EP0391067A2 - Device with a metallic crucible - Google Patents

Abstract

The invention relates to a device with a crucible (4), in which there is a metal or metal alloy, a location-dependent inductive power density being supplied to the metal or metal alloy. The crucible (4) is preferably a cooled crucible (4) with slots (12, 13) in the circumference, which is surrounded by a coil (2). The radiation pressure, which is exerted on the metal or metal alloy by the magnetic field of the coil (2), is for example location-dependent in that it corresponds to the hydrostatic pressure of the molten metal or of the molten metal alloy. <IMAGE>

Description

The invention relates to a device with a crucible in which there is a metal or a metal alloy according to the preamble of patent claim 1.

When melting substances in crucibles, make sure that the crucible has a higher melting temperature than the substance to be melted, because with conventional crucibles the inner surface of the crucible becomes as warm as the melt. In most cases, ceramic crucibles meet the requirements because they have a very high melting temperature. However, the highly heated inner surface of a ceramic crucible can react chemically with the melt, as a result of which the melt is contaminated by the crucible material. The contamination generally consists in the fact that the melt is oxidized while reducing the crucible oxide ceramic. However, it is also possible that contamination of the crucible, for. B. sulfur in solution. In addition, ceramic particles can flake off from the crucible and get into the melting material, where after the melting material solidifies, they form inclusions, which are often referred to as "low density inclusions". Such inclusions reduce the quality of the solidified melt, since they are e.g. B. Starting point of cracks.

One way to avoid these disadvantages is to make the crucible not from ceramic, but from metal. In this case, however, there arises a problem that materials having a high melting temperature cannot be melted in the crucible. Would you z. If, for example, you try to melt metals such as tantalum, tungsten or thorium in a copper crucible without special measures, the crucible would have melted long before the melting temperature of these metals was reached.

In order to be able to melt substances with a high melting temperature in crucibles with a lower melting temperature, it has long been known to cool the crucibles with water. This keeps the crucible at a temperature below its own melting point. However, the problem of heating the melting material now arises, because if the container itself is cooled, it cannot release higher temperatures to the melting material.

This problem is solved in a simple manner in that the material to be melted is heated electrically, specifically by inductive heating. Here, a coil is provided around the crucible, which supplies electrical energy through the crucible wall to the melting material.

So that a crucible made of metal is not inductively heated too much by eddy currents during inductive heating, it is already known to assemble the crucible from individual segments which are separated from one another by an insulating layer (DE-PS 518 499). For insulation z. B. Mica can be used.

Furthermore, an induction furnace without an iron core is known in which the density of the turns of the induction coil decreases from bottom to top or from top to bottom (DE-PS 563 710). This is intended to achieve regulation of the bath movement. If, for example, strong oxidation of a liquid iron or steel is to be achieved, a coil is used, the turns of which are wound more densely in the upper part than in the lower part, as a result of which a strong surface movement of the bath is achieved. On the other hand, if, for example, overheating liquid aluminum in the induction furnace, excessive bath movement, especially on the surface of the bath, is to be avoided, a coil is advantageously used, the turns of which are wound more densely in their lower part than in their upper part. However, the crucible used here is not slotted.

A similar coil is also known from US Pat. No. 1,839,801, in which the turns are closer together at the ends than in the middle.

A method for vacuum sintering powder products in an electrically heated vacuum oven is also known, in which the heating of the oven is controlled as a function of the pressure in the oven (DE-A-2 152 489). The vacuum oven used here is also not slotted.

In another known induction melting furnace for melting metals, an elongated, electrically insulated and water-cooled crucible is provided which is open at the top and bottom and which has the same cross-section over its entire length (US Pat. No. 3,775,091). This crucible is divided into at least two segments by vertical slots, each segment being electrically insulated from the other segment, so that no electrical short circuits occur. The slots serve to reduce the shielding effect of the crucible against electric fields. In order to always generate and maintain insulation between the segments and on the inside of the crucible, a ceramic lining is provided which, in the solid state, has electrical insulating properties and which has a melting point temperature which differs from the melting point temperature of the metal to be melted. This insulating lining contains, for example, an alkaline earth metal fluoride. This creates a self-generating insulation liner. The disadvantage here, however, is that the use of slag when melting reactive metals entails the risk of contamination of the metal. It has also been found that even with a partial pressure of argon or helium, the quality of the molten material left much to be desired.

