Electrocatalytically active non-carbon metal-based anodes for aluminium production cells

A non-carbon, metal-based high temperature resistant anode of a cell for the production of aluminium has a metal-based substrate coated with one or more electrically conductive adherent applied layers, at least one electrically conductive layer being electrochemically active. The electrochemically active layer contains one or more electrocatalysts fostering the oxidation of oxygen ions as well as fostering the formation of biatomic molecular gaseous oxygen to inhibit ionic and/or monoatomic oxygen attack of the metal-based substrate. The electrocatalyst can be iridium, palladium, platinum, rhodium, ruthenium, silicon, tin, zinc, Mischmetal oxides and metals of the Lanthanide series. The applied layer may further comprise electrochemically active constituents from oxides, oxyfluorides, phosphides, carbides, in particular spinels such as ferrites.

FIELD OF THE INVENTION 
This invention relates to non-carbon metal-based anodes having an 
electrocatalytically active surface for use in cells for the 
electrowinning of aluminium by the electrolysis of alumina dissolved in a 
molten fluoride-containing electrolyte, 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, at temperatures around 950.degree. 
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.degree. C. in order to have a sufficiently high solubility and rate of 
dissolution of alumina and high electrical conductivity of the bath. 
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.sub.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. 
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 an 
odes 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. 
U.S. Pat. No. 4,614,569 (Duruz et al.) describes 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 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, U.S. Pat. Nos. 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. 
A significant improvement described in U.S. Pat. No. 5,510,008, and in 
International Application WO96/12833 (Sekhar/Liu/Duruz) involved 
micropyretically producing a body from nickel, aluminium, iron and copper 
and oxidising the surface 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. 
Metal or metal-based anodes are highly desirable in aluminium 
electrowinning cells instead of carbon-based anodes. As described 
hereabove, 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. 
OBJECTS OF THE INVENTION 
An object of the invention is to reduce substantially the consumption of 
the electrochemically active anode surface of a non-carbon metal-based 
anode for aluminium electrowinning cells which is attacked by the nascent 
oxygen by enhancing the reaction of nascent oxygen to gaseous biatomic 
molecular gaseous oxygen. 
Another object of the invention is to provide a coating for a non-carbon 
metal-based anode for aluminium electrowinning cells which has a high 
electrochemical activity and also a long life and which can easily be 
applied onto an anode substrate. 
A further object of the invention is to provide a coating for a non-carbon 
metal-based anode for aluminium electrowinning cells which lowers the cell 
voltage compared to the voltage of cells having metal-based anodes which 
are not provided with this coating. 
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 high cell operating costs. 
SUMMARY OF THE INVENTION 
The invention relates to a non-carbon, metal-based high temperature 
resistant anode of a cell for the production of aluminium by the 
electrolysis of alumina dissolved in a fluoride-containing electrolyte. 
The anode has a metal-based substrate coated with one or more electrically 
conductive adherent applied layers, at least one electrically conductive 
layer being electrochemically active. The electrochemically active layer, 
which is usually the outer layer, contains one or more electrocatalysts 
fostering the oxidation of oxygen ions as well as fostering the formation 
of biatomic molecular gaseous oxygen from the monoatomic nascent oxygen 
obtained by the oxidation of the oxygen ions present at the surface of the 
anode in order to inhibit ionic and/or monoatomic oxygen attack of the 
metal-based substrate. 
In this context, metal-based substrate means that the anode substrate 
contains at least one metal as such or as alloys, intermetallics and/or 
cermets. 
The electrocatalyst(s) may be selected from iridium, palladium, platinum, 
rhodium, ruthenium, silicon, tin or zinc metals, Mischmetal and their 
oxides and metals of the Lanthanide series and their oxides as well as 
mixtures and compounds thereof. 
The electrocatalyst (s) may be applied in a layer which further comprises 
electrochemically active constituents selected from the group consisting 
of oxides, such as iron oxides, oxyfluorides, for instance cerium 
oxyfluoride, phosphides, carbides and combinations thereof. 
