尿素セッコウの単結晶成長および熱分解
1977 年 1977 巻 150 号 p. 210-214
詳細
1977 年 1977 巻 150 号 p. 210-214
Thermal decomposition of gypsum urea (GU) was studied by DTA, TG, X-ray diffraction test, IR analysis and microscopic observation. Highly pure GU crystals used here were prepared from gypsum and area by the cooling method. An aqueous solution of GU saturated at 60°C was cooled to 30°C, and a seed crystal of GU was suspended in the solution, which was kept at this temperature for one night in order to make a grown crystal. Single crystals with larger sizes than 1 cm were obtained by the repeatation of the above procedure.
A reversible transition of GU was observed at 150-155°C. The crystal of GU dissociated to urea and III-CaSO4 at 174°C, resulting in a considerably hygroscopic mixture. Furthermore, the mixture changed thermally at higher temperatures, i. e., urea changed to biuret and then volatilized passing through cyanuric acid, and the III-CaSO4 changed II-CaSO4 at 440°C. Being heated under a reduced pressure of 5 mmHg, GU dissociated into urea and III-CaSO4 at the same temperature as stated above. In this case, a large portion of the dissociated urea changed to biuret and then sublimed till up to 360°C.
The microscopic observation showed that no change of the crystal shape of GU occurred on heating, while the crystal diminished its transparency at 170-180°C owing to the thermal dissociation. It was seen by scanning electron microscopic observation that the surfaces of GU crystals heated up to 400°C were covered with the needle crystals of III-CaSO4 more loosely distributed than those of III-CaSO4 obtained from dihydrate.
