GB2542150A - Cathode assembly for electrolytic cell suitable for the Hall-Héroult process - Google Patents

Abstract

A cathode assembly for electrolytic cell suitable for the Hall-Héroult process comprises a cathode body 1 made of a carbonaceous material, and at least one cathode bar 3 made of a first conductive material. The cathode bar 3 is fitted in a groove provided in said cathode body. At least one insert 5 made of a second conductive material, having a higher electrical conductivity than that of said first conductive material, is fitted in a slot or bore provided in said cathode bar 3. The assembly is characterised by, said insert 5 having at least one first so-called electrically conductive region and at least one second so-called electrically non-conductive region L5C, where the electrically conductive contact peripheral length of said insert with said cathode body and/or said cathode bar is superior in the first region than in the second region. The non-conductive region L5C can be formed by a local cross-sectional restriction of the insert 5 or by a local widening of the cross section of the slot or bore of the cathode bar 3. Also claimed is a process for making the cathode assembly; an electrolytic cell for the production of aluminium including the cathode assembly and a process for making aluminium including the cathode assembly.

Description

Cathode assembly for electrolytic cell suitable for the Hall-Heroult process Technical field of the invention

The invention relates to an improvement of an electrolysis cell (also called “pot”) for producing aluminium by fused salt electrolysis using the Hall-Heroult-process. More precisely it relates to a cathode assembly for such electrolytic cell allowing to decrease the cathode voltage drop and modify the current distribution along the cathode assemblies in a desirable way.

In particular, the invention relates to a cathode assembly in which the electrical contact between the cathode material and the busbar to which the cathode is connected involves a copper bar.

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, a lining usually made from refractory bricks, a cathode usually covering the whole bottom of the pot (and which is 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 3.8 V to 5 V) which splits the aluminium oxide in aluminium ions and oxygen ions. The oxide ions are reduced to oxygen at the anode, said oxygen reacting with the carbon of the anode. The aluminium ions move to the cathode where they accept electrons supplied by the cathode; 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.

The electrical energy is the main operational cost in the Hall-Heroult process. Capital cost is an important issue, too. Ever since the invention of the process at the end of the 19th century much effort has been undertaken to improve the energy efficiency (expressed in kW/h per kg or ton of aluminium), and there has also be 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.

Electrolytic cells presently used for the Hall-Heroult process are rectangular and have a length usually comprised between 8 and 20 meters and a width usually comprised between 3 and 5 meters. Most newly installed pots operate at a current intensity comprised between about 400 kA and 600 kA. They are always operated in series of several tens (up to more than a hundred) pots; within each series DC currents flow from one cell to the neighbouring cell. Much effort is still being made to optimise the process in order to increase its energy efficiency.

The passage of the enormous current intensities through the electrolytic cell leads to ohmic losses at various locations of the pot. Aluminium conductors are used for the busbar systems for both anodes and cathodes. However, aluminium cannot be used in direct contact with the cathode blocks due to its low melting point (about 660°C for pure aluminium). As a consequence, steel bars are conventionally used for ensuring electrical contact with the cathode blocks; these so-called cathode bars are connected to cathode busbars (made from aluminium) by welded and/or bolted connectors. Cathode bars are typically fitted into slots machined into the lower surface of the cathode block. Electrical contact between the steel bar and the carbon material of the cathode block can be direct, or the steel bar can be embedded in cast iron.

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.

The increase in the electrical conductivity of the cathode bars implies the use of a material having a higher electrical conductivity than steel bars. All reported solutions imply the use of inserts made from a material with a higher electrical conductivity into the cathode bar, which is usually made from steel. The material with a higher electrical conductivity is usually copper. Typical solutions comprise a copper rod or bar that is inserted into a groove or slot machined into the steel cathode bar, over all or part of the length of said cathode bar. The basic concept of a copper insert fitted into a slot or groove machined in a steel cathode bar is described in WO 2001/63014 (Comalco), WO 2005/098093 (Aluminium Pechiney) and WO 2009/055844 (BHB Billington). FR 1 161 632 (Pechiney) discloses a copper insert fitted into a groove machined in a carbon cathode block using cast iron as a sealing material. The composition of cast iron used for sealing cathode bars into the grooves of carbon cathodes is known to be critical (see US 2,953,751 assigned to Pechiney), because the cast iron should not undergo any swelling due to structural transformations, as swelling could cause the carbon material to develop cracks. A large number of more specific embodiments have been described for these copper inserts, such as: A copper bar with circular cross section fitted into a steel bar with outer rectangular cross section and an inner “U” section, the “U” section being closed by a block, see US 3,551,319 (Kaiser); a copper bar welded to a lateral face of a steel bar, see US 3,846,388 (Pechiney); a copper bar with rectangular cross section inserted into a steel tube with rectangular cross sections, see US 5,976,333 (Alcoa); a copper bar with circular cross section inserted into a steel tube with rectangular external cross section and a bore with circular cross section, see WO 2005/098093 (Aluminium Pechiney).

