WO2017168310A1

WO2017168310A1 - Cathode block with copper-aluminium insert for electrolytic cell suitable for the hall-héroult process

Info

Publication number: WO2017168310A1
Application number: PCT/IB2017/051750
Authority: WO
Inventors: Bernard JONQUA; Abdalla ALZAROONI
Original assignee: Dubai Aluminium Pjsc
Priority date: 2016-03-29
Filing date: 2017-03-28
Publication date: 2017-10-05
Also published as: GB201605169D0; GB2548830A

Abstract

Cathode assembly suitable for use in a Hall-Heroult electrolysis cell (1), comprising a cathode block (12) comprising a carbonaceous material, and at least one current collector subassembly (10); - said cathode block being provided with at least one groove or bore (22), and at least one current collector subassembly being at least partially fitted into said groove or bore and protruding out of at least one end of said cathode block, - said current collector subassembly comprising a current collector bar (10) being made from a first metallic material, and provided with a groove or bore (22), - said current collector bar being provided with a metallic insert fitted into said groove or bore, wherein: o said metallic insert is a rod or bar comprising at least one first section extending over a first length, made from a second metallic material (27), and at least one second section extending over a second length, made from a third metallic material (28), o said second and third metallic materials being different from said first metallic material and having an electrical conductivity higher than said first metallic material, and said second metallic material being different from said third metallic material, and o said first section being closer to the center of said cathode block than said second section, and o said second section being spaced apart from the protruding end of said current collector.

Description

Cathode block with copper-aluminium insert for electrolytic cell suitable for the

Hall-Heroult process

Technical field of the invention

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

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 a lining (comprising refractory bricks protecting said steel potshell against heat, and cathode blocks usually made from graphite, anthracite or a mixture of both), and a plurality of anodes (usually made from carbon) that plunge into the liquid electrolyte. Anodes and cathodes are connected to external busbars. An electrical current is passed through the cell (typically at a voltage between 3.5 V to 5 V) which electrochemically reduces the aluminium oxide, split by the electrolyte into aluminium and oxygen ions, into aluminium at the cathode and oxygen at the anode; said oxygen reacting with the carbon of the anode to form carbon dioxide. The resulting metallic aluminium is not miscible with the liquid electrolyte, has a higher density than the liquid electrolyte and will thus accumulate as a liquid metal pad on the cathode surface from where it needs to be removed from time to time, usually by suction into a crucible.

The electrical energy is 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. Industrial electrolytic cells used for the Hall-Heroult process are generally rectangular in shape and connected electrically in series, the ends of the series being connected to the positive and negative poles of an electrical rectification and control substation. The general outline of these cells is known to a person skilled in the art and will not be repeated here in detail. They have a length usually comprised between 8 and 25 meters and a width usually comprised between 3 and 5 meters. The cells (also called "pots") are always operated in series of several tens (up to more than a hundred) pots (such a series being also called a "potline"); within each series DC currents flow from one cell to the neighbouring cell. For protection the cells are arranged in a building, with the cells arranged in rows either side-by-side, that is to say that the long side of each cell is perpendicular to the axis of the series, or end-to-end, that is to say that the long side of each cell is parallel to the axis of the series. It is customary to designate the sides for side- by-side cells (or ends for end-to end cells) of the cells by the terms "upstream" and "downstream" with reference to the current orientation in the series. The current enters the upstream and exits downstream of the cell. The electrical currents in most modern electrolytic cells using the Hall-Heroult process exceed 200 kA and can reach 400 kA, 450 kA or even more; in these potlines the pots are arranged side by side. Most newly installed pots operate at a current comprised between about 350 kA and 600 kA, and more often in the order of 400 kA to 500 kA.

The 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.

