Metal melting method

Abstract

To provide a metal melting method that can dissolve a molten raw material that has a smaller density than molten iron and is floating on the surface of molten iron more efficiently and in a short time.SOLUTION: A metal melting method according to the present invention is a metal melting method in an AC electric furnace comprising three upper electrodes and a raw material input tube provided on the side surface or upper surface of the furnace body, the method comprising: a step of inputting a raw material for melting having a density smaller than that of molten iron from the raw material input tube from the ceiling; a step of melting the raw material for melting using an arc as a heat source while continuously supplying the raw material for melting into the furnace; a step of forming a rotational flow field in a horizontal direction with respect to the molten iron surface; and a step of moving the raw material for melting to a high temperature region in the furnace by applying a centripetal force to the raw material for melting floating on the molten iron surface by means of the rotational flow field.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for dissolving metal. [Background technology]

[0002] The iron source produced by the blast furnace process is produced by reducing iron ore with coke, which generates a large amount of CO2. As a means of reducing CO2 emissions, there is a method of producing molten iron by melting scrap iron and DRI (direct reduced iron) in an electric furnace, and then producing molten iron using the existing steelmaking process centered on converters. Melting is performed by an arc generated from the upper electrode installed in the electric furnace to the surface of the molten iron.

[0003] Electric furnaces are broadly classified into DC electric furnaces and AC electric furnaces. It is known that in DC electric furnaces, the molten iron is actively stirred by the electromagnetic force generated by a large current flowing through the molten iron from the surface to the bottom of the furnace. In contrast, in AC electric furnaces, the current flows mainly near the surface of the molten iron, so it is known that the stirring of the molten iron by the electromagnetic force is small compared to the case of DC electric furnaces. As a countermeasure against this, Patent Document 1 discloses a technology in which an electromagnetic stirrer is provided under the bottom of an electric arc furnace to stir the molten metal in the furnace. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2020-505579 Summary of the Invention [Problem to be solved by the invention]

[0005] Direct reduced iron is produced by directly reducing iron ore or iron ore pellets with reducing gas such as hydrogen, and is called DRI. Hot briquetted iron (HBI) is produced by hot compressing the DRI after reduction. The apparent density of DRI is 3500 kg / m 3 The apparent density of HBI is about 5000 to 5500 kg / m 3 The density of molten iron is about 7000 kg / m 3 Therefore, both DRI and HBI have a lower density than molten iron and rise to the surface.

[0006] In the technology disclosed in Patent Document 1, the flow of molten iron stirred by an electromagnetic stirrer tends to move DRI and HBI to the furnace wall, which is at a relatively low temperature, instead of directly below the upper electrode, which is at a high temperature, which reduces the melting efficiency of DRI and HBI.

[0007] Therefore, an object of the present invention is to provide a metal melting method capable of more efficiently melting raw materials that have a lower density than molten iron and float to the surface of the molten metal in a short period of time in the case of a large electric furnace using three electrodes. [Means for solving the problem]

[0008] [1] A method for melting metals in an AC electric furnace equipped with three upper electrodes and a raw material charging pipe provided on a side or top surface of a furnace body, comprising the steps of: charging raw materials for melting having a lower density than molten iron from the ceiling through the raw material charging pipe; melting the raw materials using an arc as a heat source while continuously supplying the raw materials into the furnace; forming a rotational flow field at the surface of the molten iron that is horizontal to the surface of the molten iron; and applying a centripetal force to the raw materials floating on the surface of the molten iron by the rotational flow field, thereby moving the raw materials to a high-temperature region in the furnace. [2] The metal melting method according to [1], wherein in the rotating flow field, the position at which the flow velocity of the molten iron is maximum is within a range of 1.0 m or more and 2.5 m or less from the center of the furnace. [3] The metal melting method according to [2], wherein the arc length L (m), the maximum flow velocity V0 (m / s) of the molten iron, and the distance RC (m) between the center of the furnace and the center of the upper electrode satisfy the following relationship expressions (i), (ii), and (iii): R≧(V0+2.65L-1.56) / 0.4+RC-0.8 (i) R≧(V0-2.0L-0.4) / 1.48+RC-0.8 (ii) 0.4≦L≦0.6 (iii) [4] The metal melting method according to [3], wherein the arc length L (m), the maximum flow velocity V0 (m / s) of the molten iron, and the distance RC (m) between the center of the furnace and the center of the upper electrode satisfy the following relationship expressions (v) and (vi). R≧(V0+2.0L-0.95) / 0.4+RC-0.8 ···(v) R≧(V0-2.0L+1.37) / 0.94+RC-0.8 ···(vi) [5] The metal melting method according to [4], wherein the arc length L (m), the maximum value V0 (m / s) of the flow velocity of the molten iron, and the distance RC (m) between the center of the furnace and the center of the upper electrode satisfy the following relational expression (vii): R≦(V0+1.01L+2.38) / 1.8+RC-0.8 (vii) [6] The metal melting method according to [2], wherein the arc length L (m), the maximum value V0 (m / s) of the flow velocity of the molten iron, and the distance RC (m) between the center of the furnace and the center of the upper electrode satisfy the following relational expression (iv): R≦(V0+1.7L+0.88) / 1.1+RC-0.8 (iv) [7] The metal melting method according to any one of [1] to [6], wherein the AC electric furnace is provided with two electromagnetic stirrers on the underside of the furnace bottom, and the rotating flow field is formed by electromagnetic forces generated by the two electromagnetic stirrers acting on the molten iron in opposite directions. Effect of the Invention

[0009] According to the above configuration, the molten raw materials such as HBI, which have a lower density than molten iron and float to the surface of the molten metal, quickly reach the high temperature region, so that these molten raw materials can be melted more efficiently in a short period of time. [Brief description of the drawings]

