[0001] This invention relates to a method for producing non-carbon, metal-based anodes provided
with an electrochemical active surface coating for use in cells for the electrowinning
of aluminium by the electrolysis of alumina dissolved in a molten fluoride-containing
electrolyte.
[0002] The technology for the production of aluminium by the electrolysis of alumina, dissolved
in molten cryolite, at temperatures around 950°C is more than one hundred years old.
[0003] This process, conceived almost simultaneously by Hall and Héroult, has not evolved
as many other electrochemical processes.
[0004] 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.
[0005] The carbon 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 CO
2 and small amounts of CO and fluorine-containing dangerous gases. The actual consumption
of the anode is as much as 450 Kg/Ton of aluminium produced which is more than 1/3
higher than the theoretical amount of 333 Kg/Ton.
[0006] 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.
[0007] US Patent 4,614,569 (Duruz/Derivaz/Debely/Adorian) describes metal-based 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.
[0008] 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.
[0009] Likewise, US Patents 5,069,771, 4,960,494 and 4,956,068 (all Nyguen/Lazouni/Doan)
disclose aluminium production anodes with an oxidised copper-nickel surface on an
alloy substrate with a protective barrier layer. However, full protection of the alloy
substrate was difficult to achieve.
[0010] A significant improvement described in US Patent 5,510,008, and in International
Application WO96/12833 (Sekhar/Liu/Duruz) involved a micropyretically produced body
of nickel, aluminium, iron and copper whose surface is oxidised before use or in-situ.
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 and increase their life to make their use economic.
[0011] Metal or metal-based anodes are highly desirable in aluminium electrowinning cells
instead of carbon-based anodes. Many attempts were made to use metallic anodes for
aluminium production, however they were never adopted by the aluminium industry because
of their poor performance.
[0012] An object of the invention is to reduce substantially the consumption of an applied
electrochemically active anode surface coating of a metal-based non-carbon anode for
aluminium electrowinning cells which coating is in contact with the electrolyte.
[0013] Another object of the invention is to provide a surface coating for a metal-based
anode for aluminium electrowinning cells which in addition to a long life has a high
electrochemical activity and can easily be applied onto an anode substrate.
[0014] A major object of the invention is to provide an anode for the electrowinning of
aluminium which has no carbon so as to eliminate carbon-generated pollution and reduce
the high cell operating costs.
[0015] The invention relates to a method of applying a coating which is made of oxides that
comprise ferrites and/or chromites and which is electrically conductive and electrochemically
active for the oxidation of oxygen ions, onto an oxidation-resistant metal-based anode
substrate to produce a non-carbon, metal-based, high temperature resistant aluminium
production anode for the electrolysis of alumina dissolved in a fluoride-containing
electrolyte. The coating comprises: at least one ferrite selected from cobalt, manganese,
nickel, magnesium and zinc ferrite; and/or at least one chromite selected from iron,
cobalt, copper, manganese, beryllium, calcium, strontium, barium, magnesium, nickel
and zinc chromite.
[0016] According to the invention, the method comprises applying onto the anode substrate
a plurality of applied layers of the electrically conductive and electrochemically
active coating, the layers being selected from: layers of liquid solutions; layers
of liquid dispersions and pasty dispersions; layers of liquid suspensions and pasty
suspensions; and layers of pasty slurries and non-pasty slurries, and combinations
thereof, with or without heat treatment between two consecutively applied layers.
At least one of these layers contains a polymeric and/or colloidal carrier. The method
of the invention further comprises exposing the applied layers to a final heat treatment
so as to form the electrically conductive and the electrochemically active oxide coating
that comprises the ferrite(s) and/or chromite(s) from the applied layers.
[0017] At least one applied layer of the coating comprises: one or more dried colloids selected
from colloidal alumina, silica, yttria, ceria, thoria, zirconia, tin oxide and nickel-ferrite;
and/or one or more dried inorganic polymers selected from polymeric nickel aluminate,
magnesium chromite and magnesium ferrite.
[0018] During use of the anode, the oxidation of oxygen ions forms monoatomic nascent oxygen
which may as such or as biatomic molecular gaseous oxygen oxidise or further oxidise
the surface of the multi-layer coating, or part or most of the multi-layer coating
or the surface of the substrate, to form a limited barrier to ionic and nascent monoatomic
oxygen and at least a limited barrier to gaseous oxygen.
[0019] The multi-layer coating may have a slow dissolution rate in the fluoride-containing
electrolyte.
[0020] In the context of this invention:
- a metal-based anode means that the anode contains mainly one or more metals in the
anode substrate as such or as alloys, intermetallics and/or cermets.
