Technical Field of the Invention

The present invention relates to a filter element, a method for producing a filter element, a filtration apparatus comprising a filter element and to the use of a filter element in a filtration apparatus.

Background to the Invention

Filter elements that are used in filter press apparatus typically comprise a filter press plate with a recess and a textile filter cloth. The filter cloth is typically draped over each face of the filter press plate and secured to the filter press plate, usually by tying the cloth to the filter press plate in a region outside of the filtration area, by using a hook and loop fastening means such as Velcro (RTM) or by using some other simple locating means.

An example of a conventional filter element is shown in Figure 2, and it can be seen that a poor fit exists between the filter cloth and the filter press plate. This is highly disadvantageous since an ill-fitting filter cloth is known to create regions of increased wear, resulting in shorter lifetimes and premature failure of the filter cloth. A further disadvantage is that an ill-fitting filter cloth tends to comprise an increased number or folds and creases. Not only does this result in poor sealing, which is significant in view of the high pressures involved in filtration, it also provides regions where solids from the filtration slurry become trapped and build up. This in turn increases the amount of time and effort needed to release the filter cake from the filter cloth which is both practically and commercially disadvantageous. In use, the filter cloth will also become stretched in the area which covers the plate recess as a solid filter cake forms within the plate recess formed between adjacent plates. The stretching of the filter cloth results in an increased risk of the cloth tearing, stretch damage and/or the creation of wear zones, all of which can limit the working lifetime of the filter medium.

As best shown in Figure 1, the surface of the filter plate comprises a plurality of protrusions that are configured to provide drainage pathways for filtrate that is expelled through the cloth once the filter press is closed, pressure is applied and the filter cake is squeezed. Since a textile cloth is not specifically adapted to fit to the filter plate, the drainage pathways can become partially or fully blocked by the textile thereby leading to a reduction in filtration efficiency. Accordingly, it is an object of embodiments of the present invention to provide a filter element where an improved fit exists between the filter plate and a filter medium. It is also an object of embodiments of the present invention to provide a filter element where the filter medium exhibits fewer folds and creases. A further object of embodiments of the present invention is to provide a filter element where increased sealing performance exists between the filter plate and the filter medium. It is yet a further object of embodiments of the present invention to provide a method for producing a filter element that exhibits improved sealing performance and cake release properties.

Summary of the Invention According to a first aspect of the invention there is provided a filter element for a filtration apparatus comprising a filter plate and a filter medium, wherein the contours of the filter medium are complimentary to the contours of the filter plate. The filter medium may be vacuum formed on the filter plate, pressure formed on the filter plate or thermoformed on the filter plate.

The provision of a filter element with a vacuum formed, pressure formed or thermoformed filter medium gives rise to an improved fit between the filter medium and the filter plate. In this connection the contours of the vacuum formed, pressure formed or thermoformed filter medium are complimentary to the contours of the filter plate to provide a snug fit as the filter medium is fitted on the filter plate. The improved fit also means that the filter medium is substantially crease and fold-free and that the known problems of premature wear and poor sealing between the filter medium and the filter plate are avoided or at least reduced. This in turn leads to improvements in filtration efficiency and in the purity of the filtrate obtained. Moreover, because the vacuum formed filter medium, pressure formed filter medium and thermoformed filter medium are substantially free from folds and creases, the filter cake can be easily and quickly released from the filter medium, thereby reducing the amount of filtration apparatus downtime when it is necessary to inspect, clean and/or replace the filter medium.

