US20140110251A1

US20140110251A1 - Annular electrolysis cell and annular cathode with magnetic field compensation

Info

Publication number: US20140110251A1
Application number: US14/138,331
Authority: US
Inventors: Thomas Frommelt; Christian Bruch
Original assignee: SGL Carbon SE
Current assignee: SGL Carbon SE
Priority date: 2011-06-22
Filing date: 2013-12-23
Publication date: 2014-04-24
Also published as: AR086974A1; ZA201309289B; CA2838940A1; CN103764877A; DE102011078002A1; EP2723919A2; WO2012175419A2; AU2012271993A1; WO2012175419A3; RU2014101691A; JP2014517157A

Abstract

An electrolysis cell, in particular for producing aluminum, contains a cathode, a layer made up of liquid aluminum arranged on an upper side of the cathode, a melt layer, thereupon and an anode above the melt layer. 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. The electrolysis cell contains at least one further current supply arranged outside of the opening of the cathode, which current supply extends in the vertical direction at least in certain sections and which current supply is electrically connected to the cathode and/or to the anode.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation application, under 35 U.S.C. §120, of copending international application No. PCT/EP2012/061431, filed Jun. 15, 2012, which designated the United States; this application also claims the priority, under 35 U.S.C. §119, of German patent application No. DE 10 2011 078 002.5, filed Jun. 22, 2011; the prior applications are herewith incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to an electrolysis cell, in particular for producing aluminum, 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 aluminum, which is usually carried out industrially in accordance with the Hall-Héroult process. In the Hall-Héroult process, a melt composed of aluminum oxide and cryolite is electrolyzed. In this case, the cryolite Na₃[AlF₆] is used to reduce the melting point from 2,045° C. for pure aluminum oxide to approximately 950° C. for a mixture containing cryolite, aluminum oxide and additives, such as aluminum 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 conditions 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 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 aluminum located on the cathode upper side is an anode formed from individual anode blocks in particular, between which and the surface of the aluminum, the electrolyte, that is to say the melt containing the aluminum oxide and cryolite, is located. During the electrolysis carried out at approximately 1,000° C., the aluminum 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 aluminum oxide dissolved in the melt is split into aluminum and oxygen by electrical current flow. Seen electrochemically, the layer made up of liquid aluminum is the actual cathode, as aluminum ions are reduced to elementary aluminum 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 aluminum, 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-Héroult process is that it is very energy intensive. Approximately 12 to 15 kWh of electrical energy are required to create 1 kg of aluminum, which makes up 40% of the production costs. In order to be able to reduce the production costs, it is therefore 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 aluminum and the cathode material, relatively high ohmic losses in the form of Joule dissipation occur predominantly in the melt. Considering the comparatively high specific losses in the melt, there exists an endeavor to reduce the thickness of the melt layer and therefore the spacing between the anode and the layer made up of liquid aluminum to the greatest extent possible. However, due to the electromagnetic interactions present during the electrolysis and the wave formation caused thereby in the layer made up of liquid aluminum, there is the risk in the case of too small a thickness of the melt layer, that the layer made up of liquid aluminum comes into contact with the anode, which may lead to short circuits of the electrolysis cell and to undesired reoxidation of the aluminum 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 aluminum cannot be reduced arbitrarily.
The driving force for the wave formation in the layer made up of liquid aluminum and the melt layer arranged there above is the Lorentz force density generated there, which is defined as the vector product of the electric current density present at the respective point and the magnetic flux density present at the same point.
While the current density distribution in the anode and in the melt layer is comparatively homogeneous, the current density distribution in the aluminum 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 horizontally directed magnetic field to a high vertical Lorentz force density, which in turn, as illustrated, leads to a strongly pronounced wave formation, particularly in the aluminum layer. These strongly pronounced horizontal current density components in the direction of the cathode result from the effect that the current in the cathode and in the aluminum 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 supplying 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 aluminum layer, the inhomogeneous current density distribution and the increased current density at the lateral edge regions of the cathode compared to that in center 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 international patent disclosure WO 2007/118510 A2 (corresponding to U.S. Pat. No. 7,776,191) 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 aluminum is homogenized. Even in the case of an electrolysis cell of this type, a considerable wave formation takes place, particularly in the aluminum layer, however, as a consequence of which, the possibility of reducing the spacing between the anode and the layer made up of liquid aluminum is limited.
Irrespective of that, it is known for reducing wave formation in the layer made up of liquid aluminum and the melt layer to configure the current supply to the anode and to the cathode of the electrolysis cell using complex current supply geometries in such a manner that only small magnetic fields result in the region of the layer made up of liquid aluminum 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 possible. However, it proves exceptionally difficult to significantly reduce the wave formation in the layer made up of liquid aluminum 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 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.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to create an electrolysis cell which has 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 aluminum arising as a consequence of the thereby increased tendency to wave formation in the layer made up of liquid aluminum.
