EP3310499B1 - Process for the mechanical classifying of polysilicon - Google Patents

Description

The invention relates to a method for the mechanical classification of polysilicon.

Polycrystalline silicon (polysilicon for short) serves as the starting material for the production of single-crystal silicon for semiconductors according to the Czochralski (CZ) or zone melting (FZ) process, as well as for the production of single or multicrystalline silicon using various drawing and casting processes Production of solar cells for photovoltaics.

Polycrystalline silicon is usually produced using the Siemens process. In this process, support bodies, usually thin filament rods made of silicon, are heated by direct current passage in a bell-shaped reactor ("Siemens reactor") and a reaction gas containing hydrogen and one or more silicon-containing components are introduced, the polycrystalline silicon being deposited on the support bodies.

For most applications, the polycrystalline silicon rods produced in this way are broken into small fragments, which are then usually classified according to size. Screening machines are usually used to sort or classify polycrystalline silicon into different size classes after comminution.

Alternatively, polycrystalline silicon granulate is produced in a fluidized bed reactor. This is done by fluidizing silicon particles by means of a gas flow in a fluidized bed, which is heated to high temperatures by a heating device. By adding a silicon-containing reaction gas, a pyrolysis reaction takes place on the hot particle surface. Elemental silicon is deposited on the silicon particles and the individual particles increase in diameter.

After its production, the polysilicon granulate is usually divided into two or more fractions or classes (classification) by means of a screening system. The smallest sieve fraction (undersized sieve) can then be processed into seed particles in a grinding system and added to the reactor. The target fraction is usually packaged and transported to the customer. The customer uses the polysilicon granulate i.a. for growing single crystals according to the Czochralski method.

A sieving machine is generally a machine for sieving, i.e. the separation (separation) of solid mixtures according to grain size. According to the movement characteristics, a distinction is made between plane vibrating screening machines and throwing screening machines. The screening machines are mostly driven electromagnetically or by unbalance motors or gears. The movement of the screen lining is used to transport the feed material in the longitudinal direction of the screen and to allow the fine fraction to pass through the screen openings. In contrast to planar vibrating screening machines, throwing screening machines also have a vertical screening acceleration in addition to the horizontal.

A special type is the multi-deck screening machine, which can fractionate several grain sizes at the same time. They are designed for a variety of sharp separations in the medium to fine grain range. In the case of multi-deck planar screening machines, the drive principle is based on two unbalance motors working in opposite directions, which generate a linear vibration. The material to be sieved moves in a straight line over the horizontal dividing surface. The machine works with low vibration acceleration. Thanks to a modular system, a large number of screen decks can be put together to form a screen stack. This means that, if necessary, different grain sizes can be produced in a single machine without having to change the screen linings. By repeating the same screen deck sequences several times, the material to be screened can be offered a large screen area.

US 8021483 B2 , or DE60310627 T2 discloses an apparatus for sorting polycrystalline silicon pieces including a vibration motor assembly and a stepped tray classifier attached to the vibration motor assembly. The The vibration motor arrangement ensures that the silicon pieces move over a first base containing grooves. In a fluidized bed area, dust is removed by a stream of air through a perforated plate. In a profiled area of the first base, the silicon pieces are deposited in holes of grooves or remain on ridges of the grooves. At the end of the first floor, pieces of silicon that are smaller than a gap between the first and the following floor fall through this onto a conveyor belt. Larger pieces of silicon move across the gap and fall onto the second tray.

US 2007/0235574 A1 discloses an apparatus for comminuting and sorting polycrystalline silicon, comprising a feed device for a polysilicon coarse fraction into a crusher system, the crusher system, and a sorting system for sorting the polysilicon fragments, the apparatus being provided with a control which has a variable setting at least a crushing parameter in the crushing system and / or at least one sorting parameter in the sorting system. The sorting system particularly preferably consists of a multi-stage mechanical screening system and a multi-stage optoelectronic separation system.

Also US 2009/0120848 A1 describes a device that enables flexible classification of broken polycrystalline silicon, characterized in that it comprises a mechanical screening system and an optoelectronic sorting system, the polycrystalline being separated by the mechanical screening system into a fine silicon fraction and a residual silicon fraction and the silicon -Residual fraction is separated into further fractions via an optoelectronic sorting system.

The mechanical screening system is preferably a vibrating screening machine that is driven by an unbalance motor. Mesh and perforated screens are preferred as screen covering.

US 2012/0198793 A1 discloses a method for metering and packaging polysilicon fragments, wherein a product stream of polysilicon fragments is transported over a conveyor chute, separated into coarse and fine fragments by means of at least one sieve, weighed by means of a metering scale and adjusted to a target weight is metered, the at least one sieve and the metering scales at least partially comprise a hard metal on their surfaces.