Several documents disclose the use of a joint material present between the cathode material and the steel bar: WO 2013/039893 (Alcoa) describes the use of a copper insert as a joint, WO 2007/071392 (SGL Carbon) describes the use of sheets made from expanded graphite, and RU 2285764 describes the use of a carbonaceous paste. Such a joint material may improve the electrical contact between the carbon block and the steel bar. RU 2285754 proposes to secure the copper bar inserted into the slot of the steel bar by welded-on steel plates while allowing for a narrow cavity between the copper insert and the steel bar, i.e. the section of the copper insert is somewhat smaller than that of the groove into which it is fitted. The opposite approach is taken by WO 2009/055844 describing the use of roll bonding or explosion bonding in order to obtain an excellent contact between the copper insert and the steel bar over the whole length of the insert.

Another problem addressed by many inventions is the connection between the copper insert and the steel cathode bar. This contact is critical for at least three reasons: the electrical contact between the copper insert and the cathode bar should be as good as possible; the thermal expansion coefficients of steel and copper are rather different and may lead to dimensional variations during the start-up of the pot; and the thermal conductivity of copper and steel is rather different, which needs to be taken into account for designing (and minimising) the heat transfer between the pot and the aluminium busbar. For this reason, in some prior art embodiments the copper inserts do not extend along the whole length of the steel cathode bar, but a spacer section is provided at each end of the cathode bar into which the copper insert does not extend. Also, the copper bar can be made in two pieces separated in the centre of the cathode by a steel plug and/or an air gap. Such a structure is described in US 6,387,237 and US 6,231,745 (Alcoa). The opposite approach is proposed by WO 2002/42525 (Servico), namely a cathode bar comprising a steel bar into which at each end a copper bar is inserted, the copper insert extending beyond the end of the steel bar and ensuring the electrical contact with the connection to the aluminium busbar.

As can be seen, there has been a wealth of different designs of copper inserts in cathode bars.

In addition to the goal to decrease ohmic losses, the insertion of copper bars of complex shape into cathode bars has been used to fine-tune the current distribution over the length of the cathode bar. In fact, changes to the properties of cathode blocks have led to the emergence of new problems such as, for example, erosion of cathodes. For example, it has been observed that as the graphite content of cathode blocks increases, a block becomes more sensitive to erosion problems at the head of the block. In fact, the current density is not distributed uniformly over the entire width of the pot, and there is a peak current density at each end of the block, on the surface of the cathode. This peak current density causes local erosion of the cathode due to magnetohydrodynamic effects which are related to high magnetic fields and lead to stirring of the molten aluminium and generalized and/or localized wear on the cathode surface that is in contact with the molten metal pad.; this is particularly marked when the block is rich in graphite. Such local erosion can limit the lifetime of the cathode. These phenomena are well known in the art (see for a review: Sorlie and 0ye, “Cathodes in Aluminium Electrolysis”, 3rd editition (2010), in particular p.205-209 and p.527-550). In order to decrease this effect, WO 2005/98093 describes the presence of an unsealed zone at the extremity between the cathode bar and the cathode material. A similar solution is proposed in WO 2004/031452 (ALCAN) using embedding spacers. Fine tuning of the electrical conductivity of cathode blocks parallel to their length by using specially designed cathode bars with copper inserts can therefore decrease the localised cathode erosion; some of the cited documents also address the influence of thermal losses though cathode bars on magnetohydrodynamic effects, knowing that the use of copper inserts tends to lead to an increase of the thermal conductivity of the cathode bar. WO 2004/059039 (SGL) describes cathode assembly in which a marginal zone of the cathode block facing the collector that has a higher electrical resistance parallel to the length of the cathode block than in the centre of the cathode block; this goal is obtained by using copper inserts or plates with different thickness over different portions of the length of the steel bar, in conjunction with electrically insulating layers between the copper bar and the steel bar. These cathode structures are rather difficult to manufacture and to assemble. A similar effect is achieved in a simpler way by a copper insert that ends at a specified distance from the outer face of the cathode block, as described in WO 2008/062318 (Rio Tinto Alcan). Another system aiming to achieve the same effect is WO 03/014423 (Alcoa): it uses several parallel steel cathode bars in conjunction with electrically insulating layers. US 6,231,745 (Alcoa) describes a half-length copper insert that extends somewhat out of the cathode block, but does not extend outside the cell wall.

