DE102012202589A1 - Use for a crucible - Google Patents

Abstract

The present invention relates to an insert for a crucible comprising a silicon dioxide vessel, wherein the silicon dioxide vessel comprises silicon dioxide elements which are joined together and the silicon dioxide elements are connected in a planar manner to at least one graphite element layer.

Description

The present invention relates to an insert for a crucible comprising a silicon dioxide vessel and a crucible comprising at least one such insert.

Crucibles for various applications are well known. For processing refractory metals or semi-metals, these high requirements must be satisfied. When silicon solidifies, the volume increases, so that a crucible suitable for carrying out directional solidification is subjected to particular stresses.

Depending on their application, inserts for crucibles, such as composites, in particular laminated glass, in addition to high safety requirements, also have to meet special stability requirements with respect to external mechanical forces, which may amount to a few kPa. In particular, in metallurgical applications with pressure loads of typically at least about 15 kPa (about 1.5 m water column or a load of about 1.5 to / m ² ) or even up to ten times this value are extremely high demands on the mechanical properties of Inserts for crucibles, such as composite safety glass systems provided.

According to existing safety regulations, laminated glass has to withstand four times the practically occurring loads and thus values of up to 60 to / m ² and above. High mechanical stability can already be achieved with tempered glass plates provided by a thermal treatment (thermally toughened / tempered or partially tempered glass) or other known methods.

Glass containers, so-called crusibles, in which metal melts, in particular silicon melts, can be handled during processing, are known in particular. However, these glass vessels must be fitted in a structurally very complex procedure in suitable graphite crucible. This complex procedure is not only very expensive on its own, but the glass vessels used so far must be prepared beforehand in an upstream process, which in turn is complex and associated with high costs.

In view of the prior art, it is an object of the present invention to provide a technically improved use for a crucible, in particular for the technical handling of molten metal, which does not have the disadvantages of conventional glass vessels.

In particular, a cost-effective and structurally easy-to-maintain use for a crucible for the handling of silicon melts, especially for the directional solidification of silicon, to be provided.

Furthermore, an insert for a crucible is to be provided, which is suitable as a container for the purification of a silicon melt.

Furthermore, the starting materials used should be as inexpensive to produce or obtain.

Furthermore, the use of a crucible, as well as the crucible comprising such a use should be able to meet the highest purity requirements of the material.

Furthermore, a crucible is to be provided for such an insert to be used according to the invention.

Other tasks that are not explicitly mentioned emerge from the overall context of the following description, examples and claims.

These and other objects which are not explicitly mentioned, but which are readily derivable or deducible from the contexts discussed hereinabove, are solved by an insert for a crucible having all the features of patent claim 1. Advantageous modifications of the use according to the invention for a crucible are set forth in the subclaims 2 to 14 under protection. Furthermore, claim 15 has a crucible comprising at least one such insert for the subject.

The present invention accordingly relates to an insert for a crucible comprising a silicon dioxide vessel, which is characterized in that the silicon dioxide vessel comprises silicon dioxide elements which are joined together and the silicon dioxide elements are connected in a planar manner to at least one graphite element layer ; and a crucible comprising at least one such insert.

This makes it possible to provide in an unpredictable way an insert for a crucible of the type set forth above, which has a particularly good property profile, in particular, which is not associated with the disadvantages of conventional glass vessels

In particular, it has surprisingly been found that a cost-effective and structurally easy-to-use insert for a crucible for the handling of silicon melts, in particular for the directed solidification of silicon, could be provided.

Furthermore, this newly developed use for a crucible as a container for a purification of a silicon melt has been found to be suitable.

In addition, the starting materials used can be very easily and inexpensively manufactured or obtained.

Furthermore, the use of a crucible, as well as the crucible comprising such an insert, meets the highest purity requirements of the material.

Furthermore, a crucible could be provided for such an insert to be used according to the invention.

An insert to be used according to the invention for a crucible for a molten metal, in particular for a silicon melt, is furthermore suitable for being able to apply a temperature gradient for directional solidification of a silicon melt.

By individual silicon dioxide elements one understands according to the present invention individual different building blocks of silicon dioxide, which are suitable for the production of a silicon dioxide glass vessel, whereby the glass vessel forms a part of an insert for a crucible. In general, these elements have a tile shape, so that the elements have a small thickness compared to the length and width. The width is defined as the minimum extent of the silica element across the surface, each including a dome. The length is defined here as the maximum extent of the silicon dioxide element perpendicular to the width. The thickness is the minimum extent perpendicular to the plane formed by the length and the width.

The length of the silicon dioxide elements may preferably be in the range from 1 to 10 ⁴ mm, preferably from 10 to 1000 mm, particularly preferably from 10 to 250 mm. According to a particular embodiment, a silicon dioxide element may preferably have a width of 1 to 10 ⁴ mm, preferably from 10 to 1000 mm, particularly preferably from 10 to 250 mm.

Furthermore, it may be provided that the ratio of the average length of a single silicon dioxide element to the average width of a single silicon dioxide element is from 1 to 100, preferably from 1 to 3, particularly preferably about 1.

Such individual silicon dioxide elements according to the present invention may have an area of from 1 to 10 ⁸ mm ² , preferably from 10 ² to 10 ⁶ mm ² , more preferably from 10 ² to 10 ³ mm ² .

The silicon dioxide elements in the embodiments according to the invention may have a thickness in the range from 0.1 to 100 mm, preferably from 1 to 50 mm, particularly preferably from 2 to 25 mm.

In a preferred embodiment of the invention, it may be provided that the ratio of the individual area of such a silicon dioxide element in relation to the total volume of the silica glass vessel of 10 ^-10 to 10 ^-1 1 / mm, preferably from 10 ^-6 to 10 ^-2 1 / mm, more preferably from 10 ^-5 to 10 ^-3 1 / mm.

The number of individual silicon dioxide elements required to produce a silica glass vessel according to the invention should be from 1 to 100,000, preferably from 5 to 20,000, particularly preferably from 10 to 1000.

According to a preferred embodiment, the size of the individual elements may be based on the overall target geometry, which may be indicated by the approximate error given by the device at least. This applies, for example, to embodiments with round or oval sections of the insert, wherein the components do not have to have these round or oval shapes correspondingly so that circular cutouts with annular gaps, joints or the like are obtained. In this case, the size of the component can be selected at most so that this error is at most 20%, preferably at most 10%.

The shape of the individual silicon dioxide elements for the preparation of the insert may be identical or different, but a closed vessel must be obtained. Joints can be closed by a grout, such as a ceramic adhesive. For example, the silicon dioxide elements may have any shape of a polygon, the polygon should have at least three corners, preferably at least four corners. The corners may be pointed or rounded here, with a pointed shape being preferred if silicon dioxide elements having an identical shape are used to produce the vessel.

Furthermore, the individual silicon dioxide elements may have a shape which has both corners and round shaped elements, preferably at least two corners and at least one round shaped element, more preferably at least three corners and at least two round shaped elements.

In order to form a large-scale use for a crucible of the present invention, it is necessary to add a plurality of silica elements of the present invention to a silica vessel. This can be done for example via a blunt or oblique shock. Furthermore, the silicon dioxide elements may preferably be designed so that the silicon dioxide elements can be assembled in a particularly simple and secure manner, the nature of the joining per se being uncritical. Thus, the silicon dioxide elements may be provided with a connection system.

In a preferred embodiment of the insert for a crucible, it may be provided that the silicon dioxide elements are joined together via a tongue and groove system.

Furthermore, the silicon dioxide elements may have a shape at the edges which allow for an overlapping connection or bung. Furthermore, nubs and recesses can be formed, which allow a clamping connection. To increase the quality of the connection, the silicon dioxide elements can be bonded together by gluing.

