Patent Application
20240010539
The invention relates to methods of making high quality glass products as well as to high quality glass products.
There are different kinds of glass compositions. Some glass compositions are relatively easy to manufacture in good quality, others require sophisticated equipment and/or extremely well balanced production processes. Generally, the glass compositions used to make drinking glasses and ordinary windowpanes are of the former type. One reason is that the glass used for these products has rather low melting temperatures and a steep viscosity-temperature curve; in addition, quality criteria of the manufactured products are not very stringent. For example, ordinary windowpanes and drinking glasses may contain an occasional bubble, and slight variations in shape and dimension are tolerable.
The types of glass compositions used for many mass products, such as soda lime glass compositions, have low melting temperatures because of significant amounts of alkali metal oxides and alkaline earth metal oxides. The respective glass melting facilities achieve very high throughput, often more than 200 tons of glass per day, or even 400 tons per day. Of course, the amount of energy needed for making 200 tons of low melting glass is significantly lower than for a higher melting glass.
The quality required for a given product depends on its intended use. Some high-quality glasses are used to make products that do not entertain the occasional bubble and must meet stringent criteria in terms of shape and dimensional variations. Many of these glasses are rather difficult to manufacture not only because of the stringent criteria but also because of high melting temperatures. High melting temperatures may be necessary to achieve melt viscosities sufficient for homogenization and removal of bubbles from the melt.
What is needed in the art is a way to provide glass products that meet high quality criteria.
In some embodiments, the invention relates to a method of making high quality glass products from high viscosity melts, the method comprising: heating glass raw material to obtain a glass melt; heating the glass melt in a melting tank, the melting tank having a melting tank bottom, and the glass melt having a volume and a melt surface; removing bubbles from the melt; withdrawing the glass melt from the melting tank at a rate of 2.0 t per m3 melt volume per 24 h or less; and obtaining a glass product having less than 20 bubbles per kg of glass. The glass melt is heated using one or more heat sources such that at least a part of the glass melt has a viscosity of 102.5 dPas or less. Heating the glass melt includes heating the melt surface and/or directly heating the glass melt volume. The amount of thermal energy introduced directly into the glass melt volume is more than 60% of the total amount of thermal energy introduced into the melt in the melting tank. The maximum difference between a temperature at a location on the glass melt surface and a temperature at a location at the melting tank bottom vertically underneath said location is such that the difference in glass melt densities corresponding to said temperatures is less than 0.05 g/cm3 per meter distance between the two locations. The glass melt has a viscosity of 102 dPas at a temperature above 1580° C.
The method provided according to the invention is for glass compositions with very high melting temperatures, such as glass compositions comprising only limited amounts of alkali and alkaline earth metal oxides. The composition of the glass is such that the melt has a viscosity of 100 dPas at temperatures above 1580° C. The method includes heating the melt to a temperature sufficiently high that the viscosity is 102.5 dPas or less. This means that a lot of energy is required. Low viscosities and the corresponding high temperatures are desirable for bubbles to leave the melt.
The method includes that a major part of the thermal energy is introduced directly into the glass melt volume. For example, one or more electrodes may be used to heat the glass melt directly. One or more electrodes may be located partially or completely in or on a wall of the melting tank. One or more electrodes may be located partially or completely in or on a bottom plate of the melting tank. In some embodiments, one or more electrodes constitute a wall section and/or a bottom plate section of the melting tank.
It was found that if a maximum difference in glass melt densities between (a) a location on the surface of the glass melt in the melting tank and (b) a location at the melting tank bottom vertically underneath said location is limited, the occurrence of short-circuit glass flow can be avoided. Short-circuit glass flow is undesirable because it means that portions of the melt take a shortcut to an outlet of the melting tank. These portions have reduced dwelling time in the melting tank. The resulting glass products typically have large amounts of bubbles and/or striae.
The glass compositions of the method disclosed herein have high viscosities. These compositions often contain high amounts of SiO2 and Al2O3. Both components have rather high melting temperatures. In most glass melts, SiO2 is the last component in the batch that liquefies. This means that before all the components of the batch liquefy, there may be portions within the melt volume having reduced proportions of higher melting components. Of course, the mentioned portions within the melt volume have higher amounts of lower melting components, particularly of the alkalis. The method described herein avoids short-circuit glass flow by limiting a difference in glass melt densities within the melting tank. Limiting the density difference as described herein reduces undesired volume flow within the melting tank induced by differences in density. Thereby, very high quality glass products are obtainable. Particularly, glass products produced by the method described herein may have particularly low bubble counts, and/or low or no striae.
