WO2014060250A1 - Method for producing high-purity silicon nitride - Google Patents

Abstract

The present invention relates to a method for producing high-purity silicon nitride, a silicon nitride having a specific impurity profile, and use thereof, in particular for producing high-purity silicon.

Description

 Process for producing high purity silicon nitride

The present invention relates to a novel process for producing high-purity silicon nitride (S1 ₃ N ₄ )

Silicate solutions, a new high-purity S1 ₃ N ₄ with a special impurity profile and its use, in particular for the production of solar grade silicon.

The share of photovoltaic cells in the world

Electricity production has been steadily rising for several years. In order to further expand its market share, it is

It is essential that the production costs of photovoltaic cells are reduced and their effectiveness is increased.

A major cost factor in the production of

Photovoltaic cells are the cost of high purity silicon (solar silicon). This is becoming large-scale

usually developed after more than 50 years ago

Siemens process produced. In this method, silicon first with gaseous hydrogen chloride at 300- 350 ° C in a fluidized bed reactor to trichlorosilane

(Silica chloroform) reacted. After elaborate

Distillation steps, the trichlorosilane in the presence of hydrogen in a reversal of the above reaction on heated Reinstsiliziumstäben at 1000-1200 ° C again

thermally decomposed. The elemental silicon grows on the rods and the liberated hydrogen chloride is returned to the circulation. As a by-product falls

Silicon tetrachloride, which either converted to trichlorosilane and recycled in the process or in the

Oxygen flame is burned to fumed silica.

A chlorine-free alternative to the above method is the decomposition of monosilane, which also from the

Elements can be obtained and after a

Cleaning step on heated surfaces or at

Passing through fluidized bed reactors decays again. Examples of this can be found in WO 2005118474 AI. The obtained in the above-described ways polycrystalline silicon (polysilicon) is for the production of

Solar panels suitable (so-called solar silicon) and has a purity of over 99.99%. The previously described

However, processes are very expensive and energy-intensive, so that there is a high demand for more cost-effective and more effective processes for the production of solar grade silicon.

Alternatively, it is proposed in the prior art

Solar silicon by means of carbothermic reduction of

Produce silicon dioxide. In this regard suggests

 Example WO 2007/106860 Al before, high purity silica gels by reaction with a high purity carbon source in

Convert solar silicon. This direct carbothermic

Conversion of silicon dioxide to silicon has the disadvantage that the product obtained has a high proportion of carbon and silicon carbide, so that a complex decarburization of the silicon must be carried out.

JP Murray, in Solar Energy, 80, 2006, 1349-1354, proposes in his paper entitled "Silicon and solar-grade silicon production by solar dissociation of S 1 ₃ N ₄ ", highly pure

Silicon dioxide first to convert it into silicon nitride and then from this by thermal decomposition solar silicon

manufacture. However, Murray does not indicate how high-purity solar grade silicon can be obtained in sufficient quality and at economically reasonable prices.

Because silicate solutions in very large quantities as very

low-cost raw material are present, there was no lack of attempts in the past, high-purity S 1O ₂

Produce silicate solutions. Thus, in US 4,973,462 processes have been described in which highly viscous water glass at low pH of the reaction solution with a

Acid agent was converted to S 1O ₂ . This S ₁ O ₂ was then filtered, washed with water, in a mixture of acid, water and a chelating reagent

resuspended, filtered several times and washed. In JP02- 311310 a similar process was described, but this was already in the precipitation reaction

Chelating added. These two methods have the disadvantage that they are a very expensive

Reprocessing procedure. It has also been shown that the precipitates obtained after the precipitation are sometimes difficult to filter. Finally, additional costs for the chelating reagent and its separation from the

Silicon dioxide on. Thus, there is still a need for an effective and inexpensive process for producing high purity

Silicon dioxide, of high purity silicon nitride and of

Solar grade silicon.

A particular object of the present invention was to provide a process for producing high-purity silicon nitride suitable for decomposition to solar silicon, which does not have at least some disadvantages of the abovementioned processes or only in reduced form. The object of the invention was in particular also to provide novel highly pure S1 ₃ N ₄ , which

exceptionally well suited for the production of solar grade silicon. Other tasks not explicitly mentioned arise from the overall context of the following description and the claims.

The foregoing and other objects are achieved by the method and the products resulting from its application in the specification and claims.

The inventors have surprisingly found that it is possible by a simple process procedure to obtain in a simple way, without a multiplicity of additional purification steps, e.g. Calcining or chelating, and without high

expenditure on equipment to produce high-purity silicon dioxide, which, in particular using special measures, can be converted into high-purity silicon nitride, which is particularly suitable for the production of solar grade silicon.

An essential feature of the process of silica production is the control of the pH of the silica and the reaction media in which the silica is present during the respective partial process steps. Without being bound by any particular theory, the inventors believe that due to a very low pH of the reaction medium

ensures that ideally no free, negatively charged SiO groups on the silica surface

are present, can be tied to the disturbing metal ions. At very low pH, the surface is even positively charged, so that metal cations of the

 Silicon dioxide surface are repelled. Be this

Metal ions now washed out, as long as the pH is very low, it can be prevented that they attach to the surface of the silica according to the invention. If the silicon dioxide surface assumes a positive charge, then it is also prevented that silica particles adhere to one another and thus cavities are formed in which impurities could accumulate. The invention

Procedure thus comes without the use of

Chelating reagents or ion exchangers. Even calcination steps can be dispensed with. Thus, the present method is much simpler and

less expensive than prior art methods.

Another advantage of the method according to the invention is that it can be carried out in conventional apparatuses.

