Molten iron manufacturing method

Abstract

Provided is technology for melting a cold iron source with high productivity and a reduced power source unit. A method for producing molten iron according to the present invention includes melting a cold iron source through electric energy using an electric furnace comprising a melting chamber and a preheating chamber. A burner that comprises an injection hole for injecting fuel and an injection hole for injecting a combustion-supporting gas, and that sprays flames towards molten iron in the melting chamber from the injections holes is arranged in the melting chamber, and a powdered auxiliary raw material or an auxiliary material that has been processed into a powder is blown so as to pass through the flames formed by the burner. Furthermore, an exhaust gas generated by combustion in the burner is introduced into the preheating chamber, and when the cold iron source inside the preheating chamber is to be preheated, the supply rate of the fuel or the auxiliary raw material to be used by the burner is adjusted to preheat the cold iron source in the preheating chamber at a prescribed temperature.

Description

[Technical Field]

[0001] The present invention relates to a technology for melting a cold iron source with high productivity and reduced power consumption. [Background technology]

[0002] In recent years, the steel industry has been developing technologies to reduce fossil fuel consumption and CO2 emissions in order to prevent global warming. Conventional integrated steelworks produce molten pig iron by reducing iron ore with carbon. Producing this molten pig iron requires approximately 500 kg of carbon per ton of molten pig iron for the reduction of the iron ore. However, when molten steel is produced using cold iron sources such as iron scrap or solid reduced iron as the primary raw material, the carbon source required for iron ore reduction is eliminated, and only sufficient heat energy is required to melt the cold iron source. This allows for a significant reduction in CO2 emissions.

[0003] Electric furnaces such as arc furnaces and induction melting furnaces are often used in high-mix cold iron source operations. Electric power is used to provide much of the heat required to melt the cold iron source. To improve productivity and reduce power consumption, the following techniques are typically used in arc furnace operations: 1) Support burners are installed on the furnace walls and slag outlets to promote melting of the cold iron source in cold spots, etc. 2) Oxygen is supplied from an oxygen gas supply lance to provide heat for the oxidation of the iron, a process known as oxygen-enriched operation.

[0004] However, oxygen-enriched operation poses the problem of reduced yield due to iron oxidation loss. Furthermore, when using auxiliary burners, the burner flame is formed in the upper part of the furnace body above the surface of the molten iron, so the efficiency of heat transfer to the molten iron in the furnace is low, and most of the supplied heat is emitted as sensible heat in the exhaust gas. Therefore, even if the electricity consumption rate can be reduced, the effect of reducing the total energy input, including fuel, is small. Therefore, a heat-imparting means that can efficiently heat the molten iron and cold iron source in the furnace is desired.

[0005] As a highly efficient heat application method, for example, Patent Documents 1 and 2 disclose a technology in which a lance for introducing powdered ore is installed separately from a top-blowing lance for supplying oxidizing gas in an iron-bath smelting reduction furnace. In this technology, a burner with an ore flow hole and a fuel and oxygen injection hole is provided at the tip of the lance, and the ore is supplied so that it passes through the flame generated by the burner. It has been shown that the ore heated in the flame transfers heat to the molten iron in the furnace, dramatically improving the utilization of burner combustion heat. It has also been shown that the heat transfer from the gas generated by burner combustion to the powder in the burner flame also reduces the temperature of the gas generated by burner combustion, i.e., the exhaust gas temperature. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-138207 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-179876 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the above-mentioned conventional techniques have the following problems. When heating is performed using a burner alone without feeding powder into the burner flame, the efficiency of burner combustion heat transfer to the molten iron in the furnace is low, as described above. Furthermore, the increased sensible heat of the exhaust gas results in heat being discharged outside the furnace. As described in Patent Documents 1 and 2, adding powder and granular material transfers the burner combustion heat to the powder, lowering the temperature of the gas generated by the burner combustion. However, if the powder and granular material supply rate is low, the amount of heat transfer is also low, resulting in a low efficiency of heat transfer to the molten iron in the furnace and a small decrease in the combustion gas temperature. To achieve high-efficiency heat transfer to the molten iron in the furnace and reduce the exhaust gas temperature, Patent Document 2 describes a smelting reduction process in which the powder / fuel ratio S / Q is 0.3 or greater, where S is the powder supply rate (kg / min) and Q is the calorific value of the burner fuel per unit time (MJ / min). This indicates that a sufficient amount of powder and granular material must be supplied relative to the burner combustion heat.

