Impurity Regulation during Calcium Sulfate Crystallization and Phase Transformation: Purification, Resource Recovery, and Immobilization

Jie Li; Li Li; Changzhou Weng; Jiemin Qin; Zhulin Xue; Lijuan Jia; Changyong Yu; Xiaoqin Li; Weizhen Liu; Zhang Lin

Abstract

Calcium sulfate is a widely occurring inorganic salt in natural environments and industrial systems. It mainly occurs as dihydrate gypsum, hemihydrate gypsum, and anhydrite. During the nucleation and growth of calcium sulfate, impurity elements can bind to the calcium sulfate matrix through surface adsorption, coprecipitation, physical encapsulation, or lattice incorporation. Industrial processes generate large amounts of impurity-bearing calcium sulfate wastes, in which impurities are associated with the calcium sulfate matrix. The growing annual production and stockpiling of this waste pose challenges for environmental management and resource recovery. On the other hand, phase transformation in the calcium sulfate system provides possibilities for controlling impurity behavior. During dissolution-recrystallization, thermal dehydration and physical enhancement, the reconstruction of the calcium sulfate matrix can promote the release of impurities. Conversely, under specific crystallization conditions, foreign ions can also be immobilized in the calcium sulfate phase. This review summarizes the nucleation and phase transformation behavior of calcium sulfate and discusses the distribution of impurity ions in calcium sulfate. It then discusses different treatment methods based on calcium sulfate phase transformation. For calcium sulfate wastes, phase transformation can be used to promote impurity release and calcium sulfate purification through wet recrystallization, thermal dehydration–rehydration, or physical enhancement. the dissolution-recrystallization process of calcium sulfate is promoted to achieve deep extraction of impurities. Typical examples include gypsum containing heavy metals, as well as industrial by-product gypsum such as phosphogypsum and red gypsum. Then, immobilization applications based on calcium sulfate crystallization are discussed. Finally, the practical limitations and remaining challenges of these approaches are discussed. This review aims to guide the treatment of calcium sulfate waste from a microstructural perspective and promote the sustainable recycling of resources.

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