DD291537A5 - METHOD FOR THE HYDROTHERMAL PRODUCTION OF SODIUM SILICATE SOLUTIONS WITH HIGH SIO DEEP 2: NA LOW 2 O-MOL AGAINST - Google Patents

Abstract

Disclosed is a process for the hydrothermal preparation of sodium silicate solutions having a high SiO 2: Na 2 O molar ratio by reacting a crystalline silica with aqueous sodium hydroxide solution, characterized in that the crystalline silica used is a quartz-tempered quartz at temperatures above 100 ° C. Tempered quartz with aqueous sodium hydroxide solution in a concentration range of 10 to 50 * at temperatures of 150 to 300C and the corresponding pressures of saturated steam in a closed pressure reactor. {Sodium silicate preparation, hydrothermal; SiO 2: Na 2 O molar ratio, high; Reaction of silica crystal with sodium hydroxide solution

Description

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Field of application of the invention

The present invention relates to a process for the hydrothermal preparation of sodium silicate solutions with a high SiO ₂ : Na ₂ O molar ratio by reacting a crystalline silica with aqueous sodium hydroxide solutions.

Characteristic of the known state of the art

For a general overview of the preparation of aqueous sodium silicate solutions give the monographs Winnacker, Küchler, Chemical Technology, Volume 3, Inorganic Technology II, 4, Edition, 1983, pp 54-63 and Ulimann's Encyclopedia of Industrial Chemistry, Volume 21.4. Auflago, 1982, pp. 409-412.

Of the alkali metal silicates known by the name of "water glass", the most commonly used for technical purposes are sodium silicate solutions - generally referred to as sodium silicate glasses 3.5: 1 The production of soda water on an industrial scale is generally carried out by melting quartz sand and soda together in suitable ovens at temperatures in the range of 1400 to 1500 ⁰ C.

The solidifying on cooling melt, the glass, is then dissolved in a further process step using pressure and elevated temperatures V, water and the resulting solution - depending on the quality requirement - optionally filtered.

However, this high-temperature melting process is both ar aratively and in terms of the required amounts of energy very expensive and continues to lead to significant emissions, such as dust, nitrogen oxides and sulfur oxides.

In addition to this high-temperature melting process, which is mainly used in industry, there are also known hydrothermal processes for the preparation of aqueous sodium silicate solutions, which are described in a number of patent applications.

On the one hand, these processes are based on amorphous silicon dioxide, that is to say essentially on fly dusts and naturally occurring amorphous silicon dioxide modifications.

The resulting process products are of low quality due to the usual impurities of the flue dust and the natural amorphous silica compounds, which are used as input materials and can therefore be used only to a limited extent for technical products.

DE-AS 2826432 handles a process for the preparation of waterglass solutions by reacting aerosols obtained when obtaining silicon or Ferrofellicium-Leglerungen, with aqueous alkali metal hydroxide solutions at elevated temperatures and subsequent filtration is contained solutions, which is characterized in that Flue dust with a 6- to 15gew .-% aqueous alkali metal hydroxide solution at temperatures of 12O ⁰ C to 19O ⁰ C and a pressure of 2.9 to 18.6 bar autoclaved, wherein the weight ratio of alkali metal hydroxide solution to solid fly ash 2: 1 to 5: 1. The process products have a molar ratio SiO ₂ : Na ₂ O of 2.2 to 4: 1. The flue dusts used as starting materials have a silicon content of 89 to 98% by weight, which according to the exemplary embodiments is always 90% by weight; the rest is impurities. DE-OS 2609831 relates to a process for the treatment of silica-polluting polluting waste dusts from the silicon-metal and silicon-alloy production to silicas or silicates, which is characterized in that the following process steps I to III are combined:

Dissolving the fly dust in alkali hydroxide solutions to form alkali silicate solutions;

II purification of the alkali silicate solutions of organic constituents by treatment with activated charcoal and / or oxidants and separation of the non-digestible residue from the solution;

III Reaction of alkali metal silicate solutions with inorganic or organic acids and / or their salts for further purification.

The alkali silicate solutions thus obtained generally have a molar ratio SiO ₂ ) Na ₂ O in the range of 3.3 to 5.0: 1.

