Cathode pot for an aluminum electrolytic cell

A cathode pot for the electrolytic production of aluminum has composite bodies which line the sidewalls of the steel shell and are bonded to the carbon floor elements such that a seal is formed. The inner side of the said composite is of carbonaceous material and the outer side of a hard ceramic material. This conducts the electric current poorly but conducts heat well, is resistant to molten aluminum and the prevailing atmosphere of the process, and has a coefficient of thermal expansion comparable to that of carbon. The intimate joining together of the two sides permits almost unhindered flow of heat from inside to outside. The composite bodies are manufactured in layers, mechanically compacted and baked or graphitized in packing powder.

BACKGROUND OF THE INVENTION 
The invention relates to a cathode pot of a cell for producing aluminum by 
the fused salt reduction process having an outer steel shell, an 
insulating base layer and on this insulation carbon blocks which enclose 
iron cathode bars, such that the carbon pot contains the melt of 
electrolyte and aluminum, and relates too to a process for manufacturing 
the lining of the pot sidewall. 
The fused salt process for producing aluminum by electrolytic reduction of 
aluminum oxide involves dissolving the latter in a fluoride melt which is 
made up for the greater part of cryolite. The cathodically precipitated 
aluminum collects under the fluoride melt on the carbon floor of the cell. 
The surface of the molten aluminum forms the cathode. Dipping into the 
melt from above are anodes which in conventional processes are made up of 
amorphous carbon. As a result of electrolytic decomposition of the 
aluminum oxide, oxygen is formed at the carbon anode with which it reacts 
to form CO.sub.2 and CO. 
The electrolytic process takes place in a temperature range of about 
940.degree.-970.degree. C. During the course of the process the 
electrolyte becomes depleted of aluminum oxide. At a lower concentration 
of 1-2 wt % aluminum oxide in the electrolyte the anode effect occurs 
whereby the voltage increases for example from 4-5 V to 30 V and higher. 
Then at the latest the aluminum oxide concentration must be increased by 
feeding additional alumina to the cell. 
In present day smelter operations the addition of alumina is made almost 
exclusively by so called point feeding or by central feeding. The 
previously conventional periodic external feeding for example every 3-6 
hours has been replaced by feeding at intervals of only some few minutes. 
These changes in cell feeding lead to elimination of the protective 
sidewall layer of solidified electrolyte at the metal level. This layer 
normally covers the place where the carbon floor blocks meet the sidewalls 
of the pot and, depending on the form of external feeding is formed by 
sediments. In the absence of that layer the sidewalls of the pot are 
therefore exposed more to erosion and corrosive attack by the molten 
charge in the pot. Consequently the useful service life of the pot is 
markedly reduced. 
The following are the main reasons for the wearing away of the sidewalls of 
the pot. 
Movement of metal and electrolyte which contain abrasive particulate 
solids, and local turbulence produced by magneto-hydrodynamic effects. 
Corrosion of the carbon by the atmosphere produced in the process. 
Passage of the direct electric current through the sidewalls. 
Proposed in the British Pat. No. 814 038 is to line the walls of the 
reduction pot with thin ceramic tiles e.g. tiles of a material comprising 
silicon carbide bonded together with silicon nitride. Tiles of 
kaolin-bonded silicon carbide and other refractory materials can be 
employed for the same purpose. Some of the linings made up of such tiles 
feature a thermally insulating layer e.g. of alumina between the tiles and 
the sidewall of the steel shell. The floor of the pot is as before fitted 
with carbon blocks with the gaps between them filled with a rammed mass of 
non-baked carbon. The disadvantage of these tiles, which mostly contain 
silicon carbide as the main component, is that the binder used in them is 
attacked by the molten electrolyte. Also of disadvantage is that the tiles 
can usually not be bonded close enough to each other to prevent the molten 
electrolyte penetrating the gaps in time. 
Described in the U.S. Pat. No. 3,256,173 is a process for manufacturing the 
sidewalls of a reduction pot for production of aluminum by the 
electrolytic fused salt reduction process, in which silicon carbide powder 
mixed with powdered coke and pitch is employed. The lining of the walls is 
performed by ramming i.e. compacting this mass into place. The ramming 
mass described in U.S. Pat. No. 3,256,173 overcomes the disadvantages of 
preformed ceramic tiles which are bonded together, but it is a poor 
thermal and d.c. electrical conductor. 
