Electric furnace equipment

Abstract

The present invention provides electric furnace equipment that prevents overheating of molten metal by changing heat transfer between the molten metal and slag from heat conduction to forced convective heat transfer, and that improves production yield without making the equipment oversized. This electric furnace equipment stores molten metal, slag, a metal raw material, and an auxiliary raw material in layers, and enables the discharge of the molten metal and the slag. The electric furnace equipment comprises: an arc furnace having a bottom part, a furnace wall, and a furnace lid; and an electromagnetic stirrer disposed at a prescribed position on the bottom part or a side surface of the arc furnace in order to enable stirring of the molten metal that is in the arc furnace.

Description

[Technical Field]

[0001] This invention relates to an electric furnace apparatus for producing molten metal by melting a metal raw material containing reduced iron. In the following description, "t" is the unit of mass. 3 The unit "L" represents kilograms and is a unit of volume. -3 m 3 This represents the standard conditions of 0°C and 101325 Pa, indicated by "N" before the unit of gas volume. In this specification, "x~y" representing a numerical range means x or greater and y or less, including boundary values. [Background technology]

[0002] In recent years, there has been a growing demand to reduce CO2 emissions in order to lessen the environmental burden. In the steel industry, the direct reduction (DR) method for producing iron sources is attracting attention as an alternative to the blast furnace method, which emits a large amount of CO2. In the DR method, for example, iron-containing agglomerates are reduced in a shaft furnace to produce directly reduced iron (DRI). This reduced iron and other iron source materials such as iron scrap are charged into an electric arc furnace (EAF) or submerged arc furnace (SAF) and heated and melted, and molten iron is produced after separating the slag.

[0003] In Japan, studies are underway to apply the DR (Dry Reduction) method to low-grade iron ore produced in Australia, India, and other countries. Currently, it is difficult to melt and process reduced iron produced from low-grade iron ore using the EAF (Earth-Axial Fountain) method. Therefore, the use of a submerged arc furnace (SAF) is being considered.

[0004] The production of molten iron by SAF is a process in which metallic materials and auxiliary materials such as scrap, pig iron, directly reduced iron (DRI), molten iron, or hot briquetteed iron (HBI) are continuously supplied into a submerged arc furnace (SAF) without opening the furnace lid, or charged in small buckets. This SAF melting process may also be carried out in flat-bath operation. During the charging of raw materials, electrodes are inserted into the molten slag in the submerged arc furnace, and arcs are generated between multiple electrodes or between electrodes and molten iron. An electric current is also passed between the raw materials and the slag, heating and melting the raw materials through resistance heating and heat transfer.

[0005] One problem with the SAF melting process is the inefficient heat transfer to raw materials located far from the electrodes. This leads to delayed melting of the metal raw materials, causing problems such as concentration gradients, unreliable temperature measurements, unreliable process control, and over-temperature molten metal tapping. To solve this non-uniform temperature problem, the electromagnetic stirrer disclosed in Patent Document 1 has been developed. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Special Publication No. 2020-505579 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, the conventional technology had the following problems. Specifically, the technology described in Patent Document 1 is a technology applied to so-called electric arc furnaces. Electric arc furnaces are operated with a low slag ratio and a high Fe content. It cannot be directly applied to a technology in which molten metal, slag, metal raw materials, and auxiliary materials are stacked and contained in an electric furnace, and the molten metal and slag are discharged individually and continuously while operating in a reducing atmosphere.

[0008] This invention has been made in view of the above circumstances, and aims to provide an electric furnace system that prevents overheating of the molten metal by changing the heat transfer between the molten metal and the slag from thermal conduction to forced convection, thereby improving productivity without requiring excessive equipment. [Means for solving the problem]

[0009] The electric furnace equipment according to the present invention, which advantageously solves the above problems, is an electric furnace equipment that stacks and houses molten metal, slag, metal raw materials and auxiliary raw materials, and is capable of discharging molten metal and slag, and is characterized by comprising: an arc furnace having a bottom, a furnace wall and a furnace lid; and an electromagnetic stirrer positioned at a predetermined location on the bottom or side of the arc furnace in order to enable stirring of the molten metal in the arc furnace.

