JP7588655B2 - Cathode assembly in a Hall-Hellou cell for aluminum production and method for making same - Google Patents

Description

The present invention relates to a cathode block in a Hall-Heroult cell for the electrolytic production of aluminum. In particular, the present invention relates to a catholyte having a cathode block comprising a wear-resistant composite material in selected areas or parts of the surface layer, in which case the height of the cathode block can be reduced as a result. Based on the principle of cathode rodding, the height of the cathode block can be further reduced. The present invention then also relates to a method for producing such a catholyte.

Conventional cathode blocks in aluminum production cells are prepared by mixing coke aggregates with a pitch binder. This mixture is then extruded or pressed in a vibromold and calcined, after which it is graphitized. Such blocks are commonly called graphitized cathode blocks.

The graphitized cathode block has grooves/slots machined into its bottom so that current leads such as collector bars can be inserted and bonded to the cathode block, a procedure called cathode rod machining. During cathode rod machining, the space between the walls of the slot and the collector bar can typically be filled with molten cast iron or an adhesive paste or glue to secure the collector bar.

Rod machining in the collector plate is another alternative where steel shots can be filled into the spaces between the grooves in the cathode block attached to the collector plate. Another alternative is to use round Cu rods inserted into holes machined in the cathode block instead of the traditional steel collector bars.

The cathode block assembly consists of a cathode block and a cathode bar, plate, or rod that is connected to the cathode block after the rod fabrication process.

Several cathode block assemblies are placed within a cell and together form a cathode panel.

In general, Hall-Helous cells are typically operated for 5-7 years before the cathode requires relining, primarily due to the relatively high wear rate (typically 50-70 mm/year) or due to sodium penetration, swelling, or cracking. The height of the cathode block is typically in the range of 450-500 mm to ensure adequate cell life due to the relatively high wear rate.

For a typically operating 350 kA cell, the mass of the cathode block in the cathode plate of one cell corresponds to a weight of 25-30 tonnes. For a given number of cells, say a pot line of 700 cells, with a relining operation of each cell every 6 years, the mass of relined cathode block material amounts to 3000-3600 tonnes per year.

According to the applicant's own patent EP 1 239 636, the conductive particles are applied as a filler material between the current conductor (electric conductive particles) and the body of carbonaceous material in the electrode. The use of the conductive particles without a hardening matrix promotes the mobility of the conductive particles for the electric current when the geometry changes over time, for example due to thermal expansion. It has been found that the electrical resistance in the cathode element with such particles applied is improved with respect to the commonly used adhesive paste or molten cast iron. In FIG. 9 of the patent EP 1 239 636, an end view of a cathode block having recesses or slots in its lower part is disclosed. In the slots, collector bars are arranged and the remaining space is filled with conductive particles. The collector bars are fixed to a collector plate that collects the current, further ensuring the stability of the cathode element and providing an additional adhesion area.

From other prior art, various patents and patent applications are known that relate to the use of _TiB2 particles mixed with carbon ("C- _TiB2 composite") in cathode blocks used in reduction cells for the production of aluminum by electrolysis. In this case, the C- _TiB2 composite can be present as a layer on the top of the cathode block or over the entire height of the cathode block. Such cathode blocks are known as wettable cathodes and are intended to be applied in so-called drain cells, where there is no metal pool at the bottom of the electrolytic cell.

(Patent Document 2) discloses the manufacture of cathodes by producing blocks or shapes from titanium diboride-carbon eutectic.

(US Pat. No. 5,399,633) relates to a method for producing a cathode having several layers, in which at least one metal boride-containing layer overlies a graphite layer. In addition to the metal boride, the layer contains carbon. Oxidation and erosion resistance can be improved.