[0008] This means that the amount of heat generated by the burner and the amount of heat that can be imparted to the molten iron in the furnace are limited by the amount of powder and granular material that can be supplied during the refining process. If excess powder and granular material is supplied compared to the amount of powder and granular auxiliary material that is actually required for the refining process, extra sensible heat is required to heat the excess powder and granular material to the molten iron temperature, resulting in a heat loss that exceeds the amount of heat imparted by the burner.

[0009] The present invention has been made in view of the above circumstances, and aims to propose a technology for melting a cold iron source in an electric furnace with high productivity and reduced power consumption. [Means for solving the problem]

[0010] The method for producing molten iron according to the present invention, which advantageously solves the above-mentioned problems, is a method for producing molten iron in which an electric furnace equipped with a melting chamber and a preheating chamber is used to melt a cold iron source using electric energy, the method being characterized in that a burner is disposed in the melting chamber, the burner having an injection hole for ejecting fuel and an injection hole for ejecting combustion-supporting gas, and the burner injects a flame from the injection hole toward the furnace contents in the melting chamber, powdered or powdered auxiliary materials are injected so as to pass through the flame formed by the burner, and exhaust gas produced by combustion in the burner is introduced into a preheating chamber, and in preheating the cold iron source in the preheating chamber, the supply rate of the fuel or auxiliary materials used in the burner is adjusted, thereby preheating the cold iron source in the preheating chamber to a predetermined temperature.

[0011] The method for producing molten iron according to the present invention is as follows: (a) The upper limit of the preheating temperature of the cold iron source is set to 1200°C. (b) the calorific value of the fuel used in the burner per unit time is Q (MJ / min), the supply rate of the auxiliary material is S (kg / min), and the powder fuel ratio S / Q (kg / MJ) is 0.10 or more and 0.50 or less; (c) The electric furnace is an electric furnace equipped with a vertical preheating chamber; This may be a more preferable solution. [Effects of the Invention]

[0012] According to the present invention, by supplying powder and granular material via a burner flame, the powder and granular material is heated within the burner flame and becomes a heat transfer medium. This allows the burner combustion heat to be used efficiently to heat the cold iron source and molten iron in the melting chamber of an electric furnace, thereby reducing power consumption. Furthermore, the high-temperature exhaust gas generated by the burner combustion is conducted into a preheating chamber located separately from the melting chamber where the burner is installed, and is used to heat the cold iron source filled in the preheating chamber. This also allows the sensible heat of the exhaust gas to be effectively utilized. By charging the cold iron source preheated using the sensible heat of the exhaust gas into the melting chamber, a predetermined amount of cold iron source can be melted with a lower power consumption rate than when adding unheated cold iron source. This also shortens the time between tapping molten metal in an electric furnace, thereby improving productivity. Because the sensible heat of the exhaust gas from the burner combustion can be used to preheat the cold iron source, heat radiation to the system is reduced and thermal efficiency is improved.

