STEAM TREATMENT OF SILICON DIOXIDE POWDER IN THE PREPARATION OF QUARTZ GLASS

Abstract

The invention relates to a process for the preparation of a quartz glass body comprising the process steps i.) Providing a silicon dioxide granulate, ii.) Making a glass melt out of the silicon dioxide granulate and iii) Making a quartz glass body out of at least part of the glass melt, wherein the provision comprises the steps I. Providing a silicon dioxide powder with at least two particles made from a silicon-chlorine compound, II. Bringing the silicon dioxide powder into contact with steam to obtain a treated silicon dioxide powder and III. Granulating the treated silicon dioxide powder to obtain a silicon dioxide granulate, and wherein the chorine content of the silicon dioxide powder is greater than the chlorine content of the silicon dioxide granulate. The invention further relates to a quartz glass body which is obtainable by this process. The invention further relates to a process for providing a silicon dioxide granulate. The invention further relates to a light guide, an illuminant and a formed body, which are each obtainable by further processing of the quartz glass body.

Claims

1-20. (canceled)

21. A process for the preparation of a quartz glass body comprising: providing a silicon dioxide granulate, wherein the provision comprises: producing a silicon dioxide powder with at least two particles prepared from a silicon-chlorine compound; bringing the silicon dioxide powder into contact with steam to obtain a treated silicon dioxide powder; and granulating the treated silicon dioxide powder to obtain a silicon dioxide granulate; making a glass melt out of the silicon dioxide granulate; and making a quartz glass body out of at least part of the glass melt; wherein the chlorine content of the silicon dioxide powder is greater than the chlorine content of the silicon dioxide granulate.

22. The process according to claim 21, wherein the silicon dioxide powder has a particle size distribution D.sub.50 in the range from 6 to 15 m.

23. The process according to claim 21, wherein the at least two particles carry out a motion relative to one another.

24. The process according to claim 21, wherein the steam comprises at least one of: a relative moisture content of at least 70 vol.-%, based on the total volume of the steam used; a water content in a range from 0.2 to 40 kg/m.sup.3 based on the total volume of the steam; a content of a gas different from steam, for example He, Ar, air or a mixture of two or more thereof, in a range from 30 to 40 vol.-% based on the total volume of the steam; a content of inert gas, particularly helium, in a range from 1 to 3 vol.-% based on the total volume of the steam; a temperature in a range from 150 to 500 C.; and a steam pressure in a range from 0.1 to 5 bar, the pressure being stated as pressure over standard pressure in each case.

25. The process according to claim 21, wherein the weight ratio of steam to silicon dioxide is in a range from 1:1 to 100:1.

26. The process according to claim 21, wherein the ratio of the chlorine content of the silicon dioxide powder before the steam treatment to the chlorine content of the silicon dioxide powder after the steam treatment is in a range from 500:1 to 10:1.

27. The process according to claim 21, wherein the pH value of an aqueous 4% dispersion of the silicon dioxide powder after the steam treatment lies in a range from 3 to 5.5.

28. The process according to claim 21, wherein the treated silicon dioxide powder comprises at least one of: a chlorine content of less than 200 ppm; an aluminium content of less than 200 ppb; a residual moisture content of less than 10 wt.-%, based on the total weight of the silicon dioxide powder; a BET surface area in a range from 10 to 70 m.sup.2/g; a bulk density of less than 0.3 g/cm.sup.3; a carbon content of less than 50 ppm; a total content of metals different from aluminium of less than 2.5 ppm; at least 70 wt.-% of the powder particles based on the total number of powder particles have a primary particle size of less than 100 nm; a tamped density in a range from 0.001 to 0.3 g/cm.sup.3; a particle size distribution D.sub.10 in the range from 1 to 10 m; a particle size distribution D.sub.50 in the range from 6 to 15 m; and a particle size distribution D.sub.90 in the range from 5 to 40 m; wherein the ppm and ppb are each based on the total weight of the silicon dioxide powder.

29. The process according to claim 21, further comprising making a hollow body with at least one opening out of the quartz glass body.

30. A process for the preparation of a silicon dioxide granulate comprising: making a silicon dioxide powder with at least two particles prepared from silicon tetrachloride; bringing the silicon dioxide powder into contact with steam to obtain a treated silicon dioxide powder; and granulating the treated silicon dioxide powder to obtain a silicon dioxide granulate, wherein the chlorine content of the silicon dioxide powder is greater than the chlorine content of the silicon dioxide granulate.

31. The process according to claim 30, wherein the silicon dioxide powder has a particle size distribution D.sub.50 in the range from 6 to 15 m.

32. A silicon dioxide granulate obtained by a process according to claim 30.

33. The silicon dioxide granulate according to claim 32, wherein the silicon dioxide granulate has a chlorine content of less than 500 ppm.

34. The silicon dioxide granulate according to claim 32, comprising at least one of: a BET surface area of 5 to 50 m.sup.2/g; a mean particle size in a range from 50 to 500 m; a bulk density in a range from 0.5 to 1.2 g/cm.sup.3. a carbon content of less than 50 ppm; an aluminium content of less than 200 ppb; a tamped density in a range from 0.7 to 1.2 g/cm.sup.3; a pore volume in a range from 0.1 to 2.5 mL/g; an angle of repose in a range from 23 to 26; a particle size distribution D.sub.10 in a range from 50 to 150 m; a particle size distribution D.sub.50 in a range from 150 to 300 m; and a particle size distribution D.sub.90 in a range from 250 to 620 m, wherein the ppm and ppb are each based on the total weight of the silicon dioxide granulate.

35. A quartz glass body obtained by a process according to claim 21.

36. The quartz glass body according to claim 35, comprising at least one of: an OH content of less than 10 ppm; a chlorine content of less than 60 ppm; an aluminium content of less than 200 ppb; an ODC content of less than 5*10.sup.15/cm.sup.3; a metal content of metals different from aluminium of less than 1 ppm; a viscosity (p=1013 hPa) in a range from log.sub.10 (n (1250 C.)/dPas)=11.4 to log.sub.10 ( (1250 C.)/dPas)=12.9 or log.sub.10 (n (1300 C.)/dPas)=11.1 to log.sub.10 ( (1300 C.)/dPas)=12.2 or log.sub.10 (n (1350 C.)/dPas)=10.5 to log.sub.10 ( (1350 C.)/dPas)=11.5; a standard deviation of the OH content of not more than 10%, based on the OH-content of the quartz glass body; a standard deviation of the Cl content of not more than 10%, based on the Cl content of the quartz glass body; a standard deviation of the Al content of not more than 10%, based on the Al content of the quartz glass body; a refractive index homogeneity of less than 10.sup.4; a cylindrical form; a tungsten content of less than 1000 ppb; and a molybdenum content of less than 1000 ppb, wherein the ppb and ppm are each based on the total weight of the quartz glass body.

37. A process for the preparation of a light guide comprising: providing: a hollow body with at least one opening obtained by a process according to claim 29, or a quartz glass body according to claim 35, wherein the quartz glass body is first processed to obtain a hollow body with at least one opening; introducing one or multiple core rods into the hollow body through the at least one opening to obtain a precursor; and drawing the precursor in the warm to obtain a light guide with one or multiple cores and a jacket.