The sidewalls of cathode pots made of carbon or silicon carbide feature the 
following basic properties: 
TABLE I 
______________________________________ 
Property Carbon SiC 
______________________________________ 
Thermal conductivity 
excellent very good 
Electrical conductivity 
excellent low 
Corrosion resistance (gases) 
moderate good 
Wear resistance moderate very good 
Ease of shaping easy difficult 
Resistance towards liquid Al 
neutral neutral 
Resistance towards molten 
neutral contaminating 
electrolyte materials 
______________________________________ 
SUMMARY OF THE INVENTION 
The object of the present invention is to develop a cathode pot of a fused 
salt electrolytic cell for the production of aluminum having an outer 
steel shell, a layer of insulation on the floor and on this insulation 
carbon floor elements surrounding iron cathode bars and a process for 
manufacturing the lining for its sidewalls, wherein the disadvantages of 
the materials used up to now for the sidewall are overcome. 
With reference to the device this object is achieved by way of the 
invention by prefabricated composite bodies which line the sides of the 
steel shell, are joined forming a seal to the carbon elements of the floor 
and are such that, 
their inner side is of carbonaceous material and contains a fraction of 
binder, and 
their outer side is of a hard ceramic material which is a poor electrical 
conductor but a good thermal conductor, is resistant to molten aluminum 
and the process fumes, and has a coefficient of thermal expansion 
comparable to that of carbon, both sides being intimately joined and heat 
can flow almost unhindered from inside to outside. 
Trials with cathode pots having sidewalls of layer type composite bodies 
revealed the following results: 
Due to the good thermal conductivity of the composite, a layer of 
solidified electrolyte is formed on the inside of the pot. Heat transfer 
from the carbon layer to the ceramic layer is not diminished, as the bond 
between these layers remains intact. 
The electrolyzing d.c. current does not pass through the composite, as the 
ceramic layer is a poor electrical conductor. 
The ceramic layer of the composite is resistant to corrosive attack by the 
fumes produced in the process. 
Any abrasive action of the moving bath and solid particles in it can effect 
at most the carbon layer; at the latest when the ceramic layer is reached, 
no further erosion takes place. As a rule, however, pores formed in the 
carbon layer become filled with solidified electrolyte which prevents 
further attack. 
The aluminum produced is of good smelter quality i.e. the bath does not 
take up any undesired impurities. 
When installing the composite blocks the carbon part can be easily shaped 
by mechanical means, which for example permits them to be bonded to the 
carbon elements of the floor. 
It was found, therefore, that a cathode pot with sidewalls of composite 
bodies according to the invention exhibit all the advantages of materials 
known to date, without having to accept their disadvantages to any 
significant extent. The outer layer of the composite in the pot. i.e. the 
layer facing the steel shell is preferably of silicon carbide, silicon 
carbide bonded with silicon nitride, highly sintered aluminum oxide or 
ceramics with a high concentration of aluminum oxide. On heating from room 
temperature to the operating temperature of the aluminum fused salt 
electrolytic process these materials exhibit a coefficient of thermal 
expansion comparable to that of carbon, regardless whether the carbon is 
in the form of amorphous carbon, semi-graphite or graphite. 5 to 15 wt % 
binder, in particular pitch, can be mixed into the ceramic materials. 
The inner layer of the composite in the cathode pot is preferably of 
amorphous carbon, semi-graphite or graphite containing 10 to 20 wt % 
binder, in particular pitch. 
Apart from the preferred pitch, other substances employed as binding agents 
are formaldehyde resins, multicomponent adhesives which are commercially 
available or a mixture of epoxy resin and tar. Any differences in 
expansion or contraction occuring with the different materials during 
baking can be prevented by modification of the composition (ratio of 
binder to dry components, granulometry). 
The composite bodies, preferably slab or tile shaped, are made as large as 
possible in order to eliminate joints as much as possible. Usefully they 
extend in one piece over the whole height of the pot. The composite bodies 
are, for example, 100-200 mm thick depending on the construction of the 
pot; the thickness of the two layers can usefully be about the same. 
