Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
  • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
  • C25C3/08—Cell construction, e.g. bottoms, walls, cathodes

Definitions

• the present invention relates to a cathode block for an aluminum electrolytic cell.
• Such electrolysis cells are used for the electrolytic production of aluminum, which is usually carried out industrially by the Hall-Heroult process.
• a melt composed of alumina and cryolite is electrolyzed.
• the cryolite, Na 3 [AIF 6 ] serves to lower 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 method has a bottom composed of a plurality of adjacent cathode blocks forming the cathode.
• the cathode blocks are usually composed of a carbonaceous material.
• grooves are provided on the lower sides of the cathode blocks, in each of which at least one bus bar is arranged, through which the current supplied via the anodes is removed.
• the gaps between the individual walls delimiting the grooves of the cathode blocks and the busbars are often poured with cast iron in order to electrically and mechanically connect the busbars to the cathode blocks through the cast iron busbars produced thereby.
• the aluminum formed is deposited below the electrolyte layer due to its greater density compared to that of the electrolyte, ie as an intermediate layer between the upper side of the cathode blocks and the electrolyte layer.
• the aluminum oxide dissolved in the cryolite melt is split by the flow of electrical current into aluminum and oxygen.
• the layer of molten aluminum is the actual cathode because aluminum ions are reduced to elemental aluminum on its surface. Nevertheless, the term cathode will not be understood below to mean the cathode from an electrochemical point of view, ie the layer of molten aluminum, but rather the component forming the electrolytic cell bottom and composed of one or more cathode blocks.
• a major disadvantage of the Hall-Heroult process is that it is very energy intensive. To produce 1 kg of aluminum about 12 to 15 kWh of electrical energy is needed, which accounts for up to 40% of the manufacturing cost. In order to reduce the manufacturing costs, it is therefore desirable to reduce the specific energy consumption in this process as much as possible.
• graphite cathodes are increasingly used in recent times, ie those of cathode blocks, which contain graphite as the main component.
• graphitic cathode blocks the production of which is used as the starting material graphite
• graphitized cathode blocks for their preparation as a starting material, a carbon-containing graphite precursor is used, which is converted by a subsequent heat treatment at 2,100 to 3,000 ° C to graphite.
• Graphite is characterized by a significantly lower specific electrical resistance and by a significant compared to amorphous carbon higher thermal conductivity, which is why the use of graphite cathodes in the electrolysis on the one hand, the specific energy consumption of the electrolysis can be reduced and on the other hand, the electrolysis can be carried out at a higher current, which allows an increase in the productivity of each electrolysis cell.
• graphite cathode blocks, and particularly graphitized cathode blocks are subject to heavy wear during electrolysis due to surface erosion, which is significantly greater than the wear of cathode blocks of amorphous carbon.
• This removal of the cathode block surfaces is not uniform over the longitudinal direction of the cathode block, but to an increased extent at the edge regions of the cathode block, in which the operation of the cathode block, the largest local electric current density occurs. This is because, in the peripheral areas, the contacting of the bus bars with the power supply elements occurs, and therefore, the resulting electrical resistance from the power supply elements to the surface of the cathode block is less when flowing over the peripheral areas of the cathode block than when flowing over the center of the cathode block.
• the surface of the cathode blocks changes with increasing operating time, viewed in the longitudinal direction of the cathode blocks, to an approximately W-shaped profile, whereby due to the non-uniform Abtrag the service life of the cathode blocks is limited by the locations with the largest removal.
• mechanical influences increase the wear of a cathode block during electrolysis. Since the molten aluminum layer is constantly in motion due to the high magnetic fields prevailing during the electrolysis and the resulting electromagnetic interactions, a considerable erosion of particles occurs on the cathode block surface, which in the case of graphite cathode blocks leads to considerably greater wear than cathode blocks of amorphous carbon leads.
