It is widely acknowledged in the industry that disc granulators produce granules with high sphericity, smooth surfaces, and an attractive commercial appearance. However, when the same batch of material is processed using twin-roll extrusion granulation, the resulting granules typically exhibit higher compressive strength. The reason for this lies not in the quality of the equipment, but in the fundamental differences between the two forming mechanisms.
Disc granulation is a form of wet agglomeration. Materials on an inclined rotating disc are subjected to the combined forces of gravity, centrifugal force, and friction, causing them to roll and adhere layer by layer into spheres. The internal structure relies on the material’s inherent moisture and adhesiveness for bonding; without high-pressure compression, the granules remain relatively porous and loose. While this method yields excellent sphericity and uniform particle size, the granule strength is limited by the material’s natural adhesiveness and shrinkage characteristics upon drying.
Extrusion granulation is a dry, high-pressure forming process. Powdered material is forcibly compressed between two rollers, resulting in a dense internal structure with low porosity and, consequently, significantly higher compressive strength. The trade-off is that the granules are typically cylindrical or irregularly flake-shaped, lacking the high sphericity achieved by disc granulation.
Bio-organic fertilizer disc granulators are specifically optimized for this process. They utilize ambient-temperature physical forming with adjustable disc inclination and rotation speed, allowing materials to roll naturally into spheres on a low-friction surface. The temperature is maintained between 30°C and 55°C throughout the process to prevent high temperatures from killing beneficial bacteria. While the pelletization rate can reach 85%–95%, the low-temperature process limits further increases in granule strength—as the strength cannot be enhanced through high-temperature sintering or high-pressure compression. This represents an inevitable trade-off between “biological activity” and “mechanical strength” in bio-organic fertilizers.
The configuration of the entire disc granulation production line is designed around this trade-off. The line typically comprises crushing, mixing, granulation, screening, material recycling, drying, and packaging stages. The screening and material return stage is crucial; returning undersized fines and oversized granules to the granulator for re-pelletizing not only boosts the yield of finished product but also prevents material waste. The drying stage is particularly critical for bio-organic fertilizer; the temperature must be kept below 60°C to avoid massive die-off of the functional bacteria.