The invention lies in the field of filter cloths. For example for manufacturing grinding compounds and the like the mass consisting of (abrasive) grains must be removed from a liquid or viscous-liquid mass by guiding this mass with pressure through a filtering unit, for instance a so-called "plate filter", which comprises at least one filter cloth. This filter cloth is placed under strain of pressure by throughput of the mass with force, whereby tensile forces occur in the cloth. For this reason use is made for this type of application of cloths of which the threads/yarns can be placed under strain of tension. The cloth must further be very well able to withstand the heavy mechanical influence of grains being partly held back and partly allowed through. It has not proved possible up until now to design a cloth sufficiently capable of withstanding the forces which occur. If the fabrics used have too great a stretch it is not possible to prevent the pores defined by the fabric threads from becoming enlarged as the pressure force exerted by the medium on the cloth increases. For filtering purposes this is highly undesirable since it is required that the pore size is controlled within relative narrow limits.

In addition, the filter cloths according to the prior art are insufficiently wear-resistant.

The existing filters are therefore subject to a capacity loss increasing with use, a certain material loss as a result of the unintended passage of solids to be held back, whereby an environmental problem can then occur as the filtrate still contains particles with dimensions above the norm. The invention has for its object to provide a solution for the stated problems according to the prior art and provides for this purpose a filter cloth comprising a fabric of yarns consisting of polymer material and consisting of filaments/fibres with a parallel orientation of at least 80%. Claims 2, 3 and 4 give increasing degrees of preference.

The invention further provides a cloth of the stated type wherein the degree of crystallization of the polymer material amounts to at least 60%. Claims 6, 7 and 8 give degrees of increasing preference. It is essential for the invention that use is made for the threads or yarns of which the cloth consists of polymer material in the form of filaments or fibres with a parallel orientation which is in principle as high as possible but amounts to at least 80%. Suitable materials are for instance polyethylene, copolymer with ethylene, polyalkylketone. It will be apparent that the invention is not limited to these materials.

In order to enable the best possible control of the pore size the embodiment is recommended in which warp and weft are the same.

The cloth can in particular be of the type with flat pattern design.

In a particular embodiment the cloth according to the invention has the special feature that the fabric threads define pores with dimensions in the order of a maximum of 20 x 20 μm. Claims 18, 19 and 20 give increasing degrees of preference.

Finally, the invention also relates to the use of filaments/fibres of the Dyneema type (mark of the Dutch concern DSM) for the above specified cloth according to the invention.

The cloth according to the invention will generally be very fine-mesh and based on overstretched filaments/fibres. The stretching of the filaments/fibres will preferably go so far that amorphous portions substantially no longer occur therein.

Use can be made of an impregnation of the cloth according to the invention. This can take place with a ther oplast, in particular a polyolefin. A (linear) polyethylene for example can be envisaged here.

Such an impregnation has the advantage of providing sealing of the filament bundles in the yarn. This prevents adhesion, penetration into or even through by the material for filtering.

A more permanent result of the calendering treatment is also obtained in this manner.

The calendering treatment itself contributes toward discharge of the filter mass held back by the cloth because the cloth has a smoother surface.

There further occurs a compacting of the yarn. The fused material can be present in some abundance in the yarn which has for instance a substantially round sectional form. The setting of the calendering device, which can be characterized by temperature, pressure and through feed speed, is such that the fibres remain unaffected but the impregnate melts and can rearrange itself in the yarn flattened by the pressure. Due to the effective squeezing the even nature of the impregnation is improved. The shearing strength of warp and weft is also improved, which results in retaining of the adjusted filter apertures or pores.

There is further an effective sealing of needle perforations. This is important since it has been found that particular types of filter material cause erosion damage via these apertures.

The calendering treatment further contributes to an increase in the wear resistance.

The following processing sequence of the cloth can be given by way of example:

* Washing of the cloth.

* Impregnating with the dissolved polyolefin.

* Fusing and drying by means of a heat treatment.

* The cloth is treated under tension for 2-3 minutes at a temperature of 133-138*C.

* Calendering takes place at a temperature of 125*C, a pressure of about 3 MPa and a throughfeed speed of over 9 metres per second.

* Washing. Polyolefin pressed out by the calendering device is herein washed off the yarns. This washing is preferably carried out after the finishing to bring about sufficient mechanical friction.

Attention is now focussed on finishing techniques.

The basic materials used are characterized by very low stretch percentages. The small stretch prevents formation of a completely hollow space between the plates of a plate filter, which will be further discussed hereinbelow with reference to the annexed drawings.

Depending on the dimensions of the plates and the positioning of the filling opening, the cloth deficit can be calculated using goniometric calculations.

