ANODES METALLIQUES EXEMPTES DE CARBONE POUR CELLULES DE PRODUCTION D'ALUMINIUM

NON-CARBON METAL-BASED ANODES FOR ALUMINIUM PRODUCTION CELLS

Abrégé français

Anode métallique exempte de carbone pour une cellule d'extraction électrolytique d'aluminium, qui comporte un substrat métallique électriquement conducteur résistant aux hautes températures, dont la surface devient passive et pratiquement inerte à l'électrolyte, et un revêtement qui adhère au substrat métallique et qui rend la surface de l'anode électrochimiquement active pour l'oxydation des ions d'oxygène présents au niveau de l'interface de l'électrolyte. Le substrat métallique peut être choisi parmi nickel, cobalt, chrome, molybdène, tantale et la série des lanthanides. Les constituants actifs du revêtement sont par exemple des oxydes tels que des spinelles ou pérovskites, des oxyfluorures, des phosphures ou carbures, en particulier des ferrites. Les constituants actifs peuvent être déposés sur le substrat à partir d'un coulis ou d'une suspension contenant de la matière colloïdale et la matière électrochimiquement active.

Abrégé anglais

A non-carbon metal-based anode of a cell for the electrowinning of aluminium
comprising an electrically conductive metal substrate
resistant to high temperature, the surface of which becomes passive and
substantially inert to the electrolyte, and a coating adherent to
the metal substrate making the surface of the anode electrochemically active
for the oxidation of oxygen ions present at the electrolyte
interface. The substrate metal may be selected from nickel, cobalt, chromium,
molybdenum, tantalum and the Lanthanide series. The active
constituents of the coating are for example oxides such as spinels or
perovskites, oxyfluorides, phosphides or carbides, in particular ferrites.
The active constituents may be coated onto the substrate from a slurry or
suspension containing colloidal material and the electrochemically
active material.

Revendications

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We Claim:
1. A method of manufacturing a non-carbon metal-based
anode of a cell for the electrowinning of aluminium, said
method comprising coating a substrate of electrically
conductive metal resistant to high temperature and the
surface of which becomes passive and substantially inert to
the electrolyte with at least one layer of an
electrochemically active coating precursor in the form of a
slurry or suspension containing at least one
electrochemically active constituent or a precursor
thereof, and heat-treating the or each layer on the
substrate to obtain a coating adherent to the passivatable
metal substrate making the surface of the anode
electrochemically active for the oxidation of oxygen ions
present at the electrolyte interface.
2. The method of claim 1, wherein the method of
manufacturing a non-carbon metal-based anode of a cell for
the electrowinning of aluminium is by the electrolysis of
alumina dissolved in fluoride-containing electrolyte
3. The method of claim 1, wherein the passivatable metal
substrate comprises at least one metal selected from
nickel, cobalt, chromium, molybdenum, tantalum and the
Lanthanide series, and their alloys or intermetallics.
4. The method of claim 3, wherein the passivatable metal
substrate is nickel-plated copper.

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5. The method of claim 1, wherein the coating is formed
by further applying at least one electrocatalyst or a
precursor thereof for the formation of oxygen gas.
6. The method of claim 1, wherein the coating is formed
by further applying a bonding material substantially
resistant to cryolite for bonding the constituents of the
coating together and onto the passivatable metal substrate.
7. The method of claim 1, wherein the coating is obtained
from a slurry or suspension containing colloidal or
polymeric material.
8. The method of claim 1, wherein the slurry or
suspension contains at least one of alumina, ceria, lithia,
magnesia, silica, thoria, yttria, zirconia, tin oxide and
zinc oxide, and colloids containing active constituents of
the coating or precursors thereof, all in the form of
colloids or polymers.
9. The method of claim 5, wherein the or at least one of
said electrochemically active constituent(s) is selected
from the group consisting of oxides, oxyfluorides,
phosphides, carbides and combinations thereof.
10. The method of claim 9, wherein said oxides comprise
spinels and/or perovskites.
11. The method of claim 10, wherein said spinels are
doped, non-stoichiometric and/or partially substituted
spinels, the doped spinels comprising dopants selected from

