Note: Descriptions are shown in the official language in which they were submitted.
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ALUMINIUM ELECTROWINNING CELL WITH SIDEWALLS
RESISTANT TO MOLTEN ELECTROLYTE
Field of the Invention
The invention relates to drained-cathode cells
for the electrowinning of aluminium from alumina
dissolved in a molten fluoride-containing electrolyte
having sidewalls resistant to molten electrolyte, and
methods of operating the cells to produce aluminium.
Backctround of the Invention
The technology for the production of aluminium by
the electrolysis of alumina, dissolved in molten cryolite
containing salts, at temperatures around 950°C is more
than one hundred years old.
This process, conceived almost simultaneously by
Hall and Heroult, has not evolved as much as other
electrochemical processes, despite the tremendous growth
in the total production of aluminium that in fifty years
has increased almost one hundred fold. The process and
the cell design have not undergone any great change or
improvement and carbonaceous materials are still used as
electrodes and cell linings.
The electrolytic cell trough is typically made of
a steel shell provided with an insulating lining of
refractory material covered by prebaked anthracite-
graphite or all graphite carbon blocks at the cell floor
bottom which acts as cathode. The side walls are also
covered with prebaked anthracite-graphite carbon plates.
To increase the efficiency of aluminium
production numerous drained-cathode cell designs have
been developed, in particular including sloping drained
cathode surface, as for instance disclosed in US Patents
3,400,061 (Lewis/Altos/Hildebrandt), 4,602,990 (Boxall/
Gamson/Green/Stephen), 5,368,702 (de Nora), 5,683,559 (de
Nora), European Patent Application No. 0 393 816
(Stedman), and PCT application W099/02764 (de Nora/
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Duruz). These cell designs permit reduction of the inter
electrode gap and consequently reduction of the voltage
drop between the anodes and cathodes. However, drained
cathode cells have not as yet found significant
acceptance in industrial aluminium production.
It has been proposed to decrease energy losses
during aluminium production by increasing the thermal
insulation of the sidewalls of aluminium production
cells. However, suppression of the thermal gradient
through the sidewalls prevents bath from freezing on the
sidewalls and consequently leads to exposure of the
sidewalls to highly aggressive molten electrolyte and
molten aluminium.
Several proposals have been made in order to
increase the sidewall resistance for ledgeless cell
operation. US Patent 2,915,442 (Lewis) discloses inter-
alia use of silicon carbide or silicon nitride as
sidewall material. US Patent 3,256,173 (Schmitt/wittner)
describes a sidewall lining made of a honeycomb matrix of
coke and pitch in which particulate silicon carbide is
embedded. US Patent 5,876,584 (Cortellini) discloses
sidewall lining material of silicon carbide, silicon
nitride or boron carbide having a density of at least 95~
and no apparent porosity.
Sidewalls of known ledgeless cells are most
exposed to erosion at the interface between the molten
electrolyte and the molten aluminium which accumulates on
the bottom of the cell. Despite formation of an inert
film of aluminium oxide around the molten aluminium
metal, cryolite operates as a catalyst which dissolves
the protective aluminium oxide film at the
aluminium/cryolite interface, allowing the molten
aluminium metal to wet the sidewalls along the molten
aluminium level. As opposed to aluminium oxide, the
oxide-free aluminium metal is reactive at the cell
operating temperature and combines with constituents of
the sidewalls, which leads to rapid erosion of the
sidewalls about the molten aluminium level.
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While the foregoing references indicate continued
efforts to improve the operation of molten cell
electrolysis operations, none suggest the invention and
there have been no acceptable proposals for avoiding cell
sidewall erosion caused by reaction with molten aluminium
metal.
Objects of the Invention
An object of the invention is to provide a design
for an aluminium electrowinning cell in which electrolyte
is inhibited from freezing on the sidewalls.
Another object of the invention is to provide a
cell configuration for crustless or substantially
crustless molten electrolyte resistant sidewalls, in
particular carbide and/or nitride-containing sidewalls,
which leads to an increased sidewall lifetime.