• DE 197 14 433 C2 discloses a cathode block with a coating containing at least 80% by weight of titanium diboride, which is produced by plasma spraying of titanium diboride onto the surface of the cathode block.
• This coating is intended to improve the abrasion resistance of the cathode block.
• Such coatings of pure titanium diboride or with a very high content of titanium diboride are very brittle and therefore susceptible to cracking.
• the specific thermal expansion of these coatings is about twice that of carbon, which is why the coating of such a cathode block when used in a fused-salt electrolysis has only a short life.
• cathode blocks have recently been used, the sides of which, during operation of the electrolytic cell, are profiled by one or more recesses and / or elevations facing the molten aluminum and electrolyte side.
• Such cathode blocks whose tops each have between 1 and 8 and preferably 2 elevations with a height of 50 to 200 mm, are disclosed, for example, in EP 2 133 446 A1. Due to the profiled surface, the movement of the molten aluminum caused by the electromagnetic interaction in the electrolysis is reduced. This results in less corrugation and bulging of the aluminum layer.
• the distance between the molten aluminum and the anode which due to the comparatively strong and intense wave formation of the aluminum layer in the use of non-surface profiled cathode blocks to avoid short circuits and unwanted reoxidation of the aluminum formed usually 4 to 5 by the use of the surface profiled cathode blocks cm, can be reduced to 2 to 4 cm. Due to this reduction in the distance between the molten aluminum and the anode is the electrical cell resistance due to the reduction of the ohmic resistance and thus reduces the specific energy consumption.
• surface profiled cathode blocks and in particular surface profiled cathode blocks based on graphite have a number of disadvantages.
• sludge of undissolved aluminum oxide with adhering cryolite melt can settle.
• surfaces of aluminum melt consisting of graphite are only very poorly wetted.
• surface-profiled cathode blocks based on graphite in particular at the corners and edges of the elevations of their profiled surfaces, are very susceptible to wear.
• the deposited on the profiled surfaces of the cathode blocks sludge reduces the effective cathode surface and thereby impedes the flow of current, whereby the specific energy consumption is increased. This effect additionally increases the current density locally, which can lead to a shorter service life of the electrolysis cell.
• the object of the present invention is therefore to provide a cathode block which is surface-profiled in order to allow a small distance between the molten aluminum and the anode in the electrolytic cell, which has a low electrical resistivity, which is preferably wettable well with molten aluminum, and which in particular has a high abrasion resistance and wear resistance to the abrasive, chemical and thermal conditions prevailing during operation in a fused-salt electrolysis.
• a cathode block for an aluminum electrolytic cell having a base layer and a cover layer, wherein the base layer contains graphite, the cover layer has an at least partially profiled surface and the cover layer of a 1 to less is composed of 50 wt .-% hard material having a melting point of at least 1, 000 ° C containing graphite composite material.
• This solution is based on the finding that by providing an at least partially surface-profiled cover layer of a graphite composite material, which not less than 1 wt .-%, but not more than less than 50 wt .-% hard material having a melting point of at least 1, 000 ° C. contains, on a graphite-containing base layer, a cathode block is obtained, which has sufficient for an energy-efficient operation of fused electrolysis low electrical resistivity and also very abrasionsbestteil and therefore wear resistant to the prevailing in the molten state electrolysis abrasive, chemical and thermal conditions.
• the cathode block according to the present invention combines the surface profiling of the side of the cathode block facing the melt during operation of the electrolytic cell and the provision of graphite in the base layer and in the top layer of the cathode block advantages associated - in particular low electrical resistance of the cathode block and low wave and wave height of the aluminum melt in the use of the cathode block in a fused-salt electrolysis of alumina in a cryolite melt, so that the distance between the surface of the layer of molten aluminum and the anode in the electrolytic cell, for example, to 1 to 4 cm, and preferably to 2 to 3 cm can be reduced, which reduces the specific energy consumption of the electrolysis process; at the same time, however, the cathode block according to the present invention does not have the disadvantages resulting from the use of graphite, such as lack of wettability by aluminum melt and in particular a low abrasion or wear resistance and not the disadvantages resulting from the use of surface profiling, such as
• the cathode block of the present invention is comprised of performing fused-salt electrolysis with an alumina and cryolite
• hard material in accordance with the definition of this term in the art is understood to mean a material which is characterized by a particularly high hardness, especially at high temperatures of 1000 ° C. and higher.
