METHOD AND APPARATUS FOR MELTING GLASS

Abstract

A method and an apparatus for melting down glass are provided. The method includes using microwave radiation for at least part of the energy supply for melting for transforming a batch into a glass melt. The microwave radiation captures at least part of the transition between batch and primary melt. The method and apparatus include melting assembly with a melting tank which has walls within which both the batch for melting and the molten batch can be accommodated as a glass melt, where above the batch and above the glass melt there is at least one microwave-emitting source disposed.

Claims

1. A method for melting down glass, comprising: forming a glass melt using microwave radiation as at least part of an energy supply, wherein the forming step comprises: irradiating the microwave radiation at a transition between a batch and a primary melt; and coupling the microwave radiation into an upper region directly below a batch covering so that a temperature is increased.

2. The method of claim 1, further comprising supplying a batch charge to the glass melt to form a coherent batch covering lying on the glass melt.

3. The method of claim 1, wherein the batch covering covers the glass melt superficially such that a surface of the glass melt is covered completely in a region where the microwave radiation is irradiated.

4. The method of claim 1, wherein the batch covering has a part that covers the glass melt and extends on a surface of the glass melt beyond a region where the microwave radiation is irradiated.

5. The method of claim 1, wherein the step of irradiating the microwave radiation comprises irradiating from a direction of a top furnace by microwave-emitting sources.

6. The method of claim 1, wherein the microwave radiation comprises at least 10% of energy supplied to transform the batch into the glass melt.

7. The method of claim 6, wherein the microwave radiation comprises all of the energy supplied to transform the batch into the glass melt.

8. The method of claim 1, further comprising heating the glass melt with an ohmic electrical heating.

9. The method of claim 8, wherein the step of heating the glass melt with the ohmic electrical heating comprises using electrical energy that has an at least neutral CO.sub.2 balance.

10. The method of claim 1, wherein the step of irradiating the microwave radiation comprises coupling in the microwave radiation in a region of a melting tank in which no top furnace firing by burners is performed.

11. The method of claim 1, wherein the step of irradiating the microwave radiation comprises generating the microwave radiation by device selected from a group consisting of a magnetron, a semiconductor-based generator of microwave radiation, and combinations thereof.

12. The method of claim 1, wherein the step of irradiating the microwave radiation comprises generating the microwave radiation with a frequency of higher than 500 MHz and lower than 6 GHz.

13. The method of claim 12, wherein the frequency is lower than or equal to 915 MHz.

14. The method of claim 1, further comprising generating a throughput of the molten glass is more than 0.5 t/d.

15. An apparatus for melting down glass, comprising: a melting assembly having a melting tank which has walls within which both a batch for melting and a molten batch can be accommodated as a glass melt; and a microwave-emitting source disposed above the batch and above the glass melt.

16. The apparatus of claim 15, wherein the microwave-emitting source is disposed at a top furnace of the melting assembly.

17. The apparatus of claim 16, wherein the microwave-emitting source is coupled into a region of the melting tank that is free from top furnace firing by burners.

18. The apparatus of claim 15, wherein the microwave-emitting source is positioned and configured to radiate microwave radiation onto a melting reaction zone between the batch and a primary melt.

19. The apparatus of claim 15, further comprising an ohmic electrical heater positioned and configured to heat the glass melt.

20. The apparatus of claim 15, wherein the microwave-emitting source is selected from a group consisting of a magnetron, a semiconductor-based generator of microwave radiation, a microwave generator generating the microwave radiation with a frequency of higher than 500 MHz and lower than 6 GHz, a microwave generator generating the microwave radiation with a frequency of higher than 500 MHz and lower than 3 GHz, a microwave generator generating the microwave radiation with a frequency of higher than 500 MHz and lower than 2.45 GHz, and a microwave generator generating the microwave radiation with a frequency of higher than 500 MHz and lower than or equal to 915 MHz.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0094] The invention is described in more detail below with preferred embodiments and with reference to the appended drawings, in which

[0095] FIG. 1 shows a comparison of the electrical conductivities of various glasses as a function of the temperature of the respective glass,

