WO2011066248A2

WO2011066248A2 - Method and apparatus for reducing condensate related defects in a glass manufacturing process

Info

Publication number: WO2011066248A2
Application number: PCT/US2010/057729
Authority: WO
Inventors: Mark A. Cook; Raymond E. Fraley
Original assignee: Corning Incorporated
Priority date: 2009-11-30
Filing date: 2010-11-23
Publication date: 2011-06-03
Also published as: JP5739900B2; JP2013512184A; WO2011066248A3; TWI471280B; TW201130761A; KR20120117806A; CN102753490A

Abstract

The present invention is directed toward an apparatus and method of reducing contamination of a glass melt by oxide particulates, such as particulates of platinum oxide, which may condense on the inside surfaces of a stir chamber, particularly the stir shaft, and fall back into the glass melt. The apparatus includes a condensation collection vessel arranged annularly around the stir shaft of a stirring device. The collection vessel is located above the free surface of the molten glass occupying the stir chamber and serves to collect any condensed particulates that have formed on the stir chamber cover or gaps within the cover itself. By collecting the condensate, defects in the final glass product that would otherwise be created by the condensate, can be avoided. A method of stirring a glass melt is also provided.

Description

METHOD AND APPARATUS FOR REDUCING CONDENSATE RELATED DEFECTS IN

A GLASS MANUFACTURING PROCESS

CLAIMING BENEFIT OF PRIOR FILED U.S. APPLICATION

[0001] This application claims the benefit of U.S. Application Serial No. 61/265,060, filed on November 30, 2009. The content of this document and the entire disclosure of publications, patents, and patent documents mentioned herein are incorporated by reference.

FIELD

[0002] The invention relates generally to a method of reducing contaminants in a glass melt, and more specifically to reducing condensation-formed contaminants during a glass stirring process.

TECHNICAL BACKGROUND

[0003] Chemical and thermal homogeneity is a crucial part of good glass forming operations. The function of a glass melting operation is generally to produce glass with acceptable levels of gaseous or solid inclusions, but this glass usually has cord (or striae or ream) of chemically dissimilar phases. These non-homogeneous components of the glass result from a variety of normal occurrences during the melting process including refractory dissolution, melting stratification, glass surface volatilization, and temperature differences. The resulting cords are visible in the glass because of color and/or index differences.

[0004] One approach for improving the homogeneity of glass is to pass the molten glass through a vertically-oriented stir chamber located downstream of the melter. Such stir chambers are equipped with a stirrer having a central shaft which is rotated by a suitable motor. A plurality of blades extend from the shaft and serve to mix the molten glass as it passes from the top to the bottom of the stir chamber. The present invention is concerned with the operation of such stir chambers without introducing further defects into the resulting glass, specifically, defects arising from condensed oxides. [0005] Volatile oxides in a glass stir chamber can be formed from any of the elements present in the glass and stir chamber. Some of the most volatile and damaging oxides are formed from Pt, As, Sb, B, and Sn. Primary sources of condensable oxides in a glass melt include hot platinum surfaces for Pt0₂, and the glass free surface for B₂0₃, As₄06, Sb₄0₆, and Sn0₂. By glass free surface what is meant is the surface of the glass which is exposed to the atmosphere within the stir chamber. Because the atmosphere above the glass free surface, and which atmosphere may contain any or all of the foregoing, or other volatile materials, is hotter than the atmosphere outside of the stir chamber, there is a natural tendency for the atmosphere above the free glass surface to flow upward through any opening, such as through the annular space between the stirrer shaft and the stir chamber cover. Since the stir chamber shaft becomes cooler as the distance between the stirrer shaft and the glass free surface increases, the volatile oxides contained with the stir chamber atmosphere will condense onto the surface of the shaft if the shaft and/or cover temperature are below the dew point of the oxides. When the resulting condensates reach a critical size they can break off, falling into the glass and causing inclusion or blister defects in the glass product.

[0006] Heating the shaft above the glass free surface has proven only partially successful in reducing particulate contamination in the glass melt, resulting only in a stratification of the condensation.

