EP3523463B1

EP3523463B1 - Cathode assembly for electrolytic cell suitable for the hall-héroult process

Info

Publication number: EP3523463B1
Application number: EP17857916.5A
Authority: EP
Inventors: Bernard JONQUA; Abdalla ALZAROONI
Original assignee: Dubai Aluminium PJSC
Current assignee: Dubai Aluminium PJSC
Priority date: 2016-10-05
Filing date: 2017-09-19
Publication date: 2021-02-17
Anticipated expiration: 2037-09-19
Also published as: GB201616873D0; EP3523463A4; WO2018065844A1; GB2554702A; EP3523463A1

Description

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 bus bar 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 (Al₂O₃) is dissolved in molten cryolite (Na₃AlF₆), 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 bus bars. 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 19^th century much effort has been undertaken to improve the energy efficiency (expressed in kW/h per kg or ton of aluminium), and there has also 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 2001/063014 (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 Pechine, 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 2,846,388 (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 2285754 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 bus bar.
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: Sørlie and Øye, "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.

Objects of the invention

A first object of the invention is 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 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;

in that said cathode assembly is further provided with connection means intended to connect said cathode assembly with a cathodic bus bar,
in that said connection means comprise at least one first connection member extending across said slot or bore, as well as across said insert, viewed from the end,
in that the free end of said first connection member, adjacent to said slot and said insert, comprises at least one connection zone for mechanical and electrical connection to said end wall of cathode bar,
and in that said free end of said first connection member also comprises a non-connection zone which does neither mechanically nor electrically contact said insert.
Advantageously, said free end of first connection member comprises a central non-connection zone facing said insert, as well as two lateral connection zones provided on either side of non-connection zone, said two lateral connection zones being mechanically and electrically connected to said end wall of cathode bar, on either side of said slot. In one embodiment, said central non-connection zone is flush with said lateral connection zones.
In an advantageous embodiment, said end wall of said cathode bar protrudes with respect to said end wall of said insert, by a length of protrusion inferior to 10 millimetres, in particular being equal to about 5 millimetres.
Advantageously, said connection means further comprise at least one other connection member, which extends away from said insert, viewed from the end. Said connection means may comprise one single first connection member, as well as two other connection members, these three connection members being located the one under the others.
Advantageously each other connection member comprises a free end which is provided with a connection zone on said end wall of cathode bar, said connection zone extending over the whole width of said end wall. Said free end of each connection member may be provided with an abutment edge for abutment against end wall of cathode bar.
Advantageously the height of connection zone is superior to the height of abutment edge, the ratio between height of connection zone and height of abutment edge being in particular equal to 2. Said connection zones and, potentially, said non-connection zone, may be provided on a bevelled edge that extends from said abutment edge.
In one embodiment each connection zone is formed by a weld. Advantageously the ratio between the sum of the surface areas of said welds and the whole surface area of end wall of cathode bar is superior to 45%, in particular to 55%.
Each connection member may comprise a flexible member intended to be connected to said cathode bus bar, as well as a transition member attached to said flexible member, said transition member being connected to said cathode bar and being provided with said connection zones and, potentially, said non-connection zone. Advantageously each flexible member is elongated viewed from the side and is rectangular in cross section.
Each transition member may comprise a plate made of aluminium, said plate being attached with said flexible member, as well as a block made of steel, said block being attached to said plate and being provided with said connection zones and, potentially, said non-connection zone.
In one embodiment peripheral facing walls of said groove and said cathode bar define a gap that is at least partially filled with a conductive filling material and, in the vicinity of an end wall of cathode body, said peripheral facing walls of said groove and said cathode bar define a so-called electrically non-conductive region. Said electrically non-conductive region comprises an insulating member, provided at the end of said groove, which extends over at least part of the peripheral wall of said groove. Said insulating member is for example at least one layer of insulating material, in particular of insulating paint.
Advantageously said electrically non-conductive region also comprises a stopping member, provided at the end of said groove, said stopping member being adapted to prevent expansion of filling material towards the end of said groove. Stopping member may be a blanket, in particular a blanket made of ceramic fiber. In one embodiment insulating member is provided directly onto walls of the groove, whereas stopping member is provided directly onto insulating member.
Advantageously the axial length of stopping member is inferior to that of insulating member. The axial length of insulating member is for example comprised between 50 and 200 mm, typically equal to substantially 150 mm. The axial length of stopping member is for example comprised between 25 and 50 mm, typically equal to substantially 40 mm. In one embodiment, in cross section, the groove is U shaped and insulating member extends from each corner of said groove, over at least part of both lateral walls of said groove.
The cathode assembly according to the invention may be further provided with at least a first intermediate plate made of aluminium, permanently attached to facing end of said connection member, said cathode assembly further comprising means for a removable fixation of said first intermediate plate on said cathodic bus bar.
Advantageously means for removable fixation of said first intermediate plate on said cathodic bus bar comprise a second intermediate plate made of aluminium, intended to be permanently attached to said cathodic bus bar, as well as means for removable fixation, in particular bolting means, between said first intermediate plate and said second intermediate plate.
In one embodiment the cathode assembly may further comprise at least one intermediate tab made of a material different from aluminium, in particular made of copper, said intermediate tab being intercalated between facing walls of said first intermediate plate and said second intermediate plate.
In another embodiment the cathode assembly may further comprise one single first intermediate plate, as well as means for a direct removable fixation of said single plate to said bus bar in a removable way, in particular by bolting means.
Advantageously 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. In the non-conductive region, peripheral walls of said insert may 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. The slot or bore in cathode bar may have 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 an alternative, the insert may have 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. The use of such non-conductive region is a simple means to locally decrease the conductivity of the cathode assembly along its length.
Another object of the present invention is a process for making a cathode assembly as described above, 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, said cathode bar being fitted in said groove or bore provided in said cathode body;
(d) providing a slot or bore in said cathode bar, said slot or bore leading to one end wall of said cathode bar;
(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 said slot or bore provided in said cathode bar, one end wall of said insert being substantially flush with said end wall of said cathode bar;
(f) providing at least one first connection member;
(g) connecting said first connection member to said end wall of cathode bar, either side of said slot, in a way such as the free end of said first connection member, adjacent to said slot and said insert, forms at least one connection zone for mechanical and electrical connection to said end wall of cathode bar, and in that said free end of said first connection member also forms a non-connection zone which does not mechanically nor electrically contact said insert.

