CA2893476A1

CA2893476A1 - Side-wall block for a wall in an electrolytic cell for reducing aluminium

Info

Publication number: CA2893476A1
Application number: CA2893476A
Authority: CA
Inventors: Frank Hiltmann; Janusz Tomala; Ghazanfar Abbas; Thomas Frommelt; Rainer Schmitt; Markus Pfeffer
Original assignee: SGL Carbon SE
Current assignee: Tokai Cobex GmbH
Priority date: 2012-12-13
Filing date: 2013-12-13
Publication date: 2014-06-19
Anticipated expiration: 2033-12-13
Also published as: RU2015127998A; UA118098C2; RU2668615C2; WO2014091023A1; JP6457397B2; CN104854264B; CN104854264A; CA2893476C; EP2931945A1; JP2016505714A

Abstract

The invention relates to a side-wall block for a wall in an electrolytic cell, in particular for producing aluminium, a method for producing such a side-wall block, use of such a side-wall block, and an electrolytic cell having such a side-wall block. The side-wall block (28) is a layered body, comprising a layer having a higher thermal conductivity and a layer having a lower thermal conductivity, wherein the difference in the thermal conductivity is at least W/(m.cndot.K).

Description

WO 2014/091023 Al SIDE-WALL BLOCK FOR A WALL IN AN ELECTROLYTIC CELL
FOR REDUCING ALUMINIUM
The present invention relates to a side-wall brick for a wall in an electrolytic cell, in particular for producing aluminium, to a method for producing such a side-wall brick, and to a use of such a side-wall brick and to an electrolytic cell having such a side-wall brick.
Electrolytic cells are used for the production of aluminium by electrolysis, which is conventionally carried out industrially by the Hall-Heroult process. In the Hall-Heroult process, a melt composed of aluminium oxide and cryolite, preferably of approximately from 15 to 20 % aluminium oxide and approximately from 85 to 80 % cryolite, is electrolysed. The cryolite, Na3[AlF6], thereby serves to lower the melting point from 2045 C
for pure aluminium oxide to approximately 960 C for a mixture containing cryolite, aluminium oxide and added substances, such as aluminium fluoride and calcium fluoride, so that the melt electrolysis can be carried out at a reduced temperature of approximately 960 C.
The electrolytic cell used in this process has a floor which is composed of a plurality of adjacent cathode blocks, for example 24 adjacent cathode blocks, which form the cathode. A
gap is formed between the adjacent cathode blocks. The arrangement of the cathode block and the gap, which may be filled, is generally referred to as the cathode floor. The cathode floor is enclosed by a wall which is formed of a plurality of side-wall bricks and, together with the cathode floor, forms an inner trough which receives the aluminium layer and the melt layer and which is enclosed by an outer steel trough. The gaps or spaces present between adjacent cathode blocks and between the cathode blocks and the side-wall bricks are conventionally filled with ramming mass of carbon and/or material containing carbon, such as anthracite or graphite, and a binder, such as coal tar pitch. This serves as a seal against molten constituents and to compensate for mechanical stresses which occur, for example, owing to the expansion of the cathode blocks upon heating during start up of the electrolytic cell. In order to withstand the thermal and chemical conditions that prevail during operation of the electrolytic cell, the cathode blocks are conventionally composed of a uniform material containing carbon, and the side-wall bricks are conventionally composed of a uniform material containing carbon or silicon carbide. Grooves are provided on the undersides of the cathode blocks, in each of which grooves there is arranged at least one current rail through =
which the current supplied via the anodes is conveyed out of the electrolytic cell. Beneath the cathode floor, that is to say between the cathode blocks and the floor of the steel trough housing the cathode, there is conventionally provided a lining of a refractory material, which thermally insulates the floor of the steel trough from the cathode floor.
Approximately from 3 to 5 cm above the layer of molten aluminium situated on the upper side of the cathode there is arranged an anode formed of individual anode blocks, the melt containing aluminium oxide and cryolite being situated between the anode and the surface of the aluminium. During the electrolysis, which is carried out at approximately 960 C, the aluminium that forms settles, because it has a greater density as compared with that of the melt, beneath the molten layer, that is to say as an intermediate layer between the upper side of the cathode blocks and the molten layer. During the electrolysis, the aluminium oxide dissolved in the molten cryolite is cleaved into aluminium and oxygen by electrical current flow. From an electrochemical point of view, the layer of molten aluminium is the actual cathode, because aluminium ions are reduced to elemental aluminium at its surface.
Nevertheless, the term cathode is understood hereinbelow as meaning not the cathode from an electrochemical point of view, that is to say the layer of molten aluminium, but the component composed of one or more cathode blocks that forms the cathode floor.
Modern electrolytic cells are operated at high electrolysis current intensities of, for example, up to 600 kA in order to ensure a high productivity of the electrolytic cell.
These high current intensities lead to increased heat generation during the electrolysis process.
As a result of the high heat generation, it is found to be difficult to adapt the heat dissipation from the electrolytic cell in such a manner that optimum thermal conditions in terms of the stability and efficiency of the electrolysis and also in terms of the service life of the electrolytic cell are achieved throughout the electrolytic cell, the energy efficiency of the electrolysis, for example, being reduced by excessive thermal energy losses in regions of the electrolytic cell where heat generation is high. The reliability and economy of the electrolysis operation and the service life of the electrolytic cell are accordingly impaired by the unfavourable thermal conditions in the electrolytic cell.
Although the dissipation of excess heat generated in the electrolytic cell, that is to say heat that is not required to maintain the melting process, can be adjusted via the floor of the steel trough through the refractory lining arranged between the cathode floor and the steel trough, which refractory linina conventionally consists of refractory bricks or plates which are placed in the steel trough and stacked one on top of the other in the region of the floor of the steel trough, heat dissipation via the comparatively thin side wall of the inner trough formed by the cathode floor and the side-wall bricks pays an important part for the temperature conditions in the region of the layer of liquid aluminium and the molten layer in which the electrolysis takes place. The heat flows in that side wall are particularly relevant for the thermal conditions in the electrolytic cell because the side wall is conventionally in contact with various constituents and media in the electrolytic cell, that is to say in particular with the layer of liquid aluminium, the liquid molten layer arranged thereon, a layer of solidified melt, or crust, present above the liquid molten layer, and the gaseous atmosphere that develops during the electrolytic cell, with the various elements contained therein.
The amounts of thermal energy that are generated must be dissipated in part in a defined manner, but at the same time excessive heat losses, which mean energy losses and thus impair the economy of the electrolysis process, must also be avoided.
In known electrolytic cells, because of the vertical construction of the side walls and the resulting structural requirements, there is no additional lining of stacked refractory bricks in the region of the relatively thin side walls, as is provided in the region of the floor of the steel trough, so that the heat dissipation via the side walls cannot be adapted in the same simple manner as in the floor region. Uniform side walls, which conventionally consist of a material containing carbon or silicon carbide, are homogeneous in respect of the thermal conduction properties perpendicularly to the plane of the side walls and permit only limited control of the heat flows and of the profile of the isothermals in the side wall during operation. The gap between such a side wall and a cathode block is conventionally filled with ramming mass of carbon and/or material containing carbon, such as anthracite or graphite, and a binder, such as coal tar. This gap is frequently rammed manually or semi-automatically, whereby ramming faults can occur which can lead to damage to the gap or, in the worst case, even to premature failure of the electrolytic cell as a whole. Such damage often occurs only during start up or during operation of the electrolytic cell. The risk that damage will occur is all the greater, the wider or thicker the corresponding gap. A wider or thicker gap additionally also means a higher outlay in terms of work and a higher burden for the environment and the personnel responsible for the electrolytic cell, because substances that are harmful to health are found in conventional ramming masses. It is known to replace some or all of the ramming mass required between the side-wall bricks and cathode blocks by a sloping layer of pre-burned carbon or graphite. If only some of the ramming mass is replaced, the thickness of the ramming mass gap is reduced by more than 50 % to 99 %, preferably more than 75 % to 99 %, particularly preferably more than 90 % to 99 %. It is also possible that this layer does not fill the entire volume of the original ramming mass layer, for example in order to create space for enlarging the anode surface. In most cases, a thin, perpendicular peripheral ramming mass gap having a thickness of, for example, 50 mm is retained.
Vertically arranged side-wall bricks of, for example, silicon-nitride-bonded silicon carbide can be connected to this sloping layer. Such a construction comprising a side-wall brick and a sloping layer is referred to in the following as a "composite side-wall brick". A composite side-wall brick, in which the silicon-nitride-bonded silicon carbide is adhesively bonded to the sloping layer, is already in use in modern electrolytic cells. The adhesive conventionally used can comprise substances which are harmful to health, which again means a higher burden for the environment and the personnel responsible for the electrolytic cell.
In addition, application of the adhesive material requires an additional working step. If adhesion faults occur as a result of faulty adhesive material or incorrect application thereof, the adhesively bonded joints can fail. The side-wall bricks of such a composite side-wall brick are made of a uniform material and thus do not permit any differentiation in respect of the thermal conductivity in the side-wall brick itself. The adhesive material or the adhesively bonded joint can also influence the heat flow in the electrolytic cell. Because the adhesively bonded joint itself is very thin, irregularities in that joint can impair the corresponding local heat flow.
Carbonisation of the adhesive material during start up of the cell can lead to a reduction of its adhesion, which can mean a weakening of the bond between the sloping layer of pre-burned carbon or graphite and the vertical side-wall brick. If that bond fails, that is to say the above sloping layer and the vertical side-wall brick are no longer connected to one another, the heat flow is impaired in an undefined manner and the necessary heat dissipation can no longer be sufficiently ensured. This can lead to overheating of the electrolytic cell and, in the worst case, to the premature failure thereof, that is to say the lifetime or service life of the electrolytic cell is reduced. Adhesive material in the form of a thin adhesive layer can also be used between the individual side-wall bricks which form the side wall.
DE 3506200 discloses side-wall bricks for the wall of an electrolytic cell which are a composite body having a layer-wise construction, comprising an inner layer of a material containing carbon and an outer layer of a hard ceramic material, these two layers being intimately connected to one another. A virtually unhindered heat flow from the inside to the outside is thereby made possible. However, when such side-wall bricks are used, the resistance to wear, in particular abrasive and/or corrosive wear, is still not sufficient.

