ALUMINIUM ELECTROWINNING CELL WITH IMPROVED CARBON CATHODE BLOCKS

Abstract: An electrolytic cell for the electrowinning of aluminium from alumina dissolved in a halide-containing molten electrolyte, has a cathode cell bottom made of a series of carbon cathode blocks (10) connected side-by-side transversally of the cell by glue or ramming paste, each one provided with steel or other conductive bars (11) across the cell for the delivery of current. A series of anodes (15) faces a pool or layer of molten aluminium on the cathode cell bottom which is preferably coated with an aluminium-wettable refractory coating (35). Movement of the aluminium pool (40) or layer is decreased by providing the top surfaces of the carbon blocks (10) with a series of parallel channels (25) or grooves along the direction of the conductive bars, transverse to the cell. In use, these channels (25) or grooves are covered by, and restrain, the pool or the layer of molten aluminium, in particular against movement in a direction transverse to the channels or grooves, so as to greatly decrease movement of the aluminium pool or so as to facilitate the collection/evacuation of aluminium, which substantially improves cell operation and extends cell life.

Description: ALUMINIUM ELECTRO INNING CELL WITH IMPROVED CARBON CATHODE BLOCKS Field _■_ „h_ Invention The present invention concerns a cell for the electrowinning of aluminium by the electrolysis of alumina dissolved in a fluoride-based molten electrolyte such as cryolite, incorporating improved carbon cathode blocks. 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 wall and at the cell floor bottom which acts as cathode and to which the negative pole of a direct curias of steel conductor bars embedded in the carbon blocks. The anodes are still made of carbonaceous material and must be replaced every few weeks. The operating temperature is still approximately 950°C in order to have a sufficiently high rate of dissolution of alumina which decreases at lower temperatures and to have a higher conductivity of the electrolyte. The carbonaceous materials used in Hall-Heroult cells as cell lining deteriorate under the existing adverse operating conditions and limit the cell life. The anodes have a very short life because during electrolysis the oxygen which should evolve on the anode surface combines with the carbon to form CO2 and small amounts of CO. The actual consumption of the anode is approximately 450 kg/ton of aluminium produced which is more than 1/3 higher than the theoretical amount. The carbon lining of the cathode bottom has a useful life of a few years after which the operation of the entire cell must be stopped and the cell relined at great cost . Despite an aluminium pool having a thickness of 15 to 20 cm maintained over the cathode, the deterioration of the cathode carbon blocks cannot be avoided because of penetration of sodium into the carbon which by chemical reaction and intercalation causes swelling, deformation and disintegration of the cathode carbon blocks, as well as penetration of cryolite and liquid aluminium. The carbon blocks of the cell side wall do not resist oxidation and attack by cryolite and a layer of solidified cryolite has to be maintained on the cell side walls to protect them. In addition, when cells are rebuilt, there are problems of disposal of the carbon cathodes which contain toxic compounds including cyanides . Another major drawback, however, is due to the fact that irregular electromagnetic forces create waves in the molten aluminium pool and the anode-cathode distance (ACD) , also called interelectrode gap (IEG), must be kept at a safe minimum value of approximately 50 mm to avoid short circuiting between the aluminium cathode and the anode or reoxidation of the metal by contact with the CO 2 gas formed at the anode surface. The high electrical resistivity of the electrolyte, which is about 0.4 ohm. cm., causes a voltage drop which alone represents more than 40% of the total voltage drop with a resulting energy efficiency which reaches only 25% in the most modern cells. The high cost of energy together with the low efficiency, has become an even bigger item in the total manufacturing cost of aluminium since the oil crisis, and has decreased the rate of growth of this important metal . In the second largest electrochemical industry following aluminium, namely the caustic and chlorine industry, the invention of the dimensionally stable anodes (DSA®) based on noble metal activated titanium metal, which were developed around 1970, permitted a revolutionary progress in the chlorine cell technology resulting in a substantial increase in cell energy efficiency, in cell life and in chlorine-caustic purity. The substitution of graphite anodes with DSA® increased drastically the life of the anodes and reduced substantially the cost of operating the cells. Rapid growth of the chlorine caustic industry was retarded only by ecological concerns. In the case of aluminium production, pollution is not due to the aluminium produced, but to the materials and the manufacturing processes used and to the cell design and operation. However, progress has been reported in the operation of modern aluminium plants which utilize cells where the gases emanating from the cells are in large part collected and adequately scrubbed and where the emission of highly polluting gases during the manufacture of the carbon anodes and cathodes is carefully controlled. While progress has been reported in the fabrication of carbon cathodes by the application of coatings or layers using new aluminium wettable materials which are also a barrier to sodium penetration during electrolysis, no progress has been achieved in design of cathodes for aluminium production cells with a view to restraining movement of the molten aluminium in order to reduce the interelectrode gap and the rate of wear of its surface. U.S. Patent 4,560,488 (Sane et al) discloses a recent development in molten salt electrolysis cells concerning making materials wettable by molten aluminium. However, the carbon or graphite anodes and cathodes are of conventional design with no suggestion leading to the present invention. U.S. Patent 4,681,671 (Duruz) illustrates another improvement in molten salt electrolysis wherein operation at lower than usual temperatures is carried out utilizing permanent anodes, e.g. metal, alloy, ceramic or a metal- ceramic composite as disclosed in European Patent Application No. 0030834 and U.S. Patent 4,397,729. Again, while improved operation is achieved at lower temperatures, there is no suggestion of the subject matter of the present invention. PCT Application WO 89/06289 (La Camera et al) deals with an improved molten electrolysis wherein attention