Technical field of the invention
[0001] The invention relates to a cathode block for an electrolytic cell for producing aluminium
by fused salt electrolysis using the Hall-Heroult-process. In particular, the invention
relates to a cathode block in which the electrical contact between the cathode material
and the busbar to which the cathode is connected involves a copper bar.
Prior art
[0002] The Hall-Heroult process is the only continuous industrial process for producing
metallic aluminium form aluminium oxide. Aluminium oxide (Al
2O
3) is dissolved in molten cryolite (Na
3AlF
6), 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, a lining (comprising refractory bricks protecting said cell against heat loss,
and cathode blocks usually made from graphite, anthracite or a mixture of both, covering
the whole bottom of the pot), a superstructure and a plurality of anodes (usually
made from carbon) that plunge into the liquid electrolyte. Anodes and cathodes are
connected to external busbars. An electrical current is passed through the cell (typically
at a voltage between 3.5 V to 5 V) which electrochemically reduces the aluminium oxide,
split by the electrolyte into aluminium ions and oxygen ions. The oxygen ions are
reduced to oxygen at the anode, said oxygen reacting with the carbon of the anode.
The aluminium ions move to the cathode where they accept electrons supplied by the
cathode; 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 from where it needs to be removed from time to time,
usually by suction into a crucible.
[0003] The electrical energy is the main operational cost in the Hall-Heroult process. Capital
cost is an important issue, too. Ever since the invention of the process at the end
of the 19
th century much effort has been undertaken to improve the energy efficiency (expressed
in kW/h per kg or ton of aluminium), and there has also be a trend to increase the
size of the pots and the current intensity at which they are operated in order to
increase the plant productivity and bring down the capital cost per unit of aluminium
produced in the plant.
[0004] Industrial electrolytic cells presently used for the Hall-Heroult process are rectangular
and have a length usually comprised between 8 and 25 meters and a width usually comprised
between 3 and 5 meters. Most newly installed pots operate at a current intensity comprised
between about 400 kA and 600 kA. They are always operated in series of several tens
(up to more than a hundred) pots (such a series being also called a "potline"); within
each series DC currents flow from one cell to the neighbouring cell. Much effort is
still being made to optimise the process in order to increase its energy efficiency.
[0005] The passage of the enormous current intensities through the electrolytic cell leads
to ohmic losses at various locations of the pot. Aluminium conductors are used for
the busbar systems for both anodes and cathodes. However, aluminium cannot be used
in direct contact with the cathode blocks due to its low melting point (about 660°C
for pure aluminium). As a consequence, steel bars are conventionally used for ensuring
electrical contact with the cathode blocks; these so-called cathode bars are connected
to cathodic busbars (made from aluminium) by welded and/or bolted connectors. Cathode
bars are typically fitted into slots machined into the lower surface of the cathode
block. Electrical contact between the steel bar and the carbon material of the cathode
block can be direct, or the steel bar can be embedded in cast iron, as described in
GB 663 763 (assigned to Compagnie de Produits Chimiques et Electrométallurgiques Alais, Froges
& Camargue).
[0006] 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.
[0007] The increase in the electrical conductivity of the cathode bars implies the use of
a material having a higher electrical conductivity than steel bars. All reported solutions
imply the use of inserts made from a material with a higher electrical conductivity
into the cathode bar, which is usually made from steel. The material with a higher
electrical conductivity is usually copper. Typical solutions comprise a copper rod
or bar that is inserted into a groove or slot machined into the steel cathode bar,
over all or part of the length of said cathode bar. The basic concept of a copper
insert fitted into a slot or groove machined in a steel cathode bar is described in
WO 2001/63014 (Comalco),
WO 2005/098093 (Aluminium Pechiney) and
WO 2009/055855 (BHB Billington).
[0008] FR 1 161 632 (Pechiney) discloses a copper insert fitted into a groove machined in a carbon cathode
block using cast iron as a sealing material. The composition of cast iron used for
sealing cathode bars into the grooves of carbon cathodes is known to be critical (see
US 2,593,751 assigned to Pechiney), because the cast iron should not undergo any swelling due
to structural transformations, as swelling could cause the carbon material to develop
cracks.
[0009] A large number of more specific embodiments have been described for these copper
inserts, such as:
- A copper bar with circular cross section fitted into a steel bar with outer rectangular
cross section and an inner "U" section, the "U" section being closed by a block, see
US 3,551,319 (Kaiser).
- A copper bar welded to a lateral face of a steel bar, see US 3,846,388 (Pechiney).
- A copper bar with rectangular cross section inserted into a steel tube with rectangular
cross sections, see US 5,976,333 (Alcoa).
- A copper bar with circular cross section inserted into a steel tube with rectangular
external cross section and a bore with circular cross section, see WO 2005/098093 (Aluminium Pechiney).
[0010] Several documents disclose the use of a joint material present between the cathode
material and the steel bar:
WO 2013/039893 (Alcoa) describes the use of a copper insert as a joint,
WO 2007/071392 (SGL Carbon) describes the use of sheets made from expanded graphite, and
RU 2285764 describes the use of a carbonaceous paste. Such a joint material may improve the
electrical contact between the carbon block and the steel bar.
[0011] RU 2285754 proposes to secure the copper bar inserted into the slot of the steel bar by welded-on
steel plates while allowing for a narrow cavity between the copper insert and the
steel bar, i.e. the section of the copper insert is somewhat smaller that that of
the groove into which it is fitted.
[0012] The opposite approach is taken by
WO 2009/055844 describing the use of roll bonding or explosion bonding in order to obtain an excellent
contact between the copper insert and the steel bar over the whole length of the insert.
