Method for melting sponge iron and device for carrying out the method

Abstract

This invention relates to a method for melting and processing, particularly mixing, conductive materials, especially sponge iron, having a high content of gangue and slagging agent, using at least one induction furnace (1) and coupling at least one alternating magnetic field to the material via at least one first coil assembly (4) and at least one second coil assembly (5), the first and second coil assemblies surrounding a melting crucible (2) of the induction furnace (1) having a crucible volume (3) for containing the material, wherein the method comprises melting and stirring the molten material while the first coil assembly (4) and the second coil assembly (5) are operating simultaneously, wherein, particularly during operation of the induction furnace (1), the phase angle of the alternating voltage of the first coil assembly (4) and / or the second coil assembly (5) is adjusted such that, during stirring operation, the phase angle of the alternating voltage of the first coil assembly (4) is at least temporarily offset relative to the phase angle of the alternating voltage of the second coil assembly (5). The invention also relates to an induction furnace (1) for carrying out the method.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for melting and treating and / or mixing electrically conductive material, in particular sponge iron, having a high content of gangue and slag forming agents, using at least one induction crucible furnace and coupling at least one alternating magnetic field into the material by at least one first coil assembly and at least one second coil assembly, which surround a melting crucible of the induction crucible furnace having a crucible volume for receiving the material. BACKGROUND

[0002] It is generally known in the prior art to melt, hold and treat metals by means of alternating electromagnetic fields. The treatment includes keeping or stirring the molten metal to remove harmful trace elements, which are separated from the melt, for example, in the form of slag. Furthermore, it is well known that molten material exposed to a magnetic field moves when eddy currents excited in the molten material by the electromagnetic field generate a flux field opposite to the applied magnetic field. This phenomenon is commonly used for inductively stirring a melt bath to achieve homogenization of the melt and separation of impurities from the melt.

[0003] From the document WO 2002 / 071809 a device and a method for heating and mixing electrically conductive material in a container by magnetic induction are known. The container has a plurality of induction coils arranged around the container, which are connected to each other to form at least one three-phase impedance network, and a single-phase alternating current power supply having an output operating at an induction mixing frequency, wherein the mixing frequency is lower than the induction heating frequency. The method according to WO 2002 / 071809 comprises connecting the output of the single-phase alternating current power supply to the plurality of induction coils via at least one capacitive element to form a heating circuit operating at or close to a resonant frequency for providing a heating alternating current to the plurality of induction coils, wherein the heating alternating current generates a heating magnetic field and the heating magnetic field is inductively coupled to the electrically conductive material to heat the electrically conductive material. Furthermore, connecting the output of the three-phase alternating current power supply to the plurality of induction coils via at least one inductive element to form a mixing circuit for providing a mixing alternating current to the plurality of induction coils, wherein the mixing alternating current generates a mixing magnetic field and the mixing magnetic field is inductively coupled to the electrically conductive field to mix the electrically conductive material. The method is particular in that low frequency stirring and high frequency melting are performed simultaneously using two separate frequency converters.

[0004] To date, the methods described above have not been used to melt sponge iron. In the context of this invention, sponge iron should be understood as a product obtained through direct reduction (DRI) of iron ore. In the context of this invention, the term sponge iron includes sponge-like products with an iron content between 90% and 95%. In the context of this invention, sponge iron also includes material obtained through the so-called Midrex process, existing in the form of lumps or pellets (HBI). Sponge iron contains both carbon and a very large amount of slag. This necessitates melting and processing the material to separate the slag phase from the melt phase and to transfer impurities from the melt phase to the slag phase.

