Method for operating electric furnace

Abstract

A method for operating an electric furnace is provided, in which, when a raw metallic material is melted in the electric furnace to produce a molten metal, high productivity is obtained and the refractory wear is less. The method for operating an electric furnace comprises stacking and accommodating, inside the electric furnace, a molten metal, slag, a raw metallic material, and a subsidiary raw material in this order and continuously discharging a molten metal and slag, wherein: the electric furnace comprises a bottom part, furnace walls, a furnace lid, and a means for molten-metal stirring; and the input power per surface of the molten metal is 300 kW / m2 or greater and the density of the power of stirring the molten metal by the stirring means is in the range of 5-100 W / t.

Description

Electric furnace operation method

[0001] The present invention relates to a method for operating an electric furnace, particularly a submerged arc furnace, in which a metal raw material containing reduced iron is melted in the electric furnace to produce molten metal. ３ kg, and the volume unit "L" is 10 －３ m ３ The "N" attached to the unit of gas volume indicates the standard state of 0°C and 101,325 Pa. In this specification, the numerical range "x to y" indicates not less than x and not more than y, and includes the boundary value.

[0002] In recent years, CO ２ In the steel industry, CO emissions are also being reduced. ２ As an alternative to the blast furnace process, which has a large amount of iron emissions, the direct reduction (DR) process for producing iron sources has been attracting attention. In the DR process, for example, iron-containing agglomerates are reduced in a shaft furnace to produce direct reduced iron (DRI). This reduced iron or iron scrap or other iron source raw materials are charged into an electric arc furnace (EAF), submerged arc furnace (SAF), or the like, where they are heated and melted, and the slag is separated to produce molten iron.

[0003] In Japan, studies are underway to apply the DR process to low-grade iron ore produced in Australia, India, etc. However, it is difficult to melt the reduced iron produced from low-grade iron ore using the current EAF. 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, direct reduced iron (DRI), hot metal, or hot briquetted iron (HBI), are continuously fed or charged in small buckets into a submerged arc furnace (SAF) without opening the furnace lid. This SAF melting process may also be performed in flat-bath operation. During charging, electrodes are inserted into the molten slag in the SAF, and an arc is generated between the electrodes or between the electrodes and the molten iron. An electric current is passed through the material-slag connection, heating and melting the materials by resistance heating or heat transfer.

[0005] One problem with the SAF melting process is the inefficient heat transfer to the source material located away from the electrode, which slows melting of the source metal and causes problems such as concentration gradients, unreliable temperature measurement, unreliable process control, and excessively hot molten metal tapping. To solve this problem of uneven temperature, electromagnetic stirring, as disclosed in U.S. Patent No. 5,629,999, and gas stirring, as disclosed in U.S. Patent No. 5,629,999, have been developed.

[0006] JP 2020-505579 A JP 2002-317918 A

[0007] However, the prior art has 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-to-molten-metal ratio and a high Fe content. It cannot be directly applied to a technology in which molten metal, molten slag, raw metal materials, and auxiliary materials are stacked in an electric furnace, and the molten metal and slag are continuously discharged and the furnace is operated in a reducing atmosphere. Furthermore, the technology described in Patent Document 2 is an electric furnace for a waste melting furnace, and is not intended for mass production of molten metal. A method of operating an electric furnace, particularly a submerged arc furnace, that takes into account stable production of molten metal has been an issue.

[0008] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for operating an electric furnace that is highly productive and causes little wear of refractories when melting metal raw materials in the electric furnace to produce molten metal.

[0009] The method of operating an electric furnace according to the present invention, which advantageously solves the above-mentioned problems, is a method of operating an electric furnace in which molten metal, slag, metal raw materials, and auxiliary materials are stored in layers in the electric furnace in that order, and the molten metal and slag are continuously discharged, the electric furnace having a bottom, furnace walls, furnace lid, and molten metal stirring means, and an input power per surface of the molten metal is 300 kW / m ２ The above is characterized in that the power density of stirring the molten metal by the stirring means is set in the range of 5 to 100 W / t.