• the melting point of the hard material used is considerably higher than 1 .000 ° C, in particular hard materials having a melting point of at least 1, 500 ° C, preferably hard materials having a melting point of at least 2000 ° C and more preferably hard materials having a melting point of at least 2,500 ° C have been found to be particularly suitable.
• all hard materials can be used in the cover layer of the cathode block according to the invention.
• hard materials which have a Knoop hardness of at least 1 000 N / mm 2 , preferably of at least 1 500 N / mm 2 , particularly preferably of at least 2000 N / mm, measured according to DIN EN 843-4 2 and most preferably of at least 2,500 N / mm 2 .
• the covering layer of the cathode block according to the invention contains as hard material a hard carbon material having a Knoop hardness measured according to DIN EN 843-4 of at least 1 000 N / mm 2 , preferably of at least 1 500 N / mm 2 , more preferably at least 2,000 N / mm 2 and most preferably at least 2,500 N / mm 2 .
• carbon material is meant in particular a material containing more than 60% by weight, preferably more than 70% by weight, more preferably more than 80% by weight and most preferably more than 90% by weight of carbon.
• the carbon material is preferably a material selected from the group consisting of coke, anthracite, carbon black, glassy carbon and Mixtures of two or more of the aforementioned materials, and more preferably coke.
• This group of compounds is also referred to below as "non-graphitizable carbons", in the sense of non-or at least poorly graphitizable carbon according to the German patent application DE 10 2010 029 538.8, to which reference is made in this regard.
• Bad graphitizable cokes are especially hard coke, such as acetylene coke.
• the top layer of the cathode block according to the invention as a hard material carbon material, preferably selected from coke, anthracite, carbon black and glassy carbon and particularly preferably coke, with a low graphitization contains.
• Graphitized cathode blocks are made by mixing a carbon-containing graphite precursor with binder and shaping this mixture into the shape of a cathode block, then carbonizing and finally graphitizing.
• carbon material with a low graphitability is understood as meaning a carbon material which has a heat treatment according to Maire and Mehring (J.Maire, J.Mehring, Proceedings of the 4th Conference on Carbon, Pergamon Press 1960 pages 345 to 350) at 2,800 ° C from the average layer spacing c / 2 calculated graphitization degree has a maximum of 0.50.
• Good results are obtained in particular when the carbon material, preferably selected from Coke, anthracite, carbon black and glassy carbon, a degree of graphitization of not more than 0.4, and more preferably of at most 0.3.
• the cover layer of the cathode block according to the invention 1 to 25 wt .-%, particularly preferably 10 to 25 wt .-% and most preferably 10 bis Contains 20 wt .-% of the carbon material as hard material.
• the carbon material used as hard material in the cover layer of the cathode block according to the invention preferably selected from coke, anthracite, carbon black and glassy carbon and particularly preferably coke, has a particle size of up to 3 mm and preferably of up to 2 mm.
• the individual particles have an onion shell structure, which in the context of the present invention is understood to mean a multilayer structure in which an inner layer of particles with a spherical to ellipsoidal shape is completely or at least partially covered by at least one intermediate layer and one outer layer.
• a carbon material preferably selected from coke, anthracite, carbon black and glassy carbon and particularly preferably coke
• the hard material in which the apparent stack height of the carbon material after a temperature treatment of 2,800 ° C. is preferably less than 20 nm, whereas the BET specific surface area of the carbon material particles is preferably 10 to 40 m 2 / g, and more preferably 20 to 30 m 2 / g.
