Note: Descriptions are shown in the official language in which they were submitted.
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WO 2012/013769 Al
PROCESS FOR PRODUCING A CATHODE BLOCK FOR AN ALUMINIUM
ELECTROLYSIS CELL AND A CATHODE BLOCK
The present invention relates to a process for
producing a cathode block for an aluminium electrolysis
cell and a cathode block.
A known process for producing metallic aluminium is the
Hall-Heroult process. In this electrolytic process, the
base of the electrolysis cell is typically formed by a
cathode surface comprising individual cathode blocks.
The cathodes are contacted from beneath via steel bars,
which are introduced into corresponding elongated
recesses in the underside of the cathode blocks.
The production of cathode blocks conventionally takes
place by mixing coke with carbon-containing particles,
such as anthracite, carbon or graphite, compacting and
carbonising. If need be, this is followed by a
graphitising step at higher temperatures, at which the
carbon-containing particles and the coke are converted
at least partially into graphite.
As a result of the graphitising, the thermal
conductivity of the cathode material is greatly
increased and the specific electrical resistance is
greatly reduced.
Graphitized carbon and graphite are however poorly
wetted or are not wetted at all by liquid aluminium.
The power requirement and therefore also the energy
requirement of an electrolysis cell is thus increased.
In order to solve this problem, TiB2 is introduced into
an upper layer of a cathode block in the prior art.
This is described for example in DE 112006004078. Such
an upper layer, which represents a TiB2-graphite
composite, is in direct contact with the aluminium melt
and is therefore crucial for the current coupling from
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the cathode into the aluminium melt. TiB2 and similar hard-
ceramic materials produce an improvement in the wettability of
the cathode in the graphitised state and therefore a better
energy efficiency of the electrolysis process. Ceramic hard
materials can moreover increase the bulk density and the
hardness of cathodes, which leads to a better resistance to
wear especially with respect to aluminium and cryolite melts.
Hard materials are also referred to as RHM (refractory hard
material).
However, during a graphitising process, TiB2 powder and similar
hard material powders partially lose their effect of increasing
the wettability and wear resistance.
The present invention, relates to a simple process for the
production of a TiB2-graphite composite cathode which is
readily wettable with respect to aluminium melts and has good
wear properties, as well as a corresponding cathode block.
In one aspect, the invention relates to a process for the
production of a cathode block as a multi-layer block,
comprising the steps of providing starting materials, including
coke and a hard material powder, and if appropriate a carbon-
containing material, wherein a first layer contains the coke as
starting material and a second layer contains the coke and the
hard material, as starting material, mixing the starting
materials, forming a cathode block, carbonising and
graphitising, and also cooling, wherein the step for
graphitising is carried out at a temperature between 2300 and
3000 C, and wherein the second layer is produced with a
thickness of 10 to 50% of the total thickness of the cathode
block.
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A process according to the invention for the production of a
cathode block comprises the steps of providing starting
materials, including coke and a hard material powder, such as
for example TiB2, and if appropriate a further carbon-
containing material, mixing the starting materials, forming a
cathode block, carbonising and graphitising, and also, cooling,
and is characterised in that the step for graphitising is
carried out at temperatures between 2300 and 3000 C, in
particular between 2400 and 2900 C.
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Temperatures below 2900 C have proved to be
particularly advantageous, since conventional TiB2 does
not melt below 2900 C. It is true that melting
presumably does not lead to any chemical change in the
TiB2, since TiB2 is detected in a cathode block by x-
ray-diffraction analysis also after melting and
subsequent cooling. However, as a result of melting,
finely distributed TiB2 particles agglomerate to form
larger particles. There is also a certain risk of
liquid TiB2 moving in an uncontrolled manner through
open porosity.
In the temperature range according to the invention,
the graphitising process has progressed so far that a
high thermal and electrical conductivity of the carbon-
containing material is produced.
The graphitising step is preferably carried out at an
average heating rate between 90 K/h and 200 K/h.
Alternatively, or in addition, the graphitising
temperature is held for a period between 0 and 1 h.
With these heating rates and this holding period,
particularly good results are achieved in terms of
graphitising and preserving the hard material.
A period of the temperature treatment up to the time
when the cooling is started can preferably amount to 10
to 28 hours.
It may be advantageous for the composite with hard
material and graphite or graphitised carbon to form the
whole cathode block. This has the advantage that a
single green material composition is required and
accordingly only a single mixing step.
