Absorption of magnetic field lines in electrolytic reduction cells

The vertical magnetic force lines in electrolytic reduction cells, in particular in cells for the production of aluminum, are absorbed by a device comprising the steel pot shell and a covering which covers the upper part of the cell and is magnetically connected to the steel shell. The effect of this covering is uniform over the whole of the cell. The covering is made of a magnetic material which conducts well, and it can for example be in the form of a hood, frame or cross bars.

BACKGROUND OF THE INVENTION 
The invention concerns a device for the absorption of vertical magnetic 
field lines in electrolytic cells, in particular in cells for the 
production of aluminum. 
To win aluminum electrolytically from aluminum oxide, the latter is 
dissolved in a fluoride melt made up mainly of cryolite (Na.sub.3 
AlF.sub.3). The aluminum deposited at the cathode gathers under the 
fluoride melt on the carbon floor of the cell where the liquid aluminum 
forms the cathode. The level of liquid aluminum in the cell rises by about 
1.5-2 cm per day and is removed from the cell, generally once a day, using 
a suction device. 
In the conventional process anodes made of amorphous carbon dip into the 
melt from above and supply direct current to the fluoride melt. As a 
result of the electrolytic decomposition of the aluminum oxide, oxygen is 
formed at the anodes and combines with the carbon of the anodes to form CO 
and CO.sub.2. When a carbon anode has been consumed, it is replaced by a 
new one. 
The production of aluminum via molten salt electrolysis takes place in a 
temperature range of about 940.degree. to 975.degree. C. 
The electrical currents employed for the electrolytic cells (often called 
pots for short) which are connected in series, are usually of the order of 
100-200 kA (kiloampere). At such currents the surface of the liquid 
aluminum on the floor of the cell is no longer horizontal. Forces due to 
magnetic fields and horizontal components of electrical current act on the 
molten metal causing pronounced fluctuations in level and also movements 
which can be of the order of several centimeters. 
Both changes in level and movements in the molten metal are, for various 
reasons, disadvantageous to the economics of aluminum production: 
(a) The distance between the anodes and the surface of the aluminum which 
forms the cathode must be kept excessively large, which means a greater 
voltage drop and therefore a greater consumption of energy. 
(b) The lining of the cell is subject to greater consumption or wear. Also, 
cracks or holes can result making premature replacement or repair 
necessary. The costs incurred are great as, in addition to the expense of 
labour and materials, there is also a loss in production. 
In view of this efforts have been made for a long time now to reduce these 
movements and changes in level of the liquid metal to a minimum or even to 
eliminate such effects completely where possible. 
The first efforts were aimed at achieving as uniform as possible 
distribution of current between the anodes and the cathode. On route from 
the carbon anodes to the carbon floor of the cell the electrical current 
flows first through the fluoride melt which forms the electrolyte and then 
through the liquid metal. The electrical resistance of the electrolyte is 
incomparably greater than that of the carbon or, in particular, that of 
the metal. It is therefore relatively easy to keep the flow of current 
vertical through the electrolyte. In liquid metal on the other hand 
besides the desired, and for the electrolysis necessary, vertical 
components of electrical current there are also undesired horizontal 
components of current. 
Also, the busbars which conduct very large current produce magnetic fields. 
The vertical flux lines of these magnetic fields create electromotive 
forces running in the horizontal direction in the molten aluminum. 
The cells are usually constructed in a steel shell. The magnetic conductive 
material allows the interior of the pot to be screened partly from the 
magnetic fields produced outside the pot. 
In the German Pat. No. 1 083 564 an attempt is made to suppress and/or keep 
constant the vertical components of the vertical fields of current flowing 
uniformly and in the vertical direction over the whole of the surface of 
the cell. To this end the surface of the metal which acts as the cathode 
is matched to the anodes and as much as possible of the horizontal busbars 
arranged such that the surface area is as large as possible. 
From the German Pat. No. 1 143 032 it is known that the effect of the 
magnetic fields from the external conductors can be removed to a large 
extent by installing iron screening between the busbars and the pot. 
Although the heat produced in the pot can be influenced there are no 
indications that movement of the metal can be prevented. 
