Process and device for the production of aluminium by the electrolysis of a molten charge

The magnitude of the horizontal components of electrical current flowing through the aluminium on the cathode blocks of an electrolytic reduction cell are reduced by ensuring that the current transmitted decreases towards the edge of the electrolytic cell. This is achieved by making the contact resistance between the carbon lining and the cathode bars embedded in the carbon cathode blocks increase towards the edge of the cell. The result is that the tendency for the melt to bulge upwards and the stirring action in the electrolyte are considerably reduced.

The invention concerns a process and device for the production of aluminium 
via electrolysis using an electrolytic cell which has anodes dipping into 
a molten electrolyte under which, opposite the anodes and at a distance 
from them, there are cathode bars embedded in the carbon lining of the 
cell in which the liquid aluminium, produced in the process and lying on 
the carbon blocks under the anodes serves as the cathode. 
In the electrolytic production of aluminium from aluminium oxide (Al.sub.2 
O.sub.3) the aluminium oxide is usually dissolved in a fluoride melt 
which, for the main part, consists of cryolite (Na.sub.3 AlF.sub.6). The 
aluminium, which separates out at the cathode, collects on the carbon 
blocks of the cell under the fluoride melt; the surface of this liquid 
aluminium then acts as the cathode. Immersed into the fluoride melt are 
the anodes at which the oxygen ions from the aluminium oxide form oxygen 
which, in the conventional process using carbon anodes, combines with the 
carbon to form CO and CO.sub.2. 
The electrical conductivity of the fluoride melt is so poor compared with 
that of the liquid aluminium that the electrical current flowing in the 
electrolyte from the anodes in the direction of the cathodic carbon lining 
flows approximately vertically through the fluoride melt (i.e. the current 
density in the vertical direction in the electrolyte is in general uniform 
everywhere). This however does not apply to the carbon lining and the 
underlying cathode bars, which can for example be in the form of iron 
bars. The carbon lining of the floor, cathode bars and the contact 
resistance between these have different electrical properties with the 
result that the carbon lining transmits relatively more current at the 
edge of the cell than in the middle or centre of the cell. The current 
drawn from the bottom of the liquid aluminium is therefore still 
non-uniform even if there is a completely uniform supply of current to the 
upper surface of the liquid aluminium. The components of current density 
which are essentially horizontal and directed outwards in the liquid 
aluminium are very harmful. Together with the unavoidable, magnetic 
induction forces in the liquid aluminium, they produce forces which differ 
greatly from those in the electrolyte, causing the liquid aluminium to 
bulge upwards and producing a stirring effect. 
With this in mind the inventor set himself the task of eliminating the 
outward, horizontal components of electrical current flowing in the liquid 
aluminium and, in a process of the kind described at the beginning, 
achieving a uniform current density in the vertical direction also in the 
liquid aluminium. This object is achieved by way of the invention in that 
the electrical conductivity between the melt (and/or the cathode) and the 
cathode bars is reduced from the centre of the cell to the edge of the 
cell in such a way that the same current density per unit area flows 
through the electrolytically deposited aluminium to the cathode bars over 
approximately the whole width of the cell. The electrical contact between 
the cathode bars and the conventional, surrounding carbon lining should be 
made to decrease from the centre of the cell to its edge in such a way 
that the same current per unit area of carbon lining is transmitted from 
the deposited aluminium to the carbon lining over the whole width of the 
cell. This process is made possible by a device by means of which the 
electrical contact between the carbon lining and the cathode bars 
decreases from the centre of the cell to its edge and the contact 
resistance increases in the same direction. 
Thanks to this measure the forces in the liquid aluminium and in the 
electrolyte are equalised with the result that the above mentioned bulging 
and stirring action in the liquid aluminium is either markedly reduced or 
even eliminated. This is achieved by eliminating the outward, horizontal 
components of electrical current in the liquid aluminium. 
In accordance with another feature of the invention the electrical current 
flowing may be decreased stepwise from the centre of the electrolytic cell 
to the edge of the cell, with the length of the steps or regions providing 
electrical contact between the carbon lining and the cathode bars 
decreasing in the same direction, and the width of the spaces between 
these regions of electrical contact increasing. 
It is also within the scope of the invention to make the amount of 
electrical power transmitted decrease continuously from the centre of the 
cell to the edge of the cell by filling the space between the carbon 
lining and the conductor bars with a conducting medium, preferably by 
pouring cast iron into the space, and such that the said space is filled 
to a decreasing extent towards the edge of the cell. 
The carbon lining, which is usefully made up of individual prebaked carbon 
blocks, is connected discontinuously to the iron cathode or collector bars 
by a compressible mass which is a good electrical conductor or by cast 
iron, with the result that the areas where there is less contact produce 
an increase in the contact resistance towards the edge of the cell. The 
electrical current drawn by the carbon lining increases therefore towards 
the centre of the cell and decreases towards the edge of the cell, and can 
even decrease to zero current. By predetermining the size of the contact 
resistance the electrical current can be made flow vertically through the 
liquid aluminium. 
