Research Of ?-Hemihydrate Calcium Sulfate Prepared From Phosphogypsum By Salt Solution Under Atmospheric Pressure

In this paper,using phosphogypsum to prepared a-hemihydrate calcium sulfate by salt method under atmospheric pressure to realize the resource utilization of phosphogypsum.The removal efficiency of washing and sulfuric acid at different conditions to P₂O₅ and F-of phosphogypsum were compared by leaching method.Using phosphogypsum as raw materials to prepared a-hemihydrate calcium sulfate in Ca-Mg-K composite salt solution,the products under the condition of different Ca²⁺ concentration and pH value were determined by FESEM,XRD,TG-DTA and other analysis methods.The components of liquid phase and solid phase in each reaction system at different reaction time were measured,the corresponding crystal transfer rate were calculated;the phosphogypsum dissolution kinetics model established,using function to fit saturation C*and dissolution rate of phosphogypsum,the kinetic functions were obtained under the different reaction conditions.The main research contents and results are as follows:Firstly,the removing processes of impurities have been investigated.The result shows that water soluble P₂O₅ and F-of phosphogypsum can be effectively removed by single procedure of washing.Under the temperature of 65? and 60min washing time,the soluble P₂O₅ content in phosphogypsum decreased by 66.54%,and F-content decreased by 64.33%;in sulfuric acid leaching purification process,the impurity will be removal effective as the mass fraction of H₂SO₄ and reaction temperature was increased,and the content of P₂O₅ decreased by 75.6%,and F-decreased by 78.4%in the sample when the reaction temperature was 55?,the mass fraction of 35%sulfuric acid after 2h leaching time.Secondly,the effects of different pH and Ca²⁺ concentration on the microstructure and crystal morphology of the products were investigated.The product has large size and low aspect ratio when the Ca²⁺ concentration of initial reaction system augmentation.However,if the concentration of Ca²⁺ in the reaction system more than 4.2M,the conversion rate will seriously delayed.The length diameter ratio significantly influenced by pH of the solution,in neutral-alkaline environment,the products has low aspect ratio,which presents short column or rod crystal form;and the aspect ratio of product increased as the pH value of reaction system decreased,which has the tend to fine acicular crystal growth.Thirdly,the pH value and the concentration of Ca²⁺ have obvious effects on the water content and the rate of crystallization,and the moisture content of the product reflects the proportion of the aqueous phase in the products.As the initial reaction system with high concentrations of Ca²⁺,the dissolution rate of dihydrate phase decreased;when the pH value of initial reaction system was decreased,the dissociate ion of sulfate in solution will be increased,and the crystallization time will be shortened,the reaction rate will be increased at the same time.Finally,the dissolution kinetics model of phosphogypsum was established,the determination of dissolved phase liquid sulfate ion concentration changes with time were fitted on solute saturation C*and instantaneous dissolution rate of Rs,the kinetic data were obtained under corresponding conditions.The results show that the model fits well with experimental data,the dissolution process of phosphogypsum is controlled by surface reaction,as the concentration of Ca²⁺ in the range 2.5M to 3.5M in the reaction system of neutral environment,the solution apparent reaction order has little difference,and the average value is 1.533.