International Publication No. 2009/099335A1 U.S. Pat. No. 4,544,524 International Publication No. 00/36187

One drawback of such wettable cathode blocks is the high cost of the C- _TiB2 composite material, which requires a significant reduction in energy consumption during operation to support the high cost of these blocks. Although drain cells with wettable cathodes could theoretically reduce energy consumption significantly, changes in the design of the electrolysis cells and other new technological elements are required to exploit the potential of such electrolysis cells (e.g., inert side walls and inert anodes). Thus, drain cells for aluminum electrolysis have not been successfully implemented to date.

The present invention describes a concept referred to as a wear-resistant cathode block intended for conventional Hall-Helou cells, rather than a wettable cathode block for use in drain cells. Although both types of blocks can utilize C- _TiB2 composites, the block design, composite composition and properties, and cost would be quite different, so the wear-resistant cathode block solution presented in this case would not be usable as a wettable cathode in a drain cell. For example, the entire surface of a wettable cathode in a drain cell must be coated with a wettable composite material (e.g., C- _TiB2 ), whereas in accordance with the present invention, only selected portions of the surface of the wear-resistant cathode block need be coated with the wear-resistant composite material (e.g., C- _TiB2 ).

The wear-resistant composite material used on selected areas of the surface of the wear-resistant cathode block may consist of carbon and 5-80% by weight of other hard material refractory metal boride powders such as TiB ₂ , HfB ₂ , ZrB ₂ , CrB ₂ , or WB ₂ , or refractory metal carbide powders such as SiC, Cr ₃ C ₂ , B ₄ C, TiC, or Al ₂ O ₃ , or any suitable combination thereof.

Cathode blocks can be manufactured in several ways, for example, green cathode blocks can be prepared by adding a conventional cathode paste at the bottom of a conventional vibroforming mold in cathode block manufacturing, then leveled with a rake (or other similar tool), followed by using the same rake to prepare pits in selected areas of the surface, which are filled with a wear-resistant composite material before vibroforming. The green wear-resistant cathode blocks are then heat treated (i.e., calcined and graphitized) in the same manner as conventional cathode blocks.

Alternatively, the green cathode blocks can be prepared by adding the composite material in selected areas at the bottom of a conventional vibratory mold for producing cathode blocks, followed by adding a conventional cathode paste at the top, which is then smoothed with a rake (or similar tool) and then vibratory molded. Thus, the wear-resistant cathode blocks are prepared upside down. The green wear-resistant cathode blocks are then heat treated (i.e., calcined and graphitized) in a manner similar to that of conventional cathode blocks.

According to one aspect of the invention, the selected area or portion of the surface of the cathode block made from the wear-resistant composite material represents less than 100% of the total surface of the cathode block, in a second aspect, the selected area or portion of the surface of the cathode block made from the wear-resistant composite material represents less than 50% of the total surface of the cathode block, and in a third aspect, the selected area or portion of the surface of the cathode block made from the wear-resistant composite material represents less than 30% of the total surface of the cathode block. In a fourth aspect of the invention, the selected area or portion of the surface of the cathode block made from the wear-resistant composite material represents less than 10% of the total surface of the cathode block.

According to a further aspect of the invention, the wear-resistant composite has a thickness between 3 and 10 cm. According to another aspect of the invention, the wear-resistant composite is arranged in selected areas of the cathode surface where the wear rate of the cathode is high and can be predicted by modeling or determined by autopsy.

According to a further aspect of the invention, the wear-resistant composite material is disposed in a region toward the end of the cathode block (i.e., in the region 0-80 cm from the end of the cathode block).

In a preferred embodiment, by combining some principles of the applicant's own WO 2009/099335 A1 and NO 20180369, a collector plate structure with steel and Cu inserts can be advantageously applied to the busbar system instead of the steel cathode bars traditionally used as connectors between the cathode block and the flexible.