According to the invention, the object is achieved by an electrolysis cell and in particular by the provision of an electrolysis cell for producing aluminum, which contains a cathode, a layer made up of liquid aluminum on the upper side of the cathode, a melt layer e.g. containing cryolite thereupon and an anode above the melt layer. 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. The electrolysis cell contains at least one further current supply arranged outside of the opening of the cathode, which current supply extends in the vertical direction at least in certain sections and which current 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 tendency to wave formation in the aluminum layer, but also in particular a homogenizing of the magnetic field strength and therefore of the Lorentz force density distribution and the tendency to wave formation in the aluminum layer is achieved, specifically as viewed in particular via the cross section of the electrolysis cell. By 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 magnetic 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 cathode compensates the magnetic field generated by 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 magnetic fields can be optimized. In particular, if a plurality of external current 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 distribution can be achieved.
Thus, with the electrolysis cell according to the invention, individual regions with increased magnetic flux density, as are unavoidable in conventional electrolysis cells even when using complex current supply geometries, can likewise be effectively avoided as with the necessity of complex current supply geometries themselves. In particular, according to the invention, an exceptional reduction and homogenization 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 reduced wave formation in the layer made up of liquid aluminum 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 aluminum 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 increased considerably.
In the sense of the present invention, an opening extending vertically through the cathode is understood to mean an opening 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 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 truncated cone or the shape of a truncated pyramid. Equally, a current supply extending vertically through the opening is understood to mean a current 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 5° and most preferably at an angle of 0° 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 sections 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 aluminum, 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 opening of the cathode can also extend through the layer made up of liquid aluminum 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 aluminum, 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 aluminum, 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 aluminum, the melt layer arranged 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 aluminum, 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 of a cathode, a layer made up of liquid aluminum, a melt layer or an anode. At least one current supply is 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 contains not only an opening extending 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 cathode, layer made up of liquid aluminum, melt layer and anode over at least a part of its length arranged within the at least one opening and in particular over its entire length arranged within the opening, but rather electrically 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 respective length and/or be surrounded by an electrically insulating substance or medium, such as for example by air. If the at least one opening also extends through the layer made up of liquid aluminum and the melt layer, it is preferred that the inner current supply is electrically insulated from the layer made up of liquid aluminum and the melt layer at least over its entire length extending through the opening provided in the layer made up of liquid aluminum and in the melt layer and particularly preferably 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 aluminum or the melt layer, which forms a tank for the layer consisting of liquid aluminum and the melt layer, the tank preferably running annularly around the opening formed in the layer made up of liquid aluminum 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 of a refractory 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 aluminum, 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 aluminum, the melt layer and the anode, is understood 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 aluminum 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 realized 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 cathode. With this geometry, a particularly effective magnetic field compensation can be achieved within the layer made up of liquid aluminum 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 electrolysis 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 simple and cost-effective producibility and a good approximation of the preferred 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 multiple-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 six such cathode 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, particularly 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 compensation of the magnetic flux density is achieved in the region of the entire layer made up of liquid aluminum and the entire melt layer, specifically 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 aluminum, 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 understood to mean the diameter of the largest circle running in the horizontal plane which can be arranged in the opening of the respective constituent of the electrolysis cell without cutting the internal circumference of the opening. Analogously thereto, external diameter is in this case understood to mean the diameter of the smallest circle running in the horizontal plane which can be arranged around the external circumference of the respective constituent of the electrolysis cell without cutting the external circumference of the constituent.
