Granule strength is a key indicator of NPK compound fertilizer quality. Insufficient strength leads to pulverization and caking during screening, transport, and storage, compromising both appearance and application effectiveness; conversely, excessive strength slows down disintegration in the soil, thereby reducing fertilizer efficiency. Therefore, during the operation of the NPK fertilizer production line, it is essential to implement precise, end-to-end process control to stabilize granule strength and ensure product quality meets standards.

Strict control over raw material quality and pretreatment forms the foundation for enhancing granule strength. Raw material fineness directly affects granule cohesion; materials must be pulverized to a uniform fineness to prevent loose formation caused by a mix of coarse and fine particles. Simultaneously, the raw material ratio should be optimized—limiting the proportion of hard materials while incorporating eco-friendly binders like bentonite or starch to improve binding performance. Furthermore, strict control of raw material moisture content and pre-processing dehumidification are necessary to prevent issues such as loose or fragile granules at the source.
Precise regulation of granulation process parameters is central to controlling strength. Roller press granulation is the mainstream production method for NPK fertilizers; roller pressure must be stabilized between 20 and 22 MPa. Insufficient pressure results in poor compaction and weak granules, while excessive pressure accelerates equipment wear and produces granules that are too hard. Additionally, the liquid phase content during granulation should be maintained between 8% and 12%; too little liquid prevents proper binding, while too much causes granule adhesion and uneven formation. Matching equipment rotation speeds ensures sufficient material residence time in the granulation zone, resulting in well-formed, dense granules.
Optimizing the drying and cooling stages is crucial for stabilizing strength. The drying process should follow a gradient heating approach, with strict control over temperature and duration to prevent surface cracking or internal hollowing caused by rapid, high-temperature drying, thereby ensuring uniform moisture evaporation throughout the granule. The final moisture content of the product must be kept within the standard range, as excessive residual moisture can lead to caking and pulverization during subsequent handling. During the cooling phase, a constant-speed air-cooling mode is employed to gradually lower granule temperature, relieve internal stress, and enhance structural stability.
Establishing a routine quality control mechanism is essential. Throughout production, granule compressive strength is tested via periodic sampling and benchmarked against industry standards to maintain it within the 12–20 N range. Issues such as raw material fluctuations, equipment parameter deviations, and temperature or humidity anomalies are promptly identified and addressed through dynamic process parameter adjustments, creating a closed-loop control system. Precise management of the NPK fertilizer production line effectively stabilizes granule strength, reduces the defect rate, and enhances the product's market competitiveness.