Aluminium electrolysis cell with continuous anode

Abstract: Aluminium electrolysis cell comprising a cathode (13) and a continuous anode (14) being composed of carbon bodies which are glued or in some other way connected to one another. Onto the anode is added new carbon bodies to replace carbon material consumed during the electrolysis process. The anode is divided into sections in the form of easily detachable holders or cassettes (8) which are disposed close to one another in a row extending longitudinally of the cell. Each of the cassettes (8) has at its upper end, projections (23) designed to be removably connected to bearer walls or constructions at the long sides of the cell.

Description: The present invention relates to electrolysis cells for producing aluminium comprising a cathode and an anode of the continuous type and in particular having an anode which is composed of blocks of carbon, glued or mechanically attached to one another, and onto which anode is attached new blocks of carbon to replace carbon material consumed during the electrolysis process. Generally, two types of electrolytic cell are used in the electrolytic production of aluminium, namely cells provided with self-baking anodes, so called Soederberg anodes, and cells equipped with prebaked carbon anodes, which have to be exchanged with new anodes due to their consumption during the electrolysis process. Electrolysis cells with anodes of the prebaked type have the advantage that the voltage drop is less than on the Soederberg type. This mainly has to do with the fact that the electrical resistance of prebaked anodes is lower than the resistance in the coke mass of the Soederberg anode. In addition the drop in voltage between the current conductors and the carbon material is lower for the prebaked anodes than for the Soederberg anodes. This is because the current conductors for the prebaked anodes are connected to the carbon blocks before they are positioned in the cell and therefore can be firmly connected by means of gluing, screwing, casting or the like. The current conductors for the Soederberg anodes, on the other hand, are placed in position in the carbonatious anode mass during the electrolysis process in such a way that when they have reached their lowermost position they can be pulled up to be repositioned (the connection is relatively loose). On the other hand the prebaked anodes of the discontinuous type are encumbered with several disadvantages. As they have to be replaced by new anodes before they are completely used, there is an anode rest loss of about 15-25% of the total anode consumption. Further, the exchange and maintenance work is extensive and may be fairly expensive. Due to the disadvantages of traditional electrolysis cells, electrolysis cells with prebaked anodes of the continuous type have been developed. NO Patent No. 98126 discloses a cell for producing aluminium which uses a continuous prebaked anode composed of blocks of carbon attached to one another by means of gluing. The anode is disposed in a vertically sliding relation within a steel jacket and the electric current is conducted via contacts bolts provided in holes in the top side of the anode, as in the Soederberg anode. When attaching new carbon blocks to the anode, the bolts have to be pulled out, which is unpractical and time consuming, and results in high running costs. The solution has therefore not found any practical application. NO Patent No. 73535 discloses a cell for producing aluminium, wherein two anodes are arranged, side by side, in steel jackets. The anodes are composed of blocks of carbon onto which can be joined new blocks of carbon as the anodes are used. The feeding of the anodes is accomplished by means of jacks provided on top of the steel jackets. Further, for the supply of electric current to the electrodes and to provide the necessary friction to hold the electrodes in place, the lower ends of the steel jackets are provided with pressure devices in the form of weight arms, each arm acting on an exchangeable sliding contact which is influenced by a spring having individual screw adjusting means. A disadvantage with the above solution is that the pressure devices, which are constructionally complicated having screws and moveable parts, are disposed slightly above the electrolytic bath and are therefore susceptible to damage by the heat and harmful gases from the bath. A further disadvantage is that the pressure devices are large in size, which reduces the effective anode area and makes it difficult to get access to the electrolytic bath for instance in connection with the tapping of metal, crust breaking etc. Also the maintenance of such cells is difficult since only two electrodes are used, each comprising large carbon blocks which are cumbersome to handle. Also the relatively long distance between the current connectors of the blocks, causes ineffective current supply and uneven current distribution in the anode. Due to the above disadvantages the solution according to NO Patent No. 73535 has not found any practical use. US 3009870 discloses an electrolytic cell having an anode comprising several anode stacks, the space between adjacent anode stacks being filled with a packing cosisting of tightly stamped pulverulent electrolyte material, electrical current being supplied to each anode stack through bolts fastened to the ends of the anode stacks. The present invention seeks to provide an electrolysis cell for producing aluminium based on the continuous anode principle which is not encumbered with the above disadvantages, i.e. which is constructionally simple and thus cheap to build, and which at the same time is reliable and uncomplicated to maintain. In accordance with the invention, an aluminium electrolysis cell comprises a cathode and continuous anode, the anode comprising carbon block bodies connected together to form a vertical stack of carbon blocks so that new carbon blocks, to replace carbon material consumed during the electrolysis process, may be provided during use, characterised in that the anode is divided into sections, each section being mounted in an easily detachable holder or cassette, the holders or cassettes being disposed close to one another in a row extending longitudinally of the cell, each cassette having at its upper end a projection designed to be removably connected to bearer walls or constructions at the long sides of the