CA2910233C

CA2910233C - Cathode block having a slot with varying depth and a securing system

Info

Publication number: CA2910233C
Application number: CA2910233A
Authority: CA
Inventors: Frank Hiltmann; Markus Pfeffer
Original assignee: Sgl Cfl Ce GmbH
Current assignee: Tokai Cobex GmbH
Priority date: 2013-04-26
Filing date: 2014-04-25
Publication date: 2018-01-16
Anticipated expiration: 2034-04-25
Also published as: EP3546620B1; RU2015150375A; PL3546620T3; RU2727621C2; CN105247109B; EP2989235B1; JP2016516905A; DE102013207737A1; UA117481C2; EP2989235A1; RU2020114123A3; EP2989235B9; RU2020114123A; WO2014174089A1; CN105247109A; CA2910233A1; EP3546620A1; PL2989235T3; JP6808485B2

Abstract

A cathode block for an aluminum electrolysis cell based on carbon and/or graphite has at least one slot for receiving at least one bus bar, said slot extending in the longitudinal direction of the cathode block, wherein at least one of the at least one slot has a depth that varies when viewed over the length of the cathode block, and wherein at least one recess which extends horizontally in the longitudinal direction of the cathode block is provided in the cathode block wall bordering the at least one slot. According to another embodiment, a cathode block for an aluminum electrolysis cell based on carbon and/or graphite has at least one slot for receiving at least one bus bar, said slot extending in the longitudinal direction of the cathode block, wherein at least one of the at least one slot has a depth that varies when viewed over the length of the cathode block, and wherein this slot is bordered by a wall, at least one projection that projects into the slot being provided on said wall.

Description

CATHODE BLOCK HAVING A SLOT WITH VARYING DEPTH AND A SECURING SYSTEM
The present invention relates to a cathode block for an aluminum electrolysis cell, to its utilization and also to a cathode comprising it.
Electrolysis cells are for example used for the electrolytic production of aluminum which, on the industrial scale, is usually carried out according to the Hall-Heroult process. In the Hall-Heroult process, a molten mixture of aluminum oxide and cryolite is electrolyzed. Here, the cryolite, Na3[Al F6], is used to lower the melting point of 2045 C for pure aluminum oxide to approx. 950 C for a mixture containing a cryolite, aluminum oxide and additional substances, such as aluminum fluoride and calcium fluoride.
The electrolysis cell used in this process comprises a cathode bottom which is composed of a plurality of, for example, up to 28 adjacent cathode blocks forming the cathode. Here, the intermediate spaces between the cathode blocks are usually filled with a carbonaceous ramming paste in order to seal the cathode against molten constituents of the electrolysis cell and in order to compensate for mechanical stresses which arise as the electrolysis cell is put into operation. The cathode blocks are usually made of a carbonaceous material, such as graphite, in order to withstand the thermal and chemical conditions prevailing when the cell is in operation. The undersides of the cathode blocks are usually provided with slots in each of which one or two busbars are arranged through which the current supplied via the anodes is discharged. Here the intermediate spaces between the busbars and the individual cathode block walls bordering the slots are often filled with cast iron so that the encasement of the busbars with cast iron thus created connects the busbars to the cathode blocks electrically and mechanically. About 3 to 5 cm above the layer of liquid aluminum on the top side of the cathode, which is usually 15 to 50 cm thick, there is an anode, in particular formed of individual anode blocks. The electrolyte, in other words, the molten mass containing aluminum oxide and cryolite, is found between this anode and the surface of the aluminum. During electrolysis, which is carried out at approximately 1000 C, the aluminum thus formed, being denser than the electrolyte, settles below the electrolyte layer - in other words, as an intermediate layer between the top side of the cathode and the electrolyte layer. In electrolysis, the aluminum oxide dissolved in the molten mass is broken down into aluminum and oxygen by the electrical current flow. From the electrochemical point of view, the layer of liquid aluminum is the actual cathode since aluminum ions are reduced to elemental aluminum on its surface.
Nonetheless, in what follows, the term cathode will not refer to the cathode from the electrochemical point of view, in

2 other words, the layer of liquid aluminum, but rather to the component composed, for example, of one or more cathode blocks and forming the bottom of the electrolysis cell.
One major disadvantage of the cathode arrangements used in the Hall-Heroult process is their comparatively poor resistance to wear, which manifests itself as a removal of material from cathode block surfaces during electrolysis. Here, due to a heterogeneous distribution of current within the cathode blocks, material is not removed evenly from the cathode block surfaces over the length of the cathode blocks but to a greater extent at the ends of the cathode blocks with the result that after electrolysis has proceeded for a certain amount of time, the surfaces of the cathode blocks assume a W-shaped profile. Due to the uneven removal of material from the cathode block surfaces, the useful life of the cathode blocks is limited by the locations with the greatest removal of material. In order to combat this problem, a cathode block is proposed in WO 2007/118510 A2 whose slot, intended to accommodate one or more busbars, has with respect to the cathode block length a greater depth in the center than at the cathode block ends.
In the operation of the electrolysis cell, this results in an essentially homogeneous vertical current distribution over the cathode block length, whereby the higher wear at the cathode block ends is reduced and the service life of the cathode is thus extended. Here, the busbar(s) is or are encased in cast iron in the usual way, whereby this encasement is effected by pouring liquid cast iron into the intermediate space between the slot and the busbar(s). A cathode block of this kind is however encumbered with disadvantages. While the liquid cast iron is being poured into the intermediate space between the slot and the busbar(s) and afterwards, while the electrolysis cell comprising the cathode block is being put into operation and afterwards, while the electrolysis cell is being switched off and subsequently restarted and afterwards, the cathode block is exposed to comparatively large temperature changes which lead to the cast iron and the busbar(s) expanding or contracting relative to the cathode block. This effect of expansion or contraction can be amplified by the temperature gradients which occur. When the phrase 'large temperature change(s)' is used in what follows, it should be understood as indicating that one or both of the effects mentioned - in other words, expansion / contraction or temperature gradient - is or are present. Since cast iron and the material of the busbar(s) have a higher coefficient of thermal expansion than the cathode block material, when there is a temperature increase, the cast iron and the busbar(s) expand relative to the cathode block while a temperature decrease on the other hand results in them contracting relative to the cathode block. This causes a deterioration in the electrical contact between busbar, cast iron and cathode block, especially in the case of the usual slots with their rectangular cross-section, and this in turn results in a higher electrical resistance of the arrangement and thus a poor energy efficiency of the electrolytic process.
Apart from this, before the liquid cast iron is poured into the intermediate space between the slot and the busbar(s), the busbar(s) are movable not only vertically but also horizontally such that they can

