Method and apparatus for homogenizing flat glass

Method and apparatus for making flat glass

The invention relates to a method and an apparatus for manufacturing flat glass.

Glass used for glazing windows, such as windows in buildings and cars, has high requirements for optical integrity. Glass of this type, commonly known as flat glass, although this glass product may be slightly curved, is required to transmit images without distortions that could be unpleasant to the human eye. It is thus desirable that the composition of the flat glass produced be largely uniform in order to avoid local differences in refractive power which could cause distortions in the transmitted image. The requirements for flat glass are considerably higher than for other types of glass, such as compressed and blown glass products (e.g. bottles) or fibers, where the quality of the transmitted image is not a major factor in their normal embodiments.

It is known that the main reason for the inhomogeneity of glass is the fouling of molten glass by refractory materials which come into contact with the glass during the melting process. The slow but continuous wear of ceramic refractory materials caused by molten glass creates streaks of different composition in the molten glass. To minimize the amount of these inhomogeneities in the glass stream withdrawn from the melting furnace, a large amount of molten glass in the melting furnace is used in the flat glass manufacturing process, leaving little or no contact with refractory materials. This technique has achieved only limited success and is expensive due to the large size of the required melting vessels and the energy required to maintain the recycling of molten glass. It would be desirable to reduce these costs and improve the homogeneity of the flat glass.

Confusion has long been used to improve homogeneity in glass melting processes. In the manufacture of bottle glass or the like, molten glass is usually agitated in a pre-furnace immediately before the glass product is formed, but agitation at a similar stage in the flat glass manufacturing process has previously been found to degrade the quality of the glass in question. This difference is due not only to the fact that a higher distortion tolerance is allowed in the bottle glass or the like, but also to the fact that the flat glass is usually made at lower temperatures than the bottle glass and that confusion is apparently ineffective at these lower temperatures. The flat glass is occasionally agitated, as described in U.S. Patent Nos. 4,046,546 and 4,047,918, but the agitation generally occurs in the hotter areas of the melting furnace, which are considerably upstream of the area in which the formation of the flat glass begins. Thus, in such cases, a considerable distance must be provided between the agitation and planarization areas to cool the molten glass to the formation temperature, and as a result, contact with the refractory materials occurs after the agitation operation. It would be desirable to agitate the flat glass immediately prior to its formation to improve the homogeneity of the glass and not otherwise degrade the quality of the glass with respect to distortions.

The ability to shake the container glass immediately before forming it also provides the ability to add dyes to the glass at this point. This is advantageous because it allows the color of the glass to be changed quickly and cheaply, since only a small amount of residual glass is

5 580 included in this color change. Because of the difficulty of stirring flat glass downstream, as described above, this method of changing the color of the glass has generally not been possible in the manufacture of flat glass. Instead, the dyes for flat glass are usually fed to a melting furnace mixed with other raw materials. As a result, all the molten glass in the melting furnace has a certain color, and in order to change the color, essentially the entire contents of the melting furnace must be removed from the furnace. Such a color change method is time consuming and expensive, and it would be desirable to provide a more efficient way to change colors during the flat glass forming process.

The present invention relates to a process for producing flat glass, in which molten glass is mixed before its temperature has dropped below its formation temperature so that the glass is substantially homogenized, and the homogenized glass is passed to a layer of molten metal in the forming zone. . The method is characterized in that the mixing of the molten glass is performed while it is supported by the molten metal and that the glass is supported by the molten metal from the beginning of the mixing until a flat glass sheet is formed from the glass.

The invention also relates to an apparatus comprising a container for receiving and storing molten glass, means for mixing molten glass in the container, a forming chamber containing molten metal on which the glass coming from the container is made to float to form a flat glass sheet. The device is characterized in that the vessel contains a molten metal pond which communicates with the molten metal in the forming chamber.

The present invention provides a method and apparatus for mixing flat glass immediately prior to forming the glass into an optically high quality flat glass product. The inhomogeneities in the molten glass that may result from contact with surfaces made of refractory materials are thus alleviated by agitation at a point where there is little or no possibility of further inhomogeneities occurring before the glass is formed into a sheet. In the context of the present invention, the optical quality of the flat glass is improved by stirring the molten glass before its temperature has dropped from the cleaning temperature to its forming temperature. For a standard soda-lime-silicon glass mixture, it has been found advisable to mix at a glass temperature of at least 2200 ° F (1200 ° C). Since this temperature occurs significantly upstream of the point where the formation begins in a normal flat glass process, special measures are required to avoid distortive fouling between the point of confusion and the point of commencement of the formation operation. These measures comprise two options: starting a forming operation at very high temperatures or preventing dirty contact with refractory materials as the glass cools to a more conventional forming temperature.

