WO2013124166A1 - Method for the production of high-purity sio2 - Google Patents

Description

Process for producing high purity Si0 ₂

The invention relates to processes for producing high-purity SiO ₂ . Furthermore, the present invention relates to a system for carrying out the method according to the invention.

An essential cost factor in the production of electronic components, in particular of photovoltaic cells, are the expenditures for the high-purity silicon required for this purpose

Efforts are made to obtain silicon with the required degree of purity cost. A relatively inexpensive method is set forth inter alia in WO 2010/037694. 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.

For this purpose, SiO ₂ can be purified by a washing process. The purified SiO ₂ is usually ground, then with a

Carbon source, for example, added to a carbohydrate and compacted into a shaped body. The carbohydrate contained in the molded body may then be pyrolyzed to carbon to obtain a molded article which can be reduced to silicon in an electric arc furnace.

The processes known from the prior art for producing high-purity silicon already show a good property profile. However, there is a continuing need to improve these methods. Especially the production of high-purity silicon dioxide as a part of the above

presented task is a challenge.

In view of the prior art, it was an object of the present invention to provide a process for producing high purity silica (SiO ₂ ) which can be easily and inexpensively carried out. In particular, the process should be used in a plant for the purification of

Silica can be carried out over a long period of time without systemic interruptions would be necessary.

In particular, it has been an object to provide a method which can be carried out at a very high speed. In addition, the process should be able to be carried out with as few process steps as possible, whereby they should be simple and reproducible.

Furthermore, the implementation of the process should not be associated with any risk to the environment or to human health, so that the use of substances or compounds that could be harmful to the environment should be substantially avoided.

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

Furthermore, a molded body of silica should be provided, which has a particularly high strength. Further tasks not explicitly mentioned arise from the overall context of the following description, examples and

Claims.

 These tasks as well as others, which are not explicitly mentioned, are solved, however, from the contexts discussed here such as

Of course, let derive or inevitably result from these, by the method described in claim 1. Advantageous modifications of this method are referred to in claim 1

Subclaims under protection.

The subject of the present invention is accordingly a process for the production of highly pure SiO ₂ , comprising the steps

a. Preparation of an acidifying agent with a pH of less than 2;

b. Providing a silicate solution;

c. Add the silicate solution from step b. into the template from step a. such that the pH of the precipitation suspension obtained at all times remains at a value less than 2; and

d. Separating and washing the obtained silica;

which is characterized in that the silicon dioxide obtained by the precipitation is continuously or semi-continuously separated from the precipitation area and washed in a separate washing area from the precipitation area.

The inventive method can be carried out easily and inexpensively. In this case, in particular contaminants that are caused by the

Purification are minimized. Furthermore, the process can be carried out in a plant for the purification of silicon dioxide over a long period of time without system-related interruptions being necessary.

 Furthermore, a very high speed process can be performed. In addition, the process can be carried out with very few process steps, the same being simple and reproducible.

Furthermore, the implementation of the method is not associated with a risk to the environment or the health of people, so that the use of substances or compounds harmful to health, which could be associated with disadvantages for the environment, can be essentially dispensed with.

Furthermore, the starting materials used are generally inexpensive to produce or available.

According to a particular embodiment of the present invention, silica is provided, which leads to particularly stable shaped bodies.

The present process is for producing high purity silica (SiO ₂ ). In particular, the silicon dioxide obtainable by the present process can be used as a raw material for the production of metallic silicon. Furthermore, the silicon dioxide obtainable according to the present process can be used to produce SiO ₂ shaped bodies, which are advantageously used for the production of components which are used in connection with the production and further processing of metallic silicon and are familiar to the person skilled in the art. The term silicon dioxide in the context of the present invention also refers to water-containing compositions which may contain free and / or bound water. Furthermore, in small quantities, also substances which originate from the preparation or the purification may be present in these compositions. In this context, in particular on the

Requirements that the high-purity silica must meet in order to be used as intended.

The silicon dioxide to be purified according to the invention can be obtained, for example, from a silicate-containing solution, for example a water glass, by means of a

Precipitation reaction can be obtained.

A preferred precipitation of a silica dissolved in an aqueous phase, in particular completely dissolved silica, is preferably carried out with an acidulant. Upon reaction of the aqueous-phase-dissolved silica with the acidulant, preferably adding the aqueous-phase-dissolved silica 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 silica is present during the various process steps of silica production.

According to this preferred aspect, the original and the precipitation suspension into which the silica dissolved in the aqueous phase, in particular the water glass, added, preferably added dropwise, always react acid. 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 substantially constant pH is desired, then the pH should show only 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 always in significant excess to alkali silicate solution

is present to allow a pH less than 2 of the precipitation suspension at any time.

