Note: Descriptions are shown in the official language in which they were submitted.
CA 02509839 2005-06-13
r
SGL CARBON AG
- 1
02/083 SGL October 10, 2003
Dr.HD ui
Cathode systems for the electrolytic production of
aluminum
Field of the Invention
The invention relates to cathode systems for the
electrolytic production of aluminum, in particular ones
having an improved operating life.
Background of the Invention
In the electrolytic production of metallic aluminum in
the Hall-Heroult process, aluminum oxide dissolved in
about 20 times its amount of molten cryolite (Na3[A1F6])
as flux is decomposed by means of direct current (at a
voltage of from 4 to 5 V and a current of from 80 000 to
500 000 A) at a temperature of about 960 °C in
electrolysis cells. The liquid aluminum collects on the
bottom of the carbon-lined tank serving as cathode under
the melt which largely protects it from reoxidation. The
carbon electrodes acting as anode (block or Soderberg
anodes) are gradually consumed by the oxygen which is
liberated.
Suitable electrolysis cells usually comprise a steel tank
which is lined on the inside with a thermally insulating
material. The bottom of the electrolysis cells comprises
a plurality of cathode blocks which are arranged in
parallel on the insulating material and whose joins
between one another and to the outer wall are sealed by
means of ramming paste paste comprising mixtures of
carbon granules and black coal tar or black coal tar
pitch. The material for the cathode blocks usually
comprises anthracite (now also graphite or coke or
CA 02509839 2005-06-13
SGZ CARBON AG
- 2
mixtures thereof with anthracite) which is calcined at
1 200 °C or above, then milled and classified according
to particle size. A suitable particle size fraction is
mixed with pitch and shaped to produce blocks. The pitch
binder is subsequently converted at elevated temperature
into a material consisting essentially of carbon. A
distinction is made here between graphitized (treatment
at about 3 000 °C), "semigraphitized" (treatment at about
2 300 °C), "semigraphitic" (graphitic particles, but
treatment of the block at about 1 200 °C) and amorphous
blocks (particles are not graphitized or only partially
graphitized, treatment of the block at about 1 200 °C).
The electric current is conducted away from the liquid
electrolyte and the aluminum melt covering the bottom by
means of steel bars or collectors which are connected
electrically to the cathode blocks.
Consumption of the material of the cathode blocks, too,
as a result of erosion is observed, and this determines
the life of the electrolysis cell, which is usually from
1 500 to 3 000 days. This erosion is not distributed
uniformly over the length of the cathode blocks, but
instead, especially in the case of graphitized cathode
blocks, two maxima in the removal of material are
observed in the vicinity of the side blocks and a minimum
is observed in the middle of the length of the cathode
blocks (W-shaped profile). Due to the nonuniform removal
of material, the useful life is naturally determined by
the areas having the greatest removal of material.
The useful life of the cathode blocks has been the
subject of numerous studies.
CA 02509839 2005-06-13
SGL CARBON AG
- 3
In J. Appl. Electrochemistry 19 (1989), pp. 580 to 588,
M. Sorlie and H.A. dye have reported a systematic study
on the various influences on the cathode materials, seals
and side blocks and their effects on the useful life.
In EP-A 0 284 298, improved sealing materials are
described for joining the cathode blocks. They have less
tendency to suffer from crack formation than known
sealing materials and thus reduce the risk of failure.
However, this measure does not alter the nonuniform
corrosion over the length of the cathode blocks.
An improvement in the flow of electricity between the
steel collectors (in this case configured as plates) and
the cathode by use of contact pins in the interface is
described in WO-A 97/48838 and in Aluminium 72, 1996,
number 11, pages 822 to 826. However, the installation of
these contact pins and the machining of the recesses on
the adjoining part incur considerable expense.
In WO-A 00/46426 a one-piece graphite cathode block is
described which has varying specific electrical
resistances in a direction parallel to the longitudinal
axis, with the resistance near the ends of the block
being higher than in the middle. These differences are
achieved by different heat treatment in graphitization,
namely the use of temperatures of from 2 200 to 2 500 °C
in the region of the ends and from 2 700 to 3 000 °C in
the region of the middle of the cathode blocks. Such
different temperatures can be achieved by lack of
insulation of the graphitising furnaces. Another
possibility is to choose appropriately different current
densities during graphitization, and thus distribute the
Joule heat nonuniformly over the cathode block to be
CA 02509839 2005-06-13
SGL CARBON AG
_ 4
graphitized. While the first possibility is to be
rejected for economic reasons, the second possibility
results in additional complication of the graphitization
step which has to be optimized in each case to the
specific cathode shape.
