METHOD FOR PRODUCING A CATHODE STEEL BAR WITH COPPER INSERT, AND METHOD FOR REMOVING A COPPER INSERT FROM A USED CATHODE BAR

Abstract

The present invention relates to a method for producing a cathode steel bar with copper insert for use in an electrolytic cell for the electrolytic production of aluminium using the Hall-Héroult process. The present invention further relates to a method of removing a copper insert from a cathode bar used in an electrolytic cell for the electrolytic production of aluminium using the Hall-Héroult process, and reusing the copper from old copper inserts in production of new cathode bars with copper insert.

Description

TECHNICAL FIELD

The invention relates to the technical field of electrolysis in molten salts for making aluminium using the Hall-Héroult process. More precisely, the present invention relates to improved cathode steel bars with copper insert, a method for producing such cathode steel bars possibly from used cathode steel bars, and a method for removing copper inserts from used cathode steel bars.

BACKGROUND ART

Aluminium is commonly produced by electrolysis of alumina (aluminium oxide) dissolved in a molten cryolite bath. A Hall-Héroult type electrolytic cell for such production comprises a steel shell (pot shell) with a lining of refractory material, where the bottom of the cell is a carbon cathode having several current collectors embedded therein, and several anode blocks that are partly submerged in the electrolyte bath, and arranged at a distance above the cathode. In industrial production of aluminium the cells of the Hall-Héroult type are connected electrically in series, and the solution of alumina in molten cryolite is brought to a temperature up to about 980° C. by the heating effect of the current traversing through the cell. The thus formed aluminium metal accumulates in the cell bottom, on the cathode surface, and is regularly tapped from the cell.

In more detail, the cathode consists of a carbon liner with several cathode steel bars to conduct electric current out of the cell. The cathode steel often has copper inserts at least along parts of the length of the cathode steel, in order to improve the distribution of current along the cathode, see e.g. NO 343609, WO 01/63014, WO 01/27353.

The lifetime of a cathode in an electrolysis cell may be a few years before relining of the cell is needed. Used cathode steel with copper inserts usually has a low value for recycling, and must be replaced by new ones in a lined cell.

There are various ways to produce cathode steel with a copper insert. One method is to drill a longitudinal hole in the steel bar with a diameter corresponding to a copper rod to be inserted. The steel bar is heated and crimped around the copper rod. This method requires high precision as the clearance between the hole in the steel and the copper rod must be small, approx. 0.5 mm, to ensure sufficiently good contact between the steel and the copper insert, which is important to obtain good conductivity. The said method also has limitations in terms of the design of the cross section of the copper insert.

Therefore, there is a desire for an improved method of producing cathode steel bars with copper insert of good quality, e.g. in terms of conductivity, and without pores or suction in the copper inserts. Thus, it is an objective of the present invention to mitigate, alleviate or eliminate one or more of the disadvantages of today's solutions in this technical field.

SUMMARY OF INVENTION

In a first aspect, the present invention relates to a method of producing a cathode steel bar with copper insert for use in an electrolytic cell for the electrolytic production of aluminium using the Hall-Héroult process, comprising:

• • i) providing a steel portion of the cathode steel bar equipped with a cavity for the copper insert and a sleeve on the upper part,
  • providing one or more solid copper item(s) sized to be inserted into the cavity of the steel portion,
  • entering the copper item(s) into the steel portion through the sleeve, positioning at least one inductor in close proximity to at least parts of the outside of the steel portion,
  • supplying electric energy to the inductor, causing induction heating of the cathode steel bar to a temperature sufficiently high and for a time sufficiently long to molten at least the outer part of the copper item(s),
  • cooling, and solidifying the molten copper;
  • or
  • ii) providing a steel portion of the cathode steel bar equipped with a cavity for the copper insert and a sleeve on the upper part,
  • positioning at least one inductor in close proximity to at least parts of the outside of the steel portion,
  • supplying electric energy to the inductor, causing induction preheating of the cathode steel bar,
  • providing molten copper,
  • pouring the molten copper into the cavity of the steel portion through the sleeve, cooling, and solidifying the molten copper.