Without wishing to be bound by any particular theory, it can be assumed that a very low pH ensures that virtually no free, negatively charged SiO groups are present on the silicon dioxide surface to which interfering metal ions can be bound.

At very low pH, the surface is surprisingly even positively charged, so that metal cations are repelled from the silica surface. If these metal ions are now washed out, as long as the pH is very low, it can be prevented that they accumulate on the surface of the silicon dioxide according to the invention. Take that

Silica surface on a positive charge, so it also prevents Silica particles attach to each other and thereby cavities or gussets are formed, in which could store contaminants. This property of charge separation makes it possible for this silica, in the event that it is moved, to generate a current signal according to the rules of Maxwell for electrodynamics, and thus easily monitors and / or determines the quality of the silica from outside (contactlessly) can be.

Particularly preferred is a precipitation process for preparing purified silica, especially high purity silica, comprising

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 the preparation of 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 silica, 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.

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 0.3 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, respectively, 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 from an acidifier or a

Acidifier and water produced. The water is preferably distilled or demineralised water (demineralized water). The quality can be monitored or determined by simple conductivity measurement according to the known standards, for example. DIN. In all variants of the present method, not only in the particularly preferred embodiments described in detail above, may be described as

Acidifiers 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 in concentrated or diluted form or mixtures of

the aforementioned acids are used. 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 in a purity which is commonly referred to as

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 are introduced into the product

Procedures should be introduced, which do not remain dissolved in the aqueous phase of the precipitation suspension. In any case, the acidulants should not have any impurities that would precipitate with the silica in the acid precipitation, unless they could be added by means of added complexing agents or kept in the precipitation suspension by pH control or washed out with the subsequent washing 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. Due to the yellow / orange color of the reaction solution, the degree of purification during the washing step d. very well

be traced.

In fact, it has been found that titanium in particular is a very persistent contaminant, which readily attaches to the silica even at pH values above 2. It was found that when the yellow color disappeared in stage d. In general, the desired purity of the purified silicon oxide, in particular of the silica, is reached and the

From this point on, silica can be washed with distilled or deionised water until a neutral pH of the silica is reached. In order to achieve this indicator function of the peroxide, it is also possible for the peroxide not in step a., But in step b. the water glass or in step c. add as third stream. In principle, it is also possible for the peroxide to be present only after step c and before step d. or during step d. admit. 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 to a particular theory, it can be assumed that some - especially carbonaceous - contaminated by oxidation with the peroxide and from the

Reaction solution are removed. Other impurities are going through

Oxidation in a more soluble and thus leachable form brought. 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 silica preferably an aqueous silicate solution, more preferably an alkali and / or Erdalkalisilikatlosung, most preferably a water glass. Such solutions can be purchased commercially, made by liquefying solid silicates out

Made of silica and sodium carbonate or prepared for example by the hydrothermal process directly from silica and sodium hydroxide and water at elevated temperature. The hydrothermal process may be preferred over the soda process because it may result in 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, moreover, the water glasses after the Hydrothermalvefahren usually before precipitation on be concentrated. 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 im

Connection, 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 silica, from 1, 5 to 4.5, preferably 1, 7 to 4.2, particularly preferably from 2 to 4.0.

The precipitation process for the preparation of an SiO ₂ composition which can be used according to the invention is achieved without the use of chelating reagents or of ion exchange columns. Even calcination steps of the purified silicon oxide can be dispensed with. Thus, the present is

Felling process much easier and cheaper than prior art methods. Another advantage of the invention

Precipitation method is that it is used in conventional apparatus

can be carried out.

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 pretreated in accordance with WO 2007/106860, in order to advance the boron and / or phosphorus content

minimize. For this purpose, the alkali silicate solution (aqueous phase in which silica is dissolved) with a transition metal, calcium or magnesium, a Molybdenum salt or treated with a molybdate-modified ion exchanger to minimize the phosphorus content. Before the precipitation according to the method of WO 2007/106860, the

Alkalisilikatlösung the precipitation according to the invention in the acidic, in particular at a pH less than 2 are supplied. Preferably, in

However, the process according to the invention used acidifying agents and silicate solutions which were not treated by means of ion exchangers before the precipitation.

In a specific embodiment, a silicate solution according to the

Process of EP 0 504 467 B1 before the actual acidic precipitation according to the invention are pretreated as silica sol. This is the whole

The disclosure content of EP 0 504 467 B1 is expressly included in the present document. The silica sol obtainable according to the processes disclosed in EP 0 504 467 B1 is preferred after a treatment

completely dissolved again according to the methods of EP 0 504 467 B1 and subsequently fed to an acidic precipitation according to the invention in order to obtain purified silicon oxide according to the invention.

The silicate solution preferably has one prior to acid precipitation

Silica content of about at least 10 wt .-% or higher.