- a liquid solution means a liquid containing ionic species which are smaller than 5
nanometers and/or polymeric species of 5 to 10 nanometers and no larger particles;
- a dispersion means a liquid containing particles in colloidal form, wherein the size
of the largest particles is comprised between 10 and 100 nanometers;
- a suspension means a liquid containing particles in which the largest particles are
comprised between 100 and 1000 nanometers; and
- a slurry means a liquid containing particles the size of which exceeds 1000 nanometers.
[0021] Advantageously, the metal-based substrate comprises at least one metal selected from
nickel, copper, cobalt, chromium, molybdenum, tantalum and iron, and mixtures thereof,
as metals and/or oxides, in one or more layers.
[0022] Preferably, the metal-based substrate comprises a surface pre-coating or pre-impregnation.
The pre-coating or pre-impregnation may for instance comprise ceria.
[0023] The multi-layer coating may comprise one or more oxides and/or oxyfluorides, and
combinations thereof, such as spinels and/or perovskites. For instance, the electrochemically
active layer may contain doped, non-stoichiometric and/or partially substituted spinels,
the doped spinels comprising dopants selected from the group consisting Ti
4+, Zr
4+, Sn
4+, Fe
4+, Hf
4+, Mn
4+, Fe
3+, Ni
3+, Co
3+, Mn
3+, Al
3+, Cr
3+, Fe
2+, Ni
2+, Co
2+, Mg
2+, Mn
2+, Cu
2+, Zn
2+ and Li
+.
[0024] The oxide may be present in the electrochemically active multi-layer coating 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.
[0025] The multi-layer coating may comprise a ferrite, such as a ferrite selected from cobalt,
manganese, nickel, magnesium and zinc ferrite, and mixtures thereof. The ferrite may
be doped with at least one oxide selected from chromium, titanium, tin and zirconium
oxide. Nickel ferrite may be partially substituted with divalent iron (Fe
2+).
[0026] Alternatively, the multi-layer coating may comprise a chromite, such as a chromite
selected from iron, cobalt, copper, manganese, beryllium, calcium, strontium, barium,
magnesium, nickel and zinc chromite.
[0027] Advantageously, the multi-layer coating may comprise an electrocatalyst for the formation
of molecular oxygen from atomic oxygen, selected from iridium, palladium, platinum,
rhodium, ruthenium, silicon, tin and zinc, the Lanthanide series and Mischmetal, and
their oxides, mixtures and compounds thereof.
[0028] At least one layer of the multi-layer coating may also comprise one or more dried
colloids or polymers, for example selected from the group consisting of colloidal
alumina, silica, yttria, ceria, thoria, zirconia, magnesia, lithia, tin oxide, zinc
oxide, monoaluminium phosphate or cerium acetate. The colloid or polymer may be derived
from colloid or polymer precursors and reagents which are solutions of at least one
salt such as chlorides, sulfates, nitrates, chlorates, perchlorates or metal organic
compounds such as alkoxides, formates, acetates of aluminium, silicon, yttrium, cerium,
thorium zirconium, magnesium and lithium. Possibly, the solutions of metal organic
compounds, principally metal alkoxides, are of the general formula M(OR)
z where M is a metal or complex cation, R is an alkyl chain and z is a number, preferably
from 1 to 12. The colloid or polymer precursor or reagent may also contain a chelating
agent such as acetyl acetone or ethylacetoacetate.
[0029] In one embodiment at least one layer below the electrochemically active surface,
which may be a solid or liquid applied layer, constitutes a barrier to oxygen, such
as a chromium or black non-stoichiometric nickel layer. The oxygen barrier layer may
in turn be covered with a protective barrier preventing its dissolution, such as a
nickel and/or copper layer.
[0030] Several techniques may be used to apply the layers such as painting, spraying, dipping,
brush, electroplating or rollers.
[0031] Each liquid-applied layer may be allowed to dry at least partially in the ambient
air or assisted by heating before applying the next layer.
[0032] The multi-layer coating may be also formed by applying onto the metal-based substrate
a precursor containing constituents which react among themselves to form the coating,
and reacting the constituents to form the multi-layer coating. Alternatively, the
multi-layer coating may be formed by applying onto the metal-based substrate a precursor
containing at least one constituent which reacts with the metal-substrate to form
the multi-layer coating, and reacting the constituent(s) with the metal-substrate
to form the coating.
[0033] A solid-applied layer may be applied onto the metal-substrate by plasma spraying,
arc spraying, physical vapour deposition, chemical vapour deposition or calendering
rollers.
