Glass melting method and apparatus

Abstract

The present invention relates to a method for melting glass, in which microwave radiation is used for at least part of the energy supplied to the melt in order to convert the batch into a glass melt, the microwave radiation used acting on at least part of the transition between the batch and the initial melt, and in particular to an apparatus for melting glass for carrying out this method, in particular for converting the batch into a glass melt, the apparatus comprising a melting unit including a melting tank with walls, inside which not only the batch to be melted but also the melted batch as a glass melt can be accommodated, and at least one microwave radiation source, in particular at least one microwave emitter, is arranged above the batch and the glass melt.

Description

[Technical field]

[0001] The present invention relates to a method and apparatus for melting glass using microwave radiation, in particular for converting a batch into a glass melt.

[0002] Glass melting vessels on an industrial scale have traditionally been heated by burners, which use gas and / or oil in the respective burners for combustion, resulting in the emission of CO2 and, in the case of air, NOx as exhaust gases.

[0003] Some prior art work describes discontinuous crucible melting and deals with microwave heating.

[0004] WO 200200063 describes a crucible in a microwave resonator. Microwave heating can improve the chemical homogeneity of the glass melt, which can be attributed to hot spots (thermal inhomogeneities) in the volume of the melt.

[0005] In WO199700119, the melt in a cooled cavity is heated with tunable microwave radiation, the melt being present in a "solid" skull, where the uptake of the microwave radiation can be improved by the use of a plasma torch or the addition of graphite.

[0006] DE 195 41 133 A1 describes the melting of phosphate glasses in microwave-heated crucibles, but no information is given regarding the uptake of microwave radiation and its properties.

[0007] In some other prior art, continuous melting processes are carried out by microwave assisted heating. These are described for example in the following publications:

[0008] DE 200910025905 A1 discloses a melting method which requires melting of a batch by thin film melting. The thin film melting module consists of a double-walled tube which in one variant can be heated externally with microwave radiation via a susceptor. Melting of low-melting eutectic mixtures and specific raw material formulations is carried out. No direct coupling of microwaves to the batch is disclosed. The microwave radiation is introduced into the susceptor, which heats the batch.

[0009] FR 19960005084 describes an overflow crucible heated by microwave radiation. In an overflow crucible or in a V- or U-shaped tube, a standing wave is generated inside the tube. The microwave power is homogenized by modulation of the standing wave. The advantage is that a good homogenization of the melt is achieved due to a large volume mixing effect.

[0010] DE 10 2016205 845 A1 also discloses, inter alia, microwave heating for the pre-reaction of the batch, but this has the disadvantage that, especially in this temperature range, the microwave radiation is only very slightly absorbed by the batch, making the heating performance in this case very inefficient.

[0011] Japanese Patent Application 55-125514 describes a flow-through crucible in a closed resonator into which microwave energy is introduced. This document also does not focus on the initial melting region, but instead introduces microwave energy into the glass melt, dispersing the granular components to be melted.

[0012] DE 10 2016200697 A1 claims a continuously operating vessel which can be heated in various ways, in particular also by microwaves. No attention is given to the localization of the microwave energy.

[0013] Chinese Utility Model No. 204224428 discloses a gas-fired melting tank in which microwave radiation is introduced into a charge tube below the charge.

[0014] US20140255417 describes a method for producing glass pieces in a continuous furnace. In addition to various heating methods, microwave heating is also claimed. However, this is not a melting bath. The energy is incorporated into the pressed material.

[0015] China Utility Model No. 203128388 and China Patent Application Publication No. 201210552723 describe a burner-heated glass melting tank with a microwave emitter in the arch, which is used to break bubbles. The second document also claims that microwaves have a heating function. However, pure microwave top heating is not claimed. Furthermore, there is no description of energy release between the batch and the initial melt.

[0016] WO2006059576 claims a microwave assisted reduced pressure clarification chamber.

[0017] US Patent Publication No. 2004056026 describes a cascade vessel having multiple crucibles connected in series, which are placed in microwave resonators connected in series and heated by microwave radiation.

[0018] GYROTRON TECHNOLOGY INC. offers the subject of glass melting by microwave technology. The main feature is the use of gyrotrons in the frequency range from >30GHz to >100GHz. Gyrotrons are tubes with low efficiency of generating microwaves for their use, and the investment for such systems is very high. Magnetrons are an economical solution, but cannot be used for such high frequencies.

[0019] Such high frequencies (above 30 GHz) are undesirable because the penetration depth is extremely small and the risk of overheating is too high, whereas microwaves in the range of around 2 GHz present a much lower risk. Currently, with the developments in the plasma physics / nuclear fusion field, the efficiency of high power gyrotrons is comparable to that of magnetrons.

[0020] In all known cases, the microwaves act on the entire volume of the melt and essentially no directional heating occurs. What is important is the power density (W / m 3 ).

[0021] Also, all electric baths with ohmic heating are generally limited in throughput because the input power per unit melt surface is limited by the maximum allowable current density of the electrodes. The current density depends on the type of glass, so that for some glasses the value of ohmic heating is limited to less than 10% of the total energy demand. Within the scope of this disclosure, ohmic heating is understood to be heating in which a current passed through the melt generates heat at the ohmic resistance of the glass melt and this heat is introduced into the glass melt to heat it.

[0022] The invention is based on the problem of improving existing melting methods and melting apparatuses in such a way that greater efficiency is achieved in the use of energy used for heating during the melting of glass, in particular during the conversion of the batch into a glass melt, and preferably the environmental load is reduced.

[0023] This problem is solved by a method according to claim 1 and by a device according to claim 13.

[0024] According to the invention, a method for melting glass comprises using microwave radiation as at least a portion of the energy supplied to the melt to convert the batch into a glass melt, the microwave radiation being used acting at least at a portion of the transition between the batch and the initial melt.

[0025] In so doing, microwave radiation is introduced into the upper region immediately below the batch pile, and thus into the melting reaction zone, where the temperature is increased to promote melting, particularly relative to or in comparison to an otherwise identical process that does not use microwave radiation.

[0026] As will be explained in more detail below, it is precisely in this initial liquid phase bubbly initial molten region that the majority of the microwave energy is advantageously absorbed.

[0027] Therefore, the zone between the batch pile, also called the batch blanket above the glass melt, and the glass melt is preferably heated in an isotropic manner.

