Classifications

• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
  • C03B5/00—Melting in furnaces; Furnaces so far as specially adapted for glass manufacture
  • C03B5/16—Special features of the melting process; Auxiliary means specially adapted for glass-melting furnaces
  • C03B5/18—Stirring devices; Homogenisation
  • C03B5/182—Stirring devices; Homogenisation by moving the molten glass along fixed elements, e.g. deflectors, weirs, baffle plates
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
  • C03B5/00—Melting in furnaces; Furnaces so far as specially adapted for glass manufacture
  • C03B5/16—Special features of the melting process; Auxiliary means specially adapted for glass-melting furnaces
  • C03B5/18—Stirring devices; Homogenisation
  • C03B5/183—Stirring devices; Homogenisation using thermal means, e.g. for creating convection currents
  • C03B5/185—Electric means
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
  • C03B5/00—Melting in furnaces; Furnaces so far as specially adapted for glass manufacture
  • C03B5/16—Special features of the melting process; Auxiliary means specially adapted for glass-melting furnaces
  • C03B5/18—Stirring devices; Homogenisation
  • C03B5/187—Stirring devices; Homogenisation with moving elements
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
  • C03B5/00—Melting in furnaces; Furnaces so far as specially adapted for glass manufacture
  • C03B5/16—Special features of the melting process; Auxiliary means specially adapted for glass-melting furnaces
  • C03B5/225—Refining

Definitions

• the disclosure relates generally to a process and apparatus for making glass and more particularly to a process and apparatus for improving the quality of glass in a molten glass refining step, which process produces glass that may be used in a variety of products.
• molten glass a viscous molten material
• the melting process also produces unwanted by-products, that, if not removed from the molten glass, may pass through the glass making process and manifest in the finished article as visual defects.
• visual defects In the case of gaseous byproducts, such defects are variously referred to as seeds, blisters, or gaseous inclusions.
• chemical inhomogeneities in the molten glass can result in certain other visual defects, commonly referred to as striae, striations or cord.
• defects like striae, cord and seeds are unacceptable defects that can significantly impact the suitability of the final article for its intended purpose. Accordingly, there is a need to remove or prevent the formation of such defects before the glass article reaches its final form.
• the process of melting batch materials to produce molten glass generates various gaseous byproducts. These gaseous byproducts may be dissolved in the glass itself, or may be dispersed as bubbles within the glass. Such gases may include, for example and without limitation, CO 2 and SO 2 .
• gases may include, for example and without limitation, CO 2 and SO 2 .
• One method used to remove these melting related defects is by adding a fining agent, such as oxides of arsenic, antimony, tin, cerium or boron, during the initial melting process.
• the fining agent may be added to the batch material provided to the melting furnace.
• the molten glass in a later step in the manufacturing process is heated to a predetermined temperature sufficiently greater than the initial melting temperature to induce the fining agent to generate oxygen bubbles through a change in atomic valence state. In other words, the fining agent is reduced and gives off excess oxygen.
• the higher temperature reduces the viscosity of the molten glass, making it easier for the oxygen bubbles to flow upward through the molten glass.
• melting-produced gases diffuse into the oxygen bubbles and are transported to a free surface of the molten glass, where the gasses are released to an atmosphere above the free surface.
• Certain glass making processes employ precious metal delivery systems for delivering the molten glass to subsequent forming apparatus. This is particularly true for high purity glass intended for optical or other precision applications require high optical clarity.
• a problem associated with the formation of oxygen bubbles is that if these bubbles are in contact for any appreciable period of time with interior surfaces of certain glass processing and refining vessels formed from a precious metal or a precious metal alloy, the interior surfaces of the vessels can corrode. Such corrosion, if left unchecked, can weaken the vessels walls and eventually cause breaches in the vessel. Accordingly, molten glass is typically subjected to refining (or more commonly referred to simply as "fining"), wherein gaseous inclusions (bubbles) are removed from the molten glass.
• the molten glass is agitated or mixed after the fining process but before the molten glass has cooled to a temperature where a viscosity of the molten glass makes it difficult to do so.
• the molten glass is agitated or mixed after the fining process but before the molten glass has cooled to a temperature where a viscosity of the molten glass makes it difficult to do so.