[0034] The above methods may also be applied for reconditioning an anode as described above
whose electrochemically active multi-layer coating is worn or damaged. The method
comprises clearing at least worn out and/or damaged parts of the active coating from
the substrate and then reconstituting at least the electrochemically active coating.
[0035] A further object of the invention is a method of producing aluminium in a cell, comprising
manufacturing an anode by applying an electrically conductive and electrochemically
active oxide coating onto an oxidation-resistant metal-based anode substrate as described
above, introducing the anode into the cell, dissolving alumina in a fluoride-containing
electrolyte, such as cryolite, of the cell and electrolysing the dissolved alumina
to produce aluminium.
[0036] Preferably, the cell comprises an 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.
[0037] 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.
[0038] Advantageously, the cell may comprise means to circulate the electrolyte between
the anodes and facing cathodes and/or means to facilitate dissolution of alumina in
the electrolyte.
[0039] 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.
[0040] The invention will be further described in the following Examples:
Example 1
[0041] A polymeric slurry was prepared from: a non-dispersable but suspendable particulate
consisting of a nickel-ferrite powder and a nickel aluminate (NiOAl
2O
3) precursor material acting as a polymeric carrier and binder for the nickel ferrite
powder. The nickel-ferrite powder was specially prepared; however, commercially-available
products could also have been used. The precursor NiOAl
2O
3 materials, solution and gel powder reacted to form the spinel NiAl
2O
4 at < 1000°C.
[0042] When applied to a suitably prepared substrate such as nickel, this slurry produced
an oxide coating made from the pre-formed or the in-situ formed nickel ferrite which
adhered well onto the substrate and formed a coherent coating when dried and heated.
The slurry could be applied by a simple technique such as brushing or dipping to give
a coating of pre-determined thickness.
[0043] An anode was made by brushing 15 layers of this slurry onto a substrate in order
to obtain a final coating of a thickness of about 150 micron. The substrate consisted
of 74 weight% nickel, 17 weight% chromium and 9 weight% iron, such as Inconel®. Each
applied layer was allowed to dry for 10 minutes at 100°C before applying a further
layer. The slurry-brushed substrate was then submitted to a final heat treatment at
450-500°C 15 minutes. X-ray diffraction showed nickel-aluminate had formed in the
coating.
[0044] The anode was then tested in an electrolytic cell containing cryolite at 960°C wherein
alumina was dissolved in a amount of 6 weight%. After 15 hours the anode was extracted
and showed no signs of substantial corrosion.
Example 2
[0045] A carrier consisting of a nickel aluminate polymeric solution containing a non-dispersed
but suspended particulate of nickel aluminate was made by heating 75 g of Al(NO
3)
3.9 H
2O (0.2 moles Al) at 80°C to give a concentrated solution which readily dissolved 12
g of NiCO
3 (0.1 moles). The viscous solution (50 ml) contained 200 g/l Al
2O
3 and 160 g/l NiO (total oxide, >350 g/l).
[0046] This nickel-rich polymeric concentrated anion deficient solution was compatible with
commercially-available alumina sols e.g. NYACOL™.
[0047] A stoichiometrically accurate NiO.Al
2O
3 mixture was prepared by adding 5 ml of the anion deficient solution to 2.0 ml of
a 150 g/l alumina sol; this mixture was stable to gelling and could be applied to
smooth metal and ceramic surfaces by a dip-coating technique. When heated to 450-500°C,
X-ray diffraction showed nickel-aluminate had formed in the coating.
[0048] Other non-dispersable particulate than nickel aluminate could be suspended in the
anion-deficient nickel aluminate precursor solution and applied as coatings which
when heat-treated would form nickel-aluminate containing the added oxides.
[0049] An anode was then prepared and tested as described in Example 1 and showed similar
results.
Example 3
[0050] A colloidal solution containing a metal ferrite precursor (as required for NiONiFe
2O
4) was prepared by mixing 20.7 g Ni(NO
3)
2.6 H
2O (5.17 g NiO) with 18.4 g Fe(NO
3)
3.9 H
2O (4.8 g Fe
2O
3) and dissolving the salts in water to a volume of 30 ml. The solution was stable
to viscosity changes and to precipitation when aged for several days at 20°C.
[0051] An organic solvent such as PRIMENE™ JMT (R
3CNH
2 molecular weight ∼350) is immiscible with water and extracts nitric acid from acid
and metal nitrate salt solutions. An amount of 75 ml of the PRIMENE™ JMT (2.3 M) diluted
with an inert hydrocarbon solvent was mixed with 10 ml of the colloidal nickel-ferrite
precursor solution. Within a few minutes the spherical droplets of feed were converted
to a mixed oxide gel; they were filtered off, washed with acetone and dried to a free-flowing
powder. When the gel was heated in air, nickel-ferrite formed at < 800°C and the powder
could be used as a non-dispersable but suspended particulate in colloidal and/or inorganic
polymeric slurries as described in Example 1 or 2. Commercially-available nickel-ferrite
powder could also have been used.
