Glass melting process in microwave heating using a metal crucible made of platinum or a platinum alloy

Abstract

The melting of glass using microwave heating is a relatively new development and has been tested in laboratory-scale glass production. Various glass systems in the temperature range of 1200 - 1500 °C using MW heating have been implemented as alternative heating methods. These glass systems containing SiO2, B2O3, and P2O5 as the main components of various special-purpose glass matrices have been melted. In small-scale glass melting by microwave heating, the required electrical energy is significantly reduced, and 50 - 75% of the energy is saved. Furthermore, the total time of glass melting is dramatically shortened due to a faster heating rate and volumetric heating. The OH concentration has been found to be lower in barium borosilicate glass and zinc borate glass prepared by microwave heating. A higher Fe 2+ being retained may have the potential to be applied in the development of IR-absorbing glass.

Description

Technical Field

[0001] The present invention relates to a glass melting process in microwave heating using a metal crucible made of platinum or a platinum alloy. The present invention addresses the glass melting process using a platinum crucible in microwave heating, and thus enables the deployment of high-purity glass in microwave heating by providing significant advantages such as energy efficiency, reduced processing time, and further increased heating rate. The present invention clarifies the design and arrangement of microwave absorbers for using a metal pot to melt glass. Further, the present invention discloses the arrangement of the crucible and the design of the MW susceptor / absorber surrounding the crucible for suppressing the reflected power within the MW cavity, thereby heating the crucible and then heating the glass inside the crucible. Therefore, by utilizing microwave heating, it becomes possible to produce high-purity glass in a metal container such as a crucible of platinum or a platinum alloy. Low-melting-point glass can be prepared using a crucible of Inconel alloy or other metals.

Background Art

[0002] The melting of glass using microwave (MW) heating is a relatively new development and has been tested in laboratory-scale glass production. Various glass systems with SiO2, B2O3, and P2O5 as the main components have been melted under MW heating at 1100 - 1550 °C. In small-scale glass melting by microwave heating, the required electrical energy is significantly lower, and 50 - 75% of the energy is saved. All the reported glasses were carried out in microwave heating using non-metallic crucibles (i.e., quartz, alumina, refractories). Contamination from the crucible wall can have a very significant impact on glass melting because it can change the desired properties of the glass. Although contamination from the crucible material remains low in microwave heating, contamination from the crucible material into the glass during melting cannot be prevented even with microwave heating. This limits the use of such crucibles in high-quality glass preparation even at the laboratory scale. Furthermore, the melting crucible is an important component required for scaling up the melting process. Refractory / quartz crucibles often cause a number of problems including contamination from the crucible wall and cracks due to thermal shock, which limits the use of non-metallic crucibles for large-scale melting. Moreover, these non-metallic crucibles impair the possibility of achieving a higher heating rate in MW heating. Although a heating rate of over 25 °C / min can be achieved, the use of non-metallic crucibles limits the heating rate to less than 25 °C / min. In many cases, the heating rate is maintained within the range of 10 - 15 °C / min. To minimize glass contamination and achieve a higher heating rate, a crucible made of platinum or a platinum alloy for melting high-quality glass is required. However, the use of metal crucibles has not been reported so far mainly due to the reflection of microwaves from the bulk metal.

[0003] Against such a background, the present invention presents the preparation of glass melting using a metal crucible (i.e., platinum or a platinum alloy). Platinum crucibles of various dimensions are used to melt glasses of different compositions. Thus, microwave heating can be utilized to produce high-purity glasses that require strict compositional uniformity, and it is possible to prepare them in this melting method using noble metal crucibles.

