MOULDED FILTER MEDIA COMPRISING A SHEET OF POROUS NONWOVEN MATERIAL

Abstract: A moulded filter medium, e.g. a filter cloth comprises a sheet of porous nonwoven material (1) which is pre-moulded and pre-formed to conform to and snugly fit into the recesses and contours of a filter press plate. The sheet may comprise sintered partially fused particles of plastics, metal or ceramic or a cast acrylic or methacrylic resin or polymer sheet, or a coagulated polymer or microporous foam. The cloth may include a support structure such as a perforated membrane or mesh (3) or a spiral link fabric (12) embedded in the sheet.

Description: MOULDED FILTER MEDIA COMPRISING A SHEET OF POROUS NONWOVEN MATERIAL This invention relates to filter media such as filter cloths for use in filter plate presses. A filter plate press comprises a plurality of movable plates which when pressed together define a number of chambers into which slurry to be filtered can be fed. Each chamber has filtration means often in the form of filter cloths, so that solid residue is retained within the chamber and filtrate is removed from the chamber. The residue forms a filter cake and by various known means the moisture content of the cake can be reduced. When the filtration process is completed the plates are separated so that filter cake can be removed from the chambers. After cleaning and rearranging of the plates a further filtration cycle can be carried out. The filter plates typically have central feed holes via which the slurry progressively enters the chamber of the filter press. The liquid filtrate passes through the filter cloths secured on both sides of the plates extending over recessed filtration areas in the plates from which recessed areas the liquor is drained. The solids are retained and collect on the filter cloths to form the filter cake. The cloths are secured to the top surfaces of the plates and a seal is formed between the peripheries of adjacent plates defining chambers between the plates. The chambers are connected to each other by the central feed holes through the plates and the filter cloths about the feed holes may be sealed and connected by an impervious barrel neck connector. During operation the flow of slurry impinges against the cloths and forces the cloths into the recessed surfaces of the plates. This leads to distortion of the cloth, particularly at the plate corners and edges at locations where the cloth is in contact with raised bosses projecting from the plane of the filter plate towards the fabric to provide support points. This distortion of the cloth structure results in interstices in the cloth opening up, allowing larger particles than the designed maximum size to pass through the cloth which reduces filtration efficiency and performance. This problem is particularly noted with filter cloths based on woven fabrics. A further problem with standard cloths for such filter plates is that the weight and adhesion of the cake to the filter cloth causes the cloth to leave the contours of the plate and subsequently when the cake is removed and the press resealed the cloth will frequently be in a slightly different position having been displaced for example by lateral forces in scraping the cake from the cloth. This causes the cloth to become deformed into the plate recesses into a different position. As a result of this creep, over a period of time this has the effect of stretching the cloth to add further to the problems of opening interstices. It has been proposed therefore to provide woven monofilament warp/weft or monofilament warp/multifilament weft fabrics substantially comprising polypropylene yarns since the low melting point of the polyme r makes the cloths easy to thermoform into the plate recesses and contours. However, distortion-susceptible woven structures are still present. Also it has been proposed (US Patent 4931 178) to provide a porous PTFE membrane on a woven substrate to form a filter cloth. This proposal is also subject to the same disadvantages due to the effects of distortion on the woven substrate. GB-A-2283926 discloses a filter made from partially fused particles which may be polymeric or of metal, and may be located on a base structure or a reinforcement structure may be embedded therein. This structure again does not address the problem of filter medium distortions in filter presses. It is an object of the invention to provide a filter medium principally but not exclusively for use in press filters which remedies the above noted weakness of known woven based filter cloths. According to this invention a filter medium comprises a sheet of porous nonwoven material, which is pre-moulded or pre-formed to conform to and snugly fit into the recesses and contours of a standard filter press plate. The porous nonwoven material is preferably a sintered sheet made for example of partially fused particles. These may be of a suitable polymer or may be metal or a ceramic material. Polymer is preferred because the resultant material has greater flexibility. The porous nonwoven material may however be a film or sheet of coagulated polymer, a microporous foam, or a "porum" (i.e. a sheet of polymer such as synthetic or natural rubber which has been rendered porous by leaching out particles of soluble material such as sodium chloride or sugar that have been blended with the polymer prior to extrusion or moulding of the sheet) . The sheet may be made or preformed in moulds corresponding to the contours of a respective design of filter plate. The sheet material may be reinforced to improve its strength with a supporting structure such as a perforated membrane, a nonwoven mesh, a spiral link fabric, incorporation of chopped fibres, including bicomponent fibres or parallel yarns. Woven or knitted fabrics may also be used as a reinforcement without detriment as the woven mesh plays no part in the filtration action and thus any distortion encountered will have no effect in the filtration performance. By way of example, two preferred embodiments of the invention will be briefly described with reference to the accompanying drawings, wherein:- Fig. 1 is a diagrammatic cross-section of a first embodiment of filter medium fabric according to the invention; and Fig. 2 is a similar view of a second embodiment of filter medium fabric according to the invention. In Fig. 1 a filter medium fabric is shown in an enlarged diagrammatic cross-section. The cloth comprises a filter layer 1 which is made of particles 2 of a suitable synthetic plastics material, e.g. a polyolefin, which are partially adhered to one another by being subjected to heating to achieve superficial softening and to pressure. A suitable material is disclosed in GB-A-2,283,926 wherein particles of partially fused polyethylene or polypropylene form a filter material. The filter layer 1 is supported to give mechanical strength by a membrane substrate 3 which forms a mesh or lattice of a plastics material providing apertures or pores 4 for the passage of filtrate liquor and possibly reinforced internally by yarns such as 5 extending in one or both directions of the mesh of lattice. Such a membrane may be made in accordance with GB-A-2202893. The particles of the filter layer 1 are depicted as being of relatively small size forming two or three layers with pores and interstices between the solid residue particles forming flow channels through the filter layer 1 for the filtrate liquor, whilst particles above the designed size are retained by the filter layer. ln Fig. 2 a filter medium in the form of a filter cloth comprises a filter layer 1 1 , of larger particles which again are fused together to form a coherent porous layer. This layer is supported for mechanical strength on a spiral link fabric 12 which may be substantially as disclosed in GB-A- 2051 1 54. Pairs of cross-yarns 13 are connected by interdigitated spirals 14 to link the yarns 13 into a continuous open fabric. The filter layer is again preferably made by partial fusion of e.g. polyethylene, polypropylene or EVA particles in accordance with GB-A- 2283926. A porous filter cloth may alternatively be made by casting a solution of a polymer, e.g. an acrylic or methacrylic based thermoset resin or thermoplastic polymer, such as polymethyl methacrylate, in a suitable mould of appropriate shape, and driving off moisture by heat to form a porous structure. The filter cloth may alternatively be manufactured using a vibrational moulding technique, as described for example in EP-A-010914. When mounting a filter medium of the invention such as a filter cloth on a press filter, a hot former may be applied to the area around the central feed aperture to compact the medium and prevent leakage in this gasket region by rendering the cloth impermeable. The barrel neck may be welded to the cloth and/or the cloth preferred, by a similar technique to assist mating with the barrel neck in the central feed aperture area. These embodiments are described by way of example only and as already stated a wide variety of sintered materials including metals and ceramics may be used to provide the nonwoven fabric as well as other nonwoven structures. The fabric may be unsupported or supported by structures other than those particularly described. The filter media of the invention have a number of advantages, including quick and easy installation. It may also be possible to use integrally moulded barrel neck connectors, filter press seal members and/or couplings for mating with corresponding profiles on the inside and/or outside of the plate, and further it may be possible to provide a combined filter material and plate.

