リン酸セッコウに関する研究

(第7報) 鉱酸によるリン酸セッコウの脱水および水和

抄録

The constitutional diagram of gypsum in the H₃PO₄ acid (1) or mixed acid of H₃PO₄ and H₂5O₄ (2) has been reported already. But there are many questions about phosphoric acid gypsum which have not been solved.
In the present paper, we have examined the dehydration and hydration of gypsums produced by actual phosphoric acid process. We have examined 4 phosphoric acid gypsums. Their analytical comosition (3) is as Table 1, and their crystal figure is as Fig 1. And, for reference, a extra pure gypsum and a clay gypsum were also examined.
These gypsums in 10 to 40% H₂50₄ were dehydrated by heating as shown in Fig. 2. The higher acid than 50% H₂SO₄ makes the all gypsum to anhydride but hemihydrate state. When the acid is more dilute than 40%, A and C gypsum has hemihydrate zone, but B and D has not. We assumed this difference is caused by anhydrite contained in B and C, for the solubility of anhydrite is so lower than hemihydrate that hemihydrate precipitates to the anhydrate. The experimental comfirmations of this assumption shall be reported in another paper.
Both the reagent gypsum and clay gypsum dehydrate at apploximately same temperature calculated by Nordengren's (1) and Murakami's (4) equation.
In HCl and HNO₃ all gypsums dehydrate in same way as shown in Fig. 3. We suppose this cause is that the anhydrite of the gypsum is all solved out by these acid until it can not precipitate.
As per Fig. 4 the hydration velocity in H₂SO₄ is higher than in HCl. The gypsum of uniform and grane crystal as A dehydrate linearly by heating time, but another gypsum of thin and irregular form crystal, dehydrate more easily and quickly. (5, 6) These character of dehydration is the causes of difference of dehydration of Fig. 2 and 3. By hydration of hemihydrate of A in the acid to which the crystal modifier of Fe³⁺ or Al³⁺ ion, (7, 8) is added, the gypsum crystalize out to the uniform and 2 to 3 times as large as crystals.