WO2007088518A1

WO2007088518A1 - Filtration

Info

Publication number: WO2007088518A1
Application number: PCT/IB2007/050343
Authority: WO
Inventors: James Hendry Alfred Thompson
Original assignee: James Hendry Alfred Thompson
Priority date: 2006-02-03
Filing date: 2007-02-01
Publication date: 2007-08-09

Abstract

This invention relates to a filtration element (10) having a body (12) of open-cell porous material, of a polymeric plastics or resinous material, the body having an outer surface (18, 22, 24, 26) provided with a multiplicity of pores and the body having a porous interior provided with a network made up of a multiplicity of interconnected passages and drainage channels (28, 30, 32) leading to a drainage outlet (34), each drainage channel having a surface provided with a multiplicity of pores and the network of passages placing the pores of the outer surface of the body in liquid flow communication with the pores of the surface of each channel, to permit a liquid in which the element is immersed to flow to the drainage outlet. The invention also relates to a method of making such a filtration element (10).

Description

FILTRATION

THIS INVENTION relates, broadly, to filtration. More particularly, the invention relates to a filtration element suitable for industrial use, e.g. in the mining industry, and to a method of making the filtration element.

According to the invention there is provided a filtration element having a body of open-cell porous material, the body having an outer surface provided with a multiplicity of pores and the body having a porous interior provided with a network made up of a multiplicity of interconnected passages which provide the porous material with its open-cell character, the body further having, in its interior, at least one drainage channel leading to at least one drainage outlet, each drainage channel having a surface provided with a multiplicity of pores and the network of passages placing the pores of the outer surface of the body in liquid flow communication with the pores of the surface of each channel, to permit a liquid in which the element is immersed to flow, in response to a pressure drop across the body from the outer surface of the element to its drainage outlet, via the network of passages from the pores of the outer surface to the pores of each channel surface and along the associated channel to the associated drainage outlet, the filtration element being characterized in that the open-cell porous material of the body is a polymeric plastics or resinous material.

Usually the body will have, in its interior, a plurality of said drainage channels. Each drainage channel may have its own drainage outlet, there being a drainage outlet for each drainage channel. Preferably, however, the body has, in its interior, a plurality of said drainage channels which are interconnected to form a network which leads into a single common drainage outlet.

The body may be of flattened shape, having a pair of more or less parallel oppositely outwardly facing major faces interconnected by one or more edge faces. In this case, the drainage channels may be located midway between the major faces, the or each drainage outlet being through an edge face, midway between the major faces. In a particular construction, the body may be more or less in the form of a truncated sector, the major faces tapering in outline from an outer edge face towards an inner edge face, the body having a pair of side edge faces which radiate from the ends of the inner edge face to the ends of the outer edge face, the ends of the outer edge face being spaced further apart from each other than the ends of the inner edge face. More particularly, the body may be of flattened shape, having a pair of substantially parallel oppositely outwardly facing major faces interconnected by one or more edge faces, each channel being located, preferably midway, between the major faces and each drainage outlet opening through a said edge face.

In the above particular construction, each major face may have an outer edge which is curved and preferably part-circular, and an inner edge which is straight, the side edges also being straight and diverging from each other in a direction from the ends of the inner edge to the ends of the outer edge, so that a plurality of such filter elements can be placed side-by-side to provide a circular outer periphery, and an inner periphery which has the shape of a regular polygon.

Accordingly the flattened shape of the body may, as foreshadowed above, be in the form of a truncated sector, the major faces having an outline which tapers radially inwardly from an arcuate convex part-circular radially outer circumferentially extending edge face towards a tangential radially inner edge face, the body having a pair of straight side edge faces which radiate respectively from opposite ends of the inner edge face to opposite ends of the outer edge face and which diverge radially outwardly from each other so that their radially outer ends are spaced further apart than their radially inner ends.

When the major faces taper from outer edges to inner edges, the majority of the plurality of drainage channels may radiate from inner ends to outer ends, the radiating channels being located side-by-side and converging towards each other from their outer ends, which may be at or adjacent the outer edge face of the element, to their inner ends, which are spaced from the inner edge face of the element. In this case the radiating channels may be interconnected by a transverse channel, and a central one of the radiating channels may lead to the drainage outlet, which may be centrally positioned on the inner edge face.