Said process may further comprise:

providing at least one other connection member
connecting said other connection member to said end wall of cathode bar, in a way such as said other connection member extends away from said insert, viewed from the front.

Connecting said first connection member to said end wall of cathode bar may be carried out before connecting said other connection member to said end wall of cathode bar.
Another object of the present 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 assembly, each cathode assembly comprising at least one metallic cathode collector bar 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 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 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 assembly, and preferably more than 60% of said cathode assemblies and, more preferably, each of said cathode assemblies, is a cathode assembly as described above.
Another object of the present invention is an electrolytic cell for the production of aluminium by the Hall-Heroult process, comprising at least one cathode assembly as described above.
Another object of the present invention is a process for making aluminium by the Hall-Héroult process, using an electrolytic cell provided with cathode assemblies as described above.
Another object of the present invention is 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;

Said cathode assembly may be provided with at least one optional advantageous feature, amongst those above recited in connection with cathode assembly according to first object of the present invention.
Another object of the present invention is a cathode assembly suitable for a Hall-Heroult electrolysis cell, comprising

Said further cathode assembly may be provided with at least one optional advantageous feature, amongst those above recited in connection with cathode assembly according to first object of the present invention.

Figures

Figures 1 to 24 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 III-III 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 perspective view, analogous to figure 9, showing at greater scale the front end of both cathode bar and insert of figure 9.
Figure 13 is a front view, showing the cathode assembly of figure 1 which is connected to a cathodic bus bar of an electrolytic cell.
Figure 14 is an enlarged view, showing with more details zone XIV of figure 13.
Figure 15 is an end view, along arrow XV on figure 14, showing the connexion zones of connecting members, allowing the connection of cathodic bus bar and electrolytic cell both shown on this figure 13.
Figure 16 is a perspective view, showing at greater scale the connexion zones of the upper connecting member of figure 15, along arrow XVI on figure 14
Figure 17 is an end view, analogous to figure 15, showing the connexion zones of a connecting member according to another variant of the invention.
Figure 18 is a perspective view, analogous to figure 16, showing the connexion zones of a connecting member according to a variant of the invention.
Figure 19 is a front view, analogous to figure 13, showing the connection on cathodic bus bar of connecting members according to a further variant of the invention
Figure 20 is a front view, analogous to figure 13, showing two adjacent electrolytic cells, each is which is provided with a bus bar connected with a respective cathode assembly of figure 1.
Figures 21 and 22 are perspective views showing, under two different point of views and at greater scale, the front end of the cathode block of figure 1, the walls of the groove of this cathode block being provided with an insulation paint according to a first embodiment of the invention.
Figures 23 and 24 are perspective views, analogous to figures 21 and 22, showing under two different points of view and at greater scale the front end of the cathode block of figure 1, the walls of the groove of this cathode block being provided with this insulation paint according to a second embodiment of the invention.
Figures 25 and 26 are perspective views showing, analogous to figures 21 and 22, the walls of the groove of this cathode block being provided with a blanket according to a variant of the invention.
Figures 27 and 28 are perspective views, analogous to figures 23 and 24, the walls of the groove of this cathode block being provided with the blanket according to figures 25 and 26.