[0096] FIG. 2 shows tan(delta), describing the incoupling of the microwave into glasses, and measured for certain typical specialty glasses, with tan(delta) values for different glasses being represented as a function of the temperature indicated in ° C.,

[0097] FIG. 3 shows both the penetration depth D in μm and the irradiated power input P in W/cm.sup.3 at E=10 kV/m as a function of the temperature indicated in ° C. for glass A,

[0098] FIG. 4 shows both the penetration depth D in μm and the irradiated power input P in W/cm.sup.3 at E=10 kV/m as a function of the temperature indicated in ° C. for glass B,

[0099] FIG. 5 shows tan(delta) and the dielectric constant for different powder sizes as a function of the temperature indicated in ° C.,

[0100] FIG. 6 shows the swelling term p determined within a simulation (FlexPDE simulation) for an illustrative glass, determined on a layer thickness of 4 mm in x-direction of 20 MW/m.sup.3 as dropping close to 0,

[0101] FIG. 7 shows an apparatus for melting down glass, more particularly for transforming a batch into a glass melt, in a first preferred embodiment,

[0102] FIG. 8 shows an apparatus for melting down glass, more particularly for transforming a batch into a glass melt, in a second preferred embodiment, and also a vertical temperature profile Tmw resulting in this apparatus, in comparison to the vertical temperature profile Th of a conventional melting tank without top furnace heating and to the temperature profile Tob of a conventional melting tank with top furnace heating.

DETAILED DESCRIPTION

[0103] In the description which follows, identical reference symbols in the figures denote in each case identical or equivalent constituents or functional elements. For the sake of better understanding, however, the figures are not represented to scale, unless the representation is a diagrammatic two-dimensional representation of respective data quantities.

[0104] While all of the methods described before in the prior art utilize microwave radiation as a heating method in the volume, it has not been recognized that the temperature-dependent and material-dependent absorption behavior of microwave radiation can be utilized in order to pass microwave radiation, as virtually absorbing-free radiation, through cold regions of the batch cover, this absorption, as soon as it impinges on a hot glass melt, then being fully absorbed in a very short zone and converted into thermal energy.

[0105] This zone may be, for example, the melting reaction zone described in the context of the present disclosure.

[0106] This effect of temperature-dependent absorption is recognized as a problem in numerous specifications, and is described with the associated formation of hotspots. To date, however, it has not been recognized that this effect, hitherto regarded to be deleterious and detrimental, may be utilized in the area of the melting-down of batches.

[0107] No specification in the prior art has to date described how, as in the presently claimed method, the glass melt is melted continuously and the microwave heating, i.e., the microwave radiation used for heating, serves for or essentially only for melt production from the batch of the interface with the hot glass melt, also referred to essentially below as melting reaction zone, and is directed onto this zone between batch and primary melt.

[0108] Advantageously, moreover, the microwave in the melting-down region of a melt heated electrically below the batch may be employed as heating from “above”.

[0109] This means that in certain presently disclosed embodiments, the microwave radiation is able to replace the top furnace burner firing that is customary in industrial tanks.

[0110] With the presently disclosed embodiments, therefore, it is possible to circumvent a disadvantage to date of melting tanks with all-electric heating, also referred to as AE tanks, in terms of quality, throughput limitation and flow instability, owing to the vertical temperature gradients.

[0111] The invention advantageously also exploits the incoupling behavior of microwave radiation, hence of the microwave energy incoupled by absorption into glass melts, which in the case of conventional microwave applications leads to the negative and unwanted effect of formation of hotspots.

[0112] Indeed, as described in more detail below with reference in particular to FIGS. 2 to 8, the microwave radiation couples into the material to only a very low extent in the batch carpet at low batch temperatures. This means that the material in this temperature range is transparent or semitransparent to microwave radiation and hence exhibits only low absorption for such radiation.

[0113] When the microwave radiation has penetrated the batch, this radiation impinges on the first zone of the hot primary melt that lies in its path. There, the microwave absorption increases sharply and the entire energy is absorbed and converted into thermal energy in a very short section of a few millimeters or centimeters—depending on glass synthesis—substantially in this zone, which is presently also referred to as melting reaction zone, between batch and melt surface of the primary melt.