• there are limitations to how effective such conventional processes are due to the cooling already experienced by the molten glass downstream of the fining process.
• zirconium oxide (hereafter zirconia) is one such ceramic material that may be used in the construction of melting furnaces.
• the zirconia comprising the melting furnace may dissolve into the molten glass during formation of the molten glass and can remain in the glass to the end of the process, remaining an ingredient of the final glass product. If the zirconia is evenly dispersed and dissolved into the molten glass at a low concentration it does not pose a significant problem or affect the final product. However, if the zirconia is not uniformly mixed into and effectively dissolved throughout the molten glass, and significant
• the zirconia can crystalize out of solution as the molten glass cools and create visual defects in the final product. Thus, crystallization cannot be allowed to occur prior to homogenization, as the mixing that occurs during homogenization does little to remove the crystals once formed.
• the molten glass is homogenized early in the glass manufacturing process, close to the melting step, before the glass has cooled below a temperature where a viscosity of the molten glass detrimentally impacts mixing efficiency and crystalline components are allowed to form in the glass.
• a method of fining molten glass in a glass manufacturing process comprising flowing molten glass from a melting furnace to a metallic fining vessel through a first metallic conduit positioned between the fining vessel and the melting furnace, the fining vessel comprising a first portion and a second portion; flowing the molten glass in an upward vertical direction through the first portion of the fining vessel; agitating the molten glass as it flows in the upward vertical direction; increasing a temperature of the molten glass as it flows in the upward vertical direction; redirecting the flow of molten glass from an upward vertical direction to a no n- vertical direction in the second portion of the fining vessel; and wherein the molten glass in the first portion and the second portion of the fining vessel comprises a continuous free glass surface that is an interface with an atmosphere above the glass free surface to allow bubbles of gas in the molten glass to escape into the atmosphere.
• the non-vertical direction is a horizontal direction.
• the agitating can comprise actively mixing the molten glass with a rotating member. In some instances the agitating provides an upward pumping action on the molten glass.
• the step of increasing the temperature of the molten glass comprises flowing an electric current through (i.e. within) a wall of the first portion.
• the method may further comprise flowing the molten glass from the fining vessel to a stirring vessel positioned downstream of the fining vessel through a second metallic conduit positioned between the fining vessel and the stirring vessel, wherein the molten glass flowing within the second metallic conduit does not have a free glass surface, and stirring the molten glass in the stirring vessel.
• a glass processing apparatus comprising a melting furnace formed from a refractory material and configured to melt a batch material to form a molten glass; a metallic fining vessel comprising a first portion having a vertical longitudinal axis and second portion having a non-vertical longitudinal axis connected thereto; a first metallic conduit extending between the melting furnace and the fining vessel first portion such that molten glass flowing to the fining vessel from the melting furnace passes through the first metallic conduit; a stirring vessel positioned downstream from the fining vessel; a second metallic conduit extending between the fining vessel and the stirring vessel such that molten glass flowing to the stirring vessel from the fining vessel passes through the second metallic conduit; an agitating member positioned in the first portion configured to agitate the molten glass as the molten glass flows upward through the first portion and an electrode attached to the first portion configured to allow an electric current to flow through a wall of the first portion.
• the longitudinal axis of the second portion can be orthogonal to the longitudinal axis
• the agitating member may comprise a rotatable stirrer.
• the rotatable stirrer may comprise, for example, an agitation element coupled to and extending outward from the shaft, and wherein a distance between a floor of the first portion and an uppermost point on the extension member is greater than a distance between the floor of the first portion and the lowest point on an interior surface of a wall of the second portion.
• the agitating member can be configured to provide an upward pumping action to the molten glass.
• a longitudinal axis of the first conduit is orthogonal to a longitudinal axis of the first portion.
• a fining vessel for fining molten glass comprising a first portion having a first longitudinal axis and a second portion having a second longitudinal axis, wherein the first longitudinal axis is vertical and the second longitudinal axis is non- vertical; an agitating member positioned within the first portion; at least one vent passage extending through a wall of the second portion so that an interior volume of the second portion is in fluid communication with an atmosphere external to the second portion; and an electrode attached to the first portion configured to allow an electric current to flow through a wall of the first portion.
