HU183291B - Method and apparatus for casting metals - Google Patents

Abstract

Dense homogeneous metal rod (12) is continuously cast in long lengths by introducing liquid metal into the lower portion of an elongated, upwardly- extending alternating electromagnetic field (28) solidifying the metal while moving upwardly through the field, and removing solidified metal rod product from the upper portion of the field. The field may act to support the weight of the upwardly-moving metal column, to move the column upwards, to contain the column and maintain it out of contact with the wall of a surrounding tube (25), and provide a stirring effect to homogenise the solidifying metal. <IMAGE>

Description

The field of continuous casting is one of the most evolving fields of metallurgy for a long time. Accordingly, a considerable number of patent applications, patents and other technical literature deal with this subject. However, it is a fact that relatively few ideas have been put into practice in this area. The embodiments generally involve a mold which is in contact with the molten metal and takes the form defined therein and then freezes. These molds or crystallizers may be casting drums, casting strips or, in special cases, so-called casting drums. core wire which is passed through a metal melt in a crucible and deposited therein. In this case, the crystallizer is not actually located around the metal melt, but rather the metal melt surrounds the mold.

If an alternating current electromagnetic field is used as a mold, as in the case of the present invention, it does the molding and discharge of the metal melt from the metal bath, but eliminates the need for various casting drums, casting strips or core wires. It also enables the production of relatively small quantities of copper, nickel or other metal rods by continuous casting technology, due to the simplification of continuous casting technology and the significant reduction in equipment required for this purpose, using conventional and expensive ingot casting and hot rolling instead of technology.

The use of electromagnetic fields as molds has long been proposed. In these solutions, the molten metal is led from above to the electromagnetic field and, after solidification, is pulled down from the mold in the usual way. Such technology is described, for example, in U.S. Patent Nos. 3,467,166, 3,605,865, 3,735,799, 4,014,379 and 4,126,175. U.S. Pat. In each variant, the adhesion or growth is longitudinal, the melt being introduced semi-continuously or continuously by gravity into the metal bath at the upper end of the downwardly moving ingot.

One of the major disadvantages of this technology is the lack of protection against power failure. In fact, if for any reason the electrical supply fails, the liquid metal will leak out of the casting apparatus due to the downward addition of the molten metal. In addition, great care must be taken to ensure that the molten metal does not escape from the metal bath at the top of the die, which requires careful control and regulation of both the molten metal dosing and the rate at which the ingot is pulled out.

However, these parameters are severely limited by heat exchange problems, so the possibilities for this version of continuous casting are limited.

Alternatively, as disclosed in U.S. Patent Nos. 3,746,077 and 3,872,913. U.S. Patents teach that the molten metal is forced upwards into a vertically-opened vertical chill by vacuum or vacuum, where the solidified product is intermittently separated from the surface of the metal bath and removed. This eliminates the risk of metal melt leakage caused by a power outage, but reappears the disadvantages of using a conventional contact die.

It is an object of the present invention to overcome the above-mentioned difficulties and to provide a fundamentally new casting process and apparatus which allows the safe casting of relatively small quantities of metal.

The object of the present invention is to provide an elongated, vertical, alternating current electromagnetic field, to introduce a metal melt into its lower part, to move it upward and to conduct the solidified material in the upper part out of the electromagnetic field.

The apparatus for carrying out the process comprises a vertical crystallizer formed as a floating tube and surrounded by a cooling unit and an electromagnetic floating unit. The device is provided with a device for extracting solidified material.

Copper or other metal rods produced by the process of the present invention can be rolled, annealed and pulled in the conventional manner.

The process and equipment do not represent a cost increase over conventional methods, and in some applications, there may be specific cost savings.

For example, the invention can be used to produce welding rods and other products which are not subject to stringent particle size specifications and thus can be directly sized.

A further advantage of the present invention is that its use does not involve any composition limitations. It can be used for the production of copper bars as well as for casting aluminum, aluminum alloys, copper alloys.

The present invention is based on a fundamentally new discovery that continuous casting can be accomplished by moving a column of metal liquid upward in a forming zone in which the material is gradually cooled and solidified, while electromagnetic field reduces the force required to move the metal melt and stiffen rod upward. . This completely new application of the electromagnetic field according to the invention is used to float or shape the molten metal for at least a part of the length of the metal column, especially in the freezing zone.