Furthermore, by connecting these collector plates to the cathode block by metal particles such as steel balls and thermal expansion forces, and utilizing wear-resistant composites on specific areas of the cathode block, the height of the cathode block can be significantly reduced from a typical height of 450-500 mm to a height of 80-230 mm, and still maintain the same cell life or even extend the cell life. This can reduce the annual consumption of cathode material by 45-85% or more for a given electrolyzer train. Furthermore, by only applying wear-resistant composites on specific areas of the cathode block, the use of costly wear-resistant materials can be minimized. As the present invention is used in conventional electrolysis cells in aluminum metal production, it is possible to have a cathode block design where only a portion of the surface is coated with wear-resistant composites, compared to wet cathodes in drain cells where the entire surface of the cathode block must be coated with, for example, TiB ₂ -carbon composites.

In an alternative embodiment, the height of the cathode block can be reduced to a height of 130-280 mm by using round Cu rods instead of conventional cathode bars, and is made possible by utilizing specially designed cathode blocks with wear resistant composite materials in certain areas of the cathode block. This can reduce the consumption of cathode material by approximately 45-75% or more per year for a given electrolyzer train.

As yet another alternative embodiment, the height of the cathode block can be reduced to a height of 170-370 mm by using conventional cathode bars and rodding them with cast iron, and by applying specially designed cathode blocks with wear resistant composite materials in certain areas of the cathode block. This can reduce the consumption of cathode material by about 25-65% or more per year for a given electrolyzer train.

One preferred embodiment can be realized by combining the following two elements:
i) wear-resistant composite electrode material in specific regions of the cathode block;
ii) Collector plate as the connection between the cathode block and the cathode flexible which connects to the busbar system.

An alternative embodiment using Cu rods can be realized by combining the following two elements:
i) wear-resistant composite electrode material in specific regions of the cathode block;
i) Cu rods as connections between the cathode block and the cathode flexible that connect to the busbar system.

An alternative embodiment using conventional steel collector bars can be realized by combining the following two elements:
i) wear-resistant composite electrode material in specific regions of the cathode block;
i) Cathode bars as the connection between the cathode block and the cathode flexible which connects to the busbar system.

This realization of the invention has been made possible by:
i) The low erosion rate experienced with the wear-resistant composite material avoids the need to have cathode blocks of conventional height (typically 450-500 mm) to ensure sufficient cathode life. Whereas the maximum wear rate of conventional cathode blocks can be in the range of 50-70 mm per year, the maximum wear rate of the wear-resistant composite material is significantly lower (typically in the range of 10-30 mm, but can be even lower). Furthermore, the wear rate of the cathode surface is not uniformly distributed across the surface (i.e., some locations have significantly higher wear rates than others). The exact mechanism of this wear is not fully understood, but it is believed that the wear rate increases at the cathode surface due to the increased current density and magnetically induced metal flow. The cathode wear rate of the cathode surface can be determined empirically when the cell is shut down and inspected, and the results of this inspection can be used as information for modeling and designing wear-resistant cathode blocks for cell relining. Due to their high cost, wear resistant composite materials should only be used in selected areas where wear rates are high, typically, but not limited to, towards the ends of the cathode block.
ii) The collector plate geometry, which is the preferred embodiment, or the Cu rods, which is the alternative embodiment, avoids the need to have 15-18 cm of carbon around the conventional cathode bar, allowing for an even lower build height of the cathode element.

The present invention is realized by implementing the mechanical design of the cathode block assembly manufacturing process of the cathode block, the joining process between the cathode block and the collector plate or Cu rod, and the thermoelectric/thermo-mechanical design of the entire cathode shell-cathode lining including the cathode bar assembly.

It is a different cathode design than has been realized in the past. The conventional thinking based on the preconceptions of those skilled in the art is that the cathode block must be a certain height due to the risk of tapping out, sodium swelling, stability reasons, etc.

