When a seed company is harvesting seeds which are to be sold to farmers and growers, there’s always a certain fraction of the seeds that is not of the intended quality. So the seeds need to be cleaned and graded into the right weight class. And often this is done with the help of a gravity table separator, separating the heavier from the lighter seeds.

But with the use of conventional table separators, gravity and air are often insufficient to completely direct light products to the discharge. Mixing occurs and as such a middle fraction is created consisting of lighter heavy seeds and heavier light seed, called the ‘middlings’. This fraction contains too many good seed to discard, but also too many undesirable seed/particles to permit blending it into the good seed fraction. Until now there was no way around it. More or lesser amounts of middlings are always formed, and must be reprocessed to salvage the good seed it contains.

The middlings fraction can be re-separated over the gravity separator to try and salvage the good seed. A re-run allows the middlings fraction to be spread out further over the deck surface and this produces a closer separation of good seed from the undesirable material. But such re-runs comes with additional time and cost. In addition, the overloading of the table with problematic mid-range grain weights reduces the sorting accuracy and capacity. Recirculation during the separation process increases the stress on the grain and leads to mechanical damage. Consequently the middle fraction can only be seen as inferior quality seed.

Recently we have developed an innovation that provides a 99.5% separation into good seeds and bad seeds, and eliminates the need for re-runs. So the controversial question, “what to do with the medium fraction?” no longer exists. The innovation sorts precisely into heavy and light grains as the middle fraction is eliminated. The innovation that we introduced was that we modified the last third of the table surface. The new table lining has been designed as a zig-zag shaped ribbed construction, which increases the lifting force and diverts airflow to sort the heavy goods more precisely from the light. As a result, the heavy grains move upward as if they were on a staircase. Light and heavy products are clearly separated and precisely sorted. It’s sorting according to the all or nothing principle.

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