JPH0352522B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for refining metals. Electric slag remelting is a secondary melting or refining method. The primary produced ingot, known as a consumable electrode, is remelted and solidified again under precisely controlled conditions to improve the grain structure of the ingot and eliminate inclusions at a longer time than can be obtained during the primary melting. and removing impurities that form inclusions. Remelting is performed by resistive heating by passing a current between the consumable electrode and the second electrode. The consumable electrode is partially immersed in the slag layer and generates joule heat for melting in the slag layer. A puddle of molten metal forms on the underside of the slag. The slug provides a path for electrical current. It also removes inclusions and inclusion-forming impurities from the melt.

Normally current flows through the slug between the consumable electrode and the ingot. Such an approach works very well when the ingot formed has a cross-section substantially the same as or larger than the cross-section of the electrode being fused. However, such a method is undesirable when the cross-section of the ingot formed is smaller than the cross-section of the electrode to be melted, for example when the ingot formed is a bar. If the cross section of the ingot is small, it is necessary to interrupt the current frequently. Cutting off the current means cutting off the current path intermittently, and it is necessary to cut the current using sliding contacts to the ingot. Sliding contacts are problematic because they are often not smooth or clean from the surface of the ingot on which they are formed.

If the ingot being formed is smaller than the electrode being melted, the path of current through the ingot is likely to cause another problem. The large current required to melt the consumable electrode resistively heats the ingot and slows solidification.

There was a request to provide an alternative current path. An experiment was conducted in which a current was passed between the consumable electrode and the crucible through the slag. This resulted in unstable operation of the furnace and damage to the crucible. It was assumed that a layer of slag would solidify against the crucible wall and that this slag layer would promote arcing through the slag layer. The crucible was water cooled.

The present invention provides a current path that avoids arcing that occurs when current is passed between the consumable electrode and the second electrode, which has cooling means as an integral part. The invention also provides a method for passing an electric current through the slag between two non-consumable electrodes, at least one of which has cooling means as an integral part, such as when injecting molten metal through the slag. , to provide a path for current that prevents arcing from occurring. Passing current through at least one electrically conductive member interposed between the slag and the cooled electrode eliminates the arcing that would otherwise occur when passing current through the slag and between the electrodes. The electrically conductive member has a melting point higher than the temperature of the slag. For this application, the slag temperature is the temperature away from the crucible wall, and the fact that there is a temperature gradient in the slag layer.

The electric slag remelting process is covered by U.S. Patent No.
It is disclosed in many references including 4108235 and 4145563. Patent No.
No. 4,108,235 and No. 4,145,563 do not disclose the current path of the present invention. Patent No.
The current path of '4108235 is between the consumable electrode and the crucible and the mandrel used to cast the hollow ingot. The current path in Patent No. 4,145,563 includes a crucible liner. However, the crucible liner is electrically isolated from the crucible.

Many references disclose methods for purifying metals that are already molten. Included in these references is West German Patent No. 1483646. Similar to the references mentioned in the previous section, Patent No. 1,483,646 does not disclose the current path of the present invention. Patent No. 1,483,646 describes a solidified slag shell 13 which electrically insulates the slag from the crucible. The electrically conductive member of the present invention does not require such insulation.

It is therefore an object of the present invention to provide a purification method characterized by the use of an improved current path.

The foregoing and other objects of the invention are best understood from the following description, ie, the reference to the drawings herein.

The drawing is a schematic diagram of the components forming the current path.

The present invention provides a method for refining metal in a crucible containing a layer of molten slag, in which the molten metal moves downward through the slag and resolidifies as an ingot at the bottom. The slag is maintained in a molten state by passing a current between the first and second electrodes through the slag layer. The second electrode has liquid or gaseous cooling means as an integral part thereof. It is usually made from copper or copper alloys. Current is passed between the first and second electrodes through at least one electrically conductive member interposed between the slug and the second electrode. The first electrode may be a consumable electrode partially immersed in the molten slag so that it gradually melts and the droplets of metal move downwardly from the molten slag. The second electrode is typically a crucible, but may also be a non-consumable electrode partially immersed in a layer of molten slag. The current source may be direct current or alternating current, but alternating current is preferred.