[0013] Another problem addressed by many inventions is the connection between the copper
insert and the steel cathode bar. This contact is critical for at least three reasons:
the electrical contact between the copper insert and the cathode bar should be as
good as possible; the thermal expansion coefficients of steel and copper are rather
different and may lead to dimensional variations during the start-up of the pot; and
the thermal conductivity of copper and steel is rather different, which needs to be
taken into account for designing (and minimising) the heat transfer between the pot
and the aluminium busbar.
[0014] For this reason, in some prior art embodiments the copper inserts do not extend along
the whole length of the steel cathode bar, but a spacer section is provided at each
end of the cathode bar into which the copper insert does not extend. Also, the copper
bar can be made in two pieces separated in the centre of the cathode by a steel plug
and/or an air gap. Such a structure is described in
US 6,387,237 and
US 6,231,745 (Alcoa).
[0015] The opposite approach is proposed by
WO 2002/42525 (Servico), namely a cathode bar comprising a steel bar into which at each end a copper
bar is inserted, the copper insert extending beyond the end of the steel bar and ensuring
the electrical contact with the connection to the aluminium busbar.
[0016] The insertion of copper bars of complex shape into cathode bars has been used to
fine-tune the current distribution over the length of the cathode bar; such an embodiment
is described in
WO 03/014434 (Alcoa). Other structures ensuring a variation of the electrical conductivity of
the cathode bar along its length are described in
WO 2004/059039 (SGL Carbon) and in
WO 2008/062318 (Rio Tinto Alcan).
[0017] As can be seen, there has been a wealth of different designs of copper inserts in
cathode bars. However, three potential problems with copper inserts have hardly ever
been mentioned in the patent literature:
Firstly, as all these copper inserts are inlaid into grooves machined into a steel
bar, it should be noted that the machining of steel is a rather difficult and expensive
process which adds to the cost of the copper material.
Secondly, the melting point of copper (about 1080°C for pure copper) is rather close
to the temperature of the liquid phases in the pot (around 950°C to 1000°C). Knowing
that cathode blocks have a lifetime between typically 4 to 7 years, and knowing that
copper readily forms a dense oxide layer on its surface, and knowing that the melting
point of steel is much higher than that of copper, and knowing that spent cathodes
usually show a significant deformation of the steel cathode bars, there can be some
concern about the long-term behaviour of copper-inserted cathode bars in relation
with their dimensional stability (related to possible local melting or at least creep)
and contact resistance.
Thirdly, it is desirable to be able to separate the copper insert from the steel bar
in spent cathode blocks completely and easily (i.e. without intensive labour). There
are two reasons for this: copper is much more expensive than steel, and it is therefore
desirable to recover the copper insert from the steel bar for recycling. Furthermore,
if the steel bar is recycled with the copper insert or with a significant amount of
copper residues, not only the copper will be lost for separate recycling but also
the copper may poison the steel batch into which the steel bar is recycled.
[0018] RU 2209856 C1 discloses a cathode element for a Hall-Heroult electrolysis cell, comprising carbonaceous
cathode blocks provided with grooves and a current collector bar made of copper inserted
into each of the grooves. The bars are T-, L- or channel shaped and there is no metal
sealing used.
[0019] It is the objective of the present invention to come up with a new design of cathode
bars that solves all of the abovementioned problems.
Figures
[0020]
Figures 1 illustrates the prior art. Figure 1a shows a schematic perspective view
of a cathode block 1 according to prior art. The steel bar 2 is fitted into groove at the bottom surface 5 of the cathode block 1. A copper bar insert 3 is fitted into a groove machined into the higher surface 6 of the steel bar 2.
Figure 1b shows a cross section of another prior art cathode block in which the steel
bar 2 is fitted into the groove using cast iron 4; a copper bar 3 is fitted into the steel bar 2.
Figure 1c shows another variant of the prior art embodiment of figure 1a with two
parallel cathode bars 2 made from steel with a copper insert 3.
Figures 2 to 5 illustrate embodiments of the cathode block according to the invention
that will be discussed in more detail below.
Figures 6 and 7 show schematic perspective views of two examples of a connector (figure
6) or a connection plate (figure 7) used to connect the copper bar of the cathode
block according to the invention to a busbar.
Object of the invention
[0021] According to the invention the problem is solved by using a cathode bar comprising
a full copper bar.
[0022] Unlike prior art cathode bars using copper inserts, the cathode bar according to
the present invention does not use a copper bar that is inserted into a steel bar,
but uses the copper bar in direct contact with the carbonaceous material of the cathode
block or of an intermediate carbonaceous material that is in contact with the cathode
block.
[0023] The first object of the invention is a cathode element suitable for use in a Hall-Heroult
electrolysis cell, as defined in claim 1.
[0024] In an advantageous embodiment said cathode element does not comprise any other metallic
connection bar in direct contact with the carbonaceous material than connection bars
made in copper or copper alloys.
[0025] Said intermediate carbonaceous material can be compressed expanded graphite and/or
a cured carbonaceous seal. If a cured carbonaceous seal is used, a graphitized carbonaceous
seal is preferred. Said cured (and preferably graphitized) carbonaceous seal advantageously
comprises graphite particles. In a specific embodiment the connection bar is in direct
contact with compressed expanded graphite, and said expanded compressed graphite is
in direct contact with a cured (and preferably graphitized) carbonaceous seal, said
cured carbonaceous seal being in direct contact with the carbonaceous material of
the cathode block.
