WO2017072618A1

WO2017072618A1 - Superstructure for electrolytic cell, comprising means for moving anode beam with respect to the frame of this superstructure

Info

Publication number: WO2017072618A1
Application number: PCT/IB2016/056118
Authority: WO
Inventors: Mark Jordan; Mahmood ABDULMALIK
Original assignee: Dubai Aluminium Pjsc
Priority date: 2015-10-28
Filing date: 2016-10-13
Publication date: 2017-05-04
Also published as: GB201519017D0; GB2543788A

Abstract

This superstructure comprises a fixed frame (4) of substantially rectangular shape, an anode beam (6) adapted to support anode rods (9) of said cell, said superstructure comprising moving means (1A-1D, 2A-2D, 40, 60, 61A-61D, 62A-62D), for moving said beam with respect to said frame along a substantially vertical direction, said moving means comprising at least one screw jack (1A-1D, 2A-2D) and drive means (40) for driving said screw jack, said screw jack comprising a body (10) fixedly attached on said frame, and an actuation rod (14) fixedly attached on said beam, said actuation rod being mobile with respect with said body along said substantially vertical direction, According to the invention,said moving means comprise at least two rows (R1, R2) of screw jacks, each row comprising at least two jacks and extending along a respective large side (L1, L2) of fixed frame, said drive means comprise at least one motor (40), said motor being adapted to drive all the jacks (1A-1D, 2A-2D) of at least one row of jacks and said motor being located between longitudinal end jacks (1A, 2A, 1D, 2D), said superstructure also comprises a transmission assembly (60, 61A-61D, 62A-62D), adapted to transmit the movement imparted by said single motor (40) to all the screw jacks of a given row.

Description

Superstructure for electrolytic cell, comprising means for moving anode beam with respect to the frame of this superstructure

Technical field of the invention

The invention relates to the field of fused salt electrolysis, and more precisely to a superstructure of an electrolytic cell suitable for the Hall-Heroult process for making aluminium by fused salt electrolysis. In particular, the invention relates to the moving means, which enables the displacement of a mobile anode beam of this superstructure, with respect to a fixed frame of the latter.

Prior art

The Hall-Heroult process is the only continuous industrial process for producing metallic aluminium from aluminium oxide. Aluminium oxide (Al₂0₃) is dissolved in molten cryolite (Na₃AIF₆), and the resulting mixture (typically at a temperature comprised between 940 °C and 970 °C) acts as a liquid electrolyte in an electrolytic cell. An electrolytic cell (also called "pot") used for the Hall-Heroult process typically comprises a steel shell (so-called potshell), a lining (comprising refractory bricks protecting said steel potshell against heat, and cathode blocks usually made from graphite, anthracite or a mixture of both), a superstructure and a plurality of anodes (usually made from carbon) that plunge into the liquid electrolyte. Anodes and cathodes are connected to external busbars. An electrical current is passed through the cell (typically at a voltage between 3.5 V and 5 V) which electrochemically reduces the aluminium oxide, split by the electrolyte into aluminium and oxygen ions, into aluminium at the cathode and oxygen at the anode; said oxygen reacting with the carbon of the anode to form carbon dioxide. The resulting metallic aluminium is not miscible with the liquid electrolyte, has a higher density than the liquid electrolyte and will thus accumulate as a liquid metal pad on the cathode surface from where it needs to be removed from time to time, usually by suction into a crucible.

The electrical energy is a major operational cost in the Hall-Heroult process. Capital cost is an important issue, too. Ever since the invention of the process at the end of the 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.

The superstructure of the cell comprises a fixed frame and a mobile metallic anode busbar, also called "anode beam", which extends at the outer periphery of the fixed frame. Each anode is provided with a metallic rod, for mechanical attachment and electrical connection to said anode beam.

Anodes are consumables in the Hall-Heroult process. In normal use, all the anodes are gradually consumed, at speeds which may slightly vary from one to the other. In order to keep the interelectrode spacing constant, it is therefore necessary to move each anode downwards with respect to the anode beam, independently from the movement of other anodes. To this end, each anode is provided with a connector for individual removable and adjustable fixation on the anode beam. Such connectors are described for example in WO 2006/11 1649 (E.C.L.) and EP 1 876 265 A2 (NKM).

