U.S. patent application number 11/667142 was filed with the patent office on 2008-02-21 for aluminium electrowinning with enhanced electrolyte circulation.
Invention is credited to Vittorio De Nora, Thinh T. Nguyen.
Application Number | 20080041729 11/667142 |
Document ID | / |
Family ID | 39100338 |
Filed Date | 2008-02-21 |
United States Patent
Application |
20080041729 |
Kind Code |
A1 |
De Nora; Vittorio ; et
al. |
February 21, 2008 |
Aluminium Electrowinning With Enhanced Electrolyte Circulation
Abstract
A method of operating an aluminium electrowinning cell that has
one or more metal-based anodes (5). The anodes (5) comprise
metal-based foraminate anode bodies (10) which are suspended by
metal-based anode stems (20) in a molten electrolyte (50) and which
are spaced above a cathode (30). The method comprises electrolysing
alumina dissolved in the molten electrolyte (50) by passing current
via the anode stems (20) and the anode bodies (10) through the
electrolyte (50) to the facing cathode (30) whereby aluminium (60)
is cathodically produced and gas is anodically evolved. The gas
promotes an electrolyte circulation (51) through the foraminate
anode bodies (10) which facilitates dissolution of alumina. Each
anode (5) has a foraminate anode body (10) suspended by least three
anode stems (20) that are spaced apart from one another and
distributed around a foraminate stemless central part of the anode
body (10). These stems extend from the anode body (10) to above the
molten electrolyte (50), the electrolyte (50) flowing up through
and above said foraminate central part of the anode body (10) to
enhance dissolution of alumina fed thereabove.
Inventors: |
De Nora; Vittorio; (Veyras,
CH) ; Nguyen; Thinh T.; (Onex, CH) |
Correspondence
Address: |
Jayadeep R. Deshmukh
458 Cherry Hill Road
Princeton
NJ
08540
US
|
Family ID: |
39100338 |
Appl. No.: |
11/667142 |
Filed: |
October 24, 2005 |
PCT Filed: |
October 24, 2005 |
PCT NO: |
PCT/IB05/53466 |
371 Date: |
May 4, 2007 |
Current U.S.
Class: |
205/380 ;
204/243.1; 204/284 |
Current CPC
Class: |
C25C 3/12 20130101; C25C
3/06 20130101 |
Class at
Publication: |
205/380 ;
204/243.1; 204/284 |
International
Class: |
C25C 3/12 20060101
C25C003/12 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 5, 2004 |
IB |
PCT/IB2004/003642 |
Claims
1. A method of operating an aluminium electrowinning cell, the cell
having one or more metal-based anodes that comprise metal-based
foraminate anode bodies which are suspended by metal-based anode
stems in a molten electrolyte and which are spaced above a cathode,
said method comprising electrolysing alumina dissolved in the
molten electrolyte by passing current via the anode stems and the
or each anode body through the electrolyte to the facing cathode
whereby aluminium is cathodically produced and gas is anodically
evolved, the gas promoting an electrolyte circulation through the
or each foraminate anode body to facilitate dissolution of alumina
fed thereabove, wherein the or each anode has a foraminate anode
body suspended by at least three anode stems that are spaced apart
from one another and distributed around a foraminate stemless
central part of the anode body, said stems extending from the anode
body to above the molten electrolyte, electrolyte flowing up
through and above said foraminate central part to enhance
dissolution of alumina.
2. The method of claim 1, wherein alumina is supplied to the
electrolyte between the stems of at least one anode vertically
above the central part of the anode body.
3. The method of claim 1 or 2, comprising draining molten aluminium
produced on the cathode.
4. An aluminium electrowinning cell having one or more metal-based
anodes that comprise metal-based foraminate anode bodies which are
suspended by metal-based anode stems in an alumina-containing
molten electrolyte and which are spaced above a cathode, said cell
being arranged so as to permit an electrolyte circulation promoted
by anodically evolved gas through the or each foraminate anode body
to facilitate dissolution of alumina in the electrolyte fed
thereabove, wherein the or each anode has a foraminate anode body
suspended by at least three anode stems that are spaced apart from
one another and distributed around a foraminate stemless central
part of the anode body, said stems extending from the anode body to
above the molten electrolyte so as to permit an upflow of
electrolyte through and above said foraminate central part to
enhance dissolution of alumina.
5. The cell of claim 4, wherein the anode stems of one anode are
connected together by cross-members.
6. The cell of claim 5, wherein the anode stems of one anode are
connected together by cross-members above the insulating cover.
7. The cell of claim 5, wherein the anode stems of one anode are
connected together by cross-members below the insulating cover.
8. The cell of claim 5, 6 or 7, wherein the cross-members are
joined to a main current conductor that is connected to an anode
bus bar.
