EP0502083A1

EP0502083A1 - Method for producing alkali metal silicates

Info

Publication number: EP0502083A1
Application number: EP91900249A
Authority: EP
Inventors: Johannes W. Hachgenei; Rudolf Novotny; Peter Christophliemk; Hans Dolhaine; Jürgen Föll
Original assignee: Henkel AG and Co KGaA
Current assignee: Henkel AG and Co KGaA
Priority date: 1989-11-23
Filing date: 1990-11-14
Publication date: 1992-09-09
Also published as: WO1991008169A1; HU9201697D0; TR24637A; FI922320A; FI922320A0; ZA909392B; KR920703447A; US5215732A; AR242543A1; DE3938729A1; YU220590A; BR9007867A; CA2069477A1; CN1052645A; HUT61247A; AU635846B2; JPH05503066A; PL287887A1; IE904224A1; AU7895791A

Abstract

Selon un procédé de production de silicates de métaux alcalins à partir d'un matériau cristallin contenant du SiO2 et d'une solution aqueuse d'un hydroxyde de métal alcalin à une température élevée et sous une pression normale, on utilise comme matériau cristallin contenant du SiO2 de la cristobalite et/ou du sable silicieux durci par trempe et on fait réagir ce matériau dans une solution aqueuse de 20 à 50 % en poids d'hydroxyde de sodium ou de potassium, le rapport molaire entre le SiO2 et le Na2O ou le K2O dans le mélange de réaction étant compris entre 2:1 et 1:7.According to a process for producing alkali metal silicates from a crystalline material containing SiO2 and an aqueous solution of an alkali metal hydroxide at an elevated temperature and under normal pressure, as the crystalline material containing SiO2 of cristobalite and / or silica sand hardened by quenching and this material is reacted in an aqueous solution of 20 to 50% by weight of sodium or potassium hydroxide, the molar ratio between SiO2 and Na2O or K2O in the reaction mixture being between 2: 1 and 1: 7.

Description

Process for the preparation of Al alimetansilicaten

The invention relates to a process for the production of alkali metal silicates from crystalline SiO 2 -containing material and aqueous alkali metal hydroxide solution at elevated temperature and normal pressure.

Alkali metal silicates, e.g. Water glasses are produced in large quantities and used in many areas both in solution and as a solid. These include detergents and cleaning agents, adhesives, paints, ore flotation and water treatment. They also serve as raw materials for the production of zeolites and silicas, sols and gels (Büchner et al. "Industrial Inorganic Chemistry", Verlag Chemie, 1984, p. 333).

Water glass solutions are customarily characterized by two physical sizes. On the one hand by the molar ratio Siθ2 / M2θ, called module below, and the solids content, i.e. the proportion by weight of SiO 2 and M2O in the solution, where M stands for Na or K. Both factors influence the viscosity of the alkali metal silicate solution.

The maximum solubility of an alkali metal silicate with a specific module can be determined from tables and diagrams. In general, higher solids contents in the solution can be achieved at higher alkali contents, ie a lower modulus. Water glasses up to a module of 4.3 are obtained via melting processes. Such melting processes have been known for the past century; only the implementation of is common today Quartz sand with soda at temperatures around 1500 ° C (Winnacker-Küchler, "Chemical Technology", C. Hanser Verlag, 4th edition (1983), Volume 3, "Inorganic Technology II", page 58 ff). Only a small part of the alkali metal silicates represented in this way is sold as solid glass. The main amount is then dissolved in water. In the case of glasses with a modulus> 2.0, the reaction rate at the reflux temperature of the solution is unsatisfactory, so that pressure digestion at 4-6 bar and 150 ° C. is preferred.

Low molecular weight (module <2.7), alkali-rich water glasses can also be produced hydrothermally from quartz sand and concentrated aqueous sodium hydroxide solution. Because of the low reactivity of the quartz sand, elevated temperatures and increased pressure are necessary for this. Technically, two processes are used: nickel-plated rotating pressure solvents at temperatures from 200 to 220 ° C and tube reactors at temperatures from 250 to 260 ° C (Winnacker-Küchler, op. Cit., Page 61 f).

