EP0456707A1 - HYDROTHERMAL PROCESS FOR PREPARING SODIUM SILICATE SOLUTIONS WITH A HIGH SiO 2?:Na 2?O MOLAR RATIO - Google Patents

Abstract

According to a process for the hydrothermal production of sodium silicate solutions having a high molar ratio between SiO2 and Na2O, crystalline silicon dioxide is converted with an aqueous solution of sodium hydroxide. This process is characterized by the fact that one uses quartz stabilized at temperatures above 1100 ° C up to its melting point as crystalline silicon dioxide, and that this stabilized quartz is converted with a solution. aqueous sodium hydroxide having a concentration between 10 and 50% by weight in a pressurized fluid reactor at temperatures between 150 and 300 ° C and under saturated water vapor pressures corresponding to these temperatures.

Description

Process for the hydrothermal production of sodium silicate solutions with a high SiO ₂ : Na ₂ O molar ratio

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

A general overview of the production of aqueous sodium silicate solutions is given in the monographs Winnacker, Küchler, Chemische Technologie, Volume 3, Inorganische Technologie II, 4th edition, 1983, pp. 54-63 and Ullmanns Encyklopadie der Technische Chemie, Volume 21, 4th edition , 1982, pp. 409-412.

Of the alkali metal silicates known under the name "water glass", sodium silicate solutions - generally referred to as sodium water glass - are most frequently used for technical purposes. Sodium water glasses of this type predominantly have a solids content of about 30 to 40% by weight and a molar ratio of silicon dioxide to sodium oxide of 3.4 to 3.5: 1. The production of soda water glasses on an industrial scale is generally carried out by melting together quartz sand and soda in suitable ovens at temperatures in the range from 1,400 to

1 500 ° C. The melt which solidifies on cooling, the solid glass, is then dissolved in water in a further process step using pressure and elevated temperatures, and the resulting solution is filtered, depending on the quality requirement. However, this high-temperature melting process is very expensive both in terms of equipment and in terms of the amount of energy required and also leads to not inconsiderable emissions, such as dust, nitrogen oxides and sulfur oxides.

In addition to this high-temperature melting process, which is mainly used in industry, hydrothermal processes for the preparation of aqueous sodium silicate solutions are also known and 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 process products obtained in this way are of poor quality owing to the usual impurities in the fly dusts and the natural amorphous silicon dioxide compounds which are used as input materials and can therefore be used only to a limited extent for technical products.

DE-AS 2826432 relates to a process for the preparation of water glass solutions by reacting fly dusts, which are obtained when silicon or ferrosilicon alloys are obtained, with aqueous alkali metal hydroxide solutions at elevated temperatures and then filtering the solutions contained, which is characterized in that Fly dust treated with a 6 to 15 wt .-% aqueous alkali metal hydroxide solution at temperatures from 120 ° C to 190 ° C and a pressure of 2.9 to 18.6 bar in an autoclave, the weight ratio of alkali metal hydroxide solution to solid fly dust 2 : 1 to 5: 1. The process products have a SiÜ2: Na ₂ O molar ratio of 2.2 to 4: 1. The fly dusts used as starting materials have a silicon content from 89 to 98% by weight, which according to the exemplary embodiments is always 90% by weight; the rest consists of impurities.

DE-OS 26 09 831 relates to a process for the preparation of silicon dioxide-containing, environmentally harmful waste dusts from silicon metal and silicon alloy production to form silicas or silicates, which is characterized in that the following process steps I to III are combined:

 I dissolving the fly dusts in alkali hydroxide solutions to form alkali silicate solutions;

 II Purification of the alkali silicate solutions from organic components by treatment with activated carbon and / or oxidizing agents and separation of the non-digestible residue from the solution;

 III implementation of the alkali silicate solutions with inorganic or organic acids and / or their salts for the purpose of further purification.

The alkali silicate solutions obtained in this way generally have a SiO ₂ : Na ₂ O molar ratio in the range from 3.3 to 5.0: 1.