The following reference signs are used on the figures:

1,2	Cathode body	171, 172	Side walls of groove 17
3,3',4,4'	Cathode bar	173	Bottom wall of groove 17
5,5',6,6'	Insert (bar)	371,372	Side walls of slot 37
5A,5B	End regions of insert 5	373	Bottom of slot 37
5C	Intermediate region of insert 5	C	Cathode assembly
7	Clearance	F	Filling material (Cast Iron)
11,12	Front / rear wall of cathode body 1	SP	Spacer
12,14	Upper / lower wall of cathode body 1	51,52	Front / rear wall of insert 5
15,16	Side walls of cathode body 1	53,54	Upper / lower wall of insert 5
17,	Groove in cathode body 1	54A, 54B	Lower wall of insert 5 (end regions)
31,32	Front / rear wall of cathode bar 3	55,56	Side walls of insert 5
33,34	Upper / lower wall of cathode bar 3	55A, 55B	Side walls of insert 5 (end regions)
35,36	Side walls of cathode bar 3	55C	Side wall of insert 5 (intermediate)
37,37'	Slot forming housing	56A,56B	Side walls of insert 5 (end regions)
374	Blind wall of slot 37	38	Projection
39	Chamfer	58	Recess
2, 4, 6	Connectors	21,41,61	Flexible members
21a-61a	First end of 21-61	21b-61b	Second end of 21-61
21c-61c	Pocket for welding material
22,42,62	Transition members	23,43,63	Plate of transition members
24,44,64	Block of transition members	25,45,65	Upper edge of block
29,49,69	Bevelled edge of block	29a-69a	Pocket for welding material
46,66	Welds of 44,64	H45,H65	Height of 45,65
H46,H66	Height of 46,66
25a,25b	Lateral zones of 25	25c	Central zone of 25
26a,26b	Lateral welds of 29	27	Central zone of 29
100	Bus bar	102,104	Intermediate plates
102',104'	Tabs	106	Bolt
200	Pot shell
171,172	Side walls of 17	173	Upper wall of 17
174,175	Corners of 17	302,304	Layers of insulating paint
306,306'	Blanket
DX	Depth of element X : D37
HX	Height of element X : H1, H3, H5, H37, H2, H4, H6, H302, H304
LX	Length of element X : L3, L5, L5C, L38, L58, L302, L304, L306, L302', L306'
WX	Width of element X : W1, W3, W51, W5A, W5B, W7, W17, W37

