Abstract:
The present invention is directed toward a method of reducing contamination of a glass melt in a stirring apparatus by an oxide material. The oxide material, such as platinum oxide, may be volatilized by the high temperature of the glass melt, and then condense on the inside surfaces of a stirring vessel, particularly the stirrer shaft and surrounding surfaces of the stirring vessel cover. A build-up of condensed oxide material may then be dislodged and fall back into the glass melt. Accordingly, an apparatus and method is provided that includes a heating element disposed adjacent an annular gap between the stirring vessel cover and the stirrer shaft. The heating element heats a surface of the stirring vessel cover bounding the annular gap and prevents condensation of volatile oxides that may flow through the annular gap.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     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. 
     2. Technical Background 
     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. 
     One approach for improving the homogeneity of glass is to pass the molten glass through a vertically-oriented stirring apparatus located downstream of the melter. Such stirring apparatus are equipped with a stirrer having a central shaft rotated by a suitable driving force, such as a motor. A plurality of blades extends from the shaft and mix the molten glass as it passes from the top to the bottom of the stirring apparatus. The operation of such stir chambers should not introduce further defects into the resulting glass, specifically, defects arising from condensed oxides. 
     Volatile oxides in a glass stirring apparatus can be formed from any of the elements present in the glass and stirring apparatus. 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 PtO 2 , and the glass free surface for B 2 O 3 , As 4 O 6 , Sb 4 O 6 , and SnO 2 . By glass free surface what is meant is the surface of the glass which is exposed to the atmosphere within the stirring apparatus. 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 stirring apparatus, 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 stirring vessel cover. Since the stirrer shaft becomes cooler as the distance between the stirrer shaft and the glass free surface increases, the volatile oxides contained with the stirring apparatus atmosphere can 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. 
     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. 
     One prior art method of reducing contamination of the glass melt by condensates has been to dispose a disc-shaped shield between the glass free surface and upper portions of the stir chamber. However, such methods may make it difficult to control the temperature of the glass free surface, such as by heating the chamber cover above the glass. In addition, the joint between the shield and the stirrer shaft may serve as an additional source of condensate contamination. 
     SUMMARY 
     In one embodiment an apparatus  10  for stirring molten glass melt is disclosed comprising a stirring vessel  12  and a stirring vessel cover  14  positioned over the stirring vessel, a surface  40  of the stirring vessel cover  14  defining an aperture  38  through which a stirrer shaft  24  extends, thereby forming an annular gap  52  between the stirrer shaft  24  and the aperture-defining surface  40  of the stirring vessel cover  14 , a first channel  48  formed in the stirring vessel cover  14  at the aperture-defining surface  40 ; and a first heating element  56  disposed in the first channel that heats the aperture-defining surface. 
     The stirring vessel cover  14  may further comprise a second channel  60  comprising a thermocouple  58  disposed therein, and wherein a sensing end  62  of the thermocouple is positioned proximate the aperture-defining surface  40 . A sensing end of the thermocouple is preferably positioned to sense a temperature of the stirring vessel cover adjacent to the annular gap. 
     In some embodiments a platinum-containing cladding may be disposed over a surface  34  of the stirring vessel cover facing a free surface  28  of the molten glass  30 . The stirring vessel cover further includes an additional channel  44  formed in a surface  34  of the stirring vessel cover facing the molten glass  30 , and wherein a second heating element  42  is disposed in the additional channel  44 . 
     In another embodiment, a method of stirring a molten glass  30  is described comprising flowing the molten glass into a stirring vessel  12 , stirring the molten glass with a stirrer  16  extending through an aperture  38  defined by a surface  40  of a stirring vessel cover  14  positioned over the stirring vessel, thereby forming an annular gap  52  between the stirrer  16  and stirring vessel cover  14 , and heating aperture-defining surface  40  of stirring vessel cover  14  with a heating element positioned adjacent to the aperture-defining surface  40 . 
     The method may further comprise sensing a temperature within the annular gap  52  with a thermocouple  58  disposed within the stirring vessel cover  14 . The sensed temperature can then be used to control the magnitude of an electrical current supplied to the heating element, thereby regulating a temperature of the aperture-defining surface and the annular gap  52  between the surface  40  and the stirrer shaft  24 . 
     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 
         FIG. 1  is a cross sectional view of an exemplary stirring apparatus according to an embodiment of the present invention showing the stirring vessel cover and the annular gap heating elements. 
         FIG. 2  is a perspective view of an embodiment of the stirring vessel cover of  FIG. 1  formed in two segments. 
