CA2826604C - Cathode block having a top layer containing hard material - Google Patents
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- CA2826604C CA2826604C CA2826604A CA2826604A CA2826604C CA 2826604 C CA2826604 C CA 2826604C CA 2826604 A CA2826604 A CA 2826604A CA 2826604 A CA2826604 A CA 2826604A CA 2826604 C CA2826604 C CA 2826604C
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- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
- C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
- C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
- C25C3/08—Cell construction, e.g. bottoms, walls, cathodes
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Abstract
A cathode block for an aluminum electrolytic cell comprises a primary layer and a cover layer arranged thereon, the primary layer containing graphite and the cover layer being composed of a carbon composite material which contains 15 to less than 50% by weight of a hard material having a melting point of at least 1.000°C.
Description
01.08.2013 Cathode block having a top layer containing hard material The present invention relates to a cathode block for an aluminium electrolysis cell.
Such electrolysis cells are used for the electrolytic production of aluminium, which is customarily carried out in industry by the Hall-Heroult process. In the Hall-Heroult process, a melt composed of aluminium oxide and cryolite is electrolysed.
Here, the cryolite, Na3[AlF6], serves to lower the melting point of 2045 C for pure aluminium oxide to about 950 C for a mixture containing cryolite, aluminium oxide and additives, such as aluminium fluoride and calcium fluoride.
The electrolysis cell used in this process has a bottom, which is usually composed of a multiplicity of adjoining cathode blocks forming the cathode. In order to with-stand the thermal and chemical conditions which prevail during operation of the cell, the cathode blocks are customarily composed of a carbon-containing mate-rial. The undersides of each of the cathode blocks are provided with grooves, in each of which there is arranged at least one busbar through which the current fed via the anodes is discharged. In this case, the interstices between the individual walls of the cathode blocks, which delimit the grooves, and the busbars are often sealed with cast iron, in order to electrically and mechanically connect the busbars to the cathode blocks by virtue of the resulting encasement of the busbars with cast iron. An anode formed from individual anode blocks is arranged about 3 to cm above the layer of molten aluminium located on the top side of the cathode, and the electrolyte, i.e. the melt containing aluminium oxide and cryolite, is located between said anode and the surface of the aluminium. During the electrolysis carried out at about 1000 C, the aluminium which has formed settles beneath the electrolyte layer, i.e. as an intermediate layer between the top side of the cathode blocks and the electrolyte layer, on account of the fact that its density is relatively large compared to that of the electrolyte. During the electrolysis, the aluminium
Such electrolysis cells are used for the electrolytic production of aluminium, which is customarily carried out in industry by the Hall-Heroult process. In the Hall-Heroult process, a melt composed of aluminium oxide and cryolite is electrolysed.
Here, the cryolite, Na3[AlF6], serves to lower the melting point of 2045 C for pure aluminium oxide to about 950 C for a mixture containing cryolite, aluminium oxide and additives, such as aluminium fluoride and calcium fluoride.
The electrolysis cell used in this process has a bottom, which is usually composed of a multiplicity of adjoining cathode blocks forming the cathode. In order to with-stand the thermal and chemical conditions which prevail during operation of the cell, the cathode blocks are customarily composed of a carbon-containing mate-rial. The undersides of each of the cathode blocks are provided with grooves, in each of which there is arranged at least one busbar through which the current fed via the anodes is discharged. In this case, the interstices between the individual walls of the cathode blocks, which delimit the grooves, and the busbars are often sealed with cast iron, in order to electrically and mechanically connect the busbars to the cathode blocks by virtue of the resulting encasement of the busbars with cast iron. An anode formed from individual anode blocks is arranged about 3 to cm above the layer of molten aluminium located on the top side of the cathode, and the electrolyte, i.e. the melt containing aluminium oxide and cryolite, is located between said anode and the surface of the aluminium. During the electrolysis carried out at about 1000 C, the aluminium which has formed settles beneath the electrolyte layer, i.e. as an intermediate layer between the top side of the cathode blocks and the electrolyte layer, on account of the fact that its density is relatively large compared to that of the electrolyte. During the electrolysis, the aluminium
2 oxide dissolved in the cryolite melt is cleaved into aluminium and oxygen by a flow of electric current. In terms of electrochemistry, the layer of molten aluminium is the actual cathode, since aluminium ions are reduced to elemental aluminium on the surface thereof. Nevertheless, hereinbelow the term "cathode" will not be un-derstood to mean the cathode from an electrochemical point of view, i.e. the layer of molten aluminium, but rather the component which forms the electrolysis cell bottom and is composed of one or more cathode blocks.
A significant disadvantage of the Hall-Heroult process is that it requires a large amount of energy. To produce 1 kg of aluminium, about 12 to 15 kWh of electrical energy are required, which amounts to up to 40% of the production costs. To make it possible to lower the production costs, it is therefore desirable to reduce the specific energy consumption of this process as far as possible.
For this reason, graphite cathodes have increasingly been used in recent times, i.e. cathode blocks which contain graphite as the main constituent. Compared to amorphous carbon, graphite is distinguished by a considerably lower electrical resistivity and also by a significantly higher thermal conductivity, and this is why the use of graphite cathodes during the electrolysis makes it possible firstly to reduce the specific energy consumption of the electrolysis and secondly to carry out the electrolysis at a higher current intensity, which makes it possible to in-crease the aluminium production per electrolysis cell. However, graphite cathode blocks have a very low and in particular a relatively low resistance to the abrasive wear processes which occur during operation of the electrolysis cell, and therefore have a shorter service life than cathode blocks consisting of amorphous carbon. In particular, slurry made up of undissolved aluminium oxide readily settles on the surface of graphite cathode blocks, and this firstly considerably reduces the wear resistance of the cathode block, on account of the particle abrasion resulting from the slurry formation, and secondly hinders the flow of current on the cathode block surface, on account of the reduction in the effective cathode surface, resulting in
A significant disadvantage of the Hall-Heroult process is that it requires a large amount of energy. To produce 1 kg of aluminium, about 12 to 15 kWh of electrical energy are required, which amounts to up to 40% of the production costs. To make it possible to lower the production costs, it is therefore desirable to reduce the specific energy consumption of this process as far as possible.
For this reason, graphite cathodes have increasingly been used in recent times, i.e. cathode blocks which contain graphite as the main constituent. Compared to amorphous carbon, graphite is distinguished by a considerably lower electrical resistivity and also by a significantly higher thermal conductivity, and this is why the use of graphite cathodes during the electrolysis makes it possible firstly to reduce the specific energy consumption of the electrolysis and secondly to carry out the electrolysis at a higher current intensity, which makes it possible to in-crease the aluminium production per electrolysis cell. However, graphite cathode blocks have a very low and in particular a relatively low resistance to the abrasive wear processes which occur during operation of the electrolysis cell, and therefore have a shorter service life than cathode blocks consisting of amorphous carbon. In particular, slurry made up of undissolved aluminium oxide readily settles on the surface of graphite cathode blocks, and this firstly considerably reduces the wear resistance of the cathode block, on account of the particle abrasion resulting from the slurry formation, and secondly hinders the flow of current on the cathode block surface, on account of the reduction in the effective cathode surface, resulting in
3 an increase in the specific energy consumption during electrolysis. This additionally leads to an increase in the current density, which can lead to a shorter service life of the electrolysis cell.
In order to improve the wetting of the cathode block surface, it has been proposed in WO 96/07773 Al to apply a coating of pure titanium diboride, zirconium diboride or the like to the cathode block. DE 197 14 433 C2 discloses a cathode block having a similar coating which contains at least 80 % by weight of titanium diboride and is produced by plasma spraying titanium diboride onto the surface of the cathode block.
Such coatings of pure titanium diboride or having a very high titanium diboride content are very brittle and therefore susceptible to cracking, however. In addition, the specific thermal expansion of these coatings is approximately twice as high as that of amorphous carbon or graphite, which is why these have only a short service life when they are used in fused-salt electrolysis.
The present invention relates to a cathode block which has a low specific electrical resistivity, which is distinguished by a high thermal conductivity, which can be wetted well with aluminium melt, which has a high wear resistance with respect to the abrasive, chemical and thermal conditions which prevail during operation in fused-salt electrolysis, and which, in particular, is also distinguished by the fact that no slurry is deposited or at best small quantities of slurry are deposited on its surface when carrying out fused-salt electrolysis.
In one aspect, the invention relates to a cathode block for an aluminium electrolysis cell having a primary layer and having a top layer, wherein the primary layer contains graphite and the top layer is composed of a carbon composite material containing 15 to less than 50 ck by weight of hard material having a melting point of at least 1000 C, and wherein the top layer has a thickness of 50 to 400 mm.
, ,
In order to improve the wetting of the cathode block surface, it has been proposed in WO 96/07773 Al to apply a coating of pure titanium diboride, zirconium diboride or the like to the cathode block. DE 197 14 433 C2 discloses a cathode block having a similar coating which contains at least 80 % by weight of titanium diboride and is produced by plasma spraying titanium diboride onto the surface of the cathode block.
Such coatings of pure titanium diboride or having a very high titanium diboride content are very brittle and therefore susceptible to cracking, however. In addition, the specific thermal expansion of these coatings is approximately twice as high as that of amorphous carbon or graphite, which is why these have only a short service life when they are used in fused-salt electrolysis.
