GB2564456A - Electrolysis cell for Hall-Héroult process, with cooling pipes for forced air cooling - Google Patents

Abstract

The potshell (10) comprises: a bottom wall (11) and peripheral walls (12-15) extending upwards from said bottom wall, so as to define an inner reception volume (V10). The peripheral walls comprising side walls (12, 13) and end walls (14, 15), a plurality of reinforcement members (20), are provided side by side along at least part of said peripheral walls. The pot shell is further provided with cooling means (30, 40), adapted to blow a gaseous cooling medium against the outer surface of at least part of said peripheral walls. The cooling means comprises a system of rigid pipes 30, 40) extending, for the greater part of their length, parallel to the deck plate (10') of said potshell. The rigid pipes being attached to at least part of said reinforcement members (ie ribs, fans) by a permanent fixing means (24), said permanent fixing means comprising a hole (24) provided in each said reinforcement member, adapted to receive said rigid pipe (vent).

Description

Electrolysis cell for Hall-Heroult process, with cooling pipes for forced air cooling

Technical field of the invention

The invention relates to the field of fused salt electrolysis, and more precisely to electrolysis cells using the Hall-Heroult process for making aluminium by fused salt electrolysis. The invention relates to an electrolysis cell with an integrated air cooling system that can be used to avoid overheating in certain critical situations, in particular during start-up and in case of abnormal operation conditions of the electrolysis cell.

State of the art

The Hall-Heroult process is the only continuous industrial process for producing metallic aluminium from aluminium oxide. Aluminium oxide (AI2O3) is dissolved in molten cryolite (Na3AIF6), and the resulting mixture (typically at a temperature comprised between 940 °C and 970 °C) acts as a liquid electrolyte in an electrolytic cell. An electrolytic cell (also called “pot”) used for the Hall-Heroult process typically comprises a steel shell (so-called potshell), a lining (comprising refractory bricks protecting said steel potshell against heat, and cathode blocks usually made from graphite, anthracite or a mixture of both), a superstructure and a plurality of anodes (usually made from carbon) that plunge into the liquid electrolyte contained in the volume defined by the cathode bottom and a side lining made from carbonaceous material. The superstructure of the cell comprises a fixed frame and a mobile metallic anode busbar, also called an “anode beam”, which extends at the outer periphery of the fixed frame. Each anode assembly comprises at least one anode and is provided with a metallic rod, for mechanical attachment and electrical connection of said anode assembly to said anode beam. Anodes and cathodes are connected to external busbars. An electrical current is passed through the cell (typically at a voltage between 3.5 V and 5 V) which electrochemically reduces the aluminium oxide, split by the electrolyte into aluminium and oxygen ions, into aluminium at the cathode and oxygen at the anode; said oxygen reacting with the carbon of the anode to form carbon dioxide. The resulting metallic aluminium is not miscible with the liquid electrolyte, has a higher density than the liquid electrolyte and will thus accumulate as a liquid metal pad on the cathode surface from where it needs to be removed from time to time, usually by suction into a crucible.

The electrical energy is a major operational cost in the Hall-Heroult process. Capital cost is an important issue, too. Ever since the invention of the process at the end of the 19th century much effort has been undertaken to improve the energy efficiency (expressed in kW/h per kg or tonne of aluminium), and there has also been a trend to increase the size of the pots and the current intensity at which they are operated in order to increase the plant productivity and bring down the capital cost per unit of aluminium produced in the plant.

Industrial electrolytic cells used for the Hall-Heroult process are generally rectangular in shape and connected electrically in series, the ends of the series being connected to the positive and negative poles of an electrical rectification and control substation. The general outline of these cells is known to a person skilled in the art and will not be repeated here in detail. They have a length usually comprised between 8 and 25 metres and a width usually comprised between 3 and 5 metres. The cells (also called “pots”) are always operated in series of several tens (up to several hundreds) of pots (such a series being also called a “potline”); within each series DC currents flow from one cell to the neighbouring cell. For protection the cells are arranged in a building, with the cells arranged in rows either side-by-side, that is to say that the long side of each cell is perpendicular to the axis of the series, or end-to-end, that is to say that the long side of each cell is parallel to the axis of the series. It is customary to designate the sides for side-by-side cells (or ends for end-to end cells) of the cells by the terms “upstream” and “downstream” with reference to the current orientation in the series. The current enters the upstream and exits downstream of the cell. The electrical currents in most modern electrolytic cells using the Hall-Heroult process exceed 200 kA and can reach 400 kA, 450 kA or even more; in these potlines the pots are arranged side by side. Most newly installed pots operate at a current comprised between about 350 kA and 600 kA, and more often in the order of 400 kA to 500 kA.

