WO2021234211A1

WO2021234211A1 - A screen plate, a segment of a screen plate and an apparatus for screening a suspension

Info

Publication number: WO2021234211A1
Application number: PCT/FI2021/000006
Authority: WO
Inventors: Jouko Hautala; Heikki AHLSTEDT
Original assignee: Pr Pulping Oy
Priority date: 2020-05-19
Filing date: 2021-05-17
Publication date: 2021-11-25
Also published as: FI20207090A1

Abstract

The present invention is directed to a screen plate (106, 301, 400, 600, 700), a segment for a screen plate and an apparatus for screening a suspension, wherein the screening is carried out using a screen plate (106, 301, 400, 600, 700) having grooves (202, 302, 402) and a plurality of openings (203, 403) extending through said screen plate. The openings (203, 403) are located inside the grooves (202, 302, 402).

Description

A SCREEN PLATE, A SEGMENT OF A SCREEN PLATE AND AN APPARATUS FOR SCREENING A SUSPENSION

Background

This invention relates to a segment of a screen plate, a screen plate, and an apparatus for screening a suspension.

A screening process is an important step in many industrial processes. Screening is a process where impurities are separated from the wanted material. For example, in pulp and paper production pulp screening is a key process, used to enhance the quality of a wide range of pulp and paper products. In the screening process the pulp suspension, fed to the screening apparatus, is divided to an accept flow and to a reject flow. The accept flow advances further in the process. The reject flow will be retreated so that it can usually be further purified, refined, or totally removed from the process.

Screening apparatus, like pulpers used in pulp screening, can have a screen plate against which a wing of a rotating rotor pushes the material to be screened. The screen plate has holes through which the accept flow is driven. The material that is not able to penetrate the screen plate is directed to the reject flow. Patent application US 2008/0053874 shows an example of a pulper and a screen plate used for screening fiber suspension.

The prior art solutions have multiple problems. These problems include accumulation of reject material to the screen plate, variation of the screening efficiency in different parts of the screen plate, difficulty of maintenance of the screen plate as well as the rotor cutting the impurity object into smaller pieces allowing them to pass through the screen plate to the accept flow. In addition, the prior solutions also have a low refining consistency after the screen plate, requiring additional steps to increase the refining consistency. Such additional steps increase the complexity and cost, both capital as well as the operational cost. The present invention is directed to solve these prior art problems.

Summary

This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

An apparatus with a screen plate, a screen plate and a segment of a screen plate is disclosed. In some embodiments these are used for screening a suspension of fibrous material, preferably pulp or recycled fiber in a pulper. In some embodiments the apparatus is rotor unit of a pulper with rotating wings.

The screen plate or the segment of the screen plate comprises first set of grooves and a plurality of openings, which could be slots, extending through the screen plate. In some embodiments the openings are located at the grooves. In some embodiments the first set of grooves are extending to a first direction and the screen plate comprises a second set of grooves extending in a second direction, the second direction being different from the first direction.

In some embodiments the segments are connected either directly or via an intermediate part to form a closed elliptical, preferably circular, screen plate.

In some embodiments the first direction to which the first set of grooves is extending is such that, when installed under wings of a rotor, the angle between the normal of grooves in the first set of the grooves and the normal of the leading edge of the wings is between -45 and +45 degrees, preferably between -15 and +15 degrees.

In some embodiments at least some of the grooves are wider on the upside of the screen plate and narrower on the side of the openings. Furthermore, at least one side surface of the grooves could be beveled. In some embodiments this beveled side surface could be the side surface facing a leading edge of the wing of the rotor.

Short description of the figures:

Fig. 1 : An embodiment of an apparatus according the invention

Fig. 2a: An embodiment of a screen plate segment according the invention

Fig. 2b: A diagram of an embodiment of a screen plate segment according the invention

Fig. 3 : An embodiment of a screen plate according the invention Fig. 4: A detail of a cross section of a screen plate according the invention

Fig. 5: An embodiment of a screen plate according the invention

Fig. 6: An embodiment of a screen plate according the invention

Fig. 7: An embodiment of a screen plate according the invention

Fig. 8a: A detail of a cross section of a screen plate according the invention

Fig. 8b: A wired plate, from which a screen plate according some embodiment of the invention can be cut

Detailed Description

Figure 1 shows one embodiment of a screening apparatus, preferably for screening a suspension with fibrous material in it. The suspension could contain for example pulp content, paperboards, cartons, fiberboards, packing material or bio waste. The apparatus could be used for example in a pulper or more specifically in a rotor unit (100) for a pulper. In some embodiments the pulper is a tank-type open top pulper, in some other embodiments the pulper could be a closed vessel. In some embodiments the pulper comprises a circular planar screen plate.