[0028] Entrapment is understood here to mean the interaction of microwave radiation with the initial molten phase, whether this is present in the batch as a solid or in liquid form within this initial molten phase.

[0029] In some embodiments of the present disclosure, microwave heating, especially in combination with ohmic electrical heating, can generally equalize the horizontally large temperature gradient in the melting region in the zone between the pile and the initial melt, thus slowing the flow. Heat provided by convection of the melt from the deeper molten bath is dissipated throughout the pile.

[0030] The heat dissipation zone is directly generated by the microwave booster, thereby reducing or equalizing the vertical temperature gradient beneath the batch blanket with respect to the heat dissipated from the molten bath.

[0031] In the past, microwave radiation was not absorbed into the melting reaction zone formed between the batch pile and the molten glass, and therefore the microwave radiation was not utilized very efficiently in converting the batch, which exists as a solid, into a molten state when introduced into a conventional melting tank.

[0032] It is advantageous if the pile covers the glass melt in an areal manner such that the surface of the glass melt is completely covered by the area of ​​the incident microwave radiation, or if the part of the pile covering the glass melt extends further onto the surface of the glass melt beyond the area of ​​incidence of the microwave radiation, since in that case it is possible to ensure that a large part of the incident microwave power, in particular more than 90%, can be used for the conversion of the batch initially present as a solid into its liquid state, in particular without the microwave power being absorbed by the melt surface adjacent to the pile.

[0033] The adjacent particulate agglomerations of the fed batch components are considered to be a mass of batch which floats on the melt and covers the surface of the melt, preferably in an opaque manner, in particular at least in the area of ​​the incident microwave radiation, where an opaque covering is considered to be a complete covering which is present such that the remaining part of said incident microwave radiation impinges on the glass melt after the incident microwave radiation has initially passed through the batch components at each point of incidence, in particular in the area of ​​the initial melt, in particular in the melting reaction zone, if complete absorption has not yet taken place.

[0034] This batch pile can form a batch blanket, such as a blanket over the molten glass when the batch is filled, where the height and lateral surface extent of the batch pile can be determined by the batch feed rate per unit time and the microwave radiation output.

[0035] If the microwave radiation supplied is of constant power and the energy further supplied to the glass melt, which can additionally be supplied, for example by ohmic electrical heating, is constant, the size of the batch blanket resulting as a batch filling during batch supply, and the associated height and lateral extension above the glass melt, can be controlled by the batch supply rate per unit time and therefore set to the desired values, respectively. For example, with a throughput of 0.5 t / d, the batch supply rate is about 20.8 kg / h.

[0036] Since most of the gases escaping the melt reaction zone of the forming melt are recondensed on the batch blanket of the batch charge upon cooling and thus recondensed, in melting tanks operated according to the present disclosure, the batch component feed rates are also more or less consistent with the final glass composition of each melting tank because volatile emissions are strongly suppressed.

[0037] Further examples of this are described in more detail below in the description of the preferred embodiments.

[0038] It is advantageous to provide microwave radiation and to avoid fast flow paths from the batch to the vessel outlet due to slower flow, and to calm convection currents by uniform heating, especially in the lateral direction. The advantage of the present invention is that the melting reaction is optimized and the melt becomes more homogeneous.

[0039] It is generally advantageous to provide energy at the boundary between the batch and the initial melt using microwave radiation, resulting in an increase in temperature and therefore an increased melting rate, right in the melting reaction zone between the batch and the initial melt.

[0040] This can be achieved in particular by introducing microwave radiation from a microwave radiation source in the direction of the upper furnace, where the upper furnace itself is kept cooler than the glass melt and no combustion gases are used, resulting in no CO2 emissions from combustion.

[0041] Another advantage over gas-fired top furnaces is that this heating allows the gases evolved during melting of the raw materials to be evacuated very efficiently through the relatively cool porous, gas-permeable pile, since the upper regions of the batch are cooler than the regions adjacent to the melt. Additional advantages are that the batch is less dusted during melting because the melting reactions do not take place in areas of high gas velocity, and direct evaporation of volatile components from the pile is reduced, especially borates in the case of borosilicate glasses.

[0042] In contrast to gas-flame treated surfaces, microwave heating does not result in vitrification of the batch pile, and thus the released gases can escape, are not trapped in the melt, and do not need to be vented again in a subsequent energy-intensive fining step. Volatile components rising upwards from the lower surface of the batch, e.g. B- or Cl-containing components, can be recondensed in the cold batch blanket, thus limiting the proportion of material released to the environment or to the upper furnace.

[0043] Emissions of B or Cl containing components from the melt could be reduced by more than 50% in embodiments of the present disclosure, especially when microwave radiation accounts for the total energy supplied to the batch for conversion to glass melt in an all-electric melting tank.

[0044] The lower temperature on the top side of the batch blanket means that there is no conversion of the fining agents on this side and, for the same reason, essentially no decomposition of the individual ingredients, e.g., nitrates, carbonates, etc. Also, the lower temperature on the top side of the batch limits the evaporation of the borate in the case of borosilicate glasses, which is favorable for achieving the target composition and reducing dust generation.

[0045] The initial foaming of the microwave heated initial melt is much less than the initial foaming of the burner heated batch melt.The gas load of the microwave heated initial melt is much less than the gas load of the burner heated batch melt.

[0046] The kinetics of melting is determined by the melting of the sand grains. As long as this is not completed, further gas bubbles will be generated. The regions in which there is still solid, already molten glass and an upper furnace atmosphere, for example also with gases escaping from the melt, and thus having solid, liquid and gas components, correspond within the scope of the present disclosure to the regions of sand grain melting and are in contact with the glass melt flowing below.

[0047] In the ideal case, the melting method may even eliminate post-fining. Microwave-assisted all-electric melting tanks provide glasses that are good enough for many product requirements.

[0048] At the same time, by using electricity that has a zero CO2 balance, the goal of a "CO2-free melting method" can be achieved.

[0049] At relatively high current densities, depending on the glass chemistry, conventional methods can lead to secondary effects at the electrodes, which lead to contamination of the melt with electrode material that is unacceptable for special glasses and limits the service life of the electrodes. This contamination is characterized by small particles in the μm range, which can seriously disrupt production and even lead to a complete production stop. The extent of this stoppage depends heavily on the glass specifications. Typical contamination levels for molybdenum electrodes range from 5 to 100 ppm, with 30 ppm already being unacceptable for some applications. However, for some special glasses, contaminations of more than 10 ppm already lead to unacceptable problems.

