JPH0242777B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION TECHNICAL FIELD OF THE INVENTION This invention relates to the use of subatmospheric pressures to promote the fining of molten glass and the like. More specifically, the invention relates to a practical arrangement for the continuous commercial scale use of such fining techniques.

BACKGROUND OF THE INVENTION In the melting of glass, substantial amounts of gas are produced as a result of the decomposition of batch materials. Other gases are either physically entrained by the batch material or introduced into the glass during melting from a combustion heat source. Most of the gas will escape during the initial phase of melting, but some will become trapped in the melt. Some of the trapped gas dissolves in the glass, while other parts form discrete gaseous inclusions known as bubbles or "seeds" that are left in unduly high concentrations in the product glass. If this happens, it becomes a hindrance. These gas inclusions rise to the surface and, given sufficient time, escape from the melt in a step of the glass manufacturing operation known as "fining." Elevated temperatures are conventionally applied in the fining zone to reduce the viscosity of the melt and promote the rise and escape of gaseous inclusions by enlarging the cell diameter. The energy required for the high temperatures used in the fining stage and the large melting vessels required to allow sufficient time for gaseous inclusions to escape the melt are major expenses in glass manufacturing operations. Therefore, it would be desirable to improve the fining process to reduce these costs.

It has been known that reduced pressure can aid the fining process by lowering the partial pressure of dissolved gases above the melt. Reducing the pressure will also increase the volume of bubbles within the melt, causing them to rise to the surface faster. The impracticality of providing an airtight vessel to draw a vacuum therein on the scale of a conventional clarification chamber is illustrated in U.S. Pat. No. 1,564,235;
2877280; 3338694 and 3442622, the use of vacuum fining has been limited to relatively small scale batch operations.

Continuous vacuum fining processes have been proposed, but have not been accepted for large scale, continuous glass production due to various drawbacks. US Patent No.
In the continuous vacuum clarifiers shown in 805139; 1589308; and 3519412, the major drawback is the need for relatively narrow vertical passages into and out of the vacuum zone due to the pressure differential. These passageways complicate the construction of such vessels, especially in view of the need for airtight walls, increase the exposure of the extrudate to contaminant resistant contact, and are significantly viscous to extrudate flow. Add resistance.

It is recognized that substantial glass height is required to balance even moderate vacuums. Changing the production rate of such systems is also problematic, especially in view of the viscous drag factor. Production rate flexibility is important in continuous commercial operations due to variations in the product being made (thickness, width) and economic factors that influence the desired production rate. In each of the three patents mentioned above, the driving force for increasing the velocity of flow through the vacuum zone passage is determined by the depth of the melt upstream of the vacuum zone relative to the depth of the melt downstream from the vacuum zone. It can be provided by increasing.
The magnitude of this level difference is further increased by the inherent viscous resistance in these systems. Since accelerated erosion of the side walls occurs at the level of the melt surface, a significant change in the level worsens the erosion, which in turn deteriorates the quality of the product glass.

A simpler structure is shown in US Pat. No. 3,429,684, in which batch material is fed through a vacuum lock and melted at the top of a vertically elongated vacuum chamber. Changing the production rate in that equipment appears to require changing the amount of vacuum imposed in the chamber, which adversely changes the fining performance. Melting the raw material within the vacuum chamber is another drawback of the device for several reasons. First, a large volume of foam is formed by carrying out the initial decomposition of the raw material under vacuum, which requires an adjoining tank of sufficient size to accommodate the foam. Second, feed material may follow a shortcut path into the product stream, thus negating proper melting and clarification. Thirdly, carrying out the initial stages of melting in a vacuum vessel and heating the melt to the fining temperature requires a large amount of heat to be supplied to the melt in the vessel.
Inputting such high heat into the vessel induces convection within the melt which increases wall erosion and leads to contamination of the clarified product stream. Fourth, the carbon dioxide released from the decomposition of the batch carbonate creates a relatively high partial pressure of carbon dioxide in the tank;
This at least partially negates the ability of the reduced pressure to remove carbon dioxide from the melt.

US Pat. No. 4,195,982 discloses first melting the glass under elevated pressure and then refining the glass in a separate chamber at lower pressure. Both chambers are heated.

US Patent No. 4110098 discloses a method of intentionally foaming glass to aid in fining. The foaming is induced by intense heating and chemical blowing agents at atmospheric pressure.

