CA2838940A1 - Annular electrolysis cell and annular cathode with magnetic field compensation - Google Patents

Abstract

The invention relates to an electrolytic cell, especially for producing aluminum, which comprises a cathode, a liquid aluminum layer arranged on the top surface of the cathode, a melt layer on top thereof and an anode on top of said melt layer, the cathode having at least one opening that extends vertically through the cathode, through which opening at least one current supply extends vertically and is electrically connected to the anode and/or the cathode. The electrolytic cell comprises at least one further current supply outside the opening o the cathode, which current supply extends vertically at least in sections and is electrically connected to the cathode and/or the anode. The invention further relates to a cathode for an electrolytic cell.

Description

SGL CARBON SE

Annular electrolysis cell and annular cathode with magnetic field compensation The present invention relates to an electrolysis cell, in particular for pro-ducing aluminium, as well as a cathode which is suitable for use in an electrolysis cell of this type.
Electrolysis cells are for example used for the electrolytic production of aluminium, which is usually carried out industrially in accordance with the Hall-Fleroult process. In the Hall-Heroult process, a melt composed of aluminium oxide and cryolite is electrolysed. In this case, the cryolite Na3[A1F6] is used to reduce the melting point from 2,045 C for pure alu-minium oxide to approx. 950 C for a mixture containing cryolite, alumin-ium oxide and additives, such as aluminium fluoride and calcium fluoride.
The electrolysis cell used in this process has a cathode base which can be composed of a multiplicity of mutually adjacent cathode blocks which form the cathode. In order to withstand the thermal and chemical condi-tions prevailing during the operation of the cell, the cathode is usually composed of a carbon-containing material. Grooves are usually provided on the undersides of the cathode in each case, in which at least one bus bar is arranged in each case, by means of which the current supplied via the anodes is conducted away. Arranged approximately 3 to 5 cm above the, usually 15 to 150 cm high, layer made of liquid aluminium located on the cathode upper side is an anode formed from individual anode blocks in particular, between which and the surface of the aluminium, the elec-trolyte, that is to say the melt containing the aluminium oxide and cryo-lite, is located. During the electrolysis carried out at approximately 1,000 C, the aluminium formed settles below the electrolyte layer on ac-count of its greater density compared to the that of the electrolyte, that is to say as an intermediate layer between the upper side of the cathode and the electrolyte layer. During the electrolysis, the aluminium oxide dis-solved in the melt is split into aluminium and oxygen by means of electri-cal current flow. Seen electrochemically, the layer made up of liquid alu-minium is the actual cathode, as aluminium ions are reduced to elemen-tary aluminium at the surface thereof. Nevertheless, in the following the term cathode is not understood to mean the cathode from an electro-chemical viewpoint, that is to say the layer made up of liquid aluminium, but rather as the component for example composed from one or a plurality of cathode blocks, which forms the electrolyte cell base.
An important disadvantage of the Hall-Heroult process is that it is very energy intensive. Approximately 12 to 15 kWh of electrical energy are re-quired to create 1 kg of aluminium, which makes up 40% of the produc-tion costs. In order to be able to reduce the production costs, it is there-fore desirable to reduce the specific energy consumption in this process to the greatest extent possible.
Due to the relatively high electrical resistance of the melt in particular in comparison with the layer made up of liquid aluminium and the cathode material, relatively high ohmic losses in the form of Joule dissipation oc-cur predominantly in the melt. Considering the comparatively high spe-cific losses in the melt, there exists an endeavour to reduce the thickness of the melt layer and therefore the spacing between the anode and the layer made up of liquid aluminium to the greatest extent possible. How-ever, due to the electromagnetic interactions present during the electroly-sis and the wave formation caused thereby in the layer made up of liquid a aluminium, there is the risk in the case of too small a thickness of the melt layer, that the layer made up of liquid aluminium comes into contact with the anode, which may lead to short circuits of the electrolysis cell and to undesired reoxidation of the aluminium formed. Short circuits of this type further lead to increased wear and thus to a reduced service life of the electrolysis cell. For these reasons, the spacing between the anode and the layer made up of liquid aluminium cannot be reduced arbitrarily.
The driving force for the wave formation in the layer made up of liquid aluminium and the melt layer arranged thereabove is the Lorentz force density generated there, which is defined as the vector product of the elec-tric current density present at the respective point and the magnetic flux density present at the same point.
Whilst the current density distribution in the anode and in the melt layer is comparatively homogeneous, the current density distribution in the aluminium layer and on the surface of the cathode is very inhomogeneous due to strongly pronounced horizontal current density components in the direction of the cathode. In this case, the strong horizontal components of the electric current density lead with the usually likewise essentially hori-zontally directed magnetic field to a high vertical Lorentz force density, which in turn, as illustrated, leads to a strongly pronounced wave forma-tion, particularly in the aluminium layer. These strongly pronounced hori-zontal current density components in the direction of the cathode result from the effect 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 typically concentrated onto the lateral edge regions of the cathode, where the connection of the bus bars contacting the cathode with the current supplying elements takes place, as the resulting electrical resistance from the current supply-ing elements to the surface of the cathode is smaller in the case of flow via the lateral edge regions located close to the current supplying elements than in the case of flow via the middle of the cathode.
In addition to an increased wave formation in the aluminium layer, the inhomogeneous current density distribution and the increased current density at the lateral edge regions of the cathode compared to that in cen-tre of the cathode also leads to an increased wear of the cathode in these lateral edge regions, which following long-term operation of the electrolysis cell, typically leads to a characteristic wear profile, which is approximately W-shaped in cross section, of the cathode blocks in the longitudinal axis thereof.
In order to counteract this W-shaped wear profile, it has been suggested in WO 2007/118510 A2 for example, to adapt the configuration of the bus bar and the groove accommodating the bus bar in such a manner that the current density in the region of the layer made up of liquid aluminium is homogenised. Even in the case of an electrolysis cell of this type, a consid-erable wave formation takes place, particularly in the aluminium layer, however, as a consequence of which, the possibility of reducing the spac-ing between the anode and the layer made up of liquid aluminium is lim-ited.
Irrespective of that, it is known for reducing wave formation in the layer made up of liquid aluminium and the melt layer to configure the current supply to the anode and to the cathode of the electrolysis cell using com-plex current supply geometries in such a manner that only small magnetic fields result in the region of the layer made up of liquid aluminium and the melt layer, so that the amount of magnetic flux density and thus also the amount of the Lorentz force density in this region is as small as possi-ble. However, it proves exceptionally difficult to significantly reduce the wave formation in the layer made up of liquid aluminium and in the melt layer in this manner, as even when using very complex geometries of the current supplies, always at least individual regions have high magnetic 5 fields and thus a high tendency to wave formation there. Among other things, this can also be traced back to the fact that the electrolysis cell and therefore also the cathode are shaped in a rectangular manner, whereas the magnetic fields generated by the current running through the individual current supplies run in a cylindrical manner.
The object of the present invention is therefore to create an electrolysis cell which has a reduced specific energy consumption during the operation thereof and also an increased service life. In particular, an electrolysis cell should be provided, in which the thickness of the melt layer is reduced without instabilities such as short circuits or reoxidations of the formed aluminium arising as a consequence of the thereby increased tendency to wave formation in the layer made up of liquid aluminium.
According to the invention, this object is achieved by means of the provi-sion of an electrolysis cell according to Patent Claim 1 and in particular by means of the provision of an electrolysis cell for producing aluminium, which comprises a cathode, a layer made up of liquid aluminium on the upper side of the cathode, a melt layer e.g. containing cryolite thereupon and an anode above the melt layer, wherein the cathode has at least one opening extending vertically through the cathode, in which opening at least one current supply extending vertically through the opening and electrically connected to the anode and/or to the cathode is provided, and wherein the electrolysis cell comprises at least one further current supply arranged outside of the opening of the cathode, which current supply ex-tends in the vertical direction at least in certain sections and which cur-rent supply is electrically connected to the cathode and/or to the anode.
By means of the current supply provided in the opening of the cathode and running vertically through the cathode opening, in combination with the at least one external current supply arranged outside of the cathode as in conventional electrolysis cells, not only a reduction of the magnetic field strength and therefore the Lorentz force density as well as the ten-dency to wave formation in the aluminium layer, but also in particular a homogenising of the magnetic field strength and therefore of the Lorentz force density distribution and the tendency to wave formation in the alu-minium layer is achieved, specifically as viewed in particular via the cross section of the electrolysis cell. By means of the current flowing through the current supply provided in the opening of the cathode in the rectified direction - with respect to the at least one outer current supply - a mag-netic field is generated, which is opposed to the magnetic field generated by the current flowing through the at least one external current supply arranged outside of the cathode opening. For this reason, the magnetic field generated by the current supply provided in the opening of the cath-ode compensates the magnetic field generated by means of the current flow in the at least one external current supply. By setting the current intensity in the individual current supplies, the compensation of the mag-netic fields can be optimised. In particular, if a plurality of external cur-rent supplies are arranged evenly around the current supply provided in the opening of the cathode, a particularly complete compensation of the magnetic fields and/or a particularly homogeneous magnetic field distri-bution can be achieved.
