Note: Descriptions are shown in the official language in which they were submitted.
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Method for producing a cathode steel bar with copper insert, and method for
removing a copper insert from a used cathode bar
Technical field
The invention relates to the technical field of electrolysis in molten salts
for making
aluminium using the Hall-Heroult process. More precisely, the present
invention relates
to improved cathode steel bars with copper insert, a method for producing such
cathode
steel bars possibly from used cathode steel bars, and a method for removing
copper
inserts from used cathode steel bars.
Background art
Aluminium is commonly produced by electrolysis of alumina (aluminium oxide)
dissolved in a molten cryolite bath. A Hall-Heroult type electrolytic cell for
such
production comprises a steel shell (pot shell) with a lining of refractory
material, where
is the bottom of the cell is a carbon cathode having several current
collectors embedded
therein, and several anode blocks that are partly submerged in the electrolyte
bath, and
arranged at a distance above the cathode. In industrial production of
aluminium the cells
of the Hall-Heroult type are connected electrically in series, and the
solution of alumina
in molten cryolite is brought to a temperature up to about 980 C by the
heating effect
of the current traversing through the cell. The thus formed aluminium metal
accumulates in the cell bottom, on the cathode surface, and is regularly
tapped from the
cell.
In more detail, the cathode consists of a carbon liner with several cathode
steel bars to
conduct electric current out of the cell. The cathode steel often has copper
inserts at
least along parts of the length of the cathode steel, in order to improve the
distribution
of current along the cathode, see e.g. NO 343609, WO 01/63014, WO 01/27353.
The lifetime of a cathode in an electrolysis cell may be a few years before
relining of
the cell is needed. Used cathode steel with copper inserts usually has a low
value for
recycling, and must be replaced by new ones in a lined cell.
There are various ways to produce cathode steel with a copper insert. One
method is to
drill a longitudinal hole in the steel bar with a diameter corresponding to a
copper rod to
be inserted. The steel bar is heated and crimped around the copper rod. This
method
requires high precision as the clearance between the hole in the steel and the
copper rod
must be small, approx. 0.5 mm, to ensure sufficiently good contact between the
steel
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and the copper insert, which is important to obtain good conductivity. The
said method
also has limitations in terms of the design of the cross section of the copper
insert.
Therefore, there is a desire for an improved method of producing cathode steel
bars with
copper insert of good quality, e.g. in terms of conductivity, and without
pores or suction
in the copper inserts. Thus, it is an objective of the present invention to
mitigate,
alleviate or eliminate one or more of the disadvantages of today's solutions
in this
technical field.
io Summary of invention
In a first aspect, the present invention relates to a method of producing a
cathode steel
bar with copper insert for use in an electrolytic cell for the electrolytic
production of
aluminium using the Hall-Heroult process, comprising:
i) providing a steel portion of the cathode steel bar equipped with a cavity
for the copper
insert and a sleeve on the upper part,
providing one or more solid copper item(s) sized to be inserted into the
cavity of the
steel portion,
entering the copper item(s) into the steel portion through the sleeve,
positioning at least one inductor in close proximity to at least parts of the
outside of the
steel portion,
supplying electric energy to the inductor, causing induction heating of the
cathode steel
bar to a temperature sufficiently high and for a time sufficiently long to
molten at least
the outer part of the copper item(s),
cooling, and solidifying the molten copper;
or
ii) providing a steel portion of the cathode steel bar equipped with a cavity
for the
copper insert and a sleeve on the upper part,
positioning at least one inductor in close proximity to at least parts of the
outside of the
steel portion,
supplying electric energy to the inductor, causing induction preheating of the
cathode
steel bar,
providing molten copper,
pouring the molten copper into the cavity of the steel portion through the
sleeve,
cooling, and solidifying the molten copper.
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The method of the present invention may be a circular process where copper
insert of an
used cathode bar is the source of the copper insert of the cathode bar to be
prepared.
That is, the present invention provides a method enabling recyclability of
used cathode
bars with copper insert. This is advantageous in terms of flexibility,
production time and
cathode performance, as well as it is environmental friendly.
In a second aspect the present invention relates to a cathode steel bar with
copper insert
for use in an electrolytic cell for the electrolytic production of aluminium
using the
Hall-Heroult process, obtained by the process above.
The present invention provides cathode bars with improved performance in terms
of
conductivity.