[0026] In specific embodiments, said metallic connection bar can have a round or a rectangular
cross section. In a specific embodiment said metallic connection bar has a rectangular
cross section, and an intermediate carbonaceous material is used in direct contact
with the metallic connection bar, said intermediate carbonaceous material being preferably
compressed expanded graphite.
[0027] Another object of the invention is a process for manufacturing a cathode element
suitable for use in Hall-Heroult electrolysis cell as defined in claim 11.
[0028] In a variant of this process, which is particularly suitable for rectangular metallic
contact bars, said intermediate carbonaceous material may comprise a seal, preferable
containing graphite particles, said seal being applied onto said metallic contact
bar and/or onto at least one face of said sheet of compressed expanded graphite.
[0029] Still another object of the invention is a process for producing aluminium in a Hall-Heroult
electrolysis cell, as defined in claim 15.
Description
[0030] The present invention applies to cathodes used in the Hall-Heroult process that form
the bottom of an electrolytic cell, said cathodes being assembled from individual
cathode blocks, each of which bears at least one cathode bar.
[0031] Said cathode blocks
10 are generally rectangular in shape and comprise an upper surface
50 (also called "hot surface") and a lower surface
15 (also called "cold surface"). They comprise at least one cathode bar. Said cathode
bar is usually inserted into a groove machined into the cold surface
15 of the cathode block
10.
[0032] According to the invention the problem is solved by using a cathode bar comprising
a full copper bar
13 over at least part of the length of said cathode block
10. That is to say that over at least part of the length of the cathode block the full
section of said cathode bar is copper.
[0033] The present inventors have found that copper bars can be used directly as cathode
bars, replacing the steel bars, instead of using them as inserts in steel bars. More
precisely, the copper bar can be used in direct contact with a carbonaceous material.
Said carbonaceous material can be the cathode block
10 itself, or an intermediate carbonaceous material
17 (shown on figure 4a) that is in direct contact with the metallic connection bar
13 made from copper or a copper alloy and also in direct contact with the carbon cathode
block
10. No metallic seal (such as steel or cast iron) is used.
[0034] The intermediate carbonaceous material
17 in direct contact with the metallic connection bar
13 must be somewhat deformable, such as to accommodate the higher thermal expansion
of the metallic connection bar
13, made from copper or a copper alloy, with respect to the material of the cathode
block
10. As an intermediate carbonaceous material
17 in direct contact with the copper bar
13 and the cathode block
10, compressed expanded graphite (most conveniently in the form of a sheet) can be used,
and/ or carbonaceous sealing paste.
[0035] Compressed expanded graphite is available in the form of sheet of different densities
and thickness from several manufacturers and under different tradenames (such as Papyex™
manufactured by Mersen and Sigraflex™ manufactured by SGL).
[0036] Concerning the carbonaceous sealing paste, the sealing paste advantageously includes
carbonaceous particles dispersed in a binder that has a high carbon yield after baking.
Said carbonaceous particles can be graphite particles. Such sealing pastes are commercially
available from different manufacturers and under various tradenames (Sealing paste
HCF 80 from Carbone Savoie for example). The thickness of the sealing paste is calculated
according to the width of the collector bar, taking into consideration the differential
of thermal expansions between the copper and the graphitized cathode block; a thickness
comprised between 15 and 25 mm can be used.
[0037] Considering the very different thermal expansion coefficients for the copper and
the cathode block, if the copper bar
17 is sealed inside the cathode block groove, and for avoiding any mechanical stresses
when starting the pot pre-heating, between the copper bar and the carbon material
of the cathode block, the copper should be covered by a deformable intermediate carbonaceous
material such as a sheet of compressed expanded graphite. It should be avoided to
seal directly the copper bar inside the cathode block groove.
[0038] The cathode blocks
10 are usually graphitized carbon blocks.
[0039] The expression "direct contact" means that there is direct contact between the copper
bar and the carbonaceous material (the carbonaceous material being the cathode block
material itself, or an intermediate carbonaceous material such as graphite sheet or
a carbonaceous sealing paste), without any intermediate metallic material (such as
a steel bar or steel shell, or a metallic sealing material (for example cast iron)).
The presence of the natural oxide layer on the copper bar is unavoidable and does
not prevent "direct contact".
[0040] According to one embodiment which is not according to the invention shown schematically
in figure 2a, the copper bar
13 is directly inserted into a groove machined in the lower surface
15 of the cathode block
10; this surface is advantageously the bottom surface (cold surface) of the cathode.
The groove extends advantageously parallel to the length of the cathode block
10. The groove extends over the whole length of the cathode block, and so does the cathode
bar. According to the invention shown in figure 2d the groove is made on each end
of the cathode block, and these two grooves are separated by a gap
16 centred at mid-length of the cathode block.
[0041] According to another embodiment shown on figures 2b and 2c, the copper bar is inserted
into a hole drilled into the cathode block
10. The hole extends parallel to the length of the cathode block. In an embodiment not
according to the invention (shown on figure 2b) a continuous hole is made in the cathode
block, a continuous copper bar
13 is inserted. In an embodiment according to the invention (shown on figure 2c), the
hole is discontinuous, and two half copper bars
13a,
13b are inserted from each extremity of the cathode block
10; inside the cathode block they are separated by a gap
16 centred at mid-length of the cathode block
10. Said hole is closer to the bottom surface
15 of the cathode block
10 than to the upper (hot) surface.