DE-OS 26 19604 relates to a process for the production of liquid water glass from amorphous silicon dioxide and alkali metal hydroxide, which is characterized in that silicon dioxide dust in the form of fly ash which has been separated from the exhaust gases from ferroalloy industries and other industries working with silicon furnaces, Alkali hydroxide and water are mixed in a certain weight ratio and then brought to a temperature between 75 and 100 ° C with stirring, after which the liquid obtained is cooled. The silica dusts used as the starting material for this water glass production have in generally has a silicon dioxide content of 94 to 98 wt .-%, the rest consists of impurities.

DE-AS 23 28 542 relates to a process for the preparation of AT potassium silicates by treatment of perlite with an alkali metal hydroxide solution and hydrothermal treatment of the pulp obtained in the autoclave with subsequent filtration, which is characterized in that an alkali solution is treated with an alkali solution to treat the perlite Concentration of 40 to 140 g / l Na ₂ O is used in an amount in which the ratio of the liquid phase to the solid phase is 0.7 to 1.5: 1. Perlite is an essentially amorphous, glass-like mountain rock of volcanic origin, which mainly consists of (in% by weight): silicon dioxide 73, aluminum oxide 15 and other oxides 8.

As the above explanations show, the water glasses described in the patent literature and obtained from amorphous silicon dioxide always only provide process products with poor properties that have to be subjected to further cleaning.

The prior art described below relates to processes for the hydrothermal production of sodium silicate solutions from crystalline silicon dioxide, that is to say sand, and sodium hydroxide solution, which according to the processes of the prior art, however, only have an SiO ₂ : Na ₂ O molar ratio of less than 2, 89: 1 can be implemented.

DE-OS 30 02857 relates to a process for the hydrothermal production of sodium silicate solutions with a molar ratio SiO ₂ : Na2θ of 1.03 to 2.88: 1 by reacting sand with aqueous sodium hydroxide solution under pressure and at elevated temperatures and subsequent filtration, which thereby is marked that the aqueous sodium hydroxide solution has a concentration of 10 to 50% by weight with an excess of sand of up to 300%, based on the molar ratios of SiO ₂ : Na ₂ O in the batch, at temperatures in the range from 150 to 250 ° C. and converts the pressures of saturated water vapor corresponding to these temperatures, and uses the unreacted excess of sand in whole or in part as a filter medium for the sodium silicate solution formed. According to the exemplary embodiments of this published patent application, however, a maximum SiO ₂ = Na ₂ O molar ratio of 1.68: 1 is achieved.

DE-OS 34 21 158 relates to a process for the hydrothermal production of sodium silicate solutions with a molar ratio of SiO ₂ : Na2θ of 1.96 to 2.17 by reacting excess sand with aqueous sodium hydroxide solution, which is characterized in that one has an excess of sand and a reaction mixture containing preheated aqueous sodium hydroxide solution is reacted in a rotating, cylindrical, closed pressure reactor until a certain molar ratio of SiO ₂ : Na ₂ O is reached and then filtered using the excess sand and optionally an additional filter aid. In the exemplary embodiments, aqueous sodium silicate solutions having a SiO ₂ : Na ₂ O molar ratio of up to 2.27: 1 are disclosed.

DE-OS 33 13 814 relates to a process for the preparation of a clear solution of a sodium silicate, the molar ratio of silicon dioxide: sodium oxide being at most 2.58: 1, by digestion of crystalline silicon dioxide with an average grain size between 0.1 and 2 mm, in which an aqueous solution of sodium hydroxide passes through a bed of silicon dioxide which is formed in a vertical tubular reactor without mechanical movement and which is fed from top to bottom with silicon dioxide and the aqueous solution of sodium hydroxide. Belgian patent 649 739 relates to a method and an apparatus for the production of clear sodium silicate bases by dissolving a material containing silicic acid at high temperature and under pressure in aqueous caustic soda solution, which is characterized in that the product consists of the excess material containing silicic acid and / or the insoluble contaminated substances are separated by means of filter elements which are arranged in the vicinity of the reactor floor, said filtration advantageously taking place under the temperature and pressure conditions which are very similar to the reaction conditions. The aqueous sodium silicate solutions obtained in this way have a SiO ₂ : Na ₂ O molar ratio of about 2.5: 1.

