Alloy-based anode structures for aluminium production

Abstract

A long-lasting metal-based oxygen-evolving anode (10) for the electrowinning of aluminium from alumina dissolved in a molten electrolyte, has a plurality of electrochemically active anode members (15,15′) spaced apart and parallel to one another. Each anode member (15) can comprise a bottom part (15a) which has a substantially constant width over its height and which is extended upwardly by a tapered top part (15b for guiding a circulation a of electrolyte (30) thereon. The bottom part (15a) is usually made of a metal alloy with a substantially flat oxide bottom surface (16) which is electrochemically active for the oxidation of oxygen. The metal alloy can comprise an electrically conductive inert structural metal and an active diffusable metal that during electrolysis slowly diffuses to the electrochemically active bottom surface (16) where it is oxidised for maintaining the electrochemically active bottom surface (16) and slowly dissolves into the molten electrolyte (30), in which case the bottom part (15a) forms a long-lasting supply of the active metal diffusable to the electrochemically active bottom surface (16).

Description

FIELD OF THE INVENTION

[0001] This invention relates to alloy-based oxygen-evolving anodes for the electrowinning of aluminium having an improved design for increasing their lifetime, cells using them and a method of producing aluminium with such anodes.

BACKGROUND ART

[0002] 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.

[0003] Using metal anodes in aluminium electrowinning cells would drastically improve the aluminium process by reducing pollution and the cost of aluminium production.

[0004] Several attempts have been made in order to develop non-carbon anodes for aluminium electrowinning cells, resistant to chemical attacks of the bath and by the cell environment, and with an electrochemical active surface for the oxidation of oxygen ions to atomic and molecular gaseous oxygen and having a low dissolution rate. However, all attempts have failed mainly due to the anode materials which had a low electrical conductivity and caused unacceptable contamination of the aluminium produced. Many patents have been filed on non-carbon anodes but none has found commercial acceptance, also because of economical reasons.

[0005] U.S. Pat. No. 4,614,569 (Duruz/Derivaz/Debely/Adorian) describes metal anodes for aluminium electrowinning coated with a protective coating of cerium oxyfluoride, formed in-situ in the cell or pre-applied, this coating being maintained during electrolysis by the addition of small amounts of a cerium compound to the molten cryolite electrolyte so as to protect the surface of the anode from the electrolyte attack.

[0006] Several designs for oxygen-evolving anodes for aluminium electrowinning cells were proposed in the following documents. U.S. Pat. 4,681,671 (Duruz) discloses vertical anode plates or blades operated in low temperature aluminium electrowinning cells. U.S. Pat. No. 5,310,476 (Sekhar/de Nora) discloses oxygen-evolving anodes consisting of roof-like assembled pairs of anode plates. U.S. Pat. No. 5,362,366 (de Nora/Sekhar) describes non-consumable anode shapes including roof-like assembled pairs of anode plates. U.S. Pat. No. 5,368,702 (de Nora) discloses vertical tubular or frustoconical oxygen-evolving anodes for multimonopolar aluminium cells. U.S. Pat. No. 5,683,559 (de Nora) describes an aluminium electrowinning cell with oxygen-evolving bent anode plates which are aligned in a roof-like configuration facing correspondingly shaped cathodes. U.S. Pat. No. 5,725,744 (de Nora/Duruz) discloses vertical oxygen-evolving anode plates, preferably porous or reticulated, in a multimonopolar cell arrangement for aluminium electrowinning cells operating at reduced temperature.

[0007] WO00/40781 and WO00/40782 (both de Nora) both disclose aluminium production anodes with a series of parallel spaced-apart elongated anode members which are electrochemically active for the oxidation of oxygen. Various anode members with different cross-sections are disclosed in these applications, in particular anode members with a tapered upper part and a flat electrochemically active bottom surface as shown in FIG. 5 of WO00/40781 as well as in FIGS. 3 and 13 of WO00/40782.

SUMMARY OF THE INVENTION

[0008] The present invention relates to improved anode designs, in particular those disclosed in WO00/40781 and WO00/40782 mentioned above. The anode member designs of the present invention are specially adapted, to promote gas release and/or electrolyte circulation through the anode and increase the lifetime of the anode that is made from an alloy comprising an electrically conductive inert structural metal, such as nickel and/or cobalt, and an active diffusable metal, such as iron, that diffuses to the electrochemically active anode surface where it is oxidised for maintaining the electrochemically active surface.

[0009] Thus, the invention provides a long-lasting metal-based oxygen-evolving anode for the electrowinning of aluminium from alumina dissolved in a molten electrolyte. This anode has a plurality of electrochemically active anode members. Each anode member comprises a bottom part which has a substantially constant width over its height and which is extended upwardly by a tapered top part for guiding a circulation of electrolyte thereon. The bottom part of each anode member is made of a metal alloy with a substantially flat oxide bottom surface which is electrochemically active for the oxidation of oxygen.

[0010] The metal alloy of the bottom part of each anode member comprises an electrically conductive inert structural metal and an active diffusable metal that during electrolysis slowly diffuses to the electrochemically active bottom surface where it is oxidised for maintaining the electrochemically active bottom surface and slowly dissolves into the molten electrolyte. This bottom part forms a long-lasting supply of the active metal diffusable to the electrochemically active bottom surface.

[0011] For instance, the inert structural metal is nickel and/or cobalt. The active diffusable metal may be iron, the electrochemically active bottom surface being iron oxide-based. Before use, the inert structural metal/active diffusable metal atomic ratio can be up to or even above 1, in particular from 1 to 4.

