Vol.2 No.2 2009

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Research paper : A strategy to reduce energy usage in ceramic fabrication (K. Watari et al.)−134−Synthesiology - English edition Vol.2 No.2 (2009) use highly efficient firing furnace, (2) reduce heat energy by lowering the sintering temperature, and (3) reduce heat energy generated from debinding and exhaust gas decomposition.4.2 Approach from equipment developmentFor (1), taking for example the gas firing furnace, the energy needed to sinter the ceramic body at 1300 °C is about 2 % of the total energy consumed. The remainder is about 25 % to heat the furnace wall, about 17 % is heat loss from the furnace wall, and the loss from exhaust gas is over 50 %. Therefore, swift measures such as the development of a firing system for high-efficiency sintering are necessary. Recently, a microwave furnace and a regeneration furnace that recovers high temperature exhaust gas have been developed[2]. The development of a high-efficiency sintering furnace is extremely necessary for energy saving in the ceramic manufacturing line. However, this issue was removed from the scenario to achieve the objective, because the main issue here concerns the development of equipment.4.3 Approach from sintering technologyFor (2), one of the ways is to reduce the total amount of energy needed for heating by using the existing sintering equipment by developing a low-temperature sintering technology. To promote low-temperature sintering of ceramics, it is necessary to mobilize nanoparticle handling technology[3][4], low melting point sintering additive technology[5], dispersion technology[6], surface coating technology (for auxiliary agents that may accelerate the sintering reaction of auxiliaries and particles), high-density forming technology, and others. All technologies work effectively for low-temperature sintering, and enables production of dense sintered body at firing temperature 100~300 °C lower than conventional sintering. However, there are problems such as shrinkage after firing due to addition of nanoparticles, difficulty in controlling the shrinkage, changes of material property due to addition of low melting point auxiliary agent and pollution of material surface, and reduced workability by using high-density forming technology. Therefore, we withdrew from the approach based on sintering technology.4.4 Approach from binder technologyThe method of (3) is the reduction or elimination of the currently used organic binder. Addition of organic binder enables formation of complex shapes and improves the strength of the green body. However, since an organic binder has low affinity for ceramic raw particle, it causes partial binder aggregation and also weakens the bonding strength between particles. Therefore, for good formability and shape maintenance after forming, a large amount of the organic binder must be added. Although the amount of binders differs according to size, thickness, shape, and processing of the green body, in general, it is 5 wt% or less in the case of dry mold product, 10 wt% or more for sheet mold product, and 20 wt% or more for complex shape product. Since the binder is unneeded after forming, it is removed from the body by heat decomposition or evaporation in the debinding step. The organic material used as a binder normally gasifies when heated at around 600 °C. Since sintering quality decreases when any binder remains on the powder surface as ash or carbon, precise process control is necessary in the debinding step. At the same time, the gas produced may cause structural defects such as pores, flaking, and warping in the green body and sheet, so the temperature must be increased gradually[7]. If the temperature at which the binder is completely eliminated is 600 °C, 60 hours of heating is necessary at a heating rate of 10 °C/h to reach the temperature, and 20 hours at 30 °C/h. Moreover, considering the heating and cooling times, the amount of energy required for debinding is extremely high.It is known that the gas generated by heat decomposition of binders contains organic material, and therefore, it is decomposed into harmless substances such as carbon dioxide and water, normally using the afterburner in the exhaust gas decomposition step. The heat decomposition temperature of many organic gases is 600 °C or above, and if the temperature of the afterburner is set above the heat decomposition temperature, the energy required for the treatment of exhaust gas becomes fairly considerable[8]. As shown in Fig. 1 and Fig. 2, the energy consumption related to binders is extremely high. If the amount of the organic binder used can be reduced by some sort of technological development, the amounts of heat energy required for debinding and exhaust gas decomposition can be reduced. Therefore, to promote energy savings of the existing ceramic manufacturing process, we decided to make our approach from the binder technology.If the developed binder technology necessitates major changes in the existing manufacturing line, the initial objective will be Fig. 2 Percentage of consumption energy in each process (all are laboratory level; energy required for powder production is not included).Consumption energy required to sinter 1 kg alumina. Organic binder additive: 10 mass%. Degreasing step: maintain 600 °C 1 hr (12 °C/h). Exhaust gas treatment step: maintain 900 °C. Sintering step: maintain 1400 °C 4 hrs (600 °C/h). For degreasing and sintering steps, 6 KW electric furnace was used. For exhaust gas treatment step, 1.4 KW electric furnace was used.(43 %)(26 %)(27 %)(4 %)Mixing & dispersionDebindingExhaustgastreatmentSintering

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