Finely distributed, amorphous silicas dissolve exothermically in alkalis. For example, particularly pure alkali metal silicate solutions are sometimes obtained by reacting pyrogenic or precipitated silica with alkali alkali. Apart from special cases, this procedure is too expensive.

Amorphous silica is also obtained as a by-product or waste in various technical processes. Several processes for the use of such silicas have been described.

According to Przem. Chem. 67 (8) (1988) 384-6 (Chemical Abstracts 109_: 213144f), middle-modular sodium silicate solutions can be obtained from the waste silica of the AIF3 and HF production. According to JP-76 17519 (Chemical Abstracts, 86:19 116q), dust from the production of ferrosilicon contains approx. 90% by weight of highly reactive SiO2, which is mixed with 8.1% sodium hydroxide solution at temperatures around 90 ° C to form a water glass Solution with a high module can be implemented.

As a further alkali metal silicate, solid, crystalline, anhydrous sodium polysilicate (Na2Siθ2) _{oo has} technical importance as a builder component in washing and cleaning agents. This sodium silicate of the gross composition Na2Siθ3 contains infinite chains of SiOj tetrahedra which are linked by bridges to the sodium atoms. Such chain silicates are referred to in mineralogy as "inosilicates" and in chemistry as "polysilicates". The term "metasilicate" is also widespread but incorrect. In the following, the term "sodium polysilicate" is used exclusively.

On a large industrial scale, anhydrous sodium polysilicate is produced by an annealing reaction of quartz sand and soda in a rotary kiln at approx. 950 ° C (Büchner et al. "Industrial inorganic chemistry", Verlag Chemie, 1984, p. 333). The reaction time is about 45 minutes. At an even higher temperature, but then shorter reaction times, the polysilicate can also be obtained by melting sand and soda (Ullmann's Encyklopadie der Technische Chemie, Verlag Chemie, 1982, Vol. 21, p. 412).

Furthermore, US Pat. No. 3,532,458 describes the hydrothermal production of sodium polysilicate starting from quartz sand. For a complete reaction of the quartz sand with an aqueous sodium hydroxide solution, temperatures of approximately 200 ° C. at elevated pressure are necessary. DE-AS 1567 572 proposes to produce anhydrous, crystalline alkali metal silicates, preferably sodium polysilicate, by using finely divided, solid alkali metal silicate which is heated to a temperature of above 130 ° C. and is kept in constant motion Spraying an aqueous alkali metal silicate solution produces a film, and by means of an additional hot gas stream causes the water to evaporate, the coating and drying step being repeated until the size of the crystalline anhydrous alkali metal silicate particles has increased to the desired extent. In general, part of the largely anhydrous alkali metal silicate is returned to the continuous process as a starting component.

The disadvantages of this last-mentioned method are that a large number of sodium polysilicate grains are required as the inoculation basis in the sodium polysilicate which is kept in constant motion and that therefore a very large proportion of the spray granules obtained must be ground again and returned, so that ultimately the yield of these processes is low.

From DE-PS 968 034 it is also known to produce solid sodium polysilicate containing water of crystallization by finely divided silica, such as quartz sand or quartz powder, and aqueous sodium hydroxide solution in a ratio such as the ratio of alkali metal oxide: SiÜ2 in the product to be produced roughly corresponds to, mixes homogeneously, continuously feeds the mixture into a reaction tube against the pressure prevailing in it, and it leads through the reaction tube at temperatures above about 175 ° C. and at elevated pressure with the proviso that a uniform distribution of the silica in the mixture is ensured by regulating the linear flow rate. The hot reaction product from the reaction tube is then let through Relief valve exit, the originally higher water content of the reaction product formed being reduced to the desired water content of the end product as a result of the water evaporation occurring during the relaxation. In this way it is possible to produce sodium polysilicate hydrates with less than 9 moles of crystal water.

