Pilot scale preparation of α-calcium sulfate hemihydrate from FGD gypsum in Ca–K–Mg aqueous solution under atmospheric pressure

Preparation of α-calcium sulfate hemihydrate (α-HH) from flue gas desulfurization (FGD) gypsum is one of the most promising alternatives and of great value in effectively utilizing the FGD gypsum. Based upon a salt solution method and the optimum operating conditions obtained in the laboratory scale experiments, pilot scale tests have been conducted to confirm the feasibility of the process. The pilot experiments were carried out in a Ca–K–Mg chloride solution medium at 94 ± 2 °C under atmospheric pressure on a 500–1000 kg/batch setup. The crystal water content, solid phases, crystal morphology, strength and chemical composition of the products were examined and compared. It was found that short prism-shaped α-HH was successfully prepared within 5–6 h, with the purity of α-HH up to 95%, the 2 h bending strength of 6.8–7.3 MPa, 2 h compressive strength of 12.2–23.3 MPa and dry compressive strength of 29.6–37.9 MPa. It can be concluded that the process is highly feasible in the pilot scale, with high quality of the products. This research can provide necessary and useful information for the industrial production in the future.