[0052] As described in Example 1, an anode was then prepared by coating a nickel plated
copper core covered with a chromium based oxygen barrier layer and a nickel-copper
protective barrier layer preventing dissolution of the chromium layer with this slurry,
tested and showed similar results.
Example 4
[0053] An amount of 5 g of NiCO
3 was dissolved in a solution containing 35 g Fe(NO
3)
3.9 H
2O to give a mixture (40 ml) having the composition required for the formation of NiFe2O
4. The solution was converted to colloidal gel particles by solvent extracting the
nitrate with PRIMENE™ JMT as described in Example 3. The nickel-ferrite precursor
gel was calcined in air to give a non-dispersable but suspended particulate in the
form of a nickel-ferrite powder, which could be hosted into nickel-aluminate carrier
for coating applications from colloidal and/or polymeric slurries.
[0054] A 200 micron thick coating consisting of 15 superimposed layers was obtained on an
Inconel® substrate as in Example 1 by dipping the substrate in this slurry. As in
Example 1, each layer was allowed to dry before applying a further layer.
[0055] The coated substrate was then submitted to a final heat treatment at 600°C for 1
hour to consolidate the coating and form an anode.
[0056] The anode was then tested in a cell as in Example 1 and showed similar results.
Example 5
[0057] An amount of 100 g of Cr(NO
3)
3.9 H
2O was heated to dissolve the salt in its own water of crystallisation to form a solution
containing 19 g Cr
2O
3. The solution was heated to 120°C and 12.5 g of magnesium-hydroxy carbonate containing
the equivalent of 5.0 g MgO was added. Upon stirring a solution was obtained in the
form of an anion-deficient polymer mixture with a density of approximately 1.5 g/cm
3 suitable to act as a carrier. An amount of 50 g of this carrier was evaporated to
dryness to convert the solution into a fine oxide powder. The oxides were then calcined
at 600°C into a magnesium chromite powder to form a non-dispersable but suspended
particulate.
[0058] After grinding to a fine powder, the magnesium chromite particulate was suspended
in the polymer carrier to form a slurry suitable for coating treated metal substrates.
[0059] An anode was then prepared and tested as in Example 4 and showed similar results.
Example 6
[0060] An amount of 150 g of Fe(NO
3)
3.9 H
2O was heated to dissolve the salt in its own water of crystallisation to form a solution
containing 29 g Fe
2O
3. The solution was heated to 120°C and 18.9 g of magnesium hydroxy-carbonate dissolved
in the hot solution to form 7.5 g MgO in form of an inorganic polymer together with
Fe
2O
3. An amount of 50 g of the polymer solution was evaporated to dryness and then calcined
at 600°C yielding approximately 13 g of magnesium ferrite powder.
[0061] After calcination, the ferrite powder was ground in a pestle and mortar and then
dispersed in the same inorganic polymer to give a slurry that was used to coat a treated
metal substrate.
[0062] An anode was then prepared and tested as in Example 3 and showed similar results.
Example 7
[0063] A cleaned surface of an Inconel™ billet (typically comprising 76 weight% nickel -
15.5 weight% chromium - 8 weight% iron) was pre-coated with a ceria colloid as described
in US Patent 4,356,106 (Woodhead/Raw), dried and heated in air at 500°C. The pre-coated
billet was then further coated with the polymeric slurry described in Example 1 or
2, dried and heated in air at 500°C. The ferrite coating was very adherent and successive
layers of the slurry could be applied to build up a coating of ferrite/aluminate having
a thickness above 100 micron.
[0064] A similar untreated Inconel™ billet was coated with a 10 micron thick layer using
the polymeric slurry described in Example 1 or 2 but without pre-coating the billet
with ceria colloid. After heat-treatment the coating was cracked and easily broke
away from the substrate, which demonstrated the effect of the ceria pre-coat.
[0065] An anode was then prepared and tested as in Example 1 and showed similar results.
Example 8
[0066] A test anode was made by coating by electrodeposition a core structure in the shape
of a rod having a diameter of 12 mm consisting of 74 weight% nickel, 17 weight% chromium
and 9 weight% iron, such as Inconel®, first with a nickel layer about 200 micron thick
and then a copper layer about 100 micron thick.
