EP1554416B1 - Cellules d'extraction electrolytique de l'aluminium avec anodes a base de metal - Google Patents

Cellules d'extraction electrolytique de l'aluminium avec anodes a base de metal Download PDF

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Publication number
EP1554416B1
EP1554416B1 EP03751166A EP03751166A EP1554416B1 EP 1554416 B1 EP1554416 B1 EP 1554416B1 EP 03751166 A EP03751166 A EP 03751166A EP 03751166 A EP03751166 A EP 03751166A EP 1554416 B1 EP1554416 B1 EP 1554416B1
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Prior art keywords
weight
anode
aluminium
fluoride
cell
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EP1554416A1 (fr
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Vittorio De Nora
Thinh T. Nguyen
Jean-Jacques Duruz
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Rio Tinto Alcan International Ltd
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Rio Tinto Alcan International Ltd
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    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
    • C25C7/00—Constructional parts, or assemblies thereof, of cells; Servicing or operating of cells
    • C25C7/02—Electrodes; Connections thereof
    • C25C7/025—Electrodes; Connections thereof used in cells for the electrolysis of melts
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
    • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
    • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
    • C25C3/08—Cell construction, e.g. bottoms, walls, cathodes
    • C25C3/12—Anodes
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
    • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
    • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
    • C25C3/08—Cell construction, e.g. bottoms, walls, cathodes
    • C25C3/12—Anodes
    • C25C3/125—Anodes based on carbon

Definitions

  • This invention relates to aluminium electrowinning cells having metal-based anodes which contain at least one of nickel, iron and copper and which during use are inhibited from passivating and dissolving and from causing unacceptable contamination of the product aluminium.
  • the technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite, at temperatures around 950°C is more than one hundred years old and still uses carbon anodes and cathodes.
  • metal anodes in commercial aluminium electrowinning cells would be new and drastically improve the aluminium process by reducing pollution and the cost of aluminium production.
  • EP Patent application 0 306 100 (Nguyen/Lazouni/ Doan ) describes anodes composed of a chromium, nickel, cobalt and/or iron based substrate covered with an oxygen barrier layer and a ceramic coating of nickel, copper and/or manganese oxide which may be further covered with an in-situ formed protective cerium oxyfluoride layer.
  • US Patents 5,069,771 , 4,960,494 and 4,956,068 disclose aluminium production anodes with an oxidised copper-nickel surface on an alloy substrate with a protective oxygen barrier layer. However, full protection of the alloy substrate was difficult to achieve.
  • US Patent 6,248,227 discloses an aluminium electrowinning anode having a metallic anode body which can be made of various alloys, for example a nickel-iron-copper alloy.
  • a metallic anode body which can be made of various alloys, for example a nickel-iron-copper alloy.
  • the surface of the anode body is oxidised by anodically evolved oxygen to form an integral electrochemically active oxide-based surface layer.
  • the oxidation rate of the anode body is equal to the rate of dissolution of the surface layer into the electrolyte. This oxidation rate is controlled by the thickness and permeability of the surface layer which limits the diffusion of anodically evolved oxygen therethrough to the anode body.
  • WO00/06803 (Duruz/de Nora/Crottaz ) and WO00/06804 (Crottaz/Duruz ) both disclose an anode produced from a nickel-iron alloy which is surface oxidised to form a coherent and adherent outer iron oxide-based layer whose surface is electrochemically active.
  • WO00/06804 also mentions that the anode may be used in an electrolyte at a temperature of 820° to 870°C containing 23 to 26.5 weight% AlF 3 , 3 to 5 weight% Al 2 O 3 , 1 to 2 weight% LiF and 1 to 2 weight% MgF 2 .
  • the electrolyte may contain Al 2 O 3 in an amount of up to 30 weight%, in particular 5 to 10 or 15 weight%, most of which is in the form of suspended particles and some of which is dissolved in the electrolyte, i.e. typically 1 to 4 weight% dissolved Al 2 O 3 .
  • such an electrolyte is said to be useable at temperatures up to 900°C.
  • the electrolyte further contains 0.004 to 0.2 weight% transition metal additives to facilitate alumina dissolution and improve cathodic operation.
  • US Patent 5,725,744 discloses an aluminium production cell having anodes made of nickel, iron and/or copper in a electrolyte at a temperature from 680° to 880°C containing 42-63 weight% AlF 3 , up to 48 weight% NaF, up to 48 weight% LiF and 1 to 5 weight% Al 2 O 3 .
  • MgF 2 , KF and CaF 2 are also mentioned as possible bath constituents.
  • Metal or metal-based anodes are highly desirable in aluminium electrowinning cells instead of carbon-based anodes. Many attempts were made to use metallic anodes for aluminium production, however they were never adopted by the aluminium industry for commercial aluminium production because their lifetime was too short and needs to be increased.
  • One object of the invention is to provide an aluminium electrowinning cell incorporating metal-based anodes which remain substantially insoluble at the cell operating temperature and which can be operated without passivation or excessive contamination of the produced aluminium.
  • Another object of the invention is to provide an aluminium electrowinning cell operating with a crustless and ledgeless electrolyte, which can achieve high productivity, low contamination of the product aluminium, and whose components resist corrosion and wear.
  • the invention relates to a cell for electrowinning aluminium from alumina.
