Research On Preparation And Mechanism In Crystal Modifier Technology Of α-Hemihydrate Desulfurization Gypsum With The Hydrothermal Method In Salt Solution With Atmospheric Pressure

Flue gas desulfurization (FGD) gypsum is the main product from wet limestone flue gas desulfurization technology. As the energy saving and emission reduction proceeding, more and more heat-engine plants install desulfurizer, which make the output of FGD gypsum becomes larger. In order to avoid a larger amount of FGD gypsum using too much cultivated land and causing secondary pollution, applying FGD gypsum efficiently is extremely urgent. FGD gypsum which purity is high and the quality is stable is a wet powder because it forms in liquid. All the characteristics make FGD gypsum be appropriate for preparingα-hemihydrate gypsum with hydrothermal method in salt solution with atmospheric pressure.α-hemihydrate gypsum is prepared in hydrothermal method in salt solution with atmospheric pressure, and its crystal morphology is controlled by crystal modifier. Results of the experiment show that the concentration of NaCl solution and temperature influence both the process of dehydration and the crystal morphology ofα-hemihydrate gypsum. When the NaCl solution and temperature are too low, the dehydration of FGD gypsum is incapable and noα-hemihydrate gypsum is created.Α-hemihydrate gypsum could be created in 15% NaCl solution and 92.5℃at least, in which condition the dehydration rate of FGD gypsum is very low, and the crystal ofα-hemihydrate gypsum formed is big. As the increasing of the concentration of NaCl solution and temperature, the dehydration rate of FGD gypsum is faster and the crystal ofα-hemihydrate gypsum formed becomes smaller.It is found that the structure influences the effect of the crystal modifier a lot. Monobasic acids are helpless. Binary acids and polyprotic acids whose carboxyls bordered upon are at a distance of 3 C-C are very effective, but others are not. The effect of the binary acids modifier is produced by the synergistic effect of the two carboxyls. The maleinoid compound is more effective than trans heterogeneous compound when a double bond exists because the carboxyls of maleinoid compound could be a synergistic effect easily.The mechanism of crystal modifier is not solved clearly all the time. But it is proved by SEM with EDS and XPS in the thesis. The modifier adsorbs on the crystal face (111) of the crystal ofα-hemihydrate gypsum through chemical reaction. Then it inhibits the growth of C axis and alters the relative growth rate of crystal faces in different axis directions, consequently leading to the transformation of crystalline habit and crystal morphology ofα-hemihydrate gypsum. In the meanwhile, mineral salt crystal modifier could not changes the crystal morphology ofα-hemihydrate gypsum. Metal ions would not adsorb on the crystal face either (111) or (110) of the crystal ofα-hemihydrate gypsum. If mineral salt crystal modifier and organic acid crystal modifier exist at the same time, the effective one is organic acid as before.