How to operate an electric arc furnace

Abstract

The furnace vessel (1) of the arc furnace is charged with the metal (2) in the form of solid agglomerates. The energy supply (3) of the arc furnace then draws electrical energy from the supply network (4) and supplies it to the electrodes (6) of the arc furnace via the furnace transformer (5), so that an arc (12) is formed between the electrodes (6) and the metal (2) or molten metal (13), which melts the metal (2) to form the molten metal (13). Finally, the molten metal (13) is removed from the furnace vessel (1). The number of electrodes (6) is at least three. The energy supply (3) adjusts the operating frequency (fa, fb) of each electrode (6A, 6B) separately for at least two electrodes (6). The current (Ic) through the third electrode (6C) is determined by the current (Ia) through the first electrode (6A) and the current (Ib) through the second electrode (6B). Of all the electrodes (6) in the arc furnace, which electrodes are the first electrodes (6A), which electrodes are the second electrodes (6B), and which electrodes are the third electrodes (6C) are dynamically assigned to the electrodes (6).

Description

[Technical field]

[0001] The invention is based on a method for operating an electric arc furnace, in which an energy supply device of the electric arc furnace draws electric energy from a supply network between the charging of metal in solid agglomerates into the furnace vessel of the electric arc furnace and the removal of molten metal from the furnace vessel, and supplies the drawn electric energy via a furnace transformer to at least one first electrode, at least one second electrode and at least one third electrode of the electric arc furnace, whereby an arc is formed between the electrodes and the metal or molten metal, and the metal is melted by the arc to form molten metal, and the energy supply device adjusts the operating frequency of the first electrode and the operating frequency of the second electrode separately, whereby the current through the third electrode is determined by the current through the first and second electrodes.

[0002] The invention is further based on a control program for a control device of an arc furnace, the control program comprising a machine code processable by the control device, the processing of the machine code by the control device causing the control device to operate the arc furnace according to such an operating method.

[0003] The invention is further based on a control device for an electric arc furnace, which is programmed with a control program such that processing of a machine code by the control device causes the control device to operate the electric arc furnace according to such an operating method.

[0004] The invention is further based on an electric arc furnace, the electric arc furnace has a furnace vessel, into which metal can be charged and from which molten metal can be removed; the arc furnace comprises an energy supply device and electrodes as well as a furnace transformer, the energy supply device is connected on the input side to the supply grid and on the output side to the electrodes via the furnace transformer, the arc furnace comprises a control device capable of activating at least one energy supply device, The control device is constructed as described above. [Background technology]

[0005] The above-mentioned subject matter is known, for example, from US Pat. No. 5,399,633.

[0006] When melting metals, especially steel, in an electric arc furnace, the supply of electrical energy to the electrodes of the electric arc furnace takes place via a furnace transformer, which is often connected to the supply grid via a medium voltage transformer. The furnace transformer supplies several voltage steps. For a certain power range and for other high current ranges, the respective voltage steps can be selected in the furnace transformer. Fine adjustment within a certain voltage step can take place, for example, by impedance control.

[0007] In this approach, only some small voltage steps are possible, and the electrode current is subject to strong fluctuations. To reduce these fluctuations, the positioning of the electrode is controlled mechanically, usually through a hydraulic adjustment device. The mechanical adjustment of the electrode has significantly lower dynamics than the real behavior of the arc. The fluctuations can therefore only be compensated poorly. Furthermore, the fluctuations result in significant loads on components such as high-current cables, current-carrying brackets, hydraulic cylinders, etc. The fluctuations occur both in the melting phase and in the subsequent flat bath phase.

[0008] When adjusting the electrode voltage through the voltage steps of the furnace transformer, the electrode positioning must be continually readjusted. This readjustment can be done, for example, to control to a specific impedance or a specific power output. However, since the dynamics of the positioning device are relatively small compared to the changes in the electrical system of the arc, some uncompensable fluctuations remain. This results in a less than optimal energy input to the molten steel.

