GB2548565A - Busbar system for compensating the magnetic field in adjacent rows of transversely arranged electrolytic cells - Google Patents

Abstract

A cathode busbar system 1 for an electrolytic cell of substantially rectangular shape, suitable for the Hall-Héroult electrolysis process of producing aluminium, the cathode busbar system 1 comprising a ring busbar that surrounds an outer metallic shell of the electrolytic cell when viewed from above, the ring busbar being substantially rectangular and defining a main plane PR, a median longitudinal plane PX and a median transversal plane PY, both of which are orthogonal to the main plane PR. The ring busbar has two opposite and parallel longitudinal parts 2,3 each extending along the long sides of the cell, and two opposite and parallel transversal parts 4,5 extending along the ends of the cell, connection means 21,31 for connection with both electrical connection points of each cathode block (Fig 8; 805) of the cell The ring bus bar is asymmetric with respect both to the median longitudinal plane PX and to the median transversal plane PY. The parallel transversal parts 4,5 may also be asymmetric with respect to both to the median longitudinal plane PX and to the median transversal plane PY. The system may comprise a downstream electrical balancing circuit 100 of conductors 100a-f arranged parallel to the downstream longitudal part of the ring busbar. The transversal parts 4,5 may contain a derivation sector that could be U-shaped, projecting towards the bottom and being offset with respect to the median longitudal plane PX. The transversal parts 4,5 may also consist of two parallel busbars separated by an intercalary space of a least 10 mm. Further aspects of the invention relate to an electrolytic cell comprising the cathode busbar system, a potline of a plurality of electrolytic cells and an electrolysis plant comprising at least one potline.

Description

Busbar system for compensating the magnetic field in adjacent rows of transversely arranged electrolytic cells

Technical field of the invention

The invention relates to the field of fused salt electrolysis, and more precisely to an electrolytic cell suitable for the Hall-Heroult process for making aluminium by fused salt electrolysis. In particular, the invention relates to a particular arrangement of the cathode busbar system in an electrolysis plant in which electrolytic cells are arranged side by side, capable of counterbalancing the negative effect of high vertical magnetic field in the upstream corners of the cell as well as the vertical magnetic fields generated by adjacent rows of cells.

Prior art

The Hall-Heroult process is the only continuous industrial process for producing metallic aluminium form 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), and a plurality of anodes (usually made from carbon) that plunge into the liquid electrolyte. 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 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 kWh 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 a 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 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.

These enormous electrical DC currents flow through various conductors, such as electrolyte, liquid metal, anodes, cathode, connecting conductors, where they generate heat with ohmic voltage drops and where they generate significant magnetic fields. As mentioned above, electrolysis according to the Hall-Heroult process is a continuous process driven by the flow of electric current across the electrolyte, whereby said electric current reduces the aluminium atoms that are bounded in the alumina present in the molten electrolyte. Four equilibria define the optimum cell operation window, leading to the high current efficiency: electrical equilibrium, magnetic equilibrium, thermal equilibrium and chemical equilibrium. Two of these equilibria are determined by the cell design; two others can be acted upon in the process of cell operation.

Conditions of electrical equilibrium of the cell are attained when the distribution of the current is as uniform as possible throughout the electrolyte; the thickness of electrolyte between the anode and the cathode (inter-electrode spacing) in a typical Hall-Heroult cell is of the order of about two to five centimeters. The main operating parameter by which the operator can act on this equilibrium is the inter-electrode spacing; which determines the cell voltage to a large extent. The main permanent perturbation factor of the electrical equilibrium is the current path in the liquid metal, as will be explained below; this factor is determined by the cell design and cell operation. Discontinuous events such as anode change and so-called anode effects also perturb the electrical equilibrium.

The magnetic equilibrium and the thermal equilibrium of the cell are both determined to a large extent by the cell design. The magnetic equilibrium is determined to a large extent by the busbar structure. Perturbation factors are mainly related to electrical currents arising from conductors outside of the cell. The thermal equilibrium is determined by the choice and thickness of materials and components, and by the lining; perturbation factors are mainly related to specific discontinuous operations (anode change, metal tapping, adding of electrolyte) or to so-called anode effects (this term and the phenomenon that it designates are known to a person skilled in the art and need not to be explained here).

The chemical equilibrium is determined by the chemical composition of the electrolytic bath; alumina addition is the principal operational parameter.

The present invention is related to the magnetic equilibrium of an electrolytic Hall-Heroult cell. Such cells are of rectangular shape, and as such they are symmetric by construction. Asymmetry arises from asymmetry of the electric current flow in the cell. Electrical current enters the cell through anodes which cover a large part of the surface of the cell, crosses the electrolyte and the liquid metal pad, and is collected by the cathode which forms the whole surface of the cell bottom. The cathode is made from a carbonaceous material and contains steel collector bars which enable an electrical contact to be established with the cathode busbar. However, the electrical conductivity of both the cathode and the steel cathode bar is much lower than that of the liquid metal pad. As a consequence the current lines in the liquid metal are not vertical but have horizontal components, interacting with the vertical magnetic field and leading to magnetohydrodynamic (MHD) perturbations. Furthermore, at the downstream side of the cell, the cathode busbar is linked to a small number of connecting elements called anode risers, through which the current is fed into the anode beam of the downstream cell. Further perturbation of the magnetic field in the cell is due to boundary effects, the cell not having infinite dimensions.

As a consequence, the magnetic field in the cell locally has a spatial distribution which, combined with electrical currents in the cell, creates Laplace forces; these induce movement of liquid conductors (electrolyte and metal) and deform the metal-bath interface hydrostatically. Laplace forces may also induce metal-bath interface oscillations. The resulting unevenness of the metal surface leads to a local variation in anode-to-metal pad distance across the length of the pot, which is represented by small fluctuations of the overall cell voltage signal; this may even lead to a short-circuit between the anode and the cathode. These oscillations of the metal-bath interface are called magnetohydrodynamic (MHD) instabilities which are detrimental to the performance of the process; they require the distance between anode and cathode to be increased and this counter measure increases the electrical resistance of the cell, leading to ohmic losses and eventually to an increase in energy consumption. The MHD instabilities are specifically the result of the vertical magnetic field component which tends to increase with the size of the pots and with the cell current.