An 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 electrochemically active layer may in particular comprise spinels 
and/or perovskites, such as ferrite which may be selected from cobalt, 
manganese, molybdenum, nickel, magnesium and zinc ferrite, and mixtures 
thereof. Nickel ferrite may be partially substituted with Fe.sup.2+. 
Additionally, ferrites may doped with at least one oxide selected from the 
group consisting of chromium, titanium, tin and zirconium oxide. 
Optionally the electrochemically active layer may comprise a chromite, such 
as iron, cobalt, copper, manganese, beryllium, calcium, strontium, barium, 
magnesium, nickel and zinc chromite. 
The electrochemically active layer may be applied in the form of powder or 
slurry onto metal-based substrate, dried as necessary and heat-treated. 
Typically, the electrochemically active layer may advantageously be applied 
in the form a slurry or suspension containing colloidal material and then 
dried and/or heat treated. Such slurry or a suspension usually comprises 
at least one colloid selected from colloidal alumina, ceria, lithia, 
magnesia, silica, thoria, yttria, zirconia and colloids containing active 
constituents of the active filling. Ferrites and/or chromites may 
advantageously be applied with a catalyst onto the metal-based substrate 
in a slurry or suspension. 
Different techniques may be used to apply the electrochemically active 
layer such as dipping, spraying, painting, brushing, plasma spraying, 
electro-chemical deposition, physical vapour deposition, chemical vapour 
deposition or calendar rolling. 
Usually the metal-based substrate comprises a metal, an alloy, an 
intermetallic compound or a cermet. For instance, the metal-based 
substrate may comprise at least one metal selected from nickel, copper, 
cobalt, chromium, molybdenum, tantalum or iron. For instance, the core 
structure may be made of an alloy consisting of 10 to 30 weight % of 
chromium, 55 to 90% of at least one of nickel, cobalt or iron, and 0 to 
15% of aluminium, titanium, zirconium, yttrium, hafnium or niobium. 
Possibly, metal-based substrate may comprise an alloy or intermetallic 
compound containing at least two metals selected from nickel, iron and 
aluminium. 
Alternatively, the metal-based substrate can comprise a cermet containing 
copper and/or nickel as a metal. The metal-based substrate may, in 
particular, comprise a cermet containing a metal and at least one stable 
oxide selected from nickel cuprate, nickel ferrite or nickel oxide. 
In an embodiment, the a node substrate may consist of a plurality of 
superimposed, adherent, electrically conductive layers consisting of: 
a) a metal-based core layer of low electrical resistance for connecting the 
a node to a positive current supply, such as a metal, an alloy, an 
intermetallic compound and/or a cermet; 
b) at least one layer on the metal-based core layer forming a barrier 
substantially impervious to monoatomic oxygen and molecular oxygen, such 
as chromium oxide an d/or black non-stoichiometric nickel oxide; and 
c) one or more layers on the oxygen barrier to protect the oxygen barrier 
and which remain inactive in the reactions for the evolution of oxygen gas 
and inhibit the dissolution of the oxygen barrier, such as an oxidised 
interdiffusion or alloy of nickel and copper; 
the substrate outer layer being coat ed with the electrically conductive, 
electrochemically active adherent applied layer comprising the 
electrocatalyst according to the invention. 
The invention also relates to a cell for the production of aluminium by the 
electrolysis of alumina dissolved in a molten electrolyte comprising at 
least one anode as described above. 
Advantageously, the cell may comprise at least one aluminium-wettable 
cathode which can be a drained cathode on which aluminium is produced and 
from which it continuously drains. 
Usually, the cell is in a monopolar, multi-monopolar or in a bipolar 
configuration. Bipolar cells may comprise the anodes as described above as 
the anodic side of at least one bipolar electrode and/or as a terminal a 
node. 
In such a bipolar cell an electric current is passed from the surface of 
the terminal cathode to the surface of the terminal anode as ionic current 
in the electrolyte and as electronic current through the bipolar 
electrodes, thereby electrolysing the alumina dissolved in the electrolyte 
to produce aluminium on each cathode surface and oxygen on each anode 
surface. 
