EP0169765A1 - Cooled casing for a crucible with high-frequency electromagnetic induction melting - Google Patents

Abstract

Cold cage for melting crucible by electromagnetic induction at high frequency, comprising, in known manner, a series of hollow segments (2) traversed by cooling water, surrounded by an inductor winding (5) at high or medium frequency , and in which the products to be melted are confined (6), characterized in that at least part of the walls of each segment (2) of the cage (1) consists of at least two layers of adjacent materials, and in that the relative thicknesses of these layers as well as the frequency of supply of the electric inductor (5) are chosen in such a way that the eddy currents induced in the cage develop in a layer which is good conductor of electricity.

Description

Technical literature has widely described, in recent years, "cold cages" or "cold crucibles" used in physico-chemical applications or special metallurgy, in particular for the melting by induction at high or medium frequency of metals or special alloys, or insulating materials, refractory or not, often occurring at high temperature. Most of the cold crucibles described comprise a greater or lesser number of segments, either straight or arranged in a pin. Generally, these segments are made of copper, which is perfectly suitable if the melting operation is carried out without direct contact with the crucible in a protective atmosphere, or if the copper in the crucible is protected from the liquid bath by a protective layer of solid slag or again by a natural shell of the Liquid material (insulating under normal conditions) melted by induction at appropriate frequency and of which a thin film is solidified in contact with the cold crucible.

This known state of the art is shown in Figure 1 which shows in perspective-elevation a cold crucible whose cold cage 1 is composed of a number of segments such as 2 in copper. This cage 1 is cooled by a circulation of water schematically represented by inlets 3 and outlets 4 putting the interior of each segment 2 into communication with the exterior. FIG. 1 also shows the inductor winding 5 which is responsible for carrying out the melting of the material 6 by magnetic induction at high frequency. This material 6 enclosed in the crucible 1, internally sealed by a solidified crust 7, thus constitutes what the secondary of a transformer whose Inductor 5 is the primary. It is under the influence of high frequency secondary currents that develop in the material 6 that it heats up and then achieves the desired fusion.

However, in a certain number of increasingly numerous operations due to the considerable increase in the potential applications of fusion or of physico-chemical or metallurgical treatments in cold crucible, it can occur, when the crucible is immersed in a physically or chemically aggressive environment, an alteration of copper either by chemical attack, or by entrainment of copper particles or atoms by effect of erosion or surface desorption. These effects are very troublesome insofar as they are likely on the one hand to cause premature wear of the crucible or on the other hand to cause pollution of the material treated in the crucible When the latter is used for the preparation of very high purity materials.

These harmful phenomena linked to the presence of copper as the base metal of the cold cage are further amplified when it is used, and the cases of application are more and more numerous, as containment or maintenance of an inductive plasma; such inductive plasma torches are in fact increasingly used for the preparation of ultra pure metals, alloys or noble materials: silica, quartz, alumina, silicon, titanium, etc.

The present invention specifically relates to a cold cage for melting crucible by electromagnetic induction which overcomes the prior drawbacks of the prior art while con serving The well known advantages of fusion or physico-chemical treatment in a cold crucible.

This cold cage for an electric induction melting crucible which in a known manner comprises a series of hollow segments traversed by cooling water, surrounded by a high or medium frequency inductor winding, and in which are confined the products to be melted. , is characterized in that at least a part of the walls of each segment of the cage consists of at least two layers of adjacent materials, one of which is resistant to corrosion is in contact with the products to be melted, and whose The other is a good conductor of electricity, and in that the relative thicknesses of these layers as well as the frequency of supply of the electric inductor are chosen in such a way that the eddy currents induced in the cage develop mainly. in a layer that conducts electricity well.