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the group consisting of Ti4+, Zr4+, Sn4+, Fe4+, Hf4+, Mn4+,
Fe3+, Ni3+, C03+, Mn3+, A13+, Cr3+, Fe2+, Ni2+, Co2+, Mg2+,
Mn2+, Cu2+, Zn2+ and Li+.
12. The method of claim 10, wherein said spinels comprise
a ferrite and/or a chromite.
13. The method of claim 12, wherein said ferrite is
selected from the group consisting of cobalt, manganese,
molybdenum, nickel and zinc, and mixtures thereof.
14. The method of claim 13, wherein the ferrite is doped
with at least one oxide selected from the group consisting
of chromium, titanium, tantalum, tin, zinc and zirconium
oxide.
15. The method of claim 13, wherein said ferrite is
nickel-ferrite or nickel-ferrite partially substituted with
Fe2+.
16. The method of claim 12, wherein said chromite is
selected from the group consisting of iron, cobalt, copper,
manganese, beryllium, calcium, strontium, barium,
magnesium, nickel and zinc chromite.
17. The method of claim 9, wherein the or at least one of
said electrochemically active constituent(s) comprises at
least one Lanthanide as an oxide or a oxyfluoride, and
mixtures thereof.
18. The method of claim 17, wherein said oxyfluoride is

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cerium oxyfluoride.
19. The method of claim 5, wherein the or at least one of
said electrochemically active constituent(s) comprises at
least one metal selected from iron, chromium, copper and
nickel, and oxides, mixtures and compounds thereof.
20. The method of claim 6, wherein said electrocatalyst(s)
is/are selected from iridium, palladium, platinum, rhodium,
ruthenium, silicon, tin and zinc, the Lanthanide series and
mischmetal, and their oxides, mixtures and compounds
thereof.
21. The method of claim 1, comprising reacting
constituents of the coating precursor among themselves to
form the coating.
22. The method of claim 1, comprising reacting at least
one constituent of the coating precursor with the
passivatable metal substrate to form the coating.
23. The method of claim 1, wherein the coating precursor
is applied onto the substrate by rollers, brush or
spraying.
24. The method of claim 1, comprising coating the
passivatable metal substrate onto an electronically
conductive core.
25. The method of claim 24, wherein the core is selected
from metals, alloys, intermetallics, cermets and conductive
ceramics.

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26. The method of claim 24, wherein the metals of the core
are selected from copper, chromium, cobalt, iron,
aluminium, hafnium, molybdenum, nickel, niobium, silicon,
tantalum, titanium, tungsten, vanadium, yttrium and
zirconium, and combinations and compounds thereof.
27. The method of claim 26, wherein the core is an alloy
comprising 10 to 30 weight% of chromium, 55 to 90 weight%
of at least one of nickel, cobalt and/or iron and 0 to 15
weight% of at least one of aluminium, hafnium, molybdenum,
niobium, silicon, tantalum, tungsten, vanadium, yttrium and
zirconium.
28. The method of claim 24, comprising forming an oxygen
barrier layer on the core.
29. The method of claim 28, comprising oxidising the
surface of the core to form the oxygen barrier layer.
30. The method of claim 26, comprising applying a
precursor of the oxygen barrier layer onto the core and
heat treating.
31. The method of claim 26, wherein the oxygen barrier
layer comprises chromium oxide.
32. The method of claim 26, wherein the oxygen barrier
layer comprises black non-stoichiometric nickel oxide.
33. The method of claim 26, comprising covering the oxygen
barrier layer with at least one protective layer consisting

