Note: Descriptions are shown in the official language in which they were submitted.
CA 02596427 2007-08-17
- ~ -
ALUMINIUM ELECTROWINNING CELL
WITH IMPROVED CARBON CATHODE BLOCKS
Field of the nv_ntion
The present invention concerns a cell for the
electrowinning of aluminium by the electrolysis of alumina
dissolved in a fluoride-based molten electrolyte such as
cryolite, incorporating improved carbon cathode blocks.
Background 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 npt 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 wall and at the cell floor
bottom which acts as cathode and to which the negative pole
of a direct curlas of steel conductor bars embedded in the
carbon blocks.
The anodes are still made of carbonaceous material
and must be replaced every few weeks. The operating
temperature is still approximately 950 C in order to have a
sufficiently high rate of dissolution of alumina which
decreases at lower temperatures and to have a higher
conductivity of the electrolyte.
The carbonaceous materials used in Hall-Heroult
CA 02596427 2007-08-17
- 2 -
cells as cell lining deteriorate under the existing adverse
operating conditions and limit the cell life.
The anodes have a very short life because during
electrolysis the oxygen which should evolve on the anode
surface combines with the carbon to form CO2 and small
amounts of CO. The actual consumption of the anode is
approximately 450 kg/ton of aluminium produced which is more
than 1/3 higher than the theoretical amount.
The carbon lining of the cathode bottom has a useful
life of a few years after which the operation of the entire
cell must be stopped and the cell relined at great cost.
Despite an aluminium pool having a thickness of 15 to 20 cm
maintained over the cathode, the deterioration of the
cathode carbon blocks cannot be avoided because of
penetration of sodium into the carbon which by chemical
reaction and intercalation causes swelling, deformation and
disintegration of the cathode carbon blocks, as well as
penetration of cryolite and liquid aluminium.
The carbon blocks of the cell side wail do not
resist oxidation and attack by cryolite and a layer of
solidified cryolite has to be maintained on the cell side
walls to protect them. In addition, when cells are rebuilt,
there are problems of disposal of the carbon cathodes which
contain toxic compounds including cyanides.
Another major drawback, however, is due to the fact
that irregular electromagnetic forces create waves in the
molten aluminium pool and the anode-cathode distance (ACD),
also called interelectrode gap (IEG), must be kept at a safe
minimum value of approximately 50 mm to avoid short
circuiting between the aluminium cathode and the anode or
reoxidation of the metal by contact with the C02 gas formed
at the anode surface.
The high electrical resistivity of the electrolyte,
which is about 0.4 ohm. cm., causes a voltage drop which
alone represents more than 40% of the total voltage drop
with a resulting energy efficiency which reaches only 25% in
the most modern cells. The high cost of energy together with
the low efficiency, has become an even bigger item in the
total manufacturing cost of aluminium since the oil crisis,
and has decreased the rate of growth of this important
CA 02596427 2007-08-17
- 3 -
metal.
In the second largest electrochemical industry
following aluminium, namely the caustic and chlorine
industry, the invention of the dimensionally stable anodes
(DSA ) based on noble metal activated titanium metal, which
were developed around 1970, permitted a revolutionary
progress in the chlorine cell technology resulting in a
substantial increase in cell energy efficiency, in cell life
and in chlorine-caustic purity. The substitution of graphite
anodes with DSA increased drastically the life of the
anodes and reduced substantially the cost of operating the
cells. Rapid growth of the chlorine caustic industry was
retarded only by ecological concerns.
In the case of aluminium production, pollution is
not due to the aluminium produced, but to the materials and
the manufacturing processes used and to the cell design and
operation.
However, progress has been reported in the operation
of modern aluminium plants which utilize cells where the
gases emanating from the cells are in large part collected
and adequately scrubbed and where the emission of highly
polluting gases during the manufacture of the carbon anodes
and cathodes is carefully controlled.
While progress has been reported in the fabrication
of carbon cathodes by the application of coatings or layers
using new aluminium wettable materials which are also a
barrier to sodium penetration during electrolysis, no
progress has been achieved in design of cathodes for
aluminium production cells with a view to restraining
movement of the molten aluminium in order to reduce the
interelectrode gap and the rate of wear of its surface.
U.S. Patent 4,560,488 (Sane et al) discloses a
recent development in molten salt electrolysis cells
concerning making materials wettable by molten aluminium.
However, the carbon or graphite anodes and cathodes are of
conventional design with no suggestion leading to the
present invention.
U.S. Patent 4,681,671 (Duruz) illustrates another
improvement in molten salt electrolysis wherein operation at
CA 02596427 2007-08-17
- 4 -
lower than usual temperatures is carried out utilizing
permanent anodes, e.g. metal, alloy, ceramic or a metal-
ceramic composite as disclosed in European Patent
Application No. 0030834 and U.S. Patent 4,397,729. Again,
while improved operation is achieved at lower temperatures,
there is no suggestion of the subject matter of the present
invention.
