Method for producing synthetic quartz glass granules

Abstract

The production of a quartz glass grit comprises the granulation of pyrogenetically produced silicic acid, and the formation of a SiO.sub.2 granulate and the vitrification of the SiO.sub.2 granulate using a treatment gas, which contains at least 30% by volume of helium and/or hydrogen. Said process is time consuming and cost intensive. In order to provide a method which makes it possible, starting from a porous SiO.sub.2 granulate, to manufacture, in a cost effective manner, a dense, synthetic quartz glass grit, which is suitable for melting bubble-free components made of quartz glass, according to the invention the vitrification of the SiO.sub.2 granulate occurs in a rotary kiln having a mullite-containing ceramic rotary kiln, for the manufacture of which a starting powder, which contains a molar proportion of at least 45% SiO.sub.2 and Al.sub.2O.sub.3 is applied by means of a thermal powder spraying method, forming a mullite-containing layer on a mold core, and the mold core is subsequently removed, and wherein the ceramic rotary kiln is flooded with a treatment gas or rinsed with a treatment gas, and wherein the ceramic rotary kiln is flooded with a treatment gas or rinsed with a treatment gas, which contains at least 30% by volume of helium and/or hydrogen.

Claims

1. A method for producing synthetic quartz glass granules, said method comprising: vitrifying a free-flowing SiO.sub.2 granulate of porous granulate particles, which is obtained by granulation of pyrogenically produced silicic acid, and said vitrifying the SiO.sub.2 granulate taking place in a rotary kiln comprising a ceramic rotary tube containing mullite, said ceramic rotary tube being made by applying a starting powder containing SiO.sub.2 and Al.sub.2O.sub.3 with a molar fraction of at least 70% to a mold core using a thermal powder spraying method so as to form a mullite-containing layer, and subsequently removing the mold core, and wherein, during the vitrifying of the SiO.sub.2 granulate therein, the ceramic rotary tube is flooded or flushed with a treatment gas containing at least 30% by vol. of helium, hydrogen, or a mixture thereof.

2. The method according to claim 1, wherein the thermal powder spraying method used for producing the ceramic rotary tube is a plasma powder-spraying method.

3. The method according to claim 1, wherein the starting powder contains at least 75 mole % Al.sub.2O.sub.3, and wherein SiO.sub.2 and Al.sub.2O.sub.3 are present in amounts that together constitute at least 95% by wt. of the ceramic rotary tube.

4. The method according to claim 1, wherein the ceramic rotary tube is of material that has an alkali content of less than 0.5% by wt.

5. The method according to claim 1, wherein at least part of the starting powder is synthetically produced.

6. The method according to claim 1, wherein the ceramic rotary tube is of a ceramic, mullite-containing material that has a density in the range of from 2.5 to 2.9 g/cm.sup.3.

7. The method according to claim 1, wherein the ceramic rotary tube is of a ceramic, mullite-containing material that has an open porosity of less than 10% by vol.

8. The method according to claim 1, wherein the treatment gas contains at least 50% of helium, hydrogen or a mixture thereof.

9. The method according to claim 8, wherein the porous granulate particles are subjected to vibration.

10. The method according to claim 8, wherein the porous granulate particles are heated using a resistance heater surrounding the rotary tube.

11. The method according to claim 1, wherein the porous granulate particles are heated during vitrification to a temperature in the range of 1300 C. to 1600 C.

12. The method according to claim 1, wherein Al.sub.2O.sub.3 doping in the range of 1 to 15 wt. ppm is effected using the rotary tube.

13. The method according to claim 1, wherein the rotary tube consists completely of mullite-containing ceramic.

14. The method according to claim 1, wherein the porous granulate particles have a mean grain size between 100 m and 2000 m (D.sub.50 value each time).

15. The method according to claim 1, wherein the porous granulate particles have a narrow particle size distribution with a value and a value each having a respective particle diameter associated therewith, wherein the particle diameter of the D.sub.90 value is not more than twice as great as the particle diameter of the D.sub.10 value.

16. The method according to claim 1, wherein the rotary tube is a rotary kiln, and prior to vitrification the SiO.sub.2 granulate is subjected to purification by heating in a halogen-containing atmosphere, and wherein the SiO.sub.2 granulate is purified in a second rotary kiln.

17. The method according to claim 16, wherein the second rotary kiln is used for drying and purifying the SiO.sub.2 granulate and is subdivided into zones, including a drying zone and a cleaning zone, and wherein adjacent zones are subdivided by separating screens provided with openings or by labyrinth traps.

