AU2012261136A1

AU2012261136A1 - Electrolysis cell and cathode with irregular surface profiling

Info

Publication number: AU2012261136A1
Application number: AU2012261136A
Authority: AU
Inventors: Thomas Frommelt; Frank Hiltmann
Original assignee: SGL Carbon SE
Current assignee: SGL Carbon SE
Priority date: 2011-05-23
Filing date: 2012-04-25
Publication date: 2013-05-09
Also published as: BR112013029998A2; ZA201308689B; WO2012159839A3; JP2014517876A; CA2836838A1; CN103635610A; US20140076723A1; DE102011076302A1; RU2013156866A; EP2714965A2; WO2012159839A2

Abstract

The present invention relates to an electrolysis cell, in particular for the production of aluminium, comprising a cathode, a layer of liquid aluminium on the top side of the cathode, a melt layer on top of said layer of liquid aluminium, and an anode above the melt layer, wherein the cathode has, on the top side thereof, a surface profiling formed from two or more elevations, wherein the surface profiling of the cathode is configured and arranged such that in each case one elevation is provided at at least two of the twenty points on the surface of the top side of the cathode which are arranged in each case vertically below those regions of the boundary between the layer of liquid aluminium and the melt layer at which the peaks with the twenty highest maximum values are present in the reference wave forming potential distribution at the boundary, wherein a reference wave forming potential is defined as the wave forming potential which is present at a point in the boundary between the layer of liquid aluminium and the melt layer during the operation of the electrolysis cell with - instead of the cathode with surface profiling - a reference cathode without surface profiling but of otherwise identical configuration to the cathode with surface profiling.