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; such an embodiment is described in WO 03/014434 (Alcoa). Other structures ensuring a variation of the electrical conductivity of the cathode bar along its length are described in WO 2004/059039 (SGL Carbon), WO 2004/031452 (Alcan), and WO 2008/062318 (Rio Tinto Alcan). The problem that the present invention endeavors to solve is to further improve cathode collector bars, in particular regarding their current collection efficiency, to fine tune the current distribution in the cathode block (especially to obtain an even distribution of the current along the length of the cathode collector bar), to further decrease the voltage drop, and to decrease the cost of such improved collector bars. Object of the invention

According to the present invention the problem has been solved by providing a cathode block with a cathode collector bar made from a first metal with a first electrical conductivity, comprising an insert made from one or more metals (so-called "second metal") different from the first metal, said second metal each having a second electrical conductivity, wherein said first and second metals are selected such that said second electrical conductivity is higher than said first conductivity, said cathode collector bar being configured to decrease energy consumption (cathode drop) and provide an even current distribution in a cathode block.

The part of the cathode collector bar that is made from said first metal is called here "first conductor". Said first conductor of the collector bar is generally made from mild steel, which withstands the high temperatures found in the middle of the operating electrolytic cell (up to 1000°C in the middle of the cathode blocks). Steel as the first metal offers the best compromise between electrical conductivity, cost, mechanical resistance, and long- term behavior in high-temperature contact with the carbonaceous cathode material. However, its electrical conductivity at the operating temperature is rather low, thereby generating a significant voltage drop in the electrolytic cell. In order to increase the overall electrical conductivity of the cathode collector bar, an insert with a material having an electrical conductivity higher than steel is provided in the steel collector bar.

Copper as the second metal offers the highest electrical conductivity, even if its melting point (about 1080°C for pure copper) is rather close to the temperature of the liquid phases in the pot (around 950°C to 1000°C).

A first objective of the invention is to optimize current collection of the cathode collector bar and to decrease the voltage drop. This is achieved by positioning the copper bar in the upper part of the cathode collector bar, where most of the current is flowing from the aluminium metal pad. This positioning ensures that most of the current collected by the cathode block from the liquid metal pad and transmitted to the cathode collector bars is flowing only through the vertical sides of the cathode collector bars and most likely on the upper parts of vertical sides of the cathode collector bar. This positioning in the upper part can be achieved by inserting the copper bar in a groove that has been machined in the upper face of the cathode collector bar, or into a bore drilled in the upper part of the cathode collector bar.

According to one aspect of the invention, the cathode collector bar is a steel bar with a copper insert, wherein said copper insert is positioned in the upper face of the cathode collector bar. This maximizes current flow collection by said copper insert, but requires good pot control, in particular avoidance of events that might increase the temperature of the liquid phases in the pot close to the melting point of copper. In fact, as temperature is decreasing throughout the thickness of the cathode block, the upper side of the cathode collector bar is hotter than the lower side of the cathode collector bar.

According to another aspect of the invention the cathode collector bar comprises a first conductor and one second conductor made with two different metals, the first conductor being electrically connected to the bus bar system and having its external surfaces in contact with the cathode block within a groove provided inside the cathode blocks, the second conductor being composed by copper and aluminium rods having an electrical resistance less than the first conductor, the second conductor being positioned inside a bore situated on the upper part of the first conductor.

A second objective of the invention is to decrease the cost of fabrication of the cathode bar collector. This is achieved by replacing at least a part of the copper by metal having a higher conductivity than steel, but which is less expensive than copper. Aluminium is the preferred third metal. Manufacturing the bi-metallic insert from a second metal which is copper and a third metal (preferably aluminium) having a higher electrical conductivity than steel, but which is less expensive than copper, decreases the total price of the insert. However, when aluminium is used as the third metal, the design of such a bimetallic insert needs to take into account the melting point of aluminium (about 660 °C) which is much lower than that of copper, and much lower than the temperature of the liquid phases in the electrolytic cell.