US 2014/0130455 A1 discloses, in the context of a method for packaging polycrystalline silicon fragments, that in a metering system fine components, that is to say the finest particles and chippings of the polysilicon, are separated off by means of a sieve. The screen can be a perforated plate, a bar screen or an optopneumatic sorting system.
The screens used comprise at least partially a low-contamination material such. B. a hard metal or ceramic / carbide .. The screens can be partially or fully provided with a coating of titanium nitride, titanium carbide, aluminum titanium nitride or DLC (Diamond Like Carbon).

Bar screens usually comprise bars running in parallel, the screen passage being determined by the distance between the bars and the screen overflow emerging at the free end of the bars. In the known bar sieves, the sieve bars are arranged in one plane and the material to be sieved is transported in that the sieve bars are inclined downward at their free end.

Separating devices according to the state of the art, such as bar screens, tend to clog when separating fines in the packaging machines. This also applies to the well-known step floor classifiers in which fractions are attempted to be separated using gaps between the floors.
The consequence of this is that these separation devices have to be cleaned cyclically and thus do not achieve a continuous, constant separation accuracy.
In addition, this requires system downtimes and additional effort for cleaning.
It is also disadvantageous that no exact separation is achieved, especially since, in addition to the fraction to be separated, a considerable proportion of oversizes is also separated off. This also leads to an undesired reduction in the yield of the target fraction.

DE19822996 C1 discloses a separation device for elongated solid parts, which has a vibrating base with a number of longitudinal grooves extending in the conveying direction, adjoining the sieve openings for separating the elongated solid parts (18), characterized in that the groove depth of the longitudinal grooves (12) in the conveying direction (6) decreases. In order to avoid blockages and to ensure that the flow of solids is as liquid as possible, a further advantageous embodiment provides for the sieve openings to widen in the conveying direction.

The arrangement described allows metal wires to be removed from pyrolysis residues such as those from construction sites or municipal waste.

The groove depth is initially designed in such a way that particles collect in the grooves, especially the long wires from the waste to be recycled. Then the groove depth decreases up to a planar state clearly decreases. As a result, the long wires can be cut off.

The object of the invention resulted from the problem described. The object of the invention is achieved by a method for the mechanical classification of polysilicon, consisting of fragments of the size classes Fraction size 0 0.1 to 5 mm, or Fraction size 1 3 to 15 mm, or Fraction size 2 10 to 40 mm, or Fraction size 3 20 to 60 mm, or Fraction size 4 45 to 120 mm, or Fraction size 5 100 to 250 mm, where the size class of polysilicon fragments is defined as the longest distance between two points on the surface of a silicon fragment (= max. length), with a sieve system, the polysilicon on a sieve plate, comprising a feed area for polysilicon, a profiled area with peaks and valleys, an area with slots, the slots adjoining the sinks, and a removal area, the slots opening in the direction of the removal area enlarge, is abandoned, wherein the sieve plate is set in vibration in such a way that the polysilicon executes a movement in the direction of the removal area, whereby small-scale polysilicon collects in the depressions of the sieve plate and falls through the slots of the sieve plate and is thereby separated from the applied polysilicon, The sieve plate is inclined at an angle of 5 to 20 ° to the horizontal, the tips of the profiled area also continuing into the area with slots so that the entire sieve plate is profiled, but the sieve plate at its rear end in the conveying direction Has slots instead of sinks where the depth and angle of the depressions of the profiled area and the size of the slots are designed so that either finely divided silicon, which can be separated by means of a mesh screen with square meshes with a size of 8 mm x 8 mm, is separated from the applied polysilicon 3 to 5, or finely divided silicon, which can be separated by means of a mesh screen with square meshes with a size of 1 mm × 1 mm, is separated from the applied polysilicon with chunk sizes 0 to 2.

The polysilicon is polycrystalline fragments.

Small-sized polysilicon is to be understood as a subset of the applied amount of polysilicon that is to be separated off by means of the screening system. The small-sized polysilicon thus corresponds to the fraction to be separated. According to the invention, the abandoned polysilicon is polysilicon fragments with a fine fraction. The fine fraction should be separated with the sieve plate.

The size class of polysilicon fragments is defined as the longest distance between two points on the surface of a silicon fragment (= max. Length): Fraction Size (BG) 0 0.1 to 5 mm Fraction size 1 3 to 15 mm Fraction size 2 10 to 40 mm Fraction size 3 20 to 60 mm Fraction size 4 45 to 120 mm Fraction size 5 100 to 250 mm

In the following, for the chunk sizes 3 to 5, all fragments or particles of silicon that are of such a size that they can be separated by means of a mesh screen with square meshes with a size of 8 mm x 8 mm are referred to as fines.