38. A process for the preparation of an illuminant comprising: providing a hollow body obtained by a process according to claim 29; or a quartz glass body according to claim 35, wherein the quartz glass body is first processed to obtain a hollow body; optionally fitting the hollow body with electrodes; and filling the hollow body with a gas.

39. A process for the preparation of a formed body comprising: providing a quartz glass body according to claim 35 or obtained by a process according to claim 21; and forming the quartz glass body to obtain the formed body.

40. A use of a silicon dioxide powder prepared from a silicon-chlorine compound for the preparation of quartz glass and of products comprising quartz glass selected from the group consisting of a light guide, an illuminant and a formed body.

Description

FIGURES

[0965] FIG. 1 flow diagram (process for the preparation of a quartz glass body)

[0966] FIG. 2 flow diagram (process for the preparation of a silicon dioxide granulate I)

[0967] FIG. 3 flow diagram (process for the preparation of a silicon dioxide granulate II)

[0968] FIG. 4 flow diagram (process for the preparation of a light guide)

[0969] FIG. 5 flow diagram (process for the preparation of an illuminant)

[0970] FIG. 6 schematic representation of a hanging crucible in an oven

[0971] FIG. 7 schematic representation of a standing crucible in an oven

[0972] FIG. 8 schematic representation of a crucible with a flushing ring

[0973] FIG. 9 schematic representation of a spray tower

[0974] FIG. 10 schematic representation of a cross section of a light guide

[0975] FIG. 11 schematic representation of a view of a light guide

[0976] FIG. 12 schematic representation of a crucible with a dew point measuring device

[0977] FIG. 13 schematic representation of a gas pressure sinter oven (GDS oven)

[0978] FIG. 14 flow diagram (process for the preparation of a formed body)

DESCRIPTION OF THE FIGURES

[0979] FIG. 1 shows a flow diagram containing the steps 101 to 104 of a process 100 for the preparation of a quartz glass body according to the present invention. In a first step 101, a silicon dioxide granulate is provided. In a second step 102, a glass melt is made from the silicon dioxide granulate.

[0980] Preferably, moulds are used for the melting which can be introduced into and removed from an oven. Such moulds are often made of graphite. They provide a negative form for the caste item. The silicon dioxide granulate is filled into the mould and is first melted in the mould in step 103. Subsequently, the quartz glass body is formed in the same mould by cooling the melt. It is then freed from the mould and processed further, for example in an optional step 104. This procedure is discontinuous. The forming of the melt is preferably performed at reduced pressure, in particular in a vacuum. Further, it is possible during step 103 to charge the oven intermittently with a reducing, hydrogen containing atmosphere.

[0981] In another procedure, hanging or standing crucibles are preferably employed. The melting is preferably performed in a reducing, hydrogen containing atmosphere. In a third step 103, a quartz glass body is formed. The formation of the quartz glass body is preferably performed by removing at least a part of the glass melt from the crucible and cooling. The removal is preferably performed through a nozzle at the lower end of the crucible. In this case, the form of the quartz glass body can be determined by the design of the nozzle. In this way, for example, solid bodies can be obtained. Hollow bodies are obtained for example if the nozzle additionally has a mandrel. This example of a process for the preparation of quartz glass bodies, and in particular step 103, is preferably performed continuously. In an optional step 104, a hollow body can be formed from a solid quartz glass body.

[0982] FIG. 2 shows a flow diagram containing the steps 201 to 204 of a process 200 for the preparation of a silicon dioxide granulate I. In a first step 201, a silicon dioxide powder is provided. A silicon dioxide powder is preferably obtained from a synthetic process in which a silicon tetrachloride is converted into silicon dioxide in a pyrogenic process. In a second step 202, the silicon dioxide powder is brought into contact with steam to obtain a treated silicon dioxide powder. The treated silicon dioxide powder has a lower chlorine content than the silicon dioxide powder before the steam processing. In a third step 203, the treated silicon dioxide powder is mixed with a liquid, preferably with water, to obtain a slurry of the treated silicon dioxide powder. In a fourth step 204, the silicon dioxide contained in the slurry is transformed into a silicon dioxide granulate, preferably by spray granulation. For this, the slurry is sprayed into a spray tower and dried to obtain granules.

[0983] FIG. 3 Preparing silicon dioxide granulate II

[0984] FIG. 3 shows a flow diagram containing the steps 301, 302, 303 and 304 of a process 300 for the preparation of a silicon dioxide granulate II. The steps 301, 302 and 303 proceed corresponding to the steps 201, 202 and 203 according to FIG. 2. In step 304, the silicon dioxide granulate I obtained in step 303 is processed to obtain a silicon dioxide granulate II. This is preferably performed by warming the silicon dioxide granulate I in a chlorine containing atmosphere.

[0985] FIG. 4 shows a flow diagram containing the steps 401, 403 and 404 as well as the optional step 402 of the process for the preparation of a light guide. In the first step 401, a quartz glass body is provided, preferably a quartz glass body prepared according to process 100. Such a quartz glass body can be a solid or a hollow quartz glass body. In a second step 402, a hollow quartz glass body corresponding to step 104 is formed from a solid quartz glass body provided in step 401. In a third step 403, one or more than one core rods are introduced into the hollow. In a fourth step 404, the quartz glass body fitted with one or more than one core rods is processed to obtain a light guide. For this, the quartz glass body fitted with one or more than one core rods is preferably softened by warming and stretched until the desired thickness of the light guide is achieved.

[0986] FIG. 5 shows a flow diagram containing the steps 501, 503 and 504 as well as the optional step 502 of a process for the preparation of an illuminant. In the first step 501, a quartz glass body is provided, preferably a quartz glass body prepared according to process 100. Such a quartz glass body can be a solid or a hollow quartz glass body. If the quartz glass body provided in step 501 is solid, it is optionally formed in a second step 502 to obtain a hollow quartz glass body corresponding to step 104. In an optional third step, the hollow quartz glass body is fitted with electrodes. In a fourth step 504, the hollow quartz glass body is filled with a gas, preferably with argon, krypton, xenon or a combination thereof. Preferably, a solid quartz glass body is first provided (501), formed to obtain a hollow body (502), fitted with electrodes (503) and filled with a gas (504).

[0987] In FIG. 6, a preferred embodiment of an oven 800 with a hanging crucible is shown. The crucible 801 is arranged hanging in the oven 800. The crucible 801 has a hanger assembly 802 in its upper region, as well as a solids inlet 803 and a nozzle 804 as outlet. The crucible 801 is filled via the solids inlet 803 with silicon dioxide granulate 805. In operation, silicon dioxide granulate 805 is present in the upper region of the crucible 801, whilst a glass melt 806 is present in the lower region of the crucible. The crucible 801 can be heated by heating elements 807 which are arranged on the outer side of the crucible wall 810. The oven also has an insulation layer 809 between the heating elements 807 and the outer wall 808 of the oven. The space in between the insulation layer 809 and the crucible wall 810 can be filled with a gas and for this purpose has a gas inlet 811 and a gas outlet 812. A quartz glass body 813 can be removed from the oven through the nozzle 804.