[0042] The cathode bars
13,
13a,
13b can be made with full cylindrical copper bars inserted into a cylindrical hole of
corresponding section. They can also be made with full rectangular copper bars inserted
into a rectangular groove of corresponding section that has been machined at the bottom
or at the sides of the cathode bar. In that case the rectangular copper bar can be
either directly in contact with the cathode block or the copper bars can be sealed
inside the grooves using a carbonaceous sealing paste, or there can be another intermediate
carbonaceous material such as a sheet of compressed expanded graphite. Other shapes
of cross section of the copper bar and the corresponding groove or hole can be used,
but are less preferred as a good surface contact between the cathode block and the
copper bar is desirable. One cathode bar or two or more cathode bars can be used on
each extremity of the cathode block. Figures 2d(1) and 2d(2) show embodiments with
one rectangular bar
13a,
13b at each end of the cathode block
10 that is inserted into a groove machined at the bottom face
15 of the cathode block
10; figure 2a(1) shows a continuous bar
13, figure 2d(1) a discontinuous bar
13a,
13b. Figures 2a(3) and 2d(3) show corresponding embodiment with two rectangular bars at
each end of the cathode block; figure 2a(3) represents continuous bars, figure 2d(3)
discontinuous bars. Figures 2b(2) and 2c(2) show embodiments with one round bar inserted
into a hole machined into the cathode block; figure 2b(2) represents a continuous
bar, figure 2c(2) represents discontinuous bars. Figures 2b(3) and 2c(3) show embodiments
with two round bars inserted into holes machined into the cathode block; figure 2b(3)
represents continuous bars, figure 2c(3) represents discontinuous bars.
[0043] Figure 3 shows an example: two cylindrical copper bars
13a,
13b having a diameter of 90 mm and a length e of 1850 mm each are inserted in a hole
made on both sides of the lower part of the cathode block
10; the total length a of the cathode block was 3250 mm, the total height
b was 425 mm, the total width c was 420 mm, and the cathode bar extends out of the
block by a length
f of about 525 mm; the gap
d between the two embedded ends of the copper bars was about 600 mm. The axis of the
collector bars made with full copper cylindrical rod was located at 70 mm from the
bottom (cold) face of the cathode block. Figure 3 corresponds to the embodiment schematically
represented by figure 2c(2).
[0044] As the electrical conductivity of the copper (1,68 x 10
-8 Ω m) is about 6 times higher than that of steel (10 x 10
-8 Ω m), a given conductivity value of the cathode bar can be obtained with a copper
bar having a smaller diameter or section for rectangular copper bars than the steel
bar. As an example, to replace a steel bar with a cross section of about 221 cm
2 (width 170 mm, height 130 mm), a copper cross section of at least 37,14 cm
2 is needed, corresponding to a cylindrical bar of 68.77 mm in diameter.
[0045] Using copper bars
13 with larger cross section will lead to an increase in electrical conductivity. In
the above example, the increase of the cathode bar section for the copper bar beyond
37.14 cm
2 will lead to a higher conductivity of the copper bar compared to the steel bar: the
cylindrical copper bar with a diameter of 90 mm has a conductivity that has increased
by about 71% compared to that of the steel bar (63.6 cm
2 compared to 37.14 cm
2 for the equivalent resistance using steel bars). This decrease in ohmic losses in
the cell leads to a decrease in voltage drop across the cell. Such an embodiment is
shown in figure 4b; this copper rod
13 is inserted into a hole drilled into the lower part of the cathode block
10, parallel to its length.
[0046] For replacing a rectangular steel bar with a cross section of 221 cm
2 (width 170 mm, height 130 mm) by a rectangular copper bar, a copper cross section
of 37.14 cm
2 would be sufficient; such a rectangular copper bar could have a cross section of
approximately 60 mm by 65 mm.
[0047] For the case of using rectangular copper bar and to obtain the same increase of conductivity
of 71%, the copper section would have to be about 69 cm
2, which could be achieved by a rectangular copper rod of 70 by 100 mm. Such a rectangular
copper bar should be installed horizontally inside the cathode block groove, meaning
that the groove must have between 120 to 125 mm of width and between 72.5 and 75 mm
of depth if a carbonaceous sealing material (sheet of compressed expanded graphite
and carbonaceous sealing paste) is used.
[0048] Another consequence of the higher electrical conductivity of copper (and the smaller
diameter of the copper bar with respect to the steel bar) is the possibility to increase
the depth of the copper bar in the cathode with respect of that of the steel bar,
that is to say the spacing
h between the upper surface
50 of the cathode block
10 that is in contact with the liquid aluminium and the upper surface
51 of the cathode bar
13. In the above example, the full copper bar (cylindrical rod of 90 mm diameter, i.e.
a cross section of 63.6 cm
2) replaces a rectangular steel bar of much greater height (130 mm, width 170 mm, i.e.
a cross section of 221 cm
2)), and the thickness of carbonaceous material in the cathode block is increased from
h = 275 mm to
h = 310 mm (see figure 4b), taking into account the additional thickness of the cast
irons seal.
[0049] Figure 4b shows a cylindrical copper bar
13 installed on the bottom of the cathode block
10. In this example, if cylindrical copper bars of 90 mm of diameter is used, the thickness
h of carbonaceous material between the liquid aluminium and the upper surface of the
copper bar can be increased from 275 mm to 310 mm. This increase of 35 mm (or 13 %
more material) corresponds to about 8.4 months of wear for the cathode blocks, and
therefore extends the life expectancy of the pot by 0.7 year.
