Note: Descriptions are shown in the official language in which they were submitted.
CA 02826604 2013-08-06
SGL CARBON SE 1
2010/089W0
01.08.2013
Cathode block having a top layer containing hard material
The present invention relates to a cathode block for an aluminium electrolysis
cell.
Such electrolysis cells are used for the electrolytic production of aluminium,
which
is customarily carried out in industry by the Hall-Heroult process. In the
Hall-
Heroult process, a melt composed of aluminium oxide and cryolite is
electrolysed.
Here, the cryolite, Na3[AlF6], serves to lower the melting point of 2045 C for
pure
aluminium oxide to about 950 C for a mixture containing cryolite, aluminium
oxide
and additives, such as aluminium fluoride and calcium fluoride.
The electrolysis cell used in this process has a bottom, which is usually
composed
of a multiplicity of adjoining cathode blocks forming the cathode. In order to
with-
stand the thermal and chemical conditions which prevail during operation of
the
cell, the cathode blocks are customarily composed of a carbon-containing mate-
rial. The undersides of each of the cathode blocks are provided with grooves,
in
each of which there is arranged at least one busbar through which the current
fed
via the anodes is discharged. In this case, the interstices between the
individual
walls of the cathode blocks, which delimit the grooves, and the busbars are
often
sealed with cast iron, in order to electrically and mechanically connect the
busbars
to the cathode blocks by virtue of the resulting encasement of the busbars
with
cast iron. An anode formed from individual anode blocks is arranged about 3 to
5
cm above the layer of molten aluminium located on the top side of the cathode,
and the electrolyte, i.e. the melt containing aluminium oxide and cryolite, is
located
between said anode and the surface of the aluminium. During the electrolysis
carried out at about 1000 C, the aluminium which has formed settles beneath
the
electrolyte layer, i.e. as an intermediate layer between the top side of the
cathode
blocks and the electrolyte layer, on account of the fact that its density is
relatively
large compared to that of the electrolyte. During the electrolysis, the
aluminium
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2
oxide dissolved in the cryolite melt is cleaved into aluminium and oxygen by a
flow
of electric current. In terms of electrochemistry, the layer of molten
aluminium is
the actual cathode, since aluminium ions are reduced to elemental aluminium on
the surface thereof. Nevertheless, hereinbelow the term "cathode" will not be
un-
derstood to mean the cathode from an electrochemical point of view, i.e. the
layer
of molten aluminium, but rather the component which forms the electrolysis
cell
bottom and is composed of one or more cathode blocks.
A significant disadvantage of the Hall-Heroult process is that it requires a
large
amount of energy. To produce 1 kg of aluminium, about 12 to 15 kWh of
electrical
energy are required, which amounts to up to 40% of the production costs. To
make it possible to lower the production costs, it is therefore desirable to
reduce
the specific energy consumption of this process as far as possible.
For this reason, graphite cathodes have increasingly been used in recent
times,
i.e. cathode blocks which contain graphite as the main constituent. Compared
to
amorphous carbon, graphite is distinguished by a considerably lower electrical
resistivity and also by a significantly higher thermal conductivity, and this
is why
the use of graphite cathodes during the electrolysis makes it possible firstly
to
reduce the specific energy consumption of the electrolysis and secondly to
carry
out the electrolysis at a higher current intensity, which makes it possible to
in-
crease the aluminium production per electrolysis cell. However, graphite
cathode
blocks have a very low and in particular a relatively low resistance to the
abrasive
wear processes which occur during operation of the electrolysis cell, and
therefore
have a shorter service life than cathode blocks consisting of amorphous
carbon. In
particular, slurry made up of undissolved aluminium oxide readily settles on
the
surface of graphite cathode blocks, and this firstly considerably reduces the
wear
resistance of the cathode block, on account of the particle abrasion resulting
from
the slurry formation, and secondly hinders the flow of current on the cathode
block
surface, on account of the reduction in the effective cathode surface,
resulting in
CA 02826604 2015-06-05
25861-128
3
an increase in the specific energy consumption during electrolysis. This
additionally
leads to an increase in the current density, which can lead to a shorter
service life of
the electrolysis cell.
