GB2162080A

GB2162080A - Filtering

Info

Publication number: GB2162080A
Application number: GB08516246A
Authority: GB
Inventors: Henry A Fremont; Mary C Stiffler
Original assignee: Champion International Corp
Current assignee: Champion International Corp
Priority date: 1984-07-26
Filing date: 1985-06-27
Publication date: 1986-01-29
Also published as: FR2568140A1; GB2162080B; GB8516246D0; FR2568140B1; DE3523998A1; DE3523998C2; JPS6138609A; CA1260848A

Abstract

A reticulated hydrophobic filter medium, in the form of a moving belt or covering on an inward-flow rotary drum, has a thickness of 3 mm or more and has 16 or more pores per linear cm. Filter cake is removed from the medium by reverse flow, which can be induced by a suction nozzle 26, with or without flexible shroud 26a, possibly downstream of a couch roll. Alternatively the couch roll itself can apply the suction and be located on the opposite side of the medium from an air or water delivery jet. <IMAGE>

Description

SPECIFICATION Retrieving Solid Material From a Suspension The invention relates to a method of treating suspensions of solid material in liquid to separate a substantial portion of the solid material from the liquid.

The use of a hydrophobic foam filter to separate solid material from a stream has been described in U.S. Patent Nos. 4,310,424; 4,303,533 and U.S. Patent Application Serial No. 564,720 filed December 23, 1983, U.S. Patent No. 4,212,737 relies on and teaches hydrophobic foam ratherthan hydrophilic. It has previously been proposed to use a couch roll, shower, or other mechanical means to retrieve the solid material from the foam surface. Such mechanical methods have the potential for causing compaction of the solid material within the reticulated foam structure due to the pressure exerted on the surface. The ultimate result can be a reduction in flow through the filtering medium.

According to the invention there is provided a method of treating suspensions of solid material in liquid to separate a substantial portion of the solid material from the liquid which includes maximizing volumetric flow of the liquid through an open-celled reticulated hydrophobic foam separation medium and maximizing said separation for a sustained period, the method comprising: providing an open-celled reticulated hydrophobic foam separation medium of a thickness of 3.2 mm (one-eighth inch) or greater and a porosity in excess of 15.7 pores per lineal cm (forty pores per lineal inch); subjecting the foam separating medium to a pressure differential and to a stream of the suspension thereby to create a flow of liquid into and through the medium; a deposit of a major part of the solid material on the surface of the medium and a deposit of at least some of the remaining part in the reticulated foam structure; and retrieving at least some of the solid material from the foam surface and from the reticulated structure by entraining solid material in an outwardly directed fluid moving transversely in contact with the medium.

The method of the invention can minimize build-up of solid material on the surface of the medium and within the reticulated foam structure and, therefore, maintain the volumetric flow rate.

Typically, the liquid feed stream containing suspended solid material is introduced into a tank containing a rotary drum covered with an open-celled hydrophobic foam. Regardless of the specific foam medium moving support apparatus, the stream is filtered as it passes through the foam, leaving some or all of the solid material on the surface, and any remainder trapped within the reticulated foam structure. Clean filtrate is withdrawn from the interior of the drum or belt structure and either reused or discarded. Any material trapped within the foam can be retrieved by expressing methods described in U.S. Patent Nos.

4,303,533 and 4,310,424.

In the method of the invention solid material is retrieved from the foam surface and reticulated foam structure by entraining at least some of the solid material in fluid passed through the foam medium.

Embodiments of the method include: (1) vacuuming the material from the foam surface and reticulated foam structure by entrainment in a transversely directed fluid, (2) using a couch roll to remove the majority of solids from the foam surface and then vacuuming the surface and reticulated structure to remove the remainder of the solids, (3) using a vacuum nozzle or vacuum couch roll to remove solids from the surface and the reticulated foam structure, (4) using a water or air spray from inside the foam medium moving support apparatus to loosen and entrain remaining solids after first removing the major part of the solid material by means of a vacuum nozzle or couch roll.

