Cathode shell structure
The present invention relates to a cathode structure for an alunninium electrolysis cell.
Electrolysis cells for aluminium production based upon the Hall-Heroult principle commonly have prebaked anodes in its upper part and a cathode structure in its lower part. The cathode structure comprises mainly an outer steel shell provided with vertical stiffeners and horizontal beams at its outside which are made out of steel too, the cathode structure further being provided with layers of protective and insulating material at Its inside. In the mainly horizontal bottom part of the cathode structure there is commonly arranged electronic conducting carbon blocks. The cathode structure when In operation, contains a molten aluminium metal pad and above that molten bath material having temperatures that can be approximately 970° or even higher.
The main feature of the outer steel shell including Its stiffeners and beams is to maintain the geometric and dimensional configuration of the cathode structure during its lifetime of operation. Due to continuous chemical swelling of the cathode and lining as well the high temperatures involved and the effect this has on the properties of lining and steel components involved, several attempts have been carried out to make the steel shell structure and the configuration of stiffeners able to withstand this influence in a long lasting manner.
For instance, appropriate cooling may relieve some of the harmful effects of the temperature in the steel shell to prevent aging of the steel and collapsing of mechanical properties like strength, elasticity, etc. In the prior art there are examples of arrangements for natural cooling by designing the stiffening structure in a manner that allows surrounding air to pass close to the shell structure.
US 4087345 disclose a cathode structure designed to be cooled by natural cooling and where there are arranged ducts or passages between vertical stiffeners outside the cathode shell for allowing an upward draught of air. Horizontal beams arranged as a part of the stiffening structure are arranged onto the outside of the vertical stiffeners.
Due to the high thermal loads in modern electrolysis cells with amperages approaching 400 kA or even more, thermal expansion and twisting or torsion based deviation of the vertical members may cause insufficient structural stability of the steel shell.

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US 4,322,282 disclose a tank for an electrolytic cell provided with horizontal movable hollow sections secured to the sidewalls of the metal tank. The sections bend inwards as a result of the temperature gradient across them such that they counter a pressure on the sidewalls due to the dilation of the cell contents, thus acting as a thermo-spring. There is provided holes in opposing horizontal walls of the hollow section so as to reduce thermal conduction within the sections and thereby maintain the temperature gradient therein. The circulation of air which results, helps further to achieve and maintain the temperature difference.
However, due to the rather small holes placed in the two opposing horizontal walls, the hollow section will restrict heavily the natural upward draught along the steel shell. Simulations show that flow inside the hollow section will be partly vertical and partly circulating, resulting in less heat transport from the hollow section. In particular, flow close to the shell will be partly directed downward and give scarce cooling in this region. This is further illustrated in Fig. 3.
In accordance to the present invention, at least one horizontal web is arranged in contact with the steel shell without intermediate vertical members that may, as a result of thermal loads, alter their geometric configuration. To reduce or eliminate the negative effect of arranging horizontal webs in direct contact with the steel plates (shell), there are made provisions for adequate cooling of these webs and the plates itself. With the present invention there is achieved a stable and cost effective solution with regard to the negative phenomenon of ageing and geometrical/dimensional deviation in the structural components of the pot shell. The web has apertures facing the shell.
These and further advantages can be achieved in accordance to the invention as described in the accompanying patent claims.
In the following, the invention shall be further described by examples and figures where:
Fig. 1 discloses a three dimensional view of a quart pot shell structure without the
inner walls for better visibility,
Fig. 2 discloses a three dimensional view of an end structure of the pot shell shown
in Figure 1 without end wall and deck plate, Fig. 3 discloses air flow inside one hollow section of prior art type.

