Abstract:
A cell for the electrowinning of aluminium ( 50 ) form alumina dissolved in a molten electrolyte comprise a generally horizontal cell bottom ( 5 ), preferably aluminium-wettable, on which a pool of product aluminium ( 50 ) is collected from at least one electrically conductive cathodic element ( 10 ) having aluminium-wettable cathode surfaces ( 11 ). The cathodic element comprises an inclined cathodic wall ( 10 ) in the electrolyte ( 60 ) above the generally horizontal cell bottom ( 5 ). The cathodic wall ( 10 ) has an upwardly-oriented inclined face ( 11 ) that forms a sloping upper aluminum-wettable drained active cathode surface on which aluminium is produced and drains into the aluminium pool ( 50 ), and a downwardly-oriented inclined face ( 12 ) which is in contact with the molten electrolyte ( 60 ) and which overlies the aluminium pool ( 50 ). The aluminium pool ( 50 ) covers substantially the entire cell bottom ( 5 ) including underneath the cathodic wall ( 10 ). A return path for alumina-enriched electrolyte ( 60 ) towards a bottom end of the anode-cathode gap ( 40 ) may be provided behind the cathodic wall ( 10 ) along an inactive surface ( 12 ) thereof. The cell may be fitted with anodes ( 10 ) that are foraminate, e.g. an arrangement of spaced apart parallel rods, or solid plates.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to a cell for the electrowinning of aluminium from alumina provided with inclined aluminium-wettable drained cathodes.  
       BACKGROUND ART  
       [0002]     The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite containing salts, at temperatures around 950° C. is more than one hundred years old. This process and the cell design have not undergone any great change or improvement and carbonaceous materials are still used as electrodes and cell linings.  
         [0003]     U.S. Pat. No. 3,400,061 (Lewis/Hildebrandt) and U.S. Pat. No. 4,602,990 (Boxall/Gamson/Green/Traugott) disclose aluminium electrowinning cells with sloped drained cathodes facing anodes sloping across the cell. In these cells, the molten aluminium flows down the sloping cathodes into a median longitudinal groove along the centre of the cell, or into lateral longitudinal grooves along the cell sides, for collecting the molten aluminium and delivering it to a sump.  
         [0004]     In U.S. Pat. No. 5,362,366 (de Nora/Sekhar), a double-polar anode-cathode arrangement was disclosed wherein cathode bodies were suspended from the anodes permitting removal and reimmersion of the assembly during operation, such assembly also operating with a drained cathode.  
         [0005]     U.S. Pat. No. 5,368,702 (de Nora) proposed a novel multimonopolar cell having upwardly extending cathodes facing and surrounded by or in-between anodes having a relatively large inwardly-facing active anode surface area. In some embodiments, electrolyte circulation was achieved c-sing a tubular anode with openings.  
         [0006]     U.S. Pat. No. 5,651,874 (de Nora/Sekhar) proposed coating components with a slurry-applied coating of refractory boride, which proved excellent for cathode applications. This publication discloses slurry-applied applications and novel drained cathode configurations, including designs where a solid cathode body with an inclined upper drained cathode surface is placed on or secured to the cell bottom.  
         [0007]     U.S. Pat. No. 5,472,578 (de Nora) discloses an aluminium production cell comprising a grid on the cell bottom for restraining motion of the aluminium pool on the cell bottom. In some embodiments, the top end of the grid forms an aluminium-wettable drained cathode surface under an active anode surface.  
         [0008]     WO00/40782 (de Nora) discloses aluminium production anodes with a series of coplanar parallel elongated anode members which are spaced-apart by flow-through openings and which form an electrochemically active surface. In one embodiment two downwardly converging spaced apart adjacent anodes can be arranged between a pair of substantially vertical cathodes. The adjacent anodes are spaced apart by an electrolyte down-flow gap in which alumina-rich electrolyte flows downwards until it circulates via the adjacent anodes&#39; flow-through openings into the inter-electrode gaps.  
         [0009]     WO01/31088 (de Nora) discloses aluminium electrowinning cells with solid anodes having a V-shaped active surface facing sloping cathodes. The anodes and cathodes are associated with vertical passages for the circulation of alumina-rich electrolyte to a bottom part of the inter-electrode gaps spacing the anodes and cathodes.  