In some embodiments, the invention relates to a glass product having less than 20 bubbles per kg of glass, the glass having a viscosity of 102 dPas at a temperature above 1580° C., the glass having a temperature dependence of glass melt density in the temperature range of from T4 to T2 of at least 9.0 mg/cm3 per 100° C., where T4 is the temperature where the glass has a viscosity of 104 dPas and T2 is the temperature where the glass has a viscosity of 102 dPas.
The invention also provides glass products obtainable using the method. The glass products have a very low bubble count. Notably, by avoiding short-circuit glass flow, the dwelling time of the glass melt in the melting tank is sufficiently long for bubbles to rise to the glass melt surface and leave the melt volume. This is relevant for highly viscous melts because bubbles rise to the surface with lower speed in viscous melts compared to low viscosity melts. This is true even for glass compositions that have a rather high temperature dependence of glass melt density.
As discussed before, the method avoids that portions of the melt take shortcuts to the exit of the melting tank. Particularly, this applies to low viscosity portions of the melt that tend to have lower relative amounts of SiO2. The method provided according to invention does not allow low viscosity portions of the melt to leave the melting tank before the final glass composition is achieved within the melting tank. The result is a high quality glass product.
The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:
Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate embodiments of the invention and such exemplifications are not to be construed as limiting the scope of the invention in any manner.
A “glass melt” is a volume of a batch of glass raw materials that has a viscosity of less than 107.6 dPas.
A “melting tank” is a vessel used for melting glass. The vessel defines a volume that can contain a glass melt. The melting tank may have a substantially rectangular base, or bottom plate. It may have walls to keep the melt within the tank. Typically, a melting tank will not be filled to the rim. A melting tank may have a cover above the glass melt surface (“covered melting tank”). The cover may be vaulted. The “melting tank” may be a part of a larger melting facility, which may comprise further parts such as a refining tank or refining area. Some melting facilities have a combined tank with different sections, one section for melting and one for refining, in which case the “melting tank” according to this disclosure relates to the whole combined tank including the refining section.
The “bottom plate” is a part of the melting tank that forms the bottom of the tank. The bottom plate may be a single piece of material. Alternatively, the bottom plate may be composed of a plurality of parts or sections. The bottom plate may be closed, i.e. essentially impermeable to the glass melt. Alternatively, the bottom plate may have a closable opening so that glass melt may be withdrawn from the melting tank through the bottom opening.
The “hottest location on the glass melt surface” is the spot on the surface of the glass melt within the melting tank that has the hottest temperature compared to other locations on the surface of the glass melt within the melting tank. The temperatures on the glass melt surface can easily be measured using thermocouples and/or pyrometers. Temperatures on the bottom of the melting tank may for example be measured using thermocouples within the bottom plate or extending into the melt. Thermocouples may be water-cooled. The “glass melt surface” is the part of the melt that is in direct contact with the surrounding atmosphere (e.g. air). The melting tank may comprise portions where unmelted raw material swims on melted material, e.g. close to a raw material inlet port. These portions—which may correspond to 10 to 30% of the melting tank area—are not considered part of the glass melt surface.
The shortest distance between the “glass melt surface” and the melting tank bottom that is in contact with the melt, is the “glass melt depth” at the location under consideration.
A “bubble” is a gaseous inclusion within the glass or the glass melt, optionally having a diameter of at least 10 μm. The “diameter” means the largest diameter of the gaseous inclusion.