[0009] From the prior art, in particular from US Pat. No. 5,399,623 and US Pat. No. 5,493,637, and to a lesser extent from US Pat. No. 5,493,627, approaches are known that allow the electrode voltage to be adjusted continuously. These embodiments offer significant advantages compared to regulating the electrode voltage through voltage steps of the furnace transformer. For example, it is possible to vary the electrode voltage continuously, not just stepwise. Furthermore, the furnace transformer can be of simpler construction, since it does not have to provide several voltage steps. Furthermore, the adjustment of the electrode voltage is possible with significantly greater dynamics than the positioning of the electrodes. Finally, these embodiments allow an additional type of control.

[0010] Despite the flexibility afforded by the possibility of continuously adjusting the electrode voltage in the operation of an arc furnace, the operation of the arc furnace in the prior art is still often not optimal, for example suboptimal energy input into the metal or molten metal and arc breaks can still occur. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] International Publication No. 2021 / 115573 Brochure [Patent Document 2] International Publication No. 2015 / 176899 Brochure [Patent Document 3] European Patent Application Publication No. 3124903 [Patent Document 4] European Patent Application Publication No. 1026921 Summary of the Invention [Problem to be solved by the invention]

[0012] The object of the present invention is to provide a possibility to avoid the disadvantages of the prior art. [Means for solving the problem]

[0013] This problem is solved by a method having the features of claim 1. Advantageous embodiments of the method are the subject of dependent claims 2 to 11.

[0014] According to the invention, an operating method of the initially mentioned type consists in dynamically allocating to the electrodes which of the electrodes of the arc furnace are first electrodes, which are second electrodes and which are third electrodes.

[0015] Thereby, for at least the first and second electrodes, the respective operating frequencies can be optimally adjusted for the specific operation of each electrode.

[0016] It is possible for the operating frequency of the first electrode and the operating frequency of the second electrode to be the same, provided that the operating frequencies are adjusted separately and happen to be the same value, but in general the operating frequency of the first electrode and the operating frequency of the second electrode are different.

[0017] It is also possible that the operating frequency of the first electrode is constant in time. However, in general, a better operation of the arc furnace is obtained if the operating frequency of the first electrode varies in time. Preferably, the operating frequency of the first electrode varies depending on the process conditions of the arc furnace. The process conditions can be determined in particular by evaluating electrical or acoustic operating variables of the arc furnace. Self-evidently, the same approach can also be used for the second electrode.

[0018] It is also possible that the operating frequency of the second electrode is adjusted independently from the operating frequency of the first electrode. However, it is often advantageous to adjust the operating frequency of the second electrode taking into account the operating frequency of the first electrode. For example, the operating frequency of the second electrode can be adjusted to have a certain interval with respect to the operating frequency of the first electrode. Also, the operating frequency of the second electrode can first be determined independently from the operating frequency of the first electrode, but the operating frequency thus determined can be adopted only if it has at least a certain interval with respect to the operating frequency of the first electrode, and in other cases, the operating frequency of the second electrode can be adjusted to have a certain interval with respect to the operating frequency of the first electrode.

[0019] That is to say, in the present invention, the status of each of the first, second and third electrodes is not fixedly determined once and for all, considering respectively the period between the charging of the metal into the furnace vessel and the removal of the molten metal from the furnace vessel, but changes from time to time. That is to say, the same electrode of the arc furnace can be the first electrode at one time, the second electrode at another time and neither the first nor the second electrode at yet another time. If the arc furnace has three electrodes in total and at a time x, electrode a operates with frequency fa and electrode b with frequency fb, and the current through electrode c is determined by the currents through electrodes a and b, then at a later time y, it is possible, for example, to operate electrode b with frequency fa and electrode c with frequency fb, and thus the current through electrode a is determined by the currents through electrodes b and c. The transition can be abrupt or continuous, in the latter case in particular sinusoidal.

