U.S. patent application number 10/535809 was filed with the patent office on 2006-06-15 for electrolytic cell with improved feed device.
Invention is credited to Vittorio De Nora, Thinh T. Nguyen.
Application Number | 20060124471 10/535809 |
Document ID | / |
Family ID | 32448805 |
Filed Date | 2006-06-15 |
United States Patent
Application |
20060124471 |
Kind Code |
A1 |
Nguyen; Thinh T. ; et
al. |
June 15, 2006 |
Electrolytic cell with improved feed device
Abstract
A cell for the electrowinning of a metal (70) from a compound
thereof dissolved in a molten electrolyte comprises: a thermally
insulated cell trough (10,20) and a thermally insulated cell cover
(30) which are arranged to contain an electrolyte (40) and maintain
it in a substantially crustless molten state; and a feeder (50) for
feeding a particulate (60) of the metal compound to the molten
electrolyte (40). The feeder (50) comprises a feeding tube (51)
extending into the cell trough (10,20) and has a tubular end
portion (52) which is located between the molten electrolyte (40)
and the insulating cell cover (30) and which has a substantially
horizontal axial direction. The feeder (50) is arranged to feed the
particulate (60) into the feeding tube (51), along the feeding tube
(51) and through an opening (53) in the tubular end portion (52)
from where it is delivered over the molten electrolyte (40). The
opening (53) is located at an end of the tubular end portion (52)
and is arranged to deliver the particulate (60) from the feeding
tube (51) substantially along the axial direction of the tubular
end portion (52). The end opening (53) can be coaxial with the
tubular end portion (52) or off-axis.
Inventors: |
Nguyen; Thinh T.; (Onex,
CH) ; De Nora; Vittorio; (Veyras, CH) |
Correspondence
Address: |
Jayadeep R Deshmukh
458 Cherry Hill Road
Princeton
NJ
08540
US
|
Family ID: |
32448805 |
Appl. No.: |
10/535809 |
Filed: |
December 3, 2003 |
PCT Filed: |
December 3, 2003 |
PCT NO: |
PCT/IB03/05752 |
371 Date: |
May 23, 2005 |
Current U.S.
Class: |
205/389 |
Current CPC
Class: |
C25C 3/14 20130101 |
Class at
Publication: |
205/389 |
International
Class: |
C25C 3/14 20060101
C25C003/14 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 4, 2002 |
WO |
PCT/IB02/05101 |
Claims
1. A cell for the electrowinning of a metal from a compound thereof
dissolved in a molten electrolyte, comprising: a thermally
insulated cell trough and a thermally insulated cell cover which
are arranged to contain an electrolyte and maintain it in a
substantially crustless molten state; and means for feeding a
particulate of the metal compound to the molten electrolyte
comprising at least one feeding tube extending into the cell trough
and having a tubular end portion which is located between the
molten electrolyte and the insulating cell cover and which has a
substantially horizontal axial direction, the feeding means being
arranged to feed said particulate into the feeding tube, along the
feeding tube and through an opening in the tubular end portion from
where it is delivered over the molten electrolyte, characterised in
that said opening is located at an end of the tubular end portion
and is arranged to deliver the particulate from the feeding tube
over the molten electrolyte substantially along the axial direction
of the tubular end portion.
2. The cell of claim 1, wherein the end opening is coaxial with the
tubular end portion.
3. The cell of claim 1, wherein the end opening is off-axis.
4. The cell of any preceding claim, wherein the feeding tube is
substantially linear or gradually curved.
5. The cell of any preceding claim, wherein the feeding means
comprise a plurality of tubular end portions, each end portion
having a substantially horizontal axial direction and an end
opening arranged to deliver the particulate from the feeding tube
over the molten electrolyte substantially along the axial direction
of the tubular end portion.
6. The cell of claim 5, wherein several tubular end portions are
part of the same feeding tube.
7. The cell of any preceding claim, wherein the feeding means
comprise a plurality of feeding tube, each having a tubular end
portion with an end opening for delivering the particulate.
8. The cell of any preceding claim, wherein the feeding means are
arranged to fluidise the particulate in the feeding tube and to
feed the fluidised particulate through the end opening of the end
portion over the molten electrolyte.
