ALUMINUM ELECTROWINNING CELL WITH SIDEWALLS RESISTANT TO MOLTEN ELECTROLYTE

Abstract: A drained cathode cell for the electrowinning of aluminium comprises a cell bottom (20) arranged to collect product aluminium and thermic insulating sidewalls (55,55') lined with a molten electrolyte resistant sidewall lining (50) which is made of material liable to react with molten aluminium, in particular containing silicon carbide, silicon nitride or boron nitride. The thermic insulating sidewalls (55,55') inhibit formation of an electrolyte crust on the lining (50), whereby the lining (50) is exposed to molten electrolyte. The cell bottom (20) has a peripheral surface from which the insulating sidewalls (55,55') extend generally vertically to form, with the cell bottom, a trough for containing molten electrolyte and aluminium produced on at least one drained cathode (32). The peripheral surface of the cell bottom (20) is arranged to keep the product aluminium from contacting and reacting with the molten electrolyte resistant sidewall lining (50) above and around the entire peripheral surface.

Description: ALUMINIUM ELECTROWINNING CELL WITH SIDEWALLS RESISTANT TO MOLTEN ELECTROLYTE Field of the Invention The invention relates to drained-cathode cells for the electro inning of aluminium from alumina dissolved in a molten fluoride-containing electrolyte having sidewalls resistant to molten electrolyte, and methods of operating the cells to produce aluminium. Background of the Invention 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, conceived almost simultaneously by Hall and Heroult, has not evolved as much as other electrochemical processes, despite the tremendous growth in the total production of aluminium that in fifty years has increased almost one hundred fold. The process and the cell design have not undergone any great change or improvement and carbonaceous materials are still used as electrodes and cell linings. The electrolytic cell trough is typically made of a steel shell provided with an insulating lining of refractory material covered by prebaked anthracite- graphite or all graphite carbon blocks at the cell floor bottom which acts as cathode. The side walls are also covered with prebaked anthracite-graphite carbon plates. To increase the efficiency of aluminium production numerous drained-cathode cell designs have been developed, in particular including sloping drained cathode surface, as for instance disclosed in US Patents 3,400,061 (Lewis/Altos/Hildebrandt) , 4,602,990 (Boxall/ Gamson/Green/Stephen) , 5,368,702 (de Nora) , 5,683,559 (de Nora) , European Patent Application No. 0 393 816 (Stedman) , and PCT application WO99/02764 (de Nora/ Duruz) . These cell designs permit reduction of the inter- electrode gap and consequently reduction of the voltage drop between the anodes and cathodes. However, drained cathode cells have not as yet found significant acceptance in industrial aluminium production. It has been proposed to decrease energy losses during aluminium production by increasing the thermal insulation of the sidewalls of aluminium production cells. However, suppression of the thermal gradient through the sidewalls prevents bath from freezing on the sidewalls and consequently leads to exposure of the sidewalls to highly aggressive molten electrolyte and molten aluminium. Several proposals have been made in order to increase the sidewall resistance for ledgeless cell operation. US Patent 2,915,442 (Lewis) discloses inter- alia use of silicon carbide or silicon nitride as sidewall material. US Patent 3,256,173 (Schmitt/Wittner) describes a sidewall lining made of a honeycomb matrix of coke and pitch in which particulate silicon carbide is embedded. US Patent 5,876,584 (Cortellini) discloses sidewall lining material of silicon carbide, silicon nitride or boron carbide having a density of at least 95% and no apparent porosity. Sidewalls of known ledgeless cells are most exposed to erosion at the interface between the molten electrolyte and the molten aluminium which accumulates on the bottom of the cell. Despite formation of an inert film of aluminium oxide around the molten aluminium metal, cryolite operates as a catalyst which dissolves the protective aluminium oxide film at the aluminium/cryolite interface, allowing the molten aluminium metal to wet the sidewalls along the molten aluminium level. As opposed to aluminium oxide, the oxide-free aluminium metal is reactive at the cell operating temperature and combines with constituents of the sidewalls, which leads to rapid erosion of the sidewalls about the molten aluminium level. While the foregoing references indicate continued efforts to improve the operation of molten cell electrolysis operations, none suggest the invention and there have been no acceptable proposals for avoiding cell sidewall erosion caused by reaction with molten aluminium metal . Objects of the Invention An object of the invention is to provide a design for an aluminium electrowinning cell in which electrolyte is inhibited from freezing on the sidewalls. Another object of the invention is to provide a cell configuration