[0010] The method for operating an electric furnace according to the present invention comprises: (a) configuring the electric furnace as an electric furnace capable of melting without intentionally supplying oxygen gas; (b) configuring the stirring means as an electromagnetic stirrer provided at the bottom of the electric furnace; (c) configuring the stirring means as gas injection from a submerged lance or bottom gas blowing means provided at the bottom of the electric furnace; (d) configuring the molten metal outlet and the slag outlet in the furnace wall in different directions relative to the center of the hearth, and discharging the molten metal from an outlet filled with plugging material by penetrating the plugging material; (e) configuring the metal raw material as reduced iron in part or all; (f) configuring the slag as having a basicity in the range of 1.0 to 1.5, where the basicity of the slag is defined as the ratio of SiO in the slag by mass. ２ (g) an arc furnace is used as the electric furnace, and at least one of the electrode height, arc voltage, and arc current is adjusted so that the value E / √I obtained by dividing the arc voltage E (V) that is the voltage to ground by the square root of the current I (A) per arc electrode is 2.0 or less; (h) the electric furnace is a submerged arc furnace. These may be more preferable solutions.

[0011] According to the method for operating an electric furnace of the present invention, by appropriately stirring the molten metal, sufficient input power can be applied, thereby suppressing the temperature rise of the molten metal and enabling the production of molten metal with high productivity while suppressing wear of the refractory.

[0012] 1 is a schematic vertical cross-sectional view illustrating a method for operating an electric furnace according to an embodiment of the present invention, in which (a) shows an example having electromagnetic stirring equipment, and (b) shows an example of bottom-blowing stirring. 2 is a graph showing the influence of the power density for stirring molten iron on the relationship between the input power per surface of the molten iron in an electric furnace and the molten iron temperature. 3 is a schematic vertical cross-sectional view showing a submerged arc furnace without a molten metal stirring function.

[0013] The following is a detailed description of embodiments of the present invention. The following embodiments are intended to exemplify equipment 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 defined in the claims.

[0014] FIG. 1 is a schematic longitudinal cross-sectional view illustrating the configuration of a submerged arc furnace suitable for use in a method for operating an electric furnace according to one embodiment of the present invention. FIG. 1(a) shows an example equipped with an electromagnetic stirrer 6 consisting of a linear motor. FIG. 1(b) shows an example in which gas bubbles 7A are injected from a bottom-blowing tuyeres 7. The submerged arc furnace 1 contains stacked raw materials M, consisting of molten pig iron P, molten slag S, raw metal materials, and auxiliary materials, within the furnace wall 2. In the submerged arc furnace 1, electrodes 3 are inserted into the molten slag S, and arcs are generated between multiple electrodes 3 or between the electrodes 3 and the molten pig iron P to heat and melt the raw materials by radiant heating or resistance heating of the current flowing between the electrodes, raw materials, slag, and molten pig iron. In this embodiment, oxygen is not intentionally supplied; that is, electric heating is performed in a reducing atmosphere. "Intentionally not supplying oxygen" does not prevent air from being mixed in during raw material charging, through the blown gas, gaps in the refractory, etc. This means that oxidation heat is not used to heat the raw materials.

[0015] In this embodiment, it is preferable to continuously produce molten metal by adding raw materials 4 from the top. In this case, it is preferable to have a molten metal outlet and a slag outlet, and to arrange them in different directions relative to the center of the hearth. It is preferable to fill each outlet with plugging material, and when a predetermined amount of molten metal has been held, to open the plugging material and discharge the molten metal.

[0016] In this embodiment, iron source raw materials containing reduced iron are charged into a submerged arc furnace 1. At the same time, as an auxiliary raw material, molten slag S formed on molten iron P is added to the submerged arc furnace 1. The basicity (CaO / SiO ２ The basicity of the molten slag (CaO / SiO ２ ) is the mass fraction of CaO in the molten slag. ２ It refers to the ratio to.