In each case, it is important that the mechanical element, at least with regard to its rigidly fixed parts, be spaced so as to have only minimum contact, if any, with the foam medium. The moving and loaded foam medium inherently resists geometric integrity and may have areas which are variably spaced from the retrieving apparatus. If the foam medium has significant regular contact with the rigidly fixed parts of the retrieving apparatus it will unduly abrade, distort and otherwise impair its structural integrity.

In the case of a couch roll or a vacuum couch roll, a moving fibre mat on the foam surface can provide spacing, even if there is some minimal foam compression under the roll. In the case of a vacuum nozzle, for instance, a flexible shroud may extend from the nozzle and contact the foam to enhance entrainment.

The high capacity of the filter, i.e., maximum volumetric flow rate, can be maintained through efficient, continuous, and thorough retrieval of solid material from the filtration surface and reticulated structure, thus minimizing any tendency for build-up or "plugging" of solid material which would reduce capacity.

Selection of various combinations of retrieval methods allows the solids consistency to be tailored to meet the needs of the specific application. For example, solids can be retrieved from the foam at 1~4% consistency using vacuum only and up to 19% consistency using a couch roll/vacuum combination.

The invention is diagrammatically illustrated by way of example in the accompanying drawings, in which:~ Figure 1 is a schematic elevational view of a drum kind of foam separation device for carrying out the method of the invention with a vacuum nozzle for retrieving solid material from a foam surface and reticulated foam structure; Figure la is an enlarged fragmentary view of a portion of Figure 1 showing a flexible shroud on the vacuum nozzle; Figure 2 is a view similar to Figure 1 of a belt kind of foam separation device for carrying out the method of the invention with a couch roll followed by an air spray from inside the drum; and Figure 3 is a view similar to Figure 1 of a drum kind of foam separation device.

Figures 1 to 3 illustrate foam media separation devices or filters for use on a pulp or paper process stream. The devices can advantageously be utilized to reclaim and thicken fibre in streams with either occasional or continuous high fibre loading in a manner to be described hereinafter. Like numerals identify like parts in the various views.

A separation and thickening device 10 shown in Figure 1 includes a rotary drum 12 having a thin layer of open-celled resilient hydrophobic foam 14 in a thickness range of from 3.2 mm to 50.8 mm (1/8-inch to 2 inches), preferably approximately 25.4 mm (1 inch), and a porosity in the range of from 15.7 pores per lineal cm to 31.2 pores per lineal cm (40 pores per lineal inch to 80 pores per lineal inch), preferably in the range from 25.6 to 31.2 pores per cm. Other characteristics of the foam are as described in U.S. Patent Nos.

4,303,533 and 4,310,424.

The feed stream or pulp slurry is a aqueous solution of cellulosic pulp which routinely flows from the pulpmaking or papermaking processes and which contains desirable fibre, small contaminants and small undesirable fibre parts. The feed stream is introduced into a tank 16 containing the rotary drum 12 by means of an inlet pipe 18. The stream is filtered as it passes through the foam 14, leaving the majority of the larger desirable fibre 20 on the foam surface. The foam passes the majority of the contaminants and small fibre parts with a liquid filtrate 22 which flows from the drum interior through outlet pipe 24. The difference in height between the outer liquid level in the tank 16 and the inner liquid level of the filtrate 22 produces a pressure differential AP.

The drum type separation device of Figure 1 is provided with a vacuum nozzle retrieval device 26 which removes the fibre mat or major part of the solids from the foam surface and remaining fibre particles from the reticulated foam structure as fluid air and residual water in the foam pass transversely through the foam and entrain the particles in their flow stream for movement outwardly toward and into the vacuum nozzle.

Showers can be used subsequently to clean the foam surface, if desired.

The vacuum nozzle retrieval device 26 deposits the solids and residual liquid associated therewith in a separator 27 connected by means of a pipe 28 to a blower 29 and an air outlet 29a. The solids, which are typically fibre of a consistency of from 1~4% (1% equals 10,000 ppm) in the embodiment of Figure 1, are removed from the separator 27 through an outlet pipe 30. The blower 29 typically provides approximately 20.32 to 30.48 cm (8 to 12 inches) water of vacuum at the nozzle.

