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

Abstract

The invention relates to a method for melting and treating, in particular mixing, electrically conductive material with high gangue and slag-formers, in particular sponge iron using at least one induction crucible furnace (1) and by coupling at least one alternating-current magnetic field into the material by means of at least one first and at least one second coil arrangement (4,5), which surround a crucible (2) of the induction crucible furnace (1) with a crucible volume (3) for receiving the material, wherein the method comprises melting and stirring the molten material during simultaneous operation of the first and the second coil arrangements (4, 5), wherein the phase angle of the alternating voltage of the first and / or second coil arrangements (4, 5), in particular during operation of the induction crucible furnace (1), is set such that the phase angle of the alternating voltage of the first coil arrangement (4) is shifted with respect to the phase angle of the alternating voltage of the second coil arrangement (5) at least temporarily during a operation. The invention further relates to an induction crucible furnace (1) for carrying out the method.

Description

[0001] Process for melting sponge iron and device for carrying out the process

[0002] The invention relates to a method for melting and for treating and / or mixing electrically conductive material with high gangue and slag formers, in particular sponge iron, using at least one crucible induction furnace and by coupling at least one alternating current magnetic field into the material by means of at least one first and at least one second coil arrangement which enclose a melting crucible of the crucible induction furnace with a crucible volume for receiving the material.

[0003] It is generally known in the art to melt, keep hot, and treat metals using alternating electromagnetic fields. Treatment involves holding 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 generally known that a molten metal exposed to a magnetic field begins to move when eddy currents, which are excited in the molten metal by the electromagnetic field, generate a flux field that counteracts the applied magnetic field. This phenomenon is generally used to inductively stir molten baths to achieve homogenization of the melt and the separation of impurities from the melt.

[0004] A device and a method for heating and mixing an electrically conductive material by means of magnetic induction in a vessel are known from WO 2002 / 071809. The vessel comprises a plurality of induction coils arranged around the vessel, which are interconnected to form at least one three-phase impedance network, a single-phase AC power source with an output operating at an inductive mixing frequency, wherein the mixing frequency is lower than the inductive heating frequency.The method according to WO 2002 / 071809 comprises connecting the output of the single-phase alternating current source to a plurality of induction coils via at least one capacitive element to form a heating circuit operating at or near the 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 electrical material.Furthermore, the output of the three-phase AC power source is connected to a plurality of induction coils via at least one inductive element to form a mixing circuit for providing a mixing AC current to the plurality of induction coils. The mixing AC 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 particularly characterized by simultaneous low-frequency stirring and high-frequency melting using two separate converters.

[0005] Processes of the type described above have not yet been used to melt sponge iron. Sponge iron, within the meaning of the present invention, is understood to be a product obtained from the direct reduction of iron ore (DRI). The term sponge iron, within the meaning of the invention, encompasses a sponge-like product with an iron content of between 90 and 95%. Sponge iron, within the meaning of the invention, also includes a material obtained using the so-called Midrex process and in which the material is in the form of briquettes or pellets (HBI). Sponge iron comprises both carbon and a very large proportion of slag. This requires melting of the material and treatment to separate the slag phase from the melt phase and to transfer impurities from the melt phase to the slag phase.

[0006] Typically, iron products based on sponge iron, both DRI and HBI, are manufactured using electric arc furnaces (EAFs), submerged arc furnaces (SAFs), open bath furnaces (OBFs), and induction furnaces with thyristor technology. The use of these technologies for the production of iron products has the disadvantage that, when using electric arc furnaces, the melt is enriched with carbon through the burning of graphite electrodes. When using arc furnaces, the melt is enriched with oxygen, hydrogen, and nitrogen from the arc. The electrical efficiency of such units is also relatively low. The resulting exhaust gases necessitate the use of complex gas purification systems. Further disadvantages include high investment costs, high burn-up of alloying elements, and limited process flexibility.The use of induction furnaces with thyristor technology has the disadvantage that a network reaction is generated by resulting harmonics and the interfaces between slag and melt are created in the middle of the crucible.

[0007] The invention is therefore based on the object of providing a process of the type mentioned above, with which, in particular, an iron product can be easily produced with relatively low energy consumption and a high degree of purity. In particular, the metal to be melted should be treated in such a way that undesirable gases such as oxygen, hydrogen, and nitrogen do not increase and accompanying elements such as phosphorus and sulfur are separated. In particular, the invention should also enable simple homogenization of the melt and defined separation of the slag phase.