The use of insulating slags between the melt and the cooled segments is not necessary for electrical reasons, as described in DE-PS 518 499. Insulating layers between the melt and the cooled segments represent an advantage in that they represent a heat insulation layer and thus significantly reduce the heat flow from the melt to the cooled segments, so that melting can be carried out with lower electrical induction heating outputs. The size of the power supply can be smaller and the current forces limiting the process are not yet so noticeable.

A method for inductively melting reactive metals and alloys in a non-reactive environment is also known, in which the melting material is melted in a crucible divided into segments in the absence of insulating slag (EP-A-0276544). This process is intended to produce high-quality products that are not contaminated by slag or the like. Here, the wall segments of the crucible are not electrically insulated from one another, but on theirs Base connected to each other and thus electrically short-circuited, as has already been described in previous publications and in DE-PS 518 499. In addition, the crucible is provided in an evacuated room with less than 500 µm Hg.

A disadvantage of this method is that the inductively introduced electrical heating power is the same over the entire height of the crucible, which does not lead to an optimal melting time.

It is also known, however, to divide the heating area into different zones in the case of diffusion blast furnaces, vacuum furnaces or pottery furnaces, wherein a different coil can be used for each zone (US Pat. No. 3,291,969, DE-A-2 152 489, US Pat. No. 4 011 430, DE-PS 27 04 661, DE-A-3 703 108, DE-PS 10 55 715, DE-PS 974 747, DE-PS 12 25 787, GB-PS 627 507). However, these known furnaces are not suitable for melting materials whose melting temperature is above the melting temperature of the crucible.

A fundamental disadvantage of the above-described melting process with a cooled crucible is the high energy losses which the melting material suffers from the dissipation of heat to the crucible wall. The thermal process efficiency can only be kept acceptable by the fact that the melting process runs as quickly as possible and thus the amount of energy dissipated as heat losses - as a product of power loss and time - becomes small.

The invention is therefore based on the object of creating a device according to the preamble of patent claim 1, with which it is possible to produce high-purity metal by melting and to reduce the heat losses.

This object is achieved in accordance with the features of patent claim 1.

The advantage achieved by the invention is in particular that the electrical energy can be efficiently brought to the melting material without it being contaminated with electrically insulating parts, because the slots between the segments of the crucible are only in the area facing away from the melt with an insulator replenished. The area facing the melt is empty at the depth of approximately one slot width. In addition, the melting process can be carried out evenly and quickly because the radiation pressure of the inductive energy supply counteracts the gravitational pressure of the melt. The height-dependent power density achieves the maximum possible heating power at a selected operating frequency. At the same time, the heat losses from the melt to the crucible are reduced, since the mechanical contact surface between the melt and the crucible is kept as small as possible. This results in the cylindrical part of the melt due to the partially pushed back outer melt surface, due to the height-dependent optimized electromagnetic radiation pressure. This means that the cross-section of the crucible is fully utilized at all melt pool heights. If the bath top is stabilized by additional measures, the heat-radiating surface is kept as low as possible in this area. The advantages of the invention become apparent not only in the melting process, but also in so-called temperature maintenance, that is to say during the time in which the material to be melted has already melted and is to be kept in the molten state for a predetermined period of time. During the temperature holding time, the frequency of the heating induction current is reduced so much that forces are similarly high with reduced power as during melting. In order to avoid local overheating, especially in the case of large crucibles, different frequencies can be used in different heating zones or partial coils in a special embodiment of the invention. The height-dependent power distribution also has the advantage during temperature maintenance that the melt can degas quickly due to the relatively large surface area that forms. so that treatment time and losses are reduced. In addition, a large coherent vortex is formed in the melting area as the power density increases, which mixes the melt well thermally and metallurgically. In addition to melting and maintaining the temperature, the invention also has advantages when the melt solidifies. As is known, the inductive melting of materials in a cooled crucible has the general advantage over conventional induction melting that the melt does not have to be poured into a mold. Rather, it is possible to solidify the melt in the crucible, which reduces the investment costs. By simply switching off the induction current, a block quality similar to that of permanent mold casting is achieved. Due to the measure according to the invention that the lower heating zones are greatly reduced compared to the upper heating zones, based on the holding power, the solidification zone progresses slowly from bottom to top, and a directional solidification structure results. For other types of alloy, it is advantageous to produce a fine-grained primary structure. The stirring effect of the electromagnetic field maintained in the liquidus area causes fine grain to be produced in the block.

