Note: Descriptions are shown in the official language in which they were submitted.
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ELECTROLYTIC CELL WITH IMPROVED POWDER FEED DEVICE
Field of the Invention
The present invention relates to a device for
feeding powder material to different areas of a molten
electrolyte of a metal electrowinning cell. The powder
material may be a compound of the metal to be electrowon or
one or more constituents of the electrolyte.
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 and still uses carbon anodes and
cathodes.
In this process alumina is fed to the molten
electrolyte for its subsequent dissolution and electrolysis.
Conventional cells are operated with a crust of frozen
electrolyte above the molten electrolyte which crust needs
to be periodically broken to form an opening for feeding
alumina into the molten electrolyte situated underneath.
Improper alumina feeding to the electrolyte can lead
on the one hand to local oversaturation of alumina and
precipitation of undissolved alumina onto the cell bottom
with decline in efficiency, and on the other hand to
depletion of dissolved alumina which leads to anode effects
when the anodes are made of carbon or to electrolytic
dissolution of the anodes when they are made of a metal-
based material.
Usual alumina point feeders for aluminium production
cells, for instance as disclosed in US Patent 3,901,787
(Niizeki/Watanabe/Yamamoto/Takeuchi/Kubota), feed alumina to
different areas of a molten electrolyte of an aluminium
production cell using several feed pipes connected to the
same alumina reservoir. Each feed pipe has an alumina
metering device or an alumina gate for feeding batches of
alumina to each area of the molten electrolyte.
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WO00/63464 (de Nora/Berclaz) discloses another
alumina feeder of an aluminium production cell in which
batches of alumina supplied from a reservoir through several
feed pipes are sprayed over the molten electrolyte.
US Patent 3,681,229 (Lowe) discloses an alumina
feeder with an alumina metering device for supplying a
metered amount of alumina to a horizontal conveyor duct
which extends along an aluminium production cell and along
which the powder alumina is fluidised and carried by
pressurised air. The conveyor duct is provided with
apertures leading into corresponding feed pipes for feeding
alumina to a plurality of different feed locations of the
cell. The size of these apertures determines approximately
the amount of alumina supplied from the conveyor duct to the
feed pipes.
Summarir of the Invention
It is an object of the invention to provide a device
for feeding powder material to a metal electrowinning cell
which reliably divides metered amounts of powder material
into portions of constant amount and for supplying
substantially constant amounts of powder to different areas
of the molten electrolyte.
The invention relates to a device for intermittently
supplying powder material to a plurality of feed pipes, i.e.
at least 2 and usually 3 to 8 or more feed pipes, for
feeding batches of the powder material to different areas of
a molten electrolyte in a metal electrowinning cell.
For example, the device of the invention may be used
in a cell for the electrolytic production of aluminium or of
another metal, for feeding to the electrolyte of the cell a
compound of the metal to be produced or one or more
constituents of the electrolyte, for example A1203, AlF3,
NaF, CaF~, MgF~ or LiF can be fed to a cryolite-based
aluminium production electrolyte. Suitable aluminium
production cells in which such a feed device can be used are
for example disclosed in US patent 6,258,246 (Duruz/de
Nora), WO00/63463 (de Nora), W001/31086 (de Nora/Duruz),
W001/31088, PCT/IB02/00668, PCT/IB02/0670, PCT/IB01/00953
and PCT/IB01/00956 (all de Nora).
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According to the invention, the device comprises
means for metering quantities of powder material and for
intermittently dropping the metered quantities over at least
one inclined and/or upright dividing surface where each
metered quantity of the powder material is divided as it
drops over the dividing surfaces) into a plurality of
batches which correspond to the plurality of feed pipes and
which are directed to the corresponding feed pipes.
The dividing surfaces) can be inclined and
preferably upwardly converging. For example, the dividing
surfaces) is/are of generally conical or pyramidal shape.
Alternatively, the device comprises a plurality of
upright dividing surfaces. For example the upright dividing
surfaces are surfaces of concurrent upright plates radially
arranged and defining between them a plurality of
compartments into which each metered quantity of powder
material is divided after having been dropped over the
upright plates.
Usually, the dividing surfaces) is/are in a housing
having a plurality of openings in correspondence with the
plurality feed pipes.
In one embodiment, the metering means comprises a
metering chamber having a volume corresponding to the amount
of powder material of each metered quantity, the metering
chamber having an inlet for incoming powder material from a
feedstock, usually stored in a reservoir, and an outlet for
outgoing metered quantities of powder material to the
dividing surface(s). The metering means may comprise a
movable closure member for closing the inlet of the metering
chamber coupled with a movable closure member for closing
the outlet of the metering chamber such that when the inlet
is closed the outlet is open and vice-versa.