While these cathode bar systems allow some fine-tuning of electrical conductivity at the outer face of the cathode assembly, they are rather complex to manufacture, and do not offer much flexibility as to the conductivity profile than can be achieved over the length of the cathode block.

It is the objective of the present invention to come up with a new design of cathode bars that decreases ohmic losses and that allows flexible fine tuning of electrical conductivity of the cathode assembly along its length, and that is simple to manufacture.

Object of the invention

According to the invention, the problem is solved by a cathode assembly suitable for a Hall-Heroult electrolysis cell, comprising - a cathode body made of a carbonaceous material; - at least one cathode bar made of a first conductive material, said cathode bar being fitted in a groove provided in said cathode body; - at least an insert made of a second conductive material, having a higher electrical conductivity than that of said first conductive material, said insert being fitted in a slot provided in said cathode bar; characterized in that said insert has at least one first so-called electrically conductive region and at least one second so-called electrically non-conductive region, the electrically conductive contact peripheral length of said insert with said cathode body and/or said cathode bar being superior in the first region than in the second region.

This cathode assembly is the first object of the present invention.

In an embodiment, in the non-conductive region, peripheral walls of said insert define a functional clearance with the facing walls of said cathode body and/or said cathode bar, said functional clearance being filled with a solid non-conductive material or with air.

Said electrically non-conductive region is placed inside the groove in such a way that it starts at the edge of the front wall and/or rear wall of cathode body and extends towards the centre of the cathode body.

In an embodiment, the non-conductive region extends over a part of the axial length of this insert, which is comprised between 6 and 20%, in particular between 9 and 15% of the whole axial length of this insert.

In another embodiment, the slot or bore in cathode bar has a substantially constant cross section over its axial length and the non-conductive region is defined by a local restriction of the cross section of the insert.

In yet another embodiment, the insert has a substantially constant cross section over its axial length and the non-conductive region is defined by a local widening of the cross section of the slot or bore of cathode bar.

In yet another embodiment, both the slot or bore in the cathode bar and the insert have a polygonal cross section, and at least one peripheral wall of said insert is remote from facing walls of the slot or bore in the non-conductive region.

In yet another embodiment both the slot or bore in the cathode bar and the insert have a circular cross section, and peripheral wall of said insert is remote from facing wall of the slot or bore in the non-conductive region, so as to define an annular gap there between.

In all embodiments, advantageously, the axial length of said second so called non-conductive region is between 50 mm and 250 mm, in particular between 100 and 200 mm.

In another embodiment, the insert and the slot are polygonal in cross-section, the number of walls of the insert, contacting the cathode bar, being superior in the conductive region(s) than in the non-conductive region(s).

Another object of the invention is a process for making a cathode assembly according to the invention, comprising the steps of (a) providing a cathode body made of a carbonaceous material; (b) providing a groove or bore in said cathode body; (c) providing at least one cathode bar made of a first conductive material, (d) providing a slot or bore in said cathode body, (e) providing at least one insert made of a second conductive material, having a higher electrical conductivity than that of said first conductive material, said insert being fitted in a slot or bore provided in said cathode bar; (d) providing at least one cathode bar being fitted in said groove or bore provided in said cathode body; wherein that said insert has at least one first so-called electrically conductive region and at least one second so-called electrically non-conductive region, the electrically conductive contact length, in cross section, of said insert with said body and/or said cathode bar being superior in the first region than in the second region.

Yet another object of the invention is an electrolytic cell for the production of aluminium by the Hall-Heroult process, comprising at least one cathode assembly according to the invention. A last object is a process for making aluminium by the Hall-Heroult process, using an electrolysis cell having cathodes assemblies according to the invention.

Figures

Figures 1 to 26 represent various embodiments of the present invention.

Figure 1 is a perspective view, showing a first embodiment of a cathode assembly according to the invention, wherein the insert of this assembly is rectangular in cross section.

Figure 2 is a bottom view, showing the cathode assembly of figure 1.