[0020] The electrodes of an arc furnace are generally adjustable in height. The heights of the electrodes may be adjustable together or independently of each other. If independently adjustable, the dynamic assignment of the respective "roles" to the electrodes may be combined with an approach in which the position of each electrode in the arc furnace is determined depending on whether it is the first, second or third electrode. The notion of "depending" should not mean that the "role" of each electrode completely determines the positioning of each electrode. Rather, it is sufficient that the "role" of each electrode is also taken into account.

[0021] To achieve individual height adjustment, it may already be sufficient if, in addition to adjusting the electrodes together, it is possible to slightly overlap the additional movement of the individual electrodes: in this case, for n electrodes, only n-1 devices are needed to provide the additional movement.

[0022] Between the charging of the metal into the furnace vessel and the removal of the molten metal from the furnace vessel, the electric arc furnace is operated first in the melting phase and then in the flat bath phase. In the melting phase, the metal is melted to form molten metal. The melting phase can be divided into an initial phase and a final phase. The initial phase is often called the piercing phase. The majority of the melting of the metal takes place in the final phase. In the flat bath phase, the molten metal is further heated.

[0023] Furthermore, in some cases, the charging of the metal into the furnace vessel is also very uneven, with larger metal fragments in the area under one electrode and smaller metal fragments in the area under another electrode. In this case, it may be advantageous to determine the operating frequency of the first electrode in the melting phase depending on the size of the metal fragments to be melted with the first electrode. The operating frequency of the first electrode may be determined during the melting phase, in particular, in such a way that the larger the metal fragments to be melted with the first electrode, the smaller the operating frequency of the first electrode. Furthermore, preferably, the operating frequency of the first electrode during the melting phase or the final phase of the melting phase is higher than the power frequency at which the supply grid operates, in particular at least 10 Hz higher. Self-evidently, the same approach can also be used for the second electrode.

[0024] Preferably, the electrical energy supplied to the electrodes is determined in such a way that the electrodes each introduce the same amount of energy into the metal or molten metal on a time average, where the time average is not formed over the individual periods of the respective operating frequency of each electrode, but over the entire movement of the electrodes between the charging of the metal into the furnace vessel and the removal of the molten metal from the furnace vessel, or over at least one operating phase of the arc furnace (initial phase of the melting phase, final phase of the melting phase and flat bath phase, or melting phase and flat bath phase), whereby in particular an equalization of the temperature of the molten metal is obtained.

[0025] The object is furthermore achieved by a control program having the features of claim 12. According to the invention, by processing of the machine code by the control device, the control device operates the arc furnace according to the operating method according to the invention.

[0026] The object is further achieved by a control device having the features of claim 13. According to the invention the control device is programmed with a control program according to the invention, so that the control device operates the arc furnace according to the operating method according to the invention.

[0027] The object is furthermore achieved by an electric arc furnace having the features of claim 14. According to the invention, the control device is configured as a control device according to the invention.

[0028] The above mentioned features, characteristics and advantages of the invention and the manner in which they are obtained will become clearer and more easily understood in connection with the following description of an embodiment, which is described in more detail in conjunction with the drawings, in which: [Brief description of the drawings]

[0029] [Figure 1] FIG. 1 is a block diagram of an electric arc furnace. [Diagram 2] FIG. 2 shows the furnace vessel during the melting stage. [Diagram 3]FIG. [Figure 4] FIG. 2 shows the furnace vessel at the flat bath stage. [Diagram 5] FIG. [Figure 6] FIG. [Figure 7] FIG. [Figure 8] FIG. 1 is a diagram showing functional dependencies. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] According to Fig. 1, the electric arc furnace has a furnace vessel 1, which can be charged with metal 2 (see Fig. 2). During charging, the metal 2 is supplied to the furnace vessel 1 in the form of solid aggregates. The metal 2 can be, for example, steel, which in the case of steel can in particular be scrap.

