PROCESS FOR CONTINUOUS GRAPHITIZATION

PROCEDE DE GRAPHITISATION EN CONTINU

English Abstract

Process for continuous graphitization, carried out in a
tunnel furnace having a transport device, heat locks at
the inlet and outlet of the furnace and in the interior
of the furnace at least one heating section whose length
is less than the length of the bodies to-be graphitized
and whose heating power is altered according to the
movement of the bodies, in particular for cathode blocks
which are used in the electrolytic reduction of aluminum
oxide.

French Abstract

L'invention concerne un procédé de graphitisation en continu, faisant appel à un four de type tunnel comprenant un dispositif d'acheminement, des sas thermiques à l'entrée et à la sortie du four, ainsi qu'au moins une section de chauffage située à l'intérieur du four, la longueur de cette section étant inférieure à la longueur du corps devant subir le traitement de graphitisation et sa puissance de chauffage étant modifiée à la suite du déplacement du corps. Ce procédé est destiné en particulier à des blocs cathodiques utilisés pour la réduction électrolytique d'oxyde d'aluminium.

Claims

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Claims
1. A process for continuous graphitization, carried out
in a tunnel furnace having a transport device, heat
locks at the inlet and outlet of the furnace and in
the interior of the furnace at least one heating
section whose length is less than the length of the
bodies to be graphitized and whose heating power is
altered according to the movement of the bodies.
2. The process as claimed in claim 1, wherein the heat-
ing section comprises a conductive heating facility
in which the distance between the power leads is
less than the length of one of the bodies to be
graphitized.
3. The process as claimed in claim 1, wherein the
heating section comprises an inductive heating
facility.
4. The process as claimed in claim 1, wherein the
heating section comprises a combination of at least
two types of heating selected from among radiative
heating, conductive heating and inductive heating.
5. The process as claimed in claim 1, wherein the power
of the heating section is reduced by at least 10%
when the ends of one of the bodies to be graphitized
pass through the heating section.
6. The process as claimed in any one of claims 1 to 5, wherein the furnace
comprises a plurality of heating sections whose power is altered

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as a function of time so that at least the heating
section in which the middle of one of the bodies to
be graphitized is located is operated at its nominal
power while the heating sections in which one end or
a segment close to the end of one of the bodies to
be graphitized are located are operated at an at
least 5% lower power.
7. The process as claimed in any one of claims 1 to 6, wherein the bodies
to be graphitized are carbonized cathode blocks for the electrolytic
reduction of aluminum oxide.

Description

CA 02470686 2004-06-16
SGL CARBON AG
01/104 SGL 1 -
Process for continuous graphitization
The invention relates to a process for continuous
graphitization, in particular of cathode blocks 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,
effectively a solution of aluminum oxide in cryolite. The
working temperature is, for example, about 1 000 C. The
electrolytically generated molten aluminum collects 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.
Owing to the chemical resistance and thermal stability
required, the material of 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 display increased

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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.
WO 00/46426 describes a graphite cathode consisting of a

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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.
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.
These known methods have considerable disadvantages for
industrial use. A difference of 500 C between the desired

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graphitization temperatures in the middle and at the ends of the cathodes
cannot
be achieved by means of heat sinks alone. The required difference between heat
conduction to the outside results in a considerable energy loss which
significantly
increases the costs of manufacture. The higher heat loss toward the side of
the
furnace also means a higher thermal stress 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 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 some embodiments of the present
invention to provide a practical process for producing cathodes which have a
differing electrical conductivity over their length.
According to the present invention, there is provided a process for
continuous graphitization, carried out in a tunnel furnace having a transport
device, heat locks at the inlet and outlet of the furnace and in the interior
of the
furnace at least one heating section whose length is less than the length of
the
bodies to be graphitized and whose heating power is altered according to the
movement of the bodies.
Some embodiments provide a continuous graphitization process
starting from, in particular, carbonized electrode blocks, carried out in a
tunnel
furnace having a transport device, heat locks at the inlet and outlet of the
furnace
and in the interior of the furnace at least one heating section whose length
is less
than the length of the electrode blocks and whose heating power is altered
according to the movement of the electrode blocks.
Basic heating of the bodies to be graphitized is preferably ensured
by means of conductive heating. Electric power can be supplied to the
individual
blocks, and the power leads are then advantageously not attached directly to
the
ends of the blocks but the power is supplied, for example, via sleeves in the
outer
zones.

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The distance from the ends of the bodies can be up to 10%
of their length. However, it is also possible for the
electric power to be supplied via the ends or the end
faces of the bodies. The energy introduced by means of
5 conductive heating is advantageously such that on its own
it heats the bodies to temperatures of not more than
2 650 C, preferably up to 2 500 C and in particular up to
2 200 C.
For the purposes of the invention, it is also possible
for the electric power for conductive heating to be
supplied in such a way that more than one body is
conductively heated by means of this current. This
embodiment corresponds in principle to those in which the
electric power is supplied to an individual body via its
end faces. Of course, the conditions for the energy
supplied as set forth in the previous paragraph also
apply.
The additional energy necessary to reach the temperatures
required for the graphitization can be supplied by
inductive heating or by means of radiative heating. The
heating elements for this purpose preferably have a
length which is less than that of the bodies to be
graphitized. For the purposes of the present invention,
the length of a heating element is the distance in the
direction of the length of the bodies to be graphitized
which is operated at essentially constant energy output
over this length; in particular, it is not the physical
length of the heating element. For example, the addi-
tional heating can be provided using arrangements of
inductive heating elements in which the effective length
of the individual elements is not more than 50% of the
length of the bodies to be graphitized. The effective
length is preferably up to 30% of the length of the

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bodies, in particular up to 20%. In place of the induc-
tive heating elements or in combination therewith, it is
also possible to use radiative heating elements for whose
geometries the same conditions as for the inductive
heating elements apply.
The time for which the additional heating element(s)
is/are switched on and the points in time at which
switching on and off occur are, according to the inven-
tion, selected so that the middle zones (based on the
longitudinal direction of the bodies) are subjected to a
higher energy input and therefore are brought to higher
temperatures, preferably to temperatures of at least
2 650 C, preferably at least 2 700 C, in particular to
temperatures in a range from 2 700 to 3 000 C. According
to the invention, it is also possible for the additional
heating elements not to be switched off completely when
the end zones or the zones close to the end reach the
effective region of the additional heating elements, but
for them to be operated merely at a reduced energy
output. In this case, the energy output should be reduced
by at least 10% when the end zones reach the effective
region of the additional heating elements.
If the furnace has a plurality of heating sections,
preference is given to altering their power as a function
of time so that the heating section which is in each case
in the middle of a body to be graphitized is operated at
its nominal power while the heating sections in which one
end or a segment close to the end of one of the bodies to
be graphitized are located are operated at an at least 5%
lower power.