[0001] 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.
[0002] 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.
[0003] This process, conceived almost simultaneously by Hall and Héroult, and the cell design
have not undergone any great change or improvement and carbonaceous materials are
still used as electrodes and cell linings.
[0004] 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 CO
2 gas formed at the anode surface.
[0005] The high electrical resistivity of the electrolyte causes a voltage drop in the inter-electrode
gap which alone represents as much as 40% of the total voltage drop with a resulting
low energy efficiency.
[0006] 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).
[0007] 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 WO99/36591
and WO99/36592 (both in the name of de Nora), WO99/36593 and WO99/36594 (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.
[0008] 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-Héroult cells.
[0009] 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 operating with
carbon electrodes and with an electrolytic bath containing aluminium chloride instead
of alumina. However, these cell designs have not been commercially adopted.
[0010] 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 FeO). 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.
[0011] US Patent 4,374,050 (Ray) discloses bipolar electrodes for aluminium electrowinning
having a carbon cathode body and an anode layer having an active surface formed by
non-stoichiometric multiple oxides, in particular nickel-iron oxides. It is inter-alia
mentioned that such multiple oxides can be obtained by oxidising an alloy of suitable
composition.
[0012] Despite all previous attempts, the bipolar technology has never been successfully
implemented in industrial aluminium production cells due to many problems of cell
operation.
Summary of the Invention
[0013] It is an object of the invention to provide a bipolar electrode for aluminium electrowinning
bipolar cells, which has an oxygen resistant anode surface.
[0014] 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.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] 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 electrochemically active for the oxidation reaction of oxygen ions into nascent
monoatomic oxygen.
[0019] 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.
[0020] According to the invention, the anode layer is an oxidised low-carbon high-strength
low-alloy (HSLA) steel layer as described below.
[0021] 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.
[0022] 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.
[0023] 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.
[0024] In addition, the bipolar electrode may advantageously comprise an intermediate protective
layer, usually made of copper, or a copper nickel alloy, or oxide(s) thereof, which
is located between the anode layer and the oxygen barrier layer and protects the oxygen
barrier layer by inhibiting its dissolution.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] The cathode body may be made of petroleum coke, metallurgical coke, anthracite, graphite,
amorphous carbon, fullerene and low density carbon.
[0029] 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 as boron oxide, boric acid and tetraboric acid, by following
the teachings disclosed in US Patent 5,486,278 (Manganiello/Duruz/Bellò). 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.
[0030] 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.
[0031] 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.
[0032] 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).
[0033] 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, 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.
[0034] It has been observed that low-carbon HSLA steels such as Cor-Ten™, 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.
[0035] 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 conditions,
in particular in cells for the electrowinning of aluminium.
[0036] 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.
[0037] 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.
[0038] High-strength low-alloy (HSLA) steels are a group of low-carbon steels (typically
up to 0.5 weight% 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.
[0039] 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.
[0040] 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.
[0041] The high-strength low-alloy steel layer comprises 94 to 98 weight% 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.
[0042] It has been observed that iron oxides and in particular hematite (Fe
2O
3) 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.
[0043] 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.
[0044] Laboratory scale cell tests utilising a NiFe
2O
4/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.
[0045] In the case of iron oxide it has been found that lowering the temperature of the
electrolyte decreases 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.
[0046] 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.
[0047] 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).
[0048] 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.
[0049] 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 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.
[0050] 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.
[0051] 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 AlF
3 in a weight ratio NaF/AlF
3 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 weight% and
6 weight%.
[0052] 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.
[0053] 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.
[0054] The iron may be intermittently fed into the electrolyte together with alumina. Alternatively,
a sacrificial electrode may continuously feed the iron into the electrolyte.
[0055] 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 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.
[0056] In general, 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.
[0057] When 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.
[0058] The surface of the current confinement member is resistant to the electrolyte and
to oxygen where exposed 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.
[0059] The shape of the anode layer and cathode body of each bipolar may be substantially
circular or rectangular, in particular square.
[0060] The bipolar electrodes may be inclined to the vertical, substantially vertical or
substantially horizontal in the bipolar cell.
[0061] 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.
