EP3043929B1 - Classification of polycrystalline silicon - Google Patents

Description

The invention relates to a method for classifying polysilicon.

Polycrystalline silicon (polysilicon in short) serves as a starting material for the production of monocrystalline silicon for semiconductors according to the Czochralski (CZ) or zone melting (FZ) method, as well as for the production of monocrystalline or multicrystalline silicon by various drawing and casting method Production of solar cells for photovoltaics.

Polycrystalline silicon is usually produced by means of the Siemens process. In this method, in a bell-shaped reactor ("Siemens reactor") carrier body, usually thin filament rods made of silicon, heated by direct passage of current and a reaction gas containing hydrogen and one or more silicon-containing components introduced. Usually, the silicon-containing component used is trichlorosilane (SiHCl ₃ , TCS) or a mixture of trichlorosilane with dichlorosilane (SiH ₂ Cl ₂ , DCS) and / or with tetrachlorosilane (SiCl ₄ , STC). More rarely, but also on an industrial scale silane (SiH ₄₎ is used. The filament rods are mounted vertically in electrodes located at the bottom of the reactor, via which the connection to the power supply takes place. High-purity polysilicon deposits on the heated filament rods and the horizontal bridge, increasing their diameter over time. After cooling the rods, the reactor bell is opened and the rods are removed by hand or with the help of special devices, so-called removal aids for further processing or for temporary storage. For most applications, polycrystalline silicon rods are broken down into small fragments, which are usually subsequently sized.

Polycrystalline silicon granules or polysilicon granules for short is an alternative to the polysilicon produced in the Siemens process. While the polysilicon obtained in the Siemens process as a cylindrical silicon rod, the time-consuming and costly crushed before further processing into fragments and, if necessary, must be cleaned again, polysilicon granules bulk material properties and can be used directly as a raw material for example for single crystal production for the photovoltaic and electronics industry , Polysilicon granules are produced in a fluidized bed reactor. This is done by fluidization of silicon particles by means of a gas flow in a fluidized bed, which is heated by a heater to high temperatures. By addition of a silicon-containing reaction gas a pyrolysis reaction takes place on the hot particle surface. Here, elemental silicon is deposited on the silicon particles and the individual particles grow in diameter. By the regular withdrawal of grown particles and addition of smaller silicon particles as seed particles, the process can be operated continuously with all the associated advantages. Silicon-containing compounds (for example chlorosilanes or bromosilanes), monosilane (SiH ₄ ) and mixtures of these gases with hydrogen can be used as the silicon-containing educt gas.

The polycrystalline silicon granules are divided after its preparation by means of a sieve in two or more fractions.
The smallest sieve fraction (Siebunterkorn) can then be processed in a grinding plant to seed particles and added to the reactor.
The screening target fraction is usually packed.

US 2009081108 A1 discloses a workbench for manually sorting polycrystalline silicon by size and quality. An ionization system is implemented to neutralize electrostatic charges by active air ionization. Ionizers penetrate the clean room air with ions in such a way that static charges on insulators and ungrounded conductors are dissipated.

Usually screening machines are used to sort or classify polycrystalline silicon after comminution into different size classes.

A screening machine is generally a machine for screening, ie the separation (separation) of solid mixtures to particle sizes.

According to the movement characteristics, a distinction is made between planing vibrating screens and throwing machines.

The drive of the screening machines is usually electromagnetic or by unbalance motors or gearbox.

The movement of the Siebbelags serves the further transport of the feedstock in Sieblängsrichtung and the passage of the fine fraction through the mesh openings.

In contrast to Planschwangsiebmaschinen occurs in throwing machines in addition to the horizontal and a vertical Siebbeschleunigung.

In the litter screening machines, vertical throwing motions overlap with slight turning movements. As a result, the sample material spreads over the entire surface of the sieve bottom and at the same time the particles experience an acceleration in a vertical direction (thrown up). In the air they can make free turns and are compared with the mesh of the screen fabric when falling back on the screen. If the particles are smaller than these, they pass through the sieve, they are larger, they are thrown up again. The rotational movement ensures that they have a different orientation on the next sieve on the mesh and so maybe pass through a mesh opening.

In planing sifters, the sieve tower completes a horizontal circular movement in one plane. As a result, the particles on the screen fabric largely retain their orientation. Planeting machines are preferably used for needle-shaped, platelet-shaped, elongated or fibrous screening goods, in which a throwing up of the sample is not necessarily beneficial.

A special type is the Mehrdecksiebmaschine, which can fractionate several grain sizes simultaneously. They are designed for a large number of sharp separations in the mid to ultra-fine grain range.

The drive principle is based on two-deck planer on two counter-rotating unbalance motors that produce a linear vibration. The screen material moves in a straight line over the horizontal separating surface. The machine works with low vibration acceleration.

Through a modular system, a variety of screening decks can be assembled into a stack of sieves. Thus, if necessary, different grain sizes can be produced in a single machine, without having to change screen coverings. By repeated repetition of the same Siebdeckfolgen the screening material can be offered much screen area.

US 8021483 B2 discloses an apparatus for sorting polycrystalline silicon pieces including a vibratory motor assembly and a step bottom classifier attached to the vibratory motor assembly. The vibratory motor assembly causes the pieces of silicon to move over a first bottom 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 floor, the pieces of silicon settle in holes of grooves or remain on ridges of the grooves.

At the end of the first bottom, pieces of silicon smaller than a gap fall through it onto a conveyor belt. Larger pieces of silicon move across the gap and fall to the second floor. The parts of the device that come in contact with the polycrystalline silicon pieces are made of materials that minimize contamination of silicon. Examples are tungsten carbide, PE, PP, PFA, PU, PVDF, PTFE, silicon and ceramics.

US 2007235574 A1 discloses a device for crushing and sorting polycrystalline silicon, comprising a polysilicon rough-cut feeder in a crusher, the crusher, and a polysilicon-collapse classifier, the device being provided with a controller having at least one variable setting a crushing parameter in the crusher and / or at least one sorting parameter in the sorting system allows. Particularly preferably, the sorting plant consists of a multi-stage mechanical screening plant and a multi-stage optoelectronic separation plant. Preference Schwingsiebmaschinen that are driven by an unbalance motor used. As Siebbelag mesh and perforated sieves are preferred.