9. The cell of any preceding claim, wherein the feeding means are
arranged to feed and disperse the particulate over substantially
the entire molten electrolyte.
10. The cell of any preceding claim, wherein the feeding tube
extends into the cell trough through a cell sidewall.
11. The cell of any one of claims 1 to 9, wherein the feeding tube
extends into the cell trough through the cell cover.
12. The cell of any preceding claim, wherein the feeding means
comprise a fan or a blower for driving the particulate along the
feeding tube and through the end opening.
13. The cell of any preceding claim, wherein the feeding means
comprise a heater arranged to heat the particulate before it is
delivered from the end opening over the molten electrolyte.
14. The cell of any preceding claim, which is an aluminium
electrowinning cell and wherein the molten electrolyte is a
fluoride-based electrolyte.
15. The cell of claim 14, which comprises one or more
oxygen-evolving anodes, in particular metal-based or ceramic-based
anodes.
16. The cell of claim 15, wherein the or each oxygen-evolving anode
comprises an active anode structure having through-openings for the
flow of alumina-depleted electrolyte from below to above the anode
and/or through-openings for the flow of alumina-enriched
electrolyte from above to below the anode.
17. The cell of claim 16, wherein the feeding means are arranged to
deliver and disperse alumina over an expanse which includes at
least part of the perpendicular projection onto the molten
electrolyte surface of an active anode structure.
18. The cell of any one of claims 14 to 17, comprising an
aluminium-wettable cathode.
19. The cell of claim 18, wherein the cathode is a drained
cathode.
20. A method of electrowinning a metal from a compound thereof
dissolved in a substantially crustless molten electrolyte
comprising: feeding particulate of the metal compound into and
along a feeding tube having a tubular end portion with an axial
direction extending substantially horizontally over the
substantially crustless molten electrolyte and delivering the
particulate through an opening in the tubular end portion over the
molten electrolyte where it is dissolved and then electrolysed to
produce said metal, said method being characterised by delivering
the particulate over the substantially crustless molten electrolyte
from the feeding tube substantially along said axial direction of
the tubular end portion through said opening which is located at an
end of the tubular end portion.
21. The method of claim 20, comprising delivering a particulate
comprising particles, the sizes of which are in the range of 20 to
200 micron, in particular 30 to 50 micron.
22. The method of claim 20 or 21, wherein said particulate is
alumina and said metal is aluminium.
23. A cell for the electrowinning of a metal from a compound
thereof dissolved in a molten electrolyte, comprising means for
feeding a particulate of the metal compound to the molten
electrolyte, the feeding means comprising at least one feeding tube
having a tubular end portion which is located above the molten
electrolyte and which has a substantially horizontal axial
direction, said feeding means being arranged to feed said
particulate into the feeding tube, along the feeding tube and
through an opening in the tubular end portion from where it is
delivered over the molten electrolyte, characterised in that said
opening is located at an end of the tubular end portion and is
arranged to deliver the particulate from the feeding tube over the
molten electrolyte substantially along the axial direction of the
tubular end portion.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a cell for the
electrowinning of a metal from a compound thereof dissolved in a
molten electrolyte. The cell is fitted with a device for feeding
particulate of the metal compound over to the molten
electrolyte.
BACKGROUND OF THE INVENTION
[0002] The feed device of the invention can be used in various
molten salt electrolysis cells in particular for aluminium
electrowinning.
[0003] The technology for the production of aluminium by the
electrolysis of alumina, dissolved in molten cryolite containing
salts, at temperatures around 950.degree. C. is more than one
hundred years old and has not undergone any great change or
improvement, in particular in the way in which alumina is fed to
the molten electrolyte for its subsequent dissolution and
electrolysis.
[0004] Conventional cells are usually operated with a crust of
frozen electrolyte above the molten electrolyte. This crust needs
to be periodically broken to form an opening for feeding alumina
into the molten electrolyte situated underneath. Various systems
have been provided to locally break the frozen electrolyte crust
and feed alumina into the molten electrolyte, for instance as
described in U.S. Pat. No. 3,664,946 (Schaper/Springer/Kyburz),
U.S. Pat. No. 4,049,529 (Golla), U.S. Pat. No. 4,437,964
(Gerphagnon/Wolter), U.S. Pat. No. 5,045,168
(Dalen/Kvalavag/Nagell), U.S. Pat. No. 5,108,557 (Nordquist), U.S.
