Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01F—MIXING, e.g. DISSOLVING, EMULSIFYING OR DISPERSING
  • B01F27/00—Mixers with rotary stirring devices in fixed receptacles; Kneaders
  • B01F27/05—Stirrers
  • B01F27/11—Stirrers characterised by the configuration of the stirrers
  • B01F27/112—Stirrers characterised by the configuration of the stirrers with arms, paddles, vanes or blades
  • B01F27/1123—Stirrers characterised by the configuration of the stirrers with arms, paddles, vanes or blades sickle-shaped, i.e. curved in at least one direction
• • 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
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01F—MIXING, e.g. DISSOLVING, EMULSIFYING OR DISPERSING
  • B01F27/00—Mixers with rotary stirring devices in fixed receptacles; Kneaders
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01F—MIXING, e.g. DISSOLVING, EMULSIFYING OR DISPERSING
  • B01F27/00—Mixers with rotary stirring devices in fixed receptacles; Kneaders
  • B01F27/05—Stirrers
  • B01F27/11—Stirrers characterised by the configuration of the stirrers
  • B01F27/115—Stirrers characterised by the configuration of the stirrers comprising discs or disc-like elements essentially perpendicular to the stirrer shaft axis
  • B01F27/1152—Stirrers characterised by the configuration of the stirrers comprising discs or disc-like elements essentially perpendicular to the stirrer shaft axis with separate elements other than discs fixed on the discs, e.g. vanes fixed on the discs
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01F—MIXING, e.g. DISSOLVING, EMULSIFYING OR DISPERSING
  • B01F27/00—Mixers with rotary stirring devices in fixed receptacles; Kneaders
  • B01F27/50—Pipe mixers, i.e. mixers wherein the materials to be mixed flow continuously through pipes, e.g. column mixers
• • 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
  • C03B5/1875—Stirring devices; Homogenisation with moving elements of the screw or pump-action type
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01F—MIXING, e.g. DISSOLVING, EMULSIFYING OR DISPERSING
  • B01F2215/00—Auxiliary or complementary information in relation with mixing
  • B01F2215/04—Technical information in relation with mixing
  • B01F2215/0409—Relationships between different variables defining features or parameters of the apparatus or process
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01F—MIXING, e.g. DISSOLVING, EMULSIFYING OR DISPERSING
  • B01F2215/00—Auxiliary or complementary information in relation with mixing
  • B01F2215/04—Technical information in relation with mixing
  • B01F2215/0413—Numerical information
  • B01F2215/0436—Operational information
  • B01F2215/044—Numerical composition values of components or mixtures, e.g. percentage of components
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01F—MIXING, e.g. DISSOLVING, EMULSIFYING OR DISPERSING
  • B01F27/00—Mixers with rotary stirring devices in fixed receptacles; Kneaders
  • B01F27/80—Mixers with rotary stirring devices in fixed receptacles; Kneaders with stirrers rotating about a substantially vertical axis

