Note: Descriptions are shown in the official language in which they were submitted.
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Cathode assembly with metallic collector bar for electrolytic cell suitable
for the
Hall-Fleroult process
Technical field of the invention
The invention relates to the field of fused salt electrolysis using the Hall-
Heroult process
for making aluminium. More specifically it relates to the improvement of the
cathode
blocks of such an electrolysis cell, the improvement being related to the
cathode drop and
the current distribution along the cathode blocks. In particular, the
invention relates to an
improvement for cathode block provided with a cathode collector bar.
Prior art
The Hall-Heroult process is the only continuous industrial process for
producing metallic
aluminium from aluminium oxide. Aluminium oxide (A1203) is dissolved in molten
cryolite
(Na3AIF6), and the resulting mixture (typically at a temperature comprised
between 940 C
and 970 C) acts as a liquid electrolyte in an electrolytic cell. An
electrolytic cell (also
called "pot") used for the Hall-Heroult process typically comprises a steel
shell (so-called
pot shell), a lining (comprising refractory bricks protecting said steel shell
against heat,
and cathode blocks usually made from graphite, anthracite or a mixture of
both), and a
plurality of anodes (usually made from carbon) that plunge into the liquid
electrolyte
contained in the volume defined by the cathode bottom and a side lining made
from
carbonaceous material. Anodes and cathodes are connected to external busbars.
An
electrical current is passed through the cell (typically at a voltage between
3.5 V and 5 V)
which electrochemically reduces the aluminium oxide, split in the electrolyte
into
aluminium and oxygen ions, then into aluminium at the cathode and oxygen at
the anode;
said oxygen reacting with the carbon of the anode to form carbon dioxide. The
resulting
metallic aluminium is not miscible with the liquid electrolyte, has a higher
density than the
liquid electrolyte and will thus accumulate as a liquid metal pad on the
cathode surface
below the electrolyte from where it needs to be removed from time to time,
usually by
suction into a crucible.
Industrial electrolytic cells used for the Hall-Heroult process are generally
rectangular in
shape and connected electrically in series, the ends of the series being
connected to the
positive and negative poles of an electrical rectification and control
substation. The
general outline of these cells is known to a person skilled in the art and
will not be
repeated here in detail. They have a length usually comprised between 8 and 25
meters
and a width usually comprised between 3 and 5 meters. The cells (also called
"pots") are
always operated in series of several tens (up to several hundreds) of pots
(such a series
being also called a "potline"); within each series DC currents flow from one
cell to the
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neighbouring cell. The electrical currents in most modern electrolytic cells
using the Hall-
Heroult process exceed 200 kA and can reach 400 kA, 450 kA or even more; in
these
potlines the pots are arranged side by side. Most newly installed pots operate
at a current
comprised between about 350 kA and 600 kA, and more often in the order of 400
kA to
500 kA. The passage of these enormous current intensities through the
electrolytic cell
leads to ohmic losses at various locations of the pot and its environment.
Cathode assemblies for use in electrolytic cells suitable for the Hall-Heroult
process are
industrially manufactured for more than a century, and the state of the art is
summarized
in the reference book "Cathodes in Aluminium Electrolysis" by M. Sm.lie and H.
(25ye, 31d
edition (Dusseldorf 2010). They comprise a cathode body made from a carbon
material
and one or more metallic cathode bars that are fitted into slots or grooves
machined into
the lower surface of said carbon body. Said metallic cathode bar protrudes out
of each
end of the cathode block, thereby allowing to connect the cathode assembly to
the
cathode busbar system. The metallic cathode bar is usually mode from steel;
copper
inserts within the steel bar can be used in order to increase the electrical
conductivity of
the cathode bar. Said steel bars are inserted into grooves that are wider than
the steel
bars, and then fixed with electrically conductive glue (carbonaceous glue or
cement, or
ramming paste) or with cast iron that is poured into the interstitial space
between the steel
bar and the carbon body, as described in GB 663 763 (assigned to Compagnie de
Produits Chimiques et Electrometallurgiques Alais, Froges & Camargue).