Pat. No. 5,294,318 (Grant/Kristoff), U.S. Pat. Nos. 5,324,408 and
5,423,968 (both in the name of Kissane).
[0005] One drawback of feeding alumina to the molten electrolyte by
initially breaking the electrolyte crust resides in the
introduction of a mass of frozen electrolyte into the molten
electrolyte which generates a thermal shock therein. Moreover,
after the crust is broken cold alumina is added to the molten
electrolyte which freezes the bath, forming dense alumina and/or
electrolyte aggregates increasing the chance of sludging.
[0006] With the trend towards automated systems, the frequency of
feeding alumina has been increased. Feeding may take place every 20
to 90 min., sometimes even shorter, for instance every 1 to 5 min.
as described in U.S. Pat. No. 3,673,075 (Kibby), while smaller
amounts of alumina are introduced with each feed. The advantages
are in particular maintaining a more constant concentration of
dissolved alumina in the electrolyte and reducing the temperature
variation in the electrolyte. A typical automated break and feed
system comprises a pneumatically-operated crust breaker beam and an
ore bin capable of discharging a fixed amount of alumina (K.
Grjotheim & B. J. Welsh, "Aluminium: Smelter Technology", 1988,
2.sup.nd Edition, Aluminium Verlag GmbH, D-4000 Dusseldorf 1, pp.
231-232).
[0007] U.S. Pat. No. 5,476,574 (Welsh/Stretch/Purdie) discloses a
feeder arranged to continuously feed alumina to an aluminium
electrowinning cell. The feeder is associated with a point breaker
which is operated to maintain a hole in a frozen electrolyte crust
on the surface of the molten electrolyte.
[0008] In order to enhance dispersion, dissolution and control of
the amount of fine particulate alumina fed to the electrolytic
bath, various alumina feed devices have been developed involving
fluidisation of alumina powder by using compressed gas such as
compressed air, for instance as disclosed in U.S. Pat. No.
3,901,787 (Niizeki/Watanabe/Yamamoto/Takeuchi/Kubota), U.S. Pat.
No. 4,498,818 (Gudmundur/Eggertsson) and U.S. Pat. No. 4,525,105
(Jaggi).
[0009] Despite substantial efforts to enhance the feeding of
alumina as described above, feeding is still locally limited to one
or more feeding points over the electrolytic bath between suspended
carbon anode blocks using vertical point feeders. Furthermore, the
above described processes still necessitate to periodically form or
continuously maintain as many holes in the frozen electrolyte crust
above the molten bath as there are feeding points.
[0010] WO03/006717 (Berclaz/Duruz) discloses a device for feeding
alumina to a thermally insulated aluminium electrowinning cell in
which metered quantities of alumina are dropped from a dosing
system onto a divider that divides the metered quantities into
batches and that directs these batches into a plurality of feeding
tubes which guide the batches to different areas of the cell's
molten electrolyte.
[0011] Dispersive spraying of alumina has been proposed for
crustless aluminium production cells. WO00/63464 (de Nora/Berclaz)
discloses an aluminium electrowinning cell with a thermally
insulated crustless molten electrolyte and inter-alia an alumina
feeding tube extending horizontally above the molten electrolyte.
The feeding tube has a series of openings along its length for
spraying sideways alumina fed along the tube.
[0012] Despite the improvement of the different alumina spraying
systems disclosed in WO00/63464 and WO03/006717, there is still a
need to simplify and enhance the spraying of a particulate feed,
e.g. alumina, over the molten electrolyte of a cell for the
electrowinning of a metal, such as aluminium.
OBJECTS OF THE INVENTION
[0013] It is an object of the invention to provide a cell for the
electrowinning of a metal, such as aluminium, fitted with a simple
feeder of a compound of the metal, such as alumina, designed to
deliver, in particular dispersing, the compound as a particulate
over the cell's molten electrolyte.
[0014] A further object of the invention is to provide a cell for
the electrowinning a metal, such as aluminium, fitted with a feeder
of a compound of the metal, such as alumina, designed to operate
with a substantially crustless molten electrolyte.