The screening levels can be successively or in another structure, such. B. a tree structure, be arranged. Preferably, the sieves are arranged in three stages in a tree structure.
The freed from fines polysilicon fraction is preferably sorted by means of optoelectronic separation plant. Sorting of polysilicon fracture can be done according to all criteria that are state of the art in image processing. It is preferably carried out according to one to three of the criteria selected from the group length, area, shape, morphology, color and weight of polysilicon fragments, particularly preferably length and area.

• Fraktion 0: Bruchgrößen mit einer Verteilung von ca. 0 bis 3 mm
• Fraktion 1: Bruchgrößen mit einer Verteilung von ca. 1 mm bis 10 mm
• Fraktion 2: Bruchgrößen mit einer Verteilung von ca. 10 mm bis 40 mm
• Fraktion 3: Bruchgrößen mit einer Verteilung von ca. 25 mm bis 65 mm
• Fraktion 4: Bruchgrößen mit einer Verteilung von ca. 50 mm bis 110 mm
• Fraktion 5: Bruchgrößen mit einer Verteilung von ca. > 90 mm bis 250 mm

This allows the preparation of the following fractions:

• Fraction 0: fraction sizes with a distribution of about 0 to 3 mm
• Fraction 1: fraction sizes with a distribution of about 1 mm to 10 mm
• Fraction 2: fraction sizes with a distribution of about 10 mm to 40 mm
• Fraction 3: fraction sizes with a distribution of about 25 mm to 65 mm
• Fraction 4: fraction sizes with a distribution of approx. 50 mm to 110 mm
• Fraction 5: fraction sizes with a distribution of approx.> 90 mm to 250 mm

About the exact distribution of fractions within the fractions makes US 2007235574 A1 not specified. DE3811091A1 discloses a method of mechanically classifying polycrystalline silicon fracture, wherein the silicon fracture is on one or more screens, each comprising a polycrystalline silicon screen, whereby the silicon fraction is separated into various size classes.

US 5165548 A discloses an apparatus for size separating silicon pieces suitable for semiconductor applications, comprising a cylindrical screen connected to a device for rotating the cylindrical screen, wherein the surfaces of the screen contacting the silicon pieces consist essentially of silicon suitable for semiconductor applications.

US 7959008 B2 claims a method for sifting out first particles of granules comprising first and second particles by conveying the granules along a first screen surface, preferably starting from a vibration device, wherein the first particles have an aspect ratio a1 with a1> n: 1 and n = 2, 3, > 3, in particular with a1> 3: 1, and the second particles have a dimensioning which allows them to pass through the meshes of the first screen surface, characterized in that the granules are conveyed along the screen surface between this and a cover extending along the screen surface and that due to the cover the first particles are aligned with their longitudinal axes running along the screen surface, wherein the longitudinal extent of each first particle is greater than the mesh size of the screen forming the first screen surface and the longitudinal extent of the second particles is equal to or smaller than the mesh size.

EP 1454679 B1 describes a screening device with a first oscillating body, which is provided with first cross members, and a second oscillating body, which is provided with second cross members, which first and second cross members are arranged alternately and have clamping devices, so that elastic Siebbeläge between a first and one each second cross member are clamped, and a drive unit which is directly coupled to the first oscillating body and over which the first oscillating body is positively guided, so that the clamped elastic Siebbeläge between a stretched and a compressed position are moved back and forth, the second oscillating body opposite the first oscillating body is forcibly guided.

In US 6375011 B1 For example, there is proposed a method for promoting silicon fracture in which the silicon fragments are passed over a conveying surface of a vibrating conveyor made of hyperpure silicon. In this case, sharp-edged silicon fragments are rounded when they are conveyed on the vibrating conveyor surface of a vibrating conveyor. The specific surfaces of the silicon fragments are reduced, superficial adhering contaminations are abraded.

The rounded by a first vibratory conveyor unit silicon break can via a second vibratory conveyor unit are performed. Their conveying surface consists of parallel arranged hyperpure silicon plates, which are fixed by lateral fastening devices. The hyperpure silicon plates have through openings, for example in the form of openings. The conveyor edges, which serve as a lateral boundary of the conveyor surfaces, are also made of hyperpure silicon plates and are fixed, for example, by hold-down. The conveyor surfaces made of hyperpure silicon slabs are supported by steel plates and optionally damping mats.

US 2012052297 A1 discloses a process for the production of polycrystalline silicon, comprising breaking into broken pieces of polycrystalline silicon deposited on thin rods in a Siemens reactor, classifying the fragments into size classes of about 0.5 mm to greater than 45 mm, treating the silicon fragments by means of compressed air or dry ice to form silicon Remove dust from the debris without chemical wet cleaning. The polycrystalline silicon is classified as follows: fraction size 0 (BG0) in mm: about 0.5 to 5; Break size 1 (BG1) in mm: approx. 3 to 15; Fracture size 2 (BG2) in mm: approx. 10 to 40; Crack size 3 (BG3) in mm: approx. 20 to 60; Fracture size 4 (BG4) in mm: approx.>45; in each case at least 90% by weight of the fracture fraction being within the stated size range. This corresponds to the specification of the different fraction sizes into which the silicon is to be classified. The application gives no information on the actual result of the classification or sorting of the silicon and the size distributions within the individual size classes.

US 2009120848 A1 describes a device which allows a flexible classification of broken polycrystalline silicon, characterized in that it comprises a mechanical sieve and an optoelectronic sorting system, wherein the poly-break is separated by the mechanical sieve into a silicon fines and a silicon remainder and the silicon -Restanteil is separated via an optoelectronic sorting in other fractions.
The mechanical screen is preferably a vibrating screen, which is driven by an unbalance motor.

