MELTING AND REFINING OF GLASS OR THE LIKE WITH VACUUM REFINING

Abstract: Method and apparatus are provided for refining glassy material (51) or the like by vacuum. The material (51) is first melted and then introduced into a vacuum chamber (12), pre­ ferably into space above liquid (51) held in the chamber (12). Other aspects involve a two-step melting process prepara­ tory to the vacuum refining, in which the material is initially liquefied in one stage (10) and dissolution of particles is sub­ stantially completed in a second stage (11). The molten mat­ erial (51) is preferably foamed (50) immediately upon entry into the vacuum chamber (12).

Description: The present invention relates to the use of subatmospheric pressure to expedite refining of molten glass or the like. More particularly, the invention relates to a practical arrangement for a continuous commercial scale utilization of such a refining technique. In the melting of glass, substantial quantities of gas are produced as a result of decomposition of batch materials. Other gases are physically entrained by the batch materials or are introduced into the melting glass from combustion heat sources. Most of the gas escapes during the initial phase of melting, but some becomes entrapped in the melt. Some of the trapped gas dissolves in the glass, but other portions form discrete gaseous inclusions known as bubbles or "seeds" which would be objectionable if permitted to remain in unduly high concentrations in the product glass. The gas inclusions will rise to the surface and escape from the melt if given sufficient time in the stage of a glassmaking operation known as "refining" or "fining." High temperatures are conventionally provided in the refining zone to expedite the rise and escape of the gaseous inclusions by reducing the viscosity of the melt and by enlarging the bubble diameters. The energy required for the high temperatures employed in the refining stage and the large melting vessel required to provide sufficient residence time for the gaseous inclusions to escape from the melt are major expenses of a glassmaking operation. Accordingly, it would be desirable to improve the refining process to reduce these costs. It has been known that reduced pressure could assist the refining process by reducing the partial pressure over the melt of the dissolved gases. Also, reducing the pressure increases the volume of bubbles within the melt so as to speed their rise to the surface. The impracticality of providing a gas-tight vessel on the scale of a conventional refining chamber so as to draw a vacuum therein has limited the use of vacuum refining to relatively small scale batch operations such as disclosed in US-A-1,564,235;-2,781,411;-2,877,280; -3,338,694; and-3,442,622. Continuous vacuum refining processes have been proposed but have not found acceptance for large scale, continuous manufacture of glass due to various drawbacks. In the continuous vacuum refining arrangements shown in US-A-805,139;-1,598,308; and-3,519,412 a major disadvantage is the requirement for relatively narrow vertical passageways leading into and out of the vacuum zone necessitated by the pressure difference. These passageways complicate the construction of such a vessel, particularly in view of the requirement for gas-tight walls, increase the exposure of the throughput to contaminating refractory contact, and impose a significant viscous drag to the throughput flow. It may be noted that a substantial height of glass is required to balance even a moderate degree of vacuum. Varying the output of such a system is also a problem, particularly in view of the viscous drag factor. Flexibility in the output rate is important in a continuous commercial operation due to changes in the product being made (thickness, width) and economic factors that affect the rate of production desired. In each of the three patents noted above, the driving force for increasing the rate of flow through the passages of the vacuum section can be provided only by increasing the depth of the melt upstream of the vacuum section relative to the depth of the melt downstream from the vacuum section. The magnitude of this level difference is exacerbated by the viscous drag inherent in these systems. Because accelerated erosion of the side walls occurs at the elevation of the surface of the melt, significantly changing the level aggravates the erosion which, in turn, deteriorates the quality of the product glass. A simpler structure is shown in US-A-3,429,684, wherein batch materials are fed through a vacuum lock and melted at the top of a vertically elongated vacuum chamber. Varying throughput in that arrangement appears to require changing the amount of vacuum imposed in the chamber, which would disadvantageously alter the degree of refining achieved. Melting raw materials within the vacuum chamber is another disadvantage of that arrangement for several reasons. First, large volumes of foam would be created by carrying out the initial decomposition of the raw materials under vacuum, which would require a vessel large enough to contain the foam. Second, there is a danger that raw materials may follow a short circulation path to the output stream, thus avoiding adequate melting and refining. Third, carrying out the initial stages of melting and heating the melt to a refining temperature within the vacuum vessel require large amounts of heat to be supplied to the melt within the vessel. Such a major heat input to the vessel inherently induces convection currents within the melt that increase erosion of