US20140076723A1 - Electrolysis cell and cathode with irregular surface profiling - Google Patents

Abstract

An electrolysis cell for the production of aluminum has a liquid aluminum layer on a cathode, a melt layer on the liquid aluminum, and an anode above the melt layer. An upper side of the cathode has surface profiling with two or more elevations provided at at least two of the twenty points of the surface of the upper side of the cathode vertically beneath those regions of the boundary surface between the layer of liquid aluminum and the melt layer in which peaks with the twenty highest maxima are present in the distribution of a reference wave formation potential in the boundary surface. The reference wave formation potential is defined as the wave formation potential which, when the electrolysis cell is operated with a reference cathode without surface profiling is present at a point in the boundary surface between the layer of liquid aluminum and the melt layer.

Description

CROSS-REFERENCE TO RELATED APPLICATION
• This application is a continuation, under 35 U.S.C. §120, of copending international patent application No. PCT/EP2012/057524, filed Apr. 25, 2012, which designated the United States; the application also claims the priority, under 35U.S.C. §119, of German patent application No. 10 2011 076 302.3, filed May 23, 2011; the prior applications are herewith incorporated by reference in their entirety.
BACKGROUND OF THE INVENTION Field of the Invention
• The present invention relates to an electrolysis cell, in particular for the production of aluminum, 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 aluminum, which is carried out industrially usually according to the Hall-Héroult process. In the Hall-Héroult process, a melt composed of aluminum oxide and cryolite is electrolyzed. The cryolite, Na₃[AlF₆], is used to lower the melting point of 2,045° C. for pure aluminum oxide to approx. 950° C. for a mixture containing cryolite, aluminum oxide and additives, such as aluminum fluoride and calcium fluoride.
• The electrolysis cell used in this process comprises a cathode base, which can comprise a large number of cathode blocks lying adjacent to one another forming the cathode. In order to withstand the thermal and chemical 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 usually 15 to 50 cm high layer of liquid aluminum present on the upper side of the cathode is an anode constituted by individual anode blocks, the electrolyte, i.e. the melt containing aluminum oxide and cryolite, being located between the latter and the surface of the aluminum. During the electrolysis carried out at approx. 1,000° C., the formed aluminum is deposited beneath the electrolyte layer on account of its greater density 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 electrolysis, the aluminum oxide dissolved in the melt is split up by the electric current flow to form aluminum and oxygen. Viewed electrochemically, the layer of liquid aluminum is the actual cathode, since aluminum ions are reduced to elementary aluminum 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 aluminum, but rather the component forming the electrolysis cell base, for example comprising one or more cathode blocks.
• A significant drawback of the Hall-Héroult process is that it is very energy-intensive. In order to produce 1 kg of aluminum, approx. 12 to 15 kWh of electrical energy is required, which accounts for up to 40% of the production costs. In order to be able to reduce the production costs, it is therefore desirable to reduce the specific energy consumption with this process as far as possible.
• On account of the relatively high electrical resistance of the melt, particularly compared to the layer of liquid aluminum and the cathode material, relatively high ohmic losses in the form of Joule dissipation occur especially 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 aluminum. However, on account of the electromagnetic interactions present during the electrolysis and the wave formation thus produced in the layer of liquid aluminum when there is an excessively small thickness of the melt layer, there is the risk of the layer of liquid aluminum coming into contact with the anode, which can lead to short-circuits of the electrolysis cell and to undesired reoxidation of the formed aluminum. Such 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 aluminum cannot be reduced arbitrarily.
• 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 aluminum and the melt during the operation of the electrolysis cell having a surface profiling. Published patent application US 2011/0056826 A1 discloses for example a cathode with a regularly constituted surface profiling. The horizontal and vertical fluctuations in the layer of liquid aluminum are intended to be reduced by the regularly constituted surface profiling, as a result of which the stability of the layer of liquid aluminum is intended to be increased. With such a regularly constituted surface profiling, however, the wave formation in the layer of liquid aluminum is reduced only to a limited extent and in particular not uniformly over the whole cathode surface. Furthermore, this known regular surface profiling in the cathode block surface leads, due to the reduced movement in the layer of liquid aluminum, indirectly to a considerable hindrance of the mixing in the melt layer located above the latter which is required for the dissolution of the periodically supplied aluminum oxide, and this proves to have a disadvantageous effect on the achievable energy efficiency of the electrolysis.
• European patent EP 0 938 598 B1 and German patent DE 101 64 008 C1 describe electrolysis cells with 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 homogeneous as possible arises at the upper side of the cathode. In the case of these electrolysis cells, however, a comparatively marked wave formation also takes place in the layer of liquid aluminum, for which reason a reduction in the specific energy consumption in the electrolysis cell and an increase in its service life are not possible.
BRIEF SUMMARY OF THE INVENTION
• It is accordingly an object of the invention to provide a electrolysis cell and cathode which overcome the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which provides for 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 reoxidation of the formed aluminum, occurring in the layer of liquid aluminum as a result of a wave formation tendency that is thereby increased. At the same time, the electrolysis cell according to the invention should ensure sufficient mixing in the melt layer during its operation.
• With the foregoing and other objects in view there is provided, in accordance with the invention, an electrolysis cell for the production of aluminum, comprising:
• a cathode having an upper side formed with a surface profiling of two or more elevations;
• during an operation of the electrolysis cell, a layer of liquid aluminum on said upper side of said cathode, a melt layer on the layer of liquid aluminum, and an anode above the melt layer;
• said surface profiling of said cathode being configured and disposed in such a way that an elevation is in each case provided at two or more of twenty points of a surface of said upper side of said cathode that are in each case disposed vertically beneath those regions of a boundary surface between the layer of liquid aluminum and the melt layer in which peaks with the twenty highest maximum values are present in a distribution of a reference wave formation potential present in the boundary surface;
• the reference wave formation potential being defined as a wave formation potential which, during the operation of the electrolysis cell with a reference cathode having no surface profiling being used instead of said cathode with said surface profiling, but having an otherwise identical configuration as said cathode with said surface profiling, is present at a point in the boundary surface between the layer of liquid aluminum and the melt layer.
• In other words, the objects are achieved by making available an electrolysis cell as claimed, and particular an electrolysis cell for the production of aluminum which comprising a cathode, on the upper side of the cathode a layer of liquid aluminum, thereon a melt layer which contains aluminum oxide and cryolite, and above the melt layer an anode, wherein the cathode comprises at its upper side a 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 disposed vertically beneath those regions of the boundary surface between the layer of liquid aluminum 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 formation 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 cathode with surface profiling, is present at a point in the boundary surface between the layer of liquid aluminum and the melt layer.