By arranging pleat strips on the peripheral edges, wherein the outer zone is shorter than the adjoining zone in stretched situation, a convex shape of the filter cloth can be obtained on the basis of the calculated cloth deficit. The extra quantity of cloth relative to the comparatively short outer zone of the peripheral edge can form the necessary hollow space after closing of the plates.

Needle perforations cause damage to the fabric during finishing. The fibre/filament material is damaged by the needles and is thereby weakened. It is therefore desirable to work with the smallest possible yarn thickness with the highest possible strength ratio. A cloth according to the invention which for use in the plate filter comprises two cloth layers sewn to each other by sewing thread in an overlapping (filling) collar zone, can advantageously have the special feature according to the invention that the sewing thread consists of filaments/fibres of the Dyneema type (mark of the Dutch concern DSM) (Mm 52.5/3dr can for instance be envisaged) and use of so-called SPI needles.

By sewing together multiple cloth layers with so-called lock stitching at a sufficiently high tension of the yarn, the yarn and the associated knotting lie protected, as it were embedded, between the cloth layers. This reduces the erosion of the sewing yarns and thereby provides an effective increase in the lifespan of the filter cloth.

As stated in claims 17 and 18, substantially rectan- gular pores can be envisaged with dimensions of 20 x 20 μm. Smaller pores have even been contemplated since the invention offers the option of manufacturing such pores with a large degree of accuracy and stability. It would nevertheless appear probable that in order to filter products with

particles having diameters of less than 5 μm it is necessary to work with relatively elongate openings. As stated in claim 19, such elongate openings can preferably amount to 5-2 x 60-300 μm. Smaller pores of for example 5-20 x 5-20 μm are found to be insufficiently permeable.

It is of importance for the invention that the dimensions of the pores are very well controlled. The dominant relevant parameters are the degree of twisting, the warp density, the weft density, the washing shrink, impregnation, calendering treatment and stretch during use.

Applicable twistings vary between around 60 and 100 twists per metre in the case of multifilament yarn DTex 440. With regard to the occurrence of stretch and resulting loss of strength, data above 100 twists per metre is less significant. At less than 60 twists per metre the yarn is more difficult or even impossible to process in warp direction on the weaving machine.

Rotation speed

Max. count Yarn ø per cm.

60 17.5 threads 500 μ

100 19.0 threads 420 μm

Choice of the correct setting of the density of the warp results in an approximation of the filterability on the basis of particle size. The set density of the weft determines the effective permeability. Example: In the case of a warp of 17.5 threads per cm (DTex 440 with 100 twists per minute) , the opening between the warp threads varies between 20 and 25 μm after a certain period of use.

Settings with an accuracy of 0.5 threads/cm in DTex 440 are technically well controllable for both warp and weft. In the adjusting of densities of the weft, the following values were determined with these yarns and a warp of 17.5 threads per cm:

Distance Permeability *

15 threads per cm 75 μm 25 s.

19 threads per cm 10 μm 1 in. 52 s.

* =- measured throughfeed time of 1,000 ml

- at underpressure of 600 millibar constant (•= 400 millibar overpressure)

- surface of 12 cm ³

The intermediate measured settings shift by 0.5 threads per cm and give more or less proportional values (17 threads, 50 μm, 1 min. 15 s.). Due to the above described impregnation the entire amount of liquid, in particular water, runs through the openings between warp and weft. In a test on the mentioned installation it was found that cloth with a throughfeed time amounting to more than 2 minutes 30 seconds is insufficiently permeable.

Yarns of substantially round filaments have larger openings than yarns with an at least more or less rectangular cross section, this of course with corresponding cross- sectional area. A better sealing contributes to closing of the cloth against penetrating filter particles. This also reduces the dependency on the quality of the impregnation. It can be remarked generally that known aramide fibres have a round cross-sectional form. Dyneema fibres are supplied with an at least more or less rectangular cross sectional form. It should be understood in this respect that the designation "rectangular" is of rather qualitative character and should not be interpreted too absolutely. It is important to note that the thickness of the fibres is small relative to the dimension lying more or less perpendicularly thereof, wherein it is noted that Dyneema fibres have a more or less kidney- shaped cross section.

The invention will now be elucidated with reference to the annexed drawings, wherein:

Figure 1 shows a perspective view of a plate filter;

Figure 2 is a partly cut away top view of the plate filter according to figure 1;

Figure 3 is a perspective view of a plate with a double filter cloth according to the invention; Figure 4 shows in perspective view the plate and the filter cloth of figure 3 in assembled state;

Figure 5 shows a detail cross section through two assembled filter plates with a filter cloth according to the invention in assembled state; Figure 6 is a perspective view of a part of a filter cloth with a pleated edge;

Figure 7 is a perspective view of a variant of a double filter cloth with pleated edges;

Figure 8 is a partly broken away perspective view of a detail of the filter cloth according to figure 3; and

Figure 9 is a cross-sectional view of a double filter cloth according to yet another embodiment.