Initiating the formation of glass as a flat glass sheet to confuse glass at or near the temperatures used in the present invention is problematic when using a conventional flotation process because the viscosity of the glass at such temperatures is too low for efficient use of mechanical perforators. The relatively high temperatures that occur would also negatively affect the wear rate of the feed structures used in conventional flotation operations, to which molten glass is typically fed at temperatures below about 2000 ° F (1100 ° C). However, it has now been found that homogenization below the standard requirements for flat glass can be achieved without degrading the quality of the product if the molten glass is agitated at relatively high temperatures and immediately subjected to a forming operation in which the glass can be treated at such temperatures. An example of a suitable flat glass forming process is described in U.S. Patent No. 4,395,272 (Kunkle et al.), Which uses a pressurized chamber to thin the glass to the desired thickness. Because such a forming process can treat glass at or slightly below the stirring temperature, forming the glass into a flat strip can be initiated a short distance downstream of the stirring operation, eliminating the need for a long length of intermediate path where glass integrity could be compromised. According to the present invention, the support layer formed by the molten metal starts upstream of the forming chamber and preferably extends through the agitation zone into the forming chamber, whereby the main contact area with the refractory materials is eliminated. Such an arrangement also advantageously allows the lip or threshold to be removed from the inlet point of the molten glass into the forming chamber, thereby eliminating an element prone to relatively high wear during a conventional flotation forming operation.

An alternative solution according to the present invention, which is different from the formation of glass at high temperatures, is to avoid dirty contact with refractory materials downstream of the confusion zone as the glass cools to a suitable formation temperature as it flows towards the formation chamber. Thus, the channel between the agitation zone and the forming chamber can be coated with a layer of molten metal, such as tin, which is an extension of the molten metal in the forming chamber. The confusion zone can also be provided with such a protective layer, although it is not essential. In particular, it has been found advantageous to provide a support surface formed by the molten metal in the agitation zone for the glass, as it reduces the friction in the molten glass container. This results in a higher average feed rate through the blending zone, allowing product or color changes to be made more quickly.

The present invention is in principle independent of the particular flat glass forming technique used, but can nevertheless be used in a particularly advantageous manner in connection with a floating forming process in which a continuous flat glass strip is formed by casting molten glass on the surface of a molten metal pond.

The molten glass fed to the agitation chamber of the present invention can be melted and cleaned by any technique suitable for making flat glass, but since the agitation and preferably also the forming process is initiated at exceptionally high temperatures, the melting furnace temperature treatment section may be shorter than usual. It is preferred that in the context of the present invention, the agitation is applied to the forming chamber in the forward flow of glass instead of the recycled glass mass. For this reason, it is recommended to use devices to prevent the return flow of the glass from the agitation zone towards the melter. Thus, the most preferred arrangement in which there is no return flow of the glass is to pour the molten glass vertically into the agitation zone. This vertical feeding to the agitation zone avoids the material problems associated with using a barrier to counteract the backflow that would exist when using the horizontal feeding method. In addition, the vertical feed method is well compatible with cleaning techniques such as those described in U.S. Patent 4,600,426 (Schwenninger). In this arrangement, the glass passes down through a vertical elongate vessel and is removed through the base portion. By means of such an arrangement, the purified glass can advantageously be fed directly into the mixing chamber according to the present invention.

The agitation arrangement of the present invention not only improves the optical quality of the flat glass, but can also be used to homogenize dyes or other substances added to the glass after cleaning the glass. This allows the color or composition of the glass to be changed quickly and cheaply thanks to the small amount of residual glass that is affected during the product change.

The feature of the invention associated with the support of the molten metal formed by the embarrassment includes advantages that are not limited to the manufacture of higher quality glass products, such as flat glass. The low frictional resistance of the glass flow and the resulting ease of product changes are also advantageous in the manufacture of other types of glass, such as tank glass or tableware.

