Note: Descriptions are shown in the official language in which they were submitted.
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Cathode shell structure
The present invention relates to a cathode structure for an aluminium
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 9700 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.
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According to one aspect of the present invention, there is provided cathode
structure for an
electrolysis cell, the structure comprising: a shell with vertical stiffeners
and horizontal webs
at its outside, and further several cradle structures consisting of
interconnected vertical
structural elements and horizontal beams, where at least one horizontal web is
provided with
apertures facing the shell, wherein the apertures are represented by discrete
cut-outs at the
side edge of the horizontal web having bridging parts between them, the
horizontal web
being connected to the shell via said bridging parts whereby each of the
apertures are
adjacent a wall of the shell and secure sufficient ventilation along the
shell, based upon
upward draught, the apertures being sufficiently wide and deep to reduce the
presence of the
horizontal web with regard to ventilation.These and further advantages can be
achieved in
accordance to the invention as described in the accompanying patent claims.
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 wall 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"1, 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 wall 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.
