Note: Descriptions are shown in the official language in which they were submitted.
CA 02825785 2013-07-26
SGL CARBON SE 1
2010/081W0
=
Cathode arrangement and cathode block with a
groove having a guide recess
The present invention relates to a cathode arrangement for an aluminium elec-
trolysis cell, to a cathode block for such a cathode arrangement and to a
process
for producing such a cathode arrangement.
Such electrolysis cells are used for the electrolytic production of aluminium,
which
is customarily carried out in industry by the Hall-Heroult process. In the
Hall-
Heroult process, a melt composed of aluminium oxide and cryolite is
electrolysed.
Here, the cryolite, Na3[A1F6], serves to lower the melting point of 2045 C for
pure
aluminium oxide to about 950 C for a mixture containing cryolite, aluminium
oxide
and additives, such as aluminium fluoride and calcium fluoride.
The electrolysis cell used in this process has a bottom, which is composed of
a
multiplicity of adjoining cathode blocks forming the cathode. In order to
withstand
the thermal and chemical conditions which prevail during operation of the
cell, the
cathode blocks are customarily composed of a carbon-containing material. The
undersides of each of the cathode blocks are provided with grooves, in each of
which there is arranged at least one busbar through which the current fed via
the
anodes is discharged. In this case, the interstices between the individual
walls of
the cathode blocks, which delimit the grooves, and the busbars are often
sealed
with cast iron, in order to electrically and mechanically connect the busbars
to the
cathode blocks by virtue of the resulting encasement of the busbars with cast
iron.
An anode formed from individual anode blocks is arranged about 3 to 5 cm above
the layer of molten aluminium located on the top side of the cathode, and the
elec-
trolyte, i.e. the melt containing aluminium oxide and cryolite, is located
between
said anode and the surface of the aluminium. During the electrolysis carried
out at
about 1000 C, the aluminium which has formed settles beneath the electrolyte
layer, i.e. as an intermediate layer between the top side of the cathode
blocks and
CA 02825785 2013-07-26
2
the electrolyte layer, on account of the fact that its density is relatively
large com-
pared to that of the electrolyte. During the electrolysis, the aluminium oxide
dis-
solved in the cryolite melt is cleaved to form aluminium and oxygen by a flow
of
electric current. In terms of electrochemistry, the layer of molten aluminium
is the
actual cathode, since aluminium ions are reduced to elemental aluminium on the
surface thereof. Nevertheless, hereinbelow the term "cathode" will not be
under-
stood to mean the cathode from an electrochemical point of view, i.e. the
layer of
molten aluminium, but rather the component which forms the electrolysis cell
bot-
tom and is composed of one or more cathode blocks.
A significant disadvantage of the cathode arrangements used in the Hall-
Heroult
process is their relatively low wear resistance, which manifests itself by
erosion of
the cathode block surfaces during electrolysis. In this case, on account of an
in-
homogeneous current distribution within the cathode blocks, the cathode block
surfaces are not eroded uniformly over the length of the cathode blocks, but
rather
to an increased extent at the cathode block ends, and therefore the surfaces
of the
cathode blocks change to a W-shaped profile after certain electrolysis
duration. As
a result of the nonuniform erosion of the cathode block surfaces, the useful
life of
the cathode blocks is limited by the areas with the greatest erosion.
In order to counter this problem, WO 2007/118510 A2 discloses a cathode block
with a groove which is intended for receiving a busbar and has a greater depth
in
the centre than at the cathode block ends, with respect to the cathode block
length. This achieves a substantially homogeneous vertical current
distribution
over the cathode block length during operation of the electrolysis cell, as a
result
of which the increased wear on the cathode block ends is reduced and thus the
service life of the cathode is increased.
A further disadvantage of the cathode arrangement used in the Hall-Heroult
proc-
ess is its comparatively high electrical resistance. One of several reasons
for the
1
,
CA 02825785 2013-07-26
µ
3
comparatively high electrical resistance is that the contact resistance
between the
busbars and the cathode blocks of the cathode is comparatively high and this
contact resistance additionally increases as the operating time of the cathode
increases. This is caused firstly by the fact that constituents of the melt
undesira-
bly diffuse into the cathode blocks during electrolysis, which leads to the
formation
of insulating layers of for example 13-aluminium oxide, and secondly by the
fact that
the steel of the busbars, the cast iron and the carbon of the cathode blocks
start to
creep after relatively long loading, i.e. the steel of the busbars, the cast
iron and
the carbon of the cathode blocks deform irreversibly after relatively long
loading.