Another embodiment of a cathode having an improved life
is described in WO-A 00/46427. Here, a graphite cathode
is impregnated with a carbonizable substance under
reduced pressure at elevated temperature, with
temperature and time having to be selected sa that the
substance is sufficiently fluid to fill the pores of the
cathode, and the impregnated cathode is subsequently
carbonized at a temperature below 1 600 °C. This requires
additional working steps in manufacture of the cathode.
Finally, in WO-A 00/46428, a graphite cathode is
described whose specific electrical resistance is higher
in the direction perpendicular to its longitudinal axis
than in the direction of the longitudinal axis. This
difference in resistance is achieved by use of different
materials for producing the cathode, with at least some
being anisotropic, and by production under conditions
which promote orientation of particles, e.g. extrusion or
vibromoulding. This procedure requires specific
(additional) materials and adapted production processes.
All the measures mentioned thus imply particularly
increased costs in the production of the cathodes.
Summary of the invention
It is therefore an obj ect of the invention to make 'the
removal of material from the cathode blocks as uniform as
possible over the length of the blocks by means of simple
CA 02509839 2005-06-13
SGL CARBON AG
- 5
measures. In particular, it is desirable to leave the
production process for the cathodes uniform so that the
variety of production measures is not increased
unnecessarily.
This object is achieved by dividing the conduction of
electric currents from the carbon cathode into a
plurality of zones. This can be achieved by dividing the
contact composition or ramming paste providing the
electrical connection between the cathode and the
collectors into a plurality of zones in which materials
having differing conductivity or differing electrical
resistance are used, or by making up the steel bars or
collectors of a plurality of parts.
The invention accordingly provides cathode systems for
the electrolytic production of aluminum, which are
divided in the direction of their long axis on the side
of the power conduction from the cathode to the collector
into at least two parts having a differing electrical
resistances in such a way that the electrical resistance
between the free ends of the collector to the part of the
marginal zone of the cathode facing the collector is at
least 1.2 times the electrical resistance between the
free ends of the collector to the part of the middle of
the cathode facing the collector. To achieve this, either
the contact composition or the collector is divided into
zones of differing resistance.
For the present purposes, a cathode system is the
combination of cathode block, the collector and the
contact composition or ramming paste which effects the
electrical contact between cathode block and collector.
CA 02509839 2005-06-13
SGL CARBON AG
- 6
One way of realizing the solution provided by the
invention is to use different materials having a
different contact resistance between the collector and
the carbon material of the cathode along the length of
the cathode systems. A further way is to use multipart
collectors, with material and conduction cross section of
the collector parts being chosen so as to give the
desired resistance between a given point (facing the
melt) of the cathode block and the free ends of the
collectors.
The invention further provides processes for realising an
electrical contact between cathodes and collectors by
means of at least two contact compositions or ramming
I5 pastes having differing electrical conductivities,
processes for producing suitable collectors having the
multipart structure described and the use of different
contact compositions or ramming pastes or multipart
collector designs in cathode systems for the electrolytic
production of metallic aluminum.
Detailed Description of the Preferred Embodiments
The contact or tamping composition serves to provide
mechanical strength of the combination of collector and
cathode and also to provide electrical contact between
these parts of the cathode system. For example, it is
customary to fill the gap between collector and cathode
by pouring cast iron into it. An alternative is to use
ramming pastes made up of particulate carbon (anthracite
and/or graphite) and/or metal particles (powder, shoot,
fibers, whiskers or platelets; in particular of iron or
iron alloys such as steel) as filler and tars (in
particular black coal tar) or pitches (in particular
black coal pitch) as binder. The conductivity or
CA 02509839 2005-06-13
SGL CARBON AG
electrical resistance can be varied by choice of type
(composition, particle size and particle size
distribution) and amount of the conductive filler. It is
likewise possible to use adhesives, in particular two-
s component or multicomponent adhesives such as those based
on epoxy resins or phenolic resins, which are likewise
given the desired degree of conductivity by addition of
particulate metal and/or carbon in the form of anthracite
and/or graphite powders.
