Note: Descriptions are shown in the official language in which they were submitted.
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SGL CARBON AG
01/101 SGL 1
Graphitized cathode blocks
The invention relates to graphitized cathode blocks, a
process for producing them and their use, in particular
for the electrolytic production of aluminum.
In the electrolytic production of aluminum by the
Hall-Heroult process, use is made of electrolysis cells
which have a bottom which is made up of ~ plurality of
blocks and acts as cathode. The electrolyte is a melt
comprising mainly a solution of aluminum oxide in
cryolite. The working temperature is, for example, about
1 000 °C. The electrolytically generated molten aluminum
is deposited on the bottom of the cell under a layer of
the electrolyte. The cells are surrounded by a metallic
housing (preferably steel) lined with high-temperature-
resistant material.
Due to the chemical resistance and thermal stability
required, the material of choice for the cathode blocks
is preferably carbon which may have been partially or
completely graphitized by means of thermal treatment.
Such cathode blocks are produced by mixing pitches,
cokes, anthracite and/or graphite in selected particle
sizes or particle size distributions for the solids and
shaping, firing and, if appropriate, (partially) graphi
tizing the mixtures. Firing (carbonization) is usually
carried out at temperatures of about 1 200 °C, and the
graphitization is usually carried out at temperatures
above 2 400 °C.
While graphitized cathodes are preferred because of their
higher electrical conductivity, they suffer from
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increased corrosion during operation, corresponding to a
mean annual decrease in their thickness of up to 80 mm.
This corrosion is not distributed uniformly over the
length of the cathode blocks (corresponding to the width
of the cell) , but the surface of the cathode blocks is
changed to a W-shaped profile. Due to the nonuniform
removal of material, the useful life of the cathode
blocks is limited by the areas having the greatest
removal of material.
One possible way of making the removal of material more
uniform over the length of the cathode block and thus
increasing the useful life is to configure the cathode
blocks so that their electrical resistance varies over
the length in such a way that the current density (and
thus the corrosion) is uniform over their length or at
least displays a very small deviation from its mean over
the length.
One solution is described in DE 20 61 263, in which
composite cathodes are made up of either a plurality of
carbon blocks which have different electrical conduc-
tivities and are arranged so that a uniform or approxi-
mately uniform current distribution results, or of carbon
blocks whose electrical resistances increase continuously
in the direction of the cathodic terminals. The number of
carbon blocks and their electrical resistance depends in
each case on the size and type of the cell and have to be
calculated afresh for each case. Cathode blocks made up
of a plurality of individual carbon blocks are compli-
cated to construct; the joins also have to be sealed well
in order to prevent the liquid aluminum flowing out at
the joins.
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In WO 00/46426, a graphite cathode is described
consisting of a single block which has an electrical
conductivity which is varied over its length, with the
conductivity being lower at the ends of the block than in
the middle. This nonuniform distribution of electrical
conductivity is achieved by bringing the end zones to a
temperature of from 2 200 to 2 500 °C during
graphitization, while the middle zone is exposed to a
temperature of from 2 700 to 3 000 °C. This different heat
treatment can be achieved in two ways according to these
teachings: on the one hand, heat loss in the
graphitization furnace can be limited differently, or
heat sinks can be provided in the vicinity of the end
zones so as to increase the heat loss. In the case of a
transverse graphitization, the density of the thermally
insulating bed is altered so that the heat loss over the
length of the cathodes becomes nonuniform and the desired
temperatures are obtained as a result. In the case of
longitudinal graphitization, too, heat loss in the
vicinity of the ends can be increased by different
configuration of the thermally insulating bed, or bodies
which carry away the heat, preferably graphite bodies,
are installed for this purpose in their vicinity so as to
produce greater outward heat flow to the furnace wall.
According to another method, the difference in heat
treatment can be achieved by local changes in the current
density, with the result of different heat evolution.