Thus, with the electrolysis cell according to the invention, individual re-gions with increased magnetic flux density, as are unavoidable in conven-, a tional electrolysis cells even when using complex current supply geome-tries, can likewise be effectively avoided as with the necessity of complex current supply geometries themselves. In particular, according to the in-vention, an exceptional reduction and homogenisation of the magnetic flux density can be achieved just by using an individual conductor section of the current supply extending in the vertical direction through the opening of the cathode, without geometrically complex geometries of the at least one external current supply, which are expensive in terms of production and also installation, having to be used. In this manner, a markedly re-duced wave formation in the layer made up of liquid aluminium and the melt layer is achieved in the electrolysis cell, so that the anode can also be = arranged at a reduced spacing from the layer made up of liquid alumin-ium in a riskless manner, as a result of which, the service life, the stability _ and the energy efficiency during operation of the electrolysis cell are in-creased considerably.
In the sense of the present invention, an opening extending vertically through the cathode is understood to mean an opening which, with re-spect to the vertical, extends at an angle of less than 45 , preferably less than 30 , particularly preferably less than 15 , very particularly preferably less than 5' and most preferably at an angle of 0 through the cathode.
The edging of the opening can, as viewed in a cross section of the cathode, extend in an oblique or straight manner through the cathode with respect to the vertical direction, so that the opening can for example have the shape of a straight or oblique prism with an in particular polygonal base surface or in the shape of a straight or oblique cylinder. Alternatively, the opening can also have a shape which tapers in the vertical direction and can in particular be constructed approximately in the shape of a trun-cated cone or the shape of a truncated pyramid. Equally, a current supply extending vertically through the opening is understood to mean a current , a supply which, with respect to the vertical, extends at an angle of less than 45 , preferably less than 30 , particularly preferably less than 15 , very particularly preferably less than 50 and most preferably at an angle of 00 through the cathode. Analogously, a further current supply extending in the vertical direction at least in certain sections is understood to mean a current supply which, with respect to the vertical, extends at least in sec-tions at an angle of less than 45 , preferably less than 30 , particularly preferably less than 15 , very particularly preferably less than 5 and most preferably at an angle of 0 .
Preferably, the layer made up of liquid aluminium, the melt layer and the . anode have an outline essentially corresponding to the cathode, as viewed in a plan view. The opening of the cathode extends accordingly vertically through preferably the entire electrolysis cell.
Good results are in particular achieved in this case if the at least one opening in the cathode is arranged essentially centrally as viewed in a plan view. In this embodiment, it is additionally preferred that the at least one current supply extending through the opening is arranged essentially centrally in the opening and therefore at least essentially centrally in the cathode. In the case of this arrangement of the opening, a particularly even compensation of the magnetic fields can be achieved in the regions of the cathode located around the opening.
As illustrated previously, the current supply extending through the open-ing of the cathode can also extend through the layer made up of liquid aluminium arranged above the cathode, through the melt layer arranged thereupon and the anode arranged above the same. In this case, even in the layer made up of liquid aluminium, in the melt layer arranged there-upon and the anode arranged above the same, one opening is provided in each case, which extends vertically through the layer made up of liquid aluminium, the melt layer or the anode, and which is aligned with the opening of the cathode when the electrolysis cell is viewed from above; in other words, the layer made up of liquid aluminium, the melt layer ar-ranged thereupon and the anode arranged above the same are shaped in the same manner as the cathode. However, it is also possible that the current supply extending through the opening of the cathode only extends through two or one of the layers made up of liquid aluminium, the melt layer and the anode or only extends through the opening of the cathode.
Thus, the electrolysis cell can overall have one opening which extends vertically through one or a plurality of and in particular through all of the = components of the electrolysis cell selected from the group consisting of cathode, layer made up of liquid aluminium, melt layer and anode, at least one current supply being supplied in the opening, which extends vertically through this opening and is electrically connected to the anode and/or to the cathode. When the formulation "opening of the cathode" is used above or in the following, this formulation comprises not only an opening ex-tending exclusively through the cathode, but rather in particular also a previously described opening which extends through the cathode and additionally through further components of the electrolysis cell.
Preferably, the inner current supply is not directly electrically connected to the component surrounding the respective opening, such as the cath-ode, layer made up of liquid aluminium, melt layer and anode over at least a part of its length arranged within the at least one opening and in par-ticular over its entire length arranged within the opening, but rather elec-trically insulated from the respective component of the electrolysis cell.
The inner current supply can to this end be arranged in the opening spaced from the respective component of the electrolysis cell over its re-spective length and/or be surrounded by an electrically insulating sub-stance or medium, such as for example by air. If the at least one opening also extends through the layer made up of liquid aluminium and the melt layer, it is preferred that the inner current supply is electrically insulated from the layer made up of liquid aluminium and the melt layer at least 5 over its entire length extending through the opening provided in the layer made up of liquid aluminium and in the melt layer and particularly pref-erably is also electrically insulated from the cathode and anode over its entire length extending through the opening provided in the cathode and in the anode.
Basically, the cathode can be constructed in any desired manner known to the person skilled in the art. For example, the cathode can form the base of a tub carrying the layer made up of liquid aluminium or the melt layer, which forms a tank for the layer consisting of liquid aluminium and the melt layer, the tank preferably running annularly around the opening formed in the layer made up of liquid aluminium or in the melt layer. In this embodiment, the tank is preferably delimited in the direction of the opening by external walls provided in the tub, which walls form a shaft, through which the inner current supply extends, the inner current supply preferably being spaced from the external walls forming the shaft. In this case, the side walls of the tank can be constructed by means of a refrac-tory material.
In a development of the invention, it is suggested that the cathode, as viewed in a plan view, be shaped in an annular manner. In this manner, a cathode, which has an opening arranged centrally in the cathode, can be provided particularly simply. In this case, the layer made up of liquid aluminium, the melt layer and the anode of the electrolysis cell are shaped in an annular manner corresponding to the cathode as viewed in a plan view. In this case, according to the current invention, an annular shape of a constituent of the electrolysis cell, i.e. particularly of the cathode, the layer made up of liquid aluminium, the melt layer and the anode, is un-derstood to mean that the respective constituent forms the shape of a ring which may either be closed or may be shaped in an open manner at one or a plurality of places. Particularly in the case of the cathode, the layer made up of liquid aluminium and the melt layer, a shaping in the shape of a closed ring is preferred, whereas the anode may in particular also be constructed in the shape of an open ring, for example in the shape of a segmented ring which is open at a plurality of places, wherein such an open ring may for example be constructed by a plurality of anode blocks arranged annularly around the opening and spaced from one another.
In the context of the present invention, the inner and the outer current supply/current supplies are preferably electrically connected to the same electrode, which can for example be realised in that the inner and outer current supply are directly connected to the same current conductor which is connected directly to the electrode.
According to a further advantageous embodiment of the present invention, the cathode has an at least approximately circular outline, as viewed in a plan view. In this manner, the rotational symmetry of the magnetic flux density of the current supplies is recreated by the geometry of the cath-ode. With this geometry, a particularly effective magnetic field compensa-tion can be achieved within the layer made up of liquid aluminium and the melt layer, as a result of which a wave formation is reduced in an even more effective manner and the stability and energy efficiency of the elec-trolysis cell can be increased yet further. The cathode can in this case principally be constructed as a closed ring running around the opening.
Alternatively, the cathode can also be constructed as an only partially closed ring which is configured in an open manner at one or a plurality of points.
Alternatively to the present embodiment, the cathode can have an at least approximately polygonal ring-shaped outline as viewed in a plan view. As a result, particularly in the case of a polygonal ring-shaped shape with a high number of corners, an approximation of the preferred shape of a circular ring and the advantageous effects connected therewith is achieved, with the additional advantage that a polygonal ring-shaped cathode can be produced in a simpler and more cost-effective manner than a circular cathode. Good results are in particular achieved in this = case if the external circumference and/or the internal circumference of the outline of the cathode, which is polygonal ring-shaped as viewed in a plan view, has the shape of a preferably regular polygon with n corners, wherein n is preferably 3 to 100, particularly preferably 3 to 10 and very particularly preferably 3, 4, 5, 6, 7 or 8. As a compromise between a sim-ple and cost-effective producibility and a good approximation of the pre-ferred circular shape, in this embodiment the cathode is most preferably shaped as a regular polygonal ring with 6 or 8 corners.
Basically, the cathode of the electrolysis cell can be of single-piece or mul-tiple-piece configuration, a multiple-piece configuration being preferred from the viewpoint of production technology. In this case, in the multiple-piece configuration, the individual cathode blocks forming the cathode are preferably arranged around the current supply, which extends through the opening, next to one another and preferably adjoining one another, forming an annular cathode. In this case, a circular or polygonal ring-shaped shape is preferred. A segment-by-segment construction of the cathode simplifies the provision of the individual components and the composition of the electrolysis cell during the installation.

