HIGH-PURITY SILICON DIOXIDE GRANULES FOR QUARTZ GLASS APPLICATIONS AND METHOD FOR PRODUCING SAID GRANULES

Abstract

It has been found that conventional cheap waterglass qualities in a strongly acidic medium react to give high-purity silica grades, the treatment of which with a base leads to products which can be processed further to give glass bodies with low silanol group contents.

Claims

1. A process for producing a glass product, comprising: adding a silicate solution with a viscosity of 0.1 to 10 000 poise to an initial charge which comprises an acidifier and has a pH of less than 2.0, with the proviso that the pH during the adding is always below 2.0, obtaining silica from the solution and subsequently treating the silica at least once with an acidic wash medium with a pH below 2.0, subsequently washing the silica to neutrality and treating the silica with a base, removing a particle size fraction in the range of 200-1000 m and sintering the particle size fraction at a temperature of at least 600 C. to form high-purity silica granules comprising an alkali metal content between 0.01 and 10.0 ppm, an alkaline earth metal content between 0.01 and 10.0 ppm, a boron content between 0.001 and 1.0 ppm, a phosphorus content between 0.001 and 1.0 ppm, a nitrogen pore volume between 0.01 and 1.5 ml/g, and a maximum pore dimension between 5 and 500 nm, and heating the high-purity silica granules to form a glass product having a content of silicon-bonded OH groups between 0.1 and 150 ppm.

2. The process according to claim 1, wherein the pH of the initial charge comprising the acidifier is less than 1.5.

3. The process according to claim 1, wherein the pH of the initial charge comprising the acidifier is less than 1.0.

4. The process according to claim 1, wherein the pH of the initial charge comprising the acidifier is less than 0.5.

5. The process according to claim 1, wherein the viscosity of the silicate solution is 0.4 to 1000 poise.

6. The process according to claim 1, wherein the viscosity of the silicate solution is more than 5 poise.

7. The process according to claim 1, wherein the viscosity of the silicate solution is less than 2 poise.

8. The process according to claim 1, wherein the pH during the addition of the silicate solution is always below 1.5 and the pH of the wash medium is likewise below 1.5.

9. The process according to claim 1, wherein the pH during the addition of the silicate solution is always below 1.0 and the pH of the wash medium is below 1.0.

10. The process according to claim 1, wherein the pH during the addition of the silicate solution is always below 0.5 and the pH of the wash medium is -below 0.5.

11. The process according to claim 1, wherein washing the silica to neutrality is performed with demineralised water until the demineralized water has a conductivity of below 100 S, preferably below 10 S.

12. The process according to claim 1, wherein the base is a nitrogen base.

13. The process according to claim 12, wherein the nitrogen base is ammonia.

14. The process according to claim 12, wherein the nitrogen basecomprises a primary amine, a secondary amine, a tertiary amine or a combination thereof.

15. The process according to claim 1, wherein subjecting the silica to a basic treatment is effected at elevated temperature, elevated pressure or a combination thereof.

16. The process according to claim 1, wherein the silica is washed, dried and comminuted after subjecting the silica to a basic treatment.

17. The process according to claim 1, wherein a particle size fraction in the range of 200-600 m is removed.

18. The process according to claim 11, wherein a particle size fraction in the range of 200-400 m is removed.

19. The process according to claim 1, wherein a particle size fraction in the range of 250-350 m is removed.

20. The process according to claim 1, wherein the particle size fraction is sintered at a temperature of at least 1000 C.

21. The process according to claim 1, wherein the particle size fraction is sintered at a temperature of at least 1200 C.

22. The process according to claim 1, wherein the glass product comprises an impurity-sensitive quartz glass product.

23. The process according to claim 1, wherein the glass product comprises a content of silicon-bonded OH groups between 0.1 and 80 ppm.

24. The process according to claim 1, wherein the glass product comprises a content of silicon-bonded OH groups between 0.1 and 60 ppm.