for crustless or substantially crustless molten electrolyte resistant sidewalls, in particular carbide and/or nitride-containing sidewalls, which leads to an increased sidewall lifetime. A further object of the invention is to provide a cell configuration for crustless or substantially crustless molten electrolyte resistant sidewalls, in particular carbide and/or nitride-containing sidewalls, which leads to a reduced erosion, oxidation or corrosion of the sidewalls. A major object of the invention is to provide a drained cathode cell configuration with sidewalls resistant to molten electrolyte, in particular carbide and/or nitride-containing sidewalls, for crustless or substantially crustless operation. Summary of the Invention One main aspect of the invention concerns a drained-cathode cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte. The drained-cathode cell has a cell bottom which comprises an arrangement for collecting product aluminium and a peripheral upper surface that surrounds the arrangement for collecting product aluminium. At least the part of the cell bottom which is in contact with molten aluminium during operation is made of material resistant to molten aluminium. Aluminium is produced on at least one drained cathode surface from which the produced aluminium drains into said arrangement for collecting the product aluminium during operation. The drained-cathode cell further comprises one or more thermic insulating sidewalls extending generally vertically upwards from the peripheral surface of the cell bottom to form with the cell bottom a trough for containing during operation molten electrolyte and the product aluminium. Above the peripheral surface, the or each thermic insulating sidewall is lined with a sidewall lining made of material resistant to molten electrolyte but liable to react with molten aluminium, the or each thermic insulating sidewall inhibits formation of an electrolyte crust or ledge on the sidewall lining that during operation remains permanently exposed to molten electrolyte above and around said peripheral surface. The peripheral surface of the cell bottom is arranged to keep molten aluminium away from the sidewall lining above and around the entire peripheral surface, whereby the molten aluminium is prevented from reacting with the sidewall lining above and around the entire peripheral surface. The drained-cathode cell design according to the invention thus keeps the molten aluminium away from all cell sidewalls preventing it from contacting and reacting with the sidewall lining resistant to molten electrolyte, enabling use of a sidewall lining made of a carbide and/or a nitride, such as silicon carbide, silicon nitride or boron nitride, without risk of damage to the sidewall lining by reaction with molten aluminium as can occur with known designs. Usually the cell comprises four of the above mentioned insulated sidewalls in a generally rectangular arrangement. However, the invention can also be implemented with other sidewall configurations. The upper surface of the cell bottom for example comprises opposed sloping surfaces leading from opposed sidewalls down into a central channel for the continuous removal of product aluminium, the central channel extending parallel to said opposed sidewalls. This central draining channel (or a side channel or several channels in other embodiments) preferably leads into an aluminium storage sump or space which is internal or external to the cell and from which the aluminium can be tapped from time to time. Alternatively, the upper surface of the cell bottom comprises a series of oppositely sloping surfaces forming therebetween recesses or channels that extend parallel to opposed sidewalls . The recesses or channels can be of various shapes, for example generally V-shaped. Usually, the peripheral surface slopes down to a flat or sloping main surface of the cell bottom which forms the drained cathode surface or which receives produced aluminium from a drained cathode surface located thereabove . This main surface leads into the arrangement for collecting product aluminium. When the main surface is at a slope, the peripheral surface is usually inclined at a steeper slope than the main surface . In one embodiment, the main surface comprises downwardly converging inclined surfaces sloping down from first opposed sidewalls. The converging surfaces are inclined along second opposed sidewalls. The peripheral surface extends horizontally along the first opposed sidewalls and follows the inclination of the converging surfaces along the second opposed sidewalls . In this embodiment, the sloping peripheral surface can be of substantially uniform width around the entire cell bottom. In another embodiment, where the main surface also comprises downwardly converging inclined surfaces sloping down from first opposed sidewalls, the converging surfaces are inclined along second opposed sidewalls, and the peripheral surface extends horizontally along the first and second opposed sidewalls, the sloping peripheral surface extends down to the converging inclined surfaces around the entire cell