Furthermore, it can be provided that the silicon dioxide elements have a purity of preferably at least 90%, wherein the silicon dioxide elements are particularly preferably made of a high-purity material.

As a high-purity material is considered a material whose total metallic impurities are below 10,000 ppm, preferably below 1000 ppm. In a particularly preferred embodiment, the aluminum and boron contaminants are less than 20 ppm, less than or equal to 5 ppm calcium, less than or equal to 150 ppm iron, less than or equal to 25 ppm magnesium, manganese, and less chromium and less than 10 ppm phosphorus , with titanium less than 5 ppm, with zinc less than or equal to 3 ppm and the sum of impurities with sodium silicate less than 5 ppm.

Preferably, the individual silicon dioxide elements can be obtained from any high purity silica, which can be obtained by casting.

In a preferred embodiment of the invention, it may further be provided form the individual silicon elements that together the silica vessel of the use of the crucible to provide ₂ mass of a water-containing SiO.

The process necessary for the production of SiO ₂ elements comprises preparing a water-containing SiO ₂ mass, solidifying the water-containing SiO ₂ mass and drying the solidified SiO ₂ mass, the water-containing SiO ₂ mass being self-organizing.

The term "SiO ₂ mass" refers to a composition comprising SiO ₂ with different proportions of free and / or bound water, wherein the degree of condensation of the silica for this composition is not intrinsically essential. Accordingly, the term "SiO ₂ mass" also includes compounds having SiOH groups, which may also be commonly referred to as polysilicic acids.

The term "self-organizing" indicates that a water-containing SiO ₂ composition suitable for the present silicon dioxide vessel can be reversibly converted from a solidified to a flowable state. In this case, no permanent phase separation preferably takes place to a greater extent, so that the water is distributed substantially uniformly in the SiO ₂ phase on a macroscopic examination. However, it should be noted in this context that in a microscopic view, of course, there are two phases. A flowable state in the context of the present invention means that the water-containing SiO ₂ mass has a viscosity of preferably at most 30 Pas, preferably at most 20 Pas and particularly preferably at most 7 Pas, measured immediately after mass production (about 2 minutes after sampling), with a rotation rheometer at about 23 ° C, which is operated at a shear rate between 1 and 200 [1 / s]. At a shear rate of 10 [1 / s], the entry is made over a period of approx. 3 minutes. The viscosity is then about 5 Pas, determined with a Rheostress viscometer from Thermo Haake using the vane rotor 22 (diameter 22 mm, 5 blades) with a measuring range of 1 to 2.2 10 ⁶ Pas. At a shear rate of 1 [1 / s] and otherwise the same setting, a viscosity of 25 Pas is measured.

A solidified state, the water-containing SiO ₂ composition at a starting viscosity of preferably at least 30 Pas, more preferably at least 100 Pas. This value is determined by using the viscosity value of the rheometer 1 second after starting the rotary vane rotary vane at about 23 ° C and a shear rate of 10 [1 / s].

Preferably, a solidified, water-containing SiO ₂ composition for shaping by the action of shear forces can be re-liquefied. For this purpose, customary, familiar to those skilled methods and devices can be used, such as mixers, stirrers or mills with suitable tool geometry for the entry of shear forces. Among the preferred devices include intensive mixer (Eirich), continuous mixers or ring layer mixers, for example. From the company Lödige; Stirring container with mixing elements, which preferably have a sloping blade or a toothed disc; but also mills, in particular colloid mills or other rotor-stator systems that use annular gaps of different widths and different speeds. Furthermore, ultrasound-based apparatuses and tools, in particular sonotrodes and preferably ultrasound sources are suitable, which have a curved pathogen, whereby shear forces can be introduced into the SiO ₂ water mass in a particularly simple and defined way, which leads to their liquefaction. It is particularly advantageous that no particular abrasion of a tool takes place here. This ultrasonic arrangement is preferably operated in the non-linear range. The apparatus used for liquefying the water-containing SiO ₂ mass is generally dependent on the shearing force required for liquefaction. Advantages can be achieved inter alia by an apparatus whose shear rate (indicated as the peripheral speed of the tool) in the range of 0.01 to 50 m / s, in particular in the range of 0.1 to 20 m / s and particularly preferably in the range of 1 to 10 m / s. In the case of ultrasonic liquefaction, these can certainly reach ranges of the speed of sound. The time that is sheared, depending on the shear rate in a continuous process, may preferably be in the range of 0.01 to 90 minutes, more preferably in the range of 0.1 to 30 minutes.

For solidification of the water-containing SiO ₂ material, the same may preferably be allowed to stand for at least 0.1 minutes, preferably at least 2 minutes, in particular 20 minutes and more preferably at least 1 hour. The term "let stand" in this context preferably means that the mass or the composition is not subjected to shear forces. Furthermore, solidification can be carried out or accelerated, for example, by introduction of energy, preferably heating or additive addition. Additives may in this case be all crosslinkers known to the person skilled in the art, for example silanes, in particular functional silanes and here without limiting the invention, for example TEOS (Si (OC ₂ H ₅ ) ₄ ; tetraethoxysilane), which is advantageously available inexpensively in the highest purity. Additives can be far-reaching substances which bring about an increase in the pH, for example to values which are preferably in the range from 2.5 to 6.5, particularly preferably from 2.5 to 4, for example alkaline compounds, ammonia water being used with preference may be added, which is preferably added after molding.

A preferred solidified, water-containing SiO ₂ material may have a water content in the range of 2 to 98% by weight, in particular 20 to 85% by weight, preferably 30 to 75% by weight and particularly preferably 40 to 65% by weight. exhibit. The water content of a flowable SiO ₂ mass can be in the same ranges.

According to a particular embodiment, a SiO ₂ composition having a lower water content can be mixed with an SiO ₂ composition which has a higher water content in order to achieve the previously stated water content. The SiO ₂ compositions used for this purpose do not necessarily have to be self-organizing, but may have this property individually.

Furthermore, a solidified, water-containing SiO ₂ mass is preferably characterized by a pH of less than 5.0, preferably less than 4.0, in particular less than 3.5, preferably less than 3.0, particularly preferably less than 2.5.

Advantages can be achieved in particular by a solidified, water-containing SiO ₂ composition having a pH of greater than 0, preferably greater than 0.5 and particularly preferably greater than 1.0. The pH of the solidified, water-containing SiO ₂ mass can be determined by liquefying it on the basis of the flowable SiO ₂ mass obtained in this way. In this case, customary measuring methods can be used, such as those which are suitable for determining the H ⁺ ion concentration.

The SiO ₂ masses can, for example, be cast in the form of molds with any suitable geometry in a variant preferably consisting of textile-like fabrics or nets consisting of high-temperature resistant, pure plastics (silicone, PTFE, POM, PEEK), ceramic (SiC , Si ₃ N ₄ ), carbon fibers, graphite in all its forms of representation, metal fibers with suitable high-purity coating and / or quartz glass and / or in combination with glass fibers and / or carbon fibers exist and / or produced. The molds are segmented in a particularly preferred embodiment, which allows a particularly simple demoulding.

The high permeability of water vapor and / or liquid water through the fabric significantly improves the drying behavior of the Fromkörpers. The textile behavior of the mold has, surprisingly, the properties that a tension-free shrinkage of the molded body is possible during the drying process, which allows a particularly simple demolding without breakage.

