The dissolution rate, compressive strength, and hygroscopicity of fertilizer granules may appear to depend on the raw material formula, but at a deeper level, they are determined by an often-overlooked indicator: internal porosity. Even when using the same blend of urea, ammonium phosphate, and potash, different granulation methods result in vastly different porosities—and consequently, vastly different performance in the field.
Double-roller extrusion granulation: low porosity, high density, slow disintegration. The double roller press granulator uses a pair of counter-rotating rollers to compress powdered material into dense sheets under high pressure, which are then crushed and screened into granules. This forming method results in very low internal porosity and a dense structure; the compressive strength can exceed 30 N, making the granules suitable for long-distance transport and mechanical spreading. Because moisture cannot easily penetrate the granule core, the dissolution rate is relatively slow, leading to a steadier release of nutrients. However, it is important to note that double-roller extruded granules are not true “controlled-release fertilizers”; precise controlled release still requires a subsequent coating process. The stable operation of the roller press granulator production line depends on maintaining a feed particle size of ≤0.6 mm, a moisture content of 8%–15%, and regular inspection of the roller surface patterns.
Disc granulation: high porosity, easy disintegration, strong fast-acting properties. New-type organic fertilizer granulators and new-type two-in-one organic fertilizer granulators typically employ wet agglomeration. Materials roll into spheres on an inclined disc using their own moisture, retaining a large number of micropores within the granules. These micropores allow for rapid moisture penetration; the granules disintegrate quickly upon contact with water, releasing nutrients rapidly, which makes them suitable for use as top-dressing fertilizers. However, high porosity also implies lower compressive strength—typically only 10–15 N—resulting in a higher risk of crumbling during long-distance transport. For bio-organic fertilizer granulation, porosity holds additional significance: moderate porosity facilitates the entry of oxygen and moisture, providing a more stable micro-environment for beneficial microorganisms. However, the temperature throughout the granulation process must be kept below 55°C; otherwise, the microbial strains will suffer massive die-off, rendering even an optimal porosity meaningless.
BB fertilizer blending: This process involves no granulation, thereby preserving the original porosity of the granules. BB fertilizer production lines and mixers operate on a completely different principle—they do not alter granule morphology but rely solely on physical mixing. Raw materials such as urea, ammonium phosphate, and potassium chloride are already qualified industrial granules, each possessing its own inherent porosity and dissolution characteristics. The objective of the blending line is simply to ensure accurate proportioning and uniform mixing without damaging the original granule structure. BB fertilizer mixers employ a gentle, twin-shaft paddle mixing action that completes the process in 2 to 5 minutes, achieving a coefficient of variation (CV) for uniformity of ≤5% while keeping the granule breakage rate below 0.5%. Excessive mixing time or excessively high rotational speeds can cause surface abrasion and expose pores, which actually exacerbates moisture absorption and segregation.