The filter medium may be vacuum formed pressure formed or thermoformed directly on the filter plate or alternatively the filter medium may first be vacuum formed, pressure formed or thermoformed on a filter plate mould and then provided on the filter plate. One advantage of pre-forming the filter medium on a filter plate mould is that a filter medium having pre-determined dimensions can be obtained which ensures that the filter medium conforms to the contours of the filter plate without blocking any of the drainage pathways defined by the protrusions on the filter plate. The filter plate preferably comprises a filter cavity and an outer edge that surrounds the filter cavity, and in one embodiment of the invention, the pre-formed filter medium is formed to fit within the filter plate cavity. The outer edge of the filter plate may be grooved, stepped or otherwise shaped so that the filter medium fits snugly in the filter plate cavity. The outer edge of the filter plate may additionally be provided with an elastomeric seal to prevent the lateral escape of filtrate during a filtering operation. The elastomeric seal is preferably non-porous and may be formed from an elastomeric material such as rubber.

The filter plate may be formed from a material that is capable of withstanding the high pressures and forces that are typically found in conventional filter press filtration apparatus. Metals and polymers such as polypropylene have been found to be particularly suitable for this purpose.

The filter medium may be in the form of a sheet or strip, and in one embodiment the filter medium may be in form of a formed sheet or strip,

The filter medium may comprise a vacuum formable plastics material such as a thermoplastic. Accordingly, suitable filter medium materials for vacuum forming are those that are able to substantially maintain their shape at room temperature and pressure and which are formable under applied heat.

The filter medium may be a woven material, a non-woven material such as needlefelt, a link fabric, a sintered material, a coated material, a laminated material or chemically treated material. For instance, the filter medium may be chemically treated to improve the abrasion resistance of the material. The filter medium may comprise any of the following yarns, either alone or in combination: monofilament yarns, multi-filament yarns, staple yarns or fibrillated tapes.

The filter medium may comprise any of the following weave patterns, either alone or in combination: plain, twill, satin, double layer, triple layer or modified versions of said weave patterns.

The filter medium may comprise any of the following, either alone or in combination: Acrylonitrile Butadiene Styrene (ABS), Polyamide (PA), Polyimide (PI), Polyester, Polyester Copolymer (PETG), Polystyrene (PS), Polycarbonate (PC), Polypropylene (PP), Polyethylene (PE), Polyvinyl Chloride (PVC) Acrylic (PMMA), Modacrylic, Polyphenylene Sulphide, Polytetrafluoroethylene, Co-Polyamide, Polyolefin, Polyaramid, Polyvinylidene fluoride, Polyvinylidene difluoride, Fluoropolymers and Polyether ether ketone

In one embodiment the filter medium may comprise high density polyethylene (HDPE). The relatively low coefficient of friction of HDPE has the benefit that the filter medium exhibits improved cake release properties.

Preferably the filter medium is porous so as to facilitate the separation of the liquid filtrate from suspended solids present in the slurry. The pores may have a size dimension of between 15 μπι and 160 μπι. In one embodiment the filter medium may have a pore size of 15-45μπι, while in another embodiment the pore size may be 50-90 μπι. In a further embodiment of the invention the pore size may be greater than 90 μπι and up to 160 μπι. The porous filter medium may have a pore size between 5 μπι and 35 μπι after pressure forming or thermoforming, e.g. when the filter medium is a sintered porous material. On the other hand, when the filter material is a non-woven material, e.g. needle felt, the pore size after pressure forming may be between 3 μπι and 165 μπι. The filter medium may have a thickness of 0.5 - 6.0 mm and in one embodiment the filter medium has a thickness of 1.0-4.0 mm. For instance the thickness of the filter medium may be 2.0-3.0 mm.

The filter medium may have a smooth outer surface to provide better sealing and aid cake release. The filter medium may comprise two or more layers, and in one embodiment the filter medium comprises a surface layer and a drainage layer. The pores in the surface layer preferably exhibit a reduced size dimension relative to those in the drainage layer. This enables the surface layer to efficiently filter the solid particles from the filter slurry, while the relative increase in pore size in the drainage layer leads to improvements in dewatering performance.

The filter element may comprise sealing means for preventing the lateral escape of the filtrate. The sealing means may for example comprise seals, clips or clamps.