[0067] The coated structure was heat treated at 1000°C in argon for 5 hours. This heat treatment
provides for the interdiffusion of nickel and copper to form an intermediate layer.
The structure was then heat treated for 24 hours at 1000° at air to form a chromium
oxide (Cr
2O
3) barrier layer on the core structure and oxidising at least partly the interdiffused
nickel-copper layer thereby forming the intermediate layer.
[0068] A nickel-ferrite powder was made by drying and calcining at 900°C the gel product
obtained from an inorganic polymer precursor solution containing ferric nitrate and
nickel carbonate. A thick paste was made by mixing 1 g of this nickel-ferrite powder
with 0.85 g of a nickel aluminate polymer solution containing the equivalent of 0.15
g of oxide. This thick paste was then diluted with 1 ml of water and ground in a pestle
and mortar to obtain a suitable viscosity to form a nickel-based paint.
[0069] An electrochemically active oxide layer was obtained on the core structure by applying
the nickel-based paint onto the core structure with a brush. The painted structure
was allowed to dry for 30 minutes before heat treating it at 500°C for 1 hour to decompose
volatile components and to consolidate the oxide coating.
[0070] The heat treated coating layer was about 15 micron thick. Further coating layers
were applied following the same procedure in order to obtain a 200 micron thick electrochemically
active coating covering the core structure.
[0071] The anode was then tested in a cryolite melt containing approximately 6 weight% alumina
at 970°C by passing current at a current density of about 0.8 A/cm
2. After 100 hours the anode was extracted from the cryolite and showed no sign of
significant internal corrosion after microscopic examination of a cross-section of
the anode specimen.
1. A method of applying a coating which is made of oxides that comprise ferrites and/or
chromites and which is electrically conductive and electrochemically active for the
oxidation of oxygen ions, onto an oxidation-resistant metal-based anode substrate
to produce a non-carbon, metal-based, high temperature resistant aluminium production
anode for the electrolysis of alumina dissolved in a fluoride-containing electrolyte,
said coating comprising:
- at least one ferrite selected from cobalt, manganese, nickel, magnesium and zinc
ferrite; and/or
- at least one chromite selected from iron, cobalt, copper, manganese, beryllium,
calcium, strontium, barium, magnesium, nickel and zinc chromite,
said method comprising applying onto the anode substrate a plurality of precursor
layers of said electrically conductive and electrochemically active coating, said
layers being selected from:
a) layers of liquid solutions;
b) layers of liquid dispersions and pasty dispersions;
c) layers of liquid suspensions and pasty suspensions; and
d) layers of pasty slurries and non-pasty slurries,
and combinations thereof, with or without heat treatment between two consecutively
applied layers, at least one of the layers containing a polymeric and/or a colloidal
carrier, and exposing the applied layers to a final heat treatment so as to form the
electrically conductive and electrochemically active oxide coating that comprises
the ferrite(s) and/or chromite(s) from the applied layers, at least one applied layer
comprising:
- one or more dried colloids selected from colloidal alumina, silica, yttria, ceria,
thoria, zirconia, tin oxide and nickel-ferrite; and/or
- one or more dried inorganic polymers selected from polymeric nickel aluminate, magnesium
chromite and magnesium ferrite.
2. The method of claim 1, wherein at least one layer is applied by painting, spraying,
dipping, brush, electroplating or rollers.
3. The method of claim 1, comprising applying at least one layer as a suspension.
4. The method of claim 1, wherein the substrate is pre-coated or pre-impregnated by painting,
spraying, dipping or infiltration with reagents and precursors, gels and/or colloids
before application of the multi-layer coating, in particular with a solution containing
ceria or a ceria precursor.
5. The method of claim 1, wherein several liquid-containing layers are applied, each
layer being allowed to dry at least partially in the ambient air or assisted by heating
before applying the next layer.
6. The method of claim 1, comprising applying onto the metal-based substrate a precursor
containing constituents which react among themselves to form the multi-layer coating,
or which react with the metal-substrate to form the multi-layer coating, and reacting
the constituents to form the multi-layer coating.
7. The method of claim 1, wherein a further layer is applied as a solid onto the metal-substrate
by plasma spraying, arc spraying, physical vapour deposition, chemical vapour deposition
or calendering rollers.
8. The method of claim 1, wherein the metal-based substrate is selected from a metal,
an alloy, an intermetallic compound or a cermet.
9. The method claim 8, wherein the metal-based substrate comprises at least one metal
selected from nickel, copper, cobalt, chromium, molybdenum, tantalum and iron, and
mixtures thereof, as metals and/or oxides, in one or more layers.