Further details and other advantages of the present invention will become apparent from the accompanying drawings and the following detailed description. In the drawings:

Figure 1 shows a longitudinal cross-sectional view of a preferred embodiment of the present invention in which a molten metal pool forms a support surface for molten glass extending continuously through an agitation chamber into a high temperature flat glass forming chamber;

Fig. 2 is a longitudinal cross-sectional view of an embodiment of the present invention including a support surface formed by molten metal in a mixing and forming chamber and a cooling zone therebetween to reduce the temperature of molten glass to a conventional flat glass forming temperature;

Fig. 3 shows a longitudinal cross-sectional view of an alternative arrangement according to the invention for feeding purified molten glass to a mixing chamber according to the present invention, whereby a conventional tank type melting and cleaning furnace can be fed molten glass to the mixing chamber by vertical pouring;

Fig. 4 shows a longitudinal cross-sectional view of another alternative molten glass feed arrangement such as Fig. 3 using a plunger-controlled bottom outlet;

Figure 5 shows a longitudinal cross-sectional view of another arrangement for feeding molten glass into an agitation chamber according to the present invention, with the purified molten glass being fed horizontally into the agitation chamber over a threshold member.

Figure 1 shows an embodiment of the present invention including an agitation chamber 10 into which purified molten glass 11 flows continuously from any suitable melting and cleaning furnace known in the art. As shown in the figure, in this preferred arrangement, the feed flow of the glass passes vertically inside the agitation chamber 10, whereby the return flow is blocked. In a particular example similar to Figure 20, this vertical flow occurs through the bottom opening of the cleaning vessel 12 or other tank upstream. In the arrangement shown, the flow from the vessel 12 can be controlled by means of a valve member 13, which may be of the type described in U.S. Pat. No. 4,604,121 (Schwenninger). A useful detail shown in the figure, which does not form part of the present invention, is a rod 14 extending downwardly from the valve member 13 to provide a regular streamlined path for vertically flowing glass mass to avoid air entrapment in the glass as it enters the glass mass 11 inside the mixing chamber.

As with all other embodiments of the present invention, the glass is now preferably above 2200 ° F at 35 (1200 ° C) during agitation. Thus, the glass flow into the agitation chamber 15 is at least at this temperature. More suitably, no significant amount of heating is used in the agitation chamber, and thus the incoming glass stream 15 is usually at a temperature slightly above the minimum agitation temperature as the glass cools somewhat as it passes from the cleaning device 12 to the forming chamber 16. There is no actual upper limit on the temperature of the glass entering the agitator chamber, but in practice it is most likely that the glass would be slightly below the peak cleaning temperature applied to it during the upstream cleaning process, which is generally below about 2800 ° F (1500 ° C). In addition, in practice, in some cases it may be desirable to allow the glass to cool well before entering the agitation chamber, for example to about 2400 ° F (1300 ° C) or below, to increase the life of elements in contact with molten glass, such as agitators.

The present invention is not limited to any particular construction of the stirrer, but any previously proposed apparatus for stirring molten glass may be used. Some equipment arrangements may be more efficient in homogenizing the glass, but the number of agitators and their rotational speed can still be selected to compensate for efficiency variations. The particular stirrer structure shown in each drawing is a preferred example in that it has a strong mixing effect and is readily available. Another potentially suitable embodiment is described in U.S. Patent No. 4,493,557 (Nayak et al.). Each agitator 20 shown in Figure 1 comprises a helical agitator portion disposed in the bottom portion of the shaft, each portion being molded from a ceramic refractory material. To prevent air from being drawn into the molten mass, it is recommended to rotate the helical agitators in such a direction that they draw the molten glass mass upwards towards the surface. This direction of rotation also prevents additives that may have deposited on the surface of the melt in the agitation chamber from being wiped out prematurely, and when centered as streaks, into the active agitation zone. The actuators for rotating the agitators can be of any type suitable for this purpose and the agitators can be used individually or in groups. For convenience, for example, the agitators in the transverse row may be rotated in the same direction, and in order to increase the shear forces on the glass, it is recommended to rotate the adjacent transverse row in the opposite direction, as shown in the figures. However, it will be appreciated that any rotational pattern may be used in connection with the present invention as long as sufficient homogenization is achieved. In order to achieve good homogenization, it is desired that substantially the entire cross-sectional area of ​​the molten glass in the agitation chamber be agitated and the number and size of agitators may be selected accordingly. Thus, in the embodiment of Figure 1, the helical portion of each agitator substantially corresponds to the depth of the molten glass, and a series of closely spaced agitators are used which actively affect the entire width of the molten material in the agitator chamber. The degree of homogenization is also affected by the confusion factor caused by each addition of melt and the flow rate. Thus, it is recommended to use several rows of stirrers so that each addition mass of glass is repeatedly subjected to stirring forces as it passes through the stirring chamber. The number of stirrer rows depends on the desired degree of homogenization and the flow rate of the glass. As a general guide, one stirrer may be used for each daily production of 10 tonnes of average quality flat glass. Obviously, for some applications, lower quality requirements may allow the use of fewer mixers.