[0988] In FIG. 7 a preferred embodiment of an oven 900 with a standing crucible is shown. The crucible 901 is arranged standing in the oven 900. The crucible 901 has a standing area 902, a solids inlet 903 and a nozzle 904 as outlet. The crucible 901 is filled with silicon dioxide granulate 905 via the inlet 903. In operation, silicon dioxide granulate 905 is present in the upper region of the crucible, whilst a glass melt 906 is present in the lower region of the crucible. The crucible can be heated by heating elements 907 which are arranged on the outer side of the crucible wall 910. The oven also has an insulation layer 909 between the heating elements 907 and the outer wall 908. The space between the insulation layer 909 and the crucible wall 910 can be filled with a gas and for this purpose has a gas inlet 911 and a gas outlet 912. A quartz glass body 913 can be removed from the crucible 901 through the nozzle 904.

[0989] In FIG. 8 is shown a preferred embodiment of a crucible 1000. The crucible 1000 has a solids inlet 1001 and a nozzle 1002 as outlet. The crucible 1000 is filled with silicon dioxide granulate 1003 via the solids inlet 1001. In operation, silicon dioxide granulate 1003 is present as a reposing cone 1004 in the upper region of the crucible 1000, whilst a glass melt 1005 is present in the lower region of the crucible. The crucible 1000 can be filled with a gas. It has a gas inlet 1006 and a gas outlet 1007. The gas inlet is a flushing ring mounted on the crucible wall above the silicon dioxide granulate. The gas in the interior of the crucible is released through the flushing ring (with a gas feed not shown here) close above the melting level and/or the reposing cone near the crucible wall and flows in the direction of the gas outlet 1007 which is arranged as a ring in the lid 1008 of the crucible 1000. The gas flow 1010 which is produced in this way moves along the crucible wall and submerges it. A quartz glass body 1009 can be removed from the crucible 1000 through the nozzle 1002.

[0990] In FIG. 9 is shown a preferred embodiment of a spray tower 1100 for spray granulating silicon dioxide. The spray tower 1100 comprises a feed 1101 through which a pressurised slurry containing silicon dioxide powder and a liquid are fed into the spray tower. At the end of the pipeline is a nozzle 1102 through which the slurry is introduced into the spray tower as a finely spread distribution. Preferably, the nozzle slopes upward, so that the slurry is sprayed into the spray tower as fine droplets in the nozzle direction and then falls down in an arc under the influence of gravity. At the upper end of the spray tower there is a gas inlet 1103. By introduction of a gas through the gas inlet 1103, a gas flow is created in the opposite direction to the exit direction of the slurry out of the nozzle 1102. The spray tower 1100 also comprises a screening device 1104 and a sieving device 1105. Particles which are smaller than a defined particle size are extracted by the screening device 1104 and removed through the discharge 1106. The extraction strength of the screening device 1104 can be configured to correspond to the particle size of the particles to be extracted. Particles above a defined particle size are sieved off by the sieving device 1105 and removed through the discharge 1107. The sieve permeability of the sieving device 1105 can be selected to correspond to the particle size to be sieved off. The remaining particles, a silicon dioxide granulate having the desired particle size, is removed through the outlet 1108.

[0991] In FIG. 10 is shown a schematic cross section through a light guide 1200 according to the invention which has a core 1201 and a jacket M1 1202 which surrounds the core 1201.

[0992] FIG. 11 shows schematically a top view of a guide 1300 which has cable structure. In order to represent the arrangement of the core 1301 and the jacket M1 1302 around the core 1301, a part of the core 1301 is shown without the jacket M1 1302. Typically however, the core 1301 is sheathed over its entire length by the jacket M1 1302.

[0993] FIG. 12 shows a preferred embodiment of a crucible 1400. The crucible has a solids inlet 1401 and an outlet 1402. In operation, silicon dioxide granulate 1403 is present in a reposing cone 1404 in the upper region of the crucible 1400, whilst a glass melt 1405 is present in the lower region of the crucible. The crucible 1400 has a gas inlet 1406 and a gas outlet 1407. The gas inlet 1406 and the gas outlet 1407 are arranged above the reposing cone 1404 of the silicon dioxide granulate 1403. The gas outlet 1406 comprises a pipeline for the gas feed 1408 and a device 1409 for measuring the dew point of the exiting gas. The device 1409 comprises a dew point mirror hygrometer (not shown here). The separation between the crucible and the device 1409 for measuring the dew point can vary. A quartz glass body 1410 can be removed through the outlet 1402 of the crucible 1400.

[0994] FIG. 13 shows a preferred embodiment of the oven 1500 which is suitable for a vacuum sintering process, a gas pressure sinter process and in particular a combination thereof. The oven has from outside inward a pressure resistant jacket 1501 and a thermal insulating layer 1502. The space enclosed thereby, referred to as the oven interior, can be charged with a gas or a gas mixture via a gas feed 1504. Further, the oven interior has a gas outlet 1505 via which gas can be removed. According to the gas transport balance between gas feed 1504 and gas removal at 1505 an over pressure, a vacuum or also a gas flow can be produced in the interior of the oven 1500. Further, heating elements 1506 are present in the oven interior 1500. These are often mounted on the insulation layer 1502 (not shown here). For protecting the melt material from contamination, there is a so-called liner 1507 in the interior of the oven, which separates the oven chamber 1503 from the heating elements 1506. Moulds 1508 with material to be melted 1509 can be introduced into the oven chamber 1503. The mould 1508 can be open on a side (shown here) or can completely enclose the melt material 1509 (not shown).

[0995] FIG. 14 shows a flow diagram containing the steps 1601 and 1602 of a process for the preparation of a formed body. In the first step 1601, a quartz glass body is provided, preferably a quartz glass body prepared according to process 100. Such a quartz glass body can be a solid or hollow body quartz glass body. In a second step 1602, a formed body is formed from a solid quartz glass body provided in step 1601.

[0996] Test Methods

[0997] a. Fictive Temperature

[0998] The fictive temperature is measured by Raman spectroscopy using the Raman scattering intensity at about 606 cm.sup.1. The procedure and analysis described in the contribution of Pfleiderer et. al.; The UV-induced 210 nm absorption band in fused Silica with different thermal history and stoichiometry; Journal of Non-Crystalline Solids, volume 159 (1993), pages 145-153.

[0999] b. OH Content

[1000] The OH content of the glass is measured by infrared spectroscopy. The method of D. M. Dodd & D. M. Fraser Optical Determinations of OH in Fused Silica (J. A. P. 37, 3991 (1966)) is employed. Instead of the device named therein, an FTIR-spectrometer (Fourier transform infrared spectrometer, current System 2000 of Perkin Elmer) is employed. The analysis of the spectra can in principle be performed on either the absorption band at approx. 3670 cm.sup.1 or on the absorption band at approx. 7200 cm.sup.1. The selection of the band is made on the basis that the transmission loss through OH absorption is between 10 and 90%.

[1001] c. Oxygen Deficiency Centers (ODCs)

[1002] For the quantitative detection, the ODC(I) absorption is measured at 165 nm by means of a transmission measurement at a probe with thickness between 1-2 mm using a vacuum UV spectrometer, model VUVAS 2000, of McPherson, Inc. (USA).