• a. Aluminium kleiner gleich 100 ppm oder bevorzugt zwischen 10 ppm und 0,0001 ppm;
• b. Bor kleiner 10 ppm bis 0,0001 ppm;
• c. Calcium kleiner 6 ppm, bevorzugt zwischen 2 ppm und 0,0001 ppm;
• d. Eisen kleiner gleich 50 ppm, bevorzugt zwischen 10 ppm und 0,0001 ppm;
• e. Nickel kleiner gleich 10 ppm, bevorzugt zwischen 5 ppm und 0,0001 ppm;
• f. Phosphor kleiner 10 ppm bis 0,0001 ppm;
• g. Titan kleiner gleich 10 ppm, bevorzugt kleiner gleich 1 ppm bis 0,0001 ppm;
• h. Zink kleiner gleich 6 ppm, bevorzugt kleiner gleich 1 ppm bis 0,0001 ppm;
• i. Zinn kleiner gleich 20 ppm, bevorzugt kleiner gleich 3 ppm bis 0,0001 ppm

aufweist.The self-assembling SiO ₂ materials which are suitable for carrying out the present invention may, according to a preferred aspect, have a very high purity. A preferred pure silicon dioxide is characterized in that it has a content, as measured by ICP-MS and sample preparation known to the person skilled in the art:

• a. Aluminum less than or equal to 100 ppm, or preferably between 10 ppm and 0.0001 ppm;
• b. Boron less than 10 ppm to 0.0001 ppm;
• c. Calcium less than 6 ppm, preferably between 2 ppm and 0.0001 ppm;
• d. Iron less than or equal to 50 ppm, preferably between 10 ppm and 0.0001 ppm;
• e. Nickel less than or equal to 10 ppm, preferably between 5 ppm and 0.0001 ppm;
• f. Phosphorus less than 10 ppm to 0.0001 ppm;
• G. Titanium less than or equal to 10 ppm, preferably less than or equal to 1 ppm to 0.0001 ppm;
• H. Zinc less than or equal to 6 ppm, preferably less than or equal to 1 ppm to 0.0001 ppm;
• i. Tin less than or equal to 20 ppm, preferably less than or equal to 3 ppm to 0.0001 ppm

having.

A preferred high-purity silicon dioxide is characterized in that the sum of the o. G. Impurities (a-i) less than 1000 ppm, preferably less than 100 ppm, more preferably less than 10 ppm, very preferably less than 5 ppm, especially preferably between 0.5 and 4.9 ppm and very particularly preferably between 0.9 and 3, 9 ppm, wherein for each element, in particular the metal elements, a purity in the range of the detection limit can be sought. The data in ppm, refer to the weight.

The determination of impurities is carried out by ICP-MS / OES (Induction Coupling Spectrometry - Mass Spectrometry / Optical Electron Spectrometry) and AAS (Atomic Absorption Spectroscopy).

A water-containing SiO ₂ composition which can be used according to the invention can be obtained, for example, from a solution containing silicate, for example a water glass, by a precipitation reaction.

A preferred precipitation of a silicon oxide dissolved in the aqueous phase, in particular completely dissolved silicon oxide, is preferably carried out with an acidulant. Upon reaction of the silica dissolved in the aqueous phase with the acidulant, wherein the silica dissolved in the aqueous phase is preferably added to the acidulant, a precipitation suspension is obtained.

An important feature of the process is the control of the pH of the silica and of the reaction media in which the silicon dioxide is present during the various process steps of the silica production.

According to this preferred aspect, the initial charge and the precipitation suspension in which the silicon oxide dissolved in the aqueous phase, in particular the water glass, is added, preferably added dropwise, must always react more acidically. Acid is understood as meaning a pH below 6.5, in particular below 5.0, preferably below 3.5, more preferably below 2.5, and according to the invention below 2.0 to below 0.5. A pH control in the sense that the pH does not vary too much to obtain reproducible precipitate suspensions may be sought. If a constant or largely constant pH value is desired, the pH value should only show a fluctuation range of plus / minus 1.0, in particular of plus / minus 0.5, preferably of plus / minus 0.2.

In a particularly preferred embodiment of the present invention, the pH of the original and the precipitation suspension is always kept smaller than 2, preferably smaller than 1, particularly preferably smaller than 0.5. Furthermore, it is preferred if the acid is always present in significant excess to the alkali metal silicate solution to allow a pH less than 2 of the precipitation suspension at any time.

• a. Herstellen einer Vorlage aus einem Säuerungsmittel mit einem pH-Wert von kleiner 2, bevorzugt kleiner 1,5, besonders bevorzugt kleiner 1, ganz besonders bevorzugt kleiner 0,5;
• b. Bereitstellen einer Silikatlösung, wobei insbesondere die Viskosität zur Herstellung des durch Fällung gereinigten Siliziumoxids vorteilhaft in bestimmten Viskositätsbereichen eingestellt werden kann, bevorzugt ist insbesondere eine Viskosität von 0,001 bis 1000 Pas, wobei je nach Verfahrensführung sich dieser Viskositätsbereich – wie nachstehend dargelegt – bedingt durch weitere Verfahrensparameter weiter auffächern kann;
• c. Zugabe der Silikatlösung aus Schritt b. in die Vorlage aus Schritt a. derart, dass der pH-Wert der erhaltenen Fällsuspension jederzeit auf einem Wert kleiner 2, bevorzugt kleiner 1,5, besonders bevorzugt kleiner 1 und ganz besonders bevorzugt kleiner 0,5 bleibt; und
• d. Abtrennen und Waschen des erhaltenen Siliziumdioxids, wobei das Waschmedium einen pH-Wert kleiner 2, bevorzugt kleiner 1,5, besonders bevorzugt kleiner 1 und ganz besonders bevorzugt kleiner 0,5 aufweist.

Particularly preferred is a precipitation process for the preparation of purified silicon oxide, in particular high-purity silicon dioxide, comprising the subsequent steps

• a. Producing a template from an acidifier having a pH of less than 2, preferably less than 1.5, more preferably less than 1, most preferably less than 0.5;
• b. Providing a silicate solution, wherein in particular the viscosity for producing the precipitated silica can be advantageously adjusted in certain viscosity ranges, preferably a viscosity of 0.001 to 1000 Pas is preferred, depending on the process, this viscosity range - as set out below - due to further process parameters can fan out further;
• c. Add the silicate solution from step b. into the template from step a. such that the pH of the precipitation suspension obtained at any time to a value less than 2, preferably less than 1.5, more preferably less than 1 and most preferably less than 0.5; and
• d. Separating and washing the resulting silicon dioxide, wherein the washing medium has a pH of less than 2, preferably less than 1.5, more preferably less than 1 and most preferably less than 0.5.

Depending on the pH of the washing medium used, the SiO ₂ mass can be washed with water to a higher pH. In this case, the SiO ₂ mass can also be washed to pH values above the values set out above and then lowered by the addition of acid. Preferably, therefore, the resulting silica can be washed with water, wherein the pH of the SiO ₂ mass obtained preferably to a value in the range of 0 to 7.5 and / or the conductivity of the washing suspension to a value equal to or less than 100 μS / cm , preferably less than or equal to 10 μS / cm, and preferably less than or equal to 5 μS / cm is reduced.

• b. Bereitstellen einer Silikatlösung mit einer Viskosität von 0,001 bis 0,2 Pas

According to a first particularly preferred variant of this process, preference is given to a precipitation process for the preparation of purified silicon oxide, in particular high-purity silicon dioxide, which is carried out with silicate solutions of low to medium viscosity, so that step b. can be modified as follows:

• b. Providing a silicate solution with a viscosity of 0.001 to 0.2 Pas

• b. Bereitstellen einer Silikatlösung mit einer Viskosität von 0,2 bis 10000 Pas

According to a second particularly preferred variant of this process, a precipitation process for producing purified silicon oxide, in particular high-purity silicon dioxide, which is carried out with silicate solutions of high or very high viscosity, may be preferred, so that step b. can be modified as follows:

• b. Providing a silicate solution with a viscosity of 0.2 to 10,000 Pas

In the various variants of the method set out above, in step a. in the precipitation container a template made of an acidifier or an acidifier and water. The water is preferably distilled or demineralised water (demineralized water).