[0017] In this embodiment, the iron source raw material to be melted in the submerged arc furnace 1 includes reduced iron. The reduced iron may be produced in advance by a direct reduction method, or commercially available reduced iron may be purchased and used. Iron scrap or scale may also be used as the iron source raw material. The mass ratio of the reduced iron to the iron source raw material is preferably 50 to 100%.

[0018] When the metallization rate of the reduced iron is 60% or higher, the power consumption rate is low and energy efficiency is excellent. There is no particular upper limit to the metallization rate. If the metallization rate is too high, the reduced iron, which is the raw material 4 charged from above the molten slag S in the submerged arc furnace 1, may conduct electricity through the slag, resulting in a decrease in resistance heat. Therefore, it is preferable that the upper limit of the metallization rate be approximately 90%.

[0019] The slag-forming material is limestone (CaCO ３ ), quicklime (CaO), and SiO ２ The source is silica (SiO ２ It is preferable to add lumps or powders of slag S together with or mixed with the metal raw material, for example, by granulation. ２ ) is preferably in the range of 1.0 to 1.5. When the basicity of the molten slag is in this range, it has a composition similar to that of blast furnace slag, making it suitable for reuse as roadbed materials such as cement. In addition, it is preferable that the total iron content in the molten slag is 5.0 mass% or less. This can be achieved by intentionally not supplying oxygen gas, and iron yield is improved.

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

[0021] In this embodiment, electric energy is used to melt the metal raw material, for example, reduced iron. Therefore, compared to the case where the heat of combustion of carbon is used, such as in the blast furnace method, CO２ This has the effect of reducing emissions. It is preferable to use electrical energy generated by renewable energy.

[0022] When a submerged arc furnace without a molten metal stirring function, such as that shown in FIG. 3, is used in an electric furnace operating method for producing molten metal similar to that of this embodiment, the following problems arise. In such a submerged arc furnace, there is no external oxygen supply, and no solid-liquid flow occurs within the furnace. Therefore, heat transfer from the molten metal P through the molten slag S to the raw materials M is almost exclusively by thermal conduction. Furthermore, the electrode 3 is usually positioned centrally when viewed from above. Therefore, the heat supply to the raw materials M is limited, and even if the input power is increased, most of the input power is consumed in heating the molten metal P. This reduces the input power used to melt the raw materials M, particularly the heat supply to the raw materials M located far from the electrode 3. A power density of 250 kW / m per molten metal surface is required. ２ The molten iron temperature will reach 1500°C or higher. Since the wear rate of ordinary refractories increases rapidly above 1550°C, the refractories will be severely worn under the above conditions. For this reason, a normal submerged arc furnace requires a power density of 250 kW / m per molten iron surface. ２ In many cases, furnaces are operated at about this level. To produce a large amount of molten metal using such a furnace would require large-area equipment and numerous pieces of equipment, resulting in excessive investment. Therefore, the inventors conducted research and found the relationship shown in Figure 2 between the stirring power density ε and the molten iron temperature, making it possible to apply a high power density while suppressing the molten iron temperature.

[0023] As a result of extensive research, the inventors have found that the relationship between the power density per surface of the molten iron and the temperature of the molten iron P depends on the stirring power. Figure 2 shows the results of this research. For example, in a normal submerged arc furnace, the stirring power density ε = 0, and in a conventional submerged arc furnace, the stirring power density ε = 250 kW / m ２ It was also newly discovered that by adding even a small amount of stirring power, a large amount of electricity can be input to the furnace without raising the temperature of the molten metal P.

[0024] The submerged arc furnace 1 of this embodiment includes a bottom, a furnace wall 2, a furnace cover (not shown), and a means for stirring the molten metal. ２ By setting the power density ε of the stirring means for the molten metal to the range of 5 to 100 W / t, high productivity and stable operation can be achieved without excessive capital investment. ２ The stirring power density ε is preferably in the range of 10 to 30 W / t.