         FIG. 3  is a perspective view of one segment of the stirring vessel cover of  FIG. 2  showing a channel in a surface defining a central aperture of the cover for heating that surface, and channels formed in a bottom surface of the cover for receiving cover bottom heating elements. 
         FIG. 4  is a cross section view of a portion of a stirring vessel cover according to an embodiment of the present invention illustrating the channels of  FIG. 3  containing the bottom surface heating elements and the aperture-defining surface of the cover, and further showing a metallic cladding material disposed over surfaces of the cover. 
         FIG. 5  is a cross section view of a portion of a stirring vessel cover according to another embodiment of the present invention illustrating the channels of  FIG. 3  containing the bottom surface heating elements and the aperture-defining surface of the cover, wherein the channel at the surface of the aperture defining surface of the stirring vessel is positioned at a median location rather than an upper edge portion of the cover. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an exemplary apparatus for practicing a method for homogenizing a glass melt according to an embodiment of the present invention. Stirring apparatus  10  of  FIG. 1  includes stirring vessel  12 , stirring vessel cover  14  and stirrer  16 . 
     Stirring vessel  12  is preferably cylindrically-shaped and substantially vertically-oriented, although the stirring vessel may have other shapes and orientations as needed. Preferably, the stirring vessel includes inner surface  18  comprising platinum or a platinum alloy. Other materials having resistance to high temperature, including resistance to corrosion, as well as electrical conductivity, may be substituted. For example, suitable metals for forming inner surface  18  can include other platinum group metals such as rhodium, iridium, palladium, ruthenium, osmium and alloys thereof. Stirring vessel  12  comprises molten glass inlet pipe  20  located at or near the top of stirring vessel  12  and molten glass outlet pipe  22  located near the bottom of the stirring vessel. However, it will be recognized by the skilled artisan that inlet pipe  20  and outlet pipe  22  may be reversed in some embodiments, such that the molten glass flows into the stirring apparatus from the bottom and flows out through the top of the stirring apparatus. Intermediate positions for the inlet and outlet pipes may also be employed provided adequate stirring (i.e. the desired amount of homogenization) is achieved. 
     Stirrer  16  comprises stirrer shaft  24  and a plurality of stirring blades  26  extending from stirrer shaft  24 . Stirring blades  26  are typically submerged below free surface  28  of molten glass  30  during operation of the stirring apparatus. The molten glass surface temperature is typically in the range between about 1300° C. to 1500° C., but may be higher or lower depending upon the glass composition. Stirrer  16  preferably comprises platinum, and may be a platinum alloy or dispersion-strengthened platinum (e.g., a zirconia-strengthened platinum alloy). 
     Stirring vessel cover  14  covers an upper open end of stirring vessel  12 , and includes an upper surface  32  and a lower surface  34 . Lower surface  34  may further include a cladding material  36  (see  FIG. 4 ) positioned over lower surface  34  to protect lower surface  34  from the corrosive atmosphere above the free surface of the molten glass. For example, lower surface  34  may include a platinum or platinum alloy (e.g. platinum-rhodium) cladding. Stirring vessel cover  14  defines an aperture  38  ( FIG. 2 ) extending through a thickness of the stirring vessel cover and through which stirrer shaft  24  extends. Aperture  38  is bounded by an aperture-defining surface  40 , of stirring vessel cover  14 . In some embodiments, stirring vessel cover  14  may be formed in a plurality of segments to facilitate easy removal and replacement of the stirring vessel cover, such as during a rebuilding of the stirring apparatus. For example, in the embodiment illustrated in  FIG. 2 , stirring vessel cover  14  is shown having two segments, stirring vessel first cover segment  14   a  and stirring vessel second cover segment  14   b.    
     As best shown in  FIGS. 3 and 4 , stirring vessel cover  14  may also include one or more heating elements  42  embedded at lower surface  34  of the stirring vessel cover. Heating elements  42 , typically in the form of a metallic coil, rod or ribbon, are disposed in one or more channels  44  formed in lower surface  34  of stirring vessel cover  14 . In some embodiments, lower surface  34  includes a single channel  44  through which a single heating element  42  extends. For example, the channel may form a generally spiral shape. However, the use of a single heating element  42  is solely for ease in manufacture and maintenance, and the use of a plurality of heating elements, disposed in one or more channels  44 , can be employed. In the instance where stirring vessel cover  14  is formed in one or more segments, at least two heating elements  42  are disposed in at least two channels  44 , at least one channel for each segment, which simplifies removal of the stirring cover segments. The one or more heating elements  42  may be secured within channel  44  by refractory cement  46 . 