[0031] The arc furnace further comprises an energy supply device 3. The energy supply device 3 is connected on the input side to a supply network 4. The supply network 4 is typically a medium voltage network, has a nominal voltage in the double kV range and operates at a power supply frequency f0, which is typically 50 Hz or 60 Hz. As shown in Figure 1, the supply network 4 is typically a three-phase supply network.

[0032] The arc furnace further comprises a furnace transformer 5 and electrodes 6. On the output side, the energy supply device 3 is connected to the electrodes 6 via the furnace transformer 5. In the scope of the present invention, there are at least three electrodes 6. In most cases, there are exactly three electrodes 6. Furthermore, the furnace transformer 5 is generally configured as a three-phase transformer. However, regardless of the concrete embodiment, the electrode voltage U applied to the electrodes 6 is clearly lower than the nominal voltage of the supply network 4. The electrode voltage U is shown in FIG. 1 only for one of the electrodes 6. In most cases, the electrode voltage U is in the range of several hundreds of V. In individual cases, voltages of more than 1 kV are also possible. However, in general, it does not exceed 2 kV.

[0033] Generally, there is furthermore a switching device, which can disconnect the energy supply device 3 from the supply network 4. Furthermore, there may be a switching device, which can disconnect the energy supply device 3 from the furnace transformer 5 and / or the furnace transformer 5 from the electrodes 6. The switching device performs a purely binary switching process, but does not regulate the voltage and current. Furthermore, active or passive filter devices may be arranged on the primary or secondary side of the furnace transformer 5. Since the switching devices and filter devices are of secondary importance for the function according to the invention, they are not shown in FIG. 1 (and in the other figures) for the sake of clarity.

[0034] The energy supply device 3 can draw electrical energy from the supply network 4 and supply the drawn electrical energy to the electrodes 6 via the furnace transformer 5. The energy supply device 3 generally has a number of semiconductor switches for this purpose. Possible embodiments of the energy supply device 3 are described in EP 1 339 636 A1 ("Gold Standard"). Alternatively, for example, the embodiments according to EP 1 339 636 A1 or EP 1 339 636 A1 can be used. However, independently of the concrete embodiment of the energy supply device 3, the energy supply device 3 can perform, on the output side, i.e. towards the furnace transformer 5, a quasi-continuous staging of the electrode voltage U applied to the electrodes 6 and / or the electrode current I supplied to the electrodes 6. As in the case of the electrode voltage U, the electrode current I is also shown in FIG. 1 for only one of the electrodes 6. The respective operating frequency f, when the electrode voltage U applied to the electrodes 6 or the electrode current I supplied to the electrodes 6 is varied, can also be adjusted by means of the energy supply device 3. The respective operating frequency f can be above or below the power supply frequency f0 as required.

[0035] Furthermore, the arc furnace has a positioning device 7, by means of which the electrodes 6 can be positioned, as indicated in FIG. 1 by the double arrow 8 next to them. In the simplest case, a joint positioning of the electrodes 6 is performed. However, preferably, an individual positioning of the electrodes 6 is performed. This is indicated in FIG. 1 by the different lengths of the double arrows 8. The direction of movement in which the electrodes 6 are positioned may be vertical. Alternatively, the direction of movement may be slightly inclined to the vertical. However, also in this case the vertical component is the dominant component of the movement. The positioning device 7 may for example have one or more hydraulic cylinder units.

[0036] Finally, the arc furnace has a control device 9. By means of the control device 9 at least the energy supply device 3 is controlled. The control device 9 generates an operating value A1 for operating the energy supply device 3. According to the operating value A1 the energy supply device 3 is operated.

[0037] The positioning device 7 is often also controlled by the control device 9, which in this case further generates actuation values ​​A2 for actuating the positioning device 7. The positioning device 7 then operates in accordance with these actuation values ​​A2.

[0038] The controller 9 is configured as a software programmable controller, which is indicated in Figure 1 by the notation "μP" (for microprocessor controlled). The function and manner of operation of the controller 9 is determined by a control program 10 into which the controller 9 is programmed. The control program 10 includes machine code 11 processable by the controller 9. Processing of the machine code 11 by the controller 9 causes the controller 9 to operate the arc furnace according to a method of operation as will be described in detail below in connection with the other figures.

