GB2548571A - Flexible electrical connector for electrolytic cell - Google Patents

Abstract

A flexible electrical connector 10 for connecting one end 4 of a cathode collector bar 11 to a cathodic busbar (Fig 1; 5) of an electrolytic cell, suitable for the Hall-Heroult electrolysis process, the connector 10 comprising a flexible elongated body 12 of aluminium sheets or strips, means 14, 16 for attaching a first end 12' of the body 12 to the cathode collector bar 11, and means for attaching a second end 12" of the body 12 to the busbar (Fig 1; 5). The means for attaching said second end 12" of the body 12 to the busbar (Fig 1; 5) comprises a first interface member 20 of an electrical connection member 22 made of aluminium, permanently attached to the second end 12" of the body 12, and mechanical fixation member 24 adapted to be fixed in a removable way on the cathodic busbar (Fig 1; 5). There is also a cathode block comprising a cathode collector bar 11 provided with the flexible connector 10 and an electrolytic cell comprising a plurality of cathode blocks (Fig 1; 2) with cathode collector bars 11 and the connectors 10 to allow removable attachment of the connector 10 to the busbar 11. Further aspects relate to an aluminium electrolysis plant containing at least one line of the electrolysis cells, a method for making aluminium carried out in the aluminium electrolysis plant and a method for relining an electrolytic cell containing the flexible electrical connectors 10.

Description

Flexible electrical connector for electrolytic cell Technical field of the invention

The invention relates to the field of large-scale electrolytic cells, such as electrolytic cells used for the Hall-Heroult process for making aluminium, and in particular to the electrical connection between the cathode blocks and the cathodic busbar. More precisely, it relates to flexible electrical connectors used to connect the cathode collector bar to the cathodic busbar.

Prior art

The Hall-Heroult process is the only continuous industrial process for producing metallic aluminium from aluminium oxide. Aluminium oxide (Al203) 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 pot shell), a lining (comprising refractory bricks protecting said steel shell 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. Anodes and cathodes are connected to external aluminium busbars. An electrical current is passed through the cell (typically at a voltage between 3.7 V and 5 V) which electrochemically reduces the aluminium oxide, split in 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 dioxyde. 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 ton 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 meters and a width usually comprised between 3 and 5 meters. The cells (also called “pots”) are always operated in series of several tens (up to more than four hundred) 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.

In use, the current is fed into the anodic busbar, flows from the anode beam to the anode rod and to the anode in contact with the liquid electrolyte, crosses the liquid metal pad and eventually is collected at the cathode block. The cathode block is provided with a collector bar, usually made of steel, which is connected to the cathodic busbar through flexible conducting members. Each such conductive member extends between the cathode collector bar and the cathodic busbar; it is connected to the cathodic busbar and to the cathode collector bar using connecting members. Current collected by the cathodic busbar is then fed into the anodic busbar of the adjacent downstream cell through so-called anodic risers which “rise” the current from the cathodic busbar (usually located beneath the floor level of the main working area of the pot-room building) up to the anode beam (located above the pot, as a part of the superstructure).

The present invention is more particularly related to these flexible connecting members connecting the cathode collector bars to the cathodic busbar, to their structure and to the way they may be mounted on the cathode collector bar and on the cathodic busbar. Mechanical flexibility is required for these connection members for practical reasons: it facilitates their handling and mounting, and avoids mechanical tension due to thermal expansion. Furthermore, the connection of the cathodic collector bar to the cathodic busbar should be of excellent electrical quality and reliability. In particular, it should not lead to a significant voltage drop, it should not be subject to corrosion, and more generally it should not degrade over time.

Flexible conducting members connecting to busbars are known as such in the field of electrolytic cells used for the Hall-Heroult process. GB 578,026, filed in 1944, discloses a bolted connection of the flexible conductor to the anodic busbar.

