EP4139502B1

EP4139502B1 - Cathode assembly for a hall-heroult cell for aluminium production

Info

Publication number: EP4139502B1
Application number: EP20725624.9A
Authority: EP
Inventors: Asgeir BARDAL; Morten SUNDHEIM JENSEN
Original assignee: Norsk Hydro ASA
Current assignee: Norsk Hydro ASA
Priority date: 2020-04-24
Filing date: 2020-04-24
Publication date: 2024-03-13
Anticipated expiration: 2040-04-24
Also published as: JP2023530566A; WO2021213672A1; CA3179900A1; BR112022019157A2; AU2020443557A1; EP4139502A1

Description

The present invention relates to cathode block for a Hall-Héroult cell for electrolytic production of aluminium. In particular, the invention relates to cathode assembly with cathode blocks comprising a wear resistant composite material in selected areas or parts of the surface layer, and where the height of the cathode blocks can consequently be reduced. Based upon the cathode rodding principle, the cathode block height can be reduced even more.

Prior art

Conventional cathode blocks for aluminium production cells are prepared by mixing of a coke aggregate and a pitch binder. This mix is then extruded or pressed in a vibromold, before baking and subsequent graphitization. Such blocks are commonly referred to as graphitized cathode blocks.
Grooves/slots are machined in the bottom of the graphitized cathode blocks thereof allowing current leads such as collector bars to be entered into and joined to the cathode blocks, a procedure which is called cathode rodding. During cathode rodding, the space between the wall of the slots and the bars can commonly be filled with melted cast-iron or a contacting paste or glue for the fixation of said collector bars.
Rodding with collector plates is one other alternative, where steel shots can be filled in a space between groves in a cathode block attached to a collector plate. Yet another alternative is to use round Cu rods inserted in holes machined in the cathode blocks, instead of conventional steel collector bars.
The cathode block assembly consists of the cathode block and the cathode bar, plate or rod, connected to the cathode block after the rodding process.
Several cathode block assemblies are installed in the cell and form together a cathode panel.
Commonly, a Hall-Héroult cell is operated typically for 5-7 years before relining of the cathode becomes necessary mainly due a relatively high wear rate (typically 50-70 mm/year), or due to sodium penetration, swelling or cracking. Due to the relatively high wear rate the height of the cathode blocks are typically in the range of 450-500 mm, in order to ensure sufficient lifetime if the cell.
In commonly operated 350 kA cells the mass of cathode blocks in the cathode panel of one cell can represent 25-30 tons of weight. For a pot line of a given number of cells, say 700 cells and with a relining operation every 6th year on each cell, the mass of relined cathode block material amounts to 3000-3600 tons annually.
In accordance with the Applicants own WO2009/099335A1 there is applied electric conductive particles as a fill-in material between an electrical current conductor and a body of carbonaceous material in an electrode. The use of electric conductive particles without a hardening matrix facilitates mobility of the conductive particles for electric current when the geometry changes over time e.g. due to thermal expansion. The electrical resistance in a cathode element where such particles is applied has been observed to be improved with reference to commonly used contact paste or melted cast-iron. In Fig. 9 of said WO-document it is disclosed an end-view of a cathode block having recesses or slots in its lower part. There are arranged collector bars into the slots, where the remaining space is filled with electric conductive particles. The collector bars are fixed to a collector plate that collects the current and that further secures stability of the cathode element and gives additional contact area.
From other prior art there is known various patents and patent applications on use of TiB₂ particles intermixed with the carbon of the cathode blocks ('C-TiB₂ composite') used in reduction cells for production of aluminium by electrolysis. Here the C-TiB₂ composite can be either as a layer at the top of the cathode block, or throughout the full height of the cathode block. Such cathode blocks are known as wettable cathodes and they are intended for application in so called drained cells where there are no metal pool in the bottom of the electrolytic cell.
US 4,544,524 discloses production of cathodes where blocks or shaped bodies are produced from a titanium diboride - carbon eutectic.
WO 00/36187 relates to a method for producing a cathode with several layers. At least one metal boride containing layer overlies a graphite layer. In addition to metal boride the layers also contain carbon. Oxidation and erosion resistance can be improved.
One disadvantage with such wettable cathode blocks is that cost of the C-TiB₂ composite material will be high, and in order to support the high costs of these blocks a considerable reduction energy consumption during operation is required. Drained cells with a wettable cathode could theoretically reduce the energy consumption considerably, but in order to utilize the potential of such electrolysis cells a change of the electrolysis cell design as well as other new technology elements are required (e.g. inert side walls and inert anodes). Drained cells for aluminium electrolysis have, to this day, therefore not been implemented successfully.
US 4,624,766 , US 4,466,995 , DE 10 2012 201468 and WO 2019/174948 disclose further cathodes for aluminium production.