In a development of the inventive idea, it is suggested that the electrolysis cell contains 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 compensate 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 particularly good magnetic field compensation is achieved if the number of current 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 spacing's, 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 concentrically surround the current supply extending through the opening.
Generally, the entire electrical cell current used for the electrolysis preferably 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 cathode and the further current supplies are preferably adapted to one another—for example by 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 aluminum and the melt layer.
In order to further reduce the wave formation in the layer made up of liquid aluminum and the melt layer, it is suggested in a development of the inventive idea that the cathode has at least two pin-shaped contacting elements 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 aluminum arranged there above and the melt layer in such a targeted manner that a particularly homogeneous current density distribution arises over the entire surface of the cathode. In this manner, horizontal current density components in the layer made up of liquid aluminum are avoided to the greatest extent possible, for which reason, wave formation in the layer made up of liquid aluminum 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-shaped contacting elements extends and preferably all contacting elements extend at an angle of less than 30° and preferably less than 10° with respect to the vertical and particularly preferably vertically 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 underside 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 spacing from the cathode underside.
If the contacting elements extend into the cathode, the same are preferably connected to the cathode via a screw connection, the contacting elements 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, aluminum, copper and/or carbon containing 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 meter of base area of the cathode. In the case of a density of this type, the distribution 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 aluminum 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 aluminum as a consequence of the magnetic field compensation by 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 material 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 particularly preferably between 15 and 50% by weight of the hard material. In this case, hard material is, in accordance with the usual technical definition 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 the addition of such a hard material, an abrasive wearing of the cathode during the operation thereof at the surface thereof facing the layer made up of liquid aluminum can be prevented or at least substantially reduced. For this purpose, the cathode can also be structured in two layers, namely composed of a cover layer provided on the side thereof facing the layer made up of liquid aluminum and a base layer lying there below, wherein the cover layer is constructed from the carbon composite material and/or graphite composite material comprising the hard material and the base layer is composed for example of hard-material-free graphite. In this case, the hard material can for example 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 preferably of at least 2,000 N/mm²and very particularly preferably of at least 2,500 N/mm²and can for example be selected from the group which consists 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, aluminum oxide and any desired chemical combinations and/or mixtures of two or more of the previously mentioned compounds.
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 aluminum is arranged and which for example can be formed by a cover layer of the cathode which contains a hard material, as described previously. Wave formation in the layer made up of liquid aluminum can be prevented particularly effectively during operation of the electrolysis cell by 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 electrolysis cell for producing aluminum, 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 particularly 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 direction, 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 approximately circular-segment-shaped or at least approximately trapezoidal outline, as viewed in a plan view. A basic shape of this type can be produced simply and is suitable in particular for producing an at least approximately circular cathode by the corresponding assembly of the individual cathode blocks. The cathode blocks can in each case be connected to one another by 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 external 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 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-shaped contacting element in each case on its underside. As a result, the option is created to contact 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 aluminum arranged 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-shaped 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 produced between a pin-shaped 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-shaped contacting element arranged in a recess of the cathode, the recess preferably having an internal thread on its inner side for such a screw connection.
The length of the recesses for the pin-shaped contacting elements is preferably between 100 and 500 mm and the diameter of the recesses for pin-shaped contacting elements is preferably between 30 and 200 mm. The recesses for pin-shaped contacting elements can be arranged at least in certain areas in a density of 4 to 1000 recesses per square meter of base area of the cathode. In the case of a density of this type, the distribution of the contacting elements inserted into the recesses can be adapted in such a targeted manner that an at least particularly even current density distribution results at the cathode surface.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an annular electrolysis cell and an annular cathode with magnetic field compensation, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a diagrammatic, cross-sectional view of an electrolysis cell according to the prior art;