[0062] 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.
[0063] 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-based surface,
connected to a carbon cathode body as described above.
[0064] Another aspect of the invention is a method of manufacturing a bipolar electrode
as described above comprising a carbon cathode body connected to an HSLA 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.
[0065] 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.
[0066] 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 cathode body, and to produce oxygen on the active surface of the terminal anode
and of the or each anode layer.
[0067] The invention was tested in a laboratory scale bipolar cell as described in the following
Example:
Example
[0068] A bipolar electrode was made by coating one side of a graphite cathode body (3 x
7 x 1 cm) with a chromium oxide (Cr
2O
3) oxygen barrier layer having a thickness of about 50 micron and forming thereon an
anode layer consisting of iron oxide.
[0069] 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 Cr
3+ polymer solution consisting of concentrated chromium hydroxide containing 400 g/l
of Cr
2O
3 equivalent.
[0070] 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.
[0071] 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 AlF
3 in a molar ratio NaF/AlF
3 of 1.9 and approximately 6 weight% alumina, and tested at a current density of about
0.8 A/cm
2.
[0072] 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 approximately 180 ppm, which is sufficient to saturate or nearly
saturate the electrolyte with iron species.
[0073] 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.
[0074] 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.
[0075] 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.
1. 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 comprising a carbon
cathode body having on one side an active surface on which aluminium is produced and
being connected on the other side through an oxygen impermeable barrier layer to an
anode layer having a metal oxide-based outer surface which is electrochemically active
for the oxidation reaction of oxygen ions into nascent monoatomic oxygen, as well
as for subsequent reaction for the formation of gaseous biatomic molecular oxygen,
characterised in that the anode layer is an oxidised low-carbon high-strength low-alloy (HSLA) layer which
comprises 94 to 98 weight% 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.
2. The bipolar cell of claim 1, wherein the oxygen barrier layer is made of at least
one metal selected from chromium, niobium and nickel, or an oxide thereof.
3. The bipolar cell of claim 1, wherein the or each bipolar electrode comprises an inert
electrically conductive intermediate protective or bonding layer located between the
oxygen barrier layer and the anode layer or the cathode body, the intermediate layer
comprising copper, or a copper nickel alloy, or oxide(s) thereof.
4. The bipolar cell of claim 1, wherein cathode body is made of carbon, such as petroleum
coke, metallurgical coke, anthracite, graphite, amorphous carbon, fullerene and low
density carbon.
5. The bipolar cell of claim 1, wherein at least the side of the cathode body which is
connected to the anode layer is impregnated and/or coated with a phosphate of aluminium
and/or a boron compound.
6. The bipolar cell of claim 1, wherein the carbon of the cathode body is exposed to
molten cell contents.
7. The bipolar cell of claim 1, wherein the cathode body comprises a drained aluminium-wettable
outer coating, preferably comprising a refractory hard metal boride, on which aluminium
is produced.
8. The bipolar cell of claim 1, wherein the anode layer comprises a metal, an alloy,
an intermetallic compound or a cermet.
9. The bipolar cell of claim 1, wherein during normal operation in the cell the anode
layer is slowly consumable by oxidation of its surface and dissolution into the electrolyte
of the formed surface oxide.
10. The bipolar cell of claim 1, wherein the anode layer has a hematite-based outer surface.
11. The bipolar cell of claim 1, wherein during operation the anode layer remains dimensionally
stable by maintaining in the electrolyte a sufficient concentration of iron species,
the cell operating temperature being sufficiently 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 species to an acceptable level.
12. The bipolar cell of claim 1, comprising at least one inert, electrically non-conductive
current confinement member arranged to inhibit or reduce current bypass around the
edges of the anode layer and the cathode body of the bipolar electrodes.
13. The bipolar cell of claim 1, wherein the bipolar electrodes are vertical or inclined
to the vertical.
14. The bipolar cell of claim 1, wherein the bipolar electrodes are substantially horizontal.
15. 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 HSLA
anode layer having a metal oxide-based outer surface connected to a carbon cathode
body as defined in claim 1.
