Influence of the Process Parameters on the Synthesis of Urea Phosphate and the Properties of the Obtained Product

Abstract

The ever-increasing food demand associated with the growing human population poses similar challenges to both farmers and fertilizer producers. In view of climate change and the increasing area of infertile land, it is very important to use correctly balanced and highly effective fertilizers in agriculture. Water-soluble fertilizers are becoming more and more popular. It is convenient to use them together with irrigation water because this reduces the negative effects of droughts and accelerates the assimilation of nutrients needed by plants. The aim of this work was to synthesize urea phosphate (UP) (water-soluble complex nitrogen–phosphorus fertilizers NPF) through the reaction of phosphoric acid and urea. The most important moment of the work is that the synthesis was carried out using a purified wet-process phosphoric acid (PWPA) and urea by varying the stoichiometric ratio and the duration time of crystallization. Based on the results of the experiment, it was found that, in the presence of excess acid, the concentration of phosphorus pentoxide (P2O5) is too high, the concentration of amide nitrogen (Namide) is too low, and vice versa. The best ratio of P2O5 and Namide was determined when both reactants were used in a ratio of 1.0:1.0. Crystallization was carried out at 20 °C with different reaction times: 30, 60, 90 and 120 min. Analysis of the chemical composition of the synthesized urea phosphate and determination of the main components, i.e., Namide and P2O5 concentrations, were performed using standard fertilizer analysis methods. Using the optical emission spectroscopy, the concentrations of chemical elements (sulphur, aluminium, iron, calcium, magnesium, silicon, etc.) were also determined in the synthesized product. During the experiment, not only the chemical composition of the product, but also the resulting crystals of the product, and their size and shape—properties that are highly dependent on the duration of crystallization—were analysed. The thermal stability of UP crystals was investigated using simultaneous thermal analysis; the crystallinity of UP was determined using X-ray diffraction analysis; the identification of groups of chemical elements was carried out using Fourier Transform Infrared spectroscopy analysis; the shape and size of crystals were investigated using scan electron microscopy and optical microscope techniques.