AU769455B2 - Improvements in repair of aluminium smelting apparatus - Google Patents

Description

P00011 Regulation 3.2 Revised 2/98

AUSTRALIA

Patents Act, 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT *0 9 TO BE COMPLETED BY THE APPLICANT NAME OF APPLICANT: MALCOLM MANWARING ACTUAL INVENTOR: ADDRESS FOR SERVICE: INVENTION TITLE: DETAILS OF ASSOCIATED PROVISIONAL APPLICATION NO: MALCOLM MANWARING Peter Maxwell Associates Level 6 60 Pitt Street SYDNEY NSW 2000 IMPROVEMENTS IN REPAIR OF ALUMINIUM SMELTING APPARATUS PP 7580 8 December 1998 Australia The following statement is a full description of this invention including the best method of performing it known to me:- The present invention relates to improvements in the repair of aluminium smelting apparatus and, in particular, to a process for repairing a damaged stud tip for a carbon anode used in the smelting of aluminium.

Aluminium is typically smelted in an industrial Hall-Heroult reduction cell by an electrolytic process that requires a carbon electrode suspended over a carbon lined vessel, which contains alumina (Al20 3 and a mixture of salts, which act as a solute. DC electric current is caused to flow from the suspended carbon electrode, which acts as an anode, to the carbon lining of vessel, which acts as a cathode. The industrial cell used for this process is 10 termed a pot.

The rod used to suspend the carbon anode consists of a long conductive rod joined to a steel yoke to which a number of large steel studs are attached. During the manufacture of the anode, these studs are placed in holes moulded into the carbon and the studs are secured to the carbon by 15 pouring cast iron into the annulus formed between the stud and the moulded hole in the carbon.

The studs are subject to high attrition rates due to the aggressive environment within an operating pot. High oxidation rates, high temperatures (960 0 C) and electrical burning are common. The outside surface of the stud is constantly being worn away by a combination of erosion, corrosion, electrical burns and mechanical damage. The tips of the studs (being approximately the bottom half of the stud) have to be refurbished in order to restore them to their original size and shape. This work, known in the industry as "tip repair", is required at a frequency typically of one repair per year per stud and results in a quantity of repairs in a typical smelter of between 30,000 and 50,000 tip repairs per year.

28-11-03; 3:21PM:PETER MAXWELL ;612 92479945 S/ 13 3 A typical process of tip repair consists of the removal of the bottom 150 mm of the stud, which ranges in diameter from 100 mm to 200 mm, and the welding on of a cylindrical steel repair piece of the same diameter as the stud with a chamfer on the bottom and a profile on the top to facilitate rewelding.

However, that process of tip repair requires that the damaged tip currently removed must be disposed as scrap. The damaged tip has a mass of about 20 kg and so a total of about 800,000 kg of steel would be scrapped within a year in any typical smelter.

Also, the welding on of the cylindrical steel repair piece creates an 10 increased electrical resistance for the current flowing from the rod to the anode, as it is conducted through a reduced area of steel left at the weld site.

There is also a high labour consumption in the cutting of the damaged tip of the steel stud and the welding on of the new repair piece. The repair piece must also be machined to exact dimensions to provide the bottom 15 chamfer and the top profile for rewelding, which is a task involving substantial cost and time.

.It is an object of the present invention to overcome or at least substantially ameliorate these aforementioned disadvantages of prior art tip repair processes.

"20 According to the invention, there is provided a method of stud tip repair comprising the steps of: providing a mould defining a casting cavity; locating a damaged stud tip within the mould to leave one or more gap portions in the casting cavity; filling the one or more gap portions with molten metal; cooling the molten metal.

COMS ID No: SMBI-00513892 Received by IP Australia: Time 15:26 Date 2003-11-28 28-11-03: 3:21PM:PETER MAXWELL :612 92479945 6/ 13 4 In a preferred form of the invention, there is provided a method for repairing a damaged stud tip for a carbon anode of an electrolytic cell for the smelting of aluminium, comprising the steps of:providing a mould defining a casting cavity, (ii) locating the damaged stud tip within the mould to leave one or more gap portions of the casting cavity, (iii) filling the one or more gap portions with molten metal, and (iv) cooling the molten metal so that the metal solidifies and the stud S:f .tip assumes the shape defined by the mould.

In a preferred embodiment, the molten metal is hot enough to cause :i melting of the surface of the stud tip and the creation of a homogenous melt zone between the surface and the molten metal.

The mould may include a reverse taper forming structure, or a key or groove forming structure, so as to produce a reverse taper, key or groove on 15 the surface of the stud tip that could serve to positively lock the carbon anode onto the stud tip during manufacture of the anode.

Preferably, the molten metal is superheated steel.

According to another aspect of the invention, there is provided a repaired stud for smelting comprising: 20 an already used stud having formed around it a cast exterior portion, the repaired stud having an exterior shape resembling a new stud, wherein the exterior portion is cast in a mould.

In a preferred form of the invention, there is provided a stud tip for a carbon anode of an aluminium smelter electrolytic cell, the stud tip comprising a core metal portion and at least one repaired portion, the said one or more repaired portion consisting of metal that has been cast onto the core metal portion by melting the molten form of the metal with the surface of the core COMS ID No: SMBI-00513892 Received by IP Australia: Time 15:26 Date 2003-11-28 28-11-03; 3:21PM;PETER MAXWELL ;612 92479945 7/ 13 4a metal portion within a mould and thereafter cooling the molten form of the metal within the mould.