During the past decades, much effort has been devoted to the decrease of ohmic
losses
in cathode bars. Most inventions reported in prior art patents focus on the
intrinsic
conductivity of the steel cathode bar, or on the contact resistance between
the cathode
bar and the cathode block or between the cathode bar and the aluminium busbar.
A cathode with a full copper cathode bar inlaid into a groove machined in the
lower
surface of the carbon body is known from WO 2016/079605 (Kan Nak s.a.), in
particular
figures 7 and 9 of said document. The contact between the carbon body and the
copper
bar is critical for the electrical performance of the electrolysis cell.
Copper has a much
higher thermal expansion coefficient than the carbon material of the cathode
block body,
and the copper bar in direct contact with the carbon body will operate at a
temperature
that is probably less than 100 C lower than its melting point, leading to
significant thermal
expansion. As a consequence, a well-defined allowance for thermal expansion
must be
provided when machining the groove; otherwise strains and stresses caused by
the
expanding copper bar may lead to cracks in the carbon material of the cathode
block. If no
glue is used for accommodating dimensional tolerances, a very precise
machining of the
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groove is required in order to ensure a good and reliable electrical contact
between the
copper bar and the carbon body over the whole length. Reliability of this
contact is of
paramount importance, because once installed into a cell and the cell started,
a cathode
block cannot be repaired, and cannot be replaced without relining of the whole
cell. The
normal lifetime of a cathode lining is comprised between five and seven years.
Usually, large carbon products such as cathodes for use in Hall-Heroult cells
are
machined with a tolerance of 2 mm; a tolerance of 1 mm can be reached, but
at a high
cost. The applicant has found that it is very difficult to get reliable
contacts by inserting
metallic bar, in particular copper bar, directly into grooves machined into
the carbon
bodies without using glue.
The problem addressed by the present invention is therefore to improve the
quality and
reliability of the electrical contact of metallic bars, in particular copper
bars, inserted into
grooves machined into the carbon body of a cathode block.
Objects of the invention
A first object of the invention is a cathode assembly suitable for a Hall-
Heroult electrolysis
cell, comprising
- a cathode body made of a carbonaceous material, said cathode body being
provided with at least one slot, said slot being provided with side walls
parallel
to a longitudinal direction of said slot;
- at least one cathode collector bar made of a metallic material, said
cathode bar
being provided with side walls, which are in contact with said side walls of
said
slot;
characterized in that said cathode bar comprises two bar elements, each bar
element
being provided with a main side wall which is in contact with a respective
side wall of said
slot, as well as a tapered wall, the two tapered walls of said bar elements
forming a
contact line between these two bar elements.
Advantageously, said cathode assembly is provided with fixation means, in
particular
permanent fixation means, between said tapered walls of said bar elements.
Said fixation
means are advantageously welding means. In one embodiment, said welding means
comprise at least one welding line, in particular several welding lines,
extending over at
least part of said contact line.
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In an advantageous embodiment, said cathode body is provided with at least one
first and
one second slots, each slot being provided with a blind wall defining a
longitudinal end of
said slot, each slot receiving a respective cathode bar. Advantageously, said
cathode bar
is jammed against said longitudinal end of a respective slot.
In one embodiment, one first bar element has a triangular shape and comprises
a front
wall, said main side wall and said tapered wall. Advantageously, one second
bar element
comprises a further side wall, opposite to main side wall, said further side
wall protruding
with respect to main side wall of first bar element, along a transversal
direction of said
slot.
In an advantageous embodiment said front wall of first bar element is
positioned against
longitudinal end of said slot, and said second bar element comprises a front
wall, the
length of which is far inferior to that of front wall of first bar element,
front wall of second
bar element being remote from end of said slot.
In one embodiment said cathode bar has two portions, i.e. a first portion the
width of
which is equal to that of slot, as well as a second portion the width of which
is superior to
that of slot.
In an advantageous embodiment said cathode bar has a protrusion which extends
outside
said slot. Said protrusion is in particular formed by said second portion and
by a fraction of
said first portion. In one embodiment main side wall of each bar element
protrudes outside
said slot.
In one embodiment main side wall of each bar element directly contacts a
respective side
wall of said slot.
