Note: Descriptions are shown in the official language in which they were submitted.
CA 02862277 2014-07-22
WO 2013/113837 Al
Method for producing a cathode block for an aluminium electrolytic cell
The present invention relates to a method for producing a cathode block for an
aluminium
electrolytic cell and to a cathode block produced by this method.
A known method for producing metal aluminium is the Hall-Heroult process. In
this
electrolytic method, the base of an electrolytic cell is typically formed by a
cathode surface
consisting of individual cathode blocks. The cathodes are contacted from below
via steel
bars, which are introduced into corresponding elongate recesses in the
underside of the
cathode blocks.
Cathode blocks are conventionally produced by mixing coke with carbon-
containing particles
such as anthracite, carbon or graphite, compressing and carbonising. This may
optionally be
followed by a graphitisation step at higher temperatures at which the carbon-
containing
particles and the coke are converted into graphite at least in part. A carbon
cathode
consisting at least in part of graphite is obtained.
The service life of the cathode blocks is limited by a range of factors. In
particular corrosion
and erosion by liquid aluminium and electrolyte, in particular cryolite,
disrupt the cathode
blocks from the upper face over time.
Various measures have been taken in the past to increase the wear resistance
of the
cathode blocks. For example, it has been attempted to increase the bulk
density of the
cathode blocks, and this should increase the strength and thus the wear
resistance thereof.
However, only bulk densities of up to 1.68 g/cm3 can be obtained in this way
in fully
graphitised, non-impregnated cathode blocks, meaning that the wear resistance
still remains
sub-optimal.
On the other hand, carbon cathodes have been coated with titanium boride
(TiB2) (disclosed
in CN 1062008) or with a TiB2/carbon mixture, as disclosed for example in DE
112006004078. TiB2 can clearly improve the wetting properties of aluminium on
the cathode,
and additionally contributes to a higher hardness and wear resistance.
Nevertheless, the
wear resistance of a TiB2 layer on a carbon cathode and on a composite layer
of carbon and
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TiB2 is still too low, and so the wear resistance of cathode blocks provided
with layers of this
type is also too low.
Therefore, the object of the invention is to provide a carbon-based cathode
block having a
high, improved wear resistance and a method for the production thereof.
The present invention therefore relates to a method for producing a cathode
block, the
method comprising: (a) providing a mixture of starting materials comprising
coke and pitch,
wherein the coke comprises two coke types which have different volume-change
properties
during carbonisation, graphitisation, cooling, or a combination thereof; (b)
shaping the
mixture into a green body; and (c) carbonising the green body to give a
carbonised green
body, graphitising the carbonised green body without it being previously
impregnated to
obtain a graphitised body, and cooling after the graphitising.
The present invention further relates to a cathode block produced by the
method as defined
herein, wherein the bulk density in at least one layer of the cathode block is
greater than
1.68 g/cm3 up to 1.75 g/cm3 based on the carbon portion.
In this context, according to the invention the coke comprises two coke types,
which have
different volume-change properties during carbonisation and/or graphitisation
and/or cooling.
Further, unlike in conventional methods for producing a cathode block, the
carbonised green
body is not impregnated prior to graphitisation, and in particular is not
impregnated with pitch,
tar or artificial resins. In the graphitisation step, at least a portion of
carbon in the cathode
block is converted into graphite.
It has surprisingly been found that the service life of the cathode blocks
produced by a method
according to the invention is much higher than for cathode blocks produced by
conventional
methods. This is all the more surprising given that, unlike in conventional
methods, the
carbonised green body is not impregnated prior to graphitisation to produce a
cathode block. For
example, in US 4,308,115, to produce a cathode, a green mixture of coke and
pitch is prepared,
and subsequently undergoes a shaping step to produce a green body.
Subsequently, the green
body is compacted in that it is repeatedly impregnated with pitch and
subsequently burnt.