The present invention relates in a preferred embodiment to a catholyte based on collector plates, or in an alternative embodiment to a catholyte based on Cu rods or conventional collector bars, both with electrical conductors, in which in its structure, in particular for the reduction of the height, some new and inventive features are included. Some main elements relate to:
i) reduced cost of the cathode (less cathode material in general, and less of the expensive cathode material is used, since it is applied only to selective areas or portions of the cathode);
ii) reduced mV losses through the cathode (less current paths due to lower height) which reduces energy consumption;
iii) More space under the cathode block for insulation, reducing energy consumption;
iv) Increasing the cavity above the cathode block resulting in several operational advantages such as taller and therefore longer lasting anodes, less anode consumption, less anode treatment, better anode coating, more flexibility in terms of metal and cell height, less spillage of anode coating material at the base etc.

These and further advantages are achieved by the present invention as defined in the appended claims.

The invention is further illustrated below with reference to the figures and examples.

FIG. 1 is a schematic diagram showing the main parts of a Hall-Hellueux cell in cross-section. 1 shows an end view of a conventional cathode block having two slots for cathode bars. 3 discloses a side view of a cathode block similar to that of FIG. 2. 1 discloses an end view of a cathode block having two slots for cathode bars, where the cathode block has a special design with a lower height and wear-resistant composite material in selected areas of its top layer in accordance with the present invention. 5 discloses a side view of a cathode block similar to that of FIG. 4, with wear-resistant composite material disposed in selected areas toward the ends of the cathode block. 1 discloses an end view of a cathode block rodded with two Cu bars, where the cathode block has a special design with a lower height and wear-resistant composite material in selected areas of its top layer in accordance with the present invention. 7 discloses a side view of a cathode block similar to that of FIG. 6, where the wear-resistant composite material is disposed in selected areas toward the ends of the cathode block. 1 discloses an end view of a cathode block machined to a collector plate rod, where the cathode block has a special design with a lower height and wear resistant composite material in selected areas of its top layer in accordance with the present invention. 10 discloses a side view of a cathode block similar to that of FIG. 8, where the wear-resistant composite material is disposed in selected areas toward the ends of the cathode block.

In Figure 1, a cross-sectional view of the main parts of a conventional Hall-Hellou cell can be seen. The figure shows the upper structure including the alumina/fluoride hopper, anode stub, busbars, and feeders. In addition, a pair of anodes partially covered with crust are immersed in the liquid bath. Below the liquid bath, a layer of liquid aluminum is shown. A cathode is arranged below the liquid aluminum. The cathode has two collector bars embedded inward from each end.

Figure 2 shows an end view of a conventional cathode block 24, with two cathode bars 22, 22' recessed and rod-machined with cast iron 21, 21'. The width of the cathode block can range from 40 to 60 cm, and the height can range from 40 to 60 cm.

Figure 3 discloses a side view of a conventional cathode block 24 similar to that of Figure 2. The length of the block may range from 250 to 350 cm, and the height may range from 40 to 60 cm.

Figure 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 wear-resistant composite material in selected areas of its top layer 25 according to the present invention. The top layer in this embodiment can be 5-10 cm thick, while the cathode block can be 25-35 cm thick. The width can be in the range of 40-60 cm.

Figure 5 discloses a side view of a cathode block 24, similar to that of Figure 4, which in this case has a special design with a lower height and wear-resistant composite material in selected areas of its upper layers 25, 25' according to the present invention. Said areas are arranged in the end areas of the cathode block in this embodiment and are 5-10 cm thick.

Figure 6 discloses, in end view, a cathode block rodded with two Cu rods 50, 50', where the cathode block 24 has a lower height and a special design with wear-resistant composite material in selected areas of its top layer 25 according to the present invention. The thickness of the wear-resistant top layer can be 5-10 cm, while the total height of the cathode block can be 13-28 cm.

Figure 7 discloses a side view of a cathode block corresponding to that of Figure 6, where the cathode block 24 has a special design with a lower height and wear-resistant composite material in selected areas 25, 25' of its upper layer according to the present invention. The length of the cathode block can be in the range of 300-350 cm.