         [0010]     While the foregoing references indicate continued efforts to improve cell operations, none suggests the invention and there have been no entirely acceptable proposals for improving the cell efficiency, and at the same time facilitating the implementation of a drained cathode configuration with improved electrolyte circulation and large storage capacity of product aluminium.  
       OBJECTS OF THE INVENTION  
       [0011]     It is an object of the invention to provide an aluminium electrowinning cell with an aluminium-wettable drained cathode of great working area and with a great aluminium storage capacity.  
         [0012]     Another object of the invention is to provide a novel cathode design which can easily be retrofitted in existing conventional aluminium production cells.  
         [0013]     A further object of the invention is to provide an aluminium production cell, in particular a retrofitted cell, with cathodes that can be replaced or serviced during cell operation.  
         [0014]     Yet another object of the invention is to provide an aluminium production cell with low cost dimensionally stable aluminium wettable-drained cathodes.  
         [0015]     A major object of the invention is to provide an aluminium electrowinning cell which generates less pollution than conventional Hall-Héroult cells.  
       SUMMARY OF THE INVENTION  
       [0016]     The invention relates to a cell for the electrowinning of aluminium from alumina dissolved in a molten electrolyte. The cell comprises a generally horizontal cell bottom on which a pool of product aluminium is collected and at least one electrically conductive cathodic element having one or more sloping upper aluminium-wettable drained active cathode surfaces separated by an anode-cathode gap from one or more anodes with corresponding sloping active anode surfaces.  
         [0017]     According to the invention, the cathodic element comprises an inclined cathodic wall in the electrolyte above the generally horizontal cell bottom. This cathodic wall has an upwardly-oriented inclined face that forms the sloping upper aluminium-wettable drained active cathode surface(s) on which aluminium is produced and drains into the aluminium pool, and a downwardly-oriented inclined face which is in contact with the molten electrolyte and which overlies the aluminium pool. The aluminium pool covers substantially the entire cell bottom including underneath the cathodic wall.  
         [0018]     The cathodic wall can be placed into existing or new Hall-Héroult cells or into cells of new design providing the cells are fitted with sloping consumable or preferably non-consumable anodes. The cell bottom is preferably aluminium-wettable. It can be made of carbon, in particular carbon blocks, optionally coated with an aluminium-wettable material, for example as disclosed in U.S. Pat. No. 5,651,874 (de Nora/Sekhar), WO98/17842 (Sekhar/Duruz/Liu), WO01/42531 (Nguyen/Duruz/de Nora), WO01/42168 (de Nora/Duruz) and PCT/IB02/01932 (Nguyen/de Nora).  
         [0019]     The cell according to the invention can be an entirely new cell or a retrofitted cell that comprises a cell bottom of a refurbished cell retrofitted with the above described anode structure and sloping cathode.  
         [0020]     Such a cathode design on the one hand provides a great aluminium storage capacity and a great active cathode surface area, and on the other hand reduces the required cathodic material for producing cathodes having a sloping cathode surface.  
         [0021]     The active cathode surface is usually at an angle between 15 deg. and up to nearly vertical, typically 85 deg. Such a cathode configuration advantageously has active cathode surfaces with a steep slope, i.e. above 45 deg., typically from 60 deg. to 80 deg.  
         [0022]     This cathodic wall can comprise a generally flat plate. The plate can be uniformly planar or have a plurality of sloping sections, in particular in a v- or inverted v-shape arrangement in cross-section. Alternatively, the cathodic wall can be generally conical or pyramidal. Alternatively, the cathodic wall can made of a series of spaced apart generally parallel elongated cathodic members, such as bars, rods or blades. Each elongated member may be horizontal or at a slope, in particular extending along a vertical plane that is perpendicular to the sloping upper aluminium-wettable drained active cathode surface.  
         [0023]     For instance, the cathodic wall has its bottom end on the cell bottom in the aluminium pool.  
         [0024]     Alternatively, the cathodic wall may be suspended in the molten electrolyte. The cathodic wall may be suspended and spaced above the aluminium pool, in which case the cathodic wall is connected electrically above the electrolyte. Alternatively, the cathodic wall may be suspended and dip in the aluminium pool and can thus be electrically connected either above the electrolyte or through the aluminium pool.  
