Preparation of calcium sulfate hemihydrate whiskers from chlor-alkali brine sludge and industrial waste under normal pressure

Brine sludge is solid waste generated in the chlor-alkali industry. It contains high concentrations of salts and chemical substances, posing environmental and health risks. In this study, brine sludge and Glauber's salt from the chlor-alkali industry was used as raw materials to synthesize calcium sulfate hemihydrate whiskers (CSHW). This approach enables the effective resource utilization of industrial solid waste. The optimal preparation conditions were determined through single-factor and orthogonal experiments. The synthesized whiskers were an average length of 114.07 μm and an average aspect ratio of 30.30 under the conditions of a reaction temperature of 98 °C, reaction time of 4.0 h, molar ratio of Ca²⁺ to SO₄²⁻ of 1:1, and reaction pH of 1.0. Mg²⁺ and Fe³⁺ in the brine sludge exert distinct influences on CSHW growth. Specifically, Fe³⁺ inhibits the whiskers growth. When the pH was around 2.69 during acid leaching process, Fe³⁺ concentration was effectively controlled at 2.5 × 10⁻⁶ mM, which significantly suppressed Fe³⁺ release from the brine sludge and ensured whiskers with optimal aspect ratios. Low-concentration Mg²⁺ (0.025 M) demonstrates selective adsorption on the whisker surfaces, facilitating the growth of whiskers with high aspect ratios. The preferential adsorption of Mg²⁺ on the (200) crystallographic plane of CSHW was confirmed by X-ray photoelectron spectroscopy (XPS), Zeta potential measurements, and density functional theory (DFT) calculations. This study offers a viable strategy for the resource utilization of waste from the chlor-alkali industry.