Hall-Heroult pots operate normally at a temperature in the order of 940°C to 980°C as a continuous fused salt electrolytic process: the metallic, liquid aluminium produced by the process is removed from time to time, and the alumina dissolved in the electrolytic bath that is consumed during the process is replenished from time to time. When the cell is put into service for the first time after its construction or after relining (typically after five to seven years of operation), it needs to be started up using a specific start-up procedure. During start-up of the cell, liquid electrolyte (also called “liquid bath”) is poured into the cell until the electrolysis process can be started in the cell. Specific preheating procedures are needed in order to avoid thermal shock of the cell components in contact with the liquid bath or liquid metal; thermal shock may induce cracks.

The start-up of a pot, after its construction, refurbishment or repair, is known to be a critical phase for the life of a new pot (see “Cathodes in Aluminium Electrolysis” by M. Sorlie and H.A. 0ye, 3rd edition, Diisseldorf 2010, pages 111 - 158). Before starting up a cold electrolysis pot, the pot is preheated, typically to a temperature ofthe order of 850°C to 900°C. This is a critical step because preheating a cold pot will generate strong thermal gradients that may lead to abnormal thermal stresses, in particular on the sole (upper surface) of the cathode; these stresses may damage the cathode elements, in particular by inducing cracks. After preheating, liquid electrolyte (also called “’liquid bath”, a mixture of alumina and cryolite) is poured into the preheated pot, and electrical current is passed through the liquid bath in order to start the electrolysis process which then generates enough heat to keep the electrolyte liquid. During this start-up phase the pot is not operating under steady state conditions, and is usually exposed to a temperature well above its normal operating temperature of 960 °C to 970 °C. At this stage it is desirable not to reach a temperature of 1 000 °C, but it may happen that the temperature reaches 1 000 °C or even 1 050 °C for a duration of One or two days. In abnormal cases the temperature may reach 1 100 °C during start-up for short durations up to 24 h. Such a high temperature may lead to transient deformation of the steel potshell due to thermal expansion and softening; overheating can also lead to permanent deformation of the potshell.

It is known that pots with new cathodes are more sensitive to excessive heat than pots in which the side ledge, a crust formed by the solidification of electrolytic bath, is protecting the sidewall lining materials. More precisely, the side ledge provides both a chemical and thermal protection of the sidewall lining in contact with the liquid bath; said sidewall lining is a carbonaceous material. Underneath this carbonaceous material the potshell is thermally protected by refractory bricks, usually made from silicon carbide. After the startup of a new pot, the formation of side ledge usually takes several days; under normal operation conditions its thickness depends on the bath temperature and is typically in the order of 2 cm to 10 cm. During this period, the potshell sides are thermally protected only by the sidewall lining, which is less efficient than the combination ledge plus sidewall lining and the sidewall lining is not protected until the side ledge has begun to form. Temperatures higher than the normal operating temperatures may also occur after startup of the pot. In particular the so-called anode effect may lead to a significant increase in pot temperature for a duration that ranges from a few tens of minutes to a few hours and in pathological situations (possibly combined with absence of remedies or inefficient remedies applied by pot operators) this duration may exceed 24 hours.

Other reasons that make it desirable to be able to cool the potshell are related to specific designs of pots or their cathodes. In particular, WO 2016/157021 (Dubai Aluminium PJSC) describes a Hall-Heroult electrolysis cell using cathode blocks with collector bars made from plain copper, instead of steel collector bars or steel collector bars with copper inserts. These cells are particularly sensitive to overheating. Indeed, in modern Hall-

Heroult cells the temperature of the liquid aluminium pad in contact with the upper surface of the cathode blocks is normally about 950°C to 970°C, and under these operating conditions the temperature in the centre of the cathode block can reach 950°C. However, as mentioned above, in case of abnormal conditions the temperature of the aluminium pad can increase to over 1000°C, typically up to 1080°C for short duration of up to 24 hours. Such overheating may occur especially when starting the pot. While modern potlines and sophisticated pot control can avoid and/or limit overheating (above 1 000°C), this event cannot be fully excluded throughout the normal lifetime of an electrolysis pot (5 to 7 years). Knowing that the melting point of pure copper is about 1084°C, it must be ruled out that in case of abnormal overheating the copper cathode collector bar will suffer irreversible damage.