18. The method according to claim 1, wherein the starting powder contains at least 75 mole % Al.sub.2O.sub.3, and wherein the contents of SiO.sub.2 and Al.sub.2O.sub.3 together account for at least 98% by wt. of the ceramic rotary tube.

19. The method according to claim 1, wherein the ceramic rotary tube is of a ceramic mullite-containing material that has an alkali content of less than 0.1% by wt.

20. The method according to claim 1, wherein the ceramic rotary tube is of a ceramic mullite-containing material that has an open porosity of less than 5% by vol.

21. The method according to claim 1, wherein the treatment gas contains at least 95% of helium, hydrogen, or a mixture thereof.

22. The method according to claim 1, wherein the porous granulate particles have a mean grain size between 200 m and 400 m (D.sub.50 value each time).

Description

Embodiment

(1) The invention shall now be explained in more detail with reference to an embodiment and a drawing. In a schematic illustration

(2) FIG. 1 shows a system for producing a mullite tube for use in the method according to the invention by way of thermal powder spraying;

(3) FIG. 2 shows a rotary kiln for performing the vitrification step in the method according to the invention, in a side view; and

(4) FIG. 3 shows a temperature profile over the length of the rotary kiln.

(5) The system for thermal powder spraying as is schematically shown in FIG. 1 comprises a plasma burner 101, a reservoir with feed line 102 for a starting powder 103, and a metallic carrier tube 105 which is rotatable about its longitudinal axis 104 and has an outer diameter of 150 mm, a length of about 2 m and a smooth outer jacket surface. The plasma burner 101 and the starting-powder feed line 102 are reversingly movable along the longitudinal axis 104 of the carrier tube, as outlined by directional arrow 106.

(6) The starting powder 103 is a synthetically produced mixed powder which is present in the mullite structure and consists of SiO.sub.2 and of Al.sub.2O.sub.3, the latter with a molar fraction of 75%. The starting powder contains no binders or other additives. The mean particle size is 120 m. The starting powder 103 is continuously supplied to the plasma flame 107 of the plasma burner 101 while the burner is reversingly moved in the direction of the longitudinal axis 104 along the carrier tube 105. The starting powder 103 is thereby melted in the plasma flame 107 and flung due to the plasma pressure against the outer jacket surface of the carrier tube 105 which is rotating about its longitudinal axis 104. During solidification a layer 108 is formed of a ceramic, mullite-like structure. A layer having a thickness of about 150 m is produced per deposition pass. The deposition process is continued until the layer 108 has reached a thickness of 20 mm. After removal of the carrier tube 105 the mullite tube obtained thereby is subjected to a sintering treatment at a temperature of 1250 C. for the purpose of further densification.

(7) The mullite tube obtained thereby by means of the thermal powder-spraying method is almost free of undesired impurities. Its open porosity is zero; the closed porosity is about 8%, the density is 2.8 g/cm.sup.3, and its melting temperature is 1830 C. The mullite-containing rotary tube 6 (see FIG. 2) is obtained by slightly grinding the outer wall and sawing off the end caps for use in the method according to the invention.

(8) Owing to the manufacturing process the rotary tube obtained thereby is distinguished by high purity and a smooth and dense inner wall. In the intended use the input of impurities into the granulate to be vitrified and the risk of adhesions to the inner wall of the rotary tube are thereby minimized.

(9) FIG. 2 shows a rotary kiln 1 which is supported on rollers 2. The rotary kiln 1 substantially comprises a frame 5 of SiC in which a rotary tube 6 of mullite ceramic with an inner diameter of 150 mm and a length of 1.8 m is fixed. The rotary tube 6 is rotatable about a central axis 7 and heatable by means of a resistance heater 8 provided on the outer jacket.

(10) The rotary kiln 1 is slightly inclined in longitudinal direction 7 relative to the horizontal to induce the transportation of a loose material consisting of porous SiO.sub.2 granulate 9 from the inlet side 3 of the rotary kiln 1 to the removal side 10. The open inlet side 3 is closed by means of a rotatorily fixed inlet housing 4. The inlet housing 4 is equipped with an inlet 16 for the supply of porous SiO.sub.2 granulate 9 and with a further inlet (not shown) for the supply of helium and other treatment gases, such as hydrogen.

(11) The open removal side 10 of the rotary tube 6 is closed by means of an also rotatorily fixed removal housing 11. The removal housing 11 is provided with an outlet 17 for the removal of vitrified and post-treated quartz glass granules 15; gas can also flow via said outlet out of the rotary kiln 1. For the suction of helium-rich gas from the furnace atmosphere a suction nozzle 18 is provided that is arranged in the upper area of the rotary kiln 1. Furthermore, the removal housing 11 is equipped with a gas inlet nozzle 19 by means of which a helium-free gas, particularly argon, is introduced into the rotary tube 6.

