DE102014007354B4 - Process for processing residues from the mechanical treatment of silicon products - Google Patents

Abstract

A method for treating residues from the mechanical processing of silicon products for a subsequent silane synthesis, the residues comprising at least silicon, carbon-containing particles and metallic constituents, comprising the steps of: contacting the residues with an etching liquid which dissolves metallic constituents but not silicon or at least partially separating the etching liquid with metals dissolved therein from undissolved particles of at least silicon and carbon-containing particles; at least partially separating particles containing silicon and carbon using a heavy liquid with ZnBrin water and / or alcohol; at least partially removing of oxides from the surface of the silicon particles; melting the silicon particles; introducing the molten silicon into a process chamber, wherein the molten silicon is injected upon introduction i n the process chamber is atomized by a gas flow, and then falls to the bottom of the process chamber or a collecting container attached thereto, wherein the process conditions in the process chamber, in particular the gas flow for atomizing the silicon are adjusted so that the atomized silicon during the free fall at least partially solidified.

Description

The present invention relates to a method for processing residues from the mechanical processing of silicon products, in particular for a subsequent silane synthesis, wherein the residues comprise at least silicon, carbon-containing particles, such as silicon carbide and / or diamond, and metallic constituents. The term silane synthesis as used herein is intended to encompass a wide variety of syntheses, in particular to methyl silane, monosilane, chlorosilane, methylchlorosilane.

For the production of semiconductor devices and solar cells, silicon is currently used as a base material in large volumes (over 200,000 t / year). The silicon is first obtained from quartz and further processed in a carbothermic melting process to form so-called metallurgical silicon (mg-Si). The silicon blocks obtained from the melt are ground to small particles and serve as starting material for the next step, the production of polysilicon. This preparation may, for example, have a silane synthesis followed by decomposition into high purity silicon, such as the Siemens process. In this case, a reaction of the silicon with hydrogen chloride to form trichlorosilane and other products is effected for the production of polysilicon from metallurgical silicon. The trichlorosilane is decomposed into silicon and other products after distillation and separation from the other products on heated ultra-pure silicon rods, with the silicon precipitating on the high-purity silicon rods. Although trichlorosilane synthesis is mentioned here, the production of polysilicon is also known in other ways, which use a different form of silane synthesis.

While mg-Si contains metallic and other impurities in the range of about 1% to 2%, purity levels of 6N to 10N can be achieved in the production of polysilicon. These high levels of purity are the basis for the use of the basic silicon as semiconductor for applications in the semiconductor industry and in photovoltaics. On the basis of this purity, the electrical properties of the semiconductor material silicon can be changed very controlled by the targeted addition of so-called dopants in extremely small amounts, which then enables the production of electronic components such as semiconductor chips and solar cells.

In an industrial production of such components, the actual semiconductor processing steps, however, are preceded by the following process steps: The high-purity polysilicon is first melted again and further processed to form so-called ingots. In the so-called multi-process polysilicon is melted in a crucible and crystallized as a block-shaped block (multi-ingot) by cooling. This process is used in photovoltaics. In the so-called mono-process single crystals are pulled from a melt. This process provides a higher quality material and is used in both the semiconductor and photovoltaic industries. In both cases, the removal of edge pieces then produces so-called bricks, from which silicon wafers with a thickness of 180-210 μm (in photovoltaics) and 650-750 μm (in the semiconductor industry) are produced. The invention relates to the silicon material losses incurred in making these silicon wafers, for example by sawing the bricks. Reuse is economically very interesting, since annually very large quantities (over 70,000 t / year) are lost to high-purity material in this process step. Previously described methods are usually not practicable or economically viable in an industrial production scale and are therefore not used in practice.

An important key process in the production of silicon wafers for the semiconductor industry and photovoltaics is wire sawing, which uses the aforementioned bricks as the starting material. In the case of the wire sawing process, the majority of these are sawn with metal wires and a silicon carbide suspension (slurry), typically based on silicon carbide (eg SiC F600) and polyethylene glycol (PEG200). Here, in particular when producing silicon wafers in photovoltaics, a very high percentage (up to 45%) of the high-purity silicon material is mechanically eroded and is subsequently found again in the used sawing suspension. For the recovery of silicon carbide of original grain size and the liquid (PEG), different technical processes are already in use today, which mainly relate to a spreading of the slurry.

It remains an at least partially dewatered mixture of silicon and silicon carbide with other impurities, especially metallic impurities, back, which is usually no longer used in the semiconductor industry or photovoltaic use. Such a mixture is hereinafter referred to as "residue from the mechanical processing of silicon products" and represents a concrete possible starting material for the method described below. In addition to the residues from sawing still arise other silicon-containing residues from other grinding and cutting processes within the semiconductor industry, as well as in the crystalline Photovoltaics. Depending on their origin, these residues are contaminated to different degrees with varying degrees of contamination. In order to be able to reuse them, these impurities must be reduced to a degree that depends on the re-use. These grinding and cutting residues can also be referred to as a residue from the mechanical processing of silicon products, which can serve as starting material for the process described. The removal of a liquid phase from used cutting or cooling liquids also produces comparable mixtures in diamond wire sawing. The residues discussed in this section are collectively referred to herein as "mechanical silicon machining residues" and have in common that they contain silicon carbonaceous particles, such as silicon carbide and / or diamond, and metallic constituents.

Such residue, for example, when derived from slurry sawing, typically contains 25-35% residual moisture, 20-30% SiC, 34-50% silicon, 8-12% iron, 1-3% carbon (predominantly PEG). , 10-15% SiO ₂ . In addition, copper and zinc, as well as diamond, which was used as abrasive and nickel, which often serves to bond the diamonds may be included. The particle size is usually 0-5μm, but in particular larger SiC particles up to 15μm occur.

The patent literature describes some approaches to the reuse of silicon, mostly from used sawing suspension. In the US 6,780,665 A For example, a method is described which holds isolated silicon particles from used sawing suspension together with a binder. The binder should then be removed again in a further process step and the silicon be made available again as a base material for the production of solar cells after a sintering process. Since very high purities are required for the production of semiconductor components such as solar cells, but these can be achieved by such a process, if at all, only with great difficulty or only uneconomically, such processes have hitherto received no appreciable spread on the market.

