Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a potshell for an
electolytic cell for the electrolytic reduction of alumini~um,
consisting of a bottom plate, side walls and end walls of
~teel plates, and a reinforcing structure surrounding these.
Experience has shown that in electrolytic cells,
or pots, which are operated at high amperages, electromagnetie
forces are produced which can disturb operations. These
disturbances usually start to appear around currents of 40-50
~A. when even higher currents are used , for example 100-200
kA, the difficulties can become so great that it is impossible
to aehieve satisfaetory operational eonditions unless eertain,
and usually eostly, design provisions are made. These
eleetromagnetie forees oceur when the eurrent passes through
magnetic fields in the electrolytie eell, and they eause
waves, bubbles and other movements in the bath and the metal.
As the difference in the specific gravity of the molten bath
and the underlying molten metal lS very small, this can result
in considerabLe variations in the distance between the anode
and the molten metal. To prevent the molten metal from
touching the anode, thus eausing a part short circuit, the
distanee between the anode and the eathode has to be increased.
This results in a larger voltage drop between the anode and
the cathode, hotter pot operation, and higher power eonsumption
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for each kilogram of metal produeed.
Horizontal electrie eurrents in the metal pad, in
eonjunetion with the vertical magnetie fields, give rise to
eleetromagnetie forces in the longitudinal direction of the
pot. To limit the horizontal current components, and thus
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reduce the most powerful movements in metal and bath, it is
usual practice to arrange 'for there to be formed an internal
coating of frozen bath along the carbon side lining of the
pot. This coating, or side freeze, has an electrical insulating
effect, and'thus limits the horizontal current components.
This side freeze is produced by not making the side lining
too thick and by not having any thermal insulation between
the side lining and the steel shell of the pot. A further
advantage of the side freeze is that lt protects the side
lining from attack by the bath. Nevertheless, it frequently
occurs that the molten material penetrates through the si'de
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linings with highly adverse consequences'and heavy costs.
This penetration of'the side lining is primarily caused by -
hlgh operating temperatures~ either in connectlon with anode
effect or on account of poor operating conditions which can
be partly due to powerful electromagnetic forces.
' The object of the present invention is to furnish
a strong and stable si'de freeze, whilst at the same time it
becomes possible to build a stronger and more durable potshell
without increasing the amount of steel used. This is achieved,
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according to the invention, in that up against the side walls '
and the end walls on the outside of these, there are disposed
vertical stiffeners, and in that outside the~se stiffeners
there lS disposed an essentially horizontal, reinforcing
frame, in such a manner that between the stiffeners there are
formed vertical, free air passages up against the side walls
and the end walls, and in such a manner that the stiffeners
act as cooling ribs conducting and dissipating the heat from
the side walls and the'end walls.
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The advantages galned through this arrangement
are primarily:
1. Placing the reinforcing frame clear of the potshell sides:
- results in only small thermal stresses in the reinforcing
frame, thus avoiding ageing phenomena in the load-bearing
structures,
- results in the temperature of the reinforcing frame
remaining low and with only small temperature variations
as the ambient temperature varies,
- results in only small temperature difference between the
inside and the outside parts of the reinforcing frame,
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- guides~the air current in towards the sides of the potshéll.
2. Vertical stiffeners in contact with the sides of the potshell:
- act as cooling rlbs,
- offer little air resistance,
- guide the air vertically up along the sides of the potshell,
- prevents accumulations of alumina building up along the
sides of the potshell.
In other words, this design of potshell results in high and
stable heat transfer figures between the shell sides~and~the
surroundings. This is of very great importance in the
; formation of a strong and stable side freeze which in turn
protects the side lining.
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Stable side freeze results in a stable and favour-
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able cathodic current distribution, and thus good operating
conditions. Good operating conditions give higher current
efEiciency and longer cathode life.
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The invention is described below in more detail
with reEerence to the drawings, in which:
Fig. 1 depicts a vertical section through a part -
of an electrolytic cell comprising the side wall structure
in this, and
Fig. 2 depicts a horizontal section through a part
of an electrolytic cell on a somewhat smaller scale.
Fig. 3 depicts a vertical section through an ;
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electrolytic cell havlng a transverse reinforoement arrangement.
: L ~: The figures show side walls l and end walls ll
which, together with a bottom plate 10, form the potshell
proper, built of steel plates. The steel walls l and ll
require a strong peripheral relnforcement which, in this case,
primarily consists of vertical stiffeners 2 along thè side
walls l and corresponding stiffeners 22 along the end walls~
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~ These stiffeners act as vertical cooling ribs which serve
; ~ to transfer the heat from the walls of the potshell to the
air currents flowing along the outside. Stiffeners 2 and
22 can expediently be made of I-sections or T-sections in
order better to be able to resist the powerful expansion
forces from the lining 3a and 3b of the pot, as shown in Fig.l.
Outside tha stiffeners 2 and 22 there is disposed~
a reinforcing frame 4 consisting of strong steel girders
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41 and 42, lying horizontally along the side walls and the
end walls of the structure, respectively. This reinforcing-
frame 4 should preferably run, unbroken, around all the sides
of the potshell ! and can be arranged to be divided into two
by means of a fish plate connection 44 or the like in the
middle at each end.