The invention also relates to a device for
intermittently feeding batches of powder material to
different areas of a molten electrolyte in a metal
electrowinning cell. The feed device comprises a reservoir
for storing the powder material and a plurality of feed
pipes for feeding the batches of the powder material to the
different areas of the molten electrolyte. The reservoir and
the feed pipes are connected through a device as described
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above which is arranged to divide metered quantities of the
powder material into the batches of the powder material and
direct the batches to the feed pipes.
If needed, compressed air may be supplied into the
feed pipes to fluidise the powder material therein and
facilitate the transport of the powder material in the feed
pipes towards the molten electrolyte. However, when the feed
pipes are vertical and/or at a slope, gravity may be
sufficient to displace the powder material in the feed pipes
to the molten electrolyte.
A further aspect of the invention relates to a cell
for the electrowinning of a metal from ions of the metal in
a molten electrolyte, in particular a cell for the
production of aluminium, comprising a feed device for
feeding batches of a powder material to different areas of
the molten electrolyte as described above.
The device may contain and supply to the molten
electrolyte a compound of the metal to be electrowon, in
particular alumina for the electrowinning of aluminium.
Another aspect of the invention relates to a cell
for the electrowinning of a metal from ions of the metal in
a molten electrolyte, comprising a device as described above
for intermittently supplying powder material to a plurality
of feed pipes for feeding batches of the powder material to
different areas of the molten electrolyte.
Yet another aspect of the invention relates to a
method of producing a metal, in particular aluminium by the
electrolysis of an aluminium compound dissolved in a molten
electrolyte. The method comprises passing an electrolysis
current in a molten electrolyte containing a dissolved
compound of the metal between an anode and a cathode to
produce the metal at the cathode and intermittently dropping
metered quantities of powder material, in particular
selected from at least one of the metal compound and
constituents of the molten electrolyte, over at least one
inclined and/or upright dividing surface where each metered
quantity of powder material is divided as it drops over the
dividing surfaces) into a plurality of batches which
correspond to said plurality of feed pipes and which are
directed to said corresponding feed pipes that feed the
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batches of powder material to different areas of the molten
electrolyte.
Brief Description of the Drawings
The appended drawing schematically and by way of
example shows an aluminium electrowinning cell using an
alumina feed device according to the invention.
Detailed Description
The aluminium production cell shown in the appended
drawing comprises a plurality of oxygen evolving anodes 10
dipping in a molten electrolyte 5 over a cell bottom. The
cell bottom comprises a series of pairs of spaced apart
carbon cathode blocks 25 placed across the cell and having
an aluminium-wettable upper layer 22. The aluminium-wettable
layer 22 is covered with aluminium-wettable openly porous
plates 21 which are filled with molten aluminium to form an
aluminium-wetted drained active cathode surface 20 above the
upper surfaces 22 of the carbon cathode blocks 25.
The cathode blocks 25 are made of graphite and have
a reduced height, e.g. 30 cm, and are coated with the
aluminium-wettable layer 22 to protect the graphite from
erosion and wear. Suitable aluminium-wettable layers are
disclosed in W001/42168 (de Nora/Duruz), W001/42531
(Nguyen/Duruz/de Nora) and PCT/IB01/00949 (Nguyen/de Nora).
The cell bottom further comprises a centrally
located recess 35 at a level below the upper surfaces 22 of
the carbon cathode blocks 25 and which during use collects
molten aluminium 60 drained from the aluminium-wettable
drained active cathode surface 20.
The aluminium collection recess 35 is formed in a
reservoir body 30 which is placed between the blocks 25 of
each pair of cathode blocks and spaces them apart across the
cell. As shown, the recess 35 formed in the reservoir body
30 is generally U-shaped with 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 also covers the surface of
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recess 35 to protect the reservoir body 30 during use
against wear and sodium intercalation.
As shown, the reservoir body 30 extends below the
cathode blocks 25 into the refractory 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 subjected to thermal expansion during
start-up of the cell and normal operation. Longitudinally
spaced apart spacer bars (not shown) placed across an upper
part of the recess 35 may provide additional mechanical
strength to the reservoir body 30. Such spacer bars can be
made of carbon material coated with an aluminium-wettable
protective layer.
The openly porous plates 21 placed on the upper
surfaces 22 of the carbon cathode blocks 25 and located in
the central region of the cell bottom extend over part of
the aluminium collection recess 35 so that during use the
protruding part of the aluminium-wetted drained active
cathode surface 20 is located over the 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 through a conventional
tapping tube. The spacing between the openly porous plates
21 over the aluminium collection recess can be much smaller
along the remaining parts of the recess 35, thereby
maximising the surface area of the active cathode surface
20.
