Note: Descriptions are shown in the official language in which they were submitted.
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ELECTRODE TYPE SlSDERBERG FOR MAKING SILICON ALLOYS AND SILICON METAL
FIELD OF 'fHE INVENTION
This invention relates to a self baking electrode for the production of
silicon
alloys and silicon metal.
BRIEF DESCRIPTION OF 1'HE PRIOR ART
The use of self-baking electrodes (also called "Siiderberg electrodes") for
the
production of ferro-alloys has been known for about 75 years (see US Patent
no. 1,440,724 of September 1919 and US Patent no. 1,441,037 of January 1923
both in the name of Soderberg). Self baking electrodes basically consist of a
carbon-
containing material such as anthracite, pet coke, tar and hitch, which is
filled into a
steel casing held in position within an electric arc furnace by means of
contact shoes
and a suspension/slipping device. The application of high electric currents
plus the
heat of the arc struck by the electrode during tile furnace operation develops
sufficient heat to melt the material filled into the casing and form a paste,
then cokify
the so-formed paste, and finally bake the electrode.
The steel casings of the Soderberg electrodes presently in use are in majority
round in shape and provided with a series of inwardly projecting fins
extending
radially towards the center of the electrode in order to provide mechanical
strength
to the ~ electrode, heat penetration within the electrode through the
conductivity of the
fins and act as current conductor. The fins and the casing are typically made
of
regular steel, and their amount, length and physical shape depend tin what is
considered optimum for thorough baking as per each geometric design.
As the electrode is consumed during the production of silicon or ferro-alloy,
both the paste and casing have to be replaced. This is dome high on top of the
electrode column so that there is sufficient static pressure for compaction,
and for
running through the various stages of the temperature pattern from softening
of the
paste up to the heat generated by current flow.
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Consumption of the electrode is compensated by regular slipping of the
electrode l111-ough the contact shoes. The iron casing and the fins passing
down the
contact shoes at each slipping burn and oxidize or melt, and thereby fall info
the mix.
Because of this consunrhtion/oxidation, the iron pick-up is of such a
magnitude that
the Soderberg technology cannot be applied to produce commercial grade silicon
metal where, depending un tire quality grade fur Si, the Fe content has to be
below
1 % , below 0.5 %, below 0.35 % or every I~eluw 0.2 % .
Therefore, su far, silicon nrelal has been produced exclusively by using a so-
called "pre-baked" electrwde, which is an anrurhhuus car~lxm or semi-
graphitized
electrode produced in specific manufacturing units and teen supplied in
sections of
typically 2 to 2.5 rn length. These pre-baked electrodes, which are usually 4
to G
times more expensive than Soderberg electrodes, are to lie connected to each
other
by specific devices, which can lie nipples and sockets ur a system of
male/fernale
design cuts at the ends of each section of the electrode. In operation in a
silicon
metal furnace, these connections between pieces of electrodes are limiting
factors
for energy transfer from one electrode to the otter underneath ttre contact
shoe.
Because of tire heat and current transfer pattern, nipples and sockets are
prone to breaking with ain~upt changes of power in the furnace - as caused by
any
type of power shutdown - so that elecUcxle breakages are part of undesired
negative
influences on operation.
Furthermore, their strength is relatively low as compared to the Stiderberg
electrodes, which do not contain the weak Spots due tU connectors Ur nipples,
making it more solid and accepting higher specific power per square secaion.
Therefore, reduction of electrode costs using the self baking principle is one
of the main challenges of every silicon metal producer.
Many attempts have been undertaken to develop a type of Soderberg
electrode which would allow a cheaper production of silicon lnetal while
meeting all '
the criteria fur reducing the amount of iron in the produced metal.
In the 70', Nippon Denko of Japan developed a system in which the casings
and fins usually made of steel were replaced by casings and fins made of
aluminum
(see Japanese Patents nus. 951,888 and 835,59G). 'This attempt to use aluminum
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for both the casing and sins has never been used industrially, because of the
lack of
mechanical stability and the substantially different conductivity of aluminum
compared lu steel.
Another approach was undertaken by M . Cavigli (see Italian Patent
no. 606,568 of July 1960). In this parent, it was suggested to remove the fins
from
the outer casing and to adjust the relative movement of the paste with respect
to the
outside casing by sliding or extruding the inner contents of the casing as a
central
consumable member. Iron crosses were provided within the casing to support the
electrode while it baked. These iron crosses held the electrode while allowing
a
relative movement between dle casing and the electrode by either pressing or
reducing the suspension weight. Thic ty~,e~m hat been in operation in one
laryt_ in
Italy. It permits to reduce the iron contamination, as the slipping of the
casing
represents only 1/10 of llle slipping of the electrode itself. However, it
does not
permit to reach the same low level in icon impurities as obtained with
conventional
pre-baked electrcxles.