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of copper or copper and at least one of nickel and cobalt,
and/or oxides thereof to protect the oxygen barrier layer
by inhibiting its dissolution into the electrolyte.
34. The method of claim 1 for reconditioning a non-carbon
metal-based anode having a passivatable substrate with an
electrochemically active coating, when at least part of the
active coating has become non-active or worn out, said
method comprising clearing the surface of the substrate
before re-coating said surface with a coating applied from
said slurry or suspension.
35. A non-carbon metal-based anode of a cell for the
electrowinning of aluminium, comprising an electrically
conductive metal substrate resistant to high temperature,
the surface of which becomes passive and substantially
inert to the electrolyte, and an electrochemically active
coating adherent to the surface of the metal substrate
making and keeping the surface of the anode conductive and
electrochemically active for the oxidation of oxygen ions
present at the electrolyte interface, said coating
containing electrochemically active constituents in a
colloid obtainable from at least one electrochemically
active constituent or a precursor thereof in a colloid-
containing slurry or suspension.
36. The anode of claim 35, wherein the anode is for the
electrowinning of aluminium by the electrolysis of alumina
dissolved in a molten fluoride-containing electrolyte.
37. The anode of claim 35, wherein the passivatable metal
substrate comprises at least one metal selected from

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nickel, cobalt, chromium, molybdenum, tantalum and the
Lanthanide series, and their alloys or intermetallics.
38. The anode of claim of claim 37, wherein the
passivatable metal substrate is nickel-plated copper.
39. The anode of claim 35, wherein the coating further
comprises at least one electrocatalyst or a precursor
thereof for the formation of oxygen gas.
40. The anode of claim 35, wherein the coating further
comprises a bonding material substantially resistant to
cryolite for bonding the constituents of the coating
together and onto the passivatable metal substrate.
41. The anode of claim 35, wherein the coating is a heat-
treated slurry or suspension containing at least one heat-
treated colloid or polymer selected from heat-treated
colloidal or polymeric alumina, ceria, lithia, magnesia,
silica, thoria, yttria, zirconia, tin oxide, and zinc
oxide, and colloids containing active constituents of the
coating or precursors thereof, all in the form of heat
treated colloids or polymers.
42. The anode of claim 35, wherein the or at least one of
said electrochemically active constituent(s) is selected
from the group consisting of oxides, oxyfluorides,
phosphides, carbides and combinations thereof.
43. The anode of claim 42, wherein said oxides comprise
spinels and/or perovskites.

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44. The anode of claim 42, wherein said spinels are doped,
non-stoichiometric and/or partially substituted spinels,
the doped spinels comprising dopants selected from the
group consisting of Ti4+, Zr4+, Sn4+, Fe4+, Hf4+, Mn4+,
Fe3+, Ni3+, Co3+1 Mn3+, A13+, Cr3+, Fe2+, Ni2+, Co2+, Mg2+,
Mn2+, Cu2+, Zn2+ and Li+.
45. The anode of claim 44, wherein said spinels comprise a
ferrite and/or a chromite.
46. The anode of claim 45, wherein said ferrite is
selected from the group consisting of cobalt, manganese,
molybdenum, nickel and zinc, and mixtures thereof.
47. The anode of claim 46, wherein the ferrite is doped
with at least one oxide selected from the group consisting
of chromium, titanium, tantalum, tin, zinc and zirconium
oxide.
48. The anode of claim 46, wherein said ferrite is nickel-
ferrite or nickel-ferrite partially substituted with Fe2+.
49. The anode of claim 45, wherein said chromite is
selected from the group consisting of iron, cobalt, copper,
manganese, beryllium, calcium, strontium, barium,
magnesium, nickel and zinc chromite.
50. The anode of claim 42, wherein the or at least one of
said electrochemically active constituent(s) comprises at
least one Lanthanide as an oxide or an oxyfluoride, and
mixtures thereof.