• the agitating member can comprise a rotatable stirrer. In some embodiments the agitating member is configured to provide an upward pumping action to the molten glass when the rotatable stirrer is rotated.
• the rotatable stirrer can comprise an agitation element coupled to and extending outward from the shaft, and wherein a distance between a floor of the first portion and an uppermost point on the agitation element is greater than a distance between the floor of the first portion and the lowest point on an interior surface of a wall of the second portion.
• the electrode may be placed in electrical contact with an electrical power source.
• the fining vessel first portion and second portion may comprise platinum, and in some embodiments an end of the second portion intersects a wall of the first portion.
• the first longitudinal axis of the fining vessel may be orthogonal to the second longitudinal axis.
• FIG. 1 is a schematic view of the major functional parts of an exemplary glass manufacturing apparatus that incorporates a fining vessel as described herein;
• FIG. 2 is a cross sectional view of an exemplary forming body according to an embodiment of the present disclosure
• FIG. 3 is a perspective view of a fining vessel according to an embodiment of the present disclosure.
• FIG. 4 is a schematic view of a fining vessel according to an embodiment of the present disclosure
• FIG. 5 is a perspective view of a portion of a moving member comprising a fining vessel according to embodiments described herein;
• FIG. 6 is a perspective view of a portion of another moving member comprising a fining vessel according to embodiments described herein;
• FIG. 7 is a schematic view of a finning vessel according to embodiments described herein and comprising stationary agitating members.
• FIG. 1 is a schematic diagram of an exemplary glass making apparatus 10.
• Glass making apparatus 10 may be used, for example, to manufacture glass substrates for flat panel displays, such as liquid crystal displays (LCDs) or organic light emitting diode (OLED) displays.
• LCDs liquid crystal displays
• OLED organic light emitting diode
• Glass making apparatus 10 includes melting furnace 12, fining vessel 14, connecting pipe 16 that is a conduit providing fluid communication between melting furnace 12 and fining vessel 14 and through which molten glass flows between the melting furnace and the fining vessel, stirring chamber 18, connecting pipe 20 that is a conduit providing fluid communication between fining vessel 14 and stirring chamber 18 and through which molten glass flows between fining vessel 14 and stirring chamber 18, collection vessel 22, connecting pipe 24 that is a conduit providing fluid communication between stirring chamber 18 and collection vessel 22 and through which molten glass flows between the stirring chamber and collection vessel 22, downcomer pipe 26, inlet 28 and forming body 30.
• forming body 30 is a wedge-shaped body comprising a trough 32 in an upper surface thereof and external converging forming surfaces 34 that meet at a lower apex of the converging forming surfaces along a line, root 36, that extends along a length of the bottom of the forming body.
• Molten glass 38 from collection vessel 22 is supplied to trough 32 where it overflows the trough and flows separately over converging forming surfaces 34.
• a cross sectional view of the forming body is shown in FIG. 2. The separate flows of molten glass join (i.e. fuse) at root 36, thereby forming a singular continuous flow of molten glass, i.e.
• Melting furnace 12 and forming body 30 are typically constructed of a heat resistant refractory material, such as a ceramic material, capable of withstanding the temperatures needed to melt the constituent raw materials (batch) deposited into melting furnace 12 (arrow 44) into molten glass 38 and form the molten glass into a glass article, such as glass ribbon 40.
• the temperature necessary to create molten glass can vary from less than 1300°C to over 1550°C, and is dependent on the type of glass to be produced and the requisite raw materials. For example, where the glass is an aluminoborosilicate glass, such as may be used for certain display glass applications, the final melting temperature may be in the range of 1550°C to 1570°C. Other glass types may have similar or different melting temperatures.
• suitable refractory materials utilized in melting furnaces include oxides of zirconium and/or aluminum.
• the molten glass is typically heated to a temperature greater than a temperature of the molten glass within the melting furnace.
• a suitable fining temperature can be equal to or greater than about 1600°C, more typically in a range from about 1600°C to about 1650°C and in some embodiments between about 1600°C and about 1700°C.
• Suitable fining agents include without limitation oxides of arsenic, antimony, tin, cerium, and iron. However, certain fining agents, such as arsenic and antimony oxides are quite toxic. As a result, less toxic fining agents may be selected, such as an oxide of tin.