Such hydrothermal processes for producing soda water glasses from sand and caustic soda are also discussed in summary in the monographs by Winnacker, Küchler and Ullmann already cited above. At Winnacker, Küchler it says

(Pages 61 and 62) that, however, only a sodium water glass with a SiO ₂ / Na ₂ O ratio of less than 2.7 can be achieved in the hydrothermal process at the temperatures normally used. In this context, Ullmann mentions that only sodium silicate solutions with SiO ₂ / Na ₂ O molar ratios up to 2.5: 1 can be obtained in this way (page 412, left column).

On the basis of the literature cited above, there was therefore a direct prejudice regarding the production of sodium silicate solutions with higher SiO ₂ / Na ₂ O molar ratios in the hydrothermal process from sand, ie from crystalline SiO ₂ , and sodium hydroxide solution.

The present invention is based on the object of a method for hydrothermal. Production of sodium silicate solutions by reacting a crystalline silicon dioxide with to provide an aqueous sodium hydroxide solution in which sodium silicate solutions with molar ratios of SiO ₂ / Na ₂ O of more than 2.9: 1 are achieved.

The object according to the invention is achieved by using a specially tempered quartz which is reacted with sodium hydroxide solutions under special reaction conditions.

The present invention thus relates to a process for the hydrothermal production of sodium silicate solutions with a high SiO ₂ : Na ₂ O molar ratio by reacting a crystalline silicon dioxide with an aqueous sodium hydroxide solution, which is characterized in that

that one uses a quartz tempered at temperatures in the range from over 1100 ° C. to the melting point as crystalline silicon dioxide and

reacting this tempered quartz with aqueous sodium hydroxide solution in a concentration range from 10 to 50% by weight at temperatures from 150 to 300 ° C. and the pressures of saturated water vapor corresponding to these temperatures in a pressure reactor.

The process according to the invention is technically easier to handle due to its one-stage process control and is therefore more cost-effective than the technically complex, large amounts of energy which are high and pollute the prior art, that is to say the high-temperature melting process followed by a solution step.

Compared to the hydrothermal processes of the prior art, the process according to the invention has the advantage that, by using the quartz specially tempered according to the invention, sodium silicate solutions with a molar ratio of SiO ₂ : Na ₂ O of more than 2.9: 1 can be obtained, which is not possible using non-tempered quartz as the SiO ₂ component.

Furthermore, it has surprisingly been found that quartz tempered in this way, preferably a cristobalite formed in this way, is capable of direct, single-stage preparation of aqueous sodium silicate solutions which have a SiO ₂ molar ratio, even with short reaction times, as part of a hydrothermal synthesis under the conditions specified above : Na ₂ O greater 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 process according to the invention. The use of a readily soluble crystalline silicon dioxide modification allows sodium silicate solutions with a high silicon dioxide / sodium oxide molar ratio to be produced, this being done in high space / time yields with minimal energy consumption.

The sodium silicate solutions thus obtained preferably have an 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 ° C with the addition of catalytic amounts of alkali as crystalline silicon dioxide, which essentially converts to cristobalite under these tempering conditions, and in that the quartz thus tempered with aqueous sodium hydroxide solution in a concentration range from 15 to 30% by weight, preferably 20 to 30% by weight, at temperatures from 200 to 250 ° C. and these temperatures corresponding pressures of saturated water vapor in a closed pressure reactor.