[0050] Another drawback of conventional purely electrical heating baths due to the ohmic resistance of the glass is that the temperature in the contact area between the glass and the refractory wall is too high, which can limit electrical heating by electrodes. Another advantage of heating by microwave radiation is that the non-contact introduction of power can be done in the area directly under the batch, thus "far away from the refractory wall", which can be quite durable for continuous use.

[0051] Unlike electrode heating, ohmic heating has a lower temperature below the batch pile, so current flows to the hotter area, meaning that in pure ohmic heating, the energy does not reach directly below the batch zone. This significant drawback can be avoided with the technological advances over conventional melting disclosed herein.

[0052] The term microwave or microwave radiation as used within the scope of the present disclosure is initially, and therefore without further definition, the common name for electromagnetic waves having frequencies given in older literature as in the range of 1-300 GHz, which corresponds to wavelengths in vacuum of about 30 cm-1 mm. Other recent references give broader frequency band limits in the literature, for example from 300 MHz to about 1 THz. Within the scope of the present disclosure, microwaves are understood to be electromagnetic waves according to recent literature data having, by definition, frequencies from 300 MHz to about 1 THz.

[0053] Within the scope of this disclosure, the terms microwave radiation and microwaves are used interchangeably and each refer to the same electromagnetic waves as defined above.

[0054] Typically, batch is understood to be the component of the subsequent molten glass that exists as a solid before being charged into the glass melt, and initial melt is understood to be the molten batch that has not yet been subjected to further fining, in particular finish fining.

[0055] By way of example, and without limiting generality, the batch may have a glass-ceramic and / or even BS glass type, in particular a borosilicate glass type, and a cullet content of 20% to 50%.

[0056] Primary melt is a term in glass technology that refers to the melt before fining: the initial liquid phase where all the ingredients have turned into a liquid state but still contain gas bubbles.

[0057] Within the scope of this disclosure, the generic term melting also encompasses the processes of melting and dissolving.

[0058] Melting is understood to be the process of melting at least a portion of a batch body present as a solid, whereby this batch body changes from its solid aggregate state to a liquid aggregate state, as explained in more detail below and defined for the purposes of this disclosure.

[0059] Melting is understood to mean the complete conversion of a batch mass initially present as a solid into its liquid state, in particular into an incipient glass melt.

[0060] Overall, it has been found to be advantageous to use microwave radiation for heating in the area of ​​batch loading.

[0061] In this case, an increase in the melting rate can also occur in a bath with ohmic electrical heating.

[0062] Within the scope of the present disclosure, a melting reaction zone is understood to be a spatial boundary or transitional region, on one side of which the batch is still present in solid form, and on the other side of this boundary or transitional region, melting or melt-off has already occurred, changing the batch in particular to a liquid state. The initial liquid phase is formed by the melting of salts, e.g. Na2CO3, B2O3, at their respective melting points, into which the other batch components are reactively dissolved.

[0063] Various chemical reactions take place and result in silicate melts. The first reaction starts in the solid state between the partners (e.g. alkali / alkaline earth metal carbonates such as Na2CO3 and SiO2) when the temperature is reached at which a eutectic phase (e.g. Na2O-SiO2) is formed. The first low-melting alkali metal silicate compounds are formed. Concurrently with the conversion of the alkali and alkaline earth metal carbonates or hydroxides, gases are released, which can escape outside the process through the open batch pile. By further increasing the temperature, the low-melting raw materials reach their melting point. Only with the appearance of the liquid phase can the reaction rate be significantly increased. Then the so-called initial melt is present. In this initial melt, molten alkali metal silicate compounds are present, together with residual quartz grains and other sparingly soluble components. The remaining quartz grains and other sparingly soluble components are gradually dissolved in the already existing silicate melt at higher temperatures and corresponding residence times, forming the final glass composition. When microwaves are used, they are introduced already at the appearance of the first eutectic phase, and the microwave radiation is absorbed by this alkali-rich, i.e. highly conductive, phase, accelerating the reaction. The more residual quartz and remaining refractory components are dissolved in the initial melt, the less the microwave energy is absorbed. This means that microwaves specifically support the dissolution process, especially in the initial molten phase.

[0064] The depth of the melting reaction zone is usually several millimeters, and can preferably range from about 1 mm to 100 mm in the direction of microwave radiation, depending on the type of glass.

[0065] Since microwave radiation is radiated in the volume, the melting reaction zone does not have to be located outside each batch body, but can also act completely on each batch component or batch body that is initially present as a solid as the temperature advances, this is especially the case when the overall temperature increases and thus the overall heating is initially from a temperature Tg-5K to a temperature Tg+50K or especially higher. In this case, it is not necessary for a sharp local boundary to be formed in the batch body, but the melting zone is understood to be the entire location where there is a local area of ​​the batch body at a temperature Tg-5K and a melted part at a temperature Tg+50K, especially higher. This case occurs especially in the case of small particulate or powdery batches with a size in the range of the size of the melting reaction zone.

[0066] Preferably, microwave radiation is injected from the direction of the upper furnace by a microwave radiation source. Unlike thermal radiation, MW radiation does not diffuse and can be introduced directionally or partially directionally by suitable measures such as protective FF walls. Advantageously, directional radiation is generated, for example, using a Vivaldi antenna or a trumpet-shaped radiator. The radiation is introduced via an antenna in the upper furnace towards the sump or the batch surface.

[0067] The upper furnace itself is kept at a lower temperature than the glass melt and, since no combustion gases are used, no CO2 is produced from combustion.

[0068] Preferably, in embodiments of the present disclosure, microwave radiation accounts for at least 10% of the energy supplied to the batch for conversion to a glass melt.

[0069] Here, the energy supplied for conversion to a glass melt is understood to mean the total energy supplied to the batch and used to heat the glass until it is in molten form and in particular exists as an initial melt, and therefore before its fining or final fining.

[0070] The initial melt has a glass viscosity of 10 3dPas or less and at least 10 2 Above this viscosity value, and especially also for lower viscosity values, the molten glass is considered within the scope of this disclosure to be in a molten or liquid state.

