CA2854928A1

CA2854928A1 - Cathode block having a domed and/or rounded surface

Info

Publication number: CA2854928A1
Application number: CA2854928A
Authority: CA
Inventors: Thomas Frommelt; Arash RASHIDI
Original assignee: SGL Carbon SE
Current assignee: SGL Carbon SE
Priority date: 2011-11-09
Filing date: 2012-11-07
Publication date: 2013-05-16
Also published as: DE102011086044A1; WO2013068412A3; EP2776608A2; WO2013068412A2; CN103958740A; IN2014CN03395A; JP2014532816A

Abstract

The invention relates to a cathode block for an aluminum electrolytic cell, in particular based on carbon and/or graphite, having at least one groove for accommodating a bus bar, said groove being arranged on one of the sides of the cathode block and extending in the longitudinal direction of the cathode block, wherein at least one section of the surface of the side of the cathode block opposite the side having the at least one groove is curved outwardly in the shape of an arch as viewed in the cross-section of the cathode block, wherein the vertex of the at least one section curved outwardly in the shape of an arch is arranged over the at least one groove, with respect to the direction extending perpendicularly to the side having the at least one groove in the cross-section of the cathode block. The invention further relates to a cathode block for an aluminum electrolytic cell, in particular based on carbon and/or graphite, which cathode block has a tub-shaped surface as viewed in the longitudinal section of the cathode block, wherein the tub has two edge regions and a bottom region, which is arranged between the edge regions and lowered relative to the edge regions as viewed in the longitudinal direction of the cathode, wherein a respective side wall region is provided between each edge region and the bottom region, which side wall region connects the respective edge region and the bottom region, wherein at least one of the two connecting regions between the edge regions and the side wall regions and/or at least one of the two connecting regions between the bottom region and the side wall regions is curved in the shape of an arch, wherein the at least one section curved in the shape of an arch has a length of more than 2 cm.

Description

CATHODE BLOCK HAVING A DOMED AND/OR ROUNDED SURFACE
The present invention relates to a cathode block, which is suitable in particular for use in an electrolytic cell for producing aluminium.
Electrolytic cells are used for example for the electrolytic production of aluminium, which is usually performed industrially in accordance with the Hall-Heroult process. In the Hall-Heroult process, a melt composed of aluminium oxide and cryolite is electrolysed. Here, the cryolite, Na3[AIF6], is used to lower the melting point from 2,045 C for pure aluminium oxide to approximately 950 C for a mixture containing cryolite, aluminium oxide and additives, such as aluminium fluoride and calcium fluoride.
The electrolytic cell used in this process comprises a cathode base, which may be composed of a plurality of cathode blocks which are adjacent to one another and form the cathode. In order to withstand the thermal and chemical conditions prevailing during operation of the cell, the cathode is usually composed of a carbonaceous material. The undersides of the cathode are usually provided with grooves, in each of which at least one busbar is arranged, by means of which the current fed via the anodes is discharged. An anode, in particular formed from individual anode blocks, is arranged approximately 3 to 5 cm above the layer of liquid aluminium, usually 15 to 50 cm high, located on the cathode upper side, the electrolyte, that is to say the melt containing aluminium oxide and cryolite, being located between said anode and the surface of the aluminium. During the electrolysis performed at approximately 1,000 C, the formed aluminium settles beneath the electrolyte layer due to its greater density compared with the electrolyte, that is to say settles as an intermediate layer between the upper side of the cathode and the electrolyte layer. During electrolysis, the aluminium oxide dissolved in the melt is cleaved by electric current flow to form aluminium and oxygen.
Considered electrochemically, the layer of liquid aluminium is the actual cathode, since aluminium ions are reduced at the surface thereof to form elemental aluminium.
Nevertheless, the term "cathode" will not be understood hereinafter to mean the cathode from an electrochemical viewpoint, that is to say the layer of liquid aluminium, but the component part forming the electrolytic cell base, for example composed of one or more cathode blocks.
A significant disadvantage of the Hall-Heroult process is that it is very energy intensive. Approximately 12 to 15 kWh of electrical energy are required to generate 1 kg of aluminium, 'which accounts for up to 40 % of the production costs. In order to reduce the production costs, it is therefore desirable to reduce the specific energy consumption of this method where possible.
Due to the relatively high electrical resistance of the melt in particular compared with the layer of liquid aluminium and the cathode material, relatively high ohmic losses in the form of Joule dissipation occur particularly in the melt. In view of the comparatively high specific losses in the melt, there is a significant effort to reduce the thickness of the melt layer and therefore the distance between the anode and the layer of liquid aluminium as far as possible. However, due to the electromagnetic interactions present during the electrolysis process and the wave formation thus produced in the layer of liquid aluminium, there is thus the risk with an excessively thin melt layer that the layer of liquid aluminium will come into contact with the anode, which may lead to short circuits of the electrolytic cell and to undesirable re-oxidation of the formed aluminium and to the electrical instability of the electrolysis mode and in particular to a fluctuation of the call voltage. Short circuits that occur may also lead to increased wear and therefore to a reduced service life of the electrolytic cell. For these reasons, the distance between the anode and the layer of liquid aluminium cannot be reduced arbitrarily.
The driving force for the wave formation in the layer of liquid aluminium is the inhomogeneous distribution of the electric current density and the magnetic flux density over the surface of the cathode, which leads to a wave-formation-promoting distribution of the Lorentz force density in the layer of liquid aluminium. Here, the Lorentz force density is defined as the vector product of the electric current density present at a specific point and of the magnetic flux density present at this point. One reason, inter alia, for the inhomogeneous distribution of the electric current density and of the magnetic flux density at the upper side of the cathode is in turn the fact that the current in the cathode and in the aluminium bath preferably takes the path of lowest electrical resistance. For this reason, the electric current flowing through the cathode is concentrated primarily on the lateral edge regions of the cathode, at which the cathode is connected to the busbars contacting said cathode, since the resultant electrical resistance with the current flow via the edge regions to the surface of the cathode is lower than with the current flow via the centre of the cathode to the surface of the cathode, in which case a longer route or electrical path has to be covered than with the current flow via the edge regions to the surface of the cathode.
Besides an intensified wave formation in the layer of liquid aluminium, the inhomogeneous current density distribution and in particular the increased current density at the lateral edge regions of the cathode, as considered in the transverse direction of the cathode, compared with the current density in the centre of the cathode also leads to an intensified wear of the cathode in the lateral edge regions, which typically leads after relatively long operation of the electrolytic cell to a characteristic approximately W-shaped wear profile of the cathode in the cross section of the cathode.
In order to reduce the specific energy consumption of an electrolytic cell, it has been proposed recently to use, in electrolytic cells, cathodes having a profiled surface, more specifically cathodes for example of which the upper side, as considered in the cross section of the cathode, is embodied in the form of a V-shaped tub. Here, the indentation in the cathode surface configured in the form of a V-shaped tub causes the current density in the lateral edge regions of the cathode to be reduced, whereby the wave formation potential and also the wear are reduced in these regions. In WO 2011/148347, cathodes are used of which the upper side contains an indented central region, wherein this indented region may have a certain volume A. These indentations have inclined surfaces, that is to say they are provided in the shape of tubs. However, these cathodes and the cathode blocks from which these cathodes are composed also do not solve sufficiently the problems of wave formation and wear.
The object of the present invention is therefore to provide a cathode block, which, when used in the case of fused-salt electrolysis in an electrolytic cell, has a reduced specific energy consumption and an increased service life.
In particular, a cathode block is to be provided which makes it possible to reduce the thickness in the electrolytic cell of the melt layer between the aluminium and the anode, without the resultant occurrence of instabilities, such as short circuits or re-oxidations of the formed aluminium or fluctuations of the electrolytic cell voltage, caused by increased wave formation tendency in the layer of liquid aluminium.