In some cases, it is convenient to move all the anodes with respect to the fixed frame of the superstructure, in one single operation. All the anode rods being fixedly attached to the beam, the latter is moved vertically downwards or upwards with respect to the frame. To this end the superstructure is provided with mechanical moving means, comprising in particular jacks, adapted to impart a vertical upward or downward movement of the whole anode beam. During this vertical displacement of the anode beam the electrolytic cell remains in operation. The present invention is more particularly related to the above mentioned moving means. Several patents have been published which deal with prior constructive solutions, concerning the structure of these moving means. US 2006/0137972 discloses an actuator for displacing the anode beam with respect to the fixed frame, which comprises two mechanical jacks: one such jack is provided for each longitudinal side of the frame. This jack is characterized by specific numeric values, concerning the distance between the axis of the worm screw and the axis of the drive wheel, as well as the reduction ratio between this worm screw and this drive wheel. Each jack is connected to the anode beam by rods and levers, provided along a respective large side of the frame.

The solution proposed by US 2006/0137972 implies however some drawbacks. Thus, the force imparted by the jack is transmitted by above mentioned levers via a cam-like mechanism. Therefore, the value of the force actually transmitted, along a vertical direction, depends on each cam angle. In other words, the strokes may vary from one lever to another. Moreover, this solution implies the use of specific, i.e. non-standard jacks.

DE 10 200910 (VAW) describes a scissor lift jacking system with a motor placed at the short end of the cell; such scissor systems are subject to heavy wear. The present invention mainly focusses on the means, which make it possible to move the anode beam with respect to the frame of the superstructure.

These moving means should first be highly reliable, in a chemically and thermally aggressive environment altogether with high magnetic fields. In this respect, hydraulic devices should be avoided due to the risk of spill of hydraulic liquid, as any kind of liquid above a vessel containing liquid aluminium represents a potential safety hazard.

Moreover, these moving means should permit a very precise control of the force transmitted by the drive means. In this respect, a perfect positioning of the anode beam should be reached, during its movement with respect to the frame. The moving means should allow load reversal, which occurs when the anodes need to be pulled out of a partly solidified bath. Furthermore, these moving means should not be bulky, as available space in the neighborhood and above the cell is limited. Investment cost is a major issue, too. Simplicity of maintenance in such a hostile environment is also aimed by the present invention. Object of the invention

According to the invention, the problem has been solved by using a plurality of screw jacks on each long side of the cell, as well as at least one motor, adapted to drive all the jacks of at least one row of jacks. In these conditions, the moving assembly of the invention has several advantages over prior art systems. Thus, it allows the transmission of a controlled vertical force to each jack at the same time. Therefore, the strokes of all the jacks are substantially identical, so that the beam remains horizontal, during its movement. Said motor is located between longitudinal end jacks, which improves the whole quality of transmission, in particular in comparison with motor means located on the side of the cell. Moreover, the invention makes it possible to use jacks of a standard type. This permits to save global manufacture costs of the whole smelter.

A first object of the present invention is therefore a superstructure for an electrolytic cell, suitable for the Hall-Heroult electrolysis process, said superstructure comprising a fixed frame of substantially rectangular shape, an anode beam adapted to support anode rods of said cell, said superstructure comprising moving means for moving said beam with respect to said frame along a substantially vertical direction, said moving means comprising at least one screw jack and drive means for driving said screw jack, said screw jack comprising a body fixedly attached on said frame, and an actuation rod fixedly attached on said beam, said actuation rod being mobile with respect with said body along said substantially vertical direction, said superstructure being characterized in that said moving means comprise at least two rows of screw jacks, each row comprising at least two jacks and extending along a respective large side of fixed frame, said drive means comprise at least one motor, said motor being adapted to drive all the jacks of at least one row of jacks and said motor being located between longitudinal end jacks, said superstructure also comprises a transmission assembly adapted to transmit the movement imparted by said single motor to all the screw jacks of a given row.

According to a preferred embodiment, drive means comprise one single motor adapted to drive all the jacks of the two rows of jacks. This permits a better coordination and synchronicity of the whole transmission. Typically, said motor is located substantially at a midpoint between longitudinal end jacks.

