COMPOSITE CELL BOTTOM FOR ALUMINUM ELECTROWINNING

Abstract: A cell for the electrowinning of aluminum from molten salts has a cell bottom lining consisting partly of a refractory mass (4) and partly of carbon bodies (5). At least 30% and preferably 50% or more of the cell bottom area is occupied by the refractory mass (4). The carbon bodies (5) are level with the refractory mass (4) or are recessed therein.

Description: TECHNICAL FIELD The invention relates to aluminum reduction cells of the type having a cell bottom comprising a carbon body through which current is supplied to a pool of molten aluminum resting on the cell bottom, as well as to methods of fabricating and assembling such cells and methods of producing aluminum by electrolysis of a molten salt containing a dissolved aluminum compound in particular molten cryolite containing alumina, using an improved cell of this type. BACKGROUND ART Conventional Hall-Heroult cells for the electrolytic production of aluminum employ a carbon cell bottom which serves to supply current to a deep pool of molten aluminum forming the cathode. The cathodic aluminum is necessarily thick (at least 80-100 mm) because carbon is non-wettable by molten aluminum and during operation would not completely cover the carbon if the aluminum layer were thinner. In the conventional arrangement, a horizontal steel conductor bar is embedded in the lower part of the carbon cell bottom for the supply of current from an external source. Thus, the entire cell bottom in contact with the molten aluminum cathode consists of carbon which, in operation, is impregnated with sodium species and other ingredients of the cryolite leading to the formation of toxic compounds including cyanides. Despite the many disadvantages associated with carbon as cathode current feeder material (non-wettability by aluminum, necessitating deep pool operation; the relatively high electrical resistance of carbon, leading to energy losses; reactions within the cell environment necessitating disposal of large quantities of contaminated carbon when the cell bottom is renewed; swelling, which must be compensated by supporting the cell sidewalls in cradles, etc), attempts to replace carbon with theoretically more advantageous materials and employing new cell designs have not so far met with success. Thus, for example, an aluminum production cell having an electrically non-conductive refractory lining with a "bottom entry" current collector is described in U.S. Patent 3,287,247. The inner end of the current collector has a cap of TiB₂ projecting into a depression containing a deep pool of molten aluminum. U.S. Patent 3,321,392 describes a similar arrangement in which the protruding ends of TiB₂ conductor bars are rounded. U.S. Patents 3,093,570 and 3,457,158 disclose similar designs in which bottom-entry cylindrical current collector bars or posts of TiB₂ or graphite extend through a non-conductive refractory lining consisting throughout of powders of alumina and cryolite or aluminum fluoride. U.S. Patent 4,613,418 has proposed an aluminum production cell with an alumina potlining in which bottom-entry current collectors are embedded and extend to a recess in the potlining. To prevent the unwanted collection of sludge in these depressions, this patent proposes filling the depressions with balls of aluminum-wettable material. Related designs are proposed in U.S. Patent 4,612,103. These alternative cell designs, using a non-carbon cell bottom, have great promise. Replacement of the carbon cell bottom with, eg, alumina leads to potential savings in materials and operating costs. However, such proposals heretofore have generally relied on the use of a family of materials known as Refractory Hard Metals ("RHM") encompassing the borides and carbides of Ti, Zr, Hf, V, Nb, Ta. TiB₂ has been identified as the most promising RHM material. The use of these materials as part of the current supply arrangement has encountered a number of problems including cost and the difficulty of producing and machining large pieces of the materials. Such difficulties have led to the design expedients proposed in the aforementioned US Patents 4,613,418 and 4,612,103, where, for example, small pieces of TiB₂ are assembled or packed together in an environment of molten aluminum as part of the current supply arrangement. The problems experienced with RHM current collectors and further expedients for dealing with them, namely the provision of a protective barrier incorporating a molten fluoride- or chloride-containing salt mixture or a getter such as particulate aluminum, are further described in EP-A-0 215 555. In addition to the problems associated with the use of RHM materials, the cell design employing multiple current collector bars or posts of relatively small cross-section penetrating through the cell lining has many inherent drawbacks since each current collector must carry a high current and the failure of any single current collector