Note: Descriptions are shown in the official language in which they were submitted.
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ALUMINIUM ELECTROWINNTNG CELLS HAVING A DRAINED CATHODE
BOTTOM AND AN ALUMINIUM COLLECTION RESERVOIR
Field of the Invention
This invention relates to a cell for the
electrowinning of aluminium from alumina dissolved in a-
fluoride-containing molten electrolyte having an aluminium-
wettable drained cathode surface and an aluminium reservoir,
and a method to produce aluminium in such an aluminium
electrowinning cell.
Background Art
The technology for the production of aluminium by
the electrolysis of alumina, dissolved in molten cryolite,
at temperatures around 950°C is more than one hundred years
old and still uses carbon anodes and cathodes.
Only recently has it become possible to coat carbon
cathodes with a slurry which adheres to the carbon and
becomes aluminium-wettable, as disclosed in US Patent
5,316,718 (Sekhar/de Nora) and US Patent 5,651,874 (de
Nora/Sekar).
US Patent 5,683,559 (de Nora) proposed a new drained
cathode design for aluminium production cells, where grooves
or recesses were incorporated in the surface of blocks
forming the cathode surface in order to channel the drained
product aluminium. A specific embodiment provides an
enhanced anode and drained cathode geometry where aluminium
is produced between V-shaped anodes and cathodes and
collected in recessed grooves.
WO98/53120 (Berclaz/de Nora) discloses an aluminium
production cell provided with a cathode mass supported on a
cathode shell or plate, the cathode mass having a horizontal
drained cathode surface and a central channel extending
along the cell for draining molten aluminium.
WO00/63463 (de Nora) discloses an aluminium
production cell in which the drained cathode bottom is
divided into four drained cathode sections by a
longitudinally extending central aluminium evacuation groove
and a central aluminium collection reservoir extending
centrally across the cell on a spacer body located between
and parallel to cathode blocks placed across the cell.
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Objects of the Invention
An object of the invention is to provide an
aluminium electrowinning cell bottom and an aluminium
electrowinning cell having an aluminium-wettable drained
cathode which is made of conventional cell blocks which can
be easily retrofitted in existing cells.
A further object of the invention is to provide an
aluminium electrowinning cell having an aluminium collection
reservoir from which molten aluminium can be tapped without
the risk of aluminium freeing in the reservoir, and which
can be easily retrofitted in existing cells.
A major object of the invention is to provide a
modular assembly of an aluminium collection reservoir and
cathode blocks for an aluminium production cell.
Yet another object of the invention is to provide a
method to produce aluminium in an aluminium electrowinning
cell provided with such a cell bottom.
Summary of the Invention
The invention provides a cell for the electrowinning
of aluminium from alumina. This cell comprises a plurality
of anodes, in particular oxygen-evolving anodes, facing a
series of pairs of cathode blocks with aluminium-wettable
drained cathode surfaces placed across the cell and a
longitudinal aluminium collection recess which is located
between the cathode blocks and which extends along the cell
and is at a lower level than the drained cathode surfaces so
that during use aluminium produced on the drained cathode
surfaces drains into the aluminium collection recess.
According to the invention, the aluminium collection
recess is defined between the cathode blocks by a separate
reservoir body which is placed between the blocks of each
pair of cathode blocks spacing them apart across the cell
and which extends along the cell.
The cathode blocks can be made of graphite and/or
have an aluminium-wettable upper part. For example, the
cathode blocks are coated with an aluminium-wettable layer.
Alternatively, the cathode blocks are made of aluminium
wettable material.
Suitable aluminium-wettable materials and carbon
materials for cathode blocks are disclosed in US Patent
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5,651,874 (de Nora/Sekhar), and PCT applications W098/17842
(Sekhar/Duruz/liu), W001/42168 (de Nora/Duruz) and
W001/42536 (Nguyen/Duruz/de Nora).
The cathode blocks may be covered with porous
ceramic-based plates filled with molten aluminium whose
surfaces serve as aluminium-wettable drained cathode
surfaces on which aluminium is produced and from which
aluminium drains into the collection reservoir.