Typically, the two rows have the same numbers of screw jacks. According to an advantageous embodiment, viewed from above, the arrangement of the screw jacks is symmetrical with respect to a median transversal axis of the frame. For example, each row has between 2 and 6 jacks, in particular between 3 and 5 jacks, preferably 4 jacks. According to an advantageous embodiment, transmission means comprise longitudinal shafts extending each between two adjacent jacks of one row. Transmission means may further comprise at least one transversal shaft, adapted to engage said longitudinal shafts. Advantageously each row is provided with two intermediate longitudinal shafts on either side of said transversal shaft, as well as with two end longitudinal shafts. Said transversal shaft may extend along an axis parallel to said median transversal axis of the frame, while being offset with respect to said median transversal axis. Transmission means may further comprise at least one primary bevel gearbox located between the two rows of jacks, as well as two secondary bevel gearboxes, each being located on a respective row of jacks.

According to an advantageous embodiment, the superstructure comprises at least one cover member extending over at least part of at least one transmission shaft. The body of said screw jack may be fixed on a wall of a protecting housing, which projects above the main top of fixed frame. Typically said housing has protective closed top and rear walls, as well as a front wall for fixation of the jack, this front wall defining an access to the jack, from the side of the frame.

Another object of the present invention is an electrolytic cell suitable for the Hall-Heroult electrolysis process, comprising a superstructure according to the present invention.

A further object of the present 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 cathodic bus bar 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.

Still a further object of the present 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 present invention.

Figures

Figures 1 to 10 represent various embodiments of the present invention.

Figure 1 is a schematic view, showing the global arrangement of an electrolytic Hall- Heroult electrolysis cell (pot) according to prior art. Figure 2 is a perspective view, showing a superstructure according to the invention, which has been mounted on an electrolysis cell such as that of figure 1.

Figures 3 and 4 are top and front views, showing the arrangement of jacks which equip the superstructure of figure 2

Figure 5 is a side view, at a larger scale than figures 3 and 4, showing the arrangement of jacks of these figures 3 and 4;

Figures 6 and 7 are perspective and front views, showing a jack of the superstructure, mounted on the walls of a housing;

Figure 8 is a side view showing a jack of the superstructure and its drive worm screw; Figure 9 is a front view showing a jack of the superstructure and its coupling with a transmission shaft;

Figure 10 is a front view showing the bottom end of a jack of the superstructure. The following reference numbers and letters are used on the figures: s Superstructure 4 Fixed frame of S

5 Top of 4 6 Anode beam

8 Hooks 9 Anode rod

L1.L2 Large sides of 4 S1.S2 Small sides of 4

R1.R2 Rows of jacks Y-Y Transversal axis of 4

1A-1 D Jacks of row 1 2A-2D Jacks of row 2

L,L',LT Distances between jacks

10 Body of jack 12 Sleeve of jack

13 Drive wheel of jack 14 Rod of jack

15 Worm screw A15 Axis of 15

RA Rotation axis of jack 16 Lower clevis of jack

16' Bore in 16 17 Upper clevis of jack

17' Bore in 17

30 Housing 31 Recess in 5

32 Wings of 30 33 Rear wall of 30

34 Top wall of 30 35 Front wall of 30

36 Bore in 35 37 Fixation means

38 Access

40 Motor 42 Gearbox

44 Coffer of 40 45 Opening in 40

46 Output pin of 40 50 Primary bevel gearbox

51 ,52 Secondary bevel gearboxes 51 ', 52' Coffers of 51 ,52

60 Primary shaft A60 Axis of 60

61 ,62 Secondary shafts 61 ',61" Pins of 61

15' Pin of 15 70 Coupling

72 Bearing 80-82 Covers Detailed description

An aluminium smelter comprises a plurality of electrolytic cells arranged the one behind the other (and side by side), typically along two parallel lines. These cells are electrically connected in series by means of conductors, so that electrolysis current passes from one cell to the next. The number of cells in a series is typically comprised between 50 and over 100, but this figure is not substantial for the present invention. The cells are arranged transversally in reference of main direction of the line they constitute. In other words the main dimension, or length, of each cell is substantially orthogonal to the main direction of a respective line, i.e. the circulation direction of current.

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".

