AU2008324753B2

AU2008324753B2 - Filtration element

Info

Publication number: AU2008324753B2
Application number: AU2008324753A
Authority: AU
Inventors: Lesley Adele Ironside; Silvio Nicoli
Original assignee: BHP BILLITON WORSLEY ALUMINA Pty Ltd
Current assignee: South32 Aluminium Worsley Pty Ltd
Priority date: 2007-11-05
Filing date: 2008-11-04
Publication date: 2012-07-19
Anticipated expiration: 2028-11-04
Also published as: AU2008324753A1; BRPI0820060A2; WO2009059354A1

Abstract

A filtration element (10) comprises two filter supports (12) disposed in a mutually facing spaced apart relationship to form a gap (26) therebetween. Each filter support (12) comprises a plurality of openings (14) through which a liquid can flow into the gap. In addition each filter support (12) has a footprint area and an effective surface area wherein the effective surface area is greater than the footprint area. A pliable filter medium (28) surrounds the filter supports (12). Thus liquid first passes through the filter medium (28) then through the openings (14) into the gap (26). The filter medium allows the liquid to flow through while blocking passage of solid particles above a predetermined particle size, forming a filtrate that flows into the gap (26). The filtrate can be discharge thought a conduit (30) that is in fluid communication with the gap (26).