[0005] Typically, ferrous products are produced from sponge iron (whether based on direct reduced iron (DRI) or hot-pressed iron (HBI)) using electric arc furnaces (EAF), submerged arc furnaces (SAF), open-bottom furnaces (OBF), and induction furnaces employing thyristor technology. The disadvantages of using these technologies are that, in EAFs, the melt is enriched with carbon through the combustion of graphite electrodes. The melt is also enriched with oxygen, hydrogen, and nitrogen from the electric arc. Furthermore, these assemblies have relatively low electrical efficiency. The resulting exhaust gases require complex gas purification equipment. Other disadvantages include high investment costs, significant alloy element loss, and low method flexibility. The disadvantages of using induction furnaces with thyristor technology include grid feedback due to generated harmonics, and the slag-melt interface forming in the center of the melting crucible. Summary of the Invention

[0006] Therefore, the object of the present invention is to provide a method of the type described at the beginning, which enables the simple production of iron products with relatively low energy consumption and high purity. In particular, the metal to be melted should be treated in such a way that undesirable gases such as oxygen, hydrogen, and nitrogen are not increased, and associated elements such as phosphorus and sulfur can be separated. According to the invention, it should also be possible to achieve simple homogenization of the melt and clear separation of the slag phase.

[0007] Another object of the present invention is to provide an induction crucible furnace that is particularly suitable for carrying out the method.

[0008] This objective is achieved by a method having the features of claim 1 and by providing an induction crucible furnace having the features of claim 13. Advantageous embodiments of the invention are derived from the dependent claims.

[0009] One aspect of the invention relates to a method for melting and mixing conductive materials, particularly conductive materials with high gangue and slagging agent content, especially sponge iron, using at least one induction furnace and coupling at least one alternating magnetic field to the material via at least one first coil assembly and at least one second coil assembly, the first and second coil assemblies surrounding a melting crucible having a crucible volume for containing the material, wherein the method includes melting and stirring the molten material respectively while the first and second coil assemblies are operating simultaneously, and is characterized in particular by adjusting the phase angle of the alternating voltage of the first and / or second coil assemblies before and / or during operation of the induction furnace such that, during stirring operation, the phase angle of the alternating voltage of the first coil assembly is at least temporarily shifted or offset relative to the phase angle of the alternating voltage of the second coil assembly.

[0010] A key feature of the method according to the invention is that the melting and mixing and / or stirring of the material are not performed simultaneously, but rather in different operating modes of the induction crucible furnace. The method according to the invention comprises, in chronological order, first melting, followed by the treatment of the melt.

[0011] This method is advantageously implemented using an induction crucible furnace with adjustable phase shift, which allows the phase shift between the first and second coil assemblies to be pre-set for the stirring operation mode or for the processing of the melt to obtain the desired stirring mode.

[0012] Advantageously, the method involves treating the melt by a strong first stirring process or optionally by a less strong second stirring process. The use of the term "stirring process" as "first stirring process" and "second stirring process" does not affect the temporal order of the stirring processes, and they may be performed alternately.

[0013] According to a preferred variation of the method, it includes changing the phase shift of the AC voltage between the first coil assembly and the second coil assembly during operation of the induction crucible furnace. Particularly preferably, it is specified that the phase shift between the first coil assembly and the second coil assembly is adjusted during stirring of the melt to set a desired stirring mode.

[0014] Particularly preferably, the induction crucible furnace is capable of operating in different operating modes, wherein melting and stirring are carried out in the different operating modes of the induction crucible furnace, wherein these operating modes include at least a first stirring operating mode and a second stirring operating mode.

[0015] With regard to the clear phase separation between the melt and slag and the particularly good homogenization of the melt, it is particularly advantageous to adjust the first stirring mode in such a way that a molten pool flow with the center of the molten metal pointing downwards and the edges upwards is formed within the melting crucible, and an upper, preferably edge-side, slag phase is formed. The resulting molten pool flow is advantageously a circulating flow, which forms in the opposite direction to the circulating flow generated in the slag phase, thereby achieving strong exchange at the interface between the melt and slag. The melting crucible can be enclosed in a generally cylindrical volume. In the sense of the invention, a generally cylindrical shape means that the bottom of the crucible, especially in the edge region, is designed so that no flow dead zones are formed there.