1. 1) J Air Waste Manag Assoc. 2013 May; 63(5):521-7. Tsai TH, Shih YP, Wu YF , Recycling silicon wire-saw slurries, die insbesondere eine effektive Trennung von Si und SiC als schwierig erachtet;
2. 2) Yen-Chih Lin, Teng-Yu Wang, Chung-Wen Lan, Clifford Y. Tai, Recovery of silicon powder from kerf loss slurry by centrifugation, Powder Technology 200 (2010) 216-223 : die beim Zentrifugieren eine Reinheit von Si von 90.8 Gewicht% bei einer 74.1% Ausbeute für realistisch hält;
3. 3) US 2010 02 84 885 A1 (Wang): Wang beschreibt ein Trennverfahren für Si und SiC unter Einsatz eines Zentrifugierverfahrens mit einer Schwerflüssigkeit, bei dem die höchste erzielbare Reinheit des Siliziums bei 91 Gewicht% bei einer Ausbeute von 73% liegt. „However, the effect of centrifugation for separating silicon is limited. No matter how the operating variables are adjusted, such as specific gravity of liquid, centrifugation time and solid concentration, the purity of silicon cannot be efficiently improved. Thus, the highest purity of silicon by centrifugation is merely 91 weight percentage (wt %) with a recovery of 73%.“
4. 4) Final Report on EC Project, RE-Si- CLE, „Recycling of Silicon Waste from PV Production Cycle“, Project funded by the European Community under the „Competitive and Sustainable Growth Programme“, 2002 to 2004, Project No. NNE5-2001-00175: In dem Bericht wurde darauf hingewiesen, dass bei der Aufbereitung von Siliziumrückständen zwar Fortschritte bei der Reduzierung von Eisen unter <0,1% gemacht wurden, aber eine Abtrennung von insbesondere kleinen SiC Partikeln mit einer Größe von <1um noch nicht gelöst ist.

Another method, which relates to a processing of a used Sägesuspension is in the US 8,231,006 B2 described. Again, in the process, high-purity silicon is to be isolated directly from the sawing suspension in order to use it as polysilicon in the ingoting process (or a similar process). This results, inter alia, from the terms used "solar grade material" and the allegedly achievable degrees of purity of less than "50 ppma carbon content and less than 150 ppma metal content". However, with the methods described in the patent, it would be a great challenge, if at all without additional measures not mentioned, to achieve the indicated purities. This is indicated, for example, by the following publications:

1. 1) J Air Waste Manag Assoc. 2013 May; 63 (5): 521-7. Tsai TH, Shih YP, Wu YF , Recycling silicon wire-saw slurries, which in particular considers an effective separation of Si and SiC difficult;
2. 2) Yen-Chih Lin, Teng-Yu Wang, Chung-Wen Lan, Clifford Y. Tai, Recovery of silicon powder from slurry slurry, Powder Technology 200 (2010) 216-223 which, on centrifugation, considers a purity of Si of 90.8% by weight to be realistic in a 74.1% yield;
3. 3) US 2010 02 84 885 A1 (Wang): Wang describes a separation process for Si and SiC using a centrifugal process with a heavy liquid in which the highest achievable purity of the silicon is 91% by weight with a yield of 73%. However, the effect of centrifugation for separating silicon is limited. Liquidity, centrifugation time and solid concentration, the purity of silicon can not be improved. Thus, the highest purity of silicon by centrifugation is 91% (wt%) with a recovery of 73%. "
4. 4) Final Report on EC Project, RE-SICLE, "Recycling of Silicon Waste from PV Production Cycle", Project funded by the European Community under the "Competitive and Sustainable Growth Program", 2002 to 2004, Project no. NNE5-2001-00175: The report noted that, while the processing of silicon residues made progress in reducing iron below <0.1%, separation of small SiC particles as small as <1 μm still occurred is not solved.

A reprocessing of saw residue for a direct silicon recycling for an ingoting process could therefore not prevail so far. So far, such as in the patent US 8,231,006 B2 described method therefore only partially used in the field of recycling the sawing suspension (ie for the recovery of the SIC particles and the liquid), but not in the field of silicon recycling.

The patent applications US 2010 00 32 630 A1 or. US 2010 00 61 913 A1 , which also deals with a treatment of Saw residues, assume that it is practically impossible to achieve the purity required for ingoting. It is therefore proposed to carry out a further purification step by conversion into the gas phase and further process steps. However, these publications also refer to the technical challenges involved in dealing with the resulting extremely fine-grained silicon dust, without offering a solution for this.

Another problem that arises in the processing of saw residue, but is not taken up by all publications to this is the oxide formation in silicon dusts, which can prevent meaningful further use. The most common method of oxide removal is to etch silicon dusts with hydrofluoric acid (HF). This is for example also in the US 8,231,006 B2 explicitly named as an important step in the overall process for silicon recycling from sawdust. However, the use of hydrofluoric acid is known to be associated with environmentally harmful waste products and thus to avoid as possible.

Further, on the DE 10 2011 100 844 A1 which comprises a method for removing contaminants from metallurgical silicon, comprising the steps of: melting the metallurgical silicon, introducing the molten silicon into a process chamber, wherein the molten silicon is atomized when introduced via a gas flow, and then falls to the bottom of the process chamber, Generating a reactive atmosphere in the process chamber, wherein the reactive atmosphere includes a process gas that reacts with impurities in the molten silicon to remove them from the molten silicon.

Furthermore, the shows DE 33 43 406 A1 a special multi-step process for purifying silicon, wherein one of the steps combines shredded silicon with a heavy separation liquid that has a density relative to pure silicon sufficient to effect density-differential separation of the silicon from impurities has sufficiently low viscosity to allow rapid separation of the crushed particles under force.

The present invention is therefore based on the object of providing a method and a device which overcomes at least one of the above-mentioned problems. In particular, an efficient recycling of silicon from residues of mechanical silicon processing should be provided. Alternatively or additionally, even environmentally harmful waste products should be reduced or prevented in an oxide removal of silicon surfaces.

According to the invention the above object is achieved by a method for processing residues from the mechanical processing of silicon products according to claim 1. Other embodiments of the invention will become apparent inter alia from the dependent claims.