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 a cathode body 1, a cathode bar 3 and two inserts 5 and 5'.
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 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 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 front 11 or rear 12 walls 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 end walls, i.e. 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 the process of cathode casting, the gaps between facing walls of bar 3 and groove 17 are filled with a filling material F (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. As a not shown variant, this slot may be provided in another peripheral wall of cathode bar 3, in particular in side walls 35, 36; in this (less preferred) case a symmetrical configuration is preferred. As shown by this figure 4, slot 37 extends over only a part of the whole length of the cathode bar 3.
On the one hand rear wall, or blind wall 374 of this slot 37, is remote from facing rear wall 32 of cathode bar. The distance D374 between these two walls is typically between 250 millimetres (mm) and 950 mm. Viewed from above, blind wall 374 is rounded, which makes it possible to ease the slot machining. On the other hand, according to the invention, this slot 37 does lead to front wall 31 of this cathode bar. In a similar way, slot 37' provided into other cathode bar 3' is remote from rear wall thereof, but does lead to front wall 31' of this cathode bar 3'.
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 figures 3, 9, 10 and 11. 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. As shown on figure 3, a steel spacer SP may be provided into the filling material F, adjacent upper wall 53 of insert 5. In a way known as such, it allows the filling material F to go around the collector bar 3.
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. Rear wall 52 of insert 5 is in vicinity of facing rear wall 374. On the other hand, according to the invention, front wall 51 of insert 5 is placed towards the bar front wall 31, and is adjusted substantially flush with this bar front wall 31. On figure 12, the close view is showing at an enlarged scale that the copper insert end 51 is adjusted to the bar end 31.
Let us note L35 the longitudinal distance between above front walls 31 and 51. According to the invention, "substantially flush" first means that front walls 31 and 51 are exactly flush, i.e. distance L35 is equal to 0. It also means that said distance L35 is strictly superior to 0 but in any case is inferior to 10 mm, in particular to 5 mm. On figure 12, front wall 31 protrudes with respect to front wall 51, the protrusion length L35 being about 5 mm. As will be described hereafter, providing a protruding front wall 31 might be an advantageous embodiment of the invention. On this figure 12, L35 is illustrated at a greater scale for sake of clarity; bearing in mind it is inferior to 10 mm as indicated above. Providing insert 5 and bar end 31 substantially flush, according to the above definition, is advantageous since it avoids any substantial voltage drop (that could be within the steel collector bar end portion). The expected cathode voltage drop for the arrangement described would be between 145 and 160 mV at 455 kA. This is between 95 and 110 mV less compared to the designs without copper inserts (same cathode and collector bars at same current average).
Figure 13 shows connection members 2, 4, 6, also called "connectors", which allow the connection between above described cathode bar 3 and a cathodic bus bar 100. In a way known as such, this cathodic bus bar surrounds a pot shell, not shown on this figure 13. Bus bar is rectangular in shape and has two opposite longitudinal parts, as well as two opposite transversal parts. Connectors 2, 4, 6 extend between front wall 31 of cathode bar 3 and one longitudinal part of cathodic bus bar 100. These connectors are an advantageous part of the subject matter of the invention and will be described in more detail.
Each connection member 2, 4, 6 comprises a flexible member 21, 41, 61, as well as a transition member 22, 42, 62. Flexible members 21, 41, 61 extend substantially in a parallel way, the one above the other. Each flexible member, which is known as such, is for example an aluminium sheet or strip. It is elongated viewed from side and, as shown on figure 13, is substantially rectangular in cross-section. As shown by this figure 13, each flexible member has two opposite axial ends 21a and 21b, 41a and 41b, 61a and 61b. First end 21a, 41a, 61a is attached to the first intermediate plate 102, made in aluminium, whereas second end 21b, 41b, 61b is attached to cathode bar 3, via a respective transition member 22, 42, 62 which will be described more in detail hereafter.
First end 21a, 41a, 61a of each flexible member 21, 41, 61 is attached on bus bar 100 in a removable manner via the intermediate plate 102 bolted to the aluminium plate 104 itself welded to the bus bar 100. This permanent attachment is typically carried out by welding, preferably by a respective aluminium/aluminium welding seam of known type. As also shown on figure 13, a further plate 104 is provided, which is permanently attached to the side of bus bar 100. This permanent attachment is also typically carried out by welding, preferably by a respective aluminium/aluminium welding seam of known type. The two plates 102 and 104 are removably attached, by any appropriate means. In the illustrated embodiment, these two plates are provided with through bores adapted to receive removable fixation means, such as a bolt 106.