Finally, in the German Pat. No. 2 213 226 the magnetic fields at the sides 
and ends of the pot are influenced by the provision of additional magnetic 
conductors in the region of the pot. These magnetic conductors which run 
vertically are separate from each other and from the electrical system of 
the pot, and are situated in or on the pot wall between the layer of 
liquid metal and the busbars outside the pot. This is to say, they 
terminate in the magnetic, non-conductive carbon. 
All these above mentioned devices feature the disadvantage that they 
involve relatively extensive and expensive measures, and can not be 
implemented without re-building or modifying the pot. 
SUMMARY OF THE INVENTION 
The inventor therefore set himself the task of developing a device to 
absorb vertical magnetic fields in electrolytic reduction cells, whereby 
the said device would be simple in design and could be installed on 
existing reduction cells without interrupting production. 
This object is achieved by way of the invention in that the device 
comprises the steel shell of the pot and attached magnetically to it, a 
covering for the upper part of the pot made of a metal of high magnetic 
conductivity, whereby the screening effect is uniform over the whole pot. 
The magnetic attachment of the covering to the shell is of fundamental 
importance as the steel shell, which is a necessary part of all pots, can 
be used for a part of the magnetic screening and the whole screening is at 
the same potential. 
A ferromagnetic metal, in particular iron or steel, is employed for the 
screening. Although cobalt and nickel and their alloys could be used, for 
reasons of costs they are not considered in practice.

DETAILED DESCRIPTION 
Encapsulation of reduction cells is required increasingly today for reasons 
of work place hygene and for protection of the environment. In terms of 
the present invention, an existing fume hood of the conventional kind with 
central and side covering can be employed between the anode beam and the 
top of the anode, if it is magnetically connected to the shell of the pot 
and is electrically insulated from the anode conductor bars. The central 
covering can be connected directly to the shell and/or the side covering. 
In the case of cells with the centrally fed or so-called point feeding 
system the central cover between the series of anodes can be replaced 
wholly or partly by a container or silo or the likes of alumina. It is 
then understood of course that this silo must also be connected 
magnetically to the shell and/or side cover. 
In non-capsulated pots a coarse grid mesh can be provided between the tops 
of the anodes and the anode beam. This mesh is magnetically coupled to the 
steel shell of the pot, and contact with all parts at anode potential is 
avoided. For practical reasons, in particular because of the need to 
change anodes, the spacing of the mesh corresponds at least to the 
dimensions of the anodes being used. This mesh must also be strong enough 
to be able to withstand blows during the insertion and removal of anodes 
without suffering damage. 
It has been found particularly advantageous to provide a yoke which extends 
over the whole length of the pot above the space between the rows of 
anodes and runs horizontally at a level between the anode beam and the 
tops of the anode. If only one yoke is to be employed, this is usefully 
situated along a central plane between the rows of anodes. Two yokes can 
run side by side separated along this central plane. 
As with the rods of the mesh, the cross section of the yoke can be chosen 
at will. It can for example be round, rectangular or be some other form of 
solid or hollow section, sheet or plate. Preferably however, pipes are 
employed; these can have an outer diameter of 5-15 cm, in particular 7-10 
cm. The wall thickness is of the order of one to several centimeters and 
is limited by the strength required of the pipe. 
The yokes can have transverse components which may be of the same or 
different cross section. The transverse parts, which preferably run 
outwards at right angles, are designed such that they improve the magnetic 
screening but do not hinder operation of the cell e.g. feeding. Although 
these transverse arms of the yoke normally run in the same horizontal 
plane as the rest of the yoke they may lie at an angle of up to about 
45.degree. upwards or downwards. 
The yokes running the length of the pot can be replaced by others which 
extend over the whole width of the pot. These, usually single yokes, lie 
along the central plane between two neighboring anodes. They can, as 
required, be installed along all central planes between anodes, on each 
second, third or fourth plane etc. The number of yokes can be reduced to 
such an extent that the screening still takes place over the whole of the 
electrolytic cell. All the other details for the yoke running lengthwise 
e.g. height, transverse components and shape in cross section, also hold 
for the yoke running across the cell. 
All the described magnetic covers for the non-capsulated pot can be 
installed while the pot is under full production. The magnetic coupling to 
the steel shell, which can at the same time also be the means of 
mechanical fixing, is made releasably using bolts, clamps etc., or 
permanently by means of rivets, welding etc. 