(12) With the help of a separating screen 12 the interior is subdivided into a preheating and vitrification zone 13 and into an aftertreatment zone 15. The separating screen 12 is designed such that it is permeable for the loose material of the granulate particles 9 and the vitrified quartz glass granules 15, respectively, but otherwise substantially separates the gas chambers. For this purpose it is fixed to the inner wall of the rotary tube 6 and is provided on its outer edge with two radially opposite openings 20a, 20b of the same size. Whenever the one opening 20a passes due to the rotation of the rotary tube 6 into the region of the loose material of the granulate 9 and the quartz glass granules 15, respectively, it lets them pass and is substantially clogged by the loose material at the same time so that only little gas can escape there from the vitrification zone 13 into the aftertreatment zone 14. At the same time the opposite opening 20b is in its uppermost position in the rotary kiln 1. The relatively lightweight helium gas preferably exits there and is sucked off by means of the suction nozzle 18 directly positioned there and is simultaneously replaced by argon via the gas inlet 19.

(13) A substantial separation of the gas chambers of preheating/vitrification zone 13 and aftertreatment zone 14 is thereby possible. For an even more effective separation a plurality of separating screens 12 which are arranged one after the other and include openings offset in relation to one another in the manner of a labyrinth can be used, or separate rotary kilns are used for the vitrification of the granulate 9 and for the aftertreatment thereof. In the last-mentioned case the vitrified quartz glass granules which still have a temperature of at least 200 C. can be directly transferred into the rotary kiln for the aftertreatment.

(14) The resistance heater 8 does not extend over the region of the aftertreatment zone 14; apart from the heat input by convection and heat conduction from the neighboring vitrification zone, this zone is unheated.

(15) The method according to the invention will be described in more detail hereinafter with reference to embodiments:

Producing, Drying and Cleaning of SiO2 Granulate

Example A

(16) The granulate was produced by granulating a slurry with 60% by wt. of residual moisture from pyrogenic silicic acid (nanoscale SiO.sub.2 powder, SiO.sub.2 soot dust) and demineralized water in the intensive mixer. After granulation the residual moisture is <20%. The granulate was sieved to grain sizes of <3 mm.

(17) The residual moisture was lowered to <1% by drying at 400 C. in a rotary kiln (throughput: 20 kg/h) in air. Subsequently, the fines fraction with grain sizes of <100 m was removed. Sieving to the fraction 100-750 m is carried out; this means that the fines fraction with grain sizes of <100 m is removed. The grain size distribution is characterized by a D10 value of about 200 m and a D90 value of about 400 m.

(18) Subsequently, cleaning and further drying in HCl-containing atmosphere is carried out in the rotary kiln at a maximum temperature of 1040 C. (throughput: 10 kg/h). The specific surface area (BET) is here reduced by about 50%.

(19) This yielded a SiO.sub.2 granulate of synthetic undoped quartz glass of high purity. It consists essentially of porous spherical particles with a particle size distribution having a D10 value of 200 m, a D90 value of 400 m, and a mean particle diameter (D50 value) of 300 m.

Example B

(20) The granulate was produced by high-speed granulation from pyrogenic silicic acid (nanoscale SiO.sub.2 powder, SiO.sub.2 dust) and demineralized water in the intensive mixer. For this purpose demineralized water is fed into the intensive mixer and pyrogenic silicic acid is added under mixing until the residual moisture is about 23% by wt. and a granulate is produced. The granulate is sieved to grain sizes of 2 mm.

(21) The residual moisture is lowered to <1% by drying at 350 C. in a rotary kiln (throughput 15 kg/h) in air. The fines fraction with grain sizes <100 m was removed; otherwise, no further sieving operation was carried out.

(22) Subsequently, cleaning and further drying are carried out in HCl-containing atmosphere in the rotary kiln at temperatures of 1050-1150 C. (throughput: 10 kg/h).

(23) The sum of chemical contaminants is reduced during hot chlorination to less than 1/10 of the starting material (i.e., to <10 ppm). The granulate consists essentially of porous particles having a particle size distribution with a D10 value of 300 m, a D90 value of 450 m and a mean particle diameter (D50 value) of 350 m.

(24) Vitrification of the Granulate

(25) The rotary tube 6 which is rotating about its rotation axis 7 at a rotational speed of 8 rpm is continuously fed with undoped porous SiO.sub.2 granulate 9 at a feed rate of 15 kg/h.