PCT Application WO 89/06289 (La Camera et al) deals
with an improved molten electrolysis wherein attention is
directed to an electrode having increased surface area.
However, again, there is no disclosure leading one to the
present invention.
U.S. Patents 3,400,061 (Lewis et al) and 4,602,990
(Boxall et al) disclose aluminium electrowinning cells with
sloped drained cathodes arranged with the cathodes and
facing anode surfaces sloping across the cell. In these
cells, the molten aluminium flows down the sloping cathodes
into a niedian longitudinal groove along the centre of the
cell, or into lateral longitudinal grooves along the cell
sides, for collecting the molten aluminium and delivering it
to a sump.
U.S. Patent 5,203,971 (de Nora et al) discloses an
aluminium electrowinning cell having a partly refractory and
partly carbon based cell lining. The carbon-based part of
the cell bottom may be recessed in respect to the refractory
part, which assists in reducing movement of the aluminium
pool.
US Patent 3,856,650 (Kugler) proposed lining a
carbon cell bottom with a ceramic coating upon which
parallel rows of tiles are placed, in the molten aluminium,
in a grating-like arrangement in an attempt to reduce wear
due to movements of the aluminium pool.
To restrict movement in a "deep" cathodic pool of
molten aluminium, US Patent No 4,824,531 (Duruz et al)
proposed filling the cell bottom with a packed bed of loose
pieces of refractory material. Such a design has many
potential advantages but, because of the risk of forming a
sludge by detachment of particles from the packed bed, the
design has not found acceptance. US Patent No 4,443,313
(Dewing et al) sought to avoid this disadvantage of the
CA 02596427 2007-08-17
- 6 - 5 -
previously mentioned loose packed bed by providing a
monolayer of closely packed small ceramic shapes such as
balls, tubes or honeycomb tiles.
The following references disclose several other
improvements in cell operation.
European Patent Application No. 0308015 (de Nora)
discloses a novel current collector;
European Patent Application No. 0308013 (de Nora)
deals with a novel composite cell bottom; and
European Patent Application No. 0132031 (Dewing)
provides a novel cell lining.
While the foregoing references indicate continued
efforts to improve the operation of molten cell electrolysis
operations, none suggest the invention and all proposals for
means to restrain movement of the aluminium pool or layer on
the cell bottom have proven to be ineffective.
Summary of the Invention
This invention aims to overcome problems inherent in
the conventional design of cells used in the electrowinning
of aluminium via electrolysis of alumina dissolved in molten
fluoride-based melts in particular cryolite, notably by
proposing an improved cell bottom which incorporates means
for restraining the aluminium pool on the cell bottom,
without any need for added pieces which have to be placed or
secured on the cell bottom.
The present invention permits more efficient cell
operation by modifying the design of the cathode blocks on
the cell bottom. Such a modified design may then be utilized
in any conventional cell and even in drained cell
configurations.
The invention concerns an electrolytic cell for the
electrowinning of aluminium from alumina dissolved in a
fluoride-containing molten electrolyte, having a cathode
cell bottom made of a series of carbon cathode blocks each
having a top surface, side surfaces and a bottom surface,
the cathode blocks being connected side-by-side
transversally of the cell for example with ramming paste as
CA 02596427 2007-08-17
- 6 -
in conventional cells or, preferably, by glue. Each cathode
block is provided with a steel or other conductive bar for
the delivery of current. These conductive bars are generally
parallel to one another and extend transversally of the
cell. A series of anodes are suspended facing a pool or a
layer of molten aluminium atop the top surfaces of the
cathode blocks, and the cell bottom has means for
restraining movement of the pool or layer of molten
aluminium.
In the improved cell bottom according to the
invention, the means for restraining movement of the pool or
layer of molten aluminium comprise a series of parallel
channels or grooves in the top surfaces of the carbon blocks
along the direction of said conductive bars transverse to
the cell. In use, these channels or grooves 'are covered by,
and restrain, the pool or the layer of molten aluminium, in
particular against movement in a direction transverse to the
channels or grooves, so as to greatly decrease movement of
the aluminium pool or so as to facilitate the
collection/evacuation of aluminium, which substantially
improves cell operation and extends cell life.
The surfaces of the carbon blocks making up the
cathode cell bottom are most preferably coated with a layer
of aluminium-wettable refractory material, advantageously a
particulate refractory hard metal boride applied from a
slurry containing colloid, for example as disclosed in
WO 93/25731 (Sekhar et al).
The surfaces of the carbon blocks making up the
cathode cell bottom may be covered by a layer of molten
aluminium forming a drained cathode surface, the channels or
grooves forming a canal serving to guide the flow of
aluminium across the cell. In this drained configuration,
the channels or grooves are partly filled with molten
aluminium and the electrolysis takes place between the
aluminium-wetted cathode and the facing anode surface. In
this drained cathode configuration, as explained below, an
arrangement will be provided for removing aluminium from the
sides of the cell.