In an alternative embodiment main side wall of each bar element indirectly
contacts a
respective side wall of said slot, an intercalary material, in particular at
least one graphite
foil, being interposed between said main side wall and said respective side
wall of said
slot. A further intercalary material, in particular at least one further
graphite foil, may be
interposed between upper wall of said slot and facing walls of bar elements.
In an advantageous embodiment said side walls of said slot and said side walls
of said
bar elements show a slope, the value of which is in particular of about 10
degrees, so as
to retain said bar elements in the inner volume of said slot.
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Said cathode bar is advantageously made of copper. In an alternative
embodiment said
cathode bar is made of steel.
Another object of the present invention is a process for making a cathode
assembly as
5 described above, comprising the steps of
a) providing a cathode body made of a carbonaceous material;
b) providing at least one slot in said cathode body, said slot being provided
with side
walls parallel to a longitudinal direction of said slot;
c) providing at least two bar elements made of a metallic material, each bar
element
being provided with a main side wall and a tapered wall,
d) placing a first bar element into the slot, with its main side wall adjacent
to facing
first side wall of the slot;
e) urging forward, substantially along said longitudinal direction of said
slot, second
bar element, so as to urge main side wall of said first bar element against
facing
first side wall of the slot, so as to urge main side wall of said second bar
element
against facing opposite second side wall of the slot, and so as to set into
contact
the two tapered walls of said bar elements, along a contact line.
In an advantageous embodiment urging forward said second bar element also
provokes
the jamming of said first bar element against one longitudinal end of said
slot.
In an advantageous embodiment said process further comprises providing said
side walls
of said slot and said side walls of said bar elements with a slope, the value
of which is in
particular of about 10 degrees, so as to retain said bar elements in the inner
volume of
said slot.
In an advantageous embodiment second bar element is provided with a handling
portion
and said second bar is urged forward manually, by handling said handling
portion.
In an advantageous embodiment bar elements are provided by cutting a rough bar
along
a cutting line, said cutting line being tapered with respect to main axis of
said rough bar.
In an advantageous embodiment said process further comprises providing
fixation means,
in particular permanent fixation means, between said tapered walls of said bar
elements,
once said tapered walls of said bar elements are in mutual contact.
Another object of the present invention is an electrolytic cell suitable for
the Hall-Heroult
electrolysis process, comprising
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a cathode forming the bottom of said electrolytic cell and comprising a
plurality of parallel
cathode assembly, each cathode assembly comprising at least one metallic
cathode
collector bar protruding out of each of the two ends of the cathode,
a lateral lining defining together with the cathode a volume containing the
liquid electrolyte
and the liquid metal resulting from the Hall-Heroult electrolysis process,
an outer metallic potshell containing said cathode and lateral lining,
a plurality of anode assemblies suspended above the cathode, each anode
assembly
comprising at least one anode and at least one metallic anode rod connected to
an anode
beam,
a cathodic bus bar surrounding said potshell, said bus bar being connected to
at least part
of said cathode assemblies
said electrolytic cell being characterized in that
at least one of said cathode assembly, and preferably more than 60% of said
cathode
assemblies and, more preferably, each of said cathode assemblies, is a cathode
assembly as described above.
Another object of the present invention is an electrolytic cell for the
production of
aluminium by the Hall-Heroult process, comprising at least one cathode
assembly as
described above.
Another object of the present invention is a process for making aluminium by
the Hall-
HerouIt process, using an electrolytic cell provided with cathode assemblies
as described
above.
Figures
Figures 1 to 11 represent one embodiment of the present invention.
Figure 1 is a perspective view, showing one embodiment of a cathode assembly
according to the invention.
Figure 2 is a perspective view, showing upside down a cathode body which
belongs to
said cathode assembly according to the invention, said figure 2 showing in
particular slots
provided in said cathode body.
Figure 3 is a longitudinal section showing the cathode body of figure 2.
Figure 4 is a top view, showing at a greater scale a rough bar from which a
cathode bar is
formed, said cathode assembly of figure 1 being equipped with said cathode
bar.
Figure 5 is a top view, analogous to figure 4, showing a cutting operation of
said rough bar
of figure 4, in order to form two bar elements.
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Figures 6 and 7 are top views, analogous to figure 5, showing two steps of the
insertion of
said bar elements into a slot of said cathode body.
