Transient Solubilities in the Calcium Sulfate-Water System.

Abstract

The change in solubility in water with time of a and ß calcium sulfate hemihydrate and of /3-soluble anhydrite and "insoluble" anhydrite were determined over the temperature range of 35°to 110°C. The solubility of the hemihydrates goes through a maximum with time, which is related to modifications in the calcium sulfate phase present. The a-hemihydrate is less soluble but more stable than the ß form. The stability of a and ß hemihydrate increase as the transition temperature, 99°C. for gypsum-a-hemihydrate and 103°C. for gypsum-/3-hemihydrate, is approached. At temperatures above 45°C. the solute content in contact with "insoluble" anhydrite, prepared by igniting gypsum at 890°C., surprisingly increased with time well beyond the values previously reported for anhydrite solubility.Three FORMS of calcium sulfate have long been recognized, CaS04-2H20 (gypsum), CaS04-M2H2O (hemihydrate), and CaS04 (anhydrite). Various modifications of these forms have also been postulated, but after a careful study and review of previous work, Kelley, Southard and Anderson (9) in 1941 concluded that the only unique and reproducible forms were: CaS0v2H20, a-CaS04• V2H2O, ß-CaSOv V2H2O, a-and /3-soluble anhydrites, and insoluble anhydrite. The -hemihydrate is prepared, for example, by dehydration of gypsum in an atmosphere of saturated steam. The /3-hemihydrate is formed by dehydration of gypsum under a lower water vapor pressure, and it shows a higher solubility than the -hemihydrate. a-and ß-anhydrites are prepared by further dehydration of a-and /3-hemihydrates, respectively; both rehydrate

Coupling-Enhanced Control for Optical Disk Drives

“…The experimental value, S = 1.193 x 1O-2 Mcaso4 1-1 was obtained from experiments involving the addition of excess solid CaS04.2H20 to solutions with initial concentrations both above and below the equilibrium sohbility value. Literature values reported for the solubility of CaS04.2H20 at 100"C, S = 1.221 x 10-2 M 1-1 (Power et al 1964) and S= 1.175 x 10-2 M 1-1 (Marshall and Slusher 1966) are in good agreement with the value obtained in this investigation. Figure 2 Autoclave top plate with ports employed during a crystallization experiment: A, glass lined suspension sampling valve; B, suspension sampling tube; C, solution sampling tube with glass filter; D, Kel-F coated stirrer blade; E, Teflon plug for seed addition tube; F, seed addition tube; G, Kel-F coated thermowell; H, syringe seal; I, Teflon lined solution sampling valve; J, nitrogen inlet port; K, pressure release pop-off valve; L, Kel-F stirrer shaft; M, drive magnet for stirrer 3 Autoclave operation Supersaturated solutions of calcium sulphate dihydrate were prepared by heating a saturated solution prepared at lower temperatures.…”

“…This system was chosen because of the availability of CaS04.2H20 crystal growth data ov& a range of temperatures (1545°C) (Lui and Nancollas 1970) and because of interest in caso4.2H20 crystal growth near the transition temperature, 103"C, at which reaction (1) occurs (Power et al 1964):…”

Reactivity of Gypsum-Based Materials Subjected to Thermal Load: Investigation of Reaction Mechanisms

“…The kinetics, as well as the presence of water seemed to be crucial factors here. However, it appeared that the common solubility data (e.g., [24,25], summarized e.g., in [4]), which were also the diverse basis for the considerations in recent studies, could not be fully used to derive a reaction mechanism. This was supported, for example, by the recently published surprising results of the work of Van Driesche et al [26] and He et al [27] in which it was described for the crystallization of gypsum from aqueous solutions-under conditions for which it is generally assumed that gypsum is the stable phase-that the growth of gypsum was preceded by a hemihydrate phase as the precursor.…”

Solubility phenomena involving CaSO4 in hydrometallurgical processes concerning heavy metals

“…Freyer and Voigt [1] have critically evaluated some of the available solubility data [2][3][4][5][6][7][8][9][10][11][12][13][14] of the binary CaSO 4 + H 2 O system, where three crystal types exist, namely, gypsum, hemihydrate, and anhydrite. Among them, the hemihydrate is unstable in the temperature range 273-473 K. In pure water, the solubility of gypsum is relatively constant at ~0.015 mol kg −1 in the temperature range 273-373 K, but the solubility of anhydrite decreases with increasing temperature.…”