In order to reduce the electrical contact resistance between the busbars and
the
cathode blocks, and therefore to increase the energy efficiency of the
electrolysis
process, it has been proposed in WO 2007/071392 A2 to line the groove of a car-
bon-based or graphite-based cathode block with a graphite foil at least in
certain
regions. Aside from the fact that the graphite foil reduces the electrical
contact
resistance between the busbar, or the layer of solidified cast iron encasing
it, and
the cathode block on account of its good positive fit on both sides, the
elasticity of
the graphite foil means that the latter also reduces in particular the
increase in this
contact resistance as the operating time of the cathode increases, because the
graphite foil fills the gaps which form during creep of the steel of the
busbar and of
the carbon of the cathode block between the walls which delimit the groove of
the
cathode block and the busbar.
However, graphite foils have a smooth surface with very good sliding
properties. In
the case of a cathode block having a groove lined with a graphite foil, there
is
therefore the risk that the busbar accommodated therein, which usually has a
length of several metres and a weight of several hundred kilograms, will subse-
quently be displaced in the groove in an uncontrolled manner in the depth
direction
of the groove opening, or will even fall out of the groove, if for example the
cath-
ode block is raised as it is being installed or is moved for another reason.
This risk
1
CA 02825785 2013-07-26
4
=
is present in particular in the case of a groove having a rectangular cross
section,
which is virtually the only applicable form for the groove of a cathode block
with a
groove depth which varies over its length. In addition, the precisely fitting
contact
between the groove and the cast iron is lost as a result of the busbar
slipping in
the groove, and this leads to poorer current transfer from the busbar to the
cath-
ode block and therefore to a decrease in energy efficiency. Finally, graphite
foil
cannot be connected to cast iron or can be connected to cast iron only to a
very
small degree, and therefore the filling of the gap between the busbar and the
graphite foil by pouring liquid cast iron into it and subsequent hardening or
solidifi-
cation of the cast iron do not result in a connection between the graphite
foil and
the cast iron, but rather only in the busbar being encased with cast iron.
It is therefore an object of the present invention to provide a cathode
arrangement
for an aluminium electrolysis cell of the type mentioned in the introduction,
which
has a low electrical resistance, which is also in particular permanently low
over an
extended electrolysis period, and in particular also a low contact resistance
be-
tween the busbar and the cathode block, and in which undesirable subsequent
displacement of the busbar in the groove of the cathode block perpendicularly
to
the longitudinal direction of the cathode block, i.e. in the depth direction
of the
groove, and in particular falling out of the busbar from the groove is
reliably pre-
vented, to be precise in particular even in the case of a groove with a
rectangular
cross section, as is conventionally used in cathode blocks with a groove depth
which varies over the cathode block length.
According to the invention, this object is solved by a cathode arrangement for
an
aluminium electrolysis cell having at least one cathode block based on carbon
and/or graphite, which has at least one groove lined with a graphite foil at
least in
certain regions, wherein at least one busbar is provided in the at least one
groove
and has an encasement of cast iron at least in certain regions, wherein at
least
one recess is provided in the wall of the cathode block which delimits the at
least
CA 02825785 2013-07-26
one groove, and the encasement of cast iron engages into the at least one
recess
at least in certain portions.
