AU628933B2 - Hydrothermal process for preparing sodium silicate solutions with a high si02:na2o molar ratio - Google Patents

Description

1 11 1 (iPJ DATE 24/08/90 APPLN. I D 50467 pCTf AUJI 4 UATE Z//U JU VUI 1ME INTERNATIUi~r~tj- t t1J4v1i..L)u.INI. V CI'A) miuNi%,i L i..A1 I1NIr~ Lutivi vrK 1K/AAJuuntt.ufE INTERNATiONALE ZUSAMMENARBET i AUF DEM GEBIET DES PATENTWESENS (PCT) (51) Internationale Patentkilassifikation 5 (11) Internationale Veriiffentlichungsiiummer: WO 90/08735 C0iB 33/321 Al(43) Intcrnationales Vcriiffentlichungsdatum: 9. August 1990 (09.08.90)

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(21) Internationfles Aktenzeichen: PCT/EP9O/001 IS (22) iternationales Anmeldedaturn: 22. Januar 1990 (22.01.90) Prioritiitsdaten: P 3902 751.1 (81) Bestimmungsstaaten: AT (europilisehes Patent), AU, BE (europflisches Patent), BR, CH (europiiisches Patent), DE (europiiisches Patent), DK, DK (europiiisches Patent), ES (europfiisches Patent), Fl, FR (europiiisches Patent), GB3 (europfiisches Patent), HU, IT (europliisches Patent), JP, KR, LU (europ isches Patent), NL (europfiisches Patent), NO, RO, SE (europflisches Patent), SU.

Veriiffentlicht Alitl internatiotialein Reclietchenbericzt.

3 1. Januar 1989 (31.01.89) DE (71) Anmelder: H.ENKEL KOMMANDITG ESELLSCHAFT AUF AKTI17N [DE/DE]; Henkelstra~e 67, D-4000 Dasseldorf-Holthausen (DE).

(72) Erfinder: NOVOTNY, Rud\lf Am Rittersberg 14, D-4000 Dasseldorf 13 HOFF, Alfred Wacholderstrage 18, D-4130 Moers-Schwafheim SCHIJRTZ, Jost Hirtenmeiswinkeler Weg 14, D-5650 Solingen 1 (DE).

bA 2U^q33 u (54)Title: HYDROTHERMAL PROCESS FOR PREPARING SODIUM SILICATE SOLUTIONS WITH A HIGH Si0 2 :Na 2 O MOLAR RATIO (54) Bezeichnung: YERFAHREN ZUR HYDROTHERMALEN HERSTELLUNG VON NATRIUMSILIKNILOSUNGEN >1 IIT HOHEM Si0 2 :Na 2 O-MOLVERH/iLTNIS (57) Abstract In a hydrothermal procez for preparing sodium silicate solutions with a high Sid 2 :Na 2 O molar ratio, a crystalline silicon dioxide is reacted with an aqueous solution of sodium hydroxide. The process is characterized in that the cryr *-ne sil'con dlixide used is a quartz tempered itemperatures from 1 1000 C up to its melting point, and that this tempered quartz is reacted with an aqjueous solution of sodium hydroxide at a concentration between 10 and 50 in a closed pressure reactor at temperatures between 150 an'i 3000'C and under saturated water vapour pressures correspouiding to these temperatures.

(57) Zusammenfassung Roaki.onszolI (Uln.) REAcrIOl TIME (MIN.) Offenbart wird emn Verfahren zur hydrothermalen Herstellung von Natriumsilikatlbsungen mit hohem Si0 2 :Na 2 O-Molverhqlltnis durch Urnsetzung eines kristallinen Siliciumndioxids mit waglriger Natriumhydroxidibsung, das dadurch gekennzeichnet ist, dag mart als kristallines Siliciumdioxid einen bei Temperaturen von Oiber 1 1000 C bis zumn Schmelzpunkt getemperten Quarz einsetzt und diesen getemperten Quarz mit wfllliger Natriumhydroxidi~sung in einemn Konzentrationsbereich von 10 bis 50 Gew,-% bei Temperaturen vcai 150 bis 3000'C und den diesen Temperaturen entsprechenden Drilkken von gestlttigtem Wasserdampf in einemn geschlossenen Druckreaktor umsetzt.