The rotor unit (100) in Fig 1. comprises a body (103), a rotor (104), a bearing (105) and a screen plate (106) according some embodiments of the invention. The rotor unit (100) is in one embodiment installed connecting outer flanges (101) to the flanges (102) on the body (120) of the vessel inside which the rotor unit (100) is operating. The rotor unit (100) could also be installed so that it is connected to the inner flanges (107). In some embodiments the screen plate is attached to the inner flanges (107) and/or to the outer flanges (101) with connecting means, e.g. with screws and bolts.

The rotor (104) is joined to the axle (109) protruding from the bearing (105). The rotor (104) comprises at least two wings (110). The rotor (104) is rotating and the at least two wings (110) rotate with it. The rotating rotor (104) and the at least two wings (110) are configured to slush a suspension in a rotating action and to pump the suspension.

The rotating action causes the at least two wings (110) to mix the suspension. At the same time, the wings (110) push the suspension against the screen plate (106). The material in the suspension is disintegrated for example by being crushed against the screen plate (106) or by the turbulence caused by the rotating wings (104). The suspension is pressed e.g. by the pressure caused by the wings (110) or the pressure difference between the different sides of the screen plate against the screen plate (106). A part of the material in the suspension will flow through the small openings in the screen plate (106) to an accept space (114) located under the screen plate (106). The directions “above” and “under” as well as terms “before” and “after” and other similar terms in context of the screen plate are meant to be understood in reference to the direction of the suspension flow. I.e. terms “above”, “upside” and “before” are referring to the side of the screen plate (106) before performing the screening action and terms “under”, “downside” and “after” are referring to the accept space (114) side of the screen plate (106). Using a pulper as an exemplary embodiment, this means that even if the pulper is placed horizontally and the direction of the material flow is from left to right through the screen plate, term “under” would refer to the right side of the screen plate as that would be “downstream” according the direction of the material flow. In an extreme example the configuration of an apparatus could be such that the direction of the material flow would be from lower elevation to a higher elevation. In such a case the term “under” would still refer to “downstream” direction of the material flow, even if the direction of the flow would be “up” from the viewpoint of an external observer.

The openings on the screen plate (106) can take different forms. In some embodiments of the inventions the opening could be circular or elliptical by shape and in other embodiments those openings could be formed as slots. The shape of the slots could be for example rectangular or semi rectangular.

From the accept space (114) the accept flow is pumped through the outlet (115) for further processing.

The drive (not shown) provides a rotating movement that is transferred by the axle (109) to the rotor (104). The axle (109) is supported by the bearing unit (105), which in turn is installed to the body flange (117) using the bottom flange (116).

In some embodiments the screen plate can be a circular plate made for example from a single metal plate for example by drilling, carving, engraving, or cutting openings to it. In other embodiments the whole circular plate could be cut out of a bigger block of metal plate with already premade grooves and openings. In further embodiments the screen plate is made from parallel wires attached to support beams.

In one embodiment of the invention the screen plate is constructed of individual segments of screen plates, which form a complete screen plate when combined. Figs. 2a, 5 and 6 show exemplary embodiments of such segments and screen plates.

In Fig. 2a a segment (201a) of a screen plate according one embodiment of the invention is shown. The segment (201a) has parallel grooves (202) and slots or openings (203), through which the accepted material flow can flow into the accept space. In some embodiments the openings are also arranged in parallel lines and in some embodiments, like the one shown in Fig 2a, the openings (203) can be located inside the grooves (202). In some embodiments the grooves are continuous grooves across the whole segment or across the whole screen plate and in some embodiments the grooves consist of shorter sections of grooves that do not reach across the whole segment.

Fig. 2a also shows two other segments (201b, 201c), which could be connected to the first segment (201a), either by an intermediate device (204), like welding joint, or via connecting means (205) constructed into the segments themselves. These connecting means (205) could be e.g. holes allowing fasteners to connect the two segments together.