9. The cell of claim 5, wherein the molten electrolyte is
substantially free of any frozen crust.
10. An aluminium electrowinning metal-based anode for use in a cell
as defined in any one of claims 4 to 9 that comprises a metal-based
foraminate anode body and metal-based anode stems which are
connected to the anode body, wherein the or each anode has a
foraminate anode body connected by at least three anode stems that
are spaced apart from one another and distributed around a
foraminate stemless central part of the anode body, said stems
extending during use from the anode body to above the molten
electrolyte so as to permit an upflow of electrolyte through and
above said foraminate central part to enhance dissolution of
alumina.
11. The anode of claim 10, wherein the anode body has a grid-like
or plate-like foraminate structure that is parallel to the facing
cathode.
12. The anode of claim 10 or 11, wherein the anode body has an
upper face to which the stems are connected around a central point
of the upper surface, each anode stem being located at a distance
from the central point which is in the range of 1/4 to 3/4 of the
length of a segment of a line extending from the central point to a
side of the face and intercepting the anode stem, in particular 1/3
to 2/3 of said length.
13. The anode of any one of claims 10 to 12, wherein the anode body
has a square or rectangular upper face.
14. The anode of claim 13, wherein the anode body is suspended by
four anode stems.
15. The anode of claim 14, wherein said stems are located
substantially on crossing diagonals of the body's upper face, each
stem being located about half way between a corner of the body's
face and the crossing point of the diagonals.
16. The method of claim 14, wherein said four stems are located
substantially on two crossing perpendicular median lines of the
body's upper face, each stem being connected about half way between
a side of the body's face and the crossing point of the median
lines.
17. The anode of any one of claims 10 to 12, wherein the anode body
has a circular upper face.
18. The anode of claim 17, wherein each stem is located
substantially in the middle of a radius of the circular upper face,
the stems being evenly distributed on the circular upper face
around the body's central part.
19. The anode of claim 17 or 18, which comprises four anode
stems.
20. The anode of any one of claims 10 to 19, wherein the anode
stems have ends away from the anode body that are connected
together by cross-members.
21. The anode of claim 10, wherein the anode has pairs of opposite
stems that are connected by intercepting cross-members.
22. The anode of claim 20 or 21, wherein the cross-members are
joined to a main current conductor for connection to a busbar.
23. The anode of any one of claims 10 to 22, wherein the anode
stems have a transverse cross-sectional area that is sufficient for
passing a current that leads to a current density in the range of
0.5 to 1.5 A/cm.sup.2 at the surface of the anode with a voltage
drop along the anode stems below 80 mV/cm, in particular in the
range of 20 to 50 mV/cm.
24. The anode of any one of claims 10 to 23, wherein the anode body
has an active surface that has total projected surface area AA and
wherein the anode stems connected to the anode body have a
cumulated transverse cross-sectional area AS (equal to the sum of
the transverse cross-sectional area of the individual anode stems),
the area AS corresponding to a fraction of the area AA which is in
the range of 0.1% to 2% of the area AA, in particular 1 to
1.5%.
25. The anode of any one of claims 10 to 24, wherein each anode
stem has a diameter in the range of 2 to 8 cm, in particular 2.5 to
6 cm, such as 3 to 4 cm.
26. The anode of any one of claims 10 to 25, wherein the anode body
has an active face that has a total projected surface area in the
range of 0.2 to 2 m.sup.2, in particular 0.5 to 1.5 m.sup.2.
Description
FIELD OF THE INVENTION
[0001] This invention relates to electrowinning aluminium in a cell
having foraminate metal-based anodes which permit an improved
electrolyte circulation. The invention also relates to such a cell
and to the anode.
BACKGROUND ART
[0002] The technology for the production of aluminium by the
electrolysis of alumina, dissolved in molten cryolite, at
temperatures around 950.degree. C. is more than one hundred years
old and still uses carbon anodes and cathodes.
[0003] Using metal anodes in aluminium electrowinning cells would
drastically improve the aluminium process by reducing pollution and
the cost of aluminium production.
[0004] Several attempts have been made in order to develop
non-carbon anodes for aluminium electrowinning cells, resistant to
chemical attacks of the bath and by the cell environment, and with
an electrochemical active surface for the oxidation of oxygen ions
to atomic and molecular gaseous oxygen and having a low dissolution
rate. However, all attempts have failed mainly due to the anode
materials which had a low electrical conductivity and caused
unacceptable contamination of the aluminium produced. Many patents
have been filed on non-carbon anodes but none has found commercial
acceptance, also because of economical reasons.
