CONTINUOUS SOL-GEL METHOD FOR PRODUCING QUARTZ GLASS

Abstract

The invention relates to a continuous sol-gel method for producing quartz glass, comprising the following steps: (a) continuously metering a silicon alkoxide into a first reactor (R1) and carrying out an at least partial hydrolysis process by adding an aqueous mineral acid, thereby obtaining a first product flow (A); (b) continuously producing an aqueous silicic acid dispersion by continuously mixing water and silicic acid in a second reactor, thereby obtaining a second product flow (B); (c) continuously mixing the product flows (A) and (B) in a third reactor (R3) in order to produce a pre-sol, thereby obtaining a third product flow (C); (d) continuously adding an aqueous base to the product flow (C), thereby obtaining a sol; (e) continuously filling the exiting sol into moulds, thereby obtaining an aquagel; (f) drying the aquagel, thereby obtaining xerogels; and (g) sintering the xerogels, thereby obtaining quartz glass, with the proviso that at least one of the steps (a) to (e) additionally includes a degassing process of at least one feed material used in the step.

Claims

1. Continuous sol-gel method for producing quartz glass, comprising the following steps: (a) continuously metering a silicon alkoxide into a first reactor (R1) and carrying out an at least partial hydrolysis process by adding an aqueous mineral acid, thereby obtaining a first product flow (A); (b) continuously producing an aqueous silicic acid dispersion by continuously mixing water and silicic acid in a second reactor, thereby obtaining a second product flow (B); (c) continuously mixing the product flows (A) and (B) from steps (a) and (b) in a third reactor (R3) in order to produce a pre-sol, thereby obtaining a third product flow (C); (d) continuously adding an aqueous base to the product flow (C), thereby obtaining a sol; (e) continuously filling the exiting sol from step (d) into moulds, thereby obtaining an aquagel; (f) drying the aquagels from step (e), thereby obtaining xerogels; (g) sintering the xerogels from step (f), thereby obtaining quartz glass, wherein at least one of the steps (a) to (e) additionally includes a degassing process of at least one feed material used in the step.

2. Method according to claim 1, characterised in that the degassing process is carried out by ultrasound, vacuum degassing, distillation, vacuum/freezing cycles, thermal degassing, chemical methods, removing gas by means of inert gas; adding deaerating additives and centrifugation or a combination of two or more of these measures.

3. Method according to claim 1, characterised in that in step (a), silicon alkoxides that follow the formula (I) are used
Si(OR).sub.4 (I) in which R denotes an alkyl group having from 1 to 6 carbon atoms.

4. Method according to claim 1, characterised in that in step (a), tetraethyl orthosilicate (TEOS) is used as the silicon alkoxide.

5. Method according to claim 1, characterised in that in step (a), from approximately 1 to approximately 60 wt. % mineral acid is used based on the silicon alkoxides.

6. Method according to claim 1, characterised in that in step (a), the hydrolysis process of the silicon alkoxides is carried out at a temperature in the range of from approximately 1 to approximately 100 C.

7. Method according to claim 1, characterised in that in step (b), highly dispersive silicic acids are used that have BET surface areas in the range of from approximately 30 to approximately 100 m.sup.2/g.

8. Method according to claim 1, characterised in that in step (b), an aqueous dispersion is produced that contains from approximately 1 to approximately 60 wt. % silicic acid.

9. Method according to claim 1, characterised in that the product flows (A) and (B) are mixed in a volume ratio of alkoxide to silicic acid of from approximately 10:1 to approximately 1:10.

10. Method according to claim 1, characterised in that the product flows (A) and (B) are mixed at temperatures in a range of from approximately 0 to approximately 80 C.

11. Method according to claim 1, characterised in that a base is continuously added into the reactor (R3) to form a pre-sol.

12. Method according to claim 1, characterised in that at least one of the steps (a), (b) or (c) is carried out in a flow reactor, optionally with an upstream mixing element.

13. Method according to claim 12, characterised in that flow reactors are used that have a length of from approximately 50 to approximately 1000 m and a cross section of from approximately 1 to 10 mm.

14. Method according to claim 1, characterised in that the formation of gel is carried out at temperatures in the range of from 0 to 100 C.