Description

SGL CARBON SE 2011/018 WO/AU 11.04.2013 Electrolysis cell and cathode with irregular surface profiling 5 The present invention relates to an electrolysis cell, in particular for the production of aluminium, as well as a cathode which is suitable for use in such an electrolysis cell. Electrolysis cells are used for example for the electrolytic production of 10 aluminium, which is carried out industrially usually according to the Hall H6roult process. In the Hall-H6roult process, a melt composed of alumini um oxide and cryolite is elecrolysed. The cryolite, Na 3 [AlF 6 ], is used to lower the melting point of 2,045'C for pure aluminium oxide to approx. 950'C for a mixture containing cryolite, aluminium oxide and additives, 15 such as aluminium fluoride and calcium fluoride. The electrolysis cell used in this process comprises a cathode base, which can comprise a large number of cathode blocks lying adjacent to one an other forming the cathode. In order to withstand the thermal and chemical 20 conditions prevailing during operation of the cell, the cathode is usually composed of a carbon-containing material. Grooves are usually provided in each case at the undersides of the cathode, in which grooves there is disposed in each case at least one busbar through which the current fed via the anodes is carried away. Disposed approx. 3 to 5 cm above the usu 25 ally 15 to 50 cm high layer of liquid aluminium present on the upper side of the cathode is an anode constituted by individual anode blocks, the electrolyte, i.e. the melt containing aluminium oxide and cryolite, being located between the latter and the surface of the aluminium. During the electrolysis carried out at approx. 1,000 0 C, the formed aluminium is de 30 posited beneath the electrolyte layer on account of its greater density SGL CARBON SE 2011/018 WO 11.04.2013 2 compared to that of the electrolyte, i.e. as an intermediate layer between the upper side of the cathode and the electrolyte layer. During the elec trolysis, the aluminium oxide dissolved in the melt is split up by the elec tric current flow to form aluminium and oxygen. Viewed electrochemically, 5 the layer of liquid aluminium is the actual cathode, since aluminium ions are reduced to elementary aluminium at its surface. Nonetheless, the term cathode will be understood in the following not to mean the cathode from the electrochemical standpoint, i.e. be layer of liquid aluminium, but ra ther the component forming the electrolysis cell base, for example com 10 prising one or more cathode blocks. A significant drawback of the Hall-H6roult process is that the latter is very energy-intensive. In order to produce 1 kg of aluminium, approx. 12 to 15 kWh of electrical energy is required, which accounts for up to 40 % of the 15 production costs. In order to be able to reduce the production costs, it is therefore desirable to reduce the specific energy consumption with this process as far as possible. On account of the relatively high electrical resistance of the melt, particu 20 larly compared to the layer of liquid aluminium and the cathode material, relatively high ohmic losses in the form of Joule dissipation occur espe cially in the melt. In view of the comparatively high specific losses in the melt, it is endeavored to reduce as far as possible the thickness of the melt layer and thus the distance between the anode and the layer of liquid 25 aluminium. However, on account of the electromagnetic interactions pre sent during the electrolysis and the wave formation thus produced in the layer of liquid aluminium when there is an excessively small thickness of the melt layer, there is the risk of the layer of liquid aluminium coming into contact with the anode, which can lead to short-circuits of the elec 30 trolysis cell and to undesired reoxidation of the formed aluminium. Such SGL CARBON SE 2011/018 WO 11.04.2013 3 short-circuits also lead to increased wear and thus to a reduced service life of the electrolysis cell. For these reasons, the distance between the anode and the layer of liquid aluminium cannot be reduced arbitrarily. 5 In order to reduce further the specific energy consumption, electrolysis cells with cathodes have also recently been proposed, the upper side of which cathodes facing the liquid aluminium and the melt during the oper ation of the electrolysis cell having a surface profiling. US 2011/0056826 Al discloses for example a cathode with a regularly 10 constituted surface profiling. The horizontal and vertical fluctuations in the layer of liquid aluminium are intended to be reduced by the regularly constituted surface profiling, as a result of which the stability of the layer of liquid aluminium is intended to be increased. With such a regularly constituted surface profiling, however, the wave formation in the layer of 15 liquid aluminium is reduced only to a limited extent and in particular not uniformly over the whole cathode surface. Furthermore, this known regu lar surface profiling in the cathode block surface leads, due to the reduced movement in the layer of liquid aluminium, indirectly to a considerable hindrance of the mixing in the melt layer located above the latter which is 20 required for the dissolution of the periodically supplied aluminium oxide, and this proves to have a disadvantageous effect on the achievable energy efficiency of the electrolysis. EP 0 938 598 B1 and DE 101 64 008 C1 disclose electrolysis cells with 25 cathodes, which are adapted with regard to their electrical contacting from the exterior and with regard to their specific electrical material resistance in such a way that a distribution of the electric current density as homo geneous as possible arises at the upper side of the cathode. In the case of these electrolysis cells, however, a comparatively marked wave formation 30 also takes place in the layer of liquid aluminium, for which reason a re- SGL CARBON SE 2011/018 WO 11.04.2013 4 duction in the specific energy consumption in the electrolysis cell and an increase in its service life are not possible. Proceeding from this, the problem of the present invention consists in 5 creating an electrolysis cell which, during its operation, has a reduced specific energy consumption and an increased service life. In particular, an electrolysis cell is to be made available, in which the thickness of the melt layer is reduced without instabilities, such as short-circuits or reoxi dation of the formed aluminium, occurring in the layer of liquid alumini 10 um as a result of a wave formation tendency that is thereby increased. At the same time, the electrolysis cell according to the invention should en sure sufficient mixing in the melt layer during its operation. According to the invention, this problem is solved by making available an 15 electrolysis cell according to claim 1 and in particular by making available an electrolysis cell for the production of aluminium comprising a cathode, on the upper side of the cathode a layer of liquid aluminium, thereon a melt layer which contains aluminium oxide and cryolite, and above the melt layer an anode, wherein the cathode comprises at its upper side a 20 surface profiling formed by two or more elevations, wherein the surface profiling of the cathode is constituted and disposed in such a way that an elevation is in each case provided at at least two of the twenty points of the surface of the upper side of the cathode which in each case are dis posed vertically beneath those regions of the boundary surface between 25 the layer of liquid aluminium and the melt layer in which the peaks with the twenty highest maximum values are present in the distribution of the reference wave formation potential present in the boundary surface, wherein a reference wave formation potential is defined as the wave for mation potential which, during the operation of the electrolysis cell with 30 instead of the cathode with the surface profiling - a reference cathode SGL CARBON SE 2011/018 WO 11.04.2013 5 without surface profiling, but an otherwise identical configuration to the cathode with surface profiling, is present at a point in the boundary sur face between the layer of liquid aluminium and the melt layer. 5 According to the invention, the cathode of the electrolysis cell comprises a surface profiling which, particularly with regard to the position, the size and the shape of the individual components of the surface profiling, is adapted in a targeted manner such that, during the operation of the elec trolysis cell, the formation of marked peaks in the wave formation poten 10 tial is avoided in a targeted manner in the boundary surface between the layer of liquid aluminium and the melt layer and, consequently, a uniform and low distribution of the wave formation potential results, as viewed over this boundary surface, than would be the case with the use of a cor responding cathode without surface profiling. 15 In the sense of the present invention, surface profiling is understood to mean the sum of all the elevations provided on the base plane of the cath ode. The term base plane denotes the horizontal plane of the cathode which lies farthest in the direction of the anode and which runs through 20 the whole cross-sectional area of the cathode, without intersecting the surface-profiled upper side of the cathode. All the elevations provided on this base plane are therefore orientated in the direction towards the anode and are surrounded by the layer of liquid aluminium. The height of an elevation of the surface profiling is therefore the distance of the uppermost 25 point of the elevation from the point of the base plane of the cathode lying vertically thereunder. In this solution according to the invention, account is taken of the fact that the wave formation potential, as defined below, in the boundary sur 30 face between the layer of liquid aluminium and the melt layer during the SGL CARBON SE 2011/018 WO 11.04.2013 6 operation of the electrolysis cell is the driving force for the wave formation in the layer of liquid aluminium, and also in particular that distribution of the the wave formation potential in the case of conventional electrolysis cells is not uniform over the boundary surface between the layer of liquid 5 aluminium and the melt layer, but on the contrary is extremely heteroge neous. Due to the reduction in the wave formation potential provided ac cording to the invention and particularly as a result of the distribution of the wave formation potential in the boundary surface between the layer of liquid aluminium and the melt being made uniform, a wave formation in 10 the layer of liquid aluminium is reliably prevented or at least considerably reduced during the operation of the electrolysis cell according to the in vention, as a result of which the thickness of the melt layer can be re duced compared to conventional electrolysis cells and the efficiency of the electrolysis cell according to the invention is thus considerably increased. 15 A further important finding of the present invention is that the heteroge neous distribution of the wave formation potential present in the bounda ry surface between the layer of liquid aluminium and the melt layer in the case of conventional electrolysis cells can be directly influenced by the 20 provision and the specific configuration of the surface profile at the upper side of the cathode of the electrolysis cell and marked peaks of the wave formation potential at individual points of the boundary surface can in this way be avoided in a targeted manner. As explained below in detail, the wave formation potential at a specific point in the aforementioned 25 boundary surface depends on the vectorial product of the electric current density and the magnetic flux density present at this point. If a specific current path is considered, which leads from the current supply of the cathode to the anode of the electrolysis cell, the total electrical resistance along this path and consequently the current density at the point at which 30 the path crosses the boundary surface between the layer of liquid alumin- SGL CARBON SE 2011/018 WO 11.04.2013 7 ium and the melt layer depends in particular on what path length of the path runs respectively in the cathode block, in the layer of liquid alumini um and in the melt layer. Since these materials each have different specif ic electrical resistance values, the melt layer and also the cathode material 5 in particular having a higher specific electrical resistance than the liquid aluminium, and because the individual current paths have different path lengths in the cathode block, in the layer of liquid aluminium and in the melt layer, the total electrical resistances along the individual paths and thus also the individual current densities over the boundary surface be 10 tween the layer of liquid aluminium and the melt layer are heterogeneous in conventional electrolysis cells, so that individual points of the boundary surface exhibit marked current density peaks. Through the provision and suitable adaptation of the position, the shape and the length of the eleva tions of the surface profiling of the cathode, the path lengths of the indi 15 vidual current paths in the various sections, i.e. cathode block, layer of liquid aluminium and melt layer, are adjusted according to the present invention in such a way that, in the region of the boundary surface, a current density distribution is established which is adapted such that, in the boundary surface between the layer of liquid aluminium and the melt 20 layer during operation of the electrolysis cell, no marked peaks arise in the distribution of the wave formation potential present in this boundary sur face, as a result of which an essentially uniform and low distribution of the wave formation potential is guaranteed. 25 In order to optimise the position, the shape and the length of the eleva tions of the surface profiling of the cathode, the present invention pro ceeds from the distribution of the reference wave formation potential which results during the operation of the electrolysis cell with a conven tional, unprofiled reference cathode, and provides elevations in a targeted 30 manner at the points of the cathode surface which are disposed vertically SGL CARBON SE 2011/018 WO 11.04.2013 8 beneath the points of the boundary surface at which marked peaks in the distribution of the reference wave formation potential are present. During the operation of the electrolysis cell with the surface-profiled cathode, the electric current density is reduced in these regions and the wave formation 5 potential is thus reduced in these regions. As explained, the reference wave formation potential is the wave formation potential which results during the operation of the electrolysis cell with instead of the cathode with surface profiling - a reference cathode without 10 surface profiling, i.e. with a horizontal cathode surface, but with an oth erwise identical configuration to the cathode with the surface profiling. According to the embodiment specified in claim 1, the reference electroly sis cell used to determine the reference wave formation potential is identi cal to the electrolysis cell according to the invention, except for the fact 15 that, instead of the surface-profiled cathode, use is made of a reference cathode in which the surface profiling is not provided, in which the addi tional volume on the upper side of the cathode arising due to the omission of the surface profiling is filled with liquid aluminium or melt - depending on the layer in which the corresponding material is present with the sur 20 face-profiled cathode. Especially in cases where many elevations occupying a considerable vol ume are provided on the upper side of the cathode, it is proposed in an alternative embodiment of the present invention specified in claim 2 to use 25 a reference cathode without surface profiling to determine the reference wave formation potential and to adjust the height of this reference cathode in the electrolysis cell in such a way that, between the upper side of the cathode and the anode, the same bath volume is present for the layers of liquid aluminium and melt as in the case of the electrolysis cell with the 30 surface-profiled cathode. Since, in this case, the reference wave formation SGL CARBON SE 2011/018 WO 11.04.2013 9 potential relates to a reference electrolysis cell with the same bath volume as that of the electrolysis cell according to the invention, the reference wave formation potential thus determined is more meaningful than that determined according to claim 1, if the volume of the elevations of the 5 surface profiling of the cathode accounts for at least 10 %, preferably at least 20 % and particularly preferably at least 30 % of the volume of the cathode. The wave formation potential and thus the distribution of the wave for 10 mation potential can be determined by computer-supported electrical, magnetic and magneto-hydrodynamic simulation of the movement and wave formation in the layer of liquid aluminium and the melt of the re spective electrolysis cell. 15 According to the present invention, the wave formation potential at an arbitrary point of the boundary surface between the layer of liquid alumin ium and the melt is defined as the absolute value of the component of the flow rate present in the boundary surface of the melt, said component present at this point being directed in the normal direction to the bounda 20 ry surface, i.e. wave formation potential = ua - Si , wherein a is the flow rate of the melt as a vector and ii is the normal vector. The boundary surface is also assumed to be permeable, so that the wave formation potential represents a local measure of the wave-driving flow directed towards the boundary surface. In this case, the flow of the melt cannot of course be 25 determined experimentally, for which reason the wave formation potential is preferably determined by the simulation method described below. In order to calculate the flow conditions, the electric and magnetic fields are first calculated by means of simulation according to a finite element SGL CARBON SE 2011/018 WO 11.04.2013 10 method (FEM) and the resulting fields are then used in the calculation of the flow conditions, which also takes place by means of simulation accord ing to a finite element method (FEM). The software Comsol Multiphysics in the version 3.5a is used for both simulations. The boundary surface is 5 assumed to be permeable, wherein the wave formation potential repre sents a local measure of the wave-driving flow directed towards the boundary surface. The simulated electrolysis cell, which comprises bus bars, the current supplies of the electrolysis cell including a magnetic compensation geometry if applicable, the cathode, the layer of liquid alu 10 minium, the melt layer, the anode, if applicable an anode tree connecting the anodes and air as a surrounding medium, is split up geometrically component by component into finite volume elements. Insofar as the cell to the simulated, taking account of the aforementioned components, ex hibits one or more planes of symmetry, only the part of the electrolysis cell 15 located on one side of each plane of symmetry is simulated in each case and the symmetry conditions are taken into account by corresponding boundary conditions, as will be explained in greater detail below. The simulation proceeds in a simplifying manner from stationary condi 20 tions in the electrolysis cell, so that the simulation is based on the respec tive stationary physical equations. Furthermore, an isothermal electrolysis cell is assumed which is at operating temperature (970'C). The simulation is based on the following variables and parameters: 25 V: electric voltage, scalar a: electrical conductivity, scalar E (bold type): electric field, vektor A (bold type): electric vector potential, vector 30 Ax, Ay, Az: vector potential, component SGL CARBON SE 2011/018 WO 11.04.2013 11 H (bold type): magnetic field, vector J, j (bold type), j : electric current density, vector B (bold type), h: magnetic flux density, vector I (bold type): unit matrix, tensor 5 F (bold type): force density (sum of Lorentz force density and gravitational force density), vector u (bold type), i: flow rate, vector u (normal type), ux, uy, uz: flow rate, component p: pressure, scaler 10 p: viscosity, scaler p: density, scaler Lc: characteristic length, e.g. depth of aluminium bath vc: characteristic speed 15 Additional variables with turbulent flows: PT: turbulent viscosity, scaler k: turbulent kinetic energy ep, z: dissipation of turbulent kinetic energy lw: distance from solid boundary surfaces 20 LRef: reference length scale, corresponds to characteristic length Lc G: reciprocal distance from solid boundary surfaces PK: source term of turbulent kinetic energy fu: attenuation function viscosity f,: attenuation function dissipation 25 Rt: turbulent Reynold's number 1*: limited mixing length u,: turbulent dissipation rate of all grid cells n (bold type), ii: normal vector to the boundary surface between the layer of liquid aluminium and the melt layer, vector SGL CARBON SE 2011/018 WO 11.04.2013 12 t (bold type), i: tangential vector, vector e2,,5,,22:unit vectors, Cartesian coordinate system The constructed grids are sufficiently finely dimensioned, so that artifacts 5 of the grid are no longer visible when the wave potential is evaluated. These include, for example, marked peaks or conspicuous changes along the edges of the grid. Moreover, the dependence of the simulated values on adjusted grid fineness and slow and limited convergence of the simula tions indicate insufficient grid fineness in the relevant areas. 10 Furthermore, a quality factor of at least 0.15 is required as a quality factor for the overall grid when the grid is constructed, wherein quality factor q is defined according to the Manual of Comsol Multiphysics Software as follows: 15 Table 1 q 72jFV for tetrahedral grid cells (hi +h +h 4h, h +h} q 361rV for prismatic grid cells 9 I312 with V = volume of the grid cell and hi = edge lengths of the grid cell. 20 In detail, the construction of the grid takes place as follows: The air surrounding the electrolysis cell is modelled with an unlimited size of the grid cells, which can vary between fine regions (e.g. at the melt lay er) and coarse regions (e.g. surrounding edges of the overall arrangement).