3 move uncontrollably in the slot while the liquid cast iron is being poured in and then while the cast iron is cooling down and solidifying. This can also result in an uneven electrical contact between busbar, cast iron and cathode block. This too results in a higher electrical resistance of the arrangement and thus to poor energy efficiency of the electrolytic process.
Ramming paste can also be used instead of cast iron. Ramming pastes based on anthracite, graphite or any mixture thereof can be used as a ramming paste. Preferably a graphite-based ramming paste is used.
In order to prevent or at least hinder a bus bar from sliding in the slot of a cathode block, it is proposed in WO 2012/107412 A2 to provide at least one recess in the wall bordering the graphite-foil-lined slot of a cathode block and, after installation of the busbar(s) in the slot, to fill the intermediate space between the slot and the busbar(s) with liquid cast iron in such a way that the solidified cast iron meshes with the at least one recess. Should the slot vary in depth over the length of the cathode block, the at least one recess should run in parallel to the bottom of the slot - that is, obliquely with respect to the horizontal direction - in other words, maintain a constant distance from the bottom wall of the slot so as to ensure the busbar(s) can be shifted parallel to the slot bottom.
This is however disadvantageous because due to the fact that the cast iron and the material of the busbar(s) have a higher coefficient of thermal expansion than the cathode block material, shear stresses arise from the temperature changes occurring while the liquid cast iron is being poured into the intermediate space between the slot and the busbar(s) and afterwards, and also while the electrolysis cell comprising the cathode block is being put into operation, and while the electrolysis cell is being switched off and if applicable subsequently restarted, these temperature changes being between the cast iron and the busbar(s) on the one hand and the cathode block on the other hand.
These shear stresses can result in damage, such as cracking of the cathode block or even fracture of the cathode block, which impairs the function of the cathode block. Damage of this kind leads to reduced electrical conductivity between the busbar or cast iron and the cathode block and to a lower stability of the arrangement or even results in the entire arrangement failing. As has been described above, ramming paste can also be used here instead of cast iron.
When cast iron is mentioned hereinafter, it is to be understood that ramming paste can be substituted for the cast iron without this being stated explicitly each time.
An aspect of the present disclosure is directed to the provision of a cathode block suitable in particular for use in an aluminum electrolysis cell with which an essentially homogeneous vertical current distribution is achieved over the length of the cathode block while the electrolysis cell is in operation, which also, with the busbar(s) installed and encased in cast iron, has even with large

4 temperature changes, a low specific electrical resistance and in particular over long hours of operation, a permanently low specific electrical resistance and a low transition resistance between the busbar encased in cast iron and the cathode block, and which in the case of large temperature changes, is stable with respect to mechanical damages, such as cracking, even with the busbar(s) installed and encased in cast iron. Ramming paste can also be used instead of cast iron.
According to an aspect of the invention, there is provided a cathode block for an aluminum electrolysis cell on the basis of carbon and / or graphite, wherein the cathode block has at least one slot extending in the longitudinal direction of the cathode block for accommodating at least one busbar, wherein at least one of the at least one slot has a varying depth when viewed over the length of the cathode block, wherein at least one recess is provided in the cathode block wall bordering the at least one slot of varying depth, said recess extending horizontally in the longitudinal direction of the cathode block.
Within the context of the present invention, a recess extending horizontally in the longitudinal direction of the cathode block is to be understood as the recess running in parallel with the longitudinal plane of the cathode block. Here, running in parallel means that the recess at all of its locations has an angle of less than 8 , preferably of less than 5 , very preferably of less than 2 , especially preferably of less than 1 , extremely preferably of less than 0.5 and maximally preferably of less than 0.1 with respect to the longitudinal plane of the cathode block.
In this context, longitudinal plane is to be understood as the plane which extends in the direction of the longitudinal axis of the cathode block and runs parallel to the surface of the cathode block side opposite the slot.
Furthermore, within the context of the present invention, a recess as distinct from mere surface roughness is to be understood as a recess which with respect to the surface of the wall bordering the slot has a depth of at least 0.5 mm and preferably of at least 2 mm.
According to an aspect of the invention, it was realized that due to the provision in the wall bordering the cathode block slot of at least one recess extending horizontally in the longitudinal direction of the cathode block, and preferably in both of the side walls, in particular also when forming the slot of varying depth in the cathode block, a cathode block is created which, even with a busbar installed in the slot and encased in cast iron, has a low electrical resistance and a low transition resistance. Apart from this, due to the provision of the recess extending horizontally in the longitudinal direction of the cathode block in the wall bordering the slot in the cathode block, mechanical damage to the cathode block, such as cracking of the cathode block, with the busbar installed in the slot and encased in cast .81792396 iron is reliably prevented even with large temperature changes. On the one hand, the use of a slot of varying length in the longitudinal direction of the cathode block yields a current density distribution at the cathode block surface so homogeneous that when the electrolysis cell comprising the cathode block is in operation, an excess removal of cathode block material is effectively prevented in those