Like quartz, cristobalite is a crystal modification of silicon dioxide. This is practically exclusively produced synthetically by calcining quartz, by continuously converting quartz sand at temperatures of approx. 1500 ° C with the addition of catalysts (alkali compounds). With regard to more detailed information on cristobalite, reference is made to Ullmann's Encyclopedia of Industrial Chemistry, Volume 21, 4th edition, 1982, pp. 439-442.

For the purposes of the invention, it is therefore particularly preferred to use, as crystalline silicon dioxide, a quartz tempered 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, yet 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 using an excess of tempered quartz of up to 100 mol%, preferably 1 to 10 mol%, based on the desired target SiO ₂ : Na ₂ O molar ratios in the sodium silicate solution. In general, the reaction can also be carried out with excesses greater than 100 mol% of tempered quartz; however, this is generally not technically sensible. For the purposes of the invention, it is particularly preferred to carry out the reaction with an excess of 2 to 5 mol% of tempered quartz, based on the desired SiO ₂ : Na ₂ O molar ratio. In general, all reactors customary for the hydrothermal synthesis of sodium silicate can be used to carry out the process according to the invention. These include, for example, rotating solvers, standing solver arrangements, reactors with agitators, jet loop reactors, tubular reactors and, in principle, all reactors that are suitable for converting solids with liquids under pressure. Such reactors are described in detail, for example, in DE OS 30 02857, DE-OS 34 21 158, DE-AS 2826 432, BE-PS 649 739, DE-OS 33 13814 and DE-PS 968034.

The sodium silicate solutions (sodium silicate solutions) produced in the manner according to the invention can be used for all common uses known to the person skilled in the art and described in the relevant literature, for example for the production of fillers (precipitated silicas), as adhesives, as binders in Paints, foundry auxiliaries, catalyst carriers, as a component in detergents and cleaning agents, and as a component 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. In the exemplary embodiments, a cristobalite obtained by tempering at 1300 to 1600 ° C. and alkali catalysis was used as the annealed quartz.

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

A horizontally arranged cylindrical pressure vessel served as the reactor for carrying out the tests on an industrial scale made of steel with a nickel lining with an empty volume of approx. 24 m ³ . The pressure vessel rotated around 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 an attached pipe with an effective distribution directly into the reaction vessel.

The crystalline SiO ₂ used for the examples, the cristobalite obtained from tempered quartz, contained> 99.0% by weight SiO ₂ .

The aqueous sodium hydroxide solution (sodium hydroxide solution) required for the process was heated to about 103 ° C. with vapors from the precharge via a venturi nozzle above the sodium hydroxide solution storage container.

The quantities of substance (cristobalite / sodium hydroxide solution) were recorded using weighing devices. The raw materials were filled into the reactor, which was then sealed and set in rotation. The reaction mixture was heated to the reaction temperature of about 215 ° C. by direct introduction of steam and kept at this temperature. After a reaction time of 30 minutes at this temperature, the reactor was brought to a standstill and the reaction mixture was transferred via a flanged pipeline by means of its own pressure into a blow-out container. In this way, the reaction mixture was then separated into vapors and water glass solution at about 105 ° C. using a cyclone separator. The vapors were sucked in by a jet apparatus and used to preheat the mixed liquor of the next batch in a venturi nozzle up to the limit of the boiling point of the liquor of approximately 103 ° C.

The further processing of the water glass solution, which was approx. 100 ° C., was either carried out roughly via a sedimentation container Separation of solids or - with higher demands on the clarity of the solution - carried out via a filter.