(26) The rotary tube 6 is inclined in longitudinal direction 7 at the specific angle of repose of the granulate particles 9, so that a uniform thickness of the loose granulate is set over the length thereof. The uniform loose-material thickness facilitates the substantial separation of the interior of the rotary kiln into preheating and vitrification zone 13 and into the aftertreatment zone 14, respectively. The loose material shown in FIG. 2 in the inlet housing 4 shows a different angle of repose; this only serves a simplified schematic illustration.

(27) The zone 13 of the rotary tube 3 is flooded with helium. The loose granulate is continuously circulated and heated in this process by means of the resistance heater 8 within the rotary tube 6 and gradually vitrified into quartz glass particles 15. The maximum temperature shortly before approximately the rear third of the rotary tube 6 is about 1460 C. The rotary tube 6 of mullite ceramic withstands said temperature without difficulty.

(28) The loose material of the vitrified quartz glass particles 15 passes gradually into the aftertreatment zone 14 via the openings 20a; 20b of the separating screen 12. Due to the continuous introduction of argon via the gas inlet 19 and due to the approximately identical gas loss of helium-rich vitrification atmosphere, on the one hand by suction of the helium-rich gas exiting through the openings (20a; 20b) of the separating screen 12 by means of suction nozzle 18 and on the other hand by the gas loss via the removal nozzle 17, an atmosphere consisting of a mixture of helium with a considerable excess of argon is obtained in the aftertreatment zone 14; the helium content is less than 20% by vol. Since the aftertreatment zone 14 is not directly heated, the temperature continuously decreases from the separating screen 12 up to the outlet housing 11. At that place the mean surface temperature of the vitrified granules 15 is slightly more than 500 C. The mean residence time of the vitrified granules 15 in the aftertreatment zone 14 is about 40 minutes.

(29) An axial temperature profile over the length of the rotary tube 6, which has so far been considered to be ideal, is schematically illustrated in the diagram of FIG. 3. The temperature T of the surface of the loose granulate 9 (determined by means of pyrometer) is plotted on the y-axis against the axial position in the rotary tube 6. Directly after having been supplied, the granulate 9 is dried at a temperature of about 500 C. for a duration of 30 min, and it is subsequently pre-densified thermally at a gradually rising temperature at about 1000 C. to 1300 C. The gas contained in the porous granulate 9 is here replaced by helium at the same time. This densification and gas-exchange process lasts for about 60 min. Subsequently, the loose granulate 9 is heated up for complete vitrification, thereby reaching a maximum temperature of about 1460 C. By that time the mean residence time in the rotary kiln 6 is about 3 h.

(30) In this process stadium the helium content of the vitrified quartz glass particles 15 is relatively high. The gas volume of the theoretically releasable helium gas is 3 times the volume of the particles themselves (at a gas volume standardized to 25 C. and atmospheric pressure).

(31) After passing through the separating screen 12 the vitrified and highly helium-loaded quartz glass particles 15 are gradually cooling down in the aftertreatment zone and are simultaneously substantially degasified due to the atmosphere having a rather low helium content; this means that helium is allowed to diffuse out of the dense quartz glass granules in that the temperature is kept sufficiently highin the example more than 500 C.and the outgassing duration sufficiently longin the example more than 30 minutes. After completion of the aftertreatment the gas volume of helium to be released is at any rate only less than 2 times the volumes of the particles as such (standardized to 25 C. and atmospheric pressure).

(32) The above-mentioned process parameters in combination with the residence time of the granulate 9 in the rotary kiln 1 and the helium atmosphere in the vitrification zone 13 have the effect that the open porosity is mainly disappearing. The surface is dense. The quartz glass particles 15 are evidently completely transparent upon removal in this method stage.

(33) If agglomerates are arising, these will disintegrate again due to the mechanical stress in the moving loose granulate material 9 or by vibration of the rotary tube 6.

(34) At the same time one can observe a uniform abrasion of Al.sub.2O.sub.3 from the mullite of the rotary kiln 6 which passes onto the surface of the granulate particles 9 and into the pores thereof. The vitrified quartz glass granules produced thereby show a homogeneous doping with Al.sub.2O.sub.3 of about 10 wt. ppm for that reason. Adhesions to the inner wall of the rotary tube 6 are mainly avoided.

(35) The completely vitrified and homogeneously doped quartz glass granules have a density of more than 2.0 g/cm.sup.3 and a BET surface area of less than 1 m.sup.2/g, and they have a relatively low helium contentin consideration of the vitrification under helium. They are continuously removed via the discharge housing 11 and the outlet nozzle 17.

(36) The quartz glass granules are used for producing the outer layer of a quartz glass crucible; the viscosity-enhancing action of the Al.sub.2O.sub.3 doping helps to prolong the service life of the quartz glass crucible.