DE-OS 2619604 relates to a process for the preparation of liquid amorphous silica and alkali metal hydroxydic glasses, characterized in that silica dust in the form of fly ash separated from the exhaust gases of ferroalloy industry and other industries operating with silicon furnaces is alkali hydroxide and water are mixed in a certain weight ratio and then brought to a temperature of between 75 and 100 ° C with stirring, after which the liquid obtained is cooled. The silica dusts used as the starting material for this waterglass production generally have a silica content of 94 to 98 wt .-%, the remainder being based on impurities.

DE-AS 2328542 relates to a process for the preparation of alkali metal silicates by treatment of perlite with an alkali lye and hydrothermal treatment of the obtained pulp in the autoclave with subsequent filtration, which is characterized in that one for treating the perlite of an alkali solution having a concentration of 40 to 140g / l Na ₂ O used in an amount in which the ratio of the liquid phase to the solid phase is 0.7 to 1.5: 1. The perlite is a substantially amorphous, glassy mountain rock of volcanic origin which consists mainly of (in% by weight): silica 73. alumina 15 and other oxides 8.

As the foregoing shows, the water glasses described in the patent literature, which are obtained from amorphous silica, always provide only process products with inferior properties, which must be subjected to further purification.

The state of the art described in the following relates to processes for the hydrothermal preparation of sodium silicate solutions of crystalline silicon dioxide, ie sand, and sodium hydroxide solution, which according to the processes of the prior art, however, only up to a SiO ₂ : Na ₂ O molar ratio of less than 2, 89: 1 can be implemented.

DE-OS 3002857 relates to a process for the hydrothermal preparation of sodium silicate solutions having a MoWer SiO ₂ : Na ₂ O of 1.03 to 2.88: 1 by reaction of sand with aqueous sodium hydroxide solution under pressure and at elevated temperatures and subsequent filtration, the characterized in that the aqueous sodium hydroxide solution of a concentration of 10 to 50Gaw .-% with an excess of sand of up to 300%, based on the molar ratios of SiO ₂ INa ₁ O in the batch, at temperatures in the range of 150 to 25O ⁰ C and the pressures corresponding to those of saturated water vapor, and the unreacted excess sand completely or teilweii a used as a Hitermedium for the sodium silicate solution formed. According to the embodiments of this disclosure, however, a maximum SiO ₂ : Na? O molar ratio of 1.68: 1 is achieved.

DE-OS 3421158 relates to a process for the hydrothermal preparation of sodium silicate solutions having a molar ratio SiO ₂ : Na ₂ O from 1.96 to 2.17 by reaction of excess sand with aqueous sodium hydroxide solution, which is characterized in that one sand excess and a reaction mixture containing heated aqueous sodium hydroxide solution containing reaction mixture in a rotating, cylindrical, closed pressure reactor until a certain molar ratio SiO ₂ = Na ₂ O reacted and then filtered using the excess sand and optionally an additional filter aid. In the exemplary embodiments, aqueous sodium silicate solutions having a molar ratio SiO ₂ = Na ₂ O of up to 2.27: 1 are disclosed.

DE-OS 3313814 relates to a process for the preparation of a clear solution of a sodium silicate whose molar ratio of silica: sodium oxide is at most equal to 2.58: 1 by digestion of crystalline silica having an average particle size between 0.1 and 2 mm, in which an aqueous solution of sodium hydroxide passes through a bed of silica formed in a vertical tubular reactor without mechanical agitation and fed from top to bottom with silica and the aqueous solution of sodium hydroxide.

Belgian patent 649739 relates to a method and apparatus for the preparation of clear sodium silicate bases by dissolving a; siliceous material at high temperature and under pressure in an aqueous caustic soda solution, characterized in that the product is separated from the excess siliceous material and / or insoluble contaminants by means of filtration elements mounted in the vicinity of the reactor bottom, where r ' Said filtration is advantageously carried out under the conditions of temperature and pressure, which are very similar to the reaction conditions. The aqueous sodium silicate solutions obtained in this way have a content of SiO ₂ : Na ₂ O of about 2.5: 1. Such hydrothermal processes for the production of soda water glasses from sand and sodium hydroxide solution are also summarized in the monographs of Winnacker, Küchler and Ulimann already cited above. For Winnacker, Küchler it is stated (pages 61 and 62) that, however, only a sodium silicate with a ratio SiO ₂ / Na ₂ O of less than 2.7 can be achieved in the hydrothermal process at the temperatures usually used.

