Effect of pH on the Preparation of α‐Calcium Sulfate Hemihydrate from FGD Gypsum with the Hydrothermal Method

Abstract

pH is one of the most important parameters that determine the crystallization process, but it is always neglected in the preparation of α‐calcium sulfate hemihydrate (α‐HH) from calcium sulfate dihydrate (DH) with the hydrothermal method. Flue gas desulfurization (FGD) gypsum, which is mainly composed of DH, was used as raw material to obtain α‐HH through dehydration in a Ca–Mg–K–Cl‐solution medium at 95°C under atmospheric pressure. The initial pH values of the suspensions were adjusted from 1.2 to 8.0 to explore the influence of pH on the dehydration process and the product characteristics. The results showed that α‐HH crystal was the only dehydration product with the pH ranging from 1.2 to 8.0. With the increase of initial pH, the dehydration rate decreased and the formed α‐HH crystal had a larger particle size. The length/width ratio decreased markedly from 4.8 to 2.9 as the initial pH increased from 1.2 to 7.3. pH had a profound influence on the dehydration of DH and the morphology of α‐HH via its effect on the supersaturation and perhaps also the precipitation of Ca(OH)2 in an alkaline environment.