SUMMARY

[0007] In one broad aspect of the invention, a condensate collection vessel that attaches to a stirring rod of a stirring chamber within a glass melt and glass manufacturing system is provided. The condensate collection vessel compromises an annular base portion having attached thereto a cylindrical wall at a predetermined angle to the annular base. The condensate collection vessel is contained within a cylindrical stir chamber configured to hold molten glass. The stir chamber comprises a cover, the cover defining a passage therethrough, a stirrer having a shaft extending through the cover into the stir chamber, thereby forming an annular gap between the cover and the shaft. Attached to the shaft are impeller blades used to efficiently mix the molten glass within the chamber. [0008] The invention will be understood more easily and other objects, characteristics, details and advantages thereof will become more clearly apparent in the course of the following explanatory description, which is given, without in any way implying a limitation, with reference to the attached Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIG. 1 is a cross sectional view of an exemplary stir chamber according to an embodiment of the present invention showing the chamber cover and the condensate collection vessel.

[0010] FIG. 2 is a partial three dimensional view of the condensate collection vessel attached to the stirrer shaft.

[0011] FIG. 3 is a cross sectional view of an exemplary condensate collection vessel.

DETAILED DESCRIPTION

[0012] FIG. 1 illustrates an exemplary apparatus for practicing a method for homogenizing a glass melt. Stir chamber 10 of FIG. 1 includes an inlet pipe 12 and an outlet pipe 14. In the illustrated embodiment, molten glass flows into the stir chamber, as indicated by arrow 13, through inlet pipe 12, and flows out of the chamber, as shown by arrow 15, through outlet pipe 14. Stir chamber 10 includes at least one wall 16 which is preferably cylindrically- shaped and substantially vertically-oriented, although stir chamber may have other shapes such as oval or hexagonal. Preferably, the stir chamber wall includes an inner lining 18 comprising platinum, a platinum alloy or a dispersion strengthened platinum or platinum alloy (e.g., a zirconia strengthened platinum alloy). Other lining materials having similar refractory properties, including resistance to corrosion, as well as electrical conductivity, may be substituted. Glass inlet pipe 12 is located at or near the bottom of stir chamber 10 whereas glass outlet pipe 14 is located near the top of the stir chamber. However, it will be recognized by the skilled artisan that inlet pipe 12 and outlet pipe 14 may be reversed, such that the molten glass flows into the stir chamber from the top and flows out through the bottom of the stir chamber. Intermediate positions for the inlet and outlet pipes may also be employed provided adequate stirring (i.e. the desired amount of homogenization) is achieved. The stirrer preferably does not significantly pump the glass through the stir chamber since to produce a pumping effect generally requires unacceptably high levels of shear stress. The stirrer and the stir chamber wall are preferably composed of platinum, a platinum alloy or a dispersion strengthened platinum or platinum alloy (e.g., a zirconia strengthened platinum alloy).

[0013] Stir chamber 10 further includes a stirrer 20 comprising shaft 22 and a plurality of blades 24 which extend outward from the shaft towards wall 16 of the stir chamber. Shaft 22 is typically substantially vertically-oriented and rotatably mounted such that blades 24 which extend from the lower portion of the shaft rotate within the stir chamber at least partially submerged below free surface 26 of the molten glass. The molten glass surface temperature is typically in the range between about 1400°C to 1600°C, but may higher or lower depending upon the glass composition. Stirrer 20 is preferably composed of platinum, but may be a platinum alloy, or a dispersion-strengthened platinum or platinum alloy (e.g., a zirconia-strengthened platinum alloy).

[0014] As shown in FIG. 1 , stir chamber 10 may include a drain tube 28 for removing glass from the stir chamber during, for example, shut down of the system. In addition (or alternatively), the stir chamber may include an optional sump 30. Stirrer 20 is rotated by a suitable drive. For example, stirrer 20 may be rotated by an electric motor (not shown) through appropriate gearing or by a belt drive.