the walls, which leads to contamination of the refined product stream. Fourth, carbon dioxide released from decomposing the batch carbonates would create a relatively high partial pressure of carbon dioxide within the vessel, thereby at least partially negating the ability of the reduced pressure to remove carbon dioxide from the melt. US-A-4,195,982 discloses initially melting glass under elevated pressure and then refining the glass in a separate chamber at a lower pressure. Both chambers are heated. US-A-4,110,098 discloses a process of deliberately foaming glass to aid refining. The foaming is induced by intense heat and chemical foaming agents at atmospheric pressure. From US-A-3,951,635 a three step melting process of glass batch material is known wherein molten densified glass batch material is refined in a centrifugal refiner to remove small sized gaseous inclusions from the glass product. In a first step the glass batch material is heated in a chamber to form a foamy glass mixture which is then transferred into a second chamber containing a pool of molten glass. Said mixture is then forwarded to the third step of refining in the centrifugal refiner. It is the object to provide an improved method and apparatus for melting and refining batch material, especially glass batch material which results in improved glass material. This object is attained by a method for melting and refining glassy material or the like by producing a melt of the material and refining the molten material in a further step within a refining vessel and withdrawing the molten material from the body of molten material in the refining vessel, characterized by foaming the molten material as it is introduced into a space maintained at subatmospheric pressure above a body of the molten material within the refining vessel and collapsing the foam into the body of molten material. The invention includes an apparatus for melting and refining glassy materials or the like, comprising: a first chamber having means to heat a batch material to a flowable condition and means to drain liquefied batch material therefrom directly upon attaining the flowable condition; a second chamber adapted to receive the liquefied batch material from the first chamber and adapted to substantially complete dissolution of particles in the liquefied material; and a refining vessel adapted to receive the heated material from the second chamber at an upper end and means to drain material from a lower portion of the refining vessel characterized in that the refining vessel is at its upper part provided with means to support subatmospheric pressure within said vessel and above the compact body of heated material when the apparatus is in operation. With the method and apparatus of the invention vacuum refining may be employed in a commercial scale, continuous glassmaking process in a manner that advantageously and economically overcomes the drawbacks of the prior art and allows to produce glass products of different shape. Molten glass is admitted to the vacuum refining vessel after the majority of the thermal energy required for melting has been imparted to the melt so that little or no thermal energy need be supplied to the molten material contained within the vacuum chamber. Preferably, no more heat is added at the vacuum stage than is necessary to compensate for heat loss through the vessel walls. At sufficiently high throughput rates, the vacuum vessel may be completely unheated by other than the incoming molten glass itself. In preferred embodiments of the present invention, batch materials are first liquefied at a stage specifically adapted for that step of the process, and the liquefied material is transferred to a second stage where dissolution of solid particles is essentially completed and the temperature of the material may be raised to a temperature to provide a viscosity suitable for refining. Subsequently, the molten material is passed to the vacuum vessel. As a result, a large portion of the gaseous by-products of melting are driven off before the material is subjected to vacuum, and the region of greatest gas evolution is separated from the refining zone, whereby materials undergoing the early stages of melting cannot become mixed with portions of the melt undergoing refining. Because most or all of the thermal requirement for melting has been satisfied before the material enters the vacuum refining stage and heating of the refining stage can therefore be substantially avoided, excessive convection of the melt in the refining zone can be avoided. As a result, vessel erosion is reduced and the probability of incompletely refined portions of the melt becoming mixed with more refined portions is reduced. The assistance provided by the vacuum to the refining process enables lower temperatures to be used for refining. Lower temperatures are advantageous not only for less energy consumption, but also for the sake of reduced corrosive effect on the vessel. Glass normally refined at peak temperatures on the order of 1520°C (2800°F) can be refined to the same extent at temperatures no greater than about 1425°C (2600°F) or even 1370°C (2500°F) or lower, depending upon the level of vacuum employed. It is theorized that the creation of foam in the vacuum refining vessel significantly enhances removal of gases from the melt. The thin film and large surface area presented by the foam increases exposure to the low pressure conditions and expedites transport of the gases out of the liquid phase. This contrasts to conventional refining where residence time must