• a preferred example of coke having a low graphitization degree mentioned above is coke which is by-produced in the production of unsaturated hydrocarbons, especially acetylene, and subsequently, regardless of the type of unsaturated hydrocarbon which it produces, is called acetylene coke.
• Acetylene coke which is obtainable from the crude oil fractions or steam cracking residues used in the quenching of reaction gas in the synthesis of unsaturated hydrocarbons, in particular acetylene, has proven particularly suitable for this purpose.
• the quench oil or carbon black mixture is passed to a coker heated to about 500.degree.
• liquid components of the quench oil evaporate while the coke collects on the bottom of the coker.
• a corresponding method is described for example in DE 29 47 005 A1.
• a fine-grained, onion-like coke which preferably has a carbon content of at least 96 wt .-% and an ash content of at most 0.05 wt .-% and preferably of at most 0.01 wt .-%.
• the acetylene coke preferably has a crystallite size in the c direction L c of less than 20 nm, the crystallite size in the a direction L a being preferably less than 50 nm and particularly preferably less than 40 nm.
• coke which may be used as a hardstock in addition to or as an alternative to acetylene coke is coke made by fluidized bed processes such as the Flexicoking process developed by Exxon Mobile, a thermal cracking process using fluidized bed reactors. Coke with spherical to ellipsoidal shape is obtained by this method, which is constructed onion-like.
• coke coke formed by delayed coke formation The particles of this coke have a spherical morphology.
• the cover layer of the cathode block according to the invention contains graphite, preferably graphitized carbon and optionally carbonized and / or graphitized binder, such as pitch, especially coal tar pitch and / or petroleum pitch, tar, bitumen, phenolic resin or furan resin. If pitch is mentioned below, it means all pitches known to those skilled in the art.
• the graphite or preferably graphitized carbon together with the carbonized and / or graphitized binder forms the matrix in which the hard material is embedded.
• the cover layer contains 99 to 50 wt .-%, preferably 99 to 75 wt .-%, particularly preferably 90 to 75 wt .-% and most preferably 90 to 80 wt .-% carbon.
• the cover layer of the cathode block according to the invention contains as hard material a non-oxide ceramic, which preferably consists of at least one metal of the 4th to 6th subgroup and at least one element of the 3rd or 4th main group of the periodic table Elements is composed.
• a non-oxide ceramic which preferably consists of at least one metal of the 4th to 6th subgroup and at least one element of the 3rd or 4th main group of the periodic table Elements is composed.
• these include in particular metal carbides, borides, metal nitrides and metal carbonitrides with a metal of the 4th to 6th subgroup, such as titanium, zirconium, vanadium, niobium, tantalum, chromium or tungsten.
• suitable representatives of these groups are compounds selected from the group consisting of titanium diboride, zirconium diboride, tantalum boride, titanium carbide, boron carbide, titanium carbonitride, silicon carbide, tungsten carbide, vanadium carbide, titanium nitride, boron nitride, silicon nitride, zirconium dioxide, alumina, and any chemical combinations and / or mixtures of two or more of the aforementioned compounds. Good results are obtained in particular with titanium diboride, titanium carbide, titanium carbonitride and / or titanium nitride.
• the cover layer of the cathode block according to the invention most preferably contains titanium diboride as the hard material. All of the aforementioned hard materials can be used alone or any combination and / or mixture of two or more of the aforementioned compounds can be used.
• the hard material contained in the cover layer of the cathode block according to this second particularly preferred embodiment has a monomodal particle size distribution, the average volume-weighted particle size (d 3 ) determined by static light scattering in accordance with International Standard ISO 13320-1 5o) is 10 to 20 pm.
• non-oxide ceramics as a hard material in particular non-oxide titanium ceramics and especially titanium diboride, with a monomodal particle size distribution as defined above not only cause very good wettability of the surface of the cathode block, therefore sludge formation and sludge deposition on the surface of the cathode block are reliable is prevented, but in particular also leads to excellent abrasion resistance and thus wear resistance of the cathode block.