During mechanical classifying by sieving using vibrating screening machines according to the state of the art, material wear arises on the sieve lining and is introduced into the product. This leads to contamination of the polysilicon with components contained in the screen coating.

In addition, it is disadvantageous in the prior art that the fractions into which the polysilicon is classified clearly overlap.

In the prior art, some overlap was already accepted in the specifications.

at US 2012052297 A1 the overlap between fraction size 2 and fraction size 1 is max. 5 mm, between fraction size 1 and fraction size 0 max. 2 mm. This concerns the specification to classify. The actual distribution of fraction sizes usually deviates from this.

According to US 2007235574 A1 If the overlap between a fraction 1 and a fraction 0 is also max. 2mm.

Especially in the fractions with smaller fragment sizes of 30 mm or less, such an overlap is undesirable.

From this problem, the task of the invention resulted.

wobei

• r: Schwingamplitude;
• ω: Winkelgeschwindigkeit;
• α: Wurfwinkel ;
• β Siebneigungswinkel;
• g: Gravitationskonstante.

The object of the invention is achieved by a method for mechanically classifying polycrystalline silicon fracture or granules with a vibrating screen, wherein silicon fracture or granules are on one or more screens each comprising a screen lining, which are vibrated such that the silicon break or the silicon granules make a movement whereby the silicon or silicon granules are separated into different size classes, characterized in that the sieving index (K _v ) is greater than or equal to 1.6 and less than or equal to 3.0, and a litter screening machine is used. The sieve index is defined as the ratio of the acceleration generated by the sieve movement to the gravitational acceleration vertical to the sieve plane: $${\mathrm{K}}_{\mathrm{v}}=\mathrm{r}*{\mathrm{\omega }}^{\mathrm{2}}*\sin \left(\mathrm{\alpha }+\mathrm{\beta }\right)/\left(\mathrm{G}*\cos \left(\mathrm{\beta }\right)\right).$$

in which

• r: oscillation amplitude;
• ω: angular velocity;
• α: throw angle;
• β screen tilt angle;
• g: gravitational constant.

It indicates how strongly an object accelerates vertically at maximum relative to the gravitational acceleration g of the earth.

If the sieving index is <1, then there is pure sliding movement (without throwing motion), since the resulting vertical acceleration is smaller than the falling acceleration.

For a throwing motion, the sieving index must be> 1.

It has surprisingly been found that significantly worse sieving results both in processes with a sieving index of less than 0.6 and in processes with a sieve index of greater than 9.0 than in the inventive range of 1.6-3.0.

The movement of silicon fracture or granulate is particularly preferably a throwing movement, the sieving index being 1.6 to 3.0. It has been shown that this results in even improved screening results, in particular an even higher selectivity between the different size classes.

The oscillation amplitude is preferably 0.5 to 8 mm, particularly preferably 1 to 4 mm.

The rotational speed ω / 2π is preferably 400 to 2000 rpm, more preferably 600 to 1500 rpm.

The throwing angle is preferably 30 to 60 °, particularly preferably 40 to 50 °.

The screen tilt angle with respect to the horizontal is preferably 0 to 15 °, particularly preferably 0 to 10 °.

The screening machine preferably comprises a feed area in which the screenings are fed and a discharge area in which classified screenings are removed.

Preferably, the size of the sieve openings increases in the direction of discharge. Fractions / break sizes are preferably separated by successively arranged discharges.

Preferably, the screening machine comprises mutually arranged screen decks. This has the advantage that large fragments can not damage fine mesh screen coverings. Preferably, fractions / break sizes are separated by discharges arranged one below the other.

Preferably, the screening machine comprises a frame-sieve system. This allows a quick screen change. Also, the monitoring of any contamination is facilitated.

Such a frame-sieve system provides that screen coverings are screwed to the frame, glued, plugged or potted, that the frame made of wear-resistant plastic (preferably PP, PE, PU), possibly with steel reinforcement, or at least lined with wear-resistant plastic are. Preferably, the frames are sealed by vertical clamping. This can avoid contamination and material loss.

It is preferred to use screen linings made of particularly wear-resistant plastics, namely elastomers having a hardness greater than 65 Shore A, more preferably having a hardness greater than 80 Shore A. The Shore hardness is specified in the standards DIN 53505 and DIN 7868. In this case, one or more screen coverings or their surfaces may consist of such an elastomer.

Both one or more screen coverings or their surfaces as well as all product-contacting components and linings are preferably made of plastics with a total contamination (metals, dopants) of less than 2000 ppmw, preferably less than 500 ppmw and more preferably less than 100 ppmw.

The maximum contamination of the plastics with the elements Al, Ca, P, Ti, Sn and Zn should be less than 100 ppmw, more preferably less than 20 ppmw.

The maximum contamination of the plastics with the elements Cr, Fe, Mg, As, Co, Cu, Mo, Sb and W should be less than 10 ppmw, more preferably less than 0.2 ppmw.
The determination of the impurities is carried out by means of ICP-MS (mass spectrometry with inductively coupled plasma).

The screen coverings of plastics preferably comprise a reinforcement or filling of metals, glass fiber, carbon fiber, ceramic or composite materials for stiffening.

Preferably, the screenings are dedusted. The mechanical sieving mobilizes most of the fine dust adhering to the bulk material on the individual screen decks. This effect is used in the invention to dedust the bulk material during the screening process.

It is important in this case that the freed fine dust is transported away in an exhaust passage by a corresponding gas guide, so he can not get back into the product.

The gas flow can be generated either by a suction or by a gas purging.

Suitable visual gases are purified air, nitrogen or other inert gases.
In the screening machine, a gas velocity of 0.05 to 0.5 m / s, more preferably 0.2 to 0.3 m / s should be present.

A gas velocity of 0.2 m / s can be set, for example, with a gas throughput or an extraction capacity of about 720 Nm ³ / h per m ^{2 of} screen area.

As fine dust particles are understood that are smaller than 10 microns.

In addition to dedusting in the screening machine, dedusting by means of countercurrent air classification in the discharge lines of the individual sieve fractions is optionally carried out.