Figure 1 shows a plate filter 1 in which a number of filter cloths according to the invention are applied. A plate filter is generally known per se. The description of the filter 1 will therefore be kept brief.

A frame 2 comprises two supporting rods 3, 4. These supporting rods support filter plates 5 which will be further discussed hereinafter. Plate ¹ . 5 are provided with carrier supports 7, 8 (see also figure 3) which also serve as handles such that the plates 5 can be placed from above into the space between the rods 3, 4 to then be supported by the rods 3, 4 via the carrier supports 7, 8. Respective filter cloths are added beforehand to the plates 5. After placing of all plates a hydraulic cylinder 6 is energized to press the plates with great force against each other, whereby these latter, by interposing of the filter cloths (see for instance figure 5) , press substantially sealingly against each other. The plate filter 1 comprises a feed pipe 9 and four discharge pipes, all designated with 10 for the sake of convenience.

Figure 2 shows a plate filter 11 of the same type as in figure 1 in partly broken away top view. This figure shows that the plates 5 have central passages 12 through which the liquid for filtering can enter the filter via the feed 9 and

pass through respective filter cloths 13 in this filter, whereafter the filtrate can be guided via passages 14 to the discharges 10. Figure 2 shows that each filter cloth 13 is embodied such that it as it were wholly encloses the filter plate 15 and covers all surfaces thereof, including the inner surface of passage 12. Passages 14 are open.

Figure 3 shows a filter plate 5. This comprises a peripheral edge 15 in which the four passages 14 debouch. The edge 15 bounds a recessed portion 16 in which regularly grouped elevations 17 are arranged which serve to support the filter cloth 19 in order to preserve therebehind a space through which filtrate can flow. Nine broader elevations 18 have the same height as the peripheral edge 15 and serve for the pressure-transmitting contact with a subsequent plate, this all with interposing of the filter cloth 19.

The filter cloth 19 consists of two cloth parts 20, 21 which are mutually connected by a cloth tube through sewing. Reference is made in this respect to figures 5, 8 and 9. The cloth part 20 is carried through the passage 21 in the manner indicated with arrows 23, whereafter the peripheral edges of cloth parts 20, 21 protruding outside the plate 5 are mutually connected by tacking loops via the registered eyes 25 in the peripheral edges of the cloth parts 20, 21. A simple fixation of cloth 19 to filter plate 5 is thus obtained.

Figure 5 shows two plates 5 with associated cloths 19 which are placed against each other and form part of a plate filter 1 or 11.

Figure 5 shows that cloth 19 is supported in recess 16 by the outer surfaces of elevations 17. Connecting onto the bottom of this recess 16 are filtrate outlets 26 which debouch into the passages 14 for discharge of filtrate. The solid fraction of the liquid for filtering is held back by cloth 19. The tube 22 which covers the inner surface of passage 12 mutually joins the cloth parts 20 and 21. In the manner shown in figure 5 the tube 22 is joined to the cloth parts 20 and 21 by means of a number of layers placed one on top of another which are sewn together by Dyneema yarns. Figure 8 shows this structure in more detail. Also shown

schematically in figure 8 are the lock stitches 27 of the Dyneema yarns.

As discussed above, the entire filter cloth 19 consists of Dyneema material. As this material exhibits very little stretch while adaptation to the form shown in figure 5 must be obtained, it is necessary to give the filter cloth parts 20, 21 a certain space so that they can move freely to some extent without coming under strain of stretch.

Figure 6 shows the manner in which this can be achieved. This figure shows the peripheral edge 28 of cloth part 20. The outer zone 29 is given a reinforced form in the manner shown and provided with a strip 29 drawn in stretched position which is shorter than the zone 30 of cloth part 20 joining thereto in the stretched situation. "Space" is thus obtained in the cloth as shown schematically in figures 3 and 4. The cloth can thus display a certain concave or convex form such that it can adapt to the form of the plates 5 as shown in figure 5.

Figure 7 shows an alternative cloth 31 which is likewise provided with the pleated zones 32 of the type as shown in figure 6 and explained with reference thereto. In this embodiment the cloth 31 comprises only one cloth part and tube 22 is omitted. Instead the filter plates are embodied such that they co-act sealingly with the zone round the holes 32 arranged in the region of the passages 12 in cloth 31. As will be apparent, fastening of the cloth parts to each other by means of the eyes 25 and tacking loops 24 as shown in figure 4 and figure 5 can be omitted in this embodiment. Figure 8 has already been discussed with reference to figure 5.

Finally, figure 9 shows in cross section a cloth 33 wherein the respective cloth parts 34, 35 are mutually joined by means of a tube 36 which is connected to cloth parts 34 and 35 in the manner shown. The lock stitches are once again designated with the reference numeral 27.

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