Induction furnace including additional resonant circuit

Abstract

A crucible comprising at least two electric induction heating circuits, one of the two electric induction heating circuits being located around the side wall and the other being located below or in the bottom plate of the crucible. One of the circuits (18) comprises an electric generator (15) connected to an inductor (3), while the other circuit (19) has no electric generator but is electromagnetically coupled to the circuit (18) and consists of an auxiliary inductor (7) and a capacitor (17) with an adjustable total capacitance. The coupling conditions, the respective power through the two inductors (3, 7) and the heating distribution can be modified to homogenize the heating or to concentrate the heating on the bottom plate of the crucible. Good thermal efficiency can be expected.

Description

[0001] The invention relates to an induction furnace comprising an additional resonant circuit.

[0002] The invention relates to a cold crucible furnace with electromagnetic induction heating for melting, for example, in particular at least one electrically conductive material, such as an oxide or oxide mixture, or such mixtures with a metal proportion of up to about 30% by weight, which mixtures represent a molten melt. The invention is also particularly suitable for furnaces used in the glass industry for the production of enamels or other high-purity materials produced at high temperatures, for example 1200 K to more than 3000 K. Depending on the size of the crucible, the induction frequency is typically between 100 kHz and 1000 kHz.

[0003] The invention entails improving cold crucible furnaces, in particular by achieving a better homogenization of the charge temperature, thereby reducing the thickness of the crust on the bottom of the crucible, without involving substantial overheating of the pool or of the molten material or the generation of induced currents, which are harmful and can interfere with the components constituting the circuit of one or more inductors, in particular, for example, current generators. The advantages obtained are associated with better efficiency and better thermal homogeneity; they make it possible to simplify the installation, improve the mixing of the pool by convection, and promote the pouring of the liquid by gravity through the bottom plate.

[0004] Induction furnaces are well known in the foundry or metallurgical fields or in the nuclear industry for the production of materials or the formation of homogeneous mixtures. Such induction furnaces comprise a crucible containing a charge and at least one inductor, the excitation of which generates an electric current in the charge, thereby causing the heating and melting of the charge. This heating process is simple to implement and avoids any contact between the heat energy source and the crucible.

[0005] The crucible generally comprises side walls joined to a bottom plate which may be flat and constitutes the bottom. If the crucible has an electrically conductive material, the electrically conductive material is generally formed into sectors, i.e. consists of portions extending over angular sectors which are separated by electrically insulating partitions in order to avoid the occurrence of induced currents therein, the formation of eddy currents around the charge and corresponding energy losses.

[0006] Some crucibles are made of refractory materials, such as ramming mass or graphite. They have the advantage of high melting temperatures, but some charges, especially those composed of oxides, must be melted at higher temperatures or at least at temperatures sufficient to corrode them physically or chemically.

[0007] Furthermore, a very common design adopted by the present invention is that of a cold crucible equipped with a cooling circuit extending inside the crucible, cooled by water or another liquid. Thus, during operation of the process, the crucible is much cooler than the center of the charge and is protected from corrosion or other physical or chemical attack by the charge it contains by a solidified layer of material, which can be considered to constitute the inner wall of the crucible and such a crucible is called a crust crucible. These cold crucibles are capable of melting highly reactive materials at high temperatures above 1500K or even above 3000K, which materials can be metals such as titanium, steel or various alloys consisting of oxides (such as glass, titanium oxide, rare earth metals or mixtures); or, for example, materials of low electrical conductivity, such as silicon or enamel.