[0010] [Figure 1] 1 is a cross-sectional view of an AC electric furnace according to one embodiment of the present invention. [Diagram 2] 2 is a cross-sectional view taken along line II-II of the AC electric furnace shown in FIG. 1. [Diagram 3] FIG. 13 is a schematic diagram showing a comparative example of an AC electric furnace in which an electromagnetic stirring device is provided on the lower surface of the furnace bottom. [Figure 4] 4 is a cross-sectional view taken along line IV-IV of the AC electric furnace shown in FIG. 3. [Diagram 5] 1 is a graph showing conditions for distribution of flow velocity of molten iron in fluid analysis. [Figure 6] FIG. 13 is a diagram showing an example of the calculation results of the trajectory of an HBI injected into a reactor. [Figure 7] 1 is a graph showing the change over time in the position and velocity of HBI injected into a furnace. [Figure 8] This is a graph that analyzes the distance between the center of the furnace and the HBI injection position when the peak position of the molten iron flow velocity is changed, and the minimum molten iron flow velocity required for the HBI to reach the high temperature zone within a specified time. [Figure 9] This is a graph that analyzes the distance between the center of the furnace and the HBI injection position when the peak position of the molten iron flow velocity is changed, and the minimum flow velocity required for the HBI to remain in the high temperature region for a specified period of time. [Figure 10] This graph evaluates the conditions under which HBI enters the high temperature zone within 40 seconds after being added and remains there for an additional 40 seconds or more. [Figure 11] This graph evaluates the conditions under which HBI enters the high temperature zone within 20 seconds after being added and remains there for 60 seconds or more. [Figure 12] 1 is a graph showing the relationship between the distance between the furnace center and the HBI injection position and the flow velocity of molten iron when the distance between the furnace center and the HBI injection position is 1,550 mm or less. [Figure 13] 1 is a graph showing the relationship between the distance between the furnace center and the HBI injection position and the flow velocity of molten iron when the arc length is changed. [Figure 14] 13 is a graph showing the relationship between the arc length and the intercepts of equations (5) and (6). [Figure 15] 1 is a graph showing the relationship between the distance between the furnace center and the HBI injection position and the flow velocity of molten iron when the arc length is changed between 400 mm and 600 mm. [Figure 16] 1 is a schematic diagram of the bottom of an AC electric furnace according to the present embodiment, in which two electromagnetic stirrers are provided. [Figure 17] FIG. 13 is a diagram showing the results of an analysis of the electromagnetic fields generated by two electromagnetic stirrers. [Figure 18] FIG. 13 is a diagram showing the flow velocity distribution of the surface of molten iron when an electromagnetic stirring device is provided. [Figure 19] 1 is a graph showing changes in the maximum value of the flow velocity of molten iron and the position of the peak flow velocity when the current frequency of the electromagnetic stirrer is changed. [Figure 20] 1 is a graph showing changes in the maximum value of the flow velocity of molten iron and the peak position of the flow velocity of molten iron when the current value of the electromagnetic stirrer is changed. [Figure 21] 1 is a graph showing the results of applying equations (5) to (10) to the current frequency of the electromagnetic stirrer and the distance between the center of the furnace and the HBI insertion position. [Figure 22] 1 is a graph showing the results of applying formulas (5) to (10) to the current value of the electromagnetic stirrer and the distance between the center of the furnace and the position where the HBI is introduced. [Figure 23] FIG. 13 is a diagram showing the results of calculating the trajectory of the HBI when the distance between the reactor center and the HBI injection position is set to 1.7 m. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In this specification and the drawings, components having substantially the same functional configurations are designated by the same reference numerals, and redundant explanations will be omitted.

[0012] Fig. 1 is a cross-sectional view of an AC electric furnace 1 according to one embodiment of the present invention. In the figure and in the following description, three mutually orthogonal directions are defined as +x, +y, and +z directions. Although not shown in the figure, the direction opposite to the +x direction is defined as -x direction, the direction opposite to the +y direction is defined as -y direction, and the direction opposite to the +z direction is defined as -z direction.

[0013] As shown in the figure, an AC electric furnace 1 includes a ceiling 2, a furnace wall copper panel 3, a furnace wall refractory 4, and a furnace bottom refractory 5. In addition, three upper electrodes 6 are provided on the ceiling 2, and a bottom blowing tuyeres 7 are provided on the furnace bottom. A raw material input pipe 8 is provided on the side or top surface of the furnace body. The furnace wall copper panel 3 is provided with a tapping hole 9 for discharging molten iron 21 and a slag discharge hole 10 for discharging electric furnace slag 22. In addition, an electromagnetic stirrer 11 is provided on the lower surface of the furnace bottom. Two electromagnetic stirrers 11 may be provided.

[0014] The AC electric furnace 1 may be a tilting type in which the furnace body tilts, or a stationary type in which the furnace body does not tilt. In addition, one or both of iron-containing scrap and reduced iron are charged into the AC electric furnace 1 as the iron source. As the reduced iron, DRI and HBI can be used. Iron-containing dust may be charged into the furnace as the iron source. As the iron-containing dust, converter dust granulated product can be used.

[0015] Molten raw materials having a density lower than that of molten iron 21, including at least DRI or HBI, are charged into the furnace from the raw material charging pipe 8. In the following description, it is assumed that HBI is charged into the furnace from the raw material charging pipe 8, but DRI may also be charged from the raw material charging pipe 8.

[0016] Pr in the figure indicates the position where HBI charged from the raw material charging tube 8 reaches the surface of the molten iron 21. R in the figure indicates the distance between the furnace center C and the charging position Pr. L indicates the arc length, which is equal to the distance between the upper electrode 6 and the surface of the molten iron 21.

[0017] An arc 20 is emitted from the upper electrode 6 to melt raw materials such as reduced iron to produce molten iron 21. In the AC electric furnace 1, these raw materials are continuously supplied into the furnace, and the raw materials are melted using the arc 20 as a heat source. Gas is blown from the bottom blowing tuyeres 7 to generate a circulating flow in the molten iron 21 and electric furnace slag 22, thereby promoting heat transfer and melting of the surface of the molten iron 21 and the electric furnace slag 22. Normally, the temperature of the molten iron 21 in the AC electric furnace 1 is at most about 1700°C, but the internal temperature of the arc 20 generated from the upper electrode 6 to the surface of the molten iron 21 is 5000°C or higher, and even the arc spot on the surface of the molten iron 21 is about 2000°C.