• Fig. 1a eine Prinzipdarstellung eines herkömmlichen und an sich bekannten Induktionsschmelzofens;
• Fig. 1b die an sich bekannte Abhängigkeit der Eindringtiefe in einen metallischen Block;
• Fig. 2a einen wassergekühlten und in Segmente aufgeteilten Induktions­schmelzofen, dessen Schmelzgut induktiv aufgelöst wird;
• Fig. 2b eine Draufsicht auf den Schmelztiegel gemäß Fig. 2a;
• Fig. 3 eine erste Variante eines erfindungsgemäßen Induktionsschmelz­ofens;
• Fig. 4 eine zweite Variante eines erfindungsgemäßen Induktionsschmelz­ofens;
• Fig. 5 eine dritte Variante eines erfindungsgemäßen Induktionsschmelz­ofens,
• Fig. 6 und 7 Leistungsdichteverteilungen über die z-Achse.

Embodiments of the invention are shown in the drawing and are described in more detail below. Show it:

• 1a shows a schematic diagram of a conventional induction melting furnace known per se;
• 1b the known dependence of the depth of penetration into a metallic block;
• 2a shows a water-cooled induction melting furnace divided into segments, the melting material of which is dissolved inductively;
• FIG. 2b shows a top view of the crucible according to FIG. 2a;
• 3 shows a first variant of an induction melting furnace according to the invention;
• 4 shows a second variant of an induction melting furnace according to the invention;
• 5 shows a third variant of an induction melting furnace according to the invention,
• 6 and 7 power density distributions over the z-axis.

The principle of an induction melting furnace 1 is shown in FIG. 1a, which has an inductor 2 and material 3 to be melted. The inductor 2 consists of a coil which has an inductance and an ohmic resistance. A current flows through the inductor 2, which induces a voltage in the melting material 3, which consists of conductive material, which in turn causes a current flow in the melting material 3, which results in heating of the melting material. With δ is the penetration depth of the current.

Fig. 1b shows the course of the current density g depending on the distance to the center x = 0 for two different frequencies f₂> f₁. With δ₁ the depth of penetration for the frequency f₁ is called; it is the point on a flat, very thick wall at which the current density g has decreased from 1 to l / e, where e is Euler's number. It can be seen from this that the higher its frequency, the less deep the current penetrates.

The currents flowing in the melt 3 are also called eddy currents. Eddy currents always arise when there are electrically conductive substances in an alternating magnetic field. They flow on paths that are linked with the magnetic induction lines. The formation and properties of eddy currents are known (cf. K. Kupfmüller, introduction to theoretical electrical engineering, 11th edition, 1984, p. 304 ff.) And are therefore not to be described in detail.

The specific heat output also plays an important role in induction melting furnaces. H. the power converted into heat in the volume unit of the melting material 3. The distribution of this heat output is also known (K. Simonyi, Theoretical Electrical Engineering, 1956, p.304), so that it cannot be derived.

In the fig. 2a shows an induction melting furnace 1 according to the invention, which has a crucible 4 which is divided into different segments 5, 6, 7. Around the crucible 4 there is an inductor 2, ie a coil, arranged, which acts on the melting material 3. In the individual segments 5, 6, 7 of the crucible 4, cooling tubes 8, 9 run with a water inflow 10 and a water outflow 11. The crucible 4 preferably consists of a relatively good heat-conducting metal, since glass or ceramic would contaminate the melt too much. Since metals that conduct heat well are also good electrical conductors, the magnetic energy generated by the coil 2 penetrates mainly through the slots 12, 13 between the segments 5, 6, 7 of the crucible 4 to the melting material 3. This melting material is liquid in the upper region 14 and is supported on a cooled plate 16 via a solidified layer 15. The plate 16 can be moved up or down with a rod 17.

The liquid melt material 3 can be regarded as a liquid with regard to its mechanical properties. If one ignores flows of the melting material 3, ie if one assumes that the melting material 3 is at rest, then the pressure at a point of the melting material does not depend on the orientation of the surface element on which it acts: the pressure in a still liquid is the same in all directions. It follows from this that the same pressure prevails at points of the same height in a liquid column. However, the pressure depends on the height coordinate. If you think of z₀ as a fixed level and choose the coordinate system so that z₀ = 0, then the pressure p (z)
p (z) = P₀ - ρgz
where ρ is the density of the melt and g is the acceleration due to gravity. This formula, known from hydrostatics, says that in a heavy, density-resistant liquid, the pressure drops linearly with increasing height or increases linearly with depth.