Description

DETAILED DESCRIPTION

[0021] The invention can be divided into process steps a. to j., though not all process steps need necessarily be performed; more particularly, the drying of the silica obtained in step c. (step f.) can optionally be dispensed with. An outline of the process according to the invention can be given as follows: [0022] a. preparing an initial charge of an acidifier with a pH of less than 2.0, preferably less than 1.5, more preferably less than 1.0, most preferably less than 0.5 [0023] b. providing a silicate solution, it being possible to establish especially the viscosity for preparation of the silicon oxide purified by precipitation advantageously within particular viscosity ranges;

[0024] preference is given especially to a viscosity of 0.1 to 10 000 poise, though this viscosity range can be widened further according to the process regime - as detailed belowas a result of further process parameters [0025] c. adding the silicate solution from step b. to the initial charge from step a. in such a way that the pH of the resulting precipitation suspension is always below 2.0, preferably below 1.5, more preferably below 1.0 and most preferably below 0.5 [0026] d. removing and washing the resulting silica, the wash medium having a pH less than 2.0, preferably less than 1.5, more preferably less than 1.0 and most preferably less than 0.5 [0027] e. washing the silica to neutrality with demineralized water until the conductivity thereof has a value of below 100 S, preferably of below 10 S [0028] f. drying the resulting silica [0029] g. treating the silica with a base [0030] h. washing the silica with demineralized water, drying and comminuting the dried residue [0031] i. sieving the resulting silica granules to a particle size fraction in the range of 200-1000 m, preferably of 200-600 m, more preferably of 200-400 m and especially of 250-350 m [0032] j. sintering the silica fraction at at least 600 C., preferably at at least 1000 C. and more preferably at at least 1200 C.

[0033] According to the invention, the medium referred to hereinafter as precipitation acid, into which the silicon oxide dissolved in aqueous phase, especially a waterglass solution, is added dropwise in process step c., must always be strongly acidic. Strongly acidic is understood to mean a pH below 2.0, especially below 1.5, preferably below 1.0 and more preferably below 0.5. The aim may be to monitor the pH in the respect that the pH does not vary too greatly to obtain reproducible products. If a constant or substantially constant pH is the aim, the pH should exhibit only a range of variation of plus/minus 1.0, especially of plus/minus 0.5, preferably of plus/minus 0.2.

[0034] Acidifiers used with preference as precipitation acids are hydrochloric acid, phosphoric acid, nitric acid, sulphuric acid, chlorosulphonic acid, sulphuryl chloride, perchloric acid, formic acid and/or acetic acid, in concentrated or dilute form, or mixtures of the aforementioned acids. Particular preference is given to the aforementioned inorganic acids, i.e. mineral acids, and among these especially to sulphuric acid.

[0035] Repeated treatment of the precipitation product with (precipitation) acid, i.e. repeated acidic washing of the precipitation product, is preferred in accordance with the invention. The acidic washing can also be effected with different acids of different concentration and at different temperatures. The temperature of the acidic reaction solution during the addition of the silicate solution or of the acid is kept by heating or cooling at 20 to 95 C., preferably at 30 to 90 C., more preferably at 40 to 80 C.

[0036] Wash media may preferably be aqueous solutions of organic and/or inorganic water-soluble acids, for example of the aforementioned acids or of fumaric acid, oxalic acid or other organic acids known to those skilled in the art which do not themselves contribute to contamination of the purified silicon oxide because they can be removed completely with high-purity water. Generally suitable are therefore aqueous solutions of all organic (water-soluble) acids, especially consisting of the elements C, H and O, both as precipitation acids and as wash media if they do not themselves lead to contamination of the silicon oxide.

[0037] The wash medium may if required also comprise a mixture of water and organic solvents. Appropriate solvents are high-purity alcohols such as methanol, ethanol, propanol or isopropanol.

[0038] In the process according to the invention, it is normally unnecessary to add chelating agents in the course of precipitation or of acidic purification. Nevertheless, the present invention also includes, as a particular embodiment, the removal of metal impurities from the precipitation or wash acid undertaken using complexing agents, for which the complexing agents are preferablybut not necessarilyused immobilized on a solid phase. One example of a metal complexing agent usable in accordance with the invention is EDTA (ethylenediaminetetraacetate). It is also possible to add a peroxide as an indicator or colour marker for unwanted metal impurities. For example, hydroperoxides can be added to the precipitation suspension or to the wash medium in order to identify any titanium impurities present by colour.

[0039] The aqueous silicon oxide solution is an alkali metal and/or alkaline earth metal silicate solution, preferably a waterglass solution. Such solutions can be purchased commercially or prepared by dissolving solid silicates. In addition, the solutions can be obtained from a digestion of silica with alkali metal carbonates or prepared via a hydrothermal process at elevated temperature directly from silica, alkali metal hydroxide and water. The hydrothermal process may be preferred over the soda or potash process because it can lead to purer precipitated silicas. One disadvantage of the hydrothermal process is the limited range of moduli obtainable; for example, the modulus of SiO.sub.2 to Na.sub.2O is up to 2, preferred moduli being 3 to 4; in addition, the waterglasses after the hydrothermal process generally have to be concentrated before any precipitation. In general terms, the preparation of waterglass is known as such to the person skilled in the art.