In a method for processing residues from the mechanical working of silicon products for a subsequent silane synthesis, the residues contain at least silicon, carbon-containing particles and metallic constituents and the following steps are provided: contacting the residues with an etching liquid which dissolves metallic constituents but not silicon or the carbon-containing particles, at least partially separating the etchant with dissolved metals from undissolved particles of at least silicon and carbon-containing particles, at least partially separating silicon particles and carbon-containing particles, at least partially removing oxides from the surface of the silicon Particles, melting the silicon particles, introducing the molten silicon into a process chamber, wherein the molten silicon when introducing into the process chamber via a gas flow ze is then dusted, and then falls to the bottom of the process chamber or a collecting container attached thereto, wherein the process conditions in the process chamber, in particular the gas flow for atomizing the silicon are adjusted so that the atomized silicon at least partially solidified during the free fall. For the at least partial separation of silicon particles and carbon-containing particles, a heavy liquid with ZnBr ₂ in water and / or alcohol is used. This allows, for example, a sink-float separation with good separation of the particles when using a heavy liquid that is less harmful to the environment than the commonly used.

All in all, such a method provides an industrially usable method which separates silicon from the residues from mechanical silicon processing and brings it into a suitable form for silane synthesis. In particular, it is possible to ensure the necessary purity of the silicon without excessive use, which makes it possible to return the raw material silicon back into the value-added chain of production, at the same time being formed in a suitable manner for silane synthesis manageable silicon spheres.

In a preferred embodiment, the removal of oxides from the Surface of the silicon particles, the silicon particles in the vacuum to a temperature below the melting point of the silicon particles to at least 800 ° C, in particular heated to a temperature in the range of 1000 to 1100 ° C, wherein the negative pressure and the temperature are adjusted so that silicon oxides of the silicon particles sublimate and can be derived. This allows a chemical-free oxide removal. The discharge can be carried out automatically via a connected vacuum pump, which is preferably set to a pressure less than the vapor pressure of silicon oxide at the temperature. Since the particles thus cleaned can be fed to the same melting, the process is also energetically favorable. Preferably, the transfer from the oxide removal to the melt should be carried out so that a maximum cooling of 100 ° C occurs between the oxide removal and the feeding to the melting process.

Preference is given to a multi-stage heating with decreasing pressure, whereby first a removal of the residual moisture and then an oxide removal are made possible. By separate suction at the different temperature ranges, a separation of the removed substances is possible.

For an essentially continuous operation and as direct as possible a feed into the melt, in one embodiment the particles are kept in motion during the heating and in particular are conveyed through an elongated process space.

A size-dependent separation of the particles can be achieved additionally or alternatively also by a corresponding filtering.

In order additionally to remove SiC particles from the process without the use of chemicals, it is provided in one embodiment that the molten silicon is brought into contact with an element which consists of SiC or has an SiC coating before being introduced into the process chamber at least partially incorporating the melt-containing SiC to the element. Preferably, the element and the melt of silicon are moved relative to each other during the contact.

For silane synthesis, the process conditions in the process chamber, in particular the gas flow for atomizing the melt, are adjusted so that essentially spherical silicon particles having an average particle size of 20 μm to 600 μm are formed. Such silicon spheres are particularly suitable for subsequent trichlorosilane synthesis.

The process described in the invention combines several process steps, some of which are known, in a new process sequence which allows recycling of silicon from residues of mechanical processing of silicon products. Here, silicon spheres are produced with a reproducible uniform size distribution, which can be used as a replacement for ground metallurgical silicon as a starting material for a silane synthesis in the context of polysilicon production described above or, for example, a Müller-Rochow synthesis. The residual content of metal, silicon carbide, silicon oxide and other impurities permitted for these applications is high enough so that the described process sequence can be implemented on an industrial scale without any special measures with comparatively simple process steps.

The process is applicable to various residues from mechanical silicon processing, especially for sawing residues from slurry and diamond sawing processes. It allows a meaningful silicon recycling and offers the advantage in polysilicon production compared to the use of ground metallurgical silicon as a starting material, that the process is reproducible, since the input materials silicon balls have a uniform and adjustable size distribution. Since the residues come from high-purity silicon products, impurities such as boron, phosphorus or aluminum are not to be expected. Further, the method allows for targeted incorporation of a catalyst, for example, in the area of reflow and before sputtering, as is known in the art. The method avoids difficulties in the handling of very small silicon particles, for example with regard to conveying technology, safety problems (dust explosion) and strong tendency to oxidation, which were in some cases unresolved in previous preparation processes.

• 1 ein Flussdiagramm eines erfindungsgemäßen Verfahrens zum Aufbereiten von Rückständen aus der mechanischen Bearbeitung von Siliziumprodukten anhand eines spezifischen Beispiels von Sägerückständen;
• 2 eine schematische Darstellung einer Vorrichtung zum Entfernen von Oxiden von der Oberfläche von Siliziumpartikeln;
• 3 eine schematische Darstellung einer alternativen Ausführungsform einer Vorrichtung zum Entfernen von Oxiden von der Oberfläche von Siliziumpartikeln;
• 4 eine schematische Darstellung eine Vorrichtung zum Umformen von Siliziumpartikeln, wie sie in dem Verfahren zum Aufbereiten von Rückständen aus der mechanischen Bearbeitung von Siliziumprodukten eingesetzt werden kann.

The invention will be explained in more detail below with reference to the drawings, in which:

• 1 a flow chart of a method according to the invention for the treatment of residues from the mechanical processing of silicon products based on a specific example of sawing residues;
• 2 a schematic representation of an apparatus for removing oxides from the surface of silicon particles;
• 3 a schematic representation of an alternative embodiment of a device for removing oxides from the surface of silicon particles;
• 4 a schematic representation of an apparatus for forming silicon particles, as it can be used in the method for processing residues from the mechanical processing of silicon products.

The relative terms used in the following description, such as left, right, over and under, refer to the drawings and are not intended to limit the application in any way, even though they may refer to preferred arrangements.

1 shows a flow diagram of a method according to the invention for the treatment of residues from the mechanical processing of silicon products using a specific example of sawing residues. The process is primarily for the recovery of silicon for subsequent silane synthesis. The method shown in the flow chart is very specific and not all of the blocks and the associated method steps are required for carrying out the method according to the invention. Also, some of the illustrated blocks and associated method steps may be replaced by one or more blocks with other method steps. This is especially true for side strands of the flowchart.

The main strand of the flowchart can be classified into the areas of removal of metals / liquid components, separation of solids, removal of liquid residues / oxides and the transformation of silicon particles, which will be explained in more detail below.