[0114] FIG. 1 shows a comparison of the electrical conductivities as a function of the temperature for different glasses. Here, and also in the further course of the presently disclosure, glass A denotes an alkali-free glass, glasses B, C and D denote borosilicate glasses with different boron contents, and glasses E, F and G denote different alumosilicate glasses.

[0115] The coupling of the microwave into glasses may be described by tan(delta), which is proportional to the absorption of the microwave radiation, more particularly its irradiated power, and has been measured for certain typical specialty glasses, and is also represented in the graph of FIG. 2. Delta here denotes the loss angle, which indicates the angle between the complex dielectric constant and its real component. The resultant penetration depths of the microwave field of the microwave radiation are calculated by way of example below for two glasses. The penetration depth of the microwave energy into a material is described here by D, a quantity which indicates the distance within the power has dropped to 1/e relative to the value at the surface of the material on which the microwave radiation impinges.

[0116] At temperatures below 400° C., the penetration depth D according to type of glass is in the 0.1 m to 1 m range—in other words, the microwaves presently described radiate with decidedly little attenuation through a cold batch/raw material mixture.

[0117] In the region of the melting temperature of the glasses, the penetration depth is a few centimeters—in other words, at typical melting bath depths of 50-100 cm, there is complete adsorption, and conversion to thermal energy, in the upper melt region below the batch covering.

[0118] In this regard, see also FIG. 3 and FIG. 4, for example, which respectively show, as indicated in these figures, the penetration depth D in m and also the irradiated power density P (power input) in W/cm.sup.3 at E=10 KV/m as a function of the temperature indicated in ° C.

[0119] FIG. 3 describes the behavior for glass A and FIG. 4 for the glass B material. The glass material may for example be a composition which can be transformed into a glass-ceramic.

[0120] This heat is generated preferably in the hot zone facing the melt, more particularly in the melting reaction zone, where it leads at least to an acceleration of melting down or even to the entire melt production and/or melting-down process.

[0121] Another factor on which the microwave absorption is dependent is therefore that of whether the material is a solid body or in powder form. The present measurements have shown that pulverulent bodies or particles, presently having a mean diameter of less than 50 μm, exhibit volume-based absorption or incoupling that is lower by a factor of 3 than solid bodies or particles which presently had for instance a mean diameter of several mm, owing to the relatively loose fill and the volume factor. This effect helps here to locate the energy exactly at the correct point—that is, not in the relatively loose batch region of the batch covering, but instead only in the liquefying or liquid compact phase of the melting reaction zone. Advantageous process parameters are the bulk density of the batch and the bubble fraction of the melting batch.

[0122] The temperature behavior of the dielectric parameters, typical for glasses, is also readily apparent from the powder measurements shown in FIG. 5. The real and imaginary components of the dielectric constant may each be determined experimentally for a given frequency and a particular material, and so the penetration depth D can be calculated from them. These values are dependent not only on the composition of the batch or glass, but also on the temperature and the degree of transformation of the batch into glass. If the penetration depth is low relative to the dimensions of a batch body or batch particle, only an outer zone can be directly heated with the

[0123] MW radiation. The situation is different if the penetration depth is large in comparison to the dimensions of the batch body. In that case, only a small part of the MW energy is absorbed in the body or particles; the remainder passes through the batch body in the same way as visible light through a transparent glass.

[0124] In this case, in FIG. 5, the designations “solid Real Perm” indicate in each case the real component of the dielectric constant in accordance with the standard DKE-IEV 121-12-13 for solid bodies, and “solid Imag Perm” indicate in each case the imaginary component of the dielectric constant in accordance with the standard DKE-IEV 121-12-13 for solid bodies. The designation “solid tan d” indicates the value of tan(delta), determined from imaginary component and real component, for the corresponding solid bodies. The value of tan(delta) results from the ratio of the imaginary component relative to the real component of the respectively measured dielectric constants.

[0125] The microwave radiation is preferably coupled into a mixture of glass raw materials having a particle size, and thus a maximum lateral extent, in the 10 μm to 500 μm range, in which case this batch initially forms relatively low-melting primary phases, in which the higher-melting raw material grains are then dissolved. Alternatively or additionally, the batch may also be admixed with cullet having a larger lateral extent of up to a few mm.