According to another aspect of the invention said insert made from a second metal comprises at least first section made from copper (length L2) and a second section made from aluminium (length L3,) said first section being close to the center of the cathode block, and said second section being close to the end of the cathode collector bar that is protruding out of the cathode (this end is called here the " end of the cathode collector bar", and the end face of the cathode from which said end of the cathode collector bar is protruding out is called here the "end of the cathode block". This both decreases fabrication cost and minimizes the heat losses through the cathode bar compared to an insert made from a single second metal which is copper, since aluminium has a lower thermal conductivity than copper. Compared to a full copper insert with the same dimensions, replacing part of the copper bar the bar extremities by aluminium and thereby creating a bi-metallic insert minimizes the thermal exchange at the end of the collector bars. According to another aspect of the invention said copper bar has a cylindrical shape, with a round or oval cross section.

According to yet another aspect of the invention said copper bar has a rectangular cross section.

Another objective of the invention is to obtain an even distribution of the collected cathode current. This is achieved by providing a gap or non contact zone between the copper and the steel collector bar at the extremity end of the cathode block. This gap or non contact zone can be achieved by reducing the copper diameter at that end, so that there is no contact between the outer surface of the copper bar and the surface of the grove or bore machined or drilled into the steel cathode collector bar. This unsealing zone avoids current draining at that end and therefore improves the current distribution.

According to another aspect of the invention a gap (also called here "non-contact zone") is provided by decreasing the diameter of the copper bar over a certain length (preferably comprised between 100 mm and 250 mm, such as 150 mm) and having the copper rod going up to 400 mm to the pot center axis. This non-contact zone extends preferably from the extremity end of the cathode block over a certain length of recessed diameter towards the center of the cathode block. This minimizes the current density and thereby the cathode erosion near the extremity end of the cathode block, which increases the expectancy life of the pot lining.

According to yet another aspect of the invention an exhaust hole orientated towards the pot bottom is provided in the steel copper bar, close to the end of the bore provided inside the bar. It eases the installation of the insert when the insert is pushed into the bore, and in any case allows the copper to escape from the cathode collector bar in case of copper melting under possible drastic pot conditions; for this latter effect a diameter comprise between 4 mm and 6 mm is preferred for said exhaust hole.

A first object of the invention is therefore a cathode assembly suitable for use in a Hall- Heroult electrolysis cell, comprising a cathode block comprising a carbonaceous material, and at least one current collector subassembly;

- said cathode block being provided with at least one groove or bore, and at least one current collector subassembly being at least partially fitted into said groove or bore and protruding out of at least one end of said cathode block by a so-called protruding section having a protruding length L_P,

- said current collector subassembly comprising a current collector bar being made from a first metallic material, and provided with a groove or bore, - said current collector bar being provided with a metallic insert fitted into said groove or bore; wherein:

o said metallic insert is a rod or bar comprising at least one first section extending over a first length L₂, made from a second metallic material, and at least one second section extending over a second length L₃, made from a third metallic material,

o said second and third metallic materials being different from said first metallic material and having an electrical conductivity higher than said first metallic material, and said second metallic material being different from said third metallic material, and

o said first section being closer to the center of said cathode block than said second section, and

o said second section being spaced apart from the protruding end of said current collector bar by a so-called spacer section having a spacer length L_s.

Said first metallic material can be a ferrous material. Said second metallic material can be copper. Said third metallic material can be aluminium.

In an embodiment, said spacer length L_s is equal to said protruding length L_P plus or minus 20 %, and preferably equal to said protruding length L_P plus or minus 10%. In other embodiments, said spacer length L_s can be at least equal to said protruding length L_P, and/or

said current collector subassembly extends over the whole length of said cathode block and protrudes out of each end of said cathode block, and/or said current collector subassembly extends over less than half of the length of said cathode block and protrudes out of one end of said cathode block.

In an embodiment that can be combined with any of the preceding embodiments, said cathode block has a first surface intended to face the electrolyte in said electrolytic cell (so-called "hot cathode surface"), and an opposite surface (so-called « cold cathode surface »), and said groove or bore provided in said cathode block is closer to said cold surface, than to said hot surface.