In the example of figure 13, above described plates 102 and 104 are in direct contact. However, in alternative embodiments, at least one intermediate tab may be intercalated between facing walls of said plates 102 and 104. Figure 20 shows an embodiment wherein two tabs 102' and 104' are provided, each tab being attached to a respective plate 102 and 104. In an advantageous manner, each tab is made of a material different from aluminium, in particular made of copper. By way of example, the thickness of each tab is typically 5 millimetres. On figure 20, the scale of these tabs has been far enlarged, for sake of clarity.
Adjacent faces of base plate and tab are attached by a copper/aluminium welding seam, which can be manufactured by explosion welding or co-rolling. In case one single tab is provided, it may be attached to either plate 102 or 104. In a further not shown embodiment, one single plate 102 is provided. In this case, this single plate is mechanically fixed, in a removable manner, directly on bus bar 100. Typically, said single plate receives a bolt, which penetrates into a blind hole provided into top face of bus bar 100.
Figure 14 shows more in detail transition members 22, 42 and 62, which are also called "clads", the structures of which are substantially identical. Each transition member comprises a rear plate 23, 43 and 63, as well as a front block 24, 44 and 64, said plate and said block having substantially the same width as cathode bar 3, as illustrated in particular on figure 15. Each plate 23, 43 and 63 which is typically made of aluminium, is attached in a way known as such to a respective block 24, 44 and 64 which is typically made of steel.
Free face of each plate, i.e. its face opposite to said block, is attached to facing end 21b, 41b, 61b of a respective flexible member 21, 41, 61. In this respect, said end forms a bevelled edge, defining a pocket 21c, 41c and 61c for receiving a welding material. This attachment, by an aluminium/aluminium welding type, is carried out in a way known as such.
Free face of each block 24, 44, 64, i.e. its face opposite to above plate, forms a free end of the whole connection member 2, 4, 6. Said free face is attached to facing front wall 31 of cathode bar 3. In this respect, each free face first comprises a straight upper edge 25, 45, 65 which forms an abutment edge against cathode bar, in particular during mounting process. Said upper edge is extended by a bevelled lower edge 29, 49, 69, which defines a pocket 29a, 49a, 69a, for receiving a welding material. This attachment, by a steel/steel welding type, is carried out in a way known as such.
Figure 15 shows more in detail the attachment of free face of each block 24, 44, 64 on cathode bar 3. Intermediate 44 and lower 64 blocks each form a weld 46, 66 under above described upper edge 45, 65. Both welds 46, 66 and upper edges 45, 65 extend across the whole cathode bar 3. In a preferred way, the height H46, H66 of each weld is superior to the height H45, H65 of each upper edge. This makes it possible to have a great welding zone between the whole transition member and the cathode bar. By way of example, each ratio H46/H45 and H66/H65 is about 2. Also by way of example, a 33 mm by 20 mm section welding is preferred (33 mm of height and 20 mm of thickness). It is to be noted that upper edge 45 of intermediate block 44 partially extends across slot 37 provided in this cathode bar, as well as across insert 5. According to the invention, this edge 45 is not mechanically connected to facing front wall 51 of insert 5.
As shown by both figures 15 and 16, the attachment of free face of upper block 24 on cathode bar 3 differs from that of above described blocks 44 and 64. Indeed welding material is not filled into the whole pocket 29a of bevelled edge 29. Therefore, the latter forms two lateral welds 26a and 26b, which extend on either side of a central zone 27 which faces insert 5. In the same way as described above for edge 45, said central zone 27 is not mechanically attached to insert 5. In other words, there is no mechanical contact and no electrical connection between said zone 27 and insert 5. This avoids any substantial risk of having very high current going through the upper clad welded on the upper part of front wall 31. Zone 27 is indeed called "non-connection zone".
Moreover abutment edge 25 does not extend across the whole width of cathode bar. Said edge is formed by two lateral abutment zones 25a and 25b, which extend on either side of a central zone 25c which faces insert 5. In the same way as described above for edge 45 and zone 27, said central zone 25c is not mechanically attached to insert 5.
Zones 25a, 25b of clad 24 are not welded to cathode bar, but abut against this cathode bar. On the other hand, middle zone 25c, facing insert 5, is neither welded nor in contact with this insert 5; does the same apply for upper-mid zone of clad 44.