[0008] The invention is further based on the object of providing an induction crucible furnace which is particularly suitable for carrying out the method.

[0009] The object is achieved by a method having the features of claim 1 and by providing a crucible induction furnace having the features of claim 13. Advantageous embodiments of the invention emerge from the subclaims. One aspect of the invention relates to a method for melting and mixing electrically conductive material, in particular electrically conductive material with high gangue and slag formers, in particular sponge iron, using at least one crucible induction furnace and by coupling at least one alternating current magnetic field into the material by means of at least one first and at least one second coil arrangement which enclose a melting crucible with a crucible volume for receiving the material, wherein the method comprises melting and stirring the molten material in each case with simultaneous operation of the first and second coil arrangements and is characterized in particular bythat the phase angle of the alternating voltage of the first and / or the second coil arrangement is adjusted before and / or during operation of the induction crucible furnace such that the phase angle of the alternating voltage of the first coil arrangement is or will be shifted at least temporarily during the stirring operation to the phase angle of the alternating voltage of the second coil arrangement.

[0010] The method according to the invention is particularly characterized by the fact that the melting and the mixing and / or stirring of the material do not take place simultaneously, but rather during different operating modes of the crucible induction furnace. The method according to the invention comprises, in chronological order, first the melting and then the treatment of the melt.

[0011] The method is advantageously carried out using an induction crucible furnace with adjustable phase shift, so that for a stirring operating mode or for a treatment of the melt, a phase shift between the first and the second coil arrangement can be specified in order to achieve a desired stirring pattern.

[0012] It is advantageous if the process comprises treating the melt by means of a first intensive stirring process or, optionally, by means of a second, less intensive stirring process. The use of the designation of the stirring processes as first and second stirring processes is without prejudice to the chronological order of the stirring processes, which can also be carried out alternately.

[0013] According to a preferred variant of the method, this comprises changing the phase shift of the alternating voltage between the first coil arrangement and the second coil arrangement during operation of the crucible induction furnace. It is particularly preferred that the phase shift between the first and second coil arrangements be adjusted to establish a desired stirring pattern during stirring of the melt.

[0014] It is particularly preferably provided that the induction crucible furnace can be operated in different operating modes, wherein the melting and stirring are carried out during different operating modes of the induction crucible furnace, wherein these operating modes further comprise at least a first and a second stirring operating mode.

[0015] It is particularly advantageous with regard to a defined phase separation between melt and slag and with regard to particularly good homogenization of the melt if the first stirring mode is set so that a melt pool flow of the molten metal is directed downwards in the center and upwards at the edges within the crucible and that an upper slag phase, preferably at the edges, is formed. The melt pool flow achieved in this way is advantageously a circulation flow that is counter-current to a circulation flow generated in the slag phase, so that an intensive exchange is achieved at an interface between melt and slag. The crucible can enclose an approximately cylindrical volume. Approximately cylindrical in the sense of the invention means that the crucible base is designed, particularly in edge regions, so that no flow dead zones arise there.

[0016] A second stirring mode can be set so that a centrally upward and peripherally downward flow of the molten metal bath is established within the crucible and that a lower and preferably peripheral slag phase is formed.

[0017] The method may comprise switching or changing between the different stirring operating modes during the treatment of the melt, ie during stirring.

[0018] In a first stirring mode, an interface between melt and slag is preferably formed in an upper stirring zone of the crucible volume and in a second stirring mode in a lower stirring zone of the crucible volume.

[0019] Preferably, the first coil arrangement is formed as an upper coil arrangement surrounding the melting pot and the second coil arrangement is formed as a lower coil arrangement surrounding the melting pot.

[0020] In the method according to the invention, the melt can advantageously be transferred from a crucible bottom to an upper melt bath level in an operating position of the crucible and vice versa from the upper melt bath level to the crucible bottom.

[0021] Preferably, at least three coils each operated with a phase shift of 120 degrees are provided as the first and / or second coil arrangements. The coils are preferably connected in a star configuration to an output of a

[0022] inverter or converter connected.

[0023] In a preferred variant of the method, it can be provided that the first or second coil arrangement is operated with a phase shift of + 90 degrees or - 90 degrees with respect to the other coil arrangement.