[0039] According to Fig. 3, first in step S1 the furnace vessel 1 is charged with metal 2. Fig. 2 shows the state after charging of the furnace vessel 1 immediately after ignition of the arc 12 by corresponding activation of the energy supply device 3 by the control device 9 (see Fig. 2 and Fig. 4).

[0040] Then, in step S2, the actual operation of the arc furnace is performed. In step S2, an electrode voltage U is applied and an electrode current I is supplied to the electrode 6. As a result, an arc 12 is first formed between the electrode 6 and the metal 2, and the metal 2 is melted by the arc 12 to form a molten metal 13. This operation stage of the arc furnace is generally called the melting stage. The melting stage may be divided into an initial stage and a final stage, the initial stage being generally called the piercing stage and the final stage being called the main melting stage. Then, an arc 12 is formed between the electrode 6 and the molten metal 13, which is further heated. This operation stage of the arc furnace is generally called the flat bath stage. This state is shown in FIG. 4. In the flat bath stage, the molten metal 13 may be covered on its upper surface with a slag layer 14. The slag layer 14 may be foamed slag.

[0041] Finally, in step S3, the formed molten metal 13 is removed from the furnace vessel 1 and poured, for example, into a ladle (not shown).

[0042] Step S2, i.e. the actual operation of the arc furnace, is performed by corresponding activation of the energy supply device 3. Steps S1 and S3 can likewise be performed under the control of the control device 9. However, steps S1 and S3 do not necessarily have to be performed under the control of the control device 9. Steps S1 and S3 are therefore only shown by dashed lines in Fig. 3.

[0043] In the following, step S2, i.e. the actual operation of the arc furnace between the charging of the metal 2 into the furnace vessel 1 and the removal of the molten metal 13 from the furnace vessel 1, will be described in detail with reference to FIG. 5. For this purpose, step S2 is divided into steps S11 to S20. Furthermore, within the scope of the following description, the individual electrodes 6 will be distinguished. For this purpose, the electrodes 6 will be additionally assigned small letters in accordance with their order, respectively. In this case, the electrodes 6 will be designated as the first electrode 6a, the second electrode 6b and the third electrode 6c. In contrast, if the electrodes 6 are only referred to generally, only the general reference symbol 6 will be used. The same applies with regard to electrode-specific variables such as the electrode voltage U, the electrode current I and the operating frequency f.

[0044] According to FIG. 5, the control device 9 determines the state Z of the arc furnace in step S11. The state Z indicates how far the operation of the arc furnace has progressed between the charging of the metal 2 and the unloading of the molten metal 13. In the simplest case, the state Z of the control device 9 is set by the operator 15 (see FIG. 1). Alternatively, it is also possible for the control device 9 to determine the state Z directly on the basis of the time t that has elapsed since the first ignition of the arc 12 after the charging of the furnace vessel 1. However, preferably, the control device 9 evaluates measurement-technically determined current values ​​of the arc furnace in step S12. For example, the control device 9 can evaluate the electrode current I and / or the electrode voltage U, in particular the fluctuations thereof. The control device 9 can also evaluate the acoustical variables of the arc furnace, for example the noise level or the acoustical spectrum of the generated noise.

[0045] In step S12, the control device 9 determines which of the electrodes 6 is the first electrode 6a, the second electrode 6b and the third electrode 6c. This determination can be made directly dependent on the time t that has elapsed since the arc 12 first ignited after the charging of the furnace vessel 1. Alternatively, the determination can be made directly dependent on the time that has elapsed since the beginning of each state Z. That is, in the latter two cases, the electrodes 6 are dynamically assigned which of the electrodes 6 is the first electrode 6a, which is the second electrode 6b and which is the third electrode 6c. That is, the electrodes 6 can exchange their roles from time to time.

