Method for producing synthetic quartz glass granules

Patent metadata

Field	Value
Publication number	US-9409810-B2
Application number	US-201314390991-A
Country	US
Kind code	B2
Filing date	Mar 20, 2013
Priority date	Apr 5, 2012
Publication date	Aug 9, 2016
Grant date	Aug 9, 2016

Abstract

The production of quartz glass granules comprises the granulation of pyrogenically produced silicic acid and the formation of a SiO 2 granulate ( 9 ), the drying and cleaning of the SiO 2 granulate ( 9 ) by heating in an atmosphere containing halogen, and the vitrification of the SiO 2 granulate ( 9 ) under a treatment gas which contains at least 30% by volume of helium and/or hydrogen. This process is time-consuming and expensive. In order to provide a method which, starting from a porous SiO 2 granulate ( 9 ), allows the cost-effective production of dense, synthetic quartz glass granules suitable for melting bubble-free components of quartz glass, the invention proposes that the cleaning and vitrification of the SiO 2 granulate ( 9 ) and a post-treatment of the vitrified quartz glass granules are carried out in each case in a rotary tube ( 6 ) of a rotary kiln ( 1 ), said rotary tube rotating about a central axis ( 7 ), wherein the rotary tube ( 6 ) comprises an inner wall made of a ceramic material during vitrification, and wherein the vitrified quartz glass granules are subjected to a post-treatment during a treatment period of at least 10 minutes in an atmosphere which contains less than 20% of helium or hydrogen at a treatment temperature of 300° C. or more.

First claim

The invention claimed is:

1. A method for producing synthetic quartz glass granules by vitrifying a free-flowing SiO 2 granulate, the method comprising:
granulating pyrogenically produced silicic acid so as to form the SiO 2 granulate of porous granulate particles,
drying the SiO 2 granulate,
cleaning the SiO 2 granulate by heating in a halogen-containing atmosphere in a kiln,
vitrifying the cleaned SiO2 granulate in a treatment gas containing at least 30% by vol. of helium, hydrogen or a mixture of helium and hydrogen so as to form vitrified quartz glass granules, and subjecting the vitrified quartz glass granules to a post-treatment in an atmosphere containing helium or hydrogen in an amount less than 20%, and at a treatment temperature of 300° C. or more for a treatment period of at least 10 minutes,
wherein the kiln is a rotary kiln rotating about a central axis,
wherein the cleaning and the vitrifying of the SiO2 granulate and the post-treatment of the vitrified quartz-glass granules are carried out in a rotary tube of the rotary kiln rotating about the central axis, and
wherein the rotary tube comprises an inner wall consisting of a ceramic material and the vitrifying takes place inside said inner wall.

2. The method according to claim 1 , wherein the vitrified quartz glass granules have a surface temperature that, between the vitrifying and the post-treatment, does not fall below 200° C.

3. The method according to claim 1 , wherein the treatment temperature of the post-treatment is 800° C. or more.

4. The method according to claim 1 , wherein the treatment period of the post-treatment is 15 minutes or more.

5. The method according to claim 1 , wherein the quartz glass granules after completion of the post-treatment have a content of hydrogen and helium per 1 cm 3 that would occupy a gas volume of less than 2 cm 3 under normal conditions.

6. The method according to claim 1 , wherein the vitrifying of the SiO 2 granulate and the post-treatment of the quartz glass granules both take place in the rotary kiln, said rotary kiln, when viewed in a direction transverse to the central axis, being subdivided into a plurality of zones, including a vitrification zone in which the vitrifying occurs and a post-treatment zone in which the post-treatment occurs.

7. The method according to claim 6 , wherein the drying and cleaning of the SiO 2 granulate takes place in the rotary kiln, said rotary kiln, when viewed in a direction transverse to the central axis further comprising a drying zone and a cleaning zone.

8. The method according to claim 6 , wherein two of the zones of the rotary kiln are adjacent each other and subdivided by separating screens having openings or by labyrinth traps.

9. The method according to claim 6 , wherein the rotary tube for the cleaning and for the vitrifying is heated, and wherein the post-treatment takes place in a rotary tube portion of quartz glass in the post-treatment zone.

10. The method according to claim 9 , wherein the rotary tubes for the cleaning and the vitrifying are each heated using a resistance heater surrounding each rotary tube, and wherein the rotary tube used for the post-treatment or a portion of the rotary tube used for the post-treatment is not directly heated.

11. The method according to claim 1 , wherein the SiO2 granulate has a fines content with particles sizes of less than 100 μm that is preselected so as to constitute less than 10% by wt. of a total weight of the SiO 2 granulate.

12. The method according to claim 1 , wherein the cleaning in the rotary tube is carried out in a chlorine-containing atmosphere at a temperature in a range between 900° C. and 1250° C.

13. The method according to claim 1 , wherein the treatment gas during vitrification contains at least 50% helium, hydrogen, or a mixture of helium and hydrogen.

14. The method according to claim 1 , wherein the granulate particles are heated during said vitrifying to a temperature in a range of 1300° C. to 1600° C.

15. The method according to claim 14 , wherein the granulate particles or the vitrified quartz-glass granules are subjected to vibration.

16. The method according to claim 1 , wherein the ceramic material is of Al 2 O 3 , ZrO 2 or Si 3 N 4 .

17. The method according to claim 16 , wherein the inner wall of the rotary tube has an alkali content of less than 0.5%.

18. The method according to claim 16 , wherein the inner wall of the rotary tube consists of synthetically produced Al 2 O 3 .

19. The method according to any one of claim 16 , wherein an Al 2 O 3 doping of the vitrified quartz glass granules in the range of 1-20 wt. ppm is effected by using an Al 2 O 3 -containing rotary tube.

20. The method according to claim 1 , wherein the granulate particles have a mean grain size between 20 μm and 2000 μm (D 50 value each time).

21. The method according to claim 1 , wherein the granulate particles have a particle size distribution in which a particle diameter assigned to the D 90 value is not more than twice as great as a particle diameter assigned to the D 10 value.

22. The method according to claim 1 , wherein the vitrified quartz glass granules have a surface temperature that, between the vitrifying and the post-treatment, does not fall below 800° C.

23. The method according to claim 1 , wherein the porous granulate particles have a mean grain size between 100 μm and 400 μm (D 50 value each time).

24. The method according to claim 1 wherein the treatment gas during vitrification contains at least 95% helium, hydrogen, or a mixture of helium and hydrogen.

Assignees

• Heraeus Quarzglas
• Heraeus Quarzglas

Inventors

• Lehmann Walter
• Kayser Thomas

Classifications

• C03B19/107

  Forming hollow beads (chemical aspects C03C11/002) · CPC title

• C03B19/1095

  Thermal after-treatment of beads, e.g. tempering, crystallisation, annealing · CPC title

• C03B2201/02

  Pure silica glass, e.g. pure fused quartz · CPC title

• C03B2201/03

  Impurity concentration specified · CPC title

• C03B20/00 Primary

  Processes specially adapted for the production of quartz or fused silica articles {, not otherwise provided for (C03B19/01, C03B19/066, C03B19/106, C03B19/12, C03B19/14, C03B37/00 take precedence)} · CPC title