• this effect is particularly effective even with comparatively small amounts of ceramic hard material, preferably titanium dioxide.
• boride of less than 50 wt .-% and particularly preferably even in amounts of only 10 to 20 wt .-% is achieved in the cover layer.
• This can be dispensed with a high concentration of ceramic hard material in the cover layer, which leads to a brittle cathode block surface.
• ceramic hard material having a monomodal particle size distribution as defined above is also distinguished by very good processability.
• the dust tendency of such a hard material for example, when filling in a mixing container or during the transport of the hard material powder is sufficiently low and occurs, for example, when mixing at most a small agglomeration.
• such a hard material powder has a sufficiently high flowability and flowability, so that it can be conveyed for example with a conventional conveying device to a mixing device.
• a conventional conveying device to a mixing device.
• the cathode blocks according to the invention not only follows a simple and cost-effective manufacturability of the cathode blocks according to the invention, but in particular also follows a very homogeneous distribution of the hard material in the top layer of the cathode blocks.
• the hard material contained in the cover layer of the cathode block according to the second very particularly preferred embodiment of the present invention preferably titanium diboride, preferably has a monomodal particle size distribution, the average volume-weighted particle size (d 3 5 o) determined above being from 12 to 18 ⁇ m and particularly preferred 14 to 16 pm.
• the ceramic hard material contained in the cover layer of the cathode block may have a monomodal particle size distribution, wherein the average volume-weighted particle size (d 3 , 5 o) determined by static light scattering in accordance with International Standard ISO 13320-1 is 3 to 10 ⁇ m and preferably 4 to 6 pm.
• the ceramic hard material should have a volume-weighted volume as determined above
• d3.90 has particle size of 20 to 40 pm, and preferably from 25 to 30 pm.
• the ceramic hard material preferably has such a d 3 9 o value in combination with a d 3 5 o value as defined above.
• the ceramic hard material is preferably a non-oxidic titanium ceramic and more preferably titanium diboride.
• the ceramic hard material contained in the cover layer of the cathode block may have a volume-weighted d 3 9 o particle size of from 10 to 20 ⁇ m, and preferably from 12 to 18 ⁇ m, as determined above.
• the ceramic hard material has such a d3.90 value in combination with a d 3 5 o value defined above.
• the ceramic hard material has a volume-weighted d 3 i o particle size of from 2 to 7 m, and preferably from 3 to 5 m, as determined above.
• the hard material preferably has such a d 3 -m value in combination with a d 3 9 o value and / or d 3 5 o value as defined above.
• the hard material is preferably a non-oxidic titanium ceramic and more preferably titanium diboride.
• the ceramic hard material contained in the cover layer of the cathode block may have a volume-weighted d 3 - ⁇ particle size of from 1 to 3 ⁇ m, and preferably from 1 to 2 ⁇ m, as determined above.
• the hard material preferably has such a d 3 - ⁇ value in combination with a d 3 9 o value and / or d 3 5 o value as defined above.
• non-oxidic ceramic as hard material in particular a non-oxidic titanium ceramic and particularly preferably titanium diboride, has a particle size distribution which is determined by a span value calculated according to the following equation:
• Span (d 3 , 9o - d 3 , io) d 3 , 5o characterized from 0.65 to 3.80 and particularly preferably from 1, 00 to 2.25.
• the hard material to such a tension value in combination with a defined above d 3 9 o-value and / or d 3 5 o-value and / or d 3 -m value.
• non-oxidic ceramic hard materials in the cover layer of the cathode block according to the invention are in particular non-oxidic titanium ceramics, such as preferably titanium carbide, titanium carbonitride, titanium nitride and most preferably titanium diboride.
• the hard material to at least 80% by weight, preferably at least 90 wt .-%, more preferably at least 95 wt .-%, most preferably at least 99 wt. % and most important zugt completely of non-oxide ceramic, preferably non-oxidic titanium ceramic and particularly preferably consists of titanium diboride.