The electromagnetic energy, which is supplied from the coil 2 to the melting material 3, penetrates mainly through the slots 12, 13 and generates a radiation pressure in the volume of the melt. If the Local radiation pressure is the liquid pressure exerted on the walls of the crucible, so that the melting material at the point where the slits are located is pushed so far inwards that a weakening of the field results from a weakening of the field strength and / or an increase in the liquid height from the displaced material. As a result, an optimal melting process is not possible. The radiation pressure on the inside of the crucible 4 must therefore not be greater than the hydrostatic pressure of the melt 3. Since this hydrostatic pressure depends on the z coordinates, the radiation pressure is also designed according to the invention in such a way that it also depends on the z coordinates. This happens e.g. B. in such a way that the square of the amplitude of the magnetic field penetrating into the melting material 3 increases linearly from top to bottom.

worin K die elektrische Leitfähigkeit, µ die magnetische Permeabilität und

und die elektrischen bzw. magnetischen Feldstärken der Welle be­deuten (vgl. Bergmann/Schäfer: Lehrbuch der Experimentalphysik, Band II, Elektrizität und Magnetismus, 7. Auflage, S. 501; Mathematische Ableitungen des Strahlungsdrucks über den Maxwell'schen Spannungstensor finden sich z. B. in W. Greiner, Theoretische Physik, Band 3, Klassische Elektrodynamik, 4. Auflage, 1986, S. 242 bis 247).The time-averaged radiation pressure of an electromagnetic wave that strikes a conductive metal wall perpendicularly, from which it is partially reflected

where K is the electrical conductivity, µ the magnetic permeability and

and mean the electrical or magnetic field strengths of the wave (see Bergmann / Schäfer: Textbook of Experimental Physics, Volume II, Electricity and Magnetism, 7th edition, p. 501; mathematical derivations of the radiation pressure via the Maxwell stress tensor can be found e.g. in W. Greiner, Theoretical Physics, Volume 3, Classical Electrodynamics, 4th edition, 1986, pp. 242 to 247).

If the wall has a finite conductivity, which applies to the melting material 3, the incident wave is not completely, but only partially reflected, so that the electric field strength on the wall does not completely disappear; therefore in this case the electric field strength also contributes to the pressure, but the magnetic field strength is correspondingly smaller than before. If the shaft still partially penetrates the wall, so there is also pressure on the back, which is to be subtracted from the one acting on the front.

The power density or radiation power per unit area is known as the Poynting vector (cf. Simonyi, loc. Cit., P. 28 ff). This vector is defined as the vector product of the electric field strength and the magnetic field strength:

wobei E_y die Komponente der elektrischen Feldstärke in y-Richtung, δ die Eindringtiefe und E_o die maximale Amplitude der elektrischen Feldstärke
E _z = 0, wobei E_z die Komponente der elektrischen Feldstärke in z-Richtung
H _x = 0, wobei H _x die Komponente der magnetischen Feldstärke in x-­Richtung
H _y = 0, wobei H _y die Komponente der magnetischen Feldstärke in y-­Richtung

wobei E _o die maximale Amplitude der elek­trischen Feldstärke, π die elektrische Leit­fähigkeit und j = √-1 For the flat arrangement there are relatively simple mathematical expressions. Although the crucible used according to the invention is a rotationally symmetrical body, the differences from a flat arrangement are not very great in practice, which is why it is sufficient to set up the essential equations for flat conditions and to dispense with the more difficult to understand cylinder functions. Based on a flat arrangement, the following boundary conditions result:
E _x = 0, where E _{x is} the component of the electric field strength in the x direction

where E _{y is} the component of the electric field strength in the y direction, δ the penetration depth and E _o the maximum amplitude of the electric field strength
E _z = 0, where E _{z is} the component of the electric field strength in the z direction
H _x = 0, where H _{x is} the component of the magnetic field strength in the x direction
H _y = 0, where H _{y is} the component of the magnetic field strength in the y direction

where E _{o is} the maximum amplitude of the electric field strength, π is the electrical conductivity and j = √ -1

Re (Ey . H $\genfrac{}{}{0ex}{}{\text{*}}{\text{z}}$

)
Hierin bedeutet H* die zu H konjugierte Zahl. Der Zahlenfaktor 1/2 rührt von der zeitlichen Mittelwertbildung bei sich sinusförmig ändernden Vor­gängen her (Simonyi, a.a.O., S. 283, Gleichung 35).According to the rules of the complex calculation, this results in the amount of the Poynting vector in the x direction
S _x = $\frac{\text{1}}{\text{2nd}}$

Re (Ey. H $\genfrac{}{}{0ex}{}{\text{*}}{\text{e.g.}}$

)
Here, H * means the number conjugated to H. The numerical factor 1/2 comes from the temporal averaging of processes that change sinusoidally (Simonyi, loc. Cit., P. 283, equation 35).

