AU617562B2 - Filtration - Google Patents

Description

AUT'I'KAJLA.

1 617562 COMMONWEALTH OF AUSTRALIA The Patents Act 1952 Nama of Applicant(s): SURESCREEN MANUFACTURING CO. PTY.

LTD.

Address of Applicant(s): 88 BRICKYARD ROAD **GEEBUNG,

QUEENSLAND,

1 4034, AUSTRALIA S* Actual Inventor(s): CHARLES EDWARD ALLSOP GREGORY ELDRIDGE Address for Service: G.R. CULLEN COMPANY, V Patent Trade Mark Attorneys, 79 Eagle Street, SBrisbane, Qld. 4000, Australia.

t COMPLETE SPECIFICATION FOR THE INVENTION ENTITLED:

FILTRATION

The following statement is a full description of the invention including the best method of performing it known to us: 2 THIS INVENTION relates to the separation of solid and liquid phases by vacuum filtration and, in particular, to.

an improved form of vacuum filter.

Vacuum filtration usually involves the application of a suspension or slurry to a filter medium through which the filtrate passes under sucticon with the fines accumulating against the filter medium to form a cake. The cal 4 e is dried and then removed from the filter medium.

ooa The filter medium employed in vacuum filtration is 0o jo P 'isually either a stainless steel mesh or a synthetic plastic woven into a cloth known as saran.

One type of filter employing vacuum filtration is tl-.e vacuum drum filter. The filter cloth of a vacuum drum So filter is stretched over a frame which is in the form of a ylinder having a horizontal axis. The cylinder is rotated about its horizontal axis and is partially immersed in a trough containing a slurry to be filtered. The interior of the cylinder is citrcumferentially compartmentalised so that suction can be selectively applied to different circumferential locations on the cylinder. The portion of the cylinder located within the slurry is placed under suction and filtrate is drawn into the interior of the cylinder through the filter cloth and a cake is formed on the filter cloth. As this portion of the cylinder leaves the slurry the suction is maintained through a drying phase.

After the drying phase, the air flow is reversed and the cake 3 is blown from the filter cloth. The blowing phase represents about thirty degrees cf rotation. To assist in removing the cake from the filter cloth, a stationery scraper knife is employed which scrapes along the outer surface of the cylinder.

Another type of filter employing vacuum filtration is the vacuum disc filter. This filter has the advantage of a much larger filter area for the same floor space than a o *0 drum filter. It consists of a number of discs mounted S:I^P* vertically on one horizontal shaft. A filter cloth is stretched over each disc to form a suction compartment therebetween. The discs are partially immersed in and rotate through a trough containing the feed slurry. Suction is applied to the compartment to draw filtrate through the I' filter cloth and the cake is formed on the disc sides.

f Suction is applied for about 75 percent of a revolution whereafter an air blow occurs which loosens the cake from the disc to be separated by scraper blades and then fall away into a chute. In this case the filter medium is in the form of a filter cloth or a stainless steel mesh.

The filter media mentioned above are not self supporting and each requires a supporting framework. In the case of the vacuum disc filter, the supporting framework includes an apertured corrugated sheet which forms the sides of each disc. The mesh or filter cloth is then stretched over the apertured disc.

4 A major problem with these filter media is that they are easilv damaged particularly by the scraper blades which are used during the blowing phase to assist in removing the cake from the filter media. Frequently, the filter cloth or fine mesh filter stretches as it ages causing some buckling. As the drum or disc rotates the scraper blades can engage with the cloth causing further buckling and, in many so** cases, tearing. The damaged filter cloth must therefore be *a* replaced.

*s ,1p In addition, where the filter cloth is supported on an apertured disc, the application of suction is non-uriform over the surface of the filter disc. The suction is concentrated about the apertures giving rise to inefficient filtration. It would be desirable to provide a filter which has a more uniform pressure distribution across the surface of its filter medium.