[0013] In addition, the supply rate of the fuel or auxiliary materials used by the burner is adjusted to preheat the cold iron source in the preheating chamber to a predetermined temperature. Therefore, excessive auxiliary materials are not required, and the combustion heat of the burner can be appropriately distributed between heating the molten metal in the melting chamber and preheating the cold iron source in the preheating chamber. Therefore, the combustion heat of the burner can be used efficiently to melt the cold iron source. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a schematic vertical cross-sectional view showing an overview of an AC arc furnace having a melting chamber and a preheating chamber, as an electric furnace according to one embodiment of the present invention. [Figure 2] FIG. 2 is a schematic vertical cross-sectional view of the tip of the burner lance used in the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0015] The following describes in detail embodiments of the present invention. Note that the drawings are schematic and may differ from the actual embodiments. Furthermore, the following embodiments exemplify devices and methods for embodying the technical concept of the present invention, and are not intended to limit the configuration to those described below. In other words, the technical concept of the present invention can be modified in various ways within the technical scope described in the claims.

[0016] 1 is a vertical cross-sectional view showing an overview of an AC arc furnace 101 as an electric furnace according to one embodiment of the present invention, and illustrates the operating mode of an AC arc type electric furnace. In this embodiment, the electric furnace 101 includes a melting chamber 1 in which iron-based scrap, which is a cold iron source x, is melted by arc heating, and a preheating chamber 2 in which the iron-based scrap x to be supplied to the melting chamber 1 is preheated.

[0017] The top of the melting chamber 1 is covered with an openable, water-cooled furnace lid 4. Near the center of the melting chamber 1, multiple electrodes 5 are inserted from above through the furnace lid 4, forming an arc heating section A that melts the ferrous scrap by generating an arc between these electrodes 5. The electrodes 5 are typically made of graphite or the like and can be moved up and down. Agitation may also be performed by blowing gas into the furnace bottom.

[0018] A shaft-type (vertical) preheating chamber 2 is connected to the upper part of the melting chamber 1, away from the arc heating section A, and this preheating chamber 2 is connected to the melting chamber 1 in a vertical relationship. An openable scrap charging port 20 is provided at the top of this preheating chamber 2. An exhaust port 21 is provided at the upper part of the preheating chamber 2, and this exhaust port 21 is connected to an exhaust duct 6. This exhaust duct 6 is connected to a suction blower (not shown), and the high-temperature exhaust gas generated in the melting chamber 1 flows into the preheating chamber 2 by suction from this suction blower, and after passing through this preheating chamber 2, it is exhausted from the exhaust duct 6. A dust collector (not shown) is provided midway through the exhaust duct 6.

[0019] A bottom-opening supply bucket 13 suspended from a traveling cart 16 can move above the preheating chamber 2, and iron-based scrap x is charged into the preheating chamber 2 from this supply bucket 13 through a scrap charging opening 20.

[0020] A gate 22 is provided at the bottom of the preheating chamber, separating the melting chamber 1 from the preheating chamber 2. The gate 22 has a through hole so that the high-temperature exhaust gas in the melting chamber 1 can be conducted to the preheating chamber. When necessary, the gate is opened and the iron-based scrap x in the preheating chamber 2 is charged into the melting chamber. The iron-based scrap x in the space 1a is naturally pushed out toward the arc heating section A by the weight of the iron-based scrap x filled in the preheating chamber 2 and the space 1a.

[0021] The melting chamber 1 may be provided with an extruder (pusher) facing the space 1a below the preheating chamber 2, for extruding the ferrous scrap x filled in this space 1a toward the arc heating section A by the electrodes 5. This extruder 3 is preferably provided so as to be able to penetrate the side wall of the melting chamber 1 and move back and forth toward the arc heating section A (toward the furnace center in this embodiment), and is driven by a driving device (not shown), with its tip pushing the ferrous scrap x in the space 1a toward the arc heating section A.

[0022] In this embodiment, a burner lance 9 is inserted into the melting chamber 1 through a burner lance insertion hole provided in the furnace lid 4 so that it can be raised and lowered. In the example of FIG. 1, the burner lance 9 is inserted vertically from the furnace lid so that it can be raised and lowered, but this is not limiting. The burner lance 9 may also be inserted obliquely from above the furnace wall toward the furnace interior. Furthermore, the burner is not limited to a lance type that can be raised and lowered, and may have a nozzle fixed to the furnace lid or furnace wall. Furthermore, the burner may be provided with an oxygen supply function so that oxygen can be supplied from the burner. The burner lance 9 sprays a burner flame 9a toward the surface of the furnace contents, such as the cold iron source x and molten iron m, contained in the melting chamber 1.

