EP0034780B1 - Rotating sorter - Google Patents

Abstract

The disclosure concerns a rotary screening machine for moving pulp suspension through a screening slot and for separating impurities from the pulp suspension. An annular rotatable drum is surrounded by an annular porous screen basket that is spaced away to define a screening slot. Pulp suspension is delivered to one axial end of the slot and is discharged from the other axial end of the slot. Generally wedged shaped vanes, widening in the axial direction between the leading and trailing ends of the vane, are provided around the drum. The wedge defining surfaces may also be inclined with respect to a respective plane perpendicular to the axis. The space between the wedge surfaces is an open volume. A wall in that space is oriented to help create appropriate vacuum and eddy conditions behind the vane. The drum may have different zones, with the lower zone being provided with vanes and the upper zone being generally smooth. Other types of vanes or projections may be provided in the intermediate zone. Water sprayed into the slot dilutes the suspension. The configuration of the vane causes conveyance of the pulp suspension generally toward the impurities outlet and conveys the impurities to that outlet. The configuration of the vanes also causes the suspension to pulsate across the screen basket which dislodges pulp suspension fibers that otherwise might become matted on the screen.

Description

The invention relates to a rotary sorter for fiber suspensions contaminated with rejects, according to the preamble of patent claim 1.

Such a rotary sorter has become known from DE-B 2 850 385. In this rotary sorter, sorting bars which are inclined against the direction of rotation are attached to a rotating drum and rotate with a narrow gap relative to a screen basket. These strips are essentially perpendicular to the drum surface and develop conveyor components in the direction of the rejects outlet. However, these sorting strips do not achieve any other effects that are essential for a good sorting effect, i.e. Pass usable fibers through the perforation of the screen basket.

The sorter according to DE-B 2 712 715 also takes advantage of the effect that the underpressure arising at the rear edge in the flow direction (drain edge) of the projections contributes to the fact that fiber pieces and similarly shaped components of the suspension to be sorted pass through the holes of the Sieve can pass through to be fed to the accept material outlet. However, the negative pressure that arises at this tear-off edge is not very great. In addition, with this sorter, since the drum or the projections attached to it hardly contribute to conveying the suspension through the sorter in the axial direction, separate pressure generation of the fiber suspension by means of a pump or the like must be ensured.

The object of the invention is now to bring about a good separation effect of a sorter with as little power as possible.

According to the invention, this object is achieved by the characterizing features of patent claim 1.

This design of the sorting vanes, which create an overpressure or a relatively large negative pressure at the screen gap, means that when the vanes pass the screen basket, the pulp suspension enriched with rejects, which may be adhering there, is repeatedly released from the screen basket, whereby this effect is still caused by the rear end of the wing emerging vertebrae can be reinforced. This gives the parts to be sorted out, the fibers in the fiber suspension, the opportunity to reach holes in the screen basket, to pass through them and in this way to be sorted out of the suspension.

The wing profile (wedge shape) also has only a low drag coefficient, so that the acceleration of the suspension, especially in the circumferential direction, is very small, so that the sorter also works relatively poorly for this reason.

This design also provides a simple design or manufacturing option for the sorter, in that the screen gap to be maintained between the radially outer edge of the wing and the screen basket can be produced relatively easily by simply turning off the wings on a lathe. Tensioning on the wings can also be easily avoided by slightly chamfering the edge of the wing (in the direction of rotation) at the back.

The design and arrangement of the sorting vanes according to the invention is also advantageous insofar as the disadvantageous effect of pulsations is largely eliminated by the relatively large number of vanes, since they have only a small amplitude.

A preferred further development of the inventive concept lies in the twisting of the wedge surfaces of the sorting vanes in order to produce additional conveying impulse components of the fiber suspension in relation to the screen basket.

Preferred further embodiments result from patent claims 3 to 5 and 7 to 9 and 13.

• Fig. 1 eine erste Ausführungsform eines erfindungsgemässen Rotationssortierers in offener (druckloser) Bauweise im Axialschnitt;
• Fig. 1a einen Teilaxialschnitt durch die Sortiertrommel in vergrössertem Massstab;
• Fig. 2 und 3 gleiche Darstellungen weiterer Ausführungsformen wie in Fig. 1;
• Fig. 4 einen Teil-Radialschnitt durch die Sortiertrommel in vergrössertem Massstab;
• Fig. 5 einen Teil-Axialschnitt durch die Sortiertrommel in vergrössertem Massstab;
• Fig. 6 einen ähnlichen Schnitt wie Fig. 5 mit einer anderen Ausführungsform der Sortierflügel;
• Fig. 7a bis 7c eine wieder andere Flügelausführung;
• Fig. eine Sortiererbauart mit anderer Trommelausführung.