[0018] FIG. 2 is a cross-sectional view of the AC electric furnace 1 shown in FIG. 1 taken along line II-II. The AC electric furnace 1 basically has a circular cross section, but in many cases, an eccentric furnace body tapping method is adopted, in which the tapping port is a protruding furnace body shape. The shape of the tapping part does not affect the present invention, so it is shown as a circle for simplicity. In addition, the three upper electrodes 6 are provided so that the shape of the center of each upper electrode 6 is an equilateral triangle. Here, it is ideal to install the upper electrodes 6 so that they form an equilateral triangle, but due to equipment restrictions, there may be a deviation of about the radius of the upper electrode 6, and such a deviation does not significantly change the effect of the present invention. In the following description, the distance between the furnace center C and the center of the upper electrode 6 is RC, the radius of the molten iron 21 is d, and the radius of the upper electrode 6 is r.

[0019] Rh in the figure indicates a high-temperature region caused by the arc 20 on the surface of the molten iron 21. As described in, for example, Electro Heat (Nihon Electro, 153, 77-84, 2007), the arcs 20 are known to incline at about 35 degrees toward the furnace wall because they repel each other due to electromagnetic force. Therefore, when the radius of the upper electrode 6 is 400 mm, the distance from the lower end of the upper electrode 6 to the molten iron surface is L, and the center position of the upper electrode 6 is RC, the high-temperature region Rh in the radial direction is in the range of (RC+Ltan35°-400) to (RC+Ltan35°+400). rc in the figure indicates the distance between the center of the arc 20 on the surface of the molten iron 21 and the center C of the furnace when the arc 20 is inclined at 35 degrees toward the furnace wall.

[0020] For this reason, for example, if the arc length L is 500 mm, the position where the arc 20 collides with the molten iron 21 is 350 mm outside of directly below the center of the upper electrode 6. Therefore, in this case, if the distance RC between the furnace center C and the center of the upper electrode 6 is 800 mm from the furnace center C, the position where the surface temperature of the molten iron 21 is the highest is 1150 mm from the furnace center C.

[0021] If the radius of the lower end of the upper electrode 6 is 400 mm, the high temperature region Rh will be in the range of 750 mm to 1550 mm from the furnace center C if the range is about 400 mm from the center of the arc 20, which is the same as the radius of the lower end of the upper electrode 6. Therefore, in order to efficiently melt the HBI, it is desirable to have it quickly reach a range within 1550 mm from the furnace center C. It is even more desirable to have the HBI stay in the range of 750 mm to 1550 mm for a long time.

[0022] Here, the flow of molten iron S when an electromagnetic stirrer 97 is provided on the lower surface of the furnace bottom of an AC electric furnace 90 of a comparative example and the molten iron S is stirred is shown using Fig. 3 and Fig. 4. Fig. 3 is a schematic diagram of an AC electric furnace 90 according to a comparative example in which an electromagnetic stirrer 97 is provided on the lower part of the furnace bottom. Fig. 4 is a cross-sectional view of the AC electric furnace 90 shown in Fig. 3 taken along line IV-IV.

[0023] As shown in the figure, an AC electric furnace 90 according to the comparative example includes a furnace body 91 in which molten iron S is accommodated, a panel 92 surrounding the opening of the furnace body 91, and a lid 93 covering the upper part of the panel 92. Reduced iron is charged into the furnace through a raw material charging pipe 94 formed in the lid 93. Three upper electrodes 95 are provided above the furnace body 91, and an arc 96 is generated between the upper electrodes 95 and the surface of the molten iron S to heat the inside of the furnace and melt the charged reduced iron. In addition, an electromagnetic stirrer 97 is provided under the bottom of the furnace.

[0024] The arrows in the figure indicate the flow of molten iron S, which is molten reduced iron. The molten iron S is stirred in one direction at the bottom of the furnace by the electromagnetic stirring device 97. Therefore, as shown in the figure, the molten iron S has a flow pattern in which an upward flow and a return flow occur near the molten iron surface. This flow pattern is suitable for melting the scrap accumulated at the bottom of the furnace, but the HBI floating up to the molten iron 21 spends a longer time flowing near the furnace wall, which has a lower temperature, than directly below the upper electrode 95, which is the high-temperature region Rh. Therefore, the melting efficiency of the HBI decreases. In order to efficiently melt the floating molten iron S, it is ideal for the HBI to easily remain in the high-temperature region Rh.

[0025] Since the inside of the furnace is very hot, it is difficult to directly measure the trajectory of the HBI moving inside the furnace. Therefore, in this embodiment, a simulation is performed to evaluate the trajectory of the HBI. In order to concentrate the trajectory of the HBI moving inside the furnace to the furnace center C as much as possible, it is desirable to form a rotational flow field horizontal to the molten iron 21 surface. This is because the formed rotational flow field exerts a centripetal force on the molten raw materials such as HBI, which have a lower density than the molten iron 21 floating on the molten iron surface, and the molten raw materials move to the high temperature region Rh in the furnace. In order to evaluate the trajectory of such an HBI, the following analysis was performed.

[0026] The simulation was performed based on the method described in Turbulence Second Edition, HINZE, McGRAW-HILL, New York, 1975, p.460-462, and an outline of the method is given below. The motion of a sphere in a fluid can be expressed by the BBOT equation shown in the following formula (1). Note that v is the particle velocity, F D is the drag force, v f is the velocity of the fluid and g is the acceleration due to gravity.

[0027]

number

[0028] Here, the particle and fluid densities are ρ and ρ f Let d be the particle diameter and C be the drag coefficient. D Then, the coefficients C1, C2 and C3, and the drag force F D is expressed by the following equations (2) and (3).

[0029]

number

[0030] The second term on the right hand side of equation (1) is the total differential of the flow velocity. This can be expressed as equation (4) below.