Figure 3 is a cross section showing the cathode assembly of figure 1, along line Ill-Ill of figure 2.

Figure 4 is a perspective view, showing a cathode bar which belongs to the cathode assembly of the figure 1.

Figure 5 is a cross section showing the cathode bar of figure 4, along line V-V of figure 4. Figure 6 is a perspective view, showing an insert which belongs to the cathode assembly of the figure 1.

Figures 7 and 8 are cross sections showing the insert of figure 6, along lines respectively VII-VII and VIII-VIII of figure 6.

Figure 9 is a perspective view, showing the insert of figure 6 which is fitted in a slot of the cathode bar of figure 4.

Figures 10 and 11 are cross sections showing the insert fitted in the slot of the cathode bar, along lines respectively X-X and XI-XI of figure 9.

Figure 12 is a view at a larger scale, showing the detail XII of figure 10.

Figures 13 to 15 are cross sections analogous to figure 12, showing variants of this first embodiment the invention.

Figure 16 is a perspective view, analogous to figure 1, showing a second embodiment of a cathode assembly according to the invention, wherein the insert of this assembly is circular in cross section.

Figure 17 is a bottom view, analogous to figure 2, showing the cathode assembly of figure 16.

Figure 18 is a cross section showing the cathode assembly of figure 1, along line XVIII-XVIII of figure 17.

Figure 19 is a perspective view, analogous to figure 6, showing a cathode bar which belongs to the cathode assembly of the figure 16.

Figure 20 is a cross section showing the cathode bar of figure 19, along line XX-XX of figure 19.

Figure 21 is a perspective view, analogous to figure 6, showing an insert which belongs to the cathode assembly of the figure 16.

Figure 22 is a bottom view, showing the insert of figure 21 which is fitted in a hole of the cathode bar of figure 4.

Figures 23 and 24 are cross sections showing the insert fitted in the hole of the cathode bar, along line XXIII-XXIII and line XXIV-XXIV, respectively, of figure 22.

Figure 25 is an enlarged view, showing the detail XXV of figure 24.

Figure 26 is a cross section of a cathode assembly along a vertical longitudinal plane, showing more specifically the end regions of the insert and the cathode body.

The following reference signs are used on the figures:

Description

In the present description, the terms “upper” and “lower” refer to a cathode block in use, lying on a horizontal ground surface. Moreover, unless specific contrary indication, “conductive” means “electrically conductive”.

According to the terminology used in the present description and in the art, a “cathode assembly” C comprises the cathode body 1 and the cathode bar 3.

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 assembly C, each of which bears at least one cathode bar 3. 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.

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 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 2), i.e. the distance between walls 11 and 12, is between about 3100 mm and about 3950 mm. By way of example, its width W1 (see figure 2), i.e. the distance between walls 15 and 16, is between about 400 mm and about 675 mm. By way of example, its height H1 (see figure 1), i.e. the distance between walls 13 and 14, is between about 420 mm and about 580 mm.

The lower wall 14 of cathode body 1 is provided with a longitudinal groove 17 extending from one cathode body end to the other (see in particular figure 2). The free end of the groove 17 leads to fronts 11 or rear 12 wall of body 1.

The structure of groove 17 will now be described. Opposite side walls of groove 17 are referenced 171 and 172, whereas its upper wall is referenced 173 (see figure 3). By way of example, its width W17, i.e. the distance between walls 171 and 172, is between about 130 mm and about 280 mm. By way of example, its depth D17, i.e. the distance between upper wall 173 and the surface of lower wall 14, is between about 150 mm and about 240 mm.

The cathode assembly C also comprises two cathode bars 3 and 3’, each of which is accommodated in groove 17. Each cathode bar 3 or 3’ is made of a first conductive material, typically steel. The structure of bar 3 will now be described, bearing in mind that structure of the other bar 3’ is identical. This cathode bar 3, which has an elongated shape (see in particular figure 4). has opposite front 31 and rear 32 walls, as well as peripheral walls. The latter are formed by upper and lower walls 33 and 34, as well as side walls 35 and 36. Two adjacent walls form longitudinal chamfers or rounded corners 39, in a known manner. In one embodiment upper and lower wall 33,34 and / or side walls 35,36 are parallel; in an advantageous variant of this embodiment the cathode bar is essentially rectangular in cross section.