[0046] In step S13, the control device 9 determines a frequency range F within which the electrode current I and / or the electrode voltage U should be. The frequency range F may depend, inter alia, on the state Z. For example, as shown in FIG. 6, the frequency range F in the final stage of the melting stage (state Z=2) can exclusively include frequencies f greater than the power supply frequency f0. Similarly, it is also possible that the frequency range F in the flat bath stage (state Z=3) exclusively includes frequencies f lower than the power supply frequency f0. Finally, it is also possible that the frequency range F in the initial stage of the melting stage (state Z=1) alternatively exclusively includes frequencies f greater than the power supply frequency f0 or exclusively includes frequencies f lower than the power supply frequency f0.

[0047] Preferably, the frequency range F always has a certain minimum interval with respect to the power supply frequency. This minimum interval may be, for example, between 7 Hz and 13 Hz, in particular about 10 Hz. If the frequency range F is higher than the power supply frequency f0, the lower limit frequency of the frequency range F at a power supply frequency f0 of 50 Hz is preferably "somewhere" between 57 Hz and 63 Hz, for example 60 Hz. Similarly, if the frequency range F is lower than the power supply frequency f0, the upper limit frequency of the frequency range F at a power supply frequency f0 of 50 Hz is preferably "somewhere" between 37 Hz and 43 Hz, for example 40 Hz. If the power supply frequency f0 is 60 Hz, 10 Hz must be added to the aforementioned limit frequencies, respectively.

[0048] Preferably, the power P to be introduced into the metal 2 or the molten metal 13* is also adjusted as a function of time t, specifically depending on the state Z, as shown in FIG. 7. FIG. 7 shows a preferred embodiment, according to which the power P * has a relatively low value at the beginning of the melting stage (state Z=1), a relatively high value at the end of the melting stage (state Z=2), and a value between the beginning and end of the melting stage at the flat bath stage (Z=3).

[0049] In step S14, the control device 9 determines the operating frequency fa of the first electrode 6a. This determination is made so that the operating frequency fa is within the currently valid frequency range F. The operating frequency fa may be fixed. Alternatively, the operating frequency fa may be fixed within each state Z but dependent on the state Z. As a further alternative, the operating frequency fa may be dependent on time, e.g. on the progress of the process, within each state Z (see FIG. 6). In FIG. 6, the possible progression of the operating frequency fa over time is shown for both the frequency range F above and below the power supply frequency f0 in the case of state Z=1. Obviously, in practice, only one of the two operating frequencies fa is valid.

[0050] In step S15, the control device 9 determines the operating frequency fb of the second electrode 6b. This determination is made separately when the operating frequency fb does not necessarily have to coincide with the operating frequency fa. That is, the operating frequency fb may be different from the operating frequency fa.

[0051] It is also possible that the determination of step S15 is made independently of the determination of step S14. However, there may also be some dependencies. For example, it may be required that the operating frequency fb of the second electrode 6b maintains a minimum distance with respect to the operating frequency fa of the first electrode 6a. Alternatively, it may be required that the operating frequency fb of the second electrode 6b is within the currently valid frequency range F. In other cases, the embodiments for determining the operating frequency fa of the first electrode 6a may be applied as well.

[0052] It is no longer possible to determine the operating frequency of the third electrode 6c, since its operating mode is determined by the fact that the current I through the electrodes 6a, 6b, 6c must be zero at any instant (taking into account the sign of the current I).

[0053] In step S16, the control device 9 determines an operation value A1 of the energy supply device 3. When determining the operation value A1, the control device 9 takes into account the operating frequencies fa, fb of the first and second electrodes 6a, 6b.

[0054] If the control device 9 also controls the positioning device 7, it performs the positioning pa, pb, pc of the first, second and third electrodes 6a, 6b, 6c in step S17. In this case, the control device 9 determines the associated further actuation value A2 in step S18. It is also possible to determine it uniformly for all electrodes 6. However, it is also equally possible to perform the positioning pa, pb, pc individually for each electrode 6 depending on whether it is the first, second or third electrode 6a, 6b, 6c.

