The sand sieve eliminates all impurities smaller than the grain kernel, for example sand, broken kernels and foreign seeds. The next important step involves sorting the product into a heavy and a mixed fraction, with particles of the same size being separated according to their specific gravity.

Separation is based on the differences in the flow velocity of the various Product fractions: Heavy particles contact the oscillating sieve bottom and flow to the subsequent de-stoning stage, whereas the clean grain floats on a cushion of air to the product outlet.

The separation of low-density particles such as dust, hulls, chaff, shriveled kernels and foreign seeds is accomplished by air classification.