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 
co-pending application PCT/IB98/00161 (de Nora/Duruz) or by periodically 
moving the anodes as described in co-pending application PCT/IB98/00162 
(Duruz/Bello). 
The cell may be operated with the electrolyte at conventional temperatures, 
such as 950 to 970.degree. C., or at reduced temperatures as low as 
700.degree. C. 
Another aspect of the invention is a method of producing aluminium in such 
a cell, wherein alumina is dissolved in the electrolyte and then 
electrolysed to produce aluminium. 
Advantageously, during electrolysis the anodes are protected with a 
protective coating of cerium oxyfluoride on the electrochemically active 
layer. Usually, the protective coating is formed in-situ in the cell or 
pre-applied, and maintained by the addition of small amounts of cerium to 
the electrolyte as described in U.S. Pat. No. 4,614,569 (Duruz et al.). 
Alternatively, one or more constituents of the electrochemically active 
layer may be added to the electrolyte in an amount to slow down 
dissolution of the electrochemically active layer.

DETAILED DESCRIPTION 
The invention will be further described in the following Examples: 
EXAMPLE 1 
A test anode was made by apply ing on a nickel substrate an 
electrochemically active coating containing an electrocatalyst in the form 
of iridium oxide for the rapid conversion of the monoatomic oxygen formed 
into biatomic molecular gaseous oxygen. 
A slurry was prepared by mixing an amount of 1 g of commercially available 
nickel ferrite powder with 0.75 ml of an inorganic polymer containing 0.25 
g nickel-ferrite per 1 ml of water. An amount corresponding to 5 weight % 
of IrO.sub.2 in the form of IrCl.sub.4 was then added to the slurry. 
The slurry was then brush-coated onto the nickel substrate by applying 3 
successive layers of the slurry each layer being approximately 50 micron 
thick. Each slurry-applied layer was dried by heat-treating at 500.degree. 
C. for 15 minutes between each layer application. 
The anode was then tested in mol ten cryolite containing approximately 6 
weight % alumina at 970.degree. C. at a current density of about 0.8 
A/cm.sup.2. The anode was extracted from the cryolite after 100 hours and 
showed no sign of significant internal corrosion after microscopic 
examination of a cross-section of the anode specimen. Furthermore, during 
electrolysis the cell voltage was about 110 mV lower than the measured 
cell voltage of similarly prepared anodes having no electrocatalyst. 
EXAMPLE 2 
A test anode was made by coating by electro-deposition 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.RTM., 
first with a nickel layer about 200 micron thick and then a copper layer 
about 100 micron thick by plasma spraying. 
The coated structure was heat treated at 1000.degree. 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.degree. at air to form a chromium oxide (Cr.sub.2 
O.sub.3) barrier layer on the core structure and oxidising at least partly 
the interdiffused nickel-copper layer thereby forming the intermediate 
layer. 
A slurry was prepared by mixing an amount of 1 g of commercially available 
nickel ferrite powder with 0.75 ml of an inorganic polymer containing 0.25 
g nickel-ferrite per 1 ml of water. An amount corresponding to 5 weight % 
of IrO.sub.2 acting as an electrocatalyst for the rapid conversion of 
oxygen ions into monoatomic oxygen and subsequently gaseous oxygen was 
then added to the slurry as IrCl.sub.4. 
The slurry was then brush-coated onto the interdiffused nickel copper layer 
by applying 3 successive 50 micron thick layers of the slurry, each 
slurry-applied layer having been allowed to dry by heat-treating the anode 
at 500.degree. C. for 15 minutes between each layer application. 
The anode was then tested in a cryolite melt at 970.degree. C. containing 
approximately 6 weight % alumina, by passing a current at a current 
density of about 0.8 A/cm.sup.2. The anode was extracted from the cryolite 
after 100 hours and showed no sign of significant internal corrosion after 
microscopic examination of a cross-section of the anode specimen. 
As for the anode described in Example 1, the cell voltage was about 120 mV 
lower than the measured cell voltage of similarly prepared anodes having 
no electrocatalyst.