A novel approach for enhanced hydrothermal synthesis of α-calcium sulfate hemihydrate crystals from phosphogypsum by microwave irradiation

Abstract

Microwave radiation is a clean heating technology, which was utilized for efficient synthesis of high-value α-calcium sulfate hemihydrate (α-CSH) from phosphogypsum waste in salt solution. The conversion efficiency of phosphogypsum into α-CSH was compared with traditional electric heating, and the enhanced mechanism of microwave irradiation was explained. The results indicate that needle-shaped α-CSH crystals were synthesized in CaCl₂ solution without modifiers by microwave radiation. The crystal morphology of α-CSH was controlled by adding citric acid monohydrate (CAM), but the modifier could inhibit the transformation of phosphogypsum. Short columnar α-CSH crystals with an aspect ratio of 1 : 1 were synthesized in 2.47 mol L⁻¹ CaCl₂ and 8.17 × 10⁻³ mol L⁻¹ CAM solution within 210 min by microwave radiation, but phosphogypsum can not be transformed into α-CSH by electric heating under the same conditions. Phosphogypsum and CaCl₂ solution have high dielectric constants, hence the reaction slurry exhibits excellent microwave-absorbing properties, which could enhance the dissolution of phosphogypsum and crystallization into α-CSH. Compared with other heating methods, microwave radiation has an obvious advantage in shortening the reaction time. Consequently, microwave radiation is considered as an efficient synthesis method for α-CSH crystals.