In this case, the classifying gas is fed into the lower area of the flue ducts and the dust-laden exhaust gas is discharged in the upper area immediately in front of the screening machine. As a sight gas again the above media come into question.

The advantage of this dedusting method is that the visual flow can be adapted to the particle size of the sieve fraction. With a coarse sieve fraction, for example, a high visual flow can be set without fine product being carried along. This gives a very good dedusting and the desired low particulate matter in the product.

Preferably, the speed is temporarily increased up to 4000 rev / min, to free the screen coverings of Steckkorn. For this purpose, alternatively, the oscillation amplitude can be temporarily increased to up to 15 mm.

Likewise, it is preferred to use free-blow balls made of plastic or hyperpure silicon in order to free the screen coverings from sticking grain.

Preferably, the oscillation amplitude decreases towards the discharge. Particularly preferably, the ratio of the vibration amplitude at the discharge is up to 50% less than at the inlet. It has been shown that both wear and product contamination can be further reduced.

As a drive for the screening machine come linear, circular or elliptical oscillator in question. The drive preferably provides a vertical acceleration component to reduce screen wear and to avoid pinch.

It is preferable to use certain shapes of the sieve openings.
As advantageous rectangular openings have been found. It shows less wear due to smaller contact surfaces. Plug / clamp grain can be avoided more easily.

By contrast, round openings lead to a higher selectivity with regard to the particle size.

Square openings are also preferred. With them advantages of rectangular and round openings can be combined.

Preferably, the screening tray and the Siebauslässe inside are completely lined with silicon or with a thermoplastic or elastomeric plastic.

Steel body of the screening machine are preferably provided with welded PP lining segments. Preference is also the use of interior linings made of PU.

As side linings, steel-reinforced PU castings have proven to be particularly suitable.

For fixing the screen frame preferably quick release devices are used.

It is also preferable to use silicon hole strips as the screen covering. One or more screen coverings can be designed in this way. These are preferably perforated square rods made of hyperpure silicon.

These preferably have at least in part a conical hole shape, i. the cross-sectional area is smaller at the top than at the bottom. This contributes to avoiding pinch.

The cone preferably has an angle of 1 to 20 °, particularly preferably 1 to 5 °.

Preferably, an edge rounding of the holes with a radius of 0.1 to 2 mm is provided on the upper Sieboberseite to avoid breakouts and wear, which would lead to a deterioration of the selectivity.

Preferably, only the lower part of the hole is conical and the upper part is cylindrical, so that the hole is not expanded too quickly due to wear.

Plastic-sheathed metal support strips are preferably provided for stabilization in the event of breakage of the Si strips, in order to avoid contamination and to secure against loosening of fragments in case of inguinal hernia.

Preferably, individual Si strips are provided with hard metal end strips, which are braced horizontally or vertically. Thus, a cost-effective replacement of individual strips is possible depending on the wear. The hard metal used is preferably WC, SiC, SiN or TiN.

Preferably, the Si perforated screen is placed on a base, glued or screwed. This allows higher strength, larger areas and the use of thinner or thicker screens possible. Breakage is easier to avoid.

It is particularly preferred to use both Si perforated sieves and sieves made of plastic or sieves with a plastic covering.

Preferably, a Si-hole sieve with a hole diameter of 5 mm to 50 mm is used as the first sieve cut. The large fragments can clean the sprue grains and thus prevent clogging.

For further separation of the fine fractions one or more screens made of plastic or plastic coverings are used.

Preferably, for silicon fracture with particle sizes greater than 15 mm (max particle length), an additional pre-screen with a plastic coating and with a mesh ratio to the underlying screen deck of from 1.5: 1 to 10: 1 is used. This can reduce the plastic wear on the lower screen deck. The outlets of both screen decks are merged. The Vorsiebdeck preferably has a lower wire tension. This serves to minimize wear.

The method according to the invention (throwing motion, screen index number 1.6-3.0) leads to polycrystalline silicon fragments having a sharp particle size distribution without large overlap or to polycrystalline silicon granules classified with a high selectivity, which was not possible in the prior art.

Classified polycrystalline silicon fragments are also described, characterized by a particle size classification in fractional size classes 2, 1, 0 and F, whereby it applies for the fragments that at fraction size 2 max. 5% by weight smaller than 11 mm and max. 5% by weight greater than 27 mm; at fraction size 1 max. 5% by weight smaller than 3.7 mm and max. 5% by weight greater than 14 mm; at fraction size 0 max. 5% by weight less than 0.6 mm and max. 5 wt% greater than 4.6 mm; at fraction size F max. 5 wt .-% less than 0.1 mm and max. 5 wt .-% are greater than 0.8 mm.

The fracture size is defined in each case as the longest distance of two points on the surface of a silicon fragment (= maximum length).

• Bruchgröße F (BG F) in mm: 0,1 bis 0,8;
• Bruchgröße 0 (BG 0) in mm: 0,6 bis 4,6;
• Bruchgröße 1 (BG 1) in mm: 3,7 bis 14;
• Bruchgröße 2 (BG 2) in mm: 11 bis 27.

It follows:

• Fracture size F (BG F) in mm: 0.1 to 0.8;
• Fracture size 0 (BG 0) in mm: 0.6 to 4.6;
• Fracture size 1 (BG 1) in mm: 3.7 to 14;
• Break size 2 (BG 2) in mm: 11 to 27.

In each case at least 90% by weight of the fracture fraction are within the stated size range.

• Bruchgröße 2 zu Bruchgröße 1: max. 3 mm;
• Bruchgröße 1 zu Bruchgröße 0: max. 0,9 mm;
• Bruchgröße 0 zu Bruchgröße F: max. 0,2 mm.

This results in an overlap range of the 5% by weight quantile of the coarse fraction size to the 95% by weight quantile of the fine fraction size of:

• Breakage size 2 to breakage size 1: max. 3 mm;
• Break size 1 to breakage size 0: max. 0.9 mm;
• Fraction size 0 to fraction size F: max. 0.2 mm.