In a third aspect the present invention relates to a method of removing a
copper insert
from an used cathode bar used in an electrolytic cell for the electrolytic
production of
aluminium using the Hall-Heroult process, comprising:
providing an used cathode bar,
positioning an inductor in close proximity to at least parts of the cathode
bar,
supplying electric energy to the inductor, causing induction heating of the
cathode bar
to a temperature above the melting temperature of copper, forming molten
copper,
pouring the molten copper into a holding furnace; or
casting the molten copper into suitable items.
Hence, it is to be understood that the herein disclosed invention is not
limited to the
particular component parts of the device described or steps of the methods
described
since such device and method may vary. It is also to be understood that the
terminology
used herein is for purpose of describing particular embodiments only, and is
not
intended to be limiting. It should be noted that, as used in the specification
and the
appended claim, the articles "a", "an", "the", and "said" are intended to mean
that there
are one or more of the elements unless the context explicitly dictates
otherwise. Thus,
for example, reference to "a unit" or "the unit" may include several devices,
and the
like. Furthermore, the words "comprising", "including", "containing" and
similar
wordings does not exclude other elements or steps.
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The terms "cathode bar(s)", "cathode steel bar(s)", "steel bar(s)", "current
collector(s)",
"cathode collector bar(s)", "collector bar(s)" and "current collecting
bar(s)", may be
used interchangeably in the present disclosure, and should be understood to
denote the
same entity, unless other stated.
The term copper as used herein includes pure copper and copper alloys.
The term "cavity" as used herein is to be understood as a confined space that
may be
open in one or more ends. A longitudinal hole in a cathode bar, open in one or
both
ends, is included in this term.
The terms "used cathode bar(s)" and "old cathode bar(s)" or any variations of
the term
"cathode bar(s)" as defined above starting with the word "used" or "old", may
be used
interchangeably in the present disclosure, and should be understood to denote
the same
entity, unless other stated.
The terms "collar" and "sleeve" may be used interchangeably and has the
function as
feeder in the present disclosure.
Brief description of drawings
Figure 1: Illustrates a cathode bar according to the invention produced
by inserting
a solid copper item into the steel portion.
Figure 2: Illustrates a cathode bar according to the invention produced
by pouring
molten copper into the steel portion (top filling).
Figure 3: Illustrates a cathode bar according to the invention produced by
filling
molten copper into the steel portion from the bottom/lower part.
Figure 4: Illustrates a cathode bar according to the invention produced
by filling
molten copper into the steel portion from the bottom/lower part and using
counter gravity casting.
Figure 5: Illustrates a cathode bar according to the invention produced by
filling
molten copper into the steel portion from the bottom.
Figure 6: Illustrates a further cathode bar according to the invention
produced by
filling molten copper into the steel portion from the bottom.
Figure 7: Illustrates an "over the lip" filling device for filling molten
in production
of a cathode bar with copper insert.
Figure 8: Illustrates a bottom tapping device for filling molten copper
in production
of a cathode bar with copper insert.
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Figure 9: Illustrates removing of a copper insert from an old cathode bar
according
to the invention.
Figure 10: Photo of a section of a cathode bar with copper insert
according to the
invention.
5
The figures 1-6 and 9 illustrates various embodiments of corresponding cathode
bars.
Thus, the structural parts of the cathode bars have the same reference
numbers.
Detailed description
io In the following, the present method for producing a cathode steel bar
with copper insert
for use in an electrolytic cell for the electrolytic production of aluminium
using the
Hall-Heroult process, will be described and explained by way of examples and
with
reference to the accompanying drawings in which the same reference numbers
refer to
the same or technically equivalent elements, unless otherwise stated.
Fig. 1 illustrates one embodiment of the present invention, showing a cross-
section of a
cathode bar comprising a steel portion 2 equipped with a cavity for insertion
of solid
copper, and a collar or sleeve 4 of a metal material such as steel or any
other suitable
refractory material at the upper part acting as a feeder. The here illustrated
cathode bar
comprises a copper insert 1 designed as a solid rod/bar. A closing device 5 of
a suitable
material to prevent heat loss such as steel, titan etc., may be placed at the
upper end of
the cathode bar to ensure proper seal of the copper insert 1 within the steel
portion 2.
It should be understood that the solid copper which is the starting material
of the copper
insert can have several designs, e.g. a circular rod, or a rod of any
geometry, or ingots or
bars of any design. The only requirement of the solid copper item is that it
is sized to be
inserted into the cavity of the steel portion. For instance, the solid copper
may be a rod
with a smaller diameter than the cavity in the steel portion of the cathode
bar. Another
alternative is to use several copper items such as ingots and bars.