D 8485 Henkel KGaA TFP/Patente 24th January, 1989 A process for the hydrothermal production of sodium silicate solutions having a high SiO, NaO molar ratio This invention relates to a process for the hydrothermal production of sodium silicate solutions having a high SiO 2 Na20 molar ratio by reaction of a crystalline silicon dioxide 'with aqueous sodium hydroxide solutions.

A general synopsis of the production cf aqueous sodium silicate solutions can be found in the wcrks of Winnacker Kuchler, Chemische Technologie, Vol. 3, Anorganische Technologie II, 4th Edition, 1983, pages 54-63 and Ullmanns Encyklopadie der technischen Chemie, Vol. 21, 4th Edition, 1982, pages 409 412.

Of the alkali metal silicates known as "waterglass", sodium silicate solutions (commonly known as soda wat, rglass) are the most widely used for industrial purposes.

Soda waterglasses predominantly have a solids content of from about 30 to 40% by weight and a molar ratio of silicon dioxide to sodium oxide of 3.4 to 3.5 1. The industrial manufacture of soda waterglasses is generally based on the fusion of quartz sand and soda in suitable furnaces at temperatures in the range from 1400 to 1500*C. The melt which solidifies on cooling (solid glass) is dissolved in water under pressure at elevated temperature in another process step and the solution obtained is optionally filtered, depending on the quality requirements.

However, this high-temperature fusion process is very expensive both in terms of apparatus and in terms of energy consumption and, in addition, leads to considerable emissions, such as dust, nitrogen oxides and sulfur oxides.

In addition to this high-temperature fusion process,

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I i D 8485 2 which is the most widely used on an industrial scale, there are hydrothermal processes for the production of aqueous sodium silicate solutions which are described in a number of patent applications.

These processes start out from amorphous silicon dioxide, i.e. essentially from fly dusts and naturally occurring amorphous silicon dioxide modifications.

The end products of these processes are of poor quality as a result of the impurities typically present in the fly dusts and the naturally occurring amorphous silicon dioxide compounds used as starting materials and, accordingly, are of only limited use for industrial products.

DE-AS 28 26 432 relates to a process for the production of waterglass solutions by reaction of the fly dusts obtained in the production of silicon or ferrosiicon alloys with aqueous alkali metal hydroxide solutions at elevated temperature and subsequent filtration of the solutions obtained, characterized in that fly dust is treated with a 6 to 15% by weight aqueous alkali metal hydroxide solution in an autoclave at temperatures in the range from 120°C to 190°C under a pressure of 2.9 to 18.6 bar, the ratio by weight of alkali metal hydroxide solution to solid fly dust being from 2 1 to 5 1. The products of this process have a molar ratio of SiC 2 to Na20 of 2.2 to 4 1. The fly dusts used as starting materials have a silicon content of 89 to 98% by weight (in the Examples, the silicon content is always 90% by weight), the rest consisting of impurities.

DE-OS 26 09 831 relates to a process for working up environment-polluting waste fly dusts containing silicon dioxide from the manufacture of silicon metal and silicon alloys into silicas or silicates, 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; D 8485 3 II purifying the alkali silicate solutions to remove organic constituents by treatment with active carbon and/or oxidizing agent. and removing the non-digestible residue from the solution; III reacting the alkali silicate solutions with inorganic or organic acids and/or salts thereof for further purification.

The alkali silicate solutions obtained in this way generally have a molar ratio of SiO 2 to Na 2 0 in the range from 3.3 to 5.0 1.

DE-OS 26 19 604 relates to a process for the production of liquid waterglass from amorphous silicon dioxide and alkali hydroxide, characterized in that silicon dioxide dust in the form of fly ash, which has been removed from the waste gases of ferroalloy industries and other industries using silicon furnaces, alkali hydroxide and water are mixld in a certain ratio by weight and the resulting mixture is heated with stirring to a temperature in the range from 75 to 100°C, after which the liquid obtained is cooled. The silicon dioxide dusts used as starting material for this waterglass production process generally have a silicon dioxide content of 94 to 98% by weight, the rest consisting of impurities.