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51. The anode of claim 50, wherein said oxyfluoride is
cerium oxyfluoride.
52. The anode of claim 35, wherein the or at least one of
said electrochemically active constituent(s) comprises at
least one metal selected from iron, chromium, copper and
nickel, and oxides, mixtures and compounds thereof.
53. The anode of claim 39, wherein said electrocatalyst(s)
is/are selected from iridium, palladium, platinum, rhodium,
ruthenium, silicon, tin and zinc, the Lanthanide series and
mischmetal, and their oxides, mixtures and compounds
thereof.
54. The anode of claim 35, wherein the passivatable metal
substrate is coated on an electronically conductive core.
55. The anode of claim 54, wherein the core is selected
from metals, alloys, intermetallics, cermets and conductive
ceramics.
56. The anode of claim 55, wherein the metals of the core
are selected from copper, chromium, cobalt, iron,
aluminium, hafnium, molybdenum, nickel, niobium, silicon,
tantalum, titanium, tungsten, vanadium, yttrium and
zirconium, and combinations and compounds thereof.
57. The anode of claim 56, wherein the core is an alloy
comprising 10 to 30 weight% of chromium, 55 to 90 weight%
of at least one of nickel, cobalt and/or iron and 0 to 15
weight% of at least one of aluminium, hafnium, molybdenum,

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niobium, silicon, tantalum, tungsten, vanadium, yttrium and
zirconium.
58. The anode of claim 54, wherein the core is covered
with an oxygen barrier layer.
59. The anode of claim 58, wherein the oxygen barrier
layer comprises chromium oxide.
60. The anode of claim 58, wherein the oxygen barrier
layer comprises black non-stoichiometric nickel oxide.
61. The anode of claim 58, wherein the oxygen barrier
layer is covered with at least one protective layer
consisting of copper or copper and at least one of nickel
and cobalt, and/or oxides thereof to protect the oxygen
barrier layer by inhibiting its dissolution into the
electrolyte.
62. A cell for the production of aluminium by the
electrolysis of alumina dissolved in a fluoride-containing
electrolyte having at least one non-carbon metal-based
anode comprising an electrically conductive passivatable
metal substrate and a conductive coating having an
electrochemically active surface according to claim 35.
63. The cell of claim 62, wherein the electrolyte is
cryolite.
64. The cell of claim 62, comprising at least one
aluminium-wettable cathode.

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65. The cell of claim 64, which is in a drained
configuration, comprising at least one drained cathode on
which aluminium is produced and from which aluminium
continuously drains.
66. The cell of claim 64, which is in a bipolar
configuration and wherein the anodes form the anodic side
of at least one bipolar electrode and/or a terminal anode.
67. The cell of claim 64, comprising means to circulate
the electrolyte between the anodes and facing cathodes
and/or means to facilitate dissolution of alumina in the
electrolyte.
68. The cell of claim 64, wherein during operation the
electrolyte is at a temperature of 750°C to 970°C.
69. Use of the anode of claim 35 for the production of
aluminium in a cell for the electrowinning of aluminium by
the electrolysis of alumina dissolved in a fluoride-
containing electrolyte, wherein oxygen ions in the
electrolyte are oxidised and released as molecular oxygen
on the electrochemically active anode coating.
70. A method of producing aluminium in a cell as defined
in claim 62, comprising oxidising oxygen ions on the
electrochemically active anode coating of the or each anode
and aluminium on a cathode.