The method of the present invention may be a circular process where copper insert of an used cathode bar is the source of the copper insert of the cathode bar to be prepared. That is, the present invention provides a method enabling recyclability of used cathode bars with copper insert. This is advantageous in terms of flexibility, production time and cathode performance, as well as it is environmental friendly.

In a second aspect the present invention relates to a cathode steel bar with copper insert for use in an electrolytic cell for the electrolytic production of aluminium using the Hall-Héroult process, obtained by the process above.

The present invention provides cathode bars with improved performance in terms of conductivity.

In a third aspect the present invention relates to a method of removing a copper insert from an used cathode bar used in an electrolytic cell for the electrolytic production of aluminium using the Hall-Héroult process, comprising:

• • providing an used cathode bar,
  • positioning an inductor in close proximity to at least parts of the cathode bar, supplying electric energy to the inductor, causing induction heating of the cathode bar to a temperature above the melting temperature of copper, forming molten copper, pouring the molten copper into a holding furnace; or
  • casting the molten copper into suitable items.

Hence, it is to be understood that the herein disclosed invention is not limited to the particular component parts of the device described or steps of the methods described since such device and method may vary. It is also to be understood that the terminology used herein is for purpose of describing particular embodiments only, and is not intended to be limiting. It should be noted that, as used in the specification and the appended claim, the articles “a”, “an”, “the”, and “said” are intended to mean that there are one or more of the elements unless the context explicitly dictates otherwise. Thus, for example, reference to “a unit” or “the unit” may include several devices, and the like. Furthermore, the words “comprising”, “including”, “containing” and similar wordings does not exclude other elements or steps.

The terms “cathode bar(s)”, “cathode steel bar(s)”, “steel bar(s)”, “current collector(s)”, “cathode collector bar(s)”, “collector bar(s)” and “current collecting bar(s)”, may be used interchangeably in the present disclosure, and should be understood to denote the same entity, unless other stated.

The term «copper» as used herein includes pure copper and copper alloys.

The term “cavity” as used herein is to be understood as a confined space that may be open in one or more ends. A longitudinal hole in a cathode bar, open in one or both ends, is included in this term.

The terms “used cathode bar(s)” and “old cathode bar(s)” or any variations of the term “cathode bar(s)” as defined above starting with the word “used” or “old”, may be used interchangeably in the present disclosure, and should be understood to denote the same entity, unless other stated.

The terms “collar” and “sleeve” may be used interchangeably and has the function as feeder in the present disclosure.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1: Illustrates a cathode bar according to the invention produced by inserting a solid copper item into the steel portion.

FIG. 2: Illustrates a cathode bar according to the invention produced by pouring molten copper into the steel portion (top filling).

FIG. 3: Illustrates a cathode bar according to the invention produced by filling molten copper into the steel portion from the bottom/lower part.

FIG. 4: Illustrates a cathode bar according to the invention produced by filling molten copper into the steel portion from the bottom/lower part and using counter gravity casting.

FIG. 5: Illustrates a cathode bar according to the invention produced by filling molten copper into the steel portion from the bottom.

FIG. 6: Illustrates a further cathode bar according to the invention produced by filling molten copper into the steel portion from the bottom.

FIG. 7: Illustrates an “over the lip” filling device for filling molten in production of a cathode bar with copper insert.

FIG. 8: Illustrates a bottom tapping device for filling molten copper in production of a cathode bar with copper insert.

FIG. 9: Illustrates removing of a copper insert from an old cathode bar according to the invention.

FIG. 10: Photo of a section of a cathode bar with copper insert according to the invention.

The FIGS. 1-6 and 9 illustrates various embodiments of corresponding cathode bars. Thus, the structural parts of the cathode bars have the same reference numbers.

DETAILED DESCRIPTION

In the following, the present method for producing a cathode steel bar with copper insert for use in an electrolytic cell for the electrolytic production of aluminium using the Hall-Héroult process, will be described and explained by way of examples and with reference to the accompanying drawings in which the same reference numbers refer to the same or technically equivalent elements, unless otherwise stated.