The invention will now be explained in more detail with reference to some exemplary embodiments of the same, which are shown in the figures; show:

• 1 shows a first embodiment of a rotary sorter according to the invention in an open (unpressurized) design in axial section;
• 1a shows a partial axial section through the sorting drum on an enlarged scale;
• FIGS. 2 and 3 are the same representations of further embodiments as in FIG. 1;
• 4 shows a partial radial section through the sorting drum on an enlarged scale;
• 5 shows a partial axial section through the sorting drum on an enlarged scale;
• FIG. 6 shows a section similar to FIG. 5 with another embodiment of the sorting wing;
• 7a to 7c another wing design;
• Fig. A sorter type with a different drum design.

1, a rotary sorter according to the invention consists of an outer housing 20, a drum 1 mounted therein, which rotates within a sieve basket 2 with a constant gap and has end walls 41 and 42 at its axial ends. In this exemplary embodiment, the supply of the fibrous suspension takes place from below through the nozzle 6 directly into the sieving space 3 existing between the drum 1 and the sieve basket 2, where the suspension of the rotating vanes 4 which are fastened to the drum 1 on the jacket thereof, is detected and conveyed along the screen space in the axial direction to the reject outlet located at its other end. The accepted material passes through the holes of the screen basket 2, is dammed up to a certain height and is drawn off radially through an accepted material outlet 17. The rejects come out tangentially through a trough 12 from the sorter at the upper end.

The rotor, consisting of the drum 1 equipped with the vanes 4 and 40 and the shaft 39 carrying it, is flying in the lower part the sorter housing 20 stored in bearings, not shown. The drive takes place here, as shown, by means of belts 38 via drive pulleys 36 and 37, the pulley 36 on the shaft z. B. an electric motor (not shown here) can be attached.

It can be seen from the illustration that the sorting wings 4 are wedge-shaped, with an upper wedge surface 10 and a lower wedge surface 9, between which, when the drum rotates, starting from the front edge 11 of the wings, a negative pressure builds up towards the trailing end of the wings can. At the end of the wing, practically at the tear-off edge, vortices are created which, just like the negative pressure generated by the wing when the wing passes the screen basket, detach the fiber mat attached to it from the screen and thereby prevent it from clogging. The suspension is mixed again and again.

The sorting vanes 4 and the vanes 40 are arranged approximately helically, but the sorting vanes 4 only to the extent that they do not lead directly to an excessive axial conveying component of the fiber suspension along the screen space 3 to the reject outlet 12. In practice, this means that the wings 4 are mutually offset in such a way that the lower wedge surfaces 9 also hit part of the suspension when the wings rotate. In this embodiment it is indicated that the lower wedge surface 9 of the wings 4 is twisted, i.e. with increasing distance from the leading edge 11 the wing is bent downwards, i.e. that this surface 9 of the wings leads to a component of motion of the particles hitting them, which goes towards the sieve. As a result, the particles initially removed from the sieve by the vacuum generated are conveyed back against the sieve, so that there is the possibility that particles of accepted material can be sorted through the sieve holes. Of course, there is also the possibility of designing the upper wedge surface 10 of the wings in the same way (see FIG. 1a).

In this way, the two essential processes of the sorting process in this screen space zone, namely the detachment of the fiber suspension from the screen caused by the negative pressure and the advancement of the fiber particles to the screen, continuously alternate with one another, i.e. there is a pulsating movement of the fiber suspension in the screen space 3, in the lower zone where the sorting vanes 4 are located on the drum 1.

It is also shown the dirt ring 15 which is formed in the uppermost zone of the screen space 3 and which should consist predominantly of rejects.

A different design of the lower wedge surfaces of the sorting vanes of at least a certain area of the surface of the rotor drum now prevents, in particular, a rapid passage of the suspension directly on the surface of the rotor drum, in particular the suspension enriched with rejects, and stagnation directly on the screen basket the flow and possibly clogging with the associated poorer sorting effect.