[0015] In accordance with the present embodiment, stir chamber 10 is covered by chamber cover 32. Chamber cover 32 may rest directly upon wall 16, or high temperature sealing material may be disposed between the wall and the cover, the seal between the wall and the cover in any event being sufficient to prevent appreciable gas flow between the cover and the wall. Cover 32 may also include cover heater 34 for heating the chamber cover and therefore helping to control the free surface temperature of the glass melt flowing through the stir chamber. Cover heater 34 typically includes a resistance coil, typically comprising platinum, imbedded within the chamber cover refractory material. The resistance coil is supplied with an electric current, preferably alternating current, although direct current may be applied, to thereby heat the chamber cover. The chamber cover is typically between about 2 inches (5.08 cm) and 3 inches (7.62 cm) from the free surface of the glass melt, but this distance may be greater, as needed. Thus, volume 35 is defined between the stir chamber cover 32, stir chamber wall 16 and glass free surface 26. [0016] Chamber cover 32 also includes a passage through which stirrer shaft 22 passes. The inside surface of the passage may include a lining which forms casing 36. As with other components of the stir chamber, it is desirable that casing 36 be resistant to corrosion due to the high temperature and the corrosive gases and condensates which may develop from the molten glass. Casing 36 typically comprises platinum or a platinum alloy. Shaft 22 passing through the chamber cover passage forms annular gap 38 between the outside surface of shaft 22 and the inside surface of either the passage or, should casing 36 be employed, the annular gap is formed between the outside surface of the shaft and the inside surface of the casing. For the purpose of eliminating confusion, reference shall be made hereinafter only to the inside surface of the casing, but shall be construed to mean both instances, whichever applies. It is upon the surfaces defining the annular gap 38 upon which condensate (e.g. platinum) forms. Once the condensate reaches a certain size, it will break off and fall into the glass melt 26, thereby creating defects in the eventual glass product. That portion of shaft 22 above chamber cover 32 is surrounded by a refractory material containing shaft heater 40. Shaft heater 40, as in the case of cover heater 34, preferably comprises a resistance heating element. The heating element is preferably comprised of platinum, but may be a platinum alloy.

[0017] An insulation layer 42 is disposed overtop chamber cover 32. Insulation layer 44 similarly surrounds shaft heater 46. Annular gap 38 eliminates contact between the rotating shaft and the casing, heaters, insulation and cover.

[0018] Optionally, at least one flow tube 50 may extend from outside stir chamber 10 to the inside of stir chamber 10, i.e. volume 35. The flow tubes may be employed to cause gas to flow along the stirrer shaft, thereby reducing condensation of volatile oxides along the shaft.

[0019] Located on the stirrer shaft, below the cover 32 and above the glass melt 26 is a condensate collection vessel 40. The vessel contains an annular planer bottom portion 41 laying approximately orthogonal to the stir shaft 22. The condensate collection vessel further comprises vertically oriented side walls 43 around the outer periphery. The combination of the bottom portion 41 and the side wall 43 serves to contain any platinum or other condensate that may have formed on the interior surface of the annular gap 38 and that has subsequently broken off. In one embodiment, the area of the bottom portion exceeds the cross sectional area of the annular gap 38. In another embodiment, the distance from the outer surface of the shaft to the peripheral wall of the condensate collection vessel is between 0.5 - 2 inches. The height of the side wall can be any distance, but in one embodiment ranges from 0.25 inches to 1 inch.

[0020] As can be seen in a breakaway three dimensional view of the condensate collection vessel 40 (Fig. 2), the annular bottom portion is flush with and surrounds the shaft. A peripheral side wall defines an outer boundary for the vessel. There is no top such that condensate falling from above will land and become contained within the vessel as defined by the annular bottom portion 41 and the peripheral side walls 43. The condensate collection vessel 40 may be attached to the shaft in any number of ways but in one embodiment, a collar 45 is formed that contacts the shaft 22 along a prescribed length. The collar 45 may be welded or otherwise bound to the shaft. In one embodiment, the condensate collecting vessel is assembled on the shaft by joining two semi-annular parts and welding them together along a diameter weld line 47.

[0021] Fig. 3 shows a cross sectional view of the condensate collection vessel 40. In one embodiment, the angle between the bottom portion and the outer peripheral walls ΘΑ may be between 90 - 120 degrees. In a preferred embodiment, the angle between the bottom portion and the outer peripheral walls ΘΑ is 100 degrees. Since the collar 45 is flush with the stirrer shaft 22, the angle between the bottom portion and the collar ΘΒ will be consistent with the angle of the outer wall of the shaft. In one embodiment, that angle ΘΒ is between 85-90 degrees.