[0023] An oxygen blowing lance or a carbonaceous material blowing lance may be inserted into the melting chamber 1 from above through the furnace cover 4.

[0024] The carbonaceous material injection lance may inject one or more types of carbonaceous material, such as coke, char, coal, charcoal, or graphite, into the molten slag (s) using air or nitrogen as a carrier gas. Also, oxygen may be supplied (injected) from the oxygen injection lance, and this oxygen may displace the molten slag and be blown into the molten iron (m).

[0025] It is to be noted that instead of pure oxygen, an oxygen-containing gas (for example, a mixed gas of pure oxygen and air) may be blown from the oxygen blowing lance.

[0026] The melting chamber 1 has a tapping port 11 at the bottom of the furnace opposite the side where the preheating chamber 2 is located. A slag tapping port 12 is also provided on the side wall above the tapping port 11. These tapping ports 11 and slag tapping ports 12 are closed by packing sand and mud material filled inside, and by a tapping door 14 and a slag tapping door 15 that hold them down from the outside.

[0027] FIG. 1 shows the state in which iron-based scrap is charged as the cold iron source x, and electricity is turned on to melt the cold iron source x. During this process, powdered auxiliary material 9b is sprayed from burner lance 9 through burner flame 9a to promote the melting of the cold iron source x. In this operation, it is preferable to use a fuel primarily composed of hydrogen gas produced using renewable energy sources such as solar, wind, or hydropower. The term "fuel primarily composed of hydrogen gas" refers to hydrogen gas or a hydrogen-enriched gaseous fuel. The hydrogen-enriched gaseous fuel can be a mixture of hydrogen gas with methane gas, natural gas, or petroleum gas. From the perspective of reducing CO2 emissions, it is preferable to mix hydrogen gas at 50 vol% or more.

[0028] In the above embodiment, an AC arc furnace 101 having three electrodes is used as the electric furnace, but a DC arc furnace having an upper electrode and a lower electrode may also be used. When an arc furnace 101 is used as the electric furnace, the electrodes 5 and the arc are located in the center of the furnace body, and the installation position of the burner lance 9 is limited. As will be explained below, the burner of this embodiment can reduce the temperature of the burner flame 9a by appropriately injecting powdered auxiliary material 9b even when using a fuel mainly composed of hydrogen gas, so it can be operated without damaging the water-cooled panels of the furnace wall or the refractory material of the hearth.

[0029] FIG. 2 shows a schematic diagram of the tip 30 of a burner lance 9 used in the above embodiment. A powder supply pipe 31 with a central injection hole is arranged, surrounded by a fuel supply pipe 32 and a combustion-supporting gas supply pipe 33, both of which also have injection holes. An outer shell 35 with a cooling water passage 34 surrounds the lance. A fuel gas 36 and a combustion-supporting gas 37 are supplied from injection holes on the outer periphery of the powder supply pipe 31 to form a burner flame 9a. Powdered auxiliary material 9b injected from the powder supply pipe 31 is then heated in the burner flame 9a. This allows the powdered auxiliary material 9b to function as a heat transfer medium, improving the efficiency of flame heat transfer to the furnace contents, such as the cold iron source x and molten iron m. This results in reduced power consumption. The combustion-supporting gas 37 can be pure oxygen, a mixture of oxygen with CO2 or an inert gas, air, or oxygen-enriched air. Furthermore, the gas for transporting the powdery auxiliary material 9b can be an inert gas or a combustion-supporting gas.