The various areas, of which there are practically three, can be seen on the jacket of the rotor drum 1. The first area is the lowest and is equipped with sorting wings 4. This is followed by an area of the rotor drum in which sorting vanes, here designated 40, are also arranged, but which, in relation to the other sorting vanes 4 present in the lower drum region, can either have an inclination of the lower wedge surface 9 such that their conveying effect contrary to the direction of flow of the reject material to the reject material discharge is lower than in the other wings or, as can be seen from FIG. 1, this lower wedge surface 9, starting from the drum surface, is narrower than the upper wedge surface of the sorting wings 4 or 40. It is thereby achieved that a stronger braking of the material is produced directly on the drum surface as it passes through to the rejects outlet than on the sieve surface, as a result of which, due to the better conveying effect, no jam can form on the sieve basket 2 or, on the other hand, a better conveying effect in the direction to the reject outlet 12 results.

In addition, in the exemplary embodiment, these sorting vanes 40 are designed differently from the others in that their lower wedge surface 9, like the upper (10), is also non-twisted.

The third area of the rotor drum is now smooth, i.e. designed essentially without sorting wings 4 or 40 and is predominantly above the screening zone, i.e. of the sieve chamber 3. This is a necessary measure to allow the necessary thickening of the rejects (indicated at 15). In this way, reject consistencies of 20 to 25% and more are achieved at the reject outlet. For better discharge, the rotor drum has scraper blades 45 at its upper end.

Spray bores 8, which are arranged in the vicinity of this smooth drum area, are practically still below the highest edge of the last of the sorting vanes 40. This prevents excessive dilution of the rejects and also increases the friction of the rejects. The remaining spray bores 8 for dilution water lie in a central region of the rotor drum, which is opposite the screen basket 2 without any sorting holes or slots. This ensures that with relatively little spray water, good sieve clearance is achieved by diluting the suspension. It is also recommended that the area of the screen basket 2 below these spray bores with a relatively large screen hole, for example 6 mm in diameter, be compared to the upper part with a diameter of about 4 mm. The spray water is supplied via the connection head 14 into the cavity 44 of the at least partially provided double jacket of the rotor drum. A bearing 46 and a seal 47 are also provided there.

The inclination of the upper wedge surfaces 10 of the sorting vanes will be carried out at approximately 10 °, the inclination of the lower wedge surface 9, on the other hand, only at approximately 5 ° and with an even weaker inclination in certain circumstances with regard to the sorting vanes 40 adjacent to the smooth region 30 of the rotor drum, if one not narrower. The width difference between the upper and the lower wedge surface of the sorting wings 40 will then be chosen to be at most approximately half the width of the screen space 3. You can e.g. B. be about 20 mm when the screen space is about 50 mm wide.

It can be seen from the illustration of yet, that the lower drum portion with normal sorting wings about half, and that the uppermost smooth drum portion about _^1/3 of the Siebkorbhöhe is, could these, however, also be smaller, for example up to a quarter of Siebkorbhöhe run.

It has also proven to be expedient to prevent the suspension in the sieve chamber 3 from rotating excessively by attaching ram bars 28 running approximately parallel to the drum axis of rotation on the inside of the sieve basket. Only about four pieces are required. In this way a better separation effect is achieved.

8 shows a construction with a different zone division. The rotor or the screen space 3 is formed with three zones in this embodiment. In the lowest zone, the drum carries the sorting vanes 4. In a zone adjoining this, the drum carries vanes 5, which are mainly used to transport the rejects towards the outlet end of the sorter, and in the third zone 30 the rotor 1 is smooth, that is to say without projections and trained similar. In this part, the sorting residue is to be stowed so that it does not pass through the rotor too quickly with too little separation effect of the accepted material. There is also a spray water supply with a spray water connection head 14 in the interior of the drum 1, so that by means of spray water through spray openings 8, which are predominantly arranged in the central part of the rotor, but which extend into the second zone, even as far as the third Zone can extend, the suspension can be diluted again to allow the separation of the accepted material even in the upper area of the sorter.

This above-described open (unpressurized) sorting type is particularly suitable as a final stage sorter for the various sorting residues, including waste paper, collected in a paper mill, a high consistency of rejects being achievable in particular with the embodiment according to FIG. 1.