FIG. 1 illustrates one embodiment of the present invention, showing a cross-section of a cathode bar comprising a steel portion 2 equipped with a cavity for insertion of solid copper, and a collar or sleeve 4 of a metal material such as steel or any other suitable refractory material at the upper part acting as a feeder. The here illustrated cathode bar comprises a copper insert 1 designed as a solid rod/bar. A closing device 5 of a suitable material to prevent heat loss such as steel, titan etc., may be placed at the upper end of the cathode bar to ensure proper seal of the copper insert 1 within the steel portion 2.

It should be understood that the solid copper which is the starting material of the copper insert can have several designs, e.g. a circular rod, or a rod of any geometry, or ingots or bars of any design. The only requirement of the solid copper item is that it is sized to be inserted into the cavity of the steel portion. For instance, the solid copper may be a rod with a smaller diameter than the cavity in the steel portion of the cathode bar. Another alternative is to use several copper items such as ingots and bars.

In an embodiment the method comprises entering one or more solid copper item(s) 1 is/are into the steel portion 2 through the sleeve 4, and heating the steel portion 2, preferably using induction heating, until at least the outer part of the solid copper items inserted therein is molten. It may also be heated until the solid copper is fully molten.

In induction heating a work-piece is heated by eddy currents induced in the work-piece. An induction heating power supply converts alternating current (AC) line power to a higher frequency AC and delivers the higher frequency AC to an inductor wherein an electromagnetic field is created within the coil of the inductor. Eddy currents will be induced in an electrically conductive work-piece placed in the electromagnetic field, generating heat in the work-piece. The inductor is commonly water cooled copper conductors made of e.g. copper tubes, profiles, plates or machined copper parts. The design of the inductor influences inter alia energy transfer to work-piece, heating rate and heating efficiency. Inductors may be helix formed or prepared from plates geometry, and the number of windings may vary. The present invention is not limited to any specific design or type of inductor. Furthermore, one or more inductors, which can be controlled separately or as a whole, may be used. For instance, the inductor may appear as being divided.

At least one inductor 3 is positioned in close proximity to at least parts of the outside of the steel portion 2. Preferably, the inductor 3 is positioned such that it encircles at least partially the part of the steel portion 3 comprising the inserted copper item(s) 1. The inductor 3 is connected to a power supply (not shown in the drawings) which converts AC line power to an AC having a frequency of between 1 kHz and 50 kHz. The converted AC is delivered to the inductor 3, causing induction heating of at least a part of the steel portion 2 comprising copper item(s), encircled by the inductor 3. The steel induction heated part of the steel portion 2 is heated to a temperature above the melting temperature of copper or copper alloy. The induction heating is continued at least until the outer part of the solid copper items is molten, forming a partly or fully molten pool of copper within in the cavity of the steel portion 2.

In one embodiment, the induction heating is continued after the solid copper item(s) have become molten in order to increase performance of the cathode bar. The induction heating may be held for a period of time varying from 10 seconds to 12 hours, preferably a period of 1 minute to 1 hour.

After the induction heating, the thus heated part of the steel portion 2 containing the partly or fully molten copper is cooled. Preferably, the cooling is controlled such that the lower part of the heated area is cooled first leading to solidification of the molten inner portion of copper, while the upper part is kept molten. The cooling is preferably carried out directionally such that the copper in the upper part of the steel portion is lastly solidified. Directional cooling may be obtained by upward moving of the inductor length wise along the cathode bar. Optionally, the bottom part of the steel portion 2 may be subjected to additional cooling in order to initiate the directional cooling. By such controlled cooling, piping and formation of shrink holes is reduced and controlled. A copper insert of good quality in that it is virtually pore-free and with no suctions is ensured in the cathode bar.

FIG. 2 illustrates another embodiment of the present invention, showing a cross-section of a cathode bar comprising a steel portion 2 equipped with a cavity for filling of molten copper 6, and a collar or sleeve of a metal material such as steel or any other suitable refractory material at the upper part acting as a feeder.

In this embodiment, the method comprises preheating of the steel portion 2 of the cathode by residual heat from previously heated cathode bars and/or by utilizing induction heating.

At least one inductor 3 is positioned in close proximity to at least parts of the outside of the steel portion 2. The properties and design of the inductor to be used in this embodiment correspond to the aforementioned description of inductor.