is directed to an electrode having increased surface area. However, again, there is no disclosure leading one to the present invention. U.S. Patents 3,400,061 (Lewis et al) and 4,602,990 (Boxall et al) disclose aluminium electrowinning cells with sloped drained cathodes arranged with the cathodes and facing anode surfaces 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. U.S. Patent 5,203,971 (de Nora et al) discloses an aluminium electrowinning cell having a partly refractory and partly carbon based cell lining. The carbon-based part of the cell bottom may be recessed in respect to the refractory part, which assists in reducing movement of the aluminium pool . US Patent 3,856,650 (Kugler) proposed lining a carbon cell bottom with a ceramic coating upon which parallel rows of tiles are placed, in the molten aluminium, in a grating-like arrangement in an attempt to reduce wear due to movements of the aluminium pool. To restrict movement in a "deep" cathodic pool of molten aluminium, US Patent No 4,824,531 (Duruz et al) proposed filling the cell bottom with a packed bed of loose pieces of refractory material. Such a design has many potential advantages but, because of the risk of forming a sludge by detachment of particles from the packed bed, the design has not found acceptance. US Patent No 4,443,313 (Dewing et al) sought to avoid this disadvantage of the previously mentioned loose packed bed by providing a monolayer of closely packed small ceramic shapes such as balls, tubes or honeycomb tiles. The following references disclose several other improvements in cell operation. European Patent Application No. 0308015 (de Nora) discloses a novel current collector; European Patent Application No. 0308013 (de Nora) deals with a novel composite cell bottom; and European Patent Application No. 0132031 (Dewing) provides a novel cell lining. While the foregoing references indicate continued efforts to improve the operation of molten cell electrolysis operations, none suggest the invention and all proposals for means to restrain movement of the aluminium pool or layer on the cell bottom have proven to be ineffective. Summary of the Invention This invention aims to overcome problems inherent in the conventional design of cells used in the electrowinning of aluminium via electrolysis of alumina dissolved in molten fluoride-based melts in particular cryolite, notably by proposing an improved cell bottom which incorporates means for restraining the aluminium pool on the cell bottom, without any need for added pieces which have to be placed or secured on the cell bottom. The present invention permits more efficient cell operation by modifying the design of the cathode blocks on the cell bottom. Such a modified design may then be utilized in any conventional cell and even in drained cell configurations. The invention concerns an electrolytic cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte, having a cathode cell bottom made of a series of carbon cathode blocks each having a top surface, side surfaces and a bottom surface, the cathode blocks being connected side-by-side transversally of the cell for example with ramming paste as in conventional cells or, preferably, by glue. Each cathode block is provided with a steel or other conductive bar for the delivery of current. These conductive bars are generally parallel to one another and extend transversally of the cell. A series of anodes are suspended facing a pool or a layer of molten aluminium atop the top surfaces of the cathode blocks, and the cell bottom has means for restraining movement of the pool or layer of molten aluminium. In the improved cell bottom according to the invention, the means for restraining movement of the pool or layer of molten aluminium comprise a series of parallel channels or grooves in the top surfaces of the carbon blocks along the direction of said conductive bars transverse to the cell. In use, these channels or grooves are covered by, and restrain, the pool or the layer of molten aluminium, in particular against movement in a direction transverse to the channels or grooves, so as to greatly decrease movement of the aluminium pool or so as to facilitate the collection/evacuation of aluminium, which substantially improves cell operation and extends cell life. The surfaces of the carbon blocks making up the cathode cell bottom are most preferably coated with a layer of aluminium-wettable refractory material, advantageously a particulate refractory hard metal boride applied from a slurry containing colloid, for example as disclosed in WO 93/25731 (Sekhar et al) . The surfaces of the carbon blocks making up the cathode cell bottom may be covered by a layer of molten aluminium forming a drained cathode surface, the channels or grooves forming a canal serving to guide the flow of aluminium across the cell. In this drained configuration, the channels or grooves are partly filled with molten aluminium and the electrolysis takes place between the aluminium-wetted cathode and the facing anode surface. In this drained cathode configuration, as explained below, an arrangement will be provided for removing aluminium from the sides of the cell. Alternatively, the surfaces of the carbon blocks making up the cathode cell bottom are covered by a pool of molten aluminium, the channels or grooves serving to reduce motion of the aluminium making up this pool. In this case, the aluminium pool completely covers the carbon blocks so that the electrolysis takes place between the surface of the aluminium pool and the facing anode surface. Due to the shape and size of the channels and their arrangement which segments the pool into sections, the magnetohydrodynamic forces are decreased significantly and movement of aluminium in the pool is significantly reduced. The depth of the aluminium pool can thus be reduced because of the resulting reduction in turbulence. Moreover, reduction of the turbulence in the aluminium pool is important because this is the origin for the formation of aluminium particles in the electrolyte, which particles are oxidized by CO 2 with the formation of alumina. In either case, for the drained configuration or when there is a pool of aluminium, the inter-electrode distance is reduced with a concomitant reduction of cell voltage and an increase in energy efficiency. In all cases, for drained configurations and when there is a pool of aluminium, the channels are so configured and arranged, in particular as regards their depth and the shape and angle of their walls, that the molten aluminium contained in the channels is restrained from movement in the longitudinal direction of the cell. For a drained cell configuration, the aluminium can flow along the channel into a collection channel; for a pool