, .

In order to achieve a polygonal ring-shaped shaping of the cathode which sufficiently approximates the preferred circular shape with regards to the compensation of the magnetic flux density with low production outlay, it is suggested as a development of the inventive idea, that in the case of a multiple-piece configuration, at least one cathode block and preferably all of the cathode blocks of the cathode is/are shaped in an approximately hexagonal, at least approximately circular-segment-shaped or at least approximately trapezoidal manner as viewed in a plan view. In the case of at least approximately hexagonal or at least approximately trapezoidal cathode blocks, the cathode can for example be composed of 6 such cath-ode blocks which, in the circumferential direction, are arranged around the opening of the cathode next to one another. An essentially trapezoidal cathode block can be produced in a particularly simple manner in that an elongated initial body is cut up at angles transverse to the longitudinal direction thereof, the orientation of the angle alternating from cut to cut.
According to a further advantageous embodiment of the present invention, the ratio between the internal diameter and the external diameter of the cathode is between 0.01 and 0.99, preferably between 0.1 and 0.8, par-ticularly preferably between 0.2 and 0.6 and very particularly preferably between 0.3 and 0.5. In this manner, an exceptionally high degree of com-pensation of the magnetic flux density is achieved in the region of the entire layer made up of liquid aluminium and the entire melt layer, spe-cifically in the case of a simultaneously relatively low space requirement of the electrolysis cell in the horizontal direction. If the at least one opening also extends through one or a plurality of the layer made up of liquid alu-minium, the melt layer and the anode, the previous numerical ranges apply preferably also for the ratio between the internal diameter and the external diameter of these components. Internal diameter is in this case CA 02838940 2013:12-10 . =