% t s A

In accordance with a first aspect of the present invention this object is achieved by providing a cathode block, in particular for an aluminium electrolytic cell, having the features of Patent Claim 1 with at least one groove, which is arranged on one of the sides of the cathode block and runs in the longitudinal direction of the cathode block, for receiving a busbar, wherein at least one portion of the surface of the side of the cathode block opposite the side having the at least one groove is curved outwardly in the shape of an arch as viewed in the cross section of the cathode block, wherein the vertex of the at least one portion curved outwardly in the shape of an arch is arranged above the at least one groove, with respect to the direction running perpendicularly to the side having the at least one groove in the cross section of the cathode block.
It has been found in accordance with the invention that, due to an embodiment curved outwardly in the shape of an arch, as considered in the cross section of the cathode block, of the side of the cathode block opposite the side having the at least one groove, that is to say, with the use of the cathode block in a cathode of an electrolytic cell, due to an embodiment curved outwardly in the shape of an arch of the upper side of the cathode block, a homogenisation of the electric current density and of the magnetic flux density at the surface of the cathode block is achieved. This is because, as a result of the embodiment curved outwardly in the shape of an arch of the upper side of the cathode block, the distance between the upper edge of the groove and the portion, arranged vertically thereabove, on the upper side of the cathode block is adapted to the distances between the underside of the cathode block running beside the groove and the portions, arranged vertically thereabove, on the upper side of the cathode block, and the length of the electrical path of lowest resistance, which leads from the upper edge of the A

groove to the region of the upper side of the cathode block arranged vertically above the groove, is thus adapted to the length of the electrical paths of greater resistances, which lead from the lower edge of the cathode block beside the portion of the underside of the cathode block comprising the groove to the upper side of the cathode block. With a conventional rectangular embodiment of the cathode block, the length of the electrical path between the upper edge of the groove of rectangular cross section milled in centrally on the underside of the cathode block and the portion arranged vertically thereabove on the upper side of the cathode block is much shorter than the lengths of the electrical paths which lead from the side face of the groove to the upper side of the cathode block. Since the current in the cathode block follows the path of the lowest electrical resistance, the current flows in a previously described cathode block predominantly in the region between the upper edge of the groove and the portion arranged vertically thereabove on the upper side of the cathode block, whereas the current flow into the two adjacent portions of the cathode block is severely reduced, such that, as viewed over the surface of the cathode block, an inhomogeneous current density is produced, which leads during the operation of an electrolytic cell to a greater wear above the groove than at the joints between the blocks. The cathode surface thus adopts a wave-like form, which may lead to an increased wave formation in the layer of liquid aluminium arranged on the cathode block. Due to the embodiment of the cathode block surface according to the invention, the electrical path between the upper side of the groove and the cathode surface is enlarged and the electrical path between the side face of the groove and the cathode surface is reduced. The current flow is thus displaced directly above the groove to the sides, such that, as viewed over the surface of the cathode block, a uniform current density is present, such that, during operation of an electrolytic cell comprising the cathode a block, the wave formation in the layer of liquid aluminium arranged on the cathode block is reduced, and the wear of the cathode block over the surface thereof is also uniform.
On the whole, a wave formation in the layer of liquid aluminium is thus effectively avoided during the operation of an electrolytic cell comprising the cathode block according to the invention, and a high energy efficiency is attained with simultaneous high stability and reliability of the electrolysis operation. Due to the reduction of the wave formation in the layer of liquid aluminium arranged above the cathode block, the distance between the anode and the layer of liquid aluminium can be reduced, which leads to an additional energy saving during operation of an electrolytic cell comprising the cathode block. Here, it is particularly advantageous if the above-described measure used to homogenise the electric current density on the cathode upper side, specifically the provision of at least one portion curved outwardly in the shape of an arch, can be easily dimensioned such that the cathode block according to the invention can be used in an electrolytic cell such that the same bath volume as with the use of a conventional cathode is produced, even with reduced distance between the anode and the layer of liquid aluminium.
Within the scope of the present invention, the fact that the surface of the cathode block is curved outwardly in the shape of an arch as viewed in the cross section of the cathode block is understood to mean that the sectional curve defined by the cathode surface in the cross section in the cathode block has an outwardly directed curvature running in a convex manner with respect to the cathode block interior, wherein a curved portion is understood to mean a portion of the sectional curve in which the direction changes continuously, but without the presence of an angular or jagged or sharp change of direction in the curved region.

