2026-08-26
What is the impact of moisture on granulation?
Moisture is one of the most sensitive and critical control parameters in fertilizer granulation. Excessive moisture leads to excessively fast granulation speed, larger particle size, and lower strength; insufficient moisture makes it difficult for the material to agglomerate into granules, resulting in excessive powder and a significantly reduced granulation rate. Each material has its optimal granulation moisture content due to differences in its viscosity, fiber content, and water absorption, which is usually determined experimentally and strictly controlled within a fluctuation range of ±2%.
I. Negative Impacts of Excessive Moisture
When the moisture content of the granulating material exceeds the optimal value (e.g., over 40% for organic fertilizer, over 8%–10% for compound fertilizer), the following problems occur:
Excessively Fast Granulation and Uncontrolled Particle Size: High moisture content thickens the liquid film on the material surface, drastically increasing capillary forces and adhesion between particles. Fine particles rapidly agglomerate within a very short time, leading to a "snowball effect"—particles grow larger and larger, the particle size distribution widens, and a large number of oversized particles are produced.
Low particle strength and easy pulverization: When the moisture content is too high, the solid-phase bridges inside the particles are not fully formed. After drying, the evaporation of moisture leaves a large number of pores, resulting in a loose structure and a significant decrease in compressive strength, making them extremely prone to breakage during subsequent conveying and packaging.
Increased drying load: Excessive moisture requires longer drying times and higher hot air temperatures, increasing energy consumption. High-temperature drying may also damage the activity of organic matter.
Adhesion and clogging: High-moisture materials severely adhere to the inner wall of the granulator, the screen, and the conveyor belt, leading to increased equipment operating load and even shutdown for cleaning.
II. Negative impacts of excessively low moisture content: When the material moisture content is below the optimal value (e.g., below 25% for organic fertilizer and below 3% for compound fertilizer), the opposite problem arises: Difficulty in granulation: When moisture is insufficient, the liquid bridging force between powder particles is too weak, and the particles cannot effectively bond together. Most of the material is still discharged from the granulator in powder form, and the granulation rate may be as low as 40%–50%. High dust levels: A large amount of fine powder flies around the workshop, not only wasting raw materials but also increasing the burden on the dust removal system and worsening the working environment.
Rough particle surface: Even if granulation is achieved, the particle surface lacks smoothness, resulting in poor appearance quality.
III. Recommended Moisture Content Ranges for Different Materials
| Material Type | Recommended Granulation Moisture Content | Remarks |
|---|---|---|
| Organic Fertilizer (Fermented & Decomposed Material) | 30%-38% | High fiber content, slightly higher moisture is required for adhesion |
| Compound Fertilizer (Rotary Drum Granulation) | 3%-6% | Inorganic salts, high moisture easily causes caking |
| Compound Fertilizer (Disc Granulation) | 8%-12% | Appropriate amount of binder shall be added |
| Urea‑based Slow‑release Fertilizer | 1%-3% | Extremely low moisture, dry extrusion process adopted |
IV. Key Practical Points for Moisture Control In production, online moisture meters are typically used to monitor the feed moisture content in real time, or a rapid drying oven method is used to sample and check every 30 minutes. Adjustment methods include adding water or steam for humidification during the mixing process, or adding dry powder for return to absorb excess moisture. It should also be noted that ambient humidity affects open systems, and the formula should be adjusted appropriately during the rainy season.