On the other hand, the use of a large number of stirrers usually results in better results. The use of a larger number of agitators than required, on the other hand, has no significant disadvantages other than the cost involved.

The walls of the agitation chamber 10 of the embodiment shown in Figure 1 may be made of a molten ceramic refractory material whose contact may contaminate the molten glass to such an extent that significant optical distortions may occur in the flat glass product. Thus, in this embodiment, the molten glass is fed to the forming chamber 16 immediately after agitation to minimize the surface area of ​​the refractory material in contact with the glass after agitation. In Figure 1, the vertically adjustable barrier plate 21 controls the flow of molten glass from the agitation chamber to the surface of the molten metal pond 23, this pond being mostly molten tin. The glass mass forms a strip 24 of lesser thickness and cools as it is pulled along the molten metal pond until it cools sufficiently for extraction from the surface of the molten metal without damaging the surface of the glass strip. Because the molten glass is agitated at relatively high temperatures and immediately fed to the forming chamber, the glass enters the forming chamber at a temperature higher than the conventional float-type forming process. The temperature of the glass may drop slightly from the stirring temperature above 2200 ° F (1200 ° C), but the glass generally enters the forming chamber before cooling to the feed temperature of the conventional floating process of about 1900-2000 ° F (1040-1090 ° C). Typically, the glass entering the forming chamber of the embodiment of the present invention shown in Figure 1 is at a temperature of at least 2100 ° F (1150 ° C) at which the viscosity of the glass is not sufficient to thin the glass strip to the desired thickness in the forming chamber by mechanical means. Thus, a forming process using a higher thermal space inside the forming chamber, preferably the process described in U.S. Patent No. 4,395,272 (Kunkle et al.), Is useful in those embodiments of the present invention in which agitated glass is fed to the forming chamber at a relatively high temperature. Other pressurized glass-forming processes may also be used, although not as preferred, for example, those described in U.S. Patent Nos. 3,241,937 (Mivhalik et al.) Or 3,432,283 (Galey).

To add dyes or other additives to the molten glass in the mixing chamber, a screw feeder 29 may be used, which may, for example, extend horizontally from the side wall near the point where the glass flow 15 enters the mixing chamber. Suitable dyes are readily available and are usually in the form of concentrates, optionally containing a mixture of dyes, such as a metal oxide, mixed with a flux powder and bonded with sodium silicate or some other binder. Additives for purposes other than color change can be fed to the molten glass in the mixing chamber. Thus, a different glass mixture could be formed without the need to change its composition during the melting and purification steps.

In Figure 1, the molten metal 30 extends throughout the agitation chamber 10 and the forming chamber 16. This arrangement is recommended because it eliminates contact with the refractory material with the bottom and threshold of the agitation chamber, providing additional assurance that inhomogeneities cannot recur in the agitated glass. In addition, the elimination of the threshold eliminates an element that needs to be serviced and replaced from time to time, especially at the high temperatures that occur in connection with the preferred embodiments described herein. It will be appreciated that the molten metal layer 30 need not be extended to pass through the entire agitation chamber in an advantageous manner to avoid contact with the refractory material, but that only portions of the agitation chamber may be covered therein, particularly the downstream portion of the agitators 20.

In addition to forming an uncontaminated surface for the main area in contact with the molten glass, the use of molten metal as a support surface in the agitation chamber has other advantages. It has been found that the molten metal surface forms a very low frictional resistance on the molten glass moving thereon. As a result, the glass moves downstream through the agitation chamber relatively uniformly over its entire cross-sectional area, so that any changes in the color or composition of the glass can be realized relatively quickly with only a small glass loss during the transition phase.

The introduction of agitated glass into the forming chamber at high temperatures is considered a preferred feature of the preferred embodiments of the present invention, although some of the advantages of the invention can be achieved by feeding glass to the forming chamber at more conventional forming temperatures if harmful fouling due to contact with the refractory is avoided. An example of such an arrangement is shown in Figure 2, in which a cooling zone 40 is formed between the agitation zone 10 and the forming chamber 16. Bottom contact with the refractory material is avoided by forming a molten metal layer 41 (e.g., a layer of molten tin) that can continuously extend from the agitation chamber through the cooling zone to the forming chamber as shown in FIG.

