EP0248727A1 - High-frequency induction furnace - Google Patents

Abstract

Melting furnace for refractory materials by induction, the electrically conductive wall of which consists of a single cylindrical coil (1), the ends (2) of which are connected to a source of high-frequency alternating current, the said coil forming both the inductor and the actual crucible. This furnace comprises at least one elongated conductive part (5) arranged along the slot (4) delimited by the ends (2) of the coil (1) and close to the latter . This part (5) is maintained at an intermediate potential between the ends (2), which increases the breakdown voltage between the latter. The application of a higher voltage makes it possible to operate with a crucible of larger diameter or with lower frequencies.

Description

The present invention relates to a melting furnace by electromagnetic induction by circulation of high frequency alternating current, usable in particular for the melting and transformation of refractory materials such as ceramic oxides, glass and chemical salts.

The principle of the induction furnace consists in passing an inductor through an alternating current; a magnetic field is then created inside this inductor where the charge is liquefied. Induced currents are then generated, which circulate inside this load and are converted into heat energy by the Joule effect, provided that the resistivity of the load is less than a value depending on the diameter of this load and the frequency considered.

Many refractory materials can be considered as insulating at ambient temperature, but their resistivity decreases beyond a temperature here called inducibility. In this case it is necessary to provide a heating means to ensure the initiation of the induction phenomenon. When the charge is melted, the furnace can operate in continuous casting provided that it has suitable filling and emptying means.

Known devices, such as those protected by French patents 1,430,192 and 1,430,962, as well as by European patent 0 079 266, show that such melting furnaces can be produced according to various design variants:
- The inductor may consist of a simple envelope of conductive metal, generally cylindrical in shape and interrupted only by a slot at the terminals of which the voltage taps arrive. The current therefore travels a full turn only around the load. This design will be called monospire in what follows.
- The inductor can also consist of a solenoid (multispire design), the current then flows through a propeller.
- Whether monospire or multispire, the inductor can be isolated from the charge to be liquefied by a cooled or refractory wall (indirect induction mode). It can also be in contact with the liquefied charge: there is then a direct induction in a self-crucible. The cooling of the inductor must then be ensured, in principle, by a circulation of fluid: a solid layer of the refractory material, in pulverulent or granular form, then remains and isolates the inductor from the molten charge.

However, these various conceptions have drawbacks which can thus be summarized:
- Solutions in which an intermediate wall isolates the inductor from the charge have a reduced efficiency as a result of the Joule effect produced in this wall, as well as by the electromagnetic decoupling created.
- Solutions of the self-crucible type require the installation of an external envelope in the case of a multispire inductor in order to avoid the flow of the load out of the crucible. The single-coil inductor has the disadvantage of the risk of electric arc formation between the two voltage taps of the inductor, especially if the external layer of the load is brought to a temperature higher than the inducibility temperature. . This layer can no longer properly fulfill its role as an electrical insulator.
- The main disadvantage of multispire inductors is their high impedance, the inductance being proportional to the square of the number of turns as well as to the square of the diameter. It is then necessary to use crucibles of small diameter (in practice, not more than 35 cm for a winding with two turns), which poses problems of induction inside the load and on the other hand limits the surface heat exchange between the molten bath and the raw material added continuously.
- Another disadvantage of monospires is linked to the risk of forming an electric arc between the voltage taps, as mentioned above. This results in a limitation of the potential differences with which one can work.

The present invention provides an improvement on existing solutions insofar as it combines the simplest design: monospire oven in self-crucible, with a device making it possible to guard against the risks of arcing which constitute the major problem of monospire.

To this end, the subject of the present invention is a melting furnace for induction refractory materials, the electrically conductive wall of which consists of a single cylindrical turn, the ends of which are connected to a source of high frequency alternating current, said turn forming both the inductor and the crucible proper and comprising means for cooling its surface, this furnace being characterized in that it comprises at least one cooled piece of elongated shape made of electrically conductive material, disposed along the slot delimited by the ends of the turn, maintained at a floating potential and electrically isolated from said turn.

The elongated piece, which constitutes the essential means of the invention, thus fulfills a double function.

First, because it is conductive and automatically places itself at an intermediate potential between those of the ends of the turn, it practically eliminates the risks of arcing between the ends of this turn.

Then, by its position along the slot separating the ends of the coil, it allows sufficient cooling to ensure the tightness of the crucible vis-à-vis its contents.