[0004] Prior art and drawbacks References that may be cited (Assignees: Christian JH, Fox KM, and Washington II AL, "Int J Appl Glass Sci.", 2017, 8: 158 - 164. doi: 10.1111 / ijag.12222, Publication Year: 2017). According to this literature, in single - mode microwave - assisted heating, microwave - specific heating of crystal seeds in nuclear waste glass has been carried out. All heating was performed in a single - mode CEM Discover - SP model microwave reactor operating at 300 W and 2.45 GHz. Heating was carried out in a stationary 35 mL quartz container. When irradiating 500 mg of non - crystallized nuclear waste glass stimulant pieces at 300 W inside the cavity, this glass immediately began to heat at approximately 1.3 °C / s and finally reached approximately 100 °C. Above 100 °C, this heating became slightly slower at a rate of approximately 0.25 °C / s and finally reached a steady state at approximately 450 °C. At this point, the heating of the glass stopped but was maintained at a temperature of approximately 400 - 450 °C. Despite being continuously irradiated for more than 30 minutes at a fixed microwave output of 300 W, the temperature of the glass did not exceed 450 °C. Although there were some differences in the degree and rate of heating, similar results were observed in multiple samples. In fact, some samples could only be heated to approximately 100 - 200 °C regardless of the irradiation time. IR image of the sample that reached approximately 450 °C. Microwave heating of a 500 mg monolith of a tremolite crystallization analog of nuclear waste glass stimulant occurred faster than the non - crystallized version and reached a much higher temperature. Although there were some differences in the heating rate and the maximum temperature, generally these samples were heated from room temperature to approximately 400 °C at a rate of approximately 16 °C / s. Above 400 °C, the samples were heated more slowly at an average rate of approximately 12 °C / s. In IR radiation measurement, it was found that the temperature of these samples reached a plateau of approximately 1600 °C after only 2 minutes. Disadvantages - All heating was carried out in a single-mode CEM Discover-SP model microwave reactor operating at 300 W and 2.45 GHz. The single-mode microwave reactor cannot produce bulk glass due to size limitations. - Heating was carried out in a stationary 35 mL quartz container rather than a metal crucible. - A very small amount (500 mg) of the non-crystallized nuclear waste glass stimulant was irradiated at 300 W inside the microwave cavity of the applicant. Despite continuous irradiation for over 30 minutes at a fixed microwave output of 300 W, the temperature of the glass did not exceed 450 °C. A 300 W microwave output is insufficient to heat bulk glass above its melting point even in small amounts. - The microwave heating of a 500 mg monolith of thetoberite crystallization analog of the nuclear waste glass stimulant occurred faster and reached much higher temperatures than the non-crystallized version. IR radiation measurements revealed that the temperature of these samples reached a plateau of approximately 1600 °C after only 2 minutes. Selective heating was observed. It is impossible for all compositions to be heated and then melted.

[0005] Invention in the literature that can be referenced (F. Komatsu, *A. Takusagawa, R. Wada, and K. Asahina, "Waste Management", Vol. 10, pp. 211 - 215, 1990). According to this literature, non-combustible wastes such as incineration ash, thermal insulation materials, concrete blocks, and earth and sand, although different in shape and chemical composition, are mostly composed of dielectrics, so they were melted by microwave irradiation. As one of the best methods to ensure sufficient stability over a long period and reduce volume, Kobe Steel developed a new treatment technology using microwaves. In this paper, a new microwave melting furnace with a cubic metal crucible for treating non-combustible wastes was used. Disadvantages - It was expected to use a stainless - steel crucible in a microwave melting furnace for the treatment of glass fibers. However, the use of platinum alloys, as well as other precious metals and their alloys for high - temperature glass melting by microwaves, is not disclosed. The use of Inconel - type metal pots for low - melting - point glass is not disclosed. - The microwave frequency used: 915 - +25 MHz. This requires a large space for the equipment and the crucible (due to the longer wavelength). The present invention reveals glass having a frequency of 2.45 GHz, thereby reducing the space and volume of the equipment / utilities. - Various materials including inorganic insulators, dry residues of liquefied waste, residues of acid decomposition, earth and sand, and metal pieces, excluding glass, were heated. - A microwave melting furnace having a cubic metal crucible for treating non - combustible waste is presented, with microwave ports arranged on the upper part of this crucible. However, cylindrical / round - bottom or various other shapes having microwave ports on the side walls of the cavity were not used in this study.

[0006] Inventions in patents that can be referred to U.S. Patent No. 5462009A (Method and Apparatus for Producing Perovskite Compositions, The Boeing Company, October 31, 1995). In this patent, a precursor composition is irradiated with microwaves, whereby the precursor composition is heated and the precursor composition is converted into perovskite. A susceptor crucible for use in the treatment of perovskite precursor compositions. This susceptor crucible has an inner crucible, an outer crucible surrounding the inner crucible, and a susceptor material positioned between and separating the inner crucible and the outer crucible. Disadvantages The configuration of the MW susceptor around the crucible is disclosed. However, it can be seen that the configuration of the MW susceptor or absorber is different from the invention of CGCRI. The use of a metal crucible is different from the invention of CGCRI. The melting of glass is not disclosed.

[0007] Research papers that can be referred to (Ashis K. Mandal et al., "Journal of Alloys and Compounds", 2014). In this paper, transparent zinc borate glass is melted by using microwave energy as an alternative heating method. A comparative study of the properties of the glass prepared by these two methods is carried out by using X-ray diffraction (XRD), differential scanning calorimetry (DSC), UV-Vis-NIR spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and refractive index (RI). The amorphous nature of the samples is confirmed by X-ray diffraction investigation. It is found that the glass transition temperature (Tg) of the microwave-melted glass is about 7 - 9 °C higher than that of the glass prepared by conventional melting. The OH content is found to be less than 250 ppm in the microwave-melted glass, but more than 330 ppm in the conventional melted glass. Therefore, microwave treatment can be an alternative glass preparation method with high energy efficiency, time saving, and environmental friendliness. Disadvantages This study does not include the glass preparation using melting in a metal pot.

