Flexible Morphology-Controlled Preparation of α-Calcium Sulfate Hemihydrate from FGD Gypsum in Ethylene Glycol Water Solutions

Alcohol-solution-mediated preparation and morphological control of α-calcium sulfate hemihydrate (α-HH) are of great significance to high value-added utilization of flue gas desulfurization (FGD) gypsum. This study explores the preparation and morphological control of α-HH from FGD gypsum in ethylene glycol (EG) water solutions by coordinating the conversion kinetics and steric hindrance. Hexagonal prismatic α-HH was successfully transformed from FGD gypsum at atmospheric pressure. Whisker, rod-like, short-column, and plate shapes with aspect ratios ranging from 54.72 to 0.13 were flexibly regulated by controlling the EDTA, Na⁺, and solid contents. Selective adsorption of hydroxyl groups from EG on the side facets, coupled with the Na⁺-enhanced driving force, facilitated the formation of whiskers. Chelation of carboxyl groups from EDTA with Ca²⁺ ions on the head facets of α-HH exerted strong steric hindrance, reducing the aspect ratio and inhibiting conversion. Increasing the solid content accelerated the conversion rate and mitigated the steric hindrance of EDTA. An optimal molar ratio of EDTA to FGD gypsum was proposed (3.83 × 10^-4), above which the edge of the head facets extended, developing concave shapes at both ends of the crystals. These findings offer a systematic framework for α-HH morphology regulation and high value-added recycling of FGD gypsum, revealing the competitiveness of kinetic-driven effect and steric hindrance on the anisotropic crystal growth and the resulting morphology.