U.S. patent application number 12/308540 was filed with the patent office on 2010-11-25 for aluminium collection in electrowinning cells.
Invention is credited to Vittorio De Nora, Marcel Joubij, Thinh T. Nguyen, Frank Schnyder, Rene Von Kaenel.
Application Number | 20100294671 12/308540 |
Document ID | / |
Family ID | 38776293 |
Filed Date | 2010-11-25 |
United States Patent
Application |
20100294671 |
Kind Code |
A1 |
Nguyen; Thinh T. ; et
al. |
November 25, 2010 |
Aluminium collection in electrowinning cells
Abstract
A cell for the electrowinning of aluminium comprises an
electrolysis chamber (20) in which alumina is electrolysed to
produce aluminium (30) and a collection reservoir (40,40') in which
product aluminium is collected. The electrolysis chamber and the
collection reservoir are in liquid communication so that aluminium
produced in the electrolysis chamber can flow from the electrolysis
chamber into the collection reservoir. The electrolysis chamber
contains one or more metal-based anodes (15). Each anode has an
active anodic surface (16) spaced above a facing cathodic surface
(31) on which aluminium is produced. The cathodic surface is formed
on a structural body (12) by a layer made of molten aluminium into
which product aluminium is incorporated during operation. The
anodic surface and the cathodic surface have a substantially
constant operative position. The cell has means (60, 60', 61, 61',
62) for regulating the layer of molten aluminium so the layer forms
a shallow or deep continuous cathodic pool (35) that extends
continuously under the entire facing active anodic surface of at
least one anode. The layer regulating means are arranged to
maintain during operation the cathodic surface of the cathodic pool
at a substantially constant position by periodic or continuous
removal of molten aluminium from the aluminium pool to the
collection reservoir at a rate corresponding substantially to the
rate of production of the product aluminium that is incorporated
into the aluminium pool.
Inventors: |
Nguyen; Thinh T.; (Onex,
CH) ; Schnyder; Frank; (Chardonne, CH) ; Von
Kaenel; Rene; (Venthone, CH) ; De Nora; Vittorio;
(Veyras, CH) ; Joubij; Marcel; (Leiden,
NL) |
Correspondence
Address: |
J R Deshmuk
458 Cherry Hill Road
Princeton
NJ
08540
US
|
Family ID: |
38776293 |
Appl. No.: |
12/308540 |
Filed: |
June 20, 2007 |
PCT Filed: |
June 20, 2007 |
PCT NO: |
PCT/IB2007/052385 |
371 Date: |
August 2, 2010 |
Current U.S.
Class: |
205/375 ;
204/229.8; 204/250 |
Current CPC
Class: |
C25C 3/08 20130101 |
Class at
Publication: |
205/375 ;
204/250; 204/229.8 |
International
Class: |
C25C 3/08 20060101
C25C003/08; C25C 3/20 20060101 C25C003/20 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 22, 2006 |
IB |
PCT/IB2006/001795 |
Claims
1. A cell for the electrowinning of aluminium from alumina
dissolved in a fluoride-containing molten electrolyte, comprising
an electrolysis chamber in which alumina is electrolysed to produce
aluminium and a collection reservoir in which product aluminium is
collected, the electrolysis chamber and the collection reservoir
being in liquid communication so that aluminium produced in the
electrolysis chamber can flow from the electrolysis chamber into
the collection reservoir, the electrolysis chamber containing one
or more metal-based anodes, each anode having an active anodic
surface spaced by an anode-cathode gap from a facing cathodic
surface on which aluminium is produced, said cathodic surface being
formed on a structural body by a layer made of molten aluminium
into which product aluminium is incorporated during operation, said
anodic surface and said cathodic surface having a substantially
constant operative position so that the anode-cathode gap is
substantially constant, characterised in that the cell comprises
means for regulating said layer of molten aluminium so the layer
forms a shallow or deep continuous cathodic pool that extends
continuously under the entire facing active anodic surface of at
least one anode and that is contained within a cathodic cavity on
the structural cathodic body, the layer regulating means being
arranged to maintain during operation the cathodic surface of said
cathodic pool at a substantially constant position by periodic or
continuous removal of molten aluminium from the aluminium pool to
the collection reservoir at a rate corresponding substantially to
the rate of production of the product aluminium that is
incorporated into the aluminium pool.
2. The cell of claim 1, wherein the layer regulating means comprise
a wall which delimits the cathodic cavity, the wall having an upper
edge located at the level of said cathodic surface of the aluminium
pool and arranged to allow the flow over the edge of product
aluminium to the collection reservoir that extends below the
cathodic surface.
