No abstract

A novel “[MSO4](2–b)– ion pair”-directed mechanism is presented to describe the role of nonlattice cations (M b+) in the nucleation of calcium sulfate hemihydrate (CaSO4·0.5H2O, HH) in glycerol aqueous solution. By forming [MSO4](2–b)– ion pairs, univalent M+ ions (Li+, Na+, NH4 +, and K+) and bivalent M2+ ions (Cu2+, Zn2+, Mn2+, and Mg2+) significantly increased the concentration of lattice ions (Ca2+ and SO4 2–), resulting in a higher supersaturation of HH and shortening the induction time of HH nucleation. Meanwhile, [MSO4](2–b)– ion pairs replaced free SO4 2– as dominant reactants for HH nucleation. They carried SO4 2– and decoupled them at the nucleation sites for nucleus formation. The efficiency of M b+ ions on promoting HH nucleation is governed by its pairing and decoupling capability with SO4 2–. The difference between M b+ ion and SO4 2– in limiting diffusion coefficient (ΔD M b+ 0) could characterize these two capabilities and was a robust indicator of the efficiency of M b+ ion. This work provides a new understanding of cation-assisted crystallization and helps identify regulators for nucleation control.

Through vapor pressure measurements, water activities of the glycerolÀwater solution were first examined under temperatures of 30À85 °C and glycerol concentrations of 0À50 mol %. The water activity is the only determinant of phasetransition point among CaSO 4 3 2H 2 O (DH), α-CaSO 4 3 0.5H 2 O (α-HH), and CaSO 4 (AH) in glycerolÀwater solution. Accordingly, a phase-transition diagram was constructed as a function of temperature (0À140 °C) and glycerol concentration (0À50 mol %), which provided a thermodynamic preparation window of α-HH from DH. Controlling glycerol concentration and temperature within the window, direct transition of DH to α-HH can be regulated. However, the window is apt to shrink as the operating time is shortened, indicating that its available profile is directed by transition kinetics of DH to α-HH besides transition thermodynamics. Overall, the glycerolÀwater solution could act as a novel category of nonelectrolyte medium for α-HH preparation from DH.

No abstract

Flue gas desulfurization (FGD) gypsum mainly composed of calcium sulfate dihydrate (DH) was used as a raw material to obtain α‐calcium sulfate hemihydrate (α‐HH) through dehydration in a Ca–Mg–K–Cl‐solution medium at 95°C under atmospheric pressure. The effects of potassium sodium tartrate and sodium citrate on the preparation of α‐HH in the electrolyte solution were investigated. The results revealed that the addition of potassium sodium tartrate (1.0 × 10−2–2.5 × 10−2M) decreased the dehydration rate of FGD gypsum and increased the length/width (l/w) ratio of α‐HH crystals, which could yield unfavorable strength properties. Addition of sodium citrate (1.0 × 10−5– 2.0 × 10−5M) slightly increased the dehydration rate of FGD gypsum and decreased the l/w ratio of α‐HH crystals, which could be beneficial to increase strength. However, it also led to a partial formation of anhydrite (AH) crystals. AH was also the only dehydration product when the concentration of sodium citrate increased to 1.0 × 10−4M. Therefore, sodium citrate rather than potassium sodium tartrate could be used as an additive in Ca–Mg–K–Cl electrolyte solutions if α‐HH with a shorter l/w ratio is the desired product from FGD gypsum dehydration. The concentration of sodium citrate should be properly controlled to reduce the formation of AH.