Ulimann mentions in this connection that in this way only sodium silicate solutions with molar ratios of SiCVNa ₂ O up to 2.5: 1 can be obtained (page 412, left column).

Because of the literature cited above, there was therefore a direct prejudice with regard to the recovery of sodium silicate solutions with higher SiO ₂ / Na ₂ O molar ratios in the hydrothermal method from sand, ie from crystalline SiO ₂ , and sodium hydroxide solution.

Object of the invention

By dia invention is a technically easy to use and thus cost-effective method for the hydrothermal preparation of sodium silicate solutions with high SiO ₂ : Na ₂ O-Molvfi ratio provided.

Explanation of the essence of the invention

The present invention has for its object to provide a method for the hydrothermal preparation of sodium silicate solutions by reacting a crystalline silica with aqueous sodium hydroxide solution, wherein sodium silicate solutions with molar ratios SiO ₂ / Na ₂ O of more than 2.9: 1 are achieved.

The erfindungsgomäße object is achieved by the use of a specially annealed quartz, which is reacted under special reaction conditions with sodium hydroxide solutions.

The present invention thus relates to a process for the hydrothermal preparation of sodium silicate solutions having a high SiO ₂ : Na ₂ O moiety ratio by reacting a crystalline silicon dioxide mi; Aqueous sodium hydroxide solution, characterized in that the crystalline silica used is a quartz tempered at temperatures in the range from above 110O ⁰ C to the melting point and this annealed quartz with aqueous sodium hydroxide solution in a concentration range of 10 to 50Gew .-% at temperatures of 150 to 300 ⁰ C and corresponding to these pressures of saturated water vapor in a pressure reactor.

the process of the invention is technically easier to handle by its one-step process management and thus more cost-effective than the technically complex, large amounts of energy demanding and the environment highly polluting processes of the prior art, so the high-temperature melting process followed by solution step.

Compared to the hydrothermal method of the prior art, the inventive method has the advantage that by using the invention specifically tempered quartz and sodium silicate solutions are obtained with a molar ratio of Si0 ₂ : Na ₂ 0von more than 2.9: 1, which using not annealed quartz as Si0 ₂ component is not possible.

Furthermore, it has surprisingly been found that from quartz tempered in this way, preferably a cristobalite formed in this way, in the context of a hydrothermal synthesis under the conditions given above, the direct, one-stage preparation of aqueous sodium silicate is possible even at short reaction times, a SiO ₂ molar ratio = Na ₂ O of more than 2.9: 1.

Even with short reaction times, a high degree of conversion of the reaction products used can be achieved when using the method according to the invention. The use of a readily soluble crystalline silica modification makes it possible to prepare sodium silicate solutions with a high silica / sodium oxide molar ratio, with high space / time yields and minimal energy consumption.

Preferably, the sodium silicate solutions thus obtained have a SiO ₂ : Na ₂ O molar ratio of from 2.9 to 3.7: 1, particularly preferably from 3.0 to 3.6: 1, and in particular from 3.3 to 3, 5: 1.

According to a preferred embodiment of the present invention, the aqueous sodium silicate solution is obtained by using a quartz tempered at temperatures of 1200 to 1700 ⁰ C with the addition of catalytic amounts of alkali as crystalline silica, which converts under these annealing conditions substantially in cristobalite, and one converts the so-tempered quartz with aqueous sodium hydroxide solution in a concentration range of 15 to 30Gew., -%, preferably 20 to 30Gew .-%, at temperatures of 200 to 25O ⁰ C and corresponding to these temperatures pressures of saturated water vapor in a closed pressure reactor ,

Cristobalite, like quartz, is a crystal modification of the silica. This is almost exclusively produced synthetically by calcination of quartz, by continuously converting quartz sand at temperatures of about 1500 ⁰ C with the addition of catalysts (alkali compounds). For more information on cristobalite, see Ulimann's Encyclopedia of Industrial Chemistry, Volume 21,4. Edition, 1982, pp. 439-442.

For the purposes of the invention, it is therefore particularly preferred to employ as crystalline silica a quartz which is annealed at temperatures in the range from 1300 ° C. to 1600 ° C. with the addition of catalytic amounts of alkali, which essentially converts to cristobalite under these conditions. It is also particularly advantageous to use a freshly tempered, still warm cristobalite material for the process according to the invention.