• 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 electrolysis cell, the formation of marked peaks in the wave formation potential is avoided in a targeted manner in the boundary surface between the layer of liquid aluminum 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 corresponding cathode without surface profiling.
• 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 cathode. The term base plane denotes the horizontal plane of the cathode which lies farthest in the direction of the anode and which runs through 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 aluminum. The height of an elevation of the surface profiling is therefore the distance of the uppermost 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 surface between the layer of liquid aluminum and the melt layer during the operation of the electrolysis cell is the driving force for the wave formation in the layer of liquid aluminum, and also in particular that distribution of the wave formation potential in the case of conventional electrolysis cells is not uniform over the boundary surface between the layer of liquid aluminum and the melt layer, but on the contrary is extremely heterogeneous. Due to the reduction in the wave formation potential provided according 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 aluminum and the melt being made uniform, a wave formation in the layer of liquid aluminum is reliably prevented or at least considerably reduced during the operation of the electrolysis cell according to the invention, as a result of which the thickness of the melt layer can be reduced compared to conventional electrolysis cells and the efficiency of the electrolysis cell according to the invention is thus considerably increased.
• A further important finding of the present invention is that the heterogeneous distribution of the wave formation potential present in the boundary surface between the layer of liquid aluminum and the melt layer in the case of conventional electrolysis cells can be directly influenced by the 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 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 the path crosses the boundary surface between the layer of liquid aluminum 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 aluminum and in the melt layer. Since these materials each have different specific electrical resistance values, the melt layer and also the cathode material in particular having a higher specific electrical resistance than the liquid aluminum, and because the individual current paths have different path lengths in the cathode block, in the layer of liquid aluminum and in the melt layer, the total electrical resistances along the individual paths and thus also the individual current densities over the boundary surface between the layer of liquid aluminum 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 elevations of the surface profiling of the cathode, the path lengths of the individual current paths in the various sections, i.e. cathode block, layer of liquid aluminum 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 aluminum and the melt layer during operation of the electrolysis cell, no marked peaks arise in the distribution of the wave formation potential present in this boundary surface, as a result of which an essentially uniform and low distribution of the wave formation potential is guaranteed.
• In order to optimize the position, the shape and the length of the elevations of the surface profiling of the cathode, the present invention proceeds from the distribution of the reference wave formation potential which results during the operation of the electrolysis cell with a conventional, unprofiled reference cathode, and provides elevations in a targeted manner at the points of the cathode surface which are disposed vertically 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 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 surface profiling, i.e. with a horizontal cathode surface, but with an otherwise identical configuration to the cathode with the surface profiling. According to the embodiment specified in claim 1, the reference electrolysis cell used to determine the reference wave formation potential is identical to the electrolysis cell according to the invention, except for the fact 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 additional volume on the upper side of the cathode arising due to the omission of the surface profiling is filled with liquid aluminum or melt—depending on the layer in which the corresponding material is present with the surface-profiled cathode.
• Especially in cases where many elevations occupying a considerable volume 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 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 aluminum and melt as in the case of the electrolysis cell with the surface-profiled cathode. Since, in this case, the reference wave formation 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 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 formation potential can be determined by computer-supported electrical, magnetic and magneto-hydrodynamic simulation of the movement and wave formation in the layer of liquid aluminum and the melt of the respective electrolysis cell.
• According to the present invention, the wave formation potential at an arbitrary point of the boundary surface between the layer of liquid aluminum 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 boundary surface, i.e. wave formation potential=|{right arrow over (u)}·{right arrow over (n)}|, wherein {right arrow over (u)} is the flow rate of the melt as a vector and {right arrow over (n)} 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 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 method (FEM) and the resulting fields are then used in the calculation of the flow conditions, which also takes place by means of simulation according to a finite element method (FEM). The software Comsol Multiphysics in the version 3.5a is used for both simulations. The boundary surface is assumed to be permeable, wherein the wave formation potential represents a local measure of the wave-driving flow directed towards the boundary surface. The simulated electrolysis cell, which comprises busbars, the current supplies of the electrolysis cell including a magnetic compensation geometry if applicable, the cathode, the layer of liquid aluminum, 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, exhibits one or more planes of symmetry, only the part of the electrolysis cell 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 conditions in the electrolysis cell, so that the simulation is based on the respective 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:
• • V: electric voltage, scalar
  • σ: electrical conductivity, scalar
  • E (bold type): electric field, vector
  • A (bold type): electric vector potential, vector
  • A_x, A_y, A_z: vector potential, component
  • H (bold type): magnetic field, vector
  • J, j (bold type), {right arrow over (j)}: electric current density, vector
  • B (bold type), {right arrow over (B)}: magnetic flux density, vector
  • I (bold type): unit matrix, tensor
  • F (bold type): force density (sum of Lorentz force density and gravitational force density), vector
  • u (bold type), {right arrow over (u)}: flow rate, vector
  • u (normal type), u_x, u_y, u_z: flow rate, component
  • p: pressure, scalar
  • μ: viscosity, scalar
  • ρ: density, scalar
  • L_c: characteristic length, e.g. depth of aluminum bath
  • v_c: characteristic speed
• Additional variables with turbulent flows:
• • μ_T: turbulent viscosity, scalar
  • k: turbulent kinetic energy
  • ep, ε: dissipation of turbulent kinetic energy
  • lw: distance from solid boundary surfaces
  • L_Ref: reference length scale, corresponds to characteristic length L_C
  • G: reciprocal distance from solid boundary surfaces
  • P_K: source term of turbulent kinetic energy
  • f_u: attenuation function viscosity
  • f_ε: attenuation function dissipation
  • R_t: turbulent Reynold's number
  • l*: limited mixing length
  • u_ε: turbulent dissipation rate of all grid cells
  • n (bold type), {right arrow over (n)}: normal vector to the boundary surface between the layer of liquid aluminum and the melt layer, vector
  • t (bold type), {right arrow over (t)}: tangential vector, vector)
  • {right arrow over (e_x)}, {right arrow over (e_y)}, {right arrow over (e_z)}: unit vectors, Cartesian coordinate system
• The constructed grids are sufficiently finely dimensioned, so that artifacts 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 simulations indicate insufficient grid fineness in the relevant areas.
• 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:

• TABLE 1
  $q=\frac{72\sqrt{3}V}{{\left(h_1^2+{\mathrm{<figure-callout\; id="2"\; label="h"\; filenames="US20140076723A1-20140320-D00002.png"\; state="\{\{state\}\}">h</figure-callout>}}_2^2+h_3^2+h_4^2+h_5^2+h_6^2\right)}^{3/2^{\prime }}}$	for tetrahedral grid cells
  $q=\frac{36\sqrt{3}V}{{\left(\sum _{i=1}^9h_i^2\right)}^{3/2^{\prime }}}$	for prismatic grid cells
  with
  V = volume of the grid cell and
  hi = edge lengths of the grid cell.

• In detail, the construction of the grid takes place as follows:
• The air surrounding the electrolysis cell is modeled with an unlimited size of the grid cells, which can vary between fine regions (e.g. at the melt layer) and coarse regions (e.g. surrounding edges of the overall arrangement). 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 an edge length in the region of approx. 30 cm.
• The layer of liquid aluminum and the melt layer are modeled such that the grid cells that form the boundary surface between the layer of liquid aluminum and the melt each have an edge length in the region of approx. 3 cm. The melt layer is modeled 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 between the layer of liquid aluminum and the melt is not curved and therefore runs horizontally. Accordingly, normal vector n is adopted as vertical unit vector e_z and the wave formation potential is accordingly defined as the absolute value of vertical component u_z of the flow rate in the boundary surface.
• The layer of liquid aluminum and the cathode are modeled such that the grid cells that form the boundary surface between the cathode and the layer of liquid aluminum have an edge length in the region of approx. 5 cm.
• The anodes and cathodes are otherwise modeled 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 limited to at most 1.65 in order to avoid distorted grid elements.
• In the case of electrolysis cells defined below and operated under turbulent flow conditions, the solid boundary surfaces between the individual components of the electrolysis cell are modeled in the cell construction by 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 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 aluminum and the melt layer are additionally provided with values for the viscosity and density of the aluminum and the melt.
• The following values are taken as a basis for the material properties:

• TABLE 2
  Specific resistance [in Ohm · m]

  	Cathode	1.2 · 10−5
  	Busbars made of steel	7.78 · 10−7
  	Liquid aluminum	2.8 · 10−7
  	Melt	4.84 · 10−3
  	Anode	4.0 · 10−5
  	Anode tree (aluminum)	2.34 · 10−7

  Viscosity [in Pa · s]

  	Liquid aluminum	9.0 · 10−5
  	Melt	2.34 · 10−3

  Density [in kg/m3]