Present invention

The present invention relates to the cathode assembly of claim 1.
The present invention describes a concept called wear resistant cathode blocks which are intended for conventional Hall-Heroult cells rather than wettable cathode blocks for use in drained cells. Although both types of blocks may utilize a C-TiB₂ composite material, the design of the blocks, the composition and properties of the composite material as well as the cost will be quite different and thus the wear resistant cathode block solutions presented here could not be used as a wettable cathode in a drained cell. E.g. the whole surface of wettable cathodes for drained cells must be covered with a wettable composite material (e.g. C-TiB₂), whereas according to the present invention only selected parts of the surface of the wear resistant cathode block needs to be covered with the wear resistant composite material (e.g. C-TiB₂).
The wear resistant composite material used in selected areas of the surface of the wear resistant cathode block comprises a carbonaceous material and 5 - 80 wt% of refractory metal boride powders, such as TiB₂, HfB₂, ZrB₂, CrB₂, or WB₂, or refractory metal carbide powder such as SiC, Cr₃C₂, B₄C, TiC, or Al₂O₃ or any appropriate combinations of these.
The cathode block can be produced in several ways, for instance, the green cathode block can be prepared by adding the conventional cathode paste in the bottom of a conventional vibromold for cathode block production which is then evened out with a rake (or other another similar tool), before using the same rake to prepare pits in selected areas of the surface and that the wear resistant composite material is filled in these pits, before vibromolding is performed. The green wear resistant cathode block is then heat treated (i.e. baked and graphitized) in a similar manner as for conventional cathode blocks Alternatively, the green cathode block can be prepared by adding the composite material in selected areas in the bottom of a conventional vibromold for cathode block production, before adding the conventional cathode paste on top, which is then evened out with a rake (or a similar tool) and then vibromolded. This way the wear resistant cathode block is prepared upside-down. The green wear resistant cathode block is then heat treated (i.e. baked and graphitized) in a similar manner as for conventional cathode blocks
The selected areas or parts of the surface of the cathode block made of a wear resistant composite material corresponds to less than 100% of the total surface of cathode block, and in a second aspect the selected areas or parts of the surface of the cathode block made of a wear resistant composite material corresponds to less than 50% of the total surface of the cathode block while in a third aspect the selected areas or parts of the surface of the cathode block made of a wear resistant composite material corresponds to less than 30% of the total surface of the cathode block. In a fourth aspect of the invention, the selected areas or parts of the surface of the cathode block made of a wear resistant composite material corresponds to less than 10% of the total surface of the cathode block.
According to further aspects of the invention the wear resistant composite material has a thickness between 3 and 10 cm. According to another aspect of the invention the wear resistant composite material is arranged at selected areas of the cathode surface where the wear rate of the cathode is high and can be predicted by modelling or determined by autopsy.
The wear resistant composite material is placed in the areas towards the ends of the cathode block (i.e. the areas 0-80 cm from the end of the cathode block).
In a preferred embodiment, by combining some principles of the Applicant's own WO2009/099335A1 and NO 20180369 , ( WO 2019/174948 ), a collector plate construction with steel and Cu inserts can advantageously be applied instead of the steel cathode bars conventionally used as connectors between cathode block and flexibles to the busbar system.
Further, connecting those collector plates to the cathode block by metallic particles such as steel spheres and thermal expansion forces and by utilizing a wear resistant composite material in specific areas in the cathode block, it is possible to radically reduce the height of the cathode block from the typical height of 450-500 mm, to a height of 80-230 mm and still maintain the same cell life or even increase the cell life. By this, the yearly consume of cathode material can be reduced with 45-85% or more for a given potline. Further, by only applying the wear resistant composite material in specific areas of the cathode block the usage of the costly wear resistant material can be kept to a minimum. Since the present invention will be used in conventional electrolytic cells for aluminium metal production, it is possible to have a cathode block design where only parts of the surface are covered with the wear resistant composite material, as compared to wettable cathodes for drained cells where the whole surface of the cathode block must be covered with e.g. TiB₂-carbon composite material.
As an alternative embodiment, by using round Cu rods instead of conventional cathode bars, the height of the cathode block can be reduced to a height of 130-280 mm, made possible by utilizing a specially designed cathode block with a wear resistant composite material in specific areas of the cathode block. This can give approximately a yearly 45-75 % reduction or more in consume of cathode material for a given pot line.
As an yet another alternative embodiment, by using conventional cathode bars and rodding with cast iron, the height of the cathode block can be reduced to a height of 170-370 mm, made possible by applying a specially designed cathode block with a wear resistant composite material in specific areas of the cathode block. This can give approximately a yearly 25-65 % reduction or more in consume of cathode material for a given pot line.
One preferred embodiment can be put into practice by combining the following two elements:

i) A wear resistant composite material electrode material in specific areas of the cathode block.
ii) A collector plate as connection between the cathode block and the cathode flexibles connecting to the busbar system.

The alternative embodiment with Cu rods can be put into practice by combining the following two elements:

i) A wear resistant composite material electrode material in specific areas of the cathode block.
i) Cu rods as connection between the cathode block and the cathode flexibles connecting to the busbar system.

The alternative embodiment with conventional steel collector bars can be put into practice by combining the following two elements:

i) A wear resistant composite material electrode material in specific areas of the cathode block.
i) Cathode bars as connection between the cathode block and the cathode flexibles connecting to the busbar system.

This realization of the present invention was made possible by:

i) The low erosion rate experienced with the wear resistant composite material, avoiding the need to have a cathode block of conventional height (typically 450-500 mm) for ensuring sufficient cathode lifetime. Whereas the maximum wear rate of conventional cathode blocks may be in the range of 50-70 mm per year, the maximum wear rate of the wear resistant composite material will be substantially less (typically in the range 10-30 mm but may also be even lower). Moreover, the wear rate of the cathode surface is not evenly distributed over the surface (i.e. some locations have significantly higher wear rate than others). The exact mechanism for this wear is not fully understood, but it is believed that the wear rate increases with increased current density and magnetically induced metal flow on the cathode surface. The cathode wear rate on the cathode surface may be determined by experience when a cell is shut down and inspected, and the results from this inspection can be used as an input for modelling and design of wear resistant cathode blocks for the relining of the cell. Due to the high cost, the wear resistant composite material should only be used in selected areas where the wear rate is high, typically towards, but not restricted to, the ends of the cathode block.
ii) The preferred embodiment, the geometry of the collector plate, or the alternative embodiment, the Cu rod, avoiding the need to have 15 - 18 cm of carbon around the conventional cathode bars, allows for an even lower building height of the cathode element.

