## 4 Discussion  
A side-by-side double-impeller aggregate conveying device was designed at the early stage of the study to achieve the effective collection and transfer of BSF sand [35]. On this basis, a screening device should be designed in order to effectively solve the sorting problem of the BSF sand mixture and manage resource utilization. The existing screening test results suggested that [46] the BSF sand mixture had a certain moisture content. Without stratification, the stirring and beating device, the conventional trommel screener, and the round-hole mesh screener have a lower screening efficiency for BSF sand mixture, which easily forms a paste on the screen and reduces the screener penetration, resulting in a higher insect rate and impurity content. Furthermore, within a certain moisture content range, the adaptability of the trommel screener according to the separation of the BSF sand mixture could be improved further.  
The trommel screener, as a rotary motion screener, has been extensively used in the sorting of grain and other agricultural materials, and little research has been conducted on the separation of BSF mixture. Combined with the mechanical and physical properties of the BSF and mixture, the layered rotating spike teeth were added to the trommel screener to facilitate the dispersion and stratification of the material. Unlike the conventional trapezoidal spikes, the contact between the spherical spikes and the BSFL is spherical, thus increasing the impact cross-sectional area and reducing the rigid impact on the BSFL. Moreover, the spike teeth and the trommel screener rotate coaxially and reversely, which is more conducive to reducing the probability that the BSF sand mixture will adhere to the screener and to improving the screener penetration rate compared with the conventional trommel screener.  
The conventional trommel screener is a round-hole screener or mesh screener, achieving a higher screening efficiency for agricultural bulk materials with a low moisture content. However, for a BSF sand mixture with a high moisture content, the BSFL and insect sand materials exhibit a high viscidity, and the BSFL skin has a certain elasticity, thus actively blocking the sieve holes and creating a sieve paste phenomenon. The tubular screener, with an elongated gap, smooth surface, and no edges or corners, was used to solve the problem of the entanglement and clogging of the BSFL sand mixture that arises from the small sieve holes. Furthermore, the BSF sand mixture was formed in a thin layer of BSF sand on the inner side of the tubular screener due to the action of the coaxial and reverse rotating spikes, which can be effectively separated. The penetration of the BSF sand mixture was enhanced, and the insect impurity rate under the screener was reduced.  
This study created some innovative structural designs, but there are also some limitations. The details are as follows:  
1. The differential trommel screener designed to separate the mixture of BSF sand was applied under the test conditions, and the trial production test was performed. If it is essential to complete the large-scale separation of BSF sand mixture, the size parameters of the trommel screener should be optimized in accordance with the feeding amount so as to better meet the actual production requirements.
2. The moisture content of the BSF sand mixture varied because of the differences in the biotransformation of the BSFL, and the range of the moisture content fluctuated.  
The differential trommel screen designed in this study had an excellent screening effect under the test conditions. For a BSF sand mixture with a higher moisture content, some operating parameters should be further optimized to increase the screening penetration rate. Furthermore, when the feeding amount of BSF sand mixture fluctuates, the adaptability of the differential trommel screener should be studied in depth.