[0015] Another object of the invention is to provide a cell for the
electrowinning of a metal, such as aluminium, fitted with a feeder
of a compound of the metal, such as alumina, designed to deliver
the pre-heated compound to the molten electrolyte to minimise the
risk of sludging and enhance dissolution of the delivered
particulate.
[0016] Yet another object of the invention is to provide a cell for
the electrowinning of a metal, such as aluminium, fitted with a
feeder of a compound of the metal, such as alumina, designed to
continuously or intermittently deliver the compound as a
particulate to the molten electrolyte.
SUMMARY OF THE INVENTION
[0017] The invention relates to a cell for the electrowinning of a
metal from a compound thereof dissolved in a molten electrolyte.
The cell comprises: a thermally insulated cell trough and a
thermally insulated cell cover which are arranged to contain an
electrolyte and maintain it in a substantially crustless molten
state; means for feeding a particulate of the metal compound to the
molten electrolyte comprising at least one feeding tube extending
into the cell trough and having a tubular end portion which is
located between the molten electrolyte and the insulating cell
cover and which has a substantially horizontal axial direction,
these means being arranged to feed the particulate into the feeding
tube, along the feeding tube and through an opening in the tubular
end portion from where it is delivered over the molten
electrolyte.
[0018] In accordance with the invention, the opening is located at
an end of the tubular end portion and is arranged to deliver the
particulate from the feeding tube over the molten electrolyte
substantially along the axial direction of the tubular end portion.
The end opening may be coaxial with the tubular end portion or it
may be off-axis. The feeding tube can be substantially linear or
gradually curved. As explained blow, feeding tubes whose shape
cause the particulate to be driven around corners or other sharp
angles should be avoided along the feeding tube.
[0019] The axial direction of the tubular end portion is usually
horizontal or at an angle of up to .+-.15.degree. to the
horizontal. In average, the particulate exits the tubular end
portion along its axial direction or at a small angle thereto,
typically up to 15.degree. or 20.degree..
[0020] The feeding means of the invention can be used to disperse
the particulate of the metal compound over an entire expanse of the
surface of the electrolyte, which facilitates dissolution of the
particulate in the electrolyte by avoiding or reducing local
saturation of the electrolyte with the metal compound.
[0021] In other words, the particulate is delivered over at least
an entire portion of the surface of the electrolyte (hereinafter
sometimes referred to as the "feeding area") whose size is
substantially greater than that with conventional point feeders.
Thus, the particulate fed with this feeder is spread over a
substantially greater surface of molten electrolyte and can much
easier dissolve. Typically, the expanse of this portion is of at
least 0.1 m.sup.2, usually 0.5 or 1 or 2 m.sup.2 to 6 or 10 m.sup.2
or more.
[0022] According to the invention, the particulate delivered from
the feeding tube over the molten electrolyte is driven along the
tube and exits the tube through the end opening over the
electrolyte substantially along the axial direction of the tube.
This provides an improvement over the abovementioned WO00/63464's
feeding device regarding the simplified tube design and the
straight delivering of the particulate as it is not fed like in
WO00/63464 through lateral openings in a direction orthogonal to
the tube's horizontal axis, which leads to a loss of velocity of
the particulate as it exits the tube and increases the risk of
clogging the lateral openings.
[0023] Especially for large or industrial cells in which alumina is
preferably delivered at several different locations, the feeding
means may comprise a plurality of tubular end portions, each end
portion having a substantially horizontal axial direction and an
end opening arranged to deliver the particulate from the feeding
tube over the molten electrolyte substantially along the axial
direction of the tubular end portion. Several tubular end portions,
in particular fan-shaped end portions, can be part of the same
feeding tube.
[0024] For the same reason, the feeding means can comprise a
plurality of feeding tube, each having a tubular end portion with
an end opening for delivering the particulate.
[0025] The feeding means usually comprise a gas flow generator to
fluidise the particulate in the feeding tube and to feed the
fluidised particulate through the end opening of the end portion
over the molten electrolyte. Fluidising the particulate enhances
its flow along the feeding tube and its dispersion when delivered
over the molten electrolyte.
[0026] Advantageously, the feeding means are arranged to feed and
disperse the particulate over substantially the entire molten
electrolyte, if necessary using feeding means with several tubular
end portions and optionally several feeding tubes.
