NPK granulation temperature is typically controlled between 70–80℃. This range is derived from long-term practice using tubular reactors with ammoniation: within this temperature window, the liquid phase content of the material is moderate, and the pelleting rate can reach 70%–80%. Excessive temperature accelerates urea hydrolysis and ammonia volatilization, while insufficient temperature results in insufficient bed viscosity and difficulty in fine powder agglomeration. The essence of 70–80℃ is finding a balance between “rolling” and “retaining nitrogen.”
The granulation temperature is one of the most sensitive process parameters in an NPK fertilizer production line. A setting that is too high or too low does not merely affect yield—it directly changes nitrogen content, particle strength, drying load, and even the stability of the entire batch. Understanding why this window exists, how steam and water differ in temperature control, and how to adjust it for different nitrogen sources is essential for both plant operators and technical buyers.
I. The Triangular Relationship between Temperature, Pelletizing Rate, and Nitrogen Loss
The granulation temperature primarily affects the liquid phase content. The solubility of NPK salts in water increases with temperature. The more complete the liquid film formed on the bed surface, the stronger the liquid bridges between particles, and the higher the pelleting rate. The reason why the amine process can heat the material to 80–100℃ is precisely by utilizing the heat of reaction to increase the solubility of chemical salts, thereby reducing the granulation moisture content by 2–3 percentage points while maintaining the same liquid phase ratio.
However, temperature also brings nitrogen loss. Urea undergoes hydrolysis during granulation, and the hydrolysis rate accelerates dramatically above 75°C, increasing approximately 2.5 times for every 10°C increase. The released free ammonia not only causes nitrogen loss but also creates an ammonia-containing atmosphere within the granulator. In solid urea granulation, temperatures typically need to reach around 80°C to melt the urea particles and integrate them into the granulation process, further compressing the safety window.
Therefore, 70–80°C is a conservative but robust choice: it allows sufficient liquid phase for pellet formation while keeping the urea hydrolysis rate within an acceptable range. If the formulation uses ammonium sulfate as the main nitrogen source, the primary risk of nitrogen loss shifts from urea hydrolysis to ammonia volatilization, allowing for a slightly wider temperature window, but it should still not exceed 80°C for extended periods.
II. Steam vs. Water Spraying: The Fundamental Difference in Temperature Control Logic
When adjusting temperature within the granulator, steam and water follow completely different paths. Steam inputs heat, along with a small amount of moisture. Steam releases latent heat upon condensation, directly raising the bed temperature, while the added moisture is limited and does not significantly dilute the liquid phase concentration. In the amine process, steam, combined with the heat of reaction, can stabilize the material temperature at 80–100℃; in the traditional pelletizing process, relying solely on steam heating, the material temperature typically only reaches around 60℃, resulting in a correspondingly low pelletizing rate. Another advantage of steam is its rapid temperature response; the bed thermal state can be changed within minutes through valve adjustment, making it suitable for continuous production lines.
Water spraying primarily increases the liquid phase volume, not the heat. The sprayed water carries away heat during evaporation, effectively cooling the material; however, it simultaneously dilutes the slurry concentration, thickening the liquid film and making the material sticky. Using water for “temperature adjustment” essentially sacrifices moisture control for temperature reduction, leading to a corresponding increase in subsequent drying load.
A more reasonable division of labor in practice is: steam is responsible for raising and maintaining the bed temperature to the target temperature, while water or washing liquid is used for fine-tuning the liquid phase volume and slurry viscosity; the two should not be substituted for each other. In production, the liquid phase content is typically controlled at 12%–25% of the total material volume, the steam pressure is stabilized at 0.2–0.4 MPa, and the bed temperature is maintained at 60–80℃. For urea-based formulations, it is crucial to prevent localized temperatures from exceeding 90℃.
III. Temperature Window for Different Nitrogen Sources
Urea is most sensitive to temperature. The critical point for urea hydrolysis is approximately 75℃; above this temperature, the hydrolysis rate increases exponentially. Therefore, for NPK formulations using urea as the primary nitrogen source, the granulation temperature should be kept within the lower limit of 70–75℃, and localized hot spots in the bed should be avoided as much as possible. In high-tower or melt granulation routes, the urea melt temperature can reach above 130℃, but this is a process requirement for the melt stage and is not the same as the 70–80℃ bed temperature in rotary drum or disc granulators.
Ammonium sulfate has better thermal stability than urea, and its decomposition temperature is much higher than the granulation temperature window. Therefore, when using ammonium sulfate or amino acids to produce ammonium sulfate slurry as a nitrogen source, the temperature can be widened to 80℃ or even slightly higher, and the pelleting rate is more guaranteed. However, when urea and ammonium sulfate are mixed for granulation, the crystallization behavior of the mixed melt changes: studies have shown that the bed temperature of the urea-ammonium sulfate mixture needs to be about 15℃ lower than that of pure urea melt with the same moisture content to achieve successful granulation.
This means that the mixed nitrogen source formulation cannot simply apply the temperature setting of a single nitrogen source, but should adjust the temperature window downwards according to the urea ratio. The higher the urea ratio, the closer it should be to the 70℃ side.
IV. FAQ: What to do if the granulation temperature cannot be reached in winter?
Q: In winter, the ambient temperature is low, and the granulator bed temperature cannot reach 70℃, resulting in a decrease in pelleting rate. What should be adjusted first?
First, check if the heat input is sufficient, then check the heat loss. The first step is to check the steam pressure and flow rate: the steam pressure should be stable at 0.2–0.4 MPa. Saturated steam with a temperature below 130℃ carries insufficient latent heat, so the steam supply quality needs to be improved. The second step is to check the insulation of the granulator cylinder or disc. If necessary, install a steam heating coil on the back of the granulation disc to ensure uniform disc temperature and reduce heat loss from the material bed to the equipment body. The third step is to appropriately increase the utilization ratio of the amino acid reaction heat: within the limits of the formula, allow more of the neutralization heat from the tubular reactor to enter the material bed, replacing part of the steam load.
If the temperature is still too low after the above measures, the target temperature can be temporarily lowered to 65–70℃, while slightly increasing the liquid phase volume to compensate for viscosity. However, particle strength and return rate need to be closely monitored. The standard window should be restored after the ambient temperature rises. It is crucial to avoid simply increasing the water spray volume to “find viscosity,” as this will only turn the temperature problem into a drying load problem.
In summary, the basis for an NPK granulation temperature of 70–80℃ is not a single isolated rule, but rather the dual effect of temperature on liquid phase volume and nitrogen loss. Steam is responsible for providing heat, water is responsible for adjusting the liquid phase, urea and ammonium sulfate each have their own window, and mixed nitrogen sources need to be corrected downwards—only by treating these four things separately can temperature control be truly implemented.