bottom. Usually, the sloping peripheral surface is of uniform width along the first opposed sidewalls and of non-uniform width along the second opposed sidewalls where it forms generally triangular surfaces whose sides follow the second opposed sidewalls and the converging inclined surfaces . In a further embodiment, where the main surface also comprises downwardly converging inclined surfaces sloping down from first opposed sidewalls, the converging surfaces are inclined along second opposed sidewalls, and the sloping peripheral surface extends horizontally along the first and second opposed sidewalls, the sloping peripheral surface is connected by at least one substantially vertical connecting wall to the main surface, i.e. at least to the converging inclined surfaces. Such connecting wall(s) is/are resistant to molten aluminium. Usually, the drained surface (s) is/are on one or more cathodes which are part of the cell bottom and so arranged that molten aluminium produced thereon drains away from the sidewall lining into the arrangement for collecting molten aluminium. Alternatively, the drained cathode surface (s) can be on one or more cathodes located above the cell bottom, the molten aluminium draining from the cathodes onto the cell bottom and then into the arrangement for collecting molten aluminium. The cathode and/or the cell bottom can be made of carbonaceous material, such as compacted powdered carbon, a carbon-based paste for example as described in U.S. Patent No. 5,362,366 (de Nora/Sekhar) , prebaked carbon blocks, or graphite blocks, plates or tiles. Other suitable cathode materials which can also be used for the cell bottom are described in WO98/53120 (Berclaz/de Nora) and WO99/02764 (de Nora/Duruz) . The cathode and the cell bottom most preferably has /have an upper surface which is aluminium-wettable, for example the upper surface of the cathode or the cell bottom is coated with a coating of refractory aluminium wettable material as described in U.S. Patent 5,651,874 (de Nora/Sekhar) or W098/17842 (Sekhar/Duruz/Liu) . The aluminium-wettable surface usually comprises a refractory boride, in particular TiB 2 , advantageously applied as a coating from a slurry of particles of the refractory boride or other aluminium-wettable material. This aluminium-wettable surface can be obtained by applying a top layer of refractory aluminium-wettable material over the upper surface (which can already have a precoating of the refractory aluminium wettable material) and over parts of the cell surrounding the cathode. In one embodiment in which the cathode is part of the cell bottom, the electric current to the cathode, in particular a cathode mass, may arrive through an inner cathode holder shell or plate placed between the cathode and the outer shell, usually made of steel, as disclosed in WO98/53120 (Berclaz/de Nora) . The sidewall lining can be made of tiles containing carbide and/or nitride and/or can comprise a carbide and/or nitride based coating which during cell operation is in contact with the product aluminium. Alternatively, the sidewall lining may be coated and/or impregnated with one or more phosphates of aluminium, as disclosed in US Patent 5,534,130 (Sekhar). The phosphates of aluminium may be selected from: monoaluminium phosphate, aluminium phosphate, aluminium polyphosphate, and aluminium metaphosphate . The cells according to the invention can make use of traditional consumable prebaked carbon anodes, continuously-fed Søderberg-type anodes, as well as non- consumable or substantially non-consumable anodes. Non-consumable anodes may comprise an electrochemically active structure made of a series of horizontal anode members, each having an electrochemically active surface on which during electrolysis oxygen is anodically evolved. The anode members may be in a parallel arrangement connected by at least one connecting cross-member or in a concentric arrangement connected by at least one generally radial connecting member as described in WO00/40781 and WO00/40782 (both in the name of de Nora) . Suitable materials for oxygen-evolving anodes include iron and nickel based alloys which may be heat- treated in an oxidising atmosphere as disclosed in WO00/06802, WO00/06803 (both in the name of Duruz/de Nora/Crottaz) , WO00/06804 (Crottaz/Duruz) , PCT/IB99/01976 (Duruz/de Nora) and PCT/IB99/01977 (de Nora/Duruz) . Further oxygen-evolving anode materials are disclosed in W099/36593, W099/36594, WO00/06801, WO00/06805, PCT/IB00/00028 (all in the name of de Nora/Duruz), WO00/06800 (Duruz/de Nora), W099/36591 and W099/36592 (both in the name of de Nora) . Whether consumable prebaked anodes or non- consumable anodes are used, it is advantageous to preheat each anode before it is installed in the cell during operation, in replacement of a carbon anode which has been substantially consumed, or a non-consumable anode that has become disactivated or requires servicing. By preheating the anodes, disturbances in cell operation due to local cooling are avoided as when an electrolyte crust is formed whereby part of the anode is not active until the electrolyte