16. A method of manufacturing a bipolar electrode according to claim 15 comprising a carbon
cathode body connected to an HSLA anode layer having a metal oxide-based outer surface
through an oxygen impermeable barrier layer, the method comprising 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.
17. The method of claim 16, for reconditioning a bipolar electrode according to claim
15 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.
18. A method of producing aluminium in a bipolar cell according to claim 1, comprising
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 cathode body, and to produce oxygen on the active surfaces of the
terminal anode and of the or each anode layer.
19. The method of claim 18, comprising keeping the anode layer of the or each bipolar
electrode dimensionally stable during electrolysis by maintaining a sufficient concentration
of dissolved alumina and iron species in the electrolyte, and operating the cell at
a sufficiently low temperature so that the required concentration of iron species
in the electrolyte is limited by the reduced solubility thereof in the electrolyte
at the operating temperature, which consequently limits the contamination of the product
aluminium by iron species to an acceptable level.
20. The method of claim 19, wherein the bipolar cell is operated at an electrolyte temperature
in the range from 820 to 870°C.
21. The method of claim 19, wherein the amount of dissolved iron preventing dissolution
of the iron oxide-based anode layer is 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.
22. The method of claim 19, wherein iron is intermittently or continuously fed into the
electrolyte to maintain the amount of iron species in the electrolyte which prevents
at the operating temperature the dissolution of the anode iron oxide-based layer.
23. The method of claim 22, wherein the iron is fed into the electrolyte in the form of
iron oxide, iron fluoride, iron oxyfluoride and/or an iron-aluminium alloy.
24. The method of claim 22, wherein the iron is intermittently or continuously fed into
the electrolyte together with alumina.
25. The method of claim 24, wherein a sacrificial electrode continuously feeds iron into
the electrolyte.
1. Bipolare Zelle zur elektrolytischen Extraktion von Aluminium durch Elektrolyse von
Aluminiumoxid, das in einem geschmolzenen fluoridhaltigen Elektrolyten gelöst ist,
mit einer endständigen Kathode, einer endständigen Anode und mindestens einer dazwischenliegenden
bipolaren Elektrode, die einen Kohlenstoffkathodenkörper aufweist, auf dessen einen
Seite sich eine aktive Oberfläche befindet, an der Aluminium produziert wird, und
die an der anderen Seite über eine sauerstoffundurchlässige Barriereschicht mit einer
Anodenschicht verbunden ist, die eine äußere Oberfläche auf Metalloxidbasis aufweist,
die für die Oxidationsreaktion von Sauerstoffionen zu naszierendem monoatomarem Sauerstoff
sowie für die nachfolgende Reaktion zur Bildung von gasförmigem biatomarem molekularem
Sauerstoff elektrochemisch aktiv ist, dadurch gekennzeichnet, dass
die Anodenschicht eine oxidierte, kohlenstoffarme, hochfeste, niedriglegierte (HSLA)-Schicht
ist, die 94 bis 98 Gew.% Eisen und Kohlenstoff enthält, wobei die verbleibenden Bestandteile
eines oder mehrere weitere Metalle ausgewählt aus Chrom, Kupfer, Nickel, Silicium,
Titan, Tantal, Wolfram, Vanadium, Zirconium, Aluminium, Molybdän, Mangan und Niob
und gegebenenfalls eine geringe Menge von mindestens einem Additiv ausgewählt aus
Bor, Schwefel, Phosphor und Stickstoff sind.
2. Bipolare Zelle nach Anspruch 1, bei der die Sauerstoffbarriereschicht aus mindestens
einem Metall ausgewählt aus Chrom, Niob und Nickel oder einem Oxid davon hergestellt
ist.
3. Bipolare Zelle nach Anspruch 1, bei der die oder jede bipolare Elektrode eine inerte,
elektrisch leitende, schützende oder bindende Zwischenschicht aufweist, die sich zwischen
der Sauerstoffbarriereschicht und der Anodenschicht oder dem Kathodenkörper befindet,
wobei die Zwischenschicht Kupfer oder eine Kupfer-Nickel-Legierung oder Oxid(e) derselben
enthält.