[0008] Research papers that can be referred to ("Preparation of colourless phosphate glass by stabilising higher Fe[II] in microwave heating", (A.K. Mandal*, B. Mandal, Kavya I, T.G. Ajithkumar, A. Halder, P.K. Sinha, and Ranjan Sen, "Scientific Reports", (2018) 8:6195). According to this research paper, colourless phosphate glass containing iron was prepared using MW heating. Microwave (MW) heating can change the Fe redox ratio (Fe2+ / ΣFe), thus enabling the preparation of colourless phosphate glass. The effect of Sn concentration on the retention of Fe[II] in the glass melted in MW was compared with that of conventional glass. According to the colorimetric analysis study developing the Fe2+-ferrozine colour complex, it is clear that the Fe redox ratio required to obtain colourless phosphate glass is ≧0.49. In microwave heating, the desired effect can be obtained by adding 1 wt% of Sn metal powder, while in conventional heating, the addition of 1.9 wt% of Sn metal powder is required. It can be seen that the correlation formula of the Fe redox ratio with respect to the concentration of Sn metal is different in microwave heating and conventional heating. Therefore, by utilising this different redox change in MW heating, it is possible to adjust the optical properties. According to the energy consumption analysis, it is clear that it is 3.4 kWh in MW heating and 14 kWh in conventional glass melting using resistance heating. Furthermore, glass melting in MW can be completed within 2 hours, which is different from the approximately 5 hours previously required. MW heating plays an important role in improving the properties for producing colourless phosphate glass in addition to significantly saving energy and time.) Disadvantages According to this prior art, it is not clear about glass melting using a metal crucible. There is no disclosure regarding the layout design of the MW susceptor / absorber.)

[0009] Research papers that can be referred to (Ashis Kumar Mandal and Ranjan Sen, "Optimization of melting parameters and minimizing OH content in SiO2-B2O3-Na2O-BaO glass system in microwave heating.", Int J Appl Glass Sci., 2018;00:1-9). According to this research paper, the optimization of the melting process for glass composed of SiO2-B2O3-Na2O-BaO under microwave heating was investigated. The optimization of the melting time was carried out by varying the soaking time (8 - 60 minutes) at 1250 °C. The OH content could be minimized to 68.56 ppm in the glass using oxygen atmosphere melting, thereby preventing the diffusion of OH from the atmosphere into the melt. Disadvantages This prior art discusses the optimization process of glass melting using an alumina crucible / silica crucible. This prior art does not clarify the melting process using a metal crucible and has not investigated the feasibility of melting inside a precious metal crucible made of platinum. To initiate heating, an MW susceptor is placed around the crucible. However, the design of the MW susceptor / absorber is not disclosed.

[0010] Research papers that can be referred to ("Preparation of Chromium doped phosphate glass adopting microwave irradiation and comparative analysis of properties with conventional glass", Arijit Basak, Lata Ramrakhiani, Sourja Ghosh, Ranjan Sen, Ashis K Mandal *, Journal of Non-Crystalline Solids, 2018). According to this research paper, chromium-doped phosphate glass was melted using microwave (MW) heating, and its properties were compared with those of conventional glass. Since the absorption peaks in the UV-Vis spectra at wavelengths of 298 nm, 456 nm, 660 nm, and 685 nm are enhanced, it is suggested that the concentration of Cr 3+ in the MW-prepared glass is higher. From the changes in peak intensity and the shift of binding energy in XPS, it is confirmed that Cr(III) exists in different coordinations and orientations in the glass. Spectrophotometry that develops a color complex of 1,5-diphenylcarbazide and Cr[VI] suggests that Cr[VI] does not exist in the glass. Disadvantages This prior art discusses the melting and property analysis of chromium-doped phosphate glass in microwave (MW) heating using an alumina crucible / silica crucible. This prior art does not clarify the melting process using a metal crucible and has not investigated the feasibility of melting in a precious metal crucible made of platinum.

[0011] Research papers that can be referenced "An Overview on Microwave Processing of Material: A Special Emphasis on Glass melting", (Ashis K. Mandal* and Ranjan Sen, "Materials and Manufacturing Processes", 2017 32, 1, 1-20). According to this research paper, an overview of microwave synthesis of materials is presented. This study examines the use of microwave energy for applications in multiple material processing technologies separate from food processing. The use of microwave energy for glass processing is particularly emphasized. The melting of glass containing SiO2, P2O5, and B2O3 as the main components is discussed. It has been revealed that silica, a microwave-transparent material as previously reported, can be directly heated under microwave heating. The microwave absorption of raw materials and various glass systems is discussed. For some glass compositions, dielectric properties, particularly the loss tangent or loss factor, are presented. During the melting of glass using microwave heating, it can be seen that there is less evaporation of components and less contamination from the crucible wall. The increase in the iron oxidation-reduction ratio (Fe +2 / ΣFe) in microwave processing can be advantageous in the preparation of heat-absorbing filter glass. Small-scale glass melting using microwave heating has a high impact on energy and time savings. However, what has been discussed until now is the difficulties and future prospects associated with the upscaling of glass melting by microwave heating. Disadvantages This prior art examines the melting of various glass compositions in microwave (MW) heating and emphasizes significant changes in properties using conventional glass. This prior art reveals the melting process using a non-metallic crucible.