[0050] In a variant of the rectangular copper rods, shown on figure 4a, a rectangular copper
bar
13 with a width of 100 mm and an height of 70 mm (cross section = 70 cm
2) has been inserted in a rectangular groove made in the bottom of the cathode block
10. The total height
b of the cathode block is 425 mm. In this case, the thickness
h of carbon material above the copper bar (355 mm) is still higher than in the case
of a cylindrical bar inserted into a hole: plus 45 mm. This increase of 80 mm (or
29 % more material) corresponds to about 1.6 years of wear for the cathode blocks,
and therefore extends the life expectancy of the pot by 1.6 years. On the figure 4a,
the intermediate carbonaceous material
17 is used between the rectangular copper bar
13 and the carbonaceous material of the cathode block
10. This intermediate carbonaceous material might be avoided on the horizontal faces
of the copper bar as very small amount of current is going through this top surface.
[0051] In a variant of the rectangular copper rod shown in figure 4c, the rectangular copper
bar can be inserted directly in contact inside the groove made on the bottom of the
cathode block, providing that the right adjustment is made for the copper rod inside
the groove.
[0052] As there is a temperature gradient across the thickness of the cathode block, the
copper cathode bar will heat less than the steel cathode bar. Moreover, due to the
higher thermal conductivity of copper compared to steel, the copper cathode bar will
transport more heat than a steel cathode bar of identical design.
[0053] Figure 5 shows a cross section across a Hall-Heroult electrolysis pot
20 according to the invention with cylindrical copper bars; the cross section is parallel
to the width of the pot. The copper cathode bars
23a,
23b are discontinuous, with a gap
26 in the centre of the length of the cathode block
21.
[0054] A possible problem related to the use of a full copper cathode bar is related to
overheating of the cell. In modern Hall-Heroult cells the temperature of the liquid
aluminium sheath
29 in contact with the upper surface
27 of the cathode blocks
21 is normally about 960°C to 965°C, and under these operating conditions the temperature
in the centre of the cathode block can reach 950°C. However, in case of abnormal conditions
the temperature of the aluminium sheath can increase to over 1000°C, typically up
to 1080°C for short duration of up to 24 hours. Such overheating may occur especially
when starting the pot. While modern potlines and sophisticated pot control can avoid
and/or limit overheating (above 1080°C), this event cannot be fully excluded throughout
the normal lifetime of an electrolysis pot (5 to 7 years).
[0055] Knowing that the melting point of pure copper is about 1084°C, it must be ruled out
that in case of abnormal overheating the cathode bar
13 suffers irreversible damage. Increasing the carbon thickness
h over the upper surface of the copper bar
13 will decrease the temperature seen by the copper bar
13 by a few degrees. Inserting the copper bar
13 in a groove at the bottom
15 of the cathode block
10 will further increase the carbon thickness
h and gain a few degrees. In any case, using the cathode block according to the invention
in a Hall-Heroult cell for the manufacture of aluminium requires a careful control
of operating conditions in order to avoid overheating of the cell, or at least to
limit it to small overheat and/or for short period periods of time.
[0056] In a preferred embodiment of the invention the copper cathode bars
13,
23 are made from 99.99% copper that is oxygen free (OFE Copper grade).
[0057] In another embodiment copper alloys are used that have a higher melting point (by
about 15°C to 35°C) than pure copper; however, such alloys will have a lower electrical
conductivity.
[0058] Another potential problem related to the use of unsealed copper bars is the copper
- carbon interface: copper oxidation or other changes in properties of the copper
close to the carbon block may lead to higher electrical contact resistance. For this
reason, and in case of rectangular copper bars, it is preferable to wrap the rectangular
copper bars with graphite sheet (graphite "paper") before sealing it inside the cathode
block groove shown in figure 4a. This graphite sheet (representing an intermediate
carbonaceous material) separates the copper bar from the cathode block material, which
is advantageous considering the very different thermal expansion coefficients of copper
and of the cathode block material; the graphite foil can also act as a barrier preventing
the oxidation of the copper surface.
[0059] When designing cathode blocks according to the invention in which steel cathode bars
are replaced by copper cathode bars, another potential problem is related to the different
thermal expansion coefficient of steel and copper: about 16.6 x 10
-6 /°C for copper, about 12 x 10-6 /°C for average steel used in cathode bars and cast
iron used for sealing steel cathode bars. The thermal expansion coefficient for typical
graphite (such as D grade from Carbone Savoie) is about 5.3 x 10-6 /°C The thermal
expansion of copper is much higher than that of graphite, and higher than that of
steel. This needs to be taken into account when determining the width of the hole
or groove made in the cathode to insert the copper bar.
[0060] The invention has numerous advantages. A gain in voltage drop of more than 100 mV
can be reached in industrial potlines.
[0061] The increase in capital cost over prior art embodiments is not very significant.
Copper is more expensive than steel, but this difference is partially offset because
less copper is needed. While copper does add to the initial cost of the cathode bar
according to the invention, part of this initial cost is recovered by copper recycling.
Since no cast iron is needed, the cost of cast iron and necessary man hours (and possibly
the cost of the cast iron plant itself) is saved. On the other hand, the machining
operation for the groove (and especially for the hole) is more expensive than for
the groove according to prior art because the dimensional tolerance is a critical
issue, while a steel bar can be sealed with cast iron into a groove with no special
requirement of dimensional tolerance. Referring to the rectangular copper bars, the
groove to machine is not expensive as a gap is needed to seal the copper bars, the
groove machining tolerance can be the same as the genuine one for steel bars: +/-
2 mm. Even for the cylindrical copper bars that need a very tight tolerance that may
be difficult to achieve with some older machining equipment, the inventors have found
that the total additional cost of the cathode block according to the invention is
offset by the higher energy efficiency of such a cell.