This solution is based on the realization that a precisely fitting positively-
locking
connection which is resistant to displacement in the direction perpendicular
to the
longitudinal direction of the cathode block is achieved between a busbar and a
cathode block having a groove lined with graphite foil if at least one recess
is pro-
vided in at least one wall of the cathode block which delimits the groove and
a
busbar encased with cast iron at least in certain regions is introduced into
the
groove such that the encasement of cast iron engages into the recess at least
in
certain portions. According to the invention, it has been identified that,
independ-
ently of the high sliding properties of the graphite foil used, this achieves
a fixed
mechanical connection between the busbar encased with cast iron and the cath-
ode block perpendicular to the longitudinal direction of the cathode block,
which
counteracts undesirable displacement of the busbar in this direction and in
particu-
lar falling out of the busbar from the groove lined with graphite foil, to be
precise in
particular even in the case of a groove having a rectangular cross section, as
is
preferred for cathode blocks with a groove depth which varies over the cathode
block length. Therefore, the cathode arrangement according to the invention
has
the advantage, associated with the lining of the groove with graphite foil on
ac-
count of the electrical and mechanical properties of graphite foil, of
improved cur-
rent transfer between the busbar and the cathode block and therefore improved
energy efficiency, and at the same time avoids the disadvantage, associated
with
the high sliding properties of graphite, of uncontrolled mobility of the
busbar in the
groove in the direction perpendicular to the longitudinal direction of the
cathode
block and accompanying possible impairment of the electrical connection
between
the busbar and the cathode block in the event that the electrolysis cell is
operated
for a relatively long time.
CA 02825785 2013-07-26
6
In addition, the present invention makes it possible to utilize the sliding
properties
of the graphite foil in a targeted manner to ensure that the busbar can be dis-
placed longitudinally in the groove selectively, specifically in the case of
move-
ments caused by a change in temperature during start up.
In addition, the cathode arrangement according to the invention having the
above-
described advantages can be produced with extremely low expenditure and with-
out complicated additional process steps. Thus, the mechanical connection
which
is provided between the busbar and the cathode block can be achieved simply by
filling a recess of the cathode block at least partially with the cast iron
during the
already required casting of the busbar with the cast iron. This achieves very
close
contact between the busbar, the encasement of cast iron, the graphite foil and
the
cathode block, contributing to a particularly low electrical contact
resistance be-
tween the busbar and the cathode block. In addition, the graphite foil absorbs
the
mechanical pressure which arises during operation of the cathode arrangement
perpendicularly to the plane of the foil.
Within the context of the present invention, in demarcation relative to a mere
sur-
face roughness, a "recess" is understood to mean a cutout which, based on the
surface of the wall which delimits the groove, has a depth of at least 0.05 mm
and
preferably of 0.5 mm.
In addition, within the context of the present invention, a "graphite foil" is
under-
stood to mean not only thin graphite sheet, but also in particular a partially
com-
pressed blank or a flexible plate of expanded graphite.
Within the context of the present invention, a "cathode arrangement" is
understood
to mean a cathode block having at least one groove, wherein at least one
busbar,
possibly encased by cast iron, is received in each of the at least one groove.
Simi-
larly, this term denotes an arrangement of a plurality of cathode blocks each
hay-
CA 02825785 2013-07-26
7
ing at least one groove, wherein at least one busbar, possibly encased by cast
iron, is received in each of the at least one groove.
In principle, the encasement of cast iron can be in direct contact with the
graphite
foil or with the cathode block itself at least in the region of the recess.
Although this
is preferred according to the present invention, it is not absolutely
necessary. What
in fact matters primarily for producing the desired mechanical connection
between
the busbar and the cathode block is the fact that the encasement of cast iron
en-
gages into the at least one recess at least in certain portions, i.e. fills
the hollow
space formed by the at least one recess at least in certain regions.
According to a preferred embodiment of the present invention, that portion of
the
encasement of cast iron which engages into the at least one recess is
configured
complementarily to the recess. This makes it possible to achieve a
particularly
good positively-locking engagement of the encasement of cast iron into the
recess
and therefore particularly effective mechanical fastening of the cast iron
encase-
ment and the busbar connected thereto to the cathode block.
In order to achieve a particularly good positive fit between the cast iron
encase-
ment and the cathode block, it is proposed in a development of the concept of
the
invention that that portion of the encasement which engages into the at least
one
recess and, if appropriate, the busbar encased thereby fill at least 70%,
preferably
at least 80%, particularly preferably at least 90%, very particularly
preferably at
least 95% and most preferably 100% of the recess. It is thereby possible to
par-
ticularly reliably avoid undesirable displacement of the busbar in the
direction
perpendicular to the longitudinal direction of the cathode block and in
particular
falling out of the busbar from the groove.
