EP4158084A1

EP4158084A1 - Cathode assembly with metallic collector bar systems for electrolytic cell suitable for the hall-héroult process

Info

Publication number: EP4158084A1
Application number: EP21811764.6A
Authority: EP
Inventors: Bernard JONQUA; Alexander MUKHANOV; Mustafa MUSTAFA
Original assignee: Dubai Aluminium PJSC
Current assignee: Dubai Aluminium PJSC
Priority date: 2020-05-26
Filing date: 2021-05-25
Publication date: 2023-04-05
Also published as: GB202007782D0; CA3179811A1; WO2021240353A1; GB2595460A

Abstract

This cathode assembly (C) comprises a cathode body (1) made of a carbonaceous material, provided with at least one slot (17), said slot being provided with side walls (17A.17B) parallel to a longitudinal direction (A 17) of said slot, as well as at least one cathode collector bar system (3,5), also called bar system, said bar system being made of a metallic material, in particular made of copper, each bar system being at least partly received in a respective slot of said cathode body, each bar system being provided with so-called external side walls (36B.46B), which are in contact with said side walls of said slot. According to the invention, each bar system comprises two distinct bars, also called individual bars (30,40,50,60), each individual bar being provided with said external sidewall (36B,46B) in contact with said respective side wall (17A,17B) of said slot, each individual bar being also provided with a so-called internal side wall (35,45), said internal side walls of said individual bars being mutually adjacent, whereas maintaining means (8; 108) are provided, which are adapted to maintain said individual bars, so that said external side wall of each bar is firmly maintained in contact with a respective external side wall of said slot.

Description

CATHODE ASSEMBLY WITH METALLIC COLLECTOR BAR SYSTEMS FOR ELECTROLYTIC CELL SUITABLE FOR THE HALL-HEROULT PROCESS

Technical field of the invention

The invention relates to the field of fused salt electrolysis using the Hall-Heroult process for making aluminium. More specifically it relates to the improvement of the cathode blocks of such an electrolysis cell, the improvement being related to the cathode voltage drop and the current distribution along the cathode blocks. In particular, the invention relates to an improvement for cathode blocks provided with a cathode collector bar made from copper.

Prior art

The Hall-Heroult process is the only continuous industrial process for producing metallic aluminium from aluminium oxide. Aluminium oxide (AI₂O₃) is dissolved in molten cryolite (NasAIFe), 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 insulating panels and 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) wherein part of anodes is submerged into the liquid electrolyte contained in the volume defined by the cathode bottom and a side lining made from carbonaceous material. 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 4.40 V) which electrochemically reduces the aluminium oxide, split in the electrolyte into aluminium and oxygen ions, then into aluminium at the cathode and oxygen at the anode; said oxygen reacts 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 below the electrolyte from where it needs to be removed from time to time, usually by suction into a crucible (so-called “tapping” operation).

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 produced), and there has also been a trend to increase the size of the pots and the current intensity at which they are operated in order to increase the plant productivity and bring down the capital cost per unit of aluminium produced in the plant. Industrial electrolytic cells used for the Hall-Heroult process are generally rectangular in shape and connected electrically in series, the ends of the series being connected to the positive and negative poles of an electrical rectification and control substation. The general outline of these cells is known to a person skilled in the art and will not be repeated here in detail. They have a length usually comprised between 8 and 25 meters and a width usually comprised between 3 and 5 meters. The cells (also called “pots”) are always operated in series of several tens (up to more than four hundred) of pots (such a series being also called a “potline”); within each series DC currents flow from one cell to the neighbouring cell. The electrical currents in most modern electrolytic cells using the Hall-Heroult process exceed 200 kA and can reach 400 kA, 470 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 passage of these enormous current intensities through the electrolytic cell leads to ohmic losses at various locations of the pot and its environment.

Cathode assemblies for use in electrolytic cells suitable for the Hall-Heroult process are industrially manufactured for more than a century, and the state of the art is summarized in the reference book “Cathodes in Aluminium Electrolysis" by M. Sorlie and H. 0ye, 3^rd edition (Dusseldorf 2010). They comprise a cathode body made from a carbon material and one or more metallic cathode bars that are fitted into slots or grooves machined into the lower surface of said carbon body. Said metallic cathode bar protrudes out of each end of the cathode block, thereby allowing to connect the cathode assembly to the cathode busbar system. The metallic cathode bar is usually made from steel; copper inserts within the steel bar can be used in order to increase the electrical conductivity of the cathode bar. Said steel bars are inserted into grooves that are wider than the steel bars, and then fixed with electrically conductive glue (carbonaceous glue or cement, or ramming paste) or with cast iron that is poured into the interstitial space between the steel bar and the carbon body, as described in GB 663 763 (assigned to Compagnie de Produits Chimiques et Electrometallurgiques Alais, Froges & Camargue).

During the past decades, much effort has been devoted to the decrease of ohmic losses in cathode bars. Most inventions reported in prior art patents focus on the intrinsic conductivity of the steel cathode bar, or on the contact resistance between the cathode bar and the cathode block or between the cathode bar and the aluminium busbar.

