Generally in chemical gypsum it is easily supposed, especially when they are precipitated as the needle crystals with much surface or agglomeration of them, that the salts composing the mother liquid are present or remaining adsorbed, not to mention the mother-liquid and water insoluble substances from raw materials and reaction praction products. The effects of the third substances such as mentioned above are obviously not negligible, since they are chemically active compared with the impurities in the natural gypsum. In foreign countries the studies on this kinds of problem has not been interested in as described in another paper^{1), 2)} but in Japan the report had been proposed by Murakami and Other.³⁾ On the other hand, in the preparative chemistry for gypsum industry, the practical application of the effect of the third substances, that is, aridizing proposed by Riddell⁴⁾ has been known. Consequently in this paper, the dehydration characteristics of chemical gypsum as well as the effect of salts in aridizing methed were investigated.
The natural gypsum from Ras Mallap Island (combined water : 20.19 %) was used as the standard sample of calcium sulfate dihydrate. Sodium chloride and other 13 compounds were selected as the inorganic Salts to be examined. The dehydration behavior was measured by D.T.A. apparatus. The effect of potassium-, sodium-, calcium-, and magnesium chlorides are shown in Figs. 1, 2, 3 and 4. From these results, it has been found that all of these four chlorides lower the initial dehydration temperature and the potassium chloride has the weakest effect. As to the lowering effects of calcium-and magnesium chlorides, the tendency of induction by the dehydration of crystal water in this salts occurring at low temperature has been observed. When 0.10% and 1.0% of these salts were added, the first exothermic effect corresponding to the characteristics of α-calcium sulfate hemihydrate occurred. The second dehydration temperature was lowered accompanied with the first lowering effects by these salts and the tendency was observed to occur not easily these peak by the addition of small amount of the salts. No effects were observed on the third dehy-dration temperature and behavior.
As to the nitrates, only the addition of 1.0% of calcium-and magnesium nitrates had the tendency to lower the dehydration temperature of calcium sulfate. The first exothermic effect were observed by the addition of magnesium nitrate. As a rule, the addition of small amount of nitrates, similarly to the chlorides, has seemed to be difficult to raise the second dehydration and had no effect on the third dehy-dration and the second exotherm.
Three kinds of sulfate of potassium, sodium and magnesium and potassium-and sodium fluoride, potassium-and sodium carbonate, as shown in Table 1, gave no effect on the dehydration of calcium sulfate dihydtion and on the transition from III-anhydrite to II-anhydrite. As to the distinct occurrence of the first raexothermic effect by the addition of 1.0% of calcium chloride and 0.5, 1.0% of magnesium chloride and 1.0% of magnesium nitrate respectively, which had not been found by Murakami³⁾ ; calcium-and magnesium chlorides are the aridizing agents proposed by the Brookby⁵⁾ and Hogatt⁶⁾ , who pointed out the aridizing agents to be generally deliquescent. The similar effect of magnesium nitrate observed in this experiment as the former two salts may support the pointing out by them. But calcium nitrate did not indicate the first exothermic effect on the D.T.A. curve despite of its deliquescence. Since this first exothermic effect coincides with the view by kelly and others⁸⁾ whe considered the aridizing effect as the process to contain more α-calcium sulfate hemihydrate of low consistency in calcium sulfate hemihydrate produced by calcination, or with the characteristic behaviour of α-calcium