can lead to a total cell failure. A number of proposals have been made for alternative cell designs having carbon cell bottoms in conjunction with inert materials underneath and/or at the sides of the carbon. See, for example, US Patents 3 390 071, 4 592 820, 4 673 481 and 4 619 750. Side-entry current feeder designs have also been proposed, eg. in US Patent 3 370 071, but such designs have not found acceptance on account of a number of inherent drawbacks. There has been also a proposal in GB-A-2 076 021 to provide dividers of insulating material that subdivide the liquid aluminium cathode so that its effective surface area is somewhat less than that of facing dimensionally stable anodes, with a view to improving the anode lifetime. This arrangement, however, complicates the cell bottom and adds to its cost. GB-A-1 206 604 has disclosed carbon blocks which protrude above a cell lining for the purpose of collecting sludge on the cell bottom. This design is, however, confined to deep pool operation and the protruding carbon elevations are subject to erosion. DE-A-2 318 599 discloses a cell wherein a layer of refractory material is recessed in the edges or in the central part of a massive carbon cell bottom, level with the operative top surface of the carbon cathode. The total upwardly-facing area of carbon in the cell bottom occupies the entire surface of the cell bottom. The problems associated with replacing the carbon bottoms of aluminium reduction cells have thus not been resolved in a satisfactory manner, so that carbon cell bottoms continue to be the industry standard. EP-A-0 299 733, which is prior art under Art. 54(3), describes a cell where part of the carbon cell bottom of a conventional aluminium reduction cell is replaced with a non-conductive refractory lining material level with the carbon. DISCLOSURE OF THE INVENTION This invention is based on the realization that considerable savings can be made and other advantages obtained by replacing substantial portions of the carbon in the cell bottom by refractory materials in areas where the carbon was considered necessary to provide for an adequate supply of current to the pool of aluminium forming the cathode. The invention therefore provides a cell for the electrowinning of aluminium from molten salts utilizing carbon cathodes, in which the cell bottom lining consists partly of a refractory mass and partly of carbon inset in the refractory mass, the total upwardly facing surface area of the carbon cathode under the anode being smaller than the horizontal surface area of the anode. In this description, "projected anode area" or "horizontal surface area of the anodes" mean the surface area of the cell bottom defined by a line bounding the periphery of each anode projected onto the cell bottom. Also, it is understood that the term "carbon cathode" means the carbon cathode current feeder, since the carbon acts to supply current to the pool of molten aluminium which forms the effective cathode in the cell. According to the invention, a cell for the electrowinning of aluminium from molten salts of the type having a plurality of anodes disposed over a cell bottom comprises a carbon cathode through which current is supplied to a pool of molten aluminium on the cell bottom, and is characterized in that the cell bottom is lined with at least one body of carbon and at least one mass of non-conductive, refractory material juxtaposed with the carbon body or bodies to make up a composite cell bottom composed of adjacent areas of current-conducting carbon and non-conductive refractory material with flat upper surfaces of the carbon areas located lower than the flat upper surfaces of the refractory areas or of another refractory material on the non-conductive refractory mass, and with the total upwardly facing surface area of the carbon in the cell bottom located under the anodes being smaller than the horizontal surface area of the anodes. In these cells, at least a part of the surface area of the anodes projected on the cell bottom thus covers areas of the non-conducting refractory material. Typically, 20 % or more of the projected anode area will be occupied by the refractory material and in some embodiments the entire anode surface area projected onto the cell bottom is occupied by the refractory material. This is possible by locating the carbon cathodes so that they provide an adequate distribution of current to the cathodic pool of molten aluminum. The pool of aluminum itself is such a good electrical conductor that current is evenly distributed at the surface of the pool. By thus replacing a substantial fraction of the carbon (as compared to a conventional carbon cell bottoms) considerable advantages are achieved, including: initial materials cost saving of the amount of block carbon required. a considerable reduction of the amount of contaminated carbon to be disposed of when the cell bottom is renewed, this contaminated carbon being a non-reusable hazardous