• the total amount of the ceramic hard material in the cover layer is according to the invention at least 1 wt .-%, but at most less than 50 wt .-%.
• the cover layer contains sufficient hard material to impart on the one hand the cover layer to increase the wear resistance excellent hardness and abrasion resistance, and on the other hand to give a sufficiently high wettability of the cover layer surface with liquid aluminum to avoid sludge formation and sludge deposition whereby the wear resistance of the cathode block is further increased and the specific energy consumption during a fused-salt electrolysis is further reduced;
• the cover layer contains a sufficiently low amount of hard material, so that the surface of the cover layer does not have too high a brittleness due to the addition of hard material for a sufficiently high long-term stability.
• top layer in the second very particularly preferred embodiment of the present invention 5 to 40 wt .-%, particularly preferably 10 to 30 wt .-% and most preferably 10 to 20 wt .-% of a non-oxidic Ceramics, preferably a non-oxidic titanium ceramic and particularly preferably titanium diboride, as a hard material having a melting point of at least 1 000 ° C.
• the cover layer of the cathode block according to the second particularly preferred embodiment of the present invention contains graphitic or preferably graphitized carbon and optionally carbonized and / or graphitized binder such as pitch, especially coal tar pitch and / or petroleum pitch, tar, bitumen , Phenolic resin or furan resin. It forms the graphitic or preferably graphitized carbon together with the optional binder, the matrix in which the ceramic hard material is embedded. Good results are obtained, in particular, if the cover layer contains 99 to more than 50% by weight, preferably 95 to 60% by weight, particularly preferably 90 to 70% by weight and very particularly preferably 90 to 80% by weight of graphite ,
• the cover layer has a vertical specific electrical resistance at 950 ° C. of 5 to 20 .mu.m.sup.-3 and preferably of 9 to 13 .mu.m. This corresponds to a vertical specific resistance at room temperature of 5 to 25 ⁇ pm or 10 to 15 ⁇ pm.
• vertical specific electrical resistance is understood as meaning the specific electrical resistance in the installation situation in the vertical direction of the cathode block.
• the thickness of the cover layer should be as small as possible in order to keep the cost of a hard material expensive in the case of ceramics as low as possible, but sufficiently large for the cover layer to have sufficiently high wear resistance and service life.
• the good properties of the cathode base body should be impaired as little as possible by using the lowest possible cover layer. Good results are obtained with respect to these reasons, in particular, when the thickness of the cover layer 1 to 50%, preferably 5 to 40%, more preferably 10 to 30% and most preferably 15 to 25%, for example about 20%, of the total height of the cathode block is.
• the cover layer may have a thickness or height of 50 to 400 mm, preferably 50 to 200 mm, more preferably 70 to 180 mm, most preferably 100 to 170 mm and most preferably about 150 mm. Under thickness or height is the distance from the bottom the top layer understood to the point of the highest elevation of the top layer.
• the cover layer of the cathode block has an at least partially profiled surface. Due to the profiled surface, the movement of the molten aluminum caused by the electromagnetic interaction in the electrolysis is reduced, resulting in less corrugation and bulging of the aluminum layer. For this reason, by using surface-profiled cathode blocks, the distance between the molten aluminum and the anode can be further reduced so that the cell electrical resistance is further reduced due to the reduction of the ohmic resistance and thus the specific energy consumption.
• a profiled surface is understood here to mean a surface which has at least one depression and / or elevation extending in the transverse direction, in the longitudinal direction or in any other direction, for example in a direction extending at an acute or obtuse angle to the longitudinal direction, of the cathode block
• the depression has at least a depth or height of 0.05 mm and preferably of 0.5 mm.
• the at least one depression and / or elevation may be limited exclusively to the cover layer, or the at least one depression and / or elevation may extend into the base layer.
• the at least one depression and / or elevation extends exclusively in the cover layer.