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The hemihydrate-dihydrate process was industrialized at Japan Steel-Tube Corporation in June 1961 after the author's investigation and the pilot plant test by coworkers for six years. The characteristics of this process are as follows;
(1) Digesting temperature of phosphate rock must be kept usually in the temperature range at which hemihydrate was stable and also was formed predominantly.
(2) Subsequently, slurry including hemihydrate is cooled from digesting temperature to dihydrate stable temperature due to the mixing of recirculated slurry or by the use of evaporator cooler, and a small amount of dihydrate nuclei is added in it to accelerate growth of crystal during hydration.
(3) The keys of industrial operation are the perfect decomposition of phosphate rock, addition of the active dihydrate nuclei, and of the active silica gel to eliminate effectively obstructive actions by organic impurities and fluorine for the gypsum crystal growth.
To elucidate whether gypsum obtained as byproduct was higher than gypsum for plaster board or not, the titled test was operated.
It was sammerized as follows;
(1) On the washing cake by moving pan filter.
This gypsum contained a small amount phosphoric acid remained on the crystal surface, not in the crystal, because pH valus was in the range 1.95-2.70 and also it was 2.10-2.90 after calcining at 170°C. But the tensil strength was relatively low in the range 3-10 kg/cm².
Its color was brown and after calcination at 1000-1100°C, surface of sintered gypsum showed dark bluish gray.
This gypsum is not available as plaster mould's material for porcelain slip casting.
(2) On the rewashing gypsum by the sedimentation washing apparatus.
Byproduced gypsum which is washed again by the devised sedimentation washing apparatus, was white, pH value of it was in the range 3.90-4.60, tensil strength was 9.30-13.3 kg/cm² and surface of sintered gypsum after calcining also was white.
(3) It was confirmed that byproduced gypsum has higher qualities enough to use as plaster moulds material for porcelain slip casting.
Especially, qualities of byproduced gypsum that was manufactured from “florida calcined phosphate rock” as raw material, is excellent.
In this process, yield of P_2O5 was 96% and concentrations of phosphoric acid was 32% P₂O₅.
The plant for this process has been operated steadily since it started.

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Infrared absorption spectra of dihydrate, hemihydrate, soluble anhydrite and insoluble anhydrite of calcium sulfate were obtained between 4000 and 400 cm^-1, and considered in relation to the structures. In the spectra of soluble anhydrite, absorption bands of water were observed at the same position as those of hemihydrate. Comparing the absorption bands of water in hemihydrate and soluble anhydrite with those in dihydrate, OH stretching vibration bands in hemihydrate and soluble anhydrite shifted to the higher frequency and were sharper, but, HOH deformation bands in hemihydrate and soluble anhydrite shifted to the lower frequency and overlapped to the lower frequency band of deformation bands appeared in dihydrate. Hydrogen bond distances, which were calculated from the diagram of K. Nakamoto and his co-worker, were about 2.85Å in the case of dihydrate, and about 3.0Å in the case of hemihydrate and soluble anhydrite. Consequently, the water molecules in hemihydrate and soluble anhydrite were considered to be combined by weak hydrogen bond and exist as relatively free molecule, as compared with those in dihydrate.
The absorption bands of sulfate ion were observed at 1200-500 cm^-1, and showed the characteristic figure in each calcium sulphate. The absorption band of hemihydrate coincided with that of soluble anhydrite in the range of 1200-1000 cm^-1, but not in the range of 700-500 cm^-1. Consequently, it was suggested that the structure of hemihydrata differed a little from that of soluble anhydrite. The process, of dehydration of dihydrate, that was observed in dry mechanical grinding at 20°C temperature, 60% humidity, was traced by infrared absorption spectra and X-ray diffraction. In this case, after 10 min. grinding, hemihydrate appeared a little and after 1 hr. hemihydrate was almost the whole amount, after 4 hrs. it was dehydrated to insoluble anhydrite.

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Gypsum mines and other mines in Tohoku destrict were effected by the geological structure, and they are distributed under the control of Honsyu arc, WNW direction of Fossa magna, and NS drection. The northern part of Tohoku district as to Chokaizan-Shiogama tectonic line, differs from the southern part in the scale of ore, the zonal arrangement, quantity of metal, and geological structure. Chokaizan-Shiogama tectonic line is being formed peculiar group of are deposits. The are which is consisted of only gyppum are far from tertiary granite. Reporter knew the other same attractine effects of gypsum are deposits and tertiary granite, will make public them.

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