating chamber 7, and the target preheating temperature T ｐｈ The cold iron source 2 is preheated to a target preheat temperature T ｐｈThe charging speed of the cold iron sources 2 charged from the preheating chamber 7 to the electric arc furnace 8 is calculated based on the amount of cold iron sources 2 charged from the preheating chamber 7 to the electric arc furnace 8, the charging interval, and the amount of power input to the electric arc furnace 8. A control command signal corresponding to the charging speed is output from the output unit 44 to the actuator 30, and the opening and closing and the opening degree of the gate 11 by the actuator 30 are controlled.

[0058] Supplying an AC current to the coil 9 generates a magnetic field around the coil 9, and eddy currents are generated in the scrap 3 and reduced iron 4 charged as the cold iron source 2 inside the preheating chamber 7 under the influence of the magnetic field. As a result, Joule heat is generated in the scrap 3 and reduced iron 4 according to their respective electrical resistances, and the Joule heat increases the temperatures of the scrap 3 and reduced iron 4. The eddy currents described above are sometimes referred to as induced currents.

[0059] Since the specific heat of the scrap 3 is smaller than that of the reduced iron 4, the temperature of the scrap 3 rises more quickly than that of the reduced iron 4. Therefore, when the scrap 3 and the reduced iron 4 are charged into the preheating chamber 7, when preheating is started as described above, the heat of the scrap 3 is dissipated by the reduced iron 4 in contact with the scrap 3. In this way, the scrap 3 and the reduced iron 4 are heated to the target preheat temperature T ｐｈ It is preheated to.

[0060] Among the cold iron sources 2 in the preheating chamber 7, the temperature of the cold iron source 2 located at the bottom of the preheating chamber 7 is set to the target preheating temperature T ｐｈ When the temperature reaches 100°C, the gate 11 is opened and the preheated cold iron source 2 is charged into the melting furnace 14 of the electric arc furnace 8. The temperature of the cold iron source 2 can be calculated based on the temperature of the cold iron source 2 charged into the preheating chamber 7, the ambient temperature, the frequency of the AC current supplied to the coil 9, and the time for which the AC current is supplied. Alternatively, it may be measured by a thermometer (not shown).

[0061] The cold iron sources 2 are preferably charged into the melting furnace 14 in one to eight separate charges per full load of the preheating chamber 7. That is, the cold iron sources 2 are charged from the preheating chamber 7 into the melting furnace 14 at a charging rate t / min that satisfies the above-mentioned formulas (1) to (3). For example, the operation of the actuator 30 is controlled by the control device 40 so as to satisfy the above-mentioned formulas (1) to (3). In this manner, the opening degree of the gate 11, the time for which the gate 11 is open, and the time interval C for charging the cold iron sources 2 from the preheating chamber 7 into the electric arc furnace 8 are adjusted. As a result, the cold iron sources 2 that have reached the target preheating temperature Tph in the lower part of the preheating chamber 7 are charged into the electric arc furnace 8.

[0062] When the cold iron sources 2 are charged from the preheating chamber 7 into the melting furnace 14 in this way, a space is created in the upper part of the preheating chamber 7 by the amount of the cold iron sources 2 charged into the melting furnace 14. New cold iron sources 2 are charged into this space from the bottom-opening bucket 5. That is, the conveyor is operated in conjunction with the charging of the cold iron sources 2 from the preheating chamber 7 into the electric arc furnace 8, and scrap 3 and reduced iron 4 stored in the scrap yard are charged into the bottom-opening bucket 5 via the conveyor. Note that even when the cold iron sources 2 are charged into this space, the filling ratio H ＤＲＩ / H ＩＨ The cold iron source 2 is loaded into the preheating chamber 7 so that the value of the temperature coefficient of the cold iron source 2 is 0.8 or more and 1.0 or less.

[0063] When the cold iron source 2 is repeatedly charged into the melting furnace 14, the cold iron source 2 charged in the upper part of the preheating chamber 7 gradually moves downward and is further heated. As the cold iron source 2 moves downward, the temperature of the cold iron source 2 gradually increases, and the temperature of the cold iron source 2 located in the lower part of the preheating chamber 7 becomes higher than the target preheating temperature T ｐｈ In addition, when the temperature of the cold iron source 2 in the lower part of the preheating chamber 7 reaches the target preheating temperature T ｐｈ It is preferable to set the frequency of the AC current and the charging rate t / min so that the preheating is completed when the temperature reaches 100° C., and at the same time, the cold iron source 2 is continuously charged into the melting furnace 14 .