be provided to permit bubbles to rise to the surface and escape from the viscous melt, which entails retaining a large pool of the melt. Thus, vacuum refining of the present invention can achieve a given degree of refining in a considerably smaller space. in preferred embodiments of the invention the beneficial effects of exposing foamed melt to vacuum are enhanced by foaming the material as it enters the vacuum vessel, before it enters the body of molten material retained therein, and preferably before the entering stream penetrates into the foam layer. Another aspect of the invention relates to advantages in throughput control in a continuous refining operation. Liquefied material is metered into the upper end of the vacuum vessel through valve means, and refined melt is passed from the lower end of the vacuum vessel through another valve arrangement. The height of liquid maintained within the vacuum vessel is at least slightly greater than the height required to counterbalance the vacuum so that the melt can flow by gravity from the outlet. Also, by providing a liquid height greater than the minimum required for draining, the throughput rate can be controlled by means of the valves without altering the vacuum pressure in the vessel and without changing the liquid level within the vessel. Conversely, a range of vacuum pressures can be employed without changing the throughput rate. Aside from the valves, the system is provided with relatively low resistance to flow of the molten material therethrough. Not only is the throughput variable in a given installation of the present invention, but the effectiveness is relatively independent of the scale of the system, unlike conventional tank-type recirculating refiners that do not operate effectively for low volume applications. Therefore, the present invention can be applied effectively to a wide range of glassmaking operations. The preferred configuration for the vacuum refining vessel is a vertically elongated vessel, most conveniently in the shape of an upright cylinder. Liquified material is introduced into the headspace above the molten material held in the vessel. Upon encountering the reduced pressure in the headspace, at least a substantial portion of the material foams due to evolvement of gases dissolved in the material and due to enlargement of bubbles and seeds present in the material. Creating a foam greatly increases the surface area exposed to the reduced pressure, thus aiding the removal of gaseous species from the liquid phase. Producing the foam above the molten pool held in the vessel rather than from the molten pool is advantageous for collapsing foam and aiding the escape or gases. it has also been found that depositing newly generated foam onto a foam layer expedites collapse of the foam. Another advantage of the vertically elongated geometry is that stratification occurs due to the less dense foam or bubble containing material remaining at the upper end, so that the overall mass transport is away from the foam region, thereby rendering it unlikely that any of the unrefined material would become included in the product stream. Stripping gases from the melt at reduced pressure reduces the concentration of gases dissolved in the melt to below their saturation points at atmospheric pressure. As the molten material progresses downwardly toward an outlet at the bottom, the increasing pressure due to the depth of the melt in the vessel induces any residual gases to remain in solution and decreases the volume of any small seeds that may remain. Dissolution of gases may also be aided by permitting the temperature to fall as the material progresses toward the outlet. Moreover, the low concentration of gases remaining after vacuum refining reduces the probability of nucleation of bubbles in subsequent stages of the glassmaking process, as is frequently a problem with conventional refining. In commercial melting of glass, especially soda-lime-silica glass, sodium sulfate or calcium sulfate or other sources of sulfur are usually included in the batch materials to aid the melting and refining process. Antimony, arsenic, and fluorine are also known as refining aids. The presence of refining aids such as sulfur in the melt has been found to be a problem when refining with vacuum because of the large volumes of foam induced and because of attack on the ceramic refractory walls of a vacuum refining vessel. But heretofore, effective melting and refining of glass have been difficult to achieve without the refining aids. it is yet another advantageous aspect of the present invention that glass can be melted and refined to a high standard of quality with the use of little or no chemical refining aid. This is feasible in the present invention because the melting and refining steps are carried out in discrete stages, whereby each stage may be carried out by a process adapted to minimize or avoid the use of chemical refining aids. It is generally believed that chemical refining aids serve to expedite the accumulation and rise of bubbles from within a molten pool, but such a mechanism is believed to play no more than a minor role in the refining process of the present invention. Therefore, no significant effect on quality results from eliminating or substantially reducing the amount of refining aids used. Elimination or reduction of the refining aids is also desirable for the sake of reducing undesirable emissions into the environment. in the float process of manufacturing flat glass, reducing or eliminating sulfur from the glass is additionally advantageous