[0024] Preferably, the first upper coil arrangement is operated with a phase shift of -90 degrees in the first stirring operating mode and with a phase shift of +90 degrees in the second stirring operating mode with respect to the second lower coil arrangement.

[0025] In the preferred variant of the process according to the invention, the induction melting crucible is operated continuously. Within the scope of the invention, the induction melting crucible can also be operated batchwise. It is also possible to carry out the process using two induction melting crucibles operating in tandem.

[0026] The object underlying the invention is further achieved by the provision of an induction crucible furnace for melting and mixing electrically conductive material, in particular sponge iron, in particular for carrying out the method described above, with at least one first and at least one second coil arrangement, each enclosing a melting crucible with a crucible volume for receiving the material, characterized in that the coil arrangements are designed with an adjustable phase shift.

[0027] An embodiment of the induction crucible furnace according to the invention is characterized in particular in that an upper coil arrangement is provided as the first coil arrangement and a lower coil arrangement is provided as the second coil arrangement.

[0028] The induction furnace according to the invention is expediently designed as an IGBT medium-frequency induction furnace. Medium frequency, in the context of the present invention, is understood to mean a frequency between 110 and 1000 Hz.

[0029] To provide the higher frequency of the alternating current above the frequency of the mains current, the mains alternating current is preferably first converted to direct voltage. An inverter provides the desired medium-frequency voltage. According to the invention, the induction crucible furnace is designed as a transistor induction crucible furnace of the IGBT (Insulated Gate Bipolar Transistor) type.

[0030] Furthermore, the induction crucible furnace according to the invention expediently comprises at least one converter transformer which converts mains alternating current into direct voltage, a plurality of inverters which supply the induction coils of the coil arrangements with single-phase alternating current, preferably with a medium frequency, and at least one control and regulating device for controlling the inverters in such a way that alternating voltages with an adjustable phase shift are present at the outputs of the inverters.

[0031] In the preferred variant of the crucible induction furnace, the first coil arrangement and the second coil arrangement each comprise three induction coils, which are preferably each arranged in a star connection. The power supply is provided by a plurality of inverters connected to different coils arranged one above the other on the crucible and coaxial with it. The inverters are preferably controlled by a control device.

[0032] When the induction crucible furnace is operated to melt the material, alternating voltages are present in phase between a first coil arrangement and a second coil arrangement. The individual coils of a coil arrangement are preferably operated in a known manner with a 120° phase shift. This allows high energy inputs to be introduced into the crucible for melting and heating the melt.

[0033] The crucible induction furnace according to the invention preferably comprises at least one control and regulation device, which enables appropriate control of the inverters so that, during stirring operation, the alternating voltages at the outputs of the inverters are phase-shifted, unlike during melting operation. During stirring operation, the melt temperature of the already molten sponge iron is preferably kept constant. The control and regulation device controls the inverters so that the melt is electromagnetically stirred from the bottom of the crucible upwards and vice versa.

[0034] Preferably, three coils of a coil arrangement are connected in a star connection to an output of an inverter.

[0035] The crucible induction furnace according to the invention preferably comprises a first upper coil arrangement with three induction coils arranged around the circumference of the crucible, as well as a second lower coil arrangement, which also comprises three induction coils arranged coaxially with the upper induction coils. Thus, the crucible induction furnace preferably comprises a total of six insulated induction coils. Within the scope of the invention, the number of coil arrangements and the respective induction coils used is not critical.

[0036] The method according to the invention is explained below with reference to an embodiment shown in the drawings.

[0037] They show:

[0038] Figure 1 is a schematic representation of a crucible induction furnace according to the invention during a melting mode of operation,

[0039] Figure 2 is a schematic representation of the induction crucible furnace during a first stirring mode and

[0040] Figure 3 is a schematic representation of the induction crucible furnace during a second stirring operation mode.

[0041] The method according to the invention comprises melting, optionally holding and stirring sponge iron in the form of DRI or HBI (Direct Reduced Iron, Hot Briquetted Iron) for the purpose of producing an iron product using a crucible induction furnace 1 which is designed as an IGBT type crucible induction furnace, ie which is operated using IGBT type transistors.

[0042] The crucible induction furnace 1 comprises a melting pot 2 defining an approximately cylindrical crucible volume 3. The melting pot 2 is enclosed by a first upper coil arrangement 4 and a second lower coil arrangement 5, each of the coil arrangements 4, 5 comprising three induction coils 6. The first upper coil arrangement 4 and the second lower coil arrangement 5 are arranged coaxially with each other.

