EP0280094A2 - Filter segment for a filter for thickening a fibrous suspension - Google Patents

Abstract

Sector-shaped sieve segment for disc filters or folded sieve thickeners for thickening fiber suspensions, with a frame (18), two sieve carriers (16) and a sieve (10) resting on the outside of the latter, the frame and sieve carrier forming a coherent casting to simplify and reduce the cost of manufacture , in which the sieve (10) was embedded during casting.

Description

The invention relates to a sector-shaped screen segment for rotating disc filters or folded screen thickeners for thickening fiber suspensions.

In the production of wood pulp, pulp and the processing of waste paper and other fibrous materials, the treatment of a fiber suspension is often carried out at consistencies between approx. 0.3% and approx. 3%. The further processing of the raw materials by grinding or bleaching and the intermediate storage of the fiber material in laid paper require a higher concentration, which can be achieved economically with rotating disc filters or folding screen thickeners. In both cases, a roller rotating about a horizontal axis is immersed in a trough into which the fiber suspension to be thickened is continuously introduced. The roller, which is covered with a screen fabric on the outside, has an accordion-like shape in order to increase the screen area. In a known disc filter, such as that offered by the company Hedemora Pulp Machinery, the roller is composed of a plurality of disc-shaped units which are mounted on a common, rotatably mounted and driven support tube and each consist of a plurality of sector-shaped screen segments adjoining one another along radii, which are covered on the outside with the sieve fabric which, together with sieve carriers that define a cavity, define the end faces of the filter discs.

When the disc filter immersed in the fiber suspension rotates slowly, water from the fiber suspension passes through the sieve into the interior of the rotating roller, a non-woven fabric being formed on the outside of the sieve, which enhances the filter effect. The arrangement is such that a hydrostatic pressure difference results between the level of the fiber suspension in the trough and the level of the water flowing out of the inside of the roll through the support tube, which causes a continuous filtration. In order to remove the fiber layer that builds up on the sieve during the passage of the fiber suspension held in the trough and is getting thicker and thicker, rake-like scrapers are arranged on the trough above the fiber suspension to be filtered, which form a kind of slide on which the detached from the filter disks Slide layers of fiber to a location outside the trough. Under certain circumstances, the detachment of the fiber layers from the sieve is facilitated by providing a spray tube above the disc filter, with the aid of which the fiber layers are sprayed off from the sieve and thereby detached.

In the known disk filter described above, the sieve segments have a type of frame with two lateral limbs running in the radial direction and one outer limb in the circumferential direction, and the sieve carriers arranged at a distance from one another in the axial direction consist of a welded wire mesh on which a plastic sieve fabric rests. This forms a filter bag with a zipper, which has been shrunk onto the frame and the filter holder.

In known folded screen thickeners, the folds running in the circumferential direction are less deep than in the case of a disc filter.

In another known disc filter, the sieve carriers consist of perforated plates provided with holes of relatively large diameter.

The known sector-shaped sieve segments are disadvantageous in several respects: On the one hand, the support structures for the sieve fabric, consisting of frames and sieve carriers, are relatively complex to manufacture and therefore lead to relatively high costs; in addition, these sieve constructions are not particularly stable - you have to keep in mind that the filter discs have a diameter of the order of 2 m - and even a relatively small side runout of the rotating filter discs leads to a lateral run-up of the sieve fabric against the fiber layer of the roller detaching scraper, spray pipes and the like, which is why in the known disc filter described, the filter discs are guided laterally in the detaching area for the fiber layer. Finally, relative movements between the sieve fabric and the sieve carriers supporting it lead to signs of wear on the sieve and thus to a reduction in its service life.

The object of the invention was to create a sector-shaped sieve segment (in the side view the known sieve segments described have approximately the shape of a circular sector with a cut-off tip) for rotating disc filters or folded sieve thickeners for thickening fiber suspensions, which can be produced more easily and therefore more cheaply, especially when it comes to fastening the sieve.

Starting from a sieve segment with a frame which has a radially outer leg and a lateral leg on each of its two sides, in particular in the radial direction, to which two sieve carriers are fastened, which are arranged at a distance from one another in the axial direction and which are secured together with a on the sieve lying on them form the two main surfaces of the sieve segment, this object can be achieved according to the invention in that at least the frame legs are formed as cast parts in which the sieve is embedded, and in a preferred embodiment it is provided that the sieve is also made up of the components of a cast part forming filter holder is embedded. Manufacturing is particularly easy when the filter holders are cast onto the frame legs.