Floatation is performed using electromagnetic moving waves so that most of the metal column is weightless during casting.

The molding is performed using a stationary or continuous electromagnetic field in order to prevent most of the molten metal column from coming into contact with the crystallizer wall.

Of course, in the process of the present invention, floating and shaping can be applied simultaneously, i.e., the metal column is kept in a weightless condition and at the same time it is ensured that it does not come into contact with the crystallizer wall. The electromagnetic field is then used for both raising and shaping the melt.

A fundamental deviation from conventional practice is that electromagnetic levitation provides very high productivity by making the metal column virtually weightless. It is not necessary to cool the freshly stiffened metal product to be solid enough to support the weight of the underlying metal and to resist friction between the mold and recrystallizer wall and the shell of the ingot during extraction. In other words, the force required to pull the frozen billet out of the crystallizer is considerable.

183,291 can be reduced by this technology as it is a function of the friction between the log shell and the mold wall, and the friction is proportional to the pressure applied at the point of contact. According to the invention, this compression force is negligibly low, both because the metal column is weightless and because it has practically no contact with the wall of the mold. Electromagnetic molding thus offers a particular advantage without reducing the efficiency of the heat exchange, since the invention does not require the formation of a relatively large gap between the mold and the molten metal.

It also allows you to increase productivity by simultaneously shaping and floating using electromagnetic fields. Indeed, as we have said, the removal of the log is substantially facilitated by the reduction or elimination of frictional and adhesive forces. As far as cooling is concerned, it is very effective in both float, electromagnetic molding, and combined mode, because the gap between the mold and the shell of the stump is very small.

Combined with levitation and electromagnetic shaping, the advantage is that levitation can be easily and simply controlled over a wide power range. During the investigations, we found the surprising experience that in this mode, a noteworthy self-regulating phenomenon occurs: the forming forces and the floating forces have changed in relation to each other. By increasing the extraction rate at a given diameter, the cross-section of the metal column was reduced and the electromagnetic lifting force decreased accordingly. As the extraction rate was reduced and the cross-section increased correspondingly, the lifting force increased. In this way, although minor fluctuations may occur, the system remains essentially close to equilibrium and consequently the product has a constant cross-section and shape.

Experiments have shown that all variants of the new continuous casting process of the present invention are well suited for forming metals, metal alloys, and metal alloys, i.e., virtually any electrically conductive melt that can be heat-cured.

Further unexpected phenomena were observed during the investigations. With a column of metal melt at essentially 0 hydrostatic pressures, sufficient vortex flow is generated in the metal melt to provide a constant mixing process before and during curing, as the metal column passes through the floating zone, allowing for extremely homogeneous distribution of making ingots from metal mixtures which tend to selectively precipitate.

Thus, in the process of the invention, the metal melt is moved upwards in the alternating electromagnetic field and the solidified material is pulled upwards out of the electromagnetic field. In one of its most preferred embodiments, the molten metal is electromagnetically held in a substantially weight-free state while the metal column is pressurized against the mold wall.

In another embodiment, the metal column is also held in a weightless state by electromagnetic fluttering and, at the same time, electromagnetic molding ensures that the weight of the weightless metal column is not in contact with the mold wall.

It is also possible to carry out the process according to the invention in that the electromagnetic field is used only for molding, that is, the periphery of the metal column, in particular at the critical point where the freezing occurs, is separated from the mold wall during casting. In this embodiment, levitation is not used, only electromagnetic shaping.

In the process, it is expedient to perform the floatation in such a way that the hydrostatic pressure of the molten metal is practically zero, i.e. it moves in a weightless state.

The pull force required to raise the metal pole is provided by the pull head. This is essentially a simple metal rod which, at the beginning of casting, is brought into contact with the metal melt and continuously moved upward with the frozen material. The extraction head is moved continuously with the solid butt member rising with the aid of a gear used for this purpose. The extraction rate corresponds to the setting rate of the metal melt in the metal column.

In the embodiment of the process according to the invention, where electromagnetic floatation is practically not used, the metal column is formed with the electromagnetic field for most of its length, but especially in the freezing zone, i.e., a barrier-free mold. Accordingly, the cross-section of the metal column can be arbitrarily controlled by the electromagnetic field. The height of the metal column can also be arbitrarily selected and a single electromagnetic field or a series of stationary electromagnetic fields can be used.