[0031]

number

[0032] The viscosity of molten iron 21 is 0.005 Pa s and the density is 7000 kg / m 3 If the particle diameter is 5 cm and the velocity range is 0.05 to 1 m / s, the Reynolds number Re is about 420 to 84,000, and the drag coefficient C in this range is D is about 0.4 to 0.5. Here, the drag coefficient C D is 0.4, HBI is 5cm in diameter, and density is 5500kg / m 3 The fluid has a density of 7000kg / m 3 The calculation was performed assuming that the HBI floats near the surface of the molten iron 21 and does not move toward the hearth bottom.

[0033] FIG. 5 is a graph showing the conditions for the distribution of the flow velocity of the molten iron 21 in the fluid analysis. The horizontal axis of the graph is the distance R from the furnace center C, and the vertical axis is the circumferential flow velocity of the molten iron 21. The distribution of the flow velocity of the molten iron 21 in the simulation is based on the results of the fluid analysis that will be explained later using FIG. 18. The maximum value V0 of the flow velocity of the molten iron 21 is 1.0 m / s, and the peak position P v was located 2.0 m from the furnace center C.

[0034] FIG. 6 is a diagram showing an example of the calculation result of the trajectory of the HBI injected into the furnace. In FIG. 6, (a) shows the flow of the molten iron 21, and (b) shows the trajectory when the HBI is injected at the injection position Pr. In the illustrated example, the maximum flow velocity of the molten iron 21 is V0=1.0 m / s, the peak position Pv=2.0 m, and the HBI injection position Pr is at a distance R=2.0 m from the furnace center C. The calculation was performed assuming that the flow velocity of the molten iron 21 near the furnace wall is 80% of the maximum flow velocity V0, and the flow velocity at the furnace center C is 0 m / s. As shown in the figure, in the rotating flow field formed in the molten iron 21, a centripetal force acts on the HBI, which has a lower density than the molten iron 21, so that the HBI moves toward the furnace center C while rotating.

[0035] Figure 7 is a graph showing the time-dependent change in the position and velocity of the HBI injected into the furnace. In Figure 7, (a) shows the time-dependent change in the distance R from the furnace center C, and (b) shows the time-dependent change in the velocity toward the furnace center C. In the result shown in (a), the HBI was 1.55 m from the furnace center C after 8.11 seconds, and 0.785 m from the furnace center C after 120 seconds. From graphs (a) and (b), the HBI reaches the high-temperature region Rh immediately after being injected, and then moves toward the furnace center C while slowly rotating in the circumferential direction. Therefore, it can be seen that the HBI is efficiently melted under the above conditions.

[0036] However, if unmelted HBI excessively gathers at the furnace center C, there is a high possibility that an aggregate of unsolidified HBI will be formed. It is believed that an aggregate of unsolidified HBI significantly deteriorates the melting efficiency of the HBI. For this reason, it is also undesirable that the speed at which the HBI moves toward the furnace center C is too fast. Therefore, in order to efficiently melt the HBI, there are suitable circumferential flow speeds of the molten iron 21 and charging positions Pr. The suitable circumferential flow speeds of the molten iron 21 and charging positions Pr of the HBI are described below.

[0037] In an AC electric furnace 1 that mainly melts HBI, HBI is often charged from the ceiling 2. If a raw material charging tube 8 for charging HBI is installed near the upper electrode 6, thermal damage to the raw material charging tube 8 will be significant. For this reason, it is preferable to position the raw material charging tube 8 away from the furnace wall relative to the upper electrode 6. For this reason, the raw material charging tube 8 is often placed at a position 1.5 m to 3 m away from the furnace center C.

[0038] Here, the size of a typical HBI is about 30×50×130 mm. It is known that with this size, after HBI is added to the molten iron 21, one HBI melts in about 40 to 60 seconds. In the AC electric furnace 1, the temperature of the molten iron 21 at the HBI input position Pr decreases, so if the molten iron 21 is not stirred enough or the HBI is not transported properly, the melting rate decreases significantly compared to when one HBI is input. Therefore, efficient melting in a large AC electric furnace 1 means that the melting rate is about the same as when one HBI is input. Therefore, when multiple HBIs are intermittently input, the temperature of the molten iron 21 near the input position Pr decreases, so it is necessary to move the HBI to the high temperature region Rh quickly.

[0039] Here, we calculated the circumferential flow velocity of the molten iron 21 required for HBI, injected at a distance R (m) from the furnace center C, to reach the high temperature region Rh at a position 1550 mm from the furnace center C in at least 40 seconds, ideally 20 seconds, in the rotating flow field formed in the molten iron 21. Below, we will explain separately the case where the peak position Pv of the flow velocity of the molten iron 21 is at a position 1550 mm or more from the furnace center C and the case where it is at a position 1550 mm or less.

[0040] First, a case where the peak position Pv of the flow velocity of the molten iron 21 is at a position 1550 mm or more away from the furnace center C will be described. Fig. 8 is a graph showing an analysis of the distance R between the furnace center C and the HBI injection position Pr and the minimum flow velocity of the molten iron 21 required for the HBI to reach the high temperature region Rh within a specified time when the peak position Pv of the flow velocity of the molten iron 21 is changed. In Fig. 8, (a) shows the minimum flow velocity of the molten iron 21 required when the HBI reaches the high temperature region Rh within 20 seconds after injection, and (b) shows the minimum flow velocity of the molten iron 21 required when the HBI reaches the high temperature region Rh within 40 seconds after injection. Here, the peak position Pv of the flow velocity of the molten iron 21 in the fluid analysis shown in Fig. 5 was changed between 1.0 m and 2.5 m to evaluate the minimum flow velocity of the molten iron 21 required for the HBI to reach the high temperature region Rh within a target time.

[0041] As shown in the figure, it can be seen that the flow velocity of the molten iron 21 needs to be increased as the HBI charging position Pr and the flow velocity peak position Pv are farther from the furnace center C. In order to reach the high temperature region Rh within 20 seconds after charging, it is necessary to increase the flow velocity of the molten iron 21 when the distance R between the furnace center C and the charging position Pr is 2 m or more. Since the flow velocity of the molten iron 21 that can be achieved by using means such as the electromagnetic stirrer 11 is at most about 1 m / s, there is a risk that the manufacturing and operating costs of the AC electric furnace 1 will increase in order to increase the flow velocity.

