High-efficient hydrothermal synthesis of α-hemihydrate gypsum: A comparison of innovative microwave heating versus traditional electric heating

Heating method plays a crucial role in determining the conversion efficiency of phosphogypsum (PG) into α-hemihydrate gypsum (α-HH), traditional electric heating is bedeviled by a long preparation time and elevated reaction temperature. To address the issue, microwave heating is employed for high-efficient hydrothermal synthesis of α-HH in salt solution, the process parameters, such as reaction temperature, solid-liquid ratio, salt concentration and modifier concentration were systematically investigated and compared with the electric heating. Additionally, the enhanced mechanism of microwave heating was clarified by the measurements of solution conductivity and microwave absorption performance. The results demonstrate that microwave heating has a significant advantage in reducing the lower limit of reaction conditions and shortening the reaction time. Specifically, using electric heating, the initial reaction temperature, MgCl₂ concentration and solid-liquid ratio are 90 ℃, 25 % and 1:4, respectively. However, the initial reaction temperature and MgCl₂ concentration are decreased to 87 ℃ and 22.5 %, respectively, meanwhile the solid-liquid ratio is improved to 1:3 by microwave heating. In the presence of potassium hydrogen phthalate, short columnar α-HH crystals with an aspect ratio of 1:1 can be obtained within 120 min by microwave heating, whereas the reaction time is significantly delayed to 240 min by electric heating. This can be attributed to that MgCl₂ solution with a high conductivity and PG with a low reflection loss have a strong absorption ability for microwaves. Consequently, microwave heating is proven to be highly efficient for hydrothermal synthesis of α-HH.