[0025] If the stirring power density is below the lower limit, the molten metal may overheat, causing accelerated wear of refractories such as furnace walls. If the stirring power density is above the upper limit, the molten metal may flow too fast, causing accelerated wear of refractories.

[0026] As an example of a means for stirring the molten metal, an electromagnetic stirrer 6 as shown in FIG. 1( a) is preferably installed at the bottom. The bottom where the electromagnetic stirrer 6 is installed is preferably made of a non-magnetic material. The stirring power density of the electromagnetic stirring can be determined, for example, by numerical analysis or experimentation relative to the output of the electromagnetic stirring. For example, it can be approximately 0.5% of the output power of the electromagnetic stirring. The electromagnetic stirrer 6 can be configured, for example, as a linear motor, and can generate a traveling magnetic field in a direction from the center of the furnace bottom toward the furnace wall or from the furnace wall toward the center of the furnace bottom. Alternatively, the electromagnetic stirrers may be arranged opposite each other across the center of the furnace bottom to generate traveling magnetic fields in different directions, or the electromagnetic stirrers may be arranged circumferentially to generate a swirling flow in the molten metal. The flow of the molten metal due to electromagnetic stirring, for example, flows along the furnace bottom and reverses at the furnace wall, resulting in heat transfer to the molten metal surface due to forced convection.

[0027] As another example of a means for stirring the molten metal, a bottom-blowing tuyeres 7 as shown in Figure 1(b) is preferably installed at the bottom of the furnace, and gas is blown in to stir the molten metal with bubbles 7A. Instead of using a bottom-blowing tuyeres, gas may be blown from the tip of a submerged lance. The stirring power density can be calculated from the flow rate of the gas being blown in, for example, using the following equation (1).

[0028] ε = (371 × Q × Tm / Wm) × [ln{1 + (9.8 × ρm × h / P)} + (1 - Tg / Tm)] (1) where ε: stirring power density (W / t), Q: total gas flow rate (Nm ３ / sec), Tm: temperature of molten metal (K), Wm: weight of molten metal (t), ρm: density of molten steel (t / m ３ ), h: distance (m) from the lance tip to the molten metal surface for top-blown gas, and distance (m) from the molten metal surface to the bottom of the ladle for bottom-blown gas, P: atmospheric pressure (101.325 kPa), Tg: gas temperature (K).

[0029] In the above embodiment, an electric furnace having a substantially circular shape when viewed from above has been described as an example, but the shape may be any shape, such as an oval or rectangular shape.

[0030] The electric furnace 1 in this embodiment is preferably an arc furnace. In a submerged arc furnace, which is preferably used in this embodiment, current flows between the electrode, raw material, slag, and molten iron. Therefore, compared to electric furnaces that generally generate arcs in air, the voltage to ground is lower for the same current value. This is because the electrical conductivity of slag is significantly higher than that of air. Therefore, it is preferable to adjust at least one of the electrode height, arc voltage, and arc current so that the value E / √I, obtained by dividing the arc voltage E (V) (voltage to ground) by the square root of the current I (A) per arc electrode, is 2.0 or less. It is more preferable to set the index E / √I to 1.5 or less.

[0031] The investigation was carried out using a 4 to 6 ton submerged arc furnace 1 as shown in Figure 1. Table 1 shows the specifications of the three-phase AC electric arc furnace used in the investigation.

[0032]

[0033] Shredder chips, carbonaceous material, and blast furnace slag were charged into the submerged arc furnace 1, and electricity was applied to perform initial melting. The pre-charged amounts were adjusted to 750 kg of hot metal and 1,500 kg of slag. After the pre-charged materials were completely melted, a pre-mixed raw material consisting of reduced iron as the main raw material and auxiliary raw materials such as carbonaceous material, lime, alumina, and MgO source for adjusting the C concentration in the hot metal and the slag composition was continuously charged. The maximum amount of hot metal was targeted to be 4 t, and was adjusted by the amount of raw material charged. The final thickness of the hot metal was 140 mm.