The ceramic-based plates are preferably made of
materials which are resistant and inert to molten aluminium.
The inert and resistant ceramic material may comprise at
least one oxide selected from oxides of aluminium,
zirconium, tantalum, titanium, silicon, niobium, magnesium
and calcium and mixtures thereof, as a simple oxide and/or
in a mixed oxide, for example an aluminate of zinc (~nA104)
or titanium (TiA105). Other suitable inert and resistant
ceramic materials can be selected amongst nitrides, carbides
and borides and oxycompounds thereof, such as aluminium
nitride, AlON, SiAlON, boron nitride', silicon nitride,
silicon carbide, aluminium borides, alkali earth metal
zirconates and aluminiumates, and their mixtures.
Preferably, the ceramic-based plates contain an
aluminium-wetting agent. Suitable wetting agents include
metal oxides which are reactable with molten aluminium to
form a surface layer containing alumina, aluminium and metal
derived from the metal oxide and/or partly oxidised metal,
such as manganese, iron, cobalt, nickel, copper, zinc,
molybdenum, lanthanum or other rare earth metals or
combinations thereof.
Suitable materials for producing the openly porous
ceramic-based plates are described in US Patent 4,600,481
(Sane/ln~h.eeler/Gagescu/Debely/AdorianlDerivaz) and
PCT/IB02/00668 (de Nora).
The aluminium-filled ceramic-based plates may extend
from the cathode blocks over part of the aluminium
collection recess, thereby the surface area of the
aluminium-wettable drained cathode surface is increased.
The reservoir body may be made of anthracite or
other carbonaceous material.
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The reservoir body can be made of a plurality of
sections assembled end-to-end longitudinally along the cell
and/or side-by-side across the cell.
In one embodiment, the reservoir body has a
generally U-shaped cross-section across the cell. The
aluminium collection recess can have a triangular,
rectangular or curved cross-section or a variation thereof
which is suitable for the collection of aluminium. For
example, the aluminium collection recess is generally U-
shaped with rounded lower corners and/or an outwardly curved
upper part in cross-section
In another embodiment, the reservoir body has a
rectangular cross-section with a planar upper surface which
is at a level below the drained cathode surfaces and which
constitutes the bottom surface of the aluminium collection
recess . Upper parts of end faces of the cathode blocks may
form the lateral surfaces of the aluminium collection
recess.
The bottom part of the aluminium collection recess
may be horizontal or sloping longitudinally along the cell
down towards one end of the cell or the centre of the cell.
Preferably, the anodes are oxygen-evolving anodes,
such as metal-based anodes, in particular metal-based anodes
having an oxide-based outer part. Such anodes can be made of
an iron alloy comprising nickel and/or cobalt whose surface
may be oxidised.
Suitable metal-based anodes are disclosed in
W000/06802, WO00/06803 (both in the name of Duruz/de Nora/
Crottaz), WO00/06804 (Crottaz/Duruz), W001/42535 (Duruz/
de Nora), W001/42534 (de Nora/Duruz) and W001/42536 (Duruz/
Nguyen/de Nora). Further oxygen-evolving anode materials are
disclosed in W099/36593, W099/36594, WO00/06801, W000/06805,
WO00/40783 (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) .
Oxygen-evolving anodes may have an electrochemically
active part coated with a slowly soluble protective layer,
or a protective layer made of one or more cerium compounds,
in particular cerium oxyfluoride, as disclosed in US Patents
4,614,569 (Duruz/Derivaz/Debely/ Adorian), 4,680,094
(Duruz), 4,683,037 (Duruz) and 4,966,674 (Bannochie/
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Sheriff), which can be continuously replenished by in-situ
electrodeposition thereon of the cerium compound(s).
The invention also relates to a method of producing
. aluminium in a cell as described above. This method
comprises passing an electrolysis current between the anodes
and facing drained cathode surfaces in an electrolyte
containing dissolved alumina to evolve gas, such as oxygen,
at the anodes and produce aluminium on the drained cathode
surfaces. The produced aluminium draining from the drained
cathode surfaces into the aluminium collection recess
defined by the separate reservoir body.
