Note: Descriptions are shown in the official language in which they were submitted.
CA 02826597 2013-08-06
SGL CARBON SE 1
2010/086W0
31.07.2013
Surface-profiled graphite cathode block having an
abrasion-resistant surface
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 composed of
a
multiplicity of adjoining cathode blocks forming the cathode. In order to
withstand
the thermal and chemical conditions which prevail during operation of the
cell, the
cathode blocks are customarily composed of a carbon-containing material. 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
elec-
trolyte, 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 com-
CA 02826597 2013-08-06
2
pared to that of the electrolyte. During the electrolysis, the aluminium oxide
dis-
solved in the cryolite melt is cleaved into aluminium and oxygen by a flow of
elec-
tric current. In terms of electrochemistry, the layer of molten aluminium is
the ac-
tual cathode, since aluminium ions are reduced to elemental aluminium on the
surface thereof. Nevertheless, hereinbelow the term "cathode" will not be
under-
stood 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
bot-
tom 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. cathodes made of cathode blocks which contain graphite as the main
constitu-
ent. Here, a distinction is made between graphitic cathode blocks, which are
pro-
duced using graphite as the starting material, and graphitized cathode blocks,
which are produced using a carbon-containing graphite precursor as the
starting
material which is converted to graphite by a subsequent heat treatment at 2100
to
3000 C. Compared to amorphous carbon, graphite is distinguished by a consid-
erably lower specific 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
electroly-
sis and secondly to carry out the electrolysis at a higher current intensity,
which
makes it possible to increase the productivity of the individual electrolysis
cell.
During electrolysis, however, cathodes or cathode blocks of graphite and in
par-
ticular graphitized cathode blocks are subject to a high degree of wear, as a
result
of surface erosion, which is considerably greater than the wear experienced by
CA 02826597 2013-08-06
3
cathode blocks of amorphous carbon. This erosion of the cathode block surfaces
does not take place uniformly over the longitudinal direction of the cathode
block,
but instead to an increased extent at the boundary regions of the cathode
block at
which the greatest local electrical current density occurs during operation of
the
cathode block. This is because the busbars make contact with the current feed
elements in the boundary regions, which is why the resulting electrical
resistance
from the current feed elements up to the surface of the cathode block is lower
in
the event of flow over the boundary regions of the cathode block than in the
event
of flow over the centre of the cathode block. On account of this inhomogeneous
current density distribution, and as the operating time increases, the surface
of the
cathode blocks changes, as seen in the longitudinal direction of the cathode
blocks, to an approximately W-shaped profile, the nonuniform erosion meaning
that the useful life of the cathode blocks is limited by the points with the
greatest
erosion. Irrespective of this, mechanical influences increase the wear of a
cathode
block during electrolysis. Since the molten aluminium layer is in constant
move-
ment on account of the high magnetic fields prevailing during electrolysis and
the
resulting electromagnetic interactions, significant particle abrasion occurs
on the
cathode block surface, and in the case of graphite cathode blocks this leads
to a
considerably higher degree of wear compared to cathode blocks made of amor-
phous carbon.
In addition, DE 197 14 433 C2 discloses a cathode block having a coating which
contains at least 80 '3/0 by weight of titanium diboride and is produced by
plasma
spraying titanium diboride onto the surface of the cathode block. This coating
is
intended to improve the abrasion resistance 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
car-
bon, which is why the coating of such a cathode block has only a short service
life
when it is used in fused-salt electrolysis.
CA 02826597 2013-08-06
4
In order to further reduce the specific energy consumption when using the
above-
mentioned cathode blocks, use has also recently been made of cathode blocks
whose side facing towards the molten aluminium and electrolyte during
operation
of the electrolysis cell is profiled by one or more recesses and/or
elevations. Such
cathode blocks, the top sides of which each have between 1 and 8 and
preferably
2 elevations with a height of 50 to 200 mm, are disclosed in EP 2 133 446 Al,
for
example. The profiled surface reduces the movement of the molten aluminium
which is caused by the electromagnetic interaction present during the
electrolysis.