  	Liquid aluminum	2.3 · 103
  	Melt	2.08 · 103

• All the other material properties used in the simulation are selected such that they correspond to the actual properties of the respective material.
• For the numeric stabilization 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 aluminum 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 aluminum 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 essentially linear property transition results from the properties of the cells representing the aluminum layer given in above table 2 to the properties 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 contribute to the current transport.
• For the grid structure thus constructed, which reproduces the electrolysis cell in its geometry and therewith its material properties, the electromagnetic fields are calculated and the ascertained the values are then inserted into the calculation of the flow-mechanical movements of the melt of the electrolysis cell.
• The first step of the modeling of the electromagnetics is based on the known stationary Maxwell equations:
• 
  ∇·J=10
• 
  ∇×H=j
• 
  J=σE+J _e
• 
  E=−∇V
• 
  B=∇×A
• Lagrange functions (1st order for V and 2nd order for A) are used as starting functions for the finite element methods.
• These partial differential equations are solved for the whole geometry by numeric calculation. The boundary conditions to be used thereby are explained more precisely below; in particular, the operating current of the electrolysis cell fed through the cathode and anode enters into the calculation 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.
• 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
• $\mathrm{Re}=\frac{\rho v_CL_C}{\mu }$
• is used and, depending thereon, the following equation systems:
• The following equations (Navier-Stokes equations) are used for laminar and weakly turbulent problems with Re<10,000:
• $\rho \left(u·\nabla \right)u=\nabla ·\left[\rho I+\mu \left(\nabla u+{\left(\nabla u\right)}^T\right)-\frac{2}{3}\mu \left(\nabla ·u\right)I\right]+F$ $\nabla ·\left(\rho u\right)=0$
• Lagrange functions (1st order for p and 2nd order for u) are used as starting functions for the finite element methods.
• The following equations (Low Reynold's k-epsilon equations) are used for flows in the transition region with Re≧10,000 and <100,000:
• $\rho \left(u·\nabla \right)u=\nabla ·\left[-\rho I+\left(\mu +{\mu }_T\right)\left(\nabla u+{\left(\nabla u\right)}^T\right)-\frac{2}{3}\left(\mu +{\mu }_T\right)\left(\nabla ·u\right)I-\frac{2}{3}\rho \mathrm{kI}\right]+F$ $\nabla ·\left(\rho u\right)=0$ $\rho \left(u·\nabla \right)k=\nabla ·\left[\left(\mu +\frac{{\mu }_T}{{\sigma }_k}\right)\nabla k\right]+P_k-\mathrm{\rho \varepsilon }$ $\rho \left(u·\nabla \right)\varepsilon =\nabla ·\left[\left(\mu +\frac{{\mu }_T}{{\sigma }_e}\right)\nabla \varepsilon \right]+C_{\varepsilon 1}\frac{\varepsilon }{k}P_k-C_{\mathrm{\varepsilon 2}}\rho \frac{{\varepsilon }^2}{k}f_{\varepsilon }\left(\rho ,\mu ,k,\varepsilon ,I_w\right),\varepsilon =\mathrm{ep}$ $\nabla G·\nabla G+{\sigma }_wG\left(\nabla ·\nabla G\right)=\left(1+2{\sigma }_w\right)G^4,I_w=\frac{1}{G}-\frac{{\mathrm{<figure-callout\; id="2"\; label="I\; ref"\; filenames="US20140076723A1-20140320-D00002.png"\; state="\{\{state\}\}">I</figure-callout>}}_{\mathrm{ref}}}{2}$ ${\mu }_T=\rho C_{\mu }\frac{{\mathrm{<figure-callout\; id="2"\; label="k"\; filenames="US20140076723A1-20140320-D00002.png"\; state="\{\{state\}\}">k</figure-callout>}}^2}{\varepsilon }f_{\mu }\left(\rho ,\mu ,k,\varepsilon ,I_w\right),P_k={\mu }_T\left[\nabla u:\left(\nabla u+{\left(\nabla u\right)}^T\right)-\frac{2}{3}{\left(\nabla ·u\right)}^2\right]-\frac{2}{3}\rho k\nabla ·u$ $f_{\mu }={\left(1-{}^{-l^{*}/14}\right)}^2·\left(1+\frac{5}{R_t^{3/4}}{}^{-{\left(R/200\right)}^2}\right)$ $f_{\varepsilon }={\left(1-{}^{-l^{*}/3.1}\right)}^2·\left(1-0.3{}^{-{\left(R/6.5\right)}^2}\right)$ $l^{*}=\left(\rho u_{\varepsilon }l_w\right)/\mu$ $R_t=\rho {\mathrm{<figure-callout\; id="2"\; label="k"\; filenames="US20140076723A1-20140320-D00002.png"\; state="\{\{state\}\}">k</figure-callout>}}^2/\left(\mathrm{\mu \varepsilon }\right)$ $u_{\varepsilon }={\left(\mathrm{\mu \varepsilon }/\rho \right)}^{1/4}$
• 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:
• $\rho \left(u·\nabla \right)u=\nabla ·\left[-\rho I+\left(\mu +{\mu }_T\right)\left(\nabla u+{\left(\nabla u\right)}^T\right)-\frac{2}{3}\left(\mu +{\mu }_T\right)\left(\nabla ·u\right)I-\frac{2}{3}\rho \mathrm{kI}\right]+F$ $\nabla ·\left(\rho u\right)=0$ $\rho \left(u·\nabla \right)k=\nabla ·\left[\left(\mu +\frac{{\mu }_T}{{\sigma }_k}\right)\nabla k\right]+P_k-\mathrm{\rho \varepsilon }$ $\rho \left(u·\nabla \right)\varepsilon =\nabla ·\left[\left(\mu +\frac{{\mu }_T}{{\sigma }_c}\right)\nabla \varepsilon \right]+C_{\varepsilon 1}\frac{\varepsilon }{k}P_k-C_{\varepsilon 2}\rho \frac{{\varepsilon }^2}{k},\varepsilon =\mathrm{ep}$ ${\mu }_T=\rho C_{\mu }\frac{{\mathrm{<figure-callout\; id="2"\; label="k"\; filenames="US20140076723A1-20140320-D00002.png"\; state="\{\{state\}\}">k</figure-callout>}}^2}{\varepsilon },P_k={\mu }_T\left[\nabla u:\left(\nabla u+{\left(\nabla u\right)}^T\right)-\frac{2}{3}{\left(\nabla ·u\right)}^2\right]-\frac{2}{3}\rho k\nabla ·u$
• wherein C_μ0.09; C_ε1=1.44; C_ε2=1.02; σ_k=1.0 and σ_ε=1.3.
• 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 values previously calculated in the electromagnetic consideration in the form of the Lorentz force density {right arrow over (F)}_Ltz={right arrow over (j)}×{right arrow over (B)} also enter into the above equations. Lorentz force density {right arrow over (F)}_Liz forms, together with gravitational force density {right arrow over (F)}_g=−ρg{right arrow over (e)}_z external excitation F according to {right arrow over (F)}={right arrow over (F)}_Ltz+{right arrow over (F)}g contained in the above equations.
• The above flow-mechanical partial differential equations are also solved numerically.
• 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 during the electromagnetic calculation:
• • The external faces of the treated volume are regarded as an electrical insulator (−n·j=0).
  • Any symmetrical faces present are regarded as an electrical insulator (−n·j=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).
  • 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:
• • The magnetic flux is parallel to the external face (n×A=0) at the external faces of the treated volume.
  • A magnetic symmetry (n×H=0) is present at any symmetrical faces that may be present.
  • The calculated magnetic vector potential A is continuous at all the internal faces.
• The following boundary conditions relate to the flow fields calculated during the flow-mechanical calculation:
• • The following holds at the solid boundary surfaces:
    • When use is made of the laminar equations: The liquid adheres firmly to the solid boundary surface, which is also denoted as “No slip”, i.e. the speed u=0.
    • When use is made of the turbulent equations, a wall model is used which takes account of the friction between the respective liquid layer and the solid boundary surface.
  • An open boundary surface is present at any symmetrical faces that may be present, wherein the normal flow in relation to the boundary surface is calculated with f₀=0 according to the equation
• $\left[-\rho I+\mu \left(\nabla u+{\left(\nabla u\right)}^T\right)-\frac{2}{3}\mu \left(\nabla ·u\right)I\right]n=-f_0n,u·t=0$
• • The calculated flow rate u is continuous at all the internal faces (e.g. the boundary surface between the layer of liquid aluminum and the melt layer).
• As described above, electromagnetic magnitudes V, A_x, A_y, A_z, j_x, j_{y a} and j_z are first calculated with the aid of the Maxwell equations and the Lorentz force density resulting therefrom is then inserted into the flow-mechanical equations used in each case in order to calculate therefrom flow field magnitudes u_x, u_y, u_{z a} and p. The electromagnetic calculation and the flow-mechanical calculation are therefore coupled together in a unidirectional manner.
• 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 mechanics such as the Streamline Diffusion (GLS) available in Comsol Multiphysics and Crosswind Diffusion as well as calibration of the vector potential 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 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 aluminum 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. 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 boundary surface between the layer of liquid aluminum 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 preferably 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 preferably X=5 and Y=10 and very particularly preferably X=7 and Y=10 and/or
• wherein X=1 and Y=5, preferably X=2 and Y=5, particularly preferably 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.