configuration, the walls of the channel oppose motion of the aluminium, restraining the pool and reducing turbulence. An advantage obtained with the channeled cell bottom is that its life is extended in comparison with other electrolytic aluminium production cells. Moreover, the channeled cathode improves the uniformity of the current distribution and increases the current efficiency. Additionally, in particular for deep pool configurations, sludge can accumulate in the grooves or channels without disturbing the current distribution. In drained configurations, the grooves or channels can serve to eliminate sludge which collects in the grooves or channels but is flushed out with the molten aluminium. At least some of the transverse channels or grooves can be formed by bevels or cut-outs along the top edges of the carbon blocks. In particular, a channel can be defined between two beveled edges of adjacent blocks. In most embodiments, the width of the transverse channels or grooves is at least as great as their depth. These channels or grooves have a rectangular, trapezoidal, V-shaped, curved (i.e. concave at least in their bottom part; most convex sections would not restrain the molten aluminium) or an asymmetric cross-section designed to permit the evacuation and collection of aluminium when the cell operates in a drained cell configuration, or designed to act as a barrier to aluminium movement to reduce or eliminate turbulent aluminium pool movement in a pool configuration. Advantageously, there may be at least one cross channel or groove which extends along the cell and intersects with the parallel transverse channels or grooves. Such cross channels or grooves may serve to drain the aluminium down the inclined cathode surfaces, hence guiding the flow of molten aluminium down these surfaces into the main channels or grooves. In drained cell configurations, cross channels or grooves of suitable dimensions can also serve for the removal of the molten aluminium to an aluminium reservoir. When the surface of the carbon cathode cell bottom is sloping, cross channels or grooves may run down the sloping cathode bottom surface to facilitate drainage of the aluminium in a drained configuration. For example, the carbon cathode cell bottom comprises at least two sloping parts there being at the intersection of the two sloping parts a collecting channel leading to an aluminium reservoir for collection of the drained aluminium. In general, for a drained cathode cell the tranverse channels or grooves may lead into at least one channel arranged longitudinally of the cell for collecting the molten aluminium, and preferably having means such as a weir for maintaining a constant level of aluminium in the transversally-extending channels or groove. This aluminium collection channel may extend along one or both sides of the cell, or could be a deep central channel machined in the cathode blocks. Preferably, the surfaces of the carbon blocks making up the cathode cell bottom are treated to reduce sodium penetration, for example as described in WO 94/20650 or in WO 94/24337, or are coated with a layer which reduces sodium penetration, for example a refractory hard metal boride applied from a slurry containing colloid as disclosed in WO 93/25731 (all in the name of Sekhar et al) . In general, the carbon cathode blocks are made resistant to chemical attack and to mechanical attack. The surfaces of the carbon blocks making up the channeled cathode cell bottom can also be coated with a layer which prior to use or in use becomes harder than the carbon cathode block and thereby protects the surface against abrasive wear by limited movement of the molten aluminium. Moreover, the hardened cathode surface remains dimensionally stable whereas a facing carbon anode may erode and conform to the shape of the cathode. This surface-hardening effect can be achieved with the aforementioned refractory boride or other aluminium wettable refractory layers which provide an essentially dimensionally stable surface. In this way, the cathode cell bottom can remain dimensionally stable during electrolysis, and because of this, it is both possible and advantageous to provide channels in the tops of the carbon cathode blocks because these channeled blocks will remain dimensionally stable during cell operation. The cell incorporating the channeled cell bottom can employ conventional carbon anodes which wear in the normal way for the pool configuration but whose shape adapts to the channeled cathode, in the drained configuration. Specially- shaped carbon anodes designed to cooperate with the channeled cathode design, and in particular to facilitate gas release at the anode while assisting drainage of molten aluminium at the cathode can also be used. Dimensionally stable anodes can also be employed. One method of fabricating an electrolytic cell according to the invention comprises providing the channels in the carbon cathode blocks before or after assembling the blocks to form the channeled cathode cell bottom. The channels are possibly machined in the carbon cathode blocks, for example using a milling cutter. For some shapes, especially with bevels, it may, however, be convenient to provide the channels by extrusion. If the blocks include bevels or cut-outs along their edges, when the adjacent blocks are brought together, the bevels or cut-outs between the assembled adjacent blocks form channels. Machining operations such as milling/cutting in particular are simple to execute to provide a series of parallel channels or grooves of any desired shape in the carbon blocks. When the blocks are assembled side-by-side, gaps can be left between the adjacent blocks, which gaps are filled with an anthracite-based or other usual ramming paste. Preferably, however, the blocks will be assembled using a glue, as is known for bonding together carbon cathode blocks with no or only a very small gap, such as a resin-based glue, or an inorganic glue as disclosed in WO 94/20651 (Sekhar) . Either way, the assembled blocks form a continuous carbon cell bottom in the same manner as in conventional cells, apart from the fact that the surface of the cell bottom is channeled. Before or after assembly, the carbon blocks may be treated to make them resistant to chemical and mechanical attack. Before start up of the cell for producing aluminium, the channeled cell bottom is preferably treated to harden the surface of the cathode blocks and render the surface wettable by molten aluminium, whereby in use the carbon cathode blocks remain dimensionally stable and are wetted by molten aluminium. The invention also relates to a carbon cathode block of an electrolytic cell for the electrowinning of aluminium from alumina dissolved in a fluoride-based