The same definition applies to the fraction sizes 0 to 2, whereby the mesh size is defined here as 1 mm x 1 mm.

The sieve plate includes a feed area in which the polysilicon is fed.

In one embodiment, the polysilicon is conveyed to the screening system by means of a conveyor trough and delivered to the feed area of the screen plate.

The sieve plate also comprises a profiled area with grooves or grooves or generally depressions and elevations, so that the profiled area has depressions and peaks.

During the movement of the polysilicon on the profiled area, small fragments or small silicon particles (small in relation to the target fraction) or fines collect in the depressions of the profiled area.

In one embodiment, the abandoned polysilicon comprises fragments of size classes 3 to 5 and fines according to the above definition. During the movement of the polysilicon on the profiled area, fines collect in the depressions of the profiled area.

In one embodiment, the applied polysilicon comprises fragments of size classes 0 to 2 and fine fraction according to the above. Definition. During the movement of the polysilicon on the profiled area, the fine fraction contained in the polysilicon collects in the depressions of the profiled area.

The sieve plate comprises - adjoining the profiled area - an area with slots. The slots are arranged directly behind the depressions of the profiled area in the conveying direction. As a result, the fine fractions of the polysilicon located in the depressions of the profiled area are specifically guided to the slots in the area.

According to the invention, the tips of the profiled area continue into the area with slots, so that the entire screen plate is profiled, but the screen plate has slots instead of depressions at its rear end in the conveying direction.

The fine fraction or small fragments / particles are thus separated off via the slots in the sieve plate.

In one embodiment, the separated fines or small fragments / particles are taken up by a collecting container arranged below the slots in the sieve plate.

Larger fragments are guided in the profiled area over the tips to the removal area.

In one embodiment, the removal area is connected to a conveyor trough through which the larger fragments are removed. Likewise, another sieve plate can then follow in order to separate a further fraction from the polysilicon.

The slots widen in the conveying direction. Surprisingly, clogging of the openings / slots can thereby be effectively avoided. So they step with it problems associated and observed in the prior art, which meant a lot of effort, do not arise.

The separation accuracy is significantly higher than with bar screens, which means that significantly less faulty grain is separated and thus the yield increases.

The disclosure therefore provides a sieve plate that can be used in all types of sieve systems, in which the fine fraction or small-sized silicon collects in depressions in the first area of the sieve plate and is deliberately separated in the last area of the sieve plate by widening sieve slots.

In one embodiment, the sieve plate consists of one or more materials selected from the group consisting of plastic, ceramic, glass, diamond, amorphous carbon, silicon or metal.

In one embodiment, the sieve plate is lined or coated with one or more materials selected from the group consisting of plastic, polyurethane, ceramic, glass, diamond, amorphous carbon and silicon.

In one embodiment, the parts of the screen plate which come into contact with the polysilicon are lined or coated with one or more materials selected from the group consisting of plastic, polyurethane, ceramic, glass, diamond, amorphous carbon and silicon.

In one embodiment, the sieve plate consists of hard metal or is coated or lined with a hard metal.

In one embodiment, the sieve plate comprises a metallic base body and a coating or lining made of one or more materials selected from the group consisting of plastic, ceramic, glass, diamond, amorphous carbon and silicon.

In one embodiment of the invention, the plastic used in the aforementioned embodiments is selected from the group consisting of PVC (polyvinyl chloride), PP (polypropylene), PE (polyethylene), PU (polyurethane), PFA (perfluoroalkoxy), PVDF (polyvinylidene fluoride) and PTFE (polytetrafluoroethylene).

In one embodiment, the sieve plate comprises a coating of titanium nitride, titanium carbide, aluminum titanium nitride or DLC (Diamond Like Carbon).

The size of the slots depends on the fraction to be separated and can be up to 200 mm.

In one embodiment, a separation step should take place at 10 mm (sieving off polysilicon smaller than 10 mm), the slots having a width of 10 mm at their end (beginning of the removal area).

The design of the profiled area of the sieve plate depends on the fraction to be separated. The depth and angle of the depressions in the profiled area must be designed so that the fraction to be separated, e.g. the fine fraction collects there.

The angles of the depressions can be flat to extremely acute and be larger than 1 ° and smaller than 180 °.

The depth of the depressions can be from 1 to 200 mm.

For example, an angle of 45 ° and a depth of 20 mm are suitable for the separation of a 10 mm fraction.