Impregnated cathodes of this type are expensive to produce because of the many
repeated
impregnation and burning steps. In this context, the impregnation is carried
out so as to compact
the cathode green body, making it possible to reduce the penetration of molten
aluminium into
pores in the cathode and thus to increase the service life of cathodes of this
type.
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In spite of the absence according to the invention of this impregnation step,
penetration of
molten aluminium into pores in the cathode is presumably clearly reduced
because of the
use according to the invention of two coke types, which have different volume-
change
properties during carbonisation and/or graphitisation and/or cooling, and the
service life of
the cathodes manufactured by the method according to the invention is thus
increased.
It may be advantageous to machine the graphitised bodies mechanically to
obtain the
cathode block.
Preferably, a cathode block produced by the method according to the invention
has a bulk
density of a carbon portion of over 1.68 g/cm3, particularly preferably over
1.71 g/cm3, in
particular up to 1.75 g/cm3.
Presumably, a higher bulk density advantageously contributes to a longer
service life. On the
one hand, this may be because there is more mass per unit volume of a cathode
block, and
this leads to a higher residual mass after a given erosion duration at a given
mass erosion
per unit time. On the other hand, presumably a higher bulk density together
with a
corresponding lower porosity prevents infiltration of electrolyte, which acts
as a corrosive
medium.
Advantageously, the two coke types include a first coke type and a second coke
type, the
first coke type exhibiting greater contraction and/or expansion than the
second coke type
during carbonisation and/or graphitisation and/or cooling. In this context,
the greater
contraction and/or expansion is an advantageous development of different
volume-change
properties, which are presumably particularly suitable for leading to stronger
compaction
than if coke sorts having the same contraction and/or expansion are mixed. In
this context,
the greater contraction and/or expansion relates to any desired temperature
range. Thus
there may for example merely be greater contraction of the first coke during
carbonisation.
On the other hand, there may for example additionally or alternatively be
greater expansion
in a transition region between carbonisation and graphitisation. Additionally
or alternatively,
there may be different volume-change properties during cooling.
Preferably, the contraction and/or expansion of the first coke type during
carbonisation
and/or graphitisation and/or cooling is at least 10 % greater than that of the
second coke
type in terms of volume, in particular at least 25 % greater, in particular at
least 50 %
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greater. Thus for example in the case of a 10 % greater contraction of the
first coke type, the
contraction from room temperature to 2000 C may be 1.0 % by volume for the
second coke
type but 1.1 % by volume for the first coke type.
Advantageously, the contraction and/or expansion of the first coke type during
carbonisation
and/or graphitisation and/or cooling is at least 100 % greater than that of
the second coke
type in terms of volume, in particular at least 200 % higher, in particular at
least 300 %
higher. Thus for example in the case of a 300 % greater expansion of the first
coke type, the
expansion from room temperature to 1000 C may be 1.0 % by volume for the
second coke
type but 4.0 % by volume for the first coke type.
The method according to the invention also includes the case where the first
coke type
undergoes contraction but by contrast the second coke type undergoes expansion
in the
same temperature interval. For example, a 300 % greater contraction and/or
expansion thus
also includes the case where the second coke type contracts by 1.0 % by volume
but the
first coke type expands by 2.0 % by volume.
Alternatively, in at least a desired temperature interval of the method
according to the
invention, the second coke type, rather than the first coke type, may have a
greater
contraction and/or expansion, as described above for the first coke type.
Preferably, at least one of the two coke types is a petroleum- or coal tar
pitch coke.
Preferably, the proportion of the second coke type in the total amount of coke
in percent by
weight is between 50 % and 90 %, in particular between 50 and 80 %. In these
ranges, the
different volume-change properties of the first and second coke types
presumably have a
particularly good effect on the compaction during carbonisation and/or
graphitisation and/or
cooling. Conceivable ranges for the second coke type may be from 50 to 60 %,
but also from
60 to 80% and from 80 to 90 %.
Advantageously, at least one further carbon-containing material and/or
additives and/or
pulverulent hard material are added to the coke. This may be advantageous both
for the
processability of the coke and for the subsequent properties of the produced
cathode block.
