Purpose and influencing factors of compound fertilizer granulation

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Granulation: Purpose: First, it enables the gas-liquid raw materials to be adsorbed and solidified; large particles and solid masses of these raw materials are broken down into fine powder, which is then encapsulated to form particles with high density and good flowability. This prevents the powder from bridging and caking due to moisture absorption, electrostatic attraction between particles, or an excessive contact area. It also makes it easier to store and package the gas-liquid raw materials, as well as to weigh, package, and use them. Secondly: By forming the fertilizer into particles of appropriate size, it enhances its effectiveness. This approach prevents large pieces of raw material from being difficult to decompose, which would result in slow fertilizer action. It also avoids the problem with powdered fertilizers, where they tend to float and disperse, leading to rapid decomposition and loss of fertility, thus causing waste. Additionally, it eliminates the risks associated with gaseous or liquid fertilizers, such as their tendency to evaporate, which can cause poisoning in humans and environmental pollution. First, it is necessary to understand the criteria for granulation: 1) There must be masterbatches; 2) A liquid phase is required; 3) Viscosity is essential. The principles are as follows: 1) It’s better to use smaller amounts of materials rather than larger ones; 2) The drying temperature should be kept low rather than high; 3) Granules should be small in size, and it’s better to have them dry rather than wet. The granulation of urea-based compound fertilizers is closely related to the formula used – choosing the right fillers is crucial, and it is also important to control the drying temperature properly. The key elements in granulation using the pelletization method are: formula – the formula is extremely important; it isn’t necessary to use the formula with the lowest raw material costs, but rather one that results in a higher granulation rate, so as to reduce the overall cost. The proportion of basic sandy materials is around 20–30%; furthermore, it is important to ensure that the viscosity of the materials is enhanced, for which high-concentration urine is a good choice. Reducing the urine concentration can delay the freezing temperature of the urine, but this results in slower cooling of the particles, insufficient hardness, and thus they break into powder – meaning there are more granulated balls and more powder produced by the dryer. A conditioning agent is added to the ingredients to improve granulation conditions. Physical properties (such as the viscosity of the material, its temperature, the amount of its liquid phase, and its plasticity). The raw materials used in compound fertilizers include both viscous and non-viscous materials. Viscous materials can be further divided into those that are inherently viscous and those that dissolve and crystallize easily in the presence of water. During the granulation process, viscous materials use their own viscosity to bind non-viscous materials together to form particles. Materials that are inherently viscous can bind non-viscous materials together to form particles right within the granulator. As for materials that dissolve and crystallize easily, they dissolve into a liquid phase when steam is applied in the granulator, and then crystallize again during the drying process; during this crystallization process, they also bind non-viscous materials together to form particles. The proportion of viscous materials in the raw material determines the granulation efficiency. Pelletizing equipment and operator operations: Increase the amount of steam supplied appropriately, maintain the steam pressure at 0.6 MPa, and raise the operating temperature slightly ； Saturation steam is the best choice for granulation; using superheated steam can raise the temperature of the material more rapidly, but it results in a lower moisture content in the material, which is not conducive to granulation. 2) Increase the feedback ratio appropriately to bring it close to 4 ； 3) The steam pressure in the insulation jacket of the pipeline carrying the urea melt into the granulator should be no less than 0.3 MPa, to ensure that the temperature of the urea melt at the nozzle remains above 110°C. Additionally, the pressure of the urea melt at the nozzle must be controlled in order to adjust the spraying distance, thereby preventing excessive atomization and early cooling ； 4) The principle for granulating urea-based fertilizers is to keep the material dry rather than wet; otherwise, drying in the system becomes a problem, which leads to an excessive amount of large particles during granulation in the dryer. The crusher cannot handle such large particles, and the quality of the final product will not be good. Factors such as the amount of return material, the particle size distribution of the return material, the liquid phase volume, granulation temperature, and granulation time. During operation, the liquid content and temperature of the material in the granulator also have a significant impact on granulation. If there isn’t enough liquid phase, the material won’t bind together to form granules; on the other hand, an excessive amount of liquid phase can lead to the formation of large particles. The viscosity of viscous materials is closely related to temperature – generally, the higher the temperature, the greater the viscosity. For crystalline materials, higher temperatures facilitate the evaporation of moisture, thereby aiding crystallization.