Gypsum & Lime

Super-solubility data of calcium sulfate hemihydrate in pure water are scarcely available. Therfore, to determine the super-solubility conveniently, a new experimental apparatus is presented. The apparatus is concisted of a separable flask in modified Othmar's distillation apparatus, a constant temperature oil bath and a heated wire in the A. C. circuit of Wheatstone bridge.
When pool boiling is taken place in aqueous solution at 100°C of calcium sulfate in the flask by heat from the wire and the solution is gradually concentrated, scale of calcium sulfate hemihydratz is formed on the wire. Time variation of scale diameter, wall temperature of the wire and concentration of the solution are measured. Starting time of scale deposit is determined from both observation and rate of scale deposit. And at that time, wall temperature of the wire and concentration of the solution are determined from each time variation, then the concentration of the solution is super-solubility corresponding with the wall temperature.
The authors' results at 110-130°C of wall temperature are nearly in agreement with those at 110-150°C of wall temperature in boiling state of Nevstrueva et al. It seems that the new method is useful for measurment of super-solubility.

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To analize mechanism of calcium scale deposit on heated wall in evaporator, accurate solubility data of calcium sulfate in pure water are required, but satisfactory data are not available from databooks. Therefore, an attempt to determine the equations which represent accurately the solubility of calcium sulfate is performed.
All the solubility data which are taken from available original literatures are classified in five groups corresponding to modifications of calcium sulfate, i. e. dihydrate, α-and β-hemihydrate, soluble anhydrite and insoluble anhydrite. The equations for each group are determined by so-called double-minimum-square method using digital computor FACOM 230-35 of Kanazawa University.
It is found that all the data for soluble anhydrite are obviously classified in two groups. Because of this fact and the other verification, two types of α and β exist in soluble anhydrite. At last, the equations of solubility for six modifications of calcium sulfate are presented. Deviation of the data from the equations are less than ±10% for temperature ranges from 0 to 170 or 200°C, but for dihydrate ±-1%, 0 to 110°C.
It seems that the given each equation of solubility for modifications of calcium sulfate is the most accu-rate, comparing with the other's solubility equations.

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