In an embodiment that can be combined with any of the preceding embodiment, said current collector assembly has a first surface (so-called "hot collector bar surface") facing said hot cathode surface, and an opposite surface (so-called "cold collector bar surface") facing said cold cathode surface, and wherein said groove or bore provided in said current collector is closer to said cold collector bar surface than to said hot collector bar surface. Another object of the present invention is a process for manufacturing a cathode assembly suitable for use in a Hall-Heroult electrolysis cell, comprising the steps of: providing a cathode block comprising a carbonaceous material and at least one current collector subassembly according to the invention; machining at least one groove or drilling at least one bore in a direction parallel to the length of said cathode block; and fitting said current collector subassembly into said groove or bore.

Another object of the invention is an electrolysis cell suitable for the Hall-Heroult process, wherein said electrolytic cell comprises one or more cathode assemblies according to the invention.

A last object of the invention is a process for producing aluminium in a Hall-Heroult electrolysis cell, wherein said electrolysis cell comprises one or more cathode assemblies according to the invention.

Figures

The invention is described in more detail below, with reference to the accompanying drawings, labelled Figures 1 to 6. Figure 1 illustrates a prior art embodiment, while figures 2 to 6 illustrate embodiments according to the present invention.

Figure 1 shows a schematic transverse cross-sectional view of a prior art electrolytic cell for aluminium production according to the Hall-Heroult process, having a single rectangular cathode collector bar per cathode block.

Figure 2 is a schematic transverse cross-sectional view of an electrolytic cell for aluminium production according the Hall-Heroult process, having a cathode block in accordance with the present invention.

Figure 3 is a schematic longitudinal cross-sectional view of the right-hand part of the cathode collector bar in the electrolytic cell depicted on figure 2.

Figure 4 is a schematic transverse cross-sectional view of the cathode collector bar taken along the line A-A of figure 3.

Figure 5 shows a plot of the current distribution in a Hall-Heroult electrolytic cell according to Figures 1 (curve A, prior art) and 2 (curves B, C and D, according to three different embodiments of the invention: curve B with full contact between the insert and the cathode collector, curves C and D with a non-contact zone over 100 mm (curve C) or 150 mm (curve D) as shown on figure 2.

Figure 6 is a schematic longitudinal cross sectional view of a cathode with a cathode collector bar according to the invention. The following reference numbers are used in the figures:

Description

In the present document, the terms "upper" and "lower" used in relation with a cathode block and its components refer to the position of said cathode block in the electrolytic cell. As a consequence, compared to a "lower" position, an "upper" position is closer to the metal pad than the "lower " position. The terms "center" and "end" used in relation with a cathode block or a cathode collector bar refer to the length of the cathode block.

The apparatus and method of the present invention provide a novel cathode assembly comprising a novel cathode current collector assembly that minimizes horizontal electrical currents and decreases the cathode drop while controlling heat losses. These cathode assemblies may be incorporated into existing aluminium production cells having standard carbon cathode blocks.

