Screen plates are mainly used for separation of comparatively coarse contaminants from pulp suspensions, at which the width of the slots is dimensioned such that the fibres may pass through the screen slots while the coarse contaminants are hindered. In order to reach a good separation efficiency the screen slots should have a width which is not larger than ca.0,1-0,3mm. In spite of the fact that a separate wood fibre, as for example soft wood fibres, may be up to 5mm long it may pass through such a comparatively small screen slot, which is due on one hand that the fibre is only 1540 um broad, on the other that it is also more or less flexible and may therefore be bent along the screen slot.

To coarse contaminants is also counted so called shives which consist of small bundles of fibres sticking together, which also may be up to 5mm long. Shives which are broader than the width of each screen slot is of course always separated by the screen plate. Also shives which are smaller than the screen slots are separated by the screen plate if the shives cross the screen slots, since the shives are stiff. One problem in this connection is however that comparati- vely small shives may pass through the screen slots if they are oriented along the same. Remaining shives in the screened pulp suspension create indications of fracture in the paper which is produced from the pulp suspension, which impairs the quality of the paper. Another problem is that thin flexible contaminants as plastic flakes and fragments of sheets may turn an edge against the screen plate and pass the screen slots in that way.

The quality of paper is also influenced by its fibre composition. Consequently a pulp suspension which is produced from softwood contains comparatively flexible fibres from early wood, which are produced during the spring season when the trees grow rapidly, and comparatively stiff late summer fibres, which are produced during summer and autumn season when the trees grow more slowly.

Early wood fibres are suitable to use for paper of the kind which shall have a tight strong structure and an even surface, while the late summer fibres are suitable for paper of the kind which shall have a comparatively high stiffness but do not have to meet high demands on tightness and strength . For example a card- board consisting of a three layer paper laminate could advantageously be made up of two outer, thin tight paper layers of early wood fibres with an intermediate thick paper layer of sparsely distributed late summer fibres. It should also be of advantage to be able to influence the properties of the paper by mixing early wood fibre and late summer wood fibre in desired shares. At the present time there is no simple and good method to achieve the necessary introductory sepa- ration of the fibres into early wood fibre and late summer wood fibre.

Paper pulp which is produced from waste paper may comprise fibres of different kinds, both from softwood tree and hard wood tree, and from both chemical and mechanical pulp (unless the waste paper consist of high quality office paper).

Most fibres from hard wood tree as for example beech, birch and some oak kinds are comparatively stiff and short which means that a large part of fibres from hard wood tree in the waste paper impairs the same as regards among all the tear resistance. In order to increase the quality of recycled paper it is therefore desired to lower the share of stiff and short fibres in the recycled paper.

A usual kind of screen plate is manufactured by milling straight parallel grooves in a plane plate with a thickness of ca. 6 mm. In each groove, where the plate consequently has a reduced wall thickness, screen slots are milled, at which the width of the screen slot is about 0.15-0.30 mm. In later time it has also been suggested that such screen slots may be formed by water cutting or laser cutting.

Accordingly WO 93/07334 describes water cutting of screen slots, at which

among all curved screen slots are suggested. Laser cutting of screen slots is also discussed in EP -B1-287 267, in which also curved screen slots are suggested.

An aim of the present invention is to supply a screen plate, by means of which separation of elongated particles, as fibres or shives, from the pulp suspension is considerably increased in comparison with known screen plates.

This aim is achieved by a screen plate of the kind mentioned by way of intro- duction, which is characterised in that each screen slot has a width which is maximally 0,8 mm and which is curved in such a way that a straight rod, which has a breadth which is less than the width of the screen slot and a length which is 5 mm, is hindered to pass through the screen slot, when the rod extends along the screen plate, independently how the rod is oriented in the relation to the screen slot.

Preferable the curved screen slots extend in parallel with each other. This brings about the advantage that the screen slots may be arranged comparatively close to each other without making the firmness of the screen plate less, which also increases the open area, that is to say the capacity of the screen plate is improved in comparison to a conventional screen plate with straight screen slots.

According to one embodiment of the screen plate according to the invention each screen slot forms a zigzag path and comprises a number of straight portions, at which adjacent straight portions in the zigzag path is connected with each other by way of curved portions of the screen slot. Alternatively each screen slot may have only one straight portion connected to a curved portion, or have two straight portions, which extend in different directions in relation to each other, at which two straight portions are connected with each other by way of a curved portion.

According to another preferred embodiment of the screen plate according to the invention each screen slot bends along its whole length. Suitably the screen slot

has a bending radius, which is less than 5 mm. With screen slots formed in this way it is when screening shives in many applications possible to design the screen slot with a width up to 0,8 mm, without impairing the efficiency of separation more than unimportantly. The width of the screen slot is suitably in the interval 0,1 - 0,8 mm, preferably in the interval 0,2 - 0,6 mm. In this way the open area of this screen plate may be increased further in comparison with a conventional screen plate.

In order that shives which are considerably shorter than 5 mm, for example fibres from hard wood, may be separated with the sufficient efficiency the said bending radius should be less than 4 mm, preferably less than 3 mm.