The length L3 of cathode bar 3 is superior to that of length of half groove 17, so as to define a projection 38 (see in particular figure 2), which extends beyond front wall 11 of the cathode block body. By way of example, the length L38 of projection 38 is between about 350 mm and about 600 mm. Moreover the width W3 of bar 3, i.e. the distance between walls 35 and 36, is slightly inferior to the width W17 of groove 17. Finally the height H3 of bar 3, i.e. the distance between walls 33 and 34, is slightly inferior to the height H17 of groove 17. During manufacture, the gaps between facing walls of bar 3 and groove 17 are filled with cast iron, in a known way, and/or with an intermediate carbonaceous material (see figure 3) which will be described below.

Turning back to figure 4, the upper wall 33 of cathode bar 3 is provided with a housing formed by a longitudinal slot 37. This slot 37 extends over only a part of the whole length of the cathode bar 3, i.e. it does not longitudinally lead to front and rear walls thereof. As a variant, this slot may be provided in another peripheral wall of cathode bar 3, in particular in side walls 15,16 ; in this (less preferred) case a symmetrical configuration is preferred.

Opposite side walls of slot 37 are referenced 371 and 372, whereas its bottom wall is referenced 373 (see figure 5). By way of example, its width W37, i.e. the distance between walls 371 and 372, is between about 50 mm and about 100 mm. By way of example, its depth D37, i.e. the distance between walls bottom wall 373 and the surface of upper wall 33, is between about 50 mm and about 80 mm.

The cathode assembly C also comprises two inserts 5 and 5’, each of which is accommodated in a respective slot 37 and 37’, see figure 9. Each insert 5 or 5’ is made of a second conductive material, having a higher electrical conductivity than that of said first conductive material, typically copper. The structure of insert 5 will now be described, bearing in mind that structure of the other insert 5’ is identical. Referring to figures 6 to 8, this insert 5, which has an elongated shape, has opposite front 51 and rear 52 walls, as well as peripheral walls. The latter are formed by upper and lower walls 53 and 54, as well as side walls 55 and 56. In one embodiment upper and lower wall 53,54 and / or side walls 55,56 are parallel; in an advantageous variant of this embodiment the insert 5 is essentially rectangular in cross section.

The above length L5 of insert 5 is called axial length, namely along main axis of this insert. This term “axial length” also applies for above mentioned lengths L1 and L3. Let us also define the peripheral length of insert, i.e. the sum of the lengths of its sides, in cross section like on figures 10 and 11. As will be more precisely defined hereafter, the peripheral contact length of insert 5 is the sum of the lengths of its sides, which contact cathode bar 3.

For allowing the insert to expand inside the slot the axial length L5 of insert 5 is slightly smaller to that of slot 37. Over its length, this insert comprises at least three regions, the cross-sections of which are different. In the illustrated example, this insert includes two end regions 5A and 5B, which have the same cross-section, as well as an intermediate region 5C, which has a different cross-section as will be further explained. The ratio L5C/L5 between the axial length L5C of intermediate region 5C and the axial length L5 of whole insert 5 is between about 8 % and about 20 %, in particular between about 9 % and about 15 %.

The height H5 of insert, i.e. the distance between walls 53 and 54, is slightly inferior to the height H37 of slot 37. This height can differ over the whole length of insert 5, related to the longitudinal deformation of the collector bar. On the other hand, the width and/or height of insert 5, i.e. the distance between walls 55 and 56, and/or between walls 53 and 54, may not be constant over this length, which defines the above mentioned different regions.

The width W5A or W5B of end regions 5A and 5B substantially corresponds to that of slot 37. Therefore, once the insert 5 is placed into slot 37, there is a fit, first, between side wall 371 of said slot 37 and side walls 55A, 55B of said insert 5 and, moreover, between side wall 372 of said slot 37 and side walls 56A, 56B of said insert 5. This fit, which is shown on figure 11. makes it possible to create an electric contact between cathode bar 3 and regions 5A and 5B of the insert 5.

On the other hand, side walls 55C and 56C of intermediate region 5C define recesses with side walls 55A and 56A, as well as 55B and 56B, of adjacent end regions 5A and 5B. In other words, these side walls 55C and 56C are distant from facing walls 371 and 372 of slot 37, in order to define two functional gaps or clearances 7 (see figure 10). These clearances are better shown on figure 12, where their scale is enlarged towards other mechanical elements, in view of clarity. The width W7 of these clearances, i.e. the closest distances between respective walls 371, 372 of slot 37 and intermediate region 5C, is enough to avoid any electric contact between bar 3 and insert 5, in this intermediate region. In an advantageous manner, this width is greater than about 0.5 mm, in particular greater than about 0.8 mm. By way of example, this width W7 is between about 0.5 mm and about 2 mm, in particular between about 0.5 mm and about 1 mm.