The mesh or yokes of the invention can at the same time serve as the 
supporting frame for alumina silos or crust breaking devices, if installed 
on pots with central, transverse (U.S. patent application Ser. No. 
916,970) or point feeding systems. It is understood of course that such 
devices mounted on the screening mesh or yokes must be insulated from the 
parts of the cell at anode potential. 
Surprisingly, using the simple magnetic screening described here, e.g. with 
a single or double yoke running lengthwise above the space between the 
rows of anodes, improved efficiency can be attained in that at least 50 mV 
can be saved per pot, which leads to a corresponding reduction in the cost 
of producing primary aluminum. 
In summary, the arrangement proposed by the invention, in the simple case 
where the operation of the cell is not interrupted, brings the following 
advantages: 
(a) Lower energy consumption due to more stable operation of the cell 
(reduction in the occurrance of fluctuations). 
(b) Higher electrical yield due to lower temperatures and more stable 
operation. 
(c) Lower consumption of electrolyte. 
(d) Lower anode consumption. 
FIGS. 1 to 5 show a part of an electrolytic reduction cell. The steel shell 
12, which is lined with carbon 11 and thermal insulation 13 made of a heat 
resistant, insulating material, contains the fluoride melt 10 which 
constitutes the electrolyte. The deposited aluminum 14 on the floor of the 
cell is connected to the cathode, and therefore the surface 16 of the 
liquid aluminum is the cathode of the cell. Iron cathode bars 17 embedded 
in the carbon lining 11 transverse to the length of the cell conduct the 
direct electrical current from the carbon lining of the cell out of the 
cell at the sides. Anodes 18 made of amorphous carbon dip into the 
fluoride melt 10 from above and conduct the direct electrical current to 
the electrolyte. The anodes 18 are connected securely via anode conductor 
bars 19 and clamps 20 to the anode beam 21. The current flows from the 
cathode bars 17 of one cell to the anode beam or beams 21 of the next cell 
via conventional busbars which are not shown here. The current then flows 
via the anode bars 19, the anodes 18, the electrolyte 10, the liquid 
aluminum 14 and the carbon lining 11 to the cathode bars 17. The 
electrolyte 10 is covered with a crust 22 of solidified melt which is in 
turn covered with a layer 23 of aluminum oxide. In practice spaces form 
between the electrolyte 10 and the solidified crust 22. A crust of 
solidified electrolyte also forms at the sidewalls of the carbon lining 11 
to form a border there. This border determines the horizontal width of the 
bath comprised of liquid aluminum 14 and electrolyte 10. 
The distance d between the bottom face 24 of the anodes 18 and the surface 
16 of the aluminum, known as the interpolar distance, can be changed by 
raising or lowering the anode beam 21 by means of the hoists 25 mounted on 
columns 26. On setting the hoist 25 into operation all anodes are raised 
or lowered simultaneously. Furthermore, the anodes can be raised or 
lowered individually in a conventional manner by means of the clamps 20 on 
the anode beam 21. 
The busbars 30 outside the reduction cells conduct the electrical current 
to the anode beam of the next cell. 
The fume hood shown in FIG. 1 comprises sidewall covering 31 which is 
connected magnetically to the steel shell via 32 and is electrically 
insulated from the anode beams 19 via 33 and the central covering 34 which 
is likewise electrically insulated from the anode beams 19 via 33. The 
central covering 34 is connected magnetically to the shell 12 and/or to 
the sidewall covering 31. 
As has been mentioned already, the central covering 34 can be replaced 
partly or wholly by an alumina silo, but also by a suspended device or 
fume extraction pipe. 
In FIGS. 2 and 3 the magnetic screening is shown as a plate 35 bent over at 
both ends and connected to the shell via 32. This plate is positioned 
about midway between the head of the anode 18 and the anode beams 21. 
In FIGS. 4 and 5 the magnetic screening is provided by tubes 36 which 
extend over the whole width of the cell and are bent at both ends and 
result in a coarse grid mesh. The tubes are connected via 32 to the shell 
12 at both ends of the series of anodes and after each second anode. The 
pipes are again positioned midway between the top of the anode 18 and the 
anode beam 21.