Further problems exist with the prior art vacuum *4 filters. The drum and disc filters mentioned above usually employ a plurality of sector or segment shaped frames. Each frame usually includes a radially extending drainage tube through which filtrate is removed. In the prior filters, filtrate tends to flow back onto the cake, thereby reducing the drying efficiency of the filter.

It would therefore be desirable to provide a sector frame having improved capacity for removing filtrate from the filter.

It is therefore a principal object of the present invention to overcome or alleviate at least some of the abovementioned deficiencies in prior art vacuum filters.

According to the present invention there is provided a vacuum filter comprising two parallel spaced apart filter grills each consisting of a plurality of elongate elements in fixed spaced relationship, and a tube extending between the grills which has a longitudinal slot for applying an even vacuum in the space between the grills so that a pressure differential is produced across each grill.

Preferably, the tubes are made from roll formed metal of substantially box section with the slot being formed by the edges of opposed return flanges. Preferably, such flanges are :'~hngled inwardly toward the interior of the tube.

The tube or tubes preferably define at least one edge S:wAll of the vacuum filter and, most preferably, comprise part of support frame for the vacuum filter. The support frame itself ,can be made up of a plurality of sub-frames

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t MI: 6 which may assume the form of segments. Such segments can be assembled to form a disc filter or a drum filter.

The elongate filter elements comprising the filter grill are preferably wedge-shaped in transverse crosssection. The spacing between the elements depends on the size of the fines to be filtered. Such spacing may, for instance, range from 0.1 mm to 1.0 mm, or greater. For a coal slurry, a spacing of about 0.3 mm is preferred. The filter elements are preferably parallel and can be supported w b. by a number of cross members which suitably extend 4 transversely across the filter elements at spaced mm) intervals.

The vacuum filter can be fabricated from metal or plastics material, or a combination thereof. The preferred material is stainless steel. The stainless steel can be partially or fully encapsulated in a plastics material such as polyurethane.

Preferred features of the invention will now be described with reference to the accompanying drawings, in which:- Fig. 1 is a cross-sectional view illustrating a drum filter; Fig. 2 is a perspective view illustrating a disc filter; Figs. 3 and 4 are perspective views illustrating a segment of a disc filter constructed in accordance with the

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present invention; and Fig. 5 is a detailed view illuztrating a portion of the filter sector of Figs. 3 and 4.

Referring to Fig. 1, there is illustrated a rotary vacuum filter 10 including a drum_ll which is partially immersed in a slurry 12. Slurry 12 is contained within a tank 13, and an agitator 14 maintains the fines in suspension. The drum 11 is divided into a plurality of compartments 15, each of which independently communicate with Sib, a central air valve or manifold 16 so that air can be directed or suction can be applied to each of the compartments independently. A filter medium 17 covers the drum 11, and cake formed on the drum 11 is removed by a scraper blade 18 as the drum rotates in a clockwise tS direction.

Referring now to Fig. 2, there is illustrated a t rotary vacuum filter 20 comprising five filter discs 21 which S, are partially immersed in and rotate through a slurry contained in trough 22. Each of the discs 21 is assembled from a plurality of segments or sectors 23. Each segment or sector 23 is covered with its own independent filter medium which is stretched over its surface prior to being assembled to form the disc 21. It is to be understood that each disc segm6nt forms a compartment which communicates with a central valve or manifold similar to that mentioned in relation to Fig. 1 so that air can be blown through the filter medium or ii i ii CI~~ 8 suction can be applied to the filter medium. Scraper blades are employed in the disc filter in similar fashion to the scraper blades .illustrated in Fig. 1.