SUMMARY OF THE INVENTION In the present invention, there is provided a method and apparatus for using vacuum fining in a commercial scale continuous glass manufacturing process in a manner that advantageously and economically overcomes the deficiencies of the prior art. The molten glass is exposed to a vacuum after most of the thermal energy required for melting has been imparted to the melt so that little or no thermal energy needs to be supplied to the molten material contained within the vacuum chamber. You will be placed in the clearing room. Preferably, no more heat is added in the vacuum stage than is necessary to compensate for losses through the vessel walls. At sufficiently high production rates, the vacuum chamber is not completely heated by anything other than the incoming molten glass itself. In an embodiment of the invention, the batch material is first liquefied in a stage specifically adapted to that stage of the process, and the liquefied material is transferred to a second stage where dissolution of the solid particles is essentially complete;
and the temperature of the material is raised to a temperature that provides a viscosity suitable for clarification. The molten material is then sent to a vacuum chamber.
As a result, most of the gaseous products of dissolution are expelled before the material is subjected to vacuum, and the region of maximum gas evolution is separated from the fining zone, whereby the material subjected to the initial dissolution step is clarified. It cannot become mixed with the part of the melt undergoing the process. Most or all of the heat required for melting is met before the material enters the vacuum fining stage, and heating of the fining stage can be substantially avoided, thereby avoiding excessive convection of the melt in the fining zone. .

As a result, erosion of the vessel is reduced and the probability that partially clarified portions of the melt become mixed with clarified portions is reduced.

The assistance provided to the fining process by vacuum allows lower temperatures to be used for fining. Lower temperatures have utility not only for lower energy consumption but also for corrosive effects on the bath. Normally fined glass at peak temperatures on the order of 1540°C (2800〓) can reach temperatures around 1430°C (2600〓) or even 1370°C, depending on the vacuum level used.
(2500〓) or lower.

It is theorized that foam generation in the vacuum clarification chamber significantly enhances gas removal from the melt. The thin film and large surface area provided by the foam increases exposure to low pressure conditions and facilitates gas transport from the liquid phase. This is in contrast to conventional fining, in which residence time must be allowed to allow the closed cells to rise to the surface and escape from the viscous melt, which retains a large melt pool. It is necessary to do so. Thus, the vacuum fining of the present invention can achieve a given degree of fining in a fairly small space. In a preferred embodiment of the invention, the advantageous effect of exposing the foamed melt to vacuum is that when the material enters the vacuum chamber, before it enters the body of molten material held in the vacuum chamber, The incoming flow is then preferably enhanced by foaming the material before penetrating into the foam layer.

Another aspect of the invention relates to the benefits of controlling production in continuous fining operations. The liquefied material is metered into the upper end of the vacuum chamber through valve means and the clarified melt is conveyed from the lower end of the vacuum chamber through another valve arrangement. The height of the liquid maintained within the vacuum chamber is at least slightly greater than the height required to balance the vacuum so that the melt can flow out of the outlet by gravity. Also,
By providing a liquid height greater than the minimum required for withdrawal, the production rate is controlled by the valve without changing the vacuum pressure in the chamber and without changing the liquid level in the chamber. be able to.
Conversely, a range of vacuum pressures can be used without changing production rates. Apart from the valve, the system is provided with a relatively low resistance to the flow of molten material therethrough.

Not only is the production rate variable in a given installation of the present invention, but its effectiveness also varies with the size of the system, unlike conventional tank-type recirculating clarifiers, which do not operate effectively in low volume applications. are relatively independent.
Therefore, the present invention can be effectively applied to a wide range of glass manufacturing operations.

The preferred form of the vacuum clarification chamber is a vertically elongated vessel, most conveniently in the form of a vertical cylinder. The liquefied material is introduced into the head space above the molten material held in the vessel. Upon encountering reduced pressure in the headspace, at least a substantial portion of the material foams due to the release of gases dissolved in the material and the expansion of air bubbles and seeds present in the material. Formation of the foam greatly increases the surface area exposed to reduced pressure, thus aiding in the removal of gaseous species from the liquid phase. Producing the foam above the molten pool held in a bath rather than from the molten pool is advantageous to compress the foam and aid gas escape. It has also been discovered that depositing newly generated foam onto a foam layer promotes crushing of the foam. Another advantage of the vertically elongated geometry is that stratification occurs due to the remaining less dense foam or foam-containing material at the upper end, so that the overall mass transport is directed away from the foam region. and thereby make it less likely that unclarified material will become included in the product stream. Purging gases from the melt under reduced pressure reduces the dissolved gas concentrations in the melt below their saturation point at atmospheric pressure. As the molten material progresses downward toward the bottom outlet, increasing pressure based on the depth of the melt in the bath causes residual gas to remain in solution and the volume of small seeds that may remain to be reduced. reduce Gas dissolution may also be aided by reducing the temperature as the material progresses towards the outlet. Moreover, the low concentration of gas remaining after vacuum fining reduces the probability of bubble nucleation in subsequent steps of the glass manufacturing process, as is often a problem with conventional fining.