In order that the invention may be readily understood and put into practical effect, reference will be made to the accompanying drawings, in which:- Fig. 1 is a sectional side view of a stud connected to a carbon anode of an aluminium smelter electrolytic cell, Fig. 2 is a side view of a damaged stud, and Fig. 3 is a sectional side view of the tip of the damaged stud of 10 Fig. 2 located in a mould which casts molten metal onto the tip.

llllt/ oooooo COMS ID No: SMBI-00513892 Received by IP Australia: Time 15:26 Date 2003-11-28 I, 'tl The normal stud 10 shown in Fig. 1 is connected in a conventional manner to the yoke 11 of an anode rod (not shown in full) and to a carbon anode 12 by a cast iron sleeve 13.

The damaged stud 15 shown in Fig. 2 has numerous damage zones brought on by long exposure to the harsh environment within a typical pot.

There is a corrosion zone 16, an electric burning and erosion zone 17 and, overlapping therewith, an oxidation and chemical burning zone 18.

The damaged stud 15 may be repaired in the manner shown in Fig. 3.

A mould 19 defines a casting cavity having, for example, the original 10 dimensions of the stud tip to be repaired. In this instance, the mould is made of cubic zirconium but any suitable material could be used. The damaged stud is inserted into the mould 19 such that its damaged tip 20 is completely enclosed within the mould 19, thereby creating one or more gap portions between the tip 20 and mould 19.

15 These gap portions of the casting cavity are then filled with superheated molten metal which, in this instance, is molten steel at a temperature sufficient to cause at least a small degree of melting of the surface of the stud tip 15 and the creation of a homogeneous melt zone between the surface and the adjacent molten steel.

The molten steel is then subjected to cooling in the conventional manner so that the steel solidifies as a sleeve with the same homogeneous structure as the original or core tip and the new stud tip takes the shape defined by the mould 19. This shape may be the original tip shape.

The repaired stud is now ready for re-use in the electrolytic cell or pot of the aluminium smelter.

The small degree of melting of the surface of the damaged stud 15 may be achieved or aided in other ways. For example, the damaged stud 15 may 6 be preheated in any convenient way before or after assembly into the mould 19 to aid in creating a sufficiently high temperature at the surface of the tip 20 to promote melting.

Instead of introducing superheated molten steel into the casting cavity, the cavity may be filled with molten metal by employing an exothermic reaction of appropriate materials such as iron oxide and aluminium oxide. The exothermic reaction could take place in the cavity or in a nearby vessel.

Various other modifications may be made in details of design and construction without departing from the scope and ambit of the invention.

10 For instance, the mould may include a reverse taper forming structure, or a key or groove forming structure, so as to produce a reverse taper, key or groove on the surface of the stud tip that could serve to positively lock the carbon anode onto the stud tip during manufacture of the anode.

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Claims (2)

1. 28-11-03: 3:21PM:PETER MAXWELL :612 92479945 8/ 13 7 THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. A repaired stud for smelting comprising: an already used stud having formed around it a cast exterior portion, the repaired stud having an exterior shape resembling a new stud, wherein the exterior portion is cast in a mould. 2. The stud of claim 1, wherein: the exterior shape further comprises a reverse taper. 3. The stud of either of claims 1 or 2, wherein: the exterior shape further comprises a key or groove structure. 4. The stud of any one of claims 1 to 3, wherein: Sthe exterior portion is formed from cast steel. A method of stud tip repair comprising the steps of: providing a mould defining a casting cavity; locating a damaged stud tip within the mould to leave one or more gap portions in the casting cavity; filling the one or more gap portions with molten metal; cooling the molten metal. 6. The method of claim 5, wherein: the molten metal causes melting of a surface of the stud tip. 7. The method of either of claims 5 or 6, wherein: the mould comprises a reverse taper structure which is 28/11/2003 COMS ID No: SMBI-00513892 Received by IP Australia: Time 15:26 Date 2003-11-28 i transferred to the stud tip. 8. The method of any one of claims 5 to 8, wherein: the mould comprises a key structure which is imparted to the stud tip. 9. The method of any one of claims 5 to 8, wherein: the molten metal comprises steel. The method of any one of claims 5 to 9, wherein: •the mould is formed from cubic zirconium. 11. The method of claim 6, wherein: the melting at the stud tip creates a homogenous melt zone between a surface of the stud tip and the molten metal. l 12. The method of claim 9, wherein: the steel is superheated steel. 13. The method of any one of claims 5 to 12, wherein: at least a portion of the damaged stud tip is pre-heated prior to filling the gap portions. 14. The method of any of claims 5 to 13, wherein: filling is achieved by employing an exothermic reaction. A repaired stud made by the method of any one of claims 5 to 14. 8/12/99 9 1 6. A stud tip, substantially as hereinbefore described with reference to the accompanying drawings. Dated this 8 day of December 1 999. MALCOLM MANWARING Patent Attorneys for the Applicant PETER MAXWELL ASSOCIATES *fr Q S. S. S S. S@ 5 0 0 00
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