         [0025]     Advantageously, the cathodic wall has a variable section that decreases with an increasing distance to the electrical cathodic connection such that the section is adapted to the decreasing amount of current that flows through the cathodic wall to maintain a substantially uniform current density throughout the cathodic wall.  
         [0026]     When the cathodic wall is suspended in the electrolyte or when it can be otherwise accessed from above the electrolyte, for instance by having a part extending above the surface of electrolyte, it can be introduced into and removed from the cell during cell operation, i.e. without shutting down the cell.  
         [0027]     Especially when the cathodic wall rests on the cell bottom or dips in the aluminium pool, it advantageously has a passage in a bottom part for the aluminium pool. This passage may also serve for a flow of alumina-rich electrolyte from behind the active cathode surface(s) to a bottom part of the anode-cathode gap.  
         [0028]     The cathodic wall may also have an opening in a top part thereof for the flow of electrolyte from above an upper part of the anode-cathode gap to behind the active cathode surface(s). Alternatively, the cathodic wall can have an upper end that delimits a passage for the flow of electrolyte from above an upper part of the anode-cathode gap to behind the active cathode surface(s).  
         [0029]     In some embodiments, electrolyte circulating behind the cathode surface can enter the anode-cathode gap through openings in the cathode. When the cathodic wall is made of a series of spaced apart generally parallel elongated cathodic members, the circulation of electrolyte can be provided downwardly behind the elongated cathodic members and into the anode-cathode gap through passages between the elongated cathodic members.  
         [0030]     The cathodic wall can be made of an aluminium-wettable openly porous ceramic or ceramic-based material which is mechanically and chemically resistant and which is filled with molten aluminium.  
         [0031]     Suitable ceramic-based materials that are substantially resistant and inert to molten aluminium include oxides of aluminium, zirconium, tantalum, titanium, silicon, niobium, magnesium and calcium and mixtures thereof, as a simple oxide and/or in a mixed oxide, for example an aluminate of zinc (e.g. ZnAlO 4 ) or titanium (e.g. TiAlO 5 ) Other suitable inert and resistant ceramic materials can be selected amongst nitrides, carbides and borides and oxycompounds thereof, such as aluminium nitride, AlON, SiAlON, boron nitride, silicon nitride, silicon carbide, aluminium borides, alkali earth metal zirconates and aluminates, and their mixtures.  
         [0032]     Preferably, the aluminium-wettable openly porous walls contain an aluminium-wetting agent. Suitable wetting agents include metal oxides which are reactable with molten aluminium to form a surface layer containing alumina, aluminium and metal derived from the metal oxide and/or partly oxidised metal, such as manganese, iron, cobalt, nickel, copper, zinc, molybdenumn, lanthanum or other rare earth metals or combinations thereof, for instance as disclosed in PCT/IB02/00668 (de Nora).  
         [0033]     Further suitable materials for producing the openly porous walls are described in U.S. Pat. No. 4,600,481 (Sane/Wheeler/Gagescu/Debely/Adorian/Derivaz).  
         [0034]     The anodes can be made of carbon but are preferably made of oxygen evolving materials, in particular metal-based materials, such as surface oxidised alloys. The anodes can also be made of materials active for the oxidation of fluorine ions. Suitable metal-based anodes for the oxidation of oxygen ions or fluorine ions are disclosed in WO0/06802, WO00/06803 (both in the name of Duruz/de Nora/Crottaz), WO00/06804 (Crottaz/Duruz), WO01/43208 (Duruz/de Nora), WO01/42534 (de Nora/Duruz) and WO01/42536 (Duruz/Nguyen/de Nora). Further oxygen-evolving anode materials are disclosed in WO99/36593, WO99/36594, WO00/06801, WO00/06805, WO00/40783 (all in the name of de Nora/Duruz), WO00/06800 (Duruz/de Nora), WO99/36591 and WO99/36592 (both in the name of de Nora).  
         [0035]     The oxygen-evolving anodes may be coated with a protective layer made of one or more cerium compounds, in particular cerium oxyfluoride, 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) and U.S. Pat. Nos. 4,966,674, 4,966,674 (Bannochie/Sheriff), PCT/IB02/00667 (Nguyen/de Nora) and PCT/IB02/01169 (de Nora/Nguyen).  
         [0036]     Suitable oxygen-evolving anodes may comprise an electrochemically active foraminate metallic anode structure for the evolution of oxygen. The foraminate anode structure has through-openings for the circulation of electrolyte therethrough and is grid-like or plate-like.  