[0012] Research papers that can be referenced "Energy efficient melting of Glass for Nuclear Waste Immobilization using Microwave radiation" (A.K. Mandal et al., "International Journal of Green Energy", (2015) 12, 1280 - 1287). According to this research paper, a base glass suitable for nuclear waste immobilization was prepared by a melting and quenching technique using microwave radiation as the heat source. The microwave absorption behavior of the main raw materials was studied and presented. Also, glass was prepared in a conventional resistance heating furnace using the same batch composition. X-ray diffraction analysis of the samples prepared in the microwave furnace confirmed the formation of glass showing a similar match to that of the glass prepared in the conventional resistance heating furnace. Comparative property analysis suggested that the same glass was prepared by both methods. In the microwave furnace, it was found that the total power consumption was about 5 kWh and the maximum power demand was 1.5 kW. The power consumption in the resistance heating furnace was investigated in three different capacity furnaces and compared with that of the microwave furnace. Microwave heating recorded an energy saving within about 60% compared to the resistance heating furnace. Furthermore, the time required to melt the glass in the microwave furnace was recorded to be less than 2 hours compared to the 6 - 7 hours required in the resistance heating furnace. Therefore, microwave heating shows the potential for a significant reduction in the glass melting cost as it results in the same glass with significantly less power and time consumption. Disadvantages This prior art discusses the melting and characterization of a base glass suitable for nuclear waste immobilization in microwave (MW) heating using an alumina crucible / silica crucible. The energy consumption in microwave heating was investigated and compared with resistance heating. This prior art does not clarify the melting process using a metal crucible.

[0013] Research papers that can be referred to "Preparation of Homogeneous Barium Borosilicate Glass Using Microwave Energy" (Ashis Kumar Mandal*, Dinesh Agrawal, and Ranjan Sen, "Journal of Non-Crystalline Solids", 371 - 372, 2013, 41 - 46). According to this research paper, barium borosilicate glass with a composition of SiO2 (46.5 wt%), B2O3 (26.5 wt%), Na2O (16.5 wt%), and BaO (10.5 wt%) was melted at 1473 K in a microwave furnace and also in a conventional resistance heating furnace. A comparative study of the properties of these two glasses was carried out using standard property analysis methods. It was observed that the evaporation of volatile components was less in the microwave-prepared glass, resulting in a lower glass density compared to the glass prepared by the conventional method. The "self-stirring" effect was observed in the annealed glass prepared without injection into a preheated mold in the microwave-prepared glass sample. The same structure and light transmittance were seen in both of these glasses. It was estimated that the OH content was lower in the glass melted by microwave heating compared to conventional heating. Microwave heating has the potential to be an alternative glass preparation method with significant reduction in energy and processing time. Disadvantages This prior art discusses the melting and property analysis of barium borosilicate glass in microwave (MW) heating using an alumina crucible / silica crucible. This prior art does not clarify the melting process using a metal crucible.

[0014] Patent Inventions That Can Be Cited as References 0022NF2016 (Assignees: Ashis Kumar Mandal* and Ranjan Sen, Indian Patent: 0022NF2016, Patent Application Number: 201611009089, March 16, 2016). According to this patented invention, a process for melting Fe-doped aluminophosphate glass containing Fe metal powder in a microwave furnace under an air atmosphere was revealed. This invention relates to a method for manufacturing aluminophosphate glass composed of P2O5 - Al2O3 - B2O3 - MgO - ZnO added with Fe metal powder, Sn metal powder, and Al metal powder, and this glass is prepared by a melt quenching technique in a microwave furnace in air, inert, and reducing atmospheres. The glasses melted in a microwave furnace using an alumina crucible in different atmospheres all exhibit an IR transmittance of less than 0.1% and a visible transmittance of more than 80% at 550 nm. The glass melted in an air atmosphere shows a low IR transmittance of 0.03% at 1050 nm and a visible transmittance of up to 83% at 550 nm in a 3-mm-thick glass sample. This type of glass can be used for IR absorption filter applications such as, for example, projector lenses and other optical devices that need to eliminate the thermal effect of a strong light source. Disadvantages This prior art discusses the melting of IR-absorbing glass in microwave heating in air using an alumina crucible. This prior art does not reveal a melting method using a noble metal crucible.