“Dwelling time” is the time that a given portion of the glass melt spends in the melting tank before being withdrawn from the melting tank. Dwelling time can be measured using so-called tracers, i.e. components that are added to the glass melt so that they can be detected in the product, allowing conclusions as to the time spent in the melting tank. Examples of tracer compounds are Ca, Sr and Y. The “minimum dwelling time” is the time that a portion of glass melt needs to travel through the melting tank taking the fastest path, i.e. the time between addition of an amount of tracer compound into the melting tank and the first occurrence of the tracer in the product. The “average dwelling time” is defined as
The method includes heating glass raw material to obtain a glass melt. Heating takes place in the melting tank, typically having a melting tank bottom. The melting tank may have melting tank walls and/or an optionally vaulted cover. The melting tank is filled with the glass melt up to a certain level. The glass melt has a melt surface at the interface between the melt volume and the atmosphere above the melt. Heating may include high-frequency heating, such as in a melting tank having cooled walls. The walls of the melting tank may be cooled using a cooling agent, such as water. Cooling may include flowing the cooling agent through channels within melting tank walls. Cooling melting tank walls has the advantage that corrosion of melting tank wall material is reduced.
The method includes removing bubbles from the melt. Removing bubbles from the melt may occur within the melting tank and optionally outside the melting tank e.g. in a separate vessel. The separate vessel may be a fining vessel. Bubbles may be removed from the melt using chemical and/or physical methods. In some embodiments, removing bubbles from the melt includes that bubbles rise to the glass melt surface in the melting tank. In some embodiments, bubbles are removed from the melt in the melting tank by rising to the glass melt surface, and further bubble removal is performed in a separate vessel afterwards. Owing to the improved method provided according to the invention, bubble removal in the melting tank contributes to achieving excellent bubble quality in the glass product. In some embodiments, the method includes the step of removing bubbles from the melt in the melting tank and/or in a separate fining vessel after withdrawing the glass made from the melting tank. A temperature of the glass melt in the fining vessel may be higher than the highest temperature of the glass melt in the melting tank. Optionally, the method provided according to invention does not include vacuum fining and/or bubbling the melt.
The method includes withdrawing the glass melt from the melting tank. The method provided according to the invention is a relatively low throughput method focusing on glass product quality instead of quantity. In some embodiments, the glass melt is withdrawn from the melting tank at a rate of 2.0 t per m3 melt volume per 24 h (t/(m3*24 h)) or less. This means that per day not more than 2.0 t of glass melt are withdrawn from the melting tank per m3 glass melt present in the melting tank (“withdrawal rate”). Optionally, the withdrawal rate is less than 1.5 t/(m3*24 h), less than 1.3 t/(m3*24 h), less than 1.1 t/(m3*24 h), or less than 0.9 t/(m3*24 h). In some embodiments, the withdrawal rate is at least 0.1 t/(m3*24 h), at least 0.3 t/(m3*24 h), or at least 0.5 t/(m3*24 h).
The method provided according to the invention includes obtaining a glass product. Optionally, the glass product may have less than 20 bubbles per kilogram of glass. In some embodiments, the glass product has less than 10 bubbles, less than 5 bubbles, less than 3 bubbles or less than 2 bubbles per kilogram of glass.
The method includes heating the glass melt using one or more heat sources. In some embodiments, the heat sources include electrodes that contact the glass melt. The electrodes can comprise or consist of a metal, alloy or metal oxide, such as platinum, iridium, tungsten, molybdenum or tin oxide. Alternatively or additionally, the heat sources include burners such as gas burners and/or microwave heaters. The gas may be natural gas. In some embodiments, the gas used for the burners comprises or consists of non-fossil gas, such as biofuel (e.g. biogas) and/or hydrogen, particularly hydrogen obtained from renewable sources. In some embodiments, the amount of thermal energy obtained from burning fossil fuels is less than 10.0%, less than 5.0%, or less than 1.0% relative to the total amount of thermal energy introduced into the melt.
In some embodiments, the carbon dioxide footprint of the glass product is less than 500 g CO2 per kg of glass. In some embodiments, the carbon dioxide footprint of the glass product is less than 400 g, less than 300 g, less than 200 g, less than 100 g or even 0 g of CO2 per kg of glass. For example, a glass product having a carbon dioxide footprint of zero can be made using energy from renewable sources only, e.g. biofuel, hydrogen, or electricity from renewable sources. The carbon dioxide footprint refers to the CO2 emissions resulting from scope 1 emissions according to the GHG protocol. In this context, this refers to the amount of CO2 emission caused by burning fossil fuels and released by carbon containing raw materials during manufacturing of the glass product per kg of glass material. Optionally, the raw materials used in the method provided according to the invention do not contain any carbonates, i.e. carbonates may be present as impurities in the raw material (<0.1% by weight) but they are not intentionally added to the composition.