As explained above, in reference to figure 12, end wall 31 of cathode bar protrudes with respect to end wall 51 of insert. This is an advantageous feature of the invention, since it avoids any mutual contact of clad 22 and insert 5, while welding this clad 22 onto the end face 51. Moreover the inventors have noted that, surprisingly, the position of insert 5 into slot 37 does not substantially vary in use. Therefore, even after long time of operation, insert is not likely to move into abutment against clad 22, in an inopportune manner.
As far as mounting process is concerned, it is preferred to weld first upper connector 2 onto cathode bar. Then, the two other connectors 4 and 6 are welded to this bar 3. The reason is mainly that the welding operations would be done from the basement of the potline and the upper connector must be welded first, then the middle one and then the lowest.
Let us note S26a, S26b, S46 and S66 the respective surface areas of the different above described welds 26a, 26b, 46 and 66. Advantageously, the ratio between the sum of these welded surface areas and the whole surface area of 31 is superior to 45%, in particular to 55%. Providing a large welding surface area is advantageous, since it reduces resistance and therefore avoids any substantial drop of current.
In the example of figures 13 to 15, three connection members 2, 4 and 6 are provided. As variants, there may be a different number of such members. In the embodiment shown on figure 17, one single connection member 102 is provided. This single member has substantially the same structure as that 2, but it has superior transversal dimensions. As shown by figure 17, upper block 124 of member 2 has an abutment edge 125 which does not extend across the whole width of cathode bar. Said edge is formed by two lateral abutment zones 125a and 125b, which extend on either side of a central zone 125c which faces insert 5. In the same way as described above, said central zone 125c is not mechanically attached to insert 5. Moreover upper block forms a lower weld 126 which extends across cathode bar, but does not touch insert 5.
According to not shown embodiments, either two, or four or more members, may be provided. Providing two or three members is a good compromise between a satisfactory covering and a simple global structure. Three members is a preferred embodiment.
Figure 18 shows an alternative embodiment of upper transition member, which is referenced 222 on this figure. This connector has more or less the shape of a fork, with a free end formed by two branches 222a and 222b protruding beyond median recess 222c. The upper part of protruding branches 222a and 222b form abutment zones, such as 25a and 25b of figures 15 and 16, whereas the lower part of these protruding branches 222a and 222b are provided with lateral welds, such as 26a and 26b of figures 15 and 16. Moreover the upper part of recess 222c forms a central non-connection zone, such as 25c of figures 15 and 16, whereas the lower part of recess 222c forms a central non-connection zone, such as 27 of figures 15 and 16.
Figure 19 shows two collector bar ends of two adjacent pots, each of which is connected with a respective bus bar 100A and 100B. This figure 14 also illustrates the connection of each bus bar with a respective cathode assembly according to the invention. More particularly, this connection is allowed by connection members 2A, 4A, 6A and 2B, 4B, 6B which extend between each bus bar 100A and 100B and a respective cathode bar 3A and 3B. This connection is ensured the same way as described above, in reference to figures 13 and 14. Two sets of intermediate plate 102A, 104A, 102B and 104B, similar to above described plates 102 and 104, are also provided.
Turning back to figures 10 and 11, let us also define the peripheral length of insert, i.e. the sum of the lengths of its sides, in cross section like on these 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. According to an advantageous embodiment, 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 of insert 5, i.e. the distance between walls 55 and 56, 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 ). 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 accommodated 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 not shown variant, electrically conductive contact length in non-conductive region may be equal to 0, since no part of region 5C contacts cathode bar 3. As another not shown variant, electrically conductive contact length in non-conductive region may equal to (L54C+L56C). In this case, there is only one single clearance 7, between wall 55C and cathode bar 3.
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 not shown variant, both the slot 37 and the insert 5 may be non-polygonal, but may 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.
According to another not shown embodiment, both the insert and the slot of cathode bar may be circular in cross section. In this embodiment, cathode bar differs from that 3, essentially in that the housing receiving the insert is not formed by a slot 37, but by a bore. The latter longitudinally leads to the one single end wall of this bar. Moreover this bore differs from slot 37, in that it is circular in cross-section. The cathode assembly of this embodiment also comprises two inserts, each of which is accommodated in a respective bore. Each insert substantially extends to the end of the bore, the same way above insert 5 substantially extends to the end of above slot 37. According to this embodiment, facing walls of insert and bore may advantageously define an electrically non-conductive region, the same way facing walls of above insert 5 and above slot 37 define electrically non-conductive region.
