Research On Removal Of SO₄^2-,Mg²⁺,Ca²⁺ In Wastewater From Wet Flue Gas Desulfurization Process Of Coal-fired Power Plants

Wet flue gas desulfurization process in coal-fired power plants will produce a large amount of wastewater which has the characteristics of strong pollution and high salinity.The typical desulphurization wastewater treatment technologies are introduced.The treatment technologies of desulfurization wastewater in the future should integrate soften,concentration and resource utilization.Softening plays an important role in the whole process,and it could prevent the problems of decrease of permeation flux,reduction of heat transfer efficiency due to polarization of membrane,fouling of membrane and scaling of heat transfer equipment.The following study is carried out on the softening of desulphurized wastewater.The heat pipe evaporation experiments was carried out to understand the scaling factors and scale types of desulfurization wastewater.The samples obtained from the experiments are analyzed by XRD and SEM-EDS.The results show that residual gypsum particles,SO42-,Mg2+,Ca2+ in the water is the main cause of scaling,among which SO42-,Ca2+ lead to calcium sulfate hydrate type and Mg2+,Ca2+ lead to alkaline scale.The effects of molar ratio of calcium to sulfur,the molar ratio of aluminum to sulfur and pH on SO42-removal by ettringite precipitation method are studied by response surface methodology.The optimized conditions are obtained,which are[Ca]/[S]=2,[Al]/[S]=0.67,pH=12.14.Mg2+ has a negative impact on the removal of SO42-,which can be explained that Al3+ will combine priority with Mg2+ and forming magnesium aluminum hydroxide instead of ettringite.NaOH and flue gas are used to remove Mg2+ and Ca2+ respectively.The optimum mole ratio of NaOH to Mg2+ was determined in the process of Mg2+ removal.The effects of carbon dioxide concentration,wastewater temperature,wastewater initial pH,wastewater initial calcium concentration,and gas velocity on softening are investigated in detail in the process of Ca2+ removal.A pretreatment process with standard treatment and depth softening based on the Airlift Loop Reactor is proposed.The residual SO42-,Mg2+ and Ca2+ of 324mg/L,5mg/L and 44mg/L were detected in experiments by this process with desulfurization wastewater.