With the known electrolytic cells it is therefore not possible to achieve optimum process conditions, in particular when operating at high electrolysis current intensities, as a result of which the stability and economy of the electrolysis process that can be achieved are limited and the service life of the electrolytic cell is impaired.
Accordingly, it is an object of the invention to provide a side-wall brick for the wall of an electrolytic cell which, when used in the electrolytic cell, ensures optimum process conditions and a correspondingly high economy and stability during the electrolysis operation as well as a long service life of the electrolytic cell. In particular, the side-wall brick is to adjust heat dissipation via the side wall of the electrolytic cell in such a manner that optimum thermal conditions prevail in the electrolytic cell during the electrolysis and thermal losses, caused by an unfavourable heat and temperature distribution, are avoided to the greatest possible extent during operation. The operating temperature of the cell during the electrolysis is between 920 C and 1000 C, preferably between 950 C and 980 C. Furthermore, this side-wall brick is to have increased resistance to abrasive and/or corrosive wear, in particular to abrasive wear. In addition, it is to be possible to produce this side-wall brick, for example, without the use of adhesive(s). Furthermore, when this side-wall brick is in the form of a composite side-wall brick, it is also to allow some or all of the ramming mass between the side wall and the cathode block to be dispensed with.
According to the invention, the object is achieved by a side-wall brick for a wall in an electrolytic cell, in particular for producing aluminium, which is a layered body and comprises a layer having a lower thermal conductivity and a layer having a higher thermal conductivity, wherein the difference between lower and higher thermal conductivity is at least 5 W/m=K, measured at a temperature between 920 C and 1000 C, preferably between 950 C and 980 C, and wherein at least one of the layers is doped with silicon (powder), an oxidic ceramic material or a non-oxidic material. This layered body can be produced without using adhesive(s) ¨ as will be described below. As a result of the configuration of the side-wall brick, it is likewise possible to dispense with the use of adhesive(s) between the individual side-wall bricks which form the side wall. Owing to the form of this layered body, some or all of the ramming mass for filling the gap between the side-wall brick and the cathode block can be dispensed with ¨ as will likewise be explained below.