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Preferably, the further carbon-containing material contains graphite-
containing material; in
particular, the further carbon-containing material consists of graphite-
containing material,
such as graphite. The graphite may be synthetic and/or natural graphite.
Further carbon-
containing material of this type ensures that the required contraction of the
cathode mass,
which is dominated by the coke, is reduced.
Preferably, the further carbon-containing material is present at 1 to 40 % by
weight, in
particular 5 to 30 % by weight, based on the total amount of coke and further
carbon-
containing material.
Preferably, pitch can be added in amounts of 5 to 40 % by weight, in
particular 15 to 30 % by
weight (based on the weight of the total green mixture). Pitch acts as a
binder and serves to
create a dimensionally stable body during carbonisation.
Advantageous additives may be oil, such as compression oil, or stearic acid.
These facilitate
the mixing of the coke and if applicable of the further components.
In particular TiB2 powder is used as the pulverulent hard material. The use of
a hard material
of this type increases the wettability of the cathode in relation to the
aluminium melt. The
proportion of this hard material in the mixture of starting materials is
between 15 % by weight
and 60 % by weight, in particular between 20 A by weight and 50 % by weight.
Advantageously, the cathode block is produced as a multi-layer block, a first
layer containing
coke and optionally a further carbon-containing material as starting
materials, and a second
layer containing coke and a fireproof hard material, in particular TiB2, and
optionally a further
carbon-containing material as starting materials. Hard material is also known
as RHM
(refractory hard material). The further carbon-containing material may be
present as
described above for a monolithic cathode block. This variant of a multi-layer
block combines
the advantages of a multi-layer block, in which the layer facing the aluminium
melt contains a
hard material, with the use of two coke types having different volume-change
properties.
Since the second layer always has a high bulk density of for example over 1.82
g/cm3 after
graphitisation because of the addition of high-temperature-resistant hard
material, it is
advantageous for the first layer also to have a high bulk density of
advantageously over 1.68
g/cm3 after graphitisation. The small differences in the thermal expansion
properties and
bulk densities during the thermal treatment steps reduce the production times
and rejection
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rates of the cathode blocks, since large differences in the layers during
temperature
treatment can lead to thermal stresses. Further, the resistance to thermal
stresses and
damage resulting therefrom during use is therefore also advantageously
increased.
Preferably, the coke of the first and/or second layer comprises two coke
types, which have
different volume-change properties during carbonisation and/or graphitisation
and/or cooling
whilst leading to a bulk density of over 1.70 g/cms of the resulting graphite.
Further, preferably at least one of the two layers is produced with a bulk
density of over 1.68
g/cm3 of a carbon portion. Thus, depending on what is desired and/or required,
one or both
layers may be produced according to the invention using two different coke
types. This
results in the possibility of setting bulk densities and bulk density ratios
as required or
desired. For example, merely the first layer may be produced according to the
invention
using two coke types, whilst the second layer is produced with merely one coke
type but
additionally contains TiB2 as a ceramic hard material.
It may potentially be advantageous for the multi-layer block to comprise more
than two
layers. In this case, any desired number of the more than two layers may be
produced
according to the invention using two coke types having different volume-change
properties in
each case.
Advantageously, the second layer may be of a height which is 10 to 50 %, in
particular 15 to
45 %, of the total height of the cathode block. A small height of the second
layer, such as 20
%, may be advantageous, since a small amount of cost-intensive hard ceramic
material is
required. Alternatively, a large height of the second layer, such as 40 %, may
be
advantageous, since a layer which has hard ceramic material has a high wear
resistance.
The greater the height of this highly wear-resistant material in relation to
the total height of
the cathode block, the greater the wear resistance of the cathode block as a
whole.
It may be advantageous for the hard material to be in a monomodal particle
size distribution,
the average particle size d50 of the distribution being between 10 and 20 pm,
in particular
between 12 and 18 pm, in particular between 14 and 16 pm.
