Title of Invention

AN IMPROVED ENVIRONMENT FRIENDLY, ENERGY SAVING MICROWAVE PROCESS FOR FAST SINTERING OF SHAPED PORCELAIN COMPONENTS IN AIR

Abstract

This invention relates to an improved environ-friendly, energy saving microwave process for fast sintering of shaped porcelain components in air from quartz based raw materials consisting of feldspar, washed clay, raw clay, quartz, pyrophyllite and felcite comprising: firing shaped component in a microwave furnace using a suitable casket and susceptor arrangement in air at peak sintering temperature of 1200°C to 1260°C with soaking period of 30 minutes; maintaining heating cycle from 6 to 10 hour during sintering; resulting densified porcelain product with reduced porosity, higher bending strength and electrical properties through phase formation comprising of Quartz, Mullite and amorphous.

Full Text

FIELP OF THE INVENTION The present invention relates to an improved, fast, ener y saving environment friendly microwave process for the firing of porcelain components. The method is highly energy efficient and can be used in a commercial scale to fire porcelain components in air. The said method achieves better properties of the product than its conventional processed counterparts. BACKGROUND OF THE INVENTION Porcelain has been used as an electrical insulating material for more than 150 years. For a long time, it has been realized that several characteristic properties of porcelain (e.g. mechanical strength, high-power dielectric strength and corrosion resistance) as a ceramic product cannot be obtained in other materials. Porcelain is distinguished by the lack of open porosity in fired body. This ceramic is commonly referred as a triaxial whiteware primarily composed of clay, flux and filler. Potash feldspar or sodium feldspar is generally used as fluxes and quartz or alumina is used as fillers depending on their end use. Two types of porcelain insulators are used, the silica and alumina porcelain. In the disclosure these materials are referred as quartz body and alumina body porcelain. In quartz based porcelain, high temperature or long firing time leads to a reduction of solid quartz content in ceramic body because of melting of quartz grains. This reduction causes the decrease of mechanical strength of porcelain. Further, the difference between thermal expansion corresponding to quartz grains and the surrounding liquid phase causes mechanical stress, which can produce microcracks in porcelain. Intense changes of piece temperature could lead to increase in the already existing microcracks, causing reduction of mechanical strength under load. In alumina porcelain, the major portion of quartz is replaced by alumina, which leads to increase in mechanical strength (related to smaller number of cracks). During sintering, mullite and corundum are formed and porcelain is obtained with high content of glass phase that leads to non-porosity but without the melting of alumina grains (thus high temperature and long time of firing do not affect the mechanical strength). The alumina porcelain is insensitive to temperature changes and mechanical strength is mainly controlled by quantity of corundum (and not by the amount of mullite as in silica porcelain. The manufacturing of porcelain insulator involves mainly three steps: forming, drying, and firing. After forming they are dried in order to achieve the critical moisture level, below which there will not be further shrinkage of the porcelain body during drying. Ceramic processing is based on the sintering of powder compacts rather than melting / solidification / cold working (characteristic for metals). Sintering commonly refers to processes involved in the heat treatment of powder compacts at elevated temperatures, usually at T > 0.5 Tm, in the temperature range where diffusional mass transport is appreciable. Successful sintering usually results in a dense polycrystalline solid. During sintering this triaxial composition undergoes varied chemical reactions at different temperature. Fully sintered porcelain consists of a glassy matrix with quartz or alumina crystals with primary or secondary mullite, which is responsible for the strength of the fired porcelain. In regular practice, insulator is fired in the range of 1200° C to 1300° C with a cycle time exceeding 100 hrs. Therefore, the porcelain insulator manufacturing process is an energy and time intensive process, which will take 14 - 20 days of time and enormous amount of energy. The large proportion of the selling price of these items represents energy costs. While ceramic producers have always had a high awareness of energy costs, they are increasingly motivated to act, as energy prices increase and global competition intensifies. More than 70% of energy in ceramics manufacturing is accounted for the drying and firing process, with oil and gas as the predominant fuels. To solve this problem, an alternative method is adopted with microwave heating. This is a most promising technology over the conventional one with an advantage of time and energy savings. The process enables high rate of heating resulting in cycle time reduction and low energy consumption. This facilitates low cost of the product. Further, materials will experience the minimization of grain growth leading to improved mechanical strength. The volumetric heating will avoid any temperature gradient inside the body. The process is based on clean electrical energy and thus is an environment friendly technology. The microwave processing of porcelain is rather complicated since porcelain as such is a complex material and the fast heating rate by microwave processing poses further challenge in achieving the proper phase formation. The reactions occurred during firing needs to be properly controlled in order to achieve the best properties. The additional reducing atmosphere during processing complicates the overall processing. Further, the earlier experiments have not used the commercial compositions, rather concentrated on laboratory level- experiments, which is a problem in scale-up for commercial exploitation. DESCRIPTION