Waste mould of gypsum used in the pottery industries is ordinarily reclaimed to calcined gypsum by grinding and calcining to about 170-180°C in gypsum kettle in the common way. But thus reclaimed calcined gypsum is not so good as that from natural gypsum, owing to impurities from the pottery msses and the unsuitable crystalline structure. So that, it is necessary to purify and crystallize to the superior crystalline structure. For this purpose, the following two methods were systematically studied and fully discussed.
(I) Boiling Method: The first method is (1) the boiling of semihydrate gypsum (CaSO
4⋅1/2H
2O) or soluble anhydrous gypsum (CaSO
4), which were preliminary obtained by heating powdered waste mould gypsum, in dilute sulphuric acid solution to make to crystalline gypsum (CaSO
4⋅2H
2O), and then (2) the ordinary calcining (170-180°C) process under the atmospheric pressure, above mentioned.
(II) Autoclave Method: The second method is (1) the hydrothermal heating (120-150°C and 2-5kg/cm
2 pressure) in closed autoclve to semihydrate gypsum, (2) the crystrllizing to crystalline gypsum, and then (3) the ordinary calcining (170-180°C) process under the atmospheric pressure, above mentioned.
The crystal gypsum (CaSO
4⋅2H
2O) and calcined semihydrate gypsum (CaSO
4⋅1/2H
2O) obtained from these two methods were systematically compared on their various physical and chemical properties, and fully discussed the merits and demerits of these tow methods.
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