U.S. patent application number 11/628211 was filed with the patent office on 2011-05-05 for high stability flow-through non-carbon anodes for aluminium electrowinning.
Invention is credited to Vittorio De Nora, Thinh T. Nguyen.
Application Number | 20110100834 11/628211 |
Document ID | / |
Family ID | 35429260 |
Filed Date | 2011-05-05 |
United States Patent
Application |
20110100834 |
Kind Code |
A1 |
De Nora; Vittorio ; et
al. |
May 5, 2011 |
High stability flow-through non-carbon anodes for aluminium
electrowinning
Abstract
A cell for the electrowinning of aluminium from alumina
dissolved in a fluoride-containing molten electrolyte, comprises a
non-carbon metal-based anode having an electrically conductive
metallic structure. This anode structure comprises an outer part
with an electrochemically active anode surface on which, during
electrolysis, oxygen is anodically evolved, and which is suspended
in the electrolyte substantially parallel to a facing cathode. The
anode structure has one or more flow-through openings extending
from the active anode surface through the metallic structure, the
flow-through opening(s) being arranged for guiding a circulation of
electrolyte driven by the fast escape of anodically evolved oxygen.
The outer part of the anode comprises a layer that contains
predominantly cobalt oxide CoO to enhance the stability of the
anode.
Inventors: |
De Nora; Vittorio; (Veyras,
CH) ; Nguyen; Thinh T.; (Onex, CH) |
Family ID: |
35429260 |
Appl. No.: |
11/628211 |
Filed: |
May 25, 2005 |
PCT Filed: |
May 25, 2005 |
PCT NO: |
PCT/IB05/51718 |
371 Date: |
December 2, 2006 |
Current U.S.
Class: |
205/372 ;
204/245; 204/284 |
Current CPC
Class: |
C25C 3/06 20130101; C25C
3/12 20130101 |
Class at
Publication: |
205/372 ;
204/245; 204/284 |
International
Class: |
C25C 3/06 20060101
C25C003/06; C25C 3/08 20060101 C25C003/08; C25C 3/12 20060101
C25C003/12 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 3, 2004 |
IB |
PCT/IB04/01900 |
Claims
1. A cell for the electrowinning of aluminium from alumina
dissolved in a fluoride-containing molten electrolyte, comprising
at least one non-carbon metal-based anode having an electrically
conductive metallic structure that comprises an outer part with an
electrochemically active anode surface on which, during
electrolysis, oxygen is anodically evolved, and which is suspended
in the electrolyte substantially parallel to a facing cathode, said
metallic structure having one or more flow-through openings
extending from the active anode surface through the metallic
structure, said flow-through opening(s) being arranged for guiding
a circulation of electrolyte driven by the fast escape of
anodically evolved oxygen, wherein said outer part of the anode
comprises a layer that contains predominantly cobalt oxide CoO.
2. The cell of claim 1, wherein the anode structure is a foraminate
structure.
3. The cell of claim 2, wherein the anode structure comprises a
series of parallel anode members, in particular horizontal anode
members having electrochemically active surfaces in a generally
coplanar arrangement to form said active anode surface, the anode
members being spaced apart to form longitudinal flow-through
openings for the circulation of electrolyte driven by the fast
escape of anodically evolved oxygen.
4. The cell of claim 3, wherein the anode members are blades, bars,
rods or wires.
5. The cell of claim 1, wherein the active anode surface is
substantially horizontal.
6. The cell of claim 1, wherein the active anode surface is
substantially vertical or inclined to the horizontal.
7. The cell of claim 1, wherein the molten electrolyte is at a
temperature below 950.degree. C., in particular in the range from
910.degree. to 940.degree. C., and consists of: 6.5 to 11 weight %
dissolved alumina, in particular 7 to 10 weight %; 35 to 44 weight
% aluminium fluoride, in particular 36 to 42 weight % aluminium
fluoride, such as 36 to 38 weight; 38 to 46 weight % sodium
fluoride, in particular 39 to 43 weight %; 2 to 15 weight %
potassium fluoride, in particular 3 to 10 weight % potassium
fluoride, such as 5 to 7 weight %; 0 to 5 weight % calcium
fluoride, in particular 2 to 4 weight % calcium fluoride; and 0 to
5 weight % in total of one or more further constituents, in
particular up to 3 weight %.
8. The cell of claim 7, wherein the electrolyte contains 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.
9. The cell of claim 1, wherein the electrolyte contains alumina at
a concentration near saturation on the active anode surface.
10. The cell of claim 1, wherein the CoO-containing layer is
integral with a core made of cobalt or a cobalt alloy.
11. The cell of claim 1, wherein the anode comprises an
electrically conductive substrate that is covered with an applied
electrochemically active coating that comprises the CoO-containing
layer.
12. The cell of claim 11, wherein the CoO-containing layer is a
layer of sintered particles.
13. The cell of claim 11, wherein the CoO-containing layer is an
integral oxide layer on an applied Co-containing metallic layer of
the coating.
14. The cell of claim 11, which comprises an oxygen barrier layer
between the CoO-containing layer and the electrically conductive
substrate.
15. The cell of claim 14, wherein the oxygen barrier layer contains
at least one metal selected from nickel, copper, tungsten,
molybdenum, tantalum, niobium and chromium, or an oxide
thereof.
16. The cell of claim 15, wherein the oxygen barrier layer further
contains cobalt.
17. The cell of claim 16, wherein the oxygen barrier layer is a
cobalt alloy containing at least one metal selected from nickel,
tungsten, molybdenum, tantalum and niobium.
18. The cell of claim 17, wherein the cobalt alloy contains: at
least one of nickel, tungsten, molybdenum, tantalum and niobium in
a total amount of 5 to 30 wt %, in particular 10-20 wt %; and one
or more further elements and compounds in a total amount of up to 5
wt %, the balance being cobalt.
19. The cell of claim 18, containing as said further elements at
least one of aluminium, silicon and manganese.
20. The cell of claim 14, wherein the CoO-containing layer is
integral with the oxygen barrier layer.
21. The cell of claim 14, wherein the oxygen barrier layer is
integral with the electrically conductive substrate.
22. The cell of claims 14, wherein the oxygen barrier layer and the
CoO-containing layer, or precursors thereof, are distinct applied
layers.
23. The cell of claim 13, wherein the Co-containing metallic layer
contains cobalt in an amount of at least 95 wt %, in particular
more than 97 wt % or 99 wt %.
24. The cell of claim 13, wherein the Co-containing metallic layer
contains at least one additive selected from silicon, manganese,
nickel niobium, tantalum and aluminium in a total amount of 0.1 to
2 wt %.
25. The cell of claim 11, wherein the electrically conductive
substrate comprises at least one metal selected from chromium,
cobalt, hafnium, iron, nickel, copper, platinum, silicon, tungsten,
molybdenum, tantalum, niobium, titanium, tungsten, vanadium,
yttrium and zirconium, or a compound thereof, in particular an
oxide, or a combination thereof.
26. The cell of claim 25, wherein the electrically conductive
substrate has an outer part made of cobalt or a cobalt-rich alloy
to which the coating is applied.
27. The cell of claim 26, wherein the outer part is made of a
cobalt-rich alloy containing at least one of nickel, tungsten,
molybdenum, tantalum and niobium, said cobalt alloy containing in
particular: at least one of nickel, tungsten, molybdenum, tantalum
and niobium in a total amount of 5 to 30 wt %, in particular 10-20
wt %; and one or more further elements and compounds in a total
amount of up to 5 wt %, the balance being cobalt.
28. The cell of claim 11, wherein the electrically conductive
substrate contains or consists essentially of one or more
oxidation-resistant metals.
29. The cell of claim 28, wherein said one or more
oxidation-resistant metals is/are selected from nickel, cobalt,
chromium and niobium.
30. The cell of claim 25, wherein the electrically conductive
substrate is an alloy of nickel, iron and copper, in particular an
alloy containing: 65 to 85 weight % nickel; 5 to 25 weight % iron;
1 to 20 weight % copper; and 0 to 10 weight % further
constituents.
31. The cell of claim 10, wherein the core is made of the same
material as: the oxygen barrier layer of claim 16, the
Co-containing metallic layer of claim 23, or the cobalt-rich alloy
of claim 27.
32. The cell of claim 1, wherein the CoO-containing layer has an
open porosity of up to 12%, in particular up to 7%.
33. The cell of claim 1, wherein the CoO-containing layer has a
porosity with an average pore size below 7 micron, in particular
below 4 micron.
34. The cell of claim 1, wherein the CoO-containing layer contains
cobalt oxide CoO in an amount of at least 80 wt %, in particular
more than 90 wt % or 95 wt %.
35. The cell of claim 1, wherein the CoO-containing layer is
substantially free of Co.sub.2O.sub.3 and substantially free of
Co.sub.3O.sub.4.
36. The cell of claim 1, wherein the CoO-containing layer is
electrochemically active for the oxidation of oxygen ions and is
uncovered or is covered with an electrolyte-pervious layer.
37. The cell of claim 1, wherein the CoO-containing layer is
covered with an applied protective layer, in particular an applied
oxide layer.
38. The cell of claim 37, wherein the applied protective layer
contains cobalt oxide.
39. The cell of claim 37, wherein the applied protective layer
contains iron oxide.
40. The cell of claim 39, wherein the applied protective layer
contains oxides of cobalt and of iron, in particular cobalt
ferrite.