In the radial section through the drum shown in FIG. 4, it can also be seen that the edge 11 of the wing lying in the circumferential direction is chamfered somewhat to the rear in order to avoid fabric spinning on the wing. A dashed line 19 can also be seen, which extends between the upper and lower surfaces 9; namely, it is designed in such a way that the free space 7 between the surfaces mentioned increases continuously from the tip to the rear end of the wings. By arranging this wall according to requirements, i.e. the angle at which this wall runs in relation to the tangent to the drum shell, one can control the generation of the negative pressure and thus the mode of action of the blades. Another control option is provided by the inclination of the rear edge of said surfaces 9 and 10 with respect to the horizontal, i.e. through their twisting. Of course, this depends on the extent to which the helical arrangement of the vanes has been provided in such a way that they produce a more or less strong conveying component in the direction of the reject outlet.

It is, of course, important and advantageous in the case of a sorter designed in an open design according to FIG. 1, which is also referred to in the trade as pressure-free, the conveying component of the sorting wings in the direction of the rejects outlet, since this also serves to pass the accepted material.

The parameters to be taken into account here, which on the one hand promote the pulp suspension, in particular the rejects in the axial direction along the screen space towards the reject outlet, and on the other hand the movement of the pulp suspension, which lead to a good sorting out of the accept, i.e. H. of the fibers to be recycled influence, adapt to each other. It is possible by simple tests to determine the best arrangement for the existing stock suspension and sorting type.

It should also be mentioned that the partition 19 between the upper and lower surfaces of the wings 4 also contributes to the stiffening of the wings, so that the wall thickness of the wings can be chosen to be smaller. Expediently, the interior of the wings formed by the intermediate wall 19 is closed off by means of a wall extension 19a at the end of the latter.

It should also be pointed out that the screen space 3 can be made relatively narrow in the sorter according to the invention, so that a relatively low conveying capacity is required. A narrow, annular sieve space 3 is also inexpensive because essentially the sorting takes place only in the immediate vicinity of the sieve and therefore an unnecessarily large sieve space, i.e. with a large radial extension, can only be disadvantageous.

It is not shown in this figure that de-stripping can be brought about by strips running essentially vertically and fastened to the inside wall of the screen by passing them close to the screen (or the bars) brushing wings defibrate the specks in the fiber suspension.

The wings 4 will be arranged in principle, but particularly in the case of the sorters in an open design, so that when the drum rotates, the wings cover the entire height range of the first zone of the screening area provided with the sorting wings 4; however, these can also be provided in a denser drum arrangement.

In Fig. 5, in which a partial axial section through the drum is shown with another embodiment of the wing 4, it can be seen that in this case the upper wedge surface 10 and the lower wedge surface 9 of the wing opposite to the previously described example of the Wing leading edge 11 starting increasingly against the horizontal, specifically inclined in the radial direction, ie are twisted in such a way that they produce a conveying component of the fiber suspension away from the sieve. With this wing shape, the effect that leads to the detachment of the fibrous suspension from the sieve is of course brought about to an increased extent, so that this wing configuration is predominantly used in the case of closed types of the sorter, i.e. pressure sorters, in particular from the inside out, i.e. from the sieve chamber 3, sorting of the accepted material and corresponding feeding of the fiber suspension from the outside into the sieve chamber 3. The drum will then be run without zoning, i.e. similar to the exemplary embodiment according to FIG. 2. The conveying capacity for the material through the sorter in these so-called pressure sorters is predominantly applied by the conveying pump for the fiber suspension.

Finally, it is also possible to combine the two previously described inclinations of the surfaces in the radial direction, as shown in FIG. 6. In this case, a vortex is generated to an increased extent on the rear edge of the wings 4, which means that both the fiber suspension on the sieve basket is loosened and that the particles to be sorted are given the opportunity, in particular, to pass through the holes in the sieve.

The statements made in FIG. 5 also apply here with regard to the type of supply of the fiber suspension; here too, the use is predominantly for the closed construction with the drum design according to FIG. 2, likewise without dividing it into zones.

The closed construction is obtained from the embodiment according to FIG. 2 by closing the housing with a base plate and additionally arranging a nozzle in the lower part of the housing corresponding to the nozzle 17 provided for the waste material outlet.

If one builds such a sorter with passage of the reject material, as far as its axial movement component is concerned, from top to bottom, then one can advantageously dilute the sorting residue that accumulates below in the screen space 3, which would already be relatively thick, by spray water emerging from spray bores in the drum jacket and thus achieve a further sorting of good material. The spray water is then fed into an interior of the drum via a spray water connection head which seals against the stationary housing of the sorter, generally a housing cover, and the hollow drum shaft or a shaft-like extension of the drum, similar to FIGS. 1 to 3.