• 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 of liquid aluminum and the melt layer in which in each case a peak is present in the distribution of the reference wave formation potential present in the boundary surface has its maximum height at the point disposed vertically beneath the point of the boundary surface between the layer of liquid aluminum and the melt layer at which the peak of the distribution of the reference wave formation potential has its maximum value. 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 described 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 aluminum and the melt layer in which a peak is in each case present in the distribution of the reference wave formation potential present in the boundary surface have in each case their maximum height at the point disposed vertically beneath the point of the boundary surface between the layer of liquid aluminum and the melt layer at which the respective peaks of the distribution of the reference wave formation potential have their maximum value.
• 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 distribution of the reference wave formation potential in plan view or essentially 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 another by geometrical mapping, which can be composed of concentric elongations and congruence mappings, such as in particular displacements, 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 form essentially two rectangles with at least approximately equal side ratios or form essentially two ellipses with at least approximately identical numeric eccentricities.
• It is particularly preferable for the geometrical outer contours of all the 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.
• 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 reference 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 elevation viewed vertically from above can advantageously be selected such that it can be generated by a simplification of the outer contour of the respective 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 than the outer contour, viewed in plan view, of the peak of the distribution of the reference wave formation potential disposed vertically above the elevation in the boundary surface. It is preferable for the sum of the numbers 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 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 distribution 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 regions of an outer contour.
• In order to prevent particularly effectively an increased wave formation of the layer of liquid aluminum caused by a peak in the reference wave formation potential, it has proved to be advantageous if the three-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 distribution of the reference wave formation potential or essentially corresponds to the latter.
• It is particularly preferred if the three-dimensional shapes of all the elevations are at least essentially similar to the three-dimensional shape of the respective peak of the distribution of the reference wave formation potential or essentially correspond to the latter.
• 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 avoidance of wave formation in the region of a peak in the distribution of the reference wave formation potential. The at least one elevation, when 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 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 truncated pyramid.
• 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 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° and at most 180°.
• A further advantageous embodiment of the present invention is characterized 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 by geometrical extrusion of the base surface of the elevation upwards in the vertical direction. The extrusion direction is preferably at least approximately 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 extrusion for the at least one elevation to taper upwards in the vertical direction. 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 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 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 if the distance between the anode and the layer of liquid aluminum 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.
• As described above, the surface profiling of the cathode is adapted according to the invention in such a way that marked peaks of the wave formation potential at individual points of the boundary surface between the layer of liquid aluminum and the melt layer are avoided. The result is surface profilings which are adapted in their position, size and shape to the specific properties of the electrolysis cell determining the wave formation 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 wave formation potential present in each case. Instead, the surface profiling of the cathode of an electrolysis cell according to the invention is constituted in practice irregular at least in one direction.
• The present invention further relates to a cathode for an aluminum electrolysis 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.
• Within the meaning of the present invention, a web is regarded as an elevation running at least essentially straight in the longitudinal direction.
• Within the scope of the present invention, it has been shown that a cathode with such a surface profiling is suitable for achieving, when used in electrolysis cells, a distribution of the wave formation potential in the boundary surface between the layer of liquid aluminum 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 surface are effectively avoided. The specifically described surface profiling is 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 taking account of these conditions.
• Such a cathode can in particular be a component part of one of the previously 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 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 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, 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 distribution of the reference wave formation potential are usually present in these corner regions, so that the stability of the electrolysis cell during 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 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 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 preferably disposed on the surface of the essentially V-shaped depression.
• According to a further advantageous embodiment of the present invention, 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, is disposed at least approximately in the middle of the cathode. In this way, the electric current density 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 distribution of the wave formation potential.
• In the present invention, it has proved to be advantageous for the depression to extend 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. 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 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.
• 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 wave formation potential in the middle of the cathode block and thus an increased wave formation in the layer of liquid aluminum 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 particular for the production of aluminum, which comprises at least one cathode as described above, on the upper side of the cathode a layer of liquid aluminum, 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 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 wherein at least one of the first webs of the cathode is disposed vertically beneath 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 20°. According to the invention, it has been recognised that these regions between the anode blocks usually have a markedly increased wave formation potential, so that the described measure contributes towards increasing 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.
• A further subject-matter of the present invention is a method for the production of an electrolysis cell.
• The method for the production of an electrolysis cell, in particular an electrolysis cell for the production of aluminum, which comprises a cathode, on the upper side of the cathode a layer of liquid aluminum, thereon a melt layer and above the melt layer an anode, comprises the following steps: ascertaining the distribution of the reference wave formation potential present in the boundary surface between the layer of liquid aluminum and the melt layer of the electrolysis cell;