41. The cell of claim 37, wherein the applied protective layer
contains a cerium compound, in particular cerium oxyfluoride.
42. The cell of claim 37, wherein the applied protective layer is
electrochemically active for the oxidation of oxygen ions and is
uncovered or is covered with an electrolyte pervious-layer.
43. The cell of claim 1, which has an electrochemically active
surface that contains at least one dopant, in particular at least
one dopant selected from iridium, palladium, platinum, rhodium,
ruthenium, silicon, tungsten, molybdenum, tantalum, niobium, tin or
zinc metals, Mischmetal, metals of the Lanthanide series, as metals
and compounds, in particular oxides, and mixtures thereof.
44. The cell of claim 43, wherein the electrochemically active
surface is made of an active material containing the dopant(s) in a
total amount of 0.1 to 5 wt %, in particular 1 to 4 wt %.
45. The cell of claim 1, comprising a cathode that has an
aluminium-wettable surface, in particular a horizontal or inclined
drained surface.
46. The cell of claim 45, wherein the cathode has an
aluminium-wettable coating that comprises a refractory boride
and/or an aluminium-wetting oxide.
47. The cell of claim 1, wherein the anode is suspended in the
electrolyte by a stem, in particular a stem having an outer part
comprising a layer that contains predominantly cobalt oxide
CoO.
48. A method of producing aluminium in a cell as defined in claim
1, comprising passing an electric current through the or each
active anode structure to its active anode surfaces as electronic
current and through the electrolyte to the cathode as ionic
current, thereby electrolysing the dissolved alumina to produce
aluminium on the cathode(s) and oxygen on the active anode
surface(s) which is released through said flow-through
openings.
49. The method of claim 48, wherein oxygen ions are oxidised on the
anode's layer that contains predominantly cobalt oxide CoO.
50. The method of claim 48, wherein oxygen ions are oxidised on an
active layer applied to the anode's layer that contains
predominantly cobalt oxide CoO.
51. A non-carbon metal-based anode for the electrowinning of
aluminium from alumina dissolved in a fluoride-containing molten
electrolyte, comprising an electrically conductive metallic
structure that comprises an outer part with an electrochemically
active anode surface on which oxygen is anodically evolved and
which is suspended in the electrolyte substantially parallel to a
facing cathode during use, said metallic structure having one or
more flow-through openings extending from the active anode surface
through the metallic structure, said flow-through opening(s) being
arranged for guiding during use a circulation of electrolyte driven
by the fast escape of anodically evolved oxygen, wherein said outer
part of the anode comprises a layer that contains predominantly
cobalt oxide CoO.
Description
FIELD OF THE INVENTION
[0001] This invention relates to the use in a molten electrolyte
for the electrowinning of aluminium of a non-carbon anode having a
flow-through active structure with an enhanced stability.
BACKGROUND ART
[0002] Using non-carbon anodes--i.e. anodes which are not made of
carbon as such, e.g. graphite, coke, etc. . . . , but possibly
contain carbon in a compound--for the production of aluminium in
electrolytic cells should drastically improve the aluminium
production process by reducing pollution and the cost of aluminium
production.
[0003] The developments of non-carbon anode materials, in
particular metals, led to the design of new anode shapes that are
better adapted to the cell's fluid mechanisms and electromagnetic
effects than the conventional anodic solid carbon blocks.
[0004] Several designs for oxygen-evolving anodes for aluminium
electrowinning cells were proposed in the following documents. U.S.
Pat. No. 4,681,671 (Duruz) discloses vertical anode plates or
blades operated in low temperature aluminium electrowinning cells.
U.S. Pat. No. 5,310,476 (Sekhar/de Nora) discloses oxygen-evolving
anodes consisting of roof-like assembled pairs of anode plates.
U.S. Pat. No. 5,362,366 (de Nora/Sekhar) describes non-consumable
anode shapes including roof-like assembled pairs of anode plates.
U.S. Pat. No. 5,368,702 (de Nora) discloses vertical tubular or
frustoconical oxygen-evolving anodes for multimonopolar aluminium
cells. U.S. Pat. No. 5,683,559 (de Nora) 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 (de Nora/Duruz) discloses
vertical oxygen-evolving anode plates, preferably porous or
reticulated, in a multimonopolar cell arrangement for aluminium
electrowinning cells operating at reduced temperature. WO00/40781,
WO00/40782 and WO03/006716 (all 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.
[0005] For the dissolution of the raw material alumina, a highly
aggressive fluoride-based electrolyte, such as cryolite, is
required. Various modified electrolytes have been proposed to
improve cell operation and reduce wear of non-carbon metal-based
anode, particularly caused by corrosion by the electrolyte.
[0006] WO00/06804 (Crottaz/Duruz) teaches that a nickel-iron anode
can be used in an electrolyte at a temperature of 820.degree. to
870.degree. C. containing 23 to 26.5 weight % AlF.sub.3, 3 to 5
weight % Al.sub.2O.sub.3, 1 to 2 weight % LiF and 1 to 2 weight %
MgF.sub.2. U.S. Pat. Nos. 5,006,209 and 5,284,562 (both
Beck/Brooks), U.S. Pat. Nos. 6,258,247 and 6,379,512 (both
Brown/Brooks/Frizzle/Juric), U.S. Pat. No. 6,419,813
(Brown/Brooks/Frizzle) and U.S. Pat. No. 6,436,272 (Brown/Frizzle)
all disclose the use of nickel-copper-iron anodes in an aluminium
production electrolyte at 660.degree.-800.degree. C. containing
6-26 weigh % NaF, 7-33 weight % KF, 1-6 weight % LiF and 60-65
weight % AlF.sub.3. The electrolyte may contain Al.sub.2O.sub.3 in
an amount of up to 30 weight %, in particular 5 to 10 or 15 weight
%, most of which is in the form of suspended particles and some of
which is dissolved in the electrolyte, i.e. typically 1 to 4 weight
% dissolved Al.sub.2O.sub.3. In U.S. Pat. Nos. 6,258,247,
6,379,512, 6,419,813 and 6,436,272 such an electrolyte is said to
be useable at temperatures up to 900.degree. C. In U.S. Pat. Nos.
6,258,247 and 6,379,512 the electrolyte further contains 0.004 to
0.2 weight % transition metal additives to facilitate alumina
dissolution and improve cathodic operation. U.S. Pat. No. 5,725,744
(de Nora/Duruz) discloses an aluminium production cell having
anodes made of nickel, iron and/or copper in a electrolyte at a
temperature from 680.degree. to 880.degree. C. containing 42-63
weight % AlF.sub.3, up to 48 weight % NaF, up to 48 weight % LiF
and 1 to 5 weight % Al.sub.2O.sub.3. MgF.sub.2, KF and CaF.sub.2
are also mentioned as possible bath constituents. WO2004/035871 (de
Nora/Nguyen/Duruz) discloses a metal-based anode containing at
least one of nickel, cobalt and iron. The anode is used for
electrowinning aluminium in a fluoride-containing molten
electrolyte consisting of: 5 to 14 wt % dissolved alumina; 35 to 45
wt % aluminium fluoride; 30 to 45 wt % sodium fluoride; 5 to 20 wt
% potassium fluoride; 0 to 5 wt % calcium fluoride; and 0 to 5 wt %
of further constituents.
[0007] The materials having the greatest resistance to oxidation
are metal oxides which are all to some extent soluble in cryolite.
Oxides are also poorly electrically conductive, therefore, to avoid
substantial ohmic losses and high cell voltages, the use of
non-conductive or poorly conductive oxides should be minimal in the
manufacture of anodes. Whenever possible, a good conductive
material should be utilised for the anode core, whereas the surface
of the anode is preferably made of an oxide having a high
electrocatalytic activity. 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. Many patents have been filed on
non-carbon anodes but none has found commercial acceptance yet,
also because of economical reasons.
[0008] 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. Several patents disclose the
use of an electrically conductive metal anode core with an
oxide-based active outer part, in particular U.S. Pat. Nos.
4,956,069, 4,960,494, 5,069,771 (all Nguyen/Lazouni/Doan), U.S.
Pat. No. 6,077,415 (Duruz/de Nora), U.S. Pat. No. 6,103,090 (de
Nora), U.S. Pat. No. 6,113,758 (de Nora/Duruz) and U.S. Pat. No.
6,248,227 (de Nora/Duruz), U.S. Pat. No. 6,361,681 (de Nora/Duruz),
U.S. Pat. No. 6,365,018 (de Nora), U.S. Pat. No. 6,372,099
(Duruz/de Nora), U.S. Pat. No. 6,379,526 (Duruz/de Nora), U.S. Pat.
No. 6,413,406 (de Nora), U.S. Pat. No. 6,425,992 (de Nora), U.S.
Pat. No. 6,436,274 (de Nora/Duruz), U.S. Pat. No. 6,521,116
(Duruz/de Nora/Crottaz), U.S. Pat. No. 6,521,115 (Duruz/de
Nora/Crottaz), U.S. Pat. No. 6,533,909 (Duruz/de Nora), U.S. Pat.
No. 6,562,224 (Crottaz/Duruz) as well as PCT publications
WO00/40783 (de Nora/Duruz), WO01/42534 (de Nora/Duruz), WO01/42535
(Duruz/de Nora), WO01/42536 (Nguyen/Duruz/ de Nora), WO02/070786
(Nguyen/de Nora), WO02/083990 (de Nora/Nguyen), WO02/083991
(Nguyen/de Nora), WO03/014420 (Nguyen/Duruz/de Nora),
WO03/078695(Nguyen/de Nora), WO03/087435 (Nguyen/de Nora).