Alternatively, the surfaces of the carbon blocks
making up the cathode cell bottom are covered by a pool of
CA 02596427 2007-08-17
- 7 -
molten aluminium, the channels or grooves serving to reduce
motion of the aluminium making up this pool. In this case,
the aluminium pool completely covers the carbon blocks so
that the electrolysis takes place between the surface of the
aluminium pool and the facing anode surface. Due to the
shape and size of the channels and their arrangement which
segments the pool into sections, the magnetohydrodynamic
forces are decreased significantly and movement of aluminium
in the pool is significantly reduced. The depth of the
aluminium pool can thus be reduced because of the resulting
reduction in turbulence. Moreover, reduction of the
turbulence in the aluminium pool is important because this
is the origin for the formation of aluminium particles in
the electrolyte, which particles are oxidized by C02 with
the formation of alumina.
In either case, for the drained configuration or
when there is a pool of aluminium, the inter-electrode
distance is reduced with a concomitant reduction of cell
voltage and an increase in energy efficiency.
In all cases, for drained configurations and when
there is a pool of aluminium, the channels are so configured
and arranged, in particular as regards their depth and the
shape and angle of their walls, that the molten aluminium
contained in the channels is restrained from movement in the
longitudinal direction of the cell. For a drained cell
configuration, the aluminium can flow along the channel into
a collection channel; for a pool configuration, the walls of
the channel oppose motion of the aluminium, restraining the
pool and reducing turbulence.
An advantage obtained with the channeled cell bottom
is that its life is extended in comparison with other
electrolytic aluminium production cells. Moreover, the
channeled cathode improves the uniformity of the current
distribution and increases the current efficiency.
Additionally, in particular for deep pool configurations,
sludge can accumulate in the grooves or channels without
disturbing the current distribution. In drained
configurations, the grooves or channels can serve to
eliminate sludge which collects in the grooves or channels
but is flushed out with the molten aluminium.
CA 02596427 2007-08-17
- 8 -
At least some of the transverse channels or grooves
can be formed by bevels or cut-outs along the top edges of
the carbon blocks. In particular, a channel can be defined
between two beveled edges of adjacent blocks.
In most embodiments, the width of the transverse
channels or grooves is at least as great as their depth.
These channels or grooves have a rectangular, trapezoidal,
V-shaped, curved (i.e. concave at least in their bottom
part; most convex sections would not restrain the molten
aluminium) or an asymmetric cross-section designed to permit
the evacuation and collection of aluminium when the cell
operates in a drained cell configuration, or designed to act
as a barrier to aluminium movement to reduce or eliminate
turbulent aluminium pool movement in a pool configuration.
Advantageously, there may be at least one cross
channel or groove which extends along the cell and
intersects with the parallel transverse channels or grooves.
Such crbss channels or grooves may serve to drain the
aluminium down the inclined cathode surfaces, hence guiding
the flow of molten aluminium down these surfaces into the
main channels or grooves. In drained cell configurations,
cross channels or grooves of suitable dimensions can also
serve for the removal of the molten aluminium to an
aluminium reservoir.
When the surface of the carbon cathode cell bottom
I is sloping, cross channels or grooves may run down the
Isloping cathode bottom surface to facilitate drainage of the
aluminium in a drained configuration. For example, the
carbon cathode cell bottom comprises at least two sloping
parts there being at the intersection of the two sloping
parts a collecting channel leading to an aluminium reservoir
for collection of the drained aluminium.
In general, for a drained cathode cell the tranverse
channels or grooves may lead into at least one channel
arranged longitudinally of the cell for collecting the
molten aluminium, and preferably having means such as a weir
for maintaining a constant level of aluminium in the
transversally-extending channels or groove. This aluminium
collection channel may extend along one or both sides of the
cell, or could be a deep central channel machined in the
CA 02596427 2007-08-17
- 9 -
cathode blocks.
Preferably, the surfaces of the carbon blocks making
up the cathode cell bottom are treated to reduce sodium
penetration, for example as described in WO 94/20650 or in
WO 94/24337, or are coated with a layer which reduces sodium
penetration, for example a refractory hard metal boride
applied from a slurry containing colloid as disclosed in
WO 93/25731 (all in the name of Sekhar et al).
In general, the carbon cathode blocks are made
resistant to chemical attack and to mechanical attack. The
surfaces of the carbon blocks making up the channeled
cathode cell bottom can also be coated with a layer which
prior to use or in use be.comes harder than.the carbon
cathode block and thereby protects the surface against
abrasive wear by limited movement of the molten aluminium.
Moreover, the hardened cathode surface remains dimensionally
stable whereas a facing carbon anode may erode and conform
to the shape of the cathode. This surface-hardening effect
can be achieved with the aforementioned refractory boride or
other aluminium wettable refractory layers which provide an
essentially dimensionally stable surface.
In this way, the cathode cell bottom can remain
dimensionally stable during electrolysis, and because of
this, it is both possible and advantageous to provide
channels in the tops of the carbon cathode blocks because
these channeled blocks will remain dimensionally stable
during cell operation.