From NL-OS 7802697 it is also known to produce sodium silicate solutions by passing sand with sodium hydroxide solution at elevated pressure and a temperature of at least 200 ° C. through a pipe which can be used for the continuous digestion of bauxite and, for example, from the DE-OS 21 06 198 and DE-0S 25 14 339 is known. The manufacture of metasilicate products is preferably carried out at a temperature of 200 to 240 ° C; For the production of products with a higher ratio of SiO 2 ^{: Na} 2 (), temperatures of 240 to 280 ° C are preferably used. The pressure in the tube is preferably in the range between 10,000 and 20,000 kPa. According to the method described in this laid-open publication, however, only solutions and no solid products are produced.

DE-OS 31 24 893 describes a process for the production of water-free sodium polysilicate by treating quartz sand and / or quartz powder with concentrated aqueous sodium hydroxide solution under pressure at a temperature in the range from 200 to 400 ° C.

R0 75620 (Chemical Abstracts 100: 24023u) describes a process for the production of crystalline sodium polysilicate with a module of 1: 1 from waste containing silicon dioxide from the production of fertilizers. This process is characterized in that the solution containing sodium polysilicate must first be filtered in order to remove impurities before the Filtrate is concentrated. The crystallization then takes place when the solution is cooled to a temperature of 10 to 15 ° C.

SU-434060 (Chemical Abstracts 82: 45938w) describes a process for the production of sodium polysilicate from volcanic ash.

JP-73/16438 (Chemical Abstracts 80: 17050r) describes a process for the production of solutions containing sodium polysilicate from flue gas residues.

However, these three last-mentioned processes have the disadvantage that the SiO 2 sources used are contaminated and the separation of the contaminants presents considerable difficulties, so that these processes have so far not been able to establish themselves (Winnacker-Küchler, op. Cit., Page 61).

In contrast, it is an object of the present invention to develop a process for the preparation of alkali metal silicate solutions and suspensions which can be carried out using crystalline SiO 2 material at normal pressure and using relatively low temperatures with short reaction times.

The object was achieved according to the invention by a process for the production of alkali metal silicates from crystalline material containing SiO 2 and aqueous alkali metal hydroxide solution at elevated temperature and normal pressure, characterized in that cristobalite and / or tempered quartz sand were used as the crystalline material containing SiO 2 uses and this material with aqueous, 20 to 50 wt .-% sodium or potassium hydroxide solution at temperatures in the range of 100 to 150 ° C and under normal pressure, the molar ratio of SiÖ2 to Na2θ or K2O in the reaction mixture is in the range from 2: 1 to 1: 7.

Compared to quartz, cristobalite (or tridymite) has a higher reactivity because of its more open structure (density of quartz = 2.65 g / cπ.3, cristobalite / tridymite = 2.3 g / cπ.3). In addition to cristobalite, tridymite, optionally together with amorphous silicon dioxide, can also be used for the process according to the invention, which likewise has a higher reactivity because of its structure, which is more open than that of quartz. Tempered quartz sands behave similarly, i.e. Quartz sands which have been annealed above 1000 ° C, preferably at 1300 to 1600 ° C, with the addition of catalytic amounts of alkali and which are composed of cristobalite, tridymite and optionally amorphous silicon dioxide. By tempering quartz sand, as described in the unpublished German patent application P 3938 730.5, reactive SiO 2 phases are obtained which are composed, inter alia, of cristobalite, tridymite and amorphous SiO 2. The subject of this patent application is a process for the production of reactive silicon dioxide phases, which is characterized in that quartz sand is mixed with an alkali metal compound or its aqueous solution, the alkali metal compound being selected from the group of compounds which are used in Heating into the corresponding alkali metal oxides, that the molar ratio of SiO 2 to alkali metal oxide is between 1: 0.0025 and 1: 0.1 and that this mixture is heated to a temperature between 1100 ° C. and 1700 ° C.