         [0037]     For example, the foraminate anode structure comprises a perforated plate or is made of a series of spaced-apart parallel elongated anode members, for instance as disclosed in WO00/40782 (de Nora). The anode members can be horizontal or at a slope, in particular generally extending along a vertical plane that is perpendicular to the cathode surface. Preferably the elongated anode members have a cross-section that is proportional to the anodic current passed therethrough, i.e. a decreasing cross-section with a decreasing amount of current, to maintain a substantially uniform current density along the anode members. For example, the elongated anode members are elongated plates or blades, or rods, bars or wires.  
         [0038]     In one embodiment, the cell comprises at least one electrolyte guide member located above the foraminate anode structure for guiding the circulation of electrolyte.  
         [0039]     For instance, the anode has an inclined plate-like or grid-like open anode structure which has a generally v-shaped configuration in cross-section and which faces a corresponding generally v-shaped active cathode surface. In such a case, one or more electrolyte guide members can be located above the v-shaped anode structure. These guide members conveniently extend over substantially the entire v-shaped anode structure for guiding an up-flow of alumina-depleted electrolyte from the anode through-openings to a location above the anode structure where the electrolyte is enriched with alumina and then sideways over and around an upper end of the generally v-shaped anode structure from where the alumina-enriched electrolyte is fed into the anode-cathode gap. The cell may be so arranged that at least part of the alumina-enriched electrolyte is fed into an upper end of the anode-cathode gap and/or circulated outside and around the anode-cathode gap and directed towards a lower end thereof.  
         [0040]     A suitable v-shaped anode structure comprises a series of horizontal or sloping elongated anodes members, for instance as described above, each having an elongated surface which is electrochemically active for the evolution of oxygen. The anode members are connected to one another, usually by at least one connecting member for example as disclosed in WO00/40782 (de Nora). The elongated anode members are generally parallel to one another and in a generally v arrangement in cross-section to form the electrochemically active surface that has a generally v-shaped cross-section. The anode members are spaced apart from one another by inter-member gaps that form the through-passages.  
         [0041]     Another suitable anode comprises an electrochemically active metallic anode structure made of one or more solid plates facing an active cathode surface. This electrochemically active metallic anode structure may have an upper end that delimits a passage for the circulation of electrolyte above the anode structure or, alternatively, a passage in its upper part for the circulation of electrolyte through the anode structure.  
         [0042]     The anode plates may be flat and have a uniformly planar sloping active part or several sloping active parts, =or instance in a generally v-shaped or inverted v-shaped cross-sectional arrangement. Suitable anode plate structures are disclosed in WO99/02764 (de Nora/Duruz).  
         [0043]     To maintain a substantially uniform current density along the anode plates, they can have horizontal cross-section that is proportional to the anodic current passed therethrough, i.e. a decreasing horizontal cross-section smith a decreasing amount of current.  
         [0044]     The anodes may also be generally conical or pyramidal, for example as disclosed in U.S. Pat. No. 5,368,702 (de Nora), to fit correspondingly shaped cathode plates.  
         [0045]     The invention also concerns a method of electrowinning aluminium in a cell as described above. The method comprises electrolysing in the anode-cathode gap alumina dissolved in the molten electrolyte to produce gas anodically and aluminium on the upwardly-oriented inclined active cathode surface(s) of the cathodic wall(S). The product aluminium drains from the active cathode surface(s) and is collected on the cell bottom in the aluminium pool.  
         [0046]     Advantageous methods of operating the cell are disclosed in WO00/06802 (Duruz/de Nora/Crottaz), WO01/42535 (Duruz/de Nora), WO00/42536 (Duruz/Nguyen/de Nora) and PCT/IB02/01952 (Nguyen/de Nora)  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0047]     The invention will now be described by way of examples with reference to the schematic drawings, wherein:  
         [0048]      FIG. 1  shows a cross-sectional view of a drained-cathode cell according to the invention with a foraminate generally v-shaped oxygen-evolving anode;  
         [0049]      FIGS. 1   a  and  1   b  show a plan view and a front, view, respectively, of the cathode element shown in  FIG. 1 ;  
         [0050]      FIG. 2  shows a cross-sectional view of a drained-cathode cell according to the invention with another foraminate generally v-shaped oxygen-evolving anode;  
         [0051]      FIG. 3  shows a cross-sectional view of a drained-cathode cell according to the invention with vet another foraminate generally v-shaped oxygen-evolving anode;  
         [0052]      FIGS. 4 and 5  show cross-sectional views of drained-cathode cells according to the invention utilising oxygen-evolving solid anodic plates;  
         [0053]      FIG. 6  shows a cross-sectional view of a drained-cathode cells according to the invention fitted with several anodes, enlarged views of different possibilities being shown in  FIGS. 6   a  and  6   b ; and  
         [0054]      FIG. 7  shows a cross-sectional view of another drained-cathode cell according to the invention fitted with several anodes. 