[0126] Up to Tg (glass transition temperature), there is a steady increase in the dielectric losses. In the region of Tg, a very sharp rise in the losses is observed, since here the bonds become “loosened” and the mobility of the ions becomes substantially greater. For the use of microwave, the “hotspot effect” levels out in the region of the batch zone, since, while the glass does tend to form hotspots during melting, when it is softened, the absorption nevertheless losses its heavy dependency on temperature and the thermal runaway effect becomes intrinsically more mild.

[0127] The effective conductivity or the imaginary component of the relative permittivity is composed, as set out comprehensively in textbooks, of two fractions.

[0128] At high temperatures of around >1400° C., the ohmic fraction is predominant, and for typical glass melts has still not reached saturation even at 2000° C.

[0129] σ=20 S/mω∈.sub.0∈″.sub.r=0.14 S/m∈″.sub.r=1Example:

[0130] σ=20 S/mω∈.sub.0∈″.sub.r=0.14 S/m∈″.sub.r=1 at 2.45 GHz and

[0131] In this region, however, the absorption by electrical conductivity then comes to the fore, and ensures complete absorption of the microwave radiation within a few millimeters. See, for example, FIG. 1 and also the description thereof above.

[0132] From the representation in FIG. 1 it is apparent that the ohmic conductivity does not tend toward a limiting value. In order to prevent local overheating, however, a control of the microwave radiation power may also be advantageous for the glass melts, as in this case the penetration depth is likewise reduced.

[0133] In the region of the transition to the primary melt, on the basis of the characteristic data, power inputs of 10 to 100 W/cm.sup.3 (10 W/cm.sup.3=10 000 000 W/m.sup.3=10 000 kW/m.sup.3) in the case of the presently described versions of the method and in particular with the presently disclosed apparatus are readily possible in each case.

[0134] In this case, the power is absorbed at a depth of the melting reaction zone of a few millimeters. An example in this regard is indicated below.

[0135] Assumption: 50 000 kW/m.sup.3*0.1 m=5000 kW/m.sup.2 for E=10 kV/m.

[0136] By way of example:

[0137] ν=2.45 GHz

[0138] ∈′.sub.r=4 and ∈″.sub.r=0

[0139] σ=43 S/m

[0140] at a simulating field strength E=967 V/m, corresponding to an intensity of 1241 W/m.sup.2 (in air); in this regard, see also FIG. 6.

[0141] According to the representation from FIG. 6, as part of a simulation, the source term p (FlexPDE simulation), which indicates the calculated power absorbed in each case per unit volume W/m.sup.3, is determined on a layer thickness of 4 mm in x-direction of 20 MW/m.sup.3 as dropping to nearly 0 and is represented correspondingly therein.

[0142] From this it is also apparent that in a thin layer, such as a layer 4 mm in thickness mentioned above by way of example, the power densities deposited are already very high and almost complete—this means up to more than 90% of the energy of irradiated microwave radiation can be absorbed and provided as energy for heating.

[0143] Preferred temperatures for the incoupling of the microwave radiation are in the range from 50° C. to >1400° C.

[0144] Embodiments of the apparatuses are described below, with reference FIGS. 8 and 9.

[0145] First exemplary embodiment of the melting unit

[0146] Reference is made below to FIG. 7, which shows—provided overall with the reference numeral 1—an apparatus for melting down glass, more particularly for transforming a batch into a glass melt.

[0147] This apparatus, as seen in the flow direction of the molten glass 2, comprises a melting unit 3 and a refining unit 4.

[0148] Even if not explicitly represented above, the melting unit 3 comprises all of the supply facilities needed for the melting of glass, including, in particular, electrical supply facilities, which are able to supply electrical power with a neutral CO.sub.2 balance.

[0149] This apparatus 1 is suitable for implementing the presently described methods, more particularly for implementing the method of the invention.

[0150] The melting unit 3 comprises a melting tank 5, which has walls 6 consisting of refractory material, within which both the batch 7 for melting and the molten batch in the form of molten glass 2 and hence glass melt 2 is accommodated.

[0151] In the region of the melting unit 3, the glass is present in each case as batch 7 in solid form or, after melt production therefrom, in a form which is becoming liquid, going into the glass melt 2, and is liquid.