4. Bipolare Zelle nach Anspruch 1, bei der der Kathodenkörper aus Kohlenstoff hergestellt
ist, wie Petrolkoks, metallurgischem Koks, Anthrazit, Graphit, amorphem Kohlenstoff,
Fulleren und Kohlenstoff mit niedriger Dichte.
5. Bipolare Zelle nach Anspruch 1, bei der mindestens die Seite des Kathodenkörpers,
die mit der Anodenschicht verbunden ist, mit einem Phosphat von Aluminium und/oder
einer Borverbindung imprägniert und/oder beschichtet ist.
6. Bipolare Zelle nach Anspruch 1, bei der der Kohlenstoff des Kathodenkörpers geschmolzenem
Zelleninhalt ausgesetzt ist.
7. Bipolare Zelle nach Anspruch 1, bei der der Kathodenkörper eine aluminiumbenetzbare
äußere Abtropfbeschichtung aufweist, die vorzugsweise ein hitzebeständiges Hartmetallborid
enthält, auf dem Aluminium produziert wird.
8. Bipolare Zelle nach Anspruch 1, bei der die Anodenschicht ein Metall, eine Legierung,
eine Intermetallverbindung oder ein Cermet enthält.
9. Bipolare Zelle nach Anspruch 1, bei der die Anodenschicht während des normalen Betriebs
in der Zelle durch Oxidation ihrer Oberfläche und Auflösen des gebildeten Oberflächenoxids
in dem Elektrolyten langsam verbrauchbar ist.
10. Bipolare Zelle nach Anspruch 1, bei der die Anodenschicht eine äußere Oberfläche auf
Hämatitbasis hat.
11. Bipolare Zelle nach Anspruch 1, bei der während des Betriebs die Anodenschicht dimensionsstabil
bleibt, indem in dem Elektrolyten eine ausreichende Konzentration an Eisenspezies
aufrechterhalten wird, wobei die Zellenbetriebstemperatur ausreichend niedrig ist,
so dass die erforderliche Konzentration der Eisenspezies in dem Elektrolyten durch
die verringerte Löslichkeit der Eisenspezies in dem Elektrolyten bei der Betriebstemperatur
begrenzt wird, die demzufolge die Verunreinigung des Produktaluminiums durch Eisenspezies
auf ein annehmbares Niveau begrenzt.
12. Bipolare Zelle nach Anspruch 1, die mindestens ein inertes, elektrisch nicht leitendes
Stromsperrelement aufweist, das so angeordnet ist, dass Stromnebenschluss um die Ränder
der Anodenschicht und des Kathodenkörpers der bipolaren Elektroden gehemmt oder verringert
wird.
13. Bipolare Zelle nach Anspruch 1, bei der die bipolaren Elektroden vertikal oder zur
Vertikalen geneigt sind.
14. Bipolare Zelle nach Anspruch 1, bei der die bipolaren Elektroden im wesentlichen horizontal
sind.
15. Bipolare Elektrode einer bipolaren Zelle zur elektrolytischen Extraktion von Aluminium
durch Elektrolyse von Aluminiumoxid, das in einem geschmolzenen fluoridhaltigen Elektrolyten
gelöst ist, die eine HSLA-Anodenschicht mit einer äußeren Oberfläche auf Metalloxidbasis
aufweist, die mit einem Kohlenstoffkathodenkörper, wie in Anspruch 1 definiert, verbunden
ist.
16. Verfahren zur Herstellung einer bipolaren Elektrode gemäß Anspruch 15, die einen Kohlenstoffkathodenkörper
aufweist, der über eine sauerstoffundurchlässige Barriereschicht mit einer HSLA-Anodenschicht
mit einer äußeren Oberfläche auf Metalloxidbasis verbunden ist, wobei in dem Verfahren
a) entweder die Sauerstoffbarriereschicht direkt auf dem Kathodenkörper oder auf einer
bindenden Zwischenschicht gebildet wird, die auf dem Kathodenkörper gebildet ist,
und die Anodenschicht direkt auf der Sauerstoffbarriereschicht oder auf einer schützenden
Zwischenschicht gebildet wird, die auf der Sauerstoffbarriereschicht gebildet ist;
oder
b) die Sauerstoffbarriereschicht direkt auf dem Anodenkörper oder auf einer schützenden
Zwischenschicht gebildet wird, die auf der Anodenschicht gebildet ist, und der Kathodenkörper
direkt oder über eine bindende Zwischenschicht auf der Sauerstoffbarriereschicht gebildet
wird.