The filter medium may optionally be provided with a microporous layer to form a laminate. The microporous layer may comprise a thermoplastic such as ultra-high molecular weight polyethylene (UHMWPE). The thickness of the microporous layer may be between 10 μπι and 200 μπι and in one embodiment the thickness is between 20 μπι and 150 μπι. The microporous layer may have a pore size between 0.05 μπι and 15 μπι. The provision of the microporous layer on the filter medium gives rise to an improved filtration efficiency. In one embodiment of the invention the laminate may comprise a HDPE filter medium and the microporous layer.

According to a second aspect of the invention there is provided a method of producing a filter element which comprises the steps of forming a filter medium on a filter plate substrate so that the filter medium conforms to the contours of the filter plate.

The method according to the second aspect of the invention may be used to produce the filter element according to the first aspect of the invention and therefore the method according to the second aspect of the invention may, as appropriate, incorporate any or all of the features described in relation to the first aspect of the invention The method may comprise the step of heating the filter medium to a predetermined temperature rendering the filter medium mouldable; bringing the filter medium into contact with the filter plate substrate and applying a vacuum treatment to form the filter medium on the filter plate substrate.

In some embodiments the method may comprise the steps of bringing the filter medium into contact with the filter plate substrate and then thermoforming the filter medium. The filter medium may be thermoformed at a temperature of less than 50°C. In particular, the filter medium may be thermoformed at temperature between 25°C and 50°C.

The filter medium may be thermoformed at less than 25 tons of pressure. In particular, the filter medium may be thermoformed at 10 to 20 tons of pressure since improvements in the tensile strength of the material can be obtained. The application of a pressure of less than 25 tons may also be desirable since this may reduce the possibility of material "spring back" which could lead to the material returning to its original thickness after thermoforming. In other embodiments the method may comprise the steps of bringing the filter medium into contact with the filter plate substrate and then pressure forming the filter medium. In particular, the filter medium may be pressure formed under at least 25 tons of pressure since it has been found this reduces the possibility of the pressure formed material springing back to its original configuration.

The filter plate substrate may be a filter plate in which case the filter medium may be vacuum formed directly on the filter plate. Alternatively, the filter plate substrate may be a mould of a filter plate in which case the filter medium may be vacuum formed to the shape of the mould and thereafter provided on the filter plate. Irrespective of whether the filter medium is provided on the filter plate directly or indirectly, the contours of the vacuum formed filter medium conform and are complimentary to the contours of the filter plate such that a tight, close fit is obtained once the filter medium is fitted to the filter plate. This is turn gives rise to an improved filtering efficiency, and due to the absence of folds and creases in the filter medium, improved filter cake release properties.

The method may comprise the step of introducing pores in the filter medium.

The mould may be heated and can be used to effect heating of the filter medium to the pre-determined temperature prior to the step of vacuum forming. The mould may be formed from wood, metal or plastics. Preferably the mould is made from a material that enables the filter medium to be easily and quickly released from the mould once the filter medium has cooled and hardened after the vacuum forming treatment. The mould may be produced using a computer-controlled cutting machine such as a computer numerical control (CNC) router which enables moulds to be manufactured to a variety of filter plate designs with precision and accuracy.

When the filter medium comprises a thermoplastic the filter medium is preferably heated to a temperature above its glass transition temperature.

According to a third aspect of the invention there is provided a filtration apparatus, such as a filter press, comprising the filter element according to the first aspect of the invention. The filter press may comprise a vibration producing device that, in use, can be activated to aid release of the filter cake from the surface of the filter medium. When the filter element of the present invention is used in a vibrating filter press further improvements in cake release performance can be obtained due to the combined effects of vibrating the filter element and the filter medium's smooth surface.

According to a fourth aspect of the invention there is provided the use of a filter element according to the first aspect of the invention in a filtration apparatus such as a filter press apparatus.

Detailed Description of the Invention

In order that the invention may be more clearly understood embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which: Figure 1 shows a filter plate.

Figure 2 shows a filter element according to the prior art.