[0071] However, in particularly preferred embodiments, microwave radiation accounts for the total energy supplied to the batch for conversion to a glass melt.

[0072] Alternatively, in order to increase the melting or dissolving power in particular, in addition to the ohmic electrical heating of the melt, microwave energy can be applied from a microwave radiation source, in particular an upper furnace in which a microwave radiator is arranged, preferably injecting microwave energy into a zone between the batch and the initial melt for heating, in particular for absorption of microwave radiation.

[0073] In an embodiment of the present disclosure, it is possible to provide a CO2-neutral glass melting method, in which the energy introduction in the melting zone is performed by a combination of electrical heating, in particular ohmic heating, and microwave injection, and the electrical energy used for melting is provided by a current with at least a zero CO2 balance.

[0074] A CO2-neutral glass melting process refers to a process in which the total amount of CO2 present is not increased by the melting process.

[0075] Therefore, within the scope of this disclosure, a zero CO2 balance is considered to be the generation of a current that does not increase the total abundance of CO2.

[0076] Electric currents generated by solar, wind, hydro and / or nuclear power are considered to be electric currents with zero CO2 balance.

[0077] Fuels obtained by biological processes, also collectively referred to as biofuels, and substances obtained by chemical reactions, for example with the assistance of solar energy, such as methanol, also referred to as methanol solar fuel when methanol is obtained, are also considered to have a zero CO2 balance if their production and subsequent use do not increase the CO2 content of the atmosphere as a whole. Such biofuels can be used within the scope of the present disclosure in burners that have a zero CO2 balance and can be used, for example, in the clarification zone in the method and apparatus described herein.

[0078] Disclosed herein is a process, particularly a melting process, in which microwave radiation is introduced into a region of the melting vessel where no top furnace heating by burners is provided.

[0079] It is particularly advantageous that embodiments of the present disclosure do not require a vacuum or negative pressure for their implementation, nor do they rely on cooled walls, as is required, for example, in skull crucibles.

[0080] Another advantage of the disclosed embodiments is that there is no need to include multiple vessels necessarily connected to each other, e.g., cascade vessels, since the batch can be completely converted to a molten state already in the molten vessel below the batch blanket by very efficient capture of microwave radiation.

[0081] The microwave radiation may be generated by at least one magnetron and / or at least one semiconductor-based microwave radiation generator.

[0082] In generating microwave radiation, the disclosed method and the disclosed apparatus preferably provide microwave radiation at a frequency greater than 500 MHz and less than 6 GHz.

[0083] Microwave radiation may also be provided in the disclosed methods and disclosed apparatus at frequencies below 3 GHz, preferably 2.45 GHz or less, or 915 MHz or less.

[0084] In the method of the present disclosure, the throughput of molten glass is greater than or at least 0.5 t / d.

[0085] In a glass melting apparatus according to the invention, in particular for carrying out the method of the present disclosure, in particular for converting a batch into a glass melt, the apparatus comprises a melting unit comprising a melting vessel having a wall inside which, in addition to the batch to be melted, a melted batch as a glass melt can also be accommodated, and above the batch and the glass melt at least one microwave radiation source, in particular at least one microwave emitter, is arranged.

[0086] Preferably, at least one microwave radiation source is arranged in the upper furnace of the melting vessel, which ensures a planar distribution of microwave radiation to the batch blanket.

[0087] In an embodiment of the apparatus of the present disclosure, the microwave radiation of the microwave radiation source is directed at a melt reaction zone between the batch and the initial melt.

[0088] Additionally, in the apparatus of the present disclosure, in further embodiments, one or more provisions for ohmic electrical heating of the melt may be provided.

[0089] In carrying out the present method, particularly in converting the batch into a glass melt, if microwave radiation is used alone for melting and therefore for melting and dissolving the batch without the introduction of further energy from other energy sources, the microwave radiation used can act completely over the entire melting reaction zone and therefore also over the entire spatial volume of the melting reaction zone.

[0090] Alternatively, this is possible, but not necessarily, if an additional energy source for heating is provided to the melting process.

[0091] When an additional energy source for heating, such as an ohmic electrical energy source, is provided to the melting process, the localized area affected by the microwave radiation may be less than about 10% of the locally affected spatial volume of the melting reaction zone, or it may be 20%, 40% or 60% of the locally affected spatial volume of the melting reaction zone.

[0092] Furthermore, in this case, by targeted local energy introduction by microwave radiation, it is possible to set a defined flow regime, for example by introducing locally limited temperature inhomogeneities at local locations favorable for the melting process, which can also be achieved in particular by localized outgassing / bubble microturbulence, which can already provide a preliminary support for the subsequent fining process.

[0093] However, in a preferred embodiment, at least one microwave radiation source is incorporated into an area of ​​the melting vessel where no burner is provided for upper furnace heating or where no burners for upper furnace heating are located.

[0094] The present invention will now be described in more detail based on preferred embodiments with reference to the accompanying drawings. [Brief description of the drawings]

[0095] [Figure 1] FIG. 1 is a diagram showing a comparison of the electrical conductivity of various glasses versus the temperature of each glass. [Diagram 2] FIG. 1 shows tan(δ), a measure of microwave absorption in glass, measured for a number of representative specific glasses. The tan(δ) values ​​for various glasses are plotted against a given temperature (° C.). [Diagram 3] FIG. 1 shows the penetration depth D (μm) and incident power P (W / cm3) at E=10 KV / m for glass A at a given temperature (°C). [Figure 4]FIG. 1 shows the penetration depth D (μm) and incident power P (W / cm3) at E=10 KV / m for glass B at a given temperature (° C.). [Diagram 5] FIG. 1 shows tan(δ) and dielectric constant for various powder sizes at given temperatures (° C.). [Figure 6] FIG. 13 shows the source term p (FlexPDE simulation) for an exemplary glass determined over a range of simulations, which was found to drop from 20 MW / m3 to nearly 0 for a layer thickness of 4 mm in the x-direction. [Figure 7] FIG. 1 shows a glass melting apparatus of a first preferred embodiment, in particular for converting a batch into a glass melt. [Figure 8] FIG. 2 shows the second preferred embodiment of the glass melting apparatus, in particular for converting a batch into a glass melt, and the vertical temperature profile Tmw occurring in the apparatus compared with the vertical temperature profile Th of a conventional melting tank without upper furnace heating and with upper furnace heating, and the temperature profile Tob of a conventional melting tank.