Preferably, the inductor 3 is positioned such that it encircles at least partially the part of the steel portion 3 comprising the cavity for filling molten copper. The inductor 3 is connected to a power supply (not shown in the drawings) which converts AC line power to an AC having a frequency of between 1 kHz and 50 kHz. The converted AC is delivered to the inductor 3, causing induction heating of at least a part of the steel portion 2 encircled by the inductor 3. The steel induction heated part of the steel portion 2 is heated to a temperature above the melting temperature of copper or copper alloy.

Molten copper is provided, e.g. from used cathode bars or any other source, and filled into the steel portion 2 of the cathode bar, either filling from top or bottom with a suitable casting method.

In one embodiment, the induction heating of the steel portion 2 is continued after filling of molten copper into the cavity of the steel portion 2 in order to increase performance of the cathode bar. The induction heating of may be held for a period of time varying from 10 seconds to 12 hours, preferably a period of 1 minute to 1 hour.

After the induction heating, the thus heated part of the steel portion 2 containing the partly or fully molten copper is cooled. Preferably, the cooling is controlled such that the lower part of the heated area is cooled first leading to solidification of the molten inner portion of copper, while the upper part is kept molten. The cooling is preferably carried out directionally such that the copper in the upper part of the steel portion is lastly solidified. Directional cooling may be obtained upward moving of the inductor length wise along the cathode bar. Optionally, the bottom part of the steel portion 2 may be subjected to additional cooling in order to initiate the directional cooling. By such controlled cooling, piping and formation of shrink holes is reduced and controlled. A copper insert of good quality in that it is virtually pore-free and with no suctions is ensured in the cathode bar.

FIG. 3 illustrates further embodiment of the present invention where molten copper is filled into the steel portion from the bottom or through an opening in the steel portion 2 placed anywhere along its side below the sleeve 4 at the top, i.e. through a permanent steel mold with gating. FIG. 3 is showing a cross-section of a cathode bar comprising said a permanent steel mold with gating/a steel portion 2 equipped with a cavity for filling of molten copper, and a collar or sleeve 4 of a metal material such as steel or any other suitable refractory material at the upper part. A downsprue 7 for filling of molten copper is connected to the steel portion 2 so that the molten copper is filled into the cavity of the steel portion. The connection point is shown at the lower part of the steel portion, but may be anywhere along the side of the steel portion 2. Only one steel portion 2 is shown, but typically a number of steel portions 2/permanent steel molds with gating are connected to the downsprue 7 for simultaneous filling of molten copper. At least one inductor 3 is positioned in close proximity to at least parts of the outside of the steel portion 2 in a similar way as explained with regard to FIG. 2 above. Molten copper is filled at a desired level in the cavity of the steel portion 2 and the sleeve 4 acts as a prolongation of the cavity to protect against overfilling and will secure good quality copper. The collar/sleeve 4 may also be a designed as a separate part connected to the steel portion 2. Heating and cooling are performed as discussed for the embodiment of FIG. 2. The inductor 3 will direct the solidification towards the collar/sleeve 4. During cooling the copper level decreases. Solidified copper on the edge of the sleeve 4 may be removed by means of drilling, machining or any other suitable method, and a closing device 5 (not shown) may be introduced.

During filling of the molten copper, the steel portion 2 of the cathode bar is preferably kept at an angle to reduce pressure height and to improve the filling process by avoiding turbulence and/or oxide formation. After the filling the steel portion 2 with molten copper, the steel portion 2 may be tilted to a vertical position or any other position. This embodiment is illustrated in FIG. 4 showing a cathode bar according to the invention produced by filling molten copper 6 into the steel portion 2 at the lower end and equipped with an inductor 3, by using counter gravity casting.