In comparison with other drum filters, the separation device of Figure 1 offers the following advantages: 1. Significantly higher sustained area flow rates of the nature of 10.2x10-3 to 20.4x 10-3/m2/sec (15 to 30 gal./ft.2/min) compared to the 6.8x 1 #-# to 13.6x10-3 m3/m2/sec (10 to 20 galdft.2/min.) achievable with a standard or typical wire or perforated plate gravity decker or vacuum thickener; 2. The potential for sustained fibre consistencies of the nature of 12 to 19 percent compared to 12 percent maximum obtained with typical wire or perforated plate deckers or thickeners, when multiple retrieving devices are used.

These advantages are believed to result for reasons discussed in U.S. Patent Application Serial No.

564,720.

The inherent resistance flow of the typical thickener wire is higher than that of the foam within the disclosed porosity and thickness ranges, and these ranges due to their porosity and their void volume provide increased fluid takeaway and driving force throughout the process over that obtained in wire covered thickeners. The present invention permits the flow to be sustained.

Figure 1a illustrates a portion of the Figure 1 apparatus with the attachment of a flexible shrould 26a attached to the rigid vacuum nozzle. This prevents the rigid nozzle structure from damaging contact with the foam but permits an enhanced vacuum entrainment by minimizing extraneous fluid flow and directing the major fluid flow transversely through the foam. The flexible shroud 26a does little, if any, damage to the foam.

Figure 2 illustrates a foam belt filter device 110 which utilizes a foam separation medium 114 having the proper porosity and thickness. The foam medium 114 is rotationally driven about rotating members 112a and 11 2b. The schematic drawing of the device 110 shows that a feed stream tank 116 deposits a feed stream on the foam belt 114 and a vacuum draws filtrate 122 into a reservoir 123 for removal through an outlet pipe 124. The deposited fibre mat 120 is retrieved by a doctor 126 from the surface of a couch roll 125.

A vacuum nozzle 128 is provided to clean the foam belt surface and reticulated structure after retrieval. A separator 227, an air outlet 128a and an outlet pipe 130 for the fibre complete the illustrated combination. A blower (not shown) is connected as shown in Figure 1 to provide vacuum at the nozzle 128.

Figure 3 illustrates an apparatus 210 similar to the Figure 1,apparatus 10, with a tank 216, a drum 212 and foam 214. The apparatus 210 includes a vacuum couch roll 226 from which a doctor 226a removes the fibre for a gravity drop onto a chute 231 and into a tank 232 with a fibre outlet pipe 230. A fluid nozzle 240 directs a fluid, which is air, water or both, transversely through the rotating foam medium 214 to entrain solids on the surface and in the reticulated structure not picked up by the vacuum couch roll 226 and to deposit them on the chute 231.

By use of the retrieval apparatus described the advantages of foam filtration can be obtained for substantial periods of time with minimal loading or "plugging" of the reticulate structure of the foam.

A typical foam filtration test run on an alkaline bleach sewer, using a vacuum nozzle apparatus of the type illustrated in Figure 1, with 25.4 mm (one inch) 25.6 ppcm (65 ppi) stiff foam, on a drum turning at 2.5 RPM, produced the following data: Elapsed time: 47 hours (foam use) Flow (rate in): 397x10-5 m3/sec (63 gal./min.) Flow (rate through): 18.9 x 10~3 m3/m2/sec (effective area 27.9 gal./ft.2/min.) Vacuum: 20.32 cm (8 in.) (water AP) Stream: TSS ASH TS T PH (PPM) (%) (PPM) Feed: 294 14 5,092 46.6 9.6 Filtrate: 80 40 4,812 46.6 9.6 Fibre: 15,558 2 19,636 46.6 9.6 Legend:TSS=total suspended solids TS=total solids T=temperature ( C) Running the foam filtration apparatus with couch roll retrieval only can obtain fibre consistency up to 19% but this cannot be sustained easily without the use of vacuum or other fluid entrainment to remove material from the reticulated foam structure and thereby prevent loading or "plugging".