[0055] In step S19, the control device 9 activates the energy supply device 3 according to the determined activation value A1. If the control device 9 also controls the positioning device 7, the control device 9 also activates the positioning device 7 with a further activation value A2 in step S19.

[0056] In step S20, the controller 9 checks whether each cycle of operation of the arc furnace is complete, i.e. whether the metal 2 has been completely melted to form the molten metal 13 and, if necessary, whether the molten metal 13 has been further heated. If so, the controller 9 proceeds to step S3. Otherwise, the controller 9 returns to step S11.

[0057] FIG. 8 shows one possibility for determining the operating frequency fa of the first electrode 6a in the melting phase. The dependence according to FIG. 8 may alternatively be present throughout the entire melting phase or only during the initial or final phase of the melting phase. According to FIG. 8, the control device 9 determines the operating frequency fa of the first electrode 6a in dependence on the dimension G of the pieces of metal 2 to be melted with the first electrode 6. In particular, the operating frequency fa of the first electrode 6a may decrease monotonically or very monotonically the larger the dimension G of the pieces of metal 2. That is to say, the operating frequency fa of the first electrode 6a is preferably determined in the melting phase such that it decreases the larger the pieces of metal 2 to be melted with the first electrode 6a. Obviously, a similar approach can be adopted for the second electrode 6b and its operating frequency fb. In the case of the approach according to FIG. 8, different or identical operating frequencies fa, fb can result depending on the dimension G of the pieces of metal 2. As the dimension G, for example, a statistical average value of the dimensions of the individual pieces of metal 2 can be used. Alternatively, extreme values ​​(minimum or maximum) can be used.

[0058] The electrical energy supplied to the electrodes 6, i.e. the integral of the power supplied to the electrodes 6, is preferably determined by the control device 9 in such a way that the electrodes 6, on a time average, introduce the same amount of energy into the metal 2 or into the molten metal 13, respectively, where the time average is not formed over individual periods of the respective operating frequencies fa, fb, but over a number of periods. Particularly preferred is an averaging over the respective operating phases of the arc furnace, i.e. the initial phase of the melting phase, the final phase of the melting phase and the flat bath phase, or the melting phase and the flat bath phase.

[0059] The present invention has many advantages, in particular that the operation of the electric arc furnace can be flexibly adapted to the requirements of each individual case, and furthermore that the specific energy (e.g. kilowatt-hours per ton) required to produce a given amount of molten metal 13 can often be reduced, and the cycle time can often be reduced as well.

[0060] Although the present invention has been illustrated and described in more detail through preferred embodiments, the present invention is not limited to the disclosed embodiments, and other variations may be devised by those skilled in the art without departing from the scope of protection of the present invention. [Explanation of symbols]

[0061] 1 Furnace vessel 2 metal 3. Energy supply device 4 Supply network 5. Furnace transformers 6 electrodes 7 Positioning device 8 Double Arrow 9. Control Device 10 Control Program 11 Machine Code 12. Arc 13 Molten Metal 14 Slag layer 15 Driver A1, A2 operating values f, fa, fb Operating frequency f0 Power frequency F Frequency range G Dimension I, Ia, Ib, Ic Power supply current pa, pb, pc positioning P * Output to be introduced S1~S20 steps t time U Electrode Voltage Z state