[0016] The second stirring mode can be adjusted in such a way that a molten pool flows with the center of the molten metal pointing upwards and the edges pointing downwards in the melting crucible, and a slag phase appears in the lower part, preferably on the edge side.

[0017] The method may include switching or converting between different stirring operation modes during melt processing, i.e., during stirring.

[0018] In the first stirring operation mode, it is preferable to form an interface between the melt and the slag in the upper stirring zone of the crucible volume, while in the second stirring operation mode, the interface is formed in the lower stirring zone of the crucible volume.

[0019] Preferably, the first coil assembly is configured as an upper coil assembly surrounding the melting crucible, and the second coil assembly is configured as a lower coil assembly surrounding the melting crucible.

[0020] In the method according to the invention, the melt can advantageously flow from the bottom of the crucible to the upper surface of the molten pool in the crucible's operating position, and conversely from the upper surface of the molten pool to the bottom of the crucible.

[0021] Preferably, at least three coils operating with a 120-degree phase shift are respectively provided as the first coil assembly and / or the second coil assembly. The coils are preferably connected to the output of the inverter or frequency converter in a star connection.

[0022] In a preferred variation of the method, it may be specified that the first coil assembly or the second coil assembly operates with a phase shift of +90 degrees or -90 degrees relative to the corresponding other coil assembly.

[0023] Preferably, the upper first coil assembly operates with a phase shift of -90 degrees relative to the lower second coil assembly in the first stirring operation mode, and with a phase shift of +90 degrees in the second stirring operation mode.

[0024] In a preferred variation of the method according to the invention, the induction melting crucible operates continuously. Within the scope of the invention, the induction melting crucible can also operate intermittently. The method can also be implemented using two induction melting crucibles operating in series.

[0025] The objective of this invention is further achieved by providing an induction crucible furnace for melting and mixing conductive materials, particularly sponge iron, which is particularly used for carrying out the above-described method and has at least one first coil assembly and at least one second coil assembly, which respectively surround a melting crucible having a crucible volume for containing materials, characterized in that the coil assembly is configured to have an adjustable phase shift.

[0026] A key feature of the embodiment of the induction crucible furnace according to the present invention is that it is provided with an upper coil assembly as a first coil assembly and a lower coil assembly as a second coil assembly.

[0027] The induction crucible furnace according to the present invention is suitably constructed as an IGBT (Insulated Gate Bipolar Transistor) intermediate frequency induction crucible furnace. In the context of this invention, intermediate frequency should be understood as a frequency between 110 and 1000 Hz.

[0028] To provide alternating current at a higher frequency than the grid current frequency, it is preferable to first convert the grid alternating current to direct current voltage. The inverter provides the desired intermediate frequency voltage. According to the invention, the induction crucible furnace according to the invention is constructed as an IGBT (Insulated Gate Bipolar Transistor) type transistor induction crucible furnace.

[0029] Furthermore, the induction crucible furnace according to the invention suitably includes at least one rectifier transformer that converts grid AC power into DC voltage; a plurality of inverters that respectively provide single-phase AC power, preferably medium-frequency AC power, to the induction coils of the coil assembly; and at least one control and regulation device for manipulating the inverters such that an AC voltage with an adjustable phase shift is present at the output of the inverters.

[0030] In a preferred variant of the induction crucible furnace, the first and second coil assemblies each have three induction coils, preferably arranged in a star configuration. Power is supplied by multiple inverters connected to different coils, which are stacked vertically and coaxially with the melting crucible. The inverters are preferably controlled by a control device.

[0031] In the operation of the induction crucible furnace for melting materials, in-phase AC voltages exist at the first and second coil assemblies. The individual coils of the coil assemblies preferably operate with a known 120° phase shift. This allows a high energy input for melting and heating the melt to be introduced into the melting crucible.