crust has melted. The invention also relates to a cell trough for containing molten electrolyte and product aluminium, having a cell bottom fitted with insulating cell sidewalls which are protected with a molten electrolyte resistant lining as described above. A further aspect of the invention relates to a method of producing aluminium using the cell as outlined above which contains alumina dissolved in a fluoride- containing molten electrolyte. The method involves electrolysing the dissolved alumina to produce aluminium on the or each drained cathode surface and draining the produced aluminium from the or each drained cathode surface into the arrangement for collecting the product aluminium, the produced aluminium being kept from contacting and reacting with the sidewall lining above and around the entire peripheral surface . Advantageously, the surface of the cell bottom is maintained at a temperature corresponding to a paste state of the electrolyte whereby the cell bottom is protected from chemical attack. For example, when the cryolite-based electrolyte is at about 950°C, the surface of the cell bottom can be cooled by about 30°C, whereby the electrolyte contacting the cathode surface forms a viscous paste which protects the cell bottom. Brief Description of the Drawings The invention will be further described with reference to the accompanying schematic drawings, in which : Fig. 1 is a cross-sectional view of one aluminium electrowinning cell according to the invention; Fig. 2 is a cross-sectional view of another aluminium electrowinning cell according to the invention; Fig. 3 shows the bottom part of the cell of Fig. 2 during assembly of a cathode unit; Fig. 4 shows in longitudinal cross-section an embodiment of the cathode ready to be installed in a cell; Fig. 5 is a longitudinal cross-sectional view of another aluminium electrowinning cell according to the invention; and Fig.6 is a plan view of the cell bottom shown in Figure 1, 2 or 3 showing varied embodiments of the peripheral surface. Detailed Description Figs. 1 and 6 schematically show an aluminium electrowinning cell according to the invention wherein a plurality of anodes 10 are suspended by yokes 11 connected to an anode suspension and current supply superstructure (not shown) which hold the anodes 10 suspended above a cathode cell bottom 20 enclosed in an outer steel shell 21 forming, with its insulating lining of refractory bricks 40, a cell trough or cathode pot. Inside the outer steel shell 21 is housed a cathode 30 comprising an inner steel cathode holder shell 31 containing a cathode mass 32. As illustrated, the inner shell 31 has a flat bottom, sidewalls 33 and outwardly-directed side flanges 34 at its top. The inner shell 31 forms an open-topped container for the cathode mass 32. The top of the cathode mass 32 has inclined surfaces 35 extending over the cathode 30 and leading down into a central channel 36 for draining molten aluminium. The central channel 36 advantageously leads into an aluminium storage sump 36' which is centrally located in the cell, as shown in Figure 6. On top of the cathode mass 32, and also extending over the flanges 34, is a coating 37 of aluminium-wettable material, preferably a slurry-applied boride coating as described in U.S. Patent 5,651,874 (de Nora/Sekhar) or W098/17842 (Sekhar/Duruz/Liu) . Such coating 37 can also be applied to the inside surfaces of the bottom and sides 33 of the cathode holder shell 31, to improve electrical connection between the inner shell 31 and the cathode mass 32. The periphery of the cathode mass 32 extends to the top of the sidewall 33 of the inner shell 31, from where it slopes down to the central channel 36. Inside the part of the cell sidewalls at the top of the outer shell 21 facing the sides of anodes 10 is a sidewall lining 50 formed for example of plates of carbon or silicon carbide. As shown in Figures 1 to 3 , the insulating sidewalls 55 extend generally vertically upwards from the cell bottom 20. The insulating sidewalls 55 inhibit during operation formation of an electrolyte crust on the sidewall lining 50, whereby the lining is exposed to molten electrolyte 60. According to the invention, the peripheral surface 35' from which the insulating sidewalls 55,55' extend is arranged to drain molten aluminium away from the sidewall lining 50, to keep the product aluminium from contacting and reacting with the sidewall lining 50, as shown in Figures 1 to 3 and 6. For this purpose, the peripheral surface 35' is inclined at a steeper slope than the inclined cathode surfaces 35, as shown in Figures 1 to 5 , forming a small wedge sloping down from the end sidewalls 55 ' and extending across the cathode mass 32, so that the entire periphery 35' around the sloped cathode surfaces 35 slopes away from all cell sidewalls 55,55' to drain molten aluminium away from the sidewall lining 50, as shown in Figure 6. Figures 3 and 6 show different configurations of the peripheral surface 35'. As shown in Figure 3 , the sloped cathode surfaces 35 are made of downwardly converging inclined surfaces 35 sloping down from opposed lateral