Abstract

Objective To solve the problems of low coating quality and poor mixing uniformity of seed powder in the pelletization process of corn seeds.

Methods Based on the existing pelletization research， three plate structures were designed to achieve an optimal mixing efficiency by changing the contact area between the plate structure and seed powder. The study analyzed the pelletization mechanism of the three structures and simulated their pelletization process of corn seeds. A particle-filling method was adopted to make the model closer to the shape of actual corn seeds. To reduce simulation time， the number of corn seed filling particles was reduced to 33. A simulation model of the corn seed pelletization process was established using the discrete element software（DEM） software EDEM， and the Hertz-Mindlin with Johnson-Kendall-Roberts （JKR） contact model was used to describe the bonding behavior between particles. The mixed motion of the three plate structures at the early， middle， and late stages of the simulation was compared and analyzed. The effects of rotational speed， vibration frequency， and vibration amplitude on seed and powder mixing uniformity were studied using the dispersion coefficient as the evaluation index. The coating quality was evaluated based on the coordination number of the seeds， which represented the number of surrounding particles. A higher coordination number indicated better coating quality. The quadratic regression equation for corn seed dispersion coefficient was established using Design-Expert software for variance analysis. Rotational speed， vibration frequency， and vibration amplitude of uniform plate I were set at 50 rpm，10 Hz and 6 mm respectively. Using these test factors，17 sets of simulation tests were performed， and the resulting discrete coefficients were analyzed.

Results and Discussion The results showed that the critical conditions for plate speed were related to the structure， diameter，and edge angle of the plate， all of which affected the coating quality. At the initial stage of pelletization， the seed powder was subjected to centrifugal force and collided with the wall of the coating pan. At the middle stage， the seed powder underwent circular motion driven by centrifugal force， reciprocating up-and-down motion due to plate vibration， particle collisions， and radial diffusion along the pan. At the final stage， particles experienced free fall and circular movement along the wall under centrifugal force. The mixing uniformity was influenced by rotational speed， vibration frequency， and vibration amplitude. If either of them was too low， the mixing effect would be poor. If the speed was too quick， the effect would be worse. Also， excessive vibration frequency and amplitude would cause seed powder to be thrown out of the pan. In some cases， the interaction between seeds and powder was weak due to their insufficient contact and inappropriate collision force， leading to incomplete and high dispersion coefficient. The optimal process parameters for rotational speed， vibration frequency， and vibration amplitude of the three plate structures were：（1） Plate I：50 rpm，10 Hz， and 6 mm；（2） Plate II：60 rpm，15 Hz， and 6 mm；（3） Plate III：70 rpm，20 Hz， and 8 mm. Using Design-Expert software， variance analysis on the quadratic regression equation showed that plate speed and vibration frequency significantly impacted mixing uniformity， while vibration amplitude had no significant effect.

Conclusion The study highlights that adjusting the contact area between the plate structure and seed powder significantly affects mixing uniformity. The particle-filling method tailored for irregular seed shapes reduces the simulation time while accurately approximating real particle behavior. The quadratic regression equation for corn seed dispersion coefficient provides a good fit with a corrected determination coefficient of 0. 95， indicating strong reliability. The lowest dispersion coefficient and the best mixing effect are achieved with plate I rotational speed of 56. 3 rpm， vibration frequency of 11. 5 Hz， and vibration amplitude of 5. 8 mm. The research results can provide references for the design of small particle granulators and the optimization of pelletization process parameters.

Keywords：pelletization； discrete elements； coating； small particle size