The invention is realized to by carrying out mechanical design of the cathode block assembly production process of the cathode block, joining process between cathode block and collector plate or Cu rod, and thermoelectric / thermo mechanic design of the full cathode shell - cathode lining including the cathode bar assembly.
It's a different cathode design from those realized in the past. Traditional thinking, based upon the prejudice of the skilled person, is that cathode block must be of a certain height due to the risk of tap-out, Na-swelling, stability reasons and more.
The present invention relates in the preferred embodiment to cathode assemblies based upon collector plates or in the alternative embodiment cathode assemblies based upon Cu rods or conventional collector bars, both with electrically conductive bodies with reduction of height. Some main elements are related to;

i) reduced cathode cost (less cathode material in general, less use of expensive cathode material in particular since it is only applied in selective areas or parts of the cathode)
ii) reduced mV loss through the cathode (less current path due to lower height) reducing energy consumption
iii) more space under the cathode block for insulation, reducing energy consumption
iv) increased cavity above the cathode block, yielding several operational advantages such as taller and hence longer lasting anodes, less anode consumption, less anode handing, better anode covering, more flexibility with regards to metal and bath height, less spillage of anode cover material in the basement.

These and further advantages will be achieved by the invention as defined in the accompanying claims.
The present invention will in the following be further described by figures and examples where;

Fig. 1 is a principal sketch showing in a cross sectional view the main parts of a Hall-Héroult cell,
Fig. 2 discloses an end view of a conventional cathode block with two slots for cathode bars,
Fig. 3 discloses a side view of a similar cathode block as that of Fig. 2,
Fig. 4 discloses an end view of a cathode block with two slots for cathode bars, where the cathode block has a special design with a lower height and with a wear resistant composite material, in selected areas of the upper layer thereof, according to the invention,
Fig. 5 discloses a side view of a similar cathode block as that of Fig. 4, where the wear resistant composite materials are placed in selected areas towards the ends of the cathode block
Fig. 6 discloses an end view of cathode block rodded with two Cu-rods, where the cathode block has a special design with a lower height and with a wear resistant composite material, in selected areas of the upper layer thereof, according to the invention,
Fig. 7 discloses a side view of a similar cathode block as that of Fig. 6, where the wear resistant composite materials are placed in selected areas towards the ends of the cathode block
Fig. 8 discloses in an end view a cathode block rodded onto a collector plate rods, where the cathode block has a special design with a lower height and with a wear resistant composite material, in selected areas of the upper layer thereof, according to the invention,
Fig. 9 discloses a side view of a similar cathode block as that of Fig. 8, where the wear resistant composite materials are placed in selected areas towards the ends of the cathode block

