Study On Cooling Crystallization Process Of Industrial Urea Phosphate

As an important fine phosphate,urea phosphate(UP)is widely used in agriculture,industry,feed,flame retardant materials and other industries,and has an extensive prospect.Cooling crystallization is the most important separation and purification process in the industrial production of urea phosphate,which plays a decisive role in the product quality and market price of urea phosphate.Cooling crystallization of urea phosphate is to obtain urea phosphate crystals with uniform and larger particle size.This will contribute to the industrial production of urea phosphate products with high quality.Based on the relatively few domestic studies on the physicochemical properties and the crystallization process of urea phosphate,the research of this paper includes:(1)High purity industrial urea phosphate crystal is prepared from phosphoric acid and urea.According to the GB/T 27805-2011 protocol,it meets the requirements of industrial urea phosphate with a yield of 80%.By using high-resolution microscope,the crystalline shape is typical double hexagonal pyramid and its surface is transparent and smooth.The characteristic peaks of urea phosphate product correspond to those of standard cards by XRD.The melting point of urea phosphate crystal is 116.8 ? as measured by a microcomputer melting point instrument,which is in good agreement with the literature values.(2)The specific heat capacity data of urea phosphate are measured by using micro reaction calorimetry in the temperature range of 20?80 ?.The data show a linear relationship and the basic thermodynamic parameters of urea phosphate in this temperature range are calculated.The thermal stability of urea phosphate is analyzed by thermogravimetric analyzer.The activation energies of urea phosphate pyrolysis at different heating rates are obtained by integral method and differential method,respectively.(3)A novel apparatus for determination of solubility by dynamic laser scattering method has been developed.Using the measurement system,the solubility of UP in pure water with temperature range from 15 to 75 ? is measured.The results show that thesolubility of urea phosphate in pure water is large and changes exponentially with temperature,which confirms the feasibility of obtaining high yield urea phosphate by cooling crystallization.At the same time,the effect of sulfuric acid impurities on the solubility of urea phosphate in industrial production is measured.It is found that the solubility of urea phosphate decreases with the presence of phosphate group.The higher the temperature,the greater the difference.The heat of dissolution(heat of crystallization)of urea phosphate in aqueous solution is measured by micro-reaction calorimetry.It is found that the heat of dissolution decreases with the increase of temperature and has a good linear relationship.(4)The metastable regions and primary nucleation kinetics of urea phosphate in pure water and 3% sulfuric acid solution are determined by cooling experiment.With the increase of saturation temperature,the metastable zones width of urea phosphate in both solvents are getting widened and then narrowed.With the increase of cooling rate,the metastable zones of urea phosphate and urea phosphate are both getting widened.Meanwhile,the metastable zone of urea phosphate in 3% sulfuric acid solution is smaller than that in pure water.(5)The factors of the influence on the crystallization process of urea phosphate are researched by cooling crystallization experiment.With the increase of cooling rate,the average particle size of urea phosphate decreases,the crystalline shape becomes worse and the surface smoothness becomes worse.With the increase of stirring rate,the average particle size of urea phosphate increases,the crystalline shape becomes better and the surface smoothness becomes better.With the increase of the distance between the crystal seeds adding temperature and the saturated equilibrium temperature,the average particle size of urea phosphate decreases,the crystalline shape becomes worse and the smoothness of the surface becomes worse.With cooling rate of 0.3 ?/min,stirring rate of 300 r/min and crystal seeds adding temperature on saturated equilibrium temperature,the average particle size of urea phosphate can be larger than 800 ?m,whose surface is smooth and crystalline shape is double hexagonal pyramid.(6)The crystallization kinetics of urea phosphate is studied by cooling batchcrystallization experiment.The changes of liquid phase concentration,supersaturation,slurry density and particle size distribution of urea phosphate in the slurry during cooling process are measured and calculated.The partial differential particle number balance equation is solved by Laplace transformation.Then the nucleation rate and growth rate of urea phosphate crystallization in water are estimated,which provides an important basis for the design of crystallization process.