15. Method according to claim 1, characterised in that drying is carried out at temperatures in the range of from 0 to 150 C.

Description

DESCRIPTION OF THE INVENTION

[0025] The present invention relates to a continuous sol-gel method for producing quartz glass, which comprises the following steps: [0026] (a) continuously metering a silicon alkoxide into a first reactor (R1) and carrying out an at least partial hydrolysis process by adding an aqueous mineral acid, thereby obtaining a first product flow (A); [0027] (b) continuously producing an aqueous silicic acid dispersion by continuously mixing water and silicic acid in a second reactor, thereby obtaining a second product flow (B); [0028] (c) continuously mixing the product flows (A) and (B) in a third reactor (R3) in order to produce a pre-sol, thereby obtaining a third product flow (C); [0029] (d) continuously adding an aqueous base to the product flow (C), thereby obtaining a sol; [0030] (e) continuously filling the exiting sol into moulds, thereby obtaining an aquagel; [0031] (f) drying the aquagels, thereby obtaining xerogels; [0032] (g) sintering the xerogels, thereby obtaining quartz glass, with the proviso that at least one of the steps (a) to (e) additionally includes a degassing process of at least one feed material used in the step.

[0033] It has been found that the new continuous method solves all the manifold problems described at the outset simultaneously and comprehensively. Aside from the fact that the method allows the production of any desired quantities of products and therefore also of different quantities of products, the synthesis process leads to products of consistently high quality.

[0034] A particularly critical feature of the method of the invention consists in supplying the feed materials of the synthesis process in the degassed condition. Specifically, it has emerged that without this step, dissolved gases are discharged by the mixing as a result of changed solubilities in the reactants and, as explained at the outset, cause bubbles to form. In principle, degassing can occur in each of the method steps (a) to (e), i.e. at the stage of the reactants, the pre-sol, the dispersion or the sol itself. Preferably, the reactants are already degassed and used in the synthesis process in this form. As a precautionary measure, the reactants, the pre-sol, the dispersion or the sol can be degassed.

[0035] Degassing is carried out according to the invention preferably using ultrasound. Alternatively, these measures are possible: [0036] Vacuum degassing [0037] Distillation [0038] Vacuum/freezing cycles [0039] Thermal degassing [0040] Chemical methods, such as removing oxygen by chemical bonding; [0041] Removing gas by means of inert gas; [0042] Adding deaerating additives and [0043] Centrifugation or a combination of two or more of these measures.

[0044] Additionally, the reactants can optionally be used in a particle-free manner by using suction filters and each mould can be filled with a freshly produced sol. By avoiding rejects that do not conform to specifications, the profitability of the method in particular is thus significantly increased, especially as long cleaning times are dispensed with, in particular as the reactors that are preferably used can be easily cleaned with rinsing agents.

[0045] Silicon Alkoxides and Hydrolysis Process (Method Step A)

[0046] Silicon alkoxides that are considered within the meaning of the invention to be starting materials for producing quartz glass preferably follow the formula (I)

Si(OR).sub.4 (I)

in which R denotes an alkyl group having from 1 to 6 carbon atoms. Typical examples are tetrapropyl orthosilicate and tetrabutyl orthosilicate; however, tetramethyl orthosilicate (TMOS) and in particular tetraethyl orthosilicate (TEOS) are preferably used. As TEOS is insoluble in water, alcoholic, specifically ethanolic, solutions can be used, the alcohol functioning as the phase mediator. The silicon alkoxides can also comprise additional silicon compounds as additives, such as methyl triethylsilane, dimethyl diethylsilane, trimethyl ethylsilane and the like.

[0047] At this point, additional ionic compounds can also be added to the solution, for example the elements Na, Al, B, Cd, Co, Cu, Cr, Mn, Au, Ni, V, Ru, Fe, Y, Cs, Ba, Cd, Zn, Eu, La, K, Sr, TB, Nd, Ce, Sm, Pr, Er, Tm or Mo, i.e. when dyed quartz glass is desired. However, these compounds can also be added together with the silicic acid or in the course of further steps.