A further object of the invention is to provide a
cell configuration for crustless or substantially
crustless molten electrolyte resistant sidewalls, in
particular carbide and/or nitride-containing sidewalk ,
which leads to a reduced erosion, oxidation or corrosion
of the sidewalls.
A major object of the invention is to provide a
drained cathode cell configuration with sidewalk
resistant to molten electrolyte, in particular carbide
and/or nitride-containing sidewalls, for crustless or
substantially crustless operation.
Summary of the Invention
One main aspect of the invention concerns a
drained-cathode cell for the electrowinning of aluminium
from alumina dissolved in a fluoride-containing molten
electrolyte. The drained-cathode cell has a cell bottom
which comprises an arrangement for collecting product
aluminium and a peripheral upper surface that surrounds
the arrangement for collecting product aluminium. At
least the part of the cell bottom which is in contact
with molten aluminium during operation is made of
material resistant to molten aluminium.
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Aluminium is produced on at least one drained
cathode surface from which the produced aluminium drains
into said arrangement for collecting the product
aluminium during operation.
The drained-cathode cell further comprises one or
more thermic insulating sidewalls extending generally
vertically upwards from the peripheral surface of the
cell bottom to form with the cell bottom a trough for
containing during operation molten electrolyte and the
product aluminium. Above the peripheral surface, the or
each thermic insulating sidewall is lined with a sidewall
lining made of material resistant to molten electrolyte
but liable to react with molten aluminium. the or each
thermic insulating sidewall inhibits formation of an
electrolyte crust or ledge on the sidewall lining that
during operation remains permanently exposed to molten
electrolyte above and around said peripheral surface.
The peripheral surface of the cell bottom is
arranged to keep molten aluminium away from the sidewall
lining above and around the entire peripheral surface,
whereby the molten aluminium is prevented from reacting
with the sidewall lining above and around the entire
peripheral surface.
The drained-cathode cell design according to the
invention thus keeps the molten aluminium away from all
cell sidewalls preventing it from contacting and reacting
with the sidewall lining resistant to molten electrolyte,
enabling use of a sidewall lining made of a carbide
and/or a nitride, such as silicon carbide, silicon
nitride or boron nitride, without risk of damage to the
sidewall lining by reaction with molten aluminium as can
occur with known designs.
Usually the cell comprises four of the above
mentioned insulated sidewalls in a generally rectangular
arrangement. However, the invention can also be
implemented with other sidewall configurations.
The upper surface of the cell bottom for example
comprises opposed sloping surfaces leading from opposed
sidewalls down into a central channel for the continuous
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removal of product aluminium, the central channel
extending parallel to said opposed sidewalls. This
central draining channel (or a side channel or several
channels in other embodiments) preferably leads into an
aluminium storage sump or space which is internal or
external to the cell and from which the aluminium can be
tapped from time to time.
Alternatively, the upper surface of the cell
bottom comprises a series of oppositely sloping surfaces
forming therebetween recesses or channels that extend
parallel to opposed sidewalls. The recesses or channels
can be of various shapes, for example generally V-shaped.
Usually, the peripheral surface slopes down to a
flat or sloping main surface of the cell bottom which
forms the drained cathode surface or which receives
produced aluminium from a drained cathode surface located
thereabove. This main surface leads into the arrangement
for collecting product aluminium.
V~hen the main surface is at a slope, the
peripheral surface is usually inclined at a steeper slope
than the main surface.
In one embodiment, the main surface comprises
downwardly converging inclined surfaces sloping down from
first opposed sidewalls. The converging surfaces are
inclined along second opposed sidewalk . The peripheral
surface extends horizontally along the first opposed
sidewalls and follows the inclination of the converging
surfaces along the second opposed sidewalls. In this
embodiment, the sloping peripheral surface can be of
substantially uniform width around the entire cell
bottom.
In another embodiment, where the main surface
also comprises downwardly converging inclined surfaces
sloping down from first opposed sidewalls, the converging
surfaces are inclined along second opposed sidewalls, and
the peripheral surface extends horizontally along the
first and second opposed sidewalls, the sloping
peripheral surface extends down to the converging
inclined surfaces around the entire cell bottom. Usually,
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the sloping peripheral surface is of uniform width along
the first opposed sidewalls and of non-uniform width
along the second opposed sidewalls where it forms
generally triangular surfaces whose sides follow the
second opposed sidewalls and the converging inclined
surfaces.