[0064] Meanwhile, in the arc-type electric furnace 8, with the furnace lid 13 closed, electricity is applied to the upper electrode 16 and the bottom electrode 20. This generates arcs between the upper electrode 16 and the bottom electrode 20 and between the upper electrode 16 and the scrap 3 and reduced iron 4 in the melting furnace 14. The scrap 3 and reduced iron 4 are melted by the arc heat. Alternatively, the scrap 3 and reduced iron 4 can be melted by using the arc heat in combination with heating due to an oxidation reaction between oxygen gas from the oxygen blowing lance 17 and carbonaceous material from the carbonaceous material blowing lance 18 and heating from the auxiliary burner 19.

[0065] The above-described charging of the cold iron source 2 into the melting furnace 14 and melting of the cold iron source 2 in the melting furnace 14 are carried out until the amount of molten iron in the melting furnace 14 reaches a predetermined amount. The amount of molten iron that can be stored in the melting furnace 14 is determined by the design of the arc electric furnace 8. When the amount of molten iron in the melting furnace 14 reaches the predetermined amount, the tapping door 22 closing the tapping port 21 is opened, and the molten iron is tapped from the tapping port 21 into a ladle (not shown). At this time, the molten iron is sampled, and the components of the sampled molten iron are analyzed. The method of producing molten iron using the manufacturing apparatus 1 in this manner corresponds to the method of producing molten iron in this embodiment.

[0066] (Effects) According to the apparatus for producing molten iron 1 of this embodiment, the amount of cold iron source 2 filled in the preheating chamber 7 and the preheating conditions are optimized. This improves energy efficiency when preheating the cold iron source 2 in the preheating chamber 7. Furthermore, because the cold iron source 2 is preheated with the preheating chamber 7 closed, this also improves energy efficiency when preheating the cold iron source 2. Furthermore, in the preheating chamber 7 of this embodiment, the cold iron source 2 is preheated to a temperature equal to or lower than the Curie point, and the temperature of the cold iron source 2 is raised above the Curie point temperature and melted in the electric arc furnace 8. That is, in the apparatus for producing molten iron 1 of this embodiment, the preheating process and the melting process are separated. This allows for a reduction in energy supplied to the electric arc furnace 8 compared to a case without the preheating chamber 7. The reduction in energy used in the electric arc furnace 8 reduces the cost of producing molten iron. Furthermore, when the cold iron source 2 is a mixture of scrap 3 and reduced iron 4, as described above, the temperature of the scrap 3 rises more quickly than that of the reduced iron 4, and the heat of the scrap 3 is transferred to the reduced iron 4, thereby raising the temperature of the reduced iron 4. Even if it is difficult to heat the reduced iron 4 by induction heating, the temperature of the reduced iron 4 can be raised by using this principle. This also makes it possible to preheat the cold iron source 2 containing reduced iron 4 using energy more efficiently than before.

[0067] The present invention is not limited to the above-described embodiment. For example, although the arc-type electric furnace 8 shown in FIG. 1 is a DC arc-type electric furnace, it may be a three-phase AC arc-type electric furnace instead. A three-phase AC arc-type electric furnace has three electrodes penetrating the furnace lid. An AC current is passed through the electrodes, and the scrap and reduced iron charged into the melting furnace are melted by an arc generated between the electrodes. The other structural features of the three-phase AC arc-type electric furnace are the same as those of the DC arc-type electric furnace 8 described above. Even when a three-phase AC arc-type electric furnace is used instead of a DC arc-type electric furnace, the same functions and effects as those of this embodiment can be obtained. Furthermore, a bottom-blowing tuyeres for blowing stirring gas into the bottom of the arc-type electric furnace 8 may be provided. With this configuration, the molten iron in the arc-type electric furnace 8 can be stirred to homogenize its temperature, thereby suppressing the remaining unmelted cold iron source 2. Furthermore, the same functions and effects as those of the above-described embodiment can be obtained.

