Direct-current electric furnace

Abstract

To provide a direct-current electric furnace capable of efficiently melting a melting raw material such as DRI and HBI with smaller density than that of molten iron and easy to float to a molten metal surface in a short time.SOLUTION: In a direct-current electric furnace according to the present invention, when a face including a surface of molten iron is designated as an xy-plane, a straight line passing through a furnace center and an intersection point of a vertical line passing through a center of one arbitrary upper electrode among three upper electrodes and orthogonal to the surface of the molten iron and the surface of the molten iron is as an x-axis, the distance between the furnace center and the center of the upper electrode is as R, and the radius of the upper electrode is as h, a position (x, y) on an xy-coordinate of a point, at which an inputted melting raw material reaches the surface of the molten iron, is expressed by following formulae (i)-(iii).SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a direct current electric furnace.

Background Art

[0002] Since the iron source produced by the blast furnace method is produced by reducing iron ore with coke, the amount of CO2 generated is large. As a means of reducing the amount of CO2 generated, there is a method of producing hot metal by melting iron scrap, DRI (Direct Reduced Iron), etc. in an electric furnace and using the existing steelmaking process centered on converters to produce molten steel. Melting is performed by an arc generated from an upper electrode provided in the electric furnace to the surface of the molten iron.

[0003] In a direct current electric furnace, it is known that an arc is deflected by a magnetic field caused by a current flowing through a cable from a bottom electrode. As a countermeasure, a magnetic field generating coil is provided around the bottom electrode so that a winding line circulates, and a direct current magnetic field having a rotationally symmetric component about the arc generating electrode axis is generated. As a result, since the arc rotates about the furnace center, heat load in any direction can be made uniform, and a technique for realizing uniform melting is disclosed (Patent Document 1). Further, a technique for suppressing the deflection of an arc by making the vertical component of the magnetic flux density 90 gauss or more and the horizontal component 30 gauss or less in the arc generation region by a coil wound around the periphery of the furnace bottom is disclosed (Patent Document 2).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] Direct reduced iron is obtained by directly reducing iron ore or iron ore pellets with a reducing gas such as hydrogen, and is called DRI. Also, the DRI after the reduction treatment that is hot-compression molded is called HBI (Hot Briquetted Iron). The apparent density of DRI is about 3500 kg / m 3 and the apparent density of HBI is about 5000 to 5500 kg / m 3 . Since the density of molten iron is about 7000 kg / m 3 , both DRI and HBI are lighter than molten iron and float on the surface of the molten metal.

[0006] In the technologies disclosed in Patent Documents 1 and 2, in order to efficiently heat the scrap precipitated in the molten iron, it is possible to easily stir the molten iron near the scrap. However, when using reduced iron, it is necessary to more efficiently melt DRI and HBI, which have a density smaller than that of molten iron and float on the surface of the molten metal, in a shorter time.

[0007] Therefore, an object of the present invention is to provide a direct current electric furnace capable of more efficiently melting melting raw materials such as DRI and HBI, which have a density smaller than that of molten iron and are likely to float on the surface of the molten metal, in a shorter time.

Means for Solving the Problems

[0008] [1] In a direct current electric furnace comprising three upper electrodes and a raw material input pipe provided on the side surface or the upper surface of the furnace body for introducing a raw material containing a melting raw material having a density smaller than that of molten iron into the furnace, the three upper electrodes are provided such that the shape connecting the centers of the respective electrodes forms an equilateral triangle. Taking the plane including the surface of the molten iron as the xy plane, the straight line passing through the center of the furnace and the intersection of the vertical line passing through the center of any one of the three upper electrodes and perpendicular to the surface of the molten iron and the surface of the molten iron as the x-axis, and when the distance between the center of the furnace and the center of the upper electrode is R and the radius of the upper electrode is h, the position (x, y) on the xy coordinates of the point where the introduced melting raw material reaches the surface of the molten iron is represented by the following formulas (i) to (iii). A direct current electric furnace.