In order to improve the wetting of the cathode block surface, it has been
proposed in
WO 96/07773 Al to apply a coating of pure titanium diboride, zirconium
diboride or
the like to the cathode block. DE 197 14 433 C2 discloses a cathode block
having a
similar coating which contains at least 80 % by weight of titanium diboride
and is
produced by plasma spraying titanium diboride onto the surface of the cathode
block.
Such coatings of pure titanium diboride or having a very high titanium
diboride
content are very brittle and therefore susceptible to cracking, however. In
addition,
the specific thermal expansion of these coatings is approximately twice as
high as
that of amorphous carbon or graphite, which is why these have only a short
service
life when they are used in fused-salt electrolysis.
The present invention relates to a cathode block which has a low specific
electrical
resistivity, which is distinguished by a high thermal conductivity, which can
be wetted
well with aluminium melt, which has a high wear resistance with respect to the
abrasive, chemical and thermal conditions which prevail during operation in
fused-salt
electrolysis, and which, in particular, is also distinguished by the fact that
no slurry is
deposited or at best small quantities of slurry are deposited on its surface
when
carrying out fused-salt electrolysis.
In one aspect, the invention relates to a cathode block for an aluminium
electrolysis
cell having a primary layer and having a top layer, wherein the primary layer
contains
graphite and the top layer is composed of a carbon composite material
containing 15
to less than 50 ck by weight of hard material having a melting point of at
least
1000 C, and wherein the top layer has a thickness of 50 to 400 mm.
,
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4
This solution is based on the understanding that the provision of a top layer
which
is composed of a carbon composite material containing not less than 15 `)/0 by
weight, but at most less than 50 % by weight, of hard material having a
melting
point of at least 1000 C on a graphite-containing primary layer produces a
cath-
ode block which has a sufficiently low specific electrical resistivity for
energy-
efficient operation of fused-salt electrolysis and additionally has a very
high wear
resistance with respect to the abrasive, chemical and thermal conditions which
prevail during fused-salt electrolysis. Here, it was particularly surprising
that, in
such a cathode block, in particular the formation of a slurry or the
deposition of a
slurry on the surface is reliably prevented, and thus not only is the wear
resistance
of the cathode block increased considerably as a result of the reduction or
preven-
tion of particle abrasion resulting from slurry formation, but also in
particular hin-
drance of the flow of current as a result of the formation of a slurry or the
deposi-
tion of a slurry on the cathode block surface and a resulting increase in the
specific
energy consumption during electrolysis are reliably prevented.
Therefore, the cathode block according to the present invention is
distinguished by
the advantages associated with the provision of graphite in the primary layer
of the
cathode block, such as in particular by a low electrical resistance of the
cathode
block and by a high thermal conductivity of the cathode block, without however
having the disadvantages resulting from the use of graphite, such as a low
wear
resistance and a lack of wettability by aluminium melt. Instead, a good
wettability
of the cathode block surface with molten aluminium is achieved on account of
the
top layer containing hard material which is provided in the cathode block
according
to the invention, and therefore the formation of a slurry or the deposition of
a slurry
on the surface of the cathode block is reliably prevented. Furthermore, the
move-
ment of the molten aluminium is thereby reduced significantly, such that the
dis-
tance between the surface of the layer of molten aluminium and the anode in
the
electrolysis cell can be reduced, for example, to 2,5 to 4,0 cm and preferably
to 3
to 3,5 cm, which further reduces the specific energy consumption of the
electroly-
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sis process. In addition, despite the fact that a hard-material-containing top
layer is
used, the surface of the cathode block according to the invention surprisingly
does
not tend to crack and in particular is also not distinguished by a
disadvantageously
high brittleness.
All in all, the cathode block according to the invention has long-term
stability with
respect to carrying out fused-salt electrolysis using a melt containing
aluminium
oxide and cryolite for producing aluminium, and makes it possible for fused-
salt
electrolysis to be carried out with a very low specific energy consumption.
This is
achieved by the abovementioned combination of a graphite-containing primary
layer and a top layer which contains hard material in a quantity of less than
50 %
by weight and is based on a carbon composite material. This was particularly
surprising because the cathode blocks known from the prior art having a
coating
containing titanium diboride necessarily contain relatively high quantities of
tita-
nium diboride, which renders the known coatings brittle.