cell. Preferably the projections rest on vertically moveable bars disposed on each long side of the cell, the bars being provided with jacks at each of their ends to accomplish lifting, lowering and tilting of the anode. Suitably each cassette is provided with at least one guide, preferably two, each for holding a stack of carbon bodies. Jacks may be provided to feed the carbon blocks down through the guides into the electrolytic bath. The lower part of the cassettes are preferably provided with a holding arrangement including electrical contacts, the holding arrangement providing sufficient friction to hold the stack of carbon bodies and simultaneously providing electrical contacts to conduct current to the carbon bodies. The holding arrangement may comprise clamping devices which act on the corners of the carbon bodies, the devices being interconnected by means of cross stays and held in position by vertical stays. The contact force between the clamping devices and the carbon block may be adjusted by bending the cross stays or by displacement of the vertical stays relative to one another. The bending may be accomplished by moving stays which at one end are connected to the middle part of the cross stays and at their other ends are connected to a tightening arrangement or spring arrangement disposed at the upper end of the cassette. Alternatively the holding arrangement may comprise clamping devices which are held in position by means of vertical stays, the contact force between the devices and the carbon bodies being adjusted by means of an adjusting frame which can be lowered or lifted and which at its corners is provided with inclined guides which act on complementary guides on the clamping devices. In another embodiment, the carbon bodies may be provided with one or more bores, each bore being provided with a spindle which at its lower ends is threaded. The upper end of the spindle is supplied with a bearing and a driving arrangement to rotate the spindle, whereby the carbon bodies can be held in position and be lowered or lifted by means of the spindles. The carbon blocks may consist of two halves, each of which is provided with semi circular grooves, whereby the bores are created under the stacking operation when the new carbon block comprising the two halves is placed on top of the carbon stack and glued thereto. The invention will now be further described by means of example with reference to the accompanying drawings in which. Figure 1 shows a side view, partly in section, of a cell according to the invention. Figure 2 shows the same cell in cross section. Figure 3 shows in larger scale a perspective view of an anode cassette with a clamping device according to the invention. Figure 4 shows, in the area of the clamping device, a horizontal section of the anode cassette shown in Figure 3. Figure 5 shows a horizontal section of a clamping device as shown in Figure 3. Figure 6 shows an alternative arrangement for regulating the contact force for the clamping device. Figure 7 shows another alternative arrangement. Figure 8 shows a cross sectional view of a cell with an alternative holding and feeding arrangement. Figure 9 shows schematically a preferred method of attaching a carbon block to the anode of the arrangement according to Figure 8. A cell used in the electrolytic production of aluminium has a cathode 13 and anode 14. The cathode 13 may be of a known, traditional design comprising a steel shell 9, a refractory lining 15 and an inner carbon layer 16 with cathode busbars 17 and cathode collectors (not shown). The anode 14 is made of sections in the form of easily exchangeable cassettes or holders 8 which are provided for continuous feeding of segments or blocks of carbon 21. Additional cassettes 22, containing equipment for the supply to the electrolytic bath of additive materials such as aluminium oxide, are disposed between the cassettes 8. The cassettes 8,22 are provided with projections 18 which rest on vertically movable bars 20. The cassettes are placed in close relation relative to one another so that they form a closure upwardly for the cell. As previously mentioned, the cassettes rest on movable bars 20. These bars are provided with jacks 19, (hydraulic or mechanical) to lower, lift or tilt the anode (i.e. the cassettes), for instance in connection with anode effect problems. The jacks 19 are disposed on pillars 1 resting on the cathode construction or cell foundation, and the whole anode arrangement is thus carried by these pillars. Along the short ends and the sides of the cell is provided an outwardly/upwardly swingable or easily detachable covering 12. This cover, which is in the form of plates or the like, represents a tight closure for the cell when the plates are in a closed position, and gives easy access to the cell when they are in an open position. Since the cassettes provide a 'lid' for the cell and the ends and sides of the cell are covered with plates 12, the space above the cell is completely enclosed. This enables the gases produced during the electrolysis process to be evacuated through an evacuation duct 3. The cassettes 8 are provided with clamping devices 27 which are designed to hold the carbon block 21. The cassettes 8 are wholly or partly made of electrically conductive materials and are electrically connected to anode busbars via a connection 2 and flexibles 4. The constructional design of the cassettes is further shown in Figures 3-5. As can be seen in Figure 3 the cassette consists of an upper part 23 provided with two guides 24 for the anode carbon blocks or segments 21. The carbon blocks 21 are attached to one another by means of gluing or the like, and can, as they are gradually consumed from below, be "extended" at the top by gluing a new carbon block thereto. To reduce the heat loss through the cassettes, blocks 25 of isolation can be provided on top of the carbon blocks for each of the guides 24. Such blocks of isolation are most preferred when the cassettes 8 are provided with cooling equipment. It should be stressed however that the cells according to the invention can be used with or without cooling equipment. The feeding of the anode carbon down through the guides 24 is accomplished individually by means of removable jacks (indicated by the position 26) which are governed by means of a governing unit (not shown). The jacks 26 can be of the mechanical or hydraulic type, but