This invention is directed to a device for separating impurities, such as coarse particles like knots and other undefibered material. The device is designed for screening through gaps where the surfaces defining the gaps are movable relative to each other.

This type of screens, so-called gap-screens, usually is designed so as to have a cylindric housing with an inlet for the inject and outlets for reject and accept, respectively. In the housing, a stator and a rotor are located, which between themselves define one or more gaps, through which the pulp suspension is intended to pass. Coarse material, which cannot pass through the gaps, is separated from the pulp suspension in the form of reject.

A gap-screen comprising several gaps can be formed with concentric rings, every second one of which is stationary and every other second one rotary. The gaps then are located spaced differently from the centre of rotation. This implies that the flow conditions are different at different gaps. The number of gaps is restricted by the fact, that the diameter of the screen must be limited a.o. because of the circumferential speed.

If, instead, the gaps were located radially, i.e. the rings defining the gaps are located one after the other in axial direction, the diameter of the screen can be limited even with a greater number of gaps. Problems, however, arise at the mounting and dismounting of the rings defining the gaps, because these rings must be mounted and, respectively, dismounted individually one after the other, which is both complicated and tedious. It is, at the same time, difficult to maintain a definite gap width, because a wrong dimension of one ring can affect the size of all gaps.

The present invention solves the aforesaid problems and at the same time offers additional advantages at the screening of pulp suspensions. According to the invention, both

the stator and the rotor comprise a plurality of annular elements arranged in a mutually spaced relationship. The rotor is located radially outside the stator, so that the rotor elements are located directly in front of the spaces between the stator elements, and the axial length of the rotor elements is greater than the spaces between the stator elements and rotor elements.

Due to this design of the device, the stator and rotor can be mounted and dismounted separately as units. Furthermore, a definite gap width can be obtained for all gaps, because it can be defined by the radial distance between the annular elements of the stator and rotor.

The invention is described in greater detail in the following, with reference to the accompanying drawings showing two embodiments of the invention.

Fig. 1 is a cross-section of a device according to the invention.

Fig. 2 is an axial section through a portion of the device.

Fig. 3 shows a detail of the cross-section according to Fig. 1 on an enlarged scale. Fig. 4 is an axial section through a portion of a second embodiment of the invention. The screening device shown in Fig. 1 comprises a cylindric housing 1 with inject inlet 2 , reject outlet 3 and accept outlet 4. A stator 5 located in the housing consists of a number of annular stator elements 6 spaced radially equally from a centre line, but in a mutual axially spaced relation¬ ship. The cylindric outer surfaces of the elements 6 are located on the same radius, and the elements are held in place by a number of axially arranged stator holders 7.

A rotor 8 is located outside the stator and comprises a number of annular rotor elements 9, which are attached on rotor holders 10 arranged axially on the rotor. The rotor elements 9 have cylindric inner surfaces and are located directly in front of the spaces between the stator elements and to a certain extent overlap the outer surfaces of the stator elements 6 , so that a number of axial gaps 11 are formed

between the elements 6 and 9. The size of the gaps is determined by the radial distance between the outer and, respectively, inner surfaces of the elements 6 and 9. The gap width should be 1 - 12 mm, preferably 3 - 6 mm.

The axial length of the gaps which is determined by the overlapping of the rotor elements 9 over the stator elements 6 should be 1 - 15 mm, preferably 4 - 12 mm.

The inject inlet 2 preferably has spiral shape and is directed so that the inject is guided tangientially into the housing 1 without meeting the rotor 8 directly.

On every stator element 6 a number of cleansing members 12, preferably 2 - 4 members, are arranged and are formed as shoulders extending into the spaces between the rotor elements 9. The shoulders 12 should be limited in length ih the circumferential direction and have a bevelled front and rear edge.

The embodiments shown in Fig. 4 differs from the one described above merely in that the rotor holders 10 have been replaced by a perforated cylindric plate 13, to which the rotor elements 9 are attached.

When mounting the device, the stator 5 with elements 6 is inserted as one unit. Thereafter the rotor 8 with elements 9 is inserted as one unit. Finally, the cleansing members 12 are mounted on the stator elements 6 in the spaces between the rotor elements 9.

The screening is carried out from the outside inward in such a manner, that the pulp suspension is supplied tangentially to the cylindric housing 1 through the inlet 2. Large and heavy impurities are prevented by the centrifugal force from entering to the gaps 11 and, instead, are moved to the reject outlet 3. The suspension flows into the spaces between the rotor elements 9 and is there divided into accept and reject. The accept is discharged from the space inside the gaps through the accept outlet 4, and the reject is removed from the housing 1 through the reject outlet 3. The rotor holders 10 can also have a favourable effect on the separation process by throwing coarse and heavy particles outward from the gaps. At the embodiment equipped with a perforated plate 13 as

a holder of the rotor elements 9, the perforations are utilized for coarse separation.

The cleansing members 12 have the object partly to prevent fibre material from adhering in the gaps, and partly to bring about pulsations in the suspension adjacent the gaps and thereby to promote the screening process.

The screening device according to the invention has a simple and robust structure, where the screening elements can be exchanged quickly and easily. The screen has high capacity and efficiency and can advantageously be placed, for example, directly after a blow tank.

The rotation axle of the screen preferably is vertical or substantially vertical. The screen, alternatively, may have an inclined rotation axle, which even can be horizontal. The surfaces defining the gaps, i.e. the outer surfaces of the stator elements 6 and the inner surfaces of the rotor elements 9, preferably are cylindric, but may also be conical. These surfaces are smooth or can be provided with unevenesses in the gaps in order to additionally ensure that no material adheres.

The invention, of course, is not restricted to the embodiments shown, but can be varied within the scope of the invention idea.