A cathode with a full copper cathode bar inlaid into a groove machined in the lower surface of the carbon body of the cathode block is known from WO 2016/157021 (Dubai Aluminium PJSC). The contact between the carbon body and the copper bar is critical for the electrical performance of the electrolysis cell. Copper has a much higher thermal expansion coefficient than the carbon material of the cathode block body, and the copper bar in direct contact with the carbon body will operate at a temperature that is probably only about 100°C lower than its melting point, leading to significant thermal expansion. As a consequence, a well-defined allowance for thermal expansion must be provided when machining and installing the copper bars; otherwise strains and stresses caused by the expanding copper bar may lead to cracks in the carbon material of the cathode block. If no glue is used for accommodating dimensional tolerances, a very precise machining of the groove is required in order to ensure a good and reliable electrical contact between the copper bar and the carbon body over the whole length. Reliability of this contact is of paramount importance, because once installed into a cell and the cell started, a cathode block cannot be repaired, and cannot be replaced without relining of the whole cell. The normal lifetime of a cathode lining is comprised between five and seven years.

Usually, large carbon products such as cathodes for use in Hall-Heroult cells are machined with a tolerance of ± 2 mm; a tolerance of ± 1 mm can be reached, but at a high cost. The applicant has found that it is very difficult to get reliable contacts by inserting metallic bars, in particular copper bars, directly into grooves or slots machined into the carbon bodies without using glue.

In order to address this problem, the applicant has proposed to provide the cathode bar in the form of two distinct bar elements. This solution is described in WO 2018 /134754. The above-mentioned bar elements are provided with respective tapered walls that are likely to slightly glide the one with respect to the other, during their introduction into the slot. Once these bar elements are jammed against the side walls of the slot, this ensures a very satisfactory electrical contact. However this solution cannot be applied to large pots, in a simple manner. There are two reasons for this. First of all, large pots require cathode bars with a large cumulative cross section; a large cumulative cross section can be achieved either by increasing the number of cathode bars of a given cross section, or by increasing the cross section of each individual cathode bar. Increasing the number of cathode bars increases the manufacturing cost. The possibility to increase the cross section of each individual cathode bar is limited because such cross sections may not be available on the market.

Prior art has also proposed some other configurations, for what concerns the cathode bar. For example CN 10 839 6334 describes a cathode structure provided with a main longitudinal through groove, which accommodates several individual bars. Moreover said main groove leads into a series of local grooves, the length of which is far inferior. Each local short groove accommodates a claw head which cooperates with one of said individual bar. The main purpose of this document is to provide a cathode structure that reduces the horizontal current of the molten aluminium. However said cathode structure implies drawbacks, amongst which a mechanical complexity due in particular to the geometry of the network of grooves.

The problem addressed by the present invention is therefore to propose cathode bar systems with low voltage drop which can be used in large pots, while ensuring quality and reliability of the electrical contact between said metallic bars, in particular copper bars, and the walls of said slots. The present invention is also to provide cathode bar systems, which has a relatively simple structure, in particular for what concerns its geometry.

Objects of the invention

A first object of the invention is a cathode assembly (C) suitable for a Hall-Heroult electrolysis cell, comprising

- a cathode body (1) made of a carbonaceous material, said cathode body being provided with at least one slot (17), said slot being provided with side walls (17A,17E3) parallel to a longitudinal direction (A17) of said slot;

- at least one cathode collector bar system (3,5), also called bar system, said bar system being made of a metallic material, in particular made of copper, each bar system being at least partly received in a respective slot of said cathode body, each bar system being provided with so-called external side walls (36B.46B), which are in contact with said side walls of said slot; characterized in that said bar system comprises

- two distinct bars, also called individual bars (30,40,50,60), each individual bar being provided with said external sidewall (36B.46B) in contact with said respective side wall (17A.17B) of said slot, each individual bar being also provided with a so called internal side wall (35,45), said internal side walls of said individual bars being mutually adjacent,

- maintaining means (8; 108), which are adapted to maintain said individual bars, so that said external side wall of each bar is firmly maintained in contact with a respective external side wall of said slot.

According to an advantageous embodiment, the cathode assembly of the invention comprises two bar systems (3, 5), said bar systems being provided the one behind the other with reference with main axis A17 of said slot, whereas, for each bar system (3 and 5), individual bars (30, 40 and 50, 60) are provided side-by-side. In an embodiment one single slot is provided, the facing ends of said bar systems being mutually distant and forming an intercalary volume (19) of said single slot, said intercalary volume being advantageously filled with blankets of a ceramic material.

According to a typical embodiment each individual bar comprises an inside portion (30B) and an outside portion (30A), with reference to the inner volume of the slot. In this respect said external side walls (36B, 46B) are in particular provided on said inside portion (30B).

According to another embodiment said external side walls (36B 46B) and facing side walls (17A,17 B) of said slot show a slope, the value (a36,a46) of which is between 8 and 12 degrees, in particular of about 10 degrees, so as to retain said bar elements in the inner volume of said slot.

According to a particular advantageous embodiment of the invention, cross section of each individual bar is superior to 60 millimeters, in particular between 60 and 100 millimeters.

According to an advantageous embodiment, said maintaining means are spacing means provided between facing internal side walls of said individual bars. In this respect these maintaining means may comprise a maintaining member, in particular a shim (8; 108) formed by at least one insert, the length (L8) of said maintaining member being substantially equal to that of inside portion of individual bar. Said maintaining member is typically made of copper.