For emergency cooling of a potshell a portable system with a compressor and flexible tubes providing compressed air can be carried on the site; this has been done since the early days of aluminium industry. However, for safety reasons there should not be any long flexible tubes lying around the floor in the vicinity of operating pots.

Prior art offers several approaches to cool the potshell in case of overheating. US 4,087,345 (Ardal og Sundal Verk) and US 4,608,135 (Aluminium Company of America) describe two potshell designs that foster natural convection on its lateral (long) faces. This is not efficient and does not allow enhanced cooling when needed. EP 0 047 227 A1 (Schweizerische Aluminium AG) describes a pot in which a pipe carrying a heat exchange fluid is incorporated in the side lining of the pot. This requires a complex modification of the cell design. WO 99/54526 (Aluminium Pechiney) describes a piping system surrounding the potshell (and extending under the potshell), comprising protruding nozzles that blow air against the potshell. A decrease in temperature of the range of 50 °C up to 100 °C is reported. These protruding nozzles are rather bulky and need to be removed for maintenance work of the pot, and must be put in place and readjusted afterwards. This uses manpower. Moreover, as a rule, the presence in a potroom of movable long metallic objects, for whatever purpose, is undesirable because it represents an electrical hazard. WO 2004/007806 (Aluminium Pechiney) describes a cooling system in which water droplets are projected against the potshell. This system is complex, as it requires the water mist to be confined in a closed volume formed by a box-like structure together with the potshell. It is supposed to be more efficient than natural convection or forced-air cooling. Its main drawback is the use of water in a potroom, which poses three problems: liquid water in a high temperature environment is generally problematic; liquid water in contact with liquid aluminium generates an explosion hazard; and liquid water in proximity of bare electrical conductors and zones of different electrical potential presents an electrical hazard. As a rule, the presence of liquid water and water-bearing piping is totally undesirable in a potroom, for whichever purpose. WO 2006/053372 (BHP Billington) discloses a pot with an internal cooling system using fluid ducts extending horizontally along the side walls of the shell. A heat transfer fluid is circulated inside these ducts, using suitable pump means. This system requires a significant modification of the pot shell construction.

The present invention relates to an electrolysis cell with an integrated, permanently installed air cooling system that can be used whenever needed to avoid overheating in certain critical situations, in particular during start-up, and in case of abnormal operation conditions of the electrolysis cell.

The inventors have tried to find a solution as simple and inexpensive as possible, that can be adapted to virtually any existing potshell design, that does not induce any safety hazard, and that provides efficient cooling during pot start-up and anomalous pot operation conditions.

Objects of the invention A first object of the invention is a potshell for an electrolytic cell suitable for the Hall-Heroult electrolysis process, said potshell being intended to receive a cathode forming the bottom of said electrolytic cell and comprising a plurality of parallel cathode blocks, each cathode block comprising at least one metallic cathode collector bar protruding out of each of the two ends ofthe cathode block, a lateral lining defining together with the cathode a volume containing the liquid electrolyte and the liquid metal resulting from the Hall-Heroult electrolysis process, said potshell comprising - a bottom wall and peripheral walls extending upwards from said bottom wall, so as to define an inner reception volume, said peripheral walls comprising side walls and end walls, - a plurality of reinforcement members provided side by side along at least part of said peripheral walls, said potshell being further provided with cooling means adapted to blow a gaseous cooling medium against the outer surface of at least part of said peripheral walls, said potshell being characterized in that - said cooling means comprises a system of rigid pipes extending, for the greater part of their length, parallel to the deck plate of said potshell, - said rigid pipes being attached to at least part of said reinforcement members by a permanent fixing means, - said permanent fixing means comprising a hole provided in each said reinforcement member, adapted to receive said rigid pipe.

Said permanent fixing means may further comprise a welding joint between the facing walls of said rigid pipe and said hole provided in each said reinforcement member.

Said system of rigid pipes may comprise at least one so called side straight pipe, which extends over substantially all the length of a respective side wall of said potshell. Said system of rigid pipes may comprise at least one so called end straight pipe, which extends over substantially all the length of a respective end wall of said potshell. Advantageously, said potshell comprises valve means to selectively couple adjacent ends of one side pipe and one end pipe.

Said cooling means are advantageously adapted to blow a gaseous cooling medium against said peripheral walls at the height of the liquid electrolyte and the liquid metal contained in said volume. Said potshell may be provided with positioning means, adapted to position said cooling means at a substantially constant so-called blowing height, over at least part of said peripheral walls. Said constant blowing height is for example between 60 and 80% of the depth of said potshell. Said positioning means may comprise said fixing means.