In some embodiments of the invention several segments can be joined together to produce a complete screen plate. This screen plate could be of a circular shape, but in some embodiments the shape could also be of another elliptical shape or even a non-elliptical shape, e.g. rectangular or non- symmetrical shape, if needed. See Fig. 6 for an example of a non-circular screen plate. Thus, even if in this application a term segment is used, it is not used to limit the invention only to screen plates of a circular shape. As stated above, in some embodiments the segments can be modular parts of a non-circular screen plate.

A screen plate constructed of several individual segments has many benefits over a screen plate made of a single block. As the screen consists of multiple smaller and lighter pieces, it is easier to transport than a single larger and heavier screen plate, made of single block. Maintenance of a modular screen plate is also easier and more economical than with a single plate as it is possible to remove an individual segment and perform work on it rather than needing to remove the complete single screen plate. Fig. 2a also shows a silhouette of a wing (230) of a rotor, which as described above in context of Fig. 1 is rotating above the screen plate (201a) and pushing the suspension against it. The wing (230) of the rotor has a leading edge (231) and it is rotating in a clockwise direction as indicated by arrows (240). In some other embodiments the rotating direction could be counter cl ockwi se .

Geometry of the embodiment shown in Fig. 2a is discussed in detail with reference to Fig 2b. The first arrows (232) in Fig. 2b indicate the normals of the leading edge (231) of the wing (230) of the rotor. Openings (203), located within grooves (202) and their normals, indicated by second arrows (233), are also shown. Angles al and a2 indicate angles between the normals (232) of the leading edge (231) of the wing (230) of the rotor and the normals (233) of the openings (202). In this application the angle between the normal of the leading edge of the wing of the rotor and the normal of the openings, or grooves, if the openings are located inside the grooves, is referred as an angle of attack.

In embodiments where the angle of attack (al, a2) is small, the leading edge (231) of the rotor is typically more effective in removing any unwanted buildup or material blocking the openings than in embodiments where the angle of attack (al, a2) is larger. In some embodiments the angle of attack (al, a2) for a substantial part of the leading edge of the wing located above the screen plate is between —45 and +45 degrees, preferably between —15 and +15 degrees. Depending on the shape of the wing (230) in some embodiments the angle of attack (al, a2) is constant over a substantial part of the leading edge (231), i.e. the angles al and a2 are substantially the same. In other embodiments the angle of attack can vary along the leading edge (231), i.e., the angles al and a2 are different.

In embodiments where the openings (203) are located in the grooves (202), the angle of attack can also be measured between the grooves (202), i.e. between the normal of the grooves (202) and the leading edge of the wing (231), i.e. normal of the wing (231).

The geometry of the embodiment explained above could also be explained using the concept of a tangent, i.e. a straight line that touches the point in a leading edge of the rotor but does not, even if extended, intersect the line representing the leading edge. Due the laws of trigonometry, the angle between the tangent of the leading edge of the rotor and the direction of the grooves is same as the angle of attack explained above. Thus, in some embodiments the angle tangent of the leading edge of the wing and the grooves is between -45 and +45 degrees, preferably between —15 and +15 degrees.

Figure 3 shows another embodiment of the screen plate (301) according the invention. The screen plate (301) has parallel grooves (302) as well as slots (303a, 303b, 303c). The grooves (302) and thus also slots (303a, 303b, 303c), which in some embodiments can be located inside the grooves (302), can be in different angle than the ones described in context of the embodiment shown in Fig. 2a above. The shape of the wing (230) rotating in a direction shown by the arrows (240) and the direction of the grooves (302) are such that the angle of attack is close to zero degrees along the entire length of the leading edge (231) of the wing. I.e. the direction of a tangent of the leading edge (231) is similar to the grooves (302), thus maximizing the length of interaction of the leading edge (231) with each groove (302).

As mentioned above, in some embodiments of the invention the openings could have different forms. In Fig. 3 some forms of different openings are shown. Some openings are slots with rounded ends (303a), some are elliptical (303b) or even circular (303c), i.e. round holes. In some embodiments the round holes with a diameter less than 4 mm are used.

In some embodiments all the openings in a segment are of the same shape, in some embodiments a screen plate or a segment of a screen plate can have openings of variable shapes (303a, 303b, 303c), as in Fig 3.