If said bending radius is less than 0,5 mm and each screen slot has a width which is in the interval 0,1-0,2 mm, the screen plate according to the invention may be used for separation of considerably all useful fibres from a pulp suspension of waste paper, filling agents, printer's ink and very short fibres pass through the screen slot. Alternatively a cleaned pulp suspension may be dewatered by such a screen plate.

Another aim with the present invention is to present a method to separate a pulp suspension in to a first fraction comprising comparatively stiff fibres and a second fraction comprising comparatively flexible fibres. This further aim is obtained in that pulp suspension is screened with a screen plate, which is provided with a number of at least partly curved screen slots, dimensioned in such a way that each relatively stiff fibre is hindered from passing any of these screen slots, when a stiff fibre extends along the screen plate, independently of how it is orientated in relation to any of the screen slots, while each flexible fibre has a possibility to bend along some of the screen slots and pass the same.

In a screen plate with screen slots which bend in wave form or sinus form, the wavelength is not exceeding 3 mm if the screen should be used for dewatering.

Should the screen plate be used for screening, that is separating shives and

other undesirable contaminants, the wave length is at the most 5 mm. When the desired use is fractionation of fibres the wave length has a maximum valve between 3-5 mm.

The invention is described further in the following with the reference to attached drawing in which figure 1 shows an enlarged part of a preferred embodiment of a screen plate according to the invention, figure 2 shows a section along line ll-ll in figure 1, figure 3 shows an enlarged part of another embodiment of the screen plate according to the invention, and figure 4 shows a section along a line IV-IV in figure 3.

Figures land 3 are drawn in a larger scale than figures 2 and 4.

In figure 1 there is shown a plane-parallel screen plate 1 according to the invention, which is 6 mm thick. On the inlet side of the screen plane 1 there is milled a great number of straight parallel grooves 2 with V-form in cross section.

Each groove has a width of 4-5 mm. In each groove a screen slot 3 is cut through the screen plate by way of laser cutting or water cutting. The screen slots 3 extend in wave form and parallel with each other along the grooves 2.

The bending radius of each slot is less than 5 mm and the width of the slots is at the most 0,8 mm. The cross section area of each screen slot 3 increases from the inlet side in order to hinder obstruction of the screen slot 3. Alternatively the cross section area may be constant along each screen slot 3.

In this case each screen slot 3 has a constant bending radius, but the bending radius may also vary along the screen slot 3, for example may each screen slot 3 bend in sinus form.

The values of the bending radius and the width of each screen slot is choosen in dependence of the result of the screening that is desired. For example the bending radius and the width of each screen slot 3 should be comparatively large if long shives of soft wood fibres should be separated or if the fibre suspension of soft wood should be separated in fractions of long late summer fibres and long flexible early wood fibres. If both long and short stiff fibres are to be separated from a fibre suspension comprising both fibres from soft wood tree and fibres from hard wood tree the bending radius and the width of each screen slot should be comparatively small. Should all fibres be separated from a pulp suspension, for example produced from waste paper, said bending radius should be less than 0,5 mm and the width of the screen slot should not be larger than 0,2 mm.

In figure 3 and 4 there is shown a variant of the screen plate according to the invention comprising a plane parallel screen plate 4, the inlet side of the which is provided with a great number of straight parallel grooves 5 with U-formed cross section. In each groove 5 there is a screen slot 6 cut through the screen plate 4.

The screen slots 6 form paths 7 in zigzag form, which extend in parallel with each other along the groove 5. Each screen slot 6 comprises a first number of straight parallel portions 8a and another number of straight parallel portions 8b, which extend in another direction than the first mentioned portions 8a. Adjacent straight portions of 8a and 8b are connected with each other by way of curved portions 9 of the screen slot 6. Each curved part 9 may have a very small bending radius, for example corresponding to the radius of the cutting laser beam or water beam. On the other hand it is so that the bending radius of each portion 9 must be much less than 5 mm in correspondence to what has been said above in rela- tion to the embodiment according to figure 1 and 2. Each straight portion 8a, and 8b must also be less than 5 mm long.

Apart from choosing suitable values of the bending radius of the curved portions 9 and widths in accordance with what is valid for the curved screen slot 3 in the screen plate 1 according to figure 1 and 2 suitable values for the lengths and widths of the straight portions of 8a, 8b must also be choosen in dependence of the screening results that is desired. For example each straight portion 8a, 8b, must be comparatively long and its width shall be relative large if long shives of fibres of soft wood tree should be separated or if a fibre suspension of soft wood tree should be separated into fractions of long stiff late summer fibres and long flexible early wood fibres. If both long and short stiff fibres should be separated from a pulp suspension containing both soft wood fibres and hard wood fibres each straight portion 8a, 8b should be comparatively short and its width compara- tively small. If all fibres should be separated from a pulp suspension for example produced from waste paper, each straight portion 8a, 8b should be less than 0,5 mm and its width should not be larger than 0.2 mm.

The screen plate according to the invention is of cause not limited to the embodi- ments shown in figures 1-4 but may be varied within the scope of the invention as the same is defined in the attached claims. Consequently the screen plate may be bent, preferably have a cylindrical shape if it should be used in a pressure screen which has rotating impulse creating means. The inlet side of the screen plate may have groves with either V-form or U-form in cross section, indepen- dently if the screen slots is of the kind shown in figure 1 or figure 3. Alternatively the inlet side of the screen plate may miss groves and consequently have a plane surface.