The amount of thermal energy introduced into the melt by heating the surface may be less than 40.0% relative to the total amount of thermal energy introduced into the melt. Optionally, this value may be less than 30.0%, less than 25.0%, less than 15.0%, or less than 5.0%, or less than 1.0% or even 0%.
In some embodiments, heating the melt surface includes heating the surface using one or more microwave heaters, and/or burning bio-fuel and/or hydrogen. Directly heating the melt volume may include electrode heating.
The glass melt is heated such that at least a part of the glass melt has a viscosity of 102.5 dPas or less. Optionally, at least a part of the glass melt may have a viscosity of 102.3 dPas or less. Optionally, glass melt viscosity will not be less than 5 dPas or not less than 10 dPas. Heating the glass to very low viscosities may not be preferred as it increases the risk of short-circuit glass flow and increases erosion of the melting tank walls. In addition, low viscosities correspond to very high temperatures which is not preferred in terms of power consumption. Optionally, a minimum viscosity of the glass melt within the glass melt volume is not less than 10 dPas.
Heating the glass includes heating the melt surface and/or directly heating the glass melt volume. In some embodiments, heating includes directly heating the glass melt volume, or alternatively heating the melt surface and directly heating the glass melt volume. It was found that a proper balance between heating the melt surface and the glass melt volume is useful to obtain high-quality glass products of high viscosity glass compositions. Balancing heating the glass melt surface and heating the glass melt volume may result in a reduction of maximum glass melt density difference between a location on the glass melt surface and a location at the melting tank bottom vertically underneath said location as discussed in this disclosure.
The amount of thermal energy introduced directly into the glass melt volume is more than 60% of the total amount of thermal energy introduced into the melt. This refers to the thermal energy introduced into the melt in the melting tank. Introduction of thermal energy may be different in an optional fining vessel or other optional parts of the glass melting facility. In some embodiments, the introduction of thermal energy discussed herein may apply to an optional fining vessel and/or fining section in a combined melting tank as well. In some embodiments, the amount of thermal energy introduced directly into the glass melt is more than 70%, more than 76%, more than 80% or more than 90% of the total amount of thermal energy introduced into the melt. Optionally, more than 99% or essentially all of the total amount of thermal energy introduced into the melt is introduced directly into the glass melt volume. In some embodiments, all the thermal energy is introduced directly into the melt, i.e. no heating of the glass melt surface takes place. The exact balance of thermal energy introduction depends on the glass melt properties, e.g. the temperature dependence of glass melt density and viscosity characteristics, including VFT parameters. In some embodiments, a maximum density difference may be reduced by distributing electrodes over the volume of the melting tank such that introduction of heating energy is uniform enough to achieve the desired density difference. For example, if an area near the bottom of the melting tank is too cold, electrodes can be arranged such that more thermal energy reaches that cold spot. Alternatively, if a cold spot is closer to the surface, a longer electrode ranging from the bottom up to the surface of the melt may be used to avoid the cold spot. In some embodiments, the melting tank includes a plurality of electrodes. For example, a number of electrodes may be defined relative to the surface area of the glass melt surface. In some embodiments, the melting tank includes at least 1.0 electrodes per mz of glass melt surface. In some embodiments, the number of electrodes per m2 of glass melt surface may be at least 2.0, at least 3.0, or at least 6.0. Optionally, the number of electrodes per area of glass melt surface may range up to 8.0/m2 or up to 7.0/m2. Optionally, the heat sources may be arranged such that a uniform heat distribution is achieved.
For example, thermal absorption of the glass melt may influence the balance of thermal energy introduction desired to achieve best results. Higher proportions of direct heating of the glass melt may be appropriate for glass melts with higher thermal absorption. In some embodiments, the thermal absorption coefficient (x at 1600° C.) of the glass melt is at least 8.0 m−1, and optionally up to 60.0 m−1, and the amount of thermal energy introduced directly into the glass melt is more than 60%, or more than 70%. In some embodiments, the thermal absorption coefficient (x at 1600° C.) of the glass melt is at least 10.0 m−1, at least 12.0 m−1 or at least 14.0 m−1. Additionally or alternatively, the thermal absorption coefficient (x at 1600° C.) of the glass melt is at most 55.0 m−1, at most 45.0 m−1 or at most 40.0 m−1.