Number

[0009] According to the above configuration, melting raw materials such as HBI having a density smaller than that of molten iron and likely to float on the surface quickly reach the high-temperature region. Therefore, reducing iron materials such as DRI and HBI having a density smaller than that of molten iron and likely to float on the surface can be melted more efficiently and in a shorter time. [Brief Description of the Drawings]

[0010]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Embodiments for Carrying Out the Invention

[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 configuration are denoted by the same reference numerals, and redundant descriptions are omitted.

[0012] (Structure of a DC Electric Furnace According to an Embodiment) FIG. 1 is a cross-sectional view showing the structure of a DC electric furnace 1 according to an embodiment of the present invention. In the figure and the following description, three mutually orthogonal directions are defined as the +x direction, +y direction, and +z direction. Although not shown, the direction opposite to the +x direction is the -x direction, the direction opposite to the +y direction is the -y direction, and the direction opposite to the +z direction is the -z direction. The plane including the surface of the molten iron 21 is defined as the xy plane, but the surface of the molten iron does not necessarily have to be a strict plane and may have irregularities.

[0013] As shown in the figure, the furnace body of the DC electric furnace 1 includes a ceiling 2, a furnace wall copper panel 3, a furnace wall refractory 4, and a furnace bottom refractory 5. Further, three upper electrodes 6 are provided on the ceiling 2, and a furnace bottom electrode 7 and a bottom blowing tuyere 8 are provided at the bottom of the furnace body. An upper electrode cable 6a is connected to the upper electrode 6. Also, a furnace bottom cable 7a is connected to the furnace bottom electrode 7. In a large DC electric furnace 1, the current per upper electrode 6 is about 100 kA to 140 kA at maximum. Therefore, the total current value is a large current of about 300 kA to 420 kA.

[0014] The raw material input pipe 9 is provided on the side surface or the upper surface of the furnace body, and inputs raw materials including melting raw materials having a density smaller than that of the molten iron 21 such as at least DRI and HBI into the furnace. Further, hereinafter, the melting raw material having a density smaller than that of the molten iron 21 will be described as HBI, but it is not limited to HBI and may be a raw material such as DRI. A tapping hole 10 for discharging the molten iron 21 and a slag tapping hole 11 for discharging the electric furnace slag 22 are provided in the furnace wall refractory 4.

[0015] As the DC electric furnace 1, a tilting type in which the furnace body tilts can also be used, or a stationary type in which the furnace body does not tilt can also be used. Further, one or two of iron-containing scrap and reduced iron are charged into the DC electric furnace 1 as an iron source. As the reduced iron, HBI, DRI, etc. can be used. Iron-containing dust may be charged into the furnace as an iron source. As the iron-containing dust, granulated converter dust can be used. Pr in the figure indicates the point where the HBI charged from the raw material input pipe 9 reaches the surface of the molten iron 21. Hereinafter, this point will be referred to as the input position Pr. Let the distance from the furnace center C to the input position Pr of the HBI be r.

[0016] An arc 20 is discharged from the upper electrode 6 to melt raw materials such as reduced iron to produce molten iron 21. By blowing gas from the bottom blowing tuyere 8 to cause a circulation flow in the molten iron 21 and the electric furnace slag 22, heat transfer and melting of the surface of the molten iron 21 and the electric furnace slag 22 can be promoted.

[0017] Figure 2 is a cross-sectional view taken along line II-II of the DC electric furnace 1 shown in Figure 1. As shown in the figure, the DC electric furnace 1 has a circular horizontal cross-section. Also, the three upper electrodes 6 are arranged such that the shape connecting the centers of the respective upper electrodes 6 forms an equilateral triangle. Here, although it is ideal to install the upper electrodes 6 to form an equilateral triangle, due to equipment constraints, there may be a deviation on the order of the radius of the upper electrode 6, and this deviation does not cause a significant change in the effects of the present invention. In the following description, the distance between the furnace center C and the center of the upper electrode 6 is denoted as R, and the radius of the upper electrode 6 is denoted as h.