Certain perturbative events (adding alumina, anode change, metal tapping, anode effects) may increase these instabilities and metal and bath velocities. The perturbative effects of an event will be higher if the vertical component of the magnetic field, in particular in the upstream corners of the cell, is high.

It is therefore desirable, in order to reduce these magnetohydrodynamic instabilities, to decrease as far as possible the vertical component of the magnetic field (designated as Bz, z being the coordinate running upwards from the bottom to the top of the cell) in the liquid metal; a root-mean square average value of about one millitesla is a usual maximum target. Moreover, the horizontal components Bx and By (x being the longitudinal axis of the cell) should be anti-symmetrical with respect to the longitudinal and transverse axis, respectively. A required property of Bz is also the anti-symmetry with respect to the cell centre, i.e. equal and opposite values in each corner of the cell.

Several patents have been published which present a design in which the magnetic fields created by the various parts of the cell and the connecting conductors compensate one another, thus decreasing magnetohydrodynamic instabilities in the cell. The targeted result is a cell having a magnetic field in the cell which is symmetric or anti-symmetric with respect to the cell axes or the cell centre as explained above. In particular, it is desirable to reduce the vertical magnetic field in the two upstream corners of the cell. This is most often achieved by appropriate busbar design and/or compensation conductors. An example for a compensation loop for the symmetrisation of the vertical component of the magnetic field in an electrolysis cell is shown in GB 2 041 409.

Another perturbation factor of a cell operating under conditions of magnetic equilibrium is the effect of neighboring cells, as cells are usually arranged side-by-side in series of up to several hundred cells and divided in at least two potrooms. This perturbation leads to local variations of the vertical component of the magnetic field Bz, in the liquid metal pad which destroy the anti-symmetry of Bz with respect to cell centre, required for good MHD stability of the cell. More precisely, the value of the vertical magnetic field should be zero in the geometrical center of the liquid metal pad, but the contribution from the adjacent rows gives a bias that can be greater than one millitesla, depending on potline current and distance to the adjacent rows of cells.

The magnetic effect of neighboring cells can be decreased by an appropriate design of the potline, and prior art offers a wide range of such designs. An early example of such design is described in US 4,090,930 which discloses the use of a compensation loop which produces an additional magnetic field substantially equal to that created by the adjacent row and opposite to it, by diverting a portion of the current from the upstream conductor from the cathode of the downstream cell, passing the diverted current below the cell and rejoining the diverted current with the outer upstream conductor after being passed below the cell. US 4,169,034 discloses the use of two compensation loops which produce an additional compensating magnetic field substantially equal to that created by the adjacent rows. US 4,683,047 achieves the same goal with asymmetric conductors below the cell.

It should be borne in mind that the simple upscaling of a cell is usually not possible without specifically adapting the whole structure of the electrical distribution system, as MHD effects tend to increase with increasing current. The main starting point for such a design is the number and position of anode risers, and the design of the cathode busbar system. The design of the cathode busbar system aims at generating a magnetic field that compensates as far as possible the local Bz in the cells, and especially in the upstream corners where Bz is usually the highest. A typical plot of Bz over the length of the cell is shown on Figure 7; a typical three dimensional plot is given in figure 10 of US 4,976,841. The peaks are due to the risers that concentrate the current coming out of the cathodes locally: this preferred current path leads to non-vertical current lines across the liquid metal pad that covers the cathode A much smaller perturbation factor of a cell operating under conditions of magnetic equilibrium is due to magnetic fields generated by more distant electric conductors carrying high currents. These conductors belong to the rectifiers and power station feeding the cell line, and to other lines of electric cells present in the same plant. Such perturbation induces a slight magnetic dissymmetry (so-called “bias”) of the cell: the local variations of the vertical component of the magnetic field are no longer symmetric over the longitudinal direction of the cell, as they would be in the absence of an external magnetic field. In particular, the magnetic fields generated by adjacent rows of cells lead to a perturbation of the magnetic equilibrium of the row; this may lead to a decrease in overall efficiency of the plant. These second order effects are the main focus of the present invention.

It is customary to designate as the “adjacent row” the row closest to the row in question, while the “field of the adjacent row” is the resultant of the fields of all the rows other than the row in question.

Interaction between magnetic fields of adjacent rows can of course be decreased easily by increasing the center-to center distance between adjacent rows. With 200 kA pots this minimum distance was usually considered to be of the order of 50 m, but with 450 kA pots it should now be increased to 110 m or 120 m. However, this option may be expensive if the high consumption of land is an economic issue; the additional land may be unavailable when adding a new potline to an existing plant. Additional cost of superstructure and conductors may be a drawback, too. It is therefore desirable to be able to decrease or to compensate this effect by using an appropriate design of electrical conductors in the plant.

Prior art offers a wide range of approaches to deal with the various effects described above; the patent documents do not always clearly address which of the abovementioned problems they are dealing with. US 4,072,597 discloses a busbar system that is asymmetric with respect to a vertical plane orthogonal to the long axis of the pot and passing through the center of the pot; this patent applies to a cell with end risers. US 4,169,034 discloses the use of an auxiliary conductor at each end of the cell, parallel to the long axis of the pot and situated in the plane of the bath/metal interface as near as possible to the pot shell; a direct current of appropriate direction and intensity (of the order of 10 to 30 kA for a cell operating at 175 kA) is passed through this auxiliary conductor. US 4,713,161 discloses the use of two correcting conductors per potline, one on each side of the pot, that extend parallel to the axis of the potline; the total current in these corrective conductors is up to 70 % of the current through individual cells. This seems to give satisfactory results for potlines operating at very high amperage. However, as explained below, the use of external compensation loops leads to locally high magnetic fields on both sides of the potline, which may increase the interaction between the magnetic fields generated by adjacent rows; as a consequence, the distance between two rows needs to be increased when building a new plant or adding a new row to an existing plant.