Figure 8 is a top view, analogous to figures 6 and 7, showing the final
cathode bar of the
cathode assembly according to the invention, said cathode bar being formed
from said bar
elements once inserted in said slot and mutually attached.
Figures 9 and 10 are top views, showing at still a greater scale the details
IX and X of
figure 8.
Figure 11 is a cross section showing the cathode bar of figure 8, along line
XI-XI of figure
8.
The following reference signs are used on the figures:
Cathode assembly 1 Cathode body
11,12 Front/rear wall of cathode body 1 171, 172 Side walls of slot
17
13,14 Upper/lower wall of cathode body 1 173 Top wall of slot 17,17'
15,16 Side walls of cathode body 1 174,174' Blind wall of slot
17,17'
17, 17' Slots in cathode body 1 D174 Distance between 174 174'
3,3' Cathode bar 31,32 Front/rear wall of
cathode bar 3
33,34 Side walls of cathode bar 3 37 Contact line of 3
5 Rough bar 51,52 Front! rear wall of rough
bar 5
53,54 Upper! lower wall of rough bar 5 55,56 Side walls of
rough bar 5
AS Axis of rough bar 5 CL Cutting line
L6 Length of slotted bar as Angle of slotted bar
lac Angle between AS and CL A17,A17' Axis of slots 17, 17'
7, 9 Bar elements 9A Handling part of 9
9B Tip of 9 9C Shoulder of 9
71 Front wall of bar element 7 91,92 Front! rear wall of bar
element 9
73 Side wall of bar element 7 93,94 Side walls of bar element
9
77 Tapered wall of bar element 7 97 Tapered wall of bar
element 9
F9, S9 Motion of 9 F73,F93 Motion of 73,93
21-24 Welding lines 3A,3B Portions of 3
3C Protrusion of 3 77',97' Linking portion of 77,97
D17,D1 Depth of 17, 17' H1 Height of 1
7'
LX Length of element X : L1, L17, L17', L5, L7, L93, L94, L37, L3C'
WX Width of element X : W1, W5, W17, W17', W7, W91, W92
Detailed description
In the present description, the terms "upper" and "lower" refer to a cathode
block in the
position of its industrial use, lying on a horizontal ground surface, i.e. the
upper surface
being intended to be in contact with the liquid aluminium in the electrolysis
cell. Moreover,
unless specified otherwise, "conductive" means "electrically conductive".
According to the
terminology used in the art, a "cathode assembly" C comprises a cathode body 1
and at
least one cathode bar 3.
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The present invention is first applicable to cathode assemblies C comprising a
cathode
body 1 and at least one cathode bar 3 and 3'. In particular, the invention is
applicable to
cathode assemblies comprising two cathode bars 3,3', one 3 of which is
protruding out of
the front wall 11, the other 3' protruding out of the rear wall 12 of the
cathode body 1.
These cathode bars, which are half bars, form two portions of a so-called
"split bar" in the
sense that they are not through bars, i.e. each cathode bar is not extending
through the
whole length of the cathode block.
The present invention is also applicable to cathode assemblies having one (or
more)
through bar(s) instead of above defined split bars. The invention applies in
particular to
such through bars, which have a short length. However, the use of split bars
is preferred,
since it allows a better jamming of these bars, as will appear at reading the
following
description.
The present invention applies to cathodes used in the Hall-Heroult process
that form the
bottom of an electrolysis cell, said cathodes being assembled from individual
cathode
assemblies C, each of which bears at least one cathode bar 3, 3'. The Hall-
Heroult
process and the outline of an electrolysis cell (also called "pot") are known
to a person
skilled in the art and will not be described here in great detail. The
invention will be
explained in relation with embodiments comprising one split cathode bar per
cathode
assembly C, said split cathode bar comprising two portions 3, 3' but it is
understood that
the present invention can be applied to cathode assemblies C comprising any
number of
split cathode bars with portions 3, 3', such as two sets of split bars
arranged parallel to
each other. In the following the portion of a split cathode bar will be
referred to as the
"cathode bar".
The cathode assembly of the invention is designated as a whole by alphanumeric
reference C. It is suitable for a Hall-Heroult electrolysis cell, but could be
used in other
electrolytic processes.