• Wolfram (W):
  • Bruchgröße 1 ≤ 100000 pptw, besonders bevorzugt ≤ 20000 pptw;
  • Bruchgröße 0 ≤ 1000000 pptw, besonders bevorzugt ≤ 200000 pptw;
  • Bruchgröße F ≤ 10000000 pptw, besonders bevorzugt ≤ 2000000 pptw;
• Kobalt (Co):
  • Bruchgröße 2 ≤ 5000 pptw, besonders bevorzugt ≤ 500 pptw;
  • Bruchgröße 1 ≤ 50000 pptw, besonders bevorzugt ≤ 5000 pptw;
  • Bruchgröße 0 ≤ 500000 pptw, besonders bevorzugt ≤ 50000 pptw;
  • Bruchgröße F ≤ 5000000 pptw, besonders bevorzugt ≤ 500000 pptw;
• Eisen (Fe):
  • Bruchgröße 2 ≤ 50000 pptw, besonders bevorzugt ≤ 1000 pptw;
  • Bruchgröße 1 ≤ 500000 pptw, besonders bevorzugt ≤ 10000 pptw;
  • Bruchgröße 0 ≤ 5000000 pptw, besonders bevorzugt ≤ 100000 pptw;
  • Bruchgröße F ≤ 50000000 pptw, besonders bevorzugt ≤ 1000000 pptw;
• Kohlenstoff (C):
  • Bruchgröße 2 ≤ 1 ppmw, besonders bevorzugt ≤ 0,2 ppmw;
  • Bruchgröße 1 ≤ 10 ppmw, besonders bevorzugt ≤ 2 ppmw;
  • Bruchgröße 0 ≤ 100 ppmw, besonders bevorzugt ≤ 20 ppmw;
  • Bruchgröße F ≤ 1000 ppmw, besonders bevorzugt ≤ 200 ppmw;
• Cr, Ni, Na, Zn, Al, Cu, Mg, Ti, K, Ag, Ca, Mo je Einzelelement:
  • Bruchgröße 2 ≤ 1000 pptw, besonders bevorzugt ≤ 100 pptw;
  • Bruchgröße 1 ≤ 2000 pptw, besonders bevorzugt ≤ 200 pptw;
  • Bruchgröße 0 ≤ 10000 pptw, besonders bevorzugt ≤ 1000 pptw;
  • Bruchgröße F ≤ 100000 pptw, besonders bevorzugt ≤ 10000 pptw;
• Feinstaub (Siliciumpartikel mit einer Größe von weniger als 10 µm):
  • Bruchgröße 2 ≤ 5 ppmw, besonders bevorzugt ≤ 2 ppmw;
  • Bruchgröße 1≤ 15 ppmw, besonders bevorzugt ≤ 5 ppmw;
  • Bruchgröße ≤ 25 ppmw, besonders bevorzugt ≤ 10 ppmw;
  • Bruchgröße F ≤ 50 ppmw, besonders bevorzugt ≤ 20 ppmw.

The polycrystalline silicon fragments having the improved grain size classification preferably have a very low surface contamination:

• Tungsten (W):
  • Fracture size 1 ≤ 100,000 pptw, more preferably ≤ 20000 pptw;
  • Crack size 0 ≤ 1000000 pptw, more preferably ≤ 200000 pptw;
  • Fracture size F ≤ 10000000 pptw, more preferably ≤ 2000000 pptw;
• Cobalt (Co):
  • Fraction size 2 ≤ 5000 pptw, more preferably ≤ 500 pptw;
  • Fracture size 1 ≤ 50000 pptw, more preferably ≤ 5000 pptw;
  • Fraction size 0 ≤ 500,000 pptw, more preferably ≤ 50,000 pptw;
  • Fracture size F ≤ 5000000 pptw, more preferably ≤ 500000 pptw;
• Iron (Fe):
  • Crack size 2 ≤ 50000 pptw, more preferably ≤ 1000 pptw;
  • Crack size 1 ≤ 500,000 pptw, more preferably ≤ 10000 pptw;
  • Fraction size 0 ≤ 5000000 pptw, more preferably ≤ 100000 pptw;
  • Fracture size F ≤ 50,000,000 pptw, more preferably ≤ 1000000 pptw;
• Carbon (C):
  • Fraction size 2 ≦ 1 ppmw, more preferably ≦ 0.2 ppmw;
  • Fraction size 1 ≦ 10 ppmw, more preferably ≦ 2 ppmw;
  • Fraction size 0 ≦ 100 ppmw, more preferably ≦ 20 ppmw;
  • Fracture size F ≦ 1000 ppmw, more preferably ≦ 200 ppmw;
• Cr, Ni, Na, Zn, Al, Cu, Mg, Ti, K, Ag, Ca, Mo per single element:
  • Crack size 2 ≤ 1000 pptw, more preferably ≤ 100 pptw;
  • Fracture size 1 ≤ 2000 pptw, more preferably ≤ 200 pptw;
  • Fraction size 0 ≤ 10000 pptw, more preferably ≤ 1000 pptw;
  • Fracture size F ≦ 100000 pptw, more preferably ≦ 10000 pptw;
• Particulate matter (silicon particles less than 10 μm in size):
  • Fraction size 2 ≦ 5 ppmw, more preferably ≦ 2 ppmw;
  • Fracture size 1 ≦ 15 ppmw, more preferably ≦ 5 ppmw;
  • Crack size ≤ 25 ppmw, more preferably ≤ 10 ppmw;
  • Fraction size F ≦ 50 ppmw, more preferably ≦ 20 ppmw.

Described is also classified polycrystalline silicon granules, classified at least in the two size classes Siebzielkorn and Siebunterkorn, wherein a selectivity between Siebzielkorn and Siebunterkorn is more than 0.86.

It is preferably classified polycrystalline silicon granules, classified in Siebzielkorn, Siebunterkorn and Sieboberkorn, wherein a selectivity between Siebzielkorn and Siebunterkorn and between Siebzielkorn and Sieboberkorn is more than 0.86.

