Note: Descriptions are shown in the official language in which they were submitted.
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BIPOLAR CELL FOR THE PRODUCTION OF ALUMINIUM
WITH CARBON CATHODES
Field of the Invention
This invention relates to bipolar cells for the
electrowinning of aluminium by the electrolysis of alumina
dissolved in a molten fluoride-containing electrolyte
provided with bipolar electrodes having carbon cathodes
and oxygen-evolving anodes, methods for the fabrication
and reconditioning of such electrodes, and the operation
of such cells to maintain the anodes dimensionally stable.
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, and the cell design have not undergone
any great change or improvement and carbonaceous materials
are still used as electrodes and cell linings.
A major drawback of conventional cells is due to
the fact that irregular electromagnetic forces create
waves in the molten aluminium pool and the anode-cathode
distance (ACD), also called inter-electrode gap (IEG),
must be kept at a safe minimum value of approximately 5 cm
to avoid short circuiting between the aluminium cathode
and the anode or re-oxidation of the metal by contact with
the C02 gas formed at the anode surface.
The high electrical resistivity of the electrolyte
causes a voltage drop in the inter-electrode gap which
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alone represents as much as 40~ of the total voltage drop
with a resulting low energy efficiency.
All aluminium production cells commercially used
today have carbon anodes and carbon cathodes. Only
recently has it become possible to make the carbon cathode
surface aluminium-wettable by means of an applied coating
obtained from an applied slurry or colloidal dispersion
containing titanium diboride as described in US Patent
5,651,874 (de Nora/Sekhar). Making the cathode surface
aluminium-wettable allowed the design of drained cells
with reduced anode-cathode distance (ACD) and therefore to
save energy as described in US Patent 5,683,559 (de Nora).
Twenty years of intense and costly research made
it possible to design non-carbon anodes which eliminate
the severe pollution during their fabrication and
utilisation. Improvements have been achieved, as described
in co-pending applications W099/36591 and W099/36592 (both
in the name of de Nora), W099/36593 and W099/36594 (both
in the name of de Nora/Duruz) which disclose anodes having
a metal core resistant to cryolite and oxygen, and an
electrochemically active coating.
Several past attempts were made to design bipolar
cells in order to overcome the problems encountered with
conventional aluminium electrowinning cells. In order to
make their use economic, bipolar cells need electrodes
which are resistant to the products of electrolysed
aluminium salts. Using consumable electrodes in bipolar
cells is not acceptable as their replacement is much more
difficult and their consumption enlarges the anode-cathode
gap (ACG) and cannot be compensated by repositioning the
electrodes as in Hall-Heroult cells.
US Patents 3,822,195 and 3,893,899 (both in the
name of Dell/Haupin/Russel) and US Patent 4,110,178
(LaCamera/Trzeciak/Kinosz) all describe bipolar cells
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operating with carbon electrodes and with an electrolytic
bath containing aluminium chloride instead of alumina.
However, these cell designs have not been commercially
adopted.
US Patent 3,578,580 (Schmidt-Hatting/Huwyler)
discloses bipolar cells, in particular having inclined
electrodes, wherein the anodes are made of oxygen-
resistant material such as platinum or a conductive oxide
or wustite (ferrous oxide Fe0). The cathode is made of
carbon, or other electrically conductive material
resistant to fused melt, such as a carbide of titanium,
zirconium, tantalum or niobium.
US Patent 3,930,967 (Alder) describes a bipolar
cell electrode comprising an anode, an intermediate plate
and a cathode plate held together in an alumina or
magnesium oxide frame . The anode plate is made of ceramic
oxide material, the preferred material being tin oxide
with copper oxide and antimony oxide. The cathode is
graphite or made of borides, carbides, nitrides,
silicides, in particular of metals such as titanium,
zirconium or silicon. The intermediate plate, for instance
made of silver, nickel or cobalt, prevents direct contact
between the anode and the cathode plates to avoid any
reaction between them when exposed to high temperature.
US Patent 5,019,225 (Darracq/Duruz/Durmelat)
discloses a bipolar electrode for an aluminium
electrowinning cell having a cerium oxyfluoride anode
surface and a cerium hexaboride cathode surface, which was
specially designed for use in the process of US Patent
4,614,569 (Duruz/Derivaz/Debely/Adorian) wherein cerium
species dissolved in the electrolyte maintain the anode
surface stable.