It has been recognised that configuring a side-wall brick for an electrolytic cell with layers having different thermal conductivities, wherein at least one of the layers is doped with silicon (powder), an oxidic ceramic material or a non-oxidic material, allows the thermal conditions in the electrolytic cell to be adapted very simply and at the same time highly effectively during operation of the electrolytic cell, so that the stability and efficiency of the electrolysis and the service life of the electrolytic cell are optimised. In addition, the resistance to wear, in particular abrasive and/or corrosive wear, is increased. If adhesive material is used between the individual side-wall bricks, it is possible, owing to the configuration of this side-wall brick, to dispense with this adhesive material completely.
Furthermore, it has been recognised that, by means of a particular shaping of the side-wall bricks, it is also possible, in addition to adapting the thermal conditions, to dispense with some or all of the ramming mass for filling the gap between the side-wall brick and the cathode block.
Where the expression "side-wall brick" is used in the following, this expression can also include the above-mentioned composite side-wall bricks. As is described hereinbelow, a composite side-wall brick has a particular shape.
The expressions "lower" and "higher" thermal conductivity are to be understood as meaning that the particular layer that has that thermal conductivity has a "lower" or "higher" thermal conductivity in comparison with the respective other layer. In particular, one layer consists of a material having a lower thermal conductivity and the other layer consists of a material having a higher thermal conductivity, the two materials being different from one another. If the layered body comprises more than two layers, all the layers can have different thermal conductivities, or at least two layers can have the same thermal conductivity, and/or at least two groups of layers each having the same thermal conductivity can be provided. Where there are more than two layers, a thermal conductivity difference between at least two of the layers of at least 5 W/m=K ¨ measured at a temperature between 920 C to 1000 C, preferably between 950 C and 980 C ¨ is sufficient. In particular, the thermal conductivities of the layers differ in at least one direction of the side-wall brick which is preferably a direction that is in particular perpendicular to the side wall formed by the side-wall bricks.
The difference between lower and higher thermal conductivity ¨ measured at a temperature between 920 C and 1000 C, preferably between 950 C and 980 C ¨ can be between W/m=K and 80 W/m=K, preferably between 5 and 70 W/m=K, particularly preferably between 8 W/m=K and 60 W/m=K and most particularly preferably between 10 W/m=K
and 50 W/m.K.
By means of the different layers of the side-wall brick having different thermal conductivities, the heat conduction and dissipation via the side-wall bricks and the profile of the isothermals in the side wall can purposively be adjusted. Because some regions of the side-wall bricks are in direct contact with the layer of liquid aluminium and the molten layer in which the electrolysis takes place, the temperature conditions therein, which are particularly important for the stability and efficiency of the electrolysis, can be influenced directly and highly effectively, so that optimum thermal conditions for the operation of the electrolytic cell can be ensured. For example, different thermal conductivities can be provided in the regions of the side-wall brick which come into contact with the different media of the electrolytic cell when the side-wall brick is used in the electrolytic cell. Likewise, a plurality of successive layers having different thermal conductivities can be provided along the heat flow direction through the side-wall bricks outwards, in order to adjust the heat flow in the mentioned direction. The resulting optimisation of the thermal conditions in the electrolytic cell leads to a considerable increase in the stability and efficiency of the electrolysis process and in the service life of the electrolytic cell. The stability and efficiency of the electrolysis process and the service life of the electrolytic cell are also increased by doping at least one of the layers with silicon (powder), an oxidic ceramic material or a non-oxidic material.
The side-wall bricks according to the invention, which also include composite side-wall bricks, can preferably be installed in an electrolytic cell in the conventional manner, which corresponds to known side-wall bricks having homogeneous thermal conductivity, in each case in respect of a defined direction in space, and can there be used for lining the side wall of the steel trough without changes relating to the construction of the electrolytic cell being necessary or associated disadvantages having to be accepted, it being possible for the side wall of the electrolytic cell, in particular in known manner, to be comparatively thin. The side-wall bricks can be produced with a low outlay and with excellent mechanical stability and in particular very good cohesion between the different layers by firing the side-wall bricks in one piece from a single cohesive green base body in which different green mixtures corresponding to the layers that are to be produced are contained, whereby the base body can correspond to a single side-wall brick or a plurality of side-wall bricks can be separated from the fired base body. In the production of a composite side-wall brick, it is possible, for example, first to carve the desired polygonal shape out of such a fired green body over the entire length of the green body before individual composite side-wall bricks are then cut in the form of plates. The preferred polygonal shapes will be discussed in detail below.
Grooves, elevations, recesses and roughened areas can be added to a composite side-wall brick in a final processing operation. It is again pointed out here that the cohesion between the different layers of the side-wall bricks according to the invention is achieved without the use of adhesive(s).
Advantageous embodiments of the invention are described in the dependent claims, the description and the figures.
Any reference in the following description to one or more layers of the side-wall brick in the form of a layered body means layers having different thermal conductivities, which in particular each have a thermal conductivity which differs from the thermal conductivity of at least one other layer of the side-wall brick by 5 W/m=K or more ¨ measured at a temperature between 920 C and 1000 C, preferably between 950 C and 980 C.
The in particular two layers can follow one another in a specified direction, which can correspond to a heat flow direction that is relevant for the thermal conditions in the electrolytic cell and can be given, for example, by the thickness direction of the side-wall brick. Owing to the resulting variation in the thermal conductivity over the thickness of the side-wall brick, the total heat flow through the side-wall brick in that direction can be so regulated that a desired isothermal profile in the side-wall brick is ensured.
The layers can, however, also follow one another, for example, in the height direction of the side-wall brick which does not include a composite side-wall brick, wherein in particular the height regions of the side-wall brick that are covered by the different layers can be in contact, during use of the side-wall brick in an electrolytic cell, with different media of the electrolytic cell ¨ such as, for example, liquid aluminium, liquid or solidified melt, gas phase. Owing to the resulting variation in the thermal conductivity over the height of the side-wall brick, the heat dissipation can be adapted to the heat generation that takes place in the medium in question and the thermal conditions desired therein and additionally to the chemical requirements of the individual media.
The adaptation which is to be achieved according to the invention of the thermal conditions in an electrolytic cell during operation thereof can already be realised if the side-wall brick has exactly two layers having different thermal conductivities. Such a layer structure additionally has high stability and can be produced with a low outlay and high reliability and reproducibility. In principle, however, the number of different layers of the side-wall brick is not limited to exactly two. Instead, the side-wall brick can also comprise a larger number of layers, for example at least three, four, five, six or more different layers.
As a result, an even more differentiated local matching of the thermal conduction behaviour of the side-wall brick to the thermal conditions in the electrolytic cell can be achieved.
Preferably, the side-wall brick comprises from two to four layers, particularly preferably from two to three layers, most particularly preferably two layers. If, in addition to the desired adaptation of the thermal conditions in an electrolytic cell during operation thereof, some or all of the ramming mass between the cathode block and the side-wall brick is also to be replaced, that is to say if a composite side-wall brick is used, the composite side-wall brick can also comprise a larger number of layers, for example at least three, four, five, six or more different layers.
Preferably, the composite side-wall brick comprises from two to four layers, particularly preferably from two to three layers, most particularly preferably two layers.
The layers can follow one another in a specified direction, which can correspond in particular to a thickness or height direction of the side-wall brick, so that a variation of the thermal conductivity of the side-wall brick in the thickness direction or in the height direction of the side-wall brick is achieved. The side-wall brick can also have layers which follow one another in different directions, so that a variation of the thermal conductivity of the side-wall brick in different directions is achieved. For example, a plurality of layers of the side-wall brick following one another in a first direction can form a first layer sequence and a plurality of different layers following one another in the first direction can form a second layer sequence, the two layer sequences preferably following one another in a second direction which is different from the first direction and in particular perpendicular to the first direction, which would be like a chequered pattern.
According to an advantageous embodiment, the layered body has an alternating sequence of a layer having a lower thermal conductivity and a layer having a higher thermal conductivity. This alternating sequence can take place in a specified direction, which corresponds in particular to the thickness or height direction. However, it is also possible for an alternating sequence of a layer having a lower thermal conductivity and a layer having a higher thermal conductivity to take place in a first direction and an alternating sequence to take place in a second direction which is different from the first direction, in particular =
1 b perpendicular to the first direction. A particularly advantageous thermal conduction behaviour is thereby achieved if one outside layer of the layered body is a layer having a lower thermal conductivity and the other outside layer is a layer having a higher thermal conductivity. As a result, the heat absorption, distribution and dissipation via the outside faces of the side-wall brick, which are formed by the outside layers of the side-wall brick, is adapted effectively and directly. It is preferred that the outside layer of the layered body that is in contact with the liquid aluminium and/or the liquid molten layer is a layer having a lower thermal conductivity, and that the other outside layer of the layered body, which is in contact with the cathode floor and/or the trough, is a layer having a higher thermal conductivity. The direction in which the thermal conductivities differ is the direction which is perpendicular to the side wall formed by the side-wall bricks.
In principle, the layers and/or the side-wall brick can have any desired, suitable shape. It is to be understood that the shape depends significantly on the intended use of the side-wall brick, that is to say the adaptation of the thermal conditions in an electrolytic cell during operation thereof alone, or the combination of such an adaptation with the replacement of some or all of the ramming mass between the cathode block and the side-wall brick.
In an embodiment which is particularly advantageous in respect of the thermal conduction behaviour and the producibility of the side-wall brick, the layers of the side-wall brick have a block shape, in particular a cuboid shape, and are connected together via contact faces, in particular their bases, or via their sides. Such layers are particularly simple to produce and allow the thermal conductivity to be adapted and varied purposively along the principal directions of a preferably block-shaped, in particular cuboid-shaped, side-wall brick.
The side-wall brick is preferably block-shaped, in particular cuboid-shaped.
The thickness direction of one or more layers of the side-wall brick can coincide with the thickness direction of the side-wall brick, so that the orientation of the layers is adapted to the orientation of the side-wall brick and the corresponding principal thermal conduction directions in the side-wall brick. Layers connected together via their bases can accordingly follow one another in the thickness direction of the side-wall brick, and layers connected together via their sides can follow one another in the height direction of the side-wall brick.
Within the meaning of the invention, a block is understood as being a body which has six rectangular faces, eight rectangular corners and twelve edges, of which in each case at least four are of equal lengths and are parallel to one another. If the block is a cuboid, four edges are of the same length and parallel to one another. It is, however, also possible for eight of the twelve edges to be of the same length, four edges being parallel to one another, or all the edges are the same length, four edges being parallel to one another in this case too.
If the side-wall brick is used in the form of a composite side-wall brick in the electrolytic cell, in an advantageous embodiment of the side-wall brick at least one layer of the side-wall brick has a block shape, in particular a cuboid shape, and at least one layer of the side-wall brick has a polygonal shape. These layers are connected together via contact faces, in particular their bases; the base of the layer having a block shape is in contact either partially or completely with the base of the layer having a polygonal shape. When the bases are completely in contact, both layers have the same height; in the case of partial contact, the layer having a polygonal shape has a height which amounts to from 30 % to less than 100%, preferably from 40% to 80%, particularly preferably from 50% to 75%, of the height of the layer having a block shape. Such layers are likewise very easy to produce and, on the one hand, allow the thermal conductivity to be adapted and varied along the principal directions of the side-wall brick; on the other hand, such a side-wall brick allows some or all of the ramming mass between the side-wall brick and the cathode block to be replaced.
At least one layer of the composite side-wall brick has a polygonal shape.
Within the meaning of the invention, a polygon is understood as being a polygon which can preferably contain from three to six corners, particularly preferably from three to five corners. A polygon with four corners is understood as being, for example, a rectangle, square or trapezium.
These polygons can be in regular or irregular form. Within the context of the invention, a regular polygon is understood as being a polygon in which all the sides are the same length and all the internal angles are the same size. By means of the different polygonal shapes, the composite side-wall bricks can be adapted to the desired electrolytic cell design; for example, more space for anodes can be created by the corresponding design of a composite side-wall brick, that is to say the configuration of a layer in polygonal form. Larger anode surfaces permit a higher current intensity and thus a higher productivity. In addition, the shape of the composite side-wall brick can be adapted to the shape of the original peripheral rammed mass gap. Furthermore, these polygons can have normal and/or rounded corners. A normal corner is understood as being the point at which two sides of the corresponding polygon meet. A rounded corner is understood as being a corner which has a concave inward round curve, without any angular or square-edged change of direction in that curved region. Rounded corners have the advantage, compared with sharp corners, that a more uniform distribution of forces occurs at the rounded corners. This more uniform distribution of forces effects a reduction of the stresses that occur, and thus a reduced formation of cracks and/or defects at those points of the composite side-wall brick.
Preferably, the polygon contains only normal corners, or one corner of the polygon is rounded and the other corners are normal corners.
The thickness direction of one or more layers of the composite side-wall brick can coincide with the thickness direction of the side-wall brick, so that the orientation of the layers is adapted to the orientation of the side-wall brick and the corresponding principal thermal conduction directions in the side-wall brick. The layers connected together via their bases can accordingly follow one another in the thickness direction of the composite side-wall brick.
The side-wall brick, including the composite side-wall brick, can in principle have a flat structural shape with a relatively small thickness and an in particular significantly larger height and width, it being possible for the side-wall brick to have a larger height than width.
When the layers are connected together via their bases, the thickness of the side-wall brick can be, for example, between 50 and 700 mm and depends on the type of use. If the side-wall brick is used only for adapting the thermal conditions in an electrolytic cell, the thickness is preferably between 60 and 250 mm, particularly preferably between 80 and 150 mm, most particularly preferably between 90 and 110 mm. If, on the other hand, a composite side-wall brick is used in the electrolytic cell, the thickness is preferably between 150 and 600 mm, particularly preferably between 200 and 350 mm, most particularly preferably between 225 and 300 mm. The ratio of the thicknesses of the in particular two layers can be, for example, not more than 1:3, preferably not more than 1:2 and particularly preferably 1:1.
The width of the side-wall brick, including the composite side-wall brick, can be adapted as desired to the length of the side wall of the electrolytic cell, that is to say it can either occupy the entire length of the side wall or it amounts to only a portion of the length of the side wall.
The length of a side wall can be, for example, either from 3500 mm to 4000 mm or from 10,000 to 15,000 mm. If the length of the side wall is from 10,000 to 15,000 mm, the width of the side-wall brick can be that length, or the side wall is covered with, for example, from 2 to 3 side-wall bricks having a length of 5000 mm.