Just as with the known sieve segments, the sieve fabric also forms the lateral or frontal main surfaces of the sieve segment in the construction according to the invention, and not only when the sieve simply rests on the sieve carriers, but also in the preferred embodiment, in which the Sieve is also embedded in the sieve carrier designed as castings, provided that the embedding is carried out so that the material of the sieve carrier does not protrude beyond the outside of the sieve fabric. No special steps are required for attaching the screen, since it is automatically attached to these parts of the screen segment when the frame or the screen support is cast. Two separate sieve blanks can be used for the two main surfaces, but it is of course also possible to insert a filter bag into the mold, as in the known disc filter described, so that embedding takes place when the frame and / or the sieve carrier is poured.

In principle, one could manufacture the complete frame of a sieve segment and its two sieve carriers as a one-piece casting; However, in order to be able to use simpler and therefore cheaper molds, an embodiment is recommended in which the frame legs are divided along a central plane perpendicular to the axis of rotation of the filter disc and the frame leg halves form a one-piece, half-shell-like cast part together with the associated portafilter of the relevant segment half that a sector-shaped cut of the sieve is embedded. The sieve segment halves produced in this way can then be glued, welded, screwed or simply attached to one another in some other way, screw connection being preferred because a damaged or worn sieve segment or sieve segment half can then be replaced particularly easily and quickly, as a result of which the disk filter is idle or folded screen thickener can be shortened.

The cast parts could be metal die cast or injection molded parts. The sieve segments according to the invention become particularly cheap and light when the cast part is designed as a plastic injection molded part.

Any suitable material can be used for the sieve, but a fabric made of stainless steel wires or plastic threads or wires is particularly recommended.

Also for the sieve segment according to the invention, it has proven to be advantageous to design each sieve carrier as a grid formed by rods and to arrange the rods in particular so that they run radially and in the direction of rotation, the rods running in the direction of rotation then being opposite the rods running in the radial direction are set back in the direction of the segment interior and the sieve is only embedded in the radially extending rods, since the sieve fabric surfaces then become particularly dimensionally stable due to the small distance between the supports supporting the sieve.

It is particularly important to emphasize the fact that the embedding of the sieve in the sieve carrier and / or the frame prevents relative movements between the sieve and the parts carrying it, especially when the sieve is embedded in the material of the sieve carrier, so that the inventive Sieve segments have a significantly longer service life than the sieve segments known from the prior art. In this connection it should also be pointed out that it is of course not necessary for the sieve to be embedded in the cast material over the entire sieve thickness; it would suffice, for example, that in the case of a screen fabric, the wire or thread sheets facing the screen supports or frame legs are embedded in the cast material, even if embodiments are preferred in which the screen is embedded to such a depth in the cast material that the cast material is flush with the outside of the sieve.

• Figur 1 eine Stirnseitenansicht (in axialer Richtung) eines erfindungsgemässen Siebsegments für ein Scheibenfilter;
• Figur 2 einen Schnitt durch das Scheibenfilter-Sieb­segment nach der Linie 2-2 in Figur 1;
• Figur 3 einen Schnitt durch einen radial verlaufenden Stab eines der beiden Siebträger samt einge­bettetem Sieb entsprechend der Linie 3-3 in Figur 1;
• Figur 4 eine der Figur 1 entsprechende Stirnseitenan­sicht eines erfindungsgemässen Siebsegments für einen Faltensiebeindicker, und
• Figur 5 einen Schnitt durch dieses Siebsegment ent­sprechend der Linie 5-5 in Figur 4.

Further features, advantages and details of the invention result from the following description and the attached drawing of two embodiments of the sieve segment according to the invention; show in the drawing:

• 1 shows an end view (in the axial direction) of a sieve segment according to the invention for a disc filter;
• Figure 2 is a section through the disc filter screen segment along the line 2-2 in Figure 1;
• 3 shows a section through a radially extending rod of one of the two sieve carriers together with an embedded sieve along the line 3-3 in FIG. 1;
• FIG. 4 shows an end view corresponding to FIG. 1 of a screen segment according to the invention for a folding screen thickener, and
• 5 shows a section through this sieve segment along the line 5-5 in Figure 4.

The disc filter screen segment shown in FIGS. 1 to 3 essentially consists of two screen fabric cut parts 10 and two half shells 12, 14 designed as one-piece cast parts, each of which forms a screen support 16 and a frame half 18. The two frame halves together form an outer frame leg 20 in the radial direction and two lateral frame legs 22 extending in the radial direction and are along this frame leg about Screws 24 and threaded bores 26 are firmly connected to one another, 12 recesses 28 being provided in the half-shell for receiving the screw heads. As in the illustrated embodiment, the frame halves of the sieve segment according to the invention are closed and therefore also have an inner frame leg 30, whereby, as can be seen in FIG. 2, the two halves of this inner frame leg are arranged at a distance from one another in the axial direction To allow the filtrate to flow into a sieve segment carrier.