(vgl. Simonyi, a.a.O., S. 283, Gleichung 38).The power density flowing over the surface results from a conversion to:

(see Simonyi, loc. cit., p. 283, equation 38).

wird für den Fall einer über das Volumen konstanten elektrischen Leitfähigkeit und Permeabilität beschrieben durch:

wobei

die Stromdichte und

die magnetische Induktion sind.The electromagnetic power penetrating into the melting material generates mechanical forces in the melt. The volume force density

is described for the case of constant electrical conductivity and permeability over the volume by:

in which

the current density and

which are magnetic induction.

Da für ein ebenes Feld nur eine Kraftdichtenkomponente normal, d. h. senkrecht zur Oberfläche auftritt, gilt:

The volume force density is directly proportional to the amount of the Poynting vector. The quantity "pressure" forming in the volume of the melt is calculated from the integral over the dot product of the volume force density and the path:

Since only one force density component occurs normally, ie perpendicular to the surface, for a flat field:

If you insert the result for the power density on the surface into this equation, the result is

The electromagnetic radiation pressure does not appear abruptly on the surface of the material, but builds up normally via the path to the surface. Since the depths of penetration are small at the usual heating frequencies, one can assume in a first approximation for the formation of the molten bath surface that the electromagnetic pressure acts on the surface. The electromagnetic radiation pressure is therefore proportional to the power density radiated to the melt.

2b shows a plan view of the melting furnace 1, in which the segments 5 to 7 and 18 to 22 and the slots 12, 13 and 23 to 28 between the segments 5 to 7, 18 to 22 can be seen.

According to the invention, the melting process begins in the middle of the individual segments 5 to 7 and 18 to 22 and not behind the slots 12, 13 and 23 to 28. If the melting material 3 is in the liquid state, it is pushed inwards and it radial furrowing forms in the melt, which is most pronounced on the surface of the bath. The webs protruding from the melt stand outwards in a star shape and are located opposite the middle of the segments 5 to 7 and 18 to 22. A field incidence on the top of the crucible from the top of the molten bath tip must be avoided, as this would otherwise result in the tent tip being deformed comes and the formation of wrinkles is supported. The field incidence on the edge of the crucible can, for. B. can be prevented that the induction coil 2 does not reach beyond the edge of the crucible.

FIG. 3 shows a variant of the invention in which a differently arranged cooling system is provided and which has a coil with a downward gradient. The crucible 4, which again has several segments 5, 6, 7 and z. B. has a volume of 5.5 dm³, has a coolant inlet opening 10 and an outlet opening 11 for cooling water. Liquid metal, e.g. B. Na or NaK or an organic liquid, e.g. B. a flame retardant oil can be used. Likewise, it is possible to use liquid salt as a coolant, for example NaNO₂, NaNO₃ or KNO₃. The upper turns 29, 30 of the coil 2 are further apart than the lower turns 31, 32. As a result, a large current deposit occurs in the lower area of the coil 2, which exerts a large pressure on the melting material 3. At the upper edge of the crucible 4 there is a shorting bar 33, which brings about a certain linearization of the magnetic field. Such linearization is necessary because the coil ends abruptly at its upper edge and thus initially leads to a kink in the magnetic field strength, but on the other hand the far field decays only slowly. The fact that the field incidence across the crucible edge is greatly reduced by means of the shorting bar or ring 33 results in a field weakening in the area of the melting surface and thus a limitation of the bath elevation. This short-circuit ring 33 rests on the segments 5, 6 and is connected to them.

The coil 2 is connected to a power supply 34, which is an AC power source with a frequency of 1000 to 5000 Hz. The current flowing through the windings 29 to 32 is therefore the same at all points.

In the melting induction system according to the invention, the molten material 3 flows in the crucible 4. In the lower to middle coil region, it flows inwards, where it is deflected upwards and downwards and flows downwards again to the outside of the melt; the inward-looking forces are also greatest there. The material flowing upwards in the area of the center of the melt is visible on the surface of the melt pool and can cause instability of the bath tip. In the due of the melt webs formed by the radiation pressure results in a passive flow, which is generated by frictional forces of the tip flow.

bei Aluminium als Schmelzflüssigkeit zu 4,8 mm bzw. 3,4 mm und bei Titan als Schmelzflüssigkeit zu 13,3 mm bzw. 9,4 mm. Eine Frequenz­erhöhung führt zu einer Verkleinerung des Eindringmaßes.In practical embodiments of the invention, the power supply delivers a voltage with frequencies of 2500 Hz or 5000 Hz. The penetration dimensions are then calculated using the known formula

with aluminum as melt liquid to 4.8 mm or 3.4 mm and with titanium as melt liquid to 13.3 mm or 9.4 mm. Increasing the frequency leads to a reduction in the penetration dimension.