5 areas where there would be a high local current density when a cathode block is used with a constant slot depth in the longitudinal direction of the cathode block. By an appropriate adjustment of the slot depth, the current density distribution can be modified and evened out within broad limits. By the cathode block having in its slot a recess extending horizontally in the longitudinal direction of the cathode block, a vertical fixing of the busbar encased in cast iron in the slot of the cathode block is achieved, which nevertheless allows for a certain amount of movement in the horizontal direction of the cathode block. Due to this horizontal mobility of the busbar encased in cast iron, especially with the abrupt temperature changes occurring while the electrolysis cell comprising the cathode block is being put into operation or switched off and afterwards, the occurrence of shear stresses between the busbar encased in cast iron and the cathode block is reliably prevented, such as would occur with an obliquely arranged recess as a consequence of the greater expansion or contraction of the cast iron and busbar relative to the cathode block due to the cast iron and the material of the busbar(s) having a higher coefficient of thermal expansion than the cathode block material. In this way, damage to the cathode block, for example in the form of cracking, or even fracture of the cathode block, is reliably prevented even when the electrolysis cell is operating for long periods, while at the same time outstanding electrical conductivity between the busbar or cast iron and the cathode block is guaranteed. As a result of the busbar encased in cast iron being vertically fixed in the slot of the cathode block, there arises an advantageous pressing of the cathode bars /
cast-iron arrangement against the slot bottom due to the thermal expansion of the bars / cast-iron arrangement relative to the cathode block when the cell is put into operation. In this way, an improved electrical contact is achieved which results in a lower electrical resistance and thus in a higher energy efficiency. In a further advantage over the cathode block known from WO 2012/107412 A2, these outstanding properties are in particular also attained when the slot in the cathode block is not covered with an expensive graphite foil which is also costly to apply. All in all, even with large temperature changes, a control of tensile stresses, shear stresses and compressive stresses, such as arise from the different thermal expansion coefficients of cast iron, busbar and cathode block, is thus achieved which ensures an outstanding electrical conductivity and an excellent mechanical stability of the cathode block even with a busbar installed in the slot and encased in cast iron.
In some embodiments, in order to achieve an especially even vertical current density distribution at the cathode block surface during the electrolysis operation, it is proposed in a development of

6 the inventive concept that at least one of the at least one slots or preferably all of the slots of varying depth has or have at its or their longitudinal ends less depth than at its or their center(s).
In this way, an even distribution of the electrical current supplied during electrolysis operation is achieved over the entire length of the cathode block, whereby an excessively high electrical current density is avoided at the longitudinal ends of the cathode block and thus premature wear at the ends of the cathode block is prevented. Due to such an even current density distribution over the length of the cathode block, movements in the aluminum melt caused by the interaction of electromagnetic fields during electrolysis are avoided, whereby it becomes possible to arrange the anode at a lower height above the surface of the aluminum melt. This reduces the electrical resistance between the anode and the aluminum melt and boosts the energy efficiency of the fused-salt electrolysis being carried out. Another special advantage of this embodiment is that with this design of the slot, the busbar(s) provided in the recess of the slot and possibly encased in cast iron expand(s) horizontally during and after the increase in temperature occurring when the electrolysis cell is put into operation, as a result of which each of the busbar(s) is pressed against the cathode block bottom wall bordering the slot at this point, whereby the transition resistance between the busbar encased in cast iron and the cathode block is reduced.
In the aforementioned embodiment, the depth of at least one of the at least one slot of varying depth, when viewed in the longitudinal direction of the cathode block, increases with at least an essentially constant gradient preferably from one longitudinal end to the center of the cathode block and decreases with at least an essentially constant gradient preferably from the center of the cathode block to the other longitudinal end so that, when viewed in a longitudinal section of the cathode block, an at least essentially triangular slot is created. In this way, the aforementioned advantages are achieved to a greater degree.
According to a further embodiment of the present invention, the wall bordering the at least one slot of varying depth has a bottom wall and two side walls, wherein each of the two side walls has at least one recess which extends horizontally in the longitudinal direction of the cathode block. In this way, a particularly good vertical fixing of the busbar in the slot is achieved, with at the same time sufficiently great mobility of the busbar in the horizontal direction, in order to reliably prevent, even with large temperature changes, the occurrence of shear stresses as a consequence of the different coefficients of thermal expansion of cast iron, busbar and cathode block.
In some embodiments, preferably the wall bordering the at least one slot of varying depth comprises a bottom wall and two side walls, wherein each side wall has precisely one recess which extends

7 horizontally in the longitudinal direction of the cathode block. In this way, at comparatively low production cost, a particularly good vertical fixing of the busbar in the slot is achieved, with at the same time sufficiently great mobility of the busbar in the horizontal direction, in order to reliably prevent, even with large temperature changes, the occurrence of shear stresses as a consequence of the different coefficients of thermal expansion of cast iron, busbar and cathode block.
In some embodiments, it is likewise preferred for the wall bordering the at least one slot of varying depth to comprise a bottom wall and two side walls, wherein each side wall has two recesses which extend horizontally in the longitudinal direction of the cathode block. In this way, a particularly good vertical fixing of the busbar in the slot is also achieved, with at the same time sufficiently great mobility in the horizontal direction, when the depth of the individual recesses is comparatively shallow.
Here, the cathode block can have two slots arranged on the same side of the cathode block, wherein the two slots have the same dimensions and their bordering walls in each case comprise a bottom wall and two side walls, wherein each side wall has one recess which extends horizontally in the longitudinal direction of the cathode block or wherein each side wall has two recesses which extend horizontally in the longitudinal direction of the cathode block. In this way, at comparatively low production cost, a particularly good vertical fixing of the two busbars in the slots is achieved for a cathode block having two slots, with at the same time sufficiently great mobility in the horizontal direction, in order to reliably prevent, even with large temperature changes, the occurrence of shear stresses as a consequence of the different coefficients of thermal expansion of cast iron, busbar and cathode block.
As an alternative to the above embodiment, the cathode block can also have just one slot.
Basically, the at least one slot can have any known cross-sectional form, wherein however particularly good results are obtained when at least one of the at least one slot and preferably when each of the at least one slot has a cross-section which is at least essentially rectangular but preferably actually rectangular.
In some embodiments, in order to ensure that the busbar, if applicable encased in cast iron, is especially well fixed in the slot in the vertical direction while at the time time having sufficient mobility in the horizontal direction, it is proposed in a development of the inventive concept that at least one of the at least one recess and especially preferably each of the at least one recess extends continuously over at least 60%, preferably over at least 80%, especially preferably over

