Study On Mechanism Of Synthesis And Crystallization Of Urea Phosphate And Its Polymerization

According to Liebig’s theory of plant nutrition,the soil capacity will be weakened after the nutrients in the soil are absorbed by crops,and new nutrients need to be added to ensure the growth demand of crops.Fertilizers are essential for increasing yield of crop,ensuring stable and high yields of grain is the basis and prerequisite for promoting China’s rapid economic and social development.At present,the unreasonable utilization of fertilizer resources,polluting soil,decreasing agricultural comparative benefits.There is an urgent need to change the traditional fertilization mode,and achieve precise fertilizer application.The shortage of fresh water resources in China has promoted the accelerated development of water-saving irrigation,and the water-fertilizer integration technology that saveing water and fertilizer has ushered in development opportunities.As a supporting product of water fertilizer integration technology,which is green and in line with the policy of reducing fertilizer and the concept of sustainable development.But water-soluble fertilizers can’t be generally applied to crops due to the shortage of raw materials and the high price.Urea phosphate can provide nitrogen and phosphorus nutrients for crops,but its nutrient content is low.As a kind of water-soluble fertilizer raw material,ammonium polyphosphate has high content of nitrogen and phosphorus nutrients,high application safety.Domestic production is low due to technical monopoly,and has not yet formed large-scale.Due to the complex reaction crystallization process of wet process phosphoric acid and urea and the polymerization process of urea phosphate and urea,which needs to be further studied.The main research contents and conclusions are as follows:（1）Crystallization thermodynamics of urea phosphateThe solubility of urea phosphate in water increased with the increasing temperature.The solubility curve can be fitted to Apelblat equation.The dissolution process of urea phosphate is endothermic,spontaneous and fast.The metastable zone narrows with the increase of temperature and stirring rate,while the metastable zone increases with the increase of cooling rate.The crystallization induction period of urea phosphate reduces with the increase of stirring rate,temperature and supersaturation.The solid-liquid interfacial tension shows that the crystallization process is primary heterogeneous nucleation.The solubility,metastable zone and induction period increased with the addition of impurities.In order to reduce the process time and not exceed 0.2%in raw materials.（2）Crystallization kinetics of urea phosphateThe crystal growth rate and nucleation rate are calculated by moment transformation method according to the measured suspension density,supersaturation,crystal volume shape factor and crystal size distribution of the crystal solution of urea phosphate.The particle size analysis shows that the growth of urea phosphate crystals is size-independent growth.The nucleation and growth kinetics equations of crystallization are obtained by fitting and analyzing the data with 1st Opt software.Analysis of the kinetic equations shows that the nucleation and growth rate increased with the increase of temperature and supersaturation,and the nucleation rate increased faster due to the supersaturation index in the nucleation kinetic equation is larger.The nucleation of crystal is secondary.The crystal growth process is controlled by surface reaction.The nucleation rate increases with the increase of suspension density.（3）The preparation of urea phosphateThe optimum technological conditions of synthesizing urea phosphate by the wet-process phosphoric acid（w（P₂O₅）25.23%）are as follows:the molar ratio of wet-process phosphoric acid to urea is 1.05:1,reaction temperature is 80°C,reaction time is 60 min,crystallization time is 18 h.The yield of urea phosphate is 63.58%,the content of nitrogen and phosphorus in the product is 17.2%and 44.3%respectively,meets the standard of industrial grade urea phosphate GB/T 27805-2011.FTIR and XRD show that urea phosphate is synthesized successfully by wet process phosphoric acid,and there is no change in composition and structure of urea phosphate.The thermal analysis shows that urea phosphate can exist stably at room temperature and is easy to store.（4）Influence of impurities on the preparation of urea phosphateThe yield of urea phosphate decreased after the addition of Al³⁺、Fe³⁺、Mg²⁺and SO₄^2-.The content of nitrogen and phosphorus meets the national standards GB/T27805-2011 after the addition of impurities.The particle size of urea phosphate decreased and the dispersion between crystals is poor with the addition of Al³⁺,Fe³⁺and F^-,while the crystals are more dispersible,high transparent and regular in shape SO₄^2-reduced the particle size of urea phosphate crystals.Therefore,the content of Al³⁺, urea phosphate products in industrialized production,obtaining crystal with uniform particle size.（5）The preparation of ammonium polyphosphateThe favorable synthesis process of ammonium polyphosphate（APP）by urea phosphate prepared by wet-process phosphoric acid as the raw material are as follows:the molar ratio of urea to urea phosphate is 2.1:1,reaction temperature is 210°C and reaction time is 65 min respectively.The degree of polymerization of APP is 25.31,and the solubility is 3.94 g·（100 g）^-1 in water at 25°C,the APP is a soluble substance.The content of P₂O₅ and nitrogen in APP is 68.13%and 13.72%respectively.The analysis of FTIR and XRD shows that the product is APP form I.The thermal analysis shows that APP is easy to store at room temperature.SEM shows that the APP is porous.（6）The release law of nitrogen and phosphorus in APPReducing p H and increasing temperature are conducive to the release of nitrogen and phosphorus.The Parabolic equation,Elvoich equation and First-order kinetic equation can fit the release process,among them,the fitting result of the first-order kinetic equation is the best.The half-life of the release process of nitrogen and phosphorus in ammonium polyphosphate decreases with the increase of temperature and the decrease of p H value,and the hydrolysis activation energy is in the range of hydrolysis activation energy of polyphosphate.In conclusion,the mechanism research of crystallization process of urea phosphate enriches the crystallization theory of urea phosphate,providing theoretical guidance for the industrial production of urea phosphate.The urea phosphate is synthesized successfully from wet process phosphoric acid,eliminating the purification of wet process phosphoric acid,and optimizing the traditional process.Ammonium polyphosphate form I is synthesized from urea phosphate prepared by wet process phosphoric acid,which is soluble in water.The release process of nitrogen and phosphorus from APP in aqueous solution is studied,providing guidance for the application of ammonium polyphosphate as water-soluble fertilizer and improving the fertilizer utilization rate.This research increases the utilization rate of wet process phosphoric acid,alleviating the shortage of high-quality phosphorus resources,and is of great significance for the popularize and application of water-soluble fertilizer.