the apparatus of the invention shown in fig1 and 2 comprises a tubular body 1 having a centre and three circumferential strainer sections 2 forming a tubular strainer the tubular body 1 is rotatable about a horizontal axis in a container 3 which is provided with a coarse fraction outlet 4 at the bottom of the container 3 four distribution pipes 5 are connected to an inlet pipe 6 for a suspension to be fractionated each distribution pipe 5 is provided with three spray nozzles 7 adapted to spray the suspension in the form of jets 21 against the three strainer sections 2 respectively from above the strainer 1 inside the tubular body 1 there is a sloping chute 8 for receiving fine fraction and for conveying received fine fraction by gravity to a fine fraction outlet 9 situated outside the container 3 the tubular body 1 is journaled on the container 3 by means of a bearing 11 and rolls 12 and 13 a drive motor 14 is arranged to rotate the body 1 continously or intermittently within the body 1 there is a spray means 15 for cleaning the strainer sections 2 each strainer section 2 is composed by two interconnected truncated conical strainer elements 10 whereby the assembled strainer 1 has a form of a bellows as an alternative the strainer sections 2 may be exchanged by cylindrical strainer sections formed on a.