8 at least 90%, very especially preferably over at least 95% and maximally preferably at least over approximately the entire length of the at least one slot.
In some embodiments, it is for the same reason preferable that at least one of the at least one recess and especially preferably each of the at least one recess has a depth of 0.5 mm to 40 mm, preferably between 2 mm and 30 mm, and especially preferably between 5 mm and 20 mm.
In some embodiments, it is for the same reason preferable as well that at least one of the at least one recess and especially preferably each of the at least one recess has, with respect to the height of the cathode block, an opening width of 2 mm to 40 mm, preferably between 5 mm and 30 mm, and especially preferably between 10 mm and 20 mm.
Basically, the at least one recess can have any polygonal or curved cross-section. Good results with regard to good engagement of the cast iron encasement with the at least one recess and at the same time with regard to a reliable and unproblematic filling capability of the recess with cast iron during casting are in particular achieved when at least one of the at least one recess and especially preferably each of the at least one recess has a cross-section which is at least essentially semicircular, triangular, rectangular or trapezoidal, and preferably semicircular, triangular, rectangular or trapezoidal.
According to a further embodiment of the present invention, the at least one recess extends at least essentially orthogonally, preferably orthogonally, into the cathode block wall bordering the at least one slot.
According to an aspect of the present invention, the at least one recess -when viewed in the direction of the depth of the slot - is bounded at each of its ends by a transitional region between the recess and a slot wall section adjoining it. If this transitional region has an angular form, the angle between the adjoining section of the slot wall and the wall of the recess, when viewed from the inside of the cathode block, will preferably measure 900 to 160 , especially preferably 90' to 135 , and very especially preferably 100 to 120 .
In some embodiments, in the case where this transitional region has a form which is curved, possibly but not necessarily ideally circularly curved, the radius of curvature of the transitional region will be preferably 50 mm at maximum, especially preferably 20 mm at maximum and maximally especially preferably 5 mm at maximum.

9 Furthermore, an aspect of the present invention concerns a cathode arrangement which contains at least one previously described cathode block, wherein in at least one of the at least one slots of varying depth of the at least one cathode block is provided at least one busbar, which is at least partially encased in cast iron, which at least in sections engages with the at least one recess.
According to some embodiments of the present invention, the section of the cast-iron encasement engaging with the at least one recess has a shape complementary to the recess.
In this way, it is possible to achieve a particular good interlocking engagement of the cast-iron encasement with the recess and thus a particularly effective mechanical attachment of the cast-iron encasement and the associated busbar to the cathode block, which however still permits a sufficient mobility of the busbar in the horizontal direction to avoid shear stresses between cast iron, busbar and cathode block resulting from large temperature changes.
In some embodiments, preferably, the cast-iron encasement engages with the at least one recess over at least 50% of its length, more preferably over at least 80%, especially preferably over at least 90%, very especially preferably over at least 95% and maximally preferably over essentially its full length. In this way, the aforementioned advantages are achieved to a particularly high degree.
For the same reason, according to a further embodiment of the present invention, it is envisaged that the section of the encasement engaging with the at least one recess and where applicable the busbar encased by it fills out at least 70% of the recess, preferably at least 80%, especially preferably at least 90%, very especially preferably at least 95% and maximally preferably 100% of the recess. In this way, it is possible to prevent especially reliably an unwanted shift of the busbar in the vertical direction of the cathode block and in particular the busbar from falling out of the slot.
In some embodiments, in a development of the inventive concept, it is proposed that the cathode block of the cathode arrangement has a slot with an at least essentially rectangular but preferably a rectangular cross-section and that one or two adjoining busbar(s) are installed in the slot, wherein the intermediate space between the slot and the busbar(s) is filled with cast iron in such a way that the cast iron engages with the at least one recess over at least essentially its entire length.
A further subject matter of another aspect of the present invention is a cathode which comprises at least one previously described cathode block or at least one previously described cathode arrangement.

Furthermore, an aspect of the present invention relates to the use of a previously described cathode block, a previously described cathode arrangement or a previously described cathode for carrying out a fused-salt electrolysis to produce metal, namely preferably to produce aluminum.

In another aspect of the present invention, there is provided a cathode block for an aluminum electrolysis cell based on carbon and / or graphite, for example a previously described cathode block which has at least one slot extending in the longitudinal direction of the cathode block and serving to accommodate at least one busbar, wherein at least one of the at least one slot is of

10 varying depth when viewed over the length of the cathode block, wherein this slot is bounded by a wall, wherein at the wall, there is provided at least one projection which extends into the slot.
According to an aspect of the invention, it was recognized that by providing at least one projection extending into the slot which serves as a support surface for busbar ends or their cast-iron encasement, in particular also in giving the slot in the cathode block a varying depth, a cathode block is created in which the busbar installed in the slot and - in particular also in the case of two busbars installed adjacent to each other in the slot in each case of half length with respect to the length of the cathode block - the busbar(s) - depending on the design of the projection - are fixed in the vertical and / or horizontal direction such that an uncontrolled movement or shift of the busbars is reliably prevented when, during the creation of a cathode arrangement with one cathode block and with one or more busbars installed in its slot and encased in cast iron, the molten cast iron is being poured into the intermediate space between the slot and the busbar(s), and in particular also during the subsequent cooling down and solidifying of the cast iron despite the usually higher coefficient of thermal expansion of cast iron and the material of the busbar(s) than the coefficient of thermal expansion of the cathode block material, wherein this movement or shift can result in a poor or uneven electrical contact between busbar, cast iron and cathode block. The at least one projection extending into the slot is thus a support lug or a support stud on which one end piece of a busbar or two end pieces of two busbars can rest. As a result, the cathode arrangement made of a cathode block with one or more installed busbars encased in cast iron has a low specific electrical resistance and a low transition resistance between the busbar(s) encased in cast iron and the cathode block and in particular also a permanently low specific electrical resistance and a low transition resistance between the busbar(s) encased in cast iron and the cathode block, even in large temperature changes. The cathode block according to the invention is thus suitable in particular for accommodating two busbars installed adjacent to each other in the slot in each case having a half length with respect to the length of the cathode