In an embodiment the method comprises entering one or more solid copper
item(s) 1
is/are into the steel portion 2 through the sleeve 4, and heating the steel
portion 2,
preferably using induction heating, until at least the outer part of the solid
copper items
inserted therein is molten. It may also be heated until the solid copper is
fully molten.
In induction heating a work-piece is heated by eddy currents induced in the
work-piece.
An induction heating power supply converts alternating current (AC) line power
to a
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higher frequency AC and delivers the higher frequency AC to an inductor
wherein an
electromagnetic field is created within the coil of the inductor. Eddy
currents will be
induced in an electrically conductive work-piece placed in the electromagnetic
field,
generating heat in the work-piece. The inductor is commonly water cooled
copper
conductors made of e.g. copper tubes, profiles, plates or machined copper
parts. The
design of the inductor influences inter alia energy transfer to work-piece,
heating rate
and heating efficiency. Inductors may be helix formed or prepared from plates
geometry, and the number of windings may vary. The present invention is not
limited to
any specific design or type of inductor. Furthermore, one or more inductors,
which can
io be controlled separately or as a whole, may be used. For instance, the
inductor may
appear as being divided.
At least one inductor 3 is positioned in close proximity to at least parts of
the outside of
the steel portion 2. Preferably, the inductor 3 is positioned such that it
encircles at least
is partially the part of the steel portion 3 comprising the inserted copper
item(s) 1. The
inductor 3 is connected to a power supply (not shown in the drawings) which
converts
AC line power to an AC having a frequency of between 1 kHz and 50 kHz. The
converted AC is delivered to the inductor 3, causing induction heating of at
least a part
of the steel portion 2 comprising copper item(s), encircled by the inductor 3.
The steel
20 induction heated part of the steel portion 2 is heated to a temperature
above the melting
temperature of copper or copper alloy. The induction heating is continued at
least until
the outer part of the solid copper items is molten, forming a partly or fully
molten pool
of copper within in the cavity of the steel portion 2.
25 In one embodiment, the induction heating is continued after the solid
copper item(s)
have become molten in order to increase performance of the cathode bar. The
induction
heating may be held for a period of time varying from 10 seconds to 12 hours,
preferably a period of 1 minute to 1 hour.
30 After the induction heating, the thus heated part of the steel portion 2
containing the
partly or fully molten copper is cooled. Preferably, the cooling is controlled
such that
the lower part of the heated area is cooled first leading to solidification of
the molten
inner portion of copper, while the upper part is kept molten. The cooling is
preferably
carried out directionally such that the copper in the upper part of the steel
portion is
35 lastly solidified. Directional cooling may be obtained by upward moving
of the inductor
length wise along the cathode bar. Optionally, the bottom part of the steel
portion 2 may
be subjected to additional cooling in order to initiate the directional
cooling. By such
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controlled cooling, piping and formation of shrink holes is reduced and
controlled. A
copper insert of good quality in that it is virtually pore-free and with no
suctions is
ensured in the cathode bar.
Fig. 2 illustrates another embodiment of the present invention, showing a
cross-section
of a cathode bar comprising a steel portion 2 equipped with a cavity for
filling of molten
copper 6, and a collar or sleeve of a metal material such as steel or any
other suitable
refractory material at the upper part acting as a feeder.
1() In this embodiment, the method comprises preheating of the steel
portion 2 of the
cathode by residual heat from previously heated cathode bars and/or by
utilizing
induction heating.
At least one inductor 3 is positioned in close proximity to at least parts of
the outside of
is the steel portion 2. The properties and design of the inductor to be
used in this
embodiment correspond to the aforementioned description of inductor.
Preferably, the inductor 3 is positioned such that it encircles at least
partially the part of
the steel portion 3 comprising the cavity for filling molten copper. The
inductor 3 is
20 connected to a power supply (not shown in the drawings) which converts
AC line power
to an AC having a frequency of between 1 kHz and 50 kHz. The converted AC is
delivered to the inductor 3, causing induction heating of at least a part of
the steel
portion 2 encircled by the inductor 3. The steel induction heated part of the
steel portion
2 is heated to a temperature above the melting temperature of copper or copper
alloy.
Molten copper is provided, e.g. from used cathode bars or any other source,
and filled
into the steel portion 2 of the cathode bar, either filling from top or bottom
with a
suitable casting method.