According to an embodiment that is widely used in electrolytic cells for the Hall-Heroult process, the flexible packs of aluminium sheets are welded to the aluminium side (so-called aluminium stub) of an aluminium-steel transition joint (so-called clad); this operation can be carried out in a workshop rather than on-site. The steel side (steel stub) of the aluminium-steel transition joint is then welded to the end of the steel cathode collector bar. The disadvantage of this solution is that in case of cathode replacement the welding between the steel cathode collector bar and the transition joint has to be cut onsite to remove the old pot and has to be welded on-site on the new pot. This is because on the opposite end of the flexible the aluminium weld between the cathodic busbar and the flexible cannot be replaced on the energized busbar: on-site welding close to a potshell is both uncomfortable and unreliable, and it is therefore desirable to minimize routine welding operations in such an environment, for several reasons. First of all, space between two neighboring pots is limited, forcing the welder to work in a rather constrained environment. Furthermore, magnetic fields will interfere with the welding equipment and welding arc, thus making a reproducible welding quality difficult to achieve. Indeed, it is observed that welding seams produced under these conditions are usually not very good, especially those for the two or three first and last cathode collector bars in a pot (counted from the ends of the pot), where the magnetic field is maximum. Welding seams of poor quality may lead to a voltage drop which decreases the energy efficiency of the electrolytic cell. Safety is an issue, too, because neighboring pots and their busbars are energized, which leads to electrical shock hazard. And eventually, any kind of prolonged precision work at a distance of less than one meter from an electrolysis pot operating close to 1 000 °C is uncomfortable as such. As a consequence, the efficiency of the welder and the quality of the weld may be decreased when the welding operations are carried out on-site.

Another disadvantage of this solution is the time necessary for cutting the welding seam of the old pot and welding the flexible on the new pot. This reduces the operating rate of the pot, knowing that each day of pot downtime results in a loss of aluminium production of the order of two or three tons.

As an alternative, bolted connectors have been proposed. Mechanical and electrical contacts between two aluminium conductors may deteriorate with time, especially under adverse external conditions (humidity, heat). Bimetallic transition joints can be used; they are known as such and can be manufactured by co-rolling of an Al plate with a Cu plate, or by explosion welding. As a rule, transition joints manufactured by explosion welding show a better long-term reliability in connectors used in electrolytic cells than transition joints manufacturing by co-rolling; this is particularly true in the presence of mechanical stress and high temperature excursions. As an alternative, WO 2012/161594 discloses a flexible bimetallic Al-Cu connector for connection to the cathode collector bar, said bimetallic connector being manufactured by friction stir welding.

The present invention aims at simplifying the design and use of these connectors, and to render them more reliable. Another goal of the invention is to simplify the mounting and dismounting of these connectors and decrease the downtime of pots. Yet another goal is to save time during pot maintenance and exchange.

Objects of the invention

According to the invention the problem has been solved by a flexible electrical connector between the cathode collector bar and the cathodic busbar that has two ends, one preferably made from steel of which is welded onto the cathode collector bar which is usually made from steel, the other being a first interface that is mechanically fixed to the cathodic busbar, or to a second interface welded to the cathodic busbar. A first object of the invention is a flexible electrical connector for connecting one end of a cathode collector bar to a cathodic busbar of an electrolytic cell, suitable for the Hall-Heroult electrolysis process, said connector comprising a flexible elongated body comprising aluminium sheets or strips, means for attaching a first end of said body to a respective end of said cathode collector bar, means for attaching a second end of said body to said bus bar, said connector being characterized in that said means for attaching said second end of this body to said cathodic busbar comprise a first interface member which comprises: - an electrical connection member made of aluminium, permanently attached to said second end of the body, - a mechanical fixation member adapted to be fixed in a removable way on said cathodic busbar.

Advantageously said electrical connection member is permanently attached to said second end of the body by welding. It is also provided with further first through bore for insertion of bolting means.

In an embodiment, said mechanical fixation member defines a first free face adapted to contact a second free face defined by said bus bar or by a complementary mechanical fixation member attached to said cathodic bus bar.

Advantageously said mechanical fixation member is adapted to be fixed on said cathodic busbar by bolting. It can be provided with first through bore for insertion of bolting means.

Said electrical connection member and mechanical fixation member can be one single member or two different members.

Said mechanical fixation member can be a first contact tab that is permanently attached, in particular by welding, to electrical connection member.

