Note: Descriptions are shown in the official language in which they were submitted.
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WO 2013/068485 Al
ELECTROLYTIC CELL, IN PARTICULAR FOR PRODUCING
ALUMINUM, HAVING A TUB-SHAPED CATHODE
The present invention relates to an electrolytic cell,
in particular for producing aluminium, and to a cathode
which is suitable for use in such an electrolytic cell.
Electrolytic cells are used for example for the
electrolytic production of aluminium, which is usually
performed industrially in accordance with the Hall-
Heroult process. In the Hall-Heroult process, a melt
composed of aluminium oxide and cryolite is
electrolysed. Here, the cryolite, Na3[AIF6], is used to
lower the melting point from 2,045 C for pure
aluminium oxide to approximately 950 C for a mixture
containing cryolite, aluminium oxide and additives,
such as aluminium fluoride and calcium fluoride.
The electrolytic cell used in this process comprises a
cathode base, which may be composed of a plurality of
cathode blocks which are adjacent to one another and
form the cathode. In order to withstand the thermal and
chemical conditions prevailing during operation of the
cell, the cathode is usually composed of a carbonaceous
material. The undersides of each of the cathode blocks
are usually provided with grooves, in each of which at
least one busbar is arranged, by means of which the
current fed via the anodes is discharged. An anode, in
particular formed from individual anode blocks, is
arranged approximately 3 to 5 cm above the layer of
liquid aluminium, usually 15 to 50 cm high, located on
the cathode upper side, the electrolyte, that is to say
the melt containing aluminium oxide and cryolite, being
located between said anode and the surface of the
aluminium. During the electrolysis performed at
approximately 1,000 C, the formed aluminium settles
beneath the electrolyte layer due to its greater
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density compared with the electrolyte, that is to say
settles as an intermediate layer between the upper side
of the cathode and the electrolyte layer. During
electrolysis, the aluminium oxide dissolved in the melt
is cleaved by electric current flow to form aluminium
and oxygen. Considered electrochemically, the layer of
liquid aluminium is the actual cathode, since aluminium
ions are reduced at the surface thereof to form
elemental aluminium. Nevertheless, the term "cathode"
will not be understood hereinafter to mean the cathode
from an electrochemical viewpoint, that is to say the
layer of liquid aluminium, but the component part
forming the electrolytic cell base, for example
composed of one or more cathode blocks.
A significant disadvantage of the Hall-Heroult process
is that it is very energy intensive. Approximately 12
to 15 kWh of electrical energy are required to generate
1 kg of aluminium, which accounts for up to 40 % of the
production costs. In order to reduce the production
costs, it is therefore desirable to reduce the specific
energy consumption of this method where possible.
Due to the relatively high electrical resistance of the
melt in particular compared with the layer of liquid
aluminium and the cathode material, relatively high
ohmic losses in the form of Joule dissipation occur
particularly in the melt. In view of the comparatively
high specific losses in the melt, there is a
significant effort to reduce the thickness of the melt
layer and therefore the distance between the anode and
the layer of liquid aluminium as far as possible.
However, due to the electromagnetic interactions
present during the electrolysis process and the wave
formation thus produced in the layer of liquid
aluminium, there is thus the risk with an excessively
thin melt layer that the layer of liquid aluminium will
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come into contact with the anode, which may lead to
short circuits of the electrolytic cell and to
undesirable re-oxidation of the formed aluminium and to
the electrical instability of the electrolysis
operation and in particular to a fluctuation of the
call voltage. Short circuits that occur may also lead
to increased wear and therefore to a reduced service
life of the electrolytic cell. For these reasons, the
distance between the anode and the layer of liquid
aluminium cannot be reduced arbitrarily.
The driving force for the wave formation in the layer
of liquid aluminium is the inhomogeneous distribution
of the electric current density and the magnetic flux
density over the surface of the cathode, which leads to
a wave-formation-promoting distribution of the Lorentz
force density in the layer of liquid aluminium. Here,
the Lorentz force density is defined as the vector
product of the electric current density present at a
specific point and of the magnetic flux density present
at this point. One reason, inter alia, for the
inhomogeneous distribution of the electric current
density and of the magnetic flux density at the upper
side of the cathode is in turn the fact that the
current in the cathode and in the aluminium bath
preferably takes the path of lowest electrical
resistance. For this reason, the electric current
flowing through the cathode is concentrated primarily
on the lateral edge regions of the cathode, where the
cathode is connected to the busbars contacting said
cathode, since the resultant electrical resistance with
the current flow via the edge regions to the surface of
the cathode is lower than with the current flow via the
centre of the cathode to the surface of the cathode, in
which case a longer route or electrical path has to be
covered than with the current flow via the edge regions
to the surface of the cathode.
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Besides an intensified wave formation in the layer of
liquid aluminium, the inhomogeneous current density
distribution and in particular the increased current
density at the lateral edge regions of the cathode, as
considered in the transverse direction of the cathode,
compared with the current density in the centre of the
cathode also leads to an intensified wear of the
cathode in the lateral edge regions, which typically
leads after relatively long operation of the
electrolytic cell to a characteristic approximately W-
shaped wear profile of the cathode in the cross section
of the cathode.