Definitions

• This invention relates to the homogenization of molten glass and, in particular, to the homogenization of molten glass which will be used to produce high quality finished products having low levels of defects.
• a particularly important application of the invention is in the manufacture of glass substrates for liquid crystal displays, such as the substrates used in the production of AMLCDs.
• 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 and, in particular, with achieving high throughput and high mixing efficiency (mixing effectiveness) from such a chamber without introducing defects into the resulting glass, specifically, defects arising from the erosion of the wall of the stir chamber and/ or the surfaces of the stirrer as a result of the mixing process.
• a simple way of picturing what a stirrer does under laminar flow conditions is to think of the cord as lumps of off-composition glass surrounded by glass of desired, or parent, composition. Each piece of cord can be thought of as having an interface between it and the parent glass.
• a measure of the total inhomogeneity of the glass is the total interfacial surface area of the cord. The minimum interfacial surface area occurs when all of the cord is in one spherical lump. As the lumps are broken into smaller parts and are stretched out into flat planes, the interfacial surface area is increased despite the fact that the volume of cord remains the same.
• a measure of the efficiency of stirring (also referred to herein as the effectiveness of stirring) is the ratio of the increased interfacial area after stirring to that before stirring.
• a stirring system should perform the following three functions: [0006] (1) It should stretch the individual lumps of inhomogeneous glass into thin streaks. This function requires the application of shear stress to the glass.
• One of the objects of the present invention is thus to minimize the creation of precious metal inclusions during the stirring of molten glass.
• this primary object is supplemented by the objects of: (1) maintaining high glass throughput, and (2) maintaining high stirring efficiency (e.g., low levels of cord).
• These latter objects and the primary object pull in opposite directions, e.g., one can reduce stirrer speed to reduce sheer stress and thus erosion, but reduced stirrer speed means less efficient stirring and /or reduced throughput.
• the present invention is able to simultaneously achieve these seemingly contradictory goals by means of relationships between stirrer speed, stirrer/ stir chamber geometry, and glass viscosity which allow sheer stress to be reduced below the level where unacceptable levels of inclusions are formed (e.g., the sheer stress acting on the stirrer and the stir chamber wall can be made less than 3.5 x lO 3 N/m 2 ) while at the same time stirring efficiency and throughput are maintained at levels previously only achieved with high sheer stirring.
• the invention provides a method for homogenizing molten glass comprising: [0017] (a) providing a cylindrically-shaped, substantially vertically-oriented stir chamber which comprises a wall having an internal diameter Dwati;
• stirrer within the stir chamber, said stirrer comprising a substantially vertically-oriented shaft and a plurality of blades which extend outward from the shaft towards the wall of the stir chamber, the maximum diameter of the blades being
• N, T, V, D wa u, Dbiade, and ⁇ are selected to satisfy the relationships:
• NTV/ ⁇ 0 * 5 > 5.0 kilograms/ second
• the invention provides apparatus for practicing the above method.
• the invention provides apparatus for homogenizing molten glass comprising: [0024] (a) a cylindrically- shaped, substantially vertically- oriented stir chamber which comprises a wall; and [0025] (b) a stirrer within the stir chamber, said stirrer comprising a substantially vertically-oriented shaft and a plurality of blades which extend outward from the shaft towards the wall of the stir chamber;
• the stir chamber comprises a side exit port such that glass exiting the stir chamber undergoes a change in flow direction and the stirrer comprises at least one member for stirring the glass in the region of the change in flow direction, said at least one member having a longitudinal axis that is substantially parallel to, but not colinear with, the stirrer's shaft.
• the third aspect of the invention is used with the first and /or second aspects of the invention.
• a preferred application for each of the above three aspects of the invention is in the manufacture of liquid crystal display glass.
• the number of precious metal inclusions e.g., platinum-containing inclusions
• the number of precious metal inclusions is preferably less than 20 per kilogram of finished glass.
• Figure 1 is a schematic diagram illustrating various of the stirrer/ stir chamber dimensions discussed herein.
• Figure 2 is a perspective view of a stirrer which can be used in the practice of the invention.
• Figure 3 is a side view, partially in section, showing the stirrer of Figure 2 installed in a stir chamber of the type which can be used in the practice of the invention.
• Figure 4 is a front view, partially in section, of the stirrer/ stir chamber combination of Figure 3.
• Figure 5 is a schematic drawing illustrating in stylized form the flow of molten glass through the stir chamber of Figures 3 and 4.
• Figure 6 is a plot of shear stress versus the product of glass flow times stirring effectiveness
• Figure 7 is a plot of inclusions per kilogram in the finished glass versus calculated shear stress on the stirrer and stir chamber wall.
• diamond data points represent historical glass stirring systems and square data points represent systems employing the present invention.
• flow direction indicators including flow arrows, flow contours, and flow tubes
• stirrer/ stir chamber design was to generate high levels of shear stress so as to achieve high levels of stirring effectiveness. Shear stress is created near the wall 19 of a stir chamber 11 by close-clearance between the stirrer blades 15 and the wall and by blade speed.
• shear stress ⁇ acting on the surfaces of the stirrer and the inner surface of the stir chamber wall can be expressed as:
• shear stress levels can be reduced by reducing N and/ or reducing Dbiade and/ or increasing C.
• simply reducing shear stress is not commercially acceptable since in the end, stirring must produce suitably homogenized glass at practical flow rates. Accordingly, for a practical system, reductions in shear stress should not be at the expense of stirring effectiveness or flow rate.