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 Silicate solution may preferably be measured at a shear rate of 10 1 / s, the temperature preferably being 20 ° C.

In step b. and / or c. the first preferred variant of the precipitation method is a silicate solution having a viscosity of 0.001 to 0.3 Pas, preferably 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 most 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 is the silicate solution from step b. placed in the template and thus precipitated the silica. It is important to ensure that the acidifier is always present 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 can additional acidulant can 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 is therefore ensured that the

Silicate solution in the form of drops in the template and / or precipitation suspension occurs. This can be achieved, for example, by introducing the silicate solution into the original by means of drops. It may be outside the

Template / precipitation suspension attached and / or dipping in the template / precipitation suspension dosing act.

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.

Without being bound by any particular theory, it can be assumed that due to the low flow rate, the incoming silicate solution only spreads little immediately after it has entered the receiver / precipitation suspension becomes. This results in rapid gelation on the outer shell of the incoming silicate solution droplets or silicate solution streams before contaminants can be trapped inside the particles. By optimal choice of the flow rate of the template / precipitation suspension thus the purity of the product obtained can be improved.

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" by the half 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, especially 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, in processes where a high flow velocity in the

Template / precipitation suspension is additionally formed very fine particles.

In the other preferred embodiment of the precipitation process, ie when using highly viscous waterglass, dropwise addition of the silicate solution also produces particularly pure 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 well filterable precipitate arises as well as that no or only very small impurities in internal Hollow spaces of the silica particles are stored because the droplet shape of the dripping silicate solution is largely retained by the high viscosity and the droplets are not finely distributed before the

Gelation / crystallization begins at the surface of the drop. When

Silicate solution may preferably be the above-defined alkali and / or alkaline earth silicate solutions are used, preferably is a

Alkalisilikatlösung, particularly preferably Nat silicate (water glass) and / or potassium silicate solution 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, more preferably 0.3 to 9 mm and most 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 tom, also referred to herein as "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 that is "mushroom-topped" or "mushroom-topped" That is, instead of the hole of the above-described "Donuf" shaped particles, in the center of the annular base structure, there is a one-sided, preferably thin, ie thinner than the annular, layer of silica which spans the inner opening of the "ring". If one were to place these particles with the curved side down on the ground and look perpendicularly from above, the particles would correspond to a bowl with a curved bottom, rather massive, d. H thick upper edge and in the area of the bulge somewhat thinner ground.

Without being bound to any particular theory, it can be assumed that the acidic conditions in the receiver / reaction solution together with the dropwise addition of the silicate solution, in addition to the viscosity and the flow rate of the template / precipitation suspension, cause the drop of silicate solution in the Contact with the acid immediately at its surface begins to gel / precipitate, at the same time by the movement of the drop in the reaction solution / original, the drop is deformed. Depending on

Reaction conditions are formed at slower drop motion obviously the "mushroom-shaped" shaped particles, at faster

Drop movements, however, the "Donuf'-shaped particles are formed.

The obtained after the precipitation of silica is in from the remaining

Components of the precipitation suspension separated. According to the present invention, this is done continuously or semicontinuously.

Semicontinuously means that at regular intervals the silica obtained by the precipitation is transferred to a washing zone which the washing takes place. In this case, the precipitation reaction can be continued.

For example, precipitation may occur until the precipitated silica in the precipitating vessel has reached a minimum level, after which a portion of the precipitated silica is transferred to a washing zone.

In a continuous process, permanent silica is transferred to a washing area, in which case a precipitation of

Silica can be carried out to a minimum level in a precipitation tank. In this case, the addition of the silica may preferably be carried out at a constant rate.

The transfer can in this case be effected by mechanical devices, for example by scrapers, wherein the precipitated silica is transferred through an opening, which may be closed, into a washing area. Optionally, a centrifuge can be used for separation.

In this case, a supernatant can form in the precipitation container, which

optionally can be separated by an overflow.

After the precipitate has been transferred to a washing area, the same is washed, it being ensured by means of a suitable washing medium that the pH of the washing medium is less than 2, preferably less than 1.5, particularly preferably less than 1, very particularly preferably 0, at the beginning of the wash. 5, and more preferably 0.01 to 0.5. The washing medium used may preferably be aqueous solutions of organic and / or inorganic water-soluble acids, such as the abovementioned 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 silicon oxide, if they are not can be completely removed with ultrapure water. In general, therefore, all organic,

water-soluble acids, in particular consisting of the elements C, H and O, both from acidulant 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 necessary, the washing medium can also be 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 includes methods in which

Stabilization of acid-soluble metal complexes of the precipitation suspension or a wash medium, a metal complexing agent, such as EDTA is added.