Referring to Figure 1 , there is shown a prior art electrolytic cell 1 for aluminium production. The cell 1 includes a potshell comprising a first cell longitudinal wall 16 and a second longitudinal wall 17. The cell walls define a space lined on its bottom and sides with refractory materials along with the cathode blocks 12, thereby defining a volume containing the molten metal and electrolyte. Said cathode blocks 12 are equipped with conventional cathode collector bars 10. Such cathode collector bars 10 have a rectangular cross-section and are fabricated from mild steel. Electrical current enters the cell through anodes 13 (suspended above the cell by anode rods 3 attached to an anode frame 4) passes through the molten electrolytic bath 14, and the molten aluminium pad 15, and then enters the carbon cathode block 12. The current is carried out of the cell by the cathode collector bar 10 connected to the cathode busbar 18, 19 (shown on figure 2). The cell 1 is closed by a set of hoods 5. As shown in Figure 5 (curve A) for a typical embodiment of such a cell having the dimensions 3,900 mm x 9,550 mm and operating at a current of 246 kA, the electrical current distribution average value for the first 500 mm from the central axis of the cathode block is - 4,846 A/m² whereas for the last 200 mm the average is -20,274 A/m², i.e. about 4.8 times more than in the middle of the cathode block. The maximum current at cathode block extremities is about 5.8 times greater than the minimum current at cathode block center. Referring now to Figure 2, there is shown electrolytic cell 2 of the present invention, similar to that of figure 1 , except for the cathode assembly. In this embodiment, the cathode block has one single cathode collector bar 10. The two cathode collector bar ends extend from the cathode bus bars 18, 19 outside the cell walls. The cathode block 12 has an upper surface 20 supporting the molten metal (called here "hot cathode surface") and a lower face 21 (called here "cold cathode surface") defining a groove 22 extending from one end to the other end of the cathode block. The steel cathode collector bar 10 is placed inside the groove and secured by a layer of cast iron (not shown on the figures) joining the cathode collector bar 10 to the cathode block 12.

Referring now to Figures 2 and 3, the cathode collector bar 10 includes a body made from a first metallic material (typically a ferrous metal body) comprising a solid spacer 23 in its center and one sheath on each end 24 and 25 defining a cavity. Each cathode collector bar end is connected with cathode bus bars 18, 19.

Referring now to Figure 3, a cavity 26 is provided into the two ends of the cathode collector bar 10. Said cavities can be machined.

In an exemplary embodiment of a cell with dimensions 3,900 mm x 9,550 mm operating at 246 kA, said cavity has a diameter of 55.0 / +0.05 mm, and the cathode collector bar has a cross sectional area of 23.76 cm². The cavity depths are 1 ,750 mm from each cathode collector bar end.

The insert, of total length L, made from one or more metals different from the first metal is preferably made of a material having a high electrical conductivity, such as copper. However, copper also has a very high thermal conductivity, which leads to increased heat losses of the cell. It is therefore preferable to provide a section of the insert towards the outside of the cell by using a third metal having a higher electrical conductivity than said first metal, but a lower thermal conductivity than said second metal. Said third metal can be aluminium.

In an advantageous embodiment of the invention, the insert therefore comprises a second conductor 27 made from a second metal, of length L₂, and a third conductor 28 made from a third metal, of length L₃, said third conductor being a rod or bar. The higher electrical conductivity for the second conductor promotes a uniform electrical current distribution along the cathode collector bar being inside the cathode block (as will be shown below), thereby creating a uniform current density at the cathode block surface then a better stability of the operating cell. In addition, from the exit of the cathode block groove, the higher electrical conductivity of the second conductor extended by the third conductor provides a lower resistance between the cathode blocks and the external current carrier (cathode bus bar system), therefore reducing the voltage drop of the entire cathode block assembly.

Said third conductor 28 can extend until the end of the cathode collector bar, or there the first conductor can extend until the end of the cathode collector bar (as shown on figure 2). The second 27 and third 28 conductors have different metal compositions than the first conductor 10 being usually made of low carbon steel. The choice of the second conductor can be made in relation to its properties related to the electrical conductivity which is 44 times better than the first conductor but its thermal conductivity is 26.7 times greater than the first conductor. As a result the choice of the third conductor was related mainly to its thermal conductivity which is 41 % lower than the copper; whereas the electrical conductivity of aluminium is just 37 % lower than the copper metal and still 28 times better than the first conductor.