The cathode assembly C first comprises a cathode body 1, of known type, which
is made
of a carbonaceous material, typically graphitized carbon or graphite. This
cathode body 1,
which has an elongated shape, has opposite end walls, i.e. front 11 and rear
12 walls, as
well as peripheral walls. The latter are formed by parallel upper and lower
walls 13
and 14, as well as parallel side walls 15 and 16. By way of example, its
length Li (see
figure 3), i.e. the distance between walls 11 and 12, is between about 3100
millimetres
(mm) and about 4000 mm. By way of example, its width W/ (see figure 2), i.e.
the
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distance between walls 15 and 16, is between about 400 mm and about 700 mm. By
way
of example, its height H1 (see figure 3), i.e. the distance between walls 13
and 14, is
between about 375 mm and ab0ut580 mm.
As more clearly shown on figures 2 and 3, the lower wall 14 of cathode body 1
is provided
with two housings, each being formed by a respective longitudinal slot 17 and
17', the
longitudinal main axis of which is referenced A17, A17'. Figure 2 shows
cathode body
"upside down", with reference to its above defined industrial use position.
Each slot 17, 17' is provided with opposite side walls 171, 171' and 172, 172'
(see
figure 2), parallel to said main axis A17, A17', whereas its top wall is
referenced 173, 173'
(see figure 3). Moreover each slot 17, 17' is provided with a respective rear
wall, or blind
wall 174, 174' (see figure 2), each defining a respective longitudinal end of
said slot. The
distance D174 (see figure 3) between these two walls is typically between 200
millimetres
(mm) and 600 mm. Viewed from bottom, blind wall 174, 174' is rounded, which
makes it
possible to ease the slot machining. First slot 17 does lead to front wall 11
of this cathode
body, whereas second slot 17' does lead to rear wall 12 of this cathode body.
By way of example, width W17 or W17' (see figure 2) of each slot 17, 17', i.e.
the distance
between side walls, is between about 50 mm and about 250 mm. Advantageously,
as
illustrated in particular on figure 11, each side wall 171 and 172 shows a
slope, the value
as of which is typically of about 10 degrees. Therefore the above defined
width W17 or
W17' decreases from top wall 173, 173' of this slot to lower wall 14 of
cathode body. As
will be described hereafter, these slopes make it possible to maintain the bar
elements in
the inner volume of the slot, when turning over the cathode assemblies. For
sake of
clarity, the value of as is exaggerated on this figure 11.
By way of example, depth D17 or D17' of each slot 17, 17' (see figure 3), i.e.
the distance
between top wall 173, 173' and the surface of lower wall 14 of the body 1, is
between
about 50 mm and about 150 mm. By way of example, its length L17 or L17' (see
figure 3),
i.e. the distance between front wall or rear wall of cathode body and blind
wall 174, 174',
is between about 1200 mm and about 1850 mm. This length is taken from the
junction of
rounded portion of said blind wall with rectilinear part of side walls 171,
171' and 172,
172'.
The cathode assembly C also comprises two cathode bars 3 and 3' (schematically
shown
on figure 1), each of which is accommodated in a respective slot 17 and 17'.
Each
cathode bar 3 or 3' is made of a conductive material, typically able to
conduct the current
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from the cathode to the exterior Bus Bar. Advantageously, the material of
these cathode
bars is copper. However, the invention encompasses cathode bars made of other
materials, such as for example steel, or other materials usually installed
inside the
cathode assemblies. The insertion process of cathode bar 3 into its slot 17
will now be
5 described, bearing in mind that insertion process of other cathode bar 3'
into other slot 17'
is identical.
The first step of said insertion process is the provision of a so called rough
bar, which is
shown on figure 5 and is referenced 5 as a whole. As will appear more clearly
at reading
10 the next steps of this process, the dimensions of said rough bar 5 are
globally analogous
to those of final cathode bar 3, but slightly different. This bar 5, which has
an elongated
shape, is not shown with its real scale on figure 4, in order to clearly
illustrate the insertion
process. The same remark applies to mechanical elements of figures 5 to 10:
slot 17,
cathode bar 3 and bar elements 7 and 9, formed from rough bar 5. In
particular, the width
of these mechanical elements is far exaggerated with respect to their length.