  • the cell comprises: a metal-based anode having an outer part that contains at least one of nickel, cobalt and iron and that has an electrochemically active oxide-based surface; and a fluoride-containing molten electrolyte in which the active anode surface is immersed and which, during cell operation to electrowin aluminium, is at a temperature in the range of 880°C to 940°C.
  • the electrolyte consists of: 5 to 14 weight% overall of dissolved alumina; 35 to 45 weight% aluminium fluoride; 30 to 45 weight% sodium fluoride; 5 to 20 weight% potassium fluoride; 2 to 5 weight% calcium fluoride; and 0 to 5 weight% in total of one or more further constituents.
  • the electrolyte consists of: 7 to 10 weight% dissolved alumina; 38 to 42 weight% aluminium fluoride; 34 to 43 weight% sodium fluoride; 8 to 15 weight% potassium fluoride; 2 to 4 weight% calcium fluoride; and 0 to 3 weight% in total of one or more further constituents.
  • Such an electrolyte composition is well adapted for aluminium electrowinning at reduced temperature, i.e. at a temperature below the conventional aluminium electrowinning temperature of about 950°C, using a metal-based anode containing at least one of nickel, cobalt and iron, usually in metallic and/or oxide form.
  • the electrolyte is particularly adapted for anodes containing at least one of metallic nickel, metallic cobalt and oxides of iron.
  • Oxides of iron include ferrous oxide, hematite, magnetite and ferrites (e.g. nickel ferrite), in stoichiometric and non-stoichiometric form.
  • the anode has a metallic alloy body that contains one or more of these metals - nickel, cobalt and iron - and that is covered with an integral active oxide layer or film.
  • the presence in the electrolyte of potassium fluoride in the given amount has two effects. On the one hand, it leads to a reduction of the operating temperature by up to several tens of degrees without increase of the electrolyte's aluminium fluoride content or even a reduction thereof compared to standard electrolytes operating at about 950°C with an aluminium fluoride content of about 45 weight%. On the other hand, it maintains a high solubility of alumina, i.e. up to above about 14 weight%, in the electrolyte even though the temperature of the electrolyte is reduced by a few tens of degrees compared to conventional temperature.
  • a large amount of alumina in the electrolyte is in a dissolved form.
  • basic fluorine-poor aluminium oxyfluoride ions do not significantly passivate metallic nickel and cobalt, or dissolve iron oxides.
  • the weight ratio of dissolved alumina/aluminium fluoride in the electrolyte should be above 1/7, and often above 1/6.5 or even above 1/6, to obtain a favourable ratio of the fluorine-poor aluminium oxyfluoride ions and the fluorine-rich aluminium oxyfluoride ions.
  • the cell is preferably fitted with means to monitor and adjust the electrolyte's alumina content.
  • the abovementioned one or more further constituents of the electrolyte may comprise at least one fluoride selected from magnesium fluoride, lithium fluoride, cesium fluoride, rubidium fluoride, strontium fluoride, barium fluoride and cerium fluoride.
  • the cell is sufficiently insulated to be operated with a substantially crustless and/or ledgeless electrolyte.
  • Suitable cell insulation is disclosed in US Patent 6,402,928 (de Nora/Sekhar ), WO02/070784 and US Publication 2003/0102228 (both de Nora/Berclaz ) .
  • the cell can have a cathode that has an aluminium-wettable surface, in particular a drained horizontal or inclined surface.
  • Suitable cathode designs are for example disclosed in US Patents 5,683,559 , 5,888,360 , 6,093,304 (all de Nora ), 6,258,246 (Duruz/de Nora ), 6, 358, 393 (Berclaz/de Nora ) and 6,436,273 (de Nora/Duruz ), and in PCT publications WO99/02764 (de Nora/Duruz ), WO00/63463 (de Nora ), WO01/31086 (de Nora/Duruz ), WO01/31088 (de Nora ), WO02/070785 (de Nora ), WO02/097168 (de Nora ), WO02/097168 (de Nora ), WO03/023091 (de Nora ) and WO03/023092 (de Nora ).
  • the cathode can have an aluminium-wettable coating that comprises a refractory boride and/or an aluminium-wetting oxide.
  • Suitable aluminium-wettable materials are disclosed in WO01/42168 (de Nora/Duruz ), WO01/42531 (Nguyen/Duruz/de Nora ), WO02/070783 (de Nora ), WO02/096831 (Nguyen/de Nora ) and WO02/096830 (Duruz/ Nguyen/de Nora ).
  • the anode can have a metallic or cermet body and an oxide layer integral with or applied on the anode body.
  • the anode body is made from an iron alloy, in particular an alloy of iron with nickel and/or cobalt.
  • Suitable alloys are disclosed in US Patents 6,248,227 (de Nora/Duruz ), 6,521,115 (Duruz/de Nora/Crottaz ), 6,562,224 (Crottaz/Duruz ), and in PCT publications WO00/40783 (de Nora/Duruz ), WO01/42534 (de Nora/Duruz), WO01/42536 (Duruz/Nguyen/de Nora ), WO02/083991 (Nguyen/de Nora ), WO03/014420 (Nguyen/Duruz/de Nora ) and WO03/078695 (Nguyen/de Nora ).
  • the anode body is made from an alloy consisting of:
  • Such an alloy is oxidised prior to or during use. This can lead to diffusion of metals in the anode, especially at the alloy's surface, which locally changes the alloy's composition.