a Further, the term "vertex" of a curved portion within the scope of the present invention denotes the point of the curved portion distanced furthest as considered perpendicularly from the underside of the cathode block.
The cathode block is preferably formed on the basis of carbon and/or graphite, wherein the cathode block is composed in particular preferably in a proportion of at least 30 % by weight, further preferably in a proportion of at least 40 % by weight, more preferably in a proportion of at least 50 % by weight, even more preferably in a proportion of at least 60 % by weight, and most preferably completely of carbon and/or graphite.
The carbon is preferably amorphous carbon, and the graphite is preferably graphitic or graphitised carbon. It is further preferable for mixtures of amorphous carbon and graphitic carbon, of amorphous carbon and graphitised carbon, of amorphous carbon, graphitic and graphitised carbon, or of graphitic and graphitised carbon to be used.
In accordance with a preferred embodiment of the first aspect of the present invention, the cathode block has exactly one groove for receiving busbars, preferably one busbar, with a rectangular cross section, wherein at least the portion arranged above the groove of the side of the cathode block opposite the side having the groove is preferably curved outwardly in the shape of an arch, with respect to the direction running perpendicularly to the side having the groove in the cross section of the cathode block. As described above, the lengths of the electrical paths between the upper edge of the groove and on the one hand the regions of the surface of the cathode block arranged vertically above the groove and on the other hand the adjacently arranged regions of the surface of the cathode block are thus made uniform, such that the cathode block has a homogeneous current density as viewed over the surface thereof with use in an electrolytic cell.
Alternatively to the above embodiment, the cathode block may also have two grooves for receiving busbars, preferably one busbar in each case, with a rectangular cross section in each case, wherein at least the portion arranged above the grooves of the side of the cathode block opposite the side having the two grooves is curved outwardly in the shape of an arch once, with respect to the direction (z) running perpendicularly to the side having the grooves in the cross section of the cathode block, or wherein each of the two portions arranged above the grooves of the side of the cathode block opposite the side having the two grooves is curved outwardly in the shape of an arch, with respect to the direction running perpendicularly to the side having the two grooves in the cross section of the cathode block, wherein each of the vertices of the two portions curved outwardly in the shape of an arch is arranged above a respective one of the two grooves, with respect to the direction running perpendicularly to the side having the two grooves in the cross section of the cathode block. With a cathode block having two grooves, the lengths of the electrical paths between the upper edge of the closest groove and on the one hand the regions arranged vertically thereabove of the surface of the cathode block and on the other hand the adjacently arranged regions of the surface of the cathode block are thus made uniform, such that the cathode block has a homogeneous current density as viewed over the surface thereof with use in an electrolytic cell.
Alternatively, it is also possible for the cathode block to have two grooves for receiving busbars, preferably one busbar in each case, with a rectangular cross section in each case, wherein at least the portion arranged above the grooves of the side of the cathode block opposite the side having the grooves is preferably curved outwardly in the shape of an arch once, with respect to the direction running perpendicularly to the side having the grooves in the cross section of the cathode block, that is to say a portion curved outwardly in the shape of an arch is provided on the cathode side and spans the two grooves.
Good results in terms of the uniformity of the distribution of the electric current density on the cathode block surface with use of the cathode block in an electrolytic cell are achieved in particular if the vertex of the at least one portion curved outwardly in the shape of an arch of the surface of the cathode block, with respect to the direction running perpendicularly to the side having the at least one groove in the cross section of the cathode block, is arranged above a region of the at least one groove which extends from 20 to 80 %, preferably from 40 to 60 %, of the width of the at least one groove, and is more preferably arranged above the centrepoint of the at least one groove.
Here, the range from 20 to 80 % of the width of the groove denotes the portion of the groove which, as considered in the cross section of the cathode block, starts at 20 % of the extension of the groove measured from a lateral end of the groove in the width direction of the cathode block and ends at 80 % of the extension of the groove measured from this lateral end of the groove in the width direction of the cathode block. In this context, the centrepoint of the groove is understood to mean the point which, as considered in the cross section of the cathode block, is arranged in the centre of the groove, that is to say at 50 % of the extension of the groove measured from a lateral end of the groove in the width direction of the cathode block.
In order to achieve the most uniform distribution possible of the electric current density over the entire region of the cathode block arranged vertically above the groove width of the cathode block, it is proposed in accordance with a development of the inventive concept for the at least one region curved outwardly in the shape of an arch of the surface of the cathode block to extend over at least 20 %, preferably over at least 40 %, more preferably over at least 60 %, even more preferably over at least 80 % and most preferably over 100 % of the region which is arranged over the width of the groove, with respect to the direction running perpendicularly to the side having the at least one groove in the cross section of the cathode block.
In view of a uniform distribution of the electric current density over the entire cathode block surface with the use of the cathode block in an electrolytic cell, it is particularly preferable for the at least one region curved outwardly in the shape of an arch of the surface of the cathode block to extend over at least 20 %, preferably over at least 40 %, more preferably over at least 60 % and most preferably over at least 100 % of the side of the cathode block opposite the side having the at least one groove as viewed in the cross section of the cathode block.
If the cathode block has exactly one groove for receiving a busbar, the cathode block surface, as considered in the cross section of the cathode block, has preferably exactly one curved region, which complies with the values described above in respect of the width of the cathode block. If the cathode block has two grooves for receiving one busbar in each case, the cathode block surface, as considered in the cross section of the cathode block, preferably has a curved region which spans both grooves and which complies with the values specified above in respect of the width of the cathode block, or has two curved regions which, considered together, comply with the values specified above in respect of the width of the cathode block.
An extensive homogenisation of the distribution of the electric current density is then produced in particular within the scope of the invention if the at least one region curved outwardly in the shape of an arch of the surface of the cathode block extends for example over at least 60 %, preferably over at least 80 %, more preferably over at least 90 % and most preferably over at least 100 %
of the length of the cathode block.
A cathode surface that is particularly well adapted to the electrical flow conditions in the cathode block with use thereof in an electrolytic cell is achieved in accordance with a further embodiment of the first aspect of the present invention in that the at least one portion curved outwardly in the shape of an arch of the surface of the cathode block is shaped in the manner of an oval segment, in particular is shaped in the manner of a circular arc, is cosine-shaped, is in the form of a Gaussian normal distribution, is shaped in the manner of an ellipse segment, is in the form of a Bezier curve, is shaped in the manner of a parabola portion, or is curved in the form of a cosine curve of higher power, with respect to the cross section of the cathode block.
In accordance with the typically symmetrical electrical flow conditions in the cathode block with the use thereof in an electrolytic cell, it is preferable if the at least one portion curved outwardly in the shape of an arch of the surface of the cathode block is formed symmetrically about the central perpendicular plane of the at least one groove, with respect to the cross section of the cathode block. A
simple possibility for producing the cathode block and a possibility for universal use of the cathode block in an electrolytic cell are thus additionally achieved.
In order to achieve a particularly homogeneous distribution of the current density over the cathode surface and in order to thus particularly effectively reduce a wave formation in a layer of liquid aluminium arranged above the cathode block with the use of the cathode block according . .

to the invention in an electrolytic cell, it is proposed in a development of the inventive concept to embody the cathode surface such that the at least one portion curved outwardly in the shape of an arch has the form of an ellipse segment with a range of the polar angle between 100 and 180 , preferably between 30 and 160 , more preferably between 50 and 140 and even more preferably between 70 and 120 , and/or for the at least one portion curved outwardly in the shape of an arch to have the form of a cosine curve with a range of the angle between 10 and 180 , preferably between 30 and 160 , more preferably between 50 and 140 and even more preferably between 70 and 120 , and/or for the at least one portion curved outwardly in the shape of an arch to have the form of a segment of a circular arc with a range of the angle subtended by the arc at the centre between 100 and 180 , preferably between 30 and 160 , more preferably between 50 and 140 and even more preferably between 70 and 120 , and/or in that the at least one portion curved outwardly in the shape of an arch has the form of a Gaussian normal distribution with a quotient from the full width at half maximum of the Gaussian normal distribution and the width of the at least one groove from 0.5 to 1.5, preferably from 0.6 to 1.4, more preferably from 0.7 to 1.3, even more preferably from 0.8 to 1.2 and most preferably from 0.9 to 1.1.
In accordance with a further preferred embodiment of the present invention, it has proven to be advantageous if the quotient from the distance from the vertex of the at least one portion curved outwardly in the shape of an arch of the surface of the cathode block to the lowest point of the groove and the distance from the lowest point of the side of the cathode block opposite the side having the at least one groove to the lowest point of the groove is between more than 1:1 to at most 2:1, preferably 1.0 to 1.5, more preferably 1.0 to 1.3 and even more preferably 1.0 to 1.2.

