Study On The Slow Release And Anti Caking Properties Of Nitrogen Fertilizer Particles

Concerning the coating material used for the slow release of nitrogen fertilizers,natural vegetable oil has caught scholars’focus.What’s more,a lower coating amount,a thinner coating and a longer slow release period have become the technical research direction of the coated nitrogen fertilizer.The anti-caking technology of traditional granular nitrogen fertilizer is to coat inert powder or grease on the outer surface,while the water-soluble high-nitrogen fertilizer cannot be coated with any inert powder or grease,the modification of the interface components of water-soluble nitrogen fertilizers to improve the anti-caking property of high-nitrogen fertilizers attracted researchers’attention and has become a hot issue.In this study,polyurethane coated urea（PCU）was obtained by taking castor oil,carbodiimide,chain extender,polymeric MDI（PMDI）and wax as the coating materials,and nitrogen granule urea as the core.To be specific,the coating material was coated on the surface of urea granules by applying the drum coating method.The quality of the polyurethane coating accounts for 3.3 wt.%of the PCU mass,and the thickness of the polyurethane coating is about 14-15 um.Polyurethane coatings with different permeability were thereby obtained by adjusting the formulation,and the slow release period of PCU can be adjusted between 30-70 days.It should be pointed out that carbodiimide can react with the carboxyl group in the castor oil,reduce the acid value of the castor oil,and inhibit the hydrolysis of the polyurethane segment by the carboxyl group.In addition,the reaction between the amine group（-NH₂）and the isocyanate group（-NCO）of urea was investigated applying Fourier Transform Infrared Spectroscopy（FTIR）and Scanning Electron Microscopy（SEM）.The participation of-NH₂ in the polyurethane film formation reaction can thus result in the accelerated rate of coating formation.The techniques of coating the castor oil in advance can,therefore,suppress the reaction of-NH₂ with-NCO,and the coating uniformity of the film can accordingly be improved.The structural changes of PCU coating in water,soil,flooded soil and alkali solution were characterized in virtue of FTIR and SEM.According to the FTIR spectrum,no significant change has appeared concerning the content of alkanes as the polyurethane coating was immersed in water,flooded soil and alkali solution,whereas the content has significantly reduced as the polyurethane coating was buried in the soil.The main cause of wax degradation in polyurethane coating was thereby obtained by means of comparing PCU release rate in different media and the alkane content variation of polyurethane coating in different media.Specifically speaking,metabolism of the microorganisms in the soil happened by taking alkanes as the carbon sources,in which process,long-chain hydrocarbons were converted into short chains by microorganisms,resulting in the loss of alkane in the coating.Furthermore,the decomposition of alkanes in the PCU coating is mainly derived from wax,the degradation of which enhanced the coating permeability and increased the nutrient release rate.In the flooded soil,on the contrary,no obvious degradation appeared in the wax of the coating,which is due to the fact that the soil flooded by water,has severely stressed aerobic microorganisms in the soil,destroyed their breeding environment,causing a significant decrease in the number of microorganisms used to decompose wax.In addition,with the expansion of PCU volume,the polyurethane coating is stretched,but since the size of the wax（filler）does not change,the defect size in the coating becomes larger,which leads to an acceleration of the nutrient release rate.In a word,the more defects in coating,the faster the PCU releases nutrients,and the lower the volume expansion rate.Polyurethane coated slow release urea,with the hard segment content of being 26wt.%（PCU-L）and 40 wt.%（PCU-H）,was synthesized by adjusting the ratio of the soft and the hard segment phases in the coating,with PMDI and chain extender as the hard segment and castor oil as the soft one.And on the basis of that,the urea release rate,volume expansion ratio,and the urea quality permeating into the PCU coating during release of PCU-L and PCU-H were tested.The results showed that PCU-L shows a lower volume expansion and a faster urea release rate than PCU-H,what’s more,the quality of urea penetrating into the PCU-L coating is significantly higher than that in the PCU-H coating,indicating that in the formulation of the synthetic polyurethane coatings,an increase in the hard segment content will reduce the size and number of defects in the coatings.PCU-H thus requires greater volume expansion to form channels for urea release,the release period of PCU-H is,therefore,longer compared with that of PCU-L,besides,PCU-H is more stable in the aspect of nutrient release.While in the coatings with high hard segment content,the decrease of alkane components will cause the deterioration of soil degradability in the later stage.Agricultural ammonium nitrate（ANP）is one of the main raw materials for the production of AN-based high-nitrogen fertilizer,The IV-III phase transition of which is one of the causes for the caking of AN-based high-nitrogen fertilizer.Given that potassium salts used for the production of high-nitrogen fertilizer are usually K₂SO₄（KS）,this study,however,choose to use potassium nitrate（KN）to replace some KS for the purpose of investigating the change of anti-caking property of high-nitrogen fertilizer.To be specific,the crystalline and IV-III phase transitions of ANP were characterized using powder X-ray diffractometer（XRD）and differential scanning calorimetry（DSC）,on the basis of which,the inhibition effects of KN,KS,NH₄SO₄（NS）,and NH₄H₂PO₄（NP）on the solid phase transitions of ANP was evaluated.It was found that potassium ions can change the crystal form of ANP,KN and KS thereby show better phase transformation inhibition effect on ANP than NS and NP.On account of this above analysis,the study further prepared high nitrogen fertilizer samples with different KN contents taking the similar method of using KN to replace some of KS in the formula of AN-based high-nitrogen fertilizer.The anti-caking property and hygroscopicity of high-nitrogen fertilizer were then tested.The results showed that although the content of potassium in the compound fertilizer remained unchanged,the anti-caking performance and hygroscopicity of the AN-based high-nitrogen fertilizer were obviously improved with the increase of the content of KN and the decrease of the content of KS in the formula.In addition to ANP,Water-soluble ammonium polyphosphate（APP-II）was also utilized to replace part of monoammonium phosphate（MAP）and diammonium phosphate（DAP）in the formula of AN-based high-nitrogen fertilizer,and the high-nitrogen fertilizer with different contents of APP-II were hence prepared.The effect of APP-II on the anti-caking properties of high-nitrogen fertilizer was further determined,which indicated that the application of APP-II can improve the anti-caking performance of high-nitrogen fertilizer and reduce the degree of bonding between particles.Moreover,the anti-caking mechanism of APP-II for improving high-nitrogen fertilizer was discussed,that is,the surface of high-nitrogen fertilizer may be enriched with APP-II,which,on the one hand,weakened the side reactions of NP and DAP with other salts,and on the other hand,being a barrier,inhibited the side reactions among salts in NH₄NO₃-KNO₃-K₂SO₄-（NH₄）₂SO₄system.According to the XRD and DSC spectrograms,APP-II cannot inhibit the IV-III phase transition of ANP,and APP-II and KN have different mechanisms for improving the anti-caking property of high-nitrogen fertilizer.The present study,therefore,combined the two methods and investigated its effect on the anti-caking properties of high nitrogen fertilizer.The results showed that the replacement of some NP and DAP with APP-II in high-nitrogen fertilizer formula,and that of some KS with KN,as well as reducing the amount of ANP in the formulation,can further improve the anti-caking property of high-nitrogen fertilizer.To sum up,a method for determining the content of APP-IIin fertilizers was developed in this dissertation.The enterprise standard Q/SACF 05-2019“Determination of the content of polyphosphorus in compound fertilizers”was thus established,on the basis of which,the stability of APP-II in compound fertilizers was monitored.