WO2017051317A1

WO2017051317A1 - Cathode busbar system for electrolytic cells arranged side by side in series

Info

Publication number: WO2017051317A1
Application number: PCT/IB2016/055615
Authority: WO
Inventors: Vinko Potocnik; Marwan ALBASTAKI; Abdalla ALZAROONI
Original assignee: Dubai Aluminium Pjsc
Priority date: 2015-09-23
Filing date: 2016-09-21
Publication date: 2017-03-30
Also published as: GB2542588A; GB201516890D0; GB2542588B

Abstract

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, said cathode busbar system comprising a 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 each extending along the long sides of the cell, and two opposite and parallel transversal parts extending along the ends of the cell, said ring busbar being symmetric with respect to said median transversal plane (PY), 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 busbar is asymmetric with respect to said median longitudinal plane (PX).

Description

Cathode busbar system for electrolytic cells arranged side by side in series

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.

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 19^th 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 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 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.

These enormous electrical DC currents flow through various conductors, such as electrolyte, liquid metal, anodes, cathode, connecting conductors, where they generate heat with ohmic 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 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 bath metal interface hydrostatically. The 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. 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 (B_z) in the liquid metal; a root-mean square average value of about one millitesla is a usual maximum target. Moreover, the horizontal components B_x and B_y (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 B_z is also the asymmetry with respect to the cell centre, i.e. equal and opposite values in each corner of the cell.

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 B_z, (z being the coordinate running upwards from the bottom to the top of the cell) in the liquid metal pad which destroy the anti-symmetry of B_z 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. As an example, US 4, 169,034 (1979) 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 (1987) achieves the same goal with asymmetric busbars 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 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 B_z in the cells, and especially in the upstream corners where B_z is usually the highest. A typical plot of B_z over the length of the cell is shown on Figure 7;.

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 cell centre as explained above.

The present invention focuses on reducing the vertical magnetic field in the two upstream corners of the cell. Object of the invention

According to the invention, the problem of high vertical magnetic fields in the upstream corners of the cell has been solved in a surprising manner by a modification of the cathode busbar system. As explained above, cathode busbar systems according to prior art are symmetric with respect to a median transverse plane in side-by-side cells. The inventors have found that , the vertical magnetic field in the upstream corners of a cell can be decreased in a significant way by modifying the cathode busbar running along the end of the cell. The present invention applies to electrolytic cells of substantially rectangular shape, suitable for the Hall-Heroult electrolysis process, arranged side-by- side, in which the cathode busbar is connected to a plurality of anode risers through which the current is fed into the anode beam of the downstream cell. Said plurality of risers is arranged lengthwise, that is to say for a given cell said risers are arranged close to the longitudinal downstream rim of the cell.

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 ring busbar being symmetric with respect to said median transversal plane (PY),

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 busbar is asymmetric with respect to said median longitudinal plane. 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 preferred 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 is 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 should be asymmetric with respect to the 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. 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 anode risers comprises end anode risers and central anode risers, the end anode risers may collect a different current than the central anode risers or equal current to the one in central anode 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 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 anode 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 (PX). Said balancing circuits may also comprise conductors arranged in vicinity of and parallel to the downstream longitudinal part of said ring busbar; they are not part of the ring busbar. 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.

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 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 (B_z) 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.

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 104 and to the anode 101 in contact with the liquid electrolyte 102 where the electrolytic reaction takes place, crosses the liquid metal pad 103 resulting from the process and eventually will be collected at the cathode block 101. As cathode blocks are symmetric and have collector bar ends 105 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 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). 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 symmetric with respect to plane PY.

Longitudinal part 2 is called upstream part, since it is adjacent next upstream cell, i.e. cell C1. 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.

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 busbars are rectangular in cross section, with vertical large sides. By way of example, each 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.