. =

In order to maximise the cathode block surface usable for electrolysis, it is preferable if the at least one groove extends over at least 40 %, preferably over at least 60 %, more preferably over at least 80 %, even more preferably over at least 90 % and most preferably over the entire length of the cathode block.
In order to increase yet further the achievable homogenisation of the distribution of the electric current density at the cathode block surface, it is proposed in a development of the inventive concept for the at least one groove embodied in particular in a rectangular manner in cross section to have a depth varying over the length of said groove, wherein the at least one groove more preferably has a shallower depth at the longitudinal-side ends thereof than in the centre thereof. Here, the groove may have a profile that is triangular for example in the longitudinal section of the cathode block. An increase of the electric current density at the longitudinal-side end regions of the cathode surface compared with the surface regions arranged further inwardly, said increase being caused by the contacting of a busbar arranged in the groove, this usually occurring in the region of the longitudinal-side ends of the groove, is effectively avoided as a result of such an embodiment.
In order to achieve simultaneously a uniform distribution of the electric current density at the cathode surface with use thereof in an electrolytic cell, both in the transverse direction and in the longitudinal direction of the cathode block, it is preferable if the surface of the side opposite the side having the at least one groove is tub-shaped as considered in the longitudinal section of the cathode block. Here, it is particularly preferable if the cathode block with the tub-shaped surface is embodied in accordance with the second aspect of the present invention described hereinafter. In particular, the advantageous embodiments and advantages discussed hereinafter with respect to the second aspect of the present invention also apply similarly to the first aspect of the present invention.
In accordance with a second aspect of the present invention, the object explained in the introduction is achieved by providing a cathode block for an aluminium electrolytic cell, said cathode block having a tub-shaped surface as considered in the longitudinal section of the cathode block, wherein the tub has two edge regions and a base region, which is arranged between the edge regions and is lowered relative to the edge regions, as viewed in the longitudinal direction of the cathode block, wherein, between each of the two edge regions and the base region, a side wall region connecting the corresponding edge region and the base region is provided, wherein at least one of the two connection regions between the edge regions and the side wall regions and/or at least one of the two connection regions between the base region and the side wall regions is curved in the shape of an arch, wherein the at least on portion curved in the shape of an arch has a length of more than 2 cm.
Here, the cathode block is preferably formed on the basis of carbon and/or graphite, wherein the cathode block is composed in particular preferably in a proportion of at least 30 % by weight, further preferably in a proportion of at least 40 % by weight, more preferably in a proportion of at least 50 % by weight, even more preferably in a proportion of at least 60 % by weight, and most preferably completely of carbon and/or graphite.
The carbon is preferably amorphous carbon, and the graphite is preferably graphitic or graphitised carbon. It is further preferable for mixtures of amorphous carbon and graphitic carbon, of amorphous carbon and graphitised carbon, of amorphous carbon, graphitic and graphitised carbon, or of graphitic and graphitised carbon to be used.
The tub-shaped surface is preferably the surface of the cathode block which is arranged opposite the surface of the cathode block having the at least one groove for receiving busbars, preferably one busbar in each case. In other words, the tub-shaped surface of the cathode block according to the invention is located on the upper side of the cathode block with respect to the use of the cathode block in an electrolytic cell, that is to say on the side of the cathode block on which the layer of liquid aluminium is provided.
It has been found in accordance with the invention that, due to an embodiment curved in the shape of an arch of at least one of the connection regions of a tub-shaped cathode block, that is to say due to an embodiment curved in the shape of an arch of at least one of the connection regions which are provided between the edge regions and the side wall regions and between the base region and the side wall regions, the electric current density and the magnetic flux density are homogenised with the use of the cathode block in an electrolytic cell. This is because, with a tub-shaped embodiment of the cathode block, the current, in contrast to a square cathode block, does not flow primarily into the lateral edge regions of the cathode block, but the current flow is distributed homogeneously over the surface of the cathode block because, with the tub-shaped embodiment provided as described above, the electrical resistance of the edge regions is increased due to the greater height of the edge regions compared with the height of the base region with respect to the electrical resistance of the base region of the cathode block. This is in turn a consequence of the fact that the current in the cathode block follows the path of lowest electrical resistance. In addition, due to the embodiment curved in the shape of an CA 02854928 2014-05-07, arch of at least one of the connection regions, there are also no inhomogeneously distributed current densities in the region of the at least one connection region, which runs at an angle with a tub-shaped embodiment known from the prior art. Rather, the embodiment curved in the shape of an arch will homogenise the distribution of the electric current density occurring at the surface of the cathode block with the use of the cathode block in an electrolytic cell compared with a cathode block having an angular embodiment, even also in the connection regions, such that a wave formation in the layer of liquid aluminium is effectively avoided during operation of the electrolytic cell, as are associated instabilities of the electrolysis operation. This is because, due to the embodiment curved in the shape of an arch of at least one of the connection regions in contrast to an angular embodiment of the connection regions, peaks or valleys of the electric current density flowing through the surface of the cathode block, which are produced with an angular embodiment of the connection regions as a consequence of the fact that the current follows the path of lowest electrical resistance, are avoided at these regions. On the whole, with the operation of an electrolytic cell comprising the cathode block according to the invention, a wave formation in the layer of liquid aluminium is thus effectively avoided, and a high energy efficiency with simultaneous high stability and reliability of the electrolysis operation is attained.
Due to the reduction of the wave formation in the layer of liquid aluminium arranged above the cathode block, the distance between the anode and the layer of liquid aluminium can be reduced, which leads to an additional energy saving during the operation of an electrolytic cell comprising the cathode block.
As with the first aspect of the present invention, a portion curved in the shape of an arch is to be understood to mean a portion in which the sectional curve defined by =
CA 02854928 2014-05-07, the cathode surface in the cross section of the cathode block has a curvature running in a convex manner with respect to the cathode block interior, wherein a curved portion is understood to mean a portion of the sectional curve in which the direction changes continuously, but without the presence of an angular change of direction in the curved region.
Here, within the context of the present invention, the length of the portion curved in the shape of an arch denotes the extension, measured in the longitudinal direction of the cathode block, of the portion curved in the shape of an arch from the start thereof to the end thereof, that is to say from the point of the transition of the edge region preferably formed in a straight line into the connection region curved in the shape of an arch to the point of the transition thereof into a preferably straight portion of the side wall region or from the point of the transition of a preferably straight portion of the side wall region into the connection region curved in the shape of an arch to the point of transition thereof into the base region, preferably formed in a straight line.
Good results in terms of the homogenisation of the current density are obtained in particular if the at least one portion curved in the shape of an arch has a length of more than 2 cm to 100 cm, preferably from 3 to 50 cm, in particular preferably from 4 to 30 cm, more preferably from to 20 cm, even more preferably from 7 to 15 cm and most preferably of 10 cm, since peaks or valleys of the electric current density above the curved portion can be particularly reliably avoided by such a dimensioning of the curved portion.
If the portion curved in the shape of an arch is provided in at least one of the connection regions between the base region and the side wall regions of the tub-shaped surface CA 02854928 2014-05-07, of the cathode block, the portion curved in the shape of an arch is preferably curved inwardly in the shape of an arch, with respect to the cathode block considered in longitudinal section, whereas, if the portion is provided in at least one of the connection regions between the edge regions and the side wall regions of the tub-shaped surface of the cathode block, said portion is preferably curved outwardly in the shape of an arch. In accordance with a preferred embodiment of the present invention, at least one of the two connection regions between the base region and the side wall regions and preferably both connection regions between the base region and the side wall regions is/are consequently curved inwardly in the shape of an arch, with respect to the cathode block considered in longitudinal section. Alternatively or additionally, at least one of the two connection regions between the edge regions and the side wall regions and preferably both connection regions between the edge regions and the side wall regions is/are curved outwardly in the shape of an arch, with respect to the cathode block considered in longitudinal section.
In accordance with an embodiment of the cathode block according to the second aspect of the invention, said embodiment being particularly advantageous in view of a very homogeneous distribution of the electric current density, the two connection regions between the base region and the side wall regions are curved inwardly in the shape of an arch, with respect to the cathode block considered in longitudinal section, and the two connection regions between the edge regions and the side wall regions are curved outwardly in the shape of an arch, with respect to the cathode block considered in longitudinal section.
In a development of the inventive concept, it is proposed for at least one portion curved in the shape of an arch to have a minimum radius of curvature of at least 2 cm, CA 02854928 2014-05-07.