The present invention relates to a cathode block which has a low specific electrical resistivity, which is distinguished by a high thermal conductivity, which can be wetted well with aluminium melt, which has a high wear resistance with respect to the abrasive, chemical and thermal conditions which prevail during operation in fused-salt electrolysis, and which, in particular, is also distinguished by the fact that no slurry is deposited or at best small quantities of slurry are deposited on its surface when carrying out fused-salt electrolysis.
In one aspect, the invention relates to a cathode block for an aluminium electrolysis cell having a primary layer and having a top layer, wherein the primary layer contains graphite and the top layer is composed of a carbon composite material containing 15 to less than 50 ck by weight of hard material having a melting point of at least 1000 C, and wherein the top layer has a thickness of 50 to 400 mm.
, ,
4 This solution is based on the understanding that the provision of a top layer which is composed of a carbon composite material containing not less than 15 `)/0 by weight, but at most less than 50 % by weight, of hard material having a melting point of at least 1000 C on a graphite-containing primary layer produces a cath-ode block which has a sufficiently low specific electrical resistivity for energy-efficient operation of fused-salt electrolysis and additionally has a very high wear resistance with respect to the abrasive, chemical and thermal conditions which prevail during fused-salt electrolysis. Here, it was particularly surprising that, in such a cathode block, in particular the formation of a slurry or the deposition of a slurry on the surface is reliably prevented, and thus not only is the wear resistance of the cathode block increased considerably as a result of the reduction or preven-tion of particle abrasion resulting from slurry formation, but also in particular hin-drance of the flow of current as a result of the formation of a slurry or the deposi-tion of a slurry on the cathode block surface and a resulting increase in the specific energy consumption during electrolysis are reliably prevented.
Therefore, the cathode block according to the present invention is distinguished by the advantages associated with the provision of graphite in the primary layer of the cathode block, such as in particular by a low electrical resistance of the cathode block and by a high thermal conductivity of the cathode block, without however having the disadvantages resulting from the use of graphite, such as a low wear resistance and a lack of wettability by aluminium melt. Instead, a good wettability of the cathode block surface with molten aluminium is achieved on account of the top layer containing hard material which is provided in the cathode block according to the invention, and therefore the formation of a slurry or the deposition of a slurry on the surface of the cathode block is reliably prevented. Furthermore, the move-ment of the molten aluminium is thereby reduced significantly, such that the dis-tance between the surface of the layer of molten aluminium and the anode in the electrolysis cell can be reduced, for example, to 2,5 to 4,0 cm and preferably to 3 to 3,5 cm, which further reduces the specific energy consumption of the electroly-sis process. In addition, despite the fact that a hard-material-containing top layer is used, the surface of the cathode block according to the invention surprisingly does not tend to crack and in particular is also not distinguished by a disadvantageously high brittleness.
All in all, the cathode block according to the invention has long-term stability with respect to carrying out fused-salt electrolysis using a melt containing aluminium oxide and cryolite for producing aluminium, and makes it possible for fused-salt electrolysis to be carried out with a very low specific energy consumption.
This is achieved by the abovementioned combination of a graphite-containing primary layer and a top layer which contains hard material in a quantity of less than 50 %
by weight and is based on a carbon composite material. This was particularly surprising because the cathode blocks known from the prior art having a coating containing titanium diboride necessarily contain relatively high quantities of tita-nium diboride, which renders the known coatings brittle.
Within the context of the present invention, and in keeping with the customary definition in the art for this term, "hard material" is understood to mean a material which is distinguished by a particularly high hardness in particular even at high temperatures of 1000 C and higher.
It is preferable for the melting point of the hard material used to be considerably higher than 1000 C, where in particular hard materials having a melting point of at least 1500 C, preferably hard materials having a melting point of at least and particularly preferably hard materials having a melting point of at least C have proved to be particularly suitable.
In principle, all hard materials can be used in the top layer of the cathode block according to the invention. However, good results are achieved in particular with hard materials having a Knoop hardness, measured in accordance with DIN EN
=
843-4, of at least 1000 N/mm2, preferably of at least 1500 N/mm2, particularly preferably of at least 2000 N/mm2 and very particularly preferably of at least N/mm2.
Examples of suitable hard materials are metal carbides, metal borides, metal ni-trides and metal carbonitrides having a sufficiently high hardness at 1000 C.
Examples of suitable representatives from these groups are titanium diboride, zirconium diboride, tantalum diboride, titanium carbide, boron carbide, titanium carbonitride,silicon carbide, tungsten carbide, vanadium carbide, titanium nitride, boron nitride and silicon nitride. It is very particularly preferable to use a non-oxidic titanium ceramic, to be precise preferably titanium diboride, titanium carbide, tita-nium carbonitride and/or titanium nitride, as the hard material in the top layer of the cathode block according to the invention. Most preferably, the top layer of the cathode block according to the invention contains titanium diboride as the hard material. All of the abovementioned hard materials can be used alone or it is pos-sible to use any desired chemical combination and/or mixture of two or more of the abovementioned compounds.
According to a particularly preferred embodiment of the present invention, the hard material present in the top layer of the cathode block has a monomodal particle size distribution, wherein the mean volume-weighted particle size (d3,50), as de-termined by static light scattering in accordance with the international standard ISO 13320-1, is 10 to 20 pm. In this embodiment, it is particularly preferable to use a non-oxidic titanium ceramic and most preferable to use titanium diboride having a monomodal particle size distribution defined above.
Within the context of the present invention, it has been established that a hard material, in particular non-oxidic titanium ceramic and especially titanium diboride, having a monomodal particle size distribution defined above not only brings about a very good wettability of the surface of the cathode block, which is why the forma-tion of a slurry and the deposition of a slurry on the surface of the cathode block are reliably prevented, the wear resistance of the cathode block is increased and the specific energy consumption during electrolysis is reduced. In addition, it has surprisingly been established within the context of the present invention that this effect is also achieved in particular given relatively small amounts of titanium diboride, of less than 50 % by weight and particularly preferably even given tita-nium diboride quantities of only 15 to 20 % by weight in the top layer. It is thereby possible to dispense with a high concentration of titanium diboride in the top layer, which leads to a brittle cathode block surface. Furthermore, hard material, in par-ticular non-oxidic titanium ceramic and especially titanium diboride, having a monomodal particle size distribution defined above is also distinguished by a very good processability. In particular, the tendency of such a hard material to form dust, for example as it is being introduced into a mixing tank or as the hard mate-rial powder is being transported, is sufficiently low, and at best a small degree of agglomerate formation occurs during the mixing, for example. In addition, such a hard material powder has a sufficiently high flowability and pourability, and there-fore it can be conveyed to a mixing apparatus, for example, using a conventional conveying apparatus. This all results not only in a simple and cost-effective pro-ducibility of the cathode blocks according to the invention, but also in particular in a very homogeneous distribution of the hard material in the top layer of the cath-ode blocks.
The hard material, preferably titanium diboride, present in the top layer of the cathode block preferably has a monomodal particle size distribution, wherein the mean volume-weighted particle size (d3,50), determined as above, is 12 to 18 pm and particularly preferably 14 to 16 pm.
As an alternative to the embodiment mentioned above, the hard material present in the top layer of the cathode block can have a monomodal particle size distribu-tion, wherein the mean volume-weighted particle size (d3,50), as determined by static light scattering in accordance with the international standard ISO
13320-1, is 3 to 10 pm and preferably 4 to 6 pm. In this embodiment, too, it is particularly preferable to use a non-oxidic titanium ceramic and most preferable to use tita-nium diboride having a monomodal particle size distribution defined above.
In a development of the concept of the invention, it is proposed that the hard mate-rial has a volume-weighted d3,90 particle size, determined as above, of 20 to 40 pm and preferably of 25 to 30 pm. The hard material preferably has such a d3,90 value in combination with an above-defined d3,50 value. In this embodiment, too, the hard material is preferably a non-oxidic titanium ceramic and particularly preferably titanium diboride. As a result, the advantages and effects mentioned for the above embodiment are achieved to an even greater extent.
As an alternative to the embodiment mentioned above, the hard material present in the top layer of the cathode block can have a volume-weighted d3,90 particle size, determined as above, of 10 to 20 pm and preferably of 12 to 18 pm. The hard material preferably has such a d3,90 value in combination with an above-defined d3,50 value. In this embodiment, too, it is particularly preferable to use a non-oxidic titanium ceramic and most preferable to use titanium diboride having a monomo-dal particle size distribution defined above.
According to a further preferred embodiment of the present invention, the hard material has a volume-weighted d3,10 particle size, determined as above, of 2 to 7 pm and preferably of 3 to 5 pm. The hard material preferably has such a d3,10 value in combination with an above-defined d3,90 value and/or d3,50 value. In this embodiment, too, the hard material is preferably a non-oxidic titanium ceramic and particularly preferably titanium diboride. As a result, the advantages and effects mentioned for the above embodiments are achieved to an even greater extent.
As an alternative to the embodiment mentioned above, the hard material present in the top layer of the cathode block can have a volume-weighted d3,10 particle size, determined as above, of 1 to 3 pm and preferably of 1 to 2 pm. The hard material preferably has such a d3.10 value in combination with an above-defined d3,90 value and/or d3,50 value. In this embodiment, too, it is particularly preferable to use a non-oxidic titanium ceramic and most preferable to use titanium diboride having a monomodal particle size distribution defined above.