The present invention therefore relates to a process for producing high-purity silicon nitride, comprising the steps mentioned below Production of high purity silica by

 Precipitation of at least one silicate solution with

 at least one acidulant, wherein the pH of the precipitate, the precipitation suspension and the washing medium, at least in the first washing step, less than 2, preferably less than 1.5, more preferably less than 1, completely

 more preferably less than 0.5.

Reaction of the high-purity silicon dioxide from step a with at least one high-purity carbon source and at least one nitrogen source to give highly pure S 1 ₃ N ₄ .

The present invention also provides a process for producing high purity silicon comprising the above-mentioned.

Process for the preparation of S 1 ₃ N ₄ and additionally at least the process step c. thermal decomposition of high purity S 1 ₃ N ₄ to high purity silicon

Further subject of the present invention is a high-purity silicon nitride (S 1 ₃ N ₄ ), characterized in that it defines the following for highly pure S 1 ₃ N ₄

Contamination profile has.

Finally, the subject of the present invention is the use of the silicon nitride according to the invention. Preference is given to the use of the silicon nitride according to the invention as a coating material, in particular as a separating layer or to avoid contamination by the substrate coated with silicon nitride, which may in particular contain silicon dioxide or graphite. Furthermore, this is suitable

Silicon nitride according to the invention in a special way

Raw material for the production of ceramic materials for high mechanical and thermal loads. The present invention will be described in detail below

described. First, however, some important terms are defined.

As high-purity silicon nitride or high-purity silicon dioxide or high-purity silicon or high-purity carbon or

Highly pure carbon source, the respective substance is understood to have a following impurity profile: a. Aluminum less than or equal to 5 ppm or between 5 ppm and

0.0001 ppt, in particular between 3 ppm to 0.0001 ppt, preferably between 0.8 ppm to 0.0001, ppt, more preferably between 0.6 ppm to 0.0001 ppt, even better between 0.1 ppm to 0 , 0001 ppt, most preferably between 0.01 ppm and 0.0001 ppt, more preferably

1 ppb is up to 0.0001 ppt,

 b. Boron below 10 ppm to 0.0001 ppt, in particular in the range of 5 ppm to 0.0001 ppt, preferably in the range of 3 ppm to 0.0001 ppt, or more preferably in the range of 10 ppb to 0.0001 ppt, more preferably in Range from 1 ppb to 0, 0001 ppt

 c. Calcium less than or equal to 2 ppm, preferably between 2 ppm and 0.0001 ppt, in particular between 0.3 ppm and 0.0001 ppt, preferably between 0.01 ppm and 0.0001 ppt,

 more preferably between 1 ppb to 0.0001 ppt, d. Iron less than or equal to 20 ppm, preferably between 10 ppm and 0.0001 ppt, in particular between 0.6 ppm and 0.0001 ppt, preferably between 0.05 ppm and 0.0001 ppt,

 more preferably between 0.01 ppm to 0.0001 ppt, and most preferably from 1 ppb to 0.0001 ppt;

e. Nickel less than or equal to 10 ppm, preferably between 5 ppm and 0.0001 ppt, in particular between 0.5 ppm and 0.0001 ppt, preferably between 0.1 ppm and 0.0001 ppt,

 more preferably between 0.01 ppm to 0.0001 ppt, and most preferably between 1 ppb to 0.0001 ppt f. Phosphorus less than 10 ppm to 0.0001 ppt, preferably

 between 5 ppm to 0.0001 ppt, in particular less than 3 ppm to 0.0001 ppt, preferably between 10 ppb to 0.0001 ppt and very particularly preferably between 1 ppb to 0.0001 ppt

 G. Titanium less than or equal to 2 ppm, preferably less than or equal to 1 ppm to 0.0001 ppt, in particular between 0.6 ppm to

0.0001 ppt, preferably between 0.1 ppm to 0.0001 ppt, more preferably between 0.01 ppm to 0.0001 ppt, and most preferably between 1 ppb to 0.0001 ppt. H. Zinc less than or equal to 3 ppm, preferably less than or equal to 1 ppm to 0.0001 ppt, in particular between 0.3 ppm to 0.0001 ppt, preferably between 0.1 ppm to 0.0001 ppt,

 more preferably between 0.01 ppm to 0.0001 ppt and most preferably between 1 ppb to 0.0001 ppt, wherein for each element a purity in the

Detection limit can be sought.

In the case of high purity silicon or high purity

Silicon nitride or high purity carbon is the

Total contamination with the aforementioned elements preferably less than 100 ppm by weight, more preferably less than 10 ppm by weight and very particularly preferably less than 5 ppm by weight. The sum of the impurities contained in the high purity silicon refers to the immediate process product of the melt. Particularly preferably, the resulting high-purity silicon is suitable as solar grade silicon. In the case of the high-purity silicon dioxide, the sum of the abovementioned impurities plus sodium and potassium is preferably less than 100 ppm, particularly preferably less than 50 ppm, very preferably less than 1 ppm, especially preferably less than 5 ppm, very particularly preferably from 0.5 to 3 ppm and in particular 1 ppm to 3 ppm.

In step a. The process according to the invention first produces high-purity silicon dioxide, the step a. preferably comprises the subsequent steps

 a.a. Producing a Fällvorlage from a

 Acidulant or an acidulant and water having a pH of less than 2, preferably less than 1.5, more preferably less than 1, most preferably less than 0.5 a.b. Providing a silicate solution with a viscosity of 0.1 to 10,000 poise a.c. Common addition of the silicate solution from step a.b. with an acidifier in the template from step a.a or sole addition of the silicate solution

 Step a.b. in the template from step a.a such that the pH of the resulting precipitation suspension 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 remains a.d. Optional separation of the silica a.e. Washing the resulting silica, wherein the

Washing medium, at least in the first washing step has a pH of less than 2, preferably less than 1.5, more preferably less than 1 and most preferably less than 0.5 af Optional drying of the resulting silica. In the different variants of the invention

Method is added or used water. In which

Water is preferably distilled or deionized or deionized water.