[0022] The condensate collection vessel may be made from materials know for their ability to withstand the types of temperatures existent in the stir chamber. For example, the condensate collection vessel may be composed of platinum, but may be a platinum alloy, or a dispersion-strengthened platinum or platinum alloy (e.g., a zirconia-strengthened platinum alloy).

[0023] In operation, the condensate collection vessel will gradually collect condensed platinum condensate that has broken off from the annual gap as previously described. When the glass manufacturing system is taken down for maintenance procedures, the condensate in the vessel is harvested and either discarded or recycled. [0024] It will be apparent to those skilled in the art that various other modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Thus it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Claims

What is claimed is:

1. A stir chamber for use in stirring and containing molten glass as part of a glass manufacturing system comprising:

at least one wall and cover, the cover having a passage there through;

a stirrer comprising a shaft that extends through the cover passage thereby forming an annular gap between the shaft and the cover;

a volume above a free surface of the molten glass; and,

a condensate collection vessel located above the free surface of the molten glass and having a planar annular bottom portion arranged approximately orthogonal to the stirrer shaft, the bottom portion having a periphery, and a vertically oriented side wall connected proximate the periphery, the bottom portion and the side wall intersecting at an angle of between 90 and 120 degrees.

2. The stir chamber of claim 1 wherein the angle created by the intersection of the bottom portion and the side wall is 100 degrees.

3. The stir chamber of claim 1 wherein the condensate collection vessel further comprises an annual collar that attaches the vessel to the shaft.

4. The stir chamber of claim 1 further comprising at least one gas flow tube that allows for a flow of gas into the chamber and through the volume.

5. The stir chamber of claim 1 further comprising an inlet for allowing molten glass to enter the stir chamber and an outlet for allowing molten glass to flow out of the chamber.

6. The stir chamber of claim 1 wherein the stirrer further comprises blades extending outward from the shaft and toward the wall of the chamber.

7. The stir chamber of claim 1 wherein the condensate collection vessel is made from platinum or a platinum alloy.

8. The stir chamber of claim 1 wherein a cross sectional area of the bottom portion exceed a cross sectional area of the annular gap.

9. The stir chamber of claim 1 wherein a distance from the stir chamber to the side wall of the condensate collection vessel is between 0.5 and 2 inches.

10. The stir chamber of claim 1 wherein the side wall of the condensate collection vessel has a height of between 0.25 and 1 inch.

11. A method of stirring a glass melt comprising:

providing a stir chamber comprising at least one wall and cover, the cover having a passage there through; a stirrer comprising a shaft that extends through the cover passage thereby forming an annular gap between the shaft and the cover; a volume above a free surface of the molten glass; and, a condensate collection vessel having a planar annular bottom portion arranged approximately orthogonal to the stirrer shaft, the bottom portion having a periphery, and a vertically oriented side wall connected proximate the periphery, the bottom portion and the side wall intersecting at an angle of between 90 and 120 degrees; and,

stirring the glass melt.

12. The method of claim 11 wherein the stirring method is a part of a glass manufacturing process for making glass substrate for liquid crystal displays (LCDs).

13. The method of claim 11 wherein the angle created by the intersection of the bottom portion and the side wall is 100 degrees.

14. The method of claim 11 wherein the condensate collection vessel further comprises an annual collar that attaches the vessel to the shaft.

15. The method of claim 11 wherein the stir chamber further comprises an inlet for allowing molten glass to enter the stir chamber and an outlet for allowing molten glass to flow out of the chamber.

16. The method of claim 11 further comprising the step of collecting a condensate that forms in the annular gap, break off and falls into the condensate collection vessel.

17. The method of claim 16 further comprising the step of removing the condensate that has collected in the condensate collection vessel.

18. The method claim 11 wherein a cross sectional area of the bottom portion exceed a cross sectional area of the annular gap.

19. The method of claim 11 wherein a distance from the stir chamber to the side wall of the condensate collection vessel is between 0.5 and 2 inches.

20. The method of claim 11 wherein the side wall of the condensate collection vessel has a height of between 0.25 and 1 inch.