[0062] The cathode block according to the invention needs to be connected to the cathodic
bus bar. Specific connectors
30 are needed to connect cylindrical copper bars
33 to the cathodic bus bar.
[0063] For the cylindrical copper bars
33, an advantageous solution for connecting the copper bar extremities to the original
cathode flex is shown in figure 6: to connect the copper tabs
34a,
34b connected to the aluminium flexes (not shown on the figure) to the cylindrical copper
bar
33 extremities, two special copper plates
39a,
39b in contact with the copper cathode bar
33 are added: these copper plates
39a,
39b have a cylindrical recession on one side (in contact with the copper bar 33), the
other side in contact with the genuine copper pads
34a,
34b being flat. A means of pressing the two copper tabs against the coper bar can be
added (not shown on the figure). Advantageously the diameter of the cylindrical recess
is about the same as that of the copper bar
33; as an example, for a copper bar of diameter 90 mm a recess of diameter 90 mm is
used.
[0064] Said copper tabs
34a,
34b can be connected to said aluminium flexes by any appropriate means. As an example,
said copper tabs
34a,
34b can be part of (or can be in contact with the copper part of) a copper / aluminium
transition joint (clad) or a copper / titanium / aluminium tri-metallic clad, said
aluminium flexes being welded to the aluminium part of said transition joint.
[0065] As shown in figure 7, another simple way to connect the cathode flexes (not shown)
to the extremities
72 of the copper bar
33 is to add a metallic piece
70 to the extremities
72 of the copper bar
33, preferably by welding. Said metallic piece
70 comprises a steel plate providing a substantially vertical (preferably flat) contact
surface
71 onto which the steel part of a tri-metallic clads (composed of aluminium, titanium
and steel parts, not shown) can be welded. This solution is far less expensive than
to add extra copper plates between copper tabs and copper bars, and, more importantly,
the voltage drop, measured from the copper bar and the bus bar where the aluminium
cathode flex is welded, will be very low.
1. Cathode element suitable for use in a Hall-Heroult electrolysis cell, comprising
- a cathode block comprising a carbonaceous material,
- at least one metallic connection bar made in copper or copper alloys,
wherein said metallic connection bar is inserted into a groove or bore in direct contact
with a carbonaceous material,
wherein said metallic connection bars extend from both extremities towards the centre
length of the cathode block, leaving a gap in the centre of the cathode block,
and wherein said carbonaceous material can be the carbonaceous material of said cathode
block, or an intermediate carbonaceous material that is in direct contact with the
carbonaceous material of said cathode block.
2. Cathode element according to claim 1, wherein said cathode element does not comprise
any other metallic connection bar in direct contact with the carbonaceous material
than connection bars made in copper or copper alloys.
3. Cathode element according to claim 1 or 2, wherein said intermediate carbonaceous
material is compressed expanded graphite and/or a cured carbonaceous seal (and preferably
a graphitized carbonaceous seal).
4. Cathode element according to claim 3, wherein said cured (and preferably graphitized)
carbonaceous seal comprises graphite particles.
5. Cathode element according to claim 3 or 4, wherein the connection bar is in direct
contact with compressed expanded graphite, and wherein said expanded compressed graphite
is in direct contact with a cured (and preferably graphitized) carbonaceous seal,
said cured carbonaceous seal being in direct contact with the carbonaceous material
of the cathode block.
6. Cathode element according to any of claims 1 to 5, wherein said metallic connection
bar has a round or a rectangular cross section.
7. Cathode element according to any of claims 1 to 6, wherein said metallic connection
bar has a rectangular cross section, and an intermediate carbonaceous material is
used in direct contact with the metallic connection bar, said intermediate carbonaceous
material being preferably compressed expanded graphite.
8. Cathode element according to any of claims 1 to 7, wherein a metallic piece is fixed
to the extremities of the metallic connection bar, preferably by welding, said metallic
piece comprising a steel plate providing a substantially vertical, preferably flat,
contact surface.
9. Cathode element according to claim 8, wherein tri-metallic clads are welded onto said
contact surface, which can be electrically connected to a cathodic bus bar.
10. Cathode element according to claim 9, wherein said tri-metallic clads are aluminium
- titanium - steel tri-metallic clads.
11. Process for manufacturing a cathode element suitable for use in Hall-Heroult electrolysis
cell comprising the steps of:
- providing a cathode block comprising a carbonaceous material and at least one metallic
contact bar made from copper or a copper alloy,
- machining at least one groove or drilling at least one bore in a direction parallel
to the length of said cathode block, at each end of said cathode block, separated
by a gap in the centre of said carbon block,
- optionally covering at least part of the surface of said metallic bar with an intermediate
carbonaceous material,
- inserting said metallic connection bar into each of said grooves or said bores,
wherein said metallic bar is inserted in such a way that direct contact between said
carbonaceous material and the metallic bar is ensured, without the use of a metallic
sealing material, said carbonaceous material in direct contact with said metallic
contact bar being either the cathode block itself or said intermediate carbonaceous
material.
12. Process according to claim 11, wherein said intermediate carbonaceous material comprises
a sheet of compressed expanded graphite.
13. Process according to claim 11 or 12, wherein said intermediate carbonaceous material
comprises a seal, preferably containing graphite particles, said seal being applied
onto said metallic contact bar and/or onto at least one face of said sheet of compressed
expanded graphite.
14. Process according to any of claims 11 to 13, wherein said intermediate carbonaceous
material is used for rectangular metallic contact bars.
15. Process for producing aluminium in a Hall-Heroult electrolysis cell, wherein said
electrolysis cell comprises one or more cathode elements according to any of claims
1 to 10.