preferably of at least 10 cm and more preferably of at least 20 cm. A particularly gentle progression of the cathode block surface within the curved portion is thus achieved, whereby a particularly uniform distribution of the electric current density is achieved with the use of the cathode block in an electrolytic cell.
In accordance with a further preferred embodiment of the second aspect of the present invention, at least one portion curved in the shape of an arch is shaped in the manner of an oval segment, in particular is shaped in the manner of a circular arc, is cosine-shaped, is in the form of a Gaussian normal distribution, is shaped in the manner of an ellipsis segment, or is in the form of a Bezier curve, with respect to the longitudinal section of the cathode block.
In respect of the above-described effects to be achieved with the second aspect of the present invention, it has additionally proven to be particularly advantageous to set a ratio between the greatest height in the edge regions and the lowest height in the base region of the cathode block, with respect to the cross-sectional plane running in the longitudinal direction, between 1.1 and 4, preferably between 1.1 and 2.5, and more preferably between 1.1 and 2.1. Here, the cross-sectional plane running in the longitudinal direction denotes the vertical plane running perpendicularly to the width direction of the cathode block and parallel to the longitudinal direction of the cathode block.
Particularly good results in terms of the distribution of the electric current density at the surface of the cathode block are also attained if the angle between one end and the other end of the at least one portion curved in the shape of an arch of the at least one connection region of the cathode block according to the invention is 95 to 175 , CA 02854928 2014-05-07.

preferably 110 to 1600 and more preferably 125 to 150 .
Here, the angle between the two ends of the curved portion denotes the greater of the two angles, which is enclosed by two virtual lines to be arranged at the two ends of the portion and each running tangentially to the curved portion as considered in the longitudinal section of the cathode block.
In order to also achieve a uniform distribution of the electric current density in particular within the edge regions of the cathode block, it is proposed in a development of the inventive concept for at least one of the two edge regions and preferably both regions, as considered in the longitudinal section of the cathode block, to run in a manner sloping downwardly towards the centre of the cathode block in the longitudinal direction of the cathode block, wherein the angle of inclination of the edge region or of the edge regions with respect to this plane is preferably between 1 and 30 , more preferably between 2 and 15 and even more preferably between 30 and .
In this embodiment it is preferable for at least one of the two edge regions and preferably both of the edge regions to run in a manner sloping downwardly towards the centre of the cathode block in the longitudinal direction of the cathode block, as considered in the longitudinal section of the cathode block, over at least 30 %, preferably over at least 50 %, more preferably over at least 75 % and even more preferably over 100 % of the length of the edge region or edge regions measured in the longitudinal direction of the cathode block.
In accordance with a further preferred embodiment of the second aspect of the invention, the base region runs in a straight line at least in regions, wherein the surface of the base region has an angle between -20 and 20 , CA 02854928 2014-05-07.

preferably between -10 and 100 and more preferably of 0 with respect to the longitudinal direction.
A homogeneous distribution of the electric current density over the surface of the cathode block, said distribution being produced with the use of the cathode block according to the invention in an electrolytic cell, is attained in particular if each of the two edge regions extends over 5 to 40 %, preferably 10 to 35 % and more preferably 15 to 30 % of the length of the cathode block, and/or if the base region extends over 10 to 90 %, preferably 20 to 70 % and more preferably 30 to 60 % of the length of the cathode block.
In order to increase the flexibility in terms of the embodiment of the cathode surface, it is proposed in a development of the inventive concept for the cathode block to have a material composition varying in the longitudinal direction of the cathode block, wherein the material in the two edge regions preferably has a higher resistivity than the material in the base region of the cathode block. A
promotion of the current flow in the centre of the cathode block with respect to the edge regions is thus achieved already by the used material of the cathode block, such that the height difference between the edge regions and the base region of the cathode block surface can be comparatively low in order to achieve the desired homogenisation of the current density over the cathode block surface.
Good results are obtained with this embodiment in particular if the cathode block contains 5 to 50 % by weight and preferably between 10 to 30 % by weight of acetylene coke in the two edge regions. The electrical properties of the acetylene coke do not change or only change slightly with the graphitisation step performed during the production of the cathode block, such that the CA 02854928 2014-05-07, edge regions of the cathode block contain graphite having a lower degree of graphitisation and therefore having a greater resistivity than in the case of graphitisation without addition of the acetylene coke.
It is further preferable for the cathode block to contain 5 to 50 % by weight and preferably between 10 to 40 % by weight of titanium diboride, silicon oxide and/or chromium oxide in the base region. The titanium diboride, silicon oxide and/or chromium oxide promotes the formation of the graphite structure with the graphitisation step performed during the production of the cathode block, such that the base region of the cathode block contains graphite having a higher degree of graphitisation and therefore having a lower resistivity than in the case of graphitisation without addition of titanium diboride, silicon oxide and/or chromium oxide.
The second aspect of the present invention can be combined with the first aspect of the present invention, that is to say in accordance with a particularly preferred embodiment of the present invention the cathode block has the features described with respect to the first aspect of the present invention and the features described with respect to the second aspect of the present invention.
The present invention further relates to a cathode arrangement, in particular for an aluminium electrolytic cell, which comprises at least two cathode blocks embodied as described above.
In addition, the present invention relates to an electrolytic cell, in particular for producing aluminium, which comprises a cathode arrangement as described previously, a layer of liquid aluminium arranged on the upper side of the cathode arrangement, a melt layer on top thereof, and an anode above the melt layer.

CA 02854928 2014-05-07.

