Laminated carbon cathode for cells for the production of aluminium by electrolytic smelting

A laminated carbon cathode includes two layers of carbon blocks, i.e. and the upper layer of graphite of graphitised carbon, and a lower layer of a cheaper anthracite carbon. The two layers are so displaced with respect to one another that there are no vertical seams leading straight from the upper surface of the carbon cathode to the its underside. Dividing the cathode into two horizontal layers is combined with the embedding of current-carrying steel conductors in precise grooves between the layers. In order to capitalize on the good electrical conductivity of aluminum, an aluminum extension is friction-welded to each steel conductor as close to a shell enclosing the cathode while at the same time a collar is formed which provides an air-tight seal at the point where the cathode bar enters the side of the shell. The proposed arrangement facilitates a very practical and simple check on dimensional deviations in the carbon blocks/cathode bars, and of fitting accuracy, by the visual inspection of seam tolerances and the displacement of axes during the lining operation.

BACKGROUND OF THE INVENTION 
This invention relates to a laminated carbon cathode for use in the 
production of aluminum by electrolytic smelting. 
A cell, or pot, for the production of aluminum by electrolytic smelting 
usually includes of a rectangular, low steel shell. The bottom and sides 
of this shell are, on the inside, lined with heat-insulating refractory 
bricks. On the high temperature side, that is on the inside of the heat 
insulation, the shell has a carbon lining. This lining is in the form of a 
shallow vessel which holds the bath and the aluminum that is precipitated 
during smelting. Inside the carbon lining there are steel bars, so-called 
cathode bars, to provide the electrical connection between the carbon 
cathode and external busbars. 
The bath used for the electrolytic smelting of aluminum has a temperature 
of around 1000.degree. C. and is aggressive. This makes the greatest 
demands on the lining of the smelting vessel, while at the same time, the 
bottom must be a good conductor of electricity. A large number of 
compounds e.g. diodes, nitrides and carbides, have been tested as lining 
materials, but the choice is still dominated by various types of carbon. 
The selection of carbon materials for cathodes must take into account price 
and resistance against impregnation/penetration by compounds in the bath. 
Decisive for selection is the life of the cathode and the voltage drop 
through it. 
It has now been found that a more or less graphitised cathode exhibits a 
higher resistance against impregnation and penetration by bath and metal, 
while at the same time its electrical conductivity is better than that of 
traditional carbon products on an anthracite base. 
In many respects, electrodes of pure graphite would be preferable, but 
production capacity and price preclude a general adoption of pure graphite 
cathodes. 
Carbon linings are built up of carbon blocks placed alongside one another. 
They are bonded together by various types of adhesive or tamping paste 
which is pressed into the seams (slots) between the blocks. 
It is these seams which are the weakest element in the carbon lining. The 
final curing, or hardening, of these seams takes place during the starting 
of the cell, and it is difficult to achieve optimum heat treatment. The 
tamping paste also contains volatile substances, with the result that the 
paste in the slots, after the thermal treatment during the start of the 
cell, tends to shrink and become porous, and more permeable than the rest 
of the carbon lining. 
Bath and molten metal can penetrate through faulty slots between the carbon 
blocks, impairing the insulating properties of the refractory lining and 
attacking the cathode bars. When a pot produces aluminum with unwanted 
iron and silicon content, this is a warning that the cell is reaching the 
end of its operating life. 
A further process which can help to reduce the operating life of a cell is 
the oxidation of the cell's carbon side-lining caused by air entering 
through the holes in the side of the steel shell for the cathode bars.

DETAILED DESCRIPTION OF THE INVENTION 
The invention concerns a laminated carbon cathode for the production of 
aluminum by electrolytic smelting in that the carbon cathode is divided 
into two horizontal layers 1 and 2, of carbon blocks 5 and 6 made of 
different types, with a horizontal seam 3 between the layers of carbon 
blocks and at the same level as cathode bars 4. There are two cathode bars 
4 in each whole block, and the carbon blocks in the two layers are so laid 
that the vertical slots between the blocks in each layer are displaced or 
so staggered horizontally so that an upper seam 7 and a lower seam 8 are 
disposed on respective sides of each cathode bar 4. 
In a preferred embodiment of the invention, the carbon blocks in the upper 
layer 1 consist of graphite or graphitised carbon, while the blocks in the 
lower layer 2 consist of carbon blocks on an anthracite base. 
This arrangement reduces the quantity of the more expensive carbon blocks. 
Further, the staggering of the seams gives greater security against 
penetration of bath and molten metal in that there are no longer any 
vertical seams leading straight down from the upper surface of the carbon 
cathode to the refractory lining. In addition, the path is longer because 
of the horizontal seam between the upper and lower carbon layers. 
To derive the full benefit of the invention it is necessary to use an 
expedient adhesive with a high coke yield after heat treatment. In a 
preferred embodiment, this adhesive consists of a finely dispersed carbon 
aggregate and a furan-based or phenol-based resin, as for example 
described in European patent document No. EP 0075 279 B1. 
It is of course possible to use cathode bars of various cross sections, but 
in a prefered embodiment round cathode bars 4 have been selected, these 
being laid in the middle between the lower layer of carbon blocks 2 and 
the upper layer of carbon blocks 1, there being semicircular grooves in 
the upper carbon blocks 5 and in the lower carbon blocks 6. A circular 
cross section is efficient for electrical conductivity, while the circular 
surface provides good contact with the carbon lining under normal 
operating conditions. 
The choice of round cathode bars permits the friction welding, by known 
methods, of the cathode bar to an aluminum extension 10 which, once the 
cathode bar is in place, can be welded to the external aluminum busbar 
system which connects the cells together. Using aluminum as an electrical 
conductor as far as possible up to the cathode bar will reduce the voltage 
drop, and thus the total energy loss. 
The loss through the weld is lower than that through a screw connection, 
and furthermore it does not deteriorate with time. Also no subsequent 
tightening is necessary. 
In a preferred embodiment of the cathode bar, a collar 9 will automatically 
be formed by the welding operation, and this is used as a sealing flange 
against the side wall in the cathode shell where the cathode bar enters 
the side of the shell. This obviates the necessity for more costly and 
impractical separate sealing arrangements on the outside of the steel 
shell, for example, conventional welded-on stuffing box arrangements. 
Cathode bars expand considerably lengthwise when they are heated to a 
normal operating temperature, around 900.degree. C. It is therefore 
necessary to divide the cathode bar 10 into two parts, with a space 11 to 
allow for expansion away from the side wall, which would otherwise be bent 
outwards, and thus weakening the structure. 
The fitting of cathode linings is time-consuming, and results in a 
production loss if relining takes place in the cell in situ in the 
potroom. This invention simplifies the laying of carbon blocks and cathode 
bars in the cathode shell. Further, this system permits more extensive use 
of standard block dimensions, and thus better utilization of the carbon 
blocks when they are machined.