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Fig. 4 discloses air flow in accordance with the present invention, through a lower and upper web.
As can be seen from Fig. 1, there is shown in a 3 D view a quart of a pot shell structure 1 where the steel plates are not shown. One half of an end wail 2 and correspondingly one half of a side wall 3 as well as a quart of a bottom 4 haven been disclosed. The end wall comprises as its main constituents a deck plate 7, vertical stiffeners 8, 8', 8", 8'", one horizontal web 9 and a reinforcing bracing flange 10.
The side wall 3 comprises a deck plate 11, vertical structural elements 6, 6', 6", 6'", 6"", 6"""^ 6""" which in fact is a part of a cradle structure 12, together with corresponding horizontal beams 5, 5', 5", 5'", 5"". 5'"", 5""" that are fixedly interconnected with the said vertical structural elements. Further, the side wall 3 comprises a plurality of vertical stiffeners 13, 14,15 which additionally acts as cooling fins.
Figure 2 discloses in a 3 D view the end wail structure 2 as partly shown in Fig. 1, here in its full extension and where the deck plate has been removed. As indicated in Fig. 1, the reinforcing bracing flange 10 represents the outermost element of the end wall structure. The bracing flange is further attached to the side walls and the vertical structural element 6""" of the cradle structure closest to this end. Inside the bracing flange 10 there is arranged the mainly horizontal web 9 which outer periphery fits the shape of the bracing web,
As indicated in Fig. 1, the end wall structure further comprises vertical stiffeners 8, 8', 8", 8'", 8"", 8*"", 8""". The number of stiffeners is a function of the shell width and anticipating inner forces. These stiffeners are integrated in the horizontal web 9 by any appropriate manufacturing method. For instance the web 9 can have slots adapted to the stiffeners that are subsequently fixed to the web, for instance by welding. One other method can be to manufacture the web 9 in appropriate pieces or plates that abut the stiffeners and which subsequently are fixed to the stiffeners,
In this embodiment the vertical stiffeners are provided with a flange at their outer side. A part of such a flange is indicated at 8a' in stiffener 8", One purpose of this flange is to enhance bending and torsion stiffness in the stiffener with regard to occurring loads. The inner side of the stiffener is fixed to the steel plate(s) of the pot shell by appropriate means, for instance welding. Similarly, the stiffeners are connected to an extension of the bottom plate of the shell (not shown), to enhance stiffness in the end wall.

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To secure sufficient ventilation along the pot shell, based upon upward draught, the horizontal web 9 is provided with several holes or apertures 16, 16', 16", 16'", 16"", 16""',
16""", 16 These apertures are made sufficiently wide and deep to reduce the
presence of the horizontal web with regard to ventilation. Preferably the apertures are made semi-circular or semi-elliptical or polygonal and are represented by discrete cut¬outs at the side edge of the web. The bridging or protruding parts of the web between neighbouring recesses, are connected to the pot shell by appropriate means, for instance welding. In the Figure 2, there can be seen vertical fins 17, 17', 17", 17'", 17"", 17'"",
17 , 17'""" passing through corresponding apertures. The purpose of the fins is primary
to enhance cooling of the pot shell by conductance, but additional effects are guiding the flow of air as well as additional stiffening of the pot shell plates. The fins are advantageously fixed to the pot shell by appropriate means such as welding.
It should be understood that the end wall structure may have two horizontal webs of the type 9 that are spaced apart (not shown).
An air flow pattern in accordance with such arrangement is shown in Fig. 3. It can clearly be seen that the air flow is directed upwards along the shell wall and through the ventilation holes or apertures of type 16.

Claims
1. Cathode structure for an electrolysis cell, preferably a cell for production of aluminium according to the Hall-Heroult process, said structure comprises a shell with vertical stiffeners (8) and horizontal webs (9) at its outside, and further several cradle structures (12) consisting of interconnected vertical structural elements (6) and horizontal beams (5), where at least one horizontal web (9) is provided with apertures (16) arranged towards the shell, characterised in that
the horizontal web (9) has bridging parts between the apertures (16), the bridging parts are in contact with the shell and extend in a horizontal direction to the sides of the said vertical stiffeners (8), whereby the apertures are located between the stiffeners and further allow convective flow rising at the hot shell wall, the apertures being sufficiently wide and deep to reduce the presence of the horizontal web with regard to ventilation.
2. Cathode structure in accordance with claim 1,
characterised in that
the apertures (16) are represented by discrete cut-outs at the side edge of the web having bridging parts (18) between them, the web being connected to the shell via said bridging parts, where the apertures located at the middle section of the horizontal web are larger than that at the two end sections thereof.
3. Cathode structure in accordance with claim 1,
characterised in that
the apertures (16) are semi-circular or semi-elliptical or polygonal.
4. Cathode structure in accordance with claim 1,
characterised in that
cooling fins (17) are arranged at the outside the shell and runs through the apertures (16).
5. Cathode structure in accordance with claim 1,
characterised in that
a bracing flange (10) is arranged outside the horizontal web (9.

6. Cathode structure in accordance with claim 5,
characterised in that
the plane of orientation of the bracing flange (10) is mainly vertical.
7. Cathode structure in accordance with claim 5, in particular an end wall
construction thereof,
characterised in that
the bracing flange is connected to vertical structural elements (6) of a cradle (12)
close to this end.
"8. Cathbd e "structure Tri accbrdance withciaimS;" characterised in that
the bottom part of the vertical stiffeners (8) are connected with an extension of the bottom plate of the shell.
9. Cathode structure in accordance with any preceding claim 1-8,
characterised in that
the structural elements are made out of steel materials.
10. Cathode structure in accordance with claim 9,
characterised in that
the said elements are connected to each other by welding.
11. Cathode structure in accordance with claim 1,
characterised in that
it has two horizontal webs (9).
Dated this 05/03/2012 ^^
HRl^KESfCRk3f'<^krjDHURY. ... OFREMFiyf^SAGAR ATTORNEY FOR THE ^RLICANTS