According to one embodiment, said cooling means comprise a plurality of blowing ports, provided substantially at the same altitude, said blowing height corresponding to the altitude of the centre of each port. Said cooling means may comprise a plurality of blowing ports, provided at different altitudes, said blowing height corresponding to the mean altitude of the centres of said ports. Said blowing ports are advantageously adapted to blow cooling medium along different angles.

Another object of the invention is an electrolytic cell suitable for the Hall-Heroult electrolysis process, comprising a cathode forming the bottom of said electrolytic cell and comprising a plurality of parallel cathode blocks, each cathode block comprising at least one metallic cathode collector bar protruding out of each of the two ends of the cathode block, a lateral lining defining together with the cathode a volume containing the liquid electrolyte and the liquid metal resulting from the Hall-Heroult electrolysis process, an outer metallic potshell containing said cathode and lateral lining, a plurality of anode assemblies suspended above the cathode, each anode assembly comprising at least one carbon anode and at least one metallic anode rod connected to an anode beam, a cathodic bus bar surrounding said potshell, a plurality of connectors, each connecting one end of a cathode collector bar of a cathode block to said cathodic bus bar, said electrolytic cell being characterized in that said outer metallic potshell is a potshell as defined above.

Still another object of the invention is an aluminium electrolysis plant comprising at least one line of electrolysis cells of substantially rectangular shape, and said plant further comprising means for electrically connecting said cells in series and for connecting the cathodic busbar of a cell to the anode beam of a downstream cell, characterized in that more than 80% of the electrolysis cells in at least one of said line, and preferably each electrolysis cell of said line, is an electrolysis cell as defined above.

Still another object of the invention is a method for making aluminium by the Hall-Heroult electrolysis process, characterized in that said method is carried out in an aluminium electrolysis plant as defined above.

According to the invention, the above mentioned problem is solved by a single rigid tube extending horizontally on each of the two long sides of the pot (and optionally equally extending on each of the short sides). The tube is oriented parallel to the deck plate of the potshell. In a most advantageous embodiment the tube is inserted into a hole provided in the reinforcement member.

The height of the tube is advantageously chosen such that the cooled zone is close to the hottest zone of the potshell, that is to say the zone slightly above the upper surface of the cathode where the liquid metal pad accumulates.

The tube typically comprises a plurality of holes or nozzles, preferably arranged on a straight line parallel to the length of the tube.

The tube extends over most of the length (i.e. the longest dimension) of the potshell. In a variant, it extends also over at least part of the width (i.e. the shortest dimension) of the potshell.

In an embodiment the end of the tube comprises a connector allowing connecting a flexible tube.

In an embodiment the end of the tube comprises a section that is bent, for instance by 90°; the end of this bent section comprises said connector.

In an embodiment, said connector is connected to a flexible tube, the second end of which is connected to a connector provided on the superstructure of the pot, said connector being the end of another rigid tube connected to a compressed air supply. Said compressed air supply is necessary anyway for operating pneumatic crust breakers. Said flexible tube can be very short, less than two meters and preferably of the order of one meter; such short flexible tubes can be conveniently handled and stored by a single operator, and do not represent a safety risk in a potroom. As the system is intended to be used only in peculiar situations there is no need to have the compressed air permanently connected to the compressed air supply. However, this variant is within the scope of the present invention.

The advantage of the system according to the invention is mainly its simplicity: it can easily be adapted to virtually any potshell design comprising reinforcement members, such as cradles. Being integrated into the potshell there is no need to disassemble it when the potshell is removed for relining, repair or refurbishment.

Figures

Figures 1 to 12 represent two embodiments of the present invention; they do not limit the scope of the invention.

Figure 1 is a schematic view, showing the global arrangement of a series of cells in an electrolysis plant according to the invention.

Figure 2 is a top view, showing from above a potshell according to a first embodiment of the invention, said potshell belonging to one cell of the plant of figure 1.

Figure 3 is a longitudinal cross section, showing the potshell of figure 2 as well as some other elements of said cell which do not form part of the invention.

Figure 4 is a transversal cross section along line IV-IV of figure 2, showing potshell of figure 2, part of the adjacent potshell, as well as cooling means according to the invention. Figure 5 is a cross section analogous to figure 4, showing at a greater scale a cradle of one of the potshells of figure 4 as well as a cooling pipe fixed on said cradle.