Said first contact tab can be made of a second electrically conductive material, in particular copper.

The lower face of said electrical connection member can be attached both to said body and said first contact tab.

In an embodiment said means for attaching said first end of said body to a respective end of said cathode block comprises an aluminium pad welded to said body, and a steel pad welded to said aluminium pad.

Another objet of the present invention is a cathode block intended to be used in an electrolytic cell suitable for the Hall-Heroult process, said cathode block comprising an elongated body and at least one metallic cathode collector bar protruding out of each of the two ends of said elongated body, characterized in that at least one end and, preferably, both ends of this elongated body being provided with a connector according to the invention.

Advantageously said end of said cathode collector bar is connected to said connector by a steel-to-steel welding.

Another objet 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, - 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 anode and at least one metallic anode rod connected to an anode beam, - a cathode bus bar comprising 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 - at least one of said connectors, and preferably more than 60% of said connectors and, more preferably, each of said connectors, is a connector according to the invention, said cell further comprises means for removable fixation of mechanical fixation member on said cathodic bus bar or on a complementary mechanical fixation member attached to said cathodic busbar.

In an embodiment, said mechanical fixation member of said connector directly contacts a face of said cathodic bus bar, in particular a top face thereof, or it contacts a complementary mechanical fixation member, which is attached, in particular in a permanent way, to said cathodic busbar. Said complementary mechanical fixation member can also be attached to an intermediate member, which is attached to said cathodic busbar.

Said complementary mechanical fixation member can be a second contact tab which contacts said first contact tab of said connector, said second contact tab being made of a second electrically conductive material, in particular copper.

Said means for removable fixation can comprise a bolt inserted into said first through bore provided into said first mechanical fixation member as well as into a second bore provided into on said bus bar or into said complementary mechanical fixation.

Another objet 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 according to the invention.

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 according to the invention.

Still another object of the invention is a process for relining an electrolytic cell according to the invention comprising flexible electrical connectors according to the invention, said process comprising the following steps: - providing a sufficient number of new cathode blocks according to the invention, - removing said means for attaching said second end of said body from said cathode busbar by removing said mechanical fixation members from said cathode busbar; - optionally, removing said electrolytic cell from the potline; - relining said electrolytical cell, including the steps of dismounting the spent cathode blocks, and mounting said new cathode blocks into said electrolytical cell, thereby obtaining a relined electrolytic cell; - welding the steel junction pad to the end of the cathode collector bar by a steel/steel welding seam; inserting said relined cell into the potline; mechanically connecting said mechanical fixation members on said cathodic busbar.

Said flexible electrical connectors according to the invention can be recovered from said spent cathode blocks by cutting through said means for attaching a first end of said body of said connector to said end of said cathode collector bar. Said means for attaching is preferably a steel-steel welding joint. Cutting is preferably done before relining the pot with new cathode blocks.

Figures

Figures 1 to 7 represent embodiments of the present invention.

Figure 1 is a schematic view, showing from above the global arrangement of an electrolytic cell according to the invention.

Figure 2 is a side view, showing at a greater scale a connector which extends between a cathode block and a cathodic busbar of the cell of figure 1, at an intermediate stage of the mounting of this connector.

Figure 3 is a side view, analogous to figure 2, showing the connector of figure 2 in the mounted stage.

Figures 4 to 7 are side views, analogous to figure 3, showing four other embodiments of the connector according to the invention, in the mounted stage.

The following reference numbers and letters are used on the figures:

Detailed description

An aluminium smelter comprises a plurality of electrolytic cells arranged the one next to the other (generally side by side), typically along two parallel lines. These cells are electrically connected in series by means of conductors, so that electrolysis current passes from one cell to the next. The number of cells in a series is typically comprised between 50 and over 400, but this figure is not substantial for the present invention. The cells are arranged transversally in reference of main direction of the line they constitute. In other words the main dimension, or length, of each cell is substantially orthogonal to the main direction of a respective line, i.e. the circulation direction of current.