Figures 10 and 11 show cross sections of insert 5 accomodated in slot 37, respectively in intermediate region 5C and in end region 5B. On these figures, as previously mentioned, the electrically conductive contact peripheral length of insert can be defined by the length of this insert contacting cathode bar 3, in cross section like on figures 10 and 11. This contact may be direct, like on figure 11, or may be indirect, i.e. it is then ensured via a conductive material.

As shown on figure 11, side walls 55A and 56A are in electrical contact with the cathode bar, which means that electrically conductive contact peripheral length is equal to the sum of the lengths of three sides of this end region 5A, i.e. H5 + W5A + H5. The electrically conductive surface corresponds to the product between conductive peripheral length, as defined above, and axial length. For region 5A, this conductive surface is equal to (2*H5 + W5A)*L55A. In an analogous manner, for region 5B, electrically conductive peripheral length is equal to H5+W5B+H5, and conductive surface is equal to (2*H5 + W5B)*L55B.

On the other hand, as shown on figure 10, only lower wall 54C contacts the cathode bar, but the contact between insert and collector bar is not good enough to allow the current to flow. Electrically conductive peripheral length of region 5C is therefore at most equal to W5C and, in any case, far inferior to electrically conductive distance in other regions 5A and 5B. We can consider a ratio of 10 % maximum of the current flowing through the surface 54C compared to the contact surface in zone 5B or 5A.

As explained in the above paragraph, the electrically conductive contact peripheral length of insert is superior in each end region, which is therefore called electrically conductive region, than in the intermediate region, which is therefore called electrically non-conductive region. This makes it possible to tailor the electrical conductivity of the cathode bar 3, and more generally the electrical conductivity of the cathode assembly C, parallel to its length, by an appropriate choice of the different parameters indicated above. Tailoring the electrical conductivity of the cathode assembly C allows to modify the magnetic fields in the electrolysis cell that keep the molten metal in movement, and allows eventually to modify the magnetohydrodynamics of the electrolysis cell.

The non-conductive region is placed inside the cathode block groove in such a way that it starts at the edge of the front wall 11 and/or rear wall 12 of cathode body 1 and extends towards the centre of the cathode body 1.

Let us define the so-called conductive ratio, i.e. the ratio between, on the one hand, electrically conductive contact surface in non-conductive region and, on the other hand, electrically conductive contact surface in conductive region. In the shown example, this ratio is approximately equal to 10 %.

As a first variant, shown in figure 13, electrically conductive contact length in non-conductive region is equal to 0, since no part of region 5C contacts cathode bar 3. Facing walls respectively 54C and 373 define another clearance 7”. The conductive ratio is therefore equal to 0.

As another variant, shown in figure 14, electrically conductive contact length in non-conductive region is equal to (L54C+L56C). There is only one single clearance 7, between wall 55C and cathode bar 3. The conductive ratio is by far inferior to 1.

In the first shown embodiment, the insert and the slot are rectangular in cross-section, i.e. they have four peripheral walls. According to some non-illustrated variants, this insert and this slot may have different polygonal cross sections, with a different number of peripheral walls. In this case, the number of walls of the insert, contacting the cathode bar, is superior in the conductive region(s) than in the non-conductive region(s).

According to another variant, shown on figure 15, both the slot 37 and the insert 5 are non-polygonal, but define a portion of a circle in cross section, in particular a half circle. In this case, the non shown conductive region has the same radial dimension as the slot, whereas the non-conductive region is defined by a portion 5C of the insert, the radial dimension of which is reduced. In this non-conductive region, there is substantially no contact between the insert and the facing wall of the slot, which defines a clearance 7 which leads on upper wall 33. In this variant, the conductive ratio is equal to 0.

Figures 16 to 25 show another embodiment, wherein both the insert and the slot of cathode bar are circular in cross section. Hereafter, reference numerals 2, 4 and 6 will be given respectively to cathode body, cathode bar and insert. The mechanical elements of cathode body 2, which are similar to those of cathode body 1 of first embodiment, will be given the same reference numerals, the first digit “2” being used instead of “1”. The mechanical elements of cathode bar 4, which are similar to those of cathode bar 3 of first embodiment, will be given the same reference numerals, the first digit “4” being used instead of “3”. The mechanical elements of insert 6, which are similar to those of insert 5 of first embodiment, will be given the same reference numerals, the first digit “6” being used instead of “5”.