According to a further preferred embodiment of the process according to the invention, the reaction is carried out in the reactor by adding an excess of tempered quartz of up to 100 mol%, preferably 1 to 10 mol%, based on the desired SiO ₂ desired: Na ₂ O molar ratios in the sodium silicate solution. In general, the reaction can also be carried out with higher excesses than 100 mol .-% of tempered quartz; however, this is not technically in general

meaningful. For the purposes of the invention, it is particularly preferred to carry out the reaction with an excess of from 2 to 5 mol% of tempered quartz, based on the desired SiO ₂ : Na ₂ O molar ratio.

For carrying out the process according to the invention, it is generally possible to use all reactors customary for the hydrothermal synthesis of sodium silicate. These include z. As rotating solvers, standing Löseranordnungen, reactors with agitator, jet loop reactors, tube reactors and, in principle, all reactors that are suitable for the reaction of solids with liquids under pressure. Such reactors are described in detail, for example, in DE-OS 3002857, DE-OS 34 21158, DE-AS 2826432, BE-PS 649739, DE-OS 3313814 and DE-PS 968034.

Dio in the manner according to the invention prepared sodium silicate solutions (sodium silicate solutions) can be used for all customary uses, which are known in the art and described in the relevant literature, for example, for the preparation of fillers (precipitated silicas), as adhesives, as Binders in paints, foundry auxiliaries, catalyst supports, as a component in detergents and cleaners, and as a constituent for refractory materials.

The invention is explained in more detail below by examples. The examples were made on a laboratory scale and on an industrial scale. The tempered quartz used in the exemplary embodiments was a cristobalite obtained by tempering at 1300 to 1600 ° C. and alkali catalysis.

For the laboratory experiments, a cylindrical autoclave was used, which was heated from the outside by a heat transfer medium to reaction temperature. The results of these examples are summarized in Tables 1 and 2 below.

As a reactor for carrying out the experiments on an industrial scale was a horizontally arranged cylindrical pressure vessel made of steel with a Nickolauskleidunrj with a void volume of about 24 m ³ . The pressure vessel rotated about a horizontal axis at a speed of 6 rpm. The heating was carried out with steam of 20 or 25 bar via an opening in the axis and egg., Attached pipe with an effective distribution directly into the reaction vessel into it.

The crystalline SiO ₂ used for the examples, the cristobalite obtained from tempered quartz, contained> 99% by weight of SiO ₂ .

The required for the process aqueous sodium hydroxide solution (sodium hydroxide) was heated via a Venturi nozzle above the sodium hydroxide reservoir with vapors from the Vorcharge to about 103 ° C.

The amounts of substance (cristobalite / sodium hydroxide solution) were detected by weighing devices. The raw materials were introduced into the reactor, this then sealed and rotated, by direct introduction of steam, the reaction mixture was heated to the reaction temperature of about 215 ⁰ C and left at this temperature. After a reaction time of 30 min at this temperature, the reactor was brought to a standstill and transferred the reaction mixture via a flanged pipe by means of an autogenous pressure in a blow-out, In this way, the reaction mixture was then separated via a Zyclonabscheider in vapors and about 105 ⁰ C hot water glass solution , The vapors were sucked from a jet apparatus and used to preheat the mixed liquor of the next batch in a Venturi nozzle to the limit of the boiling point of the liquor of about 103 ⁰ C.

The further work-up of the about 100 ^° C. hot waterglass solution was carried out either via a sedimentation vessel for the coarse separation of solids or - in the case of higher demands on the clarity of the solution - via a filter.

The prepared sodium silicate solutions were analyzed for their SiO ₂ and Na ₂ O contents.

As reaction conditions, the conditions of Example 4 were selected on an industrial scale. The batch size was 24000kg. The obtained about 41% sodium silicate solution had a SiO ₂ : Na ₂ O molar ratio of 3.4: 1 and practically corresponded to the result of the experiment on a laboratory scale.

In a particular embodiment, the hydrothermal process can be carried out using cristobalite / NaOH solution at relatively high solids concentrations in the reactor, since even at high SiO ₂ : Na ₂ O molar ratio under reaction conditions (215 ° C / 20bar) the sodium silicate solution for having the process sufficient viscosity range. After completion of the reaction can then either

- under pressure directly into the reactor or

- In the discharge line to a storage tank during the Ausblasevorgangs

In addition, water are fed so that the obtained via the blow-off in the receiver tank sodium silicate solution is sufficiently diluted in that in the template at temperatures of about 100 ⁰ C, the sodium silicate solution before further processing by sedimentation or filtration a flowable, sufficiently low viscosity Consistency is evident.