[0008] However, defects in the uniformity of heating in the crucible can be observed. The hottest part of the charge tends to rise to the free surface by convection. However, the induced current responsible for heating tends to be concentrated in the part with the highest conductivity, especially corresponding to the hot part of the oxide charge. This convection may be enhanced by uneven heating and by reducing the electromagnetic induction near the bottom plate (if the bottom plate is metallic), with the result that the charge is more difficult to heat at the bottom of the crucible, where the temperature remains lower, and the solidified layer is usually thicker at the top of the bottom plate (sometimes tens of millimeters compared to a few millimeters of the side wall of the crucible). This makes melting more difficult to achieve, accompanied by high electrical losses of the cooling structure, which is harmful in itself and affects the quality of the process. Still greater obstacles are encountered when the molten charge is discharged from the bottom of the crucible by opening the stopper at the center of the bottom plate rather than by inverting the crucible, because inverting the crucible is delicate and often prohibited. In fact, when the stopper is removed, the solidified crust prevents tipping. It is conceivable to increase the power to melt the crust at the bottom of the crucible and concentrate the heat in the center; but there is the risk of overheating elsewhere, while increasing the volatility of certain oxides, thus changing the composition of the charge to melt, while melting the solidified layer of the charge covering the side walls or while damaging certain structures of the crucible, such as the insulation between the sectors. Heat losses can also be multiplied by a factor of perhaps 1.5 or 2 in a process already characterized by low efficiency, where radiation from the free surface of the bath, conduction on the crucible walls and convection with the surrounding atmosphere are all high.

[0009] It is also possible to act on the induction frequency in an attempt to obtain melting of the charge at the bottom of the crucible: if the inductor is arranged around the side wall (which is the most common), the heating distribution inside the crucible depends radially on the excitation frequency of the inductor: the higher the frequency, the more concentrated the heat is inside the wall; the excitation frequency can therefore be chosen or adjusted to adjust the volume of the molten pool and, to a certain extent, the temperature distribution inside the crucible; but the inhomogeneity of the temperature in the molten pool may become unacceptable. In practice, the circuits and cooling systems are usually overdimensioned, and this burden on the device is not necessarily satisfactory.

[0010] This drawback is particularly evident in the present case where the cold crucible is of an electrically conductive material, such as copper or certain grades of stainless steel, particularly in view of the increased electrical losses in these structures, but which exist regardless of the material constituting the crucible and its structure.

[0011] A final disadvantage of the simplest induction furnaces described so far is that they do not adapt well to charges of variable volume, and therefore different heights within the crucible if the inductors are of significantly different heights.

[0012] There is a large amount of prior art describing such induction furnaces. Some specific designs will now be described in detail which are assumed to improve the uniformity of heating.

[0013] In addition to the main inductor around the side wall of the crucible, document EP 1 045 216 B1 describes a supplementary inductor which is arranged around the pouring area below the bottom plate, around the plug that can be poured from the crucible. It can generate an additional induced current around the pouring hole through the bottom plate to prevent the charge from solidifying here and to ensure that pouring can be carried out when the plug is removed. However, the difficulties of implementation still exist, and this device is not necessarily suitable for melting materials with very high melting points. Its effect is only local for a limited time, so the more general problem of lack of uniformity when heating the charge to melt is not solved. The same applies if, as has been proposed, an uncooled metal tube is provided between the lower inductor and the pouring hole, hoping to obtain greater heat with the help of the tube; in addition, the latter also risks being corroded quickly, especially at high temperatures.

[0014] In some designs, such as that of document US 6 185 243 B1, the more conventional inductor arranged around the side wall of the crucible is replaced by an inductor which is located below the bottom plate and which may be in the shape of one or more spirals. This design is only suitable for a molten pool of small height relative to the diameter and it may be associated with high heat losses due to convection on the free surface of the molten pool and conduction on the side walls. The power supplied is still not uniform, being generally high at the inductor turns but much weaker near the side walls and in the center of the crucible (if there are no turns).

[0015] It is also proposed to provide a single inductor extending around the side wall and under the bottom plate (US-1645526-A) or two separate inductors each driven by a power supply (US 4 609 425, JP 10 253 260, US 4 687 646). However, the conflict between the electric fields generated by the two inductors or inductor components offsets the possibility of heating the charge more uniformly, which interferes with their operation and may even damage the control electronics. Finally, an improvement to these designs is proposed in document WO 2017 / 093165 A1, in which a part called a flux concentrator is located at the junction of the side wall and the bottom plate, thereby separating the two inductors. This part is a static part of a material with high magnetic permeability, thus reducing the interaction between the magnetic fields. The above-mentioned disadvantages are reduced and better efficiency is expected. Unfortunately, specifically for the operating range envisaged here (regarding the excitation frequency of the power supply), it is necessary to cool the flux concentrator so that it maintains its performance, and this may be technically difficult in some cases. The concentrator must be placed in an area that is difficult to access and may require the inductor to be moved relative to the charge; finally, in the case of a radioactive charge, it is very susceptible to radiation. A disadvantage of the alternative placement sequence is that this device does not allow the heating power to be distributed between the two inductors.