will not be further described in this connection. The lower part of the guides 24 comprise a holder arrangement in the form of a clamping device 27 which is fastened to the upper part of the guides by means of vertical stays/conductors 28, (see also Figure 4). The holder arrangement, by means of frictional force, holds the "stack" of carbon blocks 21 and at the same time conducts electric current to the anode carbon. A short current path is provided between the electrical contacts of the clamping device and the electrolytic bath. The clamping device 27 can withstand the corrosive environment close to the electrolytic bath and has reduced building width (not space demanding). This last mentioned advantage is important due to the short distance between the cassettes. The clamping devices 27 are connected with one another around the circumference each stack of carbon blocks by means of cross stays 5, and are pressed against the corners and swallow tail grooves 29 by shortening the effective length of the stays 5. Swallow tail grooves 29, are used with carbon blocks having a rectangular cross section and long sides in order to supply extra current contacts so as to obtain the best possible current distribution in the anode. A carbon block with a square cross section would only require clamping devices at the corners of the anode. The stays 5 are so designed that they can be subjected to bending. By pulling/lifting or pushing/lowering the tightening stays 30, the distance between the clamping devices is shortened and the pressure against the anode is increased. Under normal running conditions it is sufficient to hold the stays in a tight condition by means of a preset spring (not shown). The presetting of the spring can be governed so that small irregularities with regard to the dimension of the anode can be accepted without changing the holding forces beyond a wanted tolerance. The holding force and pressure between the clamping device and the anode, are selected according to the technical operating conditions. The design of the clamping device is shown in Figure 5. It consists of a constructional part 32, a current conducting part 33, a wear resistant layer 34 and external isolation 35. If the clamping device and the constructional elements which are interconnecting these devices are cooled down, cheaper materials can be used and improved results may be achieved in the form of increased contact pressure and reduced electric resistance between the clamping device and anode. Bores or conduits 36 for the circulation of a cooling fluid through the clamping devices may therefore be provided. Such bores are also provided in the stays 28 to cool these. The energy being extracted from the cooling fluid can be used for energy saving purposes, cfr. NO Patent No. 158511 belonging to one of the inventors. In the previously mentioned example the contact force between the clamping device 27 and the anode carbon 21 is adjusted by pulling or pushing the tightening stays 30. Figure 6 shows another example in which the clamping force is adjusted by moving the stays 28 up or down relative to one another. Further, Figure 7 reveals another example where the clamping devices are forced against the anode carbon by means of some kind of wedge arrangement. On the outside of the stays 28 is provided a frame 48 which can be lifted or lowered and which at the lower parts of its corners is provided with inclined guides 49 which abut complementary guides on the clamping device 27. Figure 8 shows a cross section of a cell including an anode cassette 8 with an alternatively designed holding and feeding arrangement. Two guides for the anode carbons 21 are shown. Instead of using clamping devices as explained above, each of the anode carbons 21 are provided with two vertical bores 41, and each of the bores are provided with spindles 40 having threads 38 at their lower ends. The spindles 40 are provided with thrust bearings at their upper ends and can be rotated by means of a gear and driving arrangement (not shown). The anode carbon 21 is held in position by means of the spindles through their threaded ends, and can be elevated or lowered by rotating the spindles. Electric current can be supplied wholly or partly through the spindles or through the guiding jackets 42. In Figure 9 is shown schematically a preferred method of adding new carbon bodies to the top of the anode as this is gradually consumed. As can be seen, each of the carbon bodies or blocks consist of two halves 43 each being provided with two parallel, semi circular grooves 44. The halves 43 are placed on top of the carbon block "stack" 21 (the cassette guides are not shown), and the semi circular grooves form the "bores" 41 after the gluing operation has taken place. The position number 45 indicates the glue layers between the carbon bodies 21. Alternatively, glue may also be used between the halves 43 (at 46). The guides 24 prevent the two halves 43 from separating after the gluing has taken place, and since the glue at this point of time (just after the two halves have been added to the top of the stack) has still not hardened, the clearance between the guide 24 and the carbon 21 should be sufficient to let the carbon slide downwards by its own weight. At the lower ends 42 of the guides, adjacent the threads 38 of the spindles, the clearance should be reduced. The clearance (tolerance) may be so narrow that all or part of the electric current is conducted to the carbon in this area. Alternatively the carbon blocks can be made in one piece and be provided with throughgoing bores, whereby the carbon blocks are placed onto the spindles from the top. Further, the carbon bodies do not need to be provided with two bores and two corresponding spindles, but can have one or more holes and a corresponding amount of spindles. Alternatively grooves can be provided at the corner of the anodes, spindles being disposed between the anode carbon and the walls/corners of the anode guide. In the figures and description examples are shown and explained in which carbon bodies of rectangular or square cross-section are used. Of course, the invention is not restricted to such shapes. Thus the carbon bodies may have a circular cross-section or other shape. The cassettes need not have two guides, but can have one or more than two such guides, and the carbon bodies may be of the prebaked type as well as of the "green carbon" type.