A further variant of the invention, in which the current strengths through the coil windings are not the same everywhere, is shown in FIG. 4. There is a first partial coil 36 with turns 29, 30 in the upper region, which are connected to a first power supply 35. The number of turns of this coil section is relatively large. In the lower region of the crucible 4, a second partial coil 37 with the turns 38 to 41 is provided. This second and shorter coil section 37 can be connected to its own power supply 42 and has a smaller number of turns than the first coil section 36.

However, it is also possible, which is indicated by the dashed lines 43, 44, to connect the partial coil 36 and the partial coil 37 in parallel to a common power supply 35 or 42. In the case of partial coils 36, 37 connected in parallel, a higher radiation pressure results in the lower region of the crucible 4 because of the higher coil currents and the higher current coverage of the lower partial coil 37 than in the upper region. If separate power supplies 35, 42 are used for the partial coils 36, 37, the currents flowing into the partial coils 36, 37 can be selected so that they apply the radiation pressure required in each case.

The melt pool tip is designated by the reference number 60 in FIG. 4. This crest 60 should be raised as little as possible and should not be furrowed by the radiation pressure. As already mentioned, the measure of reducing the field strength can be selected against an increase. The grooves 61 to 64 of the dome 60 are essentially due to the penetration of the electromagnetic radiation through the slots 12, 13 between the segments 5, 6, 7. The ratio of the segment width a to the slot width b is therefore essential for the properties of the bath tip 60. In order to optimally determine this ratio, various aspects have to be considered. On the one hand, the number of segments 5, 6, 7 should be as large as possible so that the electromagnetic field can penetrate into the melt 3 through many slots 12, 13. On the other hand, however, it is desirable that their number is not too large so that the lengths of the current path in which eddy currents can be induced do not become too large.

The circumference of the crucible 4 divided by the number of segments 5, 6, 7 should result in a segment width a such that the segment width a becomes comparable or even smaller with the depth δ of the field into the melt 3. The segment width a determines the periodicity of the field in the circumferential direction. With a small segment width a, the bulges or lamellae 65, 66, 67 on the dome 60 at its tip and bottom have such a large curvature that the surface forces for reducing the bulges 65, 66, 67 increase. In order to prevent the formation of bulges or lamellae 65, 66, 67 on the bath dome 60, the field strength in the dome area can be reduced or the frequency of the field increased, as already mentioned. However, narrow segments 5, 6, 7 can also be used.

Because the magnetic field has to penetrate radially inwards, losses occur in the metallic segments 5, 6, 7. These losses are caused by induction currents which cause undesirable heat losses. These losses can be limited by making the columns 12, 13 between the segments 5, 6, 7 as wide as possible. Since the gap width b on the the side facing the melt 3 should be as weak as possible so that no melt can escape to the outside, the gap which widens radially outwards offers a compromise. A larger distance between the segments 5, 6, 7 reduces the losses due to the mutual displacement of the current. The same construction principles apply to the cross-section of the segments 5, 6, 7 as for the inductor windings: There should be no sharp edges if possible, since large heat losses occur on them. The radius at the edges should be greater than 1.5 δ to 2 δ. The width b of the slots 12, 13 between the segments 5, 6, 7 can change in the vertical or axial direction. For example, it is advantageous if the slots between the segments widen below the melt 3, that is to say at the bottom 68.

The electrical voltages prevailing between two segments 5, 6, 7 do not depend on the width b of a slot 12, 13, but rather result from the circulating voltage divided by the number of segments 5/6/7. The segments 5, 6, 7 are bent towards the melt by the field of the induction coil 36, 37. An inward deformation of the segments 5, 6, 7 also results from the melting-side heating, the so-called furnace box effect. The segments 5, 6, 7 can z. B. are supported by insulating elements between the segments 5, 6, 7. These also prevent the melt from escaping in the event of a power failure. The insulation materials should be offset by one or two column widths. The bottom 68 of the crucible 4 is expediently designed as a radially slotted, water-cooled block. It is isolated from segments 5, 6, 7 in the upper area. It is also adjustable in height so that it can be optimally adapted to the melting height.