[0068] In winter when the outside temperature is low, the cold iron source 2 in the preheating chamber 7 is heated to the target preheating temperature T ｐｈ In winter, the energy required to preheat the iron source 2 to the target preheat temperature T ｐｈ It is preferable to charge the cold iron source 2 that has been preheated to some extent into the arc-type electric furnace 8 even if it has not reached the target preheat temperature T ｐｈTo suppress a temperature drop of the molten iron due to the cold iron source 2 not reaching the target temperature, it is preferable to slow down the charging rate of the cold iron source 2 compared to that in summer. For example, the relationship between the temperature of the cold iron source 2, the charging rate of the cold iron source 2, and the temperature of the molten iron is determined in advance, and the charging rate of the cold iron source 2 is calculated based on the determined relationship. The calculation of the charging rate of the cold iron source 2 is performed by the control device 40. Using the calculated charging rate as a target value, the control device 40 controls the operation of the actuator 30, i.e., the opening degree of the gate 11, the time the gate 11 is open, etc. By doing so, the same effects and advantages as those of this embodiment can be achieved even in the above-described case. Furthermore, the reduced iron 4 is more difficult to heat up than the scrap 3. Therefore, it is preferable to correct the target value of the charging rate described above depending on the mixing ratio of the scrap 3 and the reduced iron 4.

[0069] Furthermore, in winter, the temperature of the molten iron may not reach the target temperature. In such cases, it is preferable to slow down the charging rate of the cold iron source 2 compared to summer. For example, the relationship between the temperature of the molten iron, the temperature of the cold iron source 2, and the charging rate is calculated based on the previously determined relationship. Using the charging rate calculated in this manner as a target value, the control device 40 controls the operation of the actuator 30, i.e., the opening degree of the gate 11, the time the gate 11 is open, etc. By doing so, even in such cases, the same functions and effects as those of this embodiment can be achieved. Furthermore, it is preferable to correct the target value of the charging rate described above depending on the mixing ratio of the scrap 3 and the reduced iron 4.

[0070] Furthermore, the amount of electric power input to the arc electric furnace 8 may be changed based on the mixture ratio of the scrap 3 and the reduced iron 4. For example, because the reduced iron 4 heats up more slowly than the scrap 3, it is preferable to increase the amount of electric power input when the amount of reduced iron 4 is equal to or greater than a predetermined threshold value compared to when the amount is less than the threshold value. This makes it possible to optimize the generation of the arc that melts the cold iron source 2 according to the mixture ratio. Therefore, even in this case, the same functions and effects as those of this embodiment can be obtained.

[0071] An example conducted to confirm the operation and effects of the present invention will now be described. In this example, a cold iron source was melted to produce molten iron using a molten iron production apparatus configured similarly to the molten iron production apparatus 1 shown in FIG.

[0072] Table 1 shows the cold iron source and auxiliary raw materials charged into the melting furnace of the electric arc furnace in this example. As shown in Table 1, in this example, shredder chips or heavy H2 were used as scrap, and HBI was used as reduced iron. The oxygen blowing flow rate from the oxygen blowing lance was about 3000 Nm3. ３ The amount of coke lump charged and the amount of coke powder used were about 1000 kg / h, and the amount of quicklime charged was about 500 kg. The amount of electric power input to the arc-type electric furnace in this example was 50 MW.

[0073]

[0074] The shredded scrap used was general scrap from among the scraps specified in the "Unified Standards for Iron Scrap Inspection" of the Japan Iron Source Association. That is, steel plate processed products were mainly used as base materials and crushed by a shredder. The iron scrap was then sorted by a magnetic separator and used as the shredded scrap. The bulk density of the shredded scrap was 1.3 t / m ３ As scrap other than shredder waste, heavy H2, specified in the "Unified Standards for Iron Scrap Inspection" of the Japan Iron Source Association, was used. The bulk density of heavy H2 is 0.8 t / m ３ is.