Moreover,
top views of figures 6 to 10 have been hatched to clearly distinguish the
different
mechanical elements.
Rough bar 5 has front 51 and rear 52 walls, parallel upper and lower walls 53
and 54, as
well as parallel side walls 55 and 56. Respective length L5, width W5 and
height H5 of
rough bar 5 are defined the same way as above length L1, width W/ and height
H1 of
cathode body 1.
Length L5 of rough bar is far superior to that L17 of slot 17, so that final
cathode bar 3 will
protrude outside slot 17, above front wall 11. The value of the difference (L5
¨ L17)
implies the length of the protrusion of final cathode bar 3. Typically, this
difference (L5 ¨
L17) is between 400 and 700 mm. Side walls 55 and 56 show slopes, the angle of
which
is similar to that aS of the side walls of the slot. The lengths of slotted
parts of these side
walls are referred L6 and L6' on figure 4. Both L6 and L6' are superior to the
length L17 of
the cathode slot. Typically, the difference (L6 ¨ L17) or (L6' ¨ L17) is
between 150 and
200 mm.
In addition, width W5 of rough bar 5 is slightly superior to that W17 of slot
17, which
enables a steady jamming of final cathode bar 3 in the slot 17. Typically, the
difference
(W5¨ W17) is between 5 to 10 mm.
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Finally, height H5 of rough bar 5 is equal or slightly inferior to depth D17
of cathode slot,
so as to prevent final cathode bar from protruding outside the slot 17, above
lower wall 14.
Typically, the difference (D17¨ H5) is between 0 and 4 mm.
The insertion process then implies cutting rough bar 5, so as to form two
cathode bar
elements, or so called bar elements 7 and 9. Rough bar 5 is cut along a
cutting line
referenced CL, shown on figure 4. This line CL forms an angle, noted ac, with
the main
longitudinal axis A5 of rough bar 5. By way of example, this angle ac is
between 1 and
4 , typically of about 2 . The cutting operation can be carried out by water
jet cutting
following a machining of the two surfaces, since the cut cannot be straight
and the
roughness will not be good enough to get a good contact between the two parts.
Another
way is cutting the two parts directly by machining using a circular cutting
blade, typically of
3 to 6 mm of thickness. The latest way is preferred, since the cutting surface
will present a
good roughness and machining the cut surfaces will therefore not be necessary.
As shown on figure 5, first bar element 7 has a triangular shape. It has a
front wall 71, a
main side wall 73, as well as a tapered wall 77. It has neither a rear wall,
nor a second
side wall, since wall 77 directly connects both walls 71 and 73. Side wall 73
shows a
slope, which corresponds to that of side wall 55 of rough bar 5.
Let us note length L7 and width W7 of said bar element 7. Width W7 is inferior
to that W5
of rough bar 5, and is also slightly inferior to that W17 of the slot, so as
to enable an
insertion without jamming of said bar element 7 into slot 17. Typically, the
difference (W17
¨ W7) is between 10 and 40 mm. Length L7 is far inferior to that L5 of rough
bar 5, but is
slightly superior to that L17 of slot, so as to enable a protrusion of said
bar element 7
outside slot 17. Typically, the difference (L7 ¨ L17) is between 20 and 100
mm. Finally,
height H7 of bar element 7 is equal to that H5 of rough bar 5.
As also shown on figure 5, second bar element 9 has a shape which is different
from a
triangle. It has a short front wall 91, a long rear wall 92, a main side wall
93, a short side
wall 94, as well as a tapered wall 97. Side wall 93 shows a slope, which
corresponds to
that of side wall 56 of rough bar 5.
This second bar element can therefore be divided into two parts, namely a
handling part
9A with constant width, as well as an insertion part or tip 9B, with a tapered
shape. Tip 9B
is ended by a shoulder 9C, formed adjacent front wall 91, which eases the
jamming of
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bars 7 and 9 into the slot. The respective dimensions of above defined walls
and parts of
bar element 9 are as follows:
- W92 (width of 92) = W5;
- L93 (length of 93) = L5;
- L94 (length of 94) = L5¨ L7;
- H9 (height of 9) = H5.
