WO1998030385A1

WO1998030385A1 - Improved composition and design of a filter fabric useful in pulp and paper making apparatus

Info

Publication number: WO1998030385A1
Application number: PCT/US1998/000425
Authority: WO
Inventors: Joseph P. Fagan; R. Keith Sheets
Original assignee: Fagan Joseph P; Sheets R Keith
Priority date: 1997-01-10
Filing date: 1998-01-07
Publication date: 1998-07-16
Also published as: CA2271824A1

Abstract

Filtering mediums are disclosed composed of fabric having a plurality of fibers where the surfaces thereof are coated with an ethylene chlorotrifluoroethylene copolymer. The fibers may also be extruded ethylene chlorotrifluoroethylene copolymer.

Description

IMPROVED COMPOSITION AMD DESIGN OP A FILTER FABRIC USEFUL IN PULP AND PAPER MAKING APPARATUS BACKGROUND OF THE INVENTION 1. Field of the Invention. The present invention relates to filter fabric which is particularly useful in a rotary drum vacuum separator washer or the like for use in pulp and paper making apparatus. 2. Description of the Prior Art. For many years, rotary drum filters have been used in the pulp and paper industry to separate suspended and processed pulp particles from the fluids (black liquor) used in the manufacture of paper. These filters operate on the principle of the bottom arc of a rotating drum contacting a reservoir of pulp suspended in liquid and applying a vacuum to draw the pulp suspension onto the drum and to draw the fluid through a perforated screen surface such as a fabric or wire cloth which covers the drum. The solids are thereby deposited on the covering filter while the process liquors are drawn into the core of the drum. At a later point in the drum rotation cycle, the deposited cake or layer of pulp is removed from the surface of the filter. Removal is accomplished by means such as a doctor blade or board set in close proximity to or against the "surface" of the drum (see for example, U.S. Patent No. 4,505,137, the disclosure of which is incorporated by reference and U.S. Patent Nos. 3 , 363 , 744 ; 3 , 403 , 786 ; 3 , 409 , 139 and 4 , 138 , 313 ) . The composition and design of the drum covering plays an important role in separating the object (pulp) from the process liquids. This filtering device must have sufficient tortuosity to effectively remove the suspended particles while simultaneously allowing a large volume of the liquid fraction to flow through the medium and into the drum core. The continuous cycle of paper-making process imposes a high degree of fatigue on the filter device covering as a result of the constant flexing/compression of the rotation. In addition, the filter cover is continually abraded by hard deposits in the solids of the filtrate such as mineral deposits and other foreign material, as well as by the pulp removal mechanism. Further demands are also made on the filter cover since the process takes place at elevated temperatures and the aqueous suspension frequently contains harsh chemicals such as bleach and chlorides from bleach, peroxides, processing aids and acids. These hostile environments may shorten the useful life of the filter. The trapping of particles of mineral deposits, lignin-based products or particles from corrosion by-products within the matrix of the filter layer is especially deleterious: such precipitates tend to adhere to the filaments of the filter, thereby "blinding" or plugging the filtration process. When any of these hostile conditions render the filter wrap ineffective, the overall manufacturing process must be stopped and a replacement filter wrap must be installed. This obviously entails the high cost of lost production as well as the expense to install a new filter cover. It has been a continuing goal of the industry to extend the useful life of the filtering medium. Moreover, the trend in the pulp processing industry is towards increasingly stressful and stringent operating conditions and temperatures. Thus, advances in both the composition and design of the filter layer covering these rotating drums is desirable to minimize both the plugging and mechanical and/or corrosive wear, i.e. to extend their useful life. In terms of composition, these filter matrices are generally made of