This results in reduced wave formation and bulging of the aluminium layer. For
this
reason, the use of the surface-profiled cathode blocks makes it possible to
reduce
the distance between the molten aluminium and the anode to 2 to 4 cm, said dis-
tance usually being 4 to 5 cm owing to the relatively strong and intensive
wave
formation of the aluminium layer when using cathode blocks without surface
profil-
ing for the purpose of avoiding short circuits and undesirable reoxidation of
the
aluminium which has formed. On account of this reduction in the distance
between
the molten aluminium and the anode, the electrical cell resistance is reduced
ow-
ing to the reduction in the ohmic resistance and therefore the specific energy
con-
sumption.
However, surface-profiled cathode blocks, and in particular graphite-based sur-
face-profiled cathode blocks, have a number of disadvantages. Slurry
comprising
undissolved aluminium oxide with adherent cryolite melt can settle in the
recesses
of the profiled surface of the cathode blocks, in particular in the corners.
This prob-
lem is worsened by the fact that surfaces consisting of graphite are wetted
only
very poorly by aluminium melt. As a result, graphite-based surface-profiled
cath-
ode blocks are very susceptible to wear, particularly at the corners and edges
of
the elevations of the profiled surfaces thereof. In addition, the slurry which
is de-
posited on the profiled surfaces of the cathode blocks reduces the effective
cath-
ode surface and thereby hinders the flow of current, as a result of which the
spe-
cific energy consumption is increased. This effect additionally leads to a
local
CA 02826597 2013-08-06
increase in the current density, and this can lead to a shorter service life
of the
electrolysis cell.
It is therefore an object of the present invention to provide a cathode block
which
is surface-profiled to make it possible to achieve a small distance between
the
molten aluminium and the anode in the electrolysis cell, which has a low
specific
electrical resistivity, which can preferably be wetted well with aluminium
melt, and
which in particular has a high abrasion resistance and wear resistance with re-
spect to the abrasive, chemical and thermal conditions which prevail during
opera-
tion in fused-salt electrolysis.
According to the invention, this object is achieved by a cathode block for an
alu-
minium electrolysis cell having a primary layer and having a top layer,
wherein the
primary layer contains graphite, the top layer has a surface profiled at least
in
certain regions, and the top layer is composed of a graphite composite
material
containing 1 to less than 50 % by weight of hard material having a melting
point of
at least 1000 C.
This solution is based on the understanding that the provision of a top layer
which
is surface-profiled at least in certain regions and is composed of a graphite
com-
posite material containing not less than 1 % by weight, but at most less than
50 %
by weight, of hard material having a melting point of at least 1000 C on a
graph-
ite-containing primary layer produces a cathode block which has a sufficiently
low
specific electrical resistivity for energy-efficient operation of fused-salt
electrolysis
and additionally has a very high abrasion resistance and therefore wear
resistance
with respect to the abrasive, chemical and thermal conditions which prevail
during
fused-salt electrolysis. Here, it was particularly surprising that this
prevents or at
least greatly reduces the formation of a W-shaped profile, as occurs in the
case of
conventional cathode blocks made of graphite during electrolysis as a result
of
inhomogeneous abrasion over the longitudinal direction of the cathode block.
In
CA 02826597 2013-08-06
6
addition, it was particularly surprising that, in such a cathode block, in
particular
the formation of a slurry or the deposition of a slurry in the profiled
surface and in
particular in the corners of the profiled surface or of the recess(es) forming
the
profiling is reliably prevented, and thus not only is the wear resistance of
the cath-
ode block increased considerably as a result of the reduction or prevention of
particle abrasion resulting from slurry formation, but also in particular
hindrance of
the flow of current as a result of the formation of a slurry or the deposition
of a
slurry on the cathode block surface and an increase in the specific energy con-
sumption resulting thereof during electrolysis are reliably prevented.