  • the anode body can be covered with an integral iron oxide-based layer containing less than about 35 weight% nickel oxide and/or cobalt oxide, in particular from 5 to 10 weight% nickel oxide.
  • integral layers are usually obtained by preoxidation of the body before and/or during use in the cell.
  • the anode may also comprise an applied iron oxide-based coating.
  • Suitable iron oxide-based coatings are disclosed in US Patents 6,361,681 (de Nora/Duruz ), 6,365,018 (de Nora ), 6,379,526 (de Nora/Duruz ) and 6,413,406 (de Nora ), and in PCT applications PCT/IB03/01479 , PCT/IB03/03654 and PCT/IB03/03978 (all Nguyen/de Nora ).
  • the anode coating contains Fe 2 O 3 and optionally: at least one dopant selected from TiO 2 , ZnO and CuO and/or at least one inert material selected from nitrides and carbides.
  • the anode can comprise an applied cerium oxyfluoride-based outermost coating, for example as disclosed in the abovementioned US Patents 4,614,569 , 4,680,094 , 4,683,037 and 4,966,674 or PCT Applications WO02/070786 (Nguyen/de Nora ) and WO02/083990 (de Nora/Nguyen ).
  • a coating may be applied before or during use and maintained during use by the presence of cerium species in the electrolyte.
  • a nickel-containing stem can be used to suspend the anode in the electrolyte, in particular a stem having a nickel-containing core covered with an applied oxide coating, such as a coating containing aluminium oxide and titanium oxide.
  • the core of the stem can comprise a copper inner part and a nickel-based outer part. Further details of anode stems are disclosed in PCT/IB03/02702 (Crottaz/Duruz ).
  • Suitable anode designs are for example disclosed in WO99/02764 (de Nora/Duruz ), WO00/40781 , WO00/40782 , WO03/023091 , WO03/023092 and WO03/006716 (all de Nora ).
  • the cell comprises at least one component, e.g. the cathode, that contains a sodium-active cathodic material, such as elemental carbon.
  • a sodium-active cathodic material such as elemental carbon.
  • This sodium-active cathodic material is preferably shielded from the electrolyte by a sodium-inert layer to inhibit the presence in the molten electrolyte of soluble cathodically-produced sodium metal that constitutes an agent for dissolving the active oxide-based anode surface.
  • a sodium-active cathodic material such as elemental carbon.
  • This sodium-active cathodic material is preferably shielded from the electrolyte by a sodium-inert layer to inhibit the presence in the molten electrolyte of soluble cathodically-produced sodium metal that constitutes an agent for dissolving the active oxide-based anode surface.
  • the invention also relates to a cell that comprises:
  • a further aspect of the invention relates to a method of electrowinning aluminium in a cell as described above.
  • the method comprises electrolysing the dissolved alumina to produce oxygen on the anode and aluminium cathodically, and supplying alumina to the electrolyte to maintain therein a concentration of dissolved alumina of 5 to 14 weight%, in particular 7 to 10 weight%.
  • FIGS 1a and 1b schematically show an anode 10 which can be used in a cell for the electrowinning of aluminium according to the invention.
  • the anode 10 comprises a series of elongated straight anode members 15 connected to a cast or profiled support 14 for connection to a positive bus bar.
  • the cast or profiled support 14 comprises a lower horizontally extending foot 14a for electrically and mechanically connecting the anode members 15, a stem 14b for connecting the anode 10 to a positive bus bar and a pair of lateral reinforcement flanges 14c between the foot 14a and stem 14b.
  • the anode members 15 may be secured by force-fitting or welding the foot 14a on flats 15c of the anode members 15.
  • the connection between the anode members 15 and the corresponding receiving slots in the foot 14a may be shaped, for instance like dovetail joints, to allow only longitudinal movements of the anode members.
  • the anode members 15 have a bottom part 15a which has a substantially rectangular cross-section with a constant width over its height and which is extended upwardly by a tapered top part 15b with a generally triangular cross-section.
  • Each anode member 15 has a flat lower oxide surface 16 that is electrochemically active for the anodic evolution of oxygen during operation of the cell.
  • the anode may be covered with a coating of iron oxide-based material, for example applied from a composition as set out in Table III below, and/or a coating of one or more cerium compounds in particular cerium oxyfluoride.
  • the anode members 15, in particular their bottom parts 15a, are made of an iron alloy comprising nickel and/or cobalt as disclosed in Table II below.
  • the lifetime of the anode may be increased by a protective coating made of cerium compounds, in particular cerium oxyfluoride as discussed above.
  • the anode members 15 are in the form of parallel rods in a coplanar arrangement, laterally spaced apart from one another by inter-member gaps 17.
  • the inter-member gaps 17 constitute flow-through openings for the circulation of electrolyte and the escape of anodically-evolved gas released at the electrochemically active surfaces 16.
  • Figure 2a and 2b show an aluminium electrowinning cell having a series of metal-based anodes 10 in a fluoride-containing cryolite-based molten electrolyte 5 containing dissolved alumina according to the invention.
  • the electrolyte 5 has a composition that is selected from Table I below.
  • the metal-based anodes 10 have a composition selected from Table II below, optionally with a protective coating made of cerium compounds, in particular cerium oxyfluoride as discussed above.
  • the anodes 10 are similar to the anode shown in Figs. 1a and 1b . Suitable alternative anode designs are disclosed in WO00/40781 , WO00/40782 and WO03/006716 (all de Nora ).
