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
  • C03B5/187—Stirring devices; Homogenisation with moving elements
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P40/00—Technologies relating to the processing of minerals
  • Y02P40/50—Glass production, e.g. reusing waste heat during processing or shaping
  • Y02P40/57—Improving the yield, e-g- reduction of reject rates

Definitions

• This invention is directed to a method of homogenizing a molten glass, and in particular, a method for minimizing refractory metal inclusions incurred in a glass stirring operation.
• stir chambers 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 stirrer and the stir chamber are typically fabricated from a refractory metal, often a platinum group metal — ruthenium, rhodium, palladium, osmium, iridium, and platinum, or alloys thereof, but may be other high temperature, corrosion-resistant metals such as molybdenum.
• the present invention is concerned with the operation of such stir chambers without introducing inclusion defects into the resulting glass, specifically, defects arising from erosion of the stir chamber wall 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.
• This function can be achieved through the selection of blade shapes that push glass normal to the direction of bulk flow, i.e., blade shapes that produce at least some radial flow of the glass.
• shear stress may be increased by increasing blade speed, reducing the clearance (coupling) between the stirrer's blades and the wall of the stir chamber, reducing the temperature of the glass, or a combination of these actions.
• the present invention provides a method to reduce or eliminate refractory metal inclusions in the stirred glass.
• a method for homogenizing molten glass comprising, operating a stir chamber, the stir chamber including a rotating stirrer, the stir chamber and the stirrer including a refractory metal, rotating the stirrer at a plurality of different speeds N, as the molten glass flows through the stir chamber, where i indicates the number of speeds N and flowing molten glass through the stir chamber at a plurality of different glass temperatures T j , where j indicates the number of temperatures T.
• An actual refractory metal inclusion rate in the molten glass is determined for combinations of N 1 and T j , and a distribution of shear stress as a function of stir chamber and stirrer surface areas in contact with the molten glass is determined for the combinations of N, and T j .
• a threshold value of shear stress ⁇ 0 for each T j is calculated, as well as a refractory metal inclusion rate temperature dependence m t . From the above data a predicted refractory metal inclusion rate for a predetermined temperature and stirrer rotational speed can be calculated, and modifications to the stir chamber operation made in response to the predicted refractory metal inclusion rate to minimize refractory metal inclusions in the molten glass.
• N is between about 0 rpm and 100 rpm, more preferably between about 8 rpm and 18 rpm.
• Tj is preferably between about 1200 0 C and 1600 0 C, more preferably between about 1430 0 C and 1490 0 C.
• m t is between about 0 (zero) Tn -2 Pa "1 sec “1 and 1 (one) m ⁇ Pa ⁇ 'sec "1 in the temperature range between about 1200 0 C and 1600 0 C.
• the threshold shear stress ⁇ o may vary anywhere between about 1000 Pa and 10,000 Pa over the temperature range of 1200 0 C to 1600 0 C.
• FIG. 1 is a side elevation, cross sectional view of an exemplary stirring system, including a stir chamber and a stirrer rotatably mounted within the stir chamber.
• FIG. 2 is a diagrammatic representation of an embodiment of the inventive method.
• FIG. 3 is an exemplary plot of the distribution of shear stress vs. surface area of the stirrer and stir chamber exposed to the molten glass.
• FIG. 4 is a plot of refractory metal inclusions counted within samples of glass drawn from a stir system.
• FIG. 5 is a plot depicting refractory metal inclusion performance of several stir systems under a variety of operating conditions.
• shear stress is created near the inside wall surface 12 of a stir chamber 14 by close clearance between the stirrer blades 16 of stirrer 17 and the wall (coupling) and by the rotational speed of the blades.
• arrow 18 represents molten glass entering stir chamber 14
• arrow 20 represents molten glass leaving stir chamber 14.
• Blades 16 are attached to stirrer shaft 22, which is rotated at a predetermined rotational speed by motor 24.
• the shear stress ⁇ acting on the surfaces of the stirrer and the inner surface of the stir chamber wall can be expressed as:
• ⁇ ⁇ ⁇ ⁇ dv/ ⁇ c (1)
• ⁇ is the viscosity of the molten glass
• v is fluid velocity
• x is in a direction normal to the surface which experiences the shear stress.
• Equation (1) Applying equation (1) to a cylindrically-shaped stir chamber and a circularly symmetric stirrer having one or more blades of a common diameter D b i a de, for example, one obtains: where N is the stirrer speed in radians/second and C is the coupling distance between the blade tip and the stir chamber wall (i.e., in FIG. 1, For ⁇ in kilograms/meter-second, N in radians/second, and Dbi ade and C in meters, ⁇ is in newtons/meters squared (N/m 2 ). [0022] Through modeling of oil-flow based systems, stirring effectiveness E has been determined to be:
• an analytical model has been developed which may be applied to determine stirring operation conditions which minimize erosion of the stirrer and the stirrer chamber wall that can produce unacceptably high levels of refractory metal inclusions (e.g., levels of inclusions which are greater than, for example, 20 per kilogram of finished glass) that are of a size greater than a pre-determined size (e.g., inclusions having a size greater than 10 microns).