2 shows a rotary sorter designed in an open design, in which the drum 1 or the screen space 3 is likewise not divided into zones, but in which the drum carries the sorting vanes 4 continuously. However, it is provided that spray water is supplied through spray water bores via a spray head 14, as in the exemplary embodiment according to FIG. 1. The inlet for the fiber suspension is in this case at the top of the nozzle 6 and the accept material outlet at the bottom of the nozzle 17, i.e. the fiber suspension, in particular the rejects, is transported through the sorter in the axial direction from top to bottom, so that gravity favors the passage of the heavy rejects. In order to prevent the pulp suspension from passing too quickly in this case, the sorter in the sense of FIG. 3 can be designed with a baffle wall 23 which arises from the fact that the lower part 1a of the drum is designed with a larger diameter than the upper part 1b the drum. An overflow is also provided for the upper drum part 1b, in that the cleaned suspension is dammed up by a baffle 22. The accepted substance then emerges through a nozzle 18 behind the first zone. However, as already explained in relation to FIG. 1, a mutual offset of the sorting vanes can also be made in such a way that the upper wedge surfaces of the sorting vanes hit the pulp suspension to a large extent and thus continuously impart a movement component to the particles in the direction of the inlet. For this purpose, the inclination of the upper wedge surfaces can be chosen to be relatively steep, steeper than that of the lower ones, because the influence of gravity also contributes to the promotion of the fiber suspension by the sorter.

Another embodiment is also possible as a pressure sorter (in a closed design), but with the sorting of the accepted material (fibers) taking place radially inwards into the screen space 3 and the corresponding feeding of the fiber suspension from the outside radially outside the screen basket.

For this purpose, the wings according to FIGS. 7a and 7b or 7c are designed in such a way that the wall 19, which extends between the upper 10 and lower wedge surface 9, is arranged such that the free space between the surfaces mentioned extends to the screen gap down, from the front edge 11 to the rear end of the sorting wing 4 steadily reduced, so that in front of the wings above and below and in particular next to a pressure builds up in the wings and thus pulsates through the sieve basket 2 in the selected direction of flow of the fibers (see arrows), causing detachment of the solid particles accumulating and accumulating on them and thus keeping the sieve basket free. For this type of sorter, twisting of the wedge surfaces 9 and 10 of the sorting vanes according to FIG. 5 is particularly suitable, as shown in FIG. 7b, since here the comparatively largest lateral surface, ie the surface of the lateral boundary wall 19, which produces the pressure pulses on the screen basket is present, but the embodiment with untwisted wedge surfaces according to FIG. 7c can also be used well. 1a is also possible here, but certainly not as good as in FIG. 7b.

Since pressure sorters are used more for fine sorting and are generally arranged directly in front of the headbox to the paper machine, the aim is to have the narrowest possible gap at the end of the sorting wing "a" in FIG. 7a, which should be about 0.5 mm maximum. However, it is dependent on the sieve hole width, and consequently on the “degree of fineness” of the sorting level, i. H. on the other hand also the fiber suspension itself.

The inclination of the boundary wall 19 in relation to the circumference of the screen basket will be chosen to be approximately 10 °.

The described embodiments of the rotary sorter according to the invention achieve an extremely favorable mode of operation of a sorter by preventing the formation and fixing of fiber mats on the screen basket by the pulsating movement of the material on the screen basket. A clogging of the sieve holes, starting from an attachment of material particles at one point, namely from the side of the holes facing the direction of rotation, is progressively prevented by the addition of more and more particles that are stuck there, thereby preventing the pulsating Movement the direction of flow of the particles to the holes is always quite different. As a result, it is also possible that very long fibers of the accepted material can pass through the sieve holes to an increased extent, since the otherwise existing tendency of these fibers to align in the circumferential direction, which would largely prevent passage through the sieve holes, is avoided in this case. Due to the low resistance coefficient of the sorting wing and, in the case of the open design, the good conveying effect in a relatively narrow screen area, only a low drive power is required for conveying the fiber suspension.