In the commercial melting of glass, particularly soda-lime-silica glass, sodium or calcium sulfate or other sulfur sources are usually included in the batch material to aid in the melting and clarification process. Antimony, arsenic, and fluorine are also known as refining aids. The presence of refining aids such as sulfur in the melt is a problem with vacuum fining due to the induction of large volume bubbles and erosion of the ceramic refractory walls of the vacuum fining tank. was discovered. However, heretofore, effective melting and fining of glass has been difficult to achieve without fining aids. Yet another advantageous aspect of the present invention is that glass can be melted and refined to a high level of quality with little or no chemical refining aids. This is because the melting and clarification steps are carried out in separate stages, whereby each stage can be carried out in a manner adapted to minimize or avoid the use of chemical clarification aids. Possible in the present invention. It is generally believed that chemical fining aids serve to promote the collection and rise of air bubbles from within the melt pool; however, such mechanisms play a minor role in the fining process of the present invention. I believe that I can only accomplish this. Therefore, no significant effect on quality results from the omission or substantial reduction in the amount of refining aid used. Omission or reduction of clarification aids is also desirable to reduce undesirable emissions into the environment.

In the float process of flat glass manufacturing, the reduction or omission of sulfur from the glass is used to avoid defects caused by formation and evaporation, leading to condensation and evaporation of tin sulfide in the flat forming chamber onto the top surface of the glass. , additionally shows advantages. Sulfur and iron when combined have a coloring effect on the glass, so avoiding sulfur for fining allows for finer control of the color of some glasses.

Particularly advantageous is the separate ablating liquefaction process disclosed in U.S. Pat. liquifaction process). However, other liquefaction techniques can also be used.

DETAILED DESCRIPTION Although this detailed description is set forth in the context of a method and apparatus specifically adapted for glass melting, it will be appreciated that the invention is equally applicable to the processing of other materials.

Referring to the figure, the integrated melting process of the present invention preferably consists of three stages: a liquefaction stage 10, a dissolution stage 11, and a vacuum clarification stage 12.
Although a variety of devices can be used to initiate melting in the liquefaction stage 10, a highly effective arrangement for separating this process step and performing it economically is disclosed in U.S. Pat. No. 4,381,934. , which is incorporated herein by reference to the details of the preferred liquefaction stage implementation. The basic structure of the liquefaction tank is drum 1.
5, which is made of steel and has a generally cylindrical side wall, a generally open top, and a bottom that is closed except for the discharge outlet.

The drum 15 is, for example, surrounded by a surrounding support ring 16 that is rotatably supported by a plurality of support wheels 17 and held in place by a plurality of adjustment wheels 18.
is mounted for rotation about a substantially vertical axis. A substantially enclosed cavity is formed in the drum, for example by a lid structure provided with a stationary support by a peripheral frame 20. The lid 20 can take a variety of forms as known to those skilled in the art of refractory furnace construction. The preferred arrangement depicted in the figures is an upwardly domed curved arch structure fabricated from a plurality of refractory blocks. A monolithic or flat hanging design can be used for the lid.

Heat to liquefy the batch material may be provided by one or more burners 22 extending through the lid 20. Preferably, a plurality of burners are arranged around the lid to direct the flame to a large area within the drum. The burner is preferably water cooled to protect it from the harsh environment within the tank. Exhaust air escapes from the interior of the liquefaction tank through an opening 23 in the lid. Advantageously, the steam heat in the exhaust gases is disclosed in U.S. Patent No.
It may be used to preheat batch materials in a preheating step (not shown) as described in US Pat. No. 4,519,814.

The batch material is preferably in powdered form, but
The cavity of the liquefaction tank is fed by a chute 24, which extends through the exhaust opening 23 in the embodiment depicted. Details of the feed chute arrangement can be found in US Pat. No. 4,529,428.

The batch shoot 24 terminates close to the side wall of the drum 10, thereby depositing batch material on the inner side wall of the drum. A layer 25 of batch material is retained on the inner wall of the drum 10 with the aid of the rotation of the drum and serves as an insulating lining.