In order to reduce the specific energy consumption of
an electrolytic cell, it has been proposed recently to
use, in electrolytic cells, cathodes of which the upper
side, as considered in the cross section of the
cathode, is embodied in the form of a V-shaped tub.
Here, the indentation in the cathode surface configured
in the form of a V-shaped tub causes the current
density in the lateral edge regions of the cathode to
be reduced, whereby the wave formation potential and
also the wear are reduced in these regions.
However, even with use of such a tub-shaped cathode, an
undesirably high wave formation occurs in the layer of
liquid aluminium, and, with a low thickness of the
cryolite layer between the aluminium and the anode,
leads to instabilities and restricts the achievable
energy efficiency during electrolysis. In addition, an
undesirably high level of non-uniform wear on the upper
side of the cathode also occurs during electrolysis
with the use of such a cathode. These two factors
reduce the service life of the electrolytic cell and
therefore the economical viability thereof. This is due
to fact that, even with the use of such a cathode, as
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viewed over the surface of the cathode, a relatively
inhomogeneous current density distribution and
inhomogeneous distribution of the magnetic flux density
are still provided, even if these are lower than with a
cathode not embodied in a tub-shaped manner. This is
because, inter alia, the busbars contacting the cathode
are usually connected via collector bars to one or more
external current feeds, wherein the distance between
the end of the external current feed and the ends,
facing this end, of the individual busbars differs,
such that the electrical path from the external current
feed to the point at which an individual busbar
contacts the underside of the cathode is of a different
length for different busbars. Longer electrical paths
however have a higher electrical resistance for a
predefined material compared with shorter electrical
paths. The electric current flow through those busbars
of which the point contacting the underside of the
cathode is closer to the external current feed is thus
promoted, as a result of which a greater current flow
occurs in the edge regions or longitudinal portions of
the cathode arranged over these busbars than in the
edge regions or longitudinal portions of the cathode
arranged over a busbar of which the point contacting
the underside of the cathode is distanced further from
the external current feed.
The object of the present invention is therefore to
provide an electrolytic cell, which, during operation
thereof, has a reduced specific energy consumption and
an increased service life. In particular, an
electrolytic cell is to be provided in which the
thickness of the melt layer is reduced, without the
resultant occurrence of instabilities, such as short
circuits or re-oxidations of the formed aluminium or
fluctuations of the electrolytic cell voltage, caused
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by increased wave formation tendency in the layer of
liquid aluminium.
This object is achieved in accordance with the
invention by the provision of an electrolytic cell
according to Patent Claim 1 and in particular by the
provision of an electrolytic cell for producing
aluminium which comprises a cathode, a layer of liquid
aluminium arranged on the upper side of the cathode, a
melt layer on top thereof, an anode above the melt
layer, at least one busbar and preferably at least two
busbars contacting the cathode from the underside
thereof in a current-feeding manner, and at least one
external current feed, wherein the at least one or each
external current feed is electrically conductively
connected at a respective connection point to at least
one busbar and preferably to at least two of the
busbars, wherein the upper side of the cathode is tub-
shaped as considered in the cross section of the
cathode, wherein the tub has two edge regions and a
base region, which is arranged between the edge regions
and is lowered relative to the edge regions as viewed
in the width direction of the cathode, wherein, between
each of the two edge regions and the base region, a
side wall region connecting the corresponding edge
region and the base region is provided, wherein i) the
width of at least one of the base region and the edge
regions varies over the length of the cathode, and/or
ii) the height of the upper side of the cathode
determined from the underside of the cathode varies
over the length of the cathode.
The width of the base region or of an edge region of
the upper side of the cathode is to be understood
within the context of the present invention to mean the
extension of the base region or of the edge region
measured in the width direction of the cathode, that is
N
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to say the distance from one end of the base region or
edge region, as considered in the cross section of the
cathode or as considered in the width direction of the
cathode, to the other end of the base region or edge
region.
Further, the wording "the height of the upper side of
the cathode determined from the underside of the
cathode" within the context of the present invention
refers to the distance of any point on the upper side
of the cathode from the point on the underside of the
cathode arranged vertically below the first-mentioned
point.
Within the context of the present invention an external
current feed is understood to mean an arbitrary
electrical conductor which is arranged outside the
. cathode and which feeds current to the busbar(s) or
away therefrom. Here, the external current feed may be
connected directly to the busbar(s) via a connection
point in each case or may be connected indirectly to
the busbar(s) via a collector bar arranged between the
external current feed and the busbars. In the latter
case, a connection point is understood to mean the
point at which the external current feed is connected
to the collector bar connected to the busbars. In other
words, the wording "connection point between the at
least one busbar and preferably at least two busbars
and the external current feed" refers to the point at
which the electrical paths starting from the busbar(s)
electrically conductively connected (directly or
indirectly) to the external current feed converge and
transition into the external current feed. In this
context, the term "electrical path" refers to the
current path of lowest electrical resistance between
two points.