The sieve carriers 16 consist of radial rods 32 and transverse rods 34, which are integrally connected to one another and the frame halves 18, since the entire frame halves each form a single casting, in particular a plastic injection molded part. The cross bars 34 are set back in relation to the radial bars 32 in the direction of the interior of the sieve segment, as can be clearly seen in FIG. 2, while the outer end faces of the radial bars 32 are aligned with the outer end faces of the frame halves 18 according to the invention. Since the cross bars 34, as can also be seen in FIG. 2, bear against one another, the statement made above that the sieve carriers should be arranged at a distance from one another in the axial direction can of course only be interpreted in such a way that the supporting surfaces of the two sieve carriers support the sieves are spaced apart in the axial direction.

Each of the sieve fabric cut parts 10 is now, according to the invention, embedded in the sieve supports formed by the outer end faces of the frame legs 20, 22, 30 and the radial rods 32 at least over part of the thickness of the sieve, but preferably over the entire sieve thickness, such as this FIG. 3 shows one of the radial rods 32 and one of the fabric sections 10 in section; This section also shows that, in the preferred embodiment of the sieve segment according to the invention, the material of the supporting structure does not protrude outward beyond the outermost elevations of the sieve or sieve fabric.

The two frame halves can now be produced particularly easily and cheaply as plastic injection-molded parts, with a sieve fabric blank 10 being inserted into the injection mold and then the frame half being injected, the sieve fabric being embedded in the plastic forming the frame half and thus on the frame and on Filter holder is anchored.

Since the screen segment shown in FIGS. 4 and 5 differs from a folded screen thickener essentially only in that the radial extension of the frame and the screen support is smaller than in the case of a disc filter screen segment, the elements of the second embodiment corresponding to the first embodiment have been given the same reference numerals as in Figures 1 to 3, but with the addition of a dash, so that a description of the details shown in Figures 4 and 5 is unnecessary.

It is particularly advantageous that the bars of the two frame halves that do not support the screen fabric are arranged and dimensioned such that they abut against one another on the one hand (see FIGS. 2 and 5), which increases the stability of the screen segment, and between the edges of the screen segment facing outwards Rods and the sieve fabric still a relatively large flow cross-section remains for the filtrate.

The invention further relates to the method for producing the sieve segment, which is characterized in that the sieve is inserted into a mold which is used to manufacture the frame and / or the sieve carrier, whereupon the frame and / or the sieve carrier is or are cast .

Claims (10)

1. Sector-shaped sieve segment for rotating disc filters or folded sieve thickeners for thickening fiber suspensions, with a frame which has a radially outer leg and, on its two sides, in particular in the radial direction, in each case has a lateral leg to which two sieve carriers are fastened, which are fastened axially Direction of spaced sieve supports and, together with a sieve resting on them, form the two main surfaces of the sieve segment, characterized in that at least the frame legs (20, 22, 30) or the sieve carriers (16) are designed as castings into which the sieve (10) is embedded. 2. Sieve segment according to claim 1, characterized in that both the sieve carrier (16) and the frame legs (20, 22, 30) are formed as castings and that the sieve (10) is embedded in these castings. 3. Sieve segment according to claim 2, characterized in that the sieve carrier (16) on the frame legs (20, 22, 30) are cast. 4. Sieve segment according to claim 3, characterized in that the frame legs (20, 22, 30) are divided along a central plane of the sieve segment running perpendicular to the axis of rotation and that the frame half (18) together with the sieve carrier (16) of a segment half is a one-piece, cast shell-like part. 5. Sieve segment according to one or more of the preceding claims, characterized in that the casting is an injection molded plastic part. 6. Sieve segment according to one or more of the preceding claims, characterized in that the sieve (10) is a fabric made of stainless steel wires. 7. Sieve segment according to one or more of claims 1 to 5, characterized in that the sieve (10) is a plastic thread. 8. Sieve segment according to claim 4, characterized in that the two half-shells (16, 18) along the frame legs (20, 22, 30) are interconnected by screws (24). 9. Sieve segment according to one or more of claims 2 to 8, characterized in that each sieve carrier (16) is designed as a grid formed by bars (32, 34). 10. Sieve segment according to claim 9, characterized in that the bars (32, 34) run radially and transversely thereto, that the cross bars (34) with respect to the radial bars (32) are set back in the direction of the segment interior and that Sieve (10) is only embedded in the radial rods (32).

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