CA2835908A1

CA2835908A1 - Reduction furnace

Info

Publication number: CA2835908A1
Application number: CA2835908A
Authority: CA
Inventors: Roland Konig; Detlef Strieder; Rolf Degel; Axel Weyer; Walter Weischedel
Original assignee: SMS Siemag AG
Current assignee: SMS Group GmbH
Priority date: 2011-05-13
Filing date: 2012-02-22
Publication date: 2012-11-22
Anticipated expiration: 2032-02-22
Also published as: CA2835908C; EP2707665A1; CN103717988A; EP2707665B1; DE102011083036A1; RU2551738C1; WO2012156108A1; CN103717988B

Abstract

The invention relates to a reduction furnace, in particular for silicon production, comprising a furnace casing (3) and a plurality of electrodes (1a, 1b), in particular of a circular cross section, which are arranged in the furnace casing in a defined arrangement in relation to one another, in particular along an arc of a circle (2), wherein at least one of the electrodes (1a, 1b) is formed as a bundle of electrodes comprising a number of individual electrodes (1a, 1b), in particular as a double electrode.

Description

Reduction furnace The invention relates to a reduction furnace, particularly for production of silicon, according to the preamble of claim 1.
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 ... ) ' 2 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 stated in the introduction, by the characterising features of claim 1. 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 megawatts. Such furnaces are possible for, for example, solar silicon only with electrodes according to the invention.

2a 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.

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.
Further advantages and features are evident from the embodiments described in the following as well as the dependent claims.
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 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;

- 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 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 6 =
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.

<IMO .0 =

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)

Claims

claims

1. Reduction furnace, particularly for production of silicon, comprising a furnace vessel (3) and a plurality of electrodes (1a, 1b) which are arranged in the furnace vessel in a defined arrangement relative to one another along an arc (2), characterised in that at least one of the electrodes (1a, 1 b) is constructed as an electrode bundle of a plurality of individual electrodes (1a, 1 b), wherein the individual electrodes (1a, 1 b) are arranged on a line perpendicular to the arc (2).

2. Reduction furnace according to claim 1, characterised in that the individual electrodes (1a, 1b) each have a substantially circular cross-section.

3. Reduction furnace according to claim 1 or 2, characterised in that the individual electrodes (1a, 1b) consist of graphite, particularly of highly pure graphite.

4. Reduction furnace according to claim 3, characterised in that the individual electrodes (1a, 1b) have a diameter of not more than approximately 650 millimetres, particularly not more than approximately 450 millimetres.

5. Reduction furnace according to claim 3 or 4, characterised in that the reduction furnace has a power of more than approximately 10 megawatts, particularly more than 12 megawatts.

6. Reduction furnace according to any one the preceding claims, characterised in that the individual electrodes (1a, 1b) of the electrode bundle are electrically connected by way of exactly one electrode strand.

7. Reduction furnace according to any one of claims 1 to 5, characterised in that the individual electrodes (1a, 1b) of the electrode bundle are connected with separate electrode strands.

8. Reduction furnace according to any one of the preceding claims, characterised in that individual electrodes (1a, 1b) of the electrode bundle are movable out of the furnace vessel (3) separately from one another.

9. Reduction furnace according to any one of the preceding claims, characterised in that the electrode bundle (1a, 1b) can rotate or oscillate about an individual centre point.