It is advantageous that each of the at least one recess extends continuously
over
at least 20%, preferably over at least 40%, particularly preferably over at
least
CA 02825785 2013-07-26
8
=
60%, very particularly preferably over at least 80% and most preferably at
least
approximately over the entire length of the groove. This can prevent the
busbar
from possibly slipping out of the groove during assembly. In addition, if the
recess
extends over a considerable part of the groove length, as described above, it
is
possible to ensure good displaceability of the busbar in the longitudinal
direction of
the groove, in which case undesirable displacement of the busbar parallel to
the
depth direction of the groove is still reliably prevented.
In principle, the cathode block can also have a multiplicity of recesses which
follow
one another in the longitudinal direction of the groove and are separated from
one
another by recess-free portions of the groove. This embodiment is particularly
advantageous when longitudinal displaceability of the busbar in the cathode
block
is not desirable.
In order to ensure that the cast iron encasement and the busbar are anchored
reliably in the cathode block, the at least one recess preferably has a depth
of 2
mm to 40 mm, particularly preferably of 5 mm to 30 mm and very particularly
pref-
erably of 10 mm to 20 mm.
For the same reason, the at least one recess preferably has an opening width,
based on the height of the cathode block, of 2 mm to 40 mm, particularly
prefera-
bly of 5 mm to 30 mm and very particularly preferably of 10 mm to 20 mm.
As a consequence, the at least one recess preferably has a cross-sectional
area
of 1.5 mm2 to 1600 mm2, particularly preferably of 10 mm2to 900 mm2 andvery
particularly preferably of 40 mm2 to 400 mm2 . These values are preferred in
par-
ticular for recesses having a polygonal cross section and particularly having
a
rectangular cross section. If the at least one recess has a curved cross
section,
such as for example a substantially semicircular cross section, the at least
one
recess preferably has a cross-sectional area of 1.5 mm2 to 630 mm2,
particularly
CA 02825785 2013-07-26
9
preferably of 10 mm2 to 350 mm2 and very particularly preferably of 40 mm2 to
160
mm2 .
In principle, the at least one recess can have any polygonal or bent cross
section.
Good results in terms of a good positively-locking engagement of the cast iron
encasement into the at least one recess and at the same time in terms of
reliable
and unproblematic fillability of the recess with cast iron during casting are
achieved in particular if the at least one recess has an at least
substantially semi-
circular, triangular, rectangular or trapezoidal cross section.
In a development of the concept of the invention, it is proposed that the at
least
one recess extends substantially perpendicularly into the wall of the cathode
block
which delimits the groove. This brings about a particularly reliable fixing
action in
the depth direction of the groove.
According to the present invention - as considered in the depth direction of
the
groove - the at least one recess is delimited at each of its ends by a
transition
region between the recess and an adjoining portion of the groove wall. If this
tran-
sition region has an angled configuration, the angle between the adjoining
portion
of the groove wall and the wall of the recess, as seen from the inside of the
cath-
ode block, is preferably 90 degrees to 160 degrees, particularly preferably 90
degrees to 135 degrees and very particularly preferably 100 degrees to 120 de-
grees. If this transition region has a curved configuration, possibly but not
neces-
sarily ideally a configuration curved like a circle, the radius of curvature
of the
transition region is preferably at most 50 mm, particularly preferably at most
20
mm and most preferably at most 5 mm.
According to a further preferred embodiment of the present invention, the wall
which delimits the groove comprises a bottom wall and two side walls, each
side
wall having at least one recess, preferably a recess which extends
perpendicularly
CA 02825785 2013-07-26
to the surface of the respective side wall. In this way, the busbar is held on
both
sides in the groove, as a result of which the busbar can be fixed particularly
effec-
tively in the desired position. In principle, it is also possible for a
plurality of re-
cesses to be provided in one or in both of the side walls, for example at
least 1, at
least 2, at least 3 or at least 4 recesses per side wall, into each of which
the en-
casement of the busbar of cast iron engages at least in certain portions. A
particu-
larly strong connection between the busbar and the cathode block is achieved
as a
result. It is preferable for the depth and/or the volume of the individual
recesses to
be all the more lower as more recesses are provided in the groove.
It is preferable for the at least one recess to be at an at least
substantially constant
distance from the bottom wall of the groove over its length and to run
parallel
thereto. In such a configuration, displaceability of the busbar parallel to
the groove
bottom is ensured.