In all embodiments, the length of the electromagnetic field is generally larger, and more preferably much larger than its diameter. Similarly, the length of the floating metal column is greater than its diameter.

In the device for carrying out the process, a vertical crystallizer is formed as a floating tube. A cooling unit and an electromagnetic unit are arranged around it. The device is provided with a device for extracting solidified material.

The electromagnetic unit consists of coils connected to a multiphase electric spun and allows the "column" to be raised. Raising the metal column in this case means that the metal melt column is in constant contact with the solidified material on the extraction head. In this way, cavities, cracks and other material defects can be prevented.

The apparatus essentially consists of a casting vessel and a crystallizer or a mold. The molten vessel contains the molten metal and is located above it, into which the metal column consisting of the molten metal protrudes. The column of molten metal always extends over the lower end of the electromagnetic unit. Generally, the electromagnetic unit is designed as a floating unit capable of holding the fluid column in a weightless condition.

The products obtained by the process and apparatus of the present invention are long metal bodies: sticks, rods or wires, which are perfectly uniform in diameter throughout their length and are completely homogeneous in all their parts. The product, even when cast, has a smooth, slightly wavy mantle surface, since before, during and after curing, the mantle of the metal column is preferably electromagnetically formed, i.e. practically in contact with the mold wall. In addition, the resulting eddy currents provide a homogeneous distribution with constant agitation of the molten metal, even when casting a billet that is highly prone to phase separation.

-3183 291

Experiments with the process have shown that the floating product has a diameter dispersion of 0.02-0.05 mm, which, together with the exceptionally good surface quality, proves that the material hardly contacts the cooling surface during freezing.

Further details of the invention will be illustrated by way of example in the drawings. In the drawing it is

Figure 1 is a schematic diagram of the apparatus according to the invention and the hot rolling unit connected thereto, a

Figure 2 is a schematic sectional view of the apparatus of Figure 1, a

Figure 3 is an enlarged view of a portion of the apparatus of Figure 2 showing the float assembly,

Figure 4 is a sectional view similar to Figure 3 of another embodiment of the apparatus of the invention

FIG. Fig. 4a is a schematic diagram of a floating unit winding used in the equipment shown in Figs

Fig. 6 is a sectional view similar to Figs. 3 and 4 of a further embodiment of the device according to the invention, a

Fig. 7 is a photograph of a copper rod made with the method and apparatus of the present invention and a

Fig. 8 is a close-up of the copper rod shown in Fig. 7, showing surface characteristics.

Figure 1 shows that from a tilting casting pan (not shown), the molten metal enters a casting vessel 10. In this, the metal melt must always be maintained at the proper level to continuously flow into the crystallizer 11. The crystallizer All is mounted on the casting vessel 10 in a vertical position. From its upper part, the rod 12 is continuously discharged, which first passes through the cooling chamber 13 and then enters between the roller stands 14 and 15. After the hot rolling, the wire is cooled and fed to the coil 16.

If the rod 12 is to be used in a molded state, it is guided as indicated by the dotted line 17a. as a finished product.

The molten metal from the casting vessel 10 is introduced into the crystallizer 11 as a liquid metal column, in accordance with the laws of the transporting vessels. The molten metal in the casting vessel 10 should be charged at appropriate intervals from the casting pan. However, it is also possible to provide the casting vessel 10 with continuous melt supply.

Figure 2 shows that, in a preferred embodiment of the invention, the liquid column 20 is moved above the level of the metal melt in the casting vessel 10 by the action of an electromagnetic field. In electromagnetic wave motion, they hold the metal column and neutralize its hydrostatic pressure. Thus, the liquid column 20 is to be conveyed only to the lower part of the crystallizer 11, where at least a portion thereof becomes substantially weightless as soon as the floating unit is energized.

The crystallizer 11 is provided with a floating tube 25. This is preferably a heat-resistant tube attached to the casting vessel 10, in which the molten metal hardens, and from the upper end of which the material enters the cooling chamber 13.

Figure 3 shows that the float assembly comprises a plurality of rolls 28 arranged one above the other around the float tube 25. In the embodiment shown, twelve rolls 28 are disposed and their threads are substantially perpendicular to the geometric axis of the floating tube 25. The coils 28 are coupled to successive phases of a three-phase power source (see Figure 5), so that the resulting magnetic field induces Foucault currents in the metal melt in the floating tube 25, thereby causing the metal to be subjected to a lifting force. The six-phase floating unit shown produces moving waves that move at a distance proportional to the distance between successive closed loops and the frequency of excitation. The most important elements of the floating unit are the rolls 28, which, when positioned around the floating tube 25, hold the bulk of the solidifying metal melt, or at least the bottom of the floating tube 25, floating throughout the casting operation, preferably in complete weightlessness.