[0018] (Outline and Conditions of Simulation for Evaluating Heat Flow in DC Electric Furnace) Since the arc 20 emitted from the upper electrode 6 is at a high temperature and high speed, it is difficult to directly measure the temperature and flow velocity of the arc 20. Therefore, in the present embodiment, a simulation is performed to evaluate the heat flow in the DC electric furnace 1 due to arc discharge. The simulation is carried out based on the method described in Jonas ALEXIS, Marco RAMIREZ, Gerardo TRAPAGA and Par JONSSON, “Modeling of a DC Electric Arc Furnace-Heat Transfer from the Arc” ISIJ International, Vol.40 (2000), pp.1089-1097, and the outline thereof will be described below.

[0019] In the simulation, the arc 20 emitted from the upper electrode 6 is regarded as an electromagnetic fluid having conductivity, and a coupled calculation of electromagnetic field analysis, fluid analysis, and heat transfer analysis is performed. First, the method of electromagnetic field analysis will be described. Assuming that the current density flowing through the arc 20 is i (A / m 2 ), the relationship between the magnetic vector potential A and the current density i is expressed as in Equation (1). μ0 is the magnetic permeability of vacuum.

[0020]

Equation

[0021] The current density i is calculated using Equation (2), which is the conservation law of current density. Here, φ is the potential at the inlet and outlet sides of the current set as the boundary condition, and σ is the conductivity (S / m) of the arc 20, which is a value dependent on the temperature of the arc 20.

[0022]

Number

[0023] The magnetic vector potential A is calculated from Equation (3) based on the current density distribution calculated from Equation (2). Then, using the calculated magnetic vector potential A, the distribution of the magnetic flux density B (T) is calculated from Equation (3).

[0024]

Number

[0025] Next, the methods of fluid analysis and heat transfer analysis will be described. In a conductor through which current flows in a magnetic field, a Lorentz force F (N / m 3 ) = i × B is generated. Also, a Joule heating amount q (W / m 3 ) = i 2 ² / σ is generated. Using the calculated Lorentz force F and Joule heating amount q, fluid analysis and heat transfer analysis are performed. In fluid analysis, the flow of the arc 20 is analyzed by the mass conservation law shown in Equation (4) and the Navier - Stokes equation shown in Equation (5). Here, u is the flow velocity, ρ is the density, t is the time, p is the pressure, μ is the viscosity, and g is the acceleration due to gravity.

[0026]

Number

[0027] Heat transfer analysis is performed using Equation (6), which is the governing equation of heat transfer analysis. Here, C p is the specific heat, T is the temperature, λ is the thermal conductivity, S r is the radiant energy, Q tis the energy loss due to the Thomson effect.

[0028]

Number

[0029] By performing the electromagnetic field analysis, fluid analysis, and heat transfer analysis as described above, the temperature and flow velocity of the arc 20 can be obtained. As an example, the calculation conditions in the simulation were as follows: the diameter of the upper electrode 6 was 800 mm, the distance R between the furnace center C and the center of the upper electrode 6 was 3400 mm, and the distance between the upper electrode 6 and the molten iron surface of the molten iron 21 was 500 mm. Also, the atmospheric gas in the furnace was Ar, and the potential of the molten iron surface of the molten iron 21 was set to 0. Furthermore, the current value I flowing through each upper electrode 6 was 100 kA, and the starting point from which the arc 20 was emitted was the center of the lower end surface of the upper electrode 6.

[0030] Figure 3 is a diagram showing the temperature distribution of the arc 20 emitted from the upper electrode 6 simulated in the DC electric furnace 1 shown in Figure 1. In Figure 3, (a) shows the temperature distribution in the DC electric furnace 1 in a perspective view, and (b) shows it including the cross-section of the arc 20 emitted from the upper electrode 6. In the figure, the temperature distribution of the arc 20 is represented by a three-dimensional contour surface. In (a), a temperature surface of 3000 K is drawn, and in (b), the temperature distribution in the cross-section of the arc 20 emitted from the three upper electrodes 6 is drawn. As shown in the figure, in the DC electric furnace 1, the vicinity of the furnace center C, which is the region surrounded by the three upper electrodes 6, becomes a high-temperature region Rh. Therefore, if the HBI collects near the furnace center C due to the flow of the molten iron 21, the HBI will be efficiently melted.