[0005] U.S. Pat. No. 4,614,569 (Duruz/Derivaz/Debely/Adorian)
describes metal anodes for aluminium electrowinning coated with a
protective coating of cerium oxyfluoride, formed in-situ in the
cell or pre-applied, this coating being maintained during
electrolysis by the addition of small amounts of a cerium compound
to the molten cryolite electrolyte so as to protect the surface of
the anode from the electrolyte attack.
[0006] Several designs for oxygen-evolving anodes for aluminium
electrowinning cells were proposed in the following patents all
assigned to MOLTECH Invent S.A. U.S. Pat. No. 4,681,671 discloses
vertical anode plates or blades operated in low temperature
aluminium electrowinning cells. U.S. Pat. No. 5,310,476 discloses
oxygen-evolving anodes consisting of roof-like assembled pairs of
anode plates. U.S. Pat. No. 5,362,366 describes non-consumable
anode shapes including roof-like assembled pairs of anode plates.
U.S. Pat. No. 5,368,702 discloses vertical tubular or frustoconical
oxygen-evolving anodes for multimonopolar aluminium cells. U.S.
Pat. No. 5,683,559 describes an aluminium electrowinning cell with
oxygen-evolving bent anode plates which are aligned in a roof-like
configuration facing correspondingly shaped cathodes. U.S. Pat. No.
5,725,744 discloses vertical oxygen-evolving anode plates,
preferably porous or reticulated, in a multimonopolar cell
arrangement for aluminium electrowinning cells operating at reduced
temperature.
[0007] WO00/40781 and WO00/40782 (both de Nora) both disclose
aluminium production anodes with a series of parallel spaced-apart
elongated anode members which are electrochemically active for the
oxidation of oxygen. Various anodes are disclosed, in particular
WO00/40782 discloses an anode with a rectangular anode-member
arrangement which is held by two spaced-apart large feet, each foot
being integral with a large anode stem and located towards one end
of the anode-member arrangement.
SUMMARY OF THE INVENTION
[0008] A main object of the present invention is to enhance the
circulation of electrolyte in an aluminium electrowinning cell
having foraminate metal-based anodes and the dissolution of alumina
by using a new cell configuration.
[0009] In particular, the invention is concerned with the flow of
the electrolyte resulting inter-alia from the shape and
configuration of the anode. To achieve the present invention,
instead of having foraminate anode bodies that are suspended
usually from their centre by large prior art anode stems,
foraminate anode bodies are suspended by a greater number of anode
stems, preferably of smaller size, which are distributed around a
central part of the anode and which do not significantly interfere
with the electrolyte upflow. In comparison with the prior art anode
configuration, the electrolyte can flow up through substantially
the entire foraminate anode body and this upflow is not weakened by
being diverted into a multitude of different directions from below
a large main anode stem. Hence, during use, a strong electrolyte
circulation is produced and electrolyte circulates from and to
substantially the entire active anode surface which is
substantially uniformly operative for the oxidation of oxygen.
[0010] The present invention relates to a method of operating an
aluminium electrowinning cell. The cell has one or more metal-based
anodes that comprise metal-based foraminate anode bodies which are
suspended by metal-based anode stems in a molten electrolyte and
which are spaced above a cathode. The method comprises
electrolysing alumina dissolved in the molten electrolyte by
passing current via the anode stems and the or each anode body
through the electrolyte to the facing cathode whereby aluminium is
cathodically produced and gas is anodically evolved. The evolved
gas promotes an electrolyte circulation through the or each
foraminate anode body to facilitate dissolution of alumina fed
thereabove.
[0011] The or each anode has a foraminate anode body suspended by
at least three anode stems that are spaced apart from one another
and distributed around a foraminate stemless central part of the
anode body. These stems extend from the anode body to above the
molten electrolyte. Electrolyte flows up through and above this
foraminate central part to enhance dissolution of alumina.
[0012] As opposed to the abovementioned prior art anode disclosed
in WO00/40782, the anode of the present invention has at least
three anode stems that are located around a central part of the
foraminate anode body and are so dimensioned and positioned as to
minimise interferences with the electrolyte upflow through and
above the central part and generally through and above
substantially the entire anode body. Such a circulation also
permits the supply of alumina-rich electrolyte to substantially the
entire active anode surface so that electrolysis takes place over
substantially the entire active anode surface, i.e. without having
significant areas of the active anode surface that remain
inoperative for lack of electrolyte circulation to and from these
active areas.
[0013] The alumina is preferably supplied to the electrolyte
between the stems of at least one anode vertically above the
central part of the anode body directly into the electrolyte main
flow to maximise dissolution of the alumina due to the
electrolyte's stirring effect.
[0014] The cathode can be aluminium-wettable. The product molten
aluminium can be drained on the cathode. Various aluminium-wettable
cathodes and drained cathodes have been disclosed in the prior art
which can be used for this invention. See for example U.S. Pat.