In the block 1 of the flow chart are exemplified by a residue of mechanical working of a silicon product, such as slurry sawing of bricks using a silicon carbide suspension based on polyethylene glycol (especially PEG 200 ). Such a residue can already be known for recycling the PEG 200 and unused SiC particles pretreated. Unused SiC particles are those that do not fall below a certain size. However, a corresponding pretreatment does not form a necessary part of the subsequent process flow.

The residue can be in solid or pasty form and usually contains Si / SiO ₂ particles and SiC particles as main constituents and Fe as minor constituents, as well as PEG (not separated) 200 and water, as well as other not previously removed liquids. As minority constituents, diamond, Zn, Ni, Cu and other constituents, in particular further metallic constituents, may be present. The in the block 1 Components shown can be present in various concentrations. Usually, for example, when derived from slurry sawing, the residue contains 25-35% residual moisture, 20-30% SiC, 34-50% silicon (Si), 8-12% iron, 1-3% free carbon (predominantly as PEG), and 10-15% SiO ₂ . In addition, copper and zinc, as well as diamond that was used as an abrasive and nickel, which often serves to bond the diamonds, may be included. The particle size is usually 0-5μm, but in particular larger SiC particles up to 15μm occur. Such a residue containing at least silicon, a carbon-containing material such as SiC or diamond, and metallic constituents serves as an input to the process described below.

Removal of metals / liquid components

The residue is first fed to an etch in a suitable acid, such as through the block 2 is displayed. The acid can be any that does not impart impurities to subsequent processes that sufficiently dissolve metallic constituents but silicon does not. A known acid suitable for this purpose contains, for example, hydrogen chloride dissolved in water, also known as hydrochloric acid. The material is suspended as completely as possible in dilute acid. This can be achieved for example by vigorous stirring or a comparable mechanical agitation. Also, an ultra or megasonic application or other methods may assist the process.

The metal ions, especially iron, go into solution, which can be additionally supported by heating and adding complexing agents. If a large part or even all of the metals are in solution, the remaining solid is separated from the acid via a filter press or better a centrifuge or a decanter, which is also known as a decanter centrifuge. By washing in a pusher centrifuge or peeler centrifuge and / or re-suspending it several times in fresh acid and subsequently stripping it off, final residues of metal ions can be removed, if desired. It is possible without special additional measures to reach the component of metals to <0.3% by weight of the remaining solids. In particular, the metal components can be reduced to the range of 0.1 to 0.3% by weight. While such residual metal constituents could be considered as interfering with processes aimed at direct delivery of the recovered solids to an ingot process, this is not the case with the present process. Rather, could Metal contamination in the range of 1 to 5%, in particular in the range of 1 to 2% accepted as acceptable. This is because the recovered silicon is not to be added to an ingot process but to a silane synthesis in which metal components on this scale are not detrimental, but rather can serve as a catalyst, as known in the art. Accordingly, the process can be adjusted so that the acid etch does not have to be complete, for example, reducing the use of chemicals and / or reducing process time. But it is also possible to strive for a more or less complete removal of the metal components, which could simplify the process control. Surfactants can improve the wettability and thus the cleaning effect.

The at least partially separated acid may be conditioned in a known manner and the incorporated and dissolved constituents may be separated and optionally recovered as solid products, such as by the blocks 3 and 4 is displayed. In particular, the high iron content requires removal of the same from the etching solution. If this is precipitated, preferably as hydroxide, it can be fed back as a raw material into the industrial cycle and possibly make a contribution. Cu and nickel are strong environmental toxins and must also be brought to the legal levels before the solution can be disposed of. These are usually mitgefällt in the iron precipitation. Remaining PEG and other components which are withdrawn with the acid may optionally be separated. The blocks 3 and 4 represent a first sidestring of the flowchart which is not essential to the gist of the invention, but which is advantageous for the overall method illustrated.

After the acid etching, the withdrawn solids, which now essentially contain Si / SiO ₂ particles and SiC particles and / or diamond particles with correspondingly remaining liquid residues, are fed to a rinse and partial dehydration, which in the block 5 is indicated. In particular, a rinse with deionized water (DI water) is considered. By repeated picking in DI water and centrifuging or pressing off, rinsing in countercurrent or rinsing in a reciprocating or peeler centrifuge, residues of the metal contamination, acid residues and possibly added further auxiliaries can be removed and the solid can be partially dehydrated. Complete drainage is not required and could even complicate subsequent resuspension for solids separation. On the other hand, too low a drainage could dilute an optionally used separating solution too much and limit its effectiveness. The degree of drainage can be adjusted according to a subsequently selected separation process. The rinse water can optionally be conditioned in the block 3 or other unrepresented treatment.

Separation of solids

The partially dewatered solid is resuspended in the illustrated flow diagram in a separation solution, such as through the block 6 is displayed. This is done by vigorous stirring, walking / kneading or other mechanical agitation with increasing amount of solution or similar processes. The process can be supported by coupling ultra or megasonic, heating or other suitable processes. The separation solution preferably has a density which is between that of silicon (2.33 g / cm ³ ) and that of silicon carbide (3.21 g / cm ³ ). Classically bromine or iodine carbon compounds (bromoform, di-bromoethane, dilodomethane) or solutions of mercury salts have been used which are carcinogenic or otherwise of great concern. Even closed circuits do not really justify their use. Sodium metatungstate is a much less questionable solution that has been proposed in the art. The disadvantage of this solution, however, lies in the price that currently makes a use economically unattractive despite repeated recycling.

It is therefore preferred to use the use of zinc halides (eg ZnBr ₂ ) in water. With them, a sufficient density can be obtained, at the same time moderate risk potential and acceptable costs. Additives such as surfactants can improve the distribution of the solid in the solution, and chelants can stabilize the solution.

The solution should first be constantly circulated to avoid unwanted sedimentation at this time.

The suspension is then in the block 7 subjected to a swim - sink separation. This is done for example with the help of a Tricanters, which is able to separate the high-density material (SiC) and the low-density material (Si / SiO ₂ ) and already largely dehydrate. The separation into the different substances is through the blocks 8th and 9 shown. The separation solution is also separated and sent for recycling, in particular fed to the resuspension.

The separated solids may each be separately subjected to a rinse, such as through the blocks 10 and 11 is shown. The rinse may be substantially of the type described above. In particular, by rinsing with deionized water, either by repeated inclusion in DI water and centrifuging or pressing, rinsing in countercurrent or rinsing in a reciprocating or peeler centrifuge, residues of the separation solution can be removed from both solids fractions and the respective solid can be dewatered as far as possible.