[0009] U.S. Pat. No. 4,374,050 (Ray) discloses numerous multiple
oxide compositions for electrodes. Such compositions inter-alia
include oxides of iron and cobalt. The oxide compositions can be
used as a cladding on a metal layer of nickel, nickel-chromium,
steel, copper, cobalt or molybdenum. U.S. Pat. No. 4,142,005
(Cadwell/Hazelrigg) discloses an anode having a substrate made of
titanium, tantalum, tungsten, zirconium, molybdenum, niobium,
hafnium or vanadium. The substrate is coated with cobalt oxide
Co.sub.3O.sub.4.
[0010] U.S. Pat. No. 6,103,090 (de Nora), U.S. Pat. No. 6,361,681
(de Nora/Duruz), U.S. Pat. No. 6,365,018 (de Nora), U.S. Pat. No.
6,379,526 (de Nora/Duruz), U.S. Pat. No. 6,413,406 (de Nora) and
U.S. Pat. No. 6,425,992 (de Nora), and WO04/018731 (Nguyen/de Nora)
disclose anode substrates that contain at least one of chromium,
cobalt, hafnium, iron, molybdenum, nickel, copper, niobium,
platinum, silicon, tantalum, titanium, tungsten, vanadium, yttrium
and zirconium and that are coated with at least one of ferrites of
cobalt, copper, chromium, manganese, nickel and zinc. WO01/42535
(Duruz/de Nora) and WO02/097167 (Nguyen/de Nora), disclose
aluminium electrowinning anodes made of surface oxidised iron
alloys that contain at least one of nickel and cobalt. U.S. Pat.
No. 6,638,412 (de Nora/Duruz) discloses the use of anodes made of a
transition metal-containing alloy having an integral oxide layer,
the alloy comprising at least one of iron, nickel and cobalt.
[0011] Non-carbon anodes have not as yet been commercially and
industrially applied and there is still a need for a metal-based
anodic material and an appropriate anode shape that can be used for
electrowinning aluminium.
SUMMARY OF THE INVENTION
[0012] The present invention generally relates to aluminium
electrowinning with metal-based anodes having a shape for promoting
an electrolyte circulation and having an electrochemically active
outer part that has 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.
[0013] In particular, the invention relates to a cell for the
electrowinning of aluminium from alumina dissolved in a
fluoride-containing molten electrolyte. The cell comprises at least
one non-carbon metal-based anode having an electrically conductive
metallic structure that comprises an outer part with an
electrochemically active anode surface on which, during
electrolysis, oxygen is anodically evolved, and which is suspended
in the electrolyte substantially parallel to a facing cathode. This
metallic structure has one or more flow-through openings extending
from the active anode surface through the metallic structure, the
flow-through opening(s) being arranged for guiding a circulation of
electrolyte driven by the fast escape of anodically evolved oxygen.
The outer part of the anode comprises the abovementioned layer that
contains predominantly cobalt oxide CoO to enhance the stability of
the anode.
[0014] In other words, the invention concerns a cell having an
anode that has a shape that promotes electrolyte circulation and
that has an electrochemically active outer part that is resistant
to the circulating electrolyte and/or to anodically evolved oxygen
by the presence of a layer made predominantly of a special form of
cobalt oxide, i.e. CoO.
[0015] There are several forms of stoichiometric and
non-stoichiometric cobalt oxides which are based on: [0016] CoO
that contains Co(II) and that is formed predominantly at a
temperature above 920.degree. C. in air; [0017] 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; [0018] Co.sub.3O.sub.4 that contains Co(II) and Co(III) and
that is formed at temperatures between 300 and 900.degree. C.
[0019] 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.
[0020] 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.
[0021] 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.
[0022] 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.
[0023] 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.
[0024] 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.
[0025] 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.
[0026] The anode structure can be foraminate. For instance, the
anode structure can have any of the shapes disclosed in the
abovementioned WO00/40781, WO00/40782 and WO03/006716. For example,
the anode structure comprises a series of parallel anode members,
in particular horizontal anode members having electrochemically
active surfaces in a generally coplanar arrangement to form said
active anode surface, the anode members being spaced apart to form
longitudinal flow-through openings for the circulation of
electrolyte driven by the fast escape of anodically evolved oxygen.
Typically these anode members are blades, bars, rods or wires. The
active anode surface can be substantially horizontal, substantially
vertical or inclined to the horizontal, for example as disclosed in
WO00/40782 or WO03/023092 (both de Nora).
[0027] In one embodiment, the molten electrolyte is at a
temperature below 950.degree. C., in particular in the range from
910.degree. to 940.degree. C., and consists of: [0028] 6.5 to 11
weight % dissolved alumina, in particular 7 to 10 weight %; [0029]
35 to 44 weight % aluminium fluoride, in particular 36 to 42 weight
% aluminium fluoride, such as 36 to 38 weight; [0030] 38 to 46
weight % sodium fluoride, in particular 39 to 43 weight %; [0031] 2
to 15 weight % potassium fluoride, in particular 3 to 10 weight %
potassium fluoride, such as 5 to 7 weight %; [0032] 0 to 5 weight %
calcium fluoride, in particular 2 to 4 weight % calcium fluoride;
and [0033] 0 to 5 weight % in total of one or more further
constituents, in particular up to 3 weight %.
[0034] 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.
[0035] Hence, in contrast to prior art low temperature electrolytes
which carry large amounts of undissolved alumina in particulate
form, a large amount of alumina is in a dissolved form in the above
electrolyte.
[0036] 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 CoO and do not noticeably passivate or corrode metallic
cobalt. 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.
[0037] It follows that the use of the above described electrolyte
with metal-based anodes that contains CoO 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 CoO.
[0038] The electrolyte may 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.
[0039] 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.
[0040] Advantageously, The electrolyte contains alumina at a
concentration near saturation on the active anode surface.
[0041] 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.
[0042] The CoO-containing anode layer can be integral with a core
made of cobalt or a cobalt alloy. Such an anode core can be made of
the same materials as the Co-containing alloys described below. The
cobalt-containing anode core can advantageously be cast.
[0043] Alternatively, the anode comprises an electrically
conductive substrate that is covered with an applied
electrochemically active coating that comprises the CoO-containing
layer.
[0044] The CoO-containing layer can be a layer of sintered
particles. In particular, the CoO-containing layer can be formed by
applying a layer of particulate CoO to the anode and sintering. For
instance, the CoO-containing layer is applied as a slurry, in
particular a colloidal and/or polymeric slurry, and then heat
treated. Good results have been obtained by slurring particulate
metallic cobalt or CoO, optionally with additives such as Ta, in an
aqueous solution containing at least one of ethylene glycol,
hexanol, polyvinyl alcohol, polyvinyl acetate, polyacrylic acid,
hydroxy propyl methyl cellulose and ammonium polymethacrylate and
mixtures thereof, followed by application to the anode, e.g.
painting or dipping, and heat treating.
[0045] The CoO-containing layer can be an integral oxide layer on
an applied Co-containing metallic layer of the coating.
[0046] The CoO-containing layer can be formed by applying a
Co-containing metallic layer to the anode and subjecting the
metallic layer to an oxidation treatment to form the CoO-containing
layer on the metallic layer, the CoO-containing layer being
integral with the metallic layer.
[0047] Conveniently, the oxidation treatment can be carried out in
an oxygen containing atmosphere, such as air. The treatment can
also be carried out in an atmosphere that is oxygen rich or
consists essentially of pure oxygen.
[0048] It is also contemplated to carry out this oxidation
treatment by other means, for instance electrolytically. However,
it was found that full formation of the CoO integral layer cannot
be achieved in-situ during aluminium electrowinning under normal
cell operating conditions. In other words, when the anode is
intended for use in a non-carbon anode aluminium electrowinning
cell operating under the usual conditions, the anode should always
be placed into the cell with a preformed integral oxide layer
containing predominantly CoO.
[0049] As the conversion of Co(III) into Co(II) occurs at a
temperature of about 895.degree. C., the oxidation treatment should
be carried out above this temperature. Usually, the oxidation
treatment is carried out at a treatment temperature above
895.degree. C. or 920.degree. C., preferably above 940.degree. C.,
in particular within the range of 950.degree. C. to 1050.degree. C.
The Co-containing metallic layer can be heated from room
temperature to this treatment temperature at a rate of at least
300.degree. C./hour, in particular at least 450.degree. C./hour, or
is placed in an environment, in particular in an oven, that is
preheated to said temperature. The oxidation treatment at this
treatment temperature can be carried out for more than 8 or 12
hours, in particular from 16 to 48 hours. Especially when the
oxygen-content of the oxidising atmosphere is increased, the
duration of the treatment can be reduced below 8 hours, for example
down to 4 hours.
[0050] The Co-containing metallic layer can be further oxidised
during use. However, the main formation of CoO is preferably
achieved before use and in a controlled manner for the reasons
explained above.
[0051] The method for forming the CoO-containing layer on the
Co-containing metallic layer can be used to form the CoO-containing
layer on the previously mentioned Co-containing anode core.
[0052] The Co-containing metallic layer can contain alloying metals
for further reducing oxygen diffusion and/or corrosion through the
metallic layer.