     
    
     DETAILED DESCRIPTION  
       [0055]      FIG. 1  shows an aluminium production cell according to the invention having a horizontal cell bottom  5  covered with a pool of product aluminium  50 . The cell has two inclined cathodic plates  10  in a molten electrolyte  60 . Each plate  10  has an upwardly-orientated sloping aluminium-wettable drained cathode surface  11  separated by an anode-cathode gap  40  from a corresponding sloping active anode surface of an anode  20  having a v-shaped grid-like foraminate active structure  25  covered by an electrolyte guide member  30 , 30 ′ shown with two possible shapes as discussed below.  
         [0056]     The cathodic plates  10  also have a downwardly-orientated inclined rear face  12  in the electrolyte  60 . This rear face  12  overlies the aluminium pool  50  that covers substantially the entire cell bottom  5 . A bottom end  13  of the cathodic plates  10  rests on the cell bottom  5  in the aluminium pool  50  through which electrical current is passed from an external current supply to the cathodic plates  10 . The section of cathodic plates  10  decreases with an increasing distance to the cathodic pool  50  so as to compensate for the current passed from the drained cathode surfaces  11  to the anodes  20  and provide a substantially uniform current density in plates  10  along substantially the entire height of plates  10 .  
         [0057]     As shown in  FIGS. 1   a  and  1   b , the cathodic plate  10  has a cut-out  14  in its bottom end  13  for passage of the aluminium pool  50  and for providing a return flow of alumina-enriched electrolyte  60  to the bottom end of the anode-cathode gap  40 .  
         [0058]     Furthermore, the cathodic plate  10  has at its upper end a pair of horizontally extending flanges  16  that space the active part of plate  10  from the sidewall of the cell. A passage  15  is provided between flanges  16  for the down-flow of alumina-enriched electrolyte  60  from above the upper end  27  of active anode structure  25  and then behind the drained cathode surface  11  to the lower end of the anode-cathode gap  40 .  
         [0059]     Instead of using plates with flanges that delimit an electrolyte passage, a substantially uniformly planar cathodic plate may be provided with an opening in its upper part or, alternatively, a substantially uniformly planar cathodic plate may be placed against one or more spaced apart protrusions extending from the cell sidewall or against a recess in the sidewall at the level of the upper part of the cathodic plates.  
         [0060]     The cathodic plate  10  is made of aluminium-wettable openly porous material that is mechanically and chemically resistant and filled with molten aluminium, as described above.  
         [0061]     The anode  20  is suspended in the electrolyte  60  by a yoke  21  with the downwardly-orientated active anode surface formed by the v-shaped grid-like foraminate structure  25  substantially parallel to the upwardly-oriented cathode surfaces  11 . The v-shaped grid-like foraminate structure  25  is made of a series of parallel horizontal rods  26  (shown in cross-section) forming a downwardly-oriented generally v-shaped electrochemically active open anode surface. The anode rods  26  are electrically and mechanically connected through one or more cross-members (not shown), as disclosed in WO00/40782 (de Nora), and spaced apart from one another by inter-member gaps  45  that form passages for an up-flow  61  of alumina-depleted electrolyte  60 . Alternatively, the v-shaped plate-like foraminate anode structure can be made of inclined rods in a v configuration (see  FIG. 2 ) or a v-shaped perforated plate, such as an expanded metal mesh, or a pair of downwardly converging perforated plates.  
