Insights into the Role of Na⁺ on the Transformation of Gypsum into α-Hemihydrate Whiskers in Alcohol–Water Systems

Abstract

Alcohol–water solution-mediated transformation of gypsum into α-hemihydrate (α-HH) whiskers provides a green alternative for the high-value-added recycling of flue gas desulfurization (FGD) gypsum. However, the role of non-lattice cations during the transformation is still unclear. We report an evolution from “boosting–retarding” to “boosting–retarding–boosting” and finally to “boosting only” effect of non-lattice Na + functioned by the concentration of ethylene glycol (EG) in water solutions. The driving force increased almost linearly upon the introduction of Na + through the formation of ion pairs, and a higher slope was obtained at a higher EG concentration. Adsorption of Na + ions and solidification of eugsterite on gypsum surfaces blocked the nucleation sites of α-HH. The retarding effect first rapidly increased and gradually approached a limit, following a parabolic trend after Na + ions were introduced. Pentasalt, with a structure similar to that of α-HH, precipitated on the gypsum surface at higher c (Na + ). The interaction of the driving force and the structural evolution of calcium sulfate ionic clusters accounts for the evolution of transformation kinetics. The retardation zone was compressed with the increase in EG volume ratios, and a monotonic boosting effect upon Na + was observed at a 35.0 vol % of EG. Nucleation kinetics dominates the aspect ratio of α-HH whiskers. This study may provide a significant guidance for the utilization of FGD gypsum.

Efficient extraction of impurities from phosphogypsum during crystal regulation of α‐hemihydrate gypsum

“…l and ll crystal seeds, the average lengths of the end products increased sharply to 141.3 and 111.5 μm, respectively, and the average widths were only 28.5 and 20.3 μm, respectively. These results probably resulted from the preferential growth along the c ‐axis of α‐HH crystals 28 . The significant increase in the length and negligible increase in the width led to a greater aspect ratio of the product (5.4 and 3.2, respectively, induced by Nos.…”

“…These results probably resulted from the preferential growth along the c-axis of α-HH crystals. 28 The significant increase in the length and negligible increase in the width led to a greater aspect ratio of the product (5.4 and 3.2, respectively, induced by Nos. l and ll crystal seeds) than that of the added crystal seeds, and the average aspect ratios of the products induced by Nos.…”