SGL CARBON SE 2011/018 WO 11.04.2013 13 The magnification factor between two adjacent grid cells is limited to 1.65 in order to avoid distorted grid elements. The current supplies and discharges are reproduced with grid cells with 5 an edge length in the region of approx. 30 cm. The layer of liquid aluminium and the melt layer are modelled such that the grid cells that form the boundary surface between the layer of liquid aluminium and the melt each have an edge length in the region of approx. 10 3 cm. The melt layer is modelled such that the mean extension of a grid cell in the vertical direction corresponds at most to half the thickness of the melt layer. In the context of the simulation, it is assumed that the boundary surface 15 between the layer of liquid aluminium and the melt is not curved and therefore runs horizontally. Accordingly, normal vector n is adopted as vertical unit vector ez and the wave formation potential is accordingly de fined as the absolute value of vertical component uz of the flow rate in the boundary surface. 20 The layer of liquid aluminium and the cathode are modelled such that the grid cells that form the boundary surface between the cathode and the layer of liquid aluminium have an edge length in the region of approx. 5 cm. 25 The anodes and cathodes are otherwise modelled with an unrestricted size of the grid cells, wherein the size of the grid cells can vary between fine regions (e.g. at the melt layer) and coarse regions (e.g. at the supplies and discharges). The magnification factor between two adjacent grid cells is 30 limited to at most 1.65 in order to avoid distorted grid elements.

SGL CARBON SE 2011/018 WO 11.04.2013 14 In the case of electrolysis cells defined below and operated under turbu lent flow conditions, the solid boundary surfaces between the individual components of the electrolysis cell are modelled in the cell construction by 5 so-called Inflation Boundary Layers available in Comsol Multiphysics, which comprise prismatic cells (in contrast with, for example, tetrahedral elements). The individual grid cells of the grid structure thus constructed are then 10 provided with corresponding material properties, i.e. the grid cells are provided in particular with values for the specific electrical resistance and the grid cells representing the layer of liquid aluminium and the melt layer are additionally provided with values for the viscosity and density of the aluminium and the melt. The following values are taken as a basis for the 15 material properties: Table 2 Specific resistance [in Ohm-ml Cathode 1.2-10-5 Busbars made of steel 7.78-10-7 Liquid aluminium 2.8-10-7 Melt 4.84-10-3 Anode 4.0-10-5 Anode tree (aluminium) 2.34-10-7 Viscosity [in Pa-sl Liquid aluminium 9.0-10-5 Melt 2.34-10-3 Density [in kg/ma1 Liquid aluminium 2.3-103 Melt 2.08-103 SGL CARBON SE 2011/018 WO 11.04.2013 15 All the other material properties used in the simulation are selected such that they correspond to the actual properties of the respective material. 5 For the numeric stabilisation of the electromagnetic and flow-mechanical calculations, the - in reality - abrupt transition of the material properties at the boundary surface between the layer of liquid aluminium and the melt layer are also smoothed out in the simulated structure in a range of ± 3 cm, i.e. the cells of the grid structure representing the layer of liquid 10 aluminium and the melt layer which are located within a range of 3 cm below and above the boundary surface are provided with values for the material properties which are selected such that, in this range, an essen tially linear property transition results from the properties of the cells representing the aluminium layer given in above table 2 to the properties 15 of the cells representing the melt layer given in table 2. The air surrounding the electrolysis cell is provided with an artificially high specific electrical resistance of 1 Ohm-m, so that it does not contrib ute to the current transport. 20 For the grid structure thus constructed, which reproduces the electrolysis cell in its geometry and therewith its material properties, the electromag netic fields are calculated and the ascertained the values are then inserted into the calculation of the flow-mechanical movements of the melt of the 25 electrolysis cell. The first step of the modelling of the electromagnetics is based on the known stationary Maxwell equations: SGL CARBON SE 2011/018 WO 11.04.2013 16 V -j= 0 V X H = j j =tTE+J, E =-VV B=VXA Lagrange functions (1st order for V and 2nd order for A) are used as start ing functions for the finite element methods. 5 These partial differential equations are solved for the whole geometry by numeric calculation. The boundary conditions to be used thereby are ex plained more precisely below; in particular, the operating current of the electrolysis cell fed through the cathode and anode enters into the calcula 10 tion as an operating parameter preset from the exterior. The Lorentz force density thus calculated is then used as a basis for the calculation of the flow mechanics in the bath of the electrolysis cell. 15 Depending on the nature of the flow, the flow-mechanical calculation is based on different equations. In order to select the partial differential equations to be used, the known Reynold's number Re= pvcLc is used and, depending thereon, the following equation systems: The following equations (Navier-Stokes equations) are used for laminar 20 and weakly turbulent problems with Re < 10,000: p(u - Vu V -pl+ (Vu + (Vu)T) - -p(V. u) + F V - (pu) 0 SGL CARBON SE 2011/018 WO 11.04.2013 17 Lagrange functions (1st order for p and 2nd order for u) are used as start ing functions for the finite element methods. The following equations (Low Reynold's k-epsilon equations) are used for 5 flows in the transition region with Re > 10,000 and < 100,000: p( = -V) ±- (p + PT)(VU +(Vu)) - +PT)(V u)I - pkiJ + F V-(pu)=0 po(u - V)k =V - {p+ Vkl + Pk --pe I]I p(u -Vl)t-= V -p +2 V +C Pk -Ce2p0 e~,yk ,,c= ep VG. VG + u.G(V -VG) = (1 + 2f.)G 4 , 1 = 1 F G 2 p kVU: (VU + (VU)T)- (v u)j - ZpkV - u f (1 - /14)2 + - (R/200 1 - .3 I' = (puel1 /g Rt = ph2/pR Ue=(aepV 10 Lagrange functions (1st order for p and 2nd order for u, k and ep) are used as starting functions for the finite element methods. The following equations (k-epsilon equations) are used for turbulent flows with Re 100,000: 15 SGL CARBON SE 2011/018 WO 11.04.2013 18 p(uV)u=. -p + (p + T)(Vu + (.Vu)T) - (+ pT)(V- u) - pki] + F V -(pu)= 0 p(u V)k =V - + UVk +Pk p(u - V)e = V - + L)V] + C 'PkE C-P-- c ep Pk= Vu:(Vu + (Vu)T)- (V - U)2 - kV-u 1 * 3 V3 ,wherein C, = 0.09; Cr1 = 1.44; C, 2 = 1.02; Ok = 1.0 and ae = 1.3. Lagrange functions (1st order for p and 2nd order for u, k and ep) are 5 used as starting functions for the finite element methods. The values previously calculated in the electromagnetic consideration in the form of the Lorentz force density FLtz = J B also enter into the above equations. Lorentz force density FLtz forms, together with gravitational 10 force density , = -pge, external excitation F according to F = + Fg con tained in the above equations. The above flow-mechanical partial differential equations are also solved numerically. 15 In the context of the aforementioned calculations, use is also made of the following boundary conditions: The following boundary conditions relate to the electric fields calculated 20 during the electromagnetic calculation: SGL CARBON SE 2011/018 WO 11.04.2013 19 e The external faces of the treated volume are are regarded as an elec trical insulator ( -n -= 0 ). . Any symmetrical faces present are regarded as an electrical insula tor ( -"'=,I ). 5 0 An electric voltage V is applied at the input of the anode tree, which is adapted such that the cell current (e.g. 168 kA) intended for the normal operation of the electrolysis cell flows. . An electric voltage V of 0 volt is applied to the cathode-side current discharge (earthing). 10 e The calculated electrical potential V is continuous at all the internal faces. The following boundary conditions relate to the magnetic fields calculated during the electromagnetic calculation: 15 e The magnetic flux is parallel to the external face (!.x.-2 ) at the ex ternal faces of the treated volume. " A magnetic symmetry (nX H =3-) is present at any symmetrical faces that may be present. " The calculated magnetic vector potential A is continuous at all the 20 internal faces. The following boundary conditions relate to the flow fields calculated dur ing the flow-mechanical calculation: e The following holds at the solid boundary surfaces: 25 o When use is made of the laminar equations: The liquid ad heres firmly to the solid boundary surface, which is also de noted as ,,No slip", i.e. the speed u=0.