In all variants of the present process, not only in the particularly preferred embodiments described above in detail, as acidulants organic or inorganic acids, preferably mineral acids, more preferably hydrochloric acid, phosphoric acid, nitric acid, sulfuric acid, chlorosulfonic acid, sulfuryl chloride, perchloric acid, formic acid and / or Acetic acid can be used in concentrated or diluted form or mixtures of the aforementioned acids. Particularly preferred are the aforementioned inorganic acids. Very particular preference is given to hydrochloric acid, preferably 2 to 14 N, more preferably 2 to 12 N, very particularly preferably 2 to 10 N, especially preferably 2 to 7 N and very particularly preferably 3 to 6 N, phosphoric acid, preferably 2 to 59 N, particularly preferably from 2 to 50 N, very particularly preferably from 3 to 40 N, especially preferably from 3 to 30 N and very particularly preferably from 4 to 20 N, nitric acid, preferably from 1 to 24 N, particularly preferably from 1 to 20 N, very particularly preferably from 1 to 15 N, more preferably 2 to 10 N, sulfuric acid, preferably 1 to 37 N, more preferably 1 to 30 N, most preferably 2 to 20 N, especially preferably 2 to 10 N used. Most preferably, concentrated sulfuric acid is used.

The acidulants may be used in a purity commonly referred to as a "technical grade". It is clear to the person skilled in the art that as a result of the dilute or undiluted acidulants or mixtures of acidulants used, as far as possible, no impurities should be introduced into the process which do not remain dissolved in the aqueous phase of the precipitation suspension. In any case, the acidulants should not contain impurities which would precipitate with the silica in the acid precipitation, unless they could be held in the precipitation suspension by means of added chelating agents or by pH control or washed out with the subsequent wash media.

The acidulant used for precipitation may be the same which, e.g. B. also in step d. is used for washing the filter cake.

In a preferred variant of this method, in step a. in the template next to the acidifying agent added a peroxide, which causes a yellow / orange coloration with titanium (IV) ions under acidic conditions. This is particularly preferably hydrogen peroxide or potassium peroxodisulfate. Due to the yellow / orange color of the reaction solution, the degree of purification during the washing step d. be understood very well.

Particularly preferred are the variants in which the peroxide in step a. or b. is added, since it can perform in this case, in addition to the indicator function another function. Without being bound by any particular theory, it may be believed that some, especially carbonaceous, impurities are oxidized by reaction with the peroxide and removed from the reaction solution. Other impurities are converted by oxidation into a more soluble and thus washable form. The precipitation method according to the invention thus has the advantage that no Calcination step must be performed, although this is optional of course possible.

In all variants of the process according to the invention can be used as the aqueous phase dissolved in silica preferably an aqueous silicate solution, more preferably an alkali and / or alkaline earth silicate solution, most preferably a water glass. Such solutions may be commercially obtained, made by liquefaction of solid silicates, prepared from silica and sodium carbonate, or prepared directly from silica and sodium hydroxide and water at elevated temperature, for example, via the hydrothermal process. The hydrothermal process may be preferred over the soda process because it can lead to cleaner precipitated silica. A disadvantage of the hydrothermal process is the limited range of available modules, for example, the modulus of SiO ₂ to Na ₂ O is up to 2, with preferred modules are 3 to 4, also the water glasses must be concentrated after the hydrothermal process usually before precipitation become. In general, the skilled person is aware of the production of water glass as such.

According to an alternative, an alkali water glass, in particular sodium water glass or potassium water glass, optionally filtered and subsequently, if necessary, concentrated. The filtration of the waterglass or of the aqueous solution of dissolved silicates, for the separation of solid, undissolved constituents, can be carried out by processes known to the person skilled in the art and known to those skilled in the art.

The silicate solution used preferably has a modulus, i. Weight ratio of metal oxide to silicon dioxide, from 1.5 to 4.5, preferably 1.7 to 4.2, more preferably from 2 to 4.0.

The use of ion exchangers to purify the silicate solutions and / or acidulants prior to precipitation is not necessary, but may prove useful depending on the quality of the aqueous silicate solutions. Therefore, an alkaline silicate solution can also be used according to WO 2007/106860 pretreated to minimize the boron and / or phosphorus content in advance. For this purpose, the alkali metal silicate solution (aqueous phase in which silica is dissolved) can be treated with a transition metal, calcium or magnesium, a molybdenum salt or with an ion exchanger modified with molybdate salts to minimize the phosphorus content. Before the precipitation according to the method of WO 2007/106860 the alkali metal silicate solution of the precipitation according to the invention can be supplied in acidic form, in particular at a pH of less than 2. Preferably, however, acidulants and silicate solutions are used in the process according to the invention, which were not treated by means of ion exchangers prior to the precipitation.

In a specific embodiment, a silicate solution according to the methods of EP 0 504 467 B1 be pretreated as silica sol before the actual acidic precipitation according to the invention. For this purpose, the entire revelation content of EP 0 504 467 B1 explicitly included in this document. This according to the in the EP 0 504 467 B1 Silica sol obtainable by the method disclosed is preferably after a treatment according to the methods of EP 0 504 467 B1 completely dissolved again and then fed to an acidic precipitation according to the invention to obtain purified silicon oxide according to the invention.

The silicate solution preferably has a silica content of about at least 10% by weight or higher prior to acid precipitation.

A silicate solution, in particular a sodium water glass, can preferably be used for acid precipitation whose viscosity is from 0.001 to 1000 Pas, preferably 0.002 to 500 Pas, especially 0.01 to 300 Pas, especially preferably 0.04 to 100 Pas (at room temperature, 20 ° C). The viscosity of the silicate solution can preferably be measured at a shear rate of 10 1 / s, wherein the temperature is preferably 20 ° C.

In step b. and / or c. The first preferred variant of the precipitation process is a silicate solution having a viscosity of 0.001 to 0.2 Pas, preferably 0.002 to 0.19 Pas, especially 0.01 to 0.18 Pas and especially preferably 0.04 to 0.16 Pas and all especially preferably 0.05 to 0.15 Pas provided. The viscosity of the silicate solution can preferably be measured at a shear rate of 10 1 / s, wherein the temperature is preferably 20 ° C. Mixtures of several silicate solutions can also be used.

In step b and / or c of the second preferred variant of the precipitation process is a silicate solution having a viscosity of 0.2 to 1000 Pas, preferably 0.3 to 700 Pas, especially 0.4 to 600 Pas, more preferably 0.4 to 100 Pas, very particularly preferably 0.4 to 10 Pas and particularly preferably 0.5 to 5 Pas provided. The viscosity of the silicate solution can preferably be measured at a shear rate of 10 1 / s, wherein the temperature is preferably 20 ° C.

In step c. the main aspect and the two preferred variants of the precipitation process, the silicate solution from step b. placed in the template and thus precipitated the silica. It is Make sure that the acidifier is always in excess. The addition of the silicate solution therefore takes place in such a way that the pH of the reaction solution is always less than 2, preferably less than 1.5, particularly preferably less than 1, very particularly preferably less than 0.5 and especially preferably from 0.01 to 0.5. If necessary, further acidulant may be added. The temperature of the reaction solution is maintained at 20 to 95 ° C, preferably 30 to 90 ° C, particularly preferably 40 to 80 ° C during the addition of the silicate solution by heating or cooling the precipitation vessel.

