Study On The Microwave Synthesis Technology And Water Solubility Characteristics Of Ammonium Polyphosphate

Ammonium polyphosphate（APP）is a new type of high-concentration fertilizer,which has the advantages of slow-release,high-efficiency,good chelation,safety and easy compounding,etc.At present,the research and application of APP fertilizer is still in the promotion stage in China.The existing agricultural ammonium polyphosphate enterprises have small production capacity,and most of the synthetic processes used have various problems such as high production costs,high energy consumption,low output value,and unstable quality.However,with the gradual development of green water-saving agriculture in China and the current development trend of zero growth in the application of fertilizers,water-soluble APP as a new type of high-efficiency basic phosphate fertilizer or high-concentration liquid compound fertilizer with raw material,will gain more space for development and add new impetus to the promotion and application of water-soluble fertilizers.The research and development of water-soluble APP synthesis production technology and its related applications research are conducive to the realization of high-value utilization of phosphorus resources,realizing resource conservation and environmental protection,meeting the needs of agricultural development.This subject used agricultural grade urea phosphate and urea as raw materials to synthesize water-soluble APP by microwave method.We discussed the water solubility and hydrolysis laws of APP products with different polymerization degrees under different p H and temperature conditions,analyzed the release rule of phosphorus nutrient as a fertilizer and calculated the hydrolysis kinetics of low polymerization degree APP.This paper obtained the following research results.（1）The optimal reaction conditions for the microwave synthesis of ammonium polyphosphate obtained from the experiment were:the ratio of urea to urea phosphate was 0.6:1,the microwave power was 720 W,and reaction time is 9 minutes.Under this condition the average polymerization degree of APP obtained was 21.71,the solubility in water at 25℃is 6.03 g/100m L H₂O,the mass fraction of N is 13.28%,and the mass fraction of P₂O₅ is 69.26%.Orthogonal experiment analysis showed that the material molar ratio has the most significant impact on the performance of APP products,followed by microwave power and the reaction time.FT-IR and XRD analysis showed that the peak characteristics of ammonium polyphosphate synthesized by microwave method and electric furnace method are consistent,and the peak shape is complete.The product is mainly crystalline type I ammonium polyphosphate with good crystallinity and relatively pure.TG-FTIR combined system data showed that ammonium polyphosphate products are not easy to thermally decompose,have high stability and are convenient for storage.（2）The water solubility characteristics of ammonium polyphosphate by microwave method under different conditions showed that the solubility of crystalline type I ammonium polyphosphate decreases with the increase of the degree of polymerization.When the average degree of polymerization was higher than 10,the solubility decreased rapidly and the water solubility of APP became poor.SEM analysis showed that mechanical pulverization will destroy the surface structure of the sample,The dissolution system of ammonium polyphosphate in water belongs to the nature of positive solubility,and the solubility increases rapidly with the increase of temperature,especially for samples with low degree of polymerization.p H and K⁺concentration had little effect on the solubility of APP.（3）The study of the phosphorus release law of ammonium polyphosphate with different polymerization degrees and the hydrolysis kinetics of ammonium oligophosphate showed that APP with a degree of polymerization higher than 10 had higher hydrolytic stability,and the phosphorus release rate was slower in 90 days.The content of polymerized phosphorus in the samples with lower degree of polymerization decreased significantly.Therefore,APP fertilizer needs to have a wide molecular weight distribution（PDI index）to meet the phosphorus demand of crops in different periods.In addition,the temperature and p H have a great influence on the phosphorus release rate of the APP.In the neutral and alkaline environment below 25°C,ammonium polyphosphate has high hydrolytic stability,slower phosphorus release rate,and higher temperature.The p H value decreases,and the release amount of phosphorus increased.The results of changes of phosphorus content of different forms in solution of APP with polymerization degree of 2.91 showed that the increase in the content of orthophosphate in the solution is mainly due to the hydrolysis of trimer and tetrapolyphosphate.The hydrolysis of low polymerization degree ammonium polyphosphate obeys the first-order reaction mechanism.When the solution temperature is 35℃,the hydrolysis rate constant is 1.45～1.53 times that at 25℃and5.83～11.12 times that at 5℃.Compared with the neutral condition,the hydrolysis half-life of the solution at p H 4.5 was reduced by about 1.4 times,and compared with the alkaline condition at p H 9.0,it was reduced by 2.86 to 5.72 times.The apparent activation energy of the hydrolysis process of ammonium polyphosphate in different p H solutions is 42.594～59.359 k J·mol^-1.