Preference is given to using at least two different
contact compositions or ramming pastes for establishing
the contact between cathodes and collectors, with the
boundary between zones of different materials running
perpendicular to the long axis of the collectors. The
contact resistance between collector and cathode in the
middle of the length of the cathode is lower than the
contact resistance in the region of the ends of the
cathode.
Furthermore, it is preferred that the contact composition
in the region of the middle of the cathode length is cast
iron. The contact composition used in the region of the
ends of the cathode length is preferably selected from
the group consisting of tars, tar pitches, synthetic
resins based on epoxy resins and/or phenolic resins and
adhesives based on epoxy resins and/or phenolic resins
filled with electrically conductive particles.
As electrically conductive particles, particular
preference is given to particles comprising particulate
carbon and metal particles in the form of powders, shoot,
fibers, whiskers and/or platelets.
CA 02509839 2005-06-13
SGL CARBON AG
_ g _
In "Light Metals 1999", edited by C.E. Eckert,
Warrendale, PA, USA, Toda et al., describe studies on the
contact resistance between collector rods and cathode
material when using two different contact compositions of
differing conductivity. Both the compositions studied led
to a very low contact resistance of less than 0.1 ~2/mm2.
However, there is no suggestion of using contact
compositions of differing conductivity side-by-side and
thus setting a lower contact resistance in one zone than
in an adjacent zone.
The division of the contact composition or the collector
into zones of differing conductivity or differing
electrical resistance is preferably carried out so that
the current density at the contact interface between the
cathode and the aluminum melt covering its surface is
effectively uniform over the length of the cathode. For
the purposes of the present invention "effectively
uniform" refers to an embodiment in which the current
density alters by no more than a factor of 2 over the
length of the cathode. Preference is given to a change by
a factor of not more than 1.5, particularly preferably by
a factor of not more than 1.3.
If different contact compositions based on adhesives or
contact compositions containing tars or pitches as binder
are used, the recess on the underside of the cathode
blocks is preferably filled with the contact compositions
of differing resistance to such an extent that only small
amounts exit upon installation of the collector rods.
According to the invention, it is also possible to
provide zones with electrical contact by casting molten
metal, preferably cast iron, into the joins. The various
possible ways of providing electrical contact can also be
CA 02509839 2005-06-13
SGL CARBON AG
_ g _
realized in succession on the same cathode blocks.
The resistivity of the contact compositions chosen can
easily be set in a targeted way by varying the components
of the composition. Here, the same binders or binder
mixtures as matrix can be filled with different (in terms
of type and/or amount) conductive additives; however, it
is also possible to vary the binders or binder mixtures
as a function of the type and amount of the conductive
filler in order to achieve a similar processing viscosity
and thus even out the forces acting on the cathode block
during installation.
Division of the collectors into zones having differing
resistances can be carried out so that the collector is
divided into pieces of differing cross section, with the
metals used being able to be identical or different, or
into pieces composed of metals having differing
conductivities, for example copper and steel. Of course,
it is also possible to vary cross section and material of
the collector parts simultaneously. Owing to the fact
that different metals usually have a different thermal
expansion, preference is given to achieving the desired
different resistance by use of the same metal and
different cross sections. However, it is also possible,
according to the invention, to use different metals, and
in this case metal parts having differing resistivities
are then preferably arranged in the direction of the side
of the carbon cathode facing the melt on a common support
made of a metal having a good conductivity (for example
copper) .
The zones of the collector having differing resistances
are separated from one another by a sheet-like insulator.
CA 02509839 2005-06-13
SGL CARBON AG
- 10
Preference is given to using a mica sheet (because of its
high thermal stability). The coherence of collectors
assembled in this way is ensured by means of suitable
fasteners, in particular sheet metal sleeves.
Doing without such fasteners is also possible if a
structure in which a metal bar made of a material having
a good electrical conductivity is wrapped with an
insulating film and a sheath made of material having a
poorer conductivity. This sheathing is extended to such
a distance that the bar is in direct contact with the
carbon cathode in the middle of the cathode system
length. In the outer region, i.e. toward the ends of the
cathode system, electrical connection is effected
exclusively via the sheath.