In all variants of the process according to the invention, the acidulants used can be organic or inorganic acids, preferably mineral acids, particularly 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.

In particular, hydrochloric acid, preferably 2 to 14 N, more preferably 2 to 12 N, most preferably 2 to 10 N, especially preferably 2 to 7 N and most preferably 3 to 6 N, phosphoric acid, preferably 2 to 59 N, particularly preferred 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 1 to 24 N, more preferably 1 to 20 N, most preferably 1 to 15 N, especially 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 are used. All

Sulfuric acid is particularly preferably used.

The acidulants may be used in a purity which is commonly referred to as a "technical grade." This is one of the particular advantages of

inventive method. By the special

 Reaction procedure and reaction control in process step a., I. the production of high purity silicon dioxide, it is namely for the first time succeeded from commercially available starting materials without special purification steps such as chelation or

Ion exchange to produce high purity silicon dioxide. This leads to a considerable reduction in the production costs of the silicon dioxide. Further, a silicon dioxide can be obtained which is not available in such purity as natural quartz.

In step a.a. Acid used is preferably the acidulant, which is also used in step a.e. is used for washing the filter cake.

In a preferred variant of the method according to the invention, in step a.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 coloring of the

Reaction solution, the degree of purification during the washing step a.e. be understood very well. In fact, it has been found that titanium in particular is a very stubborn contaminant, which readily accumulates at the silicon dioxide even at pH values above 2. The inventors have found that when the yellow / orange coloration disappears in stage a.e. usually the desired purity of the silicon dioxide is achieved and the

 From this point on, silicon dioxide can be washed with distilled or demineralized water until a preferably neutral pH of the silicon dioxide is reached. Around

It is also possible that the peroxide is not reached in step a.a., but in step a.b. the water glass or in step a.e. as another

Add substance flow. In principle, it is also possible the peroxide after step a.e. and before step a.d. or a.e. or during step a.e. admit. All variants mentioned above and mixed forms thereof are objects of

present inventions. However, preference is given to the variants in which the peroxide is added in step aa or ab, since in this case it can exert a further function in addition to the indicator function. Without being bound by any particular theory, the inventors believe that some - especially carbonaceous contaminants

Oxidized reaction with the peroxide and removed from the reaction solution. Other impurities are going through

Oxidation in a more soluble and thus leachable form brought. The inventive method 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 can

commercially obtained by liquefaction of solid silicates, made of silica and sodium carbonate

made or for example the over

 Hydrothermal process directly from silicon dioxide and

Sodium hydroxide and water are prepared at elevated temperature. The hydrothermal process can be compared with the

Soda method may be preferred because it can lead to cleaner precipitated silica. A disadvantage of

 Hydrothermal process is the limited bandwidth

available modules, for example, the modulus of S1O ₂ to Na ₂ <0 in this case in the shelf up to 2, where in

inventive method but modules from 2 to 4

are preferred. In addition, the water glasses after the

Hydrothermal driving usually precedes a precipitation

be concentrated. In general, the expert is the

Production of water glass known as such. According to an alternative, an alkali water glass,

in particular sodium water glass or potassium water glass,

if necessary, filter and subsequently concentrate if necessary. The filtration of the waterglass or of the aqueous solution of dissolved silicates, for the separation of solid, undissolved constituents can, according to the person skilled in the art per se

known methods and known to those skilled in the art

Devices take place. The silicate solution used preferably has a modulus, ie weight ratio of metal oxide to silicon dioxide, of from 1.5 to 4.5, preferably from 1.7 to 4.2, particularly preferably from 2 to 4.0.

The precipitation method according to the invention comes 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 precipitation process is much simpler and less expensive than prior art processes. However, the present invention does not exclude such steps, i. also includes methods in which such steps are performed. Another advantage of

The precipitation method according to the invention is that it can be carried out in conventional apparatuses.

The use of ion exchangers to purify the silicate solutions and / or acidulants prior to precipitation is not necessary, may vary depending on the quality of the aqueous

But silicate solutions prove useful. Therefore, an alkaline silicate solution can also be pretreated according to WO 2007/106860 in order to minimize the boron and / or phosphorus content in advance. For this purpose, the alkali silicate solution (aqueous phase is dissolved in the silica) with a

 Transition metal, calcium or magnesium, a molybdenum salt or modified with a molybdate salt

Ion exchanger for minimizing the phosphorus content

be treated. Before the precipitation according to the method of WO 2007/106860, the alkali silicate solution of

Precipitation according to the invention in acid, especially at a pH less than 2 are supplied. Preferably, however, acidulants and

Silicate solutions used which were not treated by means of ion exchangers prior to precipitation.

The silicate solution preferably has a silica content of about at least 10% by weight or higher prior to acid precipitation. Preference is given to a silicate solution, in particular a

Sodium water glass, used whose viscosity is from 0.1 to 10,000 poise, preferably 0.2 to 5000 poise, especially 0.3 to 3000 poise, especially preferably 0.4 to 1000 poise (at room temperature, 20 ° C).

According to a first particularly preferred variant of

A process of the invention is a process which comprises silicate solutions of low to medium viscosity

is performed, i. wherein step a.b. as follows

is changed: a.b. Providing a silicate solution with a

 Viscosity of 0.1 to 2 poise

According to a second particularly preferred variant of

The invention relates to a method which with silicate solutions of high or very high viscosity

is performed, i. wherein step a.b. as follows

is changed: a.b. Providing a silicate solution with a

 Viscosity from 2 to 10,000 poise

In step a.b. and a.c. The first preferred variant of the main aspect method is a silicate solution with a

Viscosity of 0.1 to 2 poise, preferably 0.2 to 1.9 poise, especially 0.3 to 1.8 poise and especially preferably 0.4 to 1.6 poise and most preferably 0.5 to 1.5 poise

provided. Mixtures of several silicate solutions can also be used.

