EP0248452B1

EP0248452B1 - Anode hanger for fixedly holding carbon anode bodies in cells for electrolytic production of aluminium

Info

Publication number: EP0248452B1
Application number: EP19870108297
Authority: EP
Inventors: Paul Mikael Holmberg; Thor Georg Omli; Kai M. Vik
Original assignee: Norsk Hydro ASA
Current assignee: Norsk Hydro ASA
Priority date: 1986-06-06
Filing date: 1987-06-09
Publication date: 1991-01-23
Also published as: NO162083B; NO871133L; NO162083C; BR8702881A; AU7402487A; NO871133D0; EP0248452A1; DE3767567D1

Description

The present invention relates to anode hangers for fixedly holding prebaked carbon anode bodies in cells for the production of aluminuim by electrolysis according to the Hall-Heroult process, as described in the initial part of the present claim 1.
Aluminium is mostly produced by electrolysis of aluminium oxide dissolved in a cryolite bath. The electrolytic cells enabling this, consist of a carbon cathode which is placed in a steel container which on the inside is isolated with refractory materials. A carbon anode, or several carbon anode bodies, are arranged above the carbon cathode and are partly sunk into the cryolite bath. Thus the oxygen which descends from the decomposition of the aluminium oxide will gradually oxidize the carbon anode bodies.
Electric current is led from the top to the bottom of the cell, and the cryolite is kept in its floating condition by means of the Joule-effect at a temperature close to the solidification temperature. The most common temperature for these cells is between 930 and 980°C. The aluminium produced is therefore in a liquid state and is deposited on the carbon cathode due to gravity. The aluminium produced, or part of the aluminium produced, is regularly removed from the cell by means of a suction pump and transferred to a casting ladle, and is in turn decanted to a foundry stove.
Used anode bodies are replaced by new ones.
The amperage for these electrolytic cells today lies between 100.000 and 300.000 amps. Electric current connections and bus bars are therefore made of industrial metals with good electric conductivity, e.g. usually pure copper or aluminium. For simplicity sake metals with good electric conductivity (copper, aluminium etc.) in the following are called "metal", while iron and steel alloys are called "steel".
The carbon parts of the electrolytic apperatures are working at temperatures close to the temperature of the cryolite bath. The connections between the anode and cathode and the electric conductors (bus bars) are therefore made of intermediate parts which are resistant to the high temperatures. These intermediate parts are usually made of steel.
In the electric current connection between the bus bar and the anode carbon body, the anode hanger, only the lower part is made of steel, while the upper part of the hanger is made of metal. This due to the fact that steel has poor electrical conductivity compared to aluminium or copper (metal).
It is desirable that the electric current has the shortest possible path through the steel part of the anode hanger so that the ohmic resistance is at a minimum. However, a sufficient amount of steel has to be used so that the temperature in the metal part of the anode hanger does not get too high. Such too high temperature may result in that the metal melts or the connection between the steel and the metal becomes loose.
Different methods are known for connecting the metal part to the steel part of an anode hanger, such as welding, bimetallic soldering, mechanical connections or combinations of these.
From SU-A-1 008 283 a cast connection is known in which a complicated arrangement is used comprising a head, spindles and an intermediate metal bush between the bar and crosspiece head. The arrangement seems to provide very small contact area between the cast metal and the metal parts of the anode hanger.
This is also true with reference to SU-A-606 901, in which an anode hanger is shown the anode rod being connected to the crosspiece by positioning the lower part of the rod in a pocket in the crosspiece and thereafter by pouring a conductive alloy with a low melting point into the pocket.
The known types of steel/metal connections have, however, poor mechanical as well as thermal properties, and need frequent control to reveal possible defects. Further, the electrical conductivity properties are bad (high ohmic resistance), which also contributes to reduce the life of the anode hangers.
It is a main object of the present invention to provide an anode hanger which is not encumbered with the above disadvantages. Thus, the connection between the steel and metal part of the anode hanger according to the present invention has:

improved mechanical properties (mechanically stronger)
improved electrical conductivity (lower ohmic resistance)
a shorter electrical current path
lower maintenance costs, and
is cheaper to produce.