The present invention will be described hereinafter purely by way of example on the basis of advantageous embodiments with reference to the accompanying drawings, in which:
Fig. 1 shows a perspective cut-away view of a cathode block according to the prior art, Fig. 2 shows a perspective cut-away view of a cathode block in accordance with an embodiment of the first aspect of the present invention, Fig. 3 shows a front view of the cathode block detail shown in Fig. 2, Fig. 4 shows a cross-sectional view of a cathode block in accordance with a further embodiment of the first aspect of the present invention, Fig. 5 shows an illustration of possible embodiments of the portion curved outwardly in the shape of an arch of a cathode block in accordance with various embodiments of the first aspect of the invention, Fig. 6 shows a longitudinal sectional view of a cathode block according to the prior art, Fig. 7 shows a longitudinal sectional view of a cathode block in accordance with an embodiment of the second aspect of the present invention, Fig. 8 shows a longitudinal sectional view of a cathode block in accordance with a further embodiment of the second aspect of the present invention, CA 02854928 2014-05-07, Fig. 9a-e show schematic illustrations of the distribution of the electric current density produced at the surface of various cathode blocks with the use of the respective cathode block in an electrolytic cell, Fig. 10 shows a perspective view of the cathode block shown in Fig. 6 in accordance with the prior art, Fig. 11 shows a perspective view of a cathode block in accordance with an embodiment of the second aspect of the present invention, Fig. 12 shows a perspective view of a cathode block in accordance with a further embodiment of the second aspect of the present invention, and Fig. 13 shows a perspective view of a cathode block in accordance with a further embodiment of the second aspect of the present invention.
Fig. 1 shows a perspective cut-away view of a cathode block according to the prior art. The cathode block has a groove 12a arranged on a side 10a of the cathode block and running in the longitudinal direction y of the cathode block for receiving a busbar. The cathode block additionally has a side 14a, opposite the side 10a having the groove 12a, with a surface 16a which faces the layer of liquid aluminium arranged above the cathode block when the cathode block is used in an electrolytic cell. With the embodiment of the cathode block illustrated in Fig. 1, the surface 16a is flat. With operation of such a cathode block in an electrolytic cell, a non-uniform distribution of the electric current density at the surface 16a is produced, in particular as viewed in the width direction x of the cathode block, whereby a wave formation in the layer of liquid aluminium arranged thereabove is promoted. In Fig.

CA 02854928 2014-05-07, 1, as in the other figures concerning the prior art, the reference signs are provided with the index a in order to distinguish these reference signs from the corresponding reference signs in the figures illustrating the present invention.
Fig. 2 shows a perspective cut-away view of a cathode block in accordance with an embodiment of the first aspect of the invention. The width Bl of the cathode block shown in Fig.
2 measured in the width direction x is approximately 42 cm, and the width B2 of the groove 12 is approximately 20 cm.
The length L of the cathode block measured in the longitudinal direction y may be 2.5 to 4.0 m, for example.
The cathode block shown in Fig. 2 differs from that shown in Fig. 1 in that a portion of the surface 16 of the side 14 of the cathode block opposite the side 10 having the at least one groove 12 is curved outwardly in the shape of an arch, as viewed in the cross section of the cathode block.
The sectional curve of the surface 16 produced in the cross section of the cathode block is denoted in Fig. 1 by the reference sign 18. This sectional curve 18 is arched over its entire width Bl measured in the width direction x and thus constitutes a portion 20 curved in the shape of an arch of the surface 16 considered in cross section. As additionally illustrated in Fig. 1, the vertex 22 of the portion 20 curved outwardly in the shape of an arch is arranged vertically above the groove 12, with respect to the direction z running perpendicularly to the side 10 having the at least one groove 12 in the cross section of the cathode block. The region of the surface 16 arranged above the groove 12 in the direction z is denoted in Fig. 2 by the reference sign 24. The distribution of the electric current density with the use of the cathode block in an electrolytic cell is homogenised in comparison with the cathode block shown in Fig. 1 due to the arched embodiment of the surface 16, and a wave formation in the layer of = CA 02854928 2014-05-07 , liquid aluminium arranged above the cathode block is thus reduced.
The cathode block detail illustrated in Fig. 2 is illustrated in cross section in Fig. 3. As can be seen from Fig. 3, the quotient from the distance 1-11 from the vertex 22 of the curved portion 20 to the lowest point of the groove 12 and the distance h2 from the lowest point of the side 14 opposite the side 10 having the groove 12 to the lowest point of the groove 12 is approximately 1.4:1 in this embodiment.
A cathode block in accordance with a further embodiment of the first aspect of the invention is illustrated in cross section in Fig. 4. The cathode block shown in Fig. 4 has two grooves 12, 12' for receiving one busbar in each case, and the surface 16 of the cathode block has two portions 20, 20' curved outwardly in the shape of an arch as considered in cross section, wherein the vertex 22, 22' of each portion 20, 20' curved outwardly in the shape of an arch is arranged above the corresponding groove 12, 12' in the direction z oriented perpendicularly to the side 10 having the grooves 12, 12'. Even with a cathode block having two grooves 12, 12', a uniform distribution of the electric current density at the surface 16 of the cathode block can thus be achieved over the entire width B of the cathode block with use of the cathode block in an electrolytic cell.
Fig. 5 shows an illustration of possible embodiments of the portion curved outwardly in the shape of an arch of a cathode block in accordance with various embodiments of the first aspect of the invention. More specifically, Fig. 5 shows possible sectional curves 18a-e which can form a portion curved in the shape of an arch of the cathode block surface as considered in the cross section of the cathode block. Here, the x values range from (-0.5) times the CA 02854928 2014-05-07, cathode block width to 0.5 times the cathode block width, and the y value specify the height of the cathode surface relative to a virtual middle horizontal. Here, the sectional curve 18a has the form of a semicircle, the sectional curve 18b has the form of a parabola portion, the sectional curve 18c has the form of a half ellipse, the sectional curve 18d has the form of a cosine curve, and the sectional curve 18e has the form of a cosine curve in the fourth power, that is to say the function cos4(x).
A detail of a cathode block according to the prior art illustrated in longitudinal section is illustrated in Fig.
6. The cathode block according to Fig. 6 has a tub-shaped surface 16a as considered in the longitudinal section of the cathode block, said surface facing the layer of liquid aluminium arranged above the cathode block and carried by the cathode block with the use of the cathode block in an electrolytic cell. Here, the tub comprises two edge regions 26a, of which only one is shown in the cut-away illustration of Fig. 6, and a base region 28a, which is arranged between the edge regions 26a and is lowered relative to the edge regions 26a as viewed in the longitudinal direction y of the cathode block, wherein, between each of the two edge regions 26a and the base region 28a, a side wall region 30a connecting the corresponding edge region 26a and the base region 28a is provided. As shown in Fig. 6, each of the edge regions 26a and the base region 28a transition here via angular transitions into the side wall regions 30a. Here, with the use of the cathode block in an electrolytic cell, valleys in the distribution of the electric current density occur in the region of the angular transitions between the edge regions 26a' and the corresponding side wall region 30a of the surface 16a, and peaks in the distribution of the electric current density occur in the region of the angular transitions between the base region 28a and the corresponding side wall regions 30a. These peaks and CA 02854928 2014-05-07 .

