A reject separator is a device used in pulp and paper industries to separate out the rejects from the pulp.The reject separator is normally installed at the outlet of a pulper. There are also successful applications where the reject separator has been installed as a stand-alone upgrade to an existing wet hammer mill process. It is used to remove contaminants such as dirt, stones, and other foreign materials from the pulp. The reject separator is usually a large cylindrical tank with a rotating drum inside. The drum is filled with the pulp and the contaminants are separated out by the centrifugal force created by the rotation of the drum.

The rejects are then collected in a separate container. The reject separator efficiently removes contaminants and offers a very clean substrate, also when the incoming material includes high amounts of plastics.