Despite all previous attempts, the bipolar
technology has never been successfully implemented in
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industrial aluminium production cells due to many problems
of cell operation.
Summary of the Invention
It is an object of the invention to provide a
bipolar electrode for aluminium electrowinning bipolar
cells, which has an oxygen resistant anode surface.
Another object of the invention is to provide a
bipolar electrode for aluminium electrowinning bipolar
cells, which contains carbon but which is not exposed to
carbon oxidation so as to eliminate carbon-generated
pollution and high costs of carbon consumption.
Yet another object of the invention is to provide
a bipolar electrode for aluminium electrowinning bipolar
cells whose anodic surface has a sufficient operative
lifetime to make its use commercially acceptable.
An important object of the invention is to provide
a bipolar electrode for aluminium electrowinning bipolar
cells, which may be maintained dimensionally stable,
without excessively contaminating the product aluminium.
Yet another object of the invention is to provide
an aluminium electrowinning bipolar cell operating under
such conditions that the contamination of the product
aluminium is limited.
The invention relates to a bipolar cell for the
electrowinning of aluminium by the electrolysis of alumina
dissolved in a molten fluoride-containing electrolyte,
having a terminal cathode, a terminal anode and
thereinbetween at least one bipolar electrode. The bipolar
electrode comprises a carbon cathode body having on one
side an electrochemically active surface on which
aluminium is produced and connected on the other side
through an oxygen impermeable barrier layer to an anode
layer having a metal oxide outer surface which is
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electrochemically active for the oxidation reaction of
oxygen ions into nascent monoatomic oxygen.
More generally, the metal oxide may be present in
the electrochemically outer surface in a multi-compound
mixed oxide, in mixed crystals 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.
Oxyaen Barrier Layers & Protective Lavers
The oxygen barrier layer may be made of a metal or
an oxidised metal, an intermetallic compound, a mixed
perovskite ceramic, a phosphide, a carbide, a nitride such
as titanium nitride, or a combination thereof.
Suitable metals or oxides of metals acting as a
barrier to oxygen may be selected from chromium, chromium
oxide, niobium, niobium oxide, nickel and nickel oxide.
The oxygen barrier layer may in particular consist of a 5
to 20 micron thick layer of noble metal, such as platinum,
palladium, iridium or rhodium. Intermetallic compounds
such as silver-palladium, chromium-manganese and chromium-
molybdenum also act as a barrier to oxygen.
The oxygen barrier may contain a mixed perovskite
ceramic which may be chosen among zirconate, cobaltite,
chromite, chromate, manganate, ruthenate, niobiate,
tantalate and tungstate. The perovskite preferably
contains strontium zirconate to enhance the conductivity
of the oxygen barrier layer. A conductive phosphide
resistant to oxygen may be chosen among a phosphide of
titanium, chromium and tungsten. A suitable carbide may be
selected from a carbide of chromium, titanium tantalum,
niobium and/or molybdenum.
In addition, the bipolar electrode may
advantageously comprise an intermediate protective layer,
usually made of copper, or a copper nickel alloy, or
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oxides) thereof, which is located between the anode layer
and the oxygen barrier layer and protects the oxygen
barrier layer by inhibiting its dissolution.
The oxygen barrier layer may be bonded and secured
to the carbon body directly or through at least one inert,
electrically conductive, intermediate bonding layer such
as a nickel and/or copper layer.
The oxygen barrier layer, and when present the
intermediate bonding layer and/or the intermediate
protective layer, may be formed by chemical or
electrochemical deposition, chemical vapour deposition
(CVD), physical vapour deposition (PVD), plasma or arc
spraying, flame spraying, painting, bushing, dipping or
slurry dipcoating.
At least one layer selected from the oxygen
barrier layer, the anode layer, and when present the
intermediate bonding layer and the intermediate protective
layer, may be obtained by micropyretic reaction to form a
porous layer enhancing thermal expansion match. At least
two juxtaposed porous layers may be simultaneously
produced micropyretically. Two layers may also be joined
by a micropyretically obtained joint.