Particularly well filterable precipitates are obtained when the silicate solution enters the template and / or precipitation suspension in drop form. In a preferred embodiment, care is therefore taken to ensure that the silicate solution enters the original and / or precipitation suspension in droplet form. This can be achieved, for example, by introducing the silicate solution into the original by means of drops. This may be dispensing units mounted outside of the original / precipitation suspension and / or dipping in the original / precipitation suspension.

In the first particularly preferred variant, i. the method with low-viscosity water glass, it has proved to be particularly advantageous if the template / precipitation suspension is set in motion, z. B. by stirring or pumping around, that the flow velocity measured in a range which is limited by half the radius of the precipitation container ± 5 cm and surface of the reaction solution to 10 cm below the reaction surface from 0.001 to 10 m / s, preferably 0.005 to 8 m / s, particularly preferably 0.01 to 5 m / s, very particularly 0.01 to 4 m / s, especially preferably 0.01 to 2 m / s and very particularly preferably 0.01 to 1 m / s.

By combining an optimized flow rate with a possibly drop-shaped entry of the silicate solution, this effect can be further increased, so that an embodiment of the precipitation process in which the silicate solution in droplet form in a template / precipitation suspension with a flow velocity measured in a range d of half by Radius of the precipitation vessel ± 5 cm and the surface of the reaction solution is limited to 10 cm below the reaction surface, from 0.001 to 10 m / s, preferably 0.005 to 8 m / s, more preferably 0.01 to 5 m / s, especially 0, 01 to 4 m / s, more preferably 0.01 to 2 m / s and very particularly preferably 0.01 to 1 m / s is introduced. In this way it is also possible to produce silica particles which can be filtered very well. In contrast, very fine particles are formed in processes in which there is a high flow velocity in the receiver / precipitation suspension, and these particles are very difficult to filter.

In the second preferred embodiment of the precipitation method, i. H. When using highly viscous waterglass, dropwise addition of the silicate solution also produces particularly clean and readily filterable precipitates. Without being bound by any particular theory, it can be assumed that the high viscosity of the silicate solution together with the pH conditionally necessitates that after step c. a readily filterable precipitate is formed and that no or only very small impurities are stored in internal cavities of the silica particles, since the drop shape of the dripping silicate solution is largely retained by the high viscosity and the droplets are not finely dispersed before the gelation / crystallization on the surface of the drop begins. The silicate solution used may preferably be the above-defined alkali metal and / or alkaline earth metal silicate solutions, preferably an alkali metal silicate solution, more preferably sodium silicate (water glass) and / or potassium silicate solution is used. Mixtures of several silicate solutions can also be used. Alkali silicate solutions have the advantage that the alkali metal ions can be easily separated by washing. The viscosity can, for. B. by concentration of commercially available silicate solutions or by dissolving the silicates in water.

As stated previously, by suitably selecting the viscosity of the silicate solution and / or the stirring speed, the filterability of the particles can be improved since particles having a specific shape are obtained. Preference is therefore given to purified silicon oxide particles, in particular silicon dioxide particles, which preferably have an outer diameter of 0.1 to 10 mm, particularly preferably 0.3 to 9 mm and very particularly preferably 2 to 8 mm. In a first specific embodiment of the present invention, these silica particles have a ring shape, i. H. have a "hole" in the middle and are therefore comparable in shape to a miniature torus, also called "donut". The annular particles can assume a largely round, but also a more oval shape.

In a second specific embodiment of the present precipitation process, these silica particles have a shape comparable to a "mushroom head" or a "jellyfish". Ie. instead of the hole of the above-described "donut" -shaped particles, in the center of the annular basic structure is a vaulted, preferably thinner, ie thinner than the annular, part of the annular base structure, layer of silicon dioxide spanning the inner opening of the "ring". Would you do this Place particles with the curved side down on the ground and look vertically from above, so the particles correspond to a bowl with a curved bottom, rather solid, ie thick upper edge and in the region of the curvature slightly thinner ground.

The silica obtained after the precipitation is separated from the remaining constituents of the precipitation suspension. This can, depending on the filterability of the precipitate by conventional, known in the art filtration techniques, for. As filter presses or rotary filter, done. In the case of precipitates which are difficult to filter, the separation can also be effected by centrifugation and / or by decantation of the liquid constituents of the precipitation suspension.

After separation from the supernatant, the precipitate is washed, it being ensured by means of a suitable washing medium that the pH of the washing medium during the wash and thus also that of the purified silicon oxide, in particular of the silicon dioxide, is less than 2, preferably less than 1.5 preferably less than 1, very particularly preferably 0.5 and especially preferably 0.01 to 0.5.

The washing medium used may preferably be aqueous solutions of organic and / or inorganic water-soluble acids, e.g. of the aforementioned acids or fumaric acid, oxalic acid, formic acid, acetic acid or other organic acids known to those skilled in the art, which themselves do not contribute to the contamination of the purified silica unless they can be removed completely with ultrapure water. In general, therefore, all organic, water-soluble acids, in particular consisting of the elements C, H and O, both from acidulants and as preferred in the washing medium, because they themselves do not contribute to contamination of the subsequent reduction step. Preferably, in step a. and c. used acidulants or mixtures thereof used in diluted or undiluted form.

If desired, the washing medium may also comprise a mixture of water and organic solvents. Suitable solvents are high-purity alcohols, such as methanol or ethanol. A possible esterification does not disturb the subsequent reduction to silicon.

The aqueous phase preferably contains no organic solvents, such as alcohols, and / or no organic, polymeric substances.

In the process according to the invention, it is usually not necessary to add chelating agent to the precipitation suspension or during the purification. Nevertheless, the present invention also encompasses processes in which a metal complexing agent, such as EDTA, is added to stabilize acid-soluble metal complexes of the precipitation suspension or else to a washing medium.

Also a peroxide for color marking, as an "indicator" of unwanted metal impurities, can be added. For example, hydroperoxide can be added to the precipitation suspension or the washing medium in order to color-identify existing titanium impurities. The labeling is generally possible with other organic complexing agents, which in turn do not interfere in the subsequent reduction process. These are generally all complexing agents based on elements C, H and O; element N may also be useful in the complexing agent. For example, for the formation of silicon nitride, which advantageously decomposes again in the later process.

The washing is continued until the silica has the desired purity. This can be z. B. be recognized that the wash suspension contains a peroxide and visually shows no more yellowing.

Further notes on the precipitation and washing of SiO ₂ are in the document WO 2010/037694 , filed on 28.09.2009 with the European Patent Office, with the application number PCT / EP2009 / 062387 , which is incorporated herein by reference for disclosure purposes in the present application.

The thus washed and purified silica is preferably further washed with distilled water or demineralized water until the pH of the resulting silica in a range of 0 to 7.5 and / or the conductivity of the washing suspension is less than or equal to 100 μS / cm, preferably less than or equal to 10 μS / cm and preferably less than or equal to 5 μS / cm. In this case, the pH can particularly preferably be in the range from 0 to 4.0, preferably 0.2 to 3.5, in particular 0.5 to 3.0 and particularly preferably 1.0 to 2.5. In this case, a washing medium with an organic acid can be used. This can ensure that any interfering acid residues adhering to the silicon dioxide are adequately removed.

The separation can be carried out with customary measures known to the person skilled in the art, such as filtering, decanting, centrifuging and / or sedimentation, with the proviso that the impurity level of the acidified, purified silicon oxide does not deteriorate again as a result of these measures.

In difficult to filter precipitates, it may be advantageous to carry out the washing by flowing the precipitate in a close-meshed screen basket with the washing medium from below.