understood to mean the diameter of the largest circle running in the hori-zontal plane which can be arranged in the opening of the respective con-stituent of the electrolysis cell without cutting the internal circumference of the opening. Analogously thereto, external diameter is in this case un-derstood to mean the diameter of the smallest circle running in the hori-zontal plane which can be arranged around the external circumference of the respective constituent of the electrolysis cell without cutting the exter-nal circumference of the constituent.
In a development of the inventive idea, it is suggested that the electrolysis cell comprises a plurality of current supplies, particularly between 2 and - 10, preferably between 4 and 8, particularly preferably between 5 and 7 and very particularly preferably 6, arranged outside of the opening of the cathode. In this case, it is preferred that all of the current supplies of the electrolysis cell provided outside of the cathode opening extend in the vertical direction at least in sections and are electrically connected in each case to the cathode and/or to the anode. As a result, the magnetic flux densities generated by the electric current in the current supplies com-pensate one another more effectively, so that a further increase of the stability and energy efficiency during operation of the electrolysis cell is achieved. A high symmetry of the arrangement and as a result a particu-larly good magnetic field compensation is achieved if the number of cur-rent supplies arranged outside of the cathode opening is identical to the number of cathode blocks forming the cathode.
An optimal compensation of the magnetic flux density is achieved in this case, if the further current supplies are arranged at least approximately regularly, i.e. in particular at approximately regular angular spacings, from one another as viewed in the circumferential direction of the cathode and as viewed around the current supply extending through the opening.

, In this case, the further or external current supplies preferably concentri-cally surround the current supply extending through the opening.
Generally, the entire electrical cell current used for the electrolysis pref-5 erably flows through the at least one current supply extending through the cathode opening and also through the one or a plurality of current supplies of the electrolysis cell arranged outside of the cathode opening. In this case, the current supply extending through the opening of the cath-ode and the further current supplies are preferably adapted to one an-10 other - for example by means of suitable choice of the conductor cross section of the current supplies - in such a manner that the cell current . divides to the current supplies in such a manner that an optimal magnetic field compensation is achieved in the region of the layer made up of liquid aluminium and the melt layer.
In order to further reduce the wave formation in the layer made up of liq-uid aluminium and the melt layer, it is suggested in a development of the inventive idea that the cathode has at least two pin-like contacting ele-ments on its underside, which contact the cathode in a current-conducting manner. In contrast with a conventional bus bar extending from the side into the cathode, this type of contacting makes it possible to adapt current density distribution at the surface of the cathode and in the layer made up of liquid aluminium arranged thereabove and the melt layer in such a targeted manner that a particularly homogeneous current den-sity distribution arises over the entire surface of the cathode. In this man-ner, horizontal current density components in the layer made up of liquid aluminium are avoided to the greatest extent possible, for which reason, wave formation in the layer made up of liquid aluminium and the melt layer arranged thereupon is reduced to a minimum.

=

According to a further advantageous embodiment of the present invention, at least one of the pin-like contacting elements extends and preferably all contacting elements extend at an angle of less than 300 and preferably less than 100 with respect to the vertical and particularly preferably verti-cally into the cathode. As a result, a particularly good electrical contact is produced between the contacting elements and the cathode.
The contacting elements are electrically conductively connected on the side thereof which faces away from the cathode to a common base plate.
In this manner, on the one hand a good mechanical fixing and on the other hand a good electrical connection of all contacting elements is achieved. The base plate can for example rest directly against the under-side of the cathode at least in certain regions and in the process produce a direct electrical contact to the cathode. Alternatively, it is also possible that the base plate is arranged at a spacing from the cathode underside.
If the contacting elements extend into the cathode, the same are prefera-bly connected to the cathode via a screw connection, the contacting ele-ments preferably having an external thread of the screw connection on the external side thereof. In principle, any suitable electrically conductive material can be considered as a material for the contacting elements and the base plate, if existent, a steel, aluminium, copper and/or carbon con-taining material or also graphite preferably being used for this purpose.
The length of the contacting elements is preferably between 100 and 500 mm and the diameter of the contacting elements is preferably between 30 and 200 mm. The contacting elements can be arranged at least in certain areas in a density of 4 to 1000 contacting elements per square metre of base area of the cathode. In the case of a density of this type, the distribu-tion of the contacting elements can be adapted in such a targeted manner that an at least particularly even current density distribution results at the cathode surface.
A particularly high energy efficiency of the electrolysis cell can be achieved if the spacing between the anode and the layer made up of liquid alumin-ium is between 15 and 45 mm, preferably between 15 and 35 mm and particularly preferably between 15 and 25 mm. Although under energy efficiency aspects, principally a spacing which is as small as possible is to be striven for, a certain minimum spacing is however advantageous in order to maintain the operating temperature of the electrolysis cell via the Joule heat created there. The small spacing is enabled by reducing the tendency to wave formation in the layer made up of liquid aluminium as a consequence of the magnetic field compensation by means of the current supply extending through the opening of the cathode.
In order to further increase the wear resistance of the electrolysis cell, it is suggested as a development of the inventive idea that the cathode or at least a cathode block forming the cathode contains a graphite composite material or a carbon composite material or preferably consists thereof, wherein the graphite composite material contains at least one hard mate-rial with a melting point of at least 1,000 C in addition to graphite and/or amorphous carbon. The graphite composite material or carbon composite material can in particular contain between 1 and 50% by weight and par-ticularly preferably between 15 and 50% by weight of the hard material. In this case, hard material is, in accordance with the usual technical defini-tion of this term, understood to mean a material which is characterized by a particularly high hardness, in particular also at high temperatures of 1,000 C and higher. By means of the addition of such a hard material, an abrasive wearing of the cathode during the operation thereof at the sur-face thereof facing the layer made up of liquid aluminium can be pre-=

vented or at least substantially reduced. For this purpose, the cathode can also be structured in two layers, namely composed of a cover layer pro-vided on the side thereof facing the layer made up of liquid aluminium and a base layer lying therebelow, wherein the cover layer is constructed from the carbon composite material and/or graphite composite material com-prising the hard material and the base layer is composed for example of hard-material-free graphite. In this case, the hard material can for exam-ple have a Knoop hardness measured according to DIN EN 843-4 of at least 1,000 N/mm2, preferably of at least 1,500 N/mm2, particularly pref-erably of at least 2,000 N/mm2 and very particularly preferably of at least 2,500 N/mm2 and can for example be selected from the group which con-sists 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, zirconium dioxide, aluminium oxide and any desired chemical combina-tions and/or mixtures of two or more of the previously mentioned com-pounds.
According to a further preferred embodiment of the present invention, the cathode has a surface which is profiled at least in certain areas, on which surface the layer made up of liquid aluminium is arranged and which for example can be formed by means of a cover layer of the cathode which contains a hard material, as described previously. Wave formation in the layer made up of liquid aluminium can be prevented particularly effec-tively during operation of the electrolysis cell by means of such a surface profiling. In this case, the surface of the cathode can for example have a plurality of elevations and/or recesses, wherein the depth of a recess is preferably 10 to 90 mm, particularly preferably 40 to 90 mm, and very particularly preferably 60 to 80 mm.