3. The cell of claim 2, wherein said collection reservoir has a
generally U-shaped cross-section.
4. The cell of claim 2, wherein said collection reservoir has a
bottom part that extends underneath the cathodic surface.
5. The cell of claim 1, comprising a reservoir chamber that
contains said collection reservoir and that is adjacent to the
electrolysis chamber and communicates therewith through an
aluminium passage for the removal of molten aluminium from the
aluminium pool to the collection reservoir, said layer regulating
means comprising means for adjusting the amount of aluminium in the
collection reservoir so that the position of the cathodic surface
of the aluminium pool remains substantially constant during
operation.
6. The cell of claim 5, wherein the layer regulating means comprise
means for measuring directly or indirectly the position of the
cathodic surface.
7. The cell of claim 6, wherein the measuring means comprise means
for measuring the electrical characteristics of a current passing
between the cathodic surface and at least one metal-based
anode.
8. The cell of claim 6, wherein the measuring means comprise one or
more proximity sensors above the electrolyte for measuring the
height of the electrolyte and/or aluminium pool.
9. The cell of claim 5, wherein the layer regulating means comprise
a means to adjust the pressure above aluminium in the collection
reservoir.
10. The cell of claim 5, wherein the molten aluminium in the
collection reservoir is covered by a layer or molten
electrolyte.
11. The cell of claim 1, wherein the cathodic pool has a depth in
the range of 1 to 15 cm.
12. The cell of claim 11, wherein the cathodic pool has a depth in
the range of 2 to 5 cm.
13. A method producing aluminium in a cell that comprises: an
electrolysis chamber containing one or more metal-based anodes in a
molten electrolyte having alumina dissolved therein, each anode
comprising an active anodic surface spaced by an anode-cathode gap
from a facing cathodic surface that is formed on a structural body
by a molten aluminium layer, said anodic surface and said cathodic
surface having a substantially constant operative position so that
the anode-cathode gap is substantially constant; and a collection
reservoir that is in liquid communication with the electrolysis
chamber, said method comprising electrolysing dissolved alumina in
the anode-cathode gap to evolve gas anodically and produce
aluminium cathodically, product aluminium being incorporated into
said aluminium layer and flowing from the electrolysis chamber into
the collection reservoir, said method being characterised in that
molten aluminium layer is regulated to form a shallow or deep
continuous cathodic pool that extends continuously under the entire
facing active anodic surface of at least one anode and that is
contained within a cathodic cavity on the structural cathodic body,
the cathodic surface of the cathodic pool being maintained at a
substantially constant position by periodic or continuous removal
of molten aluminium from the aluminium pool to the collection
reservoir at a rate corresponding substantially to the rate of
production of the product aluminium that is incorporated into the
aluminium pool.
14. The method of claim 13, comprising maintaining the electrolyte
at a temperature in the range of 700.degree. to 1000.degree. C.,
800 to 970.degree. C., or 880 to 940.degree. C.
15. The method of claim 13 or 14, comprising maintaining the
aluminium in the collection reservoir in a molten state in
particular above 700.degree. C.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a cell for the electrowinning of
aluminium having a dimensionally stable anode facing a cathodic
aluminium surface spaced by an anode-cathode gap that is constant
during operation.
BACKGROUND ART
[0002] 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 still uses carbon anodes and cathodes that
are covered by a cathodic pool of molten aluminium on surface of
which aluminium is produced during cell operation.
[0003] It had been proposed to replace the carbon material of the
cathodes of aluminium production cells with ceramic material, in
particular metal borides.
[0004] U.S. Pat. No. 4,396,481 (Pavlek/Lagler) discloses an
aluminium electrowinning cell with a cathodic bottom made of
granular refractory material, such as TiB.sub.2, which is covered
with a layer of molten aluminium. The cell bottom includes a trough
for the dissolution of alumina, the trough having sidewalls
projecting above the molten aluminium layer so that molten
aluminium does not flow into the trough.
[0005] U.S. Pat. No. 4,231,853 (Rahn) discloses spaced apart
cathodic boride tiles secured on the carbon floor of an aluminium
electrowinning cell. In one embodiment, the cathodic tiles have
inclined drained cathode surfaces on which aluminium is produced
and drains to a recessed aluminium collection reservoir in the
carbon floor. The collection reservoir has walls that protrude
slightly above the carbon floor so as to maintain a protective pad
of molten aluminium on the carbon floor. The thickness of this pad
is controlled so that the pad remains below the drained cathode
surfaces.