• the buoyancy of the oxygen bubbles causes the bubbles to rise through the molten glass to the glass free surface, whereupon the bubbles release the contained gas into the atmosphere above the glass free surface.
• These bubbles serve as collection points for gases generated by the melting process (e.g. CO2 and SO2), whereupon the melting-generated gases, either in the form of additional bubbles, or dissolved gases, accumulate within the oxygen bubbles, increasing the size and buoyancy of the bubbles and facilitating their rise to the glass free surface.
• the bubbles rupture at the glass free surface and release the enclosed gases into the atmosphere above the glass free surface. The released gases are then vented from the fining vessel.
• the molten glass flowing from the melting furnace is not homogeneous.
• the molten glass flowing from the melting furnace is comprised of various thermal (e.g.
• the flow of molten glass at various points in the cross section of the stream of flowing molten glass can vary. This variation in flow can create areas of stagnate molten glass where regions of molten glass are flowing at significantly slower velocity than other regions of the molten glass, or in worst case situations, not flowing at all. As a result of stagnation, such regions of molten glass can also have different thermal and/or chemical makeup than the general molten glass flow. However, such stagnant regions of molten glass can be unexpectedly pulled into the general flow of molten glass, producing regions of molten glass with thermal and/or chemical inhomogeneities.
• the refractory material comprising the melting furnace for example zirconium or aluminum oxide, slowly dissolves into the molten glass over time. If the zirconium is not sufficiently dissolved and evenly dispersed throughout the molten glass, the zirconium can crystalize in the molten glass and affect the quality and composition of the final glass article. In some instances the final glass article may be rendered useless for the particular purpose it was intended. Thus, the molten glass should be thoroughly homogenized during the manufacturing process.
• Fining vessel 14, stirring chamber 18, collection vessel 22, downcomer pipe 26 and other associated molten glass transport conduits and vessels may be formed from a precious metal or a precious metal alloy.
• precious metals are typically selected from the platinum group metals, including ruthenium, rhodium, palladium, osmium, iridium, platinum and alloys thereof.
• the precious metal may be pure platinum or platinum alloyed with one or more other precious metals such as rhodium or iridium.
• Suitable precious metal alloys may include a platinum-rhodium alloy comprising by weight platinum in a range from about 80% to about 90% and rhodium in a range from about 10% to about 20%.
• fining vessel 14 and connecting pipes 16 and 20 can be pipes having a circular cross section.
• a circular cross sectional shape for at least a portion of fining vessel 14 facilitates the use of a rotating member disposed therein.
• the connecting pipes and the fining vessel, or portions thereof could have other cross sectional shapes, such as an oval or oblong shape, where the cross section is taken perpendicular to a longitudinal axis of the vessel or connecting pipe. Non-circular shapes may be used, for example, when passive mixing is employed.
• fining vessel 14 comprises a first portion, riser 46, in direct contact with connecting pipe 16, and a second portion, channel 48.
• Riser 46 may extend vertically between fining vessel inlet 50 and riser outlet 52.
• Riser outlet 52 also serves as the inlet to channel 48.
• Channel 48 extends outward, away from riser 46, and provides a flow path between riser outlet 52 and fining vessel outlet 54 for the molten glass flowing therethrough.
• an end of the channel 48 intersects a wall of riser 46 at riser outlet 52.
• riser 46 has a longitudinal axis 57 that is vertical.
• channel 48 has a longitudinal axis 59 that is horizontal.
• riser 46 and channel 48 may be substantially orthogonal to each other, that is and angle between longitudinal axis 57 and longitudinal axis 59 is 90° ⁇ 5°.
• riser 46 may be vertical and channel 48 may be horizontal. Accordingly, during operation of glass making apparatus 10, molten glass 38 formed in melting furnace 12 flows through connecting pipe 16 into fining vessel 14 at fining vessel inlet 50. The flow of molten glass is indicated by block arrows 53 in FIG. 4. The molten glass then flows upward through riser 46, out riser outlet 52 and through channel 48 to outlet 56 of fining vessel 14 into connecting pipe 20.
• a longitudinal axis 58 of connecting pipe 16 is parallel to a longitudinal axis 60 of connecting pipe 20 (see FIG. 4), but not necessarily so.