[0053] In one embodiment, the anode comprises an oxygen barrier
layer between the CoO-containing layer and the electrically
conductive substrate. The oxygen barrier layer can contain at least
one metal selected from nickel, copper, tungsten, molybdenum,
tantalum, niobium and chromium, or an oxide thereof, for example
alloyed with cobalt, such as a cobalt alloy containing tungsten,
molybdenum, tantalum and/or niobium, in particular an alloy
containing: at least one of nickel, tungsten, molybdenum, tantalum
and niobium in a total amount of 5 to 30 wt %, such as 10 to 20 wt
%; and one or more further elements and compounds in a total amount
of up to 5 wt % such as 0.01 to 4 weight %, the balance being
cobalt. These further elements may contain at least one of
aluminium, silicon and manganese.
[0054] Typically, the oxygen barrier layer and the CoO-containing
layer are formed by oxidising the surface of an applied layer of
the abovementioned cobalt alloy that contains nickel, tungsten,
molybdenum, tantalum and/or niobium. The resulting CoO-containing
layer is predominantly made of CoO and is integral with the
unoxidised part of the metallic cobalt alloy that forms the oxygen
barrier layer.
[0055] When the CoO layer is integral with the cobalt alloy, the
nickel, when present, should be contained in the alloy in an amount
of up to 20 weight %, in particular 5 to 15 weight %. Such an
amount of nickel in the alloy leads to the formation of a small
amount of nickel oxide NiO in the integral oxide layer, in about
the same proportions to cobalt as in the metallic part, i.e. 5 to
15 or 20 weight %. It has been observed that the presence of a
small amount of nickel oxide stabilises the cobalt oxide CoO and
durably inhibits the formation of Co.sub.2O.sub.3 or
Co.sub.3O.sub.4. However, when the weight ratio nickel/cobalt
exceeds 0.15 or 0.2, the advantageous chemical and electrochemical
properties of cobalt oxide CoO tend to disappear. Therefore, the
nickel content should not exceed this limit.
[0056] Alternatively, an oxygen barrier layer, for example made of
the above cobalt alloy that contains nickel, tungsten, molybdenum,
tantalum and/or niobium, can be covered with an applied layer of
CoO or a precursor thereof, as discussed above. In this case the
oxygen barrier layer can be an applied layer or it can be integral
with the electrically conductive substrate.
[0057] In another embodiment, the Co-containing metallic layer
consists essentially of cobalt, typically containing cobalt in an
amount of at least 95 wt %, in particular more than 97 wt % or 99
wt %.
[0058] Optionally the Co-containing metallic layer contains at
least one additive selected from silicon, nickel, manganese,
niobium, tantalum and aluminium in a total amount of 0.1 to 2 wt
%.
[0059] Such a Co-containing layer can be applied to an oxygen
barrier layer which is integral with the electrically conductive
substrate of the flow-through anode structure or applied
thereto.
[0060] The electrically conductive substrate can comprise at least
one metal selected from chromium, cobalt, hafnium, iron,
molybdenum, nickel, copper, platinum, silicon, titanium, tungsten,
molybdenum, tantalum, niobium, vanadium, yttrium and zirconium, or
a compound thereof, in particular an oxide, or a combination
thereof. For instance, the electrically conductive substrate may
have an outer part made of cobalt or an alloy containing
predominantly cobalt to which the coating is applied. For instance,
this cobalt alloy contains nickel, tungsten, molybdenum, tantalum
and/or niobium, in particular it contains: nickel tungsten,
molybdenum, tantalum and/or niobium in a total amount of 5 to 30 wt
%, e.g. 10 to 20 wt %; and one or more further elements and/or
compounds in a total amount of up to 5 wt %, the balance being
cobalt. These further elements may contain at least one of
aluminium, silicon and manganese. The electrically conductive
substrate, or an outer part thereof, may contain or consist
essentially of at least one oxidation-resistant metal, in
particular one or more metals selected from nickel, tungsten,
molybdenum, cobalt, chromium and niobium, and for example contains
less than 1, 5 or 10 wt % in total of other metals and metal
compounds, in particular oxides. Alternatively, the electrically
conductive substrate can be made of an alloy of nickel, iron and
copper, in particular an alloy containing: 65 to 85 weight %
nickel; 5 to 25 weight % iron; 1 to 20 weight % copper; and 0 to 10
weight % further constituents. For example, the alloy contains
about: 75 weight % nickel; 15 weight % iron; and 10 weight %
copper.
[0061] Advantageously, the anode's CoO-containing layer, in
particular when the CoO layer is integral with the applied
Co-containing metallic layer or the anode body, has an open
porosity of below 12%, such as below 7%.
[0062] The anode's CoO-containing layer can have a porosity with an
average pore size below 7 micron, in particular below 4 micron. It
is preferred to provide a substantially crack-free CoO-containing
layer so as to protect efficiently the anode's metallic outer part
which is covered by this CoO-containing layer.
[0063] Usually, the CoO-containing layer contains cobalt oxide CoO
in an amount of at least 80 wt %, in particular more than 90 wt %
or 95 wt % or 98 wt %.
[0064] Advantageously, the CoO-containing layer is substantially
free of cobalt oxide Co.sub.2O.sub.3 and substantially free of
Co.sub.3O.sub.4, and contains preferably below 3 or 1.5% of these
forms of cobalt oxide.
[0065] The CoO-containing layer may be electrochemically active for
the oxidation of oxygen ions during use, in which case this layer
is uncovered or is covered with an electrolyte-pervious layer.
[0066] Alternatively, the CoO-containing layer can be covered with
an applied protective layer, in particular an applied oxide layer
such as a layer containing cobalt and/or iron oxide, e.g. cobalt
ferrite. The applied protective layer may contain a pre-formed
and/or in-situ deposited cerium compound, in particular cerium
oxyfluoride, as for example disclosed in the abovementioned U.S.
Pat. Nos. 4,956,069, 4,960,494 and 5,069,771. Such an applied
protective layer is usually electrochemically active for the
oxidation of oxygen ions and is uncovered, or covered in turn with
an electrolyte pervious-layer.
[0067] The anode's electrochemically active surface can contain at
least one dopant, in particular at least one dopant selected from
iridium, palladium, platinum, rhodium, ruthenium, silicon,
tungsten, molybdenum, tantalum, niobium, tin or zinc metals,
Mischmetal and metals of the Lanthanide series, as metals and
compounds, in particular oxides, and mixtures thereof. The
dopant(s) can be present at the anode's surface in a total amount
of 0.1 to 5 wt %, in particular 1 to 4 wt %.
[0068] Such a dopant can be an electrocatalyst for fostering the
oxidation of oxygen ions on the anode's electrochemically active
surface and/or can contribute to inhibit diffusion of oxygen ions
into the anode.
[0069] The dopant may be added to the precursor material that is
applied to form the active surface or it can be applied to the
active surface as a thin film, for example by plasma spraying or
slurry application, and incorporated into the surface by heat
treatment.
[0070] The cell can have a cathode that has an aluminium-wettable
surface, in particular a horizontal or inclined drained surface.
This surface can be formed by an aluminium-wettable material that
comprises a refractory boride and/or an aluminium-wetting oxide.
Examples of such materials are disclosed in WO01/42168, WO01/42531,
WO02/070783, WO02/096830 and WO02/096831 (all in the name of
MOLTECH).
[0071] The anode can be suspended in the electrolyte by a stem, in
particular a stem having an outer part comprising a layer that
contains predominantly cobalt oxide CoO.
[0072] Another aspect of the invention relates to a method of
electrowinning aluminium in a cell as described above The method
comprises electrolysing the dissolved alumina to produce oxygen on
the anode and aluminium cathodically, and supplying alumina to the
electrolyte to maintain therein a concentration of dissolved
alumina of 6.5 to 11 weight %, in particular 7 to 10 weight %.
[0073] Oxygen ions may be oxidised on the anode's CoO-containing
layer that contains predominantly cobalt oxide CoO and/or, when
present, on an active layer applied to the anode's CoO layer, the
CoO layer inhibiting oxidation and/or corrosion of the anode's
metallic outer part.
[0074] The invention also relates to a non-carbon metal-based anode
for the electrowinning of aluminium from alumina dissolved in a
fluoride-containing molten electrolyte. This anode comprises an
electrically conductive metallic structure that comprises an outer
part with an electrochemically active anode surface on which oxygen
is anodically evolved and which is suspended in the electrolyte
substantially parallel to a facing cathode during use. This
metallic structure has one or more flow-through openings extending
from the active anode surface through the metallic structure. These
flow-through opening(s) are arranged for guiding during use a
circulation of electrolyte driven by the fast escape of anodically
evolved oxygen. The outer part of the anode comprises the
abovementioned layer that contains predominantly cobalt oxide CoO
to enhance the stability of the anode. This anode can include any
of the above described anode features or a combination thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0075] The invention will now be described with reference to the
schematic drawings, wherein:
[0076] FIGS. 1a and 1b show respectively a side elevation and a
plan view of an anode according to the invention;
[0077] FIGS. 2a and 2b show respectively a side elevation and a
plan view of another anode according to the invention;
[0078] FIGS. 3, 4, 5 and 6 show side elevations of variations of
the anode shown in FIGS. 1a and 1b;
[0079] FIGS. 7 and 8 show cross-sections of multi-part anode
members according to the invention;
[0080] FIG. 9 shows an aluminium electrowinning cell operating with
anodes according to the invention fitted with electrolyte guide
members;
[0081] FIGS. 10, 11 and 12 are enlarged views of parts of
variations of the electrolyte guide members shown in FIG. 9, FIG.
