Studies On The Preparation Of Coated Urea Microspheres By Fluidized Bed

At present,studies have shown that long-term use of traditional instant fertilizers lead to water pollution,soil deterioration and potential biological hazards due to the loss of excess nutrients in soil.Most of the traditional fertilizer can not be fully absorbed by crops due to their dissolution.In order to avoid the disadvantages of traditional fertilizers and achieve sustainable and efficient application,it is focuses on the development of a new slow-release fertilizer in order to increase the N absorption.Fluidized bed spray drying coating was used in this paper,with gelatin,alkyd resin and polyvinyl alcohol as coating materials to prepare slow release fertilizer with urea particle as core material.Preferential technological conditions were explored based on the preliminary solubility and coating quantity of the product,and the variation trend of nutrient release rate of the prepared product was investigated.The structure,morphology and thermal stability of the microspheres were characterized by FTIR,SEM and TGA.The following are the specific research contents and results.(1)Sustained-release fertilizers were prepared with natural macromolecule gelatin as coating material,glutaraldehyde as crosslinking agent,glycerol as plasticizer and magnesium stearate as anti-adhesive.The effects of gelatin concentration,glycerol dosage,glutaraldehyde concentration and magnesium stearate dosage on the preliminary solubility and coating amount of the product were investigated.Through the design of single factor experiment and orthogonal optimization experiment,the optimum technological conditions are as follows,gelatin solution concentration was 5 wt%,plasticizer glycerol content was 15 mL,cross-linking agent glutaraldehyde concentration was 4 wt%,anti-sticking agent magnesium stearate content was 10 g.Under these conditions,the product coating amount was 6.43%,preliminary solubility was 9.71%,which meets the national standard of slow-release fertilizer.FTIR results show that the cross-linking reaction between gelatin and glutaraldehyde resulted in Schiff base structure.SEM results show that the gelatin film is porous and its thickness is 46.11 ?m.(2)Using thermosetting resin alkyd resin as coating material,adding silica and glycerol to prepare slow-release fertilizer.The effects of alkyd resin concentration,embedding temperature,silica dosage and embedding time on the preliminary solubility and coating amount of the product were investigated.Through the design of single factor experiment and orthogonal optimization experiment,the optimum technological conditions were obtained as follows,alkyd resin concentration was 12 wt%,silica dosage was 6 g,embedding temperature was 70 ?,and embedding time was 120 min.The product coating amount was 8.75%,preliminary solubility was 7.80%,which met the requirements of national standard for slow-release fertilizer.FTIR results show that the characteristic peaks of the product are almost the same as those of alkyd resin,and the coating process is physical embedding method.SEM results show that the microspheres have smooth surface,compact structure and uniform film,the thickness of the film is 85.99 ?m.(3)Polyvinyl alcohol was used as coating material,carboxymethyl cellulose was added,and glutaraldehyde was used as crosslinking agent to prepare slow-release fertilizer.The effects of the concentration of polyvinyl alcohol,the amount of carboxymethyl cellulose and glutaraldehyde on the preliminary solubility and the amount of coating were investigated.Through single factor experiment and response test,the optimum technological conditions were obtained as follows,the concentration of polyvinyl alcohol was 6.13 wt%,the dosage of carboxymethyl cellulose was 3.06 g,the dosage of glutaraldehyde was 4.27 mL,and the predicted preliminary solubility was 6.46%.The actual preliminary solubility was 6.23%,which is close to the predicted value.The concentration of polyvinyl alcohol was 6.23 wt%,the dosage of carboxymethyl cellulose was 3.08 g,and the dosage of glutaraldehyde was 3.97 mL.The predicted coating amount was 9.91%,and the actual coating amount was 9.73%,which was close to the predicted value.FTIR results show that the film forms a stable structure with acetal structure through nucleophilic addition and other cross-linking reactions.SEM results showthat the surface structure of the microspheres is compact anduniform,the film thickness is 65.88 ?m.Compared with the above coating materials,gelatin has the lowest cost and the best biodegradability,alkyd resin has the best thermal stability,and the longest sustained release period of the product can reach 6 days,modified polyvinyl alcohol has the largest coating amount,which can reach 9.73%,and the fluidized bed coating efficiency is the highest.The results show that the nutrient release rate of the product can be controlled by optimizing the experimental conditions.