[0042] FIG. 9 is a graph analyzing the distance R between the furnace center C and the HBI input position Pr and the minimum flow velocity required for the HBI to stay in the high temperature region Rh for a predetermined period of time when the peak position Pv of the flow velocity of the molten iron 21 is changed. In FIG. 9, (a) is a graph analyzing the minimum flow velocity required for the HBI to stay in the high temperature region Rh for 60 seconds, and (b) is a graph analyzing the minimum flow velocity required for the HBI to stay in the high temperature region Rh for 40 seconds. Here, the peak position Pv of the flow velocity of the molten iron 21 was changed between 1.0 m and 2.5 m, and the minimum flow velocity of the molten iron 21 required for the HBI to reach the high temperature region Rh within a target time was calculated. In addition, the high temperature region Rh was set to a range where the distance R from the furnace center C was 750 mm or more and 1550 mm or less.

[0043] If the flow velocity of the molten iron 21 is too high, there is a risk that the HBI will leave the high-temperature region Rh and reach the vicinity of the furnace center C early on. As shown in the figure, it can be seen that the closer the peak position Pv of the flow velocity of the molten iron 21 is to the furnace wall, the easier it is to ensure the residence time of the HBI in the high-temperature region Rh even if the flow velocity of the molten iron 21 is high. At the same time, it can be seen that the closer the peak position Pv of the flow velocity of the molten iron 21 is to the furnace wall, the more it is necessary to reduce the flow velocity of the molten iron 21 at the surface.

[0044] FIG. 10 is a graph evaluating the conditions under which the HBI enters the high-temperature region Rh within 40 seconds after being charged and remains there for 40 seconds or more. The data are for the cases where the peak position Pv of the flow velocity of the molten iron 21 is at a distance R from the furnace center C of 1.0 m and 2.5 m, respectively. The graph shown in the figure shows the results of linear approximation of the data in the case where the peak position Pv of the flow velocity of the molten iron 21 is faster than the case where the peak position Pv of the flow velocity of the molten iron 21 is 2.5 m from the furnace center C, as shown by the dashed line in FIG. 8(b), and the data in the case where the peak position Pv of the flow velocity of the molten iron 21 is 1.0 m from the furnace center C, as shown by the dashed line in FIG. 9(b). From the graph shown in the figure, the conditions of the flow velocity of the molten iron 21 required for the HBI to reach the high-temperature region Rh within 40 seconds after being charged and to remain there for 40 seconds can be obtained.

[0045] From the graph shown in the figure, a relational expression is obtained between the maximum value V0 of the flow velocity of the molten iron 21 and the distance R between the furnace center C and the HBI injection position Pr. The flow velocity peak position Pv is in the range of 1.0 m to 2.5 m from the furnace center C, and the distance R between the furnace center C and the HBI injection position Pr is 1.6 m to 2.4 m. In this case, in order for the HBI to enter the high temperature region Rh within 40 seconds after injection, R and V0 need only satisfy the following formula (5). R≦(V0+1.75) / 1.1 (5) In addition, in order for HBI to stay in the high temperature region Rh for 40 seconds or more, it is necessary to satisfy formula (6). R≧(V0-0.24) / 0.4 (6)

[0046] FIG. 11 is a graph evaluating the conditions under which the HBI enters the high-temperature region Rh within 20 seconds after being charged and remains there for 60 seconds or more. The data are for the cases where the peak position Pv of the flow velocity of the molten iron 21 is at a distance R from the furnace center C of 1.0 m and 2.5 m, respectively. The graphs shown in the figures show the results of linear approximation of the data shown by the dashed line in FIG. 8(a) when the peak position Pv of the flow velocity of the molten iron 21 is faster than the case where the position is 2.5 m from the furnace center C, and the data shown by the dashed line in FIG. 9(a) when the peak position Pv of the flow velocity of the molten iron 21 is 1.0 m from the furnace center C. From the graphs shown in the figures, the conditions of the flow velocity of the molten iron 21 required for the HBI to reach the high-temperature region Rh within 40 seconds after being charged and to remain there for 40 seconds can be obtained. This condition is more desirable than the case shown in FIG. 10.

[0047] From the graph shown in the figure, a relational expression is obtained between the maximum value V0 of the flow velocity of the molten iron 21 and the distance R between the furnace center C and the HBI injection position Pr. The flow velocity peak position Pv is in the range of 1.0 m to 2.5 m from the furnace center C, and the distance R between the furnace center C and the HBI injection position Pr is 1.6 m to 2.4 m. In this case, in order for the HBI to enter the high temperature region Rh within 20 seconds after injection, the following formula (7) needs to be satisfied. R≦(V0+2.85) / 1.8 (7) In addition, in order for HBI to stay in the high temperature region Rh for 40 seconds or more, it is necessary to satisfy formula (8). R≧(V0-0.05) / 0.4 (8)

[0048] Next, a case where the peak position Pv of the flow velocity of the molten iron 21 is at a position 1550 mm or less from the furnace center C will be described. In this case, it is desirable for the HBI to remain in the high temperature region Rh, which is the region from 750 mm to 1550 mm, for 40 seconds or more, and it is even more desirable for it to remain in the region for 60 seconds or more. Therefore, it is necessary to reduce the flow velocity of the surface of the molten iron 21, and to reduce the maximum flow velocity as the peak position Pv of the flow velocity of the molten iron 21 is closer to the inside. For this reason, the peak position Pv of the flow velocity of the molten iron 21 was considered to be 1.0 m from the furnace center C.