either stainless steel or extruded polymer. The typical format of the filter materials is a woven textile-type mat or screen. It should also be noted that U.S. Patent No. 5,407,736 - McKeon describes the use of a monofilament formed from the combination of a polyester resin and a fluoropoly er resin for a paper making fabric having improved abrasion resistance. Each of these other prior types of composition has advantages and disadvantages. Stainless steel is heavy, and very difficult and costly to install. Stainless steel can handle a wide range of operating temperatures and conditions. However, the most serious drawback of stainless steel is that mineral deposits and other sticky contaminants readily adhere to the matrix, causing pluggage and stoppage and resulting process downtime. Polymeric filters, generally made of woven PVDF monofilament, are lightweight, less costly and easy to handle and install. While not as temperature tolerant as stainless steel, PVDF filter fabrics nevertheless withstand operating temperatures up to 275 degrees F. PVDF filter fabrics are not quite as smooth as stainless steel mesh since the brittle skin of the monofilament is frequently fibrillated during both the weaving as well as in actual use. As in the case of stainless steel, the most serious disadvantage of PVDF is the adherence of sticky components from the pulp slurry to the monofilament yarns of the fabric. The hairy fibrils may exacerbate this adhesion, which eventually leads to the plugging and shortened life of the filter. SUMMARY OF THE INVENTION It is an object of the present invention to provide a filtering medium particularly suited for use in vacuum separators of the type used in pulp and paper making processes and apparatus. It is a further object of the present invention to provide a filtering medium exhibiting an increase in useful temperature and pH range as compared to prior filtering mediums used in paper making processes . It is a still further object of the present invention to provide a filtering medium for use in paper making apparatus or the like which exhibits an extended useful life and is less susceptible to plugging of openings as compared to prior media used for that purpose. In accordance with a preferred embodiment of the present invention, a filtering medium suited for use on a rotary drum vacuum filter for processing pulp comprises a fabric comprising extruded monofilament E-CTFE fiber, preferably woven of fibers of approximately 12 mil diameter. DESCRIPTION OF DRAWING Figure 1 is a plot of test results for two filter media, a prior art filter made of KYNAR^® PVDF filter material (manufactured by ATOCHEM) and a filter made of HALAR^® E-CTFE fiber according to the present invention (manufactured by Ausi ont, U.S.A.) . DESCRIPTION OF PREFERRED EMBODIMENT. A filter medium suitable for use in vacuum separators of the type used in paper making processes comprises a fabric woven of extruded monofilament E-CTFE. The fabric is wrapped around a conventional vacuum drum assembly. In order to provide an increase in both the useful temperature and pH range of the filter, fabric constructions similar to PVDF filter fabric was woven of HALAR^® E-CTFE polymer and tested. The HALAR and KYNAR filter fabrics which were tested were made with identical yarn size and fabric construction. The fabrics were KYNAR and a HALAR Twinflex filtration fabric made by Barrday, Inc. of Cambridge, Ontario, Canada. Twinflex weave is a design developed by Barrday in which face and backing fabrics are woven together. The face side of the fabric is a plain weave and provides the filtration function. In plain weave, each weft yarn passes successively over and under each warp yarn with each row alternating. The back side of the fabric is a leno weave and it adds strength and stability to the fabric. The size of the yarn is 12 mil (12/1000") in both cases. Mesh opening is the distance between two adjacent yarns expressed in "mils." Open area in a fabric is an indication for straight through drainage. The open area of a fabric is calculated as follows: Open area (%)= [l-(warp count) x (warp diameter)] x [l-(weft count) x (weft diameter) ] x 100. The fabric specifications were as follows;