1'3 When the width of the side-wall brick occupies the entire length of the side wall of the electrolytic cell, it is possible, on the one hand, by means of such a side-wall brick to dispense with the adhesive material which may be used for the joints between the individual side-wall bricks; on the other hand, the simpler installation of this side-wall brick constitutes a time saving. Where the width of the side-wall brick according to the invention is only a portion of the length of the side wall, at least two side-wall bricks according to the invention are used. It is possible within the context of the invention to use side-wall bricks according to the invention having different widths, that is to say the width of an individual side-wall brick can be adapted as required. When the width of the side-wall brick according to the invention, including the composite side-wall brick, occupies only a portion of the length of the side wall, they can be between 300 and 600 mm, preferably between 400 and 600 mm, particularly preferably between 450 and 550 mm.
The height of the side-wall brick, including the composite side-wall brick, can be, for example, between 500 and 900 mm, preferably between 600 and 800 mm, particularly preferably between 600 and 750 mm. In the case of a composite side-wall brick, the height is taken to be the length of the layer having a block shape.
According to one embodiment, the side-wall brick, which does not include a composite side-wall brick, has two layers which follow one another in the thickness direction of the side-wall brick and in particular are connected together partially or completely via their base, which layers each cover from 30 % to 70 %, preferably 50 %, of the thickness of the side-wall brick and accordingly cover the entire thickness of the side-wall brick. It is here to be understood that the percentages of the individual layer thicknesses ¨ also in the following ¨ together are always 100%. A layer can thereby extend over the entire height of the side-wall brick.
The side-wall brick, which does not include a composite side-wall brick, can likewise have two layers which follow one another in the height direction of the side-wall brick and in particular are connected together via their sides, which layers each cover from 30 % to 70 %, preferably 50 %, of the height of the side-wall brick and accordingly cover the entire height of the side-wall brick. A layer can thereby extend over the entire thickness of the side-wall brick.
According to an advantageous embodiment, one or more and in particular all of the layers of the side-wall brick which is not a composite side-wall brick have a thickness of from 25 to 125 mm, preferably from 30 to 100 mm, particularly preferably from 40 to 75 mm and most particularly preferably from 45 to 55 mm. This is particularly preferred when the side-wall brick has two layers which are connected together via their bases and follow one another in the thickness direction and in particular each account for from 30 to 70 %, preferably 50 %, of the thickness of the side-wall brick. The layers can each extend over the entire height of the side-wall brick.
According to a further preferred embodiment, one or more and in particular all of the layers of the side-wall brick, which does not include a composite side-wall brick, when these layers are connected together via their sides and follow one another in the height direction, have a height of from 150 to 450 mm, preferably from 200 to 400 mm, particularly preferably from 250 to 350 mm and most particularly preferably from 280 to 320 mm. This is preferred in particular when this side-wall brick has two layers which are connected together via their sides and follow one another in the height direction and each extend in particular over from 30 % to 70 %, preferably 50 %, of the height of the side-wall brick. The layers can each extend over the entire thickness of the side-wall brick. The ratio of the heights of the in particular two layers can be, for example, not more than 1:3, preferably not more than 1:2 and particularly preferably 1:1.
According to one embodiment of a composite side-wall brick, this side-wall brick has two layers which follow one another in the thickness direction of the side-wall brick and are connected together partially or completely in particular via their base, which layers each cover from 30 % to 70 %, preferably 50 %, of the thickness of the side-wall brick and accordingly cover the entire thickness of the side-wall brick. It is here to be understood that the percentages of the individual layer thicknesses ¨ also in the following ¨
together are always 100 %. A layer can extend partially or completely over the entire height of the side-wall brick. It can be that the layer having a polygonal shape either extends completely over the entire height of the layer having a cuboid shape, or it extends over from 30 % to less than 100 %, preferably from 40 % to 80 %, particularly preferably from 50 % to 75 %, of the height of the layer having a cuboid shape.
According to a further advantageous embodiment, one or more ¨ and in particular all ¨ of the layers of the composite side-wall brick have a thickness of from 75 to 250 mm, preferably from 100 to 175 mm and particularly preferably from 110 to 150 mm. This is preferred especially when the composite side-wall brick has two layers which are connected together 1'5 partially or completely via their bases and follow one another in the thickness direction and in particular each amount to from 30 to 70 %, preferably 50 %, of the thickness of the composite side-wall brick. In the case of complete contact of the bases, the layers each extend over the entire height of the composite side-wall brick; if, on the other hand, there is partial contact of the bases, the layer having a polygonal shape extends over from 30 % to less than 100 %, preferably from 40 to 80 %, particularly preferably from 50 %
to 75 %, of the height of the layer having a cuboid shape.
According to a further embodiment, one or more cuboid-shaped layers, and in particular all the cuboid-shaped layers, of the composite side-wall brick have a height of from 500 to 900 mm, preferably from 650 to 850 mm, particularly preferably from 700 to 800 mm, and one or more, and in particular all, of the polygonal layers have a height of from 150 to less than 900 mm, preferably from 200 to 720 mm, most particularly preferably from 250 to 675 mm.
The side-wall brick can be in contact over its height with different constituents or media of the electrolytic cell, in particular with the layer of liquid aluminium, the molten layer, optionally a crust of solidified melt arranged on the molten layer, and with a gaseous atmosphere, with the various substances contained therein, that develops during operation of the electrolytic cell. In its lower region, the side-wall brick can be connected to the cathode floor and/or a ramming mass which can be provided for producing a tight connection between the cathode floor and the side-wall brick. The side-wall brick can have, in accordance with the preceding description, a plurality of layers having different thermal conductivities which follow one another in its height direction, the height regions of the side-wall brick in which the side-wall brick comes into contact with different media preferably being formed by different layers of the side-wall brick. As a result, the heat absorption and dissipation via the side-wall brick is adapted to the particular thermal conditions and requirements in the different media. By means of this adaptation, the side-wall bricks overall are subjected to less stress, which leads to higher wear resistance.
Alternatively or in addition, the side-wall brick can have a plurality of layers having different thermal conductivities which follow one another in the thickness direction of the side-wall brick. As a result, the heat conduction of the side-wall brick can be varied in a heat flow direction which runs perpendicularly to the side face of the side-wall brick delimiting the inside of the trough.

a le In an embodiment which is preferred in respect of the thermal, mechanical and chemical stability of the side-wall brick which is important for the use of the side-wall brick in an electrolytic cell, at least one layer, preferably all the layers, is made of a material selected from the group consisting of carbon, graphitic carbon, graphitised carbon or silicon carbide or arbitrary mixtures thereof, or contains such a material. These materials are particularly suitable for withstanding the conditions which occur when the side-wall brick is used in an electrolytic cell and the side-wall brick thereby comes into contact with a layer of liquid aluminium and the molten layer. Furthermore, the choice of suitable material compositions allows the thermal conductivity of the side-wall brick to be adapted in an advantageous value range. The thermal conductivity of one or more and in particular of all of the layers of the side-wall brick can be ¨ measured at a temperature between 920 C and 1000 C, preferably between 950 C and 980 C ¨ for example between 4 and 120 W/m=K, in particular between 4 and 100 W/m=K, preferably between 5 and 80 W/m=K, particularly preferably between 8 and 50 W/m.K.
A particularly high wear resistance of the side-wall brick, and thus a particularly long service life of an electrolytic cell equipped with the side-wall brick, is achieved when the carbon is anthracite, preferably electrically calcincd anthracite, and the silicon carbide is silicon-nitride-bonded silicon carbide.
Yet a further improvement in the thermal and mechanical properties of the side-wall brick can be achieved if the production of the side-wall brick comprises an impregnating step with pitch and subsequent carbonisation. The side-wall brick as a whole, or at least one layer of the side-wall brick, can thereby be subjected to impregnation as described above.
At least one of the layers can be doped with silicon (powder), an oxidic ceramic material, such as, for example, aluminium oxide or titanium dioxide, or a non-oxidic ceramic material, which is preferably composed of at least one metal of groups 4 to 6 and at least one element of group 13 or 14 of the periodic system of the elements. Doping is here understood as meaning addition to the green mixture, the individual amount of one or more dopants in the green mixture being from 3 to 15 wt.%, preferably from 5 to 10 wt.%.
Pulverulent particles having a diameter of less than 200 pm, particularly preferably less than 63 pm, are preferably used. Such non-oxidic materials include in particular metal carbides, metal borides, metal nitrides and metal carbonitrides with a metal of groups 4 to 6, such as, for example, titanium, zirconium, vanadium, niobium, tantalum, chromium or tungsten, titanium preferably being used. It is also possible to use arbitrary mixtures of oxidic ceramic materials, arbitrary mixtures of non-oxidic ceramic materials, arbitrary mixtures of oxidic ceramic materials and non-oxidic ceramic materials, arbitrary mixtures of oxidic ceramic materials and silicon (powder), arbitrary mixtures of non-oxidic ceramic materials and silicon (powder), or arbitrary mixtures of oxidic ceramic materials, non-oxidic ceramic materials and silicon (powder). Titanium diboride or titanium carbide can be mentioned as preferred non-oxidic materials. If silicon (powder) is used, it reacts during the firing process to silicon carbide. It is also possible to use a precursor for the production of silicon-nitride-bonded silicon carbide, a mixture of silicon carbide and silicon powder being used.
The firing process must here take place, with a controlled nitrogen content of the fuel gas, at up to 1400 C in order to ensure that the silicon reacts to form the actual binder phase silicon nitride. In general, the heat treatment here takes place by firing as in the case of conventional, ceramic materials, that is to say the firing temperature is adapted to the ceramic material used. It may accordingly be that the different requirements that are made of the firing processes of the individual materials used must be taken into consideration in the production of the layered body. This layer is in particular the layer that is in contact with the surrounding steel trough during operation and is thus exposed to an increased risk of oxidative wear.
The side-wall brick is preferably produced monolithically, so that the layers of the side-wall brick are in one piece and connected together by material bonding. Such a connection is distinguished by increased stability as compared with an adhesively bonded or mechanical connection. The side-wall brick can thereby form a composite body of the individual layers.
As a result, the side-wall brick has particularly high thermal, mechanical and chemical resistance, and an electrolytic cell equipped with the side-wall brick accordingly has a particularly long service life. In particular, the side-wall brick can be obtainable in one piece from a green block which can contain a plurality of different green mixtures corresponding to the different layers of the finished side-wall brick, which green mixtures form the starting materials for the different layers of the side-wall brick. The side-wall brick can be obtainable by firing the green block, whereby carbonisation and/or graphitisation of the green material of the green block can take place in particular.
For example, the thermal conductivity of the side-wall brick can be measured at a temperature between 920 C and 1000 C in accordance with DIN 51936. In the case of =