Figure 1 describes a typical arrangement of a Hall-Heroult electrolysis cell known from prior art, which the present invention aims to improve. Each electrolytic cell substantially comprises a potshell, a superstructure and a plurality of anodes. This superstructure is referenced in whole as S. It comprises a fixed frame 4 and a mobile metallic anode frame 6, hereafter called "anode beam", which extends to the outer periphery of the fixed frame 4 (see figure 2). Each anode A, not shown on figure 2 but only on figure 1 , is provided with a metallic rod 9 for mechanical attachment and electrical connection to the anode beam. On this figure 1 , beam 4 is provided with pairs of hooks 8, not shown on figure 2, adapted to facilitate the attachment of the anode rods in the usual way.

The general structure of a Hall-Heroult electrolysis pot is known per se and will not be explained here. It is sufficient to explain that the current is fed into the anode beam, flows from the anode beam to the plurality of anode rods and to the anodes in contact with the liquid electrolyte where the electrolytic reaction takes place. Then the current crosses the liquid metal pad resulting from the process and eventually will be collected at the cathode block.

The present invention is more particularly directed to the means for moving the anode beam with respect to the fixed frame. These means are hereafter called moving means. In the present embodiment, they generally comprise drive means, several screw jacks, as well as transmission means, adapted for transmitting the movement imparted by the motor to the different jacks. In the present embodiment, drive means comprise one single motor, whereas transmission means comprise several shafts and gearboxes. Figure 3 shows, viewed from above, the arrangement of the jacks on the flat top 5 of the fixed frame 4. On this figure, the housings and covers, which are associated to these jacks and will be described more in detail, are not shown. In a way known as such, the fixed frame is rectangular, its large sides are noted L1 and L2 whereas its small sides are noted S1 and S2. There are two rows R1 and R2 of jacks, which are provided along a respective large side of the frame.

In the illustrated example, each row is formed from 4 jacks, i.e. 1 A to 1 D, as well as 2A to 2D. However, a different number of jacks can be provided, such as two or three or five or six in each row. It is preferred to have two rows formed by the same number of jacks. More generally, it is preferred if the whole arrangement of these jacks is geometrically symmetric with respect to the main transversal axis Y-Y of the frame 4.

The structure of one jack, for example 1A, is known as such. Therefore, it will be briefly described hereafter, bearing in mind that the structure of all the jacks is substantially identical. Jack 1A essentially comprises a casing or body 10, which defines a sleeve 12 wherein an actuation rod 14 may move in translation along vertical axis. To this end, the jack is provided with a wheel 13 (see figure 8), which can drive into rotation, a not shown drive screw. Let us note, on figures 3 and 8, RA the rotation axis of all above rotating elements of the jack 1 A.

The rotation of said wheel is imparted by a worm screw 15, the axis of which is noted A15 on figure 8. The main axis A1 and A2 of each row R1 and R2 passes through the longitudinal axes of the worm screws of one given row. Rod 14 is threaded so that the rotational movement of drive screw provokes a translational movement of this rod 14. At its lower end, this rod 14 is equipped with a clevis 16 drilled with a bore 16', for attachment of the anode beam 6. Moreover, at its upper end, casing 10 is equipped with a further clevis 17 drilled with a bore 17', for attachment on a housing 30.

As shown in particular on figures 6 and 7, housing 30 upwardly projects from the periphery of a recess 31 , provided in the top 5 of frame 4. This housing comprises lateral wings 32, which extends from said top 5, as well as rear wall 33 and top wall 34, both extending between these wings. Moreover, a front wall 35 extends downwards from this top 34, but only over a slight part of the whole height of this housing. Front wall 35 is drilled with a hole 36, for fixation of the upper clevis 17, thanks to fixation means 37, of any appropriate type.

Once it is mounted on housing 30, the upper part of the jack is received in the inner volume of the housing, its sleeve extends through the recess 31 and its lower clevis is located under top 5. Top and rear walls of the housing 30 ensure the protection of the jack, whereas an operator may have an access 38 to this jack from the side of the frame, in particular for lubrication purposes. The motor, which is designated as a whole with reference 40, is of a type known as such. Preferably, an electric rotary motor is used, associated with a gearbox 42. This motor and its gearbox are received in a protecting coffer 44, which is drilled with an opening 45, for permitting access to an operator from the side of the frame 4. Single motor 40 is located between the longitudinal end jacks of each row, i.e. 1A, 2A and 1 D, 2D. This motor is provided substantially at a midpoint of the rows, i.e. in the vicinity of the middle of these rows, according longitudinal dimension.