[0075] The HBI has a total iron content of 88 to 94% by mass, a metallization ratio (M.Fe / T.Fe) of 94%, a carbon content of 1.0 to 1.5% by mass, and a bulk density of 2.5 to 3.5 t / m ３ The thickness t of the HBI was 30 mm, the width w was 50 mm, and the length l was 110 mm or less.

[0076] Table 2 shows the material balance of the iron source in the melting furnace of the electric arc furnace. As shown in Table 2, in this example, the amount of cold iron source containing reduced iron and scrap charged into the electric arc furnace was 100 tons, and the amount of molten iron obtained by melting the cold iron source was approximately 90 tons, the residual molten iron was approximately 80 tons, and the slag was approximately 10 tons.

[0077]

[0078] Table 3 summarizes the conditions for charging the cold iron source into the melting furnace of the electric arc furnace, the preheating conditions in the preheating chamber, the preheating results, and the operational results for Examples 1 to 12 of the present invention and the comparative example. Before charging the cold iron source into the melting furnace of the electric arc furnace, auxiliary raw materials, such as lump coke as an auxiliary fuel and quicklime as a slag-forming agent, were charged into the melting furnace through an auxiliary raw material charging chute (not shown). Injection of coke breeze from the carbonaceous material injection lance and injection of oxygen gas from the oxygen injection lance were carried out from the middle to the end of operation, when molten iron and molten slag had already been produced in the melting furnace to a certain extent.

[0079]

[0080] In the comparative example, the upper electrode and the furnace cover of the electric arc furnace were retracted to open the top of the furnace, and shredded chips and HBI were directly charged into the melting furnace from the bottom-opening bucket. In other words, the comparative example is an example in which the cold iron source was charged into the melting furnace of the electric arc furnace without preheating. In examples 1 to 12 of the present invention, the shredded chips and HBI were charged into the melting furnace in one to eight separate charges per full preheat chamber. The maximum amount charged per charge was 200 tons.

[0081] In Examples 1 to 5, 7, 8, 11, and 12, HBI was preheated to the target preheating temperature in a preheating chamber and then charged into the melting furnace of an electric arc furnace. In addition, shredder waste was directly charged into the melting furnace of an electric arc furnace, as in the comparative example.

[0082] Inventive Example 6 is an example in which HBI was preheated and then charged into the electric arc furnace, similar to Inventive Examples 1 to 5, 7, 8, 11, and 12, except that a portion of the HBI was directly charged into the electric arc furnace. In addition, shredded waste was directly charged into the electric arc furnace, similar to the comparative example.

[0083] In Example 9, a portion of the HBI was directly charged into the melting furnace of an electric arc furnace, and the remaining HBI was mixed with a portion of the shredded waste in a predetermined ratio, preheated to a target preheat temperature in a preheating chamber, and then charged into the melting furnace of the electric arc furnace. Similarly to the comparative example, the remaining shredded waste was directly charged into the melting furnace of the electric arc furnace.

[0084] Inventive Example 10 is an example in which only heavy H2 was used as the cold iron source. In Inventive Example 10, a portion of the heavy H2 was preheated to a target preheat temperature in a preheating chamber and then charged into the melting furnace of the electric arc furnace. The remaining heavy H2 was directly charged into the melting furnace of the electric arc furnace, as in the comparative example.

[0085] Inventive Examples 1 to 12 and the Comparative Example were evaluated using the "total power consumption index." The total power consumption index is an evaluation index for the melting experiment and indicates the ratio of the total power consumption of Inventive Examples 1 to 12 to the total power consumption of the Comparative Example. Specifically, the total power consumption was calculated as the sum of the power consumption of the arc-type electric furnace of the Comparative Example and the power consumption of the preheating chamber, and this was set to 100.0. Similarly, the total power consumption of Inventive Examples 1 to 12 was calculated, and the ratio of the total power consumption of Inventive Examples 1 to 12 to the total power consumption of the Comparative Example was calculated. If the total power consumption index was 95.0 or more and less than 100.0, the amount of power reduction was greater than that of the Comparative Example, and energy could be used more efficiently, and this was marked with "○" in Table 3. Furthermore, if the total power consumption index was less than 95.0, the amount of power reduction was greater and energy could be used even more efficiently, and this was marked with "◎" in Table 3. The electric power consumption rate refers to the amount of electric power supplied to the preheating chamber and the electric arc furnace to produce 1 ton of iron, and is expressed in kWh / t.