In the melting tank, the maximum difference between a temperature at a location on the glass melt surface and a temperature at a location at the melting tank bottom vertically underneath said location is such that the difference in glass melt densities corresponding to said temperatures is less than 0.05 g/cm3 per meter distance between the two locations (“maximum density difference”). This “maximum density difference” means that no difference between any two described locations at the surface and vertically underneath it at the bottom below in the melting tank is larger than indicated. As discussed before, minimizing vertical density differences helps to reduce or completely avoid short-circuit glass flow in the melting tank which will ultimately improve quality of the products obtained using the method. Optionally, the density difference is less than 0.04 g/cm3, less than 0.03 g/cm3, or less than 0.02 g/cm3. In some embodiments, a limited density difference may be unavoidable, such as at least 0.001 g/cm3, or at least 0.005 g/cm3. The “maximum density difference” relates to the magnitude of the difference between the locations, i.e. the density at the surface may be higher or lower than the density at the bottom. Alternatively or additionally, the “maximum density difference” applies to the first half of the melting tank, i.e. the half of the melting tank that includes the raw material inlet after (conceptually) dividing the melting tank at a cross-section perpendicular to its longitudinal axis.
Alternatively or additionally, a difference between a temperature at a hottest location on the glass melt surface and a temperature at a location at the melting tank bottom vertically underneath said hottest location is such that the difference in glass melt densities corresponding to said temperatures is less than 0.04 g/cm3 per meter distance between the two locations (“density difference”). As discussed before, minimizing vertical density differences in the melting tank helps to reduce or completely avoid short-circuit glass flow in the melting tank which will ultimately improve quality of the products obtained using the method. Optionally, the density difference is less than 0.03 g/cm3, less than 0.025 g/cm3, or less than 0.02 g/cm3. In some embodiments, a limited density difference may be unavoidable, such as at least 0.001 g/cm3, or at least 0.005 g/cm3. The “density difference” relates to the magnitude of the difference between the locations, i.e. the density at the surface may be higher or lower than the density at the bottom.
Temperature distribution and thus density distribution can be achieved using electrodes of the desired size and shape. For example, if more thermal energy is needed close to the glass melt surface, longer electrodes can be used. In some embodiments, heating the melt may include heating with one or more electrodes extending upwardly from the bottom of the melting tank up to at least 50% glass melt depth, optionally at least 60%, at least 70% or at least 80% of the glass melt depth. Optionally, the one or more electrodes may extend up to 100%, up to 95% or up to 90% of the glass melt depth from the bottom of the melting tank. Optionally, the one or more electrodes may extend from 50% to 100%, from 60% to 95%, or from 70% to 90% of the glass melt depth from the bottom of the melting tank.
In some embodiments, the melting tank has electrodes surfaces that contact the glass melt during heating the glass melt, which are referred to as “electrode surface area”. The “total electrode surface area” is the sum of the surface areas of all electrodes in the melting tank. This value may be at least 0.15 m2 per m3 of glass melt volume, at least 0.2 m2 per m3 glass melt volume or at least 0.25 m2 per m3 glass melt volume. Optionally, the total electrode surface area may range up to 1.5 m2 per m3 or up to 1.25 m2 per m3 of glass melt volume.
The temperature at which the glass melt has a viscosity of 102 dPas is herein called temperature T2. Similarly, the temperature at which the glass melt has a viscosity of 104 dPas is herein called temperature T4. Temperature T2 is less than 1500° C. for glass compositions with high contents of alkali metal oxides or alkaline earth metal oxides, such as soda lime glass and other glass compositions. The glass compositions used according to the invention have much higher T2 and/or T4 temperatures. The T2 temperature for the glass melt in the melting tank during the method of this disclosure is above 1580° C. and optionally even above 1600° C. or above 1620° C. In some embodiments, T2 temperature of the glass compositions may be less than 1800° C., less than 1750° C. or less than 1700° C. Glass compositions with very high T2 temperatures are very difficult to process and require a lot of energy for melting.
The T4 temperature of the glass composition in the melting tank during the method of this disclosure is above 1000° C. and optionally even above 1050° C. or above 1120° C. In some embodiments, T4 temperature of the glass compositions may be less than 1400° C., less than 1350° C. or less than 1300° C. Glass compositions with very high T4 temperatures are very difficult to process and require a lot of energy for melting.