Alternatively, it may be advantageous for the cathode
block to comprise at least two layers, wherein the
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composite layer forms the second layer of the cathode
block. This second layer is in direct contact with the
melt of the electrolysis cell.
The cathode block preferably comprises at least one
further layer (referred to below as the first layer),
which comprises less hard material powder than the
upper layer or comprises no hard material powder. This
can reduce the quantity of expensive hard material
powder used. When the cathode is used in an aluminium
electrolysis cell, the first layer is not in direct
contact with the aluminium melt and does not therefore
have to have good wettability and wear resistance.
The second layer can advantageously have a height which
amounts to 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% for example, may be
advantageous, since a small quantity of cost-intensive
hard material is required.
Alternatively, a greater height of the second layer,
such as 40% for example, may be advantageous, since a
layer possessing a hard material possesses a high wear
resistance. The greater the height of this highly wear-
resistant material in relation to the overall height of
the cathode block, the greater the wear resistance of
the overall cathode block.
The coke preferably comprises two types of coke which
have a different volume-change behaviour during the
carbonising and/or graphitising and/or cooling.
Surprisingly, it has been shown that the useful life of
the cathode blocks produced with such a process is much
longer than in the case of the cathode blocks produced
with conventional processes.
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The carbon fraction of the cathode block is preferably
compacted to a bulk density of over 1.68 g/cm3, in
particular of over 1.71 g/cm3, in particular up to
1.75 g/cm3.
A higher bulk density presumably contributes
advantageously to a longer useful life. This may on the
one hand be based on the fact that more mass is present
per unit volume of a cathode block, which, with a given
mass abrasion per unit of time, leads to a higher
residual mass after a given abrasion period. On the
other hand, it can be assumed that a higher bulk
density with a corresponding lower porosity prevents an
infiltration of electrolyte, which acts as a corrosive
medium.
With this variant, the advantages of the graphitising
temperature according to the invention in a range
between 2300 and 3000 C are combined with the increase
in the bulk density of the cathode block.
Advantageously, a consequence of the incomplete
graphitising is thus at least partially compensated
for.
Since the second layer always has a high bulk density
of for example over 1.80 g/cm3 after graphitising on
account of the addition of hard material, it is
advantageous if the first layer after graphitising also
has a high bulk density of, according to the invention,
over 1.68 g/cm3. The small differences in the thermal
expansion behaviour and bulk densities during the heat
treatment steps reduce production times and reject
rates of the cathode blocks. Furthermore, the
resistance to thermal stresses and to resultant damage
in use is therefore advantageously also increased.
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The two types of coke advantageously include a first
type of coke and a second type of coke, wherein the
first type of coke exhibits a greater shrinkage and/or
expansion than the second type of coke during the
carbonising and/or graphitising and/or cooling. In this
connection, the greater shrinkage and/or expansion is
an advantageous development of a different volume-
change behaviour, which presumably is particularly well
suited for leading to a greater compaction than when
types of coke are mixed that possess an identical
shrinkage and/or expansion. Thereby, the greater
shrinkage and/or expansion relates to an arbitrary
temperature range. Thus, for example, only a greater
shrinkage of the first coke may be present during
carbonising. On the other hand, for example, a greater
expansion may be present, additionally or instead, in a
transition zone between carbonising and graphitising.
Instead or in addition, a different volume-change
behaviour may be present during cooling.
The shrinkage and/or expansion of the first type of
coke during the carbonising and/or graphitising and/or
cooling related to the volume is preferably at least
10% higher than that of the second type of coke, in
particular at least 25% higher, in particular at least
50% higher. Thus, for example, in the case of a 10%
higher shrinkage of the first type of coke, the
shrinkage from room temperature to 20000C in the case
of the second type of coke is 1.0% by volume, but in
the case of the first type of coke 1.1% by volume.
The shrinkage and/or the expansion of the first type of
coke during the carbonising and/or graphitising and/or
cooling related to the volume is advantageously at
least 100% higher than that of the second type of coke,
in particular at least 200% higher, in particular at
least 300% higher. Thus, for example, in the case of a
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300% higher expansion of the first type of coke, the
expansion from room temperature to 10000C in the case
of the second type of coke is 1.0% by volume, whereas
in the case of the first type of coke it is 4.0% by
volume.
The case where the first type of coke experiences a
shrinkage, but the second type of coke experiences an
expansion in the same temperature range, is also
covered by the process according to the invention. A
300% higher shrinkage and/or expansion thus also
includes, for example, the case where the second type
of coke shrinks by 1.0% by volume, whereas the first
type of coke expands by 2.0% by volume.