Thus, in a particular embodiment, the body may have a plurality of said drainage channels, the majority of which drainage channels radiate outwardly from radially inner ends thereof to radially outer ends thereof, the radiating channels being located in series side-by-side and converging towards each other from their outer ends which are at or adjacent the outer edge face of the body to their inner ends which are at or adjacent the inner edge face of the body, the radiating channels being interconnected by at least one transverse channel and a central said channel of the series leading to a single said drainage outlet from the body.

Each major face of the element may be provided with a brace plate, which may likewise be of a metal such as stainless steel, the brace plate having a straight side edge extending along the inner edge of the associated major face, a pair of end edges which are straight and extend along the side edges of the major face, and a convexly curved side edge which extends across the major face and interconnects the ends of the end edges of the brace plate remote from its straight side edge, the convexly curved side edge of the brace plate being closer to the inner edge of the associated major face, than to the outer edge of said major face. The brace plates are respectively adhesively secured to the associated major faces of the body and the straight side edge and end edges of the brace plates are welded respectively to opposite edges of the strap. A pair of stainless steel tubular bushes is provided, passing respectively through the brace plates and through the body, at opposite ends of the brace plates, each bush being adhesively secured in position and having an annular stainless steel flange at each end thereof, each flange being adhesively secured to the adjacent end of the bush and to the associated brace plate.

In other words each major face of the body may be provided with a strengthening and supporting brace plate tightly secured to said major face, the brace plate being flattened and elongated and having a pair of spaced side edges extending along opposite sides thereof and a pair of end edges at opposite ends thereof and respectively interconnecting the adjacent ends of the side edges to each other, one of the side edges being straight and extending along the radially inner edge of the associated major face, the end edges being straight and extending radially outwardly respectively along the side edges of said associated major face, the other side edge of the brace plate interconnecting the radially outer ends of the end edges, said other side edge of the brace plate and the radially outer ends of its end edges being located closer to the radially inner edge of said associated major face of the body than to radially outer edge of said major face.

The element may be provided with a strap, e.g. of a metal such as stainless steel, which extends snugly around the periphery of the filtration element, face-to-face with each edge face of the element, to which edge face or faces it is adhesively secured, to strengthen and reinforce the porous material of the element.

The element may thus include a flattened metal strengthening and reinforcing strap which extends around the periphery of the body, being tightly secured face-to-face with the edge faces of the body.

Securing pins may be provided, securing the strap to one or more edge faces of the body, e.g. adjacent and spaced from each end of each of the side edge faces of the body, and adjacent and spaced from each end of the outer edge face of the body. Similarly, securing pins may be provided, securing the brace plates to the body, e.g. respectively adjacent the bushes and on the opposite sides of the bushes from the end edges of the brace plates, adjacent the curved side edges of the brace plates.

Each filtration element may be provided with a connector, which may be in the form of an elongated stainless steel connector plate, for side-by-side connection of two filtration elements to each other, the outer edge face of each filtration element being provided, adjacent its ends and between said ends and any pins adjacent said ends, with respective threaded sockets for bolting the connector plate to the filtration element by means of a bolt. For this purpose, each connector plate may be provided adjacent its ends respectively with a pair of openings therethrough, at least one of which may be in the form of a slot extending centrally, lengthwise along the connector plate. The pores and passages may be microscopic, respectively having pore and passage sizes or diameters in the size range 0.2 - 20 microns (μm), each drainage channel conveniently, but not necessarily, being straight and cylindrical, and having a diameter in the size range 5 - 15 mm.

An element as claimed in any one of the preceding claims, characterized in that the polymeric material of the body is selected from the group of materials consisting of methylmethracrylates, methacrylic acid ethyl esters (i.e. methacrylacidethylesters), N,N-dimethyl-p-toluidenes and suitable mixtures thereof.

An example of a suitable polymeric material is that available in South Africa under the trade mark "POR-TECH" from Advanced Materials Technology, Kempton Park, the "POR-TECH" comprising a liquid resin used together with a suitable filler such as a powder, which may also be a polymeric resin. The "POR- TECH" polymeric material is supplied as a two-component system or kit comprising the resin together with a suitable filler powder. "POR-TECH" is also available under the trademark "ALWA-MOULD P" from its manufacturers, Aiwa GmbH, Gronau, Germany. Two other suitable polymeric materials which can be used are the proprietary products available from Ventrade Eighteen (Proprietary) Limited, Benoni, South Africa, respectively under the trademark "MAMMUT 2000", which is a two- component porous resin casting system or kit, and under the trademark "MICRO" which is a three-component porous resin casting system or kit.