From Fig. 1 it can be seen a cross sectional view of the main parts of a conventional Hall-Héroult cell. The Figure shows a superstructure including alumina/fluoride hoppers, anode stubs, bus bars and feeding devices. Further, a pair of anodes partly covered by a crust is dipped into a liquid bath. Under the liquid bath there is shown a layer of liquid aluminium. The cathode is arranged below the liquid aluminium. There is shown two collector bars embedded in the cathode, from each end and inwards.
Fig. 2 discloses an end view of a conventional cathode block 24 have two cathode bars 22, 22' rodded in recesses by cast iron 21, 21'. The width of the cathode block can be in the range 40 - 60 cm and the height can be in the range 40 - 60 cm.
Fig. 3 discloses a side view of a conventional cathode block 24, similar to that of Fig. 2. The length of the block can be in the range 250 - 350 cm, the height can be in the range 40 - 60 cm.
Fig. 4 discloses an end view of a cathode block with two slots for cathode bars 22, 22', where the cathode block 24 has a special design with a lower height and with a wear resistant composite material, in selected areas of the upper layer 25 thereof, according to the invention. The upper layer in this embodiment can be of 5 - 10 cm thickness, while the cathode block can be of 25 - 35 cm thickness. The width can be in the range 40 - 60 cm.
Fig. 5 discloses a side view of a cathode block 24, where the cathode block 24 has a special design with a lower height and with a wear resistant composite material, in selected areas of the upper layer 25, 25' thereof according to the invention, similar to that of Fig. 4. The said areas are arranged at the end regions of the cathode block and being 5 - 10 cm thick in this embodiment.
Fig. 6 discloses in an end view a cathode block rodded with two Cu-rods 50, 50', where the cathode block 24 has a special design with a lower height and with a wear resistant composite material, in selected areas of the upper layer 25 thereof, according to the invention. The thickness of the wear resistant upper layer can be 5 - 10 cm while the total height of the cathode block can be 13 - 28 cm.
Fig. 7 discloses a side view of a cathode block corresponding to that of Fig. 6, where the cathode block 24 has a special design with a lower height and with a wear resistant composite material, in selected areas 25, 25' of the upper layer thereof, according to the invention. The length of the cathode block can be in the range 300 - 350 cm.
Fig. 8 discloses in an end view a cathode block 24 rodded onto a collector plate 20. The collector plate 20 is provided in this example with five collector elements, 30, 30' 30", 30'", 30ʺʺ that are in electrical contact with the collector plate 20. Preferably, these parts are made out of a steel quality that can easily be welded, and preferably the parts are welded together. A cathode block can be rodded to the collector elements, in a similar manner as disclosed in WO2009/099335A1 , where the solution may involve electric conductive metallic particles. The number of collector elements at the collector plate may differ from five as shown, for instance one to seven or even none, where the rodding material is arranged as a layer between the plate and the cathode block. The cathode block 24 has a special design with a lower height and with a wear resistant composite material, in selected areas 25 of the upper layer thereof, according to the invention. The thickness of this material can be 5 - 10 cm while the total height of the cathode block can be in the range 8 - 28 cm. The combination of rodding a collector plate with collector elements to a cathode block having a wear resistant composite material makes a particularly low cathode block height possible and is considered as a preferred embodiment of the invention.
Fig. 9 discloses a side view of a cathode block similar to that of Fig. 8, where the cathode block 24 has a special design with an even lower height and with a wear resistant composite material, in selected areas 25, 25' of the upper layer thereof, rodded onto a collector plate 20, according to the invention.

Further description of the preferred embodiment:

An appropriate collector plate can comprise at least one horizontal current outlet on at least one side and/or at least one vertical metallic current outlet connected to the collector plate.
In one embodiment, at least one thermocouple (TC) is inserted into a metallic component inside of or below the collector plate to be able to monitor the temperature at that location.
In a further embodiment, it comprises at least one horizontal current outlet on each end being integrated with the collector plate.
In one embodiment, there is arranged at least one vertical current outlet at the opposite side of the collector plate than the cathode block.
In one other embodiment, at least one metallic collector element is arranged at the upper side of a metallic collector plate, where said collector element is embedded in a corresponding recess in the bottom part of the cathode block, the recess being wider than the collector element and being filled with an electric conductive material comprising conductive particles.
In another embodiment, there are one or more collector elements, preferably 3 to 7 being separated at a distance of typically 50 mm to 150 mm.
In still another embodiment, the at least one collector element(s) is of same length or shorter length than the cathode block.
Advantageously, the collector plate can have one to five inserts of materials with higher electric conductivity, like copper.
The present cathode design is advantageous with regard to the magnetohydrodynamic stability of the cell it has been installed in, it may have an improved life cycle and less space usage and in operation, and it also represents a low cathode voltage drop with regard to a conventional cathode design.
At the ends of the collector plate 20, there can be arranged horizontal current outlets (not shown). The horizontal current outlets can be made out of conductors of a good conducting material like copper or copper alloy and further being, at least at its outlet ends, covered by a sheet material, preferably made out of a metal such as steel. The horizontal current outlets with their corresponding conductors can be integrated in slots made in the collector plate 20. This integration may be based upon press-fit tolerances or pre-heated plate sections to use thermal expansion for a tight fit. However, any appropriate fixation including welding may be applied. The conducting material in the slots may be covered by a protective steel plate on the upper and lower side.
Advantageously, the whole assembly with the cathode block 24 and the collector plate 20 are tilted somewhat during the filling procedure of the particles, to allow the particles to fill the recess in a smooth and complete manner. Additionally, some vibration might be applied to the plate or plate sections for homogeneous filling with the particles.
The recesses or slots in the bottom of the cathode block can be made in a green condition of the carbonaceous body by commonly used techniques or in a calcinated or graphitized condition by commonly available process equipment. The geometry of the slots has to fit the plates.
It should be understood that the electrical conducting solids or particles can be of any appropriate metal such as steel, iron, copper, aluminium etc., or alloys of same. Further, the shape of the solids can be spherical, oval or elliptic, flaked, or have any appropriate shape. The size and particle distribution may vary. The maximum size will in general be restricted by the width of the space to be filled. A non-homogenous distribution of particle sizes may be convenient to obtain a compact filling as possible, with little space between the particles.
Apart from having good electrical conducting properties, the applied material should have good mechanical properties (crushing properties) and be able to sustain high temperatures. As mentioned later, magnetic properties may be advantageous.
Further, the size of said solids can be from 0,1 millimetres and close to the minimum opening between the carbonaceous body and conductor plate. Commonly, the size may be up to 2 millimetres.
Preferably there can be several thermocouples attached to or inserted into the cathode plate to monitor the temperature in the cathode. For instance, holes up to the centre of the plate can be drilled in the cathode plate at appropriate locations for reception of thermocouples. The steel plate creates a protective housing for the thermocouples to survive the chemical aggressive environment during operation.
The insertion length of the horizontal outlets can preferably be limited in that it does not cover the central part of the cathode plate. The length of the insertion can for example be designed to reflect the existence of vertical outlets in that plate, and the path length of the current through the conductors to the next cell. On side-by-side arranged cells the length of the insertion can be made longer on the upstream side to balance the current pick-up in the cathode block to be more balanced.
Each cathode element can be fitted with horizontal outlets only for instance for end-to-end arranged cells or when there is no space for busbars under the cell, or with several horizontal outlets and one vertical outlet. To optimize the magnetic field, a configuration with one or two vertical outlets only and no horizontal outlet can be possible for some selected cathode block of the cathode panel as well.
A combination of different collector plate configurations can be applied in one cell to create a favourable magnetic field from the electric current distribution or enhance the thermal properties of the cell by reducing the number of outlets where a heat loss is undesired, e.g. on the short ends of the cell which tend to be colder due to the nearby corners. Vertical outlets attached to only some collector plates can be beneficial to optimize the current flow and magnetic field. This may as well reduce the costs of the installation when the current distribution and magnetohydrodynamic stability of the cell is sufficient.
The claimed collector plate cathode with the very low height cathode block has multiple advantages compared to a traditional design comprising a carbonaceous body with embedded collector bars:

In the preferred embodiment the block will be significantly lower, due to significantly lower erosion rate of the TiB₂-Carbon composite material the lifetime will still be higher.
The cathode voltage drop (CVD) is significantly lower (as low as 150mV) due to the low cathode block height, number of outlets, material electric properties, better electric contact due to initial mobility of particles, total surface of contact resistance and shorter current paths from the existence of vertical outlets
The current distribution into the top cathode block surface is more homogeneous due to the plate geometry, conductance of insertions, and existence of vertical outlets, thus avoiding undesired, instability causing horizontal currents in the liquid aluminium pad above the cathode block surface. The higher stability of the cell can be used to reduce the cell voltage and energy consumption further or increase the amperage and production volume
Due to the above mentioned better current distribution, the erosion of the carbonaceous material is more homogeneous thus increasing the life time of the cell
The vertical space usage of the arrangement is less than with conventional design, thus allowing for a lower cathode shell or - if the shell height is kept, to use the extra space for better bottom insulation, higher and longer-lasting anode blocks, or more height for liquid aluminium or bath
The design has a better ratio of electric to thermal conductivity at the most critical locations of high current density and heat flow, thus improving the energy efficiency of the cell (less heat loss and lower cathode voltage drop CVD)
When retrofitted to an existing cell design, vertical outlets according to the claims may allow to raise the amperage and thus increase production per cell
Better current distribution into the cathode surface giving improved MHD stability and thus options to reduce the ACD or increase the amperage level by up to 15%
Less heat loss because of smaller cross-section and exposed surface of HO and VO avoiding cold cathodes with bottom freeze, specifically if the technology is used to reduce energy consumption
Lower rodding cost in embodiments where there is no cast iron
No risk of cracking of carbon block in embodiments not using cast-in collector bars (cast iron)
Flexible installation of VOs after placing of the cathode blocks in the lining
Better balancing of current flow to upstream/downstream/bottom side giving better MHD stability
Considerably lower total height of the assembly giving space for more thermal bottom insulation or larger cavity. The difference could be up to 300 mm
Easier installation of thermocouples inside the plate due to less deep drilling than in collector bars, or direct access from bottom side'
Reduced amount of conventional cathode material saves costs in general, while the arrangement of the composite material only at selected areas where the wear of the surface of the cathode is expected as being most excessive, reduces the amount of expensive wear resistant material compared to conventional designs.