[0096] Detailed Description of the Invention and Its Embodiments In the following description, identical reference numbers in the figures each indicate components or functional elements with the same or similar action, although for ease of understanding, the figures are not drawn to scale, unless they are two-dimensional figures in which respective data quantities are graphed.

[0097] While all of the above mentioned methods in the prior art certainly utilize microwave radiation as a heating method in the volume, it was not previously known that, by utilizing the temperature and material dependent absorption behavior of microwave radiation, it is possible to pass microwave radiation as nearly unabsorbed radiation through the cold regions of the batch pile, and then, upon impact with the hot glass melt, this radiation is completely absorbed and converted into thermal energy within a very short zone.

[0098] This zone can be, for example, a lytic reaction zone as described within the present disclosure.

[0099] This effect of temperature-dependent absorption has been widely discussed in the literature, and the corresponding formation of hot spots has been explained, but it was not known until now that this effect, which was previously considered to be disadvantageous or harmful, could be utilized in the field of batch melting.

[0100] It has not been described in any prior art document so far that the glass melt is continuously melted as in the method claimed in the present invention, in which the microwave heating, and thus the microwave radiation used for the heating, is used substantially or essentially only for melting the batch at its interface with the hot glass melt, hereinafter also referred to as the melting reaction zone, and is directed towards this region between the batch and the initial melt.

[0101] Advantageously, microwaves can also be used as a "top" heater in the melting region of the melt which is electrically heated below the batch.

[0102] That is, in certain embodiments of the present disclosure, microwave radiation can replace the top furnace ignition of burners commonly used in industrial vessels.

[0103] Thus, the traditional shortcomings of all-electrically heated melting baths, also referred to as VE baths, with regard to quality, throughput limitations, and flow instabilities due to vertical temperature gradients can be avoided by embodiments of the present disclosure.

[0104] The present invention also takes advantage of the uptake behavior of microwave radiation, and therefore of microwave energy incorporated into the glass melt by absorption, which in classical microwave applications leads to the negative and undesirable effect of the formation of hot spots.

[0105] As will be described in more detail below with particular reference to Figures 2-8, at low batch temperatures, microwave radiation is poorly picked up by the materials in the batch blanket, i.e., materials within this temperature range are transparent or semi-transparent to microwave radiation and therefore have low absorptivity to microwave radiation.

[0106] After the microwave radiation has penetrated the batch, it hits the first zone of hot primary melt along its path, where microwave absorption is strong and the entire energy is substantially absorbed and converted into thermal energy in this zone (also called the melting reaction zone) between the batch and the molten surface of the primary melt within a very short distance of a few millimeters or centimeters, depending on the composition of the glass.

[0107] 1 shows a comparison of electrical conductivity versus temperature for various glasses, where, and further in this disclosure, glass A represents an alkali-free glass, glasses B, C and D represent borosilicate glasses with different boron contents, and glasses E, F and G represent different aluminosilicate glasses.

[0108] The absorption of microwaves into glass can be described by the absorption of microwave rays, specifically tan(δ) which is proportional to the incident power, and this was measured for some representative specific glasses and is shown in the graph in Figure 2. Here, the loss angle is called δ, which indicates the angle between the complex dielectric constant and its real part. The resulting penetration depth of the microwave radiation field of the microwave rays is calculated below for two glasses as an example. Here, the penetration depth of microwave energy into a material is described by D, which is a parameter indicating the section where the power decreases to 1 / e with respect to the value of the material surface where the microwave rays hit.

[0109] At temperatures below 400° C., the penetration depth D is in the range of 0.1 m to 1 m depending on the type of glass, i.e., said microwaves penetrate the cold batch / raw material mixture with appreciable unattenuation.

[0110] In the glass melting temperature range, the penetration depth is a few centimetres, i.e. at typical molten bath depths of 50-100 cm, the heat is completely absorbed and converted into thermal energy in the upper molten zone below the batch pile.

[0111] In this regard, see also, for example, Figures 3 and 4, which respectively show the penetration depth D (m) and the incident power density P (W / cm2) at E = 10 KV / m, as shown in these figures. 3 ) is shown versus temperature in °C.

[0112] Figure 3 illustrates the behavior of glass A, and Figure 4 illustrates the material of glass B. The glass material may be, for example, a composition that can be converted into a glass-ceramic.

[0113] This heat is preferably generated in a high temperature zone facing the melt, particularly the melting reaction zone, where it at least results in enhanced melting or even leads to the entire melting and / or dissolution process.

[0114] Thus, microwave absorption also depends on whether the material is solid or powdered. Measurements in the present invention have shown that powders or particles with an average diameter of less than 50 μm have a third lower uptake and absorption per unit volume than solids or particles with an average diameter of, for example, a few mm, due to their lower bulk density and volume factor. Here, this effect allows the energy to be placed exactly in the right place, i.e. only in the dense phase that is becoming or is liquid in the melting reaction zone, and not in the loose batch areas of the batch pile. Advantageous process parameters are the bulk density of the batch and the proportion of gas bubbles in the molten batch.

[0115] The temperature behavior of the dielectric parameters typical of glasses can also be clearly seen in the powder measurements shown in Figure 5. The real and imaginary parts of the dielectric constant can be experimentally determined for a given frequency and a given material, respectively, allowing the penetration depth D to be calculated. These values ​​depend not only on the batch and glass composition, but also on the temperature and the degree of batch-to-glass transformation.

[0116] When the penetration depth is small compared to the dimensions of the batch body or particles, only the outer zone can be heated directly by the MW radiation. However, when the penetration depth is large compared to the dimensions of the batch body, only a small part of the MW energy is absorbed by the body or particle, and the rest is transmitted through the batch body like visible light through clear glass.

[0117] Here, in FIG. 5, the notation "solid real" and "solid imaginary" respectively refer to the real part of the dielectric constant according to the standard DKE-IEV 121-12-13 for solids, and the imaginary part of the dielectric constant according to the standard DKE-IEV 121-12-13 for solids. The notation "solid tan d" refers to the value of tan(δ) determined from the imaginary and real parts for the corresponding solids. The value of tan(δ) is obtained from the ratio of the imaginary part to the real part of the respective measured dielectric constant.

