GB2262332A

GB2262332A - System for re-melting an electrode

Info

Publication number: GB2262332A
Application number: GB9216573A
Authority: GB
Inventors: Felix Muller; Michael Hohmann; Norbert Ludwig; Michael Protzmann; Hardy Weisweiler
Original assignee: Leybold Durferrit GmbH
Current assignee: Leybold Durferrit GmbH
Priority date: 1991-12-10
Filing date: 1992-08-05
Publication date: 1993-06-16
Also published as: GB9216573D0

Abstract

A pressure reservoir (9) is located on a melting pot (2) which surrounds in a pressure sealing manner the electrode (6) to be re-melted. The bottom end part of the electrode is immersed in a melting bath (7). The bottom (3) of this melting pot (2) has an outlet (5) from which the re-melted metal emerges as a stream. The outlet is surrounded by an electric coil the current through which determines the strength of an electromagnetic field and impedes to a greater or lesser extent the flow through the outlet. This control is augmented by increasing the vacuum in the pressure reservoir (9), enabling the flow of metal to be decreased or stopped. <IMAGE>

Description

SYSTEM FOR RE-MELTING AN ELECTRODE The invention relates to a re-melting system for the purpose of re-melting an electrode in a melting pot which contains an electrically heated melting bath and which has an outlet at the bottom surrounded by an electric coil. The re-melted metal flows through this outlet as a continuous stream and the coil is designed for the purpose of creating a controllable magnetic field which reduces the flow speed of the stream.

Furthermore, the invention relates to a process for the operation of such a re-melting system.

The electrodes which are produced in vacuuminduction ovens and cast in the shape of cylindrical blocks are freed from harmful components by re-melting.

For this purpose, the electroslag re-melting process (ESU) is used where the electrode is partly immersed in a slag bath inside a melting pot, which slag bath has a temperature that is higher than the melting temperature of the electrode. The drops of molten metal migrate through the slag and this process removes ceramic compounds in particular. At the end of the re-melting process, when the electrode has melted completely, the liquid metal collected in the melting pot is left to solidify in order to obtain a new metal block. If it is intended to let the liquid metal flow out of the melting pot with a metal stream during the melting of the metal, it is necessary to exercise precise control on the amount of metal flowing out.For this purpose, the outlet can be directed through a coil and the coil can produce sufficient magnetic holding forces if the metal outlet consists of a ceramic material. However, if the intention is to produce a metal that contains as little ceramic material as possible, the ceramic outlet cannot be used and the outlet must be made of metal, in particular copper, which has a shielding effect on the magnetic field. In order for a magnetic field still to have an effect on the metal stream flowing out, the outlet can be made from closely spaced palisades.

However, it has become evident that the magnetic forces are often not sufficient for regulating the flow speed of the metal stream and that they can stop the metal stream completely. Furthermore, a regulating mechanism using magnetic forces is undesirably slow.

The present invention provides apparatus for re-melting an electrode comprising a melting pot for containing a bath of re-melted metal, said pot having a bottom outlet, an electric coil surrounding said bottom outlet, said coil producing a variable magnetic field for regulating rate of flow of re-melted metal through said outlet, a pressure reservoir above said bath of re-melted metal, and means for regulating the pressure in said reservoir in order to regulate rate of flow of re-melted metal through said outlet.

This design of re-melting apparatus makes it possible to change the flow speed of the metal stream very quickly up to the point of stopping the stream by changing the pressure in the pressure reservoir. This allows for an optimum adjustment of the flow and therefore of the level of the melting bath inside the melting pot without having to produce high magnetic forces by means of the coil surrounding the metal outlet.

Considerable changes in the level of the melting bath inside the melting pot, which would lead to different flow speeds without a flow regulator and therefore would have to be compensated by the flow regulator, can be prevented in simple manner by means of a level measurement device for the purpose of measuring the level of the melting bath in the melting pot and by means of a current regulator for the purpose of regulating the power of the heating system of the melting bath in accordance with this level.

At the beginning of the process the outlet can be closed, independent of the magnetic holding forces and the holding forces created by a vacuum in the pressure reservoir, if, in accordance with another preferred feature of the invention, the outlet is equipped with a mechanical, ceramic-free, induction heated stopper which can be moved from a position completely closing the outlet into a position releasing the outlet. During the initial phase, this stopper prevents air from moving up from the metal outlet through the melting bath into the pressure reservoir and releasing any vacuum in the reservoir.

The heat loss of the re-melting system can be reduced by making the mantle surface of the melting pot from ceramic material. This can increase the melting speed of the electrode without incurring the risk of ceramic particles contaminating the molten metal since a skull always originates in this area, which prevents the ceramic material from migrating into the molten metal.

The danger of slag being included in the metal stream flowing out can be prevented in a very simple manner by staggering the axis of the outlet compared to the axis of the electrode.

An even better separation of the slag can be achieved if slag barriers are provided in the melting pot.

The flow of the molten metal can be varied quickly, up to the point of a complete stop by changing the pressure in the pressure reservoir. As a result, the coil system surrounding the metal outlet is required to produce less strong magnetic forces that act upon the metal stream so that the achievement of a ceramic-free metal outlet causes fewer problems than in the case where the flow has to be stopped by magnetic forces only.

It is also beneficial if the level of the melting bath can be measured in the melting pot and taken into account when regulating the electric power of the heating system of the melting bath. This allows a precise regulation of the melting speed of the electrode which results in a constant melting bath level and therefore constant pressure in the melting bath at the metal outlet.

It is advantageous to end the process by reducing or turning off the power of the heating system of the melting bath and by stopping the emerging liquid stream of metal by increasing the electromagnetic forces produced by the coil in the outlet and/or by increasing the vacuum in the pressure reservoir and afterwards mechanically locking the outlet by means of the stopper.