High melting point glass compositions require high melting temperatures, which may in turn require significant cooling of the melting tank walls in order to avoid excessive corrosion of the wall material. Extensive cooling of melting tank may have a negative influence on glass melt flow as it may greatly accelerate melt flow viscosity, increasing the risk of short circuit glass flow.
It was found that a minimum dwelling time in the melting tank may be set to at least 10 h, at least 12 h or at least 14 h. Sufficient dwelling time is useful for making a high quality glass product. Optionally, a minimum dwelling time may be up to 70 h, up to 65 h or up to 60 h. Additionally or alternatively, an average dwelling time may be 48 h±12 h. Too long dwelling times are not desirable considering that long dwelling times increase the carbon dioxide footprint of the glass products. In some embodiments, the minimum dwelling time is at least 20% of the average dwelling time. Optionally, the minimum dwelling time may be at least 25% or at least 35% or at least 45% of the average dwelling time. Of course, the higher the ratio minimum dwelling time to average dwelling time, the more energy efficient the process will be. The process provided according to the invention helps to achieve a desirable ratio of minimum to average dwelling time, such as from 20% to 100%, or up to 90%, up to 80% or up to 70%.
The melting tank may be made of a variety of heat resistant materials. In some embodiments, the bottom plate and/or walls of the melting tank comprise or consist of refractory material. The refractory material may be a ceramic material, a metal material or a combination thereof. A suitable metal is platinum. Platinum is very expensive and may contaminate the melt, if it dissolves due to aggressive melt components. In some embodiments, the refractory material does not comprise or consist of platinum and/or other metal. In some embodiments, the refractory material is an oxidic ceramic, i.e. a material comprising or consisting of one or more oxides, such as metal oxides. Optionally, the refractory material is temperature resistant up to at least 1400° C., at least 1600° C. or even at least 1700° C. “Temperature resistant” means in this context that the refractory material has a melting point or melting range above the indicated temperature. In some embodiments, the refractory material is selected from ZrO2, Al2O3, SiO2 and combinations thereof.
The method disclosed herein may be used for glass melts having resistivities at 1700° C. of at least 1.5 Ωcm. A minimum resistivity is useful for effective heating and reduced currents. Optionally, resistivity of the glass melt may range up to 30 Ωcm, up to 25 Ωcm or up to 20 Ωcm. In some embodiments, glass melt resistivity may be at least 2.0 Ωcm, at least 3.0 Ωcm or at least 4.0 Ωcm. Optionally, glass melt resistivity may range from 1.5 Ωcm to 30 Ωcm, from 2.0 Ωcm to 25 Ωcm, or from 3.0 Ωcm to 20 Ωcm.
Depending on the introduction of thermal energy into the glass melt, glass melt density at a location on the glass melt surface may be smaller than at the location at the melting tank bottom vertically underneath said location. This may be the case, if a considerable amount of thermal energy is introduced into the glass melt by heating the surface of the melt. Alternatively, glass melt density may be larger at a location on the glass melt surface than at the location at the melting tank bottom vertically underneath said location. In other words, the “density difference” and “maximum density difference” relate to the magnitude of difference between the locations.
Generally, density is larger at cooler temperatures. The more thermal energy is introduced directly into the glass melt volume in relation to the heating energy introduced to the surface of the melt, the hotter will be the melting tank bottom. In some embodiments, heating the glass melt volume includes introducing heat using electrodes at the bottom of the melting tank and/or electrodes in or on a bottom plate of the melting tank. The risk of occurrence of short-circuit glass flow particularly high, if glass melt density at the surface of the melt greatly exceeds the density at the bottom. In some embodiments, glass melt density at the hottest location of the glass melt surface exceeds the glass melt density at the bottom of the melting tank underneath the hottest location by not more than 0.045 g/cm3, not more than 0.035 g/cm3, or not more than 0.025 g/cm3.