Optionally, it is therefore possible to add a chelating reagent to the washing medium or to precipitate the precipitated silica in a washing medium corresponding pH less than 2, preferably less than 1, 5, more preferably less than 1, most preferably 0.5 and especially preferably 0.01 to 0.5, containing a chelating reagent to stir. Preferably, however, washing with the acidic wash medium occurs immediately after separation of the silica precipitate without further steps being taken.

Also a peroxide for color marking, as an "indicator" of unwanted metal impurities, can be added. For example, can

Hydroperoxide are added to the precipitation suspension or the washing medium to color codify 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. If the precipitation process according to the invention is carried out without the addition of a peroxide which forms a yellow / orange colored compound with Ti (IV) ions, a small sample of each can be added to each washing step

Wash suspension removed and mixed with a corresponding peroxide. This process is continued until the removed sample visually shows no yellow / orange coloration after addition of the peroxide. It must be ensured that the pH of the washing medium and thus also that of the purified silicon oxide, in particular of the silicon dioxide, up to this Time is less than 2, preferably less than 1, 5, more preferably less than 1, most preferably 0.5 and especially preferably 0.01 to 0.5.

The thus-washed and purified silica is preferably further washed with distilled water or demineralized water until the pH of the obtained silica is in a range of 0 to 7.5 and / or the

Conductivity of the washing suspension is less than or equal to 100 pS / cm, preferably less than or equal to 10 pS / cm and preferably less than or equal to 5 pS / cm. In this case, the pH value can particularly preferably be in the range from 0.5 to 4.0, preferably from 0.6 to 3.5, in particular from 0.7 to 3.0 and particularly preferably from 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 silica are adequately removed.

This washing process may preferably be in a second or further

Wash area were done. Accordingly, at least two washing areas may be provided for carrying out the process, in which silica to be washed is passed continuously or semicontinuously, wherein the

Washing medium, which is used in the first washing area, has a lower pH than the washing medium, which is used in the second washing area. In this case, the conversion of the silicon dioxide to be washed into the second or further wash area can be effected by the methods set out above.

According to a preferred embodiment, it can be provided that the

Washing medium with which the silica is washed, preferably at a flow rate of 0.001 to 100 m / s, preferably 0.001 to 30 m / s, particularly preferably 0.005 to 20 m / s, more preferably 0.01 to 15 m / s, very particularly 0.1 to 10 m / s, especially preferably 0.5 to 7 m / s and very particularly preferably 2 to 5 m / s s is introduced and passed through the silica. In this way it is also possible that the silica particles can be filtered very well. In contrast, very fine particles are additionally formed at high flow rates in the template, these particles cause a bimodal distribution curve, which is particularly advantageous for the molding at a later step for the stability of the moldings.

Furthermore, it can be provided that the ratio of the volume flow of

Washing medium, with which the silica is washed, to the volume of the silica to be washed preferably in the range of 1 to 1000, preferably in the range of 2 to 100, and more preferably in the range of 3 to 10.

Furthermore, the washing may preferably be carried out with a high shear, the mean particle size of which obtained by the precipitation

Silica particles by at least 10%, preferably reduced by 30% and more preferably by at least 50%. Here, the middle

Particle size, for example, according to ISO 13320-1 (1999), Particle size analysis - Laser diffraction methods - Part 1: General principles) preferably be determined as a volume average.

When carrying out the method, provision may be made for the washing area to have a height of preferably at most 3 m, particularly preferably at most 2 m. This embodiment is particularly preferred in the embodiments in which the precipitated silica at a high speed is washed, wherein preferably a reduction of the particle size is achieved.

The preferred parameters of the washing process set out above may be achieved in a cascaded embodiment of this step, for example over 2, 3 or more stages, in a single step, preferably the later, most preferably the last wash step, in at least 2 or more wash steps or in all wash steps be respected.

According to a preferred embodiment, the washing by

Influx of the precipitate in a close-meshed screen basket with the

Washing medium can be carried out from below.

The separation can be carried out with conventional, well-known to those skilled

Measures such as filtration, decantation, centrifugation and / or sedimentation are made, with the proviso that these measures are the

Degree of contamination of the acidified, purified silica does not deteriorate again.

In the following, a system for carrying out a method according to the invention will be explained with reference to a schematically illustrated figure, without, however, limiting the invention.

In Figure 1, a system is shown, which has a precipitation area 1 into which a precipitant is introduced via supply line 2. In the embodiment described, the precipitant is introduced from below into the precipitation area 1. introduced, wherein the precipitant has a pH less than 2. Via a further feed line 3, a silicate solution is introduced from above, wherein the above-described flow rates can be adjusted, for example, via the addition of precipitant in the precipitation area 1. In this case, a formation of solid silica particles occurs, as previously explained in more detail. Excess precipitant is removed via discharge 4 from the precipitation area. The silica particles precipitated in this way are transferred continuously or semicontinuously from the precipitation area 1 via the outlet 5 into a first washing area 6. For this purpose, the precipitation area 1 may comprise a scraper or similar internals, which, however, must not lead to contamination of the precipitated silica. Accordingly, these internals, in particular a scraper, are made of materials which are removed from silica in the production of silicon and can thus be tolerated. These internals are not shown in the schematic figure 1.