[0049] FIG. 12 is a graph showing the relationship between the distance R between the furnace center C and the HBI injection position Pr and the flow velocity of the molten iron 21 when the distance R between the furnace center C and the HBI injection position Pr is 1550 mm or less. The data in the figure is data when the HBI stays in the high temperature region Rh for 40 seconds and 60 seconds. From the graph shown, in order for the residence time of the HBI in the high temperature region Rh to be 40 seconds or more, it is necessary to satisfy the relationship formula (9) between the maximum value V0 of the flow velocity of the molten iron 21 and the distance R between the furnace center C and the HBI injection position Pr. Also, in order for the residence time of the HBI in the high temperature region Rh to be 60 seconds or more, it is necessary to satisfy the following formula (10). R≧(V0+1.4) / 1.48 (9) R≧(V0+0.89) / 0.94 (10)

[0050] When the maximum flow velocity V0 is 0 m / s, the distance R between the furnace center C and the HBI injection position Pr is approximately R = 1.0 m according to equations (9) and (10). Therefore, when the distance R between the furnace center C and the HBI injection position Pr is less than 1.0 m, it can be said that there is no need for a circumferential rotational flow velocity, and at the same time, this is a condition that results in low HBI melting efficiency.

[0051] Next, the effect of the distance between the surface of the molten iron 21 and the lower end of the upper electrode 6, which corresponds to the arc length L, was examined. The arc length L may be changed as an operating condition of the AC electric furnace 1. In addition, the arc length L may generally vary by about ±100 mm from the reference height. As described above, it is known that the arcs 20 of the AC electric furnace 1 repel each other due to electromagnetic force and are inclined by about 35 degrees toward the furnace wall. Therefore, when the arc length L changes by about ±100 mm, the range of the high temperature region Rh changes by about 70 mm (100 (mm) × tan 35). When the reference arc length L is 500 mm, the high temperature region Rh is 1480 mm to 680 mm when the arc length L is 400 mm, and the high temperature region Rh is 1620 mm to 820 mm when the arc length L is 600 mm.

[0052] FIG. 13 is a graph showing the relationship between the distance R between the furnace center C and the HBI injection position Pr and the flow velocity of the molten iron 21 when the arc length L is changed. In FIG. 13, (a) shows data when the arc length L is 400 mm, and (b) shows data when the arc length L is 600 mm, and the flow velocity peak position Pv is at distances R = 1.0 m and 2.5 m from the furnace center C, respectively. The dashed lines in the figure are linear approximations of the respective data. Comparing the graphs of FIG. 13(a) and (b) with the graph of FIG. 10, the slopes are the same, and only the intercepts are different. From this, it can be seen that when the arc length L is changed, an appropriate range of the flow velocity of the molten iron 21 can be obtained by changing only the intercepts of Equation (5) and Equation (6), which are conditions under which the HBI enters the high temperature region Rh within 40 seconds after injection and remains there for 40 seconds or more.

[0053] FIG. 14 is a graph showing the relationship between the arc length L and the intercepts of formulas (5) and (6). From the graph, it can be seen that the intercept of formula (5) is proportional to the arc length L (m) with a coefficient of -1.7, and the intercept of formula (6) is proportional to the arc length L (m) with a coefficient of -2.65. Based on this, formulas (11) and (12) are shown below, which take into account the influence of the arc length L (m) on formulas (5) and (6). Note that the distance RC between the furnace center C and the center of the upper electrode 6 is based on 0.8 m, and the position of the upper electrode 6 is taken into account the deviation in the direction parallel to the surface of the molten iron 21. R≦(V0+1.7L+0.88) / 1.1+RC-0.8 (11) R≧(V0+2.65L-1.56) / 0.4+RC-0.8 ···(12)

[0054] Using a similar method, for equations (7) and (8), which are the conditions for the HBI to enter the high temperature region Rh within 20 seconds after insertion and remain there for a further 60 seconds or more, equations (13) and (14) are given below, taking into account the effect of the arc length L (m). R≦(V0+1.01L+2.38) / 1.8+RC-0.8 ···(13) R≧(V0+2.0L-0.95) / 0.4+RC-0.8 ···(14)

[0055] 15 is a graph showing the relationship between the distance R between the furnace center C and the HBI injection position Pr and the flow velocity of the molten iron 21 when the arc length L changes between 400 mm and 600 mm. The graph shown is a result of calculating the flow velocity of the molten iron 21 at which the HBI residence time is 40 seconds or more when the distance R between the furnace center C and the HBI injection position Pr is equal to or less than 1550 mm, which is the high temperature region Rh. It can be seen from the graphs shown that when the arc length L changes, the slope of each graph is the same, so that it is sufficient to change the intercept of formula (9).

[0056] When the arc length L changes from 400 mm to 600 mm, the change in the flow velocity of the molten iron 21 is 0.4 m / s, so the coefficient of the arc length L (m) is -2.0. For this reason, the following formula (15) is obtained by converting formula (9) into a form that takes into account the arc length L. R≧(V0-2.0L-0.4) / 1.48+RC-0.8 ···(15)

[0057] Using a similar method, we calculated the case where the HBI stays in the high temperature region Rh for 60 seconds, and the formula (10) was converted to a form taking into account the arc length L, as shown in the following formula (16). R≧(V0-2.0L+1.37) / 0.94+RC-0.8 ···(16) From the above, it is possible to locate the HBI injection position Pr closer to the furnace wall as the circumferential flow velocity of the molten iron 21 increases, but as shown in Fig. 13(a), if the flow velocity of the molten iron 21 is too fast, the HBI will move prematurely to the furnace center C. Therefore, it is found that a maximum circumferential flow velocity of the molten iron 21 of 1.5 m / min is sufficient.

[0058] A possible method for rotating the molten iron 21 in the horizontal direction in the AC electric furnace 1 is to apply electromagnetic force by stirring the molten iron 21 using an electromagnetic stirrer 11 provided on the bottom of the furnace. Below, a method for forming a rotating flow field in the molten iron 21 using two electromagnetic stirrers 11a and 11b was examined.

[0059] Fig. 16 shows a schematic diagram of the bottom of the AC electric furnace 1 in this embodiment when two electromagnetic stirrers 11a and 11b are provided. In Fig. 16, (a) is a schematic diagram of the bottom surface of the AC electric furnace 1 from below the bottom, and (b) is a schematic diagram of a cross section of the bottom of the AC electric furnace 1. The electromagnetic stirrers 11a and 11b are made of a core made of laminated electromagnetic steel sheets, wrapped with a copper coil, and housed in a case. As shown in the figure, molten iron 21 is stirred by the two electromagnetic stirrers 11a and 11b.