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The cis() value indicates the average particle size, and in this case 50 % of
particles are
smaller than the indicated value. Accordingly, dlo or d90 indicates the
average particle size
when 10 or 90 % of the particles are smaller than the indicated value.
Surprisingly, it has been found in the context of the invention that for a d50
of this type,
although on the one hand the hard material powder has a large active surface,
which leads
to very.good wettability of the cathode block after graphitisation, on the
other hand it does
not have the drawbacks which negatively influence the processing of the hard
material
powder as a composite component in a graphite/hard material composite. These
potential
drawbacks, which the hard material powder used according to the invention does
not have,
are:
- a tendency to form dust, for example when added to a mixing container or
during
transportation of the powder,
.- agglomerate formation, in particular during mixing, for example wet mixing
with
coke (in this context, wet mixing means in particular mixing with pitch as the
liquid
phase),
- demixing as a result of the different material densities of the hard
material and the
coke.
Aside from the elimination of these drawbacks, the hard material powder used
according to
the invention has particularly good flowability and pourability. This makes
the hard material
powder particularly easy to convey using conventional conveying devices, for
example to a
mixing apparatus.
The good processability of the hard material powder having the d50 of between
10 and 20
pm and a monomodal particle size distribution greatly simplifies the
production of hard
material powder composites for cathode blocks. The cathode blocks obtained
have a very
good homogeneity in relation to the distribution of the hard material powder
in the coke in the
green body and in the graphite in the graphitised cathode body.
Preferably, the cis() of the fire-resistant hard material is between 20 and 40
pm, in particular
between 25 and 30 pm. This advantageously makes the wetting and processing
properties
of the hard material powder even better.
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Advantageously, the du) of the fire-resistant material is between 2 and 7 pm,
in particular
between 3 and 5 pm. This advantageously makes the wetting and processing
properties of
the hard material powder even better.
Further, it is possible to characterise the monomodal particle size
distribution by describing
its distribution width using what is known as the span value, which is
calculated as follows:
span = (do ¨ d10)/c150
Advantageously, the span of the fire-resistant hard material powder is between
0.65 and
3.80, in particular between 1.00 and 2.25. This advantageously makes the
wetting and
processing properties of the hard material powder even better.
Advantageously, the graphitisation step is carried out at temperatures of
between 2550 and
3000 C, in particular between 2600 and 2900 C.
Temperatures below 2900 C have been found to be particularly advantageous,
since
conventional TiB2 does not melt at less than 2900 C. Presumably melting does
not result in
a chemical change in the TiB2, since even after melting and subsequent cooling
TiB2 can be
found in a cathode block by X-ray diffractometry. However, melting can cause
finely
distributed TiB2 particles to agglomerate to form larger particles. There is
also some risk that
liquid TiB2 may move in an uncontrolled manner as a result of open porosity.
In the temperature range according to the invention, the graphitisation
process has
progressed sufficiently far to result in high thermal and electrical
conductivity of the carbon-
containing material.
Preferably, the graphitising step is carried out using an average heating rate
of between 90
K/h and 200 K/h. Alternatively or additionally, the graphitisation temperature
is maintained
for a period of between 0 and 1 h. At these heating rates or this heating
duration, particular
good results are achieved as regards graphitisation and obtaining the hard
material.
Advantageously, the duration of the thermal treatment until the time when
cooling starts may
be 10 to 28 hours.
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The invention is further achieved by a cathode block according to claim 15.
The cathode
block is advantageously produced by a method according to the invention.
According to the
invention, the bulk density is greater than 1.68 g/cm3, in particular greater
than 1.70 g/cm3, in
particular at least greater than 1.71 g/cm3, in particular up to 1.75 g/cm3.
In this context, the
bulk density is based on the layer as a whole when no fire-resistant hard
material is added,
in other words on the pure carbon portion. In the event that the layer
contains hard ceramic
material such as TiB2, the bulk density is a calculated bulk density of the
layer excluding the
portion of fire-resistant hard material.