[0010] Furthermore, the electric furnace equipment according to the present invention is (a) It must be possible to dissolve it without intentionally supplying oxygen gas. (b) The electromagnetic stirrer has at least one coil configured to generate a linearly moving magnetic field in one direction, a moving magnetic field directed from the center of the furnace bottom toward the furnace wall or from the furnace wall toward the center of the furnace bottom, and a magnetic field that generates a circumferential rotational flow in the molten metal, or a combination of these functions. (c) The outlet for molten metal and the outlet for slag are provided on the furnace walls in different directions with respect to the center of the furnace bottom. (d) The arc furnace has an input power of 300 kW / m² per surface of molten metal. 2 The electromagnetic stirrer has the above capabilities, and the stirring power density is in the range of 4 to 80 W / t. (e) The apparatus shall be capable of mixing the metal raw materials and the auxiliary raw materials, or supplying them individually. (f) Part or all of the metal raw materials are reduced iron, (g) The basicity of the slag is in the range of 1.0 to 1.5, where the basicity of the slag is the ratio of CaO to SiO2 in the slag on a mass basis. (h) The arc furnace has a control device that controls the arc voltage, which is the voltage to ground, and the current per electrode, and the control device is configured to adjust at least one of the electrode height, arc voltage, and arc current such that the value E / √I obtained by dividing the arc voltage E(V) by the square root of the current I(A) per arc electrode is 2.0 or less. (i) The arc furnace is a submerged arc furnace, and the like can serve as more preferable solutions. [Effects of the Invention]

[0011] According to the electric arc furnace facility of the present invention, an arc furnace and an electromagnetic stirrer provided at the bottom of the arc furnace appropriately stir molten metal, so that sufficient input power can be loaded, and an increase in temperature of the molten metal can be suppressed. In addition, molten metal can be produced with high productivity while suppressing wear of refractory. [Brief Description of Drawings]

[0012] [Figure 1] Fig. 1 is a schematic vertical sectional view illustrating an electric arc furnace facility according to an embodiment of the present invention. [Figure 2] (a) to (g) are schematic top views showing layout examples of electromagnetic stirrers in the electric arc furnace facility according to the above embodiment. [Figure 3] Fig. 3 is a graph showing the influence of the stirring power density of molten pig iron on the relationship between input power per surface area of molten pig iron in an electric arc furnace and molten pig iron temperature. [Figure 4] Fig. 4 is a schematic vertical sectional view showing a submerged arc furnace having no molten metal stirring function. [Mode for Carrying Out the Invention]

[0013] Hereinafter, embodiments of the present invention will be specifically described. The following embodiments illustrate facilities and methods for embodying the technical idea of the present invention, and do not limit the configuration to the following. That is, various modifications can be made to the technical idea of the present invention within the technical scope described in the claims.

[0014] FIG. 1 is a schematic vertical cross-sectional view illustrating the configuration of a submerged arc furnace for explaining an electric furnace facility according to an embodiment of the present invention. FIGS. 2(a) to 2(g) are schematic top views showing arrangement examples of an electromagnetic stirrer 6 formed of a linear motor. The submerged arc furnace 1 contains, stacked within a furnace wall 2, hot metal P, molten slag S, and a raw material M composed of a metal raw material and auxiliary raw materials. In the submerged arc furnace 1, electrodes 3 are inserted into the molten slag S, an arc is generated between the plurality of electrodes 3 or between the electrodes 3 and the hot metal P, and the raw material is heated and melted without intentionally supplying oxygen gas by radiant heating, or by resistance heating of current flowing between the electrode, the raw material, the slag, and the hot metal. The phrase "without intentionally supplying oxygen gas" does not preclude the mixing of air from sources such as during raw material charging, injected gas, or gaps in refractories. It means that oxidation heat is not used for heating the raw material.

[0015] In this embodiment, it is preferable to have an apparatus (not shown) for charging the raw material 4 from above, for example, a hopper, and continuously produce molten metal. The raw material charging positions are preferably arranged symmetrically with respect to the center of the furnace bottom. In this case, it is preferable to have a discharge port for molten metal and a discharge port for slag, which are arranged in different directions with respect to the center of the furnace bottom. It is preferable to discharge the molten metal by plugging each discharge port with a plugging material, and when a predetermined amount of molten metal is retained, penetrating the plugging material to open the discharge port.

