Vertical crucible pulling method for producing a glass body having a high silicic-acid component

Abstract

The present invention refers to a method for producing a glass body with high silicic-acid content by drawing a softened glass mass from an elongated, substantially cylindrical crucible in that SiO.sub.2 granules are supplied from above into the crucible, the SiO.sub.2 granules are heated to a softening temperature, so that the softened glass mass which comprises a melt surface is formed, the softened glass mass is drawn off via a bottom opening of the crucible so as to form a glass strand, and the glass strand is cut to length to obtain the glass body, wherein due to the supply of the SiO.sub.2 granules a bulk heap is formed that covers the melt surface in part while leaving a melt edge, and wherein the melt surface is optically detected. To improve the fusion behavior of the granules and to suppress or altogether prevent the formation of a sinter crust, it is suggested according to the invention that during the optical detection of the melt surface the width of at least a sub-section of the melt edge is determined consecutively and is set to a value within a target width range through the supply rate of the SiO.sub.2 granules.

Claims

1. A method for producing a glass body with high silicic-acid content, said method comprising: supplying SiO.sub.2 granules from above into an elongated, substantially cylindrical crucible, heating the SiO.sub.2 granules to a softening temperature, so that a softened glass mass comprising a melt surface is formed, drawing off the softened glass mass via a bottom opening of the crucible so as to form a glass strand, and cutting the glass strand to a length to obtain the glass body, wherein the supplying of the SiO.sub.2 granules causes a bulk heap to be formed that partly covers the melt surface while leaving a melt edge not covered by said bulk heap, and the method further comprising optically detecting the melt surface, wherein during the optical detecting of the melt surface a width of at least a sub-section of the melt edge is determined consecutively and is set to a value within a target width range by setting a supply rate of the SiO.sub.2 granules; and wherein the width of the melt edge is set to a value within the target width range by a nominal supply rate of SiO.sub.2 granules based on a throughput of the drawn-off glass mass, and wherein the nominal supply rate is changed by not more than 10%.

2. The method according to claim 1, wherein the supply rate of the SiO.sub.2 granules is set such that the melt edge formed has a width between 0.5 cm and 4 cm.

3. The method according to claim 1, wherein the optical detecting of the melt surface comprises a temperature measurement.

4. The method according to claim 3, wherein the temperature measurement is carried out using at least one pyrometer that is directed to a position on an edge of the bulk heap.

5. The method according to claim 1, wherein the optical detecting of the melt surface comprises an imaging of the melt edge with an at least one camera.

6. The method according to claim 1, wherein the optical detecting of the melt surface comprising an imaging detection combined with a temperature measurement, and wherein a camera is used with a pyrometer.

7. The method according to claim 6, wherein the camera is directed to a position on an edge of the bulk heap.

8. The method according claim 1, wherein the SiO.sub.2 granules are supplied via an individual filling tube terminating centrally above the bulk heap, and a distance ranging from 5 cm to 20 cm is set between the end of the filling tube and the bulk heap.

9. The method according to claim 1, wherein the supply rate of the SiO.sub.2 granules is adjusted using vibrations.

10. The method according to claim 1, wherein the SiO.sub.2 granules are supplied continuously.

11. The method according to claim 1, wherein the supply rate of the SiO.sub.2 granules is set such that a surrounding melt edge is formed with a width between 1 cm and 2 cm.

12. The method according claim 1, wherein the SiO.sub.2 granules are supplied via an individual filling tube terminating centrally above the bulk heap, and a distance ranging from 5 cm to 20 cm is set between the end of the filling tube and the bulk heap.

13. The method according to claim 1, wherein the nominal supply rate of the SiO.sub.2 granules is adjusted using vibrations.

14. The method according to claim 1, wherein the SiO.sub.2 granules are supplied continuously at the varying nominal supply rate.

15. A method for producing a glass body with high silicic-acid content, said method comprising: supplying SiO.sub.2 granules from above into a substantially cylindrical crucible, heating the SiO.sub.2 granules to a softening temperature, so that a softened glass mass comprising a melt surface is formed, drawing off the softened glass mass via a bottom opening of the crucible so as to form a glass strand, and cutting the glass strand to a length to obtain the glass body, wherein the supplying of the SiO.sub.2 granules causes a bulk heap to be formed that partly covers the melt surface while leaving a melt edge not covered by said bulk heap, and the method further comprising optically detecting the melt surface so as to repeatedly determine a width of at least a sub-section of the melt edge of the melt surface; and controlling a nominal supply rate of the SiO.sub.2 granules based on a throughput of the glass mass drawn off as the strand so as to maintain the determined width of the melt edge within a target width range.

16. The method according to claim 15, wherein the nominal supply rate varies within a range of plus or minus 10% of a supply rate value.

17. The method according to claim 15, wherein the nominal supply rate of the SiO.sub.2 granules is set such that the melt edge formed has a width between 0.5 cm and 4 cm.

18. The method according to claim 15, wherein the supply rate of the SiO.sub.2 granules is set such that a surrounding melt edge is formed with a width between 1 cm and 2 cm.

19. The method according to claim 15, wherein the optical detecting of the melt surface comprises a temperature measurement carried out using at least one pyrometer that is directed to a position on an edge of the bulk heap.

20. The method according to claim 15, wherein the optical detecting of the melt surface comprises an imaging of the melt edge with an at least one camera directed to a position on an edge of the bulk heap.

21. The method according to claim 15, wherein the optical detecting of the melt surface comprising an imaging detection combined with a temperature measurement, and wherein a camera is used with a pyrometer.

