Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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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
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=
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
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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
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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.
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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.
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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
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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
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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).
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 Wim=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
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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
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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
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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
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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. lf, 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.
CA 02893476 2015-06-02
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
CA 02893476 2015-06-02
14
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
CA 02893476 2015-06-02
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.
CA 02893476 2015-06-02
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
CA 02893476 2015-06-02
17
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
CA 02893476 2015-06-02
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18
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
CA 02893476 2015-06-02
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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
CA 02893476 2015-06-02
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
CA 02893476 2015-06-02
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21
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
CA 02893476 2015-06-02
22
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. lf, 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
CA 02893476 2017-01-12
25861-152
23
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. 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 embodirnent of the cathode blocks
that has
been mentioned.
CA 02893476 2017-01-12
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23a
According to one aspect of the present invention, there is provided a side-
wall brick for a wall
in an electrolytic cell, wherein the side-wall brick is a layered body,
comprising a layer having
a lower thermal conductivity and a layer having a higher thermal conductivity,
the difference
between lower and higher thermal conductivity being at least 5 Wim=K ¨
measured at a
temperature between 920 C and 1000 C, wherein at least one of the layers is
doped with
silicon, an oxidic ceramic material or a non-oxidic material.
The present invention will be described by way of example hereinbelow by means
of
advantageous embodiments with reference to the accompanying figures, in which:
CA 02893476 2015-06-02
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
CA 02893476 2015-06-02
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.
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26
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-wali 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
CA 02893476 2015-06-02
=
27
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.
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2B
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,
CA 02893476 2015-06-02
29
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.
CA 02893476 2015-06-02
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.
CA 02893476 2015-06-02
32
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.
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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
CA 02893476 2015-06-02
S4 .
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