measurements which exceed temperatures above 400 C, a pulsed laser is used.
The side-wall brick can have an at least substantially homogeneous thermal conductivity within a layer. Between a layer which has a lower thermal conductivity and a layer which has a higher thermal conductivity, a transition region can form, in which the thermal conductivity falls, for example at least substantially continuously, from the higher to the lower value. Such a transition region, which can be relatively small compared to the total extent of the layers, can be regarded as being a portion of the two layers.
The invention further provides a method for producing a side-wall brick according to the invention as described herein, which method comprises the steps:
a) providing a mixture for the layer having the lower thermal conductivity, a mixture for the layer having the higher thermal conductivity, and optionally one or more mixtures for at least one further layer, b) forming a green block having a layer structure from the mixtures according to step a), and c) firing the green block according to step b) at a temperature of from 800 to 1400 C, preferably from 1000 to 1300 C.
By producing the side-wall brick by firing a green block comprising different green mixtures that form the starting materials for the layers, a uniform side-wall brick having a high stability and a material-bonded and monolithic cohesion between the individual layers of the side-wall brick is achieved.
Formation of the green block according to step b) can include introducing the green mixtures into a mould. A plurality of layers of the green mixtures can be formed in the mould according to the layer structure of the finished side-wall brick. The layer structure can be produced in a simple manner by providing the layers of the green mixtures in succession in an opening direction of the mould. In a particularly simple manner, the layers can be introduced into the mould in such a manner that they are oriented substantially horizontally, preferably horizontally, and preferably follow one another in the vertical direction.
Formation of the green block according to step b) can further include vibration moulding and/or block pressing of the green material. This can be carried out with or without a vacuum. Voids present within the material can thereby be eliminated partially or completely, so that a desired bulk density is achieved uniformly throughout. Particularly high homogeneity in respect of the bulk density can additionally be achieved if the formation of the green block includes the application of pressure or compression of the green material in order to compact the material.
Suitable materials for the green mixtures are in particular all green materials which can be fired to one of the preferred materials mentioned above in respect of the finished side-wall brick. For example, at least one green mixture can contain a material which is selected from the group consisting of a material containing carbon, such as, for example, anthracite, a graphitic or graphitisable material, such as, for example, synthetic graphite and pitch, or an arbitrary mixture of these materials. Furthermore, a binder which in particular contains carbon, such as, for example, binding pitch, can be present in the mixture. By means of the purposive composition of the material of the individual layers of the green block, the thermal conductivity of the different layers of the resulting side-wall brick can purposively be adjusted. If a green mixture comprises a material containing carbon, carbonisation of the material of the green mixture preferably takes place during firing of the green block.
Furthermore, graphitisation of the material can take place as a further step d). For this purpose, the carbonised or green moulded body can be heated to temperatures of more than 2000 C and preferably more than 2200 C.
In order further to improve the thermal and mechanical properties of the side-wall brick, a further step e) can be provided after step c) of firing and/or after a step d) of graphitisation which is optionally provided, which step e) comprises impregnating the fired and optionally graphitised green block with pitch.
By means of the method described above there is preferably first produced a green body having a plurality of layers, from which a plurality of side-wall bricks having the desired dimensions can be separated, in particular by a cutting operation, in a step which follows the above-described method steps. This also applies to the production of a composite side-wall brick.
The invention further provides a side-wall brick obtainable by the method described herein.
When used in an electrolytic cell, the side-wall brick effects an optimisation of the thermal conditions in the electrolytic cell during the electrolysis operation and additionally has high mechanical stability and very strong cohesion between the different layers of the side-wall brick. Depending on the width of the side-wall bricks, it is possible to dispense with the adhesive material between the side-wall bricks. If a composite side-wall brick is used, it is additionally possible to dispense with some or all of the ramming mass between the side-wall brick and the cathode block.
The use of the side-wall brick according to the invention according to the present description for lining the side walls in an electrolytic cell is a further independent subject of the present invention. Within the context of the invention it is also possible that, for lining the side wall, at least one side-wall brick which is used to adapt the thermal conditions is combined with at least one composite side-wall brick. The number of side-wall bricks or composite side-wall bricks used here can be adapted as required.
The invention further provides an electrolytic cell, in particular for producing aluminium, which comprises a cathode, an anode and a wall, wherein at least a portion of the wall is formed by a side-wall brick according to the present description. This side-wall brick can, as described, also be a composite side-wall brick. The advantages and preferred embodiments described herein in relation to the side-wall brick, its production and use, and in particular its use in an electrolytic cell, are, when applied correspondingly, advantages and preferred embodiments of the electrolytic cell according to the invention. The at least one side-wall brick preferably forms a side wall of a trough in which the layer of liquid aluminium and the molten layer are housed. The side-wall brick can line a side wall of an outer steel trough of the electrolytic cell which encloses the inner trough formed by the side-wall brick.
As mentioned above, the amount of thermal energy produced in an electrolytic cell must, on the one hand, in part be dissipated in a defined manner; on the other hand, however, excessive heat losses must also be avoided in order to ensure a specific temperature distribution in the electrolytic cell. In addition to the side-wall bricks and composite side-wall bricks according to the invention described hitherto and the refractory lining which is situated between the cathode and the steel trough, the cathode also influences the heat management in the electrolytic cell. If too much heat is conveyed away from the electrolytic cell, the cryolite in the melt solidifies excessively and can extend to the cathode surface. As a result, the cathodic current flow is disrupted, which leads to an inhomogeneous current distribution along the cathode surface and thus to an increased electric resistance and accordingly to a reduced energy efficiency of the electrolytic cell. The heat management from the cathode to the refractory lining located beneath it can readily be adjusted, whereas the heat management from the cathode to the side walls is substantially more difficult to =