According to a further preferred embodiment of the present invention, each of
the
at least one recess is lined at least in certain regions and preferably over
its full
extent with the graphite foil, in which case it goes without saying that the
remaining
regions of the groove are also preferably lined over their full extent with
the graph-
ite foil. As a consequence, a particularly low electrical contact resistance
between
the cast iron and the cathode block is produced even in the region of the
recesses.
In addition, the sliding properties of the graphite foil mean that it is
possible to
ensure displaceability of the busbar, as described above, in the longitudinal
direc-
tion of the at least one recess and therefore in the longitudinal direction of
the
cathode block, if the majority of the surface and preferably at least
approximately
the entire surface of the wall which delimits the groove is lined with
graphite foil. In
this case, the graphite foil can be pressed against the boundary of the recess
by
the encasement of the busbar of cast iron, in order to bring about both
particularly
good electrical contact and also a particularly effective positive fit. This
effect be-
comes important especially during heating of the electrolysis cell for start
up, since
1
CA 02825785 2013-07-26
11
=
-
the specific thermal expansion of steel or iron is approximately three times
the
specific thermal expansion of conventional cathode materials.
The at least one recess of the groove can be lined with the graphite foil
during the
production of the cathode arrangement simply by inserting the graphite foil
into the
groove such that it fills the recess, and then pouring the cast iron into the
groove in
such a manner that the graphite foil is pressed into the recess, where it is
pressed
in particular directly against the cathode block material which delimits the
recess.
In order to achieve a vertical current density distribution which is uniform
over the
cathode block length, it is proposed in a development of the concept of the
inven-
tion that the at least one groove has a depth which varies over its length or
the
length of the cathode block, it being particularly preferable for the centre
of the
groove, with respect to the longitudinal direction, to have a greater depth
than the
two longitudinal-side ends thereof. This achieves a uniform distribution of
the elec-
tric current fed via the cathode arrangement over the entire length of the
cathode
block, as a result of which an excessive electric current density at the
longitudinal-
side ends of the cathode block and thus premature wear at the ends of the cath-
ode block is avoided. In this embodiment, virtually the only applicable cross-
sectional form for the groove is rectangular, and therefore the effect of the
present
invention, specifically that of reliably avoiding falling out of the busbar
from the
groove opening, is particularly pronounced here.
Such a uniform current density distribution over the length of the cathode
block
avoids movements in the aluminium melt which are caused by the interaction of
electromagnetic fields, and it is thereby possible to arrange the anode at a
smaller
height above the surface of the aluminium melt. This reduces the electrical
resis-
tance between the anode and the aluminium melt and increases the energy effi-
ciency of the fused-salt electrolysis which is carried out.
1
=
CA 02825785 2013-07-26
12
In the above-described embodiment, too, in which the cathode block has a
groove
of variable depth, the at least one recess of the cathode block is preferably
config-
ured such that it is at a substantially constant distance from the bottom of
the
groove over the length of the groove, in order to thereby make it possible to
dis-
place the busbar as required along the longitudinal direction of the cathode
block.
The cathode arrangement according to the invention is also suitable without
any
problems in particular for the use of conventional groove and/or busbar geome-
tries. By way of example, the groove and/or the busbar can conventionally have
a
substantially rectangular cross section. This is preferable in particular if
the groove
has a depth which varies in the longitudinal direction. The busbar, in
particular,
can also conventionally consist of steel.
In a development of the concept of the invention, it is proposed that the
graphite
foil lining the groove at least in certain regions contains expanded graphite
and
particularly preferably compressed expanded graphite, which is particularly
pref-
erably free of binders. It is very particularly preferable for the graphite
foil lining the
groove at least in certain regions to consist of expanded graphite and
particularly
preferably of compressed expanded graphite free of binders. As set forth
above,
the foil in principle can also be formed by a substantially plate-shaped
blank, which
contains expanded graphite and in this case has a sufficient elasticity to be
de-
formed elastically such that it permits the above-described filling of the
recess by
the cast iron encasement and in the process can be inserted into the recess be-
tween the cast iron and the wall which delimits the groove.