In all cases, it is possible to use two collector half-
rods or a single collector, where the single collector
and the half-rods have been divided in a suitable manner
into zones of differing resistance.
Preferred embodiments of the invention are illustrated by
the drawings. In these drawings,
fig. 1 shows a longitudinal section through a cathode
system having two contact compositions of
differing resistance; and
fig. 2 shows a cross section along the line II-II'
through a cathode system having one collector
which is connected to the cathode by means of
a contact composition; and
fig. 3 shows a cross section along the line III-III'
CA 02509839 2005-06-13
SGL CARBON AG
- 11
through a cathode system having one collector
which is connected to the cathode at this point
by the join being filled with cast iron; and
fig. 4 shows a longitudinal section through a cathode
end in which the collector (steel support) in
two-part form can be seen; and
fig. 5 shows a longitudinal section through a cathode
end in which the collector is made of different
metals having differing conductivities; and
fig. 6 shows a longitudinal section through a cathode
end in which the collector (steel support) in
two-part form can be seen, and in which the
configuration of the insulation with a right-
angle fit is shown in an enlarged form; and
fig. 7 shows an alternative embodiment to that shown
in fig. 6, here with an obtuse-angle fit; and
fig. 8 shows a longitudinal section through a cathode
having a three-part collector; and
fig. 9 shows a cross section through a collector which
is divided into two zones and whose parts are
connected mechanically in an electrically
insulating fashion by means of a sleeve; and
fig. 10 shows a cross section through the embodiment
shown in fig. 4 along the line X-X'; and
fig. 11 shows a cross section through the embodiment
shown in fig. 4 along the line XI-XI'; and
CA 02509839 2005-06-13
SGL CARBON AG
- 12
fig. 12 shows a cross section through a collector which
is divided into two zones and in which the zone
having the higher resistance is configured in
the form of a sheath.
Fig. 1 shows a longitudinal section through a cathode
system having a conventional collector rod 2 which is
connected via two contact compositions 13 and 14 having
different electrical resistances to the cathode 1. The
contact resistance from the collector 2 through the
contact composition 13 is greater than that via the
contact composition 14, according to the invention by a
factor of at least 1.2, preferably at least 1.5 and in
particular a factor of at least 2. In a preferred
embodiment, the material of the contact composition 14 is
cast iron, while the material of the contact composition
13 is a tar pitch, synthetic resin or synthetic resin
adhesive filled with carbon and/or metal particles.
Figs 2 and 3 show cross sections along the lines II-II'
and III-III', respectively, through the cathode system of
fig. 1. In these cases, too, the contact compositions 13
and 14 are chosen so that the contact resistance between
the collector 2 and the cathode 1 at the positions of the
sections II-II' (RII) and III-III' (RIII) obeys the
following relationship:
RIII.RII = 1:1.2 to 1:100; preferably from 1:2 to 1:80 and
in particular from 2:5 to 1:60.
Fig. 4 shows a longitudinal section through a cathode 1
with collector 2; the material 3 of the cathode is
selected from the group consisting of graphite,
semigraphitic carbon, semigraphitized carbon and
amorphous carbon, with preference being given to graphite
CA 02509839 2005-06-13
SGL CARBON AG
- 13
cathodes because of their better conductivity. In this
embodiment, the collector has two zones 4 and 5 which
have differing electrical resistances because of their
differing cross section. The materials 4 and 5 can be
identical or different. The two zones 4 and 5 are
electrically insulated from one another by an
intermediate layer 6 of an insulating material which has
to be able to withstand the operating temperature of the
cathode of about 960 °C without damage. Preference is
given to using mineral insulating materials such as mica
sheets. The required mechanical strength is achieved in
this embodiment by the zone 4 having the higher
conductivity also having the larger cross section. In a
further preferred embodiment, it is possible for the
parts 5 and 4 of the collector to be joined to one
another mechanically without being connected
electrically.
This can be achieved, for example, by, as shown in
fig. 9, a sleeve 15 made of a metal band, for example a
steel band, being placed around the collector comprising
the parts 4 and 5, with the sleeve 15 being insulated
from the collector parts 4 and 5 by an insulator 6', for
example an intermediate layer of mica. The parts 4 and 5
of the collector are electrically insulated from each
other by an insulating intermediate layer 6. The clamp
for the sleeve is not shown in this drawing.
