Preparation Of Calcium Sulfate Whisker From Desulfurized Gypsum And Photocatalytic Degradation Of Methylene Blue By Zn（x）/CSW

Desulphurized gypsum is the second largest solid waste in thermal power plants.Because of its low utilization potential and low added-value,desulphurized gypsum demands large area for storage,occupies land resources,harms soil and water environment,and endangers human respiratory system.For the preparation of calcium sulfate whisker,replacing natural gypsum with desulfurization gypsum could reduce the exploitation of natural gypsum,protect the ecological environment,and realize the integrated utilization of solid waste resources.In this thesis,calcium sulfate whiskers were prepared from desulfurization gypsum of thermal power plant solid waste,using the recrystallization method.The physicochemical properties of prepared calcium sulfate whiskers were analyzed by XRD,TG,and the microscope imager.The effects of preparation conditions（including raw material pretreatment,sulfuric acid concentration,aging time,desulfurization gypsum dosage,acidification reaction time,calcination temperature and time,etc.）and sulfuric acid cycle times on calcium sulfate whisker properties were studied.The Zn（x）/CSW catalyst was prepared by impregnation method using calcium sulfate whisker as the support and Zn as the main active component.The physicochemical properties of the catalyst were characterized by XRD,TG-DSC,FT-IR,UV-Vis DRS,SEM,H2-TPR and PL.The methylene blue degradation performance of Zn（x）/CSW catalyst was evaluated under visible light irradiation.The effects of Zn loading,promoter type and addition amount on the performance of the catalyst were investigated,and the photocatalytic degradation kinetics of methylene blue was studied.The following conclusions were obtained:1.Preparation of calcium sulfate whisker using desulfurization gypsum as raw material（1）After pretreatment of desulfurization gypsum,the content of CaSO4·2H2O in raw materials increased and the content of impurities was decreased.The purity of calcium sulfate whisker,whiteness and length-diameter ratio were improved.（2）The preparation conditions such as sulfuric acid concentration,aging time,and the amount of desulfurization gypsum all affected the quality of calcium sulfate whiskers.When the sulfuric acid concentration,the aging time,the amount of desulfurization gypsum,and the acidification reaction time was 2 mol/L,3 h,10 g/L,and 1 h,respectively,the whisker size was relatively uniform and the length-diameter ratio was large（about 80-130）.（3）Calcination temperature and time affected the water content of calcium sulfate whisker.When the calcination temperature was 90℃,the calcium sulfate whisker was mainly calcium sulfate dihydrate whisker,with a small amount of calcium sulfate hemihydrate whisker.When the calcination temperature was 100℃ and the calcination time was 1 h,the calcium sulfate dihydrate whisker was transformed to calcium sulfate hemihydrate whisker,and the morphology of calcium sulfate hemihydrate whisker was maintained at 300℃.When the calcination temperature was 400℃,the calcium sulfate whisker was anhydrous calcium sulfate whisker.（4）When sulfuric acid was re-used for 3 times,used sulfuric acid had no significant effect on the morphology of whiskers.2.The photocatalytic degradation of methylene blue over Zn（x）/CSW catalyst（1）The types of active components,Zn loading and firing temperature had significant impacts on the catalytic degradation of methylene blue.Zn（7）/CSW catalyst had abundant active sites,low band gap,high redox performance,low recombination rate of electron-hole pairs,When the calcination temperature is 500℃,the catalyst had good catalytic activity.Under visible light irradiation for 6 h,the degradation ratio of methylene blue was 90.92%.（2）After Ag and Sr promoters were added to Zn（7）/CSW catalyst,the band gap of the catalyst was changed,and the photocatalytic degradation activity was decreased.（3）The effects of Zn loading,initial methylene blue concentration and catalyst dosage on the reaction kinetics were studied.It was found that the reaction time was less than 3 h,and the Zn（x）/CSW catalyst followed the L-H kinetic model with high correlation coefficients.The rate expression of Zn（x）/CSW photocatalyst for methylene blue degradation was as follows:ln（C₀/C_t）=0.4485N^0.5193C₀^-0.3074P^0.4589.