DE-AS 23 28 542 relates to a process for the production of alkali metal silicates by treatment of perlite with an alkali hydroxide and hydrothermal treatment of the pulp obtained in an autoclave, followed by filtration, characterized in that an alkali solution having a concentration of 40 to 140 g/l Na 2 O is used to treat the perlite in a quantity at which the ratio of liquid phase to solid phase is 0.7 to 1.5 1. The perlite is a substantially amorphous glass-like nmountain rock of volcanic origin which consists mainly of (in by weight) silicon dioxide 73, aluminium oxide 15 and other oxides 8.

As can be seen from the foregoing observations, the A~ I 6 D 8485 4 waterglasses obtained from amorphous silicon dioxide described in the patent literature only ever yield end products having inferior properties which have to be subjected to further purification.

The prior art described hereinafter relates to processes for the hydrothermal production of sodium silicate solutions from crystalline silicon dioxide, i.e. sand, and sodium hydroxide, although unfortunately they can only be reacted to An SiO 2 Na 2 O molar ratio of less than 2.89 1 by state-of-the-art processes.

DE-OS 30 02 857 relates to a process for the hydrothermal production of sodium silicate solutions having a molar ratio of SiO 2 to Na20 of 1.03 to 2.88 1 by reaction of sand with aqueous sodium hydroxide solution under pressure and at elevated temperature, followed by filtration, characterized in that the aqueous sodium hydroxide solution having a concentration of 10 to 50% by weight is reacted with an excess of sand of up to 300%, based on the molar ratios of SiO 2 Na20 in the batch, at temperatures in the range from 150 to 250°C and under saturated steam pressures corresponding to those temperatures and the urreacted sand excess is completely or partly used as filter medium for the sodium silicate solution formed. According to the Examples of this Offenlegungsschrift, however, the maximum SiO 2 Na 2 O molar ratio reached is at most 1.68 1.

DE-OS 34 21 158 relates to a process for the hydrothermal production of sodium silicate solutions having a molar ratio of SiO 2 Na20 of 1.96 to 2.17 1 by reaction of excess sand with aqueous sodium hydroxide solution, characterized in that the reaction mixture containing an excess of sand and an aqueous sodium hydroxide solution heated by process heat is reacted in a rotating, cylindrical, closed pressure reactor to a certain molar ratio of SiO 2 Na 2 O and is then filtered using the excess sand and, optionally, an additional filter aid. Aqueous sodium a~

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D 8485 i silicate solutions having a molar ratio of SiO 2 to Na20 of up to 2.27 1 are mentioned 4n the Examples.

DE-OS 33 13 814 relates to a process for the production of a clear solution of a sodium silicate, in which the molar ratio of silicon dioxide to sodium oxide is at most 2.58 1, by digestion of crystalline silicon dioxide having an average grain size of from 0.1 to 2 mm, characterized in that an aqueous solution of sodium hydroxide is passed through a bed of silicon dioxide which is formed in a vertical tubular reactor with no mechanical agitation and which is fed downwards with silicon dioxide and an aqueous solution of the sodium hydroxide.

Belgian patent 649 739 describes a process and an apparatus for the production of clear sodium silicate solutions by dissolving a silica-containing material under pressure at elevated temperature in aqueous caustic soda, characterized in that the product is separated from the excess silica-containing material and/or from the insoluble contaminated substances by means of filtration elements arranged near the bottom of the reactor, the filtration process advantageously being carried out under temperature and pressure conditions very similar to the reaction conditions. The aqueous sodium silicate solutions obtained in this way have a molar ratio of Sio 2 to Na 2 O of approximately 2.5 i.

Hydrothermal processes of the type in question for the production of soda waterglasses from sand and sodium hydroxide are also discussed in the above-cited works of Winnacker, Kuchler and Ullmann. However, it is stated in Winnacker, Kachler (on pages 61 and 62) that it is only possible to obtain a soda waterglass with an SiO 2 ratio of less than 2.7 at the temperatures typically applied in the hydrothermal process. Ullmann mentions in this regard that it is only possible in this way to obtain sodium silicate solutions having molar ratios of SiO 2 6 of up to 2.5 1 (page 412, left-hand column).

Accordingly, on the basis of the literature cited above, there was a direct prejudice against the production of sodium silicate solutions having relatively nigh Si02/Na 2 0 molar ratios from sand, i.e. from crystalline SiO 2 and sodium hydroxide by the hydrothermal process.