molten electrolyte, ready to be installed in a cell. Such carbon block has a top surface, side surfaces and a bottom surface having therealong a longitudinally extending groove or like recess generally parallel to the block's top and side surfaces, for receiving a steel or other conductive bar for the delivery of current. According to the invention, the top surface of the cathode block is provided with at least one channel or groove; or at least one edge of the top surface of the cathode block is provided with at least one bevel or cut¬ out, or at least a part of the top surface is inclined, so that when two cathode blocks are placed side-by-side a channel or groove is formed between them. This channel, groove, bevel, cut-out or inclined part extends along the top surface of the cathode block generally parallel to the groove . or like recess in the bottom surface of the cathode block, so that when the cathode block with its current conducting bar is installed in a cell, the channels or grooves in the cathode block's top surfaces are arranged transversaly of the cell. This channel or groove in the top surface of the block, or formed between two blocks, is so shaped and dimensioned that when the cathode blocks are used to electrowin aluminium, molten aluminium collected in the channel or groove is restrained from moving transversally to the channel or groove, providing substantial advantages in operation, as explained above. These carbon blocks, which can incorporate all of the features described in relation to the complete cell, may have a flat top surface, or its top surface may be sloping or may comprise two roof-like inclined sections such that the blocks can be assembled side-by-side to form a cell bottom with alternately sloping sections which, at their lower intersections, form the aforesaid channels in the blocks' top surfaces. Description of the Drawings Reference is now made to the drawings wherein: - Figure 1 schematically shows part of a cell bottom formed of three cathode blocks provided with parallel channels in accordance with the invention, this schematic view being in longitudinal cross-section and side elevation of the cell; - Figure 2 schematically shows three different cathode blocks of cell bottoms in accordance with the invention provided with parallel channels of different shapes; - Figure 3 is a schematic sectional view though part of an electrolytic cell according to the invention, incorporating channeled cathode blocks and carbon anodes; - Figure 4 is a schematic sectional view through part of another electrolytic cell according to the invention, also incorporating channeled cathode blocks and carbon anodes; - Figure 5 is a similar view of another electrolytic cell according to the invention, incorporating channeled cathode blocks and dimensionally stable anodes; - Figure 6 is a view similar to Figure 3 of another electrolytic cell according to the invention; - Figure 7 is a view similar to Figure 4 of another electrolytic cell according to the invention; and - Figure 8 is a schematic plan view of the cell bottom of Figure 7 during operation, partly cut-away and with the anodes and cell superstructure not visible. Detailed De__ript_on of the Invention Figure 1 schematically shows part of a cell bottom formed of an assembly of channeled cathode blocks 10 according to the invention, three such blocks being shown. The blocks 10 are generally rectangular and made of carbon in the form of anthracite or graphite of the normal grade used for aluminium production cathodes. In their lower face, the blocks 10 have a recess 12 receiving a steel conductor bar 11 which is connected in the blocks by an electrically conductive bonding material for example cast iron. These steel conductor bars 11 extend externally to a negative bus bar of the cell. The rectangular cathode blocks 10 have a top surface (which will form the cathode cell bottom) side surfaces (which will be joined together) and a bottom surface in which the recess 12 is provided in the form of a rectangular groove whose faces are parallel to the block's top and side surfaces. Usually, the recess 12 extends all the way along the blocks' bottom surface. The side edges 20 of the block's top surfaces are beveled and, in the middle of the blocks' upper surfaces, generally V-shaped grooves 21 are machined. When two blocks 10 are brought together, the adjacent beveled edges 20 form also a V-shaped groove or channel similar in shape and size, and parallel to, the grooves 21. The adjacent blocks 10 are joined side-by-side by ramming paste 14, for example an anthracite-based paste, to form a continuous carbon cell bottom. Usually one block 10 extends over substantially the entire width of the cell. If this is not so, several blocks 10 are arranged end-to-end across the cell, i.e. along the direction of the conductor bars 11, and may also be joined by ramming paste. Instead of using ramming paste, the blocks can advantageously be bonded by a resin-based glue, in which case the gap between the adjacent blocks would be much smaller. When the blocks 10 are joined side-by-side to form a cell bottom, the conductive bars 11 extend across the cell and protrude from both sides of the cell (or alternate conductor bars protrude from opposite sides) for connection to the current supply. An anode 15, seen in side view, is diagramatically indicated in a dashed line. In use, the cell bottom formed by the blocks 10 with the V-shaped grooves or channels 20,21 is covered by a shallow pool of molten aluminium 40 (see Figure 3) , or just the grooves or channels 20, 21 are partly filled with molten aluminium, as will be described in detail later. Figure 2 shows three similar channeled carbon blocks 10 but with grooves or channels of different shapes, which blocks can be assembled into a cell bottom using glue or ramming paste. A first block has a parallel series of generally rectangular grooves 22 which are slightly wider than deep. Usually, the width of the grooves 22 is at least as great as their depth. The second block has generally trapezoidal grooves 23, and the third block has U-shaped grooves 24 of rounded cross-section, these shapes being given by way of example among many possible shapes. In all cases, the bevels 20 and grooves 21, 22, 23, 24 extend along the top surface of the cathode block parallel to the groove 12 receiving the conductor bar 11. All of the described bevels 20 and grooves 21, 22, 23, 24 can easily be machined in the blocks 10, for instance using a milling cutter. Alternatively, it is possible to provide grooves or bevels or other forms of channel by other methods, for example by extrusion. Figure 3 schematically shows, in longitudinal cross- section and side elevation, an aluminium production cell