In connection with the implementation of the method according to the invention, reference is expressly made to the disclosure of this patent application. The tempered quartz sands are obtained from slightly contaminated starting compounds, ie quartz sands as are also used for the production of water glass in the melting process. This has the advantage that no additional residues occur during the working up, that is to say, if appropriate, the filtration of the alkali metal silicate solutions, and therefore an already mature technology can be used. This is in contrast to the problematic processing of the alkali metal silicate solutions, which are obtained from waste silicas and have already been discussed.

The aqueous sodium or potassium hydroxide solutions used for digestion have a concentration of 20 to 50% by weight, for NaOH in particular a concentration of 40 to 50% by weight, in particular 50% by weight of sodium hydroxide, which corresponds to the technically available product corresponds. The concentration of the potassium hydroxide solution is preferably 40 to 50% by weight, in particular 47 to 50% by weight.

The tempered quartz sands are reacted with aqueous sodium hydroxide or potassium hydroxide at the boiling point of the respective alkali or the resulting alkali metal silicate solution or suspension (cf. Examples, Table 1). Depending on the salinity of the solution, the boiling temperature is between 150 and 100 ° C and is not constant since the salinity changes during the course of the reaction.

As an example of the tempered quartz sand, cristobalite was used as the SiO 2 source. The particle size was generally 0.1 to 0.8 mm.

The process according to the invention can be carried out batchwise or continuously and is suitable depending on the process from the SiO 2 / alkali metal oxide module used to produce alkali metal silicate suspensions or solutions. With sodium hydroxide, suspensions of sodium polysilicate result in the module range SiO 2: Na 2 O of 1.2: 1 to 1: 2, preferably 1: 1; ie the sodium polysilicate is obtained as a solid, crystalline phase. With a module SiO 2: Na 2 O of 2: 1, amorphous sodium silicates are formed, which are also obtained in solid form, ie as a suspension. In module ranges SiO 2: Na 2 O of 1:> 2, ie from 1:> 2 to 1: 7, on the other hand, soluble, alkali-rich sodium silicates, ie aqueous sodium silicate solutions, are obtained with sodium hydroxide.

With potassium hydroxide, when the process according to the invention is carried out over the entire module range SiO 2: K 2O from 2: 1 to 1: 7, only soluble alkali-rich potassium silicates result, i.e. aqueous potassium water glass solutions.

For the purposes of the present invention, it is preferred that the crystalline material containing SiO 2 is reacted with aqueous sodium hydroxide solution, the molar ratio of SiO 2 to Na 2 O in the reaction mixture being in the range from 1.2: 1 to 1: 2, preferably of 1: 1.

It is also preferred according to the invention that the crystalline SiO 2 -containing material is reacted with aqueous sodium hydroxide solution, the molar ratio of SiO 2 to Na 2 O in the reaction mixture being 2: 1.

Table 2 (see examples) shows the times depending on the module used and the concentration of the alkali metal hydroxide reproduced, which were necessary for the complete dissolution of the SiO 2 used.

Surprisingly, it has now been found that - according to a preferred embodiment of the invention - tempered quartz sand dissolves completely in 50% sodium hydroxide solution within three hours even at an SiO 2 / Na 2 O ratio of 2: 1. With a solids content of 65%, the solubility of the alkali metal silicate formed is clearly exceeded, and towards the end of the reaction a viscous mass is obtained which can hardly be stirred. In other words, this means that the SiO 2 used can be converted to sodium licat without pressure, which is readily soluble in water.

From the examples (Table 2) it is clearly evident that when the modulus to be achieved is reduced (that is, less SiO 2 based on Na 2 O), the time until the SiO 2 is completely dissolved decreases. On the other hand, the rate of dissolution also decreases with decreasing concentration of the alkali metal hydroxide.

