Note: Descriptions are shown in the official language in which they were submitted.
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Reduction furnace
The invention relates to a reduction furnace and more particularly a reduction
furnace
for production of silicon.
Prior art and set problem
The power of reduction furnaces, particularly in the case of silicon furnaces,
is limited
because highly pure graphite electrodes are needed for the production of
metallurgical
silicon carbon electrodes (prebaked electrodes) and for the production of
solar silicon.
The maximum diameter of carbon electrodes is 1270 millimetres to 1400
millimetres.
Highly pure graphite electrodes are graphite electrodes which are subsequently
purified.
Since the graphite has to be penetrated for that purpose, the maximum diameter
is
currently limited to approximately 400 to 450 millimetres, although graphite
electrodes of
up to 800 to 850 millimetres diameter can be supplied.
These limitations with the electrodes limit the possible powers of silicon
furnaces to 25 to
30 megawatts for metallurgical silicon and to approximately 10 to 12 megawatts
for solar
silicon.
The silicon process operates without a slag bath and the size of the reactive
zone in the
furnace depends on, apart from the power, primarily the diameter of the
electrodes and the
contact area with the raw material.
The use of economic Soderberg electrodes (electrode paste is moulded in a
steel casing
and baked only in the furnace) is not possible due to the required level of
purity (Fe from
the steel casing passes into the product).
Carbon electrodes are comparatively expensive and graphite electrodes even
more
expensive. From the viewpoint of handling, carbon electrodes are demanding due
to
length, diameter, weight and relatively low strength.
More recent developments and trends
In order to now be able to illustrate higher powers with at the same time
lower electrode
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costs the so-called composite - or compound - electrode was developed (for
example, type
'ELSA'). Soderberg paste is moulded around a graphite core by a rigid casing
and is
baked to the graphite core. However, this technology requires more complicated
electrode
strands with two replenishing devices (separately for core and actual
electrode) and costly
graphite electrodes as core. Due to conductor skin effect, however, only a
small amount
of current flows through the graphite electrode, which can be highly loaded in
terms of
current, but rather flows through the Soderberg paste less capable of loading.
The
graphite electrode is thus utilised only to a fraction of its power
capability.
It is the object of the invention to improve a reduction furnace with respect
to the
effectiveness of the electrodes.
According to the invention this object is fulfilled, for a reduction furnace
having
features described herein. Through construction of a conventional electrode as
a
bundle of several individual electrodes it is possible to achieve, in
particular, powers
of large electrodes by use of smaller individual electrodes.
The reduction furnace according to the invention can in a preferred detail
design be
constructed as a silicon furnace. With preference it is a furnace for
production of solar
silicon.
In a preferred development of the reduction furnace provision is made for the
individual
electrodes to each have a substantially circular cross-section.
Advantageously, it is provided that the individual electrodes consist of
graphite, particularly
highly pure graphite. A high level of purity of the silicon can thereby be
achieved. In a
preferred development provision is made for the individual electrodes to have
a diameter
of not more than approximately 650 millimetres, preferably not more than
approximately
450 millimetres. This applies particularly to the use of highly pure graphite
electrodes.
With general advantage it is in that case provided that the reduction furnace
has a power
of more than approximately 10 megawatts, particularly more than approximately
12
megawatts. Such furnaces are possible for, for example, solar silicon only
with electrodes
according to the invention.
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A method of operating an electric smelting furnace is known from DE 506 303 C.
DE 29 46 588 Al discloses a three-phase arc smelting or reduction furnace.
An electric arc or reduction furnace is known from DE 973 715 C.
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In the interest of an advantageous power distribution the individual
electrodes are
arranged on a line perpendicularly to the arc. In alternative forms of
embodiment of the
invention, however, other geometries can also be selected. In particular, the
furnace
can also be designed as a rectangular furnace in which the electrode bundles
are, for
example, arranged along one or more parallel straight lines.
In one possible form of embodiment it is provided that the individual
electrodes of the
electrode bundle are electrically connected by way of exactly one electrode
strand.
In a form of embodiment alternative thereto the individual electrodes of the
electrode
bundle can, however, also be connected with separate electrode strands,
whereby
higher powers and lower electrode currents can be achieved.
With general advantage it can be provided that individual electrodes of the
electrode
bundle can be moved out of the furnace vessel separately from one another.
In a preferred development of the invention it is provided that the electrode
bundle can
rotate or oscillate about an individual centre point. Incrustations of the
burden surface
could thereby be precluded. As a general rule the furnace vessel can be moved
relative
to the electrodes, for example rotated. Alternatively or additionally thereto
the
electrodes can also be arranged to be movable.
Accordingly, in one aspect the present invention resides in a reduction
furnace for
production of silicon, comprising a furnace vessel and a plurality of
electrodes which
are arranged in the furnace vessel in a defined arrangement relative to one
another
along an arc, wherein at least one of the electrodes is constructed as an
electrode
bundle of a plurality of individual electrodes, wherein the individual
electrodes are
arranged on a line perpendicular to the arc.
Further advantages and features are evident from the embodiments described
hereafter.
Embodiments of the invention are explained in more detail in the following and
by way
of the accompanying drawings.