The channels may have a cross-sectional outline which is selected from the group consisting of circular outlines and square outlines.

The invention extends to a method of making a filtration element as defined and a described above, the method comprising casting the body in a mould and allowing the material of the body to set in the form of a porous open-cell foam material, having pores in its surface interconnected by a network of microscopic passages in its interior to at least one drainage channel in its interior leading to a drainage outlet, the foam material being a polymeric plastics or resinous material. More particularly, according to another aspect of the invention there is thus provided a method of making a filtration element as defined and described above, the method comprising forming the body to provide it with a desired shape and providing, in its interior, at least one drainage channel leading to at least one drainage outlet, the method being characterized in that forming the body and providing it with the desired shape is by casting it in a mould from a settable polymeric plastics or resinous material and causing or allowing the material to set to form the body of porous open-cell material having a porous interior provided with a network made up of a multiplicity of interconnected passages which provide the porous set material with its open-cell character, the body having an outer surface provided with a multiplicity of pores, each drainage channel having a surface provided with a multiplicity of pores, and the network interconnecting the pores of the outer surface with the pores of each drainage channel.

The body may be cast into a mould shaped to provide the body with a flattened shape, having a pair of substantially parallel oppositely outwardly facing major faces interconnected by one or more side edge faces, the method including providing the body, in its interior, with a plurality of interconnected drainage channels located between the major faces, the drainage channels being interconnected and leading to a single drainage outlet through an edge face of the body.

The method may include strengthening and reinforcing the body by enclosing the periphery of the body made up by the side edge or edges of the body with a flattened metal strengthening and reinforcing strap which extends around the periphery of the body, and tightly securing the strap to the edge face or faces of the body; and the method may include supporting and bracing part of each major face of the body by means of a metal brace plate, by tightly securing the brace plate to said part of said major surface, and by welding part of the peripheral edge of each brace plate to part of the adjacent side edge of the strap.

Casting the body may be by way of a pour-casting technique under gravity into a split mould, provided with one or more cores for forming the drainage channel or channels. The method may include using cores in the form of rods enclosed in resiliently flexible sheaths or sleeves, for example spring steel rods enclosed in rubber sheaths or sleeves, for forming the radiating drainage channels, a core in the form of a stainless steel rod being used for forming the transverse channel. The method may further include using a rubber lining of the mould to facilitate release of the cast element from the mould, the rubber lining and sheaths or sleeves also respectively enhancing the durability of the mould and cores with regard to corrosion arising from the polymeric material during casting.

The invention will now be described, by way of non-limiting illustration, with reference to the accompanying diagrammatic drawings, in which: Figure 1 shows a schematic three-dimensional view of a filtration element in accordance with the present invention;

Figure 2 shows a schematic face-on view of a brace plate forming part of the filtration element of Figure 1 ;

Figure 3 shows a schematic face-on view of a connector plate forming part of the filtration element of Figure 1 ;

Figure 4 shows a schematic face-on view of the body of the filtration element of Figure 1 ;

Figure 5 shows a face-on view of a variation of the brace plate of Figure 2; Figure 6 shows a face-on view of a variation of the connector plate of Figure 3; and

Figure 7 shows a face-on view of a variation of the strap which forms part of the element of Figure 1 .

In Figure 1 of the drawings reference numeral 10 generally designates a filtration element 10 in accordance with the present invention. The filtration element has a flattened body 12 (see also Figure 4), more or less in the form of a truncated sector, the body 12 and the element 10 as a whole tapering from a broad end 14 to a narrow end 16. The body 12 has a pair of flat parallel major faces 18, which face oppositely outwardly from each other, being joined by four peripheral or edge faces, which are not visible in Figure 1 and are enclosed by a stainless steel strap 20, the edge faces being shown in Figure 4. These are a flat elongated rectangular inner edge face 22, an outer edge face 24, and a pair of flat rectangular side edge faces 26, which radiate and diverge outwardly from each other respectively from the ends of the inner edge face 22, to the ends of the outer edge face 24 which is part-cylindrical in shape, being curved about the axis (not shown) from which the side edge faces 26 radiate. In a preferred embodiment of the invention, the rectangular inner edge face 22 is not enclosed by the strap 20.