[0031] As described above, the arcs 20 emitted from the three upper electrodes 6 incline so as to attract each other. Therefore, the molten iron surface in the range directly below the upper electrode 6 and surrounded by the upper electrodes 6 becomes a high-temperature region Rh. The high-temperature region Rh is defined as the range surrounded by the tangent lines of the upper electrode 6 as shown in Figure 2.

[0032] (Fluid flow analysis of molten iron) FIG. 4 is a diagram showing the distribution of the Lorentz force density by electromagnetic field analysis in the DC electric furnace 1 according to an embodiment of the present invention. In FIG. 4, (a) is a view of the distribution of the Lorentz force density on the molten iron surface 21 as seen from the ceiling 2 side, and (b) is a view of the distribution of the Lorentz force density on the furnace bottom as seen from the furnace bottom side. Further, (c) is a diagram showing the distribution of the Lorentz force density in the molten iron 21 in the A-A line cross-sectional view directly below the upper electrode 6 in (a), and (d) is a diagram showing the direction of the electromagnetic force in the molten iron 21 in the A-A line cross-sectional view.

[0033] As shown in the figure, a large Lorentz force density is generated in the molten iron 21 directly below the upper electrode 6, and an electromagnetic force in the direction of pushing the molten iron 21 is generated. Also, on the side of the bottom electrode 7, an electromagnetic force in the direction toward the molten iron surface 21 is generated, but since the current density is lower than that directly below the upper electrode 6, it can be seen that the Lorentz force density is small.

[0034] (Evaluation of Molten Iron Flow) FIG. 5 is a diagram showing the result of analyzing the flow of the molten iron 21 based on the distribution of the Lorentz force density shown in FIG. 4. In FIG. 5, (a) is a view of the flow of the molten iron 21 on the molten iron surface as seen from the ceiling 2 side, and (b) is a view of the flow of the molten iron 21 in the B-B line cross-sectional view directly below the upper electrode 6. As shown in the figure, directly below the upper electrode 6, since the force pushing the molten iron 21 is strong, the molten iron 21 flows toward the furnace bottom, forms a flow that rises in the direction of the molten iron surface after colliding with the furnace bottom. Near the molten iron surface, the molten iron 21 rises from the furnace bottom near the furnace center C to the molten iron surface, forming a flow in the direction toward the furnace wall. The HBI introduced in such a situation where the flow of the molten iron 21 is formed will flow near the furnace wall where it is not in the high-temperature region Rh, resulting in a decrease in the melting efficiency of the HBI.

[0035] (Outline and Conditions of Simulation for Evaluation of HBI Movement) Since the inside of the furnace is extremely hot, it is difficult to directly measure the trajectory of the HBI moving inside the furnace. Therefore, in this embodiment, a simulation for evaluating the trajectory of the HBI is performed. In order to evaluate the trajectory of the HBI when the HBI is charged from the raw material charging pipe 9 of the DC electric furnace 1, the following analysis was carried out. The simulation is to be performed based on the method described in Turbulence Second Edition, HINZE, McGRAW-HILL, New York, 1975, p.460-462, and its outline will be described below. The motion of a sphere in a fluid can be represented by the BBOT equation shown in the following equation (7). Here, v is the velocity of the particle, F D is the drag force, v f is the velocity of the fluid, and g is the acceleration due to gravity.

[0036]

Equation

[0037] Here, let the density of the particle and the fluid be ρ and ρ f respectively, the particle diameter be d, and the drag coefficient be C D . Then, the coefficients C1, C2, and C3, and the drag force F D are represented by the following equations (8) and (9).

[0038]

Equation

[0039] The second term on the right side of equation (7) is the total differential of the flow velocity. Representing this by another equation gives the following equation (10).

[0040]

Equation

[0041] Also, the viscosity of the molten iron 21 is 0.005 Pa·s, and the density is 7000 kg / m 3, assuming the particle diameter is 5 cm and the fluid velocity range is from 0.05 to 1 m / s, the Reynolds number Re is about 420 to 84000. The drag coefficient C D in this region is about 0.4 to 0.5. Therefore, for HBI, the drag coefficient C D is 0.4, the diameter is 5 cm, the apparent density is 5500 kg / m 3 for the sphere, and the fluid is molten iron 21 with a density of 7000 kg / m 3 . The calculation was performed with these parameters. It was assumed that HBI floats near the surface of molten iron 21 and does not move toward the furnace bottom.