[0015] Patented inventions that can be referred to 0022NF2016 (Assignee: Ashis Kumar Mandal* and Ranjan Sen, Indian Patent: 0131NF2016). In this patented invention, a process for producing borosilicate glass with an OH content of less than 70 ppm using microwave heating has been revealed. This invention relates to a method for optimizing the OH content within 70 ppm in a glass consisting of SiO2-Na2O-B2O3-BaO, and this glass is prepared by microwave heating in a normal air atmosphere using a sintered batch under flowing oxygen gas (or dry air). The glass melted in a microwave furnace under flowing oxygen gas shows a minimum OH content of 68.8 ppm. The glass melted in an air atmosphere shows an OH content of approximately 90 ppm over a melting period of 1 hour. Disadvantages This prior art mentions the use of non-metallic crucibles, such as quartz and alumina crucibles, for melting glass in microwave heating. The use of platinum or other metal crucibles has not been investigated.

[0016] Patented inventions that can be referred to CN102276151B (Assignee: Yunnan Yinfeng New Materials Co., Ltd. and Zhou Tao, May 8, 2015). In this patented invention, a composition of glass ceramic powder (Al2O3 (5 - 50 wt%), SiO2 (0 - 60 wt%), B2O3·3H2O (0 - 40 wt%), MgO (0 - 15 wt%), ZrO2 (0 - 15 wt%), H3PO4 (0 - 25 wt%), Sb2O3 (0 - 5 wt%), BaO (0 - 40 wt%), CaO (0 - 35 wt%), K2O (0 - 5 wt%), Na2O (0 - 5 wt%)) is melted using a microwave plasma torch melting process. Disadvantages The melting of glass by microwave heating is similar. However, the use of metal crucibles is different from the invention of CGCRI.

[0017] Prior art of patented inventions that can be referred to CN102432179A (Assignee: Harbin Institute of Technology, October 31, 1995). According to this patented invention, a mixture of glass-ceramic compositions is melted by microwave heating at a temperature within the range of 950 to 1200 °C for 60 to 140 minutes. Disadvantages The melting of glass by microwave heating is similar. However, the use of a metal crucible is different from the invention of CGCRI.

[0018] Prior art that can be referred to CN107056045A (Assignee: Hefei Liyutai Glass Products Co., Ltd., August 18, 2017). According to this prior art, a glass composition is melted in a microwave muffle furnace, and excellent high-temperature resistance characteristics are obtained. The preparation method is simple, and it is easy to achieve industrial production. Disadvantages The use of a metal crucible is different from the invention of CGCRI.

[0019] Prior art that can be referred to U.S. Patent No. 10364176B1 (Assignee: Owens-Brockway Glass Container Inc., July 30, 2019). According to this prior art, a glass precursor gel is heated by microwave radiation. Disadvantages The use of a metal crucible is different from the invention of CGCRI.

[0020] Prior art that can be referred to CN109399938A (Assignee: Dongguan University of Technology, March 1, 2019). According to this prior art, ceramic glass is first prepared by microwave melting at 800 °C for 45 minutes, then the temperature is raised to 1100 °C over 20 minutes, and then cooled. Disadvantages The use of a metal crucible is different from the invention of CGCRI.

Prior art documents

Patent Documents

[0021]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

Non-Patent Documents

[0022]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Non-Patent Document 4

Non-Patent Document 5

Non-Patent Document 6

Non-Patent Document 7

Non-Patent Document 8

Non-Patent Document 9

Summary of the Invention

Problems to be Solved by the Invention

[0023] The main object of the present invention is to provide a method for melting glass in microwave heating using a metal crucible made of platinum or a platinum alloy, thereby eliminating the drawbacks of the prior art known heretofore as detailed above.

[0024] Another object of the present invention is to provide a method for melting glass using platinum, a Pt alloy for high-temperature melting in the range of 1200 to 1600 °C or higher.

[0025] Yet another object of the present invention is to provide a method for heating a material using a metal pot having a microwave port on the side wall of the cavity.

[0026] Another object of the present invention is to provide a method for melting glass using an Inconel type alloy or a crucible of another metal for low-temperature glass below 1200 °C.

[0027] Yet another object of the present invention is to provide a method for melting barium borosilicate glass and calcium borosilicate glass at 1400° C. in a platinum crucible.

[0028] Yet another object of the present invention is to provide a design and arrangement of a microwave susceptor or a microwave absorber surrounding a metal crucible to minimize the reflected power.

[0029] Yet another object of the present invention is to provide a method for melting glass using crucibles of various shapes, namely, cylindrical, conical, round-bottomed, etc.

[0030] Yet another object of the present invention is to provide various configurations of a microwave susceptor or a microwave absorber surrounding a metal crucible for melting glass.

