This is a frequent quality problem encountered in BB fertilizer production—the previous batch (15-15-15) was qualified, but after switching to 28-6-6, the total nutrient content of the first batch is always lower or higher, sometimes even exceeding the national standard’s allowable error range. Superficially, it seems like a problem with the batching system, but in reality, it’s the result of the combined effect of residues from the mixing equipment and the formula switching process.
Residual materials interfere with the formula. Small amounts of material from the previous batch inevitably adhere to the inner wall and mixing blades of the NPK blending machine (NPK blending machine / NPK bulk blending machine). When switching to a high-nitrogen formula (such as 28-6-6), if the previous batch contained residues from a low-nitrogen formula (such as 15-15-15), these residues will dilute the nitrogen content of the new formula, resulting in a 0.5%~1.0% lower total nitrogen content in the first batch of finished products. If the previous batch contained trace elements (such as zinc or boron), even small amounts of residue can significantly affect the trace components. Countermeasures: Before each formula change, a thorough cleaning procedure must be performed—first, empty all visible materials from the mixer, then use compressed air to blow away the inner walls and blade dead corners, and finally run the machine once with the next batch of base raw materials (such as urea) and discharge it.
Weighing deviation caused by the “memory effect” of the batching scale. After long-term operation of the same formula, the zero point of the electronic belt scale in the automatic batching system may drift slightly. When changing formulas, if it is not recalibrated, the system will still operate according to the deviation logic of the old formula, resulting in inaccurate feeding of the new formula. Countermeasures: Each time a formula is changed, the batching scale must be zero-point calibrated and weighted at least once to ensure that the weighing accuracy is restored to within ±0.2%.
First-order confirmation and batch isolation system. Establish a strict formula change management process: The first batch of products after the change must be stored and sampled separately. Only after the laboratory issues a nutrient test report confirming that it meets the standards can it be incorporated into the regular batch. If the initial inspection fails, trace the batching system parameters according to the degree of deviation, adjust them, and restart trial production until the standard is met. It is recommended to control the initial output of each switchover to half of the normal batch to reduce the amount of rework of defective products.