According to a secondary characteristic, the space between the cooled part and the ends of the coil is filled with an electrical insulator which must withstand maximum temperatures of approximately 200 ° C. and can therefore be made of paper, plaster, epoxy resin or thin layer refractory cement.

According to another secondary, but nevertheless important, characteristic of the invention, which preferably applies at the same time as the main characteristic, the bottom of the crucible is made of a conductive material.

The coil is then electrically isolated from the bottom of the crucible by a refractory electrical insulator.

According to one embodiment of this secondary characteristic, the conductive material constituting the bottom of the crucible is of the same nature as that of the coil. According to another secondary characteristic, the bottom of the crucible is made of an insulating material.

According to another secondary characteristic, the lower part of the turn, adjacent to the bottom of the crucible, is notched. This arrangement, in the case of a conductive crucible bottom allows, according to the invention, not to disturb the electromagnetic field in the lower part of the crucible by greatly reducing the coupling between the monospire and the bottom of the crucible. This arrangement, in the case of an insulating crucible bottom, makes it possible to limit the field of induction in the charge and thus prohibit fusion on contact with the refractory bottom. To complete this mode of implementation, it is possible to separate the turn and the bottom with a refractory electrical insulator and to seal the notches using this refractory electrical insulator.

• - la figure 1 représente une perspective générale du four à induction selon l'invention,
• - la figure 2 représente une vue de face, en coupe partielle, de la pièce qui contribue à isoler les prises de tension de la spire de la figure 1,
• - la figure 3 représente un mode particulier de réalisation de l'invention, d'après lequel l'isolation électrique de la spire est assurée par deux exemplaires, disposés en parallèle, de la pièce représentée figure 2. L'agencement est représenté en vue de dessus.
• - la figure 4 représente une coupe d'un fond de creuset non conducteur.

Other characteristics and advantages of the invention will emerge from the description which follows, given purely by way of non-limiting illustration, with reference to the appended drawings, in which:

• FIG. 1 represents a general perspective of the induction furnace according to the invention,
• FIG. 2 represents a front view, in partial section, of the part which contributes to isolating the voltage taps from the turn of FIG. 1,
• - Figure 3 shows a particular embodiment of the invention, according to which the electrical insulation of the turn is provided by two copies, arranged in parallel, of the part shown in Figure 2. The arrangement is shown in view On top.
• - Figure 4 shows a section of a non-conductive crucible bottom.

In FIG. 1, the oven therefore generally comprises a turn 1 constituted, according to a preferred embodiment of the invention, of a curved sheet of an electrically conductive metal such as copper or aluminum, at the ends 2 from which an electrical circuit 3 introduces the alternating electrical current necessary for operation.

Along the slot 4 delimited by the ends of the coil 1 and close to it is arranged at least one elongated piece 5 of electrically conductive material, maintained at a floating potential and electrically isolated from the coil 1 by a space which can optionally be filled with an insulator 6 disposed between the part 5 and the ends of the coil 1.

In the embodiment of the invention described in FIG. 1, the part 5 is unique and makes it possible to halve the value of the tension between the two ends of the turn.

In other embodiments of the invention, several pieces 5 are installed along the slot 4 and the tensions between the ends of the turn can be staggered more finely. This is particularly the case for the example which will be described later with reference to FIG. 3.

Each part 5 is subject to the action of the electromagnetic field and therefore traversed by induced heat-generating currents. It therefore essentially consists of a hollow envelope 7 inside which circulates a cooling fluid. FIG. 2 indicates a possible configuration, according to which a metal tube 8 is introduced inside the envelope 7: the fluid enters through the metal tube 8 and rises along the envelope 7.

The insulator 6, which also seals the crucible, must withstand maximum temperatures of around 200 ° C. since the part 5 is cooled. It can be made of paper, plaster or epoxy resin or refractory cement in a thin layer for example.

According to a particular embodiment of the invention, the bottom 9 of the crucible can be made of refractory material.

The device, shown in Figure 4, ensuring the emptying of the liquefied product then consists essentially of a cooled copper tube 18 by a few windings of a smaller copper tube 20 in which circulates a cooling fluid, the whole being embedded in the refractory material constituting the bottom 9 and closed by a plug 22 of copper itself cooled.