As the batch material on the surface of the lining 25 is exposed to the heat within the cavity, it forms a liquefied layer 26;
The sloping lining flows down to the bottom of the tank and into the central extraction opening. A ceramic bushing is attached to the outlet. Liquefied substance flow 2
8 falls freely from the liquefaction tank through an opening 29 leading to the second stage 11. This second stage may be referred to as a melting tank, since one of its functions is to complete the dissolution of the remaining unmelted particles of batch material in the liquefied stream 28 exiting the liquefaction tank 10. It is from. The liquefied material at that location is typically only partially molten and includes unmelted sand grains and a substantial gas phase. In a typical soda-lime-silica melting process using carbonate batch materials and sulfate as a clarifying agent, the gas phase consists primarily of carbon oxides and sulfur oxides. Nitrogen may also be present from trapped air.

The dissolution tank 11 serves to complete the dissolution of unmelted particles in the liquefied material coming from the first stage by providing residence time at a location remote from the downstream clarification stage. Soda, lime, and silica glass batches are typically liquefied at a temperature of about 1200℃ (2200℃), and the melting tank 11 is heated to about 1200℃.
(2200〓) to about 1320°C (2400〓), at which temperature the remaining undissolved particles usually become soluble when given sufficient residence time. The illustrated melting vessel 11 is in the form of a horizontally elongated refractory shallow box 30 with a corrosion-resistant roof 31 and an inlet and an outlet at each end to ensure adequate residence time. The depth of the molten material in the melting tank is relatively shallow to prevent circulation of material.

Although the addition of substantial thermal energy is not necessary to carry out the melting step, heating can accelerate the process, thus reducing the size of the melting vessel 11. More importantly, it is preferred to heat the material during the melting stage to raise the temperature for the subsequent clarification stage. Increasing the fining temperature to a maximum is advantageous because of the reduction in glass viscosity and the increase in the vapor pressure of the gases involved. Typically, about 1540
Temperatures of 2800 °C (2800 °C) are considered desirable for clarification of soda-lime-silica glasses, but when vacuum is used to aid clarification, lower peak clarification temperatures may be sacrificed at the expense of product quality. It can be used without making any difference. The amount by which the temperature can be lowered depends on the degree of vacuum. Therefore, when fining is to be carried out in accordance with the invention under vacuum, the glass temperature must be raised before fining, for example below 1480°C (2700°), optionally up to 1430°C (2600°).
You need to raise it below. When using the lower range of pressures disclosed herein, it is necessary that the temperature in the fining tank is no higher than 1370°C (2500°C).

This degree of peak temperature reduction results in a significantly longer life of the refractory vessel and at the same time saves energy. The liquefied material entering the melter needs to be heated only moderately to prepare the molten material for clarification. Although a combustion heat source can be used in the melting stage 11, it has been discovered, however, that this stage is well suited for electrical heating, thereby
A plurality of electrodes 32 may be provided horizontally through the sidewalls as shown. Heat is generated by the melt's own resistance to the electrical current passed between the electrodes, in the technique commonly used for electromelting of glass.

Electrode 32 may be carbon or molybdenum of a type well known to those skilled in the art.

A scum removal member 33 is provided in the melting tank to prevent suspended solids from approaching the outlet end.

Valve control of the flow of material from the melting stage 11 to the clarification stage 12 consists of a plunger 35 which is axially aligned with the withdrawal tube 36. Plunger shaft 3
7 extends through the melter roof 31 to adjust the gap between plunger 35 and tube 36, thereby modulating the flow rate of material into the fining stage.

Although a valve arrangement is preferred, other means are provided for controlling the flow rate of molten material to the fining stage, as is known in the art. One example is the heating and/or
by using cooling means.

The fining stage 12 preferably consists of a vertical upright vessel, generally cylindrical in form and having an internal ceramic refractory lining 40 enclosed in an airtight water-cooled casing. The refractory may be of the alumina-zirconia-silica type well known in the art. The casing includes a double-walled cylindrical side wall member 41 with an annular water passage therebetween, and circular end uncoolers 42 and 43. A heat insulating layer (not shown) is connected to the refractory 40 and the side wall 4.
It may be provided between 1 and 1. Valve tube 36 may be made of a refractory metal, such as platinum, and is hermetically fitted into orifice 44 at the upper end of the clarifier.