for the sake of avoiding defects caused by the formation and volatilization of tin sulfide in the flat forming chamber that leads to condensation and drippage onto the top surface of the glass. Sulfur in combination with iron has a coloration effect on glass, so the avoidance of sulfur for refining permits more precise control of the color of some glass. Particularly advantageous is the use of the discrete ablating liquefaction process disclosed in US-A-4,381,934 for rendering the pulverulent batch materials to the initially flowable stage prior to being refined by the discrete process stage of the present invention. However, other liquefying techniques could be employed. The Drawing The figure is a vertical cross-section through three stages of a melting operation including a liquefaction stage, dissolving stage and a vacuum refining stage in accordance with a preferred embodiment of the present invention. Detailed Description The detailed description will be set forth in conjunction with a method and apparatus specifically adapted for melting glass, but it should be understood that the invention is applicable to the processing of other materials as well. Referring to the figure, the overall melting process of the present invention preferably consists of three stages: a liquefaction stage 10, a dissolving stage 11 and a vacuum refining stage 12. Various arrangements could be employed to initiate the melting in the liquefaction stage 10, but a highly effective arrangement for isolating this stage of the process and carrying it out economically is that disclosed in US-A-4,381,934 The basic structure of the liquefaction vessel is a drum 15 which may be fabricated of steel and has a generally cylindrical sidewall portion, a generally open top, and a bottom portion that is closed except for a drain outlet. The drum 15 is mounted for rotation about a substantially vertical axis, for example, by means of an encircling support ring 16 rotatably carried on a plurality of support wheels 17 and held in place by a plurality of aligning wheels 18. A substantially enclosed cavity is formed within the drum 15 by means of a lid structure 20 which is provided with stationary support by way of a peripheral frame 21, for example. The lid 20 may take a variety of forms as may be known to those of skill in the refractory furnace construction art. The preferred arrangement depicted in the figure is an upwardly domed, sprung arch construction fabricated from a plurality of refractory blocks. It should be understood that monolithic or flat suspended designs could be employed for the lid. Heat for liquefying 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 perimeter of the lid so as to direct their flames toward a wide area of the material within the drum. The burners are preferably water cooled to protect them from the harsh environment within the vessel. Exhaust gases may escape from the interior of the liquefaction vessel through an opening 23 in the lid. Advantageously the waste heat in the exhaust gases may be used to preheat the batch material in a preheating stage (not shown) such as that disclosed in US-A-4,519,814. Batch materials, preferably in a pulverulent state, may be fed into the cavity of the liquefying vessel by means of a chute 24, which in the embodiment depicted extends through the exhaust opening 23. Details of the feed chute arrangement may be seen in US-A-4,529,428. The batch chute 24 terminates closely adjacent to the sidewalls of the drum 10, whereby batch material is deposited onto the inner sidewall portions of the drum. A layer 25 of the batch material is retained on the interior walls of the drum 10 aided by the rotation of the drum and serves as an insulating lining. As batch material on the surface of the lining 25 is exposed to the heat within the cavity, it forms a liquefied layer 26 that flows down the sloped lining to a central drain opening at the bottom of the vessel. The outlet may be fitted with a ceramic refractory bushing 27. A stream of liquefied material 28 falls freely from the liquefaction vessel through an opening 29 leading to the second stage 11. The second stage may be termed the dissolving vessel because one of its functions is to complete the dissolution of any unmelted grains of batch material remaining in the liquefied stream 28 leaving the liquefaction vessel 10. The liquefied material at that point is typically only partially melted, including unmelted sand grains and a substantial gaseous phase. in a typical soda-lime-silica melting process using carbonate batch materials and sulfates as a refining aid, the gaseous phase is chiefly comprised of carbon oxides and sulfur oxides. Nitrogen may also be present from entrapped air. The dissolving vessel 11 serves the function of completing the dissolution of unmelted particles in the liquefied material coming from the first stage by providing residence time at a location isolated from the downstream refining stage. Soda-lime-silica glass batch typically liquefies at a temperature of about 1200°C (2200°F) and enters the dissolving vessel 11 at a temperature of about 1200°C (2200°F) to about 1320°C (2400°), at which temperature residual unmelted particles usually become dissolved when provided with sufficient residence time. The dissolving vessel 11 shown is in the form of a horizontally elongated refractory basin 30 with a refractory roof 31, with the inlet and outlet at opposite ends thereof so as to assure adequate residence time. The depth of molten material in the dissolving vessel may be relatively shallow in