Alternatively, instead of the first type of coke, the
second type of coke can exhibit a greater shrinkage
and/or expansion in at least one arbitrary temperature
range of the process according to the invention, as
described above for the first type of coke.
At least one of the two types of coke is preferably a
petroleum coke or coal-tar-pitch coke.
The quantity fraction in percentage by weight of the
second type of coke in the total quantity of coke
preferably amounts to between 50% and 90%. In these
quantity ranges, the different volume-change behaviour
of the first and second type of coke has a particularly
good effect on a compaction during the carbonising
and/or graphitising and/or cooling. Conceivable
advantageous quantity ranges of the second type of coke
can be 50 to 60%, but also 60 to 80%, as well as 80 to
90%.
At least one further carbon-containing material and/or
pitch and/or additives are advantageously added to the
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coke. This can be advantageous both with regard to the
processability of the coke as well as the subsequent
properties of the produced cathode block.
The further carbon-containing material preferably
contains graphite-containing material; in particular,
the further carbon-containing material comprises
graphite-containing material, such as for example
graphite. The graphite can be synthetic and/or natural
graphite. The effect of such a further carbon-
containing material is that the necessary shrinkage of
the cathode material, which is dominated by the coke,
is reduced.
The carbon-containing material related to the total
quantity of coke and carbon-containing material is
advantageously present at 1 to 40% by weight, in
particular at 5 to 30% by weight.
In addition to the quantity of coke and, as
appropriate, carbon-containing material, said quantity
representing a total of 100% by weight, pitch in
quantities of 5 to 40% by weight, in particular 15 to
30% by weight (related to 100% by the weight of the
total green mixture), can preferably be added. Pitch
acts as a binder and serves to produce a dimensionally
stable body during the carbonising.
Advantageous additives can be oil, such as auxiliary
pressure oil, or stearic acid. These facilitate mixing
of the coke and, if appropriate, the further
components.
The coke in at least one of the two layers, i.e. in the
first and/or the second layer, preferably comprises two
types of coke, which have a different volume-change
behaviour during the carbonising and/or graphitising
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and/or cooling. This can presumably lead to a
compaction of the emerging graphite of over 1.70 g/cm3,
in particular over 1.71 g/cm3. Depending on what is
desired and/or required, both layers or one of the two
layers can therefore be produced, according to the
invention, with two different types of coke. The
possibility thus arises of adjusting bulk densities and
bulk density ratios, as required or desired. According
to the invention, solely the first layer can for
example be produced with two types of coke, whilst the
second layer is produced with only one type of coke,
but additionally contains TiB2 as a hard material. The
expansion behaviours of the two layers are thus made
similar, which can advantageously increase the useful
life of the layers.
If need be, it may be advantageous for the multi-layer
block to comprise more than two layers. In this case,
it is possible according to the invention to produce,
from the more than two layers, an arbitrary number of
layers, in each case with two types of coke having a
different volume-change behaviour.
Further advantageous embodiments and developments of
the invention are explained below with the aid of a
preferred example of embodiment.
For the production of a cathode block according to the
invention, a first and a second coke are ground
separately from one another, separated into grain size
fractions and mixed together with pitch together with,
for example, 15 to 25% by weight, such as for example
20% by weight, of TiB2. The weight proportion of the
first coke in the total quantity of coke can amount for
example to 10 to 20% by weight or 40 to 45% by weight.
The mixture is filled into a mould, which largely
corresponds to the subsequent shape of the cathode
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blocks, and compacted by vibration or block-pressed.
The emerging green body is heated up to a final
temperature in a range from 2300 to 3000 C, such as
2600 or 2800 C for example, a graphitising step taking
place, and is subsequently cooled. The emerging cathode
block has a bulk density of 1.68 g/cm3 and a very high
wear resistance to liquid aluminium and cryolite. The
thermal and electrical conductivity are high on account
of the retained average degree of graphitising. A loss
of TiB2 was not able to be established by x-ray
diffraction analysis. The wettability of the cathode
block by liquid aluminium is very good.
Alternatively, a single type of coke is used. The
wetting behaviour of the resultant cathode block is for
the most part just as good as in the first example of
embodiment. The thermal and electrical conductivity lie
in similar ranges to those in the first example of
embodiment.
,
In a further variant of the example of embodiment,
graphite powder or carbon particles are added to the
coke mixture.
All the features mentioned in the description, the
examples and claims can contribute to the invention in
any combination. The invention is not however limited
to the stated examples, but can also be performed in
modifications that are not specifically described here.