SGL CARBON SE 2011/018 WO 11.04.2013 20 o When use is made of the turbulent equations, a wall model is used which takes account of the friction between the respec tive liquid layer and the solid boundary surface. e An open boundary surface is present at any symmetrical faces that 5 may be present, wherein the normal flow in relation to the boundary surface is calculated with fo= 0 according to the equation P,+ 1(U+ (VU)T) -. P(V. ) uJI n ,M. e The calculated flow rate u is continuous at all the internal faces (e.g. 10 the boundary surface between the layer of liquid aluminium and the melt layer). As described above, electromagnetic magnitudes V, Ax, Ay, Az, jx, jy a and jz are first calculated with the aid of the Maxwell equations and the Lorentz 15 force density resulting therefrom is then inserted into the flow-mechanical equations used in each case in order to calculate therefrom flow field magnitudes ux, uy, uz a and p. The electromagnetic calculation and the flow-mechanical calculation are therefore coupled together in a unidirec tional manner. 20 Iterative solvers (GMRES) with geometrical multigrid preconditioning are used in each case to solve the partial differential equations stated above. If need be, use is made of standard stabilisation techniques for flow me chanics such as the Streamline Diffusion (GLS) available in Comsol Mul 25 tiphysics and Crosswind Diffusion as well as calibration of the vector po tential in the electromagnetic calculation. According to the invention, the surface profiling of the cathode according to the invention comprises two or more elevations, wherein an elevation is SGL CARBON SE 2011/018 WO 11.04.2013 21 provided in each case at at least two of the twenty points of the surface of the upper side of the cathode which in each case are disposed vertically beneath those regions of the boundary surface between the layer of liquid aluminium and the melt layer in which the peaks with the twenty highest 5 maximum values are present in the distribution of the reference wave formation potential present in the boundary surface. In a development of the inventive idea, it is proposed that an elevation is provided in each case at at least X of the Y points of the surface of the upper side of the cathode which in each case are disposed vertically beneath those regions of the 10 boundary surface between the layer of liquid aluminium and the melt layer in which the peaks with the Y highest maximum values are present in the distribution of the reference wave formation potential present in the boundary surface, wherein X = 4 and Y = 20, preferably X = 6 and Y = 20, particularly prefer 15 ably X = 10 and Y = 20 and very particularly preferably X = 14 and Y = 20 and/or wherein X = 2 and Y = 10, preferably X = 3 and Y = 10, particularly prefer ably X = 5 and Y = 10 and very particularly preferably X = 7 and Y = 10 and/or 20 wherein X = 1 and Y = 5, preferably X = 2 and Y = 5, particularly prefera bly X = 3 and Y = 5 and very particularly preferably X = 4 and Y = 5. In this way, marked peaks in the wave formation potential of the electrolysis cell are avoided particularly comprehensively, so that the stability of the electrolysis cell during operation is increased still further. 25 According to a further preferred embodiment of the invention, provision is made such that at least one of the elevations disposed at the points of the surface of the upper side of the cathode which in each case are disposed vertically beneath those regions of the boundary surface between the layer 30 of liquid aluminium and the melt layer in which in each case a peak is SGL CARBON SE 2011/018 WO 11.04.2013 22 present in the distribution of the reference wave formation potential pre sent in the boundary surface has its maximum height at the point dis posed vertically beneath the point of the boundary surface between the layer of liquid aluminium and the melt layer at which the peak of the dis 5 tribution of the reference wave formation potential has its maximum val ue. An excessive wave formation in the corresponding region of the boundary surface is thus particularly effectively avoided. It is particularly preferable for the essentially congruent arrangement de 10 scribed above to be guaranteed for all peak-elevation pairs, i.e. all the elevations at the points of the surface of the upper side of the cathode which in each case are disposed vertically beneath those regions of the boundary surface between the layer of liquid aluminium and the melt layer in which a peak is in each case present in the distribution of the 15 reference wave formation potential present in the boundary surface have in each case their maximum height at the point disposed vertically be neath the point of the boundary surface between the layer of liquid alu minium and the melt layer at which the respective peaks of the distribu tion of the reference wave formation potential have their maximum value. 20 A particularly effective compensation of a peak in the distribution of the reference wave formation potential is achieved if the geometrical outer contour of at least one of the elevations is at least essentially similar in plan view to the geometrical outer contour of the respective peak of the 25 distribution of the reference wave formation potential in plan view or es sentially corresponds to the latter. Similarity is understood in agreement with the commonly used specialist linguistic usage, that the two outer contours can be transferred into one 30 another by geometrical mapping, which can be composed of concentric SGL CARBON SE 2011/018 WO 11.04.2013 23 elongations and congruence mappings, such as in particular displace ments, rotations or mirroring. For example, the two outer contours can essentially correspond to a circle. The two outer contours can also form essentially two triangles which have two essentially identical angles, or 5 form essentially two rectangles with at least approximately equal side rati os or form essentially two ellipses with at least approximately identical numeric eccentricities. It is particularly preferable for the geometrical outer contours of all the 10 elevations in plan view to be at least essentially similar to the geometrical outer contour of the respective peaks of the distribution of the reference wave formation potential in plan view or essentially correspond to the latter. 15 According to a further preferred embodiment of the present invention, the geometrical outer contour of at least one of the elevations in plan view is constituted at least in sections at least approximately polygonal and/or ellipsoidal. Such elevations can be produced particularly easily and are particularly well suited for effectively compensating for a peak of the refer 20 ence wave formation potential. An elevation that is particularly easy to produce emerges when the elevation constituted as a polygon has 3, 4, 5 or 6 corners. Within the scope of the present invention, the outer contour of an eleva 25 tion viewed vertically from above can advantageously be selected such that it can be generated by a simplification of the outer contour of the respec tive peak of the distribution of the reference wave formation potential viewed vertically from above. At least one of the elevations thus preferably has an outer contour, viewed in plan view, which is geometrically simpler 30 than the outer contour, viewed in plan view, of the peak of the distribution SGL CARBON SE 2011/018 WO 11.04.2013 24 of the reference wave formation potential disposed vertically above the elevation in the boundary surface. It is preferable for the sum of the num bers of all the corners and all the differently curved regions of the outer contour area of the elevation viewed from above to be smaller than the 5 sum of all the corners and all the differently curved regions of the outer contour area, viewed from above, of the corresponding peak of the distri bution of the reference wave formation potential. All the sections of the outer contour following one another in the peripheral direction, between which a point of inflection is located, are counted as differently curved 10 regions of an outer contour. In order to prevent particularly effectively an increased wave formation of the layer of liquid aluminium caused by a peak in the reference wave for mation potential, it has proved to be advantageous if the three 15 dimensional shape of at least one of the elevations is at least essentially similar to the three-dimensional shape of the respective peak of the distri bution of the reference wave formation potential or essentially corresponds to the latter. 20 It is particularly preferred if the three-dimensional shapes of all the eleva tions are at least essentially similar to the three-dimensional shape of the respective peak of the distribution of the reference wave formation poten tial or essentially correspond to the latter. 25 According to a further advantageous embodiment of the invention, at least one of the elevations has a three-dimensional shape tapering upwards in the vertical direction. This formation leads to a particularly effective avoid ance of wave formation in the region of a peak in the distribution of the reference wave formation potential. The at least one elevation, when SGL CARBON SE 2011/018 WO 11.04.2013 25 viewed from the side, can for example have an essentially polygonal and preferably essentially trapezoidal outer contour. In a development of the inventive idea, it is proposed that at least one of 5 the elevations, viewed upwards in the vertical direction, is bounded by a top surface which, viewed in plan view, has a smaller area than the base surface of the elevation viewed in plan view. The elevation can for example be constituted at least approximately conical or in the shape of a truncat ed pyramid. 10 According to a further embodiment of the invention, at least one of the elevations has a three-dimensional shape which, proceeding from the base surface of the elevation, can be generated by rotating the base surface about an axis of rotation bordering the base surface. The axis of rotation 15 preferably runs essentially horizontally. Such elevation geometries are particularly well suited for effectively making the distribution of the wave formation potential uniform and are also particularly easy to produce. The at least one elevation can preferably be generated by rotating the base surface around the axis of rotation through an angle of at least 75 0 and at 20 most 180 . A further advantageous embodiment of the present invention is character ised in that at least one of the elevations has a three-dimensional shape which, proceeding from the base surface of the elevation, can be generated 25 by geometrical extrusion of the base surface of the elevation upwards in the vertical direction. The extrusion direction is preferably at least approx imately vertical and diverges up to 45' from the vertical direction. The elevation is preferably reduced in size in the extrusion direction by scaling in the course of the extrusion. In principle, it is preferable during the ex 30 trusion for the at least one elevation to taper upwards in the vertical direc- SGL CARBON SE 2011/018 WO 11.04.2013 26 tion. The introduction of elevations is also possible by means of vacuum vibration, uniaxial pressing or another suitable forming process. In the case of the electrolysis cell, the cathode can comprise two or more 5 cathode blocks and/or the anode can comprise two or more anode blocks. In particular, a plurality of cathode blocks can be disposed in succession beside one another viewed in the transverse direction of the cathode blocks and can be connected along their longitudinal sides by means of a tamping compound. Furthermore, it is preferable that, viewed in the width 10 direction of the cathode blocks, an anode block covers two cathode blocks and, viewed in the longitudinal direction of the cathode blocks, two anode blocks cover a cathode block. A particularly high energy efficiency of the electrolysis cell can be achieved 15 if the distance between the anode and the layer of liquid aluminium amounts to between 15 and 45 mm, preferably between 15 and 35 mm and particularly preferably between 15 and 25 mm. This small distance is achieved by the reduction in the wave formation potentials and by making the distribution of the wave formation potential uniform. 