[1003] Then:

N=/

with [1004] N=defect concentration [1/cm.sup.3] [1005] =optical absorption [1/cm, base e] of the ODC(I) band [1006] =effective cross section [cm.sup.2] [1007] wherein the effective cross section is set to =7.5.Math.10.sup.17 cm.sup.2 (from L. Skuja, Color Centers and Their Transformations in Glassy SiO.sub.2, Lectures of the summer school Photosensitivity in optical Waveguides and glasses, Jul. 13-18 1998, Vitznau, Switzerland).

[1008] d. Elemental Analysis

[1009] d.-1) Solid samples are crushed. Then, approx. 20 g of the sample is cleaned by introducing it into a HFresistant vessel fully, covering it with HF and thermally treating at 100 C. for an hour. After cooling, the acid is discarded and the sample cleaned several times with high purity water. Then, the vessel and the sample are dried in the drying cabinet.

[1010] Next, approx. 2 g of the solid sample (crushed material cleaned as above; dusts etc. without pre-treatment) is weighed into an HF resistant extraction vessel and dissolved in 15 ml HF (50 wt.-%). The extraction vessel is closed and thermally treated at 100 C. until the sample is completely dissolved. Then, the extraction vessel is opened and further thermally treated at 100 C., until the solution is completely evaporated. Meanwhile, the extraction vessel is filled 3 with 15 ml of high purity water. 1 ml HNO.sub.3 is introduced into the extraction vessel, in order to dissolve separated impurities and filled up to 15 ml with high purity water. The sample solution is then ready.

[1011] d.-2) ICP-MS/ICP-OES Measurement

[1012] Whether OES or MS is employed depends on the expected elemental concentrations. Typically, measurements of MS are 1 ppb, and for OES they are 10 ppb (in each case based on the weighed sample). The measurement of the elemental concentration with the measuring device is performed according to the stipulations of the device manufacturer (ICP-MS: Agilent 7500ce; ICP-OES: Perkin Elmer 7300 DV) and using certified reference liquids for calibration. The elemental concentrations in the solution (15 ml) measured by the device are then converted based on the original weight of the probe (2 g).

[1013] Note: It is to be kept in mind that the acid, the vessels, the water and the devices must be sufficiently pure in order to measure the elemental concentrations in question. This is checked by extracting a blank sample without quartz glass.

[1014] The following elements are measured in this way: Li, Na, Mg, K, Ca, Fe, Ni, Cr, Hf, Zr, Ti, (Ta), V, Nb, W, Mo, Al.

[1015] d.-3) The measurement of samples present as a liquid is carried out as described above, wherein the sample preparation according to step d.-1) is skipped. 15 ml of the liquid sample are introduced into the extraction flask. No conversion based on the original sample weight is made.

[1016] e. Determination of Density of a Liquid

[1017] For measuring the density of a liquid, a precisely defined volume of the liquid is weighed into a measuring device which is inert to the liquid and its constituents, wherein the empty weight and the filled weight of the vessel are measured. The density is given as the difference between the two weight measurements divided by the volume of the liquid introduced.

[1018] f. Fluoride Determination

[1019] 15 g of a quartz glass sample is crushed and cleaned by treating in nitric acid at 70 C. The sample is then washed several times with high purity water and then dried. 2 g of the sample is weighed into a nickel crucible and covered with 10 g Na.sub.2CO.sub.3 and 0.5 g ZnO. The crucible is closed with a Ni-lid and roasted at 1000 C. for an hour. The nickel crucible is then filled with water and boiled up until the melt cake has dissolved entirely. The solution is transferred to a 200 ml measuring flask and filled up to 200 ml with high purity water. After sedimentation of undissolved constituents, 30 ml are taken and transferred to a 100 ml measuring flask, 0.75 ml of glacial acetic acid and 60 ml TISAB are added and filled up with high purity water. The sample solution is transferred to a 150 ml glass beaker.

[1020] The measurement of the fluoride content in the sample solution is performed by means of an ion sensitive (fluoride) electrode, suitable for the expected concentration range, and display device as stipulated by the manufacturer, here a fluoride ion selective electrode and reference electrode F-500 with R503/D connected to a pMX 3000/pH/ION from Wissenschaftlich-Technische Werkstatten GmbH. With the fluoride concentration in the solution, the dilution factor and the sample weight, the fluoride concentration in the quartz glass is calculated.

[1021] g. Determination of Chlorine (>=50 ppm)

[1022] 15 g of a quartz glass sample is crushed and cleaned by treating with nitric acid at approx. 70 C. Subsequently, the sample is rinsed several times with high purity water and then dried. 2 g of the sample are then filled into a PTFE-insert for a pressure container, dissolved with 15 ml NaOH (c=10 mol/l), closed with a PTFE lid and placed in the pressure container. It is closed and thermally treated at approx. 155 C. for 24 hours. After cooling, the PTFE insert is removed and the solution is transferred entirely to a 100 ml measuring flask. There, 10 ml HNO.sub.3 (65 wt.-%) and 15 ml acetate buffer and added, allowed to cool and filled to 100 ml with high purity water. The sample solution is transferred to a 150 ml glass beaker. The sample solution has a pH value in the range between 5 and 7.

[1023] The measurement of the chloride content in the sample solution is performed by means of an ion sensitive (Chloride) electrode which is suitable for the expected concentration range, and a display device as stipulated by the manufacturer, here an electrode of type C1-500 and a reference electrode of type R-503/D attached to a pMX 3000/pH/ION from Wissenschaftlich-Technische Werkstatten GmbH.

[1024] h. Chlorine Content (<50 ppm)

[1025] Chlorine content <50 ppm up to 0.1 ppm in quartz glass is measured by neutron activation analysis (NAA). For this, 3 bores, each of 3 mm diameter and 1 cm long are taken from the quartz glass body under investigation. These are given to a research institute for analysis, in this case to the institute for nuclear chemistry of the Johannes-Gutenberg University in Mainz, Germany. In order to exclude contamination of the sample with chlorine, a thorough cleaning of the sample in an HF bath on location directly before the measurement was arranged. Each bore is measured several times. The results and the bores are then sent back by the research institute.

[1026] i. Optical Properties

[1027] The transmission of quartz glass samples is measured with the commercial grating- or FTIR-spectrometer from Perkin Elmer (Lambda 900 [190-3000 nm] or System 2000 [1000-5000 nm]). The selection is determined by the required measuring range.

[1028] For measuring the absolute transmission, the sample bodies are polished on parallel planes (surface roughness RMS <0.5 nm) and the surface is cleared off all residues by ultrasound treatment. The sample thickness is 1 cm. In the case of an expected strong transmission loss due to impurities, dopants etc., a thicker or thinner sample can be selected in order to stay within the measuring range of the device. A sample thickness (measuring length) is selected at which only slight artefacts are produced on account of the passage of the radiation through the sample and at the same time a sufficiently detectable effect is measured.

[1029] The measurement of the opacity, the sample is placed in front of an integrating sphere. The opacity is calculated using the measured transmission value T according to the formula: O=1/T=I.sub.0/I.