By contrast, the problem addressed by the present invention is to provide a process for the hydrothermal production of sodium silicate solutions by reaction of a crystalline silicon dioxide with aqueous sodium hydroxide solution, in which sodium silicate solutions ;iaving molar Si02/Na20 ratios of more than 2.9 1 are obtained.

The problem addressed by the invention is solved by the use of a specially conditioned quartz which is reacted with sodium hydroxide solutions under special reaction conditions.

*According to a first embodiment of this invention, there is 15 provided a process for the hydrothermal preparation of sodium silicate solutions with a high molar Si0 NaO0 ratio of more than 2.9 1 by converting a crystalline silicon dioxide with an aqueous sodium hydroxide Ssolution in the concentration range of 10 to 50% by weight, based on the amount of aqueous sodium hydroxide solution used in the process, at 20 temperatures of 150 to 300 0 C under saturated steam pressures corresponding to these temperatures in a closed pressurized reactor, characterized in that a quartz annealed at temperatures of the range from HI above 1100*C to the melting point is used as the crystalline silicon dioxide.

Because it involves only one step, the process according to the iinvention is easier to handle on an industrial scale and, hence, is less Sexpensive than the heavily polluting state-of-the-art processes with i their high energy consumption, i.e. the high-temperature fusion processes involving a subsequent dissolving step.

The process according to the invention has the advan- LMM/1310V I. D 8485 7 tage over known hydrothermal processes that, through the use of the quartz specially conditioned in accordance with the invention, it is even possible to obtain sodium silicate solutions with a molar ratio of SiO 2 to Na 2 O of more than 2.9 1, which is not possible where unconditioned quartz is used as the SiO 2 component.

It has also surprisingly been found that aqueous sodium silicate solutions which have a molar ratio of SiO 2 to Na20 of more than 2.9 1 can be directly produced in a single step from quartz conditioned in this way, preferably from a cristobalite formed in this way, by hydrothermal synthesis under the conditions described above, even with short reaction times.

Where the process according to the invention is applied, a high conversion of the reaction components used can be obtained, even with short reaction times. The use of a readily soluble crystalline silicon dioxide modification enables sodium silicate solutions having a high molar ratio of silicon dioxide to sodium oxide to be obtained in high volume/time yields with minimal energy consumption.

The sodium silicate solutions thus obtained have an SiO 2 Na20 molar ratio of preferably 2.9 to 3.7 1, more preferably 3.0 to 3.6 1 and most preferably 3.3 to 3.5 1.

In one preferred embodiment of the invention, the aqueous sodium silicate solution is obtained by using as the crystalline silicon dioxide a quartz which has been conditioned at temperatures of 1200 to 1700°C in the presence of catalytic quantities of alkali, changing largely into cristobalite under those conditions, and by reacting the quartz thus conditioned with aqueous sodium hydroxide solution in a concentration range of 15 to 30% by weight and preferably 20 to 30% by weight, the reaction being carried out in a closed pressure reactor at temper- D 8485 8 atures in the range from 200 to 250'C and under the saturated steam pressures corresponding to those temperatures.

Cristobalite, like quartz, is a crystal modification of silicon dioxide. It is produced almost entirely synthetically by calcination of quartz in a process in which quartz sand is continuously converted at temperatures of approximately 150C°C in the presence of catalysts (alkali compounds). Full information on cristobalite can be found in Ullmanns Encyklopadie der technischen Chemie, Vol. 21, 4th Edition, 1982, pages 439 442.

In the context of the invention, therefore, it is particularly preferred to use as the crystall.ne silicon dioxide a quartz which has been conditioned at temperatures in the range from 1300*C to 1600°C in the presence of catalytic quantities of alkali, changing largely into cristobalite under those conditions. In addition, it is of particular advantage to use a freshly conditioned, still warm cristobalite material for the process according to the invention.

In another preferred embodiment of the process according to the invention, the reaction is carried out in the ractor by using an excess of conditioned quartz of up to 100 mol-% and preferably from 1 to 10 mol-%, based on the desired molar ratio of SiO 2 to NaO2 in the sodium silicate solution. In general, the reaction may even be carried out with larger excesses than 100 mol-% of conditioned quartz, although this is not generally appropriate on an industrial scale. According to the invention it is particularly preferred to carry out the reaction with an excess of 2 to S 30 5 mol-% conditioned quartz, based on the desired SiO, NazO molar ratio.