According to another advantageous embodiment of the invention, said cathode assembly further comprises immobilization means (85,86,87), adapted to mutually immobilize said two individual bars. In this respect said immobilization means may be provided in the inside portion of said bars. In an advantageous manner, said immobilization means are removable fixation means. In particular said removable fixation means may comprise a plate (85) as well as screws, at least one first screw (86) being screwed in a first individual bar (30) whereas at least one second screw (87) is fixed in the second individual bar (40).

According to another advantageous embodiment of the invention at least one side wall of at least one individual bar indirectly contacts an adjacent side wall of said slot, an intercalary material, in particular at least one graphite foil (70, 71), being interposed between said side wall of said bar and said adjacent side wall of said slot.

According to another embodiment, the cathode assembly of the invention further comprises connection means for connecting each individual bar with a bus bar. In this respect said connection means may comprise at least one flexible member (90,91 ,92) each intended to be connected to said cathode bus bar, as well as a transition member (95) attached to said flexible member, said transition member being connected to each individual bar.

According to a first variant, the cathode assembly may be further provided with at least a first intermediate plate (94) made of aluminium, permanently attached to said connection means, said cathode assembly further comprising means for a removable fixation of said first intermediate plate (94) on said cathodic bus bar (200).

According to another variant said connection means may comprise at least one flex (190,191) intended to be directly connected to said bus bar, as well as one transition tab (197) fixed on a respective individual bar. In this respect said tab may be fixed on a bolt (195, 196), said bolt being introduced into a hole (138,148) provided in the individual bar.

According to an advantageous embodiment of the invention, said cathode bar may be made of copper.

A second object of the invention is a process for making a cathode assembly (C) as above defined, comprising the steps of: providing a cathode body (1) made of a carbonaceous material; providing at least one slot (17) in said cathode body, said slot being provided with side walls (17A.17B) parallel to a longitudinal direction of said slot; providing at least two bar systems made of a metallic material, each bar system comprising two individual bars; placing said individual bars into the slot, with a side wall of each individual bar adjacent to a facing side wall (17A.17B) of the slot; urging said side wall of each individual bar against said facing side wall of said slot, thereby creating a clearance (80; 180) between facing internal walls of said individual bars; providing maintaining means and inserting said maintaining means (8; 108) into said clearance, in particular substantially along said longitudinal direction of said slot, so that said external side wall of each individual bar is maintained in contact with a respective external side wall of said slot.

According to an advantageous embodiment urging step may comprise placing two wedges (75,76) between facing internal walls of said individual bars, at opposite longitudinal ends of said bars. According to another advantageous embodiment, said process comprises providing a plurality of maintaining means having different widths, so as to adapt to different widths of said clearance.

According to another embodiment, said process comprises placing one first individual bar in its substantially final position, with respect to a longitudinal axis of the slot, and thereafter placing the other individual bar in its substantially final position, with respect to said longitudinal axis of the slot.

A third object of the invention is an electrolytic cell suitable for the Hall-Heroult electrolysis process, comprising a cathode forming the bottom of said electrolytic cell and comprising a plurality of parallel cathode assemblies, each cathode assembly (C) comprising a cathode body (1) and at least one metallic cathode collector bar system (3,5) protruding out of each of the two ends of the cathode, a lateral lining defining together with the cathode a volume containing the liquid electrolyte and the liquid metal resulting from the Hall-Heroult electrolysis process, an outer metallic potshell (202) containing said cathode and lateral lining, a plurality of anode assemblies suspended above the cathode, each anode assembly comprising at least one anode and at least one metallic anode rod connected to an anode beam, a cathodic bus bar (200) surrounding said potshell, said bus bar being connected to at least part of said cathode assemblies said electrolytic cell being characterized in that at least one of said cathode assemblies, and preferably more than 60% of said cathode assemblies and, more preferably, each of said cathode assemblies, is a cathode assembly as above defined.

According to an embodiment, when said electrolytic cell comprises a cathode assembly provided with at least said first intermediate plate (94) made of aluminium, permanently attached to said connection means, and when said cathode assembly further comprises means for a removable fixation of said first intermediate plate (94) on said cathodic bus bar (200), said cell comprises a main plate (205) secured to said bus bar, as well as removable fixation means between said main plate and said intermediate plate (94) of said cathode assembly.

A fourth object of the invention is an electrolytic cell for the production of aluminium by the Hall-Heroult process, comprising at least one cathode assembly as above defined. A fifth object of the invention is a process for making aluminium by the Hall-Heroult process, using an electrolytic cell provided with cathode assemblies as above defined.

Figures

Figures 1 to 24 represent two different embodiments of the present invention.

Figure 1 is a perspective view, showing one embodiment of a cathode assembly according to the invention.

Figure 2 is a perspective view, showing upside down the cathode assembly of figure 1. Figure 3 is a perspective view, analogous to figure 2, showing a cathode body which belongs to cathode assembly according to the invention, said figure 3 showing in particular a slot provided in said cathode body.

Figure 4 is a cross section along line IV-IV of figure 3.

Figure 5 is a top view, showing two collector bars which form a collector bar system which belongs to the cathode assembly according to the invention.

Figure 6 is a side view, showing one of the collector bars illustrated on figure 5.

Figures 7 and 8 are cross sections along lines VII-VII and VIII-VIII of figure 5.

Figure 9 is a top view, showing a first step of the insertion process of the collector bars of figure 5 into the slot of figure 3.

Figure 10 is a cross-section, analogous to figure 4, also showing this first step of figure 9. Figure 11 is a top view, analogous to figure 9, showing a second step of the insertion process of the collector bars into the slot, wherein the two collector bars are pushed against the sides of the slot by means of two steel wedges.

