Phase transformation and dehydration of calcium sulphate dihydrate in solution studied by SEM

Abstract

The transformation of separate crystals of CaS04·2H20 (DH) into CaS04·0.5H20 (HR) in an aqueaus solution of H3P04 and H2S04 at 90-97°C was studied using a JSM-35CF SEM. The source DR and acids were reagent or eommereial (taken from the produetion of H3P04 by a wet process). DR was allowed to eonvert into HR and periodie samples of the suspension were drawn, filtered, and investigated in the solid phase by SEM. It was shown that the dehydration of reagent DH speeimens oeeurred through the reerystallization mechanism, whereas the transformation of eommereial DR into HH was basieally topoehemieal and developed by a relay race meehanism without any phase transformation front. The observed differenee between eommereial and reagent DR speeimens may be explained by differenees in their erystalline strueture, initial partiele size and by the presenee of impurities in DH and aeids. It is established that ehemieal impurities ean ehange the meehanism of the phase transformation of the erystals DH into HH.

Preparation of α-Calcium Sulfate Hemihydrate by Reaction of Sulfuric Acid with Lime

“…Different organics were tried, but 2-propanol was the one that was chosen. Use of acetone is reported in the literature, , but 2-propanol was found to achieve better results. The final dry samples were examined with XRD (Philips PW1710 X-ray diffractometer) as well as DSC (PerkinElmer PYRIS differential scanning calorimeter) for phase identification, and with SEM (JEOL-840A SEM, coupled with energy-dispersive X-ray spectrometer and digital imaging system) for morphology characterization.…”

Phase transformation kinetics of calcium sulfate phases in strong CaCl2HCl solutions

“…Previous research for a variety of solution compositions found that the DH to HH, DH to AH and HH to AH phase transformations typically proceeded via a dissolution-precipitation based mechanism (Austin et al, 1975;Azimi and Papangelakis, 2011;Beretka and Van Der Touw, 1989;Farrah et al, 2004;Hardie, 1967;Imahashi and Miyoshi, 1994;Kontrec et al, 2002;Müller et al, 2009;Sirota et al, 1992). Furthermore, it was found that an increase in temperature (Farrah et al, 2004), the presence of seed of the more stable phase (Azimi and Papangelakis, 2011;Dutrizac and Kuiper, 2006;Farrah et al, 2004;Hardie, 1967) and low water activity (Hardie, 1967) accelerate the transformation process.…”

“…Furthermore, it was found that an increase in temperature (Farrah et al, 2004), the presence of seed of the more stable phase (Azimi and Papangelakis, 2011;Dutrizac and Kuiper, 2006;Farrah et al, 2004;Hardie, 1967) and low water activity (Hardie, 1967) accelerate the transformation process. The presence of impurities might even lead to a change in transformation mechanism, e.g., from a dissolution-precipitation based mechanism in the case of pure materials to a solid state transformation in the case of impure materials (Sirota et al, 1992). The time signifying the onset of the conversion process (otherwise known as induction time) has been reported to vary from hours to several days depending on solution composition and temperature.…”

Preparation of α-High-Strength Hemihydrate from Flue Gas Desulfurization Gypsum in AlCl3–MgCl2 Solution at Atmospheric Pressure

“…There are many studies to explore the mechanism of transformation from DH to HH and to study the influence of cations and additives on HH crystal growth. Sirota et al reported that the impurities present in the FGD gypsum could change the transformation mechanism, where a solid-phase relay-race transformation without any front occurs and thus influences the shape and size of the formed HH crystals. The effects of metal ions such as Na + , K + , Mg 2+ , Cu 2+ , Al 3+ , and Fe 3+ on the crystal morphology and size of calcium sulfate whiskers in aqueous HCl solutions were also investigated. , The results revealed that Al 3+ ions resulted in bigger and shorter crystals of HH .…”