[0118] Preferably, microwave radiation is introduced into a mixture of glass frits having a particle size distribution, i.e., a maximum lateral extension in the range of 10 μm to 500 μm, the batch first forming a low-melting primary phase into which the higher-melting raw material particles are then dissolved. Alternatively or additionally, it is also possible to add larger cullets, extending laterally up to several mm, to the batch.

[0119] Up to Tg (glass transition temperature), the dielectric losses increase steadily. In the Tg range, a very strong increase in losses is observed, since the bonds "loosen" and the ions become much more mobile. When microwaves are used in the region of the batch zone, the "hot spot effect" is equalized, because, although it is true that glasses tend to form hot spots on melting if they are softening, the absorption loses its strong temperature dependence and the effect of thermal runaway is substantially mitigated.

[0120] The imaginary part of the effective electrical conductivity or permittivity value is composed of two parts, as detailed in the textbook.

[0121] At high temperatures above about 1400°C, the ohmic component becomes dominant, and in a typical glass melt, saturation is not yet reached even at 2000°C.

[0122] Example: σ=20S / m

number

[0123] However, in this range, absorption by electrical conduction comes to the fore and within a few millimeters microwave radiation is completely absorbed - see for example Figure 1 and the above discussion on it.

[0124] From the illustration in Figure 1 it can be seen that the ohmic conduction does not approach the limiting value, but power control of the microwave radiation to the glass melt to avoid local overheating may also be advantageous, since in this case the penetration depth is reduced as well.

[0125] In the transition region to incipient melt, characterization data indicates that embodiments of the methods described herein, and particularly the apparatus disclosed herein, can achieve a melting power of 10-100 W / cm 3 (10W / cm 3 =10,000,000W / m 3 =10,000kW / m 3 ) input power is possible without any problems.

[0126] Here, the absorption of power takes place at a depth of several millimeters in the molten reaction zone.

[0127] Assumption: 50,000kW / m at E=10kV / m 3 ×0.1m=5000kW / m 2 It becomes.

[0128] for example, ν=2.45GHz

number

[0129] According to the diagram in FIG. 6, within the simulation range, the calculated absorbed power per unit volume (W / m 3 ) shows that the source term p (FlexPDE simulation) is 20 MW / m 3 It was confirmed that the value drops to almost zero at a layer thickness of 4 mm in the x direction, and this is shown in the figure.

[0130] It is also clear from this that a thin layer of, for example, 4 mm thickness as mentioned above already provides a very high power density, so that almost all of the energy of the incident microwave radiation, i.e. up to more than 90%, is absorbed and made available as energy for heating.

[0131] Preferred temperatures for the incorporation of microwave radiation are in the range of 50°C to >1400°C.

[0132] An embodiment of the device will now be described with reference to FIGS.

[0133] First Exemplary Embodiment of the Melting Unit In the following, reference is made to Figure 7, which shows in particular a glass melting apparatus for converting a batch into a glass melt, which is given the overall reference number 1.

[0134] The apparatus includes, as viewed in the flow direction of the molten glass 2 , a melting unit 3 and a fining unit 4 .

[0135] The melting unit 3 is equipped with all supplies necessary for the melting of glass, even if not explicitly mentioned above, and in particular with an electrical supply capable of providing electric current with zero CO2 balance.

[0136] The device 1 is suitable for carrying out the methods described herein, in particular for carrying out the method according to the invention.

[0137] The melting unit 3 comprises a melting tank 5 having walls 6 made of refractory material, inside which is accommodated not only the batch 7 to be melted but also the already melted batch as molten glass 2 and thus as glass melt 2.

[0138] In the area of ​​the melting unit 3, the glass is either present as a batch 7 in solid form or, after its melting, passes into the glass melt 2, becoming more liquid, and is present in liquid form.

[0139] Above the batch 7 and above the glass melt 2 extending in molten form in the melting tank 5 from the bottom of the melting tank 5 to a height Hg, at least one microwave radiation source 8, in particular at least one microwave emitter 9, is arranged, which comprises a magnetron or a semiconductor-based microwave radiation generator.

[0140] The area above the glass melt 2 which forms the roof or arch 10 of the melting vessel 5 is referred to as the upper furnace 11 .

[0141] By applying microwave radiation as described above, the microwave radiation is absorbed in the dissolution reaction zone 13, that is, taken up into the dissolution reaction zone 13, thereby heating the dissolution reaction zone 13.

[0142] As can be seen in FIG. 7, the melting reaction zone 13 is located directly below the batch pile 17 formed by the charge of batch 7 and extends vertically between the glass melt 2 and the batch 7 which is still in solid form.

[0143] The vertical direction is understood to be the Z direction shown in Figure 8, extending perpendicularly upwards to a horizontal plane, e.g., the surface of the uncovered, non-flowable glass melt 2. Within the scope of the present disclosure, the terms "top" or "bottom" and "above" or "below" also apply to this vertical direction, insofar as they are spatial indications.

[0144] The closer the particles of Batch 7 are located to the glass melt 2, the higher their temperature and the higher their absorption capacity, which is proportional to tan(δ), as can be seen from Figure 5 and the associated discussion. This essentially results in the penetration depth D for each temperature of the particles of Batch 7, which can be seen in Figures 3 and 4, being in the negative vertical direction.

[0145] It can be seen that with an increase in the temperature of the batch 7, the penetration depth D of the microwave radiation 18 decreases significantly. This takes place in the negative Z direction according to the illustration in FIG. 8, so that the microwave radiation 18 of the microwave radiation source 8 already provides a very high power density in the region of 4 mm thickness, so that almost all of the energy of the incident microwave radiation, i.e. up to more than 90%, is absorbed and is available as energy for heating the particles of the batch 7 in particular. The energy of the microwave radiation is converted exactly in the region where this energy is needed to achieve a high specific melting power, i.e. in the melting reaction zone 13. The melt 2 is significantly hotter by the microwave radiation approximately only in this region, so that the melting process can be carried out significantly more quickly in the reaction zone without a significant temperature increase in the entire melt 2. A higher melting power can be achieved without a significant increase in the temperature of the entire melt volume, i.e. without increasing the corrosion of the walls 5 and the electrodes 14.

[0146] Here, the microwave radiation can be generated, for example, by one or more magnetrons (915 MHz and / or 2.45 GHz) arranged in the upper furnace 11 .