Nos. 5,683,559, 5,888,360, 6,093,304, 6,258,246, 6,358,393 and
6,436,273, and in PCT publications WO99/02764, WO00/63463,
WO01/31086, WO01/31088, WO01/42168, WO01/42531, WO02/070783,
WO02/070785, WO02/096830, WO02/096831, WO02/097168, WO02/097168,
WO03/023091 and WO03/023092 (all assigned to MOLTECH Invent
S.A.).
[0015] Another aspect of the invention relates to an aluminium
electrowinning cell having one or more metal-based anodes that
comprise metal-based foraminate anode bodies which are suspended by
metal-based anode stems in an alumina-containing molten electrolyte
and which are spaced above a cathode. This cell is arranged so as
to permit an electrolyte circulation promoted by anodically evolved
gas through the or each foraminate anode body to facilitate
dissolution of alumina in the electrolyte fed thereabove.
[0016] The or each anode has a foraminate anode body suspended by
at least three anode stems that are spaced apart from one another
and distributed around a foraminate stemless central part of the
anode body. These stems extend from the anode body to above the
molten electrolyte so as to permit an upflow of electrolyte through
and above the foraminate central part to enhance dissolution of
alumina.
[0017] Usually, the anode has a metal-based active surface, in
particular an oxide surface that can for example contain at least
one iron, cobalt, nickel and copper. In one embodiment the active
surface is made predominantly of cobalt oxide CoO which provides
the advantages described below.
[0018] The anode stems of one anode can be connected together by
cross-members.
[0019] Usually, the cell has a cover above the electrolyte. The
cover can be an insulating cover, in particular for cell operation
with a substantially crustless and/or ledgeless molten electrolyte.
Covers and materials are disclosed in U.S. Pat. No. 6,402,928,
WO02/070784 and WO03/102274 (all assigned to MOLTECH Invent
S.A.).
[0020] The anode stems of one anode can be connected together by
cross-members above or below the insulating cover.
[0021] The cross-members can be joined to a main current conductor
(or main stem) that is connected to an anode bus bar. The bus bar
is usually part of the cell superstructure located above the cell
cover. Such a main conductor should be located outside the
electrolyte circulation to avoid interference therewith, for
example this conductor can be located above the electrolyte.
[0022] Usually, the alumina-containing electrolyte is a
fluoride-based electrolyte, in particular an electrolyte containing
predominantly aluminium fluoride and sodium fluoride.
[0023] A further aspect of the invention relates to an aluminium
electrowinning metal-based anode for use in a cell as described
above. The anode comprises a metal-based foraminate anode body and
metal-based anode stems which are connected to the anode body.
[0024] The or each anode has a foraminate anode body connected by
at least three of said anode stems that are spaced apart from one
another and distributed around a foraminate stemless central part
of the anode body. These stems extend during use from the anode
body to above the molten electrolyte so as to permit an upflow of
electrolyte through and above this foraminate central part to
enhance dissolution of alumina.
[0025] Four to eight anode stems can be connected to an anode body,
in particular four to six stems.
[0026] Usually, the anode body has a grid-like or plate-like
foraminate structure that is parallel to the facing cathode.
Examples of such anode bodies are disclosed in WO00/40781,
WO00/40782 and WO03/006716 (all assigned to MOLTECH Invent
S.A.).
[0027] Typically, the anode body has an upper face to which the
stems are connected around a central point of the upper face. Each
anode stem can be located at a distance from the central point
which is in the range of 1/4 to 3/4 of the length of a segment of a
line extending from the central point to a side of the face and
intercepting the anode stem, in particular 1/3 to 2/3 of said
length.
[0028] The anodes stems are preferably positioned on the anode
bodies taking into account the current distribution in the anode
bodies during use so as to optimise this distribution.
[0029] In one embodiment, the anode body has polygonal, in
particular a square or rectangular upper face. For example, the
anode body is suspended by four anode stems.
[0030] These stems can be generally located on crossing diagonals
of the body's upper face, each stem being located generally on the
upper face about half way between a corner of the body's face and
the crossing point of the diagonals. In other words, the
rectangular or square upper face can be notionally divided into
four equal rectangular or square quadrants in the middle of which
an anode stem is connected to the anode body.
[0031] These four stems may be generally located substantially on
crossing perpendicular median lines of the body's upper face, each
stem being connected about half way between a side of the body's
face and the crossing point of the median lines.
[0032] In another embodiment an anode body has a generally circular
upper face. Alternatively the upper face can be generally elliptic.
The upper face may be connected to four stems evenly distributed
around the body's central part. Each stem can be located
substantially in the middle of a radius of the circular upper
face.