  • the drained cathode surface 20 is formed by tiles 21A which have their upper face coated with an aluminium-wettable layer. Each anode 10 faces a corresponding tile 21A. Suitable tiles are disclosed in greater detail in WO02/096830 (Duruz/Nguyen/de Nora ).
  • Tiles 21A are placed on upper aluminium-wettable faces 22 of a series of carbon cathode blocks 25 extending in pairs arranged end-to-end across the cell. As shown in Figures 2a and 2b , pairs of tiles 21A are spaced apart to form aluminium collection channels 36 that communicate with a central aluminium collection groove 30.
  • the central aluminium collection groove 30 is located in or between pairs of cathode blocks 25 arranged end-to-end across the cell.
  • the tiles 21A preferably cover a part of the groove 30 to maximise the surface area of the aluminium-wettable cathode surface 20.
  • the cell is thermally sufficiently insulated to enable ledgeless and crustless operation.
  • the cell comprises sidewalls 40 made of an outer layer of insulating refractory bricks and an inner layer of carbonaceous material exposed to molten electrolyte 5 and to the environment thereabove. These sidewalls 40 are protected against the molten electrolyte 5 and the environment thereabove with tiles 21B of the same type as tiles 21A.
  • the cathode blocks 25 are connected to the sidewalls 40 by a peripheral wedge 41 which is resistant to the molten electrolyte 5.
  • the cell is fitted with an insulating cover 45 above the electrolyte 5.
  • This cover inhibits heat loss and maintains the surface of the electrolyte in a molten state. Further details of suitable covers are disclosed in the abovementioned references.
  • alumina dissolved in the molten electrolyte 5 at a temperature of 880° to 940°C is electrolysed between the anodes 10 and the cathode surface 20 to produce gas on the operative anodes surfaces 16 and molten aluminium on the aluminium-wettable drained cathode tiles 21A.
  • the cathodically-produced molten aluminium flows on the drained cathode surface 20 into the aluminium collection channels 36 and then into the central aluminium collection groove 30 for subsequent tapping.
  • the cell shown in Figure 3 comprises a plurality of metal-based anodes 10 dipping in a molten electrolyte 5 according to the invention.
  • the anodes 10 are similar to the anode shown in Figs. 1a and 1b . Suitable alternative anode designs are disclosed in WO00/40781 , WO00/40782 , WO03/006716 and WO03/023092 (all de Nora ).
  • the cell bottom comprises a series of pairs of spaced apart carbon cathode blocks 25 placed across the cell and having an aluminium-wettable upper surface 22 formed by an aluminium-wettable layer.
  • the upper surfaces 22 are covered with aluminium-wettable openly porous plates 21 which are filled with molten aluminium to form an aluminium-wetted drained active cathode surface 20 above the upper surfaces 22 of the carbon cathode blocks 25. Further details of such a cathode bottom are disclosed in WO02/097168 and WO02/097169 (both de Nora ).
  • the cathode blocks 25 are made of graphite and have a reduced height, e.g. 30 cm, and are coated with an aluminium-wettable layer which forms the upper surface 22 and which protects the graphite from erosion and wear. Suitable aluminium-wettable layers are disclosed in US Patent 5,651,874 , WO98/17842 , WO01/42168 and WO01/42531 .
  • the aluminium-wettable openly porous plates 21 covering the coated cathode blocks 25 can be made of the material disclosed in WO02/070783 (de Nora ).
  • the cell bottom further comprises a centrally-located recess 35 which extends at a level below the upper surfaces 22 of the carbon cathode blocks 25 and which during use collects molten aluminium 60 drained from the aluminium-wettable drained active cathode surface 20.
  • the aluminium collection recess 35 is formed in a reservoir body 30 which is placed between the blocks 25 of each pair of cathode blocks and spaces them apart across the cell. As shown in Figure 3 , the recess 35 formed in the reservoir body 30 is generally U-shaped with rounded lower corners and an outwardly curved upper part.
  • the reservoir body 30 is made of two generally L-shaped sections 31 assembled across the cell.
  • the reservoir sections 31 are made of anthracite-based material.
  • the aluminium-wettable layer forming the upper surfaces 22 extends in the recess 35 to protect the reservoir body 30 during use against wear and sodium or potassium intercalation.
  • the reservoir body 30 extends below the cathode blocks 25 into the refractory and insulating material 26 of the cell bottom permitting maximisation of the capacity of the aluminium collection recess 35.
  • the reservoir body 30 has a solid base 32 which extends from above to below the bottom face of the cathode blocks 25 and provides sufficient mechanical resistance to keep the blocks 25 properly spaced apart across the cell when exposed to thermal expansion during start-up of the cell and normal operation.
  • a solid base 32 which extends from above to below the bottom face of the cathode blocks 25 and provides sufficient mechanical resistance to keep the blocks 25 properly spaced apart across the cell when exposed to thermal expansion during start-up of the cell and normal operation.
  • longitudinally spaced apart spacer bars 33 placed across the reservoir body 30 may provide additional mechanical strength to the reservoir body 30.
  • Such spacer bars 33 can be made of carbon material coated with an aluminium-wettable protective layer.
  • the openly porous plates 21 placed on the upper surfaces 22 of the carbon cathode blocks 25 and located in the central region of the cell bottom extend over part of the aluminium collection recess 35 so that during use the protruding part of the aluminium-wetted drained active cathode surface 20 is located over the recess 35.