1. Kathodenelement, das zur Verwendung in einer Hall-Heroult-Elektrolysezelle geeignet
ist, umfassend
- einen Kathodenblock, der ein kohlenstoffhaltiges Material umfasst,
- mindestens eine metallische Verbindungsstange, die aus Kupfer oder Kupferlegierungen
gefertigt ist,
wobei die metallische Verbindungsstange in direktem Kontakt mit einem kohlenstoffhaltigen
Material in eine Nut oder Bohrung eingefügt ist,
wobei sich die metallischen Verbindungsstangen von beiden Enden zur mittleren Länge
des Kathodenblocks hin erstrecken, wobei sie in der Mitte des Kathodenblocks einen
Spalt lassen,
und wobei es sich bei dem kohlenstoffhaltigen Material um das kohlenstoffhaltige Material
des Kathodenblocks oder ein kohlenstoffhaltiges Zwischenmaterial handeln kann, das
sich mit dem kohlenstoffhaltigen Material des Kathodenblocks in direktem Kontakt befindet.
2. Kathodenelement nach Anspruch 1, wobei das Kathodenelement keine andere metallische
Verbindungsstange in direktem Kontakt mit dem kohlenstoffhaltigen Material als aus
Kupfer oder Kupferlegierungen gefertigte Verbindungsstangen umfasst.
3. Kathodenelement nach Anspruch 1 oder 2, wobei es sich bei dem kohlenstoffhaltigen
Zwischenmaterial um verdichteten Blähgraphit und/oder eine ausgehärtete kohlenstoffhaltige
Dichtung (und vorzugsweise eine graphitisierte kohlenstoffhaltige Dichtung) handelt.
4. Kathodenelement nach Anspruch 3, wobei die ausgehärtete (und vorzugsweise graphitisierte)
kohlenstoffhaltige Dichtung Graphitpartikel umfasst.
5. Kathodenelement nach Anspruch 3 oder 4, wobei sich die Verbindungsstange in direktem
Kontakt mit verdichtetem Blähgraphit befindet, und wobei sich der verdichtete Blähgraphit
in direktem Kontakt mit einer ausgehärteten (und vorzugsweise graphitisierten) kohlenstoffhaltigen
Dichtung befindet, wobei sich die ausgehärtete kohlenstoffhaltige Dichtung in direktem
Kontakt mit dem kohlenstoffhaltigen Material des Kathodenblocks befindet.
6. Kathodenelement nach einem der Ansprüche 1 bis 5, wobei die metallische Verbindungsstange
einen runden oder einen rechteckigen Querschnitt aufweist.
7. Kathodenelement nach einem der Ansprüche 1 bis 6, wobei die metallische Verbindungsstange
einen rechteckigen Querschnitt aufweist, und ein kohlenstoffhaltiges Zwischenmaterial
in direktem Kontakt mit der metallischen Verbindungsstange verwendet wird, wobei es
sich bei dem kohlenstoffhaltigen Zwischenmaterial vorzugsweise um verdichteten Blähgraphit
handelt.
8. Kathodenelement nach einem der Ansprüche 1 bis 7, wobei an den Enden der metallischen
Verbindungsstange ein metallisches Teil befestigt ist, vorzugsweise durch Schweißen,
wobei das metallische Teil eine Stahlplatte umfasst, die eine im Wesentlichen vertikale,
vorzugsweise flache, Kontaktfläche bereitstellt.
9. Kathodenelement nach Anspruch 8, wobei an die Kontaktfläche Trimetallauflagen geschweißt
sind, die mit einer kathodischen Sammelschiene elektrisch verbunden werden können.
10. Kathodenelement nach Anspruch 9, wobei es sich bei den Trimetallauflagen um Aluminium-Titan-Stahl-Trimetallauflagen
handelt.
11. Verfahren zur Herstellung eines Kathodenelements, das zur Verwendung in einer Hall-Heroult-Elektrolysezelle
geeignet ist, umfassend die Schritte des:
- Bereitstellens eines Kathodenblocks, der ein kohlenstoffhaltiges Material umfasst,
und mindestens einer metallischen Kontaktstange, die aus Kupfer oder einer Kupferlegierung
gefertigt ist,
- Spanens von mindestens einer Nut oder Bohrens von mindestens einer Bohrung in einer
zur Länge des Kathodenblocks parallelen Richtung an jedem Ende des Kathodenblocks,
die durch einen Spalt in der Mitte des Kohlenstoffblocks getrennt sind,
- gegebenenfalls Überziehens von mindestens einem Teil der Fläche der metallischen
Stange mit einem kohlenstoffhaltigen Zwischenmaterial,
- Einfügens der metallischen Verbindungsstange in jede der Nuten oder der Bohrungen,
wobei die metallische Stange in einer derartigen Weise eingefügt wird, dass direkter
Kontakt zwischen dem kohlenstoffhaltigen Material und der metallischen Stange sichergestellt
ist, ohne der Verwendung eines metallischen Dichtungsmaterials, wobei es sich bei
dem kohlenstoffhaltigen Material, das sich mit der metallischen Kontaktstange in direktem
Kontakt befindet, um entweder den Kathodenblock selbst oder das kohlenstoffhaltige
Zwischenmaterial handelt.
12. Verfahren nach Anspruch 11, wobei das kohlenstoffhaltige Zwischenmaterial eine Lage
aus verdichtetem Blähgraphit umfasst.