Description

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NON-CARBON METAL-BASED ANODES FOR
ALUMINIUM PRODUCTION CELLS
Field of the Invention
This invention relates to non-carbon metal-based
anodes for use in cells for the electrowinning of
aluminium by the electrolysis of alumina dissolved in a
molten fluoride-containing electrolyte, and to methods
for their fabrication and reconditioning, as well as to
electrowinning cells containing such anodes and their use
to produce aluminium.
Background Art
The technology for the production of aluminium by
the electrolysis of alumina, dissolved in molten cryolite
containing salts, at temperatures around 950 C is more
than one hundred years old.
This process, conceived almost simultaneously by
Hall and Heroult, has not evolved as many other
electrochemical processes.
The anodes are still made of carbonaceous
material and must be replaced every few weeks. The
operating temperature is still not less than 950 C in
order to have a sufficiently high solubility and rate of
dissolution of alumina and high electrical conductivity
of the bath.
The anodes have a very short life because during
electrolysis the oxygen which should evolve on the anode
surface combines with the carbon to form polluting CO2
and small amounts of CO and fluoride-containing dangerous
gases. The actual consumption of the anode is as much as

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450 Kg/Ton of aluminium produced which is more than 1/3
higher than the theoretical amount of 333 Kg/Ton
The frequent substitution of the anodes in the
cells is still a clumsy and unpleasant operation. This
cannot be avoided or greatly improved due to the size and
weight of the anode and the high temperature of
operation.
Several improvements were made in order to
increase the lifetime of the anodes of aluminium
electrowinning cells, usually by improving their
resistance to chemical attacks by the cell environment
and air to those parts of the anodes which remain outside
the bath. However, most attempts to increase the chemical
resistance of anodes were coupled with a degradation of
their electrical conductivity.
US Patent 4,614,569 (Duruz/Derivaz/Debely/
Adorian) describes non-carbon 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 by the addition of cerium
compounds to the molten cryolite electrolyte. This made
it possible to have a protection of the surface only from
the electrolyte attack and to a certain extent from the
gaseous oxygen but not from the nascent monoatomic
oxygen.
EP Patent application 0 306 100 (Nyguen/Lazouni/
Doan) describes anodes composed of a chromium, nickel,
cobalt and/or iron based substrate covered with an oxygen
barrier layer and a ceramic coating of nickel, copper
and/or manganese oxide which may be further covered with
an in-situ formed protective cerium oxyfluoride layer.
Likewise, US Patents 5,069,771, 4,960,494 and
4,956,068 (all Nyguen/Lazouni/Doan) disclose aluminium
production anodes with an oxidized copper-nickel surface

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on an alloy substrate with a protective barrier layer.
However, full protection of the alloy substrate was
difficult to achieve.
A significant improvement was described in US
Patent 5,510,008, and in International Application
W096/12833 (Sekhar/Liu/Duruz) involved a anode having a
micropyretically produced body from a combination of
nickel, aluminium, iron and copper and oxidising the
surface before use or in-situ during electrolysis. By
said micropyretic methods materials have been obtained
whose surfaces when oxidised are active for the anodic
reaction and whose metallic interior has low electrical
resistivity to carry a current from high electrical
resistant surface to the busbars. However it would be
useful, if it were possible, to simplify the
manufacturing process of these materials obtained from
powders and increase their life to make their use
economic.
Metal or metal based anodes are highly desirable
in aluminium electrowinning cells instead of carbon-based
anodes. Many attempts were made to use metal-based anodes
for aluminium production, however they were never adopted
by the aluminium industry because of their poor
performance.
Ob-iects of the Invention
An object of the invention is to substantially
reduce the consumption of the active anode surface of an
aluminium electrowinning anode which is attacked by the
nascent oxygen by enhancing the reaction of nascent
oxygen to biatomic molecular gaseous oxygen.
Another object of the invention is to provide a
coating for an aluminium electrowinning anode which has a
high electrochemical activity and also a long life and