[0152] Above the batch 7 and also above the glass melt 2, which extends from the bottom of the melting tank 5 up to a height Hg in liquid-melt form in the melting tank 5, there is at least one microwave-emitting source 8 disposed, more particularly at least one microwave radiator 9, which comprises a magnetron or a semiconductor-based generator of microwave radiation.

[0153] The region above the glass melt 2, which forms the roof dome 10 of the melting tank 5, is termed the top furnace 11.

[0154] The microwave irradiation is irradiated as described above such that it is absorbed in the melting region zone 13, meaning that it is coupled into this zone and leads as a result to the heating of said zone.

[0155] As is evident from FIG. 7, the melting reaction zone 13 is disposed directly below the batch covering 17 formed by the charging of the batch 7, and extends in a vertical direction between the glass melt 2 and the batch 7 still present as a solid.

[0156] The vertical direction is understood to be the Z-direction indicated in FIG. 8, which extends upward perpendicularly to a horizontal plane, this being, for example, the surface of an uncovered, flow-free glass melt 2. It is relative to this vertical direction that, in the context of this disclosure, the designations “above” or “beneath” and also “over” or “below” are based, insofar as these are spatial indications.

[0157] The closer particles of the batch 7 to the glass melt 2, the higher their temperature and also the higher the absorption capacity proportional to tan(delta), as evident from FIG. 5 from the associated description. This then results in a negative vertical direction, essentially, in the penetration depths D, which can be seen in FIGS. 3 and 4, for the respective temperature of particles of the batch 7.

[0158] It is apparent that with increasing temperature of the batch 7, there is a sharp decrease in the penetration depth D of the microwave radiation 18, which as shown in FIG. 8 takes place in the negative Z-direction such that the microwave radiation 18 of the microwave-emitting source 8 deposits very high power densities even in the region of 4 mm thickness, and is absorbed almost completely, meaning up to 90% of the energy of irradiated microwave radiation, and is provided as energy for the heating in particular of the particles of the batch 7. The microwave radiation energy is converted exactly in the region where it is needed for a high specific melting performance, in the melting reaction zone 13. The melt 2 becomes significantly hotter almost only in this zone, as a result of the microwave radiation, and the processes of melting down are able to operate much more quickly in the reaction zone, without a marked increase in the temperature of the melt 2 as a whole. The higher levels of melt production can be achieved without a marked temperature increase of the melt volume as a whole, meaning that the corrosion of the walls 5 and the electrodes 14 is not increased.

[0159] The microwave radiation is generated in this case, for example, by one or more magnetrons (915 MHz and/or 2.45 GHz) which are disposed in the top furnace 11.

[0160] The top furnace 11 consists of ceramic material with low microwave absorption, e.g., SiO.sub.2, or comprises such material, and is surrounded by a microwave-shielding metallic casing 12.

[0161] The batch 7 is charged via screw chargers known to the skilled person or through a “microwave-impervious” opening, designed in each case such that they cannot give off any microwave energy to the outside.

[0162] The power may be irradiated by one or more magnetrons. Heating with gyrotrons and magnetrons and also other microwave frequencies would also be possible in principle.

[0163] The charging duct may be considered as a waveguide, the sizing of which may be such that for the MW frequency employed it is operated at well below its cut-off frequency, taking account of the batch dielectricity. Accordingly there is no possibility of wave propagation, and waves which want to run outward from the region above the glass melt are attenuated exponentially in said region.

[0164] In the lower region, the melting tank 5 may be heated by an electrical ancillary heater (EZH), which possesses electrode 14 and 15, providing electrical power for the ohmic electric heating of the melt 2. The EZH may be operated, for example, at 50 Hz or 10 kHz.

[0165] Possible electrode materials of the electrodes 14 and 15 are all commonly used materials such as platinum, tungsten, molybdenum, iridium or tin oxide.

[0166] After melting down, the molten glass 2 is transferred into a refining region 16 of the refining unit 4 and is then transferred for shaping.

[0167] The energy input in the melting tank 5 takes place preferably only by way of electrical resistance heating and microwave energy.