Classified polycrystalline silicon granules preferably have the following impurities with metals on the surface: Fe: <800 pptw, more preferably <400 pptw; Cr: <100 pptw, more preferably <60 pptw; Ni: <100 pptw, more preferably <50 pptw; Na: <100 pptw, more preferably <50 pptw; Cu: <20 pptw, more preferably <10 pptw; Zn: <2000 pptw, more preferably <1000 pptw.

Classified polycrystalline silicon granules preferably have a surface carbon contamination of less than 10 ppmw, more preferably less than 5 ppmw.

Classified polycrystalline silicon granules preferably have a surface particulate contamination of less than 10 ppmw, more preferably we niger than 5 ppmw, on. Fine dust is defined as silicon particles with a size of less than 10 μm.

Examples and Comparative Examples

The advantages of the invention are shown below with reference to examples and comparative examples.

Example 1 and Comparative Example 2 relate to classifying polycrystalline silicon fragments into fractions 2, 1, 0, and F.

Example 3 and Comparative Example 4 relate to classifying polycrystalline silicon granules (sieve grain size 0.75 - 4 mm).

example 1

Table 1a shows the essential parameters of the screening machine. <b> Table 1a </ b> Wire width b [mm] 600 Sieve length l [mm] 1600 Frequency n [Hz] 25 Speed [rpm] 1500 Angular velocity ω [1 / s] 157.1 Amplitude [mm] 3 Amplitude r [mm] 1.5 Screen inclination β [°] 0 Throwing angle α [°] 50 Screening number Kv [-] 2.9 Throughput [kp / h] 700 N ₂ sealing gas [Nm ³ / h] 50

Table 1b shows which sieve set was used in the example. Three sieve decks with different sieve mesh sizes were used. <b> Table 1b </ b> Mesh size [mm] material Deck 1 9 polyurethane Deck 2 1.9 polyamide Deck 3 0.3 polyamide

Table 1c shows the composition of the screen coverings. <b> Table 1c </ b> element polyurethane: Polyamide: Al [ppmw] 17 0.7 Ca [ppmw] 14 9.1 Cr [ppmw] <0.2 0.3 Fe [ppmw] 0.7 0.9 K [ppmw] 0.7 <0.2 Mg [ppmw] 0.4 0.2 Na [ppmw] 0.3 0.6 P [ppmw] 63 <20 Sn [ppmw] 5.4 <0.2 Ti [ppmw] 570 0.2 Zn [ppmw] 8.5 <0.2 As, B, Ba, Cd, Co, Cu, Li, Mn, Mo, Ni, Sr, V [ppmw] <0.2 <0.2 Be, Bi, Pb, Sb, W [ppmw] <0.2 <0.2

The sieving results achieved with regard to the particle size distribution are shown in Tables 1d and 1e . <b> Table 1d </ b> Fraction size 2 Break size 1 Fraction size 0 Fracture size F 5% by weight - length quantile: [mm] 11.3 3.9 0.65 0.12 95% by weight - length quantile: [mm] 26.7 13.9 4.4 0.72 BG 2/1 BG1 / 0 Size 0 / F Overlap 5% by weight / 95% by weight [mm] 2.6 0.5 0.07

Table 1f shows the contaminants of the classified fragments with surface metals, carbon, dopants and particulate matter. <b> Table 1f </ b> Metals, carbon, dopants, particulate matter Fraction size 2 Break size 1 Fraction size 0 Fracture size F Fe [pptw] 80 170 1200 12800 Cr [pptw] 10 60 270 7300 Ni [pptw] <10 10 110 5400 Na [pptw] 20 40 430 6300 Zn [pptw] <10 40 210 5000 Al [pptw] 30 80 40 6200 Cu [pptw] <10 <10 30 <5000 Mg [pptw] <10 20 70 5600 Ti [pptw] <10 20 170 <5000 W [pptw] 1500 6340 57600 969000 K [pptw] 20 10 160 <5000 Ag [pptw] <10 <10 <10 <5000 Ca [pptw] 60 110 350 <5000 Co [pptw] 270 730 9300 135000 V [pptw] <10 10 130 <5000 Pb [pptw] <10 <10 90 <5000 Zr [pptw] <10 <10 860 <5000 Mo, As, Be, Bi, Cd, In, Li, Mn, Sn [pptw] <10 <10 <10 <5000 C [ppbw] 72 278 896 5857 B [pptw] 6 15 41 106 P [pptw] 35 131 208 574 As [pptw] 3 7 15 51 Fine dust (<10 μm) [ppmw] 1.9 3.8 8.4 17.2

Comparative Example 2

Table 2a shows the essential parameters of the screening machine used for this purpose. <b> Table 2a </ b> Wire width b [mm] 600 Length l [mg] 1600 Frequency n [Hz] 20 Speed [rpm] 1200 Angular velocity ω [1 / s] 125.7 Amplitude [mm] 2.4 Amplitude r [mm] 1.2 Screen inclination β [°] 0 Throwing angle α [°] 45 Screening number Kv [-] 1.4 Throughput [kg / h] 700 N ₂ sealing gas [Nm ³ / h] NN

Table 2b shows which sieve set was used in Comparative Example 2 . Three sieve decks with different sieve mesh sizes were used. <b> Table 2b </ b> Mesh size [mm] material Deck 1 9 polyurethane Deck 2 1.9 polyamide Deck 3 0.3 polyamide

Table 2c shows the composition of the screen coverings used. <b> Table 2c </ b> element polyurethane: Polyamide: Al [ppmw] 43 2.3 Ca [ppmw] 35 44 Cr [ppmw] <0.2 2.0 Fe [ppmw] 4.5 4.7 K [ppmw] 5.1 0.6 Mg [ppmw] 2.6 0.8 Na [ppmw] 3.8 6.1 P [ppmw] 114 28 Sn [ppmw] 18 1.1 Ti [ppmw] 1220 0.7 Zn [ppmw] 19 1.5 Ni [ppmw] 1.2 0.8 Cu [ppmw] 0.8 0.6 B [ppmw] 4.4 1.9 As, B, Ba, Cd, Co, Li, Mn, Mo, Sr, V [ppmw] <0.2 <0.2 Be, Bi, Pb, Sb, W [ppmw] <0.2 <0.2

The sieving results obtained with regard to the particle size distribution are shown in Tables 2d and 2e . <b> Table 2d </ b> Fraction size 2 Break size 1 Fraction size 0 Fracture size F 5% by weight - length quantile [mm] 10 3 0.5 0.11 95% by weight - length quantum [mm] 40 15 5 0.81 BG 2/1 BG1 / 0 Size 0 / F Overlap 5% by weight / 95% by weight [mm] 5 2 0.31

The overlap is significantly higher than in example 1. This is due to the changed parameters of the screening machine, in particular to the lower sieve index.