The body 12 has a thickness, from one major face 18 to the other, of 30mm and the angle between the inner edge face and each side edge face is 105°. The body 12 is formed of an open-cell porous polymeric resin which is available in South Africa from Advanced Materials Technology of 6 Director Road, Spartan, Extension 2, Kempton Park under the trade mark "POR-TECH ". The body 12 has outer surfaces and interior surfaces which are porous, being provided with a multiplicity of pores of a pore size of 5μm, the body having an interior provided with a network formed of a multiplicity of interconnected microscopic passages having a diameter more or less the same size as the pore size, i.e. 5μm. In this regard it is to be noted that, by following the manufacturer's instructions, "POR-TECH" resin/filler mixed with water can be cast to have a desired pore/passage size in the range 0.7 - 20μm. The polymeric resin of "POR-TECH" is liquid methylmethacrylate and the resin is filled by means of a methacrylacidethylester powder filler, in a resin:powder mass ratio of 9:26. The powder is inert and is evenly dispersed in a matrix of the resin. The body 12 is porous throughout its interior.

In the interior of the body 12 are provided a plurality of drainage channels arranged in a network (see broken lines in Figure 4). These are a central drainage channel 28, an array of fourteen channels 30 arranged in two groups of seven, spaced in series side-by-side on each side of the central channel 28, and a cross-channel 32 extending normally to the central channel 28, transversely to the channels 28, 30. The channels 30 diverge from one another in the direction away from the narrow end 16 of the body 12 to its broad end 14, being more or less equally spaced side-by-side in series from one another, from the side edge faces 24 of the body 12, and from the central channel 28, the spaces between the channels tapering in the direction from said broad end 14 to said narrow end 16.

The central channel 28 opens out of a drainage outlet 34 (see Figure 4) centrally positioned on the inner edge face 22 of the body 12, its opposite end being blind and closed off adjacent the outer edge face 24 of the body 12. In turn the channels 30 extend from blind closed-off outer ends, spaced from and adjacent the outer edge face 24 of the body 12, to blind closed-off inner ends spaced from the inner edge face 22 of the body 12 by a spacing greater than the spacing between the outer ends of the channels 30and the outer edge face 24. The channels 30 have inner end portions 36 of reduced diameter. The channel 32 intersects the channels 28, 30 at the position, spaced from the inner ends of the channels 30, where the inner end portions 36 enlarge and merge into the remainders of the respective channels 30.

The channels 28, 30 and 32 are all straight and are located midway between, and parallel to, the major faces 18 of the body 12. The channels 30 have a diameter of 10mm, except at their inner end portions 36 which have a diameter of 4mm. The channel 32 has a diameter of 10mm; and the channel 28 has a diameter of 10mm except for an inner end portion 38 thereof which is of increased diameter. The end portion 38 tapers from the drainage outlet, which has a diameter of 20mm, to the remainder thereof, the taper of the end portion ending more or less alongside the inner ends of the end portions 36 of channels 30. The drainage outlet 34 is provided with a connector fitting 40 (Figure 1 ), adhesively secured in place, for connection to a drainage line (not shown). The outer ends of the channels 28 and 30 adjacent the outer edge face 24, and opposite ends of the channel 32, are closed off by threaded plastics plugs (not shown) available under the trade mark "Tufnol" from Meyerton Bolt & Nut These plugs are of 12mm diameter and 20mm length, adhesively secured in position, the outer ends of the channels 28 and 30, and the ends of the channel 32, being drilled and tapped to receive these plugs.

While the channels 28, 30, 32 depicted in this embodiment are substantially circular in cross-section, it will be appreciated by the person skilled in the art that other cross-sectional shapes are also possible. In particular, it is envisaged that another embodiment of the invention will feature channels 28, 30, 32 of substantially square cross-section.