order to discourage recirculation of material. Although the addition of substantial thermal energy is not necessary to perform the dissolving step, heating can expedite the process and thus reduce the size of the dissolving vessel 11. More significantly, however, it is preferred to heat the material in the dissolving stage so as to raise its temperature in preparation for the refining stage to follow. Maximizing the temperature for refining is advantageous for the sake of reducing glass viscosity and increasing vapor pressure of included gases. Typically a temperature of about 1520°C (2800°F) is considered desirable for refining soda-lime-silica glass, but when vacuum is employed to assist refining, lower peak refining temperatures may be used without sacrificing product quality. The amount by which temperatures can be reduced depends upon the degree of vacuum. Therefore, when refining is to be performed under vacuum in accordance with the present invention, the glass temperature need be raised to no more than 1480°C (2700°F), for example, and optionally no more than 1430°C (2600°F) prior to refining. When the lower range of pressures disclosed herein are used, the temperature in the refining vessel need be no higher than 1370°C (2500°F). Peak temperature reductions on this order result in significantly longer life for refractory vessels as well as energy savings. The liquefied material entering the dissolving vessel need be heated only moderately to prepare the molten material for refining. Combustion heat sources could be used in the dissolving stage 11, but it has been found that this stage lends itself well to electric heating, whereby a plurality of electrodes 32 may be provided as shown in the figure extending horizontally through the sidewalls. Heat is generated by the resistance of the melt itself to electric current passing between electrodes in the technique conventionally employed to electrically melt glass. The electrodes 32 may be carbon or molybdenum of a type well known to those of skill in the art. A skimming member 33 may be provided in the dissolving vessel to prevent any floating material from approaching the outlet end. A valve controlling the flow of material from the dissolving stage 11 to the refining vessel 12 is comprised of a plunger 35 axially aligned with a drain tube 36. The shaft 37 of the plunger extends through the roof 31 of the dissolving vessel so as to permit control over the gap of the plunger 35 and the tube 36 to thereby modulate the rate of flow of material into the refining vessel. Although the valve arrangement is preferred, other means could be provided to control the flow rate of molten material to the refining vessel as are known in the art. An example would be the use of heating and/or cooling means associated with the drain tube so as to modulate the viscosity, and thus the flow rate, of the molten material passing therethrough. The refining vessel 12 preferably consists of a vertically upright vessel that may be generally cylindrical in configuration having an interior ceramic refractory lining 40 shrouded in a gas-tight water-cooled casing. The refractory may be an alumina-zirconia-silica type well known in the art. The casing may include a double walled, cylindrical sidewall member 41 having an annular water passageway therebetween and circular end coolers 42 and 43. A layer of insulation (not shown) may be provided between the refractory 40 and the sidewall 41. The valve tube 36 may be fabricated of a refractory metal such as platinum and is sealingly fitted into an orifice 44 at the upper end of the refining vessel. As the molten material passes through the tube 36 and encounters the reduced pressure within the refining vessel, gases included in the melt expand in volume, creating a foam layer 50 resting on a body of liquid 51. As foam collapses it is incorporated into the liquid body 51. Subatmospheric pressure may be established within the refining vessel through a vacuum conduit 52 extending through the upper portion of the vessel. As used herein, "foaming" can be considered to be characterized by at least a doubling of the volume of the molten material. if the material is fully foamed, the volume increase is usually much greater than double. Distributing the molten material as thin membranes of a foam greatly increases the surface area exposed to the reduced pressure. Therefore, maximizing the foaming effect is preferred. it is also preferred that the foam be exposed to the lowest pressures in the system, which are encountered at the top of the vessel in the headspace above the liquid, and therefore exposure is improved by permitting newly introduced, foamed material to fall through the headspace onto the top of the foam layer. Also, it is more consistent with the mass transfer in the vessel to deposit freshly foamed material onto the top of the foam layer rather than generating foam from the surface of the liquid pool beneath the foam layer. Depending upon the pressure in the vacuum space and the volume flow rate of the molten material entering the refining vessel, the entering stream may either penetrate through the foam layer as a generally coherent liquid stream, whereby foaming occurs from the surface of the pool 51, or the stream may foam immediately upon encountering the reduced pressure. Either mode can be used, but for the reasons stated above, the latter mode has been found to be more effective. The heat content of the molten throughput material entering the refining vessel 12 can be sufficient to maintain suitable temperatures within