In step a.b. and a.c. The second preferred variant of the main aspect method is a silicate solution with a

Viscosity of 2 to 10,000 poise, preferably 3 to 7,000 poise, especially 4 to 6,000 poise, especially preferably 4 to 1,000 Poise, especially 4 to 100 poise and

more preferably 5 to 50 poise provided. An example of a highly concentrated water glass with high

Viscosity is the water glass 58/60 with a density of 1.690 - 1.710, a Si0 ₂ ~ content of 36 - 37 wt.%, A Na ₂ <0

Content of 17.8 - 18.4 wt.% And a viscosity at 20 ° C of about 600 poise as described in Ullmanns Encyclopedia of

Chemistry, 4th revised and expanded edition, Volume 21, Verlag Chemie GmbH, D-6940 Weinheim, 1982, page 411

is described. There are also general

Instructions for making high-viscosity water glasses. Another example is a water glass from VAN BAERLE CHEMISCHE FABRIK, Gernsheim, Germany, with a viscosity of 500 poise, 58-60 porosity, density of 1.67-1.71, Na ₂ O content of 18%, Si0 ₂ content of 37.0% water content of approx. 45.0%, weight ratio Si0 ₂ / aO approx. 2.05, molar ratio Si0 ₂ / aO approx. 2.1. PQ Corporation offers water glasses with viscosities of, for example, 15 and 21 pois. The skilled person is known to be highly concentrated

Silicate solutions by concentrating less viscous

 Silicate solutions or by dissolving solid silicates in water can produce. An example of a water glass with a viscosity of 5-6 poise is disclosed in the examples of the invention.

In step a.c. In the process according to the invention, the silicate solution from step a.b. alone or together with an acidifier in the template and thus the

Silica precipitated. The addition takes place in such a way that the pH of the reaction solution is always less than 2, preferably less than 1.5, more preferably less than 1, most preferably less than 0.5 and especially preferably 0.001 to 0.5. If necessary, additional acidulant can be added to control the pH. The temperature of the reaction solution is preferably maintained at 20 to 95 ° C, more preferably 30 to 90 ° C, most preferably 40 to 80 ° C during the addition of the silicate solution by heating or cooling the precipitation vessel. The inventors have found that especially good

Filterable precipitates are obtained when the

Silicate solution in drop form in the template and / or

Fällsuspension occurs. In a preferred embodiment of the present invention, 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 dropping the silicate solution by means of a spray applied outside the template / precipitation suspension and / or immersed in the receiver / precipitation suspension

Metering unit is introduced. Suitable aggregates such as e.g. Spraying units, drop generators, Prillteller are the

Specialist known.

In the first particularly preferred variant of the

Main aspect method, i. In the low viscous waterglass process, it has been found to be particularly advantageous to agitate the master / precipitation suspension, e.g. by stirring or pumping over that

Flow rate 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, more preferably 0.01 to 5 m / s, more preferably 0.01 to 4 m / s, especially preferably 0.01 to 2 m / s and most preferably 0.01 to 1 m / s.

Without being bound to a particular theory, the inventors believe that the low

Flow rate the incoming silicate solution

is distributed only slightly after entry into the submission / precipitation suspension. This results in the outer shell of the incoming silicate solution droplets or

Silicate solution streams for rapid gelation 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

obtained product 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 method according to the invention, in which the silicate solution in droplet form in a template / precipitation suspension at a flow rate, measured in a range of half by Radius of the precipitation tank ± 5 cm and surface of the reaction solution to 10 cm below the

Reaction surface is limited, from 0.001 to 10 m / s, preferably 0.005 to 8 m / s, more 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 is introduced. In this way it is also possible

To produce silica particles which can be filtered very well (see Figures 1 and 2). In contrast, in processes in which a high

 Flow rate in the template / precipitation suspension

if very fine particles are formed, these particles are very difficult to filter.

In the second preferred embodiment of the main aspect of the present invention, i. when using

highly viscous water glass also arise through

dropwise addition of the silicate solution particularly clean and easily filterable precipitates. Without being bound by any particular theory, the inventors believe that the high viscosity of the silicate solution, along with the pH, causes it to be readily filterable after step c)

Precipitation occurs as well as that no or only very small impurities in internal cavities of the

 Silica particles are stored, since the droplet shape of the dripping silicate solution is largely retained by the high viscosity and the droplets are not finely divided before the gelation / crystallization begins on the surface of the droplet.

As stated above, by suitable choice of the viscosity of the silicate solution and / or the stirring speed

Filterability of the particles can be improved because particles to be obtained with a special shape. The subject of the present invention are therefore purified

Silica 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 special

Embodiment of the present invention have this

Silica particles have a ring shape, i. have a "hole" in the middle (see Figure la) and are thus similar in shape to a miniature "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 according to the invention, these have

 Silica particles have a shape that is comparable to a "mushroom head" or a "jellyfish". That instead of the hole of the above-described "donut" -shaped particles is in the center of the annular base structure a curved one side, preferably thinner, ie thinner than the annular part, layer of silicon dioxide (see Figure 2a), which the inner opening of the Spans "rings". If one were to place these particles with the curved side down on the ground and look perpendicularly from above, then the particles would correspond to a bowl with a curved bottom, rather solid, i. thick upper edge and in the area of the vault slightly thinner ground.

Without being bound by any particular theory, the inventors believe that the acidic conditions in the

 Template / reaction solution together with the drop-shaped 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 on contact with the acid begins immediately on its surface

gelled / precipitate, at the same time by the movement of the drop in the reaction solution / template, the drop is deformed. Depending on the reaction conditions, the "mushroom head" evidently forms with slower drop movement. shaped "particles, with faster drop movements, however, the" donut "-shaped particles are formed.