The graphite content of the graphite foil is preferably at least 60%, further
prefera-
bly at least 70%, particularly preferably at least 80%, especially preferably
at least
90% and very particularly preferably at least approximately 100%.
CA 02825785 2013-07-26
13
=
=
Good results in terms of optimum exploitation of the mechanical and electrical
properties of the graphite are achieved in particular if the graphite foil has
a thick-
ness of between 0.2 mm and 3 mm, preferably between 0.2 mm and 1 mm and
particularly preferably between 0.3 mm and 0.5 mm.
Depending on the desired properties, the graphite foil can be inserted or adhe-
sively bonded into the groove. Adhesive bonding of the graphite foil into the
groove is preferable in particular if the graphite foil is pressed to only a
relatively
small degree against the surface of the recess, or if displacement of the
graphite
foil, no matter how small, in the longitudinal direction of the cathode block
is to be
avoided.
According to a further preferred embodiment of the present invention, the
cathode
block has one or two grooves for receiving in each case at least one busbar.
In
principle, it is possible within the context of the invention for one groove
of the
cathode block to receive exactly one busbar, but in particular also two
busbars,
which are inserted into various portions of the length of the groove. In this
case,
the busbars can be arranged so that they lie opposite one another on their
faces.
The present invention also relates to a cathode block for a cathode
arrangement of
an aluminium electrolysis cell based on carbon and/or graphite, which has at
least
one groove for receiving a busbar, wherein at least one recess is provided in
the
wall of the cathode block which delimits the at least one groove. Such a
cathode
block can advantageously be used as a component part of the cathode arrange-
ment described above. Here, the cathode block can be constructed on the basis
of
amorphous carbon, graphitic carbon, graphitized carbon or any desired mixture
of
the above carbons.
The present invention also relates to a process for producing a cathode
arrange-
ment for an aluminium electrolysis cell, comprising the following steps:
CA 02825785 2013-07-26
14
=
providing a cathode block based on carbon and/or graphite, which has at
least one groove for receiving a busbar, wherein at least one recess is pro-
vided in the wall of the cathode block which delimits the at least one groove,
lining at least a region of the at least one groove with a graphite foil,
inserting a busbar into the at least one groove,
pouring liquid cast iron into at least a portion of the at least one recess
between the graphite foil and the busbar, and
allowing the cast iron to solidify.
The static pressure of the cast iron column thrusts the graphite foil located
in the
groove into the at least one recess, where it is pressed in particular against
the
cathode block which delimits the at least one recess. It is thereby possible
with
particular ease to produce a cathode arrangement having a recess lined
partially
or completely by the graphite foil which has a particularly low electrical
contact
resistance between the busbar and the cathode block. During heating of the
elec-
trolysis cell for start up, particularly close contact is achieved by the
different ther-
mal expansions of steel or iron and the cathode material.
The graphite foil can be inserted and/or adhesively bonded into the groove
before
the busbar is inserted. A loose insertion of the graphite foil in the groove
can be
sufficient as a prefixing, since the graphite foil is preferably pressed by
the cast
iron against the at least one wall of the cathode block which delimits the
groove
during casting.
For producing the cathode block, a carbon-containing or graphite-containing
start-
ing material or a mixture of a plurality of such materials can be brought into
a
mould and then compacted to form a green body. The starting materials in this
case are preferably present in particulate or granular form. Then, the green
body
can be heated and thus carbonized and, if appropriate, graphitized. Within the
CA 02825785 2013-07-26
context of the present invention, it is possible to use both carbonized
cathode
blocks, which are understood to mean those cathode blocks which, during their
production, have been subjected to heat treatment of up to at most 1500 C and
preferably between 800 and 1200 C and have a high content of amorphous car-
bon, and also graphitized cathode blocks, which are understood to mean those
cathode blocks which, during their production, have been subjected to heat
treat-
ment of more than 2000 C and preferably between 2300 and 2700 C and have a
high content of graphite-like carbon. Finally, it is possible to use cathode
blocks
based on graphitic carbon, i.e. those which have not been graphitized but to
which
graphite has been added as starting material.
As the starting substances for carbonized cathode blocks, use is made for exam-
ple of a mixture of calcined anthracite, graphite and coal tar pitch and/or
petroleum
pitch, whereas graphitic cathode blocks are produced for example from a
mixture
containing graphite and coal tar pitch and/or petroleum pitch. Here, graphite
de-
notes both natural and synthetic graphite.