Description

EMBODIMENT

(1) The invention will now be described in more detail hereinafter with reference to a patent drawing and an embodiment. As the sole FIGURE,

(2) FIG. 1 shows an apparatus for performing the method of the invention, in a schematic representation.

(3) SiO.sub.2 granules 3 are continuously filled into a crucible 1 of tungsten via a single filling tube 2. The SiO.sub.2 granules 3 are stored above the crucible 1 in a container 6 which comprises an agitator. The crucible 1 has a bottom outlet opening 4 through which a molten quartz glass mass exits and is drawn off as a strand 16.

(4) The crucible 1 is upwardly closed with a cover 7 through which the filling tube 2 projects centrally into the crucible 1. Moreover, at least one opening is laterally provided on the cover 7; through this opening the melt surface of the softened glass mass 23 on the edge of the crucible 1 and the supplied SiO.sub.2 granules 3 can be optically detected by means of an inspection device 11.

(5) A resistance heating coil 8 is arranged around the crucible for heating the crucible 1. This coil is outwardly surrounded by a thermal insulation 9. The space between the resistance heating coil 8 and the outer wall of the crucible is flushed with a hydrogen-containing protective gas which is supplied via the nozzles 10 and 15 and discharged in the area of the lower end of the crucible 1. A helium-hydrogen gas mixture is introduced into the crucible 1 via an inlet 14 into the upper portion of the crucible interior 5.

(6) Due to the supply of the SiO.sub.2 granules 3 via the filling tube 2 a heap cone 13 which is surrounded by a surrounding melt edge 24 is formed on the melt surface 12 of the SiO.sub.2 granules 3 previously fused into a softened glass mass 23.

(7) The method of the invention is now explained in more detail with reference to an embodiment and FIG. 1.

(8) The SiO.sub.2 granules 3 are fed via the filling tube 2 from the storage container 6, which is connected to an agitator, into the crucible 1. The agitator adjusts a continuous supply of SiO.sub.2 granules 3. As a rule, the agitator may also be arranged separated from the storage container and only acts on the supply line for the SiO.sub.2 granules into the crucible. The supply rate of SiO.sub.2 granules depends on the calculated throughput for the production of quartz glass components of a given geometry and size (tubes or rods with corresponding diameters, wall thicknesses, lengths).

(9) The supplied SiO.sub.2 granules 3 form a bulk cone 13 in the crucible 1 on the SiO.sub.2 granules which have previously been molten into a softened glass mass 23. At the beginning of the drawing process the filling tube 2 is shifted vertically upwards to such an extent that a distance of about 10 cm is set between the end of the filling tube 2 and the bulk cone 13. This prevents on the one hand the formation of a sinter layer between filling tube 2 and bulk cone 13 and on the other hand the formation of a sinter crust on the inner wall of the crucible by blowing away of the SiO.sub.2 granular particles.

(10) In the crucible 1 the silica particles 3 are heated to a temperature of about 2100° C. to 2200° C. A homogeneous, bubble-free glass mass 23 on which the bulk cone 13 of SiO.sub.2 particles 3 is floating is here formed in the lower portion of the crucible 1 without said cone having any contact with the wall of the crucible 1; rather, the melt surface 12 forms a surrounding melt edge 24 of a width of about 2 cm around the base area of the bulk cone.

(11) The width of the melt edge 24 is optically detected by means of the inspection device 11. Said device comprises a camera which is arranged in the cover 7 of the crucible 1 above the melt edge 24 and by means of which an image of the melt surface 12 is recorded in the area of the melt edge 24 and measured. The camera can optionally be adjusted, deviating from an initial position, such that the edge 13.1 of the bulk cone 12 is detected. When the initial position is known, the width of the melt edge 24 can directly be determined through alignment of the camera without a picture of the full width having to be taken. The width of the melt edge 24 which is thereby determined is a control variable for the fine adjustment of the supply of the SiO.sub.2 granules 3; apart from the basic control variable, in response to the throughput this variable helps to keep a surrounding melt edge 24 around the bulk cone 13 within a width between 1 cm and 2 cm (=target width range) during the melting process. Together with the optical detection of the melt edge 23, the temperature is measured and monitored in this area. To this end a pyrometer (not shown) is used that is integrated into the inspection device 11 and the measuring spot of which is transmitted by means of an optical fiber exactly onto the melt edge 24 or the edge 13.1 of the bulk cone 12 consisting of SiO.sub.2 granules 3. A further pyrometer 21 may be directed at a defined distance from the first pyrometer onto the lateral surface of the bulk cone 13. The temperature of the granulation layer can thereby be detected in this position of the bulk cone 13, whereby temperature trends can be detected.

(12) The softened glass mass 23 flows out via the bottom outlet opening 4 and is then drawn off in the form of a cylindrical quartz-glass strand 16 downwards, as illustrated by the directional arrow 17. Subsections are cut off to the desired length from the cooled quartz glass strand 16 in the form of a rod with a diameter of 30 mm.

(13) At an average draw-off rate of the quartz glass rod of 11 m/h, one achieves a nominal supply rate of SiO.sub.2 granules of about 17 kg/h.

(14) If the width of the melt edge 24 does not reach the lower limit value of 1 cm, the supply rate of SiO.sub.2 granules is reduced by 5% based on said nominal value until the lower limit is again exceeded.

(15) Inversely, when the upper limit value of 1 cm is exceeded, the supply rate of SiO.sub.2 granules is increased by about 5% based on the nominal value until the current width is within the target width range again.

(16) This normally happens within a few minutes. This momentary increase or decrease in the nominal supply rate is so small that it has no significant impact on throughput and other process control.