valleys lead to an inhomogeneous distribution of the electric current density over the cathode surface and thus to an undesirable wave formation in the layer of liquid aluminium provided on the cathode surface.
Fig. 7 shows a detail of a cathode block according to an embodiment of the second aspect of the present invention illustrated in longitudinal section. The cathode block in Fig. 7 differs from the cathode block shown in Fig. 6 in that the connection region 32 between the edge region 26 and the side wall region 30 is curved outwardly in the shape of an arch as considered in the longitudinal section of the cathode block, wherein the portion 34 curved in the shape of an arch has a length L1 measured in the longitudinal direction y of the cathode block of more than 2 cm. Due to this curved transition between the edge region 26 and the side wall region 30, a valley of the electric current density is avoided in this region of the cathode block surface 16 with the use of the cathode block in an electrolytic cell, and the wave formation in the layer of liquid aluminium is thus reduced. In the cathode block shown in Fig. 7, the quotient from the greatest height h3 in the edge region 26 and the lowest height h4 in the base region 28 of the cathode block is approximately 2:1.
Further, the angle a between one end and the other end of the portion 34 curved in the shape of an arch is approximately 120 .
Fig. 8 shows a detail of a cathode block in accordance with a further embodiment of the second aspect of the present invention illustrated in longitudinal section. The cathode block shown in Fig. 8 differs from that shown in Fig. 7 in that, instead of the connection region 32 between the edge region 26 and the side wall region 30, the connection region 36 between the side wall region 30 and the base region 28 is embodied as a portion 34 curved inwardly in the shape of an arch, as considered in the longitudinal CA 02854928 2014-05-07 .

section of the cathode block, wherein the portion 34 curved in the shape of an arch has a length L1 measured in the longitudinal direction y of the cathode block of more than 2 cm. Due to this curved transition between the side wall region 30 and the base region 28, a peak of the electric current density in this region of the cathode block surface 16 with the use of the cathode block in an electrolytic cell is avoided, and the wave formation in the layer of liquid aluminium is thus reduced further.
The embodiments shown in Figures 7 and 8 can also be combined such that both the two transitions between the edge regions 26 and the side wall regions 30 and the transitions between the side wall regions 30 and the base region 28 are curved in the shape of an arch. A
particularly homogeneous distribution of the electric current density at the surface 16 of the cathode block with respect to the wave formation in the layer of liquid aluminium can thus be achieved.
Fig. 9a-e show a schematic illustration of the distribution of electric current density produced at the surface of various cathode blocks with the use of the respective cathode block in an electrolytic cell. More specifically, figures 9a-e each show the distribution, projected in the horizontal plane, of the electric current density occurring at the surface of the respective cathode block during operation of the electrolytic cell. The key 38 shown in Fig 9a-e specifies which hatching in Fig. 9a-e corresponds to which electric current density.
In Fig. 9a the distribution of the current density with the use of a tub-shaped cathode block according to the prior art as shown in Figures 6 and 10 is shown.
In Fig. 9b the distribution of the electric current density with the use of a cathode block as shown in Fig. 2 in = CA 02854928 2014-05-07 , accordance with the first aspect of the present invention with an outwardly curved surface 16 (see Fig. 2) and with an embodiment otherwise identical to the cathode block of Fig. 10 is illustrated. As highlighted in Fig. 9b by the dashed boxes 40, a much more homogeneous distribution of the electric current density is produced over the width (x direction) of the cathode block than with the cathode block illustrated in Fig. 9a due to the curvature of the cathode block surface predominantly in the region that is in the middle as considered in the longitudinal direction y of the cathode block.
In Fig. 9c the distribution of the electric current density with the use of a cathode block shown in Fig. 11 is shown, said cathode block being formed simultaneously in accordance with the first aspect of the invention and also in accordance with the second aspect of the invention. More specifically, the surface 16 of the cathode block in accordance with the second aspect of the invention has a tub-shaped surface 16 (see Fig. 11) as considered in longitudinal section, wherein the tub has two edge regions 26, 26' and a base region 28, which is arranged between the edge regions 26, 26' and is lowered relative to the edge regions 26, 26' as viewed in the longitudinal direction y of the cathode block, wherein, between each of the edge regions 26, 26' and the base region 28, a side wall region 30, 30' connecting the corresponding edge region 26, 26' and the base region 28 is provided. With the embodiment shown in Fig. 11, both the two connection regions 32, 32 between the edge regions 26, 26' and the side wall regions 30, 30' and the two connection regions 36, 36' between the base region 28 and the side wall regions 30, 30' are curved in the shape of an arch as considered in the longitudinal section of the cathode block, wherein the two portions 34, 34' curved in the shape of an arch have a length of more than 2 cm. The connection regions 32, 32' between the edge regions 26, 26' and the side wall regions 30, 30' are curved outwardly here in the shape of an arch, and the connection regions 36, 36' between the base region 28 and the side wall regions 30, 30' are curved inwardly in the shape of an arch. The embodiment shown in Fig. 11, as considered in the cross section of the cathode block, additionally has an outwardly directed curvature of the surface 16, which is not illustrated in Fig. 11 for the sake of simplicity. As can be seen from Fig. 9c, the embodiment of the cathode block surface 16 shown in Fig. 11 leads to a further homogenisation of the distribution of the electric current density, in particular in the region of the boxes 40 indicated in Fig. 9c.
Fig. 9d shows the distribution of the electric current density with the use of a cathode block as shown in Fig.
12. The cathode block shown in Fig. 12 corresponds substantially to the embodiment shown in Fig. 11, wherein the flat edge regions 26, 26' in Fig. 12 run at an angle p with respect to the horizontal towards the centre of the cathode block, as considered in the longitudinal section of the cathode. As can be seen from Fig. 9d, the distribution of the electric current density in the region of the boxes 40, that is to say at the longitudinal-side end of the cathode block, is thus additionally homogenised significantly.
Fig. 9e shows the distribution of the electric current density with use of a cathode block shown in Fig. 13. The cathode block shown in Fig. 13 corresponds substantially to the embodiment shown in Fig. 12, wherein the cathode block shown in Fig. 13 has a locally varying material composition, wherein specifically acetylene coke is added to the cathode block portions 42, 42' arranged beneath the edge regions 26, 26', that is to say to the regions 42, 42' of the cathode block illustrated by single hatching, and titanium diboride is added to the region 44 of the cathode block arranged beneath the base region 28 and denoted in Fig. 13 by double hatching. As shown in Fig. 9e, the distribution of the electric current density in the central region of the cathode surface (denoted by the box 40) is additionally homogenised again as a result of this measure compared with the distribution shown in Fig. 9d.

= CA 02854928 2014-05-07 .

LIST OF REFERENCE SIGNS
10, 10a (under)side of the cathode block 12, 12', 12a groove 14, 14a (upper) side of the cathode block 16, 16a surface of the cathode block 18 sectional curve 20, 20' portion curved in the shape of an arch 22, 22' vertex of the portion curved in the shape of an arch 24, 24' region arranged above the groove 26, 26', 26a edge region 28, 28a base region 30, 30', 30a side wall region 32, 32' connection region 34, 34' portion curved in the shape of an arch 36, 36' connection region 38 key 40 box 42, 42' regions with added acetylene coke 44 regions with added titanium diboride B, Bl width h1-h4 distance, height L, L1 length f y, z width, longitudinal and vertical direction cx, p angle

Claims

1. A cathode block for an aluminium electrolytic cell having at least one groove (12, 12') arranged on one of the sides (10, 14) of the cathode block and running in the longitudinal direction (y) of the cathode block for receiving a busbar, wherein at least one portion (20, 20') of the surface (16) of the side (14) of the cathode block opposite the side (10) having the at least one groove (12, 12') is curved outwardly in the shape of an arch as viewed in the cross section of the cathode block, wherein the vertex (22, 22') of the at least one portion (20, 20') curved outwardly in the shape of an arch is arranged above the at least one groove (12, 12'), with respect to the direction (z) running perpendicularly to the side (10) having the at least one groove (12, 12') in the cross section of the cathode block.