Within the context of the present invention, and in keeping with the customary
definition in the art for this term, "hard material" is understood to mean a
material
which is distinguished by a particularly high hardness in particular even at
high
temperatures of 1000 C and higher.
It is preferable for the melting point of the hard material used to be
considerably
higher than 1000 C, where in particular hard materials having a melting point
of at
least 1500 C, preferably hard materials having a melting point of at least
2000 C
and particularly preferably hard materials having a melting point of at least
2500
C have proved to be particularly suitable.
In principle, all hard materials can be used in the top layer of the cathode
block
according to the invention. However, good results are achieved in particular
with
hard materials having a Knoop hardness, measured in accordance with DIN EN
=
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6
843-4, of at least 1000 N/mm2, preferably of at least 1500 N/mm2, particularly
preferably of at least 2000 N/mm2 and very particularly preferably of at least
2500
N/mm2.
Examples of suitable hard materials are metal carbides, metal borides, metal
ni-
trides and metal carbonitrides having a sufficiently high hardness at 1000 C.
Examples of suitable representatives from these groups are titanium diboride,
zirconium diboride, tantalum diboride, titanium carbide, boron carbide,
titanium
carbonitride,silicon carbide, tungsten carbide, vanadium carbide, titanium
nitride,
boron nitride and silicon nitride. It is very particularly preferable to use a
non-oxidic
titanium ceramic, to be precise preferably titanium diboride, titanium
carbide, tita-
nium carbonitride and/or titanium nitride, as the hard material in the top
layer of the
cathode block according to the invention. Most preferably, the top layer of
the
cathode block according to the invention contains titanium diboride as the
hard
material. All of the abovementioned hard materials can be used alone or it is
pos-
sible to use any desired chemical combination and/or mixture of two or more of
the
abovementioned compounds.
According to a particularly preferred embodiment of the present invention, the
hard
material present in the top layer of the cathode block has a monomodal
particle
size distribution, wherein the mean volume-weighted particle size (d3,50), as
de-
termined by static light scattering in accordance with the international
standard
ISO 13320-1, is 10 to 20 pm. In this embodiment, it is particularly preferable
to use
a non-oxidic titanium ceramic and most preferable to use titanium diboride
having
a monomodal particle size distribution defined above.
Within the context of the present invention, it has been established that a
hard
material, in particular non-oxidic titanium ceramic and especially titanium
diboride,
having a monomodal particle size distribution defined above not only brings
about
a very good wettability of the surface of the cathode block, which is why the
forma-
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7
tion of a slurry and the deposition of a slurry on the surface of the cathode
block
are reliably prevented, the wear resistance of the cathode block is increased
and
the specific energy consumption during electrolysis is reduced. In addition,
it has
surprisingly been established within the context of the present invention that
this
effect is also achieved in particular given relatively small amounts of
titanium
diboride, of less than 50 % by weight and particularly preferably even given
tita-
nium diboride quantities of only 15 to 20 % by weight in the top layer. It is
thereby
possible to dispense with a high concentration of titanium diboride in the top
layer,
which leads to a brittle cathode block surface. Furthermore, hard material, in
par-
ticular non-oxidic titanium ceramic and especially titanium diboride, having a
monomodal particle size distribution defined above is also distinguished by a
very
good processability. In particular, the tendency of such a hard material to
form
dust, for example as it is being introduced into a mixing tank or as the hard
mate-
rial powder is being transported, is sufficiently low, and at best a small
degree of
agglomerate formation occurs during the mixing, for example. In addition, such
a
hard material powder has a sufficiently high flowability and pourability, and
there-
fore it can be conveyed to a mixing apparatus, for example, using a
conventional
conveying apparatus. This all results not only in a simple and cost-effective
pro-
ducibility of the cathode blocks according to the invention, but also in
particular in
a very homogeneous distribution of the hard material in the top layer of the
cath-
ode blocks.
The hard material, preferably titanium diboride, present in the top layer of
the
cathode block preferably has a monomodal particle size distribution, wherein
the
mean volume-weighted particle size (d3,50), determined as above, is 12 to 18
pm
and particularly preferably 14 to 16 pm.