Therefore, the cathode block according to the present invention combines the
advantages associated with the surface profiling of that side of the cathode
block
which faces towards the melt during operation of the electrolysis cell and
also the
advantages associated with the provision of graphite in the primary layer and
in
the top layer of the cathode block - such as in particular a low electrical
resistance
of the cathode block and a small degree of wave formation and a small wave
height of the aluminium melt when using the cathode block for the fused-salt
elec-
trolysis of aluminium oxide in a cryolite melt, such that the distance between
the
surface of the layer of molten aluminium and the anode in the electrolysis
cell can
be reduced, for example, to 1 to 4 cm and preferably to 2 to 3 cm, which
reduces
the specific energy consumption of the electrolysis process; at the same time,
however, the cathode block according to the present invention does not have
the
disadvantages resulting from the use of graphite, such as a lack of
wettability by
the aluminium melt and in particular a low abrasion and wear resistance, and
also
does not have the disadvantages resulting from the use of the surface
profiling,
such as the formation of a slurry or the deposition of a slurry in the
profiled surface
structure. Instead, an outstanding abrasion resistance and therefore wear
resis-
tance of the cathode block is achieved on account of the hard-material-
containing
top layer provided in the cathode block according to the invention. Since this
hard
material is present only in the top layer, but not in the primary layer,
possible dis-
CA 02826597 2013-08-06
7
advantages owing to the hard material addition, such as a reduction in the
electri-
cal conductivity of the cathode block, are avoided, however. In addition,
despite
the fact that a hard-material-containing top layer is used, the surface of the
cath-
ode 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
re-
spect 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.
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
843-4, of at least 1000 Nimm2, preferably of at least 1500 Nimm2, particularly
preferably of at least 2000 Nimm2 and very particularly preferably of at least
2500
N/mm2.
According to a first, very particularly preferred embodiment of the present
inven-
tion, the top layer of the cathode block according to the invention contains,
as the
. CA 02826597 2013-08-06
8
hard material, a hard carbon material having a Knoop hardness, measured in
accordance with DIN EN 843-4, of at least 1000 N/rnm2, preferably of at least
1500
N/rnm2, particularly preferably of at least 2000 N/mm2 and very particularly
pref-
erably of at least 2500 N/rnm2. Here, carbon material is understood to mean in
particular a material containing more than 60 % by weight, preferably more
than
70 `)/0 by weight, particularly preferably more than 80 % by weight and very
particu-
larly preferably more than 90 % by weight of carbon.
The carbon material is preferably a material selected from the group
consisting of
coke, anthracite, carbon black, vitreous carbon and mixtures of two or more of
the
abovementioned materials, and particularly preferably coke. Hereinbelow, this
group of compounds is also referred to as "non-graphitizable carbons", to be
pre-
cise within the meaning of carbon which cannot be graphitized or can be graph-
itized at least poorly as per German patent application DE 10 2010 029 538.8,
to
which reference is made in this respect. Poorly graphitizable cokes are in
particu-
lar hard coke, such as acetylene coke.
In particular for cathode blocks with primary and top layers composed of graph-
itized carbon, it is proposed in a development of the concept of the invention
that
the top layer of the cathode block according to the invention contains, as the
hard
material, a carbon material preferably selected from coke, anthracite,
vitreous
carbon and carbon black and particularly preferably coke, with a low
graphitizabil-
ity. Graphitized cathode blocks are produced by mixing a carbon-containing
graph-
ite precursor with binder and forming this mixture to give the form of a
cathode
block, then carbonizing it and finally graphitizing it. Since a carbon
material having
a low graphitizability is then added as the hard material to this mixture
containing
graphite precursor and binder, destruction of the hard material additive or
conver-
sion of the hard material to relatively soft graphite is prevented or at least
greatly
reduced during the final graphitization, and the hard material can therefore
fulfil its
task after the graphitization, i.e. that of increasing the abrasion resistance
of the
= CA 02826597 2013-08-06
9
cathode block. Within the context of the present invention, "carbon material
having
a low graphitizability" is understood to mean a carbon material having a
degree of
graphitization of at most 0,50, as calculated from the mean layer spacing c/2
ac-
cording to Maire and Mehring (J.Maire, J.Mehring, Proceedings of the 4th
Confer-
ence on Carbon, Pergamon Press 1960 pages 345 to 350) after heat treatment at
2800 C. Good results are achieved in particular if the carbon material,
preferably
selected from coke, anthracite, carbon black and vitreous carbon, has a degree
of
graphitization of at most 0,4 and particularly preferably of at most 0,3.