adjust. The cathode blocks which form the cathode conventionally consist of a uniform material, that is to say these homogeneous cathode blocks have the same thermal conductivity, so that these cathode blocks are poorly or not at all capable of assisting optimum heat management in the electrolytic cell. This is the case in particular for the adjustment of the heat management from the cathode to the side walls.
WO 02/064860 describes cathode blocks which, when viewed in the direction of the long side of the cathode, have different layers which have different electric resistances, that is to say the cathode blocks are produced using different materials (having different specific electric resistances) in layers in the direction of the long side of the cathode. With these cathode blocks, the current flow through the cell is to be brought close to the ideal current profile even without complex guiding of current guide rails.
Cathode blocks which have different layers in the direction of the long side of the cathode, as a result of the use of different materials, also have different thermal conductivities within the cathode block. Such cathode blocks can also advantageously be used to reduce the heat losses caused by the cathode, in particular in the direction of the long side of the cathode, that is to say towards the side walls. As a result, the profile of the heat flow can also be controlled in the individual cathode blocks, and thus in the cathode as a whole.
Advantageously, the respective cathode block comprises, in the direction of the long side of the cathode, at least three layers, preferably from three layers to seven layers, particularly preferably from three layers to five layers, most particularly preferably three layers. Layers having a higher thermal conductivity and layers having a lower thermal conductivity are present, it being understood that in adjacent layers, one layer has a higher thermal conductivity as compared with the other layer. The difference in thermal conductivity between a layer having a higher thermal conductivity and a layer having a lower thermal conductivity is at least 10 %, based on the material having the lower thermal conductivity, in the temperature range from 920 to 1000 C, measured in the direction of the longitudinal axis of the cathode block. The cathode block can comprise at least two layers which have the same thermal conductivity, that is to say which consist of the same material. They can be the two outside or edge layers of the cathode block. With such a cathode block it is possible, by selecting the number of layers, the sequence of the layers and by selecting the thermal conductivity values of each individual layer, purposively to control the heat flow in the cathode block. If a smaller heat flow from the electrolytic cell is desired, a cathode block which has three layers, for example, can be used. The two outside layers, that is to say the two layers which are in thermal contact with the side wall of the electrolytic cell directly or via the ramming mass, are layers having a lower thermal conductivity, whereas the third, middle layer is a layer having a higher thermal conductivity. If, on the other hand, a higher heat flow from the electrolytic cell is desired, the two outside layers, in the case of a cathode block having three layers, are those which have a higher thermal conductivity in comparison with the third, middle layer.
The length of a cathode block is normally from 2500 to 3500 mm.
The length of an above-mentioned individual layer ¨ viewed in the longitudinal direction of the cathode ¨ depends on the desired heat flow in the cathode block and can purposively be chosen in dependence on that heat flow. The length of an individual layer further depends on the number of layers in the cathode block. If there are seven layers, for example, an individual layer has a length of from 300 to 600 mm. If only three layers are used, the outside or edge layers are from 400 to 600 mm long and the inside layer has a length of from 1700 to 2300 mm. Independently of the number of layers, the outside or edge layers of the cathode block have a length of from 400 to 600 mm, preferably of 500 mm.
The individual layers of the mentioned cathode blocks are composed on the basis of carbon, that is to say of a material which contains carbon. With regard to the thermal conductivity, it has been found to be advantageous for the cathode block to be composed of a material which contains at least 50 wt.%, preferably at least 80 wt.%, particularly preferably at least 90 wt.%, most particularly preferably at least 95 wt.% and most preferably at least 99 wt.%
carbon. The mentioned carbon can be chosen from the group consisting of amorphous carbons, graphitic carbons, graphitised carbons and arbitrary mixtures of two or more of the above-mentioned carbons.
The same method as for the above-described side-wall bricks according to the invention can be used for producing the cathode blocks. Therefore, for the production of the cathode blocks, reference is made to the corresponding remarks made above for the method for producing the side-wall bricks according to the invention.
For the production of the cathode blocks too, a uniform cathode block having high stability and a material-bonded cohesion between the individual layers of the resulting monolithic cathode blocks is achieved by firing a green block comprising different green mixtures that form the starting materials for the layers.
Suitable materials for the green mixtures in the case of the production of cathode blocks are also in particular all green materials that can be fired to give one of the preferred materials mentioned above in relation to the finished cathode block. For example, at least one green mixture can contain a material which is selected from the group consisting of a material containing carbon, such as, for example, anthracite, a graphitic or graphitisable material, such as, for example, synthetic graphite and pitch, or an arbitrary mixture of those materials.
Furthermore, a binder in particular containing carbon, such as, for example, binding pitch, can be present in the mixture. By means of the purposive composition of the material of the individual layers of the green block, the thermal conductivity of the different layers of the resulting cathode block can purposively be adjusted.
The shape of the layers in a cathode block can be different. In addition to layers which occupy the full height H of the cathode block, there can also be layers which occupy only a portion of the height H, as shown, for example, in Fig. 10 and 11. This shaping of the layers can take place in dependence on the desired heat flow in the cathode block, that is to say the heat flow can purposively be controlled by means of this shaping as well as by the choice of the materials of the layers, and thus the thermal conductivity values.
By combining the side-wall bricks according to the invention with the above-mentioned cathode blocks in an electrolytic cell, that is to say using both the side-wall bricks according to the invention and the above-described cathode blocks together in an electrolytic cell, the thermal conditions in an electrolytic cell can be controlled even more purposively ¨ than by means of the side-wall bricks according to the invention alone. As a result, the process conditions in an electrolytic cell can be optimised, whereby the achievable stability and economy of the electrolysis process are improved and the service life of the electrolytic cell is increased. It is here to be understood that each embodiment of the side-wall bricks that has been mentioned can be combined with each embodiment of the cathode blocks that has been mentioned.
The present invention will be described by way of example hereinbelow by means of advantageous embodiments with reference to the accompanying figures, in which:

2'4 Fig. 1 is a cutaway perspective view of an electrolytic cell according to an embodiment of the invention;
Fig. 2 is a perspective view of a side-wall brick according to an embodiment of the invention;
Fig. 3 is a perspective view of a side-wall brick according to a further embodiment of the invention;
Fig. 4 is a perspective view of base body from which a plurality of side-wall bricks according to an embodiment of the invention can be separated;
Fig. 5 is a perspective view of a further base body from which a plurality of side-wall bricks according to an embodiment of the invention can be separated;
Fig. 6 shows, in cross section, various embodiments of a composite side-wall brick;
Fig. 7 is a perspective view of a base body from which a plurality of composite side-wall bricks according to an embodiment of the invention can be separated, and of a composite side-wall brick which has been separated;
Fig. 8 is a perspective view of a further base body from which a plurality of composite side-wall bricks according to an embodiment of the invention can be separated;
Fig. 9 shows, in cross section, a further base body from which a plurality of composite side-wall bricks according to an embodiment of the invention can be separated;
Fig. 10 is a perspective view of a cathode block; and Fig. 11 shows cathode blocks having different shapes of the layers.
Fig. 1 is a partially cutaway perspective view of an electrolytic cell for producing aluminium according to an embodiment of the invention. The electrolytic cell comprises a cathode which is composed of a plurality of cathode blocks 12 forming a cathode floor.
On the upper side of the cathode there is arranged a layer 14 of liquid aluminium on which there is arranged a liquid molten layer 16 and, above the liquid molten layer 16, a layer or crust 18 of solidified melt.
Above the molten layer 16 there is arranged an anode which consists of a plurality of anode blocks 20 immersed in the molten layer 16. During operation of the electrolytic cell, electric current is supplied via the anode blocks 20 and passed through the molten layer 16 and the layer 14 of liquid aluminium to the cathode blocks 12. The current is conveyed away via the cathode blocks 12 and via the current rails 22 which are inserted into corresponding grooves on the underside of the cathode blocks 12. The electrolysis takes place in the molten layer 16 and leads to the cleavage of elemental aluminium from the melt, the elemental aluminium accumulating on the upper side of the cathode floor to form the layer 16 of liquid aluminium.
The electrolytic cell has a steel trough 24 serving as an outer enclosure, in the floor region of which there are laid a plurality of plates 26 of a refractory material which are stacked one on top of the other and which thermally insulate the cathode blocks 12 positioned thereon from the floor of the steel trough 24.
The side walls of the steel trough 24 are lined with a plurality of cuboid-shaped side-wall bricks 28. The side-wall bricks 28 form the side walls of an inner trough, in which the layer 14 of liquid aluminium, the liquid molten layer 16 and the solidified melt layer 18 are housed and the floor of which is formed by the cathode floor formed by the cathode blocks 12. The gaps formed between a cathode block 12 and a side-wall brick 28 are plugged by a ramming mass 30. Such a ramming mass can likewise be provided for plugging the gaps between the cathode blocks 12 and for plugging the gaps between the side-wall bricks 28.
As is shown in Fig. 1, the side-wall bricks 28 are substantially cuboid-shaped and stand upright in the steel trough 24 so that the height direction of the side-wall bricks 28 is parallel to the vertical. The surfaces of the side-wall bricks 28 delimiting the inside of the trough are formed by the bases 32 thereof which are parallel to the height direction and the width direction of the side-wall bricks 28, and the side-wall bricks 28 are connected together via their sides 34 which are parallel to the height direction and the width direction. As is shown in Fig. 1, the side-wall bricks 28 are in contact in different regions of their height with different constituents or media of the electrolytic cell, namely with the ramming mass 30, optionally the layer 14 of liquid aluminium, the liquid molten layer 16 and the solidified molten layer 18.

