Urea Gypsum Preparation From FGD Gypsum And Its Slow-release In Water And Soil

A great deal of flue gas desulfuration (FGD) gypsum has been produced as industrial by-product by exhaust gas desulfuration when lime/limestone wet desulfuration processes are employed. FGD gypsum has been used as wallboards, setting retarders in cement, building plaster manufacturing, and saline soil ameliorant. However, the amount of FGD gypsum in China is up to69million tons in2012, and more than19million tons has not been utilized. In the paper, urea gypsum (UG) was prepared with FGD gypsum and urea, and then transferred into slow-release urea fertilizer by coating technology. The effect of coating recipe and coating percentage on nitrogen release was investigated, and the mechanism of slow release was elucidated.Under mild temperature and atmospheric pressure, UG was prepared with FGD gypsum and urea using saturated urea solution as crystallization medium. The dissolution-recrystallization began in0.5h from the dissolution, and finished in1.5h. The UG crystal is in prismatic shape, which is definitely different from those of urea and gypsum. Biuret is the main by-product in agricultural urea, and strong inhibit UG formation and crystal growth. By increasing biuret concentration, the transformation from raw materials to UG is prolonged, and crystal varies from prismatic shape to needle-like one with a distinct increase in aspect ratio. The elements of Hg, Cd, Cr, Pb and As are deeply concerned when FGD gypsum is used in agriculture. However UG contains fewer amounts of these elements than FGD gypsum after the transformation.UG tablet was prepared by pressing UG crystals together with starch paste. The tablet release51.3wt%nitrogen in1.0h and100wt%nitrogen in8h, showing the property of slow-release when compared with the complete nitrogen release0.5h for urea in water. However, the nitrogen-release rate of UG tablet cannot meet the National Standard of Slow Release Fertilizer (GB/T23348-2009, China) in which it must be controlled low than15%in one day. To improve the slow release property, stearic acid, paraffin wax and ceresin wax were used to coat the tablet. With10.3wt%of coating materials (the ratio of these three materials is60:20:20),0.2,75.9and 100wt%nitrogen were released respectively in1,28, and35d, meeting China Standard of Slow Release Fertilizer. The nitrogen release rate decreases with the coating percentage increase. As the coating percentage was increased to11.2wt%, not any nitrogen was released during35d. The dense film improves the waterproofness of the tablet coated with stearic acid. After adding paraffin wax into stearic acid, the tablet surface becomes wrinkled and fissured, which deteriorates water-proofing capacity and then accelerates the nitrogen release. Further addition of ceresin wax gives rise to numerous small spherical particles on tablet surface and changed the morphology of the film. This change improves the ductility of paraffin wax to avoid tablet crack and flake formation, being responsible for the excellent slow and sustainable nitrogen release.Urea gypsum was synthesized using FGD gypsum and urea as raw materials. Further, a new slow-release nitrogen fertilizer, the coated urea gypsum, was prepared by coating with stearic acid, paraffin wax. and ceresin wax. This work finds a way to reuse FGD gypsum as a new slow-release nitrogen fertilizer in agriculture area.