Seamed filter elements are used when it is necessary to provide a cylindrical or tubular filter element from a filter cloth, and achieved by joining the ends of a sheet or length of the filter cloth to form a continuous tubular member.

This seaming is carried out by stitching overlapped edges of the filter cloth using a yarn which is used to sew the edges together. Such seamed filter elements may for example be used in centrifuge filters, an example of which is outlined in US 6,063, 298 (WILKIE). The filter element therein comprises a cylindrical sleeve of filter cloth received'within 6 so-called "basket"comprising a cylinder of mesh or perforated sheet material which allows liquor to pass through when the centrifuge is rotated, the solids phase being caught by the filter cloth and forming a filter cake on the inside surface of the cloth.

It is found in practice however that such sewn seams present a number of problems, for example the punctures in the filter medium caused by sewing needle or needles are prone to leakage as they present openings many times the man pore size of the filter cloth, the threads can break and be pulled away particularly when the cake is scraped off the filter cloth, and the sewing process can result in broken needles which remain in the seams and are then hazardous to anyone installing or removing the filter cloth. The raised profile of sewn seams means that they are vulnerable to damage from the scraper blades used to detach the filter cake. Regular scraping causes stress on the seam leading to premature failure of the seam or the wearing of a hole in the substrate adjacent to the sewn seam, thereby shortening the expected life of the filter cloth.

Fragments and pulled-away threads can contaminate the filter cake which is of considerable economic importance where the solids content of the slurry initially introduced into the centrifuge is the valuable fraction of the slurry. This is particularly the case in products such as dyes, pigments or pharmaceutical where very high purity is required.

Breakage and pulling away of threads further impairs the integrity of the seam, causing leakage and thus loss of filter cake material, and in extreme cases destruction of the seam which leads to complete failure of the filter.

At present, the leakage problems are attempted to be overcome by painting or coating the stitched seams with a polymer sealant. This however entails an extra operation, increasing fabrication times. Also if the sealant is not applied correctly, or is not allowed to dry or cure, then it may also come away from the seam and contaminate the solid cake.

It is therefore an object of the invention to provide a leak-proof seam which has a low profile, is free from sewn seams, and in which polymer sealants are not used.

In accordance with the invention a method of forming a cylindrical or tubular filter cloth comprises joining the ends of a sheet or length of a woven or non-woven filter cloth containing thermoplastic material,, characterized in that the method includes the step of joining said ends by welding them together using hot air.

The ends may be overlapped and then passed under a stream of air which is heated to a temperature above the fusion temperature of the thermoplastic material in the filter cloth.

The heated air may be applied to the seam area as a point jet from a welding head incorporating a nozzle which is traversed along the seam to

cause the fusion of the thermoplastic material, or as a curtain of hot air provided for example from an elongated slit nozzle which is applied to the full length of the seam simultaneouslv.

The seam is preferably immediately subjected to pressure using pressure exerting means, for example from a roller or presser shoe immediately following the welding head, so that the cross-section profile of the welded seam is approximately the same thickness as the original substrate.

The sheet or length of filter cloth is thus formed into an endless band which is provided by joining of the hot-air welded seam. The band may then be used as a tubular filter, cartridge filter or bag filter for a range of applications, including centrifuges.

A further panel of filter or other cloth may be welded to the base of the tubular filter to form a bag filter ; this base preferably has a small disc cut from the centre to provide an opening to allow the cloth to fit over the central slurry feed of the centrifuge machine.

The tube of filter cloth may be turned inside-out after welding, so that the weld can appear on either the inside or the outside of the finished product.

This may be for cosmetic reasons, or to avoid a scraper blade catching on the seam.

Edge regions of the ends may be reduced in thickness by compression or removal of material and overlapped, the contacting faces of the overlapped edges being softened by heated air then joined by exertion of pressure.

The edge regions of two filter fabric ends may be brought together in overlapping relationship in the nip of a pair of wheels or rollers, the overlapping faces being subjected to fusion by hot air immediately prior to

being brought together in the nip by means of a flat nozzle, and the weld completed by means of pressure exerted in the nip.

This method of joining is particularly useful in making centrifuge liners for use for example in processing dyes, pigments or pharmaceuticals.