Cathode Bodies
The cathode body may be made of petroleum coke,
metallurgical coke, anthracite, graphite, amorphous
carbon, fullerene and low density carbon.
Advantageously, the side of the cathode body which
is connected to the anode layer may be impregnated and/or
coated with a phosphate of aluminium, such as
monoaluminium phosphate, aluminium phosphate, aluminium
polyphosphate and aluminium metaphosphate, as described in
US Patent 5,534,130 (Sekhar). Alternatively, the side of
the cathode body which is connected to the anode layer may
be impregnated and/or coated with a boron compound, such
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as boron oxide, boric acid and tetraboric acid, by
following the teachings disclosed in US Patent 5,486,278
(Manganiello/Duruz/Bella). The impregnation and/or coating
is usually achieved from a solution or a slurry which is
applied into/onto the surface of the cathode body,
possibly assisted by vacuum, and heat treated.
During use in the cell, the carbon of the cathode
body may be exposed to the molten cell contents, in
particular to produced aluminium. Alternatively, the
carbon cathode body may comprise a drained aluminium-
wettable outer coating on which aluminium is produced.
However, great care should be taken for designing the
electrode to prevent the produced aluminium from draining
onto or otherwise coming into contact with the oxide-based
anode layer, particularly when containing iron-oxide.
An aluminium-wettable cathode coating may for
instance comprise a refractory hard metal boride, for
example a boride selected from borides of titanium,
chromium, vanadium, zirconium, hafnium, niobium, tantalum,
molybdenum and cerium, and combinations thereof.
Preferably, the aluminium-wettable coating is a
non-reactively sintered coating of preformed particulate
refractory hard metal boride, as described in US Patent
5,651,874 (de Nora/Sekhar). However, the aluminium-
wettable coating may also be a micropyretically-reacted
coating produced from a refractory hard metal boride
precursor as described in US Patents 5,310,476 and
5,364,513 (both in the name of Sekhar/de Nora).
The aluminium-wettable coating may be a dried
and/or heat treated slurry containing refractory hard
metal boride and/or a precursor thereof. The slurry may
comprise a colloid selected from colloidal silica,
alumina, yttria, ceria, thoria, zirconia, magnesia,
lithia, tin oxide, zinc oxide, acetates and formates
thereof as well as oxides and hydroxides of other metals,
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cationic species and mixtures thereof, as described in the
patents mentioned in the previous paragraph. The
aluminium-wettable coating may advantageously be
aluminised prior to use.
Electrochemicallv Active Anode Layers
The electrochemically active anode layer may for
instance comprise a metal, alloy, intermetallic compound
or cermet which during normal operation in the cell is
slowly consumable by oxidation of its surface and
dissolution into the electrolyte of the formed surface
oxide. In this case the rate of oxidation may be
substantially equal to the rate of dissolution.
Such anode layer may contain or consist of at
least one metal selected from nickel, copper, cobalt,
chromium, molybdenum, tantalum, tungsten, iron and
combinations thereof.
Optionally, the electrochemically active layer may
further comprise at least one additive selected from
beryllium, magnesium, yttrium, titanium, zirconium,
vanadium, niobium, tantalum, chromium, molybdenum,
tungsten, manganese, rhodium, silver, hafnium, lithium,
cerium and other Lanthanides.
Advantageously, the electrochemically active layer
may also comprise at least one electrocatalyst for the
anode reaction selected from iridium, palladium, platinum,
rhodium, ruthenium, silicon, tin, mischmetal and metals of
the Lanthanide series, and mixture, oxides and compounds
thereof, for example as disclosed in W099/36592 (de Nora).
The electrochemically active layer may comprise
spinels and/or perovskites, in particular ferrites
selected from the group consisting of cobalt, copper,
manganese, magnesium, nickel and zinc ferrite, and
mixtures thereof, such as nickel ferrite partially
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substituted with Fe2+. Additionally, the ferrite may be
doped with at least one oxide selected from chromium,
titanium, tin and zirconium oxide.