The transmission means of the invention will now be described. The output pin 46 of the motor 40 drives a first so called primary bevel gearbox 50, also received in coffer 44 (see figure 2), which is placed between the two rows of jacks, but closer to the first one. This primary gearbox engages with two secondary gearboxes 51 and 52, each of which is located in a respective coffer 51' and 52' (see figure 2), on a respective row of jacks. Gearbox 50 directly engages with gearbox 51 , i.e. without any intermediate shaft, whereas it engages with other gearbox 52 via a so called primary intermediate shaft 60.

The latter extends transversally along an axis A60, which is parallel to main axis Y-Y but which is slightly remote therefrom. In other words, shaft 60 and motor 40 are in a globally central location, with respect to longitudinal direction of the whole frame, which permits a satisfactory transmission to the different jacks. However, there is an offset OFF (see figure 3) between A60 and Y-Y.

Each secondary gearbox 51 or 52 engages with all the worm screws of a respective row R1 or R2 of jacks. To this end, one given gearbox is first connected, on each side, with respective longitudinal intermediate shafts 61 B, 61 C, 62B, 62C, which drive worm screws of intermediate jacks 1 B, 1C, 2B, 2C. The rotating movement of these intermediate screws then drive longitudinal end shafts 61 A, 61 D, 62A, 62D, which drive worm screws of end jacks 1 A, 1 D, 2A, 2D. Figure 9 shows a typical connection between one shaft 61A and the facing worm screw 15. The facing pins 61 ' and 15' of the shaft and screw are linked through a coupling 70, of any appropriate type. Other couplings 70, which are provided between other worm screws and shafts, are referenced on global top view of figure 3. Detailed figure 9 also illustrates the other pin 15" of screw 15, which does not cooperate with a further shaft. This free pin 15" is mounted on a bearing 72, fixedly attached on the top 5 of the frame 4.

Transversal shaft and longitudinal shafts are protected by covers 80, 81A-81 D and 82A- 82D (see figures 2, 6 and 7). Each cover, which has a length similar to that of the corresponding shaft, is U or V shaped, so that it is opened towards the bottom. It is fixed on a respective housing or coffer, by any appropriate means.

In operation, when the motor is actuated in a first rotation direction, it drives the worm screws of all the jacks in the same direction, thanks to the transmission means, i.e. bevel gearboxes and shafts. Therefore, mobile rods of all the jacks are moved in the same translation direction, i.e. upwards or downwards. Then, if the motor is actuated in the opposite rotation direction, mobile rods of all the jacks are moved in the same opposite translation direction, i.e. respectively downwards or upwards. In a specific embodiment for the anode frame of a Hall-Heroult electrolysis cell designed to operate at a current comprised between 400 and 460 kA, the moving means comprised eight identical screw jacks and one electric motor as well as gearboxes, drive shafts and couplings, and appropriate hardware to interconnect the system. The motor was located substantially at a midpoint between longitudinal end jacks. Jacks were self-locking. Each jack had one easily accessible greasing point, allowing greasing without removal from the superstructure. The maximum total suspended load was comprised between 60 and 70 metric tons, and the moving means were designed to withstand a maximum total suspended load comprised between 80 and 100 metric tons. At normal load the design lifting speed was comprised between 30 and 150 mm/min, and preferably between 60 and 120 mm/min, all jacks working synchronously. Inner jacks carry each about 13 to 15% of the total load, outer jacks each about 10 to 12% of the total load. The working stroke was comprised between 210 mm and 300 mm, preferably between 225 mm and 280 mm. The superstructure comprised mechanical stops that prevent over travel of the screws when lowering or raising. The motor and gearbox unit used a helical bevel configuration placing the motor shaft perpendicular to the gearbox output shaft; this provides acceptable clearance to other equipment on the superstructure. Primary and secondary shafts were protected against dust by a housing. The superstructure according to the invention has several advantages. Its moving means operate well at high temperatures that may occasionally reach 100°C (during pot start up or anode effects), in a heavily dust-laden environment containing dust particles that are abrasive (alumina) and corrosive (alumina containing HF; AIF₃), and in the presence of magnetic fields (that can reach about 50 to 100 mT at the superstructure level). Said moving means are extremely reliable and are assembled from standard components. In particular the jacks can all be of the same type, even if the load is not distributed evenly across the jacks of a given superstructure. Load reversal is possible. The precision of the vertical movement of the anode beam can be better than ±2 mm, and even better than ±1 mm with respect to a fixed point, under normal load. Lifting speed can be controlled and varied easily, especially if the whole system is driven by one single electrical motor.