[0012] Usually, the metal alloy of the bottom part comprises the inert structural metal and the active diffusable metal in a total amount of at least 65 weight %, in particular at least 80 weight %, preferably at least 90 weight % of the alloy. For example, the metal alloy of the bottom part further comprises at least one metal selected from chromium, copper, silicon, titanium, tantalum, tungsten, vanadium, zirconium, scandium, yttrium, molybdenum, manganese, niobium, cerium and ytterbium in a total amount of up to 10 weight % of the alloy. Furthermore, the metal alloy of the bottom part may comprise at least one catalyst selected from iridium, palladium, platinum, rhodium, ruthenium, tin or zinc metals, Mischmetals and their oxides and metals of the Lanthanide series and their oxides as well as mixtures and compounds thereof, in a total amount of up to 5 weight % of the alloy. The metal alloy of the bottom part can comprise aluminium in an amount less than 20 weight %, in particular less than 10 weight %, preferably from 1 to 6 weight % of the alloy.

[0013] Examples of suitable metal alloys for the bottom part and conditioning are described in greater detail in WO00/06803 (Duruz/de Nora/Crottaz), WO00/06804 (Crottaz/Duruz), WO01/42534 (de Nora/Duruz), WO01/42536 (Duruz/Nguyen/de Nora) and PCT/IB02/01241 (Nguyen/de Nora).

[0014] In one embodiment, the anode is covered with a protective layer made of one or more cerium compounds, in particular cerium oxyfluoride. Such coatings and cell operation therewith are disclosed in U.S. Pat. No. 4,614,569 (Duruz/Derivaz/Debely/Adorian), U.S. Pat. No. 4,680,094 (Duruz), U.S. Pat. No. 4,683,037 (Duruz) and U.S. Pat. No. 4,966,674 (Bannochie/Sherriff). These coatings reduce the dissolution of the oxidised diffusable metal, in particular iron, and thus reduce the required diffusion of the diffusable metal to the electrochemically active bottom surface thereby extending the lifetime of the anode.

[0015] The diffusion rate of the diffusable metal at the operating conditions can be adjusted by an appropriate addition of one or more additives to the alloy of the anode bottom part as disclosed in PCT/IB02/01241 (Nguyen/de Nora).

[0016] Usually, the width of the bottom part is of the same order as the size of the height of the bottom part. For example, the height of the bottom part is in the range of about half to twice the size of the width of the bottom part.

[0017] The height of the reservoir-forming bottom part is usually at least several millimetres, typically from 5 to 25 mm, in particular from 10 to 15 mm. Such a reservoir has the capacity to provide an additional anode lifetime of 50 to 100%, for instance an additional lifetime of 5′000 to 10′000 hours to an anode member that has a lifetime of 10′000 hours without a reservoir-forming bottom part, in particular when the anode member has a composition and is operated under conditions exemplified in PCT/IB02/01241 or PCT/IB02/01952 (both in the name of Nguyen/de Nora).

[0018] The tapered top part of the or each anode member may have one or more upwardly converging inclined surfaces with a substantially constant slope, i.e. generally triangular or trapezoidal in cross-section. The top part may have a generally curved cross-section, in particular generally elliptic or semi-circular. The cross-section may be symmetric or asymmetric as explained below.

[0019] The height of the tapered top part may be greater than half the size of the width of the anode member but preferably not greater than twice the size of the width of the anode member. The surface of the tapered top part may have an average slope in the range of 30 and 75 deg, in particular 45 to 60 deg, to the horizontal.

[0020] During use, the tapered top part permits an improved up-flow of electrolyte from the electrochemically active surface by delimiting an electrolyte up-flow path with a gradually increasing section that reduces or prevents the formation of flow-inhibiting turbulences adjacent and/or above the anode members in the electrolyte.

[0021] The overall height of the anode member is usually of the same order as its width, for instance from half to three times, in particular from equal to twice, the width.

[0022] Usually, the electrochemically active anode members are spaced apart, usually parallel to one another and preferably with their electrochemically active bottom surfaces in a generally coplanar arrangement. In most embodiments, each anode member is elongated and has a substantially constant cross-section along its length. The anode members may be straight or arched or circular. Alternatively, the anode members may have a generally circular or quadratic or other polygonal base.

[0023] The spacing between the anode members should be sufficient to permit a flow of electrolyte and gas, in particular an up-flow driven by anodically released gas, between them. The spacing between the anode members can be of the same order as the height of the reservoir-forming bottom part of each anode member, for instance between half to twice the height of the bottom part. Usually, the spacing between two anode members is greater than 10 mm. To avoid substantial reduction of the overall surface area of the electrochemically active anode surfaces, the anode members should not be spaced by more than 20 mm, preferably 15 mm.

[0024] Preferably, the dimensions of the anode members and spacing between them are adapted to the hydrodynamic conditions during use in the molten electrolyte.

[0025] These spaced apart anode members can be connected through one or more electrically conductive connecting cross-members which may be embedded in the tapered top part of the anode members. A plurality of such connecting cross-members may be connected together through one or more electrically conductive connecting transverse members. Usually, the anode comprises a vertical current feeder which is mechanically and electrically connected to the or one of the above connecting members and which is connectable to a positive bus bar.

[0026] Furthermore, the anode may comprise one or more electrolyte guide members for guiding an electrolyte flow from and/or to the electrochemically active bottom surface(s), for example as disclosed in WO00/40781 (de Nora).

[0027] The shape of the tapered top part may be adapted for the down-flow of alumina-rich electrolyte or for the up-flow of alumina-depleted electrolyte. For instance, an anode member, in particular with a top part having an asymmetric cross-section, may be designed for a down-flow of electrolyte on one side and an up-flow of electrolyte on the other side of the tapered top part. In other words, the shape of the tapered top part can be arranged to promote an up-flow of electrolyte over one side of the top part and a down-flow of electrolyte over the other side of the top part.

[0028] The invention also relates to a cell for the electrowinning of aluminium from alumina, comprising at least one of the above described oxygen-evolving anodes facing a cathode in a molten electrolyte.