The respective rinsing liquids containing dilute separation solution can be processed and also recycled. In particular, this shows the block 12 an exemplary concentration of the separation solution. This can be done for example by reverse osmosis or distilling off the solvent. Finest turbidities can be removed by filtration.

That in the block 11 Rinsed and partially dewatered silicon carbide can be returned as valuable material to the industrial cycle, such as through the blocks 13 represents a valuable material that can make a significant contribution to the costs of the procedure. The further processing of SiC after the separation of Si / SiO ₂ , in turn, represents a side strand which is not essential to the invention, but which may be advantageous for the overall process.

Although the illustration according to 1 suggests that the separation of the solids in Si / SiO ₂ on the one hand and SiC on the other hand is perfect or substantially perfect, it should be noted that this will not usually be the case. A complete separation does not seem attainable, or at least not without a negligible overhead, as indicated by some of the references cited in the introduction. But this is not necessary for the further procedure here.

As an alternative solvent, ethanol or other suitable solutions could be used for the separation solution.

As will be explained in more detail below, the separation of the solids can also take place in other ways.

Removal of liquid residues / oxides

The primary Si / SiO ₂ containing, rinsed and partially dewatered solid (block 14 ) is then subjected to an oxide removal, which passes through the block 15 is shown. This preferably has a stepwise heating of the solid in vacuo. In a first stage, residual moisture is first evaporated and removed.

In a further, preferably spatially separated zone, the temperature is further increased and the pressure is lowered. In this case, the temperature is increased to at least 800 ° C but below the melting point of Si, in particular to a temperature below 1200 ° C. The pressure is correspondingly lowered to a value which is preferably below that of the vapor pressure of SiO at the set temperature. The lower temperature for the oxide removal is selected to provide sufficient vapor pressure while the upper temperature is selected to avoid sintering of the Si particles. Preferably, the temperature should be between 950 ° C and 1150 ° C, in particular between 1000 ° C and 1100 ° C. At 1000 ° C, well below the melting point of the Si of 1414 ° C, a vapor pressure of 10 ^-2 mbar can already be observed on SiO, which is replenished continuously by an equilibrium reaction of SiO ₂ until the latter is largely removed, which by continuous pumping to the desired pressure is achieved.

From the exhaust gas stream, a mixture of silicon and silicon dioxide can be condensed out on cooling. When oxygen is added to the exhaust stream, fumed silica can be obtained, which is a sought after industrial product.

This type of Oxidenfernung, which can also be used with advantage detached from the other process steps, can preferably be carried out in a continuous process. For this purpose, for example, a device can be used as they follow with reference to the 2 and 3 will be explained in more detail.

As an alternative to the oxide removal process described above, other suitable processes can be used, such as wet-chemical removal of SiO ₂ . As is known in the semiconductor industry, SiO ₂ can be removed by etching with HF. Subsequent flushing and dewatering, as described for removal of the separation solution, would also be required here. The use of hydrofluoric acid is associated with significant costs for procurement and disposal and should, however, be avoided as much as possible, which is why the above method is preferred. Environmental considerations support this view.

Forming silicon particles

The already heated, dried and oxygen-depleted silicon is finally melted by further supply of energy. This is preferably done in a so-called "cold crucible" (crucible), in which there is no contact of the molten silicon with the crucible material and thus contamination can be limited. In the melt, if any, still present SiC particles are present as a solid. These can optionally be made using the wetting properties of SiC in the Melt be removed or reduced in quantity compared to an introduced into the melt SIC element, as by the block 16 is shown. In particular, an arbitrarily shaped SiC element which can be produced with high purity can be immersed in the melt. Due to the wetting properties of SiC in the melt, SiC particles can adhere to the element and thus be effectively removed from the melt. In particular, the SiC element can have a grid or sieve shape and can also be moved in the melt, in particular rotated, for good contact. The element may optionally be replaced regularly, as will be appreciated by those skilled in the art.

For another forming (block 17 ), the liquid silicon is removed in a thin stream from the crucible, introduced into a process chamber and atomized there by a gas flow. The resulting droplets fall down in the process room and solidify at least partially before they reach the ground or a separate container. The resulting particles have a substantially spherical shape. By adjusting the process parameters, in particular the gas flow for atomizing the liquid silicon, the particle size can be adjusted. This can be optimized in particular with regard to further use. For a subsequent trichlorosilane synthesis, the particle size is preferably adjusted to an average particle size of 20 .mu.m to 600 .mu.m, in particular from 45 .mu.m to 500 .mu.m. For other syntheses, other particle sizes can be advantageous, which can be produced by appropriate adjustment of the process parameters. The gas used for the atomization is preferably an inert gas. The actual main branch of the process ends with this part of the transformation, but as shown, further steps can follow directly. The expected composition of the silicon spheres is <3% SiC, <2% SiO ₂ ,> 95% Si.

The resulting particles (block 18 ) are suitably until their further use, for example in a silane synthesis (block 19 ) for polysilicon production according to the Siemens process or other silane syntheses, such as the Müller-Rochow synthesis under inert gas, but at least dry to prevent moisture absorption. As a result, it is possible to at least reduce or even prevent a renewed formation of an oxide layer on the particle surface.

The method described above combines several known method steps in a specific new process sequence. The process steps can be implemented in plants which are in some cases conventionally obtainable, in combination with a preferred, novel apparatus for oxide removal and a specially adapted atomization plant, both of which are described below with reference to FIGS 2 to 4 be explained in more detail.

2 shows a schematic representation of an embodiment of a device 30 for removing oxides from the surface of silicon particles.

The device consists essentially of an elongated body 32 who has a transport room 34 for transporting silicon particles, a heating device 36 for introducing heat into the transport space and a vacuum unit, not shown, for generating a negative pressure in the transport space.

The elongated body 32 has a substantially tubular shape, which at the opposite ends 38 . 39 is closed and inside the transport room 34 forms. At the ends 38 . 39 in each case a passage opening is formed. The passage opening in the end 38 is formed here as an input opening, while the passage opening in the end 39 is formed as an exit opening, as shown by corresponding arrows. Consequently, a corresponding main transport direction through the transport space 34 provided through.