• the at least one depression and / or elevation, seen in the transverse direction of the cathode block can have any desired geometry.
• the at least one depression or elevation, in the transversal View of the cathode block convex, concave or polygonal, such as trapezoidal, triangular, rectangular or square, be formed.
• the surface profiling comprises at least one depression, the ratio of depth to width of the at least one depression being 1: 3 to 1: 1 and preferably 1: 2 to 1: 1.
• the depth of the at least one recess is 10 to 90 mm, preferably 40 to 90 mm and particularly preferably 60 to 80 mm, for example about 70 mm.
• the width of the at least one recess is 100 to 200 mm, more preferably 120 to 180 mm and most preferably 140 to 160 mm, such as about 150 mm.
• the at least one depression viewed in the longitudinal direction of the cathode block, to extend only in regions.
• the at least one recess extend the entire length of the cathode block to achieve the effect of reducing or completely reducing the formation of waves of liquid aluminum.
• the depth and / or width of the at least one recess varies over the length of the cathode block.
• the geometry of the recess can vary over the length of the cathode block.
• the surface profiling comprises at least one elevation, it is also for the reason of corrugation in the operation of erfindungsge- In order to avoid or at least considerably reduce the cathode block in the fused-salt electrolysis of alumina in a cryolite melt, and to drastically reduce the height of the waves that form, it is preferred that the height to width ratio of the at least one bump be 1: 2 to 2 : 1 and preferably about 1: 1.
• the height of the at least one elevation is 10 to 150 mm, preferably 40 to 90 mm and particularly preferably 60 to 80 mm, for example about 70 mm.
• the width of the at least one protrusion is 50 to 150 mm, more preferably 55 to 100 mm and most preferably 60 to 90 mm, such as about 75 mm.
• the at least one elevation viewed in the longitudinal direction of the cathode block, extends only in regions.
• the at least one projection extends over the entire length of the cathode block.
• the height and / or width of the at least one bump varies over the length of the cathode block.
• the geometry of the survey can vary over the length of the cathode block.
• the ratio of the width of the at least one recess to the width of the at least one projection is preferably 4: 1 to 1: 1, such as about 2: 1.
• the present invention is not limited. Good results are obtained, for example, when the cathode block has in its transverse direction 1 to 3 wells and preferably 2 wells.
• the base layer is at least 80% by weight, preferably at least 90% by weight, more preferably at least 95% by weight, most preferably at least 99% by weight. and most preferably entirely composed of a mixture of graphite and binder, such as carbonized graphitized pitch.
• a base layer has a suitably low electrical resistivity.
• this mixture is preferably from 70 to 95 wt .-% graphite and 5 to 30 wt .-% binder and more preferably from 80 to 90 wt .-% graphite and 10 to 20 wt .-% binder, such as 85 wt % Graphite and 15% by weight carbonized or graphitized pitch, respectively.
• both the upper side of the base layer and the lower side of the cover layer and thus also the interface between the base layer and the cover layer are configured substantially planar.
• Both layers of the catho- The blocks can be connected together by a shaking process or by a pressing process in the green state.
• substantially planar is understood in this context that the base layer is not profiled and the profile is provided with a cover layer.
• an intermediate layer may be provided between the base layer and the cover layer which, for example, is constructed like the cover layer, except that the intermediate layer has a lower concentration of hard material than the cover layer.
• the base layer has a vertical electrical resistivity at 950 ° C of 13 to 18 ⁇ ⁇ and preferably from 14 to 16 ⁇ ⁇ . This corresponds to vertical electrical resistances at room temperature of 14 to 20 ⁇ pm or 16 to 18 ⁇ m.
• a further subject of the present invention is a cathode which contains at least one cathode block described above, wherein the cathode block has at least one groove on the side of the base layer opposite the cover layer, wherein at least one bus bar is provided in the at least one groove in order to move the cathode during to supply electricity to the electrolysis.