The purified silicon dioxide obtained in this way, in particular high-purity silicon dioxide, can be dried and further processed in order to adjust the self-assembling SiO ₂ mass to the preferred proportions of water set out below. The drying can be carried out by means of all methods and devices known to the person skilled in the art, e.g. B. belt dryer, tray dryer, drum dryer, etc. take place.

Surprisingly, it is possible by the inventive method in a particularly simple and economical manner to obtain a SiO ₂ element in any shape. For this purpose, a flowable water-containing SiO ₂ mass can be poured into a mold.

In this case, the flowable water-containing SiO ₂ material can be introduced and distributed in any desired manner into a mold having the desired dimensions. For example, the entry can be done by hand or by machine via Zuteilorgane. The filled mold can be subjected to vibration in order to achieve a rapid and uniform distribution of the water-containing SiO ₂ material in the mold.

The casting molds to be used for the production of the elements are not subject to any special requirements, but their use should not result in impurities entering the SiO ₂ element. For example, suitable molds of high temperature resistant, pure plastics (silicone, PTFE, POM, PEEK), ceramic (SiC, Si ₃ N ₄ ), graphite in all its forms of representation, metal can be produced with a suitable high-purity coating and / or quartz glass. The molds are segmented in a particularly preferred embodiment, which allows a particularly simple demoulding.

In a preferred embodiment, the solid forms may instead be made of open materials such as textile-like fabrics or nets made of high temperature resistant, pure plastics (silicone, PTFE, POM, PEEK), ceramics (SiC, Si ₃ N ₄ ), carbon fibers, graphite consist in all its forms of representation, metal fibers with suitable high-purity coating and / or quartz glass and / or in conjunction with glass fibers and / or carbon fibers and / or produced. The molds are segmented in a further particularly preferred embodiment, which allows a particularly simple demolding by pulling apart. Elastic temperature-resistant materials (for example, similar to nylon stockings) allow here in particular a continuous procedure.

The high permeability of water vapor and / or liquid water through the fabric significantly improves the drying behavior of the Fromkörpers. The textile behavior of the mold also surprisingly has the properties that, for example, in the case of a tube-like shape, stress-free shrinkage of the cast molded body during the drying process becomes possible, which permits particularly simple demolding without breakage.

After shaping, the solidified, water-containing SiO ₂ mass is stabilized by means of an alkaline additive and / or by drying. For this purpose, the filled mold can be transferred without or after addition of additive in a dryer which is heated, for example, electrically, with hot air, superheated steam, IR radiation, microwaves or combinations of these heating methods. In this case, conventional devices, such as belt dryer, tray dryer, drum dryer can be used, which dry continuously or batchwise.

Advantageously, the SiO ₂ elements can be dried to a water content that allows non-destructive removal from the molds. Accordingly, the drying in the casting mold can be carried out to a water content of less than 60% by weight, in particular less than 50% by weight and particularly preferably less than 40% by weight.

Drying to a water content which is below the stated values can be carried out particularly preferably after demolding of the SiO ₂ element, it being possible to use the dryers set out above.

Surprising advantages include SiO ₂ elements which, after drying, have a water content in the range from 0.0001 to 50% by weight, preferably 0.0005 to 50% by weight, in particular 0.001 to 10% by weight, and especially preferably 0.005 to 5 wt .-%, measured by means of the generally known in the art thermogravimetry method (IR-moisture meter).

Preferably, the drying of the solidified, water-containing SiO ₂ material at a temperature in the range of 50 ° C to 350 ° C, preferably 80 to 300 ° C, in particular 90 to 250 ° C and particularly preferably 100 to 200 ° C under normal conditions ( ie at atmospheric pressure).

The pressure at which the drying takes place can be in a wide range, so that the drying is carried out under reduced or elevated pressure can be. For economic reasons, drying at ambient or atmospheric pressure (950 to 1050 mbar) may be preferred.

To increase the hardness of the dried SiO ₂ element, the same can be thermally densified or sintered. This can be carried out batchwise, for example, in conventional industrial furnaces, for example shaft furnaces or microwave sintering furnaces, or continuously, for example in so-called push-through furnaces or shaft furnaces.

The thermal densification or sintering can be carried out at a temperature in the range from 400 to 1700 ° C., in particular 500 to 1500 ° C., preferably 600 to 1200 ° C. and particularly preferably 700 to 1100 ° C.

The duration of the thermal compaction or sintering depends on the temperature, the desired density and optionally the desired hardness of the SiO ₂ element. Preferably, the thermal densification or sintering can be carried out over a period of 5 hours, preferably 2 hours, particularly preferably 1 hour.

The dried and / or sintered SiO ₂ elements with the typical dimensions described above may have, for example, a compressive strength (stated as breaking strength) of at least 10 N / cm ² , preferably of more than 20 N / cm ² , with particularly sintered SiO ₂ - Elements can show compressive strength values of at least 50 or even at least 150 N / cm ² , in each case measured by means of compression tests on an arrangement for pressure strength tests.

According to a particular embodiment, the SiO ₂ element may preferably have a density of at least 0.7 g / cm ³ , preferably a density of at least 1 g / cm ³ , particularly preferably at least 1.4 g / cm ³ . In a high-temperature sintering even a density of 2.65 (quartz glass density) can be achieved.

The connection of the SiO ₂ elements to a glass vessel is not subject to any particular limitations, but the process used for this purpose must lead to a stable, stable at the conditions set out above glass vessel. It is preferably provided that the silicon dioxide elements are connected to one another via a ceramic adhesive.

In this case, it may be preferred that the ceramic adhesive comprises a silicon compound which can be hydrolyzed to SiO ₂ . Without loss of generality, concrete examples of such silicon compounds are SiCl ₄ , HSiCl ₃ , Si (OCH ₃ ) ₄ , Si (OOCCH ₃ ) _4, and Si (OC ₂ H ₅ ) ₄ . The compounds mentioned can be used individually or as a mixture. The preferred silicon compounds include in particular alkoxysilanes, particularly preferably tetraalkoxysilanes, tetraethoxysilane (TEOS) being particularly preferred.

The ceramic adhesive used particularly preferably comprises a composition comprising silicon dioxide, at least one silicon compound which is hydrolyzable to SiO ₂ , preferably a tetraalkoxysilane, in particular TEOS and at least one curing catalyst, preferably H ₂ O ₂ , H ₂ SO ₄ or HNO ₃ . The silica may preferably be particulate, wherein the diameter of these particles is preferably in the range of 0.001 to 100 microns, more preferably in the range of 0.005 to 10 microns and more preferably in the range of 0.01 to 1 microns, measured by laser diffraction.

In a preferred embodiment, the ceramic adhesive used may comprise tetraethoxysilane, preferably TEOS and silicon dioxide based on a self-organizing, water-containing SiO ₂ mass.

Furthermore, according to the invention, it can preferably be provided that the ceramic adhesive has a temperature stability greater than or equal to 1000 ° C., preferably greater than or equal to 1200 ° C., particularly preferably greater than or equal to 1400 ° C.

It may also be provided that the ceramic adhesive has a purity greater than or equal to 75%, preferably greater than or equal to 90%, particularly preferably greater than or equal to 99.9%.

Ceramic adhesives which can be used according to the invention and which are preferably based on SiO ₂ are commercially available, for example from Polytec PT GmbH.

An insert according to the invention, in addition to the previously described SiO ₂ elements, which preferably form a glass vessel, has at least one graphite element layer. This layer is connected to the surface of the silicon dioxide elements. Accordingly, the area of the silicon dioxide elements given by the length and width are in direct or indirect contact with at least one graphite element layer. This can be done mechanically or through a bonding layer. The term "graphite element layer" means that the insert has at least one layer-shaped element which is essentially formed from graphite. According to a preferred embodiment, the graphite element layer is impermeable or dense, so that small amounts of liquid silicon can not penetrate this layer. Layers with high density are also referred to as films below designated, wherein a plurality of films can be joined together by gluing.