[0006] U.S. Pat. No. 4,544,457 (Sane/Wheeler/Kuivila) discloses an
openly porous sheath made of aluminium wettable material, in
particular TiB.sub.2, filled with molten aluminium. The sheath
covers a cathode block located on the bottom of an aluminium
electrowinning cell, aluminium being produced on the sheath and
drained therefrom onto the cell bottom where it is collected. With
this drained structure, no aluminium pool is formed on the cathode
in which wave motion might cause a short circuit between anode and
cathode.
[0007] More recently, it has become possible to coat carbon
cathodes with a slurry which adheres to the carbon and becomes
aluminium-wettable, as disclosed in U.S. Pat. Nos. 5,316,718 and
5,651,874 (both assigned to MOLTECH Invent S.A.). Further
developments of aluminium-wettable materials for use in aluminium
electrowinning cells are disclosed in WO01/42168, WO01/42531,
WO02/070783, WO02/096830, WO02/096831 WO2004/092449, WO2005/068390
(all assigned to MOLTECH Invent S.A.).
[0008] The use of aluminium-wettable cathode surfaces--on which
aluminium is drained instead of being accumulated into a shallow or
deep pool on the cathode bottom--avoids fluctuations during
operation of the cathodic active surface formed by the surface of
the accumulated produced aluminium. This is desirable in particular
when the cell operates with dimensionally stable anodes. In this
case a constant anode-cathode gap can be maintained without having
to adjust the position of the anode. To prevent variations of the
position of the active cathodic surface, the aluminium produced
cathodically can be drained from the cathode and evacuated to an
aluminium collection reservoir.
[0009] U.S. Pat. No. 6,682,643 (assigned to MOLTECH Invent S.A.)
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. WO02/070785, WO02/097168 and WO02/097169 (all assigned to
MOLTECH Invent S.A.) disclose further aluminium electrowinning
cells in which product aluminium drained is from a horizontal
cathode and collected in a recessed central reservoir.
[0010] It has also been proposed to provide drained-cathode
aluminium production cells having an electrolysis chamber
communicating with an aluminium collection chamber.
[0011] US2004/0011660 (Bradford/Barnett/Mezner) discloses an
aluminium electrowinning cell having an electrolysis chamber with a
vertical cathode on which aluminium is produced and drained. The
product aluminium is evacuated through a hole in the bottom of the
electrolysis chamber into an aluminium collection chamber that is
located underneath the electrolysis chamber.
[0012] U.S. Pat. No. 6,419,812 (Beck/Brown) discloses an aluminium
electrowinning cell having an electrolysis chamber with vertical
cathodes on which aluminium is produced and drained. Product
aluminium is conveyed along the cell by a collection tube extending
under the vertical cathodes to a bottom part of a reservoir chamber
next to the electrolysis chamber. Aluminium is forced up into the
reservoir chamber by the pressure of the electrolyte height in the
electrolysis chamber.
[0013] A drawback of known drained-cathode cells operating with
dimensionally-stable anodes lies in the difficulty to achieve
complete and permanent coverage of the drained cathode with product
aluminium. Full coverage is needed to protect the cathode against
molten electrolyte and to avoid non uniform wear of the cathode and
non uniform distribution of the electrolysis current at the surface
of the drained cathode and sub-optimal use of the anode and cathode
surfaces to produce aluminium.
OBJECTS OF THE INVENTION
[0014] It is therefore a preferred object to solve the above
problem.
[0015] A preferred object of the invention is to provide an
aluminium electrowinning cell having a cathodic surface formed of
molten aluminium which has a substantially constant position and on
which aluminium is produced.
[0016] Another preferred object of the invention is to produce
aluminium on a cathode which is covered with a shallow or deep pool
of product aluminium having a substantially non fluctuating height
(or depth).
[0017] A further preferred object of the invention is to provide an
aluminium electrowinning cell having dimensionally stable anodes
with a substantially fixed position during use in the cell.
SUMMARY OF THE INVENTION
[0018] The invention provides a cell for the electrowinning of
aluminium from alumina dissolved in a fluoride-containing molten
electrolyte. The cell comprises an electrolysis chamber in which
alumina is electrolysed to produce aluminium and a collection
reservoir in which product aluminium is collected. The electrolysis
chamber and the collection reservoir are in liquid communication so
that aluminium produced in the electrolysis chamber can flow from
the electrolysis chamber into the collection reservoir. The
electrolysis chamber contains one or more metal-based anodes. Each
anode has an active anodic surface spaced by an anode-cathode gap
from a facing cathodic surface on which aluminium is produced. The
cathodic surface is formed on a structural body by a layer made of
molten aluminium into which product aluminium is incorporated
during operation. The anodic surface and the cathodic surface have
a substantially constant operative position so that the
anode-cathode gap is substantially constant.