[0029] Suitable cell features are disclosed in U.S. Pat. No. 6,258,246 (Duruz/de Nora), WO00/63463 (de Nora), WO00/63464 (de Nora/Berclaz), WO01/31086 (de Nora/Duruz), WO01/42168 (de Nora/Duruz), WO01/42531 (Nguyen/Duruz/de Nora) and PCT/IB02/00670 (de Nora).

[0030] Another aspect of the invention relates to a method of electrowinning aluminium. The method comprises passing an electrolysis current in a molten electrolyte containing dissolved alumina between a cathode and at least one of the above described oxygen-evolving anodes to evolve oxygen on the anode(s) and produce aluminium on the cathode.

[0031] A protective layer of one or more cerium compounds, in particular cerium oxyfluoride, may be deposited and/or maintained on the anode by the presence of cerium species in the molten electrolyte, as disclosed in the abovementioned U.S. Pat. Nos. 4,614,569, 4,680,094, 4,683,037 and 4,966,674.

[0032] The molten electrolyte, usually a cryolite-based molten electrolyte, may be at a temperature in the range of 700° to 1000° C., in particular from 830° to 930° or 940° C. Preferably, the electrolyte is saturated or nearly saturated with dissolved alumina to reduce the solubility of the metal alloy of the bottom part of the oxygen-evolving anode(s).

[0033] A further inventive aspect concerns a metal-based anode for an aluminium electrowinning cell. The anode comprises a metal-based structure having an anode surface which is active for the anodic evolution of oxygen and which is arranged to be placed in the cell substantially parallel to a facing cathode. The metallic structure has a series of parallel anode members, each anode member comprising a tapered top part and an electrochemically active oxygen-evolving bottom surface below and integral with the tapered top part. The electrochemically active bottom surfaces of the metal-based structure are in a generally coplanar arrangement to form the active anode surface. The anode members are spaced laterally to form longitudinal flow-through openings for the flow of electrolyte.

[0034] The tapered top part of at least one anode member has an asymmetric cross-section adapted for an electrolyte up-flow on a first face of the tapered top part and for an electrolyte down-flow on a second face of the tapered top part. The first face delimits an up-flow through opening and the second face delimits a down-flow through opening.

[0035] In other words, the shape of the tapered top part is arranged to promote an up-flow of electrolyte over one face of the top part and a down-flow of electrolyte over the other face of the top part.

[0036] As mentioned above, at least one anode member may comprise a bottom part which has a substantially constant width over its height and which is extended upwardly by the tapered top part, the bottom part being made of a metal alloy with a substantially flat oxide bottom surface which forms said electrochemically active surface.

[0037] Such a metal alloy can comprise an electrically conductive inert structural metal and an active diffusable metal that during electrolysis slowly diffuses to the electrochemically active bottom surface where it is oxidised for maintaining the electrochemically active bottom surface and slowly dissolves into the molten electrolyte. Such a bottom part forms a long-lasting supply of the active metal diffusable to the electrochemically active bottom surface.

[0038] The bottom part can include any of the corresponding abovementioned features, in particular the features relating to the composition, shape and dimensions of the bottom part.

[0039] The electrochemically active bottom surface of at least one anode member can be joined to opposite bottom ends of the tapered top part of the anode member. In this case, the bottom surface can be generally planar or curved, in particular convex.

[0040] A pair of adjacent anode members can have their tapered top parts upwardly converging. Usually, the first faces of the pair of anode members delimit an up-flow through opening between the anode members of the pair and the second faces of the pair of anode members delimit two down-flow through openings on opposite sides of the pair of anode members. The first faces of the pair of anode members can be vertical or upwardly converging and the second faces of the pair of anode members can be upwardly converging.

[0041] At least one of these first face and second face can be generally planar and at least one of them can be curved, in particular convex. Various combinations of such shapes are described below.

[0042] As mentioned above, the height of the anode member is usually of the same order as its width, for instance from half to three times, in particular from equal to twice, the width. The first and second faces of the tapered top part may have an average slope in the range of 30 and 75 deg, in particular 45 to 60 deg, to the horizontal.

[0043] In most embodiments, each anode member is elongated and has a substantially constant cross-section along its length. The anode members may be straight or arched or circular. Alternatively, the anode members may have a generally circular or quadratic or other polygonal base.

[0044] The average spacing between the anode members can be of the same order as the height of the anode member bottom part of each anode member, for instance from a quarter to twice the height of the anode member. Usually, the average spacing between two anode members is greater than about 5 to 10 mm. To avoid substantial reduction of the overall surface area of the electrochemically active anode surfaces, active bottom surfaces of the anode members should not be spaced by more than about 20 to 30 mm.

[0045] Suitable anode materials for making the anode members are disclosed above. Further anode materials are disclosed in U.S. Pat. No. 6,077,415 (Duruz/de Nora), U.S. Pat. No. 6,113,758 (de Nora/Duruz), U.S. Pat. No. 6,248,227 (de Nora/Duruz), U.S. Pat. No. 6,372,099 (Duruz/de Nora) and WO00/40783 (de Nora/Duruz). Suitable electrochemically active anode coatings that can be maintained in-situ are disclosed in U.S. Pat. No. 4,614,569 (Duruz/Derivaz/Debely/Adorian), U.S. Pat. No. 4,680,094 (Duruz), U.S. Pat. No. 4,683,037 (Duruz), U.S. Pat. No. 4,966,674 (Bannochie/Sheriff), U.S. Pat. No. 6,372,099 (Duruz/de Nora) and PCT/IB02/01169 (de Nora/Nguyen), further suitable electrochemically active coating are for example disclosed in U.S. Pat. No. 6,103,090 (de Nora), U.S. Pat. No. 6,361,681 (de Nora/Duruz), U.S. Pat. No. 6,365,018 (de Nora) and WO99/36594 (de Nora/Duruz).

[0046] The invention also relates to an aluminium production cell comprising an anode as described above and to a method of electrowinning aluminium with such an anode.