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

The conditions of Example 4 were chosen as reaction conditions on an industrial scale. The batch size was 24,000 kg. The approximately 41% sodium water glass solution obtained had an 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 using cristobalite / NaOH solution can take place at relatively high solid concentrations in the reactor, since even at high SiO ₂ : Na ₂ O molar ratios under reaction conditions (215 ° C / 20 bar) the sodium silicate solution has sufficient viscosity range for the process. After completion of the reaction, either

- Under pressure directly into the reactor or

 - In the blow-out line to a storage container during the blow-out process

additional water are fed in, so that the sodium silicate solution that has reached the discharge container via the blow-out line is sufficiently diluted in such a way that the sodium silicate solution in the initial charge at temperatures of approximately 100 ° C. has a flowable, sufficiently low viscosity before further processing by sedimentation or filtration Has consistency.

example 1 Example 1 demonstrates a batch which is favorable with regard to the relatively low starting liquor concentration when using stoichiometric cristobalite, based on a target ratio of SiO ₂ : Na ₂ O in the sodium silicate solution of 3.46: 1.

Example 2

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

Example 3/4

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

Example 5/6

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

Table 1; Recognition ratios

Example feedstocks and quantities% NaOH molar ratio

No. Cristobalite Sodium hydroxide solution concentration Cristobalite: Na ₂ O

 (g) (g) in approach * 3

1 49 94.02 20.0 3.46: 1

 2 49 62.68 30.0 3.46: 1

3 ^{* 1} 49 91.28 20.0 3.56: 1

4 ^{* 2} 49 89.54 20.0 3.63: 1

5 ^{* 2} 49 89.54 20.0 3.63: 1

6 ^{* 2} 49 89.54 20.0 3.63: 1

* 1 cristobalite excess 3%, based on a desired molar ratio

SiO ₂ s Na ₂ O in the solution of 3.46: 1

* 2 Cristobalite excess 5%, based on a target molar ratio

SiO ₂ s Na ₂ O in the solution of 3.46: 1

* 3 taking into account all SiO ₂ - and Na ₂ O- in the reactor

Components

Table 2; Reaction conditions and product characterization

Example HT reaction conditions sodium silicate solution ^{* 1} molar ratio No. reaction time reaction temp. % SiO ₂ % Na ₂ O SiO ₂ / Na ₂ O

 (min) (° C)

1 30 215 33.22 10.31 3.32: 1

2 30 215 43.07 13.17 3.37: 1

3 30 215 33.66 10.24 3.39: 1

4 30 215 33.78 10.22 3.41: 1

5 60 215 34.14 10.17 3.46: 1

 1 120 215 37.27 10.15 3.48: 1

* 1 Composition of the end product in the reactor.

 Before filtration, the solution is diluted to a solids content of <41% solids in the sodium silicate solution.

(HT stands for "hydrothermal")

Example 7

In order to demonstrate the influence of the annealing temperature of the quartz on the product properties of the aqueous sodium silicate solutions produced, quartz was first annealed at temperatures of 850 ° C. to 1600 ° C. with the addition of catalytic amounts of alkali and then reacted hydrothermally with sodium hydroxide solution. As a comparison, untreated quartz was converted to sodium silicate in the same standard experiment of hydrothermal conversion with sodium hydroxide solution.

In the standard test of the hydrothermal conversion of the tempered quartz with sodium hydroxide solution, the following test conditions were chosen:

 Reaction temperature 215 ° C;

Reaction time 30 min;

 Sodium hydroxide concentration 20% by weight;

 Excess silicon dioxide 5% (based on the molar ratio of 3.46: 1 for soda glass)

The conversion of this tempered quartz in sodium hydroxide solution to a sodium silicate was carried out with the quantities of the starting materials listed below and led to the conversions and molar ratios shown in the table below.

Conversion of tempered sand in sodium hydroxide solution to Na glass. Molar ratio SiO ₂ / Na ₂ O = 3.46: 1 for quantities of Na glass

35.37% tempered sand 49.00 g

25.85% NaOH, 50% 35.82 g 38.78% water 53.72 g

Sales% SiO ₂ % Na ₂ O wt.