Claims

A cathode assembly for an electrolysis cell of Hall-Héroult type for producing aluminium, comprising a graphitized cathode block of an electronic conductive material rodded to steel collector bars, Cu rods or a metallic collector plate,
wherein,
selected areas or parts of the surface of the cathode block are made of a wear resistant composite material comprising 5 - 80 wt% of a refractory metal boride powder such as TiB₂, HfB₂, ZrB₂, CrB₂, WB₂, or refractory metal carbide powder such as SiC, Cr₃C₂, B₄C, TiC, or Al₂O₃, together with a carbonaceous material, the rest of the cathode block comprises conventional graphitized cathode material,

wherein the wear resistant composite material is placed in the areas towards the ends of the cathode block, i.e. the areas 0-80 cm from the end of the cathode block,

wherein the selected areas or parts of the surface of the cathode block made of a wear resistant composite material correspond to less than 100% of the total surface of the cathode block,

wherein the cathode assembly is obtainable by a method, wherein either:
(i) a green cathode block is prepared by adding a conventional cathode paste in the bottom of a conventional vibromold for cathode block production which is evened out with a rake or a similar tool before preparing pits in selected areas of the surface, wherein wear resistant composite material is filled in these pits and vibromolding is performed, followed by heat treatment such as baking or graphitization of the green cathode block in a similar manner as for conventional cathode blocks; or

(ii) a green cathode block is prepared by adding a wear resistant composite material in selected areas in the bottom of a conventional vibromold for cathode block production, before adding the conventional cathode paste on top, which is then evened out by a rake or a similar tool and then vibromolded followed by heat treatment such as baking or graphitization of the green cathode block in a similar manner as for conventional cathode blocks.
A cathode assembly according to claim 1,
wherein,
the wear resistant composite material has a thickness between 3 and 10 cm.
A cathode assembly according to claim 1,
wherein,
the selected areas or parts of the surface of the cathode block made of a wear resistant composite material corresponds to less than 50% of the total surface of the cathode block.
A cathode assembly according to claim 1,
wherein,
the selected areas or parts of the surface of the cathode block made of a wear resistant composite material corresponds to less than 30% of the total surface of the cathode block.
A cathode assembly according to claim 1,
wherein,
the selected areas or parts of the surface of the cathode block made of a wear resistant composite material corresponds to less than 10% of the total surface of the cathode block.
A cathode assembly according to claim 1-5,
wherein,
the cathode block after graphitization, is rodded to steel collector bars where the total height of the cathode block is between 25 and 35 cm.
A cathode assembly according to claim 1-5,
wherein,
the cathode block after graphitization, is rodded to Cu rods where the total height of the cathode block is between 13 and 28 cm.
A cathode assembly according to claim 1-5,
wherein,
the cathode block after graphitization, is rodded to a metallic collector plate where the total height of the cathode block is between 8 and 28 cm.