[0047] The method of electrowinning aluminium comprises passing an electrolysis current in a molten electrolyte containing dissolved alumina between the anode a facing cathode to evolve oxygen anodically and produce aluminium cathodically. The anodically evolved oxygen drives an up-flow of alumina-depleted electrolyte over the first faces of the anode members of the anode, which up-flow promotes a down-flow of alumina-rich electrolyte over the second faces of the anode members of the anode.

[0048] Suitable additional features relating to the cell and its operation are disclosed above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0049] The invention will now be described by way of example with reference to the schematic drawings, wherein:

[0050] FIG. 1a and 1b show respectively a side elevation and a plan view of an anode according to the invention;

[0051] FIGS. 2a and 2b show respectively a side elevation and a plan view of another anode according to the invention;

[0052] FIG. 3 shows an aluminium electrowinning cell operating with anodes according to the invention fitted with electrolyte guide members;

[0053] FIGS. 4, 5 and 6 are schematic views of parts of aluminium electrowinning cells operating with anodes according to the invention, FIG. 4 illustrating electrolyte circulation;

[0054] FIG. 7 is a cross section of another anode according to the invention with electrolyte guide members only one of which is shown;

[0055] FIG. 8 shows a plan view of half of an assembly of several electrolyte guide members like the one shown in FIG. 7;

[0056] FIG. 9 is a plan view of the anode shown FIG. 13 with half of an assembly of electrolyte guide members as shown in FIG. 8;

[0057] FIG. 10 is a plan view of a variation of the anode of FIG. 9; and

[0058] FIGS. 11 to 14 are schematic views of parts of aluminium electrowinning cells operating with anodes having anode members with an asymmetric cross section.

DETAILED DESCRIPTION

[0059] FIG. 1a and 1b schematically show an anode 10 for the electrowinning of aluminium according to the invention.

[0060] The anode 10 comprises a vertical current feeder 11 for connecting the anode to a positive bus bar, a transverse member 12 and a pair of connecting cross-members 13 for connecting a series of elongated straight anode members 15.

[0061] In accordance with the invention, 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 electrochemically active lower oxide surface 16 where oxygen is anodically evolved during cell operation.

[0062] The anode members 15, in particular their bottom parts 15a, are made of an alloy comprising nickel and/or cobalt as electrically conductive inert structural metal(s) and iron as an active diffusable metal that during electrolysis slowly diffuses to the electrochemically active bottom surface where it is oxidised for maintaining the electrochemically active bottom surface and slowly dissolves into the molten electrolyte.

[0063] 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.

[0064] The anode members 15 are connected by the pair of connecting cross-members 13 which are in turn connected together by the transverse member 12 on which the vertical current feeder 11 is mounted. The current feeder 11, the transverse member 12, the connecting cross-members 13 and the anode members 15 are mechanically secured together by welding, rivets or other means.

[0065] Each anode member 15 has two flats 15c at the appropriate location in the tapered top part 15b for securing the cross-members 13 thereon. For simplicity, only one flat 15c is indicated in FIG. 1a

[0066] As described above, the electrochemically active surface 16 of the anode members 15 can be iron-oxide based in particular as described in greater detail in WO00/06803, WO00/06804, WO01/42534, WO01/42536 and PCT/IB02/01241 mentioned above. Also, the anode may be covered with a coating of one or more cerium compounds in particular cerium oxyfluoride as for example disclosed in U.S. Pat. No. 4,614,569, 4,680,094, 4,683,037 and 4,966,674 also mentioned above.

[0067] The transverse member 12 and the connecting cross-members 13 are so designed and positioned over the anode members 15 to provide a substantially even current distribution through the anode members 15 to their electrochemically active surfaces 16. The current feeder 11, the transverse member 12 and the connecting cross-members 13 do not need to be electrochemically active and their surface may passivate when exposed to electrolyte. However they should be electrically well conductive to avoid unnecessary voltage drops and should not substantially dissolve in electrolyte.

[0068] When the anode members 15 and the transverse members 12 are exposed to different thermal expansion, each anode member 15 may be made into two (or more where appropriate) separate “short” anode members. The “short” anode members should be longitudinally spaced apart when the thermal expansion of the anode members 15 is greater than the thermal expansion of the transverse members 12.

[0069] Alternatively, it may be advantageous in some cases, in particular to enhance the uniformity of the current distribution, to have more than two connecting cross-members 13 and/or a plurality of transverse members 12.

[0070] Also, it is not necessary for the two connecting cross-members 13 to be perpendicular to the anode members 15 in a parallel configuration as shown in FIGS. 1a & 1b. The connecting cross-members 13 may be in an X configuration in which each connecting member 13 extends from one corner to the opposite corner of a rectangular or square anode structure, a vertical current feeder 11 being connected to the intersection of the connecting members 13.

[0071] FIGS. 2a and 2b in which the same reference numerals designate the same elements, schematically show a variation of the anode 10 shown in FIGS. 1a and 1b.

[0072] Instead of having connecting cross-members 13, a transverse member 12 and a current feeder 11 for mechanically and electrically connecting the anode members 15 to a positive bus bar as illustrated in FIGS. 1a and 1b, the anode 10 shown in FIGS. 2a and 2b comprises a pair of cast or profiled support members 14 fulfilling the same function. Each cast support member 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 horizontally extending foot 14a and stem 14b.

[0073] The anode members 15 may be secured by force-fitting or welding the horizontally extending foot 14a on the flats 15c of the anode members 15. As an alternative, the shape of the anode members 15 and corresponding receiving slots in the horizontally extending foot 14a may be such as to allow only longitudinal movements of the anode members. For instance the anode members 15 and the horizontally extending foot 14a may be connected by dovetail joints.