• producing 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 upper side of the cathode which in each case are disposed vertically beneath those regions of the boundary surface between the layer of liquid aluminum 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 formation 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 cathode with surface profiling, is present at a point in the boundary surface between the layer of liquid aluminum 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 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 a further claim, there is defined a method for the production of an electrolysis cell, in particular an electrolysis cell for the production of aluminum, which comprises a cathode, on the upper side of the cathode a layer of liquid aluminum, thereon a melt layer and above the melt layer an anode. The further method comprises the steps:
• ascertainment of the distribution of the reference wave formation potential present in the boundary surface between the layer of liquid aluminum 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 upper side of the cathode which in each case are disposed vertically beneath those regions of the boundary surface between the layer of liquid aluminum 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 formation 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 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 aluminum and the melt layer is provided between the reference electrode and the anode as in the case of the electrolysis cell with the cathode with surface profiling, is present at a point in the boundary surface between the layer of liquid aluminum and the melt layer.
• Other features which are considered as characteristic for the invention are set forth in the appended claims.
• Although the invention is illustrated and described herein as embodied in an electrolysis cell and cathode with irregular surface profiling, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
• The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
• FIG. 1 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 between the layer of liquid aluminum and the melt layer in plan view;
• FIG. 3 shows the surface-profiled cathode of the electrolysis cell of FIG. 1 in plan view;
• 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 aluminum and the melt layer of the electrolysis cell of FIGS. 1 to 4;
• FIG. 6 (in partial views 6A to 6L) shows exemplary elevations for a surface profiling according to the invention; and
• FIG. 7 (in partial views 7A to 7L) shows further exemplary elevations for a surface profiling according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
• Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown an electrolysis cell 10 for the production of aluminum comprising a cathode 12, on the upper side of cathode 10 a layer 14 of liquid aluminum, thereon a melt layer 16 and above melt layer 16 an anode 18. Layer 14 of liquid aluminum and melt layer 16 merge into one another at a boundary surface 15.
• Cathode 12 comprises a plurality of elongated cathode blocks which extend in transverse direction y of electrolysis cell 10 and which are disposed beside one another in longitudinal direction x of electrolysis cell 10 and are connected to one another via a tamping-compound joint not represented. A busbar 20, which extends through the cathode block in longitudinal direction y of the cathode block and which makes electrical contact 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 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 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 explained below, to the distribution of the reference wave formation potential 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. In the calculation of the reference wave formation potential, therefore, only 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 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, 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 electrolysis 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 equipotential lines lying inside one another.
• FIG. 3 shows one of the symmetrical halves of cathode 12 of electrolysis 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 disposed essentially congruent above one another. The correspondence between peaks 34 in FIG. 2 and elevations 30 in FIG. 3 is marked by the corresponding letter endings of reference numbers 30 and 34, i.e. elevation 30 a corresponds to peak 34 a etc.
• The shape of elevations 30 is adapted to the shape of respectively assigned peaks 34 of the reference wave formation potential, wherein elevations 30 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 34 g and 34 j with ellipsoidal outer contours, an elevation 30 h in the shape of a truncated pyramid, a plurality of elevations 34 b, c, e, l, m, n, o in the shape of a truncated half-pyramid and two elevations 34 a and 34 c 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 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 surface-profiled cathode 16 present in boundary surface 15 between layer 14 of liquid aluminum and melt layer 16 is shown in FIG. 5. As a comparison 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 reduction 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 potential in the corresponding regions of boundary surface 15 is much smaller than in the case of the distribution of the reference wave formation potential. The stability of electrolysis cell 10 during operation is thus considerably increased and a longer service life and higher energy efficiency of electrolysis cell 10 are thus achieved.
• FIG. 6 and FIG. 7 show exemplary elevations 30 which are particularly suitable for a surface profiling of an electrolysis cell 10 according to the invention. Elevations 30 shown in FIG. 6 can in each case be generated by geometrical extrusion. FIG. 6 a-c show respectively polygonal, ellipsoidal and 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. 6 d shows an elevation 30 in the shape of a truncated pyramid, extruded proceeding from the base surface of FIG. 6 a. The geometrical extrusion in vertical direction z comprises a scaling of the area with increasing vertical height, so that resultant elevation 30 is continuously tapered upwards. The reference axis of the scaling is the vertical axis proceeding from centroid point 38 of base surface 36. Elevation 30 shown in FIG. 6 d 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. 6 e also shows an elevation 30 extruded from base surface 36 of FIG. 6 a, in which however an anisotropic scaling takes place, i.e. the area is scaled very differently in different directions normal to the extrusion direction. Elevation 30 of FIG. 6 e 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. 6 e, in contrast with centroid point 28 of top surface 40 in FIG. 6 d, is accordingly horizontally displaced with respect to centroid point 38 of base surface 36.
• FIG. 6 f and FIG. 6 g show in each case an elevation 30 extruded proceeding from ellipsoidal base surface 36 shown in FIG. 6 b, wherein elevation 30 shown in FIG. 6 f results from an isotropic and the elevation shown in FIG. 6 g from an anisotropic scaling of the area in the extrusion direction.
• FIG. 6 h and FIG. 6 i show in each case an elevation 30 extruded proceeding from base surface 36 shown in FIG. 6 c, wherein elevation 30 shown in FIG. 6 h results from an isotropic and the elevation shown in FIG. 6 i from an anisotropic scaling of the area in the extrusion direction.
• FIG. 7 a-i show further exemplary elevations 30, which can be generated by geometrical rotation of a base surface 36. FIG. 7 a-c show in each case different base surfaces 36, i.e. a polygonal base surface 30 in FIG. 7 a, a semi-ellipsoidal base surface 30 in FIG. 7 b and a freely selected base surface 30 in FIG. 7 c. An edge line of base surfaces 36 in each case forms axis of rotation 42 for the rotation.
• FIG. 7 d and FIG. 7 e show in each case elevations 30, which are generated proceeding from polygonal base surface 36 in FIG. 7 a, wherein, according to FIG. 7 d, the rotation body is generated exclusively by rotation and, according to FIG. 7 e, the resultant rotation body is scaled anisotropically once again with respect to base surface 36 and the direction lying normal thereto.
• FIG. 7 f and FIG. 7 g show in each case elevations 30, which are generated proceeding from semi-ellipsoidal base surface 36 in FIG. 7 b, wherein, in FIG. 7 f, the rotation body is generated exclusively by rotation and, in FIG. 7 g, the resultant rotation body is scaled anisotropically once again with respect to base surface 36 and the direction lying normal thereto.
• FIG. 7 h and FIG. 7 i show in each case elevations 30, which are generated proceeding from base surface 36 in FIG. 7 c, wherein, in FIG. 7 h, the rotation body is generated exclusively by rotation and, in FIG. 7 i, the resultant rotation body is scaled anisotropically once again with respect to base surface 36 and the direction lying normal thereto.