In addition, it is preferable if the hard material, in particular a non-oxidic titanium ceramic and particularly preferably titanium diboride, has a particle size distribution which is characterized by a span value, as calculated in accordance with the fol-lowing equation:
Span = (d3,90 - d3,10)/c13,50 of 0,65 to 3,80 and particularly preferably of 1,00 to 2,25. The hard material pref-erably has such a span value in combination with an above-defined d3,90 value and/or d3,50 value and/or d3,10 value. As a result, the advantages and effects men-tioned for the above embodiments are achieved to an even greater extent.
As set forth above, non-oxidic titanium ceramics, such as preferably titanium car-bide, titanium carbonitride, titanium nitride and most preferably titanium diboride, are suitable in particular as the hard material in the top layer of the cathode block according to the invention. For this reason, it is proposed in a development of the concept of the invention that the hard material consists of non-oxidic titanium ce-ramic and in particular of titanium diboride to an extent of at least 80 % by weight, preferably to an extent of at least 90 % by weight, particularly preferably to an extent of at least 95 % by weight, very particularly preferably to an extent of at least 99 % by weight and most preferably completely.
The total quantity of the hard material in the top layer is, according to the inven-tion, at least 15 % by weight, but at most less than 50 % by weight. When the quantity of hard material lies in this value range, the top layer contains sufficient hard material firstly to provide the top layer with an excellent hardness and abra-sion resistance for increasing the wear resistance and secondly to provide a wet-tability of the top layer surface with liquid aluminium which is sufficiently high for avoiding the formation of a slurry and the deposition of a slurry, as a result of which the wear resistance of the cathode block is increased further and the spe-cific energy consumption during fused-salt electrolysis is reduced further; at the same time, however, the top layer contains a sufficiently small quantity of hard material such that the surface of the top layer does not have a brittleness which is too high for a sufficiently high long-term stability on account of the addition of hard material.
Here, good results are achieved in particular if the top layer contains 15 to 40 % by weight and particularly preferably 15 to 30 % by weight of a hard material having a melting point of at least 1000 C.
Apart from the hard material, the top layer contains carbon and, if appropriate, binder, such as pitch, in particular coal tar pitch and/or petroleum pitch. If pitch is mentioned hereinbelow, this means all varieties of pitch known to a person skilled in the art. Here, the carbon, together with the optional binder, forms the matrix in which the hard material is embedded. Good results are achieved in particular if the top layer contains 85 to more than 50 % by weight, preferably 85 to 60 `)/0 by weight and particularly preferably 85 to 70 % by weight of carbon.
Here, the carbon present in the top layer can be amorphous carbon, graphite or a mixture of amorphous carbon and graphite.
According to a very particularly preferred embodiment of the present invention, the carbon present in the top layer of the cathode block according to the invention is exclusively amorphous carbon or a mixture of amorphous carbon and graphite. If a mixture of amorphous carbon and graphite is used, this mixture preferably con-tains 10 to 99 % by weight, particularly preferably 30 to 95 % by weight and very particularly preferably 60 to 90 % by weight of amorphous carbon, remainder graphite, where the graphite used can be both natural graphite and also synthetic graphite.
Cathode blocks according to the invention having a top layer composed of carbon composite material which contains hard material and, as the carbon component, contains a mixture of amorphous carbon and graphite containing optionally car-bonized binder (such as for example a mixture of calcined anthracite, graphite and carbonized pitch) or very particularly preferably amorphous carbon containing optionally carbonized binder (such as for example a mixture of calcined anthracite and carbonized pitch) have a particularly high abrasion resistance. The starting material used for the amorphous carbon is preferably anthracite, which is then calcined at a temperature of between 800 and 2200 C and particularly preferably between 1200 and 2000 C.
In a development of the concept of the invention, it is proposed for the cathode block top layer containing amorphous carbon that the top layer has a vertical spe-cific electrical resistivity at 950 C of 20 to 32 Q pm and preferably of 22 to 28 pm. This corresponds to vertical specific resistivities at room temperature of 23 to 40 Q pm and of 25 to 30 Q pm. In this context, "vertical specific electrical resistiv-ity" is understood to mean the specific electrical resistivity when the cathode block is installed in the vertical direction.
In principle, the thickness of the top layer should be as small as possible, in order to keep the costs for the expensive hard material as low as possible, but should , , also be sufficiently great for the top layer to have a sufficiently high wear resis-tance and service life. Good results are achieved in this respect in particular if the thickness of the top layer amounts to 1 to 50 %, preferably 5 to 40 %, particularly preferably 10 to 30 % and very particularly preferably 15 to 25 %, for example about 20 %, of the overall height of the cathode block.
By way of example, the top layer can have a thickness or height of 50 to 400 mm, preferably of 50 to 200 mm, particularly preferably of 70 to 130 mm, very particu-larly preferably of 90 to 110 mm and most preferably of about 100 mm. Here, "thickness or height" is understood to mean the distance from the underside of the top layer to the point of the highest elevation of the top layer.
Similarly, and by way of example, the base layer can have a thickness or height of 100 to 550 mm, preferably of 300 to 500 mm, particularly preferably of 400 to mm, very particularly preferably of 425 to 475 mm and most preferably of about 450 mm.
In principle, it is possible for the top layer of the cathode block to have a surface which is profiled at least in certain regions. On account of a profiled surface, the movement of the molten aluminium brought about by the electromagnetic interac-tion present during the electrolysis is reduced, resulting in relatively small wave formation and bulging of the aluminium layer. For this reason, the use of surface-profiled cathode blocks can further reduce the distance between the molten alu-minium and the anode, and therefore the electrical cell resistance is further re-duced as a result of the reduction in the ohmic resistance, and therefore so too is the specific energy consumption.
Here, a profiled surface is understood to mean a surface having at least one re-cess and/or elevation which is arranged chaotically or extends in the transverse direction, in the longitudinal direction or in any other desired direction, such as for example in a direction running at an acute or obtuse angle to the longitudinal di-rection, of the cathode block, the recess or elevation having at least a depth or height of 0,05 mm and preferably of 0,5 mm in demarcation relative to a surface roughness, as seen transversely to the cathode block surface. In this case, the at least one recess and/or elevation can be restricted exclusively to the top layer or the at least one recess and/or elevation can extend into the primary layer. It is preferable for the at least one recess and/or elevation to extend exclusively in the top layer.
Within the context of the present invention, a recess is understood to mean a cut-out directed inwards from the surface of the cathode block, whereas the term "ele-vation" means a rise directed outwards from the surface of the cathode block.
In the case of rectangular cutouts or elevations each of equal depth or height, for example, here it can depend on the observer whether these are regarded as re-cesses or elevations. The wording "recess and/or elevation" is intended to take these ambiguities between the terms "recess" and "elevation" into account.
In principle, the at least one recess and/or elevation can have any desired geome-try, as seen in the transverse direction of the cathode block. By way of example, the at least one recess or elevation can have a convex, concave or polygonal form, for example a trapezoidal, triangular, rectangular or square form, as seen in the transverse direction of the cathode block.
In order to avoid or at least considerably reduce wave formation during operation of the cathode block according to the invention for the fused-salt electrolysis of aluminium oxide in a cryolite melt, and in order to drastically reduce the height of any waves which do possibly form, it is proposed in a development of the concept of the invention that, if the surface profiling comprises at least one recess, the depth-to-width ratio of the at least one recess is 1:3 to 1:1 and preferably 1:2 to 1:1.
Good results are achieved in particular if the depth of the at least one recess is 10 to 90 mm, preferably 40 to 90 mm and particularly preferably 60 to 80 mm, such as for example about 70 mm.
According to a further preferred embodiment, the width of the at least one recess is 100 to 200 mm, particularly preferably 120 to 180 mm and very particularly pref-erably 140 to 160 mm, such as for example about 150 mm.
In principle, it is possible for the at least one recess to extend only in certain re-gions, as seen in the longitudinal direction of the cathode block. However, it is preferable for the at least one recess to extend over the entire length of the cath-ode block, so as to achieve the effect of reducing or completely reducing wave formation of liquid aluminium. It is possible, however, for the depth and/or width of the at least one recess to vary over the length of the cathode block. It is similarly possible for the geometry of the recess to also vary over the length of the cathode block.
If the surface profiling comprises at least one elevation, it is likewise preferable, in order to avoid or at least considerably reduce wave formation during operation of the cathode block according to the invention for the fused-salt electrolysis of alu-minium oxide in a cryolite melt, and in order to drastically reduce the height of any waves which do possibly form, for the height-to-width ratio of the at least one ele-vation to be 1:2 to 2:1 and preferably about 1:1.
Good results are achieved in particular if the height of the at least one elevation is to 150 mm, preferably 40 to 90 mm and particularly preferably 60 to 80 mm, such as for example about 70 mm.
According to a further preferred embodiment, the width of the at least one eleva-tion is 50 to 150 mm, particularly preferably 55 to 100 mm and very particularly preferably 60 to 90 mm, such as for example about 75 mm.
In principle, it is possible for the at least one elevation to extend only in certain regions, as seen in the longitudinal direction of the cathode block. However, it is preferable for the at least one elevation to extend over the entire length of the cathode block, so as to achieve the effect of reducing or completely reducing cor-rugation of liquid aluminium. It is possible, however, for the height and/or width of the at least one elevation to vary over the length of the cathode block. It is similarly possible for the geometry of the elevation to also vary over the length of the cath-ode block.
