Direct synthesis of single-phase α-CaSO4·0.5H2O whiskers from waste nitrate solution

Single-phase α-CaSO4·0.5H2O whiskers were directly synthesized from waste Ca(NO3)2 solution using a hydrothermal method, and HNO3 was synchronously regenerated. The effects of reaction temperature and Ca2+ concentration on the phase composition and morphology of products were determined by X-ray diffraction and optical microscopy. On the basis of the experimental results, the formation diagram of α-CaSO4·0.5H2O was plotted within the range of 5–35 g·L−1 Ca2+ and 115 °C–150 °C. In addition, the conditions of the direct synthesis of α-CaSO4·0.5H2O were determined. Well-crystallized, single-phase α-CaSO4·0.5H2O whiskers with high aspect ratios (length, 1785 μm; diameter, 10.63 μm; aspect ratio, 168) and HNO3 (70.25 g·L−1) were obtained at the optimal conditions of 25 g·L−1 Ca2+ and 125 °C.