In a further embodiment, where the main surface
also comprises downwardly converging inclined surfaces
sloping down from first opposed sidewalls, the converging
surfaces are inclined along second opposed sidewalls, and
the sloping peripheral surface extends horizontally along
the first and second opposed sidewalls, the sloping
peripheral surface is connected by at least one
substantially vertical connecting wall to the main
surface, i.e. at least to the converging inclined
surfaces. Such connecting walls) is/are resistant to
molten aluminium.
Usually, the drained surfaces) is/are on one or
more cathodes which are part of the cell bottom and so
arranged that molten aluminium produced thereon drains
away from the sidewall lining into the arrangement for
collecting molten aluminium. Alternatively, the drained
cathode surfaces) can be on one or more cathodes located
above the cell bottom, the molten aluminium draining from
the cathodes onto the cell bottom and then into the
arrangement for collecting molten aluminium.
The cathode and/or the cell bottom can be made of
carbonaceous material, such as compacted powdered carbon,
a carbon-based paste for example as described in U.S.
Patent No. 5,362,366 (de Nora/Sekhar), prebaked carbon
blocks, or graphite blocks, plates or tiles. Other
suitable cathode materials which can also be used for the
cell bottom are described in W098/53120 (Berclaz/de Nora)
and W099/02764 (de Nora/Duruz).
The cathode and the cell bottom most preferably
has/have an upper surface which is aluminium-wettable,
for example the upper surface of the cathode or the cell
bottom is coated with a coating of refractory aluminium
wettable material as described in U.S. Patent 5,651,874
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(de Nora/Sekhar) or W098/17842 (Sekhar/Duruz/Liu). The
aluminium-wettable surface usually comprises a refractory
boride, in particular TiB2, advantageously applied as a
coating from a slurry of particles of the refractory
boride or other aluminium-wettable material.
This aluminium-wettable surface can be obtained
by applying a top layer of refractory aluminium-wettable
material over the upper surface (which can already have a
precoating of the refractory aluminium wettable material)
and over parts of the cell surrounding the cathode.
In one embodiment in which the cathode is part of
the cell bottom, the electric current to the cathode, in
particular a cathode mass, may arrive through an inner
cathode holder shell or plate placed between the cathode
and the outer shell, usually made of steel, as disclosed
in W098/53120 (Berclaz/de Nora).
The sidewall lining can be made of tiles
containing carbide and/or nitride and/or can comprise a
carbide and/or nitride based coating which during cell
operation is in contact with the product aluminium.
Alternatively, the sidewall lining may be coated
and/or impregnated with one or more phosphates of
aluminium, as disclosed in US Patent 5,534,130 (Sekhar).
The phosphates of aluminium may be selected from:
monoaluminium phosphate, aluminium phosphate, aluminium
polyphosphate, and aluminium metaphosphate.
The cells according to the invention can make use
of traditional consumable prebaked carbon anodes,
continuously-fed S~Dderberg-type anodes, as well as non
consumable or substantially non-consumable anodes.
Non-consumable anodes may comprise an
electrochemically active structure made of a series of
horizontal anode members, each having an
electrochemically active surface on which during
electrolysis oxygen is anodically evolved. The anode
members may be in a parallel arrangement connected by at
least one connecting cross-member or in a concentric
arrangement connected by at least one generally radial
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connecting member as described in WO00/40781 and
WO00/40782 (both in the name of de Nora).
Suitable materials for oxygen-evolving anodes
include iron and nickel based alloys which may be heat-
s treated in an oxidising atmosphere as disclosed in
WO00/06802, W000/06803 (both in the name of
Duruz/de Nora/Crottaz), WO00/06804 (Crottaz/Duruz),
PCT/IB99/01976 (Duruz/de Nora) and PCT/IB99/01977
(de Nora/Duruz). Further oxygen-evolving anode materials
are disclosed in W099/36593, W099/36594, WO00/06801,
WO00/06805, PCT/IB00/00028 (all in the name of
de Nora/Duruz), WO00/06800 (Duruz/de Nora), W099/36591
and W099/36592 (both in the name of de Nora).