[1030] j. Refractive Index and Distribution of Refractive Index in a Tube or Rod

[1031] The distribution of refractive index of tubes/rods can be characterised by means of a York Technology Ltd. Preform Profiler P102 or P104. For this, the rod is placed lying in the measuring chamber the chamber is closed tight. The measuring chamber is then filled with an immersion oil which has a refractive index at the test wavelength of 633 nm which is very similar to that of the outermost glass layer at 633 nm. The laser beam then goes through the measuring chamber. Behind the measuring chamber (in the direction of the of the radiation) is mounted a detector which measures the angle of deviation (of the radiation entering the measuring chamber compared to the radiation exiting the measuring chamber). Under the assumption of radial symmetry of the distribution of refractive index of the rod, the diametral distribution of refractive index can be reconstructed by means of an inverse Abel transformation. These calculations are performed by the software of the device manufacturer York.

[1032] The refractive index of a sample is measured with the York Technology Ltd. Preform Profiler P104 analogue to the above description. In the case of isotropic samples, measurement of distribution of refractive index gives only one value, the refractive index.

[1033] k. Carbon Content

[1034] The quantitative measurement of the surface carbon content of silicon dioxide granulate and silicon dioxide powder is performed with a carbon analyser RC612 from Leco Corporation, USA, by the complete oxidation of all surface carbon contamination (apart from SiC) with oxygen to obtain carbon dioxide. For this, 4.0 g of a sample are weighed and introduced into the carbon analyser in a quartz glass dish. The sample is bathed in pure oxygen and heated for 180 seconds to 900 C. The CO.sub.2 which forms is measured by the infrared detector of the carbon analyser. Under these measuring conditions, the detection limit lies at 1 ppm (weight-ppm) carbon.

[1035] A quartz glass boat which is suitable for this analysis using the above named carbon analyser is obtainable as a consumable for the LECO analyser with LECO number 781-335 on the laboratory supplies market, in the present case from Deslis Laborhandel, Flurstralk 21, D-40235 Dusseldorf (Germany), Deslis-No. LQ-130XL. Such a boat has width/length/height dimensions of ca. 25 mm/60 mm/15 mm. The quartz glass boat is filled up to half its height with sample material. For silicon dioxide powder, a sample weight of 1.0 g sample material can be reached. The lower detection limit is then <1 weight ppm carbon. In the same boat, a sample weight of 4 g of a silicon dioxide granulate is reached for the same filling height (mean particle size in the range from 100 to 500 m). The lower detection limit is then about 0.1 weight ppm carbon. The lower detection limit is reached when the measurement surface integral of the sample is not greater than three times the measurement surface integral of an empty sample (empty sample=the above process but with an empty quartz glass boat).

[1036] l. Curl Parameter

[1037] The curl parameter (also called: Fibre Curl) is measured according to DIN EN 60793-1-34:2007-01 (German version of the standard IEC 60793-1-34:2006). The measurement is made according to the method described in Annex A in the sections A.2.1, A.3.2 and A.4.1 (extrema technique).

[1038] m. Attenuation

[1039] The attenuation is measured according to DIN EN 60793-1-40:2001 (German version of the standard IEC 60793-1-40:2001). The measurement is made according to the method described in the annex (cut-back method) at a wavelength of =1550 nm.

[1040] n. Viscosity of the Slurry

[1041] The slurry is set to a concentration of 30 weight-% solids content with demineralised water (Direct-Q 3UV, Millipore, Water quality: 18.2 Mcm). The viscosity is then measured with a MCR102 from Anton-Paar. For this, the viscosity is measured at 5 rpm. The measurement is made at a temperature of 23 C. and an air pressure of 1013 hPa.

[1042] o. Thixotropy

[1043] The concentration of the slurry is set to a concentration of 30 weight-% of solids with demineralised water (Direct-Q 3UV, Millipore, water quality: 18.2 MSkm). The thixotropy is then measured with an MCR102 from Anton-Paar with a cone and plate arrangement. The viscosity is measured at 5 rpm and at 50 rpm. The quotient of the first and the second value gives the thixotropic index. The measurement is made at a temperature of 23 C.

[1044] p. Zeta Potential of the Slurry

[1045] For zeta potential measurements, a zeta potential cell (Flow Cell, Beckman Coulter) is employed. The sample is dissolved in demineralised water (Direct-Q 3UV, Millipore, water quality: 18.2 Mcm) to obtain a 20 mL solution with a concentration of 1 g/L. The pH is set to 7 through addition of HNO.sub.3 solutions with concentrations of 0.1 mol/L and 1 mol/L and an NaOH solution with a concentration of 0.1 mol/L. The measurement is made at a temperature of 23 C.

[1046] q. Isoelectric Point of the Slurry

[1047] The isoelectric point, a zeta potential measurement cell (Flow Cell, Beckman Coulter) and an auto titrator (DelsaNano AT, Beckman Coulter) is employed. The sample is dissolved in demineralised water (Direct-Q 3UV, Millipore, water quality: 18.2 Mcm) to obtain a 20 mL solution with a concentration of 1 g/L. The pH is varied by adding HNO.sub.3 solutions with concentrations of 0.1 mol/L and 1 mol/L and an NaOH solution with a concentration of 0.1 mol/L. The isoelectric point is the pH value at which the zeta potential is equal to 0. The measurement is made at a temperature of 23 C.

[1048] r. pH Value of the Slurry

[1049] The pH value of the slurry is measured using a WTW 3210 from Wissenschaftlich-Technische-Werkstatten GmbH. The pH 3210 Set 3 from WTW is employed as electrode. The measurement is made at a temperature of 23 C.

[1050] s. Solids Content

[1051] A weighed portion m.sub.1 of a sample is heated for 4 hours to 500 C. reweighed after cooling (m.sub.2). The solids content w is given as m.sub.2/m.sub.1*100 [Wt. %].

[1052] t. Bulk Density

[1053] The bug: density is measured according to the standard DIN ISO 697:1984-01 with an SMG 697 from Powtec. The bulk material (silicon dioxide powder or granulate) does not clump.

[1054] u. Tamped Density (Granulate)

[1055] The tamped density is measured according to the standard DIN ISO 787:1995-10.

[1056] v. Measurement of the Pore Size Distribution

[1057] The pore size distribution is measured according to DIN 66133 (with a surface tension of 480 mN/m and a contact angle of 140). For the measurement of pore sizes smaller than 3.7 nm, the Pascal 400 from Porotec is used. For the measurement of pore sizes from 3.7 nm to 100 m, the Pascal 140 from Porotec is used. The sample is subjected to a pressure treatment prior to the measurement. For this a manual hydraulic press is used (Order no. 15011 from Specac Ltd., River House, 97 Cray Avenue, Orpington, Kent BR5 4HE, U.K.). 250 mg of sample material is weighed into a pellet die with 13 mm inner diameter from Specac Ltd. and loaded with 1 t, as per the display. This load is maintained for 5 s and readjusted if necessary. The load on the sample is then released and the sample is dried for 4 h at 1052 C. in a recirculating air drying cabinet.

[1058] The sample is weighed into the penetrometer of type 10 with an accuracy of 0.001 g and in order to give a good reproducibility of the measurement it is selected such that the stem volume used, i.e. the percentage of potentially used Hg volume for filling the penetrometer is in the range between 20% to 40% of the total Hg volume. The penetrometer is then slowly evacuated to 50 m Hg and left at this pressure for 5 min. The following parameters are provided directly by the software of the measuring device: total pore volume, total pore surface area (assuming cylindrical pores), average pore radius, modal pore radius (most frequently occurring pore radius), peak n. 2 pore radius (m).

