DC electric furnace and method for melting solid raw materials

Abstract

To provide a DC electric furnace capable of performing flexible arc control at a low equipment cost, and a solid raw material melting method using the same.SOLUTION: A DC electric furnace includes at least a pair of DC magnetic field generation devices generating magnetic fields with respect to an arc from an upper electrode at a position higher than melted or un-melted raw material charged inside the furnace. When the furnace includes two pairs of DC magnetic field generation devices, the irradiation position with the arc can be controlled to a two-dimensional optional position. Further, when the furnace has a device capable of monitoring the conditions in the furnace, by emitting the arc to the un-melted part by furnace-inside observation, the raw material can be more efficiently melted.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a DC electric furnace that has a heating function using a DC arc, improves the heating efficiency of unmelted parts during heating, improves the efficiency of oxide reduction, and reduces damage to refractories, thereby enabling efficient operation, and a method for melting solid raw materials using the same. [Background technology]

[0002] DC electric furnaces have the advantage of being able to operate with a single electrode and having a long distance between the high-temperature spot created by the arc and the furnace wall, and are therefore widely used in the melting and reduction processes of various metals.

[0003] The problem with operating this equipment is that the arc generated between the electrode placed mainly at the bottom of the furnace and the electrode placed above the molten material to be heated is concentrated on the unmelted parts of the molten material, particularly those parts close to the electrode, making it difficult to maintain a stable arc formation at all times. As a method for solving this problem, Patent Document 1 discloses a means for stably controlling the arc to the center position in the furnace by installing a magnetic field generating coil on a moving table, and Patent Document 2 discloses a DC electric furnace with a specially designed bottom electrode for the same purpose. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 5-332679 [Patent Document 2] Japanese Patent Application Publication No. 5-13166 [Non-patent literature]

[0005] [Non-Patent Document 1] K. Trivedi et al., Applied Spectroscopy Vol.42(1998),1025-1032 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the methods described in Patent Documents 1 and 2, the direction in which the electromagnetic field is applied is limited to one direction, and a large-scale power generation device is required to form the electromagnetic field in a wide range including the periphery of the furnace. Furthermore, since a strong magnetic field space is also formed around the furnace body, there are various problems such as the possibility of malfunction of peripheral devices and the need for safety management devices for the operating operator.

[0007] In view of the above problems, an object of the present invention is to provide a DC electric furnace capable of flexible arc control at low equipment cost, and a method for melting solid raw materials using the same. [Means for solving the problem]

[0008] The present inventors first conducted intensive research into a method for easily controlling the position of an arc by generating a magnetic field. Here, since an arc discharge is formed by an electrically charged plasma, it is known that it is possible to change its direction in a laboratory using the principle of electromagnetic induction, and it has also been reported in Non-Patent Document 1 that it is possible to control the plasma by applying a magnetic field. Based on these findings, the present inventors arrived at the following invention as an application of a magnetic field to an arc plasma as a means of providing flexibility in the operation of a DC electric furnace.

[0009] The present invention is as follows. (1) A direct current electric furnace, The furnace has at least one pair of DC magnetic field generators that generate a magnetic field for the arc from the upper electrode at a position higher than the molten or unmelted raw materials charged into the furnace. death, A water-cooled panel is installed on the side of the DC electric furnace, and the DC magnetic field generator is disposed within the water-cooled panel. A DC electric furnace characterized by: (2) the above (1) A method for melting a solid raw material using the DC electric furnace described in a magnetic field is generated from the at least one pair of DC magnetic field generators, thereby melting the raw material while changing a position at which the arc is irradiated onto the molten or unmelted raw material. (3) The method for melting solid raw materials described in (2) above, characterized in that the raw materials are melted while a magnetic field is generated from the at least one pair of DC magnetic field generators, and the position irradiated by the arc is periodically moved back and forth in a straight line, or periodically moved within a circle of an arbitrary radius from the center of the furnace. (4) The method for melting solid raw materials according to (2) above, characterized in that, while observing the inside of a furnace, a magnetic field is generated from the at least one pair of DC magnetic field generators, and an arc reaching the surface of the molten steel is controlled to be irradiated onto the unmelted raw materials, thereby melting the raw materials. Effect of the Invention