20 As described above, the surface profiling of the cathode is adapted accord ing to the invention in such a way that marked peaks of the wave for mation potential at individual points of the boundary surface between the layer of liquid aluminium and the melt layer are avoided. The result is 25 surface profilings which are adapted in their position, size and shape to the specific properties of the electrolysis cell determining the wave for mation potential. The present invention abandons, in a deliberate and targeted manner, the route of defining a-priori a surface profiling which is constituted regular, but which by the same token is not adapted to the 30 wave formation potential present in each case. Instead, the surface profil- SGL CARBON SE 2011/018 WO 11.04.2013 27 ing of the cathode of an electrolysis cell according to the invention is con stituted in practice irregular at least in one direction. The present invention further relates to a cathode for an aluminium elec 5 trolysis cell, the upper side whereof comprises a surface profiling with two or more first webs running essentially in a first direction of the cathode and at least one second web running at least essentially in the direction normal to the first direction of the cathode. 10 Within the meaning of the present invention, a web is regarded as an ele vation running at least essentially straight in the longitudinal direction. Within the scope of the present invention, it has been shown that a cath ode with such a surface profiling is suitable for achieving, when used in 15 electrolysis cells, a distribution of the wave formation potential in the boundary surface between the layer of liquid aluminium and the melt layer during operation of the given electrolysis cell, wherein marked peaks of the wave formation potential at individual points of the boundary sur face are effectively avoided. The specifically described surface profiling is 20 adapted to the conditions that prevail in a large number of commonly available electrolysis cells, and is constituted such as to achieve uniform distribution of the wave formation potential in these electrolysis cells tak ing account of these conditions. 25 Such a cathode can in particular be a component part of one of the previ ously described electrolysis cells according to the invention. In addition, the cathode according to the invention is preeminently well suited, when used in electrolysis cells, for achieving the advantages of an SGL CARBON SE 2011/018 WO 11.04.2013 28 improved energy efficiency and an increased service life and at the same time ensuring sufficient mixing of the melt in the electrolysis cell. According to an advantageous embodiment of the present invention, the at 5 least two first webs run at least approximately in a transverse direction of the cathode. In a development of the inventive idea, it is proposed that the upper side of the cathode has, in plan view, an essentially rectangular outer contour, 10 wherein an elevation of the cathode is provided at least in one of the four corners of the essentially rectangular outer contour. Within the scope of the present invention, it has been shown that marked peaks in the distri bution of the reference wave formation potential are usually present in these corner regions, so that the stability of the electrolysis cell during 15 operation can be considerably increased by the measure according to the invention. The elevation disposed in the corner region preferably has, in plan view, an essentially triangular outer contour. A further advantageous embodiment of the invention makes provision 20 such that the upper side of the cathode comprises a depression in the form of a trough which is at least essentially V-shaped when viewed in the cross-section of the cathode. The depression constituted in the form of a V-shaped trough serves to reduce the current density in the lateral edge regions of the cathode, which is otherwise increased on account of the 25 contact taking place there with the busbars inserted in the cathode base, and thus to reduce the wave formation potential in these regions. The at least two first webs and that the at least one second web are pref erably disposed on the surface of the essentially V-shaped depression. 30 SGL CARBON SE 2011/018 WO 11.04.2013 29 According to a further advantageous embodiment of the present invention, the connection point between the two legs of the cross-section of the de pression constituted essentially in the form of a V-shaped trough, as viewed in the cross-section of the cathode, is disposed at least approxi 5 mately in the middle of the cathode. In this way, the electric current den sity is displaced from the lateral edge regions of the cathode cross-section into the middle, in order to reduce peaks in the current density in these edge regions when the cathode is used in an electrolysis cell and to achieve a low wave formation potential and an essentially uniform distri 10 bution of the wave formation potential. In the present invention, it has proved to be advantageous for the depres sion to extend over at least 75 %, preferably over at least 90 %, particular ly preferably over at least 95 % and very particularly preferably over 100 % 15 of the surface of the cathode. In this way, the distribution of the wave formation potential is made uniform over the whole cathode surface when the cathode is used in an electrolysis cell. The at least one second web, when viewed in the second direction of the 20 cathode, is preferably disposed at least approximately in the middle of the cathode. Since an excessively high wave formation potential is otherwise to be expected in this region, a particularly favorable effect on the wave formation potential is thus achieved. 25 According to a further advantageous embodiment, the upper edge of at least one of the first webs has a distance from the bottom of the V-shaped trough increasing towards the middle of the cathode, as viewed in the transverse direction of the cathode. This increase in the distance towards the middle of the cathode block serves to avoid excessive peaks in the 30 wave formation potential in the middle of the cathode block and thus an SGL CARBON SE 2011/018 WO 11.04.2013 30 increased wave formation in the layer of liquid aluminium in this region when the cathode block is used in an electrolysis cell. A further subject-matter of the present invention is an electrolysis cell, in 5 particular for the production of aluminium, which comprises at least one cathode as described above, on the upper side of the cathode a layer of liquid aluminium, thereon a melt layer and above the melt layer an anode. The advantages and embodiments described above with regard to the cathode also apply accordingly to the electrolysis cell according to the 10 invention. According to an advantageous embodiment of the present invention, the anode comprises at least two anode blocks disposed beside one another, wherein a joint extends between the at least two anode blocks and where 15 in at least one of the first webs of the cathode is disposed vertically be neath and at least essentially parallel to the joint constituted between the two anode blocks. The angular divergence between the orientation of the webs and the orientation of the joint preferably amounts to a maximum of 200. According to the invention, it has been recognised that these regions 20 between the anode blocks usually have a markedly increased wave for mation potential, so that the described measure contributes towards in creasing the stability of the electrolysis cell still further. The at least one first web disposed vertically beneath the joint is preferably disposed in an at least approximately centered manner in relation to the joint. 25 A further subject-matter of the present invention is a method for the pro duction of an electrolysis cell with the features of claim 31. The method for the production of an electrolysis cell, in particular an elec 30 trolysis cell for the production of aluminium, which comprises a cathode, SGL CARBON SE 2011/018 WO 11.04.2013 31 on the upper side of the cathode a layer of liquid aluminium, thereon a melt layer and above the melt layer an anode, comprises the following steps: - Ascertainment of the distribution of the reference wave formation 5 potential present in the boundary surface between the layer of liquid aluminium and the melt layer of the electrolysis cell, - production of a surface profiling comprising a plurality of elevations on the upper side of the cathode, wherein an elevation is provided in each case at at least two of the twenty points of the surface of the 10 upper side of the cathode which in each case are disposed vertically beneath those regions of the boundary surface between the layer of liquid aluminium and the melt layer in which the peaks with the twenty highest maximum values are present in the distribution of the reference wave formation potential present in the boundary sur 15 face, wherein a reference wave formation potential is defined as the wave for mation potential which, during the operation of the electrolysis cell with instead of the cathode with the surface profiling - a reference cathode without surface profiling, but an otherwise identical configuration to the 20 cathode with surface profiling, is present at a point in the boundary sur face between the layer of liquid aluminium and the melt layer. With the method according to the invention, electrolysis cells according to the invention as described above can be produced. The advantages and 25 embodiments described above in respect of the electrolysis cell according to the invention are accordingly applicable to the method according to the invention. According to claim 32, a further subject-matter of the present invention is 30 a method for the production of an electrolysis cell, in particular an elec- SGL CARBON SE 2011/018 WO 11.04.2013 32 trolysis cell for the production of aluminium, which comprises a cathode, on the upper side of the cathode a layer of liquid aluminium, thereon a melt layer and above the melt layer an anode, wherein the method com prises the steps: 5 - Ascertainment of the distribution of the reference wave formation potential present in the boundary surface between the layer of liquid aluminium and the melt layer of the electrolysis cell, - production of a surface profiling comprising a plurality of elevations on the upper side of the cathode, wherein an elevation is provided in 10 each case at at least two of the twenty points of the surface of the upper side of the cathode which in each case are disposed vertically beneath those regions of the boundary surface between the layer of liquid aluminium and the melt layer in which the peaks with the twenty highest maximum values are present in the distribution of 15 the reference wave formation potential present in the boundary sur face, wherein a reference wave formation potential is defined as the wave for mation potential which, during the operation of the electrolysis cell with instead of the cathode with the surface profiling - a reference cathode 20 without surface profiling, but an otherwise identical configuration to the cathode with surface profiling, wherein the reference cathode is disposed with regard to its height in the electrolysis cell in such a way that the same volume for the layer of liquid aluminium and the melt layer is pro vided between the reference electrode and the anode as in the case of the 25 electrolysis cell with the cathode with surface profiling, is present at a point in the boundary surface between the layer of liquid aluminium and the melt layer.