[0074] FIG. 3 in which the same numeral references designate the same elements, shows an aluminium electrowinning cell according to the invention having a series of anodes 10 which are similar to those shown in FIGS. 1a and 1b, immersed in an electrolyte 30. The anodes 10 face a cathode cell bottom 20 connected to a negative busbar by current conductor bars 21. The cathode cell bottom 20 is made of conductive material such as graphite or other carbonaceous material coated with an aluminium-wettable refractory cathodic coating 22 on which aluminium 35 is produced and from which it drains or on which it forms a shallow pool, a deep pool or a stabilised pool. The molten produced aluminium 35 is spaced apart from the facing anodes 10 by an inter-electrode gap.

[0075] Pairs of anodes 10 are connected to a positive bus bar through a primary vertical current feeder 11′ and a horizontal current distributor 11″ connected at both of its ends to an anode 10 through a secondary vertical current distributor 11′″.

[0076] The secondary vertical current distributor 11′″ is mounted on the anode structure 12,13,15, on a transverse member 12 which is in turn connected to a pair of connecting cross-members 13 for connecting a series of anode members 15. The current feeders 11′,11″11′″, the transverse member 12, the connecting cross-members 13 and the anode members 15 are mechanically secured together by welding, rivets or other means.

[0077] The anode members 15 have an electrochemically active lower surface 16 on which during cell operation oxygen is anodically evolved. The anode members 15 are in the form of parallel rods in a foraminate 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 from the electrochemically active surfaces 16.

[0078] The iron oxide surface may extend over all immersed parts 11′″,12,13,15 of the anode 10, in particular over the immersed part of the secondary vertical current distributor 11′″ which is preferably covered with iron oxide at least up to 10 cm above the surface of the electrolyte 30.

[0079] The immersed but inactive parts of the anode 10 may be further coated with zinc oxide. However, when parts of the anode 10 are covered with zinc oxide, the concentration of dissolved alumina in the electrolyte 30 should be maintained at or close to saturation to prevent excessive dissolution of zinc oxide in the electrolyte 30.

[0080] The core of the inactive anode components 11′,11″,11′″,12,13 is preferably highly conductive and can be made of copper protected with successive layers of nickel, chromium, nickel, copper and optionally a further layer of nickel.

[0081] The anodes 10 are further fitted with means for enhancing dissolution of fed alumina in the form of electrolyte guide members 5 formed of parallel spaced-apart inclined baffles 5 located above and adjacent to the foraminate anode structure 12,13,15. The baffles 5 provide upper downwardly converging surfaces 6 and lower upwardly converging surfaces 7 that deflect gaseous oxygen which is anodically produced below the electrochemically active surface 16 of the anode members 15 and which escapes between the inter-member gaps 17 through the foraminate anode structure 12,13,15. The oxygen released above the baffles 5 promotes dissolution of alumina fed into the electrolyte 30 above the downwardly converging surfaces 6.

[0082] The aluminium-wettable cathodic coating 22 of the cell shown in FIG. 3 can advantageously be a slurry-applied refractory hard metal coating as disclosed in WO01/42531 (Nguyen/Duruz/de Nora), WO01/42168 (de Nora/Duruz), WO01/42531 (Nguyen/Duruz/de Nora) and PCT/IB02/01932 (Nguyen/de Nora).

[0083] The cell also comprises sidewalls 25 of carbonaceous or other material. The sidewalls 25 are coated/impregnated above the surface of the electrolyte 30 with a boron or a phosphate protective coating/impregnation 26 as described in U.S. Pat. No. 5,486,278 (Manganiello/Duruz/Bellò).

[0084] Below the surface of the electrolyte 30 the sidewalls 25 are coated with a highly aluminium-wettable coating 23, for example as disclosed in WO01/42531, WO01/42168 and PCT/IB02/01932 mentioned above, so that molten aluminium 35 driven by capillarity and magneto-hydrodynamic forces covers and protects the sidewalls 25 from the electrolyte 35. The aluminium-wettable coating 23 extends from the aluminium-wettable cathodic coating 22 over the surface of connecting corner prisms 28 up the sidewalls 25 at least to the surface of the electrolyte 30. The aluminium-wettable side coating 23 may be advantageously made of an applied and dried and/or heat treated slurry of particulate TiB2 in colloidal silica which is highly aluminium-wettable.

[0085] The sidewalls 25 and cathode bottom 20 may also be shielded from the electrolyte 30 by an aluminium-wettable openly porous lining (not shown), as disclosed in PCT/IB02/00668, PCT/IB02/00670, PCT/IB02/01883 and PCT/IB02/01884 (all in the name of de Nora) filled with molten aluminium.

[0086] Alternatively, above and below the surface of the electrolyte 30, the sidewalls 25 may be covered with a zinc-based coating, such as a zinc-oxide coating optionally with alumina or a zinc aluminate coating. When a zinc-based coating is used to coat sidewalls 25 or anodes 10 as described above, the concentration of dissolved alumina in the molten electrolyte 30 should be maintained at of close to saturation to substantially prevent dissolution of such a coating.

[0087] In a further alternative, the cell may be operated with a conventional frozen electrolyte ledge covering and protecting the sidewalls 25.

[0088] During cell operation, alumina is fed to the electrolyte 30 all over the baffles 5 and the metallic anode structure 12,13,15. The fed alumina is dissolved and distributed from the bottom end of the converging surfaces 6 into the inter-electrode gap through the inter-member gaps 17 and around edges of the metallic anode structure 12,13,15, i.e. between neighbouring pairs of anodes 10 or between peripheral anodes 10 and sidewalls 25. By passing an electric current between anodes 10 and facing cathode cell bottom 20 oxygen is evolved on the electrochemically active anode surfaces 16 and aluminium is produced which is incorporated into the cathodic molten aluminium 35. The oxygen evolved from the active surfaces 16 escapes through the inter-member gaps 17 and is deflected by the upwardly converging surfaces 7 of baffles 5. The oxygen escapes from the uppermost ends of the upwardly converging surfaces 7 enhancing dissolution of the alumina fed over the downwardly converging surfaces 6.