Another approach has been undertaken by Bruff (see US Patent
no. 4,527,329 of July 1985). This patent suggests to separate the baking of
the
paste from the one that takes place by the application of heat through Ohm's
resistance and conductivity in and below tile contact shoes. Thus, a separate
baking
installation is located way above the contact shoes. Moreover, a device is
provided
to cut and remove the iron casing underneath the baking system, well above the
contact shoes, so that basically a shaped pre-baked-like electrode enters the
contact
shoes. This system operates in a small furnace of about IOMW at Elkem
Kiistian,~dad. However, there are severe restrictions in the use for higher
powered
furnaces with larger diameter electrodes, which are the manufacturing standard
for
cost efficiency in the developed world.
' A similar solution has also been disclosed in German Patent Application no.
4,036,133 of May 1991 in the name of E. Svana.
A further system based on a relative movement of a self baking electrode with
respect to an external casing has been disclosed by Persson in US Patent no.
4,575,856 of March 1986. In this patent, the iron crosses used by Cavigli in
his
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system are replaced by smaller graphite electrodes put concentrically into the
casing. The small electrodes are supported and moved by a separate
slipping/holding device, which allows their relative movement within the
casing.
An improved system based on a "transfer" from a conventional pre-baked
S electrode to one of the extruded type as described by Cavigli and Persson is
described in Canadian patent no. 2,081,295.
The disadvantages of this system mainly result from the physical strength
limitations of the graphite electrode core and its Limited potential to absorb
compression, tension and bending forces as the electrode core is essentially
unguided over lengths of up to 14 m and can deviate from its vertical position
for
various reasons. Furthermore, the casing which, in this system, is essentially
an
. extrusion dye, needs to be slipped down occasionally to compensate for heat
damages between and underneath the contact shoes. Without such periodic
slipping, damages would reach high up in the contact shoes, and liquid paste
would
start to drip and thereby provoke disturbances known as "green" breakages in
the
Soderherg technology. The periodic slippings of the casing do slightly
contaminate
the Si not only with the iron of the casing, hut also with the alloying
element's used
in the casing material to provide the maximum possible heat oxidation
protection.
These contaminants tend to make silicon metal produced this way unsuitable for
its
application in the chemical industry to produce methylchlorosilanes out of
silicon
metal. Casings made of regular steel also have their disadvantages as vital
properties for functioning are decreased by heat, the furnace atmosphere and
the
time they are exposed to those.
OBJECTS OF THE PRESENT INVENTION
It is an object of the present invention to provide a new and improved self '
baking electrode.
Another object of the present invention is to provide a new electrode system
which allows the production of silicon metal in a Soderherg-type furnace
without any
modification to the existing slipping system or addition of another slipping
system.
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Thanks to the electrode according to the invention, the same furnace can
produce both FeSi of any grade and Si metal without any downtime between the
gradual change from one product to the other and each time at the lowest
electrode cost.
The electrode according to the invention overcomes the problems
associated with prior art: silicon metal contamination, core breakages as a
result
of extrusion forces, casing deformation, loss of production and capital
expense
for installation of new slipping systems. It also provides a way to convert
bigger
and more efficient ferro-silicon Soderberg-type furnaces instead of existing
silicon metal furnaces with pre-baked electrode technology.
SUMMARY OF THE INVENTION
Accordingly, the present invention relates to an in situ self-baking
electrode suitable for the production of silicon metal and silicon alloys for
use in
an electric arc furnace, the electrode being characterized in that it
comprises:
an elongated open ended electrically conductive metal casing for
extending generally vertically within the furnace when in use, the casing
being
made of a material unalloyed with a metal selected from the group consisting
of
titanium, vanadium, tantalum, chrome, zirconium and nickel, whereby
contamination of the silicon metal and silicon alloys to be produced in the
furnace with any of said metal upon an ongoing consumption of the casing in
the
furnace is prevented;
a central core disposed within and spaced from the casing, the central
core being made of a heat conductive carbonaceous material;
at least one framework within the casing, the framework securing the
central core to an inner surface of the casing for holding centrally the
central
core within the casing and for preventing an extrusion of the central core
downward; and
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a carbonaceous electrode paste surrounding the central core, the paste
being devised to cure into a solid electrode upon heating and to bond to the
central core.