the vessel, but at lower throughput rates energy losses through the walls may exceed the rate at which energy is being transported into the vessel by the molten material. in such a case, it may be desirable to provide heating within the refining vessel for the sake of avoiding undue temperature reduction. The amount of heating could be relatively minor since its purpose would be merely to offset heat losses through the walls, and may be carried out by conventional electric heating arrangements whereby electrodes extend radially through the side wall and electric current is passed between the electrodes through the glass. In the embodiment depicted, refined molten material is drained from the bottom of the refining vessel 12 by way of a drain tube 55 of a refractory metal such as platinum. It would also be feasible to locate the drain in a side wall of the vessel in the region of the bottom. The drain tube 55 preferably extends above the surface of the refractory bottom section 56 within which it is mounted to prevent any debris from entering the output stream. The bottom section 56 may be provided with reduced thickness adjacent to the tube 55 so as to reduce the insulating effect on the tube, thereby permitting the temperature of the tube to be elevated to prevent freezing of material within the tube. Leakage around the tube is prevented by a water cooler 57 under the bottom section 56. The flow rate of molten material from the drain tube 55 is controlled by a conical throttle member 58 carried at the end of a stem 59. The stem 59 is associated with mechanical means (not shown) to adjust the elevation of the throttle member 58 and thus adjust the gap between the throttle member and the tube 55 so as to control the flow rate therefrom. A molten stream 60 of refined material falls freely from the bottom of the refining vessel and may be passed to a forming station (not shown) where it may be shaped to the desired glass product. Refined glass, for example, may be passed to a float glass forming chamber where the molten glass floats on a pool of molten metal to form a flat sheet of glass or may be shaped to bottles. Although various shapes could be employed, the refining vessel 12 is preferably cylindrical in configuration. The cylindrical shape is advantageous for the sake of constructing a gas-tight vessel. The ratio of interior surface contact area to volume is also minimized with a circular cross-section. Compared to a conventional open hearth type recirculating refiner, only a fraction of the refractory contact area is entailed by the cylindrical vacuum refiner of the present invention. The height of molten material 51 retained in the refiner 12 is dictated by the level of vacuum imposed in the vessel. The pressure head due to the height of the liquid must be sufficient to establish a pressure equal to or greater than atmospheric at the outlet to permit the material to drain freely from the vessel. The height will depend upon the specific gravity of the molten material, which for soda-lime-silica glass at the temperatures in the refining stage is about 2.3. A height in excess of the minimum required to offset the vacuum may be desired to account for fluctuations in atmospheric pressure, to permit variation of the vacuum, and to assure steady flow through the outlet. Conditions could be maintained so that flow through the outlet is regulated without bottom valve means. But in the preferred embodiments of the present invention, a substantial excess height is provided so that the outlet flow rate is not determined by the vacuum pressure, but rather by mechanical valve means, i.e., the throttle member 58. Such an arrangement permits the throughput rate and the vacuum pressure to be varied independently of each other. Alternatively, the pressure at the outlet could be below atmospheric if the outlet is provided with pump means to overcome the pressure differential. An example of a pump that is intended for use with molten glass is disclosed in US-A-4,083,711 The pressure equalization function of the vessel 12 is independent of its width, and therefore, the vessel could theoretically be in the form of a narrow, vertical pipe. However, a relatively wide vessel is preferred for the sake of residence time to permit reabsorption of gases, for reduced flow resistance, and for distribution of heat into the lower portion of the vessel without requiring auxiliary heating sources. For these purposes, a height to width ratio of no more than 5 to 1 is preferred. The benefits of vacuum on the refining process are attained by degrees; the lower the pressure, the greater the benefit. Small reductions in pressure below atmospheric may yield measurable improvements, but to economically justify the vacuum chamber, the use of substantially reduced pressures are preferred. Thus, a pressure of no more than one-half atmosphere is preferred for the appreciable refining improvements imparted to soda-lime-silica flat glass. Significantly greater removal of gases is achieved at pressures of one-third atmosphere or less. A standard clear soda-lime-silica flat glass composition was refined at an absolute pressure of 133 mbar and yielded a product having one seed per 100 cubic centimeters, which is a quality level acceptable for many glass products. A refining pressure below 133 mbar for example 26.7-66.7 mbar would be preferred to yield commercial float glass quality of about one seed per 1,000-10,000 cubic centimeters. Seeds less than 0.01 millimeter in diameter are considered imperceptible and are not included in the seed