8 discloses in an end view a cathode block 24 rodded to a collector plate 20. The collector plate 20 comprises, in this example, five collector elements 30, 30', 30'', 30''', 30'''' in electrical contact with the collector plate 20. Preferably, these parts are made of easily weldable steel quality and preferably the parts are welded together. The cathode block can be rodded to the collector elements in a similar manner as disclosed in WO 2009/099335 A1, in which case the liquid can be accompanied by conductive metal particles. The number of collector elements in the collector plate can be different from five as shown, for example 1 to 7 or even none at all, in which case 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 wear-resistant composite material in selected areas 25 of its upper layer 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 of 8-28 cm. The combination of rod-machining the collector plate with the collector elements to the cathode block with wear-resistant composite material allows for a particularly low cathode block height and is considered a preferred embodiment of the invention.

Figure 9 discloses a side view of a cathode block similar to that of Figure 8, where the cathode block 24 has a special design with a lower height and wear-resistant composite material in selected areas 25, 25' of its upper layer that is rodded to the collector plate 20 in accordance with the present invention.

A suitable collector plate may include 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 metal part inside or below the collector plate to monitor the temperature at that location.

A further embodiment includes at least one horizontal current outlet at each end integral with the collector plate.

In one embodiment, at least one vertical current outlet is arranged on the collector plate opposite the cathode block.

In another embodiment, at least one metal collector element is arranged on the top side of the metal collector plate, said collector element being embedded in a corresponding recess in the bottom of the cathode block, the recess being wider than the collector element and being filled with a conductive material including conductive particles.

In another embodiment, there are one or more collector elements, preferably 3 to 7, typically separated by a distance of 50 mm to 150 mm.

In yet another embodiment, at least one collector element is the same length as or shorter than the cathode block.

Advantageously, the collector plate may have one to five insertions of a material with higher electrical conductivity, such as copper.

The cathode design is advantageous in terms of magnetohydrodynamic stability of the cell in which it is installed, may provide improved life cycle in operation, may use less space, and also exhibits lower cathode voltage drop with respect to conventional cathode designs.

At the ends of the collector plate 20, horizontal current outlets (not shown) can be arranged. The horizontal current outlets can be made of conductors of a good conductive material such as copper or a copper alloy and are further covered, at least at their outlet ends, with a sheet material, preferably made of a metal such as steel. The horizontal current outlets with their corresponding conductors can be fitted into slots made in the collector plate 20. This fitting can be based on press-fit tolerances or on a preheated plate area, and can be fitted using thermal expansion. However, any suitable fixing can be applied, including welding. The conductive material in the slots can be covered on the upper and lower sides with protective steel plates.

Advantageously, the entire assembly with the cathode block 24 and the collector plate 20 is tilted somewhat during the particle filling procedure to allow the particles to fill the recesses smoothly and completely. Furthermore, some vibration can be applied to the plate or plate area to ensure uniform filling with particles.

The recesses or slots in the bottom of the cathode block can be made in the green state of the carbonaceous body by commonly used techniques, or in the sintered or graphitized state by commonly available process equipment. The geometry of the slots needs to accommodate the plates.

It should be understood that the conductive solids or particles can be any suitable metal, such as steel, iron, copper, aluminum, etc., or alloys thereof. Furthermore, the shape of the solids can be spherical, elliptical or oblong, flaky, or have any suitable shape. The size and particle distribution can vary. The maximum size is generally limited by the width of the space to be filled. A non-uniform distribution of particle sizes can be advantageous to obtain the most compact packing possible, with little space between the particles.

The materials applied must have good conductive properties, as well as good mechanical properties (fracture properties) and be able to withstand high temperatures. As will be seen later, magnetic properties can be advantageous.

Furthermore, the size of the solid can be from 0.1 millimeters to close to the smallest opening between the carbonaceous body and the conductor plate. Typically, the size can be up to 2 mm.

Preferably, there may be several thermocouples attached to or inserted into the cathode plate to monitor the temperature within the cathode. For example, holes may be drilled in the cathode plate to the center at appropriate locations to receive the thermocouples. The steel plate provides a protective housing for the thermocouples to withstand the chemically aggressive environment during operation.

