Study On Coated Slow/Controlled Release Urea

As one of controlled release fertilizers, coated urea, which can enhance the use efficiency of urea to the maximum, has been a focus of D&R of fertilizer industry. In this paper, two membrane systems: 1. fertilizer-grade CaHPO4 powder plus water solution of PVA that acted as adhesive; 2. the mixture of paraffin wax and rosin, are chosen as coating materials to prepare coated urea, which has good slow-release property, contaminates soil slightly and has compound nutrient, within an ordinary open coating equipment by a relatively simple process. For the first system, the influences of the concentration of adhesive and of coating proportion on the release property of CaHPO4 coated urea are systematically investigated. For the second, the influences of ratio of paraffin wax to natural rosin, of coating proportion and of operation conditions on the release property of paraffin wax –natural rosin coated urea are systematically investigated. The mechanism of the urea release is studied theoretically on the measurements of release rate and SEM photography to the surface and cross-section of the products. Based on the study, a simple mathematical model is proposed to describe the release rate of the urea from a coated urea granule. The results of regression of release data show that the release behavior of CaHPO4 coated urea and paraffin wax –natural rosin coated urea could be characterized by the first-order release kinetics.Experimental results indicate that the slow-release property and the crushing strength of paraffin wax –natural rosin coated urea are better than that of CaHPO4-PVA coated urea. For CaHPO4-PVA coated urea, the concentration of PVA solution (8%wt.~12%wt.) used as adhesive has little effect on the slow-release property. The continued increase of coating proportion has little effect on the reduction of urea release rate, when it is over 30%wt.. For the paraffin wax –natural rosin coated urea, the ratio of paraffin wax to natural rosin is of major importance to the release rate. When the ratio is 80: 20, the release rate of active agent from the coated urea is least compared with other coated urea with same coating proportion. Furthermore, the release rate decreases significantly while coating proportion increases. Therefore, the release rate of urea can be controlled easily by adjusting the coating proportion. Utilizing the mixture of paraffin wax and natural rosin as controlled system with the CaHPO4 powder as additive to prepare controlled release urea by the method of melt atomizing encapsulation possess advantages compared with polymer-coated urea, as follows:1) No organic solvent, which may be harmful to the environment, is used in the course of preparation.2) The mixture of paraffin wax and natural rosin will degrade in the soil completely in a limit time and will not contaminate the environment.3) CaHPO4 contains phosphor component, which is needed by crop.