If the surface profiling comprises both at least one recess and at least one eleva-tion, the ratio of the width of the at least one recess to the width of the at least one elevation is preferably 4:1 to 1:1, such as for example about 2:1.
In order to reliably avoid the deposition of slurry present in the melt in the profiled structure of the surface of the cathode block as fused-salt electrolysis is being carried out, it is proposed in a development of the concept of the invention to avoid any angled and in particular right-angled regions in the profiled surface. If, for example, a substantially rectangular cross section is chosen for the at least one recess and/or elevation, it is preferable according to a preferred embodiment of the present invention to round off the right-angled regions. The radius of curvature of these rounded-off sections can be, for example, 5 to 50 mm, preferably 10 to 30 mm and particularly preferably about 20 mm. In order to avoid sharp edges, any desired geometries which all fall under the term "rounded-off' are conceivable, in principle.
The present invention is not restricted in terms of the number of recesses or eleva-tions in the cathode block. Good results are achieved, for example, if the cathode block has 1 to 3 recesses and preferably 2 recesses in the transverse direction thereof.
According to a further very particularly preferred embodiment of the present inven-tion, the primary layer is composed of a mixture of graphite and binder, such as carbonized pitch, to an extent of at least 80 % by weight, preferably to an extent of at least 90 % by weight, particularly preferably to an extent of at least 95 %
by weight, very particularly preferably to an extent of at least 99 % by weight and most preferably completely (graphite cathode body). Such a primary layer has a suitably low specific electrical resistivity and a sufficiently high specific thermal conductivity. Here, this mixture is preferably composed of 70 to 95 A) by weight of graphite and 5 to 30 A) by weight of binder and particularly preferably of 80 to 90 % by weight of graphite and 10 to 20% by weight of binder, such as for example of 85 % by weight of graphite and 15 % by weight of carbonized pitch.
It is preferable for both the top side of the primary layer and also the underside of the top layer, and therefore also the interface between the primary layer and the top layer, to have a planar form. Although it is not preferable, an intermediate layer can be provided between the primary layer and the top layer, said intermediate layer having the same structure as the top layer, for example, with the exception that the intermediate layer has a lower concentration of hard material than the top layer.
In a development of the concept of the invention, it is proposed that the primary layer has a vertical specific electrical resistivity at 950 C of 13 to 18 pm and preferably of 14 to 16 Q pm. This corresponds to vertical specific electrical resis-tivities at room temperature of 14 to 20 1-2pm and of 16 to 18 Q pm.
The present invention also relates to a cathode, which contains at least one cath-ode block described above, wherein the cathode block has at least one groove on that side of the primary layer which lies opposite the top layer, wherein at least one busbar is provided in the at least one groove in order to feed current to the cath-ode during electrolysis.
In order to fasten the at least one busbar fixedly to the cathode block, and in order to avoid hollow spaces between the busbar and the cathode block which increase the electrical resistance, it is additionally preferable for the at least one busbar to have a cast iron casing at least in certain regions and particularly preferably over the entire circumference. This casing can be produced by inserting the at least one busbar into the groove of the cathode block and then introducing cast iron into the interstice between the busbar and the walls delimiting the groove.
The present invention also relates to the use of a cathode block described above or of a cathode described above for carrying out fused-salt electrolysis for produc-ing metal, such as in particular aluminium.
The cathode block or the cathode is preferably used for carrying out fused-salt electrolysis with a melt of cryolite and aluminium oxide for producing aluminium, the fused-salt electrolysis being carried out with particular preference as a Hall-Heroult process.
Hereinbelow, the present invention is described purely by way of example on the basis of advantageous embodiments and with reference to the attached drawing.
In the drawing:
Figure 1 shows a schematic cross section of a detail of an aluminium elec-trolysis cell, which comprises a cathode block according to an exemplary embodi-ment of the present invention.
Figure 1 shows a cross section of a detail of an aluminium electrolysis cell having a cathode 12, which at the same time forms the bottom of a tank for alu-minium melt 14 produced during operation of the electrolysis cell 10 and for a cryolite-aluminium oxide melt 16 located above the aluminium melt 14. An anode 18 of the electrolysis cell 10 is in contact with the cryolite-aluminium oxide melt 16.
At the side, the tank formed by the lower part of the aluminium electrolysis cell 10 is delimited by a carbon and/or graphite lining (not shown in Figure 1).
The cathode 12 comprises a plurality of cathode blocks 20, 20', 20", which are each connected to one another via a ramming mass 24, 24' which has been in-serted into a ramming mass joint 22, 22' arranged between the cathode blocks 20, 20', 20". Similarly, the anode 18 comprises a plurality of anode blocks 26, 26', the anode blocks 26, 26' each being approximately twice as wide and approximately half as long as the cathode blocks 20, 20', 20". In this case, the anode blocks 26, 26' are arranged above the cathode blocks 20, 20', 20" in such a way that in each case an anode block 26, 26' covers two cathode blocks 20, 20', 20" arranged alongside one another in width and in each case a cathode block 20, 20', 20"
covers two anode blocks 26, 26' arranged alongside one another in length.
Each cathode block 20, 20', 20" consists of a lower primary layer 30, 30', 30"
and a top layer 32, 32', 32" which is arranged thereabove and is fixedly connected thereto. The interfaces between the primary layers 30, 30', 30" and the top layers 32, 32', 32" are planar. Whereas the primary layers 30, 30', 30" of the cathode blocks 20, 20', 20" each have a graphite material structure, i.e. consist of graphitic carbon, containing synthetic or natural graphite and carbonized binding pitch, the top layers 32, 32', 32" are each composed of a ceramic-carbon composite material containing titanium diboride, which contains 20 A by weight of titanium diboride, amorphous carbon, specifically anthracite, and carbonized pitch as the binder.
The titanium diboride present in the top layers 32, 32', 32" has a mean volume-weighted particle size (d3,50), as determined by static light scattering in accordance with the standard ISO 13320-1, of 15 pm, a d3,90 particle size of 27 pm and a d3,10 particle size of 4 pm.
Each cathode block 20, 20', 20" has a width of 650 mm and a total height of mm, the primary layers 30, 30', 30" each having a height of 450 mm and the top layers 32, 32', 32" each having a height of 100 mm. The distance between the anode blocks 26, 26' and the cathode blocks 20, 20', 20" is approximately 200 to approximately 350 mm, the layer of cryolite-aluminium oxide melt 16 arranged therebetween having a thickness of approximately 50 mm and the layer of alumin-ium melt 14 arranged thereunder likewise having a thickness of approximately to approximately 300 mm.
Finally, each cathode block 20, 20', 20" comprises two grooves 38, 38' on its un-derside each with a rectangular, specifically substantially rectangular cross sec-tion, wherein a steel busbar 40, 40' likewise having a rectangular or substantially rectangular cross section is accommodated in each groove 38, 38'. In this case, the interstices between the busbars 40, 40' and the walls which delimit the grooves 38, 38 are each sealed with cast iron (not shown), as a result of which the busbars 40, 40' are fixedly connected to the walls which delimit the grooves 38, 38'.
It is preferable for both the grooves 38, 38' and the recesses 34, 34' to be put in the top side of the top layers 32, 32', 32" during the shaping process, to be precise for example by vibrating moulds and/or punches.
List of reference symbols 10 Aluminium electrolysis cell 12 Cathode 14 Aluminium melt 16 Cryolite-aluminium oxide melt 18 Anode 20, 20', 20" Cathode block 22, 22' Ramming mass joint 24, 24' Ramming mass 26, 26' Anode block 30, 30', 30" Primary layer 32, 32', 32" Top layer 38, 38' Groove 40, 40' Busbar
Therefore, the cathode block according to the present invention is distinguished by the advantages associated with the provision of graphite in the primary layer of the cathode block, such as in particular by a low electrical resistance of the cathode block and by a high thermal conductivity of the cathode block, without however having the disadvantages resulting from the use of graphite, such as a low wear resistance and a lack of wettability by aluminium melt. Instead, a good wettability of the cathode block surface with molten aluminium is achieved on account of the top layer containing hard material which is provided in the cathode block according to the invention, and therefore the formation of a slurry or the deposition of a slurry on the surface of the cathode block is reliably prevented. Furthermore, the move-ment of the molten aluminium is thereby reduced significantly, such that the dis-tance between the surface of the layer of molten aluminium and the anode in the electrolysis cell can be reduced, for example, to 2,5 to 4,0 cm and preferably to 3 to 3,5 cm, which further reduces the specific energy consumption of the electroly-sis process. In addition, despite the fact that a hard-material-containing top layer is used, the surface of the cathode block according to the invention surprisingly does not tend to crack and in particular is also not distinguished by a disadvantageously high brittleness.
All in all, the cathode block according to the invention has long-term stability with respect to carrying out fused-salt electrolysis using a melt containing aluminium oxide and cryolite for producing aluminium, and makes it possible for fused-salt electrolysis to be carried out with a very low specific energy consumption.
This is achieved by the abovementioned combination of a graphite-containing primary layer and a top layer which contains hard material in a quantity of less than 50 %
by weight and is based on a carbon composite material. This was particularly surprising because the cathode blocks known from the prior art having a coating containing titanium diboride necessarily contain relatively high quantities of tita-nium diboride, which renders the known coatings brittle.