11 block, wherein in this case the projection is preferably provided in the center of the cathode block so that in each case one end of the two busbars can rest on the support surface created by the projection. Furthermore, the cathode block according to the invention is also suitable in particular for busbars with a rectangular cross-section. Especially in the case where a recess extending horizontally in the longitudinal direction of the cathode block is additionally provided in the slot wall of the cathode block, mechanical damage to the cathode block in the wall bordering the slot in the cathode block, such as cracking of the cathode block, with the busbar installed in the slot and encased in cast iron is reliably prevented even with large temperature changes. Here, the use of a slot of varying length in the longitudinal direction of the cathode block yields a current density distribution at the cathode block surface so homogeneous that when the electrolysis cell comprising the cathode block is in operation, an excess removal of cathode block material is effectively prevented in those areas where there would be a high local current density when a cathode block is used with a constant slot depth in the longitudinal direction of the cathode block.
By an appropriate adjustment of the slot depth, the current density distribution can be modified and evened out within broad limits. Considered as a whole, even with large or strong temperature changes, a control of tensile stresses, shear stresses and compressive stresses, which occur as a consequence of the different thermal expansion coefficients of cast iron, busbar and cathode block, is thus achieved which ensures an outstanding electrical conductivity and an excellent mechanical stability of the cathode block even with a busbar installed in the slot and encased in cast iron.
In some embodiments, in order to achieve an especially even vertical current density distribution at the cathode block surface during the electrolysis operation, it is proposed in a development of the inventive concept that at least one slot of the at least one slot or preferably all of the slots of varying depth has or have at its or their longitudinal ends less depth than at its or their center(s).
In this way, an even distribution of the electrical current supplied during electrolysis operation is achieved over the entire length of the cathode block, whereby an excessively high electrical current density is avoided at the longitudinal ends of the cathode block and thus premature wear at the ends of the cathode block is prevented. Due to such an even current density distribution over the length of the cathode block, movements in the aluminum melt caused by the interaction of electromagnetic fields during electrolysis are avoided, whereby it becomes possible to arrange the anode at a lower height above the surface of the aluminum melt. This reduces the electrical resistance between the anode and the aluminum melt and boosts the energy efficiency of the fused-salt electrolysis being carried out. Another special advantage of this embodiment is that with this design, the busbar(s) fixed in place by the at least one projection and possibly encased

12 in cast iron expand(s) horizontally during and after the increase in temperature occurring when the electrolysis cell is put into operation as a result of which the busbar(s) is or are pressed against the bottom wall of the cathode block slot(s) which bounds the slot at this point, whereby the transition resistance between the busbar(s) encased in cast iron and the cathode block is reduced.
In the aforementioned embodiment, the depth of at least one of the at least one slot of varying depth, when viewed in the longitudinal direction of the cathode block, increases with at least an essentially constant gradient preferably from one longitudinal end to the center of the cathode block and decreases with at least an essentially constant gradient preferably from the center of the cathode block to the other longitudinal end so that, when viewed in a longitudinal section of the cathode block, an at least essentially triangular slot is created. In this way, the aforementioned advantages are achieved to a greater degree.
According to a further embodiment of the present invention, the wall bordering the at least one slot of varying depth comprises a bottom wall and two side walls, wherein at the bottom wall a projection is provided which extends into the slot and which preferably extends vertically into the at least one slot. In this way, a particularly good fixing of the busbar in the slot can be achieved, namely with at the same time sufficiently great mobility of the busbar in the horizontal direction, in order to reliably prevent, even with large temperature changes, the occurrence of shear stresses as a consequence of the different coefficients of thermal expansion of cast iron, busbar and cathode block.
According to a further embodiment of the present invention, the at least one projection in the embodiment described above comprises on its side opposite the bottom wall at least one support surface for at least one busbar, which at least in sections runs at least essentially parallel, preferably parallel, to the surface of the cathode block side opposite the slot - in other words, orthogonal to the bottom end of the side wall of the wall bordering the slot in the cathode block. A
support surface of this kind is particularly suitable as a support for one busbar or as a support for two busbars.
In some embodiments, good results in this regard are obtained in particular if at least one of the at least one support surfaces of the at least one projection is designed so as to run planar, preferably at least essentially rectangular and parallel, especially preferably rectangular and parallel, with respect to the surface of the cathode block side opposite the slot.

.81792396

13 In some embodiments, in order to achieve a support surface large enough even for two adjacent end pieces of two busbars, it is proposed in a development of the inventive concept that the side opposite the bottom wall of the at least one projection be bounded by a support surface which is designed to run entirely planar, preferably at least essentially rectangular and parallel, especially preferably rectangular and parallel, with respect to the surface of the side of the cathode block opposite the slot.
In this embodiment, the entire projection surface opposite the bottom wall of the cathode block is thus designed as a support surface for one or more end pieces of one or more busbars.
The embodiment described above can for example be realized by designing the at least one projection, when viewed in section in the longitudinal extension of the cathode block, to be over its entire height at least essentially rectangular or trapezoidal, preferably rectangular or trapezoidal, wherein the side of the at least one projection opposite the bottom wall is bounded by a support surface, which is designed to run planar, at least essentially rectangular and parallel, preferably rectangular and parallel, with respect to the surface of the side of the cathode block opposite the slot.
In some embodiments, preferably, the extension of the rectangular support surface running in the longitudinal extension of the cathode block will measure 20 to 600 mm, especially preferably 50 to 400 mm, very especially preferably 100 to 300 mm, and maximally preferably 150 to 250 mm, for example, 200 mm, while on the other hand, the extension of the rectangular support surface running in the width direction of the cathode block will preferably measure at least 50%, further preferably at least 80%, especially preferably at least 90% and very especially preferably 100% of the width of the slot measured in the plane of the rectangular support surface.
In accordance with an embodiment of the present invention which is an alternative to the aforementioned embodiment and a particularly preferred embodiment, the side of the at least one projection opposite the bottom wall is bounded by a surface which comprises, when viewed in the longitudinal direction of the cathode block, two outer sections and between them one middle section, wherein the two outer sections in each case form a support surface for one busbar and in each case are designed to run planar, preferably at least essentially rectangular and parallel, especially preferably rectangular and parallel, with respect to the surface of the side of the cathode block opposite the slot, and which with respect to the depth of the slot are at the same height, while on the other hand the middle section, unlike the two outer sections, when viewed from the bottom wall, is designed with a raised form to extend into the slot.
This embodiment is

