粗りん酸中の二水せっこう結晶の成長
1971 年 1971 巻 112 号 p. 83-92
詳細
1971 年 1971 巻 112 号 p. 83-92
A method for determining the rate of gypsum crystal growth from hemihydrate in crude phosphoric acid aqueous solution was studied. The experimental fact that the feature of original dihydrate seed is visibly preserved in the as-grown dihydrate crystal made the determination of growth rate possible.
The crystal growth of dihydrate is undergone at a constant temperature (65°C) in metastable zone of hemihydrate after added slightly colorized seed, which is prepared by reaction of calcium chloride and sodium sulphate, to crude phosphoric acid solution containing hemihydrate obtained by decomposition of Fluorida rock phosphate powder with sulphuric acid. Since the original feature of seed crystal is visibly preserved in as-grown crystal, the rate constant of dihydrate growth are determined by measuring the size of as-grown dihydrate crystals and dihydrate seed in it and by applying the rate equation (a*2= (D/ρ) (C∞-Cs) t+A) derived by Humphreys Owen. Where, a* is equivalent length in the direction of c-axis of crystal of gypsum and C∞-Cs is solubility difference between hemihydrate and dihydrate in crude phosphoric acid and is constant at definite temperature and definite concentration under present experimental condition.
By using the method, the precise determination of growth rate of dihydrate crystal is not subject to confuse by irregular neucleation in the hydration process. The rate constants of dihydrate growth in crude phosphoric acid obtained from two kinds of Fluorida rock phosphate are KFnI =2. 4×102 (μ2/ hr), KFnII=6.0 × 102 (μ2/hr) respectively. The addition of reactive silicic acid to the crude acids increases the rate constants, these are KFsI =2. 5 × 103 (μ2/hr), KFsII=1.8×103 (μ2/hr) respectively.
The relation between the equivalent length and growth rate at spontaneous length is a hyperbola, a.* (da*/dt) =K. Therefore, it indicates that the smaller the size of crystal, the larger the rate of growth and the larger the size of crystal, the smaller the rate of growth.
The present method will can be used for determining the mechanism of solution growth of gypsum crystal and the growth rate of dihydrate crystal in other chemical gypsum productions.