In some embodiments the width of the openings is such that the solid impurities, present in the suspension, are not able to penetrate the screen plate into the accept space through the openings. In embodiments where recycled paper is processed in a pulper, the width of the openings could for example be smaller than the diameter of staples used in the recycled paper material. For example, if a common house/office staple size 26/6 with wire diameter of 0.405 mm is abundant it the recycled material, the slot width could be set to 0.4 mm, which would prevent stable wires reaching the accept space. In another embodiment the width of the openings is less than the diameter of the wire used for baling recycled paper into bales, e.g. the width of the openings can be less than 3 mm if a wire with a diameter of 3 mm is used. As it is possible that parts of the wire can accidentally end up in the pulper with the recycled paper, having openings with a width less than the diameter of the wire would prevent wires or pieces of wire from passing the screen plate.

Having an opening or groove with a width smaller than the size of the impurities also limits impurities getting stuck in the screen plate. Impurities stuck in the opening could easily be cut to smaller pieces by the rotating wings of the rotor. When the impurities are not stuck in the slots or in the grooves, the leading edge of the rotating wing is more likely to push them into the reject area, rather than just cutting them into smaller pieces.

Another advantage of using small openings is that the refining consistency of the suspension after the screen plate is higher compared to configurations using larger openings in the screen plate. In some embodiments of the invention the refining consistency of the pulp before the screen plate is 7-3%, preferably 6-4% and after the screen plate 6.5-2.5%, preferably 5.8-3.8%. In one embodiment the refining consistencies of the suspension before the screen plate and after the screen plate are substantially the same, preferably the consistency of the suspension that has passed through the screen plate is at least 80% of the consistency of the suspension before passing through the screen plate. For example, the consistency before the screen plate could be 5% and 4.5-4.8% after the screen plate. Thus, compared to the solutions using larger openings, using smaller openings results in higher consistency, preferably so high that it can directly be used for grinding and also in lower levels of impurities. Because of these effects additional steps and equipment for purification and for increasing the consistency can be omitted, making the process simpler and more economical.

Fig. 4 shows a cross section of a screen plate (400) according an embodiment of the invention. A leading edge (231) of a wing of a rotor and its rotation direction (240) are indicated in the figure. As discussed above, in some embodiments the body (401) of the screen plate (400) can be made from solid metal and it can have grooves (402) and openings (403) drilled, carved, engraved or cut into it. In some embodiments the openings (403) are located in the grooves, for example at the bottom of at least some or most of the grooves (402). In some embodiments a profile of one side surface of the grooves is different from the other side surface. For example, a side surface (404) of a groove (402) facing the leading edge (231) of the wing of the rotor could be beveled or sloped. More generally, the grooves (402) could be wider at the top of the groove, facing the leading edge (231) of the wing of the rotor, and narrower at the bottom, facing the opening or the accept space. In Fig. 4 the angle of the slope is determined by the ratio of the width (B) of the slope to the height (C) of the slope. The width of the slope (B) would at maximum be equal to the distance (E) between two openings.

In some embodiments the interaction between the beveled side surface and the leading edge (231) of the rotor generates a swirl, or a vortex, inside and/or above the groove. This swirl is indicated by a circular arrow (406) in Fig. 4. Some of the suspension rotating in the swirl will return to the space where the rotor of the wing is moving and some of the suspension rotating in the swirl will flow downstream to the openings (403). This rotating swirl is helping in keeping the material from accumulating to the grooves and by doing it, it prevents the grooves and/or openings from being blocked.

In some embodiments the grooves narrow at the bottom of groove to the width that is equal to the width of the openings, marked as A in the Fig. 4. In some embodiments the width of the openings, e.g. slots, or the diameter in case of circular holes, is 0.05-4.0 mm, and the width of the groove at the top, A+B in Fig. 4, is 2- 20 mm, preferably3 — 12 mm.

In some embodiments the openings are wider at the downside of the screen plate, or segment, and narrow towards the upside. This narrowing is indicated by the dashing lines (405) in Fig. 4. In Fig. 4 the width of the openings on the downside of the screen plate is marked by (D). Such narrowing shape can be produced e.g. when the openings are water cut to the screen plate block.