It can therefore be seen that the structure of the cold cage which is the subject of the invention consists essentially in making the crucible segments, no longer solely of copper, but of a composite structure comprising at least two metals closely applied one against the other, one good conductor of electricity, therefore with low Joule losses, the other more electrically resistive but characterized by greater resistance to Corrosion: said composite or sandwich structure is applied either to the whole of the cage, or to the less to the inner wall thereof more particularly exposed to the effects of corrosion or physicochemical or even hydrodynamic aggression in the case of plasmas.

The essential arrangement of the structure according to the invention therefore consists in choosing The respective thicknesses of anti-corrosion metal and good conductive metal so that the eddy currents induced in the cage under the influence of the primary inductor winding and which we know that the depth to which they are located depends on the frequency of the Induction, for the most part, develops in a layer which is a good conductor of electricity so as to minimize the losses due to the toy effect of the system.

According to the invention, the metal playing the anti-corrosion role of the crucible and present on the face of the segment turned towards the inside of the crucible consists of a stainless steel well known for its resistance in aggressive medium. But it is also known that, if the thickness of the stainless steel is equal to or greater than the depth of penetration of the currents at the working frequency in this metal, that is to say about 5 mm at 10 kHz or 5 / 10th of a mm at 1 MHz, the Joule losses developed in the external stainless steel layer are six to seven times greater than those observed in the wall with identical dimensions which would be made of copper. This is very detrimental to obtaining the acceptable electrical efficiency of the treatment carried out in such a crucible and the invention makes it possible to avoid this pitfall.

The practical arrangements of the cage according to the invention therefore make it possible, by a judicious choice of the thicknesses of layers of copper and of stainless steel present and of the induction frequencies, to produce a cold crucible having both excellent resistance to corrosion and losses Compatible with acceptable electrical efficiency.

• - La figure 2 représente en coupe élévation (figure 2a) et en coupe vue de dessus (figure 2b) un segment rectangulaire bimétal de cage froide conforme à l'invention;
• - La figure 3 représente en coupe perpendiculaire à L'axe du creuset une partie des segments de La cage froide dans Le cas d'une structure bimétallique Limitée à La paroi interne du creuset ;
• - La figure 4 représente également en coupe partielle selon un plan perpendiculaire à L'axe une partie des segments de La cage froide dans une autre forme de structure bimétallique à deux couches ;
• - La figure 5 représente en coupe selon un plan perpendiculaire à l'axe du creuset, une partie des segments constituant La cage froide dans L'hypothèse d'une réalisation multicouches en sandwich des parois des segments.

Anyway, the invention will be better understood by referring to the following description of several examples of implementation of a cold cage for electric induction melting crucible, description which will be made with reference to Figures 2 to 5 attached in which:

• - Figure 2 shows in sectional elevation (Figure 2a) and in section view from above (Figure 2b) a bimetal rectangular segment of cold cage according to the invention;
• - Figure 3 shows in section perpendicular to the axis of the crucible part of the segments of the cold cage in the case of a bimetallic structure limited to the internal wall of the crucible;
• - Figure 4 also shows in partial section along a plane perpendicular to the axis part of the segments of the cold cage in another form of bimetallic structure with two layers;
• - Figure 5 shows in section along a plane perpendicular to the axis of the crucible, part of the segments constituting the cold cage in the hypothesis of a multilayer sandwich construction of the walls of the segments.

In Figures 2a and 2b (sectional view along the horizontal plane AA of Figure 2a) there is shown the possible constitution according to the invention of a segment 2 of a cold cage for crucible melting by electrical induction.

In this first embodiment claimed by the invention, the internal wall 8 of the crucible segments is made of copper of suitable thickness, generally located between 1 and 3 mm. The external wall 9 of stainless steel is made in the form, for example, of a regular and uniform coating of stainless steel, the thickness of which is notably less than the depth of penetration of the currents in the stainless steel. For example, a coating of 20 to 40 µm above 1 MHz will be applied, 50 to 100 µm between 500 kHz and 1 MHz. Calculation and experience show that the crucibles produced in accordance with this provision resist corrosion well while presenting JouLe losses only slightly higher than those of a cold copper crucible. The coating 9 made of stainless steel is carried out by applying the well-known physicochemical processes such as, for example, "sputtering" when deposits of thin thickness, less than 200 μm for example are involved. For widely greater thicknesses and in particular from a few tenths of a millimeter to a few mm, the methods also well known for "shooping" can be applied.