The structure of cathode body 2 is substantially identical to that of cathode body 1. The structure of each cathode bar 4 or 4’ is globally similar to that of cathode bar 3 or 3’. However, cathode bar 4 differs from cathode bar 3, essentially in that the housing 47 receiving the insert 6 is not formed by a slot 37, but by a bore 47. The latter does not transversally lead to a side wall of the bar 4, but longitudinally leads to the one single wall of this bar, namely rear wall 42, as can be seen on figures 19 and 22. Moreover this bore 47 differs from slot 37, in that it is circular in cross-section. By way of example, its length L47 (see figure 22), i.e. the distance between its bottom 473 and front wall of bar 4, is between about 1000 mm and about 1750 mm. By way of example, its diameter D47 (see figure 20) is between about 30 mm and about 70 mm.

The cathode assembly C’ of this second embodiment also comprises two inserts 6 and 6’, each of which is accommodated in a respective bore 47 and 47’. Referring to figure 21. this insert 6 has an elongated shape, as well as a circular cross-section. It has opposite front 61 and rear 62 walls, as well as a peripheral wall 63.

The axial length L6 of insert 6 can be equal to that of slot 47. Over its length, this insert comprises at least three regions, the cross-sections of which are different. In the illustrated example, this insert includes two end regions 6A and 6B, which have the same cross-section, as well as an intermediate region 6C, which has a different cross-section as will be further explained. The ratio L6C/L6 between the axial length L6C of intermediate region and the axial length L6 of whole insert is between about 6 % and about 20 %, in particular between about 9 % and about 15 %.

The diameter D6A or D6B of end regions 6A and 6B substantially corresponds to that of bore 47. Therefore, once the insert 6 is placed into bore 47, there is a fit between side wall 471 of this bore and side walls 63A, 63C of the insert (as shown on figure 23 with side wall 63A). This makes it possible to create an electric contact between bar 4 and regions 6A and 6B of the insert 6. On the other hand, the diameter D6C of intermediate region 6C is slightly inferior to the diameter D6A or D6B of end regions 6A and 6B.

In other words, peripheral walls 63C are distant from facing walls 471 of bore 47, in order to define an annular gap or clearance 9. Said clearance 9 is better shown on figure 25. where its scale is enlarged towards other mechanical elements, in view of clarity. The width W9 of this clearance 9, i.e. the closest distance between facing walls of bore 47 and intermediate region 6C, is enough to avoid any electric contact between bar 4 and insert 6, in this intermediate region. In an advantageous manner, this width is greater than about 0.5 mm, and advantageously not exceeding 2 mm. By way of example, this width W9 is between 0.5 mm and 2 mm, in particular between 0.5 mm and 1 mm. In this embodiment, the conductive ratio, as defined above, is equal to 0 since no region of tapered region 6C contacts the bar 4.

In the above shown and non-shown embodiments, the cross section of the slot is constant and the insert has at least one local tapered region, the transversal dimension of which is smaller. As a variant, the cross section of the insert may be constant, whereas the slot has at least one local widened region, the transversal dimension of which is larger. Each widened region of the slot defines a non-conductive region of the insert.

As shown on figure 26, the electrically non-conductive region 7 of the copper insert 5 can be placed inside the cathode block groove in such a way that it starts at the edge of the front wall and/or rear wall of cathode body 1 and extends towards the centre of the cathode body.

It should be noted the copper insert 5 can also extend until the outer end of the steel cathode bar 3.

The cathode assembly C according to the present invention can be of the same height or of different height, and/or can have a structured surface, and accordingly the cathode formed by these cathode assemblies can have a single flat upper surface (which is by far the most common cathode structure), or its upper surface can comprise regions of different heights or can be otherwise structured.

As mentioned above, during manufacture, the gaps between facing walls of cathode bar 3 and groove 17 are filled with cast iron, in a known way, and/or with a carbonaceous intermediate material. Said intermediate carbonaceous material in direct contact with the metallic connection bar 3 must be somewhat deformable, such as to accommodate the higher thermal expansion of the metallic connection bar 3, usually made from steel, with respect to the material of the cathode assembly C. As an intermediate carbonaceous material in direct contact with the cathode bar 3 and the cathode assembly C, compressed expanded graphite (most conveniently in the form of a sheet) can be used, and/or carbonaceous sealing paste.