[0060] As shown in the figure, the two electromagnetic stirrers 11a and 11b installed in parallel each have dimensions a×b, and the distance between them is c. Here, the analysis was performed with a=2 (m), b=8 (m), and c=3 (m). The depth h1 of the molten iron 21 was set to 1.9 m, and the thickness h2 of the hearth was set to 1.0 m. The diameter D of the molten iron 21 was set to 9.6 m. The electromagnetic stirrer 11a generates an electromagnetic force in the direction indicated by f1 in the figure, and the electromagnetic stirrer 11b generates an electromagnetic force in the direction indicated by f2 in the figure. In this way, by installing the electromagnetic stirrers 11a and 11b so that the stirring directions are opposite to each other, the molten iron 21 can be rotated in the horizontal direction.

[0061] Fig. 17 is a diagram showing the results of an analysis of the electromagnetic fields generated by the two electromagnetic stirrers 11a and 11b. In Fig. 17, (a) shows the results of the electromagnetic field analysis of the molten iron 21, and (b) and (c) show the configuration of the copper coil. As shown in (a), electromagnetic force was generated in the direction and strength indicated by the arrow, and it can be seen that the electromagnetic force was generated in a direction that rotated the molten iron 21.

[0062] As shown in (b) and (c) of FIG. 17, three copper coils are used, and three-phase AC with a phase difference of 120 degrees is applied to each of them. The three phases are U, V, and W. Each phase is spaced 60 degrees apart so that the phases of the AC current are U, -W, V, -U, W, and -V. This structure generates an electromagnetic force that moves the molten iron 21 in one direction. The current value conditions in the analysis were a current value of 1000A for each phase, 70 turns, and a current frequency of 0.8Hz.

[0063] Fig. 18 is a diagram showing the flow velocity distribution of the molten iron 21 surface when the electromagnetic stirrers 11a and 11b are provided. The current value of the electromagnetic stirrers 11a and 11b is 1000A, and the current frequency is 0.8Hz. When the molten iron 21 is stirred by the two electromagnetic stirrers 11a and 11b, it can be seen that although the flow velocity distribution is not completely circular, a rotating flow field is formed. The maximum value V0 of the flow velocity of the molten iron 21 was 0.5m / s, and the peak position Pv of the flow velocity was 2.0m from the furnace center C.

[0064] Therefore, under the above analysis conditions, from equation (7), if the distance R between the furnace center C and the HBI injection position Pr is set to within 1.86 m from the furnace center C, the HBI will reach the high temperature region Rh within 20 seconds after injection and will remain in the high temperature region Rh for 60 seconds or more. Therefore, the HBI can be melted efficiently.

[0065] Table 1 shows the results of calculating the distance R between the furnace center C and the HBI injection position Pr when the current value and current frequency of the electromagnetic stirrers 11a and 11b are changed. The maximum value V0 and peak position Pv of the flow velocity of the molten iron 21 at each level are evaluated, and the evaluation results for the distance R between the furnace center C and the HBI injection position Pr are shown using equations (5) to (10). Levels 1 to 9 show cases where the current frequency is changed with a current value of 1000A, and levels 10 to 15 show cases where the current frequency is changed with a current value of 0.8Hz. The calculation was performed with the upper electrode 6 positioned 0.8m from the furnace center C and the arc length L being 0.5m.

[0066] [Table 1]

[0067] Fig. 19 is a graph showing the change in the maximum value V0 of the flow velocity of the molten iron 21 and the peak position Pv of the flow velocity when the current frequency of the electromagnetic stirrers 11a and 11b is changed. In Fig. 19, (a) shows the relationship between the current frequency of the electromagnetic stirrers 11a and 11b and the flow velocity of the molten iron 21, and (b) shows the relationship between the current frequency of the electromagnetic stirrers 11a and 11b and the peak position Pv of the flow velocity of the molten iron 21. As shown in (a), it can be seen that the flow velocity of the molten iron 21 is maximum when the current frequency is 0.6 Hz, and that the flow velocity of the molten iron 21 decreases as the current frequency increases further. In contrast, as shown in (b), it can be seen that the peak position Pv of the flow velocity of the molten iron 21 is relatively independent of the current frequency.

[0068] 20 is a graph showing the change in the maximum value V0 of the flow velocity of the molten iron 21 and the peak position Pv of the flow velocity of the molten iron 21 when the current value of the electromagnetic stirrers 11a and 11b is changed. (a) shows the relationship between the current value of the electromagnetic stirrers 11a and 11b and the flow velocity of the molten iron 21, and (b) shows the relationship between the current value of the electromagnetic stirrers 11a and 11b and the peak position Pv of the flow velocity of the molten iron 21. As shown in (a), the larger the current value of the electromagnetic stirrers 11a and 11b, the larger the flow velocity of the molten iron 21. In contrast, as shown in (b), it can be seen that the peak position Pv of the flow velocity of the molten iron 21 is relatively independent of the current value.

[0069] As described above, the peak position Pv of the flow velocity of the molten iron 21 does not depend on the current value and current frequency of the electromagnetic stirrers 11a and 11b. Therefore, the peak position Pv of the flow velocity of the molten iron 21 is determined by the positions and sizes of the coils of the electromagnetic stirrers 11a and 11b. In the AC electric furnace 1 of this embodiment, it can be said that the positions and sizes of the coils of the electromagnetic stirrers 11a and 11b should be designed so that the peak position Pv of the flow velocity is within a range of 1.0 m to 2.5 m.

[0070] FIG. 21 is a graph showing the results of applying formulas (5) to (10) to the current frequency of the electromagnetic stirrers 11a and 11b and the distance R between the furnace center C and the HBI introduction position Pr. In FIG. 21, (a) shows a desirable range for the HBI introduction position Pr, and (a) shows a more desirable range for the HBI introduction position Pr. Note that (a) and (b) are the same graph. In order for the HBI to enter the high temperature region Rh within 40 seconds after introduction and to stay there for 40 seconds or more, it is necessary to be in the region between formulas (5) and (9) in the graph (a) (the region hatched in the figure). In addition, in order for the HBI to enter the high temperature region Rh within 20 seconds after introduction and to stay there for 60 seconds or more, it is necessary to be in the region between formulas (7) and (10) in the graph (b) (the region hatched in the figure).