• the molten glass forms a continuous free glass surface 62 within both riser 46 and channel 48.
• the free glass surface is a surface of the molten glass flowing within fining vessel 14 that is exposed to an atmosphere 64 disposed within a volume above the flowing molten glass and which volume is enclosed by walls of the fining vessel.
• the free glass surface 62 is the interface between the atmosphere 64 and the molten glass 38. It should noted that neither connecting pipe 16 nor connecting pipe 20 possess a free glass surface during operation of glass making apparatus 10 as molten glass is flowing through the connecting pipe 16 and connecting pipe 20.
• Molten glass 38 entering fining vessel 14 from connecting pipe 16 at fining vessel inlet 50 may subsequently be heated within riser 46 as the molten glass flows upward through the riser.
• the molten glass within riser 46 may be heated indirectly by resistance heating elements (not shown) positioned on or around an external surface of riser 46 that heat the riser and therefore the molten glass flowing within the riser.
• riser 46 may be heated directly by flowing an electric current through the riser itself that heats the riser directly by Joule heating.
• two or more electrodes 66 may be attached to riser 46 and/or channel 48 so that an electric current can be supplied from a source (not shown) to the riser and/or channel 48. In the embodiment of FIG.
• fining vessel 14 four electrodes are attached to fining vessel 14, including two electrodes attached to riser 46 and two electrodes attached to channel 48.
• the upper most electrode 66 attached to riser 46 could be removed, wherein fining vessel would include three electrodes 66.
• a heated riser heats the molten glass flowing therein so that the molten glass can achieve a predetermined fining temperature.
• the molten glass flowing upward through riser 46 may also be agitated.
• the molten glass may be actively agitated by a moving member 68, e.g. a stirring member, positioned within riser 46 that mixes and homogenizes the molten glass.
• the moving member 68 in riser 46 is positioned at a location in the glass making process where heating of the molten glass can occur and oxygen bubbles are produced for the fining process, concern for agitation of the free glass surface 62 can be relaxed. That is, the molten glass may be cooled considerably from the fining temperature during process steps downstream of the fining vessel, and therefore exhibit a higher viscosity. The removal of bubbles from the molten glass becomes more difficult as the viscosity increases.
• moving member 68 may comprise a rotatable shaft 70 and one or more agitating elements 72 extending outward from and coupled to the shaft.
• Shaft 70 extends from riser 46 and is coupled to a source of rotational motion, for example a hydraulic or electric motor (not shown). This source of rotational motion may be directly or indirectly coupled to shaft 70.
• shaft 70 may be directly coupled to and co-linear with a motor shaft, or indirectly coupled to the motor shaft through a drive mechanism, e.g. a gear box and/or chain drive.
• the at least one agitating element may be of a variety of designs.
• the agitating element 72 may comprise one or more vanes extending outward from the shaft, the vanes shaped to churn, circulate or rotate the molten glass as it passes up through riser 46.
• the vanes may be planar, curved, or exhibit more complex shapes as necessary to achieve a predetermined mixing efficiency.
• the vanes illustrated in FIGS. 1, 3 and 4 are planar, wherein a plane of the vanes is parallel with a longitudinal axis of the shaft 70. It should be noted that the longitudinal axis of shaft 70 may be parallel to and coincident with longitudinal axis 57 of riser 46.
• Moving member 68 may be designed with vanes that pass near the vertical wall of riser 46 to sweep bubbles away from the surface of the riser wall and into the center of the flow of molten glass.
• the flow of molten glass from melting furnace 12 through connecting pipe 16 to fining vessel inlet 50 and up riser 46 and then out riser outlet 56 through channel 48 is achieved at least in part by pressure exerted by the molten glass in melting furnace 12, which is at a higher level than the molten glass in fining vessel 14.
• movement of the molten glass can also be achieved by use of a pump to move the molten glass from melting furnace 12 through connecting pipe 16 up riser 46 and through channel 48.
• the at least one agitating element 72 may also be used to move or "pump" the molten glass by actively promoting an upward movement of the molten glass through the riser.
• the shaft and agitating element may be formed as a screw, wherein one or more helical agitating elements 72 spiral up the shaft toward the free glass surface with a suitable twist rate.