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which can be replaced as soon as such activity decreases
or when the coating is worn out.
A major object of the invention is to provide an
aluminium electrowinning anode which has no carbon so as
to eliminate carbon-generated pollution and reduce the
cost of operation.
Summary of the Invention
The invention provides a non-carbon metal-based
anode of a cell for the electrowinning of aluminium, in
particular by the electrolysis of alumina dissolved in a
molten fluoride-containing electrolyte. The anode
comprises an electrically conductive metal substrate
resistant to high temperature, the surface of which
becomes passive and substantially inert to the
electrolyte, and an electrochemically active coating
adherent to the surface of the metal substrate making and
keeping the surface of the anode conductive and
electrochemically active for the oxidation of oxygen ions
present at the electrolyte interface.
whereas conventional coatings are usually used to
protect a conductive substrate of a cell component from
chemical and/or mechanical attacks destroying the
substrate, this particular treatment is applied in the
form of a coating onto a passivatable substrate to
maintain the anode surface conductive and
electrochemically active and protect it from electrolyte
attack wherever the coating covers the surface even
though the coating may be imperfect or incomplete.
This allows the coated surfaces of the anode to
remain electrochemically active during electrolysis,
while the remaining parts of the surface of the metal
substrate become inert to the electrolyte. This
passivation property offers a self-healing effect, i.e.
when the surface of the anode is imperfectly covered,

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damaged or partly worn out, parts of the metal substrate
which come into contact with the electrolyte are
automatically passivated during electrolysis and become
inert to the electrolyte and not corroded.
Metal substrates providing for this self-healing
effect in molten fluoride-based electrolyte may be made
of one or more metals selected from nickel, cobalt,
chromium, molybdenum, tantalum and the Lanthanide series
of the Periodic Table, and their alloys or
intermetallics, such as nickel-plated copper.
The coatings usually comprise:
a) at least one electrically conductive and
electrochemically active constituent,
b) an electrocatalyst, and
c) a bonding material substantially resistant to
cryolite and oxygen for bonding these constituents
together and onto the passivatable metal substrate.
These constituents are usually co-applied though it
is possible to provide sequential application of the
different constituents.
The presence of one or more electrocatalysts is
desirable, although not essential for the invention.
Likewise the presence of bonding material is not always
necessary.
Coatings can be obtained by applying their active
constituents and their precursors by various methods
which can be different for each constituent and can be
repeated in several layers. For example, a coating can be
obtained by directly applying a powder onto the
passivatable metal substrate or constituents of the
coating may be applied from a slurry or suspension
containing colloidal or polymeric material. The colloidal

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material can be a binder solely or can be part of the
active material. The colloidal material may include at
least one colloid selected from colloidal alumina, ceria,
lithia, magnesia, silica, thoria, yttria, zirconia, tin
oxide, zinc oxide and colloid containing the active
material.
When a slurry or a suspension containing colloidal
material is applied the dry colloid content corresponds
to up to 50 weight% of the colloid plus liquid carrier,
usually from 10 to 20 weight%.
The coating can be applied on the substrate by
plasma spraying, physical vapor deposition (PVD),
chemical vapor deposition (CVD), electrodeposition or
callendering rollers. A slurry or a dispersion is
preferably applied by rollers, brush or spraying.
Usually the electrochemically active constituent(s)
is/are selected from oxides, oxyfluorides, phosphides,
carbides and combinations thereof.
The oxide may be present in the electrochemically
active layer as such, or in a multi-compound mixed oxide
and/or in a solid solution of oxides. The oxide may be in
the form of a simple, double and/or multiple oxide,
and/or in the form of a stoichiometric or non-
stoichiometric oxide.
The oxides may be in the form of spinels and/or
perovskites, in particular spinels which are doped, non-
stoichiometric and/or partially substituted. Doped
spinels may comprise dopants selected from Ti4+, Zr4+,
Sn4+, Fe4+, Hf4+, Nln4+, Fe3+, Ni3+, C03+, Mn3+, A13+, Cr3+,
Fe2+, Ni2+, Co2+, jvjg2+, Mn2+, Cu2+, Zn2+ and Li+.
Such a spinel may be a ferrite, in particular a
ferrite selected from cobalt, manganese, molybdenum,
nickel and zinc, and mixtures thereof. The ferrite may be
doped with at least one oxide selected from the group