[0019] In accordance with the invention, the cell comprises means
for regulating the layer of molten aluminium so the layer forms a
shallow or deep continuous cathodic pool that extends continuously
under the entire facing active anodic surface of at least one anode
and that is contained within a cathodic cavity on the structural
cathodic body. The layer regulating means are arranged to maintain
during operation the cathodic surface of the cathodic pool at a
substantially constant position by periodic or continuous removal
of molten aluminium from the aluminium pool to the collection
reservoir at a rate corresponding substantially to the rate of
production of the product aluminium that is incorporated into the
aluminium pool.
[0020] The cathodic surface of the aluminium pool can be slightly
curved due to surface tension effects. As a result of this slight
curvature, the pool's height (or depth) may be not strictly uniform
over its surface; however the height (or depth) of the pool at
various locations does not significantly fluctuate during
operation. In such a case, if the surface of cathodic pool extends
under several anodes, the position of individual anodes can be
adjusted to such a slightly curved profile of the cathodic surface.
In other words, the anodes located above a central part of the
cathodic surface can be positioned slightly higher than the anodes
that are situated at the periphery of the cathodic surface so as to
adjust the anodes to the position of the facing portion of the
underlying cathodic surface.
[0021] In one embodiment, the layer regulating means comprise a
wall which delimits the cathodic cavity. The wall has an upper edge
located at the level of the cathodic surface of the aluminium pool
and is arranged to allow the flow over the edge of product
aluminium to the collection reservoir that extends below the
cathodic surface. The collection reservoir can have a generally
U-shaped cross-section or a bottom part that extends underneath the
cathodic surface.
[0022] This embodiment can conveniently be retrofitted in existing
cells without major modifications of the cell's structure.
[0023] In another embodiment, the cell has a reservoir chamber that
contains the collection reservoir and that is adjacent to the
electrolysis chamber and communicates therewith through an
aluminium passage for the removal of molten aluminium from the
aluminium pool to the collection reservoir. The layer regulating
means comprise means for adjusting the amount of aluminium in the
collection reservoir so that the position of the cathodic surface
of the aluminium pool does not significantly fluctuate during
operation. The layer regulating means may include means for
measuring directly or indirectly the position of the cathodic
surface, for example means for measuring the electrical
characteristics of a current passing between the cathodic surface
and at least one metal-based anode and/or one or more proximity
sensors above the electrolyte for measuring the height of the
electrolyte and/or aluminium pool. Furthermore, the layer
regulating means can have a means to adjust the pressure above the
aluminium stored in the collection reservoir.
[0024] This embodiment permits access to the product aluminium,
e.g. for tapping, without having to access the electrolysis chamber
which reduces de risk of unwanted interferences with the
electrolysis process.
[0025] Typically, the molten aluminium in the collection reservoir
is covered by a layer or molten electrolyte. This layer protects
the product aluminium against oxidation and/or freezing.
[0026] Usually, the depth of the cathodic pool does not lie below 3
or 5 mm; in particular it can have a depth in the range of 1 to 15
cm such as 2 to 5 cm.
[0027] The cathodic cavity containing the cathodic pool and/or
other cell components exposed to molten aluminium, and possibly
also to the electrolyte, can have an aluminium-wettable surface, in
particular made of an aluminium-wettable refractory material such
as a titanium diboride or other boride based layer. The
aluminium-wettable material advantageously includes one or more
wetting agents, such as oxides of iron, copper and/or nickel.
Examples of such materials are disclosed in U.S. Pat. Nos.
5,364,513, 5,651,874, 6,436,250, and in PCT publications
WO01/42168, WO01/42531, WO02/070783, WO02/096830 and WO02/096831,
WO2004/092449, WO2005/068390 (all assigned to MOLTECH Invent
S.A.).
[0028] Components which are only exposed to molten aluminium, e.g.
the collection reservoir or non conductive parts of the cathodic
cavity, can be covered with an electrically conductive material,
e.g. aluminium-wettable material as mentioned above, and/or non
conductive material. They can have a surface made of a non
conductive ceramic material, such as aluminium oxide, silicon
carbide, silicon nitride and/or boron nitride.
