Experimental Research On The Effects Of F^-and Al³⁺ Ions Impuritis And Organic Additives On Crystallization Of Desulfurization Gypsum

In order to reduce and control the emissions of sulfur dioxide, Limestone-gypsum wetflue gas desulfurization process(FGD) has been applied most widely for coal-fired powerplants in China, with a huge amount of desulfurization gypsum being produced. Due to thefact that crystallization of gypsum is one of the most important process in the desulfuriza-tion, which may influence the mass balance and system stability, as well as thecommercial values, understanding the crystallization process and optimization of thedesulfurization parameters are essential to guarantee gypsum quality and operationstability. the influence factors of calcium sulfate dihydrate crystallization, especially theeffects of fluorine ions, aluminum ions and organic additives were investigated, focusingon the induction period、nucleation rate、crystal surface energy and crystal morphology.The thermodynamic parameters of the crystallization of calcium sulfate dihydrate suchas ion activity product(IAP)、solubility product(ksp) and supersaturation index(SI) underdifferent crystallization conditions were obtained with a geochemical aqueous modelprogram, PHREEQC2.12. With reactive crystallization of calcium chloride and sodiumsulfate, we found that temperature、[Ca2+]/[SO42-] molar ratio and concentration ofreactants were the key factors influencing the dihydrate calcium sulphate crystallization,and the influence of pH was not obvious. With the increase of temperature, the inductiontime was shortened, higher temperature(i.g.55℃) was more advantageous to the growth ofthe crystal, with more uniform and larger crystals. The change of [Ca2+]/[SO42-] molarratio and concentration of reactants causes variation of the ion activity product and thedegree of supersaturation of the reaction solution, thus affecting the crystallizationinduction time and crystal morphology. Moderate degree of supersaturation(S=2.19) wasadvantageous to the formation of uniform and larger crystals.On the basis of the classical nucleation theory, we also investigated the influence offluorine ions、aluminum ions and organic additives(formic acid、succinic acid, glutaricacid、adipic acid、mixed acid and citric acid) on the induction period、nucleation rate、crystal surface energy and crystal morphology. The results indicated that the existence offluorine and aluminum ions in the reaction solution, not only can shorten the inductiontime of crystallization, lower crystallization nucleation rate, and improve crystal surfaceenergy, but also restrain the growth of the specific directions of gypsum crystals, thuschange the morphology of crystallization product, which can be explained by the principleof induced crystallization, and fluorine and aluminum ions in the solution change the nucleation mechanism of dihydrate calcium sulphate from homogeneous nucleation toheterogeneous nucleation. Except formic acid,the other five organic acids can retard theinduction time of the crystallization of calcium sulfate dihydrate in some degree. the rateretarding effect of succinic acid and citri acid is the strongest; the organic additives in thesolution will also increase crystallization nucleation rate and lower crystal surface energy.In addition, formic acid、adipic acid and citric acid can cause the change of crystalmorphology, when the concentration of formic acid or adipic acid in the reaction solutionreaches0.025mol/L, the crystals developed thicker, with the length to diameter ratio of thecrystals significantly dicreased and the particle size distribution was more uniform, whichbenefit the dewatering performance. The presence of citric acid in the reaction solution(0.01mol/L) strongly inhibit the growth rate of the special faces of calcium sulfatedihydrate crystal,which leads to the platelike crystals compared to the blank which favorsthe formation of needle crystals.