As the molten material passes through the tube 36 and encounters the reduced pressure within the fining vessel, the gas contained in the melt expands in volume and forms a layer of foam 50 that rests on the body of liquid 51.
Create. When the foam collapses, it becomes incorporated into the body 51 of liquid. A vacuum conduit 5 extending through the top of the clarification tank to maintain pressure below atmospheric pressure
It can be established through 2. As used herein, a "foam" can be considered to be characterized by at least twice the volume of the molten material. When a substance is sufficiently foamed, its volume increases by 2
Much larger than double. Distributing the molten material as a thin film of foam greatly increases the surface area exposed to reduced pressure. Therefore, it is preferable to maximize the foaming effect. It is also preferred that the foam is exposed to the lowest pressure in the system, which is present at the top of the vessel in the headspace above the liquid, so that the newly introduced material to be foamed is passed through the headspace above the top of the foam layer. Exposure is improved by dropping. Also, depositing freshly foamed material on top of the foam layer is more compatible with material transfer within the bath than having foam formed from the surface of a liquid pool below the foam. Depending on the pressure in the vacuum space and the volumetric flow rate of the molten material entering the clarification tank, the incoming flow generally penetrates into the foam layer as a cohesive liquid stream, thereby causing the foam to pool. Either foaming occurs from the surface of 51, or foaming occurs immediately when the flow is exposed to reduced pressure. Although either format can be used, the latter format has been found to be more effective for the reasons discussed above.

The heat content of the molten extrudate entering fining vessel 12 may be sufficient to maintain a suitable temperature within the vessel, but at low exit rates,
Energy losses through the walls may exceed the rate at which energy is transferred into the vessel by the molten material. In such cases, it may be desirable to provide heating within the fining tank to avoid inappropriate temperatures.

The amount of heating can be relatively small, since its purpose is only to compensate for heat loss through the wall, and can be carried out by conventional electrical heating equipment. ,that time,
Electrodes extend radially through the sidewalls and current flows between the electrodes through the glass.

Regardless of the extrusion speed, the space above the molten body in vessel 12 is at the desired temperature due to the absence of molten mass and radiation from the molten material being blocked by foam layer 50. tends to be lower. As a result, the top of the foam layer can be relatively cold, which increases the viscosity of the foam and slows the rate at which gas is expelled. In that case it has been found advantageous to provide means for heating the space above the liquid and foam. For this purpose, it has been found possible to provide a burner 53 and maintain combustion in a vacuum space, as described in US Pat. No. 4,704,153.
(U.S. Patent Original S. No. 895647 dated August 12, 1986).

A conduit 54 may be provided at the upper end of the vacuum chamber by which small amounts of water may be periodically sprayed onto the foam. It has been discovered that a spray of water helps the foam break down, which is another U.S. patent application filed July 7, 1986, U.S. Patent No. 4,794,860.
S. No. 882647 (later U.S. patent application S. No. 882647 dated September 29, 1987).
Replaced by No.102227. )) In the embodiment depicted, the clarified molten material is withdrawn from the bottom of the fining tank 12 by a withdrawal tube 55 of a refractory metal such as platinum. It is also possible to arrange the outlet in the bottom region in the side wall of the vessel. The withdrawal tube 55 preferably extends beyond the surface of the refractory bottom portion 56, within which the withdrawal tube is mounted to prevent debris from entering the extrusion stream. The bottom portion 56 is provided with a reduced thickness portion adjacent to the tube 55 to reduce the insulation effect on the tube, thereby increasing the temperature of the tube to prevent solidification of material within the tube. let Leakage around the tubes is blocked by a water cooler 57 below the bottom part 56. The flow rate of the molten material from the withdrawal tube 55 is controlled by a fibrous restrictor 58 supported at the end of the stem 59. Stem 59, in conjunction with mechanical means (not shown), adjusts the height of throttle member 58 and thus adjusts the height of throttle member 58 and tube 5.
5 is adjusted to control the flow rate therefrom. Molten stream of clarified material 60
may fall freely from the bottom of the fining tank and be delivered to a forming station (not shown) where it is shaped into the desired product. For example, clear glass is sent to a float glass forming room,
The molten glass may then be suspended above the molten metal pool to form a glass plate.

Although various shapes can be adopted, it is preferable that the clarifying tank 12 has a cylindrical shape. The cylindrical shape is advantageous for constructing an airtight tank. The internal surface contact area to volume ratio is also minimized with a circular cross section. Compared to the conventional open furnace type circulation clarifier,
Only a fraction of the refractory contact area is required by the cylindrical vacuum fining vessel of the present invention.