[0016] The present invention relates to an improved induction furnace, the most important aspect of which is that it makes it possible to increase the efficiency of heating the molten charge and to adjust the heating distribution in the heating volume as required by means of a more advantageous configuration of two inductors, one of which is located around the side wall of the crucible and the other of which is located below (or in) the bottom plate. It is expected that a higher electrical efficiency than that of earlier devices and processes will be obtained. Since the bottom of the crucible is heated more efficiently, dumping is also expected to be easier.

[0017] In particular, it relates to an induction furnace comprising a crucible having side walls cooled by fluid circulation and a bottom plate located below the side walls, a first induction heating circuit comprising an inductor and an electric generator, and a second induction heating circuit comprising a second inductor, wherein one of the inductors is located around the side walls and the other is located below or in the bottom plate, characterized in that the second circuit does not have an electric generator but is electromagnetically coupled to the first circuit and is provided with a capacitor device with adjustable capacitance, which is connected to the second inductor.

[0018] This electrical device with a double circuit exploits the phenomenon of electrical resonance of the furnace, thanks to the free electromagnetic interaction of the two circuits. Contrary to known devices in which the inductor surrounding the side walls and the inductor assigned to the bottom plate are excited by the same power source and connected to each other at the same time, or excited by two different power sources and then separated from each other, no conflict is observed between the induced magnetic fields at their junction and there is no interference with the circuits. This promotes better electrical efficiency and lower losses.

[0019] Another advantage of the present invention is the facility for better distribution of heating, specifically the possibility of increasing heating in the center and at the bottom, just above the bottom plate, to improve a uniform process and possibly facilitate pouring. This heating distribution depends on the setting of the total capacitance (adjustable during melting), and furthermore, there is a very high sensitivity to variations in this capacitance, to the extent to which a good level of temperature uniformity may be obtained, or conversely to heat substantially at the periphery, in the center or at the bottom of the molten charge.

[0020] Since the first inductor constitutes the main inductor, it is usually arranged around the side walls, and the second circuit is then assigned to the bottom plate. However, it is undoubtedly possible to encounter the opposite arrangement, in particular for crucibles with a large surface area and a low height.

[0021] The inductor at the bottom of the crucible may form a spiral covered with an electrical insulator, which is a very good thermal conductor: it is then possible and advantageous for the floor of the crucible bottom to consist of said inductor itself, thus simplifying the design of the crucible and increasing the heating efficiency.

[0022] The induction furnace may comprise at least one additional circuit, which does not have any electrical generator and comprises an inductor and capacitor arrangement, which has an adjustable total capacitance and is electromagnetically coupled to at least the first circuit; this additional circuit or these additional circuits therefore have the same characteristics as the second circuit mentioned. The additional circuit may prove useful at the top of the furnace to improve the heating of the newly added charge. The additional circuit may then be placed above the apex of the crucible, facing the indicator situated on or in the bottom plate and parallel to it.

[0023] One or more of the additional circuits may then advantageously be equipped with a circuit breaker enabling them to be disconnected at will.

[0024] Various aspects, features and advantages of the present invention will now be described with the aid of the following drawings which show certain specific embodiments of the invention, which embodiments are illustrative only.

[0025] Figure 1 Here is the general view of the induction furnace;

[0026] Figure 2 is a view of a lateral inductor;

[0027] Figure 3 is a view of the bottom inductor;

[0028] Figure 4 It is a circuit diagram;

[0029] Figure 5 is an embodiment of a capacitor bank;

[0030] Figure 6 Another embodiment is shown;

[0031] Figure 7 showing a first heating state;

[0032] Figure 8 It is the first heating state from the bottom-up perspective;

[0033] Fig. 9 showing a second heating state;

[0034] Fig.10 It is the second heating state from the bottom-up perspective;

[0035] Fig.11 showing a third heating state;

[0036] Fig.12 is the third heating state from the bottom-up perspective;

[0037] Fig.13 Another embodiment of a crucible is shown;

[0038] Fig.14 A third embodiment of a crucible is shown;

[0039] Fig.15 A bottom plate of a third embodiment is shown;

[0040] Fig.16 is a cross section of the bottom plate of the first embodiment.