[1059] w. Primary Particle Size

[1060] The primary particle size is measured using a scanning electron microscope (SEM) model Zeiss Ultra 55. The sample is suspended in demineralised water (Direct-Q 3UV, Millipore, water quality: 18.2 Mcm), to obtain an extremely dilute suspension. The suspension is treated for 1 min with the ultrasound probe (UW 2070, Bandelin electronic, 70 W, 20 kHz) and then applied to a carbon adhesive pad.

[1061] x. Mean Particle Size in Suspension

[1062] The mean particle size in suspension is measured using a Mastersizer 2000, available from Malvern Instruments Ltd., UK, according to the user manual, using the laser deflection method. The sample is suspended in demineralised water (Direct-Q 3UV, Millipore, water quality: 18.2 Mcm) to obtain a 20 mL suspension with a concentration of 1 g/L. The suspension is treated with the ultrasound probe (UW 2070, Bandelin electronic, 70 W, 20 kHz) for 1 min.

[1063] y. Particle Size and Core Size of the Solid

[1064] The particle size and core size of the solid are measured using a Camsizer XT, available from Retsch Technology GmbH, Deutschland according to the user manual. The software gives the D.sub.10, D.sub.50 and D.sub.90 values for a sample.

[1065] z. BET Measurement

[1066] For the measurement of the specific surface area, the static volumetric BET method according to DIN ISO 9277:2010 is used. For the BET measurement, a NOVA 3000 or a Quadrasorb (available from Quantachrome), which operate according to the SMART method (Sorption Method with Adaptive dosing Rate), is used. The micropore analysis is performed using the t-plot process (p/p0=0.1-0.3) and the mesopore analysis is performed using the MBET process (p/p0=0.0-0.3). As reference material, the standards alumina SARM-13 and SARM-214, available from Quantachrome are used. The tare weight of the measuring cell (clean and dry) is weighed. The type of measuring cell is selected such that the sample material which is introduced and the filler rod fill the measuring cell as much as possible and the dead space is reduced to a minimam. The sample material is introduced into the measuring cell. The amount of sample material is selected so that the expected value of the measurement value corresponds to 10-20 m.sup.2/g. The measuring cells are fixed in the baking positions of the BET measuring device (without filler rod) and evacuated to <200 mbar. The speed of the evacuation is set so that no material leaks from the measuring cell. Baking is performed in this state at 200 C. for 1 h. After cooling, the measuring cell filled with the sample is weighed (raw value). The tare weight is then subtracted from the raw value of the weight=nett weight=weight of the sample. The filling rod is then introduced into the measuring cell this is again fixed at the measuring location of the BET measuring device. Prior to the start of the measurement, the sample identifications and the sample weights are entered into the software. The measurement is started. The saturation pressure of nitrogen gas (N2 4.0) is measured. The measuring cell is evacuated and cooled down to 77 K using a nitrogen bath. The dead space is measured using helium gas (He 4.6). The measuring cell is evacuated again. A multi-point analysis with at least 5 measuring points is performed. N2 4.0 is used as absorptive. The specific surface area is given in m.sup.2/g.

[1067] Za. Viscosity of Glass Bodies

[1068] The viscosity of the glass is measured using the beam bending viscosimeter of type 401from TA Instruments with the manufacturer's software WinTA (current version 9.0) in Windows 10 according to the DIN

[1069] ISO 7884-4:1998-02 standard. The support width between the supports is 45 mm. Sample rods with rectangular cross section are cut from regions of homogeneous material (top and bottom sides of the sample have a finish of at least 1000 grain). The sample surfaces after processing have a grain size=9 m & RA=0.15 m. The sample rods have the following dimensions: length=50 mm, width=5 mm & height=3 mm (ordered: length, width, height, as in the standards document). Three samples are measured and the mean is calculated. The sample temperature is measured using a thermocouple tight against the sample surface. The following parameters are used: heating rate=25 K up to a maximum of 1500 C., loading weight=100 g, maximum bending=3000 m (deviation from the standards document).

[1070] Zb. Dew Point Measurement

[1071] The dew point is measured using a dew point mirror hygrometer called Optidew of the company Michell Instruments GmbH, D-61381 Friedrichsdorf. The measuring cell of the dew point mirror hygrometer is arranged at a distance of 100 cm from the gas outlet of the oven. For this, the measuring device with the measuring cell is connected in gas communication to the gas outlet of the oven via a T-piece and a hose (Swagelok PFA, Outer diameter: 6 mm). The over pressure at the measuring cell is 102 mbar. The through flow of the gas through the measuring cell is 1-2 standard litre/min. The measuring cell is in a room with a temperature of 25 C., 30% relative air humidity and a mean pressure of 1013 hPa.

[1072] Zc. Residual Moisture (Water Content)

[1073] The measurement of the residual moisture of a sample of silicon dioxide granulate is performed using a Moisture Analyzer HX204 from Mettler Toledo. The device functions using the principle of thermogravimetry. The HX204 is equipped with a halogen light source as heating element. The drying temperature is 220 C. The starting weight of the sample is 10 g10%. The Standard measuring method is selected. The drying is carried out until the weight change reaches not more than 1 mg/140 s. The residual moisture is given as the difference between the initial weight of the sample and the final weight of the sample, divided by the initial weight of the sample.

[1074] The measurement of residual moisture of silicon dioxide powder is performed according to DIN EN ISO 787-2:1995 (2 h, 105 C.).

EXAMPLES

[1075] The example is further illustrated in the following through examples. The invention is not limited by the examples.

[1076] B. 1. Preparation of Silicon Dioxide Powder (Silicon Source: SiCl.sub.4)

[1077] A portion of silicon tetrachloride (SiCl.sub.4) is evaporated at a temperature T and introduced with a pressure P into a flame of a burner which is formed by igniting a mixture of oxygen enriched air and hydrogen. The mean normalised gas flow to the outlet is held constant. The process mixture is then cooled with process gas. The product was separated off at a filter. The process parameters are given in table 1 and the specifications of the resulting products are given in table 2. They are indicated with B1-x.

[1078] 2. Modification 1: Increased Carbon Content

[1079] A process was carried out as described in B.1., but the burning of the silicon tetrachloride was performed such that an amount of carbon was also formed. Experimental data for this example are indicated with B2-x.

TABLE-US-00002 TABLE 1 Example B1-1 B2-1 Aerosol formation Silicon tetrachloride feed kg/h 50 50 (kmol) (0.294) (0.294) Temperature T C. 90 90 Pressure p barO 1.2 1.2 Burner feed Oxygen enriched air, Nm.sup.3/h 145 115 O.sub.2 content therein Vol. % 45 30 Feed Carbon compound Material methane Amount Nm.sup.3/h 2.0 Hydrogen feed Nm.sup.3/h 115 60 kmol/h 5.13 2.678 Stoichiometric ratios X 0.567 0.575 Y 0.946 0.85 X = as molar ratio O.sub.2/H.sub.2; Y = molar ratio of employed O.sub.2/O.sub.2 required for stoichiometric reaction with SiCl4 + H2 + CH4); barO = Over pressure.