17. Verfahren nach Anspruch 16 zum Aufarbeiten einer bipolaren Elektrode gemäß Anspruch
15, deren Anodenschicht beschädigt ist, wobei mindestens die beschädigten Teile der
Anodenschicht beseitigt werden und danach mindestens die Anodenschicht wieder hergestellt
wird.
18. Verfahren zur Produktion von Aluminium in einer bipolaren Zelle gemäß Anspruch 1,
bei dem ein elektrischer Strom von der aktiven Oberfläche der endständigen Kathode
zu der aktiven Oberfläche der endständigen Anode als Ionenstrom in dem Elektrolyten
und als Elektronenstrom durch die oder jede bipolare Elektrode geleitet wird, wodurch
das in dem Elektrolyten gelöste Aluminiumoxid elektrolysiert wird, um auf den aktiven
Oberflächen der endständigen Kathode und dem oder jedem Kathodenkörper Aluminium zu
produzieren und auf den aktiven Oberflächen der endständigen Anode und der oder jeder
Anodenschicht Sauerstoff zu produzieren.
19. Verfahren nach Anspruch 18, bei dem die Anodenschicht von der oder jeder bipolaren
Elektrode während der Elektrolyse dimensionsstabil gehalten wird, indem in dem Elektrolyten
eine ausreichende Konzentration an gelöstem Aluminiumoxid und Eisenspezies aufrechterhalten
wird und die Zelle bei ausreichend niedriger Temperatur betrieben wird, so dass die
erforderliche Konzentration der Eisenspezies in dem Elektrolyten durch die verringerte
Löslichkeit derselben in dem Elektrolyten bei der Betriebstemperatur begrenzt wird,
das demzufolge die Verunreinigung des Produktaluminiums durch Eisenspezies auf ein
annehmbares Niveau begrenzt.
20. Verfahren nach Anspruch 19, bei dem die bipolare Zelle bei einer Elektrolyttemperatur
im Bereich von 820 bis 870°C betrieben wird.
21. Verfahren nach Anspruch 19, bei dem die Menge an gelöstem Eisen, das die Auflösung
der Anodenschicht auf Eisenoxidbasis verhindert, so ist, dass das Produktaluminium
durch nicht mehr als 2000 ppm Eisen, vorzugsweise nicht mehr als 1000 ppm Eisen und
besonders bevorzugt nicht mehr als 500 ppm Eisen verunreinigt wird.
22. Verfahren nach Anspruch 19, bei dem Eisen intermittierend oder kontinuierlich in den
Elektrolyten eingespeist wird, um die Menge an Eisenspezies in dem Elektrolyten aufrechtzuerhalten,
die bei der Betriebstemperatur die Auflösung der Anodenschicht auf Eisenoxidbasis
verhindert.
23. Verfahren nach Anspruch 22, bei dem das Eisen in den Elektrolyten in Form von Eisenoxid,
Eisenfluorid, Eisenoxyfluorid und/oder einer Eisen/Aluminium-Legierung eingespeist
wird.
24. Verfahren nach Anspruch 22, bei dem das Eisen intermittierend oder kontinuierlich
zusammen mit Aluminiumoxid in den Elektrolyten eingespeist wird.
25. Verfahren nach Anspruch 24, bei dem eine Opferelektrode kontinuierlich Eisen in den
Elektrolyten einspeist.