Description

WO 2009/059354 PCT/AU2008/001632 FILTRATION ELEMENT Field of the Invention 5 The present invention relates to a filtration element for filtering solid-liquid systems. Background of the Invention 10 Filters are commonly used in industry to remove solid particles from liquids. For example, in the Bayer process, vertical pressure filters are commonly employed to remove fine mud and filter aid particles from alumina rich caustic soda solution, while vacuum disc and drum 15 filters are used to separate aluminum tri-hydrate product from caustic soda solution. The vertical pressure filter typically comprises a pressure vessel in which is suspended a plurality of 20 filtration elements. A filtration element comprises a rectangular metal frame having two wire mesh panels welded one to each side of the frame to define a space therebetween. Each filtration element is clothed with a suitable fabric that acts as a filtration medium to trap 25 solids thereby producing a filtrate. The wire supports the fabric. The filtrate flows into the space between the wire mesh panels. A discharge pipe is coupled with a top portion of the frame to receive the filtrate from the space between the wire mesh panels. 30 A feed solution comprising solid particles entrained in a liquid is initially pumped into the vertical pressure filter under pressure. The liquid passes through the pores in the fabric and openings in the wire mesh into the 35 space between the mesh and is subsequently fed to the discharge pipe. The filtrate is discharged from the discharge pipe into a larger pipe, frequently common to a number of filtration elements, which then drains under gravity from the vessel at atmospheric pressure. Solid particles accumulate gradually on the fabric 5 forming a cake that increases resistance to filtrate flow and reducing filtration rate. After a prescribed period of time, feed flow is halted and the contents of the filter drained into a collection tank. A sluicing wash is applied to the filter el-nts to remove cake and restore 10 the filter elements and in particular fabric for another filtration cycle. Summary of the Invention 15 According to one aspect of the present invention there is provided a filtration element compprising: two filter supports each filter support shaped or configured to comprise a plurality corrugations or folds, the two filter supports disposed in a mutually facing 20 spaced apart relationship to form a constant distance therebetween wherein that the corrugations or folds are in mutual alignment, each filter support having a footprint area and an effective surface area wherein the effective surface area is greater than the footprint 25 area, and wherein each filter support comprises a plurality of openings through which a liquid can flow into the gap. The filtration element may further comprise a pliable 30 filter medium that surrounds the filter supports wherein liquid passes through the filter medium prior to flowing into the gap. The pliable filter medium may comprise respective 35 portions that overlie outside surfaces of the filter supports wherein each portion of the filter medium has, ZS ZYt 2-o Ul in use, a surface area equal to or greater than the effective surface area. The corrugations or folds may have a uniform period and 5 amplitude. The or each filter support may be formed from a plate material. However, in an alternate embodiment, the or each filter support may be formed from a mesh sheet or 10 panel. The openings may be configured to have a total surface area in the range of 40% to 70% of the effective surface area. The corrugations or folds may have a period in the 15 range of 20mm ~ 60mm. The corrugations or folds in the or each filter support may have a uniform period, although in an alternate embodiment two or more corrugations or folds may be 20 formed with different period. The corrugations may have an amplitude in the range of between 10mm - 30mm. It may be most convenient for each of the corrugations to have the same amplitude however it 25 is envisaged that embodiments of the filtration element may be formed where at least two of the corrugations have a different amplitude. The filtration element may further comprise a frame 30 defining an aperture wherein the or each filter support is coupled to the frame and overlies the aperture. Further when the filtration element comprises two spaced apart filter supports the filtration element may further 35 comprise a conduit having first and second ends wherein one or both of the ends are open, the conduit being in fluid communication with the gap at one or more locations ~4~ intermediate the ends of the conduit, wherein liquid flowing into the gap flows into the conduit at the one or more locations is discharged through the or each open end nF the conduit 5 The pliable filter element may be made of cloth, and in particular a woven or felted material. The filter medium may be pleated or gathered to ensure 10 sufficient filter medium is available to cover the or each filter supports of the filter element. The filter supports may be substantially vertical. 15 Brief Description of the Drawings An embodiment of the present invention. will now be described by way of example only with reference to the accompanying drawings in which: 20 Figure 1 is a schematic representation of an embodiment of a filtration element in accordance with the present 25 THIS SECTION IS INTENTIONALLY BLANK 30 35 WO 2009/059354 PCT/AU2008/001632 -5 invention but with a covering filter medium partially removed; Figure 2 is a view of section AA of the filtration element depicted in Figure 1; and, 5 Figure 3 is a view of detail A shown in Figure 2. Detailed Description of the Preferred Embodiment Throughout this specification including the description 10 and claims the expression "footprint area" is intended to denote the area defined or bound by the peripheral edge of an article. For example, for a rectangular sheet of material having one side of length X meters and another side of length Y meters, the footprint area is simply X.Y 15 m2. In contrast, the expression "effective surface area" is used to denote the entire surface area of one side of the article. For example, for a rectangular sheet of material having one side of length X m and another side of length Y m and n triangular corrugations of the same period with 20 the corrugations parallel to the side having length Y and each side of a corrugation having a length Z m then the effective surface area is Y.2nZ M 2 . It should be appreciated that the length 2nZ is greater than length X and thus the effective area of the corrugated material is 25 greater than the footprint area assuming identical X and Y dimensions. In qualitative terms, the effective area may be considered to be the area of a pliable sheet placed on the article which contacts the entire surface of the article and being coterminous with a peripheral edge of 30 the article. The term "filtration" refers to the act of removing solid particles greater than a predetermined size from a feed comprising a mixture of solid particles and liquid. The 35 expression "filtrate" refers to the mixture less the solid particles removed by the filtration process. It will be appreciated that this mixture may contain solid particles WO 2009/059354 PCT/AU2008/001632 -6 smaller than the predetermined particle size. The expression "filter cake" refers to residual solid material remaining on a feed side of a filtration element. 5 The term "filter medium" refers to a material that traps solid particles of a size greater than the predetermined sized while allowing a remaining fraction of the mixture to pass through. 10 Referring to the accompanying drawings, a filtration element 10 comprises a filter support 12 having a footprint area and an effective surface area where the effective surface area is greater than the footprint area. Filter support 12 further comprises a plurality of 15 openings 14 through which liquid (and indeed solids of a size smaller than the openings 14) can flow. In this particular embodiment in order for the effective surface area of the filter support 12 to be greater than 20 its footprint area the filter support 12 is shaped or configured to comprise a plurality corrugations or folds 16. The filter support 12 is in a rectangular shape having a 25 long side 18 of length Y m and a short side 20 of length X m. The corrugations 16 extend parallel to side 18. Each corrugation 16 has a period P m and has sides of equal length Z m. For a filter support 12 having n corrugations 16 the effective surface area of the filter element 12 is 30 Y.2Zn M2 . However the footprint area of the filter support 12 is Y.X M 2 . It will be appreciated that due to the configuration of the corrugations 16 the dimension 2Z will always be greater than the dimension P and accordingly the effective surface area is greater than the footprint area. 35 In this particular embodiment, the filtration element 10 comprises a rectangular frame 22 which defines or bounds WO 2009/059354 PCT/AU2008/001632 -7 an aperture 24. Two filter supports 12 are attached to the frame 22 one on each side of the frame 22 and are spaced apart by a distance S to form a space or gap 26 therebetween. The corrugations or folds on each filter 5 support 12 are aligned to provide constant (i.e. equal) spacing S at all points between the supports 12. A filter medium 28 in the form of a cloth or fabric sheet or bag covers or otherwise surrounds the frame 22 and thus 10 the outside surfaces of the filter support 12. The cloth or fabric may be a woven or felted material. The filter medium 28 will allow liquid to pass there through while blocking the flow of solid particle above a predetermined particle size. This size is dependent upon the physical 15 characteristics of the filter medium 28. The filter medium 28 is pliable and has a portion that overlays and more particularly loosely drapes over the surface of the filter support 12. This portion has a 20 surface area equal to or greater than the effective surface area of the filter support 12. In order to ensure sufficient filter medium is available to cover the or each filter supports of the filter 25 element, the filter medium may be pleated or gathered. A feed flow of a solid-liquid mixture is presented to each side of the filtration element 10. Liquid is able to pass through the filter medium 28 and the openings 14 in the 30 filter support 12. Solid particles of a size too large to pass through the filter medium 28 collect on the filter medium 28. A pipe or conduit 30 having first and second ends, one or 35 both of which are open, is associated with the filtration element 10 and is in fluid communication with the space or gap 26 so that liquid flowing into the gap 26 can be WO 2009/059354 PCT/AU2008/001632 -8 discharged from the open ends of the conduit 30. One or more openings or slots (not shown) are formed in the conduit intermediate its ends to allow the liquid in the gap to flow into the conduit. In the specific illustrated 5 embodiment the conduit 30 forms part of, or extends across, a top edge of the filter element 10 and is in fluid communication with the space 26 between the filter supports 12. Thus filtrate within the space 26 is able to flow through the conduit 30 for discharge and/or further 10 processing. The flow rate through the filtration element 10 is dependent at least in part on the effective surface area presented to the feed as it flows through the filter 15 support 12 and the rate of filtration. By forming the filter support 12 with corrugations or folds 16 the effective surface area is, as demonstrated above, greater than the footprint area of a planar (i.e. uncorrrugated) filter support. 20 The total area of the openings 14 within each filter support 12 may typically be in the range of 40-70% of the effective surface area of the filter support 12. An embodiment for the filtration element 10 through a 25 vertical pressure filter used to remove fine mud and filter particles from alumina rich caustic soda solution has a frame supporting a filter support 12 with an aperture 24 having height dimension Y = 3.97 meters, a width X = 2.3 meters and a half corrugation length Z = 30 0.0283 meters. Assuming the structure shown in Figure 2 where there are two filter supports 12 spaced apart by a distance of 0.04 meters then the effective surface area of each filter support 12 is 25.8 m 2 and the volume of the space 26 is 0.517 M 3 . For a filtration rate of 1 m 3 /m 2 /h 35 the filtration flow is 25.8 m 3 /h. This provides a velocity through the filtration element 10 of 0.055 m/s.