[0048] The acidic hydrolysis process of the silicon alkoxides takes place in the reactor R1 in the presence of aqueous mineral acids, such as sulphuric acid, nitric acid, acetic acid or hydrochloric acid. Hydrochloric acid having a concentration of 0.01 mol/l has proved to be particularly favourable. The preferred volume ratio of alkoxide to mineral acid is from 10:1 to 1:10, particularly preferably from 3:1 to 1:3 and more particularly preferably from 2.5:1 to 1:2.5.

[0049] The hydrolysis process is carried out at a suitable temperature by the two reactants being conveyed by pumps, merged and reacted in a temperature-controlled flow reactor. If the reactants are unable to mix, a slug flow forms in the flow reactor. The temperature range of the hydrolysis process ranges from 1 to approximately 100 C., the preferred temperature being from approximately 70 C. to approximately 90 C.

[0050] Silicic Acids and Production of the Dispersion (Method Step B)

[0051] In the second step of the method, an aqueous dispersion of a highly dispersive silicic acid is produced, also continuously, in a temperature-controlled reactor R2. Preferably, the silicic acids have BET surface areas in the range of from approximately 30 to approximately 100 m.sup.2/g and in particular from approximately 40 to approximately 60 m.sup.2/g. Using the product Aerosil OX 50 (EVONIK) is particularly preferred, which product is a pyrogenic, hydrophilic silicic acid that has a surface area of approximately 50 m.sup.2/g and consists of more than 99.8 wt. % SiO.sub.2. Water and OX 50 are added into a temperature-controlled reactor and homogenised by a dispersing device. The dispersion can be degassed using ultrasonic treatment. The mass proportion of OX 50 in the dispersion is approximately 1-60 wt. %, in particular 33 wt. %. OX 50 and water can be metered gravimetrically, for example.

[0052] Formation of Sol (Method Step c)

[0053] While a first continuous flow of a hydrolysed silicon alkoxide compound was produced in the first method step and a second flow of an aqueous silicic acid dispersion was produced, also continuously, in the second method step, the two flows are now mixed and the pre-sol is formed in the third step. For this purpose, the product flows (A) and (B) are merged upstream of the reactor R3 by a suitable mixing system. The volume ratios of the two flows (A) and (B) can be variably adjusted. As a result, the product properties of the finished quartz glass can be influenced. A preferred volume mixing ratio is from approximately 10:1 to approximately 1:10, particularly preferably from approximately 5:1 to approximately 1:5 and more particularly preferably from approximately 2.5:1 to 1:2.5. Here, the pre-sol can be degassed, according to the standards of quality of the quartz glass, by suitable degassing methods, for example ultrasound. The product flows (A) and (B) are merged at temperatures of from 1 to approximately 100 C., preferably from approximately 10 to approximately 50 and particularly preferably at ambient temperature.

[0054] The subsequent gelation of the sol is initiated by increasing the pH. For this purpose, a base is continuously added to the continuously produced pre-sol. Whereas the hydrolysis product has a pH of from approximately 1 to 2, said pH is increased to from approximately 2 to 3 by adding the silicic acid dispersion. However, it has been found that the tear resistance of the gel during shrinking can be further improved if the pH is further increased, for example to values in the range of from 3 to 9, preferably 4-6. The base may be for example ammonia (aqueous solution or gaseous), an organic amine compound or pyridine. Alkaline bases or alkaline earth bases are less preferred because they introduce additional cations into the product, which can be undesirable for producing highly pure quartz glass.

[0055] Reactors

[0056] Even if the choice of reactors is not critical in itself, one embodiment of the invention has, however, proven to be particularly advantageous: it is particularly preferable if at least one of the steps (a), (b) or (c) is carried out in a flow reactor, optionally with an upstream mixing element.

[0057] In the simplest embodiment, the reactors are tubes made of durable material, such as Teflon, polyamide, metal, polyethylene or polypropylene, that may have a length of from approximately 50 to approximately 1000 m, preferably from approximately 100 to approximately 800 m and particularly preferably 100-500 m and a cross section in the centre of from approximately 1 to approximately 10 mm, preferably from approximately 1 to approximately 5 mm. Said tubes may be wound up in a spiral, which considerably reduces the space required. The long distances correspond to the optimum reaction time in each case for a given flow rate. Arrangements of this kind are highly flexible, as the tube lengths can be lengthened or shortened as desired and can be cleaned with minimal effort. Carrying out the reaction in this way can significantly contribute to the profitability of the method.