After step a.c. Silica obtained is preferably in step a.d. from the remaining components of the

 Precipitated suspension separated. Depending on the filterability of the precipitate, this may be done by conventional filtration techniques known to those skilled in the art, e.g. Filter presses or rotary filter, done. In difficult to filter precipitates, the separation can also by centrifugation and / or by

 Decanting the liquid components of the precipitation suspension done.

The precipitate is in step a.e. is washed, which is ensured by a suitable washing medium, that the pH of the washing medium at least during the 1st washing step and thus that of the silicon dioxide 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. As the washing medium is preferably used in step a.a. and a.c.

used acidulants or mixtures thereof used in diluted or undiluted form.

The washing medium used may preferably be aqueous solutions of organic and / or inorganic water-soluble acids, e.g. 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 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 acidulants and as preferred in the washing medium, because they themselves do not lead to contamination of the

subsequent reduction step, ie the carbothermal nitridation contribute. Preferably, the acidulant or mixtures thereof used in step aa and ac are used in dilute 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, ethanol, a possible esterification does not interfere with the subsequent reduction to silicon.

However, 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

The present invention also provides methods in which

 Stabilization of acid-soluble metal complexes of

Fällsuspension or a washing medium

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 silica in one

 Washing medium with a 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, i. without further intermediate steps being carried out.

Also, a colorant for color marking, as an "indicator" of unwanted metal impurities, can in step a.e. be added. For example, hydroperoxide may be the

Fällsuspension or the washing medium are added to make existing titanium impurities color coded. The marker is generally synonymous with others

organic complexing agents possible, which in turn in the

subsequent reduction process does not interfere. These are generally all complexing agents based on the elements C, H and O, the element N can also be used in the

Be present complexing agent. For example, for the formation of Silicon nitride, which advantageously decomposes again in the later process.

The washing at a pH of 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, is preferably continued until the silica reaches the desired

Having purity. This can e.g. be recognized that the wash suspension contains a peroxide and visually shows no more yellowing. Will the invention

 Precipitation process without addition of a peroxide which with

Ti (IV) ions a yellow / orange-colored compound forms carried out, it can be removed at each washing step, a small sample of the washing suspension and mixed with a corresponding peroxide. This process will take so long

continued until the sample removed after the addition of the peroxide visually no yellow / orange coloration shows. Alternatively, a sample may be analyzed by methods of analysis known to those skilled in the art and then washed with the acid

Washing medium can be adjusted if adequate

Purity is achieved. By this time should

it is to be ensured that the pH of the washing medium and thus also of the purified silicon oxide, in particular of the silicon dioxide, is less than 2, preferably less than 1.5, more preferably less than 1, very preferably 0.5 and especially preferably 0.01 to 0 , 5 is.

The thus-washed silica is preferably used in an intermediate step a.e.a. between step a.e. and a.f. washed with distilled water or demineralized water until the pH of the resulting silica is 4 to 7.5 and / or until the conductivity of the washing suspension is less than or equal to 9 yS / cm, preferably less than or equal to 5 yS / cm.

This ensures that any acid residues adhering to the silica have been sufficiently removed.

In the case of difficultly filterable precipitates, it may be advantageous to carry out one or all washing steps by flow washing, For example, by flowing the precipitate in a close-meshed screen basket with the washing medium from below to perform.

The entire washing steps can preferably be carried out at temperatures of 15 to 100 ° C. In order to ensure the indicator effect of the peroxide (yellow / orange coloring), it may be useful, together with the

Wash medium add more peroxide until no

Yellow / orange coloration is more visible and only then wash with washing medium without peroxide on. The resulting high purity silica can be dried and processed further. Drying may be carried out by any method known to those skilled in the art, e.g. Belt dryer,

Hordentrockner, drum dryer, etc. done.

It is advisable to ground the dried silicon dioxide in order to obtain an optimum particle spectrum for further processing. The techniques for optional grinding of the silica of the invention are known to those skilled in the art and may be e.g. in Ullmann, 5th edition, B2, 5-20. Preferably, the grinding takes place in

Fluidized bed counter-jet mills to minimize or avoid contamination of the high purity silica with metal debris from the mill walls. The grinding parameters are

preferably chosen so that the particles obtained have an average particle size ds o of 1 to 70 ym, more preferably from 3 to 50 ym, and especially from 5 to 20 ym.

The high-purity silicon dioxides are characterized in that their content of impurities is defined above

Impurity profile corresponds because process step a. to silicon dioxides, which have very low concentrations in view of a wide range of impurities.

The high-purity silicon dioxides are used in step b. to

Silicon nitride further processed. Step b. can after all methods known to those skilled in the art are carried out;

Such methods are described for example in DE 3612162. Thus, in a first preferred variant, the high-purity silicon dioxide from step a. a carbothermic nitridation in the presence of nitrogen. In a second preferred variant, the high-purity

Silicon dioxide with a carbonaceous material and ammonia to S1 ₃ N _{4 are} reacted.

As part of step b. be as carbonic

Material preferably includes one or more pure carbon sources, an organic compound of natural origin

Carbohydrate, graphite (activated carbon), coke, coal, carbon black or thermal black, optionally in a mixture of the components used.