sidewalls 55 to the central channel 36 and which are inclined along opposed end sidewalls 55' as shown in the upper part of Figure 6. As shown in Figure 3, the peripheral surface 35' extends horizontally along the lateral sidewalls 55 and follows the inclination of the converging surfaces 35 along the opposed end sidewalls 55 ' , the sloping peripheral surface 35 being of substantially uniform width around the entire cell bottom as shown in the upper part of Figure 6. A variation of the configuration of the peripheral surface 35' is shown in Figure 3 by dotted line 35' ' and on the lower part of Figure 6. The peripheral surface 35' extends horizontally along the lateral sidewalls 55 and the end sidewalls 55'. Furthermore, the sloping peripheral surface 35' extends down to the converging inclined surfaces around the entire cell bottom. In this variation, the peripheral surface 35' is of uniform width along the lateral sidewalls 55 and of non-uniform width along the end sidewalls 55' where, as shown on the lower part of Figure 6, it forms a generally triangular surface. Another variation of the configuration of the peripheral surface 35' is shown in Figure 3 by dotted line 35' ' and 35' ' ' and on the upper part of Figure 6. The peripheral surface 35' extends horizontally along the lateral and end sidewalls 55 ' as shown by dotted line 35''. The sloping peripheral surface 35' is connected to the converging inclined cathode surfaces 35 by substantially vertical connecting walls, the top of the connecting wall being indicated by line 35''' in Figures 3 and 6. As shown in Figures 3 and 6, the sloping peripheral surface 35' is of uniform width all around the sidewalls 55,55'. This connecting wall is resistant to molten aluminium and can be coated with aluminium- wettable material as mentioned above. As shown in Figures 1 to 3 , the cathode 30 is supported as a removable unit in the cell bottom 20 in a central recess of corresponding shape in the refractory bricks 40 lining the outer steel shell 21. These refractory bricks 40 are the usual types used for lining conventional cells. Current is supplied to the cathode 30 via transverse conductor bars 41 welded to the bottom of the inner shell 31. These conductor bars 41 are connected to current collector bars 42 which protrude laterally from the sides of the outer shell 21, as shown in Figure 1, these collector bars 42 being connected to external buswork (not shown) . Alternatively, current could be supplied to the cathode 30 of Fig. 1, by a series of vertical current collector bars 41 extending down through vertical openings in the bottom of the lining formed by the refractory bricks 40 (see Figures 2 and 3) . Due to the metallic conductivity of the cathode holder shell 31, these conductor bars 41 are all maintained at practically the same electrical potential leading to uniform current distribution in the collector bars 42. Moreover, the metal inner shell 31 evenly distributes the electric current in the cathode mass 32. In use, the space between the cathode 30 and the sidewall lining 50 is filled with a molten electrolyte 60 such as cryolite containing dissolved alumina at a temperature usually about 950-970°C, and into which the anodes 10 dip. When electrolysis current is passed, aluminium is formed on the sloping cathode surfaces 35 coated with the refractory boride coating 37, and the produced aluminium continuously drains down the sloping surfaces 35 into the central channel 36 from where it is removed permanently into the storage sump 36' from which the aluminium can be tapped from time to time. The anodes 10, which are shown as being consumable prebaked carbon anodes, have sloping surfaces 12 facing the sloping cathode surfaces 35. The inclination of these anode surfaces 12 facilitates the release of bubbles of the anodically-released gases. As the anode 10 is consumed, it maintains its shape, keeping a uniform anode-cathode spacing. Alternatively, it would be possible for the same cell bottom 20 and its cathode 30 to be used with non-consumable or substantially non- consumable anodes. Periodically, when the cathode 30 needs servicing, it is possible to close down the cell, remove the molten cell contents, and disassemble the entire cathode 30 to replace it with a new or a serviced cathode 30. This operation is much more convenient and less labour intensive than the conventional cell bottom relining process, has reduced risks relating to exposure to the toxic waste materials, and simplifies disposal of the toxic waste materials. The aluminium electrowinning cell shown in Fig. 2 is similar to that of Fig. 1 and like references have been used to designate like parts. In this design, the current collector bars 42 instead of being horizontal are vertical and extend through vertical apertures 43 in the lining of bricks 40. These collector bars 42 are welded centrally to the bottom of the inner shell 31. As illustrated in Fig. 4, several collector bars 42 are spaced apart from one another along the bottom of the inner shell 31. These collector bars 42 can have any desired cross-sectional shape : circular, rectangular, T- shaped, etc. Because the inner metal shell 31 keeps the collector bars 42 at practically the same potential, fluctuations in the current supply are avoided. The assembly method is illustrated in Fig. 3. It is possible to install the entire cathode 30 by lowering it using a crane until the bottom of the cathode holder shell 31 comes to rest on the top 44 of the lining of bricks 40 and its side flanges 34 come to rest on shoulders 45 of the cell lining. Then, the plates 50 can be installed on top of the flanges 34. This assembly method is simple and labour saving, compared to the usual cell lining methods used heretofore. To dismantle the cell, the sidewall lining plates 50 are removed first, then the cathode 30, after disconnecting the collector bars 42 from the negative busbar. This dismantling of the cell is remarkably simple to carry out and considerably simplifies disposal of toxic wastes. Figure 4 shows the cathode 30 ready to be installed as a unit in an aluminium electrowinning cell (not shown) which is fitted with insulating sidewalls protected with a carbide and/or nitride containing lining according to the invention. This cathode 30 comprises a metal cathode holder shell 31 made of a flat base plate to which sidewalls 33 are welded substantially at right angles along its side edges. These sidewalls 33 can extend around the entire periphery of the base plate, or only along its opposite side edges. To the bottom of the shell 31 's base plate, a series of conductor bars 42 are welded, spaced equally apart from one another along the length of the shell 31. These conductor bars 42 protrude vertically down from the shell 31, so they can pass through corresponding vertical openings in the cell bottom, for connection to an external negative busbar. In the shell 31 is a cathode mass 32 formed of a series of blocks, for example of carbon. As shown, the cathode blocks have sloping upper surfaces 35 and are fitted together to form a series of generally V-shaped recesses. In this example, parts of the cathode blocks protrude above the top of the sidewalls 33 which are embedded in the sides of the end blocks. The upper surface 35 is made up of a series of sloping surfaces in generally V-configuration, formed by placing the adjacent blocks together. The end blocks on each side of the shell 31 are shown with a sloping peripheral surface 35' from which the insulating sidewalls extend when placed in a cell. According to the invention, the peripheral surface 35' surrounds the cathode 30 and is arranged to drain molten aluminium away from the sidewall lining 50 above and around the entire peripheral surface 35', to keep the product aluminium from contacting and reacting with the sidewall lining 50 above and around the entire peripheral surface 35'. Each conductor bar 42 corresponds to the junction between two adjacent blocks forming the lower part of each V. As shown, the conductor bars 42 protrude through the shell 31 and extend part of the way up the blocks 42. Alternatively, the conductor bars 42 could be welded externally to the bottom of the shell 31. Before use, the entire sloping upper surface 35 of the cathode mass 32 is coated with an aluminium- wettable coating typically formed of slurry-applied titanium diboride . This cathode 30 can be produced as a unit and installed in an aluminium electrowinning cell (as illustrated in Fig. 3) by lifting it with a crane, and lowering it into the cell. The aluminium electrowinning cell shown in longitudinal cross-section in Fig. 5 comprises a cathode 30 with a series of spaced-apart vertical current conductors 42 welded to the bottom of its inner cathode holder shell 31, these conductors 42 protruding from the lower face of the cell bottom 20 for connection to the cathode buswork. As in Figures 1 to 3 , the insulating sidewalls 55 shown in Figure 5 extend generally vertically from the cell bottom 20 which is arranged to drain molten aluminium away from the carbide and/or nitride containing sidewall lining 50, to keep the product aluminium from contacting and reacting with the sidewall lining 50. The cathode mass 32 is made up of several layers of a conductive material such as carbon possibly combined with materials rendering the carbon impervious to molten aluminium. The mass 32 comprises an outer layer around the bottom and sides 33 of the inner shell 31. This outer layer has a peripheral edge 32a surrounding a central recess that is coated with a flat layer 38 of carbon or other conductive material on top of which is a toplayer 39 having sloping faces 35 coated with the layer 37 of aluminium-wettable boride. As illustrated, the upwardly- sloping side parts of the faces 35 are extended by bevelled parts of the edges 32a and by ramming paste 51, forming wedges along the edges of the cathode mass 32 on which the aluminium wettable boride layer 37 extends to form with the peripheral edge 32a a peripheral surface 35' of steeper slope which is arranged to drain molten aluminium away from the sidewall lining 50 above and around the entire peripheral surface according to the invention. The sloping faces 35 of cathode