[0032] The induction crucible furnace according to the invention preferably has at least one control and regulation device, which can be used to correspondingly manipulate the inverter so that, during stirring operation, unlike during melting operation, a phase-shifted AC voltage exists at the output of the inverter. During stirring operation, it is preferable to keep the molten temperature of the already melted sponge iron constant. The control and regulation device manipulates the inverter in such a way that the melt is electromagnetically stirred from the bottom of the crucible upwards, and vice versa.

[0033] Preferably, the three coils of the coil assembly are connected to the output of the inverter in a star configuration.

[0034] The induction crucible furnace according to the invention preferably comprises: an upper first coil assembly having three induction coils arranged around the periphery of the melting crucible; and a lower second coil assembly, also having three induction coils, arranged coaxially with the upper induction coils. Therefore, the induction crucible furnace preferably has a total of six induction coils, which are isolated from each other. Within the scope of the invention, the number of coil assemblies and the corresponding number of induction coils used are not critical. Attached Figure Description

[0035] The method according to the present invention will now be explained with reference to the embodiments shown in the accompanying drawings. Wherein:

[0036] Figure 1 A schematic diagram of the induction crucible furnace according to the present invention in melting operation mode is shown.

[0037] Figure 2 A schematic diagram of the induction crucible furnace in the first stirring operation mode is shown, and

[0038] Figure 3 A schematic diagram of the induction crucible furnace in the second stirring operation mode is shown. Detailed Implementation

[0039] The method according to the invention includes melting, and if necessary, holding and stirring sponge iron in the form of direct reduced iron (DRI) or hot briquetted iron (HBI) in the presence of an induction crucible furnace 1 to produce iron products. The induction crucible furnace is constructed as an IGBT-type induction crucible furnace, i.e., it operates using IGBT-type transistors.

[0040] The induction crucible furnace 1 has a melting crucible 2, which defines a generally cylindrical crucible volume 3. The melting crucible 2 is surrounded by an upper first coil assembly 4 and a lower second coil assembly 5, wherein each of the coil assemblies 4 and 5 has three induction coils 6. The upper first coil assembly 4 and the lower second coil assembly 5 are arranged coaxially with each other.

[0041] In the induction crucible furnace 1 according to the invention, the melting current is provided by a transistor frequency converter, which converts three-phase electricity from the power grid into medium-frequency single-phase alternating current with a frequency between 110 and 1000 Hz. The power supply unit also includes an inverter connected to the various induction coils 6. By operating the inverter with a control and regulation device, the desired AC voltage for heating the melting crucible 2 can be provided. The induction crucible furnace 1 may also have one or more rectifiers, which first convert the three-phase electrical energy obtained from the power grid into controllable direct current in a known manner.

[0042] The DC power supplied by the rectifier is converted into alternating polarity currents by the inverter through alternating switching on and off, and is then fed to the induction coil 6 connected in a parallel resonant circuit. To compensate for the reactive power of the induction coil, a medium-frequency capacitor bank (Mittelfrequenzkondensator-Batterie) is provided in a known manner.

[0043] The control and regulation device for the induction crucible furnace 1 includes a three-phase power input monitoring unit, a rectifier and operation control unit, and an inverter transistor control monitoring unit. The inverter can be correspondingly controlled via the control and regulation device to set different operating modes of the induction crucible furnace 1 according to the invention. According to the invention, the induction crucible furnace 1 is constructed such that the phase shift of the upper first coil assembly 4 relative to the lower second coil assembly 5 is adjustable.

[0044] Figure 1 The diagram illustrates the operation of an induction crucible furnace in melting mode, where the upper first coil assembly 4 and the lower second coil assembly 5 operate in phase. The induction coils 6 of each coil assembly 4, 5 are correspondingly connected in a star configuration, with each induction coil 6 of the coil assemblies 4, 5 operating with a 120° phase shift relative to the other induction coil. This allows for the input of high energy into the melting crucible to melt raw materials in the form of sponge iron.