Fig 5. Shows an embodiment of a screen plate (500) according the invention. The screen plate (500) is a circular plate, which has been constructed e.g. by joining or combining separate segments (501, 502, 503) together. In Fig. 5 one of the segments (501) is separated from the other segments for demonstration purposes.

In some embodiments, like the one shown in Fig 5, all or most of the screen plate segments are identical. This makes them more economical to manufacture. If further customization of the screen plate configuration in desired, all or most of the segments could be different from each other.

In some embodiments the shape of the leading edge of the rotor and the directions of the grooves are configured so that the identical segment pieces can be used to form a screen plate so that the angle of attack is same for each segment. Even if the segments are identical, the angle of attack between the grooves and the leading edge of the wing of the rotor could however be same for every segment because in order to create a circular screen plate, each segment and thus the grooves in them would have a different orientation. As explained above, this angle of attack is preferably small, e.g. between — 15 and +15 degrees. This offers a better screening efficiency compared to the situation where the screen plate is made of a single block and the direction of the grooves is constant over the entire screen plate.

The Fig. 5 also shows outer holes (510) in the outer rim segments and smaller inner holes (511) on the inner rim of the segments. The holes (510, 511) can be used to attach the segments with connecting means to another part of the apparatus, e.g. to the body of the rotor. In some embodiments the segment is attached to the outer flange (101) and to the inner flange (107) of the rotor unit (100) as explained in connection to Fig. 1.

As explained above, in some embodiments the screen plate could be of a non-elliptical shape. Fig. 6 shows a screen plate (600) having a form of a twelve-sided polygon. A segment (601) is separated from the screen plate (600) to demonstrate the form of a segment.

Compared to a solution where the direction of the grooves is the same over the entire plane of the screen, a solution, where the direction of the grooves varies in different areas of the screen plate, would allow a better screening efficiency, assuming that the directions of the grooves have been optimized. This is because by varying the directions of the grooves or the openings so that the angle of attack in which the leading edge of a wing of a rotor is moving in relation to the slots, allows an optimized angle of attack across the entire screen plate.

In some embodiments, like the one shown in Fig. 7, the screen plate (700) could be made of a single plate, typically made of metal, into which the groove directions for different areas (701a, 701b, 701c) are for example drilled, carved, engraved or cut, e.g. by water jet cutting. In some embodiments these different areas are arranged as segments (701a, 701b, 701c) over the screen plate (700).

All the segments (701a, 701b, 701c) of the screen plate (700) are identical, but as they form a circular screen plate (700), the directions of the grooves in each segment are different in comparison to the direction of the slots of the adjacent segments. However, as this change corresponds the change of the placement of the segment in comparison to the center of the screen plate (700) and the rotational axis, the angle of attack between the wing (230) of the rotor and the normal of the grooves is identical for each segment (701a, 701b, 701c). In the embodiment shown in the Fig. 7 the angle of attack is close to zero, i.e. the interaction between the leading edge of the wing of the rotor and the grooves is maximized.

In some other embodiments, like the one shown in Figs. 8a and 8b, the screen plate or segments of a screen plate are constructed by installing wires (801) close to each other and joining them together with a support structure (802). The support structure (802) could for example consist of multiple metal beams to which the wires are attached e.g. by welding. Wires could be shaped to optimize the widths of the grooves and the slot as well as the slope of the edges of the grooves. In Fig. 8a, the width of the slot is marked by A, width of the wire on the side of the rotor is marked by B, width of the groove is A + B, the depth of the groove is marked by C and the width of the wire on the side of support structure is marked by E. The angle of the slope of the wire is determined by the ratio of lengths C and B. In some embodiments like the one shown in Fig. 8a, the shape of the wires is such that the top of the slot is wider than the bottom of the slot, marked as D in Fig 8a, i.e. the width A is narrower than width D. In some embodiments this widening is achieved by having the width of the wire on the side of the rotor, i.e. B, larger than the width of the wire on the side of the support structure, i.e. E.

In some embodiments the width of the slot (A) is between 0.05 mm and 3.0 mm, preferably 0.1-1.0 mm, and the width of the groove at the top (A+ B) is between 2 mm and 15mm, preferably 2-10 mm.

In some embodiments the segments of a screen plate are cut from the wired plate shown in Fig. 8b. One embodiment of a screen plate comprising individual sectors cut out of such wired plate in the screen plate (600) is shown in Fig. 6.