OF THE INVENTION One of the objective of the present invention is to provide an improved process for the firing of an indigenously available commercial quartz based porcelain material in air. Another objective of the invention is to reduce the firing cycle of electro porcelain materials in the range of 75 - 85 % of that employed in the present commercial conventional process. A further objective of the invention is to reduce the energy requirement of processing porcelain materials in the range of 60 - 80 % of that used in the conventional commercial process. A still another objective of the invention is to confirm the retention of the required properties of the porcelain insulator body after processing by a very rapid firing cycle. PRIOR STATE OF THE ART Porcelain is a complex material and sintering to full density with required phases need careful consideration. This is the main reason of not much literature available on the microwave sintering of porcelain materials. S. Takeyama, M. Mizuno, S. Obata, T. Shimade, K. Satake, M. Sato, T. Mutoh, S. Ito, K. Ide, T. Inoue, K. Asaki, 0. Motojima and M. Fujiwara in sintering of traditional ceramics by Microwaves (84 GHz and 2.45 GHz) and in microwave: Theory and application in materials Processing V, 305-312, (2001) have reported to have sintered clay-quartz-feldspar system both by conventional (electric furnace) and microwave using 2.45 GHz frequency in air. The authors reported that to achieve the same degree of water absorption, the temperature applied in microwave hybrid sintering was slightly higher (approximately 20° C) than those used in conventional firing. However, this result is mainly concentrated on small 1 inch sample pieces which cannot be scaled up for any property measurements like strength, modulus etc. Since, the processing of porcelain is material specific, it is pertinent to experiment using indigenously available raw materials. Further, the result demonstrating that microwave requires higher temperature than that of conventional process cannot be properly explained and that will increase the cost of the product. M. Mizuno, T. Takayama, S. Obata, T. Hirai, T. Shimada, K. Satake, M. Sato, T. Mutoh, T. Shimotsuma, S. Ito, T. Inoue, K. Esaki, 0. Motojima in analysis of microwave sintered porcelain, and in microwave: Theory and application in materials processing V, 313-319, (2001) have reported to have investigated the effect of 84 GHZ microwave heating on the sintering properties of porcelain bodies in Kaolin-feldspar-quartz (KFQ) systems which is similar to the quartz body porcelain material. The study highlighted that it is very important to consider the temperature difference between liquid amorphous phase and the solid crystalline phases during microwave processing of porcelain bodies. However the effect of microwave radiation at widely used 2.45 GHz frequency on the sintering of these materials is needed to be studied in this material, since 84 GHZ frequency is not used commercially to exploit processing of materials. Further, the porcelain after firing was porous and thus cannot be used in high voltage porcelain applications. SUMMARY OF THE INVENTION To achieve the objects, and purpose of present invention, a method has been developed which allows sintering of porcelain components in air using an environment- friendly process. The processing has been carried out using a microwave furnace operating at 2.45 GHz frequency. This method enables reduction of cycle time by 70- 80% and reduction in energy cost by 60-80% over conventional method. Further, the properties of the sintered products as measured by different analytical techniques were shown to be either comparable or in some cases better than their conventional counterparts. The process is environment friendly since it does not use the ever- depleting fossil fuels for processing. The technology developed here can be scaled up in a commercial scale in processing porcelains for high voltage applications. According to the present invention there is provided an improved environ-friendly energy saving microwave process for fast sintering of shaped porcelain components in air from quartz based raw materials consisting of feldspar, washed clay, raw clay, quartz based raw materials consisting of feldspar (11-13%), washed clay (20-25%), raw clay (25-35%), quartz (20-23%), pyrophyllite (8-10%) and felcite (5-8%) comprising firing shaped component in a microwave furnace using a suitable casket and susceptor arrangement in air at peak sintering temperature of 1200°C to 1260°C with soaking period of 300 minutes; maintaining heating cycle from 6 to 10 hour during sintering; resulting densified porcelain product with reduced porosity, higher bending strength and electrical properties through phase formation comprising of Quartz, Mullite and amorphous. The present invention will be better understood by the narrated description for an embodiment below with reference to the accompanying drawings in which Figure 1 (a) represents an arrangement of samples for firing and circular disc sample (b) for temperature measurement. Figure 2 represents a firing cycle of microwave processing according to the invention and conventional processing of porcelain insulator components. Figure 3 represents X-ray diffraction patterns of quartz body porcelains which are sintered by conventional and by improved microwave process, showing the main crystalline peaks for quartz and mullite. Figure 4 shows scanning electron micrograph of (a) microwave and (b) conventionally sintered (1240° C) with magnification of 10,000 for microwave heating time-8 hours against conventional heating time ~ 90 hours. The present invention provides an improved method for the microwave firing of commercial grade and indigenously available quartz based porcelain insulator components with following raw materials: Feldspar, washed clay, Raw clay, Quarts, Pyrophyllite and Felcite. In a conventional commercial process, the raw materials are intimately mixed with wet mixing in a ball mill followed by filter pressing to obtain the cakes of desired hardness. These