[0027] The cell cover above the molten electrolyte is placed and
spaced above the surface of the molten electrolyte, for instance as
disclosed in WO99/02763 (de Nora/Sekhar), WO02/070784 and
US2003/0102228 (both de Nora/Berclaz). Such cover thermally
insulates the surface of the molten electrolyte and substantially
prevents formation of an electrolyte crust on the molten
electrolyte. The thermally insulated cavity thereby created between
the molten electrolyte and the cover serves to house the feeding
means.
[0028] Furthermore, there is no need to remove the feeding means
from under the cell cover. Normally the means is permanently
located under the cover which can remain sealed off while the
particulate is fed to the molten electrolyte to avoid thermic
losses. Conversely, conventional feeders are located above the
crust of molten electrolyte, the crust being periodically broken to
permit alumina feeding from above the crust into the molten
electrolyte.
[0029] The feeding tube can extend into the cell trough through a
cell sidewall or through the cell cover. In the latter case, the
feeding tube preferably extends through a fixed section of the cell
cover, or between a movable cover section and a fixed cover
section, or between movable cover sections, so that the feeding
tube which extends through the cover does not require to be removed
when movable cover sections are moved away to uncover the molten
electrolyte, as disclosed in the abovementioned US2003/0102228.
[0030] The cell cover is normally arranged to inhibit formation of
an electrolyte crust on the surface of the molten electrolyte
during operation. However, the surface of the electrolyte does not
need to be entirely crust free, but at least the feeding area
should be free from any frozen electrolyte crust for optimal
operation.
[0031] Usually, the feeding means comprise a fan or a blower for
driving the particulate along the feeding tube and through the end
opening with gas, in particular hot gas, e.g. air such as hot dry
air.
[0032] Also, to overcome a prior art prejudice when electrolysis is
carried out at high temperature which is for instance the case for
aluminium electrowinning, it is preferred to supply preheated
particulate to the molten electrolyte to minimise electrolyte
freezing caused by contact with "cold" solid particulate and by the
possibly endothermic dissolution reaction of the particulate in the
molten electrolyte which for instance happens with the dissolution
of alumina. Ideally, the fed particulate supplies at least part of
the energy needed for its dissolution. Heat may be provided to the
particulate during the feeding process by contact with hot air, by
using a heater or possibly with a burner providing a flame which
may also be used to drive the particulate along the feeding tube.
The particulate may be preheated before feeding, for instance by
heating a reservoir in which it is stored and from which it is
delivered through the feeding tube to the molten electrolyte. More
generally, the particulate may be heated before and/or during
delivery.
[0033] Therefore, the feeding means preferably associated with a
heater arranged to heat the particulate before it is delivered from
the end opening over the molten electrolyte.
[0034] In one embodiment, the cell of the invention is an aluminium
electrowinning cell and the molten electrolyte is a fluoride-based
electrolyte. The aluminium electrowinning cell can have one or more
oxygen-evolving anodes, in particular metal-based or ceramic-based
anodes, or possibly consumable carbon anodes.
[0035] An oxygen-evolving anode of the aluminium production cell
may comprise an active anode structure having through-openings for
the flow of alumina-depleted electrolyte from below to above the
anode and/or through-openings for the flow of alumina-enriched
electrolyte from above to below the anode.
[0036] In this case, the feeding means can be arranged to deliver
and disperse alumina over an expanse which includes at least part
of the perpendicular projection onto the molten electrolyte surface
of an active anode structure. The size of the expanse may be at
least a tenth or a fifth of the surface area of the anode
structure, in particular from a quarter to a half of the full
surface area. Typically, the size of the expanse is at least 0.1
m.sup.2, usually 0.5 or 1 or 2 m.sup.2 to 6 or 10 m.sup.2 or
more.
[0037] Conveniently, the size of this expanse corresponds
approximately to the perpendicular projection on the surface of the
molten electrolyte of the active anode surface. For example, the
expanse covers entirely or at least partly the perpendicular
projection onto the molten electrolyte surface of an active anode
structure. The alumina feeding area may correspond to the feeding
area on the surface of the molten electrolyte of one anode or
several anodes.
[0038] In one embodiment, the anode feeding area corresponds to a
projection onto the surface of the electrolyte of the active anode
surfaces, this projection possibly being smaller or greater than
the corresponding area(s) of the active anode surfaces. This anode
feeding area is usually, but not necessarily, situated directly
above the active anode surfaces.