Copper, aluminum and bronze were continuously cast with an experimental copy of the apparatus of the invention. The floating unit of the rod casting apparatus consisted of 36 threads made of copper wire consisting of six threads per 25.4 mm. Thus, the float was about 150 mm long. The phases were offset by 60 ° and the whole unit was practically two wavelengths apart. The floating metal column was 22 mm in diameter and the entire metal column was floated at a frequency of about 1200 Hz without acceleration, i.e., the floating rate was practically one. The DC power supply supplied 7 to 10 kW of AC power to the alternator motor. The solution shown in Figure 4 was used as a cooling unit.

Although cooling units of various designs can be used in the apparatus, it has been found to be extremely advantageous in the drawings to have a cooling unit 30 made of sheet metal. The cooling unit 30 is provided with an upper annular shoulder portion 31 and a lower annular shoulder portion 32. There are 33 cylindrical sections between the two. This fits on the float tube 25 and is in contact with its outer periphery. The refrigerant, preferably tap water, was continuously introduced into the upper annular shoulder portion 31, from where it passed through the cylindrical portion 33 to the lower annular shoulder portion 32. From here the cooling water was drained continuously. The cooling water carried through the floating tube 25 the amount of heat released during the melting of the metal melt.

As shown in Figure 3, the coils 28 are disposed about the cylindrical portion 33 of the cooling unit 30 between the upper annular shoulder portion 31 and the lower annular shoulder portion 32. The individual threads are wound equally spaced on the cylindrical portion 33. The entire cooling unit 30 is preferably made of stainless steel sheet, partly for corrosion resistance and partly for proper heat conductivity.

At the commencement of casting, the casting vessel 10 is filled with a molten metal, such as copper, from which the continuously casting rod will be made. This, of course, involves the additional steps of melting the metal and transporting it to the casting vessel 10 in a suitable container. From the casting vessel 10, the molten metal enters the lower part of the crystallizer 11 in the form of a liquid column 20. After the fluid column 20 has formed, the extraction head 40 is lowered 2a. 25 float tube until it comes into contact with the metal melt. The cooling water is flushed through the cooling unit 30 at full speed, thereby causing the upper portion of the metal melt to contact the extraction head 40 to freeze. After a certain amount of molten metal has frozen on the extraction head 40, we begin to lift and gradually remove the stiffened rod from the floating tube 25. Meanwhile, the melt column 20

183,291 is kept substantially free of its entire length, whereby its mantle remains substantially in contact with the inner wall surface of the floating tube 25. Float is maintained continuously while casting is continued by pulling out the rod. The resulting smooth, shiny, slightly wavy and completely dense rod is passed through the cooling chamber 13. Here, it is further cooled by means of water injection to a temperature from which the hot rolling, or optionally the final cooling and winding, can be carried out.

As the finished product is withdrawn from the melt column 20, the amount of metal melt in the floating tube 25 gradually decreases and it is therefore necessary to ensure that a continuous supply can flow into the casting vessel 10.

Experimental specimens of the apparatus of the present invention have been subjected to numerous successful castings using various metals and metallic materials. The experiments were performed primarily by casting aluminum, copper and bronze. Of these materials, rods were primarily cast as described above. In each case, the rod was a finished product, 22 mm in diameter. Its size was perfectly uniform and its composition was homogeneous. The density of the product is quite satisfactory and its surface is smooth, shiny and slightly wavy.

Of course, during the experiments, we changed the power introduced into the electromagnetic unit in the hope of using the material we used for the experiment. The level of float is always adjusted so that the acceleration is practically 0. Contrary to expectations, as previously mentioned, there was no need for precise control of the electromagnetic field, since the process was essentially self-regulating.

