Purification of Synthetic Gypsum: Techniques and Mechanisms.

Synthetic gypsum (SG) is produced in massive quantities, yet hazardous impurities limit its reuse. This review summarized the impurity types in various SGs and the corresponding removal methods. Physical methods, such as washing, screening, magnetic separation, and others, exploit solubility and size/density differences to remove soluble salts and particulates. Chemical methods, including acid leaching, precipitation/solidification, and so on, can dissolve or immobilize phosphates, fluorides, and heavy metals. Flotation utilizes the differences in the physicochemical properties of solid surfaces to remove insoluble impurities. The thermal treatment is mainly used to decompose organics and improve whiteness. Microbial methods achieve environmentally friendly cleanup through metabolic leaching or microbially induced carbonate precipitation. The phase-transformation method is a recently developed method that can achieve synergistic effects of deep impurity removal and high-value utilization by reconstructing gypsum crystals to release co-crystallized impurities. Most impurity-removal methods target only a single type of impurity. At present, purifying SG requires a combination of multiple methods, which is not recommended from a cost perspective. Subsequent research on removing impurities from SG should focus on simultaneously removing multiple major impurities in a single process, as well as the synergistic effects between impurity removal and the high-value utilization of gypsum.