10 illustrating cell operation;
[0082] FIG. 13 is a cross section of another anode according to the
invention with electrolyte guide members only one of which is
shown;
[0083] FIG. 14 shows a plan view of half of an assembly of several
electrolyte guide members like the one shown in FIG. 13;
[0084] FIG. 15 is a plan view of the anode shown FIG. 13 with half
of an assembly of electrolyte guide members as shown in FIG. 14;
and
[0085] FIG. 16 is a plan view of a variation of the anode of FIG.
15.
DETAILED DESCRIPTION
[0086] FIGS. 1a and 1b schematically show an anode 10 of a cell for
the electrowinning of aluminium according to the invention.
[0087] The anode 10 comprises a vertical current feeder 11 for
connecting the anode to a positive bus bar, a cross member 12 and a
pair of transverse connecting members 13 for connecting a series of
anode members 15.
[0088] The anode members 15 have an electrochemically active lower
surface 16 where oxygen is anodically evolved during cell
operation. The anode members 15 are in the form of parallel rods in
a coplanar arrangement, laterally spaced apart from one another by
inter-member gaps 17. The inter-member gaps 17 constitute
flow-through openings for the circulation of electrolyte and the
escape of anodically-evolved gas released at the electrochemically
active surfaces 16.
[0089] The anode members 15 are transversally connected by the pair
of transverse connecting members 13 which are in turn connected
together by the cross member 12 on which the vertical current
feeder 11 is mounted. The current feeder 11, the cross member 12,
the transverse connecting members 13 and the anode members 15 are
mechanically secured together by welding, rivets or other
means.
[0090] In accordance with the invention, the electrochemically
active surface 16 of the anode members is formed by an outer part
that comprises a layer containing predominantly CoO. This CoO layer
can form the electrochemically active surface 16 and be directly
exposed to the electrolyte during use or the CoO layer can be
covered with a further layer, for instance a layer containing
predominantly a cerium compound such as cerium oxyfluoride.
[0091] The cross-member 12 and the transverse connecting members 13
are so designed and positioned over the anode members 15 to provide
a substantially even current distribution through the anode members
15 to their electrochemically active surfaces 16. The current
feeder 11, the cross-member 12 and the transverse connecting
members 13 do not need to be electrochemically active and their
surface may passivate when exposed to electrolyte. However they
should be electrically well conductive to avoid unnecessary voltage
drops and should not substantially dissolve in electrolyte. The
electrochemically-inactive current-carrying elements (11,12,13) can
have an outer part with a protective layer containing predominantly
CoO.
[0092] FIGS. 2a and 2b schematically show a variation of the anode
10 shown in FIGS. 1a and 1b.
[0093] Instead of having transverse connecting members 13, a
cross-member 12 and a current feeder 11 for mechanically and
electrically connecting the anode members 15 to a positive bus bar
as illustrated in FIGS. 1a and 1b, the anode 10 shown in FIGS. 2a
and 2b comprises a pair of cast or profiled support members 14
fulfilling the same function. Each cast support member 14 comprises
a lower horizontally extending foot 14a for electrically and
mechanically connecting the anode members 15, a stem 14b for
connecting the anode 10 to a positive bus bar and a pair of lateral
reinforcement flanges 14c between the horizontally extending foot
14a and stem 14b.
[0094] The anode members 15 may be secured by force-fitting or
welding in the horizontal foot 14a. As an alternative, the shape of
the anode members 15 and corresponding receiving slots in the foot
14a may be such as to allow only longitudinal movements of the
anode members. For instance the anode members 15 and the foot 14a
may be connected by dovetail joints.
[0095] FIGS. 3 to 6 show a series of anodes 10 according to the
invention which are similar to the anode 10 shown in FIGS. 1a and
1b. However the cross-sections of the anode members 15 of the
anodes 10 shown in FIGS. 3 to 6 differ to the circular
cross-section of the anode members 10 shown in FIGS. 1a and 1b.
[0096] The anode members 15 of the anode shown in FIG. 3 have in
cross-section a generally semi-circular upper part and a flat
bottom which constitutes the electrochemically active surface 16 of
each anode member 15.
[0097] FIG. 4 illustrates anode members 15 in the form of rods
which have a generally bell-shaped or pear-shaped cross-section.
The electrochemically active surface 16 of the anode members 10 is
located along the bottom of the bell-shape or pear-shape.
[0098] The anode members 15 shown in FIG. 5 are rods having a
generally rectangular cross-section. The electrochemically active
surface 16 is located along the bottom narrow side of the rod.
[0099] FIGS. 6 and 7 show an anode 10 having assembled multi-part
anode members 15 comprising a first member 15b supporting an
electrochemically active second member 15a. The electrochemically
active member 15a has an electrochemically active surface 16 and is
connected along it whole length to the electrically well-conductive
support member 15b by an intermediate connecting member 15c such as
a flange.
[0100] FIG. 7 shows an enlarged view of the assembled anode member
15 of FIG. 6, comprising a generally cylindrical electrochemically
active member 15a with an electrochemically active surface 16, a
generally cylindrical electrically conductive support member 15b
and an intermediate connecting member or flange 15c electrically
and mechanically connecting the support member 15b to the
electrochemically active member 15a. Alternatively, the connecting
member 15c may be an extension of either the electrochemically
active member 15a or the support member 15b as shown in FIG. 8.
[0101] The intermediate connecting member 15c shown in FIG. 7 may
be connected to the electrochemically active member 15a and to the
support member 15b by force-fitting or welding. However, these
parts may be mechanically connected by providing a suitable
geometry of the connecting members 15c and the corresponding
receiving slots of the electrochemically active member 15a and the
support member 15b, for instance with dovetail joints.
[0102] In accordance with the invention, the electrochemically
active member 15a shown in FIGS. 7 and 8 has an outer part that
comprises a layer containing predominantly CoO. As mentioned above,
this CoO layer can form the electrochemically active surface 16 and
be directly exposed to the electrolyte during use or the CoO layer
can be covered with a further layer, for instance a layer
containing predominantly a cerium compound such as cerium
oxyfluoride.
[0103] The support member 15b shown in FIGS. 7 and 8 and the
connecting member 15c shown in FIG. 7 are preferably highly
conductive and may comprise a metallic core, for instance nickel
covered with an oxidised cobalt layer (having a CoO-based
surface).
[0104] FIG. 9 shows an aluminium electrowinning cell according to
the invention having a series of generally horizontal anodes 10
which are similar to those shown in FIGS. 1a and 1b, immersed in an
electrolyte 30. The anodes 10 face a horizontal cathode cell bottom
20 connected to a negative busbar by current conductor bars 21. The
cathode cell bottom 20 is made of conductive material such as
graphite or other carbonaceous material coated with an
aluminium-wettable refractory cathodic coating 22 on which
aluminium 35 is produced and from which it drains or on which it
forms a shallow pool, a deep pool or a stabilised pool. The molten
aluminium 35 produced is spaced apart from the facing anodes 10 by
an inter-electrode gap.
[0105] Pairs of anodes 10 are connected to a positive bus bar
through a primary vertical current feeder 11' and a horizontal
current distributor 11'' connected at both of its ends to a
foraminate anode 10 through a secondary vertical current
distributor 11'''.
[0106] The secondary vertical current distributor 11''' is mounted
on the anode structure 12,13,15, on a cross member 12 which is in
turn connected to a pair of transverse connecting members 13 for
connecting a series of anode members 15. The current feeders
11',11'',11''', the cross member 12, the transverse connecting
members 13 and the anode members 15 are mechanically secured
together by welding, rivets or other means.
[0107] The anode members 15 have an electrochemically active lower
surface 16 on which during cell operation oxygen is anodically
evolved. The anode members 15 are in the form of parallel rods in a
foraminate coplanar arrangement, laterally spaced apart from one
another by inter-member gaps 17. The inter-member gaps 17
constitute flow-through openings for the circulation of electrolyte
and the escape of anodically-evolved gas from the electrochemically
active surfaces 16.
[0108] The cross-member 12 and the transverse connecting members 13
provide a substantially even current distribution through the anode
members 15 to their electrochemically active surfaces 16. The
current feeder 11, the cross-member 12 and the transverse
connecting members 13 do not need to be electrochemically active
and their surface may passivate when exposed to electrolyte.
However they should be electrically well conductive to avoid
unnecessary voltage drops and should not substantially dissolve in
the molten electrolyte.
[0109] In accordance with the invention, the active surface 16 of
the anode members 15 can be CoO-based.
[0110] The CoO-based surface may extend over all immersed parts
11''',12,13,15 of the anode 10, in particular over the immersed
part of the secondary vertical current distributor 11''' which is
preferably covered with a CoO-based layer at least up to 10 cm
above the surface of the electrolyte 30.
[0111] The anodes 10 are further fitted with means for enhancing
dissolution of fed alumina in the form of electrolyte guide members
5 formed of parallel spaced-apart inclined baffles 5 located above
and adjacent to the foraminate anode structure 12,13,15. The
baffles 5 provide upper downwardly converging surfaces 6 and lower
upwardly converging surfaces 7 that deflect gaseous oxygen which is
anodically produced below the electrochemically active surface 16
of the anode members 15 and which escapes between the inter-member
gaps 17 through the foraminate anode structure 12,13,15. The oxygen
released above the baffles 5 promotes dissolution of alumina fed
into the electrolyte 30 above the downwardly converging surfaces 6.