[0168] Suitable microwave frequencies are preferably 915 MHz, although 2.45 GHz or 5.8 GHz are also possible. In this frequency range, magnetrons in power ranges up to 100 kW are available on a standard basis.

[0169] Examples of power inputs are as follows:

TABLE-US-00001 Through- Load per Mt LT Micro- put unit area EZH [kW] wave Example 1 [t/d] Area [t/m2] [kW] power/gas [kW] 1.1 VE 20 2 1200 200/900 1.2 VE + 30 3 1000 200/900 300 microwave 1.3 VE + 20 2 800 200/—  200 microwave

[0170] In the table above, the designation AE denotes all-electrically operated tank with ohmic electrical heating, and AE+microwave denotes all-electrically operated tank with ohmic electrical heating and microwave radiation. Microwave [kW] denotes the irradiated microwave power in kW, MT EZH [kW] the electrical power of the electrical ancillary heating in kW,

[0171] The gas consumption reported in the table above is essentially the gas consumption in the refining tank area, for which it is also possible alternatively to use biofuel.

[0172] In the case of the power inputs of the melting tank referred to above in 1.3, a batch carpet of the batch 7 lying on the glass melt is formed in the case of the all-electrically operated melting tank 5 (hence a melting tank operated without the input of nonelectric power or energy) at, for example, a throughput of 20 t/d, with an ohmic power for the heating of the glass melt of 800 kW and with a microwave power, irradiated from above into the batch 7 of the batch carpet of 200 kW. In this case the melting tank is operated with a load per unit area of 2 t/m.sup.2, meaning that the weight of the glass 2 and of the batch 7 acting on the base of the melting tank in said tank amounts, per unit area, to about 2 t/m.sup.2.

[0173] In the case of the further all-electrically operated melting tank 1.2, it was possible to provide a batch carpet of the batch 7 lying on the glass melt at, for example, a throughput of 30 t/d, with an ohmic power for the heating of the glass melt of 1000 kW and with a microwave power, irradiated from above, into the batch of the batch carpet, of 300 kW, the batch carpet provided being likewise a corresponding batch carpet lying on the glass melt. In this case the melting tank was operated with a load per unit area of about 3 t/m.sup.2, meaning that the weight acting on the bottom of the melting tank in said tank amounted, per unit area, to about 3 t/m.sup.2.

[0174] In this context, the microwave radiation 18 was incoupled in each case such that it captured only the batch carpet itself and also the melting reaction zone located below said carpet, but not the further surface of the glass melt lying exposed next to the batch carpet.

[0175] In further embodiments (FIG. 8), the microwave radiation 18 may also capture only half or a third of the area with which the batch covering or the batch carpet 13 extends flatly, more particularly opaquely on the glass melt 2. In this case the flat region considered as being captured for the microwave radiation is the region up to which the intensity of the microwave radiation has dropped from its maximum to a value of 1/e, where 1/e in the context of the present disclosure denotes in each case the reciprocal of Euler's number e.

[0176] Represented in FIG. 8, in its right-hand half in the Z-direction, is the vertical profile of the temperature Tmw, which results in this apparatus, in the glass 2 and in the batch 7, in respect of which it is apparent that the temperature Tmw initially increases at the level of the electrodes 14 and 15, upwardly starting from the bottom of the melting tank 5, but then decreases slightly as the height goes up and increases slightly again before the melting reaction zone 13, before then transitioning to a sharply pronounced maximum in the melting reaction zone 3, which extends approximately over the entire melting reaction zone 13 and hence over a distance Se in the Z-direction that corresponds approximately to the penetration depth D of the microwave radiation 18 irradiated from above.

[0177] It is also readily apparent, in this case from a temperature profile Tmw coming from above, that the batch initially present at a low temperature is very greatly increased in its temperature Tmw over a very short distance, with the maximum of the temperature Tmw lying within the region Se of the melting reaction zone 13.

[0178] Shown in comparison to the profile described above as well, illustratively, is the vertical profile of the temperature Th from a conventional melting tank without top furnace heating, and the profile of the temperature Tob from a conventional melting tank with top furnace heating.

