In a biological organic fertilizer production line, operators often deem material acceptable if the moisture content—measured via samples taken at the outlet of the double-shaft mixer—falls within the 25%–35% range. The material is then fed directly into the biological organic fertilizer dryer. However, shortly after the dryer begins operation, issues arise: inconsistent moisture levels in the output, with some parts becoming over-dried and pulverized, while others remain insufficiently dried. The reason for this is that the mixer measures “average moisture content,” whereas the dryer primarily encounters “surface moisture and free water.”
In a double-shaft mixer, liquid is sprayed onto the material, wetting the surface before it penetrates the interior. The mixer uses two symmetrical spiral shafts to achieve convective mixing, blending the fermented material with additives while simultaneously adjusting moisture levels via a spray system. If the atomization of the sprayed liquid is not sufficiently fine, droplets adhere to the particle surfaces. If the mixing duration is too short, the moisture does not have time to migrate into the interior of the particles, resulting in a “wet surface, dry interior” condition. When a sample is taken for testing, the processes of mixing and subsampling average out the surface and internal moisture, yielding a reading that appears acceptable. Once the material enters the dryer, however, the surface free water evaporates first. This causes a sudden spike in the thermal load at the dryer’s inlet section, leading to fluctuations in discharge temperature, while the material in the latter stages of the dryer remains under-dried.
Biological organic fertilizer dryers are low-temperature, high-airflow systems that are particularly sensitive to surface moisture. Because biological organic fertilizer contains functional microorganisms, the inlet air temperature is typically controlled between 60°C and 80°C, with the discharge temperature kept below 45°C–50°C; moisture is removed primarily through high airflow rather than high heat. This low-temperature drying method responds quickly to surface free water but slowly to internally bound water. If the material exiting the mixer has excessive surface moisture, the humidity at the dryer’s inlet rises sharply, reducing the efficiency of the hot air. Conversely, if the surface moisture is too low while the internal moisture remains high, the dryer may “falsely” register the material as dry, leading to a rebound in moisture content in the final output. The key to optimizing the entire production line lies in ensuring the mixer “kneads” the moisture into the material, rather than merely “spraying” it onto the surface. The configuration logic for a fertilizer-making machine should be as follows: the atomized liquid droplets should be controlled to a size of 100–200 microns; following the liquid application, the material should continue to be mixed for 3–5 minutes to allow for the redistribution of moisture both within and across the particles. The mixer’s loading factor should be maintained at 60%–70% to prevent the material from clumping, which could lead to localized over-saturation. After mixing, a brief buffering and homogenization stage should precede the drying process. For the dryer, the airflow in the initial section should be adjusted based on the surface moisture state of the incoming material, rather than relying solely on the moisture content of the final, dried product.
Meeting moisture specifications at the mixer outlet only ensures an “average” compliance. The drying process requires that the material remains neither excessively wet on the surface nor under-processed (raw) on the inside. By treating the moisture-conditioning (mixing) phase and the thermal load of the drying phase as an integrated “moisture chain,” one can establish a stable foundation for both the survival rate of viable microorganisms in bio-organic fertilizers and the overall quality of the granules.