[0031] Yet another object of the present invention is to achieve a faster heating rate of more than 25° C. per minute during glass melting, thereby minimizing the processing time and energy consumption.

Means for Solving the Problems

[0032] One embodiment of the present invention is a multimode microwave oven for melting glass using microwave heating, comprising: (a) microwave absorbers (1), (12), (14), (16); (b) metal crucibles (2), (13), (15); (c) a microwave cavity (7); and (d) a microwave insulation box (4). The multimode microwave oven is provided with the above components.

[0033] Another embodiment of the present invention is a multimode microwave oven in which the glass in the multimode microwave oven is processed at a frequency of 2.45 GHz.

[0034] Still other embodiments of the present invention are multimode microwave ovens in which the shape of the metal crucible is selected from the group consisting of cylindrical, conical, and round-bottomed.

[0035] Still other embodiments of the present invention are multimode microwave ovens in which the metal crucible is made of pure platinum metal, a platinum alloy, another precious metal, or an alloy of another of these precious metals.

[0036] Still other embodiments of the present invention are multimode microwave ovens in which the microwave absorber is selected from the group consisting of a ring, a hollow cylinder, and either one piece or a plurality of pieces.

[0037] Still another embodiment of the present invention is a multimode microwave oven in which the microwave insulation box has an opening for temperature measurement via a thermometer.

[0038] Still another embodiment of the present invention is a multimode microwave oven in which the microwave cavity has a gas inlet port or an air inlet port for maintaining a different atmosphere by flowing a desired gas or air.

[0039] One embodiment of the present invention is a multimode microwave oven in which platinum or a platinum alloy has a heating rate of >25 °C / min.

[0040] Another embodiment of the present invention is a method for performing glass melting from a multimode microwave oven using a metal crucible, comprising: (a) designing a heat insulation setup; (b) disposing a microwave absorber around the metal crucible within the heat insulation setup of step (a); (c) heating and melting the glass within the heat insulation setup of step (b) in the microwave oven in a controlled atmosphere to obtain molten glass; (d) pouring the melt of step (c) into a preheated mold to obtain high-temperature glass. (e) To obtain the processed glass, annealing the high-temperature glass obtained in step (d) and then subjecting it to controlled cooling to room temperature; (f) To obtain the glass of the desired size, grinding, polishing, and cutting the processed glass obtained in step (e); and the method includes:

[0041] Another embodiment of the present invention is a method in which the shape of the microwave absorber is selected from the group consisting of a ring, a hollow cylinder, a single piece, or a plurality of pieces.

[0042] Another embodiment of the present invention is a method in which the glass is melted within a temperature range of 1200 to 1600 °C for 30 to 60 minutes.

[0043] The present invention is shown in FIGS. 1 to 5 of the accompanying drawings herein. In these drawings, like reference numerals / letters indicate corresponding parts in the various figures.

Brief Description of the Drawings

[0044]

Figure 1

Figure 2

[12] surrounds a metal crucible

[13] . Arrangement of a platinum crucible

[13] within another similar-shaped crucible

[12] made of a microwave absorber inside the MW insulation box inside the cavity.

Figure 3

[14] surrounds a metal crucible

[15] . Arrangement of a platinum crucible

[15] within a cylindrical-shaped microwave absorber (either a hollow cylinder type or a crucible type)

[14] .

Figure 4

[16] surrounds the melting crucible. Two pieces of quasi-cylindrical MW absorbers

[16] arranged surrounding a platinum crucible.

Figure 5

[17] , cylindrical

[18] , semi-circular round-bottom type

[19] , bottom-pouring

[20] , and many others.

Mode for Carrying Out the Invention

[0045] The present invention relates to a method for melting glass using a metal crucible in a microwave oven. In FIG. 1, a schematic configuration of a microwave absorber surrounding this metal crucible is presented. This schematic configuration includes a microwave absorber [1], and a metal crucible [2] of arbitrary shape is disposed inside a microwave transmissive heat insulation setup [4] within a multi-mode microwave cavity [7]. A rod-shaped microwave absorber [1] (an absorber made of SiC, zirconia, MoSi2, or any other strong MW absorber) having the same height as the crucible is disposed around the metal crucible [2] to absorb the MW entering the cavity. An MW port [8] is located on the side wall of the multi-mode cavity [7]. FIG. 1 schematically shows the placement of the crucible into the heat insulation box [4] within the MW cavity. The upper cover [5] of the heat insulation setup [4] has a hole or opening [6] for measuring the temperature by a pyrometer [9] mounted on the upper part of the cavity [7]. The multi-mode microwave cavity [7] has a gas / air inlet port

[10] for maintaining different atmospheres by flowing a desired gas or air. The cavity [7] further has an exhaust port

[11] provided diagonally upward. The glass [3] is melted within the crucible [2].