The electrically conductive member has a melting point higher than the temperature of the slag. The shape, thickness and conductivity of the member are such that the surface portion of the member in contact with the slag has a temperature above the solidification temperature of the slag. A member having a relatively high thermal conductivity generally needs to be thicker than a similarly shaped member having a relatively low thermal conductivity. Generally, the member projects from the second electrode into the slug, but may also be embedded in the electrode. High melting point metals such as tantalum are preferred as materials constituting the member. There is no critical minimum length for the member. They are usually at least 3/16 inch (4.8 x 10 ^-3 meters) long. In most cases more than one member will be provided. They can be interposed between the slug and the second electrode by any means known to those skilled in the art. Brazing is one special means of attaching parts.

Components forming a current path according to the invention are schematically shown in the drawing. The current flows between the consumable electrode 1 and the crucible 3 through the slag layer 5 and the electrically conductive member 7.
flow through. An ingot 9 and a reservoir 11 of molten metal are also shown.

The following examples illustrate some aspects of the invention.

Attempts to pass a direct current between the consumable electrode and the crucible through the slag layer resulted in damage to the crucible. The experiment was carried out in a research electric slag remelting furnace under normal conditions at normal current (1900 amperes) and voltage magnitudes. The consumable electrode was a nickel-based alloy and had a diameter of 3.5 inches (88.9 x 10 ^-3 meters). An ingot with a diameter of 1 inch (25.4 x 10 ^-3 meters) was obtained. The inside diameter of the crucible top was 5.25 inches (133.4 x 10 ^-3 meters). The crucible was inspected prior to testing to ensure that it was free of any defects or defects. Upon inspection after the test, it was discovered that the crucible was severely etched, which appeared to be arc scars. The etching depth is typically 1/64 inch (4×
10 ^-4 meters) to 1/16 inch (1.6 x 10 ^-3 meters). The test lasted only 10 minutes. If the etching had penetrated the crucible wall, water would have gushed into the molten slag and caused an explosion. The damage to the crucible was such that it was impossible to conduct further experiments with this current.

The current path of the present invention was tested under exactly the same conditions as the trial described in the previous section. Four tantalum protrusions were brazed to the crucible. All dimensions were 3/8 inch long (9.5 x 10 ^-3 meters) and 5/8 inch diameter (15.9 x 10 ^-3 meters). The test lasted 6.5 minutes. A current of 2000 amperes was applied. Inspection of the crucible revealed no damage. The tantalum protrusions were slightly corroded.

Nine additional tests were performed on the same tantalum projections. The same nickel-based alloy was used as the consumable electrode. The tantalum protrusions protected the crucible and did not cause any damage during the test.

By virtue of the novel principles of the invention disclosed herein,
It will be apparent to those skilled in the art that various other modifications and applications of the same invention may be suggested in connection with the specific embodiments thereof. Therefore, the principles of the invention should not be limited to the specific embodiments of the invention described herein, provided the scope of the appended claims is understood.

[Brief explanation of drawings]

The drawing shows a schematic diagram of the components forming the current path of the melting furnace. 1... Consumable electrode, 3... Crucible, 5... Slag, 7... Tantalum electrode, 9... Ingot, 1
1... Molten metal.

Claims (1)

Claims: 1. A method for refining metal in a crucible containing a layer of molten slag, wherein the molten metal moves downwardly through the slag and solidifies again as an ingot at the bottom, and further comprises: In the above method, the slag is maintained in a molten state by passing an electric current between the first electrode and the second electrode, the second electrode having cooling means as an integral part. and the second electrode through at least one electrically conductive member interposed between the slug and the second electrode, the electrically conductive member being in the bulk of the slug. A method as described above, characterized in that the portion of the surface of the electrically conductive member in contact with the slag has a temperature higher than the freezing point of the slag. 2. The method of claim 1, wherein the electrically conductive member projects into the slug. 3. The method according to claim 1, wherein the electrically conductive member is a high melting point metal. 4. The method according to claim 3, wherein the high melting point metal is tantalum. 5. The method of claim 1, wherein there are at least two electrically conductive members. 6. The method of claim 1, wherein the electrically conductive member is at least 3/16 inch (4.8 x 10 ^-3 meters) long. 7. the first electrode is a consumable electrode and the electrode is partially immersed into the molten slag layer such that it gradually melts and metal droplets are transferred downwardly from the molten slag layer; A method according to claim 1. 8. the second electrode is a liquid cooled electrode;
A method according to claim 1. 9. The method of claim 1, wherein the second electrode is a crucible. 10. The method of claim 1, wherein the second electrode is a non-consumable electrode partially immersed in the layer of molten slag. 11. The method of claim 7, wherein the cross section of the ingot is smaller than the cross section of the consumable electrode. 12 the second electrode is copper or a copper alloy;
A method according to claim 1.