waste. possible capital saving by reducing the production of the block carbon. by eliminating carbon pastes currently used to cement the carbon blocks, exposure of the workers to the fumes is eliminated. longer average cell life because of reduction of the swelling of the carbon blocks and replacement by a non-swelling refractory material. because of this reduction of swelling, there is less pressure on the sides of the cell. Consequently, the number of cradles or other devices to support the cell sidewalls is reduced, further simplifying construction of the cell and enabling a significant capital saving. when the cell is shut down and re-rebuilt, the alumina or other refractory material can be ground and re-used. This entails a significant saving in raw materials (cryolite) absorbed by the refractory because these materials can now be recycled whereas with all-carbon cell bottoms, such absorbed materials are lost with the hazardous waste. reduction of the manpower and time to construct or re-line a cell. Furthermore, this new composite cell bottom is relatively inexpensive, easy to construct, composed of tried and tested materials whose performance in the cell environment is known, and suitable for retrofit of existing cells but can also be applied to new cell designs. In these cells, preferably at least 30% and often 50% or more of the surface area of the carbon cell bottom lining is replaced by a refractory mass. Usually, no more than 80 or at most 90% of the surface area of the cell bottom is made up of the refractory mass, depending on the geometrical configuration. Also, as a general rule, the upwardly-facing carbon cathode area will be less than 50% of the active anode surface area, i.e., its horizontal area plus the operative area of the sides. In most embodiments, the refractory mass extends to the cell sides. The refractory mass advantageously comprises tabular alumina, for example it may be a mixture or layers of tabular alumina and alpha alumina as disclosed in EP-A-0 215 590, but may also consist at least in part of fused alumina, eg. slabs of fused alumina forming the upwardly-facing surface of the cell bottom. The upper surface of the refractory mass may be wettable by molten aluminium, eg. by incorporating aluminium-wettable RHM materials. The level of the refractory mass, ie. its flat, horizontal upper surface is higher than the level of the carbon cathode, ie. the carbon cathode surface is recessed or inset in the refractory mass. In this way, the depth of the pool of molten aluminium above the refractory mass can be reduced while maintaining this level sufficiently above the carbon cathode to protect the carbon from contact with the electrolyte during fluctuations of the pool level. Thus, when the level of the carbon cathode is below the level of the refractory mass, this permits a shallow aluminium pool above the refractory mass thereby reducing the fluctuation of the molten aluminium. This in turn permits the electrode gap to be reduced thanks to reduced fluctuation of the molten pool. The carbon cathode can consist of a plurality of sections usually of rectangular shape (in order to reduce the effect of the magnetic field and the fluctuation of the molten aluminium pool). These carbon cathode sections are longitudinal in the cell, or transversal. Alternatively, the carbon cathode sections recessed or inset in the refractory cell bottom are placed under the anodes and are of rectangular, round or of any convenient shape. In some embodiments, the carbon cathode sections in the cells are not placed in correspondence of the anodes and are of rectangular, round or of any convenient shape. One particularly advantageous configuration which will be described later consists of a chequer pattern. The areas of carbon may be rectangular (in plan view) and the refractory mass can occupy a space made up by multiples of the rectangular spaces corresponding to the carbon cathodes. Usually both the inset carbon and the refractory mass extend down to the cell lining or other support surface of the cell bottom, but this is not necessary and in some embodiments the carbon bodies may extend only part of the way down and be supported in a recess in the refractory mass. Electrical contact of the carbon cathode to the external bus bars can be made through conventional horizontal collector bars, ie. usually transverse in relation to the cell, but other arrangements are possible in new cell designs. Vertical pins, plates or bars of metals resisting the operating temperature of the cell may be inserted in the carbon cathode and connected to the collector bars, so reducing the electrical resistivity of the carbon bodies. Such pins, plates or bars may alternatively be connected to the conductive outer shell of the cell and from there to the bus bars. The flat, upper surface of the inset carbon cathodes in contact with the molten aluminium may also be increased by providing cuts, holes, slots or other recesses