[0039] The alumina feeding area typically occupies an expanse of
the molten electrolyte surface which can be about the same size as
the surface area of the corresponding active anode surfaces.
However, when anodes co-operate with special electrolyte
circulation means, for instance as disclosed in co-pending
application WO00/40782 (de Nora), the size of the feeding area may
be smaller than the actual size of the active anode surfaces. In
practice, powder alumina may even be supplied over substantially
the entire surface of the molten electrolyte. This is particularly
advantageous in configurations where at least part of the
alumina-rich electrolyte flows through the open anode structures to
the inter-electrode gap.
[0040] At least part of the alumina-rich electrolyte may flow down
around the open anode structures into the inter-electrode gap to be
electrolysed and then alumina-depleted electrolyte can rise to the
feeding area through the open anode structures.
[0041] Whether or not alumina flows around the anodes, alumina
dissolution is improved with such an alumina feeding device. The
improvement is not bound to a specific electrolyte circulation
path. Either alumina-rich electrolyte flows from the feeding area
down through the anode structure, or alumina-depleted electrolyte
flows through the anode structure up to the feeding area, or both
flow patterns are combined.
[0042] The concept of this invention may be adapted to any
aluminium electrowinning cell and is particularly suitable for
cells operating with metal-based anodes at reduced temperatures,
typically below 940.degree. C., such as in the range of 730.degree.
to 910.degree. C. or 850.degree. to 880.degree. C., for instance
cells as disclosed in WO00/40781, WO00/40782 and WO03/006716 (all
in the name of de Nora) operating with metal-based oxygen-evolving
grid-like anodes provided with vertical through openings for the
circulation of electrolyte and the escape of anodically produced
oxygen.
[0043] Suitable materials for oxygen-evolving anodes include iron
and nickel based alloys which may be heat-treated in an oxidising
atmosphere as disclosed in WO00/06802, WO00/06803 (both in the name
of Duruz/de Nora/Crottaz), WO00/06804 (Crottaz/Duruz), WO01/42534
(Duruz/de Nora) WO01/42535 (de Nora/Duruz) WO01/42536
(Duruz/Nguyen/de Nora), WO02/083991 and WO03/078695 (both Nguyen/de
Nora). Further oxygen-evolving anode materials are disclosed in
WO99/36593, WO99/36594, WO00/06801, WO00/06805, WO00/40783 (all in
the name of de Nora/Duruz), WO00/06800 (Duruz/de Nora), WO99/36591,
WO99/36592 (both in the name of de Nora) and WO03/087435 (Nguyen/de
Nora).
[0044] The anode can comprise an applied cerium oxyfluoride-based
outermost coating, for example as disclosed in WO02/070786
(Nguyen/de Nora) and WO02/083990 (de Nora/Nguyen). Such a coating
may be applied before or during use and maintained during use by
the presence of cerium species in the electrolyte.
[0045] Advantageously, the aluminium electrowinning cell comprises
an aluminium-wettable cathode, in particular a carbon cathode
covered with an aluminium-wettable coating to increase the lifetime
of the cathode. The cathode may be a drained cathode whereby the
anode-cathode gap and the voltage drop though the electrolyte can
be reduced.
[0046] Suitable cell bottoms for aluminium production are for
example disclosed in WO00/63463 (de Nora), WO01/31086 (de
Nora/Duruz), WO01/31087 (Duruz/de Nora), WO01/42168 (de
Nora/Duruz), WO01/42531 (Nguyen/Duruz/de Nora), WO02/096831
(Nguyen/de Nora), EP 1 146 146 (de Nora), WO02/070783, WO02/070785,
WO02/097169, WO03/023091, WO02/097168 (all de Nora) and WO03/083176
(de Nora/Nguyen).
[0047] Another aspect of the invention relates to a method of
electrowinning a metal from a compound thereof dissolved in a
substantially crustless molten electrolyte. This method comprises:
feeding particulate of the metal compound into and along a feeding
tube having a substantially horizontally tubular end portion
extending over the substantially crustless molten electrolyte, and
delivering the particulate through an opening in the tubular end
portion over the molten electrolyte where it is dissolved and then
electrolysed to produce said metal.