Figure 12 is a cross-section, analogous to figure 10, also showing the second step of figure 11.

Figures 13 and 14 are a top view and a cross-section, analogous respectively to figures 9 and 10, showing a third step of the insertion process of the collector bars into the slot. Figure 15 is a top view, analogous to figure 9, showing means for mutual fixing of the two collector bars.

Figure 16 is a top view, showing at greater scale the detail XVI of figure 15.

Figure 17 is a side view, showing the electric connection between a bus bar and the cathode assembly according to the invention.

Figure 18 is a top view along arrows XVIII of figure 17.

Figure 19 is a top view, analogous to figure 5, showing the two collector bars of a cathode assembly according to a second embodiment of the invention.

Figure 20 is a cross section along lines XX-XX of figure 19. Figures 21 and 22 are cross sections, analogous to figure 20, showing the insertion of bolts inside holes provided in the collector bars, for two different values of the spacing between these bars.

Figure 23 is a side view, analogous to figure 17, showing the electric connection between a bus bar and the cathode assembly according to this second embodiment.

Figure 24 is a top view along arrows XXIV of figure 23.

The following reference signs are used on the figures:

Detailed description

The present invention applies to cathodes used in the Hall-Heroult process that form the bottom of an electrolysis cell, said cathodes being assembled from individual cathode assemblies, each of which bears at least one cathode system according to the invention. The Hall-Heroult process and the outline of an electrolysis cell (also called “pot”) are known to a person skilled in the art and will not be described here in great detail. The cathode assembly of the invention is designated as a whole by alphanumeric reference C. It is suitable for a Hall-Heroult electrolysis cell, but could be used in other electrolytic processes.

In the present description, the terms “upper” and “lower” refer to a cathode block in the position of its industrial use, lying on a horizontal ground surface the “upper” surface thus being the surface intended to be in contact with the liquid aluminium in the electrolysis cell. Moreover, unless specified otherwise, “conductive” means “electrically conductive”. According to the terminology used in the art, a “cathode assembly” C comprises a cathode body 1 and at least one cathode bar system 3, 5. As will be explained here after in greater details, each bar system according to the invention is composed of two individual cathode collector bars.

The present invention is first applicable to cathode assemblies C comprising a cathode body 1 and at least two cathode bar systems 3, 5, one 3 of which is protruding out of the front wall 11 , the other 5 protruding out of the rear wall 12 of the cathode body 1. As will be explained hereafter in greater details, each bar system 3 and 5 comprises two individual bars, respectively 30 and 40 for system 3 and 50 and 60 for system 5. In each bar system, said individual bars respectively 30 and 40, as well as 50 and 60, are located side by side.

In the first embodiment of the invention, which is shown on the figures, these cathode bar systems incorporate half bars. In other words bars 30 and 50, which belong to different bar systems 3 and 5, are located the one behind the other so as to form a first “split bar". Moreover bars 40 and 60, which also belong to these different bar systems, are also located the one behind the other to form a second “split bar”. In other words each of said half bars 30, 40, 50 and 60 are not through bars, i.e. each cathode bar is not extending through the whole length of the cathode block, in a way known as such.

According to a not shown embodiment, the present invention is also applicable to cathode assemblies including at least one bar system, each formed of two individual through bars, instead of above defined split bars. The invention applies in particular to such through bars, which have a short length. However, the use of split bars is preferred, as there is practically no current flowing through the cathode block centre.

The invention will now be explained in relation with embodiments comprising two cathode bar systems 3 and 5 per cathode assembly C, said systems being located the one behind the other or side by side. As not shown variants, it is understood that the present invention can be applied to cathode assemblies C comprising any number of cathode bar systems arranged in a mutually parallel way. The cathode assembly C first comprises a cathode body 1 , of known type, which is made of a carbonaceous material, typically graphitized carbon or graphite. This cathode body 1, which has an elongated shape, has opposite end walls, i.e. front 11 and rear 12 walls, as well as peripheral walls. The latter are formed by parallel upper and lower walls 13 and 14, as well as parallel side walls 15 and 16. By way of example, its length L1 (see figure 1), i.e. the distance between walls 11 and 12, is between about 3500 millimetres (mm) and about 4000 mm. By way of example, its width W1 (see figure 1), i.e. the distance between walls 15 and 16, is between about 400 mm and about 650 mm. By way of example, its height H1 (see figure 1), i.e. the distance between walls 13 and 14, is between about 320 mm and about 580 mm.

As more clearly shown on figure 3, the lower wall 14 of cathode body 1 is provided with one single housing, which is formed by a longitudinal through slot 17, the longitudinal main axis of which is referenced A17. On this figure 3, as well as on figure 2 and 4, and as well as on figures 9 to 14, cathode body is shown “upside down”, respectively with and without its cathode bar systems 3 and 5, with reference to its above defined industrial use position.

Said single slot 17 is provided with opposite side walls 17A and 17B (see figure 3), parallel to said main axis A17, whereas its top wall is referenced 17C (see figure 3). As mentioned here above, said slot 17 is a through slot, namely its first end leads to front wall 11 of this cathode body, whereas its opposite end leads to rear wall 12 of said cathode body.