According to another embodiment of the invention, the bottom 9 can also be made, as is the case in FIG. 1, of the same conductive material as the turn 1, the emptying device then being limited to a hole bored in the bottom and closed by a cooled copper plug, as for the embodiment described in the previous paragraph. It is then necessary to provide electrical insulation between the bottom and the coil and to avoid an excessive modification of the lines of the electromagnetic field; this is why an insulating and refractory joint 10 separates the bottom and the turn, and moreover the part of the turn adjacent to the bottom is notched, which removes the part of the electromagnetic field which would have undergone coupling due to the presence of the bottom . These notches 11 formed in the turn are sealed by an electrical insulator which seals the crucible. They are generally arranged periodically and their length is of the order of a tenth of the height of the turn.

The cooling of the walls of the crucible is carried out by means of small copper tubes 12 which are the seat of a forced circulation of fluid brought in and collected after use by two collectors 13 and 14 of larger diameters. The tubes 12 are generally circumferential. Only the cutouts 15 limited by the notches are cooled by circulation of fluid in bent tubes 16 winding along the cutouts.

This oven can be adapted to continuous operation, the solid material being able to be introduced continuously and discharged in the form of liquid by overflow using a chute not shown here fixed in the upper part of the coil, as is described in the patent application French ^No. 8,302,328.

The advantage of the invention consists in that the part (s) 5 allow (s) to operate with a higher voltage current without fear of the formation of an electric arc between the ends of the coil 1 : this voltage can be doubled in the case of an oven comprising only one of these parts. It is then possible to work with a turn of diameter twice as large, making it possible to process products with higher resistivity, which implies a heat exchange surface four times greater.

The inductance and resistance of an inductor and therefore its impedance are proportional to the square of the number of turns. The impedance of a monospire, four times lower than that of an inductor with two turns, also makes it possible not to change the diameter and to work at a frequency four times lower, which is equivalent from an energy point of view but allows the use of much simpler and more efficient AC transformers in some cases. These new possibilities can be further extended if several examples of part 5 are inserted as shown in FIG. 3. In this figure we find the same essential elements as those described in FIG. 1 and fulfilling the same roles. Only the use of two cooled parts is planned.

It is possible to work according to the resistivity of the product in the frequency range 40 kHz-500 kHz using a triode generator of aperiodic type, and in the range 50 Hz-40 kHz with a semiconductor generator or from the network.

By way of example, the material brought to the melting point (approximately 1400 ° C.) is a borosilicate glass of the VR15F type sold by HPC. The loading of raw powder is made continuously on the surface and the evacuation of the molten glass takes place by overflow through a chute formed in the upper part of the inductor.

Table 1 presents the main characteristics and the results of two tests carried out using an inductor monospire diameter 400 mm or 600 mm. For comparison, characteristics and results of a test carried out using an inductor two turns (diameter 300 mm) of the prior art, are given in the first column of this table.

The use of a high frequency generator of aperiodic type makes it possible to adapt the capacity of the oscillating circuit to the inductor used to fall within the frequency range indicated by the manufacturer.

Claims (10)

1. Melting furnace for refractory materials by induction, the electrically conductive wall of which consists of a single cylindrical turn (1), the ends (2) of which are connected to a source of high frequency alternating current (3), said turn forming both the inductor and the crucible proper and comprising cooling means (12) of its surface, characterized in that it comprises at least one cooled piece of elongated shape made of electrically conductive material ( 5), disposed along the slot (4) delimited by the ends (2) of the turn (1), maintained at a floating potential and electrically isolated from said turn. 2. Melting furnace according to claim 1, characterized in that the space between the cooled part (5) and the ends of the coil (1) is filled with an insulator. 3. Melting furnace according to any one of claims 1 or 2, characterized in that the bottom (9) of the crucible consists of a conductive material. 4. Melting furnace according to claim 3, characterized in that the conductive material constituting the bottom (9) of the crucible is of the same nature as that of the coil (1). 5. Melting furnace according to any one of claims 1 or 2, characterized in that the bottom (9) of the crucible consists of an insulating material. 6. Melting furnace according to any one of claims 1 to 5, characterized in that the lower part of the coil, adjacent to the bottom of the crucible, is notched. 7. Melting furnace according to any one of claims 3 and 4, characterized in that the conductive bottom (9) and the turn (1) are separated by a refractory electrical insulator (10). 8. Melting furnace according to claim 6, characterized in that the notches (11) formed in the turn are blocked by a refractory electrical insulator. 9. Melting furnace according to any one of claims 6 or 8, characterized in that the cutouts (15) limited by the notches are cooled by circulation of fluid in bent tubes (16) meandering along the cutouts. 10. Oven according to claim 1, characterized in that the part (5) maintained at floating potential consists mainly of a hollow envelope (7) inside which circulates a cooling fluid.

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