  • the openly porous plates 21 are spaced apart over the aluminium collection recess 35 to leave an access for the tapping of molten aluminium through a conventional tapping tube.
  • the spacing between the openly porous plates 21 over the aluminium collection recess can be much smaller along the remaining parts of the recess 35, thereby maximising the surface area of the active cathode surface 20.
  • the cell shown in Figure 3 comprises a series of corner pieces 41 made of the same openly porous material as plates 21 and filled with aluminium and placed at the periphery of the cell bottom against sidewalls 40.
  • the sidewalls 40 and the surface of the electrolyte 5 are covered with a ledge and a small crust of frozen electrolyte 6.
  • the cell is fitted with an insulating cover 45 above the electrolyte crust 6. Further details of suitable covers are disclosed in the abovementioned references.
  • the cell is also provided with exhaust pipes (not shown) that extend through the cover 45 for the removal of gases produced during electrolysis.
  • the cell comprises alumina feeders 50 with feeding tubes 51 that extend through the insulating cover 45 between the anodes 10.
  • the alumina feeders 50 are associated with a crust breaker (not shown) for breaking the crust 6 underlying the feeding tube 51 prior to feeding.
  • the insulating material of the sidewalls 40 and cover 45 may be sufficient to prevent formation of any ledge and crust of frozen electrolyte.
  • the sidewalls 40 are preferably completely shielded from the molten electrolyte 5 like in the cell of Figs. 2a and 2b or by a lining of the aforesaid openly porous material filled with aluminium.
  • Enhanced alumina dissolution may be achieved by utilising an alumina feed device which sprays and distributes alumina particles over a large area of the surface of the molten electrolyte 5.
  • Suitable alumina feed devices are disclosed in US Patent 6,572,757 (de Nora/Berclaz ) and in WO03/006717 (Berclaz/Duruz ).
  • the cell may comprise means (not shown) to promote circulation of the electrolyte 5 from and to the anode-cathode gap to enhance alumina dissolution in the electrolyte 5 and to maintain in permanence a high concentration of dissolved alumina close to the active surfaces of anodes 10, for example as disclosed in WO00/40781 (de Nora ).
  • alumina dissolved in the electrolyte 5 is electrolysed to produce oxygen on the anodes 10 and aluminium 60 on the drained cathode surfaces 20.
  • the product aluminium 60 drains from the cathode surfaces 20 over the openly porous plates 21 that extend over part of the reservoir 30 into the reservoir 30 from where it can be tapped.
  • aluminium is produced on the drained active cathode surface 20 which covers not only the cathode blocks 25 but also part of the reservoir 30, thereby maximising the useful aluminium production area (i.e. the drained cathode surface 22) of the cell.
  • Figs. 2a, 2b and 3 show specific aluminium electrowinning cells by way of example. It is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art.
  • the cell may have a sloping cathode bottom, as disclosed in WO99/02764 (de Nora/Duruz ), and optionally one or more aluminium collection reservoirs across the cell, each intersecting the collection groove to divide the drained cathode surface into four quadrants as described in WO00/63463 (de Nora ).
  • the "other" elements refer to minor additives such as manganese, silicon and yttrium which may be present in individual amounts of 0.2 to 1.5 weight%. Usual impurities, such as carbon, have not been listed in Table 2.
  • these alloys will be surface oxidised before use and further oxidised during use, as described in the Examples below.
  • a metal-based anode was tested in a potassium fluoride-free electrolyte at 900°C.
  • the anode was manufactured from a rod of diameter 20 mm and total length 20 mm made from a cast nickel-iron alloy having the composition of sample A2 of Table 2.
  • the anode rod was supported by a stem made of an alloy containing nickel, chromium and iron, such as Inconel, protected with an alumina sleeve.
  • the anode was suspended for 16 hours over the molten fluoride-based electrolyte whereby its surface was oxidised prior to immersion into the electrolyte.
  • Electrolysis was carried out by fully immersing the anode rod in the molten electrolyte.
  • the potassium fluoride-free electrolyte contained 49 weight% aluminium fluoride (AlF 3 ), 43 weight% aluminium fluoride (NaF), 4 weight% calcium fluoride (CaF 2 ) and 4 weight% alumina (Al 2 O 3 ).
  • the saturation concentration of alumina in such an electrolyte, unattainable in practice, is at 5 weight%.
  • the current density was about 0.8 A/cm 2 and the cell voltage was at 3.6-3.8 volt for 24 hours.
  • the concentration of dissolved alumina in the electrolyte was maintained during the entire electrolysis by periodically feeding fresh alumina into the cell.
  • the anode's outer dimensions had remained substantially unchanged.
  • the anode's oxide outer part had grown from an initial thickness of about 70 micron to a thickness after use of about up to 1000 micron.
  • a yellow-green layer of nickel fluoride (NiF 2 ) was observed between the oxide outer part and the metallic inner part of the anode.
  • NiF 2 nickel fluoride
  • Such a nickel fluoride layer is substantially non-conductive and passivates the anode, which caused the voltage increase.
  • the vermicular structure was observed in the metallic inner part immediately underneath the nickel fluoride layer over a depth of about 2 to 3 mm.
  • the vermicular structure had mainly empty pores that had an average diameter of about 20 to 30 micron.