[0086] (Evaluation) As shown in Table 3, the total power consumption index was lower than that of the comparative example in all of the invention examples 1 to 12. In other words, it was found that the molten iron manufacturing apparatus according to the present invention has high energy utilization efficiency and can melt the cold iron source economically.

[0087] Specifically, the total power consumption index of Inventive Examples 2 and 3 was smaller than that of Inventive Examples 1 and 4 to 12. In Inventive Examples 2 and 3, the filling ratio in the preheating chamber, the amount of cold iron source charged, the charging interval, the frequency of the AC current in the preheating chamber, the coil inner diameter, and the coil height were more appropriately set than in Inventive Examples 1 and 4 to 12. This is thought to be because efficient induction preheating and efficient melting of the cold iron source were achieved. Furthermore, the total power consumption index of Inventive Example 3 was smaller than that of Inventive Example 2. This is thought to be because Inventive Example 3 charged a smaller amount per melt than Inventive Example 2, thereby enabling more efficient melting of the cold iron source.

[0088] In Inventive Example 10, heavy H2, which has a lower bulk density than shredded waste, was charged into the induction preheating device for preheating. In Inventive Example 10, the bulk density of heavy H2 was lower than that of shredded waste, resulting in more voids in the preheating device compared to Inventive Example 9, which used shredded waste as scrap. This is thought to be why Inventive Example 10 had a lower induction heating efficiency compared to Inventive Example 9. For the same reason, Inventive Example 10 also had a higher number of preheatings, i.e., a higher number of cold iron source chargings, compared to Inventive Example 9. As a result, Inventive Example 10 experienced a higher heat loss compared to Inventive Example 9, and the preheating time became rate-limiting, increasing the arc furnace operating time and worsening the arc furnace power consumption.

[0089] Inventive Examples 11 and 12 show the results of a comparison of the coil ratio H_Coil / D_Coil. As shown in Table 3, the coil ratio H_Coil / D_Coil for Inventive Example 11 was 0.4, and the coil ratio H_Coil / D_Coil for Inventive Example 12 was 2.6. In Inventive Example 11, the induction preheating area was excessively narrow, resulting in localized heating and a hanging problem. The power consumption rate of the arc furnace and the induction preheating device was worse than that of Inventive Example 3 due to the energization required to resolve the hanging problem and the re-energization required after the hanging problem was resolved. On the other hand, in Inventive Example 12, the coil winding height H_Coil was high, and therefore the overall coil length was long. This resulted in increased coil loss, i.e., power loss due to energizing the coil, and the power consumption rate of the induction preheating device was worse than that of Inventive Example 3.

[0090] REFERENCE SIGNS LIST 1 Molten iron manufacturing apparatus 2 Cold iron source 3 Scrap 4 Reduced iron 5 Bottom-opening bucket 6 Traveling cart 7 Preheating chamber 8 Arc-type electric furnace 9 Coil 10 Power supply unit 10A AC power supply 10B Frequency converter 11 Gate of preheating chamber 12 Chute 13 Furnace cover 14 Melting furnace 15 Through hole 16 Upper electrode 17 Oxygen blowing lance 18 Carbonaceous material blowing lance 19 Auxiliary burner 20 Hearth electrode 21 Tap hole 22 Tap door 30 Actuator 40 Control device 41 Acquisition unit 42 Calculation unit 43 Memory unit 44 Output unit 45 Calculation unit

Claims

1. An apparatus for manufacturing molten iron, comprising: a preheating chamber into which a cold iron source is charged; a gate provided in the preheating chamber; and an electric arc furnace that charges the cold iron source into the preheating chamber by opening the gate and melts the charged cold iron source with arc heat to produce molten iron, the apparatus also comprising: a coil wound around the outer periphery of the preheating chamber; and a power source that supplies a current of a predetermined frequency to the coil.