[0034] The input power of the submerged arc electric furnace was adjusted by adjusting the tap voltage and electrode position so that the power was adjusted to a predetermined condition.

[0035] The transformer of the electromagnetic stirrer used for electromagnetic stirring was set to 200 kVA, and the 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 actually measure the contribution rate of electromagnetic stirring, it was calculated in advance using a numerical analysis method and set to 0.5%. This value is a typical value for electromagnetic stirrers and does not change significantly. The operating conditions are shown in Table 2.

[0036] A porous nozzle is installed for bottom agitation, and N ２ The stirring power density was calculated using the above formula (1), and the bath density was set to 7.0 t / m ３ The calculation was made assuming that the height h of the molten metal was 0.14 m, the gas temperature Tg was 300 K, and the molten metal temperature Tm was 1800 K. The operating conditions are shown in Table 3.

[0037] In the evaluation column, the amount of refractory wear (mm) was evaluated for each level, and the amount of wear under the conditions of Test No. 1 was normalized as 1, and a value that was twice or more was evaluated as x. Productivity was evaluated as the time from the start of reduced iron charging to the completion of melting of all raw materials, and a value that did not produce 4 tons of molten pig iron within 100 minutes was evaluated as x. All other values ​​were evaluated as ◯. The results are shown in Tables 2 and 3. The values ​​of the index E / √I during operation are also shown in Tables 2 and 3.

[0038]

[0039]

[0040] The input power per surface of the molten metal is 300 kW / m ２ In addition, the inventive examples in which the power density of stirring the molten metal by the stirring means was in the range of 5 to 100 W / t were excellent in both productivity and wear of the refractory material. The comparative examples outside these conditions were inferior in either productivity or wear of the refractory material.

[0041] REFERENCE SIGNS LIST 1 Electric furnace (submerged arc furnace, SAF) 2 Furnace wall 3 Electrode 4 (Charged) raw material 5 Arc 6 Electromagnetic stirrer (linear motor) 7 Bottom-blowing tuyeres 7A Gas bubbles P Molten pig iron (molten metal) S Molten slag M Raw material F (Molten metal) flow

Claims

1. A method of operating an electric furnace in which molten metal, slag, raw metal materials and auxiliary materials are stacked in order and stored in the electric furnace, and the molten metal and slag are continuously discharged, the electric furnace is equipped with a bottom, a furnace wall, a furnace cover and a means for stirring the molten metal, and the input power per surface of the molten metal is 300 kW / m ２ The method for operating an electric furnace further comprises the steps of: setting a power density for stirring the molten metal by the stirring means in a range of 5 to 100 W / t.

2. The method of operating an electric furnace according to claim 1, wherein the electric furnace is an electric furnace capable of melting without intentionally supplying oxygen gas.

3. The method of operating an electric furnace as claimed in claim 1, wherein the stirring means is an electromagnetic stirrer provided at the bottom of the electric furnace.

4. A method for operating an electric furnace as claimed in claim 1, wherein the stirring means is gas injection from an immersion lance or a bottom gas injection means provided at the bottom of the electric furnace.

5. A method for operating an electric furnace as described in claim 1, wherein the molten metal discharge port and the slag discharge port are provided in the furnace wall in different directions relative to the center of the hearth, and the molten metal is discharged from the discharge port filled with plugging material by penetrating the plugging material and opening it.

6. The method of operating an electric furnace according to claim 1, wherein a part or all of the metallic raw material is 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 ratio of SiO ２ 6. The method of claim 5, wherein the ratio of CaO to CaO is 0.01 to 0.

01.

8. A method for operating an electric furnace as claimed in claim 1, in which the electric furnace is an arc furnace, and at least one of the electrode height, the arc voltage and the arc current is adjusted so that the value E / √I obtained by dividing the arc voltage E (V), which is the voltage to ground, by the square root of the current I (A) per arc electrode is 2.0 or less.

9. The method of operating an electric furnace according to claim 1, wherein said electric furnace is a submerged arc furnace.

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