Exemplary embodiments

example 1

Example 1 demonstrates a favorable approach with regard to the relatively low batch liquor concentration with stoichiometric cristobalite use, based on a target ratio of SiO ₂ : Na ₂ O to be achieved in the sodium silicate solution of 3.46: 1.

Example 2

An increased NaOH concentration in the batch was compared with Example 1 Beivergleichbaren reaction times adjusted to determine the influence of the NaOH concentration on reaction rate and achievable SiO ₂ : Na ₂ O ratio.

Example 3/4

In order to achieve a higher molar ratio of SiO ₂ = Na ₂ O in the reaction solution, an increasing excess (+ 3% / + 5%) of cristobalite was used in comparison with Example 1, based on the nominal ratio of 3.46: 1.

Example 5/6

With a cristobalite excess of 5%, based on a desired ratio SiC> 2: Na 2 O of 3.46: 1, the reaction times were extended.

Table 1 Approach ratios

Example no.

Feedstocks · and quantities Cristobalit sodium hydroxide solution

% NaOH molar ratio concentration Cristobaiite: Na ₂ O imAnsat '* ³ 20.0 3.46: 1 30.0 3.46: 1 20.0 3.56: 1 20.0 3.63: 1 20.0 3.63: 1 20.0 3.63: 1

4ü 49 49 49 49

94.02 62.68 ..91.28 * 89.54 89.54 89.54

1% of cfistobalic acid 3%, based on a desired molar ratio SiO ₂ = Na ₂ O in the solution of 3.46; 1 "2 Crlstobalitüberschuß 5%, based on a desired MolverhSltnie SiO ₂ = Na ₂ O in the solution of 3.46: 1 * 3, taking into account all present in the reactor SiO ₂ and NajO components

Table 2

Reaction conditions and product characterization

Example no.

HT Reaction Conditions Reaction Time Reaction Temp.

(min) ( ⁰ C)

Sodium silicate solution * ¹ % SiO ₂ % Na ₂ O

Molar ratio SiCVNa ₂ O

1 30 2 30 3 30 4 30 5 60 1 120

215 215 215 215 215 215

33.22 ¹ 10.31 3.32: 1 43.07 13.17 3.37: 1 33.66 10.24 3.39: 1 33.78 10.22 3.41: 1 34.14 10.17 3.46: 1 37.27 10.15 3.48: 1

* 1 Composition of the end product in the reactor.

Prior to filtration, dilution is carried out to a solids content of <41% solids in the sodium silicate solution. (HT stands for "hydrothermal")

Example 7

In order to detect the influence of the annealing temperature of the quartz on the product properties of the aqueous sodium silicate solutions prepared, quartz was first tempered with the addition of catalytic amounts of alkali at temperatures of 85O ⁰ C to 1600 ⁰ C and then hydrothermally reacted with sodium hydroxide solution. As a comparison, untreated quartz was added in the same standard hydrothermal reaction with sodium hydroxide solution

Sodium water glass implemented.

In the standard experiment of the hydrothermal reaction of the tempered quartz with sodium hydroxide solution were as

Test conditions chosen:

Reaction temperature 215 ° C;

Reaction time 30 min;

Nitrous hydroxide concentration 20% by weight;

Excess of silica 5% (based on the molar ratio of 3.46: 1 with soda glass)

The reaction of this tempered quartz in caustic soda to a soda water glass was carried out with the amounts of the starting materials listed below and led to the tabulated below and the molar ratios.