Whether consumable prebaked anodes or non-
consumable anodes are used, it is advantageous to preheat
each anode before it is installed in the cell during
operation, in replacement of a carbon anode which has
been substantially consumed, or a non-consumable anode
that has become disactivated or requires servicing. By
preheating the anodes, disturbances in cell operation due
to local cooling are avoided as when an electrolyte crust
is formed whereby part of the anode is not active until
the electrolyte crust has melted.
The invention also relates to a cell trough for
containing molten electrolyte and product aluminium,
having a cell bottom fitted with insulating cell
sidewalls which are protected with a molten electrolyte
resistant lining as described above.
A further aspect of the invention relates to a
method of producing aluminium using the cell as outlined
above which contains alumina dissolved in a fluoride-
containing molten electrolyte. The method involves
electrolysing the dissolved alumina to produce aluminium
on the or each drained cathode surface and draining the
produced aluminium from the or each drained cathode
surface into the arrangement for collecting the product
aluminium, the produced aluminium being kept from
contacting and reacting with the sidewall lining above
and around the entire peripheral surface.
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Advantageously, the surface of the cell bottom is
maintained at a temperature corresponding to a paste
state of the electrolyte whereby the cell bottom is
protected from chemical attack. For example, when the
cryolite-based electrolyte is at about 950°C, the surface
of the cell bottom can be cooled by about 30°C, whereby
the electrolyte contacting the cathode surface forms a
viscous paste which protects the cell bottom.
Brief Description of the Drawings
The invention will be further described with
reference to the accompanying schematic drawings, in
which .
Fig. 1 is a cross-sectional view of one aluminium
electrowinning cell according to the invention;
Fig. 2 is a cross-sectional view of another
aluminium electrowinning cell according to the invention;
Fig. 3 shows the bottom part of the cell of Fig.
2 during assembly of a cathode unit;
Fig. 4 shows in longitudinal cross-section an
embodiment of the cathode ready to be installed in a
cell;
Fig. 5 is a longitudinal cross-sectional view of
another aluminium electrowinning cell according to the
invention; and
Fig.6 is a plan view of the cell bottom shown in
Figure 1, 2 or 3 showing varied embodiments of the
peripheral surface.
Detailed Description
Figs. 1 and 6 schematically show an aluminium
electrowinning cell according to the invention wherein a
plurality of anodes 10 are suspended by yokes 11
connected to an anode suspension and current supply
superstructure (not shown) which hold the anodes 10
suspended above a cathode cell bottom 20 enclosed in an
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outer steel shell 21 forming, with its insulating lining
of refractory bricks 40, a cell trough or cathode pot.
Inside the outer steel shell 21 is housed a
cathode 30 comprising an inner steel cathode holder shell
31 containing a cathode mass 32. As illustrated, the
inner shell 31 has a flat bottom, sidewalls 33 and
outwardly-directed side flanges 34 at its top. The inner
shell 31 forms an open-topped container for the cathode
mass 32.
The top of the cathode mass 32 has inclined
surfaces 35 extending over the cathode 30 and leading
down into a central channel 36 for draining molten
aluminium. The central channel 36 advantageously leads
into an aluminium storage sump 36' which is centrally
located in the cell, as shown in Figure 6. On top of the
cathode mass 32, and also extending over the flanges 34,
is a coating 37 of aluminium-wettable material,
preferably a slurry-applied boride coating as described
in U.S. Patent 5,651,874 (de Nora/Sekhar) or W098/17842
(Sekhar/Duruz/Liu). Such coating 37 can also be applied
to the inside surfaces of the bottom and sides 33 of the
cathode holder shell 31, to improve electrical connection
between the inner shell 31 and the cathode mass 32.
The periphery of the cathode mass 32 extends to
the top of the sidewall 33 of the inner shell 31, from
where it slopes down to the central channel 36.
Inside the part of the cell sidewalls at the top
of the outer shell 21 facing the sides of anodes 10 is a
sidewall lining 50 formed for example of plates of carbon
or silicon carbide.