[0147] The upper furnace 11 is made of or contains a ceramic material with low microwave absorption, such as SiO2, and is surrounded by a metal cover 12 that blocks microwaves.

[0148] The batch 7 is inserted via a threaded insert, known to those skilled in the art, or through a "microwave-tight" opening, each of which is designed to prevent the emission of microwave energy to the outside.

[0149] Power can be injected by one or more magnetrons; gyrotron, magnetron and other microwave frequency heating are in principle possible.

[0150] The feedline can be considered as a waveguide and can be dimensioned to operate at a frequency much lower than its cutoff frequency for the MW frequencies used, taking into account the batch dielectric constant: no wave propagation is possible and waves attempting to exit the region above the glass melt are attenuated exponentially there.

[0151] The lower region of the melting vessel 5 can be heated by an electric auxiliary heater (EZH) with electrodes 14, 15 that provide electrical current for ohmic electrical heating of the melt 2. The EZH can be operated at, for example, 50 Hz or 10 kHz.

[0152] The electrode material of the electrodes 14, 15 can be any commonly used material, such as platinum, tungsten, molybdenum, iridium or tin oxide.

[0153] After being melted, the molten glass 2 is transported to a fining region 16 of the fining unit 4 and then transported to a forming section.

[0154] The energy introduction in the melting tank 5 is preferably carried out solely by electrical resistance heating and microwave energy.

[0155] The microwave frequency is preferably 915 MHz, but also 2.45 GHz and 5.8 GHz are possible. In this frequency range, magnetrons are typically used with power ranges up to 100 kW.

[0156] Input power example: [Table 1]

[0157] In the above table, the notation VE means an all-electric bath with ohmic electric heating, the notation VE+microwave means an all-electric bath with ohmic electric heating and microwave radiation, microwave [kW] stands for the incident microwave power (kW) and EW EZH [kW] stands for the power of the additional electric heating (kW).

[0158] The gas consumption shown in the above table is essentially the gas consumption in the clarifier area, and biofuels can be used as an alternative.

[0159] For the input power of the melting tank given in 1.3 above, in an all-electric melting tank 5 (i.e. a melting tank operated without input of non-electrical power or energy), for example, with a throughput of 20 t / d, the ohmic power for heating the glass melt is 800 kW, and the microwave power injected from above into the batch 7 of the batch blanket is 200 kW, resulting in a batch blanket of the batch 7 resting on the glass melt. In this case, the melting tank 5 has a load per unit area of ​​2 t / m 2 , i.e. the weight per unit area of ​​the glass 2 and batch 7 acting on its bottom in the melting tank is about 2 t / m 2 It is.

[0160] In the case of another all-electric melting tank 1.2, for example, with a throughput of 30 t / d, an ohmic power for heating the glass melt of 1000 kW and a microwave power of 300 kW incident from above on the batch of the batch blanket, a batch blanket of batch 7 resting on the glass melt can be provided, where the provided batch blanket is likewise a corresponding batch blanket resting on the glass melt. In this case, the melting tank has a load per unit area of ​​about 3 t / m 2 This means that the weight per unit area acting on the bottom of the melting tank is approximately 3 t / m 2 It was.

[0161] Here, in each case, the microwave radiation 18 was incorporated such that it affected only the batch blanket itself and the melting reaction zone lying therebelow, and not any other surface of the glass melt lying freely adjacent to the batch blanket.

[0162] In a further embodiment (FIG. 8), the microwave rays 18 can also act only on half or one third of the area over which the batch pile or batch blanket 13 extends in an especially opaque manner in an areal manner above the glass melt 2. Here, the areal area in which the intensity of the microwave rays drops from its maximum value to a value of 1 / e is considered to be acted upon by the microwave rays, with 1 / e within the scope of the present disclosure representing the reciprocal of the Euler number e, respectively.

[0163] The right half of Fig. 8 shows the vertical progression of the temperature Tmw in the glass 2 and the batch 7 occurring in this device. It can be seen here that the temperature Tmw first rises upwards, starting from the bottom of the melting tank 5 at the height of the electrodes 14 and 15, then slightly decreases with increasing height, already rises slightly again just before the melting reaction zone 13, and changes to a significantly pronounced maximum value in the melting reaction zone 3, which maximum value extends over almost the entire melting reaction zone 13, and therefore over a section Se in the Z direction that corresponds approximately to the penetration depth D of the microwave radiation 18 incident from above.

[0164] Furthermore, looking at the progression of temperature Tmw from above, it can be seen that the temperature Tmw of batch 7, which is initially at a low temperature, rises very significantly in a very short period of time, with the maximum value of temperature Tmw being within region Se of the dissolution reaction zone 13.

[0165] In comparison with the above trends, the vertical temperature trend Th of a conventional melting tank without upper furnace heating and the temperature trend Tob of a conventional melting tank with upper furnace heating are also shown as examples.

[0166] In these highly simplified illustrations, it can be seen that in the embodiments of the present disclosure, the temperature does not drop as strongly towards the surface 19 of the molten glass 2 as compared to both a conventional melting tank without top furnace heating and a conventional melting tank with top furnace heating, and therefore there is a more uniform vertical temperature distribution in the glass melt 2. In each of the above temperature profile descriptions, in the description of the temperature Tmw profile, microwave radiation accounted for at least 10% of the energy supplied to the batch for conversion to glass melt.

[0167] Exemplary embodiments of microwave emitters In this exemplary embodiment, a microwave radiator 9, in particular a magnetron or semiconductor based microwave ray generator, is used, e.g. a trumpet type radiator as presented as a horn antenna in Kraus, JD Antennas, McGraw-Hill. https: / / archive.org / details / Antennas2ndbyjohnD.Kraus1988 / page / n677 Please refer to.

[0168] The required radiation characteristics determine the structural length R and therefore the lateral length of the antenna of the microwave radiation source 8 .

[0169] In order to achieve a CO2-neutral melting process in the future, it is generally advantageous to replace the heating from hydrocarbon combustion with electrical heating, in particular with electricity from renewable energies. However, the replacement of burner technology with electrically heated emitters, especially in the melting area, has failed, since currently there are no materials that can withstand continuous use under the conditions present there, i.e. high temperatures and large amounts of dust. However, this technical problem has been solved by the above-mentioned method and device, because in the area where heat is required to melt the batch, in particular to melt it and further melt it into the initial melt, a microwave radiation source, in particular a magnetron or a semiconductor-based microwave radiation generator, is arranged, which locally radiates microwave radiation in a defined manner, which absorbs heat locally in a defined manner in the batch and in parts of the batch melt and initial melt, thereby maintaining a defined distance from the walls of the melting bath, in particular walls made of refractory material.

