Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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CATHODE BLOCK HAVING A SLOT WITH A VARYING DEPTH AND A FILLED
INTERMEDIATE SPACE
The present invention relates to a cathode block for an aluminum electrolysis
cell, to its
utilization and also to a cathode comprising it.
Electrolysis cells are for example used for the electrolytic production of
aluminum which, on
the industrial scale, is usually carried out according to the Hall-Heroult
process. In the Hall-
Heroult process, a molten mixture of aluminum oxide and cryolite is
electrolyzed. Here, the
cryolite, Na3[AlF6], is used to lower the melting point of 2045 C for pure
aluminum oxide to
approx. 950 C for a mixture containing a cryolite, aluminum oxide and
additional substances,
such as aluminum fluoride and calcium fluoride.
The electrolysis cell used in this process comprisese a cathode bottom which
is composed of a
plurality of, for example, up to 28 adjacent cathode blocks forming the
cathode. Here, the
intermediate spaces between the cathode blocks are usually filled with a
carbonaceous
ramming paste in order to seal the cathode against molten constituents of the
electrolysis cell
and in order to compensate for mechanical stresses which arise as the
electrolysis cell is put
into operation. The cathode blocks are usually made of a carbonaceous
material, such as
graphite, in order to withstand the thermal and chemical conditions prevailing
when the cell is in
operation. The undersides of the cathode blocks are usually provided with
slots in each of which
one or two busbars are arranged through which the current supplied via the
anodes is
discharged. Here, the intermediate spaces between the busbars and the
individual cathode
block walls bounding the slots are often filled with cast iron so that the
encasement of the
busbars with cast iron thus created connects the busbars to the cathode blocks
electrically and
mechanically. About 3 to 5 cm above the layer of liquid aluminum on the top
side of the
cathode, which is usually 15 to 50 cm thick, there is an anode, in particular
formed of individual
anode blocks. The electrolyte, in other words, the molten mass containing
aluminum oxide and
cryolite, is found between this anode and the surface of the aluminum. During
electrolysis,
which is carried out at approximately 1000 C, the aluminum thus formed, being
denser than the
electrolyte, settles below the electrolyte layer - in other words, as an
intermediate layer between
the top side of the cathode and the electrolyte layer. In electrolysis, the
aluminum oxide
dissolved in the molten mass is broken down into aluminum and oxygen by the
electrical current
flow. From the electrochemical point of view, the layer of liquid aluminum is
the actual cathode
since aluminum ions are reduced to elemental aluminum on its surface.
Nonetheless, in what
follows, the term cathode will not refer to the cathode from the
electrochemical point of view, in
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,
,
,
other words, the layer of liquid aluminum, but rather to the component
composed, for example,
of one or more cathode blocks and forming the bottom of the electrolysis cell.
One major disadvantage of the cathode arrangements used in the Hall-Heroult
process is their
comparatively poor resistance to wear, which manifests itself as a removal of
material from cathode
block surfaces during electrolysis. Here, due to a heterogeneous distribution
of current within the
cathode blocks, material is not removed evenly from the cathode block surfaces
over the length of
the cathode blocks but to a greater extent at the ends of the cathode blocks
with the result that after
electrolysis has proceeded for a certain amount of time, the surfaces of the
cathode blocks assume
a W-shaped profile. Due to the uneven removal of material from the cathode
block surfaces, the
useful life of the cathode blocks is limited by the locations with the
greatest removal of material.
