a process is provided for the removal from paper pulp of impurities , the pulp being contained in a pulping machine . some of the pulp is periodically transferred into an enclosed chamber annexed to the pulping machine , the chamber having a volume which is small compared to the volume of the pulping machine . the pulp in the chamber is stirred and separated by a rotor , and fibre suspension is returned to the pulping machine , dilution water being introduced into this chamber . the chamber is emptied and impurities thereafter discharged .