It was found that there are glass compositions having a relatively high temperature dependence of the glass melt density in the temperature range of from T4 to T2. This temperature range is of particular relevance for glass melting and methods as discussed herein. In some embodiments, a temperature dependence of glass melt density in the temperature range of from T4 to T2 is at least 9.0 mg/cm3 per 100° C. and optionally up to 19.0 mg/cm3 per 100° C. This value may relate to the average change of density within the indicated temperature range. Optionally, this temperature dependence of glass melt density is at least 10.0 mg/cm3, at least 11.0 mg/cm3, at least 12.0 mg/cm3, or at least 13.0 mg/cm3 per 100° C. of temperature difference. Optionally, this temperature dependence of glass melt density is up to 18.0 mg/cm3, up to 17.0 mg/cm3, up to 16.0 mg/cm3, or up to 15.0 mg/cm3 per 100° C. of temperature difference.
As discussed above, the method provided according to the invention is for making high quality products of highly viscous glass compositions. The focus is on quality not quantity, the throughput is low and accordingly the melting tank may be of limited size. Optionally, said vertical distance between a location on the glass melt surface and the location at the melting tank bottom vertically underneath said location is less than 1250 mm, or less than 1100 mm. Additionally or alternatively, the distance may be at least 750 mm, or at least 850 mm. In some embodiments, the same applies to the vertical distance between the glass melt surface and the melting tank bottom.
Viscosity can, for example, be measured using a rotational viscosimeter, e.g. as described in DIN ISO 7884-2:1998-2. The dependence of viscosity on temperature is described according to the VFT equation (Vogel-Fulcher-Tammann). The VFT equation is shown below.
In the VFT equation, t is the temperature under consideration. A, B and to are the so-called VFT constants that are specific for each glass composition. As discussed before, viscosity behavior of the glass composition used in the method is of relevance. The temperature dependence of glass melt viscosity may be described using the VFT equation. Optionally, the glass melt has a VFT constant B in the range of from 5,000 to 9,000 K and/or to in the range of from 75° C. to 240° C. Optionally, VFT constant A is from −5.0 to 0.0. In some embodiments, A is −1.0 or less, such as −4.0 to −1.0. In some embodiments, VFT constant B is from 5,000 K to 9,000 K, such as from 4,500 to 8,500 K. Optionally, to may be at least 75° C. and up to 240° C.; to may be at least 200° C.
The invention also relates to a glass product. The glass product may be obtainable by a method as described herein. Accordingly, the properties that relate to the composition of the glass melt apply to the composition of the glass product accordingly.
The glass product may have less than 20 bubbles per kilogram of glass. In some embodiments, the glass product has less than 10 bubbles, less than 5 bubbles, less than 3 bubbles or less than 2 bubbles per kilogram of glass.
The glass may have a coefficient of thermal expansion in a temperature range of from 20° C. to 300° C. of from 3.0 ppm/K to 8.5 ppm/K, optionally less than 7.0 ppm/K, or less than 5.5 ppm/K. The coefficient of thermal expansion is the average linear thermal expansion coefficient according to DIN ISO 7991:1987.
The glass product can be a sheet, wafer, plate, tube, rod, ingot or block.
The glass composition may be borosilicate, alumino-borosilicate or aluminosilicate glass. The glass composition may contain alkali metal oxides in amounts of less than 20% by weight, less than 15% by weight, less than 12% by weight, less than 10% by weight or less than 5% by weight. Optionally, the glass composition may be free of alkali metal oxides. In some embodiments, the amount of alkali metal oxides in the glass composition may be at least 1% by weight.
The glass composition may contain alkaline earth metal oxides in amounts of less than 20% by weight, less than 15% by weight, less than 12% by weight, less than 10% by weight, or less than 5% by weight. Optionally, the glass composition may be free of alkaline earth metal oxides. In some embodiments, the amount of alkaline earth metal oxides in the glass composition may be at least 1% by weight.
The glass composition may contain SiO2 in an amount of at least 48% by weight, at least 55% by weight, at least 65% by weight, at least 70% by weight or at least 75% by weight. Optionally, the amount of SiO2 may range up to 85% by weight, up to 82.5% by weight or up to 80% by weight.
Optionally, the glass composition may be that of a glass ceramic, i.e. a glass composition that can be further processed into a glass ceramic by appropriate heat treatment. In case that the glass composition is that of a glass ceramic, the composition may contain nucleating agents such as TiO2 and/or ZrO2. Optionally, the total amount of TiO2 and/or ZrO2 may be at least 2.0% by weight, such as at least 2.5% by weight. The glass composition may, for example, be a lithium aluminosilicate glass composition, e.g. containing at least 2.0% by weight of Li2O.