In order to achieve a sufficiently high electrical conductivity of the cathode
block
and in particular of the top layer of the cathode block, it is preferable for
the top
layer of the cathode block according to the invention to contain 1 to 25 % by
weight, particularly preferably 10 to 25 '3/0 by weight and very particularly
preferably
to 20% by weight of the carbon material as the hard material. A particularly
optimal balance is thus achieved between a high abrasion resistance and a
suffi-
ciently high electrical conductivity of the top layer.
In addition, it is preferable that the carbon material which is used as the
hard ma-
terial in the top layer of the cathode block according to the invention and is
pref-
erably selected from coke, anthracite, carbon black and vitreous carbon and
par-
ticularly preferably coke has a grain size of up to 3 mm and preferably of up
to 2
mm.
According to a further embodiment, the individual particles have an onion skin
structure, which, within the context of the present invention, is understood
to mean
a multilayered structure in which an inner layer of particles having a
spherical to
ellipsoidal shape is covered completely or at least partially by at least one
inter-
mediate layer and an outer layer.
CA 02826597 2013-08-06
Furthermore, it has proved advantageous if the hard material used is a carbon
material, preferably selected from coke, anthracite, carbon black and vitreous
carbon and particularly preferably coke, in which the apparent stacking height
of
the carbon material after heat treatment at 2800 C is preferably less than 20
nm,
whereas the specific BET surface area of the particles of the carbon material
is
preferably 10 to 40 m2/g and particularly preferably 20 to 30 m2/g.
A preferred example of coke having a low degree of graphitization mentioned
above is coke which is obtained as a byproduct during the production of unsatu-
rated hydrocarbons, in particular of acetylene, and is referred to
hereinbelow,
independently of the nature of the unsaturated hydrocarbon during whose produc-
tion it is obtained, as acetylene coke. Acetylene coke which is obtainable
from the
crude oil fractions or steam cracking residues used for the quenching of
reaction
gas in the synthesis of unsaturated hydrocarbons, in particular of acetylene,
has
proved particularly suitable for this purpose. To produce this coke, the
quench oil
or carbon black mixture is fed to a coker heated to about 500 C. In the
coker,
liquid constituents of the quench oil evaporate, whereas the coke collects on
the
bottom of the coker. A corresponding process is described, for example, in DE
29
47 005 Al. A fine-grain coke having the form of an onion skin is thereby
obtained,
preferably having a carbon content of at least 96 % by weight and having an
ash
content of at most 0,05 % by weight and preferably of at most 0,01 % by
weight.
The acetylene coke preferably has a crystallite size in the c direction Lc of
less
than 20 nm, the crystallite size in the a direction La preferably being less
than 50
nm and particularly preferably less than 40 nm.
A further preferred example for coke which can be used as the hard material in
addition to or as an alternative to acetylene coke is coke which is produced
in
fluidized-bed processes, such as for example in the flexicoking process
developed
by Exxon Mobil, a thermal cracking process using fluidized-bed reactors. This
CA 02826597 2013-08-06
11
process produces coke having a spherical to ellipsoidal shape which has an
onion
skin structure.