TABLE-US-00003 TABLE 2 Example B1-1 B2-1 BET m.sup.2/g 49 47 Bulk density g/ml 0.07 0.01 0.06 0.01 Tamped density g/ml 0.11 0.01 0.10 0.01 Primary particle size nm 48 43 Particle size distribution D10 m 5.0 0.5 4.5 0.5 Particle size distribution D50 m 9.3 0.6 8.7 0.6 Particle size distribution D90 m 16.4 0.5 15.8 0.7 C content ppm <4 76 Cl content ppm 280 330 Al content ppb 20 20 Total of the concentrations of Ca, Co, Cr, ppb <1300 <1300 Cu, Fe, Ge, Hf, K, Li, Mg, Mn, Mo, Na, Nb, Ni, Ti, V, W, Zn, Zr Residual moisture content wt.-% 0.02-1.0 0.02-1.0 pH value in water 4% (IEP) pH 3.8 3.8 Viscosity at 5 rpm, aqueous suspension mPas 5653 6012 30 wt-%, 23 C. Alkali earth metal content ppb 550 342

[1080] C. Steam Treatment

[1081] A particle flow of silicon dioxide powder is introduced via the top of a standing column. Steam at a temperature (A) and air are fed via the bottom of the column. The column is maintained at a temperature (B) at the top of the column and at a second temperature (C) at the bottom of the column by an internally situated heater. After leaving the column (holding time (D)) the silicon dioxide powder has in particular the properties shown in Table 4. The process parameters are given in Table 3.

TABLE-US-00004 TABLE 3 Example C-1 C-2 Educt: product of B1-1 B2-1 Educt feed kg/h 100 100 Steam feed kg/h 5 5 Steam temperature (A) C. 120 120 Air feed Nm.sup.3/h 4.5 4.5 Column Height m 2 2 Inner diameter mm 600 600 T (B) C. 260 260 T (C) C. 425 425 Holding time (D) silicon dioxide s 10 10 powder

TABLE-US-00005 TABLE 4 Example C-1 C-2 pH value in water 4% (IEP) 4.6 4.6 Cl content ppm <60 <60 C content ppm <4 36 Viscosity at 5 rpm, aqueous mPas 1523 1478 suspension 30 Wt-%, 23 C.

[1082] The silicon dioxide powders obtained in the examples C-1 and C-2 each have a low chlorine content as well as a moderate pH value in suspension. The carbon content of example C-2 is higher as for C-1.

[1083] E. 1. Preparation of Silicon Dioxide Granulate from Silicon Dioxide Powder

[1084] A silicon dioxide powder is dispersed in fully desalinated water. For this, an intensive mixer of type R from the Gustav Eirich machine factory is used. The resulting suspension is pumped with a membrane pump and thereby pressurised and converted into droplets by a nozzle. These are dried in a spray tower and collect on the floor of the tower. The process parameters are given in Table 5 and the properties of the obtained granulate in Table 6. Experimental data for this example are indicated with E1-x.

TABLE-US-00006 TABLE 5 Example E1-3 E1-4 E1-5 Educt = Product from B1-1 C-1 C-2 Amount of educt Kg 10 10 10 Carbon powder Material Max. Particle size Amount Silicon component Material Grain size (d50) Amount Carbon content Total of the concentrations of Ca, Co, Cr, Cu, Fe, Ge, Hf, K, Li, Mg, Mn, Mo, Na, Nb, Ni, Ti, V, W, Zn, Zr Water Rating* FD FD FD Litres 5.4 5.4 5.4 Dispersion Solids content Wt. % 65 65 65 Nozzle Diameter mm 2.2 2.2 2.2 Temperature C. 25 25 25 Pressure Bar 16 16 16 Installation height m 6.5 6.5 6.5 Spray tower Height m 18.20 18.20 18.20 Inner diameter m 6.30 6.30 6.30 T (introduced gas) C. 380 380 380 T (exhaust) C. 110 110 110 Air flow m.sup.3/h 6500 6500 6500 Installation height = distance between nozzle and lowest point of the spray tower interior in the direction of gravity. *FD = fully desalinated, conductance 0.1 S.

TABLE-US-00007 TABLE 6 Example E1-3 E1-4 E1-5 BET m.sup.2/g 49 49 47 Bulk density g/ml 0.8 0.1 0.8 0.1 0.8 0.1 Tamped density g/ml 0.9 0.1 0.9 0.1 0.9 0.1 Mean particle size m 255 255 255 Particle size m 110 110 110 distribution D10 Particle size m 222 222 222 distribution D50 Particle size m 390 390 390 distribution D90 SPHT3 Dim. 0.64-0.98 0.64-0.98 0.64-0.98 Less Aspect ratio W/L Dim. 0.64-0.94 0.64-0.94 0.64-0.94 (width to length) less C content ppm <4 <4 32 Cl content ppm 280 <60 <60 Al content ppb 20 20 20 Total concentrations ppb <1300 <1300 <1300 of Ca, Co, Cr, Cu, Fe, Ge, Hf, K, Li, Mg, Mn, Mo, Na, Nb, Ni, Ti, V, W, Zn, Zr Residual moisture wt.-% <3 <3 <3 content Alkaline earth metal ppb 550 550 342 content Pore volume ml/g 0.45 0.45 0.45 Angle of repose 26 26 26

[1085] The granulates are all open pored, have a uniform and spherical shape (all by microscopic investigation). They tend not to stick together or cement.

[1086] F. Cleaning of Silicon Dioxide Granulate

[1087] Silicon dioxide granulate is first optionally treated with oxygen or nitrogen (see table 7) at a temperature Ti. Subsequently, the silicon dioxide granulate is treated with a co-flow of a chlorine containing component, wherein the temperature is raised to a temperature T2. The process parameters are given in Table 7 and the properties of the obtained treated granulate in Table 8.

TABLE-US-00008 TABLE 7 Example F1-3 F1-4 F1-5 Educt = Product from E1-3 E1-4 E1-5 Rotary kiln .sup.1) length cm 200 200 200 Inner diameter cm 10 10 10 Throughput kg/h 2 2 2 Rotational speed rpm 2 2 2 T1 C. 1100 1100 1100 Atmosphere O2 pure O2 pure O2 pure Reactant O2 O2 O2 Feed 300 l/h 300 l/h 300 l/h residual moisture wt.-% <1 <1 <1 content T2 C. 1100 1100 1100 Co-flow Component 1: HCl l/h 50 50 50 Component 2: Cl2 l/h 0 0 0 Component 3: N2 l/h 50 50 50 Total co-flow l/h 100 100 100 .sup.1) For the rotary kilns, the throughput is selected as the control variable. That means that during operation the mass flow exiting from the rotary kiln is weighed and then the rotational speed and/or the inclination of the rotary kiln is adapted accordingly. For example, an increase in the throughput can be achieved by a) increasing the rotational speed, or b) increasing the inclination of the rotary kiln away from horizontal, or a combination of a) and b).