[0033] Usually, the anode stems have ends away from the anode body
that are connected together by cross-members.
[0034] In particular, the anode can have pairs of opposite stems
that are connected by intercepting cross-members. For example, the
anode has four stems that are connected by two cross-members that
form an "X" or three cross-members that form an "H".
[0035] The cross-members can be joined to a main current conductor
for connection to a busbar.
[0036] Preferably, the anode stems connected to the anode body have
a sufficient transverse cross-sectional area for passing a current
that leads to a current density in the range of 0.5 to 1.5
A/cm.sup.2 near the surface of the anode body with a voltage drop
along the anode stems below 80 mV/cm, in particular in the range of
20 to 50 mV/cm. The total voltage drop along the stems can be of
the order of 0.2 to 0.5 V. The anode body can have an active
surface that has a total projected surface area AA and the anode
stems connected to the anode body have a cumulated transverse
cross-sectional area AS (equal to the sum of the transverse
cross-sectional area of the individual anode stems), the area AS
corresponding to a fraction of the area AA which is in the range of
0.1% to 2% of the area AA, in particular 1 to 1.5%.
[0037] The diameter of each anode stem may be in the range of 2 to
8 cm, in particular 2.5 to 6 cm, such as 3 to 4 cm. This is
significantly smaller than the usual diameter of anode stems, which
is typically 10-15 cm.
[0038] The anode body has an active face that usually has a total
projected surface area in the range of 0.2 to 2 m.sup.2, in
particular 0.5 to 1.5 m.sup.2.
[0039] The total projected surface area mentioned above refers to
the overall active surface without deduction of the holes present
at the surface due to the fact that the anode body is
foraminate.
[0040] The active anode surface can be made of any prior art
non-carbon materials, in particular metal-based materials such as
materials containing at least one of iron, nickel and cobalt and
oxides thereof. For example the anode's active surface can be made
of the materials disclosed in any of the following publications:
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 and WO2004/044268, WO2004/050956 (all in the name of
MOLTECH Invent S.A.).
BRIEF DESCRIPTION OF THE DRAWINGS
[0041] The invention will now be described by way of example with
reference to the drawings, wherein:
[0042] FIG. 1 is a perspective view of an anode body of an anode
according to the invention;
[0043] FIGS. 2 to 5 show schematic plan views of anodes according
to the invention; and
[0044] FIGS. 6 and 7 are cross-sectional views of two aluminium
electrowinning cells according to the invention.
DETAILED DESCRIPTION
[0045] FIG. 1 shows an anode body 10 of an anode according to the
invention. The anode body 10 has a generally rectangular upper face
and comprises a series of parallel spaced apart anode members 11
that have a generally pentagonal cross-section with a tapered upper
part for guiding an electrolyte circulation. Examples of suitable
anode members 11 are disclosed in WO03/006717 (de Nora). Variations
for the anode members are also disclosed in WO00/40781 and
WO00/40782 (both de Nora). The anode members 11 are connected by
two end-members 12 and two intermediate cross-members 13 as shown
in FIG. 1.
[0046] The anode body 10 has in the cross-members 13 four sockets
15 each with an opening 16 for receiving anode stems (not shown).
An anode stem can be screwed, force-fitted or welded into opening
16. In a variation, an anode stem can be made integral with the
cross-member 13.
[0047] Each socket 15 is located in the middle of a notional
quadrant delimited by dashed lines A and B which divide the upper
face of the anode body into four equal quadrants. In other words,
each socket 15 is located on a diagonal of the body's upper face,
half way between a corner of the upper face and the intersection of
the diagonals (which coincides with the intersection of dashed
lines A and B). Sockets 15 are joined to cross-members 13 which
permits a better current distribution from anode stems and sockets
15 to anode members 11.
[0048] For instance, the anode body 10 has a total length of 65 cm
along the direction of the anode members 11 and a total width of 61
cm across the direction of the anode members. Each anode member 11
has a width of 20 mm and a height of 25 mm. The anode members are
spaced apart by inter-member gaps of 20 mm. The total projected
active bottom surface of the anode body (without deduction of the
inter-member gaps) is about 65 cm.times.61 cm, i.e. approximately
0.4 m.sup.2. Each socket 15 has an outer diameter 60 mm and the
opening 16 has a diameter of 40 mm. This opening can receive an
anode stem whose lower end has a corresponding diameter, i.e. 40
mm. The remaining part of the anode stems can have the same
diameter or a different diameter, in particular a larger diameter.
However, the diameter of this remaining part should not be too
large, in any case no larger than the outer diameter of socket 15,
in order to avoid noticeable interference of the anode stems with
upflowing electrolyte during operation in a cell.