Claims

1. A superstructure (S) for an electrolytic cell, suitable for the Hall-Heroult electrolysis process, said superstructure comprising a fixed frame (4) of substantially rectangular shape, an anode beam (6) adapted to support anode rods (9) of said cell, said superstructure comprising moving means (1A-1 D, 2A-2D, 40, 60, 61A-61 D, 62A-62D), for moving said beam with respect to said frame along a substantially vertical direction, said moving means comprising at least one screw jack (1A-1 D, 2A-2D) and drive means (40) for driving said screw jack,

said screw jack comprising a body (10) fixedly attached on said frame, and an actuation rod (14) fixedly attached on said beam, said actuation rod being mobile with respect with said body along said substantially vertical direction, said superstructure being characterized in that said moving means comprise at least two rows (R1 , R2) of screw jacks, each row comprising at least two jacks and extending along a respective large side (L1 , L2) of fixed frame, said drive means comprise at least one motor (40), said motor being adapted to drive all the jacks (1A-1 D, 2A-2D) of at least one row of jacks and said motor being located between longitudinal end jacks (1A, 2A, 1 D, 2D), said superstructure also comprises a transmission assembly (60, 61A-61 D, 62A- 62D), adapted to transmit the movement imparted by said motor (40) to all the screw jacks of a given row.

2. A superstructure according to claim 1 , characterized in that drive means comprise one motor (40) adapted to drive all the jacks (1A-1 D, 2A-2D) of the two rows of jacks.

3. A superstructure according to claim 1 or 2, characterized in that said motor (40) is located substantially at a midpoint between longitudinal end jacks (1A, 2A, 1 D, 2D),

4. A superstructure according to claim any of claims 1 to 3, characterized in that the two rows (R1 , R2) have the same numbers of screw jacks.

5. A superstructure according to any of claims 1 to 4, characterized in that, viewed from above, the arrangement of the screw jacks is symmetrical with respect to a median transversal axis (Y-Y) of the frame.

6. A superstructure according to any of claims 1 to 5, characterized in that each row has between 2 and 6 jacks, in particular between 3 and 5 jacks, preferably 4 jacks.

7. A superstructure according to any of claims 1 to 6, characterized in that transmission means comprise longitudinal shafts (61A-61 D, 62A-62D) extending each between two adjacent jacks of one row.

8. A superstructure according to claim 7, characterized in that transmission means further comprise at least one transversal shaft (60), adapted to engage said longitudinal shafts.

9. A superstructure according to claim 8, characterized in that each row is provided with two intermediate longitudinal shafts (61 B, 61C, 62B, 62C) on either side of said transversal shaft, as well as with two end longitudinal shafts (61 A, 61 D, 62A, 62D).

10. A superstructure according to any of claims 7 to 9, characterized in that said transversal shaft (60) extends along an axis (A60) parallel to said median transversal axis (Y-Y) of the frame, while being offset with respect to said median transversal axis.

1 1. A superstructure according to any of claims 7 to 10, characterized in that it comprises at least one cover member (80, 81 A-81 D, 82A-82D) extending over at least part of at least one transmission shaft (60, 61A-61 D, 62A-62D),.

12. A superstructure according to any of claims 1 to 1 1 , characterized in that transmission means further comprise at least one primary bevel gearbox (50), located between the two rows of jacks, as well as two secondary bevel gearboxes (51 , 52), each being located on a respective row of jacks.

13. A superstructure according to any of claims 1 to 12, characterized in that the body (10) of said screw jack is fixed on a wall (35) of a protecting housing (30), which projects above the main top (5) of fixed frame (4).

14. A superstructure according to claim 13, characterized in that said housing (30) has protective closed top (34) and rear (33) walls, as well as a front wall (35) for fixation of the jack, this front wall defining an access (38) to the jack, from the side of the frame.

15. An electrolytic cell suitable for the Hall-Heroult electrolysis process, comprising a superstructure according to any of previous claims.

16. 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 cathodic busbar of a cell to the anode beam of a downstream cell,

characterized in that more than 80% of the electrolysis cells in at least one of said line, and preferably each electrolysis cell of said line, is an electrolysis cell according to claim 15.

17. 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 claim 16.