As an alternative to the embodiment mentioned above, the hard material present
in the top layer of the cathode block can have a monomodal particle size
distribu-
tion, wherein the mean volume-weighted particle size (d3,50), as determined by
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8
static light scattering in accordance with the international standard ISO
13320-1, is
3 to 10 pm and preferably 4 to 6 pm. In this embodiment, too, it is
particularly
preferable to use a non-oxidic titanium ceramic and most preferable to use
tita-
nium diboride having a monomodal particle size distribution defined above.
In a development of the concept of the invention, it is proposed that the hard
mate-
rial has a volume-weighted d3,90 particle size, determined as above, of 20 to
40 pm
and preferably of 25 to 30 pm. The hard material preferably has such a d3,90
value
in combination with an above-defined d3,50 value. In this embodiment, too, the
hard
material is preferably a non-oxidic titanium ceramic and particularly
preferably
titanium diboride. As a result, the advantages and effects mentioned for the
above
embodiment are achieved to an even greater extent.
As an alternative to the embodiment mentioned above, the hard material present
in the top layer of the cathode block can have a volume-weighted d3,90
particle
size, determined as above, of 10 to 20 pm and preferably of 12 to 18 pm. The
hard
material preferably has such a d3,90 value in combination with an above-
defined
d3,50 value. In this embodiment, too, it is particularly preferable to use a
non-oxidic
titanium ceramic and most preferable to use titanium diboride having a monomo-
dal particle size distribution defined above.
According to a further preferred embodiment of the present invention, the hard
material has a volume-weighted d3,10 particle size, determined as above, of 2
to 7
pm and preferably of 3 to 5 pm. The hard material preferably has such a d3,10
value in combination with an above-defined d3,90 value and/or d3,50 value. In
this
embodiment, too, the hard material is preferably a non-oxidic titanium ceramic
and
particularly preferably titanium diboride. As a result, the advantages and
effects
mentioned for the above embodiments are achieved to an even greater extent.
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As an alternative to the embodiment mentioned above, the hard material present
in the top layer of the cathode block can have a volume-weighted d3,10
particle
size, determined as above, of 1 to 3 pm and preferably of 1 to 2 pm. The hard
material preferably has such a d3.10 value in combination with an above-
defined
d3,90 value and/or d3,50 value. In this embodiment, too, it is particularly
preferable to
use a non-oxidic titanium ceramic and most preferable to use titanium diboride
having a monomodal particle size distribution defined above.
In addition, it is preferable if the hard material, in particular a non-oxidic
titanium
ceramic and particularly preferably titanium diboride, has a particle size
distribution
which is characterized by a span value, as calculated in accordance with the
fol-
lowing equation:
Span = (d3,90 - d3,10)/c13,50
of 0,65 to 3,80 and particularly preferably of 1,00 to 2,25. The hard material
pref-
erably has such a span value in combination with an above-defined d3,90 value
and/or d3,50 value and/or d3,10 value. As a result, the advantages and effects
men-
tioned for the above embodiments are achieved to an even greater extent.
As set forth above, non-oxidic titanium ceramics, such as preferably titanium
car-
bide, titanium carbonitride, titanium nitride and most preferably titanium
diboride,
are suitable in particular as the hard material in the top layer of the
cathode block
according to the invention. For this reason, it is proposed in a development
of the
concept of the invention that the hard material consists of non-oxidic
titanium ce-
ramic and in particular of titanium diboride to an extent of at least 80 % by
weight,
preferably to an extent of at least 90 % by weight, particularly preferably to
an
extent of at least 95 % by weight, very particularly preferably to an extent
of at
least 99 % by weight and most preferably completely.
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The total quantity of the hard material in the top layer is, according to the
inven-
tion, at least 15 % by weight, but at most less than 50 % by weight. When the
quantity of hard material lies in this value range, the top layer contains
sufficient
hard material firstly to provide the top layer with an excellent hardness and
abra-
sion resistance for increasing the wear resistance and secondly to provide a
wet-
tability of the top layer surface with liquid aluminium which is sufficiently
high for
avoiding the formation of a slurry and the deposition of a slurry, as a result
of
which the wear resistance of the cathode block is increased further and the
spe-
cific energy consumption during fused-salt electrolysis is reduced further; at
the
same time, however, the top layer contains a sufficiently small quantity of
hard
material such that the surface of the top layer does not have a brittleness
which is
too high for a sufficiently high long-term stability on account of the
addition of hard
material.