At the base body 32 are two stubs 42 . 44 formed, which serve as vacuum connections and a connection of corresponding areas of the transport space 34 allow with a respective vacuum source or suction unit, as will be explained in more detail below. The stubs 42 . 44 are inclined against the main transport direction, so that an extraction through the stubs 42 . 44 has a component opposite to the main transport direction. When displayed in 2 is the main body 32 shown in a horizontal orientation and the stubs 42 . 44 are provided above lying. Instead of the horizontal arrangement but also an inclined arrangement is possible up to a vertical arrangement, wherein the entrance opening should be arranged above the exit opening. The stubs 42 . 44 should each be located above lying, which is of course not possible in a vertical arrangement al such.

Preference is given to the stub lines 42 . 44 each connected to its own negative pressure source, which are suitable for different pressures and / or are set to different pressures. Preferably, the stub is closer to the entrance opening 42 connected to a vacuum source which generates a first negative pressure while is set while the stub further away from the input port 44 connected to a vacuum source which generates a second negative pressure which is lower than the first negative pressure. In other words, the pressure should drop towards the exit port.

In the transport room 34 is a screw conveyor 46 arranged over a extending through the input opening shaft 47 is rotatable, as shown by a corresponding rotary arrow. The screw conveyor 46 is suitably within the transport space 34 stored and carried out so that it transported with appropriate rotation of silicon particles from the input port to the exit port. Although not shown, the auger 46 have in the interior a heating device, in particular one which is suitable for providing starting temperatures rising from the inlet opening in the direction of the outlet opening.

In the illustration according to 2 is an outdoor heater 36 shown, consisting of two separate heating coils 50 . 52 exists, which can be controlled via a corresponding control unit, or separate control units. The first heating coil 50 surrounds the body 32 in the region of the inlet opening, to about half of its longitudinal extent. The second heating coil 52 surrounds the body 32 from about half of its length to the exit end. The heating coils can be designed for different temperature ranges. In particular, the first heating coil 50 be designed so that they have the transport space 34 in particular silicon particles received therein during their transport through the transport space 34 can heat to a temperature in the range of 200 to 600 ° C. The second heating coil 50 On the other hand, it can be designed to accommodate the transport space 34 in particular silicon particles received therein during their transport through the transport space 34 can heat to a temperature in the range of 600 ° C to 1200 ° C. Instead of separate heating coils 50 . 52 Of course, other heating configurations can be selected, which is an increasing heating of the transport space 34 subsidized silicon particles allows. This should be suitably isolated from the environment as will be appreciated by those skilled in the art. In particular, an internal heating device can also be provided which, for example, is located inside the screw conveyor 46 is arranged, as indicated above. Instead of a screw conveyor 46 can also be another conveyor unit for the transport of silicon particles through the transport space 34 be provided through. Optionally, can also be dispensed with a separate conveyor unit and transport by gravity through a corresponding oblique or vertical position of the body 32 ,

3 shows a corresponding device 30 which is basically the same as the device 30 according to 2 but which has no screw conveyor and in which a corresponding longitudinal axis of the base body is vertically aligned.

The inlet opening is preferably connected to a source of silicon particles, in particular silicon dusts, as obtained by the method described above from residues. The exit port is preferably in communication with a crucible, so that out of the device 30 Essentially leaking silicon particles can be introduced directly into a crucible.

All of the above-described elements of the device 30 are made of materials that have sufficient thermal stability for the temperatures occurring. Furthermore, the elements that come in contact with the silicon particles are selected so that they do not represent interfering impurities for subsequent processing of the silicon particles. In particular, elements made of silicon carbide or with a silicon carbide coating could be used.

The operation of the device will be described below 30 for oxide removal of silicon particles prepared from the method described above (from Block 14 ) are briefly explained. The particles usually have residual moisture and the oxide removal consists of two phases. In the first phase, predominantly water is removed as an oxygen carrier in addition to other volatile undesirable substances, and it is only in the second phase that oxide is primarily removed from the particles. For this purpose, the silicon particles via the input opening in the transport space 34 introduced and by the screw conveyor 46 and / or a corresponding oblique / vertical arrangement of the main body 32 through the transport room 34 moved in the direction of the exit opening. In this case, the heating device is adjusted so that in the region of the first heating coil, the silicon particles are heated to a temperature of at least 200 ° C to 600 ° C, in particular to about 300 ° C. In this area, the pressure is set to a maximum of 10 ^-1 mbar via the first stub line and the associated vacuum source. Under these conditions, water is predominantly removed as an oxygen carrier in addition to other volatile undesirable substances. Substantial oxide removal is not yet available in this area.

The heating device is further adjusted so that in the region of the second heating coil Silicon particles to a temperature below the melting temperature of the silicon particles of at least 800 ° C to 1200 ° C, in particular heated to about 1100 ° C. In this area, the pressure is set to a maximum of 10 ^-2 mbar via the second stub line and the associated vacuum source. In particular, the pressure is set to be smaller than the vapor pressure of SiO at the set temperature. The lower temperature for the oxide removal is selected to provide sufficient vapor pressure while the upper temperature is selected to avoid sintering of the Si particles. Preferably, the temperature should be between 950 ° C and 1150 ° C, in particular between 1000 ° C and 1100 ° C. At 1000 ° C, well below the melting point of the Si of 1414 ° C, a vapor pressure of 10 ^-2 mbar can already be observed on SiO, which is replenished continuously by an equilibrium reaction of SiO ₂ until the latter is largely removed, which by continuous pumping to the desired pressure is achieved. Thus, in the second region, primarily oxide removal from the silicon particles is achieved.

The silicon particles are transported by the screw conveyor 46 (according to 2 ) and / or gravity (in particular 3 ) conveyed through the transport space and oxide can be removed continuously. The corresponding heated particles are discharged at the outlet opening and preferably fed directly to a crucible. The introduced for the Oxidefernung energy for heating is thus used directly and there is an environmentally friendly and energy-efficient oxide removal, as a chemical etching and any necessary treatment of the acid used is eliminated.

In a further embodiment, the base body 32 the device 30 tilted and additionally shaken and / or rotated about its axis to promote transport of the silicon particles through the device. This is possible both with a version with and without screw conveyor. If rotated around the axis, the vacuum connections could be over the stubs 42 . 44 However, make gaskets and material supply or discharge more complex.