Claims (13)

1. A rotary screening machine for removing impurities from a pulp suspension from a fibre suspension, comprising a screening chamber (4) formed by a drum (1) rotating within a screen basket (2), at a constant distance therefrom and about a substantially vertical axis coaxial with the screen basket (2) the outer periphery of the drum being provided with vanelike sorting protrusions (4) in staggered formation characterised by that the sorting protrusions (4) are formed as wedges, in a manner that in a view of their radial longitudinal direction with respect to the radial direction of the drum surface and all sections in that longitudinal direction the protrusions are tapering in the direction of rotation of the drum.

2. A rotary screening machine according claim 1, characterised by that the (upper 10 and/or lower 9), wedgelike tapering surfaces of the sorting protrusions are twisted for generating additional velocity components of the fibre suspension in relation to the screen basket (2) (Fig. 1a).

3. A rotary screening machine according claim 1 or 2, characterised by a pressure-pulse controlling, with respect to the radial direction outer boundary wall (19) on each sorting protrusion (4), extending between the upper (10) and lower (9) wedgelike tapering surface thereof in substantially vertical direction.

4. A rotary screening machine according claim 1 to 3, characterised by that the inclination of the wedgelike tapering surfaces (9, 10) in selected in a manner that the surfaces (9, 10) generate components of transport both towards the outlet end of the screening chamber (3) for the impurities and in the opposite direction.

5. A rotary screening machine according to one of the claims 2 to 4, characterised by that a least one of the wedgelike tapering surfaces is twisted to generate a component of transport in direction radially outward towards the screening basket (2) and increasing constantly by increasing distance from the front edge (11) of the screening protrusions (4), the screen operating as an open unit and the fibre suspension being introduced from the outside into the screening chamber (3).

6. A rotary screening machine according to one of the claims 2 to 5 characterised by that at least those vanes (40) positioned adjacent the outlet (12) for the impurities have an inclination of the lower wedge surface in circumferential direction with regard of the rest of the vanes (4) to generate a minor component of transport towards the outlet (12) for the impurities which is opposed to the direction of transport of the suspension through the screening chamber (3), wherein the apparatus is designed as an open construction having transport of the impurities from the lower to the upper end of the screen (2) through the screening space (3).

7. A rotary screening machine according to one of the claims 1 to 5 characterised by that at least those vanes (40) positioned adjacent the outlet (12) for the are provided with a lower wedge surface (9) which is narrower than the upper one (10) keeping clear a space with respect to the screen (2) which is broader there than in the region of the upper wedge surface (10).

8. A rotary screening machine according to claim 6 or 7 characterised by that the rotor drum (1) is provided with at least two zones the one of which is comprising sorting vanes (4) and the other (30), positioned adjacent the outlet (12) for the impurities is formed with a smooth surface, being free of vanes (4), wherein the section with the smooth surface (30) is extending beyond the range of screening.

9. A rotary screening machine according to claim 8, characterised by that the vanes (40) positioned adjacent the smooth surface (30) of the drum (1) having different configuration with respect to the rest of the vanes are arranged in a thread line and extend at least once around the circumference of the drum (1).

10. A rotary screening machine according to claim 8 or 9, characterised by that in the middle region of the rotor drum (1) along a certain axial distance the screen (2) has a configuration void of sorting perforations and that in this region the rotor drum (1) is provided with slushing holes (8) at its circumferencial surface.

11. A rotary screening machine according to claim 1 to 5, characterised by that, admittance of suspension from below and throughflow of the rejects fraction upwards provided, the screening chamber (3) is subdivided at least into two zones, the drum (1) in the lower one being provided with sorting vanes (4) and in the upper one with wings (5) in form of helical ribs for transport of the rejects fraction.

12. A rotary screening machine according to claim 11, characterised by that the screening chamber (3) is subdivided into three zones, the drum (1) at its end which is adjacent the rejects outlet (12) having a configuration void of vanes or other flow obstacles.

13. A rotary screening machine according to claim 3, characterised by that, admittance of suspension from the outside into the screening chamber (3) and corresponding screening of the accepts fraction radially outward from the screening space (3) provided, the wedge surfaces (9, 10) form a constant gap radially outward with respect to the screen (2) and the boundary wall (19) being arranged deviating from the circumferential direction of the drum (1) in a manner to form a cavity between itself, the screen (2) and the upper (10) and lower (9) wedge surface, whose radial and axial length is increasing from the front edge (11) of the sorting vanes (4) to their trailing edge (Fig. 4).

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