The cell incorporating the channeled cell bottom can
employ conventional carbon anodes which wear in the normal
way for the pool configuration but~dhose sha e_adapts-~Q the _.
neled cathadei.+ in the drained configuration. Specially-
shape on anodes designed to cooperate with the
channeled cathode design, and in particular to facilitate
gas release at the anode while assisting drainage of molten
aluminium at the cathode can also be used. Dimensionally
stable anodes can also be employed.
One method of fabricating an electrolytic cell
according to the invention comprises providing the channels
in the carbon cathode blocks before or after assembling the
blocks to form the channeled cathode cell bottom. The
CA 02596427 2007-08-17
I - 10 -
channels are possibly machined in the carbon cathode blocks,
for example using a milling cutter. For some shapes,
especially with bevels, it may, however, be convenient to
provide the channels by extrusion. If the blocks include
bevels or cut-outs along their edges, when the adjacent
blocks are brought together, the bevels or cut-outs between
the assembled adjacent blocks form channels.
Machining operations such as milling/cutting in
particular are simple to execute to provide a series of
parallel channels or grooves of any desired shape in the
carbon blocks.
When the blocks are assembled side-by-side, gaps can
be left between the adjacent blocks, which gaps are filled
with an anthracite-based or other usual ramming paste.
Preferably, however, the blocks will be assembled using a
glue, as is known for bonding together carbon cathode blocks
with no or only a very small gap, such as a resin-based
glue, ot an inorganic glue as disclosed in WO 94/20651
(Sekhar). Either way, the assembled blocks form a continuous
carbon cell bottom in the same manner as in conventional
cells, apart from the fact that the surface of the cell
bottom is channeled.
Before or after assembly, the carbon blocks may be
treated to make them resistant to chemical and mechanical
attack. Before start up of the cell for producing aluminium,
the channeled cell bottom is preferably treated to harden
the surface of the cathode blocks and render the surface
wettable by molten aluminium, whereby in use the carbon
cathode blocks remain dimensionally stable and are wetted by
molten aluminium.
The invention also relates to a carbon cathode block
of an electrolytic cell for the electrowinning of aluminium
from alumina dissolved in a fluoride-based molten
electrolyte, ready to be installed in a cell. Such carbon
block has a top surface, side surfaces and a bottom surface
having therealong a longitudinally extending groove or like
recess generally parallel to the block's top and side
surfaces, for receiving a steel or other conductive bar for
the delivery of current.
According to the invention, the top surface of the
CA 02596427 2007-08-17
- 11 -
cathode block is provided with at least one channel or
groove; or at least one edge of the top surface of the
cathode block is provided with at least one bevel or cut-
out, or at least a part of the top surface is inclined, so
that when two cathode blocks are placed side-by-side a
channel or groove is formed between them. This channel,
groove, bevel, cut-out or inclined part extends along the
top surface of the cathode block generally parallel to the
groove or like recess in the bottom surface of the cathode
block, so that when the cathode block with its current
conducting bar is installed in a cell, the channels or
grooves in the cathode block's top surfaces are arranged
transversaly of the cell.
This channel or groove in the top surface of the
block, or formed between two blocks, is so shaped and
dimensioned that when the cathode blocks are used to
electrowin aluminium, molten aluminium collected in the
channel or groove is restrained from moving transversally to
the channel or groove, providing substantial advantages in
operation, as explained above.
These carbon blocks, which can incorporate all of
the features described in relation to the complete cell,
may have a flat top surface, or its top surface may be
sloping or may comprise two roof-like inclined sections such
that the blocks can be assembled side-by-side to form a cell
bottom with alternately sloping sections which, at their
lower intersections, form the aforesaid channels in the
blocks' top surfaces.
Descrigtion of the Drawincj
Reference is now made to the drawings wherein:
- Figure 1 schematically shows part of a cell bottom
formed of three cathode blocks provided with parallel
channels in accordance with the invention, this schematic
view being in longitudinal cross-section and side elevation
of the cell;
- Figure 2 schematically shows three different
cathode blocks of cell bottoms in accordance with the
invention provided with parallel channels of different
shapes;
CA 02596427 2007-08-17
- 12 -
- Figure 3 is a schematic sectional view though part
of an electrolytic cell according to the invention,
incorporating channeled cathode blocks and carbon anodes;
- Figure 4 is a schematic sectional view through
part of another electrolytic cell according to the
invention, also incorporating channeled cathode blocks and
carbon anodes;
- Figure 5 is a similar view of another electrolytic
cell according to the invention, incorporating channeled
cathode blocks and dimensionally stable anodes;
- Figure 6 is a view similar to Figure 3 of another
electrolytic cell according to the invention;
- Figure 7 is a view similar to Figure 4 of another
electrolytic cell according to the invention; and
- Figure 8 is a schematic plan view of the cell
bottom of Figure 7 during operation, partly cut-away and
with the anodes and cell superstructure not visible.