SGL CARBON SE 2011/018 WO 11.04.2013 33 Merely by way of example, the invention is described below with the aid of an advantageous embodiment making reference to the appended draw ings. In the figures: 5 Fig. 1 shows an electrolysis cell according to an embodiment of the invention in perspective view; Fig. 2 shows the local distribution of the reference wave formation potential of the electrolysis cell of fig. 1 in the boundary surface 10 between the layer of liquid aluminium and the melt layer in plan view; Fig. 3 shows the surface-profiled cathode of the electrolysis cell of fig. 1 in plan view; 15 Fig. 4 shows the cathode of fig. 3 in perspective view; Fig. 5 shows the local distribution of the wave formation potential in the boundary surface between the layer of liquid aluminium and 20 the melt layer of the electrolysis cell of fig. 1 to 4; Fig. 6a-i shows exemplary elevations for a surface profiling according to the invention and 25 Fig. 7a-i shows further exemplary elevations for a surface profiling ac cording to the invention. Fig. 1 shows an electrolysis cell 10 for the production of aluminium com prising a cathode 12, on the upper side of cathode 10 a layer 14 of liquid 30 aluminium, thereon a melt layer 16 and above melt layer 16 an anode 18.

SGL CARBON SE 2011/018 WO 11.04.2013 34 Layer 14 of liquid aluminium and melt layer 16 merge into one another at a boundary surface 15. Cathode 12 comprises a plurality of elongated cathode blocks which ex 5 tend in transverse direction y of electrolysis cell 10 and which are dis posed beside one another in longitudinal direction x of electrolysis cell 10 and are connected to one another via a tamping-compound joint not rep resented. A busbar 20, which extends through the cathode block in longi tudinal direction y of the cathode block and which makes electrical con 10 tact with the cathode block, is inserted at the underside of each cathode block. Busbars 20 are grouped together electrically via a current discharge 22, which is constituted geometrically in such a way that a magnetic field 15 compensation is brought about, i.e. that the distribution of magnetic flux density B brought about by the current flow is to a certain degree made uniform. Anode 18 comprises a multiplicity of anode blocks 24 which are connected 20 to one another via a current supply 28 comprising an anode tree 26. Cathode 12 of electrolysis cell 10 comprises a surface profiling comprising a plurality of elevations 30, said surface profiling being adapted, as ex plained below, to the distribution of the reference wave formation potential 25 of electrolysis cell 10 in boundary surface 15. In the present example of embodiment, the fact that electrolysis cell 10 shown in fig. 1 is mirror-symmetrical with respect to plane of symmetry 32 can be used for the calculation of the reference wave formation potential. 30 In the calculation of the reference wave formation potential, therefore, only SGL CARBON SE 2011/018 WO 11.04.2013 35 the half of the electrolysis cell located on one side of plane of symmetry 32 of the electrolytic cell must be explicitly included in the simulated volume, wherein the symmetry is taken into account by corresponding boundary conditions at the edge of the simulation volume corresponding to plane of 5 symmetry 32. Fig. 2 shows the distribution of the reference wave formation potential present at boundary surface 15 of electrolysis cell 10 from fig. 1, as viewed from above, for one of the two symmetrical halves of electrolysis cell 10, 10 wherein equipotential lines of the reference wave formation potential are shown specifically in fig. 2. The outer contour of cathode 12 of electrolysis cell 10 is also represented. As can be seen from fig. 2, the reference wave formation potential of elec 15 trolysis cell 10 comprises a plurality of peaks 34, the maximum heights whereof can be seen in fig. 2 on the basis of the number of closed equipo tential lines lying inside one another. Fig. 3 shows one of the symmetrical halves of cathode 12 of electrolysis 20 cell 10 from fig. 1 in plan view. As a comparison of fig. 3 and fig. 2 shows, elevations 30 of the surface profiling of cathode 12 are disposed in each case vertically beneath peaks 34 of the reference wave formation potential, wherein peaks 34 and elevations 30, when viewed from above, are dis posed essentially congruent above one another. The correspondence be 25 tween peaks 34 in fig. 2 and elevations 30 in fig. 3 is marked by the corre sponding letter endings of reference numbers 30 and 34, i.e. elevation 30a corresponds to peak 34a etc. The shape of elevations 30 is adapted to the shape of respectively assigned 30 peaks 34 of the reference wave formation potential, wherein elevations 30 SGL CARBON SE 2011/018 WO 11.04.2013 36 approximate to the shape of respectively assigned peaks 34 in each case by geometrically simplified shapes, such as for example by two essentially oval elevations 34g and 34j with ellipsoidal outer contours, an elevation 30h in the shape of a truncated pyramid, a plurality of elevations 34b, c, 5 e, 1, m, n, o in the shape of a truncated half-pyramid and two elevations 34a and 34c in the shape of a truncated quarter-pyramid in the corner regions of cathode 12. Fig. 4 illustrates in a perspective representation the three-dimensional 10 shape of elevations 30 adapted to the reference wave formation potential. Grooves 37, for busbars 20, disposed at the underside of cathode 12 can also be seen here (fig. 1). The distribution of the wave formation potential of electrolysis cell 10 with 15 surface-profiled cathode 16 present in boundary surface 15 between layer 14 of liquid aluminium and melt layer 16 is shown in fig. 5. As a compari son of the distribution of the wave formation potential shown in fig. 5 with the distribution of the reference wave formation potential shown in fig. 2 shows, the creation of a considerable uniformity or smoothing and a re 20 duction in the height of peaks 34 in the distribution of the wave formation potential is achieved by means of the surface profiling. Specifically, fig. 5 shows only peaks 34 which exhibit a maximum of two closed equipotential lines lying within one another. Thus, the maximum wave formation poten tial in the corresponding regions of boundary surface 15 is much smaller 25 than in the case of the distribution of the reference wave formation poten tial. The stability of electrolysis cell 10 during operation is thus considera bly increased and a longer service life and higher energy efficiency of elec trolysis cell 10 are thus achieved.

SGL CARBON SE 2011/018 WO 11.04.2013 37 Fig. 6 and fig. 7 show exemplary elevations 30 which are particularly suit able for a surface profiling of an electrolysis cell 10 according to the inven tion. Elevations 30 shown in fig. 6 can in each case be generated by geo metrical extrusion. Fig. 6a-c show respectively polygonal, ellipsoidal and 5 other base surfaces 36, proceeding from which elevation 30 is extruded. As indicated by an arrow 39, the extrusion takes place in each case in vertical direction z. Fig. 6d shows an elevation 30 in the shape of a truncated pyramid, ex 10 truded proceeding from the base surface of fig. 6a. The geometrical extru sion in vertical direction z comprises a scaling of the area with increasing vertical height, so that resultant elevation 30 is continuously tapered up wards. The reference axis of the scaling is the vertical axis proceeding from centroid point 38 of base surface 36. Elevation 30 shown in fig. 6d 15 results from an isotropic scaling, wherein the area is contracted in all directions normal to the extrusion direction, as it were to the extrusion axis. Fig. 6e also shows an elevation 30 extruded from base surface 36 of fig. 6a, in which however an anisotropic scaling takes place, i.e. the area is scaled very differently in different directions normal to the extrusion direc 20 tion. Elevation 30 of fig. 6e corresponds, moreover, to an elevation which has been extruded along an axis diverging from the vertical by a small angular amount. Centroid point 38 of top surface 40 of resultant elevation 30 in fig. 6e, in contrast with centroid point 28 of top surface 40 in fig. 6d, is accordingly horizontally displaced with respect to centroid point 38 of 25 base surface 36. Fig. 6f and fig. 6g show in each case an elevation 30 extruded proceeding from ellipsoidal base surface 36 shown in fig. 6b, wherein elevation 30 shown in fig. 6f results from an isotropic and the elevation shown in fig. 30 6g from an anisotropic scaling of the area in the extrusion direction.