As mentioned above, the anodes may be oxygen-
evolving and can be coated with a protective layer, such as
a layer of one or more cerium compounds, in particular
cerium oxyfluoride, in which case an amount of cerium
species is preferably maintained in the electrolyte to
maintain the protective cerium-based layer.
A further aspect of the invention relates to a
bottom of a cell for the electrowinning of aluminium from
alumina. This cell bottom comprises a series of pairs of
cathode blocks with aluminium-wettable drained cathode
surfaces placed thereacross and a longitudinal aluminium
collection recess which is located between the cathode
blocks and which extends along the cell bottom and is at a
lower level than the drained cathode surfaces so that during
use aluminium produced on the drained cathode surfaces
drains into the aluminium collection recess.
According to the invention, the aluminium collection
recess is defined between the cathode blocks by a separate
reservoir body which. is placed between the cathode blocks
spacing them apart across the cell bottom and which extends
along the cell bottom.
The cell bottom may comprise any of the above cell
bottom-related features or any combination thereof.
Another aspect of the invention relates to a method
of producing aluminium in a cell comprising a cell bottom as
described above. This method comprises passing an
electrolysis current between anodes and the drained cathode
surfaces of the cell bottom in an electrolyte containing
dissolved alumina to evolve gas at the anodes and produce
aluminium on the drained cathode surfaces. The produced
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aluminium drains from the drained cathode surfaces into the
aluminium collection recess defined by the separate
reservoir.
Whereas it is preferred to use non-carbon anodes to
evolve oxygen during use as mentioned above, it is also
possible to use carbon anodes on which carbon dioxide is
produced during use.
Brief Description of the Drawings
The invention will be further described by way of
example with reference to the accompanying schematic
drawing, in which Figure 1 illustrates a drained-cathode
cell having an aluminium collection reservoir in accordance
with the invention.
Detailed Description
The cell shown in Figure 1 comprises a plurality of
pairs of oxygen-evolving anodes 10 dipping in a molten
electrolyte 5 and facing a series of pairs of cathode blocks
with aluminium-wettable drained cathode surfaces 20
spaced apart across the cell and a separate longitudinal
20 reservoir body 30 which is located between the spaced apart
blocks 25. The reservoir body 30 has an upper surface which
defines a central aluminium collection recess 35. This
recess 35 extends along and is at a lower level than the
drained cathode surfaces 20 so that during use aluminium
25 produced on the drained cathode surfaces 20 drains into the
aluminium collection recess 35.
The cathode blocks 25 are made of graphite and have a
reduced height, e.g. 30 cm, and are coated with an
aluminium-wettable layer 22 which protects the graphite from
erosion and wear. Suitable aluminium-wettable layers are
disclosed in the above mentioned US Patent 5,651,874,
W098/17842, W001/42168 and W001/42531.
As shown in Figure 1, the reservoir body 30 is
generally U-shaped and the aluminium collection recess 35
has rounded lower corners and an outwardly curved upper
part.
The reservoir body 30 is made of two generally L-shaped
sections 31 assembled across the cell. The reservoir
sections 31 are made of anthracite-based material. The
aluminium-wettable layer 22 extends in the recess 35 to
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protect the reservoir body 30 during use against wear and
sodium intercalation.
As shown in Figure 1, the reservoir body 30 extends
below the cathode blocks 25 into the refractory and
insulating material 26 of the cell bottom permitting a
maximisation of the capacity of the aluminium collection
recess 35.
Furthermore, the reservoir body 30 has a solid base 32
which extends from above to below the bottom face of the
cathode blocks 25 and provides sufficient mechanical
resistance to keep the blocks 25 properly spaced. apart
across the cell when exposed to thermal expansion during
start-up of the cell and normal operation. As shown in
dotted lines in the upper part of the reservoir body 30,
longitudinally spaced apart spacer bars 33 placed across the
reservoir body 30 may provide additional mechanical strength
to the reservoir body 30. Such spacer bars 33 can be made of
carbon material coated with an aluminium-wettable protective
layer.