Detailed Descr?j2tion of the Invention
Figure 1 schematically shows part of a cell bottom
formed of an assembly of channeled cathode blocks 10
according to the invention, three such blocks being shown.
The blocks 10 are generally rectangular and made of carbon
in the form of anthracite or graphite of the normal grade
used for aluminium production cathodes. In their lower face,
the blocks 10 have a recess 12 receiving a steel conductor
bar 11 which is connected in the blocks by an electrically
conductive bonding material for example cast iron. These
steel conductor bars 11 extend externally to a negative bus
bar of the cell.
The rectangular cathode blocks 10 have a top surface
(which will form the cathode cell bottom) side surfaces
(which will be joined together) and a bottom surface in
which the recess 12 is provided in the form of a rectangular
groove whose faces are parallel to the block's top and side
surfaces. Usually, the recess 12 extends all the way along
the blocks' bottom surface.
The side edges 20 of the block's top surfaces are
CA 02596427 2007-08-17
- 13 -
beveled and, in the middle of the blocks' upper surfaces,
generally V-shaped grooves 21 are machined. When two blocks
are brought together, the adjacent beveled edges 20 form
also a V-shaped groove or channel similar in shape and size,
5 and parallel to, the grooves 21.
The adjacent blocks 10 are joined side-by-side by
ramming paste 14, for example an anthracite-based paste, to
form a continuous carbon cell bottom. Usually one b1oc'
extends over subatantiallyRthe_.entire width of= the-cell,: if
õ~~>-=..::..,~:"_,,,. .~ =--r.;,-.:;.=....~c:..._.:=.~.c:;.~~,+r.:~:c~:-
.:c,=..-v:,a,,,,:..-,z------ '--"e..a. Ji' ~
10 this is not so, several blocks 10 are arranged end-to-end
across the cell, i.e. along the direction of the conductor
bars 11, and may also be joined by ramming paste. Instead of
using ramming paste, the blocks can advantageously be bonded
by a resin-based glue, in which case the gap between the
adjacent blocks would be much smaller.
When the blocks 10 are joined side-by-side to form a
cell bottom, the conductive bars 11 extend across the cell
and protrude from both sides of the cell (or alternate
conductor bars protrude from opposite sides) for connection
to the current supply.
An anode 15, seen in side view, is diagramatically
indicated in a dashed line. In use, the cell bottom formed
by the blocks 10 with the V-shaped grooves or channels 20,21
is covered by a shallow pool of molten aluminium 40 (see
Figure 3), or just the grooves or channels 20, 21 are partly
filled with molten aluminium, as will be described in detail
later.
Figure 2 shows three similar channeled carbon blocks
10 but with grooves or channels of different shapes, which
blocks can be assembled into a cell bottom using glue or
ramming paste. A first block has a parallel series of
generally rectangular grooves 22 which are slightly wider
than deep. Usually, the width of the grooves 22 is at least
as great as their depth. The second block has generally
trapezoidal grooves 23, and the third block has U-shaped
grooves 24 of rounded cross-section, these shapes being
given by way of example among many possible shapes.
In all cases, the bevels 20 and grooves 21, 22, 23,
24 extend along the top surface of the cathode block
parallel to the groove 12 receiving the conductor bar 11.
CA 02596427 2007-08-17
- 14 -
All of the described bevels 20 and grooves 21, 22, 23, 24
can easily be machined in the blocks 10, for instance using
a milling cutter. Alternatively, it is possible to provide
grooves or bevels or other forms of channel by other
methods, for example by extrusion.
Figure 3 schematically shows, in longitudinal cross-
section and side elevation, an aluminium production cell
incorporating a carbon cell bottom formed of channeled
carbon blocks 10 similar to those described above. The
diagramatically-shown cell structure comprises a steel shell
30 of known type containing refractory material 31 over its
bottom and extending up its sides. The blocks 10, supported
on this refractory material 31, are arranged side-by-side
and extend across the cell. The carbon blocks 10 are
connected together by ramming paste 14, or alternatively are
glued together, and the endmost blocks 10 are connected by
ramming paste 34 to an insert 32 of carbon or a refractory
carbide such as silicon carbide at the cell end. As
previously, the bottoms of blocks 10 have recesses 12
receiving steel conductor bars 11 connected in the blocks by
cast iron, which conductor bars 11 extend externally to a
negative bus bar of the cell, situated along the side of the
cell.
The top surfaces of the blocks 10 have a series of
parallel channels in the form of a wavy profile 25, though
this could be any one of the other shapes shown in Figures 1
and 2, or other channeled shapes extending across the cell.
This channeled profile 25 forms the top surface of the
carbon cell bottom which is advantageously covered with a
coating 35 of aluminium-wettable refractory material on
which, as shown, there is a pool 40 of molten aluminium
below a fluoride-based molten electrolyte 41 such as molten
cryolite containing dissolved alumina.