Claims: Aluminium electrolysis cell comprising a cathode and continuous anode, the anode comprising carbon block bodies connected together to form a vertical stack of carbon blocks so that new blocks, to carbon blocks, to replace carbon material consumed during the electrolysis process, may be provided during use, characterised in that the anode is divided into sections, each section being mounted in an easily detachable holder or cassette (8), the holders or cassettes (8) being disposed close to one another in a row extending longitudinally of the cell, each holder or cassette (8) having at its upper end a projection (18) designed to be removably connected to bearer walls or constructions at the long sides of the cell. Aluminium electrolysis cell according to claim 1, characterised in that the projections (18) rest on vertically moveable bars (20) disposed on each long side of the cell. Aluminium electrolysis cell according to claim 2, characterised in that the vertically moveable bars (20) are provided with jacks (19) at each of their ends to accomplish lifting, lowering and tilting of the anode. Aluminium electrolysis cell according to claims 1-3, characterised in that each cassette (8) is provided with at least one guide (24), each for holding a stack of carbon bodies (21). Aluminium electrolysis cell according to claim 4, characterised in that the cassettes (8) are each provided with two guides (24) arranged side by side to hold two separate stacks of carbon bodies (21). Aluminium electrolysis cell according to claims 1-5, characterised in that the lower part of the cassettes are provided with a holding arrangement (27-35) including electrical contacts, the holding arrangement providing sufficient friction to hold the stack of carbon bodies (21) and simultaneously providing electrical contacts to conduct current to the carbon bodies. Aluminium electrolysis cell according to claim 6, characterised in that the holding arrangement comprises clamping devices (27) acting on the corners of the carbon bodies (21), which devices are interconnected by means of cross stays (5) and held in position by vertical stays (28). Aluminium electrolysis cell according to claim 7, characterised in that the contact force for the clamping devices is adjusted by bending the cross stays (5), or by displacement of the vertical stays (28) relative to one another. Aluminium electrolysis cell according to claim 8, characterised in that the bending is accomplished by means of tightening stays (30) which at one end are connected to the middle part of the cross stays (5) and at their other ends are connected to a tightening arrangement or spring arrangement disposed at the upper end of the cassette (8). Aluminium electrolysis cell according to claim 6 characterised in that the holding arrangement comprises clamping devices (27) which are held in position by means of vertical stays (28) and that the contact force between the devices (27) and the carbon bodies can be adjusted by means of an adjusting frame (48) which can be lowered or lifted and which at its corners is provided with inclined guides (49) acting on complementary guides on the clamping devices (27). Aluminium electrolysis cell according to claims 1-5, characterised in that the carbon bodies (21) are provided with one or more bores (41), each bore being provided with a spindle (40) which at its lower ends is threaded, the upper end of the spindle being supplied with a bearing and a driving arrangement to rotate the spindle, whereby the carbon bodies can be held in position and be lowered or lifted by means of the spindles. Electrolysis cell according to claim 11, characterised in that the carbon blocks (21) consist of two halves (42), each of which is provided with semi circular grooves, whereby the bores (41) are created under the stacking operation when the new carbon block comprising the two halves is placed on top of the carbon stack and glued thereto.

Inventors: GJOERVEN SIGMUND, HOLMEN HANS KR, HURLEN JAN, NATERSTAD TORMOD, GJOERVEN, SIGMUND, HOLMEN, HANS KR., HURLEN, JAN, NATERSTAD, TORMOD, Gjörven, Sigmund
Applicants: NORSK HYDRO AS, NORSK HYDRO A.S.
Application year: 1990
Main classification: C25C3
IPC codes: C25C3