WO 2009/059354 PCT/AU2008/001632 -9 In comparison with a standard filtration element in which the filter supports 12 are the same length and width dimension as mentioned above but are planar and thus have no corrugations, the effective surface area is the same as 5 the footprint area which in this case would be 18.3 m 2 and the volume between the planar filter supports 12, assuming that they are maximum distance apart but within the width of the frame 22, would be 0.55 M 3 . Thus a filtration rate of 1 m 3 /m 2 /h requires a filtration flow of 18.3 m 3 /h 10 providing a slower velocity through the filter of 0.037 m/s. Thus an embodiment of the present invention provides an increased filtration flow for the same footprint area. When used in the environment of a vertical pressure filter 15 for the Bayer process, the total surface area of the openings 14 within each filter support 12 may be between 40-70% of the effective area. The corrugations or folds 16 may have a period P in the range of 20mm-60mm although in the above mentioned example, the period P is 40mm. The 20 corrugation 16 may have an amplitude A in the range of 20mm-30mm although in the above example the amplitude A is 20mm. This amplitude, for corrugation 16 having the profile of an isosolese triangle, gives a half corrugation length Z of 28.3mm. 25 Now that an embodiment of the present invention has been described in detail it will be apparent to those skilled in the relevant arts that numerous modifications and variations may be made without departing from the basic 30 inventive concepts. For example, in the described embodiment, the corrugations 12 run in a vertical direction. However this is not necessary and other orientations are possible. The vertical orientation on the corrugation 16 in this embodiment is beneficial in terms 35 of removing filtration cake by application of a sluicing wash which runs vertically down the filtration element 10. Further, the corrugations 16 are depicted as having a WO 2009/059354 PCT/AU2008/001632 - 10 uniform period and amplitude. However different corrugations may be formed with different period P or amplitude A. The filter support 12 may be made from any material suitable for the physical condition in which it 5 is to be used. In particular, the material should have sufficient rigidity so as not to lose its shape and configuration in the operating conditions and be made from a material that is not substantially damaged by the solid mixture. When used in the Bayer process, the filter 10 support 12 is typically made from mild steel or stainless steel plate with the openings 14 form by a punching or stamping process. Thereafter, the plate can be pressed to form the corrugation 16. However in an alternate form, the filter support 12 may be made from a wire mesh in which 15 the openings 14 are inherently formed and subsequently pressed to produce the corrugations 16. Further, in the illustrated embodiment, the filter support 12 has a rectangular footprint. However other shapes are 20 possible such as circular discs, squares, triangles, etc. Also the material form which the filter support 12 is made can be non metallic material such as fiberglass or a plastics material. In addition while the conduit is illustrated as running along a top edge of the filtration 25 element, it may be disposed between the filter supports, and for example intermediate upper and lower edges of the filter supports 12. All such modifications and variations, together with 30 others that could be obvious to a person of ordinary skill in the art, are deemed to be within the scope of the present invention the nature of which is to be determined by the above description and the appended claims. 35

