燃焼, 熱分解によるシアン化水素の発生

抄録

This paper deals with the amounts of HCN and NH₃ formed by combustion and/or pyrolysis and the consideration of mechanism of HCN formation.
The combustion and pyrolysis were carried out in a quartz tube held in an electric furnace and gaseous products formed were collected in a series of 5 impingers containing KOH solution for HCN absorption, H₂SO₄ solution for NH₃ absorption respectively (Fig. 1). HCN was analyzed by the Liebig-Dénigès' silver nitrate titration method and NH₃ by the Folin's Nessler method.
The amounts of HCN formed by the pyrolysis or burning in air of polyacrylonitrile and nylon-6, increased, in most cases, with increasing temperature. In most of the cases, they decreased with the increase of relative air supply rate (the ratio of the air supply rate to the sample amount carried into the furnace in one time spoon operation), though on the contrary they increased in the range of very small relative air supply rate (Fig. 2 and 3). As for urea resin, melamine resin and polyurethane, HCN yields were so little affected by the relative air supply rate ranging from 0.5x 10^-2 to 2.5x 10^-2 1/min/mg that they were only related with the heating temperature (Fig. 4-6). Urea and melamine resins had their peaks in HCN yield at 650°C. HCN yeild from polyurethane increased with increasing temperature. According to Fig. 7, in the atmosphere of N₂, HCN formation occurred only at over 500°C with one exception, and increased with increasing temperature. Only polyacrylonitrile evolved HCN even at considerably low temperature of 300°C. HCN yield was proportional to the content of nitrogen in the case of pyrolysis temperature of 900°C, as shown in Fig. 8.
The findings mentioned above suggest that HCN formation is only dependent on thermal decomposition temperature. Thermal decomposition reaction is generally more brisk, as the temperature is higher. So, the amount of HCN formed is considered to increase with increasing temperature, whether the atmosphere is N₂ or air. But, when oxygen or air exists, it oxidizes HCN once formed and thus reduces the final HCN yield, though it contributes to the flame temperature rise which results in the larger HCN yield in some cases. In the case of urea and melamine resins burned at 800°C in air, most of HCN once formed was considered to be consumed by oxidation through flaming burning. Polyurethane will show the similar pattern to those of nylon-6 and polyacrylonitrile respectively in Fig. 2 and 3, if the relative air supply rate is varied more largely.
HCN was found in the gaseous products from all the nitrogen-containing low molecular organic compounds of different kinds used in the present experiments, when they were pyrolyzed at over 600°C in the current of N₂. Unlike many other compounds, lactonitrile and dimethylglyoxime evolved HCN even at the relatively low temperature of 300°-400°C, as polyacrylonitrile did. Dimethylglyoxime is the compound which could easily be turned into a nitrile by hydration. As polyacrylonitrile is believed to release HCN through dehydrogen cyanide reaction, so the other two are. The reason why such nitriles as acetonitrile, phthalonitrile and acrylonitrile did not release any amount of HCN at lower temperature than 600°C is probably that their thermal decomposition temperatures are rather high. The larger the content of nitrogen, the larger was the HCN yield in pyrolysis at 1000°C (Fig. 10). It shows the trend that the ratio of HCN yield to nitrogen content will converge some point at higher temperature than 1000°C regardless of chemical bonding or structure of compounds.
It was found that HCN was formed when a mixture of hexane and NH₃ or that of ethanol and NH₃ was heated in the current of N₂ (Fig. 9).