[0046] Figure 2 schematically shows the arrangement of the platinum crucible within a microwave-transparent heat-insulating box inside a microwave cavity having a shape similar to that of the microwave absorber

[12] . A metal crucible

[13] is placed within a crucible

[12] having a similar shape and a larger diameter made from the MW absorber. The MW absorber crucible

[12] is placed inside the microwave-transparent heat-insulating setup within the multimode microwave cavity. The dimensions of this outer crucible are similar to those of the metal crucible. Thus, the outer crucible

[12] absorbs most of the MW entering the MW cavity, thereby causing heating of the platinum crucible

[13] that contains the glass precursor.

[0047] Figure 3 shows a schematic view where a cylindrical crucible

[14] made from a microwave absorber is placed inside the microwave-transparent heat-insulating setup within the multimode microwave cavity, and a metal crucible

[15] is placed within the cylindrical crucible

[14] . The outer cylindrical crucible

[14] is made using an MW absorber / susceptor (SiC, zirconia, MoSi2, or any other strong MW absorber). The height of the outer cylindrical crucible

[14] can be different (lower, similar, higher) from the height of the metal crucible

[15] . The outer cylindrical crucible

[14] can be in the form of a ring made from an MW absorber / susceptor (SiC, zirconia, MoSi2, or any other strong MW absorber). Thus, the outer crucible / ring-shaped body

[14] absorbs all the MW entering the MW cavity, thereby causing heating of the platinum crucible

[15] that contains the glass precursor. The glass is melted within the crucible

[15] .

[0048] Figure 4 shows a schematic diagram of another type of configuration of the microwave absorber

[16] surrounding the metal crucible. This microwave absorber

[16] is placed inside the microwave-transparent insulation setup within the multimode microwave cavity. The outer ring is made of an MW absorber / susceptor (SiC, zirconia, MoSi2, or any other strong MW absorber) and can be in two pieces. These two-piece ring-shaped microwave absorbers are arranged to cover the metal crucible. The height of the outer ring

[16] can be different from the height of the metal crucible (lower, similar, higher). Thus, this outer ring absorbs all the MW entering the MW cavity, thereby causing heating of the platinum crucible that houses the glass precursor. Further, these rings absorb the microwaves reflected from the crucible wall. This placement of the crucible within the MW absorber can be used for both glass melting and material sintering. The next figure shows a schematic diagram of the crucible with a ring-shaped MW absorber design within the insulation box inside the MW cavity.

[0049] Figure 5 shows that various shapes / types of metal crucibles can be used within the microwave cavity for glass melting. Various types of crucibles such as conical

[17] , cylindrical

[18] , semi-circular round-bottom type

[19] , bottom-pouring

[20] , etc.

[0050] Detailed description of a method including the following steps for optimizing the melting conditions of borosilicate glass. Step 1 - Prepare a batch by mixing the raw materials. Step 2 - Sinter this batch (either in pellet or powder form) to remove moisture or gas components in the raw materials. Step 3 - Design an insulation setup with small pieces of microwave absorber properly placed around the metal crucible. Step 4 - Design the arrangement and position of the metal crucible within another crucible made from the MW absorber. The outer crucible (made from the MW absorber) can be of various shapes and sizes, i.e., cylindrical, conical, round-bottomed, or of uniform shape and size. Further, a ring-shaped MW absorber (either a hollow cylindrical type, one piece or multiple pieces) can be arranged around the metal crucible. Step 5 - A platinum crucible containing the sintered batch or green batch is held within the MW absorber set up inside the insulation setup, and then heating in the microwave oven is carried out. Step 4 - In the microwave oven, heat the batch (depending on the composition) at 1200 - 1600 °C for 30 - 60 minutes in an air atmosphere, controlled atmosphere, or reducing atmosphere according to the properties of the glass, and then melt it. Step 5 - Pour the melt into the preheated mold. Step 6 - Anneal the hot glass in a muffle furnace maintained at the annealing temperature of the glass, and then control the cooling to room temperature. Step 7 - Process the glass using cutting, grinding, and polishing to make the glass sample into the desired size for property analysis.

[0051] The following examples illustrate the operation of the present invention in actual implementation and should not be construed as limiting the scope of the present invention.

Examples

[0052] 25 g of calcium borosilicate glass was melted from a sintered batch in a 30 ml pure platinum crucible in a microwave oven at 1250 °C for 1 hour in an air atmosphere. The molten glass was poured into a preheated mold and then transferred to a muffle furnace maintained at 500 - 550 °C for 1 hour for annealing, and then cooled under controlled conditions to room temperature. The glass thus obtained was transparent, and its transmittance was found to be over 80% in the range of 400 - 2500 nm for a 2 mm thick sample (the maximum transmittance was 88.7% at 1500 nm and 82.9% at 550 nm).