Figure 3 shows a schematic of a filter press operation. Figure 4 shows a filter medium according to the present invention.

Figure 5 shows a filter element comprising the filter medium of Figure 4 and the filter plate of Figure 1

Figure 1 shows an example of a filter plate 100 that may be used in a filter press filtration apparatus. The filter plate 100 comprises a central aperture 1 that in use acts as a slurry feed channel. The filter plate 100 also comprises openings 2 at each corner of the filter plate 100 which in use act as a filtrate channel. In this example four reinforcing elements 3 are grouped around the central aperture 1 to reinforce and strengthen the filter plate 100. While this embodiment of the invention makes use of a central aperture 1 it will be appreciated that the aperture 1 does not necessarily have to be located in a central region of the plate 100. Similarly, it will be appreciated that the number and the position of the openings 2 in the filter plate can be varied as needed or as desired.

Figure 2 shows a filter element 200 according to the prior art in which a filter cloth 4 is draped over each face of the filter plate 100. The filter cloth 4 comprises a central hole 5 and filter cloth openings 6 that corresponds with the central aperture 1 and openings 2 of the filter plate 100. As shown in Figure 2 a poor fit exists between the filter cloth 4 and the filter plate 100 which leads to a reduction in filtration efficiency due to poor sealing between the filter plate 100 and filter cloth 4 as well as premature wear of the filter cloth 4.

Figure 3 provides an illustration of a filter press apparatus 300. The filter press apparatus 300 comprises a series of filter plates 100 that each have a recess and a central aperture 1 through which slurry can enter. As best shown in Figure 2, each filter plate 100 is provided with a filter cloth 4. Prior to commencing the filtration operation the filter plates 100 are pushed together, typically by a hydraulic ram 7, to form a series of chambers 8. In doing so, the filter cloths 4 interposed between adjacent filter plates 100 are clamped together to form an effective sealing surface. Slurry is then fed into each of the chambers 8 under pressure via a slurry inlet 9 so that the filtrate passes through the filter cloth 4 and exits via a filtrate outlet 1 1 to leave behind the filter cake in each of the chambers 8. However, as the filter cake accumulates, the filter cloth 5 is forced to conform to the shape of the recess in the filter plate 100 which can lead to tearing, stretch damage and wear zones, all of which reduce the workable lifetime of the filter cloth 4.

In accordance with the present invention there is provided a method for producing a filter element 400 comprising a filter plate 100 and a filter medium 10 in the form of a HDPE (Porex) sheet. In a first step a 2.5 mm thick sintered HDPE sheet is provided. The HDPE sheet is porous and in this embodiment the pores have a size dimension of 50-90 μπι. The HDPE sheet is then heated to a pre-determined temperature above the glass transition temperature of HDPE which causes the thermoplastic to soften and become pliable and mouldable. Then the HDPE sheet is positioned above the filter plate 100 and thereafter the HDPE sheet and the filter plate 100 are brought into contact so that the HDPE sheet is stretched over and covers the filter plate 100. The HDPE sheet is then subj ected to a vacuum forming treatment which causes the HDPE sheet to conform to the shape of the of filter plate 100. The vacuum formed filter medium 10 is then allowed to cool and harden before the filter element 400 comprising the filter plate 100 and the vacuum formed HDPE sheet can be used in a filter press filtration apparatus 300. As best shown in Figure 4, the vacuum formed filter medium 10 comprises a central filter medium aperture 11, filter medium openings 12 at each corner of the filter medium and filter medium reinforcing zones 13 that surround the filter medium aperture 11. It can be seen that a snug fit will be obtained between the vacuum formed filter medium 10 and the filter plate 100, that the contours of the vacuum formed filter medium 10 are complimentary to the contours of the filter plate 100 and that the vacuum formed filter medium 10 is substantially fold and/or crease-free. Accordingly, the filter element 400 obtained in accordance with the present invention overcomes the known problems of prior art filter elements (Figure 2) in that a leak-free seal and a superior fit between the filter plate 100 and the filter medium 10 is obtained. This in turn means that the filter element 400 of the present invention affords the user improvements in filtration efficiency when used in a filtration apparatus such as a filter press belt. Moreover, since the vacuum formed FIDPE sheet is substantially free from creases and has a relatively low coefficient of friction, improvements in filter cake release properties are also obtained.