A further subject of the present invention is a cathode for an electrolysis cell and in particular a cathode for an electrolysis cell for producing alu-minium, which has at least one opening extending vertically through the cathode. A cathode of this type is suitable for use in an electrolysis cell according to the invention as described previously. The advantages and advantageous embodiments described previously with reference to the electrolysis cell are valid in this case insofar as they can also be applied accordingly for the cathode according to the invention.
Preferably, the cathode is shaped in an at least approximately annular and preferably at least approximately circular or polygonal ring-shaped manner as viewed in a plan view.
According to a further advantageous embodiment of the present invention, the external circumference and/or the internal circumference of the out-line of the cathode, which is polygonal as viewed in a plan view, at least essentially has the shape of a preferably regular polygon with n corners, wherein n is preferably 3 to 100, particularly preferably 3 to 10 and very particularly preferably 3, 4, 5, 6, 7 or 8. In this manner, the cathode can be approximated to the circular shape considered optimal with particu-larly simple technical means and with a particularly simple production.
The cathode according to the invention can be composed of a plurality of cathode blocks which, as preferably viewed in the circumferential direc-tion, are arranged around the opening of the cathode next to one another and adjoining one another.
In this case, it is preferred if at least one cathode block and preferably all cathode blocks have an at least approximately hexagonal, at least ap-proximately circular-segment-shaped or at least approximately trapezoidal =
outline, as viewed in a plan view. A basic shape of this type can be pro-duced simply and is suitable in particular for producing an at least ap-proximately circular cathode by means of the corresponding assembly of the individual cathode blocks. The cathode blocks can in each case be 5 connected to one another by means of a ramming mass joint or in another suitable manner.
According to a further advantageous embodiment of the present invention, provision is made for the ratio between the internal diameter and the ex-10 ternal diameter of the cathode to be between 0.01 and 0.99, preferably between 0.1 and 0.8, particularly preferably between 0.2 and 0.6 and very = particularly preferably between 0.3 and 0.5. In this manner, in the entire cathode, a particularly even and small magnetic flux density can be achieved with simultaneously good usage of space with respect to the 15 extent of the cathode in the horizontal plane.
According to a further advantageous embodiment of the present invention, the cathode has at least two recesses for one pin-like contacting element in each case on its underside. As a result, the option is created to contact 20 the cathode via pin-like contacting elements inserted into the recesses of the cathode, as a result of which, the current density distribution at the surface of the cathode and in the layer made up of liquid aluminium ar-ranged thereabove and the melt layer can be adapted in such a targeted manner that a particularly homogeneous current density distribution arises over the entire surface of the cathode.
Preferably, at least one of the recesses for a pin-like contacting element and particularly preferably all of the recesses for a pin-like contacting element extend at an angle of less than 30 and preferably less than 10 with respect to the vertical and very particularly preferably vertically into ' the cathode. As a result, a particularly good electrical contact can be pro-duced between a pin-like contacting element provided in the respective recess of the cathode and the cathode.
In this case, the cathode is preferably connected via a screw connection to a pin-like contacting element arranged in a recess of the cathode, the re-cess preferably having an internal thread on its inner side for such a screw connection.
The length of the recesses for the pin-like contacting elements is prefera-bly between 100 and 500 mm and the diameter of the recesses for pin-like . contacting elements is preferably between 30 and 200 mm. The recesses for pin-like contacting elements can be arranged at least in certain areas in a density of 4 to 1000 recesses per square metre of base area of the cathode. In the case of a density of this type, the distribution of the con-tacting elements inserted into the recesses can be adapted in such a tar-geted manner that an at least particularly even current density distribu-tion results at the cathode surface.
In the following, the present invention is described by way of example on the basis of an advantageous embodiment with reference to the attached drawings. In the figures:
Fig. 1 shows an electrolysis cell according to the prior art in cross sec-tion, Fig. 2 shows a sectioned view of an electrolysis cell according to an embodiment of the invention with vertical contacting of the cath-ode in a plan view, . ' Fig. 3 shows a segment of an electrolysis cell according to an embodi-ment of the invention in a perspective view, Fig. 4 shows a schematic illustration of the electric current flow in the segment of an electrolysis cell shown in Fig. 3 according to an embodiment of the invention, Figs 5a-c show a graphical illustration of the electrical current density distribution at the cathode surface of a segment of an electroly-sis cell as shown in the Figs 2, 3 and 4 according to an embodi-ment of the invention (Fig. 5a) and - for comparison - the electri-cal current density distribution at the surface of the cathode of a , conventional electrolysis cell (Fig. 5b), Figs 6a-c show a graphical illustration of the distribution of the magnetic flux density in the boundary surface between the layer made up of liquid aluminium and the melt layer of the segment of an elec-trolysis cell shown in the Figs 2, 3, and 4 according to an em-bodiment of the invention (Fig. 6a) and - for comparison - the distribution of the magnetic flux density in the boundary surface between the layer made up of liquid aluminium and the melt layer of an electrolysis cell with conventional cathode (Fig. 6b), Fig. 7 shows a plan view of a cathode of an electrolysis cell according to an embodiment of the invention and a clear illustration of an exemplary method for the production thereof, Fig. 8 shows a plan view of a cathode of an electrolysis cell according to a further embodiment of the invention, Fig. 9 shows a plan view of a cathode of an electrolysis cell according to a further embodiment of the invention, Fig. 10 shows a segment of an electrolysis cell according to a further embodiment of the invention with horizontal contacting of the cathode in a perspective view, Fig. 11 shows an electrolysis cell according to a further embodiment of the invention in a perspective view, Fig. 12 shows an electrolysis cell according to a further embodiment of the invention in a perspective view, Fig. 13 shows an electrolysis cell according to a further embodiment of the invention in cross section and Fig. 14 shows a further cross-sectional illustration of the electrolysis cell shown in Fig. 13 with an indication of the technical current flow direction.
Fig. 1 shows an electrolysis cell according to the prior art in cross section.