Example

[0053] 60 g of calcium cesium borosilicate glass was melted from a sintered batch in an 80 ml pure platinum crucible in a microwave oven at 1250 °C for 1 hour in an air atmosphere. The molten glass was poured into a preheated mold and then transferred to a muffle furnace maintained at 500 - 550 °C for 1 hour for annealing, and then cooled under controlled conditions to room temperature. The glass thus obtained was transparent, and its transmittance was found to be over 80% in the range of 530 - 2500 nm for a 2 mm thick sample (the maximum transmittance was 87.8% at 1500 nm and 81.8% at 650 nm).

Example

[0054] 60 g of barium borosilicate - based glass was melted from a sintered batch in an 80 ml pure platinum crucible in a microwave oven at 1250 °C for 1 hour in an air atmosphere. The molten glass was poured into a preheated mold and then transferred to a muffle furnace maintained at 500 - 550 °C for 1 hour for annealing, and then cooled under controlled conditions to room temperature. The glass thus obtained was transparent. The transmittance of this glass (2 mm thick sample) was found to be over 80% in the range of 400 - 2500 nm.

[0055] Advantages The main advantages of the present invention are as follows. - The multi-mode furnace cavity is capable of causing bulk glass melting. In the multi-mode microwave furnace cavity, various compositions can be melted, and rapid heating can be achieved. - Pure platinum or a platinum alloy can be used for a high-purity glass melting crucible in a microwave furnace. - The use of an Inconel type metal pot for low melting point glass is disclosed. - The concept of arranging an MW absorber or an MW susceptor around a ceramic crucible in microwave melting of glass can be found in prior art documents. However, the present invention discloses arranging an MW absorber or an MW susceptor around a metal crucible from the perspective of parameters such as high microwave absorption, minimizing the reflection of MW in the cavity, and thereby causing heating of the metal crucible containing glass. - It is possible to achieve a high heating rate exceeding 25 °C / min, and thus the processing time and energy consumption are minimized.

Description of Signs

[0056] 1 Microwave absorber 2 Metal crucible 3 Glass 4 Microwave transmissive heat insulation setup, heat insulation box 5 Upper cover 6 Hole, opening 7 Multi-mode microwave cavity 8 MW port 9 Pyrometer 10 Gas / air inlet port 11 Exhaust port 12 Crucible type microwave absorber, microwave absorber, MW absorber crucible, outer crucible 13 Metal crucible, platinum crucible 14 Cylindrical type microwave absorber, outer cylindrical crucible, outer crucible / ring shaped body 15 Metal crucible, platinum crucible 16 Cylindrical type microwave absorber, microwave absorber, quasi-cylindrical microwave absorber, outer ring 17 Conical 18 Cylindrical 19 Semi-circular round bottom type 20 Bottom pouring

Claims

1. A multi-mode microwave oven for melting glass using microwave heating, comprising: (a) microwave absorbers (1), (12), (14), (16); (b) metal crucibles (2), (13), (15); (c) a microwave cavity (7); (d) a microwave insulation box (4), wherein in the multi-mode microwave oven, the microwave absorber is arranged around the metal crucible made of pure platinum metal, platinum alloy, other precious metals, or alloys of other precious metals thereof. The multi-mode microwave oven is characterized by this.

2. The multi-mode microwave oven according to claim 1, wherein the glass in the multi-mode microwave oven is processed at a frequency of 2.45 GHz.

3. The multi-mode microwave oven according to claim 1, wherein the shape of the metal crucible is selected from the group consisting of cylindrical, conical, and round-bottomed.

4. The multi-mode microwave oven according to claim 1, wherein the shape of the microwave absorber is selected from the group consisting of ring, hollow cylinder, one piece, or multiple pieces.

5. The multi-mode microwave oven according to claim 1, wherein the platinum or the platinum alloy has a heating rate > 25 °C / min.

6. A method for performing glass melting from the multi-mode microwave oven according to claim 1 using a metal crucible, comprising: (a) designing a heat insulation setup; (b) arranging the microwave absorber around the metal crucible in the heat insulation setup of step (a); (c) heating and melting the glass in the heat insulation setup of step (b) in the microwave oven in a controlled atmosphere to obtain molten glass; (d) pouring the melt of step (c) into a preheated mold to obtain high-temperature glass; (e) annealing the high-temperature glass obtained as in step (d) and then performing controlled cooling to room temperature to obtain processed glass; (f) grinding, polishing, and cutting the processed glass obtained as in step (e) to obtain glass of a desired size. The method includes these steps.

7. The method according to claim 6, wherein the shape of the microwave absorber is selected from the group consisting of ring, hollow cylinder, one piece, or multiple pieces.

8. The method according to claim 6, wherein the glass is melted in a temperature range of 1200 to 1600°C for 30 to 60 minutes.

Images