counts. Melting and fining aids such as sulfur or fluorine compounds are conventionally included in glass batches, but produce a substantial portion of the undesirable emissions in exhaust gas from glass melting furnaces. Their elimination would be desirable, but to attain the highest levels of quality, particularly for flat glass standards, use of the aids has been considered necessary. Furthermore, sulfur sources (e.g., sodium sulfate, calcium sulfate) have been found to cause excessive foaming under vacuum. Typically, flat glass batch includes sodium sulfate in the amounts of about 5 to 15 parts by weight per 1000 parts by weight of the silica source material (sand), with about 10 parts by weight considered desirable to assure adequate refining. When operating in accordance with the present invention, however, it has been found preferable to restrict the sodium sulfate to two parts by weight to maintain a manageable level of foaming, and yet it has been found that refining is not detrimentally affected. Most preferably, the sodium sulfate is utilized at no more than one part per 1000 parts sand, with one-half part being a particularly advantageous example. These weight ratios have been given for sodium sulfate, but it should be apparent that they can be converted to other sulfur sources by molecular weight ratios. Complete elimination of refining aids is feasible with the present invention, although trace amounts of sulfur are typically present in other batch materials so that small amounts of sulfur may be present even if no deliberate inclusion of sulfur is made in the batch. No significant detrimental effect on physical properties of glass subjected to the vacuum refining method of the present invention has been found. However, the vacuum treatment does have some detectable effect on the composition of the glass, such that glass produced by this method can be distinguished from the same type of glass produced by a conventional commercial process. The vacuum treatment has been found to reduce the concentration of volatile gaseous components, particularly the refining aids such as sulfur, to levels lower than the equilibrium levels attained with conventional processes. Glass produced in small pot melts or the like is sometimes reported as having very little or no residual refining aid content. This is because non-continuous melting processes can provide long periods of time for refining, thereby avoiding the need for chemical refining aids. Also, small melts are often produced from chemically pure raw materials and from oxide raw materials that, unlike conventional carbonate mineral batch materials, do not produce substantial volumes of gaseous by-products. However, soda-lime-silica glass products that are mass-produced by continuous melting processes are characterized by significant amounts of residual refining aids. Such products would include glass sheets suitable for glazing vision openings in buildings or vehicles (e.g., float glass) and container ware (e.g., bottles). In such products, the residual sulfur content (expressed as SO₃) is typically on the order of 0.2% by weight and seldom less than 0.1%. Even when no deliberate addition of sulfur refining aid is made to the batch, at least 0.02% SO₃ is usually detected in a soda-lime-silica glass made in a conventional continuous melter. Flat glass for transparent vision glazing applications normally has more than 0.05% SO₃. In distinction thereto, soda-lime-silica glass can be produced continuously by the present invention at the preferred vacuum levels with less than 0.02% residual SO₃, even when relatively small amounts of sulfur refining aid are being included in the batch as described above, and less than 0.01% SO₃ when no deliberate inclusion of sulfur is being made. At the lowest pressures, with no deliberate sulfur addition, SO₃ contents less than 0.005% are attainable. Commercial soda-lime-silica glass that is commonly refined with sulfur compounds may be characterized as follows: Small amounts of colorants or other refining aids may also be present. Arsenic, antimony, fluorine, and lithium compounds are sometimes used as refining aids, and residues may be detected in this type of glass. A sheet of float glass or a bottle represent commercial embodiments of the above composition. A sheet of glass that has been formed by the float process (i.e., floated on molten tin) is characterized by measurable amounts of tin oxide that migrated into surface portions of the glass on at least one side. Typically a piece of float glass has an SnO₂ concentration of at least 0.05% by weight in the first few microns below the surface that was in contact with the tin. Because the float process entails a relatively large scale continuous melting furnace of the type that conventionally employs significant amounts of sulfur-containing refining aids, float glass is characterized by minimum SO₃ concentrations higher than those discussed above for soda-lime-silica glass in general. Therefore, float glass refined by the present process having less than 0.08% SO₃ would be distinguished from conventional commercially available float glass. Most float glass falls within the following compositional ranges: Colorants and traces of other substances may be present. Although the description of the preferred embodiments and some of the advantages of the present invention are associated with continuous processes of making glass or the like, it should be evident that non-continuous refining operations could also gain at least some of the benefits of the present application.