14 preferred in particular for cathode blocks which are designed to hold two busbars, approximately of half length in each case with respect to the length of the cathode block.
This is because due to the rise provided in the middle section of the projection, the two adjacent outer sections of the projection forming the support surface for in each case one end piece of a busbar are separated by a partition wall extending in the depth direction of the slot so that the end pieces of the two busbars rest on opposite sides of the partition wall, whereby the two busbars are fixed not only in the vertical direction but also at these two end pieces in the horizontal direction. The result hereby achieved is that the two busbars in the event of an expansion due to a temperature increase will expand in a defined direction, namely in the direction of the end of the cathode block. In this way, the two busbars, which may be encased in cast iron will, during and after the increase in temperature occurring when the electrolysis cell is being put into operation, be pressed in the horizontal direction as a result of the expansion in the direction of the cathode block end against the cathode block bottom wall bordering the slot at this point, whereby the transition resistance between the busbar encased in cast iron and the cathode block is reduced.
In some embodiments, in order to achieve a particularly high degree of the advantages described above, it is preferred that the middle section of the projection, when viewed in section in the longitudinal direction of the cathode block, be given a rectangular design -in other words, in the shape of a rectangular lug - such that a step is formed in each case between the two outer sections and the middle section. In the transitional region from the support surface to the raised part, this step can be rectangular or rounded-off.
In some embodiments, good results are in particular achieved when the height of the steps is 10 to 100 mm, preferably 40 to 80 mm and especially preferably 50 to 70 mm, while on the other hand, the extension of the steps running in the width direction of the cathode block is preferably at least 50%, further preferably at least 80%, especially preferably at least 90%
and very especially preferably 100% of the width of the slot.
The present embodiment can for example be realized by the at least one projection, when viewed in section in the longitudinal direction of the cathode block, being designed as at least essentially rectangular or trapezoidal, preferably rectangular or trapezoidal, over 20% to 80% and preferably over 30% to 50% of its height, wherein on the side of this section of the projection opposite the bottom wall, a rise or lug is provided centrally when viewed in the longitudinal direction of the cathode block and extends over the remaining height of the projection.

According to a further embodiment of the present invention, it is envisaged that the at least one projection, with respect to the longitudinal direction of the cathode block, is arranged at the place where the slot has its greatest depth, wherein the projection itself is not taken into consideration here. When, as has previously been presented as particularly preferable, the slot of varying depth 5 has a shallower depth at its longitudinal ends than in its center and in particular the depth of the slot, when viewed in the longitudinal direction of the cathode block, at least increases, at least essentially continuously, from one longitudinal end up to the center of the cathode block and decreases with at least an essentially constant gradient from the center of the cathode block to the other longitudinal end, the at least one projection is therefore preferably arranged centrally 10 with respect to the longitudinal extension of the cathode block.
In some embodiments, it has furthermore proved advantageous for the at least one projection to extend at least over 50% of the total width of the slot, preferably over 80%, especially preferably over 90% and very especially preferably over the entire width. This results firstly in the projection

15 having adequate mechanical stability and secondly in the busbar(s) resting with its or their end piece(s) on at least a major part of or the full width of the support surface(s) formed by the projection.
Basically the at least one projection can consist of any material, such as metal, for example. It is however preferred that the at least one projection consists of a material which has the same coefficient of thermal expansion as the material of the rest of the cathode block. It is especially preferred that the at least one projection consists of the same material as the remaining part of the cathode block.
According to the invention, the composition of the cathode block is based on carbon and / or graphite. With regard to a sufficiently high electrical conductivity and a sufficiently high resistance to wear, good results are in particular obtained here if the at least one projection and the rest of the cathode block are made of amorphous, graphitic and / or graphitized carbon.
In some embodiments, in a further development of the inventive concept, it is proposed that the at least one projection and the rest of the cathode block are monolithic, that is, are a single piece.
This provides a particularly high mechanical stability of the connection of the projection to the remaining part of the cathode block.

16 As an alternative to this, the at least one projection can even be attached by a connecting means to the bottom wall of the cathode block. This can for example be achieved by affixing the at least one projection to the remaining part of the cathode block by means of an adhesive such as for example synthetic resin, mastic, tar or similar substances or any mixture of the aforementioned substances or attaching said projection(s) mechanically to the remaining part of the cathode block by means of a fastening element.
Furthermore an aspect of the present invention concerns a cathode arrangement which contains at least one previously described cathode block, wherein in at least one of the at least one slot of varying depth in the at least one cathode block is provided at least one busbar, which is preferably at least partially encased in cast iron, wherein the busbar, which may be encased in cast iron, rests at least on a section of the at least one projection.
In some embodiments, the cathode arrangement preferably comprises at least one cathode block, wherein, in at least one of the at least one slot of varying depth in the at least one cathode block, two busbars preferably at least partially encased in cast iron are provided, which in each case rest with one of their end pieces at least on one section of the at least one projection.
According to a further embodiment of the present invention, the at least one busbar is encased at least in sections in cast iron and especially preferably completely in cast iron.
A further subject matter of an aspect of the present invention is a cathode arrangement which comprises at least one previously described cathode block or at least one previously described cathode arrangement.
Furthermore, an aspect of the present invention relates to the use of a previously described cathode block, a previously described cathode arrangement or a previously described cathode arrangement for carrying out a fused-salt electrolysis to produce metal, for example to produce aluminum.
In what follows, the present invention is described purely by way of example by means of advantageous embodiments and with reference to the accompanying drawings.
Here:

17 Fig. 1 shows a cross-section of a section of an aluminum electrolysis cell with a cathode arrangement in accordance with a first embodiment of the present invention, Fig. 2 shows a longitudinal section of the cathode arrangement of the aluminum electrolysis cell shown in Fig. 1, Fig. 3 shows a longitudinal section of a section of an aluminum electrolysis cell with a cathode arrangement in accordance with a second embodiment of the present invention, Fig. 4 shows a cross-section of the cathode arrangement of the aluminum electrolysis cell shown in Fig. 3, Fig. 5a-d shows examples of embodiments of cross-sections of recesses which are provided in a slot of a cathode block according to embodiments of the invention, Fig. 6 shows a longitudinal section of a cathode block in accordance with a third embodiment of the present invention, Fig. 7 shows a longitudinal section of a cathode block in accordance with a fourth embodiment of the present invention, and In Fig.1 in cross-section is shown a section of an aluminum electrolysis cell 10 with a cathode arrangement 12, which at the same time forms the bottom of a vat for an aluminum melt 14 produced during the operation of the electrolysis cell 10 and fora cryolite and aluminum oxide melt 16 located above the aluminum melt 14. An anode 18 is in contact with the cryolite and aluminum oxide melt 16. The vat formed by the lower part of the aluminum electrolysis cell 10 is laterally bounded by a carbon and / or graphite cladding not shown in Fig. 1.
The cathode arrangement 12 comprises a plurality of cathode blocks 20 which in each case are connected with each other via a ramming paste 24 inserted into a ramming paste gap 22 arranged between the cathode blocks 20. A cathode block 20 here comprises two slots 26 located on its underside having a rectangular, namely essentially rectangular cross-section,