According to an advantageous development of the process, during the production
of the cathode block, the starting material containing carbon and/or graphite
is
introduced into a mould, which has a protrusion formed complementarily to the
at
least one recess.
Similarly, the at least one recess can be produced by subsequently removing
and/or eliminating cathode block material of the at least one wall of the
cathode
block which delimits the groove. It is possible in particular for the recess
to be
introduced subsequently by a milling process, in which case a milling head
used
for introducing the recess preferably has a cross section corresponding to the
recess.
1
CA 02825785 2013-07-26
16
The present invention also relates to a cathode arrangement obtainable by a
proc-
ess as described above.
Hereinbelow, the present invention is described purely by way of example on
the
basis of advantageous embodiments and with reference to the attached drawings.
In the drawings:
Figure 1 shows a cross section of a detail of an aluminium electrolysis
cell
having a cathode arrangement according to an exemplary embodi-
ment of the present invention,
Figure 2 shows a longitudinal section of the cathode arrangement of the alu-
minium electrolysis cell shown in Figure 1, and
Figures 3a-d show exemplary cross sections of recesses which are provided in a
groove of a cathode block according to the invention.
Figure 1 shows a cross section of a detail of an aluminium electrolysis cell
10
having a cathode arrangement 12, which at the same time forms the bottom of a
tank for an aluminium melt 14 produced during operation of the electrolysis
cell 10
and for a cryolite-aluminium oxide melt 16 located above the aluminium melt
14.
An anode 18 is in contact with the cryolite-aluminium oxide melt 16. At the
side,
the tank formed by the lower part of the aluminium electrolysis cell 10 is
delimited
by a carbon and/or graphite lining (not shown in Figure 1).
The cathode arrangement 12 comprises a plurality of cathode blocks 20, which
are
each connected to one another via a ramming mass 24 which has been inserted
into a ramming mass joint 22 arranged between the cathode blocks 20. A cathode
block 20 in this case comprises two grooves 26 arranged on the underside
thereof,
CA 02825785 2013-07-26
17
having a rectangular, specifically a substantially rectangular cross section,
wherein
a busbar 28 of steel likewise having a rectangular cross section is received
in each
groove 26. Here, each wall 32, 34 delimiting the groove 26 is lined by a
graphite
foil 30, which is indicated by dashed lines in Figure 1.
The grooves 26 are each delimited by two side walls 32 and a bottom wall 34 of
the cathode block 20, with a recess 36 extending substantially perpendicularly
into
the side wall 32 and having an approximately semicircular cross section being
provided in each of the side walls 32. Each recess 36 is delimited by an upper
and
a lower transition region 37 of the cathode block 20. In the present exemplary
embodiment, the transition regions 37 have an angled configuration, with an
angle
a between the adjoining portion of the groove wall and the wall of the recess
of 90
degrees. In this case, the interstice between the busbar 28 and the groove 26
lined with the graphite foil 30 is poured out in each case with cast iron 38,
and
therefore the graphite foil 30 is fixed between the cast iron 38 and the
cathode
block 20. In this case, the graphite foil 30 is pressed against the walls 32,
34 which
delimit the respective groove 26 by the cast iron 38. In the present exemplary
embodiment, the recesses 36 are also each lined by the graphite foil 30, in
which
case the cast iron 38 positively fills the lined recesses 36 and presses the
graphite
foil 30 against the cathode block 20 which delimits the recess 36. In this
way, a
low electrical contact resistance between the busbar 28 and the cathode block
20
is ensured over the entire cross section of the groove 26. The cast iron 38
forms
an encasement 39 for the busbar 28 and is integrally connected to the busbar
28.
In addition, the cast iron 38 received in the recesses 36 in each case forms a
posi-
tively-locking connection with the material of the cathode block 20 which
delimits
the recess 36, and this prevents movement of the busbar 28 connected to the
cast
iron 38 in the direction of the arrow 40. This prevents undesirable movement
of the
busbar 28 in the depth direction of the groove 26 or prevents even the busbar
28
from falling out of the groove 26.
