Research On Preparation And Properties Of Building Gypsum From Flue Gas Desulfurization Gypsum By Crystallization And Dehydration

Flue gas desulfurization gypsum（FGD）is mainly an industrial by-product produced in thermal power,petrochemical and steel industries by using desulfurization technology.Domestic FGD gypsum has huge emissions,poor quality stability and low comprehensive utilization rate,resulting in its massive accumulation and environmental pollution.Building gypsum（β-HH）is obtained by calcining FGD,and it is an effective way to quickly consume and reuse FGD with high added-value.The preparation of building gypsum by calcination with crystal modifier can play a dual role in crystallization.One is to optimize the crystallization process and improve the quality of calcined gypsum,and the other is to assist the crystallization of gypsum dihydrate（DH）and regulate the crystal morphology of DH in the process of hydration crystallization.FGD gypsum from power plant was used as raw material,aluminum sulfate（AS）,potassium aluminum sulfate（KAS）and sodium citrate（SCN）were selected as crystal modifiers,and the desulphurized calcined gypsum was prepared by appropriate calcination process.The slurry properties and mechanical properties of desulfurized calcined gypsum were tested,and the best calcining process of calcined gypsum was determined by XRD and SEM characterization methods.The influence rule of AS,KAS,SCN content on its properties was explored,and its influence mechanism was revealed.The specific conclusions are as follows:（1）The best calcination system for FGD gypsum without the addition of crystal modifier is 170℃for 2 h,and 2-h flexural strength and 2-h compressive strength of the prepared calcined gypsum are 3.10 MPa and 6.78 MPa,respectively,meeting the requirements of Grade 3.0 in the standard of"Calcined Gypsum".In the process of calcination of FGD gypsum,a large number of slit holes and microcracks are produced,and the propagation of microcracks leads to the crystal delaminate.（2）When the content of AS is 0.6 wt%and the calcination temperature is 170℃,the flexural strength of the prepared calcined gypsum is the highest,that is,the 2-h flexural strength and 2-h compressive strength are 3.80 MPa and 9.05 MPa,respectively.As KAS content is 0.3wt%,the compressive strength of calcined gypsum is the highest,and the 2-h flexural strength and 2-h compressive strength are 3.40 MPa and 9.23 MPa,respectively.When the contents of AS and KAS increase to 1.0 wt%,the destructive gas CO₂ is generated due to the double decomposition reaction between Al³⁺and Ca Mg（CO₃）₂.This can result in the loose structure of gypsum network after condensation and hardening and the increase of pore spaces,and the mechanical properties decrease significantly.（3）In the process of FGD calcined to building gypsum,the addition of AS and KAS can promote the crystallization ofβ-HH and soluble anhydrous gypsum（AH-Ⅲ）along the a-axis,and both AS and KAS can promote the coagulation of calcined gypsum.AS inhibits the growth of（020）crystal surface of new phase DH after hydration and promotes the uniform growth and aggregation of DH along the c-axis.Different from AS,KAS promotes the growth of（020）crystal surface of new phase DH after hydration,and inhibits the growth habit of DH crystal along the c-axis,which slightly reduces the length-diameter ratio.（4）FGD gypsum added SCN is calcined to prepare calcined gypsum.The strength of calcined gypsum prepared by adding 0.05 wt%SCN to FGD gypsum and calcining at 170℃for 2 h is the highest,that is,2-h flexural strength and 2-h compressive strength are 3.10 MPa and 6.80 MPa,respectively,which has little difference with that without adding SCN.However,the addition of SCN mainly reduces the standard consistency water consumption of calcined gypsum,delays the setting time and improves its workability.SCN inhibits the growth habit of DH along the c-axis during hydration,and promotes the growth along the b-axis,so the length-diameter ratio of new phase DH crystals decreases.