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 another variant, the conductive region of the insert extends over the whole length thereof. In this case, the cross sections of both the insert and the slot are identical, over the whole length of the insert.
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 peripheral walls of cathode bar 3 and groove 17 are filled with a conductive filling material F. In a known way, the latter may be cast iron 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 steel collector bar and the graphitized cathode assembly; a thickness comprised between 15 and 25 mm can be used.
Figures 21 and 22 show another advantageous embodiment of the invention, wherein gaps between facing peripheral walls of cathode bar 3 and groove 17 are filled with above described filling material F. These figures illustrate only cathode body 1 and the groove provided therein, but do not show cathode bar 3. In the vicinity of front wall 11 of cathode body 1, said peripheral facing walls of said groove and said cathode bar define a so-called electrically non-conductive region. Said electrically non-conductive region comprises an insulating member, provided at the end of said groove, which extends over at least part of the peripheral wall of said groove and/or said bar.
Figures 21 and 22 illustrate more in detail front wall 11 of cathode body 1, as well as axial end of groove 17, i.e. the part of this groove which leads to said wall 11. These figures depict above described side walls 171 and 172, upper wall 173, as well as corners 174 and 175 formed by the junctions between upper wall 173 and a respective side wall 171 and 172. Said insulating member is formed by two layers 302 and 304 of insulating material, each of which extends from a respective corner 174 and 175 and covers a part of a respective side wall 171 and 172. Moreover, each layer 302 or 304 leads to front wall 11, i.e. cathode block extremity.
Said electrically non-conductive region also comprises a stopping member 306, provided at the front end of said groove. In a way known as such, said stopping member is adapted to prevent expansion of filling material out of said groove, towards front wall 11 of cathode body 1. In the illustrated example, said stopping member is a blanket 306, in particular a blanket made of ceramic fiber. This stopping blanket, which is provided directly onto insulating layer 302 and 304, covers the whole cross section of the groove, i.e. both side walls and upper wall.
The so-called "cross dimension", or height, H302 or H304 of each layer 302 or 304 is comprised between 70 millimeters and 100 millimeters, typically equal to substantially 85 millimeters, whereas its so-called "axial length" L302 or L304 is comprised between 50 millimeters and 200 millimeters, typically equal to substantially 150 millimeters. The axial length L306 of stopping member, which is inferior to that L302 or L304 of insulating layer, is comprised between 25 millimeters and 50 millimeters, typically equal to substantially 40 millimeters.
The thickness of insulating layers 302 and 304 is comprised between 0.3 millimeters and 2 millimeters, typically equal to 0.5 millimeter. The thickness of said blanket 306 is comprised between 20 millimeters and 35 millimeters, typically equal to substantially 25 millimeters. For sake of clarity, the thickness of blanket 306 has been far exaggerated on figures 21 and 22. Moreover layers 302 and 304, which are placed under said blanket 306, are indicated by hatching but are illustrated without any thickness on these figures.
Figures 23 and 24 illustrate an alternative embodiment of the insulating means according to the invention. This variant provides one single layer 302' of insulating paint, which covers the whole cross section of the groove, i.e. both side walls 171, 172 and upper wall 173. On the other hand, the axial dimension, as well as the thickness of this single layer, are substantially identical as those above mentioned by double layers 302 and 304 (between 0.3 mm and 2 mm, typically equal to 0.5 mm). Moreover, this single layer 302' is covered by a blanket 306', similar to that 306 shown on figures 21 and 22.
In the embodiments of figures 21 to 24, both the vertical walls 171 and 172 of the groove and the vertical walls of blanket 306 or 306' are sloped. In other words, said groove defines two slight lateral protrusions, which face each other. Moreover, the thickness of blanket 306 or 306' is substantially constant.
Figures 25 and 26 illustrate alternative embodiments showing the same insulation techniques as figures 21 and 22, whereas figures 27 and 28 illustrate alternative embodiments showing the same insulation techniques as figures 23 and 24. Figures 25 to 28 differ from figures 21 to 24, in that vertical walls 171 and 172 of the groove and vertical walls of blanket 306 or 306' are straight. The thickness of blanket 306 or 306' is substantially constant.