[0010] According to the present invention, it is possible to provide a DC electric furnace capable of flexible arc control at low equipment cost and a method for melting solid raw materials using the same. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram for explaining an overview of a DC electric furnace according to an embodiment of the present invention. [Diagram 2] 1 is a diagram for explaining an overview of a DC electric furnace according to an embodiment of the present invention, viewed from above. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing an overview of a DC electric furnace according to this embodiment, and FIG. 2 is an overview seen from above of the DC electric furnace, and is a diagram for explaining the mechanism of arc control in a DC electric furnace equipped with a DC magnetic field generating device.

[0013] As shown in FIG. 1, the DC electric furnace according to this embodiment has a structure in which a refractory material 1 and a water-cooled panel 2 are arranged at the side of the refractory material 1. A lower electrode 5 provided at the bottom of the furnace is a positive electrode, and a graphite upper electrode 6 provided at the height of the water-cooled panel 2 is a negative electrode. A current is passed between the lower electrode 5 and the upper electrode 6 by a DC power source 7, and a plasma arc 8 can be generated. By charging scrap into the furnace and generating a plasma arc 8, the scrap is melted to produce molten steel. Note that FIG. 1 shows a state in the middle of melting the scrap, in which molten steel 3 and unmelted scrap 4 are mixed in the furnace, and the molten steel 3 and unmelted scrap 4 in the furnace are heated by the plasma arc 8.

[0014] In addition, a DC magnetic field generator 9 with a maximum magnetic field of 0.5 T is provided in the water-cooled panel 2, and as shown in FIG. 2, a magnetic field 11 is generated from the DC magnetic field generator 9a toward the DC magnetic field generator 9b on the opposite side (in the y-axis direction) as shown in FIG. 2. Similarly, in the x-axis direction, the magnetic field 11 can be generated from the DC magnetic field generator 9c toward the DC magnetic field generator 9d on the opposite side. In the example of FIG. 2, two pairs of DC magnetic field generators, namely, the DC magnetic field generators 9a and 9b and the DC magnetic field generators 9c and 9d, are provided. In addition, the magnetic field can be generated in the opposite direction, and the two pairs of DC magnetic field generators can be operated simultaneously. In this way, in this embodiment, the magnetic field 11 is formed at a position higher than the molten steel 3 and the unmelted scrap 4.

[0015] Here, because the plasma arc 8 is a DC electromagnetic fluid, a magnetic field 10 is also generated by the plasma arc 8 due to the right-hand screw rule (Ampere's law). Thus, as shown in Fig. 2, a magnetic field 11 generated by a DC magnetic field generator 9 is adjusted by the magnetic field 10, and an electromagnetic force 14 acts on the plasma arc 8. Here, the magnitude and direction of the electromagnetic force 14 are calculated as the vector product of the current and the magnetic flux density according to Fleming's law. Furthermore, if the lower electrode 5 is made negative and the upper electrode 6 is made positive, the direction of the electromagnetic force 14 will be reversed, but similar control is possible.

[0016] During melting of scrap in a DC electric furnace, instability of the plasma passing position due to unmelted scrap, etc., becomes a problem. In this embodiment, however, the plasma irradiation position can be controlled to the unmelted part by the effect of the electromagnetic force acting on the plasma arc, and the scrap can be efficiently melted and the melting time can be shortened while avoiding damage to the furnace body due to long-term strong heating conditions at one place. For example, by generating a magnetic field using a pair of DC magnetic field generators 9a, 9b, the plasma arc 8 can be controlled in the x-axis direction, and the arc can be prevented from being concentrated in a part close to the electrode. In addition, since it is necessary to install at least one pair of DC magnetic field generators, the scrap can be efficiently melted at a low equipment cost.