As the corrosion resistance of carbon towards the fumes produced in the 
process at the operating temperature is not very good, the composite is 
usefully arranged such that the carbon of the composite blocks in the pot 
do not project above the surface of the molten electrolyte. The carbon is 
therefore protected by a layer of solidified electrolyte; in the upper 
part of the pot only ceramic material comes into contact with the 
surrounding atmosphere. A slab shaped composite body can be designed with 
steps from the start, or its easily machinable carbon layer can be removed 
just before or after installing the composite body in the pot. 
With respect to the process for manufacturing the composite body used in 
the cathode pot, the object is achieved by way of the invention in that 
first at least one layer of a powder material is placed in a mold and 
mechanically compacted; then at least one layer of the other powder 
material is introduced into the same mold and mechanically compacted. The 
compacted composite body is then embedded in a filler type powder and 
baked or graphitized at a temperature of 1000.degree.-2500.degree. C.; 
finally the surrounding filler powder is removed. 
The mechanical compaction takes place usefully by shaking and/or pressing 
or by ramming. 
At least one of the layers of powder can be introduced into the mold in 
stages and compacted. 
Depending on the process parameters, in particular the temperature, the 
carbonaceous material is baked or graphitized in a conventional manner to 
amorphous carbon, semi-graphite or graphite. 
The cathode pot according to the invention with the composite body as 
sidewall provides the necessary good thermal conductivity required for the 
solidification of electrolyte material, while on the other hand the 
electrolyzing current can not flow through the sidewall.

DETAILED DESCRIPTION 
The slab shaped composite body shown in FIG. 1 is made up of a layer 10 of 
carbonaceous material and a layer 12 of silicon carbide. The layer 10 of 
carbonaceous material contains 15 wt % moderately hard pitch in addition 
to anthracite and pitch coke. 
In the version shown in FIG. 2 the slab shaped composite body of FIG. 1 
features two opposite-lying, rounded side faces. On fitting these together 
a better seal can be achieved between the individual slabs. 
In the case of the versions shown in FIGS. 1 and 2 it is of no consequence 
whether the silicon carbide or the carbonaceous material is put into the 
mold first. 
In the case of the composite body shown in FIG. 3 having one layer 10 of 
carbonaceous material and one layer 12 of ceramic material a slope 16 is 
provided in order that the carbon is not exposed to the atmosphere of the 
cell. 
FIG. 4 shows a version of a composite body with slope 16, in which case the 
mold is to a certain extent filled with carbonaceous material and ceramic 
material in a dissimilar manner, and then compacted; subsequently the mold 
is filled up completely with the other material and then compacted. Thus 
the various conditions prevailing in the operation of the pot can be taken 
into account. 
FIG. 5 shows a composite body installed in a reduction cell pot; the 
composite features a carbonaceaous layer 10 and a refractory layer 12. The 
lower part of the steel shell 18 is lined with a layer of insulation 20, 
in the present case firebrick. Situated on top of this layer of insulation 
are the carbon elements 22 of the floor which surround the iron cathode 
bars 24. The composite body according to the invention which has its 
refractory layer 12 directly against the sidewall of the steel shell 18 is 
joined to the carbon floor elements 22 by means of a ramming mass 26. 
During the operation of the cell a well known sidewall or ledge of 
solidified electrolyte, which is not shown here, forms along the layer 10 
of carbonaceous material and the ramming mass 26, and extends down to the 
carbon floor elements 22. If this side ledge should be defective or form 
only incompletely, then the carbon layer 10 will be attacked there, 
forming holes in it at most however until the layer 12 of refractory 
material is reached. The deeper the localized attack of the carbonaceous 
layer 10 the greater the probability of a self-healing effect i.e. that 
the electrolyte solidifies in the hole because of the good thermal 
conductivity of the silicon carbide. 
The layer 12 of refractory material not only acts as a barrier if the layer 
10 of carbonaceous material facing the electrolyte is removed locally by 
erosion or corrosion but also, because of its poor electrical 
conductivity, prevents the steel shell 18 taking on the cathode potential. 
The version shown in FIG. 6 differs from that shown in FIG. 5 only in three 
points: 
The sloping layer 10 of carbon does not extend up to the same height as the 
layer 12 of ceramic material. As a result the layer 10 of carbonaceous 
material is attacked less by the gases produced in the cell. 
The composite body according to the invention is bonded to the carbon 
elements of the floor by an adhesive layer 28. 
The layer 10 of carbon is much thinner than the layer 12 of ceramic 
material.