in the carbon body extending vertically but not reaching the current collecting means, and filled with aluminium. Furthermore, spacings (slots) can be provided between the sides of the inset carbon cathodes and the adjacent refractory mass, these spacings or slots extending vertically and being filled with aluminium, but not reaching the current collecting means. A feature of the described cells is that the cell bottom contains no portions of carbon which are not in contact with the molten aluminium. In the cells according to the invention, all the carbon serves as current feeder. There is no carbon which serves merely as a cell lining. The cell according to the invention can operate with conventional pre-baked carbon anodes or with oxygen evolving anodes, such as dimensionally stable anodes having a cerium oxide-fluoride surface coating. A method of fabricating or renovating (retrofitting) an aluminium production cell bottom according to the invention consists of lining the cell bottom with a refractory mass and carbon, the total upwardly-facing surface area of the carbon cathode located under the anode locations being smaller than the projection of the horizontal area of the anodes to be fitted in the cell, the upper surfaces of the carbon blocks being flat and being located lower than the upper flat level of the mass of refractory material. Advantageously, the carbon may be blocks of the same shape and size as the rows of carbon blocks in an existing cell, certain of these blocks being replaced in a retrofit operation with a mass of refractory material such as alumina. The invention also relates to the production of aluminium eg. by the electrolysis of alumina in molten cryolite, using the improved cell as described herein. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be further explained with reference to the accompanying schematic drawings, in which: Figs. 1 and 2 are transverse cross-sections through cell bottoms which are not in accordance with the claimed invention, but which nevertheless incorporate special features of the claimed invention; Fig. 3 is a transverse cross-section through an aluminium electrowinning cell according to the invention; Fig. 4 is a diagrammatic plan view of one form of a cell bottom as shown in Fig. 3; Figs. 5A, 5B and 5C are views similar to Fig. 4 showing different cell bottoms; Figs. 6A-6F are diagrammatic plan views of other cell bottom configurations; and Fig. 7 is a longitudinal cross-sectional view through part of another cell. DESCRIPTION OF PREFERRED EMBODIMENTS Figs. 1 and 2 do not represent embodiments of the invention as claimed, but facilitate the understanding of particular features of the claimed invention. Fig. 1 is a transverse cross-section through a Hall-Heroult cell of generally traditional design except that it has been retrofitted with a modified cell bottom including some features according to the claimed invention. The cell comprises a heat insulating shell 1, 2 having transverse cathode current-feeder bars 3 for example of steel or other suitable high-temperature resistant alloy. This shell 1, 2 contains a cell bottom made up of a mass 4 of compacted inert refractory material such as alumina and carbon bodies 5. The current-feeder bars 3 pass through the carbon bodies 5 for the supply of electric current to a pool 6 of molten aluminium resting on the top surface of the cell bottom. On top of the molten aluminium pool 6 is a layer of molten electrolyte 7, for example cryolite containing up to about 10 % of alumina at a temperature of about 900-970°C. The electrolyte 7 is surrounded by a freeze 8 of solidified electrolyte which covers the top edges of the refractory mass 4 and also extends around the periphery of the molten aluminium pool 6. Into the electrolyte 7 dip two rows of pre-baked carbon anodes 9 suspended by a conventional anode suspension arrangement (not shown). In traditional Hall-Heroult cells, the cell bottom (ie. corresponding to parts 4 and 5) is composed substantially entirely of carbon. The cell, as shown, has a mass 4 of refractory material making up a major part of the cell bottom, and can conveniently be constructed as a retrofit operation when the existing carbon cell bottom must be replaced. The carbon bodies 5 shown in Fig. 1 lie under the anodes 9 but the upwardly-facing surface area of the carbon bodies 5 under the anodes is less than the projected area of the anodes 9. Various configurations of how the bodies 5 may be disposed in the cell bottom and how the anodes 9 project onto the top face of the cell bottom will be described later. Fig. 1 shows two different arrangements for the upper faces of bodies 5. The left-hand body 5 has a flat top face 10 flush with the flat top face of the refractory mass 4, thus making up a flat uninterrupted cell bottom covered by the molten aluminium pool 6. The right-hand body 5 has two slots 11 machined into its upper face and extending down to within several centimeters of the