The prevention of horizontal, outward oriented components of electrical 
current in the liquid aluminium diminishes the amount of electrolytically 
produced aluminium which is re-oxidised by the anode gases in that, as 
explained above, the bulging and/or stirring of the liquid aluminium is 
considerably reduced or eliminated. Furthermore, since the increase in 
contact resistance also leads to a greater barrier to heat flow between 
the carbon lining and the cathode bars, the heat losses through the iron 
cathode bars are also reduced.

Above a steel container 1 lined with a thermally insulating layer 2 and 
carbon lining 3, and running in the lengthwise direction of the container 
1 there are provided anode beams 4 which rest on spindles 6 on columns 5 
and which can be raised or lowered in the dirctions "Y" by means of the 
cogged wheels 7 engaging in the spindle or spindles 6. 
Anode rods 9, which hang approximately vertical, are suspended from the 
anode beams to which they are secured by clamps 8 and have at their lower 
ends which point towards the container 1, anodes 10 made of amorphous 
carbon. The carbon anodes can be raised or lowered by means of the anode 
rods 9 in the clamps 8 to change or adjust the distance between the under 
side 11 of the anode and the inner surface 12 of the carbon lining 3. 
As illustrated in FIG. 2, in a steel container 1 with only one anode beam 4 
spanning the middle vertical position M and supporting the transverse beam 
13 which supports the conductor rods 9, the carbon lining 3 is penetrated 
across its whole width b by steel collector bars 14 the outward projecting 
ends 15 of which are connected via flexible conductors 16 to the busbars 
17 running along the side of the cell. 
In the space J inside the steel container 1 with its carbon lining 3, there 
is provided, a fluoride melt S consisting mainly of cryolite (Na.sub.3 
AlF.sub.6) which serves as the electrolyte for the production of aluminium 
by the electrolytic decomposition of aluminium oxide. 
The cathodically deposited aluminium A collects on the carbon lining 3; the 
surface 20 of this aluminium A then acts as the cathode in the 
electrolytic process, the anodes 10 being suspended above this surface 20 
and at a distance "d" from it. 
Direct current is supplied via the anode beam or beams 4 and the anode rods 
9 to the anodes 10, then through the electrolyte S, the liquid aluminium A 
and the carbon lining 3 to the cathode bars 14. The current then flows 
from the cathode bars 14 of the above mentioned cell E the anode beam of 
the next cell in series (not shown here). This pattern can be repeated as 
desired in accordance with the number of cells in the series. 
The electrolyte S is covered with a crust 30 of solidified fluoride melt, 
similarly a side freeze 31 forms at the sides 29 of the carbon lining. 
This side freeze 31 determines the horizontal expansion "f" of the bath of 
liquid aluminium A and electrolyte S. 
On the top crust 30 there is a layer 32 of aluminium oxide and between this 
crust 30 and the fluoride melt S there is a space 33. 
The distance "d" from the bottom face 11 of the anode to the aluminium 
surface 20, also called interpolar distance, can be changed by raising or 
lowering the anode beam 4 in the direction "Y" using the jacking device 6 
- 7; this takes place either simultaneously for all anodes 10 or by means 
of the clamps 8 for each anode rod 9 individually. 
As a result of attack by the oxygen released during the electrolytic 
process, the anodes 10 are consumed at their bottom face 11 by 15-20 mm 
per day, the extent depending on the type of cell. Simultaneously, the 
surface 20 of the liquid aluminium A in the cell E rises by 15-20 mm in 
the same interval of time. After the anode 10 has been consumed, it is 
replaced by a new anode 10. 
In practice a cell E is operated in such a way that after only a few days 
there are signs of various degrees of attack to the individual anodes 10. 
The anodes 10 must therefore be changed at different times stretching over 
a period of several weeks. FIG. 1 shows that in a cell E there are anodes 
10 which have been in service for different lengths of time. 
In the course of electrolysis the aluminium oxide content of the 
electrolyte decreases. At a lower concentration limit of 1-2 % Al.sub.2 
O.sub.3 in the electrolyte S, the so called anode effect occurs whereby 
the voltage increases suddenly from the normal value of 4 to 4.5 V to 30 V 
and more. At this point of time, at the latest, the top crust 30 must be 
broken and the Al.sub.2 O.sub.3 content increased by the addition of fresh 
aluminium oxide 32. 
Usually, in the normal operation of the cell E, aluminium oxide is added at 
regular intervals, even if the above mentioned anode effect has not 
occurred. In addition, each time the anode effect occurs, as described 
above, the crust 30 must be broken and the aluminium oxide concentration 
raised by addition of Al.sub.2 O.sub.3. In practice therefore the anode 
effect is always associated with extra cell supervision. The 
electrolytically deposited aluminium which collects on the carbon lining 3 
of the cell E is normally taken out of the cell E once each day using 
conventional equipment for example by means of a suction pipe 40. 