[0049] In any case, these sockets 15 and the corresponding anode
stems are significantly smaller than prior art central anode stems
which have a typical diameter of the order of 120 mm. The cross
members 13 have a width of 20 mm.
[0050] Cell operation with an anode body 10 as shown in FIG. 1 will
be described in greater detail in connection with the cells shown
in FIGS. 6 and 7.
[0051] FIGS. 2 to 5 show schematically plan views of anodes 5
having variations of the anode body shown in FIG. 1.
[0052] The anode bodies 10 of FIGS. 2 & 3 have a circular
shape. These anode bodies 10 can be made up of concentric circular
anode members connected by radial cross-members (not shown). See
for example WO00/40781 and WO00/40782 (both de Nora). The anode
body 11 shown in FIG. 2 is suspended by four anode stems 20, each
stem 20 being usually joined to or integral with a radial
cross-member. Anode stems 20 are located on a radial cross-member
and spread around the centre of the anode body 11. The anode body
11 shown in FIG. 3 is suspended by three anode stems 20, each stem
20 being located close to the middle of a radial cross-member. FIG.
2 indicates in dotted lines intercepting members 23 for connecting
anode stems 20 above the anode as explained in FIGS. 6 and 7.
[0053] The anode bodies 11 of FIGS. 4 and 5 have a square shape.
Four anode stems 20 are located on median lines A and B to the
anode body 11 of FIG. 4. Six anode stems 20 are located on median
line A and diagonals C and D in FIG. 5.
[0054] FIG. 6 shows in cross-sectional view a drained-cathode
aluminium electrowinning cell with two different anodes 5.
[0055] The cell has a carbon cathode 30 with an inclined
aluminium-wettable drained cathode surface 31 and an aluminium
collection reservoir 35. The surface of the carbon cathode can be
made wettable by applying thereto a layer of aluminium-wettable
material, in particular a titanium diboride coating as for example
disclosed in WO02/096831 (in the name of MOLTECH Invent S.A.).
Cathode 30 is located on a bed of insulating material 36 and
electrically connected to a busbar (not shown) by collector bars 37
that are usually made of steel.
[0056] The drained cathode surface 31 is covered with a thin layer
of product aluminium 60.
[0057] The cell has sidewalls 40 which are made of or covered with
a material resistant to molten electrolyte, such as silicon carbide
or fused alumina or aluminium-wetted porous aluminium as disclosed
in WO02/070783 (de Nora). Sidewall 40 is lined with an insulating
material 41 in steel shell 42. Sidewalls 40 are joined to cathode
30 by a body 43 of solidified ramming paste.
[0058] The cell is covered with an insulating cover 45 which can be
of the type disclosed in U.S. Pat. No. 6,402,928, WO02/070784 and
WO03/102274 (all assigned to MOLTECH Invent S.A.).
[0059] The cell contains an electrolyte 50 and has a sufficient
insulation 36, 41, 45 to maintain electrolyte 50 in a molten state
substantially without crust or ledge.
[0060] A suitable molten electrolyte can be at a temperature below
950.degree. C., in particular in the range from 910.degree. to
940.degree. C., and consist of: [0061] 6.5 to 11 weight % dissolved
alumina, in particular 7 to 10 weight %; [0062] 35 to 44 weight %
aluminium fluoride, in particular 36 to 42 weight % aluminium
fluoride, such as 36 to 38 weight; [0063] 38 to 46 weight % sodium
fluoride, in particular 39 to 43 weight %; [0064] 2 to 15 weight %
potassium fluoride, in particular 3 to 10 weight % potassium
fluoride, such as 5 to 7 weight %; [0065] 0 to 5 weight % calcium
fluoride, in particular 2 to 4 weight % calcium fluoride; and
[0066] 0 to 5 weight % in total of one or more further
constituents, in particular up to 3 weight %.
[0067] The presence in the electrolyte of potassium fluoride in the
above amount has two effects. On the one hand, it leads to a
reduction of the operating temperature by up to several tens of
degrees without increase of the electrolyte's aluminium fluoride
content or even a reduction thereof compared to standard
electrolytes operating at about 950.degree. C. with an aluminium
fluoride content of about 45 weight %. On the other hand, it
maintains a high solubility of alumina, i.e. up to above about 8 or
9 weight %, in the electrolyte even though the temperature of the
electrolyte is reduced compared to conventional temperature.
[0068] Hence, in the above electrolyte, in contrast to other low
temperature electrolytes which carry large amounts of undissolved
alumina in particulate form, a large amount of alumina is in a
dissolved form.