In general, any of the re.,ators typically used for the hydrothermal synthesis of sod, waterglass may also be used to carry out the process according to the invention. Reactors such as these include, for example, rotating dissol- SD 8485 9 vers, stationary dissolver arrangements, stirrer-equipped reactors, jet loop reactors, tube reactors and, in principle, any reactors which are suitable for reacting solids with liquids under pressure. Reactors such as these are described in detail, for example, in DE-OS 30 02 857, DE- OS 34 21 158, DE-AS 28 26 432, BE-PS 649 739, DE-OS 33 13 814 and DE-PS 968 034.

The sodium silicate solutions (soda waterglass soilutions) produced in accordance with the invention may be used for all the usual applications which are known to the expert and which are described in the relevant literature, for example for the production of fillers (precipitated silicas), as adhesives, as binders in paints, foundry aids, catalyst supports, as a compone.%t of detergents and as a constituent of refractory materials.

The invention is illustrated in the following by Examples. The Examples were carried out on a laboratory scale and on an industrial scale. A cristobalite obtained by conditioning at 1300 to 1600°C in the presence of alkali as catalyst was used as the conditioned quartz in the Examples.

A cylindrical autoclave externally heated to the reaction temperature by a heat-transfer medium was used for the laboratory tests. The results of these Examples are set out in Tables 1 and 2 below.

A horizontally arranged, nickel-clad cylindrical steel pressure vessel with a volume when empty of approximately 24 m 3 was used as the reactor for the industrial-scale tests. The pressure vessel rotated about a horizontal shaft at a speed of 6 r.p.m. It was heated with steam at or 25 bar through an opening in the shaft and an attached tube with effective distribution directly into the reaction vessel.

The crystalline SiO 2 used for the Examples, namely the cristobalite obtained from conditioned quartz, contained

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I l .l I'I, D 8485 99.0% by weight Si0 2 The aqueous sodium hydroxide solution (caustic soda) required for the process was heated to around 103°C with vapors from the initial, batch through a Venturi nozzle above the caustic soda storage vessel.

The quantities of cristobalite and caustic soda were determined by weighing machines. The starting materials were introduced into the reactor which was then closed and set rotating. The reaction mixture was heated to the reaction temperature of approximately 215°C by the direct introduction of steam and was kept at that temperature.

After a reaction time of 30 mins. at that temperature, the reactor was brought to a standstill and the reaction mixture was transferred under its own pressure to a blow vessel through a flanged-on pipe. In this way, the reaction mixture was then separated via a cyclone separator into vapors and waterglass solution having a temperature of approximately 105°C. The vapors were takcn in by a jet apparatus and were used to preheat the mixed lye of the next batch in a Venturi nozzle to the limit of the boiling temperature of the lye of approximately 103°C.

The further working up of the waterglass solution with a temperature of approximately 100 0 C was carried out either in a sedimentation vessel for the coarse separation of solids or, where the clarity of the solution has to meet more stringent requirements through a filter.

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

The conditions of Example 4 were selected as reaction A 30 conditions for the industrial-scale tests. The batch size was 24,000 kg. The approximately 41% soda waterglass solution obtained had an SiO 2 Na20O molar ratio of 3.4 1 and substantially corresponded to the result of the laboratory-scale test.

In one particular embodiment, the hydrothermal process 4t Lii D 8485 11 using cristobalite/NaOH solution can take place at relatively high solids concentrations in the reactor because, even with a high SiO 2 Na20 molar ratio, the sodium silicate solution has a sufficient viscosity range for the process under the reaction conditions (215"C/20 bar). On completion of the reaction, water may be additionally introduced either under pressure directly into the reactor or into the blowline to a receiving vessel during the blowing process, so that the sodium silicate solution which has entered the receiving vessel thrc-'gh the blowline is sufficiently diluted to the extent that, before further working up by sedimentation or filtration, the sodium silicate solution in the receiving vessel has a fluid consistency of sufficiently low viscosity at temperatures of approximately 100*C.

Example 1 Example 1 illustrates a favorable bat-., in terms of the relatively low stock lye concentration, the cristobalite being used ir, a stoichiometric quantity, based on an i02 NaO2 molar ratio to be obtained in the sodium Silicate solution of 3.46 1.