In an advantageous embodiment the bi-metallic insert is composed of one copper element 27 of length L₂ (in the exemplary embodiment: of 1 ,400 mm long and 55 mm of diameter), and one aluminium element 28 of length L3 (in the exemplary embodiment: of 220 mm long with the same diameter). The machining tolerance for the two cylindrical inserts should be +0/-0.05. The two elements can be screwed together by means of prominent threads in the aluminium part and a threaded hole inside the copper part, prior to inserting them into the cathode collector bar. To avoid or minimize the possible galvanic corrosion between copper and aluminium a thin layer of graphite can be spread onto the two threaded parts before screwing them together. In a preferred embodiment the copper grade for this use is an extreme high conductivity copper known as Cu-OFE (Oxygen-Free Electronic) at 99.98 wt % copper, whereas the aluminium insert is high conductivity grade. The bi-metallic inserts are then introduced inside the holes or grooves and adjusted at a predefined distance D (typically comprised between 60 mm and 150 mm, preferably between 80 mm and 130 mm, and still more preferably between 100 and 120 mm) from the cathode collector bar end. This leaves a gap G the extremity of the copper rod end and the hole end, towards the middle of the cathode collector bar. This gap allows for thermal expansion of the insert. It can have a length comprised between 10 mm and 30 mm, and preferably between 15 mm and 25 mm. Figure 4 shows an embodiment with a bore, while Figure 6 shows an embodiment with a groove. In the case of a groove the end of the cathode collector bar can be the material of the first conductor (steel), if the length of the groove is just sufficient to accept the total length L of the insert. In the case of a bore the end of the cathode bar insert can remain empty.

In an advantageous embodiment of the invention the end of the cathode collector bar insert (more precisely: the end of its third conductor 28) is at the same level as the end of the cathode block 12 (this is shown on figure 6), knowing that the cathode collector bar 10 is always protruding out of the cathode block 12, typically by about 100 mm to 500 mm.

In the embodiment of the cathode assembly according to the invention shown on figure 6, the cathode assembly comprises two current collector subassemblies 32,33, each of them extending over less than half of the length of said cathode block 12, each of them comprising a cathode collector bar 10a, 10b that is protruding out of one end of said cathode block 12. Furthermore, it can be seen that the groove or bore provided in said cathode block is closer to said cold cathode block surface 21 , than to said hot cathode block surface 20. In general, said current collector assembly has a first surface (so-called "hot collector bar surface") 30 facing said hot cathode surface 20, and an opposite surface 31 (so-called "cold collector bar surface") facing said cold cathode surface 21 (see Figure 2). In an advantageous embodiment said groove or bore provided in said current collector is closer to said hot collector bar surface 30 than to said cold collector bar surface 31 (see Figures 3 and 4).

In another advantageous embodiment, which can be combined with any other embodiment or variant, an air event 29 is provided at the end of the hole towards the lower side of the cathode collector bar. This hole is acting as an exhaust hole when installing the inserts and in case of copper melting under possible drastic pot conditions. The diameter of the exhaust hole should be comprised between about 4 mm and about 6 mm.

The arrangement and method according to the present invention redirect the current in an electrolytic cell; this reduces inefficiencies due to non-uniform current along the cathode blocks, and results in a reduction of horizontal currents that are caused by the nonuniform current.

The molten aluminium pad 15 has an electrical conductivity and this influences the current distribution along the cathode blocks 12: most of the current is concentrated at the extremities of the cathode blocks, creating a very high current density on this zone. This leads to an increases wearing rate of the cathode block in said zone, which results in a decrease of the pot life expectancy as the cathode blocks wear is approaching the cathode collector bars.

In Figure 5, there are shown three current densities at top cathode surface from the middle of the cathodes to their extremities. Curve A is the current distribution of a prior art cell design. The highest current concentration is found directly over the steel collector bar close to the end of the cathode blocks whereas the lower current distribution is on the middle of the cathode block: for the last 100 mm of the cathode block the average current value is 21 ,422 A/m² while the average current value is 4,846 A/m² for 500 mm from the cathode block median axis. The average current value on the cathode block extremities is 4.4 times greater than the one on the median zone of the cathodes blocks.