The present invention also relates to an electric arc furnace embodying a
self-baking electrode for the production of silicon metal and silicon metal
alloys
as described hereinbefore. More particularly, the electric arc furnace
comprises
an electric arc furnace for the production of silicon metal and silicon alloys
comprising:
a furnace body containing a charge to be heated;
an In situ self-baking electrode comprising:
an elongated open ended electrically conductive casing having an
upper end and a bottom end, said casing extending generally vertically within
the furnace body and being free to slip vertically through a slipping
mechanism;
and wherein said casing is made of material unalloyed with a metal selected
from the group consisting of titanium, vanadium, tantalum, chrome, zirconium
and nickel for preventing contamination of the silicon metal and silicon
alloys to
be produced in the furnace with any of said metal upon an ongoing consumption
of the casing in the furnace;
a central core disposed within and spaced from the casing, the
central core being made of heat conductive carbonaceous material;
at least one framework within the casing, the framework securing
the central core to an inner surface of the casing for holding centrally the
central
core within the casing and for preventing an extrusion of the central core
downward through the bottom end of the casing;
a carbonaceous electrode paste surrounding the central core, the
paste being devised to cure into a solid electrode upon heating and to bond to
the central core;
means for retaining the casing in ~ generally vertical position within
the furnace body; and
electric means far generating an electric arc in the furnace, the
electric means comprising a contact on the casing.
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A further object of the present invention is to propose a process for
forming in situ a self-baking electrode for the production of silicon metal
and
silicon alloys in an electric arc furnace, the process comprising the steps
of:
a) providing an elongated open ended electrically conductive metal
casing made of a material unalloyed with a metal selected from the group
consisting of titanium, vanadium, tantalum, chrome, zirconium and nickel for
preventing contamination of the silicon metal and silicon alloys to be
produced in
the furnace with any of said metal upon an ongoing consumption of the casing
in
the furnace;
b) disposing a central core of carbonaceous heat conductive material
within and spaced from tulle casing;
c) securing the central core to an inner surface of the casing and
holding it centrally within the casing;
d} sliding generally vertically the elongated electrically conductive
casing within the furnace;
e} introducing a quantity of carbonaceous electrode paste in the
casing so that said paste surrounds the central core, the paste being devised
to
cure into a solid electrode upon heating and to bond to the central core;
f) contacting the casing to an electric power source; and
g) generating with said electric power source an electric arc into the
furnace.
The central core of the electrode preferably consists of carbon or carbidic
bars or rods connected to each other so that the heat transfer is essentially
uninterrupted in their connection. Use can also be made of metal rods or bars.
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Whatever be the material used fur the manufacture of the central core, such
a core in the form of bars or rods can be hollowed to allow inside cooling
through
injection of di-atomic or inert gases. Such is particularly useful to control
and
influence the arc at dle lip of the electrode and the baking of the electrode.
In accordance with the invention, the material forming the casing is selected
so as to he electrically conductive to transfer electric power from the
contact shoes
into the Siiderberg paste while preferably preventing undesired metallic
contamination by either Ti, V, 'ra, Cr, Zr c»' Ni. Advantageously, the casing
can be
made of Cu or brass, or of an aluminum alloy or aiuminum of sufficient
strength to
support the pressure of the filling of Seiderberg paste without deformation or
dents.
Such a possible selection makes the invention particularly useful to produce
silicon metal of suitable quality for application in the Rochow-direct
synthesis.
Indeed, one has only to select the material forming the conductive core and
supporting casing so that the resulting metallic additions to the melting
contains
suitable amounts of A1 and/or Cu and/or zinc and/or tin as are required in the
silicon
thus produced.
Advantageously, the electrode according to the invention allows a user to
switch from the production of ferrosilicon using regular Stiderberg electrodes
to the
production of silicon metal using the technology described hereinabove,
without any
downtime, and since no additional devices to guide the graphite core are
required,
switch-back to Soderberg technology is possible and only with this technology.
As can be appreciated, an important improvement in the electrode according
to the invention lies in that the central core of the electrode which is
secured to the
casing is "released" from its function of transferring compression forces for
the
extrusion as for the electrode described in prior art as indicated above.
Consequently, it does not expose the core material to the risk of buckling
when
compressed, and thereby of breaking. It furthermore eliminates the need for a
separate slipping device to perform the functions of the central core, and
thereby the
substantial costs for irreversible retro-fitting of existing furnaces from the
pre-baked
carbon-electrode design to the extruded concept as described hereinabove.
Furthermore, it allows a much safer application of a hollow core electrode,
where in
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the case of the extruded principle, the presence of such a central hole in the
central
core further weakens mechanically the core' in cross section, in particular at
the level
of the nipples or connectors, with an even more pronounced susceptibility to
breakages or damages in the column while performing the extrusion increments.
