Materials enter the front-end vertical air screen through elevator or screw conveyor, this wind selection can remove light impurities according to the different proportion of impurities and materials. Then the materials enter vibration sieve, which can clean big and small impurities by different types of high precise punching sieve in accordance with different sizes of grains. Finally materials enter the back-end air screen; the secondary wind selection can remove the remnant light impurities further.