During the electrolysis operation, considerable amounts of thermal energy are generated in the electrolytic cell. Approximately one third of this thermal energy is conventionally absorbed via the side-wall bricks 28 and dissipated to the outside. The main direction of heat flow corresponds to the thickness direction of the side-wall bricks 28.
Approximately 15 % of the thermal energy is absorbed via the cathode floor or the bars.
The side-wall bricks 28 of the electrolytic cell shown in Fig. 1 each have at least one layer having a lower thermal conductivity and one layer having a higher thermal conductivity, the difference between lower and higher thermal conductivity being at least 5 W/m=K. As a result, the heat absorption and dissipation via the side wall formed by the side-wall bricks 28 is so adapted that optimum thermal conditions are established throughout the electrolytic cell during operation thereof, as a result of which the stability, reliability and efficiency of the electrolysis operation are improved and the service life of the electrolytic cell is increased.
Fig. 2 and 3 each show a side-wall brick 28 according to an embodiment of the invention which can be used, for example, in the electrolytic cell shown in Fig. 1. The side-wall bricks 28 each have a relatively small thickness d as well as a width b and a height h which is greater than the width b.
The side-wall brick 28 shown in Fig. 2 has two cuboid-shaped layers 36, 38, the layer 36 having a lower thermal conductivity and the layer 38 having a higher thermal conductivity.
The layers 36, 38 are connected together via their bases 40, 42, which are parallel to the height direction and the width direction and each form a contact face; the layers follow one another in the thickness direction of the side-wall brick 28 and each extend over approximately half the thickness d of the side-wall brick 28. As a result, the heat flows in the thickness direction and the positions of the isothermals within the side-wall bricks 28 can be so adapted that the thermal operating conditions in the electrolytic cell are optimised during operation.
The side-wall brick 28 shown in Fig. 3 likewise has two cuboid-shaped layers 36, 38, the layer 36 having a lower thermal conductivity and the layer 38 having a higher thermal conductivity. The layers 36, 38 are connected together via their sides 44, 46, which are parallel to the width direction and to the thickness direction and each form a contact face; the layers follow one another in the height direction of the side-wall brick 28 and each extend over approximately half the height h of the side-wall brick 28. Relative to the installation =

situation in the electrolytic cell, the upper half of the height is preferably formed by the layer 36 having the lower thermal conductivity. As a result, the heat conduction via the side-wall brick 28 can be adapted to the different constituents or media of the electrolytic cell which are in contact with the side-wall brick 28 in the respective height region, and to the thermal conditions prevailing therein, as a result of which the thermal conditions prevailing in the electrolytic cell during the electrolysis are optimised. In the case described above, the heat is dissipated by the good thermal contact between the lower half of the height comprising the layer 38 having the higher thermal conductivity and the cathode, which takes place via the ramming mass 30.
In the case of a different thermal design of the electrolytic cell, a reverse arrangement of the layers in relation to their thermal conductivity may be expedient.
Fig. 4 shows a base body 48 which has been produced as an intermediate product of a method according to the invention for producing a side-wall brick. The base body 48 is cuboid-shaped and consists of a cuboid-shaped layer 36 having a lower thermal conductivity and a cuboid-shaped layer 38 having a higher thermal conductivity, which layers are connected together via their bases. By means of a cutting operation, a plurality of plates forming side-wall bricks can be cut from the base body 48, which plates have two layers 36, 38 having different thermal conductivities. For that purpose, the base body 48, as is indicated in Fig. 4 by broken lines, is cut along a plurality of cutting planes which run perpendicularly to the interface between the two layers 36, 38.
Fig. 5 shows a further base body 48 which corresponds substantially to the base body shown in Fig. 4. However, the base body 48 comprises two layers 36 having a lower thermal conductivity and a layer 38 having a higher conductivity arranged between them, which layers are connected together via their bases. As is shown in Fig. 5, in order to produce the side-wall bricks, the base body 48 is cut not only in a plurality of planes perpendicular to the interfaces between the layers 36, 38, but additionally in a midplane of the layer 38 running parallel to those interfaces, so that the resulting side-wall bricks each have two layers 36, 38 having different thermal conductivities. This production process is more economical.
Fig. 6 shows cross sections of different embodiments of a composite side-wall brick 29 according to the invention, which can be used, for example, in the electrolytic cell shown in Fig. 1.

All the composite side-wall bricks shown in Fig. 6 have a cuboid-shaped layer 36 and a polygonal layer 38, the layer 36 having a lower thermal conductivity and the layer 38 having a higher thermal conductivity. The layers 36, 38 are connected together via their bases 40, 42, which are parallel to the height direction and the width direction and each form a contact face; the layers follow one another in the thickness direction of the composite side-wall brick 29 and each extend over from 30 % to 70 %, preferably 50 %, of the thickness d of the composite side-wall brick 29. The bases 40, 42 can be partially or completely in contact with one another. By means of these different configurations of the composite side-wall bricks, it is possible, on the one hand, to adapt the heat flows in the thickness direction and the positions of the isothermals inside the composite side-wall brick 29 in such a manner that the thermal operating conditions in the electrolytic cell during operation are optimised; on the other hand, it is also possible by means of such a composite side-wall brick 29 to dispense with some or all of the ramming mass between the composite side-wall brick 29 and the cathode block.
In Fig. 6a) the layer 38 has a trapezoidal shape, in Fig. 6b) the layer 38 has a triangular shape, and in Fig. 6c) the layer 38 has the shape of an irregular pentagon with a rounded corner. In these embodiments, the bases 40, 42 are completely in contact. In Fig. 6d) and 6e), on the other hand, the bases 40, 42 are only partially in contact, the layer 38 in Fig. 6d) being a rectangle having a rounded corner, and the layer 38 in Fig. 6e) having the shape of an irregular pentagon with a rounded corner.
In the case of a different thermal design of the electrolytic cell, a reverse arrangement of the layers in respect of their thermal conductivity may be expedient.
Fig. 7 shows a base body 48 which has been produced as an intermediate product of a method according to the invention for producing a composite side-wall brick 29. This base body 48 is cuboid-shaped and consists of a cuboid-shaped layer 36 having a lower thermal conductivity and a cuboid-shaped layer 38 having a higher thermal conductivity, which layers are connected together via their bases. These layers are horizontal layers.
The layer 36 is machined so that the layer acquires the desired polygonal shape over the entire length of the base body 48. In a subsequent step, plates having the desired width are cut from the base body 48. It is thus possible to utilise and adjust in the side-wall brick the grain direction which occurs during production of the base body and accordingly the different properties, such as, for example, the thermal conductivity, which occur in the horizontal and vertical direction, by choosing the bases accordingly during processing of the base body.
Fig. 8 shows a base body 48 which has been produced as the intermediate product of a method according to the invention for producing a composite side-wall brick 29. This base body 48 is cuboid-shaped and consists of two cuboid-shaped layers 36 having a lower thermal conductivity and one cuboid-shaped layer 38 having a higher thermal conductivity, which layers are connected together via their bases. These layers are vertical layers, the layers 36 being the two outside layers. A plurality of plates which have two outside layers 36 and an inside layer 38 having different thermal conductivities can be cut from the base body 48. In a subsequent step, the layer 38 is cut so that two blocks are obtained, from which the layer 38 is cut in a further step so that the desired polygonal shape is obtained. Alternatively, the base body can first be divided into two halves in the length direction, the polygon can be carved out, and then plates of the desired length can optionally be cut.
It is here possible to influence various properties, such as, for example, the thermal conductivity, by the orientation of the layers in the production of the corresponding base body ¨ either in horizontal or vertical form. The reason for this is the differing grain orientation during the shaping process and the resulting anisotropy of the physical properties.
Fig. 9 likewise shows a base body 48 which has been produced as an intermediate product of a method according to the invention for producing a composite side-wall brick 29. This base body 48 is cuboid-shaped and, like the base body of Fig. 8, consists of two cuboid-shaped layers 36 having a lower thermal conductivity and one cuboid-shaped layer 38 having a higher thermal conductivity, which layers are connected together via their bases.
These layers are vertical layers, the layers 36 being the two outside layers.
Here too, a plurality of plates which have two outside layers 36 and an inside layer 38 having different thermal conductivities can be cut from the base body 48. In a subsequent step, two composite side-wall bricks 29 having the same shape can then be cut from such an individual cut piece by suitable cutting. The advantage of this method resides in the sequence of the processing, which means that scarcely any material is lost.

Fig. 10 shows a cathode block 12 having three layers, the two outside layers consisting of the same material A and the middle layer consisting of material B. The individual layers here extend over the full height of the cathode block.
Fig. 11a) and b) show different forms of the layers in a cathode block 12, two materials, that is to say material A and material B, being used in each of the cathode blocks.
The two layers of material A here occupy only a portion of the height H and the length L of the cathode block.