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 end riser 11a, the second downstream electric balancing circuit 100b connects the downstream cathode collector bars n° 5 to 8 to the next end riser 10a, and the third downstream electric balancing circuit 100c connects downstream cathode collector bars n° 9 to 14 to the next central riser 11 b (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). As the ring bus bar is symmetric with respect to the median transversal plane PY, the same explanation can be given for downstream balancing circuits 100d,100e,100f in relation with 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. Let us consider now Figure 4, which is a front view of transversal part 4. A4 is the main direction of this 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. 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, if the whole ring busbar is symmetrical in view of plane Y-Y, so that structures of this end parts is identical to that of 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".

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, and they are equal in cross section.

Longitudinal part 3 is called downstream part, since it is adjacent next downstream cell, i.e. cell C3. 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.

As can be seen from the figures, and in particular from figures 2 and 3, the cathode busbar system according to the invention further comprising two arms 6,7 that extend between longitudinal parts 2,3 underneath said shell and connect said longitudinal parts 2,3 together. In the embodiment shown on the figures said arms are asymmetric with respect to said median longitudinal plane PX and symmetric or asymmetric with respect to said mean transversal plane PY

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 longitudinal part 3, 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 branch, 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 symmetric 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".

While Figure 2 shows a preferred embodiment of the present invention, in other embodiments said conductive arms are symmetric with respect to said median longitudinal plane PY and asymmetric with respect to said mean transversal plane PX, or they are asymmetric with respect to said median longitudinal plane PY and asymmetric with respect to said mean transversal plane PX.

Using an embodiment of the invention with symmetric busbars with respect to the transverse cell plane PY, according to figures 1 to 4, it has been possible, in industrial pots operating at about 450 kA, to significantly decrease 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.

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

1. A cathode busbar system for an electrolytic cell of substantially rectangular shape, suitable for the Hall-Heroult electrolysis process,

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 each extending along the long sides of the cell, and two opposite and parallel transversal parts extending along the ends of the cell,

said cathode busbar system being characterized in that said ring busbar is asymmetric with respect to said median longitudinal plane (PX).

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

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 comprising conductors 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 each transversal part is provided in its 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.

5. A cathode busbar system according to claim 4, characterized in that said derivation sector is asymmetric with respect to main axis (A4) of said transversal part.

6. A cathode busbar system according to any of claims 4 to 5, characterized in that said derivation sector projects towards the bottom, with respect to said main axis (A4) of said transversal part.

7. A cathode busbar system according to any of claims 4 to 6, characterized in that said derivation sector extends in a plane parallel to the median transversal plane (PY).

8. A cathode busbar system according to any of claims 4 to 7, characterized in that both derivation sectors face each other, along an axis parallel to the longitudinal parts of the ring busbar.

9. A cathode busbar system according to any of claims 4 to 8, characterized in that said derivation sector is U-shaped, said U-shape comprising rounded and/or straight sections.

10. A cathode busbar system according to any of claims 4 to 9, characterized in that said derivation sector is formed of straight portions.

1 1. A cathode busbar system according to any of claims 1 to 10, further comprising two arms that extend between longitudinal parts of said ring busbar, underneath said shell, connecting said upstream longitudinal bars to the base of anode risers of the downstream cell.

12. A cathode busbar system according to claim 1 1 , characterized in that said arms are symmetric with respect to said median transversal plane (PY).

13. A cathode busbar system according to claim 1 1 , characterized in that said arms are asymmetric with respect to said median transversal plane (PY).

14. A cathode busbar system according to any of claims 11 to 13, characterized in that said arms are asymmetric with respect to said median longitudinal plane (PX).

15. Electrolytic cell of substantially rectangular shape suitable for the Hall-Heroult electrolysis process, comprising

a lateral lining defining together with the cathode a volume containing the liquid electrolyte and the liquid metal resulting from the Hall-Heroult electrolysis process,

a plurality of anode assemblies suspended above the cathode, each anode assembly comprising at least one 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 claims 1 to 14.

16. An aluminium electrolysis plant comprising at least one line (L1 , L2) of electrolysis cells (C1 , Cn) 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 line (L1 , L2), and preferably each electrolysis cell in said line, is an electrolysis cell according to claim 15.

17. 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 claim 16.