Curve B represents the current distribution for an electrolytic cell equipped with inserts according to the invention, having copper and aluminium rods screwed together. The copper rod is in contact with the cathode collector bar over the whole length of the hole provided inside the steel bar. The effect of using this Cu/AI insert is clearly reflected in the average value of the current density for the last 100 mm of the cathode blocks which is 14, 132 Am² while the average current value is 7,023 A/m² for 500 mm from the cathode block median axis. In this embodiment of the invention, the average current density on the cathode block extremities is only 2 times greater than on the median zone of the cathodes blocks. The wearing rate of the cathode block extremities will be reduced when using such Cu/AI inserts according to the invention.

Curve D represents the current distribution for an electrolytic cell equipped with Cu/AI inserts according to another embodiment of the present invention. In this embodiment there is a non-contact zone between copper rod and cathode collector bar for 150 mm from the cathode block groove exits. The effect of using this Cu/AI insert is clearly reflected in the average value of the current density for the last 100 mm of the cathode blocks which is 12,373 Am² while the average current value is 7,861 A/m² for 500 mm from the cathode block median axis. In this embodiment of the invention, the average current value on the cathode block extremities is only 1.58 times greater than the one on the median zone of the cathodes blocks. The wearing rate of the cathode block extremities is even further reduced with the help of the non-contact zone between the copper rod and the cathode collector bar on 150 mm from the cathode block groove extremities.

Curve C represents the same cathode collector bar as that of curve D except that the non- contact zone was 100 mm instead of 150 mm; the result is better than that represented by curve B but not as good as that of curve D.

Using an insert according to the invention leads to a reduction of the horizontal current inside the molten metal pad, improving that way the stability of the pot, and then the anode - cathode distance (ACD) of the pot can be reduced to decrease the total voltage of the electrolytic cell.

At pot operating temperatures, aluminium has an electrical conductivity of 3,470,000 (Ω/m)^"1 and steel has an electrical conductivity of 877,800 (Ω /m)^"1. The conductivity of copper is 1 ,628,000 (Ω/m)^"1 , which is considerably higher than that of steel. Considering the aluminium rod placed at the end of the insert and in relation to its operating temperature, its electrical conductivity is 2,820,000 (Ω/m)^"1 which is still very high compared to that of steel.

In reference to the thermal conductivity of the materials used for our invention, the thermal conductivity of the copper is given at 401 W/m.K while the thermal conductivity of the aluminium is given at 237 W/m.K, that is much less than the copper value. Installing an aluminium rod at the extremity of the copper rod allows to decrease the thermal losses at the cathode collector bar ends. In comparison steel has a thermal conductivity listed at 80 W/m.K. Table 1 shows results of cathode voltage drop expectation calculated by the thermo- electrical model. The cathode voltage drop calculated by the thermo-electric model is reduced by up to 69.7 mV. Table 1 : Cathode voltage drop (calculated) for a cell operating at 246 kA

Table 2 shows the actual results of cathode voltage drop for an electrolytic cell built according to the invention (with L = 150 mm), measured from the metal pad to the extremities of the cathode collector bars, in comparison with two identical cells operating of the same age in the same potline and equipped with identical cathode collector bar according to prior art. The reduction of voltage drop of -70.2 (average over 13 measurement carried out over a period of 14 consecutive days) is practically identical to the value calculated using the thermal electric model which is -69.7 mV. The quality of the liquid aluminium was the same for all three cells.

Table 2: Cathode voltage drop in mV (measured)

This gain of voltage can be utilized to reduce the production cost of the aluminium, or to increase the production of the pot by increasing the current at constant power. That is to say if 2 % of total voltage is gained by using the Cu/AI inserts according to the invention, these 2 % can be converted to 2 % more current (expressed in Ampere) going through the pots and therefore increasing the aluminium production by 2%. Indeed, in this example the specific energy per kg of aluminium produced decreased from 14.34 kW/h/kg to 14.10 kW/h/kg. The cathode collector bar according to the invention leads to an important decrease of the current density near to the cathode block ends; this significantly decreases the cathode block wear in this area, and eventually increases the life expectancy of the pot: it is known that the the main cause for a pot built with graphitized cathode blocks is the disappearance of the cathode block material close to the cathode block ends.