incorporating a carbon cell bottom formed of channeled carbon blocks 10 similar to those described above. The diagramatically-shown cell structure comprises a steel shell 30 of known type containing refractory material 31 over its bottom and extending up its sides. The blocks 10, supported on this refractory material 31, are arranged side-by-side and extend across the cell. The carbon blocks 10 are connected together by ramming paste 14, or alternatively are glued together, and the endmost blocks 10 are connected by ramming paste 34 to an insert 32 of carbon or a refractory carbide such as silicon carbide at the cell end. As previously, the bottoms of blocks 10 have recesses 12 receiving steel conductor bars 11 connected in the blocks by cast iron, which conductor bars 11 extend externally to a negative bus bar of the cell, situated along the side of the cell. The top surfaces of the blocks 10 have a series of parallel channels in the form of a wavy profile 25, though this could be any one of the other shapes shown in Figures 1 and 2, or other channeled shapes extending across the cell. This channeled profile 25 forms the top surface of the carbon cell bottom which is advantageously covered with a coating 35 of aluminium-wettable refractory material on which, as shown, there is a pool 40 of molten aluminium below a fluoride-based molten electrolyte 41 such as molten cryolite containing dissolved alumina. Several anodes 15, conventionally blocks of prebaked carbon, are suspended in the cell by the usual mechanisms (not shown) enabling their height to be adjusted. Oxygen evolving non-carbon anodes may be suspended in the cell instead of the carbon anodes 15 but do not need to be vertically adjustable because they are non-consumable. The anodes 15 dip in the molten electrolyte 41 facing the channeled cathode surface 25. The anode-cathode gap is not shown to scale. In operation, the cryolite-based electrolyte 41 is usually at a temperature of about 950°C, but the invention applies also to components used in cells with electrolytes well below 900°C, and as low as 700°C. The surface of the channeled profile 25 of the carbon cathode blocks 10 can be made dimensionally stable by applying a coating 35 of an aluminium-wettable refractory hard metal (RHM) having little or no solubility in aluminium and having good resistance to attack by molten cryolite. Note that the coating 35 also covers the ramming paste 14 and 3 . Useful RHM include borides of titanium, zirconium, tantalum, chromium, nickel, cobalt, iron, niobium and/or vanadium. Useful cathode materials are carbonaceous materials such as anthracite or graphite. It is preferred that the channeled profile 25 of the cathode of the present invention have a coating 35 of particulate refractory hard metal boride in a colloid applied from a slurry of the particulate refractory hard metal boride in a colloid carrier, wherein the colloid comprises at least one of colloidal alumina, silica, yttria, ceria, thoria, zirconia, magnesia, lithia, monoaluminium phosphate or cerium acetate. The colloidal carrier has been found to considerably improve the properties of the coating produced by non-reactive sintering. WO 93/25731 (Sekhar et al) provides a method of applying refractory hard metal boride to a carbon containing component of a cell for the production of aluminium, in particular by the electrolysis of alumina dissolved in a cryolite-based molten electrolyte, this method comprising applying to the surface of the component a slurry of particulate preformed refractory boride in a colloidal carrier as specified above, followed by drying, and by heat treatment before or after the component is installed in the aluminium production cell. The method of application of the slurry to the channeled cathodes of the present invention involves painting (by brush or roller) , dipping, spraying, or pouring the slurry onto the channeled cathode and allowing to dry before another layer is added. The coating 35 does not need to be entirely dry before the application of the next layer. It is preferred to heat the coating 35 with a suitable source so as to completely dry it and improve densification of the coating. Heating and drying take place preferably in non-oxidizing atmospheres at about 80-200°C, usually for half an hour to several hours and further heat treatments are possible. The channeled profile 25 of the cathode may be treated by sand blasting or pickled with acids or fluxes such as cryolite or other combinations of fluorides and chlorides prior to the application of the coating. Similarly the channeled cathode surface may be cleaned with an organic solvent such as acetone to remove oily products and other debris prior to the application of the coating. These treatments will enhance the bonding of the coatings to the channeled carbon cathode. After coating the channeled cathode by dipping, painting or spraying the slurry or combinations of such technique ' s in single or multi-layer coatings and drying, a final coat of the colloid alone may be applied lightly prior to use. Before or after application of the coating 35 and before use, the channeled cathode can be painted, sprayed, dipped or infiltrated with reagents and precursors, gels and/or colloids. For instance, before applying the slurry of particulate refractory boride in the colloidal carrier the channeled cathode can be impregnated with e.g. a compound of lithium to improve the resistance to penetration by sodium, as described in WO 94/20650 (Sekhar et al) . To assist rapid wetting of the channeled cathode by molten aluminium, the refractory coating 35 on the channeled cathode may be exposed to molten aluminium in the presence of a flux assisting penetration of aluminium into the refractory material, the flux for example comprising a fluoride, a chloride or a borate, of at least one of lithium and sodium, or mixtures thereof. Such treatment favors aluminization of the refractory coating by the penetration therein of aluminium. In operation of the cell illustrated in Figure 3, as shown, the coating 35 on the channeled profile 25 of the carbon blocks 10 making up the cathode cell bottom is covered by a pool of molten aluminium 40. The channels in the surface serve to restrain motion of the aluminium making up the pool 40 mainly in the longitudinal direction of the cell. As illustrated, the aluminium pool 40 completely covers the carbon blocks 10 so that the electrolysis takes place between the surface of the aluminium pool 40 and the facing surface of anode 15. An advantage is that the depth of the aluminium pool 40 can be reduced because of the reduction in turbulence. Moreover, sludge can accumulate in the channels of the channeled surface 25 without disturbing the current