As considerable progress has been achieved in recent years in the control, the stability and the homogeneity of high amperage electrolysis cells, the tolerance to perturbative effects of magnetic fields generated by adjacent rows is decreasing. Furthermore, the investment cost of a potline is so high that it is desirable not to increase, and if possible, to decrease, the distance between two adjacent rows of electrolytic cells when designing a new plant or potline comprising cells of increased amperage. WO 2015/017924 discloses a compensation circuit parallel to the axis of the series that extends underneath the potline. This circuit comprises a set of three to ten parallel conductors which carry a current of between 50 % and 150 % of the electrolysis current. It is claimed that this allows to decrease the distance between two rows of cells to less than 40 m. Such a compensation circuit carrying such a high current requires a significant amount of metal, and ohmic losses will occur. The main disadvantage of this solution seems to be however the high cost of the power supply station needed for supplying current to the compensation circuit.

The problem that the present invention endeavors to resolve is therefore to decrease the effect of interaction between magnetic fields generated by two neighboring rows belonging to the same series of electrolytic cells when decreasing the distance between the rows of cells and/or increasing the current in the cells, without using long lines of compensation conductors carrying high current that is not needed for the electrolysis process and that requires independent power supply stations.

Object of the invention A first object of the present invention is therefore a cathode busbar system for an electrolytic cell of substantially rectangular shape, suitable for the Hall-Heroult electrolysis process, said electrolytic cell comprising 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 connections points, 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 cathode and lateral lining being contained in an outer metallic shell, and said electrolytic cell further comprising 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 busbar (so-called anode beam), said cathode busbar system comprising a so called cathode ring busbar, surrounding said outer metallic shell viewed from above, said ring busbar being substantially rectangular and defining a main plane, a median longitudinal plane as well as a median transversal plane, both orthogonal to said main plane, said ring busbar comprising two opposite and parallel longitudinal parts each extending along the long sides of the cell, and two opposite and parallel transversal parts extending along the ends (short sides) of the cell, said cathode busbar system being provided with connection means for connection with both electrical connection points of each cathode block of the cell, said cathode busbar system being characterized in that said ring bus bar is asymmetric with respect both to said median longitudinal plane (PX) and to said median transversal plane (PY).

Said two parallel longitudinal parts of the ring busbar are the downstream longitudinal part, electrically connected to the anode risers, and the upstream longitudinal part connected to the said downstream anode risers via the busbars at the ends of the cell.

In an embodiment said ring busbar substantially consists in said two opposite and parallel longitudinal parts each extending along the long sides of the cell, and two opposite and parallel transversal parts extending along the ends of the cell.

In specific embodiments, the parallel transversal parts of the cathode ring busbar are asymmetric with respect to said median longitudinal plane. The asymmetry of the transversal parts of the ring busbar can be achieved by providing said transversal parts in their upstream section with a derivation sector, both derivation sectors being offset with respect to said median longitudinal plan and closer to the upstream cell than to the downstream cell. Said derivation sector can be asymmetric with respect to main axis of said transversal part. In an advantageous embodiment, said derivation sector projects towards the bottom, with respect to the level of the liquid metal pad in the cell. Both derivation sectors face each other, along an axis parallel to the longitudinal part of the ring busbar.

Said derivation sector extends in a plane parallel to the median transversal plane. Various shapes are possible for said derivation sector. Said derivation sector can be U-shaped, said U-shape possibly comprising rounded and/or straight sections. In a variant, said derivation sector is formed of straight portions.

In a specific embodiment at least one of the transversal parts of the ring busbar is provided in its upstream section with a derivation sector (and preferably a “U” shaped sector), projecting towards the bottom, both derivation sectors being offset with respect to said median longitudinal plane.

As cathode blocks are symmetric and have collector bar ends coming out on each side, in side-by-side arrangements of electrolytic cells half of the current collected by the collector bars of the cathode blocks will flow directly to the downstream longitudinal part of the cathode busbar system, while the other half flows to the upstream longitudinal part (see Figure 8). It is therefore necessary to carry the cathode current collected at the upstream side of the cathode busbar system (that is to say by the upstream longitudinal parts) back to the downstream part of the cathode busbar system. This is achieved by the transversal parts of the ring busbar. However, such a ring busbar circuit needs to be equilibrated because the path of the current collected by the upstream longitudinal parts is longer than the path of the current collected by the downstream longitudinal parts. Furthermore, it is desirable that each anode riser collects a predefined current; if said plurality of risers comprises end risers and central risers, the end risers may collect a different current than the central risers or equal current to the one in centre risers.

For these reasons the cathode busbar system may comprise additional electrical balancing circuits. Said electrical balancing circuits and the components thereof are not a part of the ring busbar as defined herein.

In an embodiment, said cathode busbar system according to the invention further comprises a downstream electrical balancing circuit comprising conductors arranged in vicinity of and parallel to the downstream longitudinal part of said ring busbar.

In one embodiment which can be combined with any of the previous ones, said cathode busbar system further comprises two or more conductive arms that extend between said longitudinal parts of said ring busbar, underneath said shell. These conductive arms extending underneath the ring busbar system connect the upstream longitudinal part of the ring busbar to the downstream longitudinal part, thereby creating an additional path for the cathode current collected upstream. They are not part of the ring busbar system as such; they act as an upstream electrical balancing circuit, achieving preferential feeding of the cathode current collected by the upstream longitudinal parts of said ring busbar to the end risers.

Said conductive arms can be symmetric or asymmetric with respect to said median longitudinal planes, and/or they can be symmetric or asymmetric with respect to said median transversal plane. In an advantageous embodiment, said arms are asymmetric with respect to said median longitudinal plane.

In another embodiment each transversal part comprises at least two parallel busbars separated by an intercalary space. Advantageously, said intercalary space is at least 10 mm, preferably at least 25 mm, and most preferably at least 50 mm.