As shown in Figures 1 to 3, the insulating
sidewalk 55 extend generally vertically upwards from the
cell bottom 20. The insulating sidewalls 55 inhibit
during operation formation of an electrolyte crust on the
sidewall lining 50, whereby the lining is exposed to
molten electrolyte 60.
According to the invention, the peripheral
surface 35' from which the insulating sidewalls 55,55'
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extend is arranged to drain molten aluminium away from
the sidewall lining 50, to keep the product aluminium
from contacting and reacting with the sidewall lining 50,
as shown in Figures 1 to 3 and 6. For this purpose, the
peripheral surface 35' is inclined at a steeper slope
than the inclined cathode surfaces 35, as shown in
Figures 1 to 5, forming a small wedge sloping down from
the end sidewalls 55' and extending across the cathode
mass 32, so that the entire periphery 35' around the
sloped cathode surfaces 35 slopes away from all cell
sidewalls 55,55' to drain molten aluminium away from the
sidewall lining 50, as shown in Figure 6.
Figures 3 and 6 show different configurations of
the peripheral surface 35'.
As shown in Figure 3, the sloped cathode surfaces
35 are made of downwardly converging inclined surfaces 35
sloping down from opposed lateral sidewalls 55 to the
central channel 36 and which are inclined along opposed
end sidewalls 55' as shown in the upper part of Figure 6.
As shown in Figure 3, the peripheral surface 35' extends
horizontally along the lateral sidewalls 55 and follows
the inclination of the converging surfaces 35 along the
opposed end sidewalls 55', the sloping peripheral surface
35 being of substantially uniform width around the entire
cell bottom as shown in the upper part of Figure 6.
A variation of the configuration of the
peripheral surface 35' is shown in Figure 3 by dotted
line 35 " and on the lower part of Figure 6. The
peripheral surface 35' extends horizontally along the
lateral sidewalls 55 and the end sidewalls 55'.
Furthermore, the sloping peripheral surface 35' extends
down to the converging inclined surfaces around the
entire cell bottom. In this variation, the peripheral
surface 35' is of uniform width along the lateral
sidewalls 55 and of non-uniform width along the end
sidewalls 55' where, as shown on the lower part of Figure
6, it forms a generally triangular surface.
Another variation of the configuration of the
peripheral surface 35' is shown in Figure 3 by dotted
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line 35 " and 35 " ' and on the upper part of Figure 6 .
The peripheral surface 35' extends horizontally along the
lateral and end sidewalls 55' as shown by dotted line
35 " . The sloping peripheral surface 35' is connected to
the converging inclined cathode surfaces 35 by
substantially vertical connecting walls, the top of the
connecting wall being indicated by line 35 " ' in Figures
3 and 6. As shown in Figures 3 and 6, the sloping
peripheral surface 35' is of uniform width all around the
sidewalls 55,55'. This connecting wall is resistant to
molten aluminium and can be coated with aluminium-
wettable material as mentioned above.
As shown in Figures 1 to 3, the cathode 30 is
supported as a removable unit in the cell bottom 20 in a
central recess of corresponding shape in the refractory
bricks 40 lining the outer steel shell 21. These
refractory bricks 40 are the usual types used for lining
conventional cells.
Current is supplied to the cathode 30 via
transverse conductor bars 41 welded to the bottom of the
inner shell 31. These conductor bars 41 are connected to
current collector bars 42 which protrude laterally from
the sides of the outer shell 21, as shown in Figure 1,
these collector bars 42 being connected to external
buswork (not shown).
Alternatively, current could be supplied to the
cathode 30 of Fig. 1, by a series of vertical current
collector bars 41 extending down through vertical
openings in the bottom of the lining formed by the
refractory bricks 40 (see Figures 2 and 3).
Due to the metallic conductivity of the cathode
holder shell 31, these conductor bars 41 are all
maintained at practically the same electrical potential
leading to uniform current distribution in the collector
bars 42. Moreover, the metal inner shell 31 evenly
distributes the electric current in the cathode mass 32.