FIG. 2 is a diagrammatic, plan view of a section of an electrolysis cell according to an embodiment of the invention with vertical contacting of a cathode;

FIG. 3 is a perspective view of a segment of the electrolysis cell according to an embodiment of the invention;

FIG. 4 is a perspective view of an electric current flow in the segment of the electrolysis cell shown in FIG. 3 according to an embodiment of the invention;

FIGS. 5A-5C are graphical illustrations of the electrical current density distribution at the cathode surface of a segment of the electrolysis cell as shown in the FIGS. 2, 3 and 4 according to an embodiment of the invention (FIG. 5A) and—for comparison—the electrical current density distribution at the surface of the cathode of a conventional electrolysis cell (FIG. 5B);

FIGS. 6A-C are graphical illustrations of the distribution of the magnetic flux density in a boundary surface between the layer made up of liquid aluminum and the melt layer of the segment of an electrolysis cell shown in the FIGS. 2, 3, and 4 according to an embodiment 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 aluminum and the melt layer of an electrolysis cell with conventional cathode (FIG. 6B);

FIG. 7 is a plan view of a cathode of the electrolysis cell according to an embodiment of the invention and a clear illustration of an exemplary method for the production thereof;

FIG. 8 is a plan view of the cathode of the electrolysis cell according to a further embodiment of the invention;

FIG. 9 is a plan view of the cathode of the electrolysis cell according to a further embodiment of the invention;

FIG. 10 is a perspective view of a segment of the electrolysis cell according to a further embodiment of the invention with horizontal contacting of the cathode;

FIG. 11 is a perspective view of an electrolysis cell according to a further embodiment of the invention;

FIG. 12 is a perspective view of an electrolysis cell according to a further embodiment of the invention;

FIG. 13 is a cross-sectional view of the electrolysis cell according to a further embodiment of the invention; and

FIG. 14 is a cross-sectional illustration of the electrolysis cell shown in FIG. 13 with an indication of the technical current flow direction.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown an electrolysis cell according to the prior art in cross section. The electrolysis cell contains a conventional square cathode 10′ which forms a cathode bottom, above which a layer 12 made up of liquid aluminum is located. The layer 12 made up of liquid aluminum borders a melt layer 14 arranged above the layer 12 made up of liquid aluminum. An anode 16 arranged above the melt layer 14 and also formed from a plurality 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 conductively connected to a bus bar 34 extending laterally into the cathode 10′.
FIG. 2 shows an electrolysis cell according to an embodiment of the present invention in a plan view. The electrolysis cell contains a cathode 10, a layer 12 (not illustrated) made up of liquid aluminum on the upper side of the cathode 10, a melt layer 14 (not illustrated) thereupon and an anode 16 (not illustrated) above the melt layer 14. The last-mentioned components 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 aluminum, 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 contains an opening 18 extending vertically, i.e. perpendicularly to the drawing plane in FIG. 2, through the cathode 10, in which an “inner” current supply 20 extending through the opening and electrically conductively connected to the anode 16 (not illustrated) is provided.
In addition to the inner current supply 20, the electrolysis cell has a plurality 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 spacing's around the opening 18.
The cathode 10 as viewed in a plan view 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 cathode 10 closely approximates a concentric circle and can be produced simply compared to a concentric circle.
The cathode 10 is in this case composed of a plurality of segments or 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 circumferential direction around the opening 18 next to one another in order to form the hexagonal ring-shaped cathode 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 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 perspective 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 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 current 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 electrical current source.
In FIG. 4, the electrical current flow in the segment of the electrolysis cell shown in FIG. 3 is visualized by arrows 31. The upwardly directed 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 being compensated for in the region of the cathode 10, the layer 12 made up of liquid aluminum, the melt layer 14 and the anode 16, so that only a very small and very homogeneously distributed magnetic flux density is present in the layer 12 made up of liquid aluminum 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 aluminum and the cathode 10 is supplied by the current supplies 20, 22. The division of the electrolysis current to the inner current supply 20 on the one hand and the external current supplies 22 on the other hand is preferably adapted in this case by the corresponding 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 contacting by the 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 aluminum and the melt layer 14 and wearing of the cathode 10 are reduced solely by 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 distribution of the vertical current density at the surface of the conventional cathode 10′ shown in FIG. 5B.
FIG. 5C 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 current 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 layers 12 made up of liquid aluminum and the melt layer 14 of a segment of an electrolysis 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 aluminum and the melt layer 14 of an electrolysis 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 aluminum 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 corresponds 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 visualized. As shown in FIG. 7, a plurality of trapezoidal cathode blocks 24 for the hexagonal ring-shaped cathode 10 can be produced simply in that an essentially square crude body 32 is cut into pieces transversely 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 embodiment 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 the pin-like contacting elements 28 (see FIGS. 3 and 4), however but rather by horizontal bus bars 34. Although in the case of this contacting of the cathode 10, such a pronounced homogenization 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 circumstances, due to the improved current supply to the anode 16 and the reduction and homogenization of the distribution of the magnetic flux density connected therewith, a considerable reduction of the wave formation in the layer 12 made of liquid aluminum and the melt layer 14 is nonetheless achieved, so that the stability and energy efficiency of the electrolysis cell is here also increased considerably.
FIG. 11 shows an electrolysis cell according to a further preferred embodiment in a perspective view, wherein the electrolysis cell is essentially composed of segments as shown in FIGS. 3 and 4. In this embodiment, the opening 18 extends vertically through the cathode 10 and additionally extends through the layer 12 made up of liquid aluminum, 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 aluminum and the melt layer 14 are located in a tank delimited by a tub, wherein the bottom of the tub is formed by the cathode 10, wherein 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 cathode 10, the layer 12 made up of liquid aluminum 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 corresponds 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 essentially corresponds to the electrolysis cells shown in the FIGS. 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 sidewall 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 material, 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 aluminum 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 aluminum and for this purpose—as viewed in plan view—are of somewhat narrower construction than the cathode 10, the layer made up of liquid aluminum 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 electrically 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 electrolysis cell additionally being illustrated in this Fig. by the arrows 44.