[0030] In the method for producing molten iron according to this embodiment, for example, first, cold iron sources x, such as ferrous scrap, serving as the main raw material, are charged from a supply bucket 13 into the melting chamber 1 and preheating chamber 2 of an AC arc furnace 101 shown in FIG. 1 . After the initial cold iron sources x are charged into the melting chamber 1, electricity is turned on. Then, a burner lance 9 installed at the top of the furnace is inserted into the melting chamber 1, and the cold iron sources x are heated by electric power and the combustion heat of a burner flame 9a. Exhaust gas is passed into the preheating chamber 2 and used to preheat the cold iron sources x in the preheating chamber 2.

[0031] As the melting of the initially charged cold iron sources x progresses further and a flat bath is formed (even unmelted cold iron sources x are immersed in the molten iron m), slag is removed from the slag outlet 12 as needed, and then the gate 22 is opened and the cold iron sources x in the preheating chamber 2 are charged into the melting chamber 1. After the cold iron sources x in the preheating chamber 2 have been charged, additional cold iron sources x are charged into the preheating chamber 2 from above the preheating chamber 2. From the viewpoint of ensuring productivity, it is preferable to continuously apply electricity and heat with a burner while the cold iron sources x are being added from the preheating chamber 2 and while new cold iron sources x are being added to the preheating chamber 2. The number of times the cold iron sources x are added may be three or more.

[0032] The inventors used an AC arc furnace equipped with a melting chamber and a preheating chamber as shown in Figure 1, and a conventional AC arc furnace without a preheating chamber, and varied the fuel gas flow rate and powder supply rate to investigate the heat transfer efficiency to the furnace contents and the preheating temperature of the cold iron source when an AC arc furnace equipped with a preheating chamber was used. Here, the ratio of the supply rate S (kg / min) of the powdered auxiliary material 9b to the calorific value Q (MJ / min) per unit time of the fuel 36 used in the burner lance 9 is defined as the powder fuel ratio S / Q.

[0033] As a result, in an AC arc furnace with a preheating chamber, when a sufficient amount of powder and granular material is supplied relative to the calorific value of the fuel gas, the burner flame temperature is below 1500°C when the powder-to-fuel ratio S / Q is in the range of 0.30 to 0.50 (kg / MJ). Furthermore, improved heat transfer efficiency to the furnace contents, reduced unit power consumption, and improved productivity were confirmed. These effects were even greater than those of an AC arc furnace without a preheating chamber at the same powder-to-fuel ratio. This is thought to be due to the effective contribution of the sensible heat of the burner combustion gas to the preheating of the cold iron source in the preheating chamber, further reducing the sensible heat of the exhaust gas discharged outside the furnace. When the powder-to-fuel ratio S / Q exceeds 0.50 (kg / MJ), i.e., when the calorific value of the fuel is insufficient relative to the amount of powder and granular material supplied, the reduction in unit power consumption and improved productivity may be diminished.