Claims: CLAIMS 1 . A filter medium comprising a sheet of porous nonwoven material which is pre-moulded or pre-formed to conform to and snugly fit into the recesses and contours of a filter press plate. 2. A filter medium according to claim 1 , wherein the porous nonwoven material is a sintered sheet made of partially fused particles. 3. A filter medium according to claim 2, wherein said particles are of a polymeric material. 4. A filter medium according to claim 3, wherein said polymeric material comprises polyethylene, polypropylene or EVA. 5. A filter medium according to claim 1 formed from a solution of an acrylic or methacrylic base thermoset resin or polymer cast in a suitable mould. 6. A filter medium according to claim 5, wherein the resin is polymethyl methacrylate. 7. A filter medium according to claim 2, wherein said particles are of a metal. 8. A filter medium according to claim 2, wherein said particles are of a ceramic material. 9. A filter medium according to claim 1 or 3, wherein said nonwoven material comprises a film or sheet of coagulated polymer. 10. A filter medium according to claim 1 , wherein said nonwoven material comprises a film or sheet of microporous foam. 1 1 . A filter medium according to claim 1 , wherein said nonwoven material is a polymer rendered porous by leaching out soluble particles blended with the polymer. 12. A filter medium according to any preceding claim, wherein said sheet of porous nonwoven material is combined with a supporting layer. 13. A filter medium according to claim 12, wherein said supporting layer is a perforated membrane. 14. A filter medium according to claim 1 2, wherein said supporting layer is a nonwoven mesh containing reinforcing yarns extending in at least one common direction. 15. A filter medium according to claim 12, wherein said supporting layer is a spiral link fabric. 16. A filter medium according to claim 1 2, wherein said supporting layer is a woven or knitted fabric. 17. A filter medium according to any of claims 1 to 9, reinforced by incorporation of chopped fibres. 18. A filter medium according to claim 17, wherein said chopped fibres include bicomponent fibres. 19. A filter medium according to claim 1 , comprising a filter layer of particles of a polyolefin which are semi-adhered to one another, on a membrane substrate in the form of a mesh or lattice of a plastics material at least partially embedded in the layer of polyolefin particles. 20. A filter medium according to claim 1 , comprising a filter layer of particles of a polyolefin which are fused together to form a coherent porous layer, and supported on a spiral link fabric which is at least partially embedded in the layer of polyolefin particles.

Inventors: SAYERS IAN CHRISTISON, SAYERS, IAN, CHRISTISON
Applicants: SAYERS IAN CHRISTISON, SCAPA GROUP PLC, SAYERS, IAN, CHRISTISON
Application year: 1998
Main classification: B01D39
IPC codes: B01D25, B01D39, B01D67, B01D69