• refractory metal inclusions e.g., levels of inclusions which are greater than, for example, 20 per kilogram of finished glass
• a pre-determined size e.g., inclusions having a size greater than 10 microns
• shear stress can be reduced by reducing N and/or reducing D b i ade and/or increasing C.
• high shear stress should induce greater erosion of the stirrer and stir chamber surfaces, and hence more refractory metal inclusions in the glass.
• simply reducing shear stress to reduce refractory metal inclusions below a specified value is not always, in and of itself, a commercially viable practice, since in the end, stirring must still produce suitably homogenized glass at practical flow rates. For a practical stirring operation, reductions in shear stress should not be at the expense of stirring effectiveness or flow rate.
• a method of optimizing stirring process parameters that does not sacrifice stirring effectiveness or flow rate is highly desirable.
• shear stress is, in part, a function of location on the surfaces of the stirring system. That is, not all surfaces experience the same shear stress for a given set of operating conditions, and therefore not all surfaces of the stirrer and stir chamber are responsible for the same level of refractory metal inclusions. Accordingly, a method of correlating a refractory metal inclusion rate to the distribution of shear stress developed by the motion of the stirrer (and the stirrer blades) through the molten glass is presented.
• step 26 of method 28 comprises calculating a shear stress vs.
• stirrer/stir chamber design For multiple operating conditions and a given stirrer/stir chamber design. This may be accomplished by utilizing commercially available software to perform computational fluid dynamic (CFD) calculations using a suitable computing device (for example, a desktop computer).
• CFD computational fluid dynamic
• the stir chamber and stirrer are reduced to a virtual mesh configuration within the software that describes the stirrer arrangement, the stir chamber geometry, etc.
• the appropriate stir system operating conditions such as stirrer rotational speed and glass temperature/viscosity are also input to the software program.
• Output of the software can be configured to produce a plot of the shear stress on the refractory metal surfaces of the chamber and stirrer as a function of surface area in contact with the glass.
• An exemplary shear stress vs. surface area distribution representing the refractory metal surface area under the influence of a particular level of shear stress is shown by curve 30 in FIG. 3.
• step 32 of method 28 a series, or matrix, of designed experiments are performed to determine an actual refractory metal inclusion rate for at least several different stirrer rotation speeds and several different temperatures. A minimum of four experiments should be run, covering two different temperatures and two different stirrer rotation speeds. Additional experiments at additional temperatures and rotation speeds may be conducted as necessary or desired.
• the same stir chamber/stirrer design is used for each experiment, as well as the same glass composition. The number of refractory metal inclusions in glass drawn (removed) from the stir system may then be counted as a function of the weight of the drawn glass in order to determine a refractory metal inclusion rate. Counting of the refractory metal inclusions is a straight forward task.
• the number of refractory metal inclusions per unit weight of glass may be determined by shining a light through a sample of the glass which has been removed from the stir chamber to illuminate the inclusions, and counting the refractory metal inclusions through a microscope. Knowing the flow rate of the glass through the stirring system allows one to describe the refractory metal inclusion rate r in terms of inclusions per unit weight, or as a function of time.
• Table 1 An exemplary matrix of the inclusion-producing experiments is shown in Table 1 below, where Ti and T 2 represent two different temperatures, Ni and N 2 represent two different stirrer rotational speeds, and ri i, ri2, r2i and T22 represent the four resulting refractory metal inclusion rates. It is not necessary to restrict the matrix experiments to a 2x2 set of experiments. Additional experiments may be run as desired, and Table 1 reflects a broader set of experiments, where N, represents the i th stirrer rotational speed and T j represents the j th glass temperature, while r ⁇ represents the refractory metal inclusion rate for the i th stirrer rotation rate and the j th glass temperature.
• i and j depend upon the size of the matrix experiment which is run and simply designate the number of stirrer speeds and glass temperatures. For example, in an p-by-q matrix, i would have a value which is a whole number between 2 and p, while j would have a value which is a whole number between 2 and q. Additionally, it is not necessary that the added temperatures or rotational speeds be added symmetrically. That is, the matrix need not be symmetric. Therefore, one might use, for example, three temperatures and two rotational speeds.
• Step 34 of method 28 comprises determining the threshold value of shear stress that causes refractory metal inclusions above a predetermined rate.