Compressed expanded graphite is available in the form of sheet of different densities and thickness from several manufacturers and under different tradenames (such as Papyex™ manufactured by Mersen and Sigraflex™ manufactured by SGL).

Concerning the carbonaceous sealing paste, the sealing paste advantageously includes carbonaceous particles dispersed in a binder that has a high carbon yield after baking. Said carbonaceous particles can be graphite particles. Such sealing pastes are commercially available from different manufacturers and under various tradenames (Sealing paste HCF 80 from Carbone Savoie for example). The thickness of the sealing paste is calculated according to the width of the collector bar, taking into consideration the differential of thermal expansions between the copper and the graphitized cathode assembly; a thickness comprised between 15 and 25 mm can be used.

Claims (13)

1. A cathode assembly suitable for a Hall-Heroult electrolysis cell, comprising - a cathode body made of a carbonaceous material; - at least one cathode bar made of a first conductive material, said cathode bar being fitted in a groove provided in said cathode body; - at least an insert made of a second conductive material, having a higher electrical conductivity than that of said first conductive material, said insert being fitted in a slot or bore provided in said cathode bar; characterized in that said insert has at least one first so-called electrically conductive region and at least one second so-called electrically non-conductive region, the electrically conductive contact peripheral length of said insert with said cathode body and/or said cathode bar being superior in the first region than in the second region.

2. A cathode assembly according to claim 1, characterized in that, in the non-conductive region, peripheral walls of said insert define a functional clearance with the facing walls of said body and/or said cathode bar, said functional clearance being filled with a solid non-conductive material or with air.

3. A cathode assembly according to claim 1 or 2, characterized in that said electrically non-conductive region is placed inside the groove in such a way that it starts at the edge of the front wall and/or rear wall of cathode body and extends towards the centre of the cathode body.

4. A cathode assembly according to any of claims 1 to 3, characterized in that the non-conductive region extends over a part of the axial length of this insert, which is comprised between 6 and 20%, in particular between 9 and 15% of the whole axial length of this insert.

5. A cathode assembly according to any of claims 1 to 4, characterized in that the slot or bore in cathode bar has a substantially constant cross section over its axial length and the non-conductive region is defined by a local restriction of the cross section of the insert.

6. A cathode assembly according to any of claims 1 to 4, characterized in that the insert has a substantially constant cross section over its axial length and the non-conductive region is defined by a local widening of the cross section of the slot or bore of cathode bar.

7. A cathode assembly according to any of claims 1 to 6, characterized in that both the slot or bore in the cathode bar and the insert have a polygonal cross section, and at least one peripheral wall of said insert is remote from facing walls of the slot or bore in the non-conductive region.

8. A cathode assembly according to any of claims 1 to 6, characterized in that both the slot or bore in the cathode bar and the insert have a circular cross section, and peripheral wall of said insert is remote from facing wall of the slot or bore in the non-conductive region, so as to define an annular gap there between.

9. A cathode assembly according to any of claims 1 to 8, characterized in that the axial length of said second so called non-conductive region is between 50 mm and 250 mm, in particular between 100 and 200 mm.

10. A cathode assembly according to any of claims 1 to 9, characterized in that the insert and the slot are polygonal in cross-section, the number of walls of the insert, contacting the cathode bar, being superior in the conductive region(s) than in the non-conductive region(s).

11. A process for making a cathode assembly according to any of claims 1 to 10, comprising the steps of (a) providing a cathode body made of a carbonaceous material; (b) providing a groove or bore in said cathode body; (c) providing at least one cathode bar made of a first conductive material, (d) providing a slot or bore in said cathode body, (e) providing at least one insert made of a second conductive material, having a higher electrical conductivity than that of said first conductive material, said insert being fitted in a slot or bore provided in said cathode bar; (d) providing at least one cathode bar being fitted in said groove or bore provided in said cathode body; wherein said insert has at least one first so-called electrically conductive region and at least one second so-called electrically non-conductive region, the electrically conductive contact length, in cross section, of said insert with said body and/or said cathode bar being superior in the first region than in the second region.

12. Electrolytic cell for the production of aluminium by the Hall-Heroult process, comprising at least one cathode assembly according to any of claim 1 to 10.

13. A process for making aluminium by the Hall-Heroult process, using an electrolysis cell having cathode assemblies according to any of claims 1 to 10.