The electrochemically active layer can also
comprise ceramic oxides containing combinations of
divalent nickel, cobalt, magnesium, manganese, copper and
zinc with divalent/trivalent nickel, cobalt, manganese
and/or iron. The electrochemically active layer may for
instance have doped, non-stoichiometric and/or partially
substituted spinels, the doped spinels comprising dopants
selected from Ti4+, Zr4+, Sn4+, Feq+, Hf4+, Mna+~ Fe3+~ Ni3+~
Co3+, Mn3+, A13+, Cr3+, Fe2+, Ni2+, Co2+, Mg2+, Mn2+~ Cu2+, Zn2+
and Li+.
Advantageously, the electrochemically active layer
is initially sufficiently thick to constitute an
impermeable barrier to gaseous oxygen penetration, and
even to nascent, mono-atomic oxygen.
Transition Metal-Based Anode Layers
In other embodiments the electrochemically active
outside layer comprises a transition metal oxide, such as
iron oxide, cobalt oxide, nickel oxide or combination
thereof .
Whereas nickel as well as cobalt on their own are
poor candidates as electrochemically active materials for
aluminium electrowinning cells, an alloy of nickel and
cobalt shows the following properties. A nickel-cobalt
alloy forms upon oxidation complex oxides, in particular
(NiXCol_X)O, having semi-conducting properties.
Furthermore, nickel-cobalt oxides provide an advantage
over conventional nickel ferrite. Whereas trivalent iron
ions of nickel ferrite are slowly replaced by trivalent
aluminium ions in the octahedral sites of the spinel
lattice, which leads to a loss of conductivity and of
mechanical stability, nickel-cobalt alloys oxidised in
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oxygen at 1000°C lead to a semi-conducting mixed oxide
structure of NiCo204 and Co304 spinels which is similar to
the NaCl lattice. In these spinets, a replacement of
trivalent cobalt ions by trivalent aluminium ions is
unlikely.
In order to form an electrochemically active layer
suitable for aluminium electrowinning anodes, the cobalt
nickel atomic ratio is preferably chosen in the range of 2
to 2.7.
In a preferred embodiment, the anode layer has an
iron oxide-based outer surface, in particular a hematite-
based outer surface.
An iron oxide-based outer surface means that the
surface contains predominately iron oxide, as a simple
oxide such as hematite, or as part of an electrically
conductive and electrochemically active double or multiple
oxide, such as a ferrite, in particular cobalt, manganese,
nickel, magnesium or zinc ferrite.
The anode layer may comprise iron oxide throughout
its thickness. Alternatively, the anode layer may comprise
an anode substrate which may be passivatable and an iron
oxide-based outside layer which either is an applied layer
or is obtainable by oxidising the surface of the anode
substrate which contains iron. Alternatively, outside
layers made of other transition metal oxide may be applied
on such a substrate.
Usually the anode substrate comprises a metal, an
alloy, an intermetallic compound or a cermet, such as
nickel, copper, cobalt, chromium, molybdenum, tantalum,
tungsten, iron, and their alloys or intermetallic
compounds, or combinations thereof, in particular an alloy
consisting of 10 to 30 weighty of chromium, 55 to 90~ of
at least one of nickel, cobalt or iron, and 0 to 15~ of
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aluminium, titanium, zirconium, yttrium, hafnium or
niobium.
The anode substrate may be made of iron, or an
alloy of iron and at least one alloying metal selected
from nickel, cobalt, molybdenum, tantalum, tungsten,
niobium, titanium, zirconium, manganese and copper. Such
an anode substrate may advantageously be surface oxidised
to form the iron-oxide-based outside layer. For instance,
the anode substrate alloy may comprise from 30 to 70
weighty iron, and from 30 to 70 weighty nickel.
The iron oxide-based layer may comprise a dense
iron oxide outer portion, a microporous intermediate iron
oxide portion and an inner portion containing iron oxide
and a metal from the surface of the anode substrate.
The iron oxide-based (outside) layer may be formed
by electrodepositing iron oxide, plasma or arc spraying
iron oxide or iron as such followed by a heat treatment,
or applying iron oxide or a precursor thereof in a slurry
and drying and/or heat treating.
The iron oxide-based layer may be applied as a
colloidal and/or polymeric slurry. The colloidal and/or
polymeric slurry may comprise alumina, ceria, lithia,
magnesia, silica, thoria, yttria, zirconia, tin oxide,
zinc oxide yr iron oxide, or a heat convertible precursor
thereof, all in the form of a colloid or a polymer.
