SE505400C2 - Barrier for dewatering fiber suspensions - Google Patents

Abstract

The invention relates to a barrier for draining at the dewatering of fibrous suspensions. The barrier has the object to retain the fibers and to permit the filtrate to be discharged through openings in the barrier. The driving force for the dewatering can be either a mechanical pressure or a hydraulic pressure difference. The problem at dewatering is to retain the fibers and at the same time to facilitate the discharge of the dewatered filtrate from the boundary layer of the formed pulp cake. The device implies that the fiber side of the barrier (1) is provided with a pattern of narrow channels (2, 3) with a width of < 1 mm, preferably < 0.5 mm, which are placed so that the surface layer of the barrier has an openness of at least 20 %, preferably at least 40 %, of the boundary surface toward the fibrous suspension. The filtrate is drained via these channels to openings (4), which permit passage to the filtrate side of the barrier. The channels on the fiber side of the barrier preferably can be intersecting and extend in at least two directions, none of which need to be perpendicular to the direction of movement of the barrier.

Description

505 400 Drilled press plates normally have holes between 0.8 and 1.4 mm in diameter. Some short-fiber masses e.g. hardwood and retuber masses have average fiber lengths in the same order of magnitude as the diameter of the holes in this type of sheet. For this reason, retention is low. This is especially true when dewatering from low pulp concentrations in short press nips.

In order to remedy these problems, drills with drilled mantle plate have been lined with woven wire cloth, usually made of a synthetic material. This has had the intended effect from a drainage and retention point of view, but the life of the fabric has been too short. It often does not tolerate the harsh treatment in the press nip. It is mangled out, elongated, pulled obliquely, folds and breaks. It is also common for the wire cloth to become clogged with fine material and small urs from the fi bers suspension and after a period of operation become so dense that the drainage capacity decreases. The drainage capacity of the wire cloth decreases with increasing operating time also due to the fact that it is mechanically compressed by the press pressure and thereby becomes denser and more difficult for the filtrate to penetrate. In several industrial processes, wires made of synthetic materials are impossible to use due to the pH value and / or high temperature of the fiber suspension. In addition, it is often of great interest, especially in the manufacture of higher grades of pulp and paper, to avoid the use of synthetic materials in the process as much as possible, due to the risk that this foreign material will contaminate the final product.

The problems described here with conventional technology can be overcome by lowering the flow rate in the boundary layer between the fiber bed and the barrier, as well as by reducing the width of the openings in the surface of the barrier.

By means of the present invention, the desired speed reduction is obtained by milling channels on the surface of the barrier, so that the surface open for drainage towards the fiber suspension increases. These channels open into and drain into through-openings that lead through the thickness of the barrier to its outlet side. The openings may be back-drilled holes facing the front system of drainage channels or cutouts in the form of channels on the back of the barrier plate 505 400 which meet the channels from the fiber side so that openings for the filtrate are obtained through the barrier.

The more detailed features of the invention appear from the claims.

Examples of known barriers intended for use in dewatering and screening fiber suspensions are described in U.S. Patent No. 4,259,136 and in European Application No. 82103801.5 U.S. Application discloses a method of constructing a barrier of rods between which parallel gaps are created in one direction. This method gives smaller openings in the interface, but also a very small open area and thereby high flow rates and poor drainage of the surface layer of the drained fiber cake. The method is therefore not useful for short intensive press nips.

The European application describes a method of manufacturing a screen plate for screening fiber suspensions by providing the surface of a plate with parallel slits and providing the other side with depressions to such a depth that continuous openings are formed. The invention is characterized in that each depression is so large that it covers many slits. A barrier of this type has the same disadvantage of small open area as that described in the US patent above. In addition, a barrier of this type does not have sufficient mechanical strength to be used for dewatering fiber suspensions by means of high mechanical pressures.

The invention will be described in more detail below with 6 embodiments illustrated in Figures 1, 2, 3, 4, 5 and 6.

The first four of its embodiments involve sawing one surface of a sheet with a pattern of fine grooves. These tracks can have different depths. The tracks run in at least two directions, so that a regular system of nodes between the tracks is formed. The surface now has a dense channel system and a high proportion of open area towards the fiber suspension to be drained. The narrow grooves thus cover a large part of the surface of the plate.

