Patent ID: 7163604

Claim:
A process of producing a fiber suspension from waste paper containing coarse and fine impurities, comprising the steps of: a) filling a pulper with a batch of waste paper in water, the pulper being provided with an extraction plate and a rotating impeller co-operating with the extraction plate to create shearing forces on the paper; b) operating the pulper by rotating the impeller at different speeds during different periods of time to partly defibrate the batch of waste paper by the action of the rotating impeller, substantially without disintegrating coarse impurities into finer impurities, wherein the impeller is rotated at a speed that increases step by step during an initial operation stage, in which the defibration of the waste paper is started; c) operating the pulper so that not more than about 50% of the entire energy required for accomplishing complete defibration of the batch of waste paper is consumed during operation of the pulper; d) diluting the partly defibrated batch of waste paper; e) separating the partly defibrated batch of waste paper by means of the extraction plate into a coarse reject fraction containing coarse impurities and an initial fiber suspension containing some waste paper fragments and fine impurities; f) discharging the initial fiber suspension from the pulper; and g) pumping the initial fiber suspension through a rotary screen apparatus provided with a tubular fine screen and a pulper type impeller rotating coaxial with the tubular screen, but devoid of any impeller co-operant extraction plate, to defibrate waste paper fragments remaining in the initial fiber suspension by the action of the pulper type impeller and to separate the initial fiber suspension by means of the tubular fine screen into a fine reject fraction containing fine impurities and a complete fiber suspension substantially free from impurities.