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Through this design, vertical air passages 12 are
formed between the stiffeners 2 along the side walls l, and ..
also corresponding vertical air passages 13 between the
stiffeners 22 along the end walls 11. As may be seen,
particularly in fig. 1, the potshell is designed to be so
installed that air enters into the air passages, for example
passage 12 in Fig. 1, from -the potrooms basement under the
floor 20, as indicated by arrows 31, and flows out into the
potroom, over the floor 20, from the upper end of the air
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passages 12, as ind~i~cated by arrows 32. In this manner use
is made of the draught which is caused by the usual fan exhaust
arrangement found in such potrooms. A particular advantage
in this connection is that the temperature in the basement is ~ .
usually.lower than in the potroom itself, so that the a.ir
which flows through the air passages 12 and 13 is comparatively
cold.
The opening or slot 7 through which the air exits
at the upper end of each air passage,can expediently be
adjustable in order to make it possible to vary the cooling
effect.
; In connection with this invention it is important
-~ that no horizontal reinforcing elements be disposed along the
walls of the shell which could obstruct the vertical circula~
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tion described or which could cause there to be bullt up a
heat-insulating layer of alumina or other dust. The positioning
of the most important and the most heavily stressed load-
bearing element,. namely the reinforcing frame ~, outside the
stiffeners 2 and 22 also has the result that this reinforcing
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frame acquires a lower temperaturej and is subjected to only
small temperature variations as the ambient temperature
varies. In this manner the danger of ageing phenomena in the
reinforcing frame is heavily reduced.
In order to preven;t the accumulation of alumina
or dust in the air passages 12 and 13, there can be disposed,
as shown in Fig. 1, a roof or cover 14, in the form of a
plate or section, which forces the air current 32 to exit
sideways from air passage 12.
Experience has shown that, when the number of
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cooling ribs in the form of the above described stiffeners
has been correctly chosen and given correct dimensioning of
their surfaces, it is possible to achieve that the side freeze
5, as shown in Fig. 1, extends approximately into the shadow
of the anode 6. This is considered to be the ideal situation
as it reduces the horizontal current components to a minimum
without reducing, too heavily, the effective cathode area.
~epending upon the time of the year or the ambient temperature,
the size of the slot 7 can be varied in order thereby to
adjust the thickness of the side freeze 5. ~ ,
As may be seen from Fig. 1, the stiffeners are
supported on base blocks 24, for example by wedging, so that
lateral forces can be transferred between the bottom end of
- the stiffeners and the bottom plate 10 of the potshell. In
this embodiment the stiffeners are neither welded nor bolted
to *he potshell walls but are loose and can easily be removed
and, for exampLe, straightened, should this be necessary.
The reinforcing frame 4 is also easily dismantled, for example
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for straightening in the event of misalignment having occurred.
In connection with the abovementioned anchoring of the lower
end of the stiffeners it is important that the reinforcing-
frame 4 be positioned comparatively far up along the sides
of the shell walls, and at-least on a level with the top of
the carbon cathode 3b. In this way one takes better advantage
of the tensile strength of the bottom plate 10.
In addition to the abovementioned advantages with
respect to thermal conditions and mechanical strength, the
embodiment described according to the invention is also
advantageous inasmuch as it is simple and rational to produce.
For example, the need for machine tools is minimal. Except
for welding, no skilled labour is required. Further, it is
not of unimportance that the structure described makes it
simple and cheap to carry out maintenance and repairs, in
the event, for example, of the completé or part dismantling
of the electrolytic cell.
Finally, on the question of the thermal conditio~s,
it must be added that the new design of potshell revealed
here results in a very stable side freeze. Furthermore, it
has a very stable heat balance, i.e. it can be regarded as
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"self regulating". In practice, this new design of potshell
has pxoved to suffer very rarely from leakage through
penetration of the side lining-. This is an advantage of very
great importance as explained in the introduction. The
improved horizontal dissipation of heat through the side walls
of the potshell also result in a more favourable ratio
between heat loss through the sides and through the bottom,
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respectively, in that a simultaneously improved bottom
insulation, working in co-operation with the good cooling
of the side and end walls, produces a side freeze which has
the right configuration and which is more stable than that
which forms in previously known types of potshell. This
improves and stabilizes the current distribution in the metal
in the electrolytic cell.
It will be understood that the design:and construc- :
tion descrlbed above can be modified in various ways wlthin
the scope of this invention, for example, through some other,
or no, dividing up.of the reinforcing frame, through the use
- of stiffeners of different sectional geometry than those
mentioned by way of example, and other methods of joining
the main components of the potshell, etc.
~ . In the embodiment shown in Fig~ 3, refèrence
: numerals corresponding to those found in. Eigures 1 and 2 are
~ used to identify the following: side walls 1, vertical
~ ~ stiffeners 2, pot linings 3a ~and 3b, reinforcing frame 4
. ~ and bottom plate 10.
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:~ As an additional reinforcement this embodiment
has vertical beams 35a and 35b on either side of the potshell,
being welded or otherwise secured at their~.upper ends to the
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~ frame 4. At.their lower ends beams 35a and 35b are anchored
in recesses 37a and 37b, respectlvely, in the basement floor
39. Intermediate their ends beams 35a and 35b are connected
to each other by means of transverse rods 36 having nuts 38
threaded at their ends for connection to the beams 35a and
35b. This relnforcement makes i~ possible to r duce t~e
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dimensions of the members forming the frame 4 since transverse
forces to a large extent will be taken up by the rods 36 and
the floor 39. Smaller dimensions of the frame 4 in turn
will reduce the space requirement for each pot.
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