Here, good results are achieved in particular if the top layer contains 15 to
40 % by
weight and particularly preferably 15 to 30 % by weight of a hard material
having a
melting point of at least 1000 C.
Apart from the hard material, the top layer contains carbon and, if
appropriate,
binder, such as pitch, in particular coal tar pitch and/or petroleum pitch. If
pitch is
mentioned hereinbelow, this means all varieties of pitch known to a person
skilled
in the art. Here, the carbon, together with the optional binder, forms the
matrix in
which the hard material is embedded. Good results are achieved in particular
if the
top layer contains 85 to more than 50 % by weight, preferably 85 to 60 `)/0 by
weight and particularly preferably 85 to 70 % by weight of carbon.
Here, the carbon present in the top layer can be amorphous carbon, graphite or
a
mixture of amorphous carbon and graphite.
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According to a very particularly preferred embodiment of the present
invention, the
carbon present in the top layer of the cathode block according to the
invention is
exclusively amorphous carbon or a mixture of amorphous carbon and graphite. If
a
mixture of amorphous carbon and graphite is used, this mixture preferably con-
tains 10 to 99 % by weight, particularly preferably 30 to 95 % by weight and
very
particularly preferably 60 to 90 % by weight of amorphous carbon, remainder
graphite, where the graphite used can be both natural graphite and also
synthetic
graphite.
Cathode blocks according to the invention having a top layer composed of
carbon
composite material which contains hard material and, as the carbon component,
contains a mixture of amorphous carbon and graphite containing optionally car-
bonized binder (such as for example a mixture of calcined anthracite, graphite
and
carbonized pitch) or very particularly preferably amorphous carbon containing
optionally carbonized binder (such as for example a mixture of calcined
anthracite
and carbonized pitch) have a particularly high abrasion resistance. The
starting
material used for the amorphous carbon is preferably anthracite, which is then
calcined at a temperature of between 800 and 2200 C and particularly
preferably
between 1200 and 2000 C.
In a development of the concept of the invention, it is proposed for the
cathode
block top layer containing amorphous carbon that the top layer has a vertical
spe-
cific electrical resistivity at 950 C of 20 to 32 Q pm and preferably of 22
to 28
pm. This corresponds to vertical specific resistivities at room temperature of
23 to
40 Q pm and of 25 to 30 Q pm. In this context, "vertical specific electrical
resistiv-
ity" is understood to mean the specific electrical resistivity when the
cathode block
is installed in the vertical direction.
In principle, the thickness of the top layer should be as small as possible,
in order
to keep the costs for the expensive hard material as low as possible, but
should
,
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,
12
also be sufficiently great for the top layer to have a sufficiently high wear
resis-
tance and service life. Good results are achieved in this respect in
particular if the
thickness of the top layer amounts to 1 to 50 %, preferably 5 to 40 %,
particularly
preferably 10 to 30 % and very particularly preferably 15 to 25 %, for example
about 20 %, of the overall height of the cathode block.
By way of example, the top layer can have a thickness or height of 50 to 400
mm,
preferably of 50 to 200 mm, particularly preferably of 70 to 130 mm, very
particu-
larly preferably of 90 to 110 mm and most preferably of about 100 mm. Here,
"thickness or height" is understood to mean the distance from the underside of
the
top layer to the point of the highest elevation of the top layer.
Similarly, and by way of example, the base layer can have a thickness or
height of
100 to 550 mm, preferably of 300 to 500 mm, particularly preferably of 400 to
500
mm, very particularly preferably of 425 to 475 mm and most preferably of about
450 mm.
In principle, it is possible for the top layer of the cathode block to have a
surface
which is profiled at least in certain regions. On account of a profiled
surface, the
movement of the molten aluminium brought about by the electromagnetic interac-
tion present during the electrolysis is reduced, resulting in relatively small
wave
formation and bulging of the aluminium layer. For this reason, the use of
surface-
profiled cathode blocks can further reduce the distance between the molten alu-
minium and the anode, and therefore the electrical cell resistance is further
re-
duced as a result of the reduction in the ohmic resistance, and therefore so
too is
the specific energy consumption.