  • a test was carried out with a cell according to the invention comprising: a molten potassium fluoride-containing electrolyte at 900°C having the composition of sample D1 of Table I, i.e. rich in dissolved alumina, and an anode made from a nickel-iron alloy having the composition of sample A2 of Table 2.
  • the anode was manufactured like in the Comparative Example and suspended for 16 hours over the molten electrolyte.
  • Electrolysis was carried out in the same potassium fluoride-containing electrolyte.
  • the current density was about 0.8 A/cm 2 and the cell voltage was stable at 3.8 volt during the entire test.
  • the dissolved alumina-content was maintained around 8 weight% by periodically feeding fresh alumina into the cell.
  • the anode's outer dimensions had remained substantially unchanged.
  • the anode's oxide outer part had grown from an initial thickness of about 70 micron to a thickness after use of about up to 500 micron, instead of the 1000 micron observed in the Comparative Example. Also, no passivating yellow-green layer of nickel fluoride (NiF 2 ) was observed.
  • the vermicular structure had pores which were partly filled with oxides, in particular iron oxides, and which had an average diameter of about 2 to 5 micron.
  • Example 1 was repeated with an anode made form the nickel-cobalt-iron alloy composition of sample D2 of Table 2 which was prepared, like in Example 1, over a potassium fluoride-containing electrolyte having the composition of sample D1 of Table 1, i.e. rich in dissolved alumina. The anode was then tested in the electrolyte like in Example 1 and showed similar results.
  • Example 1 was repeated with an anode made from the nickel-iron alloy composition of sample H2 of Table 2 prepared, like in Example 1, over a potassium fluoride-containing electrolyte having the composition of sample D1 of Table 1, i.e. rich in dissolved alumina. The anode was then tested in the electrolyte like in Example 1.
  • the anode's outer dimensions had remained substantially unchanged.
  • the anode's oxide outer part had grown from an initial thickness of about 70 micron to a thickness after use of about up to 1000 micron like in the Comparative Example. However, no passivating yellow-green layer of nickel fluoride (NiF 2 ) was observed.
  • a vermicular structure was observed in the metallic inner part immediately underneath the oxide outer part over a depth of about 1.5 to 2 mm, instead of the 2 to 3 mm of the Comparative Example.
  • the vermicular structure had pores which were partly filled with oxides, in particular iron oxides, and which had an average diameter of about 2 to 5 micron.
  • Example 1 was repeated with an anode made from the nickel-iron alloy composition of sample A2 of Table 2 which was prepared, like in Example 1, over a potassium fluoride-containing electrolyte having the composition of sample A1 of Table 1, i.e. rich in dissolved alumina. The anode was then tested in the electrolyte like in Example 1 and showed similar results.
  • Examples 1 to 4 can be repeated using different combinations of electrolyte compositions (A1-I1) selected from Table 1 and anode alloy compositions (A2-K2) selected from Table 2.
  • Another aluminium electrowinning anode was prepared as follows:
  • An anode made of the nickel-iron alloy of sample A2 of Table 2 was covered with ten layers of this slurry that were applied with a brush.
  • the applied layers were dried for 10 hours at 140°C in air and then consolidated at 950°C for 16 hours to form a protective hematite-based coating which had a thickness of 0.4 to 0.45 mm.
  • the Fe 2 O 3 particles were sintered together into a microporous matrix with a volume contraction.
  • the TiO 2 particles and CuO particles were dissolved in the sintered Fe 2 O 3 .
  • the boron nitride particles remained substantially inert during the sintering but prevented migration and agglomeration of the micropores into cracks.
  • an integral oxide scale mainly of iron oxide had grown from the anode's alloy during the heat treatment and combined with iron oxide and titanium oxide from the coating to firmly anchor the coating to the oxidised alloy.
  • the integral oxide scale contained titanium oxide in an amount of about 10 metal weight%. Minor amounts of copper, aluminium and nickel were also found in the oxide scale (less that 5 metal weight% in total).
  • Electrolysis was carried out in a potassium fluoride-containing electrolyte at 900°C having the composition of sample D1 of Table 1, i.e. rich in dissolved alumina.
  • the current density was about 0.8 A/cm 2 and the cell voltage was stable at 3.6 volt during the entire test, instead of the 3.8 volt observed in Examples 1 to 4.
  • the dissolved alumina-content was maintained around 8 weight% by periodically feeding fresh alumina into the cell.
  • the anode's outer dimensions as well as the anode's coating had remained substantially unchanged. However, TiO 2 had selectively been dissolved in the electrolyte from the coating. The anode's structure underneath the coating was similar to the structure observed in Examples 1 to 4.
  • Example 6 can be repeated using different combinations of electrolyte compositions (A1-I1) selected from Table 1, anode alloy compositions (A2-K2) selected from Table 2 and coating compositions (A3-L3) selected from Table 3.
  • A1-I1 electrolyte compositions
  • A2-K2 anode alloy compositions
  • A3-L3 coating compositions
  • Example 1-5 using the potassium-fluoride electrolyte of the invention containing about 8 weight% dissolved alumina instead of a potassium-fluoride free electrolyte containing only 4 weight% dissolved alumina, inhibits fluorination and passivation of the nickel and/or cobalt of the anode and reduces wear (oxidation and dissolution of the anode's iron).