In one embodiment, the induction heating of the steel portion 2 is continued
after filling
of molten copper into the cavity of the steel portion 2 in order to increase
performance
of the cathode bar. The induction heating of may be held for a period of time
varying
from 10 seconds to 12 hours, preferably a period of 1 minute to 1 hour.
After the induction heating, the thus heated part of the steel portion 2
containing the
partly or fully molten copper is cooled. Preferably, the cooling is controlled
such that
the lower part of the heated area is cooled first leading to solidification of
the molten
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inner portion of copper, while the upper part is kept molten. The cooling is
preferably
carried out directionally such that the copper in the upper part of the steel
portion is
lastly solidified. Directional cooling may be obtained upward moving of the
inductor
length wise along the cathode bar. Optionally, the bottom part of the steel
portion 2 may
be subjected to additional cooling in order to initiate the directional
cooling. By such
controlled cooling, piping and formation of shrink holes is reduced and
controlled. A
copper insert of good quality in that it is virtually pore-free and with no
suctions is
ensured in the cathode bar.
1() Fig. 3 illustrates further embodiment of the present invention where
molten copper is
filled into the steel portion from the bottom or through an opening in the
steel portion 2
placed anywhere along its side below the sleeve 4 at the top, i.e. through a
permanent
steel mold with gating. Fig. 3 is showing a cross-section of a cathode bar
comprising
said a permanent steel mold with gating/a steel portion 2 equipped with a
cavity for
is filling of molten copper, and a collar or sleeve 4 of a metal material
such as steel or any
other suitable refractory material at the upper part. A downsprue 7 for
filling of molten
copper is connected to the steel portion 2 so that the molten copper is filled
into the
cavity of the steel portion. The connection point is shown at the lower part
of the steel
portion, but may be anywhere along the side of the steel portion 2. Only one
steel
20 portion 2 is shown, but typically a number of steel portions 2/
permanent steel molds
with gating are connected to the downsprue 7 for simultaneous filling of
molten copper.
At least one inductor 3 is positioned in close proximity to at least parts of
the outside of
the steel portion 2 in a similar way as explained with regard to Fig. 2 above.
Molten
copper is filled at a desired level in the cavity of the steel portion 2 and
the sleeve 4 acts
25 as a prolongation of the cavity to protect against overfilling and will
secure good quality
copper. The collar/sleeve 4 may also be a designed as a separate part
connected to the
steel portion 2. Heating and cooling are performed as discussed for the
embodiment of
Fig. 2. The inductor 3 will direct the solidification towards the
collar/sleeve 4. During
cooling the copper level decreases. Solidified copper on the edge of the
sleeve 4 may be
30 removed by means of drilling, machining or any other suitable method,
and a closing
device 5 (not shown) may be introduced.
During filling of the molten copper, the steel portion 2 of the cathode bar is
preferably
kept at an angle to reduce pressure height and to improve the filling process
by avoiding
35 turbulence and/or oxide formation. After the filling the steel portion 2
with molten
copper, the steel portion 2 may be tilted to a vertical position or any other
position. This
embodiment is illustrated in Fig. 4 showing a cathode bar according to the
invention
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produced by filling molten copper 6 into the steel portion 2 at the lower end
and
equipped with an inductor 3, by using counter gravity casting.
Fig. 5 illustrates further embodiment of the present invention where molten
copper is
filled into the steel portion from the bottom of the steel portion 2. Fig. 5
is showing a
cross-section of a cathode bar comprising said a steel portion 2 equipped with
a cavity
for filling of molten copper, and a collar or sleeve 4 of a metal material
such as steel or
any other suitable refractory material at the upper part. A downsprue 7 for
filling of
molten copper is connected to the steel portion 2 so that the molten copper is
filled into
io the cavity of the steel portion from its bottom/underside. Only one
steel portion 2 is
shown, but typically a number of steel portions 2 are connected to the
downsprue 7 for
simultaneous filling of molten copper. At least one inductor 3 is positioned
in close
proximity to at least parts of the outside of the steel portion 2 in a similar
way as
explained with regard to Fig. 2 above. Molten copper is filled at a desired
level in the
is cavity of the steel portion 2 and the sleeve 4 acts as a protection
against overfilling and
will secure good quality copper. The sleeve 4 shown in Fig. 5 is placed at the
top of the
steel portion 2, but any location further down on the side of the steel
portion 2 is within
the scope of the invention. Heating and cooling are performed as discussed for
the
embodiment of Fig. 2. The inductor 3 directs the solidification towards the
sleeve 4.
