What are the differences when using a gravity separator to clean soybeans and sesame seeds?

Soybeans are a typical large-seeded legume crop; their seeds have a long axis of approximately 6–8 mm, a thousand-seed weight of 120–250 g, and a bulk density of about 720–780 kg/m³. They possess high density and a rapid settling velocity in suspension; furthermore, their seed coats are relatively tough, making them resistant to skin breakage or fragmentation under the repeated action of vibration and airflow. In contrast, sesame is one of the world's smallest oilseed crops; its seeds have a long axis of about 2–3 mm, a thousand-seed weight of only 2–4 g, and a bulk density of approximately 560–600 kg/m³, with a density significantly lower than that of soybeans. Additionally, sesame has an oil content approaching 50% and a smooth, low-friction waxy surface layer; the seeds tend to stick together and clump easily in humid conditions, and their thin seed coats rupture and release oil under even slight compression or friction.

For soybeans, the gravity separator can utilize higher airflow rates, higher vibration frequencies, and a steeper deck inclination. Because soybeans are dense and settle quickly, a sufficiently strong upward airflow is required to "lift" and fluidize the bed; simultaneously, vigorous vibration ensures that stones and clods—which sink to the bottom—can effectively migrate toward the heavy-material discharge end. The deck angle is also increased to leverage the beans' own weight, facilitating rapid flow toward the discharge outlet and ensuring high processing throughput.

In contrast, parameters must be significantly adjusted for sesame seeds. Airflow must be reduced and precisely controlled; sesame seeds are so light that excessive air would carry them into the heavy-material discharge outlet, resulting in product loss. Vibration intensity must be lowered to prevent excessive bouncing and inter-seed friction, which could damage the seed coat or the natural waxy surface layer. Additionally, the deck angle is reduced and the feed rate slowed, allowing the light, smooth seeds to spread slowly and evenly across the deck, creating a stable, thin fluidized layer. In short, processing soybeans follows a high-capacity approach characterized by strong airflow, vigorous vibration, and rapid discharge, whereas processing sesame seeds requires a gentle, precision-oriented approach involving low airflow, mild vibration, and a slow, steady flow.

Impurities in raw soybeans primarily consist of broken beans, shriveled beans, split beans, pod fragments, stalk pieces, and dust, alongside small stones, soil clods, and metal scraps of similar size to the beans. When processing soybeans, the gravity separator focuses on removing "heavy impurities"—such as stones and soil clods—because these hard objects can damage downstream equipment like dehullers, the abrasive rollers of polishers, and the glass viewing windows of color sorters; simultaneously, the machine helps blow away some shriveled beans and lighter impurities.

The impurity profile of raw sesame is more complex and presents different risks: in addition to stones and soil clods introduced from the field, there are shriveled seeds, broken seeds, and dust, as well as dust and microorganisms that adhere to the seeds due to their high oil content. For sesame, the gravity separator’s priority goes beyond stone removal to preserving seed integrity; if the separation force is too aggressive, it can cause skin breakage, oil leakage, and clumping, which ultimately reduces oil yield and compromises the product's appearance. Consequently, gravity separation for sesame emphasizes "gentle separation," prioritizing the preservation of undamaged seeds over processing speed.

The core mechanism of a specific gravity separator is "airflow fluidization combined with specific gravity-based stratification"; while both materials operate on this principle, the visible separation characteristics differ. Soybeans, being dense and heavy, form a thicker fluidized layer; this allows the specific gravity difference between the beans and stones to be effectively leveraged, resulting in distinct stratification, clean separation, and a high removal rate. In contrast, sesame seeds—small, lightweight, and smooth—form an extremely thin fluidized layer. Although a specific gravity difference exists between sesame seeds and stones, the seeds' suspension characteristics in the airflow and their frictional adhesion to the separator deck are weaker; consequently, stratification is less distinct than with soybeans, requiring precise airflow adjustment to keep the seeds suspended while allowing the stones to settle, cling to the deck, and migrate upward. Furthermore, sesame seeds tend to clump when exposed to moisture; these clumps differ in density and size from individual seeds, further complicating the separation process and necessitating stricter controls on feed moisture content compared to soybeans.

For soybeans, the gravity separator excels in high throughput, high efficiency, and thorough stone removal; a single unit can handle substantial hourly volumes while ensuring consistent product purity, making it ideal for the large-scale cleaning of soybeans destined for animal feed, food products, or seed stock. For sesame seeds, the machine's core value lies in its gentle handling—preserving seed integrity and preventing damage; although the processing rate is lower than that of soybeans, it ensures the seeds remain whole without skin breakage or oil loss, resulting in superior oil yields and visual quality—factors critical for high-value products such as sesame oil, sesame paste, and export-grade sesame. If the raw soybeans or sesame seeds originate from clay-rich soil and contain significant clods, the system should be paired with a soil separator to specifically remove magnetic soil clumps that are difficult for the gravity separator to eliminate completely, thereby creating a comprehensive, closed-loop process: air-screen cleaning, de-mudding, and gravity-based stone removal.