Advantageously, as illustrated in particular on figure 4, each side wall 17A and 17B shows a slope, the value of which is noted a17. Therefore the width of the slot 17 decreases from top wall 17C of this slot to lower wall 14 of cathode body. As will be described hereafter, these slopes make it possible to maintain the bar elements in the inner volume of the slot, when turning over the cathode assemblies.

By way of example, the following numerical values are given: the maximal width W17 (see figure 4) of slot 17, i.e. the distance between side walls at the level of top wall 17C, can be between about 130 mm and about 180 mm ; the minimal width W’17 (see figure 4) of slot 17, i.e. the distance between side walls at the level of lower wall 14, can be between about 115 mm and about 125 mm ; the depth D17 of slot 17 (see figure 4), i.e. the distance between top wall 17C and the surface of lower wall 14 of the body 1 , can be between about 65 mm and about 75 mm ; the angle a17 (see figure 4), as defined above, can be between about 8° and about 12

These values do not limit the scope of the present invention but are given only for the sake of illustration, namely to enable a person skilled in the art to carry out the invention.

As mentioned here above the cathode assembly C also comprises two cathode bar systems 3 and 5 (schematically shown on figure 1), each of which is accommodated in said slot 17. Each cathode bar system is made of a conductive material, typically able to conduct the current from the cathode to the exterior bus bar. Advantageously, the material of these cathode bars is copper. However, the invention encompasses cathode bars made of other materials, such as for example steel, or other materials usually installed inside the cathode assemblies.

The structure of individual bars 30 and 40 will now be explained, with reference to figures 5 to 8, bearing in mind that the structure of bars 50 and 60 is identical. The mechanical elements of bars 50, 60, which are analogous to those of bars 30 and 40, are given the same references increased by number 20.

Each individual bar 30 and 40, which has an elongated shape, has a main longitudinal axis respectively noted A30 and A40 (shown on figure 5). Said bar 30 or 40 is first provided with a so-called outside wall 31 and 41 (shown also on figure 5), which protrudes in use with respect to cathode body, as well as opposite inside wall 32 and 42 (shown also on figure 5). Each bar 30 and 40 is also provided with upper wall 33 and 43 (shown on figure 12), as well as lower wall 34 and 44 (shown also on figure 12), which extends parallel to a respective upper wall. In addition each bar 30 and 40 is provided with a so called internal sidewall 35 and 45, said side walls being mutually adjacent once individual bars have been introduced into slot 17. Each internal wall is vertical in use, in the sense that it is perpendicular to both upper and lower walls.

Finally each bar 30 and 40 has a so-called external side wall 36 and 46, each of which being intended to contact a respective side wall of the slot 17. For what concerns these walls 36 and 46, individual bars might be divided into two regions, namely a so-called outside region 30A and an inside region 30B. In use these regions are intended to extend respectively outside and inside slot 17. Along outside region 30A, walls 36 and 46 define zones 36A and 46A which are parallel to internal walls 35 and 45. On the contrary, along inside region 30B, said walls define zones 36B and 46B which are not perpendicular to the upper and lower walls. Indeed, they are tapered with respect to a vertical line, along angles which are respectively noted a36 and a46. Typically said angles are mutually identical, while being substantially equal to above defined angle a17. The transition between inside regions and outside regions define shoulders 30C and 40C, which have an abutment function as will be explained here after.

By way of example, each bar 30 and 40 may be manufactured from a not shown rough bar. In this respect, the latter has a rectangular shape. Then this rough bar is machined, according to any appropriate process, so as to form above explained tapered zones 36B and 46B. It is also to be noted that all the above walls are substantially straight and that two adjacent of said walls are linked by rounded portions 37 and 47.

By way of example, the following numerical value are given, with respect to individual bar

30:

- the global length L30 of said bar (see figure 2) can be between about 1800 mm and about 2100 mm;

- the length 130 of outside region 30A (see figure 6) can be between about 200 mm and about 400 mm ;

- the height H30 (see figure 9) of said bar, namely the distance between upper and lower walls, can be between about 60 mm and 100 mm ;

- the width W30 of said bar (see figure 9), namely the greatest distance between opposite side walls, can be between about 64 mm and about 100 mm ;

- the distance L19, along axis A17, of intercalary volume 19 between facing ends of respective bars 30,40 and respective bars 50, 60 (see figure 2) can be between about 400 mm and about 600 mm.

The insertion process of cathode bar system 3 into slot 17 will now be described, bearing in mind that insertion process of other cathode bar system 5 into said same slot 17 is identical. In a first not shown step, which is known as such, insulating paint is applied on the walls of slot 17. This paint is typically applied only onto a part of these walls, preferably in the vicinity of slot exit. The length of the thus applied paint layer is typically about 200 millimetres. This above step is known as such, so that it is not described in greater details.

According to a not shown embodiment, each side wall of cathode bars 30 and 40 may directly contact facing side walls of the slot 17, i.e. without any intercalary material. However, in the advantageous illustrated embodiment, side walls of said cathode bars 30 and 40 indirectly contact facing side walls of the slot. In this respect, with reference to figure 12, thin sheets 70 and 71 of an intercalary material are used. According to an advantageous embodiment of the invention, said intercalary material is a graphite foil, which can be a flexible graphite foil of compressed expanded graphite. Said foil is available from various suppliers under different trademarks, such as PAPYEX® by MERSEN. The density of the foil is typically 1 and it may have about 0.5 mm of thickness. In addition, graphite material is compressible to cope with the thermal expansion of bar elements.