[0058] Formation of Gel

[0059] The pre-sol is conveyed continuously out of the reactor R3, mixed with ammonia and filled into moulds in which the formation of gel can take place. As the aquagels obtained in this manner shrink in the mould during the ageing process, they must be able to slide in the container easily. For this reason, containers made of a hydrophobic material, such as polyethylene, polypropylene, Teflon, PVC or polystyrene, are particularly suitable.

[0060] For the purpose of processing, the aquagels must be removed from the moulds and dried to form xerogels. The aquagels may be removed from the moulds under specific conditions, for example underwater. In the case of larger aquagels, the ethanol can be partially replaced with water by remaining in water for a time. This makes it possible for larger aquagels (e.g. 888 cm) to dry without tears. Additionally, the water bath can also be used in order to allow various elements to diffuse into the aquagel. This makes coloured quartz glass possible, for example. The drying conditions are influenced by the evaporation speed of the solvent in the gel, i.e. water and alcohol. Reducing the evaporation speed while maintaining a low evaporation rate helps to prevent the gel from tearing. Long drying times, conversely, make the method more expensive, and therefore a compromise must be found.

[0061] Sintering Process

[0062] The sintering process can be carried out in a manner known per se. During the sintering process, the remaining solvents contained in the xerogels are removed and the pores in the system are closed. The sintering temperature is up to 1400 C. and can be carried out in a normal atmosphere for most products. According to the invention, the sintering process is carried out in the following manner: [0063] 1) Removing the solvent; [0064] 2) Removing optionally contained undesired organic compounds; [0065] 3) Closing the optionally available pores in order to form quartz glass.

[0066] In order to remove the solvents in partial step 1, temperatures of from approximately 20 C. to approximately 200 C. are used, preferably from approximately 70 to approximately 150 C. and particularly preferably from from approximately 90 to approximately 110 C. The removal of undesired organic compounds, which develop as a result of carbonaceous reactants/products decomposing, in step 2 is carried out at temperatures in the range of from approximately 800 to approximately 1100 C., preferably from approximately 850 to approximately 1050 C. and particularly preferably from approximately 900 to approximately 1000 C. In step 3, the pores are closed at temperatures of between approximately 1100 and approximately 1400 C., preferably from approximately 1150 to approximately 1350 C. and particularly preferably from approximately 1200 to approximately 1300 C.

EXAMPLES

Example 1

[0067] TEOS was provided through a first pump and aqueous HCl (0.010 mol/l) was provided through a second pump. Both feed materials had previously been degassed by ultrasonic treatment. The two solutions were guided through tubes and merged using a tee. The mixture had the following composition:

TABLE-US-00001 TEOS 66.66 vol. % HCl 33.33 vol. %

[0068] Mixing occurred at 75 C. in a first PA tube (reactor R1) that had a length of 300 m and an inner diameter of 2.7 mm; the residence time in the tube was approximately 30 minutes. The pH was 1.5.

[0069] The OX 50 dispersion was produced in the second reactor by 500 g OX 50 being introduced into 1000 g water. The water had previously been degassed by ultrasonic treatment. An Ultra-Turrax was used for homogenising the dispersion. The dispersion, having wOX 50 of 33.33%, obtained in this manner was then further transferred by means of a pump in the next step. The two flows were merged through an additional tee and continuously mixed by a static mixer tube, the pH being approximately 2.5. The pre-sol was then degassed with ultrasound. Aqueous ammonia was then continuously added to the pre-sol and the pH of the pre-sol was adjusted to 4-5, and the pre-sol was immediately filled into moulds made of PE (222 cm) and said moulds were sealed tightly. After approximately 10 seconds, gelation began. After a residence time of 20 hours in the sealed moulds, the aquagels were removed from the moulds underwater, and were air-dried to form xerogels after two hours in the water bath. The xerogels were then sintered to form quartz glass by means of the following temperature ramps: RT-100 C. (4 hours), 100 C. (3 hours), 100 C.-950 C. (4 hours), 950 C. (2 hours), 950 C-1250 C. (6 hours), 1250 C. (1 hour).