In specific variants of this embodiment said parallel busbars have conductive sections the sum of which is different in the duct end transversal busbar and in the tap end transversal busbar. The outer of said parallel busbars can have a conductive section which is smaller than that of the inner of said parallel busbars. The ratio of the sum of the conductive cross sections of the duct end longitudinal busbars and the sum of the conductive cross sections of the tap end longitudinal busbars is greater than 1.1, preferably greater than 1.25, and preferably greater than 1.43.

Another object of the invention is an electrolytic cell of substantially rectangular shape 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 being provided with at least one current collector bar and two electrical connections points, 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 cathode and lateral lining being and lining being contained in an outer metallic shell, 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, said electrolytic cell being characterized in that it comprises a cathode busbar system according to any of the embodiments and variants of the present invention.

Yet another objects is a potline comprising a plurality of electrolytic cells of substantially rectangular shape, suitable for the Hall-Heroult electrolysis process, characterized in that at least 50%, and preferably at least 80% of said cells are electrolytic cells according to claim this invention. A last object is an electrolysis plant comprising at least one potline according to the invention. In an embodiment, said electrolysis plant has a potline according to the invention,, said potline being arranged in a first and a second rows connected in series and operating at a current ls (in kilo-Amperes), both rows being parallel, and each row comprising a plurality of electrolytic cells connected in series, characterized in that said second row runs parallel to said first row, at a distance R (in metres) of less than ls /4.1 metres, preferably less than ls /4.5 metres, still more preferably less than ls /5 metres, and most preferably less than ls /5.6 metres. This decrease in distance R between parallel rows of the same series of electrolytic cells leads to savings in investment cost, and furthermore, compared to the use of compensation conductors carrying high current according to prior art, leads also to savings in operational cost.

Another object of the invention is an aluminium electrolysis plant comprising at least one line of electrolysis cells of substantially rectangular shape, said cells being arranged side by side, and said plant further comprising means for electrically connecting said cells in series and for connecting the cathode 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 present invention. A last object of the invention is a method for making aluminium by the Hall-Heroult electrolysis process using electrolytic cells of substantially rectangular shape, characterized in that said method is carried out in an aluminium electrolysis plant according to the invention.

Figures

Figures 1 to 6 and 9 represent various embodiments of the present invention. Figures 7 and 8 illustrate prior art.

Figure 1 is a schematic view, showing the global arrangement of a plant according to the invention.

Figure 2 is a perspective view, showing a cathode busbar according to a first embodiment of the invention, which belongs to the smelter of the figure 1.

Figure 3 is a bottom schematic view, showing an electrolytic cell provided with the cathode busbar of figure 2.

Figure 4 is a front view, showing a transversal part of cathode busbar of figure 2 with its derivation section.

Figures 5 and 6 are front views, similar to figure 4, showing further variants of the transversal part of a cathode busbar.

Figure 7 is a typical plot of the vertical magnetic field (Bz) depending on the distance from the centre point of a typical 420 kA electrolysis cell. The three curves correspond to different lines parallel to the length of the cell: curve (a) corresponds to the downstream region, curve (b) to the upstream region, curve (c) to the centre.

Figure 8 is a schematic cross section along a transversal plane across a Hall-Heroult electrolytic cell. The arrows represent the current flow across the cell.

Figure 9 shows details of the transversal part of an embodiment of the busbar system according to the invention. Figure 9(a): tap end, Figure 9(b): duct end

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

Detailed description

The present invention is directed to the global arrangement of a plant, or aluminium smelter, used in the Hall-Heroult process. As schematically shown on Figure 1, the aluminium smelter of the invention comprises a plurality of electrolytic cells C1, C2, ... , Cn-1, Cn, arranged the one behind the other (and side by side) along two parallel lines L1 and L2, each of which comprises n/2, i.e. m cells. These cells are electrically connected in series by means of conductors, which are not shown on Figure 1. The number of cells in a series is typically comprised between 50 and over 100, but this figure is not substantial for the present invention. The electrolysis current therefore passes from one cell to the next, along arrow DC. The cells are arranged transversally in reference of main direction D1 or D2 (axis of the row) of the line L1 or L2 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. Figure 1 depicts a typical “clockwise” current orientation.

The Hall-Heroult process as such, the way to operate the latter, as well as the cell arrangement are known to a person skilled in the art and will not be described here in more detail. 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”.

The general structure of a Hall-Heroult electrolysis pot is known per se and will not be explained here. It is sufficient to explain, in particular in relation with Figure 8. that the current is fed into the anode busbar (called anode beam, not shown on the figures), flows from the anode beam to the anode rod 804 and to the anode 801 in contact with the liquid electrolyte 802 where the electrolytic reaction takes place, crosses the liquid metal pad 803 resulting from the process and eventually will be collected at the cathode block 805. As cathode blocks are symmetric and have collector bar ends 806 coming out on each side, in side by side arrangements of electrolytic cells half of the current collected by the collector bars 806 of the cathode blocks 805 will flow directly to the downstream longitudinal part 2 of the cathode busbar system, while the other half flows to the upstream longitudinal part 3 (see figure 8). As will be explained later, means are provided to carry the cathode current collected at the upstream part 3 of the cathode busbar system back to the downstream longitudinal part 2 of the cathode busbar system.

The present invention is more particularly directed to the cathode busbars of the potline, each of which surrounds a respective cell (schematically shown on Figure 3 as reference number 11 designating the visible outer boundaries of the cell volume, i.e. the potshell). Hereafter, the arrangement of two embodiments of the busbar associated with cell C2 will be described, in relation with Figures 2 and following. Preferably, the arrangement of a majority of the other busbars and, most preferably, of all the busbars of the plant, is similar.

Turning now to Figure 2. cathode busbar as a whole is given the general reference 1. It rests on appropriate structural elements (not shown on the figures), such as columns, in a way known as such; in a known manner, said columns rest on insulating plots on a horizontal support (usually concrete) in order to electrically insulate them from the ground. Thus, this busbar system 1 is located on about the same horizontal level as the molten aluminium metal contained within the cell. The cell is designated as C2 on Figure 2. Busbar system 1 comprises different mechanical elements, which will be described hereafter more in detail. It first includes a ring (called here “ring busbar”) which is generally formed by two longitudinal parts 2 and 3, parallel to axis X-X, as well as two transversal parts 4 and 5. This ring busbar defines a main plane PR, which extends horizontally. Moreover, two branches 6 and 7, which are parallel to transversal parts 4 and 5, extend between opposite longitudinal parts 2 and 3. All the elements which form busbar system 1 are made of aluminium.