     Stirring vessel cover  14  further includes a second channel  48  formed around at least a portion of aperture  38 . That is, channel  48  is formed in at least a portion of stirring vessel cover  14  that defines aperture  38 . Channel  48  is preferably positioned adjacent the upper portion of aperture  38  farthest from the surface of the molten glass during operation of the stirring apparatus and adjacent to upper surface  32 . Because the temperature of inner atmosphere  50  within stirring vessel  12  and above molten glass free surface  28  is significantly higher than the temperature of external atmosphere  51  outside stirring apparatus  10 , a chimney effect is created and hot gases from inner atmosphere  50  vent through annular gap  52  formed by stirrer shaft  24  and the aperture-defining surface  40  of stirring vessel cover  14 . These gases can include volatilized materials from the molten glass itself, or volatilized materials (e.g. platinum) from the stirrer and/or the stirring vessel. These volatilized materials can condense onto the surface of the stirring vessel cover and, if allowed to grow sufficiently large, break off and become entrained in the molten glass. 
     Of course, channel  48  can be placed at any vertical position within aperture  38  ( FIG. 5 ). However, absent any other heat sources, those portions of stirring vessel cover  14  farthest from the hot molten glass are cooler than those portions of the stirring vessel cover closer to the molten glass and the upper reaches of the aperture tend to be the coolest. Thus, condensates are more likely to form at the stirring vessel cover surfaces near the upper reaches of the aperture, farthest from the molten glass surface. These stirring vessel cover surfaces can include both surface  40  defining aperture  38 , and surfaces of stirring vessel shaft  24  within aperture  38 . To heat these surfaces, one or more heating elements  56  are positioned within channel  48  to heat both surface  40 , and the outer surface of stirrer shaft  24  extending through aperture  38 . To stabilize the one or more heating elements  56 , refractory cement  46  may be included in channel  48  to hold the heating element in place. Additionally, a cladding of platinum or platinum alloy disposed over portions of the stirring vessel cover such that channel  48  and encloses heating element  56  within the channel are covered by the cladding. The cladding may be an extension of cladding  36  formed over lower surface  34 . As used herein, aperture-defining surface  40  is either a surface of the refractory material of the stirring vessel cover itself, or in the instance where a cladding material is disposed over the refractory surface, the aperture defining surface  40  is the cladding surface circumscribing the aperture. 
     To monitor temperature at annular gap  52 , and if desired assist in automatic control of heating element  56  and/or heating element  42 , one or more thermocouples  58  may be included in stirring vessel cover  14 . As shown in  FIG. 4 , in some embodiments, a channel  60  may be formed through an interior of stirring vessel cover  14  and extending to, or at least near to, surface  40 . That is, the thermocouple preferably is not exposed to the atmosphere extending through gap  52 , but is close enough to the gap atmosphere the surface exposed to the gap surface that a temperature of the surface contacting the atmosphere in gap  52  can be reasonably determined. Thermocouple  58  is disposed in channel  60 . Thermocouple channel  60  is shown in  FIG. 4  as including refractory cement  46  to secure the thermocouple within channel  60 . However, since the refractory cement may interfere with the temperature sensing performance of thermocouple  58 , and makes replacement of the thermocouple difficult, the refractory cement may be excluded from the thermocouple channel if desired. Also, as shown in  FIG. 4 , cladding material  36  is positioned between the thermocouple sensing end  62  and stirrer shaft  24 . Put more simply, thermocouple  58  can be covered by cladding material  36  in a manner similar to the way in which heating element  56  is covered by the cladding. 
     During operation, a motor  63  coupled to stirrer shaft  24  through linkage  66  rotates stirrer  16 . Linkage  66  may for example include a chain and related sprockets connected to both the motor and the stirrer shaft. Molten glass  30  supplied to stirring apparatus  10  through inlet pipe  20 , is stirred and homogenized by stirrer  16 , and flows out the stirring apparatus through outlet pipe  22 . Control of the temperature at surface  40  and within annular gap  52  can be achieved with the use of a control circuit, as illustrated in  FIG. 4 . The temperature at surface  40  is sensed by the sensing end  62  of thermocouple  58 . An electrical signal is generated by the thermocouple and delivered to controller  64  via line  68 . Controller  64  interprets the electrical signal as a temperature according to a predetermined conversion factor, and compares the resultant temperature to a predetermined temperature set point. If the sensed temperature is less than the set point temperature, controller directs power source  70  via line  72  to deliver a flow of current through line  74  to heating element  56 . Once the sensed temperature reaches the set point temperature, the controller directs the power source to reduce or extinguish the current flow. Of course other control schemes are possible, and the foregoing is but one method of implementation. Heating element  42  is similarly controlled by controller  64  via power source  70  and line  76 . 
     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.