Claims (13)

  1. Cellule d'extraction électrolytique de l'aluminium à partir de l'alumine, comprenant :
    - une anode à base métallique comportant une partie externe ayant une surface à base d'oxyde électrochimiquement active et contenant au moins l'un parmi le nickel, le cobalt et le fer ;
    - un électrolyte fondu contenant un fluorure dans lequel la surface active de l'anode est immergée et qui, pendant le fonctionnement de la cellule pour extraire l'aluminium électrolytiquement, est à une température dans la plage de 880 °C à 940 °C, en particulier inférieure à 920 °C, et qui est constitué de :
    - 5 à 14 % en poids d'alumine dissoute, en particulier de 7 à 10 % en poids ;
    - 35 à 45 % en poids de fluorure d'aluminium, en particulier de 38 à 42 % en poids ;
    - 30 à 45 % en poids de fluorure de sodium, en particulier de 34 à 43 % en poids ;
    - 5 à 20 % en poids de fluorure de potassium, en particulier de 8 à 15 % en poids de fluorure de potassium ;
    - 2 à 5 % en poids de fluorure de calcium, en particulier de 2 à 4 % en poids ; et
    - 0 à 5 % en poids au total d'un ou plusieurs autres constituants, en particulier de 0 à 3 % en poids.
  2. Cellule selon la revendication 1, dans laquelle ledit un ou plusieurs autres constituants comprend au moins un fluorure choisi dans le groupe constitué du fluorure de magnésium, du fluorure de lithium, du fluorure de césium, du fluorure de rubidium, du fluorure de strontium, du fluorure de baryum et du fluorure de cérium.
  3. Cellule selon la revendication 1 ou la revendication 2, comprenant une cathode pourvue d'une surface pouvant être mouillée par l'aluminium, en particulier une surface drainée horizontale ou inclinée, la cathode comportant de façon optionnelle un revêtement pouvant être mouillé par l'aluminium comprenant un borure réfractaire et/ou un oxyde mouillant l'aluminium.
  4. Cellule selon l'une quelconque des revendications précédentes, dans laquelle l'anode a un corps en métal ou en cermet et comporte une couche d'oxyde sur le corps de l'anode.
  5. Cellule selon l'une quelconque des revendications précédentes, dans laquelle le corps de l'anode est constitué d'un alliage de fer contenant du nickel et/ou du cobalt, l'alliage étant en particulier constitué de :
    - 40 à 80 % en poids de nickel et/ou de cobalt, en particulier de 50 à 60 % en poids ;
    - 9 à 55 % en poids de fer, en particulier de 25 à 40 % en poids ;
    - 5 à 15 % en poids de cuivre, en particulier de 6 à 12 % en poids ;
    - 0 à 4 % en poids au total d'au moins l'un parmi l'aluminium, le niobium et le tantale, en particulier de 0,5 à 2 % en poids ; et
    - 0 à 2 % en poids au total d'autres constituants, en particulier de 0,5 à 1 % en poids.
  6. Cellule selon la revendication 5, dans laquelle le corps de l'anode est intégralement recouvert d'une couche à base d'oxyde de fer contenant jusqu'à 35 % en poids d'oxyde de nickel et/ou d'oxyde de cobalt, en particulier de 5 à 10 % en poids d'oxyde de nickel.
  7. Cellule selon l'une quelconque des revendications précédentes, dans laquelle l'anode comporte un revêtement à base d'oxyde de fer appliqué, comme un revêtement contenant du Fe2O3 et, de façon optionnelle : au moins un dopant choisi dans le groupe constitué du TiO2, du ZnO et du CuO et/ou au moins un matériau inerte choisi parmi les nitrures et les carbures.
  8. Cellule selon l'une quelconque des revendications précédentes, dans laquelle l'anode comporte, un revêtement externe à base d'oxyfluorure de cérium.
  9. Cellule selon l'une quelconque des revendications précédentes, dans laquelle l'anode est suspendue dans l'électrolyte par une tige contenant du nickel, en particulier une tige dont l'âme contient du nickel et qui est recouverte d'un revêtement d'oxyde, comme un revêtement contenant de l'oxyde d'aluminium et de l'oxyde de titane.
  10. Cellule selon la revendication 9, dans laquelle l'âme de la tige comporte une partie interne en cuivre et une partie externe à base de nickel.
  11. Cellule selon l'une quelconque des revendications précédentes, comportant au moins un composant contenant un matériau cathodique actif vis-à-vis du sodium, comme du carbone élémentaire, ledit matériau cathodique actif vis-à-vis du sodium étant protégé de l'électrolyte par une couche inerte vis-à-vis du sodium afin d'inhiber la présence dans l'électrolyte fondu de sodium métallique soluble produit à la cathode, lequel est un agent susceptible de dissoudre la surface active de l'anode à base d'oxyde.