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It has been found in accordance with the invention
that, by means of a cathode having the cross sectional
shape of a tub with a varying width and/or height of
the base region and/or of the edge regions of the tub
over the length of the cathode, a homogenisation of the
electric current density and of the magnetic flux
density is achieved at the upper side of the cathode,
more specifically not only considered over the cross
section of an isolated longitudinal portion of the
cathode, but in particular also over the entire surface
of the cathode, that is to say both in the longitudinal
direction and in the width direction of the cathode.
This is because the cathode material has a low
conductivity compared with the layer of liquid
aluminium arranged thereabove, as a result of which the
current flow in the base region of the tub-shaped cross
section is favoured compared with the edge regions of
the cathode, which are raised compared with the base
region. A widening of the base region or a reduction of
the width of an edge region of the cathode in a
longitudinal portion of the cathode accordingly leads
to a promotion of the current flow in this longitudinal
portion on the whole (that is to say compared with
another longitudinal portion) and leads to a promotion
of the current flow in the base region with respect to
the current flow in the edge regions of this
longitudinal portion of the cathode, whereas the
current flow in a longitudinal portion of the cathode
in which the base region is less wide and the edge
regions of the cathode are wider is reduced on the
whole and the current flow in the base region is
reduced with respect to the current flow in the edge
regions of this longitudinal portion of the cathode.
Equally, the current flow in this longitudinal portion
is promoted on the whole for this reason by a reduction
of the height of the cathode upper side in a
longitudinal portion of the cathode, whereas the
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current flow in a longitudinal portion of the cathode
in which the upper side of the cathode has a greater
height is reduced on the whole. Due to the stepped
(with respect to the width and/or height) embodiment of
the edge regions and base regions of the cathode, the
current flow between the individual longitudinal
portions of the cathode and over the width of each
longitudinal portion can thus be adjusted such that a
more uniform current density is produced, as viewed
over the surface of the cathode. In particular, due to
the stepped embodiment of the edge regions and base
regions of the cathode, the current flow through those
longitudinal portions of the cathode which are arranged
above busbars of which the point contacting the
underside of the cathode is closer to the external
current feed can be reduced by widening the edge
regions and/or raising the edge regions and/or base
regions, and the current flow through those
longitudinal portions of the cathode which are arranged
above busbars of which the point contacting the
underside of the cathode is distanced further from the
external current feed can be increased by reducing the
width of the edge regions and/or reducing the height of
the edge regions and/or base regions, such that the
current flow and the magnetic flux through the
individual longitudinal portions of the cathode can be
homogenised independently of the distance of said
longitudinal portions from the external current feed.
Due to the uniform current density and magnetic flux
density, as viewed over the surface of the cathode, the
wave formation in the layer of liquid aluminium is
drastically reduced, and the wear of the cathode as
considered over the surface thereof is homogenised. As
a result of this, the electrolytic cell according to
the invention has a reduced specific energy consumption
and an increased service life during operation thereof.
In particular, the thickness of the melt layer can be
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reduced with the electrolytic cell according to the
invention without encountering instabilities as a
result, such as short circuits or re-oxidations of the
formed aluminium or fluctuations of the electrolytic
cell voltage, caused by increased wave formation
tendency in the layer of liquid aluminium. On the
whole, a wave formation in the layer of liquid
aluminium is thus effectively avoided during operation
of the electrolytic cell according to the invention,
and a high energy efficiency with simultaneous high
stability and reliability of the electrolysis operation
is attained. Here, it is particularly advantageous if
the above-described measures used to homogenise the
electric current density on the cathode upper side,
specifically the adjustment of the width and/or height
of individual longitudinal portions of the tub-shaped
,
cathode, can be easily adjusted to one another, such
. that the same bath volume as with the use of a
conventional cathode is produced between the cathode
and the anode used in the electrolytic cell according
to the invention, even with reduced distance between
the anode and the layer of liquid aluminium.
In accordance with a particularly advantageous
embodiment of the invention, at least one edge region
of the cathode comprises at least two longitudinal
portions each having a different width, wherein the
longitudinal portion of the edge region which is
connected via the shortest electrical path to the
connection point closest for it has the greatest width
of all longitudinal portions of the edge region. In
this context, the connection point closest for a
longitudinal portion of an edge region is the
connection point between at least one busbar, and
preferably at least two busbars, and the external
current feed, which is connected to the longitudinal
portion of the cathode via the shortest electrical
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path. Since, in this embodiment, the edge region is
widened in the longitudinal portion which has the
shortest electrical path to the closest connection
point, the electrical resistance of this longitudinal
portion of the cathode is increased compared with the
electrical resistances of the other longitudinal
portions, and therefore the current flow through this
longitudinal portion of the cathode is reduced and that
through the other longitudinal portions of the cathode
is increased, such that a uniform current density
distribution is achieved, as viewed over the individual
longitudinal portions of the cathode.
