Vol.11 no.3 2019

Research paper : High-value materials from incineration residues of burnable garbage (N. FUKAYA et al.)−129−Synthesiology - English edition Vol.11 No.3 (2018) added value for the “arterial industry,” rather than the use in materials such as concrete aggregate for the so-called “venous industry.” It was newly found that by chemically treating the molten slag produced from municipal garbage disposal plants using acidic solutions under certain conditions, the silica component in the molten slag precipitated as white solids. When this white solid was recovered by filtering or other methods, silica with purity of 93–98 % could be easily obtained (Fig. 7).The estimated mechanism of this reaction is examined. As explained in Subchapter 2.2, the main components of molten slag are SiO2, CaO, and Al2O3, and the silicon components are thought to exist as “calcium salt of composite oxide of silicon and aluminum” rather than in a pure silica form. This calcium salt is thought to go through a certain process. After undergoing neutralization by acid, aluminum and calcium become soluble in an acid aqueous solution as ions, and silicon increases molecular weight by a dehydration-condensation reaction through a catalyst action of acid, after undergoing a low molecular silica condition called silica sol or orthosilicate. Then it precipitates as insoluble white solids as silica undergoes gelling. That is, in this reaction system, acid has two roles: one is to make components other than silica soluble as ions, and the other is to act as a catalyst for silica production by a sol-gel reaction. Since condensation occurs after the sol state in which silica has low molecular weight and is highly dispersed, the silica that is ultimately obtained by gelling is expected to have a high specific surface area.The results of actual nitrogen gas adsorption measurement showed that the calculated specic surface area was about 600 m2/g, and this was equivalent or higher than that of synthetic silica material that was commercially available as high specic surface area material. Therefore, it is expected that silica with high specic surface area obtained from molten slag using this technology will have various usages as adsorbents, additives for tires and synthetic rubber, catalyst carriers, in cosmetics, as abrasives, and others, in which synthetic silica is currently used in industry. Also, silica with high specific surface area will have many reaction points that can come in contact with reaction targets such as alcohol or alkaline catalysts. Therefore, when considered as a raw material for chemical reactions, it is expected to be useful in manufacturing tetraalkoxysilane mentioned in Subchapter 2.4.Currently, silica with high specic surface area used widely in industry includes fumed silica manufactured by a gas phase reaction of silicon tetrachloride in high-temperature hydrogen re, or precipitated silica (sometimes called white carbon) manufactured by a liquid phase reaction of sodium silicate and sulfuric acid. These manufacturing methods consume much energy for silicon tetrachloride and sodium silicate, and use raw materials manufactured by processes that require a certain degree of cost. On the other hand, the newly developed technology allows the use of residues that are generated irreversibly while disposing burnable garbage produced in our daily lives, and is overwhelmingly inexpensive at about 200 yen/ton even for those with qualities that satisfy the JIS standard. Therefore, compared to the conventional processes, it is expected to contribute to energy savings during silica manufacturing, reduction of carbon dioxide emission, and cost reduction of silica products.3.2 Technology to manufacture ordered nano-mesoporous silica from molten slagRecently, a porous silica material with ordered and uniform pore sizes of 2–50 nm (mesopores) called mesoporous silica is gaining attention. Mesoporous silica is generally synthesized by using a structure made by self-assembly of surfactants in solution as the mesopore template and forming silica around this template by a sol-gel reaction. Mesoporous silica has an ordered structure and has uniform nanospace within the material, and it is expected as a highly selective reaction field that cannot be realized by existing porous materials. It is a high added-value material for which research is conducted around the world for its application as a high Fig. 7 Raw material molten slag (left) and synthesized silica with high specic surface area (right)

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