• the swept volume V will normally be less than the overall volume of the stir chamber which, as shown in Figure 1, extends from entrance port 21 to exit port 23. As also shown in this figure, the swept volume can overlap with a portion of the exit port. Similarly, although not shown in Figure 1 , the swept volume can overlap with a portion of the entrance port. When such overlapping occurs, the stirrer's blades will be coupled with the stir chamber wall through only a portion of each rotation in the area of the overlap.
• T P/N « ⁇ kNBD b iade 3 /C (5) where P is the power applied to the stirrer.
• E is preferably greater than 80, more preferably greater than 100, and most preferably greater than 120, while the Q*E product is preferably greater than 5.0 kilograms/ second, more preferably greater than 7.5 kilograms/ second, and most preferably greater than 10.0 kilograms/ second.
• T needs to be kept below a value where the stirrer' s shaft will exhibit substantial creep as a result of torsional stress at the operating temperatures employed (e.g., operating temperatures between ⁇ 1350°C and ⁇ 1500°C).
• T is preferably less than 75 newton-meters.
• FIGs 2-5 show one example of the types of stirrer systems with which the present invention can be used.
• the stirrer can be of the general type disclosed in commonly- assigned U.S. Patent No. 2,569,459 to C. F. De Voe, the contents of which are hereby incorporated by reference.
• Other stirrer designs can, of course, be used in the practice of the invention.
• 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).
• stirrer 13 is preferably equipped with members (fingers) 25 which have a longitudinal axis (long dimension) that is substantially parallel to, but not colinear with, the stirrer's shaft 17.
• the number of members 25 used can vary between, for example, 1 and 7, and depending on the particular system, their length can be between, for example, 1 and 4 inches. In practice, three members each having a working length of -2.2 inches have been found to work successfully for an exit port whose height is 6 inches. For comparison, the height of stirring blades 15 can be 2.5 inches.
• the members preferably extend below the bottom of shaft
• the stirrer 17 and preferably are composed of the same material as the stirrer's blades and shaft. All of the members preferably have the same height, although members having different heights can be used if desired.
• the circumferential width of the members can vary depending on location relative to the stirrer's shaft 17, e.g., for the stirrer of Figure 2, the width of the member closest to the shaft can be 2.0 inches, while those at the outer periphery of the stirrer can have a width of 1.5 inches.
• Members 25 serve to minimize increases in cord levels which have been found to result from (1) the slower rotational speed used in the stirring systems of the invention (e.g., rotational speeds between 3 and 15 rpm) and (2) the change in direction of flow which occurs as the glass enters exit port 23 from the main body of the stir chamber (e.g., a change in direction of 90°).
• Members 25 are distinguished from the blades 15 of the stirrer in that the members do not create substantial radial flow of the glass passing through the stir chamber while the blades do create such flow.
• stir chamber 11 can include a drain tube 31 for removing glass from the stir chamber during, for example, shut down of the system.
• the stir chamber can include a sump 27 as shown by dotted lines in Figure 1.
• members 25 preferably should extend below the top of exit port 23 but should not reach all the way to the bottom of the stir chamber.
• the ratio of the distance d between the bottom end of member 25 and the bottom of the stir chamber is preferably in the range:
• Example 1 compares two stirring systems, the first being representative of previously used stirring systems (e.g., a stirring system having a blade diameter of 6 inches, a glass viscosity of 3000 poise, and a stirrer rotational speed of 30 rpm), and the second one being representative of a stirring system designed in accordance with the invention (e.g., a stirring system having a diameter of 10 inches, a glass viscosity of 1000 poise (achieved by increasing the temperature of the molten glass by, for example, approximately 80°C), and a stirrer rotational speed of 6.3 rpm).
• a stirring system designed in accordance with the invention e.g., a stirring system having a diameter of 10 inches, a glass viscosity of 1000 poise (achieved by increasing the temperature of the molten glass by, for example, approximately 80°C), and a stirrer rotational speed of 6.3 rpm.
• the shear stress on the stirrer can be reduced almost by an order of magnitude by increasing the stirrer diameter from 6" to 10" and reducing the stirred glass viscosity from 3000 to 1000 poise. These changes allow a stirrer speed reduction by a factor of four.
• a useful byproduct is that the torsional stress on the shaft is also reduced, roughly by 25%, which helps extend stirrer life.
• Figures 6 and 7 show comparisons between historical stirring (diamond data points) and stirring performed in accordance with the present invention (square data points) .
• the data was obtained using actual production equipment. In all cases, stirring was performed using a substantially vertically-oriented stir chamber and a substantially non-pumping stirrer having a substantially vertically- oriented shaft.
• the level of inclusions is a linear function of shear stress, where shear stress is calculated using equation (2).
• the inclusion /kilogram value reaches zero when the shear stress is less than 1.1 x lO -3 N/m 2 .
• the data of Figure 7 was obtained using equipment whose resolution is limited to inclusions having a size greater than 10 microns. It is believed that the invention also reduces the level of inclusions whose size is less than 10 microns.

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• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Organic Chemistry (AREA)
• Mixers Of The Rotary Stirring Type (AREA)
• Glass Compositions (AREA)

Priority Applications (1)

Applications Claiming Priority (2)

Publications (1)

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Family Applications (1)

Country Status (6)

Cited By (5)

Families Citing this family (42)

Citations (7)

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• 2002
  • 2002-10-15 KR KR1020047008216A patent/KR100878605B1/ko not_active Expired - Fee Related
  • 2002-10-15 CN CNB02823863XA patent/CN100341806C/zh not_active Expired - Fee Related
  • 2002-10-15 WO PCT/US2002/032862 patent/WO2003048054A1/en not_active Ceased
  • 2002-10-15 JP JP2003549251A patent/JP4433382B2/ja not_active Expired - Fee Related
  • 2002-11-22 US US10/302,661 patent/US7127919B2/en not_active Expired - Lifetime
  • 2002-12-10 TW TW091136058A patent/TWI233431B/zh not_active IP Right Cessation
• 2009
  • 2009-10-16 JP JP2009239112A patent/JP5123272B2/ja not_active Expired - Fee Related

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