[0016] In this embodiment, an iron source raw material containing reduced iron is charged into the submerged arc furnace. Along with this, the method includes at least one or both of adding a slag-forming material as an auxiliary raw material to adjust the basicity (CaO / SiO₂) of the molten slag S formed on the hot metal P, and adding a carbonaceous material to adjust the C content of the hot metal P. The basicity (CaO / SiO₂) of molten slag refers to the ratio of CaO to SiO₂ in terms of mass fraction in the molten slag. The apparatus for charging the raw material 4 may be prepared for each of the iron source raw material, the slag-forming material, and the carbonaceous material, or may supply them as a mixture. When mixing, the mixture may be granulated to a predetermined size.

[0017] The reduced iron used in this embodiment may be manufactured in advance by a direct reduction method, or commercially available reduced iron may be purchased and used. Iron scrap or scale can also be used as the iron source material. The mass ratio of reduced iron to the iron source material is preferably 50 to 100%.

[0018] A metallization rate of 60% or higher for reduced iron results in a lower power consumption per unit area and excellent energy efficiency. There is no particular upper limit to the metallization rate. If the metallization rate is too high, in the submerged arc furnace 1, the reduced iron, which is the raw material 4 introduced from above the molten slag S, may conduct electricity within the slag, potentially reducing the heat of resistance. Therefore, it is preferable to set the upper limit of the metallization rate to around 90%.

[0019] It is preferable to add limestone (CaCO3) or quicklime (CaO) as a CaO source and silica (SiO2) as an SiO2 source to the slag-forming material. It is preferable to use it in the form of lumps or powders, together with or mixed with metal raw materials, for example, by granulation. It is preferable to set the basicity (CaO / SiO2) of the molten slag S to the range of 1.0 to 1.5. When the basicity of the molten slag is within this range, the composition is similar to that of blast furnace slag, making it suitable for reuse as roadbed material such as cement. It is also preferable that the total iron content in the molten slag be 5.0% by mass or less. This can be achieved by intentionally not supplying oxygen gas, and the iron yield is improved.

[0020] The carbon material can be coke or coal. Biomass charcoal may also be used. It is preferable to use it in lump form or powder form, together with or mixed with metal raw materials, for example, by granulation. The carbon content in molten iron P is preferably adjusted to a range of 2.0 to 5% by mass. Within this range, it is suitable for use as pig iron, either as is or as a raw material for steelmaking in the next process.

[0021] In this embodiment, electrical energy is used to melt the metal raw material, for example, reduced iron. Therefore, a reduction in CO2 emissions can be obtained compared to cases that use the heat of carbon combustion, such as the blast furnace method. It is preferable to use electrical energy generated from renewable energy sources.

[0022] If an electric furnace is operated in the same manner as in this embodiment using a submerged arc furnace that does not have a molten metal stirring function, as shown in Figure 4, then, because there is no external supply of oxygen in the submerged arc furnace and no solid-liquid flow occurs within the furnace body, heat transfer from the molten iron P to the raw material M via the molten slag S is almost entirely by thermal conduction. Also, the electrode 3 is usually positioned in the center when viewed from above. Therefore, the heat supply to the raw material M located far from the electrode is limited, and even if the input power is increased, most of the input power is consumed in heating the molten metal, resulting in less input power being used to melt the raw material M, especially the heat supply to the raw material M located far from the electrode 3. The power density per molten iron surface is 250 kW / m³. 2 At this rate, the molten iron temperature will exceed 1500°C. Therefore, a typical submerged arc furnace has a power density of 250 kW / m³ per molten iron surface. 2 In many cases, these furnaces are operated at a limited capacity. To produce large quantities of molten metal using such furnaces would require large-area facilities and numerous pieces of equipment, resulting in excessive investment.