Further details of such baffles are disclosed in the abovementioned
WO00/40781 and WO00/40782.
[0112] The aluminium-wettable cathodic coating 22 of the cell shown
in FIG. 9 can advantageously be a slurry-applied refractory hard
metal coating as disclosed in US Patent WO01/42534 (de Nora/Duruz),
WO01/42531 (Nguyen/Duruz/de Nora) and WO02/096831 (Nguyen/de
Nora).
[0113] During cell operation, alumina is fed to the electrolyte 30
all over the baffles 5 and the metallic anode structure 12,13,15.
The fed alumina is dissolved and distributed from the bottom end of
the converging surfaces 6 into the inter-electrode gap through the
inter-member gaps 17 and around edges of the metallic anode
structure 12,13,15, i.e. between neighbouring pairs of anodes 10 or
between peripheral anodes 10 and sidewalls 25. By passing an
electric current between anodes 10 and facing cathode cell bottom
20 oxygen is evolved on the electrochemically active anode surfaces
16 and aluminium is produced which is incorporated into the
cathodic molten aluminium 35. The oxygen evolved from the active
surfaces 16 escapes through the inter-member gaps 17 and is
deflected by the upwardly converging surfaces 7 of baffles 5. The
oxygen escapes from the uppermost ends of the upwardly converging
surfaces 7 enhancing dissolution of the alumina fed over the
downwardly converging surfaces 6.
[0114] The aluminium electrowinning cells partly shown in FIGS. 10,
11 and 12 are similar to the aluminium electrowinning cell shown in
FIG. 9.
[0115] In FIG. 10 the guide members are inclined baffles 5 as shown
in FIG. 9. In this example the uppermost end of each baffle 5 is
located just above mid-height between the surface of the
electrolyte 30 and the transverse connecting members 13.
[0116] Also shown in FIG. 10, an electrolyte circulation 31 is
generated by the escape of gas released from the active surfaces 16
of the anode members 15 between the inter-member gaps 17 and which
is deflected by the upward converging surfaces 7 of the baffles 5
confining the gas and the electrolyte flow between their uppermost
edges. From the uppermost edges of the baffles 5, the anodically
evolved gas escapes towards the surface of the electrolyte 30,
whereas the electrolyte circulation 31 flows down through the
downward converging surfaces 6, through the inter-member gaps and
around edges of the metallic anode structure 12,13,15 to compensate
the depression created by the anodically released gas below the
active surfaces 17 of the anode members 15. The electrolyte
circulation 31 draws down into the inter-electrode gap dissolving
alumina particles 32 which are fed above the downward converging
surfaces 6.
[0117] FIG. 11 shows part of an aluminium electrowinning cell with
baffles 5 operating as electrolyte guide members like those shown
in cell of FIG. 9 but whose surfaces are only partly converging.
The lower sections 4 of the baffles 5 are vertical and parallel to
one another, whereas their upper sections have upward and downward
converging surfaces 6,7. The uppermost end of the baffles 5 are
located below but close to the surface of the electrolyte 30 to
increase the turbulence at the electrolyte surface caused by the
release of anodically evolved gas.
[0118] FIG. 12 shows a variation of the baffles shown in FIG. 11,
wherein parallel vertical sections 4 are located above the
converging surfaces 6,7.
[0119] By guiding and confining anodically-evolved oxygen towards
the surface of the electrolyte 30 with baffles or other confinement
means as shown in FIGS. 11 and 12, oxygen is released so close to
the surface as to create turbulences above the downwardly
converging surfaces 6, promoting dissolution of alumina fed
thereabove.
[0120] It is understood that the electrolyte confinement members 5
shown in FIGS. 9, 10, 11 and 12 can either be elongated baffles, or
instead consist of a series of vertical chimneys of funnels of
circular or polygonal cross-section, for instance as described
below.
[0121] FIGS. 13 and 15 illustrate an anode 10' having a circular
bottom, the anode 10' being shown in cross-section in FIG. 5 and
from above in FIG. 15. On the right hand side of FIGS. 13 and 15
the anode 10' is shown with electrolyte guide members 5'. The
electrolyte guide members 5' represented in FIG. 15 are shown
separately in FIG. 14.
[0122] The anode 10' shown in FIGS. 13 and 15 has several
concentric circular anode members 15. The anode members 15 are
laterally spaced apart from one another by inter-member gaps 17 and
connected together by radial connecting members in the form of
flanges 13 which join an outer ring 13'. The outer ring 13' extends
vertically from the outermost anode members 15, as shown in FIG.
13, to form with the radial flanges 13 a wheel-like structure
13,13', shown in FIG. 15, which secures the anode members 15 to a
central anode current feeder 11.
[0123] As shown in FIG. 13, the innermost circular anode member 15
partly merges with the current feeder 11, with ducts 18 extending
between the innermost circular anode member 15 and the current
feeder 11 to permit the escape of oxygen produced underneath the
central current feeder 11.
[0124] Each electrolyte guide member 5' is in the general shape of
a funnel having a wide bottom opening 9 for receiving anodically
produced oxygen and a narrow top opening 8 where the oxygen is
released to promote dissolution of alumina fed above the
electrolyte guide member 5'. The inner surface 7 of the electrolyte
guide member 5' is arranged to canalise and promote an upward
electrolyte flow driven by anodically produced oxygen. The outer
surface 6 of the electrolyte guide member 5' is arranged to promote
dissolution of alumina fed thereabove and guide alumina-rich
electrolyte down to the inter-electrode gap, the electrolyte
flowing mainly around the foraminate structure.
[0125] As shown in FIGS. 14 and 15, the electrolyte guide members
5' are in a circular arrangement, only half of the arrangement
being shown. The electrolyte guide members 5' are laterally secured
to one another by attachments 3 and so arranged to be held above
the anode members 15, the attachments 3 being for example placed on
the flanges 13 as shown in FIG. 15 or secured as required. Each
electrolyte guide member 5' is positioned in a circular sector
defined by two neighbouring radial flanges 13 and an arc of the
outer ring 13' as shown in FIG. 15.
[0126] The arrangement of the electrolyte guide members 5' and the
anode 10' can be moulded as units. This offers the advantage of
avoiding mechanical joints and the risk of altering the properties
of the materials of the electrolyte guide members 5' or the anode
10' by welding.
[0127] The anodes 10' and electrolyte guide members 5' can be made
of the same materials, in particular they can be made of a metallic
body having an outer part with a layer containing predominantly
CoO.
[0128] FIG. 16 illustrates a square anode 10' as a variation of the
round anode 10' of FIGS. 13 and 15. The anode 10' of FIG. 16 has
generally rectangular concentric parallel anode members 15 with
rounded corners. The anode 10' shown in FIG. 16 can be fitted with
electrolyte guide members similar to those of FIGS. 13 to 15 but in
a corresponding rectangular arrangement.
[0129] The anodes 10' of FIGS. 13 and 16 can be made by casting a
metal or an alloy. Typically, an anode substrate, for example
consisting manly of nickel, can be cast to form the anode's
internal structure which is then coated as described above to form
the anode's outer part having a layer that consists predominantly
of CoO. Alternatively, cobalt or a cobalt alloy can be cast to form
the anode's internal structure which is then oxidised as described
above to form an outer part with a layer that consist predominantly
of CoO. It is also contemplated to cast cobalt or a cobalt alloy
around an anode core or skeleton having a different composition
than the cast cobalt or cobalt alloy, for example iron or steel.
The electrolyte guide members 5' can be manufactured by the same
method and with the same or different materials.
[0130] Useful variations of this anode structure are disclosed in
the abovementioned WO00/40782. Further suitable anode designs are
disclosed WO99/027064 (de Nora/Duruz), WO01/31088, WO03/006716 and
WO03/023092 and U.S. Pat. No. 5,368,702 (all de Nora)
[0131] The manufacturing and behaviour in an aluminium
electrowinning cell of the cobalt-oxide containing material used
for the anode of the present invention will be further described in
the following examples:
Comparative Example 1
[0132] A cylindrical metallic cobalt sample was oxidised to form an
integral cobalt oxide layer that did not predominantly contain CoO.
The cobalt samples contained no more than a total of 1 wt %
additives and impurities and had a diameter of 1.94 cm and a height
of 3 cm.
[0133] Oxidation was carried out by placing the cobalt sample into
an oven in air and increasing the temperature from room temperature
to 850.degree. C. at a rate of 120.degree. C./hour.
[0134] After 24 hours at 850.degree. C., the oxidised cobalt sample
was allowed to cool down to room temperature and examined.
[0135] The cobalt sample was covered with a greyish oxide scale
having a thickness of about 300 micron. This oxide scale was made
of: a 80 micron thick inner layer that had a porosity of 5% with
pores that had a size of 2-5 micron; and a 220 micron thick outer
layer having an open porosity of 20% with pores that had a size of
10-20 micron. The outer oxide layer was made of a mixture of
essentially Co.sub.2O.sub.3 and Co.sub.3O.sub.4. The denser inner
oxide layer was made of CoO.
[0136] As shown in Comparative Examples 2 and 3, such oxidised
cobalt provides poor results when used as an anode material in an
aluminium electrowinning cell.
Example 1a
[0137] A cobalt sample which can be used to manufacture for an
anode according to the invention was prepared as in Comparative
Example 1 except that the sample was oxidised in an oven heated
from room temperature to a temperature of 950.degree. C. (instead
of 850.degree. C.) at the same rate (120.degree. C./hour).