Turning to figure 9 and 10 a first sheet 70 is first deposited against side wall of slot 17, before insertion of bar 40. Said sheet 70 slightly protrudes, with respect to top wall of cathode body. Individual bar 40 is then inserted into slot 17, once intercalary sheet 70 is positioned. Said bar is pushed with respect to the slot, along a motion F40 substantially parallel to axis A17. At the end of this pushing motion, shoulder 40C abuts against facing wall of cathode block; moreover, the protruding section of sheet 70 is typically cut flush with the cathode block.

It is to be noted that the above step is carried out with a cathode block upside down. In other words, during said insertion, access to slot 17 is permitted from the end of the cathode block groove, whereas so called lower wall of cathode body is in an upper position. Once inserted in the slot, bar 40 rests by gravity against wall 17C of this slot. The bar 40 is adjusted by its protrusion outside the cathode block slot, said protrusion forming the above defined outside region 30A; the length of the protruding section is typically comprised between about 300 mm and about 500 mm.

With reference to figures 11 and 12, second sheet 71 is then deposited against opposite side wall 17B of slot 17. Thereafter, individual bar 30 is pushed with respect to the slot, along a motion F30 which is also substantially parallel to axis A17. These steps, concerning sheet 71 and bar 30, are carried out the same way as above steps, concerning sheet 70 and bar 40.

In the present embodiment, one single intercalary graphite foil 70, 71 recovers a respective side wall of the slot. As a variant, one further graphite foil may recover top wall 17C of the slot. As another variant a further intercalary material, in particular at least one further graphite foil, may also be used. In any case the adjustment will be set, so that no substantial gap or space is left between individual bars, graphite foil and cathode body.

Typically two mechanical devices, known as such, are provided. With reference to figures 11 and 12, said mechanical devices are wedges 75, 76, typically made of steel. The first one 75 may be located at the inside end of the slot, with respect to its axis A17, whereas the other wedge 76 is provided at the opposite outside end of said slot. Both wedges are typically placed in the mid region of the bars, with reference to their vertical dimension in use. These wedges make it possible to urge the individual bars against side walls of the slot, along motions f30 and f40 substantially perpendicular to axis A17. Once these bars contact said side walls of the slot, their facing internal side walls 35 and 45 define a mid space or clearance 80.

According to the next step of manufacturing process, illustrated on figure 13 and 14, a maintaining member 8 is then inserted along said space 80. This member is constituted by a shim 8, which is formed by at least one insert, the latter being typically shaped as a plate. The shim 8 is typically made of steel or copper plates. In practice, the operators advantageously have in hand a set of several such inserts, the width thereof are slightly mutually different. This makes it possible to adapt the chosen number of inserts, as well as the width of each insert, to the actual width of the clearance. By way of example, the global width W8 of the thus formed shim 8 is typically between 1 and 4 millimetres.

Moreover the length L8 of each insert is chosen, so that this insert is substantially flush with the end of the slot, once placed in its final position. In other words, this length L8 is substantially equal to that of internal region of the bars 30 and 40, as illustrated in particular on figure 13. Once shim 8 is inserted between the bars, the above described wedges 75 and 76 are removed. Providing such a shim is a most particularly advantageous feature of the invention, since it makes it possible to maintain each bar 30 and 40 against a respective side wall of the slot. Therefore, a reliable electric contact is obtained between said bars and said side walls, through the two graphite foils 70 and 71.

The next manufacturing step, shown on figures 15 and 16, is mutual fixing of the individual bars 30 and 40. To this end a plate 85, typically made of steel, is placed substantially at the centre of the bars, with reference to their longitudinal axis. This plate 85 is immobilised with respect to both bar 30 and bar 40. In this respect, removable means are advantageously used, such as two screws 86 which are screwed into bar 30, as well as two further screws

87 which are screwed into bar 40. Mutual fixing of these bars makes it possible to avoid any movement between said bars, in particular during handling and installation of the cathode blocks.

According to the next step of the manufacturing process, which is advantageous but however optional, intercalary volume 19 is filled with a filling material. To this end, pieces

88 of ceramic blankets are deposited in this volume, as shown on figure 2. By way of example, the thickness of each blanket is about 25 mm. Typically, the number of superimposed blankets is between 3 and 5.

Once cathode system bars 3 and 5 are positioned and maintained against the side walls of the slot 17, the whole cathode assembly is turned upside down, so as to be in its final position of figure 1. This overturning step is carried out in a way known as such, using a not shown device. Due to the slopes of side walls of both the slots and the individual bars, as above described, these individual bars cannot escape from the slot due to gravity, so that they are firmly retained therein.

Cathode assembly according to the invention also comprises means, which enable electric connection between individual bar and a cathodic bus bar 200 shown on figures 17 and 18. In a way known as such, this bus bar 200 surrounds a pot shell 201 , which is partly illustrated on said figure. Bus bar is rectangular in shape and has two opposite longitudinal parts, as well as two opposite transversal parts. The above-mentioned connection means are shown on these figures fix a bug on is open under the form of three aluminium flexible connection members 90, 91, 92 also called “connectors”. Each flexible member, which is known as such and is for example an aluminium sheet or strip, extends between front wall of each individual cathode bar 30, 40 and one longitudinal part of cathodic bus bar.