In the present description, the terms “upper” and “lower” refer to mechanical elements in use, with respect to a horizontal ground surface. Moreover, unless otherwise specifically mentioned, “conductive” means “electrically conductive”. A Hall-Heroult electrolytic cell, the general structure of which is known per se, first comprises a cathode forming the bottom of said electrolytic cell and comprising a plurality of parallel cathode blocks, each cathode block being provided with at least one current collector bar and two electrical connection ends. A lateral lining defines together with the cathode a volume containing the liquid electrolyte and the liquid metal resulting from the Hall-Heroult electrolysis process, said cathode and lateral lining being contained in an outer metallic potshell. Said electrolytic cell further comprises a plurality of anode assemblies suspended above the cathode, each anode assembly comprising at least one anode and one metallic anode rod connected to an anodic busbar called anode beam.

On Figure 1. the outer periphery of potshell 1, some cathode blocks 11 (which form the bottom of the pot), each of which has one or more cathode collector bars protruding from each of its ends) are schematically shown, the other above mentioned components of the cell being not illustrated. Reference numbers 3, 4 refer to the protruding ends of a cathode collector bar 2 on two opposite ends of the cathode block 2 ; they protrude out the cathode block 11 and cross the potshell 1. Figure 1 also shows a cathodic bus bar 5, forming part of a whole cathode bus bar. This cathodic bus bar 5 surrounds said potshell 1, is rectangular in shape and has two opposite longitudinal parts 6, 7, as well as two opposite transversal parts 8, 9.

The general operation of a Hall-Heroult electrolysis pot is known per se and will not be explained here. It is sufficient to explain that the current is fed into the anode bus bar, flows from the anode beam to the anode rod and to the anode in contact with the liquid electrolyte, crosses the liquid metal pad and eventually will be collected at each cathode block 11. Half of the current collected by the collector bars of the cathode blocks will flow directly to opposite longitudinal parts 6, 7 of cathodic busbar 5. Cathodic bus bar 5 is substantially rectangular in cross section (as can be seen from figure 2), with vertical large sides. Two rows of connectors 10 are provided, each connector extending between the protruding end 3 or 4 of the cathode collector bar of a respective cathode block 11 and either longitudinal part 6 or 7 of cathodic busbar 5. These connectors 10 are part of the subject matter of the invention and will be described in more detail.

Figures 2 and 3 illustrate a first embodiment of the invention. The connector 10 illustrated on Figure 2 and Figure 3 extends between the end 4 of steel cathode collector bar 11 and the longitudinal part 7 of cathodic bus bar 5. This connector 10 comprises a body 12 made of a pack of flexible aluminium sheets or strips, which is linked to the end 4 via two intermediate junction pads 14 and 16, respectively made of steel and aluminium. Steel junction pad 14 is attached to end 4 by a steel/steel welding seam 15. Aluminium junction pad 16 is attached to first end 12’ of body 12 by a respective aluminium/aluminium welding seam 17.

The steel pad 14 and the aluminium pad 16 form a so-called transition joint, known as such, in which facing parts of the pads 14 and 16 are attached by a steel/aluminium welding seam 18 ; such transition joints (or clads) are commercially available and can be manufactured for example by co-rolling or explosion welding. The above mentioned constitutive elements 12 to 18 of connector 10, as well as the way of attaching one to the other, are known as such. Therefore, they will not be described more in detail in the present description. It should be noted that on Figures 2, 3, 4 and 5, transition joints are indicated by a wavy line (for example reference number 18 on Figures 2 and 3 and reference numbers 25 and 35 on Figure 2), but these embodiments do not limit the scope of the present invention.

In a variant of the connector 10 according to this invention (not shown on the Figures), a titanium layer is provided between steel pad 14 and the aluminium pad 16, forming a steel-titanium-aluminium tri-metallic transition joint. Said titanium layer may have a thickness between 0.2 mm and 2 mm, for example, preferably between 0.5 mm and 1.0 mm. The presence of the titanium layer between the steel pad and the aluminium pad stabilizes the interface, knowing that the operating temperature of this transition joint is typically of the order of 400°C to 500°C, for a typical duration of 5 to 7 years. Such tri-metallic transition joints are generally manufactured by explosion welding. Tri-metallic transition joints manufactured by co-rolling are generally using chromium instead of titanium.