4, which has a plurality of partial coils 36, 37 one above the other, it is possible to reduce the power, starting with the lower partial coil 37, until the melt 3 solidifies from below until finally only the uppermost coil 36 is included reduced power is operated so that the melt 3 in the immediate vicinity of the molten bath surface is kept liquid for a while. This fluid holding, also called "hottopping", prevents the formation of voids in the block head. The lower coil section 37 can also be operated at a lower frequency than the upper coil section 36.

5 shows a further variant of the invention, in which only one coil 2 is provided, which is supplied with electrical energy via the alternating current source 34. A capacitor 45 is connected in parallel with this coil 2, so that the coil 2 forms an oscillating circuit with this capacitor 45. An inductance 46 is connected in series with this parallel resonant circuit 2, 45, which causes a frequency change and which can be short-circuited via a switch 50. Also connected in parallel with the power supply 34 and the parallel resonant circuit 2, 45 is a direct current source 47, which superimposes a direct current on the alternating current in the coil. With the direct current source 47 it is achieved that the melt flow is calmed and the bath tip formation is stabilized. In this case, the DC magnetic field has the same direction as the AC field. However, it is also possible to place the DC field perpendicular to the AC field and, in particular, to provide it in the upper region of the melt. It is understood that the DC field can also be generated by a separate winding or by permanent magnets.

There is an additional heating source 48 above the melting material, which is shown only symbolically in FIG. 5. This can be an electron gun, a plasma source, an externally powered resistance heater or the like.

A reactive gas can be introduced into the space between the molten bath surface 60 and the heating source 48 if, for. B. a plasma torch or a glow discharge anode is used. Nitrides, oxides or the like or undesired compounds of these which float in the melt as inclusions can thus be chemically destroyed.

During the melting of a solid batch, the bottom 68 is shifted in such a way that the melt pool tip 60 is held at approximately the same location relative to the crucible 4 or the coil 2.

The division of the set electromagnetic power can be different for the different processes "melting down", "maintaining temperature" and "block solidification".

FIG. 6 shows schematically how this power density distribution can look in the case of an arrangement with several partial coils.

The crucible 4 should be designed to be very slim in order to achieve high efficiency. However, in order to limit the thermal load on the coils and segments in the case of very high crucibles, a power distribution should be sought, as is shown in FIG. 7.

The crucible 4, as already mentioned, has a volume of 5.5 dm³, for example. However, it can also have a volume of 100 to 1000 dm³.

Claims (43)