Example 2 An increased NaOH concentration in the batch was established in relation to Example 1 for comparable reaction times to determine the effect of the NaOH concentration on the reaction velocity and the obtainable SiO 2 Na 2

O

ratio.

Examples 3/4 To obtain a relatively high molar ratio of SiO 2 to Na 2

O

in the reaction solution, cristobalite was used in an _Ily__ rn~uri~Ylan rr~ .1 l-p D 8485 12 increasing excess 3% in relation to Example 1, based on the set ratio of 3.46 1.

Examples 5/6 With a cristobalite excess of uased on a set ratio of SiO 2 to Na20 of 3.46 1, the reaction times were lengthened.

r D 8485 Table 1: Batch ratios Example Starting materials and quantities No. Cristobalite Sodium potash (g) NaOH Molar ratio in the batch3 concentration cristobalite Na2O 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 S Cristobalite excess based on a desired molar ratio of SiO. to in the solution of 3.46 1 *2 Cristobalite excess based on a desired molar ratio of SiO 2 to in the solution of 3.46 1 13 Taking into account all the SiO 2 and Na20 components present in the reactor I I I' D 8485 Table 2: Reaction conditions and product characterization Example HT reaction conditions Sodium silicate solution 1 No. reaction time reaction temp. SiO 2 NaO2 (mins.) Molar ratio SiO 2 Na2O 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 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.

Filtration is preceded by dilution to a solids content of <41% in the sodium silicate solution (HT stands for "hydrothermal")

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i D 8485 Example 7 To demonstrate the effect which the quartz conditioning temperature has on the properties of the aqueous sodium silicate solutions produced, quartz was first conditioned at temperatures of 850"C to 1600"C in the presence of catalytic quantities of alkali and then hydrothermally reacted with sodium hydroxide solution. For comparison, untreated quartz was reacted with sodium hydroxide solution to soda waterglass under the same standard hydrothermal reaction conditions.

The hydrothermal reaction of the conditioned quartz with sodium hydroxide solution was carried out under the following standard test conditions: reaction temperature 215°C; reaction time 30 mins; sodium hydroxide concentration 20% by weight; excess of silicon dioxide 5% (based on the molar ratio of 3.46 1 for soda glass) The reaction of this conditioned quartz in caustic soda to form a soda waterglass was carried out with the quantities of starting materials listed below and led to the conversions and molar ratios shown in the following table, Reaction of conditioned sand in caustic soda to Na glass, molar ratio SiO, Na20 3.46 1 i j i i i a~ For Na glass 35.37 conditioned sand 30 25.85 NaOH, 50% 38.78 water Quantities 49.00 g 35.82 g 53.72 g C _1_ IP"C

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D 8485 Con- SiO 2 version Na 2 O Ratio by Molar weight ratio Theor. quantities: 35.19 10.00 3.52:1 3.63:1 Untr. sand (Comp.) 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 0 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 addition of catalytic quantities of alkali) The results show that a quartz conditioned at temperatures above 1100*C, more especially a quartz conditioned at temperatures of 1300°C and higher, surprisingly leads to a higher conversion of the crystalline SiO 2 component and hence to a higher molar ratio of the sodium silicate solution than the corresponding untreated sand.

Example 8 The effect of the faster reaction of quartz conditioned at high temperatures, cristobalite, by comparison with an unconditioned quartz is documented in the Figure which shows the result of the reaction of conditioned quartz, i.e. cristobalite, together with 20% by weight sodium hydroxide solution for a 5% excess of silicon dioxide, based on a molar ratio of 3.46 1, in a pressure vessel at 215"° over reaction times of 15, 30, 60 and 120 minutes. The molar ratio of silicon dioxide to sodium oxide was determined in each case. This curve is denoted D 8485 17 by the reference numeral 1.

For comparison, a thermally untreated quartz, i.e. a sand, was reacted under the reaction conditions described above and samples were again taken after the reaction times described above to determine the molar ratio. This curve is denoted by the reference numeral 2.

It can clearly be seen from the Figure that, in the production process according to the invention where a conditioned quartz is used, a conversion of more than is obtained after only 15 minutes, the conversion being subs'antially quantitative, even after a reaction time of only 30 minutes.