In the above illustration, two vacuum ports are provided, but it would also be possible to provide a finer subdivision of areas with more vacuum ports and, where appropriate, corresponding heating elements. It would also be possible to dispense with a subdivision into areas and provide only a vacuum connection in the high temperature range, with a corresponding heater with only one heating element could provide continuous heating from the input to the output. But rather more vacuum connections are preferred, which are preferably inclined to the direction of movement of the material, and are in the horizontal or inclined variant on the top. This can be better achieved a declining pressure, which could also be substantially uniform over the transport space away.

Within the transport space, the material is preferably durchmgt multiple transverse to the direction of movement to ensure a better gas outlet. Such a mixing can be achieved in particular by a screw conveyor, as shown, or else by other active or passive elements. In an inclined arrangement, a plurality of inclined surfaces could be provided in the transport space over which the particles slip and at the ends of the particles fall freely.

Measures to prevent undesired site condensation of volatiles may be conveniently provided, such as filters, separators, heated lines, and the like. By deliberately metering oxygen or water into the exhaust gas stream from the second region, the sublimed SiO 2 can be converted into fumed silica, which can be separated off and sold.

The device for reshaping silicon particles, which is subsequently used as a sputtering system 100 is now referred to with reference to 4 explained in more detail. It essentially corresponds to that in the DE 10 2011 100 884 A1 described apparatus, which is hereby incorporated by reference to avoid repetition. In short, the sputtering system 100 in a known manner a melting unit 103 , a process chamber unit 105 and a gas circulation unit 107 on.

The melting unit 103 consists essentially of a housing 108 , a crucible unit 109 , In the case 108 or adjacent thereto is a device 30 for oxide removal of the type described above, as indicated schematically, such that the output thereof is a substantially direct supply of exiting silicon particles in the crucible unit 109 allowed. The interior of the housing 108 is preferably kept under a protective gas atmosphere in order to avoid re-oxidation of silicon.

The crucible unit 109 We prefer formed by a cold crucible in which contactlessly a silicon melt can be generated by magnetic fields. But it is also possible a normal To provide crucible, via a heating unit 110 is heated, as in 4 shown.

The crucible unit has an element projecting into the crucible 112 that is made of or coated with silicon carbide (SiC) and arranged so that it can contact a melt in the crucible. The element 112 can be rotated via a rotating mechanism, not shown, as indicated by a corresponding arrow. The element may be, for example, a rod, grid or plate of solid SiC. Due to the significantly higher melting point of the material, the SiC element is in constant contact with liquid silicon in the crucible. The element 112 If appropriate, SiC particles which are present in the melt can at least partially take up particles and thus escape the further process, as described in the process sequence described above. The element 112 can be used with both normal crucibles and a cold crucible. In this, the liquid silicon is kept in constant motion by means of induction heating. Solid SiC particles therefore do not settle at the bottom of the crucible but come into contact with the element 112 and stick there because of the wetting properties. In combination with a cold crucible can therefore also on a turning device for the element 112 be waived. The element 112 should be replaced or cleaned at regular intervals.

The housing 108 may have a loading / unloading, not shown in detail, for loading of additional silicon and / or admixtures, which promote a subsequent silane synthesis, in particular a chlorosilane synthesis, in the crucible unit 109 , serves. Such loading / unloading can also for a removal and / or replacement of the element 112 serve.

At the bottom of the crucible unit 109 is a conduit element 113 provided via the molten silicon from the crucible unit 109 can be removed. The pipe element 113 consists of a material that is resistant to the temperatures required to melt metallurgical silicon, and does not introduce any interfering impurities into the molten silicon. The pipe element 113 extends from the bottom of the crucible unit 109 to an upper area of the process chamber unit 105 and opens into it, as will be explained in more detail below.

The process chamber unit 105 essentially has a housing 117 on, inside a process chamber 118 forms, as well as a collection container 119 on. The housing 117 has a flat upper wall and a circumferential side wall. The upper wall has a passage opening for the conduit element 113 , which is guided in a sealed manner through the upper wall. The side wall preferably has a round cross-section, but may also have a different configuration. In particular, the side wall has a vertically extending portion and a lower tapered portion.

In the side wall in the vertically extending region is an outlet opening 123 provided, which, as will be explained in more detail below, with the gas circulation unit 107 communicates. The outlet opening 123 is preferably located above a center of the process chamber 118 (in the height direction). In the side wall is further a plurality of gas inlet openings 124 intended. These are as will be explained in more detail below, also with the gas circulation unit 107 in connection. The gas inlet openings 124 are preferably located below a center of the process chamber 118 (in the height direction). The process chamber preferably has a height of 8 to 20 meters.

At the lower end of the circumferential side wall, a mounting flange is provided, which with a mounting flange of the collecting container 119 cooperates to the collection container 119 gas-tight on the housing 117 to fix.

The collection container 119 Optionally via means not shown in detail opposite the process chamber 118 be isolated and optionally applied with a negative pressure and or an inert gas to prevent or reduce oxidation of silicon particles received therein.

Although not shown, at least one heating unit may be provided to heat the process chamber to a predetermined temperature.

The gas circulation unit 107 consists essentially of a fine dust separator 131 , a first pump 133 , a gas processing unit 135 , a second pump 137 , a first inlet unit 138 , a third pump 141 and a second intake unit 142 , cooperating a gas circulation, in particular an inert gas, through the process chamber 118 allow.

The respective elements are suitable for existing existing temperatures and of materials (at least on the surface) which do not introduce impurities into the process. The pump 133 and the process chamber 118 are preferably coordinated so that a Underpressure of less than 50 mbar and particularly preferably a negative pressure of less than 15 mbar in the process chamber 118 can be adjusted.

The first inlet unit 138 has a plurality of outlets, which on an exit region of the conduit element 113 are directed to atomize therefrom emerging molten silicon. The outlets preferably provide a tapered flow cross-section in order to provide high flow rates in the region of an outlet opening. In particular, the pump 137 and the inlet unit 138 coordinated so that in the region of the outlet openings of the outlets, a high-velocity gas flow, in particular a gas flow with a supersonic speed can be generated.

The second inlet unit 142 has a variety of inlets 124 in the side wall, which are arranged so that they can produce an obliquely upward gas flow in the process chamber. Additionally or alternatively, the inlets may also be arranged to be in the process chamber 118 create a circular or spiral flow.