Referring now to Figs. 3 and 4, the application of the present invention to the segment of a disc filter will be described in greater detail. In these drawings, there is illustrated a segment 23 of a disc filter. The segment 23 comprises two radial frame members 24 and 25 and an arcuate frame member 26 which bridges the divergent ends 27 and 28 of the frame members 24 and t ~A number of parallel cross members 29 are also employed to give additional strength to the segment. The filter grill 30 is shown in part cut-away to illustrate the cross member 29 and it will be realised that the filter grill 30 covers the entire surface. The filter grill 30 is on both sides 31 and 32 of the segment 23. The frame member 24 includes a longitudinal recess 33 along which in use a threaded rod is positioned for the purpose of securing adjacent segments together. An outlet 34 communicates with the compartment 35 formed within the segment between the sides 31 and 32.

Referring to Fig. 5, there is illustrated section A of Fig. 3 which show in more detail the frame member 24, the positioning of a threaded rod 44 in recess 33 and the filter grill 30. As can be seen the frame member 24 is of hollow general box section and has been roll formed so that the return flanges 36 and 37 form a slot 38 with their respective edges 39 and 40. The filter media 30 is made up of filter elements 41 which are welded or otherwise secured to base support members 42. Each filter element 41 is of wedge shape cross-section and is preferably made from metal or plastics material. Each element is approximately 1.1 mm across its base and the distance from the base to the apex is **o approximately 1.8 mm. The included angle between the base 000 and the tapered side walls 43 is in this case approximately 77 degrees.

*0 0 In operation, as the disc filter segments rotate, liquid is sucked in between the filter grills and passes through the hollow radial frame members 24, 25 to outlet 34.

Fines are collected on the filter grill 30 from which they are ultimatel,y removed at the appropriate stage of rotation by blowing air through the filter grill from within and/or by c.

use of a scraper blade which the filter grill passes by upon its rotation.

It will be appreciated that the filter grill employed in the present invention is of rigid construction when compared with the prior art an is therefore not susceptible to the buckling and tearing common with prior art filter cloth and filter mesh. In some cases however, the filter grill may be made to have a predetermined flexibility but this will not deleteriously detract from the overall strength and useful characteristics compared with the prior

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:j I i I art filters.

Whilst the above has been given by way of illustrative example of the invention, many modifications and variations may be made thereto by persons skilled in the art without departing from the broad scope and ambit of the invention as herein set forth in the following claims.

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Claims (7)

1. A vacuum filter comprising two parallel spaced apart filter grills each consisting of a plurality of elongate elements in fixed spaced relationship, and a tube extending between the grills which has a longitudinal slot for applying an even vacuum in the space between the grills so that a pressure differential is produced across each grill.

2. A vacuum filter -s claimed in claim 1, wherein the tube comprises at least one edge wall of the vacuum filter.

3. A vacuum fijter as claimed in claim 2, wherein there are two tubes comprising two opposed edge walls of the vacuum filter.

4. A vacuum filter as claimed in any one of claims 1 3, ::'wherein each tube comprises part of a support frame for the s..vlcuum filter. S A vacuum filter as claimed in claim 4. wherein the *pport frame comprises a plurality of sub-frames in the form of 's&gments which in toto form a disc filter or a drum filter. 44 A vacuum filter as claimed in any one of the preceding :,.claims, wherein the elongate elements forming the filter grills 0• are wedge wire.

7. A vacuum filter as claimed in any one of the preceding o Sclaims, wherein the elongate elements forming the grills are spaced from 0.1 mm to 1.0 mm from one another.

8. A vacuum filter as claimed in any one of the preceding claims wherein the elongate elements forming each filter grill are supported by supports extending across the set of elements.

9. A vacuum filter as claimed in any one of claims 1 4, S^ wherein each tube is of hollow box section which has been roll formed so that return flanges are produced along its length, said return flanges together forming the longitudinal slot. A vacuum filter substantially as herein described with reference to Figs. 3 5 of the accompanying drawings. DATED this 10th day of July 1991 SURESCREEN MANUFACTURING CO. PTY. LTD. By their Patent Attorneys CULLEN CO. 9* .4 II 0 *0 0* 9