In order to combat this problem, a cathode block is proposed in WO 2007/118510
A2 whose
slot, intended to accommodate one or more busbars, has with respect to the
cathode block
length a greater depth in the center than at the cathode block ends. In the
operation of the
electrolysis cell, this results in an essentially homogeneous vertical current
distribution over
the cathode block length, whereby the higher wear at the cathode block ends is
reduced and
the service life of the cathode thus extended. Here, the busbar(s) is or are
encased in cast
iron in the usual way, whereby this encasement is effected by pouring liquid
cast iron into the
intermediate space between the slot and the busbar(s). A cathode block of this
kind is
however encumbered with disadvantages. While the liquid cast iron is being
poured into the
intermediate space between the slot and the busbar(s) and afterwards, while
the electrolysis
cell comprising the cathode block is being put into operation and afterwards,
while the
electrolysis cell is being switched off and subsequently restarted and
afterwards, the cathode
block is exposed to comparatively large temperature changes which lead to the
cast iron and
the busbar(s) expanding or contracting relative to the cathode block. This
effect of expansion
or contraction can be amplified by the temperature gradients which occur. When
the phrase
'large temperature change(s)' is used in what follows, it should be understood
as indicating
that one or both of the effects mentioned - in other words, expansion /
contraction or
temperature gradient - is or are present. Since cast iron and the material of
the busbar(s)
have a higher coefficient of thermal expansion than the cathode block
material, when there is
a temperature increase, the cast iron and the busbar(s) expand relative to the
cathode block
while a temperature decrease on the other hand results in them contracting
relative to the
cathode block. This causes a deterioration in the electrical contact between
busbar, cast iron
and cathode block, especially in the case of the usual slots with their
rectangular cross-
section, and this in turn results in a higher electrical resistance of the
arrangement and thus a
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poor energy efficiency of the electrolytic process. Apart from this, before
the liquid cast iron
is poured into the intermediate space between the slot and the busbar(s), the
busbar(s) are
movable not only vertically but also horizontally such that they can move
uncontrollably in
the slot while the liquid cast iron is being poured in and then while the cast
iron is cooling
down and solidifying. This can also result in an uneven electrical contact
between busbar,
cast iron and cathode block. This too results in a higher electrical
resistance of the
arrangement and thus to poor energy efficiency of the electrolytic process.
Ramming paste
can also be used instead of cast iron. Ramming pastes based on anthracite,
graphite or
any mixture thereof can be used as a ramming paste. Preferably a graphite-
based ramming
paste is used.
When cast iron is mentioned hereinafter, it is to be understood that ramming
paste can be
substituted for the cast iron without this being stated explicitly each time.
An aspect of the present disclosure is directed to the provision of a cathode
block suitable in
particular for use in an aluminum electrolysis cell with which an essentially
homogeneous vertical
current distribution is achieved over the length of the cathode block while
the electrolysis cell is in
operation, which also has, even with large temperature changes, a low specific
electrical
resistance and in particular over an extended period of electrolysis a
permanently low specific
electrical resistance and a low transition resistance between the busbar and
the cathode block,
and which in the case of large temperature changes is robust with regard to
mechanical damage,
such as cracking.
According to an aspect of the present invention, there is provided a cathode
block for an
aluminum electrolysis cell on the basis of carbon or graphite, wherein the
cathode block has
at least one slot extending in the longitudinal direction of the cathode
block, wherein at least
one of the at least one slot has a varying depth when viewed over the length
of the cathode
block and in the at least one slot a busbar is provided, wherein the
intermediate space
between the at least one busbar and the wall bounding the at least one slot of
varying depth
is at least partially filled with steel, wherein the steel is selected from
the group consisting of
steel having a low carbon content of < 0.1 %, a silicon content of < 0.1 % and
a phosphorus
content of < 0.05 %, metals, alloys, composites of the aforementioned
materials, metal-
infiltrated graphite or carbon materials or electrically conductive masses.
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According to another aspect of the present invention, there is provided a
cathode
arrangement containing at least one cathode block as described above.
According to another aspect of the present invention, there is provided use of
a cathode
arrangement as described above for carrying out a fused-salt electrolysis to
produce metal.
According to an aspect of the invention, there is provided a cathode block for
an aluminum
electrolysis cell on the basis of carbon and / or graphite, wherein the
cathode block has at
least one slot extending in the longitudinal direction of the cathode block,
wherein at least
one of the at least one slot has a varying depth when viewed over the length
of the cathode
block and in the at least one slot at least one busbar is provided, wherein
the intermediate
space between the at least one busbar and the wall bounding the at least one
slot of varying
depth is at least partially filled with steel. Within the intention of the
present invention, instead
of steel another suitable metal can also be used, such as for example other
metals such as
copper or silver, alloys, composite materials of the aforementioned materials,
such as for
example steel bodies with a copper core, composite materials such as for
example metal-
infiltrated graphite or carbon materials or electrically conductive masses. As
metal, any metal
can be used in said metal-infiltrated graphite or carbon materials which has a
melting point
above the operating temperature of the electrolysis cell which is at around
1000 C. Copper
with a melting point of 1080 C constitutes a preferred metal. The proportion
of metal in the
composite material may lie between 40% and 90% by weight. The carbon in the
composite
material can be anthracite and
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the graphite composite material can contain graphitized or graphitic carbon as
graphite. The
term steel will hereinafter be used in this regard for all of these materials.