[0089] The aluminium electrowinning cells partly shown in FIGS. 4, 5 and 6 in which the same numeral references designate the same elements, are similar to the aluminium electrowinning cell shown in FIG. 3.

[0090] In FIG. 4 the guide members are inclined baffles 5 as shown in FIG. 3. In this example the uppermost end of each baffle 5 is located just above mid-height between the surface of the electrolyte 30 and the transverse connecting members 13.

[0091] Also shown in FIG. 4, an electrolyte circulation 31 is generated by the escape of gas released from the active surfaces 16 of the anode members 15 between the inter-member gaps 17 and which is deflected by the upward converging surfaces 7 of the baffles 5 confining the gas and the electrolyte flow between their uppermost edges. From the uppermost edges of the baffles 5, the anodically evolved gas escapes towards the surface of the electrolyte 30, whereas the electrolyte circulation 31 flows down through the downward converging surfaces 6, through the inter-member gaps and around edges of the metallic anode structure 12,13,15 to compensate the depression created by the anodically released gas below the active surfaces 17 of the anode members 15. The electrolyte circulation 31 draws down into the inter-electrode gap dissolving alumina particles 32 which are fed above the downward converging surfaces 6.

[0092] FIG. 5 shows part of an aluminium electrowinning cell operating with an anode 10 according to the invention having electrochemically active members 15 with a rounded tapered upper part 15b having a semi-circular cross-section. The anode 10 is covered with baffles 5 operating as electrolyte guide members like those shown in cell of FIG. 4 but whose surfaces are only partly converging. The lower sections 4 of the baffles 5 are vertical and parallel to one another, whereas their upper sections have upward and downward converging surfaces 6,7. The uppermost end of the baffles 5 are located below but close to the surface of the electrolyte 30 to increase the turbulence at the electrolyte surface caused by the release of anodically evolved gas.

[0093] FIG. 6 shows a variation of the anode members baffles shown in FIG. 5, wherein the anode members 15 have a rounded tapered upper part 15b with an elliptic cross-section and the baffles 5 have their parallel vertical sections 4 located above their converging surfaces 6,7.

[0094] By guiding and confining anodically-evolved oxygen towards the surface of the electrolyte 30 with baffles or other confinement means as shown in FIGS. 5 and 6 and as further described in WO00/40781 (de Nora), oxygen is released so close to the surface as to created turbulences above the downwardly converging surfaces 6, promoting dissolution of alumina fed thereabove.

[0095] It is understood that the electrolyte confinement members 5 shown in FIGS. 3, 4, 5 and 6 can either be elongated baffles, or instead consist of a series of vertical chimneys of funnels of circular or polygonal cross-section, for instance as described below.

[0096] FIGS. 7 and 9 where the same numeral references designate the same elements, illustrate an anode 10′ having a circular bottom, the anode 10′ being shown in cross-section in FIG. 7 and from above in FIG. 9. On the right hand side of FIGS. 7 and 9 the anode 10′ is shown with electrolyte guide members 5′ according to the invention. The electrolyte guide members 5′ represented in FIG. 9 are shown separately in FIG. 8.

[0097] The anode 10′ shown in FIGS. 7 and 9 has several concentric circular anode members 15. The anode members 15 are laterally spaced apart from one another by inter-member gaps 17 and connected together by radial connecting cross-members in the form of flanges 13 which join an outer ring 13′. The outer ring 13′ extends vertically from the outermost anode members 15, as shown in FIG. 7, to form with the radial flanges 13 a wheel-like structure 13,13′, shown in FIG. 9, which secures the anode members 15 to a central anode current feeder 11.

[0098] As shown in FIG. 7, the innermost circular anode member 15 partly merges with the current feeder 11, with ducts 18 extending between the innermost circular anode member 15 and the current feeder 11 to permit the escape of oxygen produced underneath the central current feeder 11.

[0099] Each electrolyte guide member 5′ is in the general shape of a funnel having a wide bottom opening 9 for receiving anodically produced oxygen and a narrow top opening 8 where the oxygen is released to promote dissolution of alumina fed above the electrolyte guide member 5′. The inner surface 7 of the electrolyte guide member 5′ is arranged to canalise and promote an upward electrolyte flow driven by anodically produced oxygen. The outer surface 6 of the electrolyte guide member 5′ is arranged to promote dissolution of alumina fed thereabove and guide alumina-rich electrolyte down to the inter-electrode gap, the electrolyte flowing mainly around the foraminate structure.

[0100] As shown in FIGS. 8 and 9, the electrolyte guide members 5′ are in a circular arrangement, only half of the arrangement being shown. The electrolyte guide members 5′ are laterally secured to one another by attachments 3 and so arranged to be held above the anode members 15, the attachments 3 being for example placed on the flanges 13 as shown in FIG. 9 or secured as required. Each electrolyte guide member 5′ is positioned in a circular sector defined by two neighbouring radial flanges 13 and an arc of the outer ring 13′ as shown in FIG. 9.

[0101] The arrangement of the electrolyte guide members 5′ and the anode 10′ can be moulded as units. This offers the advantage of avoiding mechanical joints and the risk of altering the properties of the materials of the electrolyte guide members 5′ or the anode 10′ by welding.

[0102] FIG. 10 where the same numeral references designate the same elements, illustrates a square anode 10′ as a variation of the round anode 10′ of FIGS. 7 and 9. The anode 10′ of FIG. 10 has generally rectangular concentric parallel anode members 15 with rounded corners. The anode 10′ shown in FIG. 10 can be fitted with electrolyte guide members similar to those of FIGS. 7 to 9 but in a corresponding rectangular arrangement.