The particular advantage of this method becomes clear when comparing the values from Table 3 (comparative examples). It can be seen that untreated quartz sand dissolves in sodium hydroxide solution much more slowly. With a module of 1: 1, however, tempered quartz sands are completely dissolved in boiling 50% sodium hydroxide solution within 2 hours. In contrast, quartz sand is only 43% dissolved after 6.5 hours, quartz powder with a larger surface is only 62% reacted after 6 hours.

To achieve the highest possible reaction rate, the process is advantageously carried out at the boiling point of the aqueous alkali metal hydroxide solution or the resulting solution Alkali metal silicate solution or suspension performed. Lower temperatures slow down the reaction. Higher temperatures would increase the reaction rate, but they require increased pressure and therefore pressure vessels, which make such a process less economical. When reference is made to normal pressure in connection with the method according to the invention, this is understood to mean the usual ambient pressure of approximately 1 bar. In other words, this means that in the sense of the present invention, work is carried out without increased pressure.

In experiments 1, 2, 4 to 7 and 10 (cf. examples, table 2) the solubility of the sodium silicate formed was exceeded. In experiments 1 and 2 with the module 2: 1, the solid that was still warm separated was radiopaque orph.

In the reactions with a molar ratio SiO 2 / Na 2 O of 1.2: 1 to 1: 2, preferably 1: 1, solid, crystalline was formed

Sodium polysilicate, Na2Siθ3, which was characterized by means of X-ray diffraction diagrams (compared to JCPDS file No. 16-818 - Joint Committee for Powder Diffraction Standards). The silicate was readily water-soluble and, after drying, had an insoluble content of 0.015 to 0.18%. The same crystalline silicate was obtained in the reactions with 50% sodium hydroxide solution and operating conditions SiO 2 / Na 2 O of 1: 1.5 and 1: 2 (experiments 7 and 10).

The sodium silicate suspensions obtained can be diluted by adding water until the solubility of the alkali metal silicates is below.

In further experiments with the higher alkali contents, ie module SiO 2: Na 2 O = 1:> 2, sodium silicate solutions with low insoluble fractions were formed, which are caused by impurities in the Originating connections. The less cloudy solutions can be clarified by filtration. Also in experiments 22 to 24 carried out with potassium hydroxide (cf. examples, Tables 1 and 2), solutions of potassium silicate were formed which were also almost clear _f

As described above, it has surprisingly been found that the increased reactivity of the compounds mentioned, i.e. of cristobalite and / or tempered quartz sand, can be used advantageously for the production of solid, crystalline sodium polysilicate. Furthermore, in the process according to the invention for the production of sodium polysilicate, it is not necessary to purify the polysilicate obtained by a cleaning step preceding the crystallization.

The preferred application of the process according to the invention for the production of sodium polysilicates is demonstrated by means of further examples (Table 4). In this preferred embodiment of the present invention, too, the process is carried out in the temperature range from 100 ° to 150 ° C. at normal pressure. In this temperature range, the process according to the invention can be carried out in an open reaction vessel, since the high salt content of the reaction mixture causes the boiling point of the aqueous reaction mixture to shift towards higher temperatures. For example, in the reaction of cristobalite with aqueous 50% by weight sodium hydroxide solution in a molecular ratio of SiÜ2 'Na2θ of 1: 1 at an initial temperature of about 150 ° C and at normal pressure (1 bar) after a reaction time of 2 hours a sodium polysil cat can be obtained which contains water-insoluble residues of only 0.015% by weight. As will be explained in more detail with reference to the examples (Table 4), cristobalite or tempered quartz sand, ie cristobalite, trydimite and amorphous silicon dioxide, was reacted with the stated amounts of aqueous alkali to carry out the process. The reactions were carried out in a glass flask at normal pressure.