2. The apparatus for producing molten iron according to claim 1, further comprising: an actuator for opening and closing the gate; and a control device for controlling the actuator, wherein the control device controls the opening degree of the gate by the actuator so that the charging speed of the cold iron source preheated in the preheating chamber satisfies the following equations (1) to (3): v Ｉ : Charging speed of cold iron source (t / min) P: Amount of electric power input to arc type electric furnace (MW) T ｐｈ : Target preheat temperature of cold iron source (℃) T ｍ W: Temperature of molten iron in the melting furnace of an electric arc furnace (℃) Ｉ W: Amount of cold iron source charged per time from the preheating chamber to the melting furnace of the electric arc furnace (t) ｍ : Amount of molten iron in the melting furnace (t) C: Charging interval of cold iron source (min) 3. The apparatus for manufacturing molten iron as described in claim 2, wherein the control device calculates the charging speed of the cold iron source from the preheating chamber to the electric arc furnace based on either the temperature of the cold iron source in the preheating chamber or the temperature of the molten iron in the electric arc furnace.

4. The apparatus for producing molten iron according to claim 2 or 3, wherein the cold iron source is scrap and reduced iron, and the control device controls the amount of power input from the power source to the electric arc furnace based on the mixing ratio of the scrap and the reduced iron.

5. A molten iron manufacturing apparatus according to any one of claims 2 to 4, wherein the control device controls the power source so that the frequency of the current supplied to the coil satisfies the following equations (4) and (5): f: frequency (Hz) σ: electrical conductivity of the cold iron source at the target preheating temperature (1 / Ωm) μ ｒ : Relative permeability μ of reduced iron at the target preheating temperature ０ : Magnetic permeability of vacuum (4π×10 －７ ) t: Thickness of the cold iron source (m) w: Width of the cold iron source (m) l: Length of the cold iron source (m) 6. The apparatus for producing molten iron according to any one of claims 1 to 5, wherein the winding height and winding diameter of the coil wound around the outer circumferential surface of the preheating chamber satisfy the following formula (6): 0.5≦H_Coil / D_Coil≦2.5 (6) The winding height H_Coil is the distance between the lowest coil in the height direction of the apparatus for producing molten iron and the highest coil in the height direction. The winding diameter D_Coil is the diameter of the coil wound around the outer circumferential surface of the preheating chamber 7.

7. A method for producing molten iron using a molten iron production apparatus having a preheating chamber into which a cold iron source is charged, a gate provided in the preheating chamber, and an electric arc furnace that charges the cold iron source into the preheating chamber by opening the gate and melts the charged cold iron source with arc heat to produce molten iron, wherein the cold iron source filled in the preheating chamber is induction heated to a target preheat temperature by supplying an electric current of a predetermined frequency from a power source to a coil wound around the outer periphery of the preheating chamber.

8. The method for producing molten iron according to claim 7, wherein the cold iron source includes scrap and reduced iron.

9. The bulk density of the cold iron source inside the preheating chamber is set to 1.0 t / m ３ 3.5t / m or more ３ 9. The method for producing molten iron according to claim 7, wherein the filling ratio of the cold iron source inside the preheating chamber is set to 0.8 or more and 1.0 or less.

10. A method for producing molten iron according to any one of claims 7 to 9, wherein the cold iron source is charged from the preheating chamber to the electric arc furnace so that the charging speed of the cold iron source from the preheating chamber to the electric arc furnace satisfies the following formulas (1) to (3): v Ｉ : Charging speed of cold iron source (t / min) P: Amount of electric power input to arc type electric furnace (MW) T ｐｈ : Target preheat temperature of cold iron source (℃) T ｍ W: Temperature of molten iron in the melting furnace of an electric arc furnace (℃) Ｉ W: Amount of cold iron source charged per time from the preheating chamber to the melting furnace of the electric arc furnace (t) ｍ : Amount of molten iron in the melting furnace (t) C: Charging interval of cold iron source (min) 11. A method for producing molten iron according to any one of claims 7 to 10, wherein the frequency satisfies the following formulas (4) and (5): f: frequency (Hz) σ: electrical conductivity of the cold iron source at the target preheating temperature (1 / Ωm) μ ｒ : Relative permeability μ of reduced iron at the target preheating temperature ０ : Magnetic permeability of vacuum (4π×10 －７ ) t: Thickness of the cold iron source (m) w: Width of the cold iron source (m) l: Length of the cold iron source (m)

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