TABLE-US-00009 TABLE 8 Example F1-3 F1-4 F1-5 BET m.sup.2/g 43 45 40 C content ppm <4 <4 <4 Cl content ppm 300-400 100-200 100-200 Al content ppb 20 20 20 Pore volume mm.sup.3/g 650 650 650 Total of the con- ppb <200 <200 <200 centrations of Ca, Co, Cr, Cu, Fe, Ge, Hf, K, Li, Mg, Mn, Mo, Na, Nb, Ni, Ti, V, W, Zn, Zr Alkaline earth metal ppb 95 115 40 content Tamped density g/cm.sup.3 0.95 0.05 0.95 0.05 0.95 0.05 SiC formation was not observed.

[1088] G. Treatment of Silicon Dioxide Granulate by Warming

[1089] Silicon dioxide granulate is subjected to a temperature treatment in a pre chamber in the form of a rotary kiln which is positioned upstream of the melting oven and which is connected in flow connection to the melting oven via a further intermediate chamber. The rotary kiln is characterised by a temperature profile which increases in the flow direction. A further treated silicon dioxide granulate was obtained. The process parameters are given in Table 9 and the properties of the obtained treated granulate in Table 10.

TABLE-US-00010 TABLE 9 Example G1-3 G1-4 G1-5 Educt = product from F1-3 F1-4 F1-5 Silicon components Material Amount Rotary kiln Length cm 200 200 200 Inner diameter cm 10 10 10 Throughput kg/h 5 5 5 Rotation speed rpm 30 30 30 T1 (Rotary kiln inlet) C. RT RT RT T2 (Rotary kiln outlet) C. 500 500 500 Atmosphere Gas, flow direction O.sub.2, O.sub.2, O.sub.2, in counter- in counter- in counter- flow flow flow Total throughput of gas Nm.sup.3/h 0.6 0.6 0.6 flow ***Grain size D.sub.50 = 8 m; carbon content 5 ppm; Total foreign metals 5 ppm; 0.5 ppm; available from Wacker Chemie AG (Munich, Germany). RT = Room temperature.

TABLE-US-00011 TABLE 10 Example G1-3 G1-4 G1-5 BET m.sup.2/g 38 42 37 Water content ppm 100 100 100 C content ppm <4 <4 <4 Cl content ppm 300- 100- 100- 400 200 200 Al content ppb 20 20 20 Total of the concentrations of Ca, Co, Cr, ppb 200 200 200 Cu, Fe, Ge, Hf, K, Li, Mg, Mn, Mo, Na, Nb, Ni, Ti, V, W, Zn, Zr Alkaline earth metal content ppb 95 115 40 angle of repose 26 26 26

[1090] H. Melting of Granulate to Obtain Quartz Glass

[1091] Silicon dioxide granulate according to line 2 of Table 11 is employed for preparing a quartz glass tube in a vertical crucible drawing process. The structure of the hanging melting crucible is shown schematically in FIG. 6. The silicon dioxide granulate is introduced via the solids feed and the interior of the melting crucible is flushed with a gas mixture. In the melting crucible, a glass melt forms upon which a reposing cone of silicon dioxide granulate sits. In the lower region of the melting crucible, molten glass is removed from the glass melt through a die (optionally with a mandrel) and is pulled vertically down in the form of a tubular thread. The output of the system results from the weight of the glass melt, the viscosity of the glass through the nozzle the size of the hole provided by the nozzle. By varying the feed rate of silicon dioxide granulate and the temperature, the output can be set to the desired level. The process parameters are given in Table 11 and the properties of the formed quartz glass body in Table 12.

TABLE-US-00012 TABLE 11 Example H1-3 H1-4 H1-5 Educt = Product from G1-3 G1-4 G1-5 Melting crucible Type Hanging Hanging Hanging metal sheet metal sheet metal sheet crucible crucible crucible Type of metal tungsten tungsten tungsten length cm 150 150 150 Inner diameter cm 25 25 25 Throughput kg/h 20 20 20 T1 (Gas compartment of the C. 300 300 300 melting crucible) T2 (glass melt) C. 2100 2100 2100 T3 (nozzle) C. 1900 1900 1900 Atmosphere/Flushing gas He Concentration Vol.- 50 50 50 % H.sub.2 Concentration Vol.- 50 50 50 % Total gas flow throughput Nm.sup.3/ 4 4 4 h O.sub.2 ppm 100 100 100

TABLE-US-00013 TABLE 12 Example H1-3 H1-4 H1-5 C content ppm <4 <4 <4 Cl content ppm 300-400 100-200 100-200 Al content ppb 20 20 20 Total of the concentrations of ppb <400 <400 <400 Ca, Co, Cr, Cu, Fe, Ge, Hf, K, Li, Mg, Mn, Mo, Na, Nb, Ni, Ti, V, W, Zn, Zr Total content of impurities Ppb <1000 <1000 <1000 OH content ppm 400 400 400 Alkaline earth metal content ppb 95 115 40 ODC content l/cm.sup.3 4*10.sup.15 4*10.sup.15 4*10.sup.15 pore volume mL/g 0.1 0.1 0.1 Outer diameter tubular cm 19.7 19.7 19.7 thread/quartz glass body Viscosity Lg(/dPas) @1250 C. 11.69 0.13 11.69 0.13 11.69 0.13 @1300 C. 11.26 0.1 11.26 0.1 11.26 0.1 @1350 C. 10.69 0.07 10.69 0.07 10.69 0.07 -value are the standard deviation.

[1092] I. Melting Granulate to Obtain Quartz Glass

[1093] I.1: Employing silicon dioxide powder with a mean particle size D.sub.50<6 m Separation tests were performed on silicon dioxide powders with different particle size distributions during the dechloration in water. It was found that the silicon dioxide powder bulk material with a D.sub.50 of <6 m produce a higher material transport in the direction of the exhaust filter. The exhaust filters consequently clog very quickly and must be cleaned often. This reduces the achievable throughput, but also the life time of the employed filters. Additionally every time the filter is cleaned, the material falling from the filter, i.e. the impurity present in the system, is increased. If the throughput of treatment gas is reduced so much that the material transport in the direction of the filter is at an acceptable level, then the corresponding treatment time is so long that such a process can not be run economically. Another possibility is to employ silicon dioxide powder with a higher D.sub.50. In that case, the material transport in the direction of the filter remains at a reasonable level with a throughput which is high enough to be economically viable and a treatment time which is short enough to be economically viable.

[1094] I.2: Employing Silicon Dioxide Powder with a Mean Particle Size D.sub.50>15 m

[1095] Filter tests were conducted on slurries of different silicon dioxide powders in water. Filtering was through a filter with a cutoff level of 50 m. The slurries were filtered before granulate formation, in order to remove impurities resulting from the storage and transport of the slurried powders (fibres, impurities from transport pipes etc.). It was found that for filtering of slurries of powders with a D.sub.50 of substantially more than 15 m, e.g. 25m, the filters clog quickly. This causes an increase in the pump pressure and eventually to tearing of the filter mesh. If this goes unnoticed, the silicon dioxide granulate which is subsequently formed will be contaminated with such impurities. These impurities are responsible for bubble formation in the quartz glass body in the downstream melting of the granulate to obtain a quartz glass. However, if a slurry of a silicon dioxide powder of 15 m is filtered, an economically viable throughput and a substantially longer life time of the filter are observed. The quartz glass bodies which are prepared therefrom have not bubbles.