[0017] Although a pair of DC magnetic field generators 9a and 9b can be used to generate a magnetic field, which can efficiently melt unmelted scrap and prevent damage to the furnace body, two pairs of DC magnetic field generators may be provided as shown in FIG. 2, and the irradiation position of the plasma arc may be controlled to any position in two dimensions by controlling two independent electromagnetic forces. In this case, the magnitude of the electromagnetic force can be controlled by controlling the magnitude of the magnetic flux density (B) from the DC magnetic field generator, and the direction of the electromagnetic force can be controlled by controlling the ratio of the magnitude of the electromagnetic force in the x direction and the y direction with the two pairs of DC magnetic field generators. This allows the plasma arc to be irradiated to the molten steel or unmelted scrap over a wide range, for example, by periodically changing the irradiation position of the arc in a circular shape or a linear shape, and the scrap can be melted more efficiently. Three or more pairs of DC magnetic field generators may be provided, but the effect is limited compared to the cost involved.

[0018] In addition, a furnace interior observation device (not shown) using multiple optical fibers may be provided in the DC electric furnace, and the direction and magnitude of the electromagnetic force may be controlled so that the plasma arc is directly irradiated onto large unmelted scrap by observing the inside of the furnace. This allows the unmelted scrap to be targeted by the unmelted scrap during furnace observation, so that the unmelted scrap is mainly irradiated with the plasma arc, allowing the scrap to be melted more efficiently. In addition, the direction and magnitude of the electromagnetic force may be controlled so that the unmelted scrap is directly irradiated with the plasma arc by observing the inside of the furnace, within a range that does not damage the refractory of the furnace wall.

[0019] Furthermore, in the example shown in FIG. 1, a DC magnetic field generator is provided in the water-cooled panel 2, and a method that is easy to maintain is adopted, similar to the DC electromagnetic brake used in continuous casting. However, in the case of a DC electric furnace that does not use a water-cooled panel, it is possible to enjoy an effect similar to that of this embodiment by embedding the DC magnetic field generator in a refractory material so as not to interfere with operation.

[0020] Furthermore, although the present embodiment has been described as an example of melting scrap, the present invention can be similarly applied to the melting of other solid metals and to a method for melting solid raw materials in a reduction process of metal oxides, such as iron ore, reduced iron (DRI) with a reduction rate of 90% or more, manganese ore, and chromium ore, and similar effects can be obtained for various operations such as phosphoric acid reduction from slag. EXAMPLES

[0021] Next, the present invention will be further described based on examples, but the conditions in the examples are merely examples of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

[0022] In order to verify the effect of the present invention, a DC electric furnace (transformer capacity: 500 kVA) with an inner diameter of 600 mm and melting capacity of 800 kg as shown in Fig. 1 was used, scrap with a size of 50 to 200 mm was charged, a voltage was applied to the upper graphite electrode (upper electrode 6: negative electrode), and a current was passed through the unmelted scrap in the furnace and the molten iron being melted or after melting, to the electrode (lower electrode 5: positive electrode) installed at the bottom of the furnace, to carry out a test production of molten steel. Note that in all of the following experiments, the temperature after the end of the melting process was set to 1610 to 1630°C, and it was evaluated that the effect of the present invention was obtained if the melting time was within 50 minutes.

[0023] [Table 1]

[0024] In the first example of the invention, the inside of the furnace was not observed, and the experiment was performed with one of two pairs of DC magnetic field generators (DC magnetic field generators 9c, 9d) installed in the DC electric furnace turned off. Before the operation using one pair of DC magnetic field generators, the characteristics of the electromagnetic force and the arc current were investigated in advance, and based on the results of the investigation, the arc irradiation position was periodically moved from the center of the furnace to a region of 150 mm in both directions at a period of 5 seconds, and the scrap was melted while the arc was irradiated to the moving part longer than usual. As a result, the melting process could be performed within 50 minutes, which was the target melting time of this experiment.