Reaction of tempered sand in sodium hydroxide to Na glass, molar ratio Si0 ₂ / Na ₂ 0 = 3.46: 1 for Na glass quantities

35.37% tempered sand 49.00g

25.85% NaOH, 50% 35.82 g

38.78% water 53.72 g

% Unsafe % SiOj ⁰ ANa ₂ O weight MoI.- Married. Married. Theor. Amounts: 35.19 10.00 3.52: 1 3.63: 1 unheated. Sand (Cf.) 58.73 24,20 11.71 2:07: 1 2.13: 1 850 ⁰ C sand 48.11 20.74 12.25 1.69: 1 1.75: 1 85O ⁰ C sand » 49.18 21.10 12.20 1.73: 1 1.78: 1 950 ⁰ C sand 42,60 18.81 12.55 1.50: 1 1.55: 1 950 ⁰ C sand » 48,00 20.70 12.26 1.69: 1 1.74: 1 1100 ⁰ C Sand » 57.50 23.82 11.77 2.02: 1 2.09: 1 1300 ⁰ C sand * 86.81 32.05 10.49 3.05: 1 3.15: 1 1600 ⁰ C sand * 90,30 32.91 10.36 3.18: 1 3.28: 1 cristobalite 93.92 33.78 10.22 3.30: 1 3.41: 1

(* <> under ZuaaU catalytic amounts of alkali)

The results show that a tempered at temperatures above 1100 ⁰ C quartz, in particular a quartz which has been annealed at temperatures of 1300 ⁰ C and higher, in a surprising manner to a higher conversion of the crystalline Si0 ₂ -Component) and thus to a higher molar ratio of the sodium silicate solution than the corresponding untreated silicide.

Examples

The effect of faster conversion of quartz tempered at high temperatures, the Christobalite, as compared to a non-tempered quartz, is documented in the figure. This figure shows the result of reacting tempered quartz, ie cristobalite, together with 20 wt% sodium hydroxide solution with a 5% excess of silica, based on a molar ratio of 3.46: 1, in a pressure vessel at 215 ° C and at reaction time from 15,30,60 and 120 min.

In each case, the molar ratio of silica to sodium oxide was determined. For comparison, a quartz untreated quartz, so a sand, was reacted under the reaction conditions described above and again taken after the reaction times described above samples to determine the molar ratio. This curve is provided with the reference numeral 2.

From the figure erc ibt clear that in the manufacturing process according to the invention, in which a tempered quartz is used, already about 15min a reaction of over 80% is carried out and the reaction is quasi quantitative even after 30min reaction time.

On the other hand, in the comparison curve according to reference numeral 2, a conversion of about 40 to 50% results after 15 minutes, and after a reaction time of 120 minutes, a maximum degree of conversion of only 70% is achieved, where - in

Correspondence with the literature - even after several hours only a maximum molar ratio SiO ₂ ZNa ₂ O of 2.8: 1 is achievable.

This aptly shows the advantages of the production process of the invention caused by the tempering of the quartz sand at high temperature.

Claims (5)

1. A process for the hydrothermal preparation of sodium silicate solutions with high SiO ₂ : Na ₂ O molar ratio by reacting a crystalline silica with aqueous sodium hydroxide solution, characterized in that as a crystalline silica at temperatures in the range of about 110O ⁰ C to the melting point tempered quartz and reacts this tempered quartz with aqueous sodium hydroxide solution in a concentration range of 10 to 50 wt .-% at temperatures of 150 to 300 ⁰ C and corresponding to these pressures of saturated water vapor in a closed pressure reactor. 2. The method according to claim 1, characterized in that the resulting sodium silicate solution has a SiO ₂ : Na ₂ 0 molar ratio of 2.9 to 3.7: 1, preferably 3.0 to 3.6: 1, particularly preferably 3.3 up to 3.5: 1. 3. The method according to claims 1 or 2, characterized in that one uses as a crystalline silica at temperatures of 1200 to 1700 ⁰ C with the addition of catalytic amounts of alkali tempered quartz, which converts under these conditions substantially in cristobalite, and that the so-tempered quartz with aqueous sodium hydroxide solution in a concentration range of 15 to 30 wt .-%, preferably 20 to 30Gew .-%, at temperatures of 200 to 25O ⁰ C and corresponding to these temperatures pressures of saturated water vapor in a closed pressure reactor. 4. The method according to any one of claims 1 to 3, characterized in that one uses as a crystalline silica at temperatures in the range of 1300 to 1600 ⁰ C with the addition of catalytic amounts of alkali tempered quartz, which converts under these conditions substantially in cristobalite , 5. The method according to any one of claims 1 to 4, characterized in that the translation with an excess of tempered quartz of up to 100 mol .-%, preferably 1 to 10 mol .-% and in particular 2 to 5 mol .-%, based on the desired -SiO ₂ : Na ₂ O molar ratio in the sodium silicate solution, carried out.