In addition to the aforementioned carbonaceous materials, in a preferred embodiment, pyrolysis products of at least one carbohydrate may also be used as exclusive or additional carbon source in step b. be used. According to this variant (partial process) of the overall process according to the invention for the production of silicon nitride, it is preferred to use a carbon source, in particular a pure carbon source, in step b. to be used, which is produced by technical pyrolysis of at least one carbohydrate or a carbohydrate mixture, in particular a crystalline sugar. A pyrolysis product which can be used in accordance with the invention contains coal, in particular with graphite constituents, and optionally fractions of other carbon forms, such as coke, and is preferably poor in impurities. A part of the present invention is therefore a process for industrial, ie industrial pyrolysis of a carbohydrate or a carbohydrate mixture, which is carried out at elevated temperature. The carbohydrate component used in pyrolysis is preferably monosaccharides, ie aldoses or ketoses, such as trioses, tetroses, pentoses, hexoses, heptoses, especially glucose and fructose, but also corresponding oligomeric and polysaccharides based on said monomers, such as lactose, maltose , Sucrose, raffinose - to name but a few - all the way to cellulose or starch, including amylose and amylopectin, glycogen, glycosans and fructosans - to name but a few polysaccharides. Also used are sugar alcohols and derivatives of usable as carbohydrate component substances.

In this case, naturally occurring carbohydrates, anomers thereof, invert sugar, but also synthetic carbohydrates can be used. Likewise, carbohydrates which have been obtained biotechnologically, for example by fermentation, can be used.

In general, all carbohydrates, derivatives of

Carbohydrates and mixtures of these substances in the

relevant variant of the overall method according to the invention are used, wherein they preferably have a high purity.

Particularly preferred in a specific embodiment is a crystalline available in economical quantities

Sugar, a sugar, such as by

Crystallization of a solution or from a juice

 Sugar cane or beets can be obtained in a conventional manner, d. H. commercial crystalline sugar, especially crystalline food-grade sugar used. Furthermore, raw materials which can generally be used as carbon source are all organic compounds known to the person skilled in the art.

In a particularly preferred embodiment, the technical pyrolysis of at least one carbohydrate or the carbohydrate component with the addition of silica, in particular of high-purity silicon dioxide.

For the conversion to silicon nitride according to process step b. becomes high purity in a particular embodiment

Silica with a mean particle size ds o less than 70 ym, preferably less than 50 ym and especially less than 20 ym used.

The thermal decomposition of high-purity S1 ₃ N ₄ to high-purity silicon according to process step c. can after all that

Expert known methods are carried out; Such processes are described, for example, in WO 2010/126927 A1

described.

The process steps b. and optionally c. can be used in any thermal reactor or industrial furnace,

For example, in furnaces, electric arc furnaces, induction furnaces and / or microwave ovens, which may each be equipped with a fluidized bed apparatus and / or a rotary tube, take place.

Measuring methods:

Determination of the pH of the precipitation suspension

The procedure based on DIN EN ISO 787-9 is used for

Determination of the pH of an aqueous suspension of

Silica or the pH of a largely SiO 2 -free washing liquid.

Before carrying out the pH measurement, the pH meter (Knick, type: 766 pH meter Calimatic with temperature sensor) and the pH electrode (combination electrode from Schott, type N7680) using the buffer solutions at 20 ° C to calibrate. The calibration function should be chosen so that the two used buffer solutions include the expected pH of the sample (for example, buffer solutions with pH 4.00 and 7.00, pH 7.00 and pH 9.00 and pH 7.00 and 12.00).

The pH is determined at 20 ° C. To measure the pH, the electrode is first deionized with

 Water, subsequently rinsed with a portion of the sample medium and then immersed in the sample medium. If the pH meter shows a constant value, the pH value is read on the display.

Determination of the mean particle size d.50 of the high-purity silicas for particle sizes smaller than 70 μm with the

Laser diffraction device Coulter LS 230

Description: The application of laser diffraction according to the Fraunhofer model for the determination of particle sizes is based on the phenomenon that particles scatter monochromatic light with different intensity patterns in all directions. This scattering is dependent on the particle size. The smaller the particles, the larger the scattering angles.

Execution :

The laser diffraction device Coulter LS 230 requires after

Turn on a warm-up time of 1.5 to 2.0 hours to get constant readings. The sample must be before the

Measurement can be shaken up very well. First of all the program "Coulter LS 230" is started with a double-click, whereby it is ensured that "Use optical bench" is activated and the display on the coulter device shows "Speed off".

Then the function "drain" is used until the water in the measuring cell has run away, then the fluid transfer pump is actuated until the water flows into the overflow of the device. operated, as a result, any existing air bubbles are removed from the system; The speed will be automatically raised and lowered again. By means of the setting

"Measurement cycle" starts the measurement Measurement without PIDS

The measuring time is 60 seconds, the waiting time 0 seconds. Subsequently, this is the basis of laser diffraction

Calculation model selected. In principle, a background measurement is automatically performed before each measurement. After the background measurement, the sample must be added to the measuring cell until a concentration of 8 to 12% is reached.

This will signal the program by displaying "OK" in the upper part. The program now carries out all necessary steps itself and generates a particle size distribution of the examined sample after the measurement has been completed.

Determination of dyn. Viscosity of water glass with the

 Falling ball viscometer The determination of dyn. Viscosity of water glass takes place with the falling ball viscometer (Höppler viscometer, Fa. Thermo Haake).

Execution :

The water glass (about 45 cm ³ ) is bubble-free in the downpipe of the falling ball viscometer (Thermo Haake, falling ball viscometer C) filled to below the pipe end and then the ball (Thermo Haake, ball set type 800-0182, ball 3, density δ _κ = 8,116 g / cm ³ , diameter d _K = 15.599 mm, ball specific

Constant K = 0.09010 mPa * s * cm ³ / g). The

The viscometer is precisely controlled to 20 ± 0.03 ° C using a circulating thermostat (Jalubo 4). Before the measurement, the ball passes once through the tube to the water glass mix thoroughly. After a break of 15 minutes, the first measurement begins.