Mating m

Theor. Quantities: 35.19 10.00 3.52: 1 3.63: 1 Sand (See) 58.73 24.20 11.71 2.07: 1 2.13: 1

850 ° C sand 48.11 20.74 12.25 1.69: 1 1.75: 1

850 ° C sand * 49.18 21.10 12.20 1.73: 1 1.78: 1

950 ° C sand 42.60 18.81 12.55 1.50: 1 1.55: 1

950 ° C sand * 48.00 20.70 12.26 1.69: 1 1.74: 1

1100 ° C sand * 57.50 23.82 11.77 2.02: 1 2.09: 1

1300 ° C sand * 86.81 32.05 10.49 3.05: 1 3.15: 1

1600 ° 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

(* = with the addition of catalytic amounts of alkali)

The results show that a quartz tempered at temperatures above 1100 ° C., in particular a quartz which has been tempered at temperatures of 1300 ° C. and higher, surprisingly leads to a higher conversion of the crystalline SiO ₂ component and thus to a leads to a higher molar ratio of the sodium silicate solution than the corresponding untreated sand.

Example 8

The effect of the faster conversion of quartz tempered at high temperatures, the cristobalite, compared to an untempered quartz, is documented in the figure. This figure shows the result of the implementation of annealed 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 with reaction times of 15, 30, 60 and 120 min. The molar ratio of silicon dioxide: sodium oxide was determined in each case. This curve has the reference number 1.

For comparison, a thermally untreated quartz, ie a sand, was reacted under the reaction conditions described above and samples were taken again after the reaction times described above in order to determine the molar ratio. This curve is provided with the reference number 2.

It is clear from the figure that in the production method according to the invention, in which a tempered quartz is used, a conversion of over 80% has already taken place after 15 minutes and the conversion is quasi quantitative even after a reaction time of 30 minutes.

In the comparison curve according to reference number 2, on the other hand, there is only a conversion of approx. 40 to 50% after 15 minutes, and even after a reaction time of 120 minutes, a maximum degree of conversion of only 70% is achieved, whereby - in accordance with the literature - also after several hours only a maximum SiO ₂ / Na ₂ O molar ratio of 2.8: 1 can be achieved.

This aptly shows the advantages of the production method according to the invention brought about by tempering the quartz sand at high temperature.

Claims

Patent claims

1. A process for the hydrothermal production of sodium silicate solutions with a high SiO ₂ : Na ₂ O molar ratio by reaction of a crystalline silicon dioxide with an aqueous sodium hydroxide solution, characterized in that

that one uses a quartz tempered at temperatures in the range from over 1100 ° C. to the melting point as crystalline silicon dioxide and this quartz quartz with aqueous sodium hydroxide solution in a concentration range from 10 to 50% by weight at temperatures from 150 to 300 ° C. and the latter Pressures of saturated water vapor corresponding to temperatures are converted in a closed pressure reactor.

2. The method according to claim 1, characterized in that the sodium silicate solution obtained has a SiO 2: Na 2 O molar ratio of 2.9 to 3.7: 1, preferably 3.0 to 3.6: 1, particularly preferably 3.3 to 3, 5: 1.

3. Process according to claims 1 or 2, characterized in that a quartz tempered at temperatures of 1200 to 1700 ° C with the addition of catalytic amounts of alkali is used as crystalline silicon dioxide, which essentially converts to cristobalite under these conditions,

and that the quartz so tempered with aqueous sodium hydroxide solution in a concentration range of 15 to 30% by weight, preferably 20 to 30% by weight, at temperatures of 200 to 250 ° C. and the pressures of saturated steam corresponding to these temperatures in one closed pressure reactor.

4. The method according to any one of claims 1 to 3, characterized in that one uses as the crystalline silicon dioxide at temperatures in the range of 1300 to 1600 ° C with the addition of catalytic amounts of alkali quartz which essentially converts to cristobalite under these conditions .

5. The method according to any one of claims 1 to 4, characterized in that the reaction with an excess of annealed quartz of up to 100 mol%, preferably 1 to 10 mol% and in particular 2 to 5 mol%, based on the target SiO ₂ : Na2θ molar ratio in the sodium silicate solution.