The above advantages are achieved by means of anode hanger according to the characterizing part of the attached claims 1 and 2.
Preferred embodiments of the invention are discussed in the subordinate claims.
The present invention will now be further described by means of the attached drawings, where Figs. 1-4 show four different examples of an anode hanger according to the invention.
In the example of Fig. 1 there is shown an anode hanger onto which is fixedly mounted a carbon anode body. The anode hanger 1 consists of an upper part 3 which is made of metal, the anode hanger rod, and a lower part 4 made of steel. The lower steel part 4 of the anode hanger comprises a cross bar 7 which has four downwardly extending nipples 5. The carbon body is affixed to the nipples 5 by means of a connecting piece which may be cast iron, carbon paste, or some kind of dry packing. The metal part is connected to the steel part of the anode hanger by casting the metal 3 around the upper part of the steel cross bar 7.
The manufacturing of the anode hanger according to Fig. 1 consists in making a steel part 4 which has a cross bar 7 in which is drilled several holes 6. The steel part is cleaned, e.g. by means of sand-blasting, and is thereafter, electrolytically or by other means, metallized with a metal having good electric conductivity, such as silver or copper. By metallizing the steel, or part of the steel around which the metal is cast, an improved connection between the steel and metal is achieved.
The steel cross bar 7 is then, with its nipples protruding upwards, mounted on top of a mould which is made on beforehand. After the steel bar 7 is placed in the right position, the liquid metal melt is poured into the mould until it reaches a level where the cross bar is wholly or partly covered with metal.
During the casting operation the metal flows into the holes 6 where it solidifies. Due to the fact that the metal has a co-efficient of thermal expansion which is larger than that of steel, the metal will shrink around the steel and thereby make a firm mechanical connection, as well as an improved electrical connection between the metal and steel part of the anode hanger.
After the anode hanger is removed from the mould, a hole 9 is drilled through the steel and metal part of the hanger, slightly above the steel cross bar 7. The hole 9 serves as a grip for a lifting/holding device in connection with the transportation of the hanger, removing of anode butts etc. This will reduce the mechanical strain/ wear of the connection between the metal and steel part of the hanger.
In Fig. 2, which shows another example of an anode hanger according to the invention, the steel cross bar consists of a hollow box type construction 10. The box type construction 10 may preferably be made of a structural steel bar and is on the top side provided with a hole for the anode hanger rod 11, and on the bottom side provided with holes for the steel nipples 12.
The lower end of the metal rod 11 and the upper end of the nipples 12 extend through said holes and into the hollow box type construction 10. The rod 11 is arranged with clearance relative to the hole on the top side of the box construction 10, while the nipples preferably are welded to the edges of the respective holes with a welding seam 14 on the outside and/or a welding seam on the inside of said box construction. By welding the nipples to the box construction, the electrical conductivity is improved, and the liquid metal melt is prevented from leaking out during the casting operation.
After the welding of the nipples is completed and the anode rod 11 is placed in the right position, the casting itself may be performed by pouring metal melt through a hole (not shown) on top of the box construction 10. The metal melt may be superheated (for aluminium about 750°C). Thereby the end of the metal anode rod 11 which is embedded in the cast metal 26 inside the box construction is partly melted together with said metal 26. This will strengthen the connection between the anode rod 11 and cast metal 26, and will reduce the ohmic resistance.
It is also possible to further strengthen the connection between the cast metal 26 and the anode rod 11, respectively nipples 12, by providing a recess 13 along the circumference at the ends of the nipples and anode rod.
This latter feature will presumably also reduce the ohmic resistance as the strain conditions in the metal around the recesses 13 are favourable. Besides, for further reduction of the ohmic resistance, it is also possible to metallize the steel part of the anode hanger with silver or copper as mentioned above.
Fig. 3 shows an anode hanger which principally is of the same construction as the one described above and shown in Fig. 2. Due to the fact that the electrical conductivity between the metal and steel part of the anode hanger is improved and the electric current path in the steel part is reduced, the hanger is provided with two anode carbon bodies. Thus, instead of three nipples the anode hanger is provided with six nipples 12.
By using two carbon bodies the load on the cross bar 18 increases. The anode hanger is therefore provided with brackets 17 on each side of the anode rod 20, on the top side of the cross bar 18. The brackets are on one side welded to the cross bar 18, and on the other side welded to a vertical box type construction 19 which is arranged around the anode rod 20.
Double anode hangers of this kind contribute to reduce the manufacturing and maintenance costs for the anode hangers.
Fig. 4 reveals a fourth example of an anode hanger according to the invention. This anode hanger is partly built up of constructional elements shown in the examples of Figs. 2 and 3, as the nipple ends are embedded in a box type construction and the anode hanger is provided with two carbon anode bodies. The difference consists, however, in that the electric current is supplied directly to the cast metal 26 through flexible conductors 24, and that the anode hanger rod is made of a non-conducting steel bar 21.
These features of the invention contribute to a reduction of the electric current path through the metal part of the anode hanger, and provides for a more simple suspension arrangement.
The steel bar 21, which also may be made of square structural steel, is welded to the box type construction 22 at its lower end. Further, the box construction 22 is provided with brackets 25, as also shown in Fig. 3.
Regarding the flexible conductors 24, the ends of these may be embedded in the cast metal 26, or connected to the metal 26 by means of a mechanical connection (not shown).
Although Fig. 4 shows an anode hanger provided with six nipples (two anodes), the invention is not limited to only this embodiment. Thus, the anode hanger may be provided with more than six nipples (more than two anodes).
As stated above the present invention relates to an anode hanger where the electric current path in the steel part of the hanger is essentially reduced (in the example shown in Fig. 4 also the metal part). The radiant heat and the heat conducted through the nipples from the electrolytic bath, represent a delimitation to this current path. The nipples will thus have to be made sufficiently long so that the metal in the anode hanger does not melt. Alternatively, the metal part of the anode hanger may be provided with some kind of cooling device, such as cooling fins or the like.