The glass composition may include one or more fining agents. The fining agent may be selected from multivalent metal oxides, halides, sulfates and combinations thereof. In some embodiments, the fining agent is selected from tin oxide, cerium oxide, chloride, sulfate and combinations thereof.
Optional glass compositions include Al2O3 in an amount of at least 1.5% by weight or at least 5.0% by weight or even at least 10.0% by weight. The amount of Al2O3 may be up to 23.0% by weight, up to 20.0% by weight or up to 18.0% by weight. In some embodiments, the amount of Al2O3 may range from 1.5% to 23.0% by weight, from 5.0% to 20.0% by weight or from 10.0% to 18.0% by weight.
Additionally or alternatively, the glass composition may include B2O3 in an amount of at least 0.0% by weight or at least 8.0% by weight or even at least 10.0% by weight. The amount of B2O3 may be up to 20.0% by weight, up to 16.0% by weight or up to 14.0% by weight. In some embodiments, the amount of B2O3 may range from 0.0% to 20.0% by weight, from 8.0% to 16.0% by weight or from 10.0% to 14.0% by weight.
Many highly viscous glass compositions contain significant amounts of SiO2, Al2O3 and B2O3. Optionally, the glass compositions used according to the invention have a total content of SiO2, Al2O3 and B2O3 of at least 75.0% by weight, at least 78.0% by weight or even at least 85.0% by weight. The total amount of SiO2, Al2O3 and B2O3 may be limited to not more than 97.0% by weight, up to 93.5% by weight or up to 90.0% by weight. Optionally, the amount of SiO2, Al2O3 and B2O3 may range from 75.0% to 95.0% by weight, from 78.0% to 92.5% by weight or from 85.0% to 90.0% by weight.
The following embodiments describe optional aspects and embodiments provided according to the invention.
The invention provides a method of making high quality glass products from high viscosity melts, the method comprising:
The invention provides a method of making high quality glass products from high viscosity melts, the method comprising:
The invention provides a method of making high quality glass products from high viscosity melts, the method comprising:
Referring now to the drawings,
Electrodes 7 heat glass melt 10 directly, whereas burners 6 heat glass melt surface 11. The proportion of thermal energy introduced directly into glass melt 10 may exceed the amount of thermal energy introduced to the melt via glass melt surface 11. In some embodiments, 60% of the total thermal energy introduced into glass melt 10 is introduced directly into the volume using electrodes 7. Optionally, burners 6 may burn hydrogen, biofuel or other combustibles of renewable origin.
Generally, electrodes 7 may be in any shape. The drawings show rod-shaped electrodes 7 that extend a long way into melt 10 from the bottom 2 of melting tank 1.
Melt flow viscosity was modelled based on a glass melt density of 2.5 g/cm3 at 1550° C. The temperature is higher on the right side of the melt volume by 100° C., i.e. Too=1550° C. on the left and 1650° C. on the right. This resembles the situation in a melting tank with the refining section on the right. The melt was assumed to have a viscosity of 80 Pas at 1650° C. Further values of the glass melt were set as follows: coefficient of volume expansion β: 50 ppm/K, specific heat capacity cp: 1400 J kg−1K−1, thermal conductivity λeff: 150 W m−1K−1, glass viscosity η at 1550° C.: 80 Pas, thermal transmittance of bottom plate at an outside temperature of 27/C: h=15 W/(m2*K), thermal transmittance of side walls at an outside temperature of 27° C.: h=15 W/(m2*K), heat loss through cover of melting tank: h=100 W/(m2*K) towards Too(x), radiant emission with ε=1 towards Too(x).
There is very fast glass flow in the first two thirds of the melt volume as the melt flows from inlet to fining section.
While this invention has been described with respect to at least one embodiment, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 21163974.5 | Mar 2021 | EP | regional |
| PCT/EP2022/057350 | Mar 2022 | WO | international |
This is a continuation of International Patent Application No. PCT/EP2022/057350 filed on Mar. 21, 2022, which is incorporated in its entirety herein by reference. International Patent Application No. PCT/EP2022/057350 claims priority to European Patent Application No. 21163974.5 filed on Mar. 22, 2021, which is incorporated in its entirety herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2022/057350 | Mar 2022 | US |
| Child | 18470909 | US |