Figure 6 is a plan view of a cooling pipe which belongs to said potshell.

Figure 7 is a transversal cross section along line VII-VII of figure 6, showing said cooling Pipe-

Figure 8 is a perspective view of a potshell according to a second embodiment of the invention

Figure 9 is a perspective view, which shows an assembly of pipes provided on peripheral walls of potshell of figure 8.

Figure 10 is a cross section analogous to figure 5, along arrow X on figure 8, showing an end wall of the potshell of figure 8 as well as a cooling pipe facing said end wall.

Figure 11 is a cross section analogous to figure 10, along arrow XI on figure 8, showing a side wall of the potshell of figure 8 as well as a cooling pipe facing said side wall.

Figure 12 is a perspective view, showing at greater scale detail XII of figure 9.

The following reference signs are used on the figures:

Detailed description

The present invention is directed to the arrangement of a plant, also called aluminium smelting plant or aluminium smelter, using the Hall-Heroult process. This plant comprises a plurality of electrolysis cells connected in series (potline). The Hall-Heroult process as such, the way to operate the latter, as well as the general structure of above electrolysis cells are known to a person skilled in the art and will not be described here. In the present description, the terms “upper” and “lower” refer to mechanical elements in use, with respect to a horizontal working surface. Moreover, unless otherwise specifically mentioned, “conductive” means “electrically conductive”.

As schematically shown on figure 1, the aluminium smelter of the invention comprises a plurality of electrolytic cells C1, C2, ... , Cn-1, Cn, typically arranged along two parallel lines L1 and L2, each of which comprises n/2, i.e. m cells. Two adjacent rectangular cells (C1,C2), (C2, C3), ..., (Cn-2, Cn-1), (Cn-1,Cn) of one given line define intercalary spaces H1, ..., Hn-1. These cells are electrically connected in series by means of conductors, which are not shown on figure 1. The electrolysis current therefore passes in a cascade fashion from one cell Ci to the next cell Ci+1, along arrow DC. The number of cells in a series is typically comprised between 50 and over 200, even over 400 in the most recent smelters, but this figure is not substantial for the present invention.

The cells are rectangular shaped and are arranged transversally (side by side), in reference of the line they constitute. In other words the main dimension, or length, of each cell is substantially orthogonal to the main direction of the line, i.e. the circulation direction of current. The large sides of two adjacent cells are parallel.

Figures 2 and 3 show more in detail the structure of one of the above cells, namely that C2, bearing in mind that the structure of the other cells is similar. Figure 2 also shows intercalary spaces H1 and H2, between said cell C2 and respective neighboring cells C1 and C3, the latter not being illustrated on figure 2..

Cell C2 first comprises a concrete support SL, which is known as such, and a potshell referenced as a whole by 10, which forms an essential part of the present invention. Said potshell comprises a bottom wall 11, from which peripheral walls extends upwards. Said peripheral walls comprise two parallel side walls 12 and 13, as well as two parallel end walls 14 and 15. The above walls 11 to 15 define an inner volume V10 of the potshell, which is opened upwards and ensure the reception of further elements ofthe cell, shown on figure 3. Let us note that these further elements are not represented on figure 2, for sake of clarity.

The above mentioned further elements of cell C2 are amongst other: - a cathode forming the bottom of said electrolytic cell and comprising a plurality of parallel cathode blocks, each cathode block comprising at least one metallic cathode collector bar protruding out of each of the two ends ofthe cathode block, - a lateral lining defining together with the cathode a volume containing the liquid electrolyte and the liquid metal resulting from the Hall-Heroult electrolysis process, - a plurality of anode assemblies suspended above the cathode, each anode assembly comprising at least one carbon anode and at least one metallic anode rod connected to an anode beam, a cathodic bus bar surrounding said potshell, - a plurality of connectors, each connecting one end of a cathode collector bar of a cathode block to said cathodic bus bar.

The above further elements are well known and do not form part of the invention, so that they will not be described more in detail. The present invention is specifically directed to metallic potshell, which will be described more in detail hereafter.

Figure 4 shows facing side walls 12 and 13, which belong respectively to above described cell C2 and adjacent cell C1. This figure 4 also illustrates intercalary space H1 between said cells C1 and C2, as well as some mechanical elements of the plant, which are accommodated in the intercalary space. These further mechanical elements of figure 4, which are known perse, will not be described in detail.