[0025] In Example 2, two pairs of DC magnetic field generators were used, but the inside of the furnace was not observed. Based on the characteristics of the electromagnetic force and the arc current that were previously investigated, the arc irradiation position was periodically moved in a cycle of 5 seconds within a circle with a radius of 150 mm from the center of the furnace, and the scrap was melted while irradiating a wider area than usual with the arc. As a result, the melting process could be performed in a shorter time than in Example 1.

[0026] In Example 3, two pairs of DC magnetic field generators were used, and the inside of the furnace was observed while controlling the irradiation of the plasma arc reaching the surface of the molten steel onto large unmelted scrap to melt the scrap. As a result, the melting time was an extremely short 32 minutes.

[0027] In Example 4, two pairs of DC magnetic field generators were used, and observation of the inside of the furnace was also carried out, but in order to avoid damage to the furnace walls, the scrap was melted by controlling irradiation of large unmelted scrap within the range where the arc would not collide with the furnace walls according to the furnace interior observation conditions, similar to Example 3. As a result, although a slightly longer melting time was required than in Example 3, the damage to the refractory after melting was completed was extremely minor, and repair work to be carried out between charges could be significantly reduced.

[0028] On the other hand, in the comparative example, the power supply of all DC magnetic field generators was turned off and the scrap was melted under the condition of no electromagnetic force being applied. In this example, the plasma arc was mainly irradiated only at the center, and the plasma arc did not reach the unmelted scrap around it, resulting in low heat transfer efficiency. In addition, spark-like arc discharge occasionally reached the unmelted parts near the furnace wall, attacking the refractory. As a result, the arc irradiation operation took a long time, the melting time was 71 minutes, which was significantly longer than the target value, and the operation also caused significant damage to the refractory.

[0029] As described above, the effects of the present invention were confirmed using a small DC electric furnace. However, in the case of a large DC electric furnace having a furnace diameter of 7 to 10 m, a transformer capacity of 100 to 400 MVA, and a capacity of 150 to 350 t, for example, if a magnetic field of more than 0.1 T can be generated from the DC magnetic field generator, it is calculated that the effects of the present invention can be fully enjoyed, and it can be evaluated that the present invention can be effectively utilized in large-scale steelmaking processes. [Industrial Applicability]

[0030] INDUSTRIAL APPLICABILITY The present invention shortens the melting time and reduces damage to refractories in operations such as the production of molten steel by melting scrap and the reduction process of metal oxides such as iron, manganese, and chromium. Similarly, the present invention enables oxide reduction treatment to be carried out more efficiently than with conventional DC electric furnaces, and is of great industrial value. [Explanation of symbols]

[0031] 1 Refractories 2 Water Cooling Panel 3. Molten Steel 4 Unmelted scrap 5 Lower electrode 6 Upper electrode 7 DC power supply 8. Plasma Arc 9. DC magnetic field generator 10, 11 Magnetic field 12 Electromagnetic force

Claims

1. A direct current electric furnace, At least one pair of DC magnetic field generating devices is provided for generating a magnetic field for an arc from an upper electrode at a position higher than the molten or unmelted raw materials charged into the furnace; A DC electric furnace, characterized in that a water-cooled panel is installed on a side of the DC electric furnace, and the DC magnetic field generator is disposed within the water-cooled panel.

2. A method for melting a solid raw material using a DC electric furnace according to claim 1, a step of forming an arc between the upper electrode and the lower electrode, and generating a magnetic field from the at least one pair of DC magnetic field generators, thereby melting the raw material while changing a position at which the arc is irradiated onto the molten or unmelted raw material.

3. A method for melting a solid raw material as described in claim 2, characterized in that the raw material is melted by generating a magnetic field from the at least one pair of DC magnetic field generating devices, and periodically moving the position irradiated by the arc back and forth in a linear manner, or by periodically moving the position within a circle of any radius from the center of the furnace.

4. A method for melting solid raw materials as described in claim 2, characterized in that while observing the inside of the furnace, a magnetic field is generated from at least one pair of DC magnetic field generating devices, and an arc reaching the molten steel surface is controlled to be irradiated onto the unmelted raw materials, thereby melting the raw materials.

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