According to next step of the process of the invention, shown on figure 6,
first bar
element 7 is inserted into slot 17. It is to be noted that this step is
carried out with a
cathode block upside down. In other words, during said insertion, access to
slot 17 is
permitted from the end of the cathode block groove, whereas so called lower
wall of
cathode body is in an upper position. Once inserted in the slot, bar element 7
rests by
gravity against wall 173 of this slot. Since W7 is inferior to W17 of slot,
this insertion can
be carried out easily, without jamming of said bar element 7 into slot 17. Bar
element 7 is
positioned in the slot, so that its front wall 71 is close to the end of the
slot, and its main
side wall 73 is adjacent side wall 172 of the slot. End of the slot is defined
by the transition
between rectilinear side walls 171, 172 and rounded wall 174.
Then an operator handles second bar element 9, at the handling part 9A
thereof. The
operator pushes bar element 9 forward, i.e. towards the end of the slot, along
arrow F9. At
an intermediate stage of this forward pushing motion, the facing tapered walls
77 and 97
come into mutual contact (see figure 7). Since first bar element 7 is axially
jammed by the
end of the slot, second bar element 9 slides with respect to bar element 7
along arrow S9,
so that the global motion of bar element 9 now comprises a tapered component.
Due to
this sliding motion, both bar elements 7 and 9 are urged against the walls of
the slot 17.
More precisely, main side wall 73 of first bar element 7 is urged against
facing side wall
172 of slot 17, along arrow F73, whereas main side wall 93 of second bar
element 9 is
urged against facing opposite side wall 171 of slot 17, along arrow F93. In
the above
paragraph, insertion operation has been described manually. However, an
automatic
operation may be considered, with any appropriate tool.
Once bar elements 7 and 9 are jammed in slot 17, their tapered walls 77 and 97
are in
mutual contact along nearly their whole length, so as to form a tapered
contact line 37
(see figure 8). The length L37 of contact line is slightly inferior to both
L77 and L97, since
one short end region of each wall 77 or 97 does not contact other wall 97 or
77. The
required adjustment of the bar inside the slot can be achieved by moving the
bar element
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13
9 with respect to the first bar element 7 already installed. When the
protrusion of the bars
outside the cathode L3C is adjusted at the required value, then the two bar
elements 7
and 9 are advantageously mutually attached, in particular with a permanent
fixation
means. In this respect, welding is preferred, such as copper to copper type
using MIG
welding machines, known as such.
Referring to figure 8, at least one and, preferably several welding lines are
provided along
the above defined contact line 37. In the illustrated example, four welding
lines 21 to 24
are provided. Let us note L21 to L24 the length of each of these lines, as
well as Lw the
so called welding length, which corresponds to the sum (L21 + L22 +L23 + L24)
of the
lengths of these lines. Each of L21 to L24 is typically between 50 and 100 mm,
whereas
the welding ratio, i.e (Lw I L37) is between 10 and 40%.
After above described welding step, bar elements 7 and 9 are mutually attached
and form
final cathode bar 3 shown on figure 8. Said global cathode bar has front 31
and rear 32
walls, as well as side walls 33 and 34. Said figure 8 also illustrates above
described
tapered line 37, separating walls 77 and 97 of bar elements 7 and 9. First,
rear wall 32
and side wall 34 are respectively constituted by those 92 and 94 of bar
element 9.
Moreover, as shown by detail 9, front wall 31 is formed by front wall 71 and
front wall 91,
as well as by a linking portion 77' of tapered wall 77. Front wall 91 is
remote from end of
slot, whereas front wall 71 is positioned against said end, as explained
above. In addition,
as shown by detail 10, side wall 33 is formed by side wall 73 and side wall
93, as well as
by a linking portion 97' of tapered wall 97. Side wall 93 protrudes laterally,
with respect to
side wall 73.
Cathode can be decomposed into two portions, i.e. a first portion 3A the width
of which
W3A is equal to that W17 of slot 17, as well as a second portion 38 the width
of which
W38 is equal to that W5 of rough bar 5. This cathode defines a protrusion 3C,
which
extends outside slot 17. This protrusion, which is formed by portion 38 as
well as by a
fraction of portion 3A, has a typical length L3C between 400 and 700 mm.
