Last reviewed 1 Oct 2026 · 7 min read
Why clean and grade?
Grain delivered from the field contains foreign material (straw, chaff, stones, dust, other crops, weed seeds, broken and shrivelled grain, insect-damaged kernels, metal pieces). Cleaning removes it; grading sorts the clean grain into uniform classes by size, shape, density or quality.
Benefits: better storage life (less moisture and infestation), higher market price, protection of machines (stones damage mills), uniform processing, seed purity, meeting quality standards (AGMARK, FSSAI limits for extraneous matter).
Basis of separation
| Property | Used to separate | Machine |
|---|---|---|
| Size (width, thickness, length) | Oversize and undersize material | Sieves, screens, graders |
| Shape | Round vs flat or long | Indented cylinders, spiral, disc separators |
| Specific gravity (density) | Heavy vs light | Gravity table, destoner, floatation |
| Aerodynamic (terminal velocity) | Light vs heavy | Aspirators, winnowers, air-screen cleaners |
| Surface texture | Rough vs smooth | Velvet roll, inclined draper belt, magnetic separator (with powdered iron) |
| Colour and reflectance | Discoloured, stained grain | Photo-electric colour sorters |
| Electrical properties | Conductive vs non-conductive | Electrostatic separators |
| Magnetic properties | Ferrous particles | Magnets and magnetic drums |
| Frictional behaviour | Rolling vs sliding | Inclined belt or table |
Machines for cleaning
1. Pre-cleaners and winnowers
- Pre-cleaners remove large and light impurities before storage or drying. Winnowing fan — traditional hand or motor-driven fan that blows chaff away while grain falls.
- Aspirators use an air stream in a duct to carry light matter upward and out, while grain falls through the stream.
2. Air-screen cleaner (the basic grain cleaner)
The commonest machine; it combines screens (to separate by size) and air (to separate by weight).
Parts: hopper and feed roll, pre-aspiration (air removing dust and light chaff at the entry), two or three oscillating screens (sieves), final aspiration, eccentric drive (shaking motion), fan and settling chamber.
Operation: the top (scalping) screen has larger holes that retain big impurities (stalks, pods, clods) while grain passes through; the second (grading) screen has holes just smaller than the grain, retaining the good grain while small seeds, sand and broken grain pass through; the air removes light chaff at the beginning and the end.
3. Screens (sieves)
| Type | Hole shape | Separates by |
|---|---|---|
| Round-hole (perforated) | Circular | Width (diameter) of the grain |
| Oblong (slotted) hole | Elongated slot | Thickness |
| Triangular | Triangular | Shape and width (for specific seeds) |
| Woven-wire mesh | Square | General sieving, flour; mesh number = openings per inch |
Sieve size is given as the hole diameter (mm) for perforated screens, or the mesh number (number of openings per linear inch) for wire mesh.
4. Indented cylinder separator (length grader)
A slowly rotating drum or disc with small pockets (indents) on the inner surface. Short grains (and broken) fit into the indents and are lifted and dropped into a trough; long grains stay in the cylinder and slide out the other end. Used for separating broken rice, oats from wheat, cockle and vetch from wheat, and length grading of rice. The pocket size is chosen for the length of the grain to be removed.
5. Disc separator
A series of discs with indents on their face on a rotating shaft; the grains are lifted by the indents according to length. Used for cockle, oats, wild buckwheat removal.
6. Spiral separator
A spiral chute down which grain rolls. Round seeds (vetch, mustard) roll faster and fly off by centrifugal force, while flat grains (wheat, oats) slide down near the centre.
7. Specific-gravity (gravity) separator
A vibrating, inclined, perforated deck with air blowing up through it. Heavy grains settle and move up the slope, while light grains float and move down. Used to separate by density — shrivelled and insect-damaged grain from sound grain, seed grading.
8. Destoner
A gravity-type machine (air-and-vibration) that removes stones of the same size as the grain — essential before milling (stones in rice, pulses and wheat). The stones, being heavier, move to the upper side of the vibrating deck.
9. Magnetic separator
Permanent magnets or drums remove iron particles and tramp metal before the grain enters a mill (protecting machines and avoiding sparks).
10. Colour sorter (optical sorter)
Grain falls in a thin stream past cameras or photo-sensors, which compare the colour (or near-IR reflectance) with a reference; discoloured or foreign grains are removed by air jets. Used for rice, pulses, coffee, nuts and plastics; very high accuracy but costly.
11. Other separators
- Velvet roll separator — removes rough seeds (dodder, thistle) from smooth ones.
- Inclined draper (belt) separator — round seeds roll off the belt, flat seeds are carried up.
- Electrostatic separator — separates by surface charge.
- Floatation (wet) cleaning — in water, to separate light and heavy or stones; used for pulses, rice, fruits, vegetables (followed by drying).
Performance of a cleaner
Let a feed of mass contain a fraction of impurities and give a clean product of mass with a fraction of impurities.
- Cleaning efficiency (impurity removal) .
- Purity of the product .
- Capacity — kg/h of grain handled; depends on the screen area, vibration, grain depth and inclination: typical air-screen cleaners 0.5–10 t/h for small to medium models.
- Loss of good grain (in the reject) .
A cleaner receives 1000 kg of wheat containing 4 % impurities. The clean output is 955 kg with 0.5 % impurities.
Impurity in feed kg; impurity in product kg.
Cleaning efficiency . Product purity . Reject = kg, containing kg impurity and kg of good grain, i.e. grain loss.