During floating, the metal melt begins to accelerate upwards if the floating force is greater than the weight of the column. As a result, the cross-section of the column will be reduced and thus the lifting force will be reduced. Otherwise, if the lifting force is less than the force of gravity, the reverse occurs, that is, the equilibrium is essentially always set. This buoyancy effect is fully effective for most of the melt column. However, at the top and bottom of the float 25, the average float force is only about half that of the former, so that the melt column is held by the extraction head 40 and the hydrostatic pressure. Thus, when the melt column 20 is formed, the buoyant force acting on the underside produces a slight acceleration, and as the liquid metal moves slowly upward, it gradually enters into an electromagnetic field of a strength that is already substantially weightless and accordingly without contact with the wall of the floating tube 25 without pressure. By increasing the hydrostatic pressure, the upward movement can be accelerated, so that the initial hydrostatic pressure can be used to control the movement velocity of the melt column. The floating unit is then required to maintain this velocity only constant over the remainder of the melt column.

Obviously, it is desirable to keep the size of the apparatus and especially of the floating unit as small as possible, and likewise minimize the energy required to maintain the melt column. This requires maximum efficiency of the heat exchange and therefore requires rapid, turbulent, but relatively small, annular coolant flows. Thus, a very efficient heat transfer between the melt column 20 and the surrounding graphite floating tube 25 and its matching stainless steel cooling unit 30 can be achieved. In the cooling unit 30 shown in Figure 3, the heat transfer efficiency is further enhanced by the use of annular ribs 43 located inside the cylindrical portion 33. They inhibit laminar flow and force the fluid flowing from the upper annular shoulder portion 31 to the lower annular shoulder portion 32 to form a turbulent flow.

Although, in theory, there are no factors limiting the size of the ingot to be cast, it is advisable, when practicing the present invention, to keep the manufacture between 5 and 50 mm in diameter for practical reasons. The preferred size range of copper bars we use is 8-30 mm. The cast bar can then be rolled or further drawn by any means. In each case, however, it is expedient to select the inside diameter of the float tube 25 and to adjust the casting parameters so that a minimum annular gap is maintained between the float tube 25 and the melt column 20. This is true for the range below the freezing point of the molten metal, although the contraction during freezing is very slight. The slit 45 shown in Figures 2 and 3 is for illustration only and is obviously not proportional to the size of the slit 45 between the float 25 and the melt column 20.

In order to investigate the possibilities of the process according to the invention, we have also tried continuous casting of alloys which tend to selectively precipitate and different cure of different components. Aluminum-bronze alloy was cast in three different batches using the apparatus of the invention using the technology described above. The difference from what was said was just that

1. The heat exchanger 30 was a simple copper tube that was screwed onto the floating tube 25 as shown in FIG.

2. The melt column 20 was not created by gravity in the casting vessel 10 but by piston action.

The resulting stumps were analyzed and the results are shown in Table I.

Table I

Element initial material 1 Experiment 2 3 Fe 2.64% 2.69% 2.65% 2.71% sn 0.01% 0.03% 0.01% 0.02% Zn 0.01% 0.03% 0.02% 0.02% al 10.35% 10.12% 10.02% 10.05% Mn 0.49% 0.76% 0.68% 0.72% Ski 0.028% 0.049% 0.039% 0.046% Ni 5.00% 4.99% 4.90% 4.99% other 0.03% 0.03% 0.03% 0.03% Cu the rest the rest the rest the rest

It can be seen from the table that the composition of the alloys proved to be completely identical within the limits of the sampling and analytical technology.

Referring now to Figure 4, a variant of the apparatus according to the invention is shown. Here, the floating tube 50 is surrounded by 12 copper cooling pipes. The cooling tubes 52 are wound directly on the floating tube 50 at a distance from each other. Each of the cooling pipes 52 is connected to a separate cooling water supply line. Electrically, the cooling conduits 52 are connected to successive phases of a multiphase electrical network

-5183 291

5. This provides the lifting effect of the electromagnetic field created. Similar to the

3, here again, A, B, and C denote the three different phases. With this connection, the apparatus, though essentially constructed of units corresponding to the floating tube 25, the cooling unit 30 and the coils 28, performs both the floating and the magnetic molding. This means that the melt column 55, like the melt column 20, is kept weightless over its entire length, but the melt column 55, unlike the melt column 20, is completely separated from the wall of the floating tube 50. An annular slot 57 is provided between the two surfaces.

Optionally, a shielding gas which does not react with the metal may be introduced into this slot 57. In the case of copper casting, it has proved useful to use a mixture of nitrogen or nitrogen, hydrogen and carbon monoxide (the mixture can be recovered after burning natural gas and selecting water and carbon dioxide from the resulting gas).