The cathode blocks 25 are covered with porous ceramic-
based plates 21 which are filled with molten aluminium and
which form the aluminium-wettable drained active cathode
surfaces 20 on the cathode blocks 25. As shown in Figure 1,
the aluminium-filled ceramic-based plates 21 extend from the
cathode blocks 25 over part of the aluminium collection
recess 35. Thus, during use projecting parts of the
aluminium-wettable drained active cathode surfaces 20 are
located above the aluminium collection recess 35.
The openly porous plates 21 are spaced apart over the
aluminium collection recess 35 to leave an access for the
tapping of molten aluminium 60 through a conventional
tapping tube. The spacing between the openly porous plates
21 over the aluminium collection recess 35 can be much
smaller along the remaining parts of the recess 35, thereby
maximising the surface area of the active cathode surfaces
20.
The cell shown in Figure 1 comprises a series of corner
pieces 42 made of the openly porous material of the plates
21 and filled with aluminium and placed at the periphery of
the cell bottom against sidewalls 40. The sidewalk 40 and
the surface of the electrolyte 5 are covered with a ledge
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and a small crust of frozen electrolyte 6. The cell is
fitted with an insulating cover 45 above the electrolyte
crust 6. Further details of suitable covers are disclosed in
WO99/02763 (de Nora/Sekhar), W001/31086 (de Nora/Duruz) and
PCT/IB02/00669 (de Nora/Berclaz).
The cell is also provided with exhaust pipes (not
shown) that extend through the cover 45 for the removal of
gases produced during electrolysis.
The cell~comprises alumina feeders 15 with feeding
tubes 16 that extend through the insulating cover 45 between
the anodes 10. The alumina feeders 15 are associated with a
crust breaker (not shown) for breaking the crust 6
underlying the feeding tube 16 prior to feeding.
In a variation, the insulating material of the
sidewalk 40 and cover 45 may be sufficient to prevent
formation of any ledge and crust of frozen electrolyte. In
such a case, the sidewalls 40 are preferably completely
shielded from the molten electrolyte 5 by a lining of the
aforesaid openly porous material filled with aluminium.
The anodes 10 are preferably made of electrolyte
resistant inert metal-based material. Suitable metal-based
anode materials include iron alloys comprising nickel and/or
cobalt which may be heat-treated in an oxidising atmosphere.
Suitable anode designs which provide optimal cell
operation are disclosed in WO00/40781 and WO00/40782 (both
in the name of de Nora).
The lifetime of the anode may be increased by a
protective coating made of cerium compounds, in particular
cerium oxyfluoride. Such coatings and cell operation
therewith are disclosed in the above mentioned US Patents
4,614,569, 4,680,094, 4,683,037 and 4,966,674.
To reduce the dissolution of the anodes 10 in the
electrolyte, the cell may be operated with an electrolyte 5
at reduced temperature, typically from about 850° to 940°C,
preferably from 880° to 930°C. Operating with an electrolyte
at reduced temperature reduces the solubility of oxides, in
particular of alumina. For this reason, it is advantageous
to enhance alumina dissolution in the electrolyte 5.
Enhanced alumina dissolution may be achieved by
utilising an alumina feed device which sprays and
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distributes alumina particles over a large area of the
surface of the molten electrolyte 5. Suitable alumina feed
devices are disclosed in greater detail in W000/63464 (de
Nora/Berclaz). Furthermore, the cell may comprise means (not
shown) to promote circulation of the electrolyte 5 from and
to the anode-cathode gap to enhance alumina dissolution in
the electrolyte 5 and to maintain in permanence a high
concentration of dissolved alumina close to the active
surfaces of anodes 10, for example as disclosed in
W000/40781 (de Nora).
During operation of the cells shown in Figure 1,
alumina dissolved in the electrolyte 5 is electrolysed to
produce oxygen on the anodes 10 and aluminium 60 on the
drained cathode surfaces 20. The product aluminium 60 drains
from the cathode surfaces 20 into the reservoir 30 from
where it can be tapped.