Preferably between 30 and 70%. Figure 1 shows a variant of a barrier according to the invention. A plate 1 has been provided in two directions with intersecting channels 2, 3 in the surface against which the bersus suspension is to be dewatered. Holes 4 have been drilled from the filtrate side of the plate to every other of these crossings to such a depth that continuous openings are formed. The holes can be straight or conical.

Figure 2 shows a variant of a barrier according to the invention with greater openness in the surface compared with the variant in figure 1. The barrier plate is in some parts similar to that shown in figure 1, but has been provided with additional channels 5, 6 in two directions. This new duct system is shallower than the previous one and is drained in the deeper ducts 7, 8 which in turn are drained in the holes 4 drilled from the filtrate side.

Figure 3 shows a variant of the barrier in Figure 2 where the shallower channels 9, 10 have been laid diagonally towards the deeper channels 11, 12 to further increase the open surface and facilitate the drainage.

An alternative to draining the fine channel system on the bare side of the barrier using drilled holes is to mill grooves from the back (filtrate side) of the barrier plate to a depth where these channels meet the deepest channels on the fiber side. Figure 4 shows a variant of this type of barrier with intersecting channels of different depths where the shallower channels 13, 14 and the deeper channels 15, 16 have been laid at a 45 degree angle to the coarser grooves 17 of the filtrate side.

Figures 5a and 5b show variants of drainage barriers made of rods 18 instead of a plate. Between these rods which are joined by welding or other technology, continuous gaps 19 are formed in which the channels 20, 21, 22 machined on the fiber side open. The machined channels can be made in one direction, figure 5b, or several directions figure Sa.

Figure 6 shows a simpler form of barrier where the holes 23 are drilled completely through the plate.

The channels 24, 25 increase the openness of the surface layer towards the fiber bed to be drained and thereby the drainage is improved. However, the pierced holes mean that the barrier provides lower fiber retention than the variants shown in Figures 1 - 5.

Si ses 400 For all the variants of barriers for dewatering fi carrier suspensions described above, the channels on the fiber side of the barrier can be designed at such an angle to the direction of movement that a continuous bearing surface is obtained against a line-shaped load, e.g. a shaver with the task of scraping off the dewatered erk berkakan.

Claims (10)

505,400 Patent claims A barrier (1) whose function is to separate the fibers from the filtrate when dewatering network-forming suspensions, e.g. pulp, characterized in that the surface against the fiber suspension is provided with a pattern of narrow channels (2,3) <1 mm wide, preferably <0.5 mm wide, which gives an openness in the surface layer which occupies at least 20%, preferably at least 40% % of the barrier interface with the fiber suspension and that the filtrate is drained in whole or in part via these channels towards the filtrate side of the barrier by means of through-openings (4) opening into the filtrate side. A barrier according to claim 1, characterized in that the surface towards the ers bers suspension is provided with a pattern of intersecting narrow channels (2,3) in at least two directions. Barrier according to claim 2, characterized in that the channels on the upper side (5-8) have different depths and that only the deeper channels (7,8,11,12) meet the openings (4) from the back of the barrier and form passages for flow. of filtrate. Barrier according to claim 2, characterized in that the channels are drained through the barrier via openings, preferably holes (4) drilled from the filtrate side to such a depth that they penetrate the bottom of the channels and create openings through the barrier to the filtrate side. Barrier according to claim 4, characterized in that the emptying holes (4) on the back of the barrier are drilled towards the points where channels (3,4) meet. Barrier according to claim 4, characterized in that the emptying holes on the back of the barrier are drilled towards the points where the deepest channels (7,8,11,12) meet. Barrier according to claim 3, characterized in that the channel side channel system (13,14,15,16) is drained via channels (17) arranged on the filtrate side in the relative channel pattern on the fiber side in a substantially diagonal direction, the deepest channels (15,16 ) on the fiber side, the emptying channels (17) on the filtrate side break through and form openings through the barrier. 505 400 Barrier according to claim 2 with channels in two directions, characterized in that the channels are directed 30 - 60 degrees to the direction of movement of the barrier so that a movable line of contact across the barrier surface always rests against the land surfaces between the channels. Barrier according to claim 1, built up of longitudinal rods (18) separated by narrow gaps (19), characterized in that the surface of the rods against the fiber suspension is provided with narrow channels (20, 21, 22) which are drained to the gaps (19). between the rods (18). 10. A barrier according to claim 9, characterized in that the rods (18) have such a cross section that the gaps (19) formed between the rods widen towards the filtrate side.