[0040] In a specific embodiment, an aqueous solution of waterglass, especially sodium waterglass or potassium waterglass, is filtered before the inventive use and then, if necessary, concentrated. Any filtration of the waterglass solution or of the aqueous solution of silicates to remove solid, undissolved constituents can be effected by known processes and using apparatuses known to those skilled in the art.

[0041] The silicate solution before the acidic precipitation has a silica content of preferably at least 10% by weight. According to the invention, a silicate solution, especially a sodium waterglass solution, is used for acidic precipitation, the viscosity of which is 0.1 to 10 000 poise, preferably 0.2 to 5000 poise, more preferably 0.3 to 3000 poise and most preferably 0.4 to 1000 poise (at room temperature, 20 C.).

[0042] To conduct the precipitation, a high-viscosity waterglass solution is preferably added to an acidifier, which forms an acidic precipitation suspension. In a particular embodiment of the process according to the invention, silicate or waterglass solutions whose viscosity is about 5 poise, preferably more than 5 poise, are used (at room temperature, 20 C.).

[0043] In a further specific embodiment, silicate or waterglass solutions whose viscosity is about 2 poise, preferably less than 2 poise, are used (at room temperature, 20 C.).

[0044] The silicon oxide or silicate solutions used in accordance with the invention preferably have a modulus, i.e. a weight ratio of metal oxide to silica, of 1.5 to 4.5, preferably 1.7 to 4.2 and more preferably 2.0 to 4.0.

[0045] A variety of substances are usable in process step g. for basic treatment of the silica. Preference is given to using bases which are either themselves volatile or have an elevated vapour pressure compared to water at room temperature, or which can release volatile substances. Preference is further given to bases containing elements of main group 5 of the Periodic Table of the chemical elements, especially nitrogen bases and among these very particularly ammonia. Additionally usable in accordance with the invention are substances or substance mixtures which comprise at least one primary and/or secondary and/or tertiary amine. In general, basic substance mixtures can be used in a wide variety of different compositions, and they preferably contain at least one nitrogen base.

[0046] Preferably, but not necessarily, the basic treatment is effected at elevated temperature and/or elevated pressure.

[0047] The apparatus configuration used to perform the different process steps is of minor importance in accordance with the invention. What is important in the selection of the drying devices, filters, etc. is merely that contamination of the silica with impurities in the course of the process steps is ruled out. The units which can be used for the individual steps given this proviso are sufficiently well known to the person skilled in the art and therefore do not require any further explanations; preferred materials for components or component surfaces (coatings) which come into contact with the silica are polymers stable under the particular process conditions and/or quartz glass.

[0048] The novel silica granules are notable in that they have alkali metal and alkaline earth metal contents between 0.01 and 10.0 ppm, a boron content between 0.001 and 1.0 ppm, a phosphorus content between 0.001 and 1.0 ppm, a nitrogen pore volume between 0.01 and 1.5 ml/g and a maximum pore dimension between 5 and 500 nm, preferably between 5 and 200 nm. The nitrogen pore volume of the silica granules is preferably between 0.01 and 1.0 ml/g and especially between 0.01 and 0.6 ml/g.

[0049] The further analysis of the inventive granules showed that the carbon content thereof is between 0.01 and 40.0 ppm and the chlorine content thereof between 0.01 and 100.0 ppm; ppm figures in the context of the present invention are always the parts by weight of the chemical elements or structural units in question.

[0050] For the further processing of the silica granules, suitable particle size distributions are between 0.1 and 3000 m, preferably between 10 and 1000 m, more preferably between 100 and 800 m. In a preferred but non-obligatory embodiment, the further processing is effected in such a way that the granules are melted by a heating step in the presence of a defined steam concentration, which is preferably at first relatively high and is then reduced, to give a glass body with a low level of bubbles.

[0051] The inventive high-purity silica granules can be used for a variety of applications, for example for the production of quartz tubes and quartz crucibles, for the production of optical fibres and as fillers for epoxide moulding compositions. The inventive products can also be used to ensure good flow properties and high packing densities in moulds for quartz crucible production; these product properties can also be useful to achieve high solids loadings in epoxide moulding compositions. The inventive silica granules have alkali metal or alkaline earth metal contents of below 10 ppm in each case and are characterized by small nitrogen pore volumes of below 1 ml/g.