10. The method of claim 1, wherein the multi-layer* applied coating comprises an electrocatalyst
for the formation of molecular oxygen from atomic oxygen, selected from iridium, palladium,
platinum, rhodium, ruthenium, silicon, tin and zinc, the Lanthanide series, Mischmetal,
and their oxides, mixtures and compounds thereof.
11. A method of producing aluminium in a cell, comprising manufacturing an anode by applying
an electrically conductive and electrochemically active oxide coating onto an oxidation-resistant
metal-based anode substrate by the method defined in claim 1, introducing the anode
into the cell, dissolving alumina in a fluoride-containing electrolyte of the cell
and electrolysing the dissolved alumina to produce aluminium.
12. The method of claim 11, wherein the cell comprises an aluminium-wettable cathode.
13. The method of claim 12, wherein the cell comprises a drained cathode on which aluminium
is produced and from which aluminium continuously drains.
14. The method of claim 11, wherein the cell is in a bipolar configuration, the anode
forming the anodic side of a bipolar electrode or a terminal anode.
15. The method of claim 11, comprising circulating the electrolyte between the anode and
a facing cathode.
16. The method of claim 11, wherein during operation the electrolyte is at a temperature
of 750°C to 970°C.
1. Verfahren zum Aufbringen einer Beschichtung, die aus Oxiden hergestellt ist, die Ferrite
und/oder Chromite enthalten, und die elektrisch leitend und elektrochemisch aktiv
für die Oxidation von Sauerstoffionen ist, auf ein oxidationsbeständiges Anodensubstrat
auf Metallbasis zur Herstellung einer nicht-Kohlenstoff hochtemperaturbeständigen
Aluminiumproduktionsanode auf Metallbasis für die Elektrolyse von in fluoridhaltigem
Elektrolyten gelöstem Aluminiumoxid, wobei die Beschichtung
- mindestens ein Ferrit ausgewählt aus Kobalt-, Mangan-, Nickel-, Magnesium- und Zinkferrit;
und/oder
- mindestens ein Chromit ausgewählt aus Eisen-, Kobalt-, Kupfer-, Mangan-, Beryllium-,
Calcium-, Strontium-, Barium-, Magnesium-, Nickel- und Zinkchromit
enthält, wobei in dem Verfahren auf das Anodensubstrat eine Vielzahl von Vorläuferschichten
aus der elektrisch leitenden und elektrochemisch aktiven Beschichtung aufgebracht
wird, wobei die Schichten ausgewählt sind aus:
a) Schichten aus flüssigen Lösungen;
b) Schichten aus flüssigen Dispersionen und pastenartigen Dispersionen;
c) Schichten aus flüssigen Suspensionen und pastenartigen Suspensionen; und
d) Schichten aus pastenartigen Aufschlämmungen und nichtpastenartigen Aufschlämmungen,
und Kombinationen davon, mit oder ohne Wärmebehandlung zwischen zwei nachfolgend
aufgebrachten Schichten, wobei mindestens eine der Schichten einen polymeren und/oder
kolloidalen Träger enthält und die aufgebrachten Schichten einer am Ende erfolgenden
Wärmebehandlung unterzogen werden, um die elektrisch leitende und elektrochemisch
aktive Oxidbeschichtung zu bilden, die das Ferrit/die Ferrite und/oder das Chromit/die
Chromite aus den aufgebrachten Schichten enthält, wobei mindestens eine aufgebrachte
Schicht
- ein oder mehrere getrocknete Kolloide ausgewählt aus kolloidalem Aluminiumoxid,
Siliciumoxid, Yttriumoxid, Ceroxid, Thoriumoxid, Zirconiumoxid, Zinnoxid und Nikkel-Ferrit,
und/oder
- ein oder mehrere getrocknete anorganische Polymere ausgewählt aus polymerem Nickelaluminat,
Magnesiumchromit und Magnesiumferrit
enthält.
2. Verfahren nach Anspruch 1, bei dem mindestens eine Schicht durch Streichen, Sprühen,
Tauchen, Bürsten, Elektroplattieren oder mit Walzen aufgebracht wird.
3. Verfahren nach Anspruch 1, bei dem mindestens eine Schicht als Suspension aufgebracht
wird.
4. Verfahren nach Anspruch 1, bei dem das Substrat durch Bestreichen, Sprühen, Tauchen
oder Infiltrieren mit Reagentien und Vorläufern, Gelen und/oder Kolloiden vor Auftragung
der Mehrschichtbeschichtung, insbesondere mit einer Ceroxid oder Ceroxidvorläufer
enthaltenden Lösung, vorbeschichtet oder vorimprägniert wird.