distribution. It should be noted that the motion restraining effect of the wavy profile 25 is due mainly to the concave lower part and to the lower part of the inclined walls which has a sufficient angle and height to obstruct movement of the molten aluminium in the direction along the cell. Alternatively, the channeled surface 25 of the carbon blocks 10 making up the cathode cell bottom, coated with a refractory aluminium-wettable coating 35, may be covered by a thin layer of molten aluminium forming a drained cathode surface, the channels serving to guide the flow of aluminium across the cell. In this drained configuration, the channels are partly filled with molten aluminium so that the electrolysis takes place between the aluminium-wetted cathode surface 25 and the facing surface of anode 15 which will tend to wear in such a way as to conform to the surface 25. A cell with such a drained configuration will be described below in connection with Figure 6. Figure 4, where the same reference numerals are used to designate the same elements, shows part of another aluminium production cell whose cell bottom is made up of a series of channeled carbon blocks 10 joined by ramming paste 14. Here, the channels are formed by juxtaposing blocks 10 having inclined surfaces 26 or 27 arranged alternately facing one another so as to form V-shaped channels 28 between adjacent blocks, these channels 28 receiving a stream or canal 40' of molten aluminium in their bottom part, running across the cell. Above the V-shaped channel 28 and aluminium canal 40' are carbon anodes 15 with corresponding V-shaped active faces 16 terminating with a lower flattened part 17 opposite the aluminium canal 40' . This flattened part 17 can be formed in use. The cell of Figure 4 operates as a drained cathode cell, wherein the aluminium produced on the inclined cathode surfaces 26 and 27 coated with the aluminium-wettable refractory coating 35 flows down the inclined surfaces, in the longitudinal direction of the cell, into the channel 28 where it is collected in the aluminium canal 40' . The channels 28 join into one or two side channels inclined in the longitudinal direction of the cell in order to permanently drain the product aluminium, at a rate to keep the level of aluminium canal 40' stable. In this drained configuration, the channels 28 are partly filled with molten aluminium so that the electrolysis takes place between the inclined aluminium-wetted cathode surfaces 26,27 and the facing inclined surfaces 16 of anode 15, as well as between the aluminium canal 40' and the facing flattened part 17 of the anode 15 which will wear in such a way as to conform to the opposing surfaces (26,27 and the flat top of canal 40") . Moreover, the inclination of the anode surfaces 16 assists in release of the anodically- formed gases. Alternatively, it is possible to operate in a fully drained configuration without an aluminium canal 40', or with a narrow and shallow canal of aluminium. Figure 5 shows a cell of similar design, where the same elements are designated by the same references, but which includes dimensionally stable anodes 18 in a "roof" configuration straddling the tops of the adjacent carbon cathode blocks 10 which also have alternately arranged inclined surfaces 26,27 coated with an aluminium-wettable refractory coating 35. The anodes 18 are made of or coated with any suitable non-consumable or substantially non-consumable, electronically-conductive material resistant to the electrolyte and to the anodically produced oxygen and other gases, vapors and fumes present in the cell. The anodes 18 may for example have a metal, alloy or cermet substrate which is protected in use by a cerium-oxyfluoride-based protective coating produced and/or maintained by maintaining a concentration of cerium in the electrolyte, as described in U.S. patent 4,614,569 (Duruz et al) . The cell of Figure 5 also operates as a drained cathode cell, wherein the aluminium produced on the inclined cathode surfaces 26 and 27 coated with an aluminium-wettable refractory coating 35 flows down to the bottom of these inclined surfaces where it is collected as an aluminium canal 40' in a groove 28' which is generally U-shaped in cross-section. This groove 28' is formed between cut-outs in the edges of the carbon blocks 10, when the blocks are fitted together. As before, the cathode surfaces 26,27 are inclined in the longitudinal direction of the cell in order to permanently drain the product aluminium into the canal 40', and the aluminium is removed from this canal 40' at a rate to keep its aluminium level stable. In this drained configuration, the grooves 28' are filled with the molten aluminium forming canal 40' . Electrolysis takes place between the inclined aluminium-wetted cathode surfaces 26,27 and the facing inclined surfaces of the dimensionally-stable anodes 18. The inclination of anodes 18 assists in release of the anodically-formed gases through a central opening 19 and this can be further assisted if needed by providing ridges on the anodes 18 or making the anodes foraminate. Figure 6 shows part of another cell according to the invention similar to the cell shown in Figure 3. Instead of having a wavy channeled surface, the blocks 10 have bevelled upper side edges 20 and may be bevelled also along their top end edges, as indicated in dotted lined at 29. The bevelled side edges 20 of the adjacent blocks form V or U-shaped channels running along the blocks 10, there being a similar V or U-shaped channel formed between the bevelled edge of the endmost block 10 and the adjacent mass of ramming paste 34. The main channels formed by the bevels 20 extend parallel to the conductor bars 11, i.e. across the cell, and the cross-channels formed by bevels 29 extend along the cell. These channels formed by the beveled edges 20 have the advantage of equalizing the current distribution in the carbon blocks 10 from the centrally located current distribution bar in the block's lower surface. This may in particular make it possible to reduce the height of the carbon blocks 10, leading to a saving in carbon which reduces the amount of toxic carbon that has to be disposed of when the carbon potlining is reconstructed. In Figure 6, the adjacent blocks 10 are shown as being bonded together by a glue 14', so there is essentially no gap between them. As shown, the aluminium-wettable refractory coating 35 covers the entire upper surfaces of blocks 10 with bevels 20, 29 and the glued joints 14', and also extends over the adjacent mass of ramming paste 34. The purpose of the cross-bevels 29 in the top end edges of the blocks 10 is to form a cross-channel perpendicular to the parallel channels formed by the V or U- shaped grooves between bevels 20, for draining