[0052] Especially in the particle size range of 50-2000 m, the products surprisingly sinter to give virtually bubble-free glass bodies with silanol group contents below 150 ppm in total. The products in question preferably have silanol group contents (parts by weight of the silicon-bonded OH groups) between 0.1 and 100 ppm, more preferably between 0.1 and 80 ppm and especially between 0.1 and 60 ppm.

[0053] Otherwise, the production of these high-quality glass bodies is possible without any need for any kind of treatment with chlorinating agents and also dispenses with the use of specific gases in the thermal treatment, such as ozone or helium.

[0054] The inventive silica granules are therefore outstandingly suitable as raw materials for production of shaped bodies for quartz glass applications of all kinds, i.e. including high-transparency applications. More particularly, the suitability includes the production of products for the electronics and semiconductor industries and the manufacture of glass or light waveguides. The silica granules are additionally very suitable for the production of crucibles, and particular emphasis is given to crucibles for solar silicon production.

[0055] Further preferred fields of use for the inventive high-purity silica granules are high-temperature-resistant insulation materials, fillers for polymers and resins which may have only very low radioactivities, and finally the raw material use thereof in the production of high-purity ceramics, catalysts and catalyst supports.

[0056] The invention is described hereinafter by examples, though this description is not intended to give rise to any restriction with regard to the range of application of the invention:

[0057] 1.) Preparation of the Silica According to Process Steps a.-f.

[0058] 1800 litres of 14.1% sulphuric acid were initially charged and 350 litres of an aqueous 37/40 waterglass solution (density=1350 kg/m.sup.3, Na.sub.2O content=8%, SiO.sub.2 content=26.8%, %SiO.sub.2/%Na.sub.2O modulus=3.35) were added to this initial charge with pump circulation within one hour. In the course of addition, millimetre-size prills formed spontaneously, which formed a pervious bed and enabled, during the continued addition of waterglass, pumped circulation of the contents of the initial charge through a sieve plate at 800 litres/hour and permanent homogenization of the liquid phase.

[0059] The temperature should not exceed a value of 35 C. during the addition of the waterglass solution; if required, compliance with this maximum temperature must be ensured by cooling the initial charge. After complete addition of waterglass, the internal temperature was raised to 60 C. and kept at this value for one hour, before the synthesis solution was discharged through the sieve plate.

[0060] To wash the product obtained, the initial charge was supplemented with 1230 litres of 9.5% sulphuric acid at 60 C. within approx. 20 minutes, which was pumped in circulation for approx. 20 minutes and discharged again. This washing operation was subsequently repeated three times more with sulphuric acid at 80 C.; first with 16% and then twice more with 9% sulphuric acid. Finally, the procedure was repeated four times more in the same way with 0.7% sulphuric acid at 25 C., and then washing with demineralized water was continued at room temperature until the wash water had a conductivity of 6 S. Drying of the high-purity silica obtained is optional.

[0061] 2.) Preparation of the Silica Granules According to Process Steps g.-j.

EXAMPLE 1

[0062] 500 g of the moist silica prepared by the process described above (solids content 23.6%) were admixed in a 5 litre canister with 500 g of demineralized water and 50 g of a 25% ammonia solution. After shaking vigorously, this mixture with the lid screwed on was left to age in a drying cabinet overnight; the temperature during the alkaline ageing process was 80 C. The next day, the product was transferred into a 3000 ml beaker (quartz glass) and washed a total of five times with 500 ml of demineralized water each time, followed by decanting off; subsequently, the product in the beaker (quartz glass) was dried overnight in a drying cabinet heated to 160 C. The dry product was comminuted and sieved off to a fraction of 250-350 m. 20 g of this fraction were heated in a 1000 ml beaker (quartz glass) to 1050 C. in a muffle furnace within four hours and kept at this temperature for one hour; it was cooled gradually by leaving it to stand in the furnace.

[0063] A further 20 g of the aforementioned sieve fraction were subjected to sintering at 1250 C.under otherwise identical conditions. The BET surface areas and the pore volumes of the two sintered products and the material obtained after the drying cabinet drying were measured; in addition, glass rods were fused from these materials, all three of which had a high transparency and a low bubble content.