The illustrated cell comprises a series of corner
pieces 41 which are made of the same openly porous material
as plates 21 filled with aluminium and which are placed at
the periphery of the cell bottom against sidewalls 40. The
sidewalls 40 and the surface of the electrolyte 5 are
covered with a ledge and a small crust of frozen electrolyte
6. The cell is fitted with an insulating cover 45 above the
electrolyte crust 6 to reduce thermal losses. Further
details of suitable covers are disclosed in W001/31086 (de
Nora/Duruz) and PCT/IB02/00669 (de Nora/Berclaz).
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The cell is also provided with an exhaust pipe 46 that
extends through the cover 45 for the removal of gases
produced during electrolysis, in particular anodically
evolved oxygen.
In accordance with the invention, the cell comprises a
device 50 for intermittently feeding batches of powder
material 65, for example alumina, to different areas of the
molten electrolyte 5. The device 50 has a reservoir 54 for
storing the alumina 65 and a plurality of feed pipes 52, for
instance four feed pipes 52 only two of which are shown, for
feeding batches of alumina 65 to the different areas of the
molten electrolyte 5 adjacent anodes 20.
The feed pipes 52 extend through the insulating cover
45 between anodes 10. It is usually sufficient to have one
feed pipe for supplying alumina to a group of two or four
anodes 10, the total number of feed pipes depending on the
size of the cell. Each feed pipe 52 is associated with a
crust breaker (not shown) for breaking the crust 6 and
forming an opening 7 underlying the feed pipe 52 prior to
feeding.
The feed device 50 has a dividing arrangement with a
housing 53 having at its bottom a plurality of openings
leading into the feed pipes 52 and a generally conical
dividing surface 51 which is located between these openings
and- which divides metered quantities of powder alumina 65
dropped thereover into the batches and which directs these
batches to the corresponding feed pipes 52.
If required, the shape of the dividing surface 51 can
be adapted to divide the metered quantities of powder
alumina 65 into unequal portions, for instance when the
different feed pipes 52 feed alumina to different groups of
anodes having an unequal number of anodes.
The feed device 50 also comprises an alumina metering
device which comprises a volumetric metering chamber 55
having a volume corresponding to the amount of powder
material of each metered quantity. The metering chamber 55
has an inlet with an upper movable closure member 56a for
controlling incoming powder alumina 65 from reservoir 54 and
an outlet with a lower movable closure member 56b for
controlling outgoing metered quantities of powder alumina 65
to the dividing arrangement 51,53.
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The upper and lower movable closure members 56a,56b are
mechanically coupled on a shaft 57 which can be vertically
reciprocated by a motor 58 simultaneously to close the inlet
and open the outlet, and vice-versa.
During operation, 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.
Alumina 65 is intermittently fed from reservoir 54 to
the molten electrolyte 5 through feed pipes 52 by raising
shaft 52 to let alumina powder 65 fill the metering chamber
55, then lowering shaft 52 to deliver a metered quantity of
alumina powder 65 over the conical dividing surface 51 where
it is divided into substantially equal batches which are
directed into the corresponding feed pipes 52. The batches
are then fed through pipes 52 to different areas of
electrolyte 5 between different groups of anodes 10 at
different locations 7 that, if necessary, are maintained
crust-free by a reciprocating crust breakers (not shown)
that penetrate the crust below the lower end of the feed
pipes 52.
In a variation, the insulating material of the
sidewalls 40 and cover 45 may be sufficient to prevent
formation of any ledge and crust of frozen electrolyte. In
such a case, the sidewalk 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
as disclosed in W000/06802, WO00/06803 (both Duruz/de Nora/
Crottaz), W000/06804 (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,
W000/40783 (all de Nora/Duruz), W000/06800 (Duruz/de Nora),
W099/36591 and W099/36592 (both de Nora). Suitable anode
designs which provide optimal cell operation are disclosed
in WO00/40781 and W000/40782 (both in the name of de Nora).
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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 US Patents 4,614,569
(Duruz/Derivaz/Debely/Adorian), 4,680,094 (Duruz), 4,683,037
(Duruz) and 4,966,674 (Bannochie/Sherriff).
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. Suitable electrolyte
compositions are disclosed in PCT/IB01/00954 (Nguyen/de
Nora). 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 by feeding it to different
areas of the electrolyte 5 as provided by the present
invention.
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) or W001/31088 (de Nora).