[0043] In the induction crucible furnace 1 according to the invention, the melting power supply is provided by a transistor converter that converts three-phase current from the mains into single-phase alternating current with a center frequency between 110 and 1000 Hz. The power supply further comprises inverters connected to different induction coils 6. By controlling the inverters via a control and regulating device, a desired alternating voltage can be provided for heating the melting crucible 2. The induction crucible furnace 1 can further comprise one or more rectifiers, which, in a known manner, first convert the three-phase energy taken from the mains into a controllable direct current.

[0044] The inverters convert the direct current provided by the rectifier into a current of alternating polarity by alternating switching on and off, and this current is fed to the induction coils 6 connected in a parallel resonant circuit. A medium-frequency capacitor bank is provided in a known manner to compensate for the reactive power of the induction coils. The control and regulating device of the crucible induction furnace 1 comprises monitoring of the three-phase current feed, rectifier and sequence control, and control monitoring of the transistors of the inverters. In particular, the control and regulating device provides appropriate control of the inverters for setting various operating modes of the crucible induction furnace 1 according to the invention. According to the invention, the crucible induction furnace 1 is designed such that the phase shift of the first upper coil arrangement 4 relative to the second lower coil arrangement 5 is adjustable.

[0045] Figure 1 illustrates the operation of the crucible induction furnace in a melting mode in which the first upper coil arrangement 4 and the second lower coil arrangement 5 are operated in phase. The induction coils 6 of each coil arrangement 4, 5 are interconnected in a star configuration, with each of the induction coils 6 of a coil arrangement 4, 5 being operated 120° out of phase with the other. This allows for a high energy input into the melting crucible for melting the starting material in the form of sponge iron.

[0046] In the figures, the melt flow resulting from the eddy currents coupled into the material is illustrated by arrows. The melt flow pattern is opposite to the magnetic field induced by the induction coils 6.

[0047] The method according to the invention comprises operating the crucible induction furnace 1 in three different operating modes, namely a melting operating mode and a first and a second stirring operating mode.

[0048] Figure 2 shows a schematic representation of the crucible induction furnace 1 during the first stirring mode, which corresponds to an intensive stirring mode. The induction coils 6 of the first upper coil arrangement 4 are operated at a phase angle that is shifted by -90° relative to the lower coil arrangement 5. This results in a stirring pattern or flow diagram, as indicated by the arrows in Figure 2. The melt 7 circulates downwards centrally within the crucible volume 3 and upwards at the edges. The slag 8 collects at the edge in the upper part of the crucible volume 3 and forms an annular slag phase, with the flow being directed in the same direction in the region of an interface 9 between the melt 7 and the slag 8.In the cross-section shown in Figure 2, the melt 7 circulates clockwise on both sides of a longitudinal central axis 10 of the crucible 2, whereas the slag circulates counterclockwise on both sides of a longitudinal central axis 10 of the crucible 2.

[0049] Figure 3 shows a schematic representation of the crucible induction furnace 1 during the second stirring mode. In this second stirring mode, the induction coils 6 of the upper coil arrangement 4 are operated at a phase angle that is shifted by +90° relative to the lower coil arrangement 5. This results in a flow pattern in which the slag 8 collects at the bottom of the crucible volume 3 at the edge, whereas the melt 7 collects above the slag phase despite its significantly higher density. The melt 7 circulates upwards centrally within the crucible volume 3 and downwards at the edge, whereas the slag 8 circulates in the opposite direction, with the flow of the slag 8 and the melt 7 again being parallel in the region of the interface 9.The melt 7 circulates counterclockwise on both sides of the longitudinal center axis of the crucible (relative to the cross section shown in Figure 3), whereas the slag 8 circulates clockwise.

[0050] By alternately operating the induction crucible furnace 1 in the first stirring mode and the second stirring mode, an intensive mass transfer between slag 8 and melt 7 can be achieved in the area of ​​the interfaces 9 between slag 8 and melt 7. Such treatment advantageously enables the removal of unwanted gases such as oxygen, hydrogen, and nitrogen, as well as their accompanying elements such as phosphorus and sulfur, from the molten bath.