Claims

A cathode assembly (C) suitable for a Hall-Heroult electrolysis cell, comprising
- a cathode body (1) made of a carbonaceous material;

- at least one cathode bar (3, 3') made of a first conductive material, said cathode bar being fitted in a groove (17) provided in said cathode body;

- at least one insert (5, 5') 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 (37) or bore provided in said cathode bar;
characterized in that said slot (37) leads to an end wall (31) of said cathode bar (3) and an end wall (51) of said insert (5) is substantially flush with said end wall (31) of said cathode bar,
in that said cathode assembly is further provided with connection means (2, 4, 6) intended to connect said cathode assembly with a cathodic bus bar (100),
in that said connection means comprise at least one first connection member (2) extending across said slot (37) or bore, as well as across said insert (5), viewed from the end,
in that the free end of said first connection member (2), adjacent to said slot and said insert, comprises at least one connection zone (26a, 26b) for mechanical and electrical connection to said end wall (31) of cathode bar,
and in that said free end of said first connection member (2) also comprises a non-connection zone (27) which does neither mechanically nor electrically contact said insert (5).
A cathode assembly according to claim 1, characterized in that said free end of first connection member (2) comprises a central non-connection zone (27) facing said insert, as well as two lateral connection zones (26a, 26b) provided on either side of non-connection zone (27), said two lateral connection zones being mechanically and electrically connected to said end wall (31) of cathode bar, on either side of said slot (37), said central non-connection zone (27) being preferably flush with said lateral connection zones (26a, 26b).
A cathode assembly according to any of above claims, characterized in that said end wall (31) of said cathode bar protrudes with respect to said end wall (51) of said insert (5), by a length of protrusion (L35) which is inferior to 10 millimetres, in particular of about 5 millimetres.
A cathode assembly according to any of above claims, characterized in that said connection means (2, 4, 6) further comprise at least one other connection member (4, 6) which extends away from said insert, viewed from the end, said connection means preferably comprising one single first connection member (2), as well as two other connection members (4, 6), these three connection members being located the one under the others.
A cathode assembly according to claim 4, characterized in that each other connection member (4, 6) comprises a free end which is provided with a connection zone (46, 66) on said end wall (31) of cathode bar, said connection zone extending over the whole width of said end wall (31).
A cathode assembly according to any of claims 4 to 5, characterized in that said free end of each connection member (2, 4, 6) is provided with an abutment edge (25, 45, 65) for abutment against end wall (31) of cathode bar, the height of connection zone (26a, 26b, 46, 66) being preferably superior to the height of abutment edge (25, 45, 65), the ratio between height of connection zone and height of abutment edge being in particular equal to 2.
A cathode assembly according to any above claim, characterized in that each connection zone is formed by a weld (26a, 26b, 46, 66), the ratio between the sum of the surface areas (S26a, S26b, S46, S66) of said welds (26a, 26b, 46, 66) and the whole surface area of end wall of cathode bar being preferably superior to 45%, in particular to 55%.
A cathode assembly according to any above claim, characterized in that each connection member (2, 4, 6) comprises a flexible member (21, 41, 61) intended to be connected to said cathode bus bar, as well as a transition member (22, 42, 62) attached to said flexible member, said transition member being connected to said cathode bar and being provided with said connection zones (26a, 26b, 46, 66) and, potentially, said non-connection zone (27), and wherein each transition member (22, 42, 62) advantageously comprises
- a plate (23, 43, 63) made of aluminium, said plate being attached with said flexible member, as well as

- a block (24, 44, 64) made of steel, said block being attached to said plate and being provided with said connection zones (26a, 26b, 46, 66) and, potentially, said non-connection zone (27).
A cathode assembly according to any above claim, characterized in that peripheral facing walls of said groove (17) and said cathode bar (3) define a gap that is at least partially filled with a conductive filling material (F) and that, in the vicinity of an end wall (11) of cathode body (1), said peripheral facing walls of said groove and said cathode bar define a so-called electrically non-conductive region (302, 304, 306; 302', 306').
A cathode assembly according to claim 9, characterized in that said electrically non-conductive region comprises an insulating member (302, 304; 302'), provided at the end of said groove, which extends over at least part of the peripheral wall of said groove, the axial length (L302, L304; L302') of said insulating member being preferably comprised between 50 and 200 mm, typically equal to substantially 150 mm, and/or said insulating member being preferably at least one layer (302, 304; 302') of insulating material, in particular of insulating paint.
A cathode assembly according to claim 9, characterized in that said electrically non-conductive region also comprises a stopping member (306; 306'), provided at the end of said groove, said stopping member being adapted to prevent expansion of filling material (F) towards the end of said groove, and said stopping member being preferably a blanket (306; 306'), in particular a blanket made of ceramic fiber.
A cathode assembly according to any above claim, 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.
A cathode assembly according to claim 12, 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, wherein preferably
• 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.
A cathode assembly according to claim 13, 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.
A process for making a cathode assembly according to any of above claims, 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, said cathode bar being fitted in said groove or bore provided in said cathode body;

d) providing a slot or bore in said cathode bar, said slot or bore leading to one end wall of said cathode bar;

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 said slot or bore provided in said cathode bar, one end wall of said insert being substantially flush with said end wall of said cathode bar;

f) providing at least one first connection member (2)

g) connecting said first connection member (2) to said end wall (31) of cathode bar, either side of said slot, in a way such as on the free end of said first connection member, adjacent to said slot and said insert, forms at least one connection zone (26a, 26b) for mechanical and electrical connection to said end wall (31) of cathode bar, and in that said free end of said first connection member (2) also forms a non-connection zone (27) which does not mechanically nor electrically contact said insert (5),
said process optionally further comprising
- providing at least one other connection member (4, 6),

- connecting said other connection member (4, 6) to said end wall (31) of cathode bar, in a way such as said other connection member (4, 6) extends away from said insert, viewed from the front,
knowing that connecting said first connection member (2) to said end wall (31) of cathode bar is advantageously carried out before connecting said other connection member (4, 6) to said end wall (31) of cathode bar.