BIPOLAR CELL FOR THE PRODUCTION OF ALUMINIUM WITH CARBON CATHODES

Description

[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 CO₂ 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 (Fe₂O₃) 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 NiFe₂O₄/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 AlF₃ in a weight ratio NaF/AlF₃ 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 (Cr₂O₃) 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 Cr³⁺ polymer solution consisting of concentrated chromium hydroxide containing 400 g/l of Cr₂O₃ 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 AlF₃ in a molar ratio NaF/AlF₃ of 1.9 and approximately 6 weight% alumina, and tested at a current density of about 0.8 A/cm².

[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.

Claims

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.

Ansprüche

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.

Revendications

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.