1. Device with a crucible made of metal, which is cooled to a temperature below the temperature of the material in it, which further has a slotted side wall at least in the region of the material therein and which is surrounded by at least one induction coil, the im Material located in the crucible is supplied with energy inductively, characterized in that the inductive power density is supplied to the material located in the crucible in a location-dependent manner. 2. Device according to claim 17, characterized in that the inductive power density is supplied to the metal or the metal alloy depending on the coordinate of gravity. 3. Device according to claims 1 or 2, characterized in that the inductive power density for the purpose of melting metal or metal alloys is supplied depending on the location. 4. Device according to claim 1, characterized in that when the metal or metal alloy is present as a melt, the inductive power density is given as a function of the respective hydrostatic pressure of the melt (3) on the melt (3). 5. Device according to claim 1, characterized in that the inductive power density for the purpose of maintaining the temperature of the metal or the metal alloy is supplied depending on the location. 6. Device according to claim 1, characterized in that the inductive power density for the purpose of directional solidification is supplied to the metal or the metal alloy. 7. Device according to claims 1 to 6, characterized in that the electrical heating power increases substantially linearly from the top of the melt (3) to the bottom (68) of the melt. 8. Device according to claim 7, characterized in that the electrical heating power remains constant from a predetermined height. 9. Device according to claims 1, 2 and 4, characterized in that the power density distribution induced in the melt (3) is selected so that the melt (3) except in the immediate area of the bath top (60) in the vicinity of the spaces ( 12, 13) lifts off the crucible (4), but rests in the central areas of the segments (5, 6, 7). 10. The device according to claim 1, characterized in that in the crucible (4) cooling channels (10, 11) are let through which a coolant flows. 11. The device according to claim 10, characterized in that the coolant is water. 12. The device according to claim 10, characterized in that the coolant is liquid metal, for. B. Na or NaK. 13. The device according to claim 10, characterized in that the coolant is an organic liquid, for example a flame-retardant silicone oil. 14. The device according to claim 10, characterized in that the coolant is liquid salt, for. B. a eutectic salt mixture of NaNO₂, NaNO₃, KNO₃. 15. The device according to claim 1, characterized in that the crucible (4) is located within an induction coil (2). 16. The device according to claim 15, characterized in that the slope of the induction coil (2) is lower towards the bottom and thereby results in a higher current in the lower region of the induction coil (2), which induces a higher electromagnetic power in the lower area of the melt (3). 17. The device according to claim 15, characterized in that the coil (2) consists of several superposed partial coils (36, 37) which are fed with different currents. 18. The device according to claim 17, characterized in that the lower partial coils (37) are shorter than the upper partial coils (36). 19. The device according to claim 17, characterized in that the lower coil sections (37) have fewer turns than the upper coil sections (36). 20. The device according to claim 17, characterized in that the lower coil sections (37) are fed with higher currents than the upper coil sections (36). 21. Device according to claim 17, characterized in that a separate power supply (34, 42) is provided for each of the partial coils (36, 37). 22. Device according to one of claims 17, 18 or 19, characterized in that a common power supply (35; 43, 44) is provided for all sub-coils (36, 37), that the sub-coils (36, 37) parallel to this power supply (35) are connected and that the partial coils (36, 37) have different numbers of turns (29, 30; 38 to 41). 23. Device according to claim 1, characterized in that the material (3) located in the crucible (4) is penetrated by a constant magnetic field. 24. The device according to claim 23, characterized in that the constant magnetic field extends coaxially to the crucible axis. 25. The device according to claim 23, characterized in that the constant magnetic field extends perpendicular to the crucible axis. 26. The device according to claim 23, characterized in that the constant magnetic field acts essentially on the head region (60) of the melt (3). 27. The device according to claim 23, characterized in that the constant field extends coaxially to the crucible axis and is divided into different sections, the successive sections each having different polarity sequences. 28. The device according to claim 27, characterized in that the different polarity sequences are generated by coils. 29. The device according to claim 27, characterized in that the different polarity sequences are generated by permanent magnets. 30. Device according to claim 23, characterized in that one or more additional coils are provided around the crucible (4) for generating the DC field. 31. The device according to claim 23, characterized in that a direct current component from a direct current source (47) is superimposed on the alternating current from an alternating current source (34) for the inductive heating of the melting material for the generation of the direct field. 32. Device according to claim 1, characterized in that an additional energy source (48) is provided for heating the surface (60) of the melting material (3). 33. Device according to claim 1, characterized in that the frequency of the inductive energy source (35) is variable. 34. Device according to claim 1, characterized in that above the surface (60) of the melting material (3) an electrode (50) is provided, and a reactive gas is introduced between the surface (60) and the electrode (50), the is brought to glow discharge by a voltage applied between the electrode (50) and the surface of the melting material (3). 35. Device according to claim 1, characterized in that a load resonant circuit (2, 46) additional capacitors (45) are added to reduce the resonance frequency. 36. Device according to claim 1, characterized in that additional inductors (46) are connected in series to the heating coil (2) in order to lower the resonance frequency. 37. A method for treating a melt over a long period, characterized in that the melt (3) is supplied with an electrical power which is required to maintain a certain temperature, the frequency of the heating alternating field is gradually reduced so much that at this holding power forces arise which compensate for a large part of the liquid pressure in the crucible, so that the thermal contact between the melt (3) and the cooled segments (5, 6, 7) is significantly reduced. 38. A method for treating a melt during solidification, characterized in that the electrical heating power of an inductive alternating field is reduced for a long time so that the melt (3) during the cooling process with constant mixing by electromagnetic forces to just below the solidus temperature cools down and only then is solidification permitted. 39. A method for treating a melt during solidification, characterized in that the electromagnetic power is controlled so that the melt (3) solidifies slowly from below and an almost constant solidification rate is established. 40. The method according to claim 39, characterized in that when using several sub-coils (36, 37), the powers of these sub-coils (36, 37) are progressively greatly reduced starting from below or completely switched off. 41. Device according to claim 1, characterized in that an induction coil (2) is displaced relative to the cooled crucible (4) upwards. 42. A method for entering a batch into a crucible, characterized in that the batch is inserted into the crucible in a fully bundled state, a wire or a sheet metal tube made of the same material as the batch being used for bundling. 43. The method according to claim 42, characterized in that the diameter of the bundle or the shell of the bundle is between 2% and 10% smaller than the diameter of the crucible (4).

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