Note: Descriptions are shown in the official language in which they were submitted.
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SGL CARBON AG
01/099 SGL
Process for producing cathode blocks
The invention relates to a process for producing cathode
blocks, particularly 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 a 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 unifarmly 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
erosion.
One possible way of making the erosion 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
c~a, ro~sion) isp uniform ~wover _-thei,x.,-1-ength or.__..at-.,-_least
--._._-.~,. -..~.--.--. - ..
displays a very small deviation from its mean over t'he
length.
One solution for this problem 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 conductivities and are arranged so that a
uniform or approximately 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 depend in each case on the size and
type of the cell and have to be adapted individually for
each case. Cathode blocks made up of a plurality of
individual carbon blocks are complicated~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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2a_
10
20 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 depend 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 complicated to construct;
the joins also have to be sealed well in order to
prevent the liquid aluminum flowing out at the joins.
AMENDED SHEET
least displays a very small deviation from its mean
over the length.
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US 4,071,604 describes the graphitization of a carbon
body in an induction furnace, with the latter being the
only source of heat.
WO 00/46426 describes a graphite cathode consisting of
a single block which has an electrical conductivity
which can be changed 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
the 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, the loss of heat by
conduction 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, the heat loss in the vicinity of
the ends can be increased by different configuration of
the thermally insulating bed, or bodies which conduct
heat away, 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 the heat
treatment can be achieved by local changes in the
current density, with the result of different heat
evolution.
AMENDED SHEET
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In WO 00/46426, a graphite cathode is described
consl ting of a single block which has an electrical
conduc 'vity which is varied over its length, with the
conducti ' ty being lower at the ends of the block than in
the middle. This nonuniform distribution of electrical
conductivity 's achieved by bringing the end zones to a
temperature o from 2 200 to 2 500 °C during
graphitization, w ile the middle zone is exposed to a
temperature of from 2 700 to 3 000 °C. This different
heat treatment can be chieved in two ways according to
these teachings : on tYi'e one hand, heat loss in the
graphitization furnace cai~, be limited differently, or
heat sinks can be providedl~'~~n the vicinity of the end
zones so as to increase the heft 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 nonur~iform and the desired
temperatures are obtained as a result. In the case of
longitudinal graphitization, too, the 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 vicini y so as to
produce greater outward heat flow to the furnad.e wall.
According to another method, the difference inv heat
treatment can be achieved by local changes in the current
r'density, with the result of different heat ev~o-lu-ti.
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 costs of manufacture. The
higher heat loss towards the furnace walls 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 or electrical conductivity after the furnace
cycle is terminated and the cathodes are removed.
It is therefore an object of the present invention to
provide a practical process for producing cathodes with
an electrical conductivity varying along their length.
This object is achieved according to the invention by an
additional inductive heating of the cathode blocks.
The invention accordingly provides a process for produc-
ing cathode blocks which can be used for the electrolytic
production of aluminum, which comprises carrying out a
longitudinal graphitization of carbonized.cathode blocks
and effecting the graphitization at least partly by
inductive heating of the cathode block, with the
inductively heated zone being located in the middle of
the length of the cathode block.
During the investigations leading to the present
invention, it was found that introduction of a heat sink
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increases the specific energy consumption, i.e. has an
adverse effect on the energy consumption. However, a
desired decrease in the specific energy consumption can
be achieved when, instead of removing the energy,
additional energy is introduced into predetermined
regions by adaption of the construction. According to the
invention, this is achieved by additiona l inductive
heating, with only the middle section of the future
cathode being located in the core region of the induction
coil. The length of the core of the induction coil can be
set by appropriate choice of type and thickness of the
packing medium and the thermal conductivity of the body
so that the induction power provides part or all of the
required heat and the region located outside the zone
where the induction coil is effective is heated only by
the applied direct current and/or by thermal conduction
of the cathode block itself.
A preferred embodiment of the invention is illustrated by
the drawing. Fig. 1 shows a side view of a~cathode block.
For reasons of clarity, the figure shows only one cathode
block 4 which is heated by the Joule heat generated by
the direct current flowing in the circuit 1 in a longi-
tudinal graphitization. By means of an AC voltage source
and leads 2, an oscillating magnetic field is generated
in the coil 3 and induces an AC voltage in the middle
zone of the cathode block 4. An electric current governed
by the electrical resistance then flows in this zone and
effects further heating of the material in this zone.
The length of the zone which is inductively heated is
preferably from 25 to 90 0, particularly preferably from
to 80 0, of the total length of the cathode block.
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Furthermore, the inductive heating preferably contributes
at least 10 o and up to 100 0 of the total heat for the
cathode block concerned. The heat generated by inductive
heating in the middle of the cathode block is at least
10 o more than that at the outer ends of the cathode
block.
It is possible and preferred for the inductive heating to
be switched on only during the course of the graphitiza-
tion process. The direct heating in the longitudinal
graphitization process can then be operated at a lower
electric power and thus more inexpensively and, in
addition, the induction frequency can be made lower and
thus cheaper. The induction frequency required is a
function of the dimensions of the cathode blocks, its
specific electrical resistance and its magnetic
susceptibility. The inductive heating is preferably
switched on only after 20 0 of the total heating time has
lapsed.
The current density in electrolysis cells using the
cathodes produced in this way displays a significantly
lower deviation from the mean over the length of the
cathodes; this deviation is preferably not more than
20 0, particularly preferably not more than 10 o and in
particular not more than 5 0.
The graphitized cathode blocks produced by the process of
the invention display more uniform corrosion over the
length of the cathode and therefore have 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.