Claims (20)

1. An electrolysis cell for the production of aluminum, comprising:

a cathode having an upper side formed with a surface profiling of two or more elevations;

during an operation of the electrolysis cell, a layer of liquid aluminum on said upper side of said cathode, a melt layer on the layer of liquid aluminum, and an anode above the melt layer;

said surface profiling of said cathode being configured and disposed in such a way that an elevation is in each case provided at two or more of twenty points of a surface of said upper side of said cathode that are in each case disposed vertically beneath those regions of a boundary surface between the layer of liquid aluminum and the melt layer in which peaks with the twenty highest maximum values are present in a distribution of a reference wave formation potential present in the boundary surface;

the reference wave formation potential being defined as a wave formation potential which, during the operation of the electrolysis cell with a reference cathode having no surface profiling being used instead of said cathode with said surface profiling, but having an otherwise identical configuration as said cathode with said surface profiling, is present at a point in the boundary surface between the layer of liquid aluminum and the melt layer.

2. The electrolysis cell according to claim 1, wherein a respective elevation is provided in each case at at least X of Y points of the surface of said upper side of said cathode which are in each case disposed vertically beneath those regions of the boundary surface between the layer of liquid aluminum and the melt layer in which the peaks with Y highest maximum values are present in the distribution of the reference wave formation potential present in the boundary surface, and

wherein at least one of the following is true:

Y=20 and X is selected from the group consisting of 4, 6, 10 and 14;

Y=10 and X is selected from the group consisting of 2, 3, 5 and 7;

Y=5 and X is selected from the group consisting of 1, 2, 3 and 4.