Each major face 18 is provided, at its narrow end, with a stainless steel brace plate 42. Each plate 42 has a straight inner edge 44 (see Figure 2) extending along the inner edge face 22 of the body 12, a pair of end edges 46 extending along the side edge faces 26 of the body 12 in a direction away from the ends of the edge 44, and an outer convexly curved edge 48, which extends, part circumferentially, between the ends of the end edges 46 remote from the side edge 44. The brace plate 42 has a pair of openings 50 which receive stainless steel bushes 52 provided with annular stainless steel flanges 54 at opposite ends thereof, and a pair of openings 56 which received stainless steel pins 58. The bushes 52 and the pins 58 are received in passages drilled therefor through the body 12 to interconnect the openings 50 and 56 of the respective brace plates 42.

The brace plates 42 are adhesively secured to the major faces 18 of the body 12, their edges 44 and 46 being welded to the edges of the strap 20. The bushes 52 and pins 58 are respectively adhesively secured in the passages drilled therefor in the body 12, and the flanges 54 are adhesively secured to the ends of the associated bushes 52 and to the outer surfaces of the brace plates 42.

The strap 20 extends, peripherally to the body 12, along the edge faces 24 and 26, snugly embracing the body 12 and being adhesively secured to said edge faces. The strap 20 is further secured to the body 12 by 5mm-diameter stainless steel securing pins 60, adhesively secured in sockets drilled therefor through the strap 20 and into the body 12, to a depth of 30mm, the strap 20 having a width of 30mm, corresponding to the thickness of the body 12. The pins 60 are located spaced 50mm from each end of each side edge face 26 and 50mm from the ends of the outer edge face 24, and midway between the faces 18 of the body 12.

In Figure 3 a connector plate is generally designated 62. The plate 62 is of stainless steel and is elongated and rectangular in outline, being 20mm wide, 100mm long and having a slot 64 and an opening 66 formed therein. The slot 64 and opening 66 are centrally positioned between the side edges of the plate 62. The opening 66 is 6mm in diameter, the slot 64 having a width of 6mm and a length of 50mm. Between the ends of the outer edge face 24 and the pins 60 are provided two 5mm drilled and tapped sockets 68 through the strap 20 and into the body 12. In use, the plate 62 is bolted to adjacent elements 10 by bolts (not shown) respectively passing through the opening 66 and the slot 64 and into the sockets 68, to hold the elements in side-by-side abutment. The side edge faces 26 of the body 12 are inclined at an angle of 30° to each other, which allows twelve elements 10 to be connected edge-to-edge to one another in series, to form an annular filtration assembly with a circular outer outline and a twelve-sided inner periphery, where their connector fittings 40 can be connected to a common drainage line.

To make the body 12 it is pour-cast under gravity into a hollow split mould (not shown) having a rubber-lined inner cavity in which are located a plurality of cylindrical straight cores (not shown) to form the channels 28, 30 and 32, the cores for forming the channels 28 and 30 are spring steel rods provided with rubber sheaths or sleeves; and the core for forming the channel 28 is of stainless steel, being made up of two portions which extend into the mould cavity through opposite side edges thereof and abut the central core for the central channel 28. The stainless steel cores for the transverse channel 32 have outer ends provided with handles to facilitate extraction thereof. The cores for the channels 30 have narrow portions for forming the narrow inner end portions 36 of the channels 30, which narrow portions of the cores for forming the narrow portions 36 of the channels 30 are received in transverse passages provided therefor in the portions of the core for providing the channel 32.

In use the "POR-TECH" resin and filler are admixed with water in a resin:filler:water mass ration of 9:26:15 according to the manufacturer's instructions. More particularly, the filler and two-thirds of the water are mixed, followed by admixing with the remainder of the water, and the mixture is allowed to stand for 30 minutes and mixed again. The resin is then admixed with the water/filler mixture until homogeneous (2 minutes) and the mixture is slowly poured into the mould under gravity to form a casting. The casting heats up and reaches a temperature of 35°C after 60 - 90 minutes, and is then de-moulded, the mould being a split mould to facilitate this. The cores are removed from the casting, which is then allowed to cure for twelve hours at 40⁰C before being further fabricated, to form the body 12 of the filter element 10 described above with reference to the drawings. In Figure 5 the same reference numerals are used as in Figure 2 to designate the same parts, unless otherwise specified. The proportions and dimensions shown in Figure 5 are somewhat different from those of Figure 2, the principal difference being that the openings 56 are substantially smaller relative to the openings 50, compared with Figure 2, wherein they are more or less the same size.