[0051] 1 induction crucible furnace

[0052] 2 crucible 3 crucible volume

[0053] 4 first coil arrangement

[0054] 5 second coil arrangement

[0055] 6 Induction coil

[0056] 7 Melt 8 Slag

[0057] 9 Interface between slag and melt

[0058] 10 Longitudinal central axis of the crucible

Claims

Patent claims 1. A method for melting and treating, in particular for mixing, electrically conductive material, in particular sponge iron, using at least one crucible induction furnace (1) and by coupling at least one alternating current magnetic field into the material by means of at least one first and at least one second coil arrangement (4, 5) which enclose a melting crucible (2) of the crucible induction furnace (1) with a crucible volume (3) for receiving the material, wherein the method comprises melting and stirring the molten material during simultaneous operation of the first and second coil arrangements (4, 5), wherein the phase angle of the alternating voltage of the first and / or second coil arrangement (4, 5) is adjusted, in particular during operation of the crucible induction furnace (1),that the phase angle of the alternating voltage of the first coil arrangement (4) is shifted at least temporarily during a stirring operation to the phase angle of the alternating voltage of the second coil arrangement (5).

2. Method according to claim 1, comprising changing the phase shift of the alternating voltage between the first coil arrangement (4) and the second coil arrangement (5) during operation of the induction crucible furnace (1).

3. Method according to one of claims 1 or 2, characterized in that by setting a phase shift between the first and the second coil arrangement (4, 5) a setting of a desired stirring pattern is carried out during the stirring of the melt (7).

4. Method according to one of claims 1 to 3, characterized in that the induction crucible furnace (1) can be operated in different operating modes, wherein the melting and stirring are carried out during different Operating modes of the induction crucible furnace (1), wherein the operating modes of the induction crucible furnace (1) further comprise at least a first and a second stirring operating mode.

5. Method according to claim 4, characterized in that the first stirring operating mode is set so that a melt bath flow of the melt (7) is set up within the crucible (2) which is directed downwards centrally and upwards at the edges and that an upper and preferably edge-side slag phase is formed.

6. Method according to one of claims 4 or 5, characterized in that the second stirring operating mode is set so that a centrally upward and edge-side downward molten bath flow of the melt (7) is established within the melting crucible (2) and that a lower and preferably edge-side slag phase is formed.

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

8. Method according to one of claims 1 to 7, characterized in that the first coil arrangement is designed as an upper coil arrangement (4) surrounding the melting pot (2) and the second coil arrangement (5) is designed as a lower coil arrangement (5) surrounding the melting pot (2).

9. Method according to one of claims 1 to 8, characterized in that at least three induction coils (6) operated with a phase shift of 120 degrees are provided as the first and / or second coil arrangements (4, 5).

10. Method according to one of claims 1 to 9, characterized in that the first or second coil arrangement (4, 5) is operated with a phase shift of + 90 degrees or - 90 degrees relative to the other coil arrangement (4, 5).

11. Method according to one of claims 1 to 10, characterized in that the first upper coil arrangement (4) is operated with a phase shift of -90 degrees in the first stirring operating mode and with a phase shift of +90 degrees in the second stirring operating mode with respect to the second lower coil arrangement (5).

12. Method according to one of claims 1 to 11, characterized in that the induction crucible furnace (1) is operated continuously.

13. Induction crucible furnace (1) for melting and mixing electrically conductive material, in particular sponge iron, in particular for carrying out the method according to one of claims 1 to 12, with at least one first and at least one second coil arrangement (4, 5), each enclosing a melting crucible (2) with a crucible volume (3) for receiving the material, characterized in that the coil arrangements (4, 5) can be operated with an adjustable phase shift.

14. Induction crucible furnace (1) according to claim 13, characterized in that an upper coil arrangement (4) is provided as the first coil arrangement (4) and a lower coil arrangement (5) is provided as the second coil arrangement (5).

15. Induction crucible furnace (1) according to one of claims 13 or 14, characterized in that it is designed as an IGBT medium-frequency induction crucible furnace.

16. Induction crucible furnace (1) according to one of claims 13 to 15, comprising at least one converter transformer which converts mains alternating current into direct voltage, several inverters which supply the induction coils (6) of the coil arrangements (4, 5) with single-phase alternating current, preferably with a medium frequency, and at least one control and Control device for controlling the inverters such that alternating voltages with an adjustable phase shift are applied to the outputs of the inverters.

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

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