current-feeder bars 3. These slots 11 are made wide enough to that they fill up with molten aluminium from the pool 6. A single slot 11, or more than two slots could be provided, as convenient, or instead of slots there could be other recesses of any other suitable shape, eg. with a round cross-section. The purpose of these slots or other recesses is to reduce the current carrying path between the bars 3 and the aluminum pool 6, thereby avoiding energy loss due to the relatively low electrical conductivity of carbon. The cell in Fig 2 is the same as that shown in Fig. 1 except for details of the current supply arrangement for the carbon anodes 5. Adjacent the left-hand carbon block 5 are channels 12 in the refractory mass 4. These channels 12 end several centimeters above the current-collector bars 3 and are filled with molten aluminium from the pool 6. Again, this serves to reduce the current-carrying path between the bars 3 and pool 6. Conveniently, the walls of the mass 4 forming channels 12 may be lined with an aluminium-wettable material such as TiB₂ or a composite containing TiB₂. The right-hand part of Fig. 2 shows a carbon block 5 incorporating a series of plates or posts 13 upstanding on the bars 3. The bars 3 and posts 13 may both be of steel or a weldable alloy such as NiAl, and joined by welding. These plates or posts 13 extend upwardly in the blocks 5 but stop several centimeters short of their upper faces. Any convenient number of plates or posts 13 can be provided. This is thus another way of reducing the current-carrying path through the carbon of blocks 5. Various combinations can be made of the features shown in Figs. 1 and 2. For example, the plates or posts 13 can be combined with external channels 12, or the external channels 12 can be combined with slots 11. In the cell according to the invention illustrated in Fig. 3, the carbon bodies 5 are located in recesses 14 in the cell bottom so that the flat top face 10 of bodies 5 is below the flat top 15 of the refractory mass 4, which has bevelled edges extending down to the top face 10 of bodies 5. By this arrangement, it is possible to lower the upper level of the pool 6 and, in turn, reduce the gap between the anodes 9 and pool 6. Fig. 4 is a schematic plan view showing one possible arrangement of how the anodes 9 are disposed over the central flat part of the cell bottom made up of the refractory mass 4 and the recessed or inset carbon bodies 5. For convenience, optional features such as the slots in or channels around the recessed carbon bodies are not shown. The current-collector bars 3 which protrude laterally from the cell are also not shown. The anodes 9 are represented in outline, ie. as projected onto the cell bottom. Fig. 4 shows carbon bodies 5 extending as two side-by-side longitudinal strips along the cell and located under the two rows of anodes 9. These anodes 9 have the same shape, dimensions and location as in a conventional cell. The projection of each anode 9 on the cell bottom extends in part over the refractory mass 4 which occupies a major part of the cell bottom area. In this particular embodiment, the recessed carbon bodies 5 are located partly under the anode projections 9. Figs. 5A, 5B and 5C show three different configurations in which the recessed carbon bodies 5 also extend partly under each anode projection. In Fig 5A, transverse carbon bodies 5 are located under each side-by-side pair of anodes 9. In Fig. 5B, a rectangular or square carbon body 5 is located centrally under a cluster of four anodes 9. In Fig 5C, a single carbon body 5 is located centrally under each anode 9; two of these bodies 5' are shown as square and two others 5'' of circular shape. However other shapes are possible. As for the other embodiments, the anodes 9 project onto the refractory mass 4. In the illustrated examples, the refractory mass 4 occupies approximately the following percentages of the projected anode area: 47 % in Fig. 4, 51 % in Fig. 5A, 76 % in Fig. 5B and 70 %/66 % in Fig. 5C. Figs. 6A-6F are schematic diagrams of the cell bottom shown subdivided into rectangles each representing the location of a carbon block 5 in a conventional cell bottom to be replaced. In the conventional procedure, the carbon blocks 5 are bonded at their interfaces by carbon pastes which release hazardous fumes. By reducing the number of these interfaces, and in some cases even by eliminating them, an important advantage is obtained. For convenience, these interface lines are shown in Figs. 6A-6F even at the locations occupied by a monolithic refractory mass, eg. of packed alumina. Figs. 6A-6D illustrate a cell bottom previously made up of rows of four rectangular carbon blocks 5 and in which some of the carbon blocks have been replaced. Typically each transverse row of four carbon blocks is associated with a transverse current feeder bar (not shown), like the bar 3 in Fig 1. In the retrofitted cell bottom of the invention shown in Fig. 6A, all of