A still further preferred example of coke which can be used as the hard
material in
addition to or as an alternative to the above-described acetylene coke and/or
coke
obtained by flexicoking processes is "shot" coke, which is produced by
"delayed
coking". The particles of this coke have a spherical morphology.
Apart from the carbon material, preferably selected from coke, anthracite,
carbon
black and vitreous carbon and particularly preferably coke, as the hard
material,
the top layer of the cathode block according to the invention contains
graphite,
preferably graphitized carbon and if appropriate carbonized and/or graphitized
binder, such as pitch, in particular coal tar pitch and/or petroleum pitch,
tar, bitu-
men, phenolic resin or furan resin. If pitch is mentioned hereinbelow, this
means
all varieties of pitch known to a person skilled in the art. Here, the
graphite or pref-
erably graphitized carbon, together with the carbonized and/or graphitized
binder,
forms the matrix in which the hard material is embedded. Good results are
achieved in particular if the top layer contains 99 to 50 % by weight,
preferably 99
to 75 % by weight, particularly preferably 90 to 75 % by weight and very
particu-
larly preferably 90 to 80 % by weight of carbon.
According to a second very particularly preferred embodiment of the present in-
vention, the top layer of the cathode block according to the invention
contains, as
the hard material, a non-oxidic ceramic, which is preferably composed of at
least
one metal belonging to transition groups 4 to 6 and at least one element
belonging
to main group 3 or 4 of the Periodic Table of the Elements. These include in
par-
ticular metal carbides, metal borides, metal nitrides and metal carbonitrides
com-
prising a metal belonging to transition groups 4 to 6, such as for example
titanium,
zirconium, vanadium, niobium, tantalum, chromium or tungsten.
= CA 02826597 2013-08-06
12
Specific examples of suitable representatives from these groups are compounds
selected from the group consisting of titanium diboride, zirconium diboride,
tanta-
lum diboride, titanium carbide, boron carbide, titanium carbonitride, silicon
carbide,
tungsten carbide, vanadium carbide, titanium nitride, boron nitride, silicon
nitride,
zirconium dioxide, aluminium oxide and any desired chemical combinations
and/or
mixtures of two or more of the abovementioned compounds. Good results are
achieved in particular with titanium diboride, titanium carbide, titanium
carbonitride
and/or titanium nitride. Most preferably, the top layer of the cathode block
accord-
ing to the invention contains titanium diboride as the hard material. All of
the
abovementioned hard materials can be used alone or it is possible to use any
desired chemical combination and/or mixture of two or more of the abovemen-
tioned compounds.
In a development of the concept of the invention, it is proposed that the hard
mate-
rial present in the top layer of the cathode block according to this second
very
particularly preferred embodiment has a monomodal particle size distribution,
wherein the mean volume-weighted particle size W3,50, as determined by static
light scattering in accordance with the international standard ISO 13320-1, is
10 to
20 pm.
Within the context of the present invention, it has been established that non-
oxidic
ceramic as the hard material, in particular non-oxidic titanium ceramic and
espe-
cially 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 formation of a slurry and the deposition of a slurry
on the
surface of the cathode block are reliably prevented, but in particular also
leads to
an outstanding abrasion resistance and therefore wear resistance of the
cathode
block. 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 ceramic hard material, preferably titanium diboride, of less than
50 %
= CA 02826597 2013-08-06
13
by weight and particularly preferably even given amounts of only 10 to 20 "Yo
by
weight in the top layer. It is thereby possible to dispense with a high
concentration
of ceramic hard material in the top layer, which leads to a brittle cathode
block
surface. Furthermore, ceramic hard material 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 material powder is being
trans-
ported, 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 therefore 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 producibility 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 cathode blocks.
The hard material, preferably titanium diboride, present in the top layer of
the
cathode block according to the second very particularly preferred embodiment
of
the present invention preferably has a monomodal particle size distribution,
wherein the mean volume-weighted particle size 03,50, determined as above, is
12 to 18 pm and particularly preferably 14 to 16 pm.