[0034] Furthermore, power consumption reductions and productivity improvements were confirmed even when the calorific value of the fuel was excessive relative to the amount of powder supplied, specifically, when the powder-to-fuel ratio S / Q was below 0.30 (kg / MJ). Typically, when an electric arc furnace without a preheating chamber is operated at this range of powder-to-fuel ratio S / Q, the exhaust gas temperature becomes too high, resulting in low heat transfer efficiency to the furnace contents and reduced power consumption reductions and productivity improvements. On the other hand, in an electric arc furnace with a preheating chamber, the efficiency of the burner combustion heat contributing to heating the powder is low. Although the burner flame and exhaust gas temperatures reach high temperatures of over 1500°C, they are still able to preheat the cold iron source in the preheating chamber. This allows for low sensible heat of the exhaust gas discharged outside the furnace. In other words, we found that highly efficient use of burner combustion heat is possible without constraints on the powder supply required for refining. However, when the pulverized fuel ratio S / Q was further reduced, the preheating temperature of the cold iron sources in the preheating chamber increased, and when the preheating temperature exceeded 1200°C, the cold iron sources in the preheating chamber were likely to weld together, making it difficult to remove them from the preheating chamber. Therefore, it is preferable to set the upper limit of the preheating temperature of the cold iron sources in the preheating chamber to 1200°C. Furthermore, as a result of investigation, it was found that the preheating temperature of the cold iron sources in the preheating chamber is 1200°C or less when the pulverized fuel ratio S / Q is 0.10 (kg / MJ) or more. While there is no specific lower limit for the preheating temperature of the cold iron sources in the preheating chamber, from the viewpoint of thermal efficiency, it is preferable that the preheating temperature exceed the temperature of the cold iron sources at the time of charging into the preheating chamber. A preheating temperature of 300°C or more is more preferable, and a preheating temperature of more than 500°C is even more preferable. In order to adjust the preheating temperature of the cold iron source in the preheating chamber to an appropriate temperature, it is preferable to adjust the flow rate of air drawn in from the surroundings by adjusting the flow rate of the exhaust fan or the width of the opening on the furnace body side of the electric furnace.

[0035] Powder species can be slag formers, dust, etc., which are powder or powdered auxiliary materials 9b. To efficiently heat the auxiliary materials in the burner flame, it is necessary to increase the specific surface area, and a particle size of approximately 100 μm or less is preferable. If the particle size of the auxiliary material is large, it is preferable to process the particle size to approximately 100 μm or less by pulverization or the like. Here, the particle size is expressed as a 50% pass rate based on volume.

[0036] As the cold iron source x, it is preferable to use solid reduced iron reduced with iron-based scrap or a reducing agent that reduces CO2 emissions. Depending on the brand, solid reduced iron contains approximately 10 to 20 mass% of gangue derived from iron ore as SiO2 or Al2O3. When the solid reduced iron is melted, it becomes slag s, which exists on the surface of the molten iron m. This slag s has a high melting point and is prone to solidification, adhering to the furnace walls and potentially interfering with operation.

[0037] Therefore, it is preferable to use lime as the powdered auxiliary material 9b heated by the burner and supplied, since this allows the basicity of the slag s, i.e., the mass ratio CaO / SiO2, to be controlled to approximately 1.0. This lowers the melting point of the slag s and prevents it from solidifying. Furthermore, the heated powder provides heat to the slag s, which promotes slag formation. After the slag s has been formed into slag as described above, a slag discharge port may be opened and the slag may be discharged during melting or before the molten metal is poured out.

[0038] Any electric furnace can be used as long as it uses electrical energy to melt a cold iron source and obtain molten iron. For example, the arc furnace may be not only an AC or DC arc furnace as described above, but also an immersion arc furnace in which heating is performed by immersing a Seeberg self-baking electrode or the like in the slag. It may also be an indirect resistance furnace in which the heated material is heated by radiation from a heating element installed in the furnace or by convection and conduction heat transfer within the furnace. It may also be a plasma arc melting furnace.

[0039] The molten iron m melted in this embodiment has a composition equivalent to that of the iron-based scrap and solid reduced iron used as the main raw material, and is usually molten steel with a relatively low C content. To adjust the composition, alloy addition may be performed directly in the electric furnace where the molten iron was melted, or a finish decarburization treatment or dephosphorization treatment using oxygen refining may be performed. Furthermore, after tapping, secondary refining such as molten steel desulfurization and vacuum degassing may be performed. Subsequently, semi-finished products such as cast pieces are produced through a casting process such as continuous casting. [Example]

[0040] A cold iron source melting test was conducted using an AC arc furnace (A) without a preheating chamber, and an AC arc furnace (B) equipped with a melting chamber and a preheating chamber similar to that shown in Figure 1. Scrap was used as the cold iron source, and the total charge amount was 100 t.

