Nucleation and crystallization studies of CaSO4.2H2O were carried out under simulated conditions of scale formation in the reverse osmosis desalination technique. Equal molar ratios of CaCl2 and NaSO4 aqueous solutions were mixed with saline solution of NaCl (40 g/L). Turbidity measurements were carried out for the mixed solutions at different time intervals to determine induction period for the crystal formation of gypsum at ambient temperature (25 °C). Induction period was measured under different high supersaturation ratios ranging from 4.47 to 6.71. Induction period decreases exponentially with increasing supersaturation ratios. Free energy barrier values, critical nucleus radius and nucleation rates were calculated with and without addition of antiscalant {(scale inhibitor) [sodium hexametaphosphate (NaPO3)6, SHMP]} using crystallization equations that relate induction period with supersaturation ratios. The radius of critical nucleus increased by about 30% with SHMP addition compared with the baseline at all the studied supersaturation ratios. Most importantly, scanning electron microscope (SEM) photomicrographs show that, the shapes of crystals of the uninhibited solutions are needle-like and plate-like crystals at supersaturation ratios of 4.47 and 6.71, respectively. Addition of 4.0 mg/L SHMP antiscalant modify the shape of crystals to be similar to that at low supersaturation ratio.

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