Preparation Of ?-high-strength Gypsum Coupled With Dihydrate Wet Phosphoric Acid Process

Phosphogypsum is a solid waste discharged during the process of dihydrate wet phosphoric acid.The main component is calcium sulfate dihydrate.At present,China's phosphogypsum has accumulated hundreds of millions of tons of accumulated gypsum,but the comprehensive utilization rate is very low.Most companies deal with phosphogypsum,which not only occupies land but also pollutes the surrounding environment.Recycling of phosphogypsum has always been the focus of researchers at home and abroad,but most of them use calcination,introducing crystallizer and other methods to prepare building gypsum or high-strength gypsum.In this paper,the theory of atmospheric pressure phosphoric acid solution gypsum phase transformation is used to directly prepare the a-type high-strength gypsum by controlling the phosphoric acid concentration,conversion temperature and conversion time,etc.,which are filtered and unwashed phosphogypsum in the dihydrate wet process phosphoric acid process.And a small amount of sulfuric acid is added to decompose the remaining phosphate rock in the phosphogypsum to recover the phosphorus resources.By simulating the crystal growth of a-type calcium sulfate hemihydrate in phosphoric acid solution,the effects of Na+ and K+ on the crystallization process of ?-type calcium sulfate hemihydrate were further explored,which provided a theoretical basis for the preparation process of a-high-strength gypsum.Firstly,by examining the effects of process parameters such as phosphoric acid concentration,sulfuric acid dosage,conversion temperature and conversion time on the product,the results show that with the concentration of phosphoric acid,the amount of sulfuric acid,the amount of admixture YD,the conversion temperature,the conversion time,and the liquid-solid increase,the standard water demand,initial setting time and final setting time of the product all decreased first and then increased.The flexural strength and compressive strength both increased first and then decreased.When the phosphoric acid concentration is 35%,the admixture YD is 40%,the conversion temperature is 90?,the conversion time is 150 min,the sulfuric acid content is 2%,the liquid-solid ratio is 2.5:1,the stirring rate is 150 rad·min-1,the standard water requirement of the product is 0.39,the initial setting time is 6 min,the final setting time is 13 min,the flexural strength is 4.21 MPa,the compressive strength is 27.80 MPa,and the physical properties are all satisfied with JC/T2038-2010 "?-High-strength gypsum plaster" grade a25 strength standard.When the amount of sulfuric acid is 0-8%,the phosphorus content in the product gradually decreases and then stabilizes as the amount of sulfuric acid increases.The morphology of the product was analyzed by SEM.It was found that the effect of admixture YD on the crystal appearance of the product was obvious.When the admixture YD was between 0%and 40%,the content of the admixture YD increased.The crystal morphology decreased from dispersion to aggregation,and the aspect ratio decreased from 11.39 to 2.46.Secondly,the response surface method is used to optimize the four parameters of phosphoric acid concentration,admixture YD dosage,conversion temperature and conversion time,and the regression fitting is performed by Design-expert8.0 software.Through the significant test analysis of the model,it was found that the interaction between the admixture YD content and the conversion temperature was significant in the interaction of the four factors,and the order of influence on the intensity among the four factors was obtained as admixture YD dosage>conversion temperature>phosphoric acid concentration>conversion time.Finally,the optimization results of four parameters were as follows:a-high stregth gypsum with compressive strength of 28.10 MPa was obtained under the conditions of phosphoric acid concentration of 34%,admixture YD content of 38%,conversion temperature of 92 0C and conversion time of 145 min.Finally,the growth process of a-type calcium sulfate hemihydrate in phosphoric acid solution was simulated.The effect of Na+ and K+ on the growth process of a-type calcium sulfate hemihydrate was investigated by introducing a certain amount of impurity ions(Na,K+)'The results showed that when Na+,K+ When the concentration is 0.01-0.10 mol"L-1,with the increase of Nv+ and K+ concentrations,the induction time of a-type calcium sulfate hemihydrate increases,the surface energy and critical nucleation radius increase,and the nucleation rate and growth efficiency decrease.The morphology of the product was analyzed by SEM.It was found that the combined Na',K+ and 5042-combined to form sulfate adhered to the surface of the crystal and affected the crystal morphology.The Nano measurer software was used to analyze the particle size of the product.It was found that when the Na+concentration was increased from 0 to 0.10 mol"L-1,the crystal length-to-diameter ratio of a-type calcium sulphate hemihydrate was reduced from 41.78 to 30.48.Similarly,when K+concentration When increasing from 0 to 0.10 mol·L-1,the crystal length to diameter ratio of a-type calcium sulfate hemihydrate formed decreased from 41.78 to 23.27.XRD analysis of the product showed that the sample component was mainly a-type calcium sulfate hemihydrate.When Na+ and K+ were introduced,the a-type calcium sulfate hemihydrate diffracted crystal plane(200),(310),(400)diffraction peak intensity The diffraction peak intensity values of a-type calcium sulfate hemihydrate prepared under Na+ ions are lower than those of ?-type calcium sulfate hemihydrate crystal formed under Na+ ions,that is,K+ is ?-type hemihydrate sulfuric acid.Calcium growth inhibition is greater.When Na+ and K+ are introduced,SO42-in the solution is consumed,which reduces the supersaturation in the solution,thereby affecting the nucleation of the a-type calcium sulfate hemihydrate crystal,prolonging the crystallization induction time,increasing the surface energy and the nucleation radius.And K+ consumes more than Na+,then the effect of K+ is more obvious.According to the minimum principle of Gibbs free energy,the equilibrium constants of Ca2+,K+,Na+ and SO42-are sequentially reduced,that is,Ca2+ and SO42-are the most easily combined,followed by K+ and finally Na'.Therefore,when Na+ and K+ ions are present in the reaction medium,Na+ and K+ will combine with SO42-in solution,reduce the probability of collision between Ca2+ and SO42-,and increase the nucleation radius and surface of ?-type calcium sulfate hemihydrate crystal.It can reduce the crystal nucleation rate and growth efficiency.Since the K+ ion has a smaller Gibbs free energy than the Na+ ion reacts with the SO42-ion.Therefore,K+ retards the nucleation and growth of a-type calcium sulfate hemihydrate crystals.When Na+ and K+ are not introduced,Ca2+ and SO42-grow into a four-prism long needle-shaped ?-type calcium sulfate hemihydrate at the active site of the seed crystal.After the introduction of Na+ and K+,Na+ and K+ are adsorbed on {002},{011},{0-11} and {1-10} crystals during the growth of a-type calcium sulphate hemihydrate due to the attraction of the hetero-ions.On the surface,Ca2+ is prevented from accumulating on the {002},{O11},{0-11} and {1-10}crystal planes,so that the growth primitives grow in the radial direction,and the resulting a-type calcium sulfate hemihydrate is formed.And the resulting a-type calcium sulfate hemihydrate has disappeared into a hexagonal column shape and the aspect ratio is lowered.