The iron oxide-based layer may be formed, or
consolidated, by heat treating the anode substrate, the
surface of which contains iron and/or iron oxide, in an
oxidising gas at a temperature which is above the
operating temperature of the cell usually at a temperature
of 950°C to 1250°C. However, the carbon cathode body
should not be exposed to an oxidation treatment. If joined
to the anode layer, the carbon cathode body may be
separately protected from oxidation. Alternatively, the
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carbon cathode body may be joined to the anode layer after
oxidation.
Any of the above-mentioned layers may be slurry
applied, for instance by applying a precursor slurry. The
layers may also be applied in the form a precursor powder
followed by heat-treating.
Several techniques may be used to apply the
layers, such as dipping, spraying, painting, brushing, arc
spraying, plasma spraying, arc spraying, electrochemical
deposition, physical vapour deposition, chemical vapour
deposition or calendar rolling.
HSLA Anode Layers
Further anode substrate materials which may be used
for forming the electrochemically active layer include
high-strength low-alloy (HSLA) steels.
It has been observed that low-carbon HSLA steels such
as Cor-TenT"', even at high temperature, form under
oxidising conditions an iron oxide-based surface layer
which is dense, electrically conductive, electrochemically
active for oxygen evolution and, as opposed to oxide
layers formed on standard steels or other iron alloys, is
highly adherent and less exposed to delamination and
limits diffusion of ionic, monoatomic and molecular
oxygen.
HSLA steels are known for their strength and
resistance to atmospheric corrosion especially at lower
temperatures (below 0°C) in different areas of technology
such as civil engineering (bridges, dock walls, sea walls,
piping), architecture (buildings, frames) and mechanical
engineering (welded/bolted/riveted structures, car and
railway industry, high pressure vessels). However, these
HSLA steels have never been proposed for applications at
high temperature, especially under oxidising or corrosive
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conditions, in particular in cells for the electrowinning
of aluminium.
It has been found that the iron oxide-based surface
layer formed on the surface of a HSLA steel under
oxidising conditions limits also at elevated temperatures
the diffusion of oxygen oxidising the surface of the HSLA
steel. Thus, diffusion of oxygen through the surface layer
decreases with an increasing thickness thereof.
If the HSLA steel is exposed to an environment
promoting dissolution or delamination of the surface
layer, in particular in an aluminium electrowinning cell,
the rate of formation of the iron oxide-based surface
layer (by oxidation of the surface of the HSLA steel)
reaches the rate of dissolution or delamination of the
surface layer after a transitional period during which the
surface layer grows or decreases to reach an equilibrium
thickness in the specific environment.
High-strength low-alloy (HSLA) steels are a group of
low-carbon steels (typically up to 0.5 weighty carbon of
the total) that contain small amounts of alloying
elements. These steels have better mechanical properties
and sometimes better corrosion resistance than carbon
steels.
The surface of a high-strength low-alloy steel
electrochemically active layer may be oxidised in an
electrolytic cell or in an oxidising atmosphere, in
particular a relatively pure oxygen atmosphere. For
instance the surface of the high-strength low-alloy steel
layer may be oxidised in a first electrolytic cell and
then transferred to an aluminium production cell. In an
electrolytic cell, oxidation would typically last 5 to 15
hours at 800 to 1000°C. Alternatively, the oxidation
treatment may take place in air or in oxygen for 5 to 25
hours at 750 to 1150°C.
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In order to prevent thermal shocks causing mechanical
stresses, a high-strength low-alloy steel layer may be
tempered or annealed after pre-oxidation. Alternatively,
the high-strength low-alloy steel layer may be maintained
at elevated temperature after pre-oxidation until
immersion into the molten electrolyte of an aluminium
production cell.
The high-strength low-alloy steel layer may
comprise 94 to 98 weighty iron and carbon, the remaining
constituents being one or more further metals selected
from chromium, copper, nickel, silicon, titanium,
tantalum, tungsten, vanadium, zirconium, aluminium,
molybdenum, manganese and niobium, and optionally a small
amount of at least one additive selected from boron,
sulfur, phosphorus and nitrogen.