         [0062]     The anode  20  comprises an electrolyte guide member  30 , 30 ′ above the v-shaped grid-like anode structure  25  to guide all the up-flowing alumina-depleted electrolyte  62  through a central opening  31  in the guide member  30 , 30 ′ to an alumina feeding area  63  where it is enriched with alumina, and then sideways over an upper end  27  of the anode structure  25  so that the alumina-enriched electrolyte  60  is mainly circulated through passage  15  at the top end of plate  10  and from there along the downwardly-orientated sloping surface  12  of plate  10  and then through the cut-out  14  in the bottom end  13  of plate  10  into a lower end of the anode-cathode gap  40 . In this embodiment, a smaller part of the alumina-enriched electrolyte  60  is fed over the upper end  27  so the anode structure  25  into an upper end of the anode-cathode gap  40 .  
         [0063]     The geometry of the cell, in particular the section of the upper end of the anode-cathode gap  40  and of the passage  15 , sets the ratio between the electrolyte  60  fed into the upper end of the anode-cathode gap  40  and the electrolyte  60  circulated through passage  15  to the lower end of the anode-cathode gap  40 .  
         [0064]     In the left-hand side of  FIG. 1 , the guide member  30  is shown in the shape of a horizontal plate with a downwardly extending peripheral flange. The right-hand side of  FIG. 1  shows the guide member  30 ′ with a sloping downwardly-orientated surface leading into the central opening  31 . Other shapes are of course possible.  
         [0065]     In a variation, the electrolyte guide member is dissociated from the anode.  
         [0066]     During operation, alumina is electrolysed in the anode-cathode gap  40  and oxygen formed on the v-shaped grid-like foraminate structure  25  of the anode  20 . The oxygen escapes upwardly through the gaps  45  promoting an upflow  61  of alum na-depleted electrolyte  60 . The electrolyte up-flow is confined as indicated by arrow  62  by the electrolyte guide member  30 , 30 ′ into the opening  31  and guided to the area  63  located thereabove where alumina is fed and enriches the circulating electrolyte  60 . The alumina-enriched electrolyte  60  is then guided sideways and passes mainly behind the cathodic plate  10  into the lower end of the anode-cathode gap  40  with the remainder into the upper end of gap  40 , as described above.  
         [0067]      FIG. 2 , where the same reference numerals designate the same elements, shows another cell according to the invention in which the generally v-shaped grid-like anode structure  25  is made of a series of parallel spaced-apart inclined rods  26 , each rod extending along a vertical plane that is perpendicular to the aluminium-wettable drained cathode surface  11 .  
         [0068]     The spacing between inclined rods  26  forms a passage for the up-flow  61  of alumina-depleted electrolyte  61  sideways around rods  26 .  
         [0069]     To provide a uniform current distribution, each inclined rod  26  has a variable cross-section (the rods  26  being downwardly tapered) so as to compensate for the current passed to the drained cathode surface  11 .  
         [0070]     In a variation, the inclined anode rods  26  are substituted with other elongated anode members, for example bars, blades or plates.  
         [0071]      FIG. 3 , where the same reference numerals designate the same elements, shows another cell according to the invention in which the generally v-shaped grid-like anode structure  25  is made of a series of parallel spaced-apart horizontal blades  26  arranged like venetian blinds.  
         [0072]     Furthermore the anode structure  25  is covered with an electrolyte guide member  30 ″ in the shape of a plate placed in-between the upper ends  27  of the anode structure  25  leaving passages  31 ′ between upper ends  27  and the guide member  30 ″ for alumina-depleted electrolyte  60 . In a variation, this guide member has a downwardly-oriented guide surface that has a general flattened u- or v-shape in cross-section leading to the passages  31 ′.  
         [0073]      FIGS. 4 and 5 , where the same reference numerals designate the same elements as before, disclose two aluminium production cells with inclined cathodic plates  10  according to the invention and anodes  20  having electrochemically active structures  25  made of inclined solid plates that are parallel to the upwardly-oriented cathode surfaces  11 .  
         [0074]     In cross-section, the cathodic plates  10  and the anode plates  25  shown in  FIG. 4  are in an inverted v-shape arrangement, whereas the cathodic plates  10  shown in  FIG. 5  are in a v-shape arrangement and the anode plates  25  form a v therebetween. The anode plates  25  are provided with openings  28  above the anode-cathode gap  40  for the circulation of electrolyte  60 .  