The silica particles transferred into the first washing area 6 are washed with a washing medium, the flow rate and the volume of the washing medium preferably being selected according to the variables set out above. In this case, the washing medium preferably has a pH of less than 2, preferably less than 1. Furthermore, a peroxide may be added to the color mark to measure the purity of the silica. In this case, the residence time of the silica can be adjusted so that the silica before being transferred to the second or third washing area comprises no measurable by a peroxide content of titanium. In the embodiment set forth in FIG. 1, the washing medium is introduced from below via supply line 7 into the first washing area 6 and removed via discharge 8 from the upper area of the first washing area 6. The cleaned in the first washing area 6 silica particles are transferred via discharge 9 in a second washing area 10, wherein the first washing area 6 of the system set forth in Figure 1 this may also include internals, which are not shown in the figure. In the embodiment set forth in FIG. 1, the washing medium is introduced from below via supply line 11 into the second washing area 10 and removed via discharge 12 from the upper area of the second washing area 10. The pH of the washing medium in the second washing area 10 may preferably be higher than the pH of the washing medium used in the first washing area 6. In this case, the flow rate and the volume of the washing medium can preferably be selected according to the variables set out above.

The cleaned in the second washing area 10 silica particles are transferred via discharge line 13 in a third washing area 14, wherein the second washing area 10 of the system set forth in Figure 1 may also include internals for this purpose, which are not shown in the figure. In the embodiment set forth in FIG. 1, the washing medium is introduced from below via supply line 15 into the third washing area 14 and removed via discharge 16 from the upper area of the third washing area 14. The pH of the washing medium in the third washing area 14 may preferably be higher than the pH of the washing medium used in the second washing area 10. In this case, the flow rate and the volume of the washing medium can preferably be selected according to the variables set out above. The silica purified in this way can be taken off via discharge 17 of the system. For transferring the particles, the third washing area 14 may also comprise internals.

The previously described washing media and the precipitant can be purified and reused by conventional methods. In a particular embodiment, the discharge line 16 of the third washing area 14 with the supply line

1 1 of the second washing area 10 may be connected. Furthermore, the derivative

12 of the second washing area 10 to be connected to the supply line 7 of the first washing area 6. With this configuration, the washing medium can be used particularly economically.

The purified silica thus obtained, particularly high purity silica, may be dried and further processed to adjust the self-assembling SiO ₂ mass to the preferred levels of water set forth below.

Surprisingly, a silica composition can be obtained by the method according to the invention, from which a large proportion of water can be separated without much effort. Thus, the aqueous SiO ₂ can be left to stand to achieve a partial separation of the water. The content of water can surprisingly take values for a

Self-assembly of the SiO ₂ mass is sufficient, as described below. Further drying can be carried out by means of all methods and devices known to those skilled in the art, e.g. B. belt dryer, tray dryer, drum dryer, etc. take place.

A plant for carrying out the present process is novel and accordingly also the subject of the present invention.

A preferred plant has at least one precipitation area, which is connected to a silicate solution reservoir, so that a silicate solution in the reservoir can be introduced from above into the precipitation area, and at least one wash area, wherein the precipitation area comprises internals, preferably at least one scraper which can transfer a solid into a region of the precipitation region, from which this solid can be directed into the wash area in a targeted manner.

Furthermore, it can be provided that the system comprises at least two cascaded wash areas, which are connected to one another such that a solid can be transferred from a first washing area into a second washing area.

Surprisingly, it is possible by the process according to the invention to obtain silicon dioxide in a particularly simple and economical manner, which can be converted in a particularly simple manner into an SiO ₂ shaped body in any desired shape. The silicon dioxide obtainable by the process according to the invention is likewise an object of the present invention. Here, a preferred Siliciunndioxid characterized by a broad particle size distribution. Furthermore, the silicon dioxide may have a bi- or multimodal particle size distribution. The particle size distribution can preferably be determined according to ISO 13320-1 (1999), particle size analysis - laser diffraction methods - Part 1: General principles).

A silica according to the invention can surprisingly be formed into particularly solid, stable shaped bodies.

In this case, the silica may be provided with a binder and compacted in a conventional manner to a shaped body. Surprisingly, that can

Silica also be self-organizing, wherein a flowable hydrous SiO ₂ mass can be poured from this self-organizing silica in a mold. The hydrous, self-assembling SiO ₂ mass preferably has a pH in the range of 0.1 to 4.0, preferably 0.2 to 3.5, in particular 0.5 to 3.0 and particularly preferably 1, 0 to 2 , 5 on.