The height of the molten material 51 held in the clarification tank 12 is dictated by the vacuum level imposed in the chamber. The pressure head based on the height of the liquid must be sufficient to establish a pressure equal to or greater than atmospheric pressure at the outlet and to allow the material to be freely removed from the vessel. Its height depends on the specific gravity of the molten material, which for soda-lime-silica glass is about 2.3 at the temperature of the fining stage. The height above the minimum value required to compensate for the vacuum takes into account fluctuations in atmospheric pressure, allows fluctuations in the vacuum,
Desired to ensure steady flow through the outlet. Conditions can be maintained such that the flow through the outlet is regulated without bottom valve means.
However, in a preferred embodiment of the invention, a substantially oversized Height provided. Such an arrangement allows production rate and vacuum pressure to be varied independently of each other. Alternatively, the pressure at the outlet can be below atmospheric pressure if the outlet is provided with pumping means to overcome the pressure difference.

An example of a pump intended for use with molten glass is disclosed in U.S. Pat. No. 4,083,711;
The disclosure is incorporated herein by reference.

The pressure equalization function of the tank 12 is independent of its width, so the tank can theoretically be in the form of a narrow vertical pipe. However, relatively wide vessels are preferred because of the residence time that allows gas reabsorption, to reduce flow resistance, and to distribute heat into the lower part of the vessel without the need for auxiliary heating sources. For these purposes, a height to width ratio of no greater than 5:1 is preferred.

The benefits of vacuum on the fining process are achieved progressively, ie the lower the pressure the greater the benefit.
Although small pressure drops below atmospheric pressure may provide measurable improvements, the use of substantial reduced pressures is preferred to economically justify the vacuum chamber. For example, pressures of less than half atmospheric pressure are preferred for significant fining improvements applied to soda-lime-silica sheet glass. Significantly greater gas removal is achieved at pressures of one-third atmosphere or less. A standard clarified soda-lime-silica sheet glass composition was clarified at an absolute pressure of 100 torr to obtain a product with 1 seed per 100 cm ³ , which is an acceptable quality level for many glass products. be. Less than 100 Torr, for example from 20
A fining pressure of 50 Torr is preferred to obtain commercial float glass quality of about 1 seed per 1000-10000 ^cm3 . Seeds with a diameter of 0.01 mm or less are considered unrecognizable and are not included in the seed count.

Melting and fining aids, such as sulfur or fluorine compounds, are conventionally included in glass batches, but produce a substantial portion of undesirable emissions in the exhaust gas from the glass melter.

Although their omission is desirable, the use of auxiliaries has been considered necessary to obtain the highest quality standards, especially for flat glass standards. Additionally, sulfur sources (eg, sodium sulfate, calcium sulfate) have been found to cause excessive foaming under vacuum. Typically, sheet glass batches contain sodium sulfate in an amount of about 5 to 15 parts by weight per 1000 parts by weight of silica source material (sand), with about 10 parts by weight considered desirable to ensure adequate fining. It will be done. However, when working in accordance with the present invention,
It has been discovered that it is preferable to limit the sodium sulfate to 2 parts by weight to maintain manageable foam levels, and furthermore, that clarification is not detrimentally affected. Most preferably, sodium sulfate is used at less than 1 part per 1000 parts of sand, with 0.5 part being a particularly advantageous example. These weight ratios are given for sodium sulfate, but it is clear that they can be converted to other sulfur sources depending on the molecular weight ratio.
Complete elimination of the clarification aid is possible with the present invention,
However, trace amounts of sulfur are typically present in other batch materials, so even if sulfur is not intentionally included in the batch, small amounts of sulfur may be present.

No significant deleterious effects on the physical properties of glasses subjected to the vacuum fining process of the present invention were found. However, the vacuum treatment does have some detectable effect on the glass composition, making the glass produced by this method distinguishable from the same type of glass produced by conventional commercial methods. . It has been discovered that this vacuum treatment reduces the concentration of volatile components, particularly refining aids such as sulfur, to levels below the equilibrium levels achieved with conventional methods. Glasses produced in pots and the like are sometimes reported to have little or no residual content of refining aids. This is because the discontinuous melting process allows for longer fining times, thereby avoiding the need for chemical fining aids. Also, small melts are often produced from chemically pure feed and oxide feed materials that, unlike conventional carbonate mineral batch materials, do not produce substantial volumes of gaseous by-products. However, soda-lime-silica glass products produced in large quantities by continuous melting processes are characterized by significant amounts of residual fining aids. Such products include flat glass (eg, float glass) and containers (eg, bottles) suitable for glazing openings in buildings or vehicles. In such products, the residual sulfur content (expressed as _SO3 ) is typically on the order of 0.2% by weight, and 0.1% by weight.
% or less. Even when carefully adding sulfur fining aid to batches, at least 0.02%
SO ₃ can usually be detected in soda, lime, and silica glass produced in conventional continuous melters.
Flat glass for transparent window applications is usually
Contains SO ₃ of 0.05% or more. In contrast, soda-lime-silica glass is produced in a continuous manner according to the present invention, at the preferred vacuum level, with less than 0.02% residual SO, even when relatively small amounts of sulfur refining aid are included in the batch as described above. ₃ , and when sulfur is not included,
It can be produced with less than 0.01% SO ₃ .
At minimum pressure without adding sulfur,
_SO3 contents below 0.005% can be achieved.