In principle, at least one of a plurality of
longitudinal portions of the edge region or of the edge
regions having different widths may have a constant
width over the respective longitudinal portion, and/or
the width of at least one longitudinal portion may
decrease gradually from the longitudinal-side end
thereof closer to the closest connection point to the
longitudinal-side end distanced further. The widths of
all longitudinal portions of the edge region or of the
edge regions having different widths are preferably
constant, such that one of the edge regions or both
edge regions of the cathode (based on the width
thereof) are step- or stair-shaped, or the widths of
all longitudinal portions of the edge region or of the
edge regions having different widths decrease gradually
from the longitudinal-side end thereof closer to the
closest connection point to the longitudinal-side end
distanced further.
A particularly uniform distribution of the electric
current density is achieved over the surface of the
cathode if at least one edge region of the cathode
comprises at least three longitudinal portions each
having a different width, wherein each of the
,
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longitudinal portions which is connected to the closest
connection point via a longer electrical path than
another longitudinal portion has a smaller width than
the other longitudinal portion. The influence of the
length of the electrical path from the closest
connection point to the respective longitudinal portion
of the edge region of the cathode on the electric
current flow is thus compensated for particularly
effectively.
In accordance with a further preferred embodiment of
the present invention, the base region of the cathode
comprises at least two longitudinal portions each
having a different width, wherein the longitudinal
portion of the base region which is connected via the
shortest electrical path to the connection point
closest for it has the smallest width of all
longitudinal portions of the base region. Due to the
reduction of the width of the base region, the
electrical resistance of this longitudinal portion of
the cathode is increased, and the current flow is thus
diverted partly from this longitudinal portion, in
which the highest current flow would otherwise occur,
into adjacent longitudinal portions of the cathode,
such that a homogenisation of the current density is
achieved on the whole over the surface of the cathode.
In principle, one or more longitudinal portions of the
base region having different widths may also have a
uniform width over the respective longitudinal portion
in this embodiment of the present invention, and/or the
width of at least one or more longitudinal portions may
decrease gradually from its longitudinal-side end
arranged closer to the closest connection point to the
longitudinal-side end distanced further. Also in this
embodiment of the present invention, it is preferable
for the widths of all longitudinal portions of the base
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region having different widths to be constant, such
that the base region of the cathode (based on the width
thereof) is step- or stair-shaped, or for the width of
all longitudinal portions of the base region having
different widths to decrease gradually from the
longitudinal-side end thereof arranged closer to the
closest connection point to the longitudinal-side end
distanced further.
The base region of the cathode preferably comprises at
least three longitudinal portions each having a
different width, wherein each of the longitudinal
portions which is connected via a longer electrical
path to the connection point closest for it than
another longitudinal portion has a greater width than
the other longitudinal portion. A particularly high
uniformity of the electric current density is thus
achieved over the surface of the cathode.
In a development of the inventive concept, it is
proposed for at least one edge region of the cathode to
comprise at least two longitudinal portions each having
a different height, wherein the longitudinal portion of
the edge region of the cathode which is connected via
the shortest electrical path to the closest connection
point has the greatest height of all longitudinal
portions of the edge region, as determined from the
underside of the cathode. The electrical resistance of
the longitudinal portion of the edge region of the
cathode which is connected via the shortest electrical
path to the closest connection point is thus increased
compared with the electrical resistances of the other
longitudinal portions, such that the current flow
through this longitudinal portion of the cathode is
reduced and that through the other longitudinal
portions of the cathode is increased, such that a
uniform current density distribution is achieved, as
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viewed over the individual longitudinal portions of the
cathode.
Here, the aforementioned embodiments can also be
combined with one another, for example more
specifically such that the edge region of the cathode
in the longitudinal portion which is connected via the
shortest electrical path to the closest connection
point has a greater width and a greater height than the
longitudinal portions which are connected via a longer
electrical path to the closest connection point.
In a further development of the inventive concept, the
base region of the cathode may comprise at least two
longitudinal portions each having a different height,
wherein the longitudinal portion of the base region of
the cathode which is connected via the shortest
electrical path to the closest connection point has the
greatest height of all longitudinal portions of the
base region, as determined from the underside of the
cathode. Also in this embodiment of the present
invention, a particularly good homogenisation of the
distribution of the electric current density over the
cathode surface is achieved.
In principle, at least one of a plurality of
longitudinal portions of one of the edge regions, the
two edge regions and/or the base region having
different heights may have a uniform height over the
respective longitudinal portion, and/or the height of
at least one longitudinal portion may gradually
decrease from the longitudinal-side end thereof closer
to the closest connection point to the longitudinal-
side end distanced further. The heights of all
longitudinal portions of one of the edge regions, the
two edge regions or the base region having different
heights are preferably constant, and therefore one of
,
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the edge regions, both edge regions or the base region
of the cathode (based on the height thereof) is/are
step- or stair-shaped, or the heights of all
longitudinal portions of one of the edge regions, the
edge regions or the base region having different
heights decrease gradually from the longitudinal-side
end thereof closer to the closest connection point to
the longitudinal-side end distanced further.