The electrical conductivity of the fluoride melt S is so low compared with 
that of the liquid aluminium that the electric current leaving the lower 
face 11 of the anode 10 flows through the fluoride melt S in an 
approximately vertical direction. If marginal effects are ignored then the 
vertical current density in the electrolyte S is consequently the same 
everywhere. 
The combination of the carbon lining 3 with the cathode bars 14 inside, and 
the contact resistance between these components, brings together elements 
which have different properties. Because of the difference in electrical 
properties, the carbon lining 3, which takes in the electric current from 
the liquid aluminium A, draws more power from the edge of the cell than 
from the middle M of the cell. If the upper surface 20 of the aluminium A 
receives a uniform supply of power and the withdrawal of power at the 
inner surface 12 of the carbon lining 3 is non-uniform, then a current 
must flow in the horizontal direction in the liquid aluminium to 
compensate for the deflection in the lower paths shown in FIG. 2. The 
current then leaves the anode 10 in an approxiately vertical direction and 
flows outwards in the liquid aluminium A i.e. towards the wall of the 
steel container 1. 
The horizontal, outward oriented components of electrical current flowing 
in the liquid aluminium A are very harmful. They combine with the magnetic 
forces which are induced by the neighbouring supply and are always present 
in the liquid aluminium and generate forces which are very different from 
those in the electrolyte S. The results of these differences in force are 
that the liquid aluminium bulges upward and/or is agitated due to a 
stirring action. Both effects impair the functioning of the cell 
considerably since they cause the aluminium A which has already been 
deposited to be brought near the anodes 10, where it is oxidised to 
Al.sub.2 O.sub.3 by the itinerant anode gases (CO.sub.2) with a consequent 
loss in production. 
This problem can be avoided as shown in FIGS. 3 and 4 by means of 
electrically conductive layers 42 and 46 which are provided between the 
carbon lining 3 and the cathode bars 14. The parts 43 of the layer 42 are 
of different length "n" in the direction transverse to the long axis of 
the cell, decreasing in length "n" towards the side wall of the steel 
container 1. The width "p" of the space 44 between the cast or compressed 
parts 43 increases accordingly in the same direction. The parts of the 
cathode bars 14 adjacent to these space 44 can be insulated from the 
carbon lining 3 by badly or non-conducting material 45. This insulation 
can be omitted in the middle of the cell and/or be completely insulating 
at the edge of the cell. 
If the lengths 43 of cast or rammed-in, electrically conductive material 
are shorter towards the outside or if, as shown in FIG. 4, the amount of 
cast iron or electrically conductive compressed mass 46 between the 
cathode bars 14 and the carbon lining 3 decreases in the same direction, 
then the contact between the cathode bars 14 and the carbon lining 3 
becomes poorer towards the edge of the cell. The accompanying increase in 
contact resistance towards the outside is determined according to the 
electrical grid calculation such that the amount of current drawn from the 
liquid aluminium A by the carbon lining 3 is the same everywhere in the 
cell E. 
FIG. 4 also shows that the carbon lining 3 is made up of individual blocks 
3a and 3b which fit together with negligably small gaps 47 between them. 
The cathode bar 14 is shown here as being in one piece although, as FIGS. 
5 and 6 show, it can also be in two parts. 
Whilst in FIG. 5 the cathode bars are incorporated in the carbon floor 48 
in the normal manner, in FIG. 6 the electrical contact between the carbon 
floor 48 and the cathode bars 14 becomes worse towards the edges of the 
cell. The electrical current, represented by the flux lines 49, flows 
through the anode rods 9, the carrier plates 50 and the body 10 of the 
anodes, the molten electrolyte S, the liquid aluminium A and the carbon 
floor 48 into the cathode bars 14 which conducts away the current, still 
represented by the flux lines 49. 
A data processing program, prepared for the EM 14 cell represented 
schematically here, enables the paths of the flux lines 49 to be plotted 
out. In FIG. 5 the flux lines in the liquid aluminium A are oriented 
strongly outwards i.e. towards the edge of the electrolytic cell, 
agitating the liquid aluminium and causing it to bulge upwards. 
In FIG. 6 on the other hand, where the electrical conductivity is reduced 
(possibly even to zero) towards the edge of the cell, for example as 
illustrated in the exemplified embodiment shown in FIGS. 3 or 4, then the 
lines of flux run approximately vertically through the liquid aluminium. 
Where the passage of electrical current between a carbon cathode block and 
cathode bar of conductive compressed mass should be prevented, the space 
between the block and the bar is filled with insulating material e.g. 
asbestos cord instead of cast iron. This way the above mentioned bulging 
and stirring action in the liquid aluminium are either markedly reduced or 
even eliminated.