The whole ring busbar 2 - 5 has a rectangular shape, the length LR of which is slightly superior to that of cell C2, whereas the width WR of which is slightly superior to that of cell C2. By way of example, length LR is between about 14 000 mm and about 25 000 mm, whereas width WR is between about 5 000 mm and about 9 000 mm. Axis X-X defines a median longitudinal direction of the cell and of the whole ring busbar 2-5, whereas axis Y-Y defines a median transversal, or lateral direction of the cell and of the whole ring busbar 2 - 5. As explained hereabove, transversal axis Y-Y of the ring busbar 2-5 corresponds to the main longitudinal direction D1 of the line L1 which includes cell C2.

Moreover, PX defines a median longitudinal plane of the cell and of the whole ring busbar 2 - 5, said plane being orthogonal to main plane PR and including axis X-X. PY defines a median transversal plane of the cell and of the whole ring busbar 2 - 5, said plane being orthogonal to main plane PR and including axis Y-Y. As explained more in detail hereafter, in the embodiment of Figures 2 to 6, the ring busbar is asymmetric with respect to plane PX; this is an essential feature of the present invention. Moreover, the ring busbar is asymmetric with respect to plane PY. As will be explained in more detail below, this asymmetry with respect to each of plane PX and plane PY can be obtained by different technical features.

Longitudinal part 2 is called upstream part, since it is on the upstream side of the cell with respect to current flow direction. It first comprises a main busbar 20, which is straight and horizontal, and which extends along the whole length of part 2. This busbar 20 is rectangular in cross section, with vertical large sides. By way of example, its height H20 is between about 500 mm and about 1 100 mm, whereas its width W20 is between about 100 mm and about 300 mm. Busbar 20 is provided with a row of connectors 21, projecting downwards. In a known manner, each connector 21 may be a flexible formed from stacked sheet and is intended to cooperate with the first end of a cathode block (not shown on the figures). Busbar 20 may be manufactured in one single piece or be assembled lengthwise from two half-bars, typically by welding; the welding seams are marked with reference number 201.

Longitudinal part 3 is called downstream part, since it is on the downstream side of the cell with respect to current flow direction. It first comprises a main busbar 30, which is straight and horizontal, and which extends along the whole length of part 3. This busbar 30 is rectangular in cross section, with vertical large sides. By way of example, its height H30 is between about 300 mm and about 700 mm, whereas its width W30 is between about 100 mm and about 150 mm. Busbar 30 is provided with a row of connectors 31, similar to those 21, each of which is intended to cooperate with the other end of a respective cathode block; these connectors are known as such and will not be discussed here in more detail. Like busbar 20, busbar 30 may be manufactured in one single piece or be assembled lengthwise from two half-busbars, typically by welding; the welding seams are marked with reference numbers 301, thus creating a zig-zag and increasing its length as required by electrical equilibrium of the busbars.

Transversal part 4 is called duct end or duct part for a potline with current circulating clockwise; it is turned towards the line L2 of cells, facing the line L1 which includes present cell C2. Duct end and tap end would be interchanged for a potline with current circulating counter-clockwise. It may be formed in full thickness by one busbar, or may be formed by two parallel “half-busbars”, i.e. an inner busbar 41 and an outer busbar 42, which extend parallel the one to the other (the description will be given here for a transversal part 4 comprising two half-busbars 41, 42). These half-busbars are mutually distant, in order to define an intercalary space 43. Said intercalary space acts as an air gap that may provide some cooling of the busbars. Both inner and outer busbars are rectangular in cross section, with vertical large sides. By way of example, each inner and outer busbar has the same height H41, which is between about 500 mm and about 1 100 mm, whereas each busbar has the same width W41 which is between about 200 mm and about 400 mm. However, their height and/or width can be different, and the term “half busbar” should not be understood as necessarily implying “equal halfs”.

Transversal part 5 is called tap end or tap part in a potline with clockwise current, since it is turned opposite the other line L2 of cells. As explained above, the whole ring busbar is asymmetric in view of plane Y-Y. However, the general shape of the structures of this end parts can be similar to that of duct part 4. As an example shown on figure 2, both transversal duct part 4 and tap part 5 can both have a U shape, and in this case the U-shaped part 5 may be different in lengths composing U said shape, but can be similar in shape to that on duct part 4. On the drawings, the references of the components of part 5 are the same as those of part 4, apart from the fact that the first digit “5” replaces the first digit “4”. If the whole ring busbar is asymmetrical in view of plane Y-Y. In other embodiments the general shapes of parts 4 and 5 are different the one from the other. Advantageously, the global shapes and structures of these two parts 4 and 5 are substantially similar.

As can be seen from figure 2, each transversal part 4 or 5 is mechanically and electrically linked to a respective end of upstream longitudinal part 2. To this end, an inner junction member 81 or 91 extends between inner rod 41 or 51 and facing parts of rod 20. Moreover, an outer junction member 82 or 92 extends between outer rod 42 or 52 and facing parts of main rod 20. Each junction member has an appropriate structure, so as to fulfil the above technical function. In the shown example, it is made of stacked sheets, the flexibility of which is sufficient to create a rounded shape.

Each transversal part 4, 5 is of uniform width. In an advantageous embodiment of the invention, the width and/or cross-section of the transversal duct-end part 4 is greater than that of the transversal tap-end part 5; this is one of the asymmetric means of the ring busbar system with respect to the median transversal plane PY that allows to achieve compensation of the vertical component of the magnetic field generated by distant conductors such as adjacent row of electrolytic cells. Other means to achieve this compensation that are asymmetric with respect to the median transversal plane PY are related to the cross-section of the transversal parts 4, 5, as can be seen from figure 2. This will also be discussed in relation with figures 9(a) and 9(b), showing transversal parts 4, 5 that are formed by two parallel half bars 41, 42, 51, 52.