• a portion of a moving member 68 is shown in FIG. 5, wherein a plurality of helical agitating elements 72 are coupled to a shaft 70.
• the agitating elements 72 may be formed as blades in a propeller or fan, being oriented to provide an upward thrust to the molten glass and move the molten glass and oxygen bubbles upward to the free glass surface.
• a plane of the blades is not parallel with the longitudinal axis of the shaft 70.
• the agitating elements 72 of moving member 68 need not be completely submerged within the flow of molten glass 38, as is desired for mixing operations downstream of fining vessel 14.
• an exposed agitating element extending through the free glass surface may result in lapping of the free glass surface, thereby capturing gases disposed above the free glass surface into the molten glass.
• the higher viscosity of molten glass downstream of the fining vessel can make removing such captured pockets of gas difficult.
• Positioning a moving member so that at least a portion of an agitating element is at or above the free glass surface 62 within fining vessel 14 may be beneficial to spread bubbles contained within the molten glass evenly across the free glass surface, and thus provide the bubbles increased interaction with the molten glass flow. As shown in FIG. 4, at least a portion of the uppermost agitating element 72 extends upward from the free glass surface 62 by a distance di.
• Moving member 68 may include a rotatable stirrer comprising shaft 70 and an agitation element 72 coupled to and extending outward from the shaft, and where the moving member is positioned within riser 46 so that a distance d2 between a floor 74 of the first portion of fining vessel 14 (riser 46) and an uppermost point 76 on the agitating element 72 is greater than a distance d 3 between floor 74 of riser 46 and the lowest point 78 on an interior surface of a wall 81 of the second portion (channel 48).
• a rotatable stirrer comprising shaft 70 and an agitation element 72 coupled to and extending outward from the shaft, and where the moving member is positioned within riser 46 so that a distance d2 between a floor 74 of the first portion of fining vessel 14 (riser 46) and an uppermost point 76 on the agitating element 72 is greater than a distance d 3 between floor 74 of riser 46 and the lowest point 78 on an interior surface of a wall 81 of the
• agitation of the molten glass within riser 46 may be accomplished by flowing the molten glass over or through stationary members 80 positioned within riser 46, wherein the stationary members 80 passively redirect the flow of the molten glass, thereby promoting agitation and mixing of the molten glass.
• the stationary members e.g. baffles
• the stationary members may be coupled to an inside surface of a wall of riser 46 and extend into the flow of molten glass, being oriented so that an otherwise substantially laminar flow of molten glass 38 is redirected into a non-laminar flow, at least within proximate vicinity of the baffles.
• the stationary components may define passages extending through the stationary components to obtain more turbulent flow.
• stationary components for directing the molten glass within the riser may have a variety of configurations as may be necessary to obtain appropriate agitation without unnecessarily restricting flow of the molten glass.
• both moving members and stationary members may be employed within riser 46.
• Moving member 68, or stationary member 80 may be formed from a precious metal or a precious metal alloy (e.g. platinum group metal or platinum group metal alloy) as previously described, such as pure platinum or a platinum-rhodium alloy.
• a precious metal or a precious metal alloy e.g. platinum group metal or platinum group metal alloy
• Agitating the molten glass within a short distance of melting furnace 12, when the molten glass has or will shortly be at its highest temperature, and thus its lowest viscosity, such as within fining vessel 14, may also: 1) eliminate uneven flow of the molten glass.
• Molten glass that has not been agitated tends to flow faster at the center of the flow thereby causing stagnate glass to form along the periphery of the flow and affecting the quality and consistency of the glass; 2) eliminate chemical inhomogeneities that can result in striae, striations or cord that may cause visual defects in the final glass article, and; 3) thoroughly mix into the molten glass ceramic materials that may have dissolved into the glass during the melting process such as zirconium or aluminum oxide, thereby preventing crystallization of such materials.
• Agitating and heating the molten glass in riser 46 can also aid in the formation of oxygen bubbles, and facilitate their movement to free glass surface 62 without resulting in significant corrosion of the riser walls when the walls of the riser are substantially vertical.
• oxygen-comprising bubbles come in contact with the metal interior surfaces of the fining vessel, or other structures of glass making apparatus 10 that are constructed from precious metals or precious metal alloys, for any length of time the metal surfaces may undergo corrosion. If left unchecked, such corrosion may weaken and eventually cause a failure of the structure. This is a particularly relevant consideration for the fining vessel, since, being closest to the melting furnace, the fining vessel is most likely to have the highest accumulation of bubbles dispersed within the molten glass.
