This method is used when :-
- Larger sized impurities need to be removed eg. rocks, pieces of dirt, husk from rice, wheat or pulses etc.
- The quantity of impurities is not large.
- Threshing is the method used to separate grains from stalks etc.
- Stalks are beaten to free the grain seeds.
- Machines are used to thresh large quantities of grain but for smaller quantities, bullock carts are used.
- Winnowing is used to separate heavier and lighter components by wind or by blowing air.
- It can be used to separate grain from husk or a mixture of dry sand with saw dust and dried leaves.
- The husk particles are carried away by wind and the grains are collected below.
- This husk is used for other purposes like feeding fodder .
- It separates material s on the basis of their.
- Smaller components are able to pass through the sieve whereas bigger ones are not.
- Eg. removing husk and stone from wheat in flour mills.