The foregoing description has provided by way of non-limiting examples of particular implementations and embodiments of the invention a full and informative description of the best mode presently contemplated by the inventors for carrying out the invention. It is however clear to a person skilled in the art that the invention is not restricted to details of the embodiments presented above, but that it can be implemented in other embodiments using equivalent means without deviating from the characteristics of the invention. Furthermore, some of the features of the above-disclosed embodiments of this invention may be used to advantage without the corresponding use of other features. As such, the foregoing description should be considered as merely illustrative of the principles of the present invention, and not in limitation thereof Hence, the scope of the invention is only restricted by the appended patent claims.

Claims

1. A screen plate for screening a suspension, the screen plate comprises: first set of grooves and a plurality of openings extending through said screen plate, wherein the openings are located at the grooves characterized in that, at least some of the grooves comprise at least one beveled side surface arranged to face a leading edge of a wing of a rotor when attached to a screening apparatus.

2. The screen plate according to the claim 1 characterized in that, the first set of grooves are extending in a first direction so that, when installed under a wing of a rotor an angle between a normal of grooves in the first set of the grooves and a normal of a leading edge of the wing is between -45 and +45 degrees, preferably between -15 and +15 degrees.

3. The screen plate according to any of the claims 1 to 2 characterized in that, the screen plate comprises a plurality of segments connected together either directly or via an intermediate part.

4. The screen plate according to any of the claims 1 to 3 characterized in that, the first set of grooves are extending in a first direction and the screen plate comprises a second set of grooves extending in a second direction, the second direction being different from the first direction.

5. The screen plate according to any of the claims 1 to 4 characterized in that, at least some of said grooves are wider on the upside of the screen plate and narrower on the side of the openings.

6. The screen plate according to any of the claims 1 to 5 characterized in that, the openings are slots.

7. A segment of a screen plate for screening a suspension, the segment comprises: first set of grooves and a plurality of openings extending through said segment, wherein the openings are located at the grooves characterized in that, at least some of the grooves comprise at least one beveled side surface arranged to face a leading edge of a wing of a rotor when attached to a screening apparatus.

8. The segment according to the claim 7 characterized in that, the first set of grooves are extending in a first direction so that, when installed under a wing of a rotor an angle between a normal of grooves in the first set of the grooves and a normal of a leading edge of the wing is between -45 and +45 degrees, preferably between -15 and +15 degrees.

9. The segment according to the claim 7 or 8 characterized in that, the openings are narrower on the upside of the screen plate and wider on the downside of the screen plate.

10. The segment according to any of the claims 7 to 9 characterized in that, the segment is arranged to be connected, either directly or via an intermediate part to at least one another segment.

11. The segment according to any of the claims 7 to 10 characterized in that, at least some of said grooves are wider on the upside of the segment and narrower on the side of the openings.

12. The segment according to any of the claims 7 to 11 characterized in that, the openings are slots.

13. An apparatus for screening a suspension, the apparatus comprises: a rotor comprising a plurality of rotating wings; a screen plate; the screen plate having a first set of grooves and the screen plate having a plurality of openings extending through said screen plate wherein the openings are at the grooves characterized in that, at least some of the grooves comprise at least one beveled side surface facing a leading edge of a wing of a rotor.

14. The apparatus according to the claim 13 characterized in that, the first set of grooves are extending in a first direction so that, when installed under the wings of the rotor an angle between a normal of grooves in the first set of the grooves and a normal of a leading edge of the wings is between -45 and +45 degrees, preferably between -15 and +15 degrees.

15. The apparatus according to the claims 13 or 14 characterized in that, the apparatus is a rotor unit for a pulper arranged to screen a suspension with fibrous material, preferably pulp or recycled fiber.

16. The apparatus according to any the claims 13 to 15 characterized in that, the screen plate comprises plurality of segments connected together either directly or via an intermediate part.

17. The apparatus according to any of the claims 13 to 16 characterized in that, the first set of grooves are extending in a first direction and the screen plate comprises a second set of the grooves extending in a second direction, the second direction being different from the first direction.

18. The apparatus according to any of the claims 13 to 17 characterized in that, at least some of said grooves are wider on the upside of the screen plate and narrower on the side of the openings.

19. The apparatus according to any of the claims 13 to 18 characterized in that, the openings are slots.