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consisting of chromium, titanium, tantalum, tin, zinc and
zirconium oxide.
Nickel-ferrite or nickel-ferrite based constituents
are advantageously used for their resistance to
electrolyte and may be present as such or partially
substituted with Fe2+.
The coating may also contain a chromite which is
usually selected from iron, cobalt, copper, manganese,
beryllium, calcium, strontium, barium, magnesium, nickel
and zinc chromite.
The electrochemically active constituents of the
coating may be selected from iron, chromium, copper and
nickel, and oxides, mixtures and compounds thereof, as
well as a Lanthanide as an oxide or an oxyfluoride such
as cerium oxyfluoride, and mixtures thereof.
When an electrocatalyst is present in the coating it
is selected preferably from noble metals such as iridium,
palladium, platinum, rhodium, ruthenium, or silicon, tin
and zinc, the Lanthanide series of the Periodic Table and
mischmetal oxides, and mixtures and compounds thereof.
Coatings can be formed with or without reaction at
low or high temperature. A reaction can either take place
among the constituents of the coating; or between the
constituents of the coating and the passivatable metal
substrate. When no reaction takes place to form the
coating the active constituents must already be present
in the applied material, for example in a slurry or
suspension applied onto the substrate.
In order to manufacture these anodes any
electrically conductive and heat-resisting materials may
be used. However, metals which do not offer the self-
healing effect can only be used as metal cores which must
be coated with a layer forming the passivatable metal
substrate having this self-healing effect particularly

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when exposed to a fluoride-containing electrolyte, such
as cryolite.
The metal core may comprise metals, alloys,
intermetallics, cermets and conductive ceramics, such as
metals selected from copper, chromium, cobalt, iron,
aluminium, hafnium, molybdenum, nickel, niobium, silicon,
tantalum, titanium, tungsten, vanadium, yttrium and
zirconium, and combinations and compounds thereof.
For instance, the core may be made of an alloy
comprising 10 to 30 weight% of chromium, 55 to 90 weight%
of at least one of nickel, cobalt and/or iron and 0 to 15
weight% of at least one of aluminium, hafnium,
molybdenum, niobium, silicon, tantalum, tungsten,
vanadium, yttrium and zirconium.
The core may be covered with an oxygen barrier
layer. This layer may be obtained by oxidising the
surface of the core when it contains chromium and/or
nickel or by applying a precursor of the oxygen barrier
layer onto the core and heat treating. Usually, the
oxygen barrier layer comprises chromium oxide and/or
black non-stoichiometric nickel oxide.
The oxygen barrier layer may be covered in turn with
at least one protective layer consisting of copper or
copper and at least one of nickel and cobalt, and/or (an)
oxide(s) thereof to protect the oxygen barrier layer by
inhibiting its dissolution into the electrolyte. For
instance, the oxygen barrier layer may be coated first
with a nickel layer and then with a copper layer, heat
treated for several hours in an inert atmosphere, such as
5 hours at 1000 C in argon, to interdiffuse the nickel
and the copper layer, and upon heat treatment in an
oxidising media, such as an air oxidation for 24 hours at
1000 C, the interdiffused and oxidised nickel-copper
layer constitutes a good a protective layer.

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The invention relates also to a method of
manufacturing the described non-carbon metal-based anode.
The method comprises coating a substrate of electrically
conductive metal resistant to high temperature the
surface of which during electrolysis becomes passive and
substantially inert to the electrolyte with at least one
layer containing electrochemically active constituents or
precursors thereof and heat-treating the or each layer on
the substrate to obtain a coating adherent to the metal
substrate making the surface of the anode
electrochemically active for the oxidation of oxygen ions
present at the electrolyte interface.
The method of the invention can be applied for
reconditioning the non-carbon metal-based anode when at
least part of the active coating has been dissolved or
rendered non-active or dissolved. The method comprises
clearing the surface of the substrate before re-coating
said surface with a coating adherent to the passivatable
metal substrate once again making the surface of the
anode electrochemically active for the oxidation of
oxygen ions.
Another aspect of the invention is a cell for the
production of aluminium by the electrolysis of alumina
dissolved in a fluoride-containing electrolyte, in
particular a fluoride-based electrolyte or a cryolite-
based electrolyte or cryolite, having non-carbon metal-
based anodes comprising an electrically conductive
passivatable metal substrate and a conductive coating
having an electrochemically active surface as described
hereabove.
Preferably, the cell comprises at least one
aluminium-wettable cathode. Even more preferably, the
cell is in a drained configuration by having at least one
drained cathode on which aluminium is produced and from
which aluminium continuously drains.