the carbon blocks along the sides and ends of the cell are replaced by a refractory mass 4. This leaves a central longitudinal cathode made up of carbon bodies 5. The arrangement shown in Fig. 6B is similar to that in Fig. 6A, except that only the lateral carbon bodies are replaced with the refractory mass 4, so that the carbon cathode made of bodies 5 extends from end-to-end of the cell. Fig. 6C shows an inverse arrangement where the carbon bodies 5 are arranged around the periphery of the cell bottom, leaving a rectangular central opening filled with the refractory mass 4. Fig 6D shows how substantially square cathodes can be made up (cf. Fig. 5B); in this example, the surface area of the carbon block 5 is less than 1/4 of the cell bottom area. Figs. 6E and 6F show further cell bottom configurations possible for retrofitting a cell made up of rows of five carbon blocks. Fig. 6E shows a checkerboard design obtained by replacing alternate carbon blocks 5 by the refractory mass 4. This design has two significant advantages. Firstly, a very uniform current distribution can be obtained using all of the existing cathode current connector bars. Secondly, there are no interfaces between the carbon blocks thereby eliminating the need for bonding with carbon paste. Fig. 6F shows a similar checker arrangement in which even more carbon is replaced, ie. around the periphery of the cell bottom. Obviously, many more designs are possible for the cell bottom, depending on the size and shape of the carbon blocks for any given cell bottom. Also, in Figs 6A-6F the locations of the anodes are not shown. It is evident that the cell bottom configuration can be set up as a function of a given anode configuration (rows of one, two or three anodes) if desired. For a retrofit operation, it is clearly advantageous to design a cell bottom based on the dimensions of the existing carbon blocks. In this way, the existing production line for the carbon blocks can be used without modification. In some cases it may however be advantageous to use smaller carbon blocks, either using a modified production line or by cutting the blocks in halves, or quarters, etc. The top surface of the refractory mass 4 can be made wettable by molten aluminium, eg. by incorporating RHM materials. The carbon blocks 5 are recessed to that their top surfaces are below the aluminium-wettable top surface of the refractory mass 4. In this way, there are deeper pools of molten aluminium over the carbon bodies 5, sufficiently deep to protect the carbon bodies from attack by the electrolyte, eg. during fluctuation of the level of the pool of molten aluminium. This recessed or stepped configuration is also very advantageous in that by having confined deeper parts of the aluminium pool unwanted motions in the aluminium pool are damped, permitting operation with a narrow gap between the anodes and the aluminium pool. These aluminium-wettable parts may advantageously be tiles or slabs of fused aluminium containing RHM inclusions in their surface, as described in EP-A-0 308 014 and as illustrated in Fig. 7. Fig. 7 is a longitudinal cross-section through part of another aluminium electrowinning cell employing carbon bodies in the form of bars 5 in a recessed shallow-pool configuration. The cell has a conductive base plate 33 eg. of steel to which the bars 5 are connected by steel or other alloy plates or posts 43 having slots 44 in their upper ends to accommodate for expansion. In this example, the bars 5 do not extend right down to the base plate 33 but are contained in recesses in the refractory mass 4. On top of the alumina or other refractory mass 4 are blocks 34 of refractory material having an upper layer 35 of RHM, for example TiB₂ particles or lumps embedded in a layer of tabular alumina or in fused alumina as described in greater detail in copending application EP-A-0 308 014. The top of refractory mass 4 is just below the level of the top 10 of the carbon bars 5, and the blocks 34 are placed alongside the bars 5 whereby they provide a recess 36 which is filled with molten aluminium 6. The walls of the recess 36 can be sloping, as shown, or vertical. Thus, the molten aluminium 6 forms a shallow pool or film about 3-30 mm thick above the aluminium-wettable surface of the RHM upper layer 35, but forms a deeper pool, eg. about 25-60 mm thick, in the recesses 36 above the top 10 or the carbon bars 5, which protects the carbon from attack by the electrolyte. Above the molten aluminium 6 is a layer of molten electrolyte 7 in which the anodes 9 dip. Typically two rows of anodes 9 are arranged side-by-side with any suitable number of anodes along the cell length according the cell capacity. Advantageously, as shown, the anodes 9 will be non consumable oxygen-evolving anodes, eg. coated with a cerium oxide-fluoride coating 39. A trough or other arrangement, not shown, is provided at the sides and/or ends of the cell for containing and tapping off the produced aluminium.