The length of the horizontal outlet insertion can preferably be limited so as not to cover the central portion of the cathode plate. The length of the insertion can be designed, for example, to reflect the presence of vertical outlets in that plate and the path length of the current through the conductor to the next cell. For side-by-side cells, the insertion length can be longer upstream to balance the current pickup at the cathode block for a more balanced arrangement.

Each cathode element may be fitted with a horizontal outlet, for example only in the case of end-to-end cells, or only when there is no space in the busbar below the cell, or with several horizontal outlets and one vertical outlet. To optimise the magnetic field, a configuration with only one or two vertical outlets and no horizontal outlet may also be possible for some selected cathode blocks of the cathode panel.

A combination of different collector plate configurations can be applied to one cell to generate favorable magnetic fields from the current distribution, or to enhance the thermal properties of the cell by reducing the number of outlets where heat loss is undesirable, for example at the short ends of the cell, which tend to be cooler due to the proximity of the corners. Vertical outlets attached to only some of the collector plates can help to optimize the current flow and magnetic field. This can also reduce the cost of installation if the current distribution and magnetohydrodynamic stability of the cell are sufficient.

The claimed collector plate cathode with its very low height cathode block has several advantages over conventional designs that include carbonaceous bodies with embedded collector bars:
In the preferred embodiment, the block is significantly lower and the life is still longer due to the significantly lower erosion rate of the TiB ₂ -carbon composite.
- The cathode voltage drop (CVD) is significantly lower (around 150 mV lower) due to the lower height of the cathode block, the number of outlets, the electrical properties of the material, better electrical contact due to the initial mobility of the particles, the full surface of the contact resistance, and the shorter current path due to the presence of vertical outlets.
- Due to the plate geometry, the conductance of the insertion, and the presence of the vertical outlet, the current distribution to the upper cathode block surface becomes more uniform, thus avoiding undesirable instabilities that cause horizontal currents in the liquid aluminum pad above the cathode block surface. The higher stability of the cell can be used to further reduce the cell voltage and energy consumption or to increase the amperage and production.
- The better current distribution leads to more uniform erosion of the carbonaceous material and therefore longer cell life.
- the array uses less vertical space than conventional designs, so the cathode shell can be lower, or - if the shell height is maintained, the extra space can be used for better bottom insulation, for taller and longer lasting anode blocks, or for higher height in the liquid aluminum or bath.
- The design has a better ratio of electrical and thermal conductivity at high current density and where heat flow is most critical, thus improving the energy efficiency of the cell (less heat loss, lower cathode voltage drop CVD).
- When retrofitted onto existing cell designs, the claimed vertical outlets can increase amperage and therefore production per cell.
- Better current distribution to the cathode surface, improving MHD stability and therefore the option to reduce ACD or increase amperage levels by up to 15%.
- Less heat loss due to smaller cross-sectional area and exposed surface of HO and VO avoiding cold cathodes that freeze at the bottom, especially when techniques are used to reduce energy consumption.
- Lower rod machining costs in embodiments where no cast iron is present.
In embodiments without cast collector bars (cast iron), there is no risk of the carbon block cracking.
- Flexible installation of VO after placing the cathode block on the lining.
- Better balance of upstream/downstream/bottomside current flow leading to better MHD stability.
- The overall height of the assembly is significantly reduced, giving space for more insulation or a larger cavity at the bottom. The difference could be up to 300mm.
- The drilling depth is shallower than in the collector bar, making it easier to place thermocouples in the plate or directly access them from the bottom.
- Reducing the amount of traditional cathode material generally saves costs, but by arranging the composite material only in selected areas of the cathode surface where wear is expected to be most excessive, the amount of expensive wear-resistant material is reduced compared to traditional designs.