The measuring part engages defined at the instrument foot in the 10 ° position. By pivoting the measuring tube by 180 °, the ball is brought to the starting position for the measurement. The fall time t through the measuring section A-B is determined by means of a manual stopwatch. The beginning of the measuring time begins when the lower sphere periphery touches the targeted upper ring mark A, which must appear to the viewer as a dash. The measurement time ends when the lower sphere periphery the lower ring mark B, which must also appear as a dash,

reached. By re-swinging the measuring part by 180 °, the ball falls back into the starting position. After a break of 15 minutes, a second measurement is performed as described. Repeatability is guaranteed if the measured values differ by a maximum of 0.5%. The falling time t of the sphere becomes accurate to one decimal place in seconds

indicated.

The dynamic viscosity of the water glass (r GL) is calculated in mPa * s according to the numerical value equation n _WGL = K * (5 _K -5 _WGL ) * t

Determination of the conductivity of the washing medium

The determination of the electrical conductivity of an aqueous silica suspension or of the electrical conductivity of a largely SiO 2 -free washing liquid is at

Room temperature based on DIN EN ISO 787-14.

Determination of the content of impurities: Method description for the determination of trace elements in silica using high-resolution inductively coupled plasma mass spectrometry (HR-ICPMS)

1 - 5 g of sample material are weighed to within ± 1 mg in a PFA beaker. 1 g of mannitol solution (about 1~6) and 25-30 g of hydrofluoric acid (about 50%) are added. After a short time

The PFA beaker is heated to 110 ° C. in a heating block so that the silicon contained in the sample is present as hexafluorosilicic acid and the excess

Hydrofluoric acid slowly evaporates. The residue is mixed with 0.5 ml of nitric acid (about 65%) and a few drops

 Dissolved hydrogen peroxide solution (about 30%) for about 1 hour and made up to 10 g with ultrapure water.

To determine the trace elements are the

Take 0.05 ml or 0.1 ml syringe solutions, transfer each to a polypropylene sample tube, add 0.1 ml indium solution (c = 0.1 mg / l) as an internal standard and dilute to 10 ml with dilute nitric acid (about 3%) filled. The preparation of these two sample solutions in different

Dilutions, serves for internal quality assurance, i.

 Check whether errors were made during the measurement or sample preparation. In principle, you can only use one

Working sample solution.

From multielement stock solutions (c = 10 mg / l) containing all elements to be analyzed except indium, four calibration solutions are prepared (c = 0.1, 0.5, 1.0, 5.0 yg / l) , and in turn with the addition of 0.1 ml of indium solution (c = 0.1 mg / 1) made up to 10 ml final volume. In addition, blank solutions are prepared with 0.1 ml indium solution (c = 0.1 mg / l) to 10 ml final volume.

The element contents in the blank value,

Calibration and sample solutions are determined by means of the High

Resolution Inductively Coupled Mass Spectrometry (HR-ICPMS) and quantified by external calibration. The measurement takes place with a mass resolution (m / Am) of at least 4000 or 10000 for the elements potassium, arsenic and selenium.

The following examples are intended to illustrate the present invention in more detail, but do not limit it in any way.

Comparative Example 1

Based on Example 1 of WO 2007/106860 Al 397.6 g of water glass (27.2 wt.% Si0 ₂ and 8.0 wt.% Na ₂ 0) were mixed with 2542.4 g of deionized water. The diluted water glass was

then through a column with 41 mm inner diameter and 540 mm length, filled with 700 ml (500 g dry weight)

Amberlite IRA 743 in water, let it run. After 13.5 minutes, a pH greater than 10 was measured at the outlet of the column, so that at that time the front of the water glass solution had passed through the column. For further experiments, a purified sample of 981 grams of waterglass solution taken between the 50th and 74th minute after the aforementioned passage was used.

The analytical data of the water glass before and after

Purification can be found in Table 1 below:

Table 1:

The data from Table 1 show that the essentially described in WO 2007/106860 AI step of the purification of the water glass over Amberlite IRA 743 in commercial

 Water glass does not show a large purification effect and only causes a significant improvement in the titanium content.

The purified water glass was further processed analogously to Example 5 of WO 2007/106860 AI to S1O ₂ . For this purpose, 700 g of the water glass were acidified in a 2000 ml round bottom flask while stirring with 10% sulfuric acid. The initial pH was 11.26. After adding 110 g of the sulfuric acid, the gelation point was reached at pH 7.62 and 100 g of demineralized water were obtained added to the stirrability of the suspension again

manufacture. After addition of a total of 113 g of sulfuric acid, a pH of 6, 9 was reached and stirred at this pH for 10 minutes. It was then filtered through a Buchner funnel with a diameter of 150 mm. The product obtained was very poorly filtered. After washing five times with 500 ml of deionized water, the conductivity was 140 yS / cm. The filter cake was 2.5 days at 105 ° C in a

Dried circulating air dryer, whereby 25.4 g of dry product could be obtained. The analytical results are shown in Table 2.

Example 1 (according to the invention)

In a 4000 ml quartz glass round bottom flask with two-necked attachment, ball cooler, Liebig condenser (each made of borosilicate glass) and 500 ml graduated cylinder - to collect the distillate - were 1808 g of water glass (27.2 wt.% Si0 ₂ and 7.97 wt.% Na ₂ 0) and 20.1 g of 50% sodium hydroxide solution submitted. The sodium hydroxide solution was added in order to achieve an increased Na ₂ <0 content in the concentrated waterglass. The solution was blanketed with nitrogen to prevent reaction with carbon dioxide from the air and then heated to boiling by means of a heated mushroom. After distilling off 256 ml of water, the

Liebig cooler replaced with a stopper and boiled for a further 100 min under reflux. Thereafter, the concentrated water glass was cooled to room temperature under a nitrogen atmosphere and allowed to stand overnight. 1569 g of concentrated waterglass with a viscosity of 537 mPa * s were obtained. In a 4000 ml quartz glass two-necked flask with KPG stirrer and

Dropping funnels (each made of borosilicate glass) were initially charged at room temperature with 2513 g of 16.3% strength sulfuric acid and 16.1 g of 35% strength hydrogen peroxide. Within 3 minutes, 1000 ml of the previously prepared concentrated waterglass solution (9.8 Wt.% Na ₂ O, 30.9 wt.% Si0 ₂ , density 1.429 g / ml) was added dropwise so that the pH remained below 1. The reaction mixture warmed to 50 ° C and turned deep orange. The suspension was stirred for 20 min and then allowed to settle the resulting solid.