[0041] Each electric furnace was fitted with a burner lance equipped with a fuel supply line and an oxygen supply line on the furnace lid, and the tip of the burner lance had a multi-tube structure similar to that shown in Figure 2. Propane gas was used as the burner fuel. Comparisons were made between no burner, heating the furnace contents with the burner flame alone while supplying burner fuel but no powder, and injecting powdered lime into the burner flame. The tapping temperature was 1650°C.

[0042] After the start of current application, the melting of the initial cold iron source progressed and the height of the charge in the furnace decreased, and when a space was created in the upper part of the furnace, the burner lance was lowered and heating by the burner flame was also used. Argon gas was used as the carrier gas for supplying the powder. A total of 50 kg / t of powdered lime was fed into the electric furnace at a feed rate of S = 100 kg / min. Propane gas was used as the fuel gas at 2.2 to 14 Nm 3 The pulverized fuel ratio (S / Q) ​​was varied from 0.08 to 0.51 kg / MJ for each heat of the electric furnace. Oxygen gas was supplied as a combustion support gas to burn the propane fuel gas in each heat. The initial cold iron source melting progressed further until a flat bath was reached (any unmelted cold iron source was immersed in the molten metal). After the slag was removed from the slag outlet, the power and burner were turned off, the furnace lid was opened, and the second and subsequent cold iron sources were charged. After the second cold iron source was charged, the power was turned on again, and operation was continued as after the initial charging. Finally, molten steel at 1650°C was obtained and tapped into the ladle.

[0043] For each processing condition, a comparison was made of the power consumption rate, electric furnace processing time, and burner combustion heat transfer efficiency. The power consumption rate was calculated by dividing the amount of power used under each processing condition by the amount of power used in Process No. 1. The electric furnace processing time was the time (minutes) from the start of power supply to the start of tapping. The burner combustion heat transfer efficiency represents the ratio of the heat generated by the burner fuel transferred to the furnace contents. In the case of the AC arc furnace (B), the temperature of the scrap at the bottom of the preheating chamber just before it was placed in the furnace was measured with a radiation thermometer, and this temperature was used as the cold iron source preheat temperature. The results are shown in Table 1, which also lists the type of electric furnace and the burner specifications.

[0044] [Table 1]

[0045] In Process No. 2, an AC arc furnace without a preheating chamber (A) was used, and the contents of the furnace were heated solely by the burner flame, compared to Process No. 1, which did not use a burner. The burner combustion heat was not effectively transferred, resulting in nearly identical power consumption and electric furnace processing time. Processes No. 3 to 12, in which the powdered lime was heated within the burner flame, resulted in reduced power consumption and electric furnace processing time. This is because the powdered lime was heated within the burner flame, and some of the burner combustion heat was transferred to the contents of the furnace. However, under conditions where the calorific value of the fuel was excessive relative to the powdered lime supply rate (Processes No. 7 to 12), the efficiency of the burner combustion heat transfer decreased, and the reduction in power consumption and electric furnace processing time was reduced. This is because the amount of heat transferred to the powdered lime reached a plateau, and an increased proportion of the burner combustion heat was emitted as sensible heat in the exhaust gas.

[0046] In Processes No. 13–22, in which powdered lime was heated in a burner flame in an AC arc furnace (B) equipped with a melting chamber and a preheating chamber, the unit power consumption and electric furnace processing time were reduced, similar to Processes No. 3–12, in which powdered lime was heated in a burner flame in an AC arc furnace (A) without a preheating chamber. Processes No. 13–22 were even more effective than Processes No. 3–12 when compared at the same powder fuel ratio S / Q. Furthermore, in Processes No. 7–12, in the AC arc furnace (A) without a preheating chamber, where the fuel calorific value was excessive relative to the powdered lime feed rate, the burner heat transfer efficiency decreased, and the reduction in unit power consumption and electric furnace processing time plateaued. In Processes No. 17–22, performed in an AC arc furnace (B) equipped with a preheating chamber, the burner heat transfer efficiency did not decrease, even at the same powder fuel ratio S / Q. Furthermore, an increase in fuel calorific value resulted in a reduction in unit power consumption and a reduction in electric furnace processing time.