FIG. 5 illustrates further embodiment of the present invention where molten copper is filled into the steel portion from the bottom of the steel portion 2. FIG. 5 is showing a cross-section of a cathode bar comprising said a steel portion 2 equipped with a cavity for filling of molten copper, and a collar or sleeve 4 of a metal material such as steel or any other suitable refractory material at the upper part. A downsprue 7 for filling of molten copper is connected to the steel portion 2 so that the molten copper is filled into the cavity of the steel portion from its bottom/underside. Only one steel portion 2 is shown, but typically a number of steel portions 2 are connected to the downsprue 7 for simultaneous filling of molten copper. At least one inductor 3 is positioned in close proximity to at least parts of the outside of the steel portion 2 in a similar way as explained with regard to FIG. 2 above. Molten copper is filled at a desired level in the cavity of the steel portion 2 and the sleeve 4 acts as a protection against overfilling and will secure good quality copper. The sleeve 4 shown in FIG. 5 is placed at the top of the steel portion 2, but any location further down on the side of the steel portion 2 is within the scope of the invention. Heating and cooling are performed as discussed for the embodiment of FIG. 2. The inductor 3 directs the solidification towards the sleeve 4. During cooling the copper level decreases. Solidified copper on the edge of the sleeve 4 may be removed by means of drilling, machining or any other suitable method, and a closing device 5 (not shown) may be introduced.

FIG. 6 illustrates still a further embodiment of the present invention where molten 2 copper is filled into the steel portion from the bottom of the steel portion 2. FIG. 6 is showing a cross-section of a cathode bar comprising said a steel portion 2 equipped with a cavity for filling of molten copper, and a collar or sleeve 4 of a metal material such as steel or any other suitable refractory material at the upper part. A reservoir of molten copper equipped with a feeder P for addition of molten copper to the reservoir and a supply pipe 8 connected to the steel portion 2, is arranged below the steel portion 2. Molten copper is filled into the cavity of the steel portion from its bottom/underside through the supply pipe 8. At least one inductor 3 is positioned in close proximity to at least parts of the outside of the steel portion 2 in a similar way as explained with regard to FIG. 2 above. Molten copper is filled at a desired level in the cavity of the steel portion 2 and the sleeve 4 acts as a protection against overfilling and secure good quality copper. The sleeve 4 shown in FIG. 6 is placed at the upper part of the steel portion 2, but may be located further down on the side of the steel portion. Heating and cooling are performed as discussed for the embodiment of FIG. 2. The inductor 3 directs the solidification towards the sleeve 4. During cooling the copper level decreases. Solidified copper on the edge of the sleeve 4 may be removed by means of drilling, machining or any other suitable method, and a closing device 5 (not shown) may be introduced.

FIG. 7 illustrates an “over the lip” filling device and FIG. 8 a bottom tapping device for filling molten copper in production of a cathode bar with copper insert. Both devices are commercial available and within the knowledge of the skilled person. “Over the lip” filling and bottom tapping are useful filling methods for carrying out the embodiments of the invention related to filling molten copper when producing the cathode bars with copper insert such as illustrated in FIGS. 2-6.

FIG. 9 illustrates removing of a copper insert 1 from a used cathode bar. A cross-section of a cathode bar comprising a steel portion 2 and a copper insert 1 is shown illustrating the embodiment of the present invention where copper is melted out for optional re-use in production of a new cathode steel bar as described in the embodiment above.

In this embodiment, a used cathode steel bar is provided and prepared for copper melt out, typically through a cutting process or similar to gain direct access to the copper insert of the cathode bar.

The cathode bar is preferably preheated by rest heat from previously heated cathode bars and subsequently heated by utilizing induction heating to at least the melting point of copper or copper alloy.

At least one inductor 3 is positioned in close proximity to at least parts of the outside of the steel portion 2. The properties and design of the inductor to be used in this embodiment correspond to the aforementioned description of inductor. Preferably, the inductor 3 is positioned such that it encircles at least partially the part of the cathode bar comprising the copper insert. The inductor 3 is connected to a power supply as described above and causes induction heating of at least a part of the cathode bar encircled by the inductor 3. When reaching the melting point of the copper, molten copper is formed and poured out of the steel portion 2 of the cathode bar into a holding furnace, or casted directly into copper ingots or similar solid copper items.

In the embodiment where molten copper is poured into a holding furnace, it may be re-used directly in molten state for production of new cathode bars as described above in relation to FIGS. 2-8.

In the embodiment where the copper is casted into solid copper items, they may be re-used for production of new cathode bars as described above in relation to FIG. 1, or FIGS. 2-6 by re-melting of the solid copper items.