Several anodes 15, conventionally blocks of prebaked
carbon, are suspended in the cell by the usual mechanisms
(not shown) enabling their height to be adjusted. Oxygen
evolving non-carbon anodes may be suspended in the cell
instead of the carbon anodes 15 but do not need to be
vertically adjustable because they are non-consumable. The
anodes 15 dip in the molten electrolyte 41 facing the
channeled cathode surface 25. The anode-cathode gap is not
CA 02596427 2007-08-17
1 - 15 -
shown to scale. In operation, the cryolite-based electrolyte
41 is usually at a temperature of about 950 C, but the
invention applies also to components used in cells with
electrolytes well below 900 C, and as low as 700 C.
The surface of the channeled profile 25 of the
carbon cathode blocks 10 can be made dimensionally stable by
applying a coating 35 of an aluminium-wettable refractory
hard metal (RHM) having little or no solubility in aluminium
and having good resistance to attack by molten cryolite.
Note that the coating 35 also covers the ramming paste 14
and 34. Useful RHM include borides of titanium, zirconium,
tantalum, chromium, nickel, cobalt, iron, niobium and/or
vanadium. Useful cathode materials are carbonaceous
materials such as anthracite or graphite.
It is preferred that the channeled profile 25 of the
cathode of the present invention have a coating 35 of
particulate refractory hard metal boride in a colloid
applied 'from a slurry of the particulate refractory hard
metal boride in a colloid carrier, wherein the colloid
comprises at least one of colloidal alumina, silica, yttria,
ceria, thoria, zirconia, magnesia, lithia, monoaluminium
phosphate or cerium acetate. The colloidal carrier has been
found to considerably improve the properties of the coating
produced by non-reactive sintering.
WO 93/25731 (Sekhar et al) provides a method of
applying refractory hard metal boride to a carbon containing
component of a cell for the production of aluminium, in
particular by the electrolysis of alumina dissolved in a
cryolite-based molten electrolyte, this method comprising
applying to the surface of the component a slurry of
particulate preformed refractory boride in a colloidal
carrier as specified above, followed by drying, and by heat
treatment before or after the component i=s installed in the
aluminium production cell.
The method of application of the slurry to the
channeled cathodes of the present invention involves
painting (by brush or roller), dipping, spraying,, or pouring
the slurry onto the channeled cathode and allowing to dry
before another layer is added. The coating 35 does not need
to be entirely dry before the application of the next layer.
CA 02596427 2007-08-17
- 16 -
It is preferred to heat the coating 35 with a suitable
source so as to completely dry it and improve densification
of the coating. Heating and drying take place preferably in
non-oxidizing atmospheres at about 80-200 C, usually for
half an hour to several hours and further heat treatments
are possible.
The channeled profile 25 of the cathode may be
treated by sand blasting or pickled with acids or fluxes
such as cryolite or other combinations of fluorides and
chlorides prior to the application of the coating. Similarly
the channeled cathode surface may be cleaned with an organic
solvent such as acetone to remove oily products and other
debris prior to the application of the coating. These
treatments will enhance the bonding of the coatings to the
channeled carbon cathode.
After coating the channeled cathode by dipping,
painting or spraying the slurry or combinations of such
techniques in single or multi-layer coatings and drying, a
final coat of the colloid alone may be applied lightly prior
to use.
Before or after application of the coating 35 and
before use, the channeled cathode can be painted, sprayed,
dipped or infiltrated with reagents and precursors, gels
and/or colloids. For instance, before applying the slurry of
particulate refractory boride in the colloidal carrier the
channeled cathode can be impregnated with e.g. a compound of
lithium to improve the resistance to penetration by sodium,
as described in WO 94/20650 (Sekhar et al).
To assist rapid wetting of the channeled cathode by
molten aluminium, the refractory coating 35 on the channeled
cathode may be exposed to molten aluminium in the presence
of a flux assisting penetration of aluminium into the
refractory material, the flux for example comprising a
fluoride, a chloride or a borate, of at least one of lithium
and sodium, or mixtures thereof. Such treatment favors
aluminization of the refractory coating by the penetration
therein of aluminium.
In operation of the cell illustrated in Figure 3, as
shown, the coating 35 on the channeled profile 25 of the
carbon blocks 10 making up the cathode cell bottom is
CA 02596427 2007-08-17
- 17 -
covered by a pool of molten aluminium 40. The channels in
the surface serve to restrain motion of the aluminium making
up the pool 40 mainly in the longitudinal direction of the
cell. As illustrated, the aluminium pool 40 completely
covers the carbon blocks 10 so that the electrolysis takes
place between the surface of the aluminium pool 40 and the
facing surface of anode 15. An advantage is that the depth
of the aluminium pool 40 can be reduced because of the
reduction in turbulence. Moreover, sludge can accumulate in
the channels of the channeled surface 25 without disturbing
the current distribution.