Table 2f shows the contaminants of the classified fragments with surface metals, carbon, dopants and particulate matter. <b> Table 2f </ b> surface contamination Fraction size 2 Break size 1 Fraction size 0 Fracture size F Fe [pptw] 200 340 1640 19800 Cr [pptw] 30 50 310 11000 Ni [pptw] <10 40 180 6800 Na [pptw] 40 50 480 7900 Zn [pptw] 20 30 360 6100 Al [pptw] 70 120 160 8400 Cu [pptw] <10 20 60 <5000 Mg [pptw] <10 30 80 9700 Ti [pptw] <10 40 160 <5000 W [pptw] 1640 5830 60700 1067000 K [pptw] 10 30 140 <5000 Ag [pptw] <10 <10 <10 <5000 Ca [pptw] 50 130 380 <5000 Co [pptw] 300 790 11300 12800 V [pptw] <10 <10 100 <5000 Pb [pptw] <10 20 80 <5000 Zr [pptw] <10 <10 670 <5000 Mo, As, Be, Bi, Cd, In, Li, Mn, Sn [pptw] <10 <10 <10 <5000 C [ppbw] 103 387 1431 7299 B [pptw] 6 16 48 133 P [pptw] 32 164 216 614 As [pptw] 2 8th 22 60 Fine dust [ppmw] 4.8 11.5 19.3 44.2

The impurities are consistently higher than in Example 1. This shows the influence of the composition of the Siebbeläge on the surface contamination of the fragments after classification.

Example 3

Table 3a shows the essential parameters of the screening machine. <b> Table 3a </ b> Wire width b [mm] 500 Sieve length l [mm] 1100 Frequency n [Hz] 24.3 Speed [rpm] 1460 Angular velocity ω [1 / s] 152.9 Amplitude [mm] 2.4 Amplitude r [mm] 1.2 Screen inclination β [°] 3 Throwing angle α [°] 40 Screening number Kv [-] 1.95 Si throughput [kg / h] 1000 N ₂ sealing gas [Nm ³ / h] 55

Table 3b shows which sieve set was used in Example 3 . Three sieve decks with different sieve mesh sizes were used. <b> Table 3b </ b> Mesh size [mm] material Deck 1 9 polyurethane Deck 2 4.0 polyamide Deck 3 0.75 polyamide

Table 3c shows the composition of the screen coverings. <b> Table 3c </ b> Element: polyurethane: Polyamide: Al [ppmw] 17.1 <0.2 Ca [ppmw] 11.3 18.6 Cr [ppmw] <0.2 <0.2 Fe [ppmw] 0.6 0.3 K [ppmw] 0.9 NN Mg [ppmw] 0.3 0.2 Na [ppmw] 0.4 0.9 P [ppmw] 53.2 <20 Sn [ppmw] 5.8 NN Ti [ppmw] 560 <0.2 Zn [ppmw] 7.5 <0.2 B, Ba, Cd, Co, Cu, Li, Mn, Mo, Ni, Sr, V [ppmw] <0.2 <0.2 As, Be, Bi, Pb, Sb, W [ppmw] <0.2 NN

The sieving results obtained with regard to the particle size distribution are shown in Tables 3d and 3e . <b> Table 3d </ b> Sieve undersize (<0.75 mm) Screen grain (0.75 - 4 mm) Sieber grain (4 - 9 mm) Waste (> 9 mm) 5% by weight - quantile [mm] 0.35 0.81 3.61 NN 95% by weight - quantile [mm] 0.79 2.86 7.68 NN Screen grain / Siebunterkorn Siebenberg grain / sieve grain Selectivity [-] 0.862 0.876

Table 3f shows the contaminants of the sized granules with surface metals, carbon, dopants and particulate matter. <b> Table 3f </ b> Surface metals: Sieve undersize (<0.75 mm) Screen grain (0.75 - 4 mm) Sieber grain (4 - 9 mm) Fe [pptw] 1700 860 380 Cr [pptw] 150 100 80 Ni [pptw] 120 80 40 Na [pptw] 390 230 150 Zn [pptw] 2620 2120 1530 Al [pptw] 260 150 140 Cu [pptw] 40 25 15 Mg [pptw] 120 70 60 Ti [pptw] 210 90 90 W [pptw] 60 50 <10 K [pptw] 70 45 40 Ca [pptw] 580 360 320 Mo, As, Sn, Ag, Co, V, Pb, Zr [pptw] <10 <10 <10 C [ppbw] 564 252 204 B [ppta] 27 25 23 P [ppta] 123 120 114 As [ppta] 8th 6 6 Fine dust [ppmw] NN 3.6 NN

Comparative Example 4

Table 4a shows the essential parameters of the screening machine. <b> Table 4a </ b> Wire width b [mm] 500 Sieve length l [mm] 1100 Frequency: n [Hz] 20 Speed [rpm] 1200 Angular velocity w [1 / s] 125.7 Amplitude [mm] 2.6 Amplitude r [mm] 1.3 Screen inclination β [°] 3 Throwing angle α [°] 40 Screening number Kv [-] 1.4 Si throughput [kg / h] 1000 N ₂ sealing gas [Nm ³ / h] 45