20 During cooling the copper level decreases. Solidified copper on the edge
of the sleeve 4
may be removed by means of drilling, machining or any other suitable method,
and a
closing device 5 (not shown) may be introduced.
Fig. 6 illustrates still a further embodiment of the present invention where
molten
25 copper is filled into the steel portion from the bottom of the steel
portion 2. Fig. 6 is
showing a cross-section of a cathode bar comprising said a steel portion 2
equipped
with a cavity for filling of molten copper, and a collar or sleeve 4 of a
metal material
such as steel or any other suitable refractory material at the upper part. A
reservoir of
molten copper equipped with a feeder P for addition of molten copper to the
reservoir
30 and a supply pipe 8 connected to the steel portion 2, is arranged below
the steel portion
2. Molten copper is filled into the cavity of the steel portion from its
bottom/underside
through the supply pipe 8. At least one inductor 3 is positioned in close
proximity to at
least parts of the outside of the steel portion 2 in a similar way as
explained with regard
to Fig. 2 above. Molten copper is filled at a desired level in the cavity of
the steel
35 portion 2 and the sleeve 4 acts as a protection against overfilling and
secure good
quality copper. The sleeve 4 shown in Fig. 6 is placed at the upper part of
the steel
portion 2, but may be located further down on the side of the steel portion.
Heating and
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cooling are performed as discussed for the embodiment of Fig. 2. The inductor
3 directs
the solidification towards the sleeve 4. During cooling the copper level
decreases.
Solidified copper on the edge of the sleeve 4 may be removed by means of
drilling,
machining or any other suitable method, and a closing device 5 (not shown) may
be
5 introduced.
Fig. 7 illustrates an "over the lip" filling device and Fig. 8 a bottom
tapping device for
filling molten copper in production of a cathode bar with copper insert. Both
devices are
commercial available and within the knowledge of the skilled person. "Over the
lip"
io filling and bottom tapping are useful filling methods for carrying out
the embodiments
of the invention related to filling molten copper when producing the cathode
bars with
copper insert such as illustrated in Figs. 2-6.
Fig. 9 illustrates removing of a copper insert 1 from a used cathode bar. A
cross-section
is of a cathode bar comprising a steel portion 2 and a copper insert 1 is
shown illustrating
the embodiment of the present invention where copper is melted out for
optional re-use
in production of a new cathode steel bar as described in the embodiment above.
In this embodiment, a used cathode steel bar is provided and prepared for
copper melt
out, typically through a cutting process or similar to gain direct access to
the copper
insert of the cathode bar.
The cathode bar is preferably preheated by rest heat from previously heated
cathode
bars and subsequently heated by utilizing induction heating to at least the
melting point
of copper or copper alloy.
At least one inductor 3 is positioned in close proximity to at least parts of
the outside of
the steel portion 2. The properties and design of the inductor to be used in
this
embodiment correspond to the aforementioned description of inductor.
Preferably, the
inductor 3 is positioned such that it encircles at least partially the part of
the cathode bar
comprising the copper insert. The inductor 3 is connected to a power supply as
described above and causes induction heating of at least a part of the cathode
bar
encircled by the inductor 3. When reaching the melting point of the copper,
molten
copper is formed and poured out of the steel portion 2 of the cathode bar into
a holding
furnace, or casted directly into copper ingots or similar solid copper items.
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In the embodiment where molten copper is poured into a holding furnace, it may
be re-
used directly in molten state for production of new cathode bars as described
above in
relation to Figs. 2-8.
In the embodiment where the copper is casted into solid copper items, they may
be re-
used for production of new cathode bars as described above in relation to
Fig.1, or Figs.
2-6 by re-melting of the solid copper items.
Refining or "dilution" of the recycled copper to desired quality might be
required. Here
io "dilution" means addition of more pure copper.
Fig. 10 shows a section of a cathode bar with copper insert prepared by the
method of
the invention. It is clear from the photo of Fig. 10 that a unique
intermetallic connection
between steel and copper has been achieved. Furthermore, no pores or suction
can be
is seen in the copper insert. Thus, it is shown that the method of the
present invention
enables production of cathode bars with copper insert where the metals (steel
and
copper) are homogeneously joined. That is, the quality of the cathodes
obtained are very
good and their performance being improved in terms of conductivity. Such
cathode bars
are not previously known.
Having described preferred embodiments of the invention it will be apparent to
those
skilled in the art that other embodiments incorporating the concepts may be
used. These
and other examples of the invention illustrated above are intended by way of
example
only and the actual scope of the invention is to be determined from the
claims.
