Preparation of alpha-calcium sulfate hemihydrate from FGD gypsum in chloride-free Ca(NO3)2 solution under mild conditions

• Preparation of α-HH from FGD gypsum in chloride-free solution.
• Well-control over the phase-transition kinetics and product quality.
• Comparable mechanical strength of the product to that of commercial α-HH.
• Outstanding superiority in control of chloride content in product.
• Avoidance of the potential risk from the excess use of chloride.

This work develops a new class of salt solution method to prepare alpha-calcium sulfate hemihydrate (α-HH) from flue gas desulfurization (FGD) gypsum in trace K2SO4-contained 4.0 M Ca(NO3)2 aqueous solution at 95 °C under atmospheric pressure. The driving force for the phase transition is provided by the concentrated Ca(NO3)2 solution through decreasing the water activity. K2SO4, as an additive, plays a key role in tuning the phase-transition kinetics as well as the morphology, crystallinity and setting property of α-HH. In the presence of 8.75 mM K2SO4, the transition finishes in 6.7 h, while the product shows a purity of 98.23% α-HH, and its 2 h and 3 d bending/compressive strengths reach 5.9/17.2 and 11.2/37.4 MPa, respectively, which are comparable to those of the commercial α-HH. Compared with the recently well-developed chloride system (CaCl2 + MgCl2 + KCl), the nitrate system provides a chloride-free environment and shows outstanding superiority in control of chloride content in product and elimination of potential risk from excessive use of chloride, such as the corrosive damage to metal-made units and the by-production of chloride-containing wastewater. The obtained α-HH have a chloride content of 34 mg/kg, much lower than the 3100 mg/kg and 88 mg/kg in those obtained from the chloride system after conventional and sufficient washes, respectively. Overall, the newly-developed nitrate system provides an improved alternative for α-HH preparation with controlled process and product quality, and helps push forward the industrial recycling of FGD gypsum.