A simple and efficient approach for the high-purity CaSO4·2H2O (DH) whiskers and α-CaSO4·0.5H2O (α-HH) whiskers derived from such phosphogypsum (PG) was proposed. The impact of different experimental parameters on supersaturated dissolution-recrystallization and preparation processes of α-CaSO4·0.5H2O was elaborated. At 3.5 mol/L HCl concentration, the dissolution temperature and time were 90 °C and 20 min, respectively. After eight cycles and 5-8 times cycles, total crystallization amount of CaSO4·2H2O was 21.75 and 9.97 g/100 mL, respectively, from supersaturated HCl solution. The number of cycles affected the shape and amount of the crystal. Higher HCl concentration facilitated CaSO4·2H2O dissolution and created a much higher supersaturation, which acted as a larger driving force for phase transformation of CaSO4·2H2O to α-CaSO4·0.5H2O. The HCl solution system's optimum experimental conditions for HH whiskers preparation involved acid leaching of CaSO4·2H2O sample, with HCl concentration 6.0 mol/L, reaction temperature 80 °C, and reaction time 30 min-60 min. Under the third cycle conditions, α-CaSO4·0.5H2O whiskers were uniform in size, clear, and distinct in edges and angles. The length range of α-CaSO4·0.5H2O whiskers was from 106 μm to 231 μm and diameter range from 0.43 μm to 1.35 μm, while the longest diameter ratio was 231. Purity of α-CaSO4·0.5H2O whiskers was approximately 100%, where whiteness reached 98.6%. The reuse of the solution enables the process to discharge no waste liquid. It provides a new reference direction for green production technology of phosphogypsum.
Keywords: CaSO4·0.5H2O (HH) whiskers; CaSO4·2H2O (DH) whiskers; cycle number; phosphogypsum (PG); sustainable development.