[0045] In the accompanying diagram, the melt flow resulting from eddies coupled into the material is indicated by arrows. The flow pattern (Fließbild) of the melt flow is opposite to the magnetic field induced by induction coil 6.

[0046] The method according to the invention includes operating the induction crucible furnace 1 in three different operating modes: a melting operating mode, a first stirring operating mode, and a second stirring operating mode.

[0047] Figure 2A schematic diagram of the induction crucible furnace 1 during a first stirring operation mode, corresponding to a high-intensity stirring mode, is shown. Here, the induction coil 6 of the upper first coil assembly 4 operates with a phase angle offset of -90° relative to the lower coil assembly 5. This results in... Figure 2 The middle arrow indicates a stirring diagram or flow diagram. Melt 7 circulates downwards from the center and upwards from the edges within crucible volume 3. Slag 8 accumulates at the upper edge of crucible volume 3, forming an annular slag phase. In the region of interface 9 between melt 7 and slag 8, the flow is unidirectional. Figure 2 In the cross section shown, the melt 7 circulates clockwise on both sides of the longitudinal central axis 10 of the melting crucible 2, while the slag circulates counterclockwise on both sides of the longitudinal central axis 10 of the melting crucible 2.

[0048] Figure 3 A schematic diagram of the induction crucible furnace 1 during a second stirring operation mode is shown. In this second stirring operation mode, the induction coil 6 of the upper coil assembly 4 operates with a phase angle offset of +90° relative to the lower coil assembly 5. This results in a flow pattern where slag 8 accumulates at the bottom edge of the crucible volume 3, while melt 7, despite its significantly higher density, accumulates above the slag phase. Melt 7 circulates upwards from the center and downwards from the edge within the crucible volume 3, while slag 8 circulates in the opposite direction, wherein the flows of slag 8 and melt 7 are again co-directional in the region of interface 9. (Reference) Figure 3 As shown in the cross section, the melt 7 circulates counterclockwise on both sides of the longitudinal central axis of the melting crucible, while the slag 8 circulates clockwise.

[0049] By alternating between the first and second stirring modes of the induction crucible furnace 1, a strong mass exchange between the slag 8 and the melt 7 can be achieved in the region of the interface 9 between the slag 8 and the melt 7. This process advantageously removes undesirable gases such as oxygen, hydrogen, and nitrogen, as well as associated elements such as phosphorus and sulfur, from the molten pool.

[0050] List of reference numerals

[0051] 1. Induction Crucible Furnace

[0052] 2. Melting crucible

[0053] 3. Crucible volume

[0054] 4 First coil assembly

[0055] 5 Second Coil Assembly

[0056] 6. Induction coil

[0057] 7. Melt

[0058] 8 scum

[0059] 9. Interface between slag and melt

[0060] 10. The longitudinal central axis of the melting crucible

Claims

1. A method for melting and processing, particularly mixing, conductive materials, especially sponge iron, comprising using at least one induction furnace (1) and coupling at least one alternating magnetic field to the material via at least one first coil assembly (4) and at least one second coil assembly (5), the first and second coil assemblies surrounding a melting crucible (2) of the induction furnace (1) having a crucible volume (3) for containing the material, wherein, The method includes melting and stirring the molten material while the first coil assembly (4) and the second coil assembly (5) are running simultaneously, wherein, particularly during the operation of the induction crucible furnace (1), the phase angle of the AC voltage of the first coil assembly (4) and / or the second coil assembly (5) is adjusted such that, during stirring operation, the phase angle of the AC voltage of the first coil assembly (4) is at least temporarily offset relative to the phase angle of the AC voltage of the second coil assembly (5).

2. The method according to claim 1, characterized in that, The method includes changing the phase shift of the AC voltage between the first coil assembly (4) and the second coil assembly (5) during operation of the induction crucible furnace (1).