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The cell may be of monopolar, multi-monopolar or
bipolar configuration. A bipolar cell may comprise the
anodes as described above as a terminal anode or as the
anode part of a bipolar electrode.
Preferably, the cell comprises means to improve
the circulation of the electrolyte between the anodes and
facing cathodes and/or means to facilitate dissolution of
alumina in the electrolyte. Such means can for instance
be provided by the geometry of the cell as described in
Application No. WO 99/41429/(de Nora/Duruz) or by
periodically moving the anodes as described in
Application No. WO 99/41430 (Duruz/Bello).
The cell may be operated with the electrolyte at
conventional temperatures, such as 950 to 970 C, or at
reduced temperatures as low as 750 C.
The invention also relates to the use of such an
anode for the production of aluminium in a cell for the
electrowinning of aluminium by the electrolysis of
alumina dissolved in a fluoride-containing electrolyte,
wherein oxygen ions in the electrolyte are oxidised and
released as molecular oxygen by the electrochemically
active anode coating.
The invention will now be described in the following
examples.
Example 1
An non-carbon metal-based anode is prepared
according to the invention by hot calendar rolling at
900 C of nickel ferrite particles having a particle size
of 10-50 micron into a nickel metal sheet of 2 mm
thickness used as an electrically conductive substrate
for the anode. The nickel ferrite particles are coated
onto the nickel sheet in an amount of 500 g/m2.

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After coating, the anode was tested in an
electrolytic cell using cryolite with 6 weight% alumina
as an electrolyte and a carbon cathode covered with
molten aluminium. The anode was polarised at 1 A/cm2 for
93 hours and sustained this current density during the
entire test, the cell voltage remaining comprised between
5.5 and 5.8 Volts.
At the end of the test, the anode was dimensionally
unchanged and no sign of corrosion could be detected at
the anode surface.
Example 2
A non-carbon metal-based anode according to the
invention was obtained from a nickel substrate which was
coated with a slurry with subsequent heat-treatment.
The slurry was made from a solution consisting of 10
ml of colloidal magnesia acting as a binder mixed with
20g of nickel ferrite powder providing the
electrochemically active constituents, as described in
Example 1.
The slurry was then applied onto the substrate by
means of a brush. 15 successive layers were applied onto
the substrate. Each time a layer had been applied onto
the substrate, the layer was cured on the substrate by a
heat treatment at 500 C for 15 minutes before applying
the next layer.
After coating the substrate with the 15 successive
layers the anode had a final coating of 0.6 to 1.0 mm
thick.
The anode was then tested in a laboratory scale cell
for the electrowinning of aluminium. 10 minutes after
immersing the anode into the electrolytic bath the anode
was extracted from the cell. The parts of the anodes
which were not protected by the coating had been

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passivated under the effect of the current by the
formation of an inert and adherent nickel oxide layer
formed on the uncoated surfaces which could be observed
by optical microscopy and scanning electron microscopy of
a cross section of the anode after test.
Example 3
Similarly to Example 2, a coating was applied onto a
nickel substrate in 10 layers, except that 0.2 g of
iridium powder acting as a catalyst were added to the
mixture of colloidal alumina with nickel-nickel ferrite.
Similar results were observed.