[0029] The active anodic surface can be the surface of an anode
body that has a plurality of through passages for the flow of
circulating electrolyte through the anode body from below to above
the anode body and/or from above to below the anode body. The anode
body may comprise a series of elongated members spaced apart by
inter-member gaps which form said through passages, or the anode
body may comprise a solid body, in particular a plate, which has
through holes that form said through passages. Suitable anodes are
disclosed in WO00/40781, WO00/40782, WO03/006716, WO03/023091,
WO03/023091 and WO2005/118916 (all assigned to MOLTECH Invent
S.A.).
[0030] Suitable materials for metal-based, in particular
oxygen-evolving, anodes include at least one metal selected from
nickel, iron, cobalt and copper. For instance the anode has a metal
oxide surface, in particular a surface containing at least one of
iron oxide, nickel oxide and cobalt oxide. Suitable anode materials
are disclosed in WO99/36591 and WO99/36592, WO99/36593 and
WO99/36594, WO00/06800, WO00/06801, WO00/06802 and WO00/06803,
WO00/06804, WO00/06805, WO00/40783 and WO01/42534, WO01/42536, and
WO01/43208, WO02/070786, WO02/083990, WO02/083991, WO03/078695,
WO03/087435, WO2004/018731, WO2004/024994, WO2004/044268,
WO2004/050956, WO2005/090641 and WO2005/090643 (all assigned to
MOLTECH Invent S.A.). Oxygen-evolving anodes may be coated with a
protective layer made of one or more cerium compounds, in
particular cerium oxyfluoride, as disclosed in U.S. Pat. Nos.
4,614,569, 4,680,094, 4,683,037 and 4,966,674 (all assigned to
MOLTECH Invent S.A.).
[0031] The invention further relates to a method of operating the
above described cell to produce aluminium.
[0032] Usually, the cell's electrolyte is maintained at a
temperature in the range of 700.degree. to 1000.degree. C., in
particular 800 to 970.degree. C. such as 880 to 940.degree. C.
[0033] The electrolyte can be a fluoride-containing electrolyte,
for example as disclosed in WO00/06802, WO01/42535, WO02/097167,
WO03/083176, WO2004/035871, WO2004/074549 and WO2005/090642 (all
assigned to MOLTECH Invent S.A.).
[0034] The product aluminium is preferably maintained in a molten
state in the collection reservoir, in particular above 700.degree.
C. When the reservoir is located in a chamber that is separate from
the electrolysis chamber, the aluminium can be stored at the same
or similar temperature as the electrolyte or at a lower
temperature; in the other cases, the aluminium will usually be
stored at substantially the same temperature as the molten
electrolyte.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] The invention will be further described by way of example
with reference to the accompanying schematic drawings, in
which:
[0036] FIG. 1 illustrates an aluminium electrowinning cell
according to the invention having an electrolysis chamber separated
from an aluminium collection reservoir chamber; and
[0037] FIGS. 2 and 3 show further aluminium electrowinning cells
according to the invention including an electrolysis chamber that
has an aluminium collection reservoir.
DETAILED DESCRIPTION
[0038] The aluminium electrowinning cell shown in FIG. 1 has an
electrolysis chamber 10 in which alumina dissolved in a molten
electrolyte 20 is electrolysed to produce aluminium 30. The cell
includes a collection reservoir in a reservoir chamber 40 in which
product aluminium 30 is collected. The electrolysis chamber 10 is
adjacent to the collection reservoir 40 and in liquid communication
therewith via passage 45 so that aluminium 30 produced in the
electrolysis chamber 10 can flow from the electrolysis chamber 10
into the collection reservoir 40. Passage 45 extends from the
electrolysis chamber 10 to the collection reservoir 40 under a wall
50 that separates the chamber 10 and reservoir 40. Aluminium 30
collected in reservoir chamber 40 is covered with a protective
layer of molten electrolyte 21.
[0039] The electrolysis chamber 10 contains dimensionally stable
anodes 15. Each anode 15 has an active anodic surface 16 spaced by
an anode-cathode gap 25 from a facing cathodic surface 31 on which
aluminium 30 is produced. The cathodic surface 31 is formed on a
cathode bottom 12 by a layer 35 made of molten aluminium 30 into
which product aluminium is incorporated during operation. The
anodic surface 16 and the cathodic surface 31 have a substantially
constant operative position so that the anode-cathode gap 25 is
substantially constant during operation.