[0042] Furthermore, the general size of HBI is about 30×50×130 mm. At this size, it is known that after HBI is charged into molten iron 21, it dissolves in about 40 to 60 seconds for one HBI. Efficient dissolution of HBI in the DC electric furnace 1 means that the same dissolution rate as when one HBI is charged can be obtained. Therefore, it is preferable to make HBI reach the high-temperature region Rh within 40 seconds, which is the dissolution time, after the HBI is charged.

[0043] (Evaluation of HBI charging position and movement) Fig. 6 is a diagram evaluating the HBI charging position Pr in the DC electric furnace 1 according to an embodiment of the present invention when the current value I of the upper electrode 6 is 100 kA, in relation to the time for HBI to reach the high-temperature region Rh and the time for HBI to stay in the high-temperature region Rh. In Fig. 6, (a) is a diagram evaluating the time for HBI to reach the high-temperature region Rh within 40 seconds after charging, and (b) is a diagram evaluating the time for HBI to stay in the high-temperature region Rh. Also, the x-axis is a straight line passing through the furnace center C and the intersection of the molten iron 21 surface and a vertical line perpendicular to the molten iron 21 surface passing through the center of any one of the three upper electrodes 6. Hereinafter, the x-axis will be described as a straight line passing through the furnace center C and A1.

[0044] As shown in Fig. 6(a), in the range of the high-temperature region Rh and in the range of the influence distance d from the center of the lower end face of the upper electrode 6, the time for the HBI to reach the high-temperature region Rh is short. The influence distance d is defined as the distance from the center of the upper electrode 6 in the vicinity directly below the upper electrode 6, within the range where the downward flow of the molten iron 21 is likely to have an impact. This is presumably due to the fact that the HBI is being attracted in the region where the molten iron 21 forms a downward flow toward the furnace bottom. When the current value I flowing through each upper electrode 6 is 100 kA, the influence distance d is 0.8 m.

[0045] (Conditions for the HBI to stay in the high-temperature region Rh for a long time) As shown in Fig. 6(b), it can be seen that the region Pr of the HBI injection position where the HBI stays in the high-temperature region Rh for 40 seconds or more is narrower compared to (a). This is presumably due to the fact that the upward flow formed by the collision of the downward flows of the molten iron 21 generated directly below the three upper electrodes 6 at the furnace center C is strong. Region M in the figure is the range where the flow of the HBI is likely to be affected by the downward flow of the molten iron 21.

[0046] The region M shown in Fig. 6(b) is calculated by the following equations (11) and (12) using the distance r from the furnace center C, the distance R from the furnace center C to the center of the upper electrode 6, and the influence distance d. Here, θ is the angle with respect to the x-axis and is expressed in radians in the range from 0 to 2π. For example, R = 2.0 m and d = 0.8 m, but it is not limited to this. r ≦ R0(1 + cos3θ) ···(11) R0 = R + d ···(12)

[0047] Fig. 7 is a graph showing the results of evaluating the relationship between the current value I per upper electrode 6 and the influence distance d of the downward flow of the molten iron 21 by numerical analysis. As shown in the figure, it can be seen that as the current value I per upper electrode 6 increases, the influence distance d increases. The result of linearly approximating the data of the calculated results of the current value I and the influence distance d in the graph is shown by the following equation (13). d = 2.2×10 -5 I 2 + 5.9×10-3 I ···(13)

[0048] From the above, in order for the charged HBI to stay in the high-temperature region Rh for a long time, it is preferable that the distance r (m) between the charging position Pr, where the charged HBI reaches the surface of the molten iron 21, and the furnace center C is at the positions represented by equations (11) and (14), and the HBI is charged there. R0 = R + 2.2×10 -5 I 2 + 5.9×10 -3 I ···(14)

[0049] (Conditions for the HBI to reach the high-temperature region Rh quickly) As shown in Fig. 6(a), the position of the furnace center C on the xy coordinates is set to (0, 0). Also, if the positions of the centers of the upper electrodes 6 are A1, A2, and A3 respectively, the positions on the respective xy coordinates are (R, 0), (-1 / 2, (R√3) / 2), and (-1 / 2, -(R√3) / 2). In order for the HBI to reach the high-temperature region Rh within 40 seconds, it is preferable that the charging position Pr is within the range surrounded by the straight lines L1, L2, and L3 shown in the figure.