For work-up, the supernatant solution was decanted off and to the residue a mixture of 500 ml of deionized water and 50 ml of 96% strength sulfuric acid was added. While stirring, the

Heated suspension to boiling, let settle the solid and the supernatant decanted again. This washing process was repeated until the supernatant showed only a very slight yellowing. It was then washed with 500 ml of deionized water until a pH of the wash suspension of 5.5 was reached. The conductivity of the washing suspension was now 3 yS / cm. The supernatant was decanted off and the

product obtained at 105 ° C overnight in one

Drying circulating air dryer. The analytical data of the product obtained are shown in Table 2 below:

Table 2

The results according to Table 2 show that although the resulting silicon dioxide of the comparative example - as disclosed in WO 2007/106860 Al - a low boron and

Phosphorus content, however, the other impurities have such high concentrations that the silicon dioxide is not suitable as a starting material for the production of solar grade silicon. The produced by the process according to the invention

Silica has a total contaminant content of only 2.6 ppm across all measured elements. Also the

Impurities with elements critical for the production of solar silicon are, according to Table 2, within an acceptable range. It thus turns out that it is with the inventive method - contrary to the teachings of

It is possible - without chelating reagent or using ion exchange columns of commercially available, concentrated waterglass and commercial sulfuric acid - to produce a silicon dioxide which, due to its impurity profile, is outstandingly suitable as a starting material for solar grade silicon.

Claims

Process for the preparation of high-purity silicon nitride, characterized in that it comprises the steps mentioned below  a. Production of high purity silica by  Precipitation of at least one silicate solution with  at least one acidulant, wherein the pH of the precipitate, the precipitation suspension and the washing medium, at least in the first washing step, less than 2, preferably less than 1.5, more preferably less than 1, completely  more preferably less than 0.5 b. Implementation of the high-purity silicon dioxide from step a. with at least one high purity carbon source and at least one nitrogen source to high purity Si _{3 4} . A method according to claim 1, characterized in that step a. the following steps include a. Preparation of a Precipitate Template from an Acidifier or Acidifier and Water with 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 with a viscosity of 0.1 to 10,000 poise c. Common addition of the silicate solution from step with an acidifier in the template from step aa or sole addition of silicate solution from step into the template from step aa 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 ad remains optional separation of the silicon dioxide ae washing of the resulting silica, wherein the  Washing medium, at least in the first washing step, a pH of less than 2, preferably less than 1.5, especially  preferably less than 1 and most preferably less than 0.5 a.f. Optionally drying the resulting silica. A method according to claim 1 or 2, characterized in that the silicate solution is an aqueous waterglass solution and acidifying agent is a mineral acid, in particular  Sulfuric acid is used. Method according to one of claims 1 to 3, characterized in that step b. using a high-purity silica with aluminum contents of less than or equal to 5 ppm, boron less than 10 ppm to 0.0001 ppt, calcium less than or equal to 2 ppm, iron less than or equal to 20 ppm, nickel less than or equal to 10 ppm, phosphorus less than 10 ppm to 0.0001 ppt, titanium less than or equal to 2 ppm, and zinc less than or equal to 3 ppm. Method according to one of claims 1 to 4, characterized in that the high-purity silicon dioxide from step a. as part of step b. a carbothermic nitridation in the presence of nitrogen is subjected. 6. The method according to any one of claims 1 to 5, characterized  characterized in that the high-purity silicon dioxide from step a. as part of step b. with a carbonaceous material and ammonia is reacted. Method according to one of claims 1 to 6, characterized in that in step b. as high purity  Carbon source is an organic compound of natural origin, especially a carbohydrate, graphite, coke coal, soot or thermal black is used. Method according to one of claims 1 to 7, characterized in that in step b. used high-purity carbon source by technical pyrolysis of at least one carbohydrate or a  Carbohydrate mixture, in particular a crystalline sugar is produced. A method according to claim 8, characterized in that the technical pyrolysis is carried out with the addition of silicon oxide, in particular of high-purity silicon dioxide. Method according to one of claims 1 to 9, characterized in that in the context of step b. high purity silicon dioxide having a mean particle size ds o less than 70 ym, preferably less than 50 ym and in particular less than 20 ym is used. 11. Use of high-purity silicon nitride, which was obtained by a process according to any one of claims 1 to 10, for the production of high-purity silicon, for the production of ceramic materials, as  Coating material or as a release layer or Barrier layer. High-purity silicon nitride, characterized in that its aluminum content is less than or equal to 5 ppm, boron less than 10 ppm to 0.0001 ppt, calcium less than or equal to 2 ppm, iron less than or equal to 20 ppm, nickel less than or equal to 10 ppm, phosphorus less than 10 ppm to 0.0001 ppt, titanium less than or equal to 2 ppm and zinc less than or equal to 3 ppm. A method of producing high purity silicon, characterized in that a high purity silicon nitride obtained according to any one of claims 10 to 10 is thermally decomposed. 14. Use of high-purity silicon, which after a  A method according to claim 13 was obtained, to  Production of solar cells or other products for the solar or electronics industry.