The electrolysis cell comprises a conventional square cathode 10' which forms a cathode bottom, above which a layer 12 made up of liquid alumin-ium is located. The layer 12 made up of liquid aluminium borders a melt layer 14 arranged above the layer 12 made up of liquid aluminium. An anode 16 arranged above the melt layer 14 and also formed from a plural-ity of anode blocks 27 dips into the melt layer 14, the anode blocks 27 being electrically conductively connected to an external current supply 22.
The cathode 10' of the electrolysis cell shown in Fig. 1 is electrically con-ductively connected to a bus bar 34 extending laterally into the cathode 10'.
Fig. 2 shows an electrolysis cell according to an embodiment of the pre-sent invention in a plan view. The electrolysis cell comprises a cathode 10, a layer 12 (not illustrated) made up of liquid aluminium on the upper side of the cathode 10, a melt layer 14 (not illustrated) thereupon and an an-ode 16 (not illustrated) above the melt layer 14. The last-mentioned com-ponents are not illustrated in Fig. 2, in order thus to expose the view onto the cathode 10 of the electrolysis cell. The layer 12 made up of liquid alu-minium, the melt layer 14 and the anode 16 which are not illustrated in Fig. 2, have a shape corresponding to the cathode 10 in a plan view.
The cathode 10 comprises an opening 18 extending vertically, i.e. perpen-dicularly to the drawing plane in Fig. 2, through the cathode 10, in which an "inner" current supply 20 extending through the opening and electri-cally conductively connected to the anode 16 (not illustrated) is provided.
In addition to the inner current supply 20, the electrolysis cell has a plu-rality of "external" current supplies 22 arranged outside of the opening 18, which are arranged laterally offset to the cathode, run vertically upwards and are likewise connected to the anode 16 as shown in the Fig. 3. The external current supplies 22 are essentially arranged annularly and at regular angular spacings around the opening 18.
The cathode 10 as viewed in a plan view essentially has the shape of a regular hexagonal ring, both the external circumference and the internal circumference of the cathode 10 forming a regular hexagon and being arranged concentrically to one another. As a result, the shape of the cath-CA 02838940 2013:12-10 . , ode 10 closely approximates a concentric circle and can be produced sim-ply compared to a concentric circle.
The cathode 10 is in this case composed of a plurality of segments or 5 cathode blocks 24 which, in each case as viewed in a plan view, have the outline of a symmetrical trapezium and are arranged in the circumferen-tial direction around the opening 18 next to one another in order to form the hexagonal ring-shaped cathode 10.
10 The cathode 10, as viewed in a plan view, has a six-fold symmetry, three vertical symmetry planes 26, as shown in Fig. 2, running centrally through the cathode blocks 24 and additionally three symmetry planes , not expressly marked in Fig. 2 in each case running along the lateral faces of the cathode blocks 24 arranged between two mutually adjacent cathode 15 blocks 24.
Fig. 3 shows a segment of an electrolysis cell formed by a trapezoidal cathode block 24 according to an embodiment of the invention, which essentially corresponds to the embodiment shown in Fig. 2 in a perspec-20 tive view. In this case, the individual conductor sections, namely an inner and an external current supply 20, 22, which are combined above the anode 16 and contact the anode 16, can be seen well. Further, it can be seen in Fig. 3 that the anode 16 also consists of a plurality of anode blocks 27, the individual anode blocks 27 in accordance with the cathode 25 blocks 24 essentially having the outline of a symmetrical trapezium.
Each anode block 27 can in principle be contacted by one or a plurality of cur-rent supplies 20, 22 and a plurality of anode blocks 27 can be electrically conductively connected to one another along the lateral faces thereof, which is not absolutely necessary however. In this case, the anode blocks =

27 are suspended on electrically conductive suspension elements 25 and are electrically contacted via the same.
The cathode 10 is electrically contacted from below by a plurality of pin-.. like contacting elements 28, which extend in each case perpendicularly to the underside of the cathode 10 into the cathode 10 and those on the side facing away from the cathode 10 are electrically connected to a common base plate 30 which is connected via a current conductor 29 to an electri-cal current source.
In Fig. 4, the electrical current flow in the segment of the electrolysis cell shown in Fig. 3 is visualised by means of arrows 31. The upwardly di-rected electric current in the inner current supply 20 and the likewise upwardly directed electric current in the external current supplies 22 in .. this case generate one magnetic field in each case, the magnetic fields generated by the inner and the external current supplies 20, 22 essen-tially being compensated for in the region of the cathode 10, the layer 12 made up of liquid aluminium, the melt layer 14 and the anode 16, so that only a very small and very homogeneously distributed magnetic flux den-.. sity is present in the layer 12 made up of liquid aluminium and the melt layer 14 in particular. As shown in Fig. 4, the entire electrolysis current flowing through the anode 16, the melt layer 14, the layer 12 made up of liquid aluminium and the cathode 10 is supplied by means of the current supplies 20, 22. The division of the electrolysis current to the inner cur-.. rent supply 20 on the one hand and the external current supplies 22 on the other hand is preferably adapted in this case by means of the corre-sponding choice of the cross sections of the current supplies 20, 22 in such a manner that an optimal cancelling of the magnetic fields in the region of the annular cathode 10 results. As can be seen in particular in =

Fig. 2, the inner current supply 20 and the external current supplies 22 have different conductor cross sections to this end.
Fig. 5a shows a graphical illustration of the electrical distribution of the vertical component of the electric current density at the cathode surface of a segment of an electrolysis cell as shown in Figs 3 and 4 in a plan view.
It can be seen from Fig. 5a that by means of the particular type of contact-ing by means of pin-like contacting elements 28 shown in Figs 2, 3 and 4, an outstanding evenness of the vertical component of the electric current density can be achieved over the entire cathode block surface. In this manner, horizontal current density components are prevented to the =
greatest possible extent, so that wave formation in the layer 12 made up of liquid aluminium and the melt layer 14 and wearing of the cathode 10 are reduced solely by means of the type of contacting of the cathode 10.
Fig. 5b is an illustration, corresponding to the illustration of Fig. 5a, of the distribution of the vertical component of the electric current density at the surface of a conventional square cathode 10 of a conventional electrolysis cell.
As a comparison of Fig. 5a and Fig. 5b shows, the electrolysis cell shown in Figs 3 and 4 has a distribution of the vertical electric current density at the cathode surface which is markedly more even than that in the distri-bution of the vertical current density at the surface of the conventional cathode 10' shown in Fig. 5b.
Fig. Sc is a legend which indicates values, corresponding to the shading shown in the Fig. 5a and Fig. 5b, of the value of the vertical electric cur-rent density at the respective point of the cathode surface.

