Liquid purification or separation – Processes – Separating
Reexamination Certificate
1998-12-09
2001-09-04
Walker, W. L. (Department: 1723)
Liquid purification or separation
Processes
Separating
C210S498000, C210S499000, C210S402000, C210S403000, C162S052000, C162S251000
Reexamination Certificate
active
06284145
ABSTRACT:
The present invention relates to a screen plate for screening of a pulp suspension, comprising a number of elongated at least partly curved screen slots. The invention also relates to a method of separating a pulp suspension into a first fraction comprising comparatively stiff fibers and a second section comprising comparatively flexible fibers.
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 fibers 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, 1-0.3 mm. In spite of the fact that a separate wood fiber, as for example soft wood fibers, may be up to 5 mm long it may pass through such a comparatively small screen slot, which is due on one hand that the fiber is only 15-40 um broad, on the other that it is also more or less flexible and may therefore be bent along the screen slot.
Among coarse contaminants are also counted so called shives which consist of small bundles of fibers sticking together, which stuck-together fibers also may be up to 5 mm long. Shives which are broader than the width of each screen slot are 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 comparatively 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 fiber 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 fibers, which are produced during summer and autumn season when the trees grow more slowly. Early wood fiber are suitable to use for paper of the kind which shall have a tight strong structure and an even surface, while the late summer fibers 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 cardboard consisting of a three layer paper laminate could advantageously be made up of two outer, thin tight paper layers of early wood fibers with an intermediate thick paper layer of sparsely distributed late summer fibers. It should also be of advantage to be able to influence the properties of the paper by mixing early wood fiber and late summer wood fiber in desired shares. At the present time there is no simple and good method to achieve the necessary introductory separation of the fibers into early wood fibre and late summer wood fibre.
Paper pulp which is produced from waste paper may comprise fibers 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 fibers 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 fibers 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 fibers 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 introduction, 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 fibers 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 fibers from a pulp suspension of waste paper, filling agents, printer's ink and very short fibers 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 fibers and a second fraction comprising comparatively flexible fibers. 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 fiber extends along the screen plate, independently of how it is orientated in relation to any of the screen slots, while each flexible fiber h
Browdy and Neimark
GL&V/Celleco AB
Ocampo Marianne
Walker W. L.
LandOfFree
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