Commercial soda-lime-silica glass, commonly refined with sulfur compounds, may be characterized as follows: wt% SiO ₂ 70-74 Na ₂ O 12-16 CaO 8-12 MgO 0-5 Al ₂ O ₃ 0-3 K ₂ O 0-3 BaO 0-1 Fe ₂ O ₃ 0-1 Small amounts of colorants or other clarifying aids may also be present. Arsenic, antimony, fluorine, and lithium compounds are sometimes used as refining aids, and residues can be detected in this type of glass. Float glass plates or bottles represent common commercial embodiments of the above compositions.

Glass sheets formed by the float process (ie, suspended on molten tin) are characterized by a measurable amount of tin oxide migrated into the glass surface area on at least one side. Typically, a piece of float glass is placed on the lower two sides of the surface in contact with the tin.
With a SnO ₂ concentration of at least 0.05% by weight within 3 microns. Because the float process requires a relatively large continuous melting vessel of the type that conventionally uses significant amounts of sulfur-yellow-containing refining aids, float glasses are generally manufactured using the methods described above for soda-lime-silica glasses. characterized by a higher minimum SO ₃ concentration than discussed in . Therefore, float glasses refined according to the present invention with less than 0.08% SO ₃ are distinguished from conventional commercially available float glasses.
Most float glasses fall into the following composition range: Sio ₂ 72-74% by weight Na ₂ O 12-14 CaO 8-10 MgO 3-5 Al ₂ O ₃ 0-2 K ₂ O 0-1 Fe ₂ O ₃ 0-1 Colorants and traces of other substances may be present.

Although the description of preferred embodiments and some of the advantages of the present invention has been made in connection with continuous processes for making glass and the like, it is clear that discontinuous fining operations can also obtain at least some of the advantages of the present invention. It should be.

Other variations known to those skilled in the art are included within the scope of the invention as defined by the claims.

[Brief explanation of the drawing]

The drawings are vertical cross-sections of three stages of the melt operation, including a liquefaction stage, a melting stage, and a vacuum clarification stage, according to a preferred embodiment of the invention.

Claims (1)