Preferred embodiments of the graphite element layer are in the document US 7,708,827 B2 described, filed on 24.01.2003 with the US Patent Office (USPTO) with the application number 10 / 351,124, which are incorporated by reference thereto for disclosure purposes in the present application

Graphite can be produced by hot isostatic pressing as a block material and is exciting editable. It is available as a high-purity material, for example from SGL Carbon, and has mechanical and chemical properties which are particularly well suited to the present task, that is, a good temperature stability and, above all, no detrimental effect on the final purity silicon.

It may further be provided that the graphite element layer is flexible within the meaning of the claims if, due to its flexibility, this (starting from a planar shape) can equally assume the curved course of walls typically used in a crucible insert Metal melts, especially for silicon melts occur. Surprisingly, preferred embodiments for a graphite element layer overcome such adaptation due to their flexibility, without disturbing cracks or breaks occurring.

In addition, a graphite element layer to be used according to the invention can advantageously be present as a self-supporting fabric, in other words as a braid of carbon fibers. The flexible graphite element layer may also alternatively consist of a combination of fabric and impermeable layer, whereby the merits of a film could be combined with those of a fabric. A film is advantageously dense to oxygen, nitrogen or liquid silicon as compared to a fabric. However, it is relatively easy for a film to peel carbon films relatively easily compared to a web. A reinforced by means of a braid of carbon fibers graphite foil on the one hand has the necessary for the purpose of mechanical stability and on the other hand, then the necessary tightness.

It can also be provided that the graphite element layer has a thickness in the range of 0.1 to 50 mm, preferably from 0.5 to 1 mm, particularly preferably about 1 mm.

In an alternative embodiment of the invention, the thickness of the graphite element layer is less than or equal to 5 mm, preferably less than or equal to 3 mm, particularly preferably less than or equal to 1.5 mm.

It can further be provided that the graphite element layer has at least one functionalized surface.

Furthermore, it can be provided that the graphite element layer has a multilayer structure.

In a preferred embodiment of the invention, the multilayer structure of the graphite element layer is formed from film webs, wherein the film webs at an angle to each other in the range of 45 to 135 °, preferably from 60 to 120 °, more preferably from 75 to 105 °.

In a preferred embodiment of the invention, these may be self-supporting, flexible film webs of a graphite element layer.

The film webs of the graphite element layer to be used according to the invention may furthermore preferably have a bulk density of the graphite of from 0.1 to 2 g / cm ³ , preferably from 0.5 to 1.5 g / cm ³ , particularly preferably from 0.7 to 1.3 g / cm ³ .

Furthermore, it can preferably be provided according to the invention that the film webs of the graphite element layer to be used according to the invention have an ash value of 0.15 to 4%, preferably 0.5 to 1.5%, particularly preferably 0.7 to 1.2% ,

In addition, the film webs of the graphite element layer to be used according to the invention may preferably have a temperature resistance of from 0 to 2000 ° C., preferably from 0 to 1700 ° C., particularly preferably from 200 to 1000 ° C.

Furthermore, it can preferably be provided that the film webs of the graphite element layer to be used according to the invention have a specific heat capacity of 0.2 to 2 kJ / (kg.K), preferably 0.5 to 1 kJ / (kg.K) from 0.6 to 0.8 kJ / (kg · K).

The film webs of the graphite element layer to be used according to the invention may furthermore preferably have a thermal expansion coefficient parallel to the stratification of 0.2 to 2 × 10 ^-6 / K, preferably of 0.5 to 1.5 × 10 ^-6 / K, particularly preferably of 0, 8 to 1.2 · 10 ^-6 / K have.

Furthermore, it can be provided that the film webs of the graphite element layer to be used according to the invention preferably have a thermal expansion coefficient perpendicular to the layering from 10 to 60 × 10 ^-6 / K, preferably from 20 to 40 × 10 ^-6 / K, particularly preferably from 25 to 35 × 10 ^-6 / K.

In addition, the film webs of the graphite element layer to be used according to the invention may preferably have a Shore hardness of from 10 to 60, preferably from 20 to 40, particularly preferably from 25 to 35.

In addition, the film webs of the graphite element layer to be used according to the invention may preferably have a chloride content of less than or equal to 30 ppm, preferably less than or equal to 10 ppm, particularly preferably less than or equal to 7 ppm.

It can also be provided that an adhesion promoter layer is arranged between the silicon dioxide elements and the graphite element layer. The adhesion promoter layer can be formed, for example, from the above-described ceramic adhesives, preferred ceramic adhesives comprising a composition comprising silicon dioxide, at least one SiO ₂ hydrolyzable silicon compound, preferably a tetraalkoxysilane, in particular TEOS and at least one curing catalyst, preferably H ₂ O ₂ , H ₂ SO ₄ or ENT ₃ included. The silicon dioxide may preferably be particulate, the diameter of these particles preferably being in the range from 0.001 μm to 10 mm, particularly preferably in the range from 0.01 μm to 0.5 mm, measured by means of laser diffraction.

Such an adhesion promoter layer to be used according to the invention may have a thickness in the range from 1 μm to 4 mm, preferably from 0.1 mm to 1.5 mm, particularly preferably from 0.5 mm to 1.2 mm.

It can be provided that the inside of the silicon dioxide vessel of the insert for a crucible, which comes into contact with the SiO ₂ -containing materials, is at least partially coated with one or more materials.

It may also be provided that the silicon dioxide vessel on the inside comprises a coating of silicon nitride (Si ₃ N ₄ ).

Here, the coating of silicon nitride (Si ₃ N ₄ ) may have a thickness in the range from 1 μm to 5 mm, preferably from 10 μm to 1 mm, particularly preferably from 100 to 500 μm.

It may also be provided in a preferred embodiment that the silicon dioxide vessel comprises at least two layers of silicon dioxide elements, between which at least one graphite element layer is arranged. According to a particular embodiment, an adhesion promoter layer can be arranged in each case between the graphite element layer and the two silicon dioxide element layers.

Furthermore, a crucible comprising at least one such insert to be used according to the invention is provided. Here, the crucible can be designed so that the insert forms an integral part.

In this case, it can preferably be provided that the crucible comprises a graphite crucible into which the present insert is fitted.

The shape of the insert or the crucible can be adapted to different requirements, wherein the crucible can take the form of a cylinder or a cuboid.

• a) Herstellung einzelner Siliziumdioxid-Elemente;
• b) Verwendung der in Schritt a) hergestellten einzelnen Siliziumdioxid-Elemente zur Bereitstellung eines Siliziumdioxid-Gefäßes mittels einer Zusammenfügung dieser einzelnen Siliziumdioxid-Elemente.
• c) Flächige Verbindung dieses Siliziumdioxid-Gefäßes umfassend eine Vielzahl zusammengefügter einzelner Siliziumdioxid-Elemente mit mindestens einer Graphit-Elementschicht.

By way of non-limiting example, a method for the production of such an insert for a crucible according to the invention to be used according to the invention will be described below, comprising the following method steps:

• a) production of individual silicon dioxide elements;
• b) using the individual silicon dioxide elements produced in step a) to provide a silicon dioxide vessel by means of an assembly of these individual silicon dioxide elements.
• c) surface bonding of said silica vessel comprising a plurality of assembled single silica elements having at least one graphite element layer.

It can also be provided in a method step d) on the outside of the silicon dioxide vessel on the graphite element layer to apply a further layer of silicon dioxide elements.