3. The method according to claim 1 or 2, characterized in that, During the stirring of the melt (7), the desired stirring mode is set by adjusting the phase shift between the first coil assembly (4) and the second coil assembly (5).

4. The method according to any one of claims 1 to 3, characterized in that, The induction crucible furnace (1) is capable of operating in different operating modes, wherein melting and stirring are performed during the different operating modes of the induction crucible furnace (1), wherein the operating modes of the induction crucible furnace (1) include at least a first stirring operating mode and a second stirring operating mode.

5. The method according to claim 4, characterized in that, The first stirring operation mode is adjusted in this way so that a molten pool flow of melt (7) with the center pointing downward and the edge side pointing upward appears in the melting crucible (2), and a slag phase appears in the upper part, preferably on the edge side.

6. The method according to any one of claims 4 or 5, characterized in that, The second stirring operation mode is adjusted in this way so that a molten pool flow with the center of the melt (7) pointing upward and the edge side pointing downward is formed in the melting crucible (2), and a slag phase appears in the lower part, preferably on the edge side.

7. The method according to any one of claims 1 to 6, characterized in that, In the first stirring operation mode, an interface (9) is formed between the melt (7) and the slag (8) in the upper stirring zone of the crucible volume (3), while in the second stirring operation mode, the interface is formed in the lower stirring zone of the crucible volume (3).

8. The method according to any one of claims 1 to 7, characterized in that, The first coil assembly is constructed as an upper coil assembly (4) surrounding the melting crucible (2), and the second coil assembly (5) is constructed as a lower coil assembly (5) surrounding the melting crucible (2).

9. The method according to any one of claims 1 to 8, characterized in that, As a first coil assembly (4) and / or a second coil assembly (5), at least three induction coils (6) are respectively provided, which operate with a 120-degree phase shift.

10. The method according to any one of claims 1 to 9, characterized in that, The first coil assembly (4) or the second coil assembly (5) operates with a phase shift of +90 degrees or -90 degrees relative to the corresponding other coil assembly (4, 5).

11. The method according to any one of claims 1 to 10, characterized in that, The upper first coil assembly (4) operates with a phase shift of -90 degrees relative to the lower second coil assembly (5) in the first stirring operation mode, and with a phase shift of +90 degrees in the second stirring operation mode.

12. The method according to any one of claims 1 to 11, characterized in that, The induction crucible furnace (1) is kept running continuously.

13. An induction crucible furnace (1) for melting and mixing conductive materials, particularly sponge iron, especially for carrying out the method according to any one of claims 1 to 12, having at least one first coil assembly (4) and at least one second coil assembly (5), the first and second coil assemblies respectively surrounding a melting crucible (2) having a crucible volume (3) for containing the material, characterized in that, The coil assemblies (4, 5) are capable of operating with adjustable phase shift.

14. The induction crucible furnace (1) according to claim 13, characterized in that, The upper coil assembly (4) is configured as the first coil assembly (4), and the lower coil assembly (5) is configured as the second coil assembly (5).

15. The induction crucible furnace (1) according to claim 13 or 14, characterized in that, The induction crucible furnace is constructed as an IGBT medium-frequency induction crucible furnace.

16. The induction crucible furnace (1) according to any one of claims 13 to 15, characterized in that, The induction crucible furnace has: at least one rectifier transformer that converts grid AC power into DC voltage; a plurality of inverters that respectively provide single-phase AC power, preferably medium-frequency AC power, to the induction coils (6) of the coil assemblies (4, 5); and at least one control and regulation device for operating the inverters such that an AC voltage with an adjustable phase shift is present at the output of the inverters.

17. The induction crucible furnace (1) according to any one of claims 13 to 16, characterized in that, The first coil assembly (4) and the second coil assembly (5) each have three induction coils (6), which are preferably arranged in a star-shaped connection.

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