Objectives Urea-formaldehyde slow-release fertilizer prepared by the traditional solution of polycondensation (SUF) usually has a high moisture content, which makes the product adhere strongly to equipment. Consequently, it is difficult to achieve continuous production due lack of automatic discharge from the reactor. Moreover, the high moisture content also increases energy consumption during drying. Therefore, we improved the process technology to overcome this bottleneck.
Methods Urea-formaldehyde slow-release fertilizer (HUF) was prepared by a high-temperature gas-solid polycondensation process of paraformaldehyde and urea. The principle is that under high temperature and sealing conditions, paraformaldehyde is depolymerized into formaldehyde gas, which combines with urea in the reaction kettle to form urea-formaldehyde and water. A small amount of urea will decompose at a high temperature to produce ammonia gas, which combines with water to form ammonia water and drives the polycondensation reaction to form a urea-formaldehyde molecular chain with a high degree of polymerization. Slow-release fertilizers were prepared by high-temperature gas-solid polycondensation and conventional solution concentration, denoted as HUF and SUF, respectively. Samples with mole ratios of urea to formaldehyde of 2, 4, and 6 were prepared for each method. We studied the drying time of the products and characterized the composition and structure of urea-formaldehyde by the Acetylacetone method, Kjeldahl method, Thermal weight loss (TWL), Thermogravimetry (TG), Fourier transform infrared spectroscopy (FTIR), Gel permeation chromatography (GPC), and X-ray diffraction (XRD). Furthermore, the performance of urea formaldehyde slow-release fertilizer was tested by immersion method.
Results In a high-temperature gas-solid polycondensation reaction, the higher the reaction temperature, the higher the formaldehyde conversion rate. The formaldehyde conversion rate slightly increased when the reaction temperature was higher than 100℃. Also, the decomposition rate of urea increased sharply beyond 100℃, leading to the reduction of urea-formaldehyde nitrogen content. Therefore, 100℃ was the best reaction temperature for the high-temperature gas-solid polycondensation process. The high-temperature gas-solid polycondensation process significantly reduced the water content of the reaction product; the highest water content was 11.72%, and the drying time was shortened by at least 1 h compared with SUF. The process also improved the raw formaldehyde conversion rate and the average chain length of urea-formaldehyde molecules. When the mole ratio of urea and formaldehyde was 2∶1, the formaldehyde conversion rate of HUF was 88.22%, which was 9.26 percentage points higher than that of SUF. The average molecular weight of HUF and SUF were 4445 and 949, respectively. HUF's slow-release available nitrogen content was 21.05%, which was 12.48 percentage points higher than SUF. The activity coefficient of HUF was 42.33%, which was 20.48 percentage points higher than SUF. HUF's nitrogen release in 24 h was 49.6%, 12.1 percentage points lower than SUF. The cumulative nitrogen release of HUF in 28 days was 73.6%, which was 3.5 percentage points higher than SUF.
Conclusions Urea formaldehyde slow-release fertilizer prepared by high-temperature gas-solid polycondensation process can effectively reduce the moisture content of the product and thus achieve continuous production of the equipment, without affecting the slow-release performance.
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