Within the scope of the invention it is also possible
for the height of the cathode upper side, as viewed in
the longitudinal direction of the cathode, to vary for
the edge region and the base region of one or more
longitudinal portions in opposite direction, that is to
say for example the height of both edge regions of a
longitudinal portion is greater than that of the edge
regions of the adjacent longitudinal portions, wherein
. however the height of the base region of this
longitudinal portion is smaller than that of the base
regions of the adjacent longitudinal portions. It is
preferable however for the height of the edge regions
and the height of the base region of each longitudinal
portion of the cathode to vary in parallel, that is to
say the heights both of the edge regions and of the
base region of each longitudinal portion of the cathode
are greater or smaller than those of the edge regions
and of the base region of the adjacent longitudinal
portions.
A particularly high uniformity of the distribution of
the electric current density at the cathode upper side
is achieved if the edge region of the cathode comprises
at least three longitudinal portions each having a
different height, wherein each of the longitudinal
portions which is connected to the connection point
closest for it via a longer electrical path than
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another longitudinal portion has a lower height than
the other longitudinal portion.
Alternatively to the above-mentioned embodiment or
preferably additionally to the above-mentioned
embodiment, the base region of the cathode may comprise
at least three longitudinal portions each having a
different height, wherein each of the longitudinal
portions which is connected to the closest connection
point via a longer electrical path than another
longitudinal portion has a lower height than the other
longitudinal portion. A particularly uniform
distribution of the current density in the base region
of the tub formed by the cathode upper side is thus
achieved.
A particularly favourable distribution of the electric
current density can be achieved if the ratio of the
maximum to minimum width of at least one of the edge
regions of the cathode is between 2:1 and 1.05:1,
preferably 1.5:1 and 1.05:1 and more preferably between
1.3:1 and 1.05:1 and/or the ratio of the maximum to
minimum height of at least one of the edge regions of
the cathode is between 2:1 and 1.05:1, preferably
between 1.5:1 and 1.05:1 and more preferably between
1.3:1 and 1.05:1 and/or the ratio of the maximum to
minimum width of the base region of the cathode is
between 2:1 and 1.05:1, preferably between 1.5:1 and
1.05:1 and more preferably between 1.3:1 and 1.05:1
and/or the ratio of the maximum to minimum height of
the base region of the cathode is between 2:1 and
1.05:1, preferably between 1:5 and 1.05:1 and more
preferably between 1.3:1 and 1.05:1.
In the embodiments of the present invention in which
the height of the cathode upper side varies over the
length of the cathode, the difference between the
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maximum height in the edge regions of the cathode and
the minimum height in the edge regions of the cathode
and/or the difference between the maximum height in the
base region of the cathode and the minimum height in
the base region of the cathode is preferably less than
30 cm, more preferably less than 20 cm and even more
preferably 10 cm at most. Equally, it is preferable if
the difference between the maximum height in the edge
regions of the cathode and the minimum height in the
base region of the cathode is at most 50 % of the
distance between the highest point on the cathode upper
side and the cathode underside.
In the embodiments of the present invention in which
the width of one of the edge regions, of both edge
regions or of the base region varies over the length of
the cathode, the difference between the maximum width
of the base region and the minimum width of the base
region, as considered over the entire longitudinal
extension of the cathode, is preferably less than 30
cm, more preferably less than 20 cm, and even more
preferably less than 10 cm. Equally, it is preferable
for the difference between the maximum width of the
base region and the minimum width of the base region,
as considered over the entire longitudinal extension of
the cathode, to be at most 20 % and more preferably at
most 10 % of the cathode length.
In accordance with a development of the inventive
concept, the electrolytic cell comprises at least two
busbars contacting the cathode from the underside
thereof in a current-feeding manner, wherein the, or
each, external current feed is electrically
conductively connected at a respective connection point
to at least two of the busbars, and the at least two
busbars contacting the cathode from the underside
thereof in a current-feeding manner are arranged
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parallel to one another and at a fixed distance from
one another, extend over the entire width of the
cathode, and contact the cathode from the underside
thereof in a current-feeding manner, wherein the
individual busbars are electrically conductively
connected via one of their ends to a collector bar or
via each of their ends to separate collector bars, and
the collector bar(s) is/are electrically conductively
connected to one or more external current feeds.
Alternatively, the individual busbars may be arranged
parallel to one another and at a fixed distance from
one another, but may not extend over the entire width
of the cathode. For example, the individual busbars may
only extend over approximately half of the width of the
cathode. In this embodiment, 2 busbars for example are
arranged in succession, as viewed in the width
direction of the cathode, that is to say the busbars
are almost formed in a number of pieces, wherein each
of these two busbars arranged in succession is
connected via its end facing the cathode to an external
current feed, possibly via a collector bar. In this
embodiment, only the adjacent busbars, as viewed in the
longitudinal direction of the cathode, are naturally
each connected at a respective connection point to an
external current feed.