         [0075]     The anode plates  25  have a horizontal cross-section that varies along its length and is proportional to the anodic current passed therethrough, i.e. a decreasing horizontal cross-section with a decreasing amount of current (the plates  25  being downwardly tapered), to maintain a substantially uniform current density along the anode plates  25 .  
         [0076]     In operation, alumina is electrolysed in the anode-cathode gap  40  and oxygen released on the anode plates  25  in the gap  40  promotes an upward circulation along the entire anode-cathode gap  40  of the electrolyte  60  which is depleted in alumina. The electrolyte  60  returns from the upper end of the anode-cathode gap  40  through anode openings  28  and then down along an inactive surface  25 ′ of the anode structure  25  to the bottom end of the anode cathode gap  40 . Alumina is intermittently or continuously fed to the surface of the electrolyte  60 , as indicated by arrow  70 , whereby the electrolyte  60  is enriched with alumina while it returns to the bottom end of the anode cathode-gap  40 .  
         [0077]     In the cells of  FIGS. 4 and 5 , the electrolyte  60  does not circulate along the rear surface  11  of cathodic plates  10 . Thus, the cathodic plates  10  do not need to be associated with a passage for the circulation of electrolyte  60 . However, these plates  10  are provided with an opening in their bottom end  13  serving only for the passage of the aluminium pool  50 .  
         [0078]      FIGS. 6 and 7 , where the same reference numerals designate the same elements, show cells with several pairs of cathode plates  10  and several anodes  20 . In  FIG. 6 , the cell is fitted with a series of anodes  20  of the type illustrated in  FIG. 3  whereas in  FIG. 7 , the cell is fitted with a series of anodes of the type disclosed in  FIG. 4 .  
         [0079]     The cells of  FIGS. 6 and 7  have a series of side-by-side pairs of cathodic plates  10  in a v- or inverted v-shaped arrangement in cross-section.  
         [0080]     The cell of  FIG. 6  is fitted with foraminate anodes  20  as shown in  FIG. 3 . Alternatively, the anodes  20  can be substituted with the anodes shown in  FIG. 1, 2  or  5 .  
         [0081]     Neighbouring upper edges of plates  10  are spaced apart by spacer members  17 , 17 ′ leaving between them a passage  15  for the circulation of alumina-enriched electrolyte  60  to a bottom end of the anode-cathode gap  40 .  
         [0082]     The spacer member  17  shown on the left-hand side of  FIG. 6  and in  FIG. 6   a  has horizontally extending upper flanges  18  on the upper edges of plates  10  and a central part  19  that holds the upper edges of plates  10  apart.  
         [0083]     The spacer member  17 ′ shown on the right-hand side of  FIG. 6  and in  FIG. 6   b  has flanges  18 ′ that surround and secure the upper edges of plates  10  against the central spacing part  19 .  
         [0084]     The cell of  FIG. 7  is fitted with plate anodes  20  as shown in  FIG. 4 . In this cell configuration, circulation of alumina-enriched electrolyte  60  takes place between the anodes  20  and no electrolyte passage is needed between the cathodic plates  10  whose upper edges are juxtaposed. However, in a variation, an electrolyte passage can also be provided between the cathodic plates in accordance with the teachings of WO01/31088 (de Nora).  
         [0085]     Like in FIGS.  1  to  5 , the bottom parts  13  of the cathodic plates  10  shown in  FIGS. 6 and 7  are provided with an openings  14  for the passage of the aluminium pool  50 .  
         [0086]     The entire cell configurations or the cathodic arrangements shown in  FIGS. 6 and 7  may be retrofitted into existing Hall-Héroult cells with corresponding anodes or may be used in cells of new design, in particular in cells operating at reduced temperatures, typically 850° to 940° C.  
         [0087]     The cathodic plates  10  are, for instance, advantageously used to replace the solid cathode bodies of the cells disclosed in WO01/31088 (de Nora).  
         [0088]     In commercial cells, for example as schematically shown in  FIGS. 6 and 7 , the level of the aluminium pool  50  may be allowed to fluctuate on the cell bottom or the aluminium may be collected, e.g. over a weir that sets a maximum level of the aluminium pool, in a separate collection reservoir of the aluminium production cell.  
         [0089]     In a variation, the cathodic plates  10  shown in FIGS.  1  to  7  may be substituted with a series of parallel elongated cathodic members as mentioned above.