Two first aluminium busbars 81 and 82, or so-called cathodic busbars, extend along respective upstream C1 and downstream C2 cells. In a way known as such, each cathodic busbar is electrically linked through not shown flexible current collectors with the respective cathode collector bars of a pot. Several risers, not shown on this figure 4, connect one of these cathodic busbars to the anode beam of the adjacent downstream cell. Moreover, two further busbars 92, 94 extend between the above first busbars, parallel to the latter. Facing walls of these busbars are parallel and vertical.

In a usual manner, a plurality of reinforcement cradles, each referenced as 20, are regularly provided on side walls 12 and 13 of potshell 10. On figure 2, only some of these cradles are illustrated, at both end of each wall 12, 13 and in the middle thereof. This makes it possible to clearly show side cooling pipes 30 and 40, described more in detail hereafter. These cradles 20 extend substantially in a perpendicular direction, with respect to main plane of each wall, away from inner volume 10. As shown more in detail on figure 5, each cradle 20 is formed by a steel plate, the shape thereof is substantially rectangular. The rim of this cradle, which is opposite to side wall, is surrounded by a flange 22, which is known as such.

By way of example: - distance or spacing S20 (see figure 2) between two adjacent cradles is between 400 and 800 mm; - thickness of each cradle steel plates is between 10 and 25 mm; - height H20 (see figure 5) of each cradle is between 1000 and 2000 mm; - width W20 (see figure 5) of each cradle is between 150 and 400.

The potshell 10 according to the invention also comprises a pipe assembly, for driving a gaseous cooling medium against the outer face of side walls 12 and 13. Said gaseous cooling medium is preferably air. In the present embodiment, said pipe assembly is formed by two single pipes 30 and 40, each extending along a respective side wall 12 and 13. The structure of pipe 30 will be described hereafter, bearing in mind that the structure of other pipe 40 is similar.

As shown on figure 6, said pipe 30 comprises a main straight part 31, facing in use wall 12, as well as two end parts 32 and 33. These end parts, which are far shorter than main part 31, are bent relative to the latter, so as to extend upwards in use. According to the invention, pipe 30 is rigid, which means that it cannot undergo a substantial downwards deformation, due to its own weight. By way of example, it is made of a metallic material, such as steel.

Figure 7 is a transversal cross of pipe 30, which shows in particular the inner structure thereof. This pipe is hollow, so as to define an inner volume V30 surrounded by a wall 34. By way of example: - thickness T34 of said wall 34 is between 3 and 5 mm; - outer diameter D 30 of the whole pipe 30 is between 40 and 80 mm.

The fixation of the main part 31 of pipe on the potshell 10 is shown, in particular on figures 2 and 5. Each cradle 20, located on the side wall 12, is provided with a through hole 24. The diameter of this hole 24, noted D24 on figure 5, is slightly larger than the outer diameter D30 of pipe 30. By way of example, the difference (D24 - D30) is between 10 and 20 mm. The fixation means of the pipe 30 on each cradle is a permanent fixation means, typically welding means. On figure 5, 36 shows the weld bead between facing walls of pipe 30 and cradle 20. The above welding is carried out in a way known perse.

As shown on both figures 6 and 7, the wall 34 of pipe 30 is provided with a row of ports, noted 38. In the illustrated example, there is one single row of ports, which are located the one beside the others, at the same altitude. By way of example: - distance D38 between two adjacent ports 38 (see figure 6) is between 80 and 200 mm; - diameter d38 of each port 38 (see figure 7) is between 2 and 5 mm.

As shown on figure 3, each pipe extends horizontally, i.e. parallel to the deck plate 10’ of said potshell 10. Turning back to figure 5, let us note H38 the altitude or height of the centre of each port 38. This altitude H38 is taken into account from the upper surface of bottom wall 11 of the potshell 10. In other words, the altitude of this upper surface is 0. Let us also note D10 the depth of the potshell 10. According to an advantageous embodiment of the invention, the ratio (H38/D10) is between 0.6 and 0.8, in particular between 0.65 and 0.75. In this manner, ports 38 face the hottest zone of the potshell, that is to say the zone in vicinity of the liquid metal and liquid bath interface, above the cathode where the metal pad accumulates.

Advantageously, the distance W38 along horizontal direction between each port 38 and facing wall 12 is between 100 and 250 mm, in particular between 150 and 200 mm.

In the shown example, ports 38 are arranged along one single straight line parallel to the length of the tube. According to a first not shown variant, ports are arranged along several straight lines. These lines are typically mutually parallel, being provided the one under the other. According to a second not shown variant, ports are arranged in staggered file.