Advantageous Operating Conditions
It has been observed that iron oxides and in
particular hematite (Fe203) have a higher solubility than
nickel in molten electrolyte. However, in industrial
production the contamination tolerance of the product
aluminium by iron oxides is also much higher (up to 2000
ppm) than for other metal impurities.
Solubility is an intrinsic property of anode
materials and cannot be changed otherwise than by
modifying the electrolyte composition and/or the operating
temperature of a cell.
Laboratory scale cell tests utilising a NiFe204/Cu
cermet anode and operating under steady conditions were
carried out to establish the concentration of iron in
molten electrolyte and in the product aluminium under
different operating conditions.
In the case of iron oxide it has been found that
lowering the temperature of the electrolyte decreases
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considerably the solubility of iron species. This effect
can surprisingly be exploited to produce a major impact on
bipolar cell operation by limiting the contamination of
the product aluminium by iron.
The solubility of iron species in the electrolyte
can even be further reduced by keeping therein a
sufficient concentration of dissolved alumina, i.e. by
maintaining the electrolyte as close as possible to
saturation with alumina. Maintaining a high concentration
of dissolved alumina in the molten electrolyte decreases
the solubility limit of iron species and consequently the
contamination of the product aluminium by cathodically
reduced iron.
Thus, it has been found that when the operating
temperature of aluminium electrowinning cells is reduced
below the temperature of conventional cells an anode
coated with an outer layer of iron oxide can be made
dimensionally stable by maintaining a concentration of
iron species and dissolved alumina, in the molten
electrolyte sufficient to suppress the dissolution of the
anode coating but low enough not to exceed the
commercially acceptable level of iron in the product
aluminium, as disclosed in co-pending application
PCT/IB99/01360 (Duruz/de Nora/Crottaz).
The solubility of iron species in the electrolyte
may be also influenced by the presence in the electrolyte
of other metal species, such as calcium, lithium,
magnesium, nickel, sodium, potassium and/or barium
species.
Based on the above observations, according to a
further aspect of the invention, during operation the
anode layer of the bipolar electrode may be kept
dimensionally stable by maintaining in the electrolyte a
sufficient concentration of iron species and dissolved
alumina, the cell operating temperature being sufficiently
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low so that the required concentration of iron species in
the electrolyte is limited by the reduced solubility of
iron species in the electrolyte at the operating
temperature, which consequently limits the contamination
of the product aluminium by iron to an acceptable level.
The amount of dissolved iron preventing
dissolution of the iron oxide-based anode layer may be
such that the product aluminium is contaminated by no more
than 2000 ppm iron, preferably by no more than 1000 ppm
iron, and even more preferably by no more than 500 ppm
iron.
The operating temperature of the electrolyte may
be in the range from 750 to 910°C, preferably from 820 to
870°C. The electrolyte may contain NaF and A1F3 in a weight
ratio NaF/A1F3 from about 0.74 to 0.82, generally from 0.7
to 0.85. The concentration of alumina dissolved in the
electrolyte is below 8 weight, preferably between 2
weighty and 6 weight .
To maintain an amount of iron species in the
electrolyte preventing the dissolution of the iron oxide-
based anode layer, the cell can comprise means for
intermittently or continuously feeding iron into the
electrolyte.
The iron may be fed in the form of iron metal
and/or an iron compound, such as iron oxide, iron
fluoride, iron oxyfluoride and/or an iron-aluminium alloy.
The iron may be intermittently fed into the
electrolyte together with alumina. Alternatively, a
sacrificial electrode may continuously feed the iron into
the electrolyte.
The dissolution of such a sacrificial electrode
may be controlled and/or promoted by applying a voltage
thereto which is lower than the voltage of oxidation of
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oxygen ions. The voltage applied to the sacrificial
electrode may be adjusted so that the resulting current
passing through the sacrificial electrode corresponds to a
current necessary for the dissolution of the required
amount of iron species into the electrolyte replacing the
iron which is cathodically reduced and not otherwise
compensated.
The teachings and principles disclosed hereabove
relating operation of cells fitted with bipolar electrodes
having a hematite anode layer are also applicable to any
bipolar electrode whose electrochemically active anode
layer comprises an oxidised transition metal, such as an
oxidised nickel-cobalt alloy, as described above.