For cathode replacement, the connector 10 can be separated from the steel cathode collector bar by cutting through the steel-steel welding seam 18.

This connector 10 further comprises a first mechanical and electrical interface 20, which comprises a base plate 22 made of aluminium, as well a tab 24 made of a different material, such as copper. Base plate 22 forms an electrical connection member, whereas tab 24 forms a mechanical connection member. In the embodiment of figures 1 to 3, electrical connection member and mechanical connection member are formed by two different members.

The lower face of base plate 22 is attached to opposite end 12” of body 12 in a permanent way, preferably by a respective aluminium/aluminium welding seam 23, of known type. Moreover, adjacent faces of base plate and tab are attached by a copper/aluminium welding seam 25, of the clad or explosion type.

This first mechanical and electrical interface 20 interacts with a second mechanical and electrical interface 30, which comprises a base plate 32 made of aluminium, as well a tab 34 made of a different electrically conductive material, such as copper. In particular, the materials of both tabs 24 and 34 are identical. The edge of base plate 32 is attached to longitudinal part 7 of cathodic bus bar 5 by a respective aluminium/aluminium welding seam 33, of known type. Moreover, adjacent faces of base plate 32 and tab 34 are attached by a copper/aluminium welding seam 35, which can be manufactured, as mentioned above, by explosion welding or co-rolling.

In a specific embodiment, said first interface 20 and/or said second interface 30 is formed by an aluminium - titanium - copper trimetallic clad, which is generally manufactured by explosion welding; the titanium thickness can be between 0.2 and 2 mm, preferably between 0.3 to 1.5 mm. This further improves the lifetime of the clad. Trimetallic clads obtained by co-rolling are generally using chromium instead of titanium.

Furthermore, both base plates 22, 32 and both tabs 24, 34 are provided with through bores 22’, 32’, 24’ and 34’, adapted to receive removable fixation means, such as a bolt 40. Therefore, once the two mechanical interfaces 20, 30 are placed one above the other, with their bores in mutual communication, bolt 40 can be inserted through said bores in view of mechanical fixation and electrical connection of both interfaces.

Adjacent free faces 24”, 34” of tabs 24, 34 define a contact zone 50. Due to the nature of the material of these tabs, a satisfactory mechanical contact is obtained between both interfaces 20 and 30. Moreover, providing a copper-copper contact induces specific advantages. In particular, this contact has high durability and reliability.

As a variant (not shown on the figures), an electrically conductive material other than copper (for example aluminium) may be used for at least one tab 24, 34. It is preferred if both tabs are made of the same material.

Figures 4 to 7, which are similar to Figure 3, illustrate further variants of the invention. On these figures, the mechanical and electrical members of connector and bus bar, which are similar to those of Figure 3, are given the same reference numbers plus respectively 100, 200, 300 and 400.

On Figure 4, the second electrical and mechanical interface does not comprise a tab, such as that represented by reference number 34 on figure 3. Therefore, the mechanical contact occurs directly between tab 124 and base plate 132. Tab 124 may be made of copper. In this case the welding joint between base plate 122 and tab 124 advantageously is a transition joint, of the type as explained above. As an alternative (less preferred), tab 124 may be made of aluminium. As a consequence, in this last case the mechanical contact at the cathodic busbar 107 side is aluminium to aluminium.

On Figure 5. the first electrical and mechanical interface does not comprise a tab, such as that represented by reference number 24 on Figure 3. Therefore, the mechanical contact occurs directly between tab 234 and base plate 222. In the embodiment of figure 5, electrical connection member and mechanical connection member of the connector 210 are formed by one single member 222. Tab 234 may be made of copper (and in this case the welding joint between base plate 232 and tab 224 advantageously is a transition joint, of the type as explained above). As an alternative, tab 234 may be made of aluminium. As a consequence, in this latest case, the mechanical contact at the cathode bus bar 207 side is aluminium to aluminium.