According to a further embodiment of the present invention, after the reaction for the production of solid, crystalline sodium polysilicate (SiO 2: Na 2 O module = 1: 1) has ended, the temperature is still 70 to 130 ° C, preferably 90 to 110 ° C, warm Filter the suspension through a suction filter. A concentration or cooling of the reaction solution to initiate or improve the crystallization is not necessary in the process according to the invention. The filtrate (mother liquor) obtained during the filtration is preferably returned to the process after concentration. The sodium polysilicate remaining as a filter residue is usually still crushed warm (at 70 to 90 ° C) and then dried under reduced pressure (1333 Pa to 26664 Pa) under elevated temperature (100 to 150 ° C) to anhydrous sodium polysilicate . The drying time can be between 5 and 15 hours. The “anhydrous sodium polysilicate” obtained with the process according to the invention is understood to mean a sodium polysilicate which contains on average not more than 5% by weight and preferably less than 3.5% by weight of water. The water content was determined by determining the loss on ignition when heated to 1000 ° C. Only crystalline anhydrous sodium polysilicate can be seen in X-ray diffraction diagrams (comparison with JCPDS file No. 16-818). The suspensions of amorphous sodium silicate obtained with a module SiO 2 • Na 2 O of 2: 1 can also be worked up in the same way.

The particular advantage of the process can be seen from Examples 25, 26 and 28, in which the tempered quartz sand was converted directly to sodium polysilicate without pressure using sodium hydroxide solution. Here too, the sodium polysilicate is obtained in a practically quantitative reaction. In Examples 25 and 26, the reaction suspension was heated to the boiling point at atmospheric pressure. The boiling point decreased with the course of the reaction since the sodium hydroxide reacted. In Example 27, the suspension was kept at a temperature of 100 ° C. This temperature is not sufficient for a complete reaction within 2 hours. Example 28 shows that a tempered quartz sand consisting of cristobalite, tridymite and amorphous silicon dioxide has the same reactivity as cristobalite.

With the process according to the invention for the production of alkali metal silicates, it is accordingly possible, without pressure and depending on the selected SiO 2 / Na 2 O module, to obtain suspensions of amorphous sodium silicate or crystalline sodium polysilicate which, if desired, can then be obtained by suitable known ones Process can be dewatered. This also opens up the possibility of unpressurized, i.e. at normal pressure to produce solutions of alkali-rich sodium or potassium licates.

The following examples illustrate the invention without restricting it in any way. Examples

The experiments were carried out in a 1 liter glass three-necked round-bottom flask equipped with a reflux condenser, paddle stirrer and thermometer at normal pressure. The flask was heated over in a patio heater.

The aqueous alkali metal hydroxide solution (for example 50% by weight of NaOH, technical or 47% by weight of KOH, technical or solutions correspondingly diluted with water) was introduced and heated to boiling. The weighed amount of cristobalite was then added. The Siedetempe¬ temperature took mi ^'t the course of the reaction, since the Al used kalimetall liquor etallsilicat to the alkali (eg sodium silicate) react. The reaction time was 30 to 350 minutes, preferably 30 to 210 minutes.

The batch ratios for the individual experiments 1 to 24 (1 to 21 with NaOH; 22 to 24 with KOH) are shown in Table 1. Table 2 shows the reaction parameters of these experiments. Table 3 contains values for corresponding comparative examples in which quartz or quartz powder was used as the SiO 2 source instead of cristobalite.

Table 1 :

Batch conditions for the production of alkali metal silicate solutions and suspensions

CONTINUED Table 1

In order to follow the course of the reaction, samples of the reaction mixture were regularly taken (approx. 3 ml). These samples were diluted to about 50 ml with water and titrated to the M2θ content with a 0.1N hydrochloric acid solution. Addition of solid NaF to these sample solutions enabled the titrimetric determination of the content of dissolved SiO 2. With these two measured variables, the module of the alkali metal silicate solution or suspension can be calculated. When the desired module, which was determined by the choice of the starting concentrations, was reached, the reaction was interrupted. Towards the end of the reaction, the boiling temperature of the reaction mixture also remained constant.