A method according to the invention will now be described by way of example with reference to the accompanying drawings, wherein:- Fig. 1 is a diagram summarising one version of a method according to the invention; Fig. 2 is a diagram of an enlarged cross-section view through a seam produced by the method of the invention; Fig. 3 is a similar sectional view of the seam before welding; Fig. 4 is a diagram of an embodiment of filter element in the form of a centrifuge liner produced by the method of the invention; A filter element is to be formed from a band of filter cloth, by joining the shorter sides of the band to form a cylindrical or tubular filter element.

An example is shown in Fig. 4 wherein a band of filter cloth 1 is joined by a seam 2 to form a tubular filter element in the form of a centrifuge liner. In the Fig. 4 embodiment, an additional base panel 3 of cloth is joined to the bottom edge 4 and has a central aperture 5. this forms a bag-like structure and the aperture 5 can be located to allow a central fed pipe in a centrifuge filter to pass into the interior of the bag.

In the present invention, a seam is formed by overlapping edges 10, 11 of a filter cloth 12 containing or consisting of woven yarns or non-woven fibers of a thermoplastic material such as polyolefins, polyesters, or polyamides for example, as in Fig. 4. The overlap zone is then heated to above the fusion point of the thermoplastic material by means of hot air.

This can be effected by a nozzle 14 or hot air lance, which is traversed

width-wise of the fabric band along the overlap zone which is advantageously supported on a firm anvil surface 15. the weld zone is then subjected to pressure for example by means of a pressure roller 16 whilst the thermoplastic material is still soft. The result is that the two fabric ends are joined by a weld zone 18 in which the two layers of fabric are effectively combined into one, and the roller treatment will tend to eliminate sharp steps or discontinuities.

The nozzle 14 or hot air lance can be replaced by a slot nozzle extending lengthwise along the seam/overlap zone which treats the whole seam at once, and the roller 16 could be replaced by a pattern press or similar pressure arrangement.

The nozzle 14 may be used in one, two or more passes along the seam before rolling, or heating and rolling may be carried out in a single pass of a combined air heating and rolling head.

Fig. 5 shows an alternative lapped seam joint, wherein each fabric edge, 20,21 is reduced in thickness by compression or removal of material, by about 50% and the facing surfaces of the reduced thickness edges locally welded I the contacting surfaces only as at 22, without general fusion of the seam area as in Figs. 2 and 3. This may be effected by localised heating of the surfaces immediately before contact, overlapping them and applying pressure as the fused contact layer sets.

The weld zone is reduced to a relatively thin region in the overlap and outside of the zone the substrate is not melted thus avoiding any degradation in fibre strength or leaving of a brittle region which might fracture easily in operation.

Figs. 6 and 7 illustrate an alternative method of lengthwise welding of a seam. Upper and lower pressure wheels 30,31 pull along the two edges

32,33. The wheels 30,31 are pressed together by an upward force acting on the lower wheel (or vice versa), the other wheel of the pair being immovable in the up or downward directions. To melt the facing surfaces 32, 33 hot air 34 is blown into the region where the wheels press together, from a flat nozzle 35. The nozzle 35 has a horizontal slot opening which matches the weld zone 36 in Fig. 7 (the nozzle is"behind"the viewer in Fig.

7). The nozzle 35 is brought into position by an arm 37 which swings into position at a predetermined distance from, and relative to the rotating wheels 30,31. Typically the wheels are about 50 mm in diameter and the nozzle 35 is brought to a distance of 10-15 mm from the point where the wheels press the fabric edges together.

The hot air may be generated by passing air over an electrical heating element, or may be exhaust gases from for example a combustion torch.

The welding process can be controlled for different substrates by altering the temperature of the heated air, the pressure applied and the relative speed of the cloth and the welding head and pressure means.

Typically a higher temperature, and/or longer exposure is required for higher melting point materials such as polyamides (nylon) or polyesters as compared to polyolefins which have much lower melting points.