[0023] After diligent research, the inventors discovered that the relationship between the power density per unit area of ​​molten iron surface and the temperature of the molten iron P depends on the stirring power ε. Figure 3 shows the results of this research. For example, in a typical submerged arc furnace, the stirring power density ε = 0, and in a conventional submerged arc furnace, 250 kW / m 2 This also explains why the furnace can be operated at this level. Furthermore, we have made a novel discovery that by adding even a small amount of stirring power, a large amount of electricity can be supplied to the furnace without raising the temperature of the molten iron P.

[0024] The submerged arc furnace 1 of the present embodiment comprises a bottom, a furnace wall 2, a furnace cover (not shown), and an electromagnetic stirrer 6 serving as a stirring means for molten metal. The submerged arc furnace 1 of the present embodiment has an input power per surface area of molten metal of 300 kW / m 2 or higher, which is preferable. Further, it is preferable that the stirring power density of the electromagnetic stirrer 6 is in the range of 4 to 80 W / t. By this arrangement, high productivity and stable operation can be achieved without excessive capital investment. The input power per surface area of molten metal is 300 to 2500 kW / m 2 , which is more preferably within this range. It is more preferable that the stirring power density is in the range of 8 to 24 W / t.

[0025] If the stirring power density is below the lower limit, the stirring power density is too low, and overheating of the molten metal within the required power density range may lead to progressing wear of refractory such as the furnace wall. If the stirring power density exceeds the upper limit, the flow velocity of the molten metal is too high, which may also lead to progressing wear of the refractory.

[0026] As an example of the stirring means for molten metal, it is preferable to install the electromagnetic stirrer 6 as shown in Fig. 1 on the bottom or side of the furnace. The bottom or side where the electromagnetic stirrer 6 is installed is preferably made of a non-magnetic material. The stirring power density obtained by electromagnetic stirring can be determined with respect to the output of the electromagnetic stirring by, for example, numerical analysis or experimental methods. For example, it can be set to approximately 0.5% of the output power of the electromagnetic stirring.

[0027] In this embodiment, it is preferable that the flow F of molten metal improves heat transfer to the slag-metal interface. In addition, it is preferable that the molten metal around the electrode, which becomes a hot spot, flows toward the furnace wall. For example, as shown in Figure 2(a), an electromagnetic stirrer 6 that generates a magnetic field can be installed at the furnace bottom or on the furnace wall to cause the molten metal at the bottom of the furnace to flow toward the furnace wall from the center of the furnace bottom. Also, as shown in Figure 2(b), an electromagnetic stirrer 6 that generates a magnetic field can be installed at the furnace bottom or on the furnace wall to cause the molten metal at the bottom of the furnace to flow toward the center of the furnace wall from the furnace wall. Furthermore, as shown in Figure 2(c), electromagnetic stirrers 6 can be placed opposite each other across the center of the furnace bottom to generate magnetic fields moving in different directions. Also, as shown in Figure 2(d), electromagnetic stirrers 6 can be placed circumferentially to generate a swirling flow in the molten metal. A linear magnetic field moving in one direction can also be generated as shown in Figures 2(e) and 2(f). The shape of the electric furnace is not limited to a circle when viewed from above, but may be an ellipse or a rectangle as shown in Figures 2(f) and (g). As shown in Figure 2(g), the magnetic fields of adjacent electromagnetic stirrers 6 may be directed in opposite directions.

[0028] The submerged arc furnace 1 of this embodiment preferably has a control device that controls the arc voltage, which is the voltage to ground, and the current per electrode. In a submerged arc furnace, since current flows between the electrode, raw material, slag, and molten iron, the voltage to ground at the same current value is generally lower compared to an electric furnace that generates an arc in air. This is because the electrical conductivity of slag is significantly higher than that of air. Therefore, the control device of the submerged arc furnace 1 of this embodiment controls the value E / √I, which is obtained by dividing the arc voltage E(V) by the square root of the current I(A) per arc electrode, as an indicator. That is, it is preferable that the control device is configured to adjust at least one of the electrode height, arc voltage, and arc current so that the indicator E / √I is 2.0 or less. More preferably, the indicator E / √I is 1.5 or less. [Examples]

[0029] The investigation was conducted using a continuous submerged arc furnace 1 with a scale of 4-6 tons, as shown in Figure 1. Table 1 summarizes the specifications of the three-phase AC electric furnace used in the investigation.