In accordance with a further preferred embodiment of
the present invention the electrolytic cell comprises 2
to 60, preferably 10 to 48, more preferably 16 to 40,
even more preferably 20 to 40, and most preferably 36
busbars arranged parallel to one another and at a fixed
distance from one another, extending over the entire
width of the cathode and contacting the cathode from
the underside thereof in a current-feeding manner, and
2 to 6 external current feeds.
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Here, the cathode of the electrolytic cell may be
composed for example of 2 to 60, preferably 10 to 48,
more preferably 16 to 40, even more preferably 20 to
40, and most preferably 36 cathode blocks arranged side
by side, wherein each of the cathode blocks on the
underside thereof has at least one groove extending in
the longitudinal direction of the cathode block or in
the width direction of the cathode, at least one busbar
being arranged in said groove.
In order to increase yet further the uniformity of the
distribution of the electric current density over the
cathode surface and in particular to prevent a
superelevation of the electric current density in the
edge regions of the cathode upper side, it is proposed
in a further development of the inventive concept for
each of the grooves to have a rectangular cross section
and a depth varying over their length, wherein each of
the grooves at the longitudinal-side end thereof has a
shallower depth than in the centre thereof. Here, a
groove, as considered in the cross section of the
cathode, may have a substantially triangular form, for
example.
In order to achieve in particular a uniform
distribution of the electric current density even
within the edge regions of the cathode, it is proposed
in a development of the inventive concept for at least
one of the two edge regions and preferably both edge
regions of the cathode to run in a downwardly sloping
manner towards the centre of the cathode, as considered
in the cross section of the cathode and in the width
direction of the cathode, wherein the angle of
inclination of the edge region or of the edge regions
with respect to the horizontal plane is preferably
between 20 and 45 , more preferably between 3 and 20 ,
and even more preferably between 10 and 15 .
CA 02854937 2014-05-07
=
- 20
Here, it is preferable if at least one of the two edge
regions and preferably both of the edge regions
extends/extend over at least 30 %, preferably over at
least 50 %, more preferably over at least 75%, and even
more preferably over 100 % of the width thereof, as
considered in the cross section of the cathode and in
the width direction of the cathode, in a manner sloping
downwardly towards the centre of the cathode. However,
at least one of the edge regions may also run
substantially horizontally.
In accordance with a further preferred embodiment of
the present invention, the base region of the cathode
runs in a planar manner, at least in regions, wherein
the surface of the base region with respect to the
plane running in the vertical direction has an angle
between -20 and 20 , preferably between -10 and 10
and more preferably of 0 .
The upper side of the cathode is preferably formed in a
tub-shaped manner over at least 25 %, preferably at
least 50 %, in particular preferably at least 75 %,
more preferably at least 90 %, and most preferably
approximately 100 % of the length of the cathode, as
considered in the cross section of the cathode, wherein
the tub has two edge regions and a base region, which
is arranged between the edge regions and is lowered
relative to the edge regions, as viewed in the width
direction of the cathode, wherein, between each of the
two edge regions and the base region, a side wall
region connecting the corresponding edge region and the
base region is provided.
Here, it is preferable if i) the width of at least one
of the base region and the edge regions varies over at
least 25 %, preferably at least 50 %, in particular
= CA 02854937 2014-05-07
- 21.-
preferably at least 75 %, more preferably at least 90 %
and most preferably approximately 100 % of the length
of the cathode, and/or ii) the height of the upper side
of the cathode, determined from the underside of the
cathode, varies over at least 25 %, preferably at least
50 %, in particular preferably at least 75 %, more
preferably at least 90 %, and most preferably
approximately 100 % of the length of the cathode.
The present invention also relates to a cathode for an
electrolytic cell, in particular for an electrolytic
cell for producing aluminium, wherein the upper side of
the cathode is tub-shaped as considered in the cross
section of the cathode, wherein the tub comprises two
edge regions and a base region, which is arranged
between the edge regions and is lowered relative to the
edge regions, as viewed in the width direction of the
= cathode, wherein, between each of the two edge regions
and the base region, a side wall region connecting the
corresponding edge region and the base region is
provided, wherein i) the width of at least one of the
base region and the edge regions varies over the length
of the cathode, and/or ii) the height of the upper side
of the cathode, as determined from the underside of the
cathode, varies aver the length of the cathode. The
preferred embodiments described above in respect of the
cathode contained in the electrolytic cell according to
the invention also apply to the cathode according to
the invention.
The invention will be described hereinafter purely by
way of example on the basis of advantageous embodiments
with reference to the accompanying drawings, in which:
Fig. 1
shows a perspective view of a portion of a
cathode of an electrolytic cell in accordance
with an embodiment of the present invention,
= CA 02854937 2014-05-07
=
- 22
Fig. 2 shows a partially cut-away perspective view
of a cathode of an electrolytic cell in
accordance with a further embodiment of the
present invention,
Fig. 3a shows a partially cut-away perspective view
of a cathode of an electrolytic cell in
accordance with a further embodiment of the
present invention,
Fig. 3b shows a front view of the cathode of Fig. 3a,
and
Fig. 4 shows a plan view of an electrolytic cell in
accordance with an embodiment of the present
invention.
