Sustainable Mining and Metallurgy

Using traditional electric heating for preparation of α-hemihydrate gypsum (α-HH) from phosphogypsum (PG) takes a long time, and the addition of modifier will further prolong the reaction time and reduce the production efficiency. This study proposed the rapid synthesis of α-HH by microwave radiation heating, and the mechanism of microwave radiation enhanced synthesis of α-HH was clarified. The results show that when the concentration of CaCl₂ is 2.70mol/L, the microwave heating can convert PG into α-HH within 60min, while the electric heating requires 180min. Maleic acid can effectively regulate the crystal morphology of α-HH and obtain short columnar α-HH in 2.47mol/L CaCl₂ solution, but adding too much maleic acid will inhibit the conversion of PG into α-HH and prolong the reaction time. Compared with other heating methods, microwave radiation heating has significant advantages in shortening reaction time, and can also reduce the concentration of the reaction medium to a certain extent. The reason is that both CaCl₂ solution and PG have a high dielectric constant, exhibiting a good absorption performance for microwaves. In addition, the higher the CaCl₂ concentration, the greater the dielectric constant, dielectric loss, and conductivity of the solution, and the better the microwave absorption property, which can quickly heat the reaction slurry to promote the conversion of PG into α-HH. Therefore, microwave radiation is an efficient and energy-saving heating method for synthesizing α-HH.