Instead of the solution shown in Figure 3, the design shown in Figure 6 may be used if electromagnetic levitation is not required during casting, only the molding of the molded material. At this point, as shown in FIG. 6, the melt column 60 is completely detached from the inner wall of the floating tube 61, at least in the region where the melting of the metal melt occurs. In practice, in the practice of the process according to the invention, this effect occurs not only during the freezing stage, but also below the slit 63 shown in FIG.

The apparatus shown in Figure 6 is provided with copper tubes 62, as shown in Figure 4. They are connected to a single-phase power supply 64 and, in addition to providing electromagnetic fields, also provide cooling. Copper tubes 62 are tightly wound to graphite floating tube 61 for proper heat transfer. During operation, cooling water from the supply line (not shown) is introduced at high speed into the copper tubes 62. The used cooling water is either discharged into a container and, after cooling, used again or discharged into the sewer.

Figures 7 and 8 are views of products made by the process and apparatus of the invention. In the apparatus, the embodiment shown in Fig. 3 was used, in the apparatus shown schematically in Figures 1 and 2. Casting was performed by electromagnetic levitation, which was used to hold the melt column in a weightless condition and simultaneously pressurized against the wall of the float tube. The figure shows a smooth, shiny, slightly wavy surface of the product due to the fact that the copper column was guided along the wall of the floating tube without pressure. The result also contributed to the formation of adequate eddy currents in the metal bath during the float. The density of the product was quite satisfactory (measured at 8.9) and its composition was perfectly homogeneous along the entire length. Its diameter was 16 mm everywhere, which corresponded to the inside diameter of the floating tube. The dark stripe at the bottom of the image is about 50μ-8ΐ larger than the diameter of an otherwise glossy perimeter. The glossy mantle surface was only in contact with the float tube without pressure, while along said thin strip the stump hardened below the float zone and therefore did not contact the surface of the die or float tube without pressure. The picture clearly shows the difference between the two sections.

Claims (14)

CLAIMS 1. A method of casting metals by introducing a molten metal into a lower portion of an elongated, vertical, alternating electromagnetic field, then moving upwardly and pulling the solidified material upwardly from the electromagnetic field. 2. A process according to claim 1, characterized in that the metal melt is introduced and 1 o is pulled out continuously. 3. A method according to claim 1 or 2, characterized in that the metal melt introduced into the electromagnetic field is floated in the electromagnetic field in such a way that at least a part is weightless 15. 4. A method according to any one of claims 1 to 3, characterized in that a rod 8 to 30 mm in diameter is withdrawn from the electromagnetic field. 5. A method according to any one of claims 1 to 6, characterized in that an extraction head is lowered into the metal bath at the top of the melt column conducted into the electromagnetic field and the solidified material is then pulled out of the electromagnetic field. 6. A method according to any one of claims 1 to 4, characterized in that a copper melt is introduced into the electromagnetic field and the pulled rod is rolled and then drawn to a finished size. 7. The method of claim 6, wherein the copper rod is 16 mm in diameter. 30 out of the electromagnetic field. 8. Figures 1-7. A method according to any one of claims 1 to 3, characterized in that the melt column is floated by means of an electromagnetic field in such a way that its surface is molded or recrystallized. 35 surfaces. 9. A method according to any one of claims 1 to 6, characterized in that the melt column is formed by means of an electromagnetic field such that there is a gap between its surface and the mold or crystallizer. Apparatus for casting metals, in particular copper, copper alloys, aluminum, aluminum alloys or steel, by means of a die and a crystallizer or a die, characterized in that the crystallizer (11) is formed as a vertical floating 45 roof tube (25, 50, 61). ) as well as an electromagnetic unit and a means for pulling out the solidified material. The apparatus of claim 10 50, characterized in that the electromagnetic unit consists of coils (28) connected to successive phases of a multiphase electric power source. 12. An embodiment of the apparatus according to claim 10 or 11, characterized in that the float tube is a heat-resistant tube of uniform internal diameter. 13. An embodiment of the apparatus according to any one of claims 1 to 5, characterized in that the lower part of the floating tube (25, 50, 61) is connected to a molten metal vessel (10) and The molten metal 60 is provided with a device for supplying the lower portion of the floating tube (25, 50, 61). 14. A 11-13. An embodiment of the apparatus according to any one of claims 1 to 4, wherein the multiphase power source is a three phase generator.