Study On The Decoloration Of FGD Gypsum And Preparation Of α-Hemihydrate Gypsum Whiskers

Nowadays, coal is the most common fuel for heat-engine plant. The application of wet flue gas desulfurization（FGD） project in coal-fired power plants is increasing to control the exhaust gas because of its high sulfur dioxide removal efficiency, reliability and cost-effectiveness. FGD gypsum is the by-product from the process of flue gas desulfurization and its output will increase with the high utilization of flue gas desulfurization devices in China. However, due to the contamination of poor quality coal and low grade limestone, FGD gypsum usually become yellow or gray. If this shortcoming can be overcome, the resource of FGD gypsum will be good expoited, and the exploitation of natural gypsum will be reduced, which is of great strategic significance.Directed at the means of color removal from FGD gypsum, a new technology to produce the dihydrate calcium sulphate with the high quality in whiteness by FGD gypsum was proposed in this article, and the experiment was conducted in the mixed solution of HCl and CaCl₂ at normal pressure. In the experiment, the FGD gypsum crystal is dewatered in the mixed solution to be convered to calcium sulfate hemihydrate, which is simple needlelike, and the inter matter of the internal structure of the crystal will be discharged. Then the calcium sulfate hemihydrate can be transformed into a crust of larger crystals. After the whole progress, the impurity will be removed, and the mother liquid can be recycled. The influence of the concentrations of HCl, CaCl₂, and the material on the FGD gypsum transition temperature to calcium sulfate hemihydrate, and the influence of tempreature, concentrations of material, reaction time on the whiteness of products in the progress was sduied in this paper. Based on it, the proper reaction condition was determined, and the whiteness of products can reach more than 90 percent. Besides, the affection of Ca2+, SO42-, Fe3+ concentration and the number of mother liquor circulatings on the quality of the product was also researched. It turned out that mother liquor circulating had no adverse effect on the process of decolorization. By using of the technology to produce α-calcium sulfate hemihydrate from the calcium sulphate dihydrate with high whiteness obtained, the influence of the concentrations of HCl, CaCl₂, and the material on the whisker aspect ratio was investigated. Under favorable conditions, the whiteness of whisker can reach 95 percent,and its whisker aspect ratio is 119.9. Ultimately, the influence of mother liquor circulating on the whisker physiognomy was discussed.Calcium sulfate hemihydrate with high whiteness was obtained in the process of decolorization using HCl and CaCl₂ as reactants under a pressurized atmosphere. Based on it, the technology to produce calcium sulfate hemihydrate during the decolorization of FGD gypsum was realised. the influence of tempreature; the concentrations of HCl, CaCl₂, and the material on the whiteness of products and the whisker shape was studied. It turned out that the whiteness of calcium sulfate hemihydrate can reach 88 percent under favor conditions.