This change in the current density can, according to the
teachings, be achieved by different resistances of the
conductive bed between two cathodes in an Acheson furnace
(transverse graphitization); no such solution is indica-
ted for a longitudinal graphitization process.
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These known methods have considerable disadvantages for
industrial use. A difference of 500 °C in the desired
graphitization temperatures in the middle and at the ends
of the cathodes cannot be achieved by means of heat sinks
alone. The required difference in heat conduction to the
outside results in a considerable energy loss which
significantly increases the cost of manufacture. The
higher heat loss toward the side of the furnace also
means a higher thermal load which makes the construction
of the furnace more expensive or reduces its life.
Finally, an inhomogeneity in the thermally insulating bed
or the conductive bed is not very practical, since the
bed material would have to be introduced in a plurality
of steps and would have to be classified again according
to its thermal conduction or electrical conductivity
after the furnace cycle is concluded and the cathodes are
removed.
It is therefore an object of the present invention to
provide graphitized cathode blocks with an electrical
conductivity varying along their length.
Graphitization of cathode blocks by the longitudinal
graphitization process results in an electrical transi-
tion at the joins between the individual cathode blocks
themselves or between the blocks and electrically conduc-
tive connecting elements located between them. This
electrical transition has a resistance higher than the
resistance in the interior of the individual cathode
blocks or the connecting element. This increased resis-
tance leads to increased generation of heat and thus to
a higher temperature, i.e. to an acceleration of the
graphitization reaction. The electrical resistance at the
ends of the cathode blocks is therefore usually lower
than that in the middle of the cathode blocks in the case
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of longitudinal graphitization. This distribution of the
resistance or the electrical conductivity over the length
of the cathode block is precisely the opposite of what is
desired.
It has now been found that cathode blocks having the
desired distribution can be produced in a simple way by
i
~~~~,_g-_._the tee.-._~escrib~d.. oath,Qde .bloc-ks. ap.art_ _i n_ h~..__,~
middle and rejoining them in the opposite direction. This
gives a profile of the electrical resistance having the
shape of a V whose legs (arms) are rounded.
The presynt invention therefore provides graphitized
cathode b~~cks for the production of. aluminum by
electrolytic'~~eduction of aluminum oxide in a bath of
molten cryolite,'..wherein the cathode blocks are composed
of at least two parts and have a V-shaped profile of
their electrical resistance over their length, with the
resistance in the middle of the cathode blocks displaying
a discontinuity and increasing monotonically toward the
ends so that the ratio of the resistance at the ends of
the parts to that in the middle is at least 1,05:1.
The cathode blocks are preferably composed of at least
two parts whose electrical resistance increases mono-
tonically over their length so that the ratio of the
resistance at the ends of the parts to that in the middle
is at least 1.15:1. This ratio is particularly preferably
1.3:1.
The invention is illustrated by the drawings: In the
drawings:
Fig. 1 shows the variation in the specific electrical
resistance p over the length of. theca~_h~~
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are rounded.
The present invention therefore provides graphitized
cathode blocks for the production of aluminum by
electrolytic reduction of aluminum oxide in a bath of
molten cryolite, wherein the cathode blocks are
composed of two parts and have a V-shaped profile of
their electrical resistance over their length, with the
resistance in the middle of the cathode blocks
displaying a discontinuity and increasing monotonically
toward the ends so that the ratio of the resistance at
the ends of the parts to that in the middle is at least
1.05:1.
The cathode blocks are preferably composed of two parts
whose electrical resistance increases monotonically
over their length so that the ratio of the resistance
at the ends of the parts to that in the middle is at
least 1.15:1. This ratio is particularly preferably
1.3:1.