In particular, nickel-cobalt active anode surfaces
may also be kept dimensionally stable by maintaining a
sufficient amount of dissolved alumina and nickel and/or
cobalt species in the electrolyte.
Cell Conficturations
In generally, a cell according to the invention
may also comprise 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 circulation and/or dissolution may be
achieved by moving the electrodes or by an adequate
geometry of the cell.
v~hen needed, the bipolar cell may comprise one or
more inert, electrically non-conductive current
confinement members arranged to inhibit or reduce current
bypass around the edges of the bipolar electrodes. The
current confinement member may be in the form of a rim
projecting from the periphery of at least one bipolar
electrode.
The surface of the current confinement member is
resistant to the electrolyte and to oxygen where exposed
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to anodically released gas or to molten aluminium where
exposed to the product aluminium and may consist of a non-
conductive ceramic and/or a non-conductive oxide, such as
silicon nitride, aluminium nitride, boron nitride,
magnesium ferrite, magnesium aluminate, magnesium
chromite, zinc oxide, nickel oxide and alumina.
The shape of the anode layer and cathode body of
each bipolar may be substantially circular or rectangular,
in particular square.
The bipolar electrodes may be inclined to the
vertical, substantially vertical or substantially
horizontal in the bipolar cell.
Cell Operatinct Temperature
Cells according to the invention may be operated
with an electrolyte at conventional temperature, i.e.
around 950 to 970°C, or preferably, as stated above, at
reduced temperature in order to maintain certain types of
anode layers, e.g. iron oxide-based anode layers,
dimensionally stable.
Furthermore, when the carbon of the cathode body
is directly exposed to the molten cell contents, to
inhibit sodium penetration the electrolyte should be
operated at reduced temperature, typically below 900°C,
preferably from 700 to 870°C, or even lower, but above the
melting point of aluminium.
Further Aspects of the Invention
The invention also relates to a bipolar electrode
of a bipolar cell for the electrowinning of aluminium by
the electrolysis of alumina dissolved in a molten
fluoride-containing electrolyte, comprising an anode layer
having an oxide-based outer surface, such as a transition
metal oxide-based surface, in particular an iron oxide-
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based surface, connected to a carbon cathode body as
described above.
Another aspect of the invention is a method of
manufacturing a bipolar electrode as described above
comprising a carbon cathode body connected to an anode
layer having an oxide-based outer surface through an
oxygen impermeable barrier layer. The method comprises
either:
a) forming the oxygen barrier layer onto the cathode
body directly or onto an intermediate bonding layer
formed on the cathode body, and forming the anode
layer onto the oxygen barrier layer directly or onto
an intermediate protective layer formed on the oxygen
barrier layer; or
b) forming the oxygen barrier layer onto the anode
body directly or onto an intermediate protective
layer formed on the anode layer, and bonding the
cathode body directly or through an intermediate
bonding layer onto the oxygen barrier layer.
This method may also be carried out for
reconditioning a bipolar electrode as described above
whose anode layer is damaged, the method comprising
clearing at least the damaged parts of the anode layer and
then reconstituting at least the anode layer.
A further aspect of the invention is a method of
producing aluminium in a bipolar cell as described above.
The method comprises passing an electric current from the
active surface of the terminal cathode to the active
surface of the terminal anode as ionic current in the
electrolyte and as electronic current through the or each
bipolar electrode, thereby electrolysing the alumina
dissolved in the electrolyte to produce aluminium on the
active surfaces of the terminal cathode and of the or each
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cathode body, and to produce oxygen an the active surface
of the terminal anode and of the or each anode layer.
Detailed Description
The invention was tested in a laboratory scale
bipolar cell as described in the following Examples:
Example 1
A bipolar electrode was made by coating one side
of a graphite cathode body (3 x 7 x 1 cm) with a chromium
oxide (Cr203) oxygen barrier layer having a thickness of
about 50 micron and forming thereon an anode layer
consisting of iron oxide.
The oxygen barrier layer was applied onto the
cathode body by brushing a precursor slurry and
consolidating by heat treatment under an argon atmosphere.