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 term "SiO ₂ mass" refers to a composition comprising SiO ₂ with different proportions of free and / or bound water, the degree of condensation of the silica for this composition not essential per se. Accordingly, the term "SiO ₂ mass" also includes compounds having SiOH groups, which are also commonly known as polysilicic acids

can be designated.

A preferred water-containing SiO ₂ composition is self-organizing. The term "self-organizing" indicates that one for the present

Process suitable water-containing SiO ₂ composition can be reversibly converted from a solidified to a flowable state. Hereby finds

Preferably, no permanent phase separation takes place to a greater extent, so that the water is distributed substantially uniformly in the SiO ₂ phase when viewed macroscopically. However, be in this

Context noted 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 viscosity meter from Thermo Haake

Use of 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 mass in a

Initial viscosity of preferably at least 30 Pas, more preferably at least 100 Pas. This value is determined by the viscosity value of the rheometer 1 second after start of the vane rotor of the

Rotation rheometer at about 23 ° C and a shear rate of 10 [1 / s] is used.

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, inter alia, 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 in accordance with this aspect of the invention for liquefying the water-containing SiO ₂ mass is known in the

Generally dependent on the shear force required for liquefaction.

Surprising advantages can be achieved inter alia by an apparatus whose shear rate (indicated as 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. These In the case of ultrasonic liquefaction, it is quite possible to achieve 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.

To solidify the water-containing SiO ₂ composition, it may 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 composition or mass is not subjected to shear forces Furthermore, solidification may be effected or accelerated, for example by introduction of energy, preferably heating or addition of additives , such as silanes, especially functional silanes and here without 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 masses used for this purpose need not necessarily

self-organizing, however, 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.

Surprising 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 more 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 purified silica which is obtainable by the present process and which in particular can serve as a self-assembling SiO 2 mass can, according to a preferred aspect, have a very high purity.

A preferred pure silica is characterized by having a content as measured by IPC-MS and known to those skilled in the art

Sample preparation at: a. Aluminum less than or equal to 1000 ppm, preferably between 100 ppm and 0.0001 ppm, more preferably between 10 ppm and 0.0001 ppm;

b. Boron less than 10 ppm to 0.0001 ppm;

c. Calcium less than 8 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 8 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.

A preferred high-purity silicon dioxide is characterized in that the sum of the o. G. Impurities (ai) less than 1000 ppm, preferably less than 100 ppm, more preferably less than 10 ppm, most preferably less than 5 ppm, more preferably between 0.5 to 3 ppm, and most preferably between 1 to 3 ppm, wherein for each Element, in particular the

Metal elements a purity in the range of detection limit can be sought. The data in ppm, refer to the weight.

The determination of impurities is carried out by means of ICP-MS / OES

(Induction Coupling Spectrometry - Mass Spectrometry / Optical

Electron spectrometry) as well as AAS (atomic absorption spectroscopy)

carried out.

For producing SiO ₂ shaped bodies which can be brought into contact with carbon-containing compounds in order to obtain metallic silicon therefrom For example, a pellet mold in sizes suitable for use in an electric arc furnace can be cast. Preferably, these pellets have no corners and edges to minimize abrasion.

Suitable pellets may, inter alia, have a cylindrical shape with rounded corners, which more preferably have a diameter in the range of 25 to 80 mm, particularly preferably 35 to 60 mm, with a length to diameter ratio (L / D) of preferably 0.01 to 100 , in particular 0.1 to 2 and particularly preferably 0.5 to 1, 2. Furthermore, preferred pellets may be in the form of truncated cones with rounded corners or hemispheres. The size of the SiO ₂ shaped bodies is preferably in the range from 0.001 to 100 000 cm ^3, in particular 0.01 to 10 000 cm ³ , more preferably 0.1 to 1 000 cm ³ , especially preferably 1 to 100 cm ³ , in particular for one 500 kW oven. The size depends directly on the process management. The molds may be adapted depending on the method and technical aspects, for example as a type of ballast or gravel, with a pebble briquette being preferred when fed through a pipe. A gravel can be an advantage if added directly.

The casting molds to be used for the production of the moldings are not subject to special requirements, but their use should not result in impurities entering the SiO ₂ shaped body. 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 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 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,

Hordentrockner, drum dryers are used, which dry continuously or batchwise.

Advantageously, the SiO ₂ shaped bodies can be dried to a water content which allows non-destructive demolding from the casting molds. Demgennäß the drying in the Gießfornn can be carried out to a water content of less than 60 wt .-%, in particular less than 50 wt .-% and particularly preferably less than 40 wt .-%.