In the present embodiment, a first end of each flexible member is attached to a first intermediate plate 94, made typically of aluminium. Moreover second end of each of these flexible members is attached to each individual cathode bar 30, 40, via a transition member 95. The latter is for example formed by a stack of copper plates, in a way known as such. As shown in particular on figure 18, outside extremities 30’ and 40’ of both individual bars 30 and 40 are attached to this single transition member 95. Moreover, in a way also known as such, intermediate plate 94 is bolted to a main aluminium plate 206, itself welded to the bus bar. The above mentioned connection means, were part of this first embodiment, are known as such. Therefore, they will not be described in greater details. By way of example, these connection means may be in accordance with the teaching of WO 2018 / 065 844 in the name of the present applicant.

Figures 19 to 24 describe a variant of the cathode assembly according to the invention. On these figures the mechanical members, which are analogous to those of the above first embodiment, are given the same references added with number 100.

This variant differs from the first embodiment, mainly for what concerns the connection of the collector bars 130,140 with peripheral bus bar. With respect to figures 19 and 20, each of said bars is drilled with a through hole 138 and 148, which extends along a substantial vertical direction in normal use. Each hole is provided in the outside region of the bar, in the vicinity of the free end thereof. By way of example, the diameters D138 and D148 of these holes are between about 30 mm and about 35 mm while being typically mutually equal. Moreover distances L138 and L148 between the free end of each bar and the centre of holes 138 and 148 is between about 50 mm and about 80 mm, while being typically mutually equal. As illustrated in particular on figure 20, it is to be noted that centre axis A138 and A148 each hole 138 and 148 is slightly offset with respect to main axis A130 and A140, opposite to the other hole.

This drilling step is typically carried out, at the same time as the machining step which makes it possible to provide the above slopes. This variant of figures 19 to 24 encompasses the other manufacturing steps which are described with reference to figures 9 to 14. This second embodiment also differs from the first one, for what concerns the connection to bus bar 200. In this respect, this second embodiment makes use of bolts 195,196, which are introduced into the above described holes 138,148.

As shown on figures 21 and 22, the cross dimension, or diameter D195,D196 of said bolts is slightly inferior to that D138,D148 of said holes. This makes it possible to adapt to various widths of the clearance, between internal side walls of the individual bars. On figure 21, this width D180 is maximal, so that bolts substantially contact internal walls 138’, 148’ of holes, namely their walls which are mutually adjacent. On the other hand, on figure 22, this width d180 is minimal, so that bolts substantially contact external walls 138”, 148” of holes, namely their walls which are mutually opposite. By way of example, the difference ( D138 - D195) or ( D148 - D196 ) between the respective diameters of one given hole and the bolt received therein, is between about 0.5 mm and about 2 mm. With reference to figures 23 and 24, tabs 197, typically made of copper, are used to tighten each bolt with steel plates. The electric connection itself is provided by flexes 190,191 , which are known as such. These flexes extend from above tabs 197 directly to bus bar 200, while being attached thereto, typically by welding. The upper face of outside extremities 130’ and 140’ of both individual bars 130 and 140 is attached to a first steel plate 198, whereas the lower face of these extremities is attached to a second steel plate 198’.

The invention brings about several advantages, with respect to above-described prior art. Indeed the amperage of current electrolytic pots tends to become higher and higher. In this respect, there is a need to use collector bars having high-dimension section. It might be considered to manufacture a single collector bar having such a section. However, in practice, such bars are not within the manufacturing capabilities of current suppliers.

With that in mind, the invention provides the use of two individual collector bars. In these conditions, the cost prices of these bars remains fully competitive, since each bar can be commonly manufactured by suppliers. In this respect it has to be noted that the slope of between 8 and 12 degrees, seen on one face of the copper bars in the above-described embodiment, can be obtained directly by the copper suppliers. Indeed the latter just have to use a special die, with the same shape as the copper bars, after machining. This process will decrease drastically the total cost of the copper collector bars of the entire pot.

Moreover, according to invention, the maintaining means make it possible to create a satisfactory electric contact between the adjacent walls of the bars and the slot.

The first embodiment, making use of connectors directly attached to the end of the individual bars, has specific advantages. It is in particular preferred in those cases where the welding between copper bars and copper part of the tri-clads can be carried out easily.

The second embodiment, making use of holes, tabs and flexes, has also specific advantages. It is in particular preferred in the cases, where the pot is installed in a place where the cathode collector bars are connected to the cathode bus bars through copper tabs or other tightening means using bolts.

Claims

1. A cathode assembly (C) suitable for a Hall-Heroult electrolysis cell, comprising

2. A cathode assembly according to claim 1 , comprising two bar systems (3, 5), said bar systems being provided the one behind the other with reference with main axis A17 of said slot, whereas, for each bar system (3 and 5), individual bars (30, 40 and 50, 60) are provided side-by-side.

3. A cathode assembly according to preceding claim, comprising one single slot, the facing ends of said bar systems being mutually distant and forming an intercalary volume (19) of said single slot, said intercalary volume being advantageously filled with blankets of a ceramic material.

4. A cathode assembly according to any preceding claim, wherein each individual bar comprises an inside portion (30B) and an outside portion (30A), with reference to the inner volume of the slot.

5. A cathode assembly according to preceding claim, wherein said external side walls (36B, 46B) are provided on said inside portion (30B).