It should be noted that the motion restraining
effect of the wavy profile 25 is due mainly to the concave
lower part and to the lower part of the inclined walls which
has a sufficient angle and height to obstruct movement of
the molten aluminium in the direction along the cell.
Alternatively, the channeled surface 25 of the
carbon blocks 10 making up the cathode cell bottom, coated
with a refractory aluminium-wettable coating 35, may be
covered by a thin layer of molten aluminium forming a
drained cathode surface, the channels serving to guide the
flow of aluminium across the cell. In this drained
configuration, the channels are partly filled with molten
aluminium so that the electrolysis takes place between the
aluminium-wetted cathode surface 25 and the facing surface
of anode 15 which will tend to wear in such a way as to
conform to the surface 25. A cell with such a drained
configuration will be described below in connection with
Figure 6.
Figure 4, where the same reference numerals are used
to designate the same elements, shows part of another
aluminium production cell whose cell bottom is made up of a
series of channeled carbon blocks 10 joined by ramming paste
14. Here, the channels are formed by juxtaposing blocks 10
having inclined surfaces 26 or 27 arranged alternately
facing one another so as to form V-shaped channels 28
between adjacent blocks, these channels 28 receiving a
stream or canal 40' of molten aluminium in their bottom
part, running across the cell. Above the V-shaped channel 28
and aluminium canal 40' are carbon anodes 15 with
corresponding V-shaped active faces 16 terminating with a
1
CA 02596427 2007-08-17
- 18 -
lower flattened part 17 opposite the aluminium canal 40'.
This flattened part 17 can be formed in use.
The cell of Figure 4 operates as a drained cathode
cell, wherein the aluminium produced on the inclined cathode
surfaces 26 and 27 coated with the aluminium-wettable
refractory coating 35 flows down the inclined surfaces, in
the longitudinal direction of the cell, into the channel 28
where it is collected in the aluminium canal 40'. The
channels 28 join into one or two side channels inclined in
the longitudinal direction of the cell in order to
permanently drain the product aluminium, at a rate to keep
the level of aluminium canal 40' stable.
In this drained configuration, the channels 28 are
partly filled with molten aluminium so that the electrolysis
takes place between the inclined aluminium-wetted cathode
surfaces 26,27 and the facing inclined surfaces 16 of anode
15, as well as between the aluminium canal 40' and the
facing flattened part 17 of the anode 15 which will wear in
such a way as to conform to the opposing surfaces (26,27 and
the flat top of canal 40'). Moreover, the inclination of the
anode surfaces 16 assists in release of the anodically-
formed gases. Alternatively, it is possible to operate in a
fully drained configuration without an aluminium canal 40',
or with a narrow and shallow canal of aluminium.
Figure 5 shows a cell of similar design, where the
same elements are designated by the same references, but
which includes dimensionally stable anodes 18 in a "roof"
configuration straddling the tops of the adjacent carbon
cathode blocks 10 which also have alternately arranged
inclined surfaces 26,27 coated with an aluminium-wettable
refractory coating 35.
The anodes 18 are made of or coated with any
suitable non-consumable or substantially non-consumable,
electronically-conductive material resistant to the
electrolyte and to the anodically produced oxygen and other
gases, vapors and fumes present in the cell. The anodes 18
may for example have a metal, alloy or cermet substrate
which is protected in use by a cerium-oxyfluoride-based
protective coating produced and/or maintained by maintaining
a concentration of cerium in the electrolyte, as described
CA 02596427 2007-08-17
- 19 -
in U.S. patent 4,614,569 (Duruz et al).
The cell of Figure 5 also operates as a drained
cathode cell, wherein the aluminium produced on the inclined
cathode surfaces 26 and 27 coated with an aluminium-wettable
refractory coating 35 flows down to the bottom of these
inclined surfaces where it is collected as an aluminium
canal 40' in a groove 28' which is generally U-shaped in
cross-section. This groove 28' is formed between cut-outs in
the edges of the carbon blocks 10, when the blocks are
fitted together. As before, the cathode surfaces 26,27 are
inclined in the longitudinal direction of the cell in order
to permanently drain the product aluminium into the canal
40', and the aluminium is removed from this canal 40' at a
rate to keep its aluminium level stable. In this drained
configuration, the grooves 28' are filled with the molten
aluminium forming canal 40'. Electrolysis takes place
between the inclined aluminium-wetted cathode surfaces 26,27
and the facing inclined surfaces of the dimensionally-stable
anodes 18. The inclination of anodes 18 assists in release
of the anodically-formed gases through a central opening 19
and this can be further assisted if needed by providing
ridges on the anodes 18 or making the anodes foraminate.
Figure 6 shows part of another cell according to the
invention similar to the cell shown in Figure 3. Instead of
having a wavy channeled surface, the blocks 10 have bevelled
upper side edges 20 and may be bevelled also along their top
end edges, as indicated in dotted lined at 29. The bevelled
side edges 20 of the adjacent blocks form V or U-shaped
channels running along the blocks 10, there being a similar
V or U-shaped channel formed between the bevelled edge of
the endmost block 10 and the adjacent mass of ramming paste
34. The main channels formed by the bevels 20 extend
parallel to the conductor bars 11, i.e. across the cell, and
the cross-channels formed by bevels 29 extend along the
cell.