Drying to a water content which is below the stated values can be carried out particularly preferably after removal of the SiO ₂ shaped body, wherein the dryers described above can be used.

Among other things, surprising advantages are exhibited by SiO ₂ shaped bodies which, after drying, have a water content in the range from 0.0001 to 50% by weight,

preferably 0.0005 to 50 wt .-%, in particular 0.001 to 10 wt .-% and particularly 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 composition 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 at

Normal conditions (ie at atmospheric pressure) take place.

The pressure at which the drying takes place can be in a wide range, so that the drying can be carried out under reduced or elevated pressure. For economic reasons, a drying at ambient or

Normal pressure (950 to 1050 mbar) may be preferred.

To increase the hardness of the dried SiO ₂ shaped body, the same can be thermally densified or sintered. This can be, for example Batchwise in conventional industrial furnaces, for example, shaft kilns or

Microwave sintering furnaces or continuously, for example, in so-called

Durchstoßöfen or shaft furnaces are performed.

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 shaped body. 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 shaped bodies with the previously

described typical dimensions, for example, a

Compressive strength (indicated as breaking strength) of at least 10 N / cm ² , preferably of more than 20 N / cm ² , wherein particularly sintered SiO ₂ shaped bodies can show compressive strength values of at least 50 or even at least 150 N / cm ² , in each case measured by Compression tests on an arrangement for pressure strength tests.

The density of the SiO ₂ shaped body can be matched to the intended use. In general, the SiO ₂ shaped body may have a density in the range of 0.6 to 2.5 g / cm ³ . In a high-temperature sintering even a density of 2.65 (quartz glass density) can be achieved. In a SiO ₂ shaped body for the production of metallic silicon, in one possible embodiment Preferably, an amorphous structure with a high inner surface of the body sought to a good and uniform contact of the later, for example.

to ensure carbon source introduced with the silica. According to this aspect of the present invention, preferred SiO ₂ shaped bodies have a density in the range of 0.7 to 2.65 g / cm ³ , in particular 0.8 to 2.0 g / cm ³ , preferably 0.9 to 1.9 g / cm ³ and more preferably 1, 0 to 1, 8 g / cm ³ . The density, as stated, refers to that of the shaped body, so that the pore volume of the shaped body is included for the determination.

Furthermore, the specific surface area of preferred SiO ₂ shaped bodies for producing metallic silicon prior to compaction can be in the range from 20 to 1000 m ² / g. After thermal densification, the specific surface area is preferably in the range from 50 to 800 m ² / g, preferably in the range from 100 to 500 m ² / g and particularly preferably in the range from 120 to 350 m ² / g, measured according to BET value. Method. The specific nitrogen surface area (referred to hereinafter as the BET surface area) of the SiO ₂ shaped body is determined according to ISO 9277 as a multipoint surface. As a measuring device that serves

Surface measuring device TriStar 3000 from Micromeritics. The BET surface area is usually in a partial pressure range of 0.05 - 0.20 of the

Saturation vapor pressure of the liquid nitrogen determined. The

Sample preparation is carried out, for example, by tempering the sample for one hour at 160 ° C. under reduced pressure in the baking station VacPrep 061 of the company

Micromeritics.

According to a further embodiment, the SiO ₂ shaped body may preferably have a higher density, preferably a density of at least 2.2 g / cm ³ , more preferably at least 2.4 g / cm ³ . This embodiment can For example, be used for the production of crucibles in which metallic silicon is purified by directional solidification.

The density and the specific surface area of the dried shaped bodies, for example the pellets, can be controlled inter alia via the shear penetration, the pH, the temperature and / or the water content in the SiO ₂ casting compound. If the proportion of water is comparable, it is possible, for example, to increase the pellet density by increasing the shear input. Furthermore, the density can be adjusted via the pH and the solids content of the SiO ₂ mass, wherein a decrease in the density is associated with a decrease in the solids content. Another significant influence on the density or porosity of the

Shaped body can be achieved in the subsequent sintering step. In this case, especially the maximum sintering temperature of importance, as well as the holding time at this temperature. With increasing sintering temperature and / or holding time, higher densities of the shaped bodies can be achieved.

Depending on the application, the SiO ₂ shaped body can be further processed.

According to a preferred embodiment, the SiO ₂ shaped body can be brought into contact after sintering with a carbon-containing compound. For this purpose, as pure carbon source one or more pure

Carbon sources may optionally be used in a mixture of an organic compound of natural origin, a carbohydrate, graphite (activated carbon), coke, coal, carbon black, carbon black, thermal black, pyrolyzed carbohydrate, in particular pyrolyzed sugar. The carbon sources, especially in pellet form, can be purified, for example, by treatment with hot hydrochloric acid solution. In addition, the inventive method a

Activator can be added. The activator may serve the purpose of a Reaction initiator, reaction accelerator as well as the purpose of

Exercise carbon source. An activator is pure silicon carbide, silicon infiltrated silicon carbide, and a pure silicon carbide having a C and / or silica matrix, for example, a carbon fiber-containing

Silicon carbide.