Table 4b shows which sieve set was used in Comparative Example 4 . Three sieve decks with different sieve mesh sizes were used. <b> Table 4b </ b> Mesh size [mm] material Deck 1 9 polyurethane Deck 2 4.0 polyamide Deck 3 0.75 polyamide

Table 4c shows the composition of the screen coverings used. <b> Table 4c </ b> element polyurethane: Polyamide: Al [ppmw] 57.2 1.3 Ca [ppmw] 45.2 32.5 Cr [ppmw] 1.5 1.3 Fe [ppmw] 14.0 3.1 K [ppmw] 6.5 0.4 Mg [ppmw] 3.6 1.4 Na [ppmw] 9.5 11.1 P [ppmw] 180 25.1 Sn [ppmw] 12.5 0.6 Ti [ppmw] 1400 0.3 Zn [ppmw] 25.3 5.8 Ni [ppmw] 0.7 0.6 Cu [ppmw] 0.5 0.3 B [ppmw] 5.3 0.4 Ba, Cd, Co, Li, Mn, Mo, Sr, V, s, Be, Bi, Pb, Sb, W [ppmw] <0.2 <0.2

The sieving results achieved with regard to the particle size distribution are shown in Tables 4d and 4e . <b> Table 4d </ b> Sieve undersize (<0.75 mm) Screen grain (0.75 - 4 mm) Sieber grain (4 - 9 mm) Waste (> 9 mm) 5% by weight - quantile: [mm] 0.38 0.74 3.56 NN 95% by weight - quantile: [mm] 0.78 2.63 7, 30 NN Screen grain / Siebunterkorn Siebenberg grain / sieve grain Selectivity [-] 0.803 0,874

The selectivity of Siebzielkorn / Siebunterkorn is worse than in Example 3. This is due to the comparison with Example 3 lower Siebkennziffer.

Table 4f shows the contaminants of the sized granules with surface metals, carbon, dopants and particulate matter. <b> Table 4f </ b> Surface metals: Sieve undersize (<0.75 mm) Screen grain (0.75 - 4 mm) Sieber grain (4 - 9 mm) Fe [pptw] 3500 1490 720 Cr [pptw] 270 210 140 Ni [pptw] 300 150 80 Na [pptw] 750 530 520 Zn [pptw] 3270 2610 2230 Al [pptw] 360 220 170 Cu [pptw] 70 60 30 Mg [pptw] 610 320 130 Ti [pptw] 340 120 130 W [pptw] 50 50 <10 K [pptw] 210 170 110 Ca [pptw] 2520 810 720 sn 40 30 <10 Mo, As, Ag, Co, V, Pb, Zr [pptw] <10 <10 <10 C [ppbw] 728 311 292 P [ppta] 202 148 133 As [ppta] 15 11 8th Fine dust [ppmw] NN 8.3 NN

The impurities are consistently higher than in example 3.

The following measuring methods were used to determine the specified parameters.

The determination of carbon contamination is carried out by means of an automatic analyzer. This is described in detail in the not yet published US application with the application number 13 / 772,756 and in the German application with the file number 102012202640.1.

The determination of the dopant concentrations (boron, phosphorus, As) is carried out according to ASTM F1389-00 on monocrystalline samples.

The determination of the metal impurities is carried out according to ASTM 1724-01 with ICP-MS.

The fine dust measurement takes place as in DE 10 2010 039 754 A1 described.

The particle sizes (minimal tendon) are determined by means of dynamic image analysis according to ISO 13322-2 (measuring range: 30 μm - 30 mm, type of analysis: dry measurement of powders and granules).

Claims (8)

1. Method for mechanically classifying polycrystalline silicon chunks or granules with a vibratory screening machine, by setting silicon chunks or granules present on one or more screens each comprising a screen lining in vibration such that the silicon chunks or silicon granules perform a movement which causes the silicon chunks or silicon granules to be separated into various size classes, characterized in that a gravity screening machine is used and the screening index (Kv) is greater than or equal to 1.6 and less than or equal to 3.0, wherein the screening index (Kv) is defined as the ratio of the acceleration generated by the screening motion to the acceleration due to gravity vertical to the screening plane, and by the following formula: $${\mathrm{K}}_{\mathrm{v}}=\mathrm{r}*{\mathrm{\omega }}^{\mathrm{2}}*\sin \left(\mathrm{\alpha }+\mathrm{\beta }\right)/\left(\mathrm{g}*\cos \left(\mathrm{\beta }\right)\right);$$

   

   where

   r: amplitude of vibration;

   ω: angular velocity;

   α: throwing angle;

   β angle of screen inclination;

   g: gravitational constant.
2. Method according to Claim 1, wherein the motion of chunk silicon or granular silicon is characterized by an amplitude of vibration of 0.5 to 8 mm, a speed of rotation of 400 to 2000 rpm and a throwing angle of 30 to 60° relative to a screen plane, with the screen plane inclined by an angle of 0 to 15° relative to the horizontal.
3. Method according to either of Claims 1 and 2, wherein the screening machine comprises a plurality of screen decks arranged one on top of another.
4. Method according to any of Claims 1 to 3, wherein the screen linings are each secured on a frame of plastic or a frame comprising a plastic lining.
5. Method according to any of Claims 1 to 4, wherein one or more of the screen linings consist of an elastomer having a Shore A hardness of greater than 65 or have a surface composed of an elastomer having a Shore A hardness of greater than 65.
6. Method according to any of Claims 1 to 5, wherein one or more of the screen linings or the surfaces of one or more of the screen linings and all the further components and linings thereof that come into contact with the chunk silicon or granular silicon consist of plastics having a total contamination of less than 2000 ppmw.
7. Method according to any of Claims 1 to 4, wherein perforated silicon fillets are used in one or more of the screen linings, the holes at least in part having a conical shape.
8. Method according to any of Claims 1 to 7, wherein both perforated silicon fillets and plastic are used as screen linings, which uses a screen with a perforated Si fillet at least in a first screening step.