Claims

1. Anode hanger for fixedly holding carbon anodes (2) in cells for the electrolytic production of aluminium according to the Hall-Heroult process, comprising an upper part (3) of metal (aluminium, copper, etc.), which is connected to an anode bar and a lower steel part (4) which is connected to the upper part (3), by means of a cast connection and which consists of a cross bar (7), provided with nipples (5, 15), onto which the carbon anode is connected, characterized in that the lower steel part (4) is connected to the upper metal part (3) by casting the metal around the cross bar (7) which is provided with holes (6) or recesses in the cast connection area.

2. Anode hanger for fixedly holding carbon anodes (2) in cells for the electrolytic production of aluminium according to the Hall-Heroult process, comprising an upper part (3, 11, 20, 21) of metal (aluminium, copper etc.) or steel, which is connected to an anode bar, and a lower steel part (4) which is connected to the upper part (3, 11, 20, 21) by means of a cast connection, and which consists of a cross bar (7,10,18,22) provided with nipples (5, 15) onto which the carbon anode is connected, characterized in that the cross bar is made of a box type construction (10) and that the upper ends of the nipples (12) and the lower end of the metal rod (11) extend through said box type construction (10) and are embedded in the cast metal (26) inside the box type construction (10).

3. Anode hanger according to claim 2, where in the upper part (21) is made of steel and is not used as a conductor, and is fixedly mounted to the box type construction (22), the upper part of the nipples (24) being embedded in the cast metal (26) inside said box type construction, and the electric current is supplied through flexible conductors which are connected to the cast metal (26) inside said box type construction (22).

4. Anode hanger according to claim 2 or 3, characterized in that the nipples (12) and the end of the metal rod (11) which are embedded in the metal (26), are provided with recesses (13).

5. Anode hanger according to claims 3 and 4, characterized in that the nipples (12), before the casting is performed, are welded to the box type construction (10, 22) with a welding seam on the outside (14) and/or the inside (15) of said box type construction.

6. Anode hanger according to claims 3,4 and 5, characterized in that the anode hanger is provided with two or more anode carbon bodies (2), whereby brackets (17) are provided on the top side of the box type construction (18) on each side of the upper metal rod (20).

7. Anode hanger according to claim 3, characterized in that the anode hanger is provided with two or more carbon anodes bodies, whereby brackets (25) are provided on the top side of the box type construction (22), or each side of the upper metal part (21).

8. Anode hanger according to any one of the preceeding claims, characterized in that the steel part of the anode hanger, which is embedded in the metal, before the casting is performed, electrolytically or by other means, is provided with a metal layer such as copper or silver.