[0138] After 24 hours at 950.degree. C., the oxidised cobalt sample
was allowed to cool down to room temperature and examined.
[0139] The cobalt sample was covered with a black glassy oxide
scale having a thickness of about 350 micron (instead of 300
micron). This oxide scale had a continuous structure (instead of a
layered structure) with an open porosity of 10% (instead of 20%)
and pores that had a size of 5 micron. The outer oxide layer was
made of CoO produced above 895.degree. C. from the conversion into
CoO of Co.sub.3O.sub.4 and glassy Co.sub.2O.sub.3 formed below this
temperature and by oxidising the metallic outer part of the sample
(underneath the cobalt oxide) directly into CoO. The porosity was
due to the change of phase during the conversion of Co.sub.2O.sub.3
and Co.sub.3O.sub.4 to CoO.
[0140] Such a material can be used for making an aluminium
electrowinning anode according to the invention. However, the
density of the CoO layer and the performances of this material can
be further improved as shown in Examples 1c and 1d.
[0141] In general, to allow appropriate conversion of the cobalt
oxide and growth of CoO from the metallic outer part of the
substrate, it is important to leave the sample sufficiently long at
a temperature above 895.degree. C. The length of the heat treatment
will depend on the oxygen content of the oxidising atmosphere, the
temperature of the heat treatment, the desired amount of CoO and
the amount of Co.sub.2O.sub.3 and Co.sub.3O.sub.4 to convert into
CoO.
Example 1b
[0142] Example 1a was repeated with a similar cylindrical metallic
cobalt sample. The oven in which the sample was oxidised was heated
to a temperature of 1050.degree. C. (instead of 950.degree. C.) at
the same rate (120.degree. C./hour).
[0143] After 24 hours at 1050.degree. C., the oxidised cobalt
sample was allowed to cool down to room temperature and
examined.
[0144] The cobalt sample was covered with a black crystallised
oxide scale having a thickness of about 400 micron (instead of 350
micron). This oxide scale had a continuous structure with an open
porosity of 20% (instead of 10%) and pores that had a size of 5
micron. The outer oxide layer was made of CoO produced above
895.degree. C. like in Example 1a.
[0145] Such a oxidised cobalt is comparable to the oxidised cobalt
of Example 1a and can likewise be used as an anode material to
produce aluminium according to the present invention.
[0146] In general, to allow appropriate conversion of the cobalt
oxide and growth of CoO from the metallic outer part of the
substrate, it is important to leave the sample sufficiently long at
a temperature above 895.degree. C. The length of the heat treatment
above 895.degree. C. will depend on the oxygen content of the
oxidising atmosphere, the temperature of the heat treatment, the
desired amount of CoO and the amount of Co.sub.2O.sub.3 and
Co.sub.3O.sub.4 (produced below 895.degree. C.) which needs to be
converted into CoO.
Example 1c (Improved Material)
[0147] Example 1a was repeated with a similar cylindrical metallic
cobalt sample. The oven in which the sample was oxidised was heated
to the same temperature (950.degree. C.) at a rate of 360.degree.
C./hour (instead of 120.degree. C./hour).
[0148] After 24 hours at 950.degree. C., the oxidised cobalt sample
was allowed to cool down to room temperature and examined.
[0149] The cobalt sample was covered with a dark grey substantially
non-glassy oxide scale having a thickness of about 350 micron. This
oxide scale had a continuous structure with an open porosity of
less than 5% (instead of 10%) and pores that had a size of 5
micron.
[0150] The outer oxide layer was made of CoO that was formed
directly from metallic cobalt above 895.degree. C. which was
reached after about 2.5 hours and to a limited extent from the
conversion of previously formed Co.sub.2O.sub.3 and
Co.sub.3O.sub.4. It followed that there was less porosity caused by
the conversion of Co.sub.2O.sub.3 and Co.sub.3O.sub.4 to CoO than
in Example 1a.
[0151] Such an oxidised cobalt sample has a significantly higher
density than the samples of Examples 1a and 1b, and is
substantially crack-free. This oxidised cobalt constitutes a
preferred material for making an improved aluminium electrowinning
anode for use in a cell according to the invention.
Example 1d (Improved Material)
[0152] Example 1c was repeated with a similar cylindrical metallic
cobalt sample. The oven in which the sample was oxidised was heated
to the same temperature (1050.degree. C.) at a rate of 600.degree.
C./hour (instead of 120.degree. C./hour in Example 1a and 1b and
360.degree. C./hour in Example 1c).
[0153] After 18 hours at 1050.degree. C., the oxidised cobalt
sample was allowed to cool down to room temperature and
examined.
[0154] The cobalt sample was covered with a dark grey substantially
non-glassy oxide scale having a thickness of about 300 micron
(instead of 400 micron in Example 1b and 350 micron in Example 1c).
This oxide scale had a continuous structure with a crack-free open
porosity of less than 5% (instead of 20% in Example 1b) and pores
that had a size of less than 2 micron (instead of 5 micron in
Example 1b and in Example 1c).
[0155] The outer oxide layer was made of CoO that was formed
directly from metallic cobalt above 895.degree. C. which was
reached after about 1.5 hours and to a marginal extent from the
conversion of previously formed Co.sub.2O.sub.3 and
Co.sub.3O.sub.4. It followed that there was significantly less
porosity caused by the conversion of Co.sub.2O.sub.3 and
Co.sub.3O.sub.4 to CoO than in Example 1b and in Example 1c.
[0156] Such an oxidised cobalt sample has a significantly higher
density than the samples of Examples 1a and 1b, and is
substantially crack-free. This oxidised cobalt constitutes a
preferred material for making an improved aluminium electrowinning
anode according to the invention.
Comparative Example 2 (Overpotential Testing)
[0157] An anode made of metallic cobalt oxidised under the
conditions of Comparative Example 1 was tested in an aluminium
electrowinning cell.
[0158] The cell's electrolyte was at a temperature of 925.degree.
C. and made of 11 wt % AlF.sub.3, 4 wt % CaF.sub.2, 7 wt % KF and
9.6 wt % Al.sub.2O.sub.3, the balance being cryolite
Na.sub.3AlF.sub.6.
[0159] The anode was placed in the cell's electrolyte at a distance
of 4 cm from a facing cathode. An electrolysis current of 7.3 A was
passed from the anode to the cathode at an anodic current density
of 0.8 A/cm.sup.2.
[0160] The electrolysis current was varied between 4 and 10 A and
the corresponding cell voltage measured to estimate the oxygen
overpotential at the anode.
[0161] By extrapolating the cell's potential at a zero electrolysis
current, it was found that the oxygen overpotential at the anode
was of 0.88 V.
Example 2 (Overpotential Testing)
[0162] A test was carried out under the conditions of Comparative
Example 2 with two anodes made of metallic cobalt oxidised under
the conditions of Example 1c and 1d, respectively, in cells
according to the invention using the same electrolyte as in
Comparative Example 2. The estimated oxygen overpotential for these
anodes were at 0.22 V and 0.21 V, respectively, i.e. about 75%
lower than in Comparative Example 2.
[0163] It follows that the use of metallic cobalt covered with an
integral layer of CoO instead of Co.sub.2O.sub.3 and
Co.sub.3O.sub.4 as an aluminium electrowinning anode material in a
cell according to the invention leads to a significant saving of
energy.
Comparative Example 3 (Aluminium Electrowinning)
[0164] Another anode made of metallic cobalt oxidised under the
conditions of Comparative Example 1, i.e. resulting in a
Co.sub.2O.sub.3 and Co.sub.3O.sub.4 integral surface layer, was
tested in an aluminium electrowinning cell. The cell's electrolyte
was at 925.degree. C. and had the same composition as in
Comparative Example 2. A nominal electrolysis current of 7.3 A was
passed from the anode to the cathode at an anodic current density
of 0.8 A/cm.sup.2.
[0165] The cell voltage at start-up was above 20 V and dropped to
5.6 V after about 30 seconds. During the initial 5 hours, the cell
voltage fluctuated about 5.6 V between 4.8 and 6.4 V with short
peaks above 8 V. After this initial period, the cell voltage
stabilised at 4.0-4.2 V.
[0166] Throughout electrolysis, fresh alumina was fed to the
electrolyte to compensate for the electrolysed alumina.
[0167] After 100 hours electrolysis, the anode was removed from the
cell, allowed to cool down to room temperature and examined.
[0168] The anode's diameter had increased from 1.94 to 1.97 cm. The
anode's metallic part had been heavily oxidised. The thickness of
the integral oxide scale had increased from 350 micron to about
1.1-1.5 mm. The oxide scale was made of: a 300-400 micron thick
outer layer containing pores having a size of 30-50 micron and
having cracks; a 1-1.1 mm thick inner layer that had been formed
during electrolysis. The inner layer was porous and contained
electrolyte under the cracks of the outer layer.
Example 3 (Aluminium Electrowinning)
[0169] An anode made of metallic cobalt oxidised under the
conditions of Example 1c, i.e. resulting in a CoO integral surface
layer was tested in an aluminium electrowinning cell under the
conditions of Comparative Example 3. A nominal electrolysis current
of 7.3 A was passed from the anode to the cathode at an anodic
current density of 0.8 A/cm.sup.2.
[0170] At start-up the cell voltage was 4.1 V and steadily
decreased to 3.7-3.8 V after 30 minutes (instead of 4-4.2 in
Comparative Example 3). The cell voltage stabilised at this level
throughout the test without noticeable fluctuations, unlike in
Comparative Example 3.