In a further embodiment of the present invention the HDPE filter medium 10 is provided indirectly on the filter plate 100. In this connection the HDPE sheet is heated to a pre-determined temperature above the glass transition temperature of HDPE to render the HDPE sheet pliable and mouldable. The HDPE sheet is then positioned over a filter plate mould (not shown) that has the same profile as the filter plate 100. In this embodiment the filter plate mould is made from wood using a CNC router but it will be appreciated that the mould can also be formed from any other suitable material, e.g. metal or plastics. The heated HDPE sheet is then brought into contact with the filter plate mould and is thereafter subjected to a vacuum treatment so that the the vacuum formed HDPE sheet conforms to the contours of the filter plate mould. After the HDPE sheet has cooled and hardened it is removed from the filter plate mould using suitable cutting means and subsequently fitted on the filter plate 100. Due to the snug fit between the vacuum formed HDPE sheet and the filter plate 100, filter elements 400 manufactured in accordance with this embodiment of the invention also offer improved filtering and cake release performance.

In accordance with another embodiment of the invention there is provided a method for producing a filter element 400 by thermoforming. In a first step, a sintered porous HDPE sheet (140 mm x 140 mm) is cut to conform to the dimensions of a filter plate mould. A bottom mould part is provided in an oven where it is pre-heated to a pre-determined temperature, e.g. 25°C so that when the HDPE sheet is provided on the bottom mould part it starts to soften. A top mould part is placed on the HDPE sheet and then the mould is placed under a press device. The mould is then subjected to a predetermined pressure, e.g. 20 tons, to produce a HDPE sheet which conforms to the contours of the filter plate 100 mould. Since a snug fit can be obtained between the thermoformed HDPE sheet and the filter plate 100, it was a found that filter elements 400 that comprised the thermoformed filter medium offered improved filtering and cake release performance properties relative to filter elements that comprise conventional textile filters. In addition, it was found that the thermoformed filter medium 10 material had a tendency to spring back after thermoforming if it had been placed under a pressure of more than 25 tons. Moreover, a decrease in air permeability was observed if the sintered porous HDPE material was heated to above 50°C since at such temperatures it was observed that pore began to close. In accordance with a further embodiment of the invention there is provided a method for producing a filter element 400 by p°ressure forming. The method comprises the steps of providing a sintered porous HDPE sheet (140 mm x 140 mm) and cutting it to conform to the dimensions of a filter plate mould. The HDPE sheet is then placed between a bottom mould part and a top mould part, after which the mould is provided in a press device and subjected to a pressure of 25 tons. The inventors found that by subjecting the material to a pressure of at least 25 tons that the material did not substantially spring back to its original configuration. In addition, it was found that the filtering and cake release properties could be improved when a pressure formed filter medium 10 was used instead of a conventional textile filter medium. This has been attributed to the pressure formed filter medium 10 conforming to the contours of the filter plate 100 which enables a better fit to be obtained between the filter plate 100 and the filter medium 10.

In another embodiment of the invention a Tuftex impregnated needlefelt material was pressure formed in the manner described above, namely by providing the needlefelt material in a mould and then placing the mould under pressure without the addition of heat. By subjecting the mould to 25 tons of pressure the shape of the press plate was clearly visible following the pressure forming treatment. Moreover, the use of the pressure formed needlefelt material enabled improved cake release and filtration properties to be obtained relative to filter elements 400 that comprised conventional textile filter mediums.

The above embodiments are described by way of example only. Many variations are possible without departing from the scope of the invention.