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a 1 Embodiments:
Embodiment 1:
A side-wall brick is produced from a mixture A, containing 58 percent by weight (wt.%) electrically calcined anthracite, 9 wt.% synthetic graphite, 17 wt.% binding pitch, 8 wt.%
silicon and 8 wt.% aluminium oxide, and a mixture B, containing 77 wt.%
synthetic graphite and 23 wt.% binding pitch. For this purpose, a vibrating mould for producing a green block is filled with the two mixtures in such a manner that two successive layers of mixture A and mixture B in the height direction of the side-wall bricks to be produced follow one another in the green block. The height of the layers in the vibrating mould is so chosen, having regard to a target bulk density given by a compaction of the green block which follows the filling, that, after compaction, the two layers each extend over half the height of the green block.
This is followed by firing of the green block in an annular kiln at 1200 C to produce a base body.
Plates having a thickness of 10 cm are then cut from the fired and pre-processed base body, which plates are processed further in a subsequent step and can be impregnated with a pitch, for example. An example of a finished side-wall brick has a width of 475 mm, a height of 640 mm and a thickness of 100 mm, wherein, based on the installation situation of the side-wall brick in the electrolytic cell, in which it is arranged vertically along its height direction, the upper 320 mm of the height of the side-wall brick are formed of material A
resulting from mixture A and the lower 320 mm are formed of material B
resulting from mixture B.
Material A has a thermal conductivity, measured at room temperature in one direction of the side-wall brick, of approximately 8 W/m=K, while material B has a thermal conductivity of approximately 45 W/m=K in the same direction of the side-wall brick, in the grain direction of materials A and B. The thermal conductivity at room temperature can be measured in accordance with ISO 12987, namely in a specific direction, in the case where pressure is applied to the starting material during production of the side-wall brick, for example, perpendicularly or parallel to the direction of the application of pressure, that is to say against or in the grain direction.

The thermal conductivity measured at a temperature between 920 C and 1000 C
is approximately 9 W/m=K in the case of material A and approximately 37 W/m=K in the case of material B. Measurement of the thermal conductivity can here take place in the grain direction in accordance with DIN 51936 using a pulsed laser.
Embodiment 2:
A composite side-wall brick is produced from a mixture A, containing 58 wt.%
electrically calcined anthracite, 9 wt.% synthetic graphite, 17 wt.% binding pitch, 8 wt.%
silicon and 8 wt.% aluminium oxide, and a mixture B, containing 65 wt.% synthetic graphite, 5 wt.%
aluminium oxide, 10 wt.% silicon powder and 20 wt.% binding pitch. For that purpose, a vibrating mould for producing a green block is filled with the two mixtures in such a manner that two successive layers of mixture A and mixture B in the height direction of the combo bricks that are to be produced follow one another in the green block. The height of the layers in the vibrating mould is so chosen, having regard to a target bulk density given by a compaction of the green block which follows the filling, that, after compaction, the two layers each extend over half the height of the green block. This is followed by firing of the green block in an annular kiln at 1300 C to produce a base body.
The layer containing material A is then so machined that it acquires the desired polygonal shape over the entire length of the green block. In a subsequent step, plates having a thickness of 50 cm are then cut from the base body. An example of a finished composite side-wall brick has a width of 500 mm, a height of 700 mm and a thickness of 250 mm.
Material A has a thermal conductivity, measured at room temperature in one direction of the side-wall brick, of approximately 8 W/rn=K, while material B has a thermal conductivity of approximately 45 W/m=K in the same direction of the side-wall brick, in the grain direction of materials A and B. The thermal conductivity at room temperature can be measured in accordance with ISO 12987, namely in a specific direction, in the case where pressure is applied to the starting material during production of the side-wall brick, for example, perpendicularly or parallel to the direction of the application of pressure, that is to say against or in the grain direction.

3.3 The thermal conductivity measured at a temperature between 920 C and 1000 C
is approximately 9 W/m=K in the case of material A and approximately 37 W/m=K in the case of material B.
Embodiment 3:
A cathode block as shown in Fig. 10 is produced by introducing into a vibrating mould, the height of which is considered to be the finished height of the green moulded body, first a mixture A, then a mixture B and then again a mixture A.
Mixture A has the following composition:
57 wt.% anthracite 24 wt.% graphite and 19 wt.% binding pitch.
Mixture B has the following composition:
80 wt.% graphite and 20 wt.% binding pitch.
The height of the layers in the vibrating mould is so chosen, having regard to a target bulk density given by a compaction of the green block which follows the filling, that, after compaction, the two layers each extend over half the height of the green block. This is followed by firing of the green block in an annular kiln at 1200 C to produce a base body.
Material A has a thermal conductivity, measured at room temperature in one direction of the cathode block, of approximately 15 W/m=K, while material B has a thermal conductivity of approximately 40 W/m=K in the same direction of the cathode block, in the grain direction of materials A and B. The thermal conductivity at room temperature can be measured in accordance with ISO 12987, namely in a specific direction, in the case where pressure is applied to the starting material during production of the cathode block, for example, perpendicularly or parallel to the direction of the application of pressure, that is to say against or in the grain direction.
A cathode block so produced can have a width of 420 mm, a height of 400 mm and a length of 3100 mm and can be used to produce a cathode floor having, for example, 24 cathode 3.4 .
blocks. Such cathode blocks can be used in an electrolytic cell together with the side-wall bricks according to the invention.

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35 .
List of reference numerals 12 cathode block 14 layer of liquid aluminium 16 layer of liquid melt 18 layer of solidified melt 20 anode block 22 current rail 24 steel trough 26 refractory plate 28 side-wall brick 29 composite side-wall brick 30 ramming mass 32 base 34 side 36 layer having lower thermal conductivity 38 layer having higher thermal conductivity 40, 42 base 44, 46 side 48 base body b width h height d thickness H height of the cathode block L length of the cathode block

Claims

Claims:

1. Side-wall brick for a wall in an electrolytic cell, in particular for producing aluminium, wherein the side-wall brick (28) is a layered body, comprising a layer (36) having a lower thermal conductivity and a layer (38) having a higher thermal conductivity, the difference between lower and higher thermal conductivity being at least 5 W/m.cndot.K ¨ measured at a temperature between 920 °C
and 1000 °C, characterised in that at least one of the layers (36, 38) is doped with silicon (powder), an oxidic ceramic material or a non-oxidic material.

2. Side-wall brick according to claim 1, characterised in that the layered body has an alternating sequence of a layer (36) having a lower thermal conductivity and a layer (38) having a higher thermal conductivity.

3. Side-wall brick according to claim 2, characterised in that one outside layer of the layered body is a layer (36) having a lower thermal conductivity and the other outside layer is a layer (38) having a higher thermal conductivity.

4. Side-wall brick according to any of claims 1 to 3, characterised in that the layers (36, 38) have a block shape, in particular a cuboid shape, and are connected together via contact faces, in particular their bases (40, 42), or via their sides (44, 46).

5. Side-wall brick according to any of claims 1 to 3, characterised in that 3'7 one of the layers (36, 38) has a block shape, in particular a cuboid shape, and the other layer (36, 38) has a polygonal shape, the two layers being connected together via contact faces, in particular their bases (40, 42).

6. Side-wall brick according to claim 5, characterised in that the layer (36, 38) having a polygonal shape is a polygon having from three to six corners.

7. Side-wall brick according to any of claims 1 to 6, characterised in that, when the layers (36, 38) are connected together via their bases (40, 42), the thickness of the layered body is from 50 to 700 mm.

8. Side-wall brick according to claim 4, characterised in that, when the layers (36, 38) are connected together via their sides (44, 46), the height of the layers is from 150 to 450 mm.

9. Side-wall brick according to any of claims 1 to 8, characterised in that at least one layer (36, 38) consists of a material selected from the group consisting of carbon, graphitic carbon, graphitised carbon or silicon carbide or arbitrary mixtures thereof, or contains such a material.

10. Side-wall brick according to any of claims 1 to 9, characterised in that the difference between lower thermal conductivity and higher thermal conductivity is from 5 to 80 W/m.cndot.K, preferably from 5 to 70 W/m.cndot.K, particularly preferably from 8 to 60 W/m.cndot.K and most particularly preferably between 10 W/m.cndot.K and 50 W/m.cndot.K.

11. Method for producing a side-wall brick (28) according to any of claims 1 to 10, comprising the following steps:

a) providing a mixture for the layer (36) having the lower thermal conductivity, a mixture for the layer (38) having the higher thermal conductivity, and optionally one or more mixtures for at least one further layer, b) forming a green block having a layer structure from the mixtures according to step a), c) firing the green block according to step b) at a temperature of from 1100 to 1400 °C, preferably 1200 °C.

12. Method according to claim 11, characterised in that the formation according to step b) comprises vibration moulding.

13. Use of the side-wall brick (28) according to any of claims 1 to 12 for lining the side walls in an electrolytic cell.

14. Electrolytic cell, in particular for producing aluminium, which comprises a cathode (12) and an anode (20) as well as a side wall, wherein at least a portion of the wall is formed by a side-wall brick (28) according to at least one of claims 1 to 11.

15. Electrolytic cell according to claim 14, wherein the cathode is formed of cathode blocks which are a layered body comprising layers having a lower thermal conductivity and layers having a higher thermal conductivity.