3. The electrolysis cell according to claim 1, wherein at least one of said 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 of liquid aluminum and the melt layer in which a respective peak is present in the distribution of the reference wave formation potential present in the boundary surface has its maximum height at the point disposed vertically beneath the point of the boundary surface between the layer of liquid aluminum and the melt layer at which the peak of the distribution of the reference wave formation potential has its maximum value.

4. The electrolysis cell according to claim 1, wherein a geometrical outer contour of at least one of said elevations is substantially similar, in plan view, to a geometrical outer contour of the respective peaks of the distribution of the reference wave formation potential in plan view.

5. The electrolysis cell according to claim 1, wherein a geometrical outer contour of at least one of said elevations, in plan view, at least partially represents a substantial polygon with 3, 4, 5 or 6 corners or a substantial ellipsoid.

6. The electrolysis cell according to claim 1, wherein a three-dimensional shape of at least one of said elevations is substantially similar to a three-dimensional shape of the respective peak of the distribution of the reference wave formation potential or the three-dimensional shape of the respective said elevation corresponds to the three-dimensional shape of the respective said peak.

7. The electrolysis cell according to claim 1, wherein at least one of said elevations is upwardly tapered in vertical direction.

8. The electrolysis cell according to claim 1, wherein said surface profiling of said cathode is irregularly configured in at least one direction.

9. An electrolysis cell for the production of aluminum, comprising:

a cathode having an upper side formed with a surface profiling of two or more elevations;

during an operation of the electrolysis cell, a layer of liquid aluminum on said upper side of said cathode, a melt layer on the layer of liquid aluminum, and an anode above the melt layer;

said surface profiling of said cathode being configured and disposed in such a way that an elevation is in each case provided at two or more of twenty points of a surface of said upper side of said cathode that are in each case disposed vertically beneath those regions of a boundary surface between the layer of liquid aluminum and the melt layer in which peaks with the twenty highest maximum values are present in a distribution of a reference wave formation potential present in the boundary surface;

the reference wave formation potential being defined as a wave formation potential which, during the operation of the electrolysis cell with a reference cathode having no surface profiling being used instead of said cathode with said surface profiling, but having an otherwise identical configuration as said cathode with said surface profiling, and with the reference cathode being disposed at a level in the electrolysis cell to provide a volume for the layer of liquid aluminum and the melt layer between said reference cathode and said anode equal to a volume provided in the electrolysis cell with said cathode with said surface profiling, is present at a point in the boundary surface between the layer of liquid aluminum and the melt layer.

10. The electrolysis cell according to claim 9, wherein a respective elevation is provided in each case at at least X of Y points of the surface of said upper side of said cathode which are in each case disposed vertically beneath those regions of the boundary surface between the layer of liquid aluminum and the melt layer in which the peaks with Y highest maximum values are present in the distribution of the reference wave formation potential present in the boundary surface, and

wherein at least one of the following is true:

Y=20 and X is selected from the group consisting of 4, 6, 10 and 14;

Y=10 and X is selected from the group consisting of 2, 3, 5 and 7;

Y=5 and X is selected from the group consisting of 1, 2, 3 and 4.

11. The electrolysis cell according to claim 9, wherein at least one of said 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 of liquid aluminum and the melt layer in which a respective peak is present in the distribution of the reference wave formation potential present in the boundary surface has its maximum height at the point disposed vertically beneath the point of the boundary surface between the layer of liquid aluminum and the melt layer at which the peak of the distribution of the reference wave formation potential has its maximum value.

12. The electrolysis cell according to claim 9, wherein a geometrical outer contour of at least one of said elevations is substantially similar, in plan view, to a geometrical outer contour of the respective peaks of the distribution of the reference wave formation potential in plan view.

13. The electrolysis cell according to claim 9, wherein a geometrical outer contour of at least one of said elevations, in plan view, at least partially represents a substantial polygon with 3, 4, 5 or 6 corners or a substantial ellipsoid.

14. The electrolysis cell according to claim 9, wherein a three-dimensional shape of at least one of said elevations is substantially similar to a three-dimensional shape of the respective peak of the distribution of the reference wave formation potential or the three-dimensional shape of the respective said elevation corresponds to the three-dimensional shape of the respective said peak.

15. The electrolysis cell according to claim 9, wherein at least one of said elevations is upwardly tapered in vertical direction.

16. The electrolysis cell according to claim 9, wherein said surface profiling of said cathode is irregularly configured in at least one direction.

17. A cathode for an aluminum electrolysis cell, the cathode comprising:

a cathode block having an upper side formed with a surface profiling on a surface of the upper side;

said surface profiling having a plurality of two or more first webs running substantially in a first direction of the cathode and at least one second web running substantially in a second direction normal to said first direction of the cathode.

18. The cathode according to claim 17, wherein said upper side of the cathode is formed with a depression in the form of a trough having a substantial V-shape when viewed in cross-section of the cathode, and wherein said first webs and said at least one second web are disposed on a surface of said substantially V-shaped depression.

19. The cathode according to claim 18, wherein said depression extends over at least 75% of a surface of said cathode block.

20. The cathode according to claim 18, wherein said depression extends substantially an entire surface of said cathode block.

Images