5. Verfahren nach Anspruch 1, bei dem mehrere flüssigkeitshaltige Schichten aufgebracht
werden, wobei jede Schicht mindestens teilweise an der Umgebungsluft oder unterstützt
durch Erwärmen getrocknet wird, bevor die nächste Schicht aufgebracht wird.
6. Verfahren nach Anspruch 1, bei dem auf das Substrat auf Metallbasis ein Vorläufer
aufgebracht wird, der Bestandteile enthält, die miteinander unter Bildung der Mehrschichtbeschichtung
reagieren, oder die mit dem Metallsubstrat unter Bildung der Mehrschichtbeschichtung
reagieren, und die Bestandteile unter Bildung der Mehrschichtbeschichtung umgesetzt
werden.
7. Verfahren nach Anspruch 1, bei dem eine weitere Schicht als Feststoff durch Plasmasprühen,
Lichtbogensprühen, physikalisches Aufdampfen, chemisches Aufdampfen oder Kalanderwalzen
auf das Metallsubstrat aufgebracht wird.
8. Verfahren nach Anspruch 1, bei dem das Substrat auf Metallbasis ausgewählt ist aus
einem Metall, einer Legierung, einer Intermetallverbindung oder einem Cermet.
9. Verfahren nach Anspruch 8, bei dem das Substrat auf Metallbasis mindestens ein Metall
ausgewählt aus Nickel, Kupfer, Kobalt, Chrom, Molybdän, Tantal und Eisen und Mischungen
derselben als Metall und/oder Oxid in einer oder mehreren Schichten enthält.
10. Verfahren nach Anspruch 1, bei dem die mehrschichtig aufgebrachte Beschichtung einen
Elektrokatalysator zur Bildung von molekularem Sauerstoff aus atomarem Sauerstoff
enthält, der aus Iridium, Palladium, Platin, Rhodium, Ruthenium, Silicium, Zinn und
Zink, den Lanthanidreihen, Mischmetall und deren Oxiden, Mischungen und Verbindungen
derselben ausgewählt ist.
11. Verfahren zur Produktion von Aluminium in einer Zelle, bei dem eine Anode hergestellt
wird, indem eine elektrisch leitende und elektrochemisch aktive Oxidbeschichtung nach
dem Verfahren gemäß Anspruch 1 auf ein oxidationsbeständiges Anodensubstrat auf Metallbasis
aufgebracht wird, die Anode in die Zelle eingebracht wird, Aluminiumoxid in einem
fluoridhaltigen Elektrolyten der Zelle gelöst wird und das gelöste Aluminiumoxid zur
Produktion von Aluminium elektrolysiert wird.
12. Verfahren nach Anspruch 11, bei dem die Zelle eine aluminiumbenetzbare Kathode aufweist.
13. Verfahren nach Anspruch 12, bei dem die Zelle eine Drain-Kathode aufweist, an der
Aluminium produziert wird und von der das Aluminium kontinuierlich abläuft.
14. Verfahren nach Anspruch 11, bei dem die Zelle eine bipolare Konfiguration hat, wobei
die Anode die anodische Seite einer bipolaren Elektrode oder eine endständige Anode
bildet.
15. Verfahren nach Anspruch 11, bei dem der Elektrolyt zwischen der Anode und einer gegenüberliegenden
Kathode zirkuliert wird.
16. Verfahren nach Anspruch 11, bei dem der Elektrolyt sich während des Betriebs auf einer
Temperatur von 750°C bis 970°C befindet.
1. Procédé pour appliquer un revêtement qui est réalisé en oxydes qui comprennent des
ferrites et/ou des chromites, et qui est électriquement conducteur et électrochimiquement
actif pour l'oxydation d'ions oxygène, sur un substrat d'anode à base de métal résistant
à l'oxydation pour produire une anode de production d'aluminium résistante aux températures
élevées, à base de métal non en carbone pour l'électrolyse d'alumine dissoute dans
un électrolyte contenant du fluorure, ledit revêtement comprenant :
- au moins un ferrite choisi à partir de ferrite de cobalt, de manganèse, de nickel,
de magnésium et de zinc ; et/ou
- au moins un chromite choisi à partir de chromite de fer, de cobalt, de cuivre, de
manganèse, de béryllium, de calcium, de strontium, de baryum, de magnésium, de nickel
et de zinc,
ledit procédé consistant à appliquer sur le substrat anodique une pluralité de couches
de précurseurs dudit revêtement électriquement conducteur et électrochimiquement actif,
lesdites couches étant choisies à partir de :
a) couches de solutions liquides ;
b) couches de dispersions liquides et de dispersions pâteuses ;
c) couches de suspensions liquides et de suspensions pâteuses ; et
d) couches de coulis pâteux et de coulis non pâteux,
et des combinaisons de celles-ci, avec ou sans traitement thermique entre deux couches
appliquées consécutivement, au moins l'une des couches contenant un support colloïdal
et/ou polymère, et exposant les couches appliquées à un traitement thermique final
de façon à former le revêtement d'oxyde électriquement conducteur et électrochimiquement
actif qui comprend les ferrite(s) et/ou chromite(s) à partir des couches appliquées,
au moins une couche appliquée comprenant :
- un ou plusieurs colloïdes séchés choisis à partir d'alumine, silice, yttria, oxyde
de cérium, thorine, zircone, oxyde d'étain et nickel-ferrite colloïdaux ; et/ou
- un ou plusieurs polymères inorganiques séchés choisis à partir d'aluminate de nickel,
de chromite de magnésium et de ferrite de magnésium polymères.