The operation of the device will be described below 100 with reference to the 4 explained in more detail. First, about the device 30 Silicon in the crucible unit 109 wherein the silicon has a temperature above 800 ° C and preferably a temperature between 1000 ° C and 1100 ° C. The silicon is in the crucible unit 109 melted, leaving in the conduit element 113 molten silicon is present. Selective admixtures, such as copper between 0.01 to 5 wt .-%, or other admixtures may be added to the silicon melt, which promote a subsequent silane synthesis, in particular a chlorosilane synthesis. If appropriate, the molten silicon can be conveyed via a corresponding valve unit (not shown) in the direction of the process chamber 118 be directed. The pipe element 113 may be such that the molten silicon would not flow through there without additional force such as, for example, a negative pressure in the process chamber and / or flow effects by the process gas introduced.

When molten silicon on the pipe element 113 is pending and in the direction of the process chamber 118 flows, is via the pump 137 Gas, in particular an inert gas, via the first inlet unit 138 in the process chamber 118 initiated. The process gas has access to the process chamber 118 a high flow rate, preferably a supersonic speed. Through the conduit element 113 passing molten silicon is atomized by this gas flow into fine droplets. The thus atomized droplets of molten silicon then fall within the process chamber 118 down, and eventually, after they have possibly solidified within the collection container 119 taken in the form of silicon particles. To the fall time within the process chamber 118 increase is via the second intake unit 142 an obliquely upward gas flow within the process chamber 118 generated. The process conditions in the process chamber 118 , in particular the gas flow for atomizing the silicon, are adjusted so that the atomized silicon at least partially solidifies during the free fall and substantially spherical silicon particles with a desired average particle size arise, which may depend on the subsequent process. For a trichlorosilane synthesis, for example, an average particle size of from 20 .mu.m to 600 .mu.m, in particular from 45 .mu.m to 500 .mu.m, is preferred.

The conventional sputtering apparatus was modified with respect to the crucible in the preferred form of cold crucible, the silicon feed via the preheating oxide removal device, and the SiC element serving as a purifying unit.

Although the invention has been described with reference to a specific embodiment, the invention is not limited to the specific embodiment. Especially in the field of separation of solids, alternative methods are considered. Here, for example, it would be possible to dispense with a special separation solution, which is designed as heavy liquid and permits a density-dependent separation. In an alternative, for example, a magnetic field based separation could be used, as described in US Pat US 8,231,006 B2 is described. For example, in a further alternative, assuming that a substantial portion of the SiC particles are larger than the Si / SiO ₂ particles, a size-dependent separation could be made. This could be achieved, for example, by appropriate filtering using di-water as the carrier. But even in a decanter centrifuge such separation would be possible by appropriate control of the centrifugal forces, which act more on large, heavier particles, in turn, di-water can be used as a carrier. As a possible device is in particular also on the Sorticanter ^® the company Flottweg SE, Industriestraße 6 - 8th . 84137 Vilsbiburg, Germany (Germany), which provides for a solid-liquid-solid separation of input materials.

These devices may utilize the principle of partial sedimentation of the solids suspended in, for example, di-water (or other suitable liquid which is not a heavy liquid) and thus achieve at least partial separation of the solids. The larger, heavier particles (SiC) sediment faster, while the smaller, and lighter Si / SiO ₂ remain suspended longer and can be discharged with the liquid. The settling speed of the particles in the suspension depends on the square of the particle size and the density difference to the fluid. The effect can be increased by a corresponding increase in the centrifugal forces and by a lowering of the dynamic viscosity of the suspension liquid, which suggests the use of di-water or alkanes. The form of the solids can also enter the sedimentation rate, but this is considered to be negligible for the solids present. Such partial sedimentation could also be used outside a decanter. Although these alternative separation processes, which can also be combined, usually provide poorer separation results than a separation in a heavy liquid and in particular a higher proportion of smaller SiC particles in the recovered Si / SiO ₂ particles entails, this is in the present Process no problem. The SiC particles can - at least partially - optionally still be removed by the SiC post-purification in the melt. In addition, they can also be easily carried in the atomizer and included in the formed particles. In a subsequent silane synthesis, the SiC particles are not contaminants and would be separated by the synthesis process and then filtered out. The only remaining particles could have a cleaning effect.

Claims (9)

A method for treating residues from the mechanical processing of silicon products for a subsequent silane synthesis, the residues comprising at least silicon, carbon-containing particles and metallic constituents, comprising the steps of: contacting the residues with an etching liquid which dissolves metallic constituents but not silicon or the carbon-containing particles; at least partially separating the etching liquid with metals dissolved therein from undissolved particles of particles containing at least silicon and carbon; at least partially separating silicon particles and carbon-containing particles using a heavy liquid with ZnBr ₂ in water and / or alcohol; at least partially removing oxides from the surface of the silicon particles; Melting the silicon particles; Introducing the molten silicon into a process chamber, wherein the molten silicon is atomized as it flows into the process chamber via a gas flow, and then to the bottom of the process chamber or a collecting container attached thereto, the process conditions in the process chamber, in particular the gas flow for atomizing the silicon be adjusted so that the atomized silicon at least partially solidified during the free fall. Method according to Claim 1 wherein the removal of oxides from the surface of the silicon particles comprises heating the silicon particles in vacuum to a temperature below the melting point of the silicon particles of at least 800 ° C, in particular to a temperature in the range of 1000 to 1100 ° C, wherein the negative pressure and the temperature is adjusted so that silicon oxides can sublime and be derived from the silicon particles. Method according to Claim 2 , wherein the heating is performed in several stages with decreasing pressure. Method according to one of Claims 2 or 3 , Wherein the particles are kept in motion during the heating and in particular are conveyed through an elongated process space. Method according to one of the preceding claims, wherein a size-dependent separation of the particles is carried out via a corresponding filtering. The method of any one of the preceding claims, wherein the molten silicon is contacted with an element comprised of SiC or having an SiC coating prior to introduction into the process chamber to at least partially receive SiC containing melt on the element. Method according to Claim 6 wherein the element and the melt of silicon are moved relative to each other during the contact. Method according to one of the preceding claims, wherein the process conditions in the process chamber, in particular the gas flow for atomizing the melt are adjusted so that substantially spherical silicon particles having an average particle size of 20 .mu.m to 600 .mu.m arise. Method according to one of the preceding claims, wherein the silicon melt targeted additions, such as copper between 0, 01 to 5 wt .-%, to be added, which promote a subsequent chlorosilane synthesis.

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