At the duct end, inner half bar 41 has a cross section S41 which is smaller than the cross section S42 of outer half bar 42.

At the tap end, inner half bar 51 has a cross section S51 which is smaller than the cross section S52 of outer half bar 52.

In a specific embodiment, in each of the transversal parts 4 and 5, both half bars have the same height and differ only by their width: at the duct end, inner half bar 41 has a width W41 which is smaller than the width W42 of outer half bar 42. At the tap end, inner half bar 51 has a width W51 which is smaller than the width W52 of outer half bar 52.

In an embodiment, the cross section S51 (or in the abovementioned specific embodiment: the width W51) of tap inner half bar 51 is substantially equal to the cross section S41 (or width W41) of duct inner half bar 41. In addition, the cross section S52 (or width W52) of tap outer half bar 52 is smaller than the cross section S42 (or width W42) of duct outer half bar 42. Advantageously, the ratio of cross sections S42/S52 (or ratio {W42IW52)) is superior to 1.1, in particular to 1.8. This ratio is typically between 1.1 and 3. The sum of the cross sections (S41 + S42) (or of widths (W41 + W42)) of the two half bars 41 and 42 of duct part 4 is superior to the sum (S51 + S52) (or W51 + W52) of the cross-sections (or widths) of the two bars 51 and 52 of tap part 5. Advantageously, ratio {S41+S42)/(S51+S52) (or (W41+W42)/(W51+ W52)) is superior to 1.1, in particular to 1.43. This ratio is typically between 1.1 and 3. Each sum (S41 + S42) or (S51 + S52) is called “conductive cross section”) (and each sum (W41 + W42) or (W51 + W52) is called “conductive width”) of the respective part 4 or 5. As variants not shown on the figures, if one transversal part is formed of one single bar, its conductive cross section (or width) corresponds to the cross section (or width) of this bar and, if one transversal part is composed of at least three bars, its conductive cross section (or width) corresponds to the sum of the cross sections (or widths) of these bars.

In an embodiment, the width of intercalary space 53 of tap part 5 is substantially equal to that of intercalary space 43 of duct part 4. Generally, this width is preferably at least 10 mm, still more preferably at least 25 mm, even more preferably 50 mm. The cooling effect is usually not significant below 10 mm, and little additional cooling effect is observed above 50 mm.

In a variant not shown on the figures, longitudinal part 2 is also split into two parallel rectangular half-bars.

Figure 2 also shows downstream balancing circuits 100; they do not form part of the ring bus bar as defined herein, and do not form part of the present invention. For the sake of completeness of the present description it is sufficient to explain that the first downstream electric balancing circuit 100a connects the downstream cathode collector bars 31 n° 1 to 4 to the next anode end riser 10a, the second downstream electric balancing circuit 100b connects the downstream cathode collector bars n° 5 to 8 to the next anode end riser 10a, and the third downstream electric balancing circuit 100c connects downstream cathode collector bars n° 9 to 14 to the next central anode riser 10b (the numbering of the cathode collector bars starts at the end of the pot, the reference number 31 corresponds to the connector to said cathode collector bar, said cathode collector bar itself not being represented on Figure 2). A similar explanation can be given for downstream balancing circuits 100d,100e,100f in relation with anode risers 10c and 10d. Figure 2 does not show rods connecting said downstream electric balancing circuits to the downstream longitudinal part 3 of the ring busbar; they are located underneath and can be seen schematically on figure 3.

According to the invention, the asymmetry of the ring busbar with respect to median transversal plane PY can be achieved in different ways and using different means. These include derivation sectors in the transversal parts 4,5, as well as different thicknesses of the transversal parts 4,5, or a different distribution of individual thicknesses of the half-bars 41,42;51,52 of each transversal part 4,5. Another means to achieve asymmetry with respect to median transversal plane PY is to choose a different distance between each of transversal parts 4,5 and the closest outer limit of the pot shell. These various ways and means to achieve asymmetry with respect to median transversal plane PY can be combined.

It should be stressed here that the asymmetry with respect to median transversal plane PY is not a goal in itself but a means to achieve compensation of the magnetic field of the adjacent row.

Let us consider now Figure 4. which is a front view of transversal part 4. It shows an example of a transversal part 4 that is asymmetric with respect to the median longitudinal plane PX of the cell by use of derivation sectors. A4 is the main direction of this transversal part 4, i.e. the axis extending between opposite ends thereof. Axis A4 is parallel to axis Y-Y described hereabove. Busbars 41 and 42 define a first sector 44 of part 4, adjacent downstream longitudinal part 3, as well as a second sector 46 of part 4, adjacent upstream longitudinal part 2. First sector 44, which is straight and horizontal, extends along main axis A4.

On the contrary, second sector 46 is U-shaped and projects downwards this axis A4. In other words, sector 46 is not symmetric with respect to axis A4. Sector 46 comprises two vertical wings 461 and 461’, as well as a horizontal core 462. The height H46 of U-shaped sector 46, which is defined by the distance between the lower faces of sector 44 and core 462, is between about 1 000 mm and about 1 600 mm. The straight length L46 of U-shaped sector 46, which is defined by the distance between the opposite front and rear faces of core 462, is typically between about 1 500 mm and about 3 700 mm; this length L46 is advantageously between about 20 % and about 75 % of the length L4 of the whole part 4.

Sector 46 forms a derivation sector of transversal part 4. Let us consider the so called developed length LD46 of this sector, i.e. the sum of lengths L461, L462 and L461’. The so called derivation ratio of the derivation sector is equal to the ratio (LD46 / L46) between developed length and straight length. Advantageously, this derivation ratio is superior to 2, which permits noticeable change of the magnetic field intensity and direction at the upstream corners of the cell.

Figures 5 and 6 show variants of the derivation sector of transversal part 4, which are referenced 46A and 46B on these further drawings. On Figure 5 the wings of the U are straight and not orthogonal to its core, like on Figure 4, but extend obliquely. On Figure 6 the wings are rounded.