By contrast, the comparison curve denoted by the reference numeral 2 shows a conversion of approx. 40 to after 15 minutes and a maximum conversion of only 70%, even after a reaction time of 120 minutes; in accordance with the literature data, it is only possible to obtain a maximum molar ratio of SiO 2 to Na20 of 2.8 1 even after several hours.

This aptly illustrates the advantages of the process according to the invention achieved by conditioning the quartz sand at elevated temperature.

Claims (8)

1. A process for the hydrothermal preparation of sodium silicate solutions with a high molar Si02 Na 2 0 ratio of more than 2.9 1 by converting a crystalline silicon dioxide with an aqueous sodium hydroxide solution in the concentration range of 10 to 50% by weight, based on the amount of aqueous sodium hydroxide solution used in the process, at temperatures of 150 to 300°C under saturated steam pressures corresponding to these temperatures in a closed pressurized reactor, characterized in that a quartz annealed at temperatures of the range from above 1100 0 C to the rmelting point is used as the crystalline silicon dioxide.

2. The process according to claim 1, characterized in that the resulting sodium silicate solution has a molar SiO 2 Na20 ratio of 2.9:1 to 3.7:1. 15 3. The process according to claim 2, characterized in that che resulting sodium silicate solution has a molar SiO Na 2 0 ratio of

3.0:1 to 3.6:1.

4. The process according to claim 3, characterized in that the resulting sodium silicate solution has a molar SiO 2 Na20 ratio of 3.3:1 to 3.5:1. The process according to any one of claims 1 to 4, characterized in that a quartz annealed at temperatures of 1200 to 1700°C under addition of catalytic amounts of alkali is used, which quartz is converted under these conditions mainly into crystobalitki, and that the quartz annealed in this way is converted with an aqueous sodium hydroxide solution in the concentration range of 15 to 30% by weight, at temperatures of 200 to 250 0 C and under saturated steam pressures i| corresponding to these temperatures in a closed pressurized reactor.

6. The process according to claim 5, characterized in that the concentration range of the aqueous sodium hydroxide solution is 20 to by weight.

7. The process according to any one of claims 1 to 6, characterized in that as the crystalline silicon dioxide one uses a quartz which was annealed at temperatures of the range of 1300 to 1600°C under addition of catalytic amounts of alkali and which under these r*'fi conditions is converted mainly into crystobalite. SN-Zi"MM/ 1310v i--l 1 101-

8. The process according to any one of claims 1 to 7, characterized in that the conversion is carried out with an excess of annealed quartz of up to 100 mole%, referred to the nominal molar SiO 2 ratio in the sodium silicate solution as claimed in claim 1.

9. The process according to any one of claims 1 to 8, characterized in that the conversion is carried out with an excess of annealed quartz from 1 to 10 mole%, referred to the nominal molar SiO 2 Na20 ratio in the sodium silicate solution as claimed in claim 1. The process according to any one of claims 1 to 9, characterized in that the conversion is carried out with an excess of annealed quartz from 2 to 5 mole%, referred to the nominal molar Si0 2 Na20 ratio in the sodium silicate solution as claimed in claim 1. 2 11. A process for the hydrothermal preparation of sodium silicata solutions with a high molar SiO 2 Na 2 0 ratio of more than 2.9 1 15 which process is substantially as herein described with reference to any one of Examples 1, 2, 3, 4, 5, 6, 7 but excluding any comparative examples therein, or 8 but excluding any comparative examples therein. S. 12. A sodium silicate solution whenever prepared by the process of any one of claims 1 to 11. DATED this SEVENTH day of JULY 1992 Henkel Konmanditgesellschaft Auf Aktien Patent Attorneys for the Applicant SPRUSON FERGUSON LMM/1310v D 8485 ABSTRACT I A process for the hydrothermal production of sodium Ssilicate solutiona with a high SiO Na0 molar ratio The invention relates to a process for the hydrother- mal production of sodium silicate solutions having a high SiO 2 Na 2 O molar ratio by reaction of a crystalline silicon dioxide with aqueous sodium hydroxide solution, charac- terized in that a quartz conditioned at temperatures above 1100*C to the melting point is used as the crystalline silicon dioxide and this conditioned quartz is reacted with aqueous sodium hydroxide solution in concentration range of 10 to 50% by weight, the reaction being carried out in a closed pressure reactor at temperatures of 150 to 300°C and under saturated steam pressures corresponding to those temperatures. ji i