In experiments 1, 2, 4 to 7 and 10, the reaction suspension was cooled down to about 90 ° C. and filtered off through a suction filter. The sodium silicate remaining as a filter residue was generally comminuted while still warm and then dried under reduced pressure at elevated temperature (100 to 150 ° C.). To determine the insoluble fractions, 10 g of the dried filter residue were stirred in 1000 ml at 60 ° C. for 5 min and filtered through an ash-free filter. This filter was ashed and the remaining residue was weighed out. Table 2:

Reaction parameters for the production of alkali silicate solutions and suspensions

00

PORTFOLIO Table 2

Temperature time [° C] [min]

13 1: 3 50 46 14 40 37 15 30 29

16 1: 5 50 43 17 40 35 18 30 27

SO

19 1: 7 40 34 20 30 26 21 20 17

Conc. The

KOH

[% By weight]

22 1: 1 47 51 138-118 330 23 30 36 113-107 (550) canceled with a module of 0.77 24 1: 3 47 47 140-131 165

Table 3: Comparative examples

Use module conc. The proportion of sodium hydroxide achieved [% by weight] time [min] module dissolved Si0 [%]

Quartz: 2: 1 50 380 0.55 28 1: 1 50 390 0.43 43 1: 2 50 360 0.31 62 1: 5 50 210 0.16 80

Quartz flour: 1: 1 50 360 0.62 62

Manufacture of sodium polysilicate

Reaction parameters of the individual examples are shown in Table 4. The reactions were carried out in two ways:

A: Three-necked flask with paddle stirrer, thermometer and reflux cooler, patio heater. B: Analog A, but heatable with an oil bath.

Experiments 25 to 27 relate to reactions with cristobalite, experiment 28 relates to the reaction of tempered quartz sand (1400 ° C., 5% by weight ^* NaOH addition), consisting of cristobalite, tridymite, amorphous silicon dioxide and small amounts of sodium silicate.

The loss on ignition was determined in all cases after the crushing and drying of the filter residue.

insoluble)

25 A 307 0.18 26 A 287 0.015 27 B 285 25.0 28 B 273 0.38

r

-) Loss on ignition at 1000 ° C determined. r

2) Insoluble components (10 g sample in 1000 ml water, 5 min 60 ° C). n.b. = not determined, since the reaction in these examples has not been completed.

Claims

Patent claims

1. A process for the preparation of alkali metal silicates from crystalline SiO 2 -containing material and aqueous alkali metal hydroxide solution at elevated temperature and normal pressure, characterized in that cristobalite and / or tempered quartz sand is used as the crystalline SiO 2 -containing material and this material with aqueous, 20 to 50 wt .-% sodium or potassium hydroxide solution at temperatures in the range from 100 to 150 ° C and under normal pressure, the molar ratio of SiO 2 to Na 2 O or K 2 O in the reaction mixture in the range from 2: 1 to 1 : 7 lies.

2. The method according to claim 1, characterized in that cristobalite, tridymite, or mixtures thereof, optionally together with amorphous silicon dioxide, are used as the tempered quartz sand.

3. The method according to any one of claims 1 or 2, characterized gekenn¬ characterized in that the aqueous sodium or potassium hydroxide solution has a concentration of 40 to 50 wt .-%.

4. The method according to any one of claims 1 to 3, characterized gekenn¬ characterized in that the crystalline SiO 2 ~ containing material is reacted with aqueous sodium hydroxide solution, the molar ratio of SiO 2 to Na 2 O in the reaction mixture in the range from 1.2: 1 to 1: 2, preferably 1: 1.

5. The method according to any one of claims 1 to 3, characterized gekenn¬ characterized in that the crystalline SiO 2 -containing material is reacted with aqueous sodium hydroxide solution, the molar ratio of SiO 2 to Na 2 O in the reaction mixture being 2: 1.

6. The method according to any one of claims 4 or 5, characterized gekenn¬ characterized in that the suspensions obtained of solid, crystalline sodium polysilicate or solid, amorphous sodium silicate at a temperature of 70 to 130 ° C, preferably 90 to 110 ° C, filtered and, if appropriate, the mother liquor obtained is concentrated and returned to the process.