On Figure 6. both electrical and mechanical interfaces do not comprise a tab, such as those represented by reference numbers 24 and 34 on Figure 3. In the embodiment of figure 6, electrical connection member and mechanical connection member of the connector 310 are formed by one single member 322. The mechanical contact occurs directly between base plates 322 and 334. As a consequence, the mechanical contact at the cathodic bus bar 307 side is aluminium to aluminium.

On Figure 7. base plate 422 of the connector 410 is mechanically fixed (preferably bolted) directly on the top face 407’ of the cathodic bus bar 407. In the embodiment of figure 7, electrical connection member and mechanical connection member of the connector 410 are formed by one single member 422. The removable fixation of member 422 on bus bar 407 is ensured by a bolt which penetrates into a blind hole 407” provided into top face 407’ of bus bar. The embodiment of this Figure 7 induces specific advantages. It saves manufacturing cost and is very simple to carry out. It brings the bolted connection to a location where temperature is lower, making it more reliable. However, it requires in general a longer aluminium body 12 and therefore leads to a higher voltage drop. The latter disadvantage could be overcome in part by attaching member 422 to the bottom face of cathodic busbar 407, in particular by bolting: while this would allow to use a shorter aluminium body 12, the bolting or welding operating would be more complex because the bottom of the cathodic busbar is less accessible than the top face 407’.

In the framework of the invention, all mechanical and electrical contact surfaces should have appropriate flatness and surface roughness.

The electrolytic cell according to the invention may be mounted according to the following steps.

The flexible electrical connector 10 according to the invention can be supplied as a single piece comprising the body 12, the first interface 20, and the junction pads 14, 16 comprising the aluminium - steel transition joint 18.

Mounting of the flexible electrical connector 10 on the cathode block 11 is carried out in a first location, typically in the cathode relining workshop. The cathode block 11 is first mounted into a potshell 1 to form part of the cathode of a pot. The flexible electrical connector 10 is then mounted. This operation comprises attaching the steel junction pad 14 to the protruding end 4 of the cathode collector bar 2; this is done by welding under good working environment in a dedicated workshop, which ensures that the best possible electrical connection is achieved. Then the fully mounted pot, including the flexible electrical connectors 10 according to the invention, is supplied to the pot-room and installed at its dedicated location. Contrary to certain prior art embodiments, no time is lost with welding operations in the potroom to link the flexible to the cathode collector bar.

In the potroom, second interface member 30 is attached to the longitudinal part 7 of cathodic busbar 5 in particular by welding. This is done on-site, during the construction of the potline before energizing the cathodic busbar, in the absence of magnetic fields. Due to the bolted connection, according to the invention, any replacement or disconnection of the flexible electrical connector 10 from the cathodic bus bar 5 does not require welding.

When the pot has been positioned in the potline, first 20 and second 30 interface members 2 are attached together, in particular using bolt 40. This operation is carried onsite in the potroom.

The invention has several advantages over prior art.

First, since the interface member is permanently attached to the aluminium flexible strips or sheets of the connector 10, the latter may be handled with a high reliability, from its manufacturing site to the potshell. Moreover, since the connector may be removably fixed to the cathodic busbar, it allows very simple mounting and dismounting process.

One of the advantages of the connector and method according to the present invention is that the bolted connection is at the cathodic busbar where the temperature is moderate rather than at the end of the cathode collector bar, where the temperature is much higher. This improves the electrical and mechanical stability of the bolted contact.

Another advantage is that the bolting takes place at a location, i.e. at the cathodic busbar, where access is easier and less dangerous, that is to say at a distance from the hot pot shell, and in a position where bolting can take place from above.

Another advantage is that when relining the pot, the welding of the flexible electrical connector onto the cathode collector bar can be carried out off-site, that is to say in the dedicated relining workshop, rather than in the potroom. Welding operations in relation with the flexible electrical connector that need to be carried out in the potroom involve only aluminium-to-aluminium welds which are carried out during the construction of the potline before energization of the busbars when no magnetic fields are present that would make welding very difficult or impossible; such aluminium-aluminium welds are conventional welds that can be done using conventional gas metal arc welding (with consumable electrode) or gas tungsten-arc welding (with non-consumable tungsten electrode) techniques under inert gas shielding. Furthermore, there is no welding between dissimilar metals to be carried out in the electrolysis plant, as aluminium-copper transition joints and aluminium - steel transition joints (as well as tri-metallic steel - titanium - aluminium transition joints and tri-metallic copper - titanium - aluminium transition joints) are commercially available products.