[0179] In these greatly simplified representations it is apparent that in the case of the embodiments presently disclosed, relative both to the conventional melting tank without top furnace heating and to a conventional melting tank with top furnace heating, increases less sharply toward the surface 19 of the molten glass 2 and hence in the glass melt 2 as well there is a more homogeneous vertical temperature distribution. In the above representation of the respective temperature profiles, on indication of the profile of the temperature Tmw, at least 10% of the energy supplied to the batch for transformation into a glass melt comprised microwave radiation.

[0180] Exemplary embodiment of a microwave radiator. In this exemplary embodiment, for the microwave radiator 9, more particularly the magnetron or the semiconductor-based generator of microwave radiation, a trumpet radiator is used, of the kind configured for example as a horn antenna and described in Kraus, J. D. Antennas, McGraw-Hill; see, for example, https://archive.org/details/Antennas2ndbyjohnD.Kraus1988/page/n677.

[0181] The emission characteristic required determines the construction length R and also the length of the side faces of the antenna of the microwave-emitting source 8.

[0182] In order in the future to enable CO.sub.2-neutral melting processes, there is a general advantage to switches from heating with hydrocarbon combustion to electrical heating systems, and in this case more particularly with the use of electrical power from renewable energy. Replacement of the burner technology by electrically heated radiators, however, has failed especially in the melting-down area because at present there is no material which has long-term operation robustness under the conditions prevailing there, hence at high temperatures with severe dusting. This technical problem, however, has been solved with the above-described methods and apparatuses, since, because the region in which heat is required for melting, more particularly for the production of melt from the batch and for the further melting down of the batch to form a primary melt, as a result of the arrangement of the microwave-emitting source, more particularly magnetrons or semiconductor-based generators of microwave radiation, and the defined local delivery of microwave radiation, which through absorption, in a locally defined way, couples in heat to the batch and also melt produced from the batch, and to a part of the primary melt, it is possible to maintain a defined distance from the walls, more particularly walls consisting of refractory material, of the melting tank.

[0183] The apparatuses described above are more robust in long-term operation than when using burners, since the location at which the microwave-emitting source, more particularly magnetron or semiconductor-based generator of microwave radiation, is disposed, in particular the top furnace of the melting tank, does not have to be heated when microwave radiation is delivered.

[0184] Further field-homogenizing and therefore temperature homogenizing measures may be that the MW frequency is not fixed, but is instead “modulated through” from the microwave source, or that a mode stirrer is positioned above the melt to homogenize the field distribution, or that a stirrer is positioned in the batch and ensures a homogenization of the MW field and at the same time homogenizes the temperature in the batch.

[0185] Furthermore, through targeted release of energy in the glass-forming zone beneath the batch carpet, when using relatively little to no top furnace heating in the melting-down region, it is possible to reduce significantly the emission of volatile constituents, such as, for example, alkali metal borate, boron, fluorine, Cl, etc. The result is a cold-top-style evaporation-condensation circuit in the batch.

LIST OF REFERENCE SYMBOLS

[0186] 1 Apparatus for melting down glass [0187] 2 Molten glass, more particularly glass melt [0188] 3 Melting unit [0189] 4 Refining unit [0190] 5 Melting tank [0191] 6 Walls of melting tank 5 [0192] 7 Batch [0193] 8 Microwave-emitting source, more particularly magnetron or semiconductor-based generator of microwave radiation [0194] 9 Microwave radiator [0195] 10 Roof or dome of melting tank 5 [0196] 11 Top furnace of melting tank 5 [0197] 12 Microwave-shielding metallic casing [0198] 13 Melting reaction zone [0199] 14 Electrode [0200] 15 Electrode [0201] 16 Refining region [0202] 17 Batch covering [0203] 18 Microwave radiation, more particularly from a magnetron or semiconductor-based generator of microwave radiation [0204] 19 Surface of molten glass 2, more particularly of glass melt 2 [0205] Hg Height of the surface 19 of molten glass 2 [0206] Th Temperature within the glass melt 2 in a conventional melting tank without top furnace heating [0207] Tob Temperature within the glass melt 2 in a conventional melting tank with top furnace heating [0208] Tmw Temperature within the glass melt 2 in one of the presently disclosed embodiments [0209] Se Temperature profile in the region of the melting reaction zone 13