Fig. 6a shows a graphical illustration of the distribution of the value of the magnetic flux density in the boundary surface between the layer 12 made up of liquid aluminium and the melt layer 14 of a segment of an electroly-sis cell as shown in Figs 3 and 4, as viewed in a plan view.
Fig. 6b is an illustration of a distribution, corresponding to Fig. 6a, of the value of the magnetic flux density in the boundary surface between the layer 12 made up of liquid aluminium and the melt layer 14 of an elec-trolysis cell with a conventional square cathode 10.
= Fig. 6c is a legend which indicates values, corresponding to the shading shown in the Fig. 6a and Fig. 6b, of the value of the magnetic flux density at the respective point in the boundary surface between the layer 12 made up of liquid aluminium and the melt layer 14.
As a comparison of Fig. 6a and Fig. 6b shows, the electrolysis cell shown in Figs 2, 3 and 4 has a distribution of the magnetic flux density, which is both smaller in terms of value and markedly more evenly distributed than the distribution in an electrolysis cell with a conventional cathode 10' shown in Fig. 6b.
As a result, in combination with the markedly more even distribution of the vertical current density components shown in Fig. 5c, a markedly higher stability and markedly higher energy efficiency of the electrolysis cell shown in Figs 2, 3 and 4 is enabled.
Fig. 7 shows an electrolysis cell in a plan view, which essentially corre-sponds to the electrolysis cell shown in Figs 2, 3 and 4, an exemplary method for producing the cathode 10 of the electrolysis cell additionally . =

being visualised. As shown in Fig. 7, a plurality of trapezoidal cathode blocks 24 for the hexagonal ring-shaped cathode 10 can be produced sim-ply in that an essentially square crude body 32 is cut into pieces trans-versely to the longitudinal direction thereof, the cuts being guided in an alternating orientation as viewed in the longitudinal direction of the crude body 32. A milling or sawing tool can be used for example as a cutting tool.
Fig. 8 shows a further embodiment of an electrolysis cell in a plan view, which essentially corresponds to the embodiment shown in Fig. 7 and in which the cathode 10 has a circular outline and is composed of circular-segment-shaped cathode blocks 24.
Fig. 9 shows a further embodiment of an electrolysis cell in a plan view, which essentially corresponds to the embodiments shown in Fig. 7 and Fig. 8 and in which the cathode 10 is composed of cathode blocks 24 with a hexagonal outline in such a manner that an approximately circular out-line of the entire cathode 10 results.
Fig. 10 shows a segment of an electrolysis cell according to a further em-bodiment of the invention in a perspective view. The embodiment shown in Fig. 10 in this case essentially corresponds to the embodiments shown in Figs 2, 3, 4 and 7, the contacting of the cathode 10 not taking place by means of pin-like contacting elements 28 (see Figs 3 and 4), however but rather by means of horizontal bus bars 34. Although in the case of this contacting of the cathode 10, such a pronounced homogenisation of the vertical component of the electric current density, as is achieved for the embodiment shown in Figs 3 and 4, is not achieved under certain circum-stances, due to the improved current supply to the anode 16 and the re-duction and homogenisation of the distribution of the magnetic flux den-sity connected therewith, a considerable reduction of the wave formation in the layer 12 made of liquid aluminium and the melt layer 14 is none-theless achieved, so that the stability and energy efficiency of the elec-trolysis cell is here also increased considerably.

Fig. 11 shows an electrolysis cell according to a further preferred embodi-ment in a perspective view, wherein the electrolysis cell is essentially com-posed of segments as shown in Figures 3 and 4. In this embodiment, the opening 18 extends vertically through the cathode 10 and additionally 10 extends through the layer 12 made up of liquid aluminium, the melt layer 14 and the anode 16, wherein these constituents in each case form a = closed ring around this opening. The layer 12 made up of liquid alumin-ium and the melt layer 14 are located in a tank delimited by means of a tub, wherein the bottom of the tub is formed by the cathode 10, wherein 15 the side walls of the tub are not illustrated in Fig. 11. In this case, the anode 16 is preferably of somewhat narrower construction than the cath-ode 10, the layer 12 made up of liquid aluminium and the melt layer 14 as viewed from above, which cannot be seen from the schematic Fig. 11, and is immersed into the melt layer 14.
Fig. 12 shows a perspective illustration of an electrolysis cell according to a further embodiment of the present invention, which essentially corre-sponds to the electrolysis cell shown in Fig. 11. However, the anode 16 of the electrolysis cell shown in Fig. 12 consists of a plurality of anode blocks 27 with an essentially trapezoidal outline as viewed in plan view in each case, which anode blocks are arranged annularly around the opening 18 and are spaced apart from one another and which are in each case slightly immersed into the melt layer 14.

Fig. 13 shows a cross-sectional illustration of an electrolysis cell according to a further preferred embodiment of the present invention, which essen-tially corresponds to the electrolysis cells shown in the Figures 11 and 12.
Also shown is a steel tub 36 which forms a frame for the electrolysis cell and - in accordance with the cathode 10 - is of annular construction as viewed in plan view. In the direction of the opening 18, the steel tub 36 is delimited by perpendicular side walls which define a shaft for the inner current supply 20 extending vertically through the electrolysis cell, through which shaft the current supply 20 extends vertically.
The steel tub 36 is lined at its base with floor stones 38 and lined at its = perpendicular side walls with side-wall stones 40, wherein the floor and side-wall stones 38, 40 in each case consist of a refractory material which = is preferably electrically insulating. Preferably, the floor and side-wall stones 38, 40 forming the lining of the steel tub 36 contain a material which is selected from the group which consists of a white ceramic mate-rial, a silicon-nitride-bound silicon carbide, carbon and graphite and any desired combinations of the same materials.
The cathode 10 is arranged on the floor stones 38, which cathode forms the bottom of a tub formed by the cathode 10 and the side-wall stones 40, which tub in turn defines a tank for accommodating the layer 12 made up of liquid aluminium and the melt layer 14.
It can also be seen from Fig. 13 that the anode blocks 27 are immersed into the melt layer 14, but not into the layer 12 made up of liquid alumin-ium and for this purpose - as viewed in plan view - are of somewhat nar-rower construction than the cathode 10, the layer made up of liquid alu-minium and the melt layer 14.