Here, a profiled surface is understood to mean a surface having at least one
re-
cess and/or elevation which is arranged chaotically or extends in the
transverse
direction, in the longitudinal direction or in any other desired direction,
such as for
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13
example in a direction running at an acute or obtuse angle to the longitudinal
di-
rection, of the cathode block, the recess or elevation having at least a depth
or
height of 0,05 mm and preferably of 0,5 mm in demarcation relative to a
surface
roughness, as seen transversely to the cathode block surface. In this case,
the at
least one recess and/or elevation can be restricted exclusively to the top
layer or
the at least one recess and/or elevation can extend into the primary layer. It
is
preferable for the at least one recess and/or elevation to extend exclusively
in the
top layer.
Within the context of the present invention, a recess is understood to mean a
cut-
out directed inwards from the surface of the cathode block, whereas the term
"ele-
vation" means a rise directed outwards from the surface of the cathode block.
In
the case of rectangular cutouts or elevations each of equal depth or height,
for
example, here it can depend on the observer whether these are regarded as re-
cesses or elevations. The wording "recess and/or elevation" is intended to
take
these ambiguities between the terms "recess" and "elevation" into account.
In principle, the at least one recess and/or elevation can have any desired
geome-
try, as seen in the transverse direction of the cathode block. By way of
example,
the at least one recess or elevation can have a convex, concave or polygonal
form, for example a trapezoidal, triangular, rectangular or square form, as
seen in
the transverse direction of the cathode block.
In order to avoid or at least considerably reduce wave formation during
operation
of the cathode block according to the invention for the fused-salt
electrolysis of
aluminium oxide in a cryolite melt, and in order to drastically reduce the
height of
any waves which do possibly form, it is proposed in a development of the
concept
of the invention that, if the surface profiling comprises at least one recess,
the
depth-to-width ratio of the at least one recess is 1:3 to 1:1 and preferably
1:2 to
1:1.
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Good results are achieved in particular if the depth of the at least one
recess is 10
to 90 mm, preferably 40 to 90 mm and particularly preferably 60 to 80 mm, such
as for example about 70 mm.
According to a further preferred embodiment, the width of the at least one
recess
is 100 to 200 mm, particularly preferably 120 to 180 mm and very particularly
pref-
erably 140 to 160 mm, such as for example about 150 mm.
In principle, it is possible for the at least one recess to extend only in
certain re-
gions, as seen in the longitudinal direction of the cathode block. However, it
is
preferable for the at least one recess to extend over the entire length of the
cath-
ode block, so as to achieve the effect of reducing or completely reducing wave
formation of liquid aluminium. It is possible, however, for the depth and/or
width of
the at least one recess to vary over the length of the cathode block. It is
similarly
possible for the geometry of the recess to also vary over the length of the
cathode
block.
If the surface profiling comprises at least one elevation, it is likewise
preferable, in
order to avoid or at least considerably reduce wave formation during operation
of
the cathode block according to the invention for the fused-salt electrolysis
of alu-
minium oxide in a cryolite melt, and in order to drastically reduce the height
of any
waves which do possibly form, for the height-to-width ratio of the at least
one ele-
vation to be 1:2 to 2:1 and preferably about 1:1.
Good results are achieved in particular if the height of the at least one
elevation is
to 150 mm, preferably 40 to 90 mm and particularly preferably 60 to 80 mm,
such as for example about 70 mm.
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According to a further preferred embodiment, the width of the at least one
eleva-
tion is 50 to 150 mm, particularly preferably 55 to 100 mm and very
particularly
preferably 60 to 90 mm, such as for example about 75 mm.
In principle, it is possible for the at least one elevation to extend only in
certain
regions, as seen in the longitudinal direction of the cathode block. However,
it is
preferable for the at least one elevation to extend over the entire length of
the
cathode block, so as to achieve the effect of reducing or completely reducing
cor-
rugation of liquid aluminium. It is possible, however, for the height and/or
width of
the at least one elevation to vary over the length of the cathode block. It is
similarly
possible for the geometry of the elevation to also vary over the length of the
cath-
ode block.