13. Verfahren nach Anspruch 11 oder 12, wobei das kohlenstoffhaltige Zwischenmaterial
eine Dichtung umfasst, die vorzugsweise Graphitpartikel enthält, wobei die Dichtung
auf die metallische Kontaktstange und/oder auf mindestens eine Seite der Lage aus
verdichtetem Blähgraphit aufgebracht wird.
14. Verfahren nach einem der Ansprüche 11 bis 13, wobei das kohlenstoffhaltige Zwischenmaterial
für rechteckige metallische Kontaktstangen verwendet wird.
15. Verfahren zum Produzieren von Aluminium in einer Hall-Heroult-Elektrolysezelle, wobei
die Elektrolysezelle ein oder mehrere Kathodenelemente nach einem der Ansprüche 1
bis 10 umfasst.
1. Élément de cathode adapté pour son utilisation dans une cellule d'électrolyse Hall-Héroult,
comprenant
- un bloc de cathode comprenant un matériau carboné,
- au moins une barre de connexion métallique faite de cuivre ou d'alliages de cuivre,
dans lequel ladite barre de connexion métallique est insérée dans une rainure ou un
alésage en contact direct avec un matériau carboné,
dans lequel lesdites barres de connexion métalliques s'étendent à partir des deux
extrémités en direction de la longueur centrale du bloc de cathode, laissant un vide
dans le centre du bloc de cathode,
et dans lequel ledit matériau carboné peut être le matériau carboné dudit bloc de
cathode, ou un matériau carboné intermédiaire qui est en contact direct avec le matériau
carboné dudit bloc de cathode.
2. Élément de cathode selon la revendication 1, dans lequel ledit élément de cathode
ne comprend pas d'autre barre de connexion métallique en contact direct avec le matériau
carboné que les barres de connexion faites de cuivre ou d'alliages de cuivre.
3. Élément selon la revendication 1 ou 2, dans lequel ledit matériau carboné intermédiaire
est du graphite expansé compressé et/ou un joint carboné durci (et de préférence un
joint carboné graphité).
4. Élément de cathode selon la revendication 3, dans lequel ledit joint carboné (et de
préférence graphité) durci comprend des particules de graphite.
5. Élément de cathode selon la revendication 3 ou 4, dans lequel la barre de connexion
est en contact direct avec le graphite expansé compressé, et dans lequel ledit graphite
expansé compressé est en contact direct avec un joint carboné (et de préférence graphité)
durci, ledit joint carboné durci étant en contact direct avec le matériau carboné
du bloc de cathode.
6. Élément de cathode selon l'une quelconque des revendications 1 à 5, dans lequel ladite
barre de connexion métallique a une section transversale ronde ou rectangulaire.
7. Élément de cathode selon l'une quelconque des revendications 1 à 6, dans lequel ladite
barre de connexion métallique a une section transversale rectangulaire, et un matériau
carboné intermédiaire est utilisé en contact direct avec la barre de connexion métallique,
ledit matériau carboné intermédiaire étant de préférence du graphite expansé compressé.
8. Élément de cathode selon l'une quelconque des revendications 1 à 7, dans lequel une
pièce métallique est fixée aux extrémités de la barre de connexion métallique, de
préférence par soudage, ladite pièce de connexion métallique comprenant une plaque
d'acier fournissant une surface de contact sensiblement verticale, de préférence plate.
9. Élément de cathode selon la revendication 8, dans lequel des joints tri-métalliques
sont soudées sur ladite surface de contact, qui peut être électriquement raccordée
à une barre omnibus cathodique.
10. Élément de cathode selon la revendication 9, dans lequel lesdits joints tri-métalliques
sont des joints tri-métalliques d'aluminium-titane-acier.
11. Procédé de fabrication d'un élément de cathode adapté pour son utilisation dans une
cellule d'électrolyse Hall-Héroult comprenant les étapes de :
- fourniture d'un bloc de cathode comprenant un matériau carboné et au moins une barre
de contact métallique faite de cuivre ou d'un alliage de cuivre,
- usinage d'au moins une rainure ou forage d'au moins un alésage dans une direction
parallèle à la longueur dudit bloc de cathode, au niveau de chaque extrémité dudit
bloc de cathode, séparé par un espace dans le centre dudit bloc de carbone,
- couverture facultative d'au moins une partie de la surface sur ladite barre métallique
avec un matériau carboné intermédiaire,
- insertion de ladite barre de connexion métallique dans chacune desdites rainures
ou chacun desdits alésages, dans lequel ladite barre métallique est insérée de telle
manière qu'un contact direct entre ledit matériau carboné et ladite barre métallique
est garanti, sans utiliser de matériau de scellement métallique, ledit matériau carboné
est en contact direct avec ladite barre de contact métallique étant soit le bloc de
cathode lui-même soit ledit matériau carboné intermédiaire.
12. Procédé selon la revendication 11, dans lequel ledit matériau carboné intermédiaire
comprend une feuille de graphite expansé compressé.
13. Procédé selon la revendication 11 ou 12, dans lequel ledit matériau carboné intermédiaire
comprend un joint, de préférence contenant des particules de graphite, ledit joint
étant appliqué sur ladite barre de contact métallique et/ou sur au moins une face
de ladite feuille de graphite expansé compressé.
14. Procédé selon l'une quelconque des revendications 11 à 13, dans lequel ledit matériau
carboné intermédiaire est utilisé pour des barres de contact métalliques rectangulaires.
15. Procédé de production d'aluminium dans une cellule d'électrolyse Hall-Héroult, dans
lequel ladite cellule d'électrolyse comprend un ou plusieurs éléments de cathode selon
l'une quelconque des revendications 1 à 10.