Since maintaining optical quality standards associated with flat glass requires that the glass be agitated at relatively high temperatures in accordance with the present invention, it is within the cooling zone 40 of Figure 2 to allow the glass temperature to drop from the agitation temperature to the forming temperature. As mentioned above, the stirring temperature for a conventional soda lime-silicon glass mixture is preferably above 2200 ° F (1200 ° C) and the forming temperature is generally below 2000 ° F (1100 ° C). Thus, the length of the cooling zone is chosen so as to achieve a sufficient duration for the glass temperature to drop the required amount of glass. The cooling provided by the refractory walls of the cooling zone alone may be sufficient, but in some cases it may be desirable to shorten the length of the cooling zone by using cooling devices such as the cooling tubes 42 shown in Figure 2 above the molten glass in the cooling zone 40. Alternatively, compressed air currents can also be used to cool the glass to the formation temperature. In this example, the process of forming the molten glass into a flat strip need not involve high pressure, but any conventional flat glass forming technique may be used, such as conventional floating forming technique using mechanical damping devices that adhere to the side edge portions of the strip. The inlet arrangement to the forming chamber 16 of Figure 2 may advantageously include only a barrier plate 21 which regulates the flow of glass over the molten metal support surface 41.

The arrangement for vertical feeding of molten glass described in connection with Figure 1 may be used in each of the embodiments described, and Figures 3, 4 and 5 show examples of alternative arrangements for feeding glass into the mixing chamber, and these arrangements may be used in any of the embodiments described above. The molten glass supply arrangement shown in Fig. 6 preferably includes directing the molten glass flow 50 vertically to the agitation chamber 10. However, unlike Fig. 1, the vertical glass flow does not originate from the bottom of the cleaning vessel. from the cleaning container 52.

Figure 4 shows a similar vertical transfer of molten glass from a conventional cleaning vessel 55 to an agitation chamber 10. In this embodiment, the glass flow is monitored via a plunger 56 which cooperates with an outlet pipe 57 passing through the bottom of the vessel 55. This outlet pipe can be made of a refractory material such as platinum.

An example of an arrangement comprising a horizontal supply of molten glass to the agitation chamber 10 is shown in Figure 5. In this arrangement, the agitation chamber 10 is aligned horizontally with the end of a conventional tank-type cleaning vessel 60 by a recessed partition 61 separating the chambers and preventing molten glass The partition 61 can be provided with cooling ducts 62. The embodiment shown in Figure 5 is otherwise similar to the embodiment shown in Figure 1.

The characteristic composition of the soda-lime silicon referred to in this context is generally as follows:

Weight%

<td>SiO₂</td><td> 70-74</td>

<td>As₂0</td><td> 12-16</td>

<td>CaO</td><td> 8-12</td>

<td>MgO</td><td> 0- 5</td>

<td>^Α^ 2θ3</td><td> 0- 3</td>

<td>κ₂ο</td><td> 0- 3</td>

<td>BaO</td><td> 0- 1</td>

<td>Fe "0"</td><td> 0- 1</td>

Small amounts of dyes, detergents or impurities may also be present in the composition of the glass. The composition of most flat glasses is within the following limits:

Weight%

<td>SiO₂</td><td> 72-74</td>

<td>^As₂°</td><td> 12-14</td>

<td>CaO</td><td> 8-10</td>

<td>MgO</td><td> 3- 5</td>

<td>^^ 2θ3</td><td> 0- 2</td>

<td>κ₂ο</td><td> 0- 1</td>

<td>Fe_of0_o</td><td> 0- 1</td>

The operating temperatures described in connection with the present invention relate to the above flat glass composition. The stirring and forming temperatures for other compositions will vary according to the relationship between the temperature and viscosity of the glass mixture in question. To extrapolate the temperatures reported in this context for other glass compositions, the relationship between the temperatures and viscosities of soda-lime plita glass is given by way of example below:

Viscosity (Off) _____ Temperature _______

<td> 100</td><td> 2630</td><td>° F</td><td> 1443</td><td>° C</td>

<td> 1000</td><td> 2164</td><td>° F</td><td> 1184</td><td>° C</td>

<td> 10000</td><td> 1876</td><td>° F</td><td> 1024</td><td>° C</td>

<td> 100000</td><td> 1663</td><td>° F</td><td> 906</td><td>° C</td>

Those skilled in the art may make modifications to the present invention within the scope of the invention as defined in the appended claims.