Crystallization of calcium sulfate dihydrate at different supersaturation ratios and different free sulfate concentrations

Abstract

Free Sulfate is a major parameter affecting gypsum crystallization during phosphoric acid production. Gypsum crystal size, shape and filtration rate are significantly affected by the concentration of free sulfate. It is, therefore, important to evaluate the effectiveness of different sulfate levels from 1.5% to 3.5% on induction time and gypsum morphology. The crystallization of gypsum was carried out under simulated conditions of phosphoric acid production by the dihydrate process. Calcium hydrogen phosphate and sulfuric acid were mixed with dilute phosphoric acid at 80 o C, and the turbidity of the reaction mixture was measured at different time periods to calculate the induction time of gypsum crystals formation. With increasing free sulfate concentration, the induction time was significantly decreased. Chemical processing of Central and South Florida phosphate concentrates under different concentrations of free sulfate from 1.5% to 5.5% was carried out. The change on crystal size distribution and filtration rate were traced with free sulfate concentrations. The results show that, filtration rate of phosphogypsum was correlated to the mean diameter of crystals. In addition, induction time and co-crystallized (lattice) P 2 O 5 % in gypsum are decreased with increasing free sulfate content from 1.5% to 3.5%. Morphology of formed gypsum crystals at different sulfate contents and different supersaturation ratios are investigated.

Influence of citric acid on calcium sulfate dihydrate crystallization in aqueous media

“…The excess of sulfate in the reaction slurry is widely used in industrial application to improve gypsum crystallization quality. It has been reported that morphology of gypsum crystals is affected by free sulfate content and supersaturation ratio [4]. They showed that large gypsum crystals are obtained in the presence of high sulfate concentration and thick crystals at low supersaturation ratio.…”

Towards the establishment of a general rate law for gypsum nucleation

“…The effect of the different parameters on the induction time for gypsum nucleation was determined in the past under variable environmental conditions (Smith and Sweett, 1971;Hileman, 1978a,b, 1980;Liu and Nancollas, 1973;Packter, 1974;Klepetsanis and Koutsoukos, 1991;He et al, 1994;Klepetsanis et al, 1999;Lancia et al, 1999;Linnikov, 1999;Hina et al, 2001;Prisciandaro et al, 2001;Abdel-Aal et al, 2004;Uchymiak et al, 2008). Although a wide range of solution compositions were studied, most can be characterized as exhibiting low ionic strengths and equivalent concentrations of Ca 2+ and SO 2À 4 .…”

Effect of supersaturation ratio and Khella extract on nucleation and morphology of kidney stones

“…They found that citrate does not significantly change the surface energy of the COM crystal nucleate, but citrate does retard the induction time by lowering the supersaturation through citrate complexes formed with the free calcium ions in the solution. The calculations of surface energy, nucleation rate, free energy barrier, and radius of nucleus have also been widely used in other systems such as calcium sulfate crystallization [7][8][9][10][11][12]. Hess also used induction time to explain the inhibitory effect of citrate and the Tamm-Horsfall protein on the COM crystallization by measuring the change of optical density over time [13].…”