Fig.1 shows a perspective view of a cathode 10 of an
electrolytic cell in accordance with an embodiment of
the invention.
The cathode 10 composed of a carbonaceous material has
an upper side 12, on which the layer of liquid
aluminium of the electrolytic cell is arranged during
operation of the electrolytic cell, for example in
accordance with the Hall-Heroult process. In practice,
the cathode 10 is composed of a plurality of cathode
blocks arranged side by side as viewed in the
longitudinal direction y of the cathode, wherein the
longitudinal directions of the individual cathode
blocks each run in the width direction x of the cathode
10. The busbars which contact the cathode 10 from the
underside 14 thereof in a current-feeding manner and
are electrically conductively connected to at least one
external current feed are not shown in Fig. 1. Here,
each busbar is preferably inserted in a groove which is
CA 02854937 2014-05-07
- 23
provided in each cathode block and runs in the width
direction x of the cathode 10, that is to say in the
longitudinal direction of the respective cathode block.
As illustrated in Fig. 1, the upper side 12 of the
cathode 10, as considered in the cross section of the
cathode 10, is tub-shaped, wherein the tub has two edge
regions 16, 16' and a base region 18, which is arranged
between the edge regions 16, 16' and is lowered
relative to the edge regions 16, 16', as viewed in the
width direction x of the cathode 10, wherein, between
each of the edge regions, 16, 16' and the base region
18, a side wall region 20, 20' connecting the
corresponding edge region 16, 16' and the base region
18 is provided. During operation of the electrolytic
cell, for example in accordance with the Hall-Heroult
process, the edge regions 16, 16', the base region 18
and the side wall regions 20, 20' of the upper side 12
are covered by the layer of liquid aluminium. Here, the
edge regions 16, 16', the base region 18 and the side
wall regions 20, 20' of the cathode 10 are preferably
dimensioned such that the bath volume, that is to say
the volume between the upper side of the cathode 10 and
the underside of the anode, corresponds at least
approximately to the bath volume of an electrolytic
cell having a conventional cathode.
The edge regions 16, 16' of the cathode 10 shown in
Fig. 1 have a width b, measured in the width direction
x of the cathode 10, which varies over the length
measured in the longitudinal direction y of the cathode
10, as can be seen from Fig. 1. More specifically, the
details of the edge regions 16, 16' shown in Fig. 1
have five longitudinal portions L1-L5, wherein the
width b of each of the edge regions 16, 16' is constant
within each longitudinal portion 1,1-L5 and varies from
longitudinal portion 1,1-L5 to longitudinal portion Ll-
,
r
CA 02854937 2014-05-07
=
- 24 "-
L5, more specifically such that a step-shaped variation
of the widths b of the edge regions 16, 16' is produced
here, as considered in the longitudinal direction y of
the cathode 10.
Here, the cathode 10 shown in Fig. 1 is symmetrical
with respect to its median longitudinal plane running
parallel to the longitudinal direction y, and the width
b of the base region 18 thus also varies in the
longitudinal direction y of the cathode 10, as does the
width of the edge regions 16, 16'.
A further embodiment of a cathode 10 for an
electrolytic cell is illustrated in a partially cut-
away perspective view in Fig. 2.
This embodiment is similar to the embodiment shown in
Fig. 1 in that the cathode 10, as considered in the
cross section of the cathode 10, is tub-shaped, wherein
the tub has two edge regions 16, of which only the left
edge region is shown in the cut-away illustration of
Fig. 2, and has a base region 18, which is arranged
between the edge regions 16 and is lowered relative to
the edge regions 16, as viewed in the width direction x
of the cathode 10, wherein, between each of the edge
regions and the base region 18, a side wall region 20
connecting the corresponding edge region 16 and the
base region 18 is provided. In contrast to the
embodiment illustrated in Fig. 1, the width of the edge
and base regions 16, 18 does not vary in the
longitudinal direction y of the cathode 10 in the
embodiment shown in Fig. 2, but instead the height h of
the upper side 12 of the cathode 10 varies in the
longitudinal direction y of the cathode 10, as measured
from the underside 14 of the cathode 10 in the vertical
direction z. More specifically, the detail of the upper
side 12 of the cathode 10 shown in Fig. 2 has three
=
CA 02854937 2014-05-07
- 25
longitudinal portions 1,1-L3, wherein the height h of
the upper side 12 of the cathode 10 is constant within
each of the longitudinal portions 1,1-L3 (in the
longitudinal direction y of the cathode 10), but varies
from longitudinal portion 1,1-L3 to longitudinal portion
1,1-L3, more specifically such that a step-shaped
variation of the height h of the upper side 12 of the
cathode 10, as considered in the longitudinal direction
y of the cathode 10, is produced here both in the edge
regions 16 and in the base region 18 and in the side
wall regions 20.