Refining or “dilution” of the recycled copper to desired quality might be required. Here “dilution” means addition of more pure copper.

FIG. 10 shows a section of a cathode bar with copper insert prepared by the method of the invention. It is clear from the photo of FIG. 10 that a unique intermetallic connection between steel and copper has been achieved. Furthermore, no pores or suction can be seen in the copper insert. Thus, it is shown that the method of the present invention enables production of cathode bars with copper insert where the metals (steel and copper) are homogeneously joined. That is, the quality of the cathodes obtained are very good and their performance being improved in terms of conductivity. Such cathode bars are not previously known.

Having described preferred embodiments of the invention it will be apparent to those skilled in the art that other embodiments incorporating the concepts may be used. These and other examples of the invention illustrated above are intended by way of example only and the actual scope of the invention is to be determined from the claims.

Claims

1. A method for producing a cathode steel bar with copper insert for use in an electrolytic cell for the electrolytic production of aluminium using the Hall-Héroult process, comprising:

i) providing a steel portion of the cathode steel bar equipped with a cavity for the copper insert and a sleeve on the upper part,

providing one or more solid copper item(s) sized to be inserted into the cavity of the steel portion,

entering the copper item(s) into the steel portion through the sleeve,

positioning at least one inductor in close proximity to at least parts of the outside of the steel portion,

supplying electric energy to the inductor, causing induction heating of the cathode steel bar to a temperature sufficiently high and for a time sufficiently long to molten at least the outer part of the copper item(s),

cooling, and solidifying the molten copper;

or

ii) providing a steel portion of the cathode steel bar equipped with a cavity for the copper insert and a sleeve on the upper part,

positioning at least one inductor in close proximity to at least parts of the outside of the steel portion,

supplying electric energy to the inductor, causing induction preheating of the cathode steel bar,

providing molten copper,

pouring the molten copper into the cavity of the steel portion through the sleeve,

cooling, and solidifying the molten copper,

wherein the cooling is carried out directionally, in a direction from the lower part of the cathode bar towards its upper part.

2. The method, according to claim 1, comprising supplying electric energy with a frequency of between 1 kHz to 50 kHz from a power unit to the inductor.

3. The method, according to claim 1, wherein the heating and preheating temperature is above the melting temperature of copper or copper alloy.

4. The method, according to claim 1, wherein the heating and preheating be held for a period of 10 seconds to 12 hours.

5. (canceled)

6. The method, according to claim 1, wherein the directional cooling is obtained by upward moving of the inductor length wise along the cathode bar.

7. The method, according to claim 1, wherein the method is circular in that the solid copper item(s) or molten copper is provided from used cathode bars.

8. The method, according to claim 1, wherein the solid copper item(s) is provided from used cathode bars.

9. The method, according to claim 1, wherein pouring of the molten copper into the cavity of the steel portion is obtained by top filling.

10. The method, according to claim 1, wherein filling of the molten copper into the cavity of the steel portion is carried out by top or bottom filling.

11. The method, according to claim 9, wherein the steel portion of the copper bar is kept at an angle during filling of molten copper.

12. The method, according to claim 1, wherein the molten copper is provided from used cathode bars.

13. A cathode steel bar with copper insert for use in an electrolytic cell for the electrolytic production of aluminium using the Hall-Héroult process, obtained by the process according to claim 1.

14. A method of removing a copper insert from a cathode bar used in an electrolytic cell for the electrolytic production of aluminium using the Hall-Héroult process, comprising:

providing an used cathode bar,

positioning an inductor in close proximity to at least parts of the cathode bar,

supplying electric energy to the inductor, causing induction heating of the cathode bar to a temperature above the melting temperature of copper, forming molten copper,

pouring the molten copper into a holding furnace; or

casting the molten copper into suitable items.

15. The method, according to claim 14, comprising supplying electric energy with a frequency of between 1 kHz to 50 kHz from a power unit to the inductor.

16. The method, according to claim 4, wherein the heating and preheating be held for a period of 1 minute to 1 hour.

17. The method, according to claim 10, wherein the steel portion of the copper bar is kept at an angle during filling of molten copper.