As an alternative to the embodiment mentioned above, the ceramic hard material
present in the top layer of the cathode block can have a monomodal particle
size
distribution, wherein the mean volume-weighted particle size (d3,50), as
determined
by 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.
= CA 02826597 2013-08-06
14
In a development of the concept of the invention, it is proposed that the
ceramic
hard material has a volume-weighted d3,90 particle size, determined as above,
of
20 to 40 pm and preferably of 25 to 30 pm. The ceramic hard material
preferably
has such a d3,90 value in combination with an above-defined d3,50 value. In
this
embodiment, too, the ceramic 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 ceramic 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 ceramic 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 monomodal particle size distribution defined above.
According to a further preferred embodiment of the present invention, the
ceramic
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.
As an alternative to the embodiment mentioned above, the ceramic 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
CA 02826597 2013-08-06
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 addition, it is preferable if the non-oxidic ceramic as the hard material,
in particu-
lar 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 following equation:
Span 7.--= (d3,90 - d3,10)/d3,50
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 non-oxidic ceramic 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 ceramic, preferably non-oxidic titanium ceramic and particularly
preferably
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.
The total quantity of the ceramic hard material in the top layer is, according
to the
invention, at least 1 % 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-
= CA 02826597 2013-08-06
16
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.
In this case, good results are achieved in particular if the top layer, in the
second
very particularly preferred embodiment of the present invention, contains 5 to
40 c'/0
by weight, particularly preferably 10 to 30 % by weight and very particularly
pref-
erably 10 to 20 % by weight of a non-oxidic ceramic, preferably of a non-
oxidic
titanium ceramic and particularly preferably titanium diboride, as the hard
material
having a melting point of at least 1000 C.
Apart from the non-oxidic ceramic as the hard material, the top layer of the
cath-
ode block according to the invention contains, according to the second very
par-
ticularly preferred embodiment of the present invention, graphitic or
preferably
graphitized carbon and, if appropriate, carbonized and/or graphitized binder,
such
as pitch, in particular coal tar pitch and/or petroleum pitch, tar, bitumen,
phenolic
resin or furan resin. Here, the graphitic or preferably graphitized carbon,
together
with the optional binder, forms the matrix in which the ceramic hard material
is
embedded. Good results are achieved in particular if the top layer contains 99
to
more than 50 % by weight, preferably 95 to 60 % by weight, particularly
preferably
90 to 70 % by weight and very particularly preferably 90 to 80 % by weight of
graphite.
CA 02826597 2013-08-06
17
In a development of the concept of the invention, it is proposed for the
graphite-
containing cathode block top layer that the top layer has a vertical specific
electri-
cal resistivity at 950 C of 5 to 20 Q pm and preferably of 9 to 13 Q pm. This
corre-
sponds to vertical specific resistivities at room temperature of 5 to 25 Q pm
and of
to 15 pm. In this
context, "vertical specific electrical resistivity" 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 a hard material, which is expensive in the case of
ceramics,
as low as possible, but should also be sufficiently great for the top layer to
have a
sufficiently high wear resistance and service life. In the case of all hard
materials,
the good properties of the main cathode body should be impaired as little as
pos-
sible by the smallest possible top layer. Good results are achieved with
respect to
these reasons 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
pref-
erably 15 to 25 A), 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 180 mm, very
particu-
larly preferably of 100 to 170 mm and most preferably of about 150 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.
According to the invention, the top layer of the cathode block has a surface
which
is profiled at least in certain regions. The profiled surface reduces the
movement
of the molten aluminium which is caused by the electromagnetic interaction pre-
sent during the electrolysis, which results in reduced wave formation and
bulging
of the aluminium layer. For this reason, the use of surface-profiled cathode
blocks
= CA 02826597 2013-08-06
18
can further reduce the distance between the molten aluminium and the anode,
and
therefore the electrical cell resistance is reduced further owing to the
reduction in
the ohmic resistance and therefore the specific energy consumption.