mass 32 are inclined alternately to form flattened V-shaped recesses above which the anodes 10 are suspended with corresponding V-shaped inclined faces 11 of the anodes facing the V-shaped recesses in the cathode mass 32. The anodes 10 are suspended by steel rods 14 held at an adjustable height in attachments 15 by an anode bus 16, enabling the anodes 10 to be suspended with a selected anode-cathode gap. Assembly and disassembly of the cathode 30 of this cell is similar to what has been described previously. The cathode 30 is assembled first, outside the cell, then lowered using a crane into the cell bottom 20, passing the conductor bars 42 through corresponding openings 43 in the bricks 40. Then the gaps around the edges of the cathode mass 32 are filled with ramming paste 51 which is formed into the side wedges. Next, a slurry of refractory boride is applied to the sloping cathode faces 35, usually on top of a pre-coating already applied thereto, and also over the sloping wedge surfaces of the edges 32a and ramming paste 51. After drying and heat treatment of the boride coating 37, the cell is ready for start-up. In operation, the central recess in the cell above the cathode mass 32 contains a molten electrolyte 60, such as cryolite containing dissolved alumina, into which the anodes 10 dip. For disassembly to service the cell bottom 20, the molten contents are removed from the cell, and the ramming paste 51 is broken to enable the entire cathode unit 30 to be lifted out of the cell using a crane, after having disconnected the conductor bars 42 from the cathode busbar . While the invention has been described in conjunction with specific embodiments thereof, it is evident that many modifications and variations will be apparent to those skilled in the art in the light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications and variations which fall within the scope of the appended claims .

Claims: CLAIMS 1. A drained-cathode cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte, comprising: - a cell bottom comprising an arrangement for collecting product aluminium and a peripheral upper surface that surrounds the arrangement for collecting product aluminium, at least the part of the cell bottom which is in contact with molten aluminium during operation being made of material resistant to molten aluminium; - at least one drained cathode surface on which aluminium is produced and from which the produced aluminium drains into said arrangement for collecting the product aluminium during operation; - one or more thermic insulating sidewalls extending generally vertically upwards from said peripheral surface to form with the cell bottom a trough for containing during operation molten electrolyte and the product aluminium; and - a sidewall lining made of material resistant to molten electrolyte but liable to react with molten aluminium and which material lines the thermic insulating sidewall (s) above said peripheral surface, the thermic insulating sidewall (s) inhibiting formation of an electrolyte crust or ledge on the sidewall lining that during operation remains permanently exposed to molten electrolyte above and around said peripheral surface, said peripheral surface being arranged to keep molten aluminium away from the sidewall lining above and around the entire peripheral surface, whereby the molten aluminium is prevented from reacting with the sidewall lining above and around the entire peripheral surface. 2. The cell of claim 1, comprising four of said sidewalls in a generally rectangular arrangement. 3. The cell of claim 2, wherein the cell bottom comprises opposed sloping surfaces leading from opposed sidewalls down into a central channel for the removal of product aluminium, the central channel extending parallel to said opposed sidewalls . 4. The cell of claim 2, wherein the cell bottom comprises a series of oppositely sloping surfaces forming therebetween a series of recesses or channels that extend parallel to opposed sidewalls. 5. The cell of any preceding claim, wherein said peripheral surface slopes down to a flat or sloping main surface of the cell bottom which forms the drained cathode surface or which receives produced aluminium from a drained cathode surface located thereabove, said main surface leading into said arrangement for collecting product aluminium. 6. The cell of claim 5 when depending on claim 2, wherein said main surface comprises downwardly converging inclined surfaces sloping down from first opposed sidewalls, said converging surfaces being inclined along second opposed sidewalls, said peripheral surface extending horizontally along said first opposed sidewalls and following the inclination of said converging surfaces along said second opposed sidewalls. 7. The cell of claim 5 when depending on claim 2, wherein said main surface comprises downwardly converging inclined surfaces sloping down from first opposed sidewalls, said converging surfaces being inclined along second opposed sidewalls, said peripheral surface extending horizontally along said first and second opposed sidewalls, said sloping peripheral surface extending down to said converging inclined surfaces around the entire cell bottom. 