A non restrictive description of a preferred embodiment will now be given with
reference to the appended drawings.
BRIEF DESCRIPT1UN OF THE DRAWINGS
Figure 1 is a side elevational view, partly in section, schematically
illustrating
an electuic arc furnace in which an electrode according to the present
invention is
used;
Figure 2 is a side elevational cross-section view of an electrode according to
a preferred emlx~diment of the invention, shown above a conventional
Siiderberg
electrc><ie; and
Figure 3 is a cross section view of the electrode of Figure 2, taken along
line
II-II in Figure 2.
DESCRIPTION OF A PREFERRED EMBODIMENT
Referring to figure 1, an electric arc furnace (2 j in which an electrode (4 j
according to tile present invention may be employed is illustrated. The
furnace (2)
is of a conventional design and may be used for smelting for example,
ferrosilicon
and silicon metal. As well known in the art, the furnace (2) comprises a
furnace body
(G) formed of an outer steel shell and a suitable refractory material. A
curtain (8) is
extending upwardly from the furnace body (G) and it has an upper end engaged
by
the hood (10) or cover of the furnace body (Gj. The electrode (4) extends
vertically
within the furnace bcxly (G) through an opening ( 12j in the hood ( l0). The
furnace
(2) comprises electric means for providing an electric arc in the furnace (2)
for
smelting a charge (14) in the furnace body (G). The electric means comprises a
contact, such as a contact shoe (1G), connected to the electrode (4). The
contact
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shoe ( 16) is mounted on the electrode (4) with a conventional half ring (
18). The
furnace (2) may also be provided with a water-cooled jacket (20) for cooling
the
electrode (4) above the contact shoe (1G). Retaining means are provided for
retaining the electrode (4) vertically within the furnace (2). The retaining
means
S preferably comprises regulation cylinders (22) and two slipping bands (24)
mounted
on an upper floor (2G) of the furnace building and supporting the electrode
(4).
Referring more particularly to figures 2 and 3, the self baking electrode (4)
according to the present invention comprises an elongated open ended
electrically
conductive casing (30) for extending generally vertically within the furnace
(2) in use.
This casing (30) has an upper end (31 ) and a bottom end (33). A central core
(32)
made of a heat conductive material, preferably made of a carbonaceous
material,
. is disposed within and spaced from the casing (30). The casing (30) and the
central
core (32) define an annular channel (34) in which a carbonaceous electrode
paste
(36), preferably Soderberg paste, can lie fed, molten and baked. In other
words, a
carbonaceous electrode paste (36) is surrounding the central core (32), the
paste
(36) being devised to cure into a solid electrode upon heating and to bond to
the
central core (32).
The central core (32) can be shaped as a bar or other defined shapes and is
held centrally within the casing (30) by at least one framework (37) which
prevents
relative movement of the central core (32) with respect to the casing (30) due
to the
paste movement between the core (32) and the casing (30).
Preferably, the casing (30) is made of a flan-walled ordinary steel or a
thicker-
walled Dural~ so that the rigidity of the walls can stand the radial pressure
of the
filled-in S<iderberg paste (36). The filling of the Stiderberg paste (3G) into
the
electrode casing (3U) is done in a quasi continuous manner so as to minimize
the
"falling" height and also the total length above the contact shoes.
In case where silicon metal is to be produced in the furnace (2), the casing
(30) is preferably made of a material unalloyed with a metal selected from the
group
consisting of titanium, vanadium, tantalum, chrome, zirconium and nickel, for
preventing contamination of the silicon metal to lie produced in the furnace
(2) with
one of said metal upon an ongoing consumption of the casing in the furnace
(2).
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More preferably in this case, the casing (30) is made of a metal selected from
the group consisting of copper, brass and aluminum.
As shown in figures 2 and 3, the framework (37) securing the central core (32)
to an inner surface of the casing (30) preferably comprises a pair of opposite
rods
5 (38), each rod (38) extending generally horizontally and having a first end
(40)
driven into the central core (32) and a second end (42) secured to an inner
surface
of the casing (30). A bar (44) is extending through the central core (32)
below the
pair of rods (38), the bar (44) having its opposite outer ends (46) projecting
out from
the central core (32). The framework (37) further comprises two lateral lwame
10 members (48), each connecting together the second end (42) of each rod (38)
to a
corresponding outer end (46) of the bar (44). Referring to figure 3, two
further rods
(60) may preferably be provided for preventing the central core (32) from
twisting or
rotating within the casing (30). Each of said rods (60) comprises a first end
(62)
secured to the central core (32) and a second end (G4) secured to the inner
wall of
the casing (30), the two rods (GU) being tangent with the central core (32).