At the beginning of the process, slag can be prevented from emerging by initially melting a metal stop in the area of the electro-magnetic guiding system which had formed in the outlet and by turning on the electricity in the heating system of the melting bath afterwards. This procedure ensures that the outlet is filled with liquid molten metal before the slag melts so that the slag floats on top of the molten metal and is therefore prevented from flowing out.

In order to prevent a short circuit between the mantle surface of the electrode through the edge of the skull to the wall of the melting pot in the case of a "cold start" with a skull, an electrode with a reduced diameter at the lower end can be used in cases where the process is started with an existing skull.

This results in greater radial distances between the electrode and the skull. The invention allows for numerous design alternatives. In order to further explain the basic principle of the invention, two melting systems designed in accordance with the invention are illustrated in the accompanying drawings in which: Figure 1 is a view partly in section of a re-melting system in accordance with the invention, Figure 2 is a part-sectional view of the stream guiding system of the re-melting system of Figure 1 on an enlarged scale, and Figure 3 shows a schematic diagram of the lower portions of a modified version of the re-melting system of Figure 1.

In Figure 1, the upper part of a frame 1 supports a melting pot 2 made from copper. This melting pot 2 has a bottom 3 with a stream guiding system 4 which has an outlet 5. Inside the melting pot is an electrode 6 to be re-melted whose lower end is partly immersed in a melting bath 7. The energy required for the re-melting process is supplied to the re-melting system by means of a current supply line 8 which consists of two coaxial current bearing pipes.

A pressure reservoir 9 sits on top of the melting pot 2 and surrounds electrode 6 in a pressure sealed manner. Exhaust gas is removed by suction from this pressure reservoir 9 by means of a fan 10 through a discharge nozzle 11. A regulator 12 makes it possible to vary the pressure in the pressure reservoir 9, and in particular to create a vacuum. Since this vacuum is applied on the surface of the melting bath 7, this slows down or prevents the flow of the metal at the outlet 5. Vice versa, the flow of metal can be increased by increasing the pressure inside the pressure reservoir.

Information about the flow rate from outlet 5 is provided by a sensor 21 which is indicated only generally. Such a sensor may be an electric coil surrounding the output opening; a change in flow velocity would change the resistance of the coil. Other possibilities for determining flow rate include continuously measuring the height of liquid metal in the container (as by device 13) and continuously measuring the weight of the container.

A level measuring device 13 is located at melting pot 2 which is capable of measuring the level of the melting bath 7 and subsequently of regulating the electric energy required for the re-melting process by means of a current regulator 14.

Figure 2 shows the stream guiding system 4 in more detail. It can be seen that the outlet 5 has the shape of a funnel and incorporates individual palisades 15, 16. The outlet 5 is surrounded by a coil 17 which slows down the metal stream flowing out by means of magnetic forces to a varying degree when current is applied. At the lower end of the outlet 5 is an induction-heated stopper 18 which mechanically locks outlet 5 but which can be moved into a position opening the outlet 5 when the re-melting system is in operation.

The variation of the invention shown in Figure 3 differs from that in Figures 1 and 2 in that the axis of the electrode 6 and the axis of outlet 5 are staggered laterally. This makes it possible to provide slag barriers 19, 20 in the melting bath 7 which provide additional security against movement of slag into outlet 5, because the slag floats on top.

In a modified arrangement the energy required for the electroslag re-melting process may be regulated by means of the level of the melting bath without using a pressure reservoir to hold back the metal stream.

Claims

1. Apparatus for re-melting an electrode comprising a melting pot for containing a bath of remelted metal, said pot having a bottom outlet, an electric coil surrounding said bottom outlet, said coil producing a variable magnetic field for regulating rate of flow of re-melted metal through said outlet, a pressure reservoir above said bath of re-melted metal, and means for regulating the pressure in said reservoir in order to regulate rate of flow of re-melted metal through said outlet.

2. Apparatus as claimed in claim 1 further comprising means for measuring the level of the melting bath in said pot, and current regulation means for controlling heating power of the melting bath depending upon the level sensed.

3. Apparatus as claimed in claim 1 or claim 2 further comprising a ceramic-free induction heated stopper in said outlet, and means for moving said stopper between a position locking said outlet and a position releasing said outlet.

4. Apparatus as claimed in any one of the preceding claims wherein said melting pot has a ceramic mantle surface.

5. Apparatus as claimed in any one of the preceding claims wherein said electrode has a vertical central axis and said outlet has a central axis which is laterally staggered from the axis of the electrode.

6. Apparatus as claimed in any one of the preceding claims further comprising slag barriers in said melting pot.

7. Process for controlling the flow of molten metal from the outlet of a melting pot having a melting bath for re-melting an electrode, said process comprising regulating current in a coil surrounding said outlet, said coil producing magnetic fields which affect said flow and regulating pressure in a pressure reservoir over said bath to achieve a desired flow.

8. Process as in claim 7 further comprising measuring the level of the melting bath in said pot, and using the measured level to regulate the current in said coil.

9. Process as in claim 7 wherein said flow is stopped by regulating the current in said coil to increase the electromagnetic fields which impede flow through said outlet, decreasing the pressure in said reservoir, and stopping said flow mechanically by means of a stopper in said outlet.

10. Process as in claim 7 further comprising melting any solid metal in said outlet in order to permit flow therethrough.

11. Process as in claim 1 wherein an existing skull is used to start said melting bath and an electrode having a bottom end of reduced diameter is lowered into said bath.

12. Apparatus for re-melting an electrode substantially as hereinbefore described with reference to and as illustrated in Figures 1 and 2 or in Figures 1 and 2 as modified by Figure 3 of the accompanying drawings.