In the example shown on figure 7, ports 38 are provided in the pipe, so that they may blow a gaseous cooling medium along a substantially horizontal direction. According to a first not shown variant, ports are provided so that they may blow said medium along a same slanting direction, with respect to horizontal direction. These ports blow said medium, either upwards or downwards. According to a second not shown variant, ports are provided so that they may blow said medium along at least two different slanting directions. For example, first ports blow said medium upwards, whereas second ports blow said medium downwards. Different blowing directions of one given port 38 are shown on figure 5, by means of arrows referenced as f38.

Each end part 32, 33 is provided with a connector (not shown on the figures), of any suitable type; such connectors are known as such. In an embodiment, said connector is connected to a flexible tube, the second end of which is connected to a connector provided on the column of the potroom or on the superstructure of the pot, said connector being the end of another rigid tube connected to a compressed air supply. If compressed air is supplied on the potroom column, then the flexible tube must be several meters long. Said compressed air supply is necessary anyway for operating pneumatic crust breakers. Said flexible tube can be very short, less than two meters and preferably of the order of one meter; such short flexible tubes can be conveniently handled and stored by a single operator, and do not represent a safety risk in a potroom. As the system is intended to be used only in peculiar situations there is no need to have the compressed air permanently connected to the compressed air supply. However, this variant is within the scope of the present invention.

Figures 8 to 12 show another embodiment of the present invention. On these figures, the mechanical elements analogous to those illustrated on figures 1 to 7 will be given the same reference numerals, increased by 100.

The pipe assembly of figures 8 to 12 differs from that of figures 1 to 7, in that it comprises further pipes. Said pipe assembly of figures 8 to 12 comprises first two so called side pipes 130 and 140, analogous to that 30 and 40, each extending along a respective side wall 112 and 113. Moreover said pipe assembly also comprises two so called end pipes 150 and 160, each extending along a respective end wall 114 and 115. As shown on figure 8, each pipe extends horizontally, i.e. parallel to the deck plate 110’ of said potshell 110.

As shown in particular on figure 9, each side pipe 130 or 140 comprises a main straight part 131 or 141, facing in use walls 112 and 113. Moreover, close to both ends of said main part, each pipe 130 or 140 also comprises respective tips 132 and 133, as well as 142 and 143, which extend upwards. Each end pipe 150 or 160 comprises a main straight part 151 or 161, facing in use walls 114 and 115, as well as two end parts 152 and 153, as well as 162 and 163. These end parts, which are far shorter than main part, are bent relative to the latter, so as to extend upwards in use.

As described hereabove for end parts 31 and 32, at least some of and, preferably, all tips 132, 133, 142, 143 and end parts 152, 153, 162, 163 are provided with a respective connector, not shown on the figures, of any suitable type. This allows the feeding of compressed air into pipes 130 to 160, as in the first embodiment.

Main part 131, 141 of each pipe 130, 140 is attached on cradles 120 provided along each side wall 112, 113. Moreover, main part 151, 161 of each pipe 150, 160 is attached on further reinforcements 121 provided along each end wall 114, 115. These reinforcements 121, which are analogous to cradles 120, are known as such. The way above main parts 131 to 161 are attached on said reinforcements 121 is similar to that described hereabove for the first embodiment. Each pipe is provided with row of ports, such as those 138 of figures 10 and 11, similar to those 38 of first embodiment.

Figure 12 shows at greater scale adjacent ends, which respectively belong to side pipe 130 and to end pipe 150. Main part 131 and end part 152 both lead into a two-way valve 170, of any suitable type. This valve is controlled by a handle 172, which allows a selective gas flow between said pipes 130 and 150.

In normal operation of the plant, cooling of the peripheral walls of the potshell is not compulsory. Therefore, there is no connection between pipes and compressed air supply.

Let us suppose now that such a cooling is necessary. This may happen in particular upon starting the plant, or in case of specific events such as anode effects. In this case, pipes are connected to compressed air supply, in order to cool the facing walls of the plant. In the first embodiment, both side pipes are in general connected at the same time, so as to cool both side walls 12 and 13. In the second embodiment, operators may close or open valves 170, whether cooling of side and/or end walls is required.