Such derivation sectors can be used to achieve asymmetry with respect to the median transversal plane PY, as shown on Figure 2. In this embodiment duct part 4 differs from tap part 5, in that its U-shaped portion 46 is less high (or deep) than U-shaped portion 56 of part 5. In other words, height H46 is inferior to that H56. Advantageously, ratio (.H56/H46) is superior to 1.2, in particular to 1.94. This ratio is typically between 1.2 and 2.5.

On the other hand, the straight lengths LS46 and LS56 of these two sectors 46 and 56 are substantially identical. However, due to the above explained difference of heights, developed length LD46 is inferior to that LD56. These various shapes and their various geometric parameters offer a range of possibilities for fine-tuning of magnetic fields generated by current flowing in the transversal parts 4,5.

In the framework of the present invention, the ring bus bar system according to the invention can also be designed in a way that all transversal parts 4,5 are straight (i.e. in particular the transversal parts 4,5 have no sections that project out of the main horizontal plane PR).

In another embodiment shown on figures 9a and 9b that can be combined with any of the other embodiments, duct part 5 differs from tap part 4 in that its two half bars 51 and 52 have different widths. Inner half bar 51 has a width W51 which is inferior to that W52 of outer half bar 52. In embodiments in which all transversal parts 4,5 are straight this is a preferred approach to the ring bus bar design.

As can be seen from the figures, and in particular from figures 2 and 3, the cathode busbar system according to the invention can further comprise two arms 6,7 that extend between longitudinal parts 2,3 underneath said shell and connect said longitudinal parts 2,3 together. They act as an upstream electrical balancing circuit and are not part of the ring bus bar system as such: these arms achieve preferential feeding of the cathode current collected by the upstream longitudinal parts to the end anode risers 10a,10d. In the embodiment shown on the figures said arms 6,7 are asymmetric with respect to said mean transversal plane PY and also asymmetric with respect to said median longitudinal plane PX.

Conductive arm 6 is called duct branch, since it is offset towards duct end 4, with respect to axis Y-Y’; it extends underneath the potshell. It comprises a main pole 61, which extends parallel to Y-Y’, under the surface of main plane PR, underneath the potshell. This pole is prolonged by two orthogonal branches 62 and 63, each of which extends under a respective longitudinal part 2 or 3 towards the head of the cell. The junctions between these branches 62, 63 and these parts 2,3 are different, depending on their downstream or upstream location.

Thus, upstream branch 62 is prolonged by an intermediate segment 64, which slopes both above and towards median axis Y-Y’. A terminal upright portion 65, made of stacked plates, links segment 64 and longitudinal upstream part 2. On the other hand, downstream branch 63 is directly linked to the cut-out wedge bar 32, via an upright portion 66, also made of stacked plates. In other words, the main difference between upstream and downstream zones of arm 6 is intermediate segment 64.

Branch 7 is called tap arm, since it is offset towards tap end 5, with respect to axis Y-Y’. Although, as explained above, duct branch 6 and tap branch 7 are asymmetric with respect to axis Y-Y, the overall structure of this branch 7 is identical to that of branch 6. On the drawings, the references of the components of branch 7 are the same as those of branch 6, apart from the fact that the first digit “7” replaces the first digit “6”.

Each of said cut-out wedge bars 32 are connected to the two anodic risers in its vicinity, as can be seen from Figure 2. It is understood here that these cut-out wedge bars 32 are not part of the ring bus bar

In the embodiment shown in Figure 2, duct arm 6 substantially differs from tap arm 7, in that its main pole 61 is closer to the axis Y-Y than main pole 71 of tap arm 7. In addition, both branches 62 and 63 are shorter than respectively branches 72 and 73. L62, L63, L72 and L73 denote the respective lengths of branches 62, 63, 72 and 73. Since poles 61 and 71 are parallel, differences (L62 - L72) and (L63 - L 73) are equal. The purpose of this asymmetry of arms 6 and 7 is to decrease the vertical magnetic field in the upstream corner and not to contribute to the compensation of the magnetic field of the adjacent row of cells.

Let us consider the whole lengths L6 and L7 of arms 6 and 7. Each of these lengths respectively corresponds to the sum of the lengths of the mechanical components of each arm, i.e. 61 - 66 and 71 - 76.

As mentioned above, the whole ring busbar is asymmetrical also in view of axis Y-Y. In addition, the general shapes of arms 6, 7 are different the one from the other. However, the general shapes of two parts 4, 5 are substantially similar.

While Figure 2 shows a preferred embodiment of the present invention, in other embodiments said conductive arms 6, 7 (which do not form part of the ring busbar system as defined herein) are symmetric with respect to said median transversal plane PY and asymmetric with respect to said mean longitudinal plane PX, or they are asymmetric with respect to said median transversal plane PY in such a way that its main pole 61 is further away from the axis Y-Y than the main pole 71 and asymmetric with respect to said mean longitudinal plane PX, or they have generally different shapes.

Using an embodiment of the invention with asymmetric busbars with respect to the transverse cell plane PY and with respect to the longitudinal cell plane PX, according to figures 1 to 4, it has been possible, in industrial pots operating at about 450 kA, to significantly decrease not only the vertical component of the magnetic field in the upstream corners of the cell without adding compensation circuits such as those known in the art, but also to significantly decrease the magnetic perturbation due to an adjacent row. Surprisingly, the new, asymmetric ring busbar (reference RBBAA= ring busbar with double asymmetry) ended up with a total mass significantly lower than the design before that was symmetric with respect to the median transversal plane PY, having to U-shaped transversal parts 4,5 of identical shape and geometry (reference RBBA = ring bus bar with simple asymmetry): for the former the mass of the complete ring busbar system plus the upstream balancing circuit for a cell operating at 450 kA was reduced by about 5.7 tons with respect to the latter.