In use, the space between the cathode 30 and the
sidewall lining 50 is filled with a molten electrolyte 60
such as cryolite containing dissolved alumina at a
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temperature usually about 950-970°C, and,into which the
anodes 10 dip. When electrolysis current is passed,
aluminium is formed on the sloping cathode surfaces 35
coated with the refractory boride coating 37, and the
produced aluminium continuously drains down the sloping
surfaces 35 into the central channel 36 from where it is
removed permanently into the storage sump 36' from which
the aluminium can be tapped from time to time.
The anodes 10, which are shown as being
consumable prebaked carbon anodes, have sloping surfaces
12 facing the sloping cathode surfaces 35. The
inclination of these anode surfaces 12 facilitates the
release of bubbles of the anodically-released gases. As
the anode 10 is consumed, it maintains its shape, keeping
a uniform anode-cathode spacing. Alternatively, it would
be possible for the same cell bottom 20 and its cathode
30 to be used with non-consumable or substantially non-
consumable anodes.
Periodically, when the cathode 30 needs
servicing, it is possible to close down the cell, remove
the molten cell contents, and disassemble the entire
cathode 30 to replace it with a new or a serviced cathode
30. This operation is much more convenient and less
labour intensive than the conventional cell bottom
relining process, has reduced risks relating to exposure
to the toxic waste materials, and simplifies disposal of
the toxic waste materials.
The aluminium electrowinning cell shown in Fig. 2
is similar to that of Fig. 1 and like references have
been used to designate like parts. In this design, the
current collector bars 42 instead of being horizontal are
vertical and extend through vertical apertures 43 in the
lining of bricks 40. These collector bars 42 are welded
centrally to the bottom of the inner shell 31. As
illustrated in Fig. 4, several collector bars 42 are
spaced apart from one another along the bottom of the
inner shell 31. These collector bars 42 can have any
desired cross-sectional shape . circular, rectangular, T-
shaped, etc. Because the inner metal shell 31 keeps the
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collector bars 42 at practically the same potential,
fluctuations in the current supply are avoided.
The assembly method is illustrated in Fig. 3. It
is possible to install the entire cathode 30 by lowering
it using a crane until the bottom of the cathode holder
shell 31 comes to rest on the top 44 of the lining of
bricks 40 and its side flanges 34 come to rest on
shoulders 45 of the cell lining. Then, the plates 50 can
be installed on top of the flanges 34. This assembly
method is simple and labour saving, compared to the usual
cell lining methods used heretofore.
To dismantle the cell, the sidewall li;iing plates
50 are removed first, then the cathode 30, after
disconnecting the collector bars 42 from the negative
busbar. This dismantling of the cell is remarkably simple
to carry out and considerably simplifies disposal of
toxic wastes.
Figure 4 shows the cathode 30 ready to be
installed as a unit in an aluminium electrowinning cell
(not shown) which is fitted with insulating sidewalls
protected with a carbide and/or nitride containing lining
according to the invention. This cathode 30 comprises a
metal cathode holder shell 31 made of a flat base plate
to which sidewalls 33 are welded substantially at right
angles along its side edges. These sidewalls 33 can
extend around the entire periphery of the base plate, or
only along its opposite side edges.
To the bottom of the shell 31's base plate, a
series of conductor bars 42 are welded, spaced equally
apart from one another along the length of the shell 31.
These conductor bars 42 protrude vertically down from the
shell 31, so they can pass through corresponding vertical
openings in the cell bottom, for connection to an
external negative busbar.
In the shell 31 is a cathode mass 32 formed of a
series of blocks, for example of carbon. As shown, the
cathode blocks have sloping upper surfaces 35 and are
fitted together to form a series of generally V-shaped
recesses. In this example, parts of the cathode blocks
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protrude above the top of the sidewalls 33 which are
embedded in the sides of the end blocks.
The upper surface 35 is made up of a series of
sloping surfaces in generally V-configuration, formed by
placing the adjacent blocks together. The end blocks on
each side of the shell 31 are shown with a sloping
peripheral surface 35' from which the insulating
sidewalls extend when placed in a cell. According to the
invention, the peripheral surface 35' surrounds the
cathode 30 and is arranged to drain molten aluminium away
from the sidewall lining 50 above and around the entire
peripheral surface 35', to keep the product aluminium
from contacting and reacting with the sidewall lining 50
above and around the entire peripheral surface 35'.
