Study On The Crystallization Process Of Calcium Sulfate Dihydrate In The Wet Process Phosphoric Acid System

This study investigated the phosphorgypsum and filtrate composition during wet phosphoric acid production using Calcium Sulphate dihydrate(CaSO4·2H2O) as the object of research. And the relevant property of phosphogypsum was also characterized. Five simulated systems were designed to investigate the crystallization behavior and crystal form transformation of Calcium Sulphate dihydrate. Based on taking into account of the morphology and size of grain, CaHPO4·2H2O- H2SO4-H3PO4 aqueous system was optimized as simulated system to discuss the crystallization behavior. CaSO4·2H2O was prepared under the optimized system with the aggregation of uniform spheral CaSO4·2H2O particles, whose diameter were about 200μm.Al3+, Fe3+, Na+, K+, Mg2+and F-were chosen to study on the effect of filtration and crystallization for Calcium Sulphate. The morphology of the samples was observed using a scanning electron microscopy instrument, the crystal forms of the samples were determined using an X-ray diffractometer and the size distribution of sample were measured by using Laser particle size analyzer. Al3+, Fe3+, Na+, K+, Mg2+and F- were added into CaHPO4·2H2O-H2SO4-H3PO4 aqueous system caused filtration rate reduced, the crystal mean particle size increased when Al3+ and Fe3+ existed in solution, but Na+, K+ and Mg2+ did not have any influence nearly to crystal mean particle size. Compared to Al3+ and Fe3+, F- had adverse action. The content of water-soluble P2O5 was influenced to a considerable degree by the aggregation of crystal particles. Actually washing speed was increased by adding a moderate amount of Fe3+, Na+ and K+ becaused the aggregation of crystal particles formed.The growth of CaSO4·2H2O conformed with the model of non-smooth abrupt interface. Impurity ions existed changed reticular density, making the growth rate different on (020) and (040) facets.Major technological parameters including the reaction temperature, the content of SO42- and P2O5 in aqueous, agitation speed and reaction time were optimized in CaHPO4·2H2O-H2SO4-H3PO4 aqueous system. The relevant parameters optimized were:reaction temperature was 80 ℃, the content of SO42- was50 g·L-1 and P2O5 was 260 g·L-1, agitation rate was 50r/min and reaction was 2h. CaSO4·2H2O prepared under the optimized system with mean diameter of 31μm. The morphologies were rods and flakes. The average particle size of CaSO4·2H2O reduced gradually in the previous 40min. After 60min, the average particle size of CaSO4·2H2O basically unchanged.