Besides the fact that welding in the relining workshop leads to better welds than on-site welding, the mounting and demounting process according to the invention allows saving time: it allows to reduce the pot downtime by one full day compared to prior art solution involving steel-to-steel welding in the potroom. This is to say that the pot can be started one day earlier, during which it will produce about three tons of aluminium (at 400 kA). The process according to the invention also avoids unpleasant working conditions for welders.

Yet another advantage is that the electrical connection established by the flexible electrical connector and method according to the invention is very reliable. More precisely, because the connection to the protruding end of the cathode collector bar is done by steel to steel welding in the dedicated relining workshop, the resulting weld gives good and reliable results. The connection to the cathode bus bar is done by a mechanical means such as bolting. Bolted connections can be inspected, tested and retightened if necessary, even when the pot is operation. A criterion for reliability is the voltage drop due to the connection element, i.e. the voltage drop between the cathode collector bar and the cathodic busbar. According to the invention, this voltage drop is initially very low, in particular less than 10 mV, compared to at least about 15 mV for prior art solutions, and does not increase significantly over time. The inventors believe that this increased reliability is due to two factors: firstly, the electrical quality of bolted copper - copper contacts is not perturbed by oxide layers growing over time (knowing that according to the invention the bolted joint is used at the busbar, where the temperature is lower than at the collector bar), and secondly, the electrical quality of explosion welded transition joints is better than that of transition joints manufactured by co-rolling of two plates (so-called clad plate) or that of bolted joints. This is particularly true in the presence of mechanical stress and high temperature excursions. Furthermore, explosion welded joints have a better resistance to galvanic corrosion which may occur between dissimilar metals.

Claims (25)

1. A flexible electrical connector (10) for connecting one end (3,4) of a cathode collector bar (11) to a cathodic busbar (5) of an electrolytic cell, suitable for the Hall-Heroult electrolysis process, said connector comprising a flexible elongated body (12) comprising aluminium sheets or strips, means (14, 16) for attaching a first end (12’) of said body to a respective end (4) of said cathode collector bar (11), means for attaching a second end (12”) of said body to said bus bar (5), said connector being characterized in that said means for attaching said second end of this body to said cathodic busbar comprise a first interface member (20) which comprises: - an electrical connection member (22; 122; 222; 322; 422) made of aluminium, permanently attached to said second end (12”) of the body (12), - a mechanical fixation member (24; 124; 222; 322; 422) adapted to be fixed in a removable way on said cathodic busbar.

2. A connector according to claim 1, characterized in that said electrical connection member (22) is permanently attached to said second end (12”) of the body (12) by welding.

3 A connector according to claim 1 or 2, characterized in that said mechanical fixation member (24) defines a first free face (24”) adapted to contact a second free face defined by said cathodic busbar or by a complementary mechanical fixation member attached to said cathodic bus bar.

4. A connector according to claim 3, characterized in that said mechanical fixation member (22; 24) is adapted to be fixed on said cathodic busbar by bolting.

5. A connector according to claim 4, characterized in that said mechanical fixation member is provided with first through bore (24’) for insertion of bolting means.

6. A connector according to any of claims 1 to 5, characterized in that said electrical connection member and said mechanical fixation member are one single member (222, 322, 422).

7. A connector according to any of claims 1 to 5, characterized in that said electrical connection member and said mechanical fixation member are two different members (22, 24; 122, 124).

8. A connector according claim 7, characterized in that said mechanical fixation member is a first contact tab (24) permanently attached, in particular welded, to electrical connection member.

9. A connector according to claim 8, characterized in that said first contact tab (24) is made of a second electrically conductive material, in particular copper.

10. A connector according to claim 5, characterized in that said electrical connection member is also provided with further first through bore (22’) for insertion of said bolting means.

11. A connector (10) according to any of claims 1 to 10, characterized in that the lower face of said electrical connection member (22) is attached both to said body (12) and to said first contact tab (24).