According to the invention, it was recognized that due to the at least partial
filling of the intermediate
space which, due to the installation of a bar-shaped busbar in the slot of a
cathode block, said slot
having a varying depth when viewed along the length of the cathode block, is
formed between the
busbar and the wall bounding the at least one slot of varying depth, a cathode
arrangement is
created simply and cheaply with steel, which due to the slot of varying depth
over the length of the
cathode arrangement is characterized by an essentially homogeneous vertical
current distribution
and has at the same time, despite the slot of varying depth, a permanently low
electrical resistance
and low transition resistance between the busbar and the cathode block, and
which in the case of
large temperature changes is robust as regards mechanical damage, such as
cracking. In the
cathode arrangements known from the prior art, the additional volume of the
intermediate space,
which arises with use of a conventional bar-shaped busbar due to the depth of
the slot varying over
the length of the cathode block, is filled entirely or partially with cast
iron. A greater quantity of cast
iron does however mean a higher heat input when the cast iron is poured in and
this results in
increased thermal stresses, due to which cracks can form in the cathode block,
namely in some
circumstances not forming until electrolysis is in progress, which can result
in poor operational
behavior or even in premature failure of the entire electrolysis cell.
Furthermore, such a voluminous
layer of cast iron results in a poor electrical contact between the busbar and
the cathode block via
the interjacent cast iron since the cast-iron layer between the time of its
solidification until the
operation of the electrolysis cell undergoes a net shrinkage since the
operating temperature of the
electrolysis cell at 850 to 950 C is considerably below the solidification
temperature of cast iron of
about 1150 C. It has admittedly already been proposed in WO 2007/118510 A2 to
overcome these
disadvantages by at least partially filling the intermediate space with steel
plates or even by using a
busbar with a geometry matched to the shape of the slot. However, these
solutions involve a
greater outlay as regards the process technology. In particular, furthermore,
manufacturing a busbar
with a geometry matched to the shape of the slot is expensive. By the
intermediate space being
filled according to the invention with steel, in other words with the material
of which conventional
busbars are made, this material behaves like the busbar in the case of
temperature changes and in
particular with abrupt temperature changes so that a net shrinkage is reliably
prevented and a poor
electrical contact in the intermediate space is thereby reliably prevented.
The intermediate space
can be filled by one or more filling materials made of steel, which can be
made separately by
casting, rolling, milling or other suitable shaping methods. This means that
the solution according to
the invention can be realized particularly simply, rapidly and inexpensively.
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According to some embodiments of the present invention, at least 50% of the
intermediate space
is filled with steel, preferably at least 75%, especially preferably at least
90%, more especially
preferably at least 95%, extremely preferably at least 98% and maximally
preferably 100%.
In some embodiments, in a development of the inventive concept, it is proposed
that the
steel with which the intermediate space is at least partially filled is
preferably the same as
that of which the at least one busbar is composed. In this case, the
coefficients of thermal
expansion of the two materials are the same so that mechanical stresses
between the
busbar and the steel with which the intermediate space is at least partially
filled are reliably
minimized during the heating up to set the operating temperature of the
electrolysis cell.
Preferably steel with a very high electrical conductivity is used here as
material for the
busbars and the shaped bodies filling the intermediate space. This steel is
characterized for
example by a low carbon content of <0.1%, a silicon content of <0.1% and a
phosphorus
content of <0.05%.
According to some embodiments of the present invention, at least 50% of the
intermediate space
is filled with steel, preferably at least 75%, especially preferably at least
90%, more especially
preferably at least 95% and extremely preferably at least 98%, and cast iron
is provided between
the steel and the wall bounding the at least one slot of varying depth. The
cast iron creates a
good mechanical connection between on the one hand the steel with which the
intermediate
space is at least partially filled and the at least one busbar and on the
other hand the cathode
block of the cathode arrangement, wherein due to the steel with which the
intermediate space is
at least 50% filled and preferably at least 90% filled, comparatively small
quantities of cast iron
are required so that the disadvantages previously described with regard to
filling the intermediate
space entirely with cast iron are at least to the greatest possible extent
overcome.
