石膏と石灰

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Calcium sulfites were synthesized in aqueous solution and thermal behaviors of the products were investigated in air and N₂ gas mainly by thermal analysis (micro-TG-DTA) and X-ray powder diffraction methods. The results obtained were summerized as follows :
(1) The X-ray diffraction patterns of the products synthesized by various methods were in close agreement with that of CaSO₃·1/2H₂O in the ASTM data cards, but the amount of H₂O in them were higher by 1 to 3 wt. % than the calculated value from the formula CaSO₃.1/2H₂O. The excess water were gradually dehydrated from 100 to 300°C, and the dehydration of the hemihydrated water of CaSO₃·1/2H₂O was occurred at 350 to 400°C.
(2) In air, the CaSO₃ was very sensitive to oxidation and rapidly oxidized into II CaSO₄ at 500 to600°C. The II CaSO₄ were decomposed to CaO and SO₃ after the transition from II CaSO₄ to I CaSO₄at 1170°C.
(3) In N₂ gas, the CaSO₃ were transformed to II CaSO₄ and CaS at 700 to 800°C according to the formula (1),
4CaSO₃700-800°C-→3II CaSO₄+CaS (1)
and the products were decomposed into CaO and SO₂ below the temperature of II CaSO₄ → I CaSO4 transition according to the formula (2).
3II CaSO₄+CaS850-1150°C-→4CaO+4SO₂ (2)
(4) When the CaSO₃.1/2H₂O crystals were large enough, the oxidation in air occurred at higher temperature than usual partially accompanied with the self oxidation-reduction reaction of formula (1).

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Laboratory tests were made to recover P₂O₅ in waste-waters from wet-process phosphoric acid plants by adding lime. Tests also were made on the products.
Prior to the precipitation of P₂O₅, most of fluorine was precipitated as calcium fluoride at pH 3-4, and then the remaining phosphoric acid was precipitated as fine crystal of apatite. The crystal size of apatite varied with the amount of fluorine in the waste-water. Although citrate solubility varied with the crystal size, all of the apatite were entirely soluble in 2%. citric acid soln.

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