• riser 46 By orienting riser 46 substantially vertically, gas bubbles rising upward through riser 46 move directly up to the free glass surface 62 without remaining in contact with the surfaces of the riser for an appreciable time. That is, bubbles contained within the molten glass occupying riser 46 rise through the riser and are released through the free glass surface into the atmosphere 64 within the riser and do not dwell on the precious metal surfaces thereof. The same is true as the molten glass travels along channel 48, as the free glass surface 62 extends throughout the length of channel 48 and is continuous with the free glass surface within riser 46.
• the molten glass comprises a free glass surface that extends from the upper portion of riser 46 and along channel 48 to outlet 54 of fining vessel 14, gas bubbles will pass through free glass surface 62 into atmosphere 64 within the fining vessel.
• vent pipe 84 may be provided that extends through a wall of the fining vessel (e.g. wall 81) and forms a passage through the fining vessel wall between atmosphere 64 contained within the fining vessel and the atmosphere external to the fining vessel. If desired, vent pipe 84 may be connected to a pollution abatement system (not shown). In other embodiments, vent pipe 84 may be coupled to a vacuum source to actively draw gases from atmosphere 64.
• Vent pipe 84 may, for example, be provided near a downstream end of fining vessel 14, such as near fining vessel outlet 54 (e.g.
• FIG. 4 illustrates a pipe 90 coupled to cover 86 and extending through cover 86 to atmosphere 64 within fining vessel 14 (e.g. riser 46) so that one or more conditioning gases may be added to atmosphere 64.
• the conditioning gas may be an inert gas such as helium, argon or other inert gases and combinations thereof.
• the length of channel 48 in some embodiments is determined by the time it takes for all, or substantially all, of the gas bubbles to escape through the free glass surface 62 as the molten glass flows along channel 48.
• the velocity of the gas bubbles as they rise through the molten glass, the depth of molten glass 38 and the flow rate or average velocity of the molten glass as it flows down channel 48 are factors that can determine how long it takes bubbles to reach and escape through free glass surface 62 and thus help determine the minimum length of channel 48.
• the velocity of the gas bubbles as they rise through the molten glass is dependent on the density difference between the bubbles and molten glass 38, the minimum radius of the bubbles (the smaller the bubble the slower it moves) and the viscosity of the molten glass.
• the following or similar equation, based on Stokes law, can be used to calculate the velocity of the gas bubbles through the molten glass: were v B is the velocity of a bubble as it rises through the molten glass, ⁇ is the dynamic viscosity of the molten glass, g is the gravitational constant, a is the radius of the bubble, p' is the density of the bubble and p is the density of the molten glass.
• Stirring chamber 18 comprises stirring vessel 92 and a stirrer 94 rotatably mounted therein.
• Stirrer 94 may comprise a shaft 96 and a plurality of stirring elements 98 (e.g. vanes or blades) coupled thereto that mix and homogenize the molten glass.
• the molten glass may flow downward through stirring chamber 18.
• Stirrer 94 may be designed to be a flow neutral stirrer positioned so that during rotation the stirring elements do not impart an appreciable pumping action on the molten glass flowing through stirring chamber 18.
• Stirring elements 98 are also a sufficient distance beneath second free glass surface 100 that stirrer 94 does not disturb second free glass surface 100 as the molten glass flows through stirring chamber 18.
• Second free glass surface 100 is an interface between molten glass 38 and a second atmosphere 102 contained within stirring vessel 92 above the molten glass. Additionally, since the molten glass flowing through stirring chamber 18 is cooler than the molten glass flowing through fining vessel 14, the viscosity of the molten glass flowing through stirring chamber 18 is greater than the viscosity of the molten glass flowing through fining vessel 14. Thus placement of stirring chamber 18 along the flow path of molten glass 38 is determined at least in part by the maximum viscosity capable of being effectively stirred by stirrer 94.
• the molten glass flows to collection vessel 22 where it is directed by downcomer pipe 26 to forming body 30, where the molten glass may be formed into glass ribbon 40.

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