[0040] Electrolyte 20 is maintained at a temperature in the range
of 900 to 950.degree. C. so that a ledge 22 can form to protect
cell sidewalls 13 and a crust 23 can be made to form so as to cover
the electrolyte surface 24.
[0041] In accordance with the invention the cell comprises means
60,61,62 for regulating the layer of molten aluminium 30 so the
layer 35 forms a shallow or deep continuous cathodic pool that
extends continuously under the entire facing active anodic surfaces
16 and that is contained within a cathodic cavity on the cathode
bottom 12.
[0042] The layer regulating means 60,61,62 are arranged to maintain
during operation the cathodic pool 35 at a substantially constant
(or non fluctuating) height by periodic or continuous removal of
molten aluminium 30 from pool 35 to the collection reservoir 40 at
a rate corresponding substantially to the rate of production of
product aluminium 30 that is incorporated into the aluminium pool
35.
[0043] As shown in FIG. 1, the layer regulating means 60,61,62
include one or more vacuum pumps and pressure release valves for
adjusting the pressure in the reservoir chamber 40 above collected
aluminium 30 and electrolyte layer 21, to thereby adjust the height
and amount of collected aluminium 30 in chamber 40 so that the
cathodic surface 31 of cathodic aluminium pool 35 remains at
constant position over the cathodic bottom 12 during the production
of aluminium 30 and its incorporation into cathodic layer 35. Pumps
and valves are controlled by a control device 60 which is itself
connected to sensors 62 for measuring the electrical
characteristics of cathodic bottom 12 and anodes 15 from which the
height of the cathodic surface 31 of cathodic pool 35 is deducted,
preferably taking into account any evolution of the bath
composition during electrolysis for example by predictive models
known in the art. Alternatively, this height can be measured using
appropriate proximity sensors.
[0044] During cell operation, aluminium 30 is produced by the
electrolysis of alumina dissolved in electrolyte 20 and the product
aluminium 30 incorporated into cathodic pool 35. The distance
between the anodic surfaces 16 and cathodic surface is continuously
monitored via sensors 62, pumps and valves 61 being operated via
control device 60 so that aluminium 30 is evacuated from cathodic
pool 35 to the reservoir chamber 40 via passage 45 at the rate at
which aluminium 30 is produced in the anode-cathode gap 25 and
incorporated into the cathodic pool 25.
[0045] The cells shown in FIGS. 2 and 3, in which the same numeric
references designate the same elements, have recessed collection
reservoirs 40' adjacent to and extending below the cathodic surface
31 of cathodic pool 35. Reservoirs 40' have a generally U-shaped
cross-section. In FIG. 2, reservoirs 40' are located along a
peripheral part of the electrolysis chamber 10. In FIG. 3, a
reservoir 40' is located in a central part of the cell.
[0046] As shown in FIGS. 2 and 3, the cells' regulating means
comprise a wall 60' that extends from reservoir 40' above cathodic
bottom 12 adjacent cathodic pool 35. Wall 60' and cathodic bottom
12 form a cathodic cavity for containing the cathodic pool 35.
[0047] Wall 60' has an upper edge 61' at about the level of
cathodic surface 31 and is arranged to allow the flow over edge 61'
of product aluminium 30 to the collection reservoir 40'.
[0048] During cell operation, aluminium 30 is produced by the
electrolysis of alumina dissolved in electrolyte 20 and the product
aluminium 30 is incorporated into cathodic pool 35. As aluminium 30
is produced and incorporated into cathodic pool 35, aluminium 30
flows over edges 61' from cathodic pool 35 into reservoirs 40 at
the same rate as the rate of production of aluminium 30 and its
incorporation into pool 35. Thus, the cathodic surface 31 formed by
cathodic pool 35 is constantly maintained at the same level which
is set by the level of edge 61'.
[0049] In other words, the cathodic surface's position does not
significantly fluctuate during operation, even though the cathodic
surface 31 of pool 35 can be slightly curved due to surface tension
effects, as shown in FIGS. 2 and 3. As a result of this slight
curvature, the pool's height (or depth) is not strictly uniform
over its surface 31; however the height (or depth) of the pool 35
at various locations does not significantly fluctuate during
operation.
[0050] When the surface 31 of cathodic pool 35 extends under
several anodes 15, the position of individual anodes can be
adjusted to the slightly curved profile of cathodic surface 31. In
other words, anodes 15 located above a central part of cathodic
surface 31 can be positioned slightly higher than anodes 15 that
are situated at the periphery of cathodic surface 31 to adjust to
the height of the facing portion of the underlying cathodic surface
31.
* * * * *