12. A connector (10) according to any of claims 1 to 11, characterized in that said means (14,16) for attaching said first end (12’) of said body to a respective end (4) of said cathode block comprises an aluminium pad (16) welded to said body (12), and a steel pad (14) welded to said aluminium pad (16).

13. A cathode block (2) intended to be used in an electrolytic cell suitable for the Hall-Heroult process, said cathode block (2) comprising an elongated body and at least one metallic cathode collector bar (11) protruding out of each of the two ends of said elongated body, characterized in that at least one end and, preferably, both ends of this elongated body being provided with a connector (10) according to any of claims 1 to 12.

14. A cathode block (2) according to claim 10, characterized in that said end of said cathode collector bar (11) is connected to said connector (10) by a steel-to-steel welding.

15. 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 (2), each cathode block (2) comprising at least one metallic cathode collector bar (11) protruding out of each of the two ends of the cathode, 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 (1) containing said cathode (2) and lateral lining, - a plurality of anode assemblies suspended above the cathode, each anode assembly comprising at least one anode and at least one metallic anode rod connected to an anode beam, - a cathodic bus bar (5) surrounding said potshell (1), - a plurality of connectors, each connecting one end (3, 4) of a cathode collector bar (11) of a cathode block (2) to said cathodic bus bar (5), said electrolytic cell being characterized in that - at least one of said connectors, and preferably more than 60% of said connectors and, more preferably, each of said connectors, is a connector (10) according to any of claims 1 to 9, - said cell further comprises means (40) for removable fixation of mechanical fixation member (24) on said cathodic bus bar or on a complementary mechanical fixation member attached to said cathodic busbar.

16. An electrolytic cell according to claim 15, wherein said mechanical fixation member (422) of said connector directly contacts a face of said cathodic bus bar (407), in particular a top face (407) thereof.

17. An electrolytic cell according to claim 15, wherein said mechanical fixation member of said connector contacts a complementary mechanical fixation member (34; 132; 234; 332) , which is attached, in particular in a permanent way, to said cathodic busbar.

18. An electrolytic cell according to claim 17, wherein said complementary mechanical fixation member (132; 332) is directly attached to said cathodic busbar (107; 307).

19. An electrolytic cell according to claim 17, wherein said complementary mechanical fixation member (34; 234) is attached to an intermediate member (32; 232), which is attached to said cathodic busbar (7; 207).

20. An electrolytic cell according to claim 19, wherein said complementary mechanical fixation member is a second contact tab which contacts said first contact tab (24) of said connector, said second contact tab being made of a second electrically conductive material, in particular copper.

21. An electrolytic cell according to claim 15, characterized in that means for removable fixation comprise a bolt (40) inserted into said first through bore (24’) provided into said first mechanical fixation member as well as into a second bore (34’) provided into on said cathodic busbar or into said complementary mechanical fixation.

22. 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 15 to 21.

23. 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 22.

24. A process for relining an electrolytic cell according to any of claims 15 to 21 comprising flexible electrical connectors (10) according to any of claims 1 to 12, said process comprising the following steps: providing a sufficient number of new cathode blocks (2) according to any of claims 13 or 14, removing said means for attaching said second end (12”) of said body from said cathode busbar (5) by removing said mechanical fixation members (24; 124; 222; 322; 422) from said cathode busbar (5); preferably, removing said electrolytic cell from the potline; - relining said electrolytical cell, including the steps of dismounting the spent cathode blocks (2), and mounting said new cathode blocks (2) into said electrolytic cell, thereby obtaining a relined electrolytic cell; - welding the steel junction pad (14) to the end (4) of the cathode collector bars by a steel/steel welding seam (15), inserting said relined cell into the potline; - mechanically connecting said mechanical fixation members (24; 124; 222; 322; 422) on said cathodic busbar.

25. Process according to claim 24, further comprising: recovering said flexible electrical connectors (10) according to any of claims 1 to 12 from said spent cathode blocks (2) by cutting through said means (14,16) for attaching a first end (12’) of said body (12) of said connector (10) to said end (4) of said cathode collector bar (11).