[0170] The above-mentioned apparatus can withstand continuous operation better than when a burner is used, since the location where the microwave radiation source, particularly a magnetron or semiconductor-based microwave ray generator, is located, particularly the upper furnace of the melting tank, does not need to be heated during microwave radiation.

[0171] Further measures to homogenize the magnetic field, and therefore the temperature, could be to not fix the MW frequency but to "modulate" it from the microwave source, or to place a mode stirrer above the melt to homogenize the magnetic field distribution, or to place a stirrer within the batch that homogenizes the MW magnetic field and at the same time homogenizes the temperature within the batch.

[0172] Furthermore, with little or no upper furnace heating in the melting zone, targeted energy release in the glass-forming zone below the batch blanket can significantly reduce emissions of volatiles such as alkali metal borates, boron, fluorine, Cl, etc. An evaporation-condensation cycle is formed in the batch similar to that in the cold top. [Explanation of symbols]

[0173] 1 Glass melting equipment 2 Molten glass, especially glass melt 3 Melting Unit 4. Clarification Unit 5 Melting tank 6 Wall of melting tank 5 7 Batches 8 Microwave radiation sources, especially magnetron or semiconductor-based microwave radiation generators 9 Microwave emitters 10 Roof or arch of melting tank 5 11 Upper furnace of melting tank 5 12 Metal cover that blocks microwaves 13 Dissolution reaction zone 14 electrodes 15 electrodes 16 Clear Region 17 Mount Bach 18 Microwave radiation, especially from magnetron or semiconductor-based microwave radiation generators 19 Molten glass 2, in particular the surface of the glass melt 2 Hg Height of the surface 19 of the molten glass 2, in particular the glass melt 2, above the bottom of the melting tank 5 Th Temperature inside glass melt 2 in a conventional melting tank without upper furnace heating Tob Temperature inside glass melt 2 in a conventional melting tank with upper furnace heating Tmw the temperature inside the glass melt 2 in one of the embodiments of the present disclosure Temperature transition in the region of Se dissolution reaction zone 13

Claims

1. A method for melting glass, comprising the steps of: using microwave radiation for at least a portion of the energy provided to melt the batch to convert it into a glass melt; 4. A method according to claim 1, characterized in that the microwave radiation used acts on at least a part of the transition between the batch and the initial melt, the microwave radiation is introduced into the upper region immediately below the batch pile, increasing the temperature in the upper region immediately below the batch pile to promote melting in comparison with an otherwise identical method without microwave radiation, the batch as a batch charge fed to the glass melt forms a coherent batch pile resting on the glass melt, power control is also provided for the microwave radiation to the glass melt to avoid local overheating, the penetration depth is reduced, the power is absorbed at a depth of a melting reaction zone of a few millimeters, and more than 90% of the microwave radiation energy is absorbed within a thickness of 4 mm and is provided as energy for heating the glass melt.

2. the pile of batch is covering the glass melt in an area such that the surface of the glass melt is completely covered by the area of ​​the microwave radiation; or The method of claim 1 , wherein the portion of the batch pile covering the glass melt extends onto the surface of the glass melt beyond the area where the microwave radiation is incident.

3. 3. The method according to claim 1, wherein the microwave radiation is incident from the top furnace direction by a microwave radiation source.

4. 3. The method of claim 1 or 2, wherein microwave radiation accounts for at least 10% of the energy supplied to the batch for conversion to a glass melt.

5. 3. The method of claim 1 or 2, wherein microwave radiation accounts for the total energy supplied to the batch for conversion to a glass melt.

6. A method as described in claim 1 or 2, wherein in addition to ohmic electrical heating of the melt, microwave energy is injected from an upper furnace having a microwave radiation source disposed therein, the microwave energy being injected into a zone between the batch and the initial melt.

7. CO ２ A method for neutral glass melting is provided, in which the energy input in the melting zone is a combination of electrical heating and microwave injection, and the electrical energy used for melting is at least CO ２ 3. The method according to claim 1 or 2, wherein the current is provided with a zero balance.

8. 3. The method of claim 1 or 2, wherein the microwave radiation is introduced into a region of the melting vessel, the region being free of upper furnace heating by a burner.

9. 3. The method according to claim 1 or 2, wherein the generation of microwave radiation is performed by at least one magnetron and / or by at least one semiconductor-based microwave radiation generator.

10. 3. The method of claim 1 or 2, wherein during the generation of microwave radiation, microwave radiation is provided at a frequency greater than 500 MHz and less than 6 GHz, or less than 3 GHz, or less than or equal to 2.45 GHz, or less than or equal to 915 MHz.

11. 3. The process of claim 1 or 2, wherein the throughput of molten glass is greater than or at least 0.5 t / d.

12. A glass melting apparatus for carrying out the method described in any one of claims 1 to 11, the apparatus comprising a melting unit including a melting tank having a wall, the inside of which can accommodate not only the batch to be melted but also an already-molten batch as a glass melt, and at least one microwave radiation source is positioned above the batch and the glass melt.

13. The apparatus of claim 12 , wherein the microwave radiation source is disposed in an upper furnace of the melting unit.

14. 14. The apparatus of claim 12 or 13, wherein the microwave radiation of the microwave radiation source is directed at a melt reaction zone between the batch and the initial melt.

15. 14. Apparatus according to any one of claims 12 to 13, comprising provision for ohmic electrical heating of the melt.

16. 14. The apparatus according to claim 12, wherein the at least one microwave radiation source is introduced into an area of ​​the melting vessel where no upper furnace heating is performed by burners or no burners for upper furnace heating are arranged.

17. 14. Apparatus according to any one of claims 12 to 13, wherein the generation of microwave radiation can be performed by at least one magnetron and / or by at least one semiconductor-based microwave radiation generator.

18. 14. Apparatus according to claim 12 or 13, wherein during the generation of microwave radiation, microwave radiation is provided at a frequency higher than 500 MHz and lower than 6 GHz, or lower than 3 GHz, or equal to or lower than 2.45 GHz, or equal to or lower than 915 MHz.

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