SGL CARBON SE 2011/018 WO 11.04.2013 38 Fig. 6h and fig. 6i show in each case an elevation 30 extruded proceeding from base surface 36 shown in fig. 6c, wherein elevation 30 shown in fig. 6h results from an isotropic and the elevation shown in fig. 6i from an 5 anisotropic scaling of the area in the extrusion direction. Fig. 7a-i show further exemplary elevations 30, which can be generated by geometrical rotation of a base surface 36. Fig. 7a-c show in each case different base surfaces 36, i.e. a polygonal base surface 30 in fig. 7a, a 10 semi-ellipsoidal base surface 30 in fig. 7b and a freely selected base sur face 30 in fig. 7c. An edge line of base surfaces 36 in each case forms axis of rotation 42 for the rotation. Fig. 7d and fig. 7e show in each case elevations 30, which are generated 15 proceeding from polygonal base surface 36 in fig. 7a, wherein, according to fig. 7d, the rotation body is generated exclusively by rotation and, ac cording to fig. 7e, the resultant rotation body is scaled anisotropically once again with respect to base surface 36 and the direction lying normal there to. 20 Fig. 7f and fig. 7g show in each case elevations 30, which are generated proceeding from semi-ellipsoidal base surface 36 in fig. 7b, wherein, in fig. 7f, the rotation body is generated exclusively by rotation and, in fig. 7g, the resultant rotation body is scaled anisotropically once again with re 25 spect to base surface 36 and the direction lying normal thereto. Fig. 7h and fig. 7i show in each case elevations 30, which are generated proceeding from base surface 36 in fig. 7c, wherein, in fig. 7h, the rotation body is generated exclusively by rotation and, in fig. 7i, the resultant rota- SGL CARBON SE 2011/018 WO 11.04.2013 39 tion body is scaled anisotropically once again with respect to base surface 36 and the direction lying normal thereto.

SGL CARBON SE 2011/018 WO 11.04.2013 40 List of reference numbers 10 electrolysis cell 5 12 cathode 14 layer of liquid aluminium 15 boundary surface 16 melt layer 18 anode 10 20 busbar 22 current discharge 24 anode block 26 anode tree 28 current supply 15 30 elevation 32 plane of symmetry 34 peak 36 base surface 37 groove 20 38 centroid point 39 arrow 40 top surface 42 axis of rotation x longitudinal direction of the electrolysis cell 25 y transverse direction of the electrolysis cell

Claims

11.04.2013 Claims 5 1. An electrolysis cell, in particular for the production of aluminium, comprising a cathode (12), on the upper side of the cathode (12) a layer (14) of liquid aluminium, thereon a melt layer (16) and above the melt layer (16) an anode (18), wherein the cathode (12) compris es at its upper side a surface profiling formed by two or more eleva 10 tions (30), wherein the surface profiling of the cathode (12) is consti tuted and disposed in such a way that an elevation (30) is in each case provided at at least two of the twenty points of the surface of the upper side of the cathode (12) which in each case are disposed vertically beneath those regions of the boundary surface (15) be 15 tween the layer (14) of liquid aluminium and the melt layer (16) in which the peaks (34) with the twenty highest maximum values are present in the distribution of the reference wave formation potential present in the boundary surface (15), wherein a reference wave for mation potential is defined as the wave formation potential which, 20 during the operation of the electrolysis cell (10) with - instead of the cathode (12) with the surface profiling - a reference cathode without surface profiling, but an otherwise identical configuration to the cathode (12) with surface profiling, is present at a point in the boundary surface (15) between the layer (14) of liquid aluminium 25 and the melt layer (16). 2. An electrolysis cell, in particular for the production of aluminium, comprising a cathode (12), on the upper side of the cathode (12) a layer (14) of liquid aluminium, thereon a melt layer (16) and above 30 the melt layer (16) an anode (18), wherein the cathode (12) compris- SGL CARBON SE 2011/018 WO 11.04.2013 2 es at its upper side a surface profiling formed by two or more eleva tions (30), wherein the surface profiling of the cathode (12) is consti tuted and disposed in such a way that an elevation (30) is in each case provided at at least two of the twenty points of the surface of 5 the upper side of the cathode (12) which in each case are disposed vertically beneath those regions of the boundary surface (15) be tween the layer (14) of liquid aluminium and the melt layer (16) in which the peaks (34) with the twenty highest maximum values are present in the distribution of the reference wave formation potential 10 present in the boundary surface (15), wherein a reference wave for mation potential is defined as the wave formation potential which, during the operation of the electrolysis cell (10) with - instead of the cathode (12) with the surface profiling - a reference cathode without surface profiling, but an otherwise identical configuration to the 15 cathode (12) with surface profiling, wherein the reference cathode is disposed with regard to its height in the electrolysis cell (10) in such a way that the same volume for the layer (14) of liquid aluminium and the melt layer (16) is provided between the reference cathode and the anode (18) as in the case of the electrolysis cell (10) with the 20 cathode (12) with surface profiling, is present at a point in the boundary surface (15) between the layer (14) of liquid aluminium and the melt layer (16). 3. The electrolysis cell according to claim 1 or 2, 25 characterised in that an elevation (30) is provided in each case at at least X of the Y points of the surface of the upper side of the cathode (12) which are in each case disposed vertically beneath those regions of the bound ary surface (15) between the layer (14) of liquid aluminium and the 30 melt layer (16) in which the peaks (34) with the Y highest maximum SGL CARBON SE 2011/018 WO 11.04.2013 3 values are present in the distribution of the reference wave for mation potential present in the boundary surface (15), wherein X = 4 and Y = 20, preferably X = 6 and Y = 20, particularly preferably X = 10 and Y = 20 and very particularly preferably X = 14 5 and Y= 20 and/or wherein X = 2 and Y = 10, preferably X = 3 and Y = 10, particularly preferably X = 5 and Y = 10 and very particularly preferably X = 7 andY = 10 and/or wherein X = 1 and Y = 5, preferably X = 2 and Y = 5, particularly 10 preferably X = 3 and Y = 5 and very particularly preferably X = 4 and Y = 5. 4. The electrolysis cell according to at least one of claims 1 to 3, characterised in that 15 at least one of the elevations (30) disposed at the points of the sur face of the upper side of the cathode (12) which in each case are disposed vertically beneath those regions of the boundary surface (15) between the layer (14) of liquid aluminium and the melt layer (16) in which in each case a peak (34) is present in the distribution 20 of the reference wave formation potential present in the boundary surface (15) has its maximum height at the point disposed vertically beneath the point of the boundary surface (15) between the layer (14) of liquid aluminium and the melt layer (16) at which the peak (34) of the distribution of the reference wave formation potential has 25 its maximum value. 5. The electrolysis cell according to claim 4, characterised in that all the elevations at the points of the surface of the upper side of the 30 cathode (12) which in each case are vertically beneath those regions SGL CARBON SE 2011/018 WO 11.04.2013 4 of the boundary surface (15) between the layer (14) of liquid alumin ium and the melt layer (16) in which a peak (34) is in each case pre sent in the distribution of the reference wave formation potential present in the boundary surface (15) have in each case their maxi 5 mum height at the point disposed vertically beneath the point of the boundary surface (15) between the layer (14) of liquid aluminium and the melt layer (16) at which the respective peaks (34) of the dis tribution of the reference wave formation potential have their maxi mum value. 10 6. The electrolysis cell according to at least one of the preceding claims, characterised in that the geometrical outer contour of at least one of the elevations (30) is 15 at least essentially similar in plan view to the geometrical outer con tour of the respective peaks (34) of the distribution of the reference wave formation potential in plan view. 7. The electrolysis cell according to claim 6, 20 characterised in that the geometrical outer contours of all the elevations (30) are at least essentially similar in plan view to the geometrical outer contour of the respective peak (34) of the distribution of the reference wave formation potential in plan view. 25 8. The electrolysis cell according to at least one of the preceding claims, characterised in that the geometrical outer contour of at least one of the elevations (30) in 30 plan view is constituted at least in sections at least approximately SGL CARBON SE 2011/018 WO 11.04.2013 5 polygonal and/or ellipsoidal, wherein the polygon has in particular 3, 4, 5 or 6 corners. 9. The electrolysis cell according to at least one of the preceding 5 claims, characterised in that at least one of the elevations (30) has an outer contour, viewed in plan view, which is geometrically simpler than the outer contour, viewed in plan view, of the peak (34) of the distribution of the refer 10 ence wave formation potential disposed vertically above the elevation (30) in the boundary surface (15), wherein the outer contour of the elevation (30), viewed in plan view, preferably has a smaller number of corners and/or a smaller number of points of inflection than the outer contour of the peak (34) viewed in plan view. 15 10. The electrolysis cell according to at least one of the preceding claims, characterised in that the three-dimensional shape of at least one of the elevations (30) is 20 at least essentially similar to the three-dimensional shape of the re spective peak (34) of the distribution of the reference wave formation potential or corresponds to the latter. 11. The electrolysis cell according to claim 10, 25 characterised in that the three-dimensional shapes of all the elevations (30) are at least essentially similar to the three-dimensional shape of the respective peak (34) of the distribution of the reference wave formation poten tial or correspond to the latter. 30 SGL CARBON SE 2011/018 WO 11.04.2013 6

12. The electrolysis cell according to at least one of the preceding claims, characterised in that at least one of the elevations (30) has a three-dimensional shape 5 tapering upwards in vertical direction (z).