These channels formed b the beveled edges~ 20_a e
the advantage of_equalizinc
r the :.current distribution in the
. +~yG~ --.w..~ --.= .r....:.........:..- . .._ .. - ~'-= - -carbon ~locks 10
from the entrally_.~1oca ed current
""~~':r~'=- _......w~.~,.r1:~.:.::-a...::..::.a..~scc.:._ .~...~ _.~
dibut~on barin , the,ahb,oc~}5,';~ower~.~Sur~ace This may in
particular make it possible to reduce the height of the
carbon blocks 10, leading to a saving in carbon which
CA 02596427 2007-08-17
- 20 -
reduces the amount of toxic carbon that has to be disposed
of when the carbon potlining is reconstructed.
In Figure 6, the adjacent blocks 10 are shown as
being bonded together by a glue 14', so there is essentially
no gap between them. As shown, the aluminium-wettable
refractory coating 35 covers the entire upper surfaces of
blocks 10 with bevels 20, 29 and the glued joints 14', and
also extends over the adjacent mass of ramming paste 34.
The purpose of the cross-bevels 29 in the top end
edges of the blocks 10 is to form a cross-channel
perpendicular to the parallel channels formed by the V or U-
shaped grooves between bevels 20, for draining off the
product aluminium to maintain the canals 40' of aluminium in
he V or U-shaped grooves at a constant level.-Such cross-
fhannels can be formed in one or both ends of the blocks
tnd, if required, one or more intermediate cross channels
an be formed by machining grooves across the blocks 10,
ntersecting with the V or U-shaped grooves formed by bevels
,z0.
.,
These cross-channels, which extend along the cell,
are connected to a reservoir of molten aluminium, possibly
with a weir in order to set the level of the aluminium
canals 40'. Operation is possible with a fluctuating level
of the aluminium canals 40' or with a steady level.
The cell shown partly in Figures 7 and 8 is similar
to that of Figure 4 in that it comprises a cell bottom made
up of a series of carbon blocks 10 joined by ramming paste
14, where V-shaped channels 28 are formed by juxtaposing
blocks 10 which, in this example, have roof-like inclined
surfaces 26,27 side-by-side with the corresponding surfaces
27,26 of the adjacent blocks. The V-shaped channels 28
receive a stream or canal 40' of molten aluminium in their
bottom part, running across the cell. Above the V-shaped
channels 28 and aluminium canals 40' are carbon anodes 15
with corresponding V-shaped active faces 16. Opposite the
aluminium canal 40', the anode faces 16 have a flattened
part 17 which can be formed in use.
The cell of Figures 7 and 8 operates as a drained
cathode cell, wherein the aluminium produced on the inclined
cathode surfaces 26 and 27 coated with the aluminium-
CA 02596427 2007-08-17
- 21 -
wettable refractory coating 35 flows down these surfaces and
into the channel 28 where it is collected in the aluminium
canal 40'. The direction of flow of the molten aluminium is
schematically indicated in Figure 8 by arrows. As shown, the
channels 28 join into two side channels 42, running along
and inclined in the longitudinal direction of the cell, in
order to permanently drain the product aluminium. The molten
aluminium is drained at a rate to keep the level of the
aluminium canals 40' stable, for example by having a weir at
the outflow end of the side channels 42.
As illustrated in the right hand part of Figure 8,
the two longitudinally sloping surfaces 26 and 27 of each
cathode block 10 can be provided with cross channels or
grooves 43 which run down these sloping surfaces 26 and 27
and lead into the main channel 28 running across the cell.
Any suitable number of cross grooves 43 can be provided with
any suitable spacing. These cross grooves 43 guide the flow
of aluminium down the sloping cathode surfaces 26,27, while
restraining movement of the aluminium across the cell as it
drains down these inclined cathode surfaces. Only once the
drained aluminium joins the canal 40' can it flow freely
=across the cell to the side channels 28.
This arrangement of the cross grooves 43 down the
inclined cathode surfaces 26,27, the main collecting grooves
28 across the cell, and the side channels 42 provides a
perfectly controlled flow of the molten aluminium over the
cell bottom, avoiding turbulence due to magnetohydrodynamic
forces.
The side channels 42 can be formed in the ends of
the cathode blocks 10, or can be made in the cell sidewalls,
or can be a combination of bevels or cut-outs in the ends of
the cathode blocks 10, cooperating with bevels or ledges in
the cell sidewalls. Additionally or alternatively, a central
channel could be provided by machining recesses in the
centre parts of the inclined walls 26, 27 of the cathode
blocks 10, or by making each cathode section from two
cathode blocks placed end-to-end, with cut-outs in the
facing ends.