AMENDED SHEET
cutting apart and rejoining them in the opposite
direction. This gives a profile of the electrical
resistance having the shape of a V whose legs (arms)
." r
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The invention is illustrated by the drawings: In the
drawings:
Fig. 1 shows the variation in the specific
electrical resistance p over the length of
the cathode block, as is obtained in
longitudinal graphitization with a high
transition resistance between the individual
cathode blocks, shown in a side view of a
cathode block,
Fig. 2 shows a side view of a cathode block which
has been cut apart in the middle and put
together the other way around, with a joining
layer of tamping composition being introduced
in the middle,
Fig. 3 shows a side view of a cathode block which
has been cut apart in the middle and put
together the other way around, with an
adhesive join in the middle connecting the
two parts, and
30
AMENDED SHEET
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w _- ...~ .._..~.. b 1 o c k~~ ".° ~a ~ ...".:i.~...-.~a.,i..ne ~ . . i
~ - ~ o n g i t a a~i n~a 1 ~g r a p
tization with a high transition r~"sistance
between the individual cathode blor~ksr, shown in
a side view of a cathode bloc.~:;'~,~
Fig. 2 shows a side view of adrlcathode block which has
been cut apart in i~e middle and put together
the other way und, with a joining layer of
ramming past,,~''~ being introduced in the middle,
Fig. 3 shows,~t°~side view of a cathode block which has
beef cut apart in the middle and put together
a other way around, with an adhesive join in
/ thew_middle connecting the twos-s;---~~
_.,__~_,..__.._____. _ _.....~....~.--..---°.
Fig. 4 shows a side view of a cathode block which has
been cut apart in the middle and put together
the other way around, with the tw, o parts merely
being butted together.
In detail, fig. 1 shows the variation of the specific
electrical resistance p, calculated as (Ra/~, where R is
the electrical resistance of a cuboidal test specimen, a
is its cross-sectional area and leis its length, depicted
in the interior of the side view of a cathode block 4
over the length of the cathode block. The ends of the
block as is obtained in longitudinal graphitization are
denoted by A. The cathode block is cut apart along the
line BB, with the end faces at A being denoted by 4-1 and
the parted surface along the line BB in the side view
being designated as 4-2. The parted cathode block is then
joined together as shown in fig. 2 to 4 so that the ends
A or the end faces 4-1 are located in the middle of the
composite cathode block.
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Fig. 2 shows an embodiment in which a layer of the
ramming paste 5 is located between the two end faces 4-1
at the ends A which are now located in the middle; this
ramming paste also serves to seal the contact areas
between the individual cathode blocks at the bottom of
the electrolysis cell. Suitable ramming pastes for this
purpose are compositions based on anthracite and graphite
and having a density of about 1 700 kg/m3,~ e.g. BST 17/1
from SGL Carbon AG.
The previously internal areas 4-2 have now become
exterior faces. The variation of the specific electrical
resistance p is now such that the lowest value is in the
middle of the cathode block and the specific electrical
resistance now increases symmetrically from the middle
point to the ends. Conversely, the electrical conduc-
tivity now displays a peak in the middle of the cathode
block and decreases towards to the ends.
Fig. 3 depicts a further preferred embodiment in which
the two half blocks are each joined together at the
ends A by a layer of an adhesive 6 having the required
thermal stability.
Suitable adhesives are cold-curing resins such as BVK6
from SGL Carbon AG.
Finally, fig. 4 shows an embodiment in which a bonding
layer or intermediate layer has been omitted and the two
half blocks have merely been butted together at their
ends A. The required pressure is in this case produced by
the thermal expansion of the half blocks which are
pressed together on heating after they have been
installed in contact with one another in the electrolysis
cells. It has been found that the pressure is sufficient
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to ensure a reliable and leak-free join between the two
half blocks provided that the end faces before cutting
were sufficiently planar.
The graphitized cathode blocks of the invention display
more uniform corrosion over the length of the cathode and
therefore a significantly increased life compared to the
conventional blocks having a homogeneous distribution of
the electrical conductivity when used in the production
of aluminum by electrolytic reduction of aluminum oxide
in a bath of molten cryolite.