  12. Cellule selon la revendication 1, comprenant :
    - une anode à base métallique comportant une partie externe pourvue d'une surface à base d'oxyde électrochimiquement active et constituée d'un alliage consistant en :
    - 50 à 60 % en poids au total de nickel et/ou de cobalt ;
    - 25 à 40 % en poids de fer ;
    - 6 à 12 % en poids de cuivre ;
    - 0,5 à 2 % en poids d'aluminium et/ou de niobium ; et
    - 0,5 à 1,5 % en poids au total d'autres constituants, l'anode comportant un revêtement à base d'hématite et, de façon optionnelle, un revêtement externe à base d'oxyfluorure de cérium ;
    - une tige d'anode contenant du nickel pour la suspension de l'anode dans l'électrolyte, la tige étant recouverte d'un revêtement en oxyde d'aluminium et en oxyde de titane ;
    - un électrolyte fondu contenant un fluorure dans lequel la surface active de l'anode est immergée et qui est à une température dans la plage de 880 °C à 930 °C et qui est constitué de :
    - 7 à 10 % en poids d'alumine dissoute ;
    - 38 à 42 % en poids de fluorure d'aluminium ;
    - 34 à 43 % en poids de fluorure de sodium ;
    - 8 à 15 % en poids de fluorure de potassium ;
    - 2 à 4 % en poids de fluorure de calcium ; et
    - 0 à 3 % en poids au total d'un ou plusieurs autres constituants ;
    et
    - une cathode comportant une surface pouvant être mouillée par l'aluminium, en particulier une surface horizontale ou inclinée pouvant être drainée, formée d'un revêtement pouvant être mouillé par l'aluminium en matériau réfractaire dur et/ou en oxyde pouvant mouiller l'aluminium.
  13. Méthode d'extraction électrolytique de l'aluminium dans une cellule telle que définie dans la revendication précédente, comprenant l'électrolyse de l'alumine dissoute pour produire de l'oxygène à l'anode et de l'aluminium à la cathode, et la fourniture d'alumine dans l'électrolyte pour y maintenir une concentration en alumine dissoute de 5 à 14 % en poids, en particulier de 7 à 10 % en poids.
EP03751166A 2002-10-18 2003-10-17 Cellules d'extraction electrolytique de l'aluminium avec anodes a base de metal Expired - Lifetime EP1554416B1 (fr)

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CA2505285C (fr) * 2003-02-20 2012-07-17 Moltech Invent S.A. Cellules d'extraction electrolytique de l'aluminium a anodes a base de metal
US7811425B2 (en) * 2004-03-18 2010-10-12 Moltech Invent S.A. Non-carbon anodes with active coatings
CN100465350C (zh) * 2005-06-24 2009-03-04 曹大力 以铁及其合金为阳极在电解槽中制备铝-铁基合金的方法
UA100589C2 (ru) * 2008-09-08 2013-01-10 Ріо Тінто Алкан Інтернешнл Лімітед Металлический анод выделения кислорода, который работае при высокой плотности тока, для электролизеров восстановления алюминия
CN101586250B (zh) * 2009-06-10 2010-12-29 中南大学 一种复合涂层及其制备方法和应用
RU2457286C1 (ru) * 2011-03-02 2012-07-27 Федеральное государственное бюджетное учреждение науки Институт высокотемпературной электрохимии Уральского отделения Российской Академии наук Способ электролиза расплавленных солей с кислородсодержащими добавками с использованием инертного анода
CN103484891B (zh) * 2012-06-11 2016-06-15 内蒙古联合工业有限公司 一种电解铝用电解槽及使用该电解槽的电解工艺
CA2876345C (fr) * 2012-06-11 2017-03-14 Inner Mongolia United Industrial Co., Ltd. Electrolyte utilise pour electrolyse d'aluminium et procede d'electrolyse utilisant l'electrolyte
CN103014769A (zh) * 2012-11-26 2013-04-03 中国铝业股份有限公司 一种铝电解用合金惰性阳极及其制备方法
RU2567429C1 (ru) * 2013-07-09 2015-11-10 Общество с ограниченной ответственностью "Объединенная Компания РУСАЛ Инженерно-технологический центр" Электролит для получения алюминия электролизом расплавов
WO2015026257A1 (fr) * 2013-08-19 2015-02-26 Общество с ограниченной ответственностью "Объединенная Компания РУСАЛ Инженерно-технологический центр" Anode à base de fer pour obtenir de l'aluminium par électrolyse de bains de fusion
CN105132952B (zh) * 2015-08-26 2017-09-29 贵州理工学院 一种能降低全氟化碳排放量的电解质体系
KR102562722B1 (ko) * 2016-02-01 2023-08-03 재단법인 포항산업과학연구원 전해용 양극, 이를 포함하는 전해조, 및 상기 전해조를 이용한 전해 방법
RU2698162C2 (ru) * 2017-03-01 2019-08-22 Общество с ограниченной ответственностью "Объединенная Компания РУСАЛ Инженерно-технологический центр" Перфорированный металлический инертный анод для получения алюминия электролизом расплава
RU2686408C1 (ru) * 2018-06-20 2019-04-25 Федеральное государственное бюджетное учреждение науки Институт высокотемпературной электрохимии Уральского отделения Российской Академии наук Способ электролитического получения алюминия
CN114717610B (zh) * 2022-05-16 2023-08-08 中国铝业股份有限公司 一种降低铝电解载氟氧化铝中钾含量的方法
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NO20052377L (no) 2005-05-13
AU2003269385B2 (en) 2009-06-04
EP1554416A1 (fr) 2005-07-20
AU2003269385A1 (en) 2004-05-04
US20110031129A1 (en) 2011-02-10
SI1554416T1 (sl) 2012-05-31
RU2005115103A (ru) 2005-10-27
CA2498622A1 (fr) 2004-04-29
RU2318924C2 (ru) 2008-03-10
CN1735717B (zh) 2011-12-28
CN1735717A (zh) 2006-02-15
ES2381927T3 (es) 2012-06-01
NZ538777A (en) 2007-02-23
ATE543927T1 (de) 2012-02-15
CA2498622C (fr) 2011-09-20

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