[0041] Will be aided by Figures 1 to 3 An embodiment is described. This embodiment of an induction furnace comprises a side wall 1 or a cylindrical sleeve consisting of vertical tubes 2 of copper or stainless steel, adjacent to each other and connected by electrically insulating partitions to prevent the appearance of circular induction currents in the side wall 1. According to known arrangements, the tubes 2 are traversed by a cooling liquid such as water. A cooling circuit 12 can circulate the liquid alternately upwards and downwards, wherein the connectors between the tubes 2 are alternately located at the upper and lower ends. Another possible structure is, among others, that of US-6 996 153-B2, wherein each tube is cooled independently by a vertical reciprocating circuit.

[0042] The transverse inductor 3 surrounds the side wall 1. Here, it is a single turn composed of parallel strands 4 that overlap in height and each of which goes around the crucible in a single turn. Their ends are joined together by two vertical connections 5 and 6, which also provide electrical and hydraulic connections to the alternating current generator, the strands 4 also being cooled by the circulation of a cooling liquid, the strands being connected to a cooling circuit 13. Moreover, other arrangements of the transverse inductor, in particular a continuous spiral arrangement of the turns, would be possible.

[0043] The crucible is completed by a bottom plate 8 consisting essentially of an auxiliary inductor 7 in the form of a spiral ( Figure 3 and Fig.16 ). The auxiliary inductor 7 is also a water-cooled tube and its ends 9 and 10 also lead to the cooling circuit 14. The turns 31 of the spiral are joined by a partition electrical insulator 32, which is an electrical insulation and a very good thermal conductor, thus separating them all and providing a continuous structure for the bottom plate 8. In addition, the auxiliary inductor 7 is covered on its upper surface with a thin layer 33, which is an electrical insulation and a very good thermal conductor, for example consisting of aluminum oxide. This design makes it possible to eliminate the bottom plate traditionally used and to store the charge of the crucible directly through the auxiliary inductor 7.

[0044] The cooling of the bottom plate 8 and the side pairs 1 also leads to the formation of a skull crucible, ie the charge layers in contact with them remain solid and protect them from corrosion.

[0045] A dump plug 11 is arranged in the center of the spiral. It is cooled by conventional means, not shown here. The cooling circuits 12, 13 and 14 of the side wall 1, the main inductor 3 and the auxiliary inductor 7 are only schematically represented here, according to known embodiments and each may specifically include, for example, a pump and a heat exchanger.

[0046] Electrical devices such as Figure 4 As shown. The main inductor 3 is connected to the terminals of the alternating current generator 15. The capacitor bank 16 is also connected to the terminals of the electrical generator 15 in parallel with the main inductor 3. The assembly forms a closed main circuit 18. The resistance of this main circuit 18 is mainly due to the resistance of the crucible structure, the charge contained therein and the resistance of the main inductor 3. The auxiliary inductor 7, together with the adjustable capacitor bank 17, forms a closed auxiliary circuit 19, which is physically separated from the electrical generator 15 and does not have its own electrical generator. The resistance of this auxiliary circuit 19 is mainly due to the resistance of the auxiliary inductor 7. The proximity of the inductors 3 and 7 means that in operation an electromagnetic coupling occurs between the circuits 18 and 19, although there is no electrical generator in the auxiliary circuit 19. This coupling varies specifically depending on the power input and the charge in the crucible and the setting of the total capacitance of the capacitor bank 17, which consists of individual capacitors that can be connected to the auxiliary circuit 19 or separated by a switch 21. As Figure 5As shown, the capacitors 20 may be placed in parallel, or in other ways. Figure 6 ), the capacitor bank 17 may be replaced by an adjustable capacitor 22, and the effect will be the same.