18 wherein each slot 26 accommodates in each case one busbar 28 made of steel and also with a rectangular cross-section.
The slots 26 are in each case bounded by two side walls 32 and a bottom wall 34 of the cathode block 20, wherein in each of the side walls 32 a recess 36 extending essentially orthogonally into the side wall 32 and having an approximately semi-circular cross-section is provided. Each recess 36 is bounded by an upper and a lower transitional region 37 of the cathode block 20. The transitional regions 37 in the present exemplary embodiment are of angular form with an angle a of 90 between the adjacent section of the slot wall and the wall of the recess. The intermediate space between the busbar 28 and the slot 26 is here filled in each case with cast iron 38. Here, the cast iron 38 forms an encasement 39 for the busbar 28 and has a bonded connection to the busbar 28.
Furthermore, the cast iron 38 received in the recesses 36 forms a mechanical interlocking connection with the cathode block 20 material bordering the recess 36, said connection preventing the busbar 28 connected to the cast iron 38 from moving in the direction of the arrow 40.
In Fig. 1 in a specific example, the cross-section of the cathode arrangement 12 at one longitudinal end of the cathode block 20 is shown. The depth of the slot 26 of the cathode block 20 here varies over the length of the slot 26. The slot cross-section in the area of the center - with respect to the longitudinal direction of the cathode block - of the slot 26 is shown in Fig.
1 by the dashed line 42.
The difference between the slot depth at the longitudinal ends of the slot 26 and at the center - with respect to the longitudinal direction of the cathode block - of the slot 26 is in the present exemplary embodiment about 5 cm. Here, the depth of the slot 26 at the two longitudinal ends of the slot 26 is about 16 cm while on the other hand the depth of the slot 26 at the center -with respect to the longitudinal direction of the cathode block - of the slot 26 is about 21 cm.
The width 44 of each slot 26 is essentially constant over the entire slot length and measures approximately 15 cm, while on the other hand the width 46 of the cathode blocks 20 in each case measures about 42 cm.
In the present exemplary embodiment, several anodes 18 and several cathode blocks 20 are arranged one above the other in such a way that each anode 18 in its width covers two adjacently arranged cathode blocks 20 and in its length covers half of a cathode block 20, wherein in each case two adjacently arranged anodes 18 cover the length of one cathode block 20.

19 Fig. 2 shows a longitudinal section of the cathode block 20 shown in Fig. 1.
As can be seen from Fig. 2, the slot 26 when viewed in its longitudinal section tapers towards the center of the cathode block 20 in the form of a triangle, whereby an essentially homogeneous vertical electrical current density is ensured over the entire length of the cathode. Here, as indicated by the correspondingly labeled line in Fig. 2, the recess 36 runs parallel to the horizontal direction - in other words, parallel to the surface of the side of the cathode block 20 opposite the slot 26. The busbar 28, which for the sake of improved clarity is not shown in Fig. 2, has the shape of a bar in the present exemplary embodiment and has a rectangular cross-section, so that between the busbar and the slot bottom 34, there is an intermediate space which becomes larger towards the center of the slot 26, which can be filled either with cast iron 38 or by additional metal plates connected to the busbar 28.
The cathode arrangement and cathode block shown in longitudinal section and cross-section in Figs. 3 and 4 in accordance with a second exemplary embodiment of the present invention differ from that in Figs. 1 and 2 by the cathode block 20 only being provided with one slot 26, which has two recesses 36, 36'.
Furthermore, Fig. 5a to d show in cross-section examples of recesses 36 provided in a slot 26 of a cathode block 20 according to the invention. Here, the recesses 36 have in each case an essentially semi-circular cross-section (Fig. 5a), an essentially trapezoidal cross-section (Fig. 5b) or an essentially triangular cross-section (Fig. 5c). Here, the angle a of the transitional regions 37 between the wall of the recess 36 and the adjacent section of the slot wall 32 when viewed from the inside of the cathode block 20 measures about 90 in Fig. 5a, about 120 in Fig. 5b and about 125' in Fig.
5c. Fig. 5d shows an embodiment in which several recesses 36 of triangular cross-section as shown in Fig. 5c are arranged in succession in the direction of depth of the slot 26 in order to obtain a particularly reliable hold for an installed busbar 28. Here, the transitional regions 48 between two adjacent recesses 36 have an angle 13 of about 70 between the walls of two adjacent recesses 36 when viewed from the inside of the cathode block 20. The recesses 36 shown in Fig. 5a to d in each case extend orthogonolly into the side wall 32 of the cathode block 20 bordering the slot 26 so that with the cast iron received into the recesses 36, they form a fixing which is effective in the direction of depth of the slot 26 and which prevents an unwanted movement of the busbar 28 parallel to the direction of depth of the slot 26 after casting the busbar 28 with cast iron 38 but which allows a horizontal movement of the busbar encased in cast iron - for example, as a result of an expansion of the busbar encased in cast iron due to a large temperature change.
Fig. 6 shows a longitudinal section of a cathode block 20 in accordance with a third exemplary embodiment of the present invention, namely in contrast to those shown in Figs. 1 to 4 in this case upside down with respect to its later installation in the electrolysis cell so as to illustrate the arrangement while liquid cast iron is being poured in. This cathode block 20 differs from those shown in Figs. 1 to 4 in not having a recess in the wall bordering the slot 26. Instead, this cathode block 20 has a projection 50 in its slot 26 which, with respect to the longitudinal 5 direction of the cathode block 20, is arranged centrally and when viewed in section in the longitudinal direction of the cathode block is trapezoidal in form. Here, the surface bordering the side of the projection 50 opposite the bottom wall 34 of the cathode block 20 is designed to run planar, rectangular and parallel to the surface of the side of the cathode block opposite the slot and thus forms a support surface for the end pieces of two busbars 28.
Naturally, one recess, 10 as shown in Figs. 1 and 2, or two recesses, as shown in Figs. 3 and 4, can also be provided in at least one or in both of the side walls of the cathode block 20 bordering the slot 26.
In addition, a cathode block 20 in accordance with a fourth exemplary embodiment of the present invention is shown in Fig. 7, namely once again shown upside down in contrast to the ones in Figs.
15 1 to 4. This cathode block 20 differs from the ones shown in Fig. 6 in that the projection 50, shown cross-hatched here, is not trapezoidal in form, when viewed in section along the longitudinal direction of the cathode block, but is rectangular in form in its lower part, wherein on the side of this part of the projection 50 which is opposite the bottom wall 34 of the cathode block 20, there is a lug 54 which is arranged centrally when viewed in the longitudinal extension of the cathode block 20