Claims

1. A filtration element comprising: two filter supports, each filter support shaped or configured to comprise a 5 plurality corrugations or folds, the two filter supports disposed in a mutually facing spaced apart relationship to form a constant distance therebetween wherein that the corrugations or folds are in mutual alignment, each filter support being corrugated having a footprint area 10 and an effective surface area wherein the effective surface area is greater than the footprint area, and wherein each filter support comprises a plurality of openings through which a liquid can flow into the gap 15

2. The filtration element according to claim 1 comprising a pliable filter medium that surrounds the filter supports wherein liquid passes through the filter medium prior to flowing into the gap. 20

3. The filtration element according to claim 2 wherein the pliable filter medium comprises respective portions that overlie oxtsd surfaces of the f i.lter supports wherein each portion of the filter medium has, in use, a surface area equal to or greater than the effective 25 surface area.

4. The filtration element according to claim 2 or 3 wherein the filter medium is pleated or gathered to ensure sufficient filter medium is available to cover the 30 filter supports.

5. The filtration element according to any one of claims 1 to 4 wherein the corrugations or folds have a uniform period and amplitude. 35 2,57S~A -12

6. The filtration element according to any one of claims 1-5 wherein the filter supports are formed from a plate material. 5

7. The filtration element according to any one of claims 1-5 wherein the filter supports are formed from a mesh sheet or panel.

8. The filtration element according to any one of claims 10 1-7 wherein the openings are configured to have a total surface area in the range of 40% to 70% of the effective surface area.

9. The filtration element according to any one of claims 15 5-8 wherein the corrugations or folds have a period in the range of 20mm - 60mm.

10. The filtration element according to any one of claims 5-9 wherein the corrugations may have an amplitude in the 20 range of between 10mm - 30mm.

11. The filtration element according to any one of claims 1-10 comprising a frame defining an aperture wherein the filter supports are coupled to the frame and overlie the 25 aperture.

12. The filtration element according to any .one of claims 1-11 comprising a conduit having first and second ends wherein one or both of the ends are open, the conduit 30 being in fluid communication with the gap at one or more locations intermediate the ends of the conduit, wherein liquid flowing into the gap flows into the conduit at the one or more locations and is discharged through the or each open end of the conduit. 35 -13

13. The filtration element according to any one of claims 1-12 wherein the pliable filter element is made of a cloth or fabric material. 5

14. The filtration element according to claim 15 wherein the cloth or fabric material is a woven or felted material.

15. The filtration element according to any one of the 10 preceding claims wherein the filter supports are substantially vertical. 2w- 1- Bow