Within the context of the present invention, and in keeping with the customary definition in the art for this term, "hard material" is understood to mean a material which is distinguished by a particularly high hardness in particular even at high temperatures of 1000 C and higher.
It is preferable for the melting point of the hard material used to be considerably higher than 1000 C, where in particular hard materials having a melting point of at least 1500 C, preferably hard materials having a melting point of at least and particularly preferably hard materials having a melting point of at least C have proved to be particularly suitable.
In principle, all hard materials can be used in the top layer of the cathode block according to the invention. However, good results are achieved in particular with hard materials having a Knoop hardness, measured in accordance with DIN EN
=
843-4, of at least 1000 N/mm2, preferably of at least 1500 N/mm2, particularly preferably of at least 2000 N/mm2 and very particularly preferably of at least N/mm2.
Examples of suitable hard materials are metal carbides, metal borides, metal ni-trides and metal carbonitrides having a sufficiently high hardness at 1000 C.
Examples of suitable representatives from these groups are titanium diboride, zirconium diboride, tantalum diboride, titanium carbide, boron carbide, titanium carbonitride,silicon carbide, tungsten carbide, vanadium carbide, titanium nitride, boron nitride and silicon nitride. It is very particularly preferable to use a non-oxidic titanium ceramic, to be precise preferably titanium diboride, titanium carbide, tita-nium carbonitride and/or titanium nitride, as the hard material in the top layer of the cathode block according to the invention. Most preferably, the top layer of the cathode block according to the invention contains titanium diboride as the hard material. All of the abovementioned hard materials can be used alone or it is pos-sible to use any desired chemical combination and/or mixture of two or more of the abovementioned compounds.
According to a particularly preferred embodiment of the present invention, the hard material present in the top layer of the cathode block has a monomodal particle size distribution, wherein the mean volume-weighted particle size (d3,50), as de-termined by static light scattering in accordance with the international standard ISO 13320-1, is 10 to 20 pm. In this embodiment, it is particularly preferable to use a non-oxidic titanium ceramic and most preferable to use titanium diboride having a monomodal particle size distribution defined above.
Within the context of the present invention, it has been established that a hard material, in particular non-oxidic titanium ceramic and especially titanium diboride, having a monomodal particle size distribution defined above not only brings about a very good wettability of the surface of the cathode block, which is why the forma-tion of a slurry and the deposition of a slurry on the surface of the cathode block are reliably prevented, the wear resistance of the cathode block is increased and the specific energy consumption during electrolysis is reduced. In addition, it has surprisingly been established within the context of the present invention that this effect is also achieved in particular given relatively small amounts of titanium diboride, of less than 50 % by weight and particularly preferably even given tita-nium diboride quantities of only 15 to 20 % by weight in the top layer. It is thereby possible to dispense with a high concentration of titanium diboride in the top layer, which leads to a brittle cathode block surface. Furthermore, hard material, in par-ticular non-oxidic titanium ceramic and especially titanium diboride, having a monomodal particle size distribution defined above is also distinguished by a very good processability. In particular, the tendency of such a hard material to form dust, for example as it is being introduced into a mixing tank or as the hard mate-rial powder is being transported, is sufficiently low, and at best a small degree of agglomerate formation occurs during the mixing, for example. In addition, such a hard material powder has a sufficiently high flowability and pourability, and there-fore it can be conveyed to a mixing apparatus, for example, using a conventional conveying apparatus. This all results not only in a simple and cost-effective pro-ducibility of the cathode blocks according to the invention, but also in particular in a very homogeneous distribution of the hard material in the top layer of the cath-ode blocks.
The hard material, preferably titanium diboride, present in the top layer of the cathode block preferably has a monomodal particle size distribution, wherein the mean volume-weighted particle size (d3,50), determined as above, is 12 to 18 pm and particularly preferably 14 to 16 pm.
As an alternative to the embodiment mentioned above, the hard material present in the top layer of the cathode block can have a monomodal particle size distribu-tion, wherein the mean volume-weighted particle size (d3,50), as determined by static light scattering in accordance with the international standard ISO
13320-1, is 3 to 10 pm and preferably 4 to 6 pm. In this embodiment, too, it is particularly preferable to use a non-oxidic titanium ceramic and most preferable to use tita-nium diboride having a monomodal particle size distribution defined above.
In a development of the concept of the invention, it is proposed that the hard mate-rial has a volume-weighted d3,90 particle size, determined as above, of 20 to 40 pm and preferably of 25 to 30 pm. The hard material preferably has such a d3,90 value in combination with an above-defined d3,50 value. In this embodiment, too, the hard material is preferably a non-oxidic titanium ceramic and particularly preferably titanium diboride. As a result, the advantages and effects mentioned for the above embodiment are achieved to an even greater extent.
As an alternative to the embodiment mentioned above, the hard material present in the top layer of the cathode block can have a volume-weighted d3,90 particle size, determined as above, of 10 to 20 pm and preferably of 12 to 18 pm. The hard material preferably has such a d3,90 value in combination with an above-defined d3,50 value. In this embodiment, too, it is particularly preferable to use a non-oxidic titanium ceramic and most preferable to use titanium diboride having a monomo-dal particle size distribution defined above.
According to a further preferred embodiment of the present invention, the hard material has a volume-weighted d3,10 particle size, determined as above, of 2 to 7 pm and preferably of 3 to 5 pm. The hard material preferably has such a d3,10 value in combination with an above-defined d3,90 value and/or d3,50 value. In this embodiment, too, the hard material is preferably a non-oxidic titanium ceramic and particularly preferably titanium diboride. As a result, the advantages and effects mentioned for the above embodiments are achieved to an even greater extent.
As an alternative to the embodiment mentioned above, the hard material present in the top layer of the cathode block can have a volume-weighted d3,10 particle size, determined as above, of 1 to 3 pm and preferably of 1 to 2 pm. The hard material preferably has such a d3.10 value in combination with an above-defined d3,90 value and/or d3,50 value. In this embodiment, too, it is particularly preferable to use a non-oxidic titanium ceramic and most preferable to use titanium diboride having a monomodal particle size distribution defined above.
In addition, it is preferable if the hard material, in particular a non-oxidic titanium ceramic and particularly preferably titanium diboride, has a particle size distribution which is characterized by a span value, as calculated in accordance with the fol-lowing equation:
Span = (d3,90 - d3,10)/c13,50 of 0,65 to 3,80 and particularly preferably of 1,00 to 2,25. The hard material pref-erably has such a span value in combination with an above-defined d3,90 value and/or d3,50 value and/or d3,10 value. As a result, the advantages and effects men-tioned for the above embodiments are achieved to an even greater extent.
As set forth above, non-oxidic titanium ceramics, such as preferably titanium car-bide, titanium carbonitride, titanium nitride and most preferably titanium diboride, are suitable in particular as the hard material in the top layer of the cathode block according to the invention. For this reason, it is proposed in a development of the concept of the invention that the hard material consists of non-oxidic titanium ce-ramic and in particular of titanium diboride to an extent of at least 80 % by weight, preferably to an extent of at least 90 % by weight, particularly preferably to an extent of at least 95 % by weight, very particularly preferably to an extent of at least 99 % by weight and most preferably completely.
The total quantity of the hard material in the top layer is, according to the inven-tion, at least 15 % by weight, but at most less than 50 % by weight. When the quantity of hard material lies in this value range, the top layer contains sufficient hard material firstly to provide the top layer with an excellent hardness and abra-sion resistance for increasing the wear resistance and secondly to provide a wet-tability of the top layer surface with liquid aluminium which is sufficiently high for avoiding the formation of a slurry and the deposition of a slurry, as a result of which the wear resistance of the cathode block is increased further and the spe-cific energy consumption during fused-salt electrolysis is reduced further; at the same time, however, the top layer contains a sufficiently small quantity of hard material such that the surface of the top layer does not have a brittleness which is too high for a sufficiently high long-term stability on account of the addition of hard material.
Here, good results are achieved in particular if the top layer contains 15 to 40 % by weight and particularly preferably 15 to 30 % by weight of a hard material having a melting point of at least 1000 C.
Apart from the hard material, the top layer contains carbon and, if appropriate, binder, such as pitch, in particular coal tar pitch and/or petroleum pitch. If pitch is mentioned hereinbelow, this means all varieties of pitch known to a person skilled in the art. Here, the carbon, together with the optional binder, forms the matrix in which the hard material is embedded. Good results are achieved in particular if the top layer contains 85 to more than 50 % by weight, preferably 85 to 60 `)/0 by weight and particularly preferably 85 to 70 % by weight of carbon.
Here, the carbon present in the top layer can be amorphous carbon, graphite or a mixture of amorphous carbon and graphite.
According to a very particularly preferred embodiment of the present invention, the carbon present in the top layer of the cathode block according to the invention is exclusively amorphous carbon or a mixture of amorphous carbon and graphite. If a mixture of amorphous carbon and graphite is used, this mixture preferably con-tains 10 to 99 % by weight, particularly preferably 30 to 95 % by weight and very particularly preferably 60 to 90 % by weight of amorphous carbon, remainder graphite, where the graphite used can be both natural graphite and also synthetic graphite.