The process for producing an insert comprising a silicon dioxide vessel is preferably completed by a heat treatment step, regardless of whether a process step d) is provided.

The joining of the individual elements provided in step b) can be carried out using a ceramic adhesive. The curing of this ceramic adhesive depends on its composition and is well known. In this case, this hardening can take place before or after the two-dimensional bonding of the vessel obtained with a graphite element layer. Depending on the configuration, a sintering step may be carried out in order to increase the strength of the connection of the ceramic adhesive with the silicon dioxide elements and / or the graphite element layer. For this purpose, the glass vessel with or without graphite element layer to a temperature in Range of 500 to 1800 ° C, preferably in the range of 1000 to 1500 ° C over a period of at least 12 h, preferably at least 24 h heated.

Preferred embodiments of the present invention are explained below by way of example by way of example. Show it:

1 : A schematic structure of an embodiment of a use for a crucible according to the present invention;

2 : A schematic cross section of an embodiment of an insert according to the invention;

3 : A schematic cross section of a preferred embodiment of an insert according to the present invention;

4 Various Embodiments for Joining Single Different Silicon Dioxide Elements to a Silica Vessel

1 describes an embodiment of a silicon dioxide vessel to be used according to the invention 1 which consists of a multiplicity of individual, different silicon dioxide elements 2 is built up or is. Here is in 1a schematically a cylindrical and in 1b a cuboid silicon dioxide vessel for a use according to the invention for a crucible shown.

2 shows a cross section of an embodiment of an insert according to the invention 3 comprising three individual layers of material: a first layer of silicon dioxide elements 4 , a primer layer 5 and a graphite element layer 6 , The silicon dioxide elements 4 are glued together.

3 FIG. 3 illustrates a cross-section of a preferred embodiment of an insert according to the invention. FIG 3 ' which consists of five individual layers of material 4 . 5 . 6 . 7 and 8th was formed. Here are compared to the embodiment in 2 two additional layers of material have been used, namely a second adhesion promoter layer 7 and a second layer of silicon dioxide elements 8th This embodiment also includes a first layer of silicon dioxide elements 4 , a primer layer 5 and a graphite element layer 6 includes. The silicon dioxide elements 4 and 8th are glued together.

In these embodiments, both the first and second layers comprise silicon dioxide elements 4 and 8th a greater thickness than the graphite element layer 6 on.

4 describes different possibilities for connecting individual different silicon dioxide elements to a silicon dioxide vessel according to the invention, wherein 4A a first silicon dioxide element 9 with a second silicon dioxide element 10 butt to be stumped, to reinforce the connection ceramic adhesive 11 can be used.

In 4B becomes an oblique butt joint of a first silicon dioxide element 9 ' with a second silicon dioxide element 10' shown, wherein to strengthen the connection ceramic adhesive 11 can be used.

In 4C is a joining of the silica elements 9 ' and 10'' shown by bouncing, with one edge of the silicon dioxide element 9'' with a spring element 12 while another edge of the silicon dioxide element 10'' with a groove element 13 is provided. In addition, a ceramic adhesive can be added 11 be introduced.

An overlap is in 4D shown. The respective edges of the silicon dioxide elements 9''' and 10''' are stepped, with the projection 14 a silicon dioxide element 9''' in the recess 15 of the next silicon dioxide element 10''' is adjusted. In addition, in the joining a ceramic adhesive 11 be introduced.

An insert to be used according to the invention and a crucible comprising such an insert can preferably be provided for a method for the purification of a molten metal, in particular a silicon melt.

Such a cleaning process can be combined with other processes for purifying silicon. Particularly preferred embodiments are, inter alia, from the dissertation "Silicon for Solar Cells" by Anne-Karin Søiland of the Norwegian University of Science and Technology, October 2004 , IMT Report 2004: 65 and the publication DE 38 02 531 A1 known, filed on 28.01.1988 at German Patent Office with the application number P 38 02 531.0 which are incorporated herein by reference for disclosure purposes in the present application.

The further steps and peculiarities of processes for producing metallic silicon are described, inter alia, in WO 2010/037694 explained. In this process, SiO ₂ is reduced to metallic silicon by carbon in an electric arc furnace. The starting material used is usually an SiO ₂ shaped body in combination with a carbon source. Accordingly, the document WO 2010/037694 , submitted on 28.09.2009 to the European Patent Office with the Application number PCT / EP2009 / 062387 , for purposes of disclosure, are incorporated herein by reference.

The use according to the invention for a crucible thus enables a cost-effective and structurally simple solution to provide a specific use for a crucible, which is particularly suitable for the directional solidification of silicon melts in the production of high-purity silicon.

The use according to the invention for a crucible, as well as a crucible comprising at least one such insert is defined by the characterizing features of the appended claims.

LIST OF REFERENCE NUMBERS

1

 Silica vessel

2

 Single silicon dioxide elements

3

 commitment

4

 First layer of silicon dioxide elements

5

 First primer layer

6

 Graphite element layer

7

 Second adhesive layer

8th

 Second layer of silicon dioxide elements

9, 9', 9'', 9'''

 First silicon dioxide element

10, 10', 10'', 10'''

 Second silicon dioxide element

11

 ceramic adhesive

12

 spring element

13

 groove element

14

 Projection of a first silicon dioxide element

15

 Recess of a second silicon dioxide element

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 2007/106860 [0072, 0072]
• EP 0504467 B1 [0073, 0073, 0073, 0073]
• WO 2010/037694 [0093, 0168, 0168]
• EP 2009/062387 [0093, 0168]
• US 7708827 B2 [0122]
• DE 3802531 A1 [0167]
• DE 3802531 [0167]

Cited non-patent literature

• "Silicon for Solar Cells" by Anne-Karin Søiland of the Norwegian University of Science and Technology, October 2004 [0167]

Claims (15)

Insert for a crucible comprising a silicon dioxide vessel, characterized in that the silicon dioxide vessel comprises silicon dioxide elements which are joined together and the silicon dioxide elements are connected in a planar manner to at least one graphite element layer. Insert according to claim 1, characterized in that the silicon dioxide vessel comprises at least two layers of silicon dioxide elements, between which at least one graphite element layer is arranged. Insert according to claim 1 or 2, characterized in that the silicon dioxide vessel on the inside comprises a coating of silicon nitride (Si ₃ N ₄ ). Insert according to claim 3, characterized in that the coating of silicon nitride (Si ₃ N ₄ ) has a thickness in the range of 1 micron to 5 mm. Insert according to at least one of the preceding claims, characterized in that the silicon dioxide elements have a thickness in the range of 0.1 to 100 mm. Insert according to at least one of the preceding claims, characterized in that the silicon dioxide elements are joined together via a tongue and groove system. Insert according to at least one of the preceding claims, characterized in that a bonding agent layer is arranged between the silicon dioxide elements and the graphite element layer. Insert according to claim 7, characterized in that the adhesive layer has a thickness in the range of 1 micron to 4 mm. Insert according to at least one of the preceding claims, characterized in that the graphite element layer has a thickness in the range of 0.1 to 50 mm. Insert according to at least one of the preceding claims, characterized in that the graphite element layer has a multilayer structure. Use according to claim 10, characterized in that the multilayer structure of the graphite element layer is formed from film webs, wherein the film webs at an angle to each other in the range of 45 to 135 °. Insert according to at least one of the preceding claims, characterized in that the silicon dioxide elements are connected to one another via a ceramic adhesive. Insert according to claim 12, characterized in that the ceramic adhesive comprises an alkoxysilane. Insert according to at least one of the preceding claims, characterized in that the silicon dioxide elements have a purity of at least 90%. Crucible comprising at least one insert according to at least one of the preceding claims 1 to 14.

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