Study On The Preparation Of Large-Grained ?-Calcium Sulfate Hemihydrate And The Whitening Process Of Flue Gas Desulfurization Gypsum

With the improvement of national environmental requirements,the coverage of flue gas desulfurization?FGD?devices increases largely.The amount of FGD gypsum which is the by-product of flue gas desulfurization produced in the wet-desulfurizing process rises perpendicularly,which will not only occupy too much land resources but also cause the secondary pollution.The application range of FGD gypsum is greatly limited due to color problems caused by the quality of limestone and collection efficiency of devices.As the main value-added product of FGD gypsum,high strength gypsum has been widely used in machinery,medicine,architecture,and so on.However,low product performance and low whiteness of products make the practical application of high strength gypsum limited.If the quality of products can be improved,exploitation of natural gypsum can be mitigated and it is significantly meaningful for resource utilization of FGD gypsum.As for the low performance of products obtained by FGD gypsum,large-grained short columnar?-calcium sulfate hemihydrate can be obtained by hydrothermal autoclave method with a low salt content in this paper.Taken the large granularity,high efficiency circulation and low length-diameter ratio as the objective function,this paper uses physical purification firstly to remove enriched impurities of FGD gypsum.And the influences of washing times,slurry concentration,the amount of succinic acid,salt content,reaction temperature and pH on product performance were investigated.And short columnar?-calcium sulfate hemihydrate with the length of 50?m and length-diameter ratio of 1 can be obtained.What's more,the influence of mother liquid circulation on product performance was investigated and this paper shows that the technology is feasible and has high efficiency circulation.Using FGD gypsum after washing as the raw material,obtaining?-hemihydrate gypsum whiskers with good performance is the first evaluation index.The influences of reaction time,MgSO₄,CaCl₂,slurry concentration and reaction temperature on the morphology and composition of products were investigated.And?-hemihydrate gypsum with the optimal length-diameter ratio can be obtained.As for the low whiteness of products obtained by FGD gypsum,the effects of aging time,dosage of sodium dodecyl sulfonate?SDS?,temperature and pH on the property of products were investigated using CaSO₄·0.5H₂O whiskers obtained in MgSO₄-CaCl₂-H₂O solution as raw material,and crystal morphology and whiteness were taken as the objective function.The research shows that:whitening and large prismatic dihydrate gypsum with average length of 350?m and whiteness of 82.57%can be obtained under the optimal conditions.And short columnar?-hemihydrate gypsum with the length of 100?m and the whiteness of 85.68%can be prepared by dihydrate gypsum obtained under optimum conditions mentioned above.To improve the whiteness of product,this paper gives a new technology to prepare short columnar?-hemihydrate gypsum with high whiteness and purity based on HCl-CaCl₂-H₂O system according to the recrystallization mechanism.The transformation processes of short columnar crystals from dihydrate gypsum with different morphologies were investigated.And short columnar?-hemihydrate gypsum with the length of 50?m and the whiteness of 94.96%can be obtained.The research above provides a theoretical basis for the expansion of application range of FGD gypsum.