Further advantageous configurations and developments of the invention are
explained in the
following by way of a preferred embodiment and the drawings, in which:
Fig. 1 is a dilatometer measurement curve as a function of the temperature for
a first
and second coke type for the method according to the invention, and
Fig. 2 is a schematic drawing of the shaping of a cathode block according to
the
invention in the form of a multi-layer block.
To produce a cathode block according to the invention, a first and a second
coke are ground
separately from one another, separated into particle-size fractions and mixed
together along
with pitch. The proportion by weight of the first coke in the overall amount
of coke may be for
example 10 to 20 % by weight or 40 to 45 % by weight. A cathode block can be
produced
from the green mixture by extrusion. Alternatively, the mixture may be added
for example to
a mould, which broadly corresponds to the subsequent shape of the cathode
block, and
vibration-compacted or block-pressed. The resulting green body is heated to a
final
temperature in the range of 2550 to 3000 C ¨ a carbonisation step and
subsequently a
graphitisation step taking place, without impregnation, for example with
pitch, tar or synthetic
resin, taking place in between ¨ and subsequently cooled. The resulting
cathode block has a
bulk density of 1.71 g/cm3 and a very high wear resistance against liquid
aluminium and
cryolite.
Fig. 1 shows a dilatometer measurement curve for the first coke type (as a
dashed line)
during the graphitisation process. Fig. 1 further shows a corresponding
measurement curve
(as a solid line) for the second coke type. It can be seen that the two coke
types have
different volume-change properties.
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From a zero line at the start of the temperature programme to a temperature of
2800 C, the
first coke of Fig. 1 initially exhibits an expansion, a rise in volume being
observed until
approximately 1200 C, and a temporary reduction in volume occurring after
approximately
1400 C. Subsequently, until approximately 2100 C, an increase in maximum
volume by
comparison with the initial volume can be seen.
In the dilatometer measurement for the second coke, a progression which is
similar in
principle to that of the first coke can be observed, but the curve rises more
steeply overall.
Accordingly, at approximately 2100 C, an increase in maximum volume can also
be seen
for the second coke, but is much smaller than for the first coke.
Only during the subsequent cooling does contraction occur in both coke types,
and it is
greater for the second coke type than for the first.
Alternatively, two coke types are used, the first of which already exhibits
contraction during
the heating phase in the carbonisation and/or graphitisation step. The second
of the two
coke types has a much greater contraction (based on the contraction after
carbonisation,
graphitisation and cooling by comparison with the initial volume) than the
other coke type.
In a further variant of the embodiment, graphite powder or carbon particles
are added to the
coke mixture.
In a further variant of the embodiment, a mould 1 is initially filled in part
with a mixture 2 of
the two coke types, graphite and TiB2, and vibration-compacted, as shown in
Fig. 2a.
Subsequently, a mixture 5 of the two coke types and graphite is filled onto
the resulting
starting layer 4, which forms the upper layer facing the anode in the
subsequent cathode and
will thus be in direct contact with the aluminium melt, and likewise compacted
(see Fig. 2b).
The resulting upper starting layer 6 forms the lower layer remote from the
anode in the
subsequent cathode. This two-layer block is carbonised and graphitised as in
the first
embodiment.
All of the features mentioned in the description, examples and claims may
contribute to the
invention in any desired combination. The invention is not limited to the
examples given, but
can also be configured in modified forms which are not specifically disclosed
heroin. In
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particular, different volume-change properties also include types of
properties other than
contraction properties. For example, at least in portions of the heating and
cooling cycle, a
volume increase may be advantageous for compaction of the cathodes. In this
way, two
coke types which ultimately exhibit the same contraction after carbonisation,
graphitisation
and cooling, but which exhibit a different contraction or volume increase at
an intermediate
temperature, may be included in the invention.
As well as coke types from different manufacturers, different coke types may
also include
cokes from the same manufacturer but with different pre-treatment, such as
differently
calcined cokes.