Another test run on paper machine white water using a vacuum nozzle similar to the apparatus of Figure 1 as a "saveall", with 25.4 mm (1 inch) 31.2 ppcm (80 ppi) stiff foam and a drum turning at 3.5 RPM, produced the following data: Elapsed time: 43 hours (foam use) Flow (rate in): 549x10-5 m3/sec (87 gal./min.) Flow (rate through): 1.90x 10-3 m3/m2/sec (effective area 2.8 gal./ft.2/min.) Vacuum: 15.2 cm (6 in.) (waterAP) Stream: TSS ASH TS T PH (PPM) (%) (PPM) Feed: 3,353 23 4,670 37.7 5.8 Filtrate: 27 29 1,369 37.7 5.8 Fibre: 38,084 20 39,950 - 6.0 This was the highest fibre recovery of the test using only a vacuum nozzle and approached 4%. The fibre at this consistency can be directly recycled to the paper machine system.Obviously, the couch roll and vacuum combination, would be the most effective way to retrieve and thicken fibre from the foam medium up to 19% consistency for storage or disposal.

A more typical run as a "saveall" using the same white water feed stream and foam medium, with the drum turning at 2.0 RPM, produced the following data: Elapse time: 24 hours (foam use) Flow (rate in): 1 10-5 m3/sec (178 gal./min.) Flow (rate through): 4.1x10-3 m3/m2/sec (effective area 6.0 gal./ft.2lmin.) Vacuum: 20.32 cm (8 in.) (water AP) Stream: TSS ASH TS T PH (PPM) (%) (PPM) (F.) Feed: 2,594 15 3,574 27.7 5.3 Filtrate: 67 63 1,537 27.7 5.4 Fibre: 24,912 13 26,372 27.2 5.1 Recovery of suspended solids from pulp mill process streams using the foam filter fibre retrieval apparatus of the invention can be sustained for many hours. This is significant because these systems typically handle between 6.5 and 20.5 tons of fibre per day. Analysis of the recovered fibre, including freeness, viscosity, permanganate number, and fibre length, shows they are all typically in the normal rangefordesirable and useablefibre.

Claims

1.A method of treating suspensions of solid material in liquid to separate a substantial portion of the solid material from the liquid which includes maximizing volumetric flow of the liquid through an open-celled reticulated hydrophobic foam separation medium and maximizing said separation for a sustained period, the method comprising: providing an open-celled reticulated hydrophobic foam separation medium of a thickness of 3.2 mm (one-eighth inch) or greater and a porosity in excess of 15.7 pores per lineal cm (forty pores per lineal inch); subjecting the foam separating medium to a pressure differential and to a stream of the suspension thereby to create a flow of liquid into and through the medium; a deposit of a major part of the solid material on the surface of the medium and a deposit of at least some of the remaining part in the reticulated foam structure: and retrieving at least some of the solid material from the foam surface and from the reticulated structure by entraining solid material in an outwardly directed fluid moving transversely in contact with the medium.

2. A method according to claim 1, in which the fluid moves within the reticulated structure.

3. A method according to claim 2, in which the fluid moves through the reticulated structure.

4. A method according to any one of claims 1 to 3, in which the fluid is moved by means of a vacuum nozzle spaced from the medium.

5. A method according to claim 4, in which the vacuum nozzle is rigid and carries a flexible shroud in contact with the part of the solid material on the surface of the medium.

6. A method according to any one of claims 1 to Sin which the part of the solid material on the surface of the medium is substantially removed by a couch roll before the fluid is moved transversely and directed outwardly and in contact with the medium.

7. A method according to claim 6, in which a doctor is used to remove the solid material from the couch roll and the fluid which entrains solid material remaining on the medium surface and within the medium reticulated structure acts after the couch roll.

8. A method according to claim 2, in which a vacuum couch roll directs the fluid outwardly from within the medium reticulated structure.

9. A method according to claim 6, in which the fluid is provided from within a rotating structure supporting the medium.

10. A method according to claim 1, in which vacuum is applied although a couch roll to move the fluid.

11. A method according to claim 1, in which the medium is mounted on an endless belt.

12. A method of treating suspensions of solid material in liquid to separate a substantial portion of the solid material from the liquid as claimed in claim 1 and substantially as hereinbefore described.