Claims (9)

1. A method of making a cathode assembly for an electrolytic cell of the Hall-Hellou cell type for producing aluminum, comprising a graphitized cathode block of conductive material rodded onto a steel collector bar, Cu rod or metal collector plate, comprising:
selected areas or portions of the surface of the cathode block are made from a wear-resistant composite material comprising 5-80 weight percent of a refractory metal boride powder of _TiB2 , HfB2, _ZrB2 , _CrB2 , or _WB2 , or a refractory metal carbide powder of _SiC , _Cr3C2 , _B4C , TiC, or _Al2O3 , along with a _carbonaceous material, and the remainder of the cathode block comprises a graphitized cathode material _;
the wear-resistant composite material is disposed in an area toward an end of the cathode block;
the selected area or portion of the surface of the cathode block made from the wear-resistant composite material represents less than 100% of the total surface of the cathode block;
The cathode assembly comprises:
(i) a green cathode block is prepared by adding cathode paste at the bottom of a vibro- molding mould for producing the cathode block, which is then levelled with a rake before preparing pits in selected areas of the surface, the wear-resistant composite material is filled into these pits and vibro-molded , followed by a sintering or graphitisation heat treatment of the green cathode block;
Or,
(ii) A method for making a cathode assembly, wherein a green cathode block is prepared by adding said wear-resistant composite material at a selected area at the bottom of a vibro- molding mould for making a cathode block before adding cathode paste at the top, then levelling with a rake , then vibro-molding , followed by a heat treatment of sintering or graphitization of said green cathode block. The method of making a cathode assembly according to claim 1, wherein the wear-resistant composite material has a thickness between 3 and 10 cm. The method of making a cathode assembly according to claim 1, wherein the selected area or portion of the surface of the cathode block made from the wear-resistant composite material represents less than 50% of the total surface of the cathode block. The method of making a cathode assembly according to claim 1, wherein the selected area or portion of the surface of the cathode block made from the wear-resistant composite material represents less than 30% of the total surface of the cathode block. The method of making a cathode assembly according to claim 1, wherein the selected area or portion of the surface of the cathode block made from the wear-resistant composite material represents less than 10% of the total surface of the cathode block. The method for making a cathode assembly according to any one of claims 1 to 5, wherein the cathode block after graphitization is rodded into a steel collector bar having a total height of the cathode block between 25 and 35 cm. The method for making a cathode assembly according to any one of claims 1 to 5, wherein the cathode block after graphitization is rodded into a Cu rod having a total height of the cathode block between 13 and 28 cm. The method for making the cathode assembly of any one of claims 1 to 5, wherein the cathode block after graphitization is rodded onto a metal collector plate such that the total height of the cathode block is between 8 and 28 cm. 1. A cathode assembly for an electrolytic cell of the Hall-Hellou cell type for producing aluminum, comprising a graphitized cathode block of conductive material rodded on a steel collector bar, Cu rod or metal collector plate,
selected areas or portions of the surface of the cathode block are made from a wear-resistant composite material comprising 5-80 weight percent of a refractory metal boride powder of _TiB2 , HfB2, _ZrB2 , _CrB2 , or _WB2 , or a refractory metal carbide powder of _SiC , _Cr3C2 , _B4C , TiC, or _Al2O3 , along with a _carbonaceous material, and the remainder of the cathode block comprises a graphitized cathode material _;
the wear-resistant composite material is disposed in an area toward an end of the cathode block;
the selected area or portion of the surface of the cathode block made from the wear-resistant composite material represents less than 100% of the total surface of the cathode block;
The cathode assembly comprises:
(i) the cathode block comprises a cathode paste placed at the bottom of a vibro- molding mold for producing the cathode block , and the wear-resistant composite material filled into pits formed in selected areas of the cathode paste and vibro-molded, the cathode block being a heat-treated product;
Or,
(ii) The cathode block is placed in a selected area of the bottom of a cathode block manufacturing vibration mold , the wear-resistant composite material being vibration molded and the cathode paste being placed thereon, and the cathode block is heat treated.