According to a further embodiment of the present invention, at least 50% of
the intermediate
space is filled with steel, preferably at least 75%, and especially preferably
at least 90%,
wherein one or more steel plates or balls are provided between the busbar and
the wall
bounding the at least one slot of varying depth.
In some embodiments, in order to achieve an especially even vertical current
density distribution
at the cathode block surface during the electrolytic process, it is proposed
in a development of
the inventive concept that at least one of the at least one slot or preferably
all of the slots of
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varying depth has or have at its or their longitudinal ends less depth than at
its or their center(s).
In this way, an even distribution of the electrical current supplied during
electrolysis operation is
achieved over the entire length of the cathode block, whereby an excessively
high electrical
current density is avoided at the longitudinal ends of the cathode block and
thus premature wear
at the ends of the cathode block is prevented. Due to such an even current
density distribution
over the length of the cathode block, movements in the aluminum melt caused by
the interaction
of electromagnetic fields during electrolysis are avoided, whereby is becomes
possible to arrange
the anode at a lower height above the surface of the aluminum melt. This
reduces the electrical
resistance between the anode and the aluminum melt and boosts the energy
efficiency of the
fused-salt electrolysis being carried out.
In some embodiments, preferably each of the at least one slot has a cross-
section which is
at least essentially rectangular, preferably rectangular.
In some embodiments, it is equally preferred that the at least one busbar is
at least essentially
cuboid or bar-shaped, preferably cuboid or bar-shaped.
According to some embodiments of the present invention, the cathode block
according to the
invention is thereby obtainable and is especially preferably obtained in that
a cathode block with
at least one slot of varying depth when viewed over the length of the cathode
block is provided,
that at least one preferably bar-shaped busbar is inserted into the at least
one slot, that the
intermediate space between the at least one busbar and the wall bounding the
at least one slot
of varying depth is at least partially filled with one or more shaped bodies
made of steel.
With this embodiment, it is especially preferable for the reasons given above
for only a part
of the intermediate space to be filled with one or more shaped bodies made of
steel, such
as for example at least 50%, preferably at least 75%, and especially
preferably at least
90%, and between them and the cathode block wall bounding the at least one
slot of
varying depth for molten cast iron to be placed and the molten cast iron
allowed to solidify.
A further subject matter of the present invention is a cathode arrangement
which comprises
at least one previously described cathode block.
Finally, the present invention relates to the use of a previously described
cathode arrangement
for carrying out a fused-salt electrolysis to produce metal, preferably to
produce aluminum.
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In what follows, the present invention is described purely by way of example
by means of an
advantageous embodiment and with reference to the accompanying drawing.
Here
Fig. 1 shows a longitudinal section of a cathode arrangement in
accordance with an
embodiment of the present invention.
Fig. 1 shows a longitudinal section of a cathode arrangement 12' in accordance
with an
embodiment of the present invention, namely inverted. The cathode arrangement
12' comprises a
cathode block 20 in whose bottom a slot 26 is provided whose depth varies over
the length of the
slot 26, namely in such a way that the slot 26 has a shallower depth at its
longitudinal end than at
its center. The difference between the slot depth at the longitudinal ends of
the slot 26 and at the
center - with respect to the longitudinal direction of the cathode block - of
the slot 26 amounts in
the present exemplary embodiment to about 5 cm. Here, the depth of the slot 26
at the two
longitudinal ends of the slot 26 is about 16 cm while on the other hand the
depth of the slot 26 at
the center - with respect to the longitudinal direction of the cathode block -
of the slot 26 is about
21 cm. The width 44 of each slot 26 is essentially constant over the entire
slot length and
measures approximately 15 cm while on the other hand the width 46 of the
cathode blocks 20 in
each case measures about 42 cm. A bar-shaped busbar 28 with a rectangular
longitudinal section
is arranged in the slot 26 wherein between the busbar 28 and the slot bottom
34, there is an
intermediate space 56 which becomes larger towards the center of the slot 26.
According to the
invention, this intermediate space 56 is at least partially and in the case
shown in Fig. 1 is entirely
filled with steel, namely with the same steel as that of which the busbar 28
is made.
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'
List of reference numbers
12, 12 cathode arrangement
20 cathode block
26 slot
28 busbar
34 bottom wall
56 intermediate space