Another embodiment of the invention with a multipart
collector 2 is shown in fig. 5; here, the collector is
composed of a thin plate 11 of a metal having a low
resistance, for example copper, and two thicker plates 9
and 10 of a metal having a higher resistance but also a
higher strength and stiffness, preferably steel. Plate 11
CA 02509839 2005-06-13
SGL CARBON AG
- 14
is electrically insulated from plate 9, but connected
electrically to plate 20. As a result, the resistance of
the path from the contact at the end 12 of the collector
to the zone of contact between plate 10 and the cathode
is lower than the resistance of the path from the contact
at the end of the collector 12 to the zone of contact of
plate 9 at the cathode 1. The specific electrical
resistance of the materials of plates 9, 10 and 11 and
their geometry (cross-sectional area) is chosen in
accordance with what has been said supra so that the
resistance from the end 12 of the collector to the zone
of contact of the cathode 1 and plates 10 and 9 has a
ratio of at least 1:1.2; in particular, the ratio of the
resistances is selected so that the current density at
the interface from the cathode 1 to the aluminum melt in
the bottom of the cell is as uniform as possible. In this
embodiment too, plates 9, 10 and 12 are mechanically
joined to one another, as is illustrated in principle in
fig. 9.
For the present purposes, "as uniform as possible" means
that the ratio of the current density in the marginal
zone to the current density in the middle zone of the
cathode 1 is not more than 2:1, preferably not more than
1.5:1 and particularly preferably not more than 1.2:1.
Figs 6 and 7 show alternative embodiments of the
insulation in the case of a two-part collector 2: in
fig. 6, the collector part 4 has a recess bounded by a
right angle, while in fig. 7 the recess in the part 4 has
an obtuse angle. The embodiment shown in fig. 7 has been
found to be advantageous for introducing the insulating
intermediate layer 6. In a further preferred embodiment
which is not shown, it is also possible to round the
CA 02509839 2005-06-13
SGL CARBON AG
- 15
angle so that a platelet-shaped mineral insulator such as
mica does not break.
Fig. 8 shows a construction of a cathode system
comprising a cathode 1 from which the electric current is
conducted away via a collector 2. In this embodiment, the
collectors 2 are each made up of three parts or zones 5,
7 and 8; once again, the sleeves are not shown for
reasons of clarity.
The resistances in the embodiment with collectors having
three zones of differing electrical resistance as shown
in fig. 8 from the end 12 to zone 5 (= Rizis) , from the end
12 to zone 7 (= Rlz,~) and from the end 12 to zone 8
(= Rlz~B) preferably have the ratios shown in the following
table:
Resistance ratio Rlzis ~ Rlzia R~zis ~ R~zi~
Maximum 100 50
Preferably 80 Preferably 45
Particularly Particularly
preferably 60 preferably 40
Minimum 1.5 1.2
Preferably 2 Preferably 1.5
Particularly Particularly
preferably 30 preferably 10
When assembling collector and cathode block, it has to be
ensured that in the region having a plurality of
collector zones as in the embodiment shown in fig. 4
(longitudinal section) and fig. 9 (cross section) the
electric current is conducted away only via the collector
zone in contact with the cathode.
CA 02509839 2005-06-13
SGL CARBON AG
- 16
To achieve this, a sheet-like insulator, for example a
mica sheet, is placed on the two sides of the collector
corresponding to the length of the divided zones so that
there is no electrical connection between the cathode and
the zone of lower resistance of the collector in this
region.
Fig. 10 shows a corresponding structure (section X-X' in
fig. 4) in which insulating sheets 6" and 6" ' are
placed on both sides of the collector in the region of
the divided zones and can be fixed in position by means
of the ramming paste 13. Otherwise, the ramming paste
provides, in a known manner, electrical contact (here
between the zone 5 and the cathode) and fixes collector
2 to cathode 1.
In the region of the section XI-XI' of the cathode system
in fig. 4, which is depicted in fig. 11, such insulation
at the side is naturally no longer necessary. For this
reason, the contact resistance between the cathode 1 and
the collector 2 via the ramming paste 13 is considerably
lower in this region because of the larger contact area,
which likewise leads to an increase in the current
density in this region.
