The synthesis of α-gypsum hemihydrate and control of its morphology were investigated by adding H
2SO
4 solution into saturated solution of II-anhydrite which was dissolved in HNO
3 solution under atmospheric pressure at 100°C. Characteristics of the hemihydrate were determined by means of X-ray diffraction, thermal analysis (TG-DTA), scanning electron microscopy and chemical analysis.
II-anhydrite was easily dissolved into HNO
3 solution at 100°C, and its maximum solubility was 3.2g/100 cm
3 in 2.9mol·dm
-3 HNO
3 solution. α-hemihydrate was synthesized by adding H
2SO
4 solution into the CaSO
4-HNO
3 system saturated solution. Supersaturation degree of II-anhydrite remarkably affected to morphology and stability of α-hemihydrate. The degree was controlled in the range of 0.6 to 1.8 by changing concentration of H
2SO
4 solution. At supersaturation degree of 0.7, α-hemihydrate with aspect ratio 26 (650μm×∅25μm) was formed after 30 min, but it was dehydrated slowly to II-gypsum anhydride after more than 90 min. The many kinds of sodium carboxylates, MgCl
2 and NH
4NO
3 were added in synthetic process to produce large blocky crystal (small aspect ratio). The aspect ratio was decreased by increasing number of carbon in alkyl group constituting additives and amount of additives. The inorganic additives were effective to the growth in the length direction of α-hemihydrate particle against that of organic additives.
The synthetic technique of α-hemihydrate from II-anhydrite under atmospheric pressure was proposed in the present paper as a recyclable process of HNO
3 and H
2SO
4 solutions.
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