Each conductor bar 42 corresponds to the junction
between two adjacent blocks forming the lower part of
each V. As shown, the conductor bars 42 protrude through
the shell 31 and extend part of the way up the blocks 42.
Alternatively, the conductor bars 42 could be welded
externally to the bottom of the shell 31.
Before use, the entire sloping upper surface 35
of the cathode mass 32 is coated with an aluminium-
wettable coating typically formed of slurry-applied
titanium diboride.
This cathode 30 can be produced as a unit and
installed in an aluminium electrowinning cell (as
illustrated in Fig. 3) by lifting it with a crane, and
lowering it into the cell.
The aluminium electrowinning cell shown in
longitudinal cross-section in Fig. 5 comprises a cathode
30 with a series of spaced-apart vertical current
conductors 42 welded to the bottom of its inner cathode
holder shell 31, these conductors 42 protruding from the
lower face of the cell bottom 20 for connection to the
cathode buswork.
As in Figures 1 to 3, the insulating sidewalls 55
shown in Figure 5 extend generally vertically from the
cell bottom 20 which is arranged to drain molten
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aluminium away from the carbide and/or nitride containing
sidewall lining 50, to keep the product aluminium from
contacting and reacting with the sidewall lining 50.
The cathode mass 32 is made up of several layers
of a conductive material such as carbon possibly combined
with materials rendering the carbon impervious to molten
aluminium. The mass 32 comprises an outer layer around
the bottom and sides 33 of the inner shell 31. This outer
layer has a peripheral edge 32a surrounding a central
recess that is coated with a flat layer 38 of carbon or
other conductive material on top of which is a toplayer
39 having sloping faces 35 coated with the layer 37 of
aluminium-wettable boride. As illustrated, the upwardly-
sloping side parts of the faces 35 are extended by
bevelled parts of the edges 32a and by ramming paste 51,
forming wedges along the edges of the cathode mass 32 on
which the aluminium wettable boride layer 37 extends to
form with the peripheral edge 32a a peripheral surface
35' of steeper slope which is arranged to drain molten
aluminium away from the sidewall lining 50 above and
around the entire peripheral surface according to the
invention.
The sloping faces 35 of cathode mass 32 are
inclined alternately to form flattened V-shaped recesses
above which the anodes 10 are suspended with
corresponding V-shaped inclined faces 11 of the anodes
facing the V-shaped recesses in the cathode mass 32. The
anodes 10 are suspended by steel rods 14 held at an
adjustable height in attachments 15 by an anode bus 16,
enabling the anodes 10 to be suspended with a selected
anode-cathode gap.
Assembly and disassembly of the cathode 30 of
this cell is similar to what has been described
previously. The cathode 30 is assembled first, outside
the cell, then lowered using a crane into the cell bottom
20, passing the conductor bars 42 through corresponding
openings 43 in the bricks 40. Then the gaps around the
edges of the cathode mass 32 are filled with ramming
paste 51 which is formed into the side wedges. Next, a
slurry of refractory boride is applied to the sloping
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WO 01/31087 PCT/IB00/01483
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cathode faces 35, usually on top of a pre-coating already
applied thereto, and also over the sloping wedge surfaces
of the edges 32a and ramming paste 51. After drying and
heat treatment of the boride coating 37, the cell is
ready for start-up. In operation, the central recess in
the cell above the cathode mass 32 contains a molten
electrolyte 60, such as cryolite containing dissolved
alumina, into which the anodes 10 dip.
For disassembly to service the cell bottom 20,
the molten contents are removed from the cell, and the
ramming paste 51 is broken to enable the entire cathode
unit 30 to be lifted out of the cell using a crane, after
having disconnected the conductor bars 42 from the
cathode busbar.
While the invention has been described in
conjunction with specific embodiments thereof, it is
evident that many modifications and variations will be
apparent to those skilled in the art in the light of the
foregoing description. Accordingly, it is intended to
embrace all such alternatives, modifications and
variations which fall within the scope of the appended
claims.