For loading, the SiO ₂ shaped body can be combined with the carbon-containing compounds mentioned, preferably carbon black (industrial carbon black, carbon black), in particular thermal black, black carbon black or carbon black according to the Kvasrner process known to those skilled in the art. and / or a carbohydrate, more preferably one or more mono- or disaccharides. The introduction of these carbon-containing compounds can be effected via solutions and / or dispersions of these carbon-containing compounds.

Preferably, a porous SiO ₂ shaped body, which preferably has a density and / or specific surface with the values set out above, can be impregnated with an aqueous composition containing at least one

Carbohydrate and / or carbon black has. To record the

To improve composition in the porous body, the same can be previously exposed to a vacuum or a vacuum to remove the gas contained in the pores. Subsequently, the thus obtained, with

At least one carbonaceous compound provided SiO ₂ shaped bodies are brought to a temperature greater than 500 ° C to pyrolyze the carbonaceous compound.

According to a further aspect of the present invention, preferred SiO ₂ shaped bodies can be used to produce crucibles in which metallic silicon can be purified by directional solidification. These Crucibles usually have a multi-layered structure, wherein the outermost layer ensures mechanical stability. This layer can be constructed, for example, of graphite. The further layer provides a chemical separation between the metallic silicon and the supporting layer. This further layer is preferably formed by silicon dioxide, which can be particularly preferably provided with a layer of Si ₃ N ₄ .

The above-described molded articles which are obtainable according to the present process are novel and likewise the subject of the present invention.

The SiO ₂ shaped bodies set forth above are preferably used in processes for the production of metallic silicon, as can be used for example for the production of solar cells.

The definitions of metallurgical and solar silicon are well known. Thus, solar silicon has a silicon content of greater than or equal to 99.999% by weight.

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

Claims

claims 1 . Process for the preparation of high purity SiO ₂ , comprising the steps a. Preparation of an acidifying agent with a pH of less than 2;  b. Providing a silicate solution;  c. Add the silicate solution from step b. into the template from step a. such that the pH of the precipitation suspension obtained at all times remains at a value less than 2; and  d. Separating and washing the obtained silica;  characterized in that  the silica obtained by the precipitation is continuously or semi-continuously separated from the precipitation area and washed in a washing area separate from the precipitation area. 2. The method according to claim 1, characterized in that the washing is carried out with a high shear, wherein the mean particle size of the silica particles obtained by the precipitation is reduced by at least 30%. 3. The method according to claim 1 or 2, characterized in that the washing area is constructed cascade-shaped, so that at least two washing area are provided, is passed into the continuously or semi-continuously to be washed silica. 4. The method according to at least one of the preceding claims, characterized in that a part of the washing medium at least temporarily has a pH of less than 2. 5. The method according to any one of the preceding claims, characterized in that at least two washing areas are provided, is passed into the continuously or semicontinuously to be washed silica, wherein the washing medium, which is used in the first washing area, having a lower pH than the washing medium used in the second washing area. 6. The method according to claim 5, characterized in that the washing medium, which is used in the second washing area, has a pH in the range of 0.5 to 4. 7. The method according to at least one of the preceding claims, characterized in that the washing area has a height of at most 2 m. 8. The method according to any one of the preceding claims, characterized in that the washing medium, with which the silica is washed, with a flow rate of 0.001 to 100 m / s is passed through the silica. 9. The method according to any one of the preceding claims, characterized in that the ratio of flow rate of the washing medium, with which the silica is washed, to the volume of the silica to be washed in the range of 1 to 1000. 10. The method according to any one of the preceding claims, characterized in that the washed, aqueous silica is allowed to stand in order to achieve a partial separation of the water. 1 1. Plant for carrying out a method according to at least one of claims 1 to 10, characterized in that the plant has at least one precipitation area which is connected to a reservoir for a silicate solution, so that a silicate solution located in the reservoir can be introduced from above into the precipitation area , and at least one washing area, wherein the precipitation area comprises internals, preferably at least one scraper, which can transfer a solid to a region of the precipitation area, from which this solid can be selectively directed into the washing area. 12. Plant according to claim 1 1, characterized in that the plant comprises at least two cascaded washing areas, which are interconnected so that a solid can be transferred from a first washing area in a second washing area. 13. A silica obtainable by a process according to any one of claims 1 to 10. 14. A silica according to claim 10, characterized in that the silica has a broad particle size distribution. 15. Siliciunndioxid according to claim 13 or 14, characterized in that the silica has a bimodal or multimodal particle size distribution.