off the product aluminium to maintain the canals 40' of aluminium in the V or U-shaped grooves at a constant level. Such cross- channels can be formed in one or both ends of the blocks and, if required, one or more intermediate cross channels can be formed by machining grooves across the blocks 10, intersecting with the V or U-shaped grooves formed by bevels 20. These cross-channels, which extend along the cell, are connected to a reservoir of molten aluminium, possibly with a weir in order to set the level of the aluminium canals 40'. Operation is possible with a fluctuating level of the aluminium canals 40' or with a steady level. The cell shown partly in Figures 7 and 8 is similar to that of Figure 4 in that it comprises a cell bottom made up of a series of carbon blocks 10 joined by ramming paste 14, where V-shaped channels 28 are formed by juxtaposing blocks 10 which, in this example, have roof-like inclined surfaces 26,27 side-by-side with the corresponding surfaces 27,26 of the adjacent blocks. The V-shaped channels 28 receive a stream or canal 40' of molten aluminium in their bottom part, running across the cell. Above the V-shaped channels 28 and aluminium canals 40' are carbon anodes 15 with corresponding V-shaped active faces 16. Opposite the aluminium canal 40', the anode faces 16 have a flattened part 17 which can be formed in use . The cell of Figures 7 and 8 operates as a drained cathode cell, wherein the aluminium produced on the inclined cathode surfaces 26 and 27 coated with the aluminium- wettable refractory coating 35 flows down these surfaces and into the channel 28 where it is collected in the aluminium canal 40'. The direction of flow of the molten aluminium is schematically indicated in Figure 8 by arrows. As shown, the channels 28 join into two side channels 42, running along and inclined in the longitudinal direction of the cell, in order to permanently drain the product aluminium. The molten aluminium is drained at a rate to keep the level of the aluminium canals 40' stable, for example by having a weir at the outflow end of the side channels 42. As illustrated in the right hand part of Figure 8, the two longitudinally sloping surfaces 26 and 27 of each cathode block 10 can be provided with cross channels or grooves 43 which run down these sloping surfaces 26 and 27 and lead into the main channel 28 running across the cell. Any suitable number of cross grooves 43 can be provided with any suitable spacing. These cross grooves 43 guide the flow of aluminium down the sloping cathode surfaces 26,27, while restraining movement of the aluminium across the cell as it drains down these inclined cathode surfaces. Only once the drained aluminium joins the canal 40' can it flow freely ■ across the cell to the side channels 28. This arrangement of the cross grooves 43 down the inclined cathode surfaces 26,27, the main collecting grooves 28 across the cell, and the side channels 42 provides a perfectly controlled flow of the molten aluminium over the cell bottom, avoiding turbulence due to magnetohydrodynamic forces . The side channels 42 can be formed in the ends of the cathode blocks 10, or can be made in the cell sidewalls, or can be a combination of bevels or cut-outs in the ends of the cathode blocks 10, cooperating with bevels or ledges in the cell sidewalls. Additionally or alternatively, a central channel could be provided by machining recesses in the centre parts of the inclined walls 26, 27 of the cathode blocks 10, or by making each cathode section from two cathode blocks placed end-to-end, with cut-outs in the facing ends.

Claims: CLAIMS 1. An electrolytic cell for the electrowinning of aluminium from alumina dissolved in a fluoride-based molten electrolyte, having a cathode cell bottom made of a series of carbon cathode blocks (10) each having a top surface, side surfaces and a bottom surface, the cathode blocks being connected side-by-side transverse to the cell and being provided with steel or other conductive bars (11) for the delivery of current, said conductive bars (11) being generally parallel to one another and transverse to the cell; and a series of anodes (15) facing a pool or a layer (40,40') of molten aluminium atop the top surfaces of the cathode blocks, the cell bottom having means for restraining movement of the pool or layer of molten aluminium, characterized in that the means for restraining movement of the pool or layer of molten aluminium comprise a series of parallel channels or grooves (20,21,22,23,24,25, 26/27/28) in the top surfaces of the carbon blocks (10) along the direction of said conductive bars (11) transverse to the cell, said channels or grooves being covered in use by and restraining movement of the pool or layer (40,40') of molten aluminium. 2. The electrolytic cell of claim 1, wherein the surfaces of the carbon blocks (10) making up the cathode cell bottom are coated with a layer (35) of aluminium- wettable refractory material. 3. The electrolytic cell of claim 2, wherein the surfaces of the carbon blocks (10) making up the cathode cell bottom are covered by a layer of molten aluminium forming a drained cathode surface, the channels or grooves forming a canal (28) serving to guide the flow of aluminium (40') across the cell. 4. The electrolytic cell of claim 1 or 2, wherein the surfaces of the carbon blocks (10) making up the cathode cell bottom are covered by a pool (40) of molten aluminium, the channels or grooves serving to reduce motion of the aluminium making up this pool. 5. The electrolytic cell of any preceding claim, wherein the top surfaces of the carbon blocks (10) further comprise at least one cross channel or groove (29,43) which intersects with the parallel channels or grooves, said cross channel or groove (29,43) extending longitudinally of the cell. 6. The electrolytic cell of any preceding claim, wherein at least some of the channels or grooves are formed by bevels (20) or cut-outs along the top edges of the carbon blocks. 7. The electrolytic cell of any preceding claim, wherein the width of the channels or grooves is at least as great as their depth. 8. The electrolytic cell of any preceding claim, wherein the channels or grooves (22) have a rectangular cross-section. 9. The electrolytic cell of any one of claims 1 to 7, wherein the channels or grooves (23) have a trapezoidal cross-section. 10. The electrolytic cell of any one of claims 1 to 7, wherein the channels or grooves (21) have a V-shaped cross- section. 11. The electrolytic cell of any one of claims 1 to 7, wherein the channels or grooves (24,25) have a curved or rounded cross-section. 12. The electrolytic cell of any one of claims 1 to 7, wherein the channels or grooves have an asymmetric cross- section. 