Fundamentals of the Wet-Process Phosphoric Acid Production. 2. Kinetics and Mechanism of CaSO₄·0.5H₂O Surface Crystallization and Coating Formation

This paper is devoted to fundamentals of CaSO4·0.5H2O surface crystallization on a being dissolved surface of natural fluorapatite (FAP) crystals under conditions simulating the industrial process of wet-process phosphoric acid production. Methods of optical and scanning electron microscopy were used for the investigations. Single crystals of CaSO4·0.5H2O on the FAP crystal surface were found to grow by an aggregation mechanism with rates decreasing until zero. The mechanism included two stages: ultramicrocrystals formation, followed by their aggregation to microcrystals. The presence of dissolved FAP crystals was found not to change the crystallization mechanism of CaSO4·0.5H2O, but it changed the kinetics of their growth. Later the crystallization process resulted in the continuous formation of a multiple-layer coating of CaSO4·0.5H2O on the FAP crystal surface. Some average characteristics of the above coating (thickness and fragility) were analyzed for different hydrodynamics. An aggregation process for FAP crystals under poor agitation intensity and a possibility for the above coating to be partly removed with mutual collisions of FAP crystals under strong agitation conditions were established for the first time. Based on the experimental results, some technological conclusions about hydrodynamics inside industrial reactors of wet-process phosphoric production were made.