Claims: 1. A method of melting and refining a glassy substance or the like, comprising: producing a melt of the substance; maintaining the molten substance at subatmospheric pressure above the body of the molten substance; collapsing the foam into the body of molten material; and drawing the molten material from the body of molten material. 2. The method of claim 1, wherein the molten material is introduced into the subatmospheric space through a valved orifice above the molten material level. 3. A method according to claim 1, wherein the transport in the body of molten material is predominantly vertical towards the withdrawal position. 4. The method according to claim 1, wherein the pressure below atmospheric pressure is less than half of atmospheric pressure. 5. The method according to claim 1, wherein the pressure below atmospheric pressure is one third or less of atmospheric pressure. 6. Claim 1, wherein the pressure in the body of molten material at the height of withdrawal is at least atmospheric pressure.
The method described in section. 7. A method according to claim 6, wherein the withdrawal rate is controlled by outlet orifice means. 8. Claim 1 in which the newly foamed material is deposited onto the previously formed foam mass.
The method described in section. 9. The method of claim 1, wherein the material being melted and refined is glass. 10. The method of claim 1, wherein the material being melted and refined is soda-lime-silica glass. 11 A method for melting and refining a glassy substance or the like, wherein the batch substance is brought to an incompletely melted, fluid state in a first chamber, and the fluid substance containing unmelted particles is brought to a substantially complete melting of the particles. a second chamber where the substance is clarified; and passing the substance from the second chamber to a third chamber where it is subjected to subatmospheric pressure to clarify the substance. 12 The substances being melted are soda, lime, and silica glass, and the substances are heated to 1480℃ from the second chamber.
12. The method according to claim 11, wherein the method is fed to a third chamber at a temperature below (2700〓). 13 Substances are transferred from the first chamber to approximately 1200℃ (2200〓)
13. The method of claim 12, wherein the second chamber is fed at 1320°C (2400°). 14. The method of claim 11, wherein the material is heated in the second chamber to a temperature suitable for clarification. 15. The method according to claim 11, wherein the pressure in the third chamber is less than half the atmospheric pressure. 16. The method according to claim 11, wherein the pressure in the third chamber is one third or less of atmospheric pressure. 17 The substance being melted is soda-lime-silica glass, and the batch substance is sent to the first chamber with a clarification aid in an amount not more than the equivalent of 2 parts by weight of sodium sulfate per 1000 parts by weight of the silica source material. Claim 1 provided with a sulfur source as
The method described in Section 1. 18. The method of claim 11, wherein the substance is delivered into the third chamber at a controlled variable rate through a variable opening orifice. 19. The method of claim 18, wherein the substance is withdrawn from the third chamber through an orifice of variable opening. 20. The method of claim 11, wherein the step of rendering the batch material in a flowable state comprises exposing the batch material to heat while supporting the batch material on an incline. 21. The method of claim 20, wherein the slope surrounds a centrally heated cavity. 22. The method of claim 11, wherein the temperature is raised in the second chamber by electrical heating. 23. The method of claim 11, wherein the substance flows in the second chamber from the inlet region to the outlet region along a predominantly horizontal flow path. 24. The method of claim 11, wherein the overall temperature of the glass is not raised in the third chamber. 25. The method of claim 11, wherein the material in the third chamber follows a predominantly vertical path from the upper inlet end towards the lower outlet end. 26 Bring the batch material to an incompletely melted fluid state in the first chamber, withdraw the fluid material to the second chamber before its temperature substantially exceeds its liquefaction temperature, and clarify the temperature of the material in the second chamber. 12. A method as claimed in claim 11, comprising: raising the material to a temperature suitable for clarification, and passing the material from the second chamber to a third chamber where it is subjected to subatmospheric pressure so as to clarify the material. 27 The substances being melted are soda, lime, and silica glass, and the substances are heated to 1480℃ from the second chamber.
27. The method according to claim 26, wherein the method is fed to the third chamber at a temperature of (2700〓) or less. 28 The substance is released from the first room from about 1200℃ (2200〓)
28. The method of claim 27, wherein the second chamber is fed at a temperature of 1320°C (2400°). 29. The method of claim 26, wherein the material entering the second chamber contains unmelted particles and sufficient residence time is provided in the second chamber to allow dissolution of the particles. 30. The method of claim 26, wherein the pressure in the third chamber is less than half atmospheric pressure. 31. The method according to claim 26, wherein the pressure in the third chamber is one third or less of atmospheric pressure. 32 The substance being melted is soda-lime-silica glass, and the batch substance is sent to the first chamber with a clarification aid in an amount not more than the equivalent of 2 parts by weight of sodium sulfate per 1000 parts by weight of the silica source material. Claim 2 provided with a sulfur source as
The method described in Section 6. 33. The method of claim 26, wherein the substance is delivered into the third chamber at a controlled variable rate through a variable opening orifice. 34. The method of claim 33, wherein the material is withdrawn from the third chamber through an orifice of variable opening. 35. The method of claim 26, wherein the step of rendering the batch material in a flowable state comprises exposing the batch material to heat while supporting the batch material on an incline. 36. The method of claim 35, wherein the slope surrounds a centrally heated cavity. 37. The method of claim 26, wherein the temperature is raised in the second chamber by electrical heating. 38. The method of claim 26, wherein the substance flows in the second chamber from the inlet area to the outlet area along a path that is primarily horizontal. 39. The method of claim 26, wherein the overall temperature of the glass is not raised in the third chamber. 40 The substance in the third chamber follows a primarily vertical path from the upper inlet end to the lower outlet end;
A method according to claim 26. 41 liquefying the batch material; directing the liquefied material through valve means in the upper part of the vessel in which a subatmospheric pressure is maintained; passing the liquefied material in a vacuum tank sufficient to substantially clarify the material; 12. A method according to claim 11, comprising: maintaining the clarified material for a period of time; and withdrawing the clarified material from the lower part of the vessel. 42. The method of claim 41, wherein the pressure in the upper portion is less than half atmospheric pressure and the pressure in the lower portion is at least atmospheric pressure. 43. A method according to claim 41, wherein the rate at which the liquefied material is withdrawn from the vessel is controlled by second valve means. 44. The method according to claim 41, wherein the glassy substance is soda/lime/silica glass. 45. A method according to claim 41, wherein after being liquefied and before passing through the valve means, the temperature of the liquefied material is raised to a temperature suitable for clarification.