[0050] The straight lines L1, L2, and L3 are represented by the following equations (15) to (17) respectively. As described above, it is preferable that the charging position Pr is within the range surrounded by the straight lines L1, L2, and L3 shown in the figure. Therefore, the position (x, y) on the xy coordinates of the charging position Pr, which is the point where the charged HBI reaches the surface of the molten iron 21, preferably satisfies the following equations (15) to (17). Here, R is the distance between the furnace center C and the center of the upper electrode 6, and h is the radius of the upper electrode 6.

[0051]

Number

[0052] As described above, in the DC electric furnace 1, if the position (x, y) on the xy coordinates of the charging position Pr of the HBI, which has a density smaller than that of the molten iron 21 and floats on the surface of the molten metal, is in the regions shown in Expressions (15) to (17), the HBI quickly reaches the high-temperature region Rh. Further, if the charging position Pr of the HBI is in the regions shown in Expressions (11) and (14), the residence time in the high-temperature region Rh becomes longer. Therefore, the DC electric furnace 1 according to the present embodiment can more efficiently melt melting raw materials such as DRI and HBI, which have a density smaller than that of the molten iron 21 and are likely to float on the surface of the molten metal, in a short time.

[0053] As described above, the preferred embodiments of the present invention have been described in detail with reference to the accompanying drawings, but the present invention is not limited to these examples. It is obvious that those skilled in the technical field to which the present invention pertains can conceive of various modification examples or correction examples within the scope of the technical idea described in the claims, and it is naturally understood that these also belong to the technical scope of the present invention.

Explanation of Reference Numerals

[0054] 1... DC electric furnace, 2... ceiling, 3... furnace wall copper panel, 4... furnace wall refractory, 5... furnace bottom refractory, 6... upper electrode, 7... furnace bottom electrode, 7a... furnace bottom cable, 9... raw material charging pipe, 20... arc, 21... molten iron, 22... electric furnace slag.

Claims

1. Three upper electrodes, a raw material input pipe provided on the side surface or the upper surface of the furnace body for charging into the furnace a raw material containing a melting raw material having a density smaller than that of molten iron, in a DC electric furnace comprising: the three upper electrodes are provided such that a shape connecting the centers of the respective electrodes forms an equilateral triangle, a plane including the molten iron surface is defined as the xy plane, a straight line passing through the furnace center and the intersection of a vertical line passing through the center of any one of the three upper electrodes and perpendicular to the molten iron surface and the molten iron surface is defined as the x-axis, the distance between the furnace center and the center of the upper electrode is denoted as R, when the radius of the upper electrode is denoted as h, a DC electric furnace, wherein the position (x, y) on the xy coordinates of the point where the charged melting raw material reaches the molten iron surface is represented by the following formulas (i) to (iii). 【Number 1】

2. Three upper electrodes, a raw material input pipe provided on the side surface or the upper surface of the furnace body for charging into the furnace a raw material containing a melting raw material having a density smaller than that of molten iron, in a DC electric furnace comprising: the three upper electrodes are provided such that a shape connecting the centers of the respective electrodes forms an equilateral triangle, a plane including the molten iron surface is defined as the xy plane, a straight line passing through the furnace center and the intersection of a vertical line passing through the center of any one of the three upper electrodes and perpendicular to the molten iron surface and the molten iron surface is defined as the x-axis, the current value per one of the upper electrodes is denoted as I (kA), the angle with respect to the x-axis is denoted as θ, when the distance between the furnace center and the center of the upper electrode is denoted as R, a DC electric furnace, wherein the distance r (m) between the point where the charged melting raw material reaches the molten iron surface and the furnace center is represented by formulas (iv) and (v). r ≤ R ０ (1 + cos 3θ) ··· (iv) R ０ = R + 2.2×10 －５ I ２ + 5.9×10 －３ I ··· (v) However, θ ranges from 0 to 2π.

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