Method for producing synthetic quartz glass granules

Patent metadata

Field	Value
Publication number	US-9580348-B2
Application number	US-201314398124-A
Country	US
Kind code	B2
Filing date	Apr 17, 2013
Priority date	Apr 30, 2012
Publication date	Feb 28, 2017
Grant date	Feb 28, 2017

Abstract

The production of a quartz glass grit comprises the granulation of pyrogenetically produced silicic acid, and the formation of a SiO 2 granulate and the vitrification of the SiO 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 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 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 2 and Al 2 O 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.

First claim

The invention claimed is:

1. A method for producing synthetic quartz glass granules, said method comprising:
vitrifying a free-flowing SiO 2 granulate of porous granulate particles, which is obtained by granulation of pyrogenically produced silicic acid, and
said vitrifying the SiO 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 2 and Al 2 O 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 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 2 O 3 , and wherein SiO 2 and Al 2 O 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 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 2 O 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 μm and 2000 μm (D 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 90 value is not more than twice as great as the particle diameter of the D 10 value.

16. The method according to claim 1 , wherein the rotary tube is a rotary kiln, and prior to vitrification the SiO 2 granulate is subjected to purification by heating in a halogen-containing atmosphere, and wherein the SiO 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 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 2 O 3 , and wherein the contents of SiO 2 and Al 2 O 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 μm and 400 μm (D 50 value each time).

Assignees

• Heraeus Quarzglas

Inventors

• Lehmann Walter
• Hofmann Achim
• Kayser Thomas
• Arndt Martin

Classifications

• C04B35/64

  Burning or sintering processes (C04B33/32 takes precedence {; powder metallurgy B22F}) · CPC title

• C04B2235/77

  Density · CPC title

• F27B7/34

  Arrangements of heating devices · CPC title

• C04B2235/5427

  millimeter or submillimeter sized, i.e. larger than 0,1 mm · CPC title

• F27B7/00

  Rotary-drum furnaces, i.e. horizontal or slightly inclined · CPC title