[0047] The invention is based on the electromagnetic coupling between the circuits 18 and 19 through the inductors 3 and 7 in order to adjust the current induced by them and act on the heat distribution in the charge contained in the crucible, in particular in order to homogenize the temperature of the charge during melting and mixing, or (among other possibilities) to increase the heating of the bottom in order to release the pouring stopper 11 and facilitate the pouring operation.

[0048] The effect obtained may be the effect given in Table 1 below:

[0049]

[0050] The first row represents the operation of the device with the auxiliary circuit 19 disconnected. The voltage at the terminals of the main inductor 3 and the current passing therein are high, the voltage at the terminals of the auxiliary inductor 7 is low, so that heating is mainly carried out by the main inductor 3, but with high losses and relatively low efficiency in the side wall 1. This mode of operation is similar to known conditions and does not represent the invention nor is it generally sought. Figure 7 and Figure 8 The temperature distribution of the molten charge from a top-down view and from a bottom-up view (light shades indicate greater local heating) is represented, which shows that the heating is higher at the periphery of the charge and weaker in the center, so the charge is cooler here, especially the heating of the electric generator 15 just above the bottom plate is very weak. Choosing different excitation frequencies may cause the distribution of heating to change in the radial direction, but in any case, the heating at the bottom of the crucible is still too weak to achieve temperature uniformity.

[0051] The following row of the table below shows the effect of increasing the capacitance of the capacitor bank 17 or the adjustable capacitor 22. The electromagnetic resonance is enhanced, the voltage and current values ​​at the terminals of the main inductor 3 decrease, while the voltage at the terminals of the auxiliary inductor 7 and the current value passing therein increase. The losses in the side wall 1 decrease drastically and with increasing magnitude, the losses in the auxiliary inductor 7 increase but remain at a much lower value, which means that the efficiency of the furnace is much higher, reaching a maximum value of 85% at a capacitance value estimated at 69.45 nF. Thus, the typical distribution diameter of the heated in the charge is as follows Fig. 9 and Fig.10 As shown, again viewed from top to bottom and from bottom to top: an excellent heating homogeneity is achieved in the charge and this time it is observed that a portion of the bottom crucible is heated while being at nearly the same temperature as the portion located above.

[0052] By further increasing the capacitance of the capacitor bank 17 or the adjustable capacitor 22, the heating of the bottom of the crucible may be intensified to the detriment of the remaining parts: Fig.11 and Fig.12 The state obtained with a capacitance of 90 nF is shown, which in this case experimentally corresponds to the maximum electrical resonance, in which the voltage and current values ​​at the terminals of the main inductor 3 are minimum, while the voltage at the terminals of the auxiliary inductor 7 and the current flowing therefrom are maximum. Fig.12 As shown, the top of the charge is rarely heated and the heating is concentrated at the bottom of the crucible. Maintaining this high total capacitance and maximum resonance of the group 17, in which the auxiliary circuit 19 works most when providing maximum power, makes it possible to concentrate the heating near the auxiliary circuit 19 (i.e., here at the bottom of the crucible) and promote the melting of the solid crust while the molten bath continues to mix. Pour-out can then be achieved by opening the stopper 11 and by gravity.

[0053] Fig.13 The use of a second auxiliary circuit 23 comprising an inductor 24 and a capacitor 25 is shown, the capacitor being placed at the terminals of the inductor 24, thus closing the circuit. The capacitor 25 may be adjustable or may be non-adjustable. The inductor 24 is located above the apex of the crucible, in a form similar to that of the auxiliary circuit 19 facing it. The second auxiliary circuit 23 does not have an electrical generator. It also operates by resonant electromagnetic coupling with the main circuit 18 and by judicious adjustment or choice of the capacitance of the capacitor 25, making it possible to establish additional heating in the upper part of the crucible, which is suitable for preheating a portion of the charge newly introduced into the crucible, for example, in particular in processes where the charge is introduced gradually, or for processes where, for example, the charge is more voluminous and its volume is high. The second auxiliary circuit 26 can be deactivated by opening the circuit breaker 26.

