Vol.11 no.3 2019
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Research paper : High-value materials from incineration residues of burnable garbage (N. FUKAYA et al.)−128−Synthesiology - English edition Vol.11 No.3 (2018) in manufacturing synthetic quartz). Ethanol and catalyst potassium hydroxide were added, the mixture was heated along with molecular sieve 3A (pore diameter 0.3 nm), a type of zeolite, as an inorganic desiccant, and the reaction was conducted for 3 hours. Table 1 shows the results of the reaction. TEOS was generated at a yield of 51 % based on the silica content. The ashes remaining after burning chaff and straw that are underused resources of agricultural by-products have relatively high silica purity, and TEOS was obtained at a yield of 72–78 % when these materials were used for synthesis. The by-products of synthetic quartz manufacturing were recovered and used as the starting material, and TEOS was obtained at a 72 % yield.The point of difficulty in the development process of this technology was the shift in the idea that allowed molecular sieves to be used in this reaction and the design of the reaction apparatus. The most ideal process to synthesize tetraalkoxysilane from silica and alcohol can be described by an extremely simple chemical reaction equation shown at the top of Fig. 5. However, in practice, this reaction is governed by chemical equilibrium, and since silica is a thermodynamically stable substance, the produced tetraalkoxysilane and water react, and a reverse reaction in which tetraalkoxysilane returns to silica and alcohol occurs much more easily. Thus, it is difficult to obtain tetraalkoxysilane at a good yield. To shift the chemical equilibrium to a desired direction, a dehydration method by which produced water is successively removed from the reaction system is the key. Molecular sieves are desiccants used universally, and normally they are used in direct contact with targets (liquid or gas) to be dehydrated. While having powerful desiccant capabilities, they can also be relatively easily reversed and reused, and high efciency can be expected if they can be used. However, molecular sieves are crystallized alkaline metal aluminosilicate (alkaline metal salt that is composite of alumina and silica), and a silica unit exists within its structure. Therefore, if molecular sieves are directly introduced to the reaction system whose purpose is to dissolve silica, the structure of the sieve will disintegrate. Therefore, at the beginning of the research, we thought we could not use the sieves for this reaction, and we searched for an organic desiccant that could be directly placed in the reaction system.[4]However, with the progress of the NEDO Project, we became aware of the necessity of developing a simple process that allowed easy separation from the target substance. The desiccant used had to be easily recoverable and reusable, in order to be feasible as the new reaction process was moved up from a laboratory scale to an industrial scale. Therefore, we shifted our way of thinking to how to use a molecular sieve that tended to disintegrate in our reaction condition. We worked on the reaction apparatus, and as shown in Fig. 6, the vessel for silica and alcohol to react under the presence of a potassium hydroxide catalyst (lower part) and the vessel in which a molecular sieve functioned as a desiccant (top part) were separated. In this reaction apparatus, alcohol and water vaporized in the lower vessel, moved to the upper molecular sieve, and only the water was absorbed and removed. Therefore, the potassium hydroxide catalyst that promoted the breaking of the Si-O-Si bond of silica did not come in contact with the molecular sieve, and the water could be removed successively without the disintegration of the structure. 3 Manufacturing of high added-value materials from molten slag raw materials3.1 Technology to manufacture silica with high specic surface area from molten slagMitsui E&S focused on the fact that the research results of the aforementioned NEDO Project involved the “advanced use of silica that was the main component of molten slag” while pointing to the attainment of green sustainable chemistry (GSC). Therefore, the company started joint research with AIST to aim for further effective use of molten slag, particularly for its use in functional materials with new Molecular sieveSilicaPotassium hydroxide catalystAlcoholPressure gageFig. 6 Schematic diagram of reaction vessel for tetraalkoxylsilane and its photograph[3]

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