• a model of refractory metal inclusions can be described by the equation: where: r is the rate that refractory metal inclusions enter the glass in inclusions/sec "1 ; m t is a temperature-dependent constant representing the ease with which refractory metal-to-refractory metal bonds may be broken, in units of In -2 Pa "1 sec "1 ; ⁇ is the shear stress applied at the stir chamber inside wall and on the stirrer blades, in Pascals (Pa); and
• A is the area under the applied shear stress ⁇ in m 2 ; ⁇ o is a threshold shear stress in Pascals (Pa).
• r may be any one of the r values determined from the matrix experiment presented in Table 1, i.e. T 1 1, ri 2 ...r, j .
• the parameter m t in equation (4) represents the contribution of the material properties of the refractory metal, and is dependent upon the strength of the refractory metal-to-refractory metal molecular bonds.
• the weaker the refractory metal the larger the value of m t .
• the m t parameter is also a function of the glass properties: the more reactive the glass is with the refractory metal, the larger the value of m t .
• T is the absolute temperature of the refractory metal in contact with the molten glass.
• Hi 0 and mi can be determined experimentally by measuring the refractory metal inclusion rate at several temperature points, while keeping other process conditions, such as the stirrer rotational speed N, constant.
• glass can be processed through the stir chamber, and the number of refractory metal inclusions in the formed glass may be counted as a function of the mass (i.e. weight) of glass at a plurality of different temperatures.
• m t is assumed to be a function of temperature only, and therefore independent of stirrer rotational speed.
• the threshold shear stress may be viewed as the shear stress below which, at a given temperature, the refractory metal inclusion rate is insignificant. Equation (4) can then be used to determine the threshold shear stress ⁇ o- That is, equation (4) may be set equal to itself, through m t , for the two different stirrer rotational speeds Ni and N 2 , but a single temperature, for example Ti :
• equation (6) may be solved for ⁇ o at the given temperature.
• equation (4) is properly an integration representing the area under each distribution curve between the shear stress for the onset of refractory metal inclusions, ⁇ o, and the maximum shear stress experienced by the system under the given conditions, Omax (where ⁇ o ⁇ Om 3x ):
• Equation (6) may then be solved for the second predetermined temperature T 2 and the two different stirrer rotational speeds Ni and N 2 , to calculate a ⁇ o value for a second temperature-stirrer speed combination.
• ITi 12 moexp(-mi/T 2 ) (9)
• Equations (8) and (9), representing the two unknown values mo and mi may then be solved for mo and mi, where m t i and m ⁇ represent the temperature dependence of refractory metal inclusion at the two different temperatures Ti and T 2 , respectively. Knowing mo and mi, an m t value for any temperature may thereafter be calculated. Once m t has been determined, equation (4) may be used to calculate a predicted refractory metal inclusion rate for any stir system fabricated using the same refractory metal and stirring the same glass composition, as shown in step 40 of method 28.
• this information may be used to modify a particular stirring operation (by adjusting the stirrer rotational speed and/or the glass temperature) to minimize the number of refractory metal inclusions.
• the refractory metal inclusion rate may be calculated for a variety of different process conditions and plotted to show the process conditions required to produce a minimum of refractory metal inclusions.
• other stir system designs may be evaluated by development of the CFD-generated stress vs.
• ⁇ 0 was determined to be in the range of 3000-3200 Pa for both 1430 0 C and 1460 0 C, and approximately 1420 Pa at 1490 0 C.
• the coefficient of temperature dependence of the refractory metal inclusion ranged from 0.18 ITf 2 Pa '1 sec "1 to 0.65 In 2 Pa *1 sec "1 over the temperature range of 1430 0 C to 1490 0 C.
• the values of m t and ⁇ o obtained by following the preceding method are largely independent of stir system physical design, for a given set of refractory metal and glass composition. That is, for a stirrer/stir chamber design different than the original modeled system, but which utilizes the same refractory metal and glass composition, the coefficient of temperature dependence of refractory metal inclusions and the shear stress threshold determined above may be used to predict the refractory metal inclusion rate for different stir system designs: one need only generate appropriate distribution plots from the flow dynamics to account for the contributions to refractory metal inclusions due to the physical design of the system (e.g. the coupling distance, blade tip areas, etc.).
• the values of m t and ⁇ o determined above may be used to predict the refractory metal inclusion performance of the stir system regardless of the system's mechanical design.
• the plot shown in FIG. 5 depicts six curves.
• Curve 44 illustrates the refractory metal inclusion performance of the stir system used to generate the data presented in Table 2 above when operated over a temperature range from 1430 0 C to 1490 0 C and a stirrer rotational speed of 12 rpm.
• curves 46 - 54 reflect the predicted performance of two additional stir systems having physical designs different than the first system but fabricated from the same refractory metal and using the same glass composition over the same range of temperatures.

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• Organic Chemistry (AREA)
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• 2006
  • 2006-07-13 US US11/485,793 patent/US7578144B2/en not_active Expired - Fee Related
• 2007
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