Cathode blocks according to the invention having a top layer composed of carbon composite material which contains hard material and, as the carbon component, contains a mixture of amorphous carbon and graphite containing optionally car-bonized binder (such as for example a mixture of calcined anthracite, graphite and carbonized pitch) or very particularly preferably amorphous carbon containing optionally carbonized binder (such as for example a mixture of calcined anthracite and carbonized pitch) have a particularly high abrasion resistance. The starting material used for the amorphous carbon is preferably anthracite, which is then calcined at a temperature of between 800 and 2200 C and particularly preferably between 1200 and 2000 C.
In a development of the concept of the invention, it is proposed for the cathode block top layer containing amorphous carbon that the top layer has a vertical spe-cific electrical resistivity at 950 C of 20 to 32 Q pm and preferably of 22 to 28 pm. This corresponds to vertical specific resistivities at room temperature of 23 to 40 Q pm and of 25 to 30 Q pm. In this context, "vertical specific electrical resistiv-ity" is understood to mean the specific electrical resistivity when the cathode block is installed in the vertical direction.
In principle, the thickness of the top layer should be as small as possible, in order to keep the costs for the expensive hard material as low as possible, but should , , also be sufficiently great for the top layer to have a sufficiently high wear resis-tance and service life. Good results are achieved in this respect in particular if the thickness of the top layer amounts to 1 to 50 %, preferably 5 to 40 %, particularly preferably 10 to 30 % and very particularly preferably 15 to 25 %, for example about 20 %, of the overall height of the cathode block.
By way of example, the top layer can have a thickness or height of 50 to 400 mm, preferably of 50 to 200 mm, particularly preferably of 70 to 130 mm, very particu-larly preferably of 90 to 110 mm and most preferably of about 100 mm. Here, "thickness or height" is understood to mean the distance from the underside of the top layer to the point of the highest elevation of the top layer.
Similarly, and by way of example, the base layer can have a thickness or height of 100 to 550 mm, preferably of 300 to 500 mm, particularly preferably of 400 to mm, very particularly preferably of 425 to 475 mm and most preferably of about 450 mm.
In principle, it is possible for the top layer of the cathode block to have a surface which is profiled at least in certain regions. On account of a profiled surface, the movement of the molten aluminium brought about by the electromagnetic interac-tion present during the electrolysis is reduced, resulting in relatively small wave formation and bulging of the aluminium layer. For this reason, the use of surface-profiled cathode blocks can further reduce the distance between the molten alu-minium and the anode, and therefore the electrical cell resistance is further re-duced as a result of the reduction in the ohmic resistance, and therefore so too is the specific energy consumption.
Here, a profiled surface is understood to mean a surface having at least one re-cess and/or elevation which is arranged chaotically or extends in the transverse direction, in the longitudinal direction or in any other desired direction, such as for example in a direction running at an acute or obtuse angle to the longitudinal di-rection, of the cathode block, the recess or elevation having at least a depth or height of 0,05 mm and preferably of 0,5 mm in demarcation relative to a surface roughness, as seen transversely to the cathode block surface. In this case, the at least one recess and/or elevation can be restricted exclusively to the top layer or the at least one recess and/or elevation can extend into the primary layer. It is preferable for the at least one recess and/or elevation to extend exclusively in the top layer.
Within the context of the present invention, a recess is understood to mean a cut-out directed inwards from the surface of the cathode block, whereas the term "ele-vation" means a rise directed outwards from the surface of the cathode block.
In the case of rectangular cutouts or elevations each of equal depth or height, for example, here it can depend on the observer whether these are regarded as re-cesses or elevations. The wording "recess and/or elevation" is intended to take these ambiguities between the terms "recess" and "elevation" into account.
In principle, the at least one recess and/or elevation can have any desired geome-try, as seen in the transverse direction of the cathode block. By way of example, the at least one recess or elevation can have a convex, concave or polygonal form, for example a trapezoidal, triangular, rectangular or square form, as seen in the transverse direction of the cathode block.
In order to avoid or at least considerably reduce wave formation during operation of the cathode block according to the invention for the fused-salt electrolysis of aluminium oxide in a cryolite melt, and in order to drastically reduce the height of any waves which do possibly form, it is proposed in a development of the concept of the invention that, if the surface profiling comprises at least one recess, the depth-to-width ratio of the at least one recess is 1:3 to 1:1 and preferably 1:2 to 1:1.
Good results are achieved in particular if the depth of the at least one recess is 10 to 90 mm, preferably 40 to 90 mm and particularly preferably 60 to 80 mm, such as for example about 70 mm.
According to a further preferred embodiment, the width of the at least one recess is 100 to 200 mm, particularly preferably 120 to 180 mm and very particularly pref-erably 140 to 160 mm, such as for example about 150 mm.
In principle, it is possible for the at least one recess to extend only in certain re-gions, as seen in the longitudinal direction of the cathode block. However, it is preferable for the at least one recess to extend over the entire length of the cath-ode block, so as to achieve the effect of reducing or completely reducing wave formation of liquid aluminium. It is possible, however, for the depth and/or width of the at least one recess to vary over the length of the cathode block. It is similarly possible for the geometry of the recess to also vary over the length of the cathode block.
If the surface profiling comprises at least one elevation, it is likewise preferable, in order to avoid or at least considerably reduce wave formation during operation of the cathode block according to the invention for the fused-salt electrolysis of alu-minium oxide in a cryolite melt, and in order to drastically reduce the height of any waves which do possibly form, for the height-to-width ratio of the at least one ele-vation to be 1:2 to 2:1 and preferably about 1:1.
Good results are achieved in particular if the height of the at least one elevation is to 150 mm, preferably 40 to 90 mm and particularly preferably 60 to 80 mm, such as for example about 70 mm.
According to a further preferred embodiment, the width of the at least one eleva-tion is 50 to 150 mm, particularly preferably 55 to 100 mm and very particularly preferably 60 to 90 mm, such as for example about 75 mm.
In principle, it is possible for the at least one elevation to extend only in certain regions, as seen in the longitudinal direction of the cathode block. However, it is preferable for the at least one elevation to extend over the entire length of the cathode block, so as to achieve the effect of reducing or completely reducing cor-rugation of liquid aluminium. It is possible, however, for the height and/or width of the at least one elevation to vary over the length of the cathode block. It is similarly possible for the geometry of the elevation to also vary over the length of the cath-ode block.
If the surface profiling comprises both at least one recess and at least one eleva-tion, the ratio of the width of the at least one recess to the width of the at least one elevation is preferably 4:1 to 1:1, such as for example about 2:1.
In order to reliably avoid the deposition of slurry present in the melt in the profiled structure of the surface of the cathode block as fused-salt electrolysis is being carried out, it is proposed in a development of the concept of the invention to avoid any angled and in particular right-angled regions in the profiled surface. If, for example, a substantially rectangular cross section is chosen for the at least one recess and/or elevation, it is preferable according to a preferred embodiment of the present invention to round off the right-angled regions. The radius of curvature of these rounded-off sections can be, for example, 5 to 50 mm, preferably 10 to 30 mm and particularly preferably about 20 mm. In order to avoid sharp edges, any desired geometries which all fall under the term "rounded-off' are conceivable, in principle.
The present invention is not restricted in terms of the number of recesses or eleva-tions in the cathode block. Good results are achieved, for example, if the cathode block has 1 to 3 recesses and preferably 2 recesses in the transverse direction thereof.
According to a further very particularly preferred embodiment of the present inven-tion, the primary layer is composed of a mixture of graphite and binder, such as carbonized pitch, to an extent of at least 80 % by weight, preferably to an extent of at least 90 % by weight, particularly preferably to an extent of at least 95 %
by weight, very particularly preferably to an extent of at least 99 % by weight and most preferably completely (graphite cathode body). Such a primary layer has a suitably low specific electrical resistivity and a sufficiently high specific thermal conductivity. Here, this mixture is preferably composed of 70 to 95 A) by weight of graphite and 5 to 30 A) by weight of binder and particularly preferably of 80 to 90 % by weight of graphite and 10 to 20% by weight of binder, such as for example of 85 % by weight of graphite and 15 % by weight of carbonized pitch.
It is preferable for both the top side of the primary layer and also the underside of the top layer, and therefore also the interface between the primary layer and the top layer, to have a planar form. Although it is not preferable, an intermediate layer can be provided between the primary layer and the top layer, said intermediate layer having the same structure as the top layer, for example, with the exception that the intermediate layer has a lower concentration of hard material than the top layer.
In a development of the concept of the invention, it is proposed that the primary layer has a vertical specific electrical resistivity at 950 C of 13 to 18 pm and preferably of 14 to 16 Q pm. This corresponds to vertical specific electrical resis-tivities at room temperature of 14 to 20 1-2pm and of 16 to 18 Q pm.
The present invention also relates to a cathode, which contains at least one cath-ode block described above, wherein the cathode block has at least one groove on that side of the primary layer which lies opposite the top layer, wherein at least one busbar is provided in the at least one groove in order to feed current to the cath-ode during electrolysis.
In order to fasten the at least one busbar fixedly to the cathode block, and in order to avoid hollow spaces between the busbar and the cathode block which increase the electrical resistance, it is additionally preferable for the at least one busbar to have a cast iron casing at least in certain regions and particularly preferably over the entire circumference. This casing can be produced by inserting the at least one busbar into the groove of the cathode block and then introducing cast iron into the interstice between the busbar and the walls delimiting the groove.