As a variant of the previous embodiment, FIG. 3 also represents a bimetallic structure for producing the segments of the cold cage in which the layer of good conductive metal 8, generally made of copper, is only coated on the side facing towards L inside the crucible, a protective deposit 9 made of stainless steel. This simplified structure compared to the previous one can be used when the internal face of the crucible located opposite the corrosive charge is the only one exposed to corrosion, due in particular to the good isolation of the other faces of the segments from molten material.

In a second embodiment represented in FIG. 4, the segments of the cold cage are made of stainless steel 9. A deposit of copper 8 of thickness equal to or slightly greater than the penetration of the currents in the copper at the working frequency is applied to the three sides of the stainless steel profile not exposed to the aggression of the environment. The thickness of the deposit is around twenty microns between 1 and 5 MHz, around around fifty microns between a few hundred Khz and 1 MHz, and a few hundred microns around 10 kHz. In this embodiment, the JouLe losses are only slightly increased compared to those of a copper crucible, only in proportion to the effective length of the stainless steel present, relative to the total periphery of the segment.

Finally, and still within the scope of the invention, it is not excluded, by more advanced embodiments and for selective applications, to combine the two previous crucible embodiments as described in FIG. 3 and FIG. make another original crucible structure, which is none other than a "trimetal" copper-stainless steel-copper or stainless-copper-stainless steel structure (Figure 5) where The copper layers are referenced 8 and The stainless steel layers 9.

The cold crucibles described are capable of Wide applications in very varied scientific or industrial fields which range from Induction fusion or "autocreuset" of insulators such as Glass, refractory oxides, to fusion in the presence of metal slag or conductive alloys : titanium, zirconium, stainless steel, etc ... as well as The realization of inductive plasma torches exploited in highly aggressive medium: plasma of hydrogenated, halogenated medium, etc ...

It goes without saying that the invention is not limited to the preceding examples only, in particular as regards the shape of the segments constituting the cold crucible; any profile other than rectangular, for example circular, trapezoidal, etc., must be considered to fall within the scope of the invention.

Finally, the invention also applies to any form of cold crucible other than the cylindrical shape as presently described for example in cold crucibles of frustoconical, cylindroconical shape etc ... allowing in particular in the case of the uses of plasma selective dynamic effects in agreement with the physicochemical or metallurgical objective sought.

Claims (5)

1. Cold cage for melting crucible by high frequency electromagnetic induction comprising, in known manner, a series of hollow segments (2) traversed by cooling water, surrounded by a high or medium inductor winding (5) frequency, and in which the products to be melted are confined (6), characterized in that at least part of the walls of each segment (2) of the cage (1) consists of at least two layers of adjacent materials, one of which corrosion resistant (9) is in contact with the products to be melted (6), and the other of which (8) is a good conductor of electricity, and in that the relative thicknesses of these layers thus that the supply frequency of the electric inductor (5) are chosen in such a way that the eddy currents induced in the cage develop mainly in a layer which is a good conductor of electricity. 2. Cold cage according to claim 1, characterized in that The corrosion-resistant material is stainless steel and The material which is a good conductor of electricity is copper. 3. Cold cage according to any one of claims 1 and 2, characterized in that each segment (2) is made of good conductive material (8) coated, on the face turned against the interior of the cage, with a layer of corrosion resistant material (9). 4. Cold cage according to any one of claims 1 and 2, characterized in that each segment is made of material resistant to corrosion (9) and coated, on the faces not in contact with the molten material, with a layer of material that conducts electricity well (8). 5. Cold cage according to claim 4, characterized in that each segment (2) further comprises an internal coating (8) made of a material which is good conductor of electricity.

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