TABLE-US-00001 BET BET PV PV measure- measure- measure- measure- ment 1 ment 2 ment 1 ment 2 [m.sup.2/g] [m.sup.2/g] [cc/g] [cc/g] Starting material 795 823 0.510 0.528 After NH.sub.3 and 131 131 0.464 0.439 160 C. treatment After 1050 C. 81.2 80.4 0.269 0.274 treatment After 1250 C. 0.1 0.0 0.006 0.007 treatment

EXAMPLE 2

[0064] 2000 g of the moist silica prepared by the process described above (solids content 35%) were admixed in a 5 litre canister with 2000 g of demineralized water and 20 g of a 25% ammonia solution. After shaking vigorously, this mixture with the lid screwed shut was left to age overnight in a drying cabinet; the temperature during the alkaline ageing process was 80 C. The next day, the product was transferred into a 5000 ml beaker (quartz glass) and washed a total of three times with 1000 ml each time of demineralized water, followed by decanting off; subsequently, the product was dried in a porcelain dish in a drying cabinet heated to 160 C. overnight. This procedure was repeated several times in order to obtain a yield of more than 2000 g. The dry product was crushed in a 3000 ml quartz glass beaker with a quartz glass flask and sieved off to a fraction of 125-500 m.

[0065] 600 g of the fraction were heated in a 3000 ml quartz glass beaker to 600 C. in a muffle furnace within eight hours and held at this temperature for four hours before being left to cool overnight. The next day, the same sample was heated to 1200 C. within eight hours and held at this temperature for a further four hours; the cooling was again effected overnight. After the sintered product had been comminuted, it was filtered once again through a 500 m sieve.

[0066] The BET surface areas and the pore volumes both of this sintered material and of the product being merely dried in a drying cabinet were measured; a glass rod was also fused from each of the products. In addition, a silanol group determination by IR spectroscopy was conducted on the sintered material. The values reported in silanol group determinations always correspond to the content of silicon-bonded OH groups in ppm (by weight).

TABLE-US-00002 BET PV Silanol Silanol measure- measure- group group ment ment content content [m.sup.2/g] [cc/g] (granules) (glass rod) Starting 828 0.545 77 400 ppm not material determinable After NH.sub.3 149 0.492 82 ppm and 160 C. treatment After 1200 C. 0.1 0.004 395 ppm 85 ppm treatment

COMPARATIVE EXAMPLE

[0067] A portion of the moist silica used in Example 2 (solids content 35%), after gentle drying at 50 C., was used to produce a fraction of 125-500 m of the material by means of vibratory sieving, which was fused to a glass rod without the inventive treatment. The attempt to measure the silanol group content failed in this case because of the high bubble content of the glass rod, i.e. the intransparency caused thereby.

[0068] Production of the Glass Rods for Determination of the Silanol Group Contents:

[0069] The silica granules to be fused are introduced into a glass tube fused at one end and evacuated under high vacuum. Once a stable vacuum has been established, the glass rod is fused at least 20 cm above the granule level. Subsequently, the powder in the tube is melted with a hydrogen/oxygen gas burner to give a glass rod. The glass rod is cut into slices of thickness approx. 5 mm and the plane-parallel end faces are polished to a shine. The exact thickness of the glass slices is measured with a slide rule and included in the evaluation. The slices are clamped in the beam path of an IR measuring instrument. The IR spectroscopy determination of the silanol group content is not effected in the edge region of the slice since this consists of the material of the glass tube enveloping the fusion material.

[0070] Determination of the BET Surface Area and of the Nitrogen Pore Volume:

[0071] The specific nitrogen surface area (BET surface area) is determined to ISO 9277 as the multipoint surface area.

[0072] To determine the pore volume, the measuring principle of nitrogen sorption at 77 K, i.e. a volumetric method, is employed; this process is suitable for mesoporous solids with a pore diameter of 2 nm to 50 nm.

[0073] First, the amorphous solids are dried in a drying cabinet. The sample preparation and the measurement are effected with the ASAP 2400 instrument from Micromeritics, using nitrogen 5.0 or helium 5.0 as the analysis gases and liquid nitrogen as the cooling bath. Starting weights are measured on an analytic balance with an accuracy of 1/10 mg.

[0074] The sample to be analysed is predried at 105 C. for 15-20 hours. 0.3 g to 1.0 g of the predried substance is weighed into a sample vessel. The sample vessel is attached to the ASAP 2400 instrument and baked out at 200 C. under vacuum for 60 minutes (final vacuum <10 pm Hg). The sample is allowed to cool to room temperature under reduced pressure, blanketed with nitrogen and weighed. The difference from the weight of the nitrogen-filled sample vessel without solids gives the exact starting weight. The measurement is effected in accordance with the operating instructions of the ASAP 2400 instrument.

[0075] For evaluation of the nitrogen pore volume (pore diameter <50 nm), the adsorbed volume is determined using the desorption branch (pore volume for pores with a pore diameter of <50 nm).