20 and which extends over the remaining height of the projection 50. In other words, the side of the at least one projection 50 opposite the bottom wall 34 is bounded by a surface which comprises, when viewed in the longitudinal direction of the cathode block, two outer sections 52, 52' and arranged between them one middle section 54, wherein the two outer sections 52, 52' in each case form a support surface for one busbar 28 and in each case are designed to run planar, rectangular and parallel with respect to the surface of the side of the cathode block 20 opposite the slot 26, and with respect to the depth of the slot 26 are at the same height, while on the other hand the middle section 54, unlike the two outer sections 52, 52', when viewed from the bottom wall, is designed with a raised form to extend into the slot 26. Here, the middle section 54, when viewed in section in the longitudinal direction of the cathode block 20, is rectangular in form so that a step is formed in each case between the two outer sections 52, 52' and the middle section 54.
Naturally, one recess, as shown in Figs. 1 and 2, or two recesses, as shown in Figs. 3 and 4, can also be provided in at least one or in both of the side walls of the cathode block 20 bordering the slot 26.

21 List of reference numbers aluminum electrolysis cell 12, 12 cathode arrangement 5 14 aluminum melt 16 cryolite and aluminum oxide melt 18 anode cathode block

22 ramming paste gap 10 24 ramming paste 26 slot 28 busbar 32 side wall 34 bottom wall 15 36, 36' recess 37 transitional region between the wall of the recess and the adjacent section of the slot wall 38 cast iron 39 encasement 40 arrow 20 42 dashed line 44 width of slot 26 46 width of cathode block 20 48 transitional region between two adjacent recesses 50 projection 52, 52' outer section of the projection 54 middle section of the projection / lug a angle between the wall of the recess and the adjacent section of the slot wall angle between the walls of two adjacent recesses

Claims

CLAIMS:

1. A cathode block for an aluminum electrolysis cell on the basis of carbon and / or graphite, wherein the cathode block has at least one slot extending in the longitudinal direction of the cathode block for accommodating at least one busbar, wherein at least one of the at least one slot has a varying depth when viewed over the length of the cathode block, wherein at least one recess is provided in the wall of the cathode block bordering the at least one slot of varying depth, said recess extending horizontally in the longitudinal direction of the cathode block.

2. A cathode block according to claim 1, wherein the at least one of the at least one slot of varying depth has a shallower depth at its longitudinal ends than at its center.

3. A cathode block according to claim 1 or 2, wherein the wall bordering the at least one slot of varying depth comprises a bottom wall and two side walls, wherein each side wall comprises at least one recess which extends horizontally in the longitudinal direction of the cathode block.

4. A cathode block according to claim 3, wherein at least one of the at least one recess extends over at least 60% of the total length of the at least one slot.

5. A cathode block according to claim 4, wherein at least one of the at least one recess has a depth between 0.5 mm and 40 mm.

6. A cathode block according to claim 5, wherein at least one of the at least one recess has an opening width between 2 mm and 40 mm relative to the height of the cathode block.

7. A cathode arrangement which contains at least one cathode block according to any one of claims 1 to 6, wherein in at least one of the at least one slot of varying depth in at least one cathode block is provided at least one busbar, which has at least partially an encasement of cast iron or ramming paste, which at least in sections engages with the at least one recess.

8. A cathode block for an aluminum electrolysis cell based on carbon and /
or graphite, which has at least one slot extending in the longitudinal direction of the cathode block serving to accommodate at least one busbar, wherein at least one of the at least one slot is of varying depth when viewed over the length of the cathode block, wherein this slot is bounded by a wall, wherein at the wall, there is at least one projection which extends into the slot.

9. A cathode block according to claim 8, wherein the wall comprises a bottom wall and two side walls, wherein at the bottom wall, there is provided at least one projection extending into the slot.

10. A cathode block according to claim 9, wherein said at least one projection extends vertically into the at least one slot.

11. A cathode block according to any one of claims 8 to 10, wherein the at least one projection has on its side opposite the bottom wall at least one support surface for at least one busbar which at least in sections runs essentially parallel to the surface of the side of the cathode block opposite the slot.

12. A cathode block according to claim 11, wherein the side of the at least one projection opposite the bottom wall is bounded by a surface which comprises, when viewed in the longitudinal direction of the cathode block, two outer sections and arranged between them one middle section, wherein the two outer sections in each case form a support surface for one busbar and in each case are designed to run planar with respect to the surface of the side of the cathode block opposite the slot, and with respect to the depth of the slot are at the same height, while on the other hand the middle section, unlike the two outer sections, when viewed from the bottom wall, is designed with a raised form to extend into the slot.

13. A cathode block according to claim 12, wherein the two outer sections that in each case form a support surface for one busbar are in each case designed to run planar and at least essentially rectangular and parallel with respect to the side of the cathode block opposite the slot.

14. A cathode block according to claim 12 or 13, wherein the middle section, when viewed in section in the longitudinal direction of the cathode block, is designed rectangular in form so that a step is formed in each case between the two outer sections and the middle section.

15. A cathode block according to claim 12, wherein the at least one projection, with respect to the longitudinal extension of the cathode block, is arranged at the place where the slot has its greatest depth.

16. A cathode block according to claim 15, wherein the at least one projection is composed of the same material as the remaining part of the cathode block.

17. Use of a cathode block according to any one of claims 1 to 6 or 8 to 16 or of a cathode arrangement according to claim 7 to carry out a fused-salt electrolysis to produce metal.

18. Use of a cathode block according to any one of claims 1 to 6 or 8 to 16 or of a cathode arrangement according to claim 7 to carry out a fused-salt electrolysis to produce aluminum.