The core principle of the double roller press granulator is "using pressure instead of moisture." Two high-strength rollers rotating in opposite directions instantly compress dry powdered material into a dense sheet at ambient temperature, bringing particles close enough for van der Waals forces, adsorption, and crystal bridge bonding to take effect. The entire process requires no added water and involves no temperature increase; the material's moisture content must be controlled between 2% and 5%. This dry processing route eliminates the drying and cooling systems required for wet granulation, reducing overall energy consumption by 30% to 50% compared to wet processes. The resulting granules are 1.5 to 3 times denser, with a crushing strength of 12 to 60 N. The primary technical challenge lies not in the extrusion action itself, but in "steadily feeding homogeneous powder into the roller gap." The feed particle size must be no larger than 0.6 mm; excessive fluctuations in particle size can destabilize the roller's "bite" on the material, leading to a sharp decline in granule strength consistency. A complete roller press granulation line typically includes a metering and feeding system, a forced feeder, the main double-roller press unit, a crusher, a multi-stage screening machine, and a packaging machine. During screening, oversized granules are crushed and recycled, while qualified granules proceed to the finished product silo; the recycle ratio is approximately 15%.

The BB fertilizer production line follows a completely different logic based on physical blending. It utilizes finished granules—such as urea, diammonium phosphate (DAP), and potassium chloride—as raw materials. The production line does not alter the chemical form of any raw material; instead, it focuses solely on precise blending and proportioning. The process flow is as follows: raw material silos → batching system metering → conveying → high-speed mixing → finished product silo → automatic weighing and packaging. The BB fertilizer mixer plays a pivotal role in this production line. Unlike organic fertilizer mixers, BB fertilizer mixers prioritize three key performance indicators: a coefficient of variation (CV) for mixing uniformity of ≤5%, a granule breakage rate of <1%, and the absence of cross-contamination between batches. Mainstream equipment falls into two categories: horizontal twin-shaft paddle mixers and rotary drum mixers. Twin-shaft paddle mixers utilize a specific blade angle to create a transient fluidized zone, achieving complete mixing within tens of seconds. Drum mixers rely on internal lifting flights to cause materials to tumble freely; with virtually no shear force, the breakage rate can be kept below 0.3%, making them better suited for high-value products like controlled-release fertilizers. A full-discharge design with large opening doors ensures material residue inside the drum remains below 0.5%, eliminating the need for manual cleaning when switching formulas.

These two approaches intersect uniquely in the field of bio-organic fertilizer production. Bio-organic fertilizer granulation faces a distinct constraint: functional bacteria are extremely temperature-sensitive. For instance, *Bacillus subtilis* retains a survival rate of approximately 70% after 30 minutes of exposure to 65°C, but this figure plummets to below 20% when the temperature rises to 80°C. The dry-method twin-roll extrusion process operates at ambient temperature throughout, thereby avoiding the lethal damage to active bacteria caused by the drying stage in wet granulation. However, a conflict arises between the strict input moisture requirement for twin-roll extrusion (2%–5%) and the typical moisture content of bio-organic fertilizer raw materials (25%–35%). Consequently, actual production lines often require a preliminary drying step for the raw materials, or the use of wet granulation methods—such as disc granulation—combined with low-temperature drying (≤65°C) to strike a balance between granule formation and bacterial viability.