[0048] In accordance with the invention, the particulate is
delivered over the substantially crustless molten electrolyte from
the feeding tube substantially along the axial direction of the
tubular end portion through the opening which is located at an end
of the tubular end portion. The particulate can be delivered
continuously or in batches to the electrolyte.
[0049] In one embodiment, the particulate is alumina and the
produced metal is aluminium.
[0050] Bayer-process alumina or other suitable grades of alumina,
may be utilised. For instance, partly dehydrated alumina particles,
modified alumina, and alumina particles of different shapes and
sizes may be used. To enhance dispersion of the alumina powder
above the molten electrolyte surface, and to facilitate its
dissolution into the molten electrolyte, the alumina powder is
preferably composed of particles in the range of 20 to 200 micron,
preferably from 30 to 50 micron.
[0051] As discussed above, aluminium production cells operated at
reduced temperatures should have an insulating cover above the
molten electrolyte, since at such temperatures, the molten
electrolyte does not usually form a rigid crust but a gel-like
layer.
[0052] However, in a modification of the invention in which the
molten electrolyte of a metal electrowinning cell is not crustless,
for instance when an aluminium production cell is operated at a
conventional temperature (i.e. around 950.degree. C.), the cell
cover can include or be made of an electrolyte crust formed by
electrolyte freezing. The crust should be sufficiently spaced from
the molten electrolyte to permit the insertion of the feeding means
between the molten electrolyte and the crust. This can be achieved
for example by removing part of the molten electrolyte from the
cell after formation of the crust to form a cavity for the feeding
means between the remaining molten electrolyte and the crust.
[0053] The invention also relates to a cell for the electrowinning
of a metal, such as aluminium, from a compound thereof, e.g.
alumina, dissolved in a molten electrolyte. The cell comprises
means for feeding a particulate of the metal compound to the molten
electrolyte. These feeding means comprise at least one feeding tube
having a tubular end portion which is located above the molten
electrolyte and which has a substantially horizontal axial
direction. Such means are arranged to feed the particulate into the
feeding tube, along the feeding tube and through an opening in the
tubular end portion from where it is delivered over the molten
electrolyte.
[0054] In accordance with the invention, this opening is located at
an end of the tubular end portion and is arranged to deliver the
particulate from the feeding tube over the molten electrolyte
substantially along the axial direction of the tubular end portion.
The cell of the invention may incorporate any of the above
described cell feature or combination of features.
BRIEF DESCRIPTION OF THE DRAWINGS
[0055] The invention will be further described by way of example
with reference to the accompanying schematic drawing, in which
[0056] FIG. 1 illustrates a drained-cathode cell having an
aluminium collection reservoir in accordance with the
invention.
DETAILED DESCRIPTION
[0057] The aluminium electrowinning cell shown in FIG. 1 comprises
a cathodic bottom 10, thermally insulated cell sidewalls 20 and a
thermally insulated cell cover 30 which are arranged to contain an
electrolyte 40 and maintain it in a substantially crustless molten
state, and alumina feeders 50 for feeding alumina 60 to the molten
electrolyte 40.
[0058] Each alumina feeder 50 has at least one feeding tube 51
extending through a sidewall 20 into the cell trough and having a
horizontal tubular end portion 52 which is located between the
molten electrolyte 40 and the insulating cell cover 30. The feeder
50 is arranged to feed particulate alumina 60 into the feeding tube
51, along the feeding tube 51 and through an opening 53 in the
tubular end portion 52 from where it is delivered over the molten
electrolyte 40.
[0059] According to the invention, the opening 53 is located at the
end of the tubular end portion 52 and is arranged to deliver the
particulate alumina 60 from the feeding tube 51 over the molten
electrolyte 40 substantially along the axial direction of the
tubular end portion 51.
[0060] The feeder 50 comprises an alumina reservoir 54 which is
connected to the feeding pipe 51 through a supply pipe 56 in which
a vertical Archimedes' screw 55 doses the particulate alumina 60
fed from the reservoir to the feeding pipe 51.
[0061] The feeding pipe 51 is also connected to a compressed hot
gas source 57, such as a fan or blower, for driving particulate
alumina 60 along the feeding tube 51 and through the opening 53 at
the end of tubular end portion 52.