Style MFN-2801 MFN-3901

Material Kynar Halar

Weave Dual Layer Dual Layer

Thread Count 42 Warp x 30 42 Warp x 30 Face Cloth Fill Fill

Yarn Diameter 12 Warp X 12 12 Warp x 12 (mil) Fill Fill

Mesh Opening 12 Warp, 12 Warp, (mil) 21 Fill, 21 Fill, 24 Diagonal 24 Diagonal

% Open Area 32 32 Shrink 10 to 12% Warp 10 to 12% Warp 2 to 3% Fill 2 to 3% Fill

Flow Rate xxxx GPM, xxxx GPM, CFM > 1000 CFM > 1000

Weight 18.5 oz/yd² 18.5 oz/yd 12² pH Operating 1 through 14 1 through 14 Range

Maximum 265'F, 129°C 300°F, 150°C

Operating

Temperature

The capability of higher temperature range was verified in actual field use, as might be expected from the higher melting point of HALAR^® E-CTFE polymer (464 degrees F) as compared to PVDF.

The useful pH range was also broadened by converting to a fabric constructed wholly of HALAR^® E-CTFE polymer.

To verify proper flow through of process liquids and filtration efficiency, identical fabric weave constructions of HALAR^® E-CTFE and PVDF were also compared in short-term testing and indeed, no significant differences were found. Thus assured that the conversion to an identical fabric construction made of HALAR^® E-CTFE could be made to take advantage of the higher temperature resistance and the wider pH tolerance offered by HALAR^® E-CTFE, the change was made and side-by-side long term field testing was initiated. Surprisingly, however, these actual field trials revealed an unexpected phenomena. Referring to Figure 1, the relative liquid flow rates (initial flow rate taken as 100%) over a period of twenty four weeks was plotted for similar filter made of Ausimont HALAR^® E-CTFE fiber and ATOCHEM KYNAR^® PVDF fiber. As the exposure time for the two fabrics lengthened, the PVDF fabric construction began to plug, or blind at a rate consistent with typical performance of PVDF fabric weaves in this end use and under these conditions. The HALAR^® fiber, however, maintained a surprisingly higher flow through rate, essentially exhibiting very little change in performance. These actual field test data, and the slopes of the flow-through performance curves are depicted graphically in Figure 1. This data shows that the same sticking phenomena that plugs PVDF and stainless steel filter fabrics is substantially minimized or eliminated when a HALAR^® fiber is employed, even under the same use conditions. This surprising result was not indicated by initial flow-through measurements, nor would this behavior have been predictable from the chemistry of the two fabric constructions. This unexpected degree of less plugging substantially extends the useful life of the E-CTFE filter fabrics. This result was completely unexpected since both the closely chemically related E-CTFE and PVDF fluoropolymers have very similar surface energy and coefficients of friction (drag) . While the monofilament yarns of the E-CTFE fabrics did have less fibrils on the surface (E-CTFE is less brittle than PVDF) , this is believed to have only a minor effect. To better understand this unexpected result, the tested fabric constructions were then dried and the fiber surface morphology was examined by Atomic Force Microscopy. This very advanced technique of examining surface roughness revealed new information not normally available to those skilled in this art. It was found that the E-CTFE fiber surfaces were discernibly smoother and they thus provide much fewer sites than the PVDF filaments for particle anchorage and buildup. The reason for the ultra-smooth extruded surface of HALAR^® E-CTFE monofilament is still not completely understood. However, it is clear that the HALAR yarn surfaces would be much easier to keep free of contamination buildup. In turn, this would maintain the E-CTFE woven fabric filter effectiveness while greatly extending its useful life.

Claims

WHAT IS CLAIMED IS;

1. A filtering medium having extended life comprising a fabric comprising a plurality of fibers, wherein at least the surface of said fibers comprises a coating of the copolymer ethylene chlorotrifluoroethylene (E-CTFE) .

2. A filtering medium according to Claim 1, wherein said fibers are monofilament fibers formed from extruded E-CTFE.

3. A filtering medium according to Claim 2, wherein said fibers are woven to provide a filter fabric having at least one layer.

4. A filtering medium according to Claim 3, wherein said filter fabric has a front layer and a back layer, said front layer being formed by a plain weave having warp and weft threads, wherein each weft thread is _ passed successively over and under each warp thread with each row alternating, and said back layer being formed by a leno weave, and wherein said filter fabric is formed by weaving said front layer and said back layer together in a double weave.

5. A filtering medium according to Claim 4, wherein said filter fabric has an open area according to the following formula: Open Area = [1- (Warp Count X Warp Diameter in mils)] X [1- (Weft Count X Weft Diameter in mils) ] .

6. A filtering medium according to Claim 5, wherein said filter fabric has an open area of less than fifty percent .

7. A filtering medium according to Claim 5, wherein said filter fabric has an open area of about one- third.

8. A filtering medium according to Claim 5, wherein said front layer has a warp count greater than its weft count of threads and mesh openings substantially equal to the diameter of individual fibers.

9. A filtering medium according to Claim 8, wherein said fibers are between about 8 mils and 30 mils in diameter.

10. A filtering medium according to Claim 9, wherein said fibers are approximately 12 mils in diameter.

11. A filtering medium according to Claim 9, wherein said filter fabric is wrapped around a vacuum drum assembly.

12. A filtering medium having extended life for use in pulp and paper making vacuum separator apparatus comprising a fabric comprising a plurality of fibers, wherein at least the surface of said fibers comprises a coating of the copolymer E-CTFE.

13. A filtering medium according to Claim 12, wherein said fibers are monofilament fibers formed from extruded E-CTFE.

14. A filtering medium according to Claim 13, wherein said fibers are woven to provide a filter fabric having at least one layer.

15. A filtering medium according to Claim 14, wherein said filter fabric has a front layer and a back layer, said front layer being formed by a plain weave having warp and weft threads, wherein each weft thread is passed successively over and under each warp thread with each row alternating, and said back layer being _ formed by a leno weave, and wherein said filter fabric is formed by weaving said front layer and said back layer together in a double weave.

16. A filtering medium according to Claim 15, wherein said filter fabric has an open area according to the following formula: Open Area = [1- (Warp Count X Warp Diameter in mils) ] X [1- (Weft Count X Weft Diameter in mils) ] .

17. A filtering medium according to Claim 16, wherein said filter fabric has an open area of less than fifty percent .

18. A filtering medium according to Claim 16, wherein said filter fabric has an open area of about one- third.

19. A filtering medium according to Claim 16, wherein said front layer has a warp count greater than its weft count of threads and mesh openings substantially equal to the diameter of individual fibers .

20. A filtering medium according to Claim 19, wherein said fibers are between about 8 mils and 30 mils in diameter.

21. A filtering medium according to Claim 20, wherein said fibers are approximately 12 mils in diameter.

22. A filtering medium according to Claim 20, wherein said filter fabric is wrapped around a vacuum drum assembly.