[0030] [Table 1]

[0031] In submerged arc furnace 1, shredder scraps, carbon, and blast furnace slag were charged and power was applied for initial melting. The pre-charged amounts were adjusted to produce 750 kg of molten iron and 1500 kg of slag. After the pre-charged materials had completely melted, reduced iron as the main raw material and pre-mixed raw materials such as carbon, lime, alumina, and MgO sources to adjust the carbon concentration in the molten iron and the composition of the slag were continuously fed in. The maximum amount of molten iron was targeted at 4 tons, and this was adjusted by the amount of raw materials fed in. The final thickness of the molten iron was 140 mm.

[0032] The input power of the submerged arc electric furnace was adjusted by tap voltage and electrode position to ensure the power met the predetermined conditions.

[0033] The transformer used for the electromagnetic stirring device was 200 kVA, and its output was adjusted. The power factor at maximum output was approximately 50%, and a maximum output of approximately 100 kW was possible. Since it is difficult to measure the contribution rate of electromagnetic stirring directly, it was calculated in advance using a numerical analysis method and set to 0.5%. This value is a typical value for electromagnetic stirring devices and does not change significantly. The arrangement of the electromagnetic stirring device was as shown in Figure 2(e). The operating conditions and results are shown in Table 2.

[0034] In the evaluation column, a "○" was given if the amount of refractory material loss (mm) was less than twice the amount of loss under the conditions of Test No. 1 (with the loss under Test No. 1 being set to 1), and if 4 tons of molten iron could be produced within 100 minutes. A "△" was given if, even with the same input power to the electric furnace, the melting time was shortened or the amount of refractory material loss decreased due to electromagnetic stirring. The value of the operational index E / √I is also shown in Table 2.

[0035] [Table 2] [Explanation of Symbols]

[0036] 1. Electric furnace (submerged arc furnace, SAF) 2 Furnace wall 3 electrodes 4 (Input) Raw materials 5 Arc 6. Electromagnetic stirrer (linear motor) P hot metal S Molten slag M Raw material

Claims

1. An electric furnace facility that stores molten metal, slag, metal raw materials, and auxiliary raw materials in layers, and is capable of discharging molten metal and slag, An arc furnace having a bottom, a furnace wall and a furnace lid, To enable stirring of the molten metal in the arc furnace, the arc furnace is equipped with an electromagnetic stirrer positioned at a predetermined location on the bottom or side of the arc furnace, The aforementioned arc furnace is a submerged arc furnace, Input power per surface area of ​​molten metal is 300 kW / m². ２ Having the above abilities, The aforementioned electromagnetic stirrer is an electric furnace facility with a stirring power density in the range of 4 to 50 W / t.

2. The electric furnace equipment according to claim 1, which is capable of dissolving without intentionally supplying oxygen gas.

3. The electric furnace equipment according to claim 1, wherein the electromagnetic stirrer has at least one coil configured to perform one of the following actions, or a combination of them, of generating a linearly moving magnetic field in one direction, generating a moving magnetic field from the center of the furnace bottom toward the furnace wall or from the furnace wall toward the center of the furnace bottom, and generating a magnetic field such that a circumferential rotational flow is obtained in the molten metal.

4. The electric furnace equipment according to claim 1, wherein the molten metal discharge port and the slag discharge port are provided on furnace walls in different directions with respect to the center of the furnace bottom.

5. The electric furnace apparatus according to claim 1, comprising a device capable of mixing the metal raw material and the auxiliary raw material, or supplying them individually.

6. The electric furnace equipment according to claim 5, wherein part or all of the aforementioned metal raw materials are reduced iron.

7. The basicity of the slag is in the range of 1.0 to 1.5, where the basicity of the slag is the amount of SiO in the slag on a mass basis. ２ The electric furnace equipment according to claim 1, wherein the ratio of CaO to [amount].

8. The arc furnace has a control device that controls the arc voltage, which is the voltage to ground, and the current per electrode. The electric furnace equipment according to claim 1, wherein the control device is configured to adjust at least one of the electrode height, arc voltage, and arc current such that the value E / √I obtained by dividing the arc voltage E(V) by the square root of the current I(A) per arc electrode is 2.0 or less.

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