If the surface profiling comprises both at least one recess and at least one
eleva-
tion, the ratio of the width of the at least one recess to the width of the at
least one
elevation is preferably 4:1 to 1:1, such as for example about 2:1.
In order to reliably avoid the deposition of slurry present in the melt in the
profiled
structure of the surface of the cathode block as fused-salt electrolysis is
being
carried out, it is proposed in a development of the concept of the invention
to avoid
any angled and in particular right-angled regions in the profiled surface. If,
for
example, a substantially rectangular cross section is chosen for the at least
one
recess and/or elevation, it is preferable according to a preferred embodiment
of
the present invention to round off the right-angled regions. The radius of
curvature
of these rounded-off sections can be, for example, 5 to 50 mm, preferably 10
to 30
mm and particularly preferably about 20 mm. In order to avoid sharp edges, any
desired geometries which all fall under the term "rounded-off' are
conceivable, in
principle.
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16
The present invention is not restricted in terms of the number of recesses or
eleva-
tions in the cathode block. Good results are achieved, for example, if the
cathode
block has 1 to 3 recesses and preferably 2 recesses in the transverse
direction
thereof.
According to a further very particularly preferred embodiment of the present
inven-
tion, the primary layer is composed of a mixture of graphite and binder, such
as
carbonized pitch, to an extent of at least 80 % by weight, preferably to an
extent of
at least 90 % by weight, particularly preferably to an extent of at least 95 %
by
weight, very particularly preferably to an extent of at least 99 % by weight
and
most preferably completely (graphite cathode body). Such a primary layer has a
suitably low specific electrical resistivity and a sufficiently high specific
thermal
conductivity. Here, this mixture is preferably composed of 70 to 95 A) by
weight of
graphite and 5 to 30 A) by weight of binder and particularly preferably of 80
to 90
% by weight of graphite and 10 to 20% by weight of binder, such as for example
of
85 % by weight of graphite and 15 % by weight of carbonized pitch.
It is preferable for both the top side of the primary layer and also the
underside of
the top layer, and therefore also the interface between the primary layer and
the
top layer, to have a planar form. Although it is not preferable, an
intermediate layer
can be provided between the primary layer and the top layer, said intermediate
layer having the same structure as the top layer, for example, with the
exception
that the intermediate layer has a lower concentration of hard material than
the top
layer.
In a development of the concept of the invention, it is proposed that the
primary
layer has a vertical specific electrical resistivity at 950 C of 13 to 18 pm
and
preferably of 14 to 16 Q pm. This corresponds to vertical specific electrical
resis-
tivities at room temperature of 14 to 20 1-2pm and of 16 to 18 Q pm.
CA 02826604 2013-08-06
17
The present invention also relates to a cathode, which contains at least one
cath-
ode block described above, wherein the cathode block has at least one groove
on
that side of the primary layer which lies opposite the top layer, wherein at
least one
busbar is provided in the at least one groove in order to feed current to the
cath-
ode during electrolysis.
In order to fasten the at least one busbar fixedly to the cathode block, and
in order
to avoid hollow spaces between the busbar and the cathode block which increase
the electrical resistance, it is additionally preferable for the at least one
busbar to
have a cast iron casing at least in certain regions and particularly
preferably over
the entire circumference. This casing can be produced by inserting the at
least one
busbar into the groove of the cathode block and then introducing cast iron
into the
interstice between the busbar and the walls delimiting the groove.
The present invention also relates to the use of a cathode block described
above
or of a cathode described above for carrying out fused-salt electrolysis for
produc-
ing metal, such as in particular aluminium.
The cathode block or the cathode is preferably used for carrying out fused-
salt
electrolysis with a melt of cryolite and aluminium oxide for producing
aluminium,
the fused-salt electrolysis being carried out with particular preference as a
Hall-
Heroult process.
Hereinbelow, the present invention is described purely by way of example on
the
basis of advantageous embodiments and with reference to the attached drawing.
In the drawing:
CA 02826604 2013-08-06
18
Figure 1 shows a schematic cross section of a detail of an aluminium elec-
trolysis cell, which comprises a cathode block according to an exemplary
embodi-
ment of the present invention.