[0062] The cathode bottom 10 is drained with the cathodic surface
coated with a slurry-applied aluminium-wettable layer 11, for
instance as disclosed in the abovementioned WO01/42168, WO01/42531
and WO02/096831. The aluminium-wettable cathode layer 11 forms a
drained cathode surface on the cathode bottom 10.
[0063] Furthermore, the cathode bottom has a recessed groove 12 for
collecting and storing product aluminium 70 that is drained on the
aluminium-wettable cathode layer 11. The collected product
aluminium 70 can be periodically tapped from the recessed groove 12
by using a conventional tapping system.
[0064] The anodes 15 comprise an electrochemically active structure
16 made of oxygen-evolving material, as disclosed above. The active
anode structure 16 is provided with a series of vertical through
openings for the fast release of anodically produced oxygen and for
the down flow of alumina-rich electrolyte into the anode-cathode
gap for electrolysis, for example as described in the
abovementioned WO00/40781, WO00/40782 and WO03/006716.
[0065] The thermally insulating cover 30 is fitted on the cell and
maintains the surface of the electrolyte 40 at a sufficient
temperature to inhibit formation of a crust thereon, for instance
as disclosed in WO99/02763 (de Nora/Sekhar) and USSN2003/0102228
(de Nora/Berclaz). Cover 30 can be made of ceramic-based materials,
such as alumina, for instance as disclosed in WO02/070784 (de
Nora/Berclaz).
[0066] The ceramic cell cover 30 comprises a support section 31
which extends centrally along the cell and lateral movable sections
34 which rest on the sidewalls 20 and the support section 31. The
lateral cover sections 34 can be moved whenever access is needed to
the molten electrolyte 40, e.g. for tapping, to the anodes 15, e.g.
when they need to be replaced, or for any other reason. The lateral
cover sections 34 can be made of a plurality of side-by-side
sections which are individually movable so that whenever the area
below the cover must be accessed, only a small section of the cover
30 can be removed which permits a reduction of the thermal
losses.
[0067] The central support section 31 is suspended from horizontal
beams 33 through suspension elements 32 made of ceramic materials,
e.g. alumina, resistant to electrolyte fumes present above the
molten electrolyte. Each suspension element 32 has a bottom part
that extends through the support section 31 and is shaped such that
the support section 31 rests thereon. As shown in FIG. 1, the
bottom part of the suspension member 32 is upwardly tapered, e.g.
generally conical or pyramidal, so that the components can be
easily assembled or disassembled. The suspension members 32 can
have various shapes.
[0068] The cell is covered with a steel shell 35 located above the
insulating cover 30. The steel shell 35 is fitted with a gas
exhaust pipe 36. The steel shell 30 collects gases, such as oxygen
and electrolyte fumes, produced during electrolysis which gases are
then evacuated through the exhaust pipe 36.
[0069] During operation, a continuous or intermittent controlled
supply of particulate alumina 60 is provided from the alumina
reservoir 54 to the feeding pipe 51 by rotating Archimedes' screw
55. Alumina 60 is then fluidised and driven by compressed gases
supplied by gas source 57 along feeding pipe 51 to tubular end
portion 52 and through end opening 53 where it exits substantially
along the horizontal axial direction of tubular end portion 52 and
is dispersed while falling under the effect of gravity over the
molten electrolyte 40. In a variation, a deflector can be placed at
the end opening 53 to raise or lower slightly the average alumina
path.
[0070] The delivered alumina 60 enters electrolyte 40 where it
dissolves to enrich it. The alumina-rich electrolyte flows down the
through-openings of the active anodes structures 16 to the gap
between the active anode structures 16 and the cathode bottom 10
where it is electrolysed to produce oxygen on the active anode
structures 16 and molten aluminium 70 on the aluminium-wettable
cathode layer 11. The produced molten aluminium 70 drains into the
aluminium collection groove 12. The alumina-depleted electrolyte
resulting from electrolysis is driven up by anodically released
oxygen from under and through the active anode structures 16
towards the surface of the molten electrolyte 40 where it is
enriched with dissolving alumina 60. Such an electrolyte
circulation is described in greater detail in the abovementioned
WO00/40781, WO00/40782 and WO03/006716.
* * * * *