[0171] After 100 hours electrolysis, the anode was removed from the
cell, allowed to cool down to room temperature and examined.
[0172] The anode's external diameter did not change during
electrolysis and remained at 1.94 cm. The metallic cobalt inner
part underneath the oxide scale had slightly decreased from 1.85 to
1.78 cm. The thickness of the cobalt oxide scale had increased from
0.3 to 0.7-0.8 mm (instead of 1-1.1 mm of Comparative Example 3)
and was made of: a non-porous 300-400 micron thick external layer;
and a porous 400 micron thick internal layer that had been formed
during electrolysis. This internal oxide growth (400 micron
thickness over 100 hours) was much less than the growth observed in
Comparative example 3 (1-1.1 mm thickness over 100 hours).
[0173] It follows that the anode's CoO integral surface layer
inhibits diffusion of oxygen and oxidation of the underlying
metallic cobalt, compared to the Co.sub.2O.sub.3 and
Co.sub.3O.sub.4 integral surface layer of the anode of Comparative
Example 3.
Example 4 (Variations)
[0174] The anode material of Examples 1a to 1d, 2 and 3 can be
covered upon formation of the integral CoO layer with a slurry
applied layer, in particular containing CoFe.sub.2O.sub.4
particulate in a iron hydroxide colloid followed by drying at
250.degree. C. to form a protective layer on the CoO integral
layer.
Example 5
[0175] A coated anode for use in a cell according to the invention
was made by covering a metallic cobalt substrate with an applied
electrochemically active coating comprising an outer CoO layer and
an inner layer of tantalum and cobalt oxides.
[0176] The coating was formed by applying cobalt and tantalum using
electrodeposition. Specifically, tantalum was dispersed in the form
of physical inclusions in cobalt electrodeposits.
[0177] The electrodeposition bath had a pH of 3.0 to 3.5 and
contained: [0178] 400 g/l CoSO.sub.4.7H.sub.2O; [0179] 40 g/l
H.sub.3BO.sub.3; [0180] 40 g/l KCl; and [0181] 7-10 g/l Ta
particles.
[0182] The tantalum particles had a size below 10 micron and were
dispersed in the electrodeposition bath.
[0183] Electrodeposition on the cobalt substrate was carried out at
a current density of 35 mA/cm.sup.2 which led to a cobalt deposit
containing Ta inclusions, the deposit growing at a rate of 45
micron per hour on the substrate.
[0184] After the deposit had reached a total thickness of 250-300
micron, electrodeposition was interrupted. The deposit contained
9-15 wt % Ta corresponding to a volume fraction of 4-7 v %.
[0185] To form a coating, the substrate with its deposit were
exposed to an oxidation treatment at a temperature of 950.degree.
C. The substrate with its deposit were brought from room
temperature to 950.degree. C. at a rate of 450-500.degree. C./hour
in an oven to optimise the formation of CoO instead of
Co.sub.2O.sub.3 or Co.sub.3O.sub.4.
[0186] After 8 hours at 950.degree. C., the substrate and the
coating that was formed by oxidation of the deposit were taken out
of the oven and allowed to cool down to room temperature. The
coating had an outer oxide layer CoO on an inner oxide layer of
Co--Ta oxides, in particular CoTaO.sub.4, that had grown from the
deposit. The innermost part of the deposit had remained unoxidised,
so that the Co--Ta oxide layer was integral with the remaining
metallic Co--Ta deposit. The Co--Ta oxide layer and the CoO layer
had a total thickness of about 200 micron on the remaining metallic
Co--Ta.
[0187] As demonstrated in Example 6, this CoO outer layer can act
as an electrochemically active anode surface. The inner Co--Ta
oxide layer inhibits oxygen diffusion towards the metallic cobalt
substrate.
Example 6
[0188] A coated anode was made of a cobalt substrate covered with a
Co--Ta coating as in Example 5 and used in a cell for the
electrowinning aluminium according to the invention.
[0189] The anode was suspended in the cell's electrolyte at a
distance of 4 cm from a facing cathode. The electrolyte contained
11 wt % AlF.sub.3, 4 wt % CaF.sub.2, 7 wt % KF and 9.6 wt %
Al.sub.2O.sub.3, the balance being Na.sub.3AlF.sub.6. The
electrolyte was at a temperature of 925.degree. C.
[0190] An electrolysis current was passed from the anode to the
cathode at an anodic current density of 0.8 A/cm.sup.2. The cell
voltage remained remarkably stable at 3.6 V throughout
electrolysis.
[0191] After 150 hours electrolysis, the anode was removed from the
cell. No significant change of the anode's dimensions was observed
by visual examination.
[0192] Example 7
[0193] Example 5 was repeated by applying a Co-Ta coating onto an
anode substrate made of a metallic alloy containing 75 wt % Ni, 15
wt % Fe and 10 wt % Cu.
[0194] The anode was tested as in Example 6 at an anodic current
density of 0.8 A/cm.sup.2. At start-up, the cell voltage was at 4.2
V and decreased within the first 24 hours to 3.7 V and remained
stable thereafter.
[0195] After 120 hours electrolysis, the anode was removed from the
cell. No sign of passivation of the nickel-rich substrate was
observed and no significant change of dimensions of the anode was
noticed by visual examination of the anode.
Example 8
[0196] Examples 5 to 7 can be repeated by substituting tantalum
with niobium.
Example 9
[0197] Another anode for use in a cell according to the invention
was made by applying a coating of Co--W onto an anode substrate
made of a metallic alloy containing 75 wt % Ni, 15 wt % Fe and 10
wt % Cu.
[0198] The coating was formed by applying cobalt and tungsten using
electrodeposition. The electrodeposition bath contained: [0199] 100
g/l CoCl.sub.2.6H.sub.2O; [0200] 45 g/l Na.sub.2WO.sub.4.2H.sub.2O;
[0201] 400 g/l KNaC.sub.4H.sub.4O.sub.6.4H.sub.2O; and [0202] 50
g/l NH.sub.4Cl.
[0203] Moreover, NH.sub.4OH had been added to this bath so that the
bath had reached a pH of 8.5-8.7.
[0204] Electrodeposition on the Ni--Fe--Cu substrate was carried
out at a temperature of 82-90.degree. C. and at a current density
of 50 mA/cm.sup.2 which led to a cobalt-tungsten alloy deposit on
the substrate, the deposit growing at a rate of 35-40 micron per
hour at a cathodic current efficiency of about 90%.
[0205] After the deposit had reached a total thickness of about 250
micron, electrodeposition was interrupted. The deposited cobalt
alloy contained 20-25 wt % tungsten.
[0206] To form a coating, the substrate with its deposit were
exposed to an oxidation treatment at a temperature of 950.degree.
C. The substrate with its deposit were brought from room
temperature to 950.degree. C. at a rate of 450-500.degree. C./hour
in an oven to optimise the formation of CoO instead of
Co.sub.2O.sub.3 or Co.sub.3O.sub.4.
[0207] After 8 hours at 950.degree. C., the substrate and the
coating that was formed by oxidation of the deposit were taken out
of the oven and allowed to cool down to room temperature. The
coating contained at its surface cobalt monoxide and tungsten
oxide.
[0208] The structure of the coating after oxidation was denser and
more coherent than the coating obtained by oxidising an
electrodeposited layer of Ta--Co as disclosed in Example 1.
[0209] As demonstrated in Example 10, this coating can act as an
electrochemically active anode surface. The presence of tungsten
inhibits oxygen diffusion towards the metallic cobalt
substrate.
Example 10
[0210] An anode was made as in Example 9 and used in a cell for the
electrowinning of aluminium according to the invention.
[0211] The anode was suspended in the cell's electrolyte at a
distance of 4 cm from a facing cathode. The electrolyte contained
11 wt % AlF.sub.3, 4 wt % CaF.sub.2, 7 wt % KF and 9.6 wt %
Al.sub.2O.sub.3, the balance being Na.sub.3AlF.sub.6. The
electrolyte was at a temperature of 925.degree. C.
[0212] An electrolysis current was passed from the anode to the
cathode at an anodic current density of 0.8 A/cm.sup.2. The cell
voltage remained stable at 3.5-3.7 V throughout electrolysis. After
100 hours electrolysis, the anode was removed from the cell. No
change of the anode's dimensions was observed by visual
examination.
Example 11
[0213] Examples 9 and 10 can be repeated with an anode substrate
made of cobalt, nickel or an alloy of 92 wt % nickel and 8 wt %
copper.
[0214] Comparative tests show that the use in a conventional
cryolite-based electrolyte at 960.degree. C. of a metal-based anode
having an electrochemically active outer part comprising a layer
that contains predominantly cobalt oxide CoO, leads to accelerated
oxidation of the anode and dissolution into the electrolyte of
oxides of the anode, in particular CoO. Moreover, use of such an
anode in an electrolyte at 910.degree.-940.degree. C. without
potassium fluoride leads to corrosion or passivation the anode.
[0215] These Examples demonstrate that a material having an outer
part with a layer that contains predominantly cobalt oxide CoO as
described above, provides an enhanced stability during use in an
aluminium electrowinning cell and is therefore suitable to protect
anodes having a flow-through structure which is exposed to the
fluoride-based molten electrolyte that is rendered more aggressive
to the anodes by its circulation through the anodes.
* * * * *