The present invention also relates to the use of a cathode block described above or of a cathode described above for carrying out fused-salt electrolysis for produc-ing metal, such as in particular aluminium.
The cathode block or the cathode is preferably used for carrying out fused-salt electrolysis with a melt of cryolite and aluminium oxide for producing aluminium, the fused-salt electrolysis being carried out with particular preference as a Hall-Heroult process.
Hereinbelow, the present invention is described purely by way of example on the basis of advantageous embodiments and with reference to the attached drawing.
In the drawing:
Figure 1 shows a schematic cross section of a detail of an aluminium elec-trolysis cell, which comprises a cathode block according to an exemplary embodi-ment of the present invention.
Figure 1 shows a cross section of a detail of an aluminium electrolysis cell having a cathode 12, which at the same time forms the bottom of a tank for alu-minium melt 14 produced during operation of the electrolysis cell 10 and for a cryolite-aluminium oxide melt 16 located above the aluminium melt 14. An anode 18 of the electrolysis cell 10 is in contact with the cryolite-aluminium oxide melt 16.
At the side, the tank formed by the lower part of the aluminium electrolysis cell 10 is delimited by a carbon and/or graphite lining (not shown in Figure 1).
The cathode 12 comprises a plurality of cathode blocks 20, 20', 20", which are each connected to one another via a ramming mass 24, 24' which has been in-serted into a ramming mass joint 22, 22' arranged between the cathode blocks 20, 20', 20". Similarly, the anode 18 comprises a plurality of anode blocks 26, 26', the anode blocks 26, 26' each being approximately twice as wide and approximately half as long as the cathode blocks 20, 20', 20". In this case, the anode blocks 26, 26' are arranged above the cathode blocks 20, 20', 20" in such a way that in each case an anode block 26, 26' covers two cathode blocks 20, 20', 20" arranged alongside one another in width and in each case a cathode block 20, 20', 20"
covers two anode blocks 26, 26' arranged alongside one another in length.
Each cathode block 20, 20', 20" consists of a lower primary layer 30, 30', 30"
and a top layer 32, 32', 32" which is arranged thereabove and is fixedly connected thereto. The interfaces between the primary layers 30, 30', 30" and the top layers 32, 32', 32" are planar. Whereas the primary layers 30, 30', 30" of the cathode blocks 20, 20', 20" each have a graphite material structure, i.e. consist of graphitic carbon, containing synthetic or natural graphite and carbonized binding pitch, the top layers 32, 32', 32" are each composed of a ceramic-carbon composite material containing titanium diboride, which contains 20 A by weight of titanium diboride, amorphous carbon, specifically anthracite, and carbonized pitch as the binder.
The titanium diboride present in the top layers 32, 32', 32" has a mean volume-weighted particle size (d3,50), as determined by static light scattering in accordance with the standard ISO 13320-1, of 15 pm, a d3,90 particle size of 27 pm and a d3,10 particle size of 4 pm.
Each cathode block 20, 20', 20" has a width of 650 mm and a total height of mm, the primary layers 30, 30', 30" each having a height of 450 mm and the top layers 32, 32', 32" each having a height of 100 mm. The distance between the anode blocks 26, 26' and the cathode blocks 20, 20', 20" is approximately 200 to approximately 350 mm, the layer of cryolite-aluminium oxide melt 16 arranged therebetween having a thickness of approximately 50 mm and the layer of alumin-ium melt 14 arranged thereunder likewise having a thickness of approximately to approximately 300 mm.
Finally, each cathode block 20, 20', 20" comprises two grooves 38, 38' on its un-derside each with a rectangular, specifically substantially rectangular cross sec-tion, wherein a steel busbar 40, 40' likewise having a rectangular or substantially rectangular cross section is accommodated in each groove 38, 38'. In this case, the interstices between the busbars 40, 40' and the walls which delimit the grooves 38, 38 are each sealed with cast iron (not shown), as a result of which the busbars 40, 40' are fixedly connected to the walls which delimit the grooves 38, 38'.
It is preferable for both the grooves 38, 38' and the recesses 34, 34' to be put in the top side of the top layers 32, 32', 32" during the shaping process, to be precise for example by vibrating moulds and/or punches.
List of reference symbols 10 Aluminium electrolysis cell 12 Cathode 14 Aluminium melt 16 Cryolite-aluminium oxide melt 18 Anode 20, 20', 20" Cathode block 22, 22' Ramming mass joint 24, 24' Ramming mass 26, 26' Anode block 30, 30', 30" Primary layer 32, 32', 32" Top layer 38, 38' Groove 40, 40' Busbar
Claims (57)
1. A cathode block for an aluminium electrolysis cell having a primary layer and having a top layer, wherein the primary layer contains graphite and the top layer contains a carbon composite material containing 15 to less than 50 % by weight of a hard material having a melting point of at least 1000 °C, and wherein the top layer has a thickness of 50 to 400 mm.
2. The cathode block according to claim 1, wherein the top layer has a thickness of 50 to 200 mm.
3. The cathode block according to claim 2, wherein the top layer has a thickness of 70 to 130 mm.
4. The cathode block according to claim 3, wherein the top layer has a thickness of 90 to 110 mm.
5. The cathode block according to claim 4, wherein the top layer has a thickness of 100 mm.
6. The cathode block according to any one of claims 1 to 5, wherein the hard material has a Knoop hardness, measured in accordance with DIN EN 843-4, of at least 1,000 N/mm2.
7. The cathode according to claim 6, wherein the Knoop hardness is at least 1,500 N/mm2.
8. The cathode according to claim 7, wherein the Knoop hardness is at least 2,000 N/mm2.
9. The cathode according to claim 8, wherein the Knoop hardness is at least 2,500 N/mm2.
10. The cathode block according to any one of claims 1 to 9, wherein the hard material is selected from the group consisting of titanium diboride, zirconium diboride, tantalum diboride, titanium carbide, boron carbide, titanium carbonitride, silicon carbide, tungsten carbide, vanadium carbide, titanium nitride, boron nitride, silicon nitride and any chemical combination and/or mixtures of two or more of said compounds.
11. The cathode block according to any one of claims 1 to 10, wherein the hard material has a monomodal particle size distribution, wherein the mean volume-weighted particle size (d3,5o), as determined by static light scattering in accordance with ISO 13320-1, is 10 to 20 pm.
12. The cathode block according to claim 11, wherein the mean volume-weight particle size (d3,50) is 12 to 18 pm.
13. The cathode block according to claim 12, wherein the mean volume-weight particle size (d3,50) is 14 to 16 pm.
14. The cathode block according to any one of claims 1 to 10, wherein the hard material has a monomodal particle size distribution, wherein the mean volume-weighted particle size (d3,50), as determined by static light scattering in accordance with ISO 13320-1, is 3 to 10 µm.
15. The cathode block according to claim 14, wherein the mean volume-weight particle size (d3,50) is 4 to 6 µm.
16. The cathode block according to any one of claims 1 to 15, wherein the d3,90 particle size of the hard material, as determined by static light scattering in accordance with ISO 13320-1, is 20 to 40 µm.
17. The cathode block according to claim 16, wherein the d3,93 particle size is 25 to 30 µm.
18. The cathode block according to any one of claims 1 to 15, wherein the d3,90 particle size of the hard material, as determined by static light scattering in accordance with ISO 13320-1, is 10 to 20 µm.
19. The cathode block according to claim 18, wherein the d3,90 particle size is 12 to 18 µm.
20. The cathode block according to any one of claims 1 to 19, wherein the d3,10 particle size of the hard material, as determined by static light scattering in accordance with ISO 13320-1, is 2 to 7 µm.
21. The cathode block according to claim 20, wherein the d3,10 particle size is 3 to 5 µm.
22. The cathode block according to any one of claims 1 to 19, wherein the d3,10 particle size of the hard material, as determined by static light scattering in accordance with ISO 13320-1, is 1 to 3 µm.
23. The cathode block according to claim 22, wherein the d3,10 particle size is 1 to 2 µm.
24. The cathode block according to any one of claims 1 to 23, wherein the hard material is a non-oxidic titanium ceramic and has a particle size distribution having a span value, as calculated in accordance with the following equation:
Span = (d3,90 - d3,10)/d3,50 of 0.65 to 3.80.
Span = (d3,90 - d3,10)/d3,50 of 0.65 to 3.80.
25. The cathode block according to claim 24, wherein the span value is 1.00 to 2.25.
26. The cathode block according to claim 24 or 25, wherein the hard material is titanium diboride.
27. The cathode block according to any one of claims 1 to 26, wherein the hard material contains at least 80 % by weight of a non-oxidic titanium ceramic.
28. The cathode block according to claim 27, which contains at least 90%
by weight of the non-oxidic titanium ceramic.
by weight of the non-oxidic titanium ceramic.
29. The cathode block according to claim 28, which contains at least 95%
by weight of the non-oxidic titanium ceramic.
by weight of the non-oxidic titanium ceramic.
30. The cathode block according to claim 29, which contains at least 99%
by weight of the non-oxidic titanium ceramic.
by weight of the non-oxidic titanium ceramic.
31. The cathode block according to claim 30, which contains 100% by weight of the non-oxidic titanium ceramic.
32. The cathode block according to any one of claims 27 to 31, wherein the non-oxidic titanium ceramic is titanium diboride.
33. The cathode block according to any one of claims 1 to 32, wherein the top layer contains 15 to 40 % by weight the hard material.