6. A cathode assembly according to any preceding claim, wherein said external side walls (36B 46B) and facing side walls (17A,17 B) of said slot show a slope, the value (a36,a46) of which is between 8 and 12 degrees, in particular of about 10 degrees, so as to retain said bar elements in the inner volume of said slot.

7. A cathode assembly according to claim 1 , wherein cross section of each individual bar is superior to 60 millimeters, in particular between 60 and 100 millimeters.

8. A cathode assembly according to any preceding claim, wherein maintaining means are spacing means provided between facing internal side walls of said individual bars.

9. A cathode assembly according to claim 2, wherein maintaining means comprise a maintaining member, in particular a shim (8; 108) formed by at least one insert, the length (L8) of said maintaining member being substantially equal to that of inside portion of individual bar.

10. A cathode assembly according to claim 3, wherein said maintaining member is made of copper.

11. A cathode assembly according to any preceding claim, further comprising immobilization means (85,86,87), adapted to mutually immobilize said two individual bars.

12. A cathode assembly according to preceding claim, wherein said immobilization means are provided in the inside portion of said bars.

13. A cathode assembly according to preceding claim, wherein said immobilization means are removable fixation means.

14. A cathode assembly according to preceding claim, wherein said removable fixation means comprise a plate (85) as well as screws, at least one first screw (86) being screwed in a first individual bar (30) whereas at least one second screw (87) is fixed in the second individual bar (40).

15. A cathode assembly according to any preceding claim, wherein at least one side wall of at least one individual bar indirectly contacts an adjacent side wall of said slot, an intercalary material, in particular at least one graphite foil (70, 71), being interposed between said side wall of said bar and said adjacent side wall of said slot.

16. A cathode assembly according to any preceding claims, further comprising connection means for connecting each individual bar with a bus bar.

17. A cathode assembly according to preceding claim, wherein said connection means comprise at least one flexible member (90,91,92) each intended to be connected to said cathode bus bar, as well as a transition member (95) attached to said flexible member, said transition member being connected to each individual bar.

18. A cathode assembly according to preceding claim, characterized in that it is further provided with at least a first intermediate plate (94) made of aluminium, permanently attached to said connection means, said cathode assembly further comprising means for a removable fixation of said first intermediate plate (94) on said cathodic bus bar (200).

19. A cathode assembly according to claim 16, wherein said connection means comprise at least one flex (190,191) intended to be directly connected to said bus bar, as well as one transition tab (197) fixed on a respective individual bar.

20. A cathode assembly according to preceding claim, wherein said tab is fixed on a bolt (195, 196), said bolt being introduced into a hole (138,148) provided in the individual bar.

21. A cathode assembly according to any above claim, characterized in that said cathode bar is made of copper.

22. A process for making a cathode assembly (C) according to any of above claims, comprising the steps of: providing a cathode body (1) made of a carbonaceous material; providing at least one slot (17) in said cathode body, said slot being provided with side walls (17A.17B) parallel to a longitudinal direction of said slot; providing at least two bar systems made of a metallic material, each bar system comprising two individual bars; placing said individual bars into the slot, with a side wall of each individual bar adjacent to a facing side wall (17A.17B) of the slot; urging said side wall of each individual bar against said facing side wall of said slot, thereby creating a clearance (80; 180) between facing internal walls of said individual bars; providing maintaining means and inserting said maintaining means (8; 108) into said clearance, in particular substantially along said longitudinal direction of said slot, so that said external side wall of each individual bar is maintained in contact with a respective external side wall of said slot.

23. A process according to preceding claim, wherein urging step comprises placing two wedges (75,76) between facing internal walls of said individual bars, at opposite longitudinal ends of said bars.

24. A process according to claim 22 or 23, further comprising providing a plurality of maintaining means having different widths, so as to adapt to different widths of said clearance.

25. A process according to any of claims 20 to 24, comprising placing one first individual bar in its substantially final position, with respect to a longitudinal axis of the slot, and thereafter placing the other individual bar in its substantially final position, with respect to said longitudinal axis of the slot.

26. An electrolytic cell suitable for the Hall-Heroult electrolysis process, comprising a cathode forming the bottom of said electrolytic cell and comprising a plurality of parallel cathode assemblies, each cathode assembly (C) comprising a cathode body (1) and at least one metallic cathode collector bar system (3,5) protruding out of each of the two ends of the cathode, a lateral lining defining together with the cathode a volume containing the liquid electrolyte and the liquid metal resulting from the Hall-Heroult electrolysis process, an outer metallic potshell (202) containing said cathode and lateral lining, a plurality of anode assemblies suspended above the cathode, each anode assembly comprising at least one anode and at least one metallic anode rod connected to an anode beam, a cathodic bus bar (200) surrounding said potshell, said bus bar being connected to at least part of said cathode assemblies said electrolytic cell being characterized in that at least one of said cathode assemblies, and preferably more than 60% of said cathode assemblies and, more preferably, each of said cathode assemblies, is a cathode assembly according to any of claims 1 to 21.

27. Electrolytic cell according to preceding claim, comprising a cathode assembly according to claim 18, said cell comprising a main plate (205) secured to said bus bar, as well as removable fixation means between said main plate and said intermediate plate (94) of said cathode assembly.

28. Electrolytic cell for the production of aluminium by the Hall-Heroult process, comprising at least one cathode assembly according to any of claims 1 to 21.

29. A process for making aluminium by the Hall-Heroult process, using an electrolytic cell provided with cathode assemblies according to any of claims 1 to 21.