8. The cell of claim 5 when depending on claim 2, wherein said main surface comprises downwardly converging inclined surfaces sloping down from first opposed sidewalls, said converging surfaces being inclined along second opposed sidewalls, said peripheral surface extending horizontally along said first and second opposed sidewalls, said sloping peripheral surface being connected by at least one substantially vertical connecting wall to said converging inclined surfaces, said at least one connecting wall being resistant to molten aluminium. 9. The cell of any preceding claim, wherein the or each drained cathode surface is on a cathode which is part of the cell bottom, the cathode being so arranged that aluminium produced thereon drains away from the sidewall lining into the arrangement for collecting product aluminium. 10. The cell of any one of claims 1 to 8, wherein the or each drained cathode surface (s) is on a cathode located above the cell bottom, the cathode being so arranged that aluminium produced thereon drains away from the sidewall lining into the arrangement for collecting product aluminium. 11. The cell of claim 10, wherein the cell bottom is coated with a coating of refractory aluminium-wettable material . 12. The cell of any preceding claim, wherein the or each drained cathode surface is coated with a coating of refractory aluminium-wettable material. 13. The cell of claim 11 or 12, wherein the coating of refractory aluminium-wettable material comprises a refractory boride. 14. The cell of claim 13, wherein the coating of refractory aluminium-wettable material comprises titanium diboride . 15. The cell of any preceding claim, wherein the sidewall lining comprises a carbide and/or a nitride. 16. The cell of claim 15, wherein the sidewall lining comprises at least one of silicon carbide, silicon nitride and boron nitride. 17. The cell of claim 15 or 16, wherein the sidewall lining is made of carbide and/or nitride containing tiles. 18. The cell of claim 15 or 16, wherein the sidewall lining is coated with a carbide and/or nitride based coating. 19. The cell of any preceding claim, wherein the sidewall lining is coated and/or impregnated with one or more phosphates of aluminium. 20. The cell of claim 19, wherein said phosphates of aluminium are selected from: monoaluminium phosphate, aluminium phosphate, aluminium polyphosphate, and aluminium etaphosphate . 21. A trough of a drained-cathode cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte, comprising: - a cell bottom comprising an arrangement for collecting product aluminium and a peripheral upper surface that surrounds the arrangement for collecting product aluminium, at least the part of the cell bottom which is in contact with molten aluminium during operation being made of material resistant to molten aluminium; - at least one drained cathode surface on which aluminium is produced and from which the produced aluminium drains into said arrangement for collecting the product aluminium during operation; - one or more thermic insulating sidewalls extending generally vertically upwards from said peripheral surface to form with the cell bottom a trough for containing during operation molten electrolyte and the product aluminium; and - a sidewall lining made of material resistant to molten electrolyte but liable to react with molten aluminium and which material lines the thermic insulating sidewall (s) above said peripheral surface, the thermic insulating sidewall (s) inhibiting formation of an electrolyte crust or ledge on the sidewall lining that during operation remains permanently exposed to molten electrolyte above and around said peripheral surface, said peripheral surface being arranged to keep molten aluminium away from the sidewall lining above and around the entire peripheral surface, whereby the molten aluminium is prevented from reacting with the sidewall lining above and around the entire peripheral surface. 22. A method of producing aluminium using a cell as defined in any one of claims 1 to 20 containing alumina dissolved in a fluoride-based molten electrolyte, the method comprising electrolysing the dissolved alumina to produce aluminium on the or each drained cathode surface and draining the produced aluminium from the or each drained cathode surface into the arrangement for collecting the product aluminium, the produced aluminium being kept from contacting and reacting with the sidewall lining above and around the entire peripheral surface. 23. The method of claim 22, comprising maintaining the surface of the cell bottom at a temperature corresponding to a paste state of the electrolyte whereby the cell bottom is protected from chemical attack.

Inventors: BERCLAZ GEORGES, DE NORA VITTORIO, DURUZ JEAN-JACQUES, BERCLAZ, GEORGES, DE NORA, VITTORIO, DURUZ, JEAN-JACQUES
Applicants: BERCLAZ GEORGES, DURUZ JEAN JACQUES, MOLTECH INVENT SA, NORA VITTORIO DE, BERCLAZ, GEORGES, DE NORA, VITTORIO, DURUZ, JEAN-JACQUES, MOLTECH INVENT S.A.
Application year: 2000
Main classification: C25C3
IPC codes: C25C3