1. Cuve bipolaire pour l'électro-obtention d'aluminium par l'électrolyse d'alumine dissoute
dans un électrolyte contenant du fluorure fondu, ayant une cathode terminale, une
anode terminale et entre elles au moins une électrode bipolaire comprenant un corps
de cathode en carbone ayant, sur un côté, une surface active sur laquelle l'aluminium
est produit, et étant reliée, de l'autre côté, par l'intermédiaire d'une couche d'arrêt
imperméable à l'oxygène, à une couche d'anode ayant une surface externe à base d'oxyde
métallique qui est électrochimiquement active pour la réaction d'oxydation d'ions
oxygène en oxygène monoatomique naissant, ainsi que pour une réaction subséquente
pour la formation d'oxygène moléculaire biatomique gazeux,
caractérisée en ce que la couche anodique est une couche faiblement alliée à haute résistance à faible teneur
en carbone (HSLA) oxydée qui comprend 94 à 98% en poids de fer et de carbone, les
constituants restants étant un ou plusieurs autres métaux choisis à partir de chrome,
cuivre, nickel, silicium, titane, tantale, tungstène, vanadium, zirconium, aluminium,
molybdène, manganèse et niobium, et éventuellement une petite quantité d'au moins
un additif choisi à partir de bore, soufre, phosphore et azote.
2. Cuve bipolaire selon la revendication 1, dans laquelle la couche d'arrêt à l'oxygène
est réalisée en au moins un métal choisi à partir de chrome, niobium et nickel, ou
un oxyde de ceux-ci.
3. Cuve bipolaire selon la revendication 1, dans laquelle le ou chaque électrode bipolaire
comprend une couche de métallisation ou de protection intermédiaire inerte électriquement
conductrice, située entre la couche d'arrêt à l'oxygène et la couche anodique ou le
corps cathodique, la couche intermédiaire comprenant du cuivre, ou un alliage cuivre-nickel,
ou un oxyde(s) de ceux-ci.
4. Cuve bipolaire selon la revendication 1, dans laquelle le corps cathodique est réalisé
en carbone, tel que du coke de pétrole, du coke métallurgique, de l'anthracite, du
graphite, du carbone amorphe, du fullerène et du carbone à basse densité.
5. Cuve bipolaire selon la revendication 1, dans laquelle au moins le côté du corps cathodique
qui est relié à la couche anodique est imprégné et/ou enrobé d'un phosphate d'aluminium
et/ou d'un composé de bore.
6. Cuve bipolaire selon la revendication 1, dans laquelle le carbone du corps cathodique
est exposé au contenu de cuve fondu.
7. Cuve bipolaire selon la revendication 1, dans laquelle le corps cathodique comprend
un revêtement externe mouillable par l'aluminium drainé, de préférence, comprenant
un borure métallique dur réfractaire, sur lequel l'aluminium est produit.
8. Cuve bipolaire selon la revendication 1, dans laquelle la couche anodique comprend
un métal, un alliage, un composé intermétallique ou un cermet.
9. Cuve bipolaire selon la revendication 1, dans laquelle, pendant le fonctionnement
normal dans la cuve, la couche anodique est lentement consumée par oxydation de sa
surface et dissolution dans l'électrolyte de l'oxyde de surface formé.
10. Cuve bipolaire selon la revendication 1, dans laquelle la couche anodique a une surface
externe à base d'hématite.
11. Cuve bipolaire selon la revendication 1, dans laquelle, pendant le fonctionnement,
la couche anodique reste stable dimensionnellement en maintenant dans l'électrolyte
une concentration suffisante d'espèces de fer, la température de fonctionnement de
la cuve étant suffisamment basse de sorte que la concentration requise d'espèces de
fer dans l'électrolyte est limitée par la solubilité réduite des espèces de fer dans
l'électrolyte à la température de fonctionnement, ce qui limite en conséquence la
contamination de l'aluminium produit par des espèces de fer à un niveau acceptable.
12. Cuve bipolaire selon la revendication 1, comprenant au moins un élément de confinement
de courant électriquement non conducteur, inerte, agencé pour inhiber ou réduire la
dérivation de courant autour des bords de la couche anodique et du corps cathodique
des électrodes bipolaires.
13. Cuve bipolaire selon la revendication 1, dans laquelle les électrodes bipolaires sont
verticales ou inclinées par rapport à la verticale.
14. Cuve bipolaire selon la revendication 1, dans laquelle les électrodes bipolaires sont
sensiblement horizontales.