Typical results are given in Table 1 for the maximum value of Bz bias of two cells of identical size with ring busbars having U-shaped transversal parts, one having a “RBBAA” design according to the invention, the other having an “RBBA” ring busbar not according to the invention (in both cases the second row of the series of pots was at a distance of 92 m).

Table 1: Overall average (a) and maximum (b) Bz bias in each quarter [millitesla] at potline current of 450 kA.

In this table the first line of values represents the overall average in the liquid zone of the cell, the second line of values represents the downstream side, the third line of values represents the upstream side, and for each example, the right-hand column represents the duct end and the left-hand column the tap end of the cell. It can be seen that with respect to prior art, the busbar design according to the invention decreases the perturbation exerted on the first row of pots by the second row of pots running at the same distance; if one accepts the perturbation level of prior art potlines, then the distance between the rows of a series of pots could be decreased significantly in a potline according to the invention with respect to prior art, thus reducing the land consumption of an electrolysis plant.

As an example, if according to prior art in a potline operating at a given row-to-row distance (for example 100 m) the perturbation level was deemed acceptable, then a potline operating using a ring busbar according to the present invention could either be designed to operate at the same row-to-row distance at an increased current, or could be designed to have a smaller row-to-row distance without increasing the perturbation level.

It should be noted that in the embodiment according to figures 1 to 3, the current is conducted clockwise, that is to say it enters the last cell Cm of line L1 upstream, crosses it downstream and then turns clockwise (in direction of the duct end) to line L2. Of course the invention applies also to counter-clockwise structures, and a person skilled in the art can easily adapt the cathode ring busbar system according as shown on the figures to counter-clockwise potlines.

The cathode ring busbar system according to the invention can be manufactured from aluminium sections of appropriate cross section. In a known way, stacked aluminium sheets or plates and stacks of flexible aluminium sheets can be used for joining sections by welding.

Claims (14)

1. A cathode busbar system for an electrolytic cell of substantially rectangular shape, suitable for the Hall-Heroult electrolysis process, said electrolytic cell comprising a cathode (805) 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 (806) and two electrical connections points, a lateral lining defining together with the cathode a volume containing the liquid electrolyte (302) and the liquid metal (803) resulting from the Hall-Heroult electrolysis process, said cathode (805) and lateral lining being contained in an outer metallic shell, and said electrolytic cell further comprising a plurality of anode assemblies suspended above the cathode, each anode assembly comprising at least one anode (801) and at least one metallic anode rod (804) connected to an anode bus bar, said cathode busbar system comprising a so-called ring busbar, surrounding said outer metallic shell viewed from above, said ring busbar being substantially rectangular and defining a main plane (PR), a median longitudinal plane (PX) as well as a median transversal plane (PY), both orthogonal to said main plane (PR), said ring busbar comprising two opposite and parallel longitudinal parts (2,3) each extending along the long sides of the cell, and two opposite and parallel transversal parts (4,5) extending along the ends of the cell, said ring busbar being provided with connection means (21,31) for connection with both electrical connection points of each cathode block of the cell, said cathode busbar system being characterized in that said ring bus bar is asymmetric with respect both to said median longitudinal plane (PX) and to said median transversal plane (PY).

2. A cathode busbar system according to claim 1, characterized in that the parallel transversal parts (4,5) are asymmetric with respect both to said median longitudinal plane (PX) and to said median transversal plane (PY).

3. A cathode busbar system according to any of claims 1 to 2, characterized in that said cathode busbar system further comprises a downstream electrical balancing circuit (100) comprising conductors (100a,b,c,d,e,f) arranged in vicinity of and parallel to the downstream longitudinal part of said ring busbar.

4. A cathode busbar system according to any of claims 1 to 3, characterized in that at least one of its transversal parts (4,5) is provided in its upstream section (46,56) with a derivation sector (and preferably a “U” shaped sector), projecting towards the bottom, both derivation sectors being offset with respect to said median longitudinal plane.

5. A cathode busbar system according to any of claims 1 to 4, characterized in that said ring busbar substantially consists in said two opposite and parallel longitudinal parts (2,3) each extending along the long sides of the cell, and two opposite and parallel transversal parts (4,5) extending along the ends of the cell.

6. A cathode busbar system according to any of claims 1 to 5, characterized in that each transversal part (4,5) comprise at least two parallel busbars (41,42;51,52) separated by an intercalary space (43,53).

7. A cathode busbar system according to claim 6, characterized in that said parallel busbars (41,42;51,52) have conductive sections S41,S42, S51,S52 the sum of which is different in the duct end transversal busbar (4) and in the tap end transversal busbar (5).

8. A cathode busbar system according to any of claims 6 or 7, characterized in that the outer of said parallel busbars (42,52) has a conductive section which is smaller than that of the inner of said parallel busbars (41,51).

9. A cathode busbar system according to claim 7 or 8, characterized in that the ratio of the sum of the conductive cross sections of the duct end longitudinal busbars (4) and the sum of the conductive cross sections of the tap end longitudinal busbars (5) is greater than 1.1, preferably greater than 1.25, and preferably greater than 1.43.

10. A cathode busbar system according to any of claims 6 to 9, characterized in that said intercalary space is at least 10 mm, preferably at least 25 mm, and most preferably at least 50 mm.

11. Electrolytic cell of substantially rectangular shape, suitable for the Hall-Heroult electrolysis process, comprising a cathode busbar system according to any of claims 1 to 10.

12. Potline comprising a plurality of electrolytic cells of substantially rectangular shape, suitable for the Hall-Heroult electrolysis process, characterized in that at least 50%, and preferably at least 80% of said cells are electrolytic cells according to claim 11.

13. Electrolysis plant comprising at least one potline according to claim 12.

14. Electrolysis plant according to claim 13, having a potline according to claim 12, said potline being arranged in a first and a second rows connected in series and operating at a current ls (In kilo-Amperes), both rows being parallel, and each row comprising a plurality of electrolytic cells connected in series, characterized in that said second row runs parallel to said first row, at a distance R (in metres) of less than ls /4.1 metres, preferably less than ls /4.5 metres, still more preferably less than ls /5 metres, and most preferably less than ls /5.6 metres.