Claims

1. An electrolysis cell, comprising:

a cathode;

a layer made up of liquid aluminum disposed on an upper side of said cathode;

a melt layer disposed on said layer made of liquid aluminum;

an anode disposed above said melt layer;

at least one current supply;

said cathode having at least one opening formed therein and extending vertically through said cathode, in said opening said at least one current supply extending vertically through said opening and electrically connected to said anode and/or to said cathode; and

at least one further current supply disposed outside of said opening (18) of said cathode, said further current supply extending in a vertical direction at least in certain sections and said further current supply being electrically connected to said cathode and/or to said anode.

2. The electrolysis cell according to claim 1, wherein said opening is disposed centrally in said cathode as viewed in a plan view and said current supply extending through said opening extends centrally through said opening of said cathode.

3. The electrolysis cell according to claim 1, wherein said cathode has an at least approximately circular outline as viewed in a plan view.

4. The electrolysis cell according to claim 1, wherein said cathode has an at least approximately polygonal ring-shaped outline as viewed in a plan view.

5. The electrolysis cell according to claim 4, wherein said cathode has at least one of an external circumference or an internal circumference of said approximately polygonal ring-shaped, which is polygonal ring-shaped as viewed in a plan view, has a shape of a regular polygon with n corners, wherein n is 3 to 100.

6. The electrolysis cell according to claim 1, wherein said cathode is composed of a plurality of cathode blocks which, as viewed in a circumferential direction, are disposed around said current supply which extends through said opening, next to one another and adjoining one another, forming an annular cathode.

7. The electrolysis cell according to claim 6, wherein at least one of said cathode blocks of said cathode is shaped in a hexagonal, circular-segment-shaped or trapezoidal manner as viewed in a plan view.

8. The electrolysis cell according to claim 1, wherein a ratio between an internal diameter and an external diameter of said cathode is between 0.01 and 0.99.

9. The electrolysis cell according to claim 1, wherein said further current supply is one of between 2 and 10 further current supplies disposed outside of said opening of said cathode, which in each case extend in the vertical direction at least in sections and which are in each case electrically connected to said cathode and/or to said anode.

10. The electrolysis cell according to claim 9, wherein said further current supplies are disposed at least approximately regularly and concentrically as viewed in a circumferential direction of said cathode and as viewed around said current supply extending through said opening.

11. The electrolysis cell according to claim 1, wherein said cathode has an underside and at least two pin-shaped contacting elements on said underside.

12. The electrolysis cell according to claim 11, wherein said pin-shaped contacting elements extend at an angle of less than 30° with respect to the vertical and particularly vertically into said cathode.

13. The electrolysis cell according to claim 11, further comprising a common base plate, said pin-shaped contacting elements are electrically conductively connected on a side thereof which faces away from said cathode to said common base plate.

14. The electrolysis cell according to claim 1, wherein a spacing between said anode and said layer made up of liquid aluminum is between 15 and 45 mm.

15. A cathode for an electrolysis cell, the cathode comprising:

a cathode body having at least one opening formed therein and extending vertically through said cathode body.

16. The cathode according to claim 15, wherein said cathode body is shaped in an annular and at least approximately circular or polygonal ring-shaped manner as viewed in a plan view.

17. The cathode according to claim 16, wherein said cathode body has an external circumference outline and/or an internal circumference outline, which is polygonal ring-shaped as viewed in a plan view, has a shape of a regular polygon with n corners, wherein n is 3 to 100.

18. The cathode according to claim 15, wherein said cathode body has a plurality of cathode blocks which, as viewed in a circumferential direction, are disposed around said opening of said cathode body next to one another and adjoining one another.

19. The cathode according to claim 18, wherein at least one of said cathode blocks has a hexagonal, circular-segment-shaped outline or trapezoidal outline as viewed in a plan view.

20. The cathode according to claim 15, wherein said cathode body has an underside and at least two recesses formed therein for receiving one pin-shaped contacting element in each case on said underside.