cakes were used for further shaping of actual porcelain insulators. The components are dried and sintered in a kiln using fossil fuels with total cycle time exceeding 100 hour in a partial reducing atmosphere. In the improved method, the shaped components of high (length / diameter ratio (10: 1) were used for firing in air in a microwave furnace using a suitable casket and susceptor arrangement. Figure 1 depicts a schematic of samples placed in a microwave furnace for sintering. The temperature was measured by emissivity corrected infrared contact less pyrometer. A typical firing curve used in the present method is depicted in Figure 2 implying drastic reduction in firing cycle than that employed in the conventional process. The advantage of this method is that the microwave power can be switched off after the soaking period at the peak temperature is over, which is not the case in the conventional method. The peak sintering temperature and the process cycle have been varied to optimize the microwave firing of porcelain components. The x-ray diffraction pattern of the sintered product is compared with that obtained by conventional method (Figure 3) of sintering in a Kiln using fossil fuel. All the required phases could be fully formed by the very fast method employed in the present invention, thus necessitating the need to use this technology in a commercial scale. The microstructure of the product after firing by the new method also confirmed the phase formation from the identical microstructure as obtained in the conventional method (Figure 4). Similar microstructure in both the processes indicates that reducing atmosphere in conventional processing is not an essential step in developing the right microstructure during fast-fired microwave processing. From this observation the processing cycle of quartz- based porcelain has been developed for commercial scale production of porcelain components. The invention is illustrated with testing results wherein Table 1 shows properties of conventional and microwave sintered cylindrical rods of Quartz based Porcelain insulator components. Table 2 shows porosity variation along the length of rod at different conditions and Table 3 shows phase content of porcelain body after processing by conventional and the present improved microwave sintering methods. Table 1 summarizes the best results achieved on the microwave fired porcelain products indicating better mechanical properties from the new method and comparable electrical properties. The electrical properties of microwave sintered porcelain material is improved which was not reported in earlier literature and confirms that microwave firing of porcelain is ideal in an industrial scale to achieve the competitiveness of the product in the market. The samples were initially fired at 1210° C with a heating cycle of 6 hour and a soaking period of 30 minute at the peak temperature. It was observed that, the samples were not fully sintered and the difference in porosity content was very high from top to bottom of the sample (table 2). The cycle time and temperature are two most important parameters to achieve uniformed fully sintered body. Few more experiments were carried out by varying the heating cycle and the peak temperature keeping the soaking period of 30 minutes constant in all the experiments. It was observed that, the heating cycle of ~ 8 hours and a peak temperature of ~ 1240° C is ideal in both reducing the porosity variation throughout the sample and simultaneously densifying the component uniformly. This series of experiments have confirmed that microwave heating of these compositions do not require higher temperature than that used in conventional process. The peak temperature, duration at peak temperature and the heating cycle are three important parameters for proper densification of samples. The peak temperature plays a very important role in final densification of the product and imparting properties to the final product. In this experiment, the peak temperature was varied from 1220° C, 1240° C and 1260° C keeping the soaking duration constant at 30 minutes. All other experimental conditions were kept constant. It was observed that the bending strength of the samples varied drastically from 85 MPa for 1220° C firing to 118 MPa for 1240° C firing to 107 MPa for 1260° C firing. This result indicated that the peak temperature of 1240° C is ideal in imparting maximum strength in this porcelain composition. It Is pertinent to mention that the same peak temperature is commonly used in the conventional commercial process, however, the strength of the microwave- sintered body is significantly higher than that of its conventional counterpart. Further, this result has established that higher sintering temperature than that used in commercial process is not suitable for achieving better properties in these materials. The amorphous phase content in porcelain plays an important role in optimizing the properties. The higher peak temperature and higher heating time are directly proportional to higher glass phase formation which translates to lower mechanical properties. Besides the amorphous content, the amount of crystalline phase like quartz and mullite and cristobalite influences the properties of porcelain material. It is always desirable to possess lower amorphorous content and higher crystalline content to obtain better properties in a quartz based porcelain material. This is limited in conventional processing due to high heating time. The present method of microwave processing is a very fast process and it contributes to the phase formation with significant improvement of characteristic properties. The quantitative x-ray diffraction (XRD) analysis was carried out on the porcelain samples sintered at different conditions and analyzed by Rietvield method. The quantified XRD results (table 3) on microwave-sintered rods (MR) were compared with that of conventionally processed rod sample. It was observed that the amorphous phase content was lower and the crystalline phase content was higher in the present microwave sintered