13. The electrolytic cell of claim 5, wherein the cathode cell bottom is sloping longitudinally and wherein the cross channels or grooves run down the sloping cathode cell bottom. 14. The electrolytic cell of claim 5, wherein the cathode cell bottom comprises a plurality of pairs of two longitudinally sloping parts (26,27) and the cross channels or grooves (43) run down these sloping parts, there being at the intersection of the two sloping parts a collecting channel (28) leading to an aluminium reservoir. 15. The electrolytic cell of claim 3, or any one of claims 5 to 14 when depending on claim 3, wherein the channels (20,28) or grooves extending transversally of the cell lead into at least one channel (29,42) arranged longitudinally of the cell for collecting the molten aluminium, preferably having means for maintaining a constant level of aluminium (40') in the transversally- extending channels or grooves. 16. The electrolytic cell of any preceding claim, wherein the surfaces of the carbon blocks (10) making up the cathode cell bottom are treated to reduce sodium penetration. 17. The electrolytic cell of any preceding claim, wherein the surfaces of the carbon blocks making up the cathode cell bottom are coated with a layer (35) which reduces sodium penetration. 18. The electrolytic cell of any preceding claim, wherein the surfaces of the carbon blocks making up the cathode cell bottom are coated with a layer (35) which prior to or in use becomes harder than the carbon cathode block. 19. The electrolytic cell of any preceding claim, wherein the carbon cathode blocks (10) remain dimensionally stable during electrolysis. 20. The electrolytic cell of any preceding claim, wherein the carbon cathode blocks (10) are made resistant to chemical and mechanical attack. 21. A method of fabricating the electrolytic cell according to claim 1, comprising providing channels, grooves, bevels, sloping sections or cut-outs in the top surfaces of the carbon cathode blocks (10) before or after assembling the blocks to form the channeled cathode cell bottom. 22. The method of claim 21, wherein channels or grooves (21,22,23,24,25) are machined in the top surfaces of the carbon cathode blocks (10) . 23. The method of claim 21, wherein the channels include bevels (20) or cut-outs along the top edges of the carbon blocks ( 10 ) . 24. The method of any one of claims 21 to 23, comprising treating the carbon cathode blocks (10) to make them resistant to chemical and mechanical attack. 25. The method of any one of claims 21 to 24, wherein before start up of the cell for producing aluminium, the channeled cell bottom is treated to harden the surface of the cathode blocks (10) and render the surface wettable by molten aluminium, rendering the carbon blocks dimensionally stable during electrolysis. 26. A carbon cathode block (10) of an electrolytic cell for the electrowinning of aluminium from alumina dissolved in a fluoride-based molten electrolyte, having a top surface, side surfaces and a bottom surface, the bottom surface of the cathode block (10) having a groove (12) or like recess extending therealong generally parallel to the top and side surfaces of the cathode block, for receiving a steel or other conductive bar (11) for the delivery of current, characterized in that the top surface of the cathode block (10) is provided with at least one channel or groove (21,22,23,24,25); or at least one edge of the top surface of the cathode block is provided with at least one bevel (20) or cut-out, or at least a part (26,27) of the top surface is inclined, so that when two cathode blocks (10) are placed side-by-side a channel or groove is formed between them; said channel, groove, bevel, cut-out or inclined part extending along the top surface of the cathode block (10) generally parallel to the groove (12) or like recess in the bottom surface of the cathode block, the channel or groove in the top surface of the block, or formed between two blocks, being so shaped and dimensioned that when the cathode blocks are used to electrowin aluminium, molten aluminium collected in the channel or groove is restrained from moving transversally to the channel or groove. 27. The carbon cathode block of claim 26, comprising at least one bevel (20) or cut-out along an edge of the block's top surface. 28. The carbon cathode block of claim 26 or 27, wherein the width of the channels or grooves is at least as great as their depth. 29. The carbon cathode block of any one of claims 26 to 28, wherein the channels or grooves (22) have a rectangular cross-section. 30. The carbon cathode block of any one of claims 26 to 28, wherein the channels or grooves (23) have a trapezoidal cross-section. 31. The carbon cathode block of any one of claims 26 to 28, wherein the channels or grooves (21) have a V-shaped cross-section. 32. The carbon cathode block of any one of claims 26 to 28, wherein the channels or grooves (24,25) have a curved or rounded cross-section. 33. The carbon cathode block of any one of claims 26 to 28, wherein the channels or grooves have an asymmetric cross-section. 34. The carbon cathode block of any one of claims 26 to 33, further comprising in its top surface at least one cross channel or groove (29,43) which intersects with the channel, groove, bevel, cut-out or inclined part. 35. The carbon cathode block of any one of claims 26 to 34, wherein its top surface other than the grooved or channeled part is flat . 36. The carbon cathode block of any one of claims 26 to 34, wherein its top surface (26,27) is sloping such that the blocks can be assembled side-by-side alternately to form a cell bottom with alternately sloping sections forming therebetween the grooves or channels (28") parallel to the grooves (12) or like recesses in the blocks' bottom surfaces . 37. The carbon cathode block of any one of claims 26 to 34, wherein its top surface comprises two inclined sections (26,27) in roof-like configuration such that the blocks can be assembled side-by-side to form a cell bottom with alternately sloping sections forming therebetween the grooves or channels (28) parallel to the grooves (12) or like recesses in the blocks' bottom surfaces. 38. The carbon cathode block of any one of claims 26 to 37, wherein its top surface is coated with a layer (35) of aluminium-wettable refractory material. 39. The carbon cathode block of any one of claims 26 to 38, comprising a steel or other conductive bar (11) secured in the groove (12) or like recess in the bottom surface of the block by cast iron or another electrically conductive bonding material.

Inventors: DE NORA VITTORIO, DE NORA, VITTORIO
Applicants: MOLTECH INVENT SA, NORA VITTORIO DE, DE NORA, VITTORIO, MOLTECH INVENT S.A.
Application year: 1995
Main classification: C25C3
IPC codes: C25C3