[0103] FIGS. 11 to 14 in which the same reference numerals designate the same elements, show anodes 10 according to the invention having anode members 15,15′ which are asymmetric in vertical cross-section. The anode members 15,15′ are arranged in pairs with their tapered upper parts 15b upwardly converging. More specifically, the tapered upper parts 15b have faces 15d′,15e′ for guiding an up-flow of alumina-depleted electrolyte indicated by arrows 31′ in an up-flow through opening 17′ and faces 15d″,15e″ for guiding a down-flow of alumina-rich electrolyte indicated by arrows 31″ in a down-flow through opening 17″ between adjacent pairs of anode members 15,15′ and around the outermost anode members 15,15′ of the anodes 10.

[0104] In FIGS. 11 and 13 faces 15d′,15d″ are planar and inclined, whereas in FIG. 12 these faces 15e′,15e″ are convex. This applies also to the second pair of anode members 15 starting from the left of FIG. 14. The remaining anode members 15 shown in FIG. 14 have one planar face 15d′ and one convex face 15e″.

[0105] On the left-hand side of FIGS. 11 and 12, each anode member 15 comprises a bottom part 15a which has a constant width over its height and which is extended upwardly by the tapered top part 15b that is integral with the bottom part 15a. The bottom part 15a is made of a metal alloy with a substantially flat oxide bottom surface which forms the electrochemically active surface 16. The metal alloy can comprise an electrically conductive inert structural metal and an active diffusable metal that during electrolysis slowly diffuses to the electrochemically active bottom surface 16 where it is oxidised for maintaining the electrochemically active bottom surface and slowly dissolves into the molten electrolyte 30. According to the invention, the bottom part forms a long-lasting supply of the active metal diffusable to the electrochemically active bottom surface 16.

[0106] On the right-hand side of FIGS. 11 and 12, the electrochemically active bottom surface 16 of each anode member 15′ is joined to opposite bottom ends of the tapered top part of the anode member 15′.

[0107] Such an anode member design can also be appropriate when the anode members are made of materials that are inhibited from dissolving in the molten electrolyte 30 under the cell operating conditions, for example when the anodes are coated with an in-situ maintained cerium oxyfluoride-based coating as disclosed in U.S. Pat. No. 4,614,569 (Duruz/Derivaz/Debely/Adorian), U.S. Pat. No. 4,680,094 (Duruz), U.S. Pat. No. 4,683,037 (Duruz), U.S. Pat. No. 4,966,674 (Bannochie/Sheriff), U.S. Pat. No. 6,372,099 (Duruz/de Nora) and PCT/IB02/01169 (de Nora/Nguyen), or when the anodes are covered with another electrochemically active coating as for example disclosed in U.S. Pat. No. 6,103,090 (de Nora), U.S. Pat. No. 6,361,681 (de Nora/Duruz), U.S. Pat. No. 6,365,018 (de Nora) and WO99/36594 (de Nora/Duruz). Further suitable anode materials are disclosed in U.S. Pat. No. 6,077,415 (Duruz/de Nora), U.S. Pat. No. 6,113,758 (de Nora/Duruz), U.S. Pat. No. 6,248,227 (de Nora/Duruz), U.S. Pat. No. 6,372,099 (Duruz/de Nora) and WO00/40783 (de Nora/Duruz).

[0108] FIGS. 13 and 14 show further anodes 10 with anode members 15 illustrating different asymmetric profiles (cross-sections). The anode members 15 have a bottom part 15a which has a constant width over its height and which is extended upwardly by a tapered top part 15b.

[0109] In FIGS. 13 and 14 the anode members 15 have vertical planar faces 15d′ (except the second pair of anode members 15 starting from the left of FIG. 14 whose faces 15e′ are convex) for guiding an up-flow of electrolyte 30 (indicated by arrows 31′). The inclined faces 15d″,15e″ for guiding a down-flow of electrolyte 30 (indicated by arrows 31″), are planar in FIG. 13 and convex in FIG. 14.

[0110] On the left-hand side of FIGS. 13 and 14 the bottom part 15a of each anode member 15 extends vertically below the tapered top parts 15b, whereas on the right-hand side of FIGS. 13 and 14, the bottom part 15a of each anode member 15 extends below the tapered top parts 15b along an inclined direction in continuation of faces 15d″,15e″.

[0111] In variations of the anode members 15,15′ shown in FIGS. 11 to 14, some or all faces 15d′,15d″,15e′,15e″ can be made concave.

Claims

1. A long-lasting metal-based oxygen-evolving anode for the electrowinning of aluminium from alumina dissolved in a molten electrolyte, having a plurality of electrochemically active anode members, each member comprising a bottom part which has a substantially constant width over its height and which is extended upwardly by a tapered top part for guiding a circulation of electrolyte thereon, wherein the bottom part is made of a metal alloy with a substantially flat oxide bottom surface which is electrochemically active for the oxidation of oxygen, the metal alloy comprising an electrically conductive inert structural metal and an active diffusable metal that during electrolysis slowly diffuses to the electrochemically active bottom surface where it is oxidised for maintaining the electrochemically active bottom surface and slowly dissolves into the molten electrolyte, said bottom part forming a long-lasting supply of the active metal diffusable to the electrochemically active bottom surface.

2. The anode of claim 1, wherein the inert structural metal of at least one bottom part is selected from nickel and cobalt and alloys thereof.

3. The anode of claim 1 or 2, wherein the active diffusable metal of at least one bottom part is iron, the electrochemically active bottom surface being iron oxide-based.

4. The anode of any preceding claim, wherein at least one bottom part has an inert structural metal/active diffusable metal atomic ratio below 1 before use.

5. The anode of any one of claims 1 to 3, wherein at least one bottom part has an inert structural metal/active diffusable metal atomic ratio above 1, in particular from 1 to 4, before use.

6. The anode of any preceding claim, wherein the metal alloy of at least one bottom part comprises the inert structural metal and the active diffusable metal in a total amount of at least 65 weight %, in particular at least 80 weight %, preferably at least 90 weight % of the alloy.