[0069] Without being bound to any theory, it is believed that
combining a high concentration of dissolved alumina in the
electrolyte and a limited concentration of aluminium fluoride leads
predominantly to the formation of (basic) fluorine-poor aluminium
oxyfluoride ions ([Al.sub.2O.sub.2F.sub.4].sup.2-) instead of
(acid) fluorine-rich aluminium oxyfluoride ions
([Al.sub.2OF.sub.6].sup.2-) near the anode. As opposed to acid
fluorine-rich aluminium oxyfluoride ions, basic fluorine-poor
aluminium oxyfluoride ions do not significantly dissolve the
anode's surface, in particular when made of predominantly of cobalt
or nickel oxide, and do not noticeably passivate or corrode the
anode's metals, in particular metallic cobalt or nickel. The weight
ratio of dissolved alumina/aluminium fluoride in the electrolyte
should be above 1/7, and often above 1/6 or even above 1/5, to
obtain a favourable ratio of the fluorine-poor aluminium
oxyfluoride ions and the fluorine-rich aluminium oxyfluoride
ions.
[0070] It follows that the use of the above described electrolyte
with metal-based anodes that contain cobalt oxide and/or nickel
oxide inhibits its dissolution, passivation and corrosion.
Moreover, a high concentration of alumina dissolved in the
electrolyte further reduces dissolution of oxides of the anode, in
particular cobalt oxide and nickel oxide.
[0071] The electrolyte may for example consist of: 7 to 10 weight %
dissolved alumina; 36 to 42 weight % aluminium fluoride, in
particular 36 to 38 weight %; 39 to 43 weight % sodium fluoride; 3
to 10 weight % potassium fluoride, such as 5 to 7 weight %; 2 to 4
weight % calcium fluoride; and 0 to 3 weight % in total of one or
more further constituents. This corresponds to a cryolite-based
(Na.sub.3AlF.sub.6) molten electrolyte containing an excess of
aluminium fluoride (AlF.sub.3) that is in the range of about 8 to
15 weight % of the electrolyte, in particular about 8 to 10 weight
%, and additives that can include potassium fluoride and calcium
fluoride in the abovementioned amounts.
[0072] The electrolyte can contain as further constituent(s) at
least one fluoride selected from magnesium fluoride, lithium
fluoride, cesium fluoride, rubidium fluoride, strontium fluoride,
barium fluoride and cerium fluoride.
[0073] Advantageously, The electrolyte contains alumina at a
concentration near saturation on the active anode surface.
[0074] In order to maintain the alumina concentration above a given
threshold in the abovementioned range during normal electrolysis,
the cell is preferably fitted with means to monitor and adjust the
electrolyte's alumina content.
[0075] The drained-cathode cell trough 30, 36, 37, 40, 41, 42, 43
shown in FIG. 6 and suitable variations are disclosed in greater
detail in the prior art, in particular in U.S. Pat. Nos. 6,682,643,
6,692,620 and 6,783,656, and in WO02/070783, WO02/070785,
WO02/097168 and WO02/097169 (all assigned to MOLTECH Invent
S.A.).
[0076] According to the invention, the cell has anodes 5 with a
foraminate anode body 10 that is suspended by four anode stems 20
distributed around a foraminate stemless central part of the anode
body and that is held above the cathode 30 parallel to the drained
cathode surface 31. The anode bodies 10 are in particular of the
type shown in FIG. 1. Anode stems 20 of each anode 5 are connected
by cross-members 23 to a main current conductor 25.
[0077] Advantageously, anodes 5 have an active surface with an
enhanced stability against corrosion by the highly aggressive
circulating electrolyte and/or against oxidation by anodically
evolved oxygen, the enhanced stability being provided by a layer
that contains predominantly cobalt oxide CoO. Such a composition is
particularly suitable for anodes 5 of the invention which during
use are exposed to a strong central electrolyte circulation.
[0078] There are several forms of stoichiometric and
non-stoichiometric cobalt oxides which are based on: [0079] CoO
that contains Co(II) and that is formed predominantly at a
temperature above 920.degree. C. in air; [0080] Co.sub.2O.sub.3
that contains Co(III) and that is formed at temperatures up to
895.degree. C. and at higher temperatures begins to decompose into
CoO; [0081] Co.sub.3O.sub.4 that contains Co(II) and Co(III) and
that is formed at temperatures between 300 and 900.degree. C.
[0082] It has been observed that--unlike Co.sub.2O.sub.3 that is
unstable and Co.sub.3O.sub.4 that does not significantly inhibit
oxygen diffusion--CoO forms a well conductive electrochemically
active material for the oxidation of oxygen ions and for inhibiting
diffusion of oxygen. Thus this material forms a limited barrier
against oxidation of the metallic cobalt body underneath.
[0083] The anode's CoO-containing layer can be a layer made of
sintered particles, especially sintered CoO particles.