[0054] The invention can be implemented in many other ways. Fig.14 and Fig.15 An embodiment is shown in which the base plate with reference numeral 27 consists of water tanks 28 in circular sectors, which are joined by arc-shaped pipes 29 to circulate the cooling fluid from one water tank 28 to the next. The end 30 of the cooling circuit is also used to connect an adjustable capacitor bank as in the previous embodiment. The device is otherwise identical to the previous one and operates similarly; however, this embodiment is not preferred because the electromagnetic coupling in favor of the auxiliary circuit is much worse.

[0055] It is still possible to place the auxiliary inductor above the refractory and electrically inert base plate, as in known devices, again at the expense of reduced efficiency.

[0056] The inductor may be of any known type. The auxiliary inductor envisaged mainly has a spiral form, but inductors with a single turn formed by several concentric strands arranged in parallel or a single strand can be envisaged and produce good results. The main inductor envisaged here with several parallel single strands can be replaced by a multi-turn spiral inductor accordingly.

[0057] The total capacitance of the capacitor bank 17 must be chosen so as to be adjustable in order to achieve different operating modes corresponding to the heating distribution that is desired to be obtained, such as good uniformity in the largest part of the volume, and sufficient concentration at the bottom of the crucible to achieve pouring. It is usually indicated that the maximum resonant state of the circuit can be reached so that the maximum power can be sent to the auxiliary circuit 19. The state corresponding to the highest efficiency will be preferred during most of the process, but this is not essential, since the efficiency is still improved within a large range of total capacitance values ​​of the capacitor bank 17.

[0058] The positions of the main inductor (connected to the electrical generator) and the auxiliary inductor (equipped with an adjustable capacitor) can be interchanged without special changes to the crucible and Figures 1 to 4 Indicates the structure of the circuit arrangement.

[0059] If necessary, other auxiliary circuits can also be added at different locations of the crucible, which are electromagnetically coupled to the main circuit and consist of inductors and capacitors and do not have their own electrical generators.

Claims

1. An induction furnace comprising a crucible having a side wall (1) cooled by fluid circulation and a bottom plate (8) located below the side wall (1), a first induction heating circuit (18) comprising a first inductor (3) and an electric generator (15), and a second induction heating circuit (19) comprising a second inductor (7), one of the inductors being located around the side wall (1) and the other being located below or in the bottom plate (8), characterized in that The second induction heating circuit (19) has no electrical generator but is electromagnetically coupled to the first induction heating circuit (18) and is provided with a capacitor device (17) with adjustable capacitance, the capacitor device (17) being connected to the second inductor (7).

2. The induction furnace according to claim 1, characterized in that The bottom plate (8) is constituted by one of the inductors, in the form of a spiral, consisting of turns (31) joined by an electrical insulator (32).

3. The induction furnace according to claim 1, characterized in that The capacitor arrangement comprises an adjustable capacitor (22) having a variable capacitance.

4. The induction furnace according to claim 1, characterized in that The capacitor arrangement comprises a capacitor (20) connected to the second induction heating circuit (19) via a switch (21).

5. The induction furnace according to claim 1, characterized in that The first inductor (3) is located around the side wall (1).

6. The induction furnace according to claim 1, characterized in that The first inductor (3) is located below or in the bottom plate (8).

7. The induction furnace according to claim 1, characterized in that The induction furnace comprises at least one additional induction heating circuit (23) which does not have any electrical generator and comprises a third inductor (24) and a capacitor arrangement with an adjustable total capacitance and is electromagnetically coupled to at least the first induction heating circuit (18).

8. The induction furnace according to claim 7, characterized in that The at least one additional induction heating circuit (23) is equipped with a circuit breaker (26) capable of disconnecting the at least one additional induction heating circuit (23).

9. The induction furnace according to claim 7, characterized in that The third inductor (24) of the at least one additional induction heating circuit (23) is located above the crucible and faces the second induction heating circuit (19).

10. The induction furnace according to any one of claims 1 to 9, characterized in that The total capacitance of the capacitor means of the second induction heating circuit (19) is adjustable to a maximum electrical resonance state of the induction furnace.

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