When the collectors are divided into more than two zones,
insulation likewise has to be provided at the sides. The
necessity for insulation at the sides can be avoided if
the zone of higher resistance in the collector is
configured not as a plate facing the cathode but rather
in the form of a sheath which encloses the collector at
least to an extent which allows contact via the ramming
paste Fig. 12 shows a cross section of an embodiment of
this type, with the inner part 4 of the collector 2 being
CA 02509839 2005-06-13
. SGL CARBON AG
- 17 -
surrounded on three sides by a sheath 5 of higher
resistance. Here, insulation 6 is necessary only in the
interior of the collector; the lower cost of assembling
the collector is balanced by the increased complexity of
construction of this form, so that one or other form of
this embodiment is preferred depending on the particular
circumstances of the cell construction.
Examples:
Example 1
Graphite cathodes of conventional construction having a
length of 3 300 mm were provided with conventional steel
supports as collectors and connected electrically by
introduction of ramming pastes having differing
resistances. Here, the ratio of the specific resistance
of the tamping composition in the region near the edge to
that in the central region is 5:1. An electrolysis cell
equipped in this way with 20 cathode blocks was operated
at a current of 220 kA and 4.4 V for 1 000 days. For
comparison, cells were operated using the same cathode
system but a uniform tamping composition.
After the period of operation indicated, the cells were
emptied and dismantled, and the cathodes were examined
for wear. While in the case of the cathodes having a
uniform construction the removal of material in the two
edge zones was about 7.5 cm and that in the middle of the
cathode was only 2.5 cm, in the case of the configuration
according to the invention the removal of material in the
edge zones was measured as about 4 cm and that measured
in the middle was about 3.5 cm.
CA 02509839 2005-06-13
SGL CARBON AG
_ 28 _
Example 2
Graphite cathodes of conventional construction having a
length of 3 300 mm were provided with conventional steel
supports as collectors and connected electrically in a
conventional way by introduction of a tamping
composition. An electrolysis cell having 20 cathode
blocks was operated at a current of 220 kA and 4.4 V for
1 000 days (comparison). According to the invention, the
same cathodes were joined to steel supports like those
shown as 2 in fig. 4 whose ends had been milled down to
5/6 of their original thickness to a distance of about
700 mm from the end of the cathode. The transition to the
unmachined middle zone had an angle of about 160 ° as
shown in fig. 7. The milled area was covered with a mica
sheet 6 having a thickness of about 0.3 mm, and a steel
plate 5 having appropriate dimensions was fastened on top
of this at each end of the support with the aid of
sleeves of the type shown in fig. 9 which were insulated
with mica sheets.
The steel supports or collectors were insulated on both
sides by insertion of mica sheets as in the construction
shown in fig. 4 as far as the multipart zone of the
collector extended, and joined to the cathode by means of
a customary tamping composition.
After the period of operation indicated, the cells were
emptied and dismantled, and the cathodes were examined
for wear. While in the case of the cathodes having a
uniform construction the removal of material in the two
edge zones was about 8 cm and that in the middle of the
cathode was only 2 cm, in the case of the configuration
according to the invention the removal of material in the
CA 02509839 2005-06-13
SGL CARBON AG
- 19
edge zones was measured as about 3.5 cm and that measured
in the middle was about 3 cm.
It was found that such cathode systems according to the
invention make it possible to achieve a considerably more
uniform corrosion of the cathodes with a significant
reduction in the corrosion in the edge zone. Since the
useful life of the cathode is limited by the region of
greatest corrosion, use of cathode systems according to
the invention gives a significant increase in the life of
the cathodes in a simple and relatively easy way.
CA 02509839 2005-06-13
SGL CARBON AG
- 20
List of reference numerals
1 Cathode
2 Collector
3 Material of the cathode (graphite,
semigraphitic, semigraphitized or
amorphous carbon)
4 Collector zone having a higher
electrical conductivity than zone 5
Collector zone
6, 6' , 6" , 6" ' Insulating intermediate layer
7, 8 Collector zones
9, 10 Metal plates having a high electrical
resistance
11 Metal plates having a low electrical
resistance
12 End of the collector
' 13 Tamping composition
14 Cast iron
Sleeve comprising a metal band