Also shown in Fig. 13 is a pin-like contacting element 28 which extends vertically into the cathode 10 and is electrically connected at its end facing away from the cathode 10 to a current supply for supplying the cathode with current, which is constructed as a horizontally running collecting bar 42. The pin-like contacting element 28 and the collecting bar 42 are elec-trically insulated from the steel tub 36.
The electrolysis cell shown in Fig. 13 is shown in Fig. 14, the technical current flow direction of the current flowing during operation of the elec-trolysis cell additionally being illustrated in this Fig. by means of the ar-rows 44.

Reference List 10 Cathode 10' Conventional cathode 12 Layer made up of liquid aluminium 14 Melt layer 16 Anode 18 Cathode opening Inner current supply 22 External current supply 24 Cathode block Suspension element 15 26 Symmetry plane 27 Anode block 28 Contacting element 29 Current conductor Base plate 20 31 Arrow 32 Crude body 34 Bus bar 36 Steel tub 38 Floor stone 25 40 Side-wall stone 42 Collecting bar 44 Arrow which shows the technical current direction

Claims (22)

1. An electrolysis cell, in particular for producing aluminium, which comprises a cathode (10), a layer (12) made up of liquid aluminium arranged on the upper side of the cathode (10), a melt layer (14) thereupon and an anode (16) above the melt layer (14), wherein the cathode (10) has at least one opening (18) extending vertically through the cathode (10), in which opening at least one current supply (20) extending vertically through the opening (18) and elec-trically connected to the anode (16) and/or to the cathode (10) is provided, and wherein the electrolysis cell comprises at least one further current supply (22) arranged outside of the opening (18) of the cathode (10), which current supply extends in the vertical direc-tion at least in certain sections and which current supply is electri-cally connected to the cathode (10) and/or to the anode (16).

2. The electrolysis cell according to Claim 1, characterised inthat the opening (18) is arranged essentially centrally in the cathode (10) as viewed in a plan view and the current supply (20) extending through the opening (18) preferably extends centrally through the opening (18) of the cathode (10).

3. The electrolysis cell according to Claim 1 or 2, characterised in that the cathode (10) has an at least approximately circular outline as viewed in a plan view.

4. The electrolysis cell according to Claim 1 or 2, characterised in that the cathode (10) has an at least approximately polygonal ring-shaped outline as viewed in a plan view.

5. The electrolysis cell according to Claim 4, characterised in that the external circumference and/or the internal circumference of the outline of the cathode (10), which is polygonal ring-shaped as viewed in a plan view, has the shape of a regular polygon with n corners, wherein n is preferably 3 to 100, particularly preferably 3 to 10 and very particularly preferably 3, 4, 5, 6, 7 or 8.

6. The electrolysis cell according to at least one of the preceding claims, characterised in that the cathode (10) is composed of a plurality of cathode blocks (24) which, as viewed in the circumferential direction, are arranged around the current supply (20), which extends through the opening (18), next to one another and adjoining one another, preferably forming an annular cathode.

7. The electrolysis cell according to Claim 6, characterised in that at least one cathode block (24) of the cathode (10) is shaped in a hexagonal, circular-segment-shaped or trapezoidal manner as viewed in a plan view.

8. The electrolysis cell according to at least one of the preceding claims, characterised in that the ratio between the internal diameter and the external diameter of the cathode (10) is between 0.01 and 0.99, preferably between 0.1 and 0.8, particularly preferably between 0.2 and 0.6 and very par-ticularly preferably between 0.3 and 0.5.

9. The electrolysis cell according to at least one of the preceding claims, characterised in that the electrolysis cell comprises between 2 and 10, preferably between 4 and 8, particularly preferably between 5 and 7 and very particu-larly preferably 6 current supplies (22) arranged outside of the open-ing (18) of the cathode (10), which in each case extend in the vertical direction at least in sections and which are in each case electrically connected to the cathode (10) and/or to the anode (16).

10. The electrolysis cell according to Claim 9, characterised in that the further current supplies (22) are arranged at least approximately regularly and preferably concentrically as viewed in the circumferen-tial direction of the cathode (10) and as viewed around the current supply (20) extending through the opening (18).

11. The electrolysis cell according to at least one of the preceding claims, characterised in that the cathode (10) has at least two pin-like contacting elements (28) on its underside.

12. The electrolysis cell according to Claim 11, characterised in that at least one of the pin-like contacting elements (28) extends at an angle of less than 30° and preferably less than 10° with respect to the vertical and particularly preferably vertically into the cathode (10).

13. The electrolysis cell according to Claim 11 or 12, characterised in that the contacting elements (28) are electrically conductively connected on the side thereof which faces away from the cathode (10) to a common base plate (30).

14. The electrolysis cell according to at least one of the preceding claims, characterised in that the spacing between the anode (16) and the layer (12) made up of liquid aluminium is between 15 and 45 mm, preferably between 15 and 35 mm and particularly preferably between 15 and 25 mm.

15. A cathode for an electrolysis cell, in particular for an electrolysis cell for producing aluminium, which has at least one opening (18) ex-tending vertically through the cathode (10).

16. The cathode according to Claim 15, characterised in that the cathode (10) is shaped in an annular and preferably at least approximately circular or polygonal ring-shaped manner as viewed in a plan view.

17. The cathode according to Claim 16, characterised in that the external circumference and/or the internal circumference of the outline of the cathode (10), which is polygonal ring-shaped as viewed in a plan view, has the shape of a regular polygon with n corners, wherein n is preferably 3 to 100, particularly preferably 3 to 10 and very particularly preferably 3, 4, 5, 6, 7 or 8.

18. The cathode according to at least one of Claims 15 to 17, characterised in that the cathode (10) is comprised of a plurality of cathode blocks (24) which, as viewed in the circumferential direction, are arranged around the opening (18) of the cathode (10) next to one another and adjoining one another.

19. The cathode according to Claim 18, characterised in that at least one cathode block (24) has a hexagonal, circular-segment-shaped or trapezoidal outline as viewed in a plan view.

20. The cathode according to at least one of Claims 15 to 19, characterised in that the ratio between the internal diameter and the external diameter of the cathode (10) is between 0.01 and 0.99, preferably between 0.1 and 0.8, particularly preferably between 0.2 and 0.6 and very par-ticularly preferably between 0.3 and 0.5.

21. The cathode according to at least one of Claims 15 to 20, characterised in that the cathode (10) has at least two recesses for one pin-like contacting element (28) in each case on its underside.

22. The cathode according to Claim 21, characterised in that at least one of the recesses extends at an angle of less than 300 and preferably less than 100 with respect to the vertical and particularly preferably vertically into the cathode (10).