Here, a profiled surface is understood to mean a surface having at least one
re-
cess and/or elevation extending in the transverse direction, in the
longitudinal
direction or in any other desired direction, such as for example in a
direction run-
ning at an acute or obtuse angle to the longitudinal direction, 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.
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.
CA 02826597 2013-08-06
19
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.
CA 02826597 2013-08-06
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. By way of
ex-
ample, the at least one elevation extends over the entire length of the
cathode
block. It is possible, however, for the height and/or width of the at least
one eleva-
tion 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 cathode 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.
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
,
CA 02826597 2013-08-06
,
21
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 or graphitized pitch, to an extent of at least 80 % by weight,
preferably
to an extent of at least 90 '3/0 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. Such a primary layer has a suitably low
specific electrical resistivity. Here, this mixture is preferably composed of
70 to 95
% by weight of graphite and 5 to 30 % by weight of binder and particularly
prefera-
bly 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
or
graphitized 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 substantially planar form. Both layers of the cathode
block can
be connected to one another by a vibrating process or by a pressing process in
the green state. In this context, "substantially planar" is understood to mean
that
the primary layer is not profiled and the profile is provided with a top
layer.
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 Q
pm and
CA 02826597 2013-08-06
,
22
preferably of 14 to 16 Q pm. This corresponds to vertical specific electrical
resis-
tivities at room temperature of 14 to 20 Q pm and of 16 to 18 Q pm.
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 drawings.
In the drawings:
CA 02826597 2013-08-06
23
Figure 1 shows a schematic cross section of a detail of an aluminium
electrolysis cell, which comprises a cathode block according
to an exemplary embodiment of the present invention, and
Figures 2A to 2E each show a schematic cross section of the surface
profiling
of a cathode block according to other exemplary embodi-
ments 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
CA 02826597 2013-08-06
24
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, which is
produced, for
example, by moulding a mixture of petroleum coke and coal tar pitch with subse-
quent heat treatment at up to 3000 C, the top layers 32, 32', 32" are each
com-
posed of a graphite composite material containing acetylene coke, which
contains
20 % by weight of acetylene coke, graphite and carbonized or graphitized pitch
as
the binder. The acetylene coke present in the top layers 32, 32', 32" has a
grain
size of 0.2 to 1 mm.
Each cathode block 20, 20', 20" has a width of 650 mm and a total height,
based
on the highest point of the top layer 32, 32', 32", of 550 mm, the primary
layers 30,
30', 30" each having a height of 450 mm and the top layers 32, 32', 32" each
hav-
ing a height, based on the highest point of the top layers 32, 32', 32", 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 aluminium melt 14 arranged thereunder likewise having a thick-
ness of approximately 150 to approximately 300 mm.
Each top layer 32, 32', 32" has a profiled surface, wherein two recesses 34,
34' of
substantially rectangular cross section are provided in each top layer 32,
32', 32"
and are separated from one another in each case by an elevation 36. Whereas
the
width of each of the recesses 34, 34' is 150 mm and the depth of each of the
re-
cesses 34, 34' is 70 mm, the elevation 36 has a width of 75 mm and a height of
70
mm. Both the corners in the two recesses 34, 34' and the corners of the
elevation
36 are rounded off each with a 20 mm radius.
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-
CA 02826597 2013-08-06
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.
Figures 2A to 2E show examples of different configurations of the recesses 34,
34'
and of the elevations 36 of the surface profiling of the top layers 32, 32',
32", spe-
cifically, in each case in cross section, a rectangular configuration with
rounded-off
corners (not shown) (Figure 2A), a substantially undulatory configuration
(Figure
2B), a triangular configuration (Figure 2C), a convex configuration (Figure
2D) and
a sinusoidal configuration (Figure 2E).
List of reference symbols
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
,
CA 02826597 2013-08-06
26
40, 40' Busbar