Similarly, in Figure 6 the same reference numerals are used as in

Figure 3 to designate the same parts, unless otherwise specified. The connector plate 62 of Figure 6 is longer than the connector plate of Figure 3, there being four slots 64 spaced end-to-end in series, instead of the single slot 64 of Figure 3, and the opening 66 of Figure 3 is omitted in Figure 6.

Finally, in Figure 7, the same reference numerals are again used to designate the same parts as in Figure 1 , unless otherwise specified. The strap 20 has various pre-formed openings 70 for receiving the pins 60 (see Figure 1 ) and various pre-formed openings 72 for registering with the sockets 68 drilled in the outer edge face 24 of the body 12. There are four openings of registering with four sockets 68, instead of the two sockets 68 of Figure 1 , there being a socket 68 for each of the four slots 64 of the plate 62 of Figure 3. The ends 74 of the strap 20 are also somewhat wider than the rest of the strap 20, unlike Figure 1 where the strap 20 is of a constant width. This is to allow the edges of the ends 74 of the strap 20 to stand proud of the faces 18 of the body 12, to facilitate welding thereto of the end edges 46 of the brace plate 42.

The finished product, for storage purposes, should be soaked in water until it is saturated, and then sealed in a plastics bag which is kept immersed in a water troth at room temperature. Water at which the product is soaked should be clean, e.g. distilled, and should be free of any algial or any other microbial contamination.

Filter elements 10 of the type shown in the drawings have been found to be suitable for vacuum filtration and indeed for pressure filtration with pressure drops across the filter element from the major faces 18 of the body 12 to the drainage outlet 34 of up to 14 bar. Depending on pore size, the elements 10 can be used for microfiltration, ultrafiltration and indeed for reverse osmosis, flow reversals under a pressure drop of up to 6 bar from the outlet 34 to the major faces 18 of the body 12 being used to back-wash the body 12 and dislodge solids accumulated on the major faces 18.

Compared with similar conventional ceramic filter elements having filtration membranes coated on their major surfaces, the elements 10 illustrated in the drawings have been found to have a number of advantages. Thus, they are less expensive and stronger, and can withstand higher pressure drops across them, while being more durable. Higher pressure drops lead to higher yields with increased efficiency, and surface damage has not been found adversely to affect filtration, which is the case with coated ceramic elements. Porosity extends throughout the body 12, and is not confined to the surfaces of the major face 25. Finally, it is to be noted that disposal of spent elements provides few if any environmental problems.

Claims

CLAIMS:

1 . A filtration element (10) having a body (12) of open-cell porous material, the body having an outer surface (18, 22, 24, 26) provided with a multiplicity of pores and the body having a porous interior provided with a network made up of a multiplicity of interconnected passages which provide the porous material with its open-cell character, the body further having, in its interior, at least one drainage channel (28, 30, 32) leading to at least one drainage outlet (34), each drainage channel having a surface provided with a multiplicity of pores and the network of passages placing the pores of the outer surface of the body in liquid flow communication with the pores of the surface of each channel, to permit a liquid in which the element is immersed to flow, in response to a pressure drop across the body from the outer surface of the element to its drainage outlet, via the network of passages from the pores of the outer surface to the pores of each channel surface and along the associated channel to the associated drainage outlet, the filtration element being characterized in that the open-cell porous material of the body is a polymeric plastics or resinous material.

2. An element as claimed in Claim 1 , characterized in that the body (12) has, in its interior, a plurality of said drainage channels (28, 30, 32) which are interconnected to form a network which leads to a single common drainage outlet (34).

3. An element as claimed in Claim 1 or Claim 2, characterized in that its body (12) is of flattened shape, having a pair of substantially parallel oppositely outwardly facing major faces (18) interconnected by one or more edge faces (22, 24, 26), each channel (28, 30, 32) being located between the major faces and each drainage outlet (34) opening through a said edge face.

4. An element as claimed in Claim 3, characterized in that the flattened shape of the body (12) is in the form of a truncated sector, the major faces (18) having an outline which tapers radially inwardly from an arcuate convex part-circular radially outer circumferentially extending edge face (24) towards a tangential radially inner edge face (22), the body (12) having a pair of straight side edge faces (26) which radiate respectively from opposite ends of the inner edge face (22) to opposite ends of the outer edge face (24) and which diverge radially outwardly from each other so that their radially outer ends are spaced further apart than their radially inner ends.