15. Electrode bipolaire d'une cuve bipolaire pour l'électro-obtention d'aluminium par
l'électrolyse d'alumine dissoute dans un électrolyte contenant du fluorure fondu,
comprenant une couche anodique HSLA ayant une surface externe à base d'oxyde métallique
reliée à un corps cathodique en carbone comme défini dans la revendication 1.
16. Procédé pour fabriquer une électrode bipolaire selon la revendication 15, comprenant
un corps cathodique en carbone relié à une couche anodique HSLA ayant une surface
externe à base d'oxyde métallique par l'intermédiaire d'une couche d'arrêt imperméable
à l'oxygène, le procédé consistant soit :
a) à former la couche d'arrêt à l'oxygène sur le corps cathodique directement ou sur
une couche de métallisation intermédiaire formée sur le corps cathodique, et à former
la couche anodique sur la couche d'arrêt à l'oxygène directement ou sur une couche
de protection intermédiaire formée sur la couche de barrière à l'oxygène ; ou
b) à former la couche d'arrêt à l'oxygène sur le corps anodique directement ou sur
une couche de protection intermédiaire formée sur la couche anodique, et métalliser
le corps cathodique directement ou par l'intermédiaire d'une couche de métallisation
intermédiaire sur la couche d'arrêt à l'oxygène.
17. Procédé selon la revendication 16, pour reconditionner une électrode bipolaire selon
la revendication 15 dont la couche anodique est endommagée, le procédé consistant
à nettoyer au moins les parties endommagées de la couche anodique et, ensuite, à reconstituer
au moins la couche anodique.
18. Procédé pour produire de l'aluminium dans une cuve bipolaire selon la revendication
1, consistant à faire passer un courant électrique de la surface active de la cathode
terminale vers la surface active de l'anode terminale comme courant ionique dans l'électrolyte
et comme courant électronique par l'intermédiaire de l'électrode bipolaire ou de chaque
électrode bipolaire, en électrolysant ainsi l'alumine dissoute dans l'électrolyte
pour produire de l'aluminium sur les surfaces actives de la cathode terminale et du
corps cathodique ou de chaque corps cathodique, et pour produire de l'oxygène sur
les surfaces actives de l'anode terminale et de la couche anodique ou de chaque couche
anodique.
19. Procédé selon la revendication 18, consistant à conserver la couche anodique de l'électrode
bipolaire ou de chaque électrode bipolaire dimensionnellement stable pendant l'électrolyse
en maintenant une concentration suffisante d'alumine dissoute et d'espèces de fer
dans l'électrolyte, et à faire fonctionner la cuve à une température suffisamment
basse de sorte que la concentration requise d'espèces de fer dans l'électrolyte est
limitée par leur solubilité réduite dans l'électrolyte à la température de fonctionnement,
ce qui limite en conséquence la contamination de l'aluminium produit par les espèces
de fer à un niveau acceptable.
20. Procédé selon la revendication 19, dans lequel la cuve bipolaire fonctionne à une
température d'électrolyte dans la plage de 820 à 870°C.
21. Procédé selon la revendication 19, dans lequel la quantité de fer dissous empêchant
la dissolution de la couche anodique à base d'oxyde de fer est telle que l'aluminium
produit est contaminé par pas plus de 2000 ppm de fer, de préférence, par pas plus
de 1000 ppm de fer et, même plus préférablement, par pas plus de 500 ppm de fer.
22. Procédé selon la revendication 19, dans lequel le fer est fourni de façon intermittente
ou continue dans l'électrolyte pour maintenir la quantité d'espèces de fer dans l'électrolyte
qui empêche, à la température de fonctionnement, la dissolution de la couche anodique
à base d'oxyde de fer.
23. Procédé selon la revendication 22, dans lequel le fer est fourni dans l'électrolyte
sous la forme d'oxyde de fer, fluorure de fer, oxyfluorure de fer et/ou alliage fer-aluminium.
24. Procédé selon la revendication 22, dans lequel le fer est fourni de façon intermittente
ou continue, dans l'électrolyte en même temps que l'alumine.
25. Procédé selon la revendication 24, dans lequel une électrode soluble fournit de façon
continue le fer dans l'électrolyte.