As can be seen from Fig. 2, the edge region 16, as
considered in the width direction x of the cathode 10,
is inclined with respect to the horizontal by an angle
0, which in the present case is slightly less than 10 .
= Fig. 3a shows a perspective, partially cut-away
illustration of a portion of a cathode 10 in accordance
with a further embodiment of the present invention
which corresponds largely to the embodiment shown in
Fig. 2, specifically in that the height h of the upper
side 12 of the cathode 10 varies as viewed in the
longitudinal direction y of the cathode 10; however,
the cathode shown in Fig. 3 differs from that
illustrated in Fig. 2 in that the height h of the edge
region 16 on the one hand and the height h of the base
region 18 on the other hand vary in the opposite
direction, specifically the height h of the edge region
16 increases in the longitudinal direction y running
from the longitudinal portion L1 to the longitudinal
portion L2f whereas the height h of the base region 18
decreases in this direction.
Fig. 3b shows the cathode detail of Fig. 3a in the
longitudinal direction y as considered from the front
side of the cathode 10 and illustrates the oppositely
CA 02854937 2014-05-07
- 26
directed change in the height h of the edge region 16
and of the base region 18. Here, the dashed line shows
the course of the longitudinal portion L2 hidden by the
longitudinal portion L1 of the cathode 10.
Fig. 4 shows a plan view of an electrolytic cell in
accordance with an embodiment of the present invention.
Here, the anode structure and the part of the current
supply connected to the anode structure are not shown
in order to uncover the view of the cathode 10 and the
components arranged beneath and beside said cathode.
In cross section, the cathode 10 has the shape of a tub
comprising two edge regions 16, 16', comprising a base
region 18 and comprising side wall regions 20, 20'
= arranged between the base region 18 and the edge
regions 16, 16'. Here, the cathode 10 per se
= corresponds to the embodiment shown in Fig. 1, more
specifically in particular in that the cathode 10 shown
in Fig. 4 has a plurality of longitudinal portions L1
to L9, wherein the widths b of the individual edge
regions 16, 16' and of the base region 18 vary in the
longitudinal direction y of the cathode 10.
The electrolytic cell comprises nine bar-shaped busbars
22, 22', which each contact the cathode 10 from the
underside thereof in a current-feeding manner and each
extend via their longitudinal direction in the width
direction x of the electrolytic cell. The electrolytic
cell additionally comprises two collector bars 24, 24',
which are arranged such that each of these collector
bars 24, 24' is connected to a respective end of all
busbars 22, 22'. The collector bars 24, 24' accordingly
run the longitudinal direction y in a manner laterally
offset in relation to the cathode 10.
,
,
CA 02854937 2014-05-07
'
- 27
Each collector bar 24, 24' is associated with two
external current feeds 26, 26' and 26", 26"'
respectively, by means of which current is fed
externally to the busbars 22, 22' arranged on the
underside 14 of the cathode 10. Here, the external
current feeds 26, 26', 26", 26"' are connected at
respective connection points 28, 28', 28", 28"' to
one of the collector bars 24, 24' and therefore at this
point also indirectly to the busbars 22, 22' connected
to said collector bar 24, 24'.
As can be seen from Fig. 4, the distances between the
individual connection points 28, 28', 28", 28"' and
the entry points of the individual busbars 22, 22' in
the cathode 10, that is to say the electrical paths P1-
P3, via which the electric current must flow from the
individual connection points 28, 28' 28", 28"' to the
entry points of the individual busbars 22, 22' into the
edge regions 16, 16' of the cathode 10, are of
different length. Here, a longitudinal portion 1,1-L9,
of the cathode 10 which has a shorter electrical path
P1-P3 to the connection point 28, 28', 28", 28"'
closest to this longitudinal portion 1,1-L9 has a
greater width b in its edge regions 16, 16' than a
longitudinal portion 1,1-L9 of the cathode 10 having a
longer electrical path to the connection point 28, 28',
28", 28"' closest to this longitudinal portion.
Equally, a longitudinal portion 1,1-L9 of the cathode 10
which has a shorter electrical path P1-P3 to the
connection point 28, 28', 28", 28"' closest to this
longitudinal portion 1,1-L9 has a smaller width b in its
base region 18 than a longitudinal portion 1,1-L9 of the
cathode 10 having a longer electrical path to the
connection point 28, 28', 28", 28"' closest to this
longitudinal portion.
,
CA 02854937 2014-05-07
- 28 '
LIST OF REFERNCE SIGNS
cathode
12 upper side
14 underside
16, 16' edge regions
18 base region
20, 20' side wall region
22, 22' busbar
24, 24' collector bar
26, 26', 26", 26"' external current feed
28, 28', 28", 28"' connection point
x, y, z width, longitudinal and
vertical direction
width
1 length
height
L1-L9 longitudinal portion
Pi-P3 electrical path
angle of inclination