Figure 1 shows a cross section of a detail of an aluminium electrolysis cell
10
having a cathode 12, which at the same time forms the bottom of a tank for alu-
minium melt 14 produced during operation of the electrolysis cell 10 and for a
cryolite-aluminium oxide melt 16 located above the aluminium melt 14. An anode
18 of the electrolysis cell 10 is in contact with the cryolite-aluminium oxide
melt 16.
At the side, the tank formed by the lower part of the aluminium electrolysis
cell 10
is delimited by a carbon and/or graphite lining (not shown in Figure 1).
The cathode 12 comprises a plurality of cathode blocks 20, 20', 20", which are
each connected to one another via a ramming mass 24, 24' which has been in-
serted into a ramming mass joint 22, 22' arranged between the cathode blocks
20,
20', 20". Similarly, the anode 18 comprises a plurality of anode blocks 26,
26', the
anode blocks 26, 26' each being approximately twice as wide and approximately
half as long as the cathode blocks 20, 20', 20". In this case, the anode
blocks 26,
26' are arranged above the cathode blocks 20, 20', 20" in such a way that in
each
case an anode block 26, 26' covers two cathode blocks 20, 20', 20" arranged
alongside one another in width and in each case a cathode block 20, 20', 20"
covers two anode blocks 26, 26' arranged alongside one another in length.
Each cathode block 20, 20', 20" consists of a lower primary layer 30, 30', 30"
and
a top layer 32, 32', 32" which is arranged thereabove and is fixedly connected
thereto. The interfaces between the primary layers 30, 30', 30" and the top
layers
32, 32', 32" are planar. Whereas the primary layers 30, 30', 30" of the
cathode
blocks 20, 20', 20" each have a graphite material structure, i.e. consist of
graphitic
carbon, containing synthetic or natural graphite and carbonized binding pitch,
the
top layers 32, 32', 32" are each composed of a ceramic-carbon composite
material
CA 02826604 2013-08-06
19
containing titanium diboride, which contains 20 A by weight of titanium
diboride,
amorphous carbon, specifically anthracite, and carbonized pitch as the binder.
The
titanium diboride present in the top layers 32, 32', 32" has a mean volume-
weighted particle size (d3,50), as determined by static light scattering in
accordance
with the standard ISO 13320-1, of 15 pm, a d3,90 particle size of 27 pm and a
d3,10
particle size of 4 pm.
Each cathode block 20, 20', 20" has a width of 650 mm and a total height of
550
mm, the primary layers 30, 30', 30" each having a height of 450 mm and the top
layers 32, 32', 32" each having a height of 100 mm. The distance between the
anode blocks 26, 26' and the cathode blocks 20, 20', 20" is approximately 200
to
approximately 350 mm, the layer of cryolite-aluminium oxide melt 16 arranged
therebetween having a thickness of approximately 50 mm and the layer of alumin-
ium melt 14 arranged thereunder likewise having a thickness of approximately
150
to approximately 300 mm.
Finally, each cathode block 20, 20', 20" comprises two grooves 38, 38' on its
un-
derside each with a rectangular, specifically substantially rectangular cross
sec-
tion, wherein a steel busbar 40, 40' likewise having a rectangular or
substantially
rectangular cross section is accommodated in each groove 38, 38'. In this
case,
the interstices between the busbars 40, 40' and the walls which delimit the
grooves
38, 38 are each sealed with cast iron (not shown), as a result of which the
busbars
40, 40' are fixedly connected to the walls which delimit the grooves 38, 38'.
It is
preferable for both the grooves 38, 38' and the recesses 34, 34' to be put in
the
top side of the top layers 32, 32', 32" during the shaping process, to be
precise for
example by vibrating moulds and/or punches.
List of reference symbols
CA 02826604 2013-08-06
10 Aluminium electrolysis cell
12 Cathode
14 Aluminium melt
16 Cryolite-aluminium oxide melt
18 Anode
20, 20', 20" Cathode block
22, 22' Ramming mass joint
24, 24' Ramming mass
26, 26' Anode block
30, 30', 30" Primary layer
32, 32', 32" Top layer
38, 38' Groove
40, 40' Busbar