Claims: A method for melting and refining glassy material or the like by producing a melt of the material and refining the molten material in a further step within a refining vessel (12) and withdrawing the molten material from the body (51) of molten material in the refining vessel, characterized by, foaming the molten material as it is introduced into a space (50) maintained at subatmospheric pressure above a body of the molten material (51 )within the refining vessel (12) and collapsing the foam into the body of molten material (51). The method of claim 1 wherein the molten material is introduced into the subatmospheric space (5O)through a valved orifice (44) above the level of the molten material (51). The method of claim 1 wherein the rate of withdrawal is controlled by outlet orifice means (35, 36, 37). The method of claim 1 wherein transport through the body of molten material (51)is predominantly in a vertical direction toward the location of withdrawing. The method of claim 1 wherein the subatmospheric pressure is no more than one half of atmospheric pressure. The method of claim 5, wherein the subatmospheric pressure is within the upper portion of the vessel (12) and the pressure at the lower portion of said refining vessel (12), is at least atmospheric. The method of claim 1 wherein the subatmospheric pressure is no more than one third of atmospheric pressure. The method of claim 1 wherein the pressure in the body of molten material (51) at the elevation of withdrawal is at least atmospheric pressure. The method of claim 1 wherein material that is newly foamed is deposited onto a mass of foam previously foamed. The method of any of claims 1 to 8 wherein the material being refined is glass. The method of any of the claims 1 to 10 wherein the melt of the material is produced by heating the material in a first chamber (10) and rendering therein batch materials to an incompletely melted flowable state, draining the flowable material including unmelted particles to a second chamber (11) where melting of the particles is substantially completed, and passing the material from the second chamber (11) to the refining vessel (12) where it is refined. The method of claim 11 wherein the material is passed from the first chamber (10) to the second chamber (11) at a temperature of 1200°C (2200°F) to 1320°C (2400°F). The method of claim 11 wherein the material is passed from the second chamber (11) to the refining vessel (12) at a temperature no greater than 1450°C (2700°F). The method of claim 11 wherein the material entering the second chamber (11) includes unmelted particles, and sufficient residence time is provided in the second chamber (11) to permit dissolution of the particles. The method of claim 11 wherein the material is heated in the second chamber (11) to a temperature suitable for refining. The method of claim 11 wherein the overall temperature of the glass is not increased in the refining vessel (12). The method of claim 11 wherein the batch material is continously melted to produce a transparent soda-lime-silica glass product without adding materials that serve primarily as refining aids. The method of claim 17 wherein the material being melted is soda-lime-silica glass, and the batch materials are fed to the first chamber (10) with a sulfur source as a refining aid in an amount no greater than an equivalent amount of 2 parts by weight sodium sulfate per 1000 parts by weight of silica source material. Apparatus for melting and refining glassy materials or the like, comprising: a first chamber (10, 15) having means (22) to heat a batch material to a flowable condition and means (27) to drain liquefied batch material (28) therefrom directly upon attaining the flowable condition; a second chamber (11) adapted to receive the liquefied batch material (28) from the first chamber (10, 15) and adapted to substantially complete dissolution of particles in the liquefied material; and a refining vessel (12) adapted to receive the heated material from the second chamber (11) at an upper end and means (55, 56) to drain material from a lower portion of the refining vessel (12) characterised in that, the refining vessel (12) is at its upper part provided with means (52) to support subatmospheric pressure within said vessel (12) above the compact body of heated material when the apparatus is in operation The apparatus of claim 19 wherein inlet means (44) are provided at an upper portion of the refining vessel (12) adapted to pass molten material to a space above the molten body in said vessel (12)and outlet means (55) are provided at a lower portion of said vessel (12)adapted to pass molten material from said vessel (12). The apparatus of claim 20 wherein the inlet means (36) has associated with it means (37) to control the flow rate of molten material therethrough. The apparatus of claim 20 wherein the outlet means (55) has associated therewith means (53, 59) to control the flow rate of molten material therethrough. The apparatus of claim 19 wherein the refining vessel (12) is generally cylindrical in configuration. The apparatus of claim 23 wherein the height of the refining vessel (12) is no more than five times its width. The apparatus of claim 19 wherein the refining vessel (12) includes a gas-tight shroud (41). The apparatus of claim 25 wherein the shroud is provided with cooling means (42, 43). Use of the method of any of claims 1 to 18 to produce a soda-lime-silica glass comprising: having a residue of a sulfur-containing refining aid in an amount less than 0.01 weight percent measured as SO₃. The use of claim 27 wherein the bulk composition consists essentially of: The use of claim 27 wherein the SO₃ content is less than 0.005 % by weight. The use of claims 27 to 29 wherein the glass has less than one gaseous inclusion per 100 cm³. The use of claim 30 wherein the glass has less than one gaseous inclusion per 1.000 cm³.

Inventors: KUNKLE, GERALD ERASMUS, SCHWENNINGER, RONALD LEE, WELTON, WRIGHT MONTGOMERY
Applicants: PPG INDUSTRIES, INC.
Application year: 1986
Main classification: C03B5
IPC codes: C03C3, C03B3, C03B5