Claims (15)

1. A potshell (10; 110) for an electrolytic cell (C1-Cn) suitable for the Hall-Heroult electrolysis process, said potshell being intended to receive a cathode forming the bottom of said electrolytic cell and comprising a plurality of parallel cathode blocks, each cathode block comprising at least one metallic cathode collector bar protruding out of each of the two ends of the cathode block, a lateral lining defining together with the cathode a volume containing the liquid electrolyte and the liquid metal resulting from the Hall-Heroult electrolysis process, said potshell comprising : - a bottom wall (11; 111) and peripheral walls (12-15; 112-115) extending upwards from said bottom wall, so as to define an inner reception volume (V10), said peripheral walls comprising side walls (12, 13; 112, 113) and end walls (14, 15; 114, 115), - a plurality of reinforcement members (20; 120, 121), provided side by side along at least part of said peripheral walls, said potshell being further provided with cooling means (30, 40; 130, 140, 150, 160), adapted to blow a gaseous cooling medium against the outer surface of at least part of said peripheral walls, said potshell being characterized in that: - said cooling means comprises a system of rigid pipes (30, 40; 130, 140, 150, 160) extending, for the greater part of their length, parallel to the deck plate (10’; 110’) of said potshell, - said rigid pipes being attached to at least part of said reinforcement members by a permanent fixing means (24), - said permanent fixing means comprising a hole (24) provided in each said reinforcement member, adapted to receive said rigid pipe.

2. Potshell according to claim 1, characterized in that said permanent fixing means further comprise a welding joint (36) between the facing walls of said rigid pipe (30, 40; 130, 140, 150, 160) and said hole (24) provided in each said reinforcement member (20; 120, 121).

3. Potshell according to any of claims 1 or 2, characterized in that said system of rigid pipes comprises at least one so called side straight pipe (30, 40; 130, 140), which extends over substantially all the length of a respective side wall (12, 13; 112, 113) of said potshell (10; 110).

4. Potshell according to any of above claims, characterized in that said system of rigid pipes comprises at least one so called end straight pipe (150, 160), which extends over substantially all the length of a respective end wall (114, 115) of said potshell (110).

5. Potshell according to claims 3 and 4, characterized in that it comprises valve means (170) to selectively couple adjacent ends of one side pipe (130, 140) and one end pipe (150, 160).

6. Potshell according to any of claims 1 to 5, characterized in that said cooling means are adapted to blow a gaseous cooling medium against said peripheral walls at the height of the liquid electrolyte and the liquid metal contained in said volume.

7. Potshell according to any of above claims, characterized in that said potshell is provided with positioning means, adapted to position said cooling means at a substantially constant so-called blowing height (H38), over at least part of said peripheral walls.

8. Potshell according to claim 7, characterized in that said constant blowing height (H38) is between 60 and 80% of the depth (D10) of said potshell.

9. Potshell according to any of claims 7 or 8, characterized in that said positioning means comprises said fixing means.

10. Potshell according to any of above claims 7 to 9, characterized in that said cooling means comprise a plurality of blowing ports (38; 138), provided substantially at the same altitude, said blowing height (H38) corresponding to the altitude of the centre of each port.

11. Potshell according to any of claims 7 to 9, characterized in that said cooling means comprise a plurality of blowing ports, provided at different altitudes, said blowing height corresponding to the mean altitude of the centres of said ports.

12. Potshell according to any of claims 10 or 11, characterized in that said blowing ports are adapted to blow cooling medium along different angles.

13. An electrolytic cell suitable for the Hall-Heroult electrolysis process, comprising a cathode forming the bottom of said electrolytic cell and comprising a plurality of parallel cathode blocks, each cathode block comprising at least one metallic cathode collector bar protruding out of each of the two ends of the cathode block, a lateral lining defining together with the cathode a volume containing the liquid electrolyte and the liquid metal resulting from the Hall-Heroult electrolysis process, an outer metallic potshell (10) containing said cathode and lateral lining, a plurality of anode assemblies suspended above the cathode, each anode assembly comprising at least one carbon anode and at least one metallic anode rod connected to an anode beam, a cathodic bus bar surrounding said potshell (10), a plurality of connectors, each connecting one end of a cathode collector bar of a cathode block to said cathodic bus bar, said electrolytic cell being characterized in that said outer metallic potshell is a potshell according to any of above claims.

14. An aluminium electrolysis plant comprising at least one line of electrolysis cells of substantially rectangular shape, and said plant further comprising means for electrically connecting said cells in series and for connecting the cathodic busbar of a cell to the anode beam of a downstream cell, characterized in that more than 80% of the electrolysis cells in at least one of said line, and preferably each electrolysis cell of said line, is an electrolysis cell according to any of claim 13.

15. A method for making aluminium by the Hall-Heroult electrolysis process, characterized in that said method is carried out in an aluminium electrolysis plant according to claim 14.