The excitation of the sieve plate can be done either with a plane vibrating or throwing sieve machine. Vibration drives (such as magnetic drives) or unbalance drives can also be provided.

According to the invention, the sieve plate is inclined to the horizontal.

The angle of inclination is between 5 and 20 °, since gravity supports the conveyance via the sieve plate.

Brief description of the figures

Fig. 1 shows the schematic structure of a sieve plate.

List of reference symbols used

1

Sieve plate

2

Task area

3

Profiled area of the sieve plate

31

Lowering the profiled area

32

Tips of the profiled area

4th

Slotted area

41

slot

5

Removal area

The sieve plate 1 comprises a feed area 2 in which the polysilicon is fed. The polysilicon can, for example, be conveyed to the screening system by means of a conveyor trough and delivered to the feed area 2 of the screen plate 1.

The sieve plate 1 also comprises a profiled area 3. This profiled area 3 provides grooves or grooves or depressions of a different type so that the profiled area 3 has depressions 31 and peaks 32.

The fine fraction contained in the polysilicon collects during the movement of the polysilicon on the profiled region 3 in the depressions 31 of the profiled region 3.

The sieve plate 1 comprises - adjoining the profiled area 3 - an area 4 with slots 41. The slots 41 are arranged immediately behind (in the conveying direction) the depressions 31 of the profiled area 3. As a result, the fine fractions of the polysilicon located in the depressions 31 of the profiled region 3 are guided in a targeted manner to the slots 41 of the region 4.

The tips 32 of the profiled area 3 also continue in the area 4, so that the entire screen plate 1 is profiled, but has slots 41 instead of depressions 31 in the area 4.

The fine fraction is thus separated off via the slots 41 in the sieve plate 1. The separated fines can be received, for example, by a collecting container arranged below the slots 41 in the sieve plate 1.

Larger fragments are guided in the profiled area via the tips 32 to the removal area 5.

The slots 41 widen in the conveying direction. It has been shown that this can effectively avoid clogging of the openings.

Claims (6)

1. A method for mechanically sizing polysilicon, consisting of Fragments of the size classes break size 0 0,1 to 5 mm, or break size 1 3 to 15 mm, or break size 2 10 to 40 mm, or break size 3 20 to 60 mm, or break size 4 45 to 120 mm, or break size 5 100 to 250 mm, whereby the size class of Polysilicon fragments as longest distance between two points on the surface of a silicon fragment (=max. length), with a screening system, wherein the polysilicon is fed onto a screen plate (1) comprising a feed area (2) for polysilicon, a profiled area (3) with peaks (32) and troughs (31), an area (4) with slots (41), wherein the slots (41) adjoin the troughs (31), and a removal area (5), wherein the slots (41) increase in size in the direction of the removal area (5), wherein the screen plate (1) is vibrated such that the polysilicon performs a movement in the direction of the removal area (5), wherein finely divided polysilicon collects in the depressions (31) of the screen plate (1) and falls through the slots (41) of the screen plate (1) and is thereby separated from the charged polysilicon, wherein the screen plate (1) is inclined at an angle of 5 to 20° with respect to the horizontal., wherein the peaks (32) of the profiled area (3) also continue into the area (4) with slots (41), so that the entire screen plate (1) is profiled, the screen plate (1) however having slots (41) instead of depressions (31) at its rear end in the conveying direction, wherein the depth and angle of the depressions (31) of the profiled area (3) and the size of the slits (41) are such that either finely divided silicon, which can be separated by means of a square mesh screen having a size of 8 mm x 8 mm separated from the polysilicon of break sizes 3 to 5 or finely divided silicon, which can be separated by means of a square mesh screen with a size of 1 mm x 1 mm, is separated from the feed polysilicon of break sizes 0 to 2.
2. A method according to claim 1, wherein the screen plate (1) is made of one or more materials selected from the group consisting of plastic, ceramic, glass, diamond, amorphous carbon, silicon and metal.
3. A method according to any one of claims 1 to 2, wherein the screen plate (1) comprises a metallic base body and a coating or lining with one or more materials selected from the group consisting of plastic, ceramic, glass, diamond, amorphous carbon and silicon.
4. A method according to any one of claims 1 to 3, wherein the screen plate (1) comprises a coating of titanium nitride, titanium carbide, aluminum titanium nitride or DLC (Diamond Like Carbon).
5. A method according to any one of claims 1 to 4, wherein the screen plate (1) is made of hard metal or is lined or coated with a hard metal.
6. A method according to any one of claims 1 to 5, wherein the size of the slots (41) is up to 200 mm.

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