34. The cathode block according to claim 33, wherein the top layer contains 15 to 30% by weight of the hard material.
35. The cathode block according to any one of claims 1 to 34, wherein the top layer contains 85 to more than 50% by weight of carbon.
36. The cathode block according to claim 35, wherein the top layer contains 85 to 60% by weight of carbon.
37. The cathode block according to claim 36, wherein the top layer contains 85 to 70% by weight of carbon.
38. The cathode block according to any one of claims 35 to 37, wherein the carbon is amorphous carbon, graphite or a mixture of amorphous carbon and graphite.
39. The cathode block according to claim 38, wherein the carbon is amorphous carbon or is a mixture of 10 to 99% by weight of amorphous carbon, with the remainder graphite.
40. The cathode block according to claim 39, wherein the carbon is amorphous carbon or is a mixture of 30 to 95% by weight of amorphous carbon, with the remainder graphite.
41. The cathode block according to claim 40, wherein the carbon is amorphous carbon or is a mixture of 60 to 90% by weight of amorphous carbon, with the remainder graphite.
42. The cathode block according to any one of claims 1 to 41, wherein the top layer has a vertical specific electrical resistivity at 950 °C of 20 to 32 .OMEGA. µm.
43. The cathode block according to claim 42, wherein the vertical specific electrical resistivity is 22 to 28 .OMEGA. µm.
44. The cathode block according to any one of claims 1 to 43, wherein the thickness of the top layer amounts to 1 to 50% of the overall height of the cathode block.
45. The cathode block according to claim 44, wherein the thickness of the top layer amounts to 5 to 40% of the overall height of the cathode block.
46. The cathode block according to claim 45, wherein the thickness of the top layer amounts to 10 to 30% of the overall height of the cathode block.
47. The cathode block according to claim 46, wherein the thickness of the top layer amounts to 15 to 25% of the overall height of the cathode block.
48. The cathode block according to any one of claims 1 to 47, wherein the primary layer is composed of graphite and a binder to an extent of at least 80% by weight.
49. The cathode block according to claim 48, wherein the primary layer is composed of graphite and the binder to an extent of at least 90% by weight.
50. The cathode block according to claim 49, wherein the primary layer is composed of graphite and the binder to an extent of at least 95% by weight.
51. The cathode block according to claim 50, wherein the primary layer is composed of graphite and the binder to an extent of at least 99% by weight.
52. The cathode according to claim 51, wherein the primary layer is completely composed of graphite and the binder.
53. The cathode block according to any one of claims 1 to 52, wherein the primary layer has a vertical specific electrical resistivity at 950 °C
of 13 to 18 .OMEGA. µm.
of 13 to 18 .OMEGA. µm.
54. The cathode according to claim 53, wherein the primary layer has a vertical electrical resistivity of 14 to 16 .OMEGA. µm.
55. A cathode, which contains at least one cathode block according to any one of claims 1 to 54, wherein the cathode block has at least one groove on a side of the primary layer which lies opposite the top layer, and wherein at least one busbar is provided in the at least one groove in order to feed current to the cathode during 1 electrolysis.
56. Use of the cathode block according to any one of claims 1 to 54, or the cathode according to claim 55, for carrying out fused-salt electrolysis for producing a metal.
57. The use according to claim 56, wherein the metal is aluminium.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE102011004014.5 | 2011-02-11 | ||
| DE102011004014A DE102011004014A1 (en) | 2011-02-11 | 2011-02-11 | Cathode block with a covering layer containing hard material |
| PCT/EP2012/051961 WO2012107401A2 (en) | 2011-02-11 | 2012-02-06 | Cathode block having a cover layer that contains a hard material |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2826604A1 CA2826604A1 (en) | 2012-08-16 |
| CA2826604C true CA2826604C (en) | 2016-11-08 |
Family
ID=45562342
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA2826604A Expired - Fee Related CA2826604C (en) | 2011-02-11 | 2012-02-06 | Cathode block having a top layer containing hard material |
Country Status (8)
| Country | Link |
|---|---|
| EP (1) | EP2673399A2 (en) |
| JP (1) | JP2014505177A (en) |
| CN (1) | CN103443331A (en) |
| CA (1) | CA2826604C (en) |
| DE (1) | DE102011004014A1 (en) |
| RU (1) | RU2584097C2 (en) |
| UA (1) | UA110367C2 (en) |
| WO (1) | WO2012107401A2 (en) |
Family Cites Families (20)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4333813A (en) * | 1980-03-03 | 1982-06-08 | Reynolds Metals Company | Cathodes for alumina reduction cells |
| US4308114A (en) * | 1980-07-21 | 1981-12-29 | Aluminum Company Of America | Electrolytic production of aluminum using a composite cathode |
| EP0085093A4 (en) * | 1981-07-27 | 1984-04-27 | Great Lakes Carbon Corp | Sintered refractory hard metals. |
| US4526669A (en) * | 1982-06-03 | 1985-07-02 | Great Lakes Carbon Corporation | Cathodic component for aluminum reduction cell |
| US4466995A (en) * | 1982-07-22 | 1984-08-21 | Martin Marietta Corporation | Control of ledge formation in aluminum cell operation |
| US4544469A (en) * | 1982-07-22 | 1985-10-01 | Commonwealth Aluminum Corporation | Aluminum cell having aluminum wettable cathode surface |
| US4481052A (en) * | 1983-01-28 | 1984-11-06 | Martin Marietta Corporation | Method of making refractory hard metal containing tiles for aluminum cell cathodes |
| US4582553A (en) * | 1984-02-03 | 1986-04-15 | Commonwealth Aluminum Corporation | Process for manufacture of refractory hard metal containing plates for aluminum cell cathodes |
| FR2566002B1 (en) * | 1984-06-13 | 1986-11-21 | Pechiney Aluminium | MODULAR CATHODE BLOCK AND LOW VOLTAGE DROP CATHODE FOR HALL-HEROULT ELECTROLYSIS TANKS |
| ATE182370T1 (en) * | 1992-12-17 | 1999-08-15 | Comalco Alu | ELECTROLYSIS CELL FOR THE PRODUCTION OF METALS |
| DE69532052T2 (en) * | 1994-09-08 | 2004-08-19 | Moltech Invent S.A. | Horizontal cathode surface drained with recessed grooves for aluminum electrical extraction |
| DE19714433C2 (en) | 1997-04-08 | 2002-08-01 | Celanese Ventures Gmbh | Process for producing a coating with a titanium boride content of at least 80% by weight |
| CA2350814C (en) * | 1998-11-17 | 2006-01-10 | Alcan International Limited | Wettable and erosion/oxidation-resistant carbon-composite materials |
| CN1091471C (en) * | 2000-05-08 | 2002-09-25 | 新化县碳素厂 | Carbon block as cathode with compound titanium boride-carbon layer and its preparing process |
| CN100366800C (en) * | 2004-12-28 | 2008-02-06 | 中国铝业股份有限公司 | Preparation method of TiB2 composite layer cathode carbon block |
| CN100491600C (en) * | 2006-10-18 | 2009-05-27 | 中国铝业股份有限公司 | Method for preparing carbon block of cathode capable of being humidified |
| CN101165217A (en) * | 2007-08-08 | 2008-04-23 | 中国铝业股份有限公司 | Humid cathode carbon block with high graphite basal body and producing method thereof |
| RU2371523C1 (en) * | 2008-06-23 | 2009-10-27 | Федеральное государственное образовательное учреждение высшего профессионального образования "Сибирский федеральный университет" | Composite material for moistened cathode of aluminium electrolytic cell |
| CN201261809Y (en) * | 2008-08-12 | 2009-06-24 | 高德金 | Cathode inner lining with molten aluminum magnetic vortex stream adjustment device |
| DE102010038669A1 (en) * | 2010-07-29 | 2012-02-02 | Sgl Carbon Se | Cathode block for an aluminum electrolysis cell and a method for its production |
-
2011
- 2011-02-11 DE DE102011004014A patent/DE102011004014A1/en not_active Ceased
-
2012
- 2012-02-06 EP EP12702278.8A patent/EP2673399A2/en not_active Withdrawn
- 2012-02-06 RU RU2013141533/02A patent/RU2584097C2/en not_active IP Right Cessation
- 2012-02-06 WO PCT/EP2012/051961 patent/WO2012107401A2/en active Application Filing
- 2012-02-06 CA CA2826604A patent/CA2826604C/en not_active Expired - Fee Related
- 2012-02-06 CN CN2012800152357A patent/CN103443331A/en active Pending
- 2012-02-06 JP JP2013552924A patent/JP2014505177A/en not_active Ceased
- 2012-06-02 UA UAA201310889A patent/UA110367C2/en unknown
Also Published As
| Publication number | Publication date |
|---|---|
| WO2012107401A2 (en) | 2012-08-16 |
| RU2584097C2 (en) | 2016-05-20 |
| EP2673399A2 (en) | 2013-12-18 |
| WO2012107401A3 (en) | 2012-10-11 |
| JP2014505177A (en) | 2014-02-27 |
| CA2826604A1 (en) | 2012-08-16 |
| DE102011004014A1 (en) | 2012-08-16 |
| RU2013141533A (en) | 2015-03-20 |
| UA110367C2 (en) | 2015-12-25 |
| CN103443331A (en) | 2013-12-11 |
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