Using cathode blocks according to the invention also improves the stability of the metal pad, which allows decreasing the anode-cathode distance (ACD) without perturbing the current efficiency of the electrolytic cell. The decrease in ACD leads to a decrease of ohmic resistance, resulting in a further reduction in voltage drop, which adds to that gained by using the cathode blocks according to the invention.

In a further embodiments of the present invention (not shown on the figures), there is no solid spacer 23 provided in the single cathode bar 10. While these embodiments are not preferred, they are within the scope of the present invention.

In another embodiment (shown on figure 6) the cathode collector bar 10 is represented by two half bars 10a, 10b spaced apart in the center of the cathode block 10, leaving a solid spacer 34 within the cathode block.

Claims

1. Cathode assembly suitable for use in a Hall-Heroult electrolysis cell, comprising a cathode block comprising a carbonaceous material, and at least one current collector subassembly;

said cathode block being provided with at least one groove or bore, and at least one current collector subassembly being at least partially fitted into said groove or bore and protruding out of at least one end of said cathode block by a so-called protruding section having a protruding length L_P,

said current collector subassembly comprising a current collector bar being made from a first metallic material, and provided with a groove or bore, said current collector bar being provided with a metallic insert fitted into said groove or bore, wherein:

o said second section being spaced apart from the protruding end of said current collector bar by a so-called spacer section having a spacer length Ls.

2. Cathode assembly according to claim 1 , wherein said first metallic material is a ferrous material.

3. Cathode assembly according to claim 1 or 2, wherein said second metallic material is copper.

4. Cathode assembly according to any of claims 1 to 3, wherein said third metallic material is aluminium.

5. Cathode assembly according to any of claims 1 to 4, wherein said spacer length L_s is equal to said protruding length L_P plus or minus 20 %, and preferably equal to said protruding length L_P plus or minus 10%.

6. Cathode assembly according to any of claims 1 to 5, wherein said spacer length L_s is at least equal to said protruding length L_P.

7. Cathode assembly according to any of claims 1 to 6, wherein said current collector subassembly extends over the whole length of said cathode block and protrudes out of each end of said cathode block.

8. Cathode assembly according to any of claims 1 to 6, wherein said current collector subassembly extends over less than half of the length of said cathode block and protrudes out of one end of said cathode block.

9. Cathode assembly according to any of claims 1 to 8, wherein said cathode block has a first surface intended to face the electrolyte in said electrolytic cell (so-called "hot cathode surface"), and an opposite surface (so-called « cold cathode surface »), and said groove or bore provided in said cathode block is closer to said cold surface, than to said hot surface.

10. Cathode assembly according to any of claims 1 to 9, wherein said current collector assembly has a first surface (so-called "hot collector bar surface") facing said hot cathode surface, and an opposite surface (so-called "cold collector bar surface") facing said cold cathode surface, and wherein said groove or bore provided in said current collector is closer to said cold collector bar surface than to said hot collector bar surface.

1 1. Process for manufacturing a cathode assembly suitable for use in a Hall-Heroult electrolysis cell, comprising the steps of:

Providing a cathode block comprising a carbonaceous material and at least one current collector subassembly according to any of claims 1 to 10,

Machining at least one groove or drilling at least one bore in a direction parallel to the length of said cathode block,

Fitting said current collector subassembly into said groove or bore.

12. Electrolysis cell suitable for the Hall-Heroult process, wherein said electrolytic cell comprises one or more cathode assemblies according to any of claims 1 to 10.

13. Process for producing aluminium in a Hall-Heroult electrolysis cell, wherein said electrolysis cell comprises one or more cathode assemblies according to any of claims 1 to 10.