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

Abstract

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 4.0M 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 8.75mM K2SO4, the transition finishes in 6.7h, while the product shows a purity of 98.23% α-HH, and its 2h and 3 d bending/compressive strengths reach 5.9/17.2 and 11.2/37.4MPa, 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 34mg/kg, much lower than the 3100mg/kg and 88mg/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.