[0047] This is because, even if the heat transfer rate to the powdered lime reaches a plateau when the calorific value of the fuel is excessive relative to the powdered lime feed rate, and the burner combustion gas temperature becomes high, the gas is passed through the preheating chamber and used to preheat the scrap in the preheating chamber. This results in a low final sensible heat of the exhaust gas. Furthermore, as the calorific value of the fuel relative to the powder feed rate increases, the preheating temperature of the cold iron source in the preheating chamber also increases. When the preheating temperature of the cold iron source exceeded 1200°C, scrap pieces in the preheating chamber welded together, preventing them from being loaded into the furnace even when the gate at the bottom of the preheating chamber was opened, resulting in operational disruptions. It is preferable to keep the preheating temperature of the scrap in the preheating chamber below 1200°C.

[0048] The unit of mass "t" used in this specification is 10 3 The unit "N" attached to the unit of gas volume represents the volume under standard conditions of 0°C temperature and 101325 Pa pressure. [Industrial Applicability]

[0049] According to the method for producing molten iron of the present invention, the heat transfer efficiency is improved, and a cold iron source can be melted using a heat source with reduced CO2 emissions, which reduces the electricity consumption rate and the environmental load, making it industrially useful. The method is suitable for application to processes such as refining furnaces that require a heat source with reduced CO2 emissions and the addition of powdered auxiliary materials. [Explanation of symbols]

[0050] 1 Melting chamber 1a Spatial part 2 Preheating chamber 3. Extruder 4 Hearth lid 5 electrodes 6 Exhaust duct 7 Oxygen blowing lance 8 Carbon injection lance 9 Burner Lance 9a Burner flame 9b (powder) auxiliary raw materials 10 Furnace wall 11 Tap 12 Slag outlet 13 Feeding bucket 14 Tap door 15 Slag discharge door 16 Traveling cart 20 Scrap loading port 21 Exhaust port Gate 22 30 Burner lance tip (nozzle) 31 Powder supply pipe 32 Fuel supply pipe 33 Combustion-supporting gas supply pipe 34 Cooling water passage 35 outer shell 36 Fuel Gas 37 Combustion-supporting gas 38 Cooling water 101 (AC) Arc furnace (electric furnace) x Cold iron source (iron scrap) m molten iron s (molten) slag A Arc heating section

Claims

1. A method for producing molten iron by melting a cold iron source using electric energy in an electric furnace equipped with a melting chamber and a preheating chamber, a burner provided in the melting chamber, the burner having an injection hole for injecting fuel and an injection hole for injecting combustion-supporting gas, the burner injecting flames from the injection holes toward the furnace contents in the melting chamber; A powdered or powdered auxiliary material is blown into the flame formed by the burner, Furthermore, exhaust gas generated by combustion of the burner is guided into a preheating chamber to preheat the cold iron source in the preheating chamber, The auxiliary raw material is at least one of a slag former and dust, a supply rate of the fuel or the auxiliary material used in the burner is adjusted to preheat the cold iron source in the preheating chamber to a predetermined temperature.

2. When the upper limit of the preheating temperature of the cold iron source is set to 1200°C, 2. The method for producing molten iron according to claim 1, wherein the preheating temperature is the temperature of the cold iron source at the bottom of the preheating chamber immediately before charging into the melting chamber.

3. 2. The method for producing molten iron according to claim 1, wherein a calorific value of the fuel used in the burner per unit time is Q (MJ / min), a supply rate of the auxiliary materials is S (kg / min), and a powder fuel ratio S / Q (kg / MJ) is set to be 0.10 or more and 0.50 or less.

4. 2. The method for producing molten iron according to claim 1, wherein the electric furnace is an electric furnace equipped with a vertical preheating chamber.

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