Three Quality Indicators for Organic Fertilizer Granulation: Granulation Rate, Sphericity, and Viable Bacteria Survival Rate

The granulation rate represents the initial benchmark in the granulation process. Disc granulators utilize a tilted, rotating pan to cause materials to roll and agglomerate under the combined forces of centrifugal action, friction, and gravity; with an adjustable tilt angle (typically 35° to 55°), they can achieve granulation rates exceeding 90%. However, disc granulation imposes specific requirements regarding raw material moisture content, with optimal results occurring between 25% and 35%. The new-type two-in-one organic fertilizer granulator demonstrates superior adaptability in this regard; by integrating stirring-tooth shearing with drum-rolling technologies, it can directly process fermented materials with moisture content ranging from 20% to 45% without pre-drying, maintaining a stable granulation rate of over 90% and a ball-formation rate exceeding 98%.

Sphericity is a key indicator of a product's visual appeal. While disc granulators produce granules with good sphericity, the particle size distribution tends to vary naturally due to fluctuations in the material's residence time within the pan. The organic fertilizer spheronizer (or rounding machine) addresses a lingering issue with granulation equipment—specifically, that while it produces granules, it fails to refine their quality. Granules emerging from extrusion or disc granulators often have microscopic sharp edges; the spheronizer uses multi-layer, continuous shaping chambers to refine them through high-speed self-rotation and mutual friction. This process significantly improves sphericity, resulting in uniform, smooth, and stronger granules. The spheronization process achieves a pelletization rate exceeding 95%. Furthermore, when moisture content is maintained between 30% and 35%, any fine powder generated during the friction process instantly adheres back to the granule surface, thereby minimizing material loss.

Bio-organic fertilizers face a unique constraint during the granulation and drying stages: the survival rate of live bacteria. Common functional bacteria, such as *Bacillus subtilis*, are extremely sensitive to temperature. Patented process data indicates that drying temperatures for bio-organic fertilizers must be controlled between 40°C and 60°C, with a staged drying approach being preferable: an initial phase at 55°C–60°C followed by a second phase at 45°C–55°C. This method maximizes the preservation of bacterial viability while effectively evaporating moisture. Consequently, production lines for bio-organic fertilizers cannot simply adopt the drying parameters used for standard organic fertilizers; they require a narrower temperature window and more precise control over residence time.

From pelletization rate to sphericity and finally to live bacterial survival, these three metrics reflect the three-tiered objectives of the granulation process: first, forming the material into pellets; second, ensuring the pellets have a high-quality appearance; and third, ensuring the pellets remain "alive" until they reach the field. The disc granulator secures the baseline pelletization rate, the spheronizer refines sphericity and strength, and the production line's drying temperature control system determines whether the functional value of the bio-organic fertilizer is preserved for the end-user—the farmer.