[0071] FIG. 22 is a graph showing the results of applying formulas (5) to (10) to the current value of the electromagnetic stirrers 11a and 11b and the distance R between the furnace center C and the HBI introduction position Pr. In FIG. 22, (a) shows a desirable range for the HBI introduction position Pr, and (a) shows a more desirable range for the HBI introduction position Pr. Note that (a) and (b) are the same graph. In order for the HBI to enter the high temperature region Rh within 40 seconds after introduction and to stay there for 40 seconds or more, it is necessary for the region in graph (a) to be below formula (5) and above formulas (8) and (9) (hatched region in the figure). In addition, in order for the HBI to enter the high temperature region Rh within 20 seconds after introduction and to stay there for 60 seconds or more, it is necessary for the region in graph (b) to be below formula (7) and above formulas (8) and (10) (hatched region in the figure).

[0072] From the graphs of Figure 21(b) and Figure 22(b), it can be seen that when the current value of the electromagnetic stirrers 11a and 11b is 1000 A and the current frequency is 0.8 Hz, it is desirable for the HBI injection position Pr to be at a distance R of approximately 1.7 m from the furnace center C.

[0073] Fig. 23 is a diagram showing the results of calculating the trajectory of the HBI when the distance R between the reactor center C and the HBI injection position Pr is 1.7 m. In Fig. 23, (a), (b), and (c) show the results of calculating the trajectory when the HBI is injected at different points that satisfy the condition that the distance R = 1.7 m. As shown in the figure, regardless of the point at which the HBI is injected, it can be seen that after injection, the HBI quickly enters the high temperature region Rh, which is within the distance R = 1.55 m between the reactor center C and the HBI injection position Pr, and remains there for a long time.

[0074] (Effects of the metal melting method according to the present embodiment) A metal melting method according to an embodiment of the present invention includes a step of feeding melting raw materials such as HBI, which have a lower density than molten iron 21, from the ceiling 2 of an AC electric furnace 1 through a raw material feeding pipe 8. The method also includes a step of melting the melting raw materials using an arc 20 as a heat source while continuously feeding the melting raw materials into the furnace. The method further includes a step of forming a rotational flow field on the surface of the molten iron 21 in a horizontal direction relative to the surface of the molten iron, and applying a centripetal force to the melting raw materials floating on the surface of the molten iron by the rotational flow field, thereby moving the melting raw materials to a high-temperature region Rh in the furnace. These steps allow the melting raw materials such as HBI, which have a lower density than the molten iron 21 and float on the surface of the molten iron, to quickly reach the high-temperature region Rh, so that the melting raw materials can be melted more efficiently in a short time.

[0075] Although the preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, the present invention is not limited to these examples. It is clear that a person skilled in the art in the technical field to which the present invention pertains can come up with various modified or altered examples within the scope of the technical ideas described in the claims, and it is understood that these also naturally fall within the technical scope of the present invention. [Explanation of symbols]

[0076] 1...AC electric furnace, 2...ceiling, 3...furnace wall copper panel, 4...furnace wall refractory, 5...furnace bottom refractory, 6...upper electrode, 8...raw material input tube, 11, 11a, 11b...electromagnetic stirrer, 20...arc, 21...molten iron, 22...electric furnace slag, L...arc length, Pr...input position, R...distance, RC...distance between furnace center and upper electrode center, Rh...high temperature region, V0...maximum flow velocity.

Claims

1. A method for melting metal in an AC electric furnace having three upper electrodes and a raw material charging tube provided on a side or top surface of a furnace body, comprising: a step of charging a molten raw material having a density lower than that of molten iron from the ceiling through the raw material charging pipe; a step of melting the raw material by using an arc as a heat source while continuously supplying the raw material into a furnace; forming a horizontal rotational flow field on the surface of the molten iron; a step of applying a centripetal force to the molten raw materials floating on the molten metal surface by the rotational flow field, thereby moving the molten raw materials to a high temperature region in the furnace; A metal melting method comprising:

2. In the rotating flow field, the position where the flow velocity of the molten iron is maximum is within a range of 1.0 m to 2.5 m from the center of the furnace. The method for dissolving metal according to claim 1.

3. Arc length L (m), maximum flow velocity V of the molten iron ０ (m / s), and the distance RC (m) between the center of the furnace and the center of the upper electrode satisfies the following relations (i), (ii), and (iii); The method for dissolving metal according to claim 2. R≧(V ０ +2.65L-1.56) / 0.4+RC-0.8 ・・・(i) R≧(V ０ -2.0L-0.4) / 1.48+RC-0.8 ・・・(ii) 0.4≦L≦0.6...(iii)

4. Arc length L (m), maximum flow velocity V of the molten iron ０ (m / s), and the distance RC (m) between the center of the furnace and the center of the upper electrode satisfies the following relations (v) and (vi); The metal dissolving method according to claim 3. R≧(V ０ +2.0L-0.95) / 0.4+RC-0.8 ・・・(v) R≧(V ０ -2.0L+1.37) / 0.94+RC-0.8 ・・・(vi)

5. Arc length L (m), maximum flow velocity V of the molten iron ０ (m / s), and the distance RC (m) between the center of the furnace and the center of the upper electrode satisfies the following relational expression (vii): The metal dissolving method according to claim 4. R≦(V ０ +1.01L+238) / 1.8+R-0.8 ・・・ (vii)

6. Arc length L (m), maximum flow velocity V of the molten iron ０ (m / s), and the distance RC (m) between the center of the furnace and the center of the upper electrode satisfies the following relational expression (iv): The method for dissolving metal according to claim 2. R≦(V ０ +1.7L+0.88) / 1.1+RC-0.8 ・・・(iv)

7. The AC electric furnace is provided with two electromagnetic stirrers on the bottom surface of the furnace. The rotating flow field is formed by electromagnetic forces acting on the molten iron in opposite directions generated by the two electromagnetic stirrers. The metal dissolving method according to any one of claims 1 to 6.

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