The precursor slurry contained a suspended particulate
chromium oxide in an inorganic Cr3+ polymer solution
consisting of concentrated chromium hydroxide containing
400 g/1 of Cr203 equivalent.
The anode layer was applied onto the oxygen
barrier layer by plasma spraying iron oxide powder to form
an iron oxide layer having a thickness of about 1 mm.
The bipolar electrode so obtained was then placed
between a terminal anode and a terminal cathode in a
fluoride-based electrolyte at 850°C containing NaF and A1F3
in a molar ratio NaF/A1F3 of 1.9 and approximately 6
weighty alumina, and tested at a current density of about
0.8 A/cm2.
To inhibit dissolution of the iron-oxide anode
layer, alumina and iron oxide were intermittently added to
the electrolyte to replace the alumina and the iron
species which were reduced at the cathode. This maintains
in the electrolyte a concentration of iron species of
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approximately 180 ppm, which is sufficient to saturate or
nearly saturate the electrolyte with iron species.
After 50 hours electrolysis, the bipolar electrode
was extracted from the cell and showed no sign of
significant internal or external corrosion after
microscopic examination of a cross-section of the
electrode specimen.
The composition of the produced aluminium was also
analysed and showed the presence of 800 ppm of iron which
is below the tolerated contamination of iron in
commercially produced aluminium.
A variation of this bipolar electrode can be
obtained by replacing the chromium oxide oxygen barrier
layer with a layer of platinum having a thickness of about
15 micron applied directly onto the cathode body by
electrochemical deposition. The bipolar electrode was
tested under the same conditions and showed similar
results.
Example 2
A bipolar electrode was made by coating one side
of a graphite cathode body with an Inconel~ alloy layer
about 500 micron thick consisting of 74 weighty nickel, 17
weighty chromium and 9 weighty iron, by arc spraying. A
chromium oxide-based oxygen barrier layer was slurry
applied onto the alloy layer and consolidated by heat
treatment under an argon atmosphere as described in
Example 1. A nickel layer about 200 micron thick and then
a copper layer about 100 micron thick were successively
applied onto the oxygen barrier layer by arc spraying. In
a modification of the Example, the arc-sprayed layers may
be electrodeposited.
The coated cathode body was heat treated at 1000°C
in argon for 5 hours. This heat treatment provides for the
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interdiffusion of nickel and copper to form an
intermediate layer.
A nickel-ferrite powder was made by drying and
calcining at 900°C the gel product obtained from an
inorganic polymer precursor solution consisting of a
mixture of molten Fe(N03)3.9 H20 with a stoichiometric
amount of Ni(C03)2.6 H20. 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 nickel 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.
An electrochemically active oxide layer was
obtained on the intermediate layer by applying thereon the
nickel-based paint 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.
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 intermediate
layer.
The bipolar electrode was then tested in a
cryolite melt containing approximately 6 weighty alumina
at 970°C by passing a current at a current density of
about 0.8 A/cm2. After 100 hours the electrode was
extracted from the cryolite and showed no significant
internal corrosion after microscopic examination of a
cross-section.
The Example was repeated, using instead an
electrochemically active layer obtained from a feed
prepared by slurrying nickel ferrite powder in an
inorganic polymer solution having the required composition
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for the formation of NiFe204. The powder to polymer ratio
was 1 to 0.25. Several layers of the coating feed were
brushed onto the nickel-copper layer and heat treated to
form the electrochemically active layer on the
intermediate layer.
Alternatively, the Example can be repeated using
instead an electrochemically active layer obtained from an
amount of 1 g of commercially available nickel ferrite
powder slurried with 1 g of an inorganic polymer
consisting of a precursor of 0.25 g equivalent nickel-
ferrite per 1 ml. An amount corresponding to 5 weighty of
Ir02 acting as an electrocatalyst for the rapid conversion
of oxygen ions into monoatomic oxygen and subsequently
gaseous oxygen can be added to the slurry as IrCl4, as
described in W099/36592 (de Nora). The slurry can be
brush-coated onto the interdiffused and at least partly
oxidised nickel copper alloy layer by applying 3
successive 50 micron thick layers of the slurry, each
slurry-applied layer should be allowed to dry by heat-
treating the anode at 500°C for 15 minutes between each
layer application.