Although not essential, spread-out sheets (47) may be fixed to the inner
surface of the casing (30) to better prevent an extrusion of the baked paste
(36)
downward. However, experiments have shown that the framework (37) alone
prevents very well any extrusion of the baked electrode (36) downward, the
baked
electrode (36) bonding against the framework (37).
Referring to figure 2, a conventional Soderberg electrode (49) is illustrated
below the electrode (4) according to the present invention. This conventional
Soderberg electrode (49) comprises a casing (SOj and fins (52) mounted on the
inner wall of the casing (50). A self baked electrode (54j is formed within
the casing
(50) and both die electrode (54) and casing (50) moved down in unison. This
type
of electrode is well known in the art and does not need further description.
As can
be appreciated, this conventional Soderberg electrode (48) may have the same
diameter as the diameter of the electrode (4) according to the invention,
showing
that it is possible to easily switch frc»n the production of ferrosilicon
using a regular
Soderberg electrode (49) to the prcxluction of a silicon metal using an
electrode
according to the invention without any downtime or shutdown of the whole
furnace.
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The particular structure of the electrode according to the invention allows
for
a great reduction in the volume of metal, such as steel, that is normally used
for
preventing the extrusion of the self baked electrode downwards. As a matter of
fact,
with the electrode according to the invention, it is possible to obtain a
silicon metal
containing less than U.5 % Fe, with a casing still made of steel.
Extensive studies of the baking pattern of both a conventional Soderberg
electrode and a compound electrode where the center of the electrode is of a
solid
material having a substantially different thermal and elect~~ica! conductivity
have
shown that when the electrode comprises a central core with a high
conductivity, the
heating and baking pattern is higher in the contact shoe area as compared to
the
conventional Siiderberg technology. More specifically, baking of the paste
occurs
from the centre of the high heat conducting solid core against the surrounding
Soderberg paste towards the casing. In contrast, with a conventional
Stiderberg
electrode, baking of the paste occurs from the casing and the fins, that is
from the
outside of the electrode, toward tile inside of the same, as this is not a
different
conductivity between the core and the Soderberg material.
The present invention uses, in a welt balanced system, the heat conductivity
of the central core (32) to bake the surrounding Siiderberg paste (36). It
does not
necessitate a relative movement of the baked electrode {36) wish respect to
its
surrounding casing (30) as is the case with the compound electrodes known in
prior
art and for use in the silicon metal production.
The process for forming in situ a self baking electrode (4) in an electric arc
furnace (2), according to the present invention, comprises the following
sequence
of steps.
a) An elongated open ended electa~ically conductive casing is provided.
b) An elongated central core (32) of conductive heat material is disposed
within and spaced from the casing (30).
c) The central core (30) is secured to an inner surface of the casing (30) and
held centrally within the casing (30).
d) The elongated electrically conductive casing (30) is slid within the
furnace
(2) for extending generally vertically therein.
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e) A duantity of carbonaceous electrode paste (36) is introduced in tile
casing
(30) surrounding the central core (32). The paste (36) is devised to cure into
a solid
electrode upon heating and to bond to the central core (32).
fj An electric arc is present in the furnace (2) in a well know manner which
do
not need further description.
Preferably in step c), the central core (32) is secured to the casing (30) by
driving respectively into two opposite sides of the central core (30), a first
end (40j
of a corresponding rod (38) of a pair of opposite rods (38) and then securing
a
second end (42) of each of said opposite rods (38) to an inner surface of the
casing
(30) such that each rod (38) is extending generally horizontally within the
casing
(30). A bar (44) is inserted through the central core (32) below the two rods
(38)
such that the opposite outer ends (46 j of the bar (44) are prpjecting out
from the
central core (32). The second end (42) of each rod (38) is respectively
connected
to a correslx~nding outer end (46) of the bar (44) with a lateral frame member
(48).
In the case where the electrode (4) formed is used for the production of
silicon
metal, tile casing (30), in step d), may preferably be slid on top of a
previous
Siiderberg-type self baking electrode (49) used for tile production of
ferrosilicon, as
shown in figure 2. In this case, the casing (30) used for the production of
silicon may
have substantially the same diameter as the outer casing (50) of the
Siiderberg
electrode (48). As mentioned before, one can see that it is possible to easily
switch
from the production of ferrosilicon using a regular S~iderberg electrode (48)
to the
production of a silicon metal using an electrode according to the invention
without
any downtime or shutdown of the whole furnace.