• the at least one busbar at least partially, and particularly preferably full-circumference has a cladding of cast iron .
• This enclosure can be made by inserting the at least one bus bar into the groove of the cathode block and then into the space between the busbar and the groove bounding walls cast iron is filled.
• Another object of the present invention is the use of a previously described cathode block or a previously described cathode for performing a fused-salt electrolysis for the production of metal, in particular of aluminum.
• the cathode block or the cathode is used for carrying out a fused-salt electrolysis with a melt of cryolite and aluminum oxide for the production of aluminum, wherein the fused-salt electrolysis is particularly preferably carried out as a Hall-Heroult process.
• FIG. 1 shows a schematic cross section of a section of a
• FIGS. 2a to 2E each show a schematic cross section of the surface profiling of a cathode block according to other embodiments of the present invention.
• FIG. 1 shows a cross-section of a section of an aluminum electrolytic cell 10 with a cathode 12, which at the same time covers the bottom of a trough for electrowetting.
• aluminum melt 14 produced during the operation of the electrolysis cell 10 and forms a cryolite-alumina melt 16 located above the molten aluminum 14. With the cryolite-alumina melt 16 is an anode 18 of the electrolytic cell 10 in contact. Laterally, the trough formed by the lower part of the aluminum electrolytic cell 10 is limited by a lining of carbon and / or graphite, not shown in FIG. 1.
• the cathode 12 comprises a plurality of cathode blocks 20, 20 ', 20 ", which are each connected to one another via a ramming mass 24, 24' inserted into a ramming mass gap 22, 22 'arranged between the cathode blocks 20, 20', 20".
• the anode 18 includes a plurality of anode blocks 26, 26 ', with the anode blocks 26, 26' being each about twice as wide and about half as long as the cathode blocks 20, 20 ', 20 ", with the anode blocks 26, 26' being such arranged above the cathode blocks 20, 20 ', 20 "such that one anode block 26, 26' in width covers two adjacent cathode blocks 20, 20 ', 20" and one cathode block 20, 20', 20 "in length two adjacent anode blocks 26, 26 'covers.
• Each cathode block 20, 20 ', 20 consists of a lower base layer 30, 30', 30" and a covering layer 32, 32 ', 32 "arranged above it and firmly connected therewith.
• the acetylene coke contained in the cover layers 32, 32 ', 32 has a particle size of 0.2 to 1 mm.
• Each cathode block 20, 20 ', 20 has a width of 650 mm and a total height of 550 mm, based on the highest point of the cover layer 32, 32', 32", the base layers 30, 30 ', 30 ".
• Each cover layer 32, 32 ', 32 has a profiled surface, wherein in each cover layer 32, 32', 32" two in cross-section substantially rectangular recesses 34, 34 'are provided, which are each separated from a survey 36 from each other. While the width of the depressions 34, 34 'is 150 mm in each case and the depth of the depressions 34, 34' is 70 mm in each case, the elevation 36 has a width of 75 mm and a height of 70 mm. Both the corners in the two recesses 34, 34 'and the corners of the elevation 36 are each rounded off with a radius of 20 mm.
• each cathode block 20, 20 ', 20 on its underside in each case two grooves 38, 38', each having a rectangular, namely substantially rectangular cross-section, wherein in each groove 38, 38 'in each case a busbar 40, 40' made of steel
• both the grooves 38, 38 'and the depressions 34, 34' are connected to the walls 38, 38 'bounding the grooves 38, 38' the top of the cover layers 32, 32 ', 32 "created during the molding process, for example by Hinttelformen and / or stamp.
• FIGS. 2A to 2E show examples of different configurations of the depressions 34, 34 'and the elevations 36 of the surface profiling of the cover layers 32, 32', 32 ", namely, in each case in cross section, rectangular with rounded corners (not shown) (FIG. Fig. 2A), substantially undulating (Fig. 2B), triangular (Fig. 2C), convex (Fig. 2D) and sinusoidal (Fig. 2E).

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