Once cathode bars 3 and 3' are positioned and jammed in their respective slot
17 and 17',
the whole cathode assembly is turned upside down, so as to be in its final
position of
figure 1. Due to the slopes of side walls of both the slots and the bar
elements, as above
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14
described in reference to figure 11, cathode bars cannot escape from their
slots due to
gravity, so that they are firmly retained therein.
In the above described embodiment, each side wall of cathode bar 3 directly
contacts
facing side walls of the slot 17, i.e. without any intercalary material. The
invention also
encompasses alternative embodiments, wherein side walls of cathode bar
indirectly
contacts facing side walls of the slot. In this respect, a thin sheet of an
intercalary material
is interposed between said facing walls of slot and cathode bar.
According to an advantageous embodiment of the invention, said intercalary
material is a
graphite foil, inserted between said facing side walls. This graphite foil is
typically placed
against the side walls of the slot, before insertion of bar elements. A
further intercalary
material, in particular at least one further graphite foil, may also be
interposed between
upper wall 173 of said slot and facing walls of the bar elements 7 and 9. As a
variant, one
single intercalary graphite foil may recover both side walls and upper wall of
the slot. The
adjustment will be set, so that no substantial gap or space is left between
bar elements,
graphite foil and cathode body. Said graphite foil can be a flexible graphite
foil of
compressed expanded graphite. Said foil is available from various suppliers
under
different trademarks, such as PAPYEXO by MERSEN. The density of the foil is
typically
0.7 and it may have 0.5 mm of thickness. In addition, graphite material is
compressible to
cope with the thermal expansion of bar elements.
Example
An industrial trial was carried out using an electrolysis cell of the so-
called D18
technology; this cell was part of an existing D18 potline. The D18 technology
has been
described in several papers such as: "Update on the development of D18 cell
technology
at DUBAL" (D. VVhitfiled et al., Light Metals 2012, p. 727-731); "D18+:
potline
modernization at DUBAL" (S. Akhmetov et al., Light Metals 2013, p. 561-656);
"From D18
to D18+: Progression of DUBAL's original potlines" (D. Whitfield et al., Light
Metals 2015,
p. 499-504). The selected electrolysis cell was provided with new cathode
blocks;
seventeen cathode assemblies were used, and the assembly n 2,7,11, and 16 had
copper cathode bars according to the invention, whereas the other ones were
provided
with conventional steel cathode bars. Cathode assemblies n 2 and 7 had a
sheet of
graphite foil between the cathode bar and the cathode block, whereas
assemblies n 11
and 16 had a direct contact between the copper cathode bar and the cathode
material. All
cathode bars were half bars. Contact tabs were made from copper.
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The cell was started up according to conventional practice and run for about
3.5 months
under production conditions (211 kA). Its overall performance was slightly
better than that
of the other cells with 100 % conventional cathode assemblies: as an example,
compared
to conventional D18 cells of the same potline, the average net voltage was
slightly lower
5 (60 mV), the aluminium purity was identical (99.873 %), aluminium
production was slightly
higher, specific energy consumption was lower (approximately 300 kW/h per
ton), bath
height and metal height as well as bath temperature (958 C) were comparable,
cell
stability was comparable, base resistance set point and cathode voltage drop
were slightly
lower (approximately 20 mV).
It was found that the cathode assemblies with copper bars according to the
invention were
pulling about 40 % more current than cathode assemblies with steel bars in the
same cell.
This demonstrates that a significant gain can be obtained by constructing a
pot equipped
with all cathodes having full copper bars. At the beginning of the operations,
a small
difference was observed between copper cathode bars in direct contact with the
cathode
block and copper cathodes bars with an intercalary graphite foil, giving the
benefit to the
latest.
The cell was then autopsied; the copper bars could be easily cut out, and it
was found that
for each half bar the two bar elements were firmly welded together at their
tapered
interface and did not separate upon removal. This shows that the electrical
contact at the
tapered interface between the two bar elements was excellent. No melting of
the copper
bar was observed, no significant visual change was observed. Slight remains of
graphite
foil were visible on the collector bar side for the two bars that had been in
contact with
graphite foil. The copper could be fully recovered for recycling.
