Grain cleaning equipment separates grains and impurities based on differences in physical properties such as particle size, specific gravity, shape, airflow characteristics, and resistance to friction and impact. The main types include equipment that separates by particle size, specific gravity, shape, airflow characteristics, and resistance to friction and impact, as well as combined equipment integrating multiple screening functions.
Equipment that separates by particle size uses woven wire mesh or perforated metal plates as the screen surface, utilizing the size of the screen openings for screening. Equipment with screen openings slightly larger than the maximum diameter of the grain particles allows grain and smaller impurities to pass through the screen openings, while larger impurities are discharged as oversize. Examples include large impurity screens and cylindrical screens, commonly used for preliminary cleaning when grain enters the warehouse. Equipment with screen openings smaller than the minimum diameter of the grain particles allows small impurities to pass through and be removed, while the grain is discharged as oversize. This includes reciprocating vibrating screens, flat rotary screens, and high-speed vibrating screens, used for cleaning after grain leaves the warehouse and before processing. Reciprocating vibrating screens typically have three layers of screen surfaces to separately discharge large impurities, different types of grain, and small impurities. A planar rotary screen has a screen body suspended by a boom that rotates in a planar motion, and it has two screen surfaces. A high-speed vibrating screen has a screen body that moves in a small-amplitude circular motion in a vertical plane, and it has two to four screen surfaces. It is effective for separating barnyard grass from rice.
Gravity separation is used to separate impurities that are similar in size and shape to grain particles but different in weight, such as stones and mud. Methods include wet and dry processes. Typical equipment for wet separation is a wheat washing and drying machine, while dry separation equipment includes a gravity destoner and a gravity grading table. The gravity destoner uses an inclined, reciprocating vibrating perforated plate and controlled airflow to make the grain flow in a semi-suspended state downwards and be discharged. Heavier impurities adhere to the plate surface, rise, and are discharged through the stone-collecting zone. The gravity grading table works similarly to the gravity destoner, first separating the grain into two parts according to their weight and then cleaning them separately. Dry separation equipment saves water and avoids wastewater pollution.