Claims

1. A method for operating an electric arc furnace, comprising the steps of: drawing electrical energy from a supply network (4) between the charging of a metal in a solid agglomerated state (2) into a furnace vessel (1) of the electric arc furnace and the removal of molten metal (13) from the furnace vessel (1) and supplying the drawn electrical energy to at least one first electrode (6a), at least one second electrode (6b) and at least one third electrode (6c) of the electric arc furnace via a furnace transformer (5), thereby providing a power supply between the electrodes (6) and the A method for operating an electric arc furnace, comprising: forming an arc between a metal (2) or the molten metal (13), the metal (2) being melted by the arc to form the molten metal (13); and the energy supply device (3) separately adjusts an operating frequency (fa) of the first electrode (6a) and an operating frequency (fb) of the second electrode (6b), whereby a current (Ic) passing through the third electrode (6c) is determined by a current (Ia) passing through the first electrode (6a) and a current (Ib) passing through the second electrode (6b), 13. The method for operating an electric arc furnace, comprising: dynamically allocating to the electrodes (6) which of the electrodes (6) of the electric arc furnace is the first electrode (6a), which of the electrodes is the second electrode (6b), and which of the electrodes is the third electrode (6c).

2. 2. The method according to claim 1, characterized in that the operating frequency (fa) of the first electrode (6a) and the operating frequency (fb) of the second electrode (6b) are different from each other.

3. 3. A method according to claim 1 or 2, characterized in that the operating frequency (fa) of the first electrode (6a) varies with time.

4. 4. A method according to claim 3, characterized in that the operating frequency (fa) of the first electrode (6a) varies depending on the process conditions of the arc furnace.

5. 5. The method according to claim 4, characterized in that the process conditions are determined through evaluation of electrical or acoustic operating variables of the arc furnace.

6. 6. The method according to claim 1, wherein the operating frequency (fb) of the second electrode (6b) is adjusted taking into account the operating frequency (fa) of the first electrode (6a).

7. 7. The method according to claim 1, characterized in that the electrodes (6) of the arc furnace are height adjustable independently of one another and that the positioning (pa, pb, pc) of each of the electrodes (6) of the arc furnace is dependent on whether the electrode (6) is a first electrode (6a), a second electrode (6b) or a third electrode (6c).

8. 8. The method according to claim 1, characterized in that the electric arc furnace is operated first in a melting phase and then in a flat bath phase between the charging of the metal (2) into the furnace vessel (1) and the removal of the molten metal (13) from the furnace vessel (1), the metal (2) melts in the melting phase to form the molten metal (13), the molten metal (13) is further heated in the flat bath phase, and the operating frequency (fa) of the first electrode (6a) is determined in dependence on the size (G) of the metal (2) pieces to be melted with the first electrode (6a) in the melting phase.

9. 9. A method according to claim 8, characterized in that the operating frequency (fa) of the first electrode (6a) is determined such that the larger the pieces of metal (2) to be melted by means of said first electrode (6a) in the melting phase, the smaller said operating frequency is.

10. 10. The method according to claim 8 or 9, characterized in that the supply network (4) operates at a mains frequency (f0) and that the operating frequency (fa) of the first electrode (6a) during the melting phase or during the final phase of said melting phase is greater than said mains frequency (f0).

11. 11. A method according to claim 1, characterized in that the electrical energy supplied to the electrodes (6) is determined in such a way that the electrodes (6) introduce, on a time average, the same amount of energy into the metal (2) or into the molten metal (13), respectively.

12. A control program for a control device (9) of an arc furnace, the control program including a machine code (11) processable by the control device (9), the control device (9) processing the machine code (11) causing the control device (9) to operate the arc furnace according to the operating method according to any one of claims 1 to 11.

13. 13. A control device for an electric arc furnace, the control device being programmed with a control program (10) according to claim 12, and by processing a machine code (11) by the control device (9), the control device (9) operates the electric arc furnace according to the operating method according to any one of claims 1 to 11.

14. 1. An electric arc furnace, comprising: the electric arc furnace has a furnace vessel (1) into which metal (2) can be charged and from which molten metal (13) can be removed, the arc furnace comprises an energy supply (3) and electrodes (6) as well as a furnace transformer (5), said energy supply device (3) is connected on the input side to the supply network (4) and on the output side to said electrodes (6) via said furnace transformer (5), the arc furnace comprises a control device (9) by means of which at least the energy supply device (3) can be operated, - an electric arc furnace, in which the control device (9) is constructed according to claim 13.

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