5. An element as claimed in Claim 4, characterized in that the body (12) has a plurality of said drainage channels (28, 30, 32), the majority (28, 30) of which drainage channels radiate outwardly from radially inner ends thereof to radially outer ends thereof, the radiating channels (28, 30) being located in series side-by-side and converging towards each other from their outer ends which are at or adjacent the outer edge face (24) of the body (12) to their inner ends which are at or adjacent the inner edge face (22) of the body (12), the radiating channels (28, 30) being interconnected by at least one transverse channel (32) and a central said channel (28) of the series leading to a single said drainage outlet (34) from the body (12).

6. An element as claimed in Claim 4 or Claim 5, characterized in that each major face (18) of the body is provided with a strengthening and supporting brace plate (42) tightly secured to said major face (18), the brace plate being flattened and elongated and having a pair of spaced side edges (44, 48) extending along opposite sides thereof and a pair of end edges (46) at opposite ends thereof and respectively interconnecting the adjacent ends of the side edges (44, 48) to each other, one (44) of the side edges being straight and extending along the radially inner edge of the associated major face (18), the end edges (46) being straight and extending radially outwardly respectively along the side edges of said associated major face (18), the other side edge (48) of the brace plate (42) interconnecting the radially outer ends of the end edges (46), said other side edge (48) of the brace plate (42) and the radially outer ends of its end edges (46) being located closer to the radially inner edge of said associated major face (18) of the body (12) than to radially outer edge of said major face (18).

7. An element as claimed in any one of Claims 3 - 6 inclusive, characterized in that it includes a flattened metal strengthening and reinforcing strap (20) which extends around the periphery (22, 24, 26) of the body (12), being tightly secured face-to-face with the edge faces (22, 24, 26) of the body (12).

8. An element as claimed in any one of the preceding claims, characterized in that the pores and passages are microscopic, respectively having pore and passage sizes or diameters in the size range 0.2 - 20 microns (μm), each drainage channel being straight and cylindrical, and having a diameter in the size range 5 - 15 mm.

9. An element as claimed in any one of the preceding claims, characterized in that the polymeric material of the body is selected from the group of materials consisting of methylmethracrylates, methacrylic acid ethyl esters, N,N-dimethyl-p- toluidenes and suitable mixtures thereof.

10. An element as claimed in any one of the preceding claims, characterised in that the channels (28, 30, 32) have a cross-sectional outline which is selected from the group consisting of circular outlines and square outlines.

1 1 . A method of making a filtration element as claimed in Claim 1 , the method comprising forming the body to provide it with a desired shape and providing, in its interior, at least one drainage channel leading to at least one drainage outlet, the method being characterized in that forming the body and providing it with the desired shape is by casting it in a mould from a settable polymeric plastics or resinous material and causing or allowing the material to set to form the body of porous open- cell material having a porous interior provided with a network made up of a multiplicity of interconnected passages which provide the porous set material with its open-cell character, the body having an outer surface provided with a multiplicity of pores, each drainage channel having a surface provided with a multiplicity of pores, and the network interconnecting the pores of the outer surface with the pores of each drainage channel.

12. A method as claimed in Claim 1 1 , characterized in that the body is cast into a mould shaped to provide the body with a flattened shape, having a pair of substantially parallel oppositely outwardly facing major faces interconnected by one or more side edge faces, the method including providing the body, in its interior, with a plurality of interconnected drainage channels located between the major faces, the drainage channels being interconnected and leading to a single drainage outlet through an edge face of the body.

13. A method as claimed in Claim 12, characterized in that it includes strengthening and reinforcing the body by enclosing the periphery of the body made up by the side edge or edges of the body with a flattened metal strengthening and reinforcing strap which extends around the periphery of the body, and tightly securing the strap to the edge face or faces of the body.

14. A method as claimed in Claim 12 or Claim 13, characterized in that it includes supporting and bracing part of each major face of the body by means of a metal brace plate, by tightly securing the brace plate to said part of said major surface, and by welding part of the peripheral edge of each brace plate to part of the adjacent side edge of the strap.