7. The anode of claim 6, wherein the metal alloy of said at least one bottom part comprises at least one further metal selected from chromium, copper, silicon, titanium, tantalum, tungsten, vanadium, zirconium, scandium, yttrium, molybdenum, manganese, niobium, cerium and ytterbium in a total amount of up to 10 weight % of the alloy.

8. The anode of claim 6 or 7, wherein the metal alloy of said at least one bottom part comprises at least one catalyst selected from iridium, palladium, platinum, rhodium, ruthenium, tin or zinc metals, Mischmetals and their oxides and metals of the Lanthanide series and their oxides as well as mixtures and compounds thereof, in a total amount of up to 5 weight % of the alloy.

9. The anode of claim 6, 7 or 8, wherein the metal alloy of said at least one bottom part further comprises aluminium in an amount less than 20 weight %, in particular less than 10 weight %, preferably from 1 to 6 weight % of the alloy.

10. The anode of any preceding claim, which is covered with a protective layer made of one or more cerium compounds, in particular cerium oxyfluoride.

11. The anode of any preceding claim, wherein the tapered top part of at least one anode member has a face that is inclined at constant slope.

12. The anode of any one of claims 1 to 10, wherein the tapered top part of at least one anode member has a curved cross-section.

13. The anode of any preceding claim, wherein the tapered top part of at least one anode member has a symmetric cross-section.

14. The anode of any one of claims 1 to 12, wherein the tapered top part of at least one anode member has an asymmetric cross-section.

15. The anode of any preceding claim, wherein the electrochemically active anode members are spaced apart, preferably parallel to one another with their electrochemically active bottom surfaces in a generally coplanar arrangement.

16. The anode of any preceding claim, wherein at least one anode member is elongated and has a substantially constant cross-section along its length.

17. The anode of claim 16, wherein at least one anode member is straight.

18. The anode of claim 16, wherein at least one anode member is circular.

19. The anode of any preceding claim, wherein a plurality of anode members are connected through one or more electrically conductive connecting cross-members in particular embedded in the tapered top part of the anode members.

20. The anode of claim 19, wherein a plurality of connecting cross-members are connected together through one or more electrically conductive connecting transverse members.

21. The anode of claim 19 or 20, comprising a vertical current feeder which is mechanically and electrically connected to the or one of said connecting members and which is connectable to a positive bus bar.

22. The anode of any preceding claim, comprising one or more electrolyte guide members for guiding an electrolyte flow from and/or to the electrochemically active bottom surfaces.

23. A cell for the electrowinning of aluminium from alumina, comprising at least one oxygen-evolving anode as defined in any preceding claim facing a cathode in a molten electrolyte.

24. A method of electrowinning aluminium comprising passing an electrolysis current in a molten electrolyte containing dissolved alumina between a cathode and an anode as defined in any one of claims 1 to 22, to evolve oxygen on the anode and produce aluminium on the cathode.

25. The method of claim 24, wherein a protective layer of one or more cerium compounds, in particular cerium oxyfluoride, is deposited and/or maintained on the anode by the presence of cerium species in the molten electrolyte.

26. A metal-based anode for an aluminium electrowinning cell, comprising a metal-based structure having an anode surface which is active for the anodic evolution of oxygen and which is arranged to be placed in the cell substantially parallel to a facing cathode, said metallic structure having a series of parallel anode members, each anode member comprising a tapered top part and an electrochemically active oxygen-evolving bottom surface below and integral with the tapered top part, the electrochemically active bottom surfaces of the metal-based structure being in a generally coplanar arrangement to form said active anode surface, the anode members being spaced laterally to form longitudinal flow-through openings for the flow of electrolyte, wherein the tapered top part of at least one anode member has an asymmetric cross-section adapted for an electrolyte up-flow on a first face of the tapered top part and for an electrolyte down-flow on a second face of the tapered top part, the first face delimiting an up-flow through opening and the second face delimiting a down-flow through opening.

27. The anode of claim 26, wherein at least one anode member comprises a bottom part which has a substantially constant width over its height and which is extended upwardly by the tapered top part, the bottom part being made of a metal alloy with a substantially flat oxide bottom surface which forms said electrochemically active surface, the metal alloy comprising an electrically conductive inert structural metal and an active diffusable metal that during electrolysis slowly diffuses to the electrochemically active bottom surface where it is oxidised for maintaining the electrochemically active bottom surface and slowly dissolves into the molten electrolyte, said bottom part forming a long-lasting supply of the active metal diffusable to the electrochemically active bottom surface.

28. The anode of claim 26, wherein the electrochemically active bottom surface of at least one anode member is joined to opposite bottom ends of the tapered top part of the anode member.

29. The anode of any one of claims 26 to 28, comprising a pair of adjacent anode members having tapered top parts that converge upwardly, said first faces of the pair of anode members delimiting an up-flow through opening between the anode members of said pair, said second faces of the pair of anode members delimiting two down-flow through openings on opposite sides of the pair of anode members.

30. The anode of claims 29, wherein the first faces of said pair of anode members are vertical or upwardly converging and the second faces said pair of anode members are upwardly converging.

31. The anode of any one of claims 26 to 30, wherein at least one of said first faces and second faces is generally planar.

32. The anode of any one of claims 26 to 31, wherein at least one of said first faces and second faces is curved, in particular convex.

33. The anode of any one of claims 26 to 32, wherein at least one electrochemically active bottom surface is generally planar.

34. An aluminium production cell comprising an anode as defined in any one of claims 26 to 33.

35. A method of electrowinning aluminium comprising passing an electrolysis current in a molten electrolyte containing dissolved alumina between an anode as defined in any one of claims 26 to 33 and a facing cathode to evolve oxygen anodically and produce aluminium cathodically, wherein anodically evolved oxygen drives an up-flow of alumina-depleted electrolyte over the first faces of said anode members, which up-flow promotes a down-flow of alumina-rich electrolyte over the second faces of said anode members.