2. The cathode block according to Claim 1, characterised in that said cathode block is composed in a proportion of at least 30 % by weight, preferably in a proportion of at least 40 % by weight, more preferably in a proportion of at least 50 % by weight, even more preferably in a proportion of at least 60 % by weight and most preferably completely of carbon and/or graphite.

3. The cathode block according to Claim 1 or 2, characterised in that the vertex (22, 22') of the at least one portion (20, 20') curved outwardly in the shape of an arch of the surface (16) of the cathode block, with respect to the direction (z) running perpendicularly to the side (10) having the at least one groove (12, 12') in the cross section of the cathode block, is arranged above a region of the at least one groove (12, 12') which extends from 20 to 80 % of the width (B1) of the at least one groove (12, 12'), and preferably from 40 to 60 % of width (B1) of the at least one groove (12, 12'), and more preferably is arranged above the centrepoint of the at least one groove (12, 12').

4. The cathode block according to at least one of the preceding claims, characterised in that the at least one region (20, 20') curved outwardly in the shape of an arch of the surface (16) of the cathode block extends over at least 20 %, preferably over at least 40 %, more preferably over at least 60 %, even more preferably over at least 80 % and most preferably over 100 % of the region (24, 24') which is arranged above the width (B1) of the groove (12, 12'), with respect to the direction (z) running perpendicularly to the side (10) having the at least one groove (12, 12') in the cross section of the cathode block.

5. The cathode block according to at least one of the preceding claims, characterised in that the at least one portion (20, 20') curved outwardly in the shape of an arch has the form of an ellipse segment with a range of the polar angle between 10°
and 1800, preferably between 30° and 160°, more preferably between 50° and 140° and even more preferably between 70° and 120°, and/or the at least one portion (20, 20') curved outwardly in the shape of an arch has the form of a cosine curve with a range of the angle between 10° and 180°, preferably between 30° and 160°, more preferably between 50° and 140° and even more preferably between 700 and 120°, and/or the at least one portion (20, 20') curved outwardly in the shape of an arch has the form of a segment of a circular arc with a range of the angle subtended by the arc at the centre between 10° and 180°, preferably between 30° and 160°, more preferably between 50° and 140° and even more preferably between 70° and 120°, and/or the at least one portion (20, 20') curved outwardly in the shape of an arch has the form of a Gaussian normal distribution with a quotient from the full width at half maximum of the Gaussian normal distribution and the width of the at least one groove from 0.5 to 1.5, preferably from 0.6 to 1.4, more preferably from 0.7 to 1.3, even more preferably from 0.8 to 1.2 and most preferably from 0.9 to 1.1.

6. The cathode block according to at least one of the preceding claims, characterised in that the quotient from the distance (h1) from the vertex (22, 22') of the at least one portion (20, 20') curved outwardly in the shape of an arch of the surface (16) of the cathode block to the lowest point of the groove (12, 12') and the distance (h2) from the lowest point of the side (14) of the cathode block opposite the side (10) having the at least one groove (12, 12') to the lowest point of the groove (12, 12') is between more than 1:1 to at most 2:1, preferably 1.0 to 1.5, more preferably 1.0 to 1.3 and even more preferably 1.0 to 1.2.

7. The cathode block according to at least one of the preceding claims, characterised in that the at least one groove (12, 12'), which is preferably rectangular in cross section, has a depth that varies over the length of said groove, wherein the at least one groove (12, 12') more preferably has a shallower depth at the longitudinal-side ends thereof than in the centre thereof.

8. A cathode block for an aluminium electrolytic cell., said cathode block having a tub-shaped surface (16) as considered in the longitudinal section of the cathode block, wherein the tub has two edge regions (26, 26') and a base region (28), which is arranged between the edge regions (26, 26') and is lowered relative to the edge regions (26, 26') as viewed in the longitudinal direction (y) of the cathode block, wherein, between each of the two edge regions (26, 26') and the base region (28), a side wall region (30, 30') connecting the corresponding edge region (26, 26') and the base region (28) is provided, wherein at least one of the two connection regions (32, 32') between the edge regions (26, 26') and the side wall regions (30, 30') and/or at least one of the two connection regions (36, 36') between the base region (28) and the side wall regions (30, 30') is/are curved in the shape of an arch, wherein the at least one portion (34, 34') curved in the shape of an arch has a length (L1) of more than 2 cm to 100 cm, preferably from 3 to 50 cm, in particular preferably from 4 to 30 cm, more preferably from 5 to 20 cm, even more preferably from 7 to 15 cm, and most preferably of 10 cm.

9. The cathode block according to Claim 8, characterised in that said cathode block is composed in a proportion of at least 30 % by weight, preferably in a proportion of at least 40 % by weight, more preferably in a proportion of at least 50 % by weight, even more preferably in a proportion of 60 % by weight, and most preferably completely of carbon and/or graphite.

10. The cathode block according to Claim 8 or 9, characterised in that at least one of the two connection regions (36, 36') between the base region (28) and the side wall regions (30, 30') and preferably both connection regions (36, 36') between the base region (28) and the side wall regions (30, 30') is/are curved inwardly in the shape of an arch, with respect to the cathode block as considered in the longitudinal section.

11. The cathode block according to at least one of Claims 8 to 10, characterised in that at least one of the two connection regions (32, 32') between the edge regions (26, 26') and the side wall regions (30, 30') and preferably both connection regions (32, 32') between the edge regions (26, 26') and the side wall regions (30, 30') is/are curved outwardly in the shape of an arch, with respect to the cathode block as considered in the longitudinal section.

12. The cathode block according to at least one of Claims 8 to 11, characterised in that the ratio, with respect to the cross-sectional plane running in the longitudinal direction (y), between the greatest height (h3) in the edge regions (26, 26') and the lowest height (1-14) in the base region (28) of the cathode block is between 1.1 and 4, preferably between 1.1 and 2.5, and particularly preferably between 1.1 and 2.1.

13. The cathode block according to at least one of Claims 8 to 12, characterised in that the cathode block has a varying material composition as viewed in the longitudinal direction (y) of the cathode block, wherein the material in two edge regions (26, 26') preferably has a higher resistivity than the material in the base region (28) of the cathode block.

14. The cathode block according to Claim 13, characterised in that the cathode block contains 5 to 50 % by weight and preferably between 10 to 30 % by weight of acetylene coke in the two edge regions (26, 26').

15. The cathode block according to Claim 13 or 14, characterised in that the cathode block contains 5 to 50 % by weight and preferably between 10 to 40 % by weight of titanium diboride, silicon oxide and/or chromium oxide in the base region (28).