samples in contrast to that processed by conventional process. This result is supportive of higher mechanical properties in these materials as described earlier. The higher quartz content in microwave-processed samples contributes significantly to the mechanical properties. The significant amount of cristobalite phase as present in conventional processed samples was not found in microwave processed samples. The results have confirmed that the new improved microwave process described in this invention is ideal for the processing of porcelain materials. The main advantages of the new improved process are; a) The improved method is extremely fast process and less energy-intensive without affecting the properties of the component. This translates into very low cost for the processing of such materials in an industrial scale. b) The new method entails the firing of porcelain in air so that a combination of reducing and oxidizing atmosphere is not required as employed in the conventional process. c) The process yields less amorphous phase content and higher crystalline phase content resulting better properties than that obtained by conventional commercial process. d) The electrical properties of the fired product by the new method are either comparable or better compared to those obtained by the conventional process. e) The new method is environment friendly process since it does not utilize fossil fuels. f) The new process does not require higher temperature than that of conventional process. The invention as narrated herein with an embodiment and illustrated with test results should not be read and construed in a restrictive manner as various adaptations, changes and modifications are possible within the limit and scope of the invention as defined and encompassed in the appended claims. WE CLAIM 1. An improved environ-friendly energy saving microwave process for fast sintering of shaped porcelain components in air from quartz based raw materials consisting of feldspar (11-13%), washed clay (20-25%), raw clay (25-35%), quartz (20-23%), pyrophyllite (8-10%) and felcite (5-8%) comprising: processing the firing for sintering by electricity for avoiding the use of already depleting fossil fuels; firing shaped component in a microwave furnace using a suitable casket and susceptor arrangement in air at peak sintering temperature of 1200°C to 1260°C with soaking period of 30 minutes; maintaining heating cycle from 6 to 10 hour during sintering; characterised in that, densified porcelain product with reduced porosity, higher bending strength and electrical properties is resulted through phase formation of 43.8% of Quartz, 1.3% of Mullite and 54.9% amorphous in microwave heating cycle of 7.5 hour, peak sintering temperature of 1240°C and soaking time of 30 minutes wherein reduction in amorphous phase content and increase in crystalline phase content results better properties of the sintered products. 2. A microwave process for fast sintering of shaped porcelain as claimed in claim 1 wherein the processing requires electricity for firing and thus avoids the use of already depleting fossil fuels. 3. A microwave process for fast sintering of shaped porcelain components as claimed in claim 1, wherein phase formation of Quratz, Mullite and amorphous are respectively of 43.8 %, 1.3 % and 54.9 % at microwave heating cycle of 7.5 hour, peak sintering temperature of 1240° C and soaking time of 30 minutes. 4. A microwave process for fast sintering as claimed in the preceding claims wherein the bending strength of the sintered samples are in the range of 85 MPa to 118 MPa in the firing peak temperature range of 1220° C to 1260° C, the optimized strength being achieved at 1240° C peak temperature. 5. A microwave process for fast sintering as claimed in the preceding claims, wherein lower amorphous content and higher crystalline phase (Quartz and Mullite) are formed in the microwave heating translating higher strength than that of the conventional heating with fossil fuel with high heating cycle of 80-90 hours, in which higher glass phase are formed translating lower mechanical properties. 6. A microwave process for fast sintering as claimed in the preceding claims wherein, in the microwave heating combination of reducing and oxidizing atmosphere is not required as employed in the conventional process. 7. A microwave process of fast sintering as claimed in the preceeding claims, wherein the microwave power is switched off after the soaking period of 30 minutes at the peak sintering temperature is over. 8. A microwave process of fast sintering as claimed in the preceeding claims, wherein in higher dielectric constant and dielectric strength are achieved than the conventional heating cycle of sintering porcelain compact indicating higher insulating property achievement from the microwave heating cycle than that of the conventional heating cycle. 9. A microwave process for fast sintering as claimed in the preceding claims wherein the energy requirement is drastically reduced due to drastic reduction in firing cycle resulting cost-effectiveness during production of porcelain product. 10. An improved environ-friendly, energy saving microwave process for fast sintering of shaped porcelain components as herein described and illustrated. ABSTRACT AN IMPROVED ENVIRONMENT FRIENDLY. ENERGY SAVING MICROWAVE PROCESS FOR FAST SINTERING OF SHAPED PORCELAIN COMPONENTS IN AIR This invention relates to an improved environ-friendly, energy saving microwave process for fast sintering of shaped porcelain components in air from quartz based raw materials consisting of feldspar, washed clay, raw clay, quartz, pyrophyllite and felcite comprising: firing shaped component in a microwave furnace using a suitable casket and susceptor arrangement in air at peak sintering temperature of 1200°C to 1260°C with soaking period of 30 minutes; maintaining heating cycle from 6 to 10 hour during sintering; resulting densified porcelain product with reduced porosity, higher bending strength and electrical properties through phase formation comprising of Quartz, Mullite and amorphous.