CA 02825785 2013-07-26
18
=
Figure 1 specifically shows the cross section of the cathode arrangement 10 at
a
longitudinal-side end of the cathode block 20. In this case, the depth of the
grooves 26 of the cathode block 20 varies over the length of the grooves 26.
The
groove cross section in the region of the centre of the groove 26 is indicated
by a
dashed line 42 in Figure 1. In the present exemplary embodiment, the
difference
between the groove depth at the longitudinal-side ends of the groove 26 and in
the
centre of the groove 26 is approximately 10 cm. The width 44 of each groove 26
is
substantially constant over the entire groove length and is approximately 15
cm,
whereas the width 46 of each of the cathode blocks 20 is approximately 65 cm.
In the present exemplary embodiment, a plurality of anodes 18 and a plurality
of
cathode blocks 20 are arranged above one another in such a way that each anode
18 covers two cathode blocks 20 arranged alongside one another in width and
covers half a cathode block 20 in length, in each case two anodes 18 arranged
alongside one another covering the length of a cathode block 20.
Figure 2 is a longitudinal section showing the cathode block 20 shown in
Figure 1.
As can be seen from Figure 2, the groove 26, considered in its longitudinal
sec-
tion, tapers towards the centre of the cathode block 20 in the form of a
triangle, as
a result of which a substantially uniform vertical electric current density is
ensured
over the entire cathode length. As indicated by a dashed line in Figure 2, the
re-
cesses 36 here run parallel to the groove bottom 34 and are at a constant
distance
from the groove bottom 34 over the length of the groove 26. In the present
exem-
plary embodiment, the busbar 28, which is not shown in Figure 2 for the sake
of
greater clarity, has a bar-like form and has a rectangular longitudinal
section, such
as to form an interstice between the busbar and the groove bottom 34, which in-
terstice increases in size towards the centre of the groove 26 and can be
filled
either by cast iron 38 or by additional metal plates connected to the busbar
28.
CA 02825785 2013-07-26
19
= =
Similarly, it would also be possible to use a busbar 28 which is matched in
its
longitudinal section to the triangular profile of the groove 26.
Finally, Figures 3a to d show exemplary recesses 36, which are provided in a
groove of a cathode block 20 according to the invention, in cross section.
Here,
the recesses 36 each have a substantially semicircular cross section (Figure
3a), a
substantially trapezoidal cross section (Figure 3b) or a substantially
triangular
cross section (Figure 3c). The angle a of the transition regions 37 between
the wall
of the recess 36 and the adjoining portion of the groove wall 32, as seen from
the
inside of the cathode block 20, is in this case about 90 degrees in Figure 3a,
about
120 degrees in Figure 3b and about 125 degrees in Figure 3c. Figure 3d shows a
configuration in which a plurality of recesses 36 with a triangular cross
section, as
shown in Figure 3c, are arranged in succession in the depth direction of the
groove 26, in order to particularly reliably hold an inserted busbar 28. In
this case,
the transition regions 48 between two adjoining recesses 36 have an angle 13
of
about 70 degrees between the walls of two adjoining recesses 36, as seen from
the inside of the cathode block 20. The recesses 36 shown in Figures 3a to d
each
extend perpendicularly into the side wall 32 of the cathode block 20 which
delimits
the groove 26, such that they form a fixing with cast iron received in the
recesses
36, which is effective in the depth direction of the groove 26 and prevents
undesir-
able movement of the busbar 28 parallel to the depth direction of the groove
26
after the busbar 28 has been cast with cast iron 38.
List of reference symbols
Aluminium electrolysis cell
12 Cathode arrangement
14 Aluminium melt
16 Cryolite-aluminium oxide melt
18 Anode
CA 02825785 2013-07-26
20 Cathode block
22 Ramming mass joint
24 Ramming mass
26 Groove
28 Busbar
Graphite foil
32 Side wall
34 Bottom wall
36 Recess
37 Transition region between the wall of the recess and the adjoining
portion of the groove wall
38 Cast iron
39 Encasement
Arrow
42 Dashed line
44 Width of the groove 26
46 Width of the cathode block 20
48 Transition region between two adjoining recesses
a Angle between the wall of the recess and the adjoining portion of
the
groove wall
f3 Angle between the walls of two adjoining recesses