2. Procédé selon la revendication 1, dans lequel au moins une couche est appliquée par
peinture, pulvérisation, immersion, brossage, galvanoplastie ou rouleaux.
3. Procédé selon la revendication 1, consistant à appliquer au moins une couche comme
une suspension.
4. Procédé selon la revendication 1, dans lequel le substrat est prérevêtu ou préimprégné
par peinture, pulvérisation, immersion ou infiltration avec des réactifs et des précurseurs,
des gels et/ou des colloïdes avant l'application du revêtement à couches multiples,
en particulier avec une solution contenant de l'oxyde de cérium ou un précurseur d'oxyde
de cérium.
5. Procédé selon la revendication 1, dans lequel plusieurs couches contenant des liquides
sont appliquées, chaque couche étant laissée sécher au moins partiellement dans l'air
ambiant ou aidée par chauffage avant l'application de la couche suivante.
6. Procédé selon la revendication 1, consistant à appliquer sur le substrat à base de
métal un précurseur contenant des constituants qui réagissent entre eux pour former
le revêtement à couches multiples, ou qui réagissent avec le substrat métallique pour
former le revêtement à couches multiples, et à faire réagir les constituants pour
former le revêtement à couches multiples.
7. Procédé selon la revendication 1, dans lequel une autre couche est appliquée comme
un solide sur le substrat métallique par projection au plasma, projection à l'arc,
dépôt physique en phase gazeuse, dépôt chimique en phase gazeuse ou rouleaux de calandrage.
8. Procédé selon la revendication 1, dans lequel le substrat à base de métal est choisi
à partir d'un métal, d'un alliage, d'un composé inter-métallique ou d'un cermet.
9. Procédé selon la revendication 8, dans lequel le substrat à base de métal comprend
au moins un métal choisi à partir de nickel, cuivre, cobalt, chrome, molybdène, tantale
et fer, et des mélanges de ceux-ci, comme métaux et/ou oxydes, dans une ou plusieurs
couches.
10. Procédé selon la revendication 1, dans lequel le revêtement appliqué à couches multiples
comprend un électrocatalyseur pour la formation d'oxygène moléculaire à partir d'oxygène
atomique, choisi à partir d'iridium, palladium, platine, rhodium, ruthénium, silicium,
étain et zinc, la série des lanthanides, de mischmétal, et leurs oxydes, mélanges
et composés de ceux-ci.
11. Procédé pour produire de l'aluminium dans une cuve, consistant à fabriquer une anode
en appliquant un revêtement d'oxyde électriquement conducteur et électrochimiquement
actif sur un substrat anodique à base de métal résistant à l'oxydation par le procédé
défini dans la revendication 1, introduire l'anode dans la cuve, dissoudre l'alumine
dans un électrolyte contenant du fluorure de la cuve et électrolyser l'alumine dissoute
pour produire l'aluminium.
12. Procédé selon la revendication 11, dans lequel la cuve comprend une cathode mouillable
par l'aluminium.
13. Procédé selon la revendication 12, dans lequel la cuve comprend une cathode drainée
sur laquelle l'aluminium est produit et à partir de laquelle l'aluminium s'écoule
de façon continue.
14. Procédé selon la revendication 11, dans lequel la cuve est dans une configuration
bipolaire, l'anode formant le côté anodique d'une électrode bipolaire ou une anode
terminale.
15. Procédé selon la revendication 11, consistant à faire circuler l'électrolyte entre
l'anode et une cathode faisant face.
16. Procédé selon la revendication 11, dans lequel, pendant le fonctionnement, l'électrolyte
est à une température de 750°C à 970°C.