This invention ^' relates to a device for screening pulp suspensions in order to separate impurities and other pulp fractions unsuitable for the final product, such as coarse particles, undefibered material and poorly process- 5 ed fibres.

At the screening of pulp suspensions a high pulp concent ration, for example 3-5? ₅ is desired in order to achieve a high production capacity and to prevent unnecessarily high liquid transports in the screening system. High con-

10 centration, however, implies great difficulties in separ¬ ating the undesirable fractions from the pulp. The apert¬ ures in the screen. plate easily get clogged, and it is difficult to separate selectively the impurities at low reject withdrawal, ϊhese difficulties are primarily ^• a

15 result of the reject thickening , which takes place due to the fact that the liquid preferably follows along with the accept fraction through the screen plate. This problem is avoided in conventional screens in that the reject is diluted by the addition of further liquid. This is undes-

20 irable for other reasons, see above.

Various screen designs have been developed for solving the aforesaid problems.

One example is the arrangement of wing sections on a rotary member to be moved along the screen member and to 25 bring about instantaneous cleaning pulses and thereby to prevent clogging of the screen apertures. Such a design is shown in US-PS 4 328 C96. The problem of reject thicken¬ ing, however, is not solved, nor is such a device applic¬ able at high pulp concentrations.

*. i 30 in the EP patent application No 206 975. for example, a screening device is shown, comprising a screening cylinder and an inner rotor, which is provided with members to bring about pulsations in the pulp suspension.

These members have a cross leading edge and behind that a curved surface, the distance of which from the screen¬ ing cylinder increases successively. The leading edge prod¬ uces a positive pressure pulse, and the curved surface produces a negative pressure pulse, in order thereby to bring about a separation of impurities over the screen plate. At this design, however, there is a risk, that the pulp to too great an extent is transported about by the cross leading edge, whereby the relative speed between rotor and pulp decreases until the suction pulse ceases and the screening process stops. The screen gets blind, the effect decreases and the accept flow ceases. The cross leading edge, moreover, yields a short strong pressure pulse, which has a negative effect on the cleaning. A similar design is shown in US-PS 4 200 537. According to this publication, the rotor can be arranged so as to rotate in different directions. The embodiment shown in Fig. 3 corresponds to the aforesaid EP-publication and has the disadvantages reported. The embodiment according to Pig. 2 implies instead a sloping leading surface and a cross trailing edge of the pulsation members. This gives rise to problems with the thickening of the reject, as stated above.

The present invention offers a solution of the aforesaid problems. The device according to the invention is design¬ ed so as to render it possible to screen pulp effectively at high concentration. Accept and reject concentration. The effect consumption, furthermore, is low.

The characterizing features of the invention are apparent from the attached claims.

The invention is described in greater detail in the foll¬ owing, with reference to the accompanying drawings showing a preferred embodiment of the invention.

Fig. 1 shows a screening device according to the invention. Fig. 2 shows the same device according to section II-II in Fig. 1.

The device comprises an air-tight casing 1 with inlet 2 for the pulp suspension and outlets 3 and, respectively, 4 for accept and reject, respectively. In the casing 1 a cylindric screening member 5 is located, preferably with the axis of symmetry being vertical. The pulp inlet 2 communicates with the inside of the screening member at the upper end, while the reject outlet 4 communicates with the lower end of the screening member. The accept outlet 3 is connected to a space 6, which extends about the screening member 5. In connection to the upper portion of the casing 1 an outlet 7 for coarse reject (scrap) is located.

Within the screening member 5 an unperforated cylindric rotor 8 is located and extends along the entire screening member. The rotor 8 is concentric with the screening member, so that a screen chamber 9 is formed extending all about between rotor and screening member. The rotor

8 alternatively may be designed slightly conic, the great¬ est diameter being closest to the reject outlet. The rotor 8 is provided with at least two wing elements 10, which are secured on the rotor by means of support members 11, so that they are located in the screen chamber

9 spaced from the rotor 8 and screening member 5. The wing elements 10 are placed at a spaced relationship to each other and extend axially along the rotor. Their length in the circumferential direction yields a relation between this length and the radial dimension of the screen chamber 9 of between 2:1 and 6:1. At a rotor diam¬ eter of ca. 1 , the length of the wing elements in the circumferential direction can be, for example, 300-600 mm. The mutual distance between the wing elements can be 150-400 mm. The wing elements, furthermore, are to be placed so that their leading edges, seen in the rotation direction, are located at a greater radial distance from the rotor axle than their trailing edges, which distance

shall decrease continuously. The distance between the leading edge and screening member 5 should be 5-40 mm.

The wing elelemts 10 can extend axially along the entire rotor 8 or in axially defined zones. These zones prefer- ably are defined by partition walls extending all about, including recesses allowing axial passage of the pulp. The wing elements in the different zones are offset in relation to each other in the circumferential direction. The wing elements 10 can be designed so as to have axially straight leading and trailing edge or axially oblique leading and trailing edge.

The rotor 8 should also be provided with a bottom ring 12, which is located downwardly on the rotor to shield the reject outlet 4 in order to prevent short circuit between the inject and reject side. The bottom ring 12 thus defines the area accessible for. the reject flow by being formed as a wall with recesses 13. These recesses should be loc¬ ated in connection to the trailing edge of the wing elem¬ ent 10 located closest thereto. The recesses 13 _s further- more, shall be formed so as to prevent oblong impurities from adhering to the edges of the recesses, i.e. the trailing edge of the recesses must incline rearward in the rotation direction.

The pulp suspension is supplied via the inlet 2 to the screen chamber 9. In the screen chamber the pulp is moved axially to the reject outlet 4 while the rotor 8 with the wing elements 10 simultaneously causes the pulp to rotate. The accept thereby:"is caused to pass through the apertures in the screening member 5. Due to the form of the wing elements 10, a relatively long suction pulse affects the screening member 5 when the wing element 10 moves along the surface of the screening member.This implies that part of the liquid having passed out through the apertures in the screening member is sucked back into the screen chamber 9. Thereby the thickening of the reject is counter¬ acted, i.e. it is possible without supply of diluting liquid to limit the concentration in the reject.

Owing to the fact that the device allows the pulp suspens¬ ion to flow also beneath the wing elements 10, a favour¬ able activation of the suspension is obtained. At the same time as the space between the wing elements and screen- ing member increases along the wing elements, the distance between the wing elements and rotor decreases. Thereby pressure and speed variations favourable for screening are produced in the pulp suspension and thereby promote the separation of the pulp suspension in accept and reject. By dividing the wing elements 10 into several axially de¬ fined zones, the pressure and suction pulses can be distrib¬ uted over the screening member, so that the strains on the screening member are reduced. This can be suitable at large dimensions of the screening device. The object of placing the wing elements inclined is to reduce the risk of impurities to adhere on the leading edge. This risk, however, has not proved so great as to render it necassary to form the wing elements in this way. The bottom ring 12 has the object to prevent short circuit between the inject inlet 2 and reject outlet 4, i.e. to prevent the pulp suspension to pass partially untreated through the screen chamber 9. The location of the recesses 13 is chosen so that they are in the position where the reject is concentrated at maximum, which should be immed¬ iately after the suction pulses produced by the wing elements 10.

The screening member 5 should be formed with a screen plate, which has unevennesses, for example, grooves, on the inside in order to facilitate the separation of the accept. This is particularly advantageous at high pulp concentration.

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