Fig. 1 shows a schematic plan view of a reduction furnace according to the
prior art and
Fig. 2 shows a schematic plan view of a reduction furnace according to the
invention.
Preferred embodiment of the invention: Double electrode
In the present embodiment provided in each instance in place of a composite
electrode 1
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of large diameter are at least two (or, depending on the respective circuitry,
even more)
graphite electrodes 1 a, lb of smaller diameter. The arrangement is effected
in a circularly
round furnace vessel in a line going out from the furnace centre in succession
at right
angles to the electrode pitch circle diameter. The outer edge of the outer
electrode la and
the inner edge of the inner electrode lb each ideally, but not absolutely
necessarily, lie
somewhat outside the edge of the comparable large electrode 1. A free space is
present
between the two electrodes la, lb. The two (or more) electrodes la, lb can be
constructed to be connected either with completely separate electrode strands
or by way
of only one electrode strand. In the case of a construction with separate
electrode strands
compensation could thus be made for different rates of electrode consumption
or also
individual electrodes could be moved out of the active furnace area.
The electrode pairs la, lb or electrode bundles are arranged in distribution
on the pitch
circle. In the case of a round, three-phase furnace there are usually three
electrode pairs
or bundles.
It can be optionally provided that depending on the respective requirements of
the process
the electrode pairs 1a, lb or electrode bundles oscillate or rotate about
their own notional
centre point (approximately on the pitch circle), i.e. that in this instance
not only in a given
case would the furnace vessel 3 rotate, but also in addition the electrode
pairs or electrode
bundles la, lb ('rotating electrode strand'). This can be realised with the
comparatively
light electrode strands more simply than with heavy strands. In this way, for
example,
incrustations of the burden surface would be precluded.
In this manner
the potential capability of the graphite electrodes to be loaded with current
can be
fully utilised;
the circumference of two electrodes is available with good distribution for
current
transfer;
the electrodes can be positioned so that the reaction space, notwithstanding
small
electrode diameters, is sufficiently large and the furnace volume is filled as
desired,
but without dispensing with protective spacings with respect to the vessel
walls;
the electrode strands or electrode strand can be constructed to be
significantly
lighter, since the electrode weights are significantly smaller;
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- the electrodes can be rapidly moved with little effort;
- operating costs can be reduced;
handling can be simplified (only one of type of electrode, no past to
replenish);
- lower energy consumption is achievable, since graphite electrodes have an
80%
smaller inherent resistance than carbon electrodes or even Soderberg
electrodes
and by comparison with a composite electrode the advantage lies at
approximately
50%, which referred to the total energy consumption comes to approximately 3
to
5%;
depending on the respective circuitry the furnace resistance can be increased,
which leads to smaller transformer sizes and at the same time to lower energy
consumption, or a power increase is achievable for given values;
the otherwise frequently usual electrode pitch-circle adjustment is already
present
in concept and can be constructed to be very variable;
- depending on the respective circuitry a form of 'electrodynamic pitch-
circle
adjustment' can also be realised by differential power intake, i.e., for
example,
more energy intake by way of the inner electrode and less by way of the outer
electrode.
The production of solar silicon in high-power furnaces (approximately greater
than 10
megawatts) is possible only with double electrodes due to the specific
limitation of
diameter as a consequence of the required purity of graphite electrodes.
Since silicon furnaces are preferably constructed to be rotationally
oscillating a certain
degree of reduction of the reaction space, so to say, can be accepted in the
direction of
the pitch circle. This means that if the reaction space should adopt, instead
of an idealised
round form, rather a slightly oval form then compensation for this is provided
again by the
rotational movements, i.e. the reaction spaces are connected again in this
manner.
Further features:
Due to the mentioned limitation of the maximum diameter up to which graphite
electrodes
can be sufficiently pure for solar silicon production (currently approximately
400
millimetres), concepts needing higher powers (greater than approximately 10
megawatts)
with a greater number of electrodes are required for solar silicon production.
The solar
silicon SAF with double electrodes here avoids six-electrode rectangular
furnaces or
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untested constructional forms.
The described electrode is lighter:
A 1700th composite electrode weighs approximately 3.4 tonnes per metre.
Two 600th graphite electrodes weight only approximately 0.95 tonnes per metre.
In the case of smaller furnaces of up to approximately 5 to 10 megawatts the
two
electrodes can also be combined (i.e. two contact jaws and only one adjusting
device are
needed).
In the case of larger furnaces, two separate adjusting devices are to be
provided, but in
light construction.
The electrical circuitry can be constructed as in the case of a three-
electrode SAF.
Alternatively, however, circuitry similar to a six-electrode rectangular
furnace is also
possible. Electrode current and furnace resistance can thereby be positively
influenced,
so that, for example, 15% higher powers or lower electrode currents can be
realised.
In ideal manner this technology is suitable for all processes which are not
suitable for
cheap Soderberg electrodes. In principle, however, all processes can be
operated with
this electrode technology, thus not only for silicon and solar silicon SAFs.
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Reference numeral list
1 composite electrode (prior art)
1a, 1b double graphite electrode, optionally highly pure
2 pitch circle
3 furnace diameter (inner side of refractory lining, furnace vessel)