Claims: A cell for the electrowinning of aluminium from molten salts having a plurality of anodes disposed over a cell bottom comprising a carbon cathode through which current is supplied to a pool of molten aluminium on the cell bottom, characterized in that the cell bottom is lined with at least one body of carbon and at least one mass of non-conductive, refractory material juxtaposed with the carbon body or bodies to make up a composite cell bottom composed of adjacent areas of current-conducting carbon and non-conducting refractory material with flat upper surfaces of the carbon areas located lower than flat upper surfaces of the refractory areas or of another refractory material on the non-conductive refractory mass, and the total upwardly facing surface area of the carbon in the cell bottom located under the anodes being smaller than the horizontal surface area of the anodes. A cell according to claim 1, in which the refractory mass occupies at least 30 % of the surface area of the cell bottom. A cell according to claim 1 or 2, in which the refractory mass extends to the cell sides. A cell according to claim 1, 2 or 3, in which the refractory mass comprises tabular alumina. A cell according to any of claims 1 to 3, in which at least part of the refractory mass consists of fused alumina. A cell according to any preceding claim, in which the surface of the refractory mass is wettable by molten aluminium. A cell according to any preceding claim, in which the pool of aluminium above the refractory mass has a minimum level above the carbon cathode such that the level of molten aluminium maintained permanently is sufficient to protect the carbon from contact with the electrolyte during fluctuations of the pool level above the refractory mass. A cell according to any preceding claim, in which the carbon cathode consists of a plurality of sections. A cell according to claim 8, in which the carbon cathode sections are longitudinal in the cell. A cell according to claim 8, in which the carbon cathode sections are transversal in the cell. A cell according to claim 8, in which the carbon cathode sections in the cell are placed under the anodes. A cell according to claim 8, in which the carbon cathode sections in the cell are not placed in correspondence with the anodes. A cell according to any preceding claim, in which the electrical contact of the carbon cathode to external bus bars is made through collector bars extending horizontally through the cell bottom. A cell according to claim 13, in which vertical pins, plates or bars of metal resisting the operating temperature of the cell are inserted in the carbon cathode and connected to the collector bars. A cell according to any of claims 1 to 12, in which vertical pins, plates of bars of metal resisting the operating temperature of the cell are inserted in the carbon cathode and connected to the cell outside shell and from there to the bus bars. A cell according to any preceding claim, in which the surface of the carbon cathodes in contact with the molten aluminium is increased to improve electrical contact by providing cuts, holes, slots or other recesses in the carbon body extending vertically but not reaching the current collecting means, these cuts, holes slots or recesses being filled with molten aluminium. A cell according to any preceding claim, in which spacings (slots) are provided between the carbon cathodes and the adjacent refractory mass, these spacings (slots) extending vertically and being filled with molten aluminium, but not reaching the means for supplying current to the carbon cathodes through the cell bottom. A cell according to any preceding claim, in which the anodes are oxygen evolving anodes. A cell according to claim 18, in which the anodes are dimensionally stable. A method of renovating a used cell bottom of an aluminum production cell which cell bottom is made up of rows of blocks of carbon connected to current supply means, which method comprises replacing some of the used blocks of carbon with new blocks of carbon and replacing the other used blocks with a mass of refractory material, the upper surfaces of the carbon blocks being flat and being located lower than the upper flat level of the mass of refractory material. A method of producing aluminium by electrolysis of a molten salt, in a cell as claimed in any one of claim 1 to 19 or as renovated by the method of claim 20, by passing current from carbon cathode areas inset in the flat refractory areas of the cell bottom, the carbon cathode areas having smaller horizontal surface area than the overlying anodes.

Inventors: CRONIN, BRIAN HAROLD JOHN, DE NORA, VITTORIO, DURUZ, JEAN-JACQUES
Applicants: MOLTECH INVENT S.A.
Application year: 1988
Main classification: C25C3
IPC codes: C25C3