Alternatively, the CoO-containing layer may be an integral oxide
layer on a Co-containing metallic layer or anode core. Tests have
shown that integral oxide layers have a higher density than
sintered layers and are thus preferred to inhibit oxygen
diffusion.
[0084] When CoO is to be formed by oxidising metallic cobalt, care
should be taken to carry out a treatment that will indeed result in
the formation of CoO. It was found that using Co.sub.2O.sub.3 or
Co.sub.3O.sub.4 in a known aluminium electrowinning electrolyte
does not lead to an appropriate conversion of these forms of cobalt
oxide into CoO. Therefore, it is important to provide an anode with
the CoO layer before the anode is used in an aluminium
electrowinning electrolyte.
[0085] The formation of CoO on the metallic cobalt is preferably
controlled so as to produce a coherent and substantially crack-free
oxide layer. However, not any treatment of metallic cobalt at a
temperature above 895.degree. C. or 900.degree. C. in an
oxygen-containing atmosphere will result in optimal coherent and
substantially crack-free CoO layer that offers better
electrochemical properties than a
Co.sub.2O.sub.3/Co.sub.3O.sub.4.
[0086] For instance, if the temperature for treating the metallic
cobalt to form CoO by air oxidation of metallic cobalt is increased
at an insufficient rate, e.g. less than 200.degree. C./hour, a
thick oxide layer rich in Co.sub.3O.sub.4 and in glassy
Co.sub.2O.sub.3 is formed at the surface of the metallic cobalt.
Such a layer does not permit optimal formation of the CoO layer by
conversion at a temperature above 895.degree. C. of Co.sub.2O.sub.3
and Co.sub.3O.sub.4 into CoO. In fact, a layer of CoO resulting
from such conversion is not preferred but still useful despite an
increased porosity and may be cracked. Therefore, the required
temperature for air oxidation, i.e. above 900.degree. C., usually
at least 920.degree. C. or preferably above 940.degree. C. should
be attained sufficiently quickly, e.g. at a rate of increase of the
temperature of at least 300.degree. C. or 600.degree. C. per hour
to obtain an optimal CoO layer. The metallic cobalt may also be
placed into an oven that is pre-heated at the desired temperature
above 900.degree. C.
[0087] Likewise, if the anode is not immediately used for the
electrowinning of aluminium after formation of the CoO layer but
allowed to cool down, the cooling down should be carried out
sufficiently fast, for example by placing the anode in air at room
temperature, to avoid significant formation of Co.sub.3O.sub.4 that
could occur during the cooling, for instance in an oven that is
switched off.
[0088] Further details regarding CoO-containing anodes and cell
operation therewith are disclosed in MOL0679, MOL0680, 681 and
682.
[0089] An anode with a CoO layer obtained by slow heating of the
metallic cobalt in an oxidising environment will not have optimal
properties but still provides better results during cell operation
than an anode having a Co.sub.2O.sub.3--Co.sub.3O.sub.4 layer and
therefore also constitutes an improved aluminium electrowinning
anode according to the invention.
[0090] The section of main current conductor 25 shown in FIG. 6
corresponds to the section of prior art stems whereas the section
of anode stems 20 is a fraction of the section of conductor 25,
usually about the size of the section of the conductor 25 divided
by the number of anode stems 20 connecting each anode body 10.
[0091] On the left-hand side of FIG. 6, anode stems 20 extend
through cell cover 45 and are connected by cross-members 23 above
cover 45. On the right-hand side of FIG. 6, anode stems 20 are
connected to cross-members 23 below cover 45. The connection can be
achieved by screwing, welding or force-fitting. In a variation the
anode stems are integral with cross-members.
[0092] During use, dissolved alumina is electrolysed between anode
bodies 10 and cathode 30 to produce aluminium 60 cathodically and
oxygen anodically. The oxygen released at the anode body 10
promotes an electrolyte upflow in the direction of arrow 51 through
anode body 10. This upflow is strongest through and above the
central part of anode body 10. However, electrolyte can circulate
through substantially the entire anode body 10 and is electrolysed
over the entire active surface of anode body 10. The main current
conductor 25 being located above electrolyte 50, it does not
interfere with this electrolyte circulation.
[0093] Alumina is fed to the electrolyte 50 vertically above the
central part of each anode body 10 between anode members 20 of each
anode 5 where the stirring effect of the electrolyte is highest.
The alumina dissolves as it enters the electrolyte and is
circulated with the electrolyte 50 to the gap spacing the anode
body 10 and cathode 30 mainly around anode body 10 and is
electrolysed substantially uniformly under anode body 10.
[0094] FIG. 7, in which the same references refer to the same
elements, shows a variation of the cell shown in FIG. 6. In FIG. 7,
the cell operates with a shallow aluminium pool 60 on a horizontal
cathode 30.
* * * * *