The following table compares the failure loads of stitched and welded seams of various woven and non-woven substrates. The test was carried out under tension on 100mm samples with 50mm wide seams. The. test speed was 100mm min' TABLE 1 Material Code Stitched or Welded? Mean Failure Load /N 5cm- 2814 S 35.4 W 168. 5 PPF 18/15 S 44.9 W 181.2 NY 547 FC S 20.8 W 53.5 DAF 18 S 26. 8 186. 4 99F S 54.2 W 254. 6 4709 T/100 28. 4 W 60.0 3670 / 13 S 21.5 W 87.4 DA322 S 20. 7 W 55.1

It will be noted that the welded seams are at least twice as strong as the sewn counterparts, frequently up to five times stronger, and in one case almost seven times stronger.

Details of the substrates used in the above tests are as follows : - POLYPROPYLENE SUBSTRATES Welding Conditions Material Substrate Weight Air Air Pressure Speed Code oz sq. perm./CFM Temp./°F/psi yod. PPF 18/15 Needlefelt 18 15 300 40 27 99FF Multi-17 1 400 50 23 Filament 4709T/100 Mono-8. 7 100 400 55 23 Filament

To achieve acceptable welds there needs to be significant control over the welding conditions depending on the properties of the substrate. If the weld has not seen enough heat then it may be incomplete, or if it has seen too much heat then the substrate may have degraded. In both cases the result weld would not be as strong as expected. The needlefelt cloth (PPF 18/15) comprises fine fibres which melt easily hence low temperature and pressure settings are required. The speed of the machine is also relatively high so that the substrate doesn't have a chance to degrade under the hot air.

In addition, there needs to be a higher pressure applied to the lightweight mono-filament (4709T/100), compared to the heavy multi- filament (99FF), which is due to the fact that the mono-filament yarns are thicker and therefore more difficult to melt.

POLYESTER SUBSTRATES Welding Conditions Material Substrate Weight Air Air Pressure Speed Code oz sq. perm./CFM Temp./°F/psi yd. 1 DAF 18 Needlefelt 18 30-50 300 50 26 DA322 Multi-8. 5 25-40 420 55 22. Filament

Again, the woven substrate (DA322) needs significantly higher temperature than the non-woven (DAF 18), despite being more lightweight Compared to the equivalent polypropylene substrate the polyester materials are slightly more difficult to melt as PET has a higher melting point than PP, this is confirmed by the slightly slower welding speed and higher pressure.

NYLON SUBSTRATES Welding Conditions Material Substrate Weight Air Air Pressure Speed Code oz sq. perm./CFM Temp./°F/psi yd. 2814 Multi-13 5-7 500 50 21 filament NY 547FC Mono-8. 3 50-70 525 55 23 Filament 3670/13 Staple 16. 5 10-13 580 55 24

The most significant change to the welding conditions in the above table is the much higher temperatures needed to melt the nylon substrates.

The higher melting point of the nylon also means that high pressures and sometimes slow machine speeds are also necessary for effective welds.

Again the mono-filament substrate is more difficult to weld than the multi- filament and requires higher air temperature and pressure.

Note also that this technique is applicable to coated fabrics as the last substrate (3670/13) can still be welded even though it has a cured phenolic resin coating. However, this tough coating means that a very high temperature is needed, in this case 580°F, along with a high pressure of 55psi.

Further advantages of the hot air welded seams are that a thinner seam is achieved with no raised edges or proud standing thread loops, no thread to come loose and contaminate the product, no needle puncture marks (and thus no leaks), and no supplementary polymer sealant. The seam is thus suitable for use where purity of the product is a prime consideration, as in the pharmaceutical field.

The seam of the invention would also be expected to have significantly longer life than the present sewn seam as it has a low profile which is no longer susceptible to damage from scraping used to dislodge the filter cake.

The method of the invention can be used with a wide range of substrates including polypropylene, polyethylene, nylon or polyester substrates in the form of spun-bonded, needlefelt or woven cloths. It is also possible to carry out the method with coated products such as cured resin or coagulated polyurethane coatings.

Seamed filters according to the invention or made by the method of the invention are of particular utility for filtration where the solid product is particularly valuable, for example as in the case of dyestuffs or pharmaceutical. Although shown in connection with a tubular or sleeve filter in Fig. 4, the method is applicable to other filters-e. g. in the pharmaceutical field, notably for example fluid bed dryer bags.

It is of course to be understood that the invention is not intended to be restricted to the details of the above embodiments which are described by way of example only.