13. The electrolysis cell according to at least one of the preceding claims, characterised in that 10 at least one of the elevations (30), viewed upwards in vertical direc tion (z), is bounded by a top surface (40) which, viewed in plan view, has a smaller area than the base surface (36) of the elevation (30) viewed in plan view. 15 14. The electrolysis cell according to at least one of the preceding claims, characterised in that at least one of the elevations (30) has a three-dimensional shape which, proceeding from the base surface (36) of the elevation (30), 20 can be generated by rotating the base surface (36) about an axis of rotation (42) bordering the base surface (36), wherein the axis of ro tation (42) preferably runs horizontally.

15. The electrolysis cell according to at least one of the preceding 25 claims, characterised in that at least one of the elevations (30) has a three-dimensional shape which, proceeding from the base surface (36) of the elevation (30), can be generated by geometrical extrusion of the base surface (36) of 30 the elevation (30) upwards in vertical direction (z). SGL CARBON SE 2011/018 WO 11.04.2013 7

16. The electrolysis cell according to claim 15, characterised in that the at least one elevation (30) tapers upwards in vertical direction 5 (z).

17. The electrolysis cell according to at least one of the preceding claims, characterised in that 10 the cathode (12) comprises two or more cathode blocks and/or that the anode (18) comprises two or more anode blocks (24).

18. The electrolysis cell according to at least one of the preceding claims, 15 characterised in that the distance between the anode (18) and the layer (14) of liquid alu minium amounts to between 15 and 45 mm, preferably between 15 and 35 mm and particularly preferably between 15 and 25 mm. 20 19. The electrolysis cell according to at least one of the preceding claims, characterised in that the surface profiling of the cathode (12) is constituted irregular at least in one direction. 25

20. A cathode for an aluminium electrolysis cell, the upper side whereof comprises a surface profiling with two or more first webs (30) run ning essentially in a first direction (y) of the cathode (12) and at least one second web (30) running at least essentially in the direction (x) 30 normal to the first direction (y) of the cathode (12). SGL CARBON SE 2011/018 WO 11.04.2013 8

21. The cathode according to claim 20, characterised in that the at least two first webs (30) run at least approximately in the 5 transverse direction (y) of the cathode.

22. The cathode according to claim 20 or 21, characterised in that the upper side of the cathode (12) has, in plan view, an essentially 10 rectangular outer contour, wherein an elevation (30) of the cathode (12) is provided at least in one of the four corners of the essentially rectangular outer contour, wherein the elevation (30) preferably has an essentially triangular outer contour in plan view. 15 23. The cathode according to at least one of claims 20 to 22, characterised in that the upper side of the cathode (12) comprises a depression in the form of a trough which is at least essentially V-shaped when viewed in the cross-section of the cathode (12), wherein the at least two first 20 webs (30) and the at least one second web (30) are preferably dis posed on the surface of the essentially V-shaped depression.

24. The cathode according to claim 23, characterised in that 25 the connection point between the two legs of the cross-section of the depression constituted essentially in the form of a V-shaped trough, as viewed in the cross-section of the cathode (12), is disposed at least approximately in the middle of the cathode (12). 30 25. The cathode according to claim 23 or 24, SGL CARBON SE 2011/018 WO 11.04.2013 9 characterised in that the depression extends over at least 75 %, preferably over at least 90 %, particularly preferably over at least 95 % and very particularly preferably over 100 % of the surface of the cathode (12). 5

26. The cathode according to at least one of preceding claims 20 to 25, characterised in that the at least one second web (30), viewed in the second direction (x) of the cathode (12), is disposed at least approximately in the middle 10 of the cathode (12).

27. The cathode according to at least one of preceding claims 20 to 26, characterised in that the upper edge of at least one of the first webs (30), viewed in the 15 transverse direction (y) of the cathode (12), has a distance from the bottom of the V-shaped trough increasing towards the middle of the cathode (12).

28. An electrolysis cell, in particular for the production of aluminium, 20 comprising a cathode (12) according to at least one of claims 20 to 27, on the upper surface of the cathode (12) a layer (14) of liquid al uminium, thereon a melt layer (16) and above the melt layer (16) an anode (18). 25 29. The electrolysis cell according to claim 28, characterised in that the anode (18) comprises at least two anode blocks (24) disposed beside one another and a joint extends between the at least two an ode blocks (24), wherein at least one of the first webs (30) of the SGL CARBON SE 2011/018 WO 11.04.2013 10 cathode (12) is disposed vertically beneath and at least essentially parallel to the joint constituted between the two anode blocks (24).

30. The electrolysis cell according to claim 29, 5 characterised in that the at least one first web (30) disposed vertically beneath the joint is preferably disposed in an at least approximately centered manner in relation to the joint. 10 31. A method for the production of an electrolysis cell (10), in particular an electrolysis cell (10) for the production of aluminium, which comprises a cathode (12), on the upper side of the cathode (12) a layer (14) of liquid aluminium, thereon a melt layer (16) and above the melt layer (16) an anode (18), wherein the method comprises the 15 steps: - ascertainment of the distribution of the reference wave for mation potential present in the boundary surface (15) between the layer (14) of liquid aluminium and the melt layer (16) of the electrolysis cell, 20 - production of a surface profiling comprising a plurality of ele vations (30) on the upper side of the cathode (12), wherein an elevation (30) is provided in each case at at least two of the twenty points of the surface of the upper side of the cathode (12) which in each case are disposed vertically beneath those 25 regions of the boundary surface (15) between the layer (14) of liquid aluminium and the melt layer (16) in which the peaks (34) with the twenty highest maximum values are present in the distribution of the reference wave formation potential pre sent in the boundary surface (15), SGL CARBON SE 2011/018 WO 11.04.2013 11 wherein a reference wave formation potential is defined as the wave formation potential which, during the operation of the electrolysis cell (10) with - instead of the cathode (12) with the surface profiling a reference cathode without surface profiling, but an otherwise iden 5 tical configuration to the cathode (12) with surface profiling, is pre sent at a point in the boundary surface (15) between the layer (14) of liquid aluminium and the melt layer (16).

32. A method for the production of an electrolysis cell (10), in particular 10 an electrolysis cell (10) for the production of aluminium, which comprises a cathode (12), on the upper side of the cathode (12) a layer (14) of liquid aluminium, thereon a melt layer (16) and above the melt layer (16) an anode (18), wherein the method comprises the steps: 15 - ascertainment of the distribution of the reference wave for mation potential present in the boundary surface (15) between the layer (14) of liquid aluminium and the melt layer (16) of the electrolysis cell (10), - production of a surface profiling comprising a plurality of ele 20 vations (30) on the upper side of the cathode (12), wherein an elevation (30) is provided in each case at at least two of the twenty points of the surface of the upper side of the cathode (12) which in each case are disposed vertically beneath those regions of the boundary surface (15) between the layer (14) of 25 liquid aluminium and the melt layer (16) in which the peaks (34) with the twenty highest maximum values are present in the distribution of the reference wave formation potential pre sent in the boundary surface (15), wherein a reference wave formation potential is defined as the wave 30 formation potential which, during the operation of the electrolysis SGL CARBON SE 2011/018 WO 11.04.2013 12 cell (10) with - instead of the cathode (12) with the surface profiling a reference cathode without surface profiling, but an otherwise iden tical configuration to the cathode (12) with surface profiling, wherein the reference cathode is disposed with regard to its height in the 5 electrolysis cell (10) in such a way that the same volume for the lay er (14) of liquid aluminium and the melt layer (16) is provided be tween the reference electrode and the anode (18) as in the case of the electrolysis cell (10) with the cathode (12) with surface profiling, is present at a point in the boundary surface (15) between the layer 10 (14) of liquid aluminium and the melt layer (16).

33. Use of a method according to claim 31 or 32 for the production of an electrolysis cell (10) according to at least one of preceding claims 1 to 19. 15