Abstract
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 paper1), 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