Documents:

00019-kol-2007 correspondence.pdf

00019-kol-2007 form-18.pdf

0019-kol-2007-abstract.pdf

0019-kol-2007-assignment.pdf

0019-kol-2007-claims.pdf

0019-kol-2007-correspondence others.pdf

0019-kol-2007-description (complete).pdf

0019-kol-2007-drawings.pdf

0019-kol-2007-form1.pdf

0019-kol-2007-form2.pdf

0019-kol-2007-form3.pdf

19-KOL-2007-(04-09-2012)-CORRESPONDENCE.pdf

19-KOL-2007-ABSTRACT 1.1.pdf

19-KOL-2007-AMENDED CLAIMS.pdf

19-KOL-2007-CANCELLED COPY.pdf

19-KOL-2007-CANCELLED PAGES.pdf

19-KOL-2007-CORRESPONDENCE.pdf

19-KOL-2007-DESCRIPTION (COMPLETE) 1.1.pdf

19-KOL-2007-DRAWINGS 1.1.pdf

19-KOL-2007-EXAMINATION REPORT.pdf

19-KOL-2007-FORM 1.1.1.pdf

19-KOL-2007-FORM 18.pdf

19-KOL-2007-FORM 2.1.1.pdf

19-KOL-2007-GPA.pdf

19-KOL-2007-GRANTED-ABSTRACT.pdf

19-KOL-2007-GRANTED-CLAIMS.pdf

19-KOL-2007-GRANTED-DESCRIPTION (COMPLETE).